Document JJ6MMYLBzKoBkNvYxg1vRgMrX

FOR DU PONT USE ONLY MARSHALL LABORATORY TRAINING SEMINAR JANUARY, 1970 ADHESION BY SOUHENG WU TABLE OF CONTENTS Page INTRODUCTION.............................................. 1 LOCUS OF FAILURE. ..................__________________ . 1 WEAK BOUNDARY LAYER............ ..................................... J FORMATION OF THE JOINT...................................................... ...................... 5 EXPERIMENTAL DETERMINATION OF SURFACE PROPERTIES.......... 10 WETTING AND MATCHING OF SURFACE FORCES............................. ............. 12 RELATIONSHIP BETWEEN STRUCTURE AND SURFACE TENSION.________ 15 PROMOTING ADHESION BY USE OF FUNCTIONAL GROUPS............ 17 COUPLING AGENTS, BONDING AGENTS, AND PRIMERS................... 19 MECHANICALLY HOOKED JOINTS...................................................................... 21 TESTING OF THE JOINT STRENGTH.______ ___________ _______________ . 22 RHEOLOGICAL EFFECTS ON THE JOINT STRENGTH................................... .. 24 LONG-TERM DURABILITY............. .... ................................................................ 27 DUP030044018 This talk is intended to introduce r basic concepts and techniques in solving adhesion problems rather than to elaborate on specific matters. Therefore, the talk will cover a wide range of aspects, while the dis cussion for each topic is much shortened. The references given in the text should be consulted for additional information on each topic. In this talk, we shall first discuss the locus of failure, the case of weak boundary layer, and then go on to the fundamental process of inter facial joint formation, testing of the joint strength, and the rheological effects on the joint strength. Finally, the long-term dura bility of the joint will be touched upon. INTRODUCTION . ADHESION is a complicated phenomenon. The mechanical strength of a joint is determined not only by the interfacial attraction, but also by the bulk properties of the system. Let us write: (JOINT STRENGTH) = (INTERFACE) X (BULK) This qualitative formula emphasizes the interrelation of the interfacial and bulk properties to the joint strength. A low joint strength may be caused by a weak link either at the interface or in the bulk. LOCUS OF FAILURE Thus, when examining an adhesion problem, it is necessary, first, to determine where the weak link Is, In DUP030044019 -2 - every testing of the joint, the appearance of the ruptured surface to the eye should be observed, and its apparent locus of failure recorded. The failure may be predominantly adhesive, cohesive, or mixedly adhesive and cohesive. The eye observation cannot detect monolayers or trace residues left on the surface. Therefore, a failure which appears to be an adhesive one may turn out to be, in fact, a cohesive one when examined by an analytical technique. Some convenient analytical techniques are: optical microscopy. Stereoscan electron microscopy, contact angle measurement, evaporative rate analysis, ATR-IR and reflectance-IR spectros copy, The optical and Stereoscan microscopy gives the topography of the ruptured surface. Residues and multiple monolayers can be detected. The contact angle and the evapora tive rate of a drop of liquid depend upon the nature of the surface on which the liquid is placed. The dependence is usually sufficiently pronounced to be discernible by these measurements. The ATR-IR and reflectance-IR spectroscopy provides a way of obtaining the IR spectrum of the surface layers or thin films on a substrate, thus enabling identifica tion of the trace residues on the surface. Recently, a new technique, IEE (Induced Electron Emission) spectroscopy, is under development. This technique is capable of analyzing O surface layers of up to about 100 A. The principle and opera tion of these techniques can be found in the following references. 1. Stereoscan electron microscopy: S. Kimoto and John Russ, American Scientist, 2L (l), 112 (1969). DUP030044020 -32. Contact angle: A. W. Adamson, '*Physical Chemistry of Surfaces,'' 2nd ed., Interscienee, New York, 1$ST. 3. Evaporative rate analysis: Symposium on Evaporative Rate Analysis, Preprint, Division of Organic Coatings and Plastics Chemistry, 2 (2), 1969, 158th ACS National Meeting, New York, September, 1969. 4. .ATR-IR spectroscopy: J. K. Barr and P. A* Flournoy, in ''Physical Methods in Macromolecular Chemistry," Volume I, B, Carroll, ed., pages 109-162, Marcel Dekker, New .York, 1969. 5. IEE spectroscopy: J. D. Lee and H. K. Herglotz, in Inter departmental Symposium on Adhesion, ERM-69-9, page 226, October 1969. WEAK BOUNDARY LAYER A Joint can only be as strong as the strength of its components. In some cases, a layer of mechanically weak material may be present at the interface causing the Joint to break cohesively in this weak layer. Whenever a Joint appears to break adhesively under the eye observation, the possible existence of a weak boundary layer should always be suspected. The locus of failure should then be carefully determined by the techniques discussed in the previous section. DUP03004402 -4 The problem of weak boundary layer can be solved by a number of approaches: removal, conversion, or reinforcement. For instance, polyethylene is said to contain a weak boundary