Materials

POLYTHENE - PE
Polythene is a semi-crystalline thermoplastic. The polymer price is very favourable, which explains why PE is one of the four main volume plastics. PE is available in a large number of qualities and as a copolymer. It retains its properties at low temperatures, and some qualities do not become brittle until temperatures as low as –70°C. At high temperatures, however, the material rapidly becomes non-rigid, for which reason the use of PE in higher temperatures is considerably limited. Its impact strength is very good over a wide range of temperatures, especially in the case of LDPE. The surface has a greasy feel to it and it is difficult to add text, glue or paint without prior treatment. However, its welding properties are excellent, and it also offers excellent resistance to chemicals although it is sensitive to strong oxidizing acids such as sulphuric acid and nitric acid. PE does not dissolve in most common solvents, but may swell when exposed to aliphatic or aromatic hydrocarbons. Its resistance to chemicals is enhanced in proportion to its crystal density. A number of qualities have been approved for use with foodstuffs. PE’s electrical properties are very good, its low electrical loss factor and high volume resistivity making it an excellent insulating material. However, static electricity builds up easily unless additives are used. PE is also broken down by ultraviolet light and burns readily unless flame retardants are used. The material’s resistance to scratching is also limited. Water uptake is low.
LDPE
LDPE is low-density polythene, with density of 0,91-0,94 g/cm3, as the crystal count is 40-50 per cent lower than that of PE. The lower crystal count means that LDPE’s strength and rigidity are also lower. LDPE is often regarded as lying somewhere between a thermoplastic and a thermoplastic elastomer.
HDPE
HDPE has a density of 0,94-0,965 g/cm3, as the crystal count is 60 per cent higher than that of PE. The higher crystal count gives greater rigidity and hardness, better resistance to chemicals and lower gas and moisture permeability. On the other hand, however, impact resistance and transparency are poorer and sensitivity to tension cracking is higher.
MDPE
MDPE lies somewhere between LDPE and HDPE, with a density of 0,926-0,940 g/cm3. MDPE can be manufactured either directly or as a mixture of LDPE and HDPE.
EBA
EBA is a copolymer of ethylene and butyl acrylate. The butyl acrylate content may vary, and be up to 30 per cent. Butyl acrylate is polar, resulting in greater adhesion to other polymers. The copolymer’s density increases in proportion to the reduction in its crystallinity, making the material non-rigid, tough and flexible. Where its butyl acrylate content is high, EBA does not become brittle until temperatures as low as –80°C, and its break elongation at –20°C is 500 per cent. By comparison with pure PE, however, resistance to chemicals decreases in proportion to butyl acrylate content.

POLYPROPYLENE - PP
Polypropylene is a semi-crystalline thermoplastic that has many of the same properties as PE. However, PP is more rigid than PE, can be used at higher temperatures, and also has better fatigue resistance. But the lower temperature limit at which it is suitable for use is only –10°C, when it begins to become brittle. PP is also available as a copolymer, and in this form has better properties at low temperatures, being impact resistant at down to –40°C. PP has good electrical properties and is highly suitable for use in water. By comparison with other engineering plastics its dimensional stability is poor, but can be improved by an admixture of talc, usually around 20 per cent. The talc also enhances rigidity and the upper temperature limits at which PP can be used. PP has excellent resistance to chemicals, though it is somewhat sensitive to strong acids such as sulphuric acid, chromic acid and hydrochloric acid. PP is also sensitive to oxidation and high-energy radiation, as a result of which its outdoor uses are limited, though they can be extended with the help of stabilizers. PP is sensitive to shrinkage, particularly where reinforcing agents are not used. The surface has a greasy feel to it, and PP is difficult to glue to itself. Its resistance to scratching is limited, and it burns readily. PP can be used in temperatures of up to 120°C.

PP WITH CHALK FILLER
Chalk is an inexpensive filler for use with PP. Adding filler enhances the rigidity of the material, which also means that it can be used at higher temperatures without risk of deformation. Impact resistance can be enhanced by adding a small quantity of chalk. If chalk is used instead of talc, the resulting material will have higher impact resistance and improved fatigue resistance but lower rigidity.

