FOAM SANDWICH CONSTRUCTION

Abstract: Laminate or sandwich suitable for structural purposes which comprises a lightweight foam core having bonded one on or both sides a lightweight structural skin. More particularly the structural skin adapted to be bonded to the foam core to form the laminate or sandwich construction comprises a composite of resin and monofilaments. This structural skin is characterized in that these monofilaments are laid to extend along the skin without directional change relative to the extent of the skin. The monofilaments are at least 10 cm in length and are neither crimped nor woven. Within the composite forming the structural skin the ratio of resin to monofilament is less than 2 : 1 by weight.

Description: Foam Sandwich Construction. This invention relates to sandwich constructions using two external skins of a material having a relatively high modulus of elasticity between which is sandwiched a light-weight core as taught for example in our prior Patent Specification No 1 555 271. In that specification there is described a sandwich structure comprising two external skins each of a material having a modulus of elasticity of at least f 2 3.8 x 10 kN/m , and having a tensile strength which is at least 9.3 x 10 - kN/m2, between which skins is sandwiched a core of foam material having a density of between 75 kg/m and 750 kg/m 3 , a shear modulus of at least 3,4-00 kN/m2 when measured at a strain of 10%, and a shear strength of not 2 less than 620 kN/m , the said external skins being uniformly fixed to said core which ' latter constitutes the only means of stress transference between the skins. Accordingly to one preferred feature of the invention described in specification No 1 555 271 the core is of expanded polyurethane foam. This is the preferred material for employment in the sandwich structure according to the present invention, other details of which will be described below. According to one practical embodiment as described in the aforementioned specification, each of the oute skins of the sandwich structure may be formed of a glass fibre reinforced resin material externally clad with a coating. This coating may be a so-called gel coat which is a coating which is often of pigmented resin intended to provide a smooth attractive finish to the product. However, any other suitable surface coating may be employed that is compatible with the -resin and fibre composite employed for the structural part of the skin. A suitable and economic form of skin construction of glass fibre reinforced resin has been chopped strand mats of glass fibres which are reinforced with polyester or epoxy resin. In such mats individual glass fibres are pressed randomly together to form a mat. Typical mats have glass contents of 150 to 950 grams per square metre. An economic layup could comprise a single layer of mat suitably overlapped where edges occur, or any number of layers of mat plyed on top of each other, the resin being applied to impregnate in the usual conventional manner. Alternatively, the mat could be formed in situ to any thickness using suitable glass and resin spray equipment. However, these glass fibres are randomly disposed and hence the spaces are also random. Also it has been determined that some resins and in particular most poly¬ ester resins, if cold cured, are liable to cold flowing when subjected to a strain as low as 0.08% in many cases. When a. given strain is exceeded some of the resin . will cold flow and fibres will move relatively to each other, and this flow will continue so long as this £ - 5 - particular load is sustained. With a good quality poly¬ ester, for example, if the load is reduced with corres¬ ponding reduction in strain a load level can be reached at which level, cold flow will cease ' or become minimal. This establishes the maximum load level for sustained static loads and is of a low order relative to the dynamic capacity of the material. It has also been conventional to employ, in addition to or instead of chopped strand mat, a woven cloth-like sheet of glass fibres. In this the monofilaments are anything but straight. Instead the fibre rovings are . sinuously woven together in a . loose corrugated weave. This certainly facilitates the manipulation of the glass fibres during the laying, up process, but the sinuou.s woven construction has lead to a disadvantage which has not received recognition hitherto. This is that when the resin has been applied, according to conventional teachings, adequately to wet the glass, the resultant structure has been found to experience permanent deformation under static