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Mechanisms of contaminant elimination by oil-accommodating adhesives Part 1: Displacement and absorption
Mechanisms of contaminant elimination by off-accommodating adhesives Part 1: Displacement and absorption M . Debski, M.E.R. Shanahan and J. Schultz (Centre de Recherches sur la Physico-Chimie des Surfaces Solides and Laboratoire de Recherches sur la Physico-Chimie des Interfaces de I'Ecole Nationale Sup6rieure de Chimie de Mulhouse, France)
Oil-accommodating adhesives are rapidly becoming important in the automotive industry. The ability of certain polymer resins to produce durable metal-metal bonds without previous cleaning of the oil contaminants from the substrate marks a notable advance in adhesives technology. Nevertheless, in certain circumstances, the presolidification process required in production-line conditions leaves the bonding prone to failure under high humidity conditions. This suggests an inadequate metal-adhesive contact, probably due to inefficient oil elimination. The present study attempts to elucidate the essential methods by which surface oil is eradicated; a necessary step to good bonding.
Key words: adhesives; sheet steel; surface contamination; oil; surface free energy; displacement; absorption
Since the introduction of structural adhesives in the automotive industry, the question of bonding steel sheet has posed problems due to the inevitable surface pollution by the oils and greases present in factory conditions. The importance of this aspect has been recognized for about 20 years: to cite an example, procedures for bonding car bonnet and boot-lid reinforcing stays were already in existence in 19661. Several advantages appear when adhesive bonding is employed instead of mechanical attachment. It has been shown that a bonded structure is 30 to 40% stronger than the equivalent spot-welded assembly because of the more even stress distribution. In addition, the use of adhesives leads to: • the absence of corrosion at welds; • the lack of need to render the surface aesthetically agreeable after weld deformation; and • the waterproofing of metal-metal joints. In industry several types of oil-accommodating adhesive have been tested, in particular vinyl plastisols, epoxies and acrylics. At the present time, singlecomponent epoxy resins seem to offer the best overall
characteristics for use on oil-contaminated steel sheet. Nevertheless, the operational bonding conditions usually require a temperature above 100°C. Since curing generally occurs on the production line only at the paint-drying stage, some pre-curing process must be employed to maintain the metallic components in contact. The favoured process involves the rapid presolidification of the adhesive at certain judicious points of the assembly in question to hold it together. This method employs local heating at up to 260°C for a maximum period of 5 seconds. Although the performance of such pre-solidified joints, whether or not the process be followed by a classic cure cycle, is acceptable in general, it would seem that presolidification leaves the joints prone to degradation due to humidity. Since the same adhesive systems show little drop in humid strength when cured classically, it would seem that the pre-solidification process is not conducive to a suitable 'digestion' or displacement of the surface oil contaminant necessary to obtain good adhesion. The objective of the present study is thus to try to elucidate the physico-chemical phenomena occurring
0 1 4 3 - 7 4 9 6 / 8 6 / 0 3 0 1 4 5 - 0 5 $03.00 © 1986 Butterworth Et Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES JULY VOL.6 NO.3 1986
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at the metal/adhesive interface and, in particular, to understand what becomes of the contaminant layer. Two essential processes leading to the elimination of the oil at the metal/adhesive interface can be envisaged:
Equation (1) can then be rewritten:
• the displacement of the oil by the adhesive; and • the absorption of the oil by the adhesive 2.
