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Process Characteristics Of Unsaturated Polyester Resin

Jun 03, 2020

Brief description

Unsaturated polyester resin is a kind of chemical raw materials. It is often used to thicken and solidify the surface of objects. It is like brushing paint when used. Layer by layer, the curing process releases styrene and other harmful gases. 

Unsaturated polyester resin is one of the most commonly used thermosetting resins. It is a linear polymer formed by polycondensation of saturated dibasic acid, unsaturated dibasic acid and glycol, which is formed by diluting with crosslinking monomer or active solvent The resin solution with a certain viscosity, referred to as UP.

Unsaturated polyester resin performance characteristics

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.

Good resin 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.

There are many varieties, wide adaptation and low price.


The disadvantage is that the shrinkage rate during curing is large, the storage period is short, it contains styrene, and there is a irritating gas.

Physical and chemical properties of unsaturated polyester resin

 Physical properties

The relative density of unsaturated polyester resin is about 1.11~1.20, and the volume shrinkage during curing is large. 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.

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 is still soluble in a suitable solvent, and has good fluidity when heated

Relationship between structure and performance of unsaturated polyester resin

So far, the unsaturated polyester (resin) matrix used as the matrix of composite materials at home and abroad is basically phthalic acid type (abbreviated as o-phenyl type), isophthalic acid type (abbreviated as m-benzene type), bisphenol A type and ethylene Basic ester type, halogenated unsaturated polyester resin, etc.

O-Phenyl unsaturated polyester and m-benzene unsaturated polyester

Phthalic acid and isophthalic acid are isomers of each other, and the unsaturated polyester molecular chains synthesized by them are o-benzene type and m-benzene type, although their molecular chains are similar in chemical structure, but m-benzene type unsaturated Compared with o-phthalic unsaturated polyester, polyester has the following characteristics: ①Using isophthalic dicarboxylic acid can produce higher molecular weight unsaturated ester of isophthalic acid, which makes the cured products have better mechanics Performance, toughness, heat resistance and corrosion resistance; ②The purity of isophthalic polyester is high, and no impurities of isophthalic acid and low molecular weight isophthalate remain in the resin; ③Polyisophthalic acid poly The ester bond on the ester molecular chain is protected by the steric hindrance effect of isophthalic acid. The ester bond on the phthalate polyester molecular chain is more susceptible to attack by water and other corrosive media. Isophthalic polyester The glass fiber reinforced plastic made of resin still has quite high performance after soaking in 71℃ saturated sodium chloride solution for one year.

Bisphenol A unsaturated polyester

 Compared with the chemical structure of macromolecular chain of bisphenol A unsaturated polyester and o-phenyl unsaturated polyacid and m-phenyl unsaturated polyester, the spacing between ester bonds in the molecular chain that is easily damaged by hydrolysis is increased, Thereby reducing the density of ester bonds; bisphenol A unsaturated polyester copolymerized with cross-linking agents such as styrene has a large space effect after curing, shielding the ester group, hindering the hydrolysis of ester bonds; and in the molecular structure Neopentyl, connected with two benzene rings, maintains the stability of the chemical melon, so this type of resin has better acid resistance, alkali resistance and hydrolysis resistance.

Vinyl resin

Vinyl resin, also known as epoxy acrylic resin, is a new type of resin developed in the 1960s. It is characterized by having unsaturated double bonds at the end of the polymer.

 Vinyl resin has good comprehensive properties: ① Because the unsaturated double bond is located at the end of the polymer molecular chain, the double bond is very active, and it is not affected by steric barriers during curing, and can be passed through the phase under the trigger of organic peroxide It can be cross-linked and cured between adjacent molecular chains, and it can also be cured with monomer styrene; ② R groups in the resin chain can shield the ester bond and improve the chemical resistance and hydrolysis resistance of the ester bond; ③ vinyl resin The ester bond per unit molecular weight is about 35% to 50% less than that of ordinary unsaturated polyester, which improves the hydrolytic stability of the resin in acid and alkali solutions; ④Secondary hydroxyl groups on the resin chain Wetting and bonding with glass fiber or other fibers to improve the strength of the composite; ⑤ epoxy resin main chain, which can impart toughness to the vinyl resin, the ether bond in the molecular main chain can make the resin have excellent acid resistance Sex.  

