Synthesis of unsaturated polyester resin - main raw material
Ethylene glycol is the simplest structure of diol. Due to its structural symmetry, the resulting polyester resin has obvious crystallinity, which limits its compatibility with styrene, so it is generally not used alone. In combination with other glycols, such as 60% ethylene glycol and 40% propylene glycol, the compatibility of the polyester resin with styrene can be improved; if used alone, the resin-forming end should be used. Acetylation or propionylation to improve its compatibility.
1,2 propylene glycol, due to structural asymmetry, can obtain a non-crystalline polyester resin, can be completely soluble in styrene, and its price is relatively low, so it is currently the most widely used diol.
Other useful glycols are:
Diethylene glycol - can improve the flexibility of polyester resin;
Dipropylene glycol - improves resin flexibility and corrosion resistance;
Neopentyl glycol - improves the corrosion resistance of resins, especially alkali and hydrolytic stability.
The above diols may lose strength due to too much flexibility of the resin, or should improve the compatibility of the resin with styrene. They are generally not used alone and should be mixed with other glycols. A polyester resin with highly durable chemical corrosion, often using bisphenol A or hydrogenated bisphenol A as a raw material. To form a glycol suitable for reaction with a dibasic acid, bisphenol A should be previously propylene oxide or epoxy. The ethane reacts to form a glycol having an alcoholic hydroxyl group at both ends, such as a D-33 glycol.
The use of chlorinated or brominated glycols not only exhibits flame retardancy but also improves corrosion resistance.
The addition of a small amount of a polyol such as glycerol and pentaerythritol improves the heat resistance of the resin to a large extent.
The chemical resistance of unsaturated polyester resins depends on the chemical structure of the resin. The ester bond is the weakest link in the polyester resin and is susceptible to hydrolysis by the action of acids and bases. The different chemical structures surrounding the ester bond have different steric hindrance protection for the ester bond, leaving the article exhibiting different corrosion resistance. The steric hindrance protection of ester bonds:
Unsaturated dibasic acid
The double bond in the unsaturated polyester resin is generally provided by an unsaturated dibasic acid material. The more unsaturated acid in the resin, the larger the ratio of double bonds, the higher the degree of crosslinking when the resin is cured, thereby making the resin have higher reactivity, and the cured product of the resin has higher heat resistance and is destroyed. There is a lower elongation rate.
In order to improve the reactivity of the resin and the properties of the cured product, an unsaturated dibasic acid and a saturated dibasic acid are generally used in combination.
Maleic anhydride (maleic anhydride) and maleic acid (maleic acid) are the most commonly used unsaturated acids. Since maleic anhydride has a lower melting point and can condense less than one molecule of water upon reaction, it is used more.
Fumaric acid (fumaric acid) is a trans isomer of cis acid. Although the cis acid is almost completely isomerizable to form a trans structure when it is polycondensed above 180 ° C, it is made of transbutene. The resin prepared by the diacid has a higher softening point and a larger tendency to crystallize.
Other unsaturated acids such as maleic acid, itaconic acid and citraconic acid may also be used, but they are more expensive and less common. In addition, the resin made with itaconic acid also has the problem of miscibility of the resin with styrene. Although chlorinated maleic acid contains 26% chlorine, it is not used as a flame retardant resin. Other flame retardant ingredients must also be added.
Saturated dibasic acid
The main function of adding a saturated dibasic acid is to effectively adjust the spacing of the double bonds in the polyester molecular chain, and in addition to improve the compatibility with styrene.
In order to reduce or avoid the crystallization of the resin, phthalic anhydride can be used as a saturated dibasic acid to prepare an unsaturated polyester resin. The obtained resin has good compatibility with styrene, has good transparency and good synthesis. performance. In addition, phthalic anhydride is readily available and inexpensive, making it the most widely used saturated dibasic acid.
Compared with phthalic anhydride, isophthalic acid improves the ester group stability problem caused by the mutual repulsion caused by the two ester groups being too close together in the o-benzene type polyester, thereby improving the resin. Corrosion resistance and heat resistance, in addition, improve the toughness of the resin. Isophthalic acid can be used to synthesize a medium corrosion resistant unsaturated polyester resin. Terephthalic acid is similar to isophthalic acid. The polyester resin prepared from terephthalic acid has good corrosion resistance and toughness, but the acid activity is not large, and it is not easy to react during synthesis, and the application is not much.
A chlorine- and bromine-containing saturated dibasic acid can be used to make a flame retardant resin. Chlorobacteric acid anhydride (HET anhydride) and tetrachlorophthalic anhydride are two commonly used chlorine-containing saturated dibasic acids. Chlorobacteric acid has a chlorine content of up to 55%, and the polyester obtained by the method has better flame retardancy than the polyester obtained by using tetrachlorophthalic anhydride (containing 49.5% chlorine), and has good properties. Corrosion resistance. It has the disadvantage that the resulting resin (which is initially colorless and transparent) gradually becomes colored and darkened over time during storage and use. Even if a UV absorber is added, this color change cannot be prevented.
The substitution of the above-mentioned saturated dibasic acid with an aliphatic dibasic acid such as adipic acid and sebacic acid partially increases the flexibility and impact resistance of the obtained resin, but is generally not used alone. The difference in the steric hindrance of the ester bond is also different for saturated dibasic acids.
Protection of ester bonds: HET>IPA>PA> adipic acid
The crosslinking agent can act as a diluent to form a resin solution having a certain viscosity, in addition to being crosslinked with a resin molecular chain to form a macromolecular structure.
Styrene is the most commonly used crosslinker and its advantages are:
(1) Styrene is a low viscosity liquid with good compatibility with resins and various auxiliary components.
