Brief discussion of epoxy resins report.pdf

Acknowledgment

I am extremely thankful to Allah Almighty, who for all of the numerous difficulties and acute frustrations enables me to complete the study and present the humble piece of scientific work. I offer my humble thanks to Hazrat Muhammad (P.B.U.H) who is the beacon of advice and the biggest benefactor of mankind ever had.

I would like to offer my sincere gratitude to my advisor Dr. SHAMSA for the continuous support of my M.Sc. study, for her patience, motivation, humbleness, and immense knowledge. His guidance helped me in all the time of writing this report. I could not imagine a better advisor and mentor for my M.Sc. Besides my advisor, I am grateful to Chairman Prof. Dr. Ijaz Ahmad Bhatti and all the faculty of the Department of Chemistry especially Dr.Haq Nawaz Bhatti for his kind support and guidance, the University of Agriculture who despite their busiest schedule act like spiritual teachers. I'm thankful to my fellows for stimulating discussion, for days we were working together before the deadline and for all the fun we had in the last.

Abstract

In this research, a siloxane-type epoxy resin (SG copolymer), which has pendant epoxide rings on the side chain of the polysiloxane polymer backbone, was synthesized by the hydrosilylation reaction of poly(methylhydrosiloxane) with allyl glycidyl ether. The polymer structures were characterized by 1H NMR. The SG resin was then blended with a commercial epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) at various ratios, using dicyandiamide (DICY) as a curing agent. The curing behaviors were studied by DSC. It was found that the initial curing temperature (Ti) and peak curing temperature (Tp) were increased by the addition of SG copolymer to the epoxy resin. Their morphology, mechanical properties and the stability of the cured piece were investigated using SEM, DMA and TGA, respectively. The results show that the addition of SG copolymer increases the mobility of the cross linked network, and increases the thermal stabilityReactive polyimide containing hydroxyl functionalities was prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 3,3′-diamino-4,4′-dihydroxybiphenyl. Commercial epoxy resin was cured in the presence of different ratios of the reactive polyimide, giving a series of polyimide modified epoxy films. The transparent films had excellent solvent resistance. The tensile measurements of the films showed that, with the increase of the polyimide content, tensile modulus of the film increased but there was almost no change in the elongation at break. Viscoelastic measurements showed that glass transition temperature shifted with the increase of the polyimide content; 127°C for 13.5%, 220°C for 29.4%, 260°C for 45.4% and 290°C for 62.5%. Thermogravimetric analysis showed the increase of the thermal stability with the increase of the polyimide content.

Table of Contents

1. Introduction: 9

2. History 13

3. Epoxy Resin (prepolymers and monomers) 13

3.1 Bisphenol-based 14

3.2 Novolac 17

3.3Aliphatic 18

3.4 Halogenated 19

3.5 Diluents 20

3.6 Glycidylamine 20

3.7 Curing 21

3.8 Homopolymerisation 22

3.9 Amines 22

3.10 Anhydrides 23

3.11 Phenols 23

3.12 Thiols 24

4 Appliccations 24

4.1 Paints and coatings 24

4.2 Adhesives 26

4.3 Industrial tooling and composites 26

4.5 Wind turbine composites 27

4.6 Leisure and marine 28

4.7 Aerospace 30

4.8 Automotive composites 31

5.1 Electrical insulation (medium and high Voltage) 31

5.2 Electronics 32

5.3 Petroleum and petrochemical 32

5.4 Biology 32

5.5 Art 33

6 industry 33

7 Health Risks 33

8 Conclusion 35

List of Figure

Sr. # Title Page #

1 The blue-colored epoxy on the left is still undergoing curing.

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2 Offshore Wind Farm

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3 Exemplary epoxy application in sports industry

28

4 Power Glider

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5 Audi space frame in multilateral construction

31

6 Assembled Printed Circuit Board

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Brief discussion of epoxy resins

Epoxy resin is a flexible usually thermosetting resin made by copolymerization of an epoxide with another compound having two hydroxyl groups and used chiefly in coatings and adhesives. — called also epoxy.(Levchik & Weil, 2004)

Difference between epoxy and resins

The main difference between both adhesive types is the drying time. Both epoxy and resin adhesives require mixing before use, but epoxy hardens much faster than resin glue. ... Resin glues take longer to cure, about 8-10 hours, while epoxy adhesive only takes about 6-30 minutes.(Lockwood & Langston Jr, 1964)

1. Introduction:

Epoxy refers to any of the basic components or cured end products of epoxy resins, as well as a colloquial name for the epoxide functional group. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups.(Barton, 1985)

