GB2161424A

GB2161424A - Energy absorbant laminates

Info

Publication number: GB2161424A
Application number: GB08516562A
Authority: GB
Inventors: John Harold Sewell
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1984-07-11
Filing date: 1985-07-01
Publication date: 1986-01-15
Also published as: GB2161424B; GB8417704D0; GB8516562D0

Abstract

Laminate materials are produced from at least two layers of wood veneer and are bonded by and impregnated with a polymeric elastomer adhesive under conditions of heat and pressure. Laminates so produced exhibit energy absorbing, vibration damping properties together with visco elastic behaviour.

Description

SPECIFICATION Energy absorbant laminates This invention relates to the field of laminate materials incorporating wood veneer layers.

Laminates constructed of wood Iaminae bonded with thermosetting resins, eg phenolic resins, are well known for example as plywood. Hitherto wood laminates have been produced with strength and rigidity in mind for construction and cladding applications. In contrast, the laminates to which this invention relates exhibit energy absorbant properties in that they are readily deformed in directions across the plane of the laminate and are slow to recover the original configuration when a deforming stress is removed. These are not qualities associated with prior art wooden laminates nor are they desirable qualities in the usual applications for which plywood are intended. Moreover the laminates of the invention are resistant to impact shattering and delamination.Hard impacts exceeding the elastic limits can be absorbed by materials of the invention with only localised indentation damage. It is anticipated that the materials of the invention will find applications in shock and vibration absorbant applications and applications requiring resistance to vandal damage.

Accordingly the invention comprises an energy absorbant laminate comprising at least two laminae of wood veneer each impregnated with, and bonded within the laminate by, a polymeric elastomer adhesive. The term "wood veneer" is intended to cover only natural wood not reconstituted wood nor artificial veneers, and only wood in thicknesses up to approximately 2 mm. The polymeric elastomer adhesive may be based on a natural or a synthetic rubber but it must have sufficient adhesive qualities to bond the laminate together. An attempt to produce satisfactory laminates using unvulcanised rubber in solution was not successful as the laminate became delaminated when flexed.

The number of laminae within the laminate will depend upon the required properties of the material. Moreover the degreeof impregnation of the wood veneer laminae may be varied between a minimal surface impregnation sufficient only to bond the layers, to a much greater degree and depth of impregnation which produces a more flexible'and impact resistant laminate. The grain orientation of the wood veneer laminae within the laminate can be arranged to either an aligned or a distributed pattern so as to respectively enhance or reduce anisotropic properties.

The laminate of the invention may be reinforced by the inclusion of a layer or layers synthetic fibres such as carbon, boron or aramid fibres (aramid is a trade mark) and these can be woven or unwoven, continuous or discontinuous. In laminates so reinforced not all wood veneer layers will be bonded directly to another such layer. Indeed the laminate of the invention may be predominantly based on a fibre-reinforced composite having a number (at least two) of elastomer-impregnated and bonded wood veneer layers as replacement for synthetic fibre layers which might reduce the cost of the laminate and confer enhanced energy absorbant properties.

Furthermore, the.laminate of the invention can include metal wires or foils for either electrical purposes (such as screening) or for reinforcement.

The invention also provides a method for the production of an energy absorbant laminate including at least two laminae of wood veneer, the method comprising impregnating laminae and bonding them together to a desired laminar configuration, with a polymeric elastomer adhesive under conditions of heat and pressure. The Iaminae may be impregnated, and subsequently bonded, with adhesive applied to individual laminae from solution. In this case the impregnation will preceed the layup of the laminae and subsequent bonding.

Alternatively the adhesive may be in the form of thin film material and in this case the impregnation and bonding is likely to be combined as a single step. The degree of impregnation can be increased by impregnating individual wood veneers under pressure prior to lay up. This is principally, but not necessarily solely, applicable to impregnation using adhesives in solution.

The method of the invention may also include a further feature, that of heating the wood Iaminae in a liquid solvent prior to impregnation in order to modify the grain structure of the wood and render it more receptive to subsequent impregnation. For the purposes of this feature water should be considered as a solvent, and glycerol is another suitable liquid. If glycerol is used then there could be advantage in leaving some of the glycerol within the wood structure at the impregnation stage in order to render the wood veneers more flexible. Otherwise the solvent will be extracted from the wood veneers (together with the dissolved constituents of the wood) by evacuation or evaporation prior to impregnation.

Materials of the invention have been produced from veneers of beech, walnut, crown oak, and both striped and crown mahogany. Different woods cbnfir differing combinations of properties and costs. Beech for example is difficult to impregnate with polymeric elastomer adhesive and results in a stiffer less damped material whilst striped mahogany is easy to impregnate and results in a more damped and impact tolerant material.

Where thin films of thermoplastic elastomer adhesives are used the principal method of wood impregnation- and bonding is to heat the stacked laminations and interleaved films to a temperature of about 210 degrees Celcius and to press at temperatures at a pressure of approximately 20 Ib/in2 Coating of the wood veneer with elastomerie adhesive solutions may be by brushing, spraying or immersion. Coated wood veneer laminations may be left to dry before being impregnated and bonded.

In order that the present invention may be more fully understood examples will now be given by way of illustration only.

Example 1 Crown mahogany veneer, 0.5 mm thick, was brush coated on each side with Dunlop S758 (trade mark), a solvent-based tubbery adhesive. Pieces of the coated veneer were placed individually (between sheets of release film) on a vacuum table and pressed at atmospheric pressure for 5 minutes to effect an initial impregnation of the wood. These pieces were then removed from the vacuum table and dried for a further 5 minutes in an oven at 60"C. Three pieces of the veneer so treated were then stacked on top of each other with the grain directions at right angles and were pressed at a pressure of 500 Ib/in2 at room temperature.

