Electrodeposition of polymer coatings

Electrodeposition of polymer coatings

e. 1, PP. a19-850 . 19ns. 000-46Na;a5 53 .g' 0 . W t: I I . . P,rgamon Pras P1, REVIEW ARTICLE ELECTRODEPOSITION OF POLYMER COATINGS* FRITZ BECK ...

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e. 1, PP. a19-850 . 19ns.

000-46Na;a5 53 .g' 0 . W t: I I . . P,rgamon Pras P1,

REVIEW ARTICLE ELECTRODEPOSITION OF POLYMER COATINGS* FRITZ BECK

University-GH-Duisburg, FB 6Elektrochemie, Lotharstr . 1, D 4100 Duisburg 1, F .R .G . Abstract-Electrochemical deposition of polymer coatings makes use of electrolytic solutions, containing monomeric and/or polymeric starting materials . Under the influence of electrochemical processes physical or chemical transformation of the dissolved material leads to the formation of a solid film of 10 nm to 100 µm thickness on the electrode . Nucleation and precipitation may he rate determining processes and are discussed in greater detail, namely : (1) electrocoagulation of waterborne polymers . (2) Electrochemical coupling, in the case of anodic electropolymerization of (hetero) aromatics. The propagation step is an electrochemical one. (3) Electrochemically initiated vinylpolymcriration . The propagation step is of chemical nature . Combinations of these methods are also known. The most important practical application is corrosion protection of iron substrates.

1 . INTRODUCTION Electrodeposition of polymer layers is of great practical interest, for it combines an electrical technique, which can be easily controlled or even automated, with the inherent possibility of coating various substrates with polymers of broadly varying properties[1] . According to Fig . 1, the polymer film has an inner (1) and an outer (2) phase boundary. In the case of an inert substrate, electrons can only cross the inner interphase, while generally canons (K' ), anions (A - ) and electrons can move through the film to be transferred through the outer phase boundary . This depends on the mechanism of the film-forming electrochemical process and the transport details in the polymer . Charged carriers are transported by diffusion and migration . Molecules can be transported along a concentration gradient by diffusion alone . The bath needs the presence of an electrolyte as well as the addition of soluble monomers or polymers to build up the polymer coating. From a historical point of view, electrochemically initiated vinyl polymerization was introduced in the early 1950s by the independent work of Kern and Quast[2], Breitenbach and Srna[3], Parravano[4] and Funt and Wilhams[5] . Initiators are generated by an electrode process . Ionic or radical propagation occurs

j Z/,

Polymer Layer K

®IMe)

' Electrolyte („Bath")

q- F_

1

thereafter. Generally, the formation of a polymer solution or dispersion competes with the deposition of a polymer film on the electrode, which has a porous structure . The electrophoretic deposition of colloids has been known for a long time[6] . However, it was only after the development of waterborne paints by the chemical industry about 1960, proposed by Buinside and Brewer at the Ford Motor Co-[7, 8], that this process gained a rapidly increasing industrial importance. The polymer is prefabricated by industrial manufacturers, and the electrochemical step is simple water electrolysis . A strong pH shift in the diffusion layer causes an electrowagulation of the polymers . Recently, the anodic deposition of the so-called synthetic metals or conducting polymers has attracted much attention. This exciting development has been initiated by Chiang et al.[9], Nigrey et al .[10] and by Diaz et al .[Il j. In this case, the monomers are aromatic or heteroaromatic molecules and the initiation, as well as the propagation steps, are of electrochemical nature . Five important features of polymer layers, electrodeposited by these processes, are summarized in Table 1, namely the electrochemical equivalent m r , the characteristic thickness L, the specific conductivity s, the nature of the mobile charge carriers and the deposition time r for a 20 µm layer, which is very important for any practical application . From this compilation it can be seen that each of these parameters are subject to a broad variation over orders of magnitude. In the following, the three methods established today are discussed in the order indicated in Table 1, making up Sections 2-4 . Two recent modifications of methods I and 2 and other methods will be presented in Sections 5-7 .

2 2 . ELECTRODEPOSITION OF PAINT

Fig . 1 . Phase scheme of polymer coated electrodes .

*Presented at the ISE Meeting, Maastricht, 1987 .

The only example of present industrial importance is the electrodeposition of paint (EDP) . Annually some 103 square miles of waterborne polymers are elec 839

u 31 :7-x



840

F.

