Polymeric materials are high molecular weight chemical compounds that consist of numerous small molecular monomers (units) of the same structure. Often, the following monomer components are used to make polymers: ethylene, vinyl chloride, vinyldene chloride, vinyl acetate, propylene, methyl methacrylate, tetrafluoroethylene, styrene, urea, melamine, formaldehyde, phenol. In this article we will consider in detail what polymeric materials are, what are their chemical and physical properties, classification and types.
Types of Polymers
The peculiarity of the molecules of this material is a large molecular weight, which corresponds to the following value: M> 5 * 103. Compounds with a lower level of this parameter (M = 500-5000) are called oligomers. In low molecular weight compounds, the mass is less than 500. The following types of polymeric materials are distinguished: synthetic and natural. The latter are commonly referred to as natural rubber, mica, wool, asbestos, cellulose, etc. However, the main place is occupied by polymers of a synthetic nature, which are obtained as a result of the process of chemical synthesis from compounds of a low molecular level. Depending on the method of manufacturing high molecular weight materials, there are polymers that are created either by polycondensation or by an addition reaction.
Polymerization
This process is the combination of low molecular weight components into high molecular weight components to produce long chains. The level of polymerization is the number of "measures" in the molecules of a given composition. Most often, polymeric materials contain from a thousand to ten thousand of their units. The following commonly used compounds are obtained by polymerization: polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, polystyrene, polybutadiene, etc.
Polycondensation
This process is a stepwise reaction, which consists in combining either a large number of monomers of the same type, or a pair of different groups (A and B) into polycapacitors (macromolecules) with the simultaneous formation of the following by-products: methyl alcohol, carbon dioxide, hydrogen chloride, ammonia, water and etc. Using polycondensation get silicones, polysulfones, polycarbonates, aminos, phenoplasts, polyesters, polyamides and other polymeric materials.
Polyaddition
This process is understood to mean the formation of polymers as a result of multiple addition reactions of monomeric components, which contain limit reaction groups, to monomers of unsaturated groups (active rings or double bonds). Unlike polycondensation, the polyaddition reaction proceeds without emission of by-products. The most important process of this technology is the curing of epoxy resins and the production of polyurethanes.
Polymer Classification
In composition, all polymeric materials are divided into inorganic, organic and organoelemental. The first of them (silicate glass, mica, asbestos, ceramics, etc.) do not contain atomic carbon. They are based on oxides of aluminum, magnesium, silicon, etc. Organic polymers are the most extensive class, they contain carbon, hydrogen, nitrogen, sulfur, halogen and oxygen atoms. Organoelement polymeric materials are compounds that, in addition to the listed ones, have silicon, aluminum, titanium and other elements that can combine with organic radicals in the main chains. In nature, such combinations do not occur. These are exclusively synthetic polymers. Characteristic representatives of this group are organosilicon compounds whose main chain is built of oxygen and silicon atoms.
To obtain polymers with the necessary properties in the technique, it is often not “pure” substances that are used, but their combinations with organic or inorganic components. A good example is polymer building materials: metal plastics, plastics, fiberglass, polymer concrete.
Polymer structure
The peculiarity of the properties of these materials is due to their structure, which, in turn, is divided into the following types: linearly branched, linear, spatial with large molecular groups and very specific geometric structures, as well as staircase. Let us briefly consider each of them.
Polymeric materials with a linearly branched structure, in addition to the main chain of molecules, have side branches. Such polymers include polypropylene and polyisobutylene.
Materials with a linear structure have long zigzag or spiral chains. Their macromolecules are primarily characterized by repetition of sites in one structural group of a unit or chemical unit of the chain. Polymers with a linear structure are distinguished by the presence of very long macromolecules with a significant difference in the nature of the bonds along the chain and between them. This refers to intermolecular and chemical bonds. The macromolecules of such materials are very flexible. And this property is the basis of polymer chains, which leads to qualitatively new characteristics: high elasticity, as well as the absence of brittleness in the hardened state.
And now we learn what polymeric materials with a spatial structure are. When combined, macromolecules form strong chemical bonds in the transverse direction. The result is a mesh structure that has a heterogeneous or spatial mesh base. Polymers of this type have greater heat resistance and stiffness than linear. These materials are the basis of many structural non-metallic substances.
The molecules of polymer materials with a ladder structure consist of a pair of chains that are connected by a chemical bond. These include organosilicon polymers, which are characterized by increased rigidity, heat resistance, in addition, they do not interact with organic solvents.
