The use of hot melt
as a bonding adhesive in the industry has been transformed over
the past few years and is gradually replacing conventional solvent
or water-based systems and flame lamination. Hot melts as bonding
adhesives are based on polymers or their compounds, which are formulated
to the specific requirement of the application, e.g. melting temperature,
initial tack, curing time and crystallization.
The absence of solvents, volatile parts and water, offers an environmentally
friendly operating process, and since no additional heating energy
is required to evaporate water or solvent and the curing takes place
by ambient air temperature or by the humidity of the ambient air,
no drying or curing ovens are necessary.
Gravure coating is an old technique whose versatility has been increased
by recent technology developments. The gravure coating process relies
on an engraved roller running in a coating bath, which fills the
engraved dots or lines of the roller with the coating material.
The excess coating on the roller is wiped off by the Doctor Blade
and the coating is then deposited onto the substrate as it passes
between the Engraved roller and a Pressure Roller. This feature
is use of conventional gravure. This results in stable bead, which
when combined with lamination application gives very good quality
and a constant coating weight. A typical configuration is shown
in the diagram above.
The gravure coating head consists of a gravure roller and a doctor
blade system where the hotmelt adhesive is placed. Both parts are
circuit oil heated for an excellent temperature distribution. The
gravure of the roller exactly determines the desired uniform add-on
and the pattern on the substrate. This is required for minimum coating
weights, good bonding strength and soft touch.
Hotmelt adhesives for HYDEN
lines are usually melted off-line, in special melting units. For
the thermoplastic adhesives an extruder or a tank melter is necessary.
The rotogravure system is the best acceptable coating method, and
a must for breathable membranes. Manufacturers of foam/textile laminates
for automotive interiors have also recently started using the rotogravure
as the best alternative to flame lamination. A tremendous choice
of gravures allows an optimal selection of coating patterns and
weights. The coating patterns range from regular geometric patterns
to random dots and nets and other optical effects.