Polymers, or macromolecules, are very large molecules formed by the bonds of many small molecules, which are called constituent units, or monomers. Molecules are so large that their properties do not change significantly when a few of these units are added or removed. The term "polymeric materials" is generic. It combines three broad groups of synthetic plastics, namely: polymers; plastics and their morphological variety - polymer composite materials (PCM) or, as they are also called, reinforced plastics. The common thing for the listed groups is that their mandatory part is the polymer component, which determines the main thermal deformation and technological properties of the material. The polymer component is an organic high-molecular substance obtained as a result of a chemical reaction between the molecules of the initial low-molecular substances - monomers.
Polymers are usually called high-molecular substances (homopolymers) with additives introduced into them, namely stabilizers, inhibitors, plasticizers, lubricants, antirads, etc. Physically, polymers are homophase materials, they retain all the physical and chemical features inherent in homopolymers.
Plastics are composite materials based on polymers containing dispersed or short-fiber fillers, pigments and other bulk components. Fillers do not form a continuous phase. They (disperse medium) are located in the polymer matrix (dispersion medium). Physically, plastics are heterophase materials with isotropic (the same in all directions) physical macroproperties.
Plastics can be divided into two main groups - thermoplastic and thermoset. Thermoplastics are those that, once shaped, can be melted down and molded again; thermoset, molded once, no longer melt and cannot take another shape under the influence of temperature and pressure. Almost all plastics used in packaging are thermoplastic, for example, polyethylene and polypropylene (members of the polyolefin family), polystyrene, polyvinyl chloride, polyethylene terephthalate, nylon (nylon), polycarbonate, polyvinyl acetate, polyvinyl alcohol and others.
Plastics can also be categorized according to the method used to polymerize them into polymers obtained by addition to polycondensation. Addition polymers are produced by a mechanism that involves either free radicals or ions, whereby small molecules are rapidly added to the growing chain without the formation of accompanying molecules. Polycondensation polymers are produced by reacting the functional groups in the molecules with each other so that a long polymer chain is formed step by step and a low molecular weight co-product such as water is typically formed during each reaction step. Most packaging polymers, including polyolefins, polyvinyl chloride and polystyrene, are addition polymers.
The chemical and physical properties of plastics are determined by their chemical composition, average molecular weight and molecular weight distribution, processing (and usage) history, and the presence of additives.
Polymer reinforced materials are a type of plastics. They differ in that they use not dispersed, but reinforcing, that is, reinforcing fillers (fibers, fabrics, ribbons, felt, single crystals), which form an independent continuous phase in PCM. Separate varieties of such PCMs are called laminated plastics. This morphology makes it possible to obtain plastics with very high deformation-strength, fatigue, electrophysical, acoustic and other target characteristics that meet the highest modern requirements.
The polymerization reaction is the sequential addition of molecules of unsaturated compounds to each other with the formation of a high molecular weight product - a polymer. Alkene molecules that undergo polymerization are called monomers. The number of elementary units repeating in a macromolecule is called the degree of polymerization (denoted by n). Depending on the degree of polymerization, substances with different properties can be obtained from the same monomers. Thus, short chain polyethylene (n = 20) is a liquid with lubricating properties. Polyethylene with a chain length of 1500-2000 links is a hard but flexible plastic material from which it is possible to obtain films, make bottles and other utensils, elastic pipes, etc. Finally, polyethylene with a target length of 5-6 thousand links is a solid substance from which cast products, rigid pipes, strong threads can be prepared.
If a small number of molecules take part in the polymerization reaction, then low-molecular substances are formed, for example, dimers, trimers, etc. The conditions for the occurrence of polymerization reactions are very different. In some cases catalysts and high pressure are needed. But the main factor is the structure of the monomer molecule. Unsaturated (unsaturated) compounds enter the polymerization reaction due to the breaking of multiple bonds. The structural formulas of polymers are briefly written as follows: the formula of the elementary unit is enclosed in brackets and the letter n is put at the bottom right. For example, the structural formula of polyethylene is (-CH2-CH2-) n. It is easy to conclude that the name of the polymer is composed of the name of the monomer and the prefix poly-, for example, polyethylene, polyvinyl chloride, polystyrene, etc.
Polymerization is a chain reaction, and in order for it to start, it is necessary to activate the monomer molecules with the help of so-called initiators. Such reaction initiators can be free radicals or ions (cations, anions). Depending on the nature of the initiator, radical, cationic, or anionic polymerization mechanisms are distinguished.
The most common hydrocarbon polymers are polyethylene and polypropylene.
Polyethylene is obtained by polymerization of ethylene: Polypropylene is obtained by stereospecific polymerization of propylene (propene). Stereospecific polymerization is the process of obtaining a polymer with a strictly ordered spatial structure. Many other compounds are capable of polymerization - derivatives of ethylene, having the general formula CH2 = = CH-X, where X are various atoms or groups of atoms.
Types of polymers:
Polyolefins are a class of polymers of the same chemical nature (chemical formula -(CH2)-n) with a diverse spatial structure of molecular chains, including polyethylene and polypropylene. By the way, all carbohydrates, for example, natural gas, sugar, paraffin and wood, have a similar chemical structure. In total, 150 million tons of polymers are produced annually in the world, and polyolefins account for approximately 60% of this amount. In the future, polyolefins will surround us to a much greater extent than today, so it is useful to take a closer look at them.
The complex of properties of polyolefins, including such as resistance to ultraviolet, oxidants, tear, piercing, shrinkage during heating and tear, varies over a very wide range depending on the degree of orientational stretching of molecules in the process of obtaining polymeric materials and products.
Especially it should be emphasized that polyolefins are environmentally cleaner than most materials used by man. In the production, transportation and processing of glass, wood and paper, concrete and metal, a lot of energy is used, the production of which inevitably pollutes the environment. Disposing of traditional materials also releases harmful substances and wastes energy. Polyolefins are produced and disposed of without emission of harmful substances and with minimal energy consumption, and when burning polyolefins, a large amount of net heat is released with by-products in the form of water vapor and carbon dioxide. Polyethylene
About 60% of all plastics used for packaging are polyethylene, mainly due to its low cost, but also due to its excellent properties for many applications. High density polyethylene (HDPE - low pressure) has the simplest structure of all plastics, it consists of repeating units of ethylene. -(CH2CH2)n- high density polyethylene. Low density polyethylene (LDPE - high pressure) have the same chemical formula, but differ in that its structure is branched. -(CH2CHR) n- low density polyethylene Where R can be -H, -(CH2)nCH3, or more complex secondary branching.
Polyethylene, due to its simple chemical structure, easily folds into a crystal lattice, and therefore tends to have a high degree of crystallinity. Chain branching interferes with this ability to crystallize, resulting in fewer molecules per unit volume, and hence lower density.
