Protective organic coatings: Membrane properties and performance

Progress in Organic Coatings, 10 (1982) 5 - 33

PROTECTIVE ORGANIC AND PERFORMANCE

COATINGS:

5

MEMBRANE

PROPERTIES

H. CORTI and R. FERNANDEZ-PRINI Departarnento Quimica de Reactores, CNEA, Av. Libertador 8250, CP-1429, Capital Federal (Argentina) D. GOMEZ Sector Electroquz'mica Aplicada, INTI, Capital Federal (Argentina)

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Electrochemical properties of paint films . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Paint films as membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Transport properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Equilibrium properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Model membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Swelling and osmotic pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Permeability of paint free films to water, oxygen and ions . . . . . . . . . . . . . . . . 5 Structure and morphology of paint films . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Free films versus applied films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Film adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Accelerated testing and paint performance . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 6 10 12 14 17 18 19 22 24 25 28 31 32

1 .'Introduction T h e r u s t i n g o f m e t a l s u b s t r a t e b e n e a t h t h e p a i n t f i l m i n d i c a t e s t h a t aggressive ions, water and/or oxygen have reached the metal surface and started the electrochemical reactions responsible for the observed corrosion. F u r t h e r m o r e , .if a n o d i c a n d c a t h o d i c r e g i o n s a r e s e p a r a t e d , t h e f i l m a c t s as a n o h m i c b a r r i e r b e t w e e n t h e m . C o n s e q u e n t l y , i t is i n t e r e s t i n g t o k n o w a b o u t the mechanism of corrosion inhibition by films containing no electrochemicaUy a c t i v e p i g m e n t s , w h i c h a r e k n o w n t o d e c r e a s e t h e r a t e o f c o r r o s i o n o f coated metals by a diffusional or ohmic inhibition. In order to understand the mechanism of protection of organic coatings t h r e e m a i n r e s e a r c h l i n e s h a v e b e e n f o l l o w e d i n g e n e r a l [1, 2 ( a ) ] : Electrochemical measurements: Characterization of paint performance b y m e a s u r e m e n t o f t h e ( r e s t o r c o r r o s i o n ) p o t e n t i a l a n d / o r p o l a r i z a t i o n resistance. Equivalent circuits have been proposed by some authors in order to clarify the electrochemical behaviour of paint films. 0033-0655/82/010005-29/$07.50

© Elsevier Sequoia/Printed in The Netherlands

Membrane properties of paint films: These involve electric resistance measurements, determination of ion-exchange capacities, permselectivity of the films and permeability of various ions through the films. Difffusional barrier and mechanical properties: Determination of water and oxygen permeabilities, water uptake, influence of the latter upon adhesion, morphology of the paint film and hardness. It is a c o m m o n feature of research in paint performance that criteria for good performance based on any of the previously mentioned lines of research are frequently in contradiction with evidence obtained from the other two and with the classical accelerated tests. It is difficult to have a global view of the problems involved in the assessment of protective coatings performance. One factor is the difficulty of preparing reproducible samples and the heterogeneous nature of the paint films [2 - 4]. This appears to play a very important role in paint deterioration. Another factor is the suitability of employing information obtained from free paint films in order to explain the properties of applied films. This review of some relevant contributions to organic coatings performance is aimed at drawing some general conclusions regarding paint film behaviour and emphasizing the points which appear more critical, and should be investigated to gain a better insight into the problem.

2. Electrochemical properties of paint films (see Appendix) It seems that the direct way of establishing the onset of corrosion on a coated metal is the determination of the electrochemical parameters which characterize the corrosion. We shall analyse some aspects of electrochemical research on paint performance. However, this section is not intended to be an exhaustive review of the electrochemical behaviour of metals protected by organic coatings. For a more thorough review of this subject, refs. [ 5, 6 ] should be consulted. Electric potentials of coated metals have been determined in order to detect the changes from a passive state to potentials corresponding to active dissolution of the protected metal [7, 8]. Another type of electrochemical studies tried to establish the rate of the corrosion of metallic substrates coated with a protective film, based on the concept of linear polarization due to Stern [9] and calculating the polarization resistance of the metal. Scantlebury and Ho [6] have pointed out that the conditions under which the Stern treatment is valid are very seldom met in the study of protective coatings. Wolstenholme [5] has evaluated these electrochemical methods and shown that determinations may yield ambiguous results due to the large ohmic barrier of the painted metal substrate. Probably the most promising way of dealing with the problem of determination of corrosion rate for coated metals [ 5 ] is to interrupt suddenly the current flowing through the system.

In this way the ohmic drop, which decays instantaneously, may be calculated and the real polarization resistance obtained. However, this is a delicate and laborious technique because the system as a whole has a very large impedance and few measurements of this kind have been reported [10]. Bureau [ 11 ] has tried to eliminate from the polarization potentials of applied films the effect of the ohmic resistance of protective coatings employing the values of the DC resistances of the films when detached from the metal surfaces. Even a simple analysis of the Evans diagrams corresponding to systems having a large ohmic polarization shows that the concept of (rest or corrosion) potential, as characteristic of a metal in a given medium, is ambiguous [5] even when the metals are on an active corrosion regime. The situation is more complex when the metals are passivated. Table 1 gives values obtained by Bureau [7] for three epoxy paint films applied on steel panels and immersed for eight days in water. The observed electrochemical behaviour is very different for the three coatings in spite of the fact that they are all epoxy paints, hence it does not seem possible to adopt a unique value characteristic of such coatings. Wolstenhome's [5] conclusion still holds true: " I t is disappointing to note that the results of electrochemical tests so far performed have not been very informative". Moreover, when corrosion is detected, it is because the protection of the film has already failed and this can hardly constitute an accelerated test of paint performance. It seems that with the present understanding of the problem, corrosion is always detected after the film has lost its protective capacity. TABLE 1 Polarization resistance Rp and rest potential steel and immersed for 8 days in water [7 ] Thickness (pro)

Rp ( ~ cm 2)

E (mV)

450 970 1000

7.5 x 105 4.2 X 107 2.7 X 109

--840 --450 --50

E vs.

SCE for three epoxy films applied on

It has been suggested that the polarization resistance of applied paint films is larger than the ohmic resistance of the same free films [3, 5, 6]. Mayne and Mills [3] have observed a higher resistance for acrylic-styrene emulsion applied films as compared with the free films when an inhibitive zinc chromate pigment is present in the paint. This observation has suggested to them that the polarization resistance may be larger due to the passivation of the metal substrate by the chromate, thus increasing the overall resistance to charge flow. The results of Mayne and Mills for alkyd applied and free films [3] agree in general with the observation on epoxy-coal tar films [4]. When the

13

I] u

Q

'L'~

R2

9

R1

I

I

I

I

1

2

3

1.5

I

30

4.:,5

OAYS

Fig. 1. Resistivity of applied and free films of epoxy-coal tar 75 p.p.m. KCI solution [4].

vs.

time of immersion in

Fig. 2. Equivalent circuit.

applied films were properly soaked, their resistances attained values of the same magnitude as the free films for those regions o f the applied films where no corrosion was observed. On the other hand, regions having lower resistance than the free film are prone to become rusted on continued immersion [4]. Figure 1 illustrates results for epoxy-coal tar films; the hatched band in the figure corresponds to the values normally found for the resistivity of free films. The two curves correspond to the resistivity measured in regions of the applied films where no corrosion was detected after eight weeks of immersion in 35 p.p.m, chloride solution; the dispersion of the resistivity values for each film is represented by the vertical bars in the figure. Curve A corresponds to films cured at 20 °C and curve B to films cured at 60 °C. It is observed that after four days of immersion, the resistivity of the applied films is never higher than that of the free films. The analysis of the frequency dependence of the impedance of applied organic coatings and the determination of the equivalent circuit has been employed to predict the electrochemical performance of coatings. Figure 2 depicts an equivalent circuit which was found to describe adequately the observed impedance behaviour of coatings [6, 12]. Scantlebury and Ho [6] considered that Rz is the ohmic resistance of the film, R2 the polarization resistance and C the double layer capacitance. The values they obtained for R1 are, however, much smaller than those observed for free films of the same paints, e.g. they report a resistivity of 1.8 × 106 ~ cm for epoxy-coal tar films, while for free films [4] of the same paint, values are in the range 101° - 1011 ~2 cm (cf. Fig. 1). It seems unlikely that an applied film shows an ohmic resistance of magnitude much smaller than that of the free film of the same paint. Menges and Schneider [12] have used the same equivalent circuit to study the performance of protective plastic coatings immersed in very aggres-

