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An Introduction to Hot-melt Adhesives


Hot Melt Adhesive are mainly used in the furniture, packaging, and electronics industries. These solid adhesives usually soften on heating; they evenly wet the substrates in contact with them, and solidify on cooling. Hot melt adhesives are applied by hand held glue guns onto a surface which is intended to be bonded with another surface. As they are economical, hot melt adhesive becomes popular in industrial areas; having an extensive shelf-life; being solvent free and non-toxic; and forming an instant bond within a minute. The adhesives are applied by either spraying or beading onto the desired surface with a glue gun. They are manufactured in glue stick or cartridge form.

Hot melt adhesives are solid thermoplastics which are heated within the glue gun to change state to a liquid and then applied to one desired surface. The other surface which is intended to stick to the first is then stuck to the adhesive. The adhesive then returns to a solid upon cooling - forming a strong, reliable bond. This is why the time between application of the adhesive and sticking the objects together has to be minimal; the shorter the time the stronger the bond. This time between application and bonding of objects is known as the "open time".

Open time also relies on the heat of the adhesive on application. Open time also measures the time until the hot melt adhesive no longer possesses a bonding effect. Open time ranges from a few seconds to 1 minute.

Open time generally falls into three categories

Short: 1-15 seconds

Medium: 15-30 seconds

Long: 30-60 seconds

Different hot melt adhesives have their own viscosity levels; this is measured by a viscometer. The viscosity is the ease in which the liquid flows; as the temperature rises, the viscosity of the adhesive declines. The units in which viscosity is measured are Centipoise (cps). Viscosity is also generally categorised by 3 classes.

Lower viscosity adhesives (500 - 3000 cps) increase glue gun output due to the ease of flow of the liquid. They also form stronger bonds and decrease stringing. They are not suitable, however for permeable surfaces such as textiles and foams.

Medium viscosity adhesives (3000 - 6000 cps) have a less powerful bond strength than lower viscosity adhesives but a greater bond strength than higher viscosity adhesives. The flow of the hot melt glue is not as free as lower viscosity adhesives so a more powerful glue gun may be required.

Higher viscosity adhesives (6000 - 15000 cps) are appropriate for bonding rough and jagged surfaces due to them being less fluid than medium or lower viscosity adhesives. They are recommended for surfaces that require more gap-filling. Higher viscosity adhesives require the most powerful glue guns of the three, due to their liquid flow being the least easy of all.

Minor Home Repairs

Compatible with a variety of materials, hot melt glue is able to fix many items in a home, including broken shoe soles, torn couch fabric or loose laminate. Choose low temperature glue for delicate materials, such as fabrics, and a higher temperature glue for more durable materials, such as wood or laminate.

Furniture Construction

Hot melt glue is widely used in woodworking. Applications include desk and chair construction, drawer assembly and cabinet production. Specialty woodworking hot melt glue sticks are a popular choice due to their low viscosity and superior strength. Woodworking glue attaches to the wood grain and forms a strong bond that dries quickly to reduce construction time.

Product Packaging and Shipping

Manufacturers use hot melt glue to seal product and shipping boxes. Hot melt glue forms a strong bond that cannot be broken without tearing the box. This helps avoid product loss because of theft or damage. Commercial vendors sell glue sticks designed for product packaging. They set quickly during product assembly.