What are the factors that influence the modification of plastic fillers?
In general, the shape of the filler, particle size, surface structure, surface treatment, stacking distribution of the filler, process conditions and equipment have an impact on the modification effect of filled modified plastics.
1. Shape of the filler
The shape of the filler can be broadly classified as spherical, granular, columnar, flake, fibrous and hollow microspheres. The shape of the filler has a strong influence on the effect of filling modification. Except for spherical and hollow microspheres, all other shapes of fillers show anisotropy and the larger the aspect ratio of these anisotropic fillers, the better the mechanical and thermal properties.
Generally speaking, fillers with large aspect ratios, such as fibrous, columnar and flake, are beneficial to improving the mechanical strength of composite materials and are therefore sometimes called reinforcing agents, but these fillers can affect the moulding and processing properties of the modified materials; fillers with small aspect ratios, such as spherical and granular, can improve the moulding and processing properties, but can reduce the mechanical strength of the materials.
The use of multi-component filler can overcome the above disadvantages, such as the addition of flake mica powder in PVC resin, rigidity increased while the impact strength has decreased, then the addition of CaCO3 below 1μm reinforcement, can prevent the decline in impact strength. If the powder filler and fibre filler are used, the mechanical properties of the modified products can be balanced, glass fibre plus inorganic mineral powder filler or carbon fibre plus aluminium hydroxide, etc., can balance the performance of the modified materials.
Add 15% to 20% EPDM and 15% to 25% CaCO3 in PP, the material has excellent impact strength and low temperature resistance, can be used in the manufacture of automotive important parts.
Adding 25% to 35% asbestos fibres and 15% to 35% CaCO3 to PVC resin, the material can be used to manufacture plastic asbestos floor tile products.
2. Particle size of the filler
The particle size of filler can be expressed either in terms of actual size (μm) or in terms of the number of mesh of the sieve through which it passes. The number of mesh refers to the number of holes per square inch of sieve mesh, which is 200 if the number of holes is 200. The relationship between mesh and micron (μm) is: Sieve diameter = 15400 μm/ mesh of sieve.
The particle size of the filler has a great influence on the performance of the filled composite. Generally speaking, within a certain dosage range, the smaller the particle size of the filler, the larger the specific surface area, the greater the adsorption of polymer molecules on the filler (due to the large contact area), the more firmly the resin is bonded to the filler interface, and the greater the improvement in the rigidity, tensile strength, impact strength, dimensional stability and appearance of the material.
Especially for tensile strength and impact strength, when the particle size of the filler is larger, both fall; when the particle size is small to a certain value, both rise, but with the increase in filling amount to a limit value, that is, began to decline; the size of the limit value, that is, the highest filling amount and particle size, the larger the particle size, the higher the limit value.
The smaller the particle size of the filler, if it can be dispersed uniformly, the better the mechanical properties of the filler material; but the smaller the particle size, the more difficult it is to achieve its uniform dispersion, requiring more additives and better processing equipment, and the finer the particle the higher the processing costs required, so we must choose the appropriate particle size filler according to the needs of use.
Table 2-10 shows the mechanical properties of the material with different particle sizes of calcium carbonate, under the same granulation process, in the same grade of HDPE filled with 25% (mass fraction) calcium carbonate.
From the specific data in Table 2-10 it can be seen that 1250 mesh bicarbonate gives the best results, followed by 800 mesh. In terms of material performance to price ratio, it is appropriate to use 800 to 1250 mesh bicarbonate.
If the production of plastic film, calcium carbonate particle size shall not be greater than 1250 mesh, large particles not only affect the mechanical properties of the film, the blown film will also cause difficulties. For ultrafine fillers, such as nanoscale materials, the particle size can be about 10nm. Ultra-fine fillers provide a very effective way of modifying materials, which can improve both the rigidity and toughness of the material. The particle size distribution of the filler affects its distribution in the filling system. When the particle size distribution is wide, it is not evenly distributed in the filling system, thus affecting the performance of the filling system.