Polypropylene

Polypropylene, abbreviated as PP and also known as polypropene, is a thermoplastic polymer. It is synthetically prepared by chemical industries and has various applications such as textiles, packaging, plastic containers, plastic components, laboratory equipments, automotive components, speaker systems, stationery, and polymer banknotes. Common applications in textile include thermal undergarments, ropes, and carpets. Products made from monomer propylene have the characteristic of being rugged and are resistant to chemical solvents, acids and bases. Market for polypropylene reached 45 million tons in year 2007 alone, or about 65 billion US dollars in turnover.

Chemical and physical properties

Polypropylene's properties are durability and flexibility, especially when copolymerized with ethylene. Commercial polypropylene is isotactic with a level of crystallinity between low- and high-density PE. It's Young's modulus also falls with the intermediate level. The aforementioned properties of PE make it a good material for engineering plastic which can compete with other materials like ABS. It is also cost-effective, can be produced in translucent when not pigmented. However, transparency cannot be achieved as compared to polystyrene, acrylic, or other plastic materials. Oftentimes, it is opaque or pigmented. Another property of polypropylene is resistance against fatigue.

Polypropylene has three general types, namely homopolymer, random copolymer, and block copolymer. Ethylene is the usual comonomer. EPDM or ethylene-propylene rubber is added to polypropylene homopolymer in order to boost its low temperature impact strength. To make the polymer more transparent, randomly polymerized ethylene monomer is added to homopolymer to reduce its crystallinity.

Degradation

Degradation of polypropylene is achieved due to exposure to heat and ultraviolet radiation such as that from sunlight. At the tertiary carbon atom with every repeat unit, oxidation commonly happens, free radicals are formed and additional reaction with oxygen occurs. Then chain scission process follows to yield aldehydes and carboxyclic acids. A network of fine cracks and crazes that eventually become deeper and bigger as time of exposure increases appear in external applications. This is precisely the reason why additives that absorb ultraviolet are added to prevent degradation and cracks. Aside from UV-absorbing additives, carbon black may also be used. Anti-oxidants are also used to prevent degradation since oxidation at high temperatures spawns troubles especially during molding operations.

Manufacturing

Through extrusion and molding, polypropylene can be melted. This process includes some common extrusion methods with by-products of melt-blown and spun-bond fibers that may be converted into large variety of useful products like face masks, diapers, filter, and wipes.

As regards molding, the most common is injection molding. This is often employed in molding cups, cutlery, vials, containers, caps, housewares, automotive supplies and parts like batteries, and many other applications. Also, blow molding and injection-stretch blow molding are used as a molding technique. This process uses both extrusion and molding techniques.

The property of polypropylene to tailor grade with specific molecular properties and additives during manufacturing process makes it possible for various applications. To illustrate this, additives to enhance antistatic capabilities of polypropylene surfaces are added to repel dust and dirt. Also, machining is applied to improve surface's physical finishing. In some applications like for printing ink and paints, some additives are added to polypropylene parts to enhance adhesion of ink and paints.

Applications:

Due to the property of polypropylene to be resistant to fatigue, plastic flip-top bottles are manufactured using polypropylene. The chain molecules are concentrated in such areas, or across the hinge in order to achieve the maximum strength.

In dielectric materials where high-performance pulse and low-loss radio frequency capacitors, polypropylene are used since it requires very thin sheets.

In high-purity piping systems, polypropylene is used. The same is used in potable plumbing, hydronic heating and cooling, and also with reclaimed water systems. This is owed to the capacity of the material to withstand corrosion and chemical leaching, resistance from damage or deformation, impact freezing, and other environmental benefits. Another great capability of it is heat fusion instead of attaching the pipes together with some special glue.

In applications where heat resistance is a requirement, like in medical or laboratory equipments, polypropylene is used. It is common material for manufacturing common-grade kettles or pans.

In food containers, polypropylene is used. This is in order for those containers not to melt in dishwashers. Plastic tubs for diary products are usually polypropylene sealed with aluminum foil. After the cooling process of the products, LDPE which is less heat resistant are used as lids for such containers.

BOPP or biaxially oriented polypropylene is another application of polypropylene. These are used in making bags, especially clear plastic bags. BOPP are clear and are good fashion accessories for their stylish and artistic look.

Polyprops as commonly known in the US military is used for making gears designed for cold-weather. But most recently, polyprops were replaced by polyester like the ECWCS. For warm weather apparels such that used under Kevlar or armor clothing, polypropylene is used as it draws the sweat away from the skin.

In nonwoven clothing like diapers and other sanitary products, polypropylene is used. This is due to the ability of the material to absorb water rather than repel water. Other applications include air filters, and even liquid filters where sheets can be as thin as 0.5 to 30 microns. This is common in household water filters and air conditioning filters. Also in oil filters to contain oil spills in rivers or lakes, polypropylene is used.