SHEET MATERIAL

Abstract: A plastics sheet material comprises a pair of opposed faces (12, 14) between which run a series of elongate voids (20), the voids (20) being arranged so that there are two series of uninterrupted interconnections (14, 18) between the two opposite faces (12, 14) characterised in that the interconnections (14, 18) of each series meet at or near the respective faces. The voids (20) will in practice be parallel to one another and run the length of the sheet material, thus the material can be produced in one pass by extrusion from a die. This means that the material can be produced potentially much cheaper than, for example, an equivalent laminated product. Furthermore, being produced in one piece, the structural integrity is greater and the variety of configurations which may be employed is very high. The opposed faces may both be external faces, or one or more may be an internal face if the sheet is of a more involved cross-sectional profile. The sheet (10) of the invention allows individual sections to be made thicker for a given gross sheet thickness and weight per unit area. The number of intersections (22) in sheet of the invention can be reduced which also reduces the weight per unit area significantly while the sheet (10) as a whole and the individual sections maintain their resistance to buckling. In the preferred form of the invention, the interconnections (16, 18) meet at the surface of the opposed faces.

Description: SHEET MATERIAL This invention relates to a sheet material and in particular relates to an extruded plastics sheet material having voids therein. Plastics material sheeting is useful for a number of end uses. Rigid or semi-rigid plastics material sheeting may be used, for example, as a packaging material or a cladding material. However, in order to get the best physical properties at least cost, it is preferable that the sheet not be solid, and profiled sheets having opposed walls separated by lateral support members have been proposed hitherto. While such structures have greater strength than a solid sheet of equivalent weight, they still have drawbacks. In particular, local failure is likely to result in complete failure of the structure either immediately or by 'creep'. In our PCT publication No. WO91/16243, there is disclosed a plastics sheet material which comprises a pair of opposed faces between which run a series of elongate voids, the voids being arranged so that there are a series of uninterrupted interconnections between the two opposite faces and the voids being of a cross sectional shape such that there is a substantial absence of sharp corners. A preferred form of that sheet is described in relation to Figure 5 and has two series of interconnections, one series at 45° to the left and the other at 45° to the right, which join where they cross approximately mid- way between the skins. This configuration is a marked improvement on the above but has some limitations in that generally it is impractical to extrude a sheet in which the interconnections are less than approximately 0.1 mm in thickness, which places a limit on the amount of lightening of weight which can be achieved compared to a conventional sheet. The invention seeks to provide a form of plastics material sheet improved in the above respects and which can be produced conveniently and economically. According to the present invention there is provided a plastics sheet material which comprises a pair of opposed faces between which run a series of elongate voids, the voids being arranged so that there are two series of uninterrupted interconnections between the two opposite faces characterised in that the interconnections of each series meet at or near the respective faces. The voids will in practice be parallel to one another and run the length of the sheet material, thus the material can be produced in one pass by extrusion from a die . This means that the material can be produced potentially much cheaper than, for example, an equivalent laminated product. Furthermore, being produced in one piece, the structural integrity is greater and the variety of configurations which may be employed is very high. The opposed faces may both be external faces, or one or more may be an internal face if the sheet is of a more involved cross-sectional profile. The stiffness of a sheet of the kind referred to is related to its gross thickness while its propensity is to fail by buckling at the local level of individual sections whose strength is primarily dependent on their thickness. The sheet of the invention allows individual sections to be made thicker for a given gross sheet thickness and weight per unit area. The number of intersections in sheet of the invention can be reduced which also reduces the weight per unit area significantly while the sheet as a whole and the individual sections maintain their resistance to buckling. As with our above-mentioned PCT publication, the interconnections are preferably arranged such that they do not meet the faces at or near right angles but are