JPS6210247B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to highly abrasion resistant plastic components, particularly conveyor equipment and dewatering elements of paper machines, in which polymeric or ultra-high molecular weight low pressure polyethylene is mixed with cadmium salts and titanium dioxide and hot pressed. The present invention relates to a method for producing a molding compound for producing wear parts for machines such as machines.

According to a known method of this type (see German Patent No. 1925408), classified granules of high-molecular low-pressure polyethylene are mixed with additives and compressed into a molded body.
This is done using melt compression methods at relatively low pressures.
After cooling, the compact is ground and mixed with 10-40% pure low-pressure polyethylene powder. This mixture, consisting partly of aged recycled material and partly of pure starting material, is then hot pressed again. After plasticization and cooling, a plastic semi-finished product is obtained, which is not only hard and therefore wear-resistant to frictional stresses, but also hydrophilic and therefore when water acts on this plastic. A permanent water film is maintained on the surface of the Hydrophilicity is obtained by the added cadmium salt, and hardness is obtained by the added titanium dioxide. Such plastic semi-finished products are particularly suitable for producing high-load plastic parts for the screen tables of paper machines in which the water-carrying screen moves at high speeds. Although the sliding properties of such plastic parts made of high-molecular, low-pressure polyethylene are significantly improved, especially by the addition of cadmium salts, the wear of these parts is still very high.

It is known to reduce the wear of plastic parts of this type by embedding glass microbeads in the plastic material. In a known method of this type for producing polyethylene semi-finished products (see French Patent No. 2 319 662), 16.7% by weight of glass microbeads are added to the starting material 79.3% by weight of polymeric polyethylene, and this mixture is further enriched with antistatic properties. 4.0% by weight of acetylene black is added. The purpose of adding acetylene black is to improve the sliding properties, but at the same time the hardness of the glass microbeads must increase the wear resistance of the plastic component. Therefore, the proportion of glass microbeads is relatively high at 16.7% by weight. However, practical results have shown that the desired abrasion resistance is not achieved with the low pressure polyethylene thus prepared for various reasons. On the one hand, a statistically uniform distribution of glass microbeads within the plastic is not achieved, resulting in zones with different densities of glass microbeads embedded within the plastic, and thus differences in the wear properties of these zones. occurs. Because of this and because of the low degree of crosslinking of the plastic molecular chains, the glass microbeads detach from the plastic when the respective plastic part is exposed to abrasion stresses. Moreover, glass beads protruding from partially worn plastics expose components that slide over plastic guides, such as paper machine screens, to extremely high stresses.

It is therefore an object of the present invention to obtain a harmonious relationship between the stability and hardness of the buried glass microbeads and the sliding properties of the supporting plastic, in which the glass microbeads are fixed within cross-linked plastic molecules and the glass microbeads are The object of the present invention is to propose a method for producing plastic semi-finished products that fully guarantees a statistically uniform distribution of beads.

The purpose was to first mix low-pressure polyethylene in the form of fine granules or powder with cadmium sulphide and titanium dioxide, bring this mixture rapidly to the temperature at which plasticization begins on the surface of the individual plastic particles, and then The problem is solved by the method described above, which is characterized in that microbeads are added, the mixture is heated under pressure to the plasticizing temperature and cooled again.

A particular advantage of the method of the invention is that by preheating the plastic particles or powder until plasticization of the plastic particles begins at their surface, a very favorable adhesion of the glass microbeads added subsequently is achieved. There are many things. Rapid heating of the plastic powder is advantageous because the temperature gradient of the individual plastic particles, which can be approximately considered as spheres, is directed mostly radially inward. Before the addition of the glass microbeads, the individual plastic particles must be softened not over a certain depth but only in their surface area to facilitate adhesion, so-called surface plasticization. In this case, agglomeration of the plastic particles should only begin to form or only slightly. When glass microbeads with a diameter significantly smaller than the plastic particles are added to the heated plastic powder in this way, the glass microbeads adhere to the surface of the plastic particles, and the number of these plastic particles increases depending on the adhesive force generated per unit surface area. It only adsorbs glass microparticles. Since it can be assumed that the particle size of the plastic powder is constant, glass microbeads are uniformly attached to the surface of each plastic particle. By finely adjusting the quantity ratio of plastic powder to glass microbeads, a statistically sufficiently homogeneous distribution of the glass microbeads in the plastic powder is achieved. Another important advantage is that the adhesive strength of the plastic particles prevents the glass microbeads from settling down in the mixer; the plastic particles or powder float on top of the glass microbeads, which have a much higher specific gravity. do not become. This fact also favors a uniform distribution of glass microbeads in the plastic grains. The invention allows powder mixtures to be obtained without the need to add known additives such as dispersants.

