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Thin-layer technology in organic coatings
PROGRESS INORGANIC COATINGS Progress in Organic Coatings 28 ( 1996) 3-7
Thin-layer technology in organic coatings W. Funke Forschungsinstirurfiir Pigmeme und Lacke e. V., Allmundring 37, D-70569 Stuttgart, Germuny
Received 2 November 1994; accepted I5 June 1995
Abstract Organic coatings of a thickness between a few micrometers and some tenths of nanometers are not very common in paint and coatings technology. Thin and very thin layers of polymeric binders, however, may increase wet adhesion of organic coating systems by orders of magnitude. This increase is explained by cooperative and conformative mechanisms ofbonding. Though good wetadhesionis a pre-requirement for protection against corrosion, it does not always improve also the resistance against underrusting. This apparent contradiction may be explained by molecular chain segments being too spacious to adsorb in gaps and fissures of the metal surface. Keywords: Adhesion; Thin layers; Corrosion
1. Introduction The film thickness of industrial organic coatings varies from millimeters for protecting metals against heavily corrosive environments up to a few microns in case of washprimers or can coatings. It is well known that the properties of some types of organic coatings depend on the film thickness. Examples are systems which dry or cure oxidatively, by mere evaporation of solvents or by reaction with water. Apart from these examples, experimental evidence developed over many years [ 1,2] has shown that coating properties in a thin layer adjacent to the substrate differ from those of the bulk film. This difference implies that composition, structure and conformation of the macromolecules at the substrate surface should be different from those of the bulk film. Obviously the substrate surface influences these parameters considerably and this has significant consequences for the most important property of organic coatings, adhesion. Not very much interest and attention has been focused on very thin coating layers in coatings science and technology in spite of their importance for adhesion. One reason for this was probably the practical experience that the film thickness of base coats should be high enough to cover the roughness of the substrate surface sufficiently well, including also protuberances, in order to secure satisfactory protection. On the other hand, it is surprising and by no means self-evident that, despite having very low film thicknesses, coatings and electrical insulating coatings can perform well in protecting the substrate even against aggressive and corrosive environ0300-9440/96/$15.00 .%SD10300-9440(95
Electrochem.
Barrier Influence
0 1996 Elsevier Science S.A. All rights reserved )00583-Z
Influence
Good Wet Adhesion
’
cooperative
conformative bonding
Thin Film Technology Fig.
1.Protection mechanisms against corrosion
ments. Meanwhile very thin polymer layers have become important in microelectronics, including protective purposes. It is well known that barrier and electrochemical mechanisms in coatings are the basic elements for protecting metals against atmospheric corrosion. Considering environmental problems, there are good reasons to assume that protection by the barrier mechanism may become preponderant in future. As the resistance of organic coatings against the delaminating action of water-wet adhesion-is an important factor of the barrier mechanism, it may be expected, that by combining both the cooperative and conformative effects which may be attained with very thin coating layers, one improves corrosion protection substantially (Fig. 1) . In order to understand why properties of very thin organic layers differ from those of thick films composed of the same material, it is useful to compare interfaces between metal surfaces and organic coating films with interfaces between
4
W. Funke / Propss
in Orgunic Coatings 28 (I 996) 3-7
inorganic oxidic pigments and the binder matrix. Both substrates have striking physical, chemical and structural similarities and pigment/binder interfaces have been extensively studied over many years to understand and improve pigment dispersion and dispersion stability. To elucidate the interaction between pigment surfaces and macromolecules of the binder, adsorption experiments are very common. However, the practical relevancy and usefulness of results from such experiments have been frequently questioned, because the concentration of binder molecules for adsorption experiments is much lower than is usual in paints. As very low concentrations of binder solutions are also required for preparing thin organic layers with a thickness comparable to or somewhat above those of adsorption layers at pigment surfaces, it is not surprising that special adhesion effects are observed. Accordingly, significant improvements of wet adhesion of coating systems had been reported when aluminum or steel substrates were pretreated with very dilute binder solutions, before after drying a subsequent layer of normal film thickness was applied [ 3,4]. Interfacial interactions of organic coatings depend on chemical, physical and structural properties of both the metal surface and the lower surface of the coating film adjacent to the substrate. These six major variables explain the complex nature of adhesion of organic coatings as well as of adhesives. Accordingly both the substrate surface and the immediately adjacent interfacial polymer layer have to be considered for improving adhesion and for explaining deficiencies of adhesion. Of course, adhesion is closely related to the protection of metals against corrosion: if coatings delaminate from a steel surface in a corrosive environment the protective property fails. As corrosion is usually electrochemical, it needs water molecules to allow the transport of ions between local corrosion electrodes. Therefore adhesion of organic coatings under dry conditions is not very significant for corrosive conditions. Accordingly it was supposed [5] that adhesion under humid or wet conditions is more meaningful for protection against corrosion than dry adhesion and that this wet adhesion should be an important factor for the barrier property of organic coatings. Wet adhesion expresses the ability of coating to resist the interfering penetrating action of water molecules at interfacial or adhesion bonds. Unfortunately in most practical cases it is not clear which type of bonding is operating and to what extent it contributes to adhesion. Covalent bonds in the coating/substrate interface would certainly provide optimal wet adhesion, if they are sufficiently stable against hydrolysis. In some, mostly special cases, the existence of covalent bonds has been proved experimentally, but most metal-organic bonds are unstable against water. It is generally agreed that, besides mechanical interlocking, polar interactions, especially acid-base interactions and hydrogen bonding, are responsible for adhesion. Again these bonds are susceptible to the disrupting action of water.
