by Scott R. Sabreen, The Sabreen Group, Inc.
Polymeric adhesion bonding problems are pervasive throughout the plastics industry. Two- and three-dimensional products often include bonding plastic-to-plastic, plastic-to-metal, plastic-to-composite, optomechanical, and more. Adhesion bonding applications are not limited only to adhesives (epoxies, urethanes, acrylics, silicones, etc.), but include adhesion of printing inks, paints and coatings, encapsulants, potting compounds, metallization, and more. Plastic substrates are difficult to bond because they are hydrophobic (not naturally wettable) materials; possess poor surface wettability (i.e., low surface energies); are non-polar, inert structures; and have poor surface chemical functionality. This article discusses the basic science of contact angles, surface wetting, and chemical activation to achieving strong adhesion bond strength.
The Contact Angle
Consider a single liquid fluid droplet on a flat solid surface at rest (equilibrium state). A cross-sectional view is demonstrated below (Figure 1). The angle formed by the solid surface and the tangent line to the upper surface at the end point is called the contact angle. The contact angle is the angle between the tangent line at the contact point and the horizontal line of the solid surface. Understanding the contact angle and its physical properties of interaction between solids and liquids provides valuable information in determining optimal adhesion bonding surface wettability conditions.
The bubble/droplet shape is due to the molecular forces by which all liquids through contraction of the surface tend to bring the contained volume onto a shape having the least surface area. The intermolecular forces to contract the surface is termed surface tension. Surface tension, a measurement of surface energy expressed in dynes/cm (or mN/m SI units), is the property (due to molecular forces) by which all liquids through contraction of the surface tend to bring the contained volume onto a shape having the least surface area.
The higher the surface energy of the solid substrate relative to the surface tension of a liquid (water, printing inks, adhesives/encapsulation, coatings, etc.), the better will be its wettability, and a smaller contact angle. As a general rule, acceptable bonding adhesion is achieved when the surface energy of a substrate (measured in dynes/cm) is approximately 8-10 dynes/cm greater than the surface tension of the liquid (Figure 2).
Surface wetting testing involves measuring the contact angle. When a liquid does not completely wet a substrate (i.e., polymeric product), a contact angle is formed. The contact angle is a quantitative measure of the wetting of a solid by a liquid. It is a direct measure of interactions taking place between the participating phases (gas/liquid/solid or liquid/liquid/solid). The shape of the drop and the magnitude of the contact angle are controlled by the three interaction forces of interfacial tension of each participating phase (gas, liquid, and solid).
For many applications it may only be necessary to examine the static equilibrium contact angle using dyne solutions in accordance to a documented test procedure such as ASTM D2578. Application kits or dyne pens/solutions provide useful information, but they are not precise measurements of surface tension. Surface tension measurements can vary considerably by individual (technique) and the interpretation of the center liquid behavior. Dyne pens/solutions are known to be directional indicators of significant differences in the surface tension and capable of identifying “good” and “bad” bondable surfaces at economical pricing.
Dynamic Contact Angles (DCA)
Testing the fluid behavior of only the static contact angle can lead to misinterpretation of the liquid/solid interface results and the resolution of bonding problems. This is because industrial manufacturing production operations are more realistically dynamic conditions, not static. Thus the dynamic contact angles (DCA) are most important to understand.
Contact angles are generally considered to be affected by both changes in surface chemistry and changes in surface topography. The advancing contact angle is most sensitive to the low-energy (unmodified) components of the substrate surface, while the receding angle is more sensitive to the high energy, oxidized groups introduced by surface pretreatments. Thus, the receding angle is actually the measurement most characteristic of the modified component of the surface following pretreatments, as measured using dyne solutions. Therefore it is important to measure both the advancing and receding contact angles on all surface-modified materials1 (Figure 3).
When a droplet is attached to a solid surface and the solid surface is tilted, the droplet will lunge forward and slide downward. The angles formed are respectively termed the Advancing Angle (θa) and the Receding Angle (θr). ASTM D724 describes methods for measuring DCAs using advanced equipment (optical tensiometers and goniometers) to analyze advancing and receding contact angles based on drop shape analysis and mass.
Chemical Surface Activation
There is a strong tendency for manufacturers to focus only on contact angle measurements as the sole predictor for bonding problems and for conducting routine surface testing. Chemical surface functionality is equally important whereby hydrophobic surfaces are activated into bondable hydrophilic surfaces. Gas-phase, glow-discharge, surface oxidation pretreatment processes are used for chemical surface activation.
Gas-phase surface oxidation process methods include electrical corona discharge, flame treatment, cold gas plasma, and ultra-violet irradiation. Each method is application-specific and possesses unique advantages and potential limitations. The basic chemical and physical reaction that occurs in free electrons, ions, metastables, radicals, and UV generated in the plasma can impact a surface with energies sufficient to break the molecular bonds on the surface of most polymeric substrates. This creates very reactive free radicals on the polymer surface which, in turn, can form, cross-link, or in the presence of oxygen, react rapidly to form various chemical functional groups on the substrate surface. Polar functional groups that can form and enhance bondability include carbonyl (C=O), carboxyl (HOOC), hydroperoxide (HOO-), and hydroxyl (HO-) groups. Even small amounts of reactive functional groups incorporated into polymers can be highly beneficial to improving surface chemical functionality and wettability. Also, chemical primers/solvents and mechanical abrasion (including mold tool texture) can be utilized alone or in conjunction with gas phase pretreatments.2
Polymeric bonding problems are widespread and not limited only to adhesives. There are significant cost implications associated with bonding failure, including poor product field performance, scrap/rework, production inefficiencies, and increased quality control inspection. Through understanding the basic science of contact angles, surface wetting, and chemical activation, virtually any bonding problem can be solved successfully, even when using the most tough-to-bond polymeric, and elastomeric materials.
1Mark Strobel, Christopher Lyons, 3M Company, 3M Center, 1995
2Scott R. Sabreen, The Sabreen Group, Inc., Surface Wetting Pretreatment Methods, Plastics Decorating Magazine, 2002
Acknowledgement: Enercon Industries Corporation photos
Scott R. Sabreen is founder and president of The Sabreen Group, Inc. The Sabreen Group is a global engineering consulting company specializing in secondary plastics manufacturing processes – surface pretreatments, adhesion bonding, decorating and finishing, laser marking, and product security. For more information, call (888) SABREEN or visit www.sabreen.com or www.plasticslasermarking.com.