What is PP?
Polypropylene, or PP for short, is a semi-crystalline thermoplastic synthesized by Giulio Natta in 1954.
It is one of the commonly used polymer materials. It has the characteristics of high impact resistance, strong mechanical properties, resistance to various organic solvents and acid and alkali corrosion, and is widely used in industry. Such as various laboratory pipette tips, centrifuge tubes and the like are made of pure PP.
Do you know why they chose PP? What are its pros and cons compared to other general purpose materials?
01 Density
The density of PP is only 0.90~0.91g/cm3, which is the smallest density among all synthetic resins, only about 60% of PVC. This means that more products of the same volume can be produced with the same weight of raw material.
02 Mechanical properties
This material has good tensile strength and stiffness, but poor impact resistance, especially at low temperatures. Additionally, if the product is formed with orientation or stress, impact resistance can be significantly reduced. Although its impact resistance is poor, after modification such as filling or reinforcement, its mechanical properties are comparable to those of higher-cost engineering plastics in many fields.
03 Surface hardness
The surface hardness of PP is lower among the five general-purpose plastics, and is only slightly better than that of PE. When the crystallinity is high, the hardness will increase accordingly, but it is still not as good as PVC, PS, ABS, etc.
04 Thermal performance
Among the five main general-purpose plastics, PP has the best heat resistance. Polypropylene plastic products can work for a long time at 100 ° C, and will not deform when heated to 150 ° C by external force. After using a nucleating agent 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 microwave ovens.
05 Stress crack resistance
The residual stress in the molded product or the product working under continuous stress for a long time can cause stress cracking. Organic solvents and surfactants can significantly facilitate this process. Therefore, stress cracking tests were all carried out in the presence of surfactants. Commonly used adjuvants are alkyl aryl polyethylene glycols.
Testing has shown that the PP material has the same good resistance to stress cracking when immersed in surfactant as it does in air. Also, the lower the melt flow rate of PP (ie, the higher the molecular weight), the more resistant it is to stress cracking.
06 Chemical stability
PP has excellent chemical stability. It is inert to most acids, bases, salts and oxidizing agents. For example, it is stable in concentrated phosphoric acid, hydrochloric acid, 40% sulfuric acid and its salt solution at 100 °C. Only a few strong oxidants such as oleum can change it. 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 easily dissolve or swell in some non-polar organic solvents
07 Air tightness (gas barrier)
PP has a certain permeability to oxygen, carbon dioxide and water vapor, which is obviously different from nylon (PA) and polyester (PET). For high-barrier plastics, such as PVDC, EVOH, etc., the situation is even worse. But compared to other non-plastic materials, its airtightness is quite good. Air tightness can be greatly improved by adding barrier material or coating the surface with barrier plastic.
08 Aging characteristics
PP molecules contain tertiary carbon atoms, which are easily destroyed and degraded by photothermal action. PP without stabilizers will become brittle when heated at 150°C for more than half an hour or exposed to sunlight for 12 days. PP without stabilizer will also be seriously degraded when stored indoors in the dark for 4 months, and emit a distinct sour smell.
Adding more than 0.2% antioxidant before granulation of PP powder can effectively prevent the degradation and aging of PP during processing. Antioxidants are divided into two categories: free radical chain reaction terminators (also known as primary antioxidants) and peroxide decomposers (also known as secondary antioxidants). The reasonable combination of the two antioxidants will play a good synergistic effect.
The currently recommended B215 antioxidant is a combination of the main antioxidant 1010 (phenols) and the auxiliary antioxidant 168 (phosphite) in a ratio of 1:2. In order to prevent photoaging, it is necessary to add a UV absorber to PP to convert the UV absorption and excitation of 290~400nm wavelength into non-destructive longer wavelength light. For PP plastic products buried in the soil or used indoors to avoid light, only the main and auxiliary antioxidants are required, and no ultraviolet absorber is required.
09 Electrical performance
PP is a non-polar polymer with good electrical insulation, and due to its low water absorption, electrical insulation is not affected by humidity. And its dielectric constant and dielectric loss factor are small, not affected by frequency and temperature. The dielectric strength of PP is high and increases with temperature. These are beneficial for electrical insulation in hot and humid environments. On the other hand, the surface resistance of PP is very high, and antistatic treatment must be carried out in some occasions.
10 Processability
PP is a crystalline polymer, and its particles require a certain temperature to start melting. Unlike PE or PVC, it softens as the temperature increases during heating. Once a certain temperature is reached, the PP particles melt rapidly and completely transform into a molten state within a few degrees.
Due to the low melt viscosity of PP, it has good fluidity during molding. Especially when the melt flow rate is high, the melt viscosity will be lower. It is suitable for injection molding of large thin-walled products, such as the inner tub of a washing machine.
After PP leaves the mold, slowly cooling in the air will form larger grains, and the product will be transparent. If it is quenched in water (as follows to make the film by the water blowing cooling method), the molecular motion of PP is quickly frozen, no crystals are formed, and the film is completely transparent. The molding shrinkage rate of PP is large, reaching more than 2%, which is much larger than that of ABS plastic (0.5%).
The molding shrinkage rate of PP will vary with the type and amount of other materials added. Careful consideration should be given when making injection molded products with matching dimensions.