sive acid media. They concluded that the equivalent circuit in Fig. 2 was appropriate to describe the frequency dependence of the impedance of the coated metals and the time dependence of the resistance u p o n immersion. Their interpretation of the equivalent circuit is, however, different; R 2 is considered to be the resistance of the film and its time dependence would describe the penetration of electrolyte through the film, C being the capacitance of the substrate-film electrolyte capacitor, which will change as the dielectric constant of the film changes due to water absorption [13]. They consider that when the electrolyte reaches the metallic substrate, corrosion starts. The protective performance of the coating depends on the time rc the electrolyte takes to get through the film of thickness l; thus they suggest that the value Tc = 1 2 / 6 D ( D = diffusion coefficient) should be taken as the characteristic time for the fim's protective performance. This approach seems to be correct for highly aggressive media. In these solutions (I) will act as the cathodic depolarizer, e+HaO ÷ > H 2 0 + ~ 1H 2 (I) consequently rapid corrosion may be observed when the acidic electrolyte reaches the metal surface. Menges and Schneider [ 12] obtain the rc values b y passing a current through the free film and following its change with time; the extrapolation of the linear steady-state part of the plot gives re.

E

(mV)

--W

/

/

_ 600 -

(a) - 400

-

56,,000 r p ~ C I

.......

3OOrr~ CI-

. . . . .

OlSTILLEO WATER

i

I .200

I.~ . ~ ' \

I •

,4

j

I

i

I

i

(b~

-400

-200

o

:-5~ - ~ ,

" , 10

.... ,

,

ff

' ,

20

,

~ 3

,

~, 4

,

, 5

DAY!

Fig. 3. Potential (vs. NCE) of steel panels coated with: (a) epoxy-chromate and shop primer, (b) same as (a) and epoxy-coal tar topcoat when immersed in aqueous medium [15].

10 The measurement of the electric potentials of coated metals is a parameter relevant to the performance of paints containing an electrochemically active pigment [14, 15]. It may be especially helpful in order to decide the adequacy of a given paint scheme to protect the metal from corrosion in a particular environment. E p o x y - p o l y a m i d e paint having chromate as inhibitive pigment has been shown [15] to afford proper protection for coated steel immersed in water having less than 500 p.p.m, of chloride. In higher chloride concentration the electric potential measurements suggest that the metal is not passivated and corrosion starts upon immersion of the coated steel panels in such media. Another example of environment and paint schemes is illustrated in Fig. 3. Figure 3(a) shows the time evolution of the corrosion potential v e r s u s NCE for steel panels covered with shop primer (epoxy-ester with chromate 50 tim thick) and with an anticorrosive e p o x y - p o l y a m i d e coating. There was no electrochemical protection of the steel when the panels were immersed in 50,000 p.p.m.chloride solutions. When the same paint is topcoated with e p o x y - p o l y a m i d e - c o a l tar [120 t~m thick), the chromate appears to afford proper electrochemical protection even when the panels were immersed in 50,000 p.p.m, chloride (Fig. 3(b)).

3. Paint films as membranes As mentioned in the previous section, before the onset of corrosion in a metallic substrate coated by paint, it is necessary that substances such as water, oxygen and aggressive ions diffuse through the film and reach the metal surface. A loss of adhesion occurs after the applied coating is exposed to high water activity; however, the coating keeps the metal surface covered and protects it from the external medium and acts as a barrier to diffusion. As regards the relevance of studies of free paint films to the applied films, it may be mentioned that at this stage the substrate does not greatly affect the membrane properties of paint films, while it modifies notably the mechanical properties of the coatings (such as hardness and adhesion). A deeper insight into paint performance may be gained by studying the behaviour of free films. This point of view was put forward by Kittelberger and Elm [16] in their pioneering work on paints as membranes. It is interesting to analyse membrane properties of paint films, such as permeability, water uptake, ion-exchange capacity, permselectivity, etc. and compare them with those observed for more typically hydrophilic and hydrophobic membranes. The behaviour of model membranes having a hydrophobic matrix to which a fixed charge was added in a controlled manner will also be briefly considered. The fact that paint films have high electric resistances implies that the rate at which charges are transported through the paint films is much smaller than through the external solution. Even when a film consisted of parallel channels of small cross-section, there would be an increase in resistivity due

11 t o the film, by a factor equal to the ratio o f the geometric area of the film to the surface o f the channels. The rate o f p erm e a t i on of water or electrolytes through a high resistivity film is affected by small imperfections in it. A small pore* of 10 pm diameter in a film o f 200 tam thickness and 1 cm 2 of area, immersed in a 0.01 molar KC1 solution, reduces its resistivity to 2 × 107 ;Z cm, which is m uch smaller than that o f a typical poreless film. There is little d o u b t t hat paint films when applied properly do n o t have pores. The flow of species through a m e m br a ne is, according to Fick's law, di= --Di(dC;/dx)

(1)

where Di is the diffusion coefficient and Ci the c o n c e n t r a t i o n of species i in the film (Fig. 4). In general the e q u i l i b r i u m value of Ci in the m e m b r a n e will be different from that in the external solution (cO); the ratio Qi = (Ci/C ° )

(2)

being the partition coefficient. Since the c o n c e n t r a t i o n gradient is usually calculated with the values of the c o n c e n t r a t i o n of the external solutions, the diffusion coefficient is n o t directly determined, but rather the permeability coefficient Pi- F r o m the two previous expressons: di = -- QiDi (dC ° / d x ) = - - P i ( d C ° / d x )

(3)

showing th at the permeability coefficient depends on t w o factors: the diffusion coefficient, a transport p r o p e r t y , and the partition coefficient, an equilibrium property. Non-Fickian diffusion, generally called case II diffusion, is sometimes observed [17] for diffusion of solvents through polymers. The p h e n o m e n o logical distinction between this case and t hat described by eqn. (1) arises because in case II the p o l y m e r film undergoes mechanical stress imbalance which may even p r oduce cracks or fractures in the p o l y m e r matrix. For this case where there is no mechanical equilibrium inside the membranes, the force producing the flow of m a t t e r has a mechanical c o m p o n e n t and the • " '(-

cl

". ":.:'::;:".]

!:: .!i-::.::::: :.::

Fig. 4. Coneentration profile through a film during diffusion. *In this work we denote by pores those channels or cavities extending through the film and having sections which are distinctly larger than the free area normally present between atomic groups in the membrane matrix.

12 flow J of diffusant may be expressed by J = - - L (grad. p -- (l/C) div (S))

(4)

where S is the partial stress tensor and p is the chemical potential of the diffusing species. The second term in eqn. (4) implies convective flow within the polymer film. The relevance of these cases on coatings performance will be discussed in the next section. 3.1 Transport properties A general expression for the diffusion of small solutes through hydrophilic membranes in an aqueous environment is given by [ 18] Di = D ° g e l f ( Y p )

(5)

where D ° denotes the diffusion coefficient of species i in pure water. The term ge~ expresses the effect of the electrostatic interactions between the charges fixed to the membrane and the diffusing species upon the diffusion coefficient and is unity when the membrane has no charged sites. It slows down the transport of counterions (having charge opposite to the membrane charge) while practically it does not alter the rate of transport of co-ions (having the same charge of the membrane), and is unity for the transport of uncharged species. For univalent counterions, gel will be in the range 0.5 - 0.1 depending on the charge density of the membrane [18, 19]. The electrostatic effect will prove more important in establishing the permselectivity of membranes than in altering the magnitude of the diffusion coefficient. The water uptake of films has a big influence on the values of Di; this is expressed by the term f(Vp) in eqn. (5), where Vp is the volume fraction of polymer. For hydrophilic membranes (ion-exchangers [18] and poly(methacrylic) [20] membranes), f(Vp) has been shown to be correctly given by f(Vp) = e x p ( - - b Vp/(1 -- Vp))

