ADHESION TESTING 1. Field of the Invention.

The present invention relates generally to the field of adhesive strength testing and in particular to testing the bonding strength of surface coatings and adhesives.

Description of the Prior Art.

Three methods are conventionally used to compare or evaluate the strength of adhesion of paint or other coatings to an underlying surface or substrate: ASTM D 2197-86, Standard Test Method for Adhesion of Organic Coatings by Scrape Adhesion; ASTM D 3359-87, Standard Test Methods for Measuring Adhesion by Tape Test; and ASTM D 4541-85, Standard Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers. Additionally, there is a conventional method of evaluation or comparing the adhesion of adhesive tapes: ASTM D 3330, Test Method for Peel Adhesion of Pressure-Sensitive Tape of 180 Degree Angle.

All of these methods are deficient in that they do not produce an independent standard of measurement for the strength of adhesion. For many years those skilled in the art have attempted to find a suitable method for measuring adhesive strength which would provide results independent of the testing process. One such approach is the so-called "blister" method. In this process a fluid, either gas or liquid, is injected under the coating through a hole in its substrate forcing the coating to lift from the substrate in the form of a blister.

A promising enhancement of the blister technique was described by Dannenberg in 1960 (H. Dannenberg, "Measurement of Adhesion by a Blister Method", J. of Applied Polymer Science, Vol. V, No. 14, pp 125-134 (1961)) . However, this process is complex and has not

been widely accepted. In 1987 Gent and Lewandowski described a further enhancement of the blister method for the evaluation of the strength of the adhesion of adhesive tape, but they reported observing several discrepancies in their results. (A.N. Gent and L.H.

Lewandowski, "Blow-Off Pressures for Adhering Layers", J. of Applied Polymer Science, Vol. 33, pp 1567-1577 (1987)). None of the known blister testing methods have been widely accepted. One of the principal problems associated with the use of the blister method to evaluate the adhesion of a coating is that the film of the coating will often rupture before the coating is debonded from the substrate. Also, the varying elasticities of most coatings will cause varying relationships between the height and the diameter of the paint blister formed producing varying test results. The blister method for testing adhesive tapes used by Gent and Lewandowski has the problem of several unquantifiable variables in the process which lead to inaccurate results.

Another limitation common with conventional blister testing methods is that they must be conducted in a laboratory at normal room temperatures and are not suitable for testing at extreme environmental conditions, such as high or low temperatures.

What is needed is a relatively simple adhesive strength testing method and system which provides consistent measurements of bonding strength, and can be operated in extreme environmental conditions.

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Summary of the Invention

The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which utilizes increasing pressure against a membrane, preferably elastic, bonded to a material to facilitate the determination of the strength Of adhesion between the material and a substrate to which it is bonded by determining the pressure required to debond the material from the substrate. The strength of adhesion may be determined at room temperatures or at extreme temperatures within an environmental chamber.

In one aspect, the present invention provides a method of testing adhesion strength by bonding a first side of a membrane to a material bonded to a substrate, the bond between the membrane and the substrate having an adhesion strength greater than the adhesive strength of the bond between the material and the substrate, and applying increasing pressure to the first side of the membrane to debond the material from the substrate surrounding the aperture. The increasing pressure may be applied to the first surface via an aperture through the material and the substrate or via an aperture in the membrane if a continuous groove, such as a circular groove, is made through the material to the adjacent surface of the substrate.

In a further aspect, the present invention provides an adhesion strength testing system using a membrane having a first side bonded to a material bonded to a substrate, the bond between the membrane and the substrate having an adhesion strength greater than the adhesive strength of the bond between the material and the substrate, and means for applying increasing pressure to the first side of the membrane to debond the material from the substrate.

Brief Description of the Drawings

FIG. 1 is a top plan view of an adhesion test device embodying principles of the current invention, used to test the strength of adhesion of a coating to a substrate.