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 (usually called mercaptans). These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction is commonly referred to as curing.(Shen, Shao, & Li, 1995).(Shaw, 1993)

Reaction of polyepoxides with themselves or with polyfunctional hardeners forms a thermosetting polymer, often with favorable mechanical properties and high thermal and chemical resistance. Epoxy has a wide range of applications, including metal coatings, use in electronics/electrical components/LEDs, high tension electrical insulators, paint brush manufacturing, fiber-reinforced plastic materials, and adhesives for structural and other purposes.(Shaw, 1993)

Epoxy resins are reactive intermediates that, before they can be useful products, must be “cured” or cross-linked by polymerization into a three-dimensional infusible network with co-reactants (curing agents). Cross-linking of the resin can occur through the epoxide or hydroxyl groups, and proceeds basically by only two types of curing mechanisms: direct coupling of the resin molecules by a catalytic homopolymerization, or coupling through a reactive intermediate. Reactions used to cure low molecular weight epoxy resins occur with the epoxide ring:(Kong et al., 1981)

Sign in to download full-size image.The capability of this ring to react by a number of paths and with a variety of reactants gives epoxy resins their great versatility. The chemistry of most curing agents currently used with epoxy resins is based on polyaddition reactions that result in coupling as well as cross-linking. The more widely used curing agents are compounds containing active hydrogen (polyamines, polyacids, polymercaptans, polyphenols, etc.) that react as shown in Reaction 1 to form the corresponding β-hydroxy -amine, ester, mercaptan, or β-phenyl ether.(Kong et al., 1981)

 

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Epoxy resins and curing agents usually contain more than one reaction site per molecule, and the process of curing to form a three-dimensional network results from multiple reactions between epoxide molecules and curing agent. The specific reactions of the various reactants with epoxide groups have, in many cases, been studied in considerable detail and have been extensively reviewed elsewhere (2).

The epoxide resins (also widely known as epoxy resins and, occasionally, as ethoxyline resins) are characterised by the possession of more than one 1,2-epoxy group (I) per molecule. This group may lie within the body of the molecule but is usually terminal.(Hergenrother et al., 2005)

 

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The three-membered epoxy ring is highly strained and is reactive to many substances, particularly by with proton donors, so that reactions of the following schematic form can occur:

 

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Such reactions allow chain extension and/or cross-linking to occur without the elimination of small molecules such as water, i.e. they react by a rearrangement polymerisation type of reaction. In consequence these materials exhibit a lower curing shrinkage than many other types of thermosetting plastics.(Hergenrother et al., 2005)

There is, quite clearly, scope or a very wide range of epoxy resins. The non-epoxy part of the molecule may be aliphatic, cycloaliphatic or highly aromatic hydrocarbon or it may be non-hydrocarbon and possibly polar. It may contain unsaturation. Similar remarks also apply to the chain extension/cross-linking agents, so that cross-linked products of great diversity may be obtained. In practice, however, the commercial scene is dominated by the reaction products of bis-phenol A and epichlorohydrin, which have some 80–90% of the market share.(Hodgkin, Simon, & Varley, 1998)

The commercial interest in epoxide (epoxy) resins was first made apparent by the publication of German Patent 676 117 by I G Farben1 in 1939 which described liquid polyepoxides. In 1943 P. Castan2 filed US Patent 2 324 483, covering the curing of the resins with dibasic acids. This important process was subsequently exploited by the Ciba Company. A later patent of Castan3 covered the hardening of epoxide resins with alkaline catalysts used in the range 0.1–5% This patent, however, became of somewhat restricted value as the important amine hardeners are usually used in quantities higher than 5%.

In the early stage of their development the epoxy resins were used almost entirely for surface coating and developments in this field are to a large extent due to the works of S.O. Greenlee and described in a number of patents. These included work on the modification of epoxy resins with glycerol4, the esterifiction of the higher molecular weight materials with drying oil acids5 and reactions with phenolic6 and amino resins.7

Before World War II the cost of the intermediates for the these resins (in most cases epichlorohydrin and bis-phenol A) would have prevented the polymers from becoming of commercial importance. Subsequent improvements in the methods of producing these intermediates and improved techniques of polymerisation have, however, led to wide commercial acceptance.(Mohan, 2013)

By the beginning of the 1980s world capacity for epoxide resins reached about 600 000 tonnes per annum but at this time plant utilisation was only about 50–60%. Thus with a global consumption of about 10 million tonnes per annum for thermosetting plastics, epoxide resins had a share of about 3%. Western Europe and the USA each had about 40% of the market and Japan a little over 10%. This situation has not greatly changed since then; but by the late 1990s the world market for epoxide resins had risen to about 750 000 t.p.a.(Mohan, 2013)

About half of epoxide resin production is used for surface coating applications, with the rest divided approximately equally between electronic applications (particularly for printed circuit boards and encapsulation), the building sector and miscellaneous uses. In tonnage terms consumption of epoxide–fibre laminates is only about one-tenth that of polyester laminates, but in terms of value it is much greater.