(6)

in the range of 0.1 - 0.9 for Vp (that is 90 to 10% of water). The values found for b in eqn. (6) are between 0.9 and 1.4; this implies a very strong dependence of Di on the water uptake of the film (e.g., when Vp increases from 0.8 to 0.9, Di will decrease a hundredfold). This description cortesponds to homogeneous hydrophilic materials, the activation energies for transport being typically 3 - 5 kcal mo1-1 [18]. In these cases the polymeric chains act merely as steric hindrance to the transport of particles through the polymeric film, excluding a fraction Vp from the space available for diffusion; for this reason it is usually called the tortuosity effect. Table 2 summarizes the effect of various factors upon the diffusion of small solutes through hydrophilic membranes; the data correspond to poly(styrene sulphonate) ion-exchange membranes [21]. The data in Table 2 show that counterions diffuse more slowly than co-ions. The effect of the water content shows that diffusion coefficients are affected nearly to the same extent by a change in the water content of the material. For the case of membranes constituted by hydrophobic polymeric chains, like cellulose derivatives [22 - 24], poly(urethane) [25] and poly-

13 TABLE 2 Influence of charge and water content in the diffusion coefficients of various substances through poly(styrene sulphonate) membranes [21 ] Degree of cross-linking

Water content (%)

THO

C1

Na +

Z n 2+

1.7 4 8

74 62 44

1.17 0.96 0.42

0.67 0.70 0.26

0.49 0.32 0.14

0.13 0.095 0.030

(%)

105.Di/cm 2 s 1

(alkyl methacrylates) [26], the dependence Of DH~o on the water c o n t e n t is opposite to that given by eqn. (6). Toprak e t al. [27] have shown by IR spectroscopy that in dense cellulose acetate membranes H20 is d i s s o l v e d essentially as isolated monomers or dimers. The decrease in its diffusion coefficient with increasing water content in the membrane is attributed to the clustering of water molecules by intermolecular hydrogen bonding [26, 27]. Since water cannot form aggregates large enough to hydrate ions in these types of membrane, the transport of ions will be further hampered. These types of material [28] may have the property of desalination. This property could also be of value for protective coatings since it would imply a diffusional barrier for aggressive ions: The transport of gases through polymers is governed by the size of the diffusing species and is facilitated by the thermal rearrangement of the chain segments [29]. In this case typical activation energies for transport are in the range 10 - 20 kcal mo1-1. It is assumed that transport of solutes through membranes is based on a type of Cohen-Turnbull [30] mechanism, in which the free volume necessary for transport is provided by two different types of energy processes: for hydrophobic materials thermal rearrangements of chain segments is effective; for hydrophilic membranes the chain displacements are too slow and the rate-determining process is the thermal density fluctuation of the more mobile water molecules within the polymer network. Correspondingly D changes in two different ways with Vp depending on which energetic proces prevails. A change of the permeability of water in paint films with water content has been observed [31, 32] and a similar mechanism for ion and water transport through protective coatings has also been found [33]. However, the application of the consequences of the two mechanisms of transport to paint films is not direct because paint films are not homogeneous materials and their behaviour will also be influenced by their chemical composition, their degree of cross-linking, the presence of pigments, the fixed charge density, etc. For these reasons the properties of some model membranes which show a behaviour akin to that of paint films are described below on the basis of the two extreme mechanisms described above.

14

3.2 Equilibrium properties M a n y p a i n t films b e a r f i x e d c h a r g e s w h i c h are c r e a t e d b y c h e m i c a l r e a c t i o n d u r i n g film s e t t i n g (e.g. e p o x y - p o l y a m i d e cross-linkages, o x i d a t i v e c u r i n g of a l k y d p a i n t s ) , r e a c t i o n o f s o m e g r o u p s in t h e p o l y m e r s w i t h environmental reagents (acrylic and cellulosic paints), presence of additives f o r p o l y m e r i z a t i o n , a n d p r e s e r v e r s [ 3 4 ] or r e a c t i v e p i g m e n t s . S o m e p a i n t s m a y be c o n s i d e r e d i n t r i n s i c i o n - e x c h a n g e r s ; t h e i o n i c g r o u p s w h i c h are c h e m i c a l l y b o u n d t o t h e p o l y m e r c h a i n s are w e a k l y a c i d i c ( - C O O H ) or w e a k l y basic ( - N R 2 H + ) . C o n s e q u e n t l y p H will have a s t r o n g i n f l u e n c e on t h o s e p r o p e r t i e s o f t h e p a i n t f i l m s w h i c h are a f f e c t e d b y t h e f i x e d c h a r g e d e n s i t y . S o m e p a i n t s h a v e l i t t l e c h a n c e o f i n c o r p o r a t i n g f i x e d c h a r g e s in t h e i r p o l y m e r i c s t r u c t u r e s , as in t h e case o f p o l y ( v i n y l c h l o r i d e ) a n d chlorin a t e d r u b b e r . K h u l l a r a n d U l f v a r s o n [ 3 5 ] f o u n d z e r o f i x e d c h a r g e in p o l y u r e t h a n e films. V a r i o u s a u t h o r s [4, 35 - 38] have e s t a b l i s h e d t h e e x i s t e n c e of f i x e d c h a r g e s in p a i n t films. T a b l e 3 s u m m a r i z e s t h e i o n - e x c h a n g e c a p a c i t y f o u n d f o r s o m e t y p i c a l p a i n t films a n d t h e i r c h a n g e w i t h pH. F o r a l k y d resin p a i n t films a r e l a t i o n b e t w e e n f i x e d c h a r g e d e n s i t y a n d w a t e r u p t a k e was o b s e r v e d [ 3 5 ] . As s h o w n in T a b l e 3, t h e v a l u e s o f C M are small w h e n c o m p a r e d w i t h t y p i c a l i o n e x c h a n g e r s , b u t t h e y are n o t n e g l i g i b l e a n d t h e y c a n s t r o n g l y i n f l u e n c e t h e p r o p e r t i e s o f t h e films. T h e p r e s e n c e o f f i x e d c h a r g e m o d i f i e s t h e v a l u e s o f t h e p a r t i t i o n coe f f i c i e n t , Q, f o r i o n i c solutes. D o n n a n e x c l u s i o n o f e l e c t r o l y t e will a l w a y s be o b s e r v e d w h e n f i x e d c h a r g e s e x i s t in t h e films; this p e r m s e l e c t i v i t y arises

TABLE 3 Concentration of fixed charges (C M) found in paint films Paint type

pH

Sign fixed charge

CM (mol kg-1 )

CH: O (%)

Ref.

1.6 2.0 - 2.3 3.8 - 4.0

35

Alkyd (33% phthalic anhydride)

4 7 9

----

0.07 - 0.08 0.37 - 0.40 4.30 - 4.90

Epoxy-polyamide (1:1 )

4 6 7 8

+ + ---

0.65 - 0.66 0.36 0.09 - 0.13 1.90

Epoxy-polyamide (2:1)

4 6 7 8 3 11 4 6

+ + --+ (+--) + +

0.10 - 0.13 0.08 0.03 - 0.04 0.56 - 0.60 0.18 0.08 0.40 0.16

Epoxy-polyamide-coal tar

35

35 0.5

5.4 - 2.4

4 35

15

in order to preserve the electroneutrality of the system at equilibrium. Since the contribution of fixed charges to the total inventory of charges in the films is negligible when the external electrolyte concentration is high, Donnan exclusion is absent in this case. The equilibrium distribution of electrolyte Y X between a membrane having C M fixed charges per unit volume and an external solution of electrolytes Y X may be expressed by the Donnan coefficient, r, which, assuming ideal solutions, becomes r

-

C

-

2

(cM) 2 +

--

(7)

employing the equilibrium condition (C + CM)C = C 2. Thus for low external electrolyte concentration, there is no electrolyte Y X in the film, which will then act as a good barrier for the transport of electrolyte through the paint membrane. In spite of the fact that typical values of C M for paint materials are less than 20% of those in ion-exchangers (4 - 5 mol kg-1, cf. Table 3), strong permselectivity has been found for paint films. A direct way of establishing the permselectivity of paint films and the type of fixed charges they bear is the determination of the EMF of a concentration cell where both compartments are separated by the paint film. Figure 5 illustrates the experimental situation. If both electrodes are assumed to be reversible to C1- ion, which is the most c o m m o n case, and at both sides of the film KCI solutions are introduced, the EMF for such a cell is given by RT

a2

E = 2tK+-- l n - F al

(8)

if the transference number is considered to be concentration independent. In eqn. (8) ai is the activity of KC1 in solution i. For a membrane having a large positive charge density, tK+ ~ 0 and no EMF would be observed since the membrane potential would be exactly equal and of opposite sign to the Nernstian potential for such electrolytes. If CM were large and negative, tK+ ------- 1 and the observed EMF would be equal to that in an identical cell but with the film replaced by an electrode reversible to potassium ions.

