Basic knowledge of epoxy resins

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Basic Knowledge of Epoxy Resins 1. A Brief Overview of Epoxy Resins Polymeric compounds whose molecular structure contains epoxy groups are collectively referred to as epoxy resins. The cured epoxy resin possesses excellent physicochemical properties. It offers strong adhesion to the surfaces of both metallic and non-metallic materials, has good dielectric properties, low shrinkage upon curing, good dimensional stability, high hardness, and decent flexibility. It is also stable against alkalis and most solvents; as a result, it is widely used in various sectors of the national economy for applications such as casting, impregnation, laminates, adhesives, and coatings. China began researching epoxy resins in 1958 and quickly put them into industrial production; since then, they have seen vigorous development across the country. In addition to producing conventional bisphenol A-epichlorohydrin-type epoxy resins, various types of new epoxy resins are also manufactured to meet the urgent needs of national defense efforts and various sectors of the economy. The methods for preparing epoxy resins generally fall into the following categories: reaction of active hydrides with epichlorohydrin ； Liquid-phase oxidation of the double bond using hydrogen peroxide or peracids (e.g., peracetic acid) ； Air oxidation of double-bonded compounds ； Others. Since its performance is not entirely perfect, and the applications for epoxy resins vary, different requirements are placed on its properties depending on the application. For example, some applications require fast drying at low temperatures, while others demand excellent insulating properties. Therefore, it is necessary to modify the epoxy resin in a targeted manner, and the main methods of modification include the following: selecting a curing agent ； Add reactive diluent ； Add filler ； Add another type of thermosetting or thermoplastic resin ； Improving the epoxy resin itself. 2. Related terms for epoxy resins (I) Average degree of polymerization: The number of times the basic structural unit repeats in a molecule is called the degree of polymerization, denoted by n. The average degree of polymerization of a mixture of homologs with the same chemical composition but different degrees of polymerization is called the average degree of polymerization of that mixture. Taking bisphenol A-type epoxy resin as an example, its general formula is as follows: where n=0-22. When n=0, the formula becomes…; when n=1, it indicates the presence of 1 unit of the said structure. When n=2, it means that there are 2 structural units. When n=i, it means that the structure in is repeated i times. The basic structure in it repeats in this manner; the more times it repeats, the higher the molecular weight of the resin, so it indicates the degree of polymerization (condensation). (II) Average molecular weight m: In industrial resins, the structure of the resin changes significantly depending on the number of times the basic structural units are repeated within the molecule. Due to the different degrees of polymerization of epoxy resins, their molecular weights also vary. The molecular weight of the resin varies with the degree of polymerization; in other words, the molecular weight is a function of the degree of polymerization. M = f(n); with pure epoxy resin, the molecular weight is Mo when n = 0, M1 when n = 1, M2 when n = 2, and so on, with Mi being the molecular weight when n = I. When the resin in industry is a mixture composed of such molecules, the molecular weight exhibited is the average molecular weight, denoted as m. (III) Epoxy value Ev: The epoxy value refers to the number of epoxy equivalent units per 100 grams of epoxy resin. The unit is equivalents/100 grams]. If the molecular weight of a monomeric epoxy resin is 340, its molecular formula is as follows: each molecule contains two equivalents of epoxy groups; therefore, its epoxy value Ev = 2/340 × 100 = 0.588 equivalents per 100 grams. It can be seen that the higher the molecular weight, the lower the epoxy value; therefore, the epoxy value is a very important indicator for assessing the quality of epoxy resins. In our country, it is expressed using the epoxy value. (IV) Epoxy equivalent En: The epoxy equivalent refers to the weight of resin that contains one equivalent of epoxy groups. The unit is grams per moment]. Theoretically, the chain of an epoxy resin is an ideal linear molecule without branches, and each chain ends with an epoxide group; therefore, the epoxy equivalent is half of the resin’s average molecular weight. The higher the molecular weight of the resin, the greater its epoxy equivalent weight as well. In foreign countries, the quality parameters of epoxy resins are generally