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The electrodeposition of non-ionically stabilised emulsions
Colloids
and Surfaces,
Elsevier
Science
17 (1986)
Publishers
13-24
13
B.V., Amsterdam
THE ELECTRODEPOSITION EMULSIONS*
- Printed
in The Netherlands
OF NON-IONICALLY
STABILISED
A. DOROSZKOWSKI Research Berkshire
Department, Imperial Chemical SL2 5DS (United Kingdom)
(Received
3 August
1984; accepted
Industries
PLC, Paints Division,
Slough,
in final form 2 June 1985)
ABSTRACT One of the major industrial paint application processes is electrodeposition (electrocoat paints). By examining the mechanism of paint electrodeposition it is seen that the salient feature of the process is the destabilisation of an emulsion by an electrode process which is independent of electrophoresis. Therefore, by extending this concept it has been shown that even sterically (non-ionic) stabilised emulsions can be electrodeposited. The mechanism of such a process and examples of such emulsions which can be deposited at either the cathode or the anode are described.
INTRODUCTION
The commercial electrodeposition process, sometimes known as electrophoretic painting, for the coating of metal objects for protective purposes was introduced simultaneously by ICI in collaboration with Pressed Steel Fisher Ltd in the U.K., and by the Ford Motor Company in the U.S.A. in 1963. The concept of electrodepositing an organic material can be traced back to 1919 [l] . Crosse and Blackwell Ltd explored the possibility of electrodepositing lacquer emulsions on food cans in 1936 [2, 31. However, it was not until the early 1960s that the anodic electrodeposition of paint was established as a viable commercial process. In 1971 cathodic electrodeposition was introduced to the appliance industry by PPG Industries Inc. and in 1976 cathodic automotive systems were introduced which have virtually supplanted the earlier anodic ones because of their general, overall better protective properties, and are now the standard primer application methods in industry. While it may seem obvious that anionic emulsions will deposit at the anode, and cationic emulsions at the cathode, in a system known as “electrophoretic painting”, the deposition mechanism is very frequently misunderstood. *Dedicated
to the memory
0166-6622/86/$03.50
of M.W. Thompson.
0 1986 Elsevier
Science
Publishers
B.V
14 THE ELECTRODEPOSITION
MECHANISM
In emulsion electrodeposition, it is possible to identify three contributing processes: (1) A mass transport process in the bulk to the electrode zone. (2) An electrode process (electrode reaction). (3) An emulsion stability-instability process in the electrode zone. Mass transport The mass transport process outside the electrode zone can be described as a flux of particles which consists of contributions from: (a) particle flow due to convection or agitation (V,); (b) diffusion of particles, which will depend on a concentration gradient de/ax and their diffusion coefficient, Di; and (c) migration of particles (electrophoresis) due to the potential gradient a U/ax. Considering only the one-dimensional flux (Ji) of particles we have: Ji = VxCi + Di g
+ z /J.ici5
where Ji is the flux of i particles
in the x direction, ci is the concentration of particles, z is the charge on the particle, and pi is the mobility of the particle. When the particles are colloidal in size (radius, a) then the diffusion coefficient can be estimated by the Stokes-Einstein equation: Di=
kT _ 6nqa
(2)
where q is the viscosity of the medium. Since there is little or negligible concentration gradient (at/ax) in the bulk of the dispersion, evaluation of Eqn (2) for a submicrometer particle (e.g. 0.1 pm) in water yields a Di value of - 2 X 10e8 cm* s- ‘, giving to all intents and purposes a diffusion contribution to Ji which cm be ignored. Migration in an electric field is well known and the mobility (zipi) of a particle can be converted into a particle velocity (u) which is dependent on the potential gradient a U/ax and the zeta potential of the particle. All these values are readily measured, and while they are dependent on the electrolyte content of the medium and surface-active groups used to stabilise the particles, typical values are in the order of a few pm s-l per V cm-‘. If an electric field of 100 V per 10 cm is applied then the electrophoretic velocity is or agitation (V,) is of - 10m3 cm s-l, while the velocity due to convection the order of 1 to 10 cm s-l. Thus, when the “bulk” mass transport [Eqn (l)] is evaluated in terms of the constituent processes, using the above values, it is clear that the process, by orders of magnitude, is due to convection currents and agitation
15
(mechanical is negligible. Electrode
stirring)
and that transport
due to electrophoresis
or diffusion
reaction
The main electrode reaction in electrocoat painting is the decomposition of water. At the anode the reaction is H,O = X0, 1 + 2H’ + 2e- and at the cathode H,O = %Hz t + OH- - e-. According to Nernst [4] and Vielstich [5], in the case of a flowing bath a “diffusion” or boundary layer must be imagined at the electrode--solution interface (see Fig. 1). In this quasistationary layer, particles can only be transported by diffusion or migration. The thickness, 6N, of this layer depends on the hydrodynamic conditions which can be expressed as [5, 61 : 6N = 3$/l v,-” ,,1!6 Dili3 (3)
where y is the coordinate parallel to electrode surface, V, is the velocity of liquid flow, v is the kinematic viscosity, and Di is the diffusion coefficient. t
Fig. 1. Model to Nernst.
of diffusion
layer
for protons
at the electrode-solution
interface
according
At the anode protons are injected into the phase boundary due to the decomposition of water and, in the steady state, an approximately constant which can be related to the electric curproton concentration is established rent density (I): I = 2F&+
(C&+- cg+) SN(H+)
where F is Faraday’s
constant.
