Various inorganic fillers, such as glass fiber, talcum powder, calcium carbonate, mica powder, wollastonite, barium sulfate, graphite, carbon fiber, etc. Used for filling and strengthening modification of polypropylene; Elastomers such as POE and EPDM, polyethylene such as HDPE and LLDPE toughen polypropylene to improve its comprehensive properties. Filling, toughening, strengthening and other technologies are the main modification methods of polypropylene at present, and also become an important method to control the shrinkage of polypropylene. Adding inorganic filler to resist the shrinkage of polypropylene through the structure of inorganic filler. The other is to add a component to interweave the molecular chains of the two components, change the crystallization of polypropylene, and achieve the purpose of controlling polymer shrinkage.
1* * mixed modification
* * * Blending modification refers to mixing one or more plastics or elastomers in the original plastic system through various mixing methods by using compatibility or reaction * * *, and finally forming a new material with macroscopic uniformity and microscopic phase separation. There are many materials used for mixed modification of PP***, such as thermoplastic elastomer POE, EPDM, SBS, and polyethylene HDPE and LLDPE. When the corresponding plastics or elastomers are added to polypropylene, foreign molecular chains will disturb the crystallization of polypropylene to varying degrees and reduce the shrinkage of polypropylene.
Wang et al. [1] mixed POE with PP***, and studied the effects of different kinds of POE on the shrinkage of PP. The results showed that ethylene-octene * * * polymer had a greater influence on the crystallinity of PP than ethylene-butene * * * polymer, showing a lower shrinkage. This is because the longer the side chain segment of POE, the stronger the winding effect on PP molecular chain and the stronger the ability to limit PP crystallization, so the smaller the shrinkage rate.
In addition, Ning Kaijun and others [2] also studied the effects of POE and POP (propylene-based elastomer) on the shrinkage of polypropylene modified by talcum powder. In the two * * * mixed systems, with the increase of the amount of elastomer, the shrinkage of the system decreases gradually. When the mass fraction of POE is 20%, the modification effect of POE is better than that of POP, and the shrinkage of * * * mixed system decreases to 0.65%~0.77%, which may be attributed to their compatibility with PP matrix. The compatibility of propylene-based elastomer POP with PP is better than that of POE, so the better the compatibility, the smaller the decrease of PP shrinkage, and the more complex the dispersed phase, the greater the contribution to reducing PP shrinkage.
Luo Zhongfu et al [3] used talcum powder as filler and POE and PE as modifiers, and studied the influence of the amount of modifiers on shrinkage. The results show that LLDPE has greater influence on shrinkage than HDPE, which may be due to the greater influence of LLDPE on the crystallization behavior of polypropylene. With the increase of POE content, the shrinkage of PP decreased gradually. When the POE content is 15%, the shrinkage of PP decreases to about 0.9%.
Li Rongqun et al. [4] disclosed a modified polypropylene composite with high gloss and low shrinkage and its preparation method. Low shrinkage modified polypropylene was prepared by alloying low shrinkage polystyrene and polypropylene. However, due to the great structural difference between PP and PS, it is easy to delaminate, which leads to poor product stability. At the same time, PS and PP have poor weather resistance, so the weather resistance is low.
2 Fill in the modification
Inorganic powders used for PP filling modification mainly include talcum powder, calcium carbonate, barium sulfate, mica powder, wollastonite and so on. Inorganic powder not only reduces the cost, but also improves the comprehensive properties of materials, such as hardness, strength and thermal deformation temperature. , and has obvious influence on the shrinkage of PP, mainly in three aspects: first, the inorganic filler itself does not shrink, and its addition reduces the molding shrinkage of PP as a whole; Second, the addition of filler reduces the crystallinity of PP, thus reducing the shrinkage; Thirdly, the addition of fine inorganic filler plays the role of nucleating agent, which changes the morphology of polypropylene and prevents the formation of larger spherulites.
Hui et al. [5] studied the factors affecting the shrinkage of PP with different forms of inorganic fillers. The results show that mineral powder can well limit the shrinkage of composites, and flake talcum powder and needle-like wollastonite have more obvious restrictions on the shrinkage of PP than granular calcium carbonate. The smaller the particle size of a single mineral, the smaller the shrinkage of the composite. With the increase of mineral filler content, the shrinkage of composites decreases. When the mass fraction of talcum powder is 30%, the shrinkage rate is 0.768%.
