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What Is Unsaturated Resin?

Jun 12, 2020

1. Introduction

The unsaturated polyester resin is generally a linear polymer compound having an ester bond and an unsaturated double bond formed by polycondensation of an unsaturated dibasic acid diol or a saturated dibasic acid unsaturated diol. Generally, the polyesterification polycondensation reaction is carried out at 190~220℃ until the expected acid value (or viscosity) is reached. After the polyesterization polycondensation reaction is completed, a certain amount of vinyl monomer is added while hot to make a thick The solution of such a polymer is called unsaturated polyester resin.


2. Preparation of raw materials

 

Unsaturated polyester has many varieties because of different raw materials. The unsaturated dibasic acids or anhydrides commonly used in industrial production include maleic anhydride, fumaric acid and tetrahydrophthalic anhydride. Commonly used saturated dibasic acids or anhydrides are phthalic anhydride, isophthalic acid and adipic acid. The most used glycols are propylene glycol, diethylene glycol and dipropylene glycol. Vinyl monomers as crosslinking agents include styrene, methyl methacrylate and diallyl phthalate. In addition to the main raw materials mentioned above, there are various additives and polymerization inhibitors, catalysts or initiators, accelerators, fillers, dyes and lubricants. The most common unsaturated polyester is synthesized from maleic anhydride, phthalic anhydride and propylene glycol.

3. Production methods


It varies by variety, but basically includes two steps of polycondensation and blending.


The polycondensation methods are:


① Melt polycondensation method. Direct melting and polycondensation with acid and alcohol without adding other components. Using the difference in boiling range between alcohol and water, the water produced by the reaction is separated through the separation column. This method has simple equipment and short production cycle, so it is widely used.


② Solvent azeotropic dehydration method. Toluene or xylene (solvent) is added during the polycondensation process, and the azeotropic point of toluene and water is lower than that of water, and the water produced by the reaction is quickly taken out to promote the polycondensation reaction. The advantage of this method is that the reaction is relatively stable, easy to master, and the product color is better, but a set of water reflux device is needed, toluene is used in the reaction process, and the polycondensation section is explosion-proof.


③ Decompression method. When the shrinkage in the polycondensation reaches 2/3~3/4, it is evacuated until the acid value reaches the required value.


④Pressure method. Pressurization can accelerate the reaction, shorten the reaction cycle, and increase productivity. Blending points intervene in mixing and wet premixing. Interventional mixing is to mix reactive solid prepolymer, solid cross-linking agent, glass fiber, catalyst, colorant into molding compound; wet pre-mixing is to use styrene as cross-linking agent, liquid unsaturated polyester, glass Fibers, catalysts, lubricants, pigments, etc. are mixed in a kneader and made into a polyester mass molding compound.

4. Purpose


Various unsaturated polyesters are liquids from low viscosity to high viscosity when uncured. After adding various additives, they are heated and cured. After curing, they become rigid or elastic plastics, which can be transparent or opaque. The main purpose of unsaturated polyester is to make glass fiber reinforced plastic with glass fiber reinforcement, which is one of the main varieties in reinforced plastics.

Its forming methods are:


① The hand lay-up method is to spray a layer of resin on the mold coated with the release agent, and then lay a layer of reinforcing material, and then repeat the operation to the required thickness after expelling the air bubbles, and finally solidify and release the mold.


②Lamination method is to make glass cloth impregnated with unsaturated polyester, and then laminated and hot-press cured. It has excellent tensile strength and impact toughness, small relative density, good thermal and electrical insulation properties, good light transmission, weather resistance, acid resistance and sound insulation. The price is much cheaper than epoxy resin glass fiber reinforced plastic, so it is widely It is used to manufacture radar radomes, aircraft parts, car shells, small boats, transparent corrugated board and other building materials, sanitary toilets, and chemical equipment and pipes.


5. Performance characteristics

 

1. Excellent process performance. This is the biggest advantage of unsaturated polyester resin. It can be cured at room temperature, molded under normal pressure, and has flexible process performance, especially suitable for large-scale and on-site manufacturing of FRP products.


2. The resin has good overall performance after curing. The mechanical performance index is slightly lower than epoxy resin, but better than phenolic resin. Corrosion resistance, electrical properties and flame retardancy can be met by choosing the appropriate grade of resin. The resin is light in color and can be made into transparent products.


3. There are many varieties, wide adaptation and low price.


4. Disadvantages are greater shrinkage during curing and shorter shelf life.


6. Physical properties


The relative density of unsaturated polyester resin is about 1.11 ~ 1.20, the volume shrinkage rate is large during curing, some physical properties of cured resin are as follows:


⑴ Heat resistance. Most unsaturated polyester resins have a heat distortion temperature of 50 to 60°C, and some resins with good heat resistance can reach 120°C. The red thermal expansion coefficient α1 is (130~150)×10-6℃.


⑵ Mechanical properties. Unsaturated polyester resin has high tensile, bending, and compression strength.


⑶ Chemical corrosion resistance. Unsaturated polyester resins have good resistance to water, dilute acids, and alkalis, and poor resistance to organic solvents. At the same time, the chemical resistance of resins can vary greatly depending on their chemical structure and geometric switch.


⑷ Dielectric performance. The dielectric properties of unsaturated polyacid resins are good.


7. Chemical properties


Unsaturated polyester is a linear polymer compound with multifunctional groups. It has a polyester chain bond and an unsaturated double bond on its backbone main chain, and has carboxyl groups and hydroxyl groups at both ends of the macromolecular chain.


The double bond on the main chain can undergo copolymerization crosslinking reaction with the vinyl monomer, so that the unsaturated polyester resin changes from soluble and meltable state to insoluble and infusible state.


The ester bond on the main chain can undergo a hydrolysis reaction, and an acid or a base can accelerate the reaction. If copolymerized with styrene and cross-linked, the occurrence of hydrolysis reaction can be greatly reduced.

In acidic medium, hydrolysis is reversible and incomplete. Therefore, polyester can resist the attack of acidic medium; in alkaline medium, due to the formation of resonance stable carboxylate anion, hydrolysis becomes irreversible, so polyester is resistant to alkali Sex is poor.


The carboxyl group at the end of the polyester chain can react with alkaline earth metal oxides or hydroxides (such as MgO, CaO, Ca(OH)2, etc.) to extend the molecular chain of the unsaturated polyester, which may eventually form a complex. The molecular chain extension can make the viscous liquid resin with an initial viscosity of 0.1-1.0 Pa·s increase rapidly to 103 Pa·s or more in a short time, until it becomes a non-flowing, non-sticky, similar gel. The resin is not cross-linked when in this state, it can still be dissolved in a suitable solvent, and has good fluidity when heated.


Eight, curing mechanism


From the perspective of the chemical kinetics of free radical polymerization, the curing of UPR belongs to free radical copolymerization. The curing reaction has the characteristics of four radical reactions: chain initiation, chain growth, chain termination, and chain transfer.

Chain initiation-from the decomposition of peroxide initiators to the formation of free radicals to the addition of radicals to unsaturated groups.


Chain growth-the process of continuous addition of monomers to newly generated radicals. Compared to chain initiation, the activation energy required for chain growth is much lower.


Chain termination-Two free radicals combine to terminate the growing polymer chain.


Chain transfer-a growing large free radical can interact with other molecules, such as solvent molecules or inhibitors, so that the original active chain disappears into a stable large molecule, and the original inactive molecule becomes a free radical

Source:http://www.up-resin.com/

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