Epoxy Resins:
INTRODUCTION
1. Epoxy resin are characterized by more than one 1, 2- epoxy group per molecule.
2. Three-membered epoxy ring is highly reactive to many substance, particularly by with proton donor ( eg: amine, anhydride).
4. Depending upon the non epoxy part of molecule there are wide range of epoxy resins.
5. Non epoxy part may be aliphatic, aromatic or it may be non-hydro carbon and possibly polar.
6. Commercially important epoxy resin is prepared by the reaction of biphenol – A with epichlorohydrin.
Historical Development
1. Commercial interest in epoxy resins was frist made apparent by the publication of German patent 676117 by I G Farben in 1939 which describe liquid polyepoxide.
2. In 1943 P.Castan filed US patent covering the curing of the resins with dibasic acids
3. About half of epoxide resin production is used for surface coating applications, with the rest is used for electronic applications (particularly for printed circuit boards and encapsulation), the building sector and miscellaneous uses.
4. Properties of the cross-linked resins depend very greatly on the curing system used and on the type of resin.
MONOMERS
1. The monomers for general purpose epoxy resins are bis-phenol A and epichlorohydrin.
Bis- Phenol A
2. Bis- phenol A is prepared by reaction of acetone and phenol.
Epichlorohydrin
Starting material for the preparation of Epichlorohydrin is propylene and chlorine. The material is a colourless liquid.
MANUFACTURE
Manufacture of Bis-phenol A Resins
1. Largest product epoxy resin called Digycidyl ether of bisphenol-A (DGEBA) is manufactured by reacting bisphenol-A with epichlorohydrin
2. Typical laboratory prepation 1 mole (228g) of bis-phenol A is dissolved in 4 moles (370g) of epichlorohydrin and the mixture heated to 105 –110o C under an inert atmosphere.
3. Solution is continuously stirred for 16 hrs. While 80g (2moles) of sodium hydroxide in the form of 30% aqueous solution is added drop wise.
4. Resulting organic layer is separated and fractionally distilled.
5. General formula for glycidyl ether resin is represented by the structure.
6. With n =0, the product reduces to diglycide ether and the molecular weight is 340; with n=10, the molecular weight is about 3000. commercial diglycide ethers mostly have a low degree of polymersiation as they hardly ever have a molecular weight greater than 4000.
Epoxide resins of diglycide ether type are characterized by six parameters
1. Resin viscosity (of liquid resin)
2. Epoxide equivalent
3. Hydrooxyl equivalent
4. Average molecular weight and molecular weight distribution
5. Melting point (of solid resin)
Epoxide Equivalent
1. This is the weight of the resin (in gram) containing 1 gram chemical equivalent epoxy.
2. Determined by reacting a known quantity of resin with hydrochloric acid and measuring unconsumed acid.
Hydroxyl Equivalent
Hydroxyl equivalent is the weight of resin containing one equivalent weight of hydroxyl groups. It may be determined by many techniques but normally by reacting the resin with acetyl chloride
Miscellaneous Epoxy Resins
Other glycidyl ether resins
Non – glycidyl ether resins
Miscellaneous Glycidyl Ether Resins
Bis-phenol F- Mixture of three isomers
Novalak
1. Novalak resins have also been epoxidised through their phenolic hydroxy group.
2. This molecule has a functionality of four
3. When cured with high temperature hardeners such as methyl “nadic” anhydride both thermal degradation stability and heat deflection temperatures are considerably improved
Resin for High Heat Distortion Temperature
1. To produce resins of high heat distortion temperature it is important to have a high density of cross-linking.
2. This is achieved by using anhydride hardeners such as pyromellitic dianhydride and with the cyclic aliphatic resins.
3. Attempts have been made to use glucidyl ether resins of higher functionality such as the tetra functional structure
Flame-Resistant Resins
1. Halogenated materials have been used. A typical example is diglycidyl ether of tetrachlorobisphenol - A.
Non-glycidyl ether epoxy resins
There are two types of Non-glycidyl ether epoxy resins.
2. Those which have an essentially linear structure on to which are attached epoxide groups – the aceylic aliphatic epoxide resins.
Cyclic aliphatic resins
Compared with the diglycidyl ether resins, cyclo aliphatic resins are characterized by
1. Light colour
2. Low viscosity
3. Low rate of reaction with amine curing agents
4. High degree of cross-linking
5. Basically brittle resins can be made flexible by use of long- chain aliphatic curing agents which lead to less cross-linking
6. High dimensional stability when heated
7. High tracking resistance and arc resistance
Acyclic aliphatic resins
1. Cured resins have heat distortion temperatures substantially higher than the conventional amine-cured diglycidyl ether resins.
