Polycarbons, impact resistance testing

Impact strength (impact resistance), 10 177 of polycarbonates, 19 810-811 of styrene-based plastics, 23 362-363 Impact testing, 19 580 Impact tests, for polymer blends, 20 352... 

Driers drop-weight test (falling-weight test) n. Any test of impact resistance in which known weights are dropped once or repeatedly on the test specimen. Examples are ASTM D 4272 (plastic films), D 3029, D 4226, and D 4495 (rigid PVC sheet and parts), and F 736, Section 15 (polycarbonate sheet). 

The lens of this headlamp was molded from polycarbonate (stiff, strong, impact resistant, and has excellent light transmission in its clear form). The lens itself was a Fresnel type lens and the part also had some molded-in mounting pins. We had a prototype mold fabricated to prove out the optics and when the first parts were tested we noticed some problems. The optics of the lens was fine, but the mounting pins were not quite strong enough. So we started exploring ways to improve the structure. 

To enhance the resistance to heat softening his-phenol A is substituted by a stiffer molecule. Conventional bis-phenol A polycarbonates have lower heat distortion temperatures (deflection temperatures under load) than some of the somewhat newer aromatic thermoplastics described in the next chapter, such as the polysulphones. In 1979 a polycarbonate in which the bis-phenol A was replaced by tetramethylbis-phenol A was test marketed. This material had a Vicat softening point of 196 C, excellent resistance to hydrolysis, excellent resistance to tracking and a low density of about l.lg/cm-. Such improvements were obtained at the expense of impact strength and resistance to stress cracking. 

Since several different wear characteristics were noted for the materials tested (e.g.y charring, flow, and brittle behavior), it can be inferred that there is no unique mechanism associated with impact wear of polymer thin films. Further, because of this aspect and the probability that the same mechanisms do not occur under impact testing conditions (Charpy and Izod), it is reasonable to infer that there is little correlation between impact wear resistance and impact strength. This latter point may be illustrated by considering polycarbonate. Even though it has the highest impact strength of any unfilled polymer (4), it exhibits the poorest wear behavior in this study. 

The failure load of a fracture toughness specimen depends on the rate of load application. If a cracked compact tension specimen of 5 mm thick polycarbonate is loaded in a tensile testing machine, there is time for a neck to develop from the crack tip, so plane stress fracture occurs at a crack velocity of 5ms . However, if the load is applied in 1ms by impact loading, plane strain fracture occurs at a low Xjc value, and the crack velocity exceeds 200 ms . Special instrumentation, having a quartz crystal force gauge that responds in 0.1ms, and a grid of resistance lines on the surface to monitor the crack velocity, are needed to measure the Kic value. 

Impact Strength - The impact behaviour of PES is good it is not as outstanding as that of Polycarbonate but it shows better impact perfoimance than that of nylons or acetals. PES cannot be broken in the unnotched Izod test, and a notched Izod (ASTM D 237) has a value of 1 6 ftlb/in notch it is particularly resistant to crack initiation and PES is ev a. tougher after it has absorbed its equilibrium amount of water. 

The choice of substrate can impact the chemical and etch resistance of a coated plastic part. Thin layers of paint (especially when thin) can be influenced by the hardness of the substrate. However, this does not generally impact the chemical resistance of clear-coated parts as measured in testing such as Jacksonville. Substrates that are sensitive to solvents such as acrylonitrile butadiene styrene (ABS) and polycarbonate/polybutylene terephthalate (PC/PBT) can be rapidly or slowly degraded when attacked with certain solvents or even basic materials. 

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