Plastic materials (from Greek plassein - to shape) are synthetic polymers. They belong to macromolecular substances because they are compounds made of a big amount of atoms. The atoms join together by chemical bonding to make long chains in which basic structure units are repeated. One macromolecule can contain several hundred, thousand or even more basic structure units called monomers.
The number of monomers in a polymer molecule is expressed by polymerization degree. Compounds with a low polymerization degree are called oligomers, the ones with a higher polymerization degree are polymers. In a polymer, monomers represent a construction or a structure unit with the same chemical composition which is regularly repeated.
According to the origin the following polymers are distinguished:
a) natural – eg. polysaccharines, proteins, nucleic acids,
b) synthetic – eg. polyethylene, polyesters, polyvinyl chloride, Teflon,
c) modified – chemically modified natural substances, eg. celluloid, viscose.
Synthetic polymers (plastic) are divided into:
1. According to the type of chemical reactions by which they are created:
a) Polymers prepared by polymerization,
b) polymers prepared by polycondensation,
2. According to the shape of macromolecule polymers are divided into:
d) spatially reticular.
Characteristics of selected plastic capable of recycling
We will focus on polyethylene, polypropylene, polyvinyl chloride and polystyrene. All these types of plastic are made by polymerization. It is a polyreaction in which two identical monomers with multiple bondings react. If different monomers with multiple bondings react, it is called copolymerization.
It consists of atoms of carbon and hydrogen exclusively, therefore it does not represent a serious environmental problem in incineration of waste.
Polyethylene, a well-known polymer, is produced by polymerization of ethene
According to the way of production, type of initiation agent, and catalyst, polyethylene with different properties is produced:
- HDPE – high density PE from English high density polyethylene
- LDPE – low density PE from English low density polyethylene
Use of HDPE
- Hollow objects in moulds, eg. different high volume vessels,
- bottles for detergents
- bottles for mineral water,
- bottle lids.
Use of LDPE
- production of foils, eg. foils for agriculture,
- used as packing material, or applied on paper or aluminium foils,
- production of rubbish bags,
- production of bags,
- production of tetrapack,
- production of cables, pipes, etc.
Polypropylene is produced by polymerization of propylene.
Use of polypropylene
- it is used as a part of machines and equipment, eg. in car making industry (parts of dashboards and fans, bumpers) and in consumer industry (parts of vacuum cleaners, kitchen appliances) because polypropylene has outstanding mechanical properties,
- it is used for making syringes and other medical aids because it is resistant to sterilizing temperatures,
- production of packing foils,
- production of toys because it can be easily coloured by adding appropriate colouring agents,
- production of buckets and wash-basins,
- production of blenders, suitcases,
- in the form of fibres it is used for production of carpets, decoration cloths, knits (stockings).
Polyvinyl chloride (PVC)
Polyvinyl chloride belongs to the most important plastic. Together with polyethylene a polypropylene it belongs to most widely produced synthetic plastic. The structural model of polyvinyl chloride and its spatial chain are in the pictures.
Preparing polyvinyl chloride
Polyvinyl chloride is synthetized based on polymerization of vinyl chloride monomer. Vinyl chloride is synthetized from natural gas, petroleum and sea water salt. Polyvinyl chloride is processed either without softeners, with stabilizers, lubricants and modifiers only, into hard products (pipes, boards, etc) or with softeners into semi-solid to elastic products (foils, vessels, toys, protective gloves, etc.).
Non-softened, hard polyvinyl chloride is known under a general trade name novodur; softened, soft polyvinyl chloride is known under the namenovoplast..
Use of polyvinyl chloride
Products made of polyvinyl chloride are used in electronics, chemical industry, light industry and packing technologies.
Polyvinyl chloride is used in two forms:
a) novoplast (flexible – softened
- production of floor covering,
- food foils,
- protective gloves, raincoats,
- hoses, bottles, tablecloths,
b) novodur (fragile) – non-softened
- production of bars, pipes,
- in making furniture,
- production of leatherette products (jackets, handbags, purses, wallets, etc.),
- production of compact discs ,
- covers for everyday use products, eg. covers for books and exercise books.
Polystyrene is one of the oldest synthetic polymers. T
Polystyrene is made by polymerization of styrene.
Use of polystyrene
- it is mainly used for production of simple consumption products such as jars, bowls, trays, children’s toys and other products of consumption industry
Use of foam polystyrene
Foam polystyrene is a white soft material with outstanding thermo-insulating properties due to a huge number of empty bubbles caught in the polymer.
It is used:
- in construction industry as an insulation material,
- for insulation of cables,
- in production of switches and reels,
- in transporting different products as a protective cover,
- it is also basis of combat substance Napalm B.
