1. Density: PP has the smallest density among all synthetic resins, only 0.90~0.91g/cm3, which is about 60% of density of PVC. This means that with same weight of raw materials, more products of same volume can be produced.
2. Mechanical properties: PP has good tensile strength and rigidity, but poor impact strength, especially poor impact resistance at low temperatures. In addition, if there is orientation or stress during molding of product, impact strength will also be significantly reduced. Although impact strength is poor, its mechanical properties can compete with higher-cost engineering plastics in many fields after modification such as filling or reinforcement.
3. Surface hardness: Surface hardness of PP is low among five general plastics, only better than PE. When crystallinity is high, hardness will increase correspondingly, but it is still not as good as PVC, PS, ABS and so on.
4. Thermal properties: Among five general plastics, PP has the best heat resistance. PP plastic products can work for a long time at 100℃. When there is no external force, PP products will not deform when heated to 150℃. After nucleating agent is used to improve crystalline state of PP, its heat resistance can be further improved, and it can even be used to make utensils for heating food in a microwave oven.
5. Resistance to stress cracking: Stress cracking will occur if there is residual stress in molded product, or product will work under continuous stress for a long time. Organic solvents and surfactants can significantly promote stress cracking. Therefore, stress cracking tests are all carried out in the presence of surfactants. Commonly used auxiliary agent is alkyl aryl polyethylene glycol. Test shows that stress cracking resistance of PP when immersed in surfactant is same as in air, it has good resistance, and the smaller melt flow rate of PP (the larger molecular weight), the stronger stress cracking resistance.
6. Chemical stability: PP has excellent chemical stability and is inert to most acids, alkalis, salts and oxidants. For example, it is stable in 100℃ concentrated phosphoric acid, hydrochloric acid, 40% sulfuric acid and their salt solutions, and only a few strong oxidants such as fuming sulfuric acid can make changes. PP is a non-polar compound, very stable to polar solvents, such as alcohols, phenols, aldehydes, ketones and most carboxylic acids will not swell, but it is easy to dissolve or swell in some non-polar organic solvents.

 

7. Air tightness (gas barrier property): PP has a certain permeability to oxygen, carbon dioxide and water vapor. Compared with nylon (PA) and polyester (PET), there is a significant gap. For high-barrier plastics, For example, PVDC, EVOH, etc. are even worse. But compared with other non-plastic materials, its air tightness is still quite good. By adding barrier materials or coating barrier plastics on the surface, air tightness can be greatly improved.
8. Aging performance: There are tertiary carbon atoms in PP molecules, which are easily broken and degraded under action of light and heat. PP without stabilizer will become obviously brittle when heated at 150℃ for more than half an hour, or exposed to 12 days in a sunny place. PP powder without stabilizer will be severely degraded when placed indoors in dark for 4 months, giving out a clear sour taste.
Adding more than 0.2% antioxidant before PP powder granulation can effectively prevent PP from degrading and aging during processing and use. Antioxidants are divided into two major categories: radical chain reaction terminator (also called primary antioxidant) and peroxide decomposer (also called secondary antioxidant). Reasonable combination of primary and secondary antioxidants will exert a good synergistic effect.
Currently recommended B215 antioxidant is a combination of primary antioxidant 1010 (phenols) and secondary antioxidant 168 (phosphite) in a ratio of 1:2. In order to prevent light aging, it is necessary to add an ultraviolet absorber to PP, which can convert ultraviolet absorption and excitation with a wavelength of 290~400nm into non-destructive longer-wavelength light. For PP plastic products that are buried in soil or used indoors to avoid light, only main and auxiliary antioxidants can be added, without need to add ultraviolet absorbers.
9. Electrical properties: PP is a non-polar polymer, has good electrical insulation, PP has very low water absorption, and electrical insulation will not be affected by humidity. Dielectric constant and dielectric loss factor of PP are very small and are not affected by frequency and temperature. Dielectric strength of PP is very high, increases with temperature rise. These are beneficial to electrical insulation materials in humid and hot environments. On the other hand, surface resistance of PP is very high, in some occasions, it must be antistatic treatment.
10. Good processing performance: PP is a crystalline polymer, and its particles will not melt below a certain temperature, unlike PE or PVC, which soften as temperature increases during heating. Once it reaches a certain temperature, PP particles melt quickly, and they can all be transformed into a molten state within a few degrees.
Melt viscosity of PP is relatively low, so molding process has good fluidity, especially when melt flow rate is high, melt viscosity is smaller, which is suitable for injection molding of large thin-walled products, such as inner barrel of a washing machine. After PP leaves die, if it is slowly cooled in air, larger crystal grains will be formed, transparency of product will be low. If it is rapidly cooled in water (film is produced by water-cooling method as follows), molecular motion of PP is rapidly frozen and crystals cannot be formed. At this time, film is completely transparent. Molding shrinkage rate of PP is relatively large, reaching more than 2%, which is much greater than that of ABS plastic (0.5%).

 

Molding shrinkage rate of PP can vary with type and amount of other materials added. This needs to be carefully considered when making injection molded products with matching dimensions.