Abstract:
A composite material comprising silica cenospheres and a resin, whereby the resin is adapted to bind the cenospheres into a solid mass and retain them in a desired shape into which the composite material is to be formed. The resin is a polymeric compound, formed in situ around the cenospheres. The composite material can be used to form constructional elements used in modular buildings. As such they can include fire retardant materials to improve the safety of the buildings.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to improved modular buildings and to methods of constructing modular buildings, particularly those which are suited to withstand earthquakes and high winds.  
           [0002]    Additionally, the invention relates to structural elements for use in the construction of modular buildings and particularly to composite materials for use in the manufacture of construction elements for buildings.  
         BACKGROUND OF THE INVENTION  
         [0003]    The humanitarian and economic impact of natural disasters such as earthquakes and extreme adverse weather conditions such as high winds is becoming of increasing concern to many nations, the recent devastation in Central America being a particular example. Apart from the damage caused to structures, such as bridges and buildings being extremely expensive to repair, the danger to human life brought about by the collapse of such structures means that stronger materials and better methods of construction are increasingly being used.  
           [0004]    Unfortunately, however, many of the areas frequently affected by natural disasters are often poor and the costs of materials and improved construction is usually prohibitive. Furthermore, the quantity of raw materials necessary to produce the volume of housing required, could cause a negative impact on the environment, firstly due to the depletion of the raw materials and secondly due to the method of production of the final material which method can be energy intensive. Moreover, a primary consideration following a large scale disaster is that any new buildings erected, should be erected as rapidly as possible. Again, with conventional materials of construction this is not usually done as rapidly as desired, which increases the misery and suffering of the victims. It is also worthy of consideration that, following a large disaster, there will most likely be insufficient skilled labour available to carry out the construction required. There is therefore a need for the rigid deployment of the means to facilitate the construction of buildings which can be constructed using predominantly unskilled labour.  
           [0005]    A further and more general problem when constructing a large number of buildings is the provision, at the construction site of the elements needed to carry out the building work. This includes, typically, not just brick, steel, sand or wood for example, but also the means for producing concrete, mortar and correctly forming walls, window frames to the correct size. Such preparation of materials requires a great deal of time which is not always desirable either from an economic or a humanitarian standpoint. Furthermore, commonly used materials, particularly steel and wood are prone to degradation such as rusting or rot. Wood also can be subject to insect and mould attack which reduces its mechanical strength.  
           [0006]    It is therefore desirable to develop new materials from which the constructional elements of a house such as the walls and roof can be made. Such new materials must be durable and also able to withstand extremes of wind and temperature.  
           [0007]    It is an object of the invention to utilise available materials to provide composite materials for use in the construction of modular housing which can be produced at minimal impact to the environment.  
           [0008]    It is a further object of the invention to provide constructional elements which can be rapidly formed and deployed when constructing a building.  
           [0009]    It is a yet further object of the present invention to provide low cost and easily constructable modular housing which is capable of withstanding extreme weather conditions or even earthquakes.  
         SUMMARY OF THE INVENTION  
         [0010]    According to the invention there is provided a composite material comprising silica cenospheres and a resin, whereby the resin is adapted to bind the cenospheres into a solid mass and retain them in the desired shape into which the composite material is to be formed. The resin is preferably present in an amount from 2-10% w/w. It is particularly advantageous that the amount of resin present is 4-6%. The low usage of resin minimises the cost of the composite material and increases its mechanical strength.  
           [0011]    The resin may be selected from one or more of the following classes of polymeric compounds, polyester, polyurethane, polyacrylic, polyphenolic, polybromophenolic, polyvinylester or an epoxy resin, or mixtures thereof.  
           [0012]    The silica cenospheres preferably include a plurality of air-spaces to minimise the weight of each cenosphere, and therefore the mass of any article formed using said composite material.  
           [0013]    Advantageously, cenospheres of differing diameters are used to maximise the packing efficiency of the cenospheres within a composite material from 0.5 to 8 mm. A distribution of 2 mm, 1 mm and 0.5 mm diameter cenospheres in a w/w ratio of 3:2:1 has been found to be particularly advantageous.  
