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Composition and properties of plastics

POLYMER MATERIALS

 

Polymer materials are obtained on the basis of macromolecular substances - polymers. Synthetic organic polymers, obtained by syntheses of the simplest substances — monomers, are mainly used in construction, Polymers molecular weight exceeds more than 5000 and culminates hundred thousand units, while the weight of common low-molecular substances’ molecules changes from units to few hundreds (usually lower 500). That truthful difference in molecular weight explains the big difference of polymers physical properties from the low-molecular substances properties.

The row of general properties is distinctive to polymer materials, as it determines their using in construction: lightness in combination with high strength, water and chemical resistance, high wearing resistance, processability, colouring ability, low thermal conduction. Low heat-resistance, considerable linear increasing, creep, degradation ability– physical-chemical deterioration because of environmental factors are general defects of the polymer materials

The most of the polymer materials are used as plastics, including polymer binder, fillers, plasticizers, stabilizers and other components.

Plastics belong to the most modern construction materials, as they prevail over traditional materials by many parameters. For example, the construction quality coefficient, which means ratio between the compressive strength and the average density achieves for plastics usually 1-2, as for light-weight metal alloys, at the same time for bricks it achieves 0.02, for heavy-weight concrete with strength 20 MPa — 0.08; for pinewood – 0.7.

At replacement of metal, concrete, reinforced concrete, wood by plastics in construction in many cases one gains high technical economic effect. Plastics manufacturing allows providing high level of the comprehensive mechanization and automation of technological processes, their using leads to the high level of construction industrialization and its quality.

Depending on plastic assignment it divides on constructing, finishing, insulating and sealing, piping materials, hygiene and sanitary products, etc.

Composition and properties of plastics

Plastic masses are materials, plastic on some manufacturing stages, when the polymers are binder. Many of plastic masses beside the polymer binder include the fillers. Such plastics are called filled. At the same time, in some cases, transparent plastics for example where are no fillers (unfilled plastic) are manufactured.

Plastics components. Synthetic polymers are classified according to different signs: process of manufacture, peculiarity of atomic location in molecule and length of main chain, relation to temperature, special physical-mechanical properties and chemical composition.

Initial materials that are used for synthetic polymers manufacture can be natural gas, coal and petroleum.

There are polymerization and polycondensation polymers according to process of manufacture.



High molecular substances at polymerization are obtained because tearing of multiple bonds or ring in cyclic substances and forming of the macromolecule like a chain (Fig.16.1) under the influence of different factors: temperature, light, substances- initiators, catalysts, etc. Accordingly, there are thermal, photochemical, initiative and other kinds of polymerization according to stimulating factor character.

The simplest example of polymerization is reaction of polyethylene formation (-ÑÍ2-ÑÍ2-)n from ethylene ÑÍ2=ÑÍ2 – inflammable colorless gas, obtained from oil refine products or coal.

There are five main ways of polymerization: block – polymerization, polymerization in solution, suspension, emulsion and in gas state. At the block polymerization polymer is obtained already as products of some shape – blocs (Fig. 16.2). Representative of such polymers is polymethylmethacrylate (plexiglass), obtained in the form of colorless plates. At the bloc polymerization monomer with additive of initiator or catalyst is inundated in a form and heated. Solution polymerization is used in products obtaining with short chains, used in varnishes and glue manufacturing (varnishes polymerization), etc. In such case monomer previously is dissolved with the help of the solvent, later mixed up with initiator. At the suspension and emulsion polymerization monomer and initiator are dispergated in the water to the smallest globules. The protective colloids (gelatin and others) are brought in to suspension to guarantee the stableness of globules, to emulsion - surfactant species – emulsifiers. Monomers are in gaseous state at the gas polymerization.

Polycondensates have the shorter chains and the consistently lower molecular mass, than polymerization polymers.

Examples of polymerization and polycondensational polymers are represented in Table 16.1.

