Epoxy resin - A113

Epoxy resin
Document Type	Article
Document Identifier	113
Relevant Class	Material
Tags	Matrix Thermoset polymers Material Object
Properties Appearance Usually clear to platinum in colour State Viscous liquid at room temperature Density 960 - 1250 kg/m3 Flash point 100 - 250+ °C Hazards Globally harmonized system (GHS) Eye irritant Skin irritant Respiratory sensitizer Flammable liquid

Introduction

Epoxies are thermosetting resins containing a reactive oxirane epoxide ring, that crosslink with a hardening agent to form an insoluble solid. They are used as coatings, adhesives, encapsulating materials in electronics, and are a popular choice as a matrix material in fibre reinforced composites (FRPs). Compared to polyesters, phenolic, and melamine resins, epoxy resins are advantageous for the following characteristics :

• Dimensional stability during cure (low shrinkage)
• Excellent mechanical properties (good hardness, impact strength and toughness)
• Excellent Adhesion
• Good chemical resistance
• Chemical inertness
• Versatility for curing agent choice and curing conditions

Scope

This page covers thermoset epoxy resins. It presents the formulation, processing and design properties, advantages and limitations when compared with other thermoset resin systems (e.g. polyesters, phenolic resins, etc.), typical applications, and key considerations. Specifics about the microstructures, thermal transitions and cure kinetics of thermoset resins are given in the Foundational Knowledge volume.

Significance

Epoxies resins are used extensively in a wide range of composite parts and structures. They are the matrix material choice for high performance structural composites because of their excellent mechanical properties, and other desirable characteristics. They are often used for glass fibre reinforced polymer composites (GFRP) and carbon fibre reinforced composites (CFRP) over polyester resins when high mechanical performance is required.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Thermoset polymers
• Degree of cure
• Heat of reaction
• Thermal phase transitions of polymers
  

Object Description

Level I

Level II

Level III

Chemically, epoxy resins are characterized by their reactive oxirane structure, referred to as “epoxy” functionality or epoxy group. The simplest epoxy, and one of the most commonly used, is diglycidyl ether of bisphenol A (referred to as DGEBA) . It is the product of the reaction between bisphenol A and epichlorohydrin and its structure is pictured below.

Illustration of the epoxy group reactive oxirane structure, and the common diglycidyl ether of bisphenol A (DGEBA) epoxy structure.

As a viscous liquid prior to curing, epoxy resins are of a relatively low viscosity, permitting reinforcement infusion and part forming. In this initial state, epoxy resins contain short low molecular weight molecules (as shown in the DGEBA example) until reactions occur at the epoxy group sites during the cure process that transforms the liquid resin into a solid. These polymerization reactions include chain growth polymerization and the formation of crosslinks between the molecular chains, increasing the molecular weight of the resin as the molecules join together. While this reaction can occur on its own for some epoxies (homopolymerization), typically this reaction is achieved through the addition of a separate curing agent molecule – often amine.

Other common epoxy curing agents include:

• Anhydrides
• Polyamides
• Catalytic agents

Epoxy Reaction Stoichiometry

During the epoxy cure process, polymerization reactions take place at the reactive epoxy group sites. For the common amine hardener reaction, the epoxy ring structure opens up and reacts with an active hydrogen of the amine curing agent.

The selection of the epoxy curing agent is a key factor in determining the final properties of the epoxy system. To obtain optimum properties, the epoxide-reactive curing agent should be added at approximately the correct stoichiometric amount. In the amine curing agent example, this is the number of available reactive hydrogens in the curing agent, to the available reactive epoxide groups in the resin. The hardener-to-epoxy resin ratio provided in the resin system data sheet is generally assumed to be the 1:1 stoichiometric amount. However, variation away from 1:1 stoichiometric ratio is sometimes applied to alter the cured epoxy properties.

For example, the following epoxy property changes that can result from amine-to-resin stoichiometry adjustments are highlighted by epoxy supplier Reichhold :

Property	Excess Amine	Excess Epoxy
Water Resistance	Reduces	Increases
Heat Distortion	Reduces	Reduces
Acid Resistance	Reduces	Increases
Alkali Resistance	No Effect	No Effect
Solvent Resistance	Reduces	Reduces
Flexibility	Increases	Reduces

Unless seeking direct guidance from the resin supplier, it is best practice to follow the resin manufacturer’s instructions regarding the amount of curing agent addition or use the 1:1 stoichiometric ratio. The cured properties of the epoxy can vary dramatically with only a few % percent difference from the exact stoichiometric amount.

Epoxy Equivalent Weight (EEW) Calculation

The correct stoichiometric amounts of the required curing agent is determined using the Epoxy Equivalent Weight (EEW) calculation, also referred to as Epoxide Equivalent Weight (EEW) in some sources. Calculation of the EEW is necessary when matrix fillers or tougheners are added to the resin, as their added mass does not contain any reactive epoxy group molecules. The amount of curing agent needs to account for this added “resin dead weight”.

