Research Progress in Pultrusion Technology of Fiber Reinforced Thermoplastic Composites

 

 

Pultrusion is a continuous production process for thermoplastic composite materials with fixed cross-sections. It has the advantages of high raw material utilization, high production efficiency, low scrap rate, strong product reproducibility, and designability. It has been widely used in lightweight automobiles, building materials, wind turbine blades and other fields. The thermoplastic resin matrix is ​​solid at room temperature and has poor fluidity in the molten state, which makes it difficult to impregnate the fiber, becoming a bottleneck in the development of this type of molding process. Therefore, the key to improving the pultrusion process is concentrated on the fiber impregnation technology.

 

This paper reviews the research progress of fiber-reinforced thermoplastic composite pultrusion process, and divides the thermoplastic composite pultrusion process into non-reactive pultrusion process and reactive pultrusion process according to different impregnation methods. The impregnation characteristics, preparation process and process optimization scheme of each molding process are introduced, and the influence of different fiber impregnation methods in the pultrusion process on the quality of the product is explained. Finally, the existing problems of the pultrusion process are discussed, and the future development trend of the fiber-reinforced thermoplastic composite pultrusion process is prospected, providing a reference for in-depth research and pioneering innovation of the pultrusion process in the future.

 

Non-Reactive Pultrusion Process

 

The non-reactive pultrusion process uses fully polymerized thermoplastic resin as raw material, impregnates the fiber by melt impregnation, solvent impregnation, powder impregnation, mixed roving method, etc., and finally pultrudes into the required composite material profile.

 

Some impregnation processes in the non-reactive pultrusion process are similar to thermosetting composite materials. The processing technology is more mature, the production efficiency is high, and the processing cost is low. It is widely used in the production of composite materials.

 

Melt Impregnation Pultrusion Process

The main purpose of the melt impregnation pultrusion process is to impregnate each fiber by infiltrating the reinforcing fiber bundle with molten polymer. The viscosity of thermoplastic polymer in the molten state is usually greater than 100Pa·s. During the melt impregnation process, the polymer melt has high viscosity, low flow permeability in the fiber bundle, and poor fiber impregnation effect. Therefore, a certain pressure and temperature need to be applied during the impregnation process to improve the fiber impregnation effect.

 

In order to solve the problem of difficulty in impregnating fibers with polymer matrix in melt impregnation, researchers have done a lot of work. [Liu et al.] confirmed that increasing the impregnation rate of resin into fibers can effectively improve the mechanical properties of composite materials through dynamic mechanical analysis, mechanical tests and scanning electron microscopy analysis of composite cross sections. In order to improve the fiber impregnation effect, [Wang et al.] prepared polyoxymethylene (POM)/basalt fiber composites using a special resin impregnation device. The pultrusion molding process is shown in Figure 1.

 

This research work prepared POM melt through a twin-screw extruder, fed the continuous basalt fiber bundle into the impregnation device including multiple thermocouples and rollers, and pulled the continuous fiber bundle impregnated with polyoxymethylene melt to a customized pelletizer after air cooling; the thermoplastic composite obtained by this process has better mechanical properties than short fiber reinforced materials and retains the ability of injection molding

 

 

 

Figure 1 Schematic diagram of melt impregnation pultrusion process for POM/basalt fiber composites

 

Combined the melt impregnation pultrusion process with 3D printing technology and compared the relationship between the structural characteristics and mechanical properties of basalt/polylactic acid composites prepared by melt impregnation pultrusion 3D printing and compression molding (as shown in Figure 2). The results showed that the porosity of the composites prepared by melt impregnation pultrusion 3D printing was higher than that of the compression molding process, but the density of the composites printed by 3D printing was lower, so the composites prepared by the two processes had similar specific strengths.

