4.6.1 Prices of polymers
Prices of bulk and speciality polymers (Table 9) broadly reflect the degree of chemical processing and treatment needed to make them. Thus the polyolefins, which are directly polymerized from cracker streams, are generally the cheapest followed by vinyl derivatives of ethylene like PS and PVC. Derived polymers which require more complex treatment, such as ABS, PET and polyester thermosets are generally more expensive by factors of between two and four. Speciality engineering polymers tend to range in price (1995) from about £2000 up to £7500 per tonne or more for a material like polysulphone (PSu). These prices reflect not only more expensive feedstocks and polymerization methods but also the manufacturers’ desire to recoup development costs through a premium for their special properties.
Box 10 Additives for polymers
Polymer products without additives in the matrix material are rare, medical products which are in intimate contact with the human body, being the exception because of the problem of leaching by bodily fluids. But in the vast majority of products, additives are used to modify properties in a controllable way. So what are the principal types of additive? It is a surprisingly long list and includes
colourants (pigments and dyes)
lubricants and process aids
flame retardants and smoke suppressants
Fillers are added where transparency or translucency are not key design factors, and where stiffness can be enhanced. Many fillers are inorganic (such as glass fibre, talc and mica) and hence of higher inherent stiffness, but strength is usually sacrificed since particles are stress concentrators and may initiate cracks. Bulking agents (e.g. chalk, sawdust) have a much smaller effect on stiffness, and the prime motive is to reduce cost. One important factor in achieving best filler action is to ensure that there is good wetting between the polymer matrix and the filler. With silicate fillers, coupling agents are used to give a good bond between the two species. Silanes (organic monosilicates) are frequently used to bond glass fibre to polymer by reaction at the surface of the filler. The other ends then either react with the polymer or blend homogeneously to form the bonded interface.
Crosslinking agents are a vital part of thermoset formation, although modification of the backbone chain may be needed to achieve the desired effect (as in butyl rubber, or EPDM). Pigments are simply added to give the product colour, giving plastics decisive advantage over other materials since they offer extra freedom for designers. Dyeability of fibres is of fundamental commercial importance, so dye retention is an important property, especially for textile fabrics which are washed repeatedly. Many of the first block copolymers were in fact developed for fibre dyeability, since if the inserted blocks react with the dye, then it is held fast by strong chemical bonds (as in PET/polyether block polymers; ICI, 1950). The same philosophy has been used with impact modifiers, where rubber chains are permanently anchored to backbone chains, as in HIPS and ABS. Plasticizers are used extensively in PVC, producing a flexible rather than rigid product. Lubricants and stabilizers are also closely connected with PVC, improving processing and stability against degradation (like antioxidants and antiozonants which have more specific functions).
|Price (tonne lots) /£ tonne−1, 1995||Price (tonne lots) /£m−3|
|PET bottle grades||900||1220|
|liquid crystal polymers, LCPs||17 500||—|
|standard Malaysian rubber (SMR)||790||730|
However, it should be noted that the prices shown in the table will vary substantially depending on current supply and demand. Plant shutdowns, for example, can cause temporary price rises because supplies are often limited to a few major petrochemical plants worldwide. On the other hand, prices may slump if plant shutdowns occur (by fire damage, for example). The specific grade chosen will also affect price, those grades having many additives attracting the necessarily higher price than the raw material. The quantity purchased will influence the unit price paid by the buyer. Clearly large quantities will attract substantial discounts, and most polymer buyers will liaise with traders worldwide to achieve the best prices.
Although increases in the traded price of crude oil can push up prices of materials derived from it, the effects have been felt on all materials because of the consequent high energy costs in reducing ore to metal and subsequent processing. In fact, the real price of crude oil is now (1997) low in real terms compared with prices in the 1970s and 1980s. Economic recession has a much more important effect on trade prices. Thus the recession of the early 1990s caused prices to drop substantially, and polymer prices have only recently recovered to pre-recession levels.
Another factor which has helped to keep the Retail Price Index (RPI) indexed polymer prices relatively low has been the over-capacity for petrochemical production. During the 1960s, ever-larger petrochemical plants were built for the economies of scale in production. But the demand was effectively halted by the OPEC price rises, with the result that major chemical companies had been losing heavily on bulk polymers until only recently (1995). With many speciality polymers, the reverse has happened – demand has risen continuously over the years, and continues to rise at a fast rate, so contributing to the rise in polymer consumption which was shown in Figure 1.
When compared on a weight basis, light metals like zinc and aluminium are similar in price to engineering polymers like polyester or ABS, although the cost on a volume basis is considerably lower for polymers than for metals. Thus the cost per unit volume of polypropylene works out at about £440 m−3 compared with a price of over £3000 m−3 for mild steel. Light metals like aluminium are considerably more expensive when costed on this basis (£2650 m−3). Speciality engineering polymers like polycarbonate at £3270 m−3 are slightly more expensive than light metals using this criterion.
In addition, most materials are used in the form of alloys, composites or mixtures which will push the alloyed price above those shown in Table 9. For example, aluminium is frequently alloyed with copper to improve its stiffness and strength, and additives such as expensive antioxidants or pigments are often mixed with polyolefins. On the other hand, fillers like chalk dust, mica and carbon black can reduce the cost of the blended product while often enhancing the valuable properties of the end product. So the prices of various grades of polypropylene will vary according to the fillers and other additives incorporated. However, it is also important to be aware of the fluctuations which occur in raw materials prices – this is particularly important for general-purpose materials subject to the market forces of supply and demand.