 


Table 16.1

Examples of polymerization and polycondensation polymers

Polymers Monomers Method of production
Polymerization:
Polyethylene (-ÑÍ2-ÑÍ2-)n Ethylene ÑÍ2=ÑÍ2 Polymerization in gas phase at the high (120-250 ÌPà) or low (0.1-0.5 MPa) pressure
Polystyrene (-Ñ6Í5-ÑÍ-ÑÍ2-)n Styrene Ñ6Í5-ÑÍ=ÑÍ2 Block, emulsion or suspension polymerization
Polyvinylchloride (-ÑÍ2-ÑÍÑ1-)n Vinyl chloride ÑÍ2=ÑÍÑ1 Suspension or emulsion polymerization
Polytetrafluoroethylene (-CF2-CF2-)n Tetrafluorethylene CF2=CF2 The same
Polycondensation:
Phenol-formaldehyde Phenol Ñ6Í5ÎÍ Formaldehyde ÑÍ2Î Phenol polycondensation with formaldehyde
Carbamide Carbamide CO(N2H)2 Formaldehyde ÑÍ2Î Carbamide polycondensation
Polyether Ethylene glycol ÍÎÑÍ2-ÑÍ2ÎÍ Glycerol and other Acid polycondensation with polyatomic alcohol
Epoxy Epichlorhydrin Diphenylpropane Epichlorhydrin polycondensation with diphenylpropane

 

Polymerization the same as polycondensation polymers may be characterized by lineal, branched and spatial molecular structure (Fig. 16.3.) Chain macromolecules have profile forks at branched structure and at spatial structure they are bonded with each other in three-dimensional net by transversal chemical bonds.

Polymers, that are iteratively capable to softening and acquire plasticity at heating, but to hardening at cooling, call thermoplastic. Thermoplastic polymers have lineal or branched structure and appear mainly after polymerization reaction.

Polymers with macromolecule spatial structure after hardening cannot become plastic at heating again. They are called thermoreactive. The most of polycondensates belong to them. The more transversal bonds in such polymer’s macromolecules (compact net) are, the higher strength is, the fluidity lower is, the higher resilience is, etc. Characteristic properties of thermoplastic and thermoreactive polymers are given in Table 16.2.

Structure of macromolecule chains can be divided according to chemical structure into the carbochained and heterochained. Only the carbon atoms enter to the chains of carbochained polymers, and other atoms can enter to the chains of heterochained polymers. Element-organic polymers are the variety of heterochained polymers, that along with elements, enter into the common organic compounds (carbon, hydrogen, nitrogen and oxygen) contain and other elements – silicon, phosphorus, aluminum, titan, tin and so on. Organic silicon compounds (silicones) refer to representatives of elementorganic polymers, main silicones chain contains siloxane bonds (-Si-O-Si). Organic silicon and other compounds due to the perculiarities of chemical structure have a series of positive properties of materials, both organic and inorganic by origin, such as thermostability, hydrophobicity, elasticity etc.

Elasticity and deformability are the characteristic physical – mechanical properties, by which polymer materials are classified. High molecular compounds capable to reverse deformation under the action of external forces are called elastics (elastomers), capable to plastic deformations which are irreversible - plastics (plastomers). Different rubbers, for example, belong to the elastics, the most of polymers form plastics.

Table 16.2

Physical-mechanical properties of synthetic polymers

Polymers Density, kg/m3 Ultimate strength at stretching, MPa Thermal resistance (according to Martens), °Ñ Hardness according to Brinnel, MPa Elongation, %
Thermoplastic:
Polyethylene of low pressure 940-960 22-32 45-58 400-1000
Polyethylene of high pressure 920-930 12-16 150-250 150-600
Polystyrene 1050-1100 30-50 75-80 200-300 1-5
Polyvinylchloride unplasticized 1380-1400 50-70 65-80 130-160 0-50
Polyvinylchloride plasticized 1200-1400 7-14 30-60 40-80 100-300
Polytetrafluoroethy-lene 2120-2250 15-30 250-500 3-4
Thermoreactive :
Phenol-formaldehyde 1250-1300 25-55 80-120 8-15  
Polyethers 1310-1420 25-55 45-80 50-100 10-26
Epoxy 1150-1240 40-100 50-180 25-80 18-35
Organic silicon 1600-2000 10-50 250-350 1-4 25-30

 

Fillers can improve mechanical and dielectric properties, increase thermal stability and atmosphere resistance, diminish contraction, etc. The prime price of plastics with entering filling agents falls down considerably.

Fillers are divided by origin into organic and mineral, powdery, by form – into fiber and leaf.

The sawdust, wood, quartz and mica powders, talc, soot, graphite, kaolin, asbestos dust and others are widespread powdery fillers. Application of powdery filling agents along with the polymers of mainly phenol-formaldehyde type allows obtaining molding powders, which are widely used for making of various technical, domestic and insulating goods, and also products of the special assignment, which have enhanceable shock resistance, chemical resistance, waterproofness and heat resistance.

