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Effect of coating stress on adherence and on corrosion prevention
Progress in Organic Coatings 43 (2001) 18–24
Effect of coating stress on adherence and on corrosion prevention M. Piens, H. De Deurwaerder∗ Coatings Research Institute — CoRI, Avenue P. Holoffe, B-1342 Limelette, Belgium Accepted 21 August 2001
Abstract Pull-off, stress and elastic modulus measurements were combined to evaluate the adhesion of polyurethane coatings on aluminium. Adhesion increases with coatings polarity while adherence, the elastic energy required to detach the coating from the substrate, decreases. The harmful effect of stress on adherence was demonstrated experimentally. Soft polymeric inclusions were used to decrease the stress that arises in an epoxy coating during immersion in water and drying. Better protection of steel against corrosion in cyclic tests was obtained with the epoxy coatings having lower stress. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Stress; Adhesion; Adherence; Corrosion prevention; Soft polymeric inclusions
1. Introduction For decorating and protecting, a coating must adhere to its substrate. Paint chemists and paint users pay great attention to adhesion and try to improve it or to ensure its durability by optimising paint formulation, surface preparation and application conditions. Adhesion is a very factor quantity to measure. The fact that numerous methods are proposed to measure adhesion is a proof by itself that the experimental approach is confusing. It is not rare for a paint chemist to obtain results of adhesion tests that are quite contrary to what he expected from the chemistry of the coating and of the substrate. Significant progress has been made in understanding the nature of coatings adhesion (and adhesion failure) by several authors; amongst them are Kendall, Croll and Perera. Kendall [1] formulated an “equilibrium theory” of adhesion between elastic solids and showed that the force required to separate two surfaces depends on the interfacial surface energy but also on the elastic constants and on the geometry of the adherent bodies. His theory can be used to interpret the well-known pull-off and peel tests. Kendall [2] also showed that the shrinkage stress present in an adhesive joint weakens its adhesive strength. Croll [3,4] applied this concept to coatings. He demonstrated that adhesion is adversely affected by the stress that appears in the coating during film formation. This stress ∗ Corresponding author. Tel.: +32-2-653-0986; fax: +32-2-653-9503. E-mail addresses: [email protected] (M. Piens), [email protected] (H. De Deurwaerder).
begins to develop at the film solidification stage because the film can no longer flow to satisfy the decrease in volume associated with the escape of the remaining solvent. Whereas the film can still contract in its thickness, its area is constrained by adhesion to the substrate, and a two-dimensional stress arises in the plane of the coating. Perera [5–10] showed that beside the internal stress due to film formation, other stresses often much larger than the former can develop in a coating when the dimensional changes expected after a variation of temperature or of relative humidity are prevented by coating adhesion to the substrate. The chemical and physical modifications that a coating suffers during weathering also proved [7,8] to be the origin of stress development. The present work shows the detrimental effect that coatings stresses has on adherence and the beneficial effect that reduction of stress has on protective coatings.
2. Theory The energy balance per unit area for detaching from a rigid substrate a coating suffering a stress is given by [10,11] WT = γ + W P − β
(1)
where WT is the work done by the external tension σ z applied to the coating (e.g. in the pull-off test) to detach it from the substrate, γ the interfacial work of adhesion, WP the work expended in plastic deformation of the coating, β the elastic energy stored in the coating owing to the existence
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 2 4 3 - 0
M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
of a stress σ . These terms can be determined by the following procedure. It is assumed [2,3,10–12] that, after solidification of a coating, the change in volume of the film (due to evaporation of the remaining solvent, to cross-linking, to coalescence or due to a variation in temperature (below Tg ) or in relative humidity), occurs only by a change in thickness. Indeed, adhesion to the substrate impedes the deformation of the coating in the XY-plane parallel to the substrate. This constraint produces a stress in the coating whenever a change in volume occurs. In the pull-off test, the coating not only adheres to the substrate but is also attached to the loading fixture on which the external tension σ z is applied. Therefore, we have to consider that a lateral contraction does not occur during the pull-off test, and by using this condition (no strain allowed in the plane of the coating thus, εx = εy = 0) in Hooke’s law which describes the relationship between tension σ and strain ε in an isotropic elastic material: εx E = σx − ν(σy + σz ), εy E = σy − ν(σx + σz ), εz E = σz − ν(σx + σy )
(2)
with ν being the Poisson coefficient and E the elastic modulus. One obtains after some algebra [4]: σz = Eεz
1−ν (1 − 2ν)(1 + ν)
and by considering ν = 0.4 σz = 2.143Eεz
(3)
This indicates a significant increase of the coating cohesive strength (apparent modulus being 2.143 times higher than E) in the pull-off test. The elastic work W = WT − WP done by the applied tension σ z to detach the coating is given by W = σz dεz with εz = c/c, where c is the coating thickness and c the increase in thickness during the pull-off test. Eq. (3) gives dσz = 2.143E dc/c and after integration: W =
cσz2 4.286E
(4)
The elastic modulus E is determined from the initial (linear) slope of stress–strain curves obtained with free films and σ z the value of the stress (tension) applied in the pull-off test that provokes the detachment. W is the elastic energy stored in the coating during the pull-off test. The value of W that provokes the detachment is equal to the energy that maintained the coating attached to the substrate. The energy W corresponds exactly to the concept of adherence as defined in this paper.
