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Adhesion between polymer substrates and plasma films from tetramethylsilane and tetramethyltin by glow discharge polymerization
Adhesion between polymer substrates and plasma films from t tramethylsflane and t tramethyltin by glow discharge polymerization N. Inagaki, M. Itami and K. Katsuura
Adhesion between various polymer substrates and plasma films, which had been prepared from either tetramethylsilane or tetramethyltin by glow discharge polymerization and deposited on the surface of the polymer, was evaluated by the Scotch tape test and by lap-shear strength. It was found that the plasma films exhibited fairly good adhesion to the polymer substrates (with the exception of polypropylene). The position where failure occurred was determined by X-ray fluorescence analysis, scanning electron microscopy and energy diffractive X-ray analysis. This position was at an inner layer of the plasma film (cohesive failure of plasma film), within the polymer substrate (material failure of polymer) or at the interface between polymer substrate and plasma film (adhesive failure) depending upon the polymer substrate. Theseresults indicate an important aspect of durability of surface modification by glow discharge polymerization. Key words: adhesion tests; polymers; surface treatment; plasma films.
Glow discharge polymerization is a useful procedure for modifying the surface of a polymer, especially a polyolefin. It can be used to advantage to change surface properties to give a desired effect. For example, by using fluorocarbons as the monomer gas for glow discharge polymerization, an inert surface like polytetrafluoroethylene can be placed upon a polar one (such as one containing amidc or carbonyl groups) 1-7. The advantages of this method offer commercial application in the fields of improved adhesion 8'9. reverse osmosis membranes 1°-~2, substrates for medical applications 13.~"etc. For these types of application, it is the durability of the surface properties which needs further investigation. It would appear, therefore, that the adhesion between a polymer substrate subjected to modification of surface properties and the plasma film prepared by glow discharge polymerization is an important subject. To obtain preliminary information on the durability of a polymer surface modified by glow discharge polymerization. the present study focussed on adhesion between polymer substrates and plasma films prepared from tetramethylsilane and tetramethyltin. Experimental Materials Polymer substrates provided tbr the measurement of
adhesive strength were: high density polyethylene (PE) and polycarbonate (PC) (provided by Mitsubishi Chemical Industries); polyvinylchloride (PVC) (trom Mitsubishi Plastic Industries); nylon 6 (from Nippon Keisozai Company): ABS (from Ube Industries); polymethylmethacrylate (PMMA) (from Sumitomo Chemical Company): polytetrafluoroethylene (PTFE) (from Nippon Asbestos Company) and polypropylene (pP) (from Chisso Company). The dimensions of the samples were 34 x 240 x 2-3 mm. To prepare the surfaces they were washed with acetone or methanol, then a detergent, rinsed with distilled water, and then stored in a desiccator over silica gel. The chemicals chosen for glow discharge polymerization were: tetramethylsilane (TMS) (provided by Tokyo Kasei Kogyo Company): and tetramethyltin (TMT) (from Ventron). These were used without further purification. Glow discharge polymerization The surfaces of the samples were coated with polymers prepared from TMS and TMT by glow discharge polymerization. The apparatus and experimental procedure used for the polymerization were essentially the same as those reported elsewhere Is. The polymer sample was placed in the reaction chamber (35 mm inner diameter, 400 mm length), and the system was evacuated to approximately 0.013 N/m 2. To eliminate water adsorbed on the polymer
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surface and the weak boundary layer ]6'~7 of the polymer. the surface of the sample was exposed to argon plasma for lOmin. The argon gas was introduced into the reaction chamber at a flow rate of 0.46 c m 3 (STP)/min at 10.66 N/m s, Again the system was evacuated to 0.013 N/m 2 and then monomer gas was injected into the reaction chamber at the flow rate of 1.0 cm 3 (STP)/min for TMT or 0.26 cm3(STP)/min for ]'MS at a pressure of 10.66 N/m 2. RF power was turned on, and glow discharge polymerization at a RF level of 40 W was performed for an adequate duration. The polyme~ substrates coated with plasma films by flow discharge polymerization were then ready for the measurement of adhesive strength.
Evaluation of adhesion
Two tests were used to evaluate the strength of adhesion between the polymer substrates and the plasma films: the Scotch tape test: and lap-shear strength. The Scotch tape test was carried out according to ASTM D-3354-76. For the measurement of lap-shear strength special specimens were prepared. The surface of an aluminium coupon (25 × 100 x I mm) was sandblasted, and the coupon was used to form a lap-joint with the coated polymer substrate. An epoxide adhesive consisting of a mixture of Epikote 828 and Tomide 235-S (mixing ratio of 100 : 70 by weight) was used to bond the two samples. There was an over-lap of 25 x 12.5 mm. The construction of the joined part, therefore, was polymer substrate/plasma film/epoxide adhesive/aluminium coupon. The specimen was cured at a temperature of 20:1: 2°C and a pressure of 0.49 MN/m 2 for 19h and then stored at temperature of 20-+ 2°C for 72h. The bonding strength of the joint was measured using an lnstron universal testing instrument at a peeling speed of 5 mm/min. Observation of fracture surfaces
Fracture surfaces from the lap-shear test were observed by electron microscopy for their topography, X-ray fluorescence analysis for the determination of relative concentrations of tin and silicon present, and energy diffractive x-ray analysis for the distribution profile of tin and silicon on these surfaces. The instruments used were a Hitachi HSM-2B Table 1.