Health and safety regulations no longer favor resins being processed in an open factory environment, especially if alternative processes are available. Closed mold processes such as resin transfer molding and vacuum infusion are therefore taking over from contact molding. Resin transfer molding involves the flow of resin, driven by a combination of pressure and/or vacuum into dry reinforcement contained within a two-part matched composite mold. Resin infusion is more cost-effective as it involves only a one-sided mold, the dry reinforcement being covered with a flexible plastic film. The space between the film and the mold is evacuated, causing resin to be drawn into the reinforcement. The infusion process can now be employed for the manufacture of large parts, such as boats and wind turbine blades.(Bagheri, Marouf, & Pearson, 2009)

Filament winding still involves open liquid processing, and enhanced vapor extraction facilities are now needed for this process. There are also restrictions on some of the amine hardeners, especially aromatic amines, used for curing. The market for corrosion-resistant pipework is steady, but the use of filament wound vessels for storage and transport of gas (natural gas, butane, and soon hydrogen) is expanding. Pressure vessels of this type are equipped with polymeric or metallic liners to act as gas permeation barriers.

Epoxy composite laminates are widely used for the repair of both composite and steel structures, mainly in marine applications. Boat repair is often required, due to impact damage or because laminate plies have had to be removed because of osmosis (blistering). For this application, epoxy is the preferred resin because of its higher reactivity, compared to polyesters and vinyl esters. Laminate repair always requires some material removal to provide a fresh surface to which the epoxy repair may be keyed. Polyester or vinyl ester-based laminates can be added on top of the epoxy while it is still reactive. It is worth noting that epoxy resin is the most osmosis-resistant of all the resins used in boat-building.(Bagheri et al., 2009)

2. History

Condensation of epoxides and amines was first reported and patented by Paul Schlack of Germany in 1934.[7] Claims of discovery of bisphenol-A-based epoxy resins include Pierre Castan[8] in 1943. (Bagheri et al., 2009)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 as Vantico in the late 1990s, which was subsequently sold in 2003 and became the Advanced Materials business unit of Huntsman Corporation of the United States.(Leven, 1963) In 1946, Sylvan Greenlee, working for the Devoe & Raynolds Company, patented resin derived from bisphenol-A and epichlorohydrin. Devoe & Raynolds, which was active in the early days of the epoxy resin industry, was sold to Shell Chemical; the division involved in this work was eventually sold, and via a series of other corporate transactions is now part of Hexion Inc. (Ivanova, Pethrick, & Affrossman, 2000; Leven, 1963)

3. Epoxy Resin (prepolymers and monomers)

Most of the commercially used epoxy monomers are produced by the reaction of a compound with acidic hydroxy groups and epichlorohydrin:(Woodside & Liebfried, 1992)

 

First a hydroxy group reacts in a coupling reaction with epichlorohydrin, followed by dehydrohalogenation.(Pietsch & Lewis, 1972)

Epoxy resins produced from such epoxy monomers are called glycidyl-based epoxy resins. The hydroxy group may be derived from aliphatic diols, polyols (polyether polyols), phenolic compounds or dicarboxylic acids. Phenols can be compounds such as bisphenol A and novolak. Polyols can be compounds such as 1,4-butanediol. Di- and polyols lead to diglycid polyethers. Dicarboxylic acids such as hexahydrophthalic acid are used for diglycide ester resins. Instead of a hydroxy group, also the nitrogen atom of an amine or amide can be reacted with epichlorohydrin.(Rimdusit, Pirstpindvong, Tanthapanichakoon, & Damrongsakkul, 2005)

The other production route for epoxy resins is the conversion of aliphatic or cycloaliphatic alkenes with peracids: (Rimdusit et al., 2005)

 

As can be seen, in contrast to glycidyl-based epoxy resins, this production of such epoxy monomers does not require an acidic hydrogen atom but an aliphatic double bond.

The epoxide group is also sometimes referred to as a oxirane group.(Rimdusit et al., 2005)

3.1 Bisphenol-based

The most common epoxy resins are based on reacting epichlorohydrin (ECH) with bisphenol A,  resulting in a different chemical substance known as bisphenol A diglycidyl ether (commonly known as BADGE or DGEBA). Bisphenol A-based resins are the most widely commercialised resins but also other bisphenols are analogously reacted with epichlorohydrin, for example Bisphenol F.(Fukui, Sonomoto, & Tanaka, 1987)

 

In this two-stage reaction, epichlorohydrin is first added to bisphenol A (bis(3-chloro-2-hydroxy-propoxy)bisphenol A is formed), then a bisepoxide is formed in a condensation reaction with a stoichiometric amount of sodium hydroxide. The chlorine atom is released as sodium chloride (NaCl), the hydrogen atom as of water.

