Low density polyethylene (LDPE). This is one of the most widely used plastics. It is characterised by a density in the range 918–935 kg/m3 and is very tough and flexible. Its major application is in packaging film although its outstanding dielectric properties means it is also widely used as an electrical insulator. Other applications include domestic ware, tubing, squeeze bottles and cold water tanks.
Linear low density polyethylene (LLDPE). LLDPE has a regular linear structure with short chain branches, as opposed to the long chain branches of LDPE (shown in Fig. 1.1(b)), and it is produced by a low pressure process. Depending on the cooling rate from the melt, the material forms a structure in which the molecules are linked together. Hence for any given density, LLDPE is stiffer than LDPE and exhibits a higher yield strength and greater ductility. For processing LLDPE has higher shear viscosity, but lower shear sensitivity than LDPE, and importantly its short side branches make it softer in extension and it can be drawn to higher strains. It is increasingly replacing LDPE in some of its traditional markets such as packaging films, where it can be drawn down to much lower thicknesses for plastic bags, shrink wrap etc.
High density polyethylene (HDPE). This material has a density in the range 935–965 kg/m3 and is more crystalline than LDPE. It has a linear chain structure, without side-branching (as shown in Fig. 1.1(a)), which makes it stronger, stiffer and harder than LDPE. It is also slightly more expensive but its greater strength and durability give it a much wider range of applications. These range from piping and furniture to kitchenware, toys and food containers.
Developments in polyethylenes and particularly the introduction of new material grades with enhanced properties has been made possible over the years through the introduction of new manufacturing technologies. A good example of this was the increasing use of metallocene catalysts in the polymerisation of polyethylenes from the 1980s onwards. The big advantage of metallocenes is that they are single site catalysts so that the polymer molecules which are produced tend to be all the same – a fact which offers an array of superior properties. Traditional catalysts for polyethylene (e.g. Ziegler Natta catalysts) are multi-sited so that they produce polymers with short, medium and long molecules. In metallocene grades of polyethylene, the absence of low molecular weight species results in low extractables, a narrow melting range and free-flowing material even at low densities. The absence of high molecular weight species contributes excellent melting point control, clarity and improved flexibility/toughness at low temperatures. Metallocene grades are generally indicated by the prefix ‘m’ as in ‘mLDPE’.
Cross-linked polyethylene (XLPE). Some thermoplastic materials such as polyethylene can have their structure altered so that the molecular chains become cross-linked and the material then behaves like a thermoset. In the case of polyethylene, a range of cross-linking methods are available. These include the use of radiation, peroxides and silanes. In some cases the cross-linking can occur during moulding whereas in other cases the end-product shape is created before the cross-linking is initiated. The action of cross-linking has a number of beneficial effects including improved stress crack resistance, improved creep resistance, better chemical resistance, improved toughness and better general thermo-mechanical stability.
Polypropylene (PP). Polypropylene is an extremely versatile plastic and is available in many grades and also as a copolymer (ethylene/propylene). It has the lowest density of all thermoplastics (in the order of 900 kg/m3) and this combined with strength, stiffness and excellent fatigue and chemical resistance make it attractive in many situations. These include crates, small machine parts, car components (fans, fascia panels etc.), chair shells, TV and computer cabinets, tool handles, etc. Its excellent fatigue resistance is utilised in the moulding of integral hinges (e.g. accelerator pedals and forceps/tweezers). Polypropylene is also available in fibre form (for ropes, carpet backing) and as a film (for packaging).
Polyamides (nylon). There are several different types of nylon (e.g., nylon 6, nylon 66, nylon 11) but as a family their characteristics of strength, stiffness and toughness have earned them a reputation as engineering plastics. Table 1.10 compares the relative merits of light metal casting alloys and nylon. Typical applications for nylon include small gears, bearings, bushes, sprockets, housings for power tools, terminal blocks and slide rollers. An important design consideration is that nylon absorbs moisture which can affect its properties and dimensional stability. Glass reinforcement reduces this problem and produces an extremely strong, impact resistant material. Another major application of nylon is in fibres which are notoriously strong. The density of nylon is about 1100 kg/m3.
Table 1.10. Comparison between die casting alloys and nylons.
Polyoxymethylene (POM or acetals). The superior properties of acetal in terms of its strength, stiffness and toughness have also earned it a place as an engineering plastic. At 1420 kg/m3 it is denser than nylon but in many respects their properties are similar and they can be used for the same types of light engineering application. A factor which may favour acetal in some cases is its relatively low water absorption. The material is available as both a homopolymer and a copolymer. The former is slightly stronger and stiffer whereas the copolymer has improved high temperature performance.
Polytetrafluoroethylene (PTFE). The major advantages of this material are its excellent chemical resistance and its extremely low coefficient of friction. Not surprisingly its major area of application is in bearings and it is particularly suited to situations where the environment is aggressive. It is also widely used in areas such as insulating tapes, gaskets, pumps, diaphragms and of course non-stick coatings on cooking utensils.
Polyethylene terephthalate (PET). PET is naturally highly crystalline and in this form (known as cPET) it exhibits toughness, strength, abrasion resistance, low friction, chemical resistance and low moisture absorption. Traditionally its main application is as a textile fibre (e.g. Terylene). However, by rapid cooling the material can be induced to remain in a largely amorphous state (known as aPET), where it retains its transparency and its processability is improved. Over the past 20 years this has been exploited to great advantage in the manufacture of plastic bottles, where aPET has largely replaced glass in most applications. PET is also relatively easy to recycle and as a result of this it is now rapidly growing as a general packaging material. PET is hygroscopic and must be dried prior to melt processing.
Polybutylene terephthalate (PBT). PBT is closely related to PET, but has slightly lower strength and rigidity, slightly higher impact resistance and better mouldability. Applications include slide bearings, valve mouldings, gears, cams, plug connectors, shower heads and LCD/LEDs.
Polyetheretherketone (PEEK). PEEK is a high performance plastics which offer the possibility of elevated service temperatures. It is crystalline in nature which accounts in part for its high resistance to attack from acids, alkalis and organic solvents. It melts at a relatively high temperature compared to most thermoplastics (343°C). It is easily processed and may be used continuously at 200°C where it offers good abrasion resistance, low flammability, toughness, strength and good fatigue resistance. Its density is 1300 kg/m3. Applications include items used in demanding applications such as bearing, valves, pistons, impellors, wire, coatings, and medical implants, as well as small load bearing parts in the aerospace, automotive and chemical processing industries.
Polylactic acid (PLA). PLA is a rigid semi-crystalline thermoplastic with a density in the range 1200–1400 kg/m3. It is the most important of the new bio-degradable plastics and it is now being produced in commercial quantities that are large enough to replace commodity plastics in key markets. PLA is not a natural material, but it is made from natural renewable sources, such as corn starch, and it is bio-degradable. The material is therefore promoted as being environmentally friendly, carbon neutral and from sustainable resources. PLA is readily mouldable and has properties similar to PET apart from its lower melting point. The principal applications for PLA are currently in disposable food packaging, but it has also been used as fibres, medical implants and the feedstock for 3D printers.
Article: Roy J. Crawford, Peter J. Martin, in Plastics Engineering (Fourth Edition), 2020
Sourced from: https://www.sciencedirect.com/science/article/pii/B9780081007099000017
