Manufacturers must closely control the working temperature ranges of adhesives to ensure proper viscosity, adhesion, set speed, open time and curing. Poor temperature control creates problems that reduce adhesive performance, increase manufacturing costs and dissatisfied consumers of the end product. Each class of adhesive exhibits its unique thermal characteristics.
Adhesives in which the carrier fluid is water include resin, cohesive, dextrin and protein glues. Their advantages include ease of handling, low odor, lack of toxicity and non-flammability. They are widely used for applications such as woodworking, packaging, films, bookbinding and envelopes.
All water-based glues, except protein glue, are applied at room temperature and have slow to fast set speeds along with medium open times. Protein glue is applied at 150 degrees Fahrenheit with medium set speeds and open times. Open times and set speeds are reduced or increased when the application temperature is lowered or raised, respectively. However, adhesive applied at too low a temperature creates a weaker bond.
Dry Blended Adhesives
Included in this category of adhesives are polyvinyl alcohol, gelatin and starch-based products. They are used for tube winding, laminates, corrugated cardboard and rigid box assembly. The optimal application temperature for all types is 150 degrees Fahrenheit. Polyvinyl alcohol-based glue has a fast set speed and short to medium open time. The other types exhibit medium set speed and open time. Manufacturers who mix this type of adhesive in-house find it to be one of the most economical adhesive options.
Hot Melt Adhesives
Hot melt glues are thermoplastic polymers that are solid at room temperature. They come in a wide variety of formulations for almost any application. Product shapes include rods, blocks, chips and other shapes that match specific application equipment. Hot melt glues liquefy at temperatures ranging from 250 to 375 degrees Fahrenheit. Specific hot melt adhesive formulations include EVA, Metallocene, pressure-sensitive and APAO.
Most hot melt glues have a fast set speed between 1 and 15 seconds. Set speed does not apply to the pressure-sensitive and APAO types of hot melt, however, since they remain tacky and bond after pressure is applied to them. They are ideal for applications such as hygiene products and credit card mailers. APAO hot melt glue has the highest working temperature range and resistance to solvents.
Hot melt glue behavior is more sensitive to application temperatures. Improper temperature control can lead to problems such as charring, gelling, stringing or smoking. To avoid charring or smoking, the adhesive tank should be kept full and covered in addition to lowering the temperature. Overheating can also cause gelling. Stringing can be avoided by pre-heating the glue stock and ensuring that the running temperature is high enough.
Viscosity of hot melt glue also depends on temperature. If nozzles become clogged, it is often an indication that either the temperature or nozzle pressure is too low.
Avoid Wasted Adhesive and Time
Poor appearance, weak bonding, clogged equipment and uneven or excessive application of adhesive can often be traced back to unstable temperature control. Long set speeds due to low temperature may require additional energy during the curing step. Such problems can be avoided by knowing the correct temperature ranges for the specific adhesive product you choose.
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