PRODUCT BASED ON POLYVINYL CHLORIDE FOAM AND PROCESS OF PRODUCING SAME

Abstract: A product based on a polyvinyl chloride and the process of producing the product, where the product is to be used for filling hollow bodies in situ. The product is mixed and pregelled by a known method in a plastic processing machine. During this part of the process the blowing agent in the product is not decomposed. The decomposition of the blowing agent occurs only in an autoclave using a temperature- and pressure-exchanging, barrier-forming medium. After the decomposition of the blowing agent has been completed, a colder medium is introduced into the autoclave to cool the product to a temperature lower than its second order transition temperature, Tg. The product can then be converted to foam material by heating in situ at the expansion site.

Description: Product based on polyvinyl chloride foam and process of producing same. The present invention relates to a product for producing a polyvinyl chloride foam and the process for producing same. Polyvinyl chloride foam, also called PVC foam, can be produced by means of a number of methods. Nitrogen can, for example, be mixed into the polymer melt under high pressure (approx. 200 bar) according to the Airex method. Another method is. the Trovipor method, where PVC paste and, for example, carbon dioxide C0 2 is mixed into the PVC mixture under high pressure. Both methods produce a pre¬ gelled, gas-propelling polymer mass which is made to expand by pressure relief. Instead of using gases such as nitrogen and carbon dioxide, solid or liquid blowing agents can also be added to the polymer. In the present case, blowing agent is understood to be material which generates gases during heating to the decomposition temperature of that material. According to a known method, a dissolution or dispersion of the blowing agent will in this way be brought about in the melted polymer when the blowing agent and the polymer for exaroole are mixed in an extruder. When the polymer melt then for example is pressed out through an extrusion nozzle and on to a moving belt, the decomposed blowing agent in the polymer melt will expand and cause the polymer to rise (swell up). Thereby the expanded product is also cooled to a temperature lower than the polymer's second order transition temperature, Tg, causing the foam to become self-supporting. A problem in the production of polyvinyl chloride' foam has consisted in keeping it in the form of an intermediate product, where the blowing agent has been decomposed and the polyvinyl chloride is present in an unexpanded, but gelled state. Such an intermediate product will be highly suitable when expansion is wanted at a later point of time and when con¬ ditions on site indicate that it is advantageous to use an intermediate product which has already been prepared for expansion. It is for example possible to picture this inter¬ mediate product being used to fill an enclosed cavity in buoyancy elements, in that a measured amount of the pre¬ fabricated intermediate product is introduced into the cavity and then subjected to the effects- of heat so that it expands and completely fills the cavity. For such applications and whereever it would be desirable to carry out the expansion of the polyvinyl chloride on site, a process has been developed, where the polyvinyl-chloride resin, blowing agent, stabilizers and any possible process-regulation agents are mixed carefully with each other in a conventional mixing device at a temperature low enough for the blowing agent not to be subjected to decomposition, after which the thoroughly mixed product is divided into small pieces or granules which are then transferred tc a high-pressure press with a mould into which the granulate is introduced and then subjected to a pressure of 150-200 bar which is exerted by a plunger against the granulate in the mould at the same time as the granulate is heated in the mould to a temperature of approx. 150°C. Thereby the blowing agent is decomposed, but because of the counter- pressure, the polymer mass will not expand. Once the blowing agent has been decomposed, the polymer mass is cooled to below the second order transition temperature, Tg, of the polymer mix, after which the pressure is released. This intermediate product in the form of pressed briquettes can then be expanded at the place of use itself, in that a measured quantity of briquettes in the cavity is heated to a temperature at which the gas pressure inside the briquettes exceeds the deformation resistance of the briquette material, and thereby causes the briquettes to expand into polyvinyl chloride foam. This known method is very laborious. The presses used have, moreover, a limited production capacity and are expensive in first cost. The aim of the present invention is