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Surface defects and surface flows in coatings
Progress in Organic Coatings, 19 (1991) 59-68
59
Surface defects and surface flows in coatings* Gordon
P. Bierwagen**
Department of Polymers & Coatings, North Dakota State University, Fargo, ND 5’8105 (U.S.A.)
Contents 1 ~trodu~tion .............................................. 2 Coating application methods. ................................... 3 Basic physical phenomena of coating processes ....................... 4 Origins of surface flows. .................................. 5 The variables controlling surface ilow phenomena, ..................... 6 Surface defects caused by surface flow effects ........................ 7 Methods of eliminating surface flow-Induced defects .................... References .................................................
. ...
59 59 61 62 64 65 66 67
1 Introduction
There have been several articles written in the past reviewing the relationship between surface defects in coatings and the surface flow phenomena. This review is an update and extension of this author’s earlier paper on this topic [ 11. That gave a review of the literature through 1974, and other more recent reviews are given in the bibliography [Z-4]. The basic premise of this review is that the creation of new surfaces in coating application processes gives nonequilibrium surfaces over which surface flows (desirable and undesirable) occur as films are transferred to the substrate and film formation processes occur (see Fig. 1 and Table 1). Even in powder coatings, surface flows are very important in film-formation processes. To understand, control and eliminate defect formation in coating application processes, one must understand the physical phenomena that occur in these processes and in the resulting thin liquid films prior to coffin fo~ation. 2 Coating
application
methods
Surface flows in coatings are the interactions of surfaces with fluid motion as applied to the conversion of a bulk liquid to a thin protective or functional organic film [5]. We will consider first the various types of coating *Presented, in part, at Friends and Students qf.Prof. R. S. Hansen Symp., Div. Colloid SUT$. Chem., Am. Chem. Sot. Meet., Los Angeles, 1988. **Part of this work was completed white the author was at Avery International Inc., Decorative Films Division, 650 W. 67th PI., Scherervihe, IN 46375 (U.S.A.).
0033-0655/91/$3.50
0 Elsevler Sequoia/Printed ln The Netherlands
60
I Before
Coating
I lD%inn CoatinE 1 ApL?lication-
Surface
in applied formation
1 )
During this zt,ep, most propertiez of the fluld are not in equilibrium --surface tension is time dependent
Flow3 may occ~ film during fii steps (COATING m<
t:
: BUBBTBATB
3 COntlnuouscoa &ysUedonsu
Fig. 1. Schematic
TABLE
:
?!!sz!z
diagram of the curtain coating process.
1
Events in coating formation bulk liquid brought into contact with substrate - wetting motion of coating device/substrate causes new surface formation dilation/extensional flow equilibratiou/leveling drying/curing
in thin liquid film
applications, the characteristics of coating flows, the variables involved, special features of the coating application methods, and some comments. One point that must be considered carefully is the high surface-to-volume ratio in coating films [ 61. Coatings are thin films that use a minimum of material to achieve a desired effect such as the protection of a substrate. There are many techniques that involve the change of a bulk liquid to a thin film of organic material on a solid substrate or a continuous sheet/film or carrier web (see Table 2). Included in the latter are roll coatings, either direct or reverse [ 7-101, and gravure applications, such as decorative films printed onto polyester web for transfer coating. These methods are strongly affected by surface flows. Other methods are hand brushed [ 111 and hand roller, flow coating, doctor blade [ lo], slide coating [ 5 1, spin coating (commonly used in the electronics area) [ 121, spray coatings [ 131 - air and airless, with and without electrostatic assist - there are several different variations on this general method. We must also consider electrodeposited coatings, organic coatings deposited by methods very similar to metal electrodeposition techniques. Again, surface flow phenomenon occur in the deposition process because the deposition involves a layer which can exhibit flow, unlike a metal. Curtain coating [ 141, slot dye coating, dip coating [ 15, 161, bead
61
TABLE 2 Characteristics of coating application methods Application type
Use/products
Advantages
Disadvantages
gravure
transfer films & precision printing continuous feed roll stock - sheet metal, film base web flat parts web feed stock flat rigid stock electronic coatings general use
pattern & color control
thin films only
speed, film thickness
flow instabilities
simplicity film thickness control simple controls uniform parts portable, use on many shapes
non-uniform films limited speeds speed, rigid parts simple shapes only lack of film thickness control
thin films ease of use easy, fast
web stock only lack of control film non-uniformity
roll coating - reverse - direct dip bead Curtain
spin spray - air, airless - electrostatic slide brush roller (hand)
multilayered films general use walls, etc.
