Abstract:
The disclosed embodiments concerns a traction pad to be bonded for coating adhesion or surface cohesion tests by tearing including a counterbore designed to control the thickness of the adhesive film. Support elements such as wedges or a border maintain the base of the counterbore at the required distance from the tested surface and openings are provided for evacuating the adhesive prior to its controlled hardening when the traction pad is applied on the tested surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the National Stage of International Application No. PCT/EP2006/066803, International Filing Date, Sep. 27, 2006, which designated the United States of America, and which international application was published under PCT Article 21(2) as WO Publication No. WO 2007/036541 A1 and which claims priority from French Application No. 05 52936, filed Sep. 28, 2005, the disclosures of each being incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The disclosed embodiments relate to devices used for measuring the adhesion qualities of coatings such as paints, surface protections, adhesive or adhesive films on substrates which they are intended to, or measuring the surface cohesion qualities in general. 
         [0004]    More particularly, the disclosed embodiments relate to an improvement of traction pads used by such devices and which are bonded on the surface, the adhesion performances of which are to be measured by exerting a tearing force through said traction pads. 
         [0005]    2. Brief Description of Related Developments 
         [0006]    Knowing the adhesion qualities of a coating on a substrate, for example paint on its intended support, or the cohesion of the surface of a material, is essential data in industry since the behaviour of parts and assemblies of such parts in their future operating environments greatly depends thereon. 
         [0007]    A currently used method for determining such adhesion qualities of a coating or surface cohesion consists in measuring the efforts required for causing the tearing of a sample of the substrate coating on a test-portion, which it has been deposited on, or parts of the surface of the material. 
         [0008]    Tests are often carried out on standardised test-portions and follow precisely defined protocols, since being able to reproduce the measures repeatedly in order to be able to have an efficient comparison of the adhesion performances of the various pairs coating-substrate is very useful. 
         [0009]    The general principle of such a measurement consists in bonding a rigid pad of the calibrated bonding surface on a sample of the surface to be estimated, i.e. a substrate coated with a coating according to the planned application method or a sample of a material having been exposed to surface treatments and, when the adhesive has set, in applying a traction on the pad until it is torn from the coating of the surface of the substrate or of the surface material. 
         [0010]    The force corresponding to such tearing gives us a measure of the qualities of adhesion or cohesion desired. 
         [0011]    The standard ISO 4624 describes the procedure to be implemented so that the results obtained by various testing laboratories can be compared. 
         [0012]    Tractions devices such as the one described in patent U.S. Pat. No. 3,821,892 can also be found, which are implemented to carry out the tests mentioned. 
         [0013]    The described device makes it possible to exert a traction force on a bobbin-shaped bonded pad, and is provided with means for measuring the traction effort applied. 
         [0014]    However, although it is clear that the pad is bonded on the tested surface, the standard only mentions that the bonded surface of the pad must be flat. Thus, in the standard ISO 4624, as well as in patent U.S. Pat. No. 3,821,892, nothing mentions that the surface for bonding the pad should be anything but plane, which is a shape which is explicitly requested. When the shape of the traction pad is mentioned, such as its bobbin shape in patent U.S. Pat. No. 3,821,892, the aim is just to provide a good gripping of the pad by the traction means. 
         [0015]    Experience shows that the quality of the results obtained upon the implementation of the standard ISO 4624 is closely related to the quality of the bonding of the pad and to the quality of the test preparation. Most of the time, such qualities of preparation and bonding are not totally satisfactory, which results in an important perturbation of the measurements of the tearing forces. 
         [0016]    First, because of the fluidity of the adhesive upon the bonding of the pad on the surface to be tested, it is very difficult to check the thickness of the adhesive film and in particular, to have a uniform thickness of such film between the lower surface of the pad and the surface tested. Because of a not-controlled thickness and of the possible side lowering of the pad, the orientation of the axis of the pad can be shifted with respect to the normal to the surface of the test portion, which is the traction direction on the pad during the test. In practice, a deviation, even a small one, of the axis of the pad with respect to the traction axis and a variation of the thickness of the adhesive film at the interface between the traction pad and the tested coating substantially affect the value of the force measured upon the tearing, and this effect is problematic, since it leads to a significant dispersion of measures. 
