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Winter  2001

UNDER PRESSURE
Pressure-Sensitive Adhesives Stand Up to Forces Presented 
                  in Extreme or Unique Environments

by Tom E. Cole Jr. 

As pressure-sensitive adhesive technology advances, its use in extreme environments and unique applications becomes more feasible. These new applications may also carry with them new forces that are not tested using conventional adhesive test methods. The old properties that once determined adhesive capabilities (shear, peel and tensile strength and tack) may not tell the whole story as to whether or not a pressure-sensitive adhesive will perform adequately. Accurately determining which adhesive best suites an extreme application requires analyzing the application and the forces at play. Additionally, the performance of each adhesive must be compared against these forces, rather than looking at standard test data.

Adhesive Overview
Before analyzing the application, it is important to review the common forces that can affect an adhesive’s performance. Then it can be determined which of these are relevant to the application in question.

Shear strength. Typically measured in hours or minutes to failure, this value determines how long an adhesive can withstand pressure dragging it along the adherend (the bonding surface) before delaminating (see figure 1).

Peel strength. Measured in ounces of force per inch of width, this value tells how much pressure is required to peel the adhesive away from the adherend at either a 90- or 180-degree angle from the surface (see figure 2).

Tack. Measured in pounds of force per inch of width, tack determines the adhesive’s quick bonding strength to the surface. There are two common tack tests—loop tack and quick tack. Although the quick tack value is important in some applications, the test methods used generally are not standardized, so there may not be a true comparison from one adhesive to another. Loop tack tests generally are tested to a standard procedure.

Tensile. Measured in pounds of force per square inch, tensile denotes the force required to pull two laminated surfaces directly apart. Unlike other separation tests, the entire adhesive layer is tested at once (see figure 3). 

Cleavage. Also measured in pounds of force per square inch, this value demonstrates the force needed to pry two bonded, rigid surfaces apart from one end. Unlike peel strength, cleavage values (see figure 4) test more than just the leading edge of an adhesive bond.

FIGURE1              FIGURE2             FIGURE3              FIGURE4
Figure 1:  Shear forces    Figure 2:  Peel strength tests  Figure 3:  Tensile               Figure 4:  In cleavage, force is 
parallel to the bond          the leading edge of an              forces are perpendicular     concentrated at the leading edge of the
plane.  Force is                adhesive bond.  At least one    to the bond plane.  As          bond.  Not all the bonding surface is 
distributed over the          surface must be flexible.         with shear, force is              is responsible for the entire load.  Both
entire bonding surface.                                                    distributed over the entire   substrates are rigid or semi-rigid.
                                                                                         bonding surface.

On a typical adhesive data sheet there will be values for tack, shear, tensile and peel strength. Often cleavage is not a standard test so you may require the adhesive supplier to test this property and give you the value. Also note that the values on a data sheet are performed under lab conditions, which means they are tested in a consistent environment and bonded to a clean aluminum plate. While this may provide a good comparison of one adhesive versus another, unless the adhesive is applied under these exact conditions, it probably won’t help in determining whether the adhesive will actually work. It’s best to provide the adhesive supplier with as much information about the actual application conditions as possible (low temperature, high temperature, typical humidity range and samples of the bonding surfaces) and have them test the adhesive under these conditions.

Some adhesives require the assistance of a surface pre-treatment, or coupling agent, to provide acceptable bonds to certain surfaces. If a coupling agent is recommended, investigate it. How much does it cost? How easy is it to apply? Will there be production delays to allow for curing? Does it pose a potential health hazard to employees handling it? If a consistent coat is not always applied, how well can the adhesive perform without it?

Analyze the Application
Analyzing an application begins with determining which of the above values are important to the application, and what the maximum reasonable loads will be for each. For example, if the application were for an adhesive-mounted picture frame, shear strength would be the most important attribute. But how much force resistance would be needed? A good estimate would be the weight of the frame itself, plus a few ounces for the picture. 

Now that the adhesive property requirements have been reviewed, the next step is to analyze and understand the application 
environment and its cycles. How hot or cold does it get? What are the humidity extremes? Do any forces vary during the extreme highs or lows? Is direct sunlight a factor?

