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Epoxies

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Epoxide Resins for B.Sc/M.Sc/B.Tech/B.E./M.E./M.Tech level.

Suranjana M / Aurangabad

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Qualification: M.Tech (IIT,Kharagpur - 2000)

Teaches: Science, Chemistry

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  1. Maharashtra Institute of Technology, Aurangabad Affiliated to Dr. Babasaheb Ambedkar Marathwada University Aurangabad Course: Polymeric Materials-I T. Y. B. Tech. (Plastic and Polymer Engineering) Semester-V
  2. Unit-Ill — Epoxies Structure-properties relationship, properties and application of Epoxies References: Plastics Materials, J.A.Brydson, Seventh Edition, Butterworth- Heinemann PLASTICS MATERIALS AND PROCESSES -A Encyclopedia, Charles A. Harper, Edward M. Petrie, Interscience PAINT AND SURFACE COATINGS Theory and Practice edition, Editors: R LAMBOURNE and T A STRIVENS Concise Wiley- Second
  3. Some Applications of Epoxy Resin Top Coating for laboratory table printed circuit board Floor coating An epoxy encapsulated hvbrid circuit on a printed circuit board.
  4. Introduction 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. o ---CH—CH— (I)
  5. 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: o OH HX —CH—CH— X Such reactions allow chain extension and for 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. There is, quite clearly, scope or a very wide range of epoxy resins. The nonepoxy 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.
  6. Epoxy resins are characterized by the epoxide group (oxirane rings), which make these resins cross- linkable. The most widely used resins are diglycidyl ethers of bisphenol A (DGEBA). These are made by reacting epichlorohydrin with bisphenol A in the presence of an alkaline catalyst. By controlling the operating conditions and varying the ratio of the epichlorohydrin to bisphenol A, products of different molecular weights can be made. ' For liquid resins, n (see Fig. Eel) is normally less than 1; for solid resins, n is 2 or greater. Solids with very high melting points have n values as high as 20.
  7. —CH—CH2 0— CH 3 0-110 2 3 Figure E.I General chemical structure of epoxy (diglycidyl ether of bisphenol A j. The resins in this group range from low-viscosity liquids to solid resins melting up to 1750C. Generally the higher the melting point, the less curing agent is needed. The cured properties of all these resins are similar, but the toughness increases as the melting point increases. Although most of the DGEBA epoxies are light amber, transparent, colorless epoxies are available for optical embedments. The electronic industry demands epoxy resins with minimum ionic contamination, particularly sodium and chlorine. Most manufacturers supply DGEBA epoxy with less than 100 ppm ionic contaminants and some with less than 1 ppm chlorine and sodium.
  8. Chlorinated or brominated bisphenol A is used to produce flame-resistant epoxies. These are nowadays used in many applications like (i) Structural laminates (ii) Electrical laminates (iii)Potting and encapsulation compounds (iv)Adhesive and coatings As the halogen content increases, the viscosity generally increases. Epoxies have also been synthesized that contain high weight percents of fluorine. They can be used with silicone amines to yield products with very low moisture pickup, 0.25% at 200c. These materials are extremely stable on high temperature exposure and during outdoor weathering. Epoxy resins based on biphenyl F have some important advantages over the bis A epoxies. Compared with bis A epoxies, bis F epoxies have a lower room-temperature viscosity; a crystallization time twice as long; better resistance to sulfuric acid, acetone, and methanol; but at twice the price. The cured epoxy bis F is generally tougher than bis A epoxy cured with the same curing agent, but it will have a lower glass transition temperature, (Tg).
  9. ' Another class of epoxy resins is the novolacs, particularly the epoxy cresols and the epoxy phenol novolacs. These are produced by reacting a novolac resin, usually formed by the reaction of o-cresol or phenol and formaldehyde with epichlorohydrin. These highly functional materials are particularly recommended for transfer molding powders, electrical laminates, and parts in which superior thermal properties, high resistance to solvents and chemicals, and high reactivity with hardeners are needed.
  10. The novolacs cure more rapidly than DGEBA resins and have higher exotherms. The cured novolacs have higher heat deflection temperatures than DGEBA resins. They have excellent resistance to solvents and chemicals. Novolacs also have excellent electrical properties, which are retained at high service temperatures. Another group of epoxy resins, the cycloaliphatics, is particularly important when superior arc track and weathering resistance are necessary requirements. A distinguishing feature of cycloaliphatic resins is the location of the epoxy group(s) on a ring structure rather than on an aliphatic chain. Cycloaliphatics can be produced by the peracetic epoxidation of cycle olefins and by the condensation of an acid such as tetrahydrophthalic anhydride with epichlorohydrin, followed by dehydrohalogenation.
