Plastics 101 : The Origins of plastic

 

1. The Origins of plastic:

Fossil fuel-based plastics are made from petroleum, natural gas, and coal. These plastics account for the majority of plastic products used today. 

1-1. Monomers, the basic building block of plastic: 

The key ingredients derived from these fossil fuels are called monomers, such as ethylene and propylene. They are small molecules that can be linked together in long chains through a process called polymerization to form plastics (polymers). 

1-2. From monomers to polymers (Polymerization): 

Monomers are small, simple molecules that can be joined together in long chains to form polymers (large molecules), which are used to make plastic, rubber for example. 

Chemical additives can be used to add different properties to the plastic.

1-3. Summary of the plastic making process:

The most common plastic-making processes involves:

• Extracting fossil fuels (oil, gas, or coal).
• Processing these fossil fuels to isolate specific chemicals (ethylene, propylene, …).
• Converting these chemicals into monomers.
• Linking monomers together to form plastic polymers.
• Adding special ingredients to give the plastic specific properties (flexibility, strength, weight, …).

2. The making fossil fuel plastic in details:

2-1. How plastics are made:

Fossil fuels go through a series of manufacturing processes to be converted into plastic products. 

• Fossil fuels are processed to extract raw materials (feedstocks) such as naphtha and ethane.

• The raw material is converted into chemicals such as ethylene and propylene.

• These chemicals are transformed into monomers, which are the basic building blocks for making plastic.

• These monomers are linked together through a process called polymerization to form long chains called polymers.

• Chemical additives are added to the polymers to improve or add properties such as flexibility, durability, and color.

• The plastic material is shaped into products using different manufacturing processes.

2-2. Simple definitions:

• Naphtha: a liquid mixture derived from crude oil, used as a raw material in the plastic production process.

• Ethane: a chemical derived from natural gas, used as a raw material in the plastic production process.

• Ethylene: a basic chemical (monomer) that is produced from Ethane,  and is used to make plastic.

• Propylene: a basic chemical (monomer) that is produced from naphtha, and used to make plastic.

3. Making bio-based plastics: 

Bio-based plastics are plastics that are made from biological sources like plants, algae, or agricultural waste, instead of fossil fuels like oil and gas.

3-1. The basic process:

• Farmers harvest crops like corn, sugarcane, or castor plants.

• After harvesting, the parts of the plants containing sugars, starches, or oils are extracted.

• These materials are then processed using either bacteria or chemicals to create the plastic’s building blocks (monomers). 

• These plant-derived monomers are linked together to form plastic (polymer) just like conventional plastics.

3-2. Some Bio-based plastics types:

3-2-1. PLA plastic (in compostable cups or 3D printing): 

PLA Plastic (Polylactic Acid) is a biodegradable plastic made from corn starch or sugarcane:

• The starch contained in the corn is broken down into sugar. 

• The bacteria consumes sugar and produces lactic acid. 

• The lactic acid molecules are linked together to make PLA plastic.

3-2-2. Bio-PE (in some plastic bags and bottles): 

Bio-PE (Bio-based Polyethylene) is made from sugarcane: 

• Sugarcane contains sugar.  

• The sugar is fermented to make Ethanol (like making alcohol).  

• The ethanol is processed to create Ethylene. 

• Ethylene molecules are linked together to make Polyethylene plastic that's identical to the conventional oil-based PE.

3-2-3. Plant-based Nylon: 

A type of nylon made partly or fully from plant sources such as castor oil:

• Castor plants produce oil. 

• The oil is processed to extract a chemical called sebacic acid. 

• This acid is combined with other ingredients.  

• They link together to form nylon that can be used in fabrics or engineering plastics.

The key difference from conventional plastics is the raw material (plants instead of fossil fuels), but the final plastic can be either identical to conventional plastics or have special properties like compostability (the ability to break down into
natural substances).

4. Making the plastic pellets:

It is the industrial process that produces plastic pellets (granules) or powders that manufacturers can melt and shape into final products. This process involves the below steps.

4-1. Polymerization:

Think of this like making a chain where each link is a small molecule called a monomer:

• Simple joining (Addition): small molecules link directly together, like snapping LEGO bricks.

• Joining with a byproduct (Condensation): molecules connect to form larger molecules, releasing water or other small molecules as a byproduct. 

Examples:

• Ethylene gas molecules link together to form Polyethylene, which is used to make plastic bags.

• Styrene liquid molecules connect to make Polystyrene, which is used to make foam cups.

4-2. Compounding:

This process includes incorporating additives into a “standard” plastic to modify its properties:

• Softeners that are used to make plastic flexible (gardening hoses).

• Protectors that are used to prevent damage from sunlight or heat.

• Colors that are used to make the plastic more attractive.

• Strengtheners that are used to make the plastic tougher.

• Flame protectors or flame retardants which are additives mixed into plastic products to slow down or prevent burning.

Examples:

• Adding softeners to PVC makes it flexible enough to make shower curtains for example.

• Adding glass fibers to plastic makes it strong enough to be used for car parts.

• Adding UV protectors makes outdoor furniture last longer.

4-3. Pelletization:

This process turns plastic into uniform pellets that manufacturers can use as their raw material:

• The plastic mixture is melted.

• The molten mixture is pushed through small holes like squeezing toothpaste.

• The mixture is cooled with water or air.

• The mixture is cut into tiny pellets.

• The tiny pellets are packaged in bags for shipping.

These pellets become the raw material that factories melt and shape into different plastic products.