CHEMICAL SAFETY TRAINING MODULES

PART IX: POLYMERES

POLYMERS

The petrochemical industry produces basic raw materials, such as ethylene, propylene and vinyl chloride. Further derivatives include basic plastics, such as polyethylene, polypropylene and polyvinyl chloride. Other chemicals, such butadiene for synthetic rubber, and solvents, such as benzene, toluene and xylene are also produced. Out of these a great number of downstream products are made, including plastics, films, detergents, fibres, coatings, and elastomers.

Polymers form a large family of substances with a wide range of properties. They are large size molecules, consisting mostly of organic material. They are artificially synthesized from "chemical unit blocks" called monomers in chemical reactions. For example polyethylene (PE) repeats a unit containing two carbon atoms and four hydrogen atoms. Ethylene gas (CH₂=CH₂), reacts at a high temperature and forms, with the help of catalytic chemicals, carbon chains that repeat such units. The mass production uses crude oil distillation fractions as raw material which, after passing the chemical production line, yields different polymers. The schematic diagram in Table 1 is an example of this process.

Plastics are mixtures containing polymers as main components. Other chemicals, such as fillers, plasticizers, flame retardants, antioxidants, lubricants, heat stabilizers and colour pigments, are added to produce plastics from same polymer with different characteristics. Fibre reinforcement is a method used in the boat industry.

Plastics and resins are divided into the following types:

• Thermoplastic type material that can be softened repeatedly by heat and which is fusible by heating. Polyethylene, polypropylene, polyvinyl chloride and polystyrene are examples of this group.
• Thermosetting materials undergo chemical change when shaped by heating and cannot be reshaped if heated. Phenolic resins, epoxy resins, melamine resins, polyesters and polyurethanes are examples of this group.
• Elastomers are considered as a separate group because of their special properties. Synthetic rubbers, such as isoprene rubber and butadiene rubber, belong to this category.

Polymer production is about 8% of the total consumption of petroleum in the world. Plastics and resins make up about 60% of the petrochemical end-products. About 50 different plastics have commercial importance. These plastics are available in numerous different formulations with different qualities.

Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS) are so called "basic plastics". They cover about three quarters of total consumption of plastics.

Western Europe, the USA and Japan use 64% of the total production of polyethylene. It is the largest volume petrochemical product in the world (1992).

Consumption and demand have risen sharply in countries where local production of plastics has started.

Styrene is part of the "family" of polymers and other substances produced from ethylene gas. It is an example of a cluster of styrene polymers where this monomer is a "building block" (Table 1).

Table 1: Ethylene production chain

1. Properties, use and health effects of some polymers

Polymers are considered to be chemically inert and do not pose a health hazard. Chemical hazards are related to the additives and/or degradation products present in plastics. Various production stages, where the preceding polymer "building blocks": monomers, dimers or intermediates are present, are also possible sources of adverse effects.

1.1. Polyethylene and polypropylene

Polyethylene and polypropylene belong to the group of major commercial plastics available in bulk. Numerous articles are made out of these raw materials. Such polymers are manufactured either using pure ethylene (or propylene) or mixtures of co-monomers, such as butene, hexene, octene, vinyl acetate or methacrylate. The properties of the polymer can be adjusted by the manufacturing process parameters, such as temperature and pressure during the polymerization. These affect the molecular weight, branching of the molecule and density of the bulk material. Additives also give specific properties to the finished products. Polypropylene is mainly used to produce fibres.

Materials on the market are divided according to their density under different names which, for example, in the case of polyethylene include the following:

Polyethylene and polypropylene are not hazardous to health, and pure ethylene and propylene, the raw materials, are not physiologically toxic but simple asphyxiants. The danger arises from the additives and degradation products, both in manufacturing and in finishing. Polyethylene may contain vinyl chloride and acrylic acid as co-monomers; acrolein and formaldehyde are released in heat induced hazardous degradation products from polyethylenes, and crotonaldehyde and formaldehyde from polypropylene. Common blowing agent is a yellow fine powder of azodicarbonamide (AC) which may cause asthma. Some dyes, benzidine based and reactive dyestuffs may also have sensitizing effects. Some fillers have been found to contain hazardous asbestos fibres, and anti-block powder may contain free crystalline silica. These hazards are avoided by substituting, e.g. AC powder may be substituted with paste to prevent dust formation, and careful planning of enclosure and/or ventilation in production line.

The raw materials, ethylene and propylene, are highly flammable gases with the potential hazard of fire and explosion.

