DE1806288A1 - Water-silica coolant systems for thermoplastics - Google Patents

Abstract

Cooling of softened thermoplastic material in the manufacture of granules, sheet, mouldings or semi-finished products, is achieved by contact with a mixture of water and hydrophobic silica particles. Water droplets are not left on the plastic as occurs with water cooling and the cooled articles do not stick together. Shorter cooling times are required than for air cooling and it is easier to maintain dimensional tolerances. The transparency is also improved since the fast cooling allows higher amorphous contents to be obtained. Specifically the coolant contains 80-95% water and 20-5% SiO2, and especially 90% water + 10% SiO2. A layer less than 1u thick is retained by the plastic and this permits printing to be done without previous oxidation of the surface. Optionally a dyestuff may be included in the coolant.

Description

Process for treating softened thermoplastics The invention relates generally to the treatment of plastics in a softened state or plastic state and particularly concerns a post-treatment of the plastic Plastic surface releasing either the residual heat or with intermittent supply of heat in the production of granulates, foils, semi-finished products and molded parts until dimensional stability or change in shape is achieved.

From the low thermal conductivity of thermoplastic plastics is often used as an advantage when used for vessels and molded parts Made use of. However, the same properties are required in the manufacture of granules, Semi-finished products and molded parts relatively complex measures and means initially for the introduction and then for the removal of the heat required for the deformation. In all manufacturing and deformation processes for bodies made from such materials therefore, the heat supply and removal determines the volume output and profitability. An increase in performance is possible before other measures through better coolants.

Air as the most common coolant is cheap and available in sufficient quantities. However, their cooling effect is small and ever variable according to outside temperature. -The essential to air. more intensive cooling effect water is used in many deformation processes, e.g. granulation of plastics. However, through additional measures, that which adheres to the bodies must Water can be removed again afterwards. With certain deformation processes however, the use of water as a coolant is limited or not at all possible.

In the Schlauohfoiien-3lasverfabren with external liquid cooling is e.g. due to hydrostatic pressure on the Hose wall that Air cushions displaced inside the hose so that the foils weld together. at other continuous processes use cooling liquids in addition. into the hose.-inner given, of which drops adhere to the blue wall and stick or block them. More complex processes in which adhering drops are removed, e.g. with ultrasound have not prevailed. - Flat films and tapes are common guided through water baths or over water-cooled metal rollers. The one aimed at Heat exchange. will ever. through on the roller surfaces or on the belts formed vapor or air cushions as well as possibly disturbed by condensates. - Other coolants than water, e.g. alcohol, silicones or mercury are uneconomical.

It has now been found that granules, films, semi-finished products and molded parts following the known manufacturing process of a dry aftertreatment subjected and can be particularly intensively cooled. when the surface the respective plastic body temporarily #it a mixture of finely divided hydrophobized amorphous silicon dioxide and water temporarily covered or brought into contact with it will. According to the invention, a dry, free-flowing miscreening of finely divided SiO2 and water as heat transfer media. Suitable for performing this procedure themselves. Mixtures of 5 to 25% silicon dioxide with 80 to 95% water, whereby in particular a mixture of about 10% by weight of amorphous hydrophobized silicon dioxide and 90% by weight of water has proven effective. The substance mixtures to be used can continue according to the invention also mixed with an additive, preferably a dye be. The choice of the optimal mixing ratio depends on the through. Additions, possibly changeable surface tension of the water.

The mixture of finely divided bydrophobic substances to be used according to the invention amorphous silicon dioxide and water in a mixture ratio of about 10 : 90 forms a free-flowing powder and has a bulk density of about 0.45 kg / l. Your: Thermal conductivity differs only slightly from that of water. The treatment of softened thermoplastic Serving areas Mixture of substances can therefore be referred to in a certain sense as "dry water". It can be. thus molded parts almost as quickly. like cool with water when one the interfaces moved relative to each other. Compared to the usual especially with foils Air cooling shortens cooling times or

Cooling sections considerably .. The dimensional tolerances can thus be adhered to better. The quality of the molded parts, in particular the transparency of films, can in the improved rapid cooling significantly. can be increased, since more amorphous proportions than remain with exclusively air-cooled foils.

It was further observed that the composition of matter on the immersed or continuous molding forms a coating less than 1 rm thick, which does not exceed the tolerance limits even with the thinnest foils. This coating prevents the gluing or blocking of the molded parts, so that e.g. much higher temperature than extruded with air cooling and with a relatively high Residual heat can be wound into film rolls. - Another advantage of the manufacturing process consists in the fact that the molded part e coated with the substance mishandling has not been used previously required limited oxidation of the surface by flame or by. Corona discharges can be dyed or printed. Soluble dyes also showed. for months Exposure to daylight results in an over-smoking resistance.

