Common Problems in Pultrusion Production Process

 

 

Due to its high degree of mechanization, continuous production and stable product quality, the FRP pultrusion process has achieved sustained and stable high-speed growth in recent years. However, since most domestic pultrusion plants are small in scale and weak in technical strength, they are unable to determine the causes of many technical problems encountered in production, and therefore are unable to find solutions to the problems. In addition, many of the production formulas they use are hearsay, lack scientific basis, and are not entirely reasonable. This article analyzes the causes of several common problems and attempts to provide solutions.

 

 

Selection of Fillers

 

Fillers are very important components in pultrusion formulas. If used properly, they can improve the processability of the resin system and the performance of the cured product, and can also significantly reduce the cost of the composite material. If used improperly, it will also seriously affect the processability and product performance. Generally speaking, any powdered mineral can be used as a filler.

 

The effect of fillers on liquid resin systems is to increase viscosity, produce thixotropy, accelerate or retard curing, and reduce heat release. Among the process factors it affects, the most important is the effect on viscosity and rheology. The main factor affecting viscosity is the oil absorption rate of the filler.

 

The increase in oil absorption rate increases the viscosity, or in other words, the lower the oil absorption rate, the greater the amount of filler can be added while maintaining the same viscosity. In addition, the specific surface area of ​​the filler will affect the polymerization rate. Light calcium carbonate, talcum powder, etc. have a large specific surface area, which will hinder curing; while heavy calcium carbonate has a small specific surface area and better curing performance.

 

By coating the surface of calcium carbonate with organic matter to form active calcium carbonate, the aggregated particles are reduced, the dispersion is improved, and the gaps between particles are reduced, thereby further reducing its oil absorption rate. In addition, the price of fillers is also an important reference for choosing which filler to use.

 

Among the common fillers, the lowest cost is heavy calcium carbonate and talcum powder, the cost of surface-activated heavy calcium carbonate is slightly higher, the cost of kaolin is in the middle, and the highest is aluminum hydroxide. Aluminum hydroxide is usually used as a flame retardant filler instead of a common filler.

 

The effects of various fillers on the mechanical and physical properties of cured products are roughly the same. The effects on flame retardancy and electrical properties are slightly different. In addition, the effect on the dynamic mechanical properties of the product is more complicated, and it is related to the particle size, particle size distribution, particle shape, hardness and filling amount of the filler.

 

In summary, the choice of filler should depend on the final performance requirements of the product. Under normal circumstances, the best performance and lowest cost fillers are heavy calcium carbonate and surface-activated heavy calcium carbonate. In addition, unless there are special requirements, it is generally not recommended to use a composite (using two or more) filler system.

 

 

Thickening Phenomenon of Resin Mixture

 

The so-called thickening phenomenon refers to the gradual increase in viscosity of the prepared resin mixture during use, and the rate of increase in viscosity gradually accelerates until the mixture is difficult or even impossible to use.

 

This phenomenon is often encountered in domestic pultrusion plants, but it is difficult to find the real cause and solution. The reason for this phenomenon is that there are highly acidic components in the formula, mainly the liquid internal mold release agent used in the formula (most of the liquid internal mold release agents currently sold in China belong to this category).

 

This component reacts chemically with alkaline fillers such as calcium carbonate or metal oxide pigments in the mixture to free calcium ions or other metal ions.

 

These freed calcium ions or other metal ions quickly react chemically with the carboxyl groups at the ends of the polyester molecular chains to form complexes, forming a mesh structure of a polyester-metal complex, thereby causing the viscosity of the mixture to increase sharply and reduce fluidity.

 

The rapid increase in viscosity makes it difficult to penetrate the glass fiber, and the resistance to drawing also increases. The resulting product has a performance degradation due to insufficient wetting. Highly acidic components will also react chemically with certain acid-sensitive pigments, causing the color of the product to drift.

 

In addition, the acidic component also corrodes non-chrome-plated molds (most of the joint molds in China are not chrome-plated at present), shortening the mold life. Its reaction products with the mold will also contaminate the product, causing some fiber-rich parts of the product to turn black. In addition, it is very important that peroxide decomposes into two free radicals when it is heated in a neutral or alkaline medium. However, in an acidic medium, its cracking reaction may be an ion cracking reaction rather than a free radical cracking reaction.

 

Cracking of Products

 

Cracking of products, including surface cracks and internal cracks, is the most troublesome process problem for pultrusion plants. This problem occurs most frequently and can occur in almost any product and at any time.

 

The main cause of cracks is the thermal stress caused by exothermic shrinkage. Many manufacturers have a consensus on this point. However, it is still difficult to find a better way to eliminate thermal stress in China. Although proper adjustment of mold temperature and pultrusion speed may temporarily alleviate this problem, cracks will reappear with changes in environmental conditions and some unknown reasons. Sometimes cracks may suddenly appear under very normal circumstances, making you unable to prevent them.

 

Just add a small amount of anti-cracking brightening powder (2%-5% of the resin amount) to have a significant effect. Since the amount added is not large, it has little effect on the performance of the product and can also enhance some properties. In addition to reducing cracking, it also has the advantages of increasing the gloss of the product surface and making the color of the product more uniform. Adding this component to the formula can greatly reduce the scrap rate and make production smoother and smoother.

 

The mechanism of DP-anti-cracking brightening powder to prevent products from cracking is as follows: Since it is insoluble in resin, when it is dispersed in the resin, it forms dispersed spherical particles of about 50-100μm in size in the continuous phase of the resin, and a small amount of liquid resin is dispersed in these spherical particles.

 

When the resin is heated, both the continuous phase and the dispersed phase expand. When heated to 120-140℃, the resin begins to solidify, cross-link and shrink, and the resin in the dispersed phase spherical particles also cross-links and solidifies.

 

However, at this time, the dispersed phase spherical particles continue to expand as the temperature rises. Therefore, stress and strain are generated at the interface between the dispersed phase spherical particles and the continuous phase. As the temperature continues to rise, the curing and cross-linking of the continuous phase continues to occur and tends to be completed, and this process is accompanied by the continuous contraction of the continuous phase.

 

In this way, greater stress and strain are generated at the interface. When this stress and strain reach a certain level, the cross-linked resin network in the dispersed phase spherical particles will crack and expand in the spherical particles, thereby releasing the stress and strain at the interface. Through such a mechanism, continuous cracks in the main skeleton network of the product are prevented.

 

The above briefly discusses the selection of pultrusion fillers and the two most difficult process problems. Of course, there are far more problems in the pultrusion process. For each problem that arises, close cooperation between the pultrusion plant, equipment manufacturers and raw material suppliers is required to solve the problem better. The product development capabilities and after-sales service capabilities of equipment manufacturers and raw material suppliers are crucial to assist in solving problems in pultrusion production.