inclined to the transverse axis of the sheet. The voids may be enlarged so as to virtually fill the space between the faces leaving only relatively thin interconnections ('flutes') which act like leafs, ribs or beams between the opposed faces. Where this construction is employed it is important that the connection between each beam and the adjacent face is radiussed to avoid sharp angles for the reasons explained in our PCT publication. In the preferred form of the invention, the interconnections meet at the surface of the opposed faces, as will be described more fully hereinafter. The plastics material employed is preferably a thermoplastic and the sheet may be formed from many suitable thermoplastics material such as a polyolefin, eg polyethylene or polypropylene. Other thermoplastic materials may be employed if desired, as may fillers, colourants and the like. The thickness of the sheet and cross sectional shape of the voids will be chosen with regard to cost constraints together with the desired mechanical attributes of the finished sheet. The size and number of the interconnections will be chosen with similar considerations in mind. As compared with a sheet of equivalent dimensions and weight made from the same polymer, the voids in the constructions of the invention enable a lighter and cheaper sheet to be made without impairing its strength owing to the presence of the interconnecting lattice work between the opposed faces. Avoiding sharp comers or angles in the cross sectional shape materially contributes to the mechanical properties of the finished sheet. In particular, it enhances resistance to torsion, buckling, particularly at right angles to the direction of production (the 'compliant' direction), and impact force while avoiding points at which fractures are likely to start and from which they are likely to be propagated. Thus, as compared with a sheet of equivalent weight and dimensions made from the same polymeric material, local failure is less likely to occur, if it does occur it is less likely to result in complete failure of the structure, and also if it should occur it is less likely to creep. The pattern of voids within the sheet material will preferably be a repeating pattern, but need not be. The interconnections between the opposed faces preferably are angled each so each interconnection runs in a generally diagonal direction making an angle of between 30° and 80°, preferably between about 55° and 65°, with the opposed faces, and adjacent interconnections run in opposite directions. A particular advantage of the structure of the invention, even in relation to the structure described above as disclosed in our PCT publication, is that the strength of the sheet is unexpectedly improved in the transverse direction. That is, the sheet of the PCT publication (hereinafter referred to as 'criss-cross' for ease of reference) is stronger in the direction of the interconnections than in the transverse direction (hereinafter referred to as the 'stiffness' and 'compliant' directions respectively) - although it is very much better in this respect than hitherto available sheets with vertical flutes. Indeed, in comparable sheets of the invention and criss-cross the strength in the stiffness direction is much the same, but strength in the compliant direction may be increased by around 10%. This allows lighter, and cheaper, sheets to be employed for a given application. Moreover, it may allow the sheets to be employed in finished boxes at either orientation, i.e. not necessarily having the stiffness direction along the longest axis. In designing sheets in accordance with the invention there are various constraints which have to be met. Firstly, the sheet cannot be too thick as this would unacceptably reduce the internal dimensions of any box made. On the other hand, bending stiffness of the sheet is related to overall sheet thickness and a thicker sheet is better in this respect. Another factor is weight. Ideally this should be at a minimum for a given set of physical properties for both lightness and cheapness. If a sheet of a given thickness, e.g. 7mm, has a weight per unit area ('grammage') of 500g/m 2 , then reducing this to, say, 400 would mean reducing the section thickness in either or both of the faces ('skins') and the interconnections ('flutes') substantially in proportion. But the sheet's ability to maintain its bending stiffness is critically dependent on the ability of its mini-panels not to buckle ("Mini-panel" may be defined as an unsupported length of a skin or flute). Each mini-panel's resistance is critically geared to their dimensions. Specifically their resistance increases/decreases related to the cube of the change in their thickness and the square of the change in their length, not directly proportionately. Buckling reistance (as contrasted with bending stiffness) is therefore closely related both to the thickness and the unsupported length of a given skin or flute. Thus if the thickness of , say, a flute is reduced its length must be reduced also to at least