 

 

Figure 2 Effect of microstructural characteristics on the mechanical properties of the final composite material: (a) void content of 3D printed and molded composite materials; (b) stress-strain curves of 3D printed and (c) molded composite materials; (d) tensile strength, (e) density and (f) specific strength curves of 3D printed and molded composite materials

 

[Sommacal et al.] also proved the above conclusions, using X-ray tomography to study the pores and fiber distribution in 3D printed carbon fiber/polyetheretherketone composite materials, and the results are shown in Figure 3. This study shows that 3D printing does not remove the pores in the raw materials and the porosity is independent of the printing parameters. Therefore, the biggest problem with the melt impregnation pultrusion process is that the internal defects caused by the poor impregnation effect limit the improvement of its mechanical properties. The researchers also improved the design of the mold and increased the flow rate of the melt in the mold to improve the impregnation effect of the fiber.

 

 

 

Figure 3 X-ray tomography of raw materials and printed samples

 

Developed an extrusion and pultrusion system as shown in Figure 4. The geometry of the die was designed to be streamlined to increase the flow rate of the melt and prevent the thermally degraded melt from flowing back into the melt; the end surface of the composite material was observed using a scanning electron microscope (SEM), and it was found that the carbon fiber and the matrix were well infiltrated. However, further research is needed on factors that affect the quality of composite materials, such as the effects of fiber processing, resin type, and process parameters on the quality of composite materials.

 

 

 

Figure 4 Schematic diagram of extrusion-pultrusion machine

 

The melt impregnation pultrusion process is a relatively economical process. This process does not require expensive funds to remove and recover solvents, there is no occasional residual solvent to form pores in the material, and it is suitable for any thermoplastic polymer matrix. However, this process has the problems of high melt viscosity, difficulty in fiber impregnation, and the polymer matrix being in a molten state for a long time, which may cause thermal degradation of the polymer. By improving the impregnation process or using a special impregnation device instead of the traditional impregnation method, optimizing the mold shape and the fiber conveying mechanism to achieve uniform fiber spreading, so as to ensure uniform fiber impregnation.

 

Solvent Impregnation Pultrusion Process

The solvent impregnation pultrusion process uses a suitable solvent to completely dissolve the resin, thereby reducing the viscosity of the resin, and finally allowing the reinforcing fiber and the resin to be fully impregnated. This process successfully solves the problem of thermal degradation of the resin in the melt impregnation pultrusion process when it is in a molten state for a long time. The most important link in the solvent impregnation pultrusion process is to remove the solvent added to the prepreg to prevent the solvent residue from causing bubbles and pores in the composite product, which ultimately affects the mechanical properties of the composite product.

 

The solvent used in the solvent impregnation pultrusion process is usually alcohol or acetone. [Akbar et al.] reduced the viscosity of the resin by adding ethanol during the prepreg pretreatment, so that the reinforcing fibers can be fully impregnated with the resin matrix, effectively improving the impregnation effect. The resin in the prepreg produced by this process is evenly distributed, which will not have an adverse effect on the alignment of the fibers, solving the problems of uneven distribution of prepreg resin and fiber dislocation in the old production method.

 

[Larson et al.] designed a laboratory-scale solvent impregnation prepreg manufacturing machine. The research team successfully realized the automation of the prepreg processing device, providing equipment support for subsequent research on new prepreg resin systems.

 

[Cogswell et al.] invented a plasticizer, using a fiber-reinforced polymer method to form a prepreg with 60 continuous filaments through a mixed melt containing a thermoplastic polymer and a plasticizer. The plasticizer remains stable at the melt temperature and has a volatile property. This type of plasticizer is a high-boiling point material. This material is usually solid at ambient temperature and easily condenses on the cooling surface and can be scraped off for recycling.

 

The solvent impregnation pultrusion process uses solvents to dissolve polymers to reduce their viscosity and thus improve the impregnation effect of the fiber. However, most thermoplastic polymers currently do not have effective solvents, which greatly limits the application of this type of molding process. In addition, this process places high demands on the volatility, recyclability and environmental protection of the solvent. Therefore, researchers focus their research on the development of solvents and the recovery of solvents.

 

Powder Impregnation Pultrusion Process

 

In view of the difficulties in fiber impregnation in the melt impregnation pultrusion process, the easy thermal degradation of the matrix, the lack of effective solvents in the solvent impregnation pultrusion process, and the volatilization of solvents, the powder impregnation pultrusion process (as shown in Figure 5) came into being. The powdered resin is evenly distributed in the reinforcing fiber, which greatly shortens the distance of the resin impregnating the internal fiber and improves the fiber impregnation effect.

