Adhesives

Adhesives are materials which form a bond with other materials (as opposed to Cohesives like rubber cement or cold seal which bond only to itself once it has cured). Adhesives are used to laminate (join) materials.

For converting purposes, adhesives can be thought of in the following terms:

Permanent adhesives are not sticky at room temperature or when they are cured. A common example of this type of adhesive is carton glue and the adhesive that holds the formica to your kitchen counter. When the glue is activated, by heat for hot melts or mixing for catalysed systems, it is sticky and will form a bond. Most permanent adhesives are applied directly to one of the surface to be bonded. The second surface is either bonded quickly afterwards or the adhesive is reactivated when it is needed.

At the other end of the scale from permanent adhesives are PSA (Pressure Sensitive Adhesives). These are tacky at room temperature. If they are applied to a surface with a little pressure, a bond will be formed (though not all PSA bonds to all surfaces). Most of the adhesive used in converting operations is PSA.

In the converting industry, much of the PSA is coated onto release liner (a paper or plastic coated with a release agent, usually silicone). The PSA stays on the liner, but when it is laminated (bonded) to a compatible surface it will form a strong bond. The release liner can then be peeled off and the material laminated to an other material or it can be left on so the end user can peel off the liner and mount the part where it is needed. Labels are a common example of a part with adhesive mounted on a release liner.

Of the PSA sold on release liner, some is unsupported, that is, there is simply a single layer of adhesive on the liner (often refered to as laminating adhesive). PSA can also be purchased as supported adehsive. This is really two layers of adhesive, one on each side of a carrier such as a thin Mylar™. Supported adhesives are more expensive, but they have several advantages:

• The adhesive can be different on each side which can be useful for bonding some difficult materials
• The carrier adds stiffness and strength to the laminate. This may help in the performance of the product and will often make converting operations such as stripping easier => faster => less expensive.
• There are fewer problems getting the adhesive to release from the liner, making it easier to work with and more forgiving in processes such as slitting where an edge flaw can render an unsupported PSA useless.

Of course, the main characteristic to look for is the correct bond strength to the materials you are working with. It turns out that the bond strength sometimes changes over time. When the PSA is fist applied its initial bond is called its 'green strength'. After 24 to 48 hours, the long term stength is achieved (usually but not always greater than the green strength). For some applications, a low green strenth is a benefit. The label industry touts this characteristic as 'repositionable'. Other applications, you will want maximum strength as soon as possible.

Now we come to the chemistry part. Without getting too deep into the science or the details, most of the PSA fall into one of the following catagories:

• Acrylic adhesives are usually solvent coated. These tend to have lower green strength but higher long term strength and are more chemically and UV resistant than rubber based adhesives.
• Rubber based adhesives tend to have higher green strength and are less expensive than acrylics. Their draw back is their lack of resistance to UV and temperature and their tendency to break down over time. When you see an old price tag turned oily and falling off, there is a good chance that it was a rubber based adhesive. None the less, there are many applications where rubber based adhesives perform better that other types.
• Hot melt adhesives are most often EVA based. They generally contain many other chemical secret sauces so it is hard to classify them. The one thing they have in common is that they soften with temperature so they cannot be used in high temperature applications (over 150 °F). There are UV curable hot melts that will handle high temperatures, but that fits under specialty items. Hot melts have the greatest range of properties and can be formulated for stronger or weaker bonds as needed. Many skin contact adhesives are hot melts. One advantage of hot melt is that it is relatively inexpensive to coat, offering the customer a lot of flexibility. Hot melt coating is one of the services offered by CRC.
• There are a number of special adhesives such as the UV currable adhesives that tend to function better at higher temperatures and under sun light. There are the low melt plastics. There are low molecular weight rubbers and urethanes (post its). And those mad chemists keep coming up with more variations to meet the evolving and demanding world we live in.