coating, and transfer coating from one solid carrier to another heat and mass transfer in which surface effects are important.
all involve
3 Basic physical phenomena of coating processes The bases of surface flow phenomena have been discussed by several authors [ 17-191. The fundamental equations needed to describe surface flows are +
+?A+-VPf?P~
(1)
and ?7(VXzJ)= -v,,c+
(2)
with the symbols used defined in Table 3. Equation (2) tells us that a surface tension gradient can induce fluid motion at a surface. Further, curvature effects can induce pressure gradients which will also induce flow [20]. At a freshly formed surface, surface tension is a time-dependent quantity, and the diffusion-controlled transport of surfactants in liquid coatings must be taken into account if an accurate description of the flows is to be made. The work of Hansen [21] and others [22-241 describes this phenomenon, and indicates a relationship such as
a(t) = cro- 2RTc can be used to approximate this effect.
(3)
62 TABLE 3 Characteristic
variables
of surface
film thickness, h bulk viscosity, 7 - shear - extensional surface tension, u temperature, T concentrations, c - solvent - surfactant relative velocities, u surface lifetimes, 7 diffusion coefficients, D edge radii of substrate, r
flows in coatings da/dT duldc dTldr contact angle, 0 surface modulus, duld lnA rate of surface area (A) formation, surface viscosities - shear, 3 - dilational radius of curvature at surface, p gradient in the surface, VII time, t
d lnA/dt
4 Origins of surface flows What are the origins of surface flows in coatings? Consider Fig. 1, a schematic diagram of the curtain coating process describing some of the steps of this application technique [5] which involve coating flows typical of many application processes, caused by wetting and surface flows of all types including those around sharp edges in which the surface pressure effects become very important. As illustrated by this diagram, the coating process itself creates new surfaces during the very creation of the film. In the formation of new surfaces, there is dilation of the new and existing surfaces. Other origins of flows are the Laplace forces on these liquids as they are forming films by surface tension curvature effects [ 171. If the film breaks, the coating film will tend to withdraw to the minimum surface under surface tension forces. Another source of surface flows are gradients in temperature and surfactant concentrations along the interfaces created by evaporation or heating effects, or impurities either in the coating or substrate. If thermal gradients are present, one must also account for the temperature dependence of both the coating surface tension and coating viscosity. Gradients in temperature may give gradients in both surface tension and viscosity, creating surface flows and instabilities and, thus, surface flows and sometimes defects. Temperature gradients will also create gradients in surface tension and viscosity which can subsequently cause surface flows to occur during film curing/drying (see Fig. 2). Local hot spots in the substrate or carrier web during a coating process can also cause defects. In most materials surface tension decreases with temperature. Thus, a temperature gradient will induce a gradient in surface tension from hot to cold which will, in turn, induce flow along the
63
Fig. 2. Surface
flows induced
by temperature
gradients.
u
1
Fig. 3. Surface
r.
‘8
flows
,I
,
b, ,i
,,
h,,
;,,j
induced
;I
il
h?gh
,L’,’ ,‘,,lul ‘I’//, 1”1 .a,~,,:I~1,,‘,;,:.~~,~,.1:~‘,il :;r;l ” (,1,li I:‘11
,#
by water evaporation
/I
8,
,/.d
b,,h
,I,
in a water-reduced
“,I> ,L
1
1,
‘i
coating.
