Lecture №13.
Polymers and plastic.
1. Classification of polymers.
Thermoplastic and thermoreactive polymers.
2 . Properties of polymers.
3 . Modifying of polymers.
4 . Polyethylene. Polypropylene. Vinyl plastic. Caprone. Polystyrene.
Classification of polymers.
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Corrosion resistance, technological effectiveness and other characteristics of
metal materials even more often don't meet increasing requirements of
automotive industry when developing new models and technologies. Therefore
modern materials science is occupied with development of new materials on the
basis of the nonconventional materials, fuller use of secondary resources. The
important role belongs to creation of new machine-building materials on the
basis of synthetic, natural and artificial binding. Among the most widespread
and perspective materials - polymers and plastics.
By origin, polymers are divided on:
- natural.
- synthetic.
- artificial.
Typical representatives of natural polymers are cellulose, starch, natural
rubber.
Synthetic polymers represent a synthesis product - purposeful receiving
difficult substances from simpler. The nomenclature of synthetic polymers
constantly replenishes.
Artificial polymers are received by processing (modifying) of the natural. For
example, artificial polymer - nitrocellulose is received by cellulose nitridation.
On a chemical composition of macromolecules, polymers are distinguished:
- organic.
- inorganic.
- organometallic.
To organic polymers we refer the connections which molecules contain atoms
of carbon, hydrogen, nitrogen, oxygen and sulfurs which are the part of the main
chain and lateral groups of polymer.
Inorganic polymers are connections which don't contain in structure of
macromolecules of carbon atoms.
In the course of receiving polymeric connection, monomeric links are built in a
certain chain. On character of a structure of polymeric chains we distinguish
polymers of linear, branched and mesh structure.
Polymeric materials change the properties under the influence of temperature.
On this sign polymers are subdivided on:
- thermoreactive.
- thermoplastic.
Thermoreactive polymers (thermosettings) when heating above certain,
characteristic for this purpose like polymer, temperatures, become not fusible
and almost insoluble.
Temroplastichnye polymers (thermolayers) possess property repeatedly to
pass when heating to the melted condition.
Thermoplastic and thermoreactive polymers.
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In relation to heating, polymers are subdivided on thermoplastic and
thermoreactive.
Thermoplastic polymers have linear or branched structure of molecules. When
heating they are softened, can melt, when cooling harden. This process is
reversible.
Thermoreactive polymers in an initial stage of education have linear structure.
When heating they are softened. Further the spatial structure is formed, polymer
comes into a thermostable firm fortune.
Features of polymers structure define their properties. High molecular weight
leads to impossibility of transition to a gaseous state or formation of low-viscous
liquids. Thermostable polymers are not softened. Polymers can be in three stable
conditions: vitreous, highly elastic and plastic.
Vitreous - is a firm amorphous condition where there is no movement of links
and movement of macromolecules. The highly elastic condition is characterized
by ability of a material to big reversible changes of a form under the influence of
insignificant loadings. The plastic condition reminds a liquid state with big
viscosity.
Polymers grow old (spontaneous and irreversible their major mechanical
properties change) under the influence of light, oxygen (ozone), humidity,
warmth, long storage. Processes of aging are accelerated under the influence of
mechanical tension. The essence of aging is concluded in difficult chain reaction
with education of free radicals. Polyethylene, polyamide fibers are steadiest
against influence of high atmospheric temperatures and moisture. For aging
delay in polymers add stabilizers. For example, service life of the polyethylene
stabilized by soot, increases till 5 years, polyvinylchloride has term of services
till 25 years.
Properties of polymers.
Only two aggregate states are peculiar to polymeric materials: firm and liquid.
Transfer of a polymeric material to a gaseous state without destruction of
communications in the main chain of macromolecules is impossible.
Polymeric materials can be in four physical conditions:
- crystal
- vitreous
- hyperelastic
- plastic.
Condition of polymers at which a front view of deformations are big elastic
deformations, call hyperelastic. At a fluidity temperature impact on polymer of
mechanical loading leads to development in it irreversible deformation -
condition of viscosity.
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Important feature of polymeric materials is the combination of high specific
durability and big reversible deformations. This property is caused by existence
in macromolecules of polymers of two types of communications - strong
intramolecular (interatomic) and weaker intermolecular.
The durability of polymeric materials is an interval of time from the moment
of load application before material destruction. Functional dependence of
polymeric materials durability on a breaking point from which follows is
established that durability of polymeric materials changes in time and depends
on temperature. Therefore at a choice of polymeric materials according to
operational characteristics it is necessary to consider not only a maximum load,
but also time during which the material doesn't collapse.
Durability of polymeric materials significantly depends on structure which
they get in the course of processing. The method of polymers hardening by
orientation of macromolecules is widely used.
Modifying of polymers.
To improvement of polymeric materials properties we apply their physical and
chemical modifying - introduction in structures:
- stabilizers,
- softeners,
- greasings,
- fire-retarding agents,
- dyes,
- alloying elements.
