Recycling of polymers. "Polymer Recycling in Europe: New and Proven Solutions". Standard list of equipment for a waste processing plant

Ministry of Education of the Republic of Belarus

educational institution

"Grodno State University named after Yanka Kupala"

Faculty of Construction and Transport

Test

in the discipline "Technology of materials"

Processing of polymers and polymeric materials

A polymer is an organic substance whose long molecules are built from the same repeatedly repeating units - monomers.

Rice. 1. Scheme of the structure of the polymer macromolecule:

a) - chain-like molecules; b) - lateral connections

Possessing the ability under certain conditions to sequentially connect with each other, the monomers form long chains (Fig. 1) with linear, branched and network bond structures, resulting in polymer macromolecules.

By origin, polymers are divided into three groups:

Natural are formed as a result of the vital activity of plants and animals and are contained in wood, wool, and leather. These are protein, cellulose, starch, shellac, lignin, latex. Typically, natural polymers are subjected to isolation, purification, modification, in which the structure of the main chains remains unchanged. The product of such processing are artificial polymers. Examples are natural rubber, made from latex, celluloid, which is nitrocellulose plasticized with camphor to increase elasticity.

Natural and artificial polymers have played a large role in modern technology, and in some areas they remain indispensable to this day, for example, in the pulp and paper industry. However, a sharp increase in the production and consumption of organic materials occurred due to synthetic polymers - materials obtained by synthesis from low molecular weight substances and having no analogues in nature. Synthetic polymers are obtained during the processing of coal, natural and industrial gas, oil and other raw materials. According to the chemical structure, polymers are divided into linear, branched, network and spatial.

Depending on the change in properties during heating, polymers are divided into two main groups: thermoplastic and thermosetting. The first of them are formed on the basis of novolac resins, and the second - on the basis of resole resins.

1. Thermoplastic polymers (thermoplastics) soften when heated, turning first into a highly elastic, and then into a viscous-fluid state; when cooled, they harden. This process is reversible, that is, it can be repeated many times. Thermoplastics include polymers with a linear and branched bond structure; their monomers are linked to each other in only one direction. When reheated, such chemical bonds are not destroyed; monomer molecules acquire flexibility and mobility. Products are made from thermoplastics by pressing, injection molding, continuous extrusion (extrusion) and other methods. The most common thermoplastics are polymerization materials (polyethylene, polypropylene, polyvinyl chloride, polystyrene, fluoroplasts, etc.) and polycondensation materials (polyamide, polyurethane, anilino-formaldehyde, phenol-formaldehyde resins, etc.), produced in the form of powders, crumbs, sheets, rods, pipes, etc.

2. Thermosetting polymers (thermosets) when heated, first soften if they were solid, and then turn into a solid state. This process is irreversible, i.e., when reheated, such polymers do not soften. Thermoplastics include polymers with a network or cross-linked bond structure. Such polymers form two- or three-dimensional bonds in giant macromolecules; their monomers or linear molecules are rigidly linked to each other and are not able to move mutually. The most common thermoplastics are polycondensation materials - phenolic plastics obtained on the basis of phenol-formaldehyde, polyester, epoxy and urea resins. Parts and products made of thermoplastics are obtained by hot pressing, injection molding, and machining.

Currently, plastic products are produced by a wide variety of methods. At the same time, the choice of a method for manufacturing products is determined by the type of polymer, its initial state, as well as the configuration and dimensions of the product.

The main task in the processing of polymeric materials is to slow down negative processes and create the necessary structure of the material. The simplest methods for achieving this goal are the regulation of temperature, pressure, heating and cooling rates of the material. In addition, stabilizers are used that increase the resistance of the material to aging, plasticizers that reduce the viscosity of the material and increase the flexibility of molecular chains, as well as various fillers.

Before moving on to a discussion of various methods for processing polymers, let me remind you that polymer materials can be thermoplastic or thermoset (thermoset). Once thermoplastic materials have been molded under heat and pressure, they must be cooled below the softening temperature of the polymer before they are released from the mold, otherwise they will lose their shape. In the case of thermosetting materials, this is not necessary, since after a single combined exposure to temperature and pressure, the product retains its acquired shape even when it is released from the mold at high temperature.

When processed into products, thermoplastics are exposed to heat, mechanical pressure, atmospheric oxygen and light. The higher the temperature, the more plastic the material and the easier it is to process. However, under the influence of high temperatures and the factors mentioned above, chemical bonds break in polymers, oxidation, the formation of new undesirable structures, the movement of individual sections of macromolecules and macromolecules relative to each other, the orientation of macromolecules in different directions, and the strength of the material in the direction of orientation increases, and in the transverse direction decreases. When obtaining films and thin-walled products, this phenomenon plays a positive role; in all other cases, it causes structural inhomogeneity and causes residual stresses.

The peculiarity of the processing of thermosets into products is the combination of molding processes with curing, i.e., with chemical reactions for the formation of a cross-linked structure of macromolecules. Incomplete curing degrades material properties. Achieving the required completeness of curing even in the presence of catalysts and at elevated temperatures requires a significant amount of time, which increases the complexity of manufacturing the part. The final curing of the material can take place outside the forming tooling, as the product acquires a stable shape before this process is completed.

When processing composite materials, the adhesion (adhesion) of the binder with the filler is of great importance. The adhesion value can be increased by cleaning the surface of the filler and making it reactive. With poor adhesion of the binder to the filler, micropores appear in the material, which significantly reduce the strength of the material.

The difference in the cross section of the product in cooling rates, in the degree of crystallization, the completeness of the relaxation processes for thermoplastics and the degree of curing for thermoplastics also leads to structural heterogeneity and the appearance of additional residual stresses in products. To reduce residual stresses, heat treatment of products, structure formation during processing, and other technological methods are used.

The ever-increasing volume of production of plastics requires further improvement of existing and development of new high-performance technological processes for processing polymers. Further progress in the field of plastics processing is associated with a sharp increase in the productivity of processing equipment, a reduction in labor intensity in the production of products and an increase in their quality. The solution of the tasks set is impossible without the use of new progressive methods of processing, which include various types of processing of polymers by pressure in a solid state of aggregation.

All processes of processing polymers in the solid state are based on plastic (forced elastic) deformation, which is reversible. Forced elastic deformations in polymers develop under the influence of high mechanical stresses. After the termination of the deforming force, at temperatures below the softening temperature, the forced elastic deformation is fixed as a result of glass transition or crystallization of the material, and the deformed polymer body does not restore its original shape.

INTRODUCTION

Polymer molecules are an extensive class of compounds, the main distinguishing characteristics of which are high molecular weight and high conformational flexibility of the chain. It can be said with confidence that all the characteristic properties of such molecules, as well as the possibilities of their application associated with these properties, are due to the above features.

In our urbanized, rapidly developing world, the demand for polymeric materials has increased dramatically. It is difficult to imagine the full-fledged operation of factories, power plants, boiler houses, educational institutions, electrical household appliances that surround us at home and at work, modern computers, cars and much more without the use of these materials. Whether we want to make a toy or create a spaceship - in both cases, polymers are indispensable. But how can the polymer be given the desired shape and appearance? To answer this question, let us consider another aspect of polymer technology, namely, their processing, which is the subject of this work.

In a broad sense, polymer processing can be viewed as a kind of engineering specialty involved in the transformation of raw polymer materials into the required end products. Most of the methods currently used in polymer processing technology are modified analogs of methods used in the ceramic and metal processing industries. Indeed, we need to understand the ins and outs of polymer processing in order to replace common traditional materials with other materials with improved properties and appearance.

About 50 years ago, there was a very limited number of processes for processing polymers into end products. Currently, there are many processes and methods, the main ones are calendering, casting, direct compression, injection molding, extrusion, blow molding, cold forming, thermoforming, foaming, reinforcement, melt forming, dry and wet forming. The last three methods are used to produce fibers from fiber-forming materials, and the rest are used to process plastic and elastomeric materials into industrial products. In the following sections, I have tried to give a general overview of these important processes. For more detailed information on these and other processes, such as dip and fluidized swirl coating, electronic and thermal sealing, and welding, refer to specific textbooks on polymer processing. Also outside the scope of this abstract are issues related to coatings and adhesives.

Before proceeding directly to the consideration of methods and methods for processing polymers into final products, it is necessary to find out: what are polymers, what they are and where they can be used, i.e. what end products can be obtained from polymers? The role of polymers is very great and we must understand the need for their processing.

1. POLYMERS AND POLYMER MATERIALS

1.1 GENERAL CHARACTERISTICS AND CLASSIFICATION

A polymer is an organic substance whose long molecules are built from the same repeatedly repeating units - monomers. By origin, polymers are divided into three groups.

Natural are formed as a result of the vital activity of plants and animals and are found in wood, wool, and leather. These are protein, cellulose, starch, shellac, lignin, latex.

Typically, natural polymers are subjected to isolation, purification, modification, in which the structure of the main chains remains unchanged. The products of this processing are artificial polymers. Examples are natural rubber, made from latex, celluloid, which is nitrocellulose plasticized with camphor to increase elasticity.

Natural and artificial polymers have played a large role in modern technology, and in some areas they remain indispensable to this day, for example, in the pulp and paper industry. However, a sharp increase in the production and consumption of organic materials occurred due to synthetic polymers - materials obtained by synthesis from low molecular weight substances and have no analogues in nature. The development of chemical technology of macromolecular substances is an integral and essential part of modern scientific and technological revolution . Not a single branch of technology, especially new ones, can do without polymers. According to the chemical structure, polymers are divided into linear, branched, network and spatial.

molecules linear polymers are chemically inert with respect to each other and are interconnected only by van der Waals forces. When heated, the viscosity of such polymers decreases and they are able to reversibly transform first into a highly elastic and then into a viscous flow state (Fig. 1).

Fig.1. Schematic diagram of the viscosity of thermoplastic polymers depending on temperature: T 1 - the transition temperature from the glassy to the highly elastic state, T 2 - the transition temperature from the highly elastic to the viscous state.

Since the only effect of heating is a change in plasticity, linear polymers are called thermoplastic. It should not be thought that the term "linear" means straight, on the contrary, they are more characteristic of a serrated or helical configuration, which gives such polymers mechanical strength.

Thermoplastic polymers can not only be melted, but also dissolved, since van der Waals bonds are easily torn under the action of reagents.

branched(grafted) polymers are stronger than linear ones. Controlled chain branching is one of the main industrial methods for modifying the properties of thermoplastic polymers.

mesh structure characterized by the fact that the chains are connected to each other, and this greatly limits the movement and leads to a change in both mechanical and chemical properties. Ordinary rubber is soft, but when vulcanized with sulfur, covalent bonds of the S-0 type are formed, and the strength increases. The polymer can acquire a network structure and spontaneously, for example, under the action of light and oxygen, aging occurs with a loss of elasticity and performance. Finally, if the polymer molecules contain reactive groups, then when heated, they are connected by many strong cross-links, the polymer turns out to be cross-linked, i.e., it acquires spatial structure. Thus, heating causes reactions that dramatically and irreversibly change the properties of the material, which acquires strength and high viscosity, becomes insoluble and infusible. Due to the high reactivity of molecules, which manifests itself with increasing temperature, such polymers are called thermosetting.