A series of measurements of 0 for different liquids was thus effected on the four metal surfaces of interest: steel and zinc-coated steel, both vapour (methylethylketone) degreased and lightly regreased with oil ASTM 3. A graph of cos 0 v s (~'bl/E/~'L will give a gradient of 2 (~'SE~1/2 for apolar liquids. An example, that of vapour degreased zinc-coated steel, is given in Fig. I. The points on the right-hand side of the graph correspond to the primarily apolar liquids diiodomethane and 1-bromonaphthalene. The points corresponding to the other liquids (water, formamide and ethylene glycol) are above the line of gradient 2 (ys~ 1/2 due to polar interactions, IsPL. However, too much importance should not be attached to these points since polar interactions can depend very strongly on the type of liquid. This summarizes the basis of the estimation of the values of yO for the four metal surfaces of this study. The values obtained are to be fdund in Table 1. It is noteworthy that these values are relatively low and similar. In the case of the degreased surfaces, this could well be a reflection of the fact that some contaminant remains after treatment. The polar interaction between a solid and a liquid is a subject of much discussion and conjecture as to the direct relationship between itself and the polar components of surface free energy of the two phases in contact. For this reason no attempt will be made here to reduce I~L to more fundamental parameters. Instead, for the interactions between the metals in question and
Either or both of these processes may take place and, as will be seen below, both in fact would seem to play a role. Displacement
Clearly any physical displacement of the oil on the metal surface is a thermodynamic process involving surface energetics. As an initial step in this study, it was therefore decided to obtain surface free energy data of the materials to be considered. The metals studied were a model steel sheet and a zinc-coated steel sheet. Oil contamination was modelled using a mineral oil of standard ASTM 3 at 3 g m -2 concentration, attained by dipping in a heptane solution of the oil, draining and evaporation of the solvent. The adhesives used were a model epoxy resin based on bisphenol A for the displacement study and a formulated version of this material containing 6% dicyandiamide, both with and without a CaCO 3 filler (70 pph), for the absorption work. Measurement of metal surface free energy characteristics
It is well known that the elucidation of surface free energy data of pure metals is a delicate and difficult task. However, as far as the present study is concerned, it is sufficient to obtain data referring to the metals in service conditions. A method based on wetting of the solid surface by known liquids was employed. In this method, the contact angles of a series of liquids on the solid in question are measured. Based on the concepts of Girifalco and Good 3, Fowkes 4 and Dann 5, an equation relating contact angle, 0, to various surface free energy terms can be derived6: cos O = 2 (yDp)l/2 (7~ 1/2 ~_IPL YL 7L
rre YL
1
2
(rs 1/2
+
YL
_ 1
(2)
7L
1
~0-
(1)
where 7Dp and 7 D are respectively the dispersive components of the surface free energies of the pure solid, Ysp, and the liquid, 7L; ISPLis the polar interaction solidAiquid; and ne is the equilibrium spreading pressure of the liquid on the solid. Now it is just this last term, he, which complicates the study of high energy solids such as metals, since it represents the effective reduction of the surface free energy of the solid due to adsorption of the liquid's vapour environment; ie, zre = 7sa - ?s where Ys is the surface free energy of the solid polluted by vapour adsorption. However, it is likely that this modification will be a fairly constant term as far as dispersive interactions are concerned when treating organic liquids. Since both the liquids for contact angle measurement and the resin to be studied are essentially organic, the simplifying assumption will be made that n e does not occur in Equation (1) but that 7SDpshould be replaced by yD the dispersive componer, t of the surface free energy of the metal contaminated by organic vapour.
146
cos 0
tNT.J.ADHESION AND ADHESIVES JULY 1986
~D = 42 mJ m - 2
-1
I 0
0.2
0,1 k/-'~LT~ (m J-½ m) 7L
Fig. 1 Cos 0 v s (7LD)1/2'yL"1 for vapour degreased zinc-coated steel
Table 1. Metal and metal/resin surface free energy characteristics
Steel Zinc-coated steel Regreased steel Regreased Zn-steel Estimated errors +_ 1 mJ m -2
yD (mJ m -2)
/PR (mJ m -2)
44 42 42 41
4 6 4 5
the resin, I~R will be evaluated directly. For this, the equilibrium contact angles of the resin on the solid surfaces were measured. The term yDis known from above and the surface free energy of the resin, YR, and its dispersive component, yD, are evaluated below. Equation (2) then applies with the subscript L replaced by R and ~R can be calculated directly. The values obtained can be found in Table 1. Measurement of resin and oil surface free energy characteristics
a
Since both the model resin and ASTM 3 oil are liquids at room temperature, evaluation of surface free energy, or surface tension, 7R and 7ti respectively, was effected directly by the Wilhelmy plate method 7 using a tensiometer. The measurement of contact angles of the liquids on a reference apolar surface, polytetrafluoroethylene (PTFE), and use of Equation (2) simplified by the absence of a polar term lead to values of the dispersive components of surface free energy, yD and yD. The polar terms yv and y~ are simply obtained by difference. The results are summarized in Table 2.