Adhesive strength is the complex result of elastic and visco-elastic properties of a joint, and thus the strength is sensitive to the direction and the rate of loading, temperature of measurement, etc. There is not always a correlation between the values of adhesive strength obtained by different methods. It is therefore necessary to discuss the results in detail. Adhesive strength is frequently evaluated in peel. shear and tensile strength, in this study, the Scotch tape test (a procedure to measure peel strength) and lap-shear strength (a procedure to measure shear strength) were chosen for the evaluation of adhesion between polymer substrates and plasma films which had been deposited on them.
Scotch tape test
The results of the Scotch tape test are given in Table 1. Adhesion between plasma films and the polymer substrates is indicated as follows: (+) plasma films which did not peel off from the polymer substrates; (+-) a part of the plasma films peeled off; and ( - ) f'flms which completely peeled off and were transferred to the adhesive surface of the Scotch tape. Table 1 indicates that adhesion between plasma films and polymer substrates is dependent upon the nature of the polyme= substrate as well as that of the plasma film. Polymer substrates possessing high critical surface tension seem to show good adhesion. In Table 1 polymer substrates are listed in order of magnitude of critical surface tension (ie highest value at the top). Adhesion of plasma film prepared from TMT appears to be better than that of plasma film formed from TMS. Lap-shear strength
Results of lap-shear strength measurements are also shown in Table 1. The magnitude of lap-shear strength ranges widely from 0.45 to 4.20 MN/m 2. When coated with
Coated with plasma films prepared from T M T (800 nm)
Coated with plasma films prepared from TMS (200 nm)
Uncoated, but exposed to Ar plasma
Scotch tape test*
Lap-shear strength (MN/m 2)
Scotch tape test"
Lap-shear strength (MN/m 2)
Lap-shear strength (MN/m 2)
+ + + + + + +
4.20 2.53 2.50 1.59 1.33 2.44 0.59 1.96
+ + + -+ + -
4.14 1.79 2.42 3.77 1.89 4.20 0.45 1.83
5.37 3.16 4.64 3.39 1.72 0.64 0.48 -
* + did not peel off, ± partly peeled o f f , - - c o m p l e t e l y peeled off.
170
Results and discussion
Adhesion between plasma films and polymer subztrates
Polymer substrate
ABS Nylon PC PVC PMMA PE pp PTFE
and a Hitachi-Akashi MINI SEM 102. a Phihps PW1450. and a Hitachi HSM-2A for scanning electron microscopy (SEM), X-ray fluorescence analysis, and energy diffractive X-ray analysis, respectively.
INT.J.ADHESION A N D ADHESIVES J U L Y 1982
plasma film prepared from TMT, ABS polymer substrate shows the highest lap-shear strength of 4.20 MN/m=, while PP polymer substrate shows the lowest with a value of 0.59 MN/m2: other polymer substrates showing values between these two extremes. When coated with plasma fihn prepared from TMS, ABS and PE polymer substrates have the highest lap-shear strength with values above 4.0 MN/m 2. The magnitude of the lap-shear strength, as shown in Fig. 1, appears to be loosely related to the critical surface tension of polymer substrates. The magnitude of the lap-shear strength also appears to be affected by the type of plasma film which is deposited on the surface of polymer substrates. In the case of Pvc, the lap-shear strength increased from 1.59 to 3.77 MN/m 2 when the gas for glow discharge polymerization was changed from "rMT to TMS. A similar increase (from 2.44 to 4.20 MN/m 2) took place with the PE polymer substrate. Nylon 6 and PMMA polymer substrates also show a similar change in lap-shear strength, but the increase is not great. The other polymer substrates do not appear to be similarly affected by the change if TMS is used instead of TMT as the gas for glow discharge polymerization. Polymers formed from TMT and TMS by glow discharge polymerization under the same operating conditions as those employed in this experiment have been chemically investigated) s'~8 Polymers prepared from TMT possess an empirical, elemental composition of C~.8~H6.9,7Oo.75No.mSn, and consist mainly of CH2, Sn-CH3, and Sn-O groups. The surface energy of the polymer is 31.1 kN/m. This is divided into a polar contribution of 5.2 kN'm and a dispersive cont ribution of 25.9 kN: m. Polvmers from TMS have an empirical elemental composition of C3.~Hsa3OLooNo.19Si, and consist mainly ofCH~. Si-H. Si-O-C, Si-O-Si and Si-CH2--CH~-Si groups. The surface energy is 33.7 kN/m, which is divided into a polar contribution of 5.8 kN/m and a dispersive contribution of 27.9 kN:m. Therefore, both the polymers formed from TMS and TMT by glow discharge polymerization possess almost equal surface energy with a high polar contribution. Such adhesive phenomena between these plasma fihns and polymer substrates cannot be ex-
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Fig. 1 R e l a t i o n s h i p b e t w e e n lap-shear strength and critical s u r f a c e tension: a coated w i t h plasma f i l m prepared f r o m T M T ; ~. coated w i t h plasma f i l m prepared f r o m T M S ; : uncoated
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plained reasonably by the concept of the work of adhesion 19 or the wettability envelope =°. From the results of the Scotch tape test and lap-shear strength, adhesion between the polymer substrate and the plasma fihns deposited on their surfaces is fairly good, in most cases. In the case of PP substrate, adhesion is poor and it is not much improved for PC' and PMMA polymer substrates.