POLYOXYMETHYLENE - POM
Polyoxymethylene (POM), also known as acetal resin, is a semi-crystalline thermoplastic with a high crystal count. POM is characterized by its toughness, hardness, rigidity and high fatigue resistance. It has very good frictional and sliding characteristics, and also excellent resistance to wear and surface hardness. Its fatigue resistance and resistance to shrinking make it very suitable for use as a spring material. POM is impact resistant at temperatures of down to –40°C, and has low water absorption and gas permeability. By comparising with other semi-crystalline thermoplastics, it is possible to manufacture POM components to narrow tolerances and with good dimensional stability. Its electrical properties are not particularly good, however, and the material should not be used in electrical applications. POM is difficult to glue to itself or to other materials, and burns at a high rate of combustion. Some qualities are resistant to ultraviolet light. At normal temperatures POM is also resistant to oil, lubricants, petroleum based fuels and many organic solvents. However, its resistance to many chlorine and chromium based organic substances is poor. It is sensitive to diluted mineral acids and strong organic acids. The highest temperatures at which POM should be used are between 85°C and 120°C.

POLYAMIDE
PA6
PA6 is a semi-crystalline thermoplastic. It is characterized by high rigidity, strength and hardness. Polyamides absorb water, and PA6 will absorb 2-3 per cent at room temperature and 50 per cent humidity. The water absorbed functions as a plasticizer, enhancing the material’s impact resistance and reducing its rigidity, resistance to shrinkage and tensile strength. Polyamides’ volume varies in relation to their water content, and narrow tolerances are therefore more difficult to meet. Resistance to wear is very good, and better than that of POM. Frictional resistance is very low, and can be improved still further by adding MoS2. PA6 has better impact resistance at low temperatures than PA6.6 does. Polyamides are resistant to aliphatic, aromatic and chlorinated carbohydrates, esters, ketones and alcohols and most base substances, but are sensitive to strong mineral acids and phenols, and some of them are also sensitive to formic acid. Most polyamides have average weather resistance properties. Exposure to ultraviolet light can make them brittle, but this can be remedied by the use of stabilizers or carbon black colouring. The highest temperatures at which polyamides should be used are between 80°C and 150°C. Polyamides do not ignite easily and burn slowly, properties that can be further enhanced by the addition of small quantities of flame retardants, to make them compliant to UL 94 V-0.

PA6.6
PA6 and PA6.6 have more or less the same properties. However, PA6.6 has greater rigidity, strength, resistance to wear and resistance to temperature than PA6 does. It is also slightly more resistant to solvents and lubricants. Polyamides have high fatigue resistance, and can be steam sterilized.

POLYPHENYLENE SULPHIDE– PPS
PPS is a metal-like high-temperature plastic with a high crystal count, and is hard and rigid. It contains fillers such as glass fibres or minerals. It has very good mechanical and electrical properties even at high temperatures. The polymer can be used at continuous high temperatures of 200°C, and at temperatures even higher than this for short intervals. It has excellent impact resistance at room temperature, approximately the same as that for PC. PPS also has excellent resistance to chemicals, and at temperatures below 180°C has no known solvent. However, it will not withstand concentrated oxidants such as sulphuric acid, certain amines, benzaldehyde, nitro methane or certain halogenated compounds. Its fire resistant properties are excellent, as it gives off very few fumes and is self-extinguishing. Standard qualities are compliant with UL 94 V-0.

POLYSTYRENE - PS
Polystyrene is an amorphous, perfectly transparent thermoplastic that is one of the four main volume plastics. It is very rigid and hard, with high surface polish, excellent optical properties and good dimensional stability. It also offers low water absorption and has excellent electrical properties. However, it has poor impact resistance and limited resistance to chemicals, especially solvents. PS will not resist strong acids, and will dissolve in aromatic ones. Internal tensions may result in cracks forming in the material, especially when it is used in unsuitable environments. However, PS can be used in hot water. It has only limited resistance to ultraviolet light, and when used outdoors will become yellow and brittle. The highest temperatures at which PS should be used are between 70°C and 80°C.

HIGH-IMPACT POLYSTYRENE - (HIPS)
HIPS is a copolymer of butadiene and styrene, Normally 4-8 per cent butadiene is used. By comparison with PS its impact resistance is five times as great. If subjected to shock stress, the formation of cracks is limited by the material’s rubber content. However, HIPS is not completely transparent but is milky white in appearance. This is caused by the rubber particles dispersed in the styrene matrix. The addition of rubber reduces its rigidity and strength, and the material softens at low temperatures. As in the case of PS, resistance to ultraviolet light is poor and, when the material is used outdoors, it turns yellow and loses its impact resistance, though this may be remedied by the use of stabilizers. HIPS’ resistance to chemicals is approximately the same as that of PS. Its fire resistant properties are poor, but may be enhanced to some extent by the addition of flame retardants.

