NO863756L - METHOD FOR CELLPLAST PREPARATION. - Google Patents

Description

"PROCESS FOR THE MANUFACTURE OF PVC - CELLULAR PLASTIC"'

STATE OF THE ART

There are today several known processes for the production of PVC - cell plastic (foam plastic). Three fundamentally different processes are to be described which are interesting in connection with the present invention for the production of cellular plastic (foam plastic) from PVC (Polyvinyl Chloride). 1. The high-pressure method that produces PVC cellular plastic with closed cells. The process consists of several steps: the PVC plastic is mixed with a chemical foaming agent, stabilizer and possibly other process-regulating substances and additives. The components are mixed well at a relatively low temperature, possibly in several stages. The well-mixed mass is placed in molds and under high pressure (approx. 180 bar) the molds are kept closed, e.g. with powerful mechanical presses while heat is added to the plastic mass so that it is heated to approx. 170 degrees C. At this temperature, the chemical foaming agent decomposes and gasifies at the same time as the plastic mixture solidifies and homogenizes (gelatinizes) to a certain extent. This takes place while the mass lies still in the mold. After the plastic mass has been sufficiently heated and has been sufficiently long under high pressure, usually 40-60 min., the molds and thus the plastic mass are cooled to below the glass transition temperature of the plastic so that the plastic does not expand after the external pressure is removed, even if it now contains blowing agent in gas phase with high internal pressure. The plastic part, which is now a rigid semi-finished product, is removed from the mold and stored for some time. The plastic part is then heated again to approx. 100 degrees C. This softens the plastic, at the same time as the internal pressure in the blowing agent increases somewhat. This causes the plastic part to expand until equilibrium is achieved between internal expansion pressure and stresses in the plastic. Because a very large part of the heat-stabilizing agent in the plastic mixture is "used up" in the decomposition phase, a higher temperature than approx. 100 degrees C is used in the expansion phase because the PVC plastic can otherwise break down.

A PVC product produced using this method has closed cells and can have a specific gravity down to approx. 40 kg per m3. Products are also produced using this method with specific weights of up to approx. 600 kg per m3. Due to the high static specific pressure required during the decomposition phase of the effervescent agent, there are major practical limitations on the size of the final product. There are also great difficulties with heating the plastic mass in the mold so that all parts of the mass receive a relatively equal amount of heat.

Normally, an exothermic foaming agent is used. This creates a great danger of "burning" and decomposition of parts of the plastic mass before sufficient heat energy has been supplied in connection with the decomposition of the foaming agent. This places limitations on the thickness of the plastic mass that is heated. In addition, the process is very labor-intensive and requires relatively large investments in technical equipment.

Another method that is very widespread for the production of PVC cellular plastic is the extrusion of a plastic mixture consisting of PVC plastic, chemical foaming agent, stabilizer and any process-regulating substances and additive is where the various components are mixed well at a relatively low temperature (e.g. 110 degrees C) before the mixture is fed to the extrusion machine, usually in the form of powder or granules. In the extruder, the components are mixed well together under a high temperature (e.g. 180 degrees C) so that the mass is converted into a relatively homogeneous melt (gelatinized), at the same time that the foaming agent is supplied with sufficient heat energy to start the decomposition process. The process is controlled so that most of the decomposition of the blowing agent takes place outside (after) the extruder (including tools). After exiting the extruder tool, the plastic mass will, due to the great pressure in the eventually decomposed blowing agent, expand until equilibrium is achieved between the internal pressure and the stresses in the plastic mass.

The process is continuous and can be automated to a very high degree. It provides large capacity with relatively small investments in technical equipment. However, a cellular plastic produced by this method will usually have a large proportion of open cells. Nor has it been possible to produce PVC cell plastic using this method with a specific gravity below approx. 450 kg. per m3. This can be explained by the fact that in a PVC - cellular plastic produced according to this method, the cells are formed from relatively large concentrations of blowing agent, which give large individual cells. This means that when too much blowing agent is added, large concentrations of blowing agent will produce expansion forces in each individual cell that are so great that they exceed the plastic mass's ability to hold the expansion gas, and the gas breaks through the surface and escapes. The end product can thus have a higher specific gravity than when using less blowing agent, and furthermore the surface will be rough and in many cases not satisfactory. Recipes and process conditions that give the lowest possible specific gravity with a satisfactory surface on the product are usually sought after.