For increase of plasticity and elasticity of polymeric material at its processing,
before operation into its structure we enter softeners (air of acids and glycols,
polyair, chlorine-containing connections). To plasticization of rubbers we apply
oil refining products (paraffin, ceresin, petroleum oils) and coal, vegetable oils,
fatty acids.
Anti-softeners providing increase of connection rigidity of macromolecules
at introduction of small additives with polar groups in vitreous polymers possess
effect opposite to plasticization.
To protection of polymeric materials against aging we apply stabilizers. The
principle of operation of stabilizers is based on suppression of destruction
processes of polymeric macromolecules under the influence of external factors.
We distinguish stabilizers of the following types:
- antioxidants (slowing-down thermal and thermooxidizing destruction),
- antiozonant (slowing down ozone aging),
- light stabilizers (interfering photooxidizing destruction under the influence of
sunshine),
- anti rads (interfering destruction of a polymeric material as a result of radiation
destruction),
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- anti tiring (slowing-down processes of fatigue phenomena in a material).
Stabilizers enter in small amounts (0,01-2% on weight) at synthesis or
processing of polymers.
Polyethylene.
Typical representative of polyolefins subgroup. Depending on polymerization
conditions (pressure, type of catalyst, temperature) we receive a product of
various molecular weight.
Distinguish polyethylene:
- high pressure and low density.
- low pressure and high density.
- average pressure.
- high-molecular low pressure.
Polyethylene possesses a number of valuable properties: moisture - and gas-
tight, does not bulk up in water, elastic in a wide interval of temperatures, steady
against effect of acids and alkalis, possesses very good dielectric properties.
Combination of high chemical firmness, satisfactory mechanical properties to
technological effectiveness of processing (it is processed in all known ways: by
molding under pressure, machining, vacuum forming, welding, etc.) and low
cost determines its broad application in mechanical engineering, radio
engineering, chemical industry.
Polyethylene of low pressure possesses the bigger mechanical durability and
rigidity and is used for production of pipes, hoses, sheets, film, details of radio
equipment, and various capacities. Molding under pressure produce gates,
cogwheels working with small loading. Polyethylene of high pressure is applied
as a packing material in the form of film or in the form of unbreakable container
(bottle, canisters, and boxes).
However in view of insufficient mechanical durability to production of details
of cars it is applied restrictedly. The main lack of polyethylene - its low heat
resistance, from it is recommended to use products at temperatures not over
80ºС.
Polypropylene.
Polypropylene - is a synthetic polymer, characterized by melting temperature
near 170ºС. In comparison with polyethylene, differs higher impact strength,
durability, wear resistance, possesses high dielectric properties, low steam and
gas permeability, it is steady against effect of boiling water and alkalis, but
possesses low thermo - and light resistance. It is applied to production of the
details working in contact with aggressive liquids.
Vinyl plastic.
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Advantages of vinyl plastics are high mechanical properties, chemical
firmness, technological effectiveness of processing in products, workability
cutting.
Working temperature of vinyl plastic is from 0 to +40ºС, at sharp fluctuations
of temperature is jarred on, and when heating above 40ºС it is softening and
loses rigidity. Vinyl plastic at lowered temperatures becomes fragile. It does not
burn, but at a temperature 120-140ºС starts being softened that is used when
welding products from vinyl plastic. Decomposition temperature 160-200ºС. It
is inclined to aging under the influence of atmospheric actions and chemical
reagents.
Vinyl plastic is let out mainly in the form of sheets and profile hire (pipes,
bars, corner, etc.). Products from vinyl plastic produce expression, stamping at a
temperature 130ºС, machining, welding, gluing together by perchlorovinyl glue.
Capacities in chemical mechanical engineering, cases and separators for
storage batteries, gates, valves, fitting for pipelines, details of pumps and fans
and other products are produced from vinyl plastic.
Caprone.
Caprone gained the greatest distribution as rather cheap and least scarce
material from many brands of polyamides. Its main advantage as constructional
material is the combination of high durability, deterioration, warm and chemical
firmness with technological effectiveness of processing in a product.
Wear resistance of caprone is several times higher, than steel, cast iron and
some non-ferrous metals. The best antifrictional properties caprone with
addition of 3-5% of graphite possesses.
For production of details from caprone and other polyamides most we widely
use molding metol under pressure. Caprone is well processed by cutting, sticks
together and weldable. Details of antifrictional appointment, bearings,
cogwheels, arms, handles, covers, cases, pipeline fittings are made from it.
Polystyrene.
Polystyrene represents a product of polymerization of styrene.
There are the following types of polystyrene and materials on its basis:
- general purpose.
- shock-resistant.
- foaming.
- styrene copolymers.
Polystyrene of the general appointment is the colourless transparent material
possessing absolute water resistance, high electroinsulating properties, light
resistance and hardness. Polystyrene is resistant to a mold, to alkaline and sour
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environments. The main application of polystyrene - radio equipment details,
irresponsible constructional details, products of a household purpose.
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