Phenolic Pultrusion Process

 

 

Phenolic resin has the remarkable characteristics of flame resistance, low smoke release, and low toxic gas content. Therefore, phenolic FRP is widely used in various fire retardant applications. However, how to use phenolic resin in pultrusion process has been a process technology issue that has attracted widespread attention from the FRP industry at home and abroad in the past decade. Here we briefly introduce some of the latest developments in phenolic pultrusion process.

 

Overview of Pultrusion Process

In theory, the pultrusion process is relatively simple. The types of reinforcement materials used are relatively wide. They can be fibrous, woven or felt. The components can be glass fiber, aramid fiber, carbon fiber or other fiber materials.

 

Reinforcement materials are usually fed continuously, such as using untwisted roving yarn balls to continuously feed fibers from a creel. During pultrusion, the fiber first passes through a hot forming die of the same size as the FRP product, and then enters a drawing mechanism. There is a glue tank between the creel and the forming die, in which the pre-prepared resin is placed. After the fiber is impregnated with the resin, it is arranged through the guide device and then enters the forming die.

 

A round hole or slit is provided on the guide device to remove the excess resin adhering to the fiber. The temperature distribution of the forming die is carefully designed to ensure that the resin has completed the curing process when the pultruded material leaves the rear port of the die. There is an air cooling section in front of the pulling mechanism and the cutting machine to cool the pultruded product with a higher temperature.

 

According to relevant reports, a new form of process arrangement for the pultrusion process has emerged. This new arrangement form is to inject the formulated and mixed resin into the mold under pressure at the front end of the forming die. This new pultrusion process not only eliminates the resin impregnation tank, but also keeps the reinforcing material in a dry state before entering the mold. This process method is also called “injection pultrusion process” (IP).

 

This Injection Pultrusion Process has the Following Two Advantages:

 

① The resin components are more accurately dosed and can be continuously metered using a metering pump to avoid errors caused by manual mixing;

 

② The resin impregnation tank is changed from an open form to a fully enclosed form, which greatly reduces the possibility of resin splashing, thereby improving the working environment of the pultrusion process

 

 

Problems Encountered in Phenolic Pultrusion Process

In the polyester resin pultrusion process, due to the use of active diluent styrene, it can act as a cross-linking agent, so no volatile substances will be generated in the pultrusion mold. However, phenolic resin is a condensation resin, and water molecules will be generated during the growth or cross-linking of the resin chain.

 

Since the temperature of the pultrusion cavity is often above 100°C, what will happen during the curing process of phenolic resin and how to discharge water molecules are the first technical key problems that need to be solved in the phenolic pultrusion process.

 

The Impact of Water Released by Phenolic Condensation Reaction on the Pultrusion Process

 

As early as the early 1980s, some European phenolic manufacturers began to develop acid-catalyzed phenolic resins, whose curing temperature is about 30°C-80°C, greatly reducing the curing temperature of 130°C-180°C required for the original phenolic resin. From then on, the research and trial production process of phenolic pultrusion process began.

 

This acid-catalyzed phenolic resin can be well applied to hand lay-up, spraying, low-pressure molding, RTM and fiber winding molding processes. Its products have excellent properties such as good fire resistance, low smoke generation and low toxicity.

 

After actual testing of the pultrusion process, the results of acid-catalyzed phenolic resin under low-temperature curing are completely opposite to the expected results. Specifically, the amount of water released is higher than that of the high-temperature phenolic system; second, according to the measured performance results, the released water does not “destruct” the pultrusion process, but improves some properties of its products.

 

In addition, the high-strength curing phenolic system will turn into water vapor when it is above 100°C during polycondensation and dehydration. Will it have any adverse effects on the pultrusion process? Therefore, can the phenolic resin be pultruded using an acid catalyst at a temperature below 100°C to avoid the boiling phenomenon of the released water?

 

In response to these problems, some people have also conducted phenolic curing tests and found that it is impossible for phenolic resin to be completely cured at a temperature below 100℃ in a molding mold less than 1 meter long.

 

For high-temperature curing phenolic systems, due to the high internal pressure of the pultrusion molding cavity, the boiling point of water will also exceed 100℃, reaching 110℃, or even higher. In the early production of phenolic pultrusion rods, it was found that when the fiberglass rod was demolded, water vapor only came out at the tail end of the mold.