LDPE - high pressure polyethylene. Plastic, slightly matte, waxy to the touch, processed by extrusion into blown tubular film or into flat film through a flat die and a chilled roller. LDPE film is strong in tension and compression, resistant to impact and tear, strong at low temperatures. It has a feature - a rather low softening temperature (about 100 degrees Celsius).
HDPE - low pressure polyethylene. HDPE film is tough, durable, less waxy to the touch compared to LDPE films. Obtained by blown sleeve extrusion or flat sleeve extrusion. The softening temperature of 121°C allows steam sterilization. The frost resistance of these films is the same as that of HDPE films. The resistance to stretching and compression is high, and the resistance to impact and tear is less than that of LDPE films. Films from HDPE are an excellent barrier to moisture. Resistant to fats, oils. The "rustling" T-shirt bag ("rustle") in which you pack your purchases is made of HDPE.
There are two main types of HDPE. The "older" type, produced first in the 1930s, polymerizes at high temperatures and pressures, conditions that are energetic enough to allow a marked occurrence of chain reactions that lead to the formation of branches, both long and short. chains. This type of HDPE is sometimes referred to as high pressure polyethylene (LDPE, HD-HDPE, because of the high pressure) if it is necessary to distinguish it from the linear low pressure polyethylene, the "younger" type of LDPE. At room temperature, polyethylene is a fairly soft and flexible material. It retains this flexibility well in cold conditions, so it is applicable in frozen food packaging. However, at elevated temperatures, such as 100°C, it becomes too soft for some applications. HDPE has a higher brittleness and softening point than LDPE, but is still not a suitable hot-fill container.
About 30% of all plastics used for packaging are HDPE. It is the most widely used bottle plastic, due to its low cost, ease of molding, and excellent performance in many applications. In its natural form, HDPE has a milky white, translucent appearance, and thus is not suitable for applications where exceptional transparency is required. One disadvantage of using HDPE in some of the applications is its tendency to stress stress cracking, defined as failure of a plastic container under conditions of both stress and contact with the product, which alone does not result in failure. Environmental stress cracking in polyethylene is related to the crystallinity of the polymer.
LDPE is the most widely used packaging polymer, representing about one third of all packaging plastics. Due to its low crystallinity, it is a softer, more flexible material than HDPE. It is the material of choice for films and bags due to its low cost. LDPE has better clarity than HDPE, but still does not have the crystal clarity that is desirable for some packaging applications.
PP - polypropylene. Excellent clarity (with rapid cooling during shaping), high melting point, chemical and water resistance. PP allows water vapor to pass through, which makes it indispensable for "anti-fogging" food packaging (bread, herbs, groceries), as well as in construction for hydro-wind insulation. PP is sensitive to oxygen and oxidants. It is processed by blown extrusion or through a flat die with pouring onto a drum or cooling in a water bath. It has good transparency and gloss, high chemical resistance, especially to oils and fats, does not crack under the influence of the environment.
PVC - polyvinyl chloride. In its pure form, it is rarely used due to fragility and non-elasticity. Inexpensive. It can be processed into a film by blown extrusion or flat slot extrusion. The melt is highly viscous. PVC is thermally unstable and corrosive. When overheated and burning, it releases a highly toxic chlorine compound - dioxin. Widespread in the 60s and 70s. It is replaced by more environmentally friendly polypropylene.
Polymer identification
Consumers of polymer films very often face the practical task of recognizing the nature of the polymeric materials from which they are made. The main properties of polymeric materials, as is well known, are determined by the composition and structure of their macromolecular chains. Hence, it is clear that, in the first approximation, it may be sufficient to estimate the functional groups that make up the macromolecules to identify polymer films. Some polymers, due to the presence of hydroxyl groups (-OH), tend to water molecules. This explains the high hygroscopicity of, for example, cellulose films and a noticeable change in their performance characteristics when moistened. Other polymers (polyethylene terephthalate, polyethylenes, polypropylene, etc.) do not have such groups at all, which explains their rather good water resistance.
The presence of certain functional groups in a polymer can be determined on the basis of existing and scientifically substantiated instrumental research methods. However, the practical implementation of these methods is always associated with relatively large time costs and is due to the availability of appropriate types of rather expensive test equipment that requires appropriate qualifications for its use. At the same time, there are quite simple and "quick" practical methods for recognizing the nature of polymer films. These methods are based on the fact that polymer films from various polymeric materials differ from each other in their external features, physical and mechanical properties, as well as in relation to heating, the nature of their combustion and solubility in organic and inorganic solvents.
In many cases, the nature of polymeric materials from which polymer films are made can be determined by external features, in the study of which special attention should be paid to the following features: surface condition, color, gloss, transparency, rigidity and elasticity, tear resistance, etc. For example , non-oriented films made of polyethylene, polypropylene and polyvinyl chloride are easily stretched. Films made of polyamide, cellulose acetate, polystyrene, oriented polyethylene, polypropylene, polyvinyl chloride do not stretch well. Cellulose acetate films are not tear resistant, split easily in a direction perpendicular to their orientation, and also rustle when crushed. More tear-resistant polyamide and lavsan (polyethylene terephthalate) films, which also rustle when crushed. At the same time, films made of low-density polyethylene, plasticized polyvinyl chloride do not rustle when crushed and have high tear resistance. The results of studying the external features of the studied polymer film should be compared with the characteristic features given in Table. 1, after which some preliminary conclusions can already be drawn.
Table 1. External signs
|
Type of polymer |
Mechanical signs |
Surface condition to the touch |
Color |
Transparency |
Shine |
|
Soft, elastic, tear resistant |
Soft, smooth |
Colorless |
transparent |
||
|
Slightly oily, smooth, sweet |
Colorless |
translucent |
|||
|
Rigid, slightly elastic, tear resistant |
Dry, smooth |
Colorless |
translucent or transparent |
||
|
Tough, tear resistant |
Dry, smooth |
Colorless |
transparent |
||
|
Soft, tear resistant |
Dry, smooth |
Colorless |
transparent |
||
|
Rigid, tear resistant |
Colorless |
transparent |
|||
|
Dry, smooth |
Colorless or light yellow |
translucent |
|||
|
Rigid, weakly tear resistant |
Dry, smooth, very rustling |
Colorless or with a bluish tinge |
transparent |
||
|
Rigid, weakly tear resistant |
Dry, smooth, very rustling |
Colorless, with a yellowish or bluish tint |
highly transparent |
||
|
Rigid, not tear resistant |
Dry, smooth |
Colorless |
highly transparent |
||
|
Cellophane |
Rigid, not tear resistant |
Dry, smooth |
Colorless |
highly transparent |
However, as it is easy to understand from the analysis of the data given in Table. 2, it is not always possible to unambiguously establish the nature of the polymer from which the film is made by external signs. In this case, it is necessary to try to quantify some physical and mechanical characteristics of the existing sample of the polymer film. As can be seen, for example, from the data given in Table. 2, the density of some polymeric materials (LDPE, HDPE, PP) is less than unity, and, therefore, the samples of these films should "float" in water. In order to clarify the type of polymeric material from which the film is made, the density of the existing sample should be determined by measuring its weight and calculating or measuring its volume. The refinement of the nature of polymeric materials is also facilitated by experimental data on such physical and mechanical characteristics as ultimate strength and elongation in uniaxial tension, as well as melting temperature (Table 2). In addition, as can be seen from the analysis of the data given in Table. 2, the permeability of polymer films with respect to various media also significantly depends on the type of material from which they are made.
