A composition containing an epoxide compound with two or more epoxide groups, a thiol ester with two or more ester groups and two or more thiol groups and/or a thiol ether with two or more thiol groups, and 0.005-2 wt. %, based on the total weight of the composition, of a tertiary amine which has a 5- or 6-membered aliphatic nitrogen heterocycle, said composition containing less than 1 wt. % of a primary amine. The composition can be used as, an embedding medium for microscopy and as an adhesive and to an optical element which comprises the composition.

A composition containing an epoxide compound with two or more epoxide groups, a thiol ester with two or more ester groups and two or more thiol groups and/or a thiol ether with two or more thiol groups, and 0.005-2 wt. %, based on the total weight of the composition, of a tertiary amine which has a 5- or 6-membered aliphatic nitrogen heterocycle, said composition containing less than 1 wt.% of a primary amine. The composition can be used as an embedding medium for microscopy and as an adhesive and to an optical element which comprises the composition.

A composition containing an epoxide compound with two or more epoxide groups, a thiol ester with two or more ester groups and two or more thiol groups and/or a thiol ether with two or more thiol groups, and 0.005-2 wt. %, based on the total weight of the composition, of a tertiary amine which has a 5- or 6-membered aliphatic nitrogen heterocycle, said composition containing less than 1 wt. % of a primary amine. The composition can be used as, an embedding medium for microscopy and as an adhesive and to an optical element which comprises the composition.

A sheet molding compound which is a thickened material of an epoxy resin composition, including a component (A), a component (B), and a component (C), in which the component (A) is an epoxy resin staying at a liquid state at 25 C., the component (B) is an acid anhydride, the component (C) is an epoxy resin curing agent, and in the thickened material, at least some of epoxy groups of the component (A) and at least some of carboxy groups derived from the component (B) form ester.

A sheet molding compound which is a thickened material of an epoxy resin composition, including a component (A), a component (B), and a component (C), in which the component (A) is an epoxy resin staying at a liquid state at 25 C., the component (B) is an acid anhydride, the component (C) is an epoxy resin curing agent, and in the thickened material, at least some of epoxy groups of the component (A) and at least some of carboxy groups derived from the component (B) form ester.