Polypropylene is one of the most commonly used varieties in general plastics. It has the advantages of small density, good rigidity, resistance to flexure, chemical resistance, and good insulation. Its disadvantages are poor low-temperature impact performance, aging, and high molding shrinkage. The modification can improve the low temperature impact properties, molding shrinkage and heat aging properties of polypropylene. The use of polypropylene has greatly expanded, replacing traditional engineering plastics in many applications. Polypropylene raw material sources are abundant, and the price is cheap. Therefore, in recent years, the polypropylene modification industry has occupied the first place in the modification of polypropylene, and has become the main varieties of modified plastics, and has received increasing attention from people.
Control of the shrinkage of polypropylene modified materials is an important aspect of polypropylene modification. The good control of the shrinkage rate has important significance for the popularization and application of polypropylene modified materials, and it is also an important aspect of ensuring product quality. In particular, the use of modified polypropylene to replace traditional engineering plastics, the shrinkage of this point is very important.
Polypropylene modification has mature technology in China, and there are many reports on the physical and chemical properties of polypropylene modification. However, there are few reports on the shrinkage problem. I have gathered many years of practical experience on polypropylene shrinkage control of the modified material to do some research.
Test section
1.1 Test Raw Material Polypropylene (PP) Liaoyang Petroleum Chemical Fiber Corporation; High-density Polyethylene (HDPE) Liaoyang Petroleum Chemical Fiber Corporation
POE USA DuPont Company; EPDM Dutch DSM Company; SBS Yueyang Petrochemical Plant Glass Fiber Shanghai Yaohua; Calcium carbonate Yingkou Dashiqiao; Talc powder Haicheng Jinxin Mica Powder Hebei; Auxiliaries available; Low Density Polyethylene (LDPE) Yanshan Petrochemical
1.2 Test Equipment and Instrument Extruder TM40MVC/D-40 Italy MARIS; Injection Molding Machine TP120T Beijing Xinguan Mechanical Equipment 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 Detection Methods Raw Material Mixing ---- Extrusion Granulation ---- Injection Proofing (Placed for 24h) - Shrinkage Detection (Ambient Temperature is 23°C)
Injection molding conditions: temperature 170 °C --- 190 °C pressure 80
2. Results Discussion The large shrinkage of polypropylene is a major drawback of polypropylene itself, which is mainly due to the high crystallinity of polypropylene. Crystallized polypropylene increases its specific gravity and reduces its volume. The specific gravity was 0.851 and 0.936 when the crystallinity was 0% and 100%, respectively. Therefore, the molding shrinkage of pure PP is generally between 1.7---2.2. Control the molding shrinkage of polypropylene is mainly to control the crystallinity of its raw material molding: the smaller the crystallinity, the smaller the molding shrinkage; otherwise, the higher the crystallinity, the greater the molding shrinkage.
In the polypropylene modified plastics, due to the addition of various modifiers, the original crystallinity of the polypropylene was destroyed to varying degrees, 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 molding shrinkage of PP modified material. It can be seen from the figure that with the increase of the rubber content, the molding shrinkage rate shows a decreasing trend. This is mainly due to the fact that the addition of rubber destroys the crystallinity of polypropylene itself, resulting in a decrease in molding shrinkage, and the effects of the three elastomers POE, EPDM, and SBS on molding shrinkage 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 polypropylene mineral fillers include calcium carbonate, talcum powder and mica powder. The effect of various mineral fillers on the molding shrinkage of polypropylene is shown in Figure 2. It can be seen from the figure that the effect of mineral fillers on the molding shrinkage of PP modified materials is obvious. The effect of mineral fillers on molding shrinkage of polypropylene modified materials is mainly in three aspects: First, the mineral filler itself does not shrink, and its addition reduces the shrinkage rate of the polypropylene modified material from the overall ratio; The addition of mineral fillers will inevitably affect the crystallinity of polypropylene, thus affecting the shrinkage rate; Third, after the addition of fine minerals, 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 The effect of glass fiber on molding shrinkage of polypropylene modified material Glass fiber has the greatest impact on 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 is decreased from 1.8 to 0.5, and the surface treated glass fiber has greater influence on the molding shrinkage than without treatment. Glass fiber. The addition of glass fiber destroys the crystallinity of polypropylene and affects the shrinkage rate. More importantly, glass fiber limits the crystallization shrinkage of polypropylene.
a: surface untreated b: surface treated a: LDPE b: HDPE
2.4 Effect of Polyethylene Addition on the Molding Shrinkage of Polypropylene Figure 4 shows the effect of polyethylene addition on the shrinkage and impact of PP. 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 is also large, but after adding polypropylene to each other to different degrees of destruction of their respective degree of crystallinity, so that the overall molding shrinkage decreased.
2.5 The effect of polypropylene's own MFI change on molding shrinkage The molding shrinkage of polypropylene is affected by its crystallinity, which in turn is affected by its own molecular weight. When the MFI increases, the molecular weight decreases, the crystallization rate increases, and the molding shrinkage increases. Figure 5 shows the impact of polypropylene MFI on the molding shrinkage.
2.6 Comparison of the effects of several modifiers on the molding shrinkage of polypropylene The effect of various modifiers on the molding shrinkage of polypropylene modified materials is shown in Table 1. Various modifiers can be seen from the table. The effect on the molding shrinkage of polypropylene modified materials is different, and glass fiber has the greatest impact.
Polypropylene modified material molding shrinkage due to the addition of the modifier is not the same, molding shrinkage is not the same. In the experiment, we found that the effect of several modifiers is not the same 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 conditions of various mineral fillers all influence the molding shrinkage. In addition, the molding process of the injection molding machine during the molding process has a great influence on the molding shrinkage. Changes in injection temperature and injection pressure have an impact on mold shrinkage.
3. in conclusion
3.1 Polypropylene modified material molding shrinkage is smaller than pure polypropylene, basically between 40% - 70% of pure polypropylene. Various modifiers have different effects on 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 molding shrinkage of polypropylene modified materials.
3.3 Molding Shrinkage control is a complex process. It is necessary to ensure product quality and control the shrinkage rate. Several aspects need to be considered at the same time.
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