Higher molecular weight diglycidyl ethers (n ≥ 1) are formed by the reaction of the bisphenol A diglycidyl ether formed with further bisphenol A, this is called prepolymerization:(Demir, Kiskan, Aydogan, & Yagci, 2013)

 

A product comprising a few repeat units (n = 1 to 2) is a viscous, clear liquid; this is called a liquid epoxy resin. A product comprising more repeating units (n = 2 to 30) is at room temperature a colourless solid, which is correspondingly referred to as solid epoxy resin.

Instead of bisphenol A, other bisphenols (especially bisphenol F) or brominated bisphenols (e. g. tetrabromobisphenol A) can be used for the said epoxidation and prepolymerisation. Bisphenol F may undergo epoxy resin formation in a similar fashion to bisphenol A. These resins typically have lower viscosity and a higher mean epoxy content per gram than bisphenol A resins, which (once cured) gives them increased chemical resistance.(Changsong & Benlian, 1998)

Important epoxy resins are produced from combining epichlorohydrin and bisphenol A to give bisphenol A diglycidyl ethers.

 

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. This route of synthesis is known as the "taffy" process. More modern manufacturing methods of higher molecular weight epoxy resins is to start with liquid epoxy resin (LER) and add a calculated amount of bisphenol A and then a catalyst is added and the reaction heated to circa 160 °C (320 °F). This process is known as "advancement".[4] 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).(Changsong & Benlian, 1998) 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.

Epoxy resins are polymeric or semi-polymeric materials or an oligomer, 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 downside of high purity liquid grades is their tendency to form crystalline solids due to their highly regular structure, which then require melting to enable processing.

An important criterion for epoxy resins is the epoxide group content. This is expressed as the "epoxide equivalent weight", which is the ratio between the molecular weight of the monomer and the number of epoxide groups. This parameter is used to calculate the mass of co-reactant (hardener) to use when curing epoxy resins. Epoxies are typically cured with stoichiometric or near-stoichiometric quantities of hardener to achieve the best physical properties.(Changsong & Benlian, 1998)

3.2 Novolac

 

General structure of epoxyphenol novolak with n usually in the range from 0 to 4. The compound is present in the form of various constitutional isomers.

Novolaks are produced by reacting phenol with methanal (formaldehyde). The reaction of epichlorohydrin and novolaks produces novolaks with glycidyl residues, such as epoxyphenol novolak (EPN) or epoxycresol novolak (ECN). These highly viscous to solid resins typically carry 2 to 6 epoxy groups per molecule. By curing, highly cross-linked polymers with high temperature and chemical resistance but low mechanical flexibility are formed due to the high functionality of these resins.[2]

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.(Benyahya et al., 2014)

3.3Aliphatic

There are two common types of aliphatic epoxy resins: those obtained by epoxidation of double bonds (cycloaliphatic epoxides and epoxidized vegetable oils) and those formed by reaction with epichlorohydrin (glycidyl ethers and esters).(Schlesinger, 1973)

 

Structural formula of 3,4-Epoxycyclohexylmethyl-3’,4’-epoxycyclohexane carboxylate

Cycloaliphatic epoxides contain one or more aliphatic rings in the molecule on which the oxirane ring is contained (e.g. 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate). They are produced by the reaction of a cyclic alkene with a peracid (see above).[5] Cycloaliphatic epoxides are characterised by their aliphatic structure, high oxirane content and the absence of chlorine, which results in low viscosity and (once cured) good weather resistance, low dielectric constants and high Tg. However, aliphatic epoxy resins polymerize very slowly at room temperature, so higher temperatures and suitable accelerators are usually required. Because aliphatic epoxies have a lower electron density than aromatics, cycloaliphatic epoxies react less readily with nucleophiles than bisphenol A-based epoxy resins (which have aromatic ether groups). This means that conventional nucleophilic hardeners such as amines are hardly suitable for crosslinking. Cycloaliphatic epoxides are therefore usually homopolymerized thermally or UV-initiated in an electrophilic or cationic reaction. Due to the low dielectric constants and the absence of chlorine, cycloaliphatic epoxides are often used to encapsulate electronic systems, such as microchips or LEDs. They are also used for radiation-cured paints and varnishes. Due to their high price, however, their use has so far been limited to such applications.(Stevens, 1986) 