         [0017]    To take this phenomenon in consideration, the test operator is generally obliged to multiply the tests and repeat them a lot of times to obtain average values of the tearing stresses which can be used. 
         [0018]    Secondly, a curl of adhesive is formed around the pad upon the bonding of said pad. Such curl is composed of the fluid adhesive which is pushed laterally when the pad is pressed on the surface to be tested to provide a good adherence of the pad. The test procedure provides that the coating submitted to a tearing test is cut along the profile of the pad, so that the surface of the adhesion of the coating deposited on the substrate opposite the pad is taken into account in the measurement of the tearing stresses. A notch is thus cut in the coating around the pad, which requires the previous elimination of the curl of adhesive. Such curl of adhesive, which has hardened at this stage of the operations of the preparation of the test-portion, requires the application of important stresses on the pad itself to be eliminated. Frequently, during such operation of elimination of the adhesive curl, the applied efforts cause the pad to start ungluing, which makes the test portion inappropriate for carrying out the test, or give non-significant results of the test and thus, it is necessary to make a new test portion. 
       SUMMARY 
       [0019]    It would be advantageous to solve such adhesion problems and the issues of the preparation of the test portion by choosing a pad having original characteristics, without such characteristics questioning the existing traction and measuring means, nor the test procedures used. 
         [0020]    According to the disclosed embodiments, the traction pad for testing the coating adhesion on a substrate includes a reference axis and a lower surface, said traction pad being fixed on the surface of the coating prior to the test, using an adhesive. Said pad is characterised in that it further includes, on the lower surface intended to be bonded on the coating, a counterbore, the depth of which substantially corresponds to the thickness of the adhesive requested for fixing the traction pad to the coating and means for spacing the bottom of the counterbore, in order to provide the stability of the traction pad and holding its axis perpendicular to the plan defined by the surface of the coating to be tested, when the lower surface of the pad is applied onto the surface of the coating. 
         [0021]    In a preferred embodiment, the counterbore has a substantially constant depth. 
         [0022]    The spacing means are constituted by a border at the lower surface of a traction pad. Such border is preferably at the periphery of said lower surface. 
         [0023]    In another embodiment, the spacing means are constituted by spacers distributed on the lower surface of the traction pad, on the perimeter of said lower surface. The lower surfaces of said wedges in contact with the surface of the coating to be tested constitute the support surface of the pad. 
         [0024]    Whatever the shape of the embodiment of the pad, the spacing means can be obtained by leaving material from the traction pad when making the counterbore. 
         [0025]    In one embodiment, the traction pad includes means capable of letting the adhesive in excess flux out of the bonding area defined by the mark of the traction pad on the surface of the coating. Thus, the adhesive in excess is evacuated out of the perimeter of the bonding of the plot without such means affecting the regularity of the thickness of the adhesive film. In a first alternative solution, such means are formed as free passages arranged in the border of the lower surface of the pad. In a second alternative embodiment, such means are arranged between the spacers. Finally, in a third alternative embodiment, such means consist in one or more exhaust stacks going up into the body of the traction pad. Preferably, at least one of said exhaust stacks opens onto the outer surface of the traction pad, in a part of said pad located above the bottom of the counterbore. At least one of the exhaust stacks can open onto an expansion chamber, in addition. 
         [0026]    In the disclosed embodiments, the lower surface of the traction pad preferably includes surface unevenness. Such voluntarily created or voluntarily left unevenness improves the adhesion of the adhesive on the lower surface of the pad. 
         [0027]    The disclosed embodiments also relate to a method for bonding a traction pad on a coating covering a substrate, using an adhesive and including the following steps: 
         [0028]    a) applying the adhesive on the lower surface of the traction pad and/or on the surface of the coating; 
         [0029]    b) prior to the hardening of the adhesive, positioning the traction pad in the position and at the location required at the surface of the coating, while exerting a sufficient pressure to have the adhesive flux; 
         [0030]    c) waiting for the hardening of the adhesive; 
         [0031]    d) cutting a notch in the coating at the periphery of the traction pad; 
         [0032]    said method being characterised in that: 
         [0033]    e) the pad complies with one of claims  1  to  13 ; 
         [0034]    f) the pressure exerted at step b) is sufficient to bring the spacing means to rest on the surface of the coating; 
         [0035]    g) the adhesive in excess, if any, which forms a curl at the surface of the coating around the traction pad, is eliminated between steps b) and c) prior to the hardening of the adhesive, and while holding the traction pad with a sufficient pressure to avoid any motion of the traction pad during the operation of elimination of the adhesive in excess in order to prevent any formation of a curl of hardened adhesive; 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0036]      FIGS. 1 to 3  show the steps of the positioning of a traction pad in a known manner. 
