“Use lots of adhesive”
More adhesive is not necessarily better. The effects of excess adhesive on cure are significant. When too much adhesive is applied with 1k systems (such a Forgeway’s Formoa range), the excess will squeeze out and cause problems by reducing the effective strength build up at the bond interface, because the initial moisture from the air is absorbed in curing the excess. Therefore, Forgeway would always recommend that excess adhesive is removed. As Formoa polymers cure by absorbing moisture in the form of atmospheric humidity, removing the excess will assist in faster through-cure at the bond interface. Recent tests show the effects of excess adhesive on bond strength after 24 hours.
A moisture curing adhesive system cures from the outside in. The graph below illustrates this. Lap Shear number 1 had the excess removed immediately after application. After 24 hours this has started to through-cure and the lap shear strength is significantly higher than the other two lap shear joints.
Adhesive should be applied in horizontal or vertical straight lines. ‘S’ beads are always a ‘no no’ as they can act as a water trap. Two beads that flatten out to around 20mm wide each, with a space in between, are better than dispensing a large mass of adhesive.
Open time is the timespan from when the adhesive is applied, to when it no longer bonds to another substrate – normally quoted in minutes. It is important to understand open time as it can mean the difference between two substrates bonding or not. A product’s open time is always stated on its technical data sheet.
A quick way of testing if a product is ‘still open’ is as follows:
Wearing disposable gloves, press your index finger into the adhesive and squash down to a 2 to 3mm bond line. Alternatively use a clean stainless steel spoon or a spatula. If you get a good transfer of adhesive to your finger, the adhesive is still ‘open’ and an acceptable bond will be achieved.
If the transfer is reduced, the adhesive has started to skin over and the ultimate tensile strength and peel forces will be reduced.
If there is no transfer of adhesive to your finger, the adhesive has skinned over and an acceptable bond will not be achieved.