Polypropylene has excellent and desirable physical, mechanical, and thermal properties when used in room-temperature applications. It is relatively stiff and has a high melting point, low density, and relatively good resistance to impact. These properties can be varied in a relatively simple manner by altering the chain regularity (tacticity) content and distribution, the average chain lengths, the incorporation of a comonomer such as ethylene into the polymer chains, and the incorporation of an impact modifier into the resin formulation.
Homopolymer PP is the most widely used polypropylene material in the HPP, RCP, and ICP family of products. It is made in several different reactor designs using catalysts that link the monomers together in a stereospecific manner, resulting in polymer chains that are crystallizable. Whether they crystallize and to what extent depends on the conditions under which the entangled mass of polymer chains transitions from the melt to the solid state or how a heat-softened solid PP material is strained during a further fabrication procedure like fiber drawing. Homopolymer PP is a two-phase system because it contains both crystalline and noncrystalline regions.
The noncrystalline, or amorphous, regions are composed of both isotactic PP and atactic PP. The isotactic PP in the amorphous regions is crystallizable, and it will crystallize slowly over time up to the limit that entanglement will allow. The extent of crystallization after the initial fabrication step of converting PP pellets or powder to a molded article will slowly increase over time, as will the stiffness.
A widely accepted model of HPP morphology likens the solid structure to a system consisting of pieces of stiff cardboard linked together by strands of softer material. In the areas represented by flat pieces of cardboard, PP polymer chains weave up and down into close-packed arrays called crystallites (‘‘little crystals’’), which are called lamella by morphologists. The soft strands linking the pieces of stiff cardboard are polymer chains that exit one crystallite, enter another, and then begin weaving up and down in another crystallite.
The crystallizability of the chains is one factor that determines how thick the crystallites will be, and the thickness of the crystallites determines how much heat energy is required to melt them (the melting temperature). A typical HPP has an array of crystallites from thick ones to very thin ones, and these manifest themselves as an array of melting points. Homopolymer PP is marketed mainly by melt flow rate (MFR) and additive formulation into fiber, film, sheet, and injection molding applications. Melt flow rate is an indicator of the weight-average molecular weight as measured by the ASTM or ISO MFR test method.