Having established the various surface free energy characteristics of the metals, oil and resin in question, it is now possible to consider the condition of displacement of the liquid oil layer on the metal surface by the resin using a thermodynamic argument8. Assuming a drop of resin to maintain a constant geometrical form, the difference in free energy between the state in which the resin rests on a thin layer of oil on the metal surface and that in which the resin displaces the oil can be assessed. The two states are schematically shown in Fig. 2. This free energy difference, AF, can be written: = Z . (YSR -- YRH -- YSH)
Yij = Yi + Yj - 2 ( y D . y D ) U 2 _ i P
b
- 3 and - 5 mJ m -2 respectively for the steel and zinccoated steel substrates. Although the magnitudes of AF are relatively small, it can nevertheless be concluded that displacement of an essentially apolar oil from both the steel and zinc-coated steel surfaces is likely to take place on thermodynamic grounds. However, this argument is based purely on interfacial free energy considerations. If the oil is physically, or geometrically, trapped in pores or asperities in the metal surface the resin will be unable to displace it. It is for this reason that the following section involving direct absorption of the oil by the adhesive must be considered. Absorption
(4)
where the superscripts D and P have their usual meaning. The polarity of the oil being negligible, use of Equation (4) in Equation (3) leads to: AF
H
(3)
where the terms 7ij represent the interfacial free energies between solid, resin and oil, and Z is the surface area of the underside of the drop of resin. A similar equation was obtained by Gledhill and Kinloch 9. The terms 7ij may be developed using the work of Fowkes4:
--
m
Fig. 2 Possible final states of epoxy on metal substrate: (a) with thin intervening oil film; and (b) oil displaced by resin
Condition of displacement
AF
H
As evoked above, it is quite possible that at least some of the oil on the metal surface is physically trapped, as shown schematically in Fig. 3. In this case, for a good adhesive/metal contact, the former must be able to absorb or 'digest' the contaminant. In order to examine this aspect, a quantitative study of the variation of the glass transition temperature, Tg, of the resins as a function of oil content and cross-linking time at 210°C was undertaken. Differential scanning calorimetry
--~ [(YB 1/2 -- (YH) 1/21 [(YH) 1/2 --
(rRD)~/2I - / ~ R
(5)
The glass transition temperatures, Tg, of both the unfilled and filled resins containing dicyandiamide
If AF is negative, displacement of the oil by the resin is favoured. Use of the data for the various surfaces from Tables 1 and 2 leads to values of A F / G of about Table 2. Epoxy resin and oil surface free energy characteristics
Epoxy Oil ASTM 3
yD (mJ m -2)
y~ (mJ m -2)
YL (mJ m -2)
28 32
10 <<1
38 -32
Estimated errors _+ 1 mJ m - 2
Trapped oil
Fig. 3
Schematic representation of oil trapped on metal surface
INT.J.ADHESION AND ADHESIVES JULY 1986
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Table 3. Tg (°C) for filled and unfilled epoxy resins as e function of oil content and cross-linking time
were studied by differential scanning calorimetry (DSC) on a Mettler TA 3000. After rapid cooling to -50°C, the enthalpy difference between the resin samples and a standard was measured at a heating rate of 10°C min -l. The graphs of enthalp~l change v s temperature show a clear change in gradient at Tg. Table 3 summarizes the results for the unfilled and filled resins containing various percentages by weight of the oil ,(added by simple physical pre-mixing) and after different periods of cross-linking at 210°C. It should perhaps be noted that the Tg of the oil ASTM 3 was found to be -72°C. Tg during cross-linking is quite seriously reduced by the oil content and, indeed, the more oil is present the more this effect is marked. Fig. 4 shows the evolution of Tg for the filled resin. Fewer results were obtained for the unfilled resin but the trend is much the same. It is perhaps noteworthy that the same final Tg of approximately 115°C was found irrespective of resin filler content. There is thus an undeniable change in cross-linking kinetics due to the presence of oil. This could be due either to absorption of the oil by the resin or by evaporation during the cross-linking reaction. The presence of oil may inhibit cross-linking, plasticize the bulk of the polymer, or both.
Cross-linking time (mins)
Percentage of ASTM 3 by weight 0
1
5
10
98 115 107 119
48 105 115 121
42 74 110 114
33 48 74 115
95 97 110 117
-----
57 103 111 107
Filledepoxy 5 15 30 60
Unfilled epoxy 5 15 30 60
10% oil a loss of 3%, the filled resin with 10% oil a loss of 1% and the oil alone c. 36%. Comparison of these figures leads to two essential comments: • the proportion of oil eliminated by evaporation in the case of the unfilled resin is virtually the same as that of the pure oil alone; whereas • by contrast, the resin filled with CaCO3 is more efficient at retaining (absorbing ?) the oil, with only about a third of the quantity of oil potentially lost by evaporation disappearing during 1 hour at 210°C.