           [0014]    According to another aspect of the invention, there is provided a method of manufacturing a composite material product utilising cenospheres, the method including:  
           [0015]    distributing cenospheres within a resin monomer to form a mixture;  
           [0016]    distributing a polymerisation catalyst within the mixture;  
           [0017]    pouring the mixture into a mould; and  
           [0018]    removing the polymerised product from the mould.  
           [0019]    Preferably, the monomer is selected from an unsaturated ester such as a vinyl ester an epoxy compound, or an unsaturated hydrocarbon, to give a strong yet partially flexible composite product.  
           [0020]    The polymerisation catalyst is selected according to the monomer and can include hexamine or methylethyl ketone peroxide. The catalyst is optionally supported on an inert matrix. The use of an inert matrix enables the user to slow down or delay the polymerisation process. Advantageously, the inert matrix is a clay such as bentonite. Ultrasound can optionally be passed through the mixture to break apart the clay structure and release the catalyst rapidly into the mixture.  
           [0021]    The mixture can be vibrated during the polymerisation process to reduce any separation due to the different density of the components or due to any phase changes which may take place so as to give a consistent product.  
           [0022]    The mixture may also be optionally physically drawn into the mould, to reduce the number of air pockets in the finally formed product. The vacuum pump is particular preferred to draw the mixture into the mould.  
           [0023]    According to a fifth aspect of the invention there is provided a construction element such as a wall or roof panel for a modular building, the construction element comprising a composite material as defined hereinabove. The composite material has bonded to and across at least part of its surface a second composite material comprising a number of layers, including a first inner nylon layer and a second outer nylon layer, a glass fibre mat layer between the first and second nylon layers; and a polymer resin distributed between the nylon layers to bond the nylon layers together.  
           [0024]    The thickness of the nylon layers is preferably between 17-21 μm, so that the nylon layers neither tears too easily during processing nor distorts during processing. A value of 19 μm has been found to be particularly preferable.  
           [0025]    The resin is preferably present at a level of 30-50% w/w of the second composite material and particularly preferably at a level of 20% w/w. The resin can be selected from one or more of the following classes of polymeric compounds, polyester, polyurethane, polyacrylic, polyphenolic, polybromophenolic, polyvinylester or an epoxy resin, or mixtures thereof.  
           [0026]    A fire retardant material is preferably included between the nylon layers. The fire retardant material is preferably aluminium trioxide which can absorb heat well, and disperse the path of a flame or heat through the second composite material. The aluminium trioxide is advantageously present at a level of 15-50% w/w with respect to the resin. The level is preferably from 30-40% w/w, and particularly preferably approximately 35% w/w. Optionally, the fire retardant material can comprise glass flakes. In a further alternative, a fire retardant chemical such as polyhalogenated phenols, for example bromophenols can be included.  
           [0027]    The second composite material advantageously includes a glass-tissue layer having a density of 14-30 g/m 2 , to reduce uneven features being introduced to the surface of the second composite material.  
           [0028]    The second composite material includes a colouring agent such as titanium dioxide to improve its appearance. The surface of the second composite material can be altered by means of an electric current passed across the surface, the surface structure then being enabled to receive, for example, paint or varnish. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The invention will now be described more particularly with reference to the accompanying drawings which show, by way of example only, constructional elements manufactured using the composite materials of the invention. In the drawings,  
         [0030]    [0030]FIG. 1 is a detailed cross-sectional side elevation through a foundation floor;  
         [0031]    [0031]FIG. 2 is a cross-sectional side elevation through a foundation floor to which there is secured a wall-retaining member;  
         [0032]    [0032]FIG. 3 is a schematic top plan view of three panels joined to form a wall;  
         [0033]    [0033]FIG. 4 is a detailed section top plan view of the panel junction shown in FIG. 3;  
         [0034]    [0034]FIG. 5 is a vertical section taken through a wall panel sitting in a wall-retaining member and secured to the foundation floor by a cable;  
         [0035]    [0035]FIG. 6 is a vertical section taken through a top wall-retainer and wall panel,  
         [0036]    [0036]FIG. 7 shows a vertical section taken through a central wall panel and connected to a ridge beam;  
         [0037]    [0037]FIG. 8 shows a vertical section taken through an outside wall and connected to a roof panel;  
         [0038]    [0038]FIG. 9 is a perspective view of a wall panel;  
         [0039]    [0039]FIG. 10 is a perspective view of a roof panel with a tile effect outer surface; and  
         [0040]    [0040]FIG. 11 is a sectional side elevation taken through a roof panel along B-B of FIG. 10.  