2. A casting made from an epoxidised polybutadiene hardened with maleic anhydride gave a heat distortion temperature 250℃.
3. Nitrogen containing epoxy resin
Manufactured by glycidylization of cyanuric acid using epichlorohydrin.
4. Combination of triglycidylisocyanurate with oil-free polyesters containing carboxyl groups leads to powder coating systems which give films resistance to external weathering.
GENERAL PROPERTIES
Variety of Form: Epoxy resin systems that is resin,hardeners, and modifiers are available permitting the selection of almost any form desired for any application, ranging from extremely low viscosities to high melting solids.
Curing Latitude: Depending upon the selection of hardener, systems can cure rapidly or slowly at almost any temperature from 5 to 180ºC.
Low Shrinkage: Unlike phenolic and polyester resin behaviour, the epoxides exhibit very low shrinkage during cure.
Toughness: Cured epoxy resins are tough materials due to distance between cross-linking points and the presence of integral aliphatic chains. They are approximately seven times tougher than cured phenolic resins.
Adhesion: Chemical nature of epoxies viz., the hydroxyl and ether groups present causes outstanding adhesion to a variety of materials.
2. Being cross-linked, the resin will not dissolve without decomposition but will be swollen by liquids of similar solubility parameter to the cured resin .
ADDITIVES
1. FILLERS
2. DILUENTS
3. FLEXIBILIZERS
4. COLORENTS AND DYES
5. RHEOLOGICAL ADDITIVES
6. THICKINING AGENTS
Fillers
1. Reinforcing fibers such as glass graphite and polyaramide improve mechanical properties.
2. Non- reinforcing fillers such as alumina powder is widely used for thermal conductivity and electrical conductive applications.
3. Mica is used for electrical insulation applications.
4. Inorganic fillers such as talc, calcium carbonate and silica are used in epoxide systems for cost reduction with some strength enhancement.
5. Carbon and graphite powders are used to have lubricity.
6. Carium sulphate is used as the density controller.
7. Use of synthetic sodium aluminium silicate in epoxy coatings provides improved opacity over titanium dioxide.
Diluents
1. Diluents are free-flowing liquids incorporated to reduce the resin viscosity and simplify handling.
2. Reactive diluents such as phenyl glycidly ether, butyl glycidyl ether and octylene oxide are used.
3. Non-reactive diluents include monomeric styrene, bisphenol, hydrocarbon oils, and phthalate ester like DOP and DBP are used.
4. Primary benefits include viscosity and cost reduction, extension of pot life and decrease in exotherm.
5. Non-reactive diluents tend to degrade mechanical, electrical and chemical resistance properties as their concentration increases
Flexibilizers
1. Non-modified epoxy resins are hard and brittle. A higher toughness and impact resistance is needed for impregnating, casting and adhesive resins.
2. Low molecular weight polyaminoamides and polysulphides are used as Flexibilizers
3. More the polysulphide the higher will be the dielectric constant and lower the volume resistivity, but an increase in flexibility and impact strength.
4. Polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications
Colourants and Dyes
1. A wide variety of colourants used with epoxides.
2. Dies are less frequently used because of the natural tendency of clear epoxies to yellow when exposed to UV light.
3. Encapsulation materials for electrical industries are coloured white to brown with TiO2 or iron oxide, respectively.
4. High performance colourants such as cadmium or phthalocyanine pigments are used in the ship building and aircraft industries frequently in combination with TiO2.
Other Additives
1. Rheological additives include viscosity depressants (usually solvents, surface activators or diluents) and thixotropic agents.
2. Hydrophobic fumed silicas are used as thickening agents.
REINFORCEMENT
Reinforcement used in epoxy systems are similar to that of unsaturated polyester resin
CURING OF EPOXY RESIN
Various forms of the epoxy resins, in their thermoplastic or uncured state, are converted or hardened into useful thermosets by reaction with a variety of curing agents.
1. Aliphatic and aromatic amines
2. Polyamides,
3. Anhydrides,
4. Dicyandiamide,
5. Isocyanate,
6. Polysulphides, mercaptans etc.,
Amine Hardening System
1. Primary and secondary amines are used as reactive curing agents whilst the tertiary amines are used as catalyst
2. Ethylene diamines are most widely used aliphatic amines for curing epoxy resins
3. Primary amino group is more reactive towards epoxy than secondary amino groups.
4. Primary amino-epoxy reaction results in linear polymerization while secondary amino-epoxy reaction lead to branching and cross-linking.
Acid Hardening System
Acid hardening systems provide cured resins with very high HDT and with good physical, electrical and chemical properties.
In practice acid anhydrides are preferred to acids,
Three classes of anhydride : Room temperature solids, room temperature liquids and chlorinated anhydrides. Examples
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