surface. The general statement describing the flows induced by surface tension differences is that these flows occur in the direction proceeding from regions of lower surface tension to regions of higher surface tension. If there is a surface tension gradient in one direction d&lx, a surface flow is induced in the opposite direction to the gradient (i.e. from low to high surface tension regions). This can be thought of in thermodynamic terms as the response induced in a liquid system moving to the condition of lowest free energy, since the term aA (A = surface area) is minimized at constant A at the lowest avalues. As shown in the schematic drawing of Fig. 3, evaporation phenomena can induce local surface tension gradients. When the water in a water-borne or latex coating evaporates, the concentration of organic materials increases in the liquid film causing a decrease in surface tension. This effect can
64
generate flow away from areas of localized evaporation giving a localized decrease in the thickness of the coating. If that film thickness decrease happens to be frozen-in by any sort of drying phenomena, one gets local defects in the film. Figure 4 also shows an inverse situation which can exist in solventborne coatings. In these types of coatings the low surface tension material may very often be the organic solvent, and as it evaporates in film drying/ curing it leaves the higher surface tension polymer. Again, one may then have a localized gradient in surface tension causing an increase in thickness due to flow from areas of low surface tension in the film to the localized area of high surface tension/high evaporation. This can give a local buildup of film material and again, if frozen-in by the drying/curing process, one has a local defect. Thus, thermal gradients or concentration gradients induced by the mass transfer of drying can cause local defects in a coating film. An important issue with respect to the origins of surface flows is that a coating film as created is very often in a non-equilibrium condition. One has to consider the diffusion control of surface tension and many other effects that will impinge on the quality of the coating which are very definitely time-dependent, dynamic phenomena. The surface tension of a solution of a surface-active material has a definite time-dependence due to the diffusion of surfactants to a fresh interface. Polymeric types of surfactants can have a significant time-dependency in their surface tension because of their molecular size. The surface tension gradients discussed above can become time-dependent phenomena as the controlling variable or coupling due to flow becomes timedependent. Thus, at a fresh surface, concentration of the surface-active species become time-dependent, and one has the relationship: da _=_._ dt
do
dc
dc
dt
(4)
The fresh surface problem has been discussed quite extensively, and there are expressions for dcldt as a function of diffusion coefficient and concentration by which one has an expression for u as a function of time, i.e. u= u(D,c,t). Further, in the coating process, when the liquid coating film is applied to a moving solid substrate, wetting phenomena are also involved, with moving contact lines, dilational flows and some extensional flows in the coating liquid. As the film forms after the application process, there are often those surface flows associated with the process of ‘leveling’ [25].
5 The variables
controlling
surface flow phenomena
Let us now consider the variables necessary for describing surface flows in coatings. There is quite a list - see Table 3. As many paint scientists know from unfortunate experience, the film thickness is a very important variable when considering the effects of surface flows and the manner in
65
which they can cause defects in films. The thinner the film, the more dependent the quality of the coating on the proper control of surface flows. The tendency toward reflow to a level surface after a surface has been disturbed or distorted in application (leveiingf proceeds as the inverse cube of the thickness [25]. A thicker coating will ‘level’ far more readily than a thin coating. A thin coating is also very sensitive to surface impurities, Other important variables are the viscosity of the bulk liquid, the shear and dilational surface viscosities, surface tension, temperature and tempera~~~~d~pendence of both surface tension and viscosity, and the concentrations of both solvents and surfactants in the system. Further, one must consider the velocity components both from the viewpoint of the substrate and the coating liquid7the age of the surface, the diffusion coefficient of the surfactants in the system, the radius of curvature of the object(s) being coated, the surface elastic modulus [ 171, the contact angle, the rate of surface area formation and the radius of curvature developed in the liquid itself during the coating process. For example, in hand roll or hand brushing the brush marks have a radius of curvature which affect the manner in which the coatings will flow to a level film. 6 Smrface defects caused by surface
flow effects
One of the major reasons for the considerable amount of time expended in the analysis of surface flows and coatings are the types of defect and coating failure caused by ~controll@d surface flows, as summ~zed in Table 4. These effects are crucial to the coatings formulation scientist. If local fluctuations in condensation and temperature due to evaporation cause undesired surface flows, variations in film thickness may occur which may destroy a coating’s performance (see Fig. 4). Cratering may occur at fresh surfaces or ‘curtain’ breakage may occur in a curtain coater. When coupled with curvature effects, wetting phenomena, and meniscus stability and shape, further problems may ensue. Thin films near sharp edges (small radii of curvature, r) on wares are closely tied to Laplace pressure effects @ = crier’) causing fluid motion. Wetting and meniscus instabilities will cause the wellTABLE 4 Defects
in coatings
caused by surface flows
cratering ribbing flows curt&n/sheet breaks edge flowa - ‘picture framing’ roll cells, convection cells, ‘orange peel’ foami.n@ir entrapment dew&t&g&&y spot formation fe~&~g problems (not enough surface flrnv] others
66
atdeh
Fig. 4. Evaporation
-
Loodized Ewapordion of Organic sdvent
effects in solvent coating.