///////I/////// normal
/
organic
coating
(cont.
solution)
\
///////I/// Cooperation
Conformation
(high TQ)
(dilut. solution)
Cooperation
Preorientation
and Confamution
(dilute solution,
(dilute solution,
high Tgi
b
I?
monolayer)
d :
J-7 I
I’
Preorientation
and
Cooperation ////////////)/
(monolayer+crosslinkIng)
Fig. 2. Structure and bonding of adhesion layers on metal substrates
There are two possible ways to counteract bond rupture by water molecules: forcing the bonds to cooperate with each other and choosing application conditions which, by a suitable chain conformation of the binder molecules, allow a maximum number of bonding groups per molecule to interact with the metal substrate. The role of such cooperative and conformative effects for improving wet adhesion is indicated schematically in Fig. 1, together with the possibility of preorientation of adhesive macromolecules (Fig. 2).
2. The cooperative mechanism Acid-base interactions and hydrogen bonds have a dynamic nature, which depends, respectively, on the degree of dissociation and the water solubility of the salt on the hydrogen bonding in liquid water. These dynamic interactions must be stabilized to improve wet adhesion. Stabilization is possible by attaching bonding groups to rigid polymer backbone chains, which force neighboring bonds to cooperate (Fig. 3). It must be observed, however, that chain rigidity must be introduced after, and not before, the adsorption process, because stiff macromolecules cannot sufficiently adapt themselves to the contours of the metal surface. Crosslinking of binders and heating above the glass transition point are means for immobilizing chain segments at the metal surface. An indication for the effect of rigidity of organic layers on adhesion is that, in many cases, wet adhesion on metal surfaces is related to the glass-transition temperature, Tg, of the organic coating, especially the Tg in the presence of water [ 6,7]. It can be shown that some thin adhesion layers, which have been prepared from low-molecular resins with a high T,
W. Funke /Progress
in Orgunic Coatings 28 (1996) 3-7
Cooperative
Non-cooperative
Dynamic equlllbrum between bonding and non-bonding chain ends Mobile chain segments at the Interface
Stationary non-dynamlc bonds lmmobk chain segments at the Interface
Fig. 3. Adhesion bonds at the coating/support interface. Arrows indicate the continuation of chain segments into the bulk of the coating. I
I
I
Top Layer
Adhesion
( ca. 45 ,um)
(ca. 200 nm)
-a
Layer
___ -____
Wet Adhesion
( W.4
I
II<1
1
’
Fig. 4. Influence of cooperatively bonded adhesion layers on wet adhesion of the top layer. * Delamination between adhesion and top layer.
after curing, significantly improved wet adhesion of alkyd melamine and epoxy coatings of normal film thickness (Fig. 4). The molecules of the thin adhesion layers have a low molecular mass before film formation, which together with the increased temperature of film formation ensures an optimal penetration in and a contact with the surface structure of the metal. The needed rigidity is obtained by the crosslinking reaction. In coating systems of this type, besides wet adhesion, also the tendency of underrusting could be distinctly improved.