(4)
17
as the composition of the attracting bodies, i.e. if A 1, and AZ2 are the Hamaker constants for material 1 and material 2, respectively, in a medium of Hamaker constant Ass, then the effective Hamaker constant (metal and polymer in water) is AIs = (All” - A33G) (Az2% - Ass”‘) [ll] and has a value 0.16 x lo-l3 erg for polymerpolymer of -1.0 X lo-l3 erg. Similarly, A,,,= particles in water. Hence, there is an order of magnitude greater attraction for the particles to flocculate on the metal electrode than with themselves. Electrodeposition of an anionic dispersion is thus seen to be a process of destabilising a charge-stabilised dispersion. The acidic boundary layer around the anode suppresses ionic dissociation on the surface of the particles, causing the particles to flocculate onto the electrode (although they may also flocculate with themselves to some degree in the boundary layer before attaching themselves to the electrode). Some idea of the rate of flocculation may be obtained from Smoluchowski rapid coagulation kinetics [12] and for typical electrocoat systems t, is of the order of milliseconds; t, is the time required for the number of particles to be reduced by half. The above argument applies to charge-stabilised emulsions, the only difference with cathodic deposition is that the electrode has an alkaline boundary layer instead of an acidic one (ignoring dissolution effects of the electrode which can enhance emulsion coagulation), this likewise inhibits the association of a proton with a weak base to produce ionisation, i.e. R3 N’H = R3N + H’ R,N+H + OH- + R3N + H,O In generic terms one can divide dispersions into two categories: (1) charge stabilised, anionic (or cationic), dispersions; and (2) sterically stabilised dispersions. If the above mechanism is correct and one can devise an electrode reaction to destroy the polymeric layer producing a sterically stabilised dispersion, then it should be possible to electrodeposit even this type of system, since the first requirement of a mass transport process to the electorde zone does not require charge, nor is electrophoresis in the electrode zone necessary . STERIC STABILISATION
The governing principle of steric (or polymer) stabilisation of colloidal particles is that a sheath of solvated polymer surrounds the colloidal particles. When two such particles approach each other, the adsorbed sheaths interact generating a repulsion energy, which is sufficient to overcome the attraction energy due to surface forces. The repulsion energy in steric stabilisation is frequently subdivided into an “osmotic” term (or free energy of mixing) and a “volume restriction”
18
term. Experimental evidence suggests that, in practice, the volume restriction term can be ignored [13, 141 and colloid stability is governed by the osmotic term ( VR M ). The nett energy of interaction between two particles of radius a and barrier thickness F is [ 151 : V nett
=Vfp
+v,
and v,M
= -
kT
ViP2
G I(1
-
‘3/T) cz f(a, 6, h)
where $ 1 is Flory’s entropy parameter; 0, theta temperature; c the polymer concentration in the stabilising barrier; T, the temperature; h, the surface-tosurface separation; f(a,6 ,h) is a function of the stabilising barrier geometry; and Vi and p are the solvent molar volume and polymer density, respectively .
When T= 8, VRM will become zero and the dispersion will flocculate as has convincingly been demonstrated by Napper [16] and, if T < 0, VR M changes sign, and becomes an attraction energy. To obtain dispersion stability, it is essential to have the stabilising polymer chains well solvated. If the solvation is decreased dispersion stability is decreased so that ultimately, when solubility is lost, the polymer chains induce flocculation. CATHODIC
NON-IONIC
ELECTROCOAT
A sterically stabilised dispersion of colloidal particles can readily be made where the solvated polymer sheath comprises polyethylene oxide (PEO) chains. The entropy parameter ($ 1) in Eqn (7) is negative in aqueous solution and dispersion stability is obtained when T < 0. Furthermore, if one examines the solubility of PEO in aqueous solution as a function of pH, then at high pH the precipitation temperature of highmolecular-weight PEO is drastically reduced as shown in Fig. 2 [17]. Examination of Eqn (7) shows that the stabilising energy of the solvated polymer layer is not only a function of the theta temperature, but also depends on the surface volume concentration (c) of the solvated polymer and the layer dimensions. The theta temperature of a polymer is defined as the temnerature when the second virial coefficient is zero. It may also be loosely described as the temperature of incipient precipitation of a polymer of infinite molecular weight. Therefore Fig. 2 gives an indication of the theta-temperature behaviour of PEO stabilisers versus pH. Hence, one can see immediately that, for a given steric barrier stabilising a dispersion, the repulsion energy is going to decrease drastically at high pH. If the other variables of the steric barrier are arranged so that the reduction in
19
75
PHI> 2 I
4 I
6
8 I
10 I
12 I
Figure Fig. 2. Upper
temperature
limit of solubility
versus pH (Ref.