Especially high mechanical strength of plastics is achieved at application of fibrous (fiberglass, earth-flax, cotton, synthetic filament and other) and sheet (paper, wood leads, foil and fabric) fillers. Fibreglasses are especially effective among fibred fillers. Plastics under the general name of glass fiber plastics can be manufactured on their basis with application of the various synthetic polymers.

Along with fillers content of which changes in wide limits, plasticizers, stabilizers and pigments can be added in plastic if it is required.

Plasticizers can be added in an amount of 10-100 % by polimer weight to increase the elasticity, to improve fire-retarding ability and frost-resistance, to increase resistance to the ultraviolet rays and to improve the terms of processing. Essence of plasticizers action consists in penetration in the polymers macromolecules and diminishing of intermolecular forces of tripping.

Stabilizers are applied for deceleration of the plastics senescence processes during their exploitation and processing. Depending on the nature of plastics senescence stabilizers divide into two groups - thermostabilizers and lightstabilizers.

Plastics are processed into constructive products by different ways, the selection of which depends on components properties and structural features of products. So, products based on thermoplastic polymers are frequently obtained by casting under pressure, which consists in the periodic injection of molten mass portions in a form by moulded machines. An extrude-squeezing out of mass through the cannon-bit of screw extrusion machines are also applied, roll-forming - forming in a gap between the revolving rollers, thermoforming, pressing and another ways.

General properties of plastics. The average density of plastics changes in a wide range of 15-2200 kg/m3. The porous plastics have the lowest density. Fillers have a considerable influence on density. Plastics can be in 6 times lighter than steel and in 2.5 lighter than aluminum.

Plastics, usually, have high strength both at a compression and at tension and bend. Ultimate compression and tensile strength of the most high-strength plastics (glass-fiber material, wood laminated plastic and other) achieves to 300 MPa and more.

Hardness of plastics does not correlate directly with their strength unlike the metals and number of other materials. Even for such the hardest plastics, as textolites (filling agent is a cotton fabric), hardness approximately is in 10 times lower than steel’s. In spite of low hardness, plastics (especially elastic) have low abrasiveness that allows their wide using in floor coatings. Abrasiveness, for example, of one-sheeted polyvinylchloride linoleum equals 0.06, multi-layered – 0.035 g/cm2, which approximately is equal to the granite.

Plastics resistance to percussive action which is determined by the proportion of shock energy for destruction to the cut transversal area of a standard arrive the high levels for dense plastics (50-150 kJ/m2) and can sharply go down according as their porosity increasing.

Lot of plastics that are stretched out may be characterized the deformability. Relative lengthening, that means increasing of materials length in the moment of breaking up to its initial length, for polyethylene tapes, achieves the level of 300, to polyvinylchloride - 150, butyl rubber - 10%.

Module of elasticity is the description of resilient materials properties. This parameter for plastics is considerably lower than others construction materials. So, it is equals for steel (2-2.2)·105, woods (0.063-0.14)·105, paper-stratified plastic (0.021-0.028)·105, polyester glass-fiber material (0.022-0.028)·105 MPa.

There are rigid, semi-rigid, soft and elastic plastics depending on the module of elasticity. Phenol-formaldehyde and alkyd (polyester) plastics are examples of rigid plastics, which fragily collapse with the insignificant lengthening at breaking up; their module of elasticity mounts to more than 1000 Pa. Soft plastics (polyethylene and other) have the module of elasticity 20-100 MPa; the high relative lengthening characteristic is for them. Semi-rigid plastics (polypropylene and other) have intermediate values of the module of elasticity 400-1000 MPa. The module of elasticity does not exceed 20 MPa for elastic plastics (rubber and similar materials). Their deformations are mainly reversible at the normal temperature.

The low values of the plastics elasticity module promote the gradual increase of irreversible deformations at permanent load - creep. The creep of plastics can be largely explained by macromolecules sliding of polymer binder. It considerably grows even at the insignificant increase of temperature. Creep is considerably less for plastics, based on spatial polymers, the molecules of which are "sewn" together by transversal copulas. Enhanceable creep limits application of plastics in structural parts which work under the large loads.

Thermal conductivity of dense plastics without any fillers is 0.116-0.348 W/(m°K). Introduction of mineral fillers increases the thermal conductivity of plastics. Heat-reflecting properties of plastics discover the possibility of wide application in the house non structural parts.