known ribbing effects in coil coating. Moving contact lines are special sources of instability related to surface flow. Electrostatic charge build-up during spraying induces surface tension gradients and contributes to, or interferes with, proper droplet break-up. Examples of surface defects are well described in ref. 6. Stabilization of coatings with respect to undesired surface-flow-related effects is a crucial task for the coating formulation scientist. This is a most difficult problem since it involves stabilizing coatings with respect to the coupling of several dynamic phenomena, each of which may have to be controlled separately. Instabilities with respect to surface flows occur in coatings during the period from application until the coating viscosity increase causes all flows to cease. Surface flows affect all types of coatings from trade sales latex coatings to powder coatings. Spray droplet formation, cratering, leveling, edge coverage phenomena and ‘curtain’ stability in curtain coatings are all effects related to surface flow. Other specific important examples of surface defects occurring due to surface flows are ‘ribbing’ [ 7-101, which is a direct dynamic effect related to surface tension forces in the deposition process in the roll coating, and what is called ‘picture framing’ [20], i.e. surface flows that are surface tension driven due to curvature near a sharp edge, thinning the film about the edge. Further examples are given in refs. 1, 2, 3, 6 and 18.
7 Methods
of eliminating
surface flow-induced
defects
There is a large litany of this type of problem which arises when surface flows occur in an uncontrolled, undesired manner in an organic coating. One must consider all of the issues discussed above and look to the guidance of the surface chemist/engineer for assistance in eliminating potential sources
67 TABLE
5
Methods of eliminating surface defects viscosity control housekeeping/environment control surface tension control through coating operation raw material purity temperature control in curing operations close control of all coating equipment careful analysis of solubility and evaporation effects
of undesired surface flows. The traditional response by coating formulators has often been to reduce surface flow effects by the addition of large amounts of surfactant. This has some logical basis when one realizes that most A versu-s c curves at high surfactant concentration (surface saturation da, dc + 0), or fluctuations in c with distance x, will not propagate to significant variations in surface tension with distance. High surfactant levels also tend to level out thermal effects on surface tension. But, there is a price to pay for these high surfactant levels - greater water sensitivity and more foaming in the film (especially in aqueous systems), plus interference effects in other coating parameters (dispersions, etc.). It would be better to match the method of eliminating a surface-flowinduced instability in a coating to its cause, and to consider the eventual effects of what is done in eliminating the instability. Adding a surfactant to reduce surface flow effects may cause destabilization of pigment dispersion, excess foaming, shear instabilities during application, and post-cure exposure sensitivity. By considering the controlling parameters of a given surface flow instability, one can focus on an ‘exact cure’ to a problem, not an ‘overkill’. One must also look for coupled phenomena, such as temperature gradients inducing surface tension gradients which, in turn, induce surface flows, or evaporation which causes concentration gradients which induce surface tension gradients which cause surface flow. This coupling of parameters of the coating behavior by physical relationships such as u= ~(t,c,Z’) forces the formulator to consider at all time the dependence between application parameters (temperature, velocity) and coating parameters (surface tension, concentration, viscosity). A summary of the methods available to eliminate surfaceflow-induced defects is given in Table 5.
References 1 2 3 4 5
G. L. C. K. S.
P. Bierwagen, Prog. Org. Coat., 3 (1975) 101. L. Komum and H. K. Raaschou Nielsen, Prog. Org. Coat., 8 (1980) 275. M. Hansen and P. E. Pierce, Ind. Eng. Chem., Prod. Res. Dev., 13 (1974) 218. J. Ruschak, Annu. Rev. F’luid Mech., 17 (1985) 65. F. Kistler and L. E. Striven, Int. J. Numerical Methods in Fluids, 4 (1984) 20’7.