It is advantageous to heat the powder by frictional heat in the high speed mixer so that the plastic powder does not have to be heated in a separate heating step before mixing the glass microbeads therein. On the one hand, this heating takes place very rapidly, and once the desired plasticization temperature of the second plastic particles has been achieved, further temperature increases are immediately inhibited by reducing the mixer speed. The respective desired temperature is determined empirically. The glass microbeads can then be added immediately to the high speed mixer without delay.

Another advantage of the method of the invention is that glass microbeads can be particularly hard embedded into plastics by hot pressing the mixture of plastic, additives and glass microbeads twice with a cooling period in between. It will be done. Despite interruptions by glass microbeads in the closed tissue,
Plastic is completely homogeneous. Therefore, aggressive media, such as acids and alkalis, cannot find a place to penetrate through the surface of the material and destroy it. Unlike the known use of acetylene black to improve the hydrophilicity of plastics, the additives used are cadmium sulfide and
Titanium oxide acts to stabilize the entire structure of the plastic and to lower the coefficient of friction of the material surface. The reinforcing effect of the glass microbeads is thereby advantageously combined with the properties produced by other additives of the polymeric low-pressure polyethylene.

Improvements in the properties of plastic molding compounds can also be achieved by adding barium sulfate and phthalocyanine as other additives to the polyethylene particles. In this way it is also possible to adjust the coloration of the plastic semi-finished product without reducing the desired properties of abrasion resistance.

In order to achieve as statistically uniform a distribution of the glass microbeads as possible in the powder mixture, it has been found that a diameter of the glass microbeads of 50 μm or less is suitable. Moreover, such a diameter of the glass microbeads is advantageous for double crosslinking of the high molecular weight low-pressure polyethylene during and after the hot pressing process.

Plus low pressure polyethylene 93.33% by weight, additives
The mixing ratio of 1.91% by weight of glass microbeads and 4.76% by weight of glass microbeads ensures that there is still sufficient plastic content on the surface of the plastic molding compound when the glass microbeads are distributed statistically uniformly, thereby allowing the plastic product parts to be easily coated. Advantageously, no glass microbeads protrude from the surface of the component, which can promote wear due to the flowing liquid. If the wear parts of the paper machine screen are manufactured from plastic semi-finished products by the method of the invention, the screen for dewatering the paper is protected from wear.

The additives ie cadmium sulfide and titanium dioxide alone or cadmium sulfide, titanium dioxide and phthalocyanine are preferably added in the same proportions, so that they each represent approximately 1/2 of the total additives.
It becomes ~1/4.

The plasticizing temperature of the two hot-pressing processes is dependent not only on the compression method applied in each case, but also on, for example, the flow resistance of the starting material to be compacted, and therefore always requires special adaptation; as a rule, intercooling is approximately 80 °C. can be established by lowering the temperature to . The intercooling, like the preceding hot compression process, is advantageously carried out under pressure. The first heating, intermediate cooling and second heating of the mixture are preferably carried out at constant pressure.

Next, the present invention will be explained by examples.

The high-speed mixer charge is made of low-pressure polyethylene.
93.33% by weight, 1.91% by weight of additives and 4.76% by weight of glass microbeads. The 1.91% by weight additive consists of equal amounts of cadmium sulfide, titanium dioxide, barium sulfate and phthalocyanine.

Initially only the plastic powder and additives are added to the high speed mixer. The rotational speed of the high speed mixer is adjusted such that, within a short time, for example 21/2 minutes, plasticization begins on the surface of the individual plastic particles due to the frictional heat acting on the powder. The required surface temperature of individual plastic particles is
It is around 120°C, and this temperature can be slightly higher for a short time as long as the core of each plastic particle is not completely heated and therefore there is continuous heat transfer from the surface of the plastic particle to the inside. It may also be temperature. This temperature is determined empirically depending on the type of plastic and is 105 DEG -118 DEG C. for high molecular weight low pressure polyethylene.