3. The conformative
mechanism
The concentration of binder molecules in paints is usually higher by sizes of magnitude than the concentration used in adsorption experiments. Due to its high concentration polymeric binder molecules, which after application happen to be at the surface of the metal substrate, have to compete with their neighbors for adsorption sites. As a consequence of this competition only a small fraction of the adhesive groups of a macromolecule has actually the chance to interact with the support. On the other hand, at excessive dilution, macromolecules at the support surface have enough sites available for adsorption, but the coverage of the metal surface is insufficient (Fig. 5). Therefore some optimum concentration exists,
5
at which a maximal number of adhesive groups per macromolecule located at the interface interacts with the substrate. Another interfacial aspect related to wet adhesion is the chemical nature of adhesive groups. Surfaces of metals, such as steel or aluminum, are hydrophilic due to their natural, very thin oxide layers. To secure good interaction with such metal surfaces polar groups are needed as structural elements of the binder molecules. The question is, how many of such groups are required in a binder molecule and whether these groups are also beneficial to the properties of the bulk of the coating film. As to polarity, one could think of macromolecules, such as polyvinylalcohol or polyacrylic acid. However this choice seems to be unreasonable, because normal base coats of these macromolecules would yield a very hydrophilic, water-sensitive layer. By hydrogen bonding between polymer groups in the bulk of such films one may expect the mechanical strength to be improved, but in the presence of water or high humidity these bonds are disrupted and the coating system is easily delaminated. This raises the question, whether it is reasonable to use the same macromolecules for achieving good, water-resistant bulk properties of coating films and simultaneously expect optimal adhesion at the substrate by their polar nature. From this consideration it follows that, for an optimal adhesion, macromolecules should have a structure different from macromolecules with optimal properties of the bulk film. Adhesion mechanisms are not limited to the interface only but are influenced by the structure and mobility of the adjacent chain segments [ 11. However the adhesion layer probably extends not far beyond the diameter of a coiled macromolecule. It is advisable, therefore, to limit the film thickness of the adhesion layer to some ten nanometers. For preparing such thin layers, a concentration of binder molecules should be chosen which is much below the usual concentration in paints. To test this assumption, a series of polyacids was applied to the usual steel panels after degreasing and cleaning, and the dependence of wet adhesion on the concentration of the dilute aqueous solutions of polyacrylic acid was measured. After drying, this adhesion layer was covered with an unpigmented top coat of - 40-50 pm film thickness (alkyd-melamine or epoxy resin). Quite interestingly wet adhesion of these coating systems could be improved by orders of magnitude and, according to what was expected (Fig. 5), a distinct maximum of wet
1
substrate
Incomplete coverage (very low concentration)
1 1
substrate
1
complete coverage, but less ordered than on water
1
substrate
)
overcoverage (high concentration, as in practical coatings)
(low concentration)
Fig. 5. Influence of the concentration of the solution on the conformation binder molecules and their coverage of the substrate.
of
6
W. Funke / Prop-ess m Orgmic
delamination
time (h)
the adsorbed macromolecules exist. In some respect this is comparable with the situation of phosphate layers on steel surfaces, which also do not perfectly cover the substrate and therefore require post-treatments, such as chromic acid rinse. On the other hand, adsorbed layers should be stabilized after their application to develop better intermolecular “cooperation”. Both ways are presently being studied. Still further improvement of wet adhesion can be expected by a preorientation and cooperation in the form of adhesive monolayers (Fig. 2). Of course, for this purpose, special macromolecules have to be synthesized to meet the requirements for wet adhesion, and the roughness of usual metal surfaces may cause local breaks in the monolayer.
10000
1000
100
10
0
0.1
0.2 1 2 concentration
Fig. 6. Wet adhesion and concentration top coats.
2.1 2.2 PAA (m%)
2.3
2.5
of PAA with alkyd/melamine
resin
adhesion appeared in the case of polyacrylic acid at a layer thickness corresponding to a concentration of the aqueous solution of l-2% by mass (Fig. 6). A similar maximum of wet adhesion was found with other polyacids, e.g. polymethacrylic acid and copolymers of acrylic acids and of maleic acid. Obviously within this range of concentration the number of bonding groups of a macromolecule at the substrate surface is optimal. The chain conformation for this optimal interaction is induced by the rigid substrate during the adsorption progress.
4. Bonding mechanism
of polyacids to steel surfaces
The remarkably strong improvement of wet adhesion of coating systems by very thin layers of polyacids can be explained by polar interaction, hydrogen bonding or saltcomplex formation with iron ions formed at the steel surface. It has been shown that salt formation is the essential factor for this improved wet adhesion [ 81. When neutralized polyacids or polyacids in organic solvents were applied, where there is no ionic dissociation, no effect or wet adhesion were found. Insolubility of the polyacids is achieved by iron ions in the highest state of oxidation, but it is still unexplained as to how these salt complexes interact with the steel surface for attaining such a remarkable improvement of wet adhesion. Moreover, it turned out that overcoating with aqueous systems impairs wet adhesion. Probably the molecular structure of the polyacid is not strong enough for a stabilizing cooperative interaction with the metal surface when applied at ambient temperature.