FIG. 2 is a sectional view of,the test device taken along line 2—2 of FIG. 1.

FIG. 3 is a sectional view a testing unit embodying principles of the current invention utilizing the test device shown in FIGS. 1 and 2.

FIG. 4 is a schematic view of a system embodying principles of the current invention.

FIG. 5 is a sectional view a test device used to test the bonding strength of an adhesive.

FIG. 6 is a sectional view a test device used to test the bonding strength of an adhesive with selected materials.

FIG. 7 is a sectional view of another embodiment of the current invention permitting noninvasive testing.

Detailed Description of the Preferred Embodiment

The present invention provides a technique for determining the strength of adhesion of a bond between selected materials. In a first embodiment one aspect of the invention provides a means of determining the adhesive strength of a self adhering material or coating applied to a substrate, such as a layer of paint applied to an underlying surface. In another aspect the invention allows the determination of the strength of an adhesive when applied between two materials. In another aspect the invention allows the determination of the strength of adhesion of an adhesive tape. In another embodiment, the invention permits these determinations to be made noninvasively.

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As generally shown in Figs. 1 and 2, when utilized to determine the strength of adhesion of a coating such as paint to a substrate, the present invention provides a test fixture 10 having a rigid substrate 12, coating 14 and membrane 16. The adhesive strength to be determined is between substrate 12 and coating 14. If substrate 12 cannot be formed into a rigid, preferably discoid, shaped body, substrate 12 may be bonded to a rigid material such as aluminum (not shown) . Membrane 16 is preferably made of a relatively strong elastic material, which can withstand the conditions of the test without exceeding its elastic limit. For example, a thin layer of steel or other metal foil may be utilized.

Substrate 12 includes aperture 18, preferably interconnecting its major surfaces. This may be accomplished by drilling through the upper and lower surface of substrate 12. Coating 14 is then applied to the upper surface of substrate 12 covering all the surface around aperture 18. Alternatively, coating 14 may be applied to the upper surface of substrate 12, and substrate 12 then drilled to form aperture 18. Membrane 16 is bonded to coating 14 utilizing an adhesive which results in there being a greater strength of adhesion between membrane 16 and coating 14 than between coating 14 and substrate 12. Suitable adhesives include cyanoacrylate and two-part epoxy adhesives.

A fluid, such as an inert gas or liquid, is applied under pressure to the open end of aperture 18 causing the portion of membrane 16 over aperture 18 to deform outward. As membrane 16 deforms, it applies tension to the portion of coating 14 surrounding aperture 18. Fluid pressure is increased until the force applied by membrane 16 exceeds the strength of adhesion of coating 14 to substrate 12 causing the portion of coating 14 surrounding aperture 18 to debond from substrate 12.

A suitable device for applying such fluid pressure is shown in Fig. 3. Test unit 20 includes body 22 and

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retainer 24. Test unit 20 is preferably cylindrical in shape about axis 19, having open end 21 and closed end 23. Retainer 24, shaped similar to that of the retainer of a mason jar, is attachable to open end 21 via threads 26.

Body 22 and test fixture 10 are proportioned to permit test fixture 10 to fit within the interior of body 22. Closed end 23 contains passageway 28 connected to an external source of pressurized fluid via line 30. Passageway 28 is positioned to provide a fluid pathway into aperture 18. The pressurized fluid is prevented from escaping between substrate 12 and closed end 23 by a seal, such as O-ring 32.

Spacer 33, preferably shaped as a ring is positioned between retainer 24 and membrane 16. Tightening retainer 24 onto body 22 causes substrate 12 to compress O-ring 32 thereby creating a fluid tight seal between substrate 12 and closed end 23, as well as between 24, 33 and 16.

The open portions of retainer 24 and spacer 33 are larger than the diameter of aperture 18 and preferably cover only a relatively small portion of membrane 16 adjacent to its perimeter.

