PP is an additional polymer with the repeating unit of 2-methyl ethylene. PP is classified into three groups: (1) isotactic PP, in which the CH3groups are on the same side of the main chain; (2) syndiotactic PP, in which the CH3groups are symmetrically arranged on the two sides of the main chain; and (3) atactic PP, where the CH3groups are randomly distributed in their spatial relationship to the main chain. Thus based on the configuration of the CH3groups, PP has different properties. For example, the atactic PP is amorphous and rubbery. In contrast, the isotactic PP is highly crystalline with a high melting point and stiffness. Generally, the greater the amount of isotactic material, the greater the crystallinity, and the greater the softening point, stiffness, tensile strength, modulus, and hardness. Therefore for structural applications, an isotactic polymer is preferable to the other ones. The chemical resistance of PP is similar to that of HDPE. However, it has lower resistance to oxidation, chemical degradation, and cross-linking than PE[36,38]. PP can be processed by injection molding, blow molding, extrusion, film casting, and thermoforming[39], and its major use is for the fabrication of bone fixation devices, specifically, ultrahigh-strength rods for bone fixation.
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