Polymeric Materials and Properties

Polypropylene (PP): Polypropylene is manufactured by polymerizing propylene monomer with a titanium based catalyst; a co-catalyst (triethylaluminum) is added to initiate the polymerization reaction and hydrogen is used in the reactor to control polymer

molecular weight.

This reaction is produced using a slurry or gas phase type of process.

Polypropylene is the fastest growing commodity thermoplastic in the world. It is a versatile polymer used in a wide range of applications from fibers, flat filaments used in industrial bags and outdoor carpets, packaging films, home appliances, electrical and medical devices to automobile bumpers, air management systems, interior panel and under-the-hood components.

There are three polypropylene structures produced during polymerization: isotactic, syndiotactic, and atactic.

The principal structure of polypropylene is isotactic, which is a semi-crystalline polymer in a helical form. This structure has good mechanical properties that can be further improved with fiberglass reinforcements and mineral fillers (talc, calcium carbonate).

Syndiotactic polypropylene is produced by the monomer units inserted alternately head-to-tail. This structure is more flexible with better impact resistance and clarity than the isotactic structure.

Atactic polypropylene (hard wax amorphous monomer) is a by-product of the manufacturing process. The atactic monomer affects the process efficiency. This product is used in roofing tars and adhesives for the shoe industry.

All forms of polypropylene are susceptible to oxidation due to the presence of a tertiary hydrogen. Polypropylene is stabilized against thermal degradation by the addition of primary and secondary antioxidants. Neutralizing agents are also added to stabilize the low levels of chloride ash generated during manufacturing. Other special additives are used such as antistatic agents, slip agents, and UV stabilizers.

Polypropylene is produced as homopolymers and copolymers. The physical properties range from good tensile strength and stiffness to a tough, flexible, and low strength polymer.

Polypropylene homopolymer has the highest melting point with a wide range of melt flow rates and stiffness.

Polypropylene copolymers incorporate small amounts of ethylene which lower the crystallinity rate, producing higher impact strength even at low temperatures, and more flexibility, but a lower melting point and low melt mass-flow rate properties.