Epoxy resins are thermosetting polymers derived from the reaction of epichlorohydrin and bisphenol A (BPA) or other polyols. They cure through cross-linking, forming rigid three-dimensional networks that do not soften when reheated, distinguishing them from thermoplastics. The epoxy group (oxirane ring) is the reactive site that opens during polymerization.
Chemical Composition and Curing
Epoxy resins are typically supplied as two-component systems: the resin (Part A) and a hardener or curing agent (Part B). Common hardeners include aliphatic amines, cycloaliphatic amines, and aromatic amines. The stoichiometric ratio between resin and hardener determines cure kinetics and final properties. Mixing ratios by weight typically range from 1:1 to 4:1, depending on the hardener type. Cure temperatures vary from ambient (20–25 °C) for slow-hardening systems to elevated temperatures (60–120 °C) for accelerated curing.
Physical and Mechanical Properties
Fully cured epoxy resins exhibit tensile strength of 40–90 MPa, compressive strength of 90–150 MPa, and flexural strength of 80–130 MPa. Elongation at break is typically 1–6 %. Glass transition temperature (Tg) ranges from 50 °C to over 200 °C depending on formulation. Density lies between 1.1 and 1.4 g/cm³. Hardness (Shore D scale) reaches 80–90. These properties make epoxy resins suitable for structural reinforcement, adhesive bonding, and impact-resistant coatings.
Chemical Resistance and Durability
Epoxy resins offer excellent resistance to alkaline solutions, petroleum hydrocarbons, and many organic solvents. Resistance to strong acids and concentrated polar solvents is moderate. Water absorption typically occurs at 0.5–2.5 % by weight under immersion at 23 °C. UV exposure degrades unmodified epoxies; UV-stabilized formulations with hindered amine light stabilizers (HALS) extend durability. Service life in submerged or damp conditions depends on epoxy type and hardener selection.
Building Applications
In construction, epoxy resins function as binders in floor coatings and screeds, delivering thickness from 1–5 mm with seamless, monolithic surfaces. Epoxy-based primers bond to concrete, masonry, and metal substrates. Resin-bound aggregates create decorative and wear-resistant floor systems. Self-leveling epoxy pours achieve compressive strength of 50–70 MPa within 24–72 hours. Injection resins for crack repair and concrete strengthening penetrate pores with viscosity from 10–500 mPa·s. Epoxy mortars repair spalls and patch defects.
Composite Reinforcement
As matrix material for fiber-reinforced polymers (FRP), epoxy resins bond glass, carbon, or aramid fibers. Fiber volume fractions of 40–60 % yield composites with tensile strength exceeding 500 MPa. Epoxy-carbon systems are used in structural reinforcement of concrete and masonry, applied via wet layup or pre-impregnated (prepreg) sheets.
Health and Safety
Uncured epoxy resins and hardeners present skin sensitization and respiratory hazards. Bisphenol A (BPA) is a known sensitizer; exposure limits vary by jurisdiction. BPA-free formulations using alternative bisphenols (BPF, BPM) are available. Adequate ventilation, personal protective equipment, and barrier creams are required during application. Cured epoxies are inert and non-hazardous.
Regulatory Compliance
Product safety and performance data are documented in technical datasheets. VOC (volatile organic compound) content in epoxy formulations typically ranges from 0–50 g/L; low-VOC and solvent-free systems meet stringent indoor air quality requirements. Hazard classification and labeling follow GHS (Globally Harmonized System) standards. EU Building Products Regulation (CPR) and national construction standards govern use in fire-rated or load-bearing applications.