Study on the curing mechanism of unsaturated polyester resin UPR

1. Introduction
Unsaturated polyester resin (UPR) curing seems to be a very thorough theoretical and practical problem, but because the factors affecting curing reaction are quite complex, and in the various applications of UPR, the quality defects of products are almost all related to "curing". Therefore, we need to explore the curing of UPR in more depth.
(exploring the curing of unsaturated polyester resins, first of all, some concepts and definitions related to the curing of unsaturated polyester resins)
2. Concepts and definitions related to unsaturated polyester resin curing
2. 1 Definition of curing
Liquid UPR under the effect of light, heat or trigger can form an insoluble body structure
C.
) in three-way crosslinking by combining unsaturated double bonds in a wired polyester chain with the double bonds of the cross-linked monomer. This process is called the curing of the UPR.
2. The curing of
unsaturated polyester resin is a co-polymerization reaction caused by free base, and how to make the reaction start is the key to the problem. Once a monosome is triggered to produce a free base, the molecular chain can rapidly grow and form a three-way cross-linking of large molecules.
unsaturated polyester resin curing is the first to make unsaturated C-C double-bond break, due to the chemical bond break the energy required to be different, for the C-C key, its key energy E-350kJ/mol, it takes 350-550 degrees C temperature to stimulate it to crack.
clearly, it is not practical to cure the resin at such a high temperature. So people have found a substance that breaks down to produce free fundamentals at lower temperatures, which are organic peroxides. The O-O keys of some organic peroxides break down to produce free fundamentals at lower temperatures. Some of these peroxides, which can be decomposed at 50-150 degrees C, are useful for the curing of resins. We can use this characteristic of organic peroxides to select some of them as resin triggers, or curing agents.
definition of a
- unsaturated polyester resin is a peroxide, also known as a trigger or catalyst, that causes the resin to crosslink under the action of a promoter or other external condition.
the term "catalyst" here is different from the traditional sense of "catalyst". Traditionally, the term "catalyst" is used to help reactants, which, while promoting reactions, do not consume themselves. In a UPR curing reaction, the peroxide must change its own structure before it "catalyticizes" the reaction, so a more appropriate name for the peroxide used for UPR curing should be called a "starter" or "trigger".
two concepts we need to understand when it comes to peroxides are reactive oxygen content and critical temperature. Wherein "reactive oxygen" or "reactive oxygen content" is a concept that is closely related to curing agents and is often misunderstood.
reactive oxygen content:
of the total amount of oxygen and peroxide molecules in peroxide is simply the active oxygen content.
the concept itself, the reactive oxygen content of a peroxide with a lower molecular weight may be relatively high. This does not mean, however, that peroxides with high reactive oxygen content are more or faster active than peroxides with low reactive oxygen content. (Because many of our application manufacturers use reactive oxygen content as an indicator to assess curing agents) In fact, reactive oxygen content is only a measure of the concentration and purity of any particular peroxide as a constant. It has been found that many peroxides with high reactive oxygen content are not suitable for curing resins because they break down or "run out" quickly at standard curing temperatures, i.e. they break down free-to-air properties too quickly. Because freeening ternates always have a strong tendency to bind to each other, when free-to-base produces faster than they are used by unsaturated double bonds, they reassemble or terminate the polymer chain, resulting in a low molecular weight polymer resulting in incomplete curing. (A typical example is hydrogen peroxide).
: In simple
, critical temperature is the minimum temperature at which peroxides break down in large quantities to produce free fundamentals. (This temperature is generally just an approximation.) Before this temperature there was also free base release, but to varying degrees. In
we can divide peroxides into medium-temperature or high-temperature triggers depending on the critical temperature of the peroxide. For extrusion molding and molding, the operating temperature is determined according to the critical temperature of the peroxide used. The operating temperature is generally set slightly higher than the critical temperature of the trigger. (e.g., the critical temperature of methamphetamine peroxide is 80 degrees C; the critical temperature of benzoyl peroxide is 70 degrees C; the critical temperature of statins is 146 degrees C; and the statins perbenzene are 194 degrees C.) The extrusion molding process uses peroxybenzene and peroxide as triggers, and the temperature used for process heating is 90 degrees C;
2. 3 Promoter
The outside temperature directly affects the rate at which peroxides produce free base, and heating allows the curing agent to release free base, which causes resin curing, which is of course feasible, but high temperature operation will also bring some inconvenience. As a result, it was further discovered that some organic peroxides can be activated with another compound, which usually works through an oxidation-reducing reaction that can crack to create a free base at ambient temperatures without heating up. This substance, which activates peroxides at ambient temperatures, is a promoter or activator.
