Polyethylene and polypropylene raw materials production process is relatively similar, the products can be used to make plastic films, injection products, plastic pipe, etc., in many cases we found that the two kinds of raw materials in the nature and use of a great similarity, but in fact , Polypropylene raw materials and polyethylene raw materials in the use or there are many differences, Xiaobian to you to analyze the performance characteristics of polypropylene and polyethylene, to explore the two different proportions of materials mixed performance differences.
PE and PE performance differences
From the heat resistance point of view, polypropylene heat resistance is higher than that of polyethylene, usually, polypropylene melting temperature higher than polyethylene about 40% -50%, about 160-170 ℃, so the product can Sterilization at a temperature above 100 ℃, no deformation under the conditions of 150 ℃. In life we will find '5' polypropylene lunch boxes are often used in microwave oven heating food (microwave heating The general temperature of 100-140 ℃), and polyethylene due to poor heat resistance can not be used as a microwave oven plastic, including lunch boxes, cling film Similarly, in the field of ordinary packaging film, polyethylene bags more suitable for 90 ℃ below the use of polypropylene bags at relatively high temperature is also possible.
From the perspective of rigidity and tensile strength, polypropylene is mainly characterized by its small density, superior mechanical properties to polyethylene, and outstanding rigidity. For example, polypropylene has gradually started to compete with engineering plastics (PA / PC) Widely used in electrical and electronic fields, automotive. At the same time due to high tensile strength of polypropylene, and then bending resistance is good, known as the '100% plastic', bending 1 million bend is not white, which is also for our identification Polypropylene products provide clues and become a tactile indicator of product recycle classification.
From the perspective of low temperature resistance, polypropylene is less resistant to low temperature than polyethylene, impact strength at 0 ℃ only half of 20 ℃, while the brittle temperature of polyethylene up to -50 ℃ generally below; and with the relative molecular mass Increase, the lowest up to -140 ° C. Therefore, if the products need to be used in low temperature environment, or to try to choose polyethylene as raw materials.Generally refrigerated food tray is made of polyethylene raw materials.
From an anti-aging point of view, polypropylene is less weatherable than polyethylene. The structure of polypropylene is similar to that of polyethylene, but due to its presence of a methyl-branching side branch, it is more susceptible to UV light and heat Oxidative Degradation The most common aging-prone polypropylene products in everyday 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, Other raw materials, its performance is not very prominent, because the polyethylene molecules contain a small amount of double bonds and ether bonds, poor weather resistance, sun, rain can cause aging.
From the perspective of flexibility analysis, although the higher strength polypropylene, but poor flexibility, technical point of view is poor impact resistance.So when used to make film products, its applications and polyethylene applications There are still differences, polypropylene film is more used for surface packaging printing.In the pipe, but also rarely use simple polypropylene production, the need to use cross-linked polypropylene, which is commonly known as PPR tube. Common polypropylene impact resistance is poor, easy to break, so in the practical application to join the impact modifier, in the bumper and other applications should use additives to improve the impact resistance.
PE and PE blend performance
Impact of PE Type on Impact Property 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 blends remains unchanged. When the mass fraction of HDPE is higher than 60%, the impact strength of blends increases.
For PP / LDPE blends, only when the mass fraction of LDPE is higher than 60%, the impact strength increases only a little.
As for the blend of PP / LLDPE, the impact strength of LDPE is obviously improved when the mass fraction of LDPE is more than 40% .The impact strength of the blend is 37.5kJ / m2 when the content of LLDPE is 70% The impact strength of 20 times the same amount of PP / HDPE and PP / LDPE blends of 10 times and 4 times.
Under the low temperature (-18 ℃), the trend of the improvement of PP toughness of the three kinds of PE is the same as that under normal temperature, or the toughening effect of LLDPE on PP is the best.When the PP / LLDPE mass ratio is 30/70, The impact strength is 23.2kJ / m2, which is 20 times of that of pure PP, while under the same conditions, the impact strength of PP / HDPE and PP / LDPE blend system is only about 5kJ / m2, which further shows that when the same impact strength is reached, The least amount of LLDPE means that the rigidity of the PP can be more maintained. At the same dosage, the impact strength of the LLDPE-modified PP is the best, which in turn gives the material more excellent toughness.
Effect of mixing method on toughening effect
The impact strength of the sample mixed with twin-screw extruder is the highest, and the impact performance of the sample obtained by the direct injection method is the worst.With the effective length of the screw of the injection machine is smaller than that of the extruder, the effect of shear mixing is very poor Under different mixing modes, the impact properties of the materials showed the same rules, that is, the content of LLDPE increased from 40%, and the impact strength of LLDPE increased sharply with the increase of the dosage of LLDPE. Affect, but the law unchanged.
Internal structure of PP / LLDPE blends
When the content 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 content of LLDPE exceeds 50%, the fracture surface of the material presents as ductile fracture, There is tear traces, and the two-phase interface tends to blur, at this time, the yield strength of the material increased rapidly; and when the amount of LLDPE increased to 70%, it can clearly be seen PP intertwined with each other, therefore, the material has macroscopically High impact strength.
The pure PP spherulites are large in size and the interface between the spherulites is clear, so the impact properties of PP are very poor. In contrast, the crystals of LLDPE are very small and the interface between the crystals is very vague, so the impact performance is very good it is good.
PP and LLDPE crystallization morphology difference is due to the different crystallization rate caused by the two: PP crystallization rate is slower (3.3X102nm / s), the crystal growth is large, less inter-crystal connections, so intergranular interface clear; and LLDPE The crystallization rate is very fast (8.3X102nm / S), the crystal is small, more connections between the crystals, so the interfacial interface blurred.
When LLDPE was added into PP, the spherulite size was obviously reduced and the interfacial interface became obscure, which could improve the impact properties of the material.When LLDPE content increased and PP spherulites decreased further, when the LLDPE content reached 70%, the PP crystal has been split into broken crystals, the interface between crystals completely disappeared, and LLDPE mixed together, it is difficult to distinguish, therefore, the impact strength of the blend system is high, not easily be punctured. This shows that the LLDPE The addition of refined spherulites of PP increases the inter-crystal connection, which is another important reason for the improved toughness of the blend.
Effect of LLDPE dosage on blending efficiency
With the increase of the amount of LLDPE, the yield stress of the blends decreased, while the elongation at break increased gradually, and showed a good linear relationship.With the increase of the amount of LLDPE, the Vicat softening point of blends decreased.When LLDPE mass fraction was 40% -60%, the Vicat softening point of the blended material is still close to 120. With the increase of the amount of LLDPE, the impact strength of the material increases, while the tensile yield strength, tensile modulus, Vicat softening point decrease.
In the LLDPE-based system, when the material is impacted, in addition to LLDPE phase consume a lot of energy to improve the toughness of the material, but also due to LLDPE PP spherulite insertion, segmentation and refinement, the PP crystal size decreases, The increase of the inter-crystal connection and the increase of the impact strength of the material.In the blends of PP / LLDPE, when the mass fraction of LL-DPE is 40% -70%, the blends gradually form interpenetrating network structure with rigid and tough properties .