1.2. Polyvinylchloride

PVC contains the repeating block of CH₂=CH- in its molecular structure. It can be produced by initiating the polymerization of vinyl monomer with peroxide initiators. The molecular weight of the polymer is controlled by the polymerization temperature; normal range is 50-70^oC and the pressure 7-12 times normal air pressure. Smaller molecular size is produced using higher temperatures. After the polymerization the monomers can be recuperated by heating the resin in a vacuum. PVC is the most versatile of all plastics because of its blending capability with plasticizers, stabilizers and many other additives. This is important as all PVC polymers need heat stabilizers in order to be able to withstand the processing temperatures. Other additives are also used: rubbery polymers to give brittle PVC better impact strength, and fillers. The final basic plastic is in the form of paste, powder or pellets.

The processing of PVC is mainly done by using extrusion, calendering and injection moulding. Two-thirds of PVC production is consumed in the construction and building industry. Pipes are the largest single articles, consuming about one third of all produced PVC. Electrical appliances and insulation are the second largest consumers of PVC by volume. Packaging materials, including bottles and "blister" films, use about one tenth of PVC. PVC, which has been post-chlorinated, has good combustion resistance and is used in sprinkler systems and other applications where engineering grade plastics are required.

Vinyl chloride monomer, the raw material for producing PVC, is carcinogenic, causing a special type of cancer called angiosarcoma. Automatic monitoring systems can be used to reduce the exposure to the monomer and to detect leaks. Detector tubes can be worn by operators to measure the exposure. It has been possible to reduce the exposure to the monomer with control measures. Typical peak concentration found in industry prior to 1955 are estimated to have been over 1 000 ppm. In the 1970s they were found to be 100-200 ppm; frequently measured concentrations being about 10 ppm. The occupational limit value of the vinyl chloride monomer is 5 ppm (ACGIH 1994).

The vinyl chloride monomer may also cause adverse effects in the central nervous system and blood, and skin lesions. This substance may cause changes to human reproduction if the gas or vapour is inhaled.

The vinyl chloride monomer is addition to all other hazardous properties, a highly flammable gas that has caused several industrial fires.

1.3. Styrene polymers

Polystyrene is a colourless, transparent, brittle parent of the styrene plastics family. Additives and formulation with copolymers provide varying properties to this polymer, already available commercially in 1930s.

Polystyrene may be produced in the following way: styrene monomer and pure water are mixed in a closed system reactor vessel equipped with a cooling system. In the 1:1 mixture, styrene is suspended in the water to form very small droplets. The chemicals which are needed to initiate the polymerization and to produce the desired grade of polymer are added. The polymerization starts in the droplets when the temperature of the reactor is raised. A batch of 10 tons takes about 10 hours until the polymerization has consumed the maximum amount of the monomer. The temperature during the polymerization is kept at 90-130^oC. When the polystyrene pearls are ready they can be developed to expandable polystyrene (EPS) by adding pentane to the reactor. Pentane is dissolved to polystyrene pearl, and expands the pearl by evaporating on exposure to heat to produce white, foamy pearls. These are widely used in packaging of delicate articles, such as cameras, and televisions. The Styrox-sheets are also good material for construction insulation.

The combination of the styrene monomer with other monomers in the polymerization reactor gives a selection of polymers which possess unique properties of toughness, rigidity and chemical resistance. These polymers, which are about half of the styrenic polymer production, include ABS (acrylonitrile-butadiene-styrene), SAN (styrene-acrylonitrile) and the blending formulations with various other polymers. Applications of SAN which is inherently clear and transparent, include both industrial and consumer products: lenses, syringes, water filter housings, garden furniture and colour concentrate carriers for other plastics as an example of industrial application.

The styrene monomer is a flammable liquid which may also be used as a solvent, for example in glass reinforced polyester polymers used in boat manufacturing. The storage is hazardous above its flash point, 32^oC, as it may also have accumulated static energy. Usually it contains inhibitors to prevent uncontrolled polymerization. It is irritating to the eyes, respiratory system and skin when present in low concentrations. High concentrations may cause changes in liver and lowering of consciousness. Chronic exposure has effects in blood, the central nervous system, liver, in human reproduction, and is classified by IARC as possibly carcinogenic to humans. The occupational threshold limit value of styrene monomer is 50 ppm (213 mg/m³) (ACGIH 1994), with a warning against skin contact as it may penetrate intact skin.

Other monomers used in copolymer production are also hazardous to health.

Acrylonitrile, a copolymer in the production of SAN, has been classified as probably carcinogenic to humans (IARC).