A third method for producing PVC - cellular plastic is the extrusion of a plastic mixture consisting of PVC - plastic, stabilizer and any process-regulating substances and additives, where the different components are mixed well at a relatively low temperature (e.g. 110 degrees C) before the plastic mixture fed to the extruder. The mixture is melted at a higher temperature in the extruder at the same time as the mixture is homogenised. After the meltdown, a gas (e.g. C02 or N2) is added to the melt under high pressure... The gas is distributed in the melt and causes expansion of the plastic mass after exiting the extruder due to the reduction of the external pressure. The method is very rational and can give very low specific weights of the PVC product (down to approx. 30 kg per m3). However, the end product has open cells, and will therefore e.g. absorb water already at very low pressures.

In most cases, it is desirable to have tight cells in PVC - cell plastic. Absorption of liquid is then minimal, and the cellular plastic also has very good thermal insulation properties. In many cases, physical properties will also be improved.

DESCRIPTION OF THE INVENTION

The present invention combines the good chemical and physical properties of PVC - cellular plastic with dense cells produced according to the described high pressure method, with the very rational production of PVC - cellular plastic in an extruder or other appropriate plastic processing machine. The method involves the use of a homogeneous PVC mass containing PVC plastic, stabilizer, chemical foaming agent and any other process-regulating substances and additives where the chemical foaming agent is finely distributed in the mass. The fine distribution of the chemical foaming agent is done by plasticizing and homogenizing the plastic mixture into a homogeneous melt so that a well-gelatinized intermediate product is obtained. In this gelatinization process, the temperature in the melt is kept below the decomposition temperature of the chemical blowing agent in combination with any activator and other components in the plastic mixture or at a temperature level which, together with the other plastic mixture components and the process parameters, does not result in decomposition/gasification of the blowing agent or only decomposition/gasification of the blowing agent to an insignificant extent. This gelatinization process can be carried out in a plastic processing machine that meets the requirements for temperature, mixing effect and pressure, e.g. an extrusion machine.

The composition of the different substances in a plastic mixture must be adapted to, among other things, to type of mixing machine, machine for further processing and requirements for the final product. However, the plastic mixture must be structured so that it is possible to produce a well-gelatinized intermediate product without the chemical blowing agent being decomposed/gasified or only decomposed/gasified to an insignificant extent. Most chemical sprays used today, e.g. azodicarbonamide, has a decomposition temperature of approx. 235 degrees C. Usually the decomposition temperature must be reduced to approx. 170 to 180 degrees C to avoid difficulties with the breakdown of the PVC material. This is done by adding an activator or "kicker" for the foaming agent. A metal compound based on Pb, Zn, Cd or Sn is most often used. Emulsion PVC (e-PVC), mass PVC (m-PVC) and suspension PVC can be used as PVC plastic

(s-PVC) or combinations thereof. A copolymer can also be used for the PVC plastic, e.g. vinyl acetate...

The gelatinized PVC mass is carried on to the next processing phase, which can take place in the same plastic processing machine or in another plastic processing machine that is suitable for the further processing, e.g. an extrusion machine or an injection molding machine. The plastic mass can be supplied in the form of granules or powder or the mass can be supplied in a hot state from the gelatinization process without the plastic mass being significantly cooled. One thereby achieves an energy saving in the production of PVC - cellular plastic.

In this processing phase, the PVC mass is quickly heated to a temperature above the decomposition temperature for the chemical foaming agent in combination with any activator and other plastic mixture components, while the mass is subjected to a large external pressure. This is to prevent the foaming agent from expanding in the plastic processing machine itself. The plastic is processed mechanically, kneaded in the machine so that all parts of the mass get approximately the same temperature. Because the mass quickly reaches a given temperature, one has a relatively good control over the possible exothermic heat development from the foaming agent, which is a major problem in e.g. known static high-pressure process which is used today and which has been described previously. Due to the rapid heating of the entire mass that is achieved by the mechanical processing, you also get a very rapid decomposition of the chemical foaming agent. This whole process can take 3-5 min. against 40-60 min. in the high-pressure process used today.