 

This type of pultrusion process has more advantages. For example, the powdered polymer requires a shorter residence time during the heating process, which reduces the risk of polymer thermal degradation and production costs and improves production efficiency. The local melt flow of the polymer during the consolidation process improves fiber wettability, reduces fiber damage, and improves the mechanical properties of composite products.

 

 

 

Figure 5 Powder Impregnation

 

The advantages of powder impregnation pultrusion process are self-evident. In order to further optimize the process parameters, [Sala et al.] established a numerical model of powder impregnation pultrusion process and verified the reliability of the model through experiments. The results show that the pultrusion force increases with the increase of pultrusion speed and the increase of curing temperature. In addition, considering the porosity factor of the product will further improve the accuracy of the numerical model.

 

[Kerbiriou et al.] further studied the influence of different process parameters on the performance of pultruded beams made of polybutylene terephthalate (PBT) powder impregnated glass fiber bundles (GF/PBT), revealing the influence of temperature in the preheating zone, temperature and pressure distribution in the heating module, cooling conditions and stretching speed on product quality. By establishing an impregnation model, the optimal process parameters were found.

 

Developed a dry coating production process using powder impregnation method, using carbon fiber reinforced polypropylene (CF/PP) prepreg and glass fiber reinforced polypropylene (GF/PP) tow, and processed them into composite materials through powder impregnation pultrusion process (as shown in Figure 6).

 

The internal structure of CF/PP and GF/PP samples was observed using a scanning electron microscope (SEM), and it was found that the carbon fibers and glass fibers had good adhesion to the polymer powder particles. The process parameters were optimized using the Taguchi experimental design method to achieve uniform distribution of the polymer powder on the fibers.

 

Figure 6 Schematic diagram of pultrusion process

 

Hybrid Untwisted Roving Pultrusion Process

 

The hybrid untwisted roving method is to spin a certain proportion of reinforcing fibers and resin fiber bundles or film tapes tightly into mixed yarns, and then melt the resin fibers through a high-temperature sealed impregnation zone to achieve full fiber impregnation.

 

There are four main types of common mixed yarns (as shown in Figure 7).

 

The first is parallel mixed yarn: it is composed of two whole bundles of reinforcing fibers and thermoplastic fibers side by side.

 

The second is blended yarn: reinforcing fibers and thermoplastic fibers are evenly mixed together.

 

The third is powder impregnated yarn: reinforcing fibers that introduce thermoplastic powder into the yarn.

 

The fourth is precured yarn: fibers pre-impregnated with thermoplastic polymers.

 

The reinforcing fibers and polymer fibers are mixed together, which shortens the impregnation distance, facilitates fiber impregnation treatment, and makes the impregnation more uniform.

Figure 6 Schematic diagram of pultrusion process

 

Hybrid Untwisted Roving Pultrusion Process

 

The hybrid untwisted roving method is to spin a certain proportion of reinforcing fibers and resin fiber bundles or film tapes tightly into mixed yarns, and then melt the resin fibers through a high-temperature sealed impregnation zone to achieve full fiber impregnation.

 

There are four main types of common mixed yarns (as shown in Figure 7).

 

The first is parallel mixed yarn: it is composed of two whole bundles of reinforcing fibers and thermoplastic fibers side by side.

 

The second is blended yarn: reinforcing fibers and thermoplastic fibers are evenly mixed together.

 

The third is powder impregnated yarn: reinforcing fibers that introduce thermoplastic powder into the yarn.

 

The fourth is precured yarn: fibers pre-impregnated with thermoplastic polymers.

 

The reinforcing fibers and polymer fibers are mixed together, which shortens the impregnation distance, facilitates fiber impregnation treatment, and makes the impregnation more uniform.