therefore to produce an intermediate product and to produce it in such a way that the resulting foam product is produced by a method which is far more simple and reasonable than the press process described above. The present invention relates to an intermediate product based on a PVC foam mix and the process of producing this product, in which the polyvinyl chloride resin is mixed with the blowing agent, stabilizer and any other possible process-regulation additives, producing a carefully worked and what is called a gelled mixture which, in the form of a shaped, solid material, is transferred to a high-pressure heating device for decomposing the blowing agent in the mix¬ ture without the mixture to expand, after which the mixture with the decomposed blowing agent is cooled under pressure. Once the pressure has been released, it will -later be possible to expand the intermediate product obtained into polyvinyl chloride foam. The process is characterized in that the mixture of polyvinyl chloride, stabilizer, blowing agent and any other possible process-regulation additives, is transferred to an extruder or other suitable plastic processing machine, in which the mixture is kneaded without the blowing agent being decomposed and, if the machine is an extruder, extruded in any shape whatsoever, its length and breadth depending on the design of the extrusion nozzle, and made to solidify, after which the extruded, solidified mix¬ ture is transferred to an autoclave in which a pressure-exchanging and barrier-forming medium is circulating at a pressure of 20-300 bar and at a temperature of 50-270°C, and heat-treated in the autoclave for a period of 5-80 minutes in order to decompose the blowing agent in the extruded and solidified product, after which the hot medium which is circulating is replaced by a colder medium of the same type in order to reduce the temperature in the autoclave to below the second order transition temperature, Tg, of the pressure- and heat-treated product, after which the pressure is released and the pressure- and heat-treated intermediate product is removed from the autoclave in order to be later subjected to expansion in a sealed space in another place, if necessary after being divided into smaller pieces, in that the product is heated in known way, until the gas pressure inside the product exceeds the deformation resistance of the product and the product thereby is caused to expand. It has been stated above that the polymer mass can be mixed and gelled in an extruder. This is however only one of the types of plastic processing machines which can be used with regard to the production of the PVC foam mixture. Injection moulding machines and kneading machines are also suitable for the said purpose. As kneading machines parti¬ cularly Banbury mixers can be mentioned as being suitable for mixing and pregelling the said PVC foam, but other types of Sigma mixers can also be used. The invention therefore does not lay down any conditions regarding the type of plastic processing machine to be used, except that it should be suited for the purpose, i.e. that it can mix the components into a PVC foam mixture and pregel it. It has also been mentioned above that both solid and liquid blowing agents can be used in the PVC foam mixture. Of such can be mentioned azodicarbonamide, sulphonyl hydrazide and n-nitroso compounds among solid blowing agents and diiso- propylazodicarboxylate and diρhenyleneoxide-4, 4'-disulpho- hydrazide among liquid blowing agents. Neither does the invention lay down any restrictions concerning the type of PVC to be used, since both suspension PVC (S-PVC) and emulsion PVC ' (E-PVC), either in the form of a homopolymer alone or mixed with each.other, can be used for the purpose. Copolymer S-PVC and copolymer E-PVC can also be used. Of such, copolymer S-PVC and E-PVC with a content of 0.1-20 weight per cent of vinylacetate or an acryl compound in particular should be mentioned. Copolymer compounds of the type mentioned above can either be used separately or mixed with each other or with homopolymer S-PVC and/or E-PVC. Post- chlorinated PVC can also be used for the purpose. In prac¬ tical life, the choice of polymer will often come about as a result of which additives are being used or the additives will be selected to adapt the polymer compounds being used. The invention is especially characterized in that, unlike conventional methods, the decomposition of the blowing agent in the extruded product does not take place in an expensive high-pressure press with its great need for sealing in order to prevent gas escaping from the decomposed blowing agent during treatment in the high-pressure press, but in an auto¬ clave or high-pressure chamber in which both the pressure and the temperature are " produced by a circulating medium. Once the