Research progress on modification of acrylate structural adhesives

In 1975, DuPont took the lead in developing the second-generation acrylic adhesive (SGA) [1], and subsequently ITW, Loctite, lord and other companies have also developed SGA products with their own characteristics. Domestic development of SGA started slightly late, but after a period of technology accumulation, product performance has been comparable to foreign brands. Such as Beijing Tianshan, Hubei Hutian, Shanghai Kangda, Yantai Xinyou, etc., also have independent intellectual property rights of SGA products. Due to its advantages of rapid curing, high bonding strength, good flexibility and strong adaptability, SGA has been widely used in electronics, aerospace and other industrial fields [2,3].

Although the 2nd generation acrylic adhesive (SGA) is widely used, it still has some problems such as irritating odor, poor flexibility, and poor resistance to moisture and heat aging. Many researchers have reported on the modification of SGA [4,5]. On this basis, combined with the author's experience in developing SGA, this paper proposes new and effective methods to improve the odor, heat resistance, water resistance and storage performance of SGA, providing beneficial technical references for researchers.

1 Improve odor

The second generation of acrylate structural adhesive is mainly composed of acrylate monomer, toughening resin, initiator, accelerator and stabilizer, etc., and other additives such as toughening agent, thickener, thixotropic agent, filler and pigment are also added according to different uses [6]. Among these components, the volatile acrylic monomer is the most important source of SGA odor, and other auxiliaries will also contain a small amount of volatile solvents to increase the odor of SGA, but due to the small amount of use, there will be no detailed analysis here.

For the odor of acrylate monomer, the conventional method is to directly perceive the size of the odor from the smell, but it will cause the judgment error due to the difference in human smell. In the past, many researchers used the boiling point of monomers to distinguish between large, medium and small odors [7,8], while this paper will consider the odors of monomers from the vapor pressure perspective.

Molecules of substances in the liquid phase can enter the gas phase from the liquid phase, which is called volatility. At the same temperature, the vapor pressure of different pure substances is different. The vapor pressure is large, it is volatile substance, and its volatility is large; On the other hand, the vapor pressure is small, and the volatile substances are less volatile. Table 1 shows the relationship between monomer vapor pressure and odor.

Table 1 Relationship between vapor pressure and odor of monomer

As can be seen from Table 1, as the monomer vapor pressure decreases, the volatilization rate of monomer decreases, and the odor gradually decreases. Styrene has the highest vapor pressure value and the highest odor, while bisphenol A dimethacrylate has the lowest vapor pressure value and the least odor. Developers can refer to the above relationship to filter the less odor monomers and improve the odor of SGA. In addition, in the selection of low-odor monomers, it is necessary to take into account the compatibility of monomers to other components in the system, the effect of colloidal curing speed and curing strength, and other properties, which need to be considered comprehensively.

2 Improved heat resistance

At high temperature (higher than Tg), when SGA is bonded to dissimilar substrates, especially those with large differences in thermal expansion coefficient, stress is often generated at the bonding interface due to the difference in thermal expansion coefficient between the adhesive layer and the substrate, resulting in a decrease in adhesive force. The greater the interfacial stress, the more obvious the decrease of bonding strength. If the adhesive is exposed to high temperature for a long time, it may lead to adhesive failure.

In order to improve the heat resistance of SGA, usually:

（1）Acrylic monomer and modified acrylic resin with high Tg were selected

Monomers or resins containing benzene rings, heterocyclic rings or substances with large side groups can improve the heat resistance of adhesives.

（2）Add temperature resistant filler and resin

Such as inorganic salt, inert soluble temperature resistant resin, etc., can also improve the heat resistance of the adhesive.

（3）Increase crosslinking density

Adding appropriate amount of crosslinking agent, increasing the reaction crosslinking point and increasing the crosslinking density can make the structure of the whole molecular network more compact and strong, so as to improve the heat resistance of the adhesive.

GUI Wubiao et al. improved the heat resistance of SGA by adding three different types of heat resistant materials such as epoxy acrylate prepolymer, maleimide resin and aluminum silicate. The results show that the shear strength of SGA increases especially at 120 ℃ with the increase of the amount of epoxy acrylate prepolymer and aluminum silicate. They believe that the bisphenol A structure contained in the molecular chain of the epoxy acrylate prepolymer makes the colloid have the heat resistance characteristics of the epoxy resin, and after the acrylic ester is cross-linked into a network structure, it forms an interpenetrating network (IPN) structure with the epoxy acrylate prepolymer, achieving a good blend and thus improving the heat resistance [9].

Nie Qisi et al. improved the thermal strength of SGA by adding tricyclodecane dimethanol diacrylate with high Tg, and replacing common nitrile butadiene rubber with high temperature resistant acrylic rubber [10].

Liu Suzi et al. increased the Tg of the polymer system by adding cyclohexyl methacrylate with annular side groups, thus improving the thermal stability of the polymer. With the increase of the amount of cyclohexyl methacrylate, the shear strength showed a trend of first increasing and then decreasing after high temperature aging, and the appropriate amount was 15%[11].

3 Improved water resistance

For conventional plastic substrates, such as ABS, PC, PMMA, SGA can form a good bonding force with it, mainly because the adhesive liquid can swell the plastic substrate, mix it into one, and finally form a tough polymer bonding layer. Since the glue/plastic 2 phase interface has completely disappeared, water vapor cannot adversely affect the bonding. For the self-bonding of plastic substrates, it is difficult to reflect the difference in water resistance of SGA. This paper no longer discusses it, but takes the self-bonding of inorganic substrates such as metal, ceramics, glass, or the bonding of inorganic substrates with plastic substrates as examples to discuss the factors affecting the water resistance of SGA and the improvement methods.

The factors affecting the water resistance of SGA are:

(1) The adhesive layer absorbs water itself, affecting the adhesive force. Hydrophilic groups in the molecular structure of SGA, such as ester groups, hydroxyl groups, carboxyl groups, etc., will increase the water absorption of the colloid. These inhaled moisture will reduce the modulus of the colloid and reduce the bulk strength, thus weakening the bonding force of the SGA.

(2) Water intrudes into the colloid/substrate interface, affecting the adhesion. For the bonding of inorganic substrates such as metal, ceramic and glass, SGA is mainly formed by the comprehensive action of chemical bonding, intermolecular force, interface electrostatic attraction, mechanical force and so on. Water can penetrate into the adhesive layer through the plastic substrate to reach the adhesive layer/inorganic substrate interface, or directly penetrate through the adhesive layer/inorganic substrate interface to destroy the formed bonding force; Water vapor can also form a weak boundary layer at the adhesive layer/inorganic substrate interface, further reducing the bonding force.

(3) The substrate is hydrophilic, which will also affect the bonding force. The surface of inorganic substrates such as metal, ceramic and glass has a strong adsorption capacity for water vapor, which can penetrate into the adhesive layer/substrate interface, oxidize and corrode the metal surface, and reduce the bonding strength. Water vapor can also destroy the formed bonding force and reduce the bonding strength. Compared with the inorganic substrate, the hydrophilicity of the plastic will be less, but it also has a certain hydrophilicity. The more hydrophilic the substrate, the easier it is for water vapor to affect the strength of the adhesive layer through the plastic substrate, and further penetrate into the interface of the colloidal/inorganic substrate, affecting the adhesion.

The main methods to improve the water resistance of SGA:

(1) Reduce colloid water absorption

The hydrophilicity of colloids can be reduced and the water resistance can be improved by reducing the content of hydrophilic functional groups and increasing the proportion of hydrophobic components. For example, reduce the proportion of hydrophilic monomers such as hydroxyethyl acrylate, acrylic acid and vinyl acetate, and use hydrophobic monomers such as butyl methacrylate and lauryl methacrylate. Hydrophobic additives containing Si and F are added [12], such as silane coupling agents and F-containing surfactants. Increase the density of molecular structure, such as adding crosslinking agents; It can reduce the colloid water absorption and improve the water resistance of SGA.

Liu Suyu et al. added 15% cyclohexyl methacrylate to make the adhesive still have the maximum shear strength at room temperature after wet and hot aging. They found that the shear strength increased first and then decreased with the increase of cyclohexyl methacrylate dosage. They believe that with the increase of cyclohexyl methacrylate dosage, the spatial network structure of the rubber layer will gradually become tighter, and the absorption of water will gradually decrease. At the same time, a certain water vapor permeability can be ensured, so that the absorption and diffusion of water can reach a balance, and the shear strength will continue to increase. When the polymer layer is too thick (the amount of cyclohexyl ester is greater than 15%), the dense polymer layer obstructs the permeability of water vapor, while the water continues to diffuse from high concentration to low concentration under the action of diffusion, and the retained water vapor destroys the internal structure and bonding surface of the adhesive layer, resulting in a continuous decline in shear strength [11].

(2) Improve the adhesion between the colloid and the substrate

Through the composition design, a strong chemical bond is formed between the colloid and the substrate, which is not easily destroyed by water vapor, so as to resist the damage of water vapor on the adhesion force. For example, adding functional phosphate monomer can improve the adhesion force on metal aluminum substrate; At the same time, reducing the curing shrinkage stress, such as reducing the amount of high energy monomers, using flexible monomers and resins, can also improve the bonding performance of SGA. Pretreatment of the substrate surface, such as cleaning and polishing of the metal substrate surface, removes the oxide layer or pollutants, improves the adhesion of SGA, and thus achieves the purpose of improving the water resistance.

GUI Wubiao et al. also increased the hydrophobicity and interfacial adhesion of colloids by adding silane coupling agents, and improved the water resistance [13].

Nie Qisi et al. increased the adhesion between colids and substrate by adding sulfhydryl coupling agents, and the strength of the colids was improved after weathering with moisture and heat [10].

4 Improve storage performance

Potential factors affecting the stability of SGA are as follows:

(1) Monomer self-polymerization. The acrylic monomer in SGA has a tendency of self-polymerization, and the active monomer free radical formed will cause other monomers to polymerize, thus reducing the storage stability of the adhesive solution.

(2) Contains an initiator to release active free radicals, causing monomer polymerization and reducing storage stability.

(3) The composition contains a small amount of metal ions, and the initiator or reducing agent to form a REDOX pair, the formation of active free radicals to trigger monomer polymerization, will also reduce storage stability;

(4) Environmental factors. Temperature will accelerate the decomposition of initiator and accelerate the formation of active free radicals. In addition, temperature will also accelerate the self-polymerization of monomers and reduce the storage stability. Light irradiation, especially ultraviolet irradiation, can also form active monomer free radicals, causing polymerization and reducing storage stability.

In order to avoid potential factors causing the deterioration of SGA storage stability, measures are often taken:

(1) Add appropriate amount of inhibitor

The inhibitor can consume the decomposed free radicals and prevent the polymerization from occurring. The highly efficient inhibitor has little impact on the performance of SGA, and only prolongs the operation time, initial solidification time and storage time [14].

(2) Add appropriate amount of ion chelating agent

Adding an appropriate amount of ion chelating agent, such as Na2EDTA, Na4EDTA salt, etc., can make metal ions stable, reduce or avoid the probability of REDOX reaction, and improve storage stability together with the inhibitor. The selection of chelating agents should pay attention to their solubility in the system and chelating ability to metal ions [15,16].

Dai Yihua et al. accelerated the aging of the homemade two-component acrylate adhesive at 80 ℃, and the adhesive liquid appeared gel in different degrees at 4 h. Then 100 mg/kg Na4EDTA chelating agent was added to the stock solution to eliminate the residual metal ions in the system, and 500 mg/kg hydroquinone inhibitor was added to improve the storage stability of the system. After accelerated aging, no gel phenomenon occurred at 12 h [17].

(3) Reduce the storage temperature

Reducing the storage temperature can reduce the energy required for the reaction, reduce the activity of the system, and extend the storage time. When selecting the storage temperature, it should be noted that in some SGA systems containing MAA, when stored at an environment below 0 ° C, the adhesive liquid will have the phenomenon of component crystallization, which will affect the use and bonding properties. In addition, it should also be noted that when the glue is stored at low temperature, the phenomenon of component precipitation and phase separation cannot occur, otherwise the curing performance of SGA will be affected.

(4) Storage away from light

Acrylate monomers will absorb light through the packaging material, such as visible or ultraviolet light, they will stimulate carbon-carbon double bonds to form active free radicals, leading to photopolymerization. Therefore, the adhesive liquid should be stored in the dark area or the use of opaque materials to avoid the impact of light on storage stability.

(5) Partial oxygen is contained in the glue solution

Anaerobic adhesives can achieve stable storage, and oxygen plays a very key role. SGA can also cleverly use the principle of oxygen inhibition to allow oxygen to remain inside the glue to help improve storage stability. For example, in the preparation of some SGA with high viscosity and thixotropy, it is not necessary to completely remove the bubbles inside the glue. Some of the small bubbles contained in the glue will not only affect the homogeneity of the mixing of the two components, but will not affect the other properties of the product, but can prevent the polymerization reaction inside the system, thereby improving storage stability.

(6) Adjust the packaging ratio of 2 components

The common two-component acrylate structural adhesive packaging ratio is 1∶1 or 2∶1, and the initiator and accelerator used are separately packaged in different A and B components. Since both components contain reactive acrylate monomers, a component added to the initiator often causes the decomposition of the initiator to generate free radicals to trigger polymerization, resulting in unstable storage of the entire component.

In order to solve the problem of storage stability, the packaging ratio of 2 components can be adjusted, such as making a ratio of 10∶1 packaging, separating the initiator, packaging together with the plasticizer, filler, pigment, etc. without reactivity carbon-carbon double bond, to form the B component, other components such as: Acrylate monomer, toughening agent, accelerator, etc., can be packaged into component A, which can solve the problem of instability of the whole system caused by monomer polymerization caused by the decomposition of initiator [10].

5 Conclusions

In recent years, China's second generation of acrylate structural adhesives have developed rapidly, and various types of adhesives have been widely used in civil, construction, machinery, ships, aviation, electronic appliances and many other fields. However, compared with European and American products, some properties of domestic adhesives, especially comprehensive properties, have a certain gap. We believe that with the emergence of a large number of technical talents and the investment of scientific research efforts of enterprises, there will be more products with excellent performance in the near future.