Thermoplastic polymers are obtained by the reaction polymerization, flowing according to the scheme pmm p(Fig. 2), where M - monomer molecule, M p- a macromolecule consisting of monomer units, P - degree of polymerization.

During chain polymerization, the molecular weight increases almost instantly, the intermediate products are unstable, the reaction is sensitive to the presence of impurities and, as a rule, requires high pressures. It is not surprising that such a process is impossible under natural conditions, and all natural polymers were formed in a different way. Modern chemistry has created a new tool - the polymerization reaction, and thanks to him a large class of thermoplastic polymers. The polymerization reaction is realized only in complex equipment of specialized industries, and the consumer receives thermoplastic polymers in finished form.

Reactive molecules of thermosetting polymers can be formed in a simpler and more natural way - gradually from monomer to dimer, then to trimer, tetramer, etc. Such a combination of monomers, their "condensation", is called the reaction polycondensation; it does not require high purity or pressures, but is accompanied by a change in the chemical composition, and often by the release of by-products (usually water vapor) (Fig. 2). It is this reaction that occurs in nature; it can be easily carried out with only a little heating in the simplest conditions, even at home. Such high manufacturability of thermosetting polymers provides ample opportunities to manufacture various products at non-chemical enterprises, including radio plants.

Regardless of the type and composition of the starting materials and production methods, materials based on polymers can be classified as follows: plastics, fibre-reinforced plastics, laminates, films, coatings, adhesives. I will not particularly focus on all these products, I will only talk about the most widely used ones. It is necessary to show how great the need for polymeric materials is in our time, and, consequently, the importance of their processing. Otherwise the problem would be simply unfounded.

1.2 PLASTICS

The word "plastic" comes from the Greek language and refers to a material that can be pressed or molded into any shape you choose. According to this etymology, even clay could be called plastic, but in reality only products made from synthetic materials are called plastics. The American Society for Testing and Materials defines what plastic is as follows: "is any member of a wide variety of materials, wholly or partly organic in composition, which can be shaped into desired shape by the application of temperature and/or pressure."

Hundreds of plastics are known. In table. 1 shows their main types and shows individual representatives of each of the species. It should be noted that at present there is no single way to describe the entire variety of plastics due to their large number.

Table 1. Main types of plastics

Type of	Typical representatives	Type of	Typical representatives
Acrylic plastics Aminoplastics	Polymethylmethacrylate (PMMA) Polyacrylonitrile (PAN) Urea-formaldehyde resin Melamine-formaldehyde resin	Polyesters	Unsaturated polyester resins Polyethyl terephthalate (PET) Polyethyl snadipate
Cellulose	Ethylcellulose Cellulose acetate Cellulose nitrate	Polyolefins Styrene plastics	Polyethylene (PE) Polypropylene (PP) Polystyrene (PS)
Epoxy resins	Epoxy resins	Copolymer of styrene with acrylonitrile
Fluoroplastics	Polytetrafluoroethylene (PTFE) Polyvinylidene fluoride	Copolymer of acrylonitrile with styrene and butadiene (ABS)
Phenoplasts	Phenol-formaldehyde resin Phenol-furfural resin	Vinyl plastics	Polyvinyl chloride (PVC) Polyvinyl butyral
Polyamide plastics (nylons)	Polycaprolactam (PA-6) Polyhexam ethylenadipamide (PA-6,6)	Vinyl chloride-vinyl acetate copolymer

The first thermoplastic that found wide application was celluloid, an artificial polymer obtained by processing natural cellulose. He played a big role in technology, especially in cinema, but due to the exceptional fire hazard (in terms of composition, cellulose is very close to smokeless powder) already in the middle of the 20th century. its production has dropped to almost zero.

The development of electronics, telephone communications, radio urgently required the creation of new electrical insulating materials with good structural and technological properties. This is how artificial polymers appeared, made on the basis of the same cellulose, named after the first letters of the fields of application, etrols. Currently, only 2 ... 3% of the world production of polymers are cellulose plastics, while approximately 75% are synthetic thermoplastics, with 90% of them accounted for by only three: polystyrene, polyethylene, polyvinyl chloride.

Expandable polystyrene, for example, is widely used as a heat and sound insulating building material. In radio electronics, it is used for sealing products when it is necessary to ensure minimal mechanical stress, create temporary insulation from the effects of heat emitted by other elements or low temperatures and eliminate their effect on electrical properties, therefore, in on-board and microwave - equipment.

1.3 ELASTOMERS

Elastomers are commonly referred to as rubbers. Balloons, shoe soles, tires, surgical gloves, garden hoses are typical examples of elastomer products. The classic example of elastomers is natural rubber.

The rubber macromolecule has a helical structure with an identity period of 0.913 nm and contains more than 1000 isoprene residues. The structure of the rubber macromolecule provides its high elasticity - the most important technical property. Rubber has the amazing ability to reversibly stretch to 900% of its original length.

A variety of rubber is less elastic gutta-percha, or balata, the juice of some rubber plants growing in India and the Malay Peninsula. Unlike rubber, the gutta-percha molecule is shorter and has a trans-1,4 structure with an identity period of 0.504 nm.

The outstanding technical significance of natural rubber, its absence in a number of countries, including the Soviet Union, of economically viable sources, the desire to have materials that are superior in a number of properties (oil resistance, frost resistance, abrasion resistance) to natural rubber, stimulated research on the production of synthetic rubber. .

Several synthetic elastomers are currently in use. These include polybutadienes, styrene-butadiene, acrylonitrile-butadiene (nitrile rubber), polyisoprene, polychloroprene (neoprene), ethylene-propylene, isoprene-isobutylene (butyl rubber), polyfluorocarbon, polyurethane, and silicone rubbers. The raw material for producing synthetic rubber according to the Lebedev method is ethyl alcohol. Now, the production of butadiene from butane through the catalytic dehydrogenation of the latter has been developed.

Scientists have been successful and today more than one third of the rubber produced in the world is made from synthetic rubber. Rubber and rubber make a huge contribution to the technological progress of the last century. Let us recall, for example, rubber boots and various insulating materials, and the role of rubber in the most important branches of the economy will become clear to us. More than half of the world's elastomer production is spent on tire production. The manufacture of tires for a small car requires about 20 kg of rubber, of different grades and brands, and for a dump truck almost 1900 kg. A smaller part goes to other types of rubber products. Rubber makes our life more convenient.

1.4 FIBER

We are all familiar with natural fibers such as cotton, wool, linen and silk. We also know synthetic fibers from nylon, polyesters, polypropylene and acrylics. The main distinguishing feature of fibers is that their length is hundreds of times greater than their diameter. If natural fibers (except silk) are staple fibers, then synthetic ones can be obtained both in the form of continuous threads and in the form of staple fibers.

From the consumer's point of view, fibers can be of three types; everyday demand, safe and industrial.

Everyday fibers are called fibers used for the manufacture of underwear and outerwear. This group includes fibers for the manufacture of underwear, socks, shirts, suits, etc. These fibers must have appropriate strength and extensibility, softness, non-flammability, absorb moisture and be well dyed. Typical representatives of this class of fibers are cotton, silk, wool, nylon, polyesters and acrylates.

Safe fibers are fibers used for the production of carpets, curtains, chair covers, draperies, etc. Such fibers must be tough, strong, durable and wear-resistant. From the point of view of safety, the following requirements are imposed on these fibers: they must ignite poorly, do not spread flame, and emit a minimum amount of heat, smoke and toxic gases during combustion. By adding small amounts of substances containing atoms such as B, N, Si, P, C1, Br or Sb to everyday fibers, it is possible to make them fire resistant and thus turn them into safe fibers. The introduction of modifying additives into the fibers reduces their combustibility, reduces the spread of flame, but does not lead to a decrease in the release of toxic gases and smoke during combustion. Studies have shown that aromatic polyamides, polyimides, polybenzimidazoles and polyoxydiazoles can be used as safe fibers. However, when these fibers are burned, toxic gases are released, since their molecules contain nitrogen atoms. Aromatic polyesters do not have this drawback.

Industrial fibers are used as reinforcing materials in composites. These fibers are also called structural fibers because they have high modulus, strength, heat resistance, stiffness, durability. Structural fibers are used to strengthen products such as rigid and flexible pipes, tubes and hoses, as well as in composite structures called fiber materials and are used in the construction of ships, cars, aircraft and even buildings. This class of fibers includes uniaxially oriented fibers of aromatic polyamides and polyesters, carbon and silicon fibers.

2. POLYMER RECYCLING

2.1 COMPOUNDING

Polymers in their pure form, obtained from industrial plants after their isolation and purification, are called "primary" polymers or "primary" resins. With the exception of some polymers such as polystyrene, polyethylene, polypropylene, virgin polymers are generally not suitable for direct processing. Virgin PVC, for example, is a horn-like material and cannot be molded without first being softened by the addition of a plasticizer. Similarly, natural rubber requires the addition of a vulcanizing agent to form natural rubber. Most polymers are protected from thermal, oxidative and photodegradation by incorporating suitable stabilizers into them. The addition of dyes and pigments to the polymer before molding makes it possible to obtain products of a wide variety of colors. To reduce friction and improve polymer flow within processing equipment, lubricants and processing aids are added to most polymers. Fillers are usually added to the polymer to give them special properties and reduce the cost of the final product.

The process involving the incorporation of ingredients such as plasticizers, curing agents, hardeners, stabilizers, fillers, dyes, flame retardants, and lubricants into a primary polymer is referred to as "compounding", and mixtures of polymers with these additives are referred to as "compounds".

Primary plastic polymers such as polystyrene, polyethylene, polymethyl methacrylate and polyvinyl chloride are usually in the form of free-flowing fine powders. Fine powder or liquid ingredients are mixed with the powdered primary polymer using planetary mixers, V-mixers, ribbon helical mixers, Z-mixers or tipper. The displacement can be carried out either at room temperature or at elevated temperature, which, however, should be well below the softening temperature of the polymer. Liquid prepolymers are mixed using simple high speed agitators.

Primary elastomeric polymers, such as natural rubber, styrene-butadiene rubber or nitrile rubber, are obtained in the form of crumbs compressed into thick plates called "bales". They are usually mixed with vulcanizing agents, catalysts, fillers, antioxidants and lubricants. Because elastomers are not free-flowing powders like virgin plastics, they cannot be mixed with the ingredients listed above using methods used for virgin plastics. Mixing of primary plastic polymers with other components of the compound is achieved by mixing, while obtaining a compound of primary elastomers involves rolling crumbs into plastic sheets and then introducing the required ingredients into the polymer. Compounding of elastomers is carried out either in a two-roll rubber mill or in a Banbury mixer with internal mixing. Elastomers in the form of latex or low molecular weight liquid resins can be mixed by simple mixing using high speed agitators. In the case of fiber-forming polymers, compounding is not carried out. Components such as lubricants, stabilizers and fillers are usually directly added to the polymer melt or solution just before the yarn is spun.

2.2 PROCESSING TECHNOLOGY

The fact that polymeric materials are used in a wide variety of forms, such as rods, pipes, sheets, foams, coatings or adhesives, as well as molded articles, implies that there are a variety of ways to process polymer compounds into end products. Most polymer products are obtained either by molding, or processing, or by casting liquid prepolymers into a mold, followed by curing or crosslinking. The fibers are obtained during the spinning process.

