Polypropylene

Polypropylene: Structure, Properties and Applications

Polypropylene (PP) is a thermoplastic material that is produced by polymerizing propylene molecules, which are the monomer units, into very long polymer molecules or chains. There are a number of different ways to link the monomers together, but PP as a commercially used material in its most widely used form is made with catalysts that produce crystallizable polymer chains. These give rise to a product that is a semicrystalline solid with good physical, mechanical, and thermal properties. Another form of PP, produced in much lower volumes as a byproduct of semicrystalline PP production and having very poor mechanical and thermal properties, is a soft, tacky material used in adhesives, sealants, and caulk products.

Properties of PP

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.

Typical of PP

Polypropylene containing ethylene as a comonomer in the PP chains at levels in about the 1–8% range is referred to as random copolymer (RCP). HPP containing a commixed RCP phase that has an ethylene content of 45–65% is referred to as an impact copolymer (ICP). Each of these product types is described below in more detail.

• Homopolymer

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. Homopolymer PP is marketed mainly by melt flow rate (MFR) and additive formulation into fiber, film, sheet, and injection molding applications.

• Random Copolymer

Random copolymers are ethylene=propylene copolymers that are made in a single reactor by copolymerizing propylene and small amounts of ethylene (usually 7% and lower). The copolymerized ethylene changes the properties of the polymer chains significantly and results in thermoplastic products that are sold into markets in which slightly better impact properties, improved clarity, decreased haze, decreased melting point, or enhanced flexibility are required. The ethylene monomer in the PP chain manifests itself as a defect in the chain regularity, thus inhibiting the chain’s crystallizability. As the ethylene content increases, the crystallite thickness gradually decreases, and this manifests itself in a lower melting point. The amount of ethylene incorporated into the chain is usually dictated by the balance between thermal, optical, and mechanical properties.

• Impact Copolymers

Impact copolymers are physical mixtures of HPP and RCP, with the overall mixture having ethylene contents on the order of (6–15% wt%. These are sold into markets where enhanced impact resistance is needed at low temperatures, especially freezer temperature and below. The RCP part of the mixture is designed to have ethylene contents on the order of 40–65% ethylene and is termed the rubber phase. The rubber phase can be mechanically blended into the ICP by mixing rubber and HPP in an extruder, or it can be polymerized in situ in a two-reactor system. The HPP is made in the first reactor and the HPP with active catalyst still in it is conveyed to a second reactor where a mixture of ethylene and propylene monomer is polymerized in the voids and interstices of the HPP polymer powder particle. The amount of rubber phase that is blended into the HPP by mechanical or reactor methods is determined by the level of impact resistance needed. The impact resistance of the ICP product is determined not only by its rubber content but also by the size, shape, and distribution of the rubber particles throughout the ICP product. Reactor products usually give better impact resistance at a given rubber level for this reason. As the rubber content of the ICP product is increased, so is the impact resistance, but this is at the expense of the stiffness (flexural modulus) of the product.

Advantages and disadvantages of polypropylene

Comparison of homopolymer and copolymer

Application of PP

• Automobiles
• Textiles
• Medicine
• Consumer goods
• Hygiene
• Pipes and profiles
• Sheets
• Toys

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