Thermogravimetric analysis
In order to understand the process of oil elimination taking place during cross-linking, a simple thermogravimetric analysis was undertaken. The weight loss during heating at 210°C for an hour was recorded using a thermobalance. Five types of sample were investigated: the resin both unfilled and filled, the resin both unfilled and filled containing 10% by weight of oil ASTM 3, and the oil alone. The results are summarized in Fig. 5 where percentage weight is shown as a function of time of heating. After an hour, both pure resins showed zero weight loss, the unfilled resin with
It should be noted that the above figures refer to the specific geometry of the test samples (discs of 10 mm diameter and about 2 mm thickness). Any change in volume/surface area could clearly modify the data. Discussion and c o n c l u s i o n
It has been shown that two mechanisms are capable of intervening in the elimination of oil contaminants at the surface of steel members during bonding. In the
100
o.. 50
~
O
~
• Filled epoxy
O Filledepoxy + 10% oil
I
I
I
5
15
30 Cross-linkingtime (rain)
Fig. 4 Evolutionof Tgfor the filled epoxyas a functionof cross-linkingtime at 210°C and oil content
148
INT.J.ADHESIONAND ADHESIVES JULY 1986
t
60
40 -
~
30--
0
°
o
20O Oil Zk Unfilled epoxy + 10% oil 13 Filled epoxy + 10% oil • Both epoxies pure
..--.--------A '13 4 F - ~
5
15
60
30 Cross-linking time (rain)
Fig. 5
Percentage weight loss of oil, pure resins and oil-containing resins (10%) as a function of cross-linking time at 210°C
case of an apolar oil, thermodynamic displacement of the contaminant on the metal surface by the resin will take place as long as this is physically or geometrically possible, ie, if the substrate is sufficiently smooth. This constitutes the first step to assuring a good contact between the metal to be bonded and the adhesive. Nevertheless, given the rough nature of metal surfaces containing pores and asperities, the possibility cannot be excluded of a certain proportion of the oil being trapped mechanically, the adhesive simply covering the contaminated patches. This mechanical trapping has indeed even been observed on glass, which constitutes a very smooth surface. Its importance will therefore be much more marked for an inhomogeneous surface such as steel or zinc-coated steel. The second mechanism contributing to the elimination of the oil is the 'digestion' performed by the adhesive. This 'digestion' takes place by two processes. In the case of an unfilled resin, the oil would seem to be absorbed such that it escapes from the metal/resin interface by diffusion and then a relatively large fraction evaporates on reaching a free surface. In the case of a resin filled with CaCO3, although some oil would seem to disappear by evaporation, a considerable proportion is retained, at least during the time necessary for cross-linking of the adhesive. The obvious assumption to make is that the filler adsorbs oil on its surface. This, however, remains to be investigated. Clearly the relative contribution of each of these mechanisms, displacement and 'digestion', will depend to some extent on the morphology of the substrate surface. A smooth substrata will tend to favour displacement and a rough one will tend to increase the importance of the role played by 'digestion' of the contaminant. In the subdivision of this second process into evaporation and absorption, it must not be overlooked that the present experiments have considered samples of relatively large free surface area thus
favouring any evaporation of oil from the bulk. However, in practice, metal-to-metal bonding will severely reduce free surface area and the role played by the filler and the absorption of the oil will presumably surpass that of evaporation. Although the actual mechanism of absorption of the contaminant is not fully understood, it is of interest to note that the addition of a filler to the adhesive not only improves its rheological properties for industrial use, but also improves the absorbative powers. The second part of this study will propose a model to explain the relative importance of these aspects. References 1
Twiss, S.B. Appl Polym Symposia 3 (1966) p 455
2
Graham, J.A. Mach Design 8 (1977) p 163
3
Girifalco, L.A. and Good, R.J. J Phys Chem 61 (1957) p 904
4
Fowkes, F.M. Ind and Eng Chem 56 No 12 (1964) p 40
5
Dann, J.R. J Coil Interface Sci 32 No 2 (1970) p 302 and 321
6
Schultz, J, andGent~A.N, JChimPhys70No5(1973) p708
7
Neumann, A.W. and Good, R.J. "Surface and Colloid Science, volume 11 edited by R.J. Good and R.R. Stromberg (Plenum Press, New York and London, 1979) p 47
8
Shanahan, M.E.R., Cazeneuva, C., Carra, A. and Schultz, J. J Chim Phys 79 No 3 (1982) p241
9
Gledhill, R.A. and Kinloch, A.J. J Adhesion 6 (1974) p 315
Authors
The authors are with the Centre de Recherches sur la Physico-Chimie des Surfaces Solides and the Laboratoire de Recherches sur la Physico-Chimie des Interfaces de l'Ecole Nationale Suptrieure de Chimie de Mulhouse. Inquiries should be directed to Professor Schultz or Dr Shanahan at Centre National de la Recherche Scientifique, 24 avenue du Prtsident Kennedy, 68200 Mulhouse, France.
