How to accelerate the rapid solidification of liquid resin without deformation and foaming

Under the adjustment of formula and process, curing dosage is very important.

Effect of polymerization inhibitor and other additives on unsaturated polyester resin

In order to stabilize unsaturated polyester resin, polymerization inhibitor or retarder is usually added to it. This is a substance that can react with chain free radicals to generate non-free radicals or low-activity free radicals that cannot be initiated again, so that the crosslinking curing rate is reduced to zero. Therefore, due to the small amount of polymerization inhibitor added, the resin with low reactivity may show high reactivity, while the resin with high reactivity may become less active due to the excessive amount of polymerization inhibitor added. In addition, adding other additives, such as flame retardant, color paste, low shrinkage agent and various fillers, and introducing phosphorus, halogen, metal ions or other factors will affect the crosslinking activity of the resin.

(6) Influence of curing agent and polymerization inhibitor dosage

Taking JX- 196 resin as the curing experiment, the effects of different curing agents and polymerization inhibitors are as follows:

Group number BPOTBHQ n-Cu gel time min exothermic peak temperature℃ curing time min

1 0.3 0 0 0 3.7 178 1.7

2 0.3 0.02 0.07 0.07 12.9 143 3.05

3 0.3 0.02 0.07 0.02 12.3 167 2.7

4 0.3 0.04 0.04 0.04 1 1.3 164 2.6

5 0.6 0.02 0.07 0.07 8.3 18 1 1.7

6 0.6 0.02 0.07 0.02 6.4 184 1.5

7 0.6 0.04 0.04 0.04 7.6 185 1.3

8 0.9 0.04 0.04 0.04 4.2 19 1 1.2

From the above experiments, it can be seen that the curing results of three groups of curing agents with different doses form three steps. The higher the dose, the faster the curing and the higher the exothermic peak. Different polymerization inhibitors and different dosage have different curing effects. Therefore, it is very important to master the reasonable matching of polymerization inhibitor and curing agent in the process of resin manufacturing and use.

2 unsaturated polyester resin curing network structure analysis

2. Crosslinking network structure of1unsaturated polyester resin

The double bond in unsaturated polyester polymerizes with the double bond in cross-linking agent to form an insoluble cross-linking network structure, which contains two polymer molecular chain structures. The main body of the network is formed by randomly curling unsaturated polyester molecular chains, in which styrene polymer molecular chains are inserted to connect and fix the unsaturated polyester molecular chains, forming a huge network. The average molecular weight of unsaturated polyester molecular chain in the network is 1000-3000. The length of styrene molecular chain connected between unsaturated polyester molecular chains is 1-3. From a certain initiation point, polyester molecule → styrene chain → polyester molecule → phenylethyl ester, and there are only 7-8 consecutive repetitions of olefin chain, so that the average molecular weight of styrene polymer molecular chain can reach 8000- 14000. The average molecular weight of the whole network structure is 10000-30000. If the molecular weight of the network is less than 10000, it will directly affect the mechanical properties of the product such as strength, elasticity and toughness.

2.2 Long-lived Free Radicals of Unsaturated Polyester Resin Crosslinked Network

During the curing process of unsaturated polyester resin crosslinked network, the polymerization process and residual rate of unsaturated polyester and styrene have certain characteristics. Experiments show that no matter whether the cross-linking network of polyester resin is perfect or not, some steric hindrance dead spots that cannot be terminated by free radicals will be produced, forming long-lived free radicals. These long-lived free radicals only exist in unsaturated polyester chains and do not appear on small molecular crosslinking agents with only two functional groups. Due to the existence of long-lived free radicals, unsaturated polyester resin can still undergo crosslinking reaction after curing. With the increase of temperature, especially near the glass transition temperature of the resin, the mobility of molecules is greatly increased, and long-lived free radicals can move and continue to crosslink with residual crosslinking monomers, which is why post-curing of the resin can improve the curing degree.

2.3 Micro-phase separation phenomenon in polyester resin network structure

The experimental analysis shows that the micro-phase separation structure also exists in the well-crosslinked unsaturated polyester resin. This microphase separation is probably due to the mutual repulsion of different molecular chains, and the polyester chain and crosslinking agent gather together in some way to produce phase separation. The exothermic peak at the initial stage of curing makes the two phases melt together, which is an important condition for unsaturated polyester resin to form a uniform network. However, the phase separation process after the exothermic peak continues and develops with time. Low temperature treatment can accelerate the development of this microphase separation, on the contrary, heat treatment can eliminate this microphase separation. When the temperature rises, the loose phase separation zone is destroyed first, and when the temperature rises again, the tight phase separation zone is destroyed. Finally, at high temperature above the glass transition temperature, all phase separation zones can be eliminated. Once the phase region is destroyed, the phase separation will be limited by the network, rather than the monomer moving and arranging freely in the polymerization process. However, high temperature treatment above the glass transition temperature of the two phases leads to further polymerization and crosslinking in the state of uniform network, which can fundamentally eliminate this microphase separation.

The existence of microphase separation has great influence on the properties of materials. The experiment shows that under the same conditions, the polyester casting sample is cured at room temperature of 25℃ for 30 days, the curing degree reaches 90.2%, and its Barker hardness is 38.5. After high temperature treatment, although the curing degree did not increase by 92.6%, the hardness of Barco reached 44.4 because the influence of phase separation was eliminated. It can be seen that microphase separation has great influence on the hardness of resin. At the same time, we can understand the reason why the stiffness of post-treated samples at high temperature is much higher than that of cured samples at room temperature. Therefore, we should attach great importance to unsaturated resin glass fiber reinforced plastic products, especially glass fiber reinforced plastic equipment for anticorrosion and food, which must be treated at high temperature to eliminate microphase separation before it can be put into use.

