Polypropylene is one of the most widely used varieties in general-purpose plastics. It has the advantages of low density, good rigidity, flex resistance, chemical resistance and good insulation. Its shortcomings are poor low temperature impact performance, easy aging, and large molding shrinkage. The low temperature impact properties, molding shrinkage and heat aging properties of polypropylene can be improved by modification. The use of polypropylene has been expanded to a large extent, replacing traditional engineering plastics in many applications. Polypropylene raw materials are abundant and cheap, so in recent years, polypropylene modification has taken the first place in the plastic modification industry, becoming the main variety of modified plastics, which has received more and more attention.
Shrinkage control of polypropylene modified materials is an important aspect of polypropylene modification. The good control of shrinkage rate is of great significance to the promotion and use of polypropylene modified materials, and it is also an important aspect to ensure product quality. In particular, the use of modified polypropylene to replace traditional engineering plastics, shrinkage is very important.
Polypropylene modification has mature technology in China, and there are many reports on the physical and chemical properties of polypropylene modification, but there are few reports on the shrinkage rate. I have gathered some years of practical experience to discuss the shrinkage rate control of polypropylene modified materials.
Test part
1.1 Test raw material polypropylene (PP) Liaoyang Petroleum Chemical Fiber Corporation; high density polyethylene (HDPE) Liaoyang Petroleum Chemical Fiber Corporation
POE DuPont Company of the United States; EPDM Netherlands DSM Company; SBS Yueyang Petrochemical Plant Glass Fiber Shanghai Yaohua; Calcium Carbonate Yingkou Dashiqiao; Talc Powder Haicheng Jinxin Mica Powder Hebei; Auxiliary Commercial; Low Density Polyethylene (LDPE) Yanshan Petrochemical
1.2 Test equipment and instrument extruder TM40MVC/D-40 Italy MARIS; Injection molding machine TP120T Beijing Xinguan Machinery Manufacturing Co., Ltd. Melt index instrument μPXRZ-400C Jilin University Science and Education Instrument Factory; Caliper; Detection method: ASTM D955
1.3 Sample preparation and testing methods Raw material mixing ---- extrusion granulation ---- injection molding (placement 24h) ---- shrinkage detection (ambient temperature is 23 ° C)
Injection conditions: temperature 170 ° C --- 190 ° C pressure 80
2. Results Discussion Shrinkage molding of polypropylene is a major drawback of polypropylene itself, mainly due to the high crystallinity of polypropylene. The polypropylene after crystallization has an increased specific gravity and a reduced volume. When the crystallinity is 0% and 100%, the specific gravity is 0.851 and 0.936, respectively. Therefore, the molding shrinkage of pure PP is generally between 1.7 and 2.2. The molding shrinkage of the controlled polypropylene is mainly to control the crystallinity of the raw material during molding: the smaller the crystallinity, the smaller the mold shrinkage ratio; on the contrary, the higher the crystallinity, the larger the mold shrinkage ratio.
In the polypropylene modified plastic, the original crystallinity of the polypropylene is changed to some extent due to the addition of various modifiers, thereby changing the original molding shrinkage of the polypropylene.
2.1 Effect of rubber on shrinkage of polypropylene Figure 1 shows the effect of rubber on the shrinkage of PP modified material. It can be seen from the figure that the molding shrinkage rate decreases with the increase of the rubber content. This is mainly because the addition of rubber destroys the crystallinity of the polypropylene itself, resulting in a decrease in the molding shrinkage rate, and the effects of the three elastomers POE, EPDM, and SBS on the molding shrinkage rate are also different.
a: POE b: EPDM c: SBS a: talc b: CaCO3 c: mica powder
2.2 Effect of mineral filling on molding shrinkage of polypropylene modified materials The mineral fillers used in polypropylene mainly include calcium carbonate, talc powder and mica powder. The effect of various mineral fillers on the shrinkage of polypropylene molding is shown in Fig. 2. It can be seen from the figure that the influence of mineral filler on the molding shrinkage of PP modified material is obvious. The influence of mineral filler on the shrinkage of polypropylene modified material has three main aspects: First, the mineral filler itself does not shrink, and its addition reduces the shrinkage of the polypropylene modified material from the overall proportion; The addition of mineral fillers will inevitably affect the crystallinity of polypropylene, thus affecting the shrinkage rate. Third, after the addition of fine mineral agents, it acts as a nucleating agent, changing the structural state of polypropylene, preventing large balls. The formation of crystals also affects the molding shrinkage of polypropylene.
2.3 Effect of glass fiber on molding shrinkage of polypropylene modified material Glass fiber has the greatest influence on the molding shrinkage of polypropylene modified material. As shown in Fig. 3, when the content of the glass fiber reaches 30% or more, the molding shrinkage of the polypropylene modified material decreases from 1.8 to 0.5, and the surface treated glass fiber has a greater influence on the molding shrinkage than the untreated. Glass fiber. The addition of glass fiber destroys the crystallinity of polypropylene, affects the shrinkage rate, and more importantly, the glass fiber limits the crystal shrinkage of polypropylene.
a: surface untreated b: surface treated a: LDPE b: HDPE
2. 4 Effect of the addition of polyethylene on the shrinkage of polypropylene molding Figure 4 shows the molding shrinkage and effect of polyethylene in polypropylene. It can be seen from the figure that the addition of polyethylene also affects the molding shrinkage of the polypropylene modified material. Although polyethylene is also a high crystallinity plastic, the molding shrinkage rate is also large, but after adding into the polypropylene, the respective crystallinity is destroyed to different degrees, and the overall molding shrinkage rate is lowered.
2.5 Effect of MFI change of polypropylene on molding shrinkage The molding shrinkage of polypropylene is affected by its crystallinity, which is affected by its own molecular weight. When the MFI is increased, the molecular weight is decreased, the crystallization rate is increased, and the molding shrinkage rate is increased. Figure 5 shows the effect of polypropylene MFI on the mold shrinkage.
2.6 Comparison of the effects of several modifiers on the shrinkage of polypropylene. The effects of various modifiers on the shrinkage of polypropylene modified materials are shown in Table 1. Various modifiers can be seen from the table. The effect on the shrinkage of the polypropylene modified material is different, and the glass fiber has the greatest influence.
The molding shrinkage of the polypropylene modified material is different due to the addition of the modifier, and the molding shrinkage rate is different. In the test, we found that several modifiers have different effects when used together, and there are many influencing factors. For example, the degree of dispersion of the modifier in the polypropylene, the particle size of the mineral modifier itself, and the surface treatment of various mineral fillers all have an effect on the molding shrinkage. In addition, the molding process of the injection molding machine has a great influence on the molding shrinkage rate during the molding process. Changes such as injection temperature and injection pressure have an effect on the molding shrinkage.
3. in conclusion
3.1 The molding shrinkage of polypropylene modified material is smaller than that of pure polypropylene, which is basically between 40% and 70% of pure polypropylene. Various modifiers have different effects on the molding shrinkage of polypropylene modified materials. The order of influence is roughly: glass fiber > mineral > elastomer > polyethylene.
3.2 Changing the crystallinity of polypropylene is the key to controlling the shrinkage of polypropylene modified materials.
3.3 The control of molding shrinkage is a complicated process. To ensure the quality of the product and to control the shrinkage rate, several aspects must be considered at the same time.
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