INT.J.ADHESION A N D ADHESIVES JULY 1 9 8 6
149
Oil-accommodating adhesives are rapidly becoming important in the automotive industry. The ability of certain polymer resins to produce durable metal-metal bonds without previous cleaning of the oil contaminants from the substrate marks a notable advance in adhesives technology. Nevertheless, in certain circumstances, the presolidification process required in production-line conditions leaves the bonding prone to failure under high humidity conditions. This suggests an inadequate metal-adhesive contact, probably due to inefficient oil elimination. The present study attempts to elucidate the essential methods by which surface oil is eradicated; a necessary step to good bonding.
Key words: adhesives; sheet steel; surface contamination; oil; surface free energy; displacement; absorption
Since the introduction of structural adhesives in the automotive industry, the question of bonding steel sheet has posed problems due to the inevitable surface pollution by the oils and greases present in factory conditions. The importance of this aspect has been recognized for about 20 years: to cite an example, procedures for bonding car bonnet and boot-lid reinforcing stays were already in existence in 19661. Several advantages appear when adhesive bonding is employed instead of mechanical attachment. It has been shown that a bonded structure is 30 to 40% stronger than the equivalent spot-welded assembly because of the more even stress distribution. In addition, the use of adhesives leads to: • the absence of corrosion at welds; • the lack of need to render the surface aesthetically agreeable after weld deformation; and • the waterproofing of metal-metal joints. In industry several types of oil-accommodating adhesive have been tested, in particular vinyl plastisols, epoxies and acrylics. At the present time, singlecomponent epoxy resins seem to offer the best overall
characteristics for use on oil-contaminated steel sheet. Nevertheless, the operational bonding conditions usually require a temperature above 100°C. Since curing generally occurs on the production line only at the paint-drying stage, some pre-curing process must be employed to maintain the metallic components in contact. The favoured process involves the rapid presolidification of the adhesive at certain judicious points of the assembly in question to hold it together. This method employs local heating at up to 260°C for a maximum period of 5 seconds. Although the performance of such pre-solidified joints, whether or not the process be followed by a classic cure cycle, is acceptable in general, it would seem that presolidification leaves the joints prone to degradation due to humidity. Since the same adhesive systems show little drop in humid strength when cured classically, it would seem that the pre-solidification process is not conducive to a suitable 'digestion' or displacement of the surface oil contaminant necessary to obtain good adhesion. The objective of the present study is thus to try to elucidate the physico-chemical phenomena occurring
0 1 4 3 - 7 4 9 6 / 8 6 / 0 3 0 1 4 5 - 0 5 $03.00 © 1986 Butterworth Et Co (Publishers) Ltd INT.J.ADHESION AND ADHESIVES JULY VOL.6 NO.3 1986
145
at the metal/adhesive interface and, in particular, to understand what becomes of the contaminant layer. Two essential processes leading to the elimination of the oil at the metal/adhesive interface can be envisaged:
Equation (1) can then be rewritten:
• the displacement of the oil by the adhesive; and • the absorption of the oil by the adhesive 2.