As the plastic mass leaves the plastic processing machine with any extension, the external pressure on the plastic mass is reduced and the mass expands until equilibrium is achieved between the internal pressure in the blowing agent and the tensions in the mass. Because the mass expands at a high temperature (160-230 degrees C), the mass is very soft and a high degree of expansion is achieved. The high temperature can be used because most of the added heat stabilizer is active at this stage of the process. It may be necessary to reduce the temperature of the plastic mass somewhat before expansion for certain recipes and shapes to avoid the mass collapsing after expansion. After expansion, the PVC mass is cooled. It can then be further processed according to requirements. The finished PVC product has very finely distributed closed cells.

DESCRIPTION OF EXAMPLES OF PLASTIC MIXTURES AND PROCESS CONDITIONS

As previously mentioned, when building up a plastic mixture/recipe, you must take into account the plastic processing machine to be used and assess the properties you want the end product to have. If an extruder is used both for the mentioned gelatinization process and an extruder for the decomposition process of the foaming agent, the following piast mixtures will produce a hard PVC - cellular plastic with closed cells.:

Example 1 Example 2 Example 3

The various components in the above plastic mixtures are mixed in a suitable mixing machine, e.g. in a so-called high-speed mixer to a temperature of approx. 110 degrees C has been achieved. The mixture is then cooled and fed to a plastic processing machine, e.g. an extrusion machine. The mixture is processed, mixed, melted and homogenized to a homogeneous melt with a maximum temperature of approx. 150 degrees C, which is slightly below the decomposition temperature for the chemical foaming agent in combination with the other components in the mixtures. This process, also called the gelatinization process/gives a homogeneous melt containing finely divided chemical blowing agent which is not decomposed/gasified or only decomposed/gasified to a negligible extent. The gelatinized material is transferred to the next phase in the processing, which can take place in the same plastic processing machine or to a separate plastic processing machine for further processing.

The gelatinized plastic material can be added to the new one

the processing phase/mash in nen either with a temperature only slightly lower than the temperature during the gelatinization process or well cooled and then preferably chopped into fine grains, so-called granulated.

In this plastic processing machine, which can be an extrusion machine, injection molding machine or other suitable plastic processing machine, the gelatinized plastic mass is processed mechanically/kneaded at the same time as the temperature is increased to above the decomposition temperature of the blowing agent in combination with the other components of the plastic mixture, for the described plastic mixtures approx. 175 degrees C. The external pressure in the machine is kept above the expansion pressure in the eventually decomposed foaming agent. In order to achieve a sufficient degree of decomposition/gasification of the foaming agent, the material is mechanically processed/kneaded under this temperature and this pressure normally for at least 3 to 5 min. The hot mass, possibly slightly cooled, is then fed out of the plastic processing machine's nozzle and the external pressure is reduced either immediately or successively to normal atmospheric pressure. The hot plastic mass thus expands until equilibrium is achieved between the internal pressure in the blowing agent and the stresses in the plastic material. The plastic is then cooled with e.g. air or water and can be processed e.g. mechanically if desired.

In order to increase the physical properties of the final product, especially at high temperatures and at the same time improve the chemical properties, it is possible to cross-link or cross-link some of the molecules of the plastic mass. This can happen e.g. by using peroxides that are cross-linked (cross-linked/cross-linked) at high temperature or by adding a component to the mass that can be cross-linked by the application of high energy, e.g. using gamma rays. Cross-linking can also be achieved by adding a component to the plastic mass which is chemically cross-linked without the application of particularly high energy. Such a component or additive can e.g. be an isocyanate. In order to avoid that the cross-linking occurs at too early a time, it will usually be correct to use a time-delayed component in connection with the use of a chemically active cross-linking agent. For the same reason, in some cases a chemically active crosslinking agent should be added as late in the process as practically possible, e.g. immediately before the decomposition phase of the emulsifier.

AREAS OF USE FOR PVC - CELLULAR PLASTIC WITH CLOSED CELLS

The invention enables the production of both hard and soft PVC - cellular plastic with closed cells depending on the amount of plasticizer that is added to the plastic mixture.

PVC - cellular plastic with closed cells can i.a. used for the following product areas: Thermal insulation (e.g. in refrigerated vans and of gas/oil pipelines)

Products for buoyancy (e.g. floats, buoys and pontoons)

Constructions (e.g. light sandwich constructions with PVC cellular plastic as core material in e.g. walls, boats and planes).