The shaping process can be compared, for example, to sculpting a figure from clay, and the processing process to carving the same figure from a bar of soap. In the molding process, a compound in the form of a powder, flakes or granules is placed in a mold and subjected to temperature and pressure, resulting in the formation of the final product. The processing process produces products in simple shapes such as sheets, rods or pipes using stapling, stamping, gluing and welding.

Before moving on to a discussion of various methods for processing polymers, we recall that polymer materials can be thermoplastic or thermoset (thermoset). Once thermoplastic materials have been molded under heat and pressure, they must be cooled below the softening temperature of the polymer before they are released from the mold, otherwise they will lose their shape. In the case of thermosetting materials, this is not necessary, since after a single combined exposure to temperature and pressure, the product retains its acquired shape even when it is released from the mold at high temperature.

2.3 CALENDING

The calendering process is commonly used to produce continuous films and sheets. The main part of the apparatus (Fig. 1) for calendering is a set of smoothly polished metal rolls rotating in opposite directions, and a device for fine adjustment of the gap between them. The gap between the rolls determines the thickness of the calendered sheet. The polymer compound is fed onto the hot rolls and the sheet coming from these rolls is cooled as it passes through the cold rolls. At the last stage, the sheets are wound into rolls, as shown in Fig. 1. However, if instead of sheets it is required to obtain thin polymer films, a series of rolls is used with a gradually decreasing gap between them. Typically, polymers such as polyvinyl chloride, polyethylene, rubber, and butadiene-styrene-acrylonitrile are calendered into sheets.

Rice. one. Scheme of the apparatus for calendering

/ - polymer compound; 2 - calender rolls: hot (3) and cold (4); 5 - calendered sheet; b - guide rolls; 7 - winder

When using profiled rolls in the calendering machine, embossed sheets of various patterns can be obtained. Various decorative effects, such as imitation marbling, can be achieved by introducing mixtures of compounds of different colors into the calender. Marbling technology is commonly used in the production of PVC floor tiles.

2.4 CASTING

MOLD CASTING. This is a relatively inexpensive process that consists of converting a liquid prepolymer into solid products of the desired shape. Sheets, pipes, rods, etc. can be obtained by this method. products of limited length. Schematically, the mold casting process is shown in Fig.2. In this case, the prepolymer, mixed in appropriate proportions with the curing agent and other ingredients, is poured into a petri dish, which serves as a mould. Then the Petri dish is placed for several hours in an oven heated to the required temperature until the curing reaction is completed. After cooling to room temperature, the solid product is removed from the mold. A solid body cast in this way will have the shape of the internal relief of a Petri dish.

Rice. 2. The simplest picture of the mold casting process

b - filling the Petri dish with prepolymer and hardener; b - heating in the furnace; b - extraction from the mold of the cooled product

If, instead of a Petri dish, a cylindrical glass tube closed at one end is used, a product in the form of a cylindrical rod can be obtained. In addition, instead of the prepolymer and hardener, a mixture of monomer, catalyst and other ingredients heated to the polymerization temperature can be poured into the mold. Polymerization in this case will proceed inside the mold until a solid product is formed. Acrylics, epoxies, polyesters, phenols and urethanes are suitable for injection molding.

Casting molds are made of alabaster, lead or glass. During curing, the polymer block shrinks, making it easier to release from the mold.

ROTATIONAL CASTING. Hollow products such as balls and dolls are produced in a process called "rotational casting". The apparatus used in this process is shown in Figure 3.

A compound of thermoplastic material in the form of a fine powder is placed in a hollow mold. The apparatus used has a special device for simultaneous rotation of the mold around the primary and secondary axes. The mold is closed, heated and rotated. This results in a uniform distribution of the molten plastic over the entire inner surface of the hollow mold. The rotating mold is then cooled with cold water. Upon cooling, the molten plastic material, uniformly distributed over the inner surface of the mold, solidifies. Now the mold can be opened and the final product removed.

A liquid mixture of a thermosetting prepolymer with a hardener may also be loaded into the mold. Curing in this case will occur during rotation under the influence of elevated temperature.

Rotational casting produces products from PVC, such as galoshes, hollow balls or heads for dolls. Hardening of PVC is carried out by physical gelation between PVC and liquid plasticizer at temperatures of 150-200°C. Fine PVC particles are uniformly dispersed in the liquid plasticizer along with stabilizers and colorants, thus forming a relatively low viscosity substance. This pasty material, called "plastisol", is loaded into a mold and the air is evacuated from it. The mold is then rotated and heated to the required temperature, which causes the polyvinyl chloride to gel. The wall thickness of the resulting product is determined by the gelation time.

Fig.3. In the rotational casting process, hollow molds filled with polymeric material are simultaneously rotated around the primary and secondary axes.

1 - primary axis; 2 - secondary axis; 3 - detachable form detail; 4 - mold cavities; 5 - gear housing; b-to the motor

After reaching the required wall thickness, the excess plastisol is removed for a second cycle. For the final homogenization of the mixture of PVC particles with a plasticizer, the gel-like product inside the mold is heated. The final product is taken out of the mold after it has been cooled with a jet of water. The rotational casting method using a liquid material is known as the "hollow molding by pouring and rotating a mold" method.

INJECTION MOLDING. The most convenient process for the production of products from thermoplastic polymers is the injection molding process. Despite the fact that the cost of equipment in this process is quite high, its undoubted advantage is high productivity. In this process, a metered amount of molten thermoplastic polymer is injected under pressure into a relatively cold mold, where it solidifies into the final product.

The injection molding apparatus is shown in Fig.6. The process consists of supplying a compounded plastic material in the form of granules, tablets or powder from a hopper at certain intervals into a heated horizontal cylinder, where it softens. A hydraulic piston provides the pressure needed to push the molten material through the cylinder into the mold at the end of the cylinder. When the polymer mass moves along the hot zone of the cylinder, a device called a "torpedo" promotes a uniform distribution of the plastic material over the inner walls of the hot cylinder, thus ensuring uniform heat distribution throughout the volume. The molten plastic material is then injected through the injection hole into the mold cavity.

In its simplest form, the mold is a system of two parts: one of the parts is moving, the other is stationary (see Fig. 6). The stationary part of the mold is fixed at the end of the cylinder, and the movable part is removed and put on it.

With the help of a special mechanical device, the mold is tightly closed, and at this time, the molten plastic material is injected under a pressure of 1500 kg/cm. The closing mechanical device must be designed to withstand high operating pressures. The uniform flow of the molten material in the internal areas of the mold is ensured by preheating it to a certain temperature. Typically, this temperature is somewhat lower than the softening temperature of the molded plastic material. After filling the mold with molten polymer, it is cooled by circulating cold water and then opened to remove the finished product. This entire cycle can be repeated many times both manually and automatically.

CASTING FILMS. The casting method is also used for the production of polymer films. In this case, the polymer solution of the appropriate concentration is gradually poured onto a metal belt moving at a constant speed (Fig. 4), on the surface of which a continuous layer of the polymer solution is formed.

Fig.4. Scheme of the film casting process

/ - polymer solution; 2 - distribution valve; 3 - the polymer solution spreads to form a film; 4 - the solvent evaporates; 5 - endless metal belt; 6 - continuous polymer film; 7 - reel

When the solvent evaporates in a controlled mode, a thin polymer film is formed on the surface of the metal belt. After that, the film is removed by simple peeling. Most industrial cellophane sheets and photographic films are produced in this way.

2.5 DIRECT PRESSING

The direct pressing method is widely used for the production of products from thermosetting materials. Figure 5 shows a typical mold used for direct compression. The form consists of two parts - upper and lower or from a punch (positive form) and a matrix (negative form). There is a notch at the bottom of the mold and a ledge at the top. The gap between the protrusion of the upper part and the recess of the lower part in a closed mold determines the final appearance of the pressed product.

In the direct compression process, the thermosetting material is subjected to a single temperature and pressure application. The use of a hydraulic press with heated plates allows you to get the desired result.

Fig.5. Schematic representation of a mold used in the direct molding process

1 - a mold cavity filled with a thermosetting material; 2 - guide spikes; 3 - burr; 4 - molded product

The temperature and pressure during pressing can reach 200 °C and 70 kg/cm2, respectively. The operating temperature and pressure are determined by the rheological, thermal and other properties of the pressed plastic material. The mold recess is completely filled with polymer compound. When the mold is closed under pressure, the material inside it is compressed and pressed into the desired shape. Excess material is forced out of the mold in the form of a thin film called "burr". Under the influence of temperature, the pressed mass hardens. Cooling is not required to release the final product from the mold.

Fig..6. Schematic representation of the injection molding process

1 - compounded plastic material; 2 - loading funnel; 3 - piston; 4 - electric heating element; 5 - stationary part of the form;

6 - movable part of the form; 7 - main cylinder; 8 - torpedo; 9 - softened plastic material; 10 - mold; 11 - product molded by injection molding

2.6 FORMING

PNEUMOFORMING. A large number of hollow plastic products are produced by blow molding: canisters, soft drink bottles, etc. The following thermoplastic materials can be blow molded: polyethylene, polycarbonate, polyvinyl chloride, polystyrene, nylon, polypropylene, acrylics, acrylonitrile, acrylonitrile butadiene styrene polymer, however In terms of annual consumption, high-density polyethylene occupies the first place.

Blow molding has its origins in the glass industry. The scheme of this process is given in Fig.7.

A hot softened thermoplastic tube, called a "blank", is placed inside a two-part hollow mold. When the form is closed, both halves of it clamp one end of the workpiece and the air supply needle located at the other end of the tube.

Fig.7. Schematic diagram explaining the stages of the blow molding process

a - a workpiece placed in an open mold; b - closed mold;

c - blowing air into the mold; d - opening the mold. 1 - blank;

2 - needle for air supply; 3 - Press form; 4 - air; 5 - air molded product

Under the action of pressure supplied from the compressor through the needle, the hot billet is inflated like a ball until it comes into tight contact with the relatively cold inner surface of the mold. Then the mold is cooled, opened and the finished solid thermoplastic product is removed.

The preform for blow molding can be obtained by injection molding or extrusion, and depending on this, the method is called injection blow molding or extrusion blow molding, respectively.

FORMING SHEET THERMOPLASTICS. The molding of thermoplastic sheets is an extremely important process for the production of three-dimensional plastic products. With this method, even such large products as submarine hulls are obtained from sheets of acrylonitrile butadiene styrene.

The scheme of this Process is as follows. The thermoplastic sheet is heated to its softening temperature. Then the punch presses a hot flexible sheet into a metal mold matrix (Fig. 9), while the sheet takes a certain shape. When cooled, the molded product solidifies and is removed from the mold.

In the modified method, under the action of vacuum, the hot sheet is sucked into the cavity of the die and takes the required shape (Fig. 10). This method is called the vacuum forming method.

2.7 EXTRUSION

Extrusion is one of the cheapest methods for producing widely used plastic products such as films, fibers, pipes, sheets, rods, hoses and belts, where the profile of these products is determined by the shape of the extruder head outlet. Molten plastic, under certain conditions, is extruded through the outlet of the extruder head, which gives the desired profile to the extrudate. The diagram of the simplest extrusion machine is shown in Fig. 8.

