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Volume Shrinkage Can Be Compensated By Epoxy Phenolic Resin

Jun 13, 2020

         HC Paint Network News: Abstract: This article analyzes the factors that cause the internal stress of the glued joint, mainly from the aspects of modified epoxy resin and the addition of reinforcing fillers, to reduce the internal stress of the joint, improve the mechanical properties, and enhance the flexibility . Through the comparative test, the reasonable content of glass fiber as filler in epoxy resin structural adhesive was obtained in the study, so as to achieve the strength and flexibility of the adhesive, and provided basic data for its application in the field of bonding.

         The widespread application of construction adhesives has made people pay high attention to this type of chemical building materials. In China, after decades of research and practice, the results of advanced technologies such as chemistry, mechanics, materials, and structures have been integrated to apply them. In the construction of reinforced steel structure reinforcement, anchor bolt reinforcement, on-site pouring construction, its advantages and important role are more prominent.

  However, due to the high internal stress of the cured epoxy resin, there are shortcomings such as brittleness, fatigue resistance, heat resistance, and poor impact toughness. It is difficult to meet the requirements of engineering technology, and its application is subject to certain restrictions. Therefore, the toughening of epoxy resin and improving its brittleness and mechanical properties are one of the important topics. In this paper, small molecular polyamine 650# is used as a toughening agent, and talc powder, sand powder, and glass fiber are used as fillers to modify epoxy resin structural adhesive.

      In the connection of building structures, adhesive bonding has been widely used. It has the advantages of simplified structure and optimized performance. However, the internal stress in the adhesive joint will have a great impact on the adhesive strength and durability, which limits the adhesive. Connected to some important occasions. Generally, there are two main sources of internal stress: ① During the curing process of the adhesive, due to solvent evaporation, small molecule products escaping during the polycondensation reaction, or the distance between the atoms during the polyaddition reaction is shortened, the volume of the adhesive layer shrinks, causing shrinking internal stress; ②The thermal expansion coefficients of the adhesive and the bonded body are different, and thermal stress will occur when the temperature changes. The presence of internal stress on the joint usually results in a significant decrease in the strength of the joint, sometimes even causing the joint to rupture automatically.

  No matter what method is used to cure the adhesive, a certain volume shrinkage will inevitably occur. When the adhesive is in a flowing state, the volume shrinkage can be compensated; if the volume has not reached the equilibrium value after losing the fluidity, further curing will generate internal stress. The shrinkage stress in the adhesive joint is affected by many factors. When the shrinkage stress is measured by a simple method, the adhesive can be coated on the metal tin foil. After curing, the foil will bend due to the shrinkage stress, and the radius of curvature p is measured. The thickness of the adhesive layer h1 and the thickness of the foil h2 can be calculated by the following formula [2.3]:

   Where E1 is the elastic modulus of the adhesive, E2 is the elastic modulus of the metal foil, m=E1/E2, n=h1/h2.

  The materials with different thermal expansion coefficients are glued together, and the temperature will cause thermal stress in the interface. The effect of thermal stress is particularly significant when gluing two materials with very different expansion coefficients I4]. In order to avoid thermal stress, materials with very different thermal expansion coefficients for bonding generally prefer a lower curing temperature and use an adhesive that cures at room temperature. For example, for the bonding between stainless steel and nylon, if a high-temperature curing epoxy resin adhesive is used, only a very low bonding strength can be obtained, while using a room temperature curing epoxy-polyphthalamide adhesive can obtain a satisfactory bonding strength. .

   For materials whose bond expansion coefficient cannot be matched, in order to prevent them from cracking when the temperature changes, adhesives with low modulus and high elongation are generally used, so that thermal stress can be released through the deformation of the adhesive.