layer. Removal of the low M.W. fraction by fractionation was reported to improve the joint strength to aluminum. Alterna tively, the weak boundary layer can be strengthened by crosslinking with electron bombardment or by crystallization against a high-energy surface. The automotive black primer (such as O 64-1948) has been shown to contain a weak layer of about 2000 A thickness consisting of uncured low M.W. fraction on its surface. The automotive lacquer does not adhere to the primer. There fore, a sealer is used as the interlayer between the lacquer and the primer. The sealer (PMMA containing amino groups, RC-934 FMMA/HAFMA 98/2) can strongly adhere to the primer, since the amino groups will graft into the alkyd resin of the primer by amine/ester interchange reaction and thus reinforcing the weak boundary layer. The weak boundary layer on the black primer can also be strengthened by using polyfunctional monomers such as dimethacrylates or trimethacrylates which will impreg nate and crosslink the weak boundary layer. Metal surfaces usually contain weak layers. Commer cial metals as received are usually covered with oxide layers, mill scales, lubricant, or protective oil. A coating or an adhesive which is applied to such surfaces often shows poor adhesion. The oxide layers are often porous and entrap oily contaminants. Therefore, coating or adhesive bonding of metals is usually preceded by thorough surface treatments, such as sand-blasting, pickling, solvent-washing, vapor-degreasing and then followed by conversion processes such as sulfochromating. DUP030044022 -5 bonderizing, and anodizing. A properly cleaned or treated metal surface should show no water break when rinsed with distilled water. Blasting and solvent cleaning will remove the oily contaminants, while the conversion process will replace the naturally occuring oxide layer with a mechanically strong and corrosion-resistant coating on the metal surface. Such coating is usually called a "chemical conversion coating," On the other hand, when thorough surface treatment is not practical, strong adhesion may, in some cases, be obtained by formulating the adhesive or the coating with appropriate plasticizers, additives, and fillers which will absorb or displace the oily layers. One such example is the vinyl filling adhesive (829-924). Additional discussions on the surface preparation of metals and plastics can be found in C. V. Cagle, "Adhesive Bonding, Techniques and Applications," McGraw-Hill, New York, 1968. R, C. Snogren, "Selection of Surface Preparation Processes," Adhesives Age, p. 26 ff and p. 36 ff, August, 1969* For more discussions on the weak boundary layer, see J. J. Bikerman, "The Science of Adhesive Joints,rt 2nd ed.. Academic Press, 1968. FORMATION OP THE JOINT Strong adhesion (or high Joint strength) requires satisfying all of the following three basic conditions: 1. Complete or extensive wetting, 2. Absence of the weak boundary layer. DUP030044023 -6 3. High or favorable mechanical strength and properties of the adhesive material. In the previous section, we have just discussed the problem of the weak boundary layer. We emphasize here that removing the weak boundary layer is necessary but not sufficient for strong adhesion. The requirements for com plete wetting and high strength of the adhesive material should also be satisfied at the same time. We shall now discuss the problem of wetting, and then go on to the problem of the mechanical properties of the adhesive material. In the discussions to follow, we assume that the weak boundary layer is absent or has been removed. Let us now ask some basic questions about the process of the formation of a joint. What types of forces acting across the interface are required to hold the two phases together strongly? What is the basic interfacial condition for strong adhesion? What factors control the process of joint formation? *** *** The attractive forces acting across the interface may involve various types of molecular forces and chemical bonds: DUP030044024 -7 - ______ Molecular Forces Dispersion force (nonspecific) Polar forces (Specific) H-bonds (specific) Typical Energy Value 2-5 kcal/mole 5-10 kcal/mole 5-12 kcal/mole Chemical Bonds Covalent 'bonds Ionic bonds Metallic bonds Typical Energy Value 15-170 kcal/mole 140-250 kcal/mole 27-85 kcal/mole The dispersion force is nonspecific, universal and the weakest of the molecular forces. It operates in every material and accounts for the condensation of nonpolar materials such as helium and polyethylene. Both theoretical calculation and our experimental measurement, however, indicate that the dispersion force amounts to 20,000 psi, or greater when two phases are in intimate molecular contact. This is at least one order of magnitude greater than the practical Joint strength. Therefore, it can be said that strong adhesion should be obtainable when two phases are in intimate molecular contact regardless of the type of interacting forces. In other words, strong adhesion requires intimate molecular contact between the two phases. How intimate should the two phases be in contact? O The answer is of the order of 10 A separation. The molecular force between two molecules decreases as the inverse seventh power of the separation distance. When this force is