Comparison of PP and PET materials

PET polyethylene terephthalate.

PP is polypropylene.
PET: Polyethylene terephthalate
A polymer of terephthalic acid and glycol.
Known by the acronym PET, it is used to make polyethylene terephthalate fibers known as polyester in China.
This kind of fiber is of high strength and good wear performance. It is currently the most productive variety of synthetic fiber. In 1980, the world output was about 5.1 million tons, accounting for 49% of the world total synthetic fiber output
Due to the high symmetry of molecular structure and the rigidity of the phenyl chain, the polymer is characterized by high crystallinity, high melting temperature and insoluble in general organic solvents. The melting temperature is 257 ~ 265℃.
Its density increases with the increase of crystallinity, the amorphous density is 1.33 g/cm ^3, after stretching due to the improvement of crystallinity, the fiber density is 1.38 ~ 1.41 g/cm ^3, from the X-ray study, the density of complete crystal is calculated as 1.463 g/cm ^3.
The glass transition temperature of amorphous polymer was 67℃.
The crystalline polymer was 81℃.
The melting heat of the polymer is 113 ~ 122 coke/g, the specific heat capacity is 1.1 ~ 1.4 coke/g, the dielectric constant is 3.0 ~ 3.8, the specific resistance is 10^11 10^14 ohm.
PET is insoluble in common solvents, but only in some highly corrosive organic solvents such as phenol, o-chlorophenol, m-cresol, trifluoroacetic acid mixed solvents. PET fibers are stable to weak acids and bases.

It is mainly used as raw material for synthetic fiber.
Staple fibers can be blended with cotton, wool and flax and made into clothing textiles or interior decorative fabrics.
Filament can be used for clothing or industrial silk, such as for press cloth, tire cord, parachute, conveyor belt, safety belt, etc.
Film can be used as a substrate for photographic film and audio tape.
Injection molding parts can be used as packaging containers.
PP is a semi - crystalline material.
It is harder than PE and has a higher melting point.
Because homopolymer PP is very brittle when the temperature is above 0℃, many commercial PP materials are random copolymers with 1~4% ethylene or clamping copolymers with higher ratio of ethylene content.
Copolymer PP materials have lower thermal distortion temperature (100℃), low transparency, low gloss, low rigidity, but have stronger impact strength.
The strength of PP increases with the increase of ethylene content.
The vica softening temperature of PP is 150℃.
Due to its high crystallinity, the material has good surface stiffness and scratch resistance.
Environmental stress cracking does not exist in PP.
Generally, PP is modified by adding fiberglass, metal additives or thermoplastic rubber.
The turnover rate of PP ranges from 1 to 40.
PP materials with low MFR have better impact resistance but lower tensile strength.
For the same MFR material, the strength of the copolymer type is higher than that of the homopolymer type.
As a result of crystallization, PP shrinkage is quite high, generally 1.8-2.5%.
And the directional uniformity of shrinkage ratio is much better than pe-HD and other materials.
The shrinkage rate can be reduced to 0.7% by adding 30% glass additive.
Homopolymer and copolymer PP materials have excellent resistance to moisture absorption, acid and alkali corrosion, and solubility.
However, it has no resistance to aromatic hydrocarbon (such as benzene) solvents, chlorinated hydrocarbon (carbon tetrachloride) solvents, etc.
PP is also not like PE at high temperature still has antioxidant properties.