Only after this temperature has been reached are the glass microbeads, which are only slightly agglomerated, added to the plastic powder. During this process, the speed of the high speed mixer is reduced to avoid excessive heat supply. Due to the adhesive action of the plastic particles with plasticized surfaces, the glass microbeads adhere uniformly to the plasticized surfaces of all plastic particles. As a result, the plastic particles dry together and the individual particles are separated again, even if agglomeration has already begun. This is based on the mechanical action of the glass microbeads rubbing against the plastic particles by continuous circulation of the entire mixture. In this case, the temperature difference between the glass microbeads and the heated plastic powder, which are generally added to the high-speed mixer at room temperature, avoids overheating and complete softening of the plastic particles, or also makes it possible to cool the particles. Overall, this measure ensures that the glass microbeads are distributed as evenly as possible on the plastic and that the plastic does not float on the glass beads due to its low specific gravity.

The particle size of the glass microbeads of course depends on their diameter. The diameter of the polymeric glass microbeads is about 50 μm, so the glass microbeads have a similar overall powdery nature.

The powder mixture of plastic particles, additives and glass powder is sent to a sintering press in the form of fleece planks. The powder is then compressed in stages to a pressure of 100 Kg/cm ² . The compressed fleece is then heated to approximately 180 DEG C., above the plasticizing temperature of the plastic, and this temperature varies significantly depending on the choice of plate thickness. The compressed fleece is suitably heated from the center to the outside so that plasticization proceeds from the inside to the outside. The plasticizing temperature is approximately
Preference is given to using high molecular weight low pressure polyethylene at 140°C.

Depending on the desired crosslinking of the plastic molecular chains, the plasticized compound can be cooled to room temperature. A so-called intercooling to approximately 80 °C after the first hot-pressing process is carried out under a pressure of 100 Kg/cm ² and then the product is heated once more to the plasticizing temperature, during which
It is advantageous to maintain a pressure of 100 kg/cm ² or to pass through the individual pressure stages one more time after the pressure has previously dropped. After cooling to 30°C, the plastic semi-finished product, which is present as a plate, can be removed from the mold.

Claims (1)

[Claims] 1. Dewatering of highly wear-resistant plastic parts, especially conveyor equipment and paper machines, by hot-pressing a mixture of high-molecular or ultra-high-molecular low-pressure polyethylene with cadmium salts and titanium dioxide as additives. In the production of plastic semi-finished products for the production of machine wear parts such as elements, low-pressure polyethylene in the form of fine particles or powder is first mixed with cadmium sulphide and titanium dioxide, and this mixture is applied to the surface of the individual plastic particles. A process for the production of plastic semi-finished products, characterized in that the temperature at which plasticization begins is brought rapidly to the temperature at which plasticization begins, then glass microbeads are added, the mixture is heated under pressure to the plasticization temperature and cooled again. 2. The method according to claim 1, wherein the low-pressure polyethylene particles or powder are brought to a temperature at which surface plasticization of the plastic particles begins by means of frictional heat in a high-speed mixer. 3. The manufacturing method according to claim 2, wherein the temperature of the low-pressure polyethylene powder or granules adjusted in a high-speed mixer is 105 to 118°C. 4. According to claim 2 or 3, the mixture of plastic, additives and glass microbeads, after initial heating and cooling to the plasticizing temperature, is subsequently brought again to the plasticizing temperature and cooled. Manufacturing method described. 5. The production method according to claim 1, wherein barium sulfate and phthalocyanine are further mixed as additives into the mixture of low-pressure polyethylene, cadmium sulfide, titanium dioxide and glass microbeads. 6. The manufacturing method according to any one of claims 1 to 5, wherein the glass microbeads have a diameter of 50 μm or less. 7 Mixing ratio: 93.33% by weight of low pressure polyethylene, 1.91% by weight of additives and 4.76% by weight of glass microbeads
The manufacturing method according to claim 5 or 6, wherein the amount is % by weight. 8. The manufacturing method according to claim 7, wherein the additives are added in the same proportions. 9. Process according to one of claims 4 to 8, with intercooling to a temperature of about 80°C. 10 Claim 9: Intermediate cooling under pressure
Manufacturing method described in section. 11 Claim 10 in which the first heating, intermediate cooling, and intermediate heating of the powder mixture are performed under constant pressure.
Manufacturing method described in section.