[Composites Information] Classification, performance characteristics and application analysis of epoxy resin adhesives

Epoxy resin adhesives are formulated with epoxy resin as the main body. There are epoxy groups at the ends of the resin macromolecules, hydroxyl groups and ether bonds between the chains, and hydroxyl groups and ether bonds will continue to be generated during the curing process. The structure contains benzene rings and heterocycles. These structures determine that epoxy resin adhesives have excellent performance. Epoxy resin adhesives are an adhesive with a long history of use and extremely wide applications. Due to its strength, diversity and excellent adhesion to a variety of bonded surfaces, epoxy resin adhesives have been widely recognized by 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 some other surfaces.
More than 10% of epoxy resins produced in the United States are used as adhesives. In the past, people's modification of epoxy resins has been limited to rubber, such as carboxyl-terminated nitrile rubber, hydroxyl-terminated nitrile rubber, polysulfide rubber, etc. In recent years, the modification of epoxy resin has been continuously deepened, and the modification methods are changing with each passing day, such as interpenetrating network method, chemical copolymerization method, etc., especially liquid crystal toughening method and nanoparticle toughening method are the hot spots of research in recent years. With the establishment of the development model of "scale, high purification, refinement, specialization, serialization, and functionalization", the modification research of epoxy resin is changing with each passing day, and it has become the focus of attention in the industry. It will promote the further and more extensive application of epoxy resin in economic construction and people's lives.
1. Epoxy adhesives with excellent performance have a wide range of applications
Adhesive bonding (bonding, bonding, bonding, bonding) refers to the technology of connecting the surfaces of homogeneous or heterogeneous objects with adhesives, which has the characteristics of stress distribution, continuous adhesive cloth, light weight, or sealing, and low process temperature in most processes. Adhesive bonding is particularly suitable for the connection of different materials, different thicknesses, ultra-thin specifications and complex components. Adhesives are developing fastest in recent generations, with a wide range of application industries, and have a significant impact on the progress of high-tech science and technology and the improvement of people's daily life. Therefore, it is very important to research, develop and produce various types of adhesives.
Epoxy resin glue refers to a general term for a compound that contains two or more epoxy groups in a molecular structure and can form a three-dimensional cross-linked curing compound under appropriate chemical reagents and conditions.
Epoxy resin adhesives are liquid or solid adhesives composed of epoxy resins, curing agents, plasticizers, accelerators, diluents, fillers, coupling agents, flame retardants, stabilizers, etc. Among them, epoxy resins, curing agents, and toughening agents are indispensable components, and others are added or not according to needs. The bonding process of epoxy adhesives is a complex physical and chemical process, including steps such as infiltration, adhesion, and curing, and finally generates a three-dimensional cross-linked structure of the cured product, which combines the adherends into a whole.
There are many types of epoxy glues. Among all types of epoxy resins, bisphenol A epoxy resin is the largest and most widely used variety. According to its molecular weight, it can be divided into low, medium, high, and ultra-high molecular weight epoxy resins (polyphenol oxide resins). Low molecular weight resins can be cured at room temperature or high temperature, but high molecular weight epoxy resins must be cured at high temperature, and ultra-high molecular weight polyphenolic resins do not require the aid of curing agents and can form tough films at high temperatures. With the successive proposals of various adhesive theories and the in-depth progress of basic research work such as adhesive chemistry, adhesive rheology and adhesive failure mechanism, the performance, variety and application of adhesives have developed by leaps and bounds. Epoxy resins and their curing systems have also become an important class of adhesives with excellent performance, numerous varieties and wide adaptability due to their unique and excellent performance and the continuous emergence of new epoxy resins, new curing agents and additives.
In recent years, high-strength and lightweight fiber-reinforced composite materials have gradually been used in ultra-low temperature environments, and research on the ultra-low temperature performance of epoxy resins has also been increasingly strengthened. my country's research has made some progress in terms of being a matrix material for composite liquid hydrogen tanks and as a matrix material for adhesives, impregnation materials and fiber-reinforced composite materials in the field of superconductivity. Pure epoxy resin has a high cross-linking density, and even at room temperature, it has the disadvantages of being brittle, having low toughness, and having poor impact resistance. As the resin matrix of the composite material, it generally needs to be cured at a very high temperature. During the cooling process after curing, thermal stress will be generated inside the resin matrix due to heat shrinkage. When the temperature drops from room temperature to ultra-low temperature (below -150°C), the internal stress generated by thermal shrinkage in the matrix will be more significant. Once the thermal stress exceeds the strength of the resin itself, it will cause the destruction of the resin matrix. Therefore, improving toughness is crucial for the use of epoxy resin at ultra-low temperatures.
