Chapter 6
How can you improve polymers’ qualities?
A look at the use of fillers and additives to modify polymers’ properties
Reinforcing additives improve the mechanical properties of plastics and reduce the influence of temperature on these properties. This brings new applications within range for plastics, for example, replacing light metal castings or housings of sheet steel.
Fillers and reinforcing additives can modify the properties of plastics within broad limits. There is no sharp dividing line between the two terms but broadly speaking, fillers denote inorganic materials which lower the price of the plastic and may also improve its modulus of elasticity, thermal resistance and surface quality.
Fillers may also reduce shrinkage, i.e. improve parts’ dimensional stability, and shorten cycle times by raising thermal conductivity, i.e. reduce processing costs. Most fillers have a spherical, square or platelet structure, their particle size is mostly in the 5-10µm range.
Typical amounts added are between 20-40%; if less is added, the cost-reduction effect is too small; if more, the material becomes too brittle.
Of the many fillers used in plastics, calcium carbonates and, especially chalk, are of major practical significance. High-quality products possess high whiteness (good colourability) and low abrasive effect. Moulded parts show high stiffness, surface quality and colour fastness.
Talcum, with a platelet structure, and similar silicates are also widely used. On account of its low hardness, talcum has no abrasive effect in processing. It improves stiffness, hardness and flexural and compressive modulus, but lowers impact resistance.
Better chances in the marketplace
Improve properties, lower costs
Graph 2: Reinforcement with short glass fibres doubles the tensile strength of engineering plastics (tensile stress at elongation for unreinforced plastics, tensile stress at break for reinforced plastics) and the modulus of elasticity in tension.
Graph 1: Stress-strain diagram shows tensile stress at yield is a more appropriate measure to measure the tensile strength of unreinforced plastics, whereas tensile stress at break should be used for reinforced plastics.
Reinforcing additives are fibrous materials. The main purpose in using them is to raise mechanical strength and stiffness, even at elevated operating temperatures.
By far the most important of these reinforcing additives are short glass fibres with a fibre diameter of between 5-25µm and a length of between 0.1-0.3mm. The amounts added vary between 15-60% by weight.
Glass fibres generally improve strength and modulus under tensile and flexural stress, lower the tendency to creep, improve heat resistance, and often also raise impact and notched impact resistance.
In practice, between 25-40% by weight of short glass fibre roughly doubles tensile strength but considerably lowers elongation. As the stress-strain diagram opposite shows, reinforced plastics have a brittle failure mode, without a pronounced strain limit.
For this reason, the tensile stress at yield of 60-90MPa of unreinforced engineering plastics may be compared with the tensile stress at break of 120-200MPa of reinforced plastics.
The modulus of elasticity, the most widely used measure of stiffness, can be raised from 1,500-3,000MPa at 23º C (this range is typical for engineering plastics) to 12,000MPa at 23º C, or even to 20,000MPa with a higher glass fibre content.
In addition, there is another advantage: higher temperatures affect reinforced plastics’ mechanical properties much less than those of unreinforced plastics.
Aramid fibres (for example, Kevlar®) improve plastics’ properties as glass fibres do. In processing they are less abrasive and in applications involving sliding friction they cause substantially less part wear than glass fibres. In line with environmental considerations, the use of natural fibres is increasing rapidly.
With long fibres (from around 10mm fibre length in the moulded part to unidirectional reinforcement with fibre content of more than 60% by weight in some cases) mainly creep resistance, fatigue resistance and energy absorption are greatly increased, while strength and stiffness can also be raised further (the modulus of elasticity can also reach 20,000MPa at 23º C). This makes it possible to replace metal even in applications where it did not seem possible until now. The other side of the coin, however, is higher costs in designing and producing the part.
Glass fibres are the most significant type used for long fibre reinforcement. An example is afforded by the “superstructural resins” – a series of especially highly stressable thermoplastics by DuPont.
Carbon fibres are used mainly where part lightness is required, for example, in aviation engineering and, in growing measure, in sports equipment and automotive applications.
Plastics reinforcement
Figure 1: Reinforced plastics were the key to the attractive, cost-effective design of the housing and interior parts for a professional steamer.
Figure 2: Reinforcement with 35% glass fibre (by weight) gives nylon 66 the mechanical strength and dimensional stability that a pedal module needs to carry a whole pedal-mounting system for a car.
Today, there is still no universally accepted theory about plastics reinforcement. Materials suppliers’ know-how and their practical experience gathered in the use of plastics reinforced with short and long fibres is therefore all the more important.
A compact professional steamer (see Figure 1) combines technical innovation and elegant design. Reinforced plastics play a key role in its manufacture. The large surface covers and the exterior door frame are made of Crastin® SK9215 polybutylene terephthalate reinforced with 15% glass fibre.
These give the relatively thin walls high strength and stiffness; in addition, they ensure low warpage and good dimensional stability at high temperatures – essential characteristics for large surface parts. This material also withstands edible fats and oils, and other products contained in foodstuffs.
The inner door frame is subject to even higher temperatures and is made of colour-stabilised Rynite® 520CS polyethylene terephthalate, with 20% glass fibre by weight.
The designers chose Zytel® HTN 51G35 high-performance polyamide reinforced with 35% glass fibre for the steamer’s interior parts, which have to withstand hot steam. In spite of the reinforcement, this material is still flexible enough to allow snap-fit assembly of the interior parts in the housing.
An injection moulded pedal module (see Figure 2) made of Zytel® 70G35HSL carries the whole pedal-mounting system in a midrange car. This nylon 66 reinforced with 35% glass fibre gives the part, which is subject to high mechanical stresses, the necessary combination of strength and stiffness together with toughness.
Other advantages typical of good plastics design are a high degree of part integration and simplified assembly. Compared to a pedal module in cast aluminium, the part is lighter and has better acoustic damping properties, which lowers noise propagation in the passenger compartment.
The module is moulded with short cycle times and with low warpage; neither painting nor finishing operations are needed before it is assembled into the vehicle.
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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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