Material Properties

Mechanical Properties  

Glass transition temperature (Tg) 120 - 130 °C

Tensile strength 85 N/mm²

Tensile Modulus 10,500 N/mm²

Elongation at break 0.8%

Flexural strength 112 N/mm²

Flexural Modulus 10,000 N/mm²

Compressive Strength 190 N/mm²

Coefficient of linear thermal expansion 34 10-6

Water absorption - 24 hours at 23°C 5-10 mg (0.06-0.068%) ISO 62 (1980)

Thermal Properties  

Thermal Shock 2000 cycles (90 sec. at 75 °C, 90 sec. dwell, 90 sec. at 15°C) No effect

  Should not be subjected to dry-ice or liquid nitrogen.

Smoke Emission Low Smoke Emission – BS 6853 1999 App. D Clause d.8.4

  Result – Ao(on) 8.75, Ao(off) 10.41

Flammability Class 0 as classified by the current Building Regulations

Thermal Decomposition 350° C

Radioactive Decontamination BS 4247 Part 1 Test

Decontamination factor (geometric mean) A5598

Deviation factor 1.25

Ease of decontamination classification “Excellent”

Chemical Resistance of Epoxy Resin

Colours Available

EpoxyFrom Wikipedia, the free encyclopedia

This article is about the thermoset plastic materials. For the chemical group, see epoxide.

[hide]This article has multiple issues. Please help improve it or discuss these issues on the talk page

This article may contain an excessive amount of intricate detail that may only interest a specific audience

This article may be too technical for most readers to understand. (February 2013)

A syringe of "5-minute" epoxy, containing separate compartments for each of the two components

Epoxy is the cured end product of epoxy resins, as well as a colloquial name for the epoxide functional group. Epoxy is also a common name for a type of strong adhesive used for sticking things together and covering surfaces,[1] typically two resins that need to be mixed together before use.

Epoxy resins, also known as polyepoxides are a class of reactive prepolymers and polymers which contain epoxide groups. Epoxy resins may be reacted (cross-linked) either with themselves through catalytic homopolymerisation, or with a wide range of co-reactants including polyfunctional amines, acids (and acid anhydrides), phenols, alcohols, and thiols. These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction is commonly referred to as curing. Reaction of polyepoxides with themselves or with polyfunctional hardeners forms a thermosetting polymer, often with strong mechanical properties as well as high temperature and chemical resistance. Epoxy has a wide range of industrial applications, including metal coatings, use in electronic and electrical components, high tension electrical insulators, fibre-reinforced plastic materials, and structuraladhesives. Epoxy resin is employed to bind gutta percha in some root canal procedures.[2]

Popular consumer two-part epoxy adhesives for home, shop, and hobby are available in stores ranging in a wide selection of properties, including slow vs. fast curing time, opaque vs. clear colors, water-proof vs water-resistant, and flexible vs. rigid. A common misconception is that all epoxies are waterproof, however many – perhaps most – are not recommended for long-term submersion (such as flower vases) nor below the water line. Also, some epoxies bond better than others to different materials – even to different metals.

Contents

  [hide] 

1   Epoxy chemistry o 1.1   Bisphenol A epoxy resin

o 1.2   Bisphenol F epoxy resin

o 1.3   Novolac epoxy resin

o 1.4   Aliphatic epoxy resin

o 1.5   Glycidylamine epoxy resin

2   Curing epoxy resins o 2.1   Homopolymerisation

o 2.2   Amines

o 2.3   Anhydrides

o 2.4   Phenols

o 2.5   Thiols

3   History 4   Applications

o 4.1   Paints and coatings

o 4.2   Adhesives

o 4.3   Industrial tooling and composites

o 4.4   Electrical systems and electronics

o 4.5   Consumer and marine applications

o 4.6   Aerospace applications

o 4.7   Biology

o 4.8   Art

5   Industry 6   Health risks 7   Accidents 8   References 9   External links

Epoxy chemistry[edit]

Epoxy resins are low molecular weight pre-polymers or higher molecular weight polymers which normally contain at least two epoxide groups. The epoxide group is also sometimes referred to as a glycidyl or oxirane group.

A wide range of epoxy resins are produced industrially. The raw materials for epoxy resin production are today largely petroleum derived, although some plant derived sources are now becoming commercially available (e.g. plant derived glycerol used to make epichlorohydrin).

Epoxy resins are polymeric or semi-polymeric materials, and as such rarely exist as pure substances, since variable chain length results from the polymerisation reaction used to produce them. High purity grades can be produced for certain applications, e.g. using a distillation purification process. One disadvantage of high purity liquid grades is their tendency to form crystalline solids due to their highly regular structure, which require melting to enable processing.

An important characteristic of epoxy resins is the epoxide content. This is commonly expressed as the epoxide number, which is the number of epoxide equivalents in 1 kg of resin (Eq./kg), or as the equivalent weight, which is the weight in grams of resin containing 1 mole equivalent of epoxide (g/mol). One measure may be simply converted to another:

Equivalent weight (g/mol) = 1000 / epoxide number (Eq./kg)

The equivalent weight or epoxide number is used to calculate the amount of co-reactant (hardener) required when curing epoxy resins. Epoxies are typically cured withstoichiometric or near-stoichiometric quantities of curative to achieve the best physical properties.

As with other classes of thermosetting polymer materials, blending different grades of epoxy resin, as well as use of additives, plasticizers or fillers is common to achieve the desired processing and/or final properties, or to reduce cost. Use of blending, additives, and fillers is often referred to as formulating.

