November 2011

Adhesive bonding is a method used to join two substrates together using an adhesive, or glue. Increasingly bonding is replacing traditional mechanical fastening in many applications ranging from automotive body and airplane parts to dental treatments. Bonding is also used in the fabrication of wafers used in microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), as well as optoelectronic systems.

In order to understand how to develop a successful bonding method, it's helpful to review the forces at play. The cohesive force, as shown in the graphic below, is the force between one molecule of adhesion and another. Where the adhesive touches the substrate is referred to as the adhesive zone. Here the structure of the adhesion is in a different state than in the cohesive zone due to a change in physical and chemical composition. If the cohesive force is weaker than the adhesive force, the result may be a cohesive fracture - when the joint breaks along the cohesive zone. When a joint breaks along the adhesive zone, it's called an interfacial fracture. For a discussion on cohesion and adhesion and how they relate to contact angle, see our February 2009 Newsletter here: www.ramehart.com/newsletters/2009-02_news.htm

There many types of bonding, but in all cases a major factor in successful bonding is good wetting on the surface of the substrate. The wetting properties of a surface are measured in terms of contact angle. A lower contact angle signifies better wetting. And while good wetting is required for successful bonding, it should be pointed out that good wetting alone does not guarantee successful adhesion. Other factors, such as cleanliness, using the correct adhesive, and environmental factors (temperature and humidity, e.g.), mechanical properties (such as flatness and smoothness of the substrates) and other factors (such as the viscosity of the adhesive and its resistance to moisture after curing) also affect the long-term quality of a bond.

Effective wetting requires an adhesive which has a lower surface tension than the critical surface tension of the substrate. This is generally characterized by a contact angle less than 90°. But for many systems a much lower contact angle is required for successful bonding. As illustrated in the graphic below, as wetting improves contact angle decreases and solid surface free energy increases.

In addition to having good wetting properties, the substrate should also have good bonding properties - that is, the adhesive must have a good molecular interaction with the substrate in the adhesive zone.

A spectrum of different surface treatments are used to improve wetting, increase surface energy, and improve overall adhesiveness. These include cleaning (including degreasing), mechanical (sanding, deburring, sandblasting, polishing, lapping, and grinding), chemical (etching, blanching), physical (plasma-treating like corona discharge), and electrical (electrical discharge). The goal of any surface treatment prior to applying an adhesive is to improve wetting, surface energy, hydrophilicity and general bonding properties of the substrates.

Researchers who develop novel bonding products and surface treatment methods increasingly rely on contact angle to direct the efficacy of their methods. As new materials are developed and deployed in myriad applications, new approaches to bonding require increased attention to wettability and adhesiveness as a function of surface energy and contact angle.

If you are developing new surface treatment or bonding methods and need a better way to characterize your substrates prior to bonding, stick with us and we can help you find a contact angle tool for your particular application.
 

Nothing compliments a powerful tool more than help when you need it. The ramé-hart name is synonymous with quality tools and world-class technical support. Here are some of the ways we are able to provide support and self-help tools for our customers who are trying to learn to master their ramé-hart instrument or are having any type of technical problem with it:

1. Our series of YouTube videos which show a variety of procedures including how to calibrate, how to measure surface tension and interfacial tension, how to level the stage, how to use features like the right line and red line options, and more. See them here: http://www.youtube.com/user/ramehart#g/u

2. Our extensive user documentation which is included with our systems. (If you own a current-generation ramé-hart tool, a PDF copy of the software documentation is in the c:\DROPimage of your PC.)

3. Our series of How To Guides.

4. Our knols which walk through step-by-step tasks such as measuring contact angle or surface tension. See them here: http://www.ramehart.com/knol_how_to_measure_surface_tension.pdf 

5. Our website which includes a Glossary and an extensive archive of newsletters.

6. Our internal knowledgebase. When we run into novel and unusual problem or questions, we solve them or find the answer but we also document it so we can help others in the future who have the same or similar issues.

7. Our skilled and professional technical support staff who can help you via phone or email.

The next time you run into a technical challenge with your ramé-hart product, don't hesitate to contact us for help with any question or challenge.