Polyethylene notch sensitivity

The radius of the notch is quite important, particularly for plastics. For example, polyvinyl chloride (PVC) is a notch-sensitive material. If the notch is blunt (2-mm radius), the impact strength is higher than that for ABS. If the notch is sharp (0.25-mm radius), the impact strength of PVC is lower than that for ABS. Other polymers that are notch brittle are high-density polyethylene (HDPE), polypropylene (PP), polyethylene teraphthalate (PET), dry polyamides (PAs), and acetals. 

Low-Density Polyethylene. The data from outdoor weathering are summarized In Figure 1. The low-temperature brittleness (LTB) measurements show that a well-dispersed carbon black of > 1% concentration and < 35 millimicrons (mu) particle size Is necessary for maximum resistance to photo-oxldatlon. The results from elongation measurements agree with those found for LTB, as would be expected, since both measurements reflect the notch sensitivity of polyethylene to micro cracks caused by photo-oxldatlon. Accelerated weathering studies on these compounds have been reported previously (2) and Indicate the same ranking as found in outdoor exposure. 

Figure 2.14 shows the relative notch sensitivities of some polyolefins. Among the polyolefins, the low-melt-index polypropylene copolymer (GMT 61) has the lowest notch sensitivity, whereas the higher-melt-index polyethylene and polypropylene homopolymers have the highest. 

FIGURE 2.15 Influence of temperature on Izod impact strength of various polymers of low and high melt indexes. GMT 61 is a low-melt-index and low-notch-sensitivity polypropylene copolymer. KMT 61 is a high-melt-index and low-notch-sensitivity polypropylene polymer. HDPE 65045 is a high-melt-index and high-notch-sensitivity high-density polyethylene. 

One non-standardized approach to testing for notch sensitivity and stress cracking resistance of polyethylene pipes is offered by the Full Notch Creep Test (FNCT) and Notched Constant Tensile Load. Here, a square test bar with a circumferential notch is loaded at 95°C In a wetting bath at constant tensile load [266]. 

Current investigations indicate that the stress-cracking induced, sloping curve branch does not appear in internal pressure creep tests. Bimodal PE-HD types are therefore quite stress-cracking resistant, which can be explained by the reverse incorporation of comonomers. Bimodal polyethylene is a reactor blend of a low-molecular, high-density homopolymer and a low-density, high-molecular copolymer. Short-term tests on notched specimens of bimodal PE-HD show considerably reduced stress-cracking sensitivity [830]. 

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