Hot melt adhesives are thermoplastic materials that are melted by heat so that they can be applied as an adhesive. Hot melt adhesives are applied with hand glue guns to a surface that is to be bonded to another surface. They are a popular choice for industrial applications because they are economical; have a long shelf life; be solvent-free and non-toxic; and forms an instant bond within a minute. Adhesives are applied to the desired surface either by spraying or beading with a glue gun. They are manufactured in glue stick or cartridge form.
Hot melt adhesives are solid thermoplastics that are: Heated in the glue gun to change state to a liquid and then applied to a desired surface. The other surface that is to adhere to the first is then glued to the adhesive. The adhesive then resolidifies as it cools, forming a strong, reliable bond. Therefore, the time between applying the glue and sticking the items together must be minimal; The shorter the time, the stronger the bond. This time between applying and gluing objects is called "open time".
Open time also depends on the heat of the adhesive when applied. The open time also measures the time it takes for the hot melt adhesive to become non-bonding. The open time is measured in seconds and can range from a few seconds to 1 minute.
Open times generally fall into three categories:
Short: 1-15 seconds
Medium: 15-30 seconds
Long: 30-60 seconds
Different hot melt adhesives are designed for different viscosity levels; this is measured with a viscometer. Viscosity is the fluidity of the liquid; as the temperature increases, the viscosity of the adhesive decreases. The units in which viscosity is measured are centipoise (cps). Viscosity is also generally divided into 3 classes.
Lower viscosity glues (500 - 3000 cps) increase the performance of the glue gun due to the easy flow of the liquid. They also form stronger bonds and reduce stringing. However, they are not suitable for permeable surfaces such as textiles and foams.
Medium viscosity adhesives (3000-6000 cps) have less strong bond strength than lower viscosity adhesives, but greater bond strength than higher viscosity adhesives. The hot melt glue does not flow as freely as with lower viscosity glues, so a more powerful glue gun may be required.
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