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What are the types of glass fibers commonly used in the pultrusion process?
 July 6, 2022
 
The reinforcing material is the supporting skeleton of the FRP product, which basically determines the mechanical properties of the pultruded product. The use of the reinforcing material also has a certain effect on reducing the shrinkage of the product and increasing the thermal deformation temperature and low temperature impact strength.

In the design of FRP products, the selection of reinforcing materials should fully consider the fiberglass molding process of the product, because the type, laying method and content of reinforcing materials have a great influence on the performance of FRP products, and they basically determine the mechanical strength and elastic modulus of FRP products. The performance of pultruded products using different reinforcing materials is also different.

In addition, while meeting the product performance requirements of the molding process, the cost should also be considered, and cheap reinforcing materials should be selected as much as possible. Generally, the untwisted roving of glass fiber strands is lower in cost than fiber fabrics; the cost of felt is lower than that of cloth, and the impermeability is good. , but the strength is low; the C-glass fiber is cheaper than the E-glass fiber, but with the increase of alkali content, its alkali resistance, corrosion resistance and electrical properties will decrease.
 
The types of commonly used reinforcing materials are as follows:
1. Untwisted glass fiber rovings
 
Using reinforced sizing agent, untwisted glass fiber rovings can be divided into three types: plied raw silk, direct untwisted roving and bulked untwisted roving.

Due to the uneven tension of the plied strands, it is easy to sag, which makes a loose loop at the feeding end of the pultrusion equipment, which affects the smooth progress of the operation.

Direct rovings have the characteristics of good bundling, fast resin penetration, and excellent mechanical properties of products, so most of them tend to use 2400 direct rovings at present.

Bulked rovings are beneficial to improve the transverse strength of products, such as crimped rovings and air-textured rovings. Bulk roving has both the high strength of continuous long fibers and the bulkiness of short fibers. It is a material with high temperature resistance, low thermal conductivity, corrosion resistance, high capacity and high filtration efficiency. Some fibers are bulked into monofilament state, so it can also improve the surface quality of pultruded products. At present, bulked roving has been widely used at home and abroad, as the warp and weft of decorative or industrial woven fabrics. Can be used to produce friction, insulation, protection or sealing materials.

Performance requirements for untwisted glass fiber rovings for pultrusion:
(1) No overhang phenomenon;
(2) The fiber tension is uniform;
(3) Good bunching;
(4) Good wear resistance;
(5) There are few broken heads, and it is not easy to fluff;
(6) Good wettability and fast resin impregnation;
(7) High strength and rigidity.
 
2. Glass fiber mat
 
In order to make pultruded FRP products have sufficient transverse strength, reinforcing materials such as chopped strand mat, continuous strand mat, combined mat, and roving fabric must be used. Continuous strand mat is one of the most commonly used glass fiber transverse reinforcement materials at present. In order to improve the appearance of products, surface mat is sometimes used.

The continuous strand mat is composed of several layers of continuous glass fibers that are randomly laid in a circle, and the fibers are bonded with adhesives. The surface felt is a thin paper-like felt formed by randomly and uniformly laying the chopped strands of fixed length and bonded with an adhesive. The fiber content is 5% to 15%, and the thickness is 0.3 to 0.4 mm. It can make the surface of the product smooth and beautiful, and improve the aging resistance of the product.

The characteristics of glass fiber mat are: good coverage, easy to be saturated with resin, and high glue content.

The requirements of the pultrusion process for the glass fiber mat:
(1)High mechanical strength; 
(2) For chemically bonded chopped strand mats, the binder must be resistant to chemical and thermal effects during dipping and preforming to ensure sufficient strength during the forming process ; 
(3) Good wettability; 
(4) Less fluff and less broken heads.
 
3. Polyester fiber surface mat
 
Polyester fiber surface mat is a new type of reinforcing fiber material in the pultrusion industry. There is a product called Nexus in the United States, which is widely used in pultrusion products to replace glass fiber surface mats. It has good effect and low cost and has been successfully used for more than 10 years.

The advantages of using polyester fiber surface felt:
(1) It can improve the impact resistance, corrosion resistance and atmospheric aging resistance of products;
(2) It can improve the surface state of the product and make the surface of the product smoother;
(3) The application and tensile properties of polyester fiber surface felt are much better than C glass surface felt, and it is not easy to break ends during the pultrusion process, reducing parking accidents;
(4) The pultrusion speed can be increased;
(5) It can reduce the wear of the mold and improve the service life of the mold.
 
4. Glass cloth tape
 
In some special pultrusion products, in order to meet some special performance requirements, fiberglass cloth tape with a fixed width and a thickness of less than 0.2mm is used, and its tensile strength and transverse strength are very good.
 
5. Fiberglass multiaxial fabric
 
The transverse mechanical properties of pultruded composite products are poor. The use of biaxial fabric effectively improves the strength and stiffness of pultruded products.

The warp and weft fibers of this woven fabric are not intertwined with each other, but are intertwined with another woven material, so it is completely different from the traditional glass cloth. The fibers in each direction are in a collimated state and do not form Any bending, and thus the strength and stiffness of the pultruded product, is much higher than that of a composite made of continuous felt.  
 
At present, triaxial braiding technology has become the most attractive and active technology development field in the composite materials industry. According to the load requirements, the reinforcing fiber is directly woven into a structure with a three-dimensional structure, and the shape is the same as that of the composite product it constitutes. The three-way fabric is used in the pultrusion process to overcome the interlaminar shear of traditional reinforcing fiber pultrusion products. It has the disadvantages of low shear strength and easy delamination, and its interlayer performance is quite ideal.
 
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