[L[CrRO ~.[S

1

f,,

U

Fig. 5. Cell for t r a n s f e r e n c e n u m b e r d e t e r m i n a t i o n .

16 Teorell [39] has derived a general treatment for the EMF considering the Donnan exclusion and giving E in terms of the transference numbers in e l e c t r o l y t e s o l u t i o n a n d C M. F i g u r e 6 is a p l o t o f r f o r p o l y ( v i n y l c h l o r i d e ) films having a fixed charge due to -COOH groups of the peroxide catalyst; the values of r were calculated by Kumins and London [34] from membrane potential measurements using Teorell's equations. They have obtained values of C M between 0.10 and 0.16 mol kg -z for poly(vinyl chloride), poly(vinyl acetate) and their mixtures.

100

L .OD~

1 .QI

I .1

I 1

rrl

Fig. 6. D o n n a n c o e f f i c i e n t for K + in PVC m e m b r a n e s against e x t e r n a l c o n c e n t r a t i o n of KCI [341.

TABLE 4 T r a n s f e r e n c e n u m b e r s o f K + ions t h r o u g h p a i n t films Paint t y p e

CKC1

pH

tK*

Sign charge

Ref.

E p o x y - p o l y a m i d e - c o a l tar

0.001 - 0 . 0 0 2

3.5 5.5 10

0 . 0 6 - 0.33 0.25 - 0.50 0.46 - 0.50

+ + nil

Alkyd

0.01 - 0.1

6.5

0.89 - 0.91

--

36

4 4 4

Epoxy polyamide (1:1)

0.08 - 0.24

+

36

E p o x y - p o l y a m i d e (2:1 )

0.68 - 0.69

--

36

0.70 - 0.71 (film in H + or K + form) 0.91 (film in Ca + form)

--

38

--

38

1.0 0.81

---

34 34

Pentaerythritol

0.01 - 0.1

neutral

neutral

PVC-PVA

0 . 0 0 5 - 0.01 0.05 - 0.1

neutral neutral

17

There is some difficulty in carrying out the determination of the apparent transference numbers in paint films due to the high resistance and the fact that occasionally persistent polarization effects are observed for these membranes [4]. Usually the permselectivity of membranes, as established by transference number measurements, disappears if very concentrated electrolyte solutions are employed, indicating that permselectivity arises by Donnan exclusion. Experimental results are summarized in Table 4. 3.3 M o d e l m e m b r a n e s

In order to use the above ideas to study the behaviour of paint films it is important to realize that these are membranes having a relatively small ionexchange capacity and a hydrophobic matrix of high resistivity. Consequently it was of interest to study model membranes having those characteristics and compare their behaviour with those of typical hydrophilic and hydrophobic membranes. The films had poly(vinyl chloride) matrices and contained variable amounts of dispersed poly(styrene sulphonate). The polyelectrolyte was entrapped in the matrix polymer by chain entanglements and the resulting films showed no heterogeneities larger than 500 h . Their properties are summarized in Table 5. The following points are relevant for paint film studies: (i) the resistivities of the model films span more than two order of magnitude, (ii) all the materials show strong permselectivity, (iii) no simple relation between the water content and Q with CM was found. A close relation was found between the reciprocal of the resistivity and the permeability of the films. The values of the permeability coefficients calculated from the resistivities are reported in parentheses in column 7, for TABLE 5 P r o p e r t i e s o f m e m b r a n e s o f P V C - p o l y ( s t y r e n e s u l p h o n a t e ) at r o o m t e m p e r a t u r e Membrane

Thickness (pro)

CM (tool kg -1)

H20(a) (%)

Q(b)

109.•(c) (S cm -1)

1011.p,(d) (cm 2 s 1)

tK+(e)

1 3 5 7 8 11 14 19

66 31 37 66 195 108 62 34

0.49 0.39 0.33 0.30 0.25 0.195 0.11 0.045

6.4 8.1 --7.9 3.6 8.3 3.0

0.022 0.038 0.055 0.016 0.017 0.024 0.017 0.012

3.4 1.5 3.1 0.6 240 49 4.2 11

1.3 (2.5) 0.40 (1.0) ----0.85 (3.0) 6.0 (7.7)

0.87 0.93 1.00 0.92 0.92 0.85 0.92 0.87

* (a) (b) (c) (d)

Values at 25 °C, e x c e p t m e m b r a n e 1 at 15 °C. A f t e r e q u i l i b r a t i o n in pure water. P a r t i t i o n c o e f f i c i e n t o f KC1. E q u i l i b r a t e d w i t h 0.05 M KC1. P e r m e a b i l i t y o f KC1 t h r o u g h the m e m b r a n e separating a 0.05 M KC1 s o l u t i o n f r o m distilled water. (e) Measured by E M F in a cell having 0.1 and 0.2 M KC1 s o l u t i o n s in each o f t h e compartments separated by the membrane.

18 the sake of comparison. The agreement between p-1 and P is relatively good if due account is taken of the complex nature of the systems. This confirms that even in paint films and model membranes all the transport properties are correlated and this feature can only be established if the same films are e m p l o y e d to measure all the properties. This was also not ed for e p o x y - c o a l tar films [ 3 3 ] . Permeabilities and conductivities, with the exception of membrane 8, show a trend towards decreasing electrolyte flow with increasing water c o n t e n t; furthermore, activation energies for the transport of KC1 were f o u n d in the range 9 - 15 kcal mo1-1 . Thus a h y d r o p h o b i c t ype of mechanism is apparently operative between the ionogenic h y d r a t e d regions of the film. The lack of correlation between water uptake and C M may be attributed to the different charge distribution in the films; the ionic p o l y e l e c t r o l y t e chains are entrapped in the PVC matrix in different ways and the swelling pressure o f the matrix when water flow into the film will also be different. Thus it is clear that for model membranes having a more complex chemical nature, there is no obvious correlation among the different membrane properties. 3.4 Swelling and osmotic pressure When a p o l y m e r film is put in contact with an aqueous solution the activity of water is usually higher in the external medium, consequently water flows into the film creating a pressure t hat counteracts the solvent inflow. When the pressure generated equals the osmotic pressure difference between internal and external media, the solvent flow from the inside balances the solvent flow from outside. Inside the films the pressure generated mechanically by the process of swelling produces an elastic stretching of the p o l y m e r network. When the external medium is pure water or very dilute saline solutions (aw ~ 1), the water uptake of the material is maximal and the film stretches to its greatest point. At the other extreme, films which have been equilibrated in dilute solutions, when placed in solutions of higher osmotic pressure, lose water to the external medium; this agrees with the observation of Mayne [40] of regions I in paint films having electrical resistances which increase with external electrolyte concentration. The osmotic and swelling behaviours of paint films are probably the membrane properties responsible for the differences observed between free and applied films. These differences appear mainly in the mechanical properties o f the films which are directly related to the mechanical response of the paint material to the external medium. If the memb r a ne has fixed charges in its matrix, the osmotic pressure (~) that will be generated when equilibrium is achieved will be aw int 71"Y n 2 0

-= - -

RTln

ext

~

RT(CM+

2(C -- C))

(9)

aw

The swelling pressure is pr oduced by the fact that the polymeric chains in the paint film are interconnected either by chemical cross-linking (epoxy,

19 alkyd, urethane) between chains or by chain entanglements (cellulose nitrate, chorinated rubber). At low water activity, water enters the spaces and holes existing in the molecular structure of the paint film and interacts with the polar or ionic groups in the paint network. As the water activity increases, the accumulation of water brings in an elastic stretching of the film. For highly cross-linked polymers, the deformation due to swelling is small because as soon as some water molecules force the chains apart, the swelling pressure reaches the value n. On the other hand, polymers which are hydrophilic, having strong polar groups or ionic sites, swell more because there is a larger free energy decrease due to stronger polar or ionic groupH 20 interactions. It has been observed that during the curing process of polymer films a contraction of the material takes place, which if the film is free or applied on thin foils leads to a measurable change of its dimensions. When the cured films are exposed to a humid environment they swell, creating swelling pressures in the order of hundreds of atmospheres, e.g. 600 kg cm -2 for a 5% water uptake in poly(styrene sulphonate) membranes [41].