5. Combination mechanisms
Coatings 28 (1996) 3-7
6. Wet adhesion and protection of steel surfaces against corrosion It has been supposed that besides permeability to water, oxygen and ions, wet adhesion is a decisive parameter for protection of metal surfaces by organic coating systems [ 9,101. The idea underlying this assumption was that any bonds to the metal surface must be disrupted by water molecules before the respective iron atoms beneath this bond may be transformed to ions as a result of the electrochemical corrosion reaction. In this respect aluminum surfaces are much easier to protect because corrosion is not as significant as in the case of iron, and accordingly the wet adhesion is very efficiently improved by such polyacid layers [ 3,7]. Actually, improved wet adhesion by cooperative adhesion layers also improves protection. However, a direct relationship between wet adhesion of organic coatings and protection against corrosion has been questioned [ 11,121. As a matter of fact, adhesion layers of polyacids failed to improve protection if corrosion resistance was measured by the salt spray test (Fig. 7). A reasonable explanation for this disappointing protective ability of polyacid layers is that, due to the topological structure of the metal surface and the limited mobility of the chain segments of the polyacid as a consequence of their molecular size and adsorptional fixation at the metal surface, small gaps and fissures of a metal surface cannot be sufficiently covered by the polyacid. NHF (d) resp. V, *lOl
(mm/h)
of cooperative and conformative
As the coverage of metal surfaces by adhesive macromolecules is statistical, it has to be assumed that, even at the optimal concentration of the solution used for adsorption of the adhesion layer, many irregularities in the arrangement of
no pret.
0.1
0.5
Concentration
1 2 of PAS (mass%)
Fig. 7. Wet adhesion (NHF) and rate of underrusting coating/PAS-adhesion system on steel.
3
(Vu) of an epoxy resin
W. Funke /Progress
diameter
1-1
in Organic
1 nm
loop
HOH
9
Fig. 8. Molecular model for the accessability of surface ‘holes’ to the narrowest possible loop of a polyacrylic acid molecule
Coatrngs 28 (I 996) 3-7
I
tion, but large enough for the formation of an ion-transporting aqueous phase, corrosion occurs, beginning at normal or intentional coating defects and proceeding with underrusting of the adjacent intact area by cathodic delamination. How can we cope with this problem without relying on traditional pretreatments? One possible way would be to prepare extremely smooth surfaces. However, as we have to deal with molecular dimensions, this is very difficult to realize and also very expensive. Moreover, any mechanical anchoring of organic layers by a suitable roughness of the metal surface, which certainly contributes to adhesion, would be excluded. Two other ways seem to be more suitable: use of reactive molecules of low molecular mass, which after physical or chemical adsorption can be connected by crosslinking or improving the molecular adaption to the metal surface structure, by small reactive, mobile binder molecules, which may be crosslinked after adsorption, eventually assisted by elevated temperature. Of course, these proposals are rather sophisticated approaches to solve the problem of combined wet adhesion and protection against corrosion, but interfacial problems are too complicated to allow simple primitive solutions for substantial improvements.
Acknowledgements
Fig. 9. Underrusting
of coating systems with good wet adhesion
In connection with other coatings systems, this concept has been already proposed [ 13,141. However, it needs to be scaled down to molecular dimensions. Fig. 8 shows schematically, but with correct molecular dimensions, that gaps or fissures at the metal surface may be small enough to prevent to access and adsorption of chain segments of polyacrylic acid even in their most compact conformation. This would still allow a large number of water molecules to fill these ‘holes’ if the humidity is high enough. As a consequence of this consideration, polymer chain segments should be able to adapt themselves to the roughness of the substrate surface. Molecular modeling and design of the polymer could be useful for achieving this adaption. It may be assumed that the preferred adsorption sites for macromolecules are the protuberant parts of the metal surface (Fig. 9). The adsorption and reaction of each carboxyl group limits the mobility of the remaining chain segments. If the cavities of the metal surface are too small for further adsorp-
I thank Dr Roy W. Tess and the Division of Polymer Materials, Science and Engineering of the American Chemical Society very much for appreciating our scientific and technological work by the Roy W. Tess Award. The long standing support of the Bundesminister fur Wirtschaft, the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen e.V. (AlF) and the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
References [ I] [2] [ 31 [4]
Ch.A. Kumins and J. Roteman, J. Polym. Sri., PurrA, I (1963) 527. Ch.A. Kumins, J. Coat. Technd.. 52 (1980) 39. V.V. Arslanov and W. Funke, Prog. Org. Cour., 15 (1988) 365. W. Funke, Farbe +Jlack, 93 (1987) 721. [5] W. Funke, J. Oil Colour Chem. Assoc.. (1985) 229. [ 61 W. Funke, Proc. The Electrochemica[Society. Vol. 89-13, Pennington, NJ, USA, 1989, p. 121. [7] U. Christ, H. Haagen and W. Funke, XIX Fatipec Congr., Aachen, 1988, Congress Handbook Vol. III, p. 427. [ 81 J. Lorenz, Thesis, University of Stuttgart, Germany, 1994. [9] W. Funke, J. Coar. Tech&.. 55 (705) (1983) 31. [lo] P. Walker, Paint Tech&., 31 ( 1967) 22. [ 1 I] P. Walker, J. Oil Colour Chem. Assoc.. 68 (1985) 319. [ 121 R.A. Dickie, Prog. Org. Coat., 25 (1994) 3. [ 131 J. Huntsberger, J. faint Technof., 39 ( 1967) 199. [ 141 H. Leidheiser and W. Funke, J. Oil Colour Chem. Assoc., 70 (1987) 121.