These monomers require good housekeeping and well-planned working and monitoring practices and equipment to reduce the exposure. Furthermore, processing and finishing may give off styrene vapour which is the main degradation product. Reactive degradation products from additives are also created in low concentrations.

1.4. Epoxy polymers

This large group of synthetic resins obtained its name from a chemical structural group called the epoxy ring or oxirane ring.

One of the common compounds, epichlorohydrin, has this ring structure as part of the molecule. Epichlorohydrin is made to react with bisphenol-A to form bisphenol-A diglycidyl ethers, often also called DGEBA epoxy resins. This pre-polymer needs to be cured to produce the final thermoset resin. These resins can be either liquid or solid depending on their molecular weight. Liquids have a molecular weight of 340-500, a resin with a molecular weight of 500-700 is increasingly viscous, and solids at room temperature have a molecular weight over 700. Solid resins can also be made available in solution with solvents. This division is related to the health effects: resins with low molecular weight are sensitizing.

Novolacs, such as cresol-novolac, also belong to epoxy resins.

Curing can occur at ambient temperature using, for example, amines, such as triethylenetetramine, or amides, such as dicyandiamide. This applies to the low- and medium-low molecular weight epoxies. Curing at elevated temperatures can be performed using, for instance, melamine and urea-formaldehyde resins.

Generally epoxy resins have good chemical and electrical resistance, and a low shrinkage in curing. They have adhesive properties suited for use in laminates and reinforced application and adhesives. Some adhesives can withstand temperatures up to 200^oC. Epoxy resins are used in the electronics industry. In the construction industry epoxy resins are used, for example, to make self-levelling floors and to repair cracks in concrete. Paints, such as shop primers in engineering works, may also belong to epoxy resins.

Health effects are related to the monomers, prepolymers and additives. Epoxy compounds may affect the skin mucous membranes, eyes, lungs, central nervous system, and liver.

Some of the monomers used have been found to cause cancer. IARC has classified epichlorohydrin as probably carcinogenic to humans.

1.5. Polyurethanes

Polyurethanes are a family of plastics, foams and elastomers. The starting materials used in polymer production are liquid polyesters or polyethers, and compounds belonging to the chemical family of isocyanates. 2,4-Toluene isocyanate (TDI) and diphenylmethane-4,4'-diisocyanate (MDI) are examples of these. Pre-polymers of polyurethane are used in many processes to give an even larger variety of polymer properties.

Table 2. Properties of some isocyanate components used in polyurethanes

Isocyanate components evaporate into the workplace atmosphere, especially during spaying operations; TDI and MDI vapours may escape to the workplace air from open vessels at room temperature. Isocyanate components react with several chemicals present in the workplace: water and humidity, alkalis, such as ammonia or sodium hydroxide, acids, alcohols and amines (primary and secondary). Heat is released which may enhance the evaporation of toxic monomers. The formation of heat may also cause pressure in the containers. The solid polymers formed in the reaction may block the vents and pipes.

Reactive molding technique is the method used widely in processing; it is applicable to practically all forms of polyurethanes. In this common method two activated compounds in liquid form are mixed and rapidly introduced into a mould, which after reaction, produces the wanted article. This method allows the manufacture of very complex articles in a single operation. The articles can have fibre-reinforced structures, such as car bumper beams and housings for electronics and business machines, or foam articles, such as shoe soles and armrests for furniture. Rigid foams are used in refrigerator insulation and packaging. Polyurethane adhesives and coatings are commercially available; they fall into the following groups:

• two component system, where the chemical reaction is glueing when the components are mixed
• one component system, "drying" when in contact with atmospheric moisture or oxygen
• one component system, with solvent-drying

In panel production the adhesive properties eliminate the separate glueing operation as the polyurethane may be administered to foil, paper, plastic film or glass fibre to laminates.

Hazardous health effects are related to the monomers and additives; the finalized articles of polyurethane are not a health hazard. The isocyanate monomer component is very reactive and has an adverse health effect which may be irritating, toxic and sensitizing. The inhalation of vapour and skin contact are common routes of exposure. Eyes are specifically in danger; skin and eye contact should be avoided. Additives, such as organic metal salts and tertiary amines used as catalysts may also penetrate readily through intact skin. Eye contact may cause severe irritation and burns.

Massive exposure to a high concentration of isocyanate vapour has caused chemical pneumonitis and even death.

MOCA (4,4'-methylene bis(2-chloroaniline)), a curing agent for polyurethanes, has been classified by IARC as probably carcinogenic to humans; MDA (methylene dianiline) has been classified by IARC as possibly carcinogenic to humans.