At present, the main method to improve the ultra-low temperature toughness of epoxy resin is to use flexible aliphatic resins, liquid rubbers, and flexible curing agents to toughen epoxy resins. Since such materials have a low glass transition temperature and a large free volume at room temperature, when the temperature drops to ultra-low temperatures, the resin system will produce a large thermal shrinkage, resulting in large thermal stress, which limits its application at ultra-low temperatures. The blending and modification of high-performance thermoplastics and epoxy resins at room temperature can make the blending system have the superior properties of both, that is, while maintaining the high modulus of thermosetting resins, it also has the high toughness of thermoplastics.
The bonding performance (strength, heat resistance, corrosion resistance, impermeability, etc.) of adhesives depends not only on their structure and performance, as well as the structure and bonding properties of the surface of the adherend, but also on the joint design, preparation of adhesives and bonding processes, and is also restricted by the surrounding environment. Therefore, the application of epoxy adhesives is a systematic project. The performance of epoxy adhesives must be adapted to the above factors that affect the bonding performance in order to obtain the best results. When using epoxy adhesives of the same formula to bond objects of different properties, or using different bonding conditions, or in different use environments, their performance will be very different, and full attention should be paid when applying them.

Epoxy adhesives are mainly composed of two parts: epoxy resin and curing agent. In order to improve certain properties and meet different uses, auxiliary materials such as tougheners, diluents, promoters, coupling agents, etc. can also be added. Due to the high bonding strength and strong versatility of epoxy adhesives, they were once known as "all-purpose glue" and "strong glue". They are widely used in aviation, aerospace, automobiles, machinery, construction, chemicals, light industry, electronics, electrical appliances, and daily life.
With the increasingly sound environmental protection laws and regulations in my country and the improvement of people's own health awareness, environmentally friendly epoxy adhesives with good quality, no pollution, and in line with international standards are gradually becoming the mainstream products of synthetic adhesives.
2. Molecular structure and variety classification of epoxy adhesives
Epoxy resin Epoxy resin is a polymer compound with two or more epoxy groups in the molecule and a relatively low molecular weight. 1. Classification There are many varieties and brands of epoxy resins, but bisphenol A glycidyl ether epoxy resin is usually called bisphenol A epoxy resin, which is the most important type. It accounts for 90% of the total output of epoxy resins. Bisphenol A epoxy resin Bisphenol A epoxy resin is also known as general epoxy resin and standard epoxy resin. It is named E-type epoxy resin in China. It is obtained by polycondensation of bisphenol (BPA or DPP) and epichlorohydrin (ECH) under sodium hydroxide: according to the raw material ratio, reaction conditions and the method adopted, low relative molecular weight viscous liquid and high relative molecular weight, high softening point solid with different polymerization degrees can be obtained. The average relative molecular weight is 300-7000. The appearance is a nearly colorless or light yellow transparent viscous liquid or a flaky brittle solid. Epoxy resin itself is a thermoplastic linear polymer. When heated, the viscosity of the liquid resin decreases, and the solid resin softens or melts. Soluble in organic solvents such as acetone, methyl ethyl ketone, cyclohexanone, ethyl acetate, benzene, toluene, xylene, anhydrous ethanol, ethylene glycol, etc. Hydrogenated bisphenol A epoxy resin The chemical name of hydrogenated bisphenol A epoxy resin is hydrogenated bisphenol A diglycidyl ether, which is obtained by condensing hexahydrobisphenol A obtained by hydrogenation of bisphenol A with epichlorohydrin under the catalysis of sodium hydroxide. It is an epoxy resin with very low viscosity, long gel time and good weather resistance.