           [0037]      FIGS. 4   a  and  4   b  show a traction pad according to the disclosed embodiments using a border. 
           [0038]      FIGS. 5   a  and  5   b  show a traction pad according to the disclosed embodiments using spacers. 
           [0039]      FIG. 6  shows a traction pad according to the disclosed embodiments using a border having interruptions. 
           [0040]      FIG. 7  shows a traction pad according to the disclosed embodiments with side exhaust stacks. 
           [0041]      FIG. 8  shows a traction pad according to the disclosed embodiments with a vertical exhaust stack. 
           [0042]      FIG. 9  shows a traction pad according to the disclosed embodiments, the exhaust stack of which includes an expansion chamber. 
       
    
    
     DETAILED DESCRIPTION  
       [0043]    The present description and the drawings show a traction pad having an essentially axisymmetric shape. Such global shape of a traction pad is more or less generalized, because it is easy to make and it is described in the standard ISO 4624, but the disclosed embodiments can be applied to any shape of traction pad intended to be bonded either on a coating applied on a substrate for adhesion tests or directly on a material for surface cohesion tests. 
         [0044]    At its base, a traction pad  1  includes a surface la intended to be bonded on a coating  3  covering a substrate  4 , using an adhesive  2 . 
         [0045]    In order to apply traction efforts F on a surface of the coating  3 , the dimension of which is accurately determined, the coating  3  is cut on the perimeter of the pad  1  with a notch  31 . The aim of such notch is that the traction F can be uniformly applied on the surface of the coating  3  to be tested while avoiding that the coating  3 , out of the bonded area which is determined by the surface la of the bonding of the traction pad  1 , interferes with the result of the measurement, since the means  5  resting around the traction plot  1  are generally used to apply stresses in reaction to the traction force F applied to the test portion  4 . 
         [0046]    In order to avoid that the axis  20  of the traction pad  1  when in bonded position deviates from the normal to the surface of the coating  3 , a counterbore  11  is made on the lower surface of the traction pad  1  while keeping the spacing elements  12 ,  13  from the bottom of the counterbore  11   a  with respect to the positioning plan and in such a way that the requested thickness of adhesive  2  is kept between the traction pad  1  and the coating  3 . The dimensions and the disposition of said spacing means  12 ,  13  are selected so as to provide the traction pad  1  with a support surface which comes in contact with the free surface of the coating  3  in a stable and perpendicular way, with respect to axis  20  of the traction pad  1 . 
         [0047]    In one embodiment shown in  FIGS. 4   a  and  4   b,  the spacing element has the shape of a continuous lip  12  which preferably follows the perimeter of the lower surface la of the traction pad  1 . Preferably, such lip  12  is made as thin as possible so that the difference in surface between the bonded surface, i.e. the surface defined by the mark of the traction pad  1  minus the surface of the mark of the lip  12  on the surface of the coating  3 , and the cut surface of the coating  3 , i.e. the surface defined by the mark of the traction pad  1  is as little as possible so as not to interfere with the traction measurement. 
         [0048]    For example, manufacturing a peripheral lip  12  in the shape of a web, 0.5 mm in thickness, is no technical problem for a traction pad  1  made of an aluminium alloy or steel which means a reduction in the surface of the bonding by 0.25% for a traction pad  1 , 20 mm in diameter complying with standard ISO 4624. Such a deviation is perfectly negligible with respect to the dispersions of the measurements, which are established in such type of tests and can be taken into account in the calculations. 