Bisphenol A epoxy resin[edit]

The most common and important class of epoxy resins is formed from reacting epichlorohydrin with bisphenol A to form diglycidyl ethers of bisphenol A. The simplest resin of this class is formed from reacting two moles of epichlorohydrin with one mole of bisphenol A to form the bisphenol A diglycidyl ether (commonly abbreviated to DGEBA or BADGE). DGEBA resins are transparent colourless-to-pale-yellow liquids at room temperature, with viscosity typically in the range of 5-15 Pa.s at 25 °C. Industrial grades normally contain some distribution of molecular weight, since pure DGEBA shows a strong tendency to form a crystalline solid upon storage at ambient temperature.

Structure of bisphenol-A diglycidyl ether epoxy resin: n denotes the number of polymerized subunits and is typically in the

range from 0 to 25

Increasing the ratio of bisphenol A to epichlorohydrin during manufacture produces higher molecular weight linear polyethers with glycidyl end groups, which are semi-solid to hard crystalline materials at room temperature depending on the molecular weight achieved. As the molecular weight of the resin increases, the epoxide content reduces and the material behaves more and more like a thermoplastic. Very high molecular weight polycondensates (ca. 30 000 – 70 000 g/mol) form a class known as phenoxy resins and contain virtually no epoxide groups (since the terminal epoxy groups are insignificant compared to the total size of the molecule). These resins do however contain hydroxyl groups throughout the backbone, which may also undergo other cross-linking reactions, e.g. with aminoplasts, phenoplasts and isocyanates.

Bisphenol F epoxy resin[edit]

Bisphenol F may also undergo epoxidation in a similar fashion to bisphenol A. Compared to DGEBA, bisphenol F epoxy resins have lower viscosity and a higher mean epoxy content per gram, which (once cured) gives them increased chemical resistance.

Novolac epoxy resin[edit]

Reaction of phenols with formaldehyde and subsequent glycidylation with epichlorohydrin produces epoxidised novolacs, such as epoxy phenol novolacs (EPN) and epoxy cresol novolacs (ECN). These are highly viscous to solid resins with typical mean epoxide functionality of around 2 to 6. The high epoxide functionality of these resins forms a highly crosslinked polymer network displaying high temperature and chemical resistance, but low flexibility. 100% solids hybrid novolac epoxy resin systems have been developed that contain no solvents and no volatile or organic compounds. These hybrid novolac epoxies have been documented to withstand up to 98% sulfuric acid.

Aliphatic epoxy resin[edit]

There are two types of aliphatic epoxy resins: glycidyl epoxy resins and cycloaliphatic epoxides.

Glycidyl epoxy resins are typically formed by the reaction of epichlorohydrin with aliphatic alcohols or polyols to give glycidyl ethers or aliphatic carboxylic acids to give glycidyl esters.[3] This reaction is normally done in the presence of an alkali, such as sodium hydroxide, to facilitate the dehydrochlorination of the intermediate chlorohydrin. The resulting resins may be monofunctional (e.g. dodecanol glycidyl ether), difunctional (diglycidyl ester of hexahydrophthalic acid), or higher functionality (e.g. trimethylolpropane triglycidyl ether). These resins typically display low viscosity at room temperature (10-200 mPa.s) and are often used as reactive diluents. As such, they are

employed to modify (reduce) the viscosity of other epoxy resins. This has led to the term ‘modified epoxy resin’ to denote those containing viscosity-lowering reactive diluents. However, they are also used without other epoxide ingredients along with anhydride curing agents such as hexahydrophthalic anhydride to make molded objects such as high voltage insulators. This is in fact the main use of the diglycidyl esters.

The cycloaliphatic epoxides contain one or more cycloaliphatic rings in the molecule to which the oxirane ring is fused (e.g. 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate). They are formed by the reaction of cyclo-olefins with a peracid, such as peracetic acid.[4] This class also displays low viscosity at room temperature, but offers significantly higher temperature resistance and correspondingly better electrical properties at high temperatures to cured resins than the glycidyl aliphatic epoxy resins. Another advantage is the complete absence of chlorine, since no epichlorohydrin is used in the manufacturing process. This is particularly useful for electronic applications such as the encapsulation of light emitting diodes. However, room temperature reactivity is rather low compared to other classes of epoxy resin, and high temperature curing using suitable accelerators is normally required.

Glycidylamine epoxy resin[edit]

Glycidylamine epoxy resins are higher functionality epoxies which are formed when aromatic amines are reacted with epichlorohydrin. Important industrial grades are triglycidyl-p-aminophenol (functionality 3) and N,N,N,N-tetraglycidyl-4,4-methylenebis benzylamine (functionality 4). The resins are low to medium viscosity at room temperature, which makes them easier to process than EPN or ECN resins. This coupled with high reactivity, plus high temperature resistance and mechanical properties of the resulting cured network make them important materials for aerospace composite applications.

Curing epoxy resins[edit]

In general, uncured epoxy resins have only poor mechanical, chemical and heat resistance properties. However, good properties are obtained by reacting the linear epoxy resin with suitable curatives to form three-dimensional cross-linked thermoset structures. This process is commonly referred to as curing. Curing of epoxy resins is an exothermic reaction and in some cases produces sufficient heat to cause thermal degradation if not controlled.

Curing may be achieved by reacting an epoxy with itself (homopolymerisation) or by forming a copolymer with polyfunctional curatives or hardeners. In principle, any molecule containing a reactive hydrogen may react with the epoxide groups of the epoxy resin. Common classes of hardeners for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols. Relative reactivity (lowest first) is approximately in the order: phenol < anhydride < aromatic amine < cycloaliphatic amine < aliphatic amine < thiol.

Whilst some epoxy resin/ hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150 °C being common, and up to 200 °C for some specialist systems. Insufficient heat during cure will result in a network with incomplete polymerisation, and thus reduced mechanical, chemical and heat resistance. Cure temperature should typically attain the glass transition temperature (Tg) of the fully cured network in order to achieve maximum properties. Temperature is sometimes increased in a step-wise fashion to control the rate of curing and prevent excessive heat build-up from the exothermic reaction.