         [0041]    [0041]FIG. 12 is a section through a bobbin;  
         [0042]    [0042]FIG. 13 is front elevation of a bobbin,  
         [0043]    [0043]FIG. 14 is a rear elevation of a bobbin;  
         [0044]    [0044]FIG. 15 is a sectional elevation of a bobbin, the section being at right angles to that shown in FIG. 12;  
         [0045]    [0045]FIG. 16 is a section through a collet;  
         [0046]    [0046]FIG. 18 is an expanded sectional view of part of a collet shown in FIG. 16;  
         [0047]    [0047]FIG. 18 a  shows a part of the collet shown in FIG. 16;  
         [0048]    [0048]FIG. 19 illustrates the entry of a collet into a bobbin when securing a cable;  
         [0049]    [0049]FIG. 20 is a detailed section taken through a wall panel along A-A of FIG. 9; and  
         [0050]    [0050]FIG. 21 is an illustration of production of a skin for a wall panel. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0051]    Referring initially to FIG. 1, the concrete foundation slab  10 , has secured therein a vertical rod  11  to provide an anchor for securing cables. The vertical rod  11  is attached at a lower end to a ground anchor  12 .  
         [0052]    [0052]FIG. 2 shows a wall retaining member  20 , secured to a foundation slab  21  by a connection bolt  22 . A layer of mastic  23  forms a waterproof sealant between the wall retaining member  20  and the foundation slab  21 . FIG. 3 shows the wall panels  30 ,  31 ,  32  fixed in position on the wall retaining member  33  to form part of an outer wall  34  and an internal wall  35 . The panels abut a box beam  36 . In FIG. 4, the horizontal cables  40 ,  41  can be seen in more detail. In use, these cables anchor the wall panels  42 ,  43 ,  44  in position. Vertical cables  50  are held in position at the anchor point  51 . The anchor point  51  is attached to the vertical rod  52  which is set into the concrete of the foundation slab  53  as shown in FIG. 5.  
         [0053]    [0053]FIG. 6 shows the top of the wall panel  60  with a top wall track  61  fixed into position. The top wall track  61  is fixed into position by means of a connecting bolt  62  secured and tensioned by a vertical flexible fibre cable  63 . Where the wall panel is to be part of the central wall of the building, an additional ridge beam  70  may also be secured into position as shown in FIG. 7.  
         [0054]    [0054]FIG. 8 illustrates a joint between a wall panel  80  and a roof panel  81 . A top wall track  82  is fitted to the top of the wall panel  80 , and secured in position by a connection bolt  83 , attached to securing cable  84 . The roof panel  81  is secured by means of a horizontal fibre cable  85 , and further horizontal cables  86 .  
         [0055]    The roof panel  81  incorporates a built-in gutter  87  which is connected to a piping system leading to a storage tank (not illustrated). The rain-water collected by this gutter and piping system can be employed for non-potable uses such as watering crops. Once the roof panel  81  and piping system is in place, a soffit panel  88  and fascia panels may be positioned in place.  
         [0056]    A wall panel  90 , suitable for use in assembling the above buildings as shown in FIG. 9. The wall panel  90  is generally rectangular cuboidal in shape. It comprises grooves  91 A,  91 B which in use receive vertical fibre cables. The wall panel  90  also comprises apertures  92 A,  92 B,  92 C leading into internal panels across the width of the wall panel to receive horizontal securing cables  93 A,  93 B,  93 C.  
