Introduction to Plastics, Polymers, and Their Properties

This chapter is in several parts starting with an introduction to polymer chemistry. It includes polymerization chemistry and the different types of polymers and how they differ from each other. Discussed are the subjects of copolymers, branching, cross-linking, steric hindrance, isomerism, crystallinity, and other factors that affect the molecular structures of the polymers used to make plastics. Since plastics are rarely “neat,” reinforcement, fillers, and additives are reviewed. The influence of molecular structure on properties is examined. A basic understanding of plastic and polymer chemistry will make the discussion of plastics easier to understand and also provides a basis for the introduction of the plastic families in later chapters.

The testing of plastics is the next part. This section focuses on the basics of plastic properties. It is in several major parts. Covered first is the mechanical testing of plastics including tensile, shear, and flexural properties. Impact tests are discussed next. Thermal properties are then discussed such as melt index and glass transition temperature.

The final section is a perspective on how to use the data in plastic selection for specific uses based on thermal and mechanical properties, in the form of selection guides.

Keywords

Polymerization; isomers; molecular attractions; amorphous; crystallinity; additives; degree of unsaturation; steric hindrance; geometric isomer; stereoisomer; syndiotactic; isotactic; atactic; van der Waals forces; molecular weight; polydispersity; thermoplastic; thermoset; tensile properties; modulus; tensile; flexural; shear; impact; Charpy; Izod; melt point; melt index; glass transition temperature; thermal stability; toughness

The most basic components of plastic and elastomer materials are polymers. The word polymer is derived from the Greek term for “many parts.” Polymers are large molecules comprised of many repeat units called monomers that have been chemically bonded into long chains. Since World War II, the chemical industry has developed a large quantity of synthetic polymers to satisfy the material needs for a diverse range of products, including paints, coatings, fibers, films, elastomers, and structural plastics. Literally thousands of materials can be called “plastics,” although the term today is typically reserved for polymeric materials, excluding fibers, which can be molded or formed into solid or semisolid objects. As of the beginning of 2014, IDES The Plastics Web® (http://www.ides.com) listed over 85,900 different grades of plastic from over 900 suppliers.

1.1 Polymer/Plastic Chemistry

This section provides a basic understanding of polymers and plastics from a chemistry point of view.

1.1.1 Polymerization

Polymerization is the process of chemically bonding monomer building blocks to form large molecules. Commercial polymer molecules are usually thousands of repeat units long. Polymerization can proceed by one of several methods. The two most common methods are called addition polymerization and condensation polymerization.

1.1.1.1 Addition Polymerization

In addition polymerization (sometimes called chain-growth polymerization), a chain reaction adds new monomer units to the growing polymer molecule one at a time through double or triple bonds in the monomer. The polymerization process takes place in three distinct steps:

Many of the plastics discussed in later chapters of this book are formed in this manner. Some of the plastics made by addition polymerization include polyethylene (PE), polyvinyl chloride (PVC), acrylics, polystyrene (PS), and polyoxymethylene (acetal).

1.1.1.2 Condensation Polymerization

The other common method is condensation polymerization (also called step-growth polymerization), in which the reaction between monomer units and the growing polymer chain end group releases a small molecule, often water, as shown in Figure 1.2. The monomers in this case have two reactive groups. This reversible reaction will reach equilibrium and halt unless this small molecular by-product is removed. Polyesters and polyamides are among the plastics made by this process.

Understanding the polymerization process used to make a particular plastic gives insight into the nature of the plastic. For example, plastics made via condensation polymerization, in which water is released, can degrade when exposed to water at high temperature. Polyesters such as polyethylene terephthalate (PET) can degrade by a process called hydrolysis when exposed to acidic, basic, or even some neutral environments, severing the polymer chains. The polymer’s properties are degraded as a result.

1.1.2 Copolymers

A copolymer is a polymer formed when two (or more) different types of monomers are linked in the same polymer chain, as opposed to a homopolymer where only one monomer is used. If exactly three monomers are used, it is called a terpolymer.

Monomers are only occasionally symmetric; the molecular arrangement is the same regardless of which end of the monomer molecule you are looking at. The arrangement of the monomers in a copolymer can be head-to-tail, head-to-head, or tail-to-tail. Since a copolymer consists of at least two types of repeating units, copolymers can be classified based on how these units are arranged along the chain. These classifications include:

In the following examples, A and B are different monomers that do not have to be present in a one-to-one ratio. When the two monomers are arranged in an alternating fashion, the polymer is called an alternating copolymer:

In a random copolymer, the two monomers may link in any order:

In a block copolymer, all monomers of one type are grouped together and all monomers of the other type are grouped together. A block copolymer can be thought of as two homopolymers joined together at the ends:

A polymer that consists of large grouped blocks of each of the monomers is also considered as a block copolymer:

When chains of a polymer made of monomer B are grafted onto a polymer chain of monomer A we have a graft copolymer:

High-impact polystyrene (HIPS) is a graft copolymer. It is a PS backbone with chains of polybutadiene grafted onto the backbone. PS gives the material strength, but the rubbery polybutadiene chains give resilience to make it less brittle.

1.1.3 Linear, Branched, and Cross-linked Polymers

Some polymers are linear—a long chain of connected monomers. PE, PVC, Nylon 66, and polymethyl methacrylate (PMMA) are some linear commercial examples found in this book. Branched polymers can be visualized as a linear polymer with side chains of the same polymer attached to the main chain. While the branches may in turn be branched, they do not connect to another polymer chain. The ends of the branches are not connected to anything. Special types of branched polymers include star polymers, comb polymers, brush polymers, dendronized polymers [1], ladders, and dendrimers. A cross-linked polymer, sometimes called a network polymer, is one in which different chains are connected. Essentially the branches are connected to different polymer chains on the ends. These three polymer structures are shown in Figure 1.3.

1.1.4 Polarity

A molecule is two or more atoms joined by a covalent bond. Basically the positively charged atom nuclei share the negatively charged electrons. However, if the atoms are different they may not share the electrons equally. The electrons will be denser around one of the atoms. This makes that end more negatively charged than the other end and that creates a negative pole and a positive pole (a dipole), and such a bond is said to be a polar bond and the molecule is polar and has a dipole moment. A measure of how much an atom attracts electrons is electronegativity. The electronegativity of common atoms in polymers follows:

>O>ClandN>Br>CandH

The polarity of a molecule affects the attraction between molecular chains, which affects the structure of the polymer and the attraction of polar molecules, so one would expect polarity to affect solubility which affects permeability.

How does one predict molecular polarity? When there are no polar bonds in a molecule, there is no permanent charge...