SPACER MATERIAL AND METHOD FOR THE MANUFACTURE OF THE SAME

Abstract: Spacer material (21) suitable for sandwich constructions (21, 22, 23) is manufactured with the help of a low-pressure chamber (9). Inside a first space (8) at atmospheric pressure air bubbles are introduced into a curing agent-compatible plastics material, for example a thixotropic laminating polyester. The material thus provided with air bubbles is placed on an exposed mould (5) or on a base inside or outside the aforementioned low-pressure chamber. After the material provided with air bubbles has been placed inside the chamber the latter is actuated in order to provide a second pressure which is considerably below the first pressure. The plastics material takes rigid shape at said second pressure inside the chamber for a short tack-free time, and the air bubbles which have been caused to expand by the low pressure are trapped in the plastics material during the solidifying process.

Description: TITLE Spacer material and method for the manufacture of the same. TECHNICAL FIELD The present invention relates to a spacer material made of plastic which is intended to be used, amongst other things, in various sandwich constructions. Then, the plastic consists of thermosetting plastic material having foamed structure. The invention also relates to a method for the manufacture of, amongst other things, the aforementioned spacer material. The manufacturing process is particularly suited to taking place inside a low-pressure chamber so arranged as to be capable of achieving a high degree of low pressure, with a vacuum of 60-95% being envisaged in this case. PRIOR ART It has been usual until now in the manufacture of foamed plastic products to utilize some form of chemical propellant, a comparatively large quantity of which is dissolved in the base material or is added to it immediately before the foaming process is to commence. Foaming can also be achieved by the use of physical propellants in the form of a volatile gas such as freon (for example freon 11) which is dissolved in (that is to say forms part of a one-phase system with) the base material. A familiar example of the last-mentioned method is provided by the manufacture of rigid polyurethane foam. Upon activation of the propellant in question, for example by means of catalysts, heat and/or pressure, the propellant will cause gas to be generated, resulting in the formation of bubbles which are apparent in the finished product. The use of a large amount of foaming additives will, however, give rise to various manufacturing problems, for example problems associated with curing and storage which, viewed as a whole, complicate the manufacturing process and impair the product. Furthermore, the choice of base materials and the range of possible variations in the production conditions, for example the variations in temperature, are restricted to a certain extent, which has a negative effect on the result. DESCRIPTION OF THE INVENTION Technical Problem A wish associated with plastic foamed products falling within this category is for technically simple manufacturing processes to be available which in addition provide conditions for mechanically sound products at comparatively low manufacturing costs. In appropriate cases it may be wished to modify the production process to suit existing or intended low-pressure chamber equipment, the environmental advantages of which may thus be utilized in the production facility in question. The Solution • The present invention proposes a spacer material and a method for its manufacture which will resolve, amongst other things, the problem outlined above on the basis that bubbles of air which are able to enter into a two-phase system with the base material are added to the base material, said bubbles being caused to expand at a pressure which is less than the pressure at which the bubbles were added. What may more precisely be regarded as being essentially characteristic of the noval spacer material is, thus, that the foamed structure is attained by means of bubbles which expand under pressure in the form of air which has adopted an undissolved state inside the thermosetting plastics material in order to form a spacer material esentially free from additives in the form of chemical foaming agents and/or physical propellants dissolved in the material. In a preferred embodiment it is proposed that the thermosetting plastics material shall consist primarily of polyester which has been provided with confined air bubbles. In certain embodiments the spacer material is also reinforced with carbon fibres or glass fibres. The spacer material can be provided in a preferred sandwich construction with a covering layer of identical material, but with a smaller number of bubbles or entirely without bubbles. What may essentially be regarded as characteristic of the novel method is that in a first space which is at a first pressure, preferably being atmospheric pressure, air bubbles are introduced into a curing agent-compatible plastics material, in that the plastics material thus provided with air bubbles is applied on whole or parts of the mould or a base inside the first space or a second space which, in each case, preferably consists of the aforementioned low-pressure chamber, in that the first or the second spaces are or are caused to be at a second pressure considerably below the first pressure so that the air bubbles introduced into the material are caused to expand, and in that the plastics material is