         [0057]    [0057]FIGS. 10 and 11 show a roof panel  110  having a tile-effect outer surface. The roof panel  110  comprises an inner core  111  formed from the same composite material as is used for the wall panel  90 . The roof panel  110  comprises a number of channels  112  within the inner core  111  to receive horizontal securing cables  113 .  
         [0058]    FIGS.  12 - 15  show a bobbin  130  used to secure the flexible cables in position. The bobbin  130  comprises an outer wall  131  defining a cable-receiving cavity  132 . One end of the cable-receiving cavity  132  is tapered. The angle of taper with respect to the longitudinal axis of the bobbin  130  being 15 degrees.  
         [0059]    [0059]FIG. 16 shows a collet  170 , which in use fits within the tapered portion of the cavity  132 . The collet  170  has an outer sheath  171  surrounding an inner core  172 . The inner core  172  has a substantially V-shaped cross-section. The outer surface of the inner core  172  has a stepped configuration enabling it to grip the outer sheath  171 . The angle subtended by the walls of the inner core  172  is 15°. The angle subtended by the walls of the outer sheath  171  is 30°.  
         [0060]    The inner surface of the collet  170 , as shown in FIGS. 18 and 18 a  also has a stepped configuration to enable it to grip the securing cables. FIG. 19 illustrates the interaction between the collet  170  and the cable  200 , as the collet  170  is moved in the direction shown by arrow A. As the collet  170  is inserted in to the bobbin  130  the collet portions are squeezed together to grip the cable  200 .  
         [0061]    In use, once the collet and bobbin are in position around the cable, the bobbin  130  is twisted which causes the cable to be gripped more tightly thus securing the cables and retaining the wall or roof panels in position. When used in this manner collet and bobbin act as a connecting bolt.  
         [0062]    When constructing a building as described above, the ground is first prepared to a suitable state to receive the building. A concrete foundation slab is formed by pouring into place a cementatious material. Vertical rods which have been placed in the concrete at distances of approximately 1.3 m from one another along the eventual position of the perimeter and centre wall provide connections at the bottom for the connection to ground anchors, and at the top of the finished concrete foundation slab for connection to the vertical flexible fibre cable system.  
         [0063]    Wall-retaining members having a U-shaped cross-section for the perimeter and centre walls are positioned onto the foundation slab. A bed of mastic sealant is added between the wall retaining member and the foundation slab in order to improve the water proofing properties and a connection bolt used to secure the wall-retaining members to the floor slab. At the intersection of the rear and centre walls, wall panels are set in place in a T-configuration. A vertical box beam is attached at the centre of the T-junction and horizontal flexible fibre cables are threaded through the wall panels. As each wall panel is put into place a vertical flexible fibre cable is connected at the floor and threaded through an alignment tube pushed into a vertical grove in the edge of the wall panel. The alignment tube has dimensions of approximately 5 cm by 5 cm.  
         [0064]    The horizontal flexible fibre cables are tensioned and locked using a cable clamp, thus holding each wall panel precisely in position. The procedure is continued until all the wall panels including any door frames and window wall panels are in place and tensioned.  
         [0065]    When all the wall panels are in position, a top wall-retainer is put on top of each wall. Additionally, a ridge beam is placed on top of the centre wall. The vertical flexible fibre cables are then tensioned and locked using a cable clamp. The structure is ready to receive roof panels.  
         [0066]    Starting from one end of the building a first roof panel is set onto the wall and ridge beam. Horizontal flexible fibre cables are threaded through the roof panel and attached in position. A spacer tube is placed into the grooved edge of the roof panel and a sealant applied. The next roof panel is slid into place and the flexible fibre cables tensioned and locked. Any excess sealant is squeezed out between the joint between the two roof panels and can be simply removed. The sealant is normally the same colour as the panel to give an impression of there being no joints present. The roof panels are subsequently anchored to the foundation slab and to the ground anchors.  
         [0067]    Door and window cartridge units are then snapped into position in the appropriate apertures within the wall and wall panels and secured.  