Hardeners which show only low or limited reactivity at ambient temperature, but which react with epoxy resins at elevated temperature are referred to as latent hardeners. When using latent hardeners, the epoxy resin and hardener may be mixed and stored for some time prior to use, which is advantageous for many industrial processes. Very latent hardeners enable one-component (1K) products to be produced, whereby the resin and hardener are supplied pre-mixed to the end user

and only require heat to initiate curing. One-component products generally have shorter shelf-lives than standard 2-component systems, and products may require cooled storage and transport.

The epoxy curing reaction may be accelerated by addition of small quantities of accelerators. Tertiary amines, carboxylic acids and alcohols (especially phenols) are effective accelerators. Bisphenol A is a highly effective and widely used accelerator, but is now increasingly replaced due to health concerns with this substance.

Homopolymerisation[edit]

Epoxy resin may be reacted with itself in the presence of an anionic catalyst (a Lewis base such as tertiary amines or imidazoles) or a cationic catalyst (a Lewis acid such as a boron trifluoride complex) to form a cured network. This process is known as catalytic homopolymerisation. The resulting network contains only ether bridges, and exhibits high thermal and chemical resistance, but is brittle and often requires elevated temperature to effect curing, so finds only niche applications industrially. Epoxy homopolymerisation is often used when there is a requirement for UV curing, since cationic UV catalysts may be employed (e.g. for UV coatings).

Amines[edit]

Structure of TETA, a typical hardener. The amine (NH2) groups react with the epoxide groups of the resin during

polymerisation.

Polyfunctional primary amines form an important class of epoxy hardeners. Primary amines undergo an addition reaction with the epoxide group to form a hydroxyl group and a secondary amine. The secondary amine can further react with an epoxide to form a tertiary amine and an additional hydroxyl group. For aliphatic amines both primary and secondary amino hydrogens have approximately the same reactivity, while for aromatic amines, the reactivity of the secondary amine is typically 2 to 5 times less than the reactivity of a primary amino hydrogen.[5] Use of a difunctional or polyfunctional amine along with a difunctional epoxy resin forms a three-dimensional cross-linked network. Aliphatic, cycloaliphatic and aromatic amines are all employed as epoxy hardeners. Amine type will alter both the processing properties (viscosity, reactivity) and the final properties (mechanical, temperature and chemical resistance) of the cured copolymer network. Thus amine structure is normally selected according to the application. Reactivity is broadly in the order aliphatic amines > cycloaliphatic amines > aromatic amines. Temperature resistance generally increases in the same order, since aromatic amines form much more rigid structures than aliphatic amines. Whilst aromatic amines were once widely used as epoxy resin hardeners due to the excellent end properties they imparted, health concerns with handling these materials means that they have now largely been replaced by safer aliphatic or cycloaliphatic alternatives.

Anhydrides[edit]

Epoxy resins may be cured with cyclic anhydrides at elevated temperatures. Reaction occurs only after opening of the anhydride ring, e.g. by secondary hydroxyl groups in the epoxy resin. A possible side reaction may also occur between the epoxide and hydroxyl groups, but this may be suppressed by addition of tertiary amines. The low viscosity and high latency of anhydride hardeners makes them suitable for processing systems which require addition of mineral fillers prior to curing, e.g. for high voltage electrical insulators.

Phenols[edit]

Polyphenols, such as bisphenol A or novolacs can react with epoxy resins at elevated temperatures (130-180 °C), normally in the presence of a catalyst. The resulting material has ether linkages and displays higher chemical and oxidation resistance than typically obtained by curing with amines or anhydrides. Since many novolacs are solids, this class of hardeners is often employed for powder coatings.

Thiols[edit]

Also known as mercaptans, thiols with the (S-H) functional group, contain an electron poor hydrogen which reacts very readily with the epoxide group, even at ambient or sub-ambient temperatures. Whilst the resulting network does not typically display high temperature or chemical resistance, the high reactivity of the thiol group makes it useful for applications where heated curing is not possible, or very fast cure is required e.g. for domestic DIY adhesives and chemical rock bolt anchors. Thiols have a characteristic odour, which can be detected in many two-component household adhesives.

History[edit]

The first commercial attempts to prepare resins from epichlorohydrin were made in 1927 in the United States. Credit for the first synthesis of bisphenol-A-based epoxy resins is shared by Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee of the United States in 1936. Dr. Castan's work was licensed by Ciba, Ltd. of Switzerland, which went on to become one of the three major epoxy resin producers worldwide. Ciba's epoxy business was spun off and later sold in the late 1990s and is now the Advanced Materialsbusiness unit of Huntsman Corporation of the United States. Dr. Greenlee's work was for the firm of Devoe-Reynolds of the United States. Devoe-Reynolds, which was active in the early days of the epoxy resin industry, was sold to Shell Chemical (now Momentive Specialty Chemicals, formerly Hexion, Resolution Polymers and others).

Applications[edit]

The shelf life of unmixed two-part epoxies is long. There are many anecdotal reports of epoxies mislaid for decades and then used successfully.[6] However, one more commonly sees one to two years published as product specifications.

The applications for epoxy-based materials are extensive and include coatings, adhesives and resin matrices for composite materials such as those using carbon fiber andfiberglass reinforcements (although polyester, vinyl ester, and other thermosetting resins are also used for glass-reinforced plastic). The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties. In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good-to-excellent mechanical properties and very good electrical insulating properties. Many properties of epoxies can be modified (for example silver-filled epoxies with good electrical conductivity are available, although epoxies are typically electrically insulating). Variations offering high thermal insulation, or thermal conductivity combined with high electrical resistance for electronics applications, are available.[7]

Paints and coatings[edit]

Two part epoxy coatings were developed for heavy duty service on metal substrates and use less energy than heat-cured powder coatings. These systems generally use a 4:1 by volume mixing ratio, and dry quickly providing a tough, protective coating with excellent hardness. Their low volatility and water cleanup makes them useful for factory cast iron, cast steel, cast aluminium applications and reduces exposure and flammability issues associated with solvent-borne coatings. They are usually used in industrial and automotive applications since they are more heat resistant than latex-based and alkyd-based paints. Epoxy paints tend to deteriorate, known as chalk out, due to UV exposure.