loading of a surprisingly low order. The inventor has found that this disadvantage is caused to a significant extent, by load being transferred into the resin which is secreted between the sinuous weaves of the glass fibres, as though filling in the undulations • between corrugations. Then due to the propensity of the resin to cold-flow at relatively low strains, as discussed - *. hereinbefore, the layer of woven fibres can be bent and can remain bent. It has been generally accepted and recommended, heretofore, that the resin/fib.re ratio should.be at least 2.5 : 1 by weight and in some cases the resin content has been considerably increased in the mistaken belief that increased stiffness or safety could be obtained by adding more resin binder. Some increases in stiffness and resistance to permanent deformation can indeed be obtained by methods of curing the resin binder which involve heating. However, this is often inconvenient or uneconomic. Accordingly, glass fibres in the forms of chopped strand mat with randomly arranged fibres, or glass fibres in woven cloth-like sheet for with the fibres exte'nding sinuously have both proved not entirely satisfactory for forming structural skins for foam sandwich constructions. In particular they lack the ability to sustain such proportion of the load as might reasonably be expected from an examination of a load/deflection curve for the particular structure. The present invention has for its principal object to provide a structural skin for a foam sandwich construction, in particular a construction of the kind defined in British Patent Specification No 1555 271, and in which, greater stiffness and resistance to undes¬ irable non-elastic deformation under static loads is achieved. This invention provides a structural skin adapted to be bonded to a foam core to form a laminate, wherein the skin includes a composite of resin and monofilaments, characterised in that these monofilaments are laid .in parallel groups to extend along the skin, without directional change relative to the extent of the skin, the monofilaments being at least 10 cm in length and being neither crimped nor woven and wherein the ratio of resin to monofilament is less than 2 : 1 by weight within the composite. Other advantageous features are defined in . claims 2 to 13 to which reference .is now made. The core is a foam material having a density of between 75 kg/m 3 and 750 kg/m3, a shear modulus of at 2 least 3,400 kN/m when measured at a strain of 10%, and a shear strength of not less than 620 kN/m 2. The core may be a foam of polyvinylchloride or preferably it is an expanded polyurethane foam. In particular the invention provides a foam sandwich construction having a central foam core having bonded to it on one or both sides a structural skin as described above In such a sandwich construction which has structural skins on both sides of the foam core, the long substantially straight mono- filaments embedded in the resin, are correctly disposed to resist load, in tension on one side of the foam core and in compression on the opposite side of the foam core. The monofilaments are, according to the invention, kept as straight as possible and are located close to the outside of the skin. Where the skin curves of necessity, the curvature of the monofilaments is kept as low as possible. As stated the ratio by weight of resin to monofilament is less than 2 : 1 according to this invention. Preferably, where the monofilament is glass and the skin is attached to a core of polyurethane foam as described below, this ratio is kept below 2 : 1 for the sandwich construction as a whole, the ratio in the skin proper being below 1.75 : 1. The resin content of a skin according to the present invention is much lower than had previously been thought necessary and the monofilaments which are advantageously laid as near as convenient to the outside of the skin, form the main structural element in the resin mono¬ filament. composite. These two factors, in combination, enable a skin according to the invention to be much stiffer than a skin composed of resin bonded chopped strand mat. If both, employing glass products and the same resin, are compared weight for weight, the skin according to the invention will have much greater load bearing capacity when incorporated in foam sandwich structures. The resin is one of phenolic resin, polyester resin, polyurethane resin or epoxy resin and the mo.no- filaments are of glass, carbon, nylon, aromatic polyamide, acrylic resin, metal or any combination of these. For example metal wire or strip -could be employed. The