Epoxidized vegetable oils are formed by epoxidation of unsaturated fatty acids by reaction with peracids. In this case, the peracids can also be formed in situ by reacting carboxylic acids with hydrogen peroxide. Compared with LERs (liquid epoxy resins) they have very low viscosities. If, however, they are used in larger proportions as reactive diluents, this often leads to reduced chemical and thermal resistance and to poorer mechanical properties of the cured epoxides. Large scale epoxidized vegetable oils such as epoxidized soy and lens oils are used to a large extent as secondary plasticizers and cost stabilizers for PVC.(Mohan, 2013) 

Aliphatic glycidyl epoxy resins of low molar mass (mono-, bi- or polyfunctional) are formed by the reaction of epichlorohydrin with aliphatic alcohols or polyols (glycidyl ethers are formed) or with aliphatic carboxylic acids (glycidyl esters are formed). The reaction is carried out in the presence of a base such as sodium hydroxide, analogous to the formation of bisphenol A-diglycidyl ether. Also aliphatic glycidyl epoxy resins usually have a low viscosity compared to aromatic epoxy resins. They are therefore added to other epoxy resins as reactive diluents or as adhesion promoters. Epoxy resins made of (long-chain) polyols are also added to improve tensile strength and impact strength.(Barton, 1985)

A related class is cycloaliphatic epoxy resin, which contains one or more cycloaliphatic rings in the molecule (e.g. 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate). This class also displays lower viscosity at room temperature, but offers significantly higher temperature resistance than the aliphatic epoxy diluents. However, reactivity is rather low compared to other classes of epoxy resin, and high temperature curing using suitable accelerators is normally required. As aromaticity is not present in these materials as it is in Bisphenol A and F resins, the UV stability is considerably improved.(Bagheri et al., 2009)

3.4 Halogenated

Halogenated epoxy resins are admixed for special properties, in particular brominated and fluorinated epoxy resins are used. 

Brominated bisphenol A is used when flame retardant properties are required, such as in some electrical applications (e.g. printed circuit boards). The tetrabrominated bisphenol A (TBBPA, 2,2-bis(3,5-dibromophenyl)propane) or its diglycidyl ether, 2,2-bis[3,5-dibromo-4-(2,3-epoxypropoxy)phenyl]propane, can be added to the epoxy formulation. The formulation may then be reacted in the same way as pure bisphenol A. Some (non-crosslinked) epoxy resins with very high molar mass are added to engineering thermoplastics, again to achieve flame retardant properties.

Fluorinated epoxy resins have been investigated for some high performance applications, such as the fluorinated diglycidether 5-heptafluoropropyl-1,3-bis[2-(2,3-epoxypropoxy)hexafluoro-2-propyl]benzene. As it has a low surface tension, it is added as a wetting agent (surfactant) for contact with glass fibres. Its reactivity to hardeners is comparable to that of bisphenol A. When cured, the epoxy resin leads to a thermosetting plastic with high chemical resistance and low water absorption. However, the commercial use of fluorinated epoxy resins is limited by their high cost and low Tg.(Demir et al., 2013)

3.5 Diluents

Epoxy resins diluents are typically formed by glycidylation of aliphatic alcohols or polyols. The resulting materials may be monofunctional (e.g. dodecanol glycidyl ether), difunctional (butanediol diglycidyl ether), or higher functionality (e.g. trimethylolpropane triglycidyl ether). These resins typically display low viscosity at room temperature (10-200 mPa.s) and are often referred to as reactive diluents. They are rarely used alone, but are rather 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.

3.6 Glycidylamine

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-bis-(4-aminophenyl)-methane (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 makes them important materials for aerospace composite applications.(Benyahya et al., 2014)

3.7 Curing

 

Structure of a cured epoxy glue. The triamine hardener is shown in red, the resin in black. The resin's epoxide groups have reacted with the hardener and are not present anymore. The material is highly crosslinked and contains many OH groups, which confer adhesive properties.

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 or gelation process.[6] 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.

While some epoxy resin/ hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150 °C (302 °F) being common, and up to 200 °C (392 °F) 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.(Changsong & Benlian, 1998)

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.

