Can Thermoplastic Resins also be Pultruded? The Molding Process and Characteristics are as Follows

 

Pultrusion is mainly used to produce composite products and is the most widely used process in the composite industry. In recent years, with the increasing prominence of environmental issues, the use of pultrusion to manufacture continuous fiber reinforced thermoplastic composites has attracted more and more attention.

 

Although thermoplastic pultrusion has the characteristics of strong flexibility and impact resistance, good damage resistance, high damage tolerance, plastic repair, weldability, good biocompatibility, recyclability, no need for curing reaction during molding, fast molding speed and reusability, it has not yet been widely used in commercial applications.

 

The Reason is that this process is restricted by the following disadvantages:

 

Such as high melt viscosity, high molding temperature, solid matrix at room temperature, precise control of cooling and large shrinkage during melt cooling, large fluctuations in product quality, etc.

 

In order to make the pultrusion of thermoplastic materials more widely used, the important task is to develop the most suitable processing technology, reduce costs and improve quality.

 

Since the pultrusion process itself is a typical manufacturing process that can economically produce composite materials continuously, and can realize automated continuous production and a wide range of products, the process has been widely valued in industrially developed countries and has developed rapidly.

 

For example, the publication of the US patent (patent number: US5091036) and Dr. Scott Taylor’s research results on thermoplastic composite materials has brought a breakthrough to the industrial application of thermoplastic composite pultrusion.

 

In general, the key problems encountered in the transition from thermosetting matrix pultrusion to thermoplastic matrix pultrusion mainly include: the matrix is ​​solid at room temperature, poor fluidity (high viscosity) at melting temperature, and large shrinkage when the melt is cooled. At present, the typical research results and progress of pultrusion of thermoplastic resin-based composite materials can be summarized as follows.

 

Production Process

 

Due to the high viscosity of thermoplastic resin melts and the difficulty of impregnation, the key point of improving research work is concentrated on the impregnation technology, and the fundamental difference between different pultrusion processes lies in the difference in impregnation methods and impregnation processes.

 

Generally, thermoplastic composite pultrusion processes can be divided into two categories according to the impregnation technology: non-reactive pultrusion processes and reactive pultrusion processes. From the current application situation, non-reactive processes are the main ones, widely used, and relatively mature. Figure 1 is a schematic diagram of thermoplastic composite pultrusion processes using two different methods.

 

 

Process and characteristics of thermoplastic resin-based composite pultrusion molding process

 

Non-reactive Pultrusion Process

 

Melt Impregnation

The impregnation method is generally to allow uniformly dispersed, pre-tensioned continuous fiber bundles to pass through a series of wheel trains to fully impregnate the fibers in the molten resin. In order to improve the permeability, a certain pressure is usually added, or a modified component (or plasticizer) of the same type with a low relative molecular weight is mixed in.

 

This process is currently relatively mature, with the advantages that the fibers are not easy to entangle during impregnation, and can process all plastic materials that can melt and flow.

 

Solvent Impregnation

This method is to select a suitable solvent, or a mixed solvent composed of several solvents, to completely dissolve the resin to obtain a low-viscosity solution, and then use it to impregnate the fibers, and then evaporate and recover the solvent to obtain prepreg. This method overcomes the disadvantage of high melt viscosity of thermoplastic resins and can impregnate fibers well.

 

This process also has some shortcomings, such as the high cost of solvent evaporation and recovery, and easy to pollute the environment; if the solvent is not completely removed, bubbles and pores will often form in the composite material.

 

Therefore, the composite material processed by this method will have some impact on its solvent resistance during use.

 

Powder Impregnation

The powder impregnation preparation technology is to adsorb the resin fine powder on the surface of the fiber filaments in the fiber bundle through electrostatic action in the vulcanized bed, and then heat the powder to fuse on the surface of the fiber, and finally make the fiber wet during the molding process. The processing process is not limited by the viscosity of the matrix, and high relative molecular mass polymers can be distributed into the fiber. This process has less fiber damage and no polymer degradation, and has the potential advantage of low cost. The diameter of the resin powder suitable for this technology is preferably 5~10 ìm.

 

Mixed Untwisted Roving Method

This method is to spin thermoplastic resin into fiber or film tape, and then according to the amount of glue content, a certain proportion of reinforcing fiber and resin fiber bundles are tightly combined into mixed yarn, and then the resin fiber is melted into a matrix through a high-temperature sealed impregnation zone. The advantage of this method is that the resin content is easy to control and the fiber can be fully impregnated.

 

Reactive Pultrusion Process

 

Prepolymer Pultrusion Process

The resin used in this process is a mixture of monomer and initiator and then heated and melted. Due to the small initial relative molecular weight, low viscosity and good fluidity of the prepolymer, the fiber is impregnated and reacted with it, so as to achieve an ideal impregnation effect. This process requires a fast monomer polymerization speed and the reaction needs to be precisely controlled. This process is suitable for pultrusion molding of glass fiber, carbon fiber, nylon and other fiber-reinforced thermoplastics, and the resin has a wide range of applications. The prepolymer pultrusion process flow is shown in Figure 2.

