The difference of PP and PE
PP and PE performance differences
From the perspective of heat resistance, the heat resistance of polypropylene is higher than that of polyethylene. Generally, the melting temperature of polypropylene is about 40%-50% higher than that of polyethylene, about 160-170 °C, so the product can It is sterilized and sterilized at a temperature of 100 ° C or higher, and is not deformed at 150 ° C under the condition of no external force. In life, we will find that the "5" polypropylene lunch box is often used for heating food in microwave ovens (the general temperature of microwave heating is 100-140 ° C), and polyethylene can not be used as plastic for microwave oven due to poor heat resistance. Including lunch boxes and plastic wrap. Also, in the field of ordinary packaging films, polyethylene bags are more suitable for use below 90 ° C, and polypropylene bags are also possible to use at relatively high temperatures. From the perspective of rigidity and tensile strength, polypropylene is mainly characterized by low density, better mechanical properties than polyethylene, and has outstanding rigidity. For example, polypropylene has gradually begun to compete with engineering plastics (PA/PC). Widely used in the field of electronic appliances and automobiles. At the same time, due to the high tensile strength of polypropylene and the good bending resistance, it is called “100% plastic”. It is not bent white when the bending is bent 1 million times. This also provides clues for us to identify polypropylene products and become a clue. The hidden mark of the product recycling classification.
From the perspective of low temperature resistance, the low temperature resistance of polypropylene is weaker than that of polyethylene. The impact strength at 0 °C is only half of that at 20 °C, while the brittleness temperature of polyethylene is generally below -50 °C; and with the relative molecular mass Increase, as low as -140 °C. Therefore, if the product needs to be used in a low temperature environment, it is still necessary to select polyethylene as a raw material. The trays used in general chilled foods are made from polyethylene raw materials.
From the perspective of aging resistance, the aging resistance of polypropylene is weaker than that of polyethylene. The structure of polypropylene is similar to that of polyethylene. However, due to the presence of a side branch of methyl group, it is easier to be exposed to ultraviolet light and heat. Oxidative degradation. The most common aging polypropylene products in daily life are woven bags, which are easily broken when exposed to the sun for a long time. In fact, although the aging resistance of polyethylene is higher than that of polypropylene, its performance is not very prominent compared to other raw materials, because it contains a small amount of double bonds and ether bonds in polyethylene molecules, and its weather resistance is not good. Sun exposure and rain can also cause aging.
From the perspective of flexibility, although polypropylene has higher strength, it has poor flexibility, and technically speaking, it has poor impact resistance. Therefore, when used as a film product, its application field is different from that of polyethylene, and polypropylene film is more used for surface packaging printing. In the case of pipes, it is rarely produced with simple polypropylene, and cross-linked polypropylene, which is a common PPR pipe, is required. Because ordinary polypropylene has poor impact resistance and is easy to be broken, it is necessary to add an impact modifier in practical applications, and an auxiliary agent is used in an application such as a bumper to improve impact resistance.
PP and PE blending properties
Effect of PE Type on Impact Properties of Blends
Different types of PE can improve the room temperature impact strength of PP, but the difference is very obvious.
For PP/HDPE blends, when the HDPE mass fraction is less than 60%, the blend strength is essentially unchanged; when the HDPE mass fraction is above 60%, the impact strength of the blend increases.
For PP/LDPE blends, the impact strength is only greatly improved when the LDPE mass fraction is higher than 60%.
For PP/LLDPE blends, when the LLDPE mass fraction is greater than 40%, the impact strength is significantly improved. When the LLDPE mass fraction reaches 70%, the impact strength of the blend is 37.5 kJ/m2, which is 20 times the impact strength of pure PP, which is 10 times and 4 times that of the same amount of PP/HDPE and PP/LDPE blends. .
At low temperature (-18 °C), the improvement trend of PP toughness of three kinds of PE is consistent with that of normal temperature, and LLDPE has the best toughening effect on PP. When the mass ratio of PP/LLDPE is 30/70, the impact strength of the blend is 23.2kJ/m2, which is 20 times that of pure PP. Under the same conditions, the impact strength of PP/HDPE and PP/LDPE blends is only It is about 5kJ/m2. This further demonstrates that the minimum amount of LLDPE is achieved when the same impact strength is achieved, meaning that the rigidity of the PP can be maintained more; and at the same level, the impact strength of the LLDPE-modified PP is the best, which in turn gives the material a More excellent toughness.
Effect of mixing method on toughening effect
The samples mixed by the twin-screw extruder had the highest impact strength, and the samples obtained by the direct injection method had the worst impact performance. Since the effective length of the screw of the injection machine is smaller than that of the extruder, the shear mixing effect is small, and the effect is of course poor. Under different mixing methods, the impact properties of the materials are consistent, that is, the LLDPE mass fraction starts from 40%. With the increase of LLDPE dosage, the impact strength increases greatly. It indicates that the mixing method has the impact performance on the blending system. Influence, but the law does not change.
