The present invention relates to a sheet material built up as a composite laminate which is primarily intended for use as a shuttering panel in concrete casting form- work. The laminated board fabricated in accordance with the invention is primarily characterized in that its carrying outer layer comprises wood fibre board or chip¬ board (hereinafter generally designated as "particle board") with moulded-in glass fibre threads, and that the inner layer or layers comprise sheet material of preferably lower specific gravity than said carrying outer layer. Other distinguishing features of the composite laminate and its method of manufacture are disclosed in the following claims.

In the selection of sheet material for concrete shutter¬ ing panels there is generally the prerequisite that very high demands are made on the rupture and bending strength of the panels, their resistance to liquids and change in shape. Furthermore the panels shall have a very high surface strength and finish (for ease of re¬ moving the panel) . All these properties shall finally be united with the demand for a reasonable specific gravity and an acceptable price.

The material at present entirely dominating in the shuttering panel field is liquid-resistant glued plywood with a surface layer coating of phenol resin film. The plywood can have a varying number of plies of different thickness/ and different wood can be used in the plies/ depending on the field of use of the panel (intended number of castings, flooring or wall casting, formwork size etc) . The higher the demands on the properties of the panel, the more difficult it is to supply a com¬ pletely satisfactory plywood panel at a reasonable

price. For large casting pressure and repeated use there is thus required a very thick and expensive plywood with up to 15 plies of high-quality birch veneer. When all these veneer layers are to be glued together with liquid-resistant glue and the surface layer finished to the highest possible smoothness and hardness it is quite obvious that such a panel will be very expensive. Furthermore, it has through the years been more and more difficult to obtain the high-quality veneers required for these panels.

A method seeking the solution of the above mentioned problems has been to manufacture shuttering panels of lower quality veneer material and accept a smaller number of casting occasions for each panel. The thus necessary, closer exchange of panels in the formwork costs money, however, and furthermore, there occur other problems such as less bending stiffness in the panels. A further serious problem in the use of "poorer" veneers in the surface layer is that an unsatisfactory casting surface is obtained. The surface veneer is quite simply too uneven to give good adhesion to the surface layer coating and is furthermore so soft that the panel surface can be easily damaged by even light pressure and shocks.

A conceivable possibility of producing a shuttering panel of the desired kind would be to laminate a "panel core" of lower quality material with an outer layer of a high quality material, as seen from the aspects of strength and hardness. No such material, which can be readily laminated with a wooden core material and still meet strength, weight and cost requirements has so far been available. Retaining wood as the core material is naturally dependent on it being suitable for the purpose and that it can also be obtained for a reasonable price

if the quality requirements are not made too high. As a compromise, shuttering panels have also been produced with a core of plies of a lower quality and the surface layer of high quality birch veneer. However, the problem remains of obtaining in such a way a high-quality panel since the better properties of the outer veneer are not sufficient for completely counteracting the lower rupture strength, stiffness and hardness of the lower- quality core. With regard to hardness, it may be _. mentioned, for example, that the panel surface is easily damaged by pressure stresses, since the outer veneer is not sufficiently stiff for taking up the pressure by itself, but is pressed into the underlying, softer core laminate.

It may thus be established that an outer laminate layer of wooden material, but with greater stiffness, hardness and rupture strength than the birch veneer would be required to enable a lower-quality laminated wooden core to be used in high-quality shuttering panels. Such a wood-based board material could be wood particle board, the stiffness and hardness of which are sufficient per se. However, the board has the disadvantage that its rupture strength is indeed very much higher than that of the wood veneer, measured in the transverse direction of the grain thereof, but only half as great as that of the veneer measured in its grain direction. Furthermore, the boards are usually relatively sensitive to moisture, and swell somewhat when they are subjected to it. If the fact that the boards have a higher specific gravity than wood veneer is added to this, wood particle board may be excluded in principle as a starting material for shuttering panels.

However, it has now been found that wood particle board which has been reinforced with moulded-in glass fibre

threads during manufacture has surprisingly good properties if it is used as the surface layer in shuttering panels with a core of one or more layers of wooden material of poorer quality. In the right combina- tion, a shuttering panel fabricated with an outer surface of glass fibre reinforced particle board has equally as good or better strength properties and surface quality than a high-quality plywood shuttering panel manufactured in the conventional way described above.

The invention will now be described in detail with reference to the accompanying drawing figures. Fig. 1, 2 and 3, which illustrate three embodiments of shuttering panels in accordance with the invention.