taken rigid state/is polymerized with epanded air bubbles at the aforementioned second pressure inside the space concerned for a comparatively short tack-free time, for example 0.5 - 20 minutes, and preferably 5 - 10 minutes. In further developments of the idea of invention it is proposed that measures shall be adopted in order to prevent the situation arising in this context in which an excessively rapid rate of ascent is achieved by the gas bubbles mixed in at the first pressure. Accordingly, the base material must, for example, be essentially free from wetting agents. Also, the admixture of small bubbles is preferable in the interests of reducing the rate of ascent. The material selected will preferably be a thixotropic thermosetting plastics material having a viscosity at high shear rates of preferably 140-300 mPa«s. The curing agent is added preferably at the end of the air admixture period and/or in conjunction with the. pouring operation onto the mould or the base. It is preferable to utilize for the purpose of expanding the air bubbles mixed in at the aforementioned first pressure a low-pressure chamber capable of operating at a vacuum of 60-95% and preferably 70-90%. A chamber of this type can include a suitable space for the product and can be equipped with, amongst other things, openable and closable hatches and, if appropriate, material application organs, etc. OMPI Advantages By means of the invention it is possible to obtain a material which contains neither chemical foaming additives nor physical propellants dissolved in the base material, on condition that, in the last-mentioned case, the small quantity of air which is always present in the base material right from the start is ignored. In this way products which exhibit high mechanical characteristics can be obtained at a low price. The working environment can be designed to a high standard. The density and/or the reinforcement of the finished product is easily controlled by means of the equipment which is used. The curing of the product may be made simple and distinct, thereby providing the necessary conditions for high-quality products. By usage of heavy negative pressure in order to expand the bubbles, a low pressure is attained in said bubbles, which implies advantages in the curing process. The negative pressure having advantages in the usage of polyester. DESCRIPTION OF THE FIGURES Preferred embodiments of a spacer material and a method for its manufacture are described below by means of two examples, with simultaneous reference to the accompanying drawing, in which; Figure 1 illustrates the principle behind the manufacture of a spacer material in accordance with the first example; and Figure 2 illustrates the principle behind the manufacture of a spacer material in accordance with the second example; and Figure 3 illustrates examples of a sandwich construction. DETAILED EMBODIMENT Example 1 (Figure 1) In a container 1 is placed unsaturated polyester, for example laminating polyester (suitable for spray-up operation) of the type used within the small boat industry. The polyester is thixotropic and exhibits a viscosity at high shear rates of, for example, 150 mPa « s. The polyester should preferbly be at least essentially free from the wetting agents which are otherwise normally used to facilitate the admixture of the filler. The unsaturated polyester should preferably be modified in such a way that rapid bilding can be achieved after the addition of curing agents, for example peroxides. The curing time/tack-free time or the gel time should preferably be selected within the range 30 seconds - 20 minutes, and preferably 5 - 10 minutes. The tack-free time can be adjusted in a previously disclosed fashion by means of accelerators, inhibitors and curing agents. A high-speed agitator 2, for example an agitator rotating at a speed of about 2000 r/min, is utilized for stirring the unsaturated polyester in the container 1 in such a way that the admixture of the air takes place via the opening la in the container. The quantity and the size of the air bubbles which are mixed in is determined by the speed of agitation, the design of the agitator, the period of agitation and the chemical structure of the unsaturated polyester. Bubbles of sizes within the range 0.1 mm ' - 0.001 mm or smaller should preferably be aimed at, with the total quantity of air mixed in representing 10-30 per cent by volume, and preferably 15-20 per cent when mixed in at atmospheric pressure. The smaller the sizes of the bubbles introduced into the polyester, the smaller and denser will be the cells which are trapped in the final material. Small air bubbles exhibit a low rate of ascent, moreover, with the result that the air bubbles will take longer to rise to the upper surface and to pass into the surrounding atmosphere. This means that, the smaller the air bubbles which are obtained in a given unsaturated polyester, the longer will be the permissible gel time. It is worth mentioning in this respect that longer gel times will result in less critical manufacturing processes and in the possibility of increased quality in the finished product. Immediately prior to the end of the process of mixing in the air bubbles a curing agent 3 is added, for example via a pipe 4, and the polyester thus provided with a curing agent is tapped off into a "BUREA OMPI < r^ WIPO mould 5 via a pipe 6 equipped with a tapping cock 7. Agitation, the addition of the curing agent and tapping into the moulds should preferably take place