Chemicals & Additives In Plastic

The first stage in plastic production, the polymerisation of raw material.

Then substances such as fillers and chemicals (sometimes called monomeric ingredients), are added to give color, texture and a whole range of other qualities. Reinforcing fibers for example make the base polymer stronger while man-made organic chemicals, such as phthalates are added to make plastic flexible, resilient and easier to handle.

These give the plastic an additional range of qualities. There are thousands of addatives used in making plastic.

Plastic additives

Include
Reinforcing fibers to make the base polymer stronger.  For example baron, carbon, fibrous minerals, glass, Kevlar all Increases tensile strength. Others increase flexibility, heat-deflection temperature (HDT) or help resists shrinkage and warpage.
Extender fillers such as calcium carbonate and silica, clay reduces material cost.
Conductive fillers means electromagnetic shielding property can be built into plastics, which are normally poor electrical conductors include  aluminum powders, carbon fiber, graphite Improves electrical and thermal conductivity.
Coupling agents such as Silanes, titanates  improve the bonding of the plastic matrix and the reinforcing fibres.
Plasticizers – man-made organic chemicals, such as phthalates added to make plastic flexible, resilient and easier to handle. Some are considered unsafe – read more here.
Stabilizers (halogen stabilizers, antioxidants, ultraviolet absorbers, and biological preservatives) to stop it breaking down over time>Protects from thermal and UV degradation (with carbon blacks).
Processing aids (ie lubricants to reduce the viscosity of the molten plastic and others)
Flame retardants Chlorine, bromine, phosphorous, metallic salts Reduces the occurrence and spread of combustion.
Peroxides
Anti-static agents can be used to attract moisture, reducing the build-up of static charge.
Colorants (pigments and dyes) Metal oxides, chromates, carbon blacks.
Blowing agents Gas, azo compounds, hydrazine derivatives Generates a cellular form to obtain a low-density

Concerns

As you can see that is a lot of additives. So many that  we do not know what they all are. Also manufacturers are not obliged to reveal what they use in their plastic mixes. So while the polymers used in base plastics are mostly considered to be harmless, the potential toxicity of the additives is often unknown.

It is claimed that many of the additives used have not been passed as fit for human consumption and that more research needs to be done on the safe handling and ultimate disposal of these plastics.

Rather worryingly, some of the chemicals used in plastic seem to be mobile and can leach from the plastic product into the contents. For example from the plastic packaging wrapped round your cheese or the epoxy resin lining of your can of beans into your food. The jury is still out on wether this is dangerous or not but add that to a brown toast cancer scare and cheesy beans don’t look so tasty!

Halogenated plastics like PVC will, when burnt, release dioxin one of the most powerful carcinogens known.

More animals are being found with plastic in their stomachs having mistaken for food and microplastics are being ingested by bottom feeders and plankton. Some reports claim that chemicals from plastic are being absorbed by animals with ill effects.You can read more on microplastic here and read reports on animals eating plastic.

Plastic particles attract persistent organic Pollutants (POPs). POPs are a small set of toxic chemicals that remain intact in the environment for long periods and accumulate in the fatty tissues of animals. Bottom feeders eat the plastic pellets and so the POPs enter the food chain.

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Naptha & Oil Derived Plastic

Crude oil is a mixture of different hydrocarbons each with a different boiling point. These substances are separated from each other  in a distillation tower.
This results in the separation of heavy crude oil into lighter groups called fractions. Each fraction is a mixture of hydrocarbon chains (chemical compounds made up of carbon and hydrogen), which differ in terms of the size and structure of their molecules.

How Stuff Works puts it like this
Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.
There are two things that make hydrocarbons exciting to chemists:
Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.
Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH4), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon chains are very versatile!
Find out more about hydrocarbons here.

 

oil refinery

Petroleum Oil Refinery Process Diagram

From crude oil you can distill a whole load of products including;
gasoline
lubricating oils
kerosene
jet fuel
diesel fuel
heating oil
Naptha a feedstock for plastic

How much in a barrel?
Oil is sold between countries in quantities called barrels.
One barrel of oil is 42 US gallons 159 litres or 35 gallons or 280 pints
The weight of a barrel depends on where the oil comes from. However, there are about 8 barrels in a tonne
A barrel of crude oil can make about

  • 7.27 gallons (27.5 liters): Other products (feedstocks for petrochemical plants, asphalt, bitumen, tar, etc.)
  • 1.72 gallons (6.5 liters): Liquefied Petroleum Gases (LPG)
  • 3.82 gallons (14.5 liters): Jet Fuel
  • 1.76 gallons (6.6 liters): Heavy Fuel Oil (Residual)
  • 1.75 gallons (6.6 liters): Other Distillates (Heating Oil)
  • 9.21 gallons (35 liters): Diesel
  • 19.15 gallons (72.5 liters): Gasoline
  • Approximate figures because every barrel of crude is different.
  • A flight from San Francisco to Tokyo may take about 9,000 US gallons of jet fuel which requires about 2,250 barrels of crude oil to extract. From Econtrader

    Naptha

    Definitions of naptha vary but
    the fraction that boils between 27 °C and 93 °C (5 – 7 C atoms) is often called light naphtha.
    the fraction that boils between 93 °C and 177 °C (6 – 10 C atoms) is heavy naphtha.
    Crude oils from different sources contain different percentages of naphtha.
    Naptha cannot be refined into gasoline or motor oil.
    Naptha is the plastic feedstock of choice for many but in the US, most plastic is made from natural gas.

    Google says you can get anything from 27 to 54L of naptha from 1 barrel of crude.

    Naptha to Plastic

    Cracking & Polymerisation
    Hydrocarbon chains can be further refined by cracking and polymerising.
    Very basically cracking breaks the existing chains and polymerisation is remixing them into something new. You can read more about it here.

    Ethane and Propane are derived from Naptha
    Using high-temperature furnaces
    Ethane is cracked into ethylene
    Propane is cracked into propylene,
    Using a catalyst, a reactor and some heat these are now remade into plastic polymers
    Ethylene becomes polyethylene also called polythene, the world’s most widely used plastic,
    Propylene joins together to create polymers called polypropylene.
    Most of the plastics we use are derived from polyethylene and polypropylene
    Polypropylene and polyethylene were discovered in  1951 by two chemists working for Phillips Petroleum Company.

    There are enough petrochemicals in one barrel of oil to make one of the following

    • 39 polyester shirts
    • 750 pocket combs
    • 540 toothbrushes
    • 65 plastic dustpans
    • 23 hula hoops
    • 65 plastic drinking cups
    • 195 one-cup measuring cups
    • 11 plastic telephone housings
    • 135 four-inch rubber balls

    Addatives

    Processing can include the addition of plasticizers, dyes and flame-retardant chemicals – see  additives….

    Product

    The polymers are now melted, cooled then cut into small pellets called nurdles.
    These pellets are now shipped to manufacturers who make plastic products by using processes such as extrusion, injection molding, blow molding, etc.

    Qualities & Biodegradability

    These plastics are chemically inert and will not react chemically with other substances which makes them very useful. It also means that they do not break down chemically so do not biodegrade. This has a huge environmental impact as plastic trash lasts forever. See plastic lifespan.

    These plastics can be recycled
    Useful links