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On the other hand, β, the elastic energy per unit area stored in the coating owing to the presence of an internal biaxial stress σx = σy = σ , is given by [11] β = cσ 2
1−ν E
(5)
This relationship is similar to the one discussed by Perera [10,13]: β = cSε
(6)
where ε is the strain and S = σ with σ = Eε/(1 − ν) which is obtained from Eq. (2) in the case of a two-dimensional (2D) plane stress state with σx = σy and εx = εy . Eq. (1) can be written as γ =W +β or, by using Eqs. (4) and (5): c γ = (σ 2 + 2.572σ 2 ) 4.286E z
(7)
(8)
Since the stress σ that develops in the coating can be determined experimentally [5,9], Eq. (8) can be used to calculate coating adhesion γ . Eq. (8) rewritten as Eq. (8 ) c 0.6c 2 σz2 = γ − σ 4.286E E
(8 )
shows clearly that for a given value of adhesion γ which is determined by the chemistry of the interface coating–substrate, the externally applied stress σ z (or adherence W) required to detach the coating from its substrate will decrease with increasing coating stress σ (or the stored elastic energy β). When the coating stress σ is high enough for the stored elastic energy 0.6cσ 2 /E to be equal to γ , delamination occurs spontaneously (with σz = 0). Eq. (8 ) also shows that even with a constant value of adhesion γ , the adherence of a coating changes whenever the coating stress changes (temperature or relative humidity variations, immersion, drying, ageing, etc.).
3. Experimental 3.1. Materials 3.1.1. Solvent-borne polyurethanes Seven polyacrylic resins were polymerised for this study. Their compositions are given in Table 1. The resins were cross-linked with an aromatic polyisocyanate (Desmodur L 75, Bayer) in a stoichiometric ratio. After the application and a flash off period of 1 h, the varnishes were cured at 80◦ C for 24 h and stored in an air-conditioned room (21.0 ±1.5◦ C and RH = 48 ± 2%) during 7 days exactly. The dry film thickness was about 50 m. The amounts of oxygen and nitrogen atoms in the seven polyurethanes were calculated based on their chemical compositions (Table 1).
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M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
Table 1 Composition of the acrylic resins, amounts of heteroatoms of oxygen and C=O bonds in the polyurethane varnishes (mol%)
Styrene Butyl acrylate Ethyl acrylate 2-Ethylhexyl acrylate Hydroxyethyl acrylate 2-Hydroxypropyl acrylate O + N/C + H + O + N (mol%) O/C + H + O + N (mol%) –C=O (mol%)
A
B
C
D
E
G
H
56.3 37.9 – – 5.7 – 6.5 5.9 2.9
52.8 34.8 – – 12.4 – 8.4 7.2 3.6
50.0 33.0 – – 17.0 – 9.7 8.1 4.1
47.1 31.0 – – 21.8 – 10.8 8.8 4.4
52.6 – 35.1 – 12.4 – 9.5 8.1 4.1
53.3 – – 34.7 12.0 – 7.2 6.1 3.1
54.0 35.6 – – – 10.3 7.7 6.7 3.4
The varnishes were applied on thoroughly cleaned substrates: Al 5052 for pull-off measurements, stainless steel for stress measurements and polypropylene to prepare free films. 3.1.2. Waterborne epoxy The 2-K waterborne epoxy is based on a commercial dispersion (Beckopox EP 384w, Vianova) cross-linked with a commercial amine hardener (Beckopox EH 613w, Vianova) in an epoxy/hardener ratio 1.0/0.7. Different concentrations (4, 8 and 15% (v/v) on dry material) of a polyurethane dispersion (Bayhydrol PR 340, Bayer) of low Tg (−46◦ C) were added to the epoxy. The varnishes were applied on Q-panels (QD-36, The Q-panel company) for the cyclic weathering and on stainless steel for stress measurements. They were dried for 4 h at 80◦ C and then stored in an air-conditioned room.
of 0.6% was obtained. The time-dependent change in stress (tension) at constant deformation was then recorded. The decrease in stress after a time equal to the duration of the pull-off test was determined for each coating. 3.2.4. Stress measurements The equations and the experimental setup used to determine the stress are described in Refs. [5,9]. In this study, the coated substrates are clamped vertically at one end. 3.2.5. Weathering Samples were immersed every day in water (21◦ C) for 7 h and then allowed to dry for 17 h in an air-conditioned room (45% RH and 21◦ C). They were also allowed to dry during the weekend. The test was carried out for 792 h.