Polyester epoxies are used as powder coatings for washers, driers and other "white goods". Fusion Bonded Epoxy Powder Coatings (FBE) are extensively used for corrosion protection of steel pipes and fittings used in the oil and gas industry, potable water transmission pipelines (steel), and concrete reinforcing rebar. Epoxy coatings are also widely used as primers to improve the adhesion of automotive and marine paints especially on metal surfaces where corrosion (rusting) resistance is important. Metal cans and containers are often coated with epoxy to prevent rusting, especially for foods like tomatoes that are acidic. Epoxy resins are also used for decorative flooring applications such asterrazzo flooring, chip flooring, and colored aggregate flooring. Epoxy flooring has been proven to be an environmentally friendly alternate to other types of flooring, reducing the facility's impact on the environment through less water consumption and less pesticides needed.[8]

Adhesives[edit]

Special epoxy is strong enough to withstand the forces between asurfboard fin and the fin mount. This epoxy is waterproof

and capable ofcuring underwater. The blue-coloured epoxy on the left is still undergoing curing.

Epoxy adhesives are a major part of the class of adhesives called "structural adhesives" or "engineering adhesives" (that includespolyurethane, acrylic, cyanoacrylate, and other chemistries.) These high-performance adhesives are used in the construction of aircraft, automobiles, bicycles, boats, golf clubs, skis, snowboards, and other applications where high strength bonds are required. Epoxy adhesives can be developed to suit almost any application. They can be used as adhesives for wood, metal, glass, stone, and some plastics. They can be made flexible or rigid, transparent or opaque/colored, fast setting or slow setting. Epoxy adhesives are better in heat and chemical resistance than other common adhesives. In general, epoxy adhesives cured with heat will be more heat- and chemical-resistant than those cured at room temperature. The strength of epoxy adhesives is degraded at temperatures above 350 °F (177 °C).[9]

Some epoxies are cured by exposure to ultraviolet light. Such epoxies are commonly used in optics, fiber optics, and optoelectronics.

Industrial tooling and composites[edit]

Epoxy systems are used in industrial tooling applications to produce molds, master models, laminates, castings, fixtures, and other industrial production aids. This "plastic tooling" replaces metal, wood and other traditional materials, and generally improves the efficiency and either lowers the overall cost or shortens the lead-time for many industrial processes. Epoxies are also used in producing fiber-reinforced or composite parts. They are more expensive than polyester resins and vinyl ester resins, but usually produce stronger and more temperature-resistant composite parts.

Electrical systems and electronics[edit]

An epoxy encapsulated hybrid circuit on a printed circuit board

The interior of a pocket calculator. The dark lump of epoxy in the center covers the processor chip.

Epoxy resin formulations are important in the electronics industry, and are employed in motors, generators, transformers, switchgear, bushings, and insulators. Epoxy resins are excellent electrical insulators and protect electrical components from short circuiting, dust and moisture. In the electronics industry epoxy resins are the primary resin used in overmolding integrated circuits, transistors and hybrid circuits, and making printed circuit boards. The largest volume type of circuit board—an "FR-4 board"—is a sandwich of layers of glass cloth bonded into a composite by an epoxy resin. Epoxy resins are used to bond copper foil to circuit board substrates, and are a component of the solder mask on many circuit boards.

Flexible epoxy resins are used for potting transformers, inductors and LED's. By using vacuum impregnation on uncured epoxy, winding-to-winding, winding-to-core, and winding-to-insulator air voids are eliminated. The cured epoxy is an electrical insulator and a much better conductor of heat than air. Transformer and inductor hot spots are greatly reduced, giving the component a stable and longer life than unpotted product.

The LED industry is an up and coming area in which epoxy resins are playing a big role. The potted LED's are protected from the environment, while the resin can be either clear to protect the lights aesthetics, or hazy to offer light diffraction.[10]

Epoxy resins are applied using the technology of resin dispensing.

Consumer and marine applications[edit]

Epoxies are sold in hardware stores, typically as a pack containing separate resin and hardener, which must be mixed immediately before use. They are also sold in boat shops as repair resins for marine applications. Epoxies typically are not used in the outer layer of a boat because they deteriorate by exposure to UV light. They are often used during boat repair and assembly, and then

over-coated with conventional or two-part polyurethane paint or marine-varnishes that provide UV protection.

There are two main areas of marine use. Because of the better mechanical properties relative to the more common polyester resins, epoxies are used for commercial manufacture of components where a high strength/weight ratio is required. The second area is that their strength, gap filling properties and excellent adhesion to many materials including timber have created a boom in amateur building projects including aircraft and boats.

Normal gelcoat formulated for use with polyester resins and vinylester resins does not adhere to epoxy surfaces, though epoxy adheres very well if applied to polyester resin surfaces. "Flocoat" that is normally used to coat the interior of polyester fibreglass yachts is also compatible with epoxies.

Epoxy materials tend to harden somewhat more gradually, while polyester materials tend to harden quickly, particularly if a lot of catalyst is used. The chemical reactions in both cases are exothermic. Large quantities of mix will generate their own heat and greatly speed the reaction, so it is usual to mix small amounts which can be used quickly.

While it is common to associate polyester resins and epoxy resins, their properties are sufficiently different that they are properly treated as distinct materials. Polyester resins are typically low strength unless used with a reinforcing material like glass fibre, are relatively brittle unless reinforced, and have low adhesion. Epoxies, by contrast, are inherently strong, somewhat flexible and have excellent adhesion. However, polyester resins are much cheaper.

