Introduction: As one of the widely used general plastics, PP can be seen everywhere in daily life. It has higher purity than general PC. Although it does not have the high coloring of ABS, PP has higher purity and color rendering. This article shares the performance comparison between PP and several other major general-purpose plastics, and the content is for your reference:
PP is a crystalline polymer. Among the commonly used plastics, PP is the lightest, with a density of only 0.91g/cm3 (less than water). Among the general-purpose plastics, PP has the best heat resistance. Its heat distortion temperature is 80-100°C and can be boiled in boiling water. PP has good stress cracking resistance and high bending fatigue life, commonly known as “100% plastic”
PP has the lowest density among all synthetic resins, only 0.90~0.91g/cm3, which is about 60% of the density of PVC. This means that with the same weight of raw materials, more products of the same volume can be produced.
The tensile strength and rigidity of PP are relatively good, but the impact strength is poor, especially the impact resistance at low temperature. 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, after modification such as filling or reinforcement, its mechanical properties can compete with higher-cost engineering plastics in many fields.
The surface hardness of PP is low among the five general plastics, only better than PE. When the crystallinity is high, the hardness will increase correspondingly, but it is still not as good as PVC, PS, ABS and so on.
Among the five general plastics, PP has the best heat resistance. PP plastic products can work for a long time at 100°C. When there is no external force, PP products will not deform when heated to 150°C. After the nucleating agent is used 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 a microwave oven.
Stress crack resistance
The residual stress in the molded product, or the product working under continuous stress for a long time, will cause stress cracking. 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.
The test shows that the stress cracking resistance of PP when immersed in surfactant is the same as in air, and it has good resistance, and the smaller the melt flow rate of PP (the larger the molecular weight), the stronger the stress cracking resistance.
PP has excellent chemical stability and is inert to most acids, alkalis, salts and oxidants. For example, it is stable in 100°C 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.
PP has a certain degree of permeability to oxygen, carbon dioxide and water vapor. Compared with nylon (PA) and polyester (PET), there is a significant gap. For high barrier plastics, such as PVDC, EVOH, etc., it is 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, the air tightness can be greatly improved.
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 obviously brittle when heated at 150°C for more than half an hour, or exposed to 12 days in a sunny place. The PP powder without stabilizer will be severely degraded when placed indoors in the 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). The reasonable combination of primary and secondary antioxidants will Will play a good synergistic effect.
In order to prevent light aging, it is necessary to add an ultraviolet absorber to the PP, which can convert the ultraviolet absorption and excitation with a wavelength of 290~400nm into non-destructive longer-wavelength light. For PP plastic products that are buried in the soil or used indoors to avoid light, only the main and auxiliary antioxidants can be added, without the need to add ultraviolet absorbers.
PP is a non-polar polymer, has good electrical insulation, and PP has very low water absorption, and electrical insulation will not be 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 temperature rise. These are beneficial to electrical insulation materials in humid and hot environments. On the other hand, the surface resistance of PP is very high, in some occasions, it must be antistatic treatment.
PP is a crystalline polymer, and its particles will not melt below a certain temperature, unlike PE or PVC, which soften as the temperature increases during heating. Once it reaches a certain temperature, the PP particles melt quickly, and they can all be transformed into a molten state within a few degrees.
The melt viscosity of PP is relatively low, so the molding process has good fluidity, especially when the melt flow rate is high, the melt viscosity is smaller, which is suitable for injection molding of large thin-walled products, such as the inner barrel of a washing machine.
After PP leaves the die, if it is slowly cooled in the air, larger crystal grains will be formed, and the transparency of the product will be low. If it is rapidly cooled in water (the film is produced by the water-cooling method as follows), the molecular motion of PP is rapidly frozen and crystals cannot be formed. At this time, the film is completely transparent. The molding shrinkage rate of PP is relatively large, reaching more than 2%, which is much greater than that of ABS plastic (0.5%).
The molding shrinkage rate of PP can vary with the type and amount of other materials added. This needs to be carefully considered when making injection molded products with matching dimensions.