Fig 8. Schematic representation of the simplest extrusion machine

1 - loading funnel; 2 - auger; 3 - main cylinder; 4 - heating elements; 5 - outlet of the extruder head, a - Loading Zone; b - compression zone; in ~ homogenization zone

In this machine, the powder or granules of the compounded plastic material is loaded from a hopper into an electrically heated cylinder to soften the polymer. A spiral-shaped rotating screw ensures the movement of hot plastic mass along the cylinder. Since friction occurs between the rotating screw and the barrel during the movement of the polymer mass, this leads to the release of heat and, consequently, to an increase in the temperature of the processed polymer. During this movement from the hopper to the outlet of the extruder head, the plastic mass passes through three clearly separated zones: the loading zone (a), the compression zone (b) and the homogenization zone (in)(See Figure 9).

Each of these zones contributes to the extrusion process. The loading zone, for example, takes the polymer mass from the hopper and sends it to the compression zone, this operation takes place without heating.

Rice. 9. Scheme of the molding process of sheet thermoplastics

1 - sheet of thermoplastic material; 2 - clamp; 3 - punch; 4 - heat-softened sheet; 5 - matrix; 6 - product obtained by molding sheet thermoplastics

Fig.10. Diagram of the vacuum forming process for thermoplastics

1 - clamp; 2 - thermoplastic sheet; 3 - Press form; 4 - product obtained by vacuum forming of thermoplastics

In the compression zone, the heating elements ensure the melting of the powdered charge, and the rotating screw compresses it. Then the paste-like molten plastic material enters the homogenization zone, where it acquires a constant flow rate due to the screw thread of the screw.

Under the action of the pressure created in this part of the extruder, the polymer melt is fed to the outlet of the extruder head and exits with the desired profile. Due to the high viscosity of some polymers, it is sometimes necessary to have another zone, called a working zone, where the polymer is subjected to high shear loads to improve mixing efficiency. The extruded material of the desired profile leaves the extruder in a very hot state (its temperature is from 125 to 350°C), and rapid cooling is required to maintain its shape. The extrudate enters a conveyor belt passing through a vat of cold water and solidifies. Cold air blowing and cold water spraying are also used to cool the extrudate. The shaped product is further either cut or wound into coils.

The extrusion process is also used to cover wires and cables with polyvinyl chloride or rubber, and rod-like metal rods with suitable thermoplastic materials.

2.8 FOAMING

Foaming is a simple method for obtaining foam and sponge-like materials. The special properties of this class of materials - shock-absorbing ability, light weight, low thermal conductivity - make them very attractive for use in various purposes. Common foaming polymers are polyurethanes, polystyrene, polyethylene, polypropylene, silicones, epoxies, PVC, etc. The foam structure consists of isolated (closed) or interpenetrating (open) voids. In the first case, when the voids are closed, they can contain gases. Both types of structures are shown schematically in Fig. 11.

Fig.11. Schematic representation of open and closed cell structures formed during the foaming process

1- discrete (closed) cells; 2 - interpenetrating (open) cells;

3 - cell walls

There are several methods for producing foamed or cellular plastics. One of them is that air or nitrogen is blown through the molten compound until it is completely foamed. The foaming process is facilitated by the addition of surface active agents. Upon reaching the desired degree of foaming, the matrix is ​​cooled to room temperature. In this case, the thermoplastic material solidifies in a foamed state. Thermoset liquid prepolymers can be cold-foamed and then heated until fully cured. Foaming is usually achieved by adding foam or blowing agents to the polymer mass. Such agents are low molecular weight solvents or certain chemical compounds. The process of boiling of such solvents as n-pentane and n-hexane at the temperatures of curing of polymeric materials is accompanied by an intense vaporization process. On the other hand, some chemical compounds at these temperatures can decompose with the release of inert gases. So, azo-bis-isobutyronitrile thermally decomposes, while releasing a large amount of nitrogen, released into the polymer matrix as a result of the reaction between isocyanate and water, and is also used to produce foamed materials, such as polyurethane foam:

Since polyurethanes are obtained by the reaction of a polyol with a diisocyanate, additional small amounts of diisocyanate and water must be added to foam the reaction product.

So, a large amount of vapors or gases emitted by foam and gas formers leads to foaming of the polymer matrix. The polymer matrix in the foamed state is cooled to temperatures below the softening temperature of the polymer (in the case of thermoplastic materials) or subjected to a curing or crosslinking reaction (in the case of thermoset materials), as a result, the matrix acquires the rigidity necessary to maintain the foam structure. This process is called the "foam stabilization" process. If the matrix is ​​not cooled below the softening temperature or cross-linked, the gases filling it leave the pore system and the foam collapses.

Foams can be obtained in flexible, rigid and semi-rigid forms. In order to obtain foam products directly, foaming should be carried out directly inside the mold. Styrofoam sheets and rods can also be used to produce various products. Depending on the nature of the polymer and the degree of foaming, the density of the foams can range from 20 to 1000 kg/cm 3 . The use of foams is very diverse. For example, the automotive industry uses large quantities of PVC and polyurethane foams for upholstery. These materials play an important role in the manufacture of furniture. Rigid polystyrene foams are widely used for packaging and thermal insulation of buildings. Foam rubbers and polyurethane foams are used for filling mattresses, etc. Rigid polyurethane foams are also used for thermal insulation of buildings and for the manufacture of prostheses.

2.9 REINFORCEMENT

By reinforcing the plastic matrix with high-strength fiber, systems called "fiber-reinforced plastics" (FRPs) are obtained. WUAs have very valuable properties: they are distinguished by a high strength-to-weight ratio, significant corrosion resistance, and ease of manufacture. The method of fiber reinforcement makes it possible to obtain a wide range of products. For example, when creating artificial satellites in AUAs, designers and creators of spacecraft are primarily attracted by the amazingly high strength-to-weight ratio. Beautiful appearance, light weight and corrosion resistance make it possible to use WUA for ship plating. In addition, WUA is even used as a material for tanks in which acids are stored.

Let us now dwell in more detail on the chemical composition and physical nature of these unusual materials. As noted above, they are a polymeric material, the special properties of which are due to the introduction of reinforcing fibers into it. The main materials from which reinforcing fibers are made (both finely chopped and long) are glass, graphite, aluminum, carbon, boron and beryllium. The most recent developments in this field are the use of fully aromatic polyamide as reinforcing fibers, which provides more than a 50% weight reduction compared to traditional fiber reinforced plastics. Natural fibers are also used for reinforcement, such as sisal, asbestos, etc. The choice of reinforcing fiber is primarily determined by the requirements for the final product. However, glass fibers remain widely used to this day and still make the main contribution to the industrial production of WUA. The most attractive properties of glass fibers are low thermal expansion coefficient, high dimensional stability, low production cost, high tensile strength, low dielectric constant, non-flammability and chemical resistance. Other reinforcing fibers are used mainly in cases where some additional properties are required for the operation of ARP in specific conditions, despite their higher cost compared to glass fibers.

HDPE is produced by bonding fibers to a polymer matrix and then curing it under pressure and temperature. Reinforcing additives can be in the form of finely chopped fibers, long threads and fabrics. The main polymer matrices used in ARP are polyesters, epoxides, phenols, silicones, melamine, vinyl derivatives, and polyamides. Most WUAs are produced on the basis of polyester polymers, the main advantage of which is their low cost. Phenolic polymers are used in cases where high temperature resistance is required. Extremely high mechanical properties of AVP are acquired when epoxy resins are used as a polymer matrix. The use of silicone polymers gives WUAs excellent electrical and thermal properties.

Currently, there are several methods of plastic reinforcement. The most commonly used of these are: 1) hand lamination method, 2) fiber winding method, and 3) spray impregnation method.

METHOD OF LAYERING SHEETS MANUALLY. It is likely that this is the simplest method of reinforcing plastics. In this case, the quality of the final product is largely determined by the skill and skill of the operator. The whole process consists of the following steps. First, the mold is covered with a thin layer of adhesive lubricant based on polyvinyl alcohol, silicone oil or paraffin. This is done to prevent the end product from sticking to the mold. Then the form is covered with a layer of polymer, on top of which a fiberglass or mat is placed. This fiberglass is in turn coated with another layer of polymer.

Fig.12. Schematic representation of the manual layering method

1 - alternating layers of polymer and fiberglass; 2 - Press form; 3 - rolling roller

All this is tightly rolled with rollers to uniformly press the fiberglass to the polymer and remove air bubbles. The number of alternating layers of polymer and fiberglass determines the thickness of the sample (Fig. 12).

Then, at room or elevated temperature, the system cures. After curing, the reinforced plastic is removed from the mold and stripped and finished. This method produces sheets, car body parts, ship hulls, pipes and even building fragments.

WINDING METHOD OF FIBER. This method is very widely used for the production of reinforced plastic products such as high-pressure cylinders, chemical storage tanks, and rocket motor casings. It consists in the fact that a continuous monofilament, fiber, fiber bundle or woven tape is passed through a bath of resin and hardener. As the fiber exits the bath, the excess resin is squeezed out. The resin-impregnated fibers or tape are then wound onto a core of the desired shape and cured under the action of temperature.

Fig.13. Schematic representation of the fiber winding method

1- supply coil; 2 - continuous thread; 3 - unit for fiber impregnation and resin pressing; 4 - core; 5 - resin-impregnated fibers wound on a core

The winding machine (Fig. 13) is designed so that the fibers can be wound around the core in a certain way. The tension of the fiber and the method of winding it are very important from the point of view of the final deformation properties of the finished product.

SPRAYING METHOD. In this method, a spray gun with a multi-strand head is used. Jets of resin, hardener and chopped fiber are simultaneously fed from the spray gun to the surface of the mold (Fig. 14), where they form a layer of a certain thickness. Chopped fiber of a certain length is obtained by continuous supply of fibers to the grinding head of the apparatus. After reaching the required thickness, the polymer mass is cured by heating. Spraying is an express method for covering large surfaces. Many modern plastic products, such as cargo platforms, storage tanks, truck bodies and ship hulls, are made using this method.

Fig.14. Schematic representation of the spraying method

1 - form; 2 - sprayed mixture of chopped fiber and resin; 3 - a jet of chopped fiber; 4 - continuous fiber; 5- resin; 6- hardener; 7 - node for cutting fiber and spraying; 8 - resin jet

OTHER METHODS. In addition to the methods described above, others are known in the production of reinforced plastics, each of which has its own specific purpose. Thus, the method of manufacturing continuous laminates is used for the production of continuous sheets of reinforced laminates of various thicknesses. In this process, each individual layer of woven tape coming from rolls is impregnated with resin and hardener and then pressed together through a hot roll system. After curing under the influence of temperature, a laminate I of the required thickness is obtained (Fig. 15). The thickness of the material can be varied by changing the number of layers.

Fig.15. Schematic representation of the production method for continuous laminated materials

1- feed coils; 2 - continuous sheets of fiberglass; 3 - bath for impregnation in a mixture of resin and hardener; 4 - continuous laminate; 5 - laminated plastic, cut into pieces of the required size

Another method, known as the plywood method, makes it possible to manufacture products such as hollow rods or fishing rods from continuous bundles of fibers. This process is relatively simple. A continuous bundle of fibers, previously treated with resin and hardener, is pulled through a die of the corresponding profile (Fig. 16), heated to a certain temperature. At the exit from the die, the profiled product continues to be heated. The cured profile is pulled out of the die by a system of rotating rolls. This process is somewhat similar to extrusion, with the only difference being that in extrusion the polymeric material is pushed through the die from the inside by means of a rotating screw, while in the described method the material is pulled through the die outlet from the outside.

