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Toughening Modification And Mechanism Of Unsaturated Polyester

Jun 18, 2020

 1. Status of toughening and modification of unsaturated polyester matrix and BML materials

The toughening of unsaturated polyesters often follows the toughening methods of epoxy resins and other thermosetting resins, which is the most common and simplest

The convenient method is to use the method of constructing the second-phase elastomer to toughen, such as blending liquid rubber, but the toughening method and effect are not as good as epoxy resin. For example, after the epoxy resin is toughened with some liquid rubber, its apparent fracture energy can be increased by about 50 times, and the highest in the literature reported for toughening unsaturated polyester is only 2 5 times. For high-crosslinking unsaturated polyester and BML composites, the effect is even less pronounced. One reason may be the structural factors of the unsaturated polyester itself, and the other reason is that the compatibility of the unsaturated polyester with general liquid rubber is not good enough.

This article mainly explains the toughening method and principle of unsaturated polyester resin matrix. In principle, all can be used for unsaturated polyester

The methods of resin toughening can be further applied to composite materials such as BML or SML.

2. Synthesis of new tough unsaturated polyester by changing chemical composition

This method belongs to the design and synthesis of polymers at the molecular level. Generally, saturated dibasic fatty acids are used to replace saturated aromatic dibasic acids in the synthesis of unsaturated polyesters. The flexibility and flexibility of the resin increase with the increase of the saturated acid chain length. Similarly, increasing the chain length of the saturated diol can also achieve the purpose of increasing the toughness of the resin. In addition, by increasing the ratio of saturated acid to unsaturated acid and reducing the crosslink density, flexibility can also be improved. However, the effect of using this method to improve the toughness of the resin is limited, and because the molecular polarity is reduced, the number of unsaturated bonds is reduced, so that the mechanical strength and heat resistance of the product will be seriously reduced. Furthermore, the cost of toughening and modifying by synthesizing new unsaturated polyester varieties is relatively high, and the flexibility is not enough, so it cannot meet the requirements of diversified product performance in practical applications.

3. Change the structure of unsaturated polyester crosslinking network

As mentioned above, the method of using chemical synthesis modification has its limitations. Since the most commonly used crosslinking monomer for unsaturated polyesters is styrene, the rigid polystyrene segment in the network after crosslinking and curing is a cause of brittleness of the resin. For example, it is easier to use intramolecular rotation The monomer of the polymer actually changes the structure of the cross-linked network after curing, which can increase the deformability of the network and thus improve the toughness. The monomers that can be replaced now include vinyl toluene, diallyl phthalate, methyl methacrylate, triallyl cyanurate, etc. However, the production cost and process operability of these monomers are inferior to styrene, and the glass transition temperature of polymers composed of these monomers is still above room temperature, and the elasticity of their networks is still limited.

4. Modification by adding a second component

It is a goal that people are willing to pursue to achieve the toughening and modification of unsaturated polyester through the simple and practical blending method of adding the second component. The modification method by adding the second component will produce two microstructures: one is to form a two-phase structure, and the other is to form a single-phase structure or a semi-interpenetrating network structure.

(1) Two-phase structure

Introducing the rubber phase into the unsaturated polyester matrix is the most commonly used toughening method for unsaturated polyester and BMC' materials. After curing, the rubber component is generally dispersed in the matrix resin in the form of particles. When subjected to external load, it can effectively induce silver streaks and resist the development of cracks than the silver streaks, or consume a large amount of external energy by generating shear bands, thereby playing a toughening role.

For unsaturated polyesters, in the early days, people mostly concentrated on the use of small chunks of natural styrene-butadiene and nitrile rubber for toughening, but there were problems such as poor compatibility between rubber and unsaturated polyester and difficulty in processing. Later, more rubber oligomers were used to toughen unsaturated polyester, that is, dissolve with uncured unsaturated polyester in styrene), and then add initiator to initiate crosslinking and curing of unsaturated polyester. During the curing process, the molecular weight of the polyester gradually increases, and the rubber phase precipitates from the matrix, eventually forming a micro-phase separated island structure to achieve the purpose of toughening.

L. Suspene et al. used epoxy-terminated nitrile-butadiene rubber (ETBN) produced by the reaction of shuttle-terminated nitrile-butadiene rubber (C'TBN) to toughen unsaturated polyesters. A triblock polymer produced by the reaction of diglycidyl ether (DUEBA). It can effectively improve the compatibility of nitrile rubber and unsaturated polyester, so it strengthens the connection between the rubber phase and the matrix resin, and reduces the size of the precipitated rubber particle phase, thereby improving the impact strength. The size of the rubber phase particles can be observed by SEM. Large particles seem to contain substructures, so the volume fraction of the precipitated rubber phase is generally greater than the total amount of rubber added, and the substructure contains polyester. In addition, it can also be directly used as a phase solvent of C'TBN and unsaturated polyester, which can effectively reduce the boundary tension between the two. M. Abbate et al. used the reaction of amino-terminated liquid nitrile rubber (ATBN) with maleic acid to convert it into maleic-terminated terminal butyl butyl butyl (ITBN) to toughen unsaturated polyester (UP). The mechanical properties and morphology of the ITBN/UP system have been studied. Whether it is impact strength or elastic modulus, the latter is superior to the former. This is because the liquid nitrile rubber containing maleic acid end groups, the active double bonds at both ends can co-crosslink with unsaturated polyester under the initiator. After curing, the rubber phase has a strong chemical bond with the unsaturated polyester matrix, which improves the impact strength and modulus.

In addition to NBR, people also tried other liquid rubber varieties. For example, M. Abbate et al. used maleic acid with polyisobutylene (PIB) to obtain maleic acid-terminated polyisobutylene (PIBSA). This product can react with the light end groups of unsaturated polyester to form an Segment copolymer UP-PIB. Compared with the use of polyisobutylene to toughen unsaturated polyester systems, the use of maleic acid-terminated polyisobutylene liquid rubber to toughen unsaturated polyesters results in a smaller rubber phase particle size and larger particle size distribution after curing Narrow, but also strengthens the connection between the two phases. The experiment also found that it is not necessary to convert all PIB to PIS-BA. Only a small amount of PISBA can have a significant effect on the UP/PIB toughening system, so this "in situ" block copolymer can be used to increase Tough unsaturated polyester, simple and effective.

Polyurethane is widely used in toughening unsaturated polyesters as a very tunable elastomer. For example, E. Martus-celli et al. used light-end polybutadiene (HTPB) to convert into isocyanate-end polybutadiene (ITPB) as the toughening component of unsaturated polyester resin. As ITPB mixes with the uncured unsaturated polyester, the isocyanate end groups can react with the light or shuttle groups of the unsaturated polyester, thus strengthening the chemical interface between the rubber phase and the matrix after curing, improving Impact strength.