BECK

Table 1 . Typical parameters for electrocoating of polymers No. I

2 3

Type Electrodeposition of paint (EDP) Anodic deposition of synthetic metals (intrinsically conducting polymers) Film-forming electrochemical vinylpolymerization (M = l0')

in, (mgC - ')

L (AM)

(S cm ')

Charge carriers

Time t for deposition (s*)

20 0.5

20 0.01-100

10 - s 1-10'

H - , OH e , (A )

100 4000

0.1-10

10 "

(K', A - )

2

1000

K

'd=20µm,s - 1gcm -3,j=1mAcm - ' . trodeposited onto metal mass ware as car bodies to provide the first paint layer by an electrochemical process. This corresponds to some 10° tons per year or < 0.1'% of the annual world production of polymers . However, due to the high specific cost, a much higher fraction in terms of economic value is represented by these polymer materials . The structure of these polymers is like an ionomer, but with long neutral portions between the ionogenic groups . Their density is about I equivalent per kilogram, of the same order as ion exchange resins and one order below that of polyelectrolytes . This leads to a micellar structure in the aqueous solutions . In 1979, anionic systems were nearly totally exchanged by cationic ones, which offer pronounced advantages in processing, adherence and corrosion protection . The chemical nature of these special polymers was described eisewhere[t2, 13] .

b)

2 .1 . Mechanism of electrucoagulation The electrochemical reaction is water electrolysis, eg at the cathode : H,O+e--*OH-+1/2Hr .

(I)

The OH - ions injected into the cathodic diffusion layer neutralize the polymeric rations which are transported towards the cathode mainly via electrophoresis : p-NR 2 H -'+OH -.p-NR,j+H 2 O.

(2)

p means a section of the polymer backbone carrying one ionic group, The neutral form is insoluble and tends to coagulate. Thus the mechanism of EDP is an indirect one. Some complication arises due to the fact that the only electrolyte to be present in the solution is the solubilized polymer, corresponding to the following acid,/base equilibrium : p-NR, +HAtp-NR r li'+A - .

(3)

Electrochemical experiments of electrodeposition of anionic[15, 16] or cationic[17, 18] systems onto a rotating disc electrode reveal a proportionality beween the square of the current density and a critical upper rotation speed n i,,„ where clectrocoagulation becomes impossible within a present time period . This has been explained in terms of a critical concentrationn of H + or OH - ions at the phase boundary, which must he attained to force the electrocoagulation[ 16, 19] . On the basis of the model Fig. 2, the free acid HA . which is present in an equilibrium concentration c u , governed by Equation (3), is competing with the ionomer to neutralize the electrogenerated OH - ions . If the degree of neutralization a is high (a > 1) (cf. Fig . 2a) free

Fig . 2 . Concentration profiles in the diffusion layer for 011 ions and acid molecules for : (al high degree of neutralization ; (b) low degree of neutralization . Two cases of rotation speeds ry of the rotating disc electrode arc shown, namely - - low n and ---- 'iii . concentration of HA is high, and a small increase of rotation speed n (lowering d) is sufficient to decrease con - to the critical value c°OU -, where no further clectroeoagulation occurs . On the other hand, a low c HA , due to a low a (in a < I), leads to quite another situation (cf Fig . 2b). In this case, rotation speed has to be enhanced distinctly up to the point, where the critical surface concentration of OH - ions is accomplished again. Indeed, this behaviour has been observed experimentally[16, 19], and it follows from this model, with the help of simple acid,/base equilibrium considerations (cf. Equation (6) in[16]) that k Join)-F(2ceoH-Dou-+cHADHA)

(4)

(k = constant in Lewitch equation, D = diffusion coefficient) . The second term in the denominator should be much larger than the first for a > 1 . Thus it can be readily seen that n i „„ must be nearly constant at a > l,



841

Electrodeposition of polymer coatings but it should increase rapidly with a < 1, as observed experimentally and as it follows from Fig . 2. Supporting electrolytes must be absent in an EDP bath. Thus the only cations in the bath are protons arising from the weak acid HA and the very weak acid p-NR,,H' . No inert counter ions to OH - are present in the system . From this it must be concluded that the diffusion layer of the OH - ions must be built up without the usual counter ions . This leads, however, to the establishment of a negative space charge, according to Fig. 3 . The potential decay Atp in this space charge can be calculated by integration of Poisson's equation : d'ty = -p (x) .

dx'

et,

(5)

2 .2 .

Thickness growth of EDP

Ap

p (x) can be obtained with the diffusion law, where S is the slope of the concentration profilep(x) = SFx dc dx

j F DQH

(6) (7)

Combining the last three equations, it follows after two-fold integration; (8) sso Do"As experimentally shown, no large voltage effects can be seen in the course of the induction period in the galvanostatic experiment[16, 19] . Thus Atp must be in the order of 1 V . This leads to x - I pm (for j = I mA cm - ' ), corresponding to Co.x- - 10 - 4 M, in accordance with our former calculation[16], where the space charge effect had not yet been addressed . It should be stressed that in the neighbourhood of nrm, where critical concentrations of about cOOor,- are established, the induction period at the rotating disc experiment may become rather long ; we have performed an experimment, where only after one hour an electrocoagulation occurred[16, 19] . This time constant is orders of magnitude larger than the time constant for the establishment of a diffusion layer : App

62

r

4D

(9)

layer

The presence of pronounced space charges must also be assumed in the electrodeposited EDP layer itself . This was pointed out by Cooke et al .[20, 21] and by Reck[22, 23] . For anionic paints, a model of a weak acid ion exchanger has been developed, taking into account that the pK value of the polyacid, evaluated by a special titration method[19, 22], was as low as about 12 . That means that the proton concentration in the film should be only about c = 1 MM, It is clear that charge separation will occur in the polymer layer . If we assume a constant space charge density of thickness L, the voltage drop in this space charge can be calculated by the two-fold integrated Poisson equation : CF

2eeo

, L .

