H.B. Fuller EH9641

The chemical reaction that results in cross-linking begins in the presence of low levels of atmospheric moisture. The rate of cross-linking will vary depending on temperature, humidity, and thickness. Please consult with your HB Fuller technical representative for additional recommendations.

As today’s electronics get smaller and denser they rely less on yesterday’s screws and bolts and more on high-tech industrial adhesives. And the performance of those adhesives is measured in many dimensions. Take for example the glass touchscreen of a cellphone. The touchscreen itself is, by design, resilient enough to handle the rough-and-tumble of carrying, handling, and occasional mishandling to which it will be subjected. The connection of the screen to the body of the phone has to be just as tough. Bond strength varies from adhesive to adhesive, but an individual adhesive’s bond strength varies depending on the substrates to which it is bonding. Since many bonds are between unlike materials it is important to know how well the adhesive works with both. You may not need to use the adhesive with the highest bond strength for your materials, but it is important to know that the bond will be sufficient for your needs. “Wet out,” the degree to which an adhesive flows and covers a surface to maximize the contact area and the adhesion to the bonding surface, is a primary determinant of the strength of its bonds. Technically, effective wet out occurs when the surface energy of the adhesive is as low or lower than the surface energy of the substrate. This is why an adhesive’s bond strength varies depending on the substrate. For example, the bond strength of a particular adhesive, measured in MPa (MegaPascals or million newtons per square meter), may be over twice as high with polycarbonate as with glass. Finally there are all the same characteristics you would look for in a plastic resin. While devices like mobile phones do tend to have a relatively short product life, you will want an adhesive with the durability to last the life of the device. It will have to stand up to extremes of temperature, not just the heat of a summer day, but the extreme heat of a car left out in the sun on a summer day. The same is true for the extreme cold of a night out in the car in mid-winter, the UV exposure of a day at the beach, or the humidity of a muggy day in July. And while few electronic devices are likely to get dipped in gasoline or acetone, they may well be exposed to any number of chemicals — perfume, suntan lotion, sweat, and more — both individually or in combination. And then there are the cycles, hot and cold, humid and dry, that can wear down the resistance of a poorly chosen adhesive.

The characteristics of an adhesive can have a tremendous impact on the efficiency of a manufacturing operation. The more “coddling” an adhesive requires, the higher your labor costs. And while the ideal adhesive should allow enough time for necessary handling, any avoidable wait time for setup and cure will significantly reduce the throughput of your entire operation.

• As screens get larger and move closer to the edges of the device face, the adhesive used to adhere the screen to the phone body will have to be laid down in a thinner, more precise line. This will affect aspects of the adhesive itself such as viscosity along with the type of application equipment used.
• Some materials are heated for application. Others can be destabilized by heat. Some, hybrid materials must be heated but can also be destabilized by too much heat. It is critical to understand the nature of the material and how it is applied to achieve good adhesion through proper heating without overheating and reducing the “pot life” of the material.
• Open time, the time after adhesive application during which parts can be put together and positioned while still being able to achieve full bond strength, helps determine the efficiency of your assembly operation. Achievable bond strength can drop sharply after rated open time has elapsed.
• Wet out of the adhesive helps determine its appropriateness for the assembly. As described earlier, the adhesive’s ability to bond to the substrates being used is important, but the amount of pressure that can be applied to a component like a touchscreen — low in this case — requires better wet out than would be required if more pressure could be applied.
• Green strength, the immediate grip of an adhesive after application, determines how an assembly can be handled to keep the assembly process moving. In the case of electronic devices superior green strength is desirable.
• Clamp time can be the biggest bottleneck in this kind of electronic assembly. Typically it can take hours, tying up space and equipment.
• A faster curing adhesive with a shorter clamp time would free up those resources and significantly improve throughput.
• Finally, can you rely on the provider to support your application and help troubleshoot any issues that may arise. Nobody knows you application better than you do, but there are times when you’ll need adhesive expertise to ensure that you are getting the performance you need.