ACRYLONITRILE BUTADIENE STYRENE - ABS
With a view to enhancing its impact resistance, polystyrene can be copolymerized with butadiene. The result is a two-phase system in which pellets of butadiene grafted with styrene are held in a styrene matrix. ABS has an acrylonitrile content of 15-35 per cent and a butadiene content of 5-30 per cent. ABS is available in a large number of different qualities whose properties vary since the components of the material can also be varied in many ways. ABS is characterized by its good impact resistance, surface hardness and surface finish. It is not normally transparent although certain qualities can be made transparent. Its acrylonitrile content enhances its strength and resistance to chemicals and reduces the risk of tension cracks forming, by comparison with PS. The material’s impact resistance is very good, even at low temperatures, down to –30°C in the case of some qualities. ABS is resistant to acids, alkaline solutions, alcohols, fats, oil, saline solutions and saturated carbohydrates. However, it is sensitive to concentrated mineral acids, aromatic and chlorinated carbohydrates, esters and ketones, and will dissolve in acetone. Water absorption is higher than in the case of PS, and as a result ABS’ electrical properties are somewhat poorer. Because of its butadiene content, ABS is sensitive to ultraviolet light, making it unsuitable for outdoor applications, where it will turn yellow, age and become brittle. This can be remedied by varnishing it or adding carbon black colouring. One ABS quality for outdoor use is ASA, in which the butadiene is replaced by an acrylic rubber. ABS can be electroplated or chromed. The highest temperatures at which it should be used are somewhat higher than in the case of PS.

POLYCARBONATE– PC
Polycarbonate is a clear, amorphous thermoplastic characterized by very good impact resistance and good resistance to high temperatures. The material retains its impact resistance even at low temperatures. It is still impact resistant at –40°C, and there is a certain amount of resistance at temperatures of right down to –150°C. PC can also be used at high temperatures of up to 135°C and, with glass fibres added, even higher than this. Its dimensional stability is good, even at high temperatures. PC has excellent electrical properties, and is often used in electrical applications. It is self-extinguishing and compliant with UL 94 V-2. When flame retardants are added, these properties are enhanced even further and the resultant material is compliant with UL 94 V-0. PC’s thermal expansion is low and, with glass fibres added, is the same as for light alloys. However, PC is sensitive to water at high temperatures, and long-term use in water at 60°C or higher may result in decomposition and consequential loss of properties. PC will, however, withstand machine washing. When used outdoors the material absorbs ultraviolet light, turns yellow and becomes brittle, but can be used in outdoor applications if it is first stabilized. PC is resistant to weak acids, aliphatic hydrocarbons, paraffin, alcohols, and animal or vegetable oils and fats. However, it is sensitive to oxidants, base substances, ammonia, methanol, and aromatic or chlorinated carbohydrates. Exposure to certain solvents may cause tension cracks to form. PC is also used as an alloy together with ABS, ASA, PBT and PET.

POLYPHENYLENE ETHER – PPE
PPE is an amorphous thermoplastic. In its pure form it is difficult to work with because of the high risk of internal tensions, so it is normally used modified with HIPS. PPE is characterized by its good dimensional stability and resistance to wear, even at high temperatures and humidity. It is suitable for use at continuous temperatures ranging from –40°C to 100°C, and may be used at temperatures higher than this for short intervals. When glass fibres are added the risk of shrinkage is reduced, especially at high temperatures. PPE can be used in water with absolutely no problems, and will withstand repeated steam sterilization. Its excellent electrical properties are independent of temperature and humidity, and its frequency dependence is also low. By comparison with PC, PPE/HIPS offers better chemical and better hydrolytic characteristics. PPE/HIPS is resistant to diluted acids, alcohols, diluted alkalis and dishwashing agents. However, it is less resistant to a number of solvents, ketones and low-molecule esters. There is also a risk of tension cracks forming when organic solvents are used. In outdoor applications the material turns yellow, but its mechanical properties are otherwise very little affected. Its fire resistant properties are good, but not as good as those of PC. However, there are some qualities that are compliant with UL 94 V-0.

POLYOLEFIN THERMOPLASTIC ELASTOMER - TPG
A semi-soluble mixture of PE or PP and an elastomer, usually EPDM rubber. TPG is highly resistant to polar solvents and chemicals, but its resistance to oils and non-polar solvents is not particularly good. It can be varnished if suitably pre-treated. TPG is resistant to ozone, thermo-oxidative ageing and ultraviolet light, making it suitable for outdoor use. It can be used at temperatures of between –40°C and 100°C, but remains flexible at temperatures of down to –60°C. TPG has good electrical properties, but resistance to wear and elasticity are not so good.