summed over all the pairs of the molecules in the bulk, the attractive force between two semi-infinite parallel plates (F ) is found DUP030044025 -3 to be inversely proportional to the third power of the separa tion distance (R ): Pab - A/E:b (1) o An attractive force of 20,000 psi. at 10 A will be decreased O by 8 times to 2,500 psi. at 20 A. Thus, the adhesion problem becomes one of "how to bring two phases into intimate molecular contact." It should be emphasized that the process of interfacial contact is con trolled by combinations of different types of interacting forces. The dispersion force alone is not always sufficient to ensure attainment of complete interfacial contact, although it is sufficiently strong to hold the two phases together once the interfacial contact is attained. For more information on the interfacial contact said adhesion, see <J. R. Huntsberger, "Mechanisms of Adhesion," J. Paint Tech. 52, 199 t (1967). D. D. Eley, "Adhesion," Oxford University Press, 1961. The solid surfaces are almost never perfectly smooth. When an adhesive or a coating is applied on the solid surface, interfacial contact will occur initially only at the tops of the surface irregularities. Subsequent wetting will proceed only to the extent of establishing a capillary equilibrium between the adhesive (or coating) and the substrate. The equilibrium configuration of a liquid having a nonzero contact angle on a solid having a spherical pit is shown in Figure 1. DUP030044026 -9The unwet bed voids at the interface will cause stress con centration under mechanical load. Thus, rupture of the joint will occur at low loading, giving poor strength. Only when the liquid shows zero contact angle on the solid can complete wetting be obtained. The contact angle is a measure of the wettability of a solid by a liquid. It is related to the surface and interfacial tensions of the system by the well-known YoungDupre equation: 7S * 57L* COS + rLS' > where 0 is the contact angle (nonzero), y_b is the surface tension of the solid, yJj is the surface tension of the liquid, and is the interfacial tension between the liquid and the solid. This equation is valid only for nonzero contact angles The equilibrium contact angle of a liquid on a solid is shown in Figure 2, An alternative and universal measure of the wetta bility is the spreading coefficient, defined by 5l /s * " 7-, LS Positive values of indicate zero contact angles and spreading, while negative values indicate nonzero contact angles and nonspreading. Thus, the condition for complete wetting and strong adhesion Is > 0, The spreading coefficient is the driving force for wetting. Adhesives (or coatings) may come in the form of solutions, dispersions (aqueous or nonaqueous), pastes. DUP030044027 - 10 powders, or films. It should he noted that the surface quantities which we refer to here are those for the liquidcarrier-free polymer components of the adhesive (or the coating), not those for the polymer solutions or dispersions as a whole. The liquid carriers will aid the Initial con tact, hut the final wetting state is determined primarily by the liquid-carrier-free properties* EXPERIMENTAL DETERMINATION OF SURFACE PROPERTIES As discussed above, the process of Joint formation is controlled by surface properties. The important surface quantities are contact angle and spreading coefficient. These are determined by the surface and the interfacial tensions. In fact, the surface and the interfacial tensions are the two basic quantities from whieh many other surface quantities can be calculated. Direct measurement of the surface tension of a solid is difficult, if not impossible, since reversible defor mation of a solid is Impractical. However, two convenient methods are available for estimating the surface tension of a solid, especially for a polymer. One is the Zisman's contactangle method, and the other is the molten state method. In the Zisman's method, the contact angles of a series of ordinary liquids of known surface tensions on the solid are determined. A plot of cos 0 vs. the surface tension of the liquids usually gives a straight line. The intercept of the straight line with cos Q = 1 gives an approximate value of the surface tension of the solid. This value is commonly called the critical surface tension of wetting. The liquids having surface tensions smaller than the critical surface DUP030044028 - 11 tension of the solid will spread on it (i.e., showing zero contact angle). An example of the cos 9 vs. y plot is given in Figure 3. The critical surface tension will he equal to the surface tension only when the interfacial tension between f. the testing liquids and the solid is zero and the adsorption of the vapors of the testing liquids on the solid is negligible. The critical surface tensions for some polymers are listed in Table I. In the molten state method, the techniques for determining the surface tensions of liquids are used on molten polymers at elevated temperatures. The molten polymers are protected from oxidation in an inert-gas cell. A plot of the surface tension vs. temperature usually gives a straight line, linear extrapolation gives good estimates of the surface tension at lower temperatures. This method usually gives quite reliable surface tension values for