68 6 P. E. Pierce and C. K. Schoff, in T. Miranda and D. Bresinski (eds.), Coating Film Defects: Federation Series on Coatings Technology, Federation of Societies for Coating Technology,
Philadelphia, PA, 1988. 7 D. J. Coyle, C. W. Macosko and L. E. Striven, ACS Symp. Ser. No. 197, Am. Chem. Sot., Washington, DC, 1982, pp. 252-264. 8 H. Benkreira and M. F. Edwards, J. Non-Newtonian Fluid Mech., 14 (1984) 377. 9 J. R. A. Pearson, J. fluid Mech., 7 (1960) 481. 10 T. Matsuda and W. H. Brendly, J. Coat. Technol., 51 (1979) 46. 11 F. A. Saita and L. E. Striven, Lat. Am. J. Chem. Eng. Appl. C&m., 13 (1983) 121. 12 P. H. Steen, J. K. Carpenter and H. Yu, AXhE J., 34 (1988) 1673. 13 T. C. Papanastatiou, A. N. Alexandrou and W. P. Graebel, J. Rheol., 32 (1988) 485. 14 D. R. Brown, J. Fluid Mech., IO (1961) 297. 15 S. D. R. Wilson, AlChE J., 34 (1988) 1732. 16 H. Nassenstein, Be?-. Bunsenges Phys. Chem., 85 (1981) 842. 17 J. Lucassen and M. Van Den Tempel, Chem. Eng. Sci., 27 (1972) 1283. 18 J. B. Keller and M. J. M&is, SLAM J. AppZ. Math., 42 (1983) 268. 19 M. Van Den Tempel, J. Non-Newtonian FZuid Mech., 2 (1977) 205. 20 L. Weh, Plaste Kautsch., 20 (1973) 138. 21 R. S. Hansen, J. Phys. Cherrk, 64 (1960) 637. 22 R. Defay and G. Petre, in E. Matejivic (ed.), Surface and Colloid Science, Vol. 3, WileyInterscience, New York, 1971, p. 27. 23 K. Ueberreiter, S. Morimoto and R. SteuImann, Kolloid-Z., 252 (1974) 18. 24 J. Van Havenbergh and P. Joos, J. Colbid Interface Sci., 95 (1983) 172. 25 S. E. Orchard, J. Appl. Sci. Res., All (1962) 451.
59
Surface defects and surface flows in coatings* Gordon
P. Bierwagen**
Department of Polymers & Coatings, North Dakota State University, Fargo, ND 5’8105 (U.S.A.)
Contents 1 ~trodu~tion .............................................. 2 Coating application methods. ................................... 3 Basic physical phenomena of coating processes ....................... 4 Origins of surface flows. .................................. 5 The variables controlling surface ilow phenomena, ..................... 6 Surface defects caused by surface flow effects ........................ 7 Methods of eliminating surface flow-Induced defects .................... References .................................................
. ...
59 59 61 62 64 65 66 67
1 Introduction
There have been several articles written in the past reviewing the relationship between surface defects in coatings and the surface flow phenomena. This review is an update and extension of this author’s earlier paper on this topic [ 11. That gave a review of the literature through 1974, and other more recent reviews are given in the bibliography [Z-4]. The basic premise of this review is that the creation of new surfaces in coating application processes gives nonequilibrium surfaces over which surface flows (desirable and undesirable) occur as films are transferred to the substrate and film formation processes occur (see Fig. 1 and Table 1). Even in powder coatings, surface flows are very important in film-formation processes. To understand, control and eliminate defect formation in coating application processes, one must understand the physical phenomena that occur in these processes and in the resulting thin liquid films prior to coffin fo~ation. 2 Coating
application
methods
Surface flows in coatings are the interactions of surfaces with fluid motion as applied to the conversion of a bulk liquid to a thin protective or functional organic film [5]. We will consider first the various types of coating *Presented, in part, at Friends and Students qf.Prof. R. S. Hansen Symp., Div. Colloid SUT$. Chem., Am. Chem. Sot. Meet., Los Angeles, 1988. **Part of this work was completed white the author was at Avery International Inc., Decorative Films Division, 650 W. 67th PI., Scherervihe, IN 46375 (U.S.A.).
0033-0655/91/$3.50
0 Elsevler Sequoia/Printed ln The Netherlands
60
I Before
Coating
I lD%inn CoatinE 1 ApL?lication-
Surface
in applied formation
1 )
During this zt,ep, most propertiez of the fluld are not in equilibrium --surface tension is time dependent
Flow3 may occ~ film during fii steps (COATING m<
t:
: BUBBTBATB
3 COntlnuouscoa &ysUedonsu
Fig. 1. Schematic
TABLE
:
?!!sz!z
diagram of the curtain coating process.
