Epoxy resin adhesive classification, performance characteristics, and application analysis
Epoxy resin adhesive is formulated with epoxy resin as the main body. Resin macromolecules end with the epoxy group, between the chain of hydroxyl and ether bonds, and in the curing process will continue to produce hydroxyl and ether bonds, the structure contains a benzene ring and heterocyclic, the structure of these determines the epoxy resin adhesive has an excellent performance. Epoxy resin adhesive is a kind of adhesive with a long history of use and an extremely wide range of uses. Due to their strength, versatility, and excellent adhesion to a wide variety of surfaces to be bonded, epoxy adhesives are recognized by a wide range of users. They have participated in and accelerated the technological revolution in certain industrial sectors. Epoxy resins can be used to bond metals, glass, ceramics, many plastics, wood, concrete, and several other surfaces.
More than 10 percent of the epoxy resins produced in the United States are used as adhesives. In the past, the modification of epoxy resins has been limited to rubber, such as end-carboxy nitrile butadiene rubber, end-hydroxy nitrile butadiene rubber, polysulfide rubber, and so on. In recent years, the modification of epoxy resin is constantly deepened, and modification methods are changing day by day, such as the interpenetrating network method, chemical copolymerization method, etc., especially liquid crystal toughening method and nanoparticle toughening method is the hot spot of research in recent years. With the establishment of ‘scale, high purity, refinement, specialization, serialization, functionalisation’ development mode, epoxy resin modification research is changing day by day, and has formed the focus of attention in the industry, which will promote the epoxy resin in the construction of the economy and people’s lives further more and more widely used.
1. Excellent performance of epoxy adhesives in a wide range of applications
Gluing (bonding, adhesion, gluing, gluing) refers to the homogeneous or heterogeneous object surfaces connected with adhesive technology, with stress points adhesive fabric continuous, lightweight or sealing, most of the process temperature is low, and so on. Gluing is particularly suitable for different materials, different thicknesses, ultra-thin specifications, and complex components of the connection. Glued joints in recent times the fastest growing, extremely wide range of application industries, and the progress of high-tech science and technology and the improvement of people’s daily lives have a significant impact. Therefore, research, development, and production of various types of adhesives are very important.
Epoxy resin adhesive refers to a molecular structure, containing two or more two epoxy groups, and in the appropriate chemical reagents and under the right conditions, can form a three-dimensional crosslinking-like curing compounds in general.
Epoxy resin is adhesive by the epoxy resin, curing agent, plasticizer, accelerator, diluent, filler, coupling agent, flame retardant, stabilizer, and other components of the liquid or solid adhesive. Among them, epoxy resin, curing agents, and toughening agents are indispensable components, the other according to the need to decide whether to add or not. The epoxy adhesive process is a complex physical and chemical process, including infiltration, adhesion, curing, and other steps, and finally generates a three-dimensional crosslinked structure of the cured material, the adhesive combined into a whole.
There are many types of epoxy adhesive, in all kinds of epoxy resin, bisphenol A epoxy resin is the largest production, and the most widely used variety. According to its different molecular weights can be divided into low, medium, high, and ultra-high molecular weight epoxy resin (polyphenol oxygen resin). Low molecular weight resins can be cured at room temperature or high temperature, but high molecular weight epoxy resins must be cured at high temperatures, while ultra-high molecular weight polyphenoxy resins do not require the aid of a curing agent, and are capable of forming a tough film at high temperatures. With a variety of adhesive theories put forward one after another, as well as adhesive chemistry, adhesive rheology adhesive damage mechanism, and other basic research work of in-depth progress, the adhesive properties, varieties, and applications have made rapid progress. Epoxy resin and its curing system also with its unique, excellent performance and new epoxy resin, new curing agents, and additives continue to emerge, and become an important class of adhesives with excellent performance, many varieties, and wide adaptability.
In recent years, high-strength lightweight fiber-reinforced composites have been gradually used in ultra-low temperature environments, and the research on the ultra-low temperature performance of epoxy resins has been increasingly strengthened. As a composite liquid hydrogen storage tank matrix material and in the field of superconductivity as an adhesive, impregnation material, and fiber-reinforced composite matrix material, China’s research has made some progress. Pure epoxy resin has a high cross-linking density, even at room temperature there are shortcomings such as brittle, low toughness, and poor impact resistance. As the resin matrix of the composite material, it generally needs to be cured at a very high temperature. In the cooling process after curing, thermal stresses will be generated inside the resin matrix due to thermal contraction. When the temperature is reduced from room temperature to ultra-low temperature (below -150°C), the internal stresses generated inside the matrix due to thermal contraction will be more significant, and once the thermal stresses exceed the strength of the resin itself, it will lead to the destruction of the resin matrix. Therefore, improving toughness is crucial for the use of epoxy resins at ultra-low temperatures.
