Polymercaptans are used when the speed of cure is important. Even the “fast”-cure amine systems are slow in comparison, especially when the mass of epoxy is small and a thin film is used. This has led to the use of the mercaptans in a wide variety of applications:
A number of commercial mercaptan hardeners are available. They are sold with and without amine accelerators. The polymercaptan without the accelerator (Capcure 3-800) allows the formulator to have more control over the cure speed and flexibility of the product. This material is also used to accelerate amine, amidoamine and polyamide curing of epoxies. The standard five-minute cure is obtained with the accelerated Capcure® 3830-81. The fastest polymercaptan (Capcure 40 sec HV) has a gel time of 40 seconds with a 25-gram mass. There is a more water-resistant version (Capcure WR-6) for exterior uses.
The chemistry of the mercaptan reactions is fairly straightforward. The tertiary amine catalyst forms a salt with the mercaptan to generate a mercaptide anion, which is a strong nucleophile. The mercaptide will readily open the epoxy. Reaction with another mercaptan group can regenerate the mercaptide anion, as illustrated in Figure 1.
The low activation energy for the process allows the reaction to go at or below room temperature. Since heat is not necessary for the reaction to occur, it can cure in thin films even on metal substrates. The cure rate does slow as the mass gets smaller, and the film is thinner, but the decrease is not as dramatic as when using amine-based hardeners.
Another aspect of the mercaptan chemistry is the formulation flexibility. At a 1:1 weight ratio, there is an excess of epoxy compared to mercaptan groups. However, the tertiary-amine accelerator will cause the epoxy to self-cure. Thus, some variation in ratio is self-compensating:
Of course, deviating greatly from the 1:1 ratio will cause problems of incomplete cure.
In addition, the polymercaptans can be used to accelerate the curing of polyamides, amidoamines or amines. The other curatives serve as the base to accelerate mercaptans, and the mercaptans react rapidly, generating the heat to accelerate the cure with the other hardener. Commercial applications of the mercaptan accelerators are in coatings, adhesives and encapsulants.
We have made progress in the area of odor reduction and skinning resistance. The lower heat-deflection temperature is inherent in the structure of the mercaptan. In addition, improved heat resistance would be at the expense of the rapid room-temperature cure. The cost of mercaptans is also difficult to reduce due to the complex nature of the chemistry involved, safety in handling the chemicals used to produce the mercaptans, and the high, consistent quality of the products.
For example, both pyridine and benzenethiol have bad odors and are about the same size. However, the detection threshold for benzenethiol is 1/100 of the pyridine. Thus, a major problem of the mercaptan is that a person is much more likely to detect and identify it as a mercaptan at lower levels in the air. Opening a container gives a more noticeable odor since the volatiles build up in the headspace.
In addition, standard tertiary-amine accelerators also have a dimethylamine odor. There is one tertiary amine (Versamine® EH-50) that does not use the dimethylamine/formaldehyde addition to phenol. Thus, it does not give off these compounds by the reverse reaction.
In addition, there are a number of other factors involved in the odor evaluation. For example, non-laboratory personnel tend to react more negatively toward mercaptan odors. Secondly, for odor masks, florals were rated well by women, but not by men. Thirdly, one’s odor perception can vary from day to day, so one control sample is always run as a reference point, and the samples are evaluated on a relative basis. Since there is some desensitization of the evaluator upon smelling the samples, the order of samples is scrambled for each evaluation. To give time to develop a representative headspace, we waited four to 24 hours between evaluations. Based upon these techniques, we were able to improve the reliability of the odor evaluations.
Four ways to address malodor problems are shown in Figure 2. Removal can be used to eliminate some malodorous compounds. This route results in a low odor and usually does not alter the performance properties of the product. This route may be difficult to accomplish, especially if the malodor has a low odor threshold, the removal process causes some further degradation, or malodors generate slowly over time. Thus, people look for other solutions.
The most common method is to use a mask to overpower the odor, but this increases the total odor and may hurt the performance of the product. A variation of this is the newer technology of using a counteractant to reduce odor perception. The counteractant does not add to total odor the way a mask does. It is hard to find effective malodor counteractants since they are generally of low odor and tend to be very specific in their performance.
Another route to eliminating odors is to find an absorber. This is a molecule that binds specifically to the malodorous compound so that it does not volatilize. Therefore, the malodor is not perceived. A classic example of the last two methods is cat litter. The better-quality cat litter not only has an absorbent but also a malodor counteractant.
We have explored all these technologies in trying to keep the odor of the mercaptans as low as possible. We find that a combination of methods can generate the lowest possible malodor. Our new technology does not eliminate the odor but in side-by-side tests gives us about a 75% reduction in odor.
The skin forms by oxidation of the surface mercaptans to form disulfides. The first step of the process is the oxidation of the mercaptans to produce sulfide radicals. The radicals couple together to give the disulfides. As more molecules are linked by disulfide formation, a skin eventually forms on the surface. This is a surface effect and does not affect the bulk of the material.
The disulfide formation is extremely slow in the pure mercaptan. On the other hand, there are two common ways that the skin formation is promoted in formulated products. The first is by metal catalysis, with the metals coming from the fillers. The second is catalysis by amines and other bases, which are the accelerators or co-curatives with the mercaptans.
Among metal compounds, iron compounds are good oxidation catalysts because of their ability to cycle between the ferrous and ferric states. The ferric ion is known to convert mercaptans to sulfide radicals. The oxygen then re-oxidizes the ferrous compound to the ferric state, as illustrated in Figure 3.
Commercial polymercaptans are treated to remove iron so that the level is about 1 ppm of iron. However, oxidative catalysts, such as iron, can come from some fillers. The iron content of fillers can range up into the hundreds of parts per million. Some iron chelators are known to form such strong complexes that they will not undergo the oxidation-reduction cycle.
We have demonstrated in the lab that iron compounds, such as iron nitrate and iron acetylacetonate, can promote skinning and that the proper chelator can stop the iron-promoted skinning. Other metals, such as copper, cobalt or nickel, can also promote this process.
Since the mercaptide anion is important in curing the epoxies, we cannot stop the process at this step. Blanketing with nitrogen is effective in slowing the skinning by removing the oxygen from the process. Another method is to inhibit the free-radical process by adding antioxidants. We have found certain antioxidants are more effective than others.
The one disadvantage of metal chelators is that they also tie up some of the amine accelerator. As shown in the table in Figure 5, the gel time is slower for formulation C than A, but the hardness after half an hour is similar for the two samples. Comparing B, C and D shows that adding more amine does compensate for the chelator tying up some of the amine accelerator. Again, the effect is on the rate of cure and not on the degree of cure since the hardness is similar for all of the products. The tensile and lap-shear adhesive strengths are similar for the new product and the standard Capcure 3-800.
It is interesting to note that Accelerator 2 (Versamine EH-50) is less sensitive than Accelerator 1 (Versamine EH-30) to the addition of the chelator. Because Accelerator 2 does not have the dimethylamine odor, the combination with the modified polymercaptan gives the lowest total odor.
In summary, we have, with this technology, introduced advantages for epoxy formulators by offering a mercaptan curing agent with less skinning and lower odor than comparable products on the market.
This article is based on a presentation made at the Fall 2000 Adhesive and Sealant Council Convention in Minneapolis. Additional information on Capcure LOF is available from Coatings & Inks Div., Cognis Corp., 300 Brookside Ave., Ambler, PA 19002-3498; and Cognis Deutschland GmbH, Building C 02, D-40551 Düsseldorf, Germany. Call 800-445-2207, fax 215-628-1111, or visit the Web site at www.cognisci.com.