 

Figure 7 Commonly used hybrid yarns, parallel hybrid yarns, mixed yarns, powder impregnated yarns, and precured yarns for pultrusion

 

The used carbon fiber/polyetheretherketone (PEEK) composites for compression molding. The flexural strength and stiffness of the parts were 55% and 32% higher than those of the pultruded parts, respectively, which indicates that the fibers were damaged during the pultrusion process and the fiber impregnation had a high porosity. In order to further study the fiber damage and fiber impregnation rate,

 

Studied the effect of the uniformity of the distribution of the reinforcing fibers in the polymer matrix of the blended structure on the mechanical properties of the composite material. [Lapointe et al.] also prepared carbon/polyetheretherketone composites by hybrid yarn pultrusion and studied the manufacturing parameters that affect fiber damage. The pultrusion equipment used is shown in Figure 8. It was found that the yarn tension had the greatest effect on fiber damage. Keeping the yarn under a certain tension can avoid the dislocation of the carbon fiber filaments caused by the increase in volume after heating of the polymer and cooling crystallization, thereby reducing fiber damage.

 

Through experiments, it was concluded that the best rod material is to use blended yarn, which is made by a multi-mode system at a pultrusion speed of 50mm/min and a mold temperature of 400℃. The porosity of the composite material is 1%. Figure 9 is an electron microscope image of the best impregnation cross section of the multi-mode test and the pre-cured tape test. Subsequent research work can be carried out around high traction speed and improved vacuum system to test whether better rods can be made under higher levels of vacuum conditions, while ensuring product quality and improving the production efficiency of the pultrusion process.

 

With the development of computer technology, modeling and simulation methods have been applied to the optimization of experimental parameters to further improve molding efficiency and reduce R&D costs.

 

It used geometric modeling methods to study the geometric structure of braided yarns inside thermoplastic composites (as shown in Figure 10), simulated the friction between fibers, the braiding angle and other factors on the molding of composite materials, predicted the mesoscopic morphology of reinforcing fibers, and verified the model by X-ray tomography, and determined the parameters affecting the performance of composite products.

 

This method reduces research costs, provides data support for experimental results, and can be widely used in the study of composite pultrusion process parameters.

 

 

Fig.8 Schematic cross-section of the laboratory-scale pultrusion equipment used during the experiment

 

 

Cross-sectional areas at 50× magnification showing the best impregnation results from the multi-mold trials and (b) the pre-cured tape experiment.

 

 

 

Figure 10 Effect of fiber friction on entanglement state and microscopic morphology: Relationship between friction dissipation energy and (a) fiber elongation, (b) fiber diameter and (c) braiding angle; (d-f) Comparison of braided sections in the middle of the braiding pitch; (g-i) Quantitative analysis of yarns in a certain section in the middle of the braiding pitch; (j) Comparison of yarn aspect ratios; (k) Comparison of clamping angles (μ is 0.1, 0.3 and 0.6)

 

The advantage of the hybrid roving pultrusion process is that the resin content is easy to control and the fibers can be fully impregnated. In this method, the process parameters such as yarn tension, mold temperature, and pultrusion speed have a great influence on the performance of the final composite product. The process parameters can be optimized with the help of heat transfer, finite element and other methods to improve product quality.

 

Reactive Pultrusion Process

 

In order to solve the problem of high viscosity of thermoplastic polymer and difficulty in fiber impregnation in non-reactive pultrusion process, researchers have combined thermoplastic composite pultrusion and reaction injection molding (RTM) to form a reactive pultrusion process based on the pultrusion process of thermosetting composite pultrusion and injection molding.

 

The main difference between reactive pultrusion and non-reactive pultrusion is that the non-reactive pultrusion process mainly changes the viscosity of the polymer matrix by physical means (heating, pressurization, etc.), while the polymer matrix in the reactive pultrusion process is polymerized in real time by chemical reaction. The polymerized thermoplastic polymer matrix can be directly injected into the mold for impregnation with the fiber due to its advantages of high temperature, low viscosity and good fluidity. The reactive pultrusion process is divided into reactive injection pultrusion and in-situ pultrusion according to the different positions of resin polymerization.