 

Corrosion Resistance of Acid-Catalyzed Phenolic Pultrusion Molds

In the industrial production of phenolic pultrusion molding technology, the first problem encountered is the acid corrosion resistance of the mold. In production practice, it often takes only a few hours for the chrome-plated surface layer to be corroded by acid and peel off from the surface of the tool steel. Some people have attempted to solve the corrosion resistance problem of the mold by adding a suitable internal release agent to the phenolic resin. However, the test results show that after using the internal release agent, the chromium layer and the tool steel mold will still peel off, but the peeling time is extended.

 

Curing and High Viscosity of High Temperature Curing Phenolic Resin

In order to avoid the corrosion of acid catalyzed phenolic resin to the mold, some people have tested the use of high temperature curing phenolic resin in pultrusion process. Some phenolic resins can be cured quickly at 130℃-150℃.

 

For example, the resin layer used for sandpaper can be cured in 5-6 minutes at 130℃. The pultrusion temperature of acid catalyzed phenolic resin is about 180℃. According to the empirical rule that the curing time can be shortened by half if the curing temperature rises by 10℃, the resin cured at 130℃ for 6 minutes can be cured in just a few seconds at 180℃. Therefore, it is entirely possible to use high temperature curing phenolic resin in pultrusion process.

 

Usually, the viscosity of high temperature curing phenolic resin is relatively high, about 4000-6000cP. If fillers need to be added to improve the surface quality of the product, the viscosity will increase, which will have an adverse effect on the pultrusion process.

 

Research and Development of Phenolic Resin Pultrusion Process

 

As mentioned above, there are still many technical problems in the phenolic pultrusion process. In addition, the phenolic pultrusion products are not perfect. However, after continuous exploration and research, it has been found in recent years that single-component phenolic resin is completely suitable for pultrusion molding process.

 

At present, there are two main ways to research and develop phenolic resin pultrusion process:

 

One is to improve the comprehensive properties of single-component phenolic resin, such as heat curing time, viscosity and dehydration, so that it can adapt to the needs of rapid pultrusion process as much as possible.

 

The second is to change the chemical composition of phenolic resin, increase the ratio of phenol/formaldehyde to accelerate its cross-linking speed, and conduct special research on related properties such as viscosity and dehydration of phenolic resin.

 

It has been proved in practice that for phenolic resin, if its curing speed needs to be accelerated, its viscosity and dehydration must also be increased accordingly. This situation is not desirable for pultrusion molding process. For this reason, some people have attempted to find various monomers to change the chemical composition structure of phenolic resin.

 

One of the more successful examples is the use of resorcinol, which accelerates the curing speed without increasing the viscosity and dehydration of phenolic resin. In addition, the research work on phenolic pultrusion process is also looking for a catalyst that can accelerate the curing process in the mold cavity but will not corrode the mold steel.

 

Ideally, a catalyst with very low activity or even zero activity at room temperature can extend the storage time of phenolic resin in the glue tank. In actual use, the catalyst is first added to the glue tank, and then hydrolyzed or decomposed by other reactions under the high temperature conditions of the pultrusion mold to produce the free acid required for the reaction.

 

Since the acid catalyzes the phenolic resin, it will have a certain corrosive effect on the pultrusion mold and the storage tank, thereby affecting the service life of the mold and the storage tank. For this reason, whether alkaline catalysts can be used has also attracted widespread attention in the industry.

 

Since strong alkalis such as caustic soda have strong solubility, they cannot be adapted to the phenolic pultrusion process. After testing, some weak bases that are insoluble or difficult to dissolve at room temperature, but have strong solubility and activity under high temperature conditions in the pultrusion cavity, are very suitable for use as catalysts in the phenolic pultrusion process.

 

At present, some European and American countries have successfully applied phenolic resins to pultrusion molding processes and have formed a certain scale of industrial production. Australia, Japan and some other countries have also begun to enter the industrial trial stage of phenolic pultrusion technology.

 

In industrial production, these countries generally use pultrusion release agents, whether they use high-temperature phenolic molding or acid-catalyzed phenolic molding, which can effectively reduce the acid erosion of pultrusion dies. In addition, some manufacturers often perform necessary hardness treatment on the inner surface of 316# stainless steel molds to achieve the requirements of high smooth surface and wear resistance.

 

In Europe, most of the acid-catalyzed phenolic pultrusion processes are used, and some high-temperature cured phenolic pultrusion processes are also used. In the United States, most of the high-temperature cured phenolic pultrusion processes are used, and the resins used are two varieties, long-chain phenolic resins and modified phenolic resins.