[ 171).
the ~5, (1 - 8 /T) term due to high pH is sufficient for V, to be overcome by the attractive forces, flocculation will occur leading to electrodeposition of the dispersion by the alkaline boundary layer around the cathode, thus fulfilling the third requirement. However, Eqn (7) also suggests that, if the other variables, e.g. surface concentration, barrier thickness, etc., are dominantly large, a lowering of the theta value due to alkalinity may be insufficient to bring about electrocoagulation of the emulsion. It is now possible to make non-ionically stabilised emulsions deposit on the cathode [18]. Since we are dealing with a lowering of theta conditions,
Fig. 3. Structure
of self-emulsifiable
non-ionic
polymer.
20 .D
E
o
I,_
O£"4 C~'C
,_¢-~0 ~o
.~_
o.s 7 o~
0 o I
I
I
60 120 180 Deposition time (secs) Fig. 4. Weight of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) deposited on cathode at a constant applied voltage (80 V) with varying deposition times.
volts
~20C -
o100 -
P,/
>f o,
,,o
i,, p A" o i 1 I 10 20 30 2 Weight of deposit ( m g / c m ) after cure at 180°C
~,2 /
,~
(b}
(a)
>100t-
iI C
I I I I 0.1 0.3 0.5 0.7 Coulomb Yield (g/coulomb) after cure at 180"C
Fig. 5. Weight of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) deposited at varying applied voltages on: (a) cathode; (b) anode (after addition of 0.025% poly(ammonium acrylate) to the same emulsion). Fig. 6. Coulomb yields of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) on: (a) cathode; (b) anode (after addition o f 0.025% poly(ammonium acrylate) to the same emulsion).
21
and not a complete insolubilisation, not all non-ionic emulsions will deposit at the cathode. A typical non-ionic, cathodically electrodepositable composition is made by emulsifying, in water, a polymer made from two 'Epikote' molecules coupled by a polyester and the remaining oxirane groups reacted with PEO and p-nitrobenzoic acid, which may be schematically represented b y the molecule shown in Fig. 3. Emulsions made from this polymer have excellent dispersion stability and narrow particle size distributions. The electrodeposition properties in terms of applied voltage and coulomb yield are given elsewhere [ 1 8 ] , and some examples of electrodeposition of this type of emulsion (15% w/w) are given in Figs 4--6. ANODIC NON-IONIC ELECTROCOAT
It has been known for a long time that association reactions between polyalkylene oxides and polycarboxylic acids occur to give water-insoluble polymer mixtures b y an instantaneous, room-temperature reaction [19], i.e. by acidification. The interpolymer complex formed between PEO chains and a polycarboxylic acid in acidic aqueous solution has been closely studied and there are many papers in the literature describing the mechanism and the finer points of the association complex [19, 20]. The salient feature is that., at low pH, polycarboxylic acids are undissociated and hydrogen bonds may be formed with the ethereal oxygens of the PEO chain, as shown schematically in Fig. 7, to form a water-insoluble complex. Hence, if a non-ionic, sterically stabilised emulsion, in which the solvated sheath is composed of PEO chains has a polycarboxylic acid added to it, at neutral or alkaline pH, there should be negligible association between the freely soluble acid and the PEO chains. The particles should show no de/ -OOC~cH I CH~ \ C~2 \ H C --COOI CH 2 / C Hz
I
H C -- COO\ CH 2
I
CH= 1 H C--COOL
I CH2 1 O \ CH 2 I CH2 / O / CH z I CH 2
I I lC H 2 H C-- C O O H .....O / ] CHz CH~ \CHz I CH2 \ I HC~cooH"..O I I CH z CH2 I / CH z
> p H 3.9 .,, ~ < pH 3.9
~
I
O \ CH
J
CH 2 I O I
I
ICH2
H C ~ COOH-.. 'O I I CH z CHz I I CH z / CH7 HC I I ~ COOH....O J
Fig. 7. I n t e r p o l y m e r c o m p l e x f o r m a t i o n p A A - - P E O .