Mode of failure
The joint construction used in this experiment is illustrated in Fig. 2. The position where failure will occur is either at interfaces or at innerlayers. The possible interfaces are: between polymer substrate and plasma film (failure mode 1): between plasma film and epoxide adhesive (I1): and between epoxide adhesive and aluminium coupon (lit). Innerlayer failure can occur within the polymer substrate (failure mode IV), the plasma film (V), or the epoxide adhesive (Vl). Of these failure modes, mode Ill is not considered because the adhesive strength between the epoxide adhesive and the aluminium coupon is more than 6.9 MN/m 2. Polymers formed from TMT and TMS by glow discharge polymerization under the same operating conditions as those employed in this experiment, as mentioned in previous section, contain a large amount of tin (Sn) or silicon (Si). Accordingly, the position where failure occurred can be determined by the presence of either Sn or Si. For example, if failure occurred at the interface between the polymer substrate and the plasma film (mode l) Sn and Si elements will not be detected on the surface of the polymer substrate but on that ofepoxide adhesive. For failure mode II, all Sn and Si elements will be detected on the surface of the polymer substrate. However, if failure occurred at the inner-layer of the plasma film (mode V) then Sn and Si elements would exist on both the surfaces of the polymer substrate and the epoxide adhesive. Table 2 shows the relative concentrations of Sn and Si present on fracture surfaces (polymer substrate and aluminium coupon sides), which were determined by X-ray fluorescence analysis. This relative concentration of Sn and Si varies from 0 to 73 atomic %, depending upon the combination of plasma film and polymer substrate. This indicates that at least three failure modes occurred: ie modes 1, IV and V. To ascertain the failure modes assumed by X-ray fluorescence analysis, these surfaces were observed by scanning electron microscopy. There appeared four topographically different surfaces, and typical pictures are shown in Figs 3-6.
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171
Table 2. Relative concentrations of tin and silicon on the fracture surfaces Polymer substrate
ABS Nylon 6 PC PVC PMMA PE PP PTFE
Coated with plasma film prepared from TMT
Coated with plasma film prepared from TMS
Sn concentration (%)
Failure + Si concenmode tration (%)
Failure t mode
46/54* 11/89 66/34 9/91 0/100 0/100 0/100 4/96
V and II and II and II
63/37* 69/31 48/52
V I and II I and II
31/69 57/43 48/52 73/27
I and II I and II I and II IV and II
IV
*Polymer substrate side/aluminium coupon side tpositions where failure occurs, as indicated in Fig. 2
Fig. 3 shows the SEM picture of the fracture surfaces from the lap construction of ABS/plasma film from TMT/epoxide adhesive/aluminium coupon. All over the two surfaces there are scaly protuberances. It was found by energy diffractive X-ray analysis that Sn existed evenly on these surfaces, although this result is not presented here. Therefore, there is no doubt that failure occurred at the innerlayer of the plasma films from TMT (mode V). A F=g. 4
Fracture surface (polymer s=de) ~rom lap struCture o r
PE/plasma film from TMS/epoxide adhesivefaluminium coupon: (a) SEM picture; (b) showing distribution profile of Si elemen~
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-~ 4"-"
Fig. 3 SEM picture of the fracture surfaces from lap structure of ABS/plasma film from TMT/elooxide adhesive/aluminium coupon: (a) polymer side; (b] aluminium side
172
INT.J.ADHESION AND ADHESIVES JULY 1982
similar picture was obtained from the structural system of ABS/plasma film from TMS/epoxide adhesive/aluminium coupon. Fig. 4a shows the SEM picture of the fracture surface from the lap structure: PE/plasma film fronl rMs/epoxide adhesive/aluminium coupon. In this systcnl, Hie failure at the innerlayer of plasma film had been assumed by X-ray fluorescence analysis. The SEM picture of the fracture surface shows heterogeneity with smooth and hill-like surfaces. Energy diffractive X-ray analysis (Fig. 4bt indicates that Si occurs on the poltion of hill-like surface. but not on the portion of the smooth surtacc. 111 Fig. 4b the position of Si is indicated by a white spot. Therefore. it Call be concluded that in this system, failure occuz.~ simultaneously at two interfaces: between tile polymei substrate and the plasma film (mode I ): and between the plasma fihn and the epoxide adhesive (mode IlL rather than at the innerlayer of plasma film (mode V). A similar failure is observed in many systems such as nylon 6, PC and PVC which are coated with TMT and TMS. Fig. 5 shows the SEM picture of the fracture surtacc fronl the structural system: PP/plasma tilm from TMT/epoxide adhesive/aluminium coupon. The picture shows a surface as smooth as a virgin surface that has not been coated with a plasma film. indicating that the plasma has completely peeled off from the interface between the polymer substrate and the plasma film (mode I). Such a smooth surface is obtained also in the systems of PE and PMMA coated with T M T .