The current methods for improving the toughness of epoxy resins at ultra-low temperatures are mainly the use of flexible aliphatic resins and liquid rubbers as well as flexible curing agents to toughen the epoxy resins. Due to the low glass transition temperature of such materials, they have a large free volume at room temperature, and when the temperature drops to ultra-low temperatures, the resin system produces a large thermal contraction, resulting in large thermal stresses, which limits their application at ultra-low temperatures. At room temperature, high-performance thermoplastics and epoxy resin blending modification, can make the blended system at the same time with the superior performance of both, that is, to maintain the high modulus of thermosetting resins at the same time, and both high toughness of thermoplastics.
Adhesive bonding performance (strength, heat resistance, corrosion resistance, impermeability, etc.) depends not only on its structure and performance, as well as the structure of the surface of the object to be adhered to and adhesive properties, but also on the joint design, adhesive preparation, and bonding process is closely related to, but also by the constraints of the surrounding environment. Therefore, the application of epoxy adhesive is a systematic project. Epoxy adhesive performance must be compatible with the above factors affecting the performance of the joint, to obtain the best results. With the same formula of epoxy adhesive bonding objects of different nature, or using different bonding conditions, or in different environments, its performance will have a great difference, the application should pay full attention.
Epoxy adhesive mainly consists of epoxy resin and curing agent two major components. To improve certain properties, different uses can also be added to toughening agents, diluents, accelerators, coupling agents, and other auxiliary materials. Due to the high bonding strength of epoxy adhesives, and versatility, had ‘universal adhesive’, and ‘strong adhesive’, in aviation, aerospace, automotive, machinery, construction, chemical industry, light industry, electronics, electrical appliances, as well as daily life and other fields are widely used.
With the increasingly sound environmental regulations in China, as well as people’s health awareness, good quality, pollution-free, environmentally friendly epoxy adhesives in line with international standards are gradually becoming the mainstream of synthetic adhesives.
2. The molecular structure of epoxy adhesives and species classification
Epoxy resin is a molecule containing two or more two epoxy groups and the relative molecular mass of low polymer compounds, a classification of epoxy resin varieties, many brands, but the bisphenol A glycidyl ether epoxy resin is usually referred to as the bisphenol A epoxy resin is the most important class. It accounts for 90% of the total production of epoxy resins. Bisphenol A-type epoxy resin bisphenol A-type epoxy resin, also known as general-purpose epoxy resin and standard epoxy resin, China’s name is E-type epoxy resin, by the bisphenol (BPA or DPP) and epichlorohydrin (ECH) in the sodium hydroxide under the polymerization of: according to the ratio of raw materials, reaction conditions and the method of use of different methods, can be produced in different degrees of polymerization of the viscous liquid with a low relative molecular mass and high relative molecular mass, High softening point solid. The average relative molecular mass is 300-7000, and the appearance is a nearly colorless or light yellow transparent viscous liquid or flaky brittle solid. The epoxy resin itself is a thermoplastic linear polymer, when heated, the liquid resin viscosity becomes low, and the solid resin softens or melts. Soluble in acetone, methyl ethyl ketone, cyclohexanone, ethyl acetate, benzene, toluene, xylene, anhydrous ethanol, ethylene glycol, and other organic solvents. Hydrogenated Bisphenol A Epoxy Resin Hydrogenated Bisphenol A Epoxy Resin is chemically known as Hydrogenated Bisphenol A Diglycidyl Ether, which is obtained by the hydrogenation of Bisphenol A. It is obtained by the polymerization of Hexahydrogen Bisphenol A obtained from the hydrogenation of Bisphenol A with Epichlorohydrin under the catalyst of sodium hydroxide. It is a kind of epoxy resin with very low viscosity, long gel time, and quite good weather resistance.