Advantages and disadvantages of plastic
Advantages of plastic
- objects made of plastic are very light
- they show high solidity and consistency,
- reduce noise well,
- their density is half of that of aluminium, glass or porcelain, therefore they are increasingly used in production of cars, gliders, parachutes, etc.,
- they are nonconductors,
- they are good electrical and thermal insulators. Conductors of electric current are coated with a layer of plastic, walls of buildings are protected from cold by plastic siding boards,
- they do not corrode or decay, eg. outside building sidings, garden furniture, boats,
- many types of plastic are even resistant to chemical substances such as acids and hydroxides,
- they can be easily shaped, coloured or treated. These qualities are important in developing aerodynamic shapes of cars, airplanes and boats,
- as a consequence of these advantages, plastic is called “material of wish“.
Disadvantages of plastic
- they are toxic,
- they are not very hard, therefore they must be protected from mechanical damage,
- a lot of plastic is not thermally stable. It manifests in unqualified treating of textiles containing synthetic fibres, for example,
- some plastic becomes soft when exposed to heat so that it can be shaped, some decomposes when exposed to heat,
- many types of plastic are combustible,
- is damaged by some organic compounds,
- does not conduct electricity, therefore it gets charged with static electricity by friction,
- can become fragile when exposed to light and breaks easily then. Cheap products made of plastic are not worth repairing, they become objects for throwing away,
- some chemicals from plastic get into food. An example of food contaminated by different plastic is an evidence of styrene released from polystyrene, an evidence of softeners from polyvinyl chloride, acetyladehyde released from PET bottles (polyetylene terephtalate),
- disposal of polyvinyl chloride by incineration is complicated by the presence of chlorine in the structure of macromolecules ( from 1 kg of PVC as much as 600 g of HCl is produced); by incineration of polyvinyl chloride carcinogenic products of incineration– dioxins – are produced as well. Scientists consider dioxins most poisonous chemicals a man has ever produced. There are about 210 kinds of them. They are produced as useless side products of industrial activities in which chlorine is used in incineration,
- plastic is one of the worst materials for recycling and it pollutes the environment most.
Plastic and environment
Plastic is made of petroleum by rather simple chemical procedures. In spite of the fact that production of plastic is a simple process, the same cannot be said about its disposal. Polymers produced by man cannot be decomposed by organisms naturally present in the environment as they do not have enzymes needed for the decomposition of the polymers. It can be said that plastic in the environment does not undergo any decomposition. Therefore biodegradable plastic representing less burden to the environment is searched for.
Production of new kinds of plastic decomposition process of which is accelerated, eg. plastic enriched with starch, has started. Along with this development of biogradable (capable of composting) plastic which is nontoxical, has nice appearance and is less combustible has also started.
Plastic waste is a serious problem as each of us produces several kilos of it weekly. A huge amount of plastic waste is produced in households daily and a problem with its disposal arises.
Ways of plastic waste disposal
- Landfilling is the easiest and cheapest way of waste disposal. About half the plastic waste is disposed of this way. Plastic similarly to glass and porcelain unlike other materials do not undergo significant chemical changes. This way of treating waste is not efficient at all, it is only the last resort solution which has to be further enhanced.
- Incineration is a radical way of plastic waste disposal. It is easily feasible and quite common because all plastic materials can be incinerated easily. This way of plastic waste disposal has not, however, been generally accepted the most convenient solution because part of exhalation may be aggressive (eg. in polyvinyl chloride incineration toxic emissions, e.i. polychloro dibenzo-p-dioxins, polychloro dibenzofurans are released).
- Recycling is the best way today how to avoid surplus of plastic waste in the environment. It is not, however, an easy task because there are a lot of plastic materials to be tackled. The first step towards recycling is separation of waste by a consumer into categories of metals, paper and glass. A different method done in special facilities is magnetic separation and separation based on density. Physical properties of plastic are used here as magnets do not effect it, though they effect steel objects, and plastic has lower density than aluminium and glass objects. To ensure its repeated use, recycled plastic is usually all melted and at the same time a mixture of plastic of a low class is produced. The mixture can be used in production of cheap plastic products such as rubbish bags or tiles used for pavements or roads. Not all plastic, however, can be recycled.
Advantages of plastic recycling
- used plastic is used so that it does not end up in landfil sites and incineration plants,
- natural resources are conserved (petroleum)
- not used material is repeatedly minced and melted,
- products are resistant to water, wind, ice,
- products do not require maintenance, can be cleaned easily,
- products have a wide range of use.
- Disadvantages of plastic recycling
- high costs,
- quality of recycled material many times cannot reach that of the primary one as materials are degraded in processing
- big weight (difficult to carry), eg. a plastic bench costs €150 and weighs 65 kg.