Epoxy resin

The composite properties come from fibre and resin. In theory, fibre has the dominant properties, but without any resin, the properties of the fibre alone are useless. Also, resin alone the required properties are away too far. But when combined, resin and fibre achieve superior properties that they cannot achieve alone.
The matrix or the resin transfers the load or tension from fibre to fibre. The better the resin is, the higher strength can be achieved for the final composite part. The perfect resin has the right balance between adhesion to fibre, strength and elongation.
The strength of the composite is related also to other properties of composites like good electrical insulation properties, low water absorption, high chemical resistance, fatigue properties, long lifetime, and weathering performance.
There are many different marketing materials available from different suppliers. Based on my experience, this is the order, shown below, how common resins behave from a strength point of view in composites.
Unsaturated polyester resins < Vinyl ester resins /< Polyurethane resins < Epoxy resins
Complete epoxy systems for pultrusion contain:● Epoxys● Hardeners● Accelerators/catalysts● Additives to improve processing● Additives to improve properties● Internal release agent(s)