Bisphenol F-type epoxy resin chemical name for the bisphenol F diglycidyl ether, referred to as DGEBF or BPF, is by the reaction of phenol and formaldehyde in the acid catalyst to generate bisphenol F, and then with epichlorohydrin in the sodium hydroxide catalytic polycondensation reaction of colorless or yellowish transparent viscous liquid; bisphenol S-type epoxy resin chemical name for the bisphenol S glycidyl ether of glycyrrhiza glabra oil, referred to as BPS or KGEBS Bisphenol S-type epoxy resin is produced by bisphenol S and epoxy propane under the catalysis of sodium hydroxide. Bisphenol S-type epoxy resin has high heat resistance, the heat distortion temperature is higher than that of bisphenol A epoxy resin by 60~700C. The curing object is stable and has good solvent resistance. Bisphenol P-type epoxy resin is synthesized from 3-chloropropene and phenol as the main raw material of bisphenol P and then polycondensed with epichlorohydrin under the catalysis of sodium hydroxide. Bisphenol P-type epoxy resin molecular chain flexibility, good mobility at low temperatures, viscosity is lower than the bisphenol A epoxy resin, and compression strength, and impact strength is higher than the bisphenol A epoxy resin.
Phenolic epoxy resin mainly has phenol linear phenol vinegar epoxy resin o-cresol linear phenolic epoxy resin, and resorcinol phenolic epoxy resin. In addition, tetraphenyl ethane epoxy resin also belongs to phenolic epoxy resin; phenol linear phenolic epoxy resin (EPN) is a linear phenolic resin obtained by the polycondensation reaction between phenol and formaldehyde in an acidic medium, and then condensed with an excess of epichlorohydrin in the presence of sodium hydroxide to produce a line of brownish-yellow viscous liquid or semi-solid; o-cresol linear phenolic epoxy resin is a linear phenolic epoxy resin obtained by polycondensation of o-cresol and formaldehyde, and then condensed with o-cresol. O-cresol linear phenolic epoxy resin is a linear o-cresol condensed with formaldehyde and then reacted with epichlorohydrin in the presence of sodium hydroxide, and the yellow to amber solid produced after multi-step treatment; epoxy resorcinol formaldehyde resin chemical name is resorcinol formaldehyde tetrakis glycidyl ether, it is a quadruple-functional phenolic resin produced by the reaction of resorcinol and formaldehyde with oxalic acid as the catalyst. Then epichlorohydrin in the presence of sodium hydroxide condensation and the orange viscous liquid; tetraphenol ethane epoxy resin chemical name for tetraphenol ethane glycidyl ether (PGEE), is made from phenol and ethylene di-formaldehyde in the presence of acid catalyst reaction to tetraphenol ethane, and then with epichlorohydrin in the reaction catalyzed by sodium hydroxide system; naphthol phenol formaldehyde epoxy resin (EEPN) is a naphthol and formaldehyde solution condensation, the synthesis of linear phenolic resins. EEPN is made by polycondensation of a-naphthol and formaldehyde solution, synthesizing a linear cai phenolic resin, and then reacting with epichlorohydrin under the catalysis of sodium hydroxide; Fluorinated epoxy resin is made by the introduction of fluorine atoms, which makes the molecular structure dense, and the fluorine atoms of the carbon are closely arranged around the main chain of the resin. As a result, the central tension, friction coefficient, and refractive index are very low, with excellent corrosion resistance, abrasion resistance, heat resistance, pollution resistance, and durability. However, it is expensive and cannot be used for general purposes.
Polyurethane epoxy resin, also known as epoxy urethane resin, is made of polyester (or ether) polyol and epichlorohydrin under the action of BF3 and NaOH to generate polyol glycidyl ether polyol and then polycondensation with diisocyanate adducts; silicone epoxy resin is the molecular structure of silicon epoxy is made of poly(methyl phenyl siloxane) with the epoxy resin and the polycondensation of poly(methyl phenyl siloxane). Toluene is a solution, light yellow uniform liquid; organotitanium epoxy resin is made from bisphenol A-type epoxy resin in the hydroxyl group and n-butyl titanate reaction. Due to the introduction of the metal element titanium in the resin, not only to solve the water absorption caused by the presence of hydroxyl, moisture resistance and electrical properties of the problem are reduced, and because the resin has a P electron oxygen atom and has a D electron deficiency of titanium directly connected to the original, resulting in the existence of the macromolecular chain of P-D conjugate effect, and make the heat aging performance significantly improved better dielectric properties. Appearance is yellow to amber high viscosity transparent liquid.