monofilaments are each at least 10 cm in length and they may be very much longer up to say 20 m or more. For maximum efficiency the monofilaments must extend over the full span of the beam between strong points supporting the beam. Where a beam continues over a number of strong points or supports and it is not convenient to span all the supports with one continuous monofilament, sets of the straight monofilaments should be overlapped at the location of a support. Preferably to enable manipulation during construction, for example laying up in or on a mould, the monofilaments may be bound by light thread, so that there are formed bundles of parallel monofilaments which are still neither crimped nor woven but are still kept as straight as possible so that they can be laid to lie parallel with the surface of the resin monofilament composite skin. The monofilaments may be formed into a laminate with one set of monofilaments extending longitudinally and with these being crossed by other sets of monofilaments laid closely adjacent and it may be, extending transversely of the first set. In such a laninate these longitudinal and transverse monofilaments will be as straight as. possible. To this end they will not be interwoven although additional sinuous light threads may be provided which effect inter-location between the longitudinal and trans¬ verse straight monofilaments arranged .together to constitute the straight monofilament mat. These light threads do not perform any structural purpose. They are provided simply to give some relative location to the structural monofilaments prior to and in particular during laying up where they facilitate manipulation. Such a straight monofilament arrangement may be formed, on one side of a light mat of randomly arranged short monofilaments. In this latter case the mat advantageously be laid against the decorative outer gel coat, when laying up a female mould and the straight long load supporting monofilaments will then be disposed inside the mat. Resin will then be applied -to fill the inter- stices between the straight long load supporting mono- filaments. This resin will then form a bond with the mat laid against the gel coat, this bond bridging the straight monofilaments and linking with the foam which is then laid up against the straight monofilaments. Preferably however, a further mat of randomly arranged monofilaments is laid between the layer of straight filaments and the foam to result in a more homogenious and more level surface with less resin content. The aforementioned chopped strand mat can be attached to the monofilaments during the manufacture of the monofilament laminate but it is usually more convenient when using female moulds to .apply a thin layer of chopped strand mat to the gel coat. Where the monofilaments are faced on one side with chopped strands the latter would b e placed next to the foam if other layers were not included in the skin, the final requirement being that the roughness of the monofilaments be smoothed out on both sides. As a substitute, just inside the gel coat there may be laid small balls or balloons of glass. When using glass products a preferred composite mat consists of a layer of straight glass monofilaments disposed alongside an adjacent layer of randomly arranged glass filaments. There may be 500 to 2000 grams per square metre of glass in the straight monofilament layer, and more than 20 but less than 500 grams per square metre of glass in the mat layer. It will be obvious from the foregoing that the long monofilaments are intended to take the major tensile and compressive stresses in the final sandwich structure. However, this construction allows also for a substantially reduced resin : monofilament ratio and there being less resin between the layers of monofilaments, the shear stiffness will be correspondingly higher and permanent deformation due to movement in shear, will be less. One further aspect of the invention, broadly stated, comprises a structural skin which has bundles of parallel monofilaments, the individual monofilaments being substantially straight a ι~ d being neither woven nor crimped and having a length of at least 10 cm and the bundles extending along the skin and over the span to which load is to be applied, the bundles of parallel straight monofilaments being wholly embedded in a thermosetting resin. Such skins are usually arranged to bridge spans between strong points, that is supporting structural members analgous to joists. These joists may be as little as 10 cm apart . or as much as 1 metre or more apart. Preferably the individual monofilaments