1. Density
PP has the lowest density among all synthetic resins, ranging from 0.90 to 0.91g/cm3, which is about 60% of the density of PVC. This means that using the same weight of raw materials can produce more products of the same volume.
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 the molding of the product, the impact strength will also be significantly reduced. Although the impact strength is poor, its mechanical properties can compete with higher cost engineering materials in many fields after modification such as filling or reinforcement.
3. Surface hardness
The surface hardness of PP is lower among the five types of general plastics, only slightly better than PE. When the crystallinity is high, the hardness also increases slightly, but it is still inferior to PVC, PS, ABS, etc.
4. Thermal properties
Among the five general plastics, PP has the best heat resistance. PP plastic products can work for a long time at 100℃ and will not be deformed when heated to 150℃without external force.After using nucleating agents to improve the crystalline state of PP, its heat resistance can be further improved and it can even be used to make utensils for heating food in microwaves.
5. Stress cracking resistance
Residual stress in formed products or prolonged work under sustained stress can cause stress cracking. Organic solvents and surfactants can significantly promote stress cracking. Therefore, stress cracking tests were conducted in the presence of surfactants.
The experiment shows that the stress cracking resistance of PP when immersed in surfactants is the same as in air and it has good resistance. Moreover, the smaller the melt flow rate of PP (with larger molecular weight), the stronger the 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 concentrated phosphoric acid, hydrochloric acid, 40% sulfuric acid and their salt solutions at 100℃. Only a few strong oxidants such as oleum may cause them to change. PP is a non-polar compound that is very stable to polar solvents, such as alcohols, phenols, aldehydes, ketones, and most carboxylic acids, but it is easily dissolved or swelled in some non-polar organic solvents.
7. Gas tightness (gas barrier)
PP has a certain degree of permeability to oxygen, carbon dioxide and water vapor, which is significantly inferior to PA and PET. For high barrier plastics such as PVDC, EVOH, etc., it is even worse. But compared to other non plastic materials, its air tightness is still quite good. By adding barrier materials or applying barrier plastics on the surface, its gas tightness can be greatly improved.
8. Aging performance
There are tertiary carbon atoms in PP molecules, which are easily broken and degraded under the action of light and heat. PP without stabilizer will become significantly brittle when heated at 150℃ for more than half an hour or exposed to sunlight for 12 days. PP powder without stabilizers can also undergo severe degradation and emit a noticeable sour odor when stored indoors in dark for 4 months.
Adding more than 0.2% antioxidant before granulation of PP powder can effectively prevent the degradation and aging of PP during processing and use.
9. Electrical performance
PP is a non-polar polymer with good electrical insulation and its water absorption is extremely low, so its electrical insulation is not affected by humidity. The dielectric constant and dielectric loss factor of PP are very small and are not affected by frequency and temperature. The dielectric strength of PP is very high and increases with increasing temperature. These are all beneficial for electrical insulation materials in wet and hot environments. On the other hand, the surface resistance of PP is very high, and in some situations, anti-static treatment must be carried out first.
10. Processing performance
PP belongs to crystalline polymers and its particles will not melt below a certain temperature. Unlike PE or PVC, which softens with increasing temperature during the heating process. Once a certain temperature is reached, PP particles quickly melt and can all transform into a molten state within a few degrees.
The melt viscosity of PP is relatively low, so the molding process has good flow ability, especially when the melt flow rate is high, the melt viscosity is smaller. And it is suitable for injection molding of large thin-walled products, such as the inner barrel of a washing machine.
After leaving the mold, if PP is slowly cooled in air, larger grains will be generated, resulting in low transparency of the product. If it is rapidly cooled in water (as shown in the following water cooling method for thin films), the molecular motion of PP is rapidly frozen and cannot generate crystals. At this point, the thin film is completely transparent. The molding shrinkage rate of PP is relatively large, reaching over 2%, far greater than that of ABS plastic (0.5%).
