The voids, yellowish, precipitation, and color change of PVC pipes are related to several indexs of calcium carbonate.
Let’s take a look at the product picture, what a scary!
Inner wall precipitation
Calcium carbonate is the main filling of PVC pipes, and its properties are very important for the influence of fluidity and physical properties of plastics in plastic processing. Mainly the particle size, specific surface area, and surface free energy are three major, manufacturers can not control.
1 Geometric features
Usually the powder material is used as a filler in the form of granules. The shape of the particles is not very regular, but for the properties of the plastic, the geometry of the filler particles has an important influence on the physical and mechanical properties of the filling system. Therefore, the particle shape of the powder material needs to be paid attention first.
For flake particles, the concept of the ratio of diameter to thickness is used, that is, the ratio of the planar size (longitudinal or transverse) of the flake particles to the thickness; for fibrous particles, the concept of the aspect ratio is often used, that is, the length of the fibrous particles and The ratio of diameters.
The particle shape of calcium carbonate is mostly tetragonal, hexagonal, polyhedral and irregular square granules, and its shape is very important for the fluidity and physical properties of the product in plastic processing.
2 particle size
An important point of the filling modification technique is to disperse the powder particles uniformly and as one by one into the plastic matrix, like the large and small islands in the sea, called island structures. In general, the smaller the particle size of the filler particles, the better the mechanical properties of the filling system if it is uniformly dispersed; however, the smaller the particle size, the higher the processing cost, and the more difficult it is to achieve uniform dispersion. Therefore, it is very important to understand the particle size and distribution of the powder particles and to select them according to actual needs.
At present, there are many expressions on the particle size and distribution of powder particles. Before there is no uniform naming method and regulation, the plastic industry often uses the mesh method, that is, how many purposes can be used for the particles of the powder material. The mesh number of the sieve indicates the particle size. In fact, the number of meshes measured by this method refers to the largest size of the largest-sized particles in this size in the three-dimensional direction.
In plastic products, it is required that the particle size distribution of calcium powder is as narrow as possible, that is, the minimum particle size and the maximum particle size are classified, and the products having a particle size within a certain range are collected according to the use requirements to ensure the dispersibility and transparency of the product. Mechanical properties and oil absorption.
3 specific surface area
The filler particles have different surface roughness. That is, the same volume of particles, the surface area is not only related to the geometry of the particles (the spherical surface area is the smallest), but also related to the roughness of the surface. The specific surface area is the surface area per unit mass of the filler, and its size is directly related to the affinity between the filler and the resin, the difficulty of the surface activation treatment of the filler, and the cost.
4 surface free energy
The amount of free energy on the surface of the filler particles is related to the difficulty in dispersing the filler in the matrix resin. When the specific surface area is constant, the surface free energy is large, and the particles are more likely to aggregate with each other, and the more difficult to disperse. Reducing the surface free energy of the filler surface is one of the main goals.
5 oil absorption value
The amount by which the unit mass of filler can absorb the plasticizer dioctyl ester (DOP) is called the oil absorption value. In plastic articles using plasticizers, if the oil absorption of the filler is high, the plasticizer consumption is increased. The size of the filler oil absorption is related to the size and distribution of the filler and the structure of the particle surface. The absorption of light calcium carbonate is often several times that of heavy calcium carbonate. Therefore, the use of heavy calcium can reduce the amount of plasticizer in the case of the same effect on resin plasticization. Generally, heavy calcium requires an oil absorption value of less than 65 mL/100 g.
The whiteness of the filler has a crucial influence on the color and even the appearance of the filled plastic material and the product. Generally, the higher the whiteness, the less the effect on the coloration of the filled plastic, only affecting the vividness of the color. Filled plastics tend to be opaque because they are not yet completely transparent. If the color of the filler is not white or otherwise, only black or dark plastics can be made.
Calcium carbonate itself is not easy to absorb water, does not contain structural water and crystal water, but in the process of collection, storage, processing and storage of ore, the powder particles are very small and easily absorb water. In the use of plastics, the requirements for moisture content are extremely high. The powder standard requires ≤0.5%. In practical applications, the water content of the powder should be ≤0.3%.
The smaller the water content, the smaller the influence on the plastic products.