3.8 Homopolymerisation

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 for the curing process, 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).(Grenier-Loustalot & Sanglar, 1996)

3.9 Amines

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. Kinetic studies have shown the reactivity of the primary amine to be approximately double that of the secondary amine. Use of a difunctional or polyfunctional amine forms a three-dimensional cross-linked network. Aliphatic, cycloaliphatic and aromatic amines are all employed as epoxy hardeners. Amine type hardeners will alter both the processing properties (viscosity, reactivity) and the final properties (mechanical, temperature and heat resistance) of the cured copolymer network. Thus amine structure is normally selected according to the application. Overall reactivity potential for different hardeners can roughly be ordered; aliphatic amines > cycloaliphatic amines > aromatic amines, though aliphatic amines with steric hindrance near the amino groups may react as slowly as some of the aromatic amines. Slower reactivity allows longer working times for processors. Temperature resistance generally increases in the same order, since aromatic amines form much more rigid structures than aliphatic amines. Aromatic amines were widely used as epoxy resin hardeners, due to the excellent end properties when mixed with a parent resin. Over the past few decades concern about the possible adverse health effects of many aromatic amines has led to increased use of aliphatic or cycloaliphatic amine alternatives. Amines are also blended, adducted and reacted to alter properties and these amine resins are more often used to cure epoxy resins than a pure amine such as TETA. increasingly, WATER BASED polyamines are also used to help reduce the toxicity profile among other reasons.(Grenier-Loustalot & Sanglar, 1996; Hillmyer, Lipic, Hajduk, Almdal, & Bates, 1997)

 

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

3.10 Anhydrides

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 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.(Guo, Bao, Song, Yuan, & Hu, 2011)

3.11 Phenols

Polyphenols, such as bisphenol A or novolacs can react with epoxy resins at elevated temperatures (130–180 °C, 266–356 °F), 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.

3.12 Thiols

Also known as mercaptans, thiols contain a sulfur which reacts very readily with the epoxide group, even at ambient or sub-ambient temperatures. While 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.(Nakamura et al., 1986).

4 Appliccations

The applications for epoxy-based materials are extensive and include coatings, adhesives and composite materials such as those using carbon fiber and fiberglass 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.(Barton, 1985; Wang & Shieh, 1999) 

As with other classes of thermoset polymer materials, blending different grades of epoxy resin, as well as use of additives, plasticizers or fillers is common to achieve the desired processing or final properties, or to reduce cost. Use of blending, additives and fillers is often referred to as formulating (see: formulation).(Ho & Wang, 2001)

4.1 Paints and coatings

Two part epoxy coatings were developed for heavy duty service on metal substrates and use less energy than heat-cured powder coatings. These systems provide a tough, protective coating with excellent hardness. One part epoxy coatings are formulated as an emulsion in water, and can be cleaned up without solvents.(Streitberger & Dossel, 2008)

Epoxy coatings are often 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 "chalking out", due to UV exposure.(Streitberger & Dossel, 2008) 

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 have several flooring applications. They are used for protection again mechanical wear, chemical attack and thermal stressing in places such as industrial facilities, car parks or retail establishments. They can also have decorative applications, such as terrazzo flooring, chip flooring, and colored aggregate flooring.(Wijewardane & Goswami, 2012)

Epoxies were modified in a variety of ways, Reacted with fatty acids derived from oils to yield epoxy esters, which were cured the same way as alkyds . Typical ones were L8(80% linseed, D4 (40% Dehydrated castor oil). These were often reacted with styrene to make styrenated epoxy esters, used as primers. Curing with phenolics to make drum linings, curing esters with amine resins and pre-curing epoxies with amino resins to make resistant top coats. One of the best examples was a system of using solvent free epoxies for priming ships during construction, this used a system of hot airless spray with premixing at the head. This obviated the problem of solvent retention under the film, which caused adhesion problems later on.

4.2 Adhesives

 

Figure 1  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 includes polyurethane, 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.(Abdullah, 2012) 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).[13]

Some epoxies are cured by exposure to ultraviolet light. Such epoxies are commonly used in optics, fiber optics, and optoelectronics.(Hergenrother et al., 2005)

Recent developments are coatings for onshore, marine and offshore protection, as well as shipping containers. Traditionally solvent-borne, solvent-free or high-solids, state of the art products are waterborne epoxy systems to protect customers' metal assets from corrosion. This includes waterborne epoxy resin system for making zinc-rich primers.(Bagheri et al., 2009)

4.3 Industrial tooling and composites

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 thermoset polymer matrix composite parts.(Leven, 1963)

 

Figure 2 Offshore Wind Farm

4.5 Wind turbine composites

Epoxy resins are used as bonding matrix along with glass or carbon fiber fabrics to produce composites with very high strength to weight characteristics, allowing longer and more efficient rotor blades to be produced. In addition, for offshore and onshore wind energy installations, epoxy resins are used as protective coatings on steel towers, base struts and concrete foundations. Aliphatic polyurethane top coats are applied on top to ensure full UV protection, prolong operational lifetimes and lowering maintenance costs. Electric generators, connected via the drivetrain with the rotor blades, convert mechanical wind energy in usable electric energy, and rely on epoxies electrical insulation and high thermal resistance properties. The same applies to transformers, bushings, spacers, and composites cables connecting the windmills to the grid In Europe, wind energy components account for the largest segment of epoxy applications, about 27% of the market.(Pietsch & Lewis, 1972) 