measured and expressed in terms of epoxy equivalent. The epoxy equivalent and the epoxy value are essentially the same; the epoxy equivalent is obtained by dividing the epoxy value by 100. That is, En = 100/EV. (5) Epoxy group content Ec: The epoxy group content refers to the percentage of epoxy groups per molecule in the epoxy resin. The unit is %]. If the molecular weight of a monomeric epoxy resin is 340, and it contains two epoxy groups, with each epoxy group having a molecular weight of 43, then the proportion of epoxy groups is: (2×43)/340×100% = 25.3%. The epoxy groups and the epoxy value are essentially the same; the epoxy group content is obtained by multiplying the epoxy value by 43%. (VI) Hydroxyl equivalent En(OH): The hydroxyl equivalent refers to the mass of resin that contains one equivalent of hydroxyl groups. Also known as the esterification equivalent. The unit is grams per equivalent]. It is also one of the important indicators for measuring the quality of epoxy resins. Abroad, it is commonly expressed in terms of hydroxyl equivalent. (7) Hydroxyl value Ev (OH): The hydroxyl value refers to the number of hydroxyl equivalents per 100 grams of epoxy resin, expressed in equivalents/100 grams. In our country, it is usually expressed using the hydroxyl value. The hydroxyl value and the completion equivalent are essentially the same. The hydroxyl equivalent is obtained by dividing the hydroxyl value by 100; En(OH) = 100/EV(OH). (8) Organic chlorine value Ecl: The organic chlorine value refers to the number of equivalents of organic chlorine atoms per 100 grams of epoxy resin, with the unit being equivalents/100 grams. Organic chlorine mainly denotes hydrolyzable chlorine. Hydrolyzable chlorine is a 1,2-chlorohydrin ether formed by α-addition, which remains as a residue due to incomplete cyclization upon dehydrochlorination. Its presence affects the high-temperature electrical properties of epoxy resins; therefore, it is also an important indicator for assessing the quality of epoxy resins. (IX) Inorganic chlorine value: The inorganic chlorine value refers to the number of equivalent chloride ions remaining per 100 grams of epoxy resin, expressed in equivalents/100 grams. Chloride ions are residues resulting from NaCl that was not completely removed during the epoxy resin reaction process. Its presence affects the electrical properties of epoxy resin cured products at room temperature. Generally, during the resin production process, by strengthening the washing process, its content can be kept very low, so as not to affect its performance. (10) Volatiles: The volatiles in resins refer mainly to the trace amounts of solvents remaining from the resin production process, expressed as %]. (11) Viscosity: Like the molecular weight of epoxy resin, viscosity is also a useful indicator of its processing properties. The viscosity of a liquid refers to the shear resistance that occurs when adjacent layers of the liquid move relative to each other. The unit is usually . The viscosity of liquid resins is generally measured using a spindle or shaft viscometer. The viscosity of solid resins is typically determined using a 40% solution prepared from butyl catechol on a weight basis. (12) Softening point: The softening point refers to the temperature at which, under the influence of heat, the epoxy resin experiences an increase in internal energy that enables it to overcome its cohesive forces and flow. The high or low softening point of epoxy resin indirectly indicates the molecular weight of the resin. In China, the Universal Law is commonly used to determine the softening point of resins. 3. Chemical synthesis of epoxy resins (I) Synthesis of bisphenol A-type epoxy resins 1. Synthesis pathway of bisphenol A-type epoxy resins: ① Low-molecular-weight liquid bisphenol A-type epoxy resins are formed by the polycondensation of excess epichlorohydrin (ECH) with bisphenol A (BA) in the presence of a base (NaOH) ； Another method is to obtain alicyclic epoxides by chlorinating the double bonds of unsaturated cyclic hydrocarbons to form epoxy rings. ② Routes for high-molecular-weight solid epoxy resins: a. “Toffee method”: Direct reaction of bisphenol A with epichlorohydrin in the presence of a base, as calculated chemically. b. “Melting method”: Using low-molecular-weight liquid bisphenol A-type epoxy resin as a starting material, polymerization takes place in the presence of a catalyst, resulting in products with a high degree of polymerization (n>2000). This process yields a high-purity product without by-products, with a narrow molecular weight distribution, primarily by suppressing side reactions and reducing chain branching. 2. Synthesis reaction of bisphenol A epoxy resin Bisphenol A epoxy resin is prepared from ECH and BA through two steps: etherification and ring closure. 