Analysis on Pultrusion Process, Raw Materials and Application Areas

 

Pultrusion is a method for continuous production of composite material profiles. It is an automated production process that impregnates the untwisted glass fiber roving and other continuous reinforcement materials, polyester surface felt, etc. on the creel with resin, and then passes through a molding mold that maintains a certain cross-sectional shape, and continuously ejects the mold after curing in the mold, thereby forming a pultruded product.

 

The tensile strength of the products produced by the pultrusion process is higher than that of ordinary steel. The resin-rich layer on the surface also makes it have good corrosion resistance, so it is the best product to replace steel in projects with corrosive environments. It is widely used in transportation, electrical, electrical, electrical insulation, chemical, mining, ocean, boats, corrosive environments and various fields of life and civil use.

 

Pultrusion Molding Process

 

There are many forms of pultrusion molding processes and many classification methods. Such as intermittent and continuous, vertical and horizontal, wet and dry, crawler traction and clamping traction, in-mold curing and in-mold gel and out-mold curing, heating methods include electric heating, infrared heating, high-frequency heating, microwave heating or combined heating, etc.

 

The Typical Process Flow of Pultrusion is:

 

Glass fiber roving arrangement – impregnation – preforming – extrusion molding and curing – traction – cutting – finished product

 

Composition of Pultrusion Equipment

1. Reinforcement Material Delivery System: such as yarn rack, felt spreading device, yarn hole, etc.

 

2. Resin Impregnation: The straight groove impregnation method is most commonly used. During the entire impregnation process, the fibers and felts should be arranged very neatly.

 

3. Preforming: The impregnated reinforcement material passes through the preforming device and is carefully transferred in a continuous manner to ensure their relative position, gradually approaching the final shape of the product, and extrude excess resin, and then enters the mold for molding and curing.

 

4. Mold: The mold is designed under the conditions determined by the system. According to the resin curing exothermic curve and the friction performance between the material and the mold, the mold is divided into three different heating zones, and its temperature is determined by the performance of the resin system. The mold is the most critical part of the pultrusion process, and the length of the typical mold ranges from 0.6 to 1.2m.

 

5. Traction Device: The traction device itself can be a crawler-type puller or two reciprocating clamping devices to ensure continuous movement.

 

6. Cutting device: The profile is cut to the required length by an automatically synchronously moving cutting saw.

 

The function of the forming mold is to achieve the compaction, forming and curing of the blank. The cross-sectional size of the mold should take into account the molding shrinkage of the resin. The mold length is related to the curing speed, mold temperature, product size, pultrusion speed, and the properties of the reinforcement material, and is generally 600-1200mm.

 

The mold cavity should have a high finish to reduce friction, extend service life, and facilitate demolding. Electric heating is usually used, and microwave heating is used for high-performance composite materials. A cooling device is required at the mold entrance to prevent the glue from curing prematurely. The dipping process mainly controls the relative density (viscosity) and impregnation time of the glue. Its requirements and influencing factors are the same as those of prepreg.

 

The curing molding process mainly controls the molding temperature, mold temperature distribution, and the time (pultrusion speed) of the material passing through the mold. This is the key process of the pultrusion molding process. During the pultrusion molding process, a series of physical, chemical, and physicochemical complex changes occur when the prepreg passes through the mold, which is still not very clear.

 

Generally speaking, the mold can be divided into three areas according to the state of the prepreg when it passes through the mold. The reinforcement passes through the mold at a constant speed, while the resin is different. At the entrance of the mold, the behavior of the resin is similar to that of a Newtonian fluid. The viscous resistance between the resin and the inner wall of the mold slows down the forward speed of the resin, and gradually returns to a level equivalent to that of the fiber as the distance from the inner surface of the mold increases.

 

During the forward movement of the prepreg, the resin undergoes a cross-linking reaction when heated, the viscosity decreases, the viscous resistance increases, and it begins to gel and enter the gel zone.

 

Gradually hardens, shrinks and separates from the mold. The resin and the fiber move forward evenly at the same speed. It continues to cure when heated in the curing zone, and ensures that the specified degree of curing is reached when the mold is removed. The curing temperature is usually greater than the peak value of the exothermic peak of the glue, and the temperature, gel time and traction speed are matched. The temperature in the preheating zone should be low, and the temperature distribution should be controlled so that the curing exothermic peak appears later in the middle of the mold, and the detachment point is controlled in the middle of the mold.