Thin-layer technology in organic coatings W. Funke Forschungsinstirurfiir Pigmeme und Lacke e. V., Allmundring 37, D-70569 Stuttgart, Germuny
Received 2 November 1994; accepted I5 June 1995
Abstract Organic coatings of a thickness between a few micrometers and some tenths of nanometers are not very common in paint and coatings technology. Thin and very thin layers of polymeric binders, however, may increase wet adhesion of organic coating systems by orders of magnitude. This increase is explained by cooperative and conformative mechanisms ofbonding. Though good wetadhesionis a pre-requirement for protection against corrosion, it does not always improve also the resistance against underrusting. This apparent contradiction may be explained by molecular chain segments being too spacious to adsorb in gaps and fissures of the metal surface. Keywords: Adhesion; Thin layers; Corrosion
1. Introduction The film thickness of industrial organic coatings varies from millimeters for protecting metals against heavily corrosive environments up to a few microns in case of washprimers or can coatings. It is well known that the properties of some types of organic coatings depend on the film thickness. Examples are systems which dry or cure oxidatively, by mere evaporation of solvents or by reaction with water. Apart from these examples, experimental evidence developed over many years [ 1,2] has shown that coating properties in a thin layer adjacent to the substrate differ from those of the bulk film. This difference implies that composition, structure and conformation of the macromolecules at the substrate surface should be different from those of the bulk film. Obviously the substrate surface influences these parameters considerably and this has significant consequences for the most important property of organic coatings, adhesion. Not very much interest and attention has been focused on very thin coating layers in coatings science and technology in spite of their importance for adhesion. One reason for this was probably the practical experience that the film thickness of base coats should be high enough to cover the roughness of the substrate surface sufficiently well, including also protuberances, in order to secure satisfactory protection. On the other hand, it is surprising and by no means self-evident that, despite having very low film thicknesses, coatings and electrical insulating coatings can perform well in protecting the substrate even against aggressive and corrosive environ0300-9440/96/$15.00 .%SD10300-9440(95
Electrochem.
Barrier Influence
0 1996 Elsevier Science S.A. All rights reserved )00583-Z
Influence
Good Wet Adhesion
’
cooperative
conformative bonding
Thin Film Technology Fig.
1.Protection mechanisms against corrosion
ments. Meanwhile very thin polymer layers have become important in microelectronics, including protective purposes. It is well known that barrier and electrochemical mechanisms in coatings are the basic elements for protecting metals against atmospheric corrosion. Considering environmental problems, there are good reasons to assume that protection by the barrier mechanism may become preponderant in future. As the resistance of organic coatings against the delaminating action of water-wet adhesion-is an important factor of the barrier mechanism, it may be expected, that by combining both the cooperative and conformative effects which may be attained with very thin coating layers, one improves corrosion protection substantially (Fig. 1) . In order to understand why properties of very thin organic layers differ from those of thick films composed of the same material, it is useful to compare interfaces between metal surfaces and organic coating films with interfaces between
4
W. Funke / Propss
in Orgunic Coatings 28 (I 996) 3-7
inorganic oxidic pigments and the binder matrix. Both substrates have striking physical, chemical and structural similarities and pigment/binder interfaces have been extensively studied over many years to understand and improve pigment dispersion and dispersion stability. To elucidate the interaction between pigment surfaces and macromolecules of the binder, adsorption experiments are very common. However, the practical relevancy and usefulness of results from such experiments have been frequently questioned, because the concentration of binder molecules for adsorption experiments is much lower than is usual in paints. As very low concentrations of binder solutions are also required for preparing thin organic layers with a thickness comparable to or somewhat above those of adsorption layers at pigment surfaces, it is not surprising that special adhesion effects are observed. Accordingly, significant improvements of wet adhesion of coating systems had been reported when aluminum or steel substrates were pretreated with very dilute binder solutions, before after drying a subsequent layer of normal film thickness was applied [ 3,4]. Interfacial interactions of organic coatings depend on chemical, physical and structural properties of both the metal surface and the