The chemical name of bisphenol F epoxy resin is bisphenol F diglycidyl ether, referred to as DGEBF or BPF, which is a colorless or light yellow transparent viscous liquid obtained by reacting phenol and formaldehyde under an acid catalyst to generate bisphenol F, and then reacting with epichlorohydrin under the catalysis of sodium hydroxide; the chemical name of bisphenol S epoxy resin is bisphenol S diglycidyl glycyrrhizin oil ether, referred to as BPS or KGEBS, which is obtained by bisphenol S and epichlorohydrin under the catalysis of sodium hydroxide. Bisphenol S epoxy resin has high heat resistance, and its heat deformation temperature is 60-700C higher than that of bisphenol A epoxy resin. The cured product is stable and has good solvent resistance. Bisphenol P epoxy resin is synthesized from 3-chloropropylene and phenol as the main raw materials, and then polycondensed with epichlorohydrin in the presence of sodium hydroxide. Bisphenol P epoxy resin has high molecular chain flexibility, good fluidity at low temperatures, lower viscosity than bisphenol A epoxy resin, and higher compression strength and impact strength than bisphenol A epoxy resin.
Novolac epoxy resins mainly include phenol linear phenolic ester epoxy resins and o-cresol linear phenolic phenolic epoxy resins, as well as resorcinol type novolac epoxy resins. In addition, tetraphenol ethane epoxy resin also belongs to phenolic epoxy resin; phenol novolac epoxy resin (EPN) is a linear phenolic resin obtained by condensation reaction of phenol and formaldehyde in an acidic medium, and then condensed with excess epichlorohydrin in the presence of sodium hydroxide to obtain a linear brown viscous liquid or semi-solid; o-cresol novolac epoxy resin is a linear o-cresol phenolic resin obtained by condensation of o-cresol and formaldehyde, and then reacted with epichlorohydrin in the presence of sodium hydroxide, and obtained after multi-step treatment to obtain a yellow to amber solid; epoxy resorcinol formaldehyde resin has the chemical name of resorcinol formaldehyde tetraglycidyl ether, which is a tetrafunctional phenolic resin obtained by the reaction of resorcinol and formaldehyde with oxalic acid as a catalyst. Then it is polycondensed with epichlorohydrin in the presence of sodium hydroxide to obtain an orange-yellow viscous liquid; the chemical name of tetraphenol ethane epoxy resin is tetraphenol ethane glycidyl ether (PGEE), which is obtained by reacting phenol with glyoxal in the presence of an acidic catalyst to obtain tetraphenol ethane, and then reacting with epichlorohydrin under the catalysis of sodium hydroxide; naphthol phenolic epoxy resin (EEPN) is synthesized by polycondensing a-naphthol with formaldehyde solution to produce linear phenolic resin, and then reacting with epichlorohydrin under the catalysis of sodium hydroxide; fluorinated epoxy resin has a dense molecular structure due to the introduction of fluorine atoms, and carbon-fluorine atoms are closely arranged around the main chain of the resin. Therefore, the surface tension, friction coefficient, and refractive index are very low, and it has excellent corrosion resistance, wear 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 by reacting polyester (or ether) polyol with epichlorohydrin in the presence of BF3 and NaOH to generate polyol glycidyl ether, which is then polycondensed with diisocyanate; silicone epoxy resin is an epoxide containing silicon in its molecular structure, which is polycondensed with polymethylphenylsiloxane and epoxy resin. Toluene is a solution, a light yellow uniform liquid; organic titanium epoxy resin is obtained by reacting the hydroxyl group in bisphenol A epoxy resin with n-butyl titanate. Since the metal element titanium is introduced into the resin, it not only solves the problems of increased water absorption, reduced moisture resistance and electrical properties caused by the presence of hydroxyl groups, but also because the oxygen atoms with P electrons in the resin are directly connected to the titanium atoms with D electron vacancies, resulting in the P-D conjugation effect in the macromolecular chain, which significantly improves the heat aging resistance and has better dielectric properties. The appearance is a yellow to amber high-viscosity transparent liquid.
With the continuous development of high-tech and technology. In recent years, the modification of epoxy resin has been continuously deepened, and methods such as interpenetrating network, chemical copolymerization and nanoparticle toughening have been widely used. There are more and more varieties of high-performance adhesives made from epoxy resin.
There are many varieties of epoxy resin adhesives, and the classification methods and classification indicators have not yet been unified. Usually classified according to the following methods. Classification by the form of adhesives: such as solvent-free adhesives, (organic) solvent-based adhesives, water-based adhesives (which can be divided into water-emulsion type and water-soluble type), paste adhesives, film adhesives (epoxy film), etc.