solids. The best technique is the pendent drop method which allows check of equilibrium of the viscous melts. Some surface tension values of polymers obtained by this method are listed in Table I. For details of the experimental techniques, the readers are referred to the following references. W. A. Zisman, in "Adhesion and Cohesion," P. Weiss, ed., 176-208, 1962, Elsevier, New York. S, Wu, "How to Solve Adhesion Problems," ERM-69-7, August, 1969, Experimental Station. DUP030044029 - 12 The interfacial tension between two solids can be estimated by extrapolating the molten state data. The extrapolation is usually linear and reliable. Measurement of the interfacial tension between two viscous molten polymers is quite difficult. The pendent drop method is probably the only reliable technique to date for measuring it. Knowledge of the interfacial tension allows dissolution of the surface forces into polar and nonpolar components. The interfacial tension values and the components of surface forces for some polymers are listed in Tables II and III, respectively. For details of the experimental technique, see S. Wu, Surface and Interfacial Tensions of Polymer Melts, Journal of Colloid and Interface Science, 31, 153 (19^9). WETTING AND MATCHING OF SURFACE FORCES Complete wetting requires the following condition as discussed before: sA/S 7S ~ 7A " 7AS ^ The surface and the interfacial tensions are all positive quantities for the case of physical interactions. Therefore, If 7a > 7g, then will be negative and complete wetting is impossible. On the other hand, if 7A < y^s then SA^S will be either positive or negative, depending on the magnitude of the interfacial tension 7Ag. The smaller the interfacial tension, the more likely the wetting will be complete. The magnitude of the interfacial tension is deter mined primarily by the "matching" of the components DUP030044030 - 13 - r of the surface forces. In terms of the components of the surface force, the condition for complete wetting (and thus strong adhesion) can be written as: - ^ + ^A)1/2 (Vn)1/2 i1 ^j P TD where xa 7/7 and xr = Tr/y,, i.e., the dispersion force and the polar force components, xu + xr =* 1. In the following, two examples are given to illustrate the utility of the above equation and show that appropriate "matching" of the different types of surface forces is necessary for complete wetting. EXAMPLE I: Polyethylene and Poly(methyl methacrylate) do not adhere to each other. The surface tensions and the components are given below. Calculate the Q value to see what the theory predicts. ANSWER: At l40C. Polyethylene Poly (me thyl met hac rylate ) 7 28.8 52.0 x 1.0 0.71 x* 0 0,29 Q (1.0 x 0.71)1/2 + (0 x 0.29)1/2 (32.0/28.8)1/2 - 0.89 < 1 The theory predicts incomplei DUP030044031 14 - EXAMPLE II: Poly(methyl methacrylate) and Poly- (vinyl acetate) adhere strongly to each other. The surface tensions and the components are given "below. Calculate the Q value and see what the theory predicts. ANSWER: At l40C. y xd a? Poly(methyl methacrylate) 52.0 0.71 0.29 Poly(vinyl acetate) 28.6 0.64 0.56 Q (0.71 x 0.64)1/2 + (0.29 x 0.36)1/21 (32.0/28.6)1/2 = 1.06 > 1 The theory predicts complete wetting and strong adhesion. In example I, the Q value is less than unity primarily "because the force components are unmatched. The value of the first bracket is too small since polyethylene is nonpolar, i.e., x? - 0. In example II, the Q value is greater than unity primarily because the force components are matched. The value of the first bracket is sufficiently large because the polar and the nonpolar components are well matched. The property of the quantity Qxjtg )1//2 + (x^x| is such that when x^cTl = X(q3, the value is unity, and when the difference between *A "d '4 are the Sweater (i.e., the more ill-matched), the smaller the value. For more details, see S, Wu, "How to Solve Adhesion Problems," EHM-69-7, August, 1969, Experimental Station. DUP030044032 - 15 RELATIONSHIP BETWEEN STRUCTURE AHD SURFACE TENSION Surface tension is an important property controlling the process of Joint formation. Its experimental determina tion is, however, difficult. Recently, methods for theoretical prediction of the surface tension of polymers have been developed. It is now possible to calculate the surface tension of homopolymers fairly accurately from group contributions and density data. Recently, a method for predicting the density of amorphous polymers has also been developed. Prediction of the surface tensions of polymers can be made by the following methods: 1. Parachor 2. Modified Hildebrand-Scott equation 5. Solubility parameter 4. Refractive index and dielectric constant 5* Atomic refractivity For details of the methods, see S. Wu, "How to Solve Adhesion Problems," ERM-69-7, August, 1969, Experimental Station. S. Nu, unpublished results* For the method of predicting the density of polymers, see . D. W, Van Krevelen and P. J. Hoftyzer, Prediction of Polymer Densities, Journal of Applied Polymer Science, 12, 871 (1969), The surface tension is numerically equal to the energy required to bring a molecule from the bulk to the DUP030044033 - 16 - surface. Qualitatively, this energy is proportional to the intermolecular forces and the density of the material (i.e., the number of atoms or molecules