Impact strength of plastics

Impact As reviewed although impact strength of plastics is widely reported, the properties have no particular design values and can be used only to compare relative... 

Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

The subject of impact strength of plastics has received considerable attention in official standards, material data sheets and literature at large. 

The impact strength of plastics falls into tw o basic categories the pendulum tests and the falling weight tests. Each of these is then further subdivided into more specific classes. 

Temperature has a pronounced effect on the impact strength of plastics. In common with metals, many plastic materials exhibit a transition from ductile behavior to brittle as the temperature is reduced. The variation of impact strength with temperature for several common thermoplastics is shown in Figure 3.38. 

ASTM D 4508-98. Standard test method for chip impact strength of plastics. 

Figure 10.18 shows that impact strength of plasticized PVC increases with increase in the amoimt of the plasticizer. The impact improvement is not gradnal bnt requires a certain critical concentration of the plasticizer. This critical concentration of the plasticizer depends on properties of pair polymer/plasticizer For example in plasticization of polyamide-11 ductile-brittle transition occurred at 4-6% of one plasticizer and in vicinity of 15% of another plasticizer. 

Izod impact test One of the most common ASTM tests for testing the impact strength of plastic materials. 

ASTM 4508-06, Standard Test Method for Chip Impact Strength of Plastics,... 

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. 

Impact strength of plastics is commonly measured by tests in which a pendulum with a massive striking edge is allowed to hit the specimen. From the travel of the pendulum after breaking the specimen, impact strength can be calculated as the energy required to cause the break. The plastic specimen is often notched in an effort to improve the reproducibility of the mode of failure. 

Although impact strength of plastics is widely reported, the properties have no particular design values and can be used only to compare relative response of materials (toughness, etc.). Even this comparison is not completely valid because it does not solely reflect the capacity of the material to withstand shock loading, but can pick up discriminatory response to notch sensitivity. 

Equation (122) was reported as an empirical equation in the literature [61,62]. The impact strength of plastics is enhanced by dispersion of rubbery particles as explained in the preceding section. 

Uniform distribution of stress is more likely rather than that of strain unlike the conventional concept of uniform strain for rubber elasticity. Especially for inhomogeneous polymer blend, stress tends to be uniform among domains. For rupture, the degree of uniformity in stress is a determining factor. Active filler enhances strength of rubber, and the rubbery dispersion improves impact strength of plastics when the particle size is very small. 

BS2782 (1979). Method 352B, Falling weight impact strength of plastic film. 

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