The Surface Veil  (see Figure 8.3) is denser than the chopped strand felt. The resin-rich layer enhanced by the surface felt has better permeability resistance, which can prevent the protruding burrs of the chopped strand felt from extending to the surface of the resin-rich layer to form a corrosion channel with a capillary effect, which can play a better barrier effect and avoid cracking and crazing.

 

The most commonly used glass fiber surface felt is C-type glass fiber surface felt. In some special occasions (mainly fluorine-containing media, hypochlorite media, sodium hydroxide and other alkaline media), organic fibers, mainly polyester polyester fiber surface felt, are used.

 

The heat resistance temperature of polyester surface felt is lower than that of glass fiber surface felt. In the occasions where the curing exothermic peak temperature of chlorinated bridge acid unsaturated polyester resin and phenolic vinyl ester resin is high, the polyester organic surface felt will shrink more, so it is necessary to reduce the influence of stress when laying the surface felt.

 

In some occasions where corrosion is particularly severe, it is recommended to use multiple layers of surface felt to increase corrosion resistance, but the surface density is high. If it is used too much, it will hinder the overflow and compression of bubbles under the surface felt, so special attention should be paid.

 

Carbon Fiber Surface Felt has conductive function and its wear resistance is much better than C-type glass fiber surface felt and polyester surface felt, so it is also used in some special occasions. Other surface felts such as A-type (high alkali) glass fiber and ECR-type (boron-free alkali-free glass) glass fiber are rarely used except in specific occasions, and corrosion resistance tests are also required before use.

 

C-Type Glass Fiber Surface Felt

 

Polyester Surface Felt

Carbon Fiber Surface Felt

 

Polyester Surface Felt is used more and more, so it is introduced separately.

 

Most of the polyester fibers used in composite material processing are polyester surface felt, which are used in processing technologies such as pultrusion, winding, hand lay-up, RTM, molding and vacuum injection. Due to the limitations of strength and cost, polyester surface felt is mostly used as an auxiliary material. It mainly has the following characteristics: wear resistance, corrosion resistance (especially for fluorine-containing media and alkaline media), UV resistance, mechanical damage resistance, surface finish, high-quality surface resin-rich layer, protection of molds during production, etc.

 

Usually, the surface resin-rich layer can be obtained by surface coating, but it can also be done directly by using polyester surface felt. The use of surface felt can also bring convenience and advantages in the following aspects: saving coating time, no risk of stratification, forming a fine surface layer, simple and fast operation method.

 

In the pultrusion process, in order to obtain more perfect surface properties, improve the smooth surface effect of the resin-rich surface layer, enhance the product’s wear resistance, mechanical damage resistance, and UV resistance, and protect the mold during processing to extend the service life of the mold, the use of polyester surface felt is a good choice;

 

In the winding process, the use of polyester surface felt can greatly improve the product’s chemical corrosion resistance, especially in the field of pipeline production, and is widely used in hydrofluoric acid-resistant products;

 

In the hand lay-up process, the use of polyester surface felt can help improve the product’s physical and mechanical properties, improve corrosion resistance, especially resistance to fluorine-containing media, improve surface roughness, and protect operators from glass fiber damage; in the RTM process, the use of polyester surface felt and polyester reinforcement felt will improve the product’s mechanical strength and corrosion resistance.

 

Polyester Surface Felt has the Following Chemical Resistance Properties:

 

①It has good resistance to oxidants, and is particularly resistant to fluorine-containing and alkaline media;

②Under normal circumstances, it can resist high-temperature weak acid aqueous solutions and strong acid aqueous solutions at room temperature, but can be corroded by concentrated sulfuric acid with a concentration of 95%;

③It has good corrosion resistance to strong alkali aqueous solutions at room temperature, but can be corroded by boiling strong alkali solutions.

 

Polyester Surface Felt has good pre-composite properties with glass fiber (glass fiber surface felt, chopped strand mat, continuous mat, multi-axial cloth). The biggest defects of polyester surface felt are insufficient heat resistance and insufficient wrinkle resistance. For this reason, glass fiber and polyester fiber can be pre-compositely formed by sewing to make composite fiber surface felt to meet some special needs. At present, the typical specifications of polyester surface felt on the market are 20g/m2, 30g/m2, 38g/m2, 45g/m2, 60g/m2, etc.