Referring now to Fig. 4, a schematic of a typical testing system is shown. An external source of fluid (not shown) is provided to line 30. The pressure of the fluid is adjustable by regulator 34. Valve 35 controls the flow of the fluid to test unit 20. Pressure gage 36 measures the pressure of the fluid within the test system. Pressure gage 36 may be a mechanical or digital device, preferably with the capability of recording pressure measurements and/or indicating scale pressure.

In operation, test fixture 10 is partially enclosed within test unit 20 as described above. Pressurized fluid is supplied to test fixture 10 and pressure is increased until coating 14 debonds from substrate 16. The pressure at the moment of debonding is recorded as the critical pressure.

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The measurement of the strength of adhesion by the system is calculated as a function of the system design and the critical pressure. The following formula is used to determine the bonding strength: Specific work of debonding, G = P ⁿ , where "P" is the critical pressure and "m" and "n" are constants that are functions of the system design. After system check is completed, it is necessary only to observe the critical pressure. Critical pressure may be directly translated on pressure gage 36, or through a digital system, to the specific work of debonding using any system of units, e.g., English, metric or SI.

Test unit 20 may be placed in chamber 38 during testing to determine the effects of environmental conditions, such as extreme temperatures or corrosive atmospheres, upon the adhesive strength of a material. To further evaluate the effects of corrosive environment upon adhesive strength, a corrosive gas or liquid may be used as the pressurizing fluid. In addition to determining the strength of adhesion of a coating to a substrate, the invention may additionally be used to evaluate the bonding strength of an adhesive to various materials. Referring to Fig. 5, adhesive 40 is applied directly between two selected materials formed as membrane 42 and substrate 44. The resulting structure is tested as described above.

If the selected materials are not sufficiently elastic and rigid for use as membrane 42 and substrate 44, respectively, the following structure is used. Referring to Fig. 6, selected materials 50 and 52 are formed into rings and bonded together with the adhesive to be tested. Selected material 52 is bonded to rigid substrate 54, into which aperture 48 has been formed equal in size to the openings in selected materials 50 and 52. Rigid substrate 54 may be made of aluminum or other similarly rigid material. Membrane 56, which may be made of steel or other strong metallic foil, is bonded

to selected material 50 completing the structure. The bond between membrane 56 and selected material 50 and the bond between substrate 54 and selected material 52 are formed to be stronger than the bonding strength of the adhesive to be tested. This structure is then tested as described above.

The invention may additionally be used to evaluate the bonding strength of adhesive tape. In this aspect, adhesive tape is substituted for the coating 14 in Fig. 3.

In another embodiment the invention may be used to noninvasively test the strength of adhesion of paint and other coatings to surfaces where it is not practical to form an aperture through the surface. Referring to Fig. 7, surface 60 is coated with paint layer 62. To test the bonding strength of paint layer 62 to surface 60 a substantially circular groove 64 is cut through the paint layer 62 without damaging surface 60.

Membrane 66 is placed over paint layer 62 covering the area of paint layer 62 inside groove 64, and covering a portion of paint layer 62 outside groove 64, enclosing the entire perimeter of groove 64. The portion of membrane 66 covering paint layer 62 outside groove 64 is bonded to the corresponding portion of paint layer 62. Membrane 66 is preferably circular in shape, having tube 68 fixed at or near its center permitting the space between membrane 66 and paint layer 62 to be filled with a fluid and pressurized. A testing system similar to that described above and shown in Fig. 4, is connected to tube 68 and the pressure increased until the critical pressure is reached. The strength of adhesion may then be determined as previously described.

Fig. 7 depicts this embodiment of the invention just after completion of a test, showing the failure of the bond between paint layer 62 and surface 60.

Having now described the invention in accordance with the requirements of the patent statutes, those

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skilled in this art will have no difficulties making changes and modifications in the embodiment of the individual elements of the invention in order to meet their specific requirements or conditions. Such changes and modifications may be made with out departing from the scope and spirit of the invention as set forth in the following claims.