All About Polypropylen

Polypropylene, known by the acronym “PP”, is a very tough and durable plastic. It is opaque and highly resistant to heat, softening only at temperatures higher than 150º C. It is very impact resistant but has low density and can be bent easily. It can be folded numerous times so it is used as material for hinges. It also resists corrosive products very well.

 

Structurally, it is a vinyl polymer that is similar to polyethylene, except that one of the carbons of the monomer unit has a methyl group attached. Polypropylene belongs to the group of thermoplastics that are long polymer chains made from the propylene monomer. After exposure to heat and pressure with an active metal catalyst, the propylene monomer combines to form a long polymer chain called "propylene", from the Greek "poly" meaning “many” and "mero" meaning “units”.

 

The most important way to classify polypropylene is based on its chemical structure:

 

- atactic

- isotactic

- syndiotactic

 

The atactic polymer is sticky, amorphous and has a low molecular weight. These polymers provide the same effect as a plasticizer, reducing the crystallinity of polypropylene. A small amount of atactic polymer at the end of the polymer can be used to provide certain mechanical properties such as performance at lower temperatures, elongation, and optical and processable properties. Its formula is:

 

 

From a commercial point of view, isotactic polypropylene is the most important in comparison with the atactic and syndiotactic. Isotactic propylene has the most stereo-regular structure of polypropylenes and therefore achieves a high degree of crystallinity. As a result, many mechanical properties and the processability of polypropylene are highly determined by the level of isotacticity and crystallinity; however, the increase in the crystallinity of polypropylene material makes it a less hard polyethylene. The isotactic polypropylene formula is:

 

 

The syndiotactic polypropylene has recently been developed commercially, with the methyl radicals staggered along the chain in an orderly manner stereochemically, as shown below:

 

 

The formula of the monomer and polymer is as follows:

 

 

Polypropylene is produced by the polymerization of propylene in the presence of metal alkyl catalysts:

 

 

Polypropylene can be obtained from the monomer propylene by Ziegler-Natta polymerization and metallocene catalysis polymerization:

 

 

The different processes applied to polypropylene are primarily injection, extrusion, blow molding and calendering. It is suitable for both thermal forming and cold forming.

 

Listed below are the main properties of polypropylene:

- The density of polypropylene is between 0.90 and 0.93 gr/cm3. This low density enables the manufacture of light products.

- The material is more rigid than most thermoplastics. A load of 25.5 kg/cm2 applied for 24 hours produces no noticeable deformation at room temperature and is withstood up to 70º C.

- It has very high elastic resilience.

- It has excellent compatibility with its surroundings.

- It is easy to recycle this material.

- It has high impact resistance.

- Polypropylene can be used as material for nonlubricated, sliding elements.

- It has good surface resistance.

- It has good chemical resistance to moisture and heat without deforming.

- It has good surface hardness and dimensional stability.