A series of measurements of 0 for different liquids was thus effected on the four metal surfaces of interest: steel and zinc-coated steel, both vapour (methylethylketone) degreased and lightly regreased with oil ASTM 3. A graph of cos 0 v s (~'bl/E/~'L will give a gradient of 2 (~'SE~1/2 for apolar liquids. An example, that of vapour degreased zinc-coated steel, is given in Fig. I. The points on the right-hand side of the graph correspond to the primarily apolar liquids diiodomethane and 1-bromonaphthalene. The points corresponding to the other liquids (water, formamide and ethylene glycol) are above the line of gradient 2 (ys~ 1/2 due to polar interactions, IsPL. However, too much importance should not be attached to these points since polar interactions can depend very strongly on the type of liquid. This summarizes the basis of the estimation of the values of yO for the four metal surfaces of this study. The values obtained are to be fdund in Table 1. It is noteworthy that these values are relatively low and similar. In the case of the degreased surfaces, this could well be a reflection of the fact that some contaminant remains after treatment. The polar interaction between a solid and a liquid is a subject of much discussion and conjecture as to the direct relationship between itself and the polar components of surface free energy of the two phases in contact. For this reason no attempt will be made here to reduce I~L to more fundamental parameters. Instead, for the interactions between the metals in question and
Either or both of these processes may take place and, as will be seen below, both in fact would seem to play a role. Displacement
Clearly any physical displacement of the oil on the metal surface is a thermodynamic process involving surface energetics. As an initial step in this study, it was therefore decided to obtain surface free energy data of the materials to be considered. The metals studied were a model steel sheet and a zinc-coated steel sheet. Oil contamination was modelled using a mineral oil of standard ASTM 3 at 3 g m -2 concentration, attained by dipping in a heptane solution of the oil, draining and evaporation of the solvent. The adhesives used were a model epoxy resin based on bisphenol A for the displacement study and a formulated version of this material containing 6% dicyandiamide, both with and without a CaCO 3 filler (70 pph), for the absorption work. Measurement of metal surface free energy characteristics
It is well known that the elucidation of surface free energy data of pure metals is a delicate and difficult task. However, as far as the present study is concerned, it is sufficient to obtain data referring to the metals in service conditions. A method based on wetting of the solid surface by known liquids was employed. In this method, the contact angles of a series of liquids on the solid in question are measured. Based on the concepts of Girifalco and Good 3, Fowkes 4 and Dann 5, an equation relating contact angle, 0, to various surface free energy terms can be derived6: cos O = 2 (yDp)l/2 (7~ 1/2 ~_IPL YL 7L
rre YL
1
2
(rs 1/2
+
YL
_ 1
(2)
7L
1
~0-
(1)
where 7Dp and 7 D are respectively the dispersive components of the surface free energies of the pure solid, Ysp, and the liquid, 7L; ISPLis the polar interaction solidAiquid; and ne is the equilibrium spreading pressure of the liquid on the solid. Now it is just this last term, he, which complicates the study of high energy solids such as metals, since it represents the effective reduction of the surface free energy of the solid due to adsorption of the liquid's vapour environment; ie, zre = 7sa - ?s where Ys is the surface free energy of the solid polluted by vapour adsorption. However, it is likely that this modification will be a fairly constant term as far as dispersive interactions are concerned when treating organic liquids. Since both the liquids for contact angle measurement and the resin to be studied are essentially organic, the simplifying assumption will be made that n e does not occur in Equation (1) but that 7SDpshould be replaced by yD the dispersive componer, t of the surface free energy of the metal contaminated by organic vapour.
146
cos 0
tNT.J.ADHESION AND ADHESIVES JULY 1986
~D = 42 mJ m - 2
-1
I 0
0.2
0,1 k/-'~LT~ (m J-½ m) 7L
Fig. 1 Cos 0 v s (7LD)1/2'yL"1 for vapour degreased zinc-coated steel
Table 1. Metal and metal/resin surface free energy characteristics
Steel Zinc-coated steel Regreased steel Regreased Zn-steel Estimated errors +_ 1 mJ m -2
yD (mJ m -2)
/PR (mJ m -2)
44 42 42 41
4 6 4 5
the resin, I~R will be evaluated directly. For this, the equilibrium contact angles of the resin on the solid surfaces were measured. The term yDis known from above and the surface free energy of the resin, YR, and its dispersive component, yD, are evaluated below. Equation (2) then applies with the subscript L replaced by R and ~R can be calculated directly. The values obtained can be found in Table 1. Measurement of resin and oil surface free energy characteristics
a
Since both the model resin and ASTM 3 oil are liquids at room temperature, evaluation of surface free energy, or surface tension, 7R and 7ti respectively, was effected directly by the Wilhelmy plate method 7 using a tensiometer. The measurement of contact angles of the liquids on a reference apolar surface, polytetrafluoroethylene (PTFE), and use of Equation (2) simplified by the absence of a polar term lead to values of the dispersive components of surface free energy, yD and yD. The polar terms yv and y~ are simply obtained by difference. The results are summarized in Table 2.
