February, 2018
www.us-tech.com
Page 63
Precise Measurement of Surface Free Energy Promotes Superior Adhesion
By Jeff Elliott T
he process of determining the ideal plasma treatment that will resolve a challenging bond-
ing or adhesion issue is an exact sci- ence, often the domain of chemists and physicists. However, with multi- ple options and chemistries at their disposal, coming up with the ideal plasma “recipe” often involves having a precise understanding of the sur- face free energy.
Barden. “However, it can only deter- mine if bonding is working within a specific range.” Barden says customers regular-
ly ask for dyne tests to determine the surface energy is, for example, “55 dyne/cm or better.” However, deriv- ing a quantitative number can be important if a plasma treatment works best at, say, precisely 60 dyne/cm. “By more completely measuring
the surface free energy, we can come up with a measure of the quantita- tive change that occurs before and after plasma treatment,” says Barden. “This allows the customer to correlate those measurements with their actual bonding experiments to ensure the best possible bond is being achieved.” To do this, designers and manu-
facturers of plasma systems like PVA TePla America often turn to the most
advanced drop shape analyzers from global manufacturers such as Krüss.
Drop Shape Analyzers Drop shape analyzers measure
these contact angles visually. A droplet is deposited by a syringe, which is positioned above the sample surface, and a high-resolution camera captures the image from the profile or side view. The image is then meas-
Continued on page 65
Krüss handheld mobile surface analyzer. Unfortunately, many of the tra-
ditional devices used to measure sur- face energy are limited in the extent of information that they provide. In addition, many fail to measure the polar and dispersive components of surfaces that otherwise have the same surface energy. This is critically important,
says Dr. Michael Barden, head of research and development at PVA TePla America, because different types of adhesives will form a superi- or bond with a surface that is polar vs. dispersive. Organic adhesives, for example, often bond better to disper- sive surfaces, whereas two-compo- nent epoxies, carbonates or ureas work better with a polar surface. “When all these factors can be
more precisely measured, plasma experts are able to dial in the best treatment solutions faster, with more consistent results,” says Barden.
Dyne Tests Plasma treatments are often
used to create a high surface energy to increase the “wettability” of sur- faces and assist the spreading of chemical adhesives. To measure this surface energy, dyne tests are often performed to derive a value in dynes/centimeter. These tests come in many
forms, but include dyne pens that come with a specific dyne level listed on them. When run across a solid surface, the liquid from the pen will spread or wet-out if the dyne level is lower than the material’s surface energy. If the ink’s dyne level is equal or higher, it tends to remain in droplets. While, economical and sim- ple to perform, dyne pen and ink tests provide only an approximation of the surface energy. “In about 70 percent of the proj-
ects we work on, dyne testing rough- ly correlates to the adhesion,” says
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