Classification by curing conditions: cold curing adhesive (non-heat curing adhesive). It is 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 about 80~120℃; high temperature curing adhesive, curing temperature >150℃; other curing adhesives, such as light curing adhesives, wet surface and water curing adhesives, latent curing adhesives, etc.
Classification by bonding strength: Structural adhesives have high shear and tensile strength, and should also have high uneven pull-off strength, so that the bonded joints can withstand loads such as vibration, fatigue and impact for a long time. At the same time, it should also have high heat resistance and weather resistance; secondary stress structural adhesives can withstand medium loads, usually with a shear strength of 17-25Mpa and an uneven pull-off strength of 20-50kN/m; non-structural adhesives, that is, general-purpose adhesives. Its room temperature strength is still relatively high, but with the increase of temperature, the bonding strength decreases rapidly. It can only be used in parts with little stress.
Classification by use: general-purpose adhesives, special adhesives, such as high-temperature resistant adhesives (using temperature ≥150℃), low-temperature resistant adhesives (resistant to -50℃ or lower temperatures), strain adhesives (for pasting strain gauges), conductive adhesives, sealants (vacuum sealing, mechanical sealing), optical adhesives (colorless and transparent, light aging resistant, refractive index matching optical parts), corrosion-resistant adhesives, structural adhesives, etc. It can also be classified by the type of curing agent, such as amine-cured epoxy adhesive, anhydride-cured adhesive, etc. It can also be divided into two-component adhesive and one-component adhesive, pure epoxy adhesive and modified epoxy adhesive.

3. Performance characteristics of epoxy adhesives
Generally, epoxy resin contains hydroxyl and ether bonds in its structure, which makes it highly adhesive. Due to these polar groups, electromagnetic forces can be generated on adjacent interfaces. During the curing process, with the chemical reaction with the curing agent, it can further generate ether groups and ether bonds. It not only has high cohesion, but also produces strong adhesion. Therefore, epoxy adhesives have strong bonding strength to many materials such as metals, plastics, glass, wood, fibers, etc., commonly known as "universal glue".
The molecules of epoxy resin are closely arranged, and low molecular weight substances are not precipitated during the curing process. Moreover, it can be formulated into solvent-free adhesives, so its shrinkage rate is generally low. If appropriate fillers are selected, the shrinkage rate can be reduced to 0.1-0.2%.
The existence of stable benzene rings and ether chains in the epoxy resin structure and the dense structure after curing determine that epoxy adhesives have strong resistance to the effects of atmosphere, moisture, chemical media, bacteria, etc., so they can be used in many harsh environments.
Epoxy adhesives have strong bonding force and high bonding strength; small shrinkage and stable dimensions. Epoxy resin adhesives release almost no low molecular weight products during curing. The linear expansion coefficient is less affected by temperature, so the dimensional stability of the bonded parts is good; the cured product of epoxy resin glue has excellent electrical insulation properties, the volume resistivity is 1013~1016Ω.cm, and the dielectric strength is 30~50KV.Mm-1. The epoxy resin molecules contain ether bonds, and the molecular chains are closely arranged, and the cross-linking density is large, so it has good solvent resistance, oil resistance, acid resistance, alkali resistance, water resistance and other properties, especially strong alkali resistance; epoxy resin has good compatibility with many rubbers (elastomers) and thermoplastic resins, and even chemical reactions occur; it has good dispersibility with fillers, and can change the properties of epoxy resin glue in a wide range; it has good processability, is easy to use, has low toxicity, and is less harmful; the resin contains many benzene rings and heterocycles, the molecular chain is less flexible, and the cross-linked structure after curing is not easy to deform. The untoughened epoxy resin glue has poor toughness, is relatively brittle, has very low peel strength, and is not resistant to impact and vibration.