 

Reaction Injection Pultrusion Process

 

The reaction injection pultrusion process can continuously produce composite materials of unlimited length with high production efficiency (as shown in Figure 11). The process uses continuous fibers as reinforcements, mixes reactive thermoplastic resins and adds them to the resin impregnation cavity or the entrance of the pultrusion die. The fibers are impregnated and polymerized to form macromolecular polymers, which are then cooled at the rear end of the die and directly pultruded into profiles of the desired shape

 

 

 

Figure 11 Schematic diagram of reaction injection pultrusion process

 

The characteristics of the reaction injection pultrusion process are: the reactive resin is divided into two components, A and B. The two components A and B will not react separately. After they are heated and sent into the resin mixing unit for full mixing, they are impregnated and polymerized with the reinforcing fibers. At this time, the resin is at high temperature and has low viscosity, so it is easier to impregnate the reinforcing fibers.

 

In order to achieve complete impregnation of the fibers and high-speed pultrusion, [Chen et al.] studied and optimized the process parameters such as injection chamber temperature, pultrusion die temperature, and pultrusion speed, and developed a new thermoplastic reaction injection pultrusion test line, successfully preparing glass fiber reinforced polyamide composites with a fiber content of up to 70%.

 

[Epple et al.] used the reaction injection pultrusion molding process to manufacture glass fiber reinforced polyamide (PA6) composites. The results show that by selecting appropriate concentrations of activators and catalysts, the reaction injection pultrusion process can produce continuous glass fiber reinforced (PA6) composite parts with good surface quality, as shown in Figure 12.

 

However, the optimization of process parameters often requires a lot of manpower and material resources. In order to better analyze the effect of additives on the curing dynamics of resins, [Su-ratno et al.] proposed a heat transfer model for the pultrusion process of carbon/epoxy composites, and used this model to analyze the heat transfer problem in the pultrusion process.

 

[Vedernikov et al.] conducted DSC analysis and numerical simulation on the resin composition, established a kinetic model, and simulated and analyzed the effects of process parameters and additives on the polymerization behavior of resins.

 

[Liu et al.] used the finite element method to numerically simulate the thermochemical problems in the pultrusion process of materials, optimized the heating system of the mold, and achieved uniform curing of the pultruded profiles. The development of computer technology and finite elements has successfully realized the simulation analysis of process parameters, greatly saving the time and cost of process design and improving the molding quality of composite materials.

 

Figure 12 Glass fiber reinforced PA-6 composite material prepared by reaction injection pultrusion

 

The reaction injection pultrusion process is characterized by simultaneous impregnation and polymerization, which requires the resin matrix to react quickly to form a polymer. The researchers successfully improved the reaction rate of the matrix by optimizing factors such as the concentration of additives in the matrix, and obtained composite products with high surface quality. With the rapid development of computer technology, modeling and simulation technology has begun to be applied to the analysis of the influence of process parameters on resin polymerization reaction, effectively reducing the cost of optimizing process parameters and improving production efficiency.

 

In-situ Pultrusion Process

 

In-situ pultrusion process is another common pultrusion process for high-efficiency manufacturing of composite materials. This type of composite material is easily affected by the surrounding humidity during the molding process, so the fiber cannot be impregnated through the resin bath, and the reaction must be carried out in the pultrusion die, as shown in Figure 13.

 

During the pultrusion process, the resin is directly injected into the mold, and a polymerization reaction occurs inside the mold and is fully impregnated with the fiber. Usually, heating zones of different temperatures are provided along the mold direction according to the type of resin, the drawing speed and the length of the mold, and finally pultrusion forms a composite product with the same cross-sectional profile as the mold.

 

The in-situ pultrusion process is similar to the reaction injection pultrusion process, but the difference is that the resin in the reaction injection pultrusion process is impregnated with the fiber outside the mold, while the in-situ pultrusion process injects the resin prepared online directly into the pultrusion mold, which greatly improves the impregnation efficiency and production rate.

 

Figure 13 In-situ pultrusion process flow scheme

 

In order to overcome the high viscosity of the thermoplastic matrix in the molten state, [Zoller et al.] used a new liquid thermoplastic resin in-situ pultrusion process to prepare continuous glass fiber reinforced acrylic composites. The mechanical properties and characterization test analysis of the obtained unidirectional pultruded profiles showed that the fibers showed good mechanical properties along the 0° direction, with bending, tensile and compressive strengths exceeding 1000MPa, and the compressive strength was particularly outstanding, as shown in Table 1.

