Thermoplastic Polymers

Table of Contents

• What are Thermoplastic Polymers?
• Recommended Videos
• Thermoplastic Polymers Properties
• Thermoplastic and Thermosetting
• Thermoplastic Polymers Applications
• Uses Thermoplastic Polymers
• Frequently Asked Questions on Thermoplastic Polymers

What are Thermoplastic Polymers?

All the plastic materials which can be softened and melted by heating, but they set again when cool are called thermoplastics.

Thermoplastic polymers can be very broadly classified as amorphous or crystalline. Most thermoplastics suitable for use as matrices for high performance composite exhibit some degree of crystallinity because this type of structure has better resistance to chemical attack by field, hydraulic oil and paint stripper.

With regard to behaviour at elevated temperatures, polymers are classified as either thermoplastics or thermosetting. Thermoplastic polymers have linear and branched structures they soften when heated and harden when cooled. In contrast, thermosetting polymers once they have hardened, will not soften upon heating; their structures are cross-linked and network.

Recommended Videos

Thermoplastic Polymers Properties

Many thermoplastic polymers are reinforced with fibres. Reinforcement is used to improve physical properties – specifically heat deflection temperature. Glass fibres are the most commonly used reinforcing material. The wear resistance and abrasion resistance of thermoplastic polymers are improved by the use of aramid reinforcing. Although fibres can be used with any thermoplastic polymer, the following are the most important.

• Polyamide polymers use glass fibres to control brittleness. Tensile strengths are increased by a factor of three, and heat deflection temperature increases from 150 to 500^oF.
• Polycarbonate compounds using 10, 20, 30 and 40% glass fibre loading have their physical properties greatly improved.
• Other polymers benefiting from the addition of glass fibres include polyphenylene sulfide, polypropylene and polyethersulfone.

Polymers chosen for structural application are usually selected as a replacement for metal. Usually a like replacement of a polymer section for a metallic section will result in a weight saving. In addition polymers can be easily formed into shapes that are difficult to achieve with metals. By using a polymer, the engineer can design an attractive shape that favours plastic forming and achieve a saving in cost and weight and a cosmetic improvement.

An additional cost saving is realised since the polymer part does not require painting for corrosion protection as would the comparable metal part. Selection of the specific polymer will be based on mechanical requirements and the temperature and chemical end-use environment.

Thermoplastic and Thermosetting

Thermoplastic polymers have been known for a long time. It is only recently that the newer so-called high temperature or high performance thermoplastics have been introduced. The early thermoplastic polymers had predominantly aliphatic carbon backbones in which flexible carbon chains could be extended and rotated into many configurations with relative ease.

Rigidity was obtained by restricting the movement of the backbones chain either by crystallinity such as in polyethylene and polypropylene or by the introduction of side groups as in polystyrene or polymethyl methacrylate. The major limitations with these early thermoplastics which are still on the market are their low elastic modulus, low glass transition temperature and poor solvent resistance.

Thermoplastic compared with thermosetting polymers, absorb much less moisture with less consequential reduction in elevated temperature mechanical properties. Thermoplastics are much tougher than thermosets, therefore they have much better inter laminar strength and resistance to impact. Because no chemical reaction is required, they have very short processing times, although the temperatures and pressure are much greater than those required for thermosetting systems with a concomitant increase in costs.

Thermoplastic Polymers Applications

Thermoset composites are used therefore for the most structurally demanding applications particularly if high temperatures are involved.

In choosing a resin system the suitability of the physical and chemical properties of the resin for the chosen processing route, matching to the reinforcements and also how the properties of the cured resin will suit the end use of the composites.

Factors to consider are:

• Resin viscosity – sufficiently low to penetrate the reinforcement.
• Size of moulding – cure reactions are often exothermic, therefore if the moulding is thick, it may be that the rate of curve of the moulding must be reduced to prevent an uncontrolled rise in temperature during the curing process which may be sufficient to actually damage the moulding.
• Speed of reaction – has an important bearing on the rate of article production.
• Compatibility with reinforcement – the resin must wet and adhere to the reinforcement.
• Moisture level – some polymers are not as satisfactory as others in wet conditions, one of the reasons to use vinyl ester rather than polyester resins. Moisture ingress into a laminate can lead to severe loss of properties.

Uses Thermoplastic Polymers