Impact behaviour

Ramsteiner et al. [32] investigated the rubber toughening of PS AN. In ASA-containing particles with a diameter of ca 0.5 xm and at a graft rubber concentration of 50 wt %, the distance between the rubber particles is small enough for 

To compensate for the low toughness in ASA when using small rubber particles, large particles prepared using the microsuspension polymerization process can be added to the products (700 nm to 100 xm) [22]. Further work showed that the use of particles with a diameter of 0.15-0.8 p,m brings better toughness than a particle size of less than 0.1 xm [33]. 

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During service the impact behaviour of a plastic article will be influenced by the combined effects of the applied stress system and the geometry of the article. Although the applied stress system may appear simple (for example, uniaxial) it may become triaxial in local areas due to a geometrical discontinuity. Fig. 2.78... 

In the laboratory the impact behaviour of a material could be examined by testing plain samples, but since brittle failures are of particular interest it is more useful to ensure that the stress system is triaxial. This may be achieved most conveniently by means of a notch in the sample. The choice of notch depth and tip radius will affect the impact strengths observed. A sharp notch is usually taken as 0.25 mm radius and a blunt notch as 2 mm radius. 

In most cases thermoplastic components are designed for use at room temperature. It might appear, therefore, that data on the impact properties at this temperature (approximately 20°C) would provide sufficient information for design. However, this approach would be rather naive since even indoors, temperatures may vary by an amount which can have a significant effect on impact behaviour. For components used outdoors of course, the situation can be much worse with conditions varying from sub-zero to tropical. In common with metals, many plastics exhibit a transition from ductile behaviour to brittle as the temperature is reduced. 

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

ISO 6603-1, Plastics - Determination of puncture impact behaviour of rigid plastics - Part 1 Non-instrumented impact testing, 2000. 

Thermal analysis techniques (DSC,TGA, DMTA...) operating on mini or micro samples can detect pinpoint heterogeneities in final parts that bulk analysis methods such as rheom-etry are unable to do. Transient variations of moulding parameters, local design mistakes, internal stresses that influence the properties of the final product, notably impact behaviour, dimensional stability, warpage. .. can be displayed. 

Alloying with elastomers improves impact behaviour and processing. 

In all cases, carbon fibres lead to the highest mechanical performances compared to glass and aramid fibres. Nevertheless, their impact behaviour and price restrict their consumption. Glass fibres yield the cheapest composites but performances are more limited. Table 6.10 compares the properties of the main fibre types and shows some examples of properties for a nylon matrix reinforced with short fibres of the three types. 

MBS impact modifiers are used in a wide range of applications, particularly for clear packaging, as the refractive indices are similar between PVC and the modifier. They are not suitable for outdoor applications. Their impact behaviour has been studied in terms of the ductile/brittle transition (450). 

The impact behaviour of pure and impact modified PVC is studied in terms of the ductile-brittle transition. These transitions show an Arrhenius dependence on temperature related to the beta motions of the PVC matrix. A model based on Eyring theory is proposed. Beyond its theoretical interest, the model predicts the impact performance of PVC at various temperature and impact modifier contents. 6 refs. 

MODELLING THE DROP IMPACT BEHAVIOUR OF FLUID-FILLED POLYETHYLENE CONTAINERS... 

Modelling the Drop Impact Behaviour of Fluid-Filled Polyethylene Containers... 

There is a transition in the impact behaviour of both HIPS and ABS at low temperatures. At 20 °C, the entire fracture surface is stress-whitened, indicating a tou fracture which cannot be analysed by LEFM methods. At lower temperatures, however, whitening is confined to a small area near the base of the notch, and experiments have shown that this type of fracture can be described by LEFM. The point is illustrated in Fig. 14, which presents data obtained by Bucknall and Reid in Charpy tests on razor-notched HIPS specimens. At -25 °C, impact strength / is proportional to BWZ, as predicted by Eq. (15). At 40 °C, on the other hand, there... 

ISO 6603-1. Determination of multiaxial impact behaviour of rigid plastics—Part 1 Falling... 

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