in a unit volume). The inter molecular forces and the density should be relatable to atomic and group contributions. This is why the surface tension can be predicted from the molecular structure. The greater the intermolecular forces and the greater the density, the higher the surface tension will be. Thus, since a crystalline structure usually has a higher density than an amorphous structure, we expect the surface tension of a crystal to be higher than that of the amorphous isomorph. For instance, the surface tension of amorphous polyethylene is 35 dynes/cm. at room temperature, but that of the crystalline surface of polyethylene is as high as 69 dynes/cm. at the same temperature. It should be noted that the surface layer of the ordinary partially crystalline polyethylene is composed of amorphous fractions. The following example illustrates the application of parachor in predicting the surface tension of polymers, EXAMPLE: Calculate the surface tension of. PMMA at 20C. by the parachor method and compare the calculated value with the experimental value. ANSWER: The structural formula for the repeat unit of PMMA is CHS - CH2- C- C ch3 \ 00 DUP030044034 - 17 - The parachor value for the repeat unit is ( GH3 2 x 55.5 111,0 c h 2 1 40.0 11 0 0 -3 - COO 1 x 63.8 - 63.8 C 1 x 9.0 9.0 Branch point 2 X (-3.7) = -7.4 P * 216.4 The density at 20 C. is d = 1.178 gm/cc. The formula weight per repeat unit is M = 100.1 Substituting into the equation 7 - (Pd/M)4 =* 41.4 dyne/cm. The experimental value at 20C. is 41.1 dyne/cm PROMOTING ADHESION BY USE OF FUNCTIONAL GROUPS Adhesion can he improved greatly by incorporating appropriate functional groups into the polymer. Only a very small amount (usually less than 1-5#) of the functional groups is required. For instance, primary amino groups in the amount of about 0.02# nitrogen can drastically improve the adhesion of PMMA to the automotive black primer (64-1948). The most effective functional groups are usually polar, H-bonding, chelating, or otherwise chemically active groups. Their effectiveness is quite specific to the substrate and in some cases may also be specific to the polymer. A search for effective functional groups should be based on the specific chemical Interactions possible for the system in question. For instance, acid groups usually promote adhesion DU P030044035 - 18 to metals, but is totally ineffective for adhesion to the automotive black primer This is because acid groups are capable of forming ionic bonds (or ionic interactions) with metal oxides, but are incapable of forming chemical bonds (or chemical interactions) with the automotive black primer. The mechanism of promoting adhesion of PMMA to the automotive black primer by use of amino groups was found to involve formation of amide linkages between the PMMA/NHa resin and the alkyd resin of the primer by the amine/ester interchange reaction. Other effective functional groups which will react with hydroxyl groups of the alkyd resin of the primer include alkoxy silanes, acyl groups, and isocyanate groups. For ionic interactions with metal oxides, it is found that selection of the effective functional groups may be based on the' isoelectric point of the metal oxide (Ig) and the acidity or basicity constant of the functional groups (pKa or pK^). The more positive the value of E& or E^, the more effective the functional group will be: E- "b - I*b - V Thus, for aeidic surfaces (such as SiOa), amino groups should be effective, while for basic surfaces (such as MgO), acid groups should be effective. For neutral surfaces (such as A1203 and Fe203), amino and acid groups should be about equally effective. On the other hand, hydroxyl groups should be rather ineffective. These predictions are qualitatively correct in fact. DUP030044036 - 19 For more details, see S. Wu, "Adhesion Studies," ESR-69-39, September, 1969, Experimental Station COUPLING AGENTS, BONDING AGENTS, AND PRIMERS Coupling agents and bonding agents commonly refer to monomeric compounds which promote adhesion when deposited on the substrate surface or blended into the polymer as additives. Difunctional Organosilanes are well-known coupling agents. They are widely used for promoting adhesion between polymer/polymer, polymer/metal oxide, polymer/glass, and polymer/minerals. Various silane coupling agents are commercially available. They are usually difunctional and have the following structural formula: 0CH3 R- Si--- 0CH3 where R = vinyl, methacryloxy, glycidoxylpropyl, epoxycyclohexyl, aminopropyl, and N-(aminoethyl)aminopropyl groups. The methoxy groups are thought to react with the hydroxyl groups of the glass or metal surfaces to form Si--0--SI or SI-- 0-- M bonds. The functional groups R are thought to react with appropriate sites in the polymer to form covalent bonds. Thus, the polymer and the substrate are "coupled" together by chemical bonds. This interpretation of the effectiveness of the coupling agent Is not without controversy. However, it is commonly accepted. Selection of appropriate silanes depends on the specific nature of the system. 