lower surface of the coating film adjacent to the substrate. These six major variables explain the complex nature of adhesion of organic coatings as well as of adhesives. Accordingly both the substrate surface and the immediately adjacent interfacial polymer layer have to be considered for improving adhesion and for explaining deficiencies of adhesion. Of course, adhesion is closely related to the protection of metals against corrosion: if coatings delaminate from a steel surface in a corrosive environment the protective property fails. As corrosion is usually electrochemical, it needs water molecules to allow the transport of ions between local corrosion electrodes. Therefore adhesion of organic coatings under dry conditions is not very significant for corrosive conditions. Accordingly it was supposed [5] that adhesion under humid or wet conditions is more meaningful for protection against corrosion than dry adhesion and that this wet adhesion should be an important factor for the barrier property of organic coatings. Wet adhesion expresses the ability of coating to resist the interfering penetrating action of water molecules at interfacial or adhesion bonds. Unfortunately in most practical cases it is not clear which type of bonding is operating and to what extent it contributes to adhesion. Covalent bonds in the coating/substrate interface would certainly provide optimal wet adhesion, if they are sufficiently stable against hydrolysis. In some, mostly special cases, the existence of covalent bonds has been proved experimentally, but most metal-organic bonds are unstable against water. It is generally agreed that, besides mechanical interlocking, polar interactions, especially acid-base interactions and hydrogen bonding, are responsible for adhesion. Again these bonds are susceptible to the disrupting action of water.
///////I/////// normal
/
organic
coating
(cont.
solution)
\
///////I/// Cooperation
Conformation
(high TQ)
(dilut. solution)
Cooperation
Preorientation
and Confamution
(dilute solution,
(dilute solution,
high Tgi
b
I?
monolayer)
d :
J-7 I
I’
Preorientation
and
Cooperation ////////////)/
(monolayer+crosslinkIng)
Fig. 2. Structure and bonding of adhesion layers on metal substrates
There are two possible ways to counteract bond rupture by water molecules: forcing the bonds to cooperate with each other and choosing application conditions which, by a suitable chain conformation of the binder molecules, allow a maximum number of bonding groups per molecule to interact with the metal substrate. The role of such cooperative and conformative effects for improving wet adhesion is indicated schematically in Fig. 1, together with the possibility of preorientation of adhesive macromolecules (Fig. 2).
2. The cooperative mechanism Acid-base interactions and hydrogen bonds have a dynamic nature, which depends, respectively, on the degree of dissociation and the water solubility of the salt on the hydrogen bonding in liquid water. These dynamic interactions must be stabilized to improve wet adhesion. Stabilization is possible by attaching bonding groups to rigid polymer backbone chains, which force neighboring bonds to cooperate (Fig. 3). It must be observed, however, that chain rigidity must be introduced after, and not before, the adsorption process, because stiff macromolecules cannot sufficiently adapt themselves to the contours of the metal surface. Crosslinking of binders and heating above the glass transition point are means for immobilizing chain segments at the metal surface. An indication for the effect of rigidity of organic layers on adhesion is that, in many cases, wet adhesion on metal surfaces is related to the glass-transition temperature, Tg, of the organic coating, especially the Tg in the presence of water [ 6,7]. It can be shown that some thin adhesion layers, which have been prepared from low-molecular resins with a high T,
W. Funke /Progress
in Orgunic Coatings 28 (1996) 3-7
Cooperative
Non-cooperative
Dynamic equlllbrum between bonding and non-bonding chain ends Mobile chain segments at the Interface
Stationary non-dynamlc bonds lmmobk chain segments at the Interface
Fig. 3. Adhesion bonds at the coating/support interface. Arrows indicate the continuation of chain segments into the bulk of the coating. I
I
I
Top Layer
Adhesion
( ca. 45 ,um)
(ca. 200 nm)
-a
Layer
___ -____
Wet Adhesion
( W.4
I
II<1
1
’
Fig. 4. Influence of cooperatively bonded adhesion layers on wet adhesion of the top layer. * Delamination between adhesion and top layer.
after curing, significantly improved wet adhesion of alkyd melamine and epoxy coatings of normal film thickness (Fig. 4). The molecules of the thin adhesion layers have a low molecular mass before film formation, which together with the increased temperature of film formation ensures an optimal penetration in and a contact with the surface structure of the metal. The needed rigidity is obtained by the crosslinking reaction. In coating systems of this type, besides wet adhesion, also the tendency of underrusting could be distinctly improved.