         [0068]    As described, the building is easy to construct as many of the individual elements are pre-formed and need only to be fitted together. The building is also structurally reinforced with a comprehensive network of cables throughout the entire structure which are anchored to the ground through a concrete slab foundation. In this manner the buildings are able to withstand even hurricane force winds. In addition, the construction and materials from which the wall panels are formed mean that the building as a whole has superior thermal and sound insulation. In addition it is also resistant to mildew and insect damage, as well as being extremely fire resistant. Furthermore the building does not require a great deal of maintenance and is durable and long lasting.  
         [0069]    The glass beads from which the core material for the wall panels is made can be formed from recycled glass thus reducing the environmental load of materials used in the construction of the building.  
         [0070]    In a further embodiment, not illustrated, the ground anchors and vertical rods are not required, and the wall panels are secured to the floor by passing the securing members through the hollow tubes within the foundation floor. The fibre cables can be flexible or stiff according to the use to which they are put.  
         [0071]    Due to the modular nature of the buildings, the individual elements can be transported from one central location to the required site and because of the flat nature of the wall panels, the materials needed for the building can occupy a small space within the particular vehicle being used for transportation. Moreover, once the materials have reached the proposed location of the building, the building can be assembled using a relatively unskilled work force which again assists the rapid construction of the building, particularly useful following a natural disaster.  
         [0072]    A composite material, which is suitable for use in the construction of building is formed as follows. A bonding material, often referred to as bonding slip, is prepared by the dissolution of water glass (sodium silicate) in water, until the water glass solution has a sodium oxide content of 40% w/w. A blowing agent, sodium nitrate (6% w/w of the water glass concentration) is dissolved in the water glass solution and the solution produced thereby, heated to a temperature of 80C. The solution is maintained at 80C. for a period of time in order to allow excess water to evaporate. Care must be taken that the temperature does not greatly exceed 80C as this leads to the water vapour being removed too quickly, and to decomposition of the sodium nitrate. Water is removed until the sodium silicate/sodium nitrate-containing solution has a viscosity of from 1.5-2.0 poise.  
         [0073]    Silica powder, having an average particle size of 300μ—with 60% of the particles having a particle size of from 270-330μ—is fed onto a granulating tray. The particle size of the silica powder is important. Too high a percentage of fines requires, in subsequent steps, more filler resin and can also clog up the mixer. Too high a percentage of coarse material reduces the production of cenospheres.  
         [0074]    The bonding slip—i.e. the above described solution of water glass and sodium nitrate—is sprayed onto the silica powder through an oscillating arm positioned above the granulating tray. When the correct amount of bonding slip has been added the glass powder begins to form an agglomerate with the bonding slip and the surface tension within the agglomerate forms it into small beads. As the bonding slip interacts with the silica powder the viscosity of the fluid gradually increases. The particles of the silica powder gradually increase in size primarily due to hydration, reaching about 400-500μ. When the viscosity of the fluid is high enough, (approximated to the viscosity of putty or plasticine) the agglomerate is extruded through holes in the granulating tray and cut into pre-blown granules.  
         [0075]    The pre-blown granules then enter a rotating kiln heated to a temperature of 750C., the rotary action of the rotary kiln forming the pre-blown granules into spherical beads. When, due to the heating action of the rotary kiln, the beads reach a temperature of approximately 650C. the sodium nitrate begins to decompose producing a gas (NO x ). The pressure of the gas forms the internal volume of the beads into a foam like structure, which structure is maintained upon drying the bead, forming what is often referred to as a cenosphere.  
         [0076]    Using the above process, cenospheres of different sizes may be produced by employing different sized meshes during extrusion of the pellets. Typical values for the size of the cenospheres are 0.5 mm-8.0 mm. During the drying process, care should be taken that the temperature in the kiln does not exceed 800C., as the sodium nitrate consequently decomposes too quickly. The temperature of the kiln should also be above 700C. in order that the cenospheres formed are of a satisfactory quality. As an alternative or as an additional blowing agent to sodium nitrate, sodal/urea/hydrogen peroxide or soda lye/manganese IV oxide/sugar or a suitable mixture thereof can be used.  
