Polypropylene homopolymer impact strength

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

The crystallinity of isotactic polypropylene homopolymer, PP, however, leads to its well known brittle behavior at low temperatures or when impacted under conditions of stress concentration, i.e., in the presence of sharp notches. For example, the low temperature drop weight (Gardner) impact strength of unmodified PP is... 

Chun and Woodhams [39] reported significant increases in tensile strength but little effect on impact strength by using a maleated polypropylene wax with mica in a PP homopolymer. Lower levels of additive were needed than in the work with AA-g-PP described above. The authors also found a synergistic effect with some silane treatments. 

Table 18.4 Influence of type of polypropylene (homopolymer versus copolymer) and conductive filler on surface resistivity and impact strength of filled composites.

TABLE 11.17 Multiple Test Standards Employed in Reporting Impact Strengths of Polypropylene (PP) Homopolymer... 

Figure 18.29 Effect of temperature on falling weight impact strength of 0.060 inch compression moulded specimens of various polymers. GMT 61 and KMT 61 = polypropylene copolymer. ABS = acrylonitrile-butadiene-styrene terpolymer, KM 61 polypropylene homopolymer. Source Author s own files)...

Impact copolymers, also known as heterophasic copolymers or rTPOs, usually contain up to about 40% ethylene-propylene rubber (EPR), distributed inside the semicrystalline PP homopolymer matrix. This copolymer phase is added to increase the impact strength of the product at low temperatures. As a result of its glass transition temperature the impact strength of polypropylene homopolymer is often unacceptable for use in low temperature applications, such as packaging or automotive applications. 

Lusenac America offers a broad line of talc products for use In polypropylene homopolymer. Talc increases the stiffness and heat distortion temperature (HOT) of polypropylene. It also reduces the coefficient of thermal expansion. Typical talc loadings are 20 to 40%. Because homopolymers do not have good impact strength, the talc products recoaunended are relatively coarse and vary based on price, talc content, color and longterm heat aging (LTHA). 

---