With L = 20 pm and e = 10, one obtains Ao = 230 V, which is in the order of the voltages which must be applied to overcompensate this potential drop . As Fig. 4 shows, a negative space charge is formed at the electrode side due to the fixed ions . The protons accumulate at the surface of the EDP layer, where a positive space charge is established . In the course of further thickness growth, water molecules diffuse towards the anode, where water electrolysis occurs, (11)

H 2 O.1/20,+2H'+2e - ,

injecting protons at the left phase boundary . These are transported back to the bath with different mechanisms in the different zones (1)-(4), namely via diffusion and migration in (I), migration in (2) and (3) and diffusion (4). In the first stages of the electrocoagulation, most of the final coating has already been electrodeposited . A large amount of the external constant voltage is absorbed in the electric space charges and j decreases to a small residual current density, cf. Fig. 5 . This is below the critical current density, corresponding to c$ H < c0,a , and no further electrodeposition occurs . Pierce discussed this situation as a balance of elec trodeposition and dissolution[24] . However, according to our view, dissolution is negligible in a polarized film .

This effect is well known in colloid chemistry . Under these conditions, nucleation and phase formation become the rate determining steps .

e

IMe)

0 (1)

Fig . 3 . Concentration profile for OH - ions and the establishment of a negative space charge region in front of the electrode in case of cathodic electrodeposition .

(2)

131

141

Fig . 4. Schematic representation of concentration profile of H ` and counter potential profile q, in the anionic film on an anodically polarized electrode . The zones (1)+(2) are negatively charged, and (3) + (4) bear a positive space charge. For transport mechanisms cf. text .



842

F. BECK 30 IIA (2) 2D 11) 1.0 tlmin

11 0

2

Fig . 5 . Current-time curve for the anodic EDP of acrylate resin on iron at constant voltage (l .1 = 150 V, A . Three different carbon black concentrations were applied in the bath[ -(w/w) with respect to the = 70 cm') solid] : (1) 0 or t4% (w/w) Corax L; (2) 23Y (w/w) Corax L (eff section 5).

The space charge behaviour is very important in connection with practical features such as throwing power and energy dissipation . This has been discussed elsewhere[19, 231-

3 . FILM FORMING ANODIC ELECTROPOLYMERIZATION OF (HETERO) AROMATICS If one starts with monomers, two different routes are possible to come to an adhering polymer film on electrochemical polymerization . The one is to oxidize aromatic or heteroaromatic molecules at the anode to yield polymers with conjugated double bonds . The propagation step is an electrochemical one . Further oxidation of the polymer backbone to the polymer radical cation, with the parallel insertion of anions to compensate the charge (doping) leads to conducting polymers . I n many cases, resistivity is rather low, in the order of 1-10 - ' D cm, thus approaching that of metals (synthetic metals). In section 4, another possibility will be discussed briefly, namely the electrochemical generation of radicals or radical ions at the electrode, which initiate the polymerization of reactive monomers to be present in solution . In most of the cases, this is a vinylmonomer . The propagation step is a chemical one. ]'he polymeric product has a saturated backbone, it is relatively stable towards further (over) oxidation, quite in contrast to the case of conducting polymers . There has been an increasing level of interest in these polymers over the last 10 years . Table 2 compiles some

data for the most important conducting polymers today[ l l b . 25] . Graphite and the two conducting polymers polyacetylene (PA) and poly-p-phenylene (PPP) were introduced into Table 2 for comparison . P A is synthesized by chemical polymerization of acetylene in the presence of Ziegler-Natta catalysts, ef[25] . PPP is produced after the Kovacic method by oxidative polymerization of benzene in the presence of CuCI,/AIC1 3 [26] . Anodic depositions have been described as well[27, 28] . However, the last three exampies are typical for eleetrodeposition . It can he seen that the maximum degree of insertion y,,,„, corresponds to the formation of a radical cation at every second or third ring . Thus, the theoretical capacity is not very high, it is indeed below that of Pb0 s (K,, = 224 Ah kg - `). In the last column, the reversible switching potential for the reversible redox process is shown . A wide variation over nearly 2 V can be recognized . Polymers from derivatives of pyrrole etc ., eg N-phenylpyrrole or 3-methoxy-thiophene, lead to a further diversification[I I b] . This is important for the electrocatalytical behaviour of these electrodes . 3 .1 . Formation of a polymer laver upon electropol ymerization Polymers from the last three monomers in Table 2 are typically electrodeposited on inert anodes . The solvent--electrolyte systems are aqueous (acid) for PANI, aprotic for PThio and acetonitrile with I M water for PPv . depending on the reactivity of the