Epoxy resins typically require a precise mix of two components which form a third chemical. Depending on the properties required, the ratio may be anything from 1:1 or over 10:1, but in every case they must be mixed in exactly the right proportions, and thoroughly to avoid unmixed portions. The final product is then a precise thermo-setting plastic. Until they are mixed the two elements are relatively inert, although the 'hardeners' tend to be more chemically active and should be protected from the atmosphere and moisture. The rate of the reaction can be changed by using different hardeners, which may change the nature of the final product, or by controlling the temperature.

By contrast, polyester resins are usually made available in a 'promoted' form, such that the progress of previously-mixed resins from liquid to solid is already underway, albeit very slowly. The only variable available to the user is to change the rate of this process using a catalyst, often Methyl-Ethyl-Ketone-Peroxide (MEKP), which is very toxic. The presence of the catalyst in the final product actually detracts from the desirable properties; just enough catalyst to harden fast enough is preferable. The rate of cure of polyesters is controlled by the amount and type of catalyst, and the temperature.

As adhesives, epoxies bond in three ways: a) mechanically, because the bonding surfaces are roughened; b) by proximity, because the cured resins are physically so close to the bonding surfaces that they are hard to separate; c) ionically, because the epoxy resins form ionic bonds at an atomic level with the bonding surfaces. This last is substantially the strongest of the three. By contrast, polyester resins can only bond using the first two of these, which greatly reduces their utility as adhesives and in marine repair.

Aerospace applications[edit]

In the aerospace industry, epoxy is used as a structural matrix material which is then reinforced by fiber.[11] Typical fiber reinforcements include glass, carbon, Kevlar, and boron. Epoxies are also used as a structural adhesive; wood and other 'low-tech' materials are glued with epoxy resin. In 1968, Apollo 8's capsule reentered Earth's atmosphere, protected by a heat shield of epoxy resins manufactured by Dow Chemical Company.[12]

Biology[edit]

Cells (black) embedded in epoxy resin (amber) for transmission electron microscopy

Water-soluble epoxies such as Durcupan [13] [14]  are commonly used for embedding electron microscope samples in plastic so they may be sectioned (sliced thin) with a microtome and then imaged.[15]

Art[edit]

Epoxy resin, mixed with pigment, may be used as a painting medium, by pouring layers on top of each other to form a complete picture.[16]

Industry[edit]

As of 2006, the epoxy industry amounts to more than US$5 billion in North America and about US$15 billion worldwide. The Chinese market has been growing rapidly, and accounts for more than 30% of the total worldwide market. It is made up of approximately 50–100 manufacturers of basic or commodity epoxy resins and hardeners.

These commodity epoxy manufacturers mentioned above typically do not sell epoxy resins in a form usable to smaller end users, so there is another group of companies that purchase epoxy raw materials from the major producers and then compounds (blends, modifies, or otherwise customizes) epoxy systems from these raw materials. These companies are known as "formulators". The majority of the epoxy systems sold are produced by these formulators and they comprise over 60% of the dollar value of the epoxy market. There are hundreds of ways that these formulators can modify epoxies—by adding mineral fillers (talc, silica, alumina, etc.), by adding flexibilizers, viscosity reducers,colorants, thickeners, accelerators, adhesion promoters, etc.. These modifications are made to reduce costs, to improve performance, and to improve processing convenience. As a result a typical formulator sells dozens or even thousands of formulations—each tailored to the requirements of a particular application or market.

Impacted by the global economic slump, the epoxy market size declined to $15.8 billion in 2009, almost to the level of 2005. In some regional markets it even decreased nearly 20%. The current epoxy market is experiencing positive growth as the global economy revives. With an annual growth rate of 3.5 - 4% the epoxy market is expected to reach $17.7 billion by 2012 and $21.35 by 2015. Higher growth rate is foreseen thereafter due to stronger demands from epoxy composite market and epoxy adhesive market.[17]

Health risks[edit]

The primary risk associated with epoxy use is often related to the hardener component and not to the epoxy resin itself. Amine hardeners in particular are generally corrosive, but may also be classed toxic and/or carcinogenic or mutagenic. Aromatic amines present a particular health hazard (most

are known or suspected carcinogens), but their use is now restricted to specific industrial applications, and safer aliphatic or cycloaliphatic amines are commonly employed.

Liquid epoxy resins in their uncured state are mostly classed as irritant to the eyes and skin, as well as toxic to aquatic organisms. Solid epoxy resins are generally safer than liquid epoxy resins, and many are classified non-hazardous materials. One particular risk associated with epoxy resins is sensitization. The risk has been shown to be more pronounced in epoxy resins containing low molecular weight epoxy diluents.[18] Exposure to epoxy resins can, over time, induce an allergic reaction. Sensitization generally occurs due to repeated exposure (e.g. through poor working hygiene and/or lack of protective equipment) over a long period of time. Allergic reaction sometimes occurs at a time which is delayed several days from the exposure. Allergic reaction is often visible in the form of dermatitis, particularly in areas where the exposure has been highest (commonly hands and forearms). Epoxy use is a main source of occupational asthma among users of plastics.[19] Bisphenol A, which is used to manufacture a common class of epoxy resins, is a known endocrine disruptor.

Accidents[edit]

The Big Dig ceiling collapse, which occurred on July 10, 2006, is an accident which was caused by the use of fast-set epoxy which had a higher rate of creep than standard-set epoxy. The accident resulted in one fatality and cost $54 million to redesign, repair, and inspect all of the tunnels in the D Street connector. The cost of the fast-set epoxy used to secure the bolts was $1,287.60.

