In practical applications, there are several factors that affect the selection of resin:
1. Adhesive performance 2. Mechanical properties 3. Hydrolysis resistance 4. Permeability
1. Adhesive performance
The bonding performance of resin determines whether the mechanical properties can be fully utilized. The bonding performance between resin, fibers, and core materials is very important in sandwich structures. Among the three main resins mentioned above, polyester resin has the lowest bonding performance, vinyl ester resin has higher bonding performance than polyester resin, and epoxy resin has the best bonding performance. It is widely used in the manufacturing of high-strength adhesives. The high viscosity of epoxy resin comes from the presence of polar hydroxyl groups. Due to the low shrinkage rate of epoxy resin, the resin and bonding surface will not separate during the curing process. The excellent bonding performance of epoxy resin is very suitable for the manufacturing of honeycomb structures, because the bonding area in honeycomb structures is small, so the strength requirements for the adhesive are higher. The bonding performance between resin and fibers does not entirely depend on the adhesion of the resin, but also on the surface treatment method of the fibers.
2. Mechanical performance
The two most important mechanical performance indicators for resin matrix are tensile strength and modulus. The following figure shows the performance comparison of common commercial polyester resins, vinyl ester resins, and epoxy resins, cured at temperatures of 20 ° C and 80 ° C, respectively.
For designers, the shrinkage rate of the resin is also a factor that must be considered. The reason for shrinkage is the rearrangement and reorientation of the resin molecular structure when the resin is in the liquid or semi gel state. Polyester and vinyl ester require significant changes in molecular structure during curing to achieve complete curing, resulting in a high shrinkage rate after curing, up to 8%. However, the cured molecular structure of epoxy resin undergoes minimal changes and no volatile by-products are produced, resulting in a shrinkage rate of only about 2%. Low shrinkage also helps to improve mechanical properties, as the internal stress generated by high shrinkage can seriously weaken the material's load-bearing capacity. In addition, a high shrinkage rate will also form a texture on the surface of the cured component, thereby increasing the work of removing the texture and leading to an increase in production costs.
3. Hydrolysis resistance
The resistance to degradation after water absorption is crucial for all resin materials, especially for the shipbuilding industry. In practical use, all resin materials will adsorb a certain amount of water vapor, and the key lies in how the adsorbed water vapor affects the bonding performance and mechanical properties of fibers and resins. Vinyl esters and polyesters significantly reduce their water resistance due to the presence of ester groups. For example, after being immersed in water for one year, the interlayer shear strength of thin layer polyester fiberglass can only maintain 65%, while epoxy resin can maintain 90%.
4. Permeability
In humid environments, a small amount of water will pass through the fiberglass laminate in the form of water vapor. When water vapor passes through, it reacts with hydrolyzable components to form a small concentrated solution. In repeated permeation cycles, more water vapor enters the laminate through the semi permeable membrane to dilute the concentrated solution. The entry of water vapor increases the pressure inside the pores to 700 psi, and the final release of this pressure leads to cracking of the laminate or bubble defects in the adhesive layer, resulting in raspy defects on the surface. The hydrolyzable components include foreign objects in the fabric and ester bonds in polyester resin and vinyl ester resin.
In order to prevent water vapor from penetrating into the interior, it is necessary to choose a resin with low water transfer rate and high hydrolysis resistance. After sufficient waterproof treatment is applied to the surface of the reinforced fabric, this problem can be eliminated. Polymers containing epoxy segments typically have high waterproofing and mechanical properties.
Post time: Apr-30-2024