         [0077]    Wall and roof panels, having a composite structure and which can be used, for example, in the construction of housing are produced as follows. Cenospheres formed of an expanded silica and produced by the above process are mixed with an epoxy resin monomer bonding material. The size distribution of the silica cenospheres is a ratio from, by weight, of 3:2:1 for beads having a particle size of 2:1˜0.5 mm. The distribution of sizes enables an efficient close packing of the cenospheres, within the eventually formed panel and requires the minimal amount of resin. The close packing moreover contributes to the strength of the panel and reduces possible warping during polymerisation of the resin monomer and during use. A typical value for the weight ratio of silica cenospheres to resin is 95:5.  
         [0078]    As the density of the beads is 0.7-0.8 g/cm 3  and that of the resin is far denser, often in the region of 1.1 g/cm 3 , care must be taken that the viscosity of the resin is sufficiently high in order to slow down the rate at which the beads float to the top of the resin. When the cenospheres and the resin monomer have been thoroughly mixed together, a polymerisation catalyst, hexamine, at a level of 5-6% w/w of the resin monomer is added. Once the mixture has reached a viscosity of 30-50 poise, preferably 40 poise, the mixture is poured into a mould to cure and harden. The temperature at which the curing process takes place is ambient temperature. The mould can include a number of PVC tubes which pass through the body of eventually formed block. The tubes result in the presence of channels through the block, which channels facilitate the passage of cables used during the construction of a building.  
         [0079]    In order to assist the maintenance of an even distribution of the cenospheres throughout the resin, the mould has vibrating means to vibrate the mixture. Typically the vibrating means are located ⅓ and ⅔ of the way along the length of the mould. A vacuum pump can be used to draw the mixture down the mould and reduce any residual air pockets. After curing, which typically requires about an hour, the hardened core material is removed from the mould.  
         [0080]    As an alternative, or, where applicable, in combination with the polyester monomer above, a vinyl ester or an unsaturated polyester can be used. Vinyl esters are preferred as they improve the fire resistance characteristics of the material produced. The catalyst for the vinyl ester or unsaturated polyester polymerisation is methylethyl ketone peroxide (MEKP). The MEKP is normally used at a level of 2% w/w of the resin monomer. The MEKP can be absorbed onto a support such as bentonite, or other clay material. When using bentonite, the catalyst is mixed in more slowly than is normally the case in order to reduce damage to the bentonite particles. During the polymerisation process, ultrasound is used to break apart the bentonite and release the MEKP into the mixture which commences polymerisation. By absorbing the catalyst therefore, the onset of polymerisation can be delayed.  
         [0081]    As shown in FIG. 20 a wall panel  210  comprises a central core  211 . The central core  211  is a composite material comprising expanded glass beads bound together by a polyester resin. The wall panel  210  further comprises a skin  212 , and a channel  213  along which a cable can pass. The skin is also a composite material comprising 35% v/v aluminium oxide trihydrate and approximately 65% v/v polymeric resin.  
         [0082]    Alternatively, the core material is formed by pouring the precursor mixture of resin monomer into a forming tray. Once polymerisation has taken place, the block of core material thus formed is cut up to the appropriate size. The outer skin is then bonded to the core material using a bonding paste.  
         [0083]    As an alternative to using the above resins, the resin may be selected from one or more of the following classes of polymeric compounds, polyurethane, polyacrylic, polyphenolic, polybromophenolic, polyvinylester or an epoxy resin.  
         [0084]    Wall panels for use in, for example, the construction of a building are formed as follows. An inner core material, for example a composite form from resin bound glass beads is shaped in the form of a rectangular block or other desired shape. In order to improve the aesthetic, constructional and safety characteristics of the block, a skin is overlaid on one or both of the in use inner and outer surfaces of the block. The skin is often formed separately from the block and bonded subsequently to the block.  
