BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI - Universitatea ...

112 Birgit Kjærside Storm
rate of crosslinking the higher strength will be obtained. Opposite will a low
density of reactive groups for crosslinking gives a lower strength and a higher
flexibility of the adhesive. Bonding with a high strength and stiffness is
necessary under a shear stress and especially if a peeling is a risk. If the load
will be a dynamical load the bonding need to have some flexibility as well as
strength and the adhesive need certain flexibility.
The epoxy adhesives can general obtain the highest strength, because I most
epoxy adhesives the density of aromatic groups is high. Most of the epoxy
resins are made on basis of bis-phenol A, which contains two aromatic groups.
In Fig. 6 the formation of the epoxy resion bisphenol-A-diglycidylether
(DGEBA) is shown. Aliphatic groups can be added and they will change the
strength and the flexibility of the cured adhesive. Adding aliphatic groups can
also be used for changing the opening time of the adhesive, because a longer
aliphatic chain will decrease the density of reactive epoxide groups and there
fore change the reaction time. Epoxy adhesives are normal two component
systems. With mixing the two reactants the curing process will start. Increasing
temperature will decrease the reaction time. With changing in reaction
temperature the curing process – the cross linking process – can change and the
properties of the adhesive can change.
Fig. 6 – Formation of DGEBA from bisphenol A and epichlorhydrin [4].
The polyurethane adhesives can as well obtain a good strength. Most of the
polyurethane adhesives can obtain a good flexibility, especially if they are made
on basis of an aliphatic isocyanate. The polyurethane adhesives can be one- or
two component systems. Two component systems will start the reaction, when
the isocyanate and the polyol – the two active components – are mixed with
each other. Again increased temperature will decrease the reaction time and
increased temperature can change the properties of the adhesives. One
component polyurethane adhesives will start the reaction under influence of
humidity in the atmosphere. The humidity will react with one of the
components in the adhesive and make it active for reacting with the other
component. If the humidity is low the reaction will not take place as expected.