Table 2. Physical and mechanical characteristics at 20°C
|
Type of polymers |
Density kg / m 3 |
Tensile strength, MPa |
Elongation at break, % |
Water vapor permeability, g/m 2 for 24 hours |
Oxygen permeability, cm 3 / (m 2 hatm) for 24 hours |
Permeability to CO 2, cm 3 / (m 2 ham) in 24 hours |
Melting point, 0 С |
|
Cellophane |
In addition to the distinctive features in the physical and mechanical characteristics, the existing differences in the characteristic features of various polymers during their combustion should be noted. This fact makes it possible to use in practice the so-called thermal method of identification of polymer films. It consists in the fact that a sample of the film is set on fire and kept in an open flame for 5-10 seconds, while fixing the following properties: the ability to burn and its nature, the color and nature of the flame, the smell of combustion products, etc. The characteristic signs of combustion are most clearly are observed at the moment of ignition of the samples. To establish the type of polymer material from which the film is made, it is necessary to compare the results of the test with the data on the characteristic features of the behavior of polymers during combustion, given in Table. 3.
Table 3. Combustion characteristics. Chemical resistance
|
Type of polymer |
combustibility |
Flame coloring |
The smell of combustion products |
Chem. acid resistance |
Chem. alkali resistance |
|
Inside bluish, no soot |
burning paraffin |
Excellent |
|||
|
Burns in flame and when removed |
Inside bluish, no soot |
burning paraffin |
Excellent |
||
|
Burns in flame and when removed |
Inside bluish, no soot |
burning paraffin |
Excellent |
||
|
Greenish with soot |
hydrogen chloride |
||||
|
Difficult to ignite and extinguish |
Greenish with soot |
Hydrogen chloride |
Excellent |
Excellent |
|
|
Illuminates and burns out of flame |
Yellowish with strong soot |
Sweet, unpleasant |
Excellent |
||
|
Burns and self-extinguishes |
Blue, yellowish around the edges |
Burnt horn or feather |
|||
|
Difficult to ignite and extinguish |
Glowing |
Sweetish |
Excellent |
Excellent |
|
|
Difficult to ignite and extinguish |
Yellowish with soot |
Burnt paper |
|||
|
Burning in flame |
sparkling |
Acetic acid |
|||
|
Cellophane |
Burning in flame |
Burnt paper |
As can be seen from the data given in table. 3, according to the nature of combustion and the smell of combustion products, polyolefins (polyethylene and polypropylene) resemble paraffin. This is quite understandable, since the elemental chemical composition of these substances is the same. This makes it difficult to distinguish between polyethylene and polypropylene. However, with a certain skill, polypropylene can be distinguished by the sharper smells of combustion products with hints of burnt rubber or burning sealing wax.
Thus, the results of a comprehensive assessment of individual properties of polymer films in accordance with the methods outlined above make it possible in most cases to fairly reliably establish the type of polymer material from which the studied samples are made. If difficulties arise in determining the nature of the polymeric materials from which the films are made, it is necessary to conduct additional studies of their properties by chemical methods. To do this, samples can be subjected to thermal decomposition (pyrolysis), while the presence of characteristic atoms (nitrogen, chlorine, silicon, etc.) or groups of atoms (phenol, nitro groups, etc.) prone to specific reactions, as a result of which a well-defined indicator effect is detected. The above practical methods for determining the type of polymeric materials from which polymeric films are made are subjective to a certain extent, and, therefore, cannot guarantee their 100% identification. If such a need nevertheless arises, then you should use the services of special testing laboratories, the competence of which is confirmed by the relevant certification documents.
Melt flow index
The melt flow index of a polymer material is the mass of polymer in grams extruded through a capillary at a certain temperature and a certain pressure drop in 10 minutes. The determination of the value of the melt flow index is carried out on special devices called capillary viscometers. The dimensions of the capillary are standardized: length 8.000±0.025 mm; diameter 2.095±0.005 mm; the inner diameter of the viscometer cylinder is 9.54±0.016 mm. The non-integer values of the capillary sizes are due to the fact that for the first time the method for determining the melt flow index appeared in countries with the English system of measures. The conditions recommended for determining the melt flow index are regulated by the relevant standards. GOST 11645-65 recommends loads of 2.16 kg, 5 kg and 10 kg and temperatures in multiples of 10°C. ASTM 1238-62T (USA) recommends temperatures from 125°C to 275°C and loads from 0.325 kg to 21.6 kg. Most often, the melt flow index is determined at a temperature of 190°C and a load of 2.16 kg.
The value of the flow index for various polymeric materials is determined at various loads and temperatures. Therefore, it must be borne in mind that the absolute values of the flow index are comparable only for the same material. So, for example, you can compare the value of the melt flow index of low density polyethylene of different grades. Comparison of the values of the flow rates of high and low density polyethylene does not make it possible to directly compare the flow of both materials. Since the first is determined with a load of 5 kg, and the second with a load of 2.16 kg.
It should be noted that the viscosity of polymer melts depends significantly on the applied load. Since the flow index of a particular polymer material is measured only at one load value, this index characterizes only one point on the entire flow curve in the region of relatively low shear stresses. Therefore, polymers that differ slightly in macromolecular branching or molecular weight, but with the same melt flow index, may behave differently depending on the processing conditions. However, despite this, according to the melt flow rate for many polymers, the boundaries of the recommended technological parameters of the processing process are set. Considerable distribution of this method is explained by its speed and availability. Film extrusion processes require high melt viscosities; therefore, raw material grades with a low melt flow rate are used.
According to the materials of the company "NPL Plastic"
The author of this article is Academician Viktor Aleksandrovich Kabanov, an outstanding scientist in the field of macromolecular chemistry, a student and successor of Academician V.A. Kargin, one of the world leaders in polymer science, the founder of a large scientific school, the author of a large number of works, books and teaching aids.
Polymers (from the Greek polymeres - consisting of many parts, diverse) are chemical compounds with a high molecular weight (from several thousand to many millions), the molecules of which (macromolecules) consist of a large number of repeating groups (monomeric units). The atoms that make up the macromolecules are connected to each other by the forces of the main and (or) coordination valences.
Classification of polymers
By origin, polymers are divided into natural (biopolymers), such as proteins, nucleic acids, natural resins, and synthetic, such as polyethylene, polypropylene, phenol-formaldehyde resins.