Fig.16. Schematic representation of the method for obtaining pultruded fiber plastic

1 - a continuous bundle of fibers impregnated with resin and hardener; 2 - heating element; 3 - die; 4 - rotating draw rolls; 5 - finished product, cut into pieces; 6 - finished product profile

In addition, the mixture containing cut fibers, resin and hardener may be formed by any other suitable method, such as direct compression. Thermoplastic materials filled with cut fibers can be molded by direct compression, injection molding, or extrusion to produce end products with improved mechanical properties.

2.10 SPINNING FIBERS

Polymer fibers are obtained in a process called spinning. There are three fundamentally different spinning methods: melt spinning, dry spinning and wet spinning. In the melt spinning process, the polymer is in a molten state, and in other cases, in the form of solutions. However, in all these cases, the polymer, in a molten or dissolved state, flows through a multi-channel mouthpiece, which is a plate with very small holes for the exit of fibers.

SPINTING FROM THE MELT. In its simplest form, the spunmelt process can be represented as follows. Initially, the polymer flakes are melted on a heated grid, turning the polymer into a viscous mobile liquid. Sometimes, during the heating process, lumps are formed due to the processes of crosslinking or thermal destruction. These lumps can be easily removed from the hot polymer melt by passing through a block filter system. In addition, to prevent oxidative degradation, the melt should be protected from atmospheric oxygen. This is achieved mainly by creating an inert atmosphere of nitrogen, CO2, and water vapor around the polymer melt. The dosing pump delivers the polymer melt at a constant rate to the multi-channel die. The polymer melt passes through a system of fine holes in the mouthpiece and exits from there in the form of continuous and very thin monofilaments. Upon contact with cold air, the fibers emerging from the spinnerets instantly harden. Cooling and hardening processes can be greatly accelerated by blowing cold air. The solid monofilaments emerging from the spinnerets are wound onto spools.

An important feature to consider in the melt spinning process is that the diameter of the monofilament is highly dependent on the speed at which the molten polymer passes through the spinneret and the rate at which the monofilament is drawn from the spinneret and wound onto spools.

Fig.17. Schematic representation of dry spinning processes (a) and melt spinning (b)

1 - hopper; 2 - polymer flakes; 3 - heated grate; 4 - hot polymer; 5 - dosing pump; b - melt; 7- multi-channel mouthpiece, 8 - freshly spun fiber; 9 - coil; 10 - polymer solution; 11 - filter;

12 - dosing pump; 13 - multichannel mouthpiece; 14 - freshly spun fiber; 15 - on the coil

DRY SPINNING. A large number of traditional polymers such as PVC or polyacrylonitrile are processed into fibers on a large scale in the dry spinning process. The essence of this process is shown in Fig.17. The polymer is dissolved in an appropriate solvent to form a highly concentrated solution. The viscosity of the solution is adjusted by increasing the temperature. The hot, viscous polymer solution is forced through the spinnerets, thus producing thin continuous streams. The fiber from these streams is formed by simple evaporation of the solvent. Evaporation of the solvent can be accelerated by blowing with a counter flow of dry nitrogen. The fibers formed from the polymer solution are finally wound onto spools. The fiber spinning speed can reach 1000m/min. Industrial cellulose acetate fibers obtained from a 35% polymer solution in acetone at 40°C are a typical example of fiber production by dry spinning.

WET SPINNING. In wet spinning, as in dry spinning, highly concentrated polymer solutions are used, the high viscosity of which can be reduced by increasing the spinning temperature. Details of the wet spinning process are shown in Figure 18. In the wet spinning process, a viscous polymer solution is processed into thin strings when passed through spinnerets. Then these polymer jets enter the coagulation bath with a precipitant, where the polymer is precipitated from the solution in the form of thin filaments, which, after washing, drying, etc., are collected on coils. Sometimes, during wet spinning, lumps are formed instead of continuous filaments, which occurs as a result of the breakage of a trickle flowing from a spinneret under the action of surface tension forces.

Fig.18. Schematic representation of the wet spinning process

1 - polymer solution; 2 - filter; 3 - dosing pump; 4 - multichannel mouthpiece; 5 - precipitant; 6 - freshly spun fiber; 7 - bath for coagulation and sedimentation; 8 - washing bath; 9 - drying; 10 - on the coil

This can be avoided by increasing the viscosity of the polymer solution. Coagulation, which is the limiting stage of wet spinning, is a rather slow process, which explains the low solution spinning speed of 50 m/min compared to others. In industry, the wet spinning process is used to produce fibers from polyacrylonitrile, cellulose, viscose fiber, etc.

SINGLE-AXIS ORIENTATION. In the process of spinning fibers from a polymer melt or solution, the macromolecules in the fiber are not oriented and, therefore, their degree of crystallinity is relatively low, which undesirably affects the physical properties of the fiber. To improve the physical properties of the fibers, they are subjected to an operation called uniaxial drawing using some type of stretching apparatus.

The main feature of the device is the presence of a system of two rollers BUT and AT(Fig. 19), rotating at different speeds. Video clip AT rotates 4-5 times faster than the roller BUT. The spun yarn is passed successively through a roller BUT, tensile hairpin 3 and roller AT. Since the roller AT rotates at a speed greater than the roller BUT, the fiber is pulled out under the load given by the pin 3. The fiber is drawn in the zone 2. After going through the roller AT elongated polymer thread is wound on a metal reel. Despite the fact that the diameter of the thread decreases during drawing, its strength properties are significantly improved due to the orientation of macromolecules parallel to the fiber axis.

Fig.19. Schematic representation of the device for uniaxial orientation

1 - unstretched thread; 2 - exhaust zone; 3 - stretching pin; 4- drawn fiber

SUBSEQUENT PROCESSING OF FIBERS. To improve the useful properties of the fibers, they are often subjected to additional special processing: cleaning, lubrication, sizing, dyeing, etc.

Soaps and other synthetic detergents are used for cleaning. Cleaning is nothing more than the removal of dirt and other impurities from the surface of the fiber. Lubrication consists in processing the fibers in order to protect

them from friction with neighboring fibers and rough metal surfaces during processing. Natural oils are mainly used as lubricating agents. Lubrication also reduces the amount of static electricity that builds up on the fibers.

Sizing refers to the process of protective coating of fibers. Polyvinyl alcohol or gelatin are used as sizing materials for most fibers. The sizing keeps the fibers within a compact bundle and thus ensures uniform weaving. Before dyeing the fabric, the gluing should be removed by rinsing in water.

For dyeing, the fibers are placed in a dye solution, the molecules of which usually penetrate only into the amorphous regions of the fiber.

Fibers based on cellulose or proteins quickly adsorb acidic dyes, which easily bind to the amino or hydroxyl groups of the polymers. The dyeing process for synthetic fibers such as polyesters, polyamides or acrylics is much slower. In this case, the dyeing rate can be increased by increasing the temperature. Dyeing of fibers based on polyvinyl chloride, polyethylene, etc. is practically impossible without the introduction of active absorption centers into them during copolymerization and chemical oxidation.

CONCLUSION

As previously noted, polymers include numerous natural compounds: proteins, nucleic acids, cellulose, starch, rubber, and other organic substances. A large number of polymers are obtained synthetically based on the simplest compounds of elements of natural origin through polymerization, polycondensation, and chemical transformations.

In the early 1960s, polymers were considered only cheap substitutes for scarce natural raw materials - cotton, silk, and wool. But soon the understanding came that polymers, fibers and other materials based on them are sometimes better than traditionally used natural materials - they are lighter, stronger, more heat-resistant, able to work in aggressive environments. Therefore, chemists and technologists directed all their efforts to the creation of new polymers with high performance characteristics and methods for their processing. And they achieved results in this business, sometimes surpassing the results of similar activities of well-known foreign firms.

Polymers are widely used in many areas of human activity, satisfying the needs of various industries, agriculture, medicine, culture and everyday life. At the same time, it is appropriate to note that in recent years, the function of polymeric materials in any industry and the methods for their production have somewhat changed. More and more responsible tasks began to be trusted to polymers. More and more relatively small, but structurally complex and critical parts of machines and mechanisms began to be made from polymers, and at the same time, more and more polymers began to be used in the manufacture of large body parts of machines and mechanisms that carry significant loads.

The boundary of the strength properties of polymeric materials was overcome by the transition to composite materials, mainly glass and carbon fiber. So now the expression “plastic is stronger than steel” sounds quite reasonable. At the same time, polymers have retained their positions in the mass production of a huge number of those parts that do not require particularly high strength: plugs, fittings, caps, handles, scales and measuring instrument cases. Another area specific to polymers, where their advantages over any other materials are most clearly manifested, is the area of ​​interior and exterior decoration.

By the way, the same advantages stimulate the widespread use of polymeric materials in the aviation industry. For example, replacing an aluminum alloy with graphite plastic in the manufacture of an aircraft wing slat makes it possible to reduce the number of parts from 47 to 14, fasteners from 1464 to 8 bolts, reduce weight by 22%, and cost by 25%. At the same time, the safety margin of the product is 178%. Helicopter blades, jet engine fan blades are recommended to be made from polycondensation resins filled with aluminosilicate fibers, which makes it possible to reduce aircraft weight while maintaining strength and reliability.

All these examples show the huge role of polymers in our life. It is difficult to imagine what materials based on them will still be obtained. But it is safe to say that polymers will take, if not the first, then at least one of the first places in production. It is quite obvious that the quality, characteristics and properties of the final products directly depend on the polymer processing technology. The importance of this aspect forces us to look for more and more new ways of processing to obtain materials with improved performance. In this essay, only the main methods were considered. Their total number is not limited to this.

BIBLIOGRAPHY

1. Pasynkov V.V., Sorokin V.S., Materials of electronic technology, - M .: Higher School, 1986.

2.A. A. Tager, Physicochemistry of polymers, M., chemistry, 1978.

3. Tretyakov Yu.D., Chemistry: Reference materials. – M.: Enlightenment, 1984.

4. Materials Science / Ed. B.N. Arzamasov. - M .: Mashinostroenie, 1986.

5. Dontsov A. A., Dogadkin B. A., Shershnev V. A., Chemistry of elastomers, - M .: Chemistry, 1981.

Thermoplastics are plastics that, once molded, are recyclable. They can repeatedly soften when heated and harden when cooled without losing their properties. This is the reason for the huge interest in the recycling of thermoplastic waste - both domestic and industrial.

The composition of municipal solid waste (MSW) in the capital differs markedly from the average for Russia. About 110,000 tons of municipal solid waste are generated annually in Moscow. Of these, polymers make up 8-10%, and in the commercial waste of large enterprises this figure reaches 25%.

Separately, plastic bottles should be singled out in the structure of MSW. About 50 thousand tons of them are thrown away annually in Moscow alone. According to the results of the International Scientific and Practical Conference "Packaging and the Environment", 30% of all polymer waste is bottles made of polyethylene and polyvinyl chloride. However, at present, according to the State Unitary Enterprise "Promothody", no more than 9 thousand tons of polymer waste isolated from solid waste are processed annually in Moscow and the region. And half of them - in the territory of the Moscow region. What are the reasons for such insignificant recycling of thermoplastic waste?