~
/]arrl ~ : " 4
Fig. 5 SEM picture of the fracture surface (polymer side) from lap structure of PP/plasma film from TMT/epoxide adhesive/aluminium COUpon
The SEM picture of the fracture surface from the structural system of PTFE/plasma film from TMT/epoxide adhesive/aluminium coupon is shown in Fig. 6. This picture gives evidence that the PTFE substrate was stripped off here and there, indicating that failure mode IV occurred. It can be concluded that adhesion between PTFE polymer substrate and the plasma film prepared from TMT is good, even though the lap-shear strength is not so high. A similar SEM picture was obtained in the case of PTFE polymer substrate coated with plasma film from TMS.
Conclusions A combination of SEM pictures and X-ray fluorescence analysis can be used successfully to determine the position of failure in joints which consist of: polymer substrate/plasma film/epoxide adhesive/aluminium coupon. Failure appears to depend upon the nature of the polymer substrate. Most failure occur at the interface between the polymer substrate and the plasma film and also at the interface between plasma film and epoxide adhesive. The assumed mode of failure is presented in Table 2. The reason why a PP substrate exhibits poor adhesion to plasma fihns whereas a PE substrate, which is chemically analogous to PP, has good adhesion is not clear. However, the SEM picture shown in Fig. 7 seems to suggest a solution to this problem.
Fig. 7 SEM picture of the cross-section of PC polymer substrate coated with plasma film from TMS
Fig. 7 presents the cross-section of a W polymer substrate which is coated with a plasma film from TMS. The picture shows that a plasma film of approximately 0.3/am thickness covers the surface of the PC substrate but that a part of this film protudes in a mound of about 3.5/am diameter and 1.4/am height. It can be speculated that this mound may have been formed by evolution of gas from the vicinity of the polymer surface caused by degradation initiated by the action of the plasma, ie vacuum ultravielet rays. Therefore, plasma susceptibility of the polymer substrates may be an important factor when considering adhesion between polymer substrates and plasma films. The experimental evidence that adhesion between PE polymer substrates and plasma films prepared from TMS is good, having a strength as high as 4.2 MN/m 2 suggests that this procedure has practical applications for surface modification of PE polymer substrates.
Acknowledgements
The authors acknowledge Nippon Gakki Co. for the time generously made available on the lnstron tester, and Mitsubishi Chemical Industries for measurements of X-ray fluorescence analysis and energy diffractive X-ray analysis.
References
Fig. 6 SEM picture of the fracture surface (polymer side) from lap structure of PTFE/plasma fdm from TMT/epoxide adhesive/aluminium coupon
1
Washo, B.D. JMacromolSciChemAlO(1976) p559
2
O'Kane, D.F. and Rice, D.W. J Macromo/Sci Chem A10 (1976) p567
3
Yasuda, H. end Hsu, T. J Polym Sci Polym Chem Edn 15 (1977) p2411
4
Nakajima, K., Bell, A.T. and Shen, M. J App/Po/ym Sci 23 (1979) p2627
5
Moro=off, N. and Yasuda, H. J Appl Po/ym Sci 23 (1979) p 3449
6
Yasuda, H. and Lamaze, C.E. J Appl Polym Sci 17 (1973) p 201
7
Hinman, P.V., Bell, A.T. and Shen, M. J Appl Po/ym Sci 23 (1979) p3651
8
Moihonov, A. and Avny, Y. J Appl Po/ym Sci 25 (1980) p771
9
Inagaki, N. and Yasuda, H.JApplPolymSci 26 (1981) p3333
INT.J.ADHESlON
AND ADHESIVES
JULY
1982
173
10
Y~uda, H. and Lemaze, L.E. J App/Polyrn Sci 15 (1971) p2277 and 17 (1973) 10201 ,.1533
11
Bell, A.T., Wyd~ten, T. and Johnson, C.C. J ApDI Polym Sci 19, (1975) p 1911
12
Hollehan, J.R. and WydeYen, T. J Appl Polvm Sci 21 (1977) p923
13
Ylsuda, H., Bumgirner, M.O. end Morosoff, N. Annual Reloort NIH4VHL 1-73-2913 ( 1973)
14
Hollahan, J.R.andCarllon, G.L. J A p p l PolymSci14 (1970) 102499 I nagaki, N., then, S. and Kat=uura, K. Kobunshi Ronbunshu 38 (1981) 10665
15 16
Bikerman, J.J. Adhesives Age 2 (1959) p 23
17
Brewis, DJ~. end Brigill, D. Polymer 22 (1981) p 7
174
I N T . J . A D H E S I O N A N D A D H E S I V E S J U L Y 1982
18
Inlgaki, N., ¥~ii, T. and Kalumura, K. (unpubtished)
19
Van Krevekm, DoW. and Hoftyzer, D J. Properties of Polymers (Elsevier, Amsterdam. 1976)
20
Cirlin, E.H. and Kaelble, D.H. J Po/ym Sci , Po/yrn Phys Edn 11 (1973) p785
Authors The authors are with the Laboratory of Polymer Chemistry, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, 432 Japan. Inquiries should be directed to Dr Inagaki.
