Understanding the Composite Pultrusion Process

Pultrusion is a method for continuously producing composite profiles. Untwisted glass fiber roving, along with other continuous reinforcement materials and polyester surface mat, is impregnated with resin on a creel. The pultruded product is then passed through a mold that maintains a defined cross-sectional shape, where it solidifies and forms within the mold before being continuously ejected from the mold. This automated process creates pultruded products.

Products produced using pultrusion have higher tensile strength than ordinary steel. The resin-rich surface layer also imparts excellent corrosion resistance, making them an ideal alternative to steel in projects operating in corrosive environments. They are widely used in transportation, electrical engineering, electrical insulation, the chemical industry, mining, marine engineering, marine applications, corrosive environments, and various other areas of daily life and civil engineering.

Pultrusion Process

Pultrusion processes vary widely, and are categorized in a variety of ways. These include batch and continuous processes, vertical and horizontal processes, wet and dry processes, crawler-type and clamp-type processes, in-mold curing and in-mold gel curing, and heating methods such as electric heating, infrared heating, high-frequency heating, microwave heating, or a combination of these.

The typical pultrusion process is as follows:

Glass fiber roving arrangement - impregnation - preforming - extrusion molding and curing - pulling - cutting - finished product

Pultrusion Equipment Components

1. Reinforcement Material Delivery System: Such as creels, mat spreading devices, and yarn holes.

2. Resin Impregnation: The straight trough impregnation method is most commonly used. The fibers and mats should be perfectly aligned throughout the impregnation process.

3. Preforming: The impregnated reinforcements are carefully conveyed through the preforming device in a continuous process to ensure their relative positioning, gradually approaching the final shape of the product,

and extruding excess resin. The preform then enters the mold for molding and curing.

4. Mold: The mold is designed based on the conditions specified by the system. Based on the resin curing exotherm and the friction between the material and the mold, the mold is divided into three different heating

zones, whose temperatures are determined by the properties of the resin system. The mold is the most critical component of the pultrusion process, and typical mold lengths range from 0.6 to 1.2 meters.

5. Tractor: The tractor itself can be a crawler-type puller or two reciprocating clamping devices to ensure continuous motion.

6. Cutting Device: The profile is cut to the required length by an automatically synchronized saw.

The function of the forming die is to compact, shape, and cure the blank. The mold's cross-sectional dimensions should take into account the resin's molding shrinkage. The mold length is dependent on the curing

speed, mold temperature, part size, pultrusion speed, and the properties of the reinforcement material, and is generally between 600 and 1200 mm. The mold cavity should be smooth to reduce friction, extend

service life, and facilitate demolding. Electric heating is typically used, with microwave heating used for high-performance composites. A cooling device is required at the mold inlet to prevent premature curing of the

adhesive. The impregnation process primarily controls the relative density (viscosity) and impregnation time of the adhesive. These requirements and influencing factors are the same as for prepreg.

The curing process, which focuses on controlling the molding temperature, mold temperature distribution, and the material flow time (pultrusion speed), is a key step in the pultrusion process. During the pultrusion

process, the prepreg undergoes a series of complex physical, chemical, and physicochemical changes as it passes through the die, which remain largely unknown. Generally speaking, the mold can be divided into

hree zones based on the state of the prepreg as it passes through the die. The reinforcement material passes through the die at a constant speed, while the resin does not. At the mold entrance, the resin behaves

similarly to a Newtonian fluid. Viscous resistance between the resin and the mold inner surface slows the resin's forward movement, gradually returning to a level comparable to that of the fiber as the distance from

the mold inner surface increases.

As the prepreg advances, the resin undergoes a cross-linking reaction due to heat, reducing viscosity and increasing viscous resistance. It then begins to gel and enters the gel zone. Gradually, it hardens, shrinks,

and releases from the mold. The resin and fiber move forward evenly at the same speed. Curing continues in the curing zone, ensuring the specified degree of cure is achieved by the time the mold is ejected. The

curing temperature is typically greater than the peak of the adhesive's exothermic peak, and the temperature, gel time, and pulling speed are aligned. The preheating zone should be kept low, and the temperature

distribution should be controlled so that the curing exotherm peak occurs slightly later in the mold, with the release point controlled in the middle. The temperature difference between the three zones should be

maintained at 20-30°C, with minimal temperature gradients. The effects of the curing reaction exotherm should also be considered. Typically, three pairs of heating systems are used to control the temperature of each

zone.

Pulling force is crucial for ensuring smooth part ejection. The magnitude of the pulling force depends on the interfacial shear stress between the part and the mold. Shear stress decreases with increasing pulling speed, with three peaks occurring at the mold entrance, middle, and exit. The peak at the entrance is caused by the viscous resistance of the resin at that location. Its magnitude depends on the properties of the resin's viscous fluid, the inlet temperature, and the filler content. Within the mold, resin viscosity decreases with increasing temperature, and shear stress decreases. As the curing reaction proceeds, viscosity and shear stress increase. The second peak corresponds to the release point and decreases significantly with increasing pulling speed. The third peak at the exit is caused by friction between the cured part and the mold wall and has a smaller value. Pulling force is crucial in process control. A smooth part surface requires low shear stress at the release point (the second peak) and early mold release. Changes in pulling force reflect the reaction state of the part within the mold and are related to factors such as fiber content, part shape and size, release agent, temperature, and pulling speed.