Epoxy coatings or chemical sealer usually have a slick finish presenting a potential slip and fall hazard risk, however there are anti skid additives which can help mitigate this and provide increased traction.[20]

Strength & StiffnessTwo important mechanical properties of any resin system are its tensile strength and stiffness. The two figures below

show results for tests carried out on commercially available polyester, vinylester and epoxy resin systems cured at

20°C and 80°C.

 

 

After a cure period of seven days at room temperature it can be seen that a typical epoxy will have higher properties

than a typical polyester and vinylester for both strength and stiffness. The beneficial effect of a post cure at 80°C for

five hours can also be seen.

Also of importance to the composite designer and builder is the amount of shrinkage that occurs in a resin during and

following its cure period. Shrinkage is due to the resin molecules rearranging and re-orientating themselves in the

liquid and semi-gelled phase. Polyester and vinylesters require considerable molecular rearrangement to reach their

cured state and can show shrinkage of up to 8%. The different nature of the epoxy reaction, however, leads to very

little rearrangement and with no volatile by-products being evolved, typical shrinkage of an epoxy is reduced to

around 2%. The absence of shrinkage is, in part, responsible for the improved mechanical properties of epoxies over

polyester, as shrinkage is associated with built-in stresses that can weaken the material. Furthermore, shrinkage

through the thickness of a laminate leads to 'print-through' of the pattern of the reinforcing fibres, a cosmetic defect

that is difficult and expensive to eliminate.

Epoxy plastics’ general chemical and physical properties

An epoxy resin is defined as a molecule with more than one epoxy group, which can be hardened into a usable

plastic. The epoxy group, which is also called the glycidyl group, has through its characteristic appearance given the

name to epoxy.

Epoxy group

What one sees is an oxygen atom on the outside of the carbon chain. Epi means “on the outside of” and the second

part of the word comes from oxygen.

There are two spellings, namely epoxi and epoxy. The first comes from the oxygen’s bond with the carbon chain

being called an oxide. Epoxy resin is manufactured from simple basic chemicals that are readily available.

With the help of chemical formulas, the last stage out is as follows:

Bisphenol A + Epichlorohydrin

Diglycidylether Bisphenol A (DGEBA)

epoxy resin

By varying the relationship between bisphenol A and epichlorohydrin, various molecular weights are obtained for the

completed epoxy resin. The lowest molecular weight an epoxy resin of the DGEBA type can have is 340, but if two

elements together can form different molecular weights when they react, the epoxy resin will contain a mixture of

epoxy molecules of varying lengths. One therefore does not refer to the epoxy resins’ molecular weight, but rather to

their mean molecular weight.

Epoxy resin with a mean molecular weight of over 700 is called high molecular, and epoxy resin with a mean

molecular weight of under 700 low molecular. Epoxy resins can be allergens, and it is the molecular weight that

determines how great the risk. The higher molecular weight, the lower the probability for allergies.

In the formula for epoxy resin, the letter “n” is seen after the brace. If n=0, i.e. that which is inside the brace does not

exist, we then have the shortest epoxy molecule with a molecular weight of 340. It has the highest reactivity and thus

also constitutes the greatest allergy risk. If n=1, the molecular weight is 624, for n=2 it is 908, etc.

For each time n increases by 1, the molecular weight increases by 284. In a low molecular epoxy resin with the mean

molecular weight of 380, the distribution is approximately 88% n=0, 10% n=1 and 2% n=2. A pure epoxy resin with

n=2 is not an allergen, but if we look at a commercial epoxy resin with the molecular weight of 1080, the distribution is

approximately 20% n=0, 15% n=1, 15% n=2 and 50% n=3, 4 and 5. This means the even a high molecular epoxy

resin can be an allergen.

A low molecular epoxy resin with a mean molecular weight of 380 is fluid at room temperature, while an epoxy resin

with a mean molecular weight of 1000 is solid at room temperature. The molecular weight determines what the epoxy

resin can be used for.

The low molecular can be handled without solvent additives, which evaporate and are therefore used for casting,

thick coatings, gap-filling glues, etc.

The high molecular epoxy resins must as a rule be dissolved in organic solvents to be manageable, which limits

usage to paints and lacquers.

To convert epoxy resin to epoxy plastic, a reaction with a suitable substance is required. Such a substance in this

context is called a hardener.

Examples of substance groups that function as epoxy hardeners are: amines, amides, acid anhydride’s, imidazoles,

boron trifluoride complexes, phenols, mercaptans and metal oxides.

For hardening at room temperature, amines and amides are primarily used, and to a certain extent mercaptans. The

other hardener types generally require temperatures above +150°C to react with the epoxy. From this point forward,

only amines and amide hardeners will be further described.

Amines are substances that are closely related to ammonia (NH3). Depending on how many hydrogen atoms that are

replaced by alkyl groups, primary amines NH2-R, secondary amines NH-R1 or tertiary amines N-R2 arise. The total

number of amino groups determines if the amine is a monoamine (NH2-R), a diamine (NH2-R-NH2) or a polyamine

(NH2-R-NH-R-NH-R-NH2).

Furthermore, the amines are divided into aliphatic, i.e. a straight carbon chain, cycloaliphatic with a ring-shaped

carbon chain and finally aromatic where the amino group is bonded to a benzene ring.

As hardener for the epoxy resin, primarily diamines and polyamines are used.

The primary amino group NH2 contains as seen, two hydrogen atoms and one nitrogen atom. It is the hydrogen that

constitutes the reactive part, and the reaction occurs with the oxygen in the epoxy group.

With somewhat simplified chemical formulas, it looks like this:

Primary amine + Epoxy group     gives     Secondary

amino group

In the first reaction phase, one of the amine’s hydrogen atoms reacts with the epoxy group’s oxygen, causing the

formation of a hydroxyl group (OH-) at the same time as the primary amine is reduced to a secondary amine. The

reaction continues:

The secondary amine reacts with yet another epoxy group and the reaction is complete.