1
Events in coating formation bulk liquid brought into contact with substrate - wetting motion of coating device/substrate causes new surface formation dilation/extensional flow equilibratiou/leveling drying/curing
in thin liquid film
applications, the characteristics of coating flows, the variables involved, special features of the coating application methods, and some comments. One point that must be considered carefully is the high surface-to-volume ratio in coating films [ 61. Coatings are thin films that use a minimum of material to achieve a desired effect such as the protection of a substrate. There are many techniques that involve the change of a bulk liquid to a thin film of organic material on a solid substrate or a continuous sheet/film or carrier web (see Table 2). Included in the latter are roll coatings, either direct or reverse [ 7-101, and gravure applications, such as decorative films printed onto polyester web for transfer coating. These methods are strongly affected by surface flows. Other methods are hand brushed [ 111 and hand roller, flow coating, doctor blade [ lo], slide coating [ 5 1, spin coating (commonly used in the electronics area) [ 121, spray coatings [ 131 - air and airless, with and without electrostatic assist - there are several different variations on this general method. We must also consider electrodeposited coatings, organic coatings deposited by methods very similar to metal electrodeposition techniques. Again, surface flow phenomenon occur in the deposition process because the deposition involves a layer which can exhibit flow, unlike a metal. Curtain coating [ 141, slot dye coating, dip coating [ 15, 161, bead
61
TABLE 2 Characteristics of coating application methods Application type
Use/products
Advantages
Disadvantages
gravure
transfer films & precision printing continuous feed roll stock - sheet metal, film base web flat parts web feed stock flat rigid stock electronic coatings general use
pattern & color control
thin films only
speed, film thickness
flow instabilities
simplicity film thickness control simple controls uniform parts portable, use on many shapes
non-uniform films limited speeds speed, rigid parts simple shapes only lack of film thickness control
thin films ease of use easy, fast
web stock only lack of control film non-uniformity
roll coating - reverse - direct dip bead Curtain
spin spray - air, airless - electrostatic slide brush roller (hand)
multilayered films general use walls, etc.
coating, and transfer coating from one solid carrier to another heat and mass transfer in which surface effects are important.
all involve
3 Basic physical phenomena of coating processes The bases of surface flow phenomena have been discussed by several authors [ 17-191. The fundamental equations needed to describe surface flows are +
+?A+-VPf?P~
(1)
and ?7(VXzJ)= -v,,c+
(2)
with the symbols used defined in Table 3. Equation (2) tells us that a surface tension gradient can induce fluid motion at a surface. Further, curvature effects can induce pressure gradients which will also induce flow [20]. At a freshly formed surface, surface tension is a time-dependent quantity, and the diffusion-controlled transport of surfactants in liquid coatings must be taken into account if an accurate description of the flows is to be made. The work of Hansen [21] and others [22-241 describes this phenomenon, and indicates a relationship such as
a(t) = cro- 2RTc can be used to approximate this effect.
(3)
62 TABLE 3 Characteristic
variables
of surface
film thickness, h bulk viscosity, 7 - shear - extensional surface tension, u temperature, T concentrations, c - solvent - surfactant relative velocities, u surface lifetimes, 7 diffusion coefficients, D edge radii of substrate, r
flows in coatings da/dT duldc dTldr contact angle, 0 surface modulus, duld lnA rate of surface area (A) formation, surface viscosities - shear, 3 - dilational radius of curvature at surface, p gradient in the surface, VII time, t
d lnA/dt
4 Origins of surface flows What are the origins of surface flows in coatings? Consider Fig. 1, a schematic diagram of the curtain coating process describing some of the steps of this application technique [5] which involve coating flows typical of many application processes, caused by wetting and surface flows of all types including those around sharp edges in which the surface pressure effects become very important. As illustrated by this diagram, the coating process itself creates new surfaces during the very creation of the film. In the formation of new surfaces, there is dilation of the new and existing surfaces. Other origins of flows are the Laplace forces on these liquids as they are forming films by surface tension curvature effects [ 171. If the film breaks, the coating film will tend to withdraw to the minimum surface under surface tension forces. Another source of surface flows are gradients in temperature and surfactant concentrations along the interfaces created by evaporation or heating effects, or impurities either in the coating or substrate. If thermal gradients are present, one must also account for the temperature dependence of both the coating surface tension and coating viscosity. Gradients in temperature may give gradients in both surface tension and viscosity, creating surface flows and instabilities and, thus, surface flows and sometimes defects. Temperature gradients will also create gradients in surface tension and viscosity which can subsequently cause surface flows to occur during film curing/drying (see Fig. 2). Local hot spots in the substrate or carrier web during a coating process can also cause defects. In most materials surface tension decreases with temperature. Thus, a temperature gradient will induce a gradient in surface tension from hot to cold which will, in turn, induce flow along the
63
Fig. 2. Surface
flows induced
by temperature
gradients.
u
1
Fig. 3. Surface
r.