The variety and properties of vinyl resins vary widely with different raw materials. The molecular structure can be designed according to the requirements of composite materials on resin properties.


Halogenated unsaturated polyester

Halogenated unsaturated polyester refers to a chlorinated unsaturated polyester synthesized from chlorinic anhydride (HET anhydride) as a saturated dibasic acid (anhydride).  

Chlorinated unsaturated polyester resins have been used as resins with excellent self-extinguishing properties. However, recent studies have shown that chlorinated unsaturated polyester resins also have very good corrosion resistance, and its corrosion resistance in the above media is basically equivalent to bisphenol A unsaturated polyester resins and vinyl resins. Examples (such as wet chlorine) have better corrosion resistance than vinyl resins and bisphenol A unsaturated polyester resins.  

Hot and humid chlorine will react after contact with unsaturated polyester resin to produce chlorinated unsaturated polyester resin or "chlorine cream". The "chlorine cream" produced by bisphenol A unsaturated polyester resin and vinyl ester resin is soft, and wet chlorine can be further (corroded) penetrated by this "chlorine cream" layer, but the "chlorine cream" is produced by chlorinated unsaturated polyester "The character is hard and can prevent further (corrosive) penetration of wet chlorine.

The curing mechanism of unsaturated polyester resin

Analysis from the perspective of 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 large, growing 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 molecules become free radicals.

Changes in molecular structure of unsaturated polyester resin during curing

The curing process of UPR is a process in which the unsaturated double bond in the UPR molecular chain and the double bond of the cross-linking monomer (usually styrene) undergo cross-linking polymerization to form a three-dimensional network structure from linear long-chain molecules. During this curing process, there are three possible chemical reactions, namely   

1. The reaction between styrene and polyester molecules;   

2. The reaction between styrene and styrene;   

3. The reaction between polyester molecules and polyester molecules.  

The occurrence of these three reactions has been confirmed by various experiments.  

It is worth noting that when there is a trans double bond in the polyester molecular structure, the third reaction is easy to occur, that is, the reaction between the polyester molecule and the polyester molecule. This reaction can make the molecules combine It is more compact and can improve various properties of the resin.

Changes in apparent characteristics of unsaturated resin during curing

The curing process of unsaturated polyester resin can be divided into three stages, namely:   

1. Gel stage (stage A): the stage from the addition of the curing agent and accelerator until the resin coagulates into a jelly and loses its fluidity. In this junction, the resin can melt and be soluble in certain solvents (such as ethanol, acetone, etc.). This stage takes about a few minutes to tens of minutes.  

2. Hardening stage (stage B): It starts from the resin gel until it has enough hardness to reach the state of not sticking hands. In this stage, the resin can swell but not dissolve when contacted with certain solvents (such as ethanol, acetone, etc.), and can soften but not completely melt when heated. This stage takes about tens of minutes to several hours.  

3. Aging stage (stage C): it is placed at room temperature, after hardening, it reaches the required hardness of the product, and has stable physical and chemical properties available for use. At this stage, the resin neither dissolves nor melts. What we usually refer to as post-curing refers to this stage. This knot is usually a very long process. It usually takes days, weeks or even longer.

Factors affecting the curing degree of resin

The curing of unsaturated polyester resin is the process of linear macromolecules forming a three-dimensional network through the crosslinking agent, but the curing process cannot consume all the active double bonds in the resin to achieve 100% curing degree. In other words, the degree of curing of the resin is difficult to achieve. The reason is that in the late stage of the curing reaction, the viscosity of the system increases sharply, which hinders the diffusion. Generally, it can only be considered that the curing is complete when the material properties tend to be stable. The degree of resin curing has a great influence on the performance of FRP. The higher the degree of curing, the mechanical properties, physical and chemical properties of FRP products are fully exerted. (Someone has done experiments to test the physical properties of UPR resin at different stages after curing, and the results show that its bending strength does not increase with time, and it does not stabilize until one year later. In fact, for FRP products that have been put into use, one year later, due to the effects of heat, light and other aging and corrosion of the medium, the mechanical properties have gradually declined.

There are many factors that affect the degree of curing. The composition of the resin itself, the amount of initiator and accelerator, curing temperature, post-curing temperature and curing time can all affect the degree of curing of the polyester resin.