Organization of the collection

To date, there are several channels for collecting plastic waste.

The first and main one is the collection and disposal of waste from large shopping malls. This raw material is predominantly used packaging and is considered the most "clean" and best suited for further use.

The second way is selective garbage collection. In the south-west of Moscow, the city administration, together with the State Unitary Enterprise Promothody, is conducting such an experiment. Special German eurocontainers have been installed in the yards of several residential buildings. Lids for containers with holes: round - for PET bottles, a large slot - for paper. Containers are locked and constantly monitored. In two years, 12 tons of plastic bottles were collected. Today the project includes only 19 residential buildings. According to experts, when covering a territory with a population of more than 1 million inhabitants, the benefits of such a system become obvious.

The third option is the sorting of solid waste at specialized enterprises (the Kotlyakovo pilot waste sorting center, the MSK-1 private enterprise, and other waste sorting complexes). It is still quite difficult to accurately determine the volume of sorted waste, but the share of this source of secondary raw materials is already noticeable. Some commercial organizations, under the control of municipal authorities, organize their own collection points for secondary raw materials (including polymer waste) from the population. Primary sorting and pressing usually take place there. However, there are very few such places in the city.

A significant proportion of recycled materials going for processing is illegally collected at landfills. This is done by private firms, and sometimes by the management of the landfills themselves. The collected and sorted materials are sold to resellers or directly to manufacturers.

When processing thermoplastics, the uniformity of the polymers used, the degree of contamination, the color and type (film, bottles, scrap), the form of the supplied waste (compression, packaging, etc.) are very important. Depending on these and a number of other parameters, the suitability of a particular batch for further processing (and, therefore, its market value) can fluctuate markedly. Waste paper costs the most.

Sorting, crushing and pressing can be carried out by numerous intermediaries, waste sorting complexes, processors themselves, structures of the State Unitary Enterprise "Promotkhody".

In most cases, manual sorting is used, since the appropriate equipment is expensive and not always efficient.

Polymer recycling

The collected and sorted waste can be recycled into secondary granulate or immediately go to the production of new products (shopping bags and bags, disposable tableware, video cassette cases, country furniture, polymer pipes, wood-polymer boards, etc.).

The processing of polymer household waste on an industrial scale in Moscow is carried out only by OAO NII PM (production of products for the needs of the municipal economy as part of the program for separate waste collection in the South-Western Autonomous Okrug and by order of the capital's mayor's office). State Unitary Enterprise "Promotkhody" carries out crushing, washing and drying, then the flakes at a price of $ 400 per ton are transported for further processing to the Research Institute of PM.

Other processors of secondary raw materials are either too small (capacity up to 20 tons per month), or under the guise of processing they are engaged in crushing and further resale, at best they add crushed raw materials to their products. Almost no one is engaged in large-scale production of secondary granulate and agglomerate in Moscow.

According to other sources (N.M. Chalaya, NPO Plastic), many small firms are engaged in the processing of polymers contained in Moscow waste, for which this activity is not the main one. They try not to advertise it, since it is generally believed that the use of recycled materials in the production of products worsens its quality.

A typical company for this market is the production cooperative Vtorpolimer, which works directly with the city's landfill. Homeless people living in the landfill collect everything plastic there: bottles, toys, broken buckets, film, etc. For a fee, the “goods” are handed over to intermediaries, and they deliver it to Vtorpolymer. Here, things that have served their time are washed and sent for recycling. They are sorted by color, crushed and added to plastic, which is used to make installation pipes (they are used in the construction of new houses to insulate electrical wiring). The purchase price of dirty plastic scrap is 1 thousand rubles. per ton, pure - 1.5 thousand. Smaller lots are accepted at a price of 1 and 1.5 rubles. per-kg respectively.

Sorting of polymeric waste is carried out manually. The main selection criterion is the appearance of the product or the corresponding labeling. Without marking, packaging made of polystyrene, polyvinyl chloride or polypropylene cannot be visually distinguished. Bottles are most often considered PET, film - polyethylene (the specific type of PE is usually not determined), although it may well be PP or PVC. Linoleum - mainly PVC, expanded polystyrene (polystyrene) is easily identified visually, nylon fibers and technical products (spools, bushings) are usually made of polyamide. The probability of coincidences with this sorting is about 80%.

An analysis of the activities of firms operating in the secondary materials market allows us to draw the following conclusions:

1) the prices of secondary materials on the market are determined by the degree of their preparation for processing. If we take the cost of virgin low density polyethylene granulate as 100%, then the price of pure shredded polyethylene film prepared for processing is from 8 to 13% of the cost of virgin polymer. The price of polyethylene agglomerate is from 20 to 30% of the cost of the primary polymer;

2) the price of most granular secondary polymers, averaged by composition, ranges from 45 to 70% of the price of primary polymers;

3) the price of secondary polymers strongly depends on their color, that is, on the quality of the preliminary sorting of polymer waste by color. The difference in the price of recycled polymers of pure and mixed colors can reach 10-20%;

4) the prices for products obtained from primary and secondary polymers are, as a rule, almost the same, which makes the use of secondary polymers in production extremely profitable.

On average, the price of polymer waste isolated from MSW, depending on the degree of preparation, batch and type, ranges from 1 to 8 rubles / kg. Purchase prices from processors, depending on the batch and the level of contamination, are shown in table 1.

Type of polymer	Price for dirty waste, rub. /kg	Price for clean waste, rub. /kg	Prices for clean waste, $/t (as of April 2002)
Polystyrene
Polyamide
			Table 1

The price of clean MSW waste is usually equal to the price of industrial and commercial waste.

The market price for the purchase of polymer waste from MSW by a processor consists of the price of purchase by an intermediary from the population (approximately 25% of the cost), the fee for the formation of large-tonnage batches of waste, sorting, pressing and even washing for the most expensive (pure) raw materials.

Prices for products such as agglomerate and granulate average 12-24 rubles/kg (polyamide is more expensive than the others - 35-50 rubles/kg, PET - from 20 rubles/kg). Further processing increases the surplus value depending on the type of product by 30-200 %.

Investment attractiveness

According to most experts, it is profitable to invest in the processing of polymer waste, but only when relying on state support and a legislative framework focused on the interests of processors of secondary raw materials.

Today, the Moscow market consists of 20-30 small companies involved in the processing of polymer waste, mainly of industrial origin. The market as a whole is characterized by informal relations between processors and suppliers, a large share of companies for which this business is a side business, as well as low processing volumes (12-17 thousand tons per year). It can be assumed that if there is a stable demand on the part of processors for such waste, the volume of offers will grow.

It should be noted that the amount of polymer waste that is actually recycled today is a very small part of urban MSW. And this despite the fact that the demand for polymers and products from them is constantly increasing, and the problem of waste disposal is increasingly worrying the city authorities.

The constraining factor in the construction of new processing plants is the underdevelopment of the waste collection system and the lack of serious suppliers. The coincidence of interests of private business and the state in this area should inevitably lead to the adoption of laws that meet the interests of recyclers.

Present and future

1. The annual volume of PET processing in the capital is 4-5 thousand tons per year. The plans of the Moscow authorities include the organization by 2003 of a system for the selective collection of PET containers and the creation of two production complexes for its processing with a capacity of 3,000 tons per year. At present, the construction of two private PET processing plants with a total capacity of 6,000 tons annually is being completed.

In the coming months, the Moscow government should adopt regulations regulating the activities of polymer processors (their exact content is not yet known). The existing and under construction facilities are sufficient to meet the needs of the market. The possibility of state support for the projects of the State Unitary Enterprise "Promotkhody" and the company "Inteko" (potential processing capacity - 7-8 thousand tons per year) is being considered.

2. The volume of PP processing in Moscow is 4-5 thousand tons per year, although about 50-60 thousand tons are thrown away annually in the city - mainly film and big bags. After processing, PP in the form of granules is added to the primary raw materials or is entirely used for the production of plastic utensils, shopping bags, etc.).

The lack of large-scale recycling projects for this polymer (as is the case with PET) opens up great investment opportunities. The most profitable at this stage is the processing of recyclable materials into granules, since competition is much tougher in the field of consumer goods production.

3. The volume of PE processing is also 4-5 thousand tons per year. The main type of raw material is film, including agricultural film. In total, about 60-70 thousand tons of polyethylene waste is thrown out in the city every year. As a rule, enterprises involved in the processing of PE also deal with PP. One of the large companies through which about 2.5 thousand tons per year passes is Plastpoliten.

PE is highly resistant to pollution. However, the existing ban on the use of recycled polymer raw materials in the manufacture of food packaging limits the possibility of marketing.

Thus, the most rational for today seems to be the construction of an industrial complex for the processing of polyethylene, polypropylene and PET waste into granules.

This production must include:

a) sorting (requires special training of personnel to reduce the proportion of another type of polymer, which is very important for product quality);

b) washing (the largest potential volumes of raw materials are usually not sorted and not washed);

c) drying, crushing, agglomeration.

It is economically most profitable to locate this complex in the near Moscow region, since the prices for electricity, water, rent of land and industrial space are significantly lower there than in the capital (see Table 2).

Type of polymer	Price for clean waste, $/t	Price for secondary granulate, $/t	Volume in MSW thousand tons per year
			table 2

For the effective operation of such production, state support is necessary. Perhaps it makes sense to partially revise the existing sanitary standards for the processing of solid waste, as well as to oblige manufacturers of polymer products to make deductions for the processing of polymer waste. In addition, comprehensive measures should be taken at the level of the Moscow government and individual housing and communal services aimed at developing a system of selective collection and creating a network of recycling points.

The increased interest of the state in waste disposal is already reflected in the budget: from 2002 to 2010. it is planned to spend 519.2 million rubles for these purposes. from the federal budget. The budgets of the subjects of the federation are expected to allocate until 2010. 11.4 billion rubles for the implementation of the withdrawal program.

In 2001, Moscow spent 3.1 billion rubles on environmental protection. To date, the cost of already implemented projects for the processing of household waste is 115.5 million rubles.

Andrey Goliney,

The 20th century is considered the century of steel and non-ferrous metals. Aluminum, copper, iron alloys could be found everywhere - in bed headboards, bridges, mechanisms of all types, cladding panels. However, as a result of mechanical processing, 50–80% of the melted material went into chips. Experts pinned great hopes on the chemical industry associated with a decrease in material consumption. And yet, despite the growth in the use of polymers, the results of the industry by the 80s were about the same: half of the resources were wasted.

Obviously, the apparent availability of polymers is an illusion. The raw material used for their manufacture is a natural rarity. Access to its sources is a daily and invariable cause and cause of trade, diplomatic and other wars. The geography of extraction of natural resources is increasingly shifting to places not so remote. Therefore, today they are increasingly talking about the need to introduce resource-saving business models.

The uniqueness of the technological methods of modern chemical production lies not only in the ability to synthesize materials that successfully replace metal, paper or wood.

Most of today's industrial complexes of developed economies are able to recycle obsolete polymer products into new ones that are in demand by the user.

Recycled plastics

The main classes of polymers include:

• polyethylenes,
• polypropylenes,
• PVC,
• polystyrenes (including copolymers - ABS plastics),
• polyamides,
• polyethylene terephthalate.