3. The conformative
mechanism
The concentration of binder molecules in paints is usually higher by sizes of magnitude than the concentration used in adsorption experiments. Due to its high concentration polymeric binder molecules, which after application happen to be at the surface of the metal substrate, have to compete with their neighbors for adsorption sites. As a consequence of this competition only a small fraction of the adhesive groups of a macromolecule has actually the chance to interact with the support. On the other hand, at excessive dilution, macromolecules at the support surface have enough sites available for adsorption, but the coverage of the metal surface is insufficient (Fig. 5). Therefore some optimum concentration exists,
5
at which a maximal number of adhesive groups per macromolecule located at the interface interacts with the substrate. Another interfacial aspect related to wet adhesion is the chemical nature of adhesive groups. Surfaces of metals, such as steel or aluminum, are hydrophilic due to their natural, very thin oxide layers. To secure good interaction with such metal surfaces polar groups are needed as structural elements of the binder molecules. The question is, how many of such groups are required in a binder molecule and whether these groups are also beneficial to the properties of the bulk of the coating film. As to polarity, one could think of macromolecules, such as polyvinylalcohol or polyacrylic acid. However this choice seems to be unreasonable, because normal base coats of these macromolecules would yield a very hydrophilic, water-sensitive layer. By hydrogen bonding between polymer groups in the bulk of such films one may expect the mechanical strength to be improved, but in the presence of water or high humidity these bonds are disrupted and the coating system is easily delaminated. This raises the question, whether it is reasonable to use the same macromolecules for achieving good, water-resistant bulk properties of coating films and simultaneously expect optimal adhesion at the substrate by their polar nature. From this consideration it follows that, for an optimal adhesion, macromolecules should have a structure different from macromolecules with optimal properties of the bulk film. Adhesion mechanisms are not limited to the interface only but are influenced by the structure and mobility of the adjacent chain segments [ 11. However the adhesion layer probably extends not far beyond the diameter of a coiled macromolecule. It is advisable, therefore, to limit the film thickness of the adhesion layer to some ten nanometers. For preparing such thin layers, a concentration of binder molecules should be chosen which is much below the usual concentration in paints. To test this assumption, a series of polyacids was applied to the usual steel panels after degreasing and cleaning, and the dependence of wet adhesion on the concentration of the dilute aqueous solutions of polyacrylic acid was measured. After drying, this adhesion layer was covered with an unpigmented top coat of - 40-50 pm film thickness (alkyd-melamine or epoxy resin). Quite interestingly wet adhesion of these coating systems could be improved by orders of magnitude and, according to what was expected (Fig. 5), a distinct maximum of wet
1
substrate
Incomplete coverage (very low concentration)
1 1
substrate
1
complete coverage, but less ordered than on water
1
substrate
)
overcoverage (high concentration, as in practical coatings)
(low concentration)
Fig. 5. Influence of the concentration of the solution on the conformation binder molecules and their coverage of the substrate.
of
6
W. Funke / Prop-ess m Orgmic
delamination
time (h)
the adsorbed macromolecules exist. In some respect this is comparable with the situation of phosphate layers on steel surfaces, which also do not perfectly cover the substrate and therefore require post-treatments, such as chromic acid rinse. On the other hand, adsorbed layers should be stabilized after their application to develop better intermolecular “cooperation”. Both ways are presently being studied. Still further improvement of wet adhesion can be expected by a preorientation and cooperation in the form of adhesive monolayers (Fig. 2). Of course, for this purpose, special macromolecules have to be synthesized to meet the requirements for wet adhesion, and the roughness of usual metal surfaces may cause local breaks in the monolayer.
10000
1000
100
10
0
0.1
0.2 1 2 concentration
Fig. 6. Wet adhesion and concentration top coats.
2.1 2.2 PAA (m%)
2.3
2.5
of PAA with alkyd/melamine
resin
adhesion appeared in the case of polyacrylic acid at a layer thickness corresponding to a concentration of the aqueous solution of l-2% by mass (Fig. 6). A similar maximum of wet adhesion was found with other polyacids, e.g. polymethacrylic acid and copolymers of acrylic acids and of maleic acid. Obviously within this range of concentration the number of bonding groups of a macromolecule at the substrate surface is optimal. The chain conformation for this optimal interaction is induced by the rigid substrate during the adsorption progress.