3.2. Methods
4. Results and discussion
3.2.1. Pull-off tests Pull-off measurements were performed on the polyurethane varnishes applied on aluminium 5052 (thickness = 5 mm) with an Instron 1122 testing machine operating at a crosshead speed of 0.05 mm/min, 7 days after application. The experimental setup allows a perfect alignment of the jaws and impedes any bending of the substrate during the test. The measurements were performed in an air-conditioned room (45% RH and 21◦ C).
The characteristics of the seven polyurethanes investigated are presented in Table 2. In Fig. 1, the stress (tension) σ z required to detach polyurethane varnishes from aluminium is plotted as a function of the percentage of oxygen atoms in the coatings. This percentage of oxygen atoms is an image of the polar component of the coating/substrate interactions. It is expected that the adhesion γ increases with the increase of polarity of the coatings. This is, however, not reflected in Fig. 1 where σ z decreases with polarity. The same evolution as in Fig. 1 is observed when the mole percentage of heteroatoms O + N, or the mole percentage of double bonds –C=0 are considered (see Table 2). Fig. 1 indicates that the detachment becomes easier with increasing coating polarity. Obviously, this is not due to the polarity itself. As demonstrated hereafter, this phenomenon is explained by the fact that the polarity was increased by increasing the concentration of –OH in the acrylic resins (see Table 1). Thus by increasing the cross-link density of the polyurethanes and, as a consequence, the stress σ which develops in the coating during film formation and which acts against adherence, increases with polarity. Fig. 2 shows the stress σ due to film formation of a homologous series of polyurethanes (A–D) in function of the mole
3.2.2. Stress–strain measurements These measurements were carried out on free films (5 mm width, 50 mm distance between jaws) in the air-conditioned room (45% RH and 21◦ C) using an Instron 1122 testing machine operating at a crosshead speed of 50 mm/min. Thus, the relative rate of elongation is the same for the pull-off tests and for the stress–strain measurements. The elastic modulus E was obtained from the linear part of the stress–strain curves. 3.2.3. Stress relaxation Free films fixed between the jaws of the Instron were elongated at a crosshead speed of 50 mm/min until a strain
M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
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Table 2 Pull-off σ z , elastic modulus E, adherence W, stress relaxation, energy spent in plastic deformation, energy spent to detach the coating, stress σ , stored energy, adhesion γ , concentration in heteroatoms, oxygen and –C=O bonds of the polyurethane varnishes (mol%)
Pull-off, σ z (MPa) Elastic modulus, E (MPa) Adherence, W (mJ/m2 ) Relaxation (%) WP (mJ/m2 ) WT (mJ/m2 ) Stress, σ (MPa) Stored energy, β (mJ/m2 ) Adhesion, γ (mJ/m2 ) O + N/C + H + O + N (mol%) O/C + H + O + N (mol%) –C=O (mol%)
A
B
C
D
E
G
H
5.4 1500 209 5.8 13 222 1.0 19 228 6.5 5.9 2.9
4.1 1660 130 8.0 11 141 4.9 497 627 8.4 7.2 3.6
2.4 1780 35 10.2 4 39 8.2 1110 1145 9.7 8.1 4.1
1.8 1940 18 12.1 2.5 20.5 10.5 1669 1687 10.8 8.8 4.4
2.5 2100 31 6.0 2 33 8.2 881 912 9.5 8.1 4.1
3.6 790 180 22.0 51 231 0.0 0 180 7.2 6.1 3.1
4.1 1620 114 11.4 15 129 4.8 418 532 7.7 6.7 3.4
energy decreases adherence W as seen from Eq. (7) rewritten as W =γ −β
Fig. 1. Pull-off versus concentration of oxygen.
percentage in hydroxyethyl acrylate of the acrylic resin. For this series, the stress increases linearly with the cross-link density. From Eq. (5), it is obvious that to a higher stress σ corresponds a higher stored elastic energy β. This stored elastic
Fig. 2. Stress versus amount of hydroxyethyl acrylate.
For a given adhesion γ , the adherence W of the coating to its substrate decreases as β increases. In Fig. 3, the values of γ are plotted as a function of the percentage in oxygen atoms in the polyurethanes. The value corresponding to polyurethane A is not presented in the graph because during the pull-off test, a cohesive failure was observed. For the other polymers, adhesive failures were observed. Fig. 3 shows that, as expected, adhesion (W + β) of the coatings increases with the polarity. However, it must be again emphasised that, as seen in Fig. 3, a high adhesion (γ ) does not mean a good attachment (W) of the coating to its substrate. If the stress is high, i.e. if the elastic energy stored in the coating is high, adherence will be poor and even a small external stress (σ z in the pull-off tests, but also a variation in temperature or in relative humidity [9]) will provoke coating detachment. Percentages of relaxation (Table 2) were determined by stress-relaxation measurements (see Section 3.2.3). These values allow us to calculate WP and WT easily. The energy WP used for plastic deformation of the coating is beneficial for adherence. Indeed, even if adherence equals W only, a total amount of energy W +WP = WT has to be spent before the coating delaminates. The value WT can be considered as a “practical adherence”. Generally, WP is small compared to WT (see Table 2) because most of the polymers used in coatings have a glass-transition temperature Tg higher than the ambient temperature. In the glassy state, plastic deformation of the coating is limited and, of course, internal, thermal and hygroscopic stresses can develop. Since there is no doubt that stress can affect coating adherence and provoke spontaneous delamination, the aim of paint manufacturers should be to formulate their coatings in such a way that stress magnitudes are as low as possible
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M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
Fig. 3. Adhesion γ = W + β versus concentration of oxygen.
