In recent years, hot melt adhesives have developed rapidly and have a wide range of uses. In particular, EVA hot-melt adhesives have a large demand and wide application area, accounting for about 80% of the total consumption of hot-melt adhesives. The development of hot melt adhesives is quick because the hot melt adhesives are different from thermoset, solvent, and water-based adhesives in that they contain no solvent, no pollution, no heat curing, no drying process, low energy consumption, and easy operation. Can be used for high-speed continuous production line to increase production efficiency. And because it is solid at room temperature, it can be processed into film-like, rod-like, strip-like, block-shaped or granular according to the user's requirements; different formulations can also be used to modulate different formulations to meet the softening point, viscosity, embrittlement point and Use temperature and other performance requirements. Hot melt adhesive materials and formulations determine the properties and use of hot melt adhesives. For different performance requirements, choosing the right materials and designing a reasonable hot melt adhesive formulation is crucial.
2 Material, ratio and performance
2.1 EVA resin EVA hot melt adhesive is composed of copolymer EVA resin, tackifiers, waxes and antioxidants. If you want to deploy a good hot melt adhesive, you should first choose the main resin. The main resin is the main component of the hot melt adhesive, which has a great influence on the properties of the hot melt adhesive. Its microstructure determines the macro performance. EVA resin structure is as follows:
EVA resin vinyl acetate content (VA% content), the molecular weight of the copolymer and the molecular degree of branching determine the performance of the resin. Since the vinyl acetate monomer is introduced into the EVA resin molecular chain, the crystallinity is lowered compared to the polyethylene resin, and the flexibility and impact resistance are improved. The EVA resin for hot melt adhesives generally has a VA content of between 18% and 40%. The VA content in the resin increases, the toughness, impact resistance, softness, stress crack resistance, viscosity, heat sealability, and repeated bending of the resin in the cold state increase, the peel strength of the bonding increases, and the rubber elasticity increases. The strength, hardness, melting point, and heat distortion temperature also decrease. This can be based on the performance requirements of hot melt adhesives to select the appropriate VA percentage of EVA resin as the main material. For example, in the introduction of the floor block production line, the hot melt adhesive formula used for the floor block splicing is as follows: EVA (VA28%) 100g; tackifier resin 115g; wax class 35g; antioxidant 2g.
In this formula, EVA resin with a VA content of 28% was used, and the formulated hot melt adhesive had a better overall performance. If the EVA resin with a relatively high VA content is used in the formulation, the prepared hot melt adhesive has a large elasticity and insufficient hardness, and the spliced floor board is not straight. If the EVA resin with a relatively low VA content is used, the prepared hot melt adhesive has poor flexibility, low temperature performance, brittleness, and low adhesive strength. Can not meet the process requirements. Therefore, it is very important to choose the appropriate VA content EVA resin. In addition to the effect of VA content and molecular structure on the performance of EVA, the molecular size and molecular weight distribution of the copolymer are also related. There are many manufacturers of EVA in various countries in the world. Manufacturers have given product grades, VA content, density, melt flow rate, characteristics, and applications. For example, the grade 28/150 produced by the Beijing Organic Chemical Plant in China and the grade 220 produced by Mitsui Corporation in Japan all give the VA content of 28% and the melt flow rate of 95%. (MI) Melt flow rate is related to molecular structure and molecular weight. According to data reported, there is the following functional relationship between them: MI = K M-2, where K - constant; M - average molecular weight of the polymer. The MI value refers to the weight of the polymer extruded from a fixed-diameter nozzle per 10 minutes at a certain temperature and pressure. It can macroscopically reflect the mechanical properties, rheological properties and stress crack resistance of EVA resin. Interdependence. MI value increases, melt fluidity increases; molecular weight, melting melt viscosity, toughness, tensile strength and stress crack resistance decrease, and flexion and extension stress, elongation at break, strength and hardness do not change, so that in the design of EVA heat Melt flow rate (MI) value becomes a very important reference data in melt formula. In general, the MI value is large, the molecular weight is relatively small, the melt viscosity of the resin is low, and the prepared hot melt adhesive has low viscosity and good fluidity, which is favorable for diffusion and infiltration on the adherend surface. On the bonding process, there is an EVA resin with a relatively large MI that can be selected in this aspect. The disadvantage is poor oil resistance.
The MI value is 15 to 400. The hot melt adhesive formula used for rigid foam bonding in automobile manufacturing is as follows: EVA resin (VA28%MI=400) 100g; Tackifier resin 200g; Wax type 143g; Antioxidant 3g . The EVA resin used in this formula has a large MI value, and the prepared hot melt adhesive has a low melting viscosity and good fluidity, which meets the production process requirements. MI value is small, molecular weight is relatively large, resin melt viscosity is greater, the material itself has high cohesive strength, the prepared hot melt adhesive also has high strength and improves the bonding strength; the disadvantages are large viscosity, poor flowability and process performance. difference. EVA resin due to different VA content, MI value is different, the manufacturers produce a lot of product models, design hot melt adhesive formula can be based on the performance requirements of hot melt adhesives, select the appropriate VA content and MI value of EVA resin to debug the formula, you can also use two kinds EVA resin adjustment formula with different VA content and MI value. In this way, various properties can be combined to complement each other, and then the required recipes can be debugged.