When heated to the point of decomposition, polyurethanes give off toxic and corrosive gases. Hot wire cutting requires adequate local ventilation to remove the fumes and gases developed.

Dust formation should be given special attention. The dust should be removed by vacuuming and not by blowing. Under certain conditions the finely divided dust may explode.

The empty isocyanate containers should be cleaned and the remaining substance rendered harmless by letting it react to polymerize. One possibility is to use wash solution containing 9 parts of water and 1 part strong ammonia solution (30%). The amount of the wash solution must be large enough to cover the container - or the spillage - under treatment. The containers are left in the wash solution for a sufficient period to let all the isocyanate react. The reaction vessel should be left open to allow the carbon dioxide gas to escape.

1.6. Synthetic elastomers

Synthetic elastomers are used in the rubber industry together with natural rubber or as a substitute. Thermoplastic elastomers combine the easy processing of thermoplastic polymers and the properties of conventional thermoset rubber. Polyurethane rubber became available in the 1950s, styrenic elastomers came to the market in the 1960s, and in the 1970s there was a wide range of synthetic elastomers from different polymer families.

Synthetic elastomers are both rubber and plastic: in processing they behave like plastics but in functional performance they are like rubber. Fabrication of articles using injection moulding for these formulations is a rapid and highly economical method. Extrusion may also be used successfully. Thermoforming, heat welding, and blow moulding may also be applied.

Applications include the same type of products as those made of thermoset rubber with the exception of pneumatic tires. So far, no synthetic elastomer has been found suitable for that purpose.

The raw materials used to produce synthetic elastomers include styrenic polymers, such as styrene-butadiene rubber, polyethylene or polypropylene blends and polyurethane elastomers, such as urethane rubber. Synthetic elastomers may be cured by vulcanizing, exactly as natural rubber.

Polybutadiene production uses 1,3-butadiene as a monomer, and the polyisoprene rubber uses isoprene (2-methyl-1,3-butadiene) monomer, which is a flammable liquid.

Butyl rubber, a commercially important synthetic elastomer, is a polymerization product of a mixture of isobutylene and some isoprene. It is manufactured in reactors by mixing pure isobutylene and isoprene monomer with methyl chloride, which acts as a solvent, in the presence of the catalyzing agent aluminium chloride. The polymerization reaction creates heat, which has to be removed by cooling. Additives and dispersants (e.g., di-n-butyl dithiocarbamate, zinc stearate and sodium bicarbonate) may be added to the polymer slurry before water is removed.

1.7. Additives and other chemicals used in polymers

Additives are combined with the basic resins and polymers to obtain the various properties: as extenders, to facilitate their processing, and/or to produce wished colour and finish. Thousands of chemicals and mixtures are used in production and are commercially available. The large volume plastic additives are plasticizers, colorants and blowing agents. Flame retardants, heat stabilizers and lubricants are also important. Under development are additive chemicals which allow setting of the time span for use and degradation to the plastic product.

The most widely used additives may be divided into following classes:

• plasticizers
• flame retardants/smoke suppressants
• stabilizers: heat stabilizers, UV stabilizers/light absorbers
• antioxidants
• blowing/foaming agents
• initiators
• lubricants and flow control agents
• antistatic agents
• curing agents
• colorants
• fillers and reinforcements
• solvents
• optical brighteners
• preservative materials, such as antifouling agent

Antifogging agents

Antifogging agents can be applied externally or incorporated into the polymer. These are used in food packaging films and polymer films used in agriculture. In these applications it is important to have a fogless, easy penetration of light through the film. The additive encourages water to spread as an even layer on the film instead of forming droplets, which reduce the transparency of the film.

Polyvinyl chloride, polyethylene, polystyrene and polyester film containing antifogging agents are commercially available.

Antioxidants

Antioxidants prevent deterioration effects in the polymer and oxidation by contact with air. There are different types of antioxidants. Often a combination of five or more antioxidants is used in applications; the compatibility and effectiveness is specific to the formulation. There are about two hundred commercially available antioxidant trademarks for different applications.

Polypropene, polyethylene, styrene resins, such as ABS, and polyvinyl chlorides are among the resins which contain antioxidants. So-called hindered phenols and phosphite are used to prevent colour changes and deterioration in polypropylenes and polyethylenes. Amine antioxidants and thiobisphenols can be used where staining is tolerated, for example in carbon-black-containing polyethylene. The effective concentration of antioxidant varies. ABS articles deteriorate quickly in the open air and during thermal exposure without an antioxidant; usually an antioxidant combination containing organic phosphites and alkylidene bisphenols is added up to 2.5% of the total weight.

