Impact-resistant materials Subject

From an engineering point of view, the need is to control the miscibility and the phase domain size. Very small sizes (200 A) serve to make good damping compositions, while domains of the order of 1000 A make better impact-resistant materials. The extent of mixing, particularly at the interface between the domains, has now been found to be critical in determining many properties. All these are subjects of current research (26,30,41-46) see Chapter 12. 

Likewise, dead sharp corners or notches subjected to tensile loads during impact may decrease the impact resistance of a product by acting as stress concentrators, whereas generous radii in these areas may distribute the tensile load and enhance the impact resistance. This point is particularly important for products comprised of materials whose intrinsic impact resistance is a strong function of a notch radius. Such notch sensitive materials are characterized by an impact resistance that decreases drastically with notch... 

Colorless clear polycarbonate has a distinct bluish tint which is added to mask its natural straw color. Several medium-density tints are available as well as a number of translucent colors. Over the years the weatherability of polycarbonate has been the subject of much discussion and conflicting claims. Efforts are continuously being made to improve or extend its retention of appearance after prolonged exposure outdoors. The use of coatings holds some promise in this regard. Nonetheless, polycarbonate remains the material of choice when impact resistance is required, for example, to withstand attack by vandals or for safety guards and other industrial plant applications that must meet OSHA requirements [86,95]. 

Impact Resistance, Spencer Impact The method of measuring impact resistance of a film while the film is under the strain rate conditions that closely approximate the strain rates that the material is subject to in end-use applications. 

Frequently, a transition in major deformation mechanism from shear yielding to crazing or vice versa is accompanied by a sudden change in crack resistance. This is often referred to as a ductile brittle transition (DBT). Hence, low temperature impact resistance of polypropylene-based materials is a key factor in material selection for molded parts subjected the ambient temperatures as low as 40 C. In this regard, plastomers described in Chapter 7 provide reduction of DBT well below end use requirements. 

Standardized notched impact tests such as the Izod and Charpy tests (ASTM, ISO, DIN) are the most commonly used to characterize the impact strength of plastic materials. It is very difficult to use measured data from tests using idealized laboratory specimens to predict impact behavior of end-use polymeric material. The apparent lack of good correlation between measured impact fracture energy and end-use impact resistance is due to the extreme complexity of microscopic fracture processes. In particular, the influence of specimen geometry is sometimes poorly matched with the type of failure mechanism of defects present in the actual molded part subjected to end-use impact forces. 

In practice, only in very few cases do materials work in tensile mode more often they are subjected to flexure or impact. On the other hand, fiber-reinforced composites are usually applied as laminates with different orientation and alignment of the fibrous reinforcement. That is why the CPC laminates were used to study their flexural stiffness and impact resistance. The flexural tests were performed by the threopoint support test method used by Nunes et al, as shown in Figure 14.6 [74]. The support was mounted in the same Instron machine used for the tensile tests, this time operating in compression mode. Rectangular samples (155 x 100 mm) were cut out from the CPC MFC plates and placed upon the sup-... 

Application of the coupling agent causes considerable reduction in the Izod impact strength of the composites. Improved filler-matrix adhesion does not favour the impact resistance of the RHA-polypropylene composites. The tradeoff in impact strength (lower value) for modulus (higher value) seems to be inevitable. The probable reason is the inability of the resin material to slip over the surface of filler particles when the composite is subjected to the impact force. 

The toughness of a material is a design driver in many structures subjected to impact loading. For those materials that must function under a wide range of temperatures, the temperature dependence of the various material properties is often of primary concern. Other structures are subjected to wear or corrosion, so the resistance of a material to those attacks is an important part of the material choice. Thermal and electrical conductivity can be design drivers for some applications, so materials with proper ranges of behavior for those factors must be chosen. Similarly, the acoustical and thermal insulation characteristics of materials often dictate the choice of materials. 

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