Epoxy Adhesives: Low-Odor Thiol Curing Agent for Fast, Low-Temperature Cure

Epoxy adhesives are used across a wide range of applications, prized for their strong adhesion together with water and chemical resistance. Performance requirements vary by application, but fast cure is a near-universal need—shorter cure times improve both process efficiency and workability. Heat is required for cure, yet lower cure temperatures are increasingly demanded to limit direct thermal damage and the stress build-up that comes with thermal history.
Selecting a polyfunctional thiol as the curing agent is an effective way to achieve fast, low-temperature cure in epoxy adhesives. Primary thiols, however, carry a characteristic odor, and improving the working environment has been a practical challenge.

Resonac offers a lineup of multifunctional secondary thiol monomers that overcome the odor of primary thiols while enabling short-time, low-temperature curing of epoxy adhesives.

Resonac's polyfunctional secondary thiol monomers

Grade	Chemical name	Structure
Karenz MT PE1	Pentaerythritol tetrakis(3-mercaptobutyrate)
Karenz MT BD1	1,4-Bis(3-mercaptobutyryloxy)butane
Karenz MT NR1	1,3,5-Tris(2-(3-sulfanylbutanoyloxy)ethyl)-1,3,5-triazinane-2,4,6-trione
Karenz MT TPMB	Trimethylolpropane tris(3-mercaptobutyrate)

Solution

A low-odor secondary thiol for fast, low-temperature curing of epoxy adhesives

Compared by odor concentration, a primary thiol required dilution of more than 10,000× before the odor became undetectable, whereas the secondary thiol Karenz MT PE1 needed only a few hundred–fold (Figure 1). Switching from a primary to a secondary thiol thus cuts the odor that matters most in actual use.

As shown in Figure 2, thiols also serve as effective epoxy curing agents. Under a base catalyst such as an amine or imidazole, the thiol is deprotonated to a thiolate, which then adds nucleophilically to the epoxy group to form a carbon–sulfur bond. Because the thiolate is a stronger nucleophile than an alkoxide, phenolate or acetate, this addition proceeds rapidly (Table 1). Finally, the alkoxide formed on epoxy ring-opening abstracts a proton to regenerate the base catalyst, and this catalytic cycle drives the epoxy cure forward.

Comparing epoxy/curing-agent compositions at 5 °C, the amine showed no temperature change, while the thiol showed a temperature rise—evidence that cure was progressing. In short, PE1 works as an epoxy curing agent that solves the odor of primary thiols while enabling both fast and low-temperature cure.

Figure 1. Odor concentration: PE1 vs. primary thiol^{1) 2)}

1) Dilution factor at which an odor first becomes undetectable when diluted with odorless air. Measured with Shimadzu FF-2A.
2) Primary thiol : Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP)
Note:The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

Figure 2. Thiol–epoxy reaction mechanism

Figure 3. Temperature change of the compositions at 5 °C

Composition

Component		Curing-agent
		Secondary thiol PE1	Primary thiol4)	Amine5)
Epoxy resin3)		100	100	100
Curing-agent		70	70	80
Base catalyst6)		10	10	0

Features

Excellent Storage Stability

Two-part epoxy/thiol compositions using PE1 or a primary thiol were held at 60 °Cand monitored for viscosity. The secondary thiol PE1 showed no viscosity change over 30 days, whereas the primary thiol began to thicken by day 8 (Figure 4). Together with Figure 3, this indicates that PE1 can deliver both low-temperature cure and storage stability in epoxy systems.

Figure 4. Viscosity change of the compositions at 60 °C⁷⁾⁸⁾

7) Two-part epoxy (jER828, Mitsubishi Chemical)/thiol composition; primary thiol is PETMP
8) Viscosity measured at 25 °C

Updated：September 14, 2026

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