22
crease in stability on the addition of the polycarboxylic acid salt. However, if this dispersion mixture is exposed to an anode in an electrocoat cell, when current is passed the acidic boundary layer created around the anode collapses the stabilising layer around the particles (due to loss of solvation by complex formation) leading to enhanced flocculation and deposition onto the electrode. This concept was first tested on a non-ionically stabilised latex, prepared by Thompson et al. [22] , whose stabilising groups were PEO chains. When the latex, per se, was placed in an electrocoat cell and current was passed, no deposition occurred. However, when a little polymethacrylic acid salt was added and the experiment repeated, a thick deposit of latex particles was obtained on the anode. Nature of complexing
agent (polycarboxylic
acid)
It is expected that the complexing properties with PEO will depend on the solubility of the polycarboxylic acids. Ikawa et al. [20] studied the complex formation between PEO and polyacrylic acid (pAA), polymethacrylic acid (pMAA) and styrene-maleic acid (pSMA) copolymer. Their results are summarised in Fig. 8 where it is seen that pMAA complexes the most readily and pAA the least readily. The association between ether oxygens and carboxylic groups through hydrogen bonding approaches a 1: 1 stoichiometry. Ikawa et al. suggest that the variation in the effectiveness of the polyacids to complex with PEO as shown in Fig. 7 is reflected in their ability to form hydrophobic bonding to supplement the hydrogen bonding in complex formation, and electrodeposition experiments reflected this. The effect of the molecular weight of the complexing acid on the efficiency of electrodeposition of a non-ionic emulsion was studied by taking a
1
Fig. 8. Interpolymer plex (Ref. [ZO]).
2
3
complex:
4 PH
pH dependence
of precipitate
yield in polyacid-PEO
com-
23
series of pAA and adding them, on an equal weight basis, to a non-ionic emulsion, carrying out the electrodeposition and assessing the quality of the electrodeposit. It was found that the higher the molecular weight of the pAA, the better the electrodeposited layer. Non-ionic
electrodeposition
of emulsions
stabilised
by other steric stabilisers
Having shown that it is possible to electrodeposit, or more correctly electrocoagulate, PEO sterically stabilised aqueous emulsions, it was also found possible to electrodeposit on the anode sterically stabilised emulsions based on other non-ionic groups such as poly(viny1 pyrrolidone) and poly(viny1 alcohol), etc., in a similar way [21]. Thus showing that the collapse of the steric stabilising layer was achievable by loss of solvation (complex formation) and was not just restricted to PEO stabilised dispersions. Again, experimental details as to electrodeposition voltage and coulomb yield are given elsewhere [ 211, and also in Figs 5 and 6. CONCLUSION
Previously, it was thought that only charge-stabilised emulsions would electrodeposit. It has now been shown that non-ionically stabilised emulsions will also electrodeposit. Suitable emulsions will deposit at the cathode and, by the addition of polycarboxylic acid, these same emulsions can be made to switch from cathodic to anodic deposition. Indeed, simultaneous deposition at both the cathode and the anode can also be achieved under special conditions. It follows then that “electrophoretic painting” is a misnomer and that using the electrodeposition of colloids as a measure of electrophoretic mobility [23] is fundamentally unsound. Finally, it should be remarked that the virtual absence of ions in the sterically stabilised emulsions leads the single most obvious difference between non-ionic electrocoating systems and conventional systems, that is, in their conductivity (see Table 1). TABLE Emulsion
1 type (15%
Conductivity
w/w)
Non-ionic (cathodic) Non-ionic (anodic) with complexing Ionic (anodic or cathodic)
agent
630 40140 l,OOO-5,000
at 25” C (pS cm-‘)
24
REFERENCES
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Davey, USP 1,294,627. UK Patent 455,810. C.G. Summer, Trans. Faraday Sot., 36 (1940) 272. W. Nerst, Phys. Chem. 47 (1904) 52, 56. W. Vielstich, Z. Electrochem., 57 (1953) 646. F. Beck, Prog. Org. Coat., 4 (1976) l-60. G. Kortiim and JO’M. Bockris, in Electrochemistry, Vol. 2, Elsevier, Amsterdam, 1951, p. 404. H.C. Hamaker, Physica, 4 (1937) 1058. Th.G. Overbeek, Adv. Colloid Interface Sci., 16 (1982) 17-30. A.L. Smith, in G.D. Parfitt (Ed.), Dispersions of Powders in Liquids, 3rd edn, Ap plied Science Pub]., Barking, 1981, p. 127. J. Visser, Adv. Colloid Interface Sci., 3 (1972) 331-363. M. Smoluchowski, Z. Phys., 17 (1916) 557; Z. Phys. Chem., 92 (1917) 129. A. Doroszkowski and R. Lambourne, J. Colloid Interface Sci., 43 (1973) 97. D.H. Napper and R. Evans, Kolloid Z. Z. Polym., 251 (1973) 329-336, 409-414 A. Doroszkowski and R. Lambourne, J. Polym. Sci., C34 (1971) 253. D.H. Napper, J. Colloid Interface Sci., 32 (1970) 106-114. F.E. Bailey and R.W. Callard, J. Appl. Polym. Sci., 1 (1959) 56-62. A. Doroszkowski and A.C. Barlow, EP 109760. K.L. Smith A.E. Winston and D.E. Petersen, Ind. Eng. Chem., 51 (1959) 1361. T. Ikawa, K. Abz, K. Honda and E. Tsuchida, J. Polym. Sci., 13 (1975) 1505-1514. A. Doroszkowski, EP 15655. C. Graetz, M.W. Thompson, F.A. Waite and J.A. Waters, EP 13478. M.J.B. Franklin, J. Oil Colour Chem. Assoc., 51 (1968) 499-523.