Polymer phase composition
These materials are systems that consist of amorphous and crystalline regions. The first of them helps to reduce stiffness, makes the polymer flexible, that is, capable of large deformations of a reversible nature. The crystalline phase increases their strength, hardness, elastic modulus, as well as other parameters, while reducing the molecular flexibility of the substance. The ratio of the volume of all such regions to the total volume is called the degree of crystallization, where polypropylene, fluoroplastics, and high density polyethylene have a maximum level (up to 80%). Polyvinyl chloride and low density polyethylene have a lower crystallization level.
Depending on how polymer materials behave when heated, they are usually divided into thermosetting and thermoplastic.
Thermoset Polymers
These materials primarily have a linear structure. When heated, they soften, but as a result of chemical reactions in them, the structure changes to spatial, and the substance turns into solid. In the future, this quality is maintained. Polymer composite materials are built on this principle . Their subsequent heating does not soften the substance, but only leads to its decomposition. The finished thermosetting mixture does not dissolve and does not melt, so its re-processing is unacceptable. This type of material includes epoxy organosilicon, phenol-formaldehyde and other resins.
Thermoplastic polymers
These materials, when heated, first soften and then melt, and during subsequent cooling they harden. Thermoplastic polymers do not undergo chemical changes during this treatment. This makes the process completely reversible. Substances of this type have a linearly branched or linear structure of macromolecules, between which small forces act and there are absolutely no chemical bonds. These include polyethylenes, polyamides, polystyrenes, etc. The technology of polymer materials of thermoplastic type provides for their manufacture by injection molding in water-cooled forms, pressing, extrusion, blowing and other methods.
Chemical properties
Polymers can undergo the following conditions: solid, liquid, amorphous, crystalline phase, as well as highly elastic, viscous and glassy deformation. The widespread use of polymeric materials is due to their high resistance to various aggressive environments, such as concentrated acids and alkalis. They are not susceptible to electrochemical corrosion. In addition, with an increase in their molecular weight, the solubility of the material in organic solvents decreases. And polymers with a spatial structure are generally not affected by the mentioned liquids.
Physical properties
Most polymers are dielectrics; in addition, they are non-magnetic materials. Of all the structural substances used, only they have the lowest thermal conductivity and the highest heat capacity, as well as thermal shrinkage (about twenty times more than that of metal). The cause of leakage by various sealing units under low temperature conditions is the so-called glass transition of rubber, as well as a sharp difference between the expansion coefficients of metals and rubbers in the vitrified state.
Mechanical properties
Polymeric materials are characterized by a wide range of mechanical characteristics, which are highly dependent on their structure. In addition to this parameter, various external factors can have a great influence on the mechanical properties of a substance. These include: temperature, frequency, duration or speed of loading, type of stress state, pressure, nature of the environment, heat treatment, etc. A feature of the mechanical properties of polymeric materials is their relatively high strength with very low stiffness (compared with metals).
Polymers are usually divided into solid, the elastic modulus of which corresponds to E = 1–10 GPa (fibers, films, plastics), and soft highly elastic substances, the elastic modulus of which is E = 1–10 MPa (rubber). The patterns and mechanism of destruction of both are different.
Polymeric materials are characterized by a pronounced anisotropy of properties, as well as a decrease in strength, the development of creep under long-term loading. Together with this, they have a fairly high fatigue resistance. Compared with metals, they differ in a sharper dependence of mechanical properties on temperature. One of the main characteristics of polymeric materials is deformability (ductility). According to this parameter, in a wide temperature range, it is customary to evaluate their main operational and technological properties.
Polymer materials for the floor
Now consider one of the options for the practical use of polymers, revealing the full possible range of these materials. These substances are widely used in construction and repair finishing works, in particular in flooring. The huge popularity is explained by the characteristics of the substances under consideration: they are resistant to abrasion, have little heat conductivity, have little water absorption, are sufficiently strong and solid, and have high paint and varnish qualities. The production of polymeric materials can be divided into three groups: linoleums (rolled), tiled products and mixtures for the installation of seamless floors. Now we will briefly consider each of them.
Linoleums are made on the basis of different types of fillers and polymers. They may also include plasticizers, processing aids and pigments. Depending on the type of polymer material, polyester (glyphthalic), polyvinyl chloride, rubber, colloxylin and other coatings are distinguished. In addition, according to their structure, they are divided into baseless and with a sound-, heat-insulating base, single-layer and multi-layer, with a smooth, fleecy and corrugated surface, as well as single and multi-color.
Tile materials made on the basis of polymer components have very low abrasion, chemical resistance and durability. Depending on the type of raw material, this type of polymer product is divided into coumaronopolivinyl chloride, coumaron, polyvinyl chloride, rubber, phenolitic, bituminous tiles, as well as chipboards and fiberboards.
Materials for seamless floors are the most convenient and hygienic in operation, they have high strength. These mixtures are usually divided into polymer cement, polymer concrete and polyvinyl acetate.