Curing (Hardening) Reactions of Epoxy Resins

Epoxy resins are cured to be converted into useful thermoset products. During the curing process, shrinkage can be observed. As a result, both density and refractive index increase. It should be noted that shrinkage in solid or gel form can cause stress. The cure rate and shrinkage can be measured by monitoring with refractometric or dilatometric methods. Cured epoxy resins must be used below their glass transition temperature. Because thermal decomposition can occur at these temperatures. Cured epoxy products have high mechanical, chemical and thermal resistance. These products also have high electrical insulation properties. Their dielectric constants range from 3 to 6. They have good arc, surface and volume resistances. However, these properties are affected by moisture and temperature increases. These resins can be made conductive or semi-conductive with appropriate fillers and curing agents.

Curing Agents Used in Curing:

Curing is the process of creating three-dimensional thermoset structures through cross-linking of epoxy groups with various chemical agents such as hardeners and catalysts. Different curing agents are used depending on the application of the final thermoset product and the processing technique to be applied to create this product. The bulk properties of the thermoset end product depend on the type of epoxy resin and curing agent and their interaction. It should be noted that solvents, plasticizers, fillers and pigments added for structural modification also significantly change product properties. In curing, epoxy groups in the resin cross-link through two mechanisms:

Catalyzation:

Here, epoxy molecules bond directly to each other through a reaction initiated by a catalyst. This type of curing process can also be called homopolymerization. Polyadditional: Here, epoxy molecules cross-link with the help of a hardener containing active groups capable of reacting with epoxide groups in the resin. In this way, the cured epoxy resin contains hardener molecules in its final structure. The most important combinations of epoxy resins and hardeners are summarized below. Depending on their molecular weights, bisphenol-A type epoxy resins can be hardened through polyadditional reactions via epoxy or hydroxyl groups. Polyamines, polythiols and polyisocyanates are suitable for curing at room temperature. Polyanhydrides, polyphenols, acids and carboxylic functional polyesters can be cured by heat. Epoxy resins can also be cured by polycondensation with amino resins or phenolic resins. Epoxy resins can also be polymerized with catalysts such as tertiary amines, boron trifluoride complexes, ferrocenes and triarylsulfonium salts. The selection of hardener is very important in an epoxy resin system designed for a specific application. Epoxy resins have two main reactive groups. The first is epoxide groups and the second is hydroxyl groups. These groups can react with many chemical products containing hardener additives. Some hardeners cure with epoxy resin at room temperature, while others cure through heat. Epoxy resins contain two reactive groups. These groups provide the basic cross-linking reactions of the epoxy resin. These are: • Terminal epoxide groups, • Hydroxyl groups located along the chain.

A- Reaction of Epoxide Groups 1- Reaction of Epoxide Groups with Amine Groups

Terminal epoxide groups in epoxy resins react with active hydrogen groups in primary and secondary amines. This reaction takes place at room temperature. Many amine-hardened systems are formulated for curing at room temperature. Aliphatic amine and polyamide hardeners are used with bisphenol A and epoxy novolac resins. These resins are generally cured at room temperature. Reactions of epoxide groups with amine groups can be summarized with the following different amine groups. • Reactions of epoxide groups with polyamines, • Reactions of epoxide groups with polyaminoamides, • Reactions of epoxide groups with polyamides, • Reactions of epoxide groups with fenalkylamines. Systems cured using these materials show short pot life and low thermal decomposition temperature. Products obtained with polyamide hardeners have longer pot lives compared to aliphatic amines.

Polyamine Type Hardeners:

They are small molecules and therefore have low viscosity. The high number of amine hydrogens gives high cross-link density, thus providing good acid, solvent and heat resistance, but gives poor flexibility. It also gives a low AHEW (gr / eq) value, therefore the risk of incorrect mixing ratios increases. They are highly reactive, therefore they have short pot life and short drying times. They are prone to rubbery feel and blooming. Water resistance is low. They are also corrosive and toxic. Applications:

Polyamine Type Hardeners

Polyamide Type Hardeners:

Polyamide hardeners are reaction products of dimeric fatty acids and amines. Polyamidoamine hardeners are reaction products of mono-functional fatty acids and amines. Polyamide hardeners produce flexible coatings with high vibration, impact and shock resistance. However, their electrical properties are not as good as other hardeners. They are not irritating. Coatings produced from them have high surface adhesion. Surface preparation and cleaning are not as critical for adhesion as with other hardeners. Moisture absorption and permeability are higher than polyamines. Pot life is longer than amine-cured products and peak exothermic temperature is lower than amine-cured products. Polyamide hardened epoxy resin systems are used in maintenance paints. Polyamide hardeners normally used in epoxy resin systems are reaction products of dimeric plant fatty acids with polyamines. These products are amine terminated and are found in a very wide range of amine values. In addition to these properties, polyamides are not toxic or irritating. They allow for the preparation of paint compositions with longer pot life (such as several days). The most commonly used range in epoxy paints is approximately 85 to 350. Those around 240 amine value are preferred. These products vary in physical form from liquid to low melting point solids. Polyamide 115 is a high viscosity general purpose polyamide type hardener. It is used with appropriate epoxy resins in the manufacture of primer and topcoat paints. It is also used in thermoset adhesives prepared for different and various surfaces. The basic characteristic of this hardener is excellent strength properties and excellent adhesion to metallic surfaces. It is suitable for general purpose resin combinations that require particularly good exterior durability and excellent strength. On the other hand, it is compatible with many synthetic resins, varnishes, oils and other binders. When polyamide resins react with epoxy resins, they create coatings that provide excellent adhesion, flexibility, corrosion resistance and chemical resistance. Like other amine hardened systems, they are used with epoxy resins with approximately 500 EEA value. Among all amine hardened systems, polyamide systems show the least tendency to blooming. To such an extent that the pre-reaction time recommended for polyamine and polyamine adduct systems can be eliminated.

Polyamide Type Hardeners

Phenalkamine Curing Agents: A type of hardener group generically known as phenalkylamines. Phenalkamine type hardeners consist of aliphatic polyamines bonded to an aromatic ring structure with a straight aliphatic chain. This special structure of phenalkylamines gives hardeners produced from them an unusual combination of desired properties in paint coatings, casting and adhesives. Phenalkamine hardeners have certain advantages. These include: curing properties at room temperature and low temperatures, fast curing and long pot life, along with excellent chemical resistance particularly to acids and alkalis, excellent water and sea water resistance, good flexibility, moisture resistance during curing, non-critical mixing ratios. They are used in solvent-based, high solids and solvent-free epoxy systems. Phenalkamine type hardeners have many applications. Some of these are as follows: heavy industrial coatings, marine and coastal paints, concrete coatings, casting and molding, anticorrosive protection systems, adhesives (especially in automotive and construction industries) and in laminates.

2- Reaction of Epoxide Groups with Carboxylic Acid Groups

The epoxy-carboxylic acid reaction requires the presence of a basic catalyst such as a tertiary amine. Otherwise, other more effective reactions occur. Curing takes place between 160-190°C. Curing of epoxy resins with acids and carboxylic functional polyesters requires heat. Industrially, these systems are used in powder coatings. More than 70% of powder coatings are based on epoxy resins and carboxylic functional polyesters.

3- Reaction of Epoxide Groups with Polymercaptan Groups

When polymercaptans are catalyzed with tertiary amines, they become highly reactive with epoxy resins and can cure even at temperatures below room temperature. They are mostly used in fast-curing adhesive formulations. Thiols have an extremely unpleasant odor and are industrially important only for adhesives and sealants. They provide high and lasting flexibility in epoxy resins.

4- Reaction of Epoxide Groups with Dicyandiamide Groups

The crosslinking reactions of dicyandiamides have not yet been fully understood. It is known that their primary and secondary amine and nitrile groups react with epoxide groups. However, it is also known that hydroxyl groups in the resin contribute to the crosslinking reaction. Dicyandiamides and their modified forms; • Powder coating systems, • Powder casting systems, • Important curing agents for epoxy in the production of electrical laminates. Curing takes place at high temperature (160-190°C).

5- Reaction of Epoxide Groups with Imidazole Groups

Like dicyandiamides, the crosslinking reactions of imidazoles have not yet been fully understood. It can be accepted that secondary amine groups undergo addition reactions, and anionic epoxide polymerization reaction (through tertiary N atoms) takes place. Imidazoles are used in: • Electrical laminates, • Powder coating systems, • Powder casting production. • Curing temperatures are 160 - 190°C.