2.4 Influence of Crosslinking Agent on Network Structure

As mentioned above, when the two monomers are crosslinked and cured, the reactivity ratio plays a key role in affecting the uniformity of the crosslinked network of unsaturated polyester trees. Therefore, we must pay attention to the reactivity ratio when selecting crosslinking agent, so that crosslinking agent and unsaturated polyester can be polymerized alternately to form a uniform network structure. In addition, the crosslinking agent should have smaller molecular weight, lower functionality and excellent compatibility with polyester. Generally speaking, if the amount of crosslinking agent is too small, the double bonds of unsaturated polyester can't react completely, and if the amount is too large, a large number of plastic chains will inevitably be formed, which can't make the resin form a uniform and compact network. The experiment shows that the dosage of crosslinking agent styrene is usually about 35%, that is, the ratio of styrene to polyester double bond is between 1: 1.6-2.4.

2.5 Influence of molecular weight of unsaturated polyester on cross-linked network

The larger the molecular weight of polyester, the longer the molecular chain, the smaller the molecular weight and the shorter the molecular chain. Experiments show that with the increase of polyester molecular weight, the greater the probability of forming a complete network, the smaller the molecular weight, and the more difficult it is to form a complete network. With the increase of molecular weight, the end groups in the network decrease, the nodes increase, and the heat resistance is better. Therefore, the resin with large molecular weight has high heat resistance.

2.6 Influence of molecular structure of unsaturated polyester on network performance

The molecular structure between the cross-linking points of unsaturated polyester has a direct influence on the thermal properties of the network. The molecular structural unit of unsaturated polyester is composed of double bond, ester bond, ether bond, methylene and aromatic ring. Generally, the shorter the connection between double bonds, the higher the thermal denaturation temperature of the resin. The extension of the segment between double bonds will reduce the thermal denaturation temperature.

Bending strength is the comprehensive embodiment of tensile strength and compressive strength of materials, and it is an important index of material properties. The higher the crosslinking density of the resin, the more molecular chains that bear the load, and the higher the bending strength. But sometimes this is not the case. This is because the resin network is extremely uneven, and the uniform density decreases with the increase. Therefore, under the action of external force, the stress of each molecular chain is also uneven. Moreover, the molecules of high crosslinking density resin are tense, difficult to move, and the amount of denaturation is small, so they will bend rather than bend under the action of external force. It can be seen that the bending strength of highly crosslinked resin is not high because of its poor uniformity and the molecular chain is not easy to relax. The ideal molecular structure of resin with high temperature use value should be to introduce a series of asymmetric aromatic heterocyclic structures on the main chain between double bonds, preferably with a small number of polar bonds.

2.7 Influence of initiator and curing conditions on network structure of resin

Different kinds of (1) initiators have different crosslinking and curing properties. Taking cyclohexanone peroxide (HCH)/ cobalt naphthenate (CoN) and benzoyl peroxide (BPO)/ dimethylaniline (DMA) as two redox systems, we can see that the percentage content of styrene-based resin initiated by BPO/DMA system slowly decreased to 24.9% during the curing process of 80h. However, the curing rate of unsaturated polyester resin initiated by HCH/ Kang system also using styrene as crosslinking agent decreased to 24.5% after 4.5h. It can be seen that the curing of unsaturated polyester resin initiated by HCH/ Kang system is more effective than that initiated by BPO/DMA system. At the same time, it is found that the number of long-lived free radicals in the resin network cured by HCH/CoN initiation system is still not lower than that after curing for 80 days after 10. In contrast, the number of long-lived free radicals in the resin network cured by BPO/DMA initiation system disappears rapidly, which fully shows that the system has great influence on the formation of resin network. Especially in the later stage of curing, it is difficult to achieve higher curing degree.

(2) Under different curing conditions, the properties of resin curing network will be different. Take JX- 196 resin of Tianxing Company as an example: Take JX- 196 resin, add HCH/CoN initiation system, divide it into two parts, and put it in a constant temperature water bath at 25℃ and an air bath at 25℃ respectively, and record the temperature changes of each sample during curing. It can be seen that the temperature of the resin at the initial stage of curing is different from that in the water bath.

The air bath is basically the same, but after gelation, the exothermic peak of the air bath cured sample is higher, while the exothermic peak temperature of the water bath cured sample is 20-30℃ lower than the former. After post-curing, the performance parameters of air-bath cured samples are obviously better than those of water-bath cured samples. This shows that the initial curing degree of the same resin is obviously different after different curing conditions. Although the final curing degree will be consistent as long as there are enough initiators and after high temperature post-treatment, there are significant differences in curing properties. That is to say, the initial curing conditions laid the foundation of cross-linking network structure, so the physical properties of the material were determined to a considerable extent. Therefore, in the pickling process, there is a saying that cooked raw rice can no longer be cooked. After the resin is cured, it is difficult for molecules to penetrate each other, so the key moment to affect the network structure is a period of gel time. During this period, in order to ensure the uniformity and continuity of the resin network structure, the crosslinking agent needs to continue to penetrate and swell, and the exothermic peak at this time plays this role. Although the final curing degree of the crosslinked product is not high, its performance is better than that of the product without exothermic peak.

Comparison of curing properties of JX- 196 resin in air bath and water bath

Gel time minimum exothermic peak temperature℃ Barco hardness bending strength KPa

Air bath temperature 9.7184 43 211

Water bath temperature11.616330188