extend over the whole of the span between these joists and are straight or have a low degree of curvature. Other bundles of straight mortofilaments may be * • arranged to extend transversely of the first longitudinally arranged bundles. These transverse monofilaments will also be non-sinuous and embedded in the resin. It will be appreciated that the invention provides a structure in which the resin, instead of being reinforced by randomly arranged fibres, or fibres which are initially sinuous and interwoven as in woven rovings, is reinforced by straight monofilaments bridging the span where the skin extends between strong points. OMPI , σo , By way of Example two beams were compared. Both had the same overall length and same width the thickness varying slightly because of the differences in* construction. The weights were almost identical as shown below. In both cases an identical polyurethane foam core of 12.7 mm was used. Beam 1 containing chopped strand mat was carefully prepared with a low resin content. If more resin had been employed the result would be worse. The resin to glass ratios of the two beams are shown below following other construction details as follows:- Beam 1 a) 2 layers of randomly arranged glass monofilaments 2 each layer having a glass content of 450 gm/m . b) 1 core layer of polyurethane foam. a) 2 layers of randomly arranged glass monofilaments each layer having a. glass content of 450 gm/m . Beam 2 -V 1 layer of randomly arranged glass monofilaments with a glass content of 150 gm/m 2. c) 1 layer of generally straight monofilaments with a 2 glass content of 800 gm/m .. a 2 > 1 layer of randomly arranged glass monofilaments 2 with a glass content of 300 gm/m . .. b) 1 core layer of polyurethane foam. -2 > 1 layer of randomly arranged glass monofilaments with a glass content * of 300 gm/m . o) 1 layer of generally straight monofilaments with a 2 glass content of 800 gm/m . 1 layer of randomly arranged glass monofilaments •i- 2 with a glass content of 150 gm/m . Comparisons Beam 1 Beam 2 Resin : Glass Ratio - by weight 2:1 1.6:1 Weight-kg/sq metre 7.08 7.01 Spring rate in N/mm on 254 mm span (beam width 25.4 mm) 72.89 168.81 Max. central load with load still proportional to deflection under dynamic conditions. (254 mm span, beam width 25.4 mm) 30.63 50.99 Max, permissable static central load 254 mm span, 25.4 mm wide Kg 2.96 13.71 To illustrate that these improved results were not due merely to the increase in the ratio of glass to resin a further beam was constructed as follows and with a resin to glass ratio of about-1.6:l by weight. Beam 3 ** 3> 1 layer of randomly arranged glass monofilaments * with a glass content of 450 gm/m . d) 1 layer of glass filaments each more than 10 cm in length but which were sinuously woven together 2 to. form a cloth-glass content 542 gm/m . a 3 ) 1 layer of randomly arranged glass monofilaments with a glass content of 450 gm/m . bl) 1 foam core layer with a thickness of 19.25mm. a 3 ) 1 layer of randomly arranged glass monofilaments 2 with a glass content of 450 gm/m__. d) 1 layer of glass filaments each more than 10 cm in length but which were sinuously woven together 2 to form a cloth-glass content 542 gm/m . 1 layer of randomly arranged glass mono ilaments 2 with a glass content of 450 gm/m . Weight - kg/sq metre 10.092 Spring rate in N/mm on 254 span (beam width 25.4 mm) 111.89 Max. central load with load still proportional to deflection under dynamic conditions (254 mm span, beam width 25.4 mm) Kg 21.71 Max. permiss ' able static central load 254 mm span, 25.4 mm wide Kg 7.51 In Beam 2, the two layers a, were substantially non structural. Their main function was to provide a smooth external finish. The layers c) in Beam 2, had a glass content of 800 gm/m 2 and of this 400 gms ran in one direction along the beam tested while the remaining 400 gms ran across the beam and hence took hardly any structural part in supporting the load during th.e particular test. In Beam 2, the layer a_) makes little contribution structurally, its main purpose being to facilitate production. It is of interest to compare the three Beams in terms of stiffness per unit weight and permissable static load per unit weight. g^ 1 " 2 3 Stiffness/Unit Weight .9.02 24.08 11.09 Permissable Static Load/Unit Weight 0.37 1.96 0.74 Even if in Beam 3 the resin content and the overall weight had been increased, there would be a reduction in the ability to support static load. If its foam core had been reduced in thickness to 12.7 mm, as with the cores of Beams 1 and 2 its resistance to static load would have been lower.