partly compensate. Thus in the example given above, the number of flutes must be increased and the individual flutes made more upright so that they are shorter. The invention will be described further, by way of example, with reference to the accompanying drawings, in which Figures 1 is a cross sectional views of a sheet material constructed in accordance with the invention; and Figures 2 (a) and (b) illustrate typical boxes made from the sheets. Referring to the drawings, and in particular Figure 1, a portion of a sheet material generally designated 10 extruded from a thermoplastic substance such as polypropylene is shown in cross section. The sheet comprises opposed faces, or 'skins', 12,14 and two series of interconnections, or 'flutes', 16,18 between them defining generally triangular voids 20. Sharp comers are avoided by means of radiussing. The flutes 16,18 form an angle of approximately 60° and so the voids 20 are essentially equilateral triangles. The flutes are integral with the faces 12,14 at joins 22 and the spacing between adjacent joins 22, which can be thought of as the "cell" width, is equivalent to the gross thickness of the sheet. If an angle less than 60° between the flutes and the faces 12,14 is chosen the cell width will increase and, if a greater angle is chosen, the cell width will decrease. As can be seen from Figure 1 , cells 20 include three angles of 60° and the effect of these is reduced by the provision of radii 24. The thickness of the individual walls 12 and 14 and beams 16,18 are chosen within the overall width of the sheet 10 in accordance with manufacturing needs and operating parameters, and as discussed hereinbefore. With the superior structure of the sheet of the invention it is not always necessary, but the comers 24 should preferably have a minimum radius of 0.1mm and preferably a radius of 0.25mm. It has been found that the provision of a radius above this minimum effectively avoids the disadvantages relating to sharp comers discussed hereinbefore. It is preferable that the radius does not exceed, for example, 0.3mm or 0.35mm for cost considerations. In Figure 1 the interconnections 16 and 18 have been notionally extended by dotted lines so that a diamond-shaped 'cross-over' 22' is shown, and each of these is bisected by the inner surface of faces 12, 14. This is the currently preferred cross-over point. However, it is envisaged that this point could be moved a relatively small amount either inwardly towards the opposite face or outwardly towards the outer surfaces of the faces 12,14. If the latter, then part or all of cross-over 22' (in the latter case virtual) may, but preferably should not, be outside the envelope of the relevant face 12,14; that is, the interconnections 16 and 18 should preferably meet physically. Turning now to Figure 2, typical boxes 26a, 26b are illustrated made from the sheet material 10 of Figure 1. Owing to the fact that the strength of this sheet in the compliant direction is nearly 90% of that in the stiffness direction, the sheet can be oriented as desired, not necessarily with the stiffness direction parallel to the longest axes. This has the additional advantage of better utilisation of extruded sheet from a given die width. The following example illustrates the invention further. Comparison of 7mm criss-cross with 7mm "N" structure: 7mm Structure "XX" "V" Skin Thickness - mm 0.170 0.170 Flute Thickness - mm 0.050 0.140 Gms/metre squared 500 500 Bending Stiffness - raw materials having same Youngs Modulus :- (Index) (Index) - Stiffness direction -Ν.mm 9.630 (100) 9.773 (101) - Compliant direction -Ν.mm 7.627 (100) 8.507 (112) Max deflection - Given box sample and load - mm 19.3 18.6 Flute length -mm c.10.0 7.5 Unsupported Skin mini panel length - mm c. 7.0 10.1 The above is chosen to allow comparison of a sheet of the invention with the 'criss-cross' formation of our earlier patent with each sheet in gross terms (e.g. weight/sq. metre, and thickness) being physically the same. The figures show that in the stiffness direction the sheets are comparable, but in the compliant direction the stiffness of the sheet of the invention is some 12% higher while the sheet' s stiffness is overall more even with the compliant direction at 87% as against 79% of its own stiffness direction. Accordingly, the sheet of the invention gives an overall improvement in stiffness with the two sheets at the same weight and thickness. In practice, considerations of manufacture would mean that neither sheet could be made with mini-panels of less than 0.1 mm thickness. Thus in the above comparison in practice the 7mm criss-cross would need to be made with flutes of 0.1mm and this would increase its weight to 690 g/sq. metre. In broad terms, therefore, the invention allows a lighter and therefore less costly sheet to be used for a given application. Reverting to the above mini-panel data it can be seen that to produce a 7mm, 500gm sheet in the criss-cross design the flute length is 10mm while the flute thickness can only be 0.05mm. If such a sheet