desired decomposition of the blowing agent in the extruded mixture has been achieved, this medium is replaced by the same medium in a colder state. The pressure- and heat-treated extruded mixture is then cooled to below the temperature at which the gas from the blowing agent would have been able to expand the polyvinyl chloride after the pressure release of the autoclave. In the mixture.which is introduced into the extruder, there should be a relatively large amount of blowing agents, 2-50 parts per 100 parts of polyvinyl chloride. The polyvinyl chloride used can be a homopolymer PVC and/or a copolymer an /or a postchlorinated PVC of the S-PVC and/or E-PVC type. As stabilizer in the mixture, a tin stabilizer as conven¬ tionally used for synthetic polymers is preferred, but other metalliferous stabilizers can also be used. It is also advantageous that the mixture contains a polyacrylate, preferably in a quantity of approx. 6-30 parts per 100 parts of polyvinyl chloride. As blowing agent, azodicarbonamide has turned out to be very suitable although other gas-generating blowing agent, which are decomposed in the temperature range of 130-250 C and at a pressure of 50-300 bar, also can be used. Some zinc oxide as a "kicker" together with some lubri¬ cant in the form of a mixture of polyethylene wax, paraffin wax or calcium-stearate is particularly advantageous. The mixture may also contain a certain amount of softener and also a certain amount of isocyanate to increase the heat resistance of the foam. A particular requirement of the invention is that the decom¬ position of the blowing agent is carried out under high pressure and at high temperatures using a medium which should have low solubility for the gases generated by the blowing agent. In other words it is important that the therτιo-regulating medium used has a barrier-forming effect, in that the gases formed from the blowing agent are either insoluble or soluble to a particularly small extent in the medium. As medium in this connection is meant the use of for example glycerol or brine with a certain salt content. These are particularly suitable when the blowing agent gives off nitrogen (N 2 ) and carbon monoxide (CO), as is the case with azodicarbonamide, but other liquids, which have the advantageous properties stated above, can also be used, for example synthetic oils (Shell N-8401). If azodicarbonamide is used as a blowing agent, glycerol could advantageously be used as a pressure- and heat-exchanging medium in the autoclave, as glycerol has a high boiling point and is particularly resistant to decom¬ position at high temperatures and possesses, moreover, low solubility for nitrogen (N 2 ) which is one of the products generated during the decomposition of the azodicarbonamide. The autoclave itself may have any length and section since possible limitations only will be due to design and space consideration. The homogeneously mixed, pregelled product which for example comes from the extruder during the execution of the present method, can thus be extruded in the form of a continuous length with a breadth and thickness suitable for autoclaving in an autoclave. The continuous length can also be divided into bits before the autoclaving. After treatment in the autoclave and cooling to the ambient temperature, the pressure- and heat-treated intermediate product can be divided into pieces of any size for subsequent expansion in already known way at any place where this might be desired. The expansion itself is carried out by subjecting the pressure- and heat-treated pieces, which contain the decom¬ posed blowing agent, to increased temperature. In this way, the pressure of the gas in these pieces increases, so that the pieces by the temperature used will swell up, thereby achieving an expanded state. An advantageous way of carrying out expansions in a limited cavity is by introducing super¬ heated water vapour at approx. 105 C. The intermediate product containing the decomposed blowing agent will thereby expand as a result of the internally increased gas pressure and the reduced deformation resistance of the polyvinyl chloride, and with suitable control of the expansion process, it is easy to obtain polyvinyl chloride foam , with a relative density of for example 0.08 to 0.5 kg/1. The polyvinyl chloride foam has a structure with relatively small, non-interconnected pores. It is relatively hard and has a substantial deformation resistance after being deformed (compression set), for example as a result of compression. It has above all, due to its special pore structure and the polymer's hydrophobia, a very low absorption level with regard to water. Its level of heat insulation is also very good, and because of its relative low density and low absorption level with regard to water, it is particularly suitable