The resulting material had a high degree of flexibility and toughness. When bent the material did not spring back to its original shape on release but slowly resumed its original shape exhibiting visco elastic properties.

Example 2 Crown mahogany veneer was again used with Dunlop S758 (trade mark) adhesive but this time the adhesive was diluted with trichloroethane in a ratio of approximately 2:1 (solvent:adhesive). The veneer was given four brush coats on each face, drying in an oven at 600C for five minutes between each coat. A final coat of undiluted adhesive was given and dried to a tack-free state. Three pieces of coated veneer were then stacked together and pressed at a pressure of 500 ib/in2 at room temperature.

The laminate thus produced again exhibited a high degree of toughness and flexibility with pro nounced vibration damping qualities and the viscoelastic characteristics mentioned in Example 1.

Example 3 Crown mahogany veneer was again treated in a similar fashion to Example 1 but in-this case ten laminations of veneer were stacked and pressed.

The laminate so produced though more rigid than the materials of Examples 1 and 2, again exhibited visco elastic properties.

It was very noticeable that all-the materials pro duced were not springy but exhibited energy ab sorbing, vibration damping properties In the examples given above densities of the ma terials produced were measured. In all cases the measured density was approximately 0.8 gm/ml, a low density compared to plastics materials.

The method of Example 2 appeared to give better impregnation of the wood veneer due partly to the lower viscosity of the diluted adhesive.

Example 4 Laminates were made from striped and crown mahogany, beech and teak veneers (all approxi mately 0.6 mm in thickness) with a thermoplastic polyester elastomer adhesive. The adhesive used was Du Pont Hytrel (trade name) in type-designa tions 4055, 5526, 6346 and 7246 l listed in order of increasing stiffness. Both 3 and 5 ply laminates were produced with the grain directions staggered at right angles between layers.

One partricular laminate of this kind is descri.bed below in more detail to illustrate the method of preparation and the properties exhibited. This Lam- inate was made from five pieces of striped mahog any veneer 0.6 mm thick and cut to an overall size of 10 cm x 6 cm with the grain parallel to the longer edge in 3 pieces and with the grain parallel to the shorter edge in the others. Hytrel (trade name) 4055 adhesive, received in the form of grain ules was made into a film of about 0.3 mm thick ness by pressing the granules between the plates of a press at about 210 degrees Celcius. Four pieces of the film were cut, each to dimensions of 11 cm x 7 cm and interleaved between the cut wood veneers.The veneers were stacked so that the central and outermost layers were the ones having grain direction parallel to their longer edges. The assembly of stacked veneers and adhe sive films was pressed under pressure of approxi mately 20 Ib/in2 at about 210 degrees Celcius for 30 seconds, removed from the hot press and cooled under a similar pressure I to prevent warpage. The flexural modulus of this laminate, when flexed as a beam across the grain of the surface veneers, was 790 KN/cm2.

All the laminates produced, of this kind, exhibit a degree of energy absorbance but this is less pro nounced than in laminates produced from solvent based adhesives, because the adhesive does not impregnate the veneers to a similar extent. More over the laminates exhibited flexure and toughness dependent I all other parameters being equal I on the grade of adhesive utilised.

The examples described in this specification have been of simple flat laminates, however the in vention is not restricted in application to flat prod ucts. As will be apparent to one of ordinary skill in the art, a significant degree of curvature may be introduced at the lamination stage by conventional wood processing techniques. Laminates made with thermoplastic adhesives may also be susceptable to post-lamination forming by hot pressing. In re gard to laminates made from thermoplastic adhe-- sive, care needs to be exercised with regard to selection of a suitable laminating temperature and a suitable lamination thickness. Otherwise the tem erature gradient established by hot press laminat ing, between the outside and interior of the laminate might result in either insufficient melting of adhesive in the interior region or charring of the outermost veneers.

Claims

1. An energy absorbant laminate comprising at least two laminae of wood veneer each impreg nated with and bonded within the laminate by, a polymeric elastomer adhesive.

2. An energy absorbant laminate as claimed in Claim 1 including at least one lamina of synethic fibre reinforcement material.

3. An energy absorbant laminate as claimed in Claim 1 or Claim 2 including a metal wire or wires or a metal foil.

4. A method for production of an energy ab sorbant laminate including at least two Iaminae of wood veneer, the method comprising impregnating and bonding together to a desired laminar configuration, laminae with a polymeric elastomer adhesive under conditions of heat and pressure.

5. A method as claimed in Claim 4 wherein the polymeric elastomer adhesive is applied to laminae from a solvent solution prior to lay up of the laminae.

6. A method as claimed in Claim 4 wherein the polymeric elastomer adhesive is applied to the laminae in film form.

7. A method as claimed in any one of claims 46 in which at least one of the wood veneer Iaminae is impregnated with the polymeric elastomer adhesive under pressure, prior to inclusion in the lay-up of the laminae.

8. A method as claimed in any one of Claims 47 in which at least one wood veneer lamina is heated under liquid solvent prior to impregnation with the polymeric elastomer adhesive.

9. An energy absorbant laminate as claimed in Claim 1 and substantially as hereinbefore described with reference to the foregoing examples.

10. A method for production of an energy absorbant laminate as claimed in Claim 4 and substantially as hereinbefore described with reference to the foregoing examples.