Five factors affecting the application effect of fillers in plastics

1. Oil absorption value

The oil absorption value is also called the resin adsorption, which is an index of the resin absorption by the filler. In practical applications, most fillers use the oil absorption value as an indicator to roughly predict the filler's demand for resin. For fillers with the same particles, the oil absorption value of filler particles with voids is higher than that of filler particles without voids, so the amount of fillers with small oil adsorption can be increased in the resin.

The oil absorption value has a certain guiding significance for the selection of fillers, and it directly affects the cost and processing performance of molding compounds. The oil absorption value of the filler is high, and it may "eat" the resin several times or even dozens of times its own price, which virtually increases the cost of the material. As the oil absorption value increases, the viscosity of the resin increases, which will seriously affect its impregnation of fibers, and even change the rheological properties of the molding compound, making its molding process performance worse. Therefore, in order to increase the content of fillers in molding compounds. The selected packing is better with lower oil absorption value.

In order to reduce the hygroscopicity of the filler to the resin and increase the amount of the filler used, the surface treatment of the filler should be carried out.

2. Particle size and distribution

Particles are the basic unit of fillers. The particle size of the filler is generally classified by the percentage given by a certain number of sieves. As 99.8% of the particles pass through a sieve with 127.95 meshes (325 mesh), the fineness of this filler is called 325 mesh. Corresponding to the mesh mesh, the fineness of the filler is also expressed in microns. If the distance between the metal filaments constituting the mesh screen is 44um, then the filler passing through the mesh screen can also be called a filler with a diameter of 44um. Particles with a diameter larger than 44um cannot pass through the mesh screen, but particles smaller than 44um can pass through the mesh screen and be mixed together. Therefore, the particle sizes of the fillers used are actually different.

There are two requirements for filler particle size: one is the average particle size; the other is the particle size distribution. Generally, the average particle size should be about 5um, the maximum particle size should not exceed 20um, and the particle surface should be smooth. Particles exceeding 20um will adversely affect the performance of the product. The particle size of the filler has a certain relationship with the oil absorption value. Larger particles, the average particle size is 8um, the total surface area of the filler is small, the oil absorption value is also low, it is easy to be infiltrated by the resin, and can be added in a high amount, such as calcium carbonate, silicon dioxide and coarse talcum powder.

Finer fillers, fillers with an average particle size of 5um or smaller have high surface area and oil absorption value, which will increase the viscosity of the resin system with a given amount of fillers, and the amount added must be small, such as kaolin, fine talc, precipitated calcium carbonate, etc. The particle size distribution of the particles also has an important effect on the filler application. If the particle size distribution of the filler is broad, the smaller particles can be embedded in the medium-sized particles, and the medium-sized particles can also be embedded in the larger particles, so that the filler can be arranged relatively tightly, so that only a minimum amount of resin can be used to fill the gaps between the particles. Filling the particles with an appropriate amount of resin (not too much, so as not to separate the particles) is the most cost-effective economically, and at the same time obtains the best mechanical properties.

3. Filling volume

Many fillers are very cheap raw materials, which can greatly reduce the cost of molding materials and their products, so people often hope to add more fillers to molding materials as much as possible to make the filling rate of fillers higher. However, the different types, particle size and dispersibility of fillers will affect the fluidity of the resin mixture, thus affecting the addition of various fillers. In fact, the filling rate has a direct relationship with the oil absorption value. Under the condition of constant viscosity, the smaller the value, the higher the filling rate. Of course, the actual filling rate is limited, and it is impossible to achieve the maximum filling rate.

When considering the filling rate, the amount should be determined according to the viscosity of the resin mixture and the oil absorption value of the filler. For fillers with high oil absorption value, the surface can be chemically treated. After the filler is treated, the oil absorption value can be significantly reduced and the amount added can be increased. Although the surface treatment of the filler increases the cost, the cost is reduced more due to the increase in the filling volume, so the cost can still be saved in the end.

4. Thixotropy

Thixotropy is a physical phenomenon, that is, when a material is subjected to stress, its viscosity decreases significantly, and when the action stops, the material returns to its original viscosity. For thixotropic sensitive materials, under the action of molding pressure, the viscosity of the material will be too low, the material loss will be large, and even the resin and the reinforcing material will be separated. When the filler content is high, a moderate degree of thixotropy should be produced.

5. The combination of fillers

In the process of use, two or more kinds of fillers can also be mixed to learn from each other to obtain a more ideal effect. This combination can be a combination of different types of fillers, or a combination of different types, fineness, or different oil absorption values. For example, the combination of low oil absorption value and high fluidity filler can make the whole system have high filler content and good fluidity.

The type, particle size and dosage of the filler have a great influence on the shrinkage of the product. In general, the amount of filler is large and the shrinkage rate is low. Changing the type and amount of filler can adjust the viscosity of the molding compound, control the molding process of the material, and obtain satisfactory molded products. Combining various factors, you should choose low relative density, low oil absorption value, wide particle size distribution (1-20um), average particle size about 5um, low moisture content, and no grinding effect (otherwise the mold will be worn during processing) , low-cost filler.

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