100-200$ increase in the mechanical strength DUP030044037 - 20 (both dry and wet strengths) of fiber-reinforced plastics can be achieved by using appropriate silanes. For more information, see S. Sterman and J. G. Marsden, "Silane Coupling Agents," IEC 58, 53 (1966). + While it is probably true that chemical bonding 9 between the mineral substrate and the coupling agent is neces- sary for strong wet strength, chemical bonding between the coupling agent and the organic polymer is probably unnecessary. Du Pont * s products which are useful as coupling or bonding agents are: "Volan" methacrylato chromic chloride (a Werner type complex of methacrylic acid to chromium) and "Tyzor" organic titanates. Primers are frequently used for corrosion protection, or improving adhesive or coating receptivity. When an adhesive or a coating does not adhere strongly to a substrate, an appropriate primer may be used so that the adhesive or the coating will adhere strongly to the primer, while the primer also adheres strongly to the substrate. Thus, coupling agents or bonding agents may be regarded as one type of primer. The commonly preferred primers are usually formulated systems based on adhesive polymers or polymers with coupling agents. Corrosion-inhibitive fillers or additives are often added in the primers. A "wash primer" is a mixture of an acetal polymer such as "Formvar", phosphoric acid, and zinc chromate. The acetal polymer provides the adhesive polymer, the phosphoric acid provides a conversion coating, and the zinc chromate DUP030044038 - 21 provides the corrosion-inhibiting filler. Wash primers are useful for metal or mineral surfaces. MECHANICALLY HOOKED JOINTS Mechanical hooking is an important mechanism in the adhesion to porous substrates, such as paper and buffed Corfara poromerics. The Joint strength to porous substrates has been found to correlate well with the penetration of the adhesive into the pores. The driving force causing the adhesive to penetrate into the pores is the capillary pressure P. For cylindrical capillaries, it is given by P 2?a cos 0/r where Is the surface tension of the adhesive or the coating. cos 0 is the contact angle of the adhesive on the capillary wall and r is the radius of the capillary. Since the penetra tion will occur mostly when the adhesive Is in liquid state, the surface tension yii is for the adhesive in the fluid state. That is to say, the polymer plus liquid-carrier as a whole, or the melt in the case of hot-melt adhesives. The rate of capillary penetration for open-end, circular capillaries is given by d = (7a cos 0 rt/2T|)^*^ where d is the depth of penetration, t is the time and *) Is the viscosity. The degree of penetration can be Increased by using wetting agents to decrease the contact angle. DUP030044039 22 An important application of the mechanical hooking effect in F & F's products is in the adhesion of Teflon cookware enamel to the primed aluminum. The primer is a blend of binder and Teflon resin. The binder adheres to the aluminum, while the Teflon resin is Imbeded in the binder matrix and forms "tree-root-like" structures to which the Teflon resin topcoat is fused and anchored. TESTING OF THE JOINT STRENGTH The three basic types of adhesive Joints commonly used for mechanical testing are: 1, Butt Joints; see ASTM D2094-62T and B2095-62T, 2. Lap Joints; see ASTM DIOOS-S^. 5, Peel Joints; see ASTM D90>49 (180 peel, one flexible member and one rigid member), ASTM D18T6-61T (T-peel test), and ASTM D1781-62 (climbing drum peel test). There are a number of variations in the Joint design. In reporting the test results, the curing condition, surface preparation, conditioning of the Joint, type of failure, standard deviations of the measured strength, testing rate, testing temperature, adhesive thickness and the dimensions of the substrates should be noted. Depending on the intended service condition, the testing may be performed at high or low temperatures, or after exposure to water or humidity. For more Information, see G. V, Cagle, Adhesive Bonding, Techniques and Applications, McGraw-Hill, New York, 1968 DUP030044040 - 23 - The adhesive strength of a coating is more difficult to measure. In principle, the three basic types of joints used in testing adhesives may be adapted for testing the coatings. For instance, Budium can coating material is used to bond two strips of the tin-free steel substrate, and the peel strength of such a joint is measured* The Gardner Labis "Elcometer" adhesion tester appears to be a convenient tester for coatings. This is a butt joint test. An aluminum "dolly" is glued to the coating surface with a suitable adhesive (such as epoxy or cyanoacrylate, etc.) and the force required to pull off the coating is measured by pulling off the "dolly" with a portable, calibrated, spring-tester. It should be borne in mind that mechanical tests measure not only the attractive force between the two phases, but also the rheological response of the system (the coating and the substrate). Thus, the measured mechanical strength should not be used directly for the purpose of comparing the "strength of adhesion" of different coatings without specifying the nature of the system and the conditions of the test. Other "qualitative" adhesion tests include: 1. Razor blade or knife test, 2. Scratch or scrape test, 3. Mandrel bending test, 4. Impact or bump test, 5. Scotch tape test (3M No. 610 High Tack Scotch Tape). o DU P030044041 - 24 RHEOLOGICAL EFFECTS QN THE