 

The temperature difference between the three sections is controlled at 20-30°C, and the temperature gradient should not be too large. The effect of the exothermic reaction of the curing reaction should also be considered. Usually, three pairs of heating systems are used to control the temperature in the three zones.

 

Traction is the key to ensuring the smooth demolding of the product. The magnitude of the traction force depends on the interfacial shear stress between the product and the mold. The shear stress decreases with the increase of the traction speed, and three peaks appear at the entrance, middle and exit of the mold.

 

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 viscous fluid, the temperature at the entrance and the filler content. In the mold, the viscosity of the resin decreases with the increase of temperature, and the shear stress decreases. As the curing reaction proceeds, the viscosity and shear stress increase. The second peak corresponds to the detachment point and decreases significantly with the increase of the traction speed. The third peak is at the exit, which is caused by the friction between the product and the inner wall of the mold after curing, and its value is smaller.

 

Traction force is very important in process control. To make the surface of the product smooth, the shear stress (second peak) at the detachment point is required to be small and to detach from the mold as soon as possible. The change in traction force reflects the reaction state of the product in the mold and is related to the fiber content, product shape and size, release agent, temperature, traction speed, etc.

 

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, thermoplastic resins are also selected according to needs.

 

Fiber Reinforcement Materials

The fiber reinforcement materials used in pultruded FRP are mainly E glass fiber untwisted 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 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 (bis(4-tert-butylcyclohexyl peroxydicarbonate)

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

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

 

(2) Epoxy Resin Curing Agent

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

 

(3) Colorants

Colorants in pultrusion generally appear in the form of pigment paste.

 

(4) Fillers

Fillers can reduce the shrinkage rate of products, improve the dimensional stability, surface finish, smoothness and Flat or matte, etc.; effectively adjust the viscosity of the resin; can 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 the material, but cannot improve the tensile strength; can improve the coloring effect of the pigment; some fillers have excellent light stability and chemical corrosion resistance; can reduce costs.

It is best to have a gradient when selecting the particle size of the filler to achieve the best use effect. Now there is also surface treatment of the filler to increase the dosage.

 

(5) Release Agent

The release agent has extremely low surface free energy and can evenly wet the mold surface to achieve a demolding effect. Excellent demolding effect is the main condition to ensure the smooth progress of the pultrusion process.

 

The early pultrusion process used an external release agent, commonly used silicone oil, etc. But the dosage is large and the surface quality of the product is not good. Ideal, now internal mold release agent is used.

 

Internal mold release agent is directly added to the resin, under certain processing temperature conditions, it seeps out from the resin matrix and diffuses to the surface of the cured product, forming a layer of isolation film between the mold and the product, which plays a demolding role.

 

Internal mold release agents generally include phosphates, egg phosphoric acid, stearates, triethanolamine oil, etc. Among them, zinc stearate has a better demolding effect. In pultrusion production, people usually prefer to use internal mold release agents that are liquid at room temperature. Currently, most of the internal mold release agents on the market are mixtures of primary amines, secondary amines, and copolymers of organic phosphates and fatty acids.

 

Application of Pultruded Products

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

 

1. Electrical Market

This is the earliest application of pultruded fiberglass Market, currently the products successfully developed and applied include: cable tray, ladder rack, bracket, insulating ladder, transformer isolation rod, motor slot wedge, street lamp post, electric railway third rail guard plate, optical fiber cable core material, etc. There are many products worth further development in this market.

 

2. Chemical and Anti-Corrosion Market

Chemical anti-corrosion is a major user of pultruded FRP, and the successful applications include: FRP sucker rod, cooling tower bracket, offshore oil production equipment platform, walking grille, stair handrail and bracket, structural bracket in various chemical corrosion environments, water treatment plant cover, etc.

 

3. Consumer Entertainment Market

This is a market with huge potential. Currently, the products developed and applied include: fishing rods, tent poles, umbrella frames, flagpoles, tool handles, lamp posts, railings, handrails, stairs, radio antennas, yacht docks, garden tools and accessories.

 

4. Construction Market

In the construction market, pultruded FRP has penetrated into the market of traditional materials, such as: doors and windows, concrete formwork, scaffolding, stair handrails, house partition wall panels, reinforcement materials, decorative materials, etc. It is worth noting that reinforcement materials and decorative materials will have great room for growth.

 

5. Road Traffic Market

Successful applications include: barriers on both sides of highways, road signs, pedestrian bridges, sound insulation walls, refrigerated truck components, etc.