Hot Melt Coating

Hot melt adhesive is solid glue at room temperature which is heated until it reaches the consistency of honey (about 10,000 cp) and coated onto various materials.  While there are a variety of hot melt adhesive chemistries and tackiness quality, it is useful to categorize them as pressure sensitive adhesive (PSA) or permanent adhesive.

Permanent adhesives are generally used for permanent laminations or to be reactivated with heat for later bonding. Typical uses are closing card board boxes and bonding Formica sheeting to cabinets and counter tops. At room temperatures, permanent adhesives are not tacky. Most are hard to the touch, but some can be rubbery such as the traction strips on the bottom of some socks or gloves.

More commonly used in the converting industry are PSAs. These remain tacky at room temperature and will form a bond to a variety of materials when applied with pressure. These are coated onto materials which will be processed further and bonded to some surface in its final use.

There is no clear demarcation between permanent adhesives and PSA, rather, there is a continuum from the simpler permanent adhesives to which various plasticizers and tackifier can be added to enable bonding at room temperature.

There are two basic mechanisms of bonding. Most hot melts bond mechanically by flowing into the micro roughness of the surface and by wetting out the surface: a kind of suction cup. Some adhesives will interact chemically with the surface it is bonding, creating a very strong interface. There are processes, such as corona treating that can enhance a material's tendency to bond with adhesive.

Generally, the strength of a PSA bond increases over time (72 hours is often cited for maximum strength) since the viscous adhesive needs time to flow into the pores of the material it is contacting. The bond strength also increases with thickness, up to the point where the adhesive becomes so thick (4 or 5 mils) that it becomes the weak link and fail due to shear flow.

While there are many methods for coating hot melt adhesives, we use two continuous processes:

• Roll coating involves a pattern roller dipped in a bath of hot adhesive and transferred directly to the material or to a release liner which will be laminated to the material.  We use metering blades on the pattern roller and the material to control the thickness and  the smoothness of the coating.  This process is good for laying down thick coatings on porous materials and for high volume patterns in line with other narrow web converting processes.  We currently have 10" capacity for roll coating.
• Slot coating involves extruding the adhesive through a slot.  This process permits controlled thickness of adhesive to be applied over wider widths at high speeds.  The slot can be modified to lay down strips of material   of any width desired in the web direction.  The entire unit can be turned on and off rapidly to achieve patterns of rectangles with the adhesive.  Our current capacity is 40".

While it is possible to purchase PSA coated onto release liner and laminate it yourself, CRC can not only coat the adhesive, but laminate the material in wide widths and slit it in the same pass.  This means savings in delivery time and costs.