B- Reaction of Hydroxyl Groups 1- Reaction of Hydroxyl Groups with Methylol Groups

Resins containing methylol groups are: • Phenol-Formaldehyde Resins (PF), • Melamine-Formaldehyde Resins (MF), • Urea-Formaldehyde Resins (UF). If the methylol groups in PF, MF, UF resins are etherified (as butyl or methyl), similar reactions occur and similar transesterification reactions proceed under similar conditions. Both of these reactions are usually accelerated by the addition of a small amount of an acid catalyst (such as phosphoric acid). They cure at 150°C and above. They are used in the production of can coatings and coil coatings. Urea, melamine and benzoguanamine resins react with high molecular weight epoxy resins when heated. Hydroxymethyl groups react to form ether bonds with each other and with hydroxyl groups of epoxy resins. These types of systems are used in household appliances as well as in packaging. Phenolic resins also react with high molecular weight epoxy resins under heat. These systems are used as "gold lacquer" for coating food cans. Similarly synthesized bisphenol A resols produce colorless lacquers with higher chemical resistance, less odor during curing and less taste change in food when in contact with lacquer. However, flexibility is lower than standard phenolic resins.

2- Reaction of Hydroxyl Groups with Anhydride Groups

They are cured at high temperature with anhydride hardeners. Like aromatic amines, they also provide products with better chemical resistance and high thermal decomposition temperature. Particularly when pyromellitic dianhydride is used with cycloaliphatic epoxies, thermal decomposition temperature can be increased to 316°C. Phenolic-novolac hardeners are generally used with epoxy novolacs. With these hardeners, high temperature curing is performed. These products have long pot life and high mechanical and chemical resistance. In the reaction of epoxy resins with anhydride groups, a basic catalyst is used at temperatures above 100°C. Well crosslinked structures with high heat resistance are obtained. These reactions are complex as they contain esterification and etherification reactions. Polyanhydrides and monoanhyd rides should not be used to cure epoxy surface coatings. They are used in powder form in powder coatings and in solution form for can coatings; both forms are cured with heat. Films have good acid resistance and do not impart an undesirable taste to food. Esterification Reactions: The anhydride first reacts with the OH group in the resin to form a half ester. Then the carboxylic acid group in the anhydride reacts with the epoxide group in the resin. • Etherification Reactions: These reactions occur between epoxide groups and hydroxyl groups. Anhydride-cured systems provide good heat resistance and good chemical resistance. Curing temperatures are between 130 - 180°C. Uses of anhydride-cured systems: • Largely in the electrical industry (casting, impregnants), • In the production of structural composites.

Polyamine Type Hardeners

They are cured at high temperature with anhydride hardeners. Like aromatic amines, they also provide products with better chemical resistance and high thermal decomposition temperature. Particularly when pyromellitic dianhydride is used with cycloaliphatic epoxies, thermal decomposition temperature can be increased to 316°C. Phenolic-novolac hardeners are generally used with epoxy novolacs. With these hardeners, high temperature curing is performed. These products have long pot life and high mechanical and chemical resistance. Acid and anhydride hardeners can be used with all three: bisphenol A, epoxy-novolac and cycloaliphatic epoxies. However, when to be used with epoxy novolac and cycloaliphatic resins, selection of special types is recommended.

3- Reaction of Hydroxyl Groups with Isocyanate Groups

This type of reaction takes place at room temperature and is used as a two-component system with good acid resistance. Only the use of blocked isocyanates reacting at high temperatures is one-component. These are used as binders for automotive primers applied by electrodeposition. Curing temperatures are around 160 -200°C. While polyamines and thiols cure epoxy resins' epoxy groups, isocyanates cross-link with hydroxyl groups with epoxy resins to form polyurethanes. These combinations cure faster and at lower temperatures than epoxy resins cured with polyamines.

C- Copolymerization of Epoxide Groups

This type of reaction requires the presence of a suitable initiator or catalyst. These catalysts are: 1. They can be acidic through acidic interaction (Lewis acids like boron trifluoride), 2. They can be basic through basic interaction (tertiary amines like benzyldimethylamine or 2,4,6-tri [di-ethylaminomethyl phenol]).

1- Copolymerization of Epoxide Groups with Acidic Initiators (With Boron Trifluoride)

The Lewis acid activates the oxygen atom in the epoxide group to give a carbonium ion that will react with a second epoxide group. As a result, a crosslinked polyether type polymer network is formed. Boron trifluoride is generally used in mixed forms to increase the stability of the resin-initiator mixture at room temperature. For example, BF3.400 is a mixture of ethylamine and BF3. Other suitable initiators include: • Tin chloride SnCl4, • Phosphorus pentafluoride PF5.

