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Published on 22nd October, 2015
by Testing Department

Adhesives for Furniture

Highlights:

A look as the types of adhesives used in the furniture industry and their uses:

  • Urea formaldehyde adhesive (UF)
  • Polyvinyl acetate adhesives (PVAC)
  • Hot melt adhesives
  • Aqueous polyurethane adhesives
  • Bonding processes
  • Veneering
  • Edgebanding
  • Joint assembly
  • Adhesive bond quality

Introduction

The rapid development of synthetic adhesives over the last few decades has virtually eliminated the natural animal glues used for centuries. Each adhesive type is now engineered to a particular operation – be it laminating, edgebanding or jointing.

These new adhesives have had a marked effect on today’s furniture industry. In particular, the development of particle boards for cabinet making followed by the introduction of urea formaldehyde (UF) adhesives which are much better suited to automatic high speed production processes. Along with UF, the most widely used products are polyvinyl acetate (PVAC) and hot melt adhesives and within each range there are those best suited to different applications.

Urea formaldehyde adhesive (UF)

Urea formaldehyde is a thermosetting material resulting from the chemical reaction between urea and formaldehyde under acidic conditions. The reaction is accelerated by heat but once completed, continued heating within limits has no further affect on the material – hence its good heat resistance. It also has reasonable moisture resistance and these two properties, combined with good adhesion to wood and wood-based materials, together with low cost, has ensured its extensive use.

The UF resins are normally supplied as solutions in water to which a hardener is added. This creates acidic conditions, causing the chemical reaction which converts the liquid adhesive into its solid state. Some UF adhesives are available in powder form containing both resin and hardener, and the reaction is triggered by adding water.

To obtain a reasonable pot life, hardeners usually contain a buffering agent which extends the pot life by slowing down the rate of reaction. Unfortunately, any extension of pot life will result in a corresponding extension of curing time, and so a compromise is necessary between the two. The speed of reaction can be accelerated by applying heat, and a UF adhesive which takes several hours to set at normal ambient temperatures will require only a few minutes at 80° C and less than one minute at about 120° C. This family of adhesives is, therefore, ideal when hot pressing conditions are employed for fast production rates. Pot life is also affected by heat, and if factory temperatures vary considerably throughout the year, different hardeners may be required for winter and summer working.

One method of overcoming problems associated with impractical pot life or long cure times at ambient temperatures is to apply resin and hardener separately to the two parts of the joint; curing then only starts when the two surfaces are brought together. In this way, fast acting hardeners can be used without the associated problems of short pot life.

Another way is to use the automatic metering and mixing method. Resin and hardener in the required proportions are fed to a mixing head by separate metering units. The adhesive is then dispensed into the spreader. The hardener must be in liquid form, the flow of adhesive to and from the spread is carefully balanced and the amount of mixed adhesive in the system at any one time is quite low.

Polyvinyl acetate adhesives (PVAC)

Polyvinyl acetate adhesives are dispersions of small droplets of PVAC in water which set by loss of moisture, either by evaporation into the air or diffusion into the adherents, leaving the droplets to coalesce into a coherent film. Setting times will depend on the moisture content of the surrounding materials and the thickness of the adhesive film but, with sound bonding conditions, a reasonable bond strength can develop in about ten minutes when wood pieces are joined at normal ambient temperatures.

As with UF adhesives, setting times can be reduced by applying heat. Because of their thermoplastic nature, however, pressing temperatures for conventional PVACs are normally restricted to about 70° C. As one-part systems, they have the advantage of an indefinite pot life.

Some of the early PVAC adhesives were susceptible to long-term loading, particularly at high ambient temperature and relative humidity, but the creep resistance of present-day PVACs has been considerably improved.

Conventional PVAC adhesives have only limited moisture resistance although for most domestic furniture applications, bonds made with them are quite satisfactory. Improved moisture and temperature resistance is obtained by cross-linking PVACs, a technique which is well established – particularly for laminating but also where damp conditions or occasional wetting are likely.