  11. Cycloaliphatic epoxy resins have — superior arc-track resistance, — good electrical properties under adverse conditions, — good weathering properties, — high heat deflection temperatures — good color retention. Brominated epoxies have all the other characteristics of the resins in the epoxy family, and in addition they are relatively flame resistant because of the bromine constituents in the molecular structure.
  12. ???? - ??- ?? ?? -, ??-?? ????'?? — ??—?? ?-?? - ? ?—?? ?? ?? CycIce'i9hatic ????? ?- ?? - ????? ??? ?? ?—
  13. Typical reactions of the epoxide group ? ??—??2 ? ??—??2 ? ??— ??2 ? ?? —??2 + ????— + H2N + ??—- + ?? ?? ??—??2 ??? ?? ??— NH ?? ?? ??
  14. Typical grades of bisphenol epoxide resins and properties Number of repeat units (n) 0.5 2 4 9 12 Melting point CC) Viscous liquid 64—76 95—105 125-132 140-155 Epoxide equivalent 225-290 2400-4000
  15. Monomers and chemical commonly used for preparing epoxy prepolymer 1.Bisphenol —Bisphenol A —Bisphenol F (higher crystallinity, better heat resistance,but also more expensive) 2.Epichlorohydrin 3.Catalyst (such as NaOH)
  16. PREPARATION OF RESINS FROM BIS-PHENOL A CHi HO O + + O OH CHi CH The first, and still the most important, commercial epoxide resins are reaction products of bis-phenol A and epichlorhydrin. Other types of epoxide resins were introduced in the late 1950s and early 1960s, prepared by epoxidising unsaturated structures. The bis-phenol A is prepared by reaction of the acetone and phenol as shown above.
  17. Phenol and acetone are available and the bis-phenol A is easy to manufacture. Bisphenol A is comparatively inexpensive. This is one of the reasons why it has been the preferred dihydric phenol employed in epoxide resins manufacture. Since most epoxide resins are of low molecular weight and because colour is not particularly critical the degree of purity of the bis-phenol A does not have to be so great as when used in the polycarbonate resins. Bis-phenol A with a melting point of 1530C is considered adequate for the most applications whilst less pure materials may often be employed
  18. compound is derived from Epichlorohydrin, the more expensive propylene by the sequence of reactions shown below. The material is available commercially at 98% purity and is a colourless mobile liquid. CH2--CH + C12--—--) CH2=CH + HCI Cl—CH2 Propylene CH2 CH + H20/C12 cl CH2Cl CH2•C1 Chloride "Dichlotohydrin o Elev. + NaOH CHECH Temp.
  19. Many of the commercial liquid resins consist essentially of the low molecular weight diglycidyl ether of bis-phenol A together with small quantities of higher molecular weight polymers. The formation of the diglycidyl ether is believed to occur in the manner shown in the below mentioned figure, the hydrochloric acid released reacting with the form sodium chloride. caustic soda to OH CI—CH,— NaOH CFI, CH—CHFHO 6) C OH + CH OH CHO C C H r O O—CH —CH—CH. + 2HCl
  20. Typical laboratory scale preparation 1 mole (228g) of bis-phenol A is dissolved in 4 moles (370g) of epichlorohydrin and the mixture heated to 105-1100C under an atmosphere of nitrogen. The solution is continuously stirred for 16 hours while 80g (2 moles) of sodium hydroxide in the form of 30% aqueous solution is added dropwise. A rate of addition is maintained such that reaction mixture remains at a pH which is insufficient to colour phenolpthalein. The resulting organic layer is separated, dried with sodium sulphate and may then be fractionally distilled under vacuum.
  21. 0verall Reaction ?—??, ? —?? ?? Bisghenol ? ??? Epichloohydirt ? —??? ? ?—??2 0iylycidyl ???.? of bisphenoI ? Cu1*'ysf ??? ?? ?? ? ?? ?—??,—
  22. HO Polymerization mechanism CH3 ö Na HO Na
  23. BPA anion react with epichlorohydrin CH3 CH3 CH3 o .CHz-C{1z-CH2
  24. The reaction is continued ??? Na ??? ??*?-??2 —0 ?1—???-???-??: ??? ?—??? + Na + ??
  25. Formation of ????? prepolymer ??? ??? ?-??—??-• ? ??--??-??-? ??—??-??-? —?? ?? ?- ??--?? —??-? ?-??—??-?? + ??-
  26. ' It appears, at first glance, that diglycidyl ether would be prepared by a molar ratio of 2: 1 epichlorohydrin-bis- phenol A, probability considerations indicate that some higher molecular weight species will be produced. ' Experimentally it is in fact found that when a 2:1 ratio is employed, the yield of the diglycidyl ether is less than 10%. Therefore in practice two to three times the stoichiometric quantity of epichlorhydrin may be employed. The diglycidyl ether has a molecular weight of 340. Many of the well-known commercial liquid glycidyl ether resins have average molecular weights in the range 340-400 and it is therefore obvious that these materials are composed largely of the diglycidyl ether.