could be made commercially it would have unacceptable buckling resistance owing to the thinness of the flute. The corresponding flute in the 'V design is only 7mm and is 0.14mm thick - much more buckle resistant. The skin mini-panels are somewhat the other way: the criss-cross design has only 7mm lengths at 0.17mm which is good; the 'V design of the invention has 10mm lengths of 0.17mm which is still acceptable. Thus the overall buckling resistance of the 'V is improved relative even to the 'criss-cross', which itself is much better than conventional sheet. In addition, the following factor is material. Owing to the construction of the present invention the length of the "flutes" 16,18 is less than that of an equivalent criss-cross sheet. Moreover, the thickness may be greater without adding materially to the overall weight of the sheet. Thus there is a much less likelihood of the flutes failing for a given load. Because the structure of the present invention has a very high ratio of strength in the compliant/stiffness directions, the ability to reduce the weight of a box made from such a material is enhanced. There is an inherent load re-distribution within a box. For example, in a typical box having a base size of 600 x 400mm and having comer posts, with a structure of the type traditionally used in the market (with the flutes being vertical and parallel to one another) some 90%) of the total load is carried by the comer posts and the short sides, whereas with the structure of the invention only 62% of the total load needs to be carried by these. This "re-distribution" occurs automatically as each part of the box comes under strain approaching unacceptable levels, with the comer posts always picking up the strain not supported by the sides. Accordingly, because the bending stiffness of both structures in the stiffness direction is comparable, it is clear that the total capability of the structure of the present invention is much higher. The sheet material of the invention may be used in a variety of end uses. It is particularly useful in packaging, especially packaging for food products. It is also useful in a variety of other end uses such as wall panels for construction purposes. The sheet material of the invention has high bending stiffness while the risk of failure owing to buckling is carefully controlled and in particular it is resistant to torsional stresses, compression and/or extension and, furthermore, has good impact resistance. The structure is less subject to complete local failure partly by the avoidance of shaφ angles and keeping the length/thickness ratio of mini-panels within bounds, and any local failure which may occur in practice is more likely to be limited in extent and not spread to adjacent areas of the sheet. For any given thickness of sheet 10 (or for any given strength of sheet) the properties are improved in comparison with conventional sheet. Furthermore they can be produced often more economically than previous profiled sheet products. The heat and sound insulation properties of the sheet are good whilst its impermeability to water is excellent. The exact number, shape and size of interconnections employed in relation to the thickness of sheet and material used is chosen to meet particular circumstances or end use.

Claims: CLAIMS 1. A plastics sheet material which comprises a pair of opposed faces between which run a series of elongate voids, the voids being arranged so that there are two series of uninterrupted interconnections between the two opposite faces characterised in that the interconnections of each series meet at or near the respective faces. 2. A sheet as claimed in claim 1 in which the voids are parallel to one another and run the length of the sheet material. 3. A sheet as claimed in either of claims 1 or 2 , wherein the interconnections are arranged such that they do not meet the faces at or near right angles but are inclined to the transverse axis of the sheet. 4. A sheet as claimed in any of claims 1 to3 wherein the voids are enlarged so as to virtually fill the space between the faces leaving only relatively thin interconnections ('flutes') which act like leafs, ribs or beams between the opposed faces. 5. A sheet as claimed in any of claims 1 to 4 wherein the interconnections meet at the surfaces of the opposed faces. 6. A sheet as claimed in any of claims 1 to 5 , wherein the plastics material employed is a polyolefm, eg polyethylene or polypropylene. 7. A sheet as claimed in any of claims 1 to 6 wherein the interconnections between the opposed faces are angled so each interconnection ns in a generally diagonal direction making an angle of between 30° and 80°, preferably between about 55° and 65°, with the opposed faces, and adjacent interconnections run in opposite directions.

Inventors: MACKINNON ROBIN, WILLIS IAN, MACKINNON, ROBIN, WILLIS, IAN
Applicants: KORPAK LTD, MACKINNON ROBIN, WILLIS IAN, KORPAK LIMITED, MACKINNON, ROBIN, WILLIS, IAN
Application year: 1999
Main classification: B29D24
IPC codes: B29D24, B32B3, B65D1, B65D65