Claims (11)

1. Process for the production of cellular plastic with closed cells containing polyvinyl chloride (PVC) and/or copolymer with polyvinyl chloride, chemical blowing agent, stabilizer and any other process-regulating substances and additives, characterized in that the plastic mixture is processed in a plastic processing machine and supplied to this processing phase in a well-gelatinized state and that the chemical foaming agent has been finely distributed in the plastic mass during the gelatinization process at a temperature which, together with other process parameters, gives the plastic mass a good degree of gelatinization, at the same time that the chemical foaming agent has not been decomposed/gasified or only decomposed/gasified to an insignificant extent, and that the plastic mixture is processed mechanically (kneaded) in the processing machine at a temperature above the decomposition/gasification temperature for the blowing agent in combination with the other components in the plastic mixture for a sufficient time so that at least 75% of the blowing agent is decomposed/gasified during processing the iding machine and any extension thereof and that the processing takes place at an external pressure on the plastic mass that is high enough so that the decomposed/gasified emulsifier does not form gas cells larger than 0.06 millimeters in cross-section in any part of the plastic mass up to at least 75% of the blowing agent is decomposed/gasified and that the processed plastic mixture, after at least 75% of the blowing agent has been decomposed/gasified, by reducing the external pressure on the plastic mass and possibly after reducing the temperature of the plastic mass, expands to the desired shape. 2. Method as stated in claim 1, characterized in that the plastic mixture contains a sufficient amount of chemical blowing agent so that the plastic mixture, after the processing and reduction of the external pressure, achieves an expansion which gives the plastic mixture a specific weight of less than 350 kg. per m3. 3. Method as stated in claim 1, characterized in that the mechanical processing (kneading) of the plastic mixture in the plastic processing machine is completely continuous or takes place for at least 60% of the residence time of the plastic mixture in the plastic processing part of the plastic processing machine. 4. Method as stated in claim 1, characterized in that the chemical blowing agent in the plastic mass consists of a blowing agent system which develops an amount of heat that is less than 150 calories per gram of the blowing agent system in the plastic mass during the decomposition/gasification. 5. Method as stated in claim 1, characterized in that an extruder or an injection molding machine is used as the plastic processing machine. 6. Method as stated in claim 1, characterized in that the plastic processing machine is an extruder or an injection molding machine and that the external pressure on the plastic mass is achieved by the screw(s) in the extruder/injection molding machine being designed so that the necessary compression of the plastic mass and thus the necessary external the pressure on the plastic mass is achieved in the screw(s) independently of the pressure on the plastic mass after the screw(s) in the extruder/injection molding machine. 7. Method as specified in claim 1, characterized in that two or more linked plastic processing machines are used as plastic processing machines, where the first one is an extruder/extruders and that the plastic mixture is supplied in a non-gelatinized state and that the plastic mixture is processed in the first extruder/s extrude in such a way that the plastic mixture is well gelatinized at a temperature somewhat below the decomposition temperature of the blowing agent in combination with the other components in the plastic mixture, so that the blowing agent is finely distributed in the plastic mass without the blowing agent decomposing/gasifying or only decomposing/gassing to an insignificant extent, and that the plastic mixture in a hot state is transported to the next processing phase and processed as described in claim 1. 8. Method as specified in claim 1, characterized in that the plastic processing machine is used, for example an extruder or an injection molding machine and that the plastic mass is fed to the machine in a non-gelatinized state and that the gelatinization and fine distribution of the foaming agent takes place in an early phase of the processing without the foaming agent is decomposed/gasified or only decomposed/gasified to an insignificant degree, and that the plastic mass is then transported further in the machine and processed as described in claim 1. 9. Method as stated in claim 1, characterized in that the plastic mass, after it has come out of the plastic processing machine, is brought in a warm and expanded state into a shaping tool where the plastic mass achieves its final shape. 10. Method as stated in claim 1, characterized in that the external pressure on the plastic mass after the processing is reduced step by step. 11. Method as stated in claim 1, characterized in that the plastic mass immediately before the decomposition/gasification phase of the emulsifier, a cross-linking agent, for example isocyanate, is added as an additional substance.