         [0085]    The skin is formed as follows, with reference to FIG. 21. A moving bed  220  of a skin forming machine, moves at approximately walking pace. A nylon sheet  221  is unfurled from a roller  222  and laid flat onto the bed  220 . As the bed  220  moves, a resin dispenser  223  dispenses resin—in respect of which the nylon sheet  221  is impermeable—onto the surface of the sheet  221 . A glass fibre mat  224  is unfurled from the roller  225  and overlaid over the nylon sheet  221 . The glass fibre mat  224  incorporates chopped glass strands and is permeable to the applied resin. A further nylon sheet  226  is unfurled from a roller  227  and overlaid over the glass fibre mat  224  to form a layered pre-sheet. The layered pre-sheet passes through the compression rollers  228  which compress the layers together and ensure that the resin is spread out sufficiently to bond the sheets  221 ,  224 ,  226  together. Overall, the resin is present in the finished skin at a level of 15-30% w/w, although a level of 20% w/w has been found to be ideal. The sheet then passes to a curing region in which the resin is polymerised, bonding the sheets together to form a uniform structure. The skin produced thereby can be either cut into lengths or rolled up and transported or stored.  
         [0086]    The nylon sheets  221 ,  226  employed are between 17-21μ in thickness, although the thickness of 19μ has been found to give optimum performance. Above 19μ, the nylon sheets can distort, particularly during rolling and curing, which gives the skin a poor profile.  
         [0087]    The method of polymerisation used depends upon the resin used to bond the sheets together, but is typically either achieved through the use of heat or by irradiation with ultra-violet radiation. The length of the curing (polymerisation) station is typically approximately 12 m, to enable the temperature and/or radiation level to be set to give the optimum polymerisation.  
         [0088]    In addition to the layers described above, a number of the other features can also be included. A gel coat incorporating a colouring agent can be incorporated. A gel coat can be distributed over the nylon sheet  221 , prior to the addition of the bonding resin from the resin dispenser  223 . The colouring agent is added as part of the gel coat and can be, for example, titanium dioxide, which in the finished sheet imparts a white colour to the skin and eventually to the finished panel.  
         [0089]    A further sheet of glass tissue (having a density of 14-30 g/m 2 ) may also be, during the production, overlaid onto the nylon mat  121 . The glass tissue minimises the risk of surface features from the individual sheets being imparted to the finally produced skin.  
         [0090]    A fire-retardant filler can also be included in the skin, by the incorporation of a fire-retardant material into the resin dispenser  223 . One example of a fire-retardant material is aluminium oxide, having a mean particle size of less than 0.25μ and ideally a mean particle size of less than 10μ. The smaller size enables the fire-retardant material to pack more efficiently. The aluminium oxide fire-retardant material is typically present at a level of 15-50% w/w of the resin. A level of 30-40% has been found to be particularly suitable, and 35% w/w especially suitable. The level should be sufficient to impart adequate fire retardant properties to the skin produced. However, above a level of 50% w/w the aluminium oxide causes the viscosity of the resin to be too high.  
         [0091]    As an alternative, or in addition to aluminium oxide, a number of other fire retardant materials can be used. For example glass flake can be used. Chemicals, which are at least partially soluble in the resin can also be employed such as polyhalogenated phenols, for example polybromophenols.  
         [0092]    The glass fibre mat  224  can be obtained either incorporating pre-chopped strands or with the strands in tact. In the latter case, the mat  224  ideally first undergoes a chopping process which breaks the strands into small pieces. A typical density for the glass fibre mat  224  is from 280-320 g/m 2 . A value of approximately 300 g/m 2  has been found to be particularly beneficial, imparting some rigidity into the finished skin.  
         [0093]    The finished skin can be added, by the use of a suitable resin both to a pre-formed composite block or other suitable core material. Alternatively, the skin can be incorporated into a mould in which a block has been formed, in which case the core material can bond directly to the skin during the core materials production.  
         [0094]    The skin can also be corona treated, whereby an electric current is passed across its outer surface. A corona treatment modifies the overall structure of the outer surface and enables the surface to receive, for example, paint or varnish.  
         [0095]    The wall panel formed by the above process is highly durable and resistant to attack by water, fire and insects such as termites common to those areas which also suffer from extreme weather conditions.  
         [0096]    It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the appended claims.