and Surfaces,
Elsevier
Science
17 (1986)
Publishers
13-24
13
B.V., Amsterdam
THE ELECTRODEPOSITION EMULSIONS*
- Printed
in The Netherlands
OF NON-IONICALLY
STABILISED
A. DOROSZKOWSKI Research Berkshire
Department, Imperial Chemical SL2 5DS (United Kingdom)
(Received
3 August
1984; accepted
Industries
PLC, Paints Division,
Slough,
in final form 2 June 1985)
ABSTRACT One of the major industrial paint application processes is electrodeposition (electrocoat paints). By examining the mechanism of paint electrodeposition it is seen that the salient feature of the process is the destabilisation of an emulsion by an electrode process which is independent of electrophoresis. Therefore, by extending this concept it has been shown that even sterically (non-ionic) stabilised emulsions can be electrodeposited. The mechanism of such a process and examples of such emulsions which can be deposited at either the cathode or the anode are described.
INTRODUCTION
The commercial electrodeposition process, sometimes known as electrophoretic painting, for the coating of metal objects for protective purposes was introduced simultaneously by ICI in collaboration with Pressed Steel Fisher Ltd in the U.K., and by the Ford Motor Company in the U.S.A. in 1963. The concept of electrodepositing an organic material can be traced back to 1919 [l] . Crosse and Blackwell Ltd explored the possibility of electrodepositing lacquer emulsions on food cans in 1936 [2, 31. However, it was not until the early 1960s that the anodic electrodeposition of paint was established as a viable commercial process. In 1971 cathodic electrodeposition was introduced to the appliance industry by PPG Industries Inc. and in 1976 cathodic automotive systems were introduced which have virtually supplanted the earlier anodic ones because of their general, overall better protective properties, and are now the standard primer application methods in industry. While it may seem obvious that anionic emulsions will deposit at the anode, and cationic emulsions at the cathode, in a system known as “electrophoretic painting”, the deposition mechanism is very frequently misunderstood. *Dedicated
to the memory
0166-6622/86/$03.50
of M.W. Thompson.
0 1986 Elsevier
Science
Publishers
B.V
14 THE ELECTRODEPOSITION
MECHANISM
In emulsion electrodeposition, it is possible to identify three contributing processes: (1) A mass transport process in the bulk to the electrode zone. (2) An electrode process (electrode reaction). (3) An emulsion stability-instability process in the electrode zone. Mass transport The mass transport process outside the electrode zone can be described as a flux of particles which consists of contributions from: (a) particle flow due to convection or agitation (V,); (b) diffusion of particles, which will depend on a concentration gradient de/ax and their diffusion coefficient, Di; and (c) migration of particles (electrophoresis) due to the potential gradient a U/ax. Considering only the one-dimensional flux (Ji) of particles we have: Ji = VxCi + Di g
+ z /J.ici5
where Ji is the flux of i particles
in the x direction, ci is the concentration of particles, z is the charge on the particle, and pi is the mobility of the particle. When the particles are colloidal in size (radius, a) then the diffusion coefficient can be estimated by the Stokes-Einstein equation: Di=
kT _ 6nqa
(2)
where q is the viscosity of the medium. Since there is little or negligible concentration gradient (at/ax) in the bulk of the dispersion, evaluation of Eqn (2) for a submicrometer particle (e.g. 0.1 pm) in water yields a Di value of - 2 X 10e8 cm* s- ‘, giving to all intents and purposes a diffusion contribution to Ji which cm be ignored. Migration in an electric field is well known and the mobility (zipi) of a particle can be converted into a particle velocity (u) which is dependent on the potential gradient a U/ax and the zeta potential of the particle. All these values are readily measured, and while they are dependent on the electrolyte content of the medium and surface-active groups used to stabilise the particles, typical values are in the order of a few pm s-l per V cm-‘. If an electric field of 100 V per 10 cm is applied then the electrophoretic velocity is or agitation (V,) is of - 10m3 cm s-l, while the velocity due to convection the order of 1 to 10 cm s-l. Thus, when the “bulk” mass transport [Eqn (l)] is evaluated in terms of the constituent processes, using the above values, it is clear that the process, by orders of magnitude, is due to convection currents and agitation
15
(mechanical is negligible. Electrode
stirring)
and that transport
due to electrophoresis
or diffusion
reaction
The main electrode reaction in electrocoat painting is the decomposition of water. At the anode the reaction is H,O = X0, 1 + 2H’ + 2e- and at the cathode H,O = %Hz t + OH- - e-. According to Nernst [4] and Vielstich [5], in the case of a flowing bath a “diffusion” or boundary layer must be imagined at the electrode--solution interface (see Fig. 1). In this quasistationary layer, particles can only be transported by diffusion or migration. The thickness, 6N, of this layer depends on the hydrodynamic conditions which can be expressed as [5, 61 : 6N = 3$/l v,-” ,,1!6 Dili3 (3)
where y is the coordinate parallel to electrode surface, V, is the velocity of liquid flow, v is the kinematic viscosity, and Di is the diffusion coefficient. t
Fig. 1. Model to Nernst.
of diffusion
layer
for protons
at the electrode-solution
interface
according
At the anode protons are injected into the phase boundary due to the decomposition of water and, in the steady state, an approximately constant which can be related to the electric curproton concentration is established rent density (I): I = 2F&+
(C&+- cg+) SN(H+)
where F is Faraday’s
constant.