PVACs are classified into four groups, D1 – D4. D1 refers to a conventional PVAC with no cross-linking, whilst the higher numbered groups indicate cross-linked products with increasing moisture resistance. D2 and D3 are still one-part systems but D4 are two-part products.

Weight for weight, PVAC adhesives are more expensive than UFs but the cost difference can narrow if cost/unit area of panel is considered because lighter coating weights are generally possible with PVAC.

Hot melt adhesives

Although not used by the UK industry to the same extent as UF and PVAC adhesives, hot melt products play an important part in furniture production, particularly cabinets. They are solid at normal ambient temperatures, need to be heated to a liquid state before application, and must remain sufficiently fluid to be able to wet out the uncoated surface when the two surfaces are brought together. As the adhesive cools, it reverts to its solid state, completing the bond. The rate of cooling is initially high and so the bond forms rapidly which makes this type of adhesive ideally suited to flow-line production.

Care must be taken, however, to ensure that the adhesive does not over chill before the two surfaces are brought together – the rapid cooling rates are a disadvantage in this respect. If over chilling does occur, the adhesive will not wet out the second surface and the bond, if formed at all, will be inherently weak and liable to fail prematurely.

With hot melt adhesives, the change from solid to liquid is reversible and controlled solely by the temperature of the adhesive. This can be advantageous because it enables a surface to be pre-coated with adhesive and later heat reactivated just before bonding. It also means, however, that a hot melt bond is sensitive to heat and will lose strength at elevated temperatures.

Hot melt adhesives are used extensively and successfully on continuous edgebanders. They will bond most types of surface provided sound bonding procedures are used and so are also often useful for small assembly applications where the joint is not load bearing. A number of hot melt applicator guns are available which heat and dispense controlled amounts of adhesive.

Aqueous polyurethane adhesives

With the rapid development of membrane pressing on 3-D lamination using PVC foil as the decorative overlay and profiled MDF as substrate, aqueous polyurethane adhesives are now used in considerable quantities. To obtain a bond with good high temperature resistance, the adhesive is used as a two-part system. It is sprayed over the profiled surface and edges of the MDF and allowed to dry. During the pressing cycle the adhesives are heat reactivated and so able to bond the PVC overlay to the MDF.

Bonding processes

These can be divided into three main groups – veneering, edgebanding and jointing.

Veneering

Veneering used only to apply to surfacing substrates with wood veneers but today the term encompasses a wide range of surfacing materials such as plastics laminates, decorative paper and PVC foils.

For high volume production, surfacing materials such as wood veneer and the heavy papers are normally bonded in heated presses using single daylight, flow-feed machines. Platen temperatures of around 125° C give cure times of less than a minute and a fast throughput. UF adhesives are well suited to these pressing requirements and are used extensively, although the cross linked PVAC adhesives are now an alternative.

With plastics laminates, problems of bowing can occur after laminating at high temperature and so pressing temperatures are normally restricted to about 70 to 80° C. At this temperature, PVAC adhesives have the advantage over UF materials in pressing times and so are generally used for surfacing substrates with plastics laminates. Cold pressing, although not suitable for high volume production, is still carried out by small manufacturers and PVAC adhesives would normally be used.

Modern machines for laminating decorative papers on to particle board substrates work on the hot roll principle and use UF adhesives. Resin and hardener (in liquid form) are applied successively to the substrate panel and the decorative paper foil is then pressed on to it using heated rollers. This form of application, known as the ‘wet-on-wet’ process allows fast acting hardeners to be used.

Membrane presses differ from traditional veneering presses in that they have one solid platen and one flexible one, normally a thin silicone membrane. Pre-machined and pre-glued MDF panels are fed into the press and the PVC foil is placed over them. During the pressing cycle, heat and pressure acting through the membrane soften the PVC and cause it to flow in to the machined profiles on the surface and around the edges of the MDF. The adhesive on the MDF panel is also heat reactivated to a state where it can bond PVC and MDF together.