Adhesion between various polymer substrates and plasma films, which had been prepared from either tetramethylsilane or tetramethyltin by glow discharge polymerization and deposited on the surface of the polymer, was evaluated by the Scotch tape test and by lap-shear strength. It was found that the plasma films exhibited fairly good adhesion to the polymer substrates (with the exception of polypropylene). The position where failure occurred was determined by X-ray fluorescence analysis, scanning electron microscopy and energy diffractive X-ray analysis. This position was at an inner layer of the plasma film (cohesive failure of plasma film), within the polymer substrate (material failure of polymer) or at the interface between polymer substrate and plasma film (adhesive failure) depending upon the polymer substrate. Theseresults indicate an important aspect of durability of surface modification by glow discharge polymerization. Key words: adhesion tests; polymers; surface treatment; plasma films.
Glow discharge polymerization is a useful procedure for modifying the surface of a polymer, especially a polyolefin. It can be used to advantage to change surface properties to give a desired effect. For example, by using fluorocarbons as the monomer gas for glow discharge polymerization, an inert surface like polytetrafluoroethylene can be placed upon a polar one (such as one containing amidc or carbonyl groups) 1-7. The advantages of this method offer commercial application in the fields of improved adhesion 8'9. reverse osmosis membranes 1°-~2, substrates for medical applications 13.~"etc. For these types of application, it is the durability of the surface properties which needs further investigation. It would appear, therefore, that the adhesion between a polymer substrate subjected to modification of surface properties and the plasma film prepared by glow discharge polymerization is an important subject. To obtain preliminary information on the durability of a polymer surface modified by glow discharge polymerization. the present study focussed on adhesion between polymer substrates and plasma films prepared from tetramethylsilane and tetramethyltin. Experimental Materials Polymer substrates provided tbr the measurement of
adhesive strength were: high density polyethylene (PE) and polycarbonate (PC) (provided by Mitsubishi Chemical Industries); polyvinylchloride (PVC) (trom Mitsubishi Plastic Industries); nylon 6 (from Nippon Keisozai Company): ABS (from Ube Industries); polymethylmethacrylate (PMMA) (from Sumitomo Chemical Company): polytetrafluoroethylene (PTFE) (from Nippon Asbestos Company) and polypropylene (pP) (from Chisso Company). The dimensions of the samples were 34 x 240 x 2-3 mm. To prepare the surfaces they were washed with acetone or methanol, then a detergent, rinsed with distilled water, and then stored in a desiccator over silica gel. The chemicals chosen for glow discharge polymerization were: tetramethylsilane (TMS) (provided by Tokyo Kasei Kogyo Company): and tetramethyltin (TMT) (from Ventron). These were used without further purification. Glow discharge polymerization The surfaces of the samples were coated with polymers prepared from TMS and TMT by glow discharge polymerization. The apparatus and experimental procedure used for the polymerization were essentially the same as those reported elsewhere Is. The polymer sample was placed in the reaction chamber (35 mm inner diameter, 400 mm length), and the system was evacuated to approximately 0.013 N/m 2. To eliminate water adsorbed on the polymer
0143-7496/82/030169-06 $03.00 © 1982 Butterworth & Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES JULY 1982
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surface and the weak boundary layer ]6'~7 of the polymer. the surface of the sample was exposed to argon plasma for lOmin. The argon gas was introduced into the reaction chamber at a flow rate of 0.46 c m 3 (STP)/min at 10.66 N/m s, Again the system was evacuated to 0.013 N/m 2 and then monomer gas was injected into the reaction chamber at the flow rate of 1.0 cm 3 (STP)/min for TMT or 0.26 cm3(STP)/min for ]'MS at a pressure of 10.66 N/m 2. RF power was turned on, and glow discharge polymerization at a RF level of 40 W was performed for an adequate duration. The polyme~ substrates coated with plasma films by flow discharge polymerization were then ready for the measurement of adhesive strength.
Evaluation of adhesion
Two tests were used to evaluate the strength of adhesion between the polymer substrates and the plasma films: the Scotch tape test: and lap-shear strength. The Scotch tape test was carried out according to ASTM D-3354-76. For the measurement of lap-shear strength special specimens were prepared. The surface of an aluminium coupon (25 × 100 x I mm) was sandblasted, and the coupon was used to form a lap-joint with the coated polymer substrate. An epoxide adhesive consisting of a mixture of Epikote 828 and Tomide 235-S (mixing ratio of 100 : 70 by weight) was used to bond the two samples. There was an over-lap of 25 x 12.5 mm. The construction of the joined part, therefore, was polymer substrate/plasma film/epoxide adhesive/aluminium coupon. The specimen was cured at a temperature of 20:1: 2°C and a pressure of 0.49 MN/m 2 for 19h and then stored at temperature of 20-+ 2°C for 72h. The bonding strength of the joint was measured using an lnstron universal testing instrument at a peeling speed of 5 mm/min. Observation of fracture surfaces
Fracture surfaces from the lap-shear test were observed by electron microscopy for their topography, X-ray fluorescence analysis for the determination of relative concentrations of tin and silicon present, and energy diffractive x-ray analysis for the distribution profile of tin and silicon on these surfaces. The instruments used were a Hitachi HSM-2B Table 1.