4. Bonding mechanism
of polyacids to steel surfaces
The remarkably strong improvement of wet adhesion of coating systems by very thin layers of polyacids can be explained by polar interaction, hydrogen bonding or saltcomplex formation with iron ions formed at the steel surface. It has been shown that salt formation is the essential factor for this improved wet adhesion [ 81. When neutralized polyacids or polyacids in organic solvents were applied, where there is no ionic dissociation, no effect or wet adhesion were found. Insolubility of the polyacids is achieved by iron ions in the highest state of oxidation, but it is still unexplained as to how these salt complexes interact with the steel surface for attaining such a remarkable improvement of wet adhesion. Moreover, it turned out that overcoating with aqueous systems impairs wet adhesion. Probably the molecular structure of the polyacid is not strong enough for a stabilizing cooperative interaction with the metal surface when applied at ambient temperature.
5. Combination mechanisms
Coatings 28 (1996) 3-7
6. Wet adhesion and protection of steel surfaces against corrosion It has been supposed that besides permeability to water, oxygen and ions, wet adhesion is a decisive parameter for protection of metal surfaces by organic coating systems [ 9,101. The idea underlying this assumption was that any bonds to the metal surface must be disrupted by water molecules before the respective iron atoms beneath this bond may be transformed to ions as a result of the electrochemical corrosion reaction. In this respect aluminum surfaces are much easier to protect because corrosion is not as significant as in the case of iron, and accordingly the wet adhesion is very efficiently improved by such polyacid layers [ 3,7]. Actually, improved wet adhesion by cooperative adhesion layers also improves protection. However, a direct relationship between wet adhesion of organic coatings and protection against corrosion has been questioned [ 11,121. As a matter of fact, adhesion layers of polyacids failed to improve protection if corrosion resistance was measured by the salt spray test (Fig. 7). A reasonable explanation for this disappointing protective ability of polyacid layers is that, due to the topological structure of the metal surface and the limited mobility of the chain segments of the polyacid as a consequence of their molecular size and adsorptional fixation at the metal surface, small gaps and fissures of a metal surface cannot be sufficiently covered by the polyacid. NHF (d) resp. V, *lOl
(mm/h)
of cooperative and conformative
As the coverage of metal surfaces by adhesive macromolecules is statistical, it has to be assumed that, even at the optimal concentration of the solution used for adsorption of the adhesion layer, many irregularities in the arrangement of
no pret.
0.1
0.5
Concentration
1 2 of PAS (mass%)
Fig. 7. Wet adhesion (NHF) and rate of underrusting coating/PAS-adhesion system on steel.
3
(Vu) of an epoxy resin
W. Funke /Progress
diameter
1-1
in Organic
1 nm
loop
HOH
9
Fig. 8. Molecular model for the accessability of surface ‘holes’ to the narrowest possible loop of a polyacrylic acid molecule
Coatrngs 28 (I 996) 3-7
I
tion, but large enough for the formation of an ion-transporting aqueous phase, corrosion occurs, beginning at normal or intentional coating defects and proceeding with underrusting of the adjacent intact area by cathodic delamination. How can we cope with this problem without relying on traditional pretreatments? One possible way would be to prepare extremely smooth surfaces. However, as we have to deal with molecular dimensions, this is very difficult to realize and also very expensive. Moreover, any mechanical anchoring of organic layers by a suitable roughness of the metal surface, which certainly contributes to adhesion, would be excluded. Two other ways seem to be more suitable: use of reactive molecules of low molecular mass, which after physical or chemical adsorption can be connected by crosslinking or improving the molecular adaption to the metal surface structure, by small reactive, mobile binder molecules, which may be crosslinked after adsorption, eventually assisted by elevated temperature. Of course, these proposals are rather sophisticated approaches to solve the problem of combined wet adhesion and protection against corrosion, but interfacial problems are too complicated to allow simple primitive solutions for substantial improvements.
Acknowledgements
Fig. 9. Underrusting
of coating systems with good wet adhesion
In connection with other coatings systems, this concept has been already proposed [ 13,141. However, it needs to be scaled down to molecular dimensions. Fig. 8 shows schematically, but with correct molecular dimensions, that gaps or fissures at the metal surface may be small enough to prevent to access and adsorption of chain segments of polyacrylic acid even in their most compact conformation. This would still allow a large number of water molecules to fill these ‘holes’ if the humidity is high enough. As a consequence of this consideration, polymer chain segments should be able to adapt themselves to the roughness of the substrate surface. Molecular modeling and design of the polymer could be useful for achieving this adaption. It may be assumed that the preferred adsorption sites for macromolecules are the protuberant parts of the metal surface (Fig. 9). The adsorption and reaction of each carboxyl group limits the mobility of the remaining chain segments. If the cavities of the metal surface are too small for further adsorp-
I thank Dr Roy W. Tess and the Division of Polymer Materials, Science and Engineering of the American Chemical Society very much for appreciating our scientific and technological work by the Roy W. Tess Award. The long standing support of the Bundesminister fur Wirtschaft, the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen e.V. (AlF) and the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged.
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