3. Side reactions during the synthesis of bisphenol A-type epoxy resins ① Hydrolysis: ECH is hydrolyzed to glycerol, and the epoxy group is hydrolyzed to 2-ethanediol. ②Epichlorohydrin reacts with the acidic hydroxyl groups present in the intermediate to form trapped chlorine. ③Abnormal addition (β-addition) of epichlorohydrin to phenol hydroxyl groups. ④ Chain branching reaction: Chain branching in bisphenol A-type epoxy resins. ⑤ Incomplete HCl removal; residual chlorine in the resin can be hydrolyzed. The occurrence of these reactions leads to increased ECH consumption and a decrease in the epoxy content; hydrolyzable chlorine increases the total chlorine content, the purity of the resin is reduced, and its viscosity or melting point rises. (II) Effect of chlorine content: The residual chlorine in epoxy resins exists in three forms: chloride ions (Cl-), hydrolyzable chlorine, and non-hydrolyzable chlorine. Cl- are residual NaCl ions, while hydrolyzable chlorine and non-hydrolyzable chlorine are by-product ions. 1. Effect of chlorine content: ① Hydrolyzable chlorine causes fat amines to solidify the epoxy system, thereby significantly reducing its workability period. Hydrolyzable chlorine content (wt%), peak exothermic temperature (°C), gelation time (minutes): 0.056 – 142 – 80; 0.134 – 153 – 75; 0.550 – 163 – 45; 0.821 – 172 – 39. ② The liquid epoxy resin used to prepare high-molecular-weight solid epoxy resins by melting method is susceptible to hydrolysis, and the chlorine content reduces the rate of the reaction. Therefore, the hydrolyzable chlorine content in the raw resin should preferably be below 0.03 wt% (300PPm). Ratio: Liquid epoxy resin/BA = 3/2 (by volume), temperature: 110°C. Catalyst amount: 80 PPm of NaOH. Weight percentage of hydrolyzable chlorine in the liquid epoxy resin. Reaction time (hours): 0.01 – 1 hour; 0.03 – 3 hours; 0.05 – 6 hours; 0.80 – 15 hours. The epoxy resin/dicyandiamide/benzyldimethylamine combination, which is widely used in laminated materials, often suffers from prolonged gelation times and large variations in those times, and this is due to the high content of hydrolyzable chlorine in the resin as well as significant fluctuations in that chlorine content. ④Hydrolyzable chlorine can corrode the connections on integrated circuit boards; therefore, the content of hydrolyzable chlorine in the resin is required to be ≤0.03 wt%. ⑤ Under humid and warm conditions, hydrolyzable chlorine causes a **decline in the electrical properties of the resin. 2. Ways to reduce chlorine content: ① In the synthetic industry, a mixed solvent of inert solvents (benzene, toluene, etc.) and water-soluble solvents is used to enhance the contact between the organic phase (resin solution) and the inorganic phase (alkaline water), thereby improving the efficiency of HCl removal. ②By adding quaternary ammonium salts or bases when treating the crude resin with dilute NaOH, the hydrolysis chlorine can be kept below 0.05 wt%. 4. Production and preparation of epoxy resins There are many raw materials used to manufacture epoxy resins, but they can be classified into two main categories: those compounds that contain epoxy groups in their molecular structure, or those that can form epoxy groups during the reaction process. The other are compounds containing multiple hydroxyl groups. The commonly seen epoxy resins are produced from epichlorohydrin and bisphenol A as raw materials. 1. Preparation of epichlorohydrin-bisphenol A type resin: Due to differences in the amounts of materials used and the order in which they are added, the molecular weight of the resulting epoxy resin varies; moreover, its state at room temperature can be either liquid or solid. Series 618, 6101, and 634 are low-molecular-weight liquid resins; series 601 and 604 are medium-molecular-weight solid resins; series 607 and 609 are high-molecular-weight solid resins. ①Preparation of low-molecular-weight epoxy resin: Bisphenol A – 1 mole; epichlorohydrin – 2–6 moles; base (30%) – 2–2.4 moles. The procedure is as follows: Bisphenol A and epichlorohydrin are mixed under stirring to dissolve the bisphenol A. Then, the required amount of base is added to the mixture at a certain temperature. After adding the first portion of base, the reaction is allowed to continue; once epichlorohydrin has been recovered, the remaining base is added to the reaction mixture, and the reaction continues again. An appropriate amount of benzene is then added to dissolve the resin solution, followed by stirring and allowing it to settle into layers. Pour the resin-benzene solution from the upper layer back into the separatory funnel, stir, heat, maintain the temperature, allow it to stand, remove the salt precipitate, then stir again and heat to bring the resin-benzene solution to a boil and induce