The common defects of FRP products are shrinkage and deformation
No matter what kind of products you make or what process you choose, as long as it is FRP products, you will find an unavoidable problem, that is, shrinkage and deformation
The reason cause FRP products shrink is the resin shrinkage , there are 3 main reasons
- When the resin is cured, the chemical bond breaks and the cross-linking reaction occurs. The distance between the original molecules becomes the bond length, and the occupied volume becomes smaller.
- In the viscous flow state, the molecules of the resin are disorderly distributed, and gradually change into ordered distribution during the curing process. The molecules are arranged closely, so that the occupied volume becomes smaller.
- During the curing process, the exothermic reaction causes the temperature of the system to rise. After reaching equilibrium, the temperature begins to decrease, and the thermal movement of the molecular chain gradually weakens, and the free volume decreases.
Take an example. In a room, originally full of 30 people, at first everyone stood, sat or lay down at will, but the room was full. Later, we ordered them all in a unified way. 30 people stood in line one by one. The crowded room suddenly became spacious. In the sorting process, the confusion caused by walking is boring, but as everyone calms down and starts to cool down, the space is more open at this time.
The shrinkage of the resin will lead to the internal stress of the product, which is the root of the deformation of the FRP products. The root cause of shrinkage and deformation of products was found out all at once, which was caused by resin shrinkage. The next step is to find a way to solve the problem from the root.
Since the resin is the root reason cause the FRP products shrink, can we improve the shrinkage of the resin itself? The answer is yes. For example, the volume shrinkage of common unsaturated polyester resin is 7-10%, while that of epoxy resin is about 2%. Visible, choose different resin, shrinkage rate has very big gap.
For the same unsaturated polyester resin, the shrinkage can be fundamentally controlled by selecting different raw materials in the synthesis stage. The volume shrinkage of some new up resins can be controlled at 2-3%, or even lower.
But most of the time, due to the process, cost and other reasons, we do not have much space when choosing resin. When the shrinkage rate of resin is too large, we need to see the following suggestions.
The problem of resin shrinkage is that after curing, its space volume becomes larger, so we can reduce the shrinkage by adding some fillers.
The common inorganic fillers are calcium carbonate, talcum powder, aluminum hydroxide, etc. these fillers generally do not participate in chemical reactions and belong to inert materials. The shrinkage can be reduced by reducing the resin content per unit volume and occupying pits.
In addition, the shrinkage can be controlled by adding low shrinkage agent (LPA), which is mainly composed of some thermoplastic resins, elastomers or composite polymers. In addition to pit occupation, in the process of resin exothermic, thermoplastic resin will have a certain expansion, which can offset the volume shrinkage caused by the curing process of thermosetting resin, so as to reduce the shrinkage rate more effectively.
Polystyrene (PS), which is commonly used in SMC sheet production, belongs to thermoplastic resin.
Although the addition of fillers or LPA can effectively reduce the shrinkage of the resin, it can also reduce the mechanical properties of the products to a certain extent.
In the face of raw materials can not be changed, we can also control the shrinkage from the process.
It is very important to control the curing rate. If the curing speed is too fast and the reaction is violent, the exothermic peak will be too high. If the temperature of the system changes too much, it is easy to cause thermal shrinkage. Therefore, under the premise of ensuring the production efficiency, the curing speed must be controlled well, not too fast.
Control the resin content. The root cause of shrinkage is resin, so the shrinkage can be controlled by controlling the content of resin. Especially in the hand paste process, in order to speed up the work, workers often increase the amount of resin, resulting in excessive resin content of products, which is more likely to shrink and deform.
Control the number of single layer laying. This is mainly reflected in the hand paste process, a single paste of a layer and a single paste of five layers, the shrinkage rate is very obvious difference, the more layers, the more concentrated reaction exothermic, which is more likely to lead to thermal shrinkage.
Post curing treatment. The curing degree of the product is improved by heating and curing, and the monomer reaction after demoulding is prevented, resulting in shrinkage and deformation. This link is ignored in many room temperature processes (hand paste, vacuum lead-in, RTM). In order to improve the production efficiency, the demoulding is often too fast. Therefore, it is easy to cause the deformation of the products before they are put out of the warehouse for a period of time.
This is mainly for reinforced materials and sandwich materials design.
The selection of different reinforcement materials has a certain impact on the content and distribution of resin, so this problem should be considered in the design of overlay.
Whether the products choose sandwich materials or not will also affect the content of resin. For example, 20mm products, one is pure glass fiber reinforced, another design is the use of 10mm sandwich foam +10mm skin, the shrinkage rate is also different.
Of course, all choices should be based on the requirements of the product