(4)
17
as the composition of the attracting bodies, i.e. if A 1, and AZ2 are the Hamaker constants for material 1 and material 2, respectively, in a medium of Hamaker constant Ass, then the effective Hamaker constant (metal and polymer in water) is AIs = (All” - A33G) (Az2% - Ass”‘) [ll] and has a value 0.16 x lo-l3 erg for polymerpolymer of -1.0 X lo-l3 erg. Similarly, A,,,= particles in water. Hence, there is an order of magnitude greater attraction for the particles to flocculate on the metal electrode than with themselves. Electrodeposition of an anionic dispersion is thus seen to be a process of destabilising a charge-stabilised dispersion. The acidic boundary layer around the anode suppresses ionic dissociation on the surface of the particles, causing the particles to flocculate onto the electrode (although they may also flocculate with themselves to some degree in the boundary layer before attaching themselves to the electrode). Some idea of the rate of flocculation may be obtained from Smoluchowski rapid coagulation kinetics [12] and for typical electrocoat systems t, is of the order of milliseconds; t, is the time required for the number of particles to be reduced by half. The above argument applies to charge-stabilised emulsions, the only difference with cathodic deposition is that the electrode has an alkaline boundary layer instead of an acidic one (ignoring dissolution effects of the electrode which can enhance emulsion coagulation), this likewise inhibits the association of a proton with a weak base to produce ionisation, i.e. R3 N’H = R3N + H’ R,N+H + OH- + R3N + H,O In generic terms one can divide dispersions into two categories: (1) charge stabilised, anionic (or cationic), dispersions; and (2) sterically stabilised dispersions. If the above mechanism is correct and one can devise an electrode reaction to destroy the polymeric layer producing a sterically stabilised dispersion, then it should be possible to electrodeposit even this type of system, since the first requirement of a mass transport process to the electorde zone does not require charge, nor is electrophoresis in the electrode zone necessary . STERIC STABILISATION
The governing principle of steric (or polymer) stabilisation of colloidal particles is that a sheath of solvated polymer surrounds the colloidal particles. When two such particles approach each other, the adsorbed sheaths interact generating a repulsion energy, which is sufficient to overcome the attraction energy due to surface forces. The repulsion energy in steric stabilisation is frequently subdivided into an “osmotic” term (or free energy of mixing) and a “volume restriction”
18
term. Experimental evidence suggests that, in practice, the volume restriction term can be ignored [13, 141 and colloid stability is governed by the osmotic term ( VR M ). The nett energy of interaction between two particles of radius a and barrier thickness F is [ 151 : V nett
=Vfp
+v,
and v,M
= -
kT
ViP2
G I(1
-
‘3/T) cz f(a, 6, h)
where $ 1 is Flory’s entropy parameter; 0, theta temperature; c the polymer concentration in the stabilising barrier; T, the temperature; h, the surface-tosurface separation; f(a,6 ,h) is a function of the stabilising barrier geometry; and Vi and p are the solvent molar volume and polymer density, respectively .
When T= 8, VRM will become zero and the dispersion will flocculate as has convincingly been demonstrated by Napper [16] and, if T < 0, VR M changes sign, and becomes an attraction energy. To obtain dispersion stability, it is essential to have the stabilising polymer chains well solvated. If the solvation is decreased dispersion stability is decreased so that ultimately, when solubility is lost, the polymer chains induce flocculation. CATHODIC
NON-IONIC
ELECTROCOAT
A sterically stabilised dispersion of colloidal particles can readily be made where the solvated polymer sheath comprises polyethylene oxide (PEO) chains. The entropy parameter ($ 1) in Eqn (7) is negative in aqueous solution and dispersion stability is obtained when T < 0. Furthermore, if one examines the solubility of PEO in aqueous solution as a function of pH, then at high pH the precipitation temperature of highmolecular-weight PEO is drastically reduced as shown in Fig. 2 [17]. Examination of Eqn (7) shows that the stabilising energy of the solvated polymer layer is not only a function of the theta temperature, but also depends on the surface volume concentration (c) of the solvated polymer and the layer dimensions. The theta temperature of a polymer is defined as the temnerature when the second virial coefficient is zero. It may also be loosely described as the temperature of incipient precipitation of a polymer of infinite molecular weight. Therefore Fig. 2 gives an indication of the theta-temperature behaviour of PEO stabilisers versus pH. Hence, one can see immediately that, for a given steric barrier stabilising a dispersion, the repulsion energy is going to decrease drastically at high pH. If the other variables of the steric barrier are arranged so that the reduction in
19
75
PHI> 2 I
4 I
6
8 I
10 I
12 I
Figure Fig. 2. Upper
temperature
limit of solubility
versus pH (Ref.