4. Permeability of paint free films to water, oxygen and ions The corrosion of iron or steel may be described by the following reaction: 4Fe + 202 + 2H20

* 2Fe2Oa-H20

(II)

In the most c o m m o n situation in natural media, oxygen and water are the cathodic reagents, according to 4e + 02 + 2H20

) 4OH-

(III)

Water is also necessary for the formation of the various hydroxocomplexes and hydrates as corrosion products. For the coated metals, the rate of corrosion is conditioned by the rate at which the reactants of (II) are made available to the metal at the interface with the paint film [42, 43]. Table 6 gives various estimates of the rate of corrosion of steel in natural environments, and the corresponding requirements of 02 and H20 according to reaction (II). If aggressive species such as CI- or H ÷ ions are present, their permeation through the protective films will also be important for the performance of the protective coating under such conditions. Permeation is especially relevant for an evaluation of the performance of inert paint films having no electrochemically- active pigments. There are many data in the literature for the permeation rate, diffusion coefficients or flow of oxygen and water through paint films. Unfortunately, most results are given in an ambiguous manner and they cannot always be brought to a c o m m o n dimensional basis to allow comparison between them. Some results considered to have no ambiguities are given in Table 7.

2O TABLE 6 Rate of corrosion of steel in natural environments Rate of corrosion (mg/cm 2 year)

H20 consumed

02 consumed

(mg/cm 2 day)

(mg/cm 2 day)

70

0.030

0.082

7 - 128 13.3 - 333

0.003 - 0.056 0.006 - 0.147

0.008 - 0.151 0.016 - 0.392

7.9 - 79

0.004 - 0.035

0.009 - 0.093

Conditions

Ref.

Sea water saturated with 02 and industrial atmosphere Atmospheric corrosion Atmospheric corrosion (extreme cases have been eliminated) Atmospheric corrosion

42

43 44

45

It m a y be o b s e r v e d f r o m d a t a r e p o r t e d in T a b l e 7 t h a t t h e rate o f w a t e r p e r m e a t i o n is large e n o u g h to sustain t h e n o r m a l c o r r o s i o n r a t e o f steel; t h u s the p a i n t film is n o t an e f f e c t i v e b a r r i e r t o w a t e r p e r m e a t i o n . On t h e c o n t r a r y , the rate o f availability o f o x y g e n t h r o u g h p a i n t films is clearly r e d u c e d b y t h e film a n d this f a c t o r will slow d o w n the rate o f c o r r o s i o n . T h e d a t a o f Yaseen and F u n k e [31] a n d F u n k e e t al. [ 3 2 ] s h o w t h a t in s o m e instances t h e f l o w o f o x y g e n increases w i t h t h e w a t e r c o n t e n t in t h e film, p r o b a b l y due to t h e swelling o f t h e p o l y m e r film or, in o t h e r words, b e c a u s e w a t e r acts as a plasticizer. F o r a l k y d a n d m o d i f i e d cellulose p a i n t films, t h e y observed an increase o f 30 - 40% in w a t e r p e r m e a b i l i t y f o r clear varnishes a n d u p to 100% increase f o r p a i n t c o n t a i n i n g TiO2 p i g m e n t w h e n the g r a d i e n t o f w a t e r activity was at 100 - 50% c o m p a r e d t o t h a t at 50 - 0% relative h u m d i t y . As a l r e a d y m e n t i o n e d , t h e w a t e r c o n t e n t o f p o l y m e r films is an i m p o r t a n t p a r a m e t e r in d e t e r m i n i n g m a n y o f t h e film p r o p e r t i e s as m e m b r a n e barriers. T h e values n o r m a l l y f o u n d f o r w a t e r u p t a k e o f clear p a i n t films equil i b r a t e d in a high relative h u m i d i t y e n v i r o n m e n t {close to 100%) are in t h e range o f 0.5 t o 6%, o c c a s i o n a l l y t h e y m a y be a r o u n d 10 - 15% f o r r a t h e r h y d r o p h i l i c m e m b r a n e s like cellulose a c e t a t e or e p o x y - p o l y a m i d e ( 1 : 1 ) [ 4, 31, 3 6 ] . F r o m t h e practical p o i n t o f view, it is i m p o r t a n t to n o t e t h a t t h e actual o b s e r v a t i o n o f p a i n t p e r f o r m a n c e indicates t h a t f r e q u e n t l y in t h e c o a t e d metals, t h e c a t h o d i c a n d a n o d i c regions are s e p a r a t e d . W h e n e v e r a region o f an i m m e r s e d c o a t e d m e t a l s t r u c t u r e is d a m a g e d a n d t h e c o a t i n g peeled off, t h a t site will have a t e n d e n c y to b e c o m e a c a t h o d i c region b e c a u s e t h e r e 0 2 is m o r e freely available t h a n in t h e surface o f t h e c o a t e d metal. In this case t h e p e r m e a b i l i t y o f t h e film t o o x y g e n will n o t be a r e l e v a n t p a r a m e t e r f o r a g o o d p r o t e c t i v e p e r f o r m a n c e o f t h e p a i n t film. In this s i t u a t i o n it is of u t m o s t i m p o r t a n c e t h a t a high resistance exists b e t w e e n c a t h o d i c and a n o d i c regions in t h e film t o l i m i t t h e rate o f c o r r o s i o n ; i.e. a high resistivity c o a t i n g will a f f o r d a b e t t e r p r o t e c t i o n in this case.

21 TABLE 7 Water and oxygen flow through paint free films of 100 pm Paint

J/(mg cm -2 day-1) H20

Alkyd (15% PVC Fe203) Alkyd (35% PVC Fe203) Epoxy-coal tar Epoxy-polyamide (35% PVC Fe203) Chlorinated rubber (35% PVC Fe203) Cellulose acetate Cellulose nitrate Chlorinated rubber Copolymer (vinylchloride-vinylacetate) Copolymer (12% PVC TiO2) Alkyd-melamine Epoxy-melamine Polyurethane

02 0.0069 0.0081 0.0041 0.0064 0.0168 (a) (b) 0.013 0.026 0.069 0.115 0.0013 0.0013 0.0022* 0.0048 0.0041

2.37 2.07 1.07 4.96 3.48 36 4.8 1.0 8.0 1.0 1.2 1.0 1.8 1.4*

Chlorinated rubber Epoxy Epoxy-polyester Polyurethane Vinyl Styrene-acrylic latex Vinyl lacquer Alkyl-melamine Cellulose nitrate Alkyd

(e) 1.19 1.52 2.74 1.09

Epoxy-coal tar (a) (c) (e) (g)

46

32(*47)

0.0014 0.0011 0.0076*

0.6 - 1.7 (c) (d) 0.11 0.28 0.85 0.96 1.06 1.15 1.13 1.14 0.69 0.88 25.3

Alkyd

Ref.

(f) 1.51 2.54 3.95 1.45

0.55 (g)

42 13

47 31

33

0% relative humidity and (b) 95% r.h. capacity method and (d) gravimetric method. 50 -~ 0% r.h. and (f)100 -+ 50% r.h. tracer diffusion of THO.

Less i n f o r m a t i o n is available f o r e l e c t r o l y t e or salt p e r m e a t i o n . The permeabilities o f NaC1, 22Na+ a n d T H O t h r o u g h e p o x y - p o l y a m i d e - c o a l tar films [ 3 3 ] indicate t h a t all these species have t h e same t r a n s p o r t m e c h a n i s m and t h a t their m o b i l i t y is t h r e e orders o f m a g n i t u d e smaller t h a n t h a t in an e l e c t r o l y t e solution. T h e observed decrease o f D is, h o w e v e r , smaller t h a n given b y eqn. (6), w h i c h predicts f o r a 5% w a t e r c o n t e n t a r e d u c t i o n o f D o f 10 -1° times as c o m p a r e d with t h e e l e c t r o l y t e s o l u t i o n value. This d i s c r e p a n c y

22 indicates t h a t w a t e r is quite p r o b a b l y n o t h o m o g e n e o u s l y d i s t r i b u t e d in the p a i n t films. M u r r a y [49] o b t a i n e d values o f t h e s a m e o r d e r o f m a g n i t u d e f o r t h e p e r m e a b i l i t y c o e f f i c i e n t s o f NaC1 t h r o u g h cellulose a c e t a t e a n d e p o x y p o l y a m i d e films; his d a t a are, h o w e v e r , s o m e w h a t u n c e r t a i n b e c a u s e the diffusion c o e f f i c i e n t s o f chloride ion w e r e n o t o b s e r v e d to be c o n s t a n t . M u r r a y o b s e r v e d an increase in the d i f f u s i o n o f C1- w h e n the films were applied on a s u b s t r a t e ; t h e d i f f e r e n c e s were, h o w e v e r , n o t v e r y large a n d m a y reflect the n a t u r a l s c a t t e r o f the data. A t all events, if t h e d i f f e r e n c e s are real t h e y are small e n o u g h t o s u p p o r t t h e view o f K i t t e l b e r g e r and Elm [16, 50] t h a t the s u b s t r a t e does n o t alter f u n d a m e n t a l l y t h e p r o p e r t i e s o f t h e p a i n t films. K i t t e l b e r g e r and E l m [51] d e t e r m i n e d the flow o f NaC1 t h r o u g h p h e n o lic, p o l y v i n y l b u t y r a l a n d a l k y d clear varnish films. T h e i r values increased r e m a r k a b l y w h e n p i g m e n t s w e r e a d d e d t o t h e p a i n t films. T h e d a t a f o r NaC1 t r a n s p o r t t h r o u g h p a i n t films are s u m m a r i z e d in T a b l e 8. TABLE 8 Permeability of NaCl through paint films Type of paint

101°.P/cm 2 s-1

J*/mg cm 2 day-1

Ref.

