4.6 Polymer grades
Polymers synthesised by a variety of routes are available in many grades from the large polymer manufacturing companies. Naturally enough, the grades of bulk tonnage polymers, such as LDPE, PVC, HDPE and PP, run into the hundreds simply because of the multiplicity of different process routes and end functions. So what are the basic differences between grades of just one polymeric material? The most important distinguishing characteristics are structure and molecular mass.
Most suppliers of polypropylene offer grades ranging from isotactic polypropylene to crystalline ethylene propylene partial block copolymers with up to about 10 wt% ethylene comonomer. The copolymer grades offer greater toughness over a wider temperature range (particularly below 0 °C) at the expense of stiffness. The applications for copolymer grades are, by and large, more demanding than those for equivalent homopolymer grades. Each polymer is available in several different melt flow grades. Melt flow index (MFI or MFR) is a widely adopted practical way of measuring the ease of flow of a polymer grade, and so is of use in indicating the relative magnitude of process parameters for shaping the polymer granules or powder to create a finished article. It is inversely related to molecular mass, so that high MFI grades correspond to low molecular masses and vice versa. High molecular mass polymers often possess the best physical properties, which is why the most demanding uses of PP such as safety helmets and pipe fittings require low MFI grades.
Beyond the standard grades are filled and special grades which use the basic range of polymers as a matrix for other materials, such as talc, mica and glass fibre, to modify physical properties in other ways (Box 10). Chemical additives are also used (in smaller proportions) to improve resistance to sunlight or oxidation (AVI). Special grades have, in many cases, been developed by polymer manufacturers for very specific functions and may include, for example, added pigments and flame-retardant compounds.