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Featured Article

Epoxy Curing Agents – Mercaptans, The Ultimate Quick Ambient Cure

August 8, 2022 By Jeffrey Gotro

Guest Post by Jeremy Pasatta, Huntsman Corporation

In the last two blog posts we looked at amines as curing agents for epoxies, and one downside of amines is that they can take up to a couple of hours to cure at ambient temperature depending on the structure, with the fastest amines still only curing in 30 minutes to an hour.  What if you wanted to develop an adhesive or coating that cured in minutes, or possibly even under a minute?  That is where mercaptans come into the picture as curing agents for epoxies.  Mercaptans cure epoxies through an active hydrogen on a sulfur group (S-H), and typical mercaptan curing agents that are sold commercially have multiple S-H groups available for curing.  A representative structure of a commercial polymercaptan curing agent is shown in Figure 1.

Figure 1.  Representative Structure of a Polymercaptan Curing Agent

Polymercaptans by themselves do not cure epoxies though, and require the use of an accelerator, which is typically a tertiary amine such as 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30), benzyl dimtheylamine (BDMA), polyamidoamines and ethyleneamines.  Figure 2 shows the effect of increasing catalyst concentration on the cure time of a polymercaptan with a standard bisphenol A based epoxy resin.  One hundred parts of epoxy were mixed with 100 parts of polymercaptan and either 5, 7.5 or 10 parts of DMP-30.  At 10 parts of DMP-30, hardness development was rapid, on the matter of minutes, with 7.5 parts DMP-30 giving similar gel times but not as high of ultimate hardness.  At 5 parts of DMP-30, gel was slow and ultimate hardness was low compared to the higher concentrations.

Figure 2.  Effect of Catalyst Concentration on Polymercaptan Cure Times

In addition to the gel time being tunable by the amount of catalyst that is added, mercaptans have other advantages that make them extremely useful curing agents for epoxy resins, and the main highlight are listed below:

Main Advantages of Polymercaptan Curing Agents

  • Convenient 1:1 mix ratio with bisphenol A epoxy
  • Excellent bonding to wood, metal and plastic
  • Gel time adjustable to < 1 minute to over 2 hours
  • Rapid build of hardness and other physical properties
  • Outstanding for low temperature applications
  • Lower toxicity than other curatives

Not only are mercaptans easy to formulate because of the 1:1 mix ratio with bisphenol A epoxy, they are very forgiving in their stoichiometry.  They are so forgiving in fact that the mix ratio can be off by up to as much as +/- 25% and not have this affect the properties.  Figure 3 shows the cure time of 100 parts of a bisphenol A epoxy mixed with varying parts of mercaptan and 10 parts of DMP-30.  It is only at 0 parts and 150 parts of mercaptan that the properties differ significantly.

Figure 3.  Effect of Polymercaptan Concentration on Cure Times

Not only do mercaptans give rapid curing at room temperature, but they also give rapid curing at temperatures as low as -9°C.  Even at these temperatures, mercaptans will cure epoxies in under a minute depending on the catalyst concentration.

One of the down sides of polymercaptan curing agents though is their relatively low Tg when cured with bisphenol A epoxy, giving a Tg only slightly above room temperature.  This can be overcome though by the formulator using a multifunctional epoxy or epoxy novolac, where Tg’s of 50 to 60°C are easily achievable with a room temperature cure, which significantly improves the temperature resistance.

No blog post on mercaptans would be complete without mentioning the odor of mercaptans.  Because of the sulfur groups present in the molecule, mercaptans have a characteristic sulfur-based odor.  Suppliers of mercaptan will often add masking agents, which when combined with the mercaptan give an odor profile that most users find less objectionable.

Overall, it is easy to see why mercaptans are so well suited for DIY and consumer applications – fast gel times, forgiving stoichiometry, 1:1 mix ratios, good performance at low temperature, ability to bond different substrates and relatively low toxicity.  Mercaptans do have their downside though, with low chemical, thermal and water resistance, which limits their use in some structural applications.  In the next blog post we will look at the other end of the performance spectrum with anhydrides, which can give extremely long pot life and very high thermal and chemical resistance.

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Epoxy Curing Agents – Part 2: Tailoring Properties with Amines

July 25, 2022 By Jeffrey Gotro

Guest Post by Jeremy Pasatta, Huntsman Corporation In the Part 1 blog post on amine curing agents for epoxies, we reviewed in general how amines react with epoxies, how to formulate with amine curing agents, and some of the general structure-property relationships of amines.  In this post, we will ... [Click to Continue...]

Epoxy Curing Agents – Part 1: Amines

July 11, 2022 By Jeffrey Gotro

Guest Post by Jeremy Pasatta, Huntsman Corporation This is the first in a series of six guest posts by Jeremy Pasatta. Curing agents for epoxies are one of the most critical, and often overlooked, parts of a formulation.  The correct choice of curing agent can dramatically improve the properties ... [Click to Continue...]

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Dielectric Cure Monitoring Part Six: What to Know about AC and DC Cure Monitoring

May 23, 2022 By Jeffrey Gotro

Guest Post by Huan Lee, Lambient Technologies Both AC and DC measurements can probe the cure state of thermosets and composites, and it is important to understand their characteristics when deciding which to use. Dielectric cure monitoring, also called Dielectric Analysis (DEA) is an AC technique ... [Click to Continue...]

Dielectric Cure Monitoring Part Five: Choose the Optimum Frequency for Cure Monitoring

March 7, 2022 By Jeffrey Gotro

Guest Post by Huan Lee, Lambient Technologies The dielectric response of thermosets and resins depends on a number of factors that vary with frequency, so it is important to select an optimum frequency for dielectric cure monitoring (DEA). To inform this choice, we must look at the material ... [Click to Continue...]

Dielectric Cure Monitoring Part Four: Use Log Scales to Get the Most from Ion Viscosity

February 21, 2022 By Jeffrey Gotro

Guest Post by Huan Lee, Lambient Technologies Instead of ion viscosity (electrical resistivity), many users of older cure monitoring equipment measure a material’s conductance and plot the results on a linear scale. Why conductance? Why a linear scale? Because that is an old-school method based on ... [Click to Continue...]

Dielectric Cure Monitoring Part Three: Real-Time Monitoring of a UV Cured Resin

February 7, 2022 By Jeffrey Gotro

Guest Post by Huan Lee, Lambient Technologies One advantage of dielectric cure monitoring (DEA) is the ability to observe rapid polymerization of UV curable adhesives like H.B. Fuller U3345, [Ref. 1] the topic of this post. In this case the use of techniques such as DSC is not possible, or at least ... [Click to Continue...]

Dielectric Cure Monitoring Part Two: Cure Monitoring of Epoxy Molding Compound (Part 2)

January 24, 2022 By Jeffrey Gotro

Guest Post by Huan Lee, Lambient Technologies In the first post of this series, we saw in theory how ion viscosity changes as a thermoset cures, and how Critical Points can characterize the cure. Now let’s see how epoxy molding compound cures in reality. Samples of EMC were placed on a Lambient ... [Click to Continue...]

Polymer Innovation Blog

I’m Jeff Gotro and welcome to the Polymer Innovation Blog. Over the last 30 years I have been involved in polymers, innovation, along with product and process development. I acquired invaluable experience working for IBM, AlliedSignal, Honeywell and Ablestik Labs (now Henkel). In this blog, I will share some of my experiences, tips, and “tricks of the trade” to get more out of your innovation efforts and discuss new trends and technical advances in the polymer industry.
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