  27. Higher molecular weight products may be obtained by reducing the amount of excess epichlorohydrin and reacting the more strongly alkaline conditions which favour reaction of the epoxide groups with bis- phenol A. ' If the diglycidyl ether is considered as a diepoxide and represented as o o CHI—CH —R—CH —CH this will react with further hydroxyl groups OH NaOH & R--CH— CH,— O
  28. ' It will be observed that in these cases hydroxyl groups will be formed along the chain of the molecule. The general formulae for glycidyl ether resins may thus be represented by the structure shown. When n = 0, the product is the diglycidyl ether, and the molecular weight is 340. OH -OHO- c-/O- When n - 10 molecular weight is about 3000. Since commercial resins seldom have average molecular weights exceeding 4000 it will be realised that in the uncured stage the epoxy resins are polymers with a low degree of polymerisation.
  29. Care should be taken to remove residual caustic soda and other contaminates when preparing the higher molecular weight resins and in order to avoid the difficulty of washing highly viscous materials these resins may be prepared by a two-stage process. This involves first the preparation of lower molecular weight polymers with a degree of polymerisation of about three. These are then reacted with bis-phenol A in the presence of a suitable polymerisation catalyst such that the reaction takes place without the evolution of by-products.
  30. Characteristics The epoxide resins of the glycidyl ether type are usually characterised by six parameters : (1) Resins viscosity (of liquid resin) (2) Epoxide equivalent. (3) Hydroxyl equivalent. (4) Average molecular weight (and molecular weight distribution). (5) Melting point (of solid resin). (6) Heat distortion temperature (deflection temperature under load) of cured resin.
  31. Resin viscosity an important property to consider in handling the resins. depends on the molecular weight, molecular weight distribution chemical constitution of the resin presence of any modifiers or diluents. Diglycidyl ethers are highly viscous materials with viscosities of about 40-100 poise at room temperature it will be appreciated that the handling of such viscous resins can present serious problems. Epoxide equivalent It is a measure of the amount of epoxy groups. This is the weight of resin (in grammes) containing 1 gram chemical equivalent epoxy. For a pure diglycidyl ether with two epoxy groups per molecule the epoxide equivalent will be half the molecular weight (i.e. epoxide equivalent = 170). The epoxy equivalent is determined by reacting a known quantity of resin with hydrochloric acid and measuring the unconsumed acid by back titration. The reaction involved is o CH—- + HC'I OH
  32. Hydroxyl equivalent The hydroxyl equivalent is the weight of resin containing one equivalent weight of hydroxyl It may be determined by many groups. techniques but normally by reacting the resin with acetyl chloride. ' Molecular weight and molecular weight distribution May be determined by conventional techniques. As the resins are of comparatively low molecular weight it is possible to measure this by ebullioscopic and by end-group analysis techniques.
  33. Melting point of the solid resins This can be done either by the ring and ball technique (https://www.youtube.com/watch?v=-yBX14z70mI) or by Durrans mercury method. Durrans Mercury Method a known weight of resin is melted in a test tube of fixed dimensions. The resin is then cooled and it solidifies. A known weight of clean mercury is then poured on to the top of the resin and the whole assembly heated, at a fixed rate, until the resin melts and the mercury runs through the resin. The temperature at which this occurs is taken as the melting point. The ASTM heat distortion temperature (deflection under load) test may be used to characterise a resin. however, be compared using identical hardeners conditions. temperature Resins must, and curing
  34. Effect of varying the reactant ratios on the molecular weight of the epoxide resins, Mol, patio epichlorohydrinj bis•phenol A 2.0 1.4 1.33 125 1.2 Mol, ratio vNaOH/ epichlorohydrin 1.1 1.3 1.3 Softening point CC) 43 84 90 100 112 Molecular weigh} 451 791 802 1133 1420 Epoxide equivalent 314 592 730 862 1176 Eptm,i groups per molecule 1.39 1.34 I .32 1.21 'Solid resins have been prepared having a very closely controlled molecular weight distribution. These resins melt sharply to give low-viscosity liquids. 'It is possible to use larger amounts of filler with the resin with a consequent reduction in cost and coefficient of expansion, so that such resins are useful in casting operations.
