| What Happens when Heat Degrades the Hot Melt |
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| By Robert C. Schmidt, Development Associate |
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| Most bookbinders understand that standard 350°F hot melts are sensitive to thermal stress. Heat degradation occurs when any hot melt stays exposed to air and high heat for a prolonged time. The higher the temperature, the shorter the time to char formation. |
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| Excessive heat degrades a typical 350°F hot melt, impairing the adhesive's physical/mechanical and esthetic properties, as well as the book's quality. Operation of the application equipment and bookbinding productivity also suffer. |
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| Conditions that Degrade Hot Melts |
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| It's not unusual for hot melt to sit in the glue pot at standard 350°F temperature for 24 hours or more, allowing char to develop. Paper fibers and other material may enter the glue pot, contaminating the adhesive and further increasing its tendency to char. Uneven temperatures in the glue pot also cause the glue to char and degrade. Hot spots, as high as 500°F, can occur around the heaters in the bottom of the glue pot, overheating and degrading the hot melt. In a typical glue pot, char tends to build up most rapidly around the heaters. As it builds, it insulates the area, causing the heaters to stay on longer and charring more glue. The heating elements continually cycling on and off also stress the adhesive, causing further degradation. Because of hot melts' sensitivity to thermal stress, a plant that cranks up the hot melt temperature for higher throughput will see a counter-productive effect on the adhesive. |
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| The Enemy: Char |
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| When the hot melt degrades, char, i.e., small, unmeltable solid pieces of oxidized adhesive, builds up in the glue pot. Char is much like the gristle that forms on a roast that's been left in the oven too long. As the hot melt overheats and contacts the air, it becomes unstable, and forms the crust-like char. On the chemical level, several things happen as high, prolonged heat causes char to form. In the early stage of a hot melt's breakdown, molecules become smaller; the molecular weight falls. As the heat continues, the molecular weight rises again, and the small molecules combine to form larger molecules. First a gel forms. As the process continues, the molecules keep cross-linking, and a solid, rigid, burned-looking, glassy char develops. |
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| Impact on Equipment |
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| Char has a negative impact on equipment and plant productivity. Heat-degraded hot melt impairs the functioning of equipment, interfering with smooth, even application of the adhesive to the book. Char formation can cause the machine to apply the hot melt irregularly or in the improper amount. Char often interferes with flow of the hot melt in the application equipment, as lumps clog passageways through which glue flows. A pump might plug to the point of needing replacement. A clogged hose might need to be cleaned out. Other equipment may foul to the point where maintenance people must tear it down to scrape off the char, adding to production costs. |
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| Production stops for these repairs, while costs mount for maintenance man-hours, replace- ment parts, and lost production time. In this way, a heat-degraded hot melt exerts a detrimental effect on a plant's bottom line. |
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| Char impacts book quality by weakening final adhesion of pages. The glue film loses flexibility as char causes running viscosity to rise. Char also creates unsightly appearance of the spine. |
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| Formulation |
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| Depending on the formulation, char tends to form in certain 350°F hot melts more than others. A good stabilizer in the formulation impedes char formation. But if the formulator tries to save money by using fewer or no stabilizers, the hot melt becomes more likely to degrade in high, prolonged heat. A hot melt functioning at a lower running temperature, such as COOL-BIND at 250°F, avoids most of these problems, since lower temperature exerts less stress on the hot melt. But the hot melt's adhesion and other key qualities need to remain effective at the lower temperature. Retaining these key properties is what's hardest to achieve in a low temperature hot melt, and is what COOL-BIND succeeds in doing. |
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| Conclusion |
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| If bookbinders stay with a standard 350°F hot melt, they can avoid problems by making adjustments and correcting the above mentioned conditions that invite heat degradation of the adhesive. Don't crank up the hot melt temperature. If the glue has been standing in the hot pot for hours or overnight, it's best to clean out the pot. By knocking out char, the byproduct of heat degradation, plant productivity and operation of equipment will improve. |
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