[ 171).
the ~5, (1 - 8 /T) term due to high pH is sufficient for V, to be overcome by the attractive forces, flocculation will occur leading to electrodeposition of the dispersion by the alkaline boundary layer around the cathode, thus fulfilling the third requirement. However, Eqn (7) also suggests that, if the other variables, e.g. surface concentration, barrier thickness, etc., are dominantly large, a lowering of the theta value due to alkalinity may be insufficient to bring about electrocoagulation of the emulsion. It is now possible to make non-ionically stabilised emulsions deposit on the cathode [18]. Since we are dealing with a lowering of theta conditions,
Fig. 3. Structure
of self-emulsifiable
non-ionic
polymer.
20 .D
E
o
I,_
O£"4 C~'C
,_¢-~0 ~o
.~_
o.s 7 o~
0 o I
I
I
60 120 180 Deposition time (secs) Fig. 4. Weight of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) deposited on cathode at a constant applied voltage (80 V) with varying deposition times.
volts
~20C -
o100 -
P,/
>f o,
,,o
i,, p A" o i 1 I 10 20 30 2 Weight of deposit ( m g / c m ) after cure at 180°C
~,2 /
,~
(b}
(a)
>100t-
iI C
I I I I 0.1 0.3 0.5 0.7 Coulomb Yield (g/coulomb) after cure at 180"C
Fig. 5. Weight of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) deposited at varying applied voltages on: (a) cathode; (b) anode (after addition of 0.025% poly(ammonium acrylate) to the same emulsion). Fig. 6. Coulomb yields of non-ionically stabilised polymer containing curing agent poly(alkoxy methylamino triazine) on: (a) cathode; (b) anode (after addition o f 0.025% poly(ammonium acrylate) to the same emulsion).
21
and not a complete insolubilisation, not all non-ionic emulsions will deposit at the cathode. A typical non-ionic, cathodically electrodepositable composition is made by emulsifying, in water, a polymer made from two 'Epikote' molecules coupled by a polyester and the remaining oxirane groups reacted with PEO and p-nitrobenzoic acid, which may be schematically represented b y the molecule shown in Fig. 3. Emulsions made from this polymer have excellent dispersion stability and narrow particle size distributions. The electrodeposition properties in terms of applied voltage and coulomb yield are given elsewhere [ 1 8 ] , and some examples of electrodeposition of this type of emulsion (15% w/w) are given in Figs 4--6. ANODIC NON-IONIC ELECTROCOAT
It has been known for a long time that association reactions between polyalkylene oxides and polycarboxylic acids occur to give water-insoluble polymer mixtures b y an instantaneous, room-temperature reaction [19], i.e. by acidification. The interpolymer complex formed between PEO chains and a polycarboxylic acid in acidic aqueous solution has been closely studied and there are many papers in the literature describing the mechanism and the finer points of the association complex [19, 20]. The salient feature is that., at low pH, polycarboxylic acids are undissociated and hydrogen bonds may be formed with the ethereal oxygens of the PEO chain, as shown schematically in Fig. 7, to form a water-insoluble complex. Hence, if a non-ionic, sterically stabilised emulsion, in which the solvated sheath is composed of PEO chains has a polycarboxylic acid added to it, at neutral or alkaline pH, there should be negligible association between the freely soluble acid and the PEO chains. The particles should show no de/ -OOC~cH I CH~ \ C~2 \ H C --COOI CH 2 / C Hz
I
H C -- COO\ CH 2
I
CH= 1 H C--COOL
I CH2 1 O \ CH 2 I CH2 / O / CH z I CH 2
I I lC H 2 H C-- C O O H .....O / ] CHz CH~ \CHz I CH2 \ I HC~cooH"..O I I CH z CH2 I / CH z
> p H 3.9 .,, ~ < pH 3.9
~
I
O \ CH
J
CH 2 I O I
I
ICH2
H C ~ COOH-.. 'O I I CH z CHz I I CH z / CH7 HC I I ~ COOH....O J
Fig. 7. I n t e r p o l y m e r c o m p l e x f o r m a t i o n p A A - - P E O .
