Polypropylene is one of the most widely used varieties of general plastics, which has the advantages of low density, good rigidity, deflection resistance, chemical corrosion resistance and good insulation. Its shortcomings are poor low temperature impact performance, easy aging, molding shrinkage. The low temperature impact properties, forming shrinkage and thermal aging properties of polypropylene can be improved by modification.
The use of polypropylene is greatly expanded, and the traditional engineering plastics are replaced in many occasions. The source of polypropylene raw materials is sufficient and the price is cheap, so in recent years, polypropylene modification occupies the first place in the plastic modification industry, becoming the main variety of modified plastics, and more and more people's attention.
The shrinkage control of polypropylene modifier is an important aspect of polypropylene modification. Good shrinkage control is of great significance to the promotion and use of polypropylene modifier, and it is also an important aspect to ensure product quality. In particular, using modified polypropylene to replace traditional engineering plastics, the shrinkage rate is very important.
Polypropylene modification technology has been mature in China, and there are many reports on the physicochemical properties of polypropylene modification, but there are few special reports on the shrinkage rate. Based on many years of practical experience, I have made some discussion on the shrinkage control of polypropylene modified materials.
1. Test part
1.1 Test materials
Polypropylene (PP) Liaoyang Petroleum Chemical Fiber Corporation;
High density polyethylene (HDPE) Liaoyang Petroleum Chemical Fiber Corporation
POE DuPont, USA;
EPDM DSM, Netherlands;
SBS Yueyang Petrochemical Complex
Glass fiber Shanghai Yaohua;
Calcium carbonate Yingkou Dashiqiao;
Talc powder Haicheng Gold new
Mica powder Hebei;
Low density polyethylene (LDPE) : Yanshan Petrochemical
1.2 Test equipment and instruments
Extruder TM40MVC/D-40 Italy MARIS;
Injection molding machine TP120T Beijing Xinguan Machinery Equipment Manufacturing Co., LTD
Melt index instrument μPXRZ-400C Jilin University Science and Education Instrument Factory;
Calipers;
Test method ASTM D955
1.3 Sample preparation and testing methods
Raw material mixing ---- extruded granulation ---- injection molding proofing (24h) ---- shrinkage detection (ambient temperature: 23℃)
Injection molding conditions: temperature 170℃ -190℃ pressure 80
2. Result discussion
The shrinkage molding of polypropylene is a major shortcoming of polypropylene itself, which is mainly due to the high crystallinity of polypropylene. After crystallization, the specific gravity of polypropylene increases and the volume decreases. 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. Controlling the molding shrinkage of polypropylene is mainly to control the crystallinity of its raw materials when forming: the smaller the crystallinity, the smaller the molding shrinkage; Conversely, the higher the crystallinity, the greater the shrinkage of the molding.
In polypropylene modified plastics, due to the addition of various modifiers have destroyed the original crystallinity of polypropylene to varying degrees, thus changing the original molding shrinkage of polypropylene.
2.1 Influence of rubber on the shrinkage of polypropylene
With the increase of rubber content, the shrinkage rate of molding decreased. This is mainly due to the addition of rubber to destroy the crystallinity of polypropylene itself, resulting in a decline in the molding shrinkage, and the three elastomers POE, EPDM, SBS also have different effects on the molding shrinkage.
2.2 Influence of mineral filling on molding shrinkage of polypropylene modified material
The mineral additives used in polypropylene mainly include calcium carbonate, talc powder, mica powder and so on. The effect of various mineral additives on the shrinkage of polypropylene molding can be seen that the effect of mineral additives on the shrinkage of PP modification is obvious.
There are three main aspects of the effect of mineral filler on the shrinkage of polypropylene modified material: first, the mineral filler itself does not shrink, and its addition reduces the shrinkage of polypropylene modified material from the overall proportion; Secondly, the addition of mineral additives will inevitably affect the crystallinity of polypropylene, thus affecting the shrinkage rate; Third, after the fine mineral agent is added, it plays the role of a nucleating agent, changes the structural state of polypropylene, prevents the formation of large spherulites, and also affects the molding shrinkage rate of polypropylene.
2.3 Influence of glass fiber on molding shrinkage of polypropylene modified material
Glass fiber has the greatest influence on the shrinkage of polypropylene modified material. When the glass fiber content reaches more than 30%, the molding shrinkage of the polypropylene modified material decreases from 1.8 to 0.5, and the surface treated glass fiber has a greater effect 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 crystallization shrinkage of polypropylene.
2.4 Influence of the addition of polyethylene on the shrinkage of polypropylene molding
The addition of polyethylene also affects the molding shrinkage of polypropylene modified material. Although polyethylene is also a kind of high crystallinity plastic, the molding shrinkage rate is also very large, but after adding polypropylene to each other to varying degrees of destruction of the respective crystallinity, so that the overall molding shrinkage rate decreased
2.5 Influence of MFI variation of polypropylene on molding shrinkage
The molding shrinkage of polypropylene is affected by its crystallinity, which in turn is affected by its own molecular weight. When MFI increases, the molecular weight decreases, the crystallization rate increases, and the shrinkage rate increases. Effect of polypropylene MFI variation on molding shrinkage.
2.6 Comparison of the effects of several modifiers on the shrinkage of polypropylene molding
The influences of various modifiers on the molding shrinkage of polypropylene modified materials are shown in Table 1. It can be seen from the table that the influences of various modifiers on the molding shrinkage of polypropylene modified materials are different, and the glass fiber has the greatest influence.
The molding shrinkage of polypropylene modifier is different due to the addition of different modifiers, the molding shrinkage is different. In the test, we found that the effects of several modifiers are not the same, and there are many influencing factors.
Such as the degree of dispersion of the modifier in polypropylene, the particle size of the mineral modifier itself, the surface treatment of various mineral additives, etc., all have an impact on the molding shrinkage. In addition, the molding process of the injection molding machine also has a great impact on the molding shrinkage. For example, the change of injection temperature and injection pressure has an impact on the molding shrinkage.
3. 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. The effect of various modifiers on the forming shrinkage of polypropylene modifiers is different. 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 Control of molding shrinkage rate is a complex process, both to ensure product quality, but also to control shrinkage rate, need to consider several aspects at the same time.
