Hot Melt Adhesives
The base polymer or "binder" typically makes up 40-70% of the hot melt adhesive formulation. The majority of hot melt adhesive formulations utilize ethylene vinyl acetate (EVA), block copolymers, or polyolefins such as polyethylene and polypropylene as the base polymer. When formulating a hot melt adhesive a base polymer should be chosen that provides good adhesion to the substrates being bonded, low melt viscosity, good molten stability, and resistance to moisture. Direct comparison of common base polymers can be difficult because there are many ways to formulate with them. However, certain properties that play a vital role in the success of the adhesive such as molten stability, peel strength, and melt viscosity are effected by the base polymer and should be evaluated to ensure maximum performance and cost benefit.
Epolene® vs. EVAEthylene vinyl acetate is the most common polymer used in hot melt adhesive formulations today for packaging and nonwovens. Westlake has performed a comparison study of hot melt adhesive formulations based on their Epolene® polyethylene polymers versus two EVA polymers with varying vinyl acetate content. The formulations are listed in the table below. Epolene® N-21 was included in formulations A and B as an additive to reduce the setting time of the adhesive. Eastotac™ H-130 was used as a tackifying resin for all of the formulations. The antioxidants used were Irganox™ 1010 and Cyanox™ 1212 at 0.10% and 0.35% respectively.
|Epolene® C-15, %||72||-||-||-|
|Epolene® C-18, %||-||72||-||-|
|EVA (18% VA, 150 melt index) %||-||-||35||-|
|EVA (28% VA, 150 melt index) %||-||-||-||35|
|Eastotac™ H-130 Resin %||22||22||45||45|
|Epolene® N-21, %||6||6||-||-|
|Paraflint H-1, %||-||-||10||10|
|Parrafin Wax (152° F melt), %||-||-||10||10|
|Viscosity @ 177° C, cP||1200||1200||1500||1550|
|Softening Point, °C||106||107||105||108|
|Set Time, sec||3.8||3.3||3.6||4.4|
|Peel Strength (0.1 in / min) on Kraft, gm||760||890||700||990|
|Peel Strength (20 in / min) on Kraft, gm||920||650||850||990|
|Peel Strength (0.1 in / min) on Mylar, gm||18||45||5||14|
|Peel Strength (20 in / min) on Mylar, gm||79||290||4||9|
|Peel Fail Temperature, °C||101||114||109||116|
|Viscosity Change (100 hrs @ 177° C), %||7||15||30||40|
Based on the testing results above, Epolene® polyethylene polymers provide similar properties to EVA when used as a binder in hot-melt adhesives. However, in the following areas, Epolene® is superior to EVA.
Hot-melt adhesive formulations based on Epolene® have a lower melt viscosity than formulations based on EVA. This allows for easier application to substrates and better, more complete wetting of surfaces.
Hot-melt adhesives that use Epolene® as a binder have a more compatible density for repulping processes than EVA-based adhesives. Adhesives with densities that fall outside the range of 0.98–1.05 g/mL are easier to separate with centrifugal cleaners, making them better suited for recyclable packaging.
Epolene® polymers are much more stable in the molten phase than EVA polymers. This is likely due to the simplicity of the polymer structure. Adhesive formulations based on Epolene® have a much more stable viscosity when molten, as illustrated in the graph below. This, in turn, leads to a longer pot life and less downtime due to plugged nozzles.