A multi-layer shuttering panel about 21 mm thick, is illustrated in Fig. 1 and is intended for use in vertical concrete casting formwork. This panel has a core comprising five layers of veneer 11-15, of which the middle one 13 and the two exterior ones 11 and 15 have been laid with their grain direction standing, while both intermediate layers 12 and 14 have been oriented with lying grain direction, i.e. at 90° to the adjacent veneer layers. Outmost on either side of the veneer core there is a ^* glass fibre reinforced particle board layer 16 and 17, in turn provided with a phenol resin surface film of a kind known per ^' se. The glass fibre reinforced board comprises a relatively thin, 2.5-3.5 mm thick, wood particle board, in which long¬ itudinal, parallel glass fibre threads 18 have been moulded in with 20-60 mm spacing during the manufacture of the board. The outer board layers illustrated in Fig. 1 have been oriented in this case with the reinforcing threads 18 standing, i.e. so that they run at right

angles to the direction of the grain in the underlying veneer layer.

Several advantages are gained by the construction of the shuttering panel described here. As previously mention- ed, the particle board has per se a rupture and tensile strength which is about half as high as that of the wood veneer in its grain direction. In the transverse direction of the grain of the veneer, the strength of the veneer is only a fraction of that in the direction of the grain of the veneer, and in principle it may be counted on that the particle board strength in this case is at least five times as great. In general, in a. com¬ parison between plywood (cross laminated veneers) and particle boards of the same thickness it may be said that these materials have similar rupture and tensional strength properties. However, the glass fibre reinforced particle board has been given an increased rupture and tensional strength as well as form stability in the reinforcing direction which makes it comparable with the strength properties of the veneer in the grain direction. By laminating, as mentioned, the reinforced particle board layers with the reinforcing direction transverse the grain direction of the underlying veneer layer, the laminate obtains in its outer layer a ten- sional and rupture strength which is well up to that of two birch veneers which have been cross laminated in the outer layers. Since it is the rupture and tensional strength of the outer layer which is mainly decisive for the total rupture and bending stiffness of the laminated board, there is obtained a shuttering panel having very good strength properties and form stability in spite of the low quality of the core layers. Furthermore, there is afforded the essential improvement that the surface of the panel will be noticeably better both with regard

to smoothness and surface strength than what can be obtained even with the best kind of wood veneer.

While the panel illustrated in Fig. 1 has a core of a plurality of veneer layers of low density and quality, the panel illustrated in Fig. 2 comprises a particle board core 21 with glued-on outer layers 22-25 of glass fibre reinforced particle board. In this case two boards with crossed reinforcing direction are applied to either side of the core board, whereby cross reinforcing is obtained on both sides of the panel. The particle board core used in this case suitably comprises a more moisture resistant chipboard of V-313 quality. A shuttering panel with this construction is given very good form stability, although its relatively high specific gravity can be a disadvantage in some connections.

A third variant of the shuttering panel in accordance with the invention is shown in Fig. 3. This panel, which is particularly suitable for casting floor structures, comprises a core of three veneer layers where the middle one 31 is an approximately 4 mm thick pine veneer and both the outer ones 32 and 33 are approximately 1.5 mm thick birch veneers oriented with the grain direction at 90° to that of the middle layer.. Fibre boards 34 and 35 with a thickness of 2.5 mm have been placed on either side of the core. The total ready thickness of the panel with its surface coating for casting flooring is thus approximately 12 mm.

In manufacturing the laminated boards in accordance with the invention, joining together the participating layers can be carried with liquid glue applied with a suitable glue spreader, and with use .of glue films known per se. Press pressures, press times and curing temperature for

bonding are suited to the thickness of the laminate. A

2 press pressure of 10-30 kp/cm has been found to be suitable, while press times can vary between 5 and 30 minutes, depending on the total thickness of the panel. It has been found that the curing temperature should be between 125-225°C. To obtain the necessary edge pro¬ tection/moisture barrier, the finished laminated boards are provided with a suitable edge protection coating of the type used in the manufacture of conventional plywood shuttering panels.

A phenol resin film of a known kind is suitably selected as an outer layer coating, and it is either applied directly in conjunction with putting the laminated structure together under heat and pressure, or in a separate press operation on the laminate prepared according to the above. It should be noted here that different surface structures may easily be obtained on the shuttering panel by the outer layer fibre board either being applied with the rough .side inwards, its smooth outer surface then giving a very uniform and smooth surface for the surface film, or with the rough side outwards which gives a somewhat structured but still smooth casting surface.