Surface energy probe

#A clean surface has a surface energy, which is the analog of the surface tension of a liquid. The surface energy of a solid, for example, affects how well the surface of the solid is wetted by adhesives and sealants.

#The ability to monitor surface energy is useful in a variety of development and manufacturing applications, and several techniques to monitor surface energy have been established. Static and dynamic contact angle measurements between a standard solvent and the test surface are described in the measurement standard ASTM D5725-99 from the American National Standards Institute. It equates the surface energy of a test surface, or equivalently its surface free energy, to the surface tension of a liquid contacting the test surface with a zero degree contact angle. This technique is implemented in the OCA 15 video-based optical contact angle measurement system from Future Digital Scientific in Long Island, N.Y. The test surface is placed on the horizontal stage of the OCA 15, a liquid drop is placed on the surface, and images of the drop and surface are analyzed.

#Peel test measurements are commonly done to measure adhesive joint quality. The ASTM D 1876 standard from the American National Standards Institute is a peel test for evaluating a bond between two flexible substrates. This and related techniques are implemented in the Motorized Peel Tester from Imada of Northbrook, Ill. Generally peel test results are weakly dependent on the surface energy of the adherends prior to bonding, since bond failure predominantly occurs in the adhesive and not at the interfaces between the adhesive and the adherends. However, for the case where the two adherends are of the same material, and when they are peeled apart they separate at their interface with no work being expended to distort the material, the work required to separate the two materials is twice the surface energy of the material.

#FIG. 1 is a perspective view of a preferred embodiment of the apparatus.

#FIG. 2a is a perspective view of a manipulator and a sampler.

#FIG. 2b is a shows cut-away view A—A of a manipulator and a sampler specified in FIG. 2a.

#This invention is an improved peel test for applications where the test surface is not a small disposable sample. Examples of test surfaces to which this invention is applicable are surfaces of machine and process tools, manufactured parts, and materials prior to application of paints or adhesives.

#The invention measures relative surface energy for solid test surfaces. The test surface can be rigid or flexible. The test surface need not be planar. We consider variation of the test surface from planarity on three length scales. The longest length scale is the test surface curvature, and is determined by averaging the deviations of the surface from planarity over lengths of more than roughly a centimeter; the surface curvature must be small enough so the sampler can travel parallel to the local surface while uniformly contacting the surface. The shortest length scale is the test surface roughness, and is determined by averaging the deviations of the surface from planarity over lengths of less than roughly 10 microns; the test surface roughness must be small enough that the tacky surface of the sampler will deform and intimately contact the test surface within the dwell time that the sampler is adhered to the test surface. The intermediate length scale is the asperities of the test surface, covering the range between test surface curvature and test surface roughness; asperities are accommodated to the extent possible by conformality of the sampler, discussed below. Localized features of the test surface are considered asperities, such as holes, edges, and bosses. It is desirable to sample as much of the test surface as possible, however there may be obscured surface in the vicinity of an asperity.

#The sampler in this invention is a conformal tacky surface mountable on a mandrel so that it can be applied and removed from a test surface. Since a sampler will be placed in intimate contact with surfaces of uncertain cleanliness, it will probably become contaminated and require cleaning or replacing. It is preferable, therefore, that the sampler be a relatively inexpensive and disposable.