 

Figure 3 Exemplary epoxy application in sports industry

4.6 Leisure and marine

High strength and weight reduction are the main performance criteria for manufacturers of sport components. Skis, snowboards, surf and kite boards are fiber reinforced epoxy applications. Epoxy resin systems in combination with multi-axial, unidirectional, woven fabrics and variety of core materials play also a key role to manufacture high performance boats, yachts and commercial and military vessels. composite boats

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.(Rimdusit et al., 2005)

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.[15] 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.(Fukui et al., 1987)

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.(Demir et al., 2013)

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 exactly. 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, so that small amounts of catalyst are preferable, so long as the hardening proceeds at an acceptable pace. The rate of cure of polyesters can therefore be controlled by the amount and type of catalyst as well as by the temperature.(Lockwood & Langston Jr, 1964)

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.(Levchik & Weil, 2004)

 

Figure 4 Power Glider

4.7 Aerospace

In the aerospace industry, epoxy is used as a structural matrix material which is then reinforced by fiber. Typical fiber reinforcements include glass, carbon, Kevlar, and boron. Epoxies are also used as a structural glue. Materials like wood, and others that are 'low-tech' are glued with epoxy resin. Epoxies generally out-perform most other resin types in terms of mechanical properties and resistance to environmental degradation.[16] Glider and small aircraft designers considered epoxy based composites in an early date. Akaflieg Stuttgart fs29 Epoxy composite structures play an important role in today’s civil aviation industry to reduce the airframe weight. As a consequence, modern aircraft show better fuel economy and lower CO2 emissions.environmental-impact Jetliners like Boeing 787 and Airbus A 350 consist of more than 50% composites where epoxy materials play the dominant role. A 350 materials B 787 materials Processing technologies are prepregging prepreg and Resin-Transfer-Moulding (RTM) [[Composite material] in particular in the civil aviation and military aviation industry. Resin-infusion and hand lay-up processing technologies are applied when manufacturing small aircraft and gliders. Resin Infusion(Luft, 1961)

 

Figure 5 Audi space frame in multimaterial construction

4.8 Automotive composites

Major automotive and truck manufacturers consider glass and carbon-fiber composites for mass production. This comprises parts which support the structural parts of the car body such as fenders, trunk lids and hoods. The use of composite leaf springs in particular in light trucks increases the cargo load. Composite LEaf Springs 50 % mass reduction is achievable when replacing steel for the manufacture of coil springs by fiber reinforced Epoxy. New generation, in particular of electrically driven cars, require chassis and other significant load bearing structures made from CFRP. But, mass-production requires adequate automotive manufacturing. Traditionally, the poly-addition reaction of epoxy with curatives is more time consuming than metal forming or unsaturated polyester cure. But fast processing and higher thermo-mechanical resistance is required. In numbers, coming from low double digit minutes for cure to single digits and increasing the thermo-mechanical resistance from 130°C to 180°C and higher. 

5. Composite Materials

5.1 Electrical insulation (medium and high Voltage) 

Electrical Casting Materials are formulated polymeric materials used in equipment for the generation, transmission and distribution of electrical power and electrical and electronic devices and components. Such epoxy resin formulations are relevant to the electrical and electronics industry in voltage ranges from 400 kV down to >1V.The purpose of the products is to insulate, protect or shield the environment from electric current or the devices from the environment. Formulated bisphenol-based epoxy resins, cured with dicarboxylic anhydrides or amines are used for indoor applications. Cycloaliphatic epoxy resins, preferably 1,2-Cyclohexanedicarboxylic acid, bis(2,3-epoxypropyl) ester, in combination with cycloaliphatic dicarboxylic anhydrides and silanised silica flour are the base for outdoor-resistant electrical insulation, e.g. production of insulators, bushings, instrument transformers. Most common processing technologies are Vacuum Pressure Impregnation for power generators, Vacuum Casting and Automatic Pressure Gelation for medium voltage equipment and potting under vacuum for e.g. ignition coils and small transformers or potting under ambient pressure for e.g. capacitors.(Shaw, 1993)

Epoxy resins are applied using the technology of resin dispensing.

