Pultrusion molding process is a special process in the glass fiber reinforced plastic molding process. It is to continuously produce fiberglass products of unlimited length by extruding and curing continuous fiber bundles (glass fibers, carbon fibers, etc.), felt, etc. soaked in resin adhesive solution through molds under the action of traction force. Internal release agent is directly added to the resin, which is compatible with liquid resin but incompatible with cured 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, playing a role in demolding.
Internal release agents generally include phosphoric acid esters, phosphoric acid esters, stearic acid salts, triethanolamine oil, etc. Among them, zinc stearate has a better demolding effect. Due to the high viscosity of the resin, it is difficult to stir evenly when directly adding powdered zinc stearate, and the zinc stearate is loose, bulky, and carries a lot of air, resulting in many bubbles in the resin. So, usually before use, zinc stearate is added to the crosslinking agent to make it a uniform paste, and then added to the resin. The use of zinc stearate as an internal release agent has no significant effect on the color, curing speed, and resin viscosity of the product. In extrusion production, it is usually preferred to use internal release agents that are liquid at room temperature. At present, most of the commercially available internal release agents are mixtures of primary amines, secondary amines, and copolymers of organic phosphates and fatty esters.
The function of release agent is to create strong adhesive force between the formed product and the mold surface during the molding process of the product. In addition, since the pultrusion material enters the mold mouth, with the temperature rising, the resin viscosity decreases and the volume swells. The pressure acting on the mold wall gradually forms, increases and accumulates, and reaches the maximum in the gel zone. In order to prevent the additional load caused by the adhesion of formed fiberglass products on the mold, a layer of isolation film (i.e. release agent) must be applied between the product and the mold so that the product can easily detach from the mold, ensuring the surface quality of the product and the integrity of the mold. All material surfaces have surface free energy. The size varies with different substances.
Generally speaking, the surface free energy of metals is relatively high. Organic matter is also a solid, so the liquid will diffuse and evenly distribute on the surface of the solid. Release agents require extremely low surface free energy to uniformly wet the surface of the mold, forming a low surface energy coating on the mold surface to achieve easy demolding. Because the production of extrusion molding process is a continuous process, excellent demolding effect is the main condition to ensure the smooth progress of extrusion molding process. There are two types of release agents according to their usage: external release agents and internal release agents. The early pultrusion molding process used external release agents, commonly including silicone oil. In use, the release agent is placed in a dedicated groove. When the product is pulled, the release agent is brought to the surface of the glass fiber molded object immersed in resin, and then enters the molding mold for solidification to achieve the release effect. However, the amount of release agent is large and the surface quality of the product is not ideal. Therefore, internal release agent has been used instead. Liquid internal release agents have many advantages in the pultrusion process:
1. Easy to disperse in resin;
2. Has the function of cleaning molds;
3. Can protect metal molds from corrosion;
4. Improve production efficiency while reducing extrusion resistance and mold loss;
5. It can reduce the surface tension of resin mixtures, lower resin viscosity, improve the resin's wettability with reinforcing materials and fillers, and enhance the resin's flowability;
6. Assist in defoaming and improve the surface quality of components to make the product surface smooth;
7. Does not affect the curing characteristics of the resin, does not alter the shelf life of the mixed resin, does not affect the physical and mechanical properties of the product, does not affect the weather resistance of the product, etc.
Due to the fact that most liquid release agents are acidic, the following issues should be noted during use:
1. The use of acid sensitive pigments can cause color changes;
2. When using alkaline fillers such as calcium carbonate, acidic release agents will react with them, causing an increase in the viscosity of the mixture, but it will not affect the release effect;
3. If the filler is aluminum hydroxide, the acidic release agent will not only increase the viscosity of the mixture, but also release water during the curing process, causing problems such as bubbles and cracks. Usually, the initial dosage of internal release agent is 1% of the resin content, and the effective addition range is 0.75-2% based on the resin weight.
Adjustments should be made appropriately according to the actual situation.
1. Thin walled simple profiles can be used in smaller quantities, such as 0.8% or less.
2. Thick walled or complex shaped profiles need to be added more.
3. In high filler systems, the amount of internal release agent added should be increased, but excessive addition of internal release agent can delay curing.
4. In extrusion production, if the resistance is too high and the cause cannot be found, it is necessary to increase the amount of release agent appropriately. When using, attention should be paid to the feeding sequence. Before adding curing agents, fillers, and other resin additives, the internal release agent should be added to the resin system and mixed evenly. This can achieve the best demolding effect.