without affecting the other main properties. Decreasing the stress is a way to improve the adherence. One way to decrease the stress that develops in a coating is to incorporate into the coating soft polymeric inclusions able to relax or to relieve stresses. This idea has been tested by incorporating a soft polyurethane in emulsion (Tg = −46◦ C) into a two-component waterborne epoxy polyamine coating (Tg = 54◦ C). Different amounts (4, 8 and 15% (v/v) dry material) of the soft polyurethane were added to render the stress relaxation easier. The effect of these inclusions on the hygroscopic stress that develops in the epoxy coating during immersion in water and drying is shown in Fig. 4. Immediately after immersion, a compressive stress develops (Fig. 4) followed by its decrease as the plasticising effect of water ingress allows relaxation. When the coating is removed from water, a tensile stress develops and reaches an amplitude higher than the stress that arises during the film formation. As expected under dry conditions, the stress relaxation that follows is slower than in immersion. A comparison of Fig. 4a–d, shows that an increase in the concentration of soft inclusions reduces the maxi-
mum of the compressive and tensile stresses to a great extent and accelerates their relaxation rate both in water immersion and during drying. With the increase of soft inclusions, the coatings are under a lower stress and for a shorter period. In other words, they always adhere better to their substrate than coatings having a higher stress. Varnishes based on the epoxy polyamine resin with different amounts of soft inclusions were applied on steel and weathered in a cyclic test immersion–drying (see Section 3.2.5). The extension of corrosion from the scribe was always more limited with the coatings developing a lower stress, i.e. having a better adherence (see Fig. 5). From a practical point of view, this result opens new possibilities for the paint formulator to reduce the coating stress and improve the performance and the durability of protective coatings. From a fundamental point of view, this result indicates the important role of coating adherence in the protection against corrosion. To afford protection, a minimum adherence is obviously necessary for the coating to remain attached to the substrate, but higher adherence seems to result in better protection.
M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
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M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
and as a consequence, the external force is required in a pull-off test to detach the coating from the substrate. Incorporation of soft polymeric inclusions in a rigid-epoxy polyamine coating allows reduction of compressive and tensile hygroscopic stresses that develop during immersion in water and drying, respectively. The stress amplitude is reduced and the rate of relaxation is increased. Coatings with lower hygroscopic stress, i.e. with better adherence, exhibit better protection.
Acknowledgements The authors express their thanks to Dr. P. Janssen Bennynck, Managing Director of CoRI, and Dr. D. Perera for their helpful and stimulating discussions, and to P. Vandermies, M.-E. Debrue, G. Penades Bilbao and A. Lourtie for their technical assistance. They also thank Dr. J. Braeken and Dr. M. Belladone (Sigma Coatings) for their interest in the study and the polymerisation of the acrylic resins. This work was supported by the National and Regional Public Authorities and by the members of CoRI. References
Fig. 5. Weathering of two varnishes (triplicate); above the reference varnish (without inclusion) and below the same varnish with the highest concentration in inclusion: 15% by volume (dry) [20% by weight (wet)].
5. Conclusions A procedure has been described to determine adhesion from pull-off, stress and elastic modulus measurements. A series of polyurethane varnishes with increasing polarity and cross-link density was studied. Adhesion on aluminium increases with polarity. However, together with polarity and cross-link density the coating stress increases. The stored elastic energy associated with this stress reduces adherence
[1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13]
K. Kendall, J. Phys. D 4 (1971) 1186. K. Kendall, J. Phys. D 6 (1973) 1782. S.G. Croll, J. Coat. Technol. 51 (648) (1979) 64. S.G. Croll, J. Coat. Technol. 52 (665) (1980) 35. D.Y. Perera, D. Vanden Eynde, J. Coat. Technol. 53 (677) (1981) 39. D.Y. Perera, D. Vanden Eynde, J. Coat. Technol. 59 (748) (1987) 55. M. Oosterbroek, R.J. Lammers, L.G.T. van der Ven, D.Y. Perera, J. Coat. Technol. 63 (797) (1991) 55. D.Y. Perera, M. Oosterbroek, J. Coat. Technol. 66 (833) (1994) 55. D.Y. Perera, Stress phenomena in organic coatings, in: Koleske (Ed.), Paint and Coating Testing Manual, Gardner–Sward Handbook, Vol. 585, XIVth Edition, ASTM, Philadelphia, 1995. D.Y. Perera, Prog. Org. Coat. 28 (1996) 21. R.J. Farris, M.A. Maden, J. Goldfarb, Proceedings of the 14th Annual Meeting of the Adhesion Society, Vol. 138 Clearwater, FL, 1991. G.P. Bierwagen, J. Coat. Technol. 51 (658) (1979) 117. D.Y. Perera, D. Vanden Eynde, Proceedings of the 16th FATIPEC Congress, Vol. 129, Li`ege, Belgium, 1982.
