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MANUFACTURING & MATERIALS INNOVATION
T
he first adhesives capable of
successfully bonding a sole to
the upper entirely on their own
appeared in the 1940s and
were
based
on
nitrile
or
polychloroprene rubbers dissolved in
solvents. The stuck-on construction, as
it was named, really took off in the
1950s when it was found that
polychloroprene adhesives could be
heat reactivated some days after being
applied and then bonded with short
press times. This provided a high
degree of flexibility as soles and sole
units could be prepared in advance and
stored ready for use.
One thing that became apparent
from the very beginning was that good
bond strength also required careful
preparation of both upper and sole. In
the case of the latter, it was simple
roughing with a rotating wire brush to
remove loose fibres from the flesh side
of leather soles and clean and increase
the surface area on rubber ones. In the
case of the upper, however, it involved
removing the grain surface to expose
the fibres of the corium to which the
adhesive could anchor itself to and this
was less straightforward.
The best bond is achieved when the
grain is evenly removed over the entire
lasted margin. Where it is not, there
will be weak spots as the adhesive is
unable to penetrate sufficiently to
anchor properly; go too deep and
looser fibres that lie further down are
exposed and offer a weaker anchorage
for the adhesive. Different leathers also
exhibit different fibre densities and
strengths and, while this can be
overcome through choice of upper
leather, those variations will still occur
to some degree depending from where
on a hide the different upper
components have been cut.
SIMPLE REVOLVING BRUSH
The favoured method was to use a
revolving wire brush that was
periodically sharpened using a
carborundum stone with the brush
rotation reversed. Hand roughing is still
widely practised today but the machines
18
WORLD FOOTWEAR | MAY/JUNE 2012
From roughing
brush to laser
DESMA Laser Processing Cell
CREDIT: DESMA