reflux; remove the water that forms from time to time until no more bubbles appear, continue the reflux process, then cool, allow it to stand, and filter under pressure. The filter cake is placed in a debenzing tank, where debenzing is carried out first at atmospheric pressure and then under reduced pressure, until no more benzene-containing liquid comes out, resulting in a light yellow liquid resin. ②Preparation of medium-molecular-weight epoxy resin: Bisphenol A – 1 mole; epichlorohydrin – 1.5–2 moles; alkali (30%) – 1.6–2.2 moles. The procedure is as follows: Add bisphenol A, liquid alkali, and water to the reaction vessel, heat while stirring to ensure that all of the bisphenol A dissolves. Then cool the mixture, add epichlorohydrin, maintain the temperature while heat is released, and raise the temperature again. Take samples for titration until the amount of HCl used remains constant, indicating that the reaction is complete. Once the reaction is finished, stop stirring, remove the aqueous layer, wash several times with water, then dehydrate under normal pressure and subsequently under reduced pressure until the softening point meets the required specifications. After cooling, the solid resin is obtained. ③For high-molecular-weight epoxy resins: Medium-molecular-weight epoxy resin – 100; the amount of bisphenol A used is calculated as Q = W/. Where: Q is the amount of bisphenol A added; W is the amount of medium-molecular-weight epoxy resin used; E1 is the epoxy value of the medium-molecular-weight epoxy resin used; E2 is the epoxy value of the high-molecular-weight epoxy resin to be prepared. The procedure is as follows: Put the medium-molecular-weight epoxy resin into a pot, raise the temperature to melt it, stir it, then add bisphenol A; reaction at a certain temperature will yield the desired product. 3. Liquid-phase oxidation of double bonds to produce epoxy resins: Resins of the 6201 and 6207 series in China are produced using this method. 1 mole of diene compound, 1–3 moles of peracetic acid, 1 mole of sodium carbonate, solvent (benzene) – as appropriate. The procedure is as follows: Mix the diene compound, an appropriate amount of solvent (benzene), and sodium carbonate; then add peracetic acid drop by drop. After that, gradually add the remaining sodium carbonate, and maintain the mixture. Once the water becomes neutral, remove it. Then add the remaining peracetic acid, continue to maintain the mixture, add an appropriate amount of solvent and water, and stir. When the water is neutral, the water is removed, followed by vacuum desolvation; then, while still hot, the resin is filtered out. 4. Cyanuric acid epoxy resin (695) is produced by the reaction of cyanuric acid with epichlorohydrin in the presence of a base. 5. Phenol epoxy resin (690) is produced by the reaction of phenol with epichlorohydrin in the presence of a base. 6. Phenolic epoxy resin (644, etc.): 1 mole of phenol, 5 moles of water, 0.03 moles of concentrated sulfuric acid, 0.6 moles of formaldehyde, 5 moles of epichlorohydrin, 1.6 moles of base; an appropriate amount of benzene is also used. The procedure is as follows: Mix phenol and water, add concentrated sulfuric acid, raise the temperature, then add formaldehyde drop by drop. After adding it all, maintain the temperature at this level. Next, wash with water until the solution reaches neutrality, then remove excess water. Add epichlorohydrin and raise the temperature to facilitate dissolution; cool the mixture while adding base at regular intervals to control the temperature until all the base has been added. Maintain the conditions, then recover epichlorohydrin under reduced pressure. Add benzene to dissolve the residue, add more base as needed, maintain the conditions again, allow the mixture to settle, cool it, filter it, and remove benzene to obtain the resin. 7. Polyester epoxy resin: Bisphenol A – 15.16 parts; Base (10%) – 7.975 parts; Butanol solution of terephthaloyl chloride – 11.125 parts; Butanol solution of epichlorohydrin – 11.25 parts. The above components are mixed, reacted, allowed to settle, and the organic phase is separated. It is then washed until no chlorine ions remain, and the solvent is removed to obtain a resin with a softening point of 41–48°C and an epoxy group content of 1.6%. 8. Sulfone-containing epoxy resin: Under a nitrogen stream, with an alkali as a catalyst, 4,4-dihydroxydiphenylsulfone and epichlorohydrin are reacted at a certain temperature for a desired period of time; after washing and removing the solvent under reduced pressure, the product is obtained. Another method: A mixture of bis-(4-aminophenyl)sulfone, epichlorohydrin, 2-methoxyethanol, and water is heated; then butylamine and an aqueous KOH solution are added to the mixture. The mixture is stirred vigorously, water is added, the organic phase is separated, it is diluted with dichloroethylene, the solution is washed with water, and vacuum evaporation is carried out to obtain a solid resin with a softening point of 49°C. 9. Polyphenol epoxy resins are obtained by reacting phenol and dichloropropane in an acidic medium to yield 1,2,2,3-tetrahydroxy-(4-hydroxyphenyl)propane, which is then reacted with a base in the presence of an excess of epichlorohydrin. 