Adhesive strength is the complex result of elastic and visco-elastic properties of a joint, and thus the strength is sensitive to the direction and the rate of loading, temperature of measurement, etc. There is not always a correlation between the values of adhesive strength obtained by different methods. It is therefore necessary to discuss the results in detail. Adhesive strength is frequently evaluated in peel. shear and tensile strength, in this study, the Scotch tape test (a procedure to measure peel strength) and lap-shear strength (a procedure to measure shear strength) were chosen for the evaluation of adhesion between polymer substrates and plasma films which had been deposited on them.
Scotch tape test
The results of the Scotch tape test are given in Table 1. Adhesion between plasma films and the polymer substrates is indicated as follows: (+) plasma films which did not peel off from the polymer substrates; (+-) a part of the plasma films peeled off; and ( - ) f'flms which completely peeled off and were transferred to the adhesive surface of the Scotch tape. Table 1 indicates that adhesion between plasma films and polymer substrates is dependent upon the nature of the polyme= substrate as well as that of the plasma film. Polymer substrates possessing high critical surface tension seem to show good adhesion. In Table 1 polymer substrates are listed in order of magnitude of critical surface tension (ie highest value at the top). Adhesion of plasma film prepared from TMT appears to be better than that of plasma film formed from TMS. Lap-shear strength
Results of lap-shear strength measurements are also shown in Table 1. The magnitude of lap-shear strength ranges widely from 0.45 to 4.20 MN/m 2. When coated with
Coated with plasma films prepared from T M T (800 nm)
Coated with plasma films prepared from TMS (200 nm)
Uncoated, but exposed to Ar plasma
Scotch tape test*
Lap-shear strength (MN/m 2)
Scotch tape test"
Lap-shear strength (MN/m 2)
Lap-shear strength (MN/m 2)
+ + + + + + +
4.20 2.53 2.50 1.59 1.33 2.44 0.59 1.96
+ + + -+ + -
4.14 1.79 2.42 3.77 1.89 4.20 0.45 1.83
5.37 3.16 4.64 3.39 1.72 0.64 0.48 -
* + did not peel off, ± partly peeled o f f , - - c o m p l e t e l y peeled off.
170
Results and discussion
Adhesion between plasma films and polymer subztrates
Polymer substrate
ABS Nylon PC PVC PMMA PE pp PTFE
and a Hitachi-Akashi MINI SEM 102. a Phihps PW1450. and a Hitachi HSM-2A for scanning electron microscopy (SEM), X-ray fluorescence analysis, and energy diffractive X-ray analysis, respectively.
INT.J.ADHESION A N D ADHESIVES J U L Y 1982
plasma film prepared from TMT, ABS polymer substrate shows the highest lap-shear strength of 4.20 MN/m=, while PP polymer substrate shows the lowest with a value of 0.59 MN/m2: other polymer substrates showing values between these two extremes. When coated with plasma fihn prepared from TMS, ABS and PE polymer substrates have the highest lap-shear strength with values above 4.0 MN/m 2. The magnitude of the lap-shear strength, as shown in Fig. 1, appears to be loosely related to the critical surface tension of polymer substrates. The magnitude of the lap-shear strength also appears to be affected by the type of plasma film which is deposited on the surface of polymer substrates. In the case of Pvc, the lap-shear strength increased from 1.59 to 3.77 MN/m 2 when the gas for glow discharge polymerization was changed from "rMT to TMS. A similar increase (from 2.44 to 4.20 MN/m 2) took place with the PE polymer substrate. Nylon 6 and PMMA polymer substrates also show a similar change in lap-shear strength, but the increase is not great. The other polymer substrates do not appear to be similarly affected by the change if TMS is used instead of TMT as the gas for glow discharge polymerization. Polymers formed from TMT and TMS by glow discharge polymerization under the same operating conditions as those employed in this experiment have been chemically investigated) s'~8 Polymers prepared from TMT possess an empirical, elemental composition of C~.8~H6.9,7Oo.75No.mSn, and consist mainly of CH2, Sn-CH3, and Sn-O groups. The surface energy of the polymer is 31.1 kN/m. This is divided into a polar contribution of 5.2 kN'm and a dispersive cont ribution of 25.9 kN: m. Polvmers from TMS have an empirical elemental composition of C3.~Hsa3OLooNo.19Si, and consist mainly ofCH~. Si-H. Si-O-C, Si-O-Si and Si-CH2--CH~-Si groups. The surface energy is 33.7 kN/m, which is divided into a polar contribution of 5.8 kN/m and a dispersive contribution of 27.9 kN:m. Therefore, both the polymers formed from TMS and TMT by glow discharge polymerization possess almost equal surface energy with a high polar contribution. Such adhesive phenomena between these plasma fihns and polymer substrates cannot be ex-
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30 40 50 Surfoce tension (raN/m)
60
70
Fig. 1 R e l a t i o n s h i p b e t w e e n lap-shear strength and critical s u r f a c e tension: a coated w i t h plasma f i l m prepared f r o m T M T ; ~. coated w i t h plasma f i l m prepared f r o m T M S ; : uncoated
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Fig. 2 C o n s t r u c t i o n of lap-joint, I - V l ,ndicate positions o f each mode of failure
plained reasonably by the concept of the work of adhesion 19 or the wettability envelope =°. From the results of the Scotch tape test and lap-shear strength, adhesion between the polymer substrate and the plasma fihns deposited on their surfaces is fairly good, in most cases. In the case of PP substrate, adhesion is poor and it is not much improved for PC' and PMMA polymer substrates.