Table 2 . Characteristic data for intrinsically conducting polymers

Polymer Graphite Polyacetylcne Poly-p-phenytene Polypyrrole Polyaniline Polythiophene

Symbol C, unit C PA PPP PPy PANI PThio

C CH CH ., 7 CHo .7, N ., CHu .e7Nu .,1 CH,,., S, .,,

yn,o, for C, 0 .042 0 .1 -0.08 0.083 0.1 0.125

i for one monomer K unit (Ah - 0 .2 0.33 0 .6 0 .5

69 - 117 - 104 91 148 102

U 90 (V) r = I M, 25'C 1 .90 0.90

1 .80 0.10

0.40 1 .00



Flectrodeposition of polymer coatings primarily generated radical cation : -e XH 2 - (XH2)„d (XH2)g •

( 12)

XH2 symbolizes a monomer, H i means the two ahydrogen atoms . The reaction order in case of pyrrole is found to be nearly zero[29], indicating this primary step to proceed in the adsorbed state . It is followed by the dimerization of two radical cations[30, 31], assisted by the presence of counterions and water molecules, which lower the repulsive forces : 2(XH,) ,-(HX-XH),d+2H- .

(13)

The C-C bond is formed in the x, a'- position . Further oxidation of the dimers lead totetramers etc . However, the intermediates must be able to leave the electrode, for they can be rereduced at the ring in the rotating disc ring experiment at relatively negative potentials[29] . We believe that hydroxylation proceeds to some extent in the f7-position, thus increasing the solubility of the oligomers . This would explain the presence of substantial amounts of oxygen in nearly all polymers of that kind . Finally, the degree of polymerization is so high that solubility decreases again, and the polymer precipitates. For this final step, nucleation phenomena are important . Thickness growth of the film is regarded to proceed after a semisphere mechanism (with radial growth of chains)[32], while PPy seems to grow via chains in parallel to the substrate[33J .

84 3

Thus the overall mechanism of electrodeposition of layers of conducting polymers seems to be relatively complex, and the only parallelism to the galvanotechnical deposition of metal layers (electrocrystallization) may be the good electronic conductivity, allowing a nearly unlimited thickness growth . Figure 6 shows the voltammetric curve along the galvanostatic electrodeposition of polypyrrole films . An interesting prewave is found, increasing with the water concentration, cf.[34] . The limiting current density is at least one order of magnitude below diffusion control . It is not only water, which catalyses the process, but also the redox system 2 Br - /Br 2 in water or in acetonitrile[35] . In the latter solvent, a prewave at even more negative potentials is observed due to the formation of the inactive tribromide anion[35J : 3Br--+(By)-+2e- .

(14)

Substrates other than Pt, such as Au, glassy carbon, stainless steel, Cu, Ti and Fe were tested successfully for PPy[36] . Iron is of special interest As Table 3 shows, electrodeposition of PPy is even possible from aqueous electrolytes, but Fe remains in the passive state only when nitrate is the anion, cf.[36] . With many other electrolytes, iron dissolves anodically . Adherence of PPy layers is excellent for thin films (L c 1, µm), but it becomes poor for thicker layers . The texture and surface roughness of the coating depends on many parameters, among others current density,

Fig . 6. V oltammctric curves for the electrodeposition, reversible cycling and overoxidation of polypyrrole in McCN, cu,o = 0 .03 M, 0 .1 M NRu,BF„ Pt wire electrode without stirring, At, 20`C . (1) Electrodeposition, v, = 2 m V s - ' ; (2) first discharge, v, - 2 mV s - ' ; (3) 2 1/2 reversible cycles, v, = 20 m V s ' ; (4) irreversible overoxidation, q = 20 mV s ' ; (5) second cycle of (4). Step (I) in the presence of 0.1 M pyeole, all other parts without pyrrole; (a) system without any bromide, d . = 0.61 pin, (b) in the presence of 1 mM Br - , d„=0.35 pro.



84 4

F. BECK Table 3. Electrodepnsition of PPy from aqueous electrolytes at 2 mAcm -2 , 20'C, .1 0 M pyrrole, no stirring. V2A = stainless steel . Nominal thickness of PPy = 10 4m. Negative current efficiencies due to iron dissolution Fe - Fe" f 2e -

0 .1 M Electrolyte

Anode

Current efficiency (°.)