In polyvinyl chloride resins the most common stabilizers are epoxies, phosphites, and organometallic salts of tin, lead, barium, lead, zinc, and cadmium.

Antistatic agents

Plastics and resins are electrical insulators and tend to generate static electrical charges from friction during extrusion or even from movement of ambient air. Static electricity on plastic products may have effects from nuisance to serious hampering of production, and even to dangerous situations from spark formation in surroundings containing flammable vapours. Antistatic agents can be applied to the surface by dipping the plastic object in dilute water or alcohol solution. Drying leaves an antistatic material layer on the surface. External antistatics form an invisible water cover by retaining the air humidity on the surface; they are usually applied on textiles and cosmetic packaging. Antistatic agents may also be mixed to the formulation in the production process. Dosage level (by weight) varies from 0.05% for LDPE to 2.5 for styrenic resins. Crystalline polystyrene and flexible PVC need even higher levels. An internal antistatic agent cannot be applied for many plastics used in engineering applications, as these do not survive the high processing temperatures needed; such plastics are, for example, polycarbonates used in safety helmets, bullet-proof windows and lenses, and polymethyl methacrylate, used for glassy articles, signs and shields.

Over two hundred commercial antistatic agents are available; they are formulations containing substances of anionic or cationic compounds, such as quaternary ammonium salts, sodium alkyl sulphonate, organic phosphates and dithiocarbamate salt. Non-ionic antistatic agents are substances such as ethoxylates, amines, ethanolamides, esters or ethers of polyethylene glycol and ethoxylated tertiary amines for low-humidity environments.

Colorants

Colorants are available as organic or inorganic pigments, or dyes, in the form of precolour, dry colour, liquid colour or colour concentrate. Precolour is a coloured resin ready for processing. Dry colour is in powder form; it is inexpensive but poses a problem in dispersing and cleaning the processing equipment, and it is dusty. Liquid colours are easy to use but need special pumps in processing and are not suitable for a number of resins. Colour in concentrates is attached to a carrier resin and available in pellets or dices. The wanted colour can seldom be obtained by using one colorant but requires a mixture of several colorants. Plastic processors may choose the product suitable for their application from several colouring products which are commercially available. Inorganic colorants are widely used; most of them contain titanium dioxide, although iron oxide is also used. Organic pigments are available in a variety of colours too, but these are more difficult to disperse, and may be destabilized by heat and light.

Curing agents

The usefulness of polymers that have a linear modular structure increases if chemical cross-linking between the long molecules can be produced. This process is called curing. It can be done at room temperature or it may need elevated temperatures. Peroxides are a group of chemicals which may be used as well as some acid anhydrides, such as trimellitic anhydride in epoxy. Benzoyl peroxide is used to cure polyesters over a wide range of temperatures. A tertiary amine, such as dimethylaniline, is added to the mixture to activate benzoyl peroxide. Methyl ethyl ketone peroxide is extensively used in curing glass-reinforced polyester resins; a common way is to use a mixture of methyl ethyl ketone and hydrogen peroxide. One example of a methyl ethyl ketone activator is cobalt naphtenate.

For phenolic resins such as Novolac A (formed from phenol and formaldehyde in the presence of sulphuric acid) curing can be performed by adding hexamethylene tetramine.

Rotational moulding of HDPE (high density polyethylene) used for storage tanks, produces cross-links with a peroxide suitable for use in plastics with high melting points, such as 2,5-dimethyl-2,5-di(t-butylperoxy)-hexyne-3.

Degradability additives

These additives have been developed for controlling the useful life span of plastics and hence reducing the problems of waste disposal. Molecules of plastics with these additives disintegrate into non-toxic, smaller fragments through degradation by natural processes.

Photodegradation additives accelerate the natural degradation of plastics by sunlight or UV light. When the single component vinyl ketone polymer is added to polyethylene or polystyrene formulations, the product degrades after about three months of outdoor light exposure. Organometallic compound, such as calcium stearate, promote break-down in sunlight. For polyethylene and polypropylene applications a method combining stabilizing and accelerating additives in a single masterbatch has been developed, which allows precise time-control of the initiation of the photodegradation reaction. After the period of intended use has elapsed the plastic undergoes a rapid, self-accelerating degradation. This type of plastics can be used for compost and trash bags.

Photodegradable plastic continues to degrade with water, oxygen and micro-organisms when buried. Photodegradable plastics can also be recycled.