Atoms or atomic groups can be arranged in a macromolecule in the form:
- an open chain or a sequence of cycles stretched in a line (linear polymers, such as natural rubber);
- branched chains (branched polymers, eg amylopectin);
- 3D mesh (cross-linked polymers, such as cured epoxy resins).
Polymers whose molecules consist of identical monomer units are called homopolymers, for example polyvinyl chloride, polycaproamide, cellulose.
Macromolecules of the same chemical composition can be built from units of different spatial configurations. If macromolecules consist of the same stereoisomers or of different stereoisomers alternating in a chain at a certain frequency, the polymers are called stereoregular (see Stereoregular polymers).
What are copolymers
Polymers whose macromolecules contain several types of monomer units are called copolymers. Copolymers in which links of each type form sufficiently long continuous sequences that replace each other within the macromolecule are called block copolymers. One or more chains of another structure can be attached to the internal (non-terminal) links of a macromolecule of one chemical structure. Such copolymers are called graft copolymers (see also Copolymers).
Polymers in which each or some of the stereoisomers of a link form sufficiently long continuous sequences that replace each other within one macromolecule are called stereoblock copolymers.
Heterochain and homochain polymers
Depending on the composition of the main (main) chain, polymers are divided into: heterochain, the main chain of which contains atoms of various elements, most often carbon, nitrogen, silicon, phosphorus, and homochain, the main chains of which are built from identical atoms. Of the homochain polymers, the most common are carbon chain polymers, the main chains of which consist only of carbon atoms, for example, polyethylene, polymethyl methacrylate, polytetrafluoroethylene. Examples of heterochain polymers. - polyesters (polyethylene terephthalate, polycarbonates, etc.), polyamides, urea-formaldehyde resins, proteins, some organosilicon polymers. polymers whose macromolecules, along with hydrocarbon groups, contain atoms of inorganic elements are called organoelement polymers (see Organoelement polymers). a separate group of polymers. form inorganic polymers, such as plastic sulfur, polyphosphonitrile chloride (see Inorganic polymers).
Properties and key characteristics of polymers
Linear polymers have a specific complex and . The most important of these properties are: the ability to form high-strength anisotropic highly oriented fibers and films; the ability to large, long-term developing reversible deformations; the ability to swell in a highly elastic state before dissolution; high viscosity solutions (see Polymer Solutions, Swelling). This set of properties is due to the high molecular weight, chain structure, and flexibility of macromolecules. With the transition from linear chains to branched, sparse three-dimensional grids and, finally, to dense network structures, this set of properties becomes less and less pronounced. Highly cross-linked polymers are insoluble, infusible and incapable of highly elastic deformations.
Polymers can exist in crystalline and amorphous states. A necessary condition for crystallization is the regularity of sufficiently long segments of the macromolecule. in crystalline polymers. the appearance of various supramolecular structures (fibrils, spherulites, single crystals, etc.) is possible, the type of which largely determines the properties of the polymer material. Supramolecular structures in non-crystallized (amorphous) polymers are less pronounced than in crystalline ones.
Non-crystallized polymers can be in three physical states: glassy, highly elastic and viscous. polymers with a low (below room) transition temperature from a glassy to a highly elastic state are called elastomers, and those with a high temperature are called plastics. Depending on the chemical composition, structure and mutual arrangement of macromolecules, the properties of polymers. can vary over a very wide range. So, 1,4-cis-polybutadiene, built from flexible hydrocarbon chains, at a temperature of about 20 degrees C is an elastic material, which at a temperature of -60 degrees C goes into a glassy state; polymethyl methacrylate, built from more rigid chains, at a temperature of about 20 degrees C is a solid glassy product that passes into a highly elastic state only at 100 degrees C.
Cellulose, a polymer with very rigid chains connected by intermolecular hydrogen bonds, cannot exist at all in a highly elastic state up to the temperature of its decomposition. Large differences in the properties of P. can be observed even if the differences in the structure of macromolecules are at first glance small. So, stereoregular polystyrene is a crystalline substance with a melting point of about 235 degrees C, and non-stereoregular (atactic) is not able to crystallize at all and softens at a temperature of about 80 degrees C.
Polymers can enter into the following main types of reactions: the formation of chemical bonds between macromolecules (the so-called crosslinking), for example, during the vulcanization of rubbers, leather tanning; the breakdown of macromolecules into separate, shorter fragments (see Degradation of polymers); reactions of side functional groups of polymers. with low molecular weight substances that do not affect the main chain (the so-called polymer-analogous transformations); intramolecular reactions occurring between functional groups of one macromolecule, for example, intramolecular cyclization. Cross-linking often proceeds simultaneously with degradation. An example of polymer analogous transformations is the saponification of polyvinyl acetate, leading to the formation of polyvinyl alcohol.
The rate of polymer reactions. with low molecular weight substances is often limited by the rate of diffusion of the latter into the polymer phase. This is most clearly manifested in the case of cross-linked polymers. The rate of interaction of macromolecules with low molecular weight substances often depends significantly on the nature and location of neighboring units relative to the reacting unit. The same applies to intramolecular reactions between functional groups belonging to the same chain.
Some properties of polymers, such as solubility, viscous flow, stability, are very sensitive to the action of small amounts of impurities or additives that react with macromolecules. So, in order to turn linear polymers from soluble to completely insoluble, it is enough to form 1-2 cross-links per macromolecule.
The most important characteristics of polymers are chemical composition, molecular weight and molecular weight distribution, degree of branching and flexibility of macromolecules, stereoregularity, etc. Properties of polymers. strongly dependent on these characteristics.
Preparation of polymers
Natural polymers are formed during biosynthesis in the cells of living organisms. With the help of extraction, fractional precipitation, and other methods, they can be isolated from plant and animal raw materials. Synthetic polymers are obtained by polymerization and polycondensation. Carbochain polymers are usually synthesized by polymerization of monomers with one or more multiple carbon-carbon bonds or monomers containing unstable carbocyclic groups (for example, from cyclopropane and its derivatives). Heterochain polymers are obtained by polycondensation, as well as polymerization of monomers containing multiple carbon-element bonds (for example, C \u003d O, C º N, N \u003d C \u003d O) or weak heterocyclic groups (for example, in olefin oxides, lactams).
Application of polymers
Due to mechanical strength, elasticity, electrical insulation and other valuable properties, polymer products are used in various industries and in everyday life. The main types of polymeric materials are plastics, rubber, fibers (see Textile fibers, Chemical fibers), varnishes, paints, adhesives, and ion-exchange resins. The importance of biopolymers is determined by the fact that they form the basis of all living organisms and are involved in almost all life processes.
History reference. The term "polymeria" was introduced into science by I. Berzelius in 1833 to designate a special type of isomerism, in which substances (polymers) having the same composition have different molecular weights, for example, ethylene and butylene, oxygen and ozone. Thus, the content of the term did not correspond to modern ideas about polymers. "True" synthetic polymers were not yet known at that time.
