Making polymers and plastic

To make plastic, the recently formed or extracted monomers must undergo several physical and chemical transformations, allowing them to become the building blocks of plastic making: polymers. These macromolecules, with various chemical properties allowing the creation of distinctly different types of resins, are also submitted to different treatments depending on the desired final product. Let us take a look at the origins of plastic.

POLYMERS

Before looking at the actual manufacturing process, let us describe in more detail the polymers, these long chains of basic molecules (monomers) made of carbon atoms onto which other elements can be fused through certain chemical reactions, elements such as hydrogen, chlorine, nitrogen, fluorine, or oxygen.
Linear or cross-linked (these qualify the type of bonds between monomers), these precious macromolecules are divided into three main types: thermoplastics, thermosetting polymers, and elastomers. Without going further into a deeper classification of these plastic compounds, let us have a look at the main characteristics of these three fundamental plastic families:
 
1/ Thermoplastics
These linear chained compounds have a unique ability: they become malleable and easy to shape when heated above a certain temperature, and return to their original rigidity when cooled, without no degradation. This characteristic is very useful to the recycling industry, allowing it to treat the plastic without changing its molecular structure. This category of linear polymers includes polyamides such as nylon.
 
2/ Thermosetting plastic
These compounds are fundamentally different to thermoplastics. When heated, these cross-linked organic polymers set irreversibly, and their shape can not be changed without degrading their structure. Thermosetting materiel keeps its shape thanks to the many and solid chemical bonds between its chains. This family of materials includes silicon and phenol formaldehyde resins (PF) such as Bakelite.
 
3/ Elastomers
Cross-linking is a chemical reaction where one or several networks are created within a three-dimensional polymer by linking macromolecular chains together. When conformational polymers are created, the highly-bendable elastomers (a basic component of rubber) can reach up to 8 times their own size without rupturing.

POLYMERIZATION

Polymerization is the chemical reaction that allows different monomers to assemble via covalent bonds (bonding of the different atoms’ electrons), resulting in the transformation of liquid or gaseous raw materials into plastomers. There are two main types of reactions: step-growth polymerization and chain-growth polymerization.
 
1/ Step-growth polymerization
In chemistry, the structure of certain monomers is called a functional groups. These are an atom or a group of atoms within a compound that have retained similar chemical properties, and can thus react together.
During step-growth polymerization, monomers with functional groups randomly react, aided by a catalyst (an acid, for example) and a series of condensation steps that eliminate unwanted molecules like water. Monomers then form little chains that will gradually bond together, transforming into dimers, trimers, or other oligomers (molecules with 2, 3, or several parts), or finally into artificial long-chained polymers. The molar-mass (quantity of material) increases during the reaction, and decreases through capillarity the monomer concentration.
Currently, only 10% of plastic in the world is obtained through step-growth polymerization, but due to their many qualities (versatility, high chemical and thermal resistance, high fibre content…), these polymers are considered very noble and are on the list of recyclable material.
Here are some of the compounds made using this chemical reaction, a process also used to recover plastic material: polyamide fibres (such as nylon or Kevlar), polyesters (such as Tergal), polyurethane (used to make insulation foam), and certain resins (such as the one to make Formica).
 
2/ Chain-growth polymerisation
Chain-growth polymerization is a technique whereby monomers are added one by one to the end of the macromolecular chain, forming a polymer. The last monomer of the chain gives the type of reaction used. If that last one is a free radical (an atom with one or more unpaired electrons), the reaction is called radical polymerisation. On the other hand, if the monomer on the chain’s extremity is ionized, the polymerization is described as cationic or anionic.
Chain-growth polymerization happens in three main phases:

  • Chain initiation, usually by means of a chemical initiator that bonds to a monomer, creates a starting point for the polymerization.
  • Chain propagation, when the active monomer bonds with a new monomer at the end of the chain, increasing the forming-polymer’s mass (it this stage, it still has the same structure as the monomers, since no molecules are eliminated during the process).
  • Chain termination, when a chemical agent deactivates the last monomer in the chain, ending the polymerization reaction.

Over 90% of the world’s plastic production uses chain-growth polymerization. This large family of polymers includes polyethylene (used in recyclable products such as protective films, food-packaging, and certain insulation products), polystyrene (this well-known and semi-rigid plastic is used to make toys or yoghurt pots, and is one of the waste products recycled by Paprec), polyvinyl chloride (PVC is used to make bottles, door and window frames, etc), poly-vinyl acetate (used for varnishes and paints), and polypropylene (PP, manufactured as film or pieces, used for automobile parts as well as food-packaging). PP comes in different grades, such as ‘injection-moulding’ PP which is very easily recycled, or the not-so-easily recycled ‘film’ PP.

ADDITIVES

 To finalise the treatment process of plastic, and prepare it for future use, additives are incorporated to the synthesized polymers. These additives allow industries to obtain materials that are perfectly adequate to their needs. Here are some examples of additives used to treat plastic:

  • Mineral, metal, or organic fillers (such as glass, aluminium, flour, or wood) can improve the plastic’s quality and resistance (to heat or chemicals). Depending on the material needed, more or less filler is added, with the idea that a plastic resin made of only polymers is more expensive than a composite one.
  • Plasticisers (or dispersants) are chemical additives that increase a products fluidity (decreasing rigidity). Naphthalene and melamine are two such molecules compatible with certain polymers.
  • Chemical stabilizers, such as antioxidants, that prevent the polymer’s degradation during its transformation.
  • Antistatic agents, chemicals that prevent dust from settling on the plastic (due to static electricity).
  • Lubricants, such as wax or paraffin, that stop organic material from sticking to machines, as well as prevent the wear and tear of plastic.
  • Dyes and pigments, that (obviously) change the colour of the resin.
  • Flame retardants, that inhibit the production of flames on the surface of the plastic, and are used primarily in construction materials.

 
Once synthesized and added-to, polymers can come in the form of powder or granulated resin, ready to be transformed, used, and sold. They then become plastic waste, and will be treated and processed to be re-used. 
 
The plastic recycling cycle is a step-by-step process. Discover the different steps :
 

 

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