 

Usually, the pultrusion speed of phenolic resin is slower than that of unsaturated polyester resin. After some chemical modification, the pultrusion speed of heat-cured phenolic resin can reach the same level as polyester resin. It can be seen that the length of phenolic pultrusion molding die is much longer than that of other resins, such as polyester, vinyl ester, epoxy, acrylic-urethane system, etc.

 

This will undoubtedly increase some production investment costs, but when a certain production batch is reached, it is appropriate to increase the length of the pultrusion die. Because if a shorter molding die is used, the production efficiency of the pultrusion process will be reduced due to the slower pultrusion speed.

 

At present, it is difficult to accurately count the amount of phenolic resin used in the pultrusion process in various countries around the world. However, according to a recent order for 140,000 square meters of phenolic pultruded corrosion-resistant grilles by a manufacturer, this figure can fully illustrate that the prospects for phenolic pultruded fiberglass products are very broad.

 

 

Performance Test of Phenolic Pultruded FRP Products

According to relevant reports, some of the various properties of phenolic pultruded FRP products have approached, reached, or exceeded the various performance indicators of unsaturated polyester resin pultruded products. The relevant performance indicators are listed as follows:

 

Physical Properties of Acid-Catalyzed Phenolic Pultruded FRP Products

When untwisted roving is used as a reinforcing material, the flexural strength of phenolic pultruded FRP is 675MPa and the flexural modulus is 20GPa. If multi-directional reinforcing materials are used, its flexural strength is 259MPa and the flexural modulus is 9.57GPa; the tensile strength is 329MPa and the tensile modulus is 18.75GPa; the elongation at break is about 1.84%.

 

Physical Properties of Phenolic Pultruded FRP Products Cured by Heating at High Temperature

The glass fiber content of phenolic pultruded FRP products is about 76-77%, which is about 3% higher than the 73-74% of unsaturated polyester resin pultruded products. Its compressive strength is 641MPa, much higher than the 517MPa of unsaturated polyester pultruded products; the interlaminar shear strength is 39MPa, slightly lower than the 41.4MPa of polyester pultruded products; the short arm shear strength is 45.5MPa; the flexural strength (16:1) is 1.12GPa, slightly higher than the 1.06GPa of polyester pultruded products; the flexural modulus (16:1) is about 45GPa; the flexural strength (32:1) is 1.21GPa, higher than the 1.15GPa of polyester pultruded products; the flexural modulus (32:1) is 48GPa.

 

It has been proved that the mechanical properties of phenolic pultruded FRP mainly depend on the matching of glass fiber reinforcement and resin. According to the latest reports, some glass fiber manufacturers have successfully solved the production technology problems in this area, making the performance of phenolic composite materials even better. However, it must be noted that a glass fiber may not be suitable for both phenolic resin systems mentioned above.

 

Some glass fiber coupling agents suitable for high-temperature phenolic resins can be destroyed by acid catalysts and lose their surface treatment effect on glass fibers. Another significant performance of phenolic pultruded FRP products is that they have a very high heat deformation temperature, usually in the temperature range of about 250-300℃. This outstanding performance has opened up broad application prospects for phenolic pultruded products in grid floor panels.

 

Some people believe that although the phenolic pultrusion molding process has been put into practice, it has not yet formed an independent technical system, because the addition of various catalysts, different flame retardants, and the use of different chemical modification methods, these changing factors will lead to various changes in the pultrusion conditions and even the performance of FRP products.

 

The performance of phenolic pultruded FRP products meets relevant international standards

 

After nearly ten years of research and development on phenolic pultrusion technology and its products, in addition to opening up many application fields of phenolic FRP, a relatively consistent understanding has been achieved on the flame retardancy and temperature resistance of phenolic products.

 

Practice has proved that phenolic pultruded FRP products, as long as their resin formula is reasonable, can fully meet the requirements of relevant international standards, such as the requirements of British Standard BS476 Article 6 and Article 7 on building specifications, and can reach the level 0 specified therein; the level 1 standard requirements of ASTM 84 wind tunnel test; the standard requirements of French standards M1 and F1; the standard requirements of German standard DIN131, and the requirements of British standard BS6853 Class I on the critical temperature of smoke, etc.

 

However, for other types of resins, it is difficult to meet the above standards at the same time without necessary chemical modification.