1. Preparation of the surface
Before applying the adhesive, make sure the surface is
- clean and dry
- free of dirt
- free of oil and grease
- free of wax and silicone (spray).
For cleaning smooth, non-absorbent surfaces, we recommend the use of adhesion-promoting cleaners
If the surface has been thoroughly cleaned with an adhesion-promoting cleaner, a separate sanding step is not necessary in most cases. However, if preliminary adhesion tests reveal inadequate surface strength, a cleaning-sanding procedure (clean – sand – clean) can improve adhesion. Preliminary tests should always be carried out when bonding large objects, working with difficult substrates or handling time- and labour-consuming applications.
As a general rule, it is not recommended to sand untreated or anodized aluminium surfaces. In such cases, careful cleaning with adhesion-promoting cleaners is more effective.
3. Application of the adhesive
Make sure that the adhesive, the work piece and the ambient temperature are as recommended to ensure optimum bond formation. Water-based systems should never be applied below +10°C and hybrid- or PUR-based construction adhesives never below +5°C.
- Spot application: only recommended for light-weight components.
- Bead application: straight, vertical (!) beads are recommended especially for bonding large areas.
- Application by notched trowel: unproblematic with absorbent surfaces. In the case of non-absorbent surfaces, make sure to proceed as follows: vertically skim off the adhesive to produce small grooves with the trowel through which the air can circulate. This allows the adhesive to cure.
- Do not apply the adhesive horizontally or in horizontal waves.
- If the layer thickness is below 1 mm, the bonded materials should not differ too much in their expansion properties as the adhesive is not able to compensate these differences.
The following table provides an overview of the average expansion rates of different materials:
Thermal expansion of window materials Epsilon in mm/m*
Rigid PVC (white) 4.8 mm/m
Rigid PUR – applied as spray foam 4.2-6.0 mm/m
Solid wood, long pieces 0.18-0.36 mm/m
Aluminium 1.44 mm/m
Steel 0.72 mm/m
Glass 0.48-0.72 mm/m
Acrylic glass 4.2-4.8 mm/m
* Thermal expansion in mm/m within a temperature range of +20°C to +80°C.
- In order to produce an elastic, vibration-damping bond, a hybrid-based construction adhesive requires an adequate layer thickness. Layers below 2 mm thickness are not sufficient.
- If a defined layer thickness is required, it is possible to embed suitable spacers into the adhesive bed (toothpicks, veneer strips, setting blocks).
- Remove excess adhesive while still fresh. When using hybrid-based products, first scrape off the adhesive and then clean with a solvent (e.g. spirit). Excess amounts of water-based construction adhesives can be simply removed with a moist cloth
4. Securing and pressing
Only heavy work pieces and pieces under tension need to be secured. Fastening devices do not need to be of mechanical nature (e.g. clamps); often, it is sufficient to use heavy weights or fabric tapes. When working with PUR-based construction adhesives, the process of securing and pressing is crucial for the bonding result. A slight joining pressure is sufficient.
5. Aligning and repositioning
The work pieces can only be aligned and repositioned within the open time of the adhesive. Make sure no skin has formed on the adhesive before joining the parts. During the curing process, the work pieces must no longer be moved!
6. Curing / Build-up of bonding strength
The curing process is influenced by the following factors:
- Ambient temperature
- Moisture content of the work pieces
- Air humidity
- Diameter of the bonded joint
- Air circulation
Favourable curing conditions:
- For water-based construction adhesives
Temperatures between 20 and 40 °C and low humidity (below 50 %). Low temperatures and high humidity considerably slow the curing process.
When working on extremely dry substrates and at high temperatures, the adhesive may sometimes "bake" onto the surface, since the moisture of the adhesive is extracted by the substrate too quickly, and thereby impair bond formation. In these cases, pre-/wetting the substrate can help.
- For chemically curing construction adhesives
Temperatures between 20 and 40 °C and relatively high humidity (above 50 %) accelerate the curing process.
Tips for accelerating the curing process of hybrid-based construction adhesives applied in relatively thick layers or between two non-absorbent surfaces:
- Add moist sawdust to the bead (max 10 % of the bead volume).
- Place moistened gauze bandages (dressing material) into the middle of the adhesive bed.
- Moisten the surface to be bonded with a damp cloth.
The curing speed of 1-component PUR adhesives is considerably slowed down when a skin forms quickly on top of relatively thick adhesive layers. Under certain circumstances, full curing – down to the core of the adhesive bead – may be completely prevented. When coming into contact with moisture/water, these adhesives tend to foam strongly (post-expansion); occasionally, this may even result in a loss in strength.