A number of polymers were apparently obtained as early as the first half of the 19th century. However, chemists then usually tried to suppress polymerization and polycondensation, which led to the "tarring" of the products of the main chemical reaction, i.e., in fact, to the formation of a polymer. (Until now, polymers were often referred to as "resins"). The first references to synthetic polymers date back to 1838 (polyvinylidene chloride) and 1839 (polystyrene).
The chemistry of polymers arose only in connection with the creation by A. M. Butlerov of the theory of chemical structure (early 60s of the 19th century). A. M. Butlerov studied the relationship between the structure and relative stability of molecules, which manifests itself in polymerization reactions. The science of polymers received its further development (until the end of the 1920's) mainly due to intensive searches for methods for the synthesis of rubber, in which leading scientists from many countries participated (G. Bouchard, W. Tilden, German scientist C. Harries , I. L. Kondakov, S. V. Lebedev and others). In the 30s. the existence of free radical (H. Staudinger and others) and ionic (American scientist F. Whitmore and others) mechanisms of polymerization was proved. The work of W. Carothers played an important role in the development of ideas about polycondensation.
From the beginning of the 20s. 20th century theoretical ideas about the structure of polymers are also being developed. Initially, it was assumed that such biopolymers as cellulose, starch, rubber, proteins, as well as some synthetic polymers similar to them in properties (for example, polyisoprene), consist of small molecules that have an unusual ability to associate in solution into colloidal complexes due to non-covalent connections (the theory of "small blocks"). The author of a fundamentally new idea of polymers as substances consisting of macromolecules, particles of unusually large molecular weight, was G. Staudinger. The victory of the ideas of this scientist (by the beginning of the 1940s) forced us to consider polymers as a qualitatively new object of study in chemistry and physics.
Literature .: Encyclopedia of polymers, vol. 1-2, M., 1972-74; Strepikheev A. A., Derevitskaya V. A., Slonimsky G. L., Fundamentals of chemistry of macromolecular compounds, 2nd ed., [M., 1967]; Losev I. P., Trostyanskaya E. B., Chemistry of synthetic polymers, 2nd ed., M., 1964; Korshak V. V., General methods for the synthesis of macromolecular compounds, M., 1953; Kargin V. A., Slonimsky G. L., Brief essays on the physics and chemistry of polymers, 2nd ed., M., 1967; Oudian J., Fundamentals of polymer chemistry, trans. from English, M., 1974; Tager A. A., Physical Chemistry of Polymers, 2nd ed., M., 1968; Tenford Ch., Physical chemistry of polymers, trans. from English, M., 1965.
V. A. Kabanov. Source www.rubricon.ru
Foreword
All types of polymeric materials are substances in which each molecule is a chain of tens or hundreds of thousands of identical groups of atoms connected in series, and the same group of atoms is rhythmically repeated many times.
Content
The main polymeric materials are resins and plastics. Depending on whether it is a thermoplastic polymer or a thermosetting material, the material can either soften and harden repeatedly, or turn into a solid state with a single heating and permanently lose its ability to melt. The most commonly used modern polymeric materials are dispersions, latexes and adhesives.
What are building polymer materials
What are polymeric materials and how are they used in construction? All types of polymeric materials are substances in which each molecule is a chain of tens or hundreds of thousands of identical groups of atoms connected in series, and the same group of atoms is rhythmically repeated many times.
The main types of polymeric materials are divided into thermoplastic and thermosetting. Thermoplastic polymers are able to repeatedly soften and harden with temperature changes, as well as easily swell and dissolve in organic solvents. These include polystyrene, polyethylene and polyvinyl chloride (polyvinylchloride) resins and plastics.
The main property of thermosetting polymeric materials is the transition to an insoluble solid state when heated and the irreversible loss of the ability to melt. Such polymers include phenol-formaldehyde and urea-formaldehyde, polyester and epoxy resins.
Certain types of polymeric materials in construction under the influence of heat, light and air oxygen change their properties over time: they lose flexibility, elasticity, in other words, they age.
To prevent the aging of modern building polymeric materials, special stabilizers (anti-aging agents) are used, which are various organometallic compounds of lead, barium, cadmium, etc. For example, tinuvine P is used as a stabilizer.
What are polymeric materials, and what are their main characteristics, you will learn on this page.
Polymeric plastic materials and their properties
One of the main types of polymeric materials is plastics. They are a group of organic materials, which are based on synthetic or natural resinous high-molecular substances that are capable of being molded under heating and pressure, stably retaining the shape given to them.
Polymeric plastic materials have good thermal and electrical insulation properties, corrosion resistance and durability. The average density of plastics is 15-2200 kg/m3; compressive strength - 120-160 MPa. Plastics are endowed with good electrical and thermal insulation properties, corrosion resistance and durability. Some of them are transparent and highly adhesive, and tend to form thin films and protective coatings. Due to their properties, these polymeric materials are widely used in construction, mainly in combination with binders, metals and stone materials.
Plastics consist of a binder - a polymer, a filler, a plasticizer and a curing accelerator. In the manufacture of colored plastics, mineral dyes are also used.
As fillers in the manufacture of this type of polymeric materials, organic and mineral powders, asbestos, wood and glass fibers, paper, glass and cotton fabrics, wood veneer, asbestos cardboard, etc. are used. Fillers not only reduce the cost of the material, but also improve individual properties of plastics : increase hardness, strength, acid resistance and heat resistance. They must be chemically inert, non-volatile and non-toxic. Plasticizers in the manufacture of plastics are zinc acid, aluminum stearate and others, which give the material greater plasticity. Catalysts (accelerators) are used in plastics to speed up curing. An example of a catalyst is lime or urotropine, which are used to cure a phenol-formaldehyde polymer.
Synthetic polymer materials and their applications
According to the method of production, synthetic polymeric materials are divided into two classes: class A - polymers obtained by chain polymerization; class B - polymers obtained by polycondensation and stepwise polymerization.
The polymerization process is a combination of the same and different molecules. By-products during polymerization are not formed.
The polycondensation process is a combination of a large number of identical and different polyreactive molecules of low molecular weight substances, resulting in the formation of a high molecular weight substance. During the polycondensation process, water, hydrogen chloride, ammonia and other substances are released.
Silicone resins is a special group of macromolecular compounds. The peculiarity of these polymer building materials is that they have the properties of both organic and inorganic substances.
The physical and mechanical characteristics of these polymeric materials are practically independent of temperature fluctuations compared to conventional resins, in addition, they have high hydrophobicity and heat resistance. Silicone resins are used to obtain various products that are resistant to elevated temperatures (400-500°C).
The main area of application of these synthetic polymeric materials is the manufacture of concretes and mortars to increase their durability. They are also used as protective coatings on natural and artificial stone materials (concrete, limestone, travertine, marble, etc.). Impregnation has a protective effect for 6-10 years, after which it should be renewed.
For impregnation surfaces of products made of natural stone and other building structures, hydrophobizing organosilicon liquids (GCL) are used, which are dissolved with organic solvents before use, as well as an aqueous 50% emulsion (milky white), which is mixed with water before use in a ratio of 1 :ten.