Effect of coating stress on adherence and on corrosion prevention M. Piens, H. De Deurwaerder∗ Coatings Research Institute — CoRI, Avenue P. Holoffe, B-1342 Limelette, Belgium Accepted 21 August 2001
Abstract Pull-off, stress and elastic modulus measurements were combined to evaluate the adhesion of polyurethane coatings on aluminium. Adhesion increases with coatings polarity while adherence, the elastic energy required to detach the coating from the substrate, decreases. The harmful effect of stress on adherence was demonstrated experimentally. Soft polymeric inclusions were used to decrease the stress that arises in an epoxy coating during immersion in water and drying. Better protection of steel against corrosion in cyclic tests was obtained with the epoxy coatings having lower stress. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Stress; Adhesion; Adherence; Corrosion prevention; Soft polymeric inclusions
1. Introduction For decorating and protecting, a coating must adhere to its substrate. Paint chemists and paint users pay great attention to adhesion and try to improve it or to ensure its durability by optimising paint formulation, surface preparation and application conditions. Adhesion is a very factor quantity to measure. The fact that numerous methods are proposed to measure adhesion is a proof by itself that the experimental approach is confusing. It is not rare for a paint chemist to obtain results of adhesion tests that are quite contrary to what he expected from the chemistry of the coating and of the substrate. Significant progress has been made in understanding the nature of coatings adhesion (and adhesion failure) by several authors; amongst them are Kendall, Croll and Perera. Kendall [1] formulated an “equilibrium theory” of adhesion between elastic solids and showed that the force required to separate two surfaces depends on the interfacial surface energy but also on the elastic constants and on the geometry of the adherent bodies. His theory can be used to interpret the well-known pull-off and peel tests. Kendall [2] also showed that the shrinkage stress present in an adhesive joint weakens its adhesive strength. Croll [3,4] applied this concept to coatings. He demonstrated that adhesion is adversely affected by the stress that appears in the coating during film formation. This stress ∗ Corresponding author. Tel.: +32-2-653-0986; fax: +32-2-653-9503. E-mail addresses: [email protected] (M. Piens), [email protected] (H. De Deurwaerder).
begins to develop at the film solidification stage because the film can no longer flow to satisfy the decrease in volume associated with the escape of the remaining solvent. Whereas the film can still contract in its thickness, its area is constrained by adhesion to the substrate, and a two-dimensional stress arises in the plane of the coating. Perera [5–10] showed that beside the internal stress due to film formation, other stresses often much larger than the former can develop in a coating when the dimensional changes expected after a variation of temperature or of relative humidity are prevented by coating adhesion to the substrate. The chemical and physical modifications that a coating suffers during weathering also proved [7,8] to be the origin of stress development. The present work shows the detrimental effect that coatings stresses has on adherence and the beneficial effect that reduction of stress has on protective coatings.
2. Theory The energy balance per unit area for detaching from a rigid substrate a coating suffering a stress is given by [10,11] WT = γ + W P − β
(1)
where WT is the work done by the external tension σ z applied to the coating (e.g. in the pull-off test) to detach it from the substrate, γ the interfacial work of adhesion, WP the work expended in plastic deformation of the coating, β the elastic energy stored in the coating owing to the existence
0300-9440/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 2 4 3 - 0
M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
of a stress σ . These terms can be determined by the following procedure. It is assumed [2,3,10–12] that, after solidification of a coating, the change in volume of the film (due to evaporation of the remaining solvent, to cross-linking, to coalescence or due to a variation in temperature (below Tg ) or in relative humidity), occurs only by a change in thickness. Indeed, adhesion to the substrate impedes the deformation of the coating in the XY-plane parallel to the substrate. This constraint produces a stress in the coating whenever a change in volume occurs. In the pull-off test, the coating not only adheres to the substrate but is also attached to the loading fixture on which the external tension σ z is applied. Therefore, we have to consider that a lateral contraction does not occur during the pull-off test, and by using this condition (no strain allowed in the plane of the coating thus, εx = εy = 0) in Hooke’s law which describes the relationship between tension σ and strain ε in an isotropic elastic material: εx E = σx − ν(σy + σz ), εy E = σy − ν(σx + σz ), εz E = σz − ν(σx + σy )
(2)
with ν being the Poisson coefficient and E the elastic modulus. One obtains after some algebra [4]: σz = Eεz
1−ν (1 − 2ν)(1 + ν)
and by considering ν = 0.4 σz = 2.143Eεz
(3)
This indicates a significant increase of the coating cohesive strength (apparent modulus being 2.143 times higher than E) in the pull-off test. The elastic work W = WT − WP done by the applied tension σ z to detach the coating is given by W = σz dεz with εz = c/c, where c is the coating thickness and c the increase in thickness during the pull-off test. Eq. (3) gives dσz = 2.143E dc/c and after integration: W =
cσz2 4.286E
(4)
The elastic modulus E is determined from the initial (linear) slope of stress–strain curves obtained with free films and σ z the value of the stress (tension) applied in the pull-off test that provokes the detachment. W is the elastic energy stored in the coating during the pull-off test. The value of W that provokes the detachment is equal to the energy that maintained the coating attached to the substrate. The energy W corresponds exactly to the concept of adherence as defined in this paper.