Mode of failure
The joint construction used in this experiment is illustrated in Fig. 2. The position where failure will occur is either at interfaces or at innerlayers. The possible interfaces are: between polymer substrate and plasma film (failure mode 1): between plasma film and epoxide adhesive (I1): and between epoxide adhesive and aluminium coupon (lit). Innerlayer failure can occur within the polymer substrate (failure mode IV), the plasma film (V), or the epoxide adhesive (Vl). Of these failure modes, mode Ill is not considered because the adhesive strength between the epoxide adhesive and the aluminium coupon is more than 6.9 MN/m 2. Polymers formed from TMT and TMS by glow discharge polymerization under the same operating conditions as those employed in this experiment, as mentioned in previous section, contain a large amount of tin (Sn) or silicon (Si). Accordingly, the position where failure occurred can be determined by the presence of either Sn or Si. For example, if failure occurred at the interface between the polymer substrate and the plasma film (mode l) Sn and Si elements will not be detected on the surface of the polymer substrate but on that ofepoxide adhesive. For failure mode II, all Sn and Si elements will be detected on the surface of the polymer substrate. However, if failure occurred at the inner-layer of the plasma film (mode V) then Sn and Si elements would exist on both the surfaces of the polymer substrate and the epoxide adhesive. Table 2 shows the relative concentrations of Sn and Si present on fracture surfaces (polymer substrate and aluminium coupon sides), which were determined by X-ray fluorescence analysis. This relative concentration of Sn and Si varies from 0 to 73 atomic %, depending upon the combination of plasma film and polymer substrate. This indicates that at least three failure modes occurred: ie modes 1, IV and V. To ascertain the failure modes assumed by X-ray fluorescence analysis, these surfaces were observed by scanning electron microscopy. There appeared four topographically different surfaces, and typical pictures are shown in Figs 3-6.
INT.J.ADHESION A N D A D H E S I V E S J U L Y 1982
171
Table 2. Relative concentrations of tin and silicon on the fracture surfaces Polymer substrate
ABS Nylon 6 PC PVC PMMA PE PP PTFE
Coated with plasma film prepared from TMT
Coated with plasma film prepared from TMS
Sn concentration (%)
Failure + Si concenmode tration (%)
Failure t mode
46/54* 11/89 66/34 9/91 0/100 0/100 0/100 4/96
V and II and II and II
63/37* 69/31 48/52
V I and II I and II
31/69 57/43 48/52 73/27
I and II I and II I and II IV and II
IV
*Polymer substrate side/aluminium coupon side tpositions where failure occurs, as indicated in Fig. 2
Fig. 3 shows the SEM picture of the fracture surfaces from the lap construction of ABS/plasma film from TMT/epoxide adhesive/aluminium coupon. All over the two surfaces there are scaly protuberances. It was found by energy diffractive X-ray analysis that Sn existed evenly on these surfaces, although this result is not presented here. Therefore, there is no doubt that failure occurred at the innerlayer of the plasma films from TMT (mode V). A F=g. 4
Fracture surface (polymer s=de) ~rom lap struCture o r
PE/plasma film from TMS/epoxide adhesivefaluminium coupon: (a) SEM picture; (b) showing distribution profile of Si elemen~
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Fig. 3 SEM picture of the fracture surfaces from lap structure of ABS/plasma film from TMT/elooxide adhesive/aluminium coupon: (a) polymer side; (b] aluminium side
172
INT.J.ADHESION AND ADHESIVES JULY 1982
similar picture was obtained from the structural system of ABS/plasma film from TMS/epoxide adhesive/aluminium coupon. Fig. 4a shows the SEM picture of the fracture surface from the lap structure: PE/plasma film fronl rMs/epoxide adhesive/aluminium coupon. In this systcnl, Hie failure at the innerlayer of plasma film had been assumed by X-ray fluorescence analysis. The SEM picture of the fracture surface shows heterogeneity with smooth and hill-like surfaces. Energy diffractive X-ray analysis (Fig. 4bt indicates that Si occurs on the poltion of hill-like surface. but not on the portion of the smooth surtacc. 111 Fig. 4b the position of Si is indicated by a white spot. Therefore. it Call be concluded that in this system, failure occuz.~ simultaneously at two interfaces: between tile polymei substrate and the plasma film (mode I ): and between the plasma fihn and the epoxide adhesive (mode IlL rather than at the innerlayer of plasma film (mode V). A similar failure is observed in many systems such as nylon 6, PC and PVC which are coated with TMT and TMS. Fig. 5 shows the SEM picture of the fracture surtacc fronl the structural system: PP/plasma tilm from TMT/epoxide adhesive/aluminium coupon. The picture shows a surface as smooth as a virgin surface that has not been coated with a plasma film. indicating that the plasma has completely peeled off from the interface between the polymer substrate and the plasma film (mode I). Such a smooth surface is obtained also in the systems of PE and PMMA coated with T M T .
