Chapter 1

Choosing plastics to match the application.

In practically all application areas, plastics have come of age in the last 50 to 100 years like no other material. World production in 2004 was close on 225 million tonnes, a tenfold increase in less than 40 years, and the rate of increase is still about 5% per year. Two reasons explain this rapid growth:

Choosing Plastics to Match the Application

The term ‘plastics,’ just like ‘metals,’ groups a broad palette of materials with very different properties. All plastics are composed of long molecular chains. These macromolecules are made up of smaller building blocks called monomers, which are chemically linked to each other. Unlike metals, however, all plastics fall within a narrow range of specific weights—between 0,9 and 1,5 g/cm3. This makes them indispensable for weight-saving designs.

Classification of Plastics

Thermoplastics are those that can be melted repeatedly. They are grouped into semicrystalline and amorphous, according to their molecular arrangement when they solidify. Semi-crystalline plastics, such as polyethylene (PE), polyoxymethylene (POM) or polyamide (PA), have crystalline regions with molecules that run parallel. These regions give the material its strength and stiffness. Between the crystalline regions are amorphous regions which give it toughness and, in most cases, high tensile strength.

Some plastics’ macromolecules have bulky side chains which prevent them from crystallising. These are the completely amorphous ones. In an unstretched state they exhibit lower toughness and tensile strength. The difference between semi-crystalline and amorphous plastics also determines their thermal behaviour. The transition of semi-crystallines from the molten to the solid state (and back) takes place at the melting point (TM), which lies within a narrow temperature band of only a few degrees. When amorphous plastics reach their glass transition temperature (TG), on the other hand, they gradually become stiffer over a broader temperature range (20 to 40 °C); when they are solid, they are in fact “hardened liquids,” like glass. Semi-crystallines also have a glass transition temperature: in the solid state, they become brittle below TG. The normal temperature range for applications of semi-crystalline plastics is thus between TG and TM (hence their combination of toughness and strength), whereas amorphous plastics’ applications lie below their TG (hence their brittleness).

Apart from thermoplastics, there are also elastomers with rubber-like behaviour, and non-melting thermosets. When materials in these two groups are processed into shapes, a chemical process called cross-linking also takes place as well as a physical one, as in the case of the thermoplastics.

The Pyramid of Plastics: from Commodity to High-Performance

Like metals, plastics, too, can be arranged into a pyramid of materials. The lowest segment is composed of commodity plastics, which are produced in large quantities. Engineering plastics are in the middle segment, and high-performance plastics are at the top of the pile. Strength, stiffness and, in most cases, chemical resistance increase as one goes up the pyramid. Also, the higher they are in the pyramid, the higher their thermal resistance, their heat deflection temperature and their price.

Typical values for the strength of unreinforced plastics are between about 15 (e.g. LDPE) and more than 100 N/mm2 (e.g. polyetherimide or PEI). The modulus of elasticity, which describes their stiffness, lies in the range from 300 (e.g. LDPE) to 3600 N/mm2 (e.g. polyether-ether-ketone or PEEK). Addition of reinforcing materials, such as glass fibre, can raise stiffness to 20 000 N/mm2. Amorphous polystyrene (PS) has an elongation at break of about 3%, whereas that of semi-crystalline polyethylene (PE) can be as high as 1000%. Typical values for impact resistance are in the 1 to 130 kJ/m2 range.

These mechanical properties hold good at room temperature. As for all materials, they are temperature-dependent: more so with plastics than with metals, because plastics’ melting point is lower, ranging from about 120 °C for LDPE up to about 380 °C for polyimide (PI). The respective values for processing temperatures are between 190 and 400 °C. For applications, the Vicat softening point, from 30 to 300 °C is also important; so is the thermal deformation temperature, which is between 30 and 300 °C at a loading of 0,45 N/mm2.

Nearly all plastics are electrical insulators. Values for specific volume resistivity are nearly always over 1013 W.m, for surface resistivity over 1011 W. There are differences, however, in the dielectric constant and the dielectric loss factor. In this respect polar plastics like PA show higher values than non-polar ones like PE or PS, which therefore make better insulators for wire and cable.

The rule that ‘like attacks like’ applies to chemical resistance. That is why there are differences between polar and non-polar plastics. For example, PA, which is polar, withstands non-polar fuels (petrol and diesel) better than non-polar PE, which swells in petrol. Most thermoplastics show good resistance to many acids and alkalis as well. The guideline for optical properties is that amorphous plastics have high transparency and a glossy, relatively hard surface, whereas semi-crystalline plastics are white and opaque or milky, and have a softer, matt surface, on account of their many crystallite interfaces. There are ways to give semicrystallines a degree of transparency, for example in order to allow polyethylene terephthalate (PET) or polypropylene to be turned into transparent bottles.

Additives Expand Range of Properties

All these properties can be changed, within generous limits, by making blends (alloys of plastics) and with a multitude of additives and modifiers, so as to better meet application requirements. The addition of glass fibres, for example, increases strength, stiffness and thermal deformation temperature. The addition of an elastomer can improve toughness and impact resistance, and a flame retardant can make a flammable plastic self-extinguishing.

All these possibilities help to explain the triumphal advance of plastics mentioned earlier. Since the 1990s, the worldwide production of plastics in terms of volume is greater than that of steel.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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