The production processes of polyethylene and polypropylene raw materials are relatively similar, and the products can be used to make plastic films, injection molded products, plastic pipes, etc. In many cases, we find that the two raw materials have great similarities in properties and uses. But in fact, there are still many differences in the application of polypropylene and polyethylene raw materials. The editor will give you an analysis of the performance characteristics of polypropylene and polyethylene, and discuss the difference in material properties after the two are mixed in different proportions. In terms of the material used for ton bags, polypropylene can be used as an outer bag, while polyethylene can only be used as an inner film bag.
1. From the perspective of heat resistance, the heat resistance of polypropylene is higher than that of polyethylene. Under normal circumstances, the melting temperature of polypropylene is about 40%-50% higher than that of polyethylene, which is about 160-170℃, so The product can be sterilized at a temperature above 100°C, and will not be deformed at 150°C under the condition of no external force. In daily life, we will find that "5" polypropylene lunch boxes are often used to heat food in microwave ovens (the general temperature of microwave oven heating is 100-140℃), and polyethylene cannot be used as plastic for microwave ovens due to its poor heat resistance. , Including lunch boxes and plastic wrap. Similarly, in the field of ordinary packaging films, polyethylene packaging bags are more suitable for use at temperatures below 90°C, while polypropylene packaging bags can be used at relatively high temperatures.
2. From the perspective of rigidity and tensile strength, the main characteristics of polypropylene are its low density, better mechanical properties than polyethylene, and outstanding rigidity. For example, polypropylene has gradually expanded to compete with engineering plastics (PA/PC). Competition is widely used in the fields of electronics, electrical appliances and automobiles. At the same time, because polypropylene has high tensile strength and good bending resistance, it is called "100-fold plastic". It is bent 1 million times and does not become white when bent. This also provides a clue for us to distinguish polypropylene products. Hidden signs for recycling and sorting of products.
3. From the perspective of low temperature resistance, polypropylene has weaker low temperature resistance than polyethylene. The impact strength at 0°C is only half of that at 20°C, while the brittleness temperature of polyethylene can generally reach below -50°C; The increase in mass can be as low as -140°C. Therefore, if the product needs to be used in a low temperature environment, it is still necessary to choose polyethylene as the raw material. Generally, trays used for refrigerated food are made of polyethylene raw materials.
4. 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. Under the action of oxidative degradation. The most common polypropylene product that is prone to ageing in daily life is the woven bag. The woven bag is 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, compared with other raw materials, its performance is not very outstanding, because the polyethylene molecule contains a small amount of double bonds and ether bonds, and its weather resistance is not good. , Sun and rain can also cause aging.
5. From the perspective of flexibility, although polypropylene has high strength, it has poor flexibility, and technically speaking, it has poor impact resistance. Therefore, when it is used to make film products, its application field is still different from that of polyethylene. Polypropylene film is more used for surface packaging printing. In terms of pipes, simple polypropylene is rarely used for production, and cross-linked polypropylene, which is a common PPR pipe, is needed. Because ordinary polypropylene has poor impact resistance and is easy to crack, it is necessary to add impact modifiers in practical applications, and additives should be used to improve impact resistance in applications such as bumpers.
PE and PE blending performance
The effect of PE type on the impact performance of blending system
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 mass fraction of HDPE is less than 60%, the strength of the blend is basically unchanged; when the mass fraction of HDPE is higher than 60%, the impact strength of the blend increases.
For PP/LDPE blends, only when the mass fraction of LDPE is higher than 60%, the impact strength can be greatly improved.
For PP/LLDPE blends, when the mass fraction of LDPE is greater than 40%, the impact strength is significantly improved. When the mass fraction of LLDPE reaches 70%, the impact strength of the blend is 37.5kJ/m2, which can reach 20 times the impact strength of pure PP, which is 10 times and 4 times that of PP/HDPE and PP/LDPE blends with the same amount. .
At low temperature (-18°C), the trend of improvement of PP toughness by the three types of PE is the same as that at room 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 system 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 shows that when the same impact strength is achieved, the amount of LLDPE is the least, which means that the rigidity of PP can be maintained more; and at the same amount, the impact strength of LLDPE-modified PP is the best, which in turn makes the material obtain Better toughness.
Effect of mixing method on toughening effect
The impact strength of the sample mixed with the twin-screw extruder is the highest, and the impact performance of the sample obtained by the direct injection method is the worst. Since the effective length of the screw of the injection machine is smaller than that of the extruder, the shearing and mixing effect is small, and the effect is of course very poor. Under different mixing methods, the impact performance of the material exhibits the same law, that is, the mass fraction of LLDPE starts from 40%, and as the amount of LLDPE increases, its impact strength increases greatly; it shows that the mixing method has a significant impact on the impact performance of the blending system. Influence, but the law remains the same.
The internal structure of PP/LLDPE blend
When the mass fraction of LLDPE is less than 50%, the impact cross section of the blend system is smooth and flat, showing typical brittle fracture characteristics; when the mass fraction of LLDPE exceeds 50%, the material cross section exhibits ductile fracture characteristics, filaments appear, and the cross section is uneven. There are tearing marks and the two-phase interface tends to be blurred. At this time, the yield strength of the material rises rapidly; when the amount of LLDPE is increased to 70%, it can be clearly seen that the PP is interwoven into a network. Therefore, the material has a macroscopic view. Very high impact strength.
The size of pure PP spherulites is very large, and the interface between the spherulites is clear, so the impact performance of PP is extremely poor. In contrast, the crystals of LLDPE are very small, and the interface between the crystals is also very fuzzy, so its impact performance is very good.
The difference in the crystalline morphology of PP and LLDPE is caused by the difference in the crystallization rate of the two: PP has a slower crystallization rate (3.3X102nm/s), larger crystal growth, and fewer connections between crystals, so the crystal interface is clear; while LLDPE The crystallization rate is very fast (8.3X102nm/S), the crystals are small and there are many connections between the crystals, so the interface between the crystals is blurred.
When LLDPE is added to PP, it can be observed that the size of PP spherulites is obviously reduced, and the interface between the crystals becomes blurred, which is beneficial to improve the impact performance of the material. When the amount of LLDPE increases, the PP spherulites are further reduced. When the mass fraction of LLDPE reaches 70%, the PP crystals have been divided into smashed crystals, and the interface between the crystals disappears completely. It is mixed with LLDPE and is difficult to distinguish. Therefore, the blend system The impact strength is very high and it is not easy to be broken. This shows that the addition of LLDPE refines the spherulites of PP and increases the connection between the crystals, which is another important reason for the improvement of the toughness of the blended materials.
Influence of LLDPE dosage on blending effect
As the amount of LLDPE increases, the yield stress of the blend system decreases, while the elongation at break gradually increases, showing a good linear relationship. As the amount of LLDPE increases, the Vicat softening point of the blended material decreases. When the mass fraction of LLDPE is 40%-60%, the Vicat softening point of the blended material is still close to 120 degrees. As the amount of LLDPE increases, the impact strength of the material increases, while the tensile yield strength, tensile modulus, and Vicat softening point decrease.
In the LLDPE-based system, when the material is impacted, in addition to the LLDPE phase consumes a lot of energy and improves the toughness of the material, it also reduces the size of the PP crystal due to the insertion, segmentation and refinement of the PP spherulites by LLDPE. The number of connections between the crystals increases, thereby increasing the impact strength of the material. In the PP/LLDPE blend system, when the mass fraction of LL-DPE is 40%-70%, the blend gradually forms an interpenetrating network structure, which has the characteristics of rigidity and toughness.