The curing of unsaturated polyester resin (UPR) seems to have been thoroughly studied in theory and practice, but because the factors affecting the curing reaction are quite complex, the quality of products in various application fields of UPR Defects are largely related to "curing". Therefore, we have the need for a more in-depth discussion on the curing of UPR.
2. Concepts and definitions related to the curing of unsaturated polyester resins
2. 1 Definition of curing
Liquid UPR can form a three-way cross-linked insoluble and infusible body structure through the combination of unsaturated double bonds in the linear polyester chain and the double bonds of the cross-linking monomer under the action of light, heat or initiator. This process is called UPR curing.
2. 2 curing agent
The curing of unsaturated polyester resin is a free radical initiated copolymerization reaction. How to make the reaction start is the key to the problem. Once the monomer is triggered to generate free radicals, the molecular chain can grow rapidly to form a three-way cross-linked macromolecule.
The start of the curing of unsaturated polyester resin is to first break the unsaturated C—C double bond. Due to the different energy required for the chemical bond to break, for the C—C bond, the bond energy E=350kJ/mol, 350-550℃ The temperature can be cracked.
Obviously, curing the resin at such high temperatures is not practical. Therefore, people have found a substance that can decompose to generate free radicals at a lower temperature, which is organic peroxide. The O—O bonds of some organic peroxides can decompose at lower temperatures to generate free radicals. Some of these peroxides, which can decompose at 50-150°C, are very useful for resin curing. We can take advantage of this feature of organic peroxides and choose some of them as initiators for resins, or curing agents.
Definition of curing agent: The curing agent for unsaturated polyester resins is a peroxide that initiates resin crosslinking under the action of accelerators or other external conditions. It is also called initiator or catalyst.
The "catalyst" mentioned here is different from the traditional "catalyst". In the traditional sense, the term "catalyst" is used to help reactants. They promote the reaction without consuming it. In the UPR curing reaction, the peroxide must change its own structure before it "catalyzes" the reaction, so for the peroxide used for UPR curing, a more appropriate name should be called "initiator" Or "initiator".
When it comes to peroxides, the two concepts that we need to understand are active oxygen content and critical temperature. Among them, "active oxygen" or "active oxygen content" is a concept that has a close relationship with the curing agent and is often misunderstood.
Active oxygen content: The active oxygen content is simply the percentage of the total amount of oxygen and peroxide molecules in the peroxide.
From the concept itself, a peroxide with a lower molecular weight may have a relatively high active oxygen content. But this does not mean that peroxides with high active oxygen content have more or faster activity than peroxides with low active oxygen content. (Because many of our application manufacturers use active oxygen content as an indicator for evaluating curing agents.) In fact, active oxygen content is only a constant measure of the concentration and purity of any particular peroxide. It is found that many peroxides with higher active oxygen content are not suitable for curing resins because they will decompose or "deplete" quickly at standard curing temperatures, that is, the rate at which they decompose free radicals is too high fast. Since free radicals always have a strong tendency to combine with each other, when free radicals are generated faster than they are utilized by unsaturated double bonds, they will recombine or terminate the polymer chain, resulting in low molecular weight polymers This results in incomplete curing. (The typical example is hydrogen peroxide).
Critical temperature: Simply put, the critical temperature is the lowest temperature at which a large amount of peroxide decomposes to generate free radicals. (This temperature is generally only an approximation. Before this temperature, free radicals were also released, but to varying degrees.)
We can classify peroxides as medium-temperature initiators or high-temperature initiators according to their critical temperature. For pultrusion and compression molding, the working temperature is determined according to the critical temperature of the peroxide used. Generally set the working temperature to be slightly higher than the critical temperature of the initiator. (For example: the critical temperature of methyl ethyl ketone peroxide is 80 ℃; the critical temperature of benzoyl peroxide is 70 ℃; di-tert-butyl peroxide is 146 ℃; tert-butyl perbenzoate is 194 ℃. Pultrusion molding process selection Dibenzoyl peroxide and di-tert-butyl peroxide are used as initiators, and the temperature used for temperature programming is 90℃; 160℃.)