Having established the various surface free energy characteristics of the metals, oil and resin in question, it is now possible to consider the condition of displacement of the liquid oil layer on the metal surface by the resin using a thermodynamic argument8. Assuming a drop of resin to maintain a constant geometrical form, the difference in free energy between the state in which the resin rests on a thin layer of oil on the metal surface and that in which the resin displaces the oil can be assessed. The two states are schematically shown in Fig. 2. This free energy difference, AF, can be written: = Z . (YSR -- YRH -- YSH)
Yij = Yi + Yj - 2 ( y D . y D ) U 2 _ i P
b
- 3 and - 5 mJ m -2 respectively for the steel and zinccoated steel substrates. Although the magnitudes of AF are relatively small, it can nevertheless be concluded that displacement of an essentially apolar oil from both the steel and zinc-coated steel surfaces is likely to take place on thermodynamic grounds. However, this argument is based purely on interfacial free energy considerations. If the oil is physically, or geometrically, trapped in pores or asperities in the metal surface the resin will be unable to displace it. It is for this reason that the following section involving direct absorption of the oil by the adhesive must be considered. Absorption
(4)
where the superscripts D and P have their usual meaning. The polarity of the oil being negligible, use of Equation (4) in Equation (3) leads to: AF
H
(3)
where the terms 7ij represent the interfacial free energies between solid, resin and oil, and Z is the surface area of the underside of the drop of resin. A similar equation was obtained by Gledhill and Kinloch 9. The terms 7ij may be developed using the work of Fowkes4:
--
m
Fig. 2 Possible final states of epoxy on metal substrate: (a) with thin intervening oil film; and (b) oil displaced by resin
Condition of displacement
AF
H
As evoked above, it is quite possible that at least some of the oil on the metal surface is physically trapped, as shown schematically in Fig. 3. In this case, for a good adhesive/metal contact, the former must be able to absorb or 'digest' the contaminant. In order to examine this aspect, a quantitative study of the variation of the glass transition temperature, Tg, of the resins as a function of oil content and cross-linking time at 210°C was undertaken. Differential scanning calorimetry
--~ [(YB 1/2 -- (YH) 1/21 [(YH) 1/2 --
(rRD)~/2I - / ~ R
(5)
The glass transition temperatures, Tg, of both the unfilled and filled resins containing dicyandiamide
If AF is negative, displacement of the oil by the resin is favoured. Use of the data for the various surfaces from Tables 1 and 2 leads to values of A F / G of about Table 2. Epoxy resin and oil surface free energy characteristics
Epoxy Oil ASTM 3
yD (mJ m -2)
y~ (mJ m -2)
YL (mJ m -2)
28 32
10 <<1
38 -32
Estimated errors _+ 1 mJ m - 2
Trapped oil
Fig. 3
Schematic representation of oil trapped on metal surface
INT.J.ADHESION AND ADHESIVES JULY 1986
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Table 3. Tg (°C) for filled and unfilled epoxy resins as e function of oil content and cross-linking time
were studied by differential scanning calorimetry (DSC) on a Mettler TA 3000. After rapid cooling to -50°C, the enthalpy difference between the resin samples and a standard was measured at a heating rate of 10°C min -l. The graphs of enthalp~l change v s temperature show a clear change in gradient at Tg. Table 3 summarizes the results for the unfilled and filled resins containing various percentages by weight of the oil ,(added by simple physical pre-mixing) and after different periods of cross-linking at 210°C. It should perhaps be noted that the Tg of the oil ASTM 3 was found to be -72°C. Tg during cross-linking is quite seriously reduced by the oil content and, indeed, the more oil is present the more this effect is marked. Fig. 4 shows the evolution of Tg for the filled resin. Fewer results were obtained for the unfilled resin but the trend is much the same. It is perhaps noteworthy that the same final Tg of approximately 115°C was found irrespective of resin filler content. There is thus an undeniable change in cross-linking kinetics due to the presence of oil. This could be due either to absorption of the oil by the resin or by evaporation during the cross-linking reaction. The presence of oil may inhibit cross-linking, plasticize the bulk of the polymer, or both.
Cross-linking time (mins)
Percentage of ASTM 3 by weight 0
1
5
10
98 115 107 119
48 105 115 121
42 74 110 114
33 48 74 115
95 97 110 117
-----
57 103 111 107
Filledepoxy 5 15 30 60
Unfilled epoxy 5 15 30 60
10% oil a loss of 3%, the filled resin with 10% oil a loss of 1% and the oil alone c. 36%. Comparison of these figures leads to two essential comments: • the proportion of oil eliminated by evaporation in the case of the unfilled resin is virtually the same as that of the pure oil alone; whereas • by contrast, the resin filled with CaCO3 is more efficient at retaining (absorbing ?) the oil, with only about a third of the quantity of oil potentially lost by evaporation disappearing during 1 hour at 210°C.