NaCIO,

Pt V2A Fe

90 90 -96

NaBF4

Pt V2A F'e

87 85 -95

NaHSO4

Pt V2A Fe

70 6 -103

KNO,

Pt V2A Fe

88 88 88

NaH,PO,

Pt V2A Fe

50 56 - 58

NaH 2 PO, + NaBF 4

Pt V2A Fe

82 78 - 95

solvent and electrolyte . Two systematic studies for aqueous electrolytes, including polymeric anions, were published recently[37. 38] . From PANT it is reported that it must he electrodeposited potentiostatically or potentiodynatnically rather than galvanostatically . Electrodeposition of thick PPy layers onto slowly rotating cylindrical electrodes was used to fabricate continuously PPy films[39].2 Doping and undoping 3 . Table 2 includes already the most important features of the doping ,/undoping process . The overall reaction

can be written as: [(X .)'A+Pe-~[X]y F PA n n

-,

(15)

X has the same meaning as in the previous equations (12) and (13), A - is an anion, p is the degree of polymerization . A section of n monomer units in the polymer backbone bears one radical cation, called polaron in the physical literature . Thus the degree of insertion for the anions to compensate this charge with regard to one ring is given by y=1/n .

(16)

The redox process represented by Equation (13) is reversible, which means that no large potential difTerence is between forward and reverse reaction . Figure 6 reveals that a preferential process concentrates to a narrow potential region, but that further processes are smeared over a wider potential range . This has been explained in terms of a superposition of individual redox processes in the solid . The importance of a slow first discharge in connection with the relaxation of polymer chains should be noted, ef.[40] . It is clear from Fig . 6 that PPy will be deposited in the doped state . The radical cation in the solid is rather stable and it was proposed to use these systems as positive active masses in rechargeable batteries . This point will be discussed more precisely in the final section . Cyclic behaviour is very satisfactory even in aqueous electrolytes . Figure 7 shows an example[41] . Transport of anions in the film may become rate determining for thicker layers . As the diffusion coefficient is in the order of 10 -10 cm 2 s - t[30, 42], the time constant L' 4D

(17)

for a I tan layer is only 25 s, but it increases distinctly for thicker coatings . The importance of the doping/undoping process for electrocoatings with PPy etc must be seen in connection with the chemical stability of the layer . It may be

Fig . 7. Galvanostatic cycling of a polypyrrole layer, nominal thickness d„ = 65 pot, on a Pt wire electrode, A = 1 cm 2, with Joe =ldiseh = 2 mA cm -2 , Q E = 1 .56 C cm -2 , p (degree of mass utilization) = 65 %. Numbers are cycle numbers . Electrolyte : 0 .1 M NaBF4 in 11 2 0 U. C, means potential vs the sodium chloride saturated calomel electrode .



Electrodeposition of polymer coatings reduced by the underlying metal or by a reductive environment . On the other hand, undoped PPy is relatively instable towards autoxidation . 3 .3 . Overaxidation The polymer film can undergo a further alteration by anodic overoxidation in the course of the electrodeposition . Figure 6 includes a typical voltammetric overoxidation curve[43, 44] . Coulometric and analytic evaluation of these results reveal the irreversible formation of substitution products at that ring, which carries originally the radical cation state . For PPy, the final product is a 4-hydroxy-pyrrolinon at that part of the chain, if overoxidation is performed in the presence of water . Strong nucleophiles as OH - ions attack directly the radical cationic site, under the formation of a 3hydroxy addition product[45] .

845

formation of a polymer with a degree of polymerization of p = 1000 would lead to m, x 1 g C - ', cf. Table 1 . This means, that the initiation process must be very ineffective, and the reactive intermediates must disappear by side reactions . Further informations are to be expected only after an extended analytical evaluation of the film as well as of the solution . Table 4 shows in addition, that the current density has nearly no influence, as it is the case for a 10% H,PO 4 electrolyte . m, increases with increasing H,,SO, concentration, but it remains nearly constant (again in the order of 10 pg C') for H 3 PO, electrolytes between 0 .5 and 10 o (w/w) . A strong increase of m e is observed at 60'C . Deposition in the presence of air leads to similar results, at least at higher current densities, cf Fig . 8 . This has been explained in terms of reduction of oxygen at the cathode with limiting current densities,