Epoxy-coal tar Cellulose acetate Epoxy polyamide Alkyd Phenolic Polyvinylbutyral

0.2 - 2.6 2 2 0.72 0.08 0.04

0.2 - 2.2 X 10-4 1.7 X 10 -7 1.7 × 10 -4 6.2 X 10 -5 7 X 10 -6 3 × 10-6

33 49 49 51 51 51

*The flow has been calculated for a film 100 pm thickness and a concentration gradient of 100 p.p.m. NaCl. It is i n t e r e s t i n g to n o t e t h a t t h e higher f l o w o f w a t e r as c o m p a r e d w i t h e l e c t r o l y t e s (cf. T a b l e s 7 a n d 8) is n o t d u e t o a higher p e r m e a b i l i t y o f the p e r m e a n t or to a d i f f e r e n t m e c h a n i s m o f t r a n s p o r t , b u t r a t h e r to a larger gradient of concentration for H20.

5. S t r u c t u r e a n d m o r p h o l o g y o f p a i n t films F u n k e [ 2 ]~ has r e c e n t l y s u m m a r i z e d t h e view prevailing y e a r s ago a b o u t t h e s t r u c t u r e o f p a i n t coatings: " F o r m a n y years it has b e e n a c o m m o n opinion in t h e p a i n t field t h a t b i n d e r materials as used in p a i n t f o r m u l a t i o n s lead to films w h i c h h a v e h o m o g e n e o u s a m o r p h o u s s t r u c t u r e s . " This r a t h e r simplistic view has b e e n d i s p r o v e d b y F u n k e a n d c o w o r k e r s [2, 52] a n d also b y M a y n e a n d c o w o r k e r s [ 5 3 ] . T h e first g r o u p has s h o w n t h e i n f l u e n c e o f m a n y processes o n the final s t r u c t u r e o f t h e films especially during the

23 process of drying or curing. For example, solvent evaporation may induce phase separation in the system; water condensation produces 'blushing', the formation of porous external film structure [52]. Some polymeric components of the paint may either settle preferentially close to the metal interface or move away from it. Oxidative curing, as in alkyd and e p o x y - e s t e r paints, will start from the air-film interface and later reaches the metal-film interface; finally, some cross-linking reactions may be affected by the metal substrate. As a consequence of these findings it is now clear that appreciable morphological differences exist in paint films, and that the resulting coating will be heterogeneous. These heterogeneities are very important for the actual performance of the protective coatings. A similar pattern emerges from the important work of Mayne and coworkers [53], who have studied the resistivity of paint films. Their high values are considered responsible for the good protective capacity of paints having no electrochemically active pigments. Mayne [40] has found that the resistance of clear paint films is of two types: regions D, having resistances that decrease with the concentration of electrolyte, and regions I where the resistance increases with the concentration of electrolyte. It appears that clear varnish films of pentaerythritol, alkyd, tung oil and phenol-formaldehyde are intrinsically heterogeneous and the distribution of I and D regions has been mapped [54] to 0.1 cm 2. Regions havingD conduction are flooded by external electrolyte showing no Donnan exclusion. I regions have a conduction which depends on the osmotic pressure of the external solution; in concentrated solutions the osmotic solvent depletion of the film converts D regions into I regions. This behaviour and the fact that I regions have a larger microhardness and less swelling t h a n D regions [55] have led to the conclusion t h a t / r e g i o n s have a higher degree of cross-linking than D regions. Two types of ionic transport mechanisms would correspond to the two different types of resistance behaviours, according to Cherry and Mayne [56]. In regions of type I high cross-linking and hardness, low swelling -- the ions would be transported by activated jumps between sites having ionogenic or strongly polar groups where water is preferentially absorbed. This corresponds to our description of hydrophobic and model membrane behaviour having large activation energies for transport. Regions of type D would be closer to the behaviour typical of ion-exchange polymers and correspond to transport through hydrophilic membranes. Mayne and Mills [3] have observed that pigments alter the distribution of D and I regions in the films, but have no effect on other properties. E p o x y - p o l y a m i d e coal tar films also show an extensive degree of heterogenity [4] ; however, for such paint the most c o m m o n behaviour encountered corresponds to that of a charged membrane with permselectivity and Donnan exclusion. Some areas had D type behaviour but no I regions were found in e p o x y - c o a l tar films. The heterogeneities, which are intrinsic to paint films, do n o t support the view that "films have homogeneous amorphous structures", and there is a -

-

24 s t r o n g a r g u m e n t f o r a d v o c a t i n g t h a t all t h e film p r o p e r t i e s should be evaluated o n t h e s a m e piece o f film to facilitate m e a n i n g f u l c o r r e l a t i o n o f t h e i r properties. P r o l o n g e d testing o f c o a t e d panels in saline w a t e r s h o w s t h a t t h e coatings s t a r t failing in regions w h i c h have low resistance [4] ( e p o x y - c o a l tar) o r in D regions (alkyd, t u n g oil a n d e p o x y - p o l y a m i d e films) [ 3 ] . T h e e f f e c t o f the t e m p e r a t u r e at w h i c h e p o x y - p o l y a m i d e films are c u r e d is v e r y i m p o r t a n t f o r p a i n t p e r f o r m a n c e a n d its beneficial e f f e c t can be e v a l u a t e d f r o m the m e a s u r e m e n t s o f resistance o f c o a t e d panels [ 4 ] .

6. F r e e films

versus

a p p l i e d films

T h e q u e s t i o n o f h o w r e l e v a n t are t h e studies o f free films f o r actual a p p l i e d coatings is v e r y i m p o r t a n t . M u r r a y [49] has f o u n d s o m e d i f f e r e n c e s in t h e p e r m e a b i l i t y o f NaC1 t h r o u g h free a n d a p p l i e d p a i n t films; h o w e v e r , t h e s e are r a t h e r small a n d m a y be d u e to the n o r m a l s c a t t e r o f p a i n t film samples. D i f f e r e n c e s in w a t e r u p t a k e o f a p p l i e d a n d free films have b e e n report e d b y s o m e w o r k e r s [15, 5 7 ] . In o n e instance p a i n t s c o n t a i n i n g c h r o m a t e p i g m e n t s h o w e d large d i f f e r e n c e s in w a t e r a b s o r p t i o n u n d e r t h e t w o conditions. F u n k e [2, 58] has d e s c r i b e d a p p r e c i a b l e d i f f e r e n c e s in w a t e r u p t a k e f o r s o m e p a i n t films He related t h e s e findings to t h e loss o f a d h e s i o n o f the film in high h u m i d i t y c o n d i t i o n s and to its p r o t e c t i v e c a p a c i t y . We have s t u d i e d t h e e f f e c t o f s u b s t r a t e s (steel a n d glass) on the w a t e r u p t a k e o f e p o x y - p o l y a m i d e (CIBA) clear varnish; the m e a s u r e m e n t s w e r e d o n e on a large n u m b e r o f s a m p l e s so t h a t statistically m e a n i n g f u l averages c o u l d be o b t a i n e d . T h e w a t e r a b s o r p t i o n was achieved b y isopiestic equil i b r a t i o n o f the film s a m p l e s a n d weighing. T h e results, w i t h t h e i r s t a n d a r d deviations, are r e p o r t e d in T a b l e 9. TABLE 9 Water uptake of free and applied epoxy-polyamide films Type of film

Free Applied on glass (a) Applied on glass (b) Applied on steel

No. of samples

Average water content, g/100 g dry film aw = 1.0

aw = 0.84

aw = 0.63

22 19 21 10

6.90 4.40 3.65 3.45

2.71 2.22 2.16 1.81

2.23 ± 1.12 1.94 ± 0.12 1.76 ± 0.73 --

± 4.10 ± 1.30 ± 0.66 ± 0.30

-+ 1.73 ± 0.50 -+ 0.12 ± 0.12

(a) Stoichiometric ratio of polyamide to epoxy. (b) Excess polyamide. The time of equilibration varied between 10 and 20 days, when no more changes in weight were observed. The films had a thickness between 120 and 250/~m for applied films and between 70 and 170 pm for the free films.

