Langmuir 1992,8, 1857-1860
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Contact Charging Characteristics of Styrene-Butyl Methacrylate Containing Dodecametallophosphate Salts? A. R. Gutierrez, D. Fenzel-Alexander, R. Jagannathan, and A. F. Dim' IBM Almaden Research Center, K931801, 650 Harry Road, San Jose, California 95120 Received December 6, 1991. In Final Form: March 24, 1992 Dodecatungstophosphateand dodecamolybdophmphatetetraalkylammonium salts, [R4N13[P04(M03)121 where M is W or Mo, are reported to function as charge control additives in toner powders used for electrophotographic printing. With these salts the toner develops a positive contact charge (against iron powder) and the charge is reported to have improved uniformity and stability. We now find that the dodecatungstophosphate and dodecamolybdophosphate salts with a stearylammonium cation induce a negative contact charge to the styrene-co-butylmethacrylate copolymer powder when it is charge activated with metal beads, and the beads acquire the positive countercharge. Thus, a simple change in the structure in the cation of the salt reverses the sign of the charge. X-ray photoelectron spectroscopy (XPS) analysis of these powders reveals that the stearylammonium cation content on the surface is 60 times greater than the bulk content. XPS analysis of the metal beads after contact with and removal of each of the powders shows N and W or Mo signals for the presence of both ions from the salt which were originally added to the powder. Also,the cation content on the beads is in excess of the anion content and this ion imbalance must contribute to the resulting positive charge on the beads.
Introduction Dodecatungstophosphate and dodecamolybdophosphate tetraalkylammonium salts, [R4Nl3[P04(M03)121 where M is W or Mo, are described as charge control additives for toners for electrophotographic printing in two patents by Suzuki and Okamura.lS2 In the printing process the charge on the surface of the toner particles is induced by multiple contacts with a second surface (carrier beads or roller) and the predictable control of the resulting charge is important for proper performance of the toner in the printer. Salts with widely different long chain alkyl and benzyl groups on the ammonium cation and including NH4+ were found to induce a positive contact charge on the toner (against iron powder).lS2 With the addition of 1 % of the salts (ca. 4 pmol/g) to a styrene-co-butyl methacrylate resin (S-BMA)also containing 5% carbon, the contact charge is raised to 20-30 pC/g above the charge for the resin alone (ca. zero). These salts are described as providing improved charge uniformity and stability to the toner powder, and the use of these toners in a commercial photocopying machine produced copies with good image quality up to 10 OOO copies. To improve our understanding of the charging process with these materials, we measured the surface compositions and contact charge of S-BMA powders containing these salts.
Experimental Section The stearylammoniumsaltsof dodecatungstophosphate(MW 3698), [CleH3,NH~+13[P04(W03)1~3-l, and dodecamolybdophoswere O ~prepared );Z~], phate (MW 2642), [ C ~ ~ H ~ ~ N H ~ + ] ~ [ P O I ( M O
by neutralizing the corresponding acids (FlukaChemicals,Inc.) in refluxing THF. The reaction is shown in eq 1 for the molybdenum derivative. The salts were purified by recrystallization. Solid blends of the salts with the resin were prepared using the previously described pro~edure.~~~ The salts (0.25t Presented as a poster at the SPSE 5th International Congress on Advances in Non-Impact Printing Technologies,San Diego, CA, November 12, 1989. (1)Suzuki, N.;Okamura, K.; Sugiyama, G.; Suzuka, S. US Patent
4,683,188,July 28, 1987,Hodagaya Chemical Co. (2)Suzuki, N.; Okamura, K.; Sugiyama, G.; Suzuka, S. US Patent 4,780,553,October 25, 1988,Hodagaya Chemical Co. (3) Dim, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. J. Polym.Sci., Polym. Lett. 1990,28,75.
1.0% by wt) were melt mixed with carbon black pigment (8%) and a 65/35 styrene-co-butyl methacrylate random copolymer (91-92%), at 160-180 "C. The mixture was then milled and size classified to yield irregularly shaped particles typically 8-10 pm in diameter. The particlesizes were determinedusing the Coulter Multianalyzer and using 100 OOO particle counts per analysis.
H,PO~(MOO~)~, + 3C,&NHZ
+
(Ci,Hs,NH,),PO,(MoO,);~ (1)
The charge was measured using the total blow off method after gently rolling the powder with metal beads.314 The beads used were 140pmspherical iron, 2Wpm irregularlyshaped beads, and 200 pm irregularly shaped beads coated with a f i i of epoxy. The powder (2.5% by wt) was mixed with the beads and the mixture was gently rolled in a metal can for 30 min which is sufficient time for the charge to reach a constant value. The ambient temperature was 69-70 OF and the relative humidity varied from 49 to 53 % . In triplicate runs, 3.0 f 0.2 g of the mixture were carefully weighed into a Faraday cage having 44 pm screens at each end. The charge on the cage was grounded then the powder was blown away from the beads (>97%) with forced air at 56 psi during 2 min. The net charge in the cage was recorded on a Keithley 616 electrometer and was typically of the order of 4 pC. The Q/Mvalues were calculated from the charge and the weight loss of the cage. The values are averages of several measurements and have a 6 8 % error. The X-ray photoelectron spectroscopy (XPS) analyses were performed on toner particles supported on indium foil using an HP ESCA Model 5950 spectrometer. Low-energyelectronswere supplied to the sample,as required, to neutralize surfacecharging. The surface atomic concentrationswere estimated from the curve fit areas, corrected for the relative photoionization cross sections.b The binding energies were referenced to the main carbon peak taken as 284.6 eV. The binding energies (cross sections) for the signals are as follows: C, 284.6 eV (1.00);0,532.2 eV (2.494); N, 401.7 eV (1.678); S, 167.4 eV (