(2) When copolymerized with an unsaturated polyester resin, a copolymer having a uniform composition can be formed. For example, when styrene and fumarate (close to the activity of the double bond in the maleic anhydride type unsaturated polyester) are copolymerized, the reactivity ratios are r1 = 0.30 and r2 = 0.07, respectively, due to r1. R2 is less than 1, and its copolymer composition curve must pass through the diagonal line, that is, there must be a constant ratio of copolymerization points. It can be known from the copolymer composition equation that when passing through the constant ratio copolymerization point, then:
Substituting the values of r1 and r2 to obtain [M1]/[M2]=1.33, which indicates that when the double bond equivalent ratio of the two components of styrene and unsaturated polyester resin in the polyester resin solution is 1.33, it can be passed. By constant copolymerization, at this time, the composition of the two constituent raw materials and the composition of the crosslinked product of the copolymer do not change with time, and the formed copolymer has a uniform composition. When the amount of styrene that can be added is 35-40% of the polyester resin, [M1]/[M2] can be around 1.33, and the amount of styrene added is just enough to make the viscosity of the system moderate, so The cured product obtained has the best performance.
(3) Styrene raw materials are readily available and inexpensive, which is beneficial to reduce the cost of resin and FRP products.
The disadvantages of styrene are high vapor pressure, low boiling point (145 ° C), easy to volatilize, certain odor, resulting in poor construction conditions, and certain labor protection measures should be taken. At present, some countries have proposed that the content of styrene in air should be reduced to below 100 ppm (420 mg/m3). Adjusting the amount of styrene within a certain range can also affect other properties. The amount of styrene is increased, the viscosity of the resin solution is lowered, and the double bond content of the resin system is increased, so the gel time is shortened, the softening point is increased, the corrosion resistance of the resin is increased, and curing is performed. The shrinkage rate increases and vice versa. Generally, the amount of styrene added should be such as to ensure the viscosity required during construction.
Many other types of crosslinkers (such as vinyl toluene, divinylbenzene, etc.) can be used depending on the particular needs of the resin's properties and uses, but they are used in much less styrene.
Methyl methacrylate is used as a cross-linking agent because it has a low refractive index, close to the refractive index of glass fiber, and has good weathering resistance, so it is mainly used for the manufacture of transparent FRP products. The disadvantage of methyl methacrylate is that the boiling point is lower (1000C - 1010C), the volatility is larger; the price is higher; the corresponding r1 and r2 in the copolymerization system formed with the unsaturated polyester resin are 17 and close to 0 respectively. Therefore, the copolymerization tendency of the unsaturated double bond in the methyl methacrylate and the polyester resin is small, the degree of cross-linking of the product is low, the structure is loose, and the surface hardness of the product is also relatively low.
When the diallyl phthalate unit is used as the crosslinking agent, the heat resistance and electrical properties of the obtained uniform are good, the heat release is less during curing, and the shrinkage rate is low, which is suitable for large parts and dimensional stability. Good product. The disadvantage is that it is heated and cured, the viscosity is higher, and the price is higher.
In addition, a-methylstyrene has a low shrinkage and good product toughness when cured, and is suitable for use in casting and sealing formulations; chlorinated and brominated styrene are suitable for flame retardant products; Tripropylene cyanurate as a crosslinking agent can improve the heat resistance of the article. However, these crosslinkers limit their use due to their high price.
In the free radical polymerization, the addition of some trace substances can delay or slow down the polymerization in a certain time range. These substances are called polymerization inhibitors. The polymerization inhibitor is usually added after the end of the polycondensation reaction, which avoids gelation when the resin is miscible with the styrene monomer at a higher temperature, and also prolongs the storage period of the resin solution product.
Like the polymerized monomer, the polymerization inhibitor also reacts with the free radicals in the resin system to produce new radicals, but the difference is that the new radicals generated by the reaction of the free radicals with the polymerization inhibitor generally no longer undergo chain-growth reactions. They are either relatively stable, or interact with a chain termination reaction, which essentially acts to absorb and deplete the free radicals produced in the system, thereby exhibiting significant inhibition. Hydroquinone is the most commonly used polymerization inhibitor, and its amount varies depending on the type of resin. The usual amount is 0.5/10000 to 5/10000 of the total amount of the resin.
Different polymerization temperatures have different polymerization inhibitory effects. For example, tert-butyl catechol acts as a polymerization inhibitor at moderate temperatures (about 60 ° C); copper naphthenate acts at room temperature. These two compounds are also commonly used polymerization inhibitors. In another example, oxygen in the air has a significant inhibition effect on the resin at normal temperature (this is the reason why the surface of the unsaturated polyester resin is sticky at room temperature curing), but at high temperatures, oxygen exhibits a significant polymerization-promoting effect. effect. The effect of the polymerization inhibitor is different for different crosslinking agent monomers. For example, hydroquinone has a good inhibition effect on styrene monomer, but the effect of blocking and limiting the methyl methacrylate monomer is poor.
The storage period of the unsaturated polyester resin is required to be greater than 6 months, which can be determined by a heating test. It is considered that, at 80 ° C, the polyester resin liquid does not gel within 24 hours, which corresponds to a storage period of the resin liquid at room temperature for 6 months. The minimum amount of polymerization inhibitor added should be 6 months storage at room temperature.
QuanZhou Neweco High Polymer Material Co.,Ltd.
Neweco is one of the leading unsaturated polyester resin manufacturers and suppliers,widely used in automobiles, ships, rail transit, wind power generation, pipe cans, heat and corrosion protection, building materials, stone repair, sanitary ware, artificial stone and other industries. Suitable for hand lay-up, spray, pultrusion, winding, molding, vacuum introduction / RTM, casting and other molding processes. Neweco Resin is committed to excellence and focuses on the application of composite materials to provide customers with comprehensive solutions.