Hotmelt-zp Hot Melt Adhesive Innovations II
As the name implies, a hot melt must be heated to allow the Industrial Adhesives to flow. The application temperature can typically range from 110 to 133 ºC, depending on the formulation and environmental conditions. And the application equipment can be either a roll coater or glue gun. Using a roll coater requires the formula to exhibit a residence time. The residence time is the time that a hot melt can be exposed on a roll coater without reacting with the atmospheric moisture. Conversely, the reactivity can be quite fast when using a glue gun since exposure to atmospheric moisture prior to application is minimized. The initial adhesion is the second phase. This is where the hot melt rapidly solidifies and the green strength is invoked.
Green strength is the one property that distinguishes hot melts from all other adhesives. The immediate adhesive strength prior to curing is green strength. It is a cohesive strength that exhibits a resistance to external forces. A hot melt’s green strength can be high enough that the substrates do not need to be clamped or mechanically held together. Open time refers to the time that the adhesive remains tacky after application. Generally, the green strength and the open time are diametrically opposed, such that the quicker the hot melt changes from the amorphous liquid state to the crystalline solid, the greater the green strength and the shorter the open time.
The green strength and open time of the adhesive depend on the morphology of the polymer and the building blocks used to put the molecule together. Some polyester polyols provide the crystalline nature, green strength and adhesion. Polyether polyols typically are amorphous in nature, providing low-temperature flexibility and decreased viscosity, allowing the adhesive to flow. Besides, as described earlier, the isocyanate is necessary for the moisture-induced cure.
Of course, using various components in hot melt formulas allows varying degrees of green strength and open time.
The cure profile resulting in the development of green strength and the final cure properties can be measured analytically and qualitatively. The physical properties of the hot melt adhesive performance are determined by Dynamic Mechanical Analysis (DMA), Differential Scanning Calorimetry (DSC) and Instron for tensile. Both DMA and DSC methods identify the glass transition temperature. And the chemist must determine which method produces the best resolution of a particular formula.
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