Modification of the hottest unsaturated polyester

2022-08-09
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Unsaturated polyester composite modified

unsaturated polyester resin (UPR) has good mechanical properties, electrical properties and chemical resistance, and its raw materials are easy to obtain and cheap. Its composites are widely used in transportation, building materials, electronics and other industries, and have developed rapidly in the world in the past 20 years. With the development of science and technology and the needs of various applications, the requirements for the properties of UPR composites are higher and higher, which also promotes its development and application. This paper briefly introduces several newly developed and applied UPR composites

1. Surface fluorination modification of UPR composites

fluorinated materials have unique chemical inertia, solvent resistance, high temperature resistance, good barrier property, low friction coefficient and surface tension. After adding fluorinated materials, thermosetting resin has excellent corrosion resistance, water resistance and solvent resistance

1.1. Copolymerization of UPR and fluorine-containing monomers

adding fluorine-containing low molecular weight comonomers during UPR synthesis can significantly improve surface properties, but usually the amount of fluorinated copolymer monomers is relatively large, and the price is high, and the resulting materials are very expensive. In the synthesis of UPR, PFPE with hydroxyl end group can be used as comonomer to obtain fluorine modified UPR, and the toughness of fluorine modified UPR (fupr) containing low molecular weight PFPE is significantly improved. Pilati uses perfluoropolyether (PFPE) with active end groups to prepare multiblock copolymers with fluorinated blocks. When the amount of PFPE is small, the PFPE segments in polyether PFPE multiblock copolymer can migrate to the surface to obtain a fluorine rich surface. The main disadvantage of this method is that after the polymerization reaction, part of PFPE has not reacted, which will cause waste

1.2. Blending UPR and fluorinated block copolymer

blending block copolymer containing fluorinated segments with UPR, fluorinated block copolymer can preferentially migrate to the surface, and most fluorides will be concentrated on the surface, which can reduce the amount of fluorinated polymer. Poly e-caprolactam perfluoropolyether poly e-caprolactam block copolymer (pcl-pfpe-pcl) for short, the PCL block in TXC has good compatibility with PVC, ABS, San, PC and unsaturated polyester. Pcl-pfpe-pcl block copolymer can be used to form a quasi IPN system, that is, UPR and pcl-pfpe-pcl form a three-dimensional complex. Adding this material to UPR matrix can improve surface properties and mechanical properties and reduce water absorption. The molecular weight and curing conditions of PCL and PFPE segments affect the morphology and modification effect of the composites. When the amount of txcl copolymer is less than 5% (mass), the uncured txcl/upr system is completely transparent. Compared with PFPE, the compatibility between txcl and UPR is greatly improved, but if the content of txcl continues to increase, their compatibility will be reduced. Due to thermodynamic reasons, fluorinated polymers are incompatible with non fluorinated polymers, and the interfacial adhesion is very poor, so the mechanical properties are low. The compatibility or miscibility between the block copolymer and the matrix polymer can be controlled by selecting the appropriate type and length of the non fluorinated block copolymer

2. Interface modification of upr/glass fiber composites

most unsaturated polyester (UPR) resins are reinforced with glass fibers. The mechanical properties of the composites are determined by the fiber, matrix and fiber matrix interface adhesion. The adhesion between fiber and matrix may be affected by the following factors: adsorption, wetting, physical adhesion, interface diffusion, electrostatic attraction and chemical bonds. The strength of interfacial adhesion affects the final properties of composites. If the adhesion between the matrix and reinforcement in the composite is low, the stress cannot be effectively transferred from the matrix to the reinforcement phase, resulting in premature fracture at the weak interface. Therefore, the research of glass fiber reinforced unsaturated polyester mainly focuses on the behavior characteristics of the interface, interface modification and the effect of fiber matrix interface on the mechanical properties of composites

2.1 adding silane coupling agent as surfactant

silane coupling agent is usually used to improve the adhesion between UPR and filler or reinforcing material. On the one hand, the coupling agent reacts with glass fiber, on the other hand, it is coupled with matrix resin through one or more reaction groups. Soft polysiloxane can also be grafted to improve the compatibility between glass fiber and unsaturated polyester, which greatly improves the adhesion between glass fiber and resin. For Lee γ- Methyl propylene trimethoxysilane( γ- MPS) as a surface modifier, the surface energy of glass fiber is reduced, the number of pores of glass fiber/upr composite is reduced, the bending strength and bending modulus are improved, and the poor bending performance of the inlet and outlet parts of the mold is reduced

2.2. Improve fiber wettability

fiber wettability is an important factor in RTM Processing. If the wettability of the fiber is poor, it is easy to form pores, and the fiber matrix adhesion is low, resulting in the reduction of mechanical properties and surface quality. The number of pores in RTM composites is affected by the following factors: the original bubbles in the matrix resin, the pores formed during resin injection, and the growth and contraction of pores during curing. The number of pores is also affected by the macro flow and micro flow of liquid resin. Micro flow is related to surface tension and capillary pressure. In order to produce good micro flow, it is necessary to ensure good wettability of fibers, so as to make the composites have less pores and good mechanical properties. Especially in the case of low pressure, the capillary pressure caused by the surface tension of the matrix becomes more and more important

2.3. Improve the flexibility of unsaturated polyester

rot the effect of interfacial properties of glass fiber unsaturated polyester resin on the tensile properties of composites was studied. The unsaturation of unsaturated polyester and the amount of linear glycol ether have a great influence on the tensile strength. The lower the unsaturation and the higher the content of linear glycol ether, the better the flexibility, which is more conducive to improving the adhesion between UPR and glass fiber and improving the tensile strength of the composite. The improvement of interfacial adhesion may be due to the reduction of the spatial hindrance of UPR and the improvement of the interaction between glass fiber and matrix