2- Copolymerization of Epoxide Groups with Basic Initiators (With Tertiary Amines)

In this case, the epoxide group is activated by one of the carbon atoms and again forms a polyether network. Both copolymerization reactions are fast. However, because the reactive intermediates are subject to different side reactions, the degree of crosslinking can be low and the final products show poor flexibility. Therefore, these types of products are used only in special applications. For example; in fast-curing adhesives or mixed with other hardeners as in powder coatings. The curing temperature in powder coatings is between 160- 190°C. Although tertiary amines can polymerize epoxies even at room temperature, the degree of polymerization is too low to obtain usable coatings. Boron trifluoride complexes polymerize epoxies with heat. These combinations are sometimes used in powder coatings when high solvent resistance is required. Ultraviolet curing of epoxy resins has become an important development. Cycloaliphatic epoxy resins are combined with substances that initiate polymerization under UV radiation or electron beam. Among industrially important products are triarylsulfonium salts (UVE 1014, 1016, General Electric) and arenferrocenyl compounds (CG 24-061, Ciba-Geigy). To provide good film flexibility and chemical resistance, polyols should be added. Films cure in approximately 30-120 seconds at 100°C. Polyphenols react with epoxy resins with heat, but require an accelerator (such as tertiary amines, imidazoles). Due to poor color stability, this combination-based powder coatings are not used for decorative purposes, but are used for functional purposes where high thermal, mechanical and chemical resistance are required (for example, pipe coatings).

D- Modification Reactions of Epoxy Resins 1- Modification of Epoxy Resins with Vegetable Fatty Acids (Epoxy Resin Esters)

In the presence of an appropriate catalyst, it is possible to control competing reactions, namely the effective epoxide-carboxylic and hydroxyl-carboxylic reactions. Depending on the degree of unsaturation of the fatty acids, these types of paints can be air or oven dried. These epoxy esters are mostly used in protective coatings. Air or oven dried esters can be produced by esterification of epoxy resins (mr1000-2000) with fatty acids or oleorezin acids (linseed oil, soybean oil, tall oil, castor oil or dehydrated castor oil acids). Plasticized epoxy resins can be produced by reacting low molecular weight epoxy resins with dimeric fatty acids. These resins (for example Epicote 872, Shell) generally show better substrate wetting than unmodified epoxy resins. However, this reaction is related to the formation of ester bonds and therefore the resins have poor resistance to alkaline solutions. Instead of fatty acids, carboxylic functional polybutadiene acrylonitrile elastomers can also be used to flexibilize epoxy resins.

2- Reactions Producing Water-Soluble Epoxy Resins:

The presence of hydroxyl and epoxide groups in the resin molecule allows the resin to be converted to either polybasic acid anionic binders or polyamine cationic binders. These types of binders are used in the production of water-soluble primer coatings applied by electrodeposition process in the automotive industry.

3- Modification of Epoxy Resins with Unsaturated Groups (Esters of Epoxy Resins with Acrylic Acid, Methacrylic Acid):

Diacrylated epoxy resins. Epoxy vinyl ester resins. Diacrylated epoxy resins are produced by reacting epoxy resin with acrylic acid. These types of resins are interesting because they are cured using peroxides or by radiation. If semi-solid epoxy resins with low molecular weight liquid resins are heated with acrylic acid at approximately 120°C, epoxy acrylates are obtained. These resins are soluble in low molecular weight acrylate esters and can be used as raw materials for end coatings that can be cured with UV. Generally used for paper and cardboard and also for metals. Epoxy resins can also be reacted with methacrylic acid when heated. The resulting methacrylate ester resins are soluble in solvents that can polymerize (such as styrene) and are cured by catalyzing with peroxides (such as Derakane, Dow Chemical). These catalyzed resins can be applied as thick-layer laminates by combining them with glass fiber, fabrics or felts. Laminates are generally cured at approximately 60°C and have extremely good chemical resistance. Therefore, they are used in the chemical industry for the internal lining of reactors and storage tanks. Mehmet Namık Kayaalp / Chemical Engineer / Ecelak Boya Kimya Ltd. Şti. References 1. Finished Paint Products, 1974 Selection and Industrial Training Administration Ltd.-London 2. Epoxy Polymers, Edited by Jean-Pierre Pascault and Roberto J. J. Williams, 010 WILEY-VCH Verlag GmbH & Co. KGaA, 3. Coatings Technology Handbook,edited by D. Satas, 2001 by Marcel Dekker, Inc. 4. Paınt And Surface Coatıngs, Editors:R. Lambourne and T.A.Strivens, by Woodhead Publishing Ltd,1999 5. Epoxy Resins In Coatings, Edited by Willard H. Madson, Federation of Societes for Paint Technology, Philadelphia, 1972 6. Shell Epikote Resins, Technical Bulletin 1975 7. Schering Industrie-Chemikalien AG, Bergkamen W.Germany, 1978