Biodegradation can be enhanced by adding (from 6% to 15%) a cornstarch-based additive. A family of bio-plastics is under development, but these are not yet available in-large scale production.

Flame retardants and smoke suppressants

Flame retardants may develop a protective char. Others quench and cool the burning object by releasing water upon heating. Antimony compounds are used in fibres; they do not interfere in the spinning and they are inexpensive. Aluminum trihydrate releases water during a fire; it is used in some wire and cable formulations and in unsaturated polyester, epoxy and urethane plastics. Phosphate esters are widely used for flexible PVC. Alkyl and halogenated phosphates can also be added to epoxy, flexible urethane and polyester products. Ammonium polyphosphates are suitable materials for a variety of polymers, including polyethylene, polypropylene, elastomers, epoxy, and urethanes.

Foaming agents

Plastic that contain voids, or cells, are regarded as foams. Foaming agents, or blowing agents form the cells by releasing gas during the processing. Foams may be and are manufactured using any of the types of plastics including high-heat engineering and thermosetting polymers. The foam production has four steps:

• foaming agent is thoroughly dispersed into the polymer material, usually in liquid or molten form;
• gas-containing pockets are formed, often initiated by specific material serving as nucleus centres for gas formation;
• cells are allowed to grow to the desired size by collecting the gas in the formed pockets;
• cells are stabilized by increasing the viscosity so that the cell growth is no longer possible, for example, by cooling or cross-linking.

Common foaming agents include chlorofluorohydrocarbons, such as HCFC-22, hydrocarbons, such as butane and n-pentane, and compressed gases, such as nitrogen and carbon dioxide. These substances form foam as the temperature or pressure changes, which causes the agent to evaporate.

Another method is to use additives that decompose and produce gas when heated (mostly nitrogen); other decomposition products remain in the polymer mass. The large majority of foams produced in this way are manufactured using azodicarbonamide (AZ). In its pure state this substance is a yellow-orange powder that decomposes above 180^oC. The decomposition temperature can be modified within a range that is compatible to many polymers. Usually less than 1% AZ by weight of the polymer is needed for foaming.

In low-temperature processes for the production of LDPE, flexible PVC and some types of epoxies, toluene sulphonyl hydrazide (TSH) a cream-coloured powder is used. Another sulphonyl hydrazide is also widely used: oxybis(benzene sulphonyl hydrazide) (OBSH). The first sulphonyl hydrazide, TSH, decomposes at 115^oC and the second, OBSH at 160^oC.

High-temperature foaming agents are suitable for high-heat ABS, rigid PVC, polycarbonate and nylon. Toluene sulphonyl semicarbazide (TSSC) and 5-phenyl tetrazole (5-PT) are foaming agents that decompose at temperatures exceeding 215^oC.

Contact with organic foaming agents should be limited as many of them are irritants.

Lubricants

Lubricants are used to reduce the adhesion and viscosity of the plastic formulations and facilitate trouble-free processing. Lubricants can be added in small amounts to the plastic mass or used externally. They should be chosen so that they are compatible with the polymer. Chemically lubricants are metal stearates, fatty acids, esters, paraffin waxes, amides or a combination of these. Calcium stearate acts as an internal lubricant for PVC, ethylene-bis-stearamide (EBS) offers metal or mould release and is used with rigid PVC and ABS formulation.

Most lubricants can help the production process as they may have also other effects besides that of lubrication. The effects are determined by testing in order to find optimal combinations of the lubricants for specific applications.

Plasticizers

Polymers are often inherently rigid and brittle. Plasticizers are added to improve flexibility, softness and suitability for different processing applications. Plasticizers are chemically and thermally stable; their addition also enhances the stability and reduces the degradation of the host polymers. They are of various chemical types, some being used in specified products only. Plasticized polymers can be moulded, sprayed, and used in a coating in an uncured liquid form. Calendering, extrusion and moulding of the polymer melt are other methods used in processing. Polyvinyl chloride accounts for about 80% of plasticizer production and use since it is especially suitable for plasticizing. Plasticizers can also be applied to cellulose-based, polyvinyl acetate, urethanes and acrylic formulations.

Phthalates, such as di(2-ethylhexyl)phthalate, trimellitates, benzoates and terephthalates are monomeric plasticizers and examples of the hundreds of different chemicals used as plasticizers. Some polymeric plasticizers are high performance special products.

Coating is a special case in applying plasticizers. Plasticizers are in a solution or latex emulsion with resins to prevent the thin film of coating becoming brittle.