‘8
flows
,I
,
b, ,i
,,
h,,
;,,j
induced
;I
il
h?gh
,L’,’ ,‘,,lul ‘I’//, 1”1 .a,~,,:I~1,,‘,;,:.~~,~,.1:~‘,il :;r;l ” (,1,li I:‘11
,#
by water evaporation
/I
8,
,/.d
b,,h
,I,
in a water-reduced
“,I> ,L
1
1,
‘i
coating.
surface. The general statement describing the flows induced by surface tension differences is that these flows occur in the direction proceeding from regions of lower surface tension to regions of higher surface tension. If there is a surface tension gradient in one direction d&lx, a surface flow is induced in the opposite direction to the gradient (i.e. from low to high surface tension regions). This can be thought of in thermodynamic terms as the response induced in a liquid system moving to the condition of lowest free energy, since the term aA (A = surface area) is minimized at constant A at the lowest avalues. As shown in the schematic drawing of Fig. 3, evaporation phenomena can induce local surface tension gradients. When the water in a water-borne or latex coating evaporates, the concentration of organic materials increases in the liquid film causing a decrease in surface tension. This effect can
64
generate flow away from areas of localized evaporation giving a localized decrease in the thickness of the coating. If that film thickness decrease happens to be frozen-in by any sort of drying phenomena, one gets local defects in the film. Figure 4 also shows an inverse situation which can exist in solventborne coatings. In these types of coatings the low surface tension material may very often be the organic solvent, and as it evaporates in film drying/ curing it leaves the higher surface tension polymer. Again, one may then have a localized gradient in surface tension causing an increase in thickness due to flow from areas of low surface tension in the film to the localized area of high surface tension/high evaporation. This can give a local buildup of film material and again, if frozen-in by the drying/curing process, one has a local defect. Thus, thermal gradients or concentration gradients induced by the mass transfer of drying can cause local defects in a coating film. An important issue with respect to the origins of surface flows is that a coating film as created is very often in a non-equilibrium condition. One has to consider the diffusion control of surface tension and many other effects that will impinge on the quality of the coating which are very definitely time-dependent, dynamic phenomena. The surface tension of a solution of a surface-active material has a definite time-dependence due to the diffusion of surfactants to a fresh interface. Polymeric types of surfactants can have a significant time-dependency in their surface tension because of their molecular size. The surface tension gradients discussed above can become time-dependent phenomena as the controlling variable or coupling due to flow becomes timedependent. Thus, at a fresh surface, concentration of the surface-active species become time-dependent, and one has the relationship: da _=_._ dt
do
dc
dc
dt
(4)
The fresh surface problem has been discussed quite extensively, and there are expressions for dcldt as a function of diffusion coefficient and concentration by which one has an expression for u as a function of time, i.e. u= u(D,c,t). Further, in the coating process, when the liquid coating film is applied to a moving solid substrate, wetting phenomena are also involved, with moving contact lines, dilational flows and some extensional flows in the coating liquid. As the film forms after the application process, there are often those surface flows associated with the process of ‘leveling’ [25].
5 The variables
controlling
surface flow phenomena
Let us now consider the variables necessary for describing surface flows in coatings. There is quite a list - see Table 3. As many paint scientists know from unfortunate experience, the film thickness is a very important variable when considering the effects of surface flows and the manner in
65
which they can cause defects in films. The thinner the film, the more dependent the quality of the coating on the proper control of surface flows. The tendency toward reflow to a level surface after a surface has been disturbed or distorted in application (leveiingf proceeds as the inverse cube of the thickness [25]. A thicker coating will ‘level’ far more readily than a thin coating. A thin coating is also very sensitive to surface impurities, Other important variables are the viscosity of the bulk liquid, the shear and dilational surface viscosities, surface tension, temperature and tempera~~~~d~pendence of both surface tension and viscosity, and the concentrations of both solvents and surfactants in the system. Further, one must consider the velocity components both from the viewpoint of the substrate and the coating liquid7the age of the surface, the diffusion coefficient of the surfactants in the system, the radius of curvature of the object(s) being coated, the surface elastic modulus [ 171, the contact angle, the rate of surface area formation and the radius of curvature developed in the liquid itself during the coating process. For example, in hand roll or hand brushing the brush marks have a radius of curvature which affect the manner in which the coatings will flow to a level film. 6 Smrface defects caused by surface
flow effects
One of the major reasons for the considerable amount of time expended in the analysis of surface flows and coatings are the types of defect and coating failure caused by ~controll@d surface flows, as summ~zed in Table 4. These effects are crucial to the coatings formulation scientist. If local fluctuations in condensation and temperature due to evaporation cause undesired surface flows, variations in film thickness may occur which may destroy a coating’s performance (see Fig. 4). Cratering may occur at fresh surfaces or ‘curtain’ breakage may occur in a curtain coater. When coupled with curvature effects, wetting phenomena, and meniscus stability and shape, further problems may ensue. Thin films near sharp edges (small radii of curvature, r) on wares are closely tied to Laplace pressure effects @ = crier’) causing fluid motion. Wetting and meniscus instabilities will cause the wellTABLE 4 Defects
in coatings
caused by surface flows
cratering ribbing flows curt&n/sheet breaks edge flowa - ‘picture framing’ roll cells, convection cells, ‘orange peel’ foami.n@ir entrapment dew&t&g&&y spot formation fe~&~g problems (not enough surface flrnv] others
66
atdeh
Fig. 4. Evaporation
-
Loodized Ewapordion of Organic sdvent
effects in solvent coating.