as buoyancy material for objects which are to be used in contact with water. As an example of such objects -can be mentioned mooring buoys, seamarks, net floats and hollow offshore structures where the buoyancy must be ensured and where at the same time any possible damage to the structure surrounding the polyvinyl chloride should entail as little absorption of water by the buoyancy material as possible. For this reason the product manufactured by the present method is particularly suitable for use as insulation and buoyancy material in platform legs for use in offshore structures. Experts have of course been aware of the advantageous properties of polyvinyl chloride foam to be used for the above mentioned purposes, but because of the expensive production process of the previously mentioned intermediate product, the price of such a polyvinyl chloride intermediate product for subsequent expansion on the application site itself has been virtually prohibitive as compared with other plastics foam such as for example polyurethane. Polyurethane, however, has poorer durability, poorer deformation resistance and a far higher absorption level with regard to water than polyvinyl chloride foam. There has therefore been a .great need for a method of producing such a polyvinyl chloride intermediate product at a competitive price and with a competitive production capacity. This need has been fulfilled by the present invention. The invention is in accordance with the claims. For a better understanding of the invention, reference is made to examples 1-8 where detailed instructions for the production of the product according to the invention are given. Example 1 The following components are mixed in a powder mixer or other, suitable mixing device: Suspension PVC (S-PVC) 100.0 parts Calciumstearate 1.2 parts Tin stabilizer 2.0 parts Azodicarbonamide 8.0 parts Zinc oxide 1.5 parts Polymethylmethacrylate 4.8 parts Dioctylphthalate 15.0 parts After premixing, the powder mixture is added to a single-screw extruder or an injection moulding machine, where the PVC mass is heated to approx. 160 C. The kneaded and pregelled mass is then extruded in a strip which is cooled. The strip can be wound up into a roll, cut into suitable lengths or granulated and used' in such a form for the subse¬ quent autoclaving. 25 kg pregelled PVC granulate and 20 litres of glycerol or synthetic oil (Shell N-8401) are added to a 50 litre auto¬ clave, after which the autoclave is closed and the air there¬ in replaced with nitrogen. The glycerol or oil is then heated to 180°C in a separate heating vessel outside the autoclave and circulates, to and from the autoclave through this. The blowing agent in the pregelled product will be decomposed as soon as the decomposition temperature is reached. This will be within the temperature range of 155-200 C, depending on the amount of "kicker". Without "kicker", decom¬ position can only be expected at temperatures above 235 C. During heating of the PVC foam-mixture there will be a pressure increase in the autoclave, depending on the amount of spare capacity, the initial pressure and the quantity of blowing agent added. After 30 minutes, the hot glycerol or oil is forced out of the autoclave and into a tank while, at the same time, cold glycerol or oil is added and the pressure in the autoclave and appurtenant connecting equipment is maintained. When the decomposed PVC mass is cooled to a temperature well below the second order transition temperature (Tg) of the PVC mass, the autoclave is relieved and the pressure in it brought down to the normal pressure outside the autoclave. The glycerol or oil is drawn off and the PVC mass poured into a centrifuge for separation of the additional glycerol. Thereafter the PVC mass is washed with clean water in the same device. The PVC foam is then dried ' and packed in bags for later use in situ. Example 2 Instead of the mixture according to Example 1, the tin stabilizer is replaced with Ba and Pb stabilizer with 2.0 parts of each and 2.0 parts of toluene di-isocyanate. The zinc oxide is removed from the formulation. Then the raw materials are mixed and the decomposed, unexpanded PVC foam will be produced according to Example 1. Example 3 Suspension PVC (S-PVC) 100.0 parts Calciumstearate 1.5 parts Tin stabilizer 2.0 parts Azodicarbonamide 8.0 parts Zinc oxide 1.5 parts Dioctylphthalate 10.0 parts The above components are mixed in a suitable device to be subsequently added to an injection moulding machine. After the mass has been mixed homogeneously and heated to approx. 