the definition of a
promoter is a substance that promotes the curing agent to form a free base (i.e., room temperature curing) below its critical temperature.
2. 4 Photo-curing
Another substance that causes resin curing is light, the most energetic ultraviolet light in the spectrum that produces the active energy, which breaks the C-C bond of the resin, producing free-forming materials that cure the resin. For example, we've done experiments where the resin can gel in a day, even if it's below 0C, if it's placed in a place where the sun shines directly.
when a photosensitive agent is added to the UPR, it is triggered by ultraviolet or visible light as an energy source, allowing the resin to cross-link quickly.
To this point, we can learn that, depending on the way in which it is triggered, there are three types of unsaturated polyester resin curing:
thermo cure:
external heating allows the curing agent to release the free base, thus triggering the process of resin curing. (also known as heat-triggered curing)
cold curing:
The process by which the curing agent releases a free base by adding a promoter to release the free base at room temperature or at a low curing temperature. (also known as chemical decomposition causes curing)
photo curing:
by adding photosensitive agents, using ultraviolet light as energy, triggering the process of resin cross-curing. (also known as light-triggered curing)
below we mainly discuss the commonly used curing systems in cold curing
3. The types of curing agents commonly used in cold curing systems
3.1, cyclohetogen peroxide (a mixture of a variety of hydrogen peroxides)
which are dominated by the first (I.) structure.
peroxide cyclohetogen dissolves in diclones and becomes 50% paste, called a curing agent
3.2, peroxide (a type of peroxide, referred to as BPO)
structure:
peroxide is dissolved in ditrate become 50% paste, called 2 s curing agent
3.3, methyl peroxide (MEKP)
this is a liquid curing agent, generally with an effective ingredient of 50% of the methamphetamine solution, is the commercially available 5 s curing agent.
In the active ingredients, again, not a single compound, but a mixture of hydrogen peroxides from a variety of molecular structures:
these compounds have different activities, hydrogen peroxygen (-OOH) increases activity, and hydroxyl (-OH) reduces activity.
the most commonly used curing agent in China is the 5-curing agent. It is worth noting that at present, the quality of domestic 5-curing agent has decreased, there are low molecular content in curing agent is too high, water content is too high and so on. Because the production process is not closed, explosion accidents occur frequently, many manufacturers of the current production process does not use distillation method to remove water, and the use of low-temperature cooling static separation method, the disadvantage of this method is not endless water removal, curing agent water content is too high, if the use of multiple frozen separation method, but also will cause low yield, high cost. Some merchants in order to improve the reactive oxygen content of curing agents, to the curing agent directly added hydrogen peroxide, for such curing agents, the use of the following phenomena will occur:
(1), curing agents, promoters added to the resin to produce a large number of bubbles, low reaction activity or polymer content of the resin phenomenon is particularly obvious.
(2) and summer temperatures rise, the blistering phenomenon is more serious.
is caused by the rapid decomposition of hydrogen peroxide in the curing agent and the failure to react with the resin in a timely manner.
4. Types of promoters commonly used in cold curing systems
Strictly speaking, promoters can be divided into three categories:
(1), hydroxide such as cyclohexone peroxide, methyl peroxide and other effective, such as cobalt cyclane acid, cobalt octane and so on. The former is commonly used abroad.
(2), which is effective against peroxides such as benzoyl peroxide BPO, such as shamylamine: methamphetamine, diethylamine, etc.