in a first space 8 at a first pressure, preferably being atmospheric pressure. The mould thus filled with the polyester containing a curing agent is introduced into a second space 9 which can be placed under a second pressure considerably below the first pressure. The second space should preferably consist of a chamber, which may be of a previously disclosed type, operating at a very high degree of low pressure. The chamber is equipped with an openable and closable unit 9a and with air evacuation equipment (not shown), and can be positioned inside or outside the first space. The mould is placed inside the chamber at atmospheric pressure, whereupon the chamber is sealed and the air evacuation equipment is actuated until a partial vacuum of 50-400 mbar has been achieved, and preferably 100-300 mbar. The bubbles of air which have been stirred in will expand thanks to the partial vacuum which has been created, and thanks to the aforementioned short curing/ ake-free time and ascent time the expanded air bubbles will become trapped in the plastics material, producing a polyester foamed material. The density of the material is controlled by the quantity of air bubbles which are mixed in and caused to expand, by the degree of partial vacuum, and by the length of the curing time. By the appropriate choice of polyester quality, agitation and degree of partial vacuum, it is possible to achieve a density of about 3 170-600 kg/m and above for the finished product. The product is suitable to be included in sandwich constructions in which the covering layer consists of more or less bubble-free material of the same kind. When manufacturing sandwich constructions a first bubble-free layer can be placed on the mould 5, whereupon the material provided with bubbles followed by more bubble-free material are added in turn. Expansion and curing can take place in accordance with the above. It is also possible for the curing of one or both of the covering layers to take place outside the chamber at the first pressure, that is to say at atmospheric pressure. Example 2 (Figure 2) The air bubbles are mixed in as in Example 1 by means of an agitator 2' inside the container 1'. with polyester of a similar kind. Following the admixture of the air bubbles the material is pumped by means of a pump 10 to a material application arrangement 11, for example a spray pistol of a previously disclosed type. A similarly previously disclosed cutter 12, for example a glass fibre cutter, may be mounted on the pistol. With the help of the cutter chopped fibres 13 are thrown into the jet of polyester 14 which is directed towards the mould 5'. Curing agents, for example peroxides, for the polyester are added from a container 15 and are mixed in the pistol 11 with the polyester in a previously disclosed fashion. The curing agent may alternatively be mixed with the jet outside the pistol. In the case of non-reinforced products the cutter is not activated or may be omitted. Spraying of the mould can take place at atmospheric pressure or at partial vacuum inside a low-pressure chamber 9 in accordance with Figure 1, in which case, in the event of spraying taking place inside the chamber, the chamber is also provided with material application organs and, if appropriate, with cutter equipment. Curing takes place in accordance with Example 1. A sandwich construction may also be produced in this instance, in which case the spacer material is cured under a partial vacuum, and the covering layer is cured preferably at atmospheric pressure. Reinforcement of a product will result in a product with relatively excellent properties. The admixture of, for example, 30 per cent by weight of chopped fibres (glass fibre) at a density of about 3 3 800 kg/ , which should be compared with the density of 1500 kg/ for non-foamed products, will result in a bending strength of about 22 Pa, a modulus of elasticity on bending of about 1000 mPa, and a co pressive strength of about 14 mPa. It may be mentioned from a purely general point of view that the degree of reinforcement can be selected from within the range 10-40 per cent by weight, and preferably 20-30 per cent by weight. Curing or the process of attaining a rigid (tack-free) shape takes place in accordance with Example 1. Figure 3 shows examples of a sandwich construction in which the spacer material (the core) is identified by the reference designation 16, the covering layer by 17 and 18, and the expanded bubbles in the spacer material by 19. The fibre reinforcement is identified by the reference designation 20. As an alternative to the polyester in the aforementioned example, epoxy or phenol-formaldehyde and their compatible curing agents may be used. The invention is not restricted to the embodiments shown above by way of example, but may undergo modifications within the context of the following Patent Claims and the idea of invention. The admixture of air bubbles (agitation), the addition of the hardener, the pouring onto the mould or onto the base, and the curing process (both for the spacer material and for the covering layers, if present) may thus take place inside one and the same space, in the form of the aforementioned low-pressure chamber, to which the first and the second pressures are applied depending on what pressure is required in order to execute the Idea of invention. The application of plastic material and curing agent may, alternatively, take place under a partial vacuum. Either all or parts of the manufacturing process may be automated.