Polyvinyl acetate dispersion (PVA) is a product of polymerization of vinyl acetate in an aqueous medium in the presence of an initiator and a protective colloid. It is a viscous liquid of white color, homogeneous, without screams and foreign inclusions.
PVA, depending on the viscosity, is produced in three grades: H - low viscosity, C - medium viscosity, B - high viscosity. It is used in the manufacture of polymer cement mortars, mastics, pastes, which are used in facing works.
Synthetic latex SKS-65GP- a product of joint polymerization of butadiene with styrene in a ratio of 35:65 (by weight) in an aqueous emulsion using nekal and sodium soap as synthetic fatty acids as an emulsifier. Latex SKS-65GP is used in the manufacture of polymer concrete, emulsion paints, mastics and pastes used in facing works. Latex is also used in the application of various coatings.
Physical and chemical properties of this polymer building material latex SKS-65GP:
- dry matter content, %, not less than 47;
- content of unpolymerized styrene, %, not more than 0.08;
- concentration of hydrogen ions (pH), not less than 11;
- surface tension, dyne/cm2, no more than 40;
- viscosity, s - 11-15;
- ash content, %, no more than 1.5.
Synthetic latex SKS-ZOSHR is a product of joint polymerization of butadiene with styrene in an aqueous emulsion, used as a binder or adhesive material in facing works.
Physical and chemical properties of SKS-ZOSHR latex:
- dry matter content, %, not less than 33;
- gelatinization temperature, °С, not higher than 14;
- content of free alkali, %, not more than 0.15.
Characteristics of polymer adhesives
Polymer adhesives are produced in the form of liquids, powders and films.
Liquid adhesives are of two types. The first type of adhesive compositions are rubbers, resins or cellulose derivatives dissolved in an organic volatile solvent (alcohol or acetone). After evaporation of the solvent, a solid adhesive bond is formed. The second type of adhesive compositions are aqueous solutions of resins specially prepared for adhesives. Such solutions, when properly stored, do not thicken for several months. Liquid adhesives contain 40-70% solid adhesive.
Of the liquid adhesives, the most common are melamine-formaldehyde, phenol-formaldehyde, urea-formaldehyde, rubber, epoxy, polyvinyl acetate, and adhesives with the addition of silicones.
CMC glue (sodium salt of carboxymethyl cellulose) is used in the manufacture of mastics and solutions used in.
Carbinol glue (vinylacetylene carbolene)- It is a viscous transparent liquid of light orange color, which has a high adhesive power. Therefore, it is called universal. It is able to glue various materials, even such as concrete, stone, metal, wood. Hardened carbinol adhesive is resistant to oils, acids, alkalis, gasoline, acetone and water.
Concentrated nitric acid or benzoyl peroxide are used as catalysts to accelerate the hardening of carbinol glue. The latter is an explosive powder, so it should be stored away from fire.
Carbinol glue is produced on the basis of carbinol syrup (100 wt.h) of two compositions: in the 1st benzoyl peroxide (1-3 wt.h.) is added as a hardener, in the 2nd - concentrated nitric acid (1-2 wt.h.). h.).
Carbinol glue is stored at a temperature of 20°C and in the dark, as it loses its adhesive ability under the influence of light.
Epoxy adhesive It is a clear, viscous, light brown liquid with a high tackiness. It is used for bonding stone, concrete, ceramic tiles. The hardened epoxy adhesive joint is resistant to acids, alkalis, solvents, water, as well as to high mechanical loads. Epoxy resin hardeners are polyethylenepolyamine or hexamethylenediamine, and dibutyl phtholate is a plasticizer.
Introduction
1. Features of polymers
2. Classification
3. Types of polymers
4. Application
5. Polymer science
Conclusion
List of sources used
Introduction
Chains of polypropylene molecules.
Polymers(Greek πολύ- - many; μέρος - part) - inorganic and organic, amorphous and crystalline substances obtained by repeated repetition of various groups of atoms, called "monomeric units", connected into long macromolecules by chemical or coordination bonds. A polymer is a high molecular weight compound: the number of monomeric units in a polymer (degree of polymerization) must be large enough. In many cases, the number of units can be considered sufficient to classify a molecule as a polymer, if the molecular properties do not change when the next monomer unit is added. As a rule, polymers are substances with a molecular weight of several thousand to several million.
If the bond between macromolecules is carried out with the help of weak Van der Waals forces, they are called thermoplastics, if with the help of chemical bonds - thermoplastics. Linear polymers include, for example, cellulose; branched polymers, for example, amylopectin, have polymers with complex spatial three-dimensional structures.
In the structure of the polymer, a monomeric link can be distinguished - a repeating structural fragment that includes several atoms. Polymers consist of a large number of repeating groups (units) of the same structure, for example, polyvinyl chloride (-CH2-CHCl-) n, natural rubber, etc. High-molecular compounds whose molecules contain several types of repeating groups are called copolymers or heteropolymers.
The polymer is formed from monomers as a result of polymerization or polycondensation reactions. Polymers include numerous natural compounds: proteins, nucleic acids, polysaccharides, rubber and other organic substances. In most cases, the concept refers to organic compounds, but there are many inorganic polymers. A large number of polymers are obtained synthetically from the simplest compounds of elements of natural origin by polymerization, polycondensation, and chemical transformations. The names of polymers are formed from the name of the monomer with the prefix poly-: polyethylene, polypropylene, polyvinyl acetate, etc.
1. Features of polymers
Special mechanical properties:
elasticity- the ability to high reversible deformations with a relatively small load (rubbers);
low brittleness of glassy and crystalline polymers (plastics, organic glass);
the ability of macromolecules to orient under the action of a directed mechanical field (used in the manufacture of fibers and films).
Features of polymer solutions:
high solution viscosity at low polymer concentration;
the dissolution of the polymer occurs through the stage of swelling.
Special chemical properties:
the ability to dramatically change its physical and mechanical properties under the action of small amounts of a reagent (rubber vulcanization, leather tanning, etc.).
The special properties of polymers are explained not only by their large molecular weight, but also by the fact that macromolecules have a chain structure and are flexible.
2. Classification
According to the chemical composition, all polymers are divided into organic, organoelement, inorganic.
organic polymers.
organoelement polymers. They contain inorganic atoms (Si, Ti, Al) combined with organic radicals in the main chain of organic radicals. They don't exist in nature. An artificially obtained representative is organosilicon compounds.
It should be noted that combinations of different groups of polymers are often used in technical materials. These are composite materials (for example, fiberglass).
According to the shape of macromolecules, polymers are divided into linear, branched (a special case - star-shaped), ribbon, flat, comb-shaped, polymer networks, and so on.
