by Vaughan Taylor, Cabinet Materials Technologist, FIRA. Published in TRADA Timber Industry Yearbook 2002.
Flexible veneer is a general term used to describe sheets of wood veneer which have been backed with paper or fibre reinforcement. The process is not particularly new and indeed these veneers were seen many years ago, although demand at that time was low.
Fleece-backed veneer is a similar type of product used extensively for wood veneered profiled components such as cornice and pelmet trims in kitchen furniture. The fleece reinforcement of the veneer allow the original delicate veneer to be formed by a series of machine rollers into tightly contoured shapes – an impressive process. The primary function of the backing is to make the wood easier to handle and to form and bond to shaped components. Such veneers can be rolled up like wallpaper without the risk that they split apart. This cannot be easily done with the veneer in its basic form, which is a far more fragile product.
Flexible veneers supplied as 8’ x 4’ sheets have gained popularity since they enable smaller joinery shops, bespoke furniture makers and shopfitters to produce shaped wood veneered panels easily. Importantly, this can be achieved without the need for specialised equipment, such as bag or membrane presses, or the use of male and female formers. The current demand for shaped panel work has to some extent been initiated by architects and designers who have become more adventurous in their designs and who now specify curved arrangements of products such as wall panelling, specialised reception desks and so on. Indeed, the combination of flexible veneer with substrates such as flexible plywood and grooved flexible MDF allows complex curved panels to be fabricated.
Whilst the use of flexible veneers has simplified the production of shaped components and enabled greater freedom on a cost effective design basis, care is needed in the production process, particularly in respect of adhesive application, forming the bond and most importantly in controlling the moisture content of the flexible veneer. Problems can arise with dimensional movements of the veneer following exposure to drier or damper conditions than those prevailing at the time of manufacture. This can result in faults such as veneer cracking or more seriously, rippling of the veneer.
Ideally, PVAc (polyvinyl acetate) and UF (urea formaldehyde) type adhesives, typically used for high pressure flat veneering, should be selected as these give firm bonds capable of performing over a wide range of environmental conditions. However, these adhesives require some form of press to hold the veneer in contact with the substrate whilst the adhesive sets. Therefore, for hand production of shaped panels involving flexible veneer, the adhesive used will almost inevitably be a contact type. Contact adhesives rely on the grab between tacky adhesive films to hold the veneer onto the substrate. Typically, a contact-type adhesive is applied to the back of the veneer and also to the substrate surface. The adhesive is then allowed to cure partially to leave a nominally dry, but tacky film. The components are then brought together and the adhesive films on the veneer and substrate are pressed into each other. The bond is formed initially as the rubbery, tacky films grab and combine and the bond then improves as the adhesive fully cures. However, and this is a critical factor, contact adhesives do not set as rigidly as the UF or PVAc classes of adhesive. Consequently the adhesive remains in a partially elastic state and susceptible to ‘give’ when stressed. The elastic nature of contact adhesives means that they are less effective at constraining moisture induced movement of wood veneers and tend to be more susceptible to faults such as veneer cracking and rippling.
Additional factors affecting the quality of bond concern adhesive coating and application of pressure to form the bond. Where comb spreaders are used for coating the component surfaces, the bond occurs primarily at the peaks of the adhesive bands and thus a bond significantly below optimum is inevitable when only hand pressure is applied. If the adhesive is applied too thickly or allowed to overcure, a lumpy appearance of the veneer can also be a problem. Roller application and spray-applied adhesive systems give thinner gluelines and generally more even and consistent coverage.
Timing of the bond formation between the two components is also important. If the adhesive is too wet, there will be a lack of grab and the veneer will not be held sufficiently in place. Conversely, if the adhesive is overly dry, the two films will not meld together satisfactorily and the veneer will be susceptible to delamination or bubbling. Timing can also be complicated by the size of panel to be produced and local environmental conditions affecting the cure of the adhesive. For example, hot conditions will rapidly dry the adhesive films, reducing the open time for producing optimum bond strength in the veneered panel.
As so often with timber and wood-based products, moisture content is the least understood, yet most important factor in the successful production and subsequent service performance of wood veneered panels. It affects all wood veneered panel work but is especially important in the case of panels produced with contact adhesives due to the adhesives’ limited resistance to movement of the veneer, as explained above.
In order to minimise subsequent movement, it is essential that panels are fabricated with the veneers at a moisture content close to that expected in the service environment. The likelihood of cracking and rippling faults developing will be considerably reduced if close control over moisture content is observed. In modern UK office buildings, dry conditions are common and manufacture with a veneer moisture content of around 9% is recommended. Recent surveys of office environments have shown that air relative humidity levels can fall as low as 20 – 25% - equivalent to a moisture content of around 6%. Some form of climate control, such as dehumidification may therefore be required in joinery shops to maintain tight control of wood moisture conditions all year round.
However, efforts to maintain this level of control during manufacture will be negated if veneered panels are exposed to damp conditions on site or when installed into new buildings. Ideally such panels should only be installed when conditions within a building are dry.
Photographs courtesy Silverman Specialist Products.