The inventive treatment of thermoplastic plastics results even more advantages. It was found that oily or sticky exudations from molded parts when immersed in or when passing through a mixture of substances Dry off about 10 ß SiO2 with 90% water immediately so that you can spend a long time without to stick or block to remain storable. - The manufacture of thin-walled Molded parts, in particular tubular films made of Polyäthglen, are not only made by. the good thermal conductivity of the mixture of substances to be regarded as ~ dry water "improved, but also. in that, despite the 90% water content, this is like a bulk material behaves, which forms an angle of repose of approx. 250 against a flat surface. In a column of dry water used as a treatment room is gone the Horizontal pressure is much lower than the vertical pressure, in contrast to the hydrostatic one Conditions in pure water. Tubular films or thin-walled films running through the column Hollow bodies are hardly subjected to side pressures. You can without essentials Deformation much deeper than immersing in water, so that relatively long cooling distances and times can be provided.

The use of the mixture of substances is not due to the heat dissipation limited. It is also possible, for example, for film stretching and stretching processes, as well as for the production of semi-finished products and moldings, the amount of heat to be carried out at times in a treatment room under defined conditions without the disadvantages of wetting with a liquid heat transfer medium by means of the mixture of substances according to the invention Improve procedures.

Example 1 polyethylene of density 0.918 with a melt index of At a melt temperature of 2100, 0.2 becomes tubular film after the blow molding process with a wall thickness of 200 μm and a width of 55 cm that is twice as flat. -With air cooling the maximum take-off speed is 8.5 m / min, with internal and external cooling with water 30 m / min.

When using a mixture of 10 c} o hydrophobic amorphous Si02 and 90% water as external coolant become 18 m / l at a take-off speed achieved, with simultaneous internal and external cooling 26 m / min. The treated in this way Foil does not block and can be used with conventional printing inks without oxidizing the surface to print.

Example 2 Homogeneously mixed molding compounds made from approx. 50% by weight bitumen the penetration 80 according to DIN 1995 and 50 wt.% 'of a copolymer of ethylene as a melt at approx. 18,000 from a perforated plate with nozzle bores of 2.5 mm ~ extruded and made into cylinders of approx. 5 mm ~ and 5 mm in length granulated. - The granulate is placed behind the knock-off point in a container with a pourable Mixture of 10% hydrophobized amorphous SiO2 spun with 90 ffi water, in which it gives off its remaining heat.

At the same time, it is covered with a layer of the same material of approx. 5 Fm thickness coated and thereby. made pourable for about three months. - That dry, flowable treatment agent is used in a heat exchanger of the usual type cooled and fed back to the treatment tank, with the granules discharged amount is continuously supplemented.

Example 3 Flat film extruded from a slot die from 2 mm thick and 1.0 m wide from molding compounds according to Example 2, the surface with a layer formed from the oily or resinous components of the bitumen is covered, is passed through a tub in which. a mixture at about 30 ° C from 10% SiO2 with 90 ffi water. The foil-comes with a layer coated by less than 5 pm thickness of the mixture of substances and thereby. dry. the subsequently wound rolls of film no longer stick together even under tension.

Example 4 Molding compositions made of polyethylene in the form of granules essentially according to Example 1, tub-shaped linings of approx. 1 m2 Area and 3 mm wall or floor thickness in a heatable and coolable plate press pressed at approx. 130, partially cooled in the mold and with considerable residual heat shaped. Your surface is slightly sticky after dipping into the according to the invention Applicable mixture of substances, however. completely dry.

- Stacked tubs also stick together. no longer if stored for a long time.

The treatment process according to. of the invention for in the plastic state existing plastic and plastic mixtures, such as those according to. the examples 2 and 3, can still be modified to the effect that the amorphous bydrophobic Silicon dioxide mixed with water to form a giant powder is, Additives such as lubricants, dyes and stabilizers, as used for the following Treatment procedures are common, to be added.

Claims (5)

Claims 1. Process for treating, in particular cooling, softened thermoplastics Plastics in the production of granulates, foils, semi-finished products and molded parts, characterized in that the surface of the respective plastic body with a Mixture of finely divided, hydrophobicized amorphous silicon dioxide and water at times covered or is brought into contact with it. 2. The method according to claim 1, characterized. marked that the surface the softened or in still. plastic body located in the Meaning of a heat exchange with a mixture of finely divided hydrophobized amorphous SiO2 and water is brought into contact. 3. The method according to claim 1 and 2, characterized in that a dry, free-flowing mixture of finely divided, hydrophobicized amorphous Si02 and Water is used as a heat transfer medium. 4. The method according to claim 1 to 3, characterized in that a Mixture of about 5 to 20% hydrophobized amorphous Si02 with 80 to 95 ß water is used, preferably one made of 10% SiO2 with 90 Vo water. 5. The method according to claim 1 to 4, characterized in that the Mixture of SiO2 with water with an additive, preferably a dye, is offset.