 

The tensile section of the pultruded glass fiber/acrylic thermoplastic composite was studied by scanning electron microscopy, and it was observed that the fiber bundles were evenly distributed in the acrylic thermoplastic matrix, with a porosity of only 0.42%, and the acrylic thermoplastic resin was recyclable and could be used again to manufacture composite parts of this type of pultrusion process, meeting the standards of the circular economy.

 

In order to study the effect of fiber heating method on the performance of composite materials, [Thieleke et al.] prepared a fiber preheating device (as shown in Figure 14), and produced flat profiles by conducting experiments with and without preheating the fibers.

 

The study showed that the mechanical properties of the flat profile with fiber preheating increased from 46% to 79%, the residual low polymer content was significantly reduced by 80%, and the geometric accuracy of the cross-sectional shape was enhanced. The mechanical properties and surface quality of the tubular pultruded product can be significantly improved by controlling the fiber preheating temperature and pulling speed, as shown in Figure 15

 

 

Table 1 Mechanical and thermal properties of ELIUM© C595 E-glass pultruded UD profiles (flexural strength at 0° and 90° to fiber direction; tensile and compressive strength at 0° to fiber direction; glass transition temperature (Tg); residual monomer content; fiber content; and porosity)

 

Figure 14 In-situ pultrusion process with fiber preheating

 

Figure 15 Uniform fiber distribution on the tubular section of sample PR-5

 

The in-situ pultrusion process can use a variety of reinforcing fibers, especially unidirectional fibers. The process requires that the reinforcing fibers can quickly form polymers after being infiltrated with low-viscosity liquids through chemical reactions, and the reaction must be carried out in a closed space to prevent impurities from interfering with the chemical reaction. Therefore, the research direction of the in-situ pultrusion process focuses on developing high-performance reactive polymers to reduce production costs. The quality of composite products can also be improved by optimizing process parameters. For example, the use of fiber preheating can significantly improve the mechanical properties and surface quality of composite products.

 

Conclusion and Outlook

 

Compared with other composite manufacturing processes, the advantages of the pultrusion process are high production efficiency, low cost, good surface quality and the ability to produce almost infinitely long profiles. Combining thermoplastic composites with pultrusion technology is one of the most effective methods for preparing composite materials with predetermined cross-sections.

 

At present, the research from raw material research and development and pultrusion process optimization to composite material performance characterization and application is becoming more and more in-depth, but there are still some problems to be solved, mainly including the following aspects:

 

For Melt Impregnation Pultrusion Process

Most thermoplastic resins have high viscosity and need to be heated and pressurized to improve the fiber impregnation efficiency. In order to reduce the process conditions, appropriate solvents are added to the resin to reduce the resin viscosity, but it will cause the problem of difficulty in removing the solvent in the later stage. At present, the existing solvents need to be further improved and upgraded.

 

For Reactive Pultrusion Molding Process

Multiple factors such as process parameters and initiator concentration have a great influence on the performance of composite materials. The quantification and optimization of such factors often require a lot of manpower and material resources. With the development of machine learning algorithms, integrated analysis of various types of data has become possible. How to effectively use modeling and simulation methods to quickly obtain effective related factor parameters is the key to significantly reduce time and material costs and significantly improve design quality. Therefore, the research on the method of simulation analysis of reactive pultrusion process parameters can provide a strong basic support for the further widespread application of pultrusion molding process.

 

Recyclability and Weldability

The recycling technology of thermoplastic composites is more mature than that of thermosetting composites, but there is still a big gap between my country and developed countries in the recycling of thermoplastic composites. Under the strategic background of my country’s green economic development with carbon peak and carbon neutrality, the production of composite materials faces the challenges of low-carbon, environmental protection and recyclability. In the future, the demand for high-performance and high-quality thermoplastic composites will continue to increase, so how to make full use of the recyclability and weldability of thermoplastic composites is a difficult problem that needs to be overcome in the future.