Hot Melt Adhesives
Hot melt adhesives provide a number of benefits when used for packaging and non-woven applications. Fast set times, strong adhesion, and excellent moisture resistance make them ideally suited for sealing packaging and assembling disposable non-wovens like diapers and feminie hygiene products. Formulators must balance performance with cost in this highly competitive market making the selection of components extremely important. The largest component of a hot melt adhesive is the base polymer and thus it has the largest contribution to both cost and perfromance.
Base Polymers
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. EVA
Ethylene 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.Formulations
| Formulation | A | B | C | D |
|---|---|---|---|---|
| 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 |
| Antioxidants, % | 0.45 | 0.45% | 0.45 | 0.45 |
Various physical properties of the adhesive formulations were tested including viscosity, density, Tg, and softening point. Peel strength was tested on both Kraft paper and Mylar film at low and high peel rates. Molten stability was tested by observing the viscosity change of adhesive stored for 100 hours at 177° Celsius The results of the testing are listed below.
Testing Results
| Property | A | B | C | D |
|---|---|---|---|---|
| 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 |
| Tg, °C | -23 | -23 | -12 | -16 |
| Peel Fail Temperature, °C | 101 | 114 | 109 | 116 |
| Density, g/ml | 0.93 | 0.93 | 0.98 | 1.02 |
| Viscosity Change (100 hrs @ 177° C), % | 7 | 15 | 30 | 40 |
Adhesive Performance
Based on the testing results above, Epolene® polyethylene polymers provide similar properties to EVA when used as a base polymer in hot melt adhesives. In some areas, however, Epolene® is superior to EVA.Viscosity
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.
Density
Epolene® based hot melt adhesives 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 are easier to separate with centrifugal cleaners. This makes them better suited for use in recyclable packaging.
Molten Stability
Epolene® polymers are much more stable in 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 is illustrated in the graph below. This in turn leads to a longer pot life and less downtime due to plugged nozzles.