With the continuous development of high technology and technology. In recent years, the modification of epoxy resin continued to deepen, interpenetrating network, chemical copolymerization, and nanoparticle toughening, and other methods are widely used, formulated by the epoxy resin into a variety of high-performance adhesive varieties are also more and more.
Epoxy resin adhesive varieties, their classification method, and their classification index have not been unified. Usually classified according to the following methods. Classification by the form of adhesive: such as solvent-free adhesives, (organic) solvent-based adhesives, water-based adhesives (which can be divided into water emulsion and water-soluble two kinds), paste adhesive, film adhesive (epoxy film), and so on.
Classification by curing conditions: cold curing adhesive (no heat curing adhesive). Also divided into low-temperature curing adhesive, curing temperature <15 ℃; room temperature curing adhesive, curing temperature 15 ~ 40 ℃; heat-curing adhesive can be divided into medium temperature curing adhesive, curing temperature of about 80 ~ 120 ℃; high-temperature curing adhesive, curing temperature of > 150 ℃; other ways of curing adhesive, such as light-curing adhesive, wet surface and water curing adhesive, latent curing adhesive.
Classification by adhesive strength: structural adhesive shear and tensile strength, but also should have a high uneven tear strength, so that the joints can withstand vibration, fatigue, and impact over a long period, such as planting loads. At the same time should also have a high degree of heat resistance and weather resistance; secondary stress structural adhesive can withstand moderate loads, usually with shear strength of 17 ~ 25Mpa, and uneven tear strength of 20 ~ 50kN/m; non-structural adhesive, that is, general-purpose adhesive. Its room temperature strength is still relatively high, but with the increase in temperature, the adhesive strength decreases faster. It can only be used in parts that are not subject to much force.
Classification by use: general-purpose adhesives, special adhesives, such as high-temperature adhesive (use temperature ≥ 150 ° C), low-temperature adhesive (can withstand -50 ° C or lower temperatures), strain adhesive (paste strain gauges), conductive adhesive, sealant (vacuum sealing, mechanical sealing), optical adhesive (colorless and transparent, resistant to light aging, refractive index matched with optical parts), corrosion-resistant adhesive, structural adhesive, etc.. Can also be classified according to the type of curing agent, such as amine-cured epoxy adhesive, or anhydride-cured adhesive. Can also be divided into two-component adhesive and one-component adhesive, pure epoxy adhesive, and modified epoxy adhesive.
3. Epoxy adhesive performance characteristics
General epoxy resin structure contains hydroxyl, and ether bonds so that it has high adhesion, because of these polar groups, so can make the adjacent interface electromagnetic force, less in the curing process, accompanied by the chemical action of the curing agent and curing agent, but also to further generate by the base and ether bond, not only a high cohesion but also produces a very strong adhesion, so the epoxy adhesive on many kinds of materials such as metal, plastic, glass, wood, fibers, etc. Have a strong bonding strength, commonly known as ‘universal adhesive’.
The molecular arrangement of epoxy resin is tight, in the curing process does not precipitate low molecular matter, and it can be formulated into a solvent-free adhesive, so its shrinkage rate is generally low. If appropriate fillers are used, the shrinkage rate can be reduced to 0.1 to 0.2%.
Epoxy tree finger structure in the presence of a stable benzene ring, ether chain, and cured structure dense, determines the epoxy adhesive has a strong resistance to the atmosphere, moisture, chemical media, bacteria, and other roles, so it can be applied in many more demanding environments.
Epoxy adhesive adhesive force, high bonding strength; small shrinkage, dimensional stability, epoxy resin adhesive in the curing of almost no release of low molecular products. The coefficient of linear expansion is affected by temperature is small, therefore, the dimensional stability of the bonded parts is good; epoxy resin adhesive curing product has excellent electrical insulation properties, volume resistivity of 1013 ~ 1016Ω.cm, and dielectric strength of 30 ~ 50KV.Mm-1 epoxy resin molecules contain ether bonds, and the molecular chain arrangement between the close, crosslinking density is large, so there is a good resistance to solvents, oil, acid, alkali, water, and other properties, in particular, the epoxy resin molecules, the epoxy resin molecules, and the epoxy resin molecules are very good. Epoxy resin has good solvent resistance, oil resistance, acid resistance, alkali resistance, water resistance, and other properties, especially alkali resistance; epoxy resin and a lot of rubber (elastomers) and thermoplastics.
