A Look at Adhesion Increase in Pressure-Sensitive Adhesives

Last month, this column addressed the subject of permanent and removable pressure-sensitive adhesives. A prime consideration in the difference between the two types is the internal strength of the substrate carrying the adhesive. A substrate with a weak internal strength gives permanency to the adhesive, and a substrate with strong internal strength makes an adhesive removable. This column addresses the tendency of a pressure-sensitive adhesive to increase in adhesion with time.

In normal use for a pressure-sensitive adhesive, the user removes the release paper. With a tape wound with no separate release liner, the user simply unwinds the tape. These steps provide the user with a substrate coated on one side with an adhesive. The next step is to apply the adhesive side of the substrate to the surface of an item. The final operation usually involves application of thumb pressure or similar means of pressure to ensure sufficient initial adhesion.

At this point, the pressure-sensitive adhesive provides some level of bond strength to hold the adhesive-coated substrate to the item. This initial bond is a function of various factors that include the specific adhesive itself, the adhesive and cohesive strengths of the pressure-sensitive product, the degree of pressure applied during application, etc.

The bond produced in the situation described above now begins to increase. Everyone is familiar with the phenomenon. Most people have applied a pressure-sensitive material onto something and find it improperly aligned. They try to remove it to correct their misapplication.

Even for a permanent pressure-sensitive adhesive, this removal may be relatively easy if done immediately. After waiting an hour, a day, a week, or longer, the removal may be very difficult. It actually may be impossible without tearing, marring, or otherwise damaging either the adhesive-coated substrate or the item to which it had been applied.

What is happening here? The pressure-sensitive adhesive bond simply is increasing with time. By their very nature, such adhesives are relatively soft. Their ability to adhere relies on this characteristic. The softness means the adhesive can flow gradually with time so it wets out on or more intimately contacts the surface to which it has been adhered.

Various features contribute toward the degree of wetting to influence the rate of bond increase and the final bond value.

Heat enhances the effect, and cold detracts from it. High initial application pressure is a positive factor, and low initial application pressure has a negative influence.

The adhesive composition also plays an important role. Pressure-sensitive products formulated by their manufacturer to be very soft and have high adhesive strength and tack will increase more rapidly in bond values than those that have lower application strength.

This increase in bond value is the reason pressure-sensitive adhesive suppliers and users test a product under various conditions.

Tests of pressure-sensitive products normally involve adhesion of a test strip that is 1 in. wide to a standard surface such as glass or metal. Using a standard rubber roller having a specific weight, the tester applies the same pressure to a variety of test strips on different test panels.

A test measured in 30 min or 1 hr gives the initial value for adhesion. Tests on strips aged at room temperature and at 150 deg F or another elevated temperature for 24 hr and 1 wk give an indication of the rate of increase of bond and the final value.

David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service Contact him at dbentley@unm.edu.


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