Products that are complex in composition are first of all separated. For physical cleaning, various mechanisms are used - vacuum, thermal, cryogenic.

The most common and economically justified technologies are flotation and dissolution.

In the first case, the plastic is crushed, immersed in water. There are also added compounds that affect the ability of various plastics to absorb moisture. After separation, separated polymers are obtained.

In the second method, complex compressed parts are crushed and successively exposed to various solvents. To restore materials in their pure form, the resulting compounds are exposed to water vapor. As a result of a precisely executed process, finished products of a high degree of purity are obtained. Further processing of various plastics may have its own characteristics associated with the individual properties of polymers.

Polyethylene of high and low pressure (LDPE and HDPE).

The group of these compounds is also called polyolefins. They have found wide application in all types of industry, medicine, and the agricultural sector. PE are thermoplastics - materials suitable for remelting. This feature is successfully used by the industry, processing its own technological waste in order to reduce operating costs.

The complexity of the recycling of used plastic is due to the partial destruction of its surfaces caused by sunlight. Products obtained by the usual processing of products: grinding, mechanical cleaning, remelting, are not of high quality. Most often, such polyethylene is used for the manufacture of auxiliary household equipment.

Secondary polyethylene, which has undergone chemical modification, turns out to be more perfect. Various additives placed in the polymer melt bind the changed molecular units and even out the structure of the substance. Dicumyl peroxide, wax, lignins, slates are used as modifiers. Additives of certain types lead to a change in certain properties of recycled PE. Combining them allows you to get a material with the necessary parameters.

Polypropylene (PP)

This material is rarely recycled. Most often, plastic has one life, despite its excellent consumer characteristics that allow the use of the polymer in the food industry. Despite good remeltability, the high cost of maintaining hygiene deters processors. Nevertheless, in the United States every fifth ton of PP is reused.

According to chemists, PP can withstand no more than four remelts. With each heating, a certain amount of deformed molecular units accumulates, affecting the physical characteristics of the material. Secondary granules are easily processed in extruders and injection molding machines.

Recycled plastic does not require special modification. Its parameters are comparable with the original material, only slightly reduced frost resistance. Again, the polymer finds use in battery cases, garden tools, containers and films.

Polyvinyl chloride PVC

The material is used for the manufacture of linoleums, finishing films. Plastic is subject to thermal degradation. At temperatures above 100°, the oxidation of macromolecules begins to pick up speed, leading to a deterioration in the thermoplastic properties of the material.

The technology of extrusion using recycled PVC requires special preparation: the initial raw material mixture in the melt may be inhomogeneous. Solid modifications of PVC containing recycled plastic will have uneven internal stress. In order to minimize negative impacts, dry processing of granules in compactors is carried out before extrusion. As a result of this operation, fibers are formed that reinforce the walls of new products.

More often recycled polyvinyl chloride is used to obtain plastisols, vinyl plastics. Pastes, solutions, injection molded products are obtained from these materials. Among the new technologies, multi-layer casting is gaining popularity. A feature of the method is the production of a multi-component sheet, each layer of which has different characteristics.

The outer surface of the composite is formed by a high-quality polymer, the inner layers are recycled plastic.

Polystyrene (UPS, PSM) ABS plastic

Various types of polystyrene are recycled in one mass - impact-resistant modifications, copolymers, acrylonitrile butadiene styrene. The versatility of products made from PS is often the reason why industrialists refuse to process it. The price of cleaning, sorting, modification is too high.

Prospects for plastics recycling.

In developed economies, the share of plastic processing reaches 26% of the generated amount - up to 90 million tons. At the same time, the volume the world market is 600 billion dollars. The domestic segment of polymer recycling looks somewhat more modest: 5.5 million tons. According to experts, the demand of the Russian industry for monomers and full-fledged modified thermoplastics significantly exceeds their supply. The presence of these two factors leads to an increase in national capacities for polymer processing. Moreover, the growth rates of industrial volumes in this area are ahead of European ones. Existing market trends are taken into account in government forecasts. The priority of re-equipment of the processing industry is laid down in the twenty-year sectoral plan for the development of gas and petrochemistry.

During the operation of products made of polymers, wastes appear.

Used polymers under the influence of temperature, environment, air oxygen, various radiation, moisture, depending on the duration of these influences, change their properties. Significant volumes of polymer materials that have been used for a long time and are thrown into landfills pollute the environment, so the problem of recycling polymer waste is extremely relevant. At the same time, these wastes are good raw materials with appropriate adjustment of the compositions for the manufacture of products for various purposes.

Used polymeric building materials include polymeric films used for covering greenhouses, for packaging building materials and products; barn flooring: rolled and tiled polymeric materials for floors, finishing materials for walls and ceilings; heat and sound insulating polymeric materials; containers, pipes, cables, molded and profile products, etc.

In the process of collection and disposal of secondary polymeric raw materials, various methods for identifying polymers are used. Among the many methods, the following are the most common:

· IR-spectroscopy (comparison of the spectra of known polymers with recyclable ones);

Ultrasound (US). It is based on the attenuation of US. Index is determined HL the ratio of the attenuation of the sound wave to the frequency. The ultrasonic device is connected to a computer and installed on the technological line of waste disposal. For example, index HL LDPE 2.003 10 6 sec with a deviation of 1.0%, and HL PA-66 - 0.465 10 6 sec with a deviation of ± 1.5%;

· X-rays;

laser pyrolysis spectroscopy.

The separation of mixed (domestic) waste thermoplastics by type is carried out by the following main methods: flotation, separation in liquid media, aero separation, electro separation, chemical methods and deep cooling methods. The most widely used method is the flotation method, which allows the separation of mixtures of industrial thermoplastics such as PE, PP, PS and PVC. Separation of plastics is carried out by adding surfactants to water, which selectively change their hydrophilic properties. In some cases, an effective way to separate polymers may be to dissolve them in a common solvent or in a mixture of solvents. By treating the solution with steam, PVC, PS and a mixture of polyolefins are isolated; purity of products - not less than 96%. Flotation and separation methods in heavy media are the most efficient and cost-effective of all those listed above.

Recycling of used polyolefins

Waste from agricultural PE film, fertilizer bags, pipes for various purposes, out of service, waste from other sources, as well as mixed waste are to be disposed of with their subsequent use. For this, special extrusion plants are used for their processing. When polymer waste is received for processing, the melt flow rate must be at least 0.1 g/10 min.

Before starting processing, a rough separation of waste is carried out, taking into account their distinctive features. After that, the material is subjected to mechanical grinding, which can be either at normal (room) temperature or in a cryogenic method (in an environment of refrigerants, for example, liquid nitrogen). Shredded waste is fed into the washing machine for washing, which is carried out in several stages with special washing mixtures. The mass wrung out in a centrifuge with a moisture content of 10–15% is fed for final dehydration to a dryer, to a residual moisture content of 0.2%, and then to an extruder. The polymer melt is fed by the extruder screw through the filter into the strand head. The cassette or rewind filter is used to clean the polymer melt from various impurities. The purified melt is pressed through the strand holes of the head, at the exit of which the strands are cut with knives into granules of a certain size, which then fall into the cooling chamber. Passing through a special installation, the granules are dehydrated, dried and packed into bags. If it is necessary to process thin PO films, then an agglomerator is used instead of an extruder.

Drying of waste is carried out by various methods, using shelf, belt, bucket, fluidized bed, vortex and other dryers, the productivity of which reaches 500 kg/h. Due to the low density, the film floats, and the dirt settles on the bottom.

Dehydration and drying of the film is carried out on a vibrating screen and in a vortex separator, its residual moisture content is not more than 0.1%. For ease of transportation and subsequent processing into products, the film is granulated. During the granulation process, the material is compacted, its further processing is facilitated, the characteristics of secondary raw materials are averaged, as a result of which a material is obtained that can be processed on standard equipment.

For plasticization of crushed and purified polyolefin waste, single-screw extruders with a screw length (25–33) are used. D, equipped with a continuous filter for melt purification and having a degassing zone, allowing to obtain granules without pores and inclusions. When processing contaminated and mixed waste, disk extruders of a special design are used, with short multi-thread worms (3.5–5) long D having a cylindrical nozzle in the extrusion zone. The material melts in a short period of time, and fast homogenization of the melt is ensured. By changing the gap between the cone nozzle and the shell, you can adjust the shear force and friction force, while changing the mode of melting and homogenization of processing. The extruder is equipped with a degassing unit.

Granules are produced mainly in two ways: head granulation and underwater granulation. The choice of granulation method depends on the properties of the thermoplastic being processed and, in particular, on the viscosity of its melt and adhesion to the metal. During granulation on the head, the polymer melt is squeezed out through a hole in the form of strands, which are cut off by knives sliding along the spinneret plate. The resulting granules with a size of 4–5 mm (in length and diameter) are discarded with a knife from the head into the cooling chamber, and then fed into the moisture extraction device.

When using equipment with a large unit capacity, underwater granulation is used. With this method, the polymer melt is extruded in the form of strands through the holes of the die plate on the die. After passing through a cooling bath with water, the strands enter the cutting device, where they are cut into pellets by rotating cutters.

The temperature of the cooling water entering the bath along the countercurrent of the strands is maintained within 40–60 °C, and the amount of water is 20–40 m 3 per 1 ton of granulate.

Depending on the size of the extruder (the size of the screw diameter and its length), the productivity varies depending on the rheological characteristics of the polymer. The number of outlet holes in the head can be in the range of 20–300.

From the granulate, packages for household chemicals, hangers, construction parts, pallets for transporting goods, exhaust pipes, lining of drainage channels, non-pressure pipes for melioration and other products are obtained, which are characterized by reduced durability compared to products obtained from virgin polymer. Studies of the mechanism of degradation processes occurring during the operation and processing of polyolefins, their quantitative description allow us to conclude that the products obtained from recycled materials must have reproducible physical, mechanical and technological indicators.

More acceptable is the addition of secondary raw materials to the primary in the amount of 20–30%, as well as the introduction of plasticizers, stabilizers, fillers up to 40–50% into the polymer composition. Chemical modification of recycled polymers, as well as the creation of highly filled recycled polymer materials, allows even wider use of used polyolefins.

Modification of recycled polyolefins

Methods for modifying recycled polyolefin raw materials can be divided into chemical (crosslinking, the introduction of various additives, mainly of organic origin, treatment with organosilicon liquids, etc.) and physical and mechanical (filling with mineral and organic fillers).

For example, the maximum content of the gel fraction (up to 80%) and the highest physical and mechanical properties of cross-linked HLDPE are achieved with the introduction of 2–2.5% dicumyl peroxide on rollers at 130°C for 10 min. The relative elongation at break of such a material is 210%, the melt flow rate is 0.1–0.3 g/10 min. The degree of crosslinking decreases with an increase in temperature and an increase in the duration of rolling as a result of a competing degradation process. This allows you to adjust the degree of crosslinking, physical, mechanical and technological characteristics of the modified material. A method has been developed for molding products from HLDPE by introducing dicumyl peroxide directly in the process of processing, and prototypes of pipes and molded products containing 70–80% of the gel fraction have been obtained.

The introduction of wax and elastomer (up to 5 parts by mass) significantly improves the processability of VPE, increases the physical and mechanical properties (especially elongation at break and crack resistance - by 10% and from 1 to 320 hours, respectively) and reduces their spread, which indicates an increase in the homogeneity of the material.