Polymers are classified according to polarity (affecting solubility in different liquids). The polarity of the polymer units is determined by the presence of dipoles in their composition - molecules with a disconnected distribution of positive and negative charges. In nonpolar links, the dipole moments of the bonds of atoms are mutually compensated. Polymers whose units have significant polarity are called hydrophilic or polar. Polymers with non-polar links - non-polar, hydrophobic. Polymers containing both polar and non-polar units are called amphiphilic. Homopolymers, each link of which contains both polar and non-polar large groups, are proposed to be called amphiphilic homopolymers.
In relation to heating, polymers are divided into thermoplastic and thermoset. Thermoplastic polymers (polyethylene, polypropylene, polystyrene) soften when heated, even melt, and harden when cooled. This process is reversible. Thermosetting polymers, when heated, undergo irreversible chemical degradation without melting. Molecules of thermoset polymers have a non-linear structure obtained by cross-linking (for example, vulcanization) of chain polymer molecules. The elastic properties of thermosetting polymers are higher than those of thermoplastics, however, thermosetting polymers practically do not flow, as a result of which they have a lower fracture stress.
Natural organic polymers are formed in plant and animal organisms. The most important of these are polysaccharides, proteins and nucleic acids, which make up the bodies of plants and animals to a large extent and which ensure the very functioning of life on Earth. It is believed that the decisive stage in the emergence of life on Earth was the formation of more complex, high-molecular molecules from simple organic molecules (see Chemical evolution).
3. Types of polymers
synthetic polymers. Artificial polymer materials
Man has been using natural polymeric materials in his life for a long time. These are leather, furs, wool, silk, cotton, etc., used for the manufacture of clothing, various binders (cement, lime, clay), which, upon appropriate processing, form three-dimensional polymeric bodies widely used as building materials. However, the industrial production of chain polymers began at the beginning of the 20th century, although the prerequisites for this appeared earlier.
Almost immediately, the industrial production of polymers developed in two directions - by processing natural organic polymers into artificial polymeric materials and by obtaining synthetic polymers from organic low molecular weight compounds.
In the first case, large-capacity production is based on cellulose. The first polymeric material from physically modified cellulose - celluloid - was obtained at the beginning of the 20th century. Large-scale production of cellulose ethers and esters was organized before and after World War II and continues to this day. On their basis, films, fibers, paints and varnishes and thickeners are produced. It should be noted that the development of cinema and photography was possible only due to the appearance of a transparent film of nitrocellulose.
The production of synthetic polymers began in 1906, when L. Baekeland patented the so-called bakelite resin - a condensation product of phenol and formaldehyde, which turns into a three-dimensional polymer when heated. It has been used for decades in the manufacture of housings for electrical appliances, batteries, televisions, sockets, etc., and is now more commonly used as a binder and adhesive.
Thanks to the efforts of Henry Ford, before the First World War, the rapid development of the automotive industry began, first based on natural, then also synthetic rubber. The production of the latter was mastered on the eve of World War II in the Soviet Union, England, Germany and the USA. In the same years, the industrial production of polystyrene and polyvinyl chloride, which are excellent electrically insulating materials, was mastered, as well as polymethyl methacrylate - without organic glass called "plexiglass", mass aircraft construction during the war years would have been impossible.
After the war, the production of polyamide fiber and fabrics (kapron, nylon), which had begun before the war, resumed. In the 50s. 20th century polyester fiber was developed and the production of fabrics based on it called lavsan or polyethylene terephthalate was mastered. Polypropylene and nitron - artificial wool made from polyacrylonitrile - close the list of synthetic fibers that modern people use for clothing and industrial activities. In the first case, these fibers are very often combined with natural cellulose or protein fibers (cotton, wool, silk). An epochal event in the world of polymers was the discovery in the mid-50s of the XX century and the rapid industrial development of Ziegler-Natta catalysts, which led to the emergence of polymeric materials based on polyolefins and, above all, polypropylene and low-pressure polyethylene (before that, the production of polyethylene at a pressure of about 1000 atm.), as well as stereoregular polymers capable of crystallization. Then polyurethanes were introduced into mass production - the most common sealants, adhesive and porous soft materials (foam rubber), as well as polysiloxanes - organoelement polymers that have higher heat resistance and elasticity compared to organic polymers.
The list is closed by the so-called unique polymers synthesized in the 60-70s. 20th century These include aromatic polyamides, polyimides, polyesters, polyester ketones, etc.; an indispensable attribute of these polymers is the presence of aromatic cycles and (or) aromatic condensed structures. They are characterized by a combination of outstanding values of strength and heat resistance.
Refractory polymers
Many polymers, such as polyurethanes, polyesters and epoxy resins, tend to ignite, which is often unacceptable in practice. To prevent this, various additives are used or halogenated polymers are used. Halogenated unsaturated polymers are synthesized by incorporating chlorinated or brominated monomers, such as hexachloracid (HCEMTFA), dibromoneopentyl glycol, or tetrabromophthalic acid, into the condensation. The main disadvantage of such polymers is that when burned, they are able to release gases that cause corrosion, which can have a detrimental effect on nearby electronics. Given the high requirements of environmental safety, special attention is paid to halogen-free components: phosphorus compounds and metal hydroxides.
The action of aluminum hydroxide is based on the fact that under high temperature exposure, water is released, which prevents combustion. To achieve the effect, it is necessary to add large amounts of aluminum hydroxide: by weight 4 parts to one part of unsaturated polyester resins.
Ammonium pyrophosphate works on a different principle: it causes charring, which, together with a glassy layer of pyrophosphates, insulates the plastic from oxygen, inhibiting the spread of fire.
A new promising filler is layered aluminosilicates, the production of which is being created in Russia.
4. Application
Due to their valuable properties, polymers are used in mechanical engineering, the textile industry, agriculture and medicine, automobile and shipbuilding, aircraft manufacturing, and in everyday life (textiles and leather products, dishes, glue and varnishes, jewelry and other items). Based on macromolecular compounds, rubber, fibers, plastics, films and paint coatings are produced. All tissues of living organisms are macromolecular compounds.
5. Polymer science
The science of polymers began to develop as an independent field of knowledge by the beginning of the Second World War and was formed as a whole in the 50s. XX century, when the role of polymers in the development of technical progress and the vital activity of biological objects was realized. It is closely related to physics, physical, colloidal and organic chemistry and can be considered as one of the basic foundations of modern molecular biology, the objects of study of which are biopolymers.
List of sources used
1. Encyclopedia of polymers, vol. 1 - 3, ch. ed. V. A. Kargin, M., 1972 - 77;
2. Makhlis F. A., Fedyukin D. L., Terminological reference book on rubber, M., 1989;
3. Krivoshey V. N., Packaging made of polymeric materials, M., 1990;
4. Sheftel V. O., Harmful substances in plastics, M., 1991;
Abstract on the topic “Polymers” updated: January 18, 2018 by: Scientific Articles.Ru
The article below will try to answer the question of what a polymer is. Here we will look at the definition of such a term, the features of the relationships that arise in molecules, education, historical data, and much more.