OH /H OH [181 ~N~N~ H H

These electrochemical or chemical alterations may be eventually used to modify the polymer film in a positive way. Adherence, for example, may be improved . 4 . FILMFORMING ELECTROCHEMICAL VINYLPOLYMERIZATION Electrochemically generated radicals or radical ions may initiate a vinylpolymerization, if vinylmonomers are present in the electrolyte . Under special conditions, a film-building electrodeposition is observed rather than the formation of polymers precipitated in the bulk . This electrocoating principle has been known since the early 1950s[2-5] . The electrochemical production of initiators for polymerization was reviewed by Silvestri et aL[46] . Only thin or porous films seem to be possible . The film itself is an insulator for electronic charge transport and further thickness growth is only possible via film diffusion of electroactive materials and/or electrolysis products through the film . CH, = CH-CONHDiacetone acrylamide C(CH,) iCH 2-CO-CH, is a monomer with high solubility in aqueous electrolytes . It forms polymer films of good adherence upon cathodic electropolymerization[47-49] . A typical set of potential-time curves for galvanostatic electrodeposition at iron in 0 .5 H,SO 4 in the presence of air is shown in Fig . 8[49]. After an induction period ab, potential increases by a few volts along be, where a film grows from the rims of the electrode towards the center . Further thickness growth proceeds along cd, accompanied with only a slight further increase in voltage . Table 4 demonstrates that the electrochemical equivalent m, is rather low, in the order of 10 pg C' The simple model of electrochemical generation of initiators by a one electron transfer reaction and the

thus leading to an oxygen concentration of zero at the interface, where the initiators are generated[49] . The SEM micrographs represented in Fig . 9 show an polymer film uneven surface, with larger uncoated areas in some cases . As expected, pores in the film allow limited thickness growth . For iron, the maximum values of about 0 .1 mg cm -2 correspond to an average of about 1 pm . Metals with a higher hydrogen overpotential like Al or Zn give thicker films up to a factor of 50[47, 48] . This contradicts the assumption, that adsorbed hydrogen atoms act as initiators. It should be mentioned, that the electrochemical equivalent m,, shown in Table 1 (1000 mg C' ), is a theoretical one . As stated above, the experimental values are lower by a factor of 10' . Thus, in reality, the EDP process (20 mg C - ') is much more beneficial . This holds for a comparison of deposition times, 10 2 vs 103 s, cf. Figs 5 and 8 . The only advantage seems to be a better adherence. Grcev et al-[50, 51] have reported the growth of thick (L z 10 ,um) polyacrylamide films by electropo6 5

1 .30 Acm 2 J-20mAa„i 2 c

d

4 7 I . IOmAcni 2 1 0

I 1 1 o 20

la

t Imin)

Fig . 8 . Potential-time curves for the film forming galvanostatic electrodeposition of poly[diacetoneacrylamide, 10 (w/w) monomers in 0 .5 % (w/w) H,SO4 , air saturated, iron ; U, means potential vs Hg/Hg2 SO,/I M H,SO4[49] .



846

Y . BECK Table 4. Galvanostatic electrodeposition of diacetone acrylamide (DAA) from aqueous H 2 SO 4 with 10 (w/w) DAA onto carbon steel substrate (mechanical pretreatment with wet SiO 2 powder), cj.[49] . t t corresponds to section ab, t L to bed in Fig. 8 . U o is the potential during t,, U e„ a is the potential at the end of r z . All experimental results in the last six columns are the average of 2-3 runs J (mAcm - ')

0,

T (2 C)

Electrolyte cone .

I . Current density

4 10 20 30

N, N, N2 N,

20 20 20 20

2. 11 2 S04

10

N,, N, N, N, N, N, N, N, N,

Parameler

concentration

3 . Temperature

10 10 10 10 l0 10 10

to

tr tL (min) (min)

Uo (V)

Uen4 (V)

0.5 0.5 0.5 0.5

100 13 10 4

20 15 10 6

1 .4 .5 1 1 .8 2.3

6 6 7 5

1 .7 3 .0 3 .5 1 .8

20 18 9 8

20 20 20 20 20

0 .1 0 .2 0 .5 1 .0 2 .0

2 .5 9 15 47

13 22 15 14 7

2.6 1 .9 1 .5 1 .2 1 .2

3 .6 3 .0 6 .0 8 .4 8 .8

1 .1 1 .4 3 .2 5 .6 5 .7

6 5 .5 17 33 68

10 20 40 60

0 .5 0 .5 0 .5 0 .5

27 28 14 28

20 I8 14 28

1 .7 1 .4 1 .4 .2 t

3 .7 3 .5 3 .0 2 .6

1 .7 1 .3 0.9 28 .0

7 .0 6 .0 5 .0 75

20

rn (mg)

n4 (1+gC -r )

Fig. 9. SEM micrograph for a polydiaeetoneacrylamide layer, 0 .61 mg cm - ', electrodeposited on iron from 20 (w/wt 5 % H J PO 4 , at 20 mA cm -2 . t, = 7 min, t z = 15 min, N„ (b) shows a portion with disordered film[49],

lymerization in the presence of zinc electrolytes . Extended swelling of the films allows relatively fast mass transport through the electrodeposited layer .

