A peek into the plastic industry

Made to last forever – designed to throw away

Manufacturing process

Plastic types and their properties

CORNER: Bio-plastics and biodegrable plastics

References

Made to last FOREVER  –  designed to throw away

The mass production of plastics started in 1950s and has since increased nearly exponentially: when in 1950 plastics were produced 1.7 million tons per year, in 2014 the annual global production reached 311 million tons. In reality, the total quantity is even higher since these numbers do not include fibers made of polyethylene terephthalate (PET), polyamide (PA), polypropylene (PP) and polyacryl. It is estimated that the plastic production could approach to almost 2 000 million tons by 2050 if the production and use trends do not decline.

Plastic is a light-weight, durable, cheap and easily modified material, which is probably why its usage increased rapidly and is still growing. Plastics are extensively used in our everyday life – wherever you look, you will probably find something made of plastic. In Europe the largest sectors utilizing plastics are packaging (39.5 %), building and construction (20.1 %) and automotive industry (8.6 %). In addition, plastic is also used in electrical and electronic industry (5.7 %) and agriculture (3.4 %). Other uses form a large part of plastic usage (22.7 %) and include sectors such as consumer and household appliances, furniture, sport, health and safety.

However, the same properties that make plastic a popular raw-material for a wide variety of products have also disadvantages when it comes to the environment: as a light-weighted material it can end up far from the source, durability ensures it will last long in the environment and low cost makes it more likely to be discarded. The quantity of plastic entering the environment has been increasing as new uses of plastic materials have been developed and products made available to more people. It has been suggested that as much as 10 % of all plastic litter eventually ends up in the sea and becomes marine litter.

Manufacturing process

Plastic consists of polymers, which are large organic molecules composed of repeating carbon-based units or chains. Polymers are produced when molecules called monomers form long chains in a process called polymerization. Monomers can hence be thought as building blocks of polymers. The polymer is called a homopolymer, if it constitutes of repeating identical monomers, or copolymer if it has different types of monomers. The monomers used determine the basic properties, structure and size of polymers.

Some common monomers used in plastic production are ethylene, propylene, vinyl chloride and styrene. These monomers are usually obtained from petroleum or other fossil fuels, and currently approximately 4–6 % of the world’s oil production is exploited to produce plastics. In addition to fossil fuels also biomass, such as plant oils, can be used to produce bio-plastics; their share is still very small, but slowly growing. However, the oil or biomass only provide the basic components for a polymer and hence the properties of final product are not influenced by which raw material is utilized.

When plastics are produced, a variety of chemicals are used as solvents, initiators and catalysts of the manufacturing process. Initiators and catalysts aid polymerization and are only added in small quantities. The catalysts are usually based on metals, such as zinc, tin, magnesium, titanium or aluminum, and include for example peroxides.

Later additives are mixed with the polymer to aid their manufacturing processes or to modify the properties of the end product. Plastic production is quite dependent on additives, since they are essential ingredients in generating or greatly improving many of the vital properties of plastics. Their importance is also seen in the diversity of additives – there are several thousand additives that are used in plastic industry. Additives can for example improve the flexibility or durability of the polymer or make it more resistible to UV-degradation and burning. They are also used to add color to the finished product. Additives can also include fillers, such as chalk, talc and clay, which are added to reduce costs or to alter for example the conductivity of plastic.

Some examples of additives

Additive	Function	Examples
Antioxidants	protect polymer against oxidation	phenolic and aminic antioxidants
Fillers	particulate additives which can change physical properties or reduce costs	chalk, talk, clay
Flame retardants	reduce or prevent combustion	brominated flame retardants (e.g. PBDEs)
Heat stabilizers	prevent thermal degradation	lead stabilizers, calcium-zinc stabilizers
Light stabilizers	reduce reactions caused by visible or UV-light	hindered amine light stabilizers (HALS)
Odour modifiers	mask an undesirable odour or add a desirable one	vanilla, lavender
Plasticizers	enhance flexibility	phthalates (e.g. DEHP, DIDP, DINP)
Smoke suppressants	reduce smoke formation when combusting	tin compounds
UV-stabilizers	preserve from UV-radiation	benzophenones

The amount of additive ingredients is dependent on the polymer type: for example polyvinyl chloride (PVC) may contain over 40 % by weight of plasticizers, which are mostly phthalates, to make it more flexible. Additives and other substances may be released from plastics over time, when the plastics start to degrade and be potentially hazardous in the environment.

Plastic types and their properties

Different plastic products have distinct properties, which can be seen for example in their thermal-resistance, density and structure, which are largely dependent on the additives used in manufacturing. In general, plastics can be divided into thermoplastic and thermoset materials. When heated, thermoplastics can be repeatedly molded and deformed, whereas thermoset materials cannot be remolded after their formation. Thermoplastics are more common and include for example polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polystyrene (PS). Common examples of thermosets are polyurethane (PUR) and epoxy resins or coatings.

The most common polymer type is polyethylene (PE), which accounts for 29.3 % of total plastic demand in Europe followed by polypropylene (PP) with its 19.2 % share. They are commonly used for example in food packaging.

Common plastic types, their share of the plastic production in Europe in 2015 and examples of usage

Production	Abbreviation	Name	Examples of usage
19.2 %	PP	polypropylene	folders, food packaging, car bumpers
17.2 %	PE-LD, PE-LLD	polyethylene	films for food packaging, reusable bags
12.1 %	PE-HD, PE-MD	polyethylene	toys, milk bottles, pipes
10.3 %	PVC	polyvinyl chloride	window frames, flooring, pipes
7.5 %	PUR	polyurethane	mattresses and insulation panels
7 %	PS, PS-E	polystyrene	spectacle frames, packaging, plastic cups
7 %	PET	polyethylene terephthalate	bottles
19.7 %	Others (PFTE, ABS, PC etc.)	Polytetrafluoroethylene, Acrylonitrile butadiene styrene, polycarbonate	teflon coating, hub caps, roofing sheets

Additional letters associated with common polymer types indicates that there are multiple forms available from the basic polymer. For example a common polymer type, polyethylene (PE), has a lighter low-density form (PE-LD or LDPE) and denser high-density from (PE-HD or HDPE). In addition there is also medium-density polyethylene PE-MD (MDPE), which has a density between LDPE and HDPE.

To be more accurate, the densities are usually expressed as specific gravities, which tell the ratio of the density of a certain plastic type to the density of water (1 g/cm³) which acts as a reference substance. Densities of different plastic types can vary since the additives used in manufacturing processes alter the density of the final product. Specific gravities of plastics are important to understand when considering the distribution of plastic litter in the sea; the density of debris related to seawater density determines largely the vertical position of plastic litter in the marine ecosystem.

Specific gravities of some common classes of plastics

Plastic type	Specific gravity (g/cm3)
polypropylene (PP)	0.83–0.85
low-density polyethylene (LDPE, LLDPE)	0.91–0.93
high-density polyethylene (HDPE)	0.94
polystyrene (PS)	1.05
nylon (PA)	1.13
cellulose acetate (CA)	1.29
polyethylene terephthalate (PET)	1.37
polyvinyl chloride (PVC)	1.38

Besides density, different forms of certain polymer types can be further divided into groups according to their other properties. One form of low-density polyethylene are said to be linear, which is included in their abbreviation as one additional L (PE-LLD). Additional E in the abbreviation of polystyrene (PS-E or EPS) stands for expanded, foam-like structure of this form of polystyrene.