  35. Epoxies are one of the most versatile and widely used thermosetting resins, especially in the electrical/electronic industries. This is primarily because of the wide variety of formulations possible and the ease with which these formulations can be made and utilized with minimal equipment requirements. ' Formulations range from flexible to rigid in the cured state and from thin liquids to thick pastes and molding powders in the uncured state. ' Conversion from uncured to cured state is made by use of hardeners, heat, or both.
  36. Application The largest application of epoxies is in (potting, — embedding applications casting, encapsulating, and impregnating), — in molded parts, and in laminated constructions ' such as metal-clad laminates for printed circuits and unclad laminates for various types of insulating and terminal boards. ' Molded epoxy parts have excellent dimensional stability. Ref: Plastics Materials, J Brydson, 7th Edition
  37. Epoxy resins may be cooked into alkyd formulations, replacing part of the polyol. They also find a place unmodified as a third component in alkyd/MF compositions to upgrade the resistance properties of the films. Epoxy resins react with PF resins to form insoluble coatings, and well-formulated high molecular weight epoxy/PF coatings meet the highest standards of chemical resistance. These products are suitable for the linings of food cans and collapsible tubes, coatings for steel and aluminium containers, and wire enamels. Curing probably involves the formation of polyether links between the hydroxyl groups of the epoxy, and methylol groups present in PF resins of the resole type; the epoxy also reacts with phenolic hydroxyl groups on the PF. With some PF resins, compatibility problems on cold blending may be solved by pre-condensation, involving refluxing the epoxy and PF resins together in solution, when some reactive groups combine, leaving the remainder free to react in the curing process. Urea/formaldehyde resins or melamine/formaldehyde resins may be used to cure epoxy resins, giving stoved films of paler colour, but with a reduced level of chemical resistance compared with phenol/formaldehyde resins. Again, the higher molecular weight epoxy resins are preferred. Two-pack epoxy/isocyanate finishes require separate solutions of high molecular weight epoxy resin and polyisocyanate adduct as the two components; the epoxy resin must be in alcohol-free solvent since the curing reaction is predominantly with in-chain hydroxyl groups on the epoxy resin. One-pack finishes can be formulated with blocked isocyanates. Epoxy resins may also be used in two-pack compositions with polyamines or polyamides [134].The films obtained possess outstanding chemical resistance, hardness, abrasion resistance, flexibility, and adhesion. Low molecular weight solid epoxy resins are used most.Though primary or secondary amines such as triethylenetetramine may be used, in order to avoid the toxic hazards involved in handling amines, amine adducts with low molecular weight solid epoxy resin will nowadays be used as hardeners. These adducts are prepared by the reaction of excess of an amine such as diethylene triamine with an epoxy resin to produce fully amine- terminated adduct. Reactive polyamide resins formed from dimerized fatty acids with diamine are also used. Reactive coal tar pitches may be incorporated into an epoxy resin base for curing with amine, amine adduct, or with a polyamide resin. The derived coatings have excellent chemical resistance and are not brittle; they hence find use as pipeline, tank and marine coatings. Epoxy resin modification with a silicone resin is possible to enhance water resistance; epoxy/silicone combinations are used in blends with other polymers with the cure mechanisms mentioned above.
  38. Epoxy resins are commercially used in coating and structural applications. Through the proper selection of resin, modifier and curing agent, the cured epoxy resin system can be tailored to specific performance characteristics. The choice depends on cost, processing and performance requirements. ' Cured epoxy resins exhibit — excellent adhesion to a variety of substrates; — outstanding chemical and corrosion resistance; — excellent electrical insulation; — high tensile, flexural and compressive strength, — thermal stability; — a wide range of curing temperatures and low shrinkage upon cure.
  39. ' Versatile epoxy resins are used in various industrial applications: — Anticorrosion & antifouling Coatings — honeycomb structure — for paint brush bristles and for concrete topping compounds — body solders and caulking compounds for the repair of plastics and metal boats — for automotive springs and casting compounds — abrasive & water lubricated conditions for pump applications — electrochemical sensing — conducting adhesives as a lead free alternative in electronic packaging — electro conductive resins filled with graphite for casting application
  40. Electronic packaging potentiometric sensor for perchlorate ions for fabrication of stamping dies patterns tooling Caulking (Caulk or caulking is a material used to seal joints or seams against leakage in various structures and piping. The oldest form of caulk consisted of fibrous materials driven into the wedge-shaped seams between boards on wooden boats or ships) salient compounds in building and highway construction applications for cryogenic use high voltage composite insulator in applications where high orders of chemical resistance are required.