25 It may be seen t hat there is a t e n d e n c y towards a greater water c o n t e n t in the free films than in applied films, especially at 100% relative humidity. It is interesting to n ot e the large scatter f ound for the water contents of the free film samples when compared with the applied coatings. However, the reproducibility of each individual m e a s ur e m ent was better than 1% for all cases. This means that relatively little weight can be attached to conclusions of film behaviour drawn from a reduced n u m b e r o f samples. In spite of the large scatter, only free films were f ound to have m ore than 6% water. Free films at 63% relative humidity were subjected to two cycles of a b s o r p t i o n - d e s o r p t i o n of water and no significant differences were observed. Applied films were detached from their substrates by prolonged immersion in water, and the water upt a ke of the detached films was then measured again; no difference in water absorption was observed for the films t hat had been applied over steel, and only a small increase for those on glass (just above the standard deviation). These observations led to the conclusion that differences in water absorption behaviour n o t e d between free and applied films may be influenced by the p r o n o u n c e d scatter in the results f o u n d for individual samples. The differences between free and applied films are n o t eliminated by film detachm e n t and consequently appear to be generated during the process of curing of the films, which affects the elastic stress of the film n e t w o r k in different ways. This c o n t e n t i o n is also supported by the fact t hat differences are especially notable at high humidities when water c o n t e n t is m ore d e p e n d e n t on the elastic stresses of the polymer. 7. Film adhesion An i m p o r t a n t p r o p e r t y of applied films is the adhesion between the coating and the metal substrate. There is evidence t hat when an applied prot~ctive coating is exposed to high humidity or immersed in an aqueous medium, a loss of adhesion takes place [47, 57, 5 8 ] , which is very severe in some cases. Walker [ 57 ] clarified m a ny points Concerning the effect of water upon the adhesion o f paint films. He f o u n d that after a short period of exposure to a humid e nv ir o n men t the adhesion of coatings d r o p p e d sharply. This p h e n o m e n o n occurred even before water could be absorbed to any significant e x t e n t in the paint film. Films having a large water absorption and a high water permeability lose adhesion faster. Walker has shown the importance of the curing process in the way adhesion is lost and recovered after the coating is dried. Protective films which cure chemically ( e p o x y - p o l y a m i d e and p o l y e s t e r isocyanate) are greatly affected by humidity. T he loss of adhesion u p o n exposure o f various types of paints runs in the following order: larger (

loss of adhesion

~ smaller

Chemical cure > Oxidative cure > Solvent drying > Thermosetting

26 Upon drying, adhesion was recovered in the order: larger (

recovery of adhesion

~ smaller

Oxidative cure > Solvent drying > Chemical cure > T herm oset t i ng Polyurethane films did n o t recover at all on drying; thermosetting coatings recovered up to 60% of the initial adhesion when dried at high temperatures. The pigment c o n t e n t of the paint had no effect on the change of adhesion of the films, but osmotic pressure of the external solutions did. Walker observed no relation between loss of adhesion and the state of the metallic substrate after exposure, suggesting that adhesion is n o t directly c o n n e c t e d to the protective action of organic coatings. Walker concludes that "high initial adhesion values are quite immaterial to long term durability". Funke and coworkers [2(a), 5 8 ] , on the basis of their work on film adhesion and the effect of substrates upon water uptake, suggested t hat an accelerated test for film performance could consist of determining the differences of water uptake of free and applied films. T h e y have observed a close correlation between excess water absorbed in the applied film and the loss of adhesion of the coating and indicated that the time at which the applied films has a larger water c o n t e n t than the free film (crossover time} should be taken as the quantitative criterion for quality of p r o t e c t i o n of the paint. T h e y consider t hat the observed excess water goes to the m et al -fi l m interface and causes a decrease in adhesion. A calculation shows t hat for a coating having a density of 1.2 g cm -a, 1% increase in water c o n t e n t corresponds to an equivalent of 800 water monolayers for films of 300 p m thickness and 130 water monolayers if the film is only 50 t~m thick. Consequently, it appears t hat the excess water may n o t accumulate at the interface b u t spreads over all the polymeric film. It may be somewhat more concentrated in the vicinity of the metallic substrate. The results of the work of Walker and of Funke and coworkers agree with our observations t ha t the loss of adhesion of coated panels upon exposure to high h u mid i t y is a rather general p h e n o m e n o n , while all coatings when applied do n o t absorb m or e water than when they are free. The fact t h a t the films of e p o x y - p o l y a m i d e when detached from the substrates had the same water c o n t e n t as when t he y were applied, does n o t agree with the view that water accumulation occurs at the m e t a l - f i l m interface. The lack o f correlation between loss of adhesion and the corrosion of metallic substrates, and between the initial adhesion strength of a coating and its performance on prolonged exposure [ 5 7 ] , shows that adhesion is not simply related to the protective capacity but to ot her factors also. Th e fact that the curing or cross-linking procedure is i m p o r t a n t for paint p e r f o r m a n c e [3, 4] and determines the changes in adhesion when the coatings are exposed to high humidity and then dried [57] suggests that the whole p h e n o m e n o n may be related to the mechanical properties of the films and the changes occurring during curing and weathering.

27 When films are applied on a substrate and then put in c o n t a c t with water, an elastic stress due to swelling develops on the m e t a l - f i l m interface which can be held responsible for the generally observed loss of adhesion for films exposed t o a high water activity. Internal stress has been observed [59] in coatings o f e p o x y - p o l y a m i d e applied on thin metal foils. T he stress p r o d u ced when the films are applied on a rigid substrate are in a direct relation to the a m o u n t of water absorbed by the film. Films of 30 - 60 ~m thickness absorbed up to 5% water and elongated 1.75% when equilibrated as free films. During curing, a p o l y m e r usually undergoes a contraction and develops internal stress, the e x t e n t of which depends on whether the p o l y m e r film is in the applied state or is free. In a coating applied over a rigid metal substrate, strong stresses may appear. The swelling (elastic) pressure in the films ultimately determines the water uptake o f the films. F o r a film anchored to the metal surface u n d er strain, the swelling stress will be different from t hat in the unstrained state as free film. In other words, it is possible to expect a dependence o f the water uptake on the internal stress distribution in the film. F u r t h e r m o r e , this effect will have a longer range in the paint film than other mechanisms suggested to explain the different water uptake of free and applied films. It appears to be a more likely explanation for the observed loss of adhesion o f the humid film and its recovery u p o n drying as well as of the dependence o f this p h e n o m e n o n on the t y p e of curing process of the film. These arguments suggest that it would be interesting to measure the change o f water uptake in free films subjected to different mechanical stresses applied externally. Th e existence o f internal stresses in swelled p o l y m e r films is well k n o w n and it is reflected in the properties of transport through membranes by the occurrence o f two different types of limiting diffusional behaviour which have been observed in p o l y m e r films (cf. Section 3). For case I of F4ckian transport the kinetics of absorption of a p e n e t r a n t into the film follows the classical expression: M ( t ) = K ' . t 112

(10)

where M ( t ) is the fraction of diffusant t ha t has penetrated at time t (valid for initial stages of absorption). In case II diffusion it has been shown [60] t hat the initial kinetics of absorption follows a linear dependence with time: M(t) =g".t

(11)

This t y p e of behaviour occurs when there are swelling pressure gradients in the films. Consequently, the inhomogeneous distribution of p e n e t r a n t generates a convective flow within the p o l y m e r film during absorption. Alfrey et al. [60] have r e p o r t e d case II diffusion processes in some polymer films where the stresses generated during diffusion were so large t hat the films cracked in several places. Kwei and Zupko [61] have also observed

28 this t y p e o f diffusional behaviour and have determined the way it is influenced by various penetrants. Case II diffusion is frequently accompanied by fractures o f the material which may be either macroscopic or microscopic. The two types of transport behaviour would explain the differences rep o r t e d in the water uptake kinetics for free and applied paint films. Figure 7 is a schematic representation of the different reported behaviours [ 2, 15, 47 ]. It is clear from Fig. 7 that the dependence of M(t) with t is different in both cases.