2. 3 accelerator
The external temperature directly affects the rate at which free radicals are generated by the peroxide. Heating to release the free radicals from the curing agent to initiate resin curing is certainly feasible, but high temperature operation will also cause some inconveniences. As a result, it was further discovered that some organic peroxides can be activated with another compound, which usually works through oxidation-reduction reactions, without heating, and can be cleaved at ambient temperature to generate free radicals. Such substances that can activate peroxides at ambient temperature are accelerators or may be called accelerators or activators.
Definition of accelerator: An accelerator is a substance that can promote the formation of free radicals (that is, room temperature curing) below the critical temperature of the curing agent.
2. 4 Light curing
Another substance that initiates resin curing is light. The activation energy generated by the ultraviolet light with the highest energy in the spectrum can break the C—C bond of the resin and generate free radicals to cure the resin. For example, we have done experiments, even if it is below 0 ℃, if the resin is placed in direct sunlight, the resin can gel in a day.
When the photosensitizer is added to the UPR, ultraviolet or visible light is used as the energy source to initiate the crosslinking reaction of the resin.
So far, we can understand that there are three types of unsaturated polyester resin curing according to different initiation methods:
Thermal curing: external curing causes the curing agent to release free radicals, thereby initiating the curing process of the resin. (Also known as thermally initiated curing)
Cold curing: The process of curing the resin by adding accelerators to release the free radicals of the curing agent at room temperature or at a low curing temperature. (Also called chemical decomposition initiated curing)
Light curing: by adding a photosensitizer and using ultraviolet as an energy source, the process of resin crosslinking and curing is initiated. (Also called photo-initiated curing)
Below we mainly discuss the commonly used curing systems in cold curing
3. Types of curing agents commonly used in cold curing systems
3.1. Cyclohexanone peroxide (a mixture of multiple hydroperoxides)
Among them, the main structure is (Ⅰ).
Cyclohexanone peroxide is dissolved in dibutyl ester and becomes a 50% paste, called 1# curing agent
3.2 Dibenzoyl peroxide (is a peroxide, referred to as BPO)
Dibenzoyl peroxide is dissolved in dibutyl ester and becomes a 50% paste, called 2# curing agent
3.3, methyl ethyl ketone peroxide (MEKP for short)
This is a liquid curing agent, generally formulated into a 50% dimethyl ester solution, which is the commercially available 5# curing agent.
Among the active ingredients, similarly, it is not a single compound, but a mixture of hydroperoxides with multiple molecular structures:
These compounds have different activities. Hydroperoxy (-OOH) increases the activity and hydroxyl (-OH) decreases the activity.
At present, the most commonly used curing agent in China is 5# curing agent. It is worth noting that the quality of the domestically produced 5# curing agent has declined, and there are shortcomings such as too high content of low molecular substances in the curing agent and too high water content. Because the production process is not close, and explosion accidents occur frequently, many manufacturers' current production processes do not use distillation to remove water, but use low-temperature cooling and static separation. The disadvantage of this method is that the water is not exhausted, and the water content in the curing agent is too high. If multiple freezing separation methods are used, it will result in low yield and high cost. In order to increase the active oxygen content of the curing agent, some businesses directly add hydrogen peroxide to the curing agent. For such curing agent, the following phenomena will occur during use:
① After curing agent and accelerator are added into the resin, a large number of bubbles are generated. The phenomenon of resin with low reactivity or high inhibitor content is particularly obvious.
② The temperature rises in summer and the blistering phenomenon is more serious.
This is caused by the rapid decomposition of hydrogen peroxide in the curing agent and failure to react with the resin in a timely manner.
4. Types of accelerators commonly used in cold curing systems
Strictly speaking, accelerators can be divided into three categories:
① It is effective for hydroperoxides such as cyclohexanone peroxide and methyl ethyl ketone peroxide, such as cobalt naphthenate and cobalt octoate. The former is commonly used abroad.
② Effective for peroxides such as dibenzoyl peroxide BPO, such as tertiary amines: dimethylaniline, diethylaniline, etc.
③, effective for both, such as dodecyl mercaptan and so on.