  41. Other applications include potting and encapsulating compounds as impregnating resins for aerospace applications filament wound structure and tooling fixtures adhesives for aircraft aerospace textile composites composite materials in space environment for development of a moon base in high performance vehicle Epoxy based solutions are used as maintenance and product finishes, marine finishes structural steel coatings tank coatings aircraft finishes automotive primers and furniture finishes decorative floor applications; chemical resistant mortar floor topping compounds in printing ink, dental and surgical applications for cholesterol level lowering agents for voltammetric sensors for humidity sensors for manufacturing the jet-printed LCD color filter in fuel cells for optical ammonia sensor for marine primer steel coating applications light weight chemical resistant foams
  42. Major suppliers Akzo Nobel Chemical Inc.; Ciba Specialty Chemicals; Cytec Fiberite Inc.; Dow Plastics; Epic Resins; Magnolia Plastics Inc.; Resyn Corp.; Shell Chemicals; Thermoset Plastics, Inc.; Union Carbide Corp.; United Resin Corp
  43. CURING OF GLYCIDYL ETHER RESINS Epoxy resins must be cured with crosslinking agents (hardeners) or catalysts to develop desirable properties. The epoxy and hydroxyl groups are the reaction sites through which cross-linking occurs. Careful selection ' of the proper curing agent is required to achieve a balance of application properties and initial handling characteristics. Major types of curing agents are aliphatic amines, aromatic amines, catalytic curing agents, and acid anhydrides.
  44. ? ? ? ; ? . ? ( ? ? ? ? ? ? ? . ? 0 ? ? ? ) ? ? r ? ? ? ? ? ? ? u ? ? ? ? ? ? 0 ? - ? 3 ? ? ? ? pa ? . ? H ? ? ' 0 ' ? 0 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 0 ? ? ? ? ? ? ? ? ? ? ? , 0 ? ? ? ? : ? au ! 1 ? ? ? ? duo ) ? 1 ? ? ? ? q ? ? ? ? ? ? ? ? ? ? q ? ? ? ? ? ? q 1 ? ? ? ? ? U ? ? ? ? ? ? ? 0 ? ? . ? ? ? ? ? ? Lua ? ? •q -0 ? •d ? ? 01 tu ? ? ? ? ? q ? ? ? 0 ? ? ? Lld ? p ? •q ? H ? ? ? ? ? - ? . ? ? 0 ? ? ? . 7 ? ? 0 ? ? 0 ? L' ? ? ? 1 ? ? ? ? 1 ? ? 0 ? ? d IL ? LJO ? ? ? 1 00 ? 0 ? ? ? ? U ? ? ? 0 ? ? ? 1 ? ? 3 ? ? 4 ? ? ? ? ? 0 ? ? N ( ? ? ? ? ( ? ? ? ? 0 ? ? ? ? ? ? ? ? ? ? ? ? - ? 4 ? ? ? 0 0q1 ? 0 ? " ? ? 1 ? ? ? ? 0 ? 0 ? ? " p ? ? ? ? ? ? 1 ? 1 ? ? ? ? ? 1 ? 0 - ? 0q1 4 pat ? ? . ? " . ? ? 0 ? ? ? ? ? ? 3 ? ? ? ? ? ? ? . ? 0 ? ? ? = ? 0 ? ? ? ? ? 0 ? ? ? ? 40 ? ? ? ? ? ? ? ? JO 0 ? ? p ? .- ? ? ? & ? 0 ? ? ? 0 ? ? ? ? 1 ? ? ? ? ? ? d ? ? ? ? ? ? ? ? ' ( ? ? ? 1101 ? ? ? -1 ? ? ? ? q10 ? ? ? 4 ? ? ? 0 ? ? ? 4 ? q ? ? 0 ? 41 ? ? ? ? 1 ? ? 0 ? ? ? ? d ? ) ? 0 ? ? ? ? ? 1 ? ? 0 ? ? ? ; 0 ? - ? ? ? ? OJ ? ? ? 3 ? ? ? ( ? ? I " ? 0 ? ? —N ? ? ? ? 0 ? ? ? qd = ? ? . ? ? ? ?
  45. Extra Compulsory Reading on Curing ' Curing Agents for Epoxy Resin, Three Bond Technical News, Issued December 20, 1990 ' Epoxy Resin — Introduction ' Raiu Thomas, Christophe Sinture/, Sabu Thomas and Elham Mosta a Sadek El Akiab

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