Claims: PATENT CLAIMS 1. Spacer material (21) intended primrily for sandwich constructions (21, 22, 23) and comprising thermosetting plastic material and having a foamed structure, c ha ra c t e r i z e d in that the foamed structure is attained by means of air bubbles (24) which have expanded under pressure in order to create a thermosetting plastics material which is essentially free from additives in the form of chemical foaming agents and/or physical propellants dissolved in the material. 2. Spacer material in accordance with Patent Claim 1, cha r a c t e r i z ed in that the thermosetting plastics material consists of polyester and the curing agent of peroxide. 3. Spacer material in accordance with Patent Claims 1 or 2, c ha r a c t e r i z ed in that it is fibre-reinforced, for example with chopped glass fibres (25) in a proportion of 10-40 per cent by weight, and preferably 20-30 per cent by weight. 4. Spacer material in accorcance with Patent Claims 1, 2 or 3, charac t e r i zed in that the density of the non-reinforced material 3 is from about 170 kg/m upwards. 5. Method for the manufacture of, amongst other things, the spacer material indicted in Claim 1 and intended to be used primarily in conjunction with the low-pressure chamber (9), c h a r a c t e r i z e d in that inside a first space (8) which is at a first pressure, preferably atmospheric pressure, air bubbles are introduced into a curing agent-compatible plastics material, preferably in the form of unsaturated polyester, in that the plastics material thus provided with air bubbles is placed on whole or parts of a mould (5) or a base inside the first space or a second space which, in each case, preferably comprises the aforementioned low-pressure chamber, in that the first or the second spaces are or are caused to be at a second pressure considerably below the first pressure so that the air bubbles introduced into the material are caused to expand, and in that the plastics material is taken a rigid shape/is polymerized at the aforementioned second pressure inside the space concerned for a comparatively short tack-free time, for example 0.5 - 20 preferably 5 - 10 minutes. b 6. Method in accordance with Patent Claim 5, c h a r a c t e r i z e d in. that into a thixotropic and preferably unsaturated thermosetting plastics material, essentially free from wetting agents, are introduced air bubbles in a quantity which represents 10-30 per cent by volume at the first pressure and with a bubble diameter of the order of magnitude of 0.1 mm or less, and in that the plastics material is placed with or without the curing agent inside a chamber (9) which is then actuated until a high degree of low pressure is reached, in conjunction with which the plastics material, in the event of it being placed inside the chamber without a curing agent, is then added to the chamber. 7. Method in accordance with Patent Claims 5 or 6, c h ar a c t e r i z e d in that, inside a vessel (1) containing preferably unsaturated plastics material, for example a thixotropic laminating polyester which has been modified by the choice of accelerators, inhibitors and curing agent, is activated a high-speed agitator (2) for the purpose of mixing air bubbles in the plastics material, in that immediately prior to the-conclusion of the desired mixing of the air bubbles a curing agent (3) is added, and in that the agitator is switched off and the resulting mixture is placed in a mould (5) situated inside the low-pressure chamber (9) which is then evacuated in order to achieve the second pressure. 8. Method in accordance with Patent Claims 5 or 6, cha ra c t e r i z e d In that, inside a vessel (1*) containing preferably unsaturated plastics material, being preferably a thixotropic laminating polyester, is activated a high-speed agitator (2 1 ) at atmospheric pressure so that the surrounding air is mixed in during agitation, in that the material provided with air bubbles is pumped by means of a pump (10) to a material appliction arrangement (11), for example a spray pistol, and in that inside or outside a low-pressure chamber (9) the material is placed on part of or on the whole of a mould by means of the aforementioned arrangement (11), in conjunction with which the curing agent and any reinforcement (13) is added to the mould together with or separately from the plastics material (14) provided with air bubbles, and in that for the purpose, of being sprayed at atmospheric pressure the mould is placed inside the chamber and the latter is so activated as to achieve the second pressure.

Inventors: BAKKELUNN TERJE, BAKKELUNN, TERJE
Applicants: ROVAC AB, AB ROVAC
Application year: 1984
Main classification: B29D27
IPC codes: B29C44