Polypropylene and Recycling

PP stands for PolyPropylene. It’s made of propylene gas with help of catalyst (such as titanium chloride). Its internal structure is very similar to PE so the macro characteristics are also close. Normally it’s semi-transparent and the hardness approximates HDPE. The density of PP is 0.895- 0.92g/cm³, which is the lowest density among all the high-molecular polymers. It’s usually used as engineering plastics and is more economical compared with ABS. It’s good for food-contact containers.

In common practice we also classify PP into PPH and PPC.

PPH (homopolymerization, powder) is pure propylene and has three forms as per the location of branch chain: IPP, SPP and APP.

IPP: methyl is in the same side of main chain. The output volume takes 95% of total PP.

SPP: methyl is alternately arranged on both sides of main molecular chain. It has low crystallinity.

APP: unordered methyl groups arrange on both sides. It has no crystallinity, it is sticky material and is not plastics. It’s called PP wax and is commonly used as carrier for filling extrusion or acts as plasticizer during twin screw extrusion.

PPC (copolymerization, granule) is mixed with ethylene or butylenes and has two categories: PPB and PPR.

PPB: 5-20% ethylene with good rigidness and low-temperature resistance. It’s used for big containers, mechanical parts, cables etc.

PPR: 1-7% ethylene with good transparency, good impact-resistance and convenient for thermal welding. It’s mostly used for clear film, water pipe etc.

When the polypropylene is extruded in both directions, it’s called BOPP (Biaxially Oriented Polypropylene). BOPP film has a better strength and clearness and is easy to print on. So it’s widely used for packaging.

PP fiber is light weighted and is floatable in water and doesn’t absorb water. So it’s used for fish net. It’s also made into ropes. In earthquake areas PP fiber is added into the soil to enhance the stability.

The recycling code of PP is 5 and PP is 100% recyclable.