The amount of the plasticizer in formulations can vary considerably; 50% by weight is used in some applications.

Some plasticizers have been classified by IARC. For example, acetamide and di(2-ethylhexyl)phtalate are classified as possibly carcinogenic to humans.

Preservatives

These additives protect the polymer against microbiological attack. Polymers are normally resistant to microbes, mildew, fungi and bacteria when they are pure materials. However, formulations containing additives, such as plasticizers and lubricants, may support the growth of micro-organisms. Formulations of flexible polyvinyl chloride, which is most often found to be susceptible to micro-organism attacks, need to contain antimicrobials. Acrylic and silicon based adhesives also require antimicrobials in their formulations. The end-products for such formulations include marine upholstery, pool and pit liners, wall coverings, wire and cable jackets, shower curtains, awning blinds and roof membranes. The preservatives used are considered as pesticides and are in many countries subject to the registration procedures and other specific regulatory measures.

Stabilizers

Stabilizers are needed to protect the structure of polymers and pigments against degradation induced by physical conditions, such as heat and light, or chemical attacks, such as that of atmospheric oxygen. Heat stabilizers can be organotin compounds, metal salts, epoxies, pentavalent phosphorous compounds, and their mixtures. Chemicals that absorb ultraviolet light include benzophenones, benzotriazoles, salicylates, acrylates, and organonickel compounds.

Heat stabilizers

PVC polymers and co-polymers have poor resistance to heat, which is a major disadvantage. The stabilizers used in PVC formulations are organic compounds of metals. Commonly used heat stabilizers are tin compounds, such as dibutyltin bis(alkyl maleate) and dibutyltin (isooctyl mercaptoacetate), used for plastics which come into contact with food products, and lead compounds, such as dibasic lead stearate and dibasic lead phosphite. In some countries the use of stabilizers containing cadmium are banned because of their toxicity.

UV stabilizers

Hindered amine light stabilizers (HALS) can be used in pigmented polymers and are specifically suitable for the protection of white or blue PP fibres. They also act as antioxidants.

A high concentration of titanium dioxide can also serve as a stabilizer against the degrading effects of UV light.

2. Occupational exposure

Occupational hazards should be viewed in connection with processing and production activities:

• the manufacturing of basic polymers and resins;
• processing and converting the basic polymer raw materials to plastic articles; this can be achieved using formulations containing additives and processing this material to articles and products;
• fabrication and finishing the products and articles, which include mechanical machining, forming and decorating.

Major processing methods used in the manufacture of plastic are compression moulding, injection moulding, extrusion, and foaming. Injection moulding, extrusions, and production in the form of beads are predominant ways of manufacturing styrenic plastics. Extrusion is used for polypropylene and polyethylene. Compression moulding is widely used for phenolic, melamine and urea resins. The casting process where liquid is poured into a mould and then solidified, can be used to produce films, sheets, rods and tubing. The plastic may be in the form of a melt, in solution with a solvent, latex, paste or lacquer in the coating process. This method is used in the manufacture of paper, fibres and fabrics. Paper laminates are produced by impregnating the paper sheets with a thermosetting resin, compressing them and curing. Paper laminates impregnated with phenolic resins are used, for example, for durable table coverings. Lamination methods are also used in the production of glass-fibre-reinforced plastic articles, such as boats, tubes and pipes.

Plastic foam production methods include gas generation to a polymer in liquid form.

Plastic may also be pumped under high pressure through small holes and solidified, after which the fibres formed are spun to thread.

Occupational exposure to residual monomers of some polymers poses recognized risks: vinyl chloride monomers in polyvinyl chloride and acrylonitrile in ABS resins. IARC has classified vinyl chloride monomer as a substance causing cancer to humans and acrylonitrile as probably carcinogenic to humans. Likewise, some additives are probably carcinogenic (IARC). Examples of these include styrene oxide used in epoxy resin, and MOCA, used as a curing agent in the polyurethane industry. Epichlorohydrin, which is a raw material in the epoxy industry, is also probably carcinogenic to humans. Styrene and toluene isocyanates, raw materials for the production of polymers, are classified as possibly carcinogenic to humans.

Dust is produced in many operations in the plastics industry. Although not always containing toxic components it may be of general nuisance. The occupational limit value for nuisance dust has in many countries been set at 10 mg/m³.

Elevated temperatures of polymer processing cause decomposition of the polymers yielding airborne materials which are irritating to the eyes and respiratory tract. Styrenic polymers, acrylics, nylon, polyurethane and PVC are examples of polymer materials which will produce degradation products (at sufficiently high temperatures) that are toxic and irritating if inhaled.