 

 

Process and Characteristics of Thermoplastic Resin-based Composite Pultrusion Molding Process

 

Reaction Injection Pultrusion Process

Compared with the usual pultrusion process, the reaction injection (RIM) pultrusion process is characterized by: during the pultrusion process, the resin component is directly injected into the resin impregnation cavity or the entrance of the pultrusion die to be impregnated with the reinforcing material, and then formed by the heated mold. It is actually a very distinctive process formed by combining the pultrusion process with the (R I M) molding process.

 

During the RIM pultrusion process, the resin system is generally divided into two components, A and B, and each component will not react by itself. After preheating, the two components A and B of the resin system are sent to the resin mixing unit through a metering pump. After being fully mixed, they are directly introduced into the resin impregnation cavity or the mold entrance to impregnate the reinforcing material.

 

The reinforcing material is also preheated, which ensures that the resin impregnation is carried out under high temperature conditions. At this time, the viscosity of the resin system is extremely low, so the fiber permeability is extremely good; the mixing and use of the resin components are carried out at the same time, and there is no problem of the resin life. Since the resin mixing unit is close to the mold entrance, it is possible to use a fast-curing or solid resin system at room temperature.

 

The RIM pultrusion process is shown in Figure 3

 

 

Process and Characteristics of Thermoplastic Resin-based Composite Pultrusion Process

 

In-situ Pultrusion Process

This pultrusion process is actually similar to the RIM process. The difference is that the resin prepared online is directly injected into the pultrusion mold.

 

Production Equipment

Compared with thermosetting resin pultrusion equipment, thermoplastic resin pultrusion requires the addition of some specific equipment: such as impregnation station (powder or melt tank), special mold and cooling section. The powder impregnation and melt pultrusion processes in thermoplastic resin reactive pultrusion and non-reactive pultrusion also require preheating of the reinforcing fibers (as shown in Figure 4).

 

The University of Ghent, Department of textiles abroad recently designed and developed a complete set of production lines for thermoplastic fiber reinforced pultrusion, in which the added preheating device can effectively improve the dispersion performance of the reinforcing fibers and resin, and the melt viscosity, the number of reinforcing fibers, the initial temperature of the material and the size of the traction force are the determining factors of the length and temperature of the preheating device.

 

 

Figure 4 Schematic diagram of improved thermoplastic resin pultrusion production process flow

 

Comparison of pultrusion process equipment for thermoplastics and thermosetting plastics

 

(1) Material aspect Thermosetting plastic pultrusion:

The reinforcing fiber and resin are mixed evenly during the pultrusion process through the impregnation tank; Thermoplastic pultrusion: The fiber and resin that have been pre-impregnated in advance are directly pultruded.

 

(2) Mold aspect Thermosetting plastic pultrusion:

Thermoplastic pultrusion:

Thermoplastic pultrusion: Thermoplastic pultrusion: Thermoplastic pultrusion is cured by heating in the mold; Thermoplastic pultrusion: Thermoplastic pultrusion is cured by cooling in the mold, and there is a heating device in the mold to ensure that the fiber and resin are evenly dispersed.

 

Characteristics of thermoplastic pultrusion molds

 

Thermoplastic pultrusion molds are the core components of the entire production line. The process steps such as product hardening, molding and cooling are all carried out in the mold through pressure and temperature transfer. The requirements for thermoplastic pultrusion molds are generally derived from the basic requirements for thermosetting pultrusion molds, such as good wear resistance, high surface finish of the mold cavity, dimensional stability, easy operation and zoned temperature control. These goals can currently be achieved by using chrome-plated tool steel, double-layer design and electric heating elements.

 

It is usually required that the mold cavity should be polished and hard chrome-plated, the mold hardness should reach HRC70, and the roughness Rz<0.4. The length of the hardening zone depends on the thermal compensation process or the required hardening time. According to literature reports, the mold hardening zone can range from 50, 500 to 1000mm.

 

Studies have shown that a mold length of 300 to 500mm can give the pultruded profile the best shape and mechanical properties.

 

Usually, the thermoplastic pultrusion mold should be divided into different temperature zones (as shown in Figure 5), which are mainly divided into heating and cooling parts. The heating part is to ensure uniform distribution of fibers and resins. The longer heating section can ensure sufficient pressure to make the resin/glass fiber flow and mix fully, while the cooling part is to ensure the dimensional stability of the product. This thermal isolation can ensure a suitable temperature gradient along the length of the mold, and mandrels of different diameters can be used to adapt to the difference.

 

Figure 5 Schematic diagram of thermoplastic resin pultrusion die structure

 

Thermoplastic resin-based pultrusion molding has entered the practical stage abroad, but it is still in the initial exploration and application stage in my country. To accelerate the development and application of this molding technology, the key point is to fully master the impregnation of reinforcing fibers and the correct selection of process parameters, and to improve the stability of production equipment.