What is the difference between Polyethylene (PE) vs Polypropylene (PP)?

When buying plastic products, many people have heard of the two plastics Polyethylene (PE) vs Polypropylene (PP). If you don’t understand plastics, it is difficult to tell the difference between these two plastics, PP and PE. What are the characteristics of pe plastic? These two kinds of plastics have their characteristics, and the application occasions are not too the same. Only after a clear distinction can you know which one you want to buy.

What is Polyethylene(PE)?

Polyethylene, abbreviated as PE, is a thermoplastic resin obtained by the polymerization of ethylene. In industry, it also includes copolymers of ethylene and small amounts of alpha-olefins. Polyethylene is odorless, non-toxic, feels like wax, has excellent low-temperature resistance (minimum operating temperature can reach -70 ~ -100 ℃), has good chemical stability, and can resist the erosion of most acids and alkalis (not resistant to oxidation properties). acid), insoluble in general solvents at room temperature, low water absorption, and excellent electrical insulation performance; but polyethylene is very sensitive to environmental stress (chemical and mechanical effects) and has poor heat aging resistance. The properties of polyethylene vary from variety to variety, mainly depending on the molecular structure and density. Different production methods can be used to obtain products with different densities (0.91-0.96g/cm3). Polyethylene can be processed by the usual thermoplastic molding methods (see Plastics Processing). It has a wide range of uses, mainly used to manufacture films, containers, pipes, monofilaments, wires and cables, daily necessities, etc., and can be used as high-frequency insulating materials for TVs, radars, etc.

Types of PE:

(1) LDPE: low-density polyethylene, high-pressure polyethylene

(2) LLDPE: Linear Low-Density Polyethylene

(3) MDPE: medium-density polyethylene, bimodal resin

(4) HDPE: high-density polyethylene, low-pressure polyethylene

(5) UHMWPE: ultra-high molecular weight polyethylene

(6) Modified polyethylene: CPE, cross-linked polyethylene (PEX)

(7) Ethylene copolymer: ethylene-propylene copolymer (plastic), EVA, ethylene-butene copolymer, ethylene-other olefin (such as octene POE, cyclic olefin) copolymer, ethylene-unsaturated ester copolymer ( EAA, EMAA, EEA, EMA, EMMA, EMAH)

Features of PE:

Corrosion resistance, and excellent electrical insulation (especially high-frequency insulation), can be modified by chlorination, chemical cross-linking, radiation cross-linking, and can be reinforced with glass fiber. The melting point, rigidity, hardness, and strength of low-pressure polyethylene are high, with Low water absorption, good electrical properties, and radiation resistance; high-pressure polyethylene has good flexibility, elongation, impact strength, and permeability; ultra-high molecular weight polyethylene has high impact strength, fatigue resistance, and wear resistance. Low-pressure polymerization Ethylene is suitable for making corrosion-resistant parts and insulating parts; high-pressure polyethylene is suitable for making films, etc.; ultra-high molecular weight polyethylene is suitable for making shock-absorbing, wear-resistant, and transmission parts.

Molding characteristics of PE:

1. Crystalline material, low moisture absorption, does not need to be fully dried, excellent fluidity, fluidity is sensitive to pressure, the high-pressure injection should be used during molding, material temperature is uniform, filling speed is fast, and pressure is sufficient. The direct gate should not be used to prevent shrinkage Uneven, the internal stress increases. Pay attention to the selection of the gate position to prevent shrinkage and deformation.
2. The shrinkage range and shrinkage value are large, the directionality is obvious, and it is easy to deform and warp. The cooling speed should be slow, and the mold is equipped with a cold material cavity and a cooling system.
3. The heating time should not be too long, otherwise, decomposition will occur.
4. When the soft plastic parts have shallow side grooves, they can be forcibly demolded.
5. Melt cracking may occur, and should not be contacted with organic solvents to prevent cracking.
High-pressure polyethylene: more than half is used for film products, followed by pipes, injection molding products, wire wrapping layers, etc. Medium and low-pressure polyethylene: mainly injection molding products and hollow products.
Ultra-high pressure polyethylene: Due to the excellent comprehensive properties of ultra-high molecular polyethylene, it can be used as engineering plastics.

What is Polypropylene (PP)?

Polypropylene, abbreviated as PP, is a thermoplastic resin obtained by the polymerization of propylene. There are three configurations of isotactic, random, and syndiotactic, and industrial products are mainly composed of isotactic. Polypropylene also includes copolymers of propylene with small amounts of ethylene. Usually translucent colorless solid, odorless, and non-toxic. Due to the regular structure and high crystallization, the melting point is as high as 167 ℃, heat resistance, and the products can be sterilized by steam. With a density of 0.90g/cm3, it is the lightest general-purpose plastic. Corrosion resistance, tensile strength 30MPa, strength, rigidity, and transparency are better than polyethylene. The disadvantage is that it has poor low-temperature impact resistance and is easy to age, but it can be overcome by modification and the addition of antioxidants respectively.

Features of PP?

Non-toxic, odorless, low density, strength, stiffness, hardness, and heat resistance are better than low-pressure polyethylene and can be used at about 100 degrees. It has good electrical properties and high-frequency insulation not affected by humidity but becomes brittle at low temperatures is Not wear-resistant and is easy to age. Suitable for making general mechanical parts, corrosion-resistant parts, and insulating parts. Common acid and alkali organic solvents have little effect on it and can be used in tableware.

Molding characteristics of PP:

1. Crystalline material, low hygroscopicity, prone to melt rupture, and easy to decompose after long-term contact with hot metal.

2. The fluidity is good, but the shrinkage range and shrinkage value are large, and shrinkage holes, dents, and deformations are prone to occur.

3. The cooling speed is fast. The pouring system and cooling system should dissipate heat slowly, and pay attention to controlling the molding temperature. It is easy to orientate when the material temperature is low and high pressure. When the mold temperature is lower than 50°C, the plastic parts are not smooth, and it is easy to produce poor welding and flow marks. , Warping deformation occurs easily above 90°C.

4. The thickness of the plastic wall should be uniform to avoid a lack of material and sharp corners to prevent stress concentration.

What is the difference between Polyethylene (PE) vs Polypropylene (PP)?

Similarity: PE VS PP

1. PE and PP are both milky white, translucent, waxy polymers that are not colored.
2. They are all non-polar crystalline polymers with good compatibility.
3. Non-toxic, good chemical stability, easy to shape and process.
4. Poor weather resistance.

Difference: PE VS PP

1. PP has better rigidity, strength, and heat resistance than PE, and PE is easily decomposed by photooxidation, thermal oxidation, and ozone.
2. The density of PP is smaller than that of PE, and the density of PP is only 0.90~0.91g/cm3.
3. The toughness of PP is poor at low temperatures.