3. Preventive measures

Preventive measures should commence with accurate information about the polymer, the monomers and the additives used in compounding them. The variety of chemicals is large and the health effects very different depending on the raw material and the method used.

General measures should consist of the following:

Substitution a variety polymers of different origin with same qualities are available.

Technical preventive measures

Workplace:

• workplaces with hazardous materials should be separated from other type of work, e.g., spray painting with epoxy containing sensitizing and carcinogenic components. This requires complete separation, ventilation and protective clothing including fresh air breathing apparatus.
• working with some monomer and prepolymers requires complete or partial enclosure. Reactions producing polymers need closed reaction vessels and pipes to the monomers
• local ventilation
• consideration of methods for cleaning the tools used in the processing
• planned and organized waste collection and disposal
• preventing dust formation, considering the possible degradation products, solvent and additive vapours from finishing and working with polymers. For example, the dust may be just a nuisance or a cause of asthma, depending on the polymer used.

Storage

• storage should be considered; raw materials are highly flammable and reactive; some raw materials used in polymer production belong to the group of substances with potential major hazards, such as the curing agents MOCA in amounts exceeding 10 kg and methyl ethyl ketone peroxide if more than 50 metric tons is in the storage area
• planning and training of response to emergency situations, providing the necessary equipment and solutions.

Protective equipment

• Personnel several monomers, additives and solvents are potential sensitizers. People with a medical history of allergic reaction should not work with these chemicals.
• training of workers to recognize the chemical hazards involved in the use of monomers, prepolymers and additives. Training should also include choosing the right working method and protective equipment, at least the gloves, aprons and eye protection.

4. Effects on the environment

Waste arising from increased plastic and resin consumption may be a problem: in industrial countries it accounts for 7-8% by weight and nearly 20% by volume of municipal solid waste. Increased plastic recycling is needed also for this reason, not forgetting the high energy content and other possibilities of recycling.

DEFINITIONS, EXPLANATIONS

Monomer unit element, "building block", which are chemically joint together to form chains with various lengths. Example of the unit is styrene.

Polymer unit element of polymer is a chain, branched chain or net structure formed from monomers in a chemical reaction. The substance is defined by the size of the formed cluster: average molecular weighed. Polystyrene is formed from styrene molecules.

Homopolymer this type of polymer consists of only one kind of monomer unit (e.g. polystyrene is homopolymer).

Copolymer is a polymer consisting more than one kind of monomer units (such as SAN - styrene acrylonitrile - has a second monomer with styrene)

Family of polymers is a group of polymers, either homopolymers or copolymers, with different number-average molecular weights or different compositions resulting from different ratios of monomer units.

Molecular weight numerical figure describing the molecule. This figure is the sum of the weights of atoms which are listed in the table of elements: hydrogen has a weight of 1, carbon 12 and oxygen 16.

Plastic, resin products and preparations formed from polymers with additives: fillers, stabilizers, plasticizers, colorants, etc.

CHECKLIST FOR PLASTIC MATERIAL

Mark areas where more information/measures are needed:

1. INFORMATION OF THE SUBSTANCE

• Name, molecular weight, degree of purity, identity and amount of non-reacted monomers
• Proposed uses, advice of wastes resulting from the proposed uses
• Recommendations of handling, storage, transport
• Precautions for fire, dangers of chemical reactions (such as reaction with water)

2. COMMON ADDITIVES USED IN POLYMERS AND RESINS

• Plasticizers
• Flame retardants/smoke suppressants
• Stabilizers: heat stabilizers, UV stabilizers/light absorbers
• Antioxidants
• Blowing/foaming agents
• Initiators
• Lubricants and flow control agents
• Antistatic agents
• Curing agents
• Colorants
• Fillers and reinforcements
• Solvents
• Optical brighteners
• Preservative materials, such as antifouling agent

3. ACTIVITIES WITH HAZARD OF OCCUPATIONAL EXPOSURE

• Manufacturing of basic polymers and resins
• Processing and converting the basic polymer raw materials to plastic articles, use of formulations containing additives and processing this material to articles and products
• Fabrication and finishing resin or polymer products and articles includes mechanical machining, forming and decorating

4. PREVENTIVE MEASURES

• Technical measures at workplace
  • separation, enclosure, ventilation
• Storage
• Disposal
• Working methods
  • for example, cleaning the tools used in the processing
  • preventing dust formation
  • considering the possible degradation products, solvent and additive vapours from finishing and working with polymers
• Planning and training
• Protective equipment