Made to last forever – designed to throw away
Plastic types and their properties
CORNER: Bio-plastics and biodegrable plastics
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.
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
|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.
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)|
|low-density polyethylene (LDPE, LLDPE)||0.91–0.93|
|high-density polyethylene (HDPE)||0.94|
|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.
BIO-PLASTICS AND BIODEGRADABLE PLASTICS
The term bio-plastics is adopted to refer to plastics that are produced from biomass, such as organic waste or plant oils. The name does not however tell anything about the biodegradability of the material, which means the capability to completely break down to naturally occurring compounds. Thus, bio-plastics can be as persistent as plastics produced from fossil fuels and only deteriorate to smaller plastic fragments. Truly biodegradable plastics can be transformed biochemically by micro-organisms and therefore slowly disappear from the environment entirely. Biodegradable plastics have been proposed to resolve the problem with increasing amounts of plastic litter. These biodegradable plastics are designed to be more susceptible to degradation in specific environmental conditions, but these conditions may vary widely. For example biodegradable single-use plastic bags may biodegrade completely when exposed to the temperature of 50°C for a long period. However, suitable conditions for biodegradation can be rare in marine environment and hence even biodegradable materials might not rapidly degrade in the ocean.