(3) and are effective for both, such as terse alkyl thiol.
is actually commonly used in the first two, the latter of which is of little significance.
commonly used promoter
(1), cobalt cyclane acid, generally 1% styrene solution, called a promoter. It is often used in conjunction with the 1-curing agent peroxide cyclohetogen. For decades, cobalt salt preservers have been considered to have good curing properties and are widely used in room temperature curing of unsaturated polyester resins. Due to the influence of cobalt salt color, it has been widely recognized in recent years that its gel curing effect and color can not meet the needs.
(2), N, N-methamphetamine, usually 10% styrene solution, called a 2-booster. Often used in conjunction with a 2-curing agent (benzoyl peroxide). In the resin contains a large number of free phenol or polyester molecular chain containing the molecular structure of the large molecular branch chain, is a very effective curing system. (e.g. for vinyl resin curing, bisphenol Class A polyester resin curing, chlorpyric acid polyester resin, etc.).
(3), cobalt isoic acid, commonly used in pre-promoting resins, especially with a stronger cobalt isoic acid pre-promoter, can get a better drying effect. Usually isoenic acid is better than cobalt cyclane acid, because cyclane acid is a molecular weight is not fixed (molecular weight range 180-350) cyclane hydrocarbon-based derivatives, so its cobalt content is difficult to do very accurate, and because it is a by-product of petroleum refining, usually dark color, so the current market is cobalt instead of cobalt cyclane.
(4), cobalt -potassium-calcium-transition metal compound promoter, (often referred to as a 5-promoter) with alkali metal salts, alkali earth metal salts and variable prices of transition metal salts and cobalt salts, can achieve the effect of using cobalt salts alone as a promoter can not be achieved. This is one of the most common types of promoters on the market in recent years and can be divided into three categories:
(1) cobalt-potassium compound promoter, potassium salts have a greater synergy to the estimator. Its use can be used as an FRP product promoter. Potassium content should not be too high, cobalt content should not be too low. Failure to do so affects the strength of FRP products.
(2) cobalt-potassium-transition metal compound promoter, transition metal salts have great synergy with potassium salts, but there is no synergy with cobalt salts, and even delay. The addition of transition metal salts can greatly shorten the gel time and curing time, and can greatly reduce the temperature of the UPR heating peak, cobalt-potassium-transition metal salt compound promoter can be used in FRP products, but also for UPR casting system products.
(3) Cobalt - Potassium - Calcium - Transition Metal Compound Promoter, Calcium Salt on Cobalt - Potassium - Transition Metal Salt does not act as a synergetic promoter, but only acts as a whitening effect, making the casting body look lighter or close to colorless. Can be used in the casting process of upR.
need to be explained that the current commercially available promoter is mostly a compound promoter, compared with the traditional single cobalt salt type promoter, with low cost, low color number, fast curing characteristics. That is to say, at present, the quality of domestic promoter has been greatly improved, in the curing speed, curing on the color of products and many other aspects are better than imported materials.
addition, the promoter (N, N-methamphetamine) also has a strong co-effect on cobalt salts, often used in winter low temperature construction.
5. The curing process of unsaturated polyester resin
5.1 Analysis of the chemical dynamics of free-based polymerization
UPR curing is a free-based co-polymerization reaction. The curing reaction has the characteristics of chain trigger, chain growth, chain termination and chain transfer.
chain-triggering process from the decomposition of peroxide triggers to the formation of free base to the addition of this free base to unsaturated groups.
chain growth - the process by which monosomes are constantly added to the newly created free base. Chain growth requires much less activity than chain triggering.
chain - the combination of two free-base, ending the growing polymerization chain.
chain transfer - a growing large free base can interact with other molecules, such as solvent molecules or inhibitors, so that the original active chain disappears into a stable large molecule, while the original inactive molecule becomes a free base.
5. 2 Changes in molecular structure during curing of unsaturated polyester resins
The curing process of UPR is the process of cross-linking polymerization between unsaturated double bonds and cross-linked monosylbodies (usually styrene) in the UPR molecular chain, and the formation of three-dimensional network structures by long-chain molecules. In this curing process, there are three possible chemical reactions, namely,
(1),