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Fig. 5 SEM picture of the fracture surface (polymer side) from lap structure of PP/plasma film from TMT/epoxide adhesive/aluminium COUpon
The SEM picture of the fracture surface from the structural system of PTFE/plasma film from TMT/epoxide adhesive/aluminium coupon is shown in Fig. 6. This picture gives evidence that the PTFE substrate was stripped off here and there, indicating that failure mode IV occurred. It can be concluded that adhesion between PTFE polymer substrate and the plasma film prepared from TMT is good, even though the lap-shear strength is not so high. A similar SEM picture was obtained in the case of PTFE polymer substrate coated with plasma film from TMS.
Conclusions A combination of SEM pictures and X-ray fluorescence analysis can be used successfully to determine the position of failure in joints which consist of: polymer substrate/plasma film/epoxide adhesive/aluminium coupon. Failure appears to depend upon the nature of the polymer substrate. Most failure occur at the interface between the polymer substrate and the plasma film and also at the interface between plasma film and epoxide adhesive. The assumed mode of failure is presented in Table 2. The reason why a PP substrate exhibits poor adhesion to plasma fihns whereas a PE substrate, which is chemically analogous to PP, has good adhesion is not clear. However, the SEM picture shown in Fig. 7 seems to suggest a solution to this problem.
Fig. 7 SEM picture of the cross-section of PC polymer substrate coated with plasma film from TMS
Fig. 7 presents the cross-section of a W polymer substrate which is coated with a plasma film from TMS. The picture shows that a plasma film of approximately 0.3/am thickness covers the surface of the PC substrate but that a part of this film protudes in a mound of about 3.5/am diameter and 1.4/am height. It can be speculated that this mound may have been formed by evolution of gas from the vicinity of the polymer surface caused by degradation initiated by the action of the plasma, ie vacuum ultravielet rays. Therefore, plasma susceptibility of the polymer substrates may be an important factor when considering adhesion between polymer substrates and plasma films. The experimental evidence that adhesion between PE polymer substrates and plasma films prepared from TMS is good, having a strength as high as 4.2 MN/m 2 suggests that this procedure has practical applications for surface modification of PE polymer substrates.
Acknowledgements
The authors acknowledge Nippon Gakki Co. for the time generously made available on the lnstron tester, and Mitsubishi Chemical Industries for measurements of X-ray fluorescence analysis and energy diffractive X-ray analysis.
References
Fig. 6 SEM picture of the fracture surface (polymer side) from lap structure of PTFE/plasma fdm from TMT/epoxide adhesive/aluminium coupon
1
Washo, B.D. JMacromolSciChemAlO(1976) p559
2
O'Kane, D.F. and Rice, D.W. J Macromo/Sci Chem A10 (1976) p567
3
Yasuda, H. end Hsu, T. J Polym Sci Polym Chem Edn 15 (1977) p2411
4
Nakajima, K., Bell, A.T. and Shen, M. J App/Po/ym Sci 23 (1979) p2627
5
Moro=off, N. and Yasuda, H. J Appl Po/ym Sci 23 (1979) p 3449
6
Yasuda, H. and Lamaze, C.E. J Appl Polym Sci 17 (1973) p 201
7
Hinman, P.V., Bell, A.T. and Shen, M. J Appl Po/ym Sci 23 (1979) p3651
8
Moihonov, A. and Avny, Y. J Appl Po/ym Sci 25 (1980) p771
9
Inagaki, N. and Yasuda, H.JApplPolymSci 26 (1981) p3333
INT.J.ADHESlON
AND ADHESIVES
JULY
1982
173
10
Y~uda, H. and Lemaze, L.E. J App/Polyrn Sci 15 (1971) p2277 and 17 (1973) 10201 ,.1533
11
Bell, A.T., Wyd~ten, T. and Johnson, C.C. J ApDI Polym Sci 19, (1975) p 1911
12
Hollehan, J.R. and WydeYen, T. J Appl Polvm Sci 21 (1977) p923
13
Ylsuda, H., Bumgirner, M.O. end Morosoff, N. Annual Reloort NIH4VHL 1-73-2913 ( 1973)
14
Hollahan, J.R.andCarllon, G.L. J A p p l PolymSci14 (1970) 102499 I nagaki, N., then, S. and Kat=uura, K. Kobunshi Ronbunshu 38 (1981) 10665
15 16
Bikerman, J.J. Adhesives Age 2 (1959) p 23
17
Brewis, DJ~. end Brigill, D. Polymer 22 (1981) p 7
174
I N T . J . A D H E S I O N A N D A D H E S I V E S J U L Y 1982
18
Inlgaki, N., ¥~ii, T. and Kalumura, K. (unpubtished)
19
Van Krevekm, DoW. and Hoftyzer, D J. Properties of Polymers (Elsevier, Amsterdam. 1976)
20
Cirlin, E.H. and Kaelble, D.H. J Po/ym Sci , Po/yrn Phys Edn 11 (1973) p785
Authors The authors are with the Laboratory of Polymer Chemistry, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, 432 Japan. Inquiries should be directed to Dr Inagaki.