known ribbing effects in coil coating. Moving contact lines are special sources of instability related to surface flow. Electrostatic charge build-up during spraying induces surface tension gradients and contributes to, or interferes with, proper droplet break-up. Examples of surface defects are well described in ref. 6. Stabilization of coatings with respect to undesired surface-flow-related effects is a crucial task for the coating formulation scientist. This is a most difficult problem since it involves stabilizing coatings with respect to the coupling of several dynamic phenomena, each of which may have to be controlled separately. Instabilities with respect to surface flows occur in coatings during the period from application until the coating viscosity increase causes all flows to cease. Surface flows affect all types of coatings from trade sales latex coatings to powder coatings. Spray droplet formation, cratering, leveling, edge coverage phenomena and ‘curtain’ stability in curtain coatings are all effects related to surface flow. Other specific important examples of surface defects occurring due to surface flows are ‘ribbing’ [ 7-101, which is a direct dynamic effect related to surface tension forces in the deposition process in the roll coating, and what is called ‘picture framing’ [20], i.e. surface flows that are surface tension driven due to curvature near a sharp edge, thinning the film about the edge. Further examples are given in refs. 1, 2, 3, 6 and 18.
7 Methods
of eliminating
surface flow-induced
defects
There is a large litany of this type of problem which arises when surface flows occur in an uncontrolled, undesired manner in an organic coating. One must consider all of the issues discussed above and look to the guidance of the surface chemist/engineer for assistance in eliminating potential sources
67 TABLE
5
Methods of eliminating surface defects viscosity control housekeeping/environment control surface tension control through coating operation raw material purity temperature control in curing operations close control of all coating equipment careful analysis of solubility and evaporation effects
of undesired surface flows. The traditional response by coating formulators has often been to reduce surface flow effects by the addition of large amounts of surfactant. This has some logical basis when one realizes that most A versu-s c curves at high surfactant concentration (surface saturation da, dc + 0), or fluctuations in c with distance x, will not propagate to significant variations in surface tension with distance. High surfactant levels also tend to level out thermal effects on surface tension. But, there is a price to pay for these high surfactant levels - greater water sensitivity and more foaming in the film (especially in aqueous systems), plus interference effects in other coating parameters (dispersions, etc.). It would be better to match the method of eliminating a surface-flowinduced instability in a coating to its cause, and to consider the eventual effects of what is done in eliminating the instability. Adding a surfactant to reduce surface flow effects may cause destabilization of pigment dispersion, excess foaming, shear instabilities during application, and post-cure exposure sensitivity. By considering the controlling parameters of a given surface flow instability, one can focus on an ‘exact cure’ to a problem, not an ‘overkill’. One must also look for coupled phenomena, such as temperature gradients inducing surface tension gradients which, in turn, induce surface flows, or evaporation which causes concentration gradients which induce surface tension gradients which cause surface flow. This coupling of parameters of the coating behavior by physical relationships such as u= ~(t,c,Z’) forces the formulator to consider at all time the dependence between application parameters (temperature, velocity) and coating parameters (surface tension, concentration, viscosity). A summary of the methods available to eliminate surfaceflow-induced defects is given in Table 5.
References 1 2 3 4 5
G. L. C. K. S.
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