3. Low shrinkage modification of UPR composites

the volume shrinkage of unsaturated polyester during curing is about 5% - 10%, which seriously affects the warpage resistance, dimensional accuracy, stress cracking resistance and surface smoothness of FRP products. Therefore, the study of low shrinkage or non shrinkage UPR has become an important research direction

3.1. Low shrinkage additive

the study found that adding low shrinkage additive (LPA) to UPR can reduce shrinkage. LPA is some thermoplastic polymers such as polystyrene, polymethylmethacrylate, polyvinyl acetate, thermoplastic polyurethane and polyester. The type, relative molecular weight and concentration of LPA have certain effects on the shrinkage, surface quality and size control of molded polymer materials of unsaturated polyester resin. The results show that in order to obtain better shrinkage control, LPA with high molecular weight is better than LPA with low molecular weight, and the content of LPA should not be too high. LPA is very effective in reducing the shrinkage of unsaturated polyester in high temperature molding processes such as sheet molding, bulk molding, injection molding, etc

3.2 factors affecting the shrinkage effect

most LPA is used in high temperature and high pressure processes, and its shrinkage reduction effect is related to the following factors, such as thermal expansion, phase separation, phase transition between LPA and cured UPR resin, pore structure formed along the interface or in the LPA phase, etc. In the process of molding, the mixture is first heated to the temperature of the model, the unsaturated polyester resin and LPA produce thermal expansion, the initiator decomposes and triggers the free radical copolymerization of UPR molecule and styrene monomer, and at the same time, it carries out cooperative radical copolymerization with China Academy of space technology. The increase of molecular weight and the change of polarity of the reactive UPR resin make the original homogeneous system become locally heterogeneous, UPR tends to phase separation, and the thermal expansion produced by LPA partially compensates for the shrinkage of the polymerization process. As the curing process continues, UPR becomes a continuous phase and LPA becomes a dispersed phase. After curing, pore structure or microcrack structure is formed, and the pores generated by such pores or microcracks can make up for the shrinkage of UPR curing

3.3 low shrinkage control during high temperature processing

in the high temperature sheet molding (SMC) process, the mixture is first heated from room temperature to the temperature of the model, generally 150 ℃, then the strong reaction temperature rises to about 200 ℃, after reaching the exothermic peak, the temperature decreases, and finally the parts are cooled during demoulding. The temperature changes greatly during the whole molding process. The volume change of the corresponding molding material is divided into three stages: thermal expansion during heating; Shrinkage during curing; Further thermal shrinkage during demoulding and cooling. To obtain the most effective shrinkage control in the processing process, LPA should have large thermal expansion during heating and reaction, and form micropores during cooling

3.4. Low shrinkage control in low-temperature processing

at present, processing methods such as low-pressure/low-temperature sheet molding (SMC), resin transfer molding (RTM) and vacuum injection liquid composite molding, such as Seemann composite resin injection molding (scrimp), are receiving more and more attention. The thermal history of resin processed at low temperature is completely different from that processed at high temperature. LPA has different properties in different processing processes. When processing at room temperature, there is almost no temperature change in the curing process, and the curing cycle is usually relatively long, so the thermal expansion of LPA does not play a role in the low-temperature molding process

4. Natural fiber reinforcement

compared with traditional matrix fillers such as glass fiber and carbon fiber, natural fiber has the advantages of low price, renewable, biodegradable and so on. It is more and more used in the reinforcement of thermosetting and thermoplastic resins. However, except for cotton fiber, most natural fibers are mainly used in traditional products such as production lines, ropes, mats, clothing and indoor decorations, and have not been reasonably used. In recent years, the traditional application of natural fibers has been threatened by plastics and synthetic fibers such as glass fibers and nylon fibers

4.1. Advantages of cotton fiber reinforcement

cotton fiber/unsaturated polyester composites have good bending strength and bending modulus. People use natural fibers as reinforced polymer matrix, such as using natural fibers to produce plastic composites to make roofs, plates, food or grain storage warehouses and cheap household products. Natural fiber reinforced composites are regarded as promising and low-cost building materials

4.2. Treatment of cotton fiber

after breaking up, cotton fiber is alkalized with 5% sodium hydroxide to extract non cellulose such as wax and pectin, which can increase the friction and adhesion between fiber and matrix. Alkalization can improve the uniformity of fiber strength and reaction activity without stretching. However, the free water in cellulose and the binding water in amorphous region will affect the mechanical properties of composites. Therefore, water must be completely removed, and the water absorption of cellulose can be reduced by replacing hydroxyl groups through reaction

5. Upr/inorganic composites

in recent 30 years, there have been more and more studies on improving the mechanical, electrical, thermal, optical and processing properties of polymers with inorganic substances. People are looking for new methods that can not only improve the properties of filled plastic materials, but also reduce the cost: such as using fibrous fillers to improve the tensile strength, using sheet fillers to improve the rigidity, or combining particle fillers with different shapes to produce the so-called hybrid structure, and combining the effects of two or more components

5.1. Floating dust and its mixture reinforced composites

floating dust is a by-product of coal combustion collected from flue gas by electrostatic deposition. These by-products cause many environmental problems. People use floating dust/sand mixture to prepare unsaturated polyester mortar, which has strong tensile strength

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