Quick answer: Epoxy–polythiol curing is a thiol–epoxy addition reaction, commonly accelerated by a suitable catalyst. Cure rate and properties depend on functional-group equivalents, catalyst, temperature, moisture, mixing, exotherm and network design.
Reaction mechanism of epoxy resin and polythiol monomer?
The reaction mechanism between epoxy resins and polythiols mainly involves the interaction between the thiol group and the epoxy group. The following is the detailed reaction mechanism:
Reaction initiation:
During the reaction between polythiols and epoxy resins, the unshared electron pair on the tertiary amine will first capture the active hydrogen of the thiol group, resulting in an anionic charge on the sulfur atom.
Nucleophilic addition reaction:
The negatively charged sulfur atom reacts with the alpha carbon atom on the epoxy resin in a process called a nucleophilic addition reaction.
This reaction results in the opening of the ternary epoxy group of the epoxy resin to form two hydroxyl groups.
Thiol-oxygen bonds are formed:
The sulfur atom in the thiol group reacts with the oxygen atom in the epoxy group in a nucleophilic addition reaction to form a sulfur-oxygen bond, which is a key step in the curing process.
Crosslink structure formation:
The two hydroxyl groups formed in the epoxy resin further react with the mercaptan groups contained in the polythiol to form a crosslinked structure.
This cross-linking structure binds the polythiol to the epoxy resin.
Curing Results:
Through the above reaction, the polythiol and the epoxy resin are combined to form a solid material that is resistant to abrasion, corrosion, and high temperatures.
Reaction Characteristics:
The curing time of polysulfide-cured epoxy is generally between a few minutes and a few hours, and does not require high temperature or high pressure treatment, which makes it widely used in industrial production.
The cured material has excellent physical and chemical properties, such as abrasion resistance, corrosion resistance, good insulation and other characteristics.
In summary, the reaction mechanism of epoxy resin and polythiol is a complex but highly efficient process, which realizes the close bonding and curing of the two materials through nucleophilic addition reaction and the formation of cross-linked structure.
Control points in an epoxy–polythiol formulation
Start from functional-group equivalents rather than simple weight percentages. The thiol equivalent weight, epoxy equivalent weight and functionality distribution determine the theoretical ratio; fillers, pigments and other reactive ingredients must be included in the calculation where applicable.
Catalyst type and level strongly affect latency, pot life and cure speed. Mixing temperature, batch size and insulation can also change the exotherm, so a small laboratory cup result should not be transferred directly to a large mass. Record temperature versus time and check whether viscosity rise limits application before the nominal gel point.
Validate cure with more than surface touch. Depending on the use, useful checks include conversion or residual functionality, hardness development, adhesion, tensile or lap-shear performance, chemical resistance, glass-transition behavior and aging. Moisture, amines, acidic contaminants and storage history can alter the result, so raw-material controls and a defined mixing sequence are part of the cure design.
Use a documented polythiol as the formulation reference
The current CHLUMICRYL® Polythiol PM839 page provides a relevant polythiol reference. Request current equivalent-weight and handling data, then set the epoxy-to-thiol balance and catalyst level from controlled cure, pot-life and final-property trials.
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