JOINT STRENGTH The phenomenology of testing and the rheological effect on the mechanical strength of the lap joint and the peel joint are discussed below, LAP JOINTS The mechanical strength of a lap joint when the failure is cohesive or at least partly cohesive is given by (SW) p-5 where f = lap shear strength per unit overlap area, rm - maximum shear strength of the adhesive, 6 = thickness of the substrate, hQ * thickness of the adhesive layer, L = overlap length, E *3 tensile modulus of the substrate Gi = shear modulus of the adhesive, a = numerical constant characteristic of the system. This equation summarizes the important factors influ encing the mechanical strength of a lap joint. It is noted that the pounds/unit area strength decreases with increasing overlap length, as shown in Figure 4. This Is because the front edges of the overlap bear most of the load. The stress con centration factor Is roughly parabolic with the minimum at the center of the overlap. It Is also noted that the strength increases with decreasing adhesive thickness and increasing DUP030044042 - 25 r substrate thickness. The strength also increases with increasing tensile modulus of the substrate. It is impor tant to point out that the strength per unit area is pro portional to Tm/(Gi)*" ', indicating that the higher the maximum shear strength and the lower the shear modulus of the adhesive, the higher the strength of a lap joint will be. The condition for high shear strength and the condition for low shear modulus are mutually Opposing. Best results are usually obtained with tough polymers. PEEL JOINTS Peel tests are usually performed at constant rates and constant peel angle. Peel angles of 90 end 180, and the T-peel are most commonly used. The peel strength Is dependent on the peel angle and the peel rate, as well as the thickness, modulus, and ultimate strength of the adhesive and the sub strate. The edge effects are negligible when the sample width is greater than about 1 cm. The dependence of peel strength on the peel rate Is quite sensitive and deserves elaborate mention here. The peel behavior of a typical sample joint may have three distinct regions as a function of the peel rate, as shorn In Figure 5* (1) At Low latess The peel force is steady with random, Gaussian fluctuations. The mean value of the fluctua tion is the same as the median. The failure is cohesive and increases with increasing peel rate and adhesive thickness. The failure is continuously initiated at the same rate as the peeling proceeds. The peel force is rate-dependent. For the rate-dependent cohesive failure, the mean peel fore 2 can be superimposed with the peel rate and the DUP030044043 - 26 adhesive thickness. The mean peel force, F, is a single function of Rta where R is the peel rate (length/unit time), St ta is the adhesive thickness*, and a is a numerical constant depending on the nature of the system, F = <{> (Rt) vw An example of the superposition of the T-peel strength of cellophane tapes bonded with Rhoplex HA-8 acrylic adhesive where a - 0.7 is given in Figure 6. (2) At Intermediate Rates: The peel force oscillates between well-defined maxima and minima, as shown in Figure 7. The distance between the two maxima or minima is independent of the testing rate. The failure propagates faster than the rate by which the sample is pulled apart, and the failure is initiated periodically. This oscillating peel behavior is also called "slip-stick" behavior. At present, it is not clearly known why the "slip-stick" behavior occurs with some materials, but not with other materials. Increasing the thickness of the adhesive tends to shift the onset of the "slip-stick" behavior to higher peel rate and increase the mean peel force. (3) At High Rates: The failure changes to adhesive and the peel strength becomes rate-independent. The peel force is steady with random, Gaussian fluctuation, and increases with increasing adhesive thickness. The rate-independent adhesive failure for 90 peel angle has been analyzed by Gardon. The peel force is given by: DUP030044044 - 27 F/t *'25 = a - p0*5 ^(F/ta)(X - sin )J 0.5 where 9 tan"^ (a/p) t/3'75 - (1/p jF/tJ3 *5J a - 0-319 * W^s^a^'25 ^s0*75 P = 0.409 ft (E /E )0#5 t 1,5 sa s Ea and Es = moduli of the adhesive and substrate, amax - maximum stress in the adhesive,' ta and t s = thickness of the adhesive and the substrate, 0 - the negative slope angle of the substrate the the point of failure. For more information on the peel analysis, see J. L. Gardon, "Peel Adhesion, " Journal of Applied Polymer Science, Vol* 7j 625-641, and 645-66$ (196$). LONG-TERM DURABILITY The strength of an adhesive Joint may degrade when exposed to water, high humidity, or UV light, etc,, especially when the substrate is hydrophilic. Qualitatively, when the affinity of the substrate for water is greater than the affinity of the substrate for the adhesive, water will migrate to the interface and eventually displace the adhesive, forming a weak boundary layer and causing a drastic decrease of the Joint strength. DUP030044045 - 28 Humectic materials at the interface or in the adhesive will greatly increase the transport of water to the interface. On the other hand, fillers usually improve the wet durability of the Joint, probably through retardation of the