Viscosity Levels of Hot Melt Adhesives
Hot melt adhesive are thermoplastic materials which get melted by heat so they can be applied as an adhesive. Hot melt adhesives are applied by hand held glue guns onto a surface which is intended to be bonded with another surface. They are a popular choice for industrial applications due to them being economical, 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. Generally, the shorter the time, the stronger the bond. And this time between application and bonding of objects is known as the "open time".
Open time also depends 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 is measured in seconds and can be anything from a few seconds up to 1 minute.
Different hot melt adhesives are designed to have various viscosity levels, this is measured by a viscometer. The viscosity is the ease in which the liquid flows, and as the temperature rises, the viscosity of the adhesive declines. The units in which viscosity is measured are Centipoise (cps). Viscosity is also generally categorized 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.
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