Don’t limit long-term thinking to just the adhesive and how it may change over time. It is also important to consider that even the bonding surfaces may change. Some surfaces tend to shrink or expand during temperature changes, adding additional strain to the shear strength of the adhesive. Others may warp entirely, bringing cleavage requirements into play that were not considered important during the application-analyzing phase. It is important to review these changes and identify what demands they will make on the adhesive. What forces are present during or after these changes that were not present before? How will the adhesive handle these changes? Can these conditions be replicated accurately in the lab? Review all these with the adhesive supplier.

External Grid Mounting
External muntin mounting tapes are an excellent example of an extreme environment application. There are a number of forces at play trying to separate the wood, vinyl or plastic muntins from the glass fascia. There are extreme environment exposure issues for the adhesive, its carrier and all bonding surfaces. In-depth analysis is needed to fully understand the complex nature of this application.

First, let’s evaluate each adhesive property and determine its relevance to the application.

Shear. Will there be any sliding forces placed against the grid once it has been mounted? Sure, if someone attempts to slide open the sash by using the grid instead of the lift rail. So how much force are we talking about? That would depend on the weight of the sash and the resistance of the balance system in use. First determine how much force in pounds is required to open the sash. Since shear typically is measured in minutes—or hours-to-failure using with a standard load, you will likely need to provide this load information to your adhesive supplier for evaluation. The adhesive supplier may already have performed tests using a similar load for comparison. Plastic grid, like vinyl, may have a tendency to shrink and expand under extreme temperature conditions, also adding to the shear load (see figure 5).

Peel. Often used by adhesive companies as the premier indicator of their adhesive’s bonding power, peel strength is relatively useless in this application. Muntin bar and glass do not exhibit the kind of flexibility required to bring peel forces into play. 

Tack. This value, while it may not apply to the actual application, may be important in tape application. If the grid always will be applied in a plant environment, the standard test values may be adequate. If there will be application in the field, ask that the tack values be tested at higher and lower temperatures to determine what the minimum and/or maximum installation temperatures may be. Remember that the standard values will be based on tack to aluminum, so the adhesive supplier should have test values against glass as well.

Tensile. Will there be forces trying to pull the muntin straight off the glass surface? There are several. Some are obvious, others are not. Grid warping could contribute to some tensile forces over time (see figure 6).

Cleavage. Given this application, this value, as well as shear, may be the largest indicator of a tape’s overall performance. As the two bonding surfaces are mostly rigid, this value replaces peel as the premier indicator (see figure 7).

FIGURE5                             FIGURE6                                         FIGURE7
Figure 5:  Shear forces on muntin      Figure 6:  Tensile forces would             Figure 7:  Cleavage forces would include
bar would include upward and             include pulling the entire muntin            prying the muntin bar from one edge.
downward forces.                                bar straight from the glass.                    


Other Factors
Most of the current muntin tapes available require or promote the use of a silane or silane isopropanol pre-wash to perform as advertised. The silane performs the actual task of bonding to the glass, while providing a more bond-friendly surface for the adhesive. While this does increase the performance of the foam tape greatly, it also provides a more aggressive surface for undesirable contaminants like dust and dirt to adhere. One proposed solution to this problem has been to only coat the glass in thin strips where the tape actually will be mounted. Unfortunately this also increases the likelihood that the adhesive will come in contact with an untreated surface, thereby reducing its effectiveness greatly. Many, if not all, silane products add considerable cost and time to the window manufacturing process, requiring additional personnel to apply the treatment, cure time before application and other 
hazardous material handling requirements.

Another item to note is that adhesive failure may not be the only cause for application failure. Discoloration of the adhesive or a foam carrier may render an otherwise perfect tape useless.

When evaluating adhesives it is important to remember that there are a variety of factors to consider. So unless the application is to bond sheets of aluminum together in a lab, a standard data sheet may not give you the whole story. 

Tom E. Cole Jr. serves as general manager of Lamatek Inc. in Edgewater Park, N.J.

 

DWM

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