19
On the other hand, β, the elastic energy per unit area stored in the coating owing to the presence of an internal biaxial stress σx = σy = σ , is given by [11] β = cσ 2
1−ν E
(5)
This relationship is similar to the one discussed by Perera [10,13]: β = cSε
(6)
where ε is the strain and S = σ with σ = Eε/(1 − ν) which is obtained from Eq. (2) in the case of a two-dimensional (2D) plane stress state with σx = σy and εx = εy . Eq. (1) can be written as γ =W +β or, by using Eqs. (4) and (5): c γ = (σ 2 + 2.572σ 2 ) 4.286E z
(7)
(8)
Since the stress σ that develops in the coating can be determined experimentally [5,9], Eq. (8) can be used to calculate coating adhesion γ . Eq. (8) rewritten as Eq. (8 ) c 0.6c 2 σz2 = γ − σ 4.286E E
(8 )
shows clearly that for a given value of adhesion γ which is determined by the chemistry of the interface coating–substrate, the externally applied stress σ z (or adherence W) required to detach the coating from its substrate will decrease with increasing coating stress σ (or the stored elastic energy β). When the coating stress σ is high enough for the stored elastic energy 0.6cσ 2 /E to be equal to γ , delamination occurs spontaneously (with σz = 0). Eq. (8 ) also shows that even with a constant value of adhesion γ , the adherence of a coating changes whenever the coating stress changes (temperature or relative humidity variations, immersion, drying, ageing, etc.).
3. Experimental 3.1. Materials 3.1.1. Solvent-borne polyurethanes Seven polyacrylic resins were polymerised for this study. Their compositions are given in Table 1. The resins were cross-linked with an aromatic polyisocyanate (Desmodur L 75, Bayer) in a stoichiometric ratio. After the application and a flash off period of 1 h, the varnishes were cured at 80◦ C for 24 h and stored in an air-conditioned room (21.0 ±1.5◦ C and RH = 48 ± 2%) during 7 days exactly. The dry film thickness was about 50 m. The amounts of oxygen and nitrogen atoms in the seven polyurethanes were calculated based on their chemical compositions (Table 1).
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M. Piens, H. De Deurwaerder / Progress in Organic Coatings 43 (2001) 18–24
Table 1 Composition of the acrylic resins, amounts of heteroatoms of oxygen and C=O bonds in the polyurethane varnishes (mol%)
Styrene Butyl acrylate Ethyl acrylate 2-Ethylhexyl acrylate Hydroxyethyl acrylate 2-Hydroxypropyl acrylate O + N/C + H + O + N (mol%) O/C + H + O + N (mol%) –C=O (mol%)
A
B
C
D
E
G
H
56.3 37.9 – – 5.7 – 6.5 5.9 2.9
52.8 34.8 – – 12.4 – 8.4 7.2 3.6
50.0 33.0 – – 17.0 – 9.7 8.1 4.1
47.1 31.0 – – 21.8 – 10.8 8.8 4.4
52.6 – 35.1 – 12.4 – 9.5 8.1 4.1
53.3 – – 34.7 12.0 – 7.2 6.1 3.1
54.0 35.6 – – – 10.3 7.7 6.7 3.4
The varnishes were applied on thoroughly cleaned substrates: Al 5052 for pull-off measurements, stainless steel for stress measurements and polypropylene to prepare free films. 3.1.2. Waterborne epoxy The 2-K waterborne epoxy is based on a commercial dispersion (Beckopox EP 384w, Vianova) cross-linked with a commercial amine hardener (Beckopox EH 613w, Vianova) in an epoxy/hardener ratio 1.0/0.7. Different concentrations (4, 8 and 15% (v/v) on dry material) of a polyurethane dispersion (Bayhydrol PR 340, Bayer) of low Tg (−46◦ C) were added to the epoxy. The varnishes were applied on Q-panels (QD-36, The Q-panel company) for the cyclic weathering and on stainless steel for stress measurements. They were dried for 4 h at 80◦ C and then stored in an air-conditioned room.