Yang Lichen [6] also studied the effect of inorganic rigid particles on the shrinkage of modified polypropylene. Different fillers have different effects on the shrinkage of modified PP, which is mainly due to the different structures of different fillers. Talcum powder and mica powder are both flaky structures. During the molding process, the molecular chains will be aligned with the thin layers of talcum powder and mica powder in a certain direction. The lamellar structure limits the orientation shrinkage. Wollastonite has a needle-like structure, and its orientation arrangement is relatively small during the molding process, so its influence on shrinkage is less than that of talcum powder, while calcium carbonate has a granular structure, and its molecular chain will not be oriented during the molding process, so its influence on shrinkage is relatively small.
Zhou Chunhuai et al. [7] used inorganic fillers to enhance the rigidity of materials and reduce the shrinkage rate, and used activated superfine heavy calcium carbonate and talcum powder for reinforcement for the first time. The conclusions are as follows: when 20% talcum powder and calcium carbonate are used alone, the shrinkage of the material is 0.82% and 0.87% respectively, while when 10% talcum powder and 10% calcium carbonate are used as composite fillers, the shrinkage of the material is 0.87%.
When Liu et al. [8] studied the effect of talcum powder on the shrinkage of polypropylene/talcum powder composites, it was found that with the increase of talcum powder content, the shrinkage of the materials gradually decreased; Under the same conditions, the smaller the particle size of talcum powder, the lower the shrinkage of the material. By compounding two kinds of talcum powder with different particle sizes, it is concluded that when the total addition is 27.5% and the ratio of the two kinds of talcum powder is 1:2, the shrinkage of the material is the lowest, reaching 0.556%. When it is a single coarse particle or fine particle, the gap between particles is large, forming a so-called "hole effect" and the packing density becomes smaller; Coarse particles form a larger cavity, and the shrinkage rate of the material is larger during molding. When coarse and fine particles are compounded, the gap between coarse particles is filled with fine particles, forming a so-called "secondary filling effect", which increases the density of filler and reduces the overall shrinkage.
Xinya Zhang et al. [9] introduced powdered polyethylene and inorganic filler talcum powder into polypropylene matrix, and at the same time added calcium salt whisker through side feeding, powdered polyethylene obviously improved the dispersion ability of inorganic filler in the system, and then improved the fluidity of polypropylene composite materials; The use of filler with smaller particle size improves the defect of insufficient impact toughness of materials. Adding calcium salt whiskers into the side feed keeps the original aspect ratio and gives full play to the ability of calcium salt whiskers to reduce shrinkage.
3. Strengthen modification
Glass fiber has the greatest influence on the molding shrinkage of polypropylene modified materials. When the content of glass fiber reaches more than 30%, the molding shrinkage of polypropylene modified material decreases from 65438 0.8% to 0.5%, and the influence of surface-treated glass fiber on molding shrinkage is greater than that of untreated glass fiber. On the one hand, the addition of glass fiber destroys the crystallinity of polypropylene and affects the shrinkage. More importantly, glass fiber limits the crystallization shrinkage of polypropylene.
When Chen Yanan et al. [10] studied the special material for automobile bumper, chopped flat glass fiber was used to replace part of superfine talcum powder, which not only significantly improved the rigidity of polypropylene composite, but also significantly reduced the shrinkage and post-shrinkage. Moreover, because the section of flat glass fiber is flat, the fiber as a whole presents a flaky structure similar to talcum powder, and its fluidity is much higher than that of ordinary glass fiber with circular section. There is no floating fiber on the surface of the part, and the internal distribution of the part tends to be isotropic during injection molding, so that ordinary glass fiber will not warp.
prospect
Low shrinkage modified polypropylene is gradually applied to modern industry because of its excellent properties. By selecting suitable raw materials, toughening, filling, strengthening and other modification methods, modified polypropylene with different low shrinkage can be controlled and prepared. In order to apply low shrinkage modified polypropylene to more fields, there are special requirements for modified polypropylene, such as high gloss, high fluidity, high hardness, antistatic, high heat resistance and high impact resistance, which will continue to be the research and development direction of technicians in the future.
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