Claims: C L A I S 1. A structural skin adapted to be bonded to a foam core to form a laminate wherein the skin includes a composite of resin and monofilaments, characterised in that these monofilaments are laid in parallel groups to extend * along the skin, without directional change relative to the extent of the skin, the monofilaments being at least 10 cm in lenσth and being neither crimped nor woven and wherein the ratio of resin to monofilament is less than 2 : 1 by weight within the composite. 2. A skin according to claim 1 wherein the resin is one of phenolic resin, polyester resin, polyurethane resin or epoxy resin and wherein the monofilaments are of glass, carbon, nylon, acrylic resin, aromatic poly- amide or metal or any combination of these. 3. A skin according to any preceding claim wherein a group of monofilaments are laid parallel to one another and formecl into a bundle by being bound with light thread to facilitate manipulation of the monofilaments as and before the resin is applied prior to curing of the latter. 4. A skin according to any preceding claim wherein groups of straight monofilaments are laid parallel to one another and fixed to form a layer on one side of a mat of randomly disposed mono ilaments which are shorter than said straight monofilaments. 5. A skin according to ^ claim 4 wherein each layer of parallel laid monofilaments fixed to one side of the mat is crossed over by another layer of parallel laid monofilaments, all said monofilaments being substantially straight and being neither crimped nor woven, light threads being provided to inter-locate said layers during and prior to application of resin. 6. A skin according to either of claims 5 or 6 wherein the resultant composite mat has 500 to 2000 grams per square metre of glass monofilaments as against •randomly arranged glass filaments of a weight which is more than 20 and less than 500 grams per square metre. 7. A foam sandwich construction including, on at least one side of a core of foam having a density of at least 75 kg/m , a skin as defined in claim 1, the skin comprising one or more resin bonded layers of substantially straight monofilaments laid parallel to and closely adjacent to outside of the sandwich structure, and with the resin to monofilament content of each skin being less than 1.75 : 1 by weight. 8. A sandwich structure according to claim 7 wherein a mat of randomly arranged glass monofilaments is laid between the foam core ?_nd a layer of the substantially straight glass monofilaments, the mat being of a weight of more than 20 but less " than 500 grams per square metre and the layer of substantially straight monofilaments being of a weight of at least 500.grams per square metre, and with resin binder and monofilaments being present in each skin in a resin to monofilament ratio of less than 1.75 : 1 weight. 9. A foam sandwich construction according to claim 7 wherein the construction includes on the outside of at least one of the structural skins, a layer of resin which may contain a filler material. 10. A foam sandwich construction comprising a central core of foam having a density of, at least 75 kg/m 3 and, bonded on each side of said core a skin according to claim 1, further characterised in that each skin is . made up of a first layer next to the foam core and consisting of randomly arranged glass monofilaments and next outside it, a second layer consisting of glass monofilaments, these extending along the skin and being at least 10 cm in length and being non-sinuous, non-woven and non-crimped, and outside this a third layer consisting of randomly arranged glass monofilaments, said three layers being infiltrated by resin binding agent to form a resin/filament composite in which the ratio of resin to glass filament is less than 1.75 : 1 by weight. 11. A foam sandwich construction according to claim 10 wherein the glass is present a) in said first layer of randomly arranged monofilaments, in an amount of 250 to 350 grams per square metre; b) in said second layer of generally straight monofilaments, in an amount of 500 to 1500 grams per square metre; and c) in said third and outer layer of randomly arranged filaments in an amount of 100 to 200 grams per square metre. 12. A foam sandwich construction according to either of claims 10 or 11 wherein in said second layer the generally straight monofilaments w.hich are each at least 10 cm long, are arranged in two adjacent sub-layers, the monofilaments of each sub-layer ' being parallel to one another but not with the monofilaments of the other sub-layer. 13. A foam sandwich construction according to either of claims 10 or 11 wherein in said second layer, the generally straight monofilaments which are at least 10 cm long, are arranged in at least three adjacent sub-layers, the monofilaments • of each sub-layer being parallel to one another but not with the monofilaments of the next adjacent sub-layer. ' c - * 14. A foam sandwich construction according to any one of claims 10 to 13 wherein, the resin employed is polyester resin and including a decorative surface coating applied- to the outside of the sandwich construction. 15. A .laminate consisting of a core of foam having 3 a density of at least 75 kg/m , and a structural skin bonded thereto, the structural skin comprising filamental material embedded in a resin material, characterised in that the ratio of the resin to the filamental material is less than 2 : 1 by weight, and wherein the filamental material consists wholly or mainly of filaments which, individually, are at least 10 cm in length, the filaments being formed in superimposed layers running in different directions along the skin, with the filaments of each layer being substantially parallel to one another and extending along the skin without change of direction relative to the surface of the skin and without being interwoven with the filaments of an adjacent layer, and including threads for holding the, superimposed layers in assembled interlocation prior to the setting of the resin.

Inventors: SEDDON R
Applicants: SEDDON R, UNITEX LTD
Application year: 1980
Main classification: B29D27
IPC codes: B29C44, B32B5