of 0.6% was obtained. The time-dependent change in stress (tension) at constant deformation was then recorded. The decrease in stress after a time equal to the duration of the pull-off test was determined for each coating. 3.2.4. Stress measurements The equations and the experimental setup used to determine the stress are described in Refs. [5,9]. In this study, the coated substrates are clamped vertically at one end. 3.2.5. Weathering Samples were immersed every day in water (21◦ C) for 7 h and then allowed to dry for 17 h in an air-conditioned room (45% RH and 21◦ C). They were also allowed to dry during the weekend. The test was carried out for 792 h.
3.2. Methods
4. Results and discussion
3.2.1. Pull-off tests Pull-off measurements were performed on the polyurethane varnishes applied on aluminium 5052 (thickness = 5 mm) with an Instron 1122 testing machine operating at a crosshead speed of 0.05 mm/min, 7 days after application. The experimental setup allows a perfect alignment of the jaws and impedes any bending of the substrate during the test. The measurements were performed in an air-conditioned room (45% RH and 21◦ C).
The characteristics of the seven polyurethanes investigated are presented in Table 2. In Fig. 1, the stress (tension) σ z required to detach polyurethane varnishes from aluminium is plotted as a function of the percentage of oxygen atoms in the coatings. This percentage of oxygen atoms is an image of the polar component of the coating/substrate interactions. It is expected that the adhesion γ increases with the increase of polarity of the coatings. This is, however, not reflected in Fig. 1 where σ z decreases with polarity. The same evolution as in Fig. 1 is observed when the mole percentage of heteroatoms O + N, or the mole percentage of double bonds –C=0 are considered (see Table 2). Fig. 1 indicates that the detachment becomes easier with increasing coating polarity. Obviously, this is not due to the polarity itself. As demonstrated hereafter, this phenomenon is explained by the fact that the polarity was increased by increasing the concentration of –OH in the acrylic resins (see Table 1). Thus by increasing the cross-link density of the polyurethanes and, as a consequence, the stress σ which develops in the coating during film formation and which acts against adherence, increases with polarity. Fig. 2 shows the stress σ due to film formation of a homologous series of polyurethanes (A–D) in function of the mole
3.2.2. Stress–strain measurements These measurements were carried out on free films (5 mm width, 50 mm distance between jaws) in the air-conditioned room (45% RH and 21◦ C) using an Instron 1122 testing machine operating at a crosshead speed of 50 mm/min. Thus, the relative rate of elongation is the same for the pull-off tests and for the stress–strain measurements. The elastic modulus E was obtained from the linear part of the stress–strain curves. 3.2.3. Stress relaxation Free films fixed between the jaws of the Instron were elongated at a crosshead speed of 50 mm/min until a strain
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Table 2 Pull-off σ z , elastic modulus E, adherence W, stress relaxation, energy spent in plastic deformation, energy spent to detach the coating, stress σ , stored energy, adhesion γ , concentration in heteroatoms, oxygen and –C=O bonds of the polyurethane varnishes (mol%)
Pull-off, σ z (MPa) Elastic modulus, E (MPa) Adherence, W (mJ/m2 ) Relaxation (%) WP (mJ/m2 ) WT (mJ/m2 ) Stress, σ (MPa) Stored energy, β (mJ/m2 ) Adhesion, γ (mJ/m2 ) O + N/C + H + O + N (mol%) O/C + H + O + N (mol%) –C=O (mol%)
A
B
C
D
E
G
H
5.4 1500 209 5.8 13 222 1.0 19 228 6.5 5.9 2.9
4.1 1660 130 8.0 11 141 4.9 497 627 8.4 7.2 3.6
2.4 1780 35 10.2 4 39 8.2 1110 1145 9.7 8.1 4.1
1.8 1940 18 12.1 2.5 20.5 10.5 1669 1687 10.8 8.8 4.4
2.5 2100 31 6.0 2 33 8.2 881 912 9.5 8.1 4.1
3.6 790 180 22.0 51 231 0.0 0 180 7.2 6.1 3.1
4.1 1620 114 11.4 15 129 4.8 418 532 7.7 6.7 3.4
energy decreases adherence W as seen from Eq. (7) rewritten as W =γ −β
Fig. 1. Pull-off versus concentration of oxygen.
percentage in hydroxyethyl acrylate of the acrylic resin. For this series, the stress increases linearly with the cross-link density. From Eq. (5), it is obvious that to a higher stress σ corresponds a higher stored elastic energy β. This stored elastic
Fig. 2. Stress versus amount of hydroxyethyl acrylate.