Along with low thermal conductivity plastics can be characterized by large thermal expansion. Coefficient of linear thermal expansion of polyethylene (160-230)10-6, of polyvinylchloride (80-90)·10-6, phenol-formaldehyde polymers (10-30)10-6, of steel 12·10-6°C-1. Thermal expansion of plastics should be taken into account at constructions planning and exploitation to shut out deformations and cracks formation.

Destruction of plastics develops to the extent of temperature increasing that means the polymers destruction, or beginning of their melting. Initial melting temperature of most thermoplastic polymers is 105-165°C. Plastics thermostability, which is characterized by the temperature of the maximum possible deformation, is mainly in a range of 60-180°Ñ. The minimum possible performance temperature, when plastics become fragile changes in a wide range: from -10°C for vinyl plastic to -270°C for polytetrafluoroethylene materials.

Most of the plastics are highly inflammable and may be burned; they burn with the opened flame both in the area of fire and outside of it. Plastics based on polyvinylchloride, phenol-formaldehyde, carbamide, organic silicon polymers belong to hardly burned plastic materials. Introduction of the special additives – fire-retardants to the burned polymers also transfers plastics in the group of hardly burned. Fluoroplastic and perchlorovinyl plastics do not burn and do not smolder under the action of the fire.

Dense plastics are waterproof and steam-tight. Hydrophobic polymer materials (polyethylene and polyvinylchloride tapes, glass fiber plastics, vinyl plastic and other) are characterized with the least water absorption (0.1-0.5%).

Content of the large amount of hydrophilic fillers for example wood shavings, sharply increases the water absorption.

Number of non-filled plastics has high transparency. It allows production of organic glasses on their basis, used for panning of greenhouse, hothouses and buildings of the medical assignment. Organic glass – acrylic resin is the most widespread – skips up to 94% of radiated visible particle of spectrum and 73.5% of ultraviolet rays, in that time as ordinary silicate glass approximately 84-87% and 0.3-0.6%.

Usually, polymer materials are good dielectrics. It is necessary to take into account the possibility of accumulation of electrostatic charge on their surface which appears under the action of friction forces. The degree of electrization of such roll materials, as polyvinylchloride linoleum, can achieve 65 V/cm2. It is necessary to take into account at prevention of the fire especially in apartments, where the vapors of inflammable liquids concentrate. Ability to the electrification of plastics diminishes by introduction to their composition of the special matters - antistatik and filling agents which conducts a current.

Synthetic polymers and plastics based on them have high resistance to the aggressive environments. Carbon-chain polymers are the most proof to the action of acids, alkalies, salts and different oxidants. Heterochain polymers are easier affected the action of chemical reagents. Presence in the macromolecules of hydroxyl-group, for example, in the polivinil alcohol molecules, reduces resistance of polymer to the action of water and acids. Opposite, replacement of hydrogen atoms by the fluorine micromoles in composition increases chemical resistance of polymers. Fluorocarbon polymer on the chemical resistance prevail noble metals, special alloys, rust ceramics.

The most of plastics are corrosion-proof not only to the action of chemical reagents but also to the action of fungus, bacterium, insects and rodents. Plastics that consist of wood fillers (wood chipboards and fibreboards), some high-porous plastics (microporous rubber), and the polyethylene products are biologically unstable. Elements of buildings with application of wood chipboards and fibreboards, and also microporous rubber can be deteriorated by fungus and bacterium at the enhanced humidity and temperature. Pipes, tapes and other products, based on a polyethylene may be strucked by rodents. The plastics biostability is improved with wood preservative additives. Tar pitch and some other matters are added in polyethylene products to prevent the rodent damages.

It is important to take into account the sanitary-hygienic properties of floors plastics, internal revetment of walls at their application. Row of plastics, especially on the basis of phenol-formaldehyde, polyester, epoxy polymers, at incomplete processes of polymerization or polycondensation, containing toxic plasticizers, hardening agents, solvents can secrete matters to the environment, insalubrious for people and animals. Static electricity which accumulates on plastics can have a stimulant influence on a microflora.

The change of operating properties - ageing - passes in plastics in a certain degree under the heat, light and air oxygen action. The process of ageing is accelerated due to the action of the mechanical loadings. The ageing of plastics is sharply retarded by introduction of stabilizing additives.

 


Date: 2015-12-18; view: 983


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