METHOD FOR PRODUCTION OF CROSSLINKED PLASTIC FOAM

Abstract: Method for production of a crosslinked, foamed vinyl chloride containing polymer. A copolymer is produced where vinyl chloride and a glycidyl (epoxy) containing monomer and eventually another monomer are used in the polymerisation. A blowing agent is added to the polymer where the decomposition products of the blowing agent or the decomposition products of the reaction products from the blowing agent are chemically reactive with the epoxy groups or their reaction products. Crosslinking is formed when the blowing agent or the reaction product of the blowing agent is decomposed. There is preferably used a monomer composition with 30-99.95 % vinyl chloride. The glycidyl containing monomer preferably is an acrylate, methacrylate or buthylacrylate compound added in an amount of 0,05-30 %. Blowing agents with for example ammonia, cyanic acid or isocyanic acid as decomposition product can be used. A preferred blowing agent is unmodified or modified azodicarbonamide.

Description: Method for production of crosslinked plastic foam The present invention concerns a new method for production of crosslinked foamed plastics, based on a copolymer where the monomer composition mainly consists of vinyl chloride. A relatively great part of the PVC comsumption is used for foamed products. To obtain a foamed product, a blowing agent is added to the composition. At high temperatures the blowing agent is decomposed giving gas as one of the products, which result in foaming of the product. The foaming can be carried out during the processing for example gelling of paste or free foaming after extrusion. The process also could be carried out in several stages. First the material can be processed below the decomposition temperature for the blowing agent, and then be decomposed in a separate stage. This often is done under pressure in an autoclave in such a way that the material is not foamed. After cooling the pressure can be reduced without foaming to occur. The high viscosity of the material at low temperatures is the cause of this. The foaming can be regulated by heating the blowing agent to a suitable temperature where the wanted properties are obtained, for example density and cell struc¬ ture. Foamed products are interesting because of their low den¬ sity. Based on volume this gives a cheaper product, because the material demanded simultanously decreases. Rigid, foamed PVC products often are used for "sandwich" constructions in automobile-, boat- and air industry. Soft products can be used as acoustic insulation, vibration damper, insulation, sealings etc. Both rigid and soft products are used in buoy¬ ancy bodies. If the mechanical stability for the PVC foam is improved, especially at high temperatures, the range of application will increase. This can be obtained by crosslinking the foamed product. Many methods are avaiable for crosslinking of foam, for example radiation, use of peroxide which results in the formation of free radicals, the addition of low molecular substances which can be crosslinked etc. Crosslinking during radiation is an expensive process which often results in disintegration of PVC during processing. The resulting pro¬ duct often will be a brittle material. A method much in use for production of crosslinked PVC foam, is a process developed by the French company Kleber Colombes (French patent No. 1 366 979) . By this method isocyanate is added both as plasticiser and crosslinking agent. The method gives a network which reinforce the PVC foam, but PVC is not a part of this network. There are several drawbacks with this system, but first of all it concerns the handling of isocya¬ nate, which demands great care. In addition the resulting product can be brittle and can have a reduced resistance to solvents. The object of the invention is to develope a new method for production of crosslinked polymer foam with good mechanical stability. It is essential to obtain a method with is en- vironmental friendly, simple and cheap. It is an object to avoid extra process steps as crosslinking for example by radiation. These and other objects of the invention are obtained by the method as described below and the invention is characterized and defined by the accompanying patent claims. By this invention one has found a new method for production of crosslinked vinyl chloride containing foam. It is simple, cheap, environmental friendly and does not demand irridation or extra addition of chemicals (compared to production of non-crosslinked foam) . The method is based on that one sur¬ prisingly has found that the decomposition products from the blowing agent which is added to the polymer, can be used as crosslinking agent, provided that the polymer contains a glycidyl compound. Therefore a copolymer is produced where vinyl chloride and a glycidyl (epoxy) containing monomer or eventually another monomer is used in