180°C, suitable moulded objects are injection moulded, which are either autoclaved as they are or granulated. * The decomposed, unexpanded PVC foam mass is then produced according to Example 1. Example 4 The formulation according to Example 1 is added to a Banbury mixer and kneaded and pregelled therein. The decomposed, unexpanded PVC foam mass is then produced according to Example 1. Example 5 The formulation according to Example 1 is mixed, pregelled, granulated and autoclaved, but instead of glycerol as a heat-exchanging and barrier-forming medium, brine (approx. 30 g common salt per 100 g water) is used. Example 6 Emulsion PVC (E-PVC) 100.0 parts Polyethyleneglycol 2.0 parts Tin stabilizer 2.0 parts Sulphonyl hydrazide 6.0 parts Pb-phosphite 2.0 parts Polymethylmethacrylate 6.0 parts Dioctylphthalate 10.0 parts The above components are mixed and the decomposed, unexpanded PVC foam mass produced according to Example 1. Example 7 A copolymer emulsion polyvinyl chloride with 97 weight per cent polyvinyl chloride and 3 weight per cent vinyl acetate is used instead of homopolymer polyvinyl chloride. Otherwise one may proceed as described in Example 1. Example 8 Suspension PVC (S-PVC) 80 parts Postchlorinated polyvinyl chloride 20 parts are used. Otherwise one may proceed as described in Example 1. When the polyvinyl-chloride intermediate product which results from embodiments 1-8 is used in a measured amount for expansion on site, it will, as a result of the temperature increase during expansion, soften to such an extent that each one of the expanded PVC pieces will fuse together, thereby forming a whole in the cavity to be filled with PVC foam. It is of course possible to choose a formulation which produces PVC foam which does not soften enough for fusing together. In such cases the intermediate product can on beforehand be surface-treated with a fusing aid, for example hot-melt glue or a glue of an ordinary type (for example water-based emulsion glue).

Claims: Patent Claims A product which comprises a polyvinyl chloride, stabilizer, decomposed or otherwise gasified blowing agent and other possible process-regulation additives for the production of a polyvinyl chloride foam, where the product, as far as the blowing agent is concerned, is in a decomposed or gasified form and where the product is, moreover, in a pregelled but still unexpanded form, c h a r a c t e r i z e d i n t h a t the product is put in its gasified but unexpanded form, using a pressure- and temperature-regulating, barrier-forming medium, in an autoclave and volume-stabilized in its gasified form at a temperature level lower than the product's second order transition temperature (Tg) . Method.for producing the product according to claim 1, c h a r a c t e r i z e d i n t h a t the mixture of polyvinyl chloride, stabilizer, blowing agent and other possible process-regulation additives are transferred to a plastic processing machine in which the mixture is kneaded under the effect of pressure and heat without the blowing agent being decomposed, and leaves this machine in any form, length and breadth whatsoever, depending on the way in which pregelled, homogeneously mixed poly¬ vinyl-chloride product leaves the plastic processing machine, after which the set mixing is tranferred to a pressure chamber in which a temperature- and pressure-exchanging, barrier-forming medium at a pressure of 20-30 bar and at a temperature of 50-270 C is circulating, and is pressure- and heat-treated in the pressure chamber for a period of 5-80 minutes to decompose or gasify the blowing agent in the pregelled product, after which the circulating hot medium is replaced with a colder medium of the same or similar type to lower the temperature in the pressure chamber to below the second order transition temperature -(Tg) of the pressure- and heat-treated product, and that the pressure then is relieved and the pressure- and heat-treated product removed from the pressure chamber. 3. Product and method for producing the product according to claims 1 and 2, c h a r a c t e r i z e d in t h a t homopolymerized and/or copolymerized suspension poly¬ vinyl chloride (S-PVC) or homopolymerized and/or copolymerized emulsion polyvinyl chloride (E-PVC) or a mixture of both S-PVC and E-PVC in homopolymerized and/or copolymerized state is used as polyvinyl chloride. 4. Product- and method for producing the product according to claims 1 and 2, c h a r a c t e r i z e d i n t h a t a tin stabilizer is used advantageously as stabilizer for the mixture. 5. Product and method for producing the product according to claims 1 and 2, c h a r a c t e r i z e d i n t h a t azodicarbonamide is used advantageously as blowing agent. 6. Product and method for producing the product according to claims 1 and 2, c h a r a c t e r i z e d i n t h a t among others media, glycerol is used advantageously as pressure- and heat-exchanging medium.

Inventors: SPYDEVOLD BAARD, SPYDEVOLD, BAARD
Applicants: MASTER MARINE AS, MASTER MARINE A.S
Application year: 1985
Main classification: B29C67
IPC codes: C08J9