An epoxy molecule normally contains two epoxy groups, and one primary diamine has four reactive hydrogen atoms.

A schematic of the epoxy plastic then looks like this:

The epoxy plastic molecule is of course three-dimensional in reality.

Examples of amines that are used as hardeners for epoxy resins

Aliphatic amines

Diethylenetriamine

Triethylenetetramine

Aminoethyl piperazine

Trimethyl hexamethylenediamine

Cycloaliphatic amines

Isophorondiamine

Diamino-dicyclohexylmethane

Aromatic amines

Diaminodiphenylmethane

m-Phenylendiamine

It is very common to pre-react the amines with a certain portion of epoxy resin. The purpose of this is to attain a

hardener that is less fluid than the pure amine and that has a somewhat higher reactivity.

This type of hardener is called amine adducts, and reacts largely in the same way as previously shown.

The next large group of hardeners is the amides, or more correctly expressed, the polyaminoamides. An amide is

formed when a polyamine reacts with a fatty acid.

This generally applies to all polyamides, even nylon. By varying the relationship between fatty acid and amine, one

can decide if the polyamide will be acid-terminated (of the type nylon) or amine-terminated, i.e. which end groups the

polyamide will have. Only amine-terminated polyamides can be used as hardeners for epoxy resin. As fatty acid,

most often used is tall fatty acid, linolic acid or olein. The fatty acids use either monomer (one carboxylic group) or

dimer (two carboxylic groups). As polyamines, diethylenetriamine, triethylenetetramine and tetraethylenepentamine

are used. The hydrogen in the amide group (CONH) is not reactive, but it is rather the hydrogen in the primary amino

groups at the polyamide’s ends and the secondary amino groups that are derived from the polyamine, which are

reactive with the epoxy resin in the same way as previously described.

Both amine adducts and polyamides can be made water soluble. Such solutions have the capacity to emulsify low

molecular epoxy resin, which in turn provides the possibility to manufacture water soluble epoxy paints.

The reaction between an epoxy resin and a hardener is an irreversible poly-addition, i.e. no by-products are formed,

and the epoxy plastic cannot be decomposed into epoxy resin and hardener. The reaction is exothermic, which

means that heat is released. Depending on the type of hardener one uses, one can achieve very large differences in

reaction speeds. This has significant practical importance when working with epoxy. The time it takes to consume a

mixture of epoxy resin and hardener is called potlife. Depending on the mixture’s reactivity, potlife can vary from a

few seconds to several years.

Potlife can be determined in several ways.

One method is to temper the epoxy resin and hardener to +20°C. Thereafter an amount of 100 grams is mixed in a

plastic cup. The time for the mixture’s temperature to reach +50°C is set as the mixture’s potlife. For systems with low

reactivity (long potlife), one most often chooses to measure the viscosity or consistency, and measure the time until

the initial viscosity is doubled.

Both of these methods are unusable for aqueous emulsified or aqueous dispersed systems. In this case, one instead

performs a lay-up test and measures the time to gloss reduction.

Most chemical reactions follow Arrhenius’ law, which states that the reaction speed doubles for each tenth of a

degree the temperature is increased. This means that a reaction is twice as fast at +30°C as at +20°C.

The larger the amount that is mixed together, the greater the amount of exothermic heat that is created. There is not

enough time for this heat to be dissipated through the mixture vessel’s surface, so instead it heats the mixture. As the

temperature increases, the reaction speed also increases, which entails that the potlife is shorter, the larger the

amount that is mixed.

Example of potlife for 100 and 500 grams of the same epoxy resin/hardener mixture

Potlife for an epoxy system consequently provides certain information to the user on working time after mixture, but

one must consider the amount of material mixed, and the material’s initial temperature. The epoxy’s hardening time is

defined as the time from the epoxy being applied until the formed epoxy plastic has achieved its final properties

pertaining to strength and chemical resistance.

For epoxy that is applied in thin layers, the exothermic heat will not increase the temperature in the layer to any

significant degree, but instead, the epoxy quickly assumes the temperature of the substrate. The reaction between

the epoxy resin and hardener then goes relatively quick at the beginning because of the large availability of reactive

molecules, and because the mobility of the molecules is high as long as the viscosity is low.

As complete epoxy molecules are formed, the number of reactive molecules is reduced at the same time as the

viscosity increases. The reaction speed gradually slows.

As a rule of thumb, room temperature hardened epoxy needs about 7 days at +20°C to attain maximum properties,

but already after 24 hours, one can have attained about 70–80% of the final properties.

Arrhenius’ law naturally applies even if the reaction occurs at a constant temperature. This means that if the substrate

is at +10°C, it takes about 14 days to attain final properties.

Example of epoxy’s hardening process at constant temperature.

Epoxy plastic’s characteristic basic properties

Currently more than 50 different substances fulfil the definition for an epoxy resin. If one also adds that there are

several hundred different hardeners, it is easy to understand that epoxy plastic’s properties can be modified to satisfy

the most varied requirements. Nevertheless, certain basic properties are always present.

Adhesion

One of epoxy plastic’s most characteristic properties is the capacity to adhere to most substrates. The reason for this

is the presence of polar hydroxyl groups, and the ether bonds. The negligible shrinkage also entails that contact

between the epoxy plastic and the substrate is not disturbed by tensions. The epoxy plastic’s surface tension is most

often under the critical surface energy for most materials. This is one of the requirements for adhesion to be

achieved.

Mechanical strength

No other hard plastic can display as high mechanical strength as correctly formulated epoxy plastic. Again, it is

largely because of the minimal shrinkage that built-in tensions are avoided. The tensile strength can exceed 80 MPa.

Chemical resistance

Thanks to the possibility of varying the epoxy plastics’ properties, one can make epoxy plastic resistant to most

chemicals. In general, epoxy plastic is very resistant to alkali, which is of importance in surface-treating concrete.