Thermogravimetric analysis
In order to understand the process of oil elimination taking place during cross-linking, a simple thermogravimetric analysis was undertaken. The weight loss during heating at 210°C for an hour was recorded using a thermobalance. Five types of sample were investigated: the resin both unfilled and filled, the resin both unfilled and filled containing 10% by weight of oil ASTM 3, and the oil alone. The results are summarized in Fig. 5 where percentage weight is shown as a function of time of heating. After an hour, both pure resins showed zero weight loss, the unfilled resin with
It should be noted that the above figures refer to the specific geometry of the test samples (discs of 10 mm diameter and about 2 mm thickness). Any change in volume/surface area could clearly modify the data. Discussion and c o n c l u s i o n
It has been shown that two mechanisms are capable of intervening in the elimination of oil contaminants at the surface of steel members during bonding. In the
100
o.. 50
~
O
~
• Filled epoxy
O Filledepoxy + 10% oil
I
I
I
5
15
30 Cross-linkingtime (rain)
Fig. 4 Evolutionof Tgfor the filled epoxyas a functionof cross-linkingtime at 210°C and oil content
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INT.J.ADHESIONAND ADHESIVES JULY 1986
t
60
40 -
~
30--
0
°
o
20O Oil Zk Unfilled epoxy + 10% oil 13 Filled epoxy + 10% oil • Both epoxies pure
..--.--------A '13 4 F - ~
5
15
60
30 Cross-linking time (rain)
Fig. 5
Percentage weight loss of oil, pure resins and oil-containing resins (10%) as a function of cross-linking time at 210°C
case of an apolar oil, thermodynamic displacement of the contaminant on the metal surface by the resin will take place as long as this is physically or geometrically possible, ie, if the substrate is sufficiently smooth. This constitutes the first step to assuring a good contact between the metal to be bonded and the adhesive. Nevertheless, given the rough nature of metal surfaces containing pores and asperities, the possibility cannot be excluded of a certain proportion of the oil being trapped mechanically, the adhesive simply covering the contaminated patches. This mechanical trapping has indeed even been observed on glass, which constitutes a very smooth surface. Its importance will therefore be much more marked for an inhomogeneous surface such as steel or zinc-coated steel. The second mechanism contributing to the elimination of the oil is the 'digestion' performed by the adhesive. This 'digestion' takes place by two processes. In the case of an unfilled resin, the oil would seem to be absorbed such that it escapes from the metal/resin interface by diffusion and then a relatively large fraction evaporates on reaching a free surface. In the case of a resin filled with CaCO3, although some oil would seem to disappear by evaporation, a considerable proportion is retained, at least during the time necessary for cross-linking of the adhesive. The obvious assumption to make is that the filler adsorbs oil on its surface. This, however, remains to be investigated. Clearly the relative contribution of each of these mechanisms, displacement and 'digestion', will depend to some extent on the morphology of the substrate surface. A smooth substrata will tend to favour displacement and a rough one will tend to increase the importance of the role played by 'digestion' of the contaminant. In the subdivision of this second process into evaporation and absorption, it must not be overlooked that the present experiments have considered samples of relatively large free surface area thus
favouring any evaporation of oil from the bulk. However, in practice, metal-to-metal bonding will severely reduce free surface area and the role played by the filler and the absorption of the oil will presumably surpass that of evaporation. Although the actual mechanism of absorption of the contaminant is not fully understood, it is of interest to note that the addition of a filler to the adhesive not only improves its rheological properties for industrial use, but also improves the absorbative powers. The second part of this study will propose a model to explain the relative importance of these aspects. References 1
Twiss, S.B. Appl Polym Symposia 3 (1966) p 455
2
Graham, J.A. Mach Design 8 (1977) p 163
3
Girifalco, L.A. and Good, R.J. J Phys Chem 61 (1957) p 904
4
Fowkes, F.M. Ind and Eng Chem 56 No 12 (1964) p 40
5
Dann, J.R. J Coil Interface Sci 32 No 2 (1970) p 302 and 321
6
Schultz, J, andGent~A.N, JChimPhys70No5(1973) p708
7
Neumann, A.W. and Good, R.J. "Surface and Colloid Science, volume 11 edited by R.J. Good and R.R. Stromberg (Plenum Press, New York and London, 1979) p 47
8
Shanahan, M.E.R., Cazeneuva, C., Carra, A. and Schultz, J. J Chim Phys 79 No 3 (1982) p241
9
Gledhill, R.A. and Kinloch, A.J. J Adhesion 6 (1974) p 315
Authors
The authors are with the Centre de Recherches sur la Physico-Chimie des Surfaces Solides and the Laboratoire de Recherches sur la Physico-Chimie des Interfaces de l'Ecole Nationale Suptrieure de Chimie de Mulhouse. Inquiries should be directed to Professor Schultz or Dr Shanahan at Centre National de la Recherche Scientifique, 24 avenue du Prtsident Kennedy, 68200 Mulhouse, France.
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