Main raw materials for pultruded FRP

Resin matrix

Pultruded FRP mainly uses unsaturated polyester resin and vinyl ester resin, and other resins also use phenolic resin, epoxy resin, methacrylic acid and other resins. In recent years, due to the advantages of phenolic

resin such as fire resistance, foreign countries have developed phenolic resin suitable for pultruded FRP, called second-generation phenolic resin, which has been widely used. In addition to thermosetting resins,

hermoplastic resins are also selected according to needs.

Fiber reinforcement

The fiber reinforcement used in pultruded FRP is mainly E-glass fiber roving. According to the needs of the product, C-glass fiber, S-glass fiber, T-glass fiber, AR-glass fiber, etc. can also be selected. In addition, for

the needs of special-purpose products, synthetic fibers such as carbon fiber, aramid fiber, polyester fiber, vinylon, etc. can also be selected. In order to improve the lateral strength of hollow products, continuous fiber

mats, cloth, tapes, etc. can also be used as reinforcement materials.

Auxiliary materials

(1) Initiator

The characteristics of the initiator are usually expressed by active oxygen content, critical temperature, and half-life. The commonly used initiators at present are:

MEKP (methyl ethyl ketone peroxide)

TBPB (tert-butyl perbenzoate)

BPO (benzoyl peroxide)

Lm-P (special curing agent for pultrusion)

TBPO (tert-butyl peroxyethyl octanoate)

BPPD (diphenoxyethyl peroxydicarbonate)

P-16 [bis(4-tert-butylcyclohexyl peroxydicarbonate)]

In actual applications, single components are rarely used. Usually, two or three components are used in combination according to different critical temperatures.

(2) Epoxy resin curing agents

Commonly used curing agents include anhydrides, tertiary amines, and imidazoles.

(3) Colorants

Colorants in pultrusion are generally pigments. It appears in the form of a paste.

(4) Fillers

Fillers can reduce the shrinkage rate of products, improve the dimensional stability, surface finish, smoothness, and matte or matte properties of products; effectively adjust the viscosity of resins; meet different

performance requirements, improve wear resistance, improve electrical conductivity and thermal conductivity, etc. Most fillers can improve the impact strength and compression strength of materials, but cannot

improve tensile strength; can improve the coloring effect of pigments; some fillers have excellent light stability and chemical corrosion resistance; can reduce costs. It is best to select a gradient in the particle size

of the filler to achieve the best use effect. Now there are also surface treatments for fillers to increase the dosage.

(5) Release agents

Release The release agent has extremely low surface free energy and can evenly wet the mold surface, achieving a demolding effect. Excellent demolding performance is crucial for a smooth pultrusion process.

In the early days of pultrusion, external release agents, such as silicone oil, were commonly used. However, these agents required large amounts and resulted in unsatisfactory surface quality. Internal release agents

are now being adopted.

Internal release agents are added directly to the resin. Under certain processing temperatures, they seep out of the resin matrix and diffuse onto the surface of the cured part, forming a barrier between the mold and

the part, thus providing a release agent.

Internal release agents typically include phosphate esters, phosphophosphate, stearates, and triethanolamine oil. Zinc stearate offers the best release performance. In production, people generally prefer to use

internal mold release agents that are liquid at room temperature. Currently, commercially available internal mold release agents are mostly mixtures of primary and secondary amines, and copolymers of organic

phosphates and fatty acids.

Applications of Pultruded Products

Pultruded products include various rods, flat plates, hollow tubes, and profiles, with a wide range of applications, including the following:

1. Electrical Market

This is the earliest market for pultruded fiberglass. Successful applications include cable trays, ladder racks, brackets, insulating ladders, transformer spacers, motor slot wedges, streetlight poles, third rail guards,

and fiber optic cable core materials. There are many products in this market worthy of further development.

2. Chemical and Anti-corrosion Markets

Chemical anti-corrosion is a major user of pultruded FRP, with successful applications including: FRP sucker rods, cooling tower supports, offshore oil production equipment platforms, walking gratings, stair railings

and supports, structural supports for various chemically corrosive environments, and water treatment plant covers.

3. Consumer Entertainment Market

This market has enormous potential. Currently developing applications include: fishing rods, tent poles, umbrella frames, flagpoles, tool handles, lampposts, railings, handrails, stairs, radio antennas, yacht docks,

and garden tools and accessories.

4. Construction Market

In the construction market, pultruded FRP has penetrated traditional material markets such as doors and windows, concrete formwork, scaffolding, stair railings, partition wall panels, reinforcement, and decorative

materials. It is worth noting that reinforcement and decorative materials will have significant growth potential.

5. Road Transportation Market

Successful applications include: highway barriers, road signs, pedestrian overpasses, soundproofing walls, and refrigerated truck components.