Modification of HLDPE with maleic anhydride in a disk extruder also leads to an increase in its strength, heat resistance, adhesiveness and resistance to photoaging. In this case, the modifying effect is achieved at a lower concentration of the modifier and a shorter duration of the process than with the introduction of elastomer. A promising way to improve the quality of polymer materials from recycled polyolefins is thermomechanical treatment with organosilicon compounds. This method allows to obtain products from recycled materials with increased strength, elasticity and resistance to aging.

The modification mechanism consists in the formation of chemical bonds between the siloxane groups of the organosilicon liquid and unsaturated bonds and oxygen-containing groups of secondary polyolefins.

The technological process for obtaining a modified material includes the following stages: sorting, crushing and washing of waste; waste treatment with organosilicon liquid at 90±10 °C for 4–6 hours; drying of modified waste by centrifugation; regranulation of modified waste.

In addition to the solid-phase modification method, a method for modifying VPE in solution is proposed, which makes it possible to obtain an VLDPE powder with a particle size of not more than 20 μm. This powder can be used for processing into products by rotational molding and for coating by electrostatic spraying.

Filled polymer materials based on recycled polyethylene raw materials

Of great scientific and practical interest is the creation of filled polymeric materials based on recycled polyethylene raw materials. The use of polymeric materials from recycled materials containing up to 30% filler will make it possible to release up to 40% of primary raw materials and send it to the production of products that cannot be obtained from secondary raw materials (pressure pipes, packaging films, reusable transport containers, etc.).

To obtain filled polymeric materials from recycled materials, it is possible to use dispersed and reinforcing fillers of mineral and organic origin, as well as fillers that can be obtained from polymeric waste (crushed thermoset waste and rubber crumb). Almost all thermoplastic waste can be filled, as well as mixed waste, which for this purpose is also preferable from an economic point of view.

For example, the expediency of using lignin is associated with the presence of phenolic compounds in it, which contribute to the stabilization of WPE during operation; mica - with the production of products with low creep, increased heat and weather resistance, and also characterized by low wear of processing equipment and low cost. Kaolin, limestone, oil shale ash, coal spheres and iron are used as cheap inert fillers.

With the introduction of finely dispersed phosphogypsum granulated in polyethylene wax into WPE, compositions with increased elongation at break were obtained. This effect can be explained by the plasticizing effect of polyethylene wax. Thus, the tensile strength of VPE filled with phosphogypsum is 25% higher than that of VPE, and the tensile modulus is 250% higher. The reinforcing effect when mica is introduced into the HPE is associated with the features of the crystalline structure of the filler, a high characteristic ratio (the ratio of the flake diameter to the thickness), and the use of crushed, powdery WPE makes it possible to preserve the structure of the flakes with minimal destruction.

Among polyolefins, along with polyethylene, significant volumes fall on the production of products from polypropylene (PP). The increased strength properties of PP in comparison with polyethylene and its resistance to the environment indicate the relevance of its recycling. The secondary PP contains a number of impurities, such as Ca, Fe, Ti, Zn, which contribute to the formation of crystal nuclei and the creation of a crystalline structure, which leads to an increase in the rigidity of the polymer and high values ​​of both the initial elastic modulus and the quasi-equilibrium modulus. To assess the mechanical performance of polymers, the method of relaxation stresses at various temperatures is used. Secondary PP under the same conditions (in the temperature range of 293–393 K) withstands much higher mechanical stresses without destruction than the primary one, which makes it possible to use it for the manufacture of rigid structures.

Recycling of used polystyrene

Used polystyrene plastics can be used in the following areas: recycling of technological waste of high impact polystyrene (HIPS) and acrylonitrile butadiene styrene (ABS) - plastic by injection molding, extrusion and pressing; disposal of used products, EPS waste, mixed waste, disposal of heavily polluted industrial waste.

Significant volumes of polystyrene (PS) fall on foamed materials and products made from them, the density of which is in the range of 15–50 kg/m 3 . These materials are used to make mold matrices for packaging, cable insulation, boxes for packing vegetables, fruits and fish, insulation for refrigerators, refrigerators, pallets for fast food restaurants, formwork, heat and sound insulation boards for insulating buildings and structures, etc. In addition, when transporting used such products, transportation costs are sharply reduced due to the low bulk density of foamed PS waste.

One of the main methods of recycling foamed polystyrene waste is a mechanical recycling method. For agglomeration, specially designed machines are used, and for extrusion, twin-screw extruders with degassing zones are used.

The consumer point is the main location for the mechanical recycling of used EPS products waste. Contaminated foamed PS waste is subject to inspection and sorted. At the same time, impurities are removed in the form of paper, metal, other polymers and various inclusions. The polymer is crushed, washed and dried. The polymer is dehydrated by centrifugation. The final grinding is carried out in a drum, and from it the waste enters a special extruder, in which the polymer prepared for processing is compressed and melted at a temperature of about 205–210 °C. For additional purification of the polymer melt, a filter is installed, which operates on the principle of rewinding the filter material or a cassette type. The filtered polymer melt enters the degassing zone, where the screw has a deeper thread compared to the compression zone. Next, the polymer melt enters the strand head, the strands are cooled, dried and granulated. In the process of mechanical regeneration of PS waste, processes of destruction and structuring occur, so it is important that the material is subjected to minimal shear stress (a function of screw geometry, speed and melt viscosity) and a short residence time under thermomechanical load. The reduction of destructive processes is carried out due to the halogenation of the material, as well as the introduction of various additives into the polymer.

The mechanical recycling of expanded polystyrene is regulated based on the area of ​​application of the recycled polymer, for example, for the production of insulation, cardboard, cladding, etc.

There is a method for depolymerization of polystyrene waste. To do this, PS or foamed PS waste is crushed, loaded into a hermetic vessel, heated to the decomposition temperature, and the released secondary styrene is cooled in a refrigerator and the monomer thus obtained is collected in a hermetic vessel. The method requires complete sealing of the process and significant energy consumption.

Recycling of used polyvinyl chloride (PVC)

Recycling of recycled PVC involves the processing of used films, fittings, pipes, profiles (including window frames), containers, bottles, plates, roll materials, cable insulation, etc.

Depending on the composition of the composition, which may consist of vinyl plastic or plastic compound and the purpose of recycled PVC, recycling methods may be different.

For recycling, PVC product waste is washed, dried, crushed and separated from various inclusions, incl. metals. If products are made from compositions based on plasticized PVC, cryogenic grinding is most often used. If the products are made of rigid PVC, then mechanical crushing is used.

The pneumatic method is used to separate the polymer from the metal (wires, cables). The separated plasticized PVC can be processed by extrusion or injection molding. The magnetic separation method can be used to remove metallic and mineral inclusions. To separate the aluminum foil from the thermoplastic, heating in water at 95–100 °C is used.

Separation of labels from unusable containers is carried out by immersion in liquid nitrogen or oxygen at a temperature of about -50 ° C, which makes the labels or adhesive brittle and then allows them to be easily crushed and separate a homogeneous material, such as paper. For the processing of artificial leather (IR) waste, PVC-based linoleums, a method for the dry preparation of plastic waste using a compactor is proposed. It includes a number of technological operations: grinding, separation of textile fibers, plasticization, homogenization, compaction and granulation, where additives can also be introduced.

Cable waste with PVC insulation enters the crusher and is fed by a conveyor to the loading hopper of the cryogenic mine, which is a sealed container with a special transport screw. Liquid nitrogen is supplied to the mine. The cooled crushed waste is unloaded to the grinding machine, and from there it enters the metal separation device, where the brittle polymer is deposited and passed through the electrostatic corona of the separator drum and copper is extracted there.

Significant volumes of used PVC bottles require different methods of their disposal. Noteworthy is the method of separating PVC from various impurities according to the density of the calcium nitrate solution in the bath.

The mechanical process of recycling PVC bottles provides for the main stages of the process of processing waste of secondary thermoplastics, but in some cases it has its own distinctive features.

During the operation of various buildings and structures, significant volumes of metal-plastic window frames based on PVC compositions that were in use are formed. Recycled PVC frames with frames, which were in use, contain approximately 30% wt. PVC and 70% wt. glass, metal, wood and rubber. On average, a window frame contains about 18 kg of PVC. The incoming frames are unloaded into a container 2.5 m wide and 6.0 m long. Then they are pressed on a horizontal press and turned into sections up to an average of 1.3–1.5 m long, after which the material is additionally pressed using a roller and fed to chopper in which the rotor rotates at an adjustable speed. A large mixture of PVC, metal, glass, rubber and wood is fed to the conveyor, and then to the magnetic separator, where the metal is separated, and then the material enters the rotating metal separation drum. This mixture is classified into particle sizes<4 мм, 4–15 мм, 15–45 мм, >45 mm.

Fractions (>45 mm) larger than usual are returned for re-crushing. A fraction of 15–45 mm in size is sent to a metal separator, and then to a rubber separator, which is a rotating drum with rubber insulation.

After removing the metal and rubber, this coarse fraction is sent back for grinding for further size reduction.

The resulting mixture with a particle size of 4-15 mm, consisting of PVC, glass, fine residue and wood waste from the silo is fed through a separator to a drum sieve. Here, the material is again divided into two fractions with particle sizes: 4–8 and 8–15 mm.

Two separate processing lines are used for each particle size range, for a total of four processing lines. The separation of wood and glass takes place in each of these processing lines. The wood is separated by using inclined vibrating air sieves. Wood, which is lighter than other materials, is transported downward by the airflow, while heavier particles (PVC, glass) are transported upward. The separation of the glass is done in a similar manner on subsequent sieves, where the lighter particles (ie PVC) are transported downwards while the heavy particles (ie glass) are transported upwards. After the removal of wood and glass, PVC fractions from all four processing lines are combined. Metal particles are detected and removed electronically.

Purified polyvinyl chloride enters the workshop, where it is moistened and granulated to a size of 3–6 mm, after which the granules are dried with hot air to a certain moisture content. Polyvinyl chloride is divided into four fractions with a particle size of 3, 4, 5 and 6 mm. Any oversized granules (i.e. > 6 mm) are returned to the area for regrinding. Rubber particles are separated from PVC on vibrating sieves.

The final step is an optoelectronic color sorting process that separates the white PVC particles from the colored ones. This is done for fractions of each size. Since the amount of colored PVC is small compared to white PVC, white PVC fractions are sized and stored in separate bins while the colored PVC streams are mixed and stored in one bin.

The process has some special features that make the operations environmentally friendly. Air pollution does not occur as the grinding and air separation is equipped with a dust extraction system that collects dust, paper and foil in the air stream and feeds them to the microfilter trap. The grinder and drum sieve are insulated to reduce the occurrence of noise.

During wet grinding and washing of PVC from contaminants, water is supplied for re-cleaning.

Recycled PVC is used in the production of new co-extrusion window profiles. To achieve the high surface quality required for co-extrusion profiled window frames, the inside of the frames is made from recycled PVC and the outside is made from virgin PVC. The new frames contain 80% by weight recycled PVC and are comparable in mechanical and performance properties to frames made from 100% virgin PVC.

The main methods for recycling PVC plastic waste include injection molding, extrusion, calendering, and pressing.

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