Introduction
What is a polymer? This is a substance that has an inorganic or organic nature and is formed through chemical bonds that cause and give them an amorphous or crystalline form. A polymer arises by combining a large number of units of simple monomers, and the resulting structure is called a macromolecule. The type of bond can be: coordination or chemical type. The concept of a polymer is closely related to plastics.
Communication of molecules
When answering the question of what a polymer is, it is important to know how the molecules in such a substance are interconnected. In the case when macromolecules are combined by means of a weak van der Waals force, they are referred to as thermoplastics. If the bond by which they are connected is of a chemical nature, then it is a thermoplastic.
There are linear forms of polymers (cellulose) and branched ones (amylopectins). The latter has a complex three-dimensional structure. The structure of the polymer predetermines the presence of a monomer unit. This is a chain fragment that is regularly repeated and consists of several atoms.
Education
A polymer (polymer) is a substance that is formed in a number of different phenomena during the polymerization reaction, as well as polycondensation. This group of compounds includes many natural food components, among which are: proteins (protein), polysaccharide carbohydrates, I will teach, a number of nucleic acids, etc. Despite the fact that these are mainly organic substances, inorganic compounds also have a huge number of similar chemical entities. Many of them are obtained by artificial synthesis.
Specificity
The substances considered in this article have many characteristics that cause a great need for their use by humans.
The peculiarities of mechanical properties include their elasticity, low brittleness of the glassy and crystalline series of polymers, as well as the ability by which macromolecules are oriented in the compound through the activity of directed mechanical fields.
Polymer solutions have a high viscosity at low concentrations. They can dissolve after passing through the swelling stage.
The main property of the chemical type is their ability to quickly change the set of their physical and mechanical properties under the influence of a small amount of reagents. Molecules are characterized by high flexibility.
Kinds
The classification of polymers is determined in accordance with several parameters.
Considering them from the point of view of chemistry allows us to distinguish non- and organic, as well as organoelement. The latter include substances containing sets of inorganic type radicals at the base of the chain. Here, the ability of polymers to form relationships between substances of different nature is traced. An example is an organosilicon compound obtained artificially. Inorganic types of polymers dispense with carbon in repeating units, but may include it in side substituents.
In accordance with the shape, several main types of connections are distinguished: linear, mesh, comb-shaped, flat, branched, sometimes star-shaped (part of the branched group) and others.
Other types of polymers can be distinguished by determining their polarity, the value of which can be found by calculating the number of dipoles. What's this?
A dipole is a molecule that has a disconnected form of the distribution of "+" and "-" charges. The non-polar link mutually compensates the dipole moment of the bond between the atoms. Polymers, which are characterized by the presence of a significant degree of polarity, are classified as hydrophilic. An amphiphilic substance is a compound of monomers that has both non-polar and polar units.
The reactions of polymers to heating make it possible to distinguish among them thermosetting and thermoplastic. The former include substances that soften when heated and harden when exposed to low temperatures. The process is reversible. Thermosetting polymers do not recover under the influence of high temperatures, and the reaction is considered irreversible.
Development process
What is a polymer? This question stems from antiquity. However, in this form it was formulated relatively recently. Such substances have been used by man since ancient times. Silks, cotton materials, leather, wool and much more were used by our ancestors to create elements of clothing, as binding compounds, during various treatments, etc. The wording of the question changed over the course of human evolution, but was always of a general nature.
In industrial enterprises, chain polymers began to be produced at the beginning of the 20th century. Since the inception of the industry for their production, the paths for the formation of compounds have been divided into two branches. The first was engaged in the processing of polymers, organic and natural forms. With their help, artificial species were created. The synthesis process, as a rule, takes place with the participation of a low molecular weight series of compounds.
Currently, one of the largest and large-scale production uses cellulose as the basis. The process did not get better right away. The first material that was obtained by physically modifying cellulose is a celluloid polymer. However, his first discovery was made before the twentieth century - in the middle of the nineteenth. The possession of the patent for bakelite resin, which was created by Leo Baekeland, gave impetus to the beginning of the rapid development of industries in which polymers were made. This happened in 1906. The mentioned resin is a product of the condensation process of formaldehyde paired with phenol. It was possible to observe the transformation during heating, and as a result of this phenomenon, three-dimensional compounds were formed. For decades, this resin has been used in the manufacture of housings for various mechanisms, such as batteries, TVs, sockets, etc.
Contribution of Henry Ford
The production of polymers is largely due to the efforts made by G. Ford. Before the outbreak of the First World War, he actively developed the automotive industry. Initially, he used natural rubbers, and then began to synthesize them artificially. The manufacture of the latter was vigorously studied and mastered in 1937-1939. The main countries that have invested a lot of time, money and other means in this are the USSR, England, the United States of America and Germany. In the same period, polystyrene and polyvinyl chloride were mastered, which perfectly insulated electrical wiring. The discovery of polymethyl methacrylate made it possible to establish a large-scale production of aircraft during the war years.
After the war ended, the synthesis of polyamide fabrics and fibers began to resume. Their production began to develop even before the second conflict between the countries. In the fifties of the 20th century, methods were developed for obtaining polyester fibers, and the manufacture of materials such as lavsan and polyethylene tereflatate was also mastered. Polypropylene substances (artificially obtained wool) are another striking example of the exploitation of fibers obtained during the reaction of polycondensation and polymerization.
Refractory structure
Polymer - what is it? Considering such a question, we mentioned their ability to respond to heat treatment.
Going into this, it is important to know that many polymers are flammable. Such substances are easily combustible. However, this is unacceptable in most cases during their manufacture and operation. In order to prevent the likelihood of such an incident, a special series of additives are added to the polymer composition.
There is a concept of halogenated polymers, which are created by including in condensation reactions, a different set of chlorinated or brominated monomers. Such compounds have high refractoriness, but their disadvantage is that when exposed to high temperatures, they begin to form gases that give rise to corrosion processes. This negatively affects electrical equipment located nearby.
Operating methods
Reviewing polymers and plastics, we can say that they have a common quality characteristics. Both compounds are used in various branches of human activity, for example, in the manufacture of cars, for agricultural purposes, in medicine, in the manufacture of aircraft, in shipbuilding, etc. A person's everyday environment cannot do without these substances. Thanks to high-molecular type compounds, it is possible to produce various fibers, rubber and, in fact, plastics. We also do not forget that our body functions due to the presence of a large number of polymers in it, which not only build organs and tissues, but also serve as a means of extracting energy resources, for example, ATP or NADP, formed during biological oxidation and digestion.
Study of polymers
The definition of polymers was formulated over 150 years ago. However, the science studying them became independent only before the outbreak of World War II, which began in 1939. It received a stronger development already in the fifties of the twentieth century and then was studied in detail. At this time, the role of polymers was determined, their relationship with the development of progress in a technical nature, their influence on biological objects, etc. The branch of science that studies such compounds is closely connected with various sections of chemistry, physics and biology.