In hygiene industry, hot melt adhesives are
widely used in construction of disposable diapers, feminine care products
and adult incontinence.
Hot melt adhesives are solvent-free thermoplastic
adhesives, that are characteristically solid at low temperatures (usually
below 180 degrees F), however, at elevated temperatures they become
low viscosity fluids with excellent adhesive properties, and rapidly
set upon cooling to form strong and durable bonds.
There are a number of hot melt adhesives. Chemically
the most common hot melt adhesives are made from:
Ethylene vinyl acetate (EVA) copolymers, compatible with paraffin;
Styrene-isoprene-styrene (SIS) copolymers;
Styrene-butadiene-styrene (SBS) copolymers;
Ethylene ethyl acrylate copolymers (EEA);
Polyurethane reactive (PUR)
Generally, these polymers do not exhibit the full range of performance
characteristics required for an end product by themselves. Thus,
tackifying resins, waxes, antioxidants, plasticizers, and other
additives are added to the adhesive formulation to enhance the polymer
performance.
Typically a hot melt adhesive consists of:
Base (back bone) polymers, 20-40wt%, provides cohesive strength.
Tackifying resin(s), 40 - 60wt%, provides adhesion (wetting/spreading)
to surface.
Antioxidant , <0.5wt%, protects components from oxidation.
Function of tackifying resins
are to reduce the processing viscosity, increasing polarity to enhance
the wetting, adjusting adhesive's temperature resistance. There are
mainly two classes of tackifying resins:
Natural resins based on , such as rosin acids and their derivatives,
terpene resins, etc.,
Petroleum hydrocarbons which are normally short chain polyolefins.
Hot melt adhesives are compatible with many substrates,
form a strong bond quickly simply by cooling, and are clean and easy
to handle. In general, hot melt adhesives are less water sensitive
than other type of adhesives, and are unaffected by water, moisture,
or humidity, although if applied to a damp or wet surface the bonding
may be poor.
In hygiene industry, hot melt adhesives have some limitations that
must be recognized. Hot melts cannot be used, or need to be very careful
to use with heat sensitive substrates, such as some low basis weight
films or nonwovens which are made from low melting point resins.
Hot melt adhesives are widely used to join
or bond nonwovens, films, tissues, elastic threads or sheets, etc.
to construct disposable diapers, feminine care products and adult
incontinence products.
The application of holt melt adhesives in
hygiene industry are usually in the areas showed below:
Chassis glue
Hot melt adhesives are often
used to bond nonwovens, films, scrims, perforated films, laminates,
etc. to construct the chassis. Application methods can be slot,
spray, spiral, beads, control coating, etc. When bonding low
basis weight nonwovens, glue bleed-though is a watch-out.
Cited from Nordson's HP
Core glue
Hot melt adhesives are unaffected by
water/urine, moisture, or humidity, so they are used in core bonding
to maintain the integrity of pulp fluff/AGM/wrapping tissues etc.
in dry or wet conditions. Uniform application is important to ensure
good integrity without negatively impacting the absorbency properties.
Elastic glue
Elastic hot melt
adhesives are used to bond elastic strands, films, scrims etc.
onto the chassis to form the waist band, leg gather, side panels,
etc. Usually spiral application is used to bond elastic threads
(for example Lycra), to wrap around the elastic threads with
the elastic glue filaments to provide mechanical bonding in
addition to the adhesion bonding.