EEW can be calculated as follows (Example: amine curing agent):

Equation 1

Calculating the Amine H equivalent: (this value may already be provided on the curing agent datasheet)

\(Amine\ H\ eq\ wt = \frac{MW\ of\ amines}{no.\ of\ active\ hydrogens}\)

Where,

\( MW =\) Molecular Weight

\( no.\ =\) Physical number

Equation 2

Calculating the stoichiometric ratio of curing agent to EEW\[phr\ of\ amine = \frac{Amine\ H\ eq\ wt\ \times 100}{Epoxy\ eq\ wt\ of\ resin}\]

Where,

\( phr\ =\) Parts per hundred

Equation 3

Calculating the Epoxy Equivalent Weight of epoxy blends (including filler additives): (A value for resin EEW is typically provided on the resin’s data sheet. Otherwise, it can often be provided upon request from the resin supplier)

\(EEW\ of\ mix = \frac{Total \ Wt}{\frac{Wt \ a}{EEW \ a} + \frac{Wt \ b}{EEW \ b}}\)

Where,

\( Total \ Wt =\) Total weight of all reactive and non-reactive materials (e.g. fillers) together

\( EEW \ =\) Epoxy Equivalent Weight of resin a, Epoxy Equivalent Weight of resin b, etc.

\( Wt \ =\) Weight of resin a, weight of resin b, etc.

Amine Reaction

The commonly used amine curing agent reacts with the epoxy group through the active amine hydrogen. A walk through of this reaction process follows, summarized from Dow Epoxy literature .

A typical epoxy/amine reaction is shown below.

Schematic of the primary amine reaction with epoxy that occurs with an amine curing agent.

Primary amine is capable of reacting with two epoxide groups. After this initial reaction, a secondary amine is formed and reacts again as shown.

Schematic of the secondary amine reaction with epoxy that occurs with an amine curing agent.

The formed hydroxyl groups are theoretically able to further react with the epoxy groups. However, this reaction is catalyzed by tertiary amine – which in reality is restricted as the amine reaction is too immobile and hindered to act as a catalyst. Instead, the formed hydroxyl groups function to assist in the opening of the epoxide ring, acting to accelerate subsequent primary and secondary amine reactions .

Amine Reaction Accelerators

As described, hydroxyls can assist in opening the epoxy ring structure for primary and secondary amine reactions. Alcoholic and phenolic hydroxyls can be added as amine reaction accelerators to reduce gel time, and for higher molecular weight resins.

Anhydride Reaction

Coming soon.

Properties

The material property values for epoxy vary widely between different epoxy resin systems. Both the liquid and solid properties are heavily dependent on the specific epoxy resin/curing agent combination and the resulting cured epoxy crosslink network that forms.

A range of typical values is given in the table below.

Liquid resin		Notes:	Ref.
Viscosity at room temperature	0.5 - 40+ Pa.s	Viscosity typically lowers with addition of curing agent (mix viscosity), and with elevated temperature.
Cure shrinkage	2 - 7 vol%
Solid resin
Flexural strength	75 - 1890 MPa	Average: 907 MPa
Flexural modulus	2.4 - 205 GPa	Average: 58.5 GPa
Tensile strength	5.2 - 97 MPa	Average: 33.1 MPa
Tensile modulus	0.02 - 215 GPa	Average: 35.2 GPa
Elongation at break	0 - 50 %	Average: 9.35 %
Glass transition temperature	1 - 285 oC	Average: 123 oC

Applicable Processing Methods

Liquid epoxy resins can be processed with the following manufacturing methods:

• Hand lay-up
• LCM
• Press forming
• Filament winding
• Pultrusion

Applications

Typical sectors or products that use this material include:

• Wind energy
• Automotive
• Infrastructure (slurry transport piping)
• Other structural infrastructure applications (construction)
• Adhesives

Key considerations During Use

Preparation

While utilizing this material, the following are some of the key aspects to focus on to ensure that the quality of the final part is as high as possible. Epoxy resins should be stirred mechanically before use to ensure good mixing between the resin components. It is also recommended to degas the resin when processed with vacuum-based processes, such as light resin transfer molding. Epoxy resins and particularly many of the curing agents, are prone to moisture absorption during storage that can outgas during processing and form porosities.

Storage & Handling

Epoxy resins should be stored in tightly closed containers when not in use, in a dry and well-ventilated area preferably between 2-43°C (35-110°F) . They should be kept away from heat, sparks, flame and other sources of ignition. The shelf-life of epoxy resins varies, but 12-24 months is common with proper storage (exact shelf-life will be indicated on the resin data sheet). Crystallization can occur if stored below 25°C , seeded by dust particles or epoxy fillers , however, this physical change is reversible. Crystallization build up can be removed by simply heating to temperatures around 50°C or higher for a short period of time with no negative effects to the epoxy resin.

Suppliers

Product suppliers

• Olin (added Dow epoxy systems in 2015)
• Huntsman
• Evonik
• Sika
• Hexion
• Reichhold
• WEST SYSTEM

Expert support providers

A selection of people and companies capable of providing support for using this material to manufacture composite parts include:

• Composites Research Network
• Composites Knowledge Network
• Polyvations

References

About	Help

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Liquid Composite Moulding (LCM), a family of infusion processes refers to processes that saturate a dry reinforcement that is on/in the mould by means of a pressure differential (injection pressure, vacuum, combination of both).