Epoxy resin contains a variety of polar groups and highly active epoxy groups, so it has strong adhesion to various polar materials such as metal, glass, cement, wood, plastic, especially materials with high surface activity. At the same time, the cohesive strength of epoxy cured products is also very large, so its bonding strength is very high. When epoxy resin is cured, basically no low molecular volatiles are produced. The volume shrinkage of the adhesive layer is small, about 1% to 2%, which is one of the varieties with the smallest curing shrinkage among thermosetting resins. After adding fillers, it can be reduced to less than 0.2%. The linear expansion coefficient of epoxy cured products is also very small. Therefore, the internal stress is small and has little effect on the bonding strength. In addition, the creep of epoxy cured products is small, so the dimensional stability of the adhesive layer is good. There are many varieties of epoxy resins, curing agents and modifiers. Through reasonable and ingenious formula design, the adhesive can have the required processability (such as fast curing, room temperature curing, low temperature curing, curing in water, low viscosity, high viscosity, etc.) and the required performance (such as high temperature resistance, low temperature resistance, high strength, high flexibility, aging resistance, electrical conductivity, magnetic conductivity, thermal conductivity, etc.). It has good compatibility and reactivity with a variety of organic substances (monomers, resins, rubbers) and inorganic substances (such as fillers, etc.), and is easy to copolymerize, crosslink, blend, fill and other modifications to improve the performance of the adhesive layer. It can resist corrosion from a variety of media such as acids, alkalis, salts, and solvents.
Depending on the type of curing agent selected, epoxy adhesives can be cured at room temperature, medium temperature or high temperature. Generally, only a contact pressure of 0.1 to 0.5 MPa is required for curing. Most epoxy resin adhesives do not contain solvents and are easy to operate. The construction viscosity of general epoxy adhesives. The applicable period and curing speed can be adjusted through the formula to meet various requirements. This not only makes it easy to ensure the quality of bonding, but also simplifies the curing process and equipment. After the epoxy resin is cured, good electrical insulation properties can be obtained; the breakdown voltage is >35kV/mm, the volume resistance is >1015Ω.cm, the dielectric constant is 3 to 4 (50Hz), and the arc resistance is 100 to 140s. By changing the composition of epoxy resin adhesive (curing agent, toughening agent, filler, etc.), a series of adhesive formulas with different properties can be obtained to meet various needs, and various varieties with different properties can be produced by mixing with many modifiers. The general use temperature of bisphenol A epoxy resin ranges from -60 to 175°C, sometimes up to 200°C for a short time. If a new type of epoxy resin resistant to high and low temperatures is used, the use temperature can be higher or lower, and the water absorption of epoxy resin is low.
General-purpose epoxy resins, curing agents and additives have many origins and large outputs, are easy to prepare, can be contact-pressed, and can be used on a large scale. The main disadvantages of epoxy adhesives: when not toughened, the cured product is generally brittle, with poor peeling, cracking and impact resistance; the adhesion to materials with low polarity (such as polyethylene, polypropylene, etc.) is low. Surface activation treatment must be carried out first; some raw materials such as active diluents and curing agents have different degrees of toxicity and irritation. When designing the formula, it should be avoided as much as possible, and ventilation protection should be strengthened during construction.
As can be seen from the above, epoxy resin has good comprehensive mechanical properties, especially high adhesion, small shrinkage, good stability, and excellent electrical insulation properties, which provide a material basis for adhesives, composite matrix, powder coatings and other products.
4. Progress in the application technology of epoxy adhesives
Heat-resistant epoxy resin adhesive is an adhesive made of modified epoxy resin, which can be used intermittently at 250°C, or even for a long time at 400°C, and for a short time at 460°C. The base resin of this adhesive generally introduces more rigid groups or increases the cross-linking density of the cured product. For example, epoxy resins with fluorene groups, naphthalene rings and multifunctional epoxy resins, or epoxy resin adhesives modified with maleimide and silicone can meet the requirements of short-term high temperature resistance and high strength at 460°C. In recent years, with the development of electronic appliances and aerospace industries, the requirements for high temperature resistance and ablation resistance have become higher and higher. When an aircraft flies at high speed in the atmosphere, the temperature can sometimes reach thousands of degrees due to aerodynamic heating, and even the most heat-resistant metal materials will be melted. Therefore, in order to reduce weight, high temperature resistant composite materials are generally used to replace metal materials. Even in the electronic and electrical industry, sealants that can withstand high temperatures of 350°C and even flame-resistant insulating adhesives that can withstand 500-1000°C have been proposed. The F series epoxy curing agent developed by my country Aviation Corporation and the recently developed B, H, and HE series epoxy curing agents can make the epoxy resin resistant to high temperatures of 500°C and have excellent flame retardant properties, ablation resistance and good process performance.
The modified epoxy resin adhesive and preparation method overcome the shortcomings of brittleness and poor temperature resistance of general epoxy adhesives. Its main technical feature is that the polyurethane prepolymer modified epoxy resin (component A) and the homemade curing agent (component B) are formulated in a ratio of 10:1 to 1:1 (weight ratio) to form a high temperature resistant, tough and highly reactive curing system. The polyurethane prepolymer is a polysiloxane polyurethane prepolymer terminated with isocyanate groups, which is made by reacting terminal hydroxyl polysiloxane and diisocyanate in a certain proportion under certain conditions. The polyurethane prepolymer is then used to modify the epoxy resin. The homemade curing agent is composed of diamine, imidazole compound, silane coupling agent, inorganic filler and catalyst. This modified epoxy resin adhesive can be cured at room temperature and can be used for a long time at 200℃, or cured at -5℃ with a temperature resistance of 150℃; the bonding strength is 15-30Mpa; the T-peel strength is 35-65N/cm, and it has excellent oil resistance, water resistance, acid resistance, alkali resistance, and organic solvent resistance. It can bond wet surfaces, oily surfaces, metals, plastics, ceramics, hard rubber, wood, etc.

To improve the strength of epoxy resin, the resin is generally toughened by adding a second component to improve the toughness of epoxy resin. According to reports, there are mainly liquid toughening, toughening, elastic microsphere toughening, thermotropic liquid crystal (TLCP) toughening and polymer blending, copolymerization modification, etc.
Liquid rubber toughening modification generally refers to liquid nitrile rubber, poly, etc. containing terminal carboxyl, amine, hydroxyl, thiol, and epoxy groups, which are miscible with epoxy resin and precipitate during the curing process to form a two-phase structure of the "island model". Through the interaction of active groups, chemical bonds are formed at the interface of the two phases to play a toughening role. In recent years, in addition to the use of pre-reacted adducts of pure active liquid rubber, it has developed to the second generation using high-functionality epoxy resins and the third generation using metallocene catalysts to prepare block copolymers to modify epoxy prepolymers. After such modification, not only the peel strength is improved, but also the overall mechanical and thermal properties are not significantly reduced.
Polyurethane toughened epoxy adhesive is formed by polyurethane and epoxy resin to form semi-permeable network polymer (SIPN) and interpenetrating network polymer (IPN), which plays a forced miscibility and synergistic effect, so that the highly elastic polyurethane and the epoxy resin with good adhesion are organically combined, and a good toughening effect is achieved through complementarity and reinforcement.
Single-component room temperature moisture-curing epoxy adhesive is an epoxy adhesive cured with modified ketimine as a curing agent. Its characteristics are that it can be cured under humid and low temperature conditions, and can improve the temperature resistance and corrosion resistance of epoxy resin cured products. Phenolic modified ketimine curing agent, it is first reacted with formaldehyde and m-phenylenediamine to form phenolic amine, and then reacted with methyl isobutyl ketone to form phenolic modified ketimine. At present, China is working hard to study the rapid curing technology of fast curing epoxy adhesives under low temperature and low humidity. At present, the two-component room temperature curing epoxy adhesive developed in China can withstand temperatures of 200-260℃, up to 275℃, and can gel in 2-6 minutes at 25℃, fully cure in 3-8 hours, and the peel strength of polyether diamine curing can reach 4-5kN/m. Low-temperature fast curing epoxy adhesive is made of bisphenol F epoxy resin. It is combined with diphenyl decyl phosphite, DMP-30, etc., and can be quickly cured at -5℃. It has been developed and applied in the field of civil engineering. It is mainly used for concrete "integral engineering" bonding, building repair, product repair and building material bonding. In construction engineering, it can replace rivets, welding and other structural connection processes, and is used to bond various, marble and artificial boards.
High-strength composite material repair technology is the future trend of the development of external anti-corrosion layer repair technology for oil and gas pipelines. It is a technology that uses high-performance resin matrix to bond reinforced materials to form a protective structure, so it has high compressive and tensile strength and bonding force. During the repair construction, there is no need to stop the pipeline or reduce the pressure. At the same time, it has the advantages of simple and convenient operation, easy training of construction personnel, good reinforcement effect and significant economic benefits. Composite material repair technology can be carried out on-site winding construction and in-situ curing. The construction process is open flame, safe and convenient. Thirdly, the strength of composite materials reinforced by glass fiber, carbon fiber or fabric is much higher than that of ordinary steel, which makes the efficiency of composite material repair and reinforcement higher; composite materials are designable and can be targeted in thickness, number of layers, fiber distribution and other aspects according to the degree of defect damage and stress conditions, and the reliability of repair is high; the interlayer adhesive of glass fiber or carbon fiber reinforced resin-based composite materials has good interface adhesion, sealing and excellent corrosion resistance with metal, which can greatly reduce secondary corrosion damage during the operation of the pipeline. In composite material repair technology, the selection of adhesive has a vital influence on its protective performance.
When polyurethane is used to toughen epoxy resin adhesive, the polyurethane chain segment penetrates into the epoxy resin chain segment to form an interpenetrating polymer network structure (IPN) or a semi-interpenetrating polymer network structure (SIPN). Because polyurethane and epoxy resin have different solubilities, IPN materials show different degrees of phase separation, but due to the mutual entanglement between the networks, "forced miscibility" occurs, which increases the compatibility; and once the polymer is cross-linked, the mutually entangled network fixes the phase region. Since the polyurethane particles are dispersed in the continuous epoxy resin phase, the toughness of the system is increased, the stress concentration of the solidified material is dispersed, and the shear strength is increased. With the increase of the amount of polyurethane added, the shear strength gradually increases, but when the content of polyurethane exceeds 13.04%, the interpenetration degree of the interpenetrating polymer network structure formed by polyurethane/epoxy resin has reached saturation. Further increase the amount of polyurethane, the interpenetrating polymer network will have excessive interpenetration, polyurethane and epoxy resin will separate, cracks will form, and the compatibility of polyurethane and epoxy resin will drop sharply. Therefore, in terms of shear strength, the optimal amount of polyurethane is 13.04%. The peel strength is mainly related to the bonding performance and flexibility of the epoxy resin adhesive. The change law of the interpenetrating polymer network structure system formed by polyurethane and epoxy resin shows that with the increase of the amount of polyurethane added, the flexibility of the cured product increases first and then decreases, so the peel strength of the epoxy resin adhesive will increase first and then decrease with the increase of the amount of polyurethane added. When the polyurethane reaches 20%, the peel strength begins to decrease with the increase of the amount of polyurethane added. Therefore, for the peel strength, the best polyurethane dosage is 20%.
Among the many epoxy resin toughening technologies, the toughening effect of elastomers represented by polyurethane is the most significant. However, epoxy resin is a linear thermoplastic resin and will not harden by itself. Only by adding a curing agent to make it cross-linked from a linear structure to a mesh or body structure can it be cured. Therefore, while using polyurethane to toughen epoxy resin, a curing agent needs to be added to make it meet the requirements for curing performance during construction. Epoxy resin contains multiple benzene rings or heterocyclic rings, and the molecular chain is not flexible. The cured epoxy resin has a high cross-linking structure, which is not easy to deform. As a result, epoxy resin adhesives have shortcomings such as insufficient toughness, easy brittle cracking, low peel strength and poor impact resistance, which greatly limits their application. Therefore, the toughening modification of epoxy resin has important practical significance and application prospects for its application in pipeline repair.
In practice, adhesives that can be cured at room temperature and used in high temperature environments are often required. For example, structural adhesives used in construction are not only required to be able to withstand high temperatures to prevent the overall collapse of the building in a fire, but also cannot be heated and cured because of the large bonding area. However, room temperature curing EP adhesives generally cannot be used at high temperatures, and heat-resistant EP adhesives often need to be heated to fully cure. The so-called room temperature curing usually refers to a curing method that can gel within a few minutes or hours at room temperature (20-30°C), and fully cure within 7 days, and reach usable strength. Although some progress has been made in adhesives that cure at room temperature and are used at high temperatures, there is still a considerable gap between the needs and the future. In the future, we should strengthen the research on the curing mechanism of EP adhesives, develop multifunctional active curing agents, synthesize new multifunctional EP matrix resins, explore new modification methods and new fillers for EP resins, and develop adhesive research and development in the direction of resource conservation and environmental friendliness on the basis of improving performance.
Article source: Global Polyurethane
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Original title: "[Composites Information] Classification, performance characteristics and application analysis of epoxy resin adhesives"