10. Polydehydrated hexitol epoxy resin 1,4 ； 3. 6-Anhydro-D-sorbitol and epichlorohydrin are heated; NaOH (50% solution) is added, and epichlorohydrin and water are distilled off. After adding the NaOH, heating is continued to remove all the water, after which epichlorohydrin is distilled off again. The residue is then mixed with propanol. The salt was filtered out, the precipitate was washed with acetone, and the acetone was evaporated to obtain a resin with an epoxy value of 0.467 and a chlorine content of 0.11%. 11. Fluorescein epoxy resin: Fluorescein, epichlorohydrin, and isopropanol are heated in a water bath; caustic soda is added, and the mixture is heated to boiling. The filtrate is then dried under vacuum to yield a resin containing 122% of the amount of fluorescein used. The epoxy group content is 25.6%, the softening point is 95°C, the hydroxyl group content is 3.7%, and its heat resistance according to the Martin test is 250°C. 12. Chlorinated epoxy resin: Benzene chloride is used as a solvent to react bisphenol A with chlorine, thereby attaching 1–4 chlorine atoms to bisphenol A; thereafter, hydrochloric acid and chlorine are removed, and the compound then reacts with epichlorohydrin in the presence of a base. 13. Self-extinguishing epoxy resin: α-chloroglycerol is mixed with bis-(p-hydroxyphenyl)trichloroethane; NaOH is added dropwise to this mixture, and subsequent reactions are used to dissolve the resin in benzene. The salts are removed and benzene is distilled off to obtain the final product. 14. Aminophenol epoxy resin: n-aminophenol, epichlorohydrin, ethanol, and LiOH·H2O are mixed together; the mixture is then heated, and a NaOH solution is added. Water, ethanol, and excess epichlorohydrin are evaporated away. The residue is dissolved in toluene, washed with water to remove salts and bases, and then toluene is evaporated to obtain the resin, whose epoxy equivalent is 104. 15. Brominated epoxy resin: Tetra-bromobisphenol A is combined with epichlorohydrin, alkali and water are added, and the mixture is heated; a exothermic reaction occurs until boiling. After cooling, more alkali is added, and the same process is repeated until sufficient alkali is present. The mixture is then boiled again, filtered to remove epichlorohydrin and water, diluted with benzene and filtered once more. The residue is washed with alkali, then with water, and finally the benzene is removed, resulting in a resin with a heat stability of 123°C. 16. Phosphorus epoxy resin: ethylene glycol diglycidyl ether (boiling point 120–126°C/4 mm); a mixture of methyl dichlorophosphite (boiling point 66–70°C/4 mm) and ferric chloride is added drop by drop. The mixture is dissolved in chloroform, the chloroform solution is washed with soda water and water, dried over anhydrous sodium sulfate, and then chloroform is removed under vacuum or the product is precipitated from the solution using petroleum ether to obtain the resin. 17. Titanium-modified epoxy resin: The epoxy resin is heated, and butyl titanate is added under stirring; after treatment for several hours, the distilled butanol is separated to obtain the modified resin. There are other epoxy resins both domestically and internationally. 2. Epichlorohydrin–alcohol epoxy resins: The properties of the resulting resins vary depending on the type of alcohol used. In China, there are glycerol epoxy resins (662), ethylene glycol epoxy resins (669), chloroglycerol epoxy resins (600, 630), butanol epoxy resins (660), and polyethylene glycol epoxy resins (6690), among others. Alcohols – 1 mole; Epichlorohydrin – 1–3 moles; BF3–ether – 0.4 cc per mole of alcohol; Ethanol – as required; Base (45%) – 1–1.5 moles. The procedure is as follows: Add the alcohols, solvents (ethanol, benzene, etc.), and BF3–ether to a reactor, stir to mix them together, raise the temperature, then add epichlorohydrin drop by drop, and further increase the temperature to facilitate etherification. An appropriate amount of solvent is added to the etherate and stirred; NaOH is then added while continuously monitoring the pH value to maintain it at the desired level. After that, the mixture is allowed to settle into layers, and the upper clear liquid is filtered under pressure. This liquid is then returned to a vessel where the solvent is recovered under vacuum, and once the recovery is complete, the desired resin is obtained.