22
crease in stability on the addition of the polycarboxylic acid salt. However, if this dispersion mixture is exposed to an anode in an electrocoat cell, when current is passed the acidic boundary layer created around the anode collapses the stabilising layer around the particles (due to loss of solvation by complex formation) leading to enhanced flocculation and deposition onto the electrode. This concept was first tested on a non-ionically stabilised latex, prepared by Thompson et al. [22] , whose stabilising groups were PEO chains. When the latex, per se, was placed in an electrocoat cell and current was passed, no deposition occurred. However, when a little polymethacrylic acid salt was added and the experiment repeated, a thick deposit of latex particles was obtained on the anode. Nature of complexing
agent (polycarboxylic
acid)
It is expected that the complexing properties with PEO will depend on the solubility of the polycarboxylic acids. Ikawa et al. [20] studied the complex formation between PEO and polyacrylic acid (pAA), polymethacrylic acid (pMAA) and styrene-maleic acid (pSMA) copolymer. Their results are summarised in Fig. 8 where it is seen that pMAA complexes the most readily and pAA the least readily. The association between ether oxygens and carboxylic groups through hydrogen bonding approaches a 1: 1 stoichiometry. Ikawa et al. suggest that the variation in the effectiveness of the polyacids to complex with PEO as shown in Fig. 7 is reflected in their ability to form hydrophobic bonding to supplement the hydrogen bonding in complex formation, and electrodeposition experiments reflected this. The effect of the molecular weight of the complexing acid on the efficiency of electrodeposition of a non-ionic emulsion was studied by taking a
1
Fig. 8. Interpolymer plex (Ref. [ZO]).
2
3
complex:
4 PH
pH dependence
of precipitate
yield in polyacid-PEO
com-
23
series of pAA and adding them, on an equal weight basis, to a non-ionic emulsion, carrying out the electrodeposition and assessing the quality of the electrodeposit. It was found that the higher the molecular weight of the pAA, the better the electrodeposited layer. Non-ionic
electrodeposition
of emulsions
stabilised
by other steric stabilisers
Having shown that it is possible to electrodeposit, or more correctly electrocoagulate, PEO sterically stabilised aqueous emulsions, it was also found possible to electrodeposit on the anode sterically stabilised emulsions based on other non-ionic groups such as poly(viny1 pyrrolidone) and poly(viny1 alcohol), etc., in a similar way [21]. Thus showing that the collapse of the steric stabilising layer was achievable by loss of solvation (complex formation) and was not just restricted to PEO stabilised dispersions. Again, experimental details as to electrodeposition voltage and coulomb yield are given elsewhere [ 211, and also in Figs 5 and 6. CONCLUSION
Previously, it was thought that only charge-stabilised emulsions would electrodeposit. It has now been shown that non-ionically stabilised emulsions will also electrodeposit. Suitable emulsions will deposit at the cathode and, by the addition of polycarboxylic acid, these same emulsions can be made to switch from cathodic to anodic deposition. Indeed, simultaneous deposition at both the cathode and the anode can also be achieved under special conditions. It follows then that “electrophoretic painting” is a misnomer and that using the electrodeposition of colloids as a measure of electrophoretic mobility [23] is fundamentally unsound. Finally, it should be remarked that the virtual absence of ions in the sterically stabilised emulsions leads the single most obvious difference between non-ionic electrocoating systems and conventional systems, that is, in their conductivity (see Table 1). TABLE Emulsion
1 type (15%
Conductivity
w/w)
Non-ionic (cathodic) Non-ionic (anodic) with complexing Ionic (anodic or cathodic)
agent
630 40140 l,OOO-5,000
at 25” C (pS cm-‘)
24
REFERENCES
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Davey, USP 1,294,627. UK Patent 455,810. C.G. Summer, Trans. Faraday Sot., 36 (1940) 272. W. Nerst, Phys. Chem. 47 (1904) 52, 56. W. Vielstich, Z. Electrochem., 57 (1953) 646. F. Beck, Prog. Org. Coat., 4 (1976) l-60. G. Kortiim and JO’M. Bockris, in Electrochemistry, Vol. 2, Elsevier, Amsterdam, 1951, p. 404. H.C. Hamaker, Physica, 4 (1937) 1058. Th.G. Overbeek, Adv. Colloid Interface Sci., 16 (1982) 17-30. A.L. Smith, in G.D. Parfitt (Ed.), Dispersions of Powders in Liquids, 3rd edn, Ap plied Science Pub]., Barking, 1981, p. 127. J. Visser, Adv. Colloid Interface Sci., 3 (1972) 331-363. M. Smoluchowski, Z. Phys., 17 (1916) 557; Z. Phys. Chem., 92 (1917) 129. A. Doroszkowski and R. Lambourne, J. Colloid Interface Sci., 43 (1973) 97. D.H. Napper and R. Evans, Kolloid Z. Z. Polym., 251 (1973) 329-336, 409-414 A. Doroszkowski and R. Lambourne, J. Polym. Sci., C34 (1971) 253. D.H. Napper, J. Colloid Interface Sci., 32 (1970) 106-114. F.E. Bailey and R.W. Callard, J. Appl. Polym. Sci., 1 (1959) 56-62. A. Doroszkowski and A.C. Barlow, EP 109760. K.L. Smith A.E. Winston and D.E. Petersen, Ind. Eng. Chem., 51 (1959) 1361. T. Ikawa, K. Abz, K. Honda and E. Tsuchida, J. Polym. Sci., 13 (1975) 1505-1514. A. Doroszkowski, EP 15655. C. Graetz, M.W. Thompson, F.A. Waite and J.A. Waters, EP 13478. M.J.B. Franklin, J. Oil Colour Chem. Assoc., 51 (1968) 499-523.