2 Overview Of Plastics Conversion Processes

The sheer number and variety of processes used for converting plastic raw materials into components can be daunting. Thankfully, it is possible to make sense of it all by understanding the nature of the raw materials that can be used and by viewing processes in terms of the basic physical mechanics that are involved for each. This approach will make it relatively easy to understand which materials can be used with what processes and what a given process is able to accomplish.

2.1 Raw Material Considerations

Plastic materials are based on hydrocarbons, a class of organic compounds that contain hydrogen and carbon. The primary source of hydrocarbons today is crude oil, although it is possible to produce them from coal, shale, or other forms of fossil fuel. It is also possible to produce hydrocarbons from other organic matter, such as cereal grains.

Hydrocarbons are interesting compounds because some of them lend themselves to reaction by polymerization. This type of reaction produces plastic materials from simple molecular building blocks. The building blocks combine into chains that result in polymer molecules that are very large (in atomic terms). The term polymers is from the Latin poly (meaning many) and mers (meaning units). So plastics are described as hydrocarbons that are composed of "many units."

2.1.1 Raw Material Form

The first source of processing variety comes into play when the question of when and how this polymerization reaction takes place. Raw materials may be liquid components or they may be solids in the form of powders, granules, or pellets. The raw materials may be presented for processing in a prepolymerized form (polyethylene or acrylonitrile/butadiene/styrene [ABS]), they may be in a partially reacted form (urethanes = polyols + isocyanates), or they may be in the state of their precursor raw materials (phenolics and alkyds).

The general path from hydrocarbons to molding materials is shown in graphical form in Figure 1. There are several considerations that determine the .a form that raw materials take.

Physical Form of the Polymer Building Blocks. The raw materials or 5

units used to produce plastics are normally compounds that are in the form of is

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