For a given adhesion γ , the adherence W of the coating to its substrate decreases as β increases. In Fig. 3, the values of γ are plotted as a function of the percentage in oxygen atoms in the polyurethanes. The value corresponding to polyurethane A is not presented in the graph because during the pull-off test, a cohesive failure was observed. For the other polymers, adhesive failures were observed. Fig. 3 shows that, as expected, adhesion (W + β) of the coatings increases with the polarity. However, it must be again emphasised that, as seen in Fig. 3, a high adhesion (γ ) does not mean a good attachment (W) of the coating to its substrate. If the stress is high, i.e. if the elastic energy stored in the coating is high, adherence will be poor and even a small external stress (σ z in the pull-off tests, but also a variation in temperature or in relative humidity [9]) will provoke coating detachment. Percentages of relaxation (Table 2) were determined by stress-relaxation measurements (see Section 3.2.3). These values allow us to calculate WP and WT easily. The energy WP used for plastic deformation of the coating is beneficial for adherence. Indeed, even if adherence equals W only, a total amount of energy W +WP = WT has to be spent before the coating delaminates. The value WT can be considered as a “practical adherence”. Generally, WP is small compared to WT (see Table 2) because most of the polymers used in coatings have a glass-transition temperature Tg higher than the ambient temperature. In the glassy state, plastic deformation of the coating is limited and, of course, internal, thermal and hygroscopic stresses can develop. Since there is no doubt that stress can affect coating adherence and provoke spontaneous delamination, the aim of paint manufacturers should be to formulate their coatings in such a way that stress magnitudes are as low as possible
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Fig. 3. Adhesion γ = W + β versus concentration of oxygen.
without affecting the other main properties. Decreasing the stress is a way to improve the adherence. One way to decrease the stress that develops in a coating is to incorporate into the coating soft polymeric inclusions able to relax or to relieve stresses. This idea has been tested by incorporating a soft polyurethane in emulsion (Tg = −46◦ C) into a two-component waterborne epoxy polyamine coating (Tg = 54◦ C). Different amounts (4, 8 and 15% (v/v) dry material) of the soft polyurethane were added to render the stress relaxation easier. The effect of these inclusions on the hygroscopic stress that develops in the epoxy coating during immersion in water and drying is shown in Fig. 4. Immediately after immersion, a compressive stress develops (Fig. 4) followed by its decrease as the plasticising effect of water ingress allows relaxation. When the coating is removed from water, a tensile stress develops and reaches an amplitude higher than the stress that arises during the film formation. As expected under dry conditions, the stress relaxation that follows is slower than in immersion. A comparison of Fig. 4a–d, shows that an increase in the concentration of soft inclusions reduces the maxi-
mum of the compressive and tensile stresses to a great extent and accelerates their relaxation rate both in water immersion and during drying. With the increase of soft inclusions, the coatings are under a lower stress and for a shorter period. In other words, they always adhere better to their substrate than coatings having a higher stress. Varnishes based on the epoxy polyamine resin with different amounts of soft inclusions were applied on steel and weathered in a cyclic test immersion–drying (see Section 3.2.5). The extension of corrosion from the scribe was always more limited with the coatings developing a lower stress, i.e. having a better adherence (see Fig. 5). From a practical point of view, this result opens new possibilities for the paint formulator to reduce the coating stress and improve the performance and the durability of protective coatings. From a fundamental point of view, this result indicates the important role of coating adherence in the protection against corrosion. To afford protection, a minimum adherence is obviously necessary for the coating to remain attached to the substrate, but higher adherence seems to result in better protection.
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and as a consequence, the external force is required in a pull-off test to detach the coating from the substrate. Incorporation of soft polymeric inclusions in a rigid-epoxy polyamine coating allows reduction of compressive and tensile hygroscopic stresses that develop during immersion in water and drying, respectively. The stress amplitude is reduced and the rate of relaxation is increased. Coatings with lower hygroscopic stress, i.e. with better adherence, exhibit better protection.
Acknowledgements The authors express their thanks to Dr. P. Janssen Bennynck, Managing Director of CoRI, and Dr. D. Perera for their helpful and stimulating discussions, and to P. Vandermies, M.-E. Debrue, G. Penades Bilbao and A. Lourtie for their technical assistance. They also thank Dr. J. Braeken and Dr. M. Belladone (Sigma Coatings) for their interest in the study and the polymerisation of the acrylic resins. This work was supported by the National and Regional Public Authorities and by the members of CoRI. References
Fig. 5. Weathering of two varnishes (triplicate); above the reference varnish (without inclusion) and below the same varnish with the highest concentration in inclusion: 15% by volume (dry) [20% by weight (wet)].
5. Conclusions A procedure has been described to determine adhesion from pull-off, stress and elastic modulus measurements. A series of polyurethane varnishes with increasing polarity and cross-link density was studied. Adhesion on aluminium increases with polarity. However, together with polarity and cross-link density the coating stress increases. The stored elastic energy associated with this stress reduces adherence
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