the polymerisation. A blowing agent is added to the polymer where the decomposi- tion products from the blowing agent or the decomposition products of the reaction products from the blowing agent are chemically reactive with the epoxy-groups or their reaction products. Crosslinking takes place when the blowing agent or the reaction products of the blowing agent are decomposed. Preferably there is used a monomer mixture with 30-99,95% vinyl chloride. The glycidyl containing monomer preferably is an acrylate, methacrylate or buthylacrylate composition added in an amount of 0,05-30%. Blowing agents with for example ammonia, cyanic acid or isocyanic acid as decomposition products can be used. It was found that the crosslinking could be controlled by the use of certain blowing agents. A preferred blowing agent is unmodified or modified azodicar¬ bonamide. The method is applicable for copolymers both pro¬ duced with suspension-, microsuspension-, emulsion- or mass- polymerisation. Other features of the invention are described more in detail below. A blowing agent which can be used for a variety of purposes, for example together with PVC, is azodicarbonamide in pure or modified form. This is a product which decomposes at about 160-215° C, dependent on the environment. Azodicarbonamide can be decomposed in different ways: H 2 N-CO-N=N-CO-NH 2 ■ N 2 + CO + H 2 N-CO-NH 2 I (azodicarbonamide) H 2 N-CO-NH 2 > NH 3 + HNCO II (cyanic- or isocyanic acid) 2 H 2 N-CO-N=N-CO-NH 2 * H 2 N-CO-HN-NH-CO-NH 2 + N 2 + 2 HNCO III H 2 N-CO-HN-NH-CO-NH 2 • > HN-NH + NH 3 IV I I I I 0=C C=0 \ / \ / N I H Cyanic acid can be trimerizised to cyanuric acid similarly as isocyanuric acid can be formed from isocyanic acid. In addition to azodicarbonamide there are several blowing agents which liberate ammonia, for example trihydrazino- triazine and p-toluene sulphonyl semicarbazide. Regardless decomposition route for the azodicarbonamide reactive decomposition products as ammonia and/or isocyanic acid and urea are formed. From experience one know that these products does not give crosslinking of PVC. By this invention surprisingly one has found that a polymer where epoxy groups have been incorporated (grafted) through copolymerisation, can react with decomposition products as for example urea, ammonia and isocyanic acid from a blowing agent, and form crosslinking. The epoxy groups are introduced by copolymerisation of a glycidyl containing monomer, for example glycidyl methacry¬ late (GMA) , vinyl chloride (VCM) and eventually other mono- mers. Of the total monomer content there shall be 0,05-30 weight % glycidyl containing monomer, preferably 0,2-5 weight %. This is a practical limit because below 0.05% the effect will be too low and above 30% the product will be too expen¬ sive. The amount of VCM should make at least 30 % of the monomer content. The polymerisation can be performed by suspension-, microsuspension-, emulsion- or masspolymeri- sation. By copolymerisation of VCM and GMA, especially by suspension polymerisation at high temperatures, it has turned out to be favourable to add GMA after about 30 min. polymerisation. This is done to obtain a stable suspension. By copolymeri¬ sation of VCM and GMA, GMA will be consumed relatively faster than VCM (shown by the reactivity/ratios ) . Because it is favourable to have such a uniform distribution of GMA in the polymer as possible, this is preferably added in intervals during polymerisation. As GMA is rapidly consumed, it will not be enriched in the monomer phase, something obviously being an advantage if one wants to use a PVC production plant. There are several possible reactions between the epoxy con¬ taining polymer and the decomposition products from the blowing agent. One probable reaction is that ammonia reacts with two or three epoxy groups and form crosslinking. Other possible reactions are where isocyanic acid or isocyanuric acid is involved. These can react with epoxy or with hydro- xyl groups. The reaction patterns however will be similar to the reaction between epoxy and ammonia. The reaction between hydroxyl and isocyanate is catalyzed by tertiary amines. These are the reaction products if ammonia reacts with three epoxy groups. It can not be excluded that urea takes part in the reaction either. The following examples will illustrate the invention: EXAMPLE 1 Three different polymer formulations were produced. These are shown in table 1. Table 1. Recipe HP1 CP1 CP2 1 commercial leadstab. (Akzo Chemicals) 2 azodicarbonamide (Schering Industrial Chemicals) 3 modified azodicarbonamide (Schering Industrial Chemical) processing aid (Rohm and Haas Company) The compounds were roll-milled for 3 minutes at about 150° C. Blowing on the roll-mill did not occur. Ribbons of the roll- milled sheet were placed in a Werner-Mathis oven at 185° C in 5 respectively 10 minutes. The following analysis were performed: Gel content: (=gel) Qualitative (continous gel or not) or quantitative gel content (i.e. part of gel in solvent being insoluble in PVC) was measured in tetrahydrofuran (THF) at room temperature. Deformation: Deformation as a function of temperature was measured on a Mettler TMA. An indentor with a cross sec¬ tional area of 7 mm 2 was loaded to the sample with a force of 0,02N. The start temperature was 35° C and the heating rate was 10 C/min. The final temperature in the chamber was 210° C, which gave the sample a final temperature of 200° C. The results given in table 3 are deformation at final tempera¬ ture. Calorimetry (DSC) : The decomposition progress is measured with differential scancalorimetry (DSC) . The start tempera- ture was 35° C and the heating rate was 10 C/min. The results of the tests are shown in the following tables. Table 2 Recipe Blowing temp. Starting tempera- (Peak) , (°C) ture before blow¬ ing (°C) HP1 175 160 CP1 180 160 CP2 173 150 Table 2 shows that there are no significant difference bet¬ ween the blowing temperature for the copolymer (VCM:GMA) and PVC homopoly er. = W. Mathis oven at 185°C. The results for HP1 confirm earlier experience which indi- cate that PVC homopoly ers will not be crosslinked by the decomposition products. The fact that CP1 is not crosslinked after 5 minutes al¬ though it is foamed, can indicate that the decomposition follows chemical equation (I) and (II), i.e. in the first step the non-reactive decomposition products are formed and in the next step reactive products are formed. The impor¬ tance of this is that foam can be formed before the cross- linking takes place. This is favourable as a crosslinking increases the viscosity and normally also make the foaming process more difficult. CP2 was crosslinked faster than CP1 which is due tc that azodicarbonamide is modified in this case leading to quicker decomposition, which also should mean that the reactive decomposition products are liberated faster. The much improved deformation results for CP2 is very logi¬ cal, when an improvement of the deformation properties is to be expected when crosslinking is introduced. This is one of the most important improvement of properties obtained by crosslinking. EXAMPLE 2 Extrusion was carried out on a twin screw extruder, where the recipe was based on PVC and copolymer (VCM:GMA 99:1), see table 4. The experiment was carried out with two temperature profiles, compare table 5. After extrusion a part of the samples was placed in a W.Mathis oven where the temperature was 185° C. Table 4: Recipe EH1 EC1 Table 5: Sample Density (kg/m 3 ) EH1 130 140 155 200 0 no 500 130 140 155 200 5 no It 130 140 195 206 0 no 650 Table 5 shows as expected that the use of PVC homopolymer does not give crosslinking. The table also shows that when the copolymer is used, one can choose method to obtain cross- linking. For EC1 it is shown that when the temperature is kept below the decomposition temperature of the blowing agent, blowing (foaming) and crosslinking can be carried out in a separate step after the extrusion, see number 1 and 2. If the temperature in the extruder is chosen to be kept above the the decomposition temperature of the blowing agent for a short time interval, foaming can be carried out, but not crosslinking, see number 3 and 4. By keeping the temperature high over a long section of the extruder, the time interval is increased and both crosslinking and foaming during ex¬ trusion is obtained, see number 5. Having these possibilities, it should not be difficult to adapt this process to multi-stage processes where decompo- sition is carried out under pressure. Dependent on how long time the material is kept above the decomposition tempe¬ rature of the blowing agent, one can choose crosslinking before or after foaming. In special cases where a high degree of foaming is wanted, it can be preferable to modify the melth strenght to an optimum level by introducing a certain degree of crosslinking before foaming. In the above examples, soft products are used. In principle there is no difference in crosslinking rigid or medium flexi- ble product according to this method. If not foaming during processing is wanted, however, the selection of kickers (activator) will be reduced, because the temperature during processing will be higher. By use of vinyl chloride contain¬ ing polymers, the choice of thermostabilizer will be more critical. If calcium/zinc based or tin based stabilizers are used, the material can be processed without having foaming on roller or extruder. Crosslinking in connection with foam ing is easiest obtained if modified azodicarbonamide is used (gives foaming at lower temperatures) . EXAMPLE 3 The materials in table 6 were roll-milled for three minutes at about 165° C. This gives a mass temperature of about 175° C. The materials were foamed in an oven at 185° C. In table 7 is given if the crosslinking has taken place after foaming. Table 6: Recipe CP3 CP4 CP5 CP6 CP7 Buthylbenzyl- phtalate 5 10 10 - 10 Irgastab CZ-130 6 2 2 2 2 2 Edenol D-81 7 2 - 2 - 2 5 Plasticiser 6 Commercial Calsium/Zink stabilizer (Ciba-Geigy) 7 Epoxidated soyaoil In table 6 and 7 are shown that in certain formulations it is necessary to use a modified azodicarbonamide, for example Genitron SCE, to obtain crosslinking. EXAMPLE 4 By using the method for plastisols the following formulations are used, refer to the recipe in table 8. Table 8 : Recipe PCI PC2 PHI Emulsionpolymer (PVC) - - 100 Emulsionspolymer 100 (VCM:GMA 99:1) 8 Dibasic leadphosphite (Ther ostabilizer) 9 Azodicarbonamide in finely divided dispersion. These formulations are mixed and the paste is coated onto a release paper. The plastisol is gelled and foamed at 190° C. In table 9 it is given whether crosslinking has taken place after foaming or not. Table 9: Sample Time in oven Gel in THF at 190°C Table 9 shows that the use of PVC homopolymer does not give crosslinked foam, but that the copolymer (VCM:GMA) gives crosslinking in connection with foaming of the plastisol when azodicarbonamide is used as foaming agent. It is thereby shown that the method also can be used for plastisols. By this invention one has developed a method which make it possible to crosslink foamed vinyl chloride polymers through an addition of epoxygroups which are introduced via a copoly- mer. The crosslinking takes place by help of the decomposi¬ tion products from the blowing agent. The use of other chemi¬ cal compositions than those used for foaming is not neces¬ sary. As long as the processing temperature is lower than the decomposition temperature there is no risk for unwanted crosslinking to take place for example during extrusion or rolling. By the use of certain blowing agents the cross- linking can be controlled in such a way that the crosslinking starts after the foaming has started. By use of polymers containing mainly vinyl chloride, the process works by processing of flexible, semiflexible and rigid suspension polymers. The process also functions for processing of plas¬ tisols. Besides, it is not much sensitive to amendments in the stabilizer systems.

Claims: Patent claims 1. Method for production of a crosslinked, foamed vinyl chloride containing polymer, c h a r a c t e r i z e d i n t h a t a blowing agent is added to a copolymer produced by copolymerisation of vinyl chloride, glycidyl (epoxy) containing monomer and eventually other monomers, for foaming of the copolymer and where the decomposition products of the blowing agent or the decomposition produ¬ cts of the reaction products from the blowing agent, are chemically reactive with the epoxy groups of the copoly¬ mer or its reaction products, and where crosslinking takes place when the blowing agent or the reaction pro¬ duct of the blowing agent is decomposed. 2. Method according to claim 1, c h a r a c t e r i z e d i n t h a t first the polymer compound is foamed by being exposed to temperatures above the decomposition temperature of the blowing agent in a short time interval, for thereafter to be crosslinked by heat treatment above the decomposition temperature of the blowing agent for a longer periode. 3. Method according to claim 1, c h a r a c t e r i z e d i n t h a t foaming and crosslinking is performed in one process step. 4. Method according to claim 1, c h a r a c t e r i z e d i n t h a t there is used a glycidyl- containing monomer which is an acrylate- or methacrylate or buthylacrylate composition. 5. Method according to claim 4, c h a r a c t e r i z e d i n t h a t a polymer containing 0,05-30% glycidyl containing acry¬ late is used. 6. Method according to claim 1, c h a r a c t e r i z e d i n t h a t it is used a monomer composition with 30-99,95% vinyl chloride. 7. Method according to claim 1, c h a r a c t e r i z e d i n t h a t it is used a copolymer of vinyl chloride and glycidyl methacrylate (GMA) produced by suspension-, microsus- pension-, emulsion- or masspolymerisation. 8. Method according to claim 6, c h a r a c t e r i z e d i n t h a t a monomer composition is used where VCM makes 70-99,95 % and preferably 99,98 % of the composition, and that GMA makes 0,05-30 % and preferably 0,2-5 % of the monomer composition. 9. Method according to claim 1, c h a r a c t e r i z e d i n t h a t it is used a blowing agent having ammonia or cyanic acid and/or isocyanic acid as decomposition products. 10. Method according to claim 1, c h a r a c t e r i z e d i n t h a t it is used azodicarbonamide, unmodified or modified, as blowing agent. 11. Method according to claim 10, c h a r a c t e r i z e d i n t h a t it is used azodicarbonamide which is surface treated with zinc oxide.

Inventors: BOERENG RAYMOND, DAHL ROGER, PEDERSEN STEINAR, BOERENG, RAYMOND, DAHL, ROGER, PEDERSEN, STEINAR
Applicants: NORSK HYDRO AS, NORSK HYDRO A.S
Application year: 1990
Main classification: C08J9
IPC codes: C08J9