Edgebanding

The extensive use of chipboard as a substrate, and the introduction of flow-line laminating processes have led to a demand for a fast and efficient method for edging panels with decorative materials. The modern continuous hot melt edgebanders, often containing trimming and even sanding facilities for cleaning up the applied edgings, certainly meet these requirements.

Two techniques are used. Most of the larger machines apply the hot, liquid adhesive to the edge of the panel, and then press the edging strip on to the coated edge whilst the adhesive is still fluid. Application temperatures are normally around 200° C but it is imperative to follow the adhesive manufacturer’s recommendations. The second method uses pre-glued edgings which are fed past hot air blowers to reactivate the adhesive just before the pressure zone.

After some early problems, hot melt adhesives have been used extensively and successfully to bond many types of edging material. Some concern is still felt, however, about the performance of hot melt bonds when using rigid materials whose dimensions are affected by moisture content – such as plastics laminates and wood lippings – on panels which will be subjected to high ambient temperatures and humidity. To meet the demand for an edgebanding system for bonds in relatively severe environmental conditions, new machines using PVAC adhesives have been developed.

With PVAC machines, adhesive is applied to the edge of the panel and partially dried as the panel is fed past an IR heating zone. At the same time, the edging material (which has been pre-glued with PVAC and dried) passes across a hot air blower to reactivate the adhesive. Panel and edging are then brought together in a pressure zone where the two hot tacky gluelines fuse to form a bond. Inherently weak hot melt bonds will occur if the adhesive is not sufficiently fluid to wet out the surface of the edging strip.

For the bonding of substantial solid wood lippings to chipboard panels, RF heating presses are used with UF or PVAC adhesive. Whilst this is not a continuous process, throughput of panels can be high with pressing times of around one minute.

Joint assembly

Assembling joints for both cabinets and chair frames is usually carried out at room temperature. When jointing chipboard panels, dowels or other rigid inserts are normally used to reinforce the joints, whilst dowel or mortise and tenon joints are used with wood pieces for frames. PVAC adhesives, being one-part systems with comparatively short set times at room temperature, are employed extensively for this operation.

Automatic glueing and dowel driving machines are available to help reduce assembly times and PVAC is the obvious choice with these machines as limited pot life adhesives would be inappropriate. For edge-jointing wood pieces into larger panels, RF heating is normally employed with a UF adhesive, although some cross-linked PVAC adhesives are an alternative.

The adhesive film in all joints made from wood is likely to become stressed if the moisture levels in the wood change after bonding and it is important to select an adhesive with good creep resistance. This is particularly true when joining hardwood panels or lippings with a substantial thickness.

Adhesive bond quality

An adhesive bond develops in two stages. First, adhesion must be established between adhesive and adherend, and then cohesive strength must develop within the adhesive as it sets. Adhesive and application procedures must be chosen to ensure that both stages take place or else optimum bond strength will not be realised – even though the bond may appear satisfactory when first bonded.

To assess bond quality, tests are carried out at FIRA using elevated temperatures and high relative humidity to stress the glue-line internally. Badly made bonds will fail prematurely and examination of the failed surfaces generally reveals the reason.

Many of the bonding problems sent to FIRA for investigation are due to poor application procedures. Factors which can cause unsatisfactory bonds include incorrect adhesive coating weights, poor coverage, insufficient pressure during bond formation, poor machining and/or fit of parts, and incorrect moisture content of adherends.

Incorrect selection of adhesive can also contribute to bond failure in service and standard adhesive tests are then useful to determine quality. In particular, it is not advisable to use a PVAC adhesive for jointing if it does not meet the British Standard with respect to resistance to sustained load.

Bond failures in service result in customer complaints so it is important to evaluate new bonding systems at the design stage before full production procedures are initiated.

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