         [0049]    The spacing elements can also have the shape of spacers  13 , as shown in  FIGS. 5   a  and  5   b,  the height of which provides the requested thickness of adhesive  2 . The spacers  13  will be at least three, not aligned and advantageously as spaced as possible, so that the support surface of the traction pad  1  is perfectly stable. A greater number of spacers  13  can be used and their shape has little importance, as long as the surface they occupy on the coating  3  remains negligible with respect to the surface of the bonding of the traction pad  1 . 
         [0050]    Whether the spacing elements are made with a lip  12 , with spacers  13 , or any other equivalent means, it is essential for the support surface defined by the lower surfaces of such spacing elements to be perfectly stable and perpendicular to the axis  20  of the traction pad  1 , which corresponds to the direction of traction during the test proper. Such a quality is easily obtained when the spacing elements  12 ,  13  are made while leaving the corresponding material of the traction pad  1  upon the manufacturing of the counterbore  11 , for example, by machining. 
         [0051]    The bottom  11   a  of the counterbore  11  is substantially parallel to the support surface to obtain a homogeneous adhesive film with a substantially constant thickness. However, the unevenness of the surface of the bottom  11   a  of the counterbore  11 , for example, scratches left by the passage of tools during the machining of the counterbore  11 , will advantageously be kept or created to improve the qualities of adhesion of the adhesive on the traction pad  1 , the connection between the pad  1  and the adhesive  2  having to be more resistant than the connection to be tested between the coating  3  and the substrate  4 . 
         [0052]    The traction pad  1  can further be equipped with means for facilitating the evacuation of the adhesive  2  in excess during the bonding operation of said pad  1  on the test-portion when the adhesive  2  is still fluid. 
         [0053]    Thus, the lip  12 , as shown in  FIG. 6 , can be interrupted by free passages  14  on a portion of its length and on the whole or a part of its height, at one or more locations. 
         [0054]    In other embodiments, as shown in  FIGS. 7 ,  8  and  9 , such means can have the shape of one or more exhaust stacks  15 ,  16  which can be through-holes or not, which in their lower parts are opened in the space defined by the inside of the peripheral lip  12 , the bottom  11   a  of the counterbore  11  and the support surface of the traction pad  1 . 
         [0055]    The exhaust stacks  15 ,  16  can open onto the outside of the traction pad  1 , the top of the traction pad  1 , or the side wall of said pad. 
         [0056]    When the exhaust stacks  15 ,  16  are not through-holes (these solutions are not shown in the drawings), attention should be paid for giving them a sufficient volume so that they can contain the adhesive which can flux into said exhaust stacks  15 ,  16  without the air trapped between the adhesive  2  and the blind bottom of the exhaust stacks  15 ,  16  to interfere with the up-going of the adhesive  2  into the stacks  15 ,  16 . 
         [0057]    In addition, an expansion chamber  17  can be provided to collect a more important quantity of adhesive  2  in excess, without the latter overflowing the traction pad  1  through the end of the exhaust stacks  15 ,  16  when these are through-holes. 
         [0058]    When the traction pad  1  and the coating to be tested  3  are bonded with the adhesive  2 , the traction pad  1  is applied onto the coating to be tested  3  with a pressure which is sufficient to have the adhesive  2  in excess flux out while it is still fluid. 
         [0059]    The adhesive  2  fluxing out through the edges of the traction pad  1  can easily be wiped before it dries without affecting the position of the traction pad  1 , which can be held in its position during such operation using a moderate effort, thanks to the stability given by the support surface of the spacing means  12 ,  13 . The curl of the not-hardened adhesive  21  is thus easily eliminated and the notch  31  can be made in the coating  3  to be tested very easily, when the adhesive has hardened. 
         [0060]    When the traction pad  1  is provided with exhaust stacks  15 ,  16 , the up-going of the adhesive  22  into the stacks  15 ,  16  and into the expansion chambers  17 , if any, prevents or at least limits the need for eliminating the adhesive  2  fluxing out through the lower edge of the traction pad  1 . 
         [0061]    Tests carried out according to the standard ISO 4624 with traction pads complying with the disclosed embodiments, including a continuous peripheral lip  12  and a vertical exhaust stack  16  showed that less than ten tests suffice to obtain a reliable value of the tearing force F of the coating  3  tested, when over ninety tests were required with the prior art traction pads to obtain an equivalent reliability of measurement.