+

t

Fig. 7. Schematic change of the fraction of water absorbed, M(t): vs. time. An equation o f the form

M(t) =K.t"

(12)

describes with more generality the observed kinetics of water absorption in paint films. Th e e x p o n e n t n is in the range 0.5 - 1.0 and is always closer to n = 1.0 (case II diffusion) for t he film (either applied or free) which has the larger water content. Since the substrate alters the mechanical state of the paint film, the kinetics of absorption in the free film may be different from that in applied films. This is because the penetration of water may introduce more stresses in the film or release the existing ones depending on the previous history o f the coating. A simple calculation shows that the magnitude of the stress produced by p e n e t r a n t diffusion affects the adhesion forces between film and substrate. Alfrey et al. [60] give an expression t hat enables the calculation of the stress p r o d u ced on a thin disk that absorbs water. When this expression is used for an e p o x y - p o l y a m i d e film it is f o u n d that the stresses are in the range of several h u n d r e d kg cm 2, which is the order of the adhesion forces between paint films and metallic substrates. 8. Accelerated testing and paint

performance

The most c o m m o n m e t h o d s of assessing paint perform ance involve the use o f salt sprays, h u m i di t y cabinets and i m m e r s i o n o f coated panels in dif-

29 ferent liquid media (mostly aqueous). In spite of their general application, it is frequently observed [48] that the results of these tests are contradictory. It seems reasonable n o t to expect better agreement among them, because the aggressiveness of the environment is very different for each test and it has been shown that many different factors affect the properties of the protective films. These traditional tests are being subjected at present to strong criticism and revision [2(a), 62]. Efforts are being devoted to suggesting more reliable methods for accelerated testing based on the mechanism of protection and deterioration of the paint films. Various correlations have been tested on the basis of different parameters and characteristics of the paint film and its performance as protective coating [35, 46, 48, 57]. Only the correlation between ion-exchange capacity and paint performance, assessed by Khullar and Ulfvarson [35], appears to offer some possibility of generalization of paint behaviour. They observed that good protection was afforded by coatings having an ion-exchange capacity smaller than 0.3 - 0.4 meq g-1 ; it may be added here that a good number of polymeric coatings in environments having no extreme values of pH have even lower values of the ionexchange capacity (cf. Table 3). Based on the results of his extensive work, Funke has recently suggested [2(a)] that the crossover time be employed as an indication of the loss of adhesion of the coatings and the beginning of deterioration. Funke recommends that this parameter be complemented with determinations of the permeability of oxygen, which would control the rate of deterioration when the adhesion is reduced. One difficulty normally encountered in accelerated tests lies in the rather indiscriminate way the results of the test are applied without due regard to the environment where the actual protective coating is supposed to be used. It is important to consider the different places at which the coatings are to be exposed in order to evaluate those properties which are more relevant for the protective power of paint films under similar situations. At least three situations should be clearly distinguished. (a) immersion in low salinity water, (b) immersion in saline or aggressive solutions, (c) humid atmospheric exposure. For case {a), there is a large osmotic swelling of the paint film when it is actually exposed to the medium. This will reduce its adhesion strength and may in some cases give rise to blistering. However, Walker [57] has pointed out that there is no direct correlation between loss of adhesion and the deterioration of the metallic substrate covered by the coating. As suggested by Funke [2(a)], the permeability of oxygen may then be the ratedetermining step for the corrosion of the metallic substrate, because due to the reduced adhesion the paint film is n o t so tightly bound to the metal. In some areas of a coated structure (weldings, bolts, regions with high mechanical wastage), it is likely that metal gets exposed and acts as cathodic areas, in which case the electrical resistance of the paint film controls the rate of corrosion. Another indication of good film performance is when the electri-

30 cal resistance of the immersed coating [3, 4] does n o t change throughout the film surface, suggesting minimum heterogeneity of the applied protective coating. For case (b), it is important to know the permeability of the aggressive species, eg. CI-, H ÷, etc. through the protective paint films. In cases where active pigments are essential to protect the substrates, their performance should be tested in conditions similar to those corresponding to the actual paint performance. Electrochemical tests may be very valuable in this case [14, 151. For case (c}, the coatings are usually subjected to swelling and drying cycles. Humid tropical atmospheric exposure is one case where the coatings are subjected to severe conditions of natural environment and they develop mechanical strains. In this case oxygen permeability and swelling characteristics of the films appear to be important. From these considerations and the fact that so far [2(a)] the classic accelerated tests have no obvious alternatives, the following lines of procedure are recommended: Case (a): Coated panels should be tested after immersion in the medium for a few weeks, and the electrical resistance of the soaked film should be measured in different areas of the coated panels to evaluate its magnitude and also its homogeneous distribution. These measurements should be complemented with oxygen permeability determinations and study of mechanical properties (adhesion, microhardness). Case (b): Coated panels shoud be immersed in the aggressive medium and the action of active pigments evaluated electrochemically. Permeability to aggressive species of the soaked coating should be determined. The panels should be exposed to sprays of the aggressive medium (similar to salt spray tests). Case (c): After the coated panels are exposed to the humid environment, oxygen permeability and adhesion strength should be determined. For this case the humidity cabinet test is a good acelerated test. In all cases visual observation is a valuable guide to evaluate the performance of protective paints. It seems important to mention that according to our experience, the real performance of the coatings is not closely mimicked by any of the known accelerated tests. An epoxy-anticorrosive primer and epoxy-coal tar topcoat is a paint scheme very c o m m o n l y adopted to protect immersed steel structures which are usually subjected to a high flow of water (in the region of 5 m s-1), and consequently a strong mechanical stress is applied on them. These paint schemes are observed to perform remarkably well in practice. We have followed the service life of various coated structures for periods of several years from the m o m e n t of initial immersion. In spite of the fact that some of the structures were subjected to periodic drying and wetting, the coatings appeared extremely hard and strongly adherent. This suggests that adhesion improves upon pro!onged exposure, thus reversing the loss of adhesion generally found in the laboratory when coatings are exposed to humid

31 environments. The long-term increased adhesion could be due to some very slow reaction (in terms of l abor a t or y tests) in the coating or in the m e t a l film interface. This final c o m m e n t shows that m a ny points still remain to be k n o w n a b o u t the mechanism of protective paint performance even after t horough investigations on the subject. A small b u t nevertheless i m p o r t a n t poi nt in order to increase our knowledge of paint performance is the need to assess how representative of a given paint the tested samples are. This can be done either by employing the same sample for all the determinations or by obtaining statistically meaningful average properties of the paints.

Appendix There is much confusion regarding the units in which the electrical resistance (R) o f paint films and membranes should be expressed. The quantity that is characteristic" of a material, and that has a clear physical meaning, is the resistivity p which is related to R by R = p (l/A) where A is the area of the film and l its thickness. Since l is usually reported, the quantity (R-A) is a logical alternative to p because it will always be possible to calculate the resistivity of the film. In this way values r epor t e d by different authors may be c o mp ar ed with each other. On the ot he r hand, from the quant i t y R / A or R, which is often reported, there is no way of establishing the resistivity of the film unless the area is also precisely know n for each experimental situation and this is n o t usually the case. F u r t h e r m o r e , the use of R / A should be strongly discouraged on the basis that it is a physically meaningless quantity. A similar ambiguity is f ound in the way the permeation of species through films is reported. In this case the flow o f m a t t e r J =--P(AC/l) is related to the gradient o f c o n c e n t r a t i o n (AC/I) by the permeability coefficient P which is the physical relevant quantity. Both J and P are usually rep o rt e d ; however, whenever J is given, AC and l should also be reported. An ef f o r t should be made to a d o p t c o m m o n units for expressing the quantities, otherwise m a n y valuable data c a n n o t be e m p l o y e d for comparison. We r e c o m m e n d t he following set of quantities:

Recommended quantities

Alternative quantities

Quantities to be avoided

Flow of charge

p / ~ cm /S cm -1

(R.A)/V~ cm 2 In this case l must be given

R/~2 (R/A)/~2 cm -2

Flow o f m a t t e r

P or D cm 2 s-1

J/mass am -2 S- 1 (A C and l should also be given)

(J/l)/mass cm -2 s-1 am-1

32 References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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40 41 42 43 44

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