rate of water transport. The wet durability of aluminum laminates bbnded with PVAc adhesive improves by more than four-fold when the adhesive is filled with alumina or silica powders. Specific interactions or chemical bonding between the adhesive and the substrate will also improve the wet durability greatly. The loss of Joint strength is even more severe when the Joint is exposed to water or humidity under applied tensile stress. The phenomenon is related to the stress-corrosion cracking of materials. For the surface aspects of water invasion, see S. Wu, Adhesion Studies, E3R-69-39, Experimental Station, September, 1969. This talk covers only some basic concepts and tech niques. The references given in- the text should be consulted for more details. I hope this material may be of use to you. SW/mcc I/9/70 DU P030044046 TABLE I SURFACE TENSIONS & CRITICAL SURFACE TENSIONS FOR SOME POLYMERS Polymers Surface Tension Dynes/cm., 20C. Critical Surface Tension , Dynes/cm., 20C. Polydime thylsiloxane Polytetrafluoroethylene Polypropylene Poly(n-butyl methacrylate) Polyisobutylene Polyethylene Poly(vinyl acetate) Polystyrene Poly(methyl methacrylate ) Poly(ethylene oxide) Poly(2-chloro-l,5butadiene), neoprene Poly(hexamethylene adipamide), nylon 20.4 26.5 29.4 51.2 55.6 55*7 56.5 40.7 41,1 42.8 45.6 ------ ---- 18.5 * 29 52 5 27 51 56 55 59 54 58 46 DUP030044047 v:. TABLE II INTERFACIAL TENSIONS OF SOME POLYMER PAIRS* 7ia ** A - B (t - 20) Where 7 is ** interfacial tension, dyne s/cm. t temperature, C. (20-200*0.) 1. Poly(methyl methacrylate)/polye thylene 2. Poly(n-buty 1 methacryla te)/polye thylene 3- Polys tyrene/polye thylene 4. Poly(vinyl acetate)/polyethylene 5- Poly(vinyl acetate)/polyisobuty1ene 6. Poly(methyl methacrylate)/poly- s tyrene 7- Po ly (n-b u ty1 methac ryla t e ) /p 0ly(vinyl acetate) 8. Poly(me thyl methacrylate)/poly(n-butyl methacrylate) A 11.9 7.1 80 14.5 9.9 3-2 4.2 3.4 B 0.018 0.015 0.020 0.027 0.020 0.013 0.010 0.012 Data from S. Wu, (l) ESR-69-30, (2) ESR-68-34, (3) proceed ings of the 43rd National Colloid Symposium, 1969, pp. 30-38. t DUP030044048 TABLE III COMPONENTS OP SURFACE TENSIONS FOR SOME POLYMERS Poly(methyl methacrylate) Poly(n-butyl methacrylate) Polystyrene Poly(vinyl ac state) Polyethylene Xd 0.71 0.82 0.82 0.64 1.00 xP 0.29 0.18 0.18 0.36 0 (1) xd = fraction of the dispersion force componentj xr*n fraction of the polar force component. (2) Data from S. Wu, ESR-69-IO, ESR-68-34, Journal of Colloid and Interface Science, 51, 153 (1969), Journal of Physical Chemistry, March, 1970 (in press). DUP030044049 LIQUID X I I / / / X FIGURE I EQUILIBRIUM PENETRATION OF A LIQUID INTO AN AIR-FILLED SHALLOW SPHERICAL PIT DU P030044050 I FIGURE 2 EQUILIBRIUM CONTACT ANGLE OF A LIQUID ON A SOLID DUP030044051 FIGURE 3: CRITICAL SURFACE TENSION PLOT FOR POLYETHYLENE. (1) Water; (2) Glycerol; (3) Formamide; (4) Methylene iodide; (5) a-BromcnaphLhalene; (6) Tr.icr.esyl phosphate; if) Benzyl phenylundecanoate; (8) Sym.-tetrachloroethane; ( 9) t-Butyl napht.halane; (10) Di (2-ethylhf.ccyl) phthalate. DUP030044052 ( BREAKING UOAO, pounds I in- width/ 4000 3000 2000 1000 OVERLAP LENGTH, in FIGURE 4 THE EFFECT OF OVERLAP LENGTH ON THE BREAKING LOAD AND BREAKING STRESS OF A STEEL OVERLAP JOINT BONDED WITH POLY (VINYL FORMAL)/ PHENOL FORMALDE HYDE BLEND DUP030044053 PEEL FORCE I FIGURE 5 TYPICAL PEEL BEHAVIOR, SHOWING THREE DIFFERENT MODES OF PEEL. DUP030044054 FIGURE 6 SUPERPOSITION OF THE EFFECTS OF PEEL RATE AND ADHESIVE THICKNESS WHEN THE FAILURE IS COHESIVE. CELLOPHANE (40.5/a THICKNESS) TAPES COATED WITH RHOPLEX HA-- 8 ADHESIVE. DUP030044055 FIGURE 7: OSCILLATING PEEL FORCE AT SLIP-STICK FAILURE SHOWN ON AN INSTRON CHART. T-PEEL OF CELLOPHANE TAPES BONDED WITH RHOPLEX B-60 ACRYLIC ADHESIVE AT 1 IN./MIN. PEEL RATE. DUP030044056 r FIGURE 8: RANDOMLY (GAUSSIAN) FLUCTUATING PEEL FORCE ON AN INSTRON CHART FOR ADHESIVE FAILURE AT 5 IN./MIN. PEEL RATE* CELLOPHANE TAPES BONDED WITH RHOPLEX HA-8 ACRYLIC ADHESIVE. DU P030044057 SUMMARY PROBLEM ANALYSIS RECOGNIZE PROBLEMS Survey the situation Compare expectations (should) with observations (actual) for deviations ESTABLISH PRIORITY Assess seriousness, urgency, and potential payoff Select top priority problem for further analysis SPECIFY THE PROBLEM Describe precisely1 What the problem is - - - -- and - what it is not What -Unit(s), person(s), or thingCs) are involved in the deviation ? - Unit(s), person(s), or thingCs) are not involved in the deviation? - Is the trouble ? -Is not the trouble? Where1 - Or in what place(s) is the deviation observed to occur? - Or in what piace(s) is the deviation not observed to occur ? When1 Poes the deviation occur -In sequence of process? Does the deviation not occur -in sequence of process? -In clock or calendar time? -In clock or calendar time? Magnitude1 -How many unit(s), person(s), or thing(s) are involved ? -How much of each is involved ? -What is the trend? Identify what there is about each "is" that makes it different from the corresponding "is not" DEVELOP POSSIBLE CAUSES Identify changes related to differences Generate hypotheses based on changes identified DETERMINE MOST LIKELY CAUSE Consider logic,simplicity and completeness of each hypothesis Test each hypothesis against the specification VERIFY DUP030044058