Demanding surface lamination: High-tension bonding

Like in any relationship, tensions are not desired when it comes to laminating surfaces. Tensions, i.e. mechanical stress, acting in and on the joint can lead to lower efficiency and pose a threat to end product quality. Reactive PUR hot melt is the adhesive system of choice to handle that stress.

Like any other bonding, high-tension bonding is basically a multi layer compound of at least two substrates. Tensions occur only when the bondline is exposed immediately after joining to high mechanical stress, like tensile, shear, or torsion forces. To prevent those forces from impairing efficiency and diminishing quality, the adhesive must provide sufficient adhesion to the substrates and the initial strength (i.e. cohesion of the adhesive) must be higher than the acting forces. This is essential to ensure a durable and superior compound.
Therefore, the challenge for adhesive manufacturers regarding the tension between the parts to be bonded lies in launching products on the market that provide a wide range of adhesion to the different materials as well as high initial strength.

Challenging material diversity
Continuous research and developments in material science and technology driven by economic, ecological and technological factors (energy, climate and environmental protection, resource conservation, etc.) have provided an almost infinite number of different materials for which “bonding” is the most suitable and efficient joining method available.
Even wood-processing companies now also manufacture compound materials with the most different combinations of metallic (e.g. iron, steel, aluminium) and non-metallic inorganic or organic substrates (e.g. glass, ceramic, plastics) in addition to standard wood-based substrates (MDF, HDF, veneer plywood, solid wood, …).

The constantly growing diversity of substrates and laminates combined with more specialised manufacturing technologies and increasing quality demands from the consumer are met with an accordingly wide range of specialised adhesives. Adhesives have different setting processes depending on their formulation and can be classified into three categories based on their general bonding behaviour. Either completely physical setting (cooling of the melt and hardening, or evaporation of solvents or water and solidification), physical setting and chemical curing (cooling or evaporation plus a chemical reaction (crosslinking)), or completely chemical curing adhesives (polymerisation, polycondensation, etc.).
Whether rolled, thick laminating materials, resin-impregnated papers and laminates, or thermoplastic foils – moisture-curing PUR hot melt adhesives are able to compensate for the forces acting on the bondline and are ideally suited for high-tension bonding. After complete adhesive crosslinking, the compound can no longer be separated and tests of materials bonded with reactive PUR hot melts usually result in material failure. In addition, the end product will show high resistance to water and heat.
The processing properties of reactive PUR hot melts have been adapted to the high requirements of surface lamination. High initial strength is necessary to handle the high stress that may act upon the bondline. The Jowat portfolio also includes adhesives with bespoke technical characteristics (e.g. open time) – adapted to specific manufacturing processes. Combined with the high initial strength after joining, this makes PUR hot melt adhesives suitable for continuous inline production. These surface adhesives not only increase the quality of the end products that were laminated with difficult materials, they also ensure reliable processes and therefore make significantly contribute to efficiency.
In addition, numerous adhesives are tested according to IMO (International Maritime Organization) standards regarding flame spread resistance and have wheel mark certification for applications in shipbuilding interiors. Therefore, they are ideally suitable for the increasing demands on surface lamination in the furniture industry as well as in interior finishing.

Processing parameters
Successful and reliable bonding of high-tension substrates with reactive PUR hot melts requires suitable adhesive and optimally adapted processing parameters. Sufficiently good substrate wetting is an essential prerequisite. Adhesives that have to absorb strong forces immediately after joining (e.g. restoring forces) are generally formulated with high viscosity, which ensures a cohesion that is sufficiently strong. The back pressure during bonding plays a key role in this process. If the pressure during joining of e.g. sanded HPL is too low, the surface of the substrate is not wetted completely and the adhesive will not build up the necessary and optimum amount of adhesion. In the worst case, this may lead to delamination in the compound. Tension is force per unit area of a surface. The larger the bonded surface, the better and more even are the acting forces distributed. Then the adhesive will absorb the tensions optimally and provide a durable compound.
The amount of back pressure and the distribution of pressure during calendering depend on the configuration of the machine. Defining factors are among others diameter and width, wall thickness, structural design (spring, pneumatics, hydraulics), and coating (steel or rubber) of the calender.


Excursus 1: Parquet manufacturing
The manufacture of two layer parquet is a good example of high tension bonding that is carried out for health and economic reasons with modern, reactive adhesive systems with reduced emissions. Reactive hot melts like Jowatherm-Reaktant® products cover all relevant requirements in manufacturing: no warping or wood discolouring in the finished product after bonding, no formaldehyde emissions from the bondline, high resistance to moisture and heat, and considerably reduced footfall noise. Fast adhesive setting facilitates short production cycles and significantly increases productivity.

Excursus 2: Compound / Sandwich elements
Due to the broad spectrum of processed materials with sometimes very different characteristics, high tension bonding is also an issue in the production of sandwich elements. These products are usually manufactured either in continuous processes or in a combination of continuous and stationary processes. The adhesive is applied with a roller coater onto the carrier material after which the laminate is pressed to the substrate by a roller and/or a flat press (stacking pressure). The suitable adhesive and processing parameters are chosen depending on the surface properties of the materials and the manufacturing process

Excursus 3: Laminating with high-gloss materials
Another application with high tensions is the lamination of high-gloss films and other materials with PUR hot melts.
Whether by roller or by nozzle, for this special application it is essential that the adhesive be applied at the lowest possible temperature. The challenge with these adhesives is to find a balance between good flowing properties at low temperatures (in general, this means a low viscosity) and very high initial strength immediately after the lamination process. Therefore, a short so-called “open time” of the adhesive is necessary.
Apart from the energy input, the back pressure of the foil also has to be kept as low as possible, especially when thin (approx. 200 to 300 µm thick) thermoplastic high gloss decor foils are bonded, to avoid offsetting the film. At the same time, the adhesive must be applied homogeneously to the bonding partner and irregularities in the substrate surface have to be evened out with the adhesive to create a mirror-like surface. Regardless of the low back pressure, the bond strength achieved immediately after joining has to be high enough to cause fibre tear if the laminate is peeled off from the wood based substrate.
The adhesive also has to build up high hardness quickly to prevent flaws resulting from handling during and after manufacturing, from stacking during storing, and from transportation of the high-gloss panels. Flaws in the surface of these products would inevitably lead to an increased rate of rejections.

Conclusion:
From the perspective of adhesive technology and manufacturing processes, health, and efficiency, the innovative developments in reactive PUR hot melts have provided a reliable and safe solution for high-tension bonding. Safe for consumers as well as for employees in the production of furniture and parts – safe for manufacturing processes and the economic result.

The professional article is published in the magazine 'möbelfertigung 5|2015' in German language. You can download the article down below.