Diffusion density

Epoxy generally has relatively high vapour transmission resistance, but with a special technique, epoxy plastic can be

made open to diffusion. Epoxy that is open to diffusion can be applied on, for example, wet concrete and provide

adhesion higher than the concrete’s tensile strength.

Water tightness

The epoxy plastics are to be considered as watertight and they are often used to protect against water.

Electrical insulation capacity

Epoxy plastics are excellent electrical insulators. Volume resistivity is normally 1015Ohm·cm. This in combination

with high moisture resistance and chemical resistance makes the epoxy suitable for both the manufacture of

electronics components and the embedment of transformers.

Shrinkage

The epoxy plastics have very slight shrinkage during hardening. This is because the epoxy molecule has a rather

small reorientation during the hardening process compared with, for example, polyester and methylmetacrylate.

Heat resistance

When it comes to heat resistance, room temperature hardened epoxy plastic differs very little from heat hardened.

One often specifies heat resistance with HDT (Heat Deflection Temperature) or Tg (Glass transition temperature).

At HDT, the mechanical strength declines quickly. Room temperature hardened epoxy seldom attains HDT above

70°C, while heat hardened can reach 250°C.

Modifiable

Perhaps the chief property of epoxy is the nearly unlimited capability to modify the final properties of the epoxy plastic

to meet special requirements. It is primarily the hardener that influences the plastic’s properties, but as presented in

the next chapter, there are many other substances that influence epoxy products.

Stability in light

Epoxy plastics based on aromatic epoxy resins are sensitive to light in the UV range. Direct radiation with ultraviolet

light quickly causes yellowing. Even normal sunlight contains enough ultraviolet radiation for yellowing to occur. Most

resistant are aliphatic epoxy resins with anhydride or amine hardener.

Modification of epoxy resins

Viscosity at 25°C of an unmodified low molecular epoxy resin of the DGEBA type, is about 10 Pa·s, at 20°C about 24

Pa·s and at 15°C about 68 Pa·s.

It is easy to understand that an unmodified epoxy resin cannot always be used outdoors or where the temperature is

low. The first reason for modification is thus viscosity reduction to a suitable working consistency. Several options are

available here.

First, we have non-reactive diluents, which refers to such substances that can be mixed with the epoxy resin but that

do not participate in the reaction between the epoxy and hardener. Included here are the ordinary solvents such as

xylene, toluole, glycol ethers, ketones, lower alcohols, etc.

Other non-reactive diluents include a large number of substances with sufficiently low vapour pressure that they do

not evaporate from the hardened plastic under normal conditions. Examples of these are benzyl alcohol and

coumarone resins. The substances that are not chemically bonded in the epoxy plastic molecule are to be imagined

as deposited between the plastic molecular chains.

In this group are the “ordinary” solvents that have the strongest influence on viscosity, but which often entail a

dangerous path.

Epoxy resin has very high solvent retention, i.e. the capacity to retain solvents. This means that it will take a long time

before the solvent has evaporated. In a 1 mm thick layer, solvent residue can be detected after several months at

room temperature.

Damage that can arise from solvents is usually blistering, either through high heating or because of osmosis when

moisture occurs in concrete. Another reason to avoid solvents is the shrinkage effect that can result in layers

detaching from weak substrates.

The diluents that under normal conditions do not evaporate affect the epoxy plastic’s properties in other ways than

viscosity reduction. Positive changes can be flexibility, improved resistance to water and salt solutions because of

hydrophobing, improved reactivity, in part because the mobility of the epoxy and the hardener molecule is better at

lower viscosity, and in part because of the catalytic effect from hydroxyl groups in, for example, benzyl alcohol.

Negative changes are degraded heat resistance and larger thermal expansion.

At higher temperatures, volatility can be substantial with several of these substances. This can result in shrinkage

with cracking and subsequent reduced adhesion. Another aspect that sometimes must be considered is the

compatibility with bitumen. A minority of the non-reactive diluents do not bleed in bitumen.

Furthermore, many diluents in this group are esters that can saponify upon contact with concrete under the influence

of water.

Reactive diluent (thinners)

These substances have, as the name implies, the capacity to react with the epoxy resin or hardener, so that in this

way, bond in the plastic molecule.

The most used types are those that contain one or more epoxy groups. There is a very large group of such

substances that all can give the final plastic different properties.

What characterizes these substances is that they cannot migrate, evaporate or be extracted from the epoxy plastic.

Heat resistance declines somewhat in comparison to unmodified epoxy, but not at all as much as in the case of the

non-reactive diluent. A valuable property of the reactive diluent is that they reduce the surface tension of the epoxy

resin, which in turn can improve wetting capabilities and thus adhesion. Depending on the reactive diluent

composition, both flexibility and chemical resistance can be affected.

Other substances used to modify the epoxy plastic’s properties are, for example, high molecular isocyanates, which

via the hardener, can react with the epoxy resin. The result is a plastic with rubber-elastic properties, which are

retained down to about -40°C.

Such modification changes the epoxy plastic’s fundamental properties.

Heat resistance and chemical resistance decline with increased elasticity. At the same time, the viscosity increases

because the isocyanates in themselves are very highly viscous. This means that further modification is necessary to

reduce viscosity.

The next group of modification substances is pigments and fillers.

Pigment is used to colour the epoxy material, and the filler to increase the mechanical strength and to reduce costs.

The pigments used are most often metal oxides such as titanium dioxide, iron oxide and chromium oxide.

As a rule, fillers are finely ground minerals and quartz sand. It is important that both pigments and fillers are properly

dispersed in the epoxy binding agent.

In addition to the listed modification substances, there are a number of aids to affect such things as rheology, i.e.

consistency, flow, air bubbles and adhesion.

It is easy to understand that an epoxy product has a relatively complex composition, where the included components

shall interact for the results to be as intended.