5.2 Electronics

 

Figure 6 Assembled Printed Circuit Board

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. Since these applications require self-extinguishing properties, flame retardant epoxy resins or epoxy formulations with flame retardant additives are utilized.(Hergenrother et al., 2005)

5.3 Petroleum and petrochemical

Epoxies can be used to plug selective layers in a reservoir which are producing excessive brine. The technique is named "water shut-off treatment".(Hodgkin et al., 1998)

5.4 Biology

Water-soluble epoxies such as Durcupan are commonly used for embedding electron microscope samples in plastic so they may be sectioned (sliced thin) with a microtome and then imaged.[19]

5.5 Art

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.[20] It is also used in jewelry, as a doming resin for decorations and labels, and in decoupage type applications for art, countertops, and tables.(Bagheri et al., 2009)

6 industry

The global epoxy resin market was valued at approximately $10 billion in 2018. The epoxy resin market is dominated by the Asia-Pacific region. China is the major producer and consumer globally, contributing to almost 50% of the global resin global capacity in 2018. The global market is made up of approximately 50–100 manufacturers of basic or commodity epoxy resins and hardeners. In Europe, the largest markets for epoxy resins are Germany, Italy, France and the UK.(Bagheri et al., 2009)

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.

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).(Mohan, 2013)

7 Health Risks

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 as toxic or carcinogenic/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. Another group of hardeners which have come into the regulatory focus over recent years are dicarboxylic anhydrides, namely Hexahydropthalic-Anhydride (HHPA) and Methyl-Hexahydrophthalic-Anhydride (m-HHPA). Both substances have been identified under REACH as substances of equivalent concern due to the potential risk of respiratory sensitization. This SVHC identification implies that both substances are listed on the REACH candidate list for further regulatory measures. 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.[21] Exposure to epoxy resins can, over time, induce an allergic reaction. Sensitization generally occurs due to repeated exposure (e.g. through poor working hygiene 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). There is some incidence that use of epoxy resin systems may cause sensitizer-induced occupational asthma in some exposed workers. However, this may apply to specific application technologies, e.g. spray application, insufficient use of appropriate Personal Protective Equipment (PPE) and the use of volatile components, e.g. amine-based curing agents. Most epoxy resins have due to their high(er) molecular weight a low vapor pressure, thus do not vaporize easily. Bisphenol-A, which is a major building block to manufacture Bis-A- epoxy resins, has been identified as an endocrine disruptor for human health and the environment under REACH. The concept of endocrine disruption is not without controversy within the scientific community. Substances which are typically quoted in the context of endocrine disruption are xeno- and phytoestrogens, i.e. substances which mimic the effects of the human female sex hormone estrogen. The strength of a physiological response to natural estrogen or xenoestrogen is dependent on the affinity to bind to estrogen receptors on the cell surface of target organs or tissues. Human estrogen has a high binding affinity to estrogen receptors and thus elicits a physiological response at very low concentrations at nanomolar or sub-nanomolar level. Xeno- or phytoestrogens however, have a much lower binding affinity to the estrogen receptors and as such require a much higher concentration to elicit a similar physiological response as natural estrogen. The uptake of such concentrations via food ingestion does not seem plausible for most xenoestrogens such as BPA, as it would require ingesting vast quantities of canned food stuffs or beverages to induce an appreciable effect. Consequently, the risk of adverse effects for BPA exposure via a normal diet is considered to be relatively insignificant. {Shaw, 1993 #477}

8. Conclusion

In this research, a siloxane-type epoxy resin (SG copolymer), which has pendant epoxide rings on the side chain of the polysiloxane polymer backbone, was synthesized by the hydrosilylation reaction of poly(methylhydrosiloxane) with allyl glycidyl ether. The polymer structures were characterized by 1H NMR. The SG resin was then blended with a commercial epoxy resin (diglycidyl ether of bisphenol-A, DGEBA) at various ratios, using dicyandiamide (DICY) as a curing agent. The curing behaviors were studied by DSC. It was found that the initial curing temperature (Ti) and peak curing temperature (Tp) were increased by the addition of SG copolymer to the epoxy resin. Their morphology, mechanical properties and the stability of the cured piece were investigated using SEM, DMA and TGA, respectively. The results show that the addition of SG copolymer increases the mobility of the cross linked network, and increases the thermal stabilityReactive polyimide containing hydroxyl functionalities was prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 3,3′-diamino-4,4′-dihydroxybiphenyl. 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 as toxic or carcinogenic/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. Another group of hardeners which have come into the regulatory focus over recent years are dicarboxylic anhydrides, namely Hexahydropthalic-Anhydride (HHPA) and Methyl-Hexahydrophthalic-Anhydride (m-HHPA). Both substances have been identified under REACH as substances of equivalent concern due to the potential risk of respiratory sensitization.

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