5 . EDP IN CARBON BLACK FILLED SYSTEMS Conventional EDt' leads to a wet film with a low ionic conductivity. After stoving, the polymer layer is practically an insulator, and the electrodeposition of a second layer is not possible . However, electrodeposition in carbon black filled systems leads, after stoving, to a filet ofgoud electronic conductivity. Figure 10 shows clearly that the resistivity-carbon black concentration curve deviates distinctly from the percolation curve of standard plastic material filled with carbon hlack[52, 53] . At relatively low critical carbon black concentrations c KR , the resistivity 5 decreases from 10" to 10'-10 7 0 cm, and it is only at high e 5 , when d decreases again to the usual level of about I 0 cm . At that point, the wet film itself attains a good electronic conductivity, and electrodeposition at constant voltage goes on at high current densities, cf. Fig . 5 . On the other hand, lower carbon black concentrations practically do not influence the conductivity behaviour of the wet film, and

Fig. 10. Semilogarithmic plot of resistivity p of staved EDP films in the system acrylate resin-carbon black Corax L vs carbon black concentration c a : •- -- first layer; 1-A second layer on top of the stoved first layer ; conventional polyvinylchloride-carbon black system ; c Kr , critical percolation concentration ; a pp , concentration for first partial deposition ; c HL , concentration for a homogeneous layer; c,., concentration for disturbed wet film . After [52] .

throwing power does not break down . The percolation curve of Fig . 10 was explained in terms of a formation of transversal chains of polymer wetted carbon black particles . Transport of electrons proceeds along these routes, and is limited by tunneling processes through very thin polymer films between the carbon black particles[54] . A consequence of this model is the establishment of point Contacts for low c o at the surface ofthe first layer . No perfect second electrodeposition of a second layer can he realized due to the fact that the precipitating ions are transported very rapidly due to spherical diffusion . A further increase of c a is needed, and at C P1 a partial coating, but only at c at a homogeneous second layer is observed, when the point contacts merge to patches and finally to an overall layer with planar diffusion[55],



Electrodeposition of polymer coatings

847

These results are believed to provide the basis for a further rationalization of the industrial paint process, for it allows the electrodeposition of other layers than the primary one.

rotation speed n 11m vs the square of the current densities leads to the usual straight lines, which coincide for both cases . Electrodeposition at n, im leads to the PPy film alone.

6. COMPOSITION OF POLYPYRROLE AND POLYACRYLATES

7 . OTHER ELECTRODEPOSITIONS

If polyacrylates are anodically electrodeposited from the usual aqueous bath in the presence of pyrrole, a codeposition of both PPy (through anodic oxidation of the monomer) and polyacrylates (ria electrocoagulation, initiated by the protons from the electrooxidation) was observed[56] . The overall equation for the formation of PPy can be written as : nXH t -. HX-(X)„ r -XH

Cathodic electropolymerization of acrylonitrile with a low water concentration and a quaternary ammonium salt as supporting electrolyte leads to yellow polymer films in unstirred solutions[57, 58] . This was interpreted in terms of an anionic polymerization with the formation of naphthiridine units after prototropic rearrangement steps[57] . Phenol and its derivatives undergo easy anodic polymerization in basic[59-6l] or acid[62-63] aqueous electrolytes. Current efficiencies are quantitative (2 F/mole) for the former conditions, but they are poor in acid solutions . The mechanism is a radical one:

+(2n-2)H'+(2n-2)e'

(19)

A black PPy layer is initially observed, and later on, the polyacrylate deposits. Figure t I shows two deposition

OH -H +

Polymer

curves at the rotating disc electrode in the neighbourhood of the critical rotation speed . The one was measured in the pure polyacrylale system, and a normal potential time curve is the response . In the presence of pyrrole, a similar result was obtained and the only difference is the formation of a black PPy film in the course of the induction period . It should be pointed out that this PPy has the polyacrylate as the counter ion in the solid state . A plot of the limiting

Fig. 11 . Galvanostatic codeposition of polyacrylate (10% Luhydran E 33) with polypyrrole (0 . 1 M , pyrrole) at a rotating Pt disc electrode (0 .5 emr) at 2 mA cm in the vicinity of the critical rotation speed . The dashed curve represents the experiment in the absence of pyrrole[56] .

Layers of polyphenylene oxide an iron have a good corrosion protection ability. However, deposition time and current densities are too high at the present state to compete with EDP . Cathodes dipping in molten caprolactam with sodium benzoate were covered with well adhering nylon-6-films under the influence of an anionic mechanism[64] . The electrochemical equivalent was asr high as 400 g Ah - ' . Thick layers up to 40 mg cm could be attained . The cationic electropolymerization of cyclic ethers etc like tetrahydrofuran (without solvent, LiCIOa[65, 66]) or ethylene sulfide (in McCN+NBu 4 ClO 4 [67]) leads to well adhering polymer films in conjunction with dissolved polymer materials . The electropolymerization of bifunctional monomers with two activated olefinic C--C double bonds like butadiol diacrylate leads through electrochemical propagation steps (a hydrogenating coupling) to polymers[68] . In the example mentioned above, one obtains a polyester (identical to the polyester from butadiol and adipic acid!). Other examples with electrochemical propagation steps at the cathode (eg dehalogenation of a C-Hal bond) or at the anode (eg Kolbe condensation of dicarboxylic acids) are collected in[69] . These reactions are in analogy to the anodic electropolymerization of aromatic and of heteroaromatic compounds .



848

F . BECK 8- CONCLUSIONS AND PRACTICAL APPLICATIONS

An overview on the development of electrochemical coating processes is given in this paper . In preparative organic synthesis, polymers were regarded as unwanted side products for a long time, and it was only since the 1920s that chemical polymerization attained a continuously increasing interest . The same could be observed in organic electrosynthesis, where polymeric side products, especially if they are coating the electrode, are absolutely undesired . However, since the 1950s, in this field, too, polymers as products in solution or even as an electrode layer became increasingly interesting[69, 70], It should be pointed out that even after an optimization of the electrocoating process, the competition of the film-forming electrodeposition and the formation of a dispersion or solution of polymeric products in the bulk of the solution remains important . Up until now, nearly nothing was known with respect to the balance of the competing routes. The polymer layer itself behaves like a membrane, and analogous features (charge transport, mass transport, membrane potentials) must be considered as those for free-standing membranes separating two electrolytic solutions . A very important difference, however, is the inherent possibility of electrode processes at the inner phase boundary, cf. Fig. 1 . The mechanism of film formation is rather complex in most cases, and it is not yet fully understood . Nucleation and solid phase formation may become a limiting factor . It was shown that in case of anodic deposition of poly(hetero)aromatics, the solid film is obtained in the doped form . Cathodic undoping is possible . In contrast to the formulation according to equation (15), the coions seem to play an important role, and an alternative process must be discussed according to : [(X.) ~A

]po,+ne

+?K*

t[X ;"K' "A

]. (21)

Coions were detected in conducting polymers such as polypyrrole by electrochemical[71], radio tracer[72] and combined XPS/AAS[73] methods, in poly-3-

methyl thiophene by SIMS[74] and in PANT by XPS[75] . The most important application of electrodeposited polymer film is surely for corrosion protection . Twenty years ago electrodeposition of paint was introduced on a large industrial scale. Good adherence of the polymer layer is very important . Inherent competitors are all other methods, but presently at least one of the practical features, such as electrochemical equivalent, deposition time, current density and the need for aqueous systems do not meet the conditions for an industrial application . The deposition time of 2 s for electropolymerization, mentioned in Table 1, is a theoretical one ; in reality, it is about 10 5 seconds due to poor current efficiency . Besides corrosion protection, other tasks of surface techniques that may be realized by electrodeposited layers include decoration, electrical insulation and improvement of mechanical properties . Electronic conductivity of the primary layer is a precondition to provide a second, third, etc . layer by electrochemical techniques. Electrodeposition in carbon black tilled systems is one way to solve this problem . Electrodeposition of poly(hetero)aromatic compounds leads to a concurrent doping of the material, and an intrinsic electronic conductivity is achieved . Thin layers may serve as interlayers for composites, to modify catalytically the electrode surface or to fabricate electrochromic displays . Thick electrodeposits may be continuously removed from the electrode surface, to provide the material as freestanding films . The reversible doping-undoping process of such electropolymerized materials together with the chemically synthesized polymers as polyacetylene or poly-p-phenylene, may he applied as positive active masses in rechargeable batteries . However, at present, an only limited application can be seen . As Fig . 12 clearly shows, a comparison of the Li PPy battery[76, 77] with the conventional lead-acid battery, performed by Passiniemi and Osterholm[78], leads to very similar energy densities . This is also seen from the data compiled in Table 2 . The long-term behaviour is not yet satisfying, and ovcroxidation remains a problem[78] . We conclude from this that the major role of polymer layers in the future will be the direct coating of substrate materials . 18

400-

Wh/Kg

- Wh/dm 3

335

210 200167 110 80

90

V A

Li/PPy

Pb/Pb02

45 I LiIPPy

Pb/Pb0 2

Fig . 12. Energy densities (with reference to mass and volume, respectively) for a Li-PPy and a lead acid battery . The shaded areas describe the practicaly available values . After[78] .



Electrodeposilion of polymer coatings Acknowledgements-Financial support of this work by AIF (Arbeitsgemeinschaft Industrieller Forschungsvereinigungen), MWF (Ministerium fur Wissensehaft and Forschung des Landes Nordrhein-Westfalen, Landesinitiative Zukunftstechnologien) and BASF Aktiengesellschaft as well as good cooperation within the AGEF is gratefully acknowledged .

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