Thermoplastics - Additives and Alloys

Additives

Plasticisers

A large number of additives are available for the modification of polymer properties, with plasticisers the most dominant additives in terms of tonnage and value. Plasticisers are typically added to PVC compounds for increased elastomeric properties. A typical formulation for a colourless, flame retardant PVC sheet is: 53.5% PVC suspension, 32.0% plasticisers, 8.5% extenders, 5.0% flame retardants, 0.5% lubricants and 0.5% stabilisers.

Flame Retardants

The fastest growing additives are the flame retardants, largely a result of the increasingly stringent legislation on the flame and smoke toxicity of materials used in public places. Of the halogen-free flame retardants, aluminium trihydrate (ATH) is probably the most common.

UV Stabilisers

While certain monomer units that go to make up polymers do not absorb ultraviolet light (UV) (e.g. the monomer units of PVC, PMMA and PS), these polymers almost invariably contain traces of UV absorbing impurities. Hence, for prolonged outdoor use, most polymers contain light stabilisers. Hindered amine light stabilisers (HALS) and carbon black are frequently used. Long term exposure to light causes ageing of the polymer due to the breaking of polymer bonds, cross linking or oxidation (see Table 1). The consequences are, embrittlement and cracking and changes in colour or transparency. Coatings are also frequently applied to prevent attack from sunlight as well as other environmental agents.

Table 1. Resistance to weathering (data for guidance only) (PE – Polyethylene, PTFE – Polytetrafluoroethylene, PP- Polypropylene, POM- Polyoxymethylene, PS – Polystyrene, PA – Polyamide, PC – Polycarbonate, CAB – Cellulose Acrylo Butyrate, ABS – Acrylonitrile Butadiene Styrene, SAN – Styrene Acrylonitrile, PVC – Polyvinylchloride, PMMA – Polymethylmethacrylate).

Material

Not Stabilised

Stabilised

Rating

Resist. to weather+ (years)

Rating

Resist. to weather+ (years)

PE

Poor

1

Good*

>1

PTFE

Very Good

-

-

-

PP

Poor

1

Good*

10-20

POM

Poor

0.25

Good*

-

PS

Poor

0.5

-

-

PA#

Poor

1

Good*

>1

PC

Moderate

2

Good

>5

CAB

-

-

Good

>1

ABS

Moderate

0.5-2

Good*

>1

SAN

Moderate

1-2

Moderate
/Good

-

PVC

Poor

1

Good

>1

PMMA

Very Good

5-25

-

-

* = stabilised with carbon black, # = weathering restricted to surface layer, + = resistance to weathering.

Antioxidants

Antioxidants, typically hindered phenolic compounds, are used to retard thermal and oxidative degradation of polyolefins during polymerisation, processing and end use. Some suppliers offer combined UV stabiliser and antioxidant packages.

Other Additives

Other additives include impact modifiers, antistatic agents, blowing agents, pigments and dyes.

Fillers

While fillers were originally added to polymers primarily to reduce the cost of the finished product, they are now also used to enhance mechanical properties, and there is often little distinction between fillers and reinforcement. Mineral fillers such as calcium carbonate, mica and silica have been used for many years as low cost fillers that improve mechanical properties. Other mineral fillers include talc, clay and wollastonite.

Reinforcements

Reinforcements are typically high strength, high modulus, inert, fibrous materials that form a good bond with the polymer, either neat or with the aid of wetting agents. Glass fibres are the most common reinforcing material. For more demanding automotive and aerospace applications, carbon fibre reinforcement may be more appropriate. They are used primarily to improve strength, stiffness and dimensional and thermal stability.

Blends and Alloys

Considering that ABS, polyamides and polycarbonates account for about 75% of the applications of engineering thermoplastics, in some respects there is little scope for new polymers. Polysulphone, polyethersulphone, polyetherimide, polyamideimide and polyetheretherketone only account for a few percent of the total market.

One way of developing new materials without all the risks of formulating an entirely new polymer is to blend two or more polymers together. Hopefully, the blend will display the best qualities of each polymer and none of their worst. In practice, blends do not often work very well as polymers are not generally miscible. However, a number of miscible polymer blends are available commercially, including, PPO/PS, PVC/PA66 and PS/PC.

PPO/PS Blends

In the case of PPO/PS blends, the PPO imparts high-temperature resistance and some toughness; PS contributes to a lower cost. This blend is used in the automotive industry and extensively for business machine housings. Reinforced grades have replaced metals in many areas because of low cost and ease of processibility.

ABS/PC Blends

ABS is only partially miscible in PC, yet ABS/PC blends have found commercial success as there is good interfacial adhesion between the different phases. ABS is in itself a two phase mixture, basically consisting of rubber modified SAN. ABS/PC blends are used for computer, household appliance and power tool housings. Because of the plateable characteristics of ABS, ABS/PC blends are also used for car trim.

Source : Materials Information Service, edited by Justin Furness

For more information on this Materials Information Source please visit The Institute of Materials

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