2.2 Tackifiers In order to increase the surface adhesion to the adherends, bonding strength and heat resistance, most EVA hot melt adhesive formulations require tackifiers. The tackifier is generally added in an amount of 20 to 200 parts. Both EVA and tackifier formulations have a wide range of ratios, depending on performance requirements. Generally, as the amount of EVA increases, the softness, low temperature resistance, cohesive strength, and viscosity increase. As the amount of the tackifier increases, the fluidity and diffusibility become better, and the wettability and initial tackiness of the adhesive surface can be improved. However, if the amount of tackifier is too much, the adhesive layer becomes brittle and the cohesive strength decreases. When designing a hot melt adhesive formulation, the softening point of the tackifier and the EVA softening point are preferably synchronized so that the hot melt adhesive prepared has a narrow melting point range and good performance. In order to improve the heat resistance of the hot melt adhesive, it is necessary to select a material with a high softening point, and the softening point of the hot melt adhesive formulation increases as the softening point of the material increases. There are many types of tackifiers, commonly used tackifiers include rosin, polymerized rosin, hydrogenated rosin, C5 and C9 petroleum resins, thermoplastic phenolic resins, polyisobutylene, and the like. The required thickener and EVA resin should have good melting property, and have good thermal stability at the melting temperature of the hot melt adhesive. The same formulation system with different tackifiers has different tackifying effects, and its softening point directly affects the softening point of the hot melt adhesive. Therefore, the tackifier also plays an important role in the hot melt adhesive.
2.3 Waxes Waxes are also commonly used in EVA hot melt adhesive formulations. The addition of waxes to the formulation can reduce the melt viscosity, shorten the curing time, and reduce the phenomenon of spinning. It can further improve the flowability and moisturization of the hot melt adhesive, and can prevent agglomerates and stickiness on the surface of the hot melt adhesive, but the dosage Too much will make the bond strength drop, and generally the amount added does not exceed 30%.
2.4 Other Additives In order to prevent the hot melt adhesive from being oxidized and thermally decomposed during construction under high temperature and the deterioration of the adhesive and adhesive strength, in order to extend the service life of the adhesive, generally 0 to 2% of antioxidant is added. In order to reduce the cost, change the color of the glue, reduce the shrinkage at the time of curing and excessive permeability, sometimes add no more than 15% of the filler. In order to reduce the melt viscosity and increase the melting speed and increase the flexibility and cold resistance, sometimes no more than 10% of the plasticizer is added. Can also be added according to performance requirements of various improvers, additives to complete the formula performance requirements. The earliest literature on the direct catalytic hydrogenation of o-nitrochlorobenzene to DHB has been found in U.S. patents. In this reaction, 2,3-dichloro-l,4-fluorene (DCNO) derivative was added as a reduction accelerator to obtain DHB. The rate is 80% to 90%. In the late 1970s and early 1980s, reports on catalytic hydrogenation gradually increased, but some literature results were not satisfactory. Patent considers that when β-hydroxyanthraquinone or 2,6-dihydroxyanthraquinone is used as a reduction accelerator, the quality of o-chloronitrobenzene is reduced to DHB in one step in an alkaline medium in the presence of benzene, toluene or xylene. Higher, the melting point is 85-86°C, but the yield is not more than 84%. Entered the European patent applied by Japan's Toyo Ink Manufacturing Co. in the 1990s. The yield of DHB was 91.5%. Tetrahydrin was used as the solvent. In addition, the research done by Dalian University of Technology used the improved Pd/c as the catalyst and toluene as the solvent. The yield of DHB was 93%. The result of another Japanese patent is better, with alkaline water as the reducing medium, DCNO as the reduction accelerator, and sodium dodecylbenzenesulfonate as the emulsifier. The reduction from o-chloronitrobenzene to DHB is clearly divided into two stages, namely reduction to DOB first and then DOB to DHB, but these two stages can be accomplished by changing the alkali concentration in one vessel. When Pd/c was selected as the catalyst, the DHB yield was 964%, while the yield decreased to 94 2% with Pd/c. Catalytic hydrogenation is increasingly favored by people because of its many advantages: it can eliminate the use of organic solvents, eliminating the hassle of post-processing and product separation; the reducing agent is hydrogen-free from environmental pollution; high product yield; reactor pressure Not high, the equipment requirements are not engraved; the reaction cycle is short; the product is easy to separate. However, its technical requirements are relatively high; the preparation of catalysts is not disclosed in the literature; because of the use of precious metal catalysts, it must be considered for repeated use in order to reduce costs. The above factors have added difficulties to the industrialization of catalytic hydrogenation
3 Conclusion As can be seen from the above review, there are many preparation methods for DHB, but their advantages and disadvantages are different. Hydrazine hydration method, iron powder method, sodium hydrosulfide method can only be reduced from DOB to DHB, the process is incomplete, more waste; formaldehyde, formic acid, zinc powder can be achieved from the reduction of o-chloronitrobenzene to DHB. As more wastes are used, they limit the popularization and application of these methods; electrolysis and catalytic hydrogenation can significantly reduce the three wastes and have greater promotion value. In fact, the domestic zinc powder method and the formaldehyde-hydrazine hydrate combination method are used to produce DHB in small batches. Because the three wastes are severe and the quality is unstable, most of them are open, closed and stopped. It is reported that foreign countries already have the production of DHB catalytic hydrogenation, and domestic companies want to introduce two sets of 1,000 tons of catalytic hydrogenation production technology. The electrolytic reduction method has not been reported industrially. It may be because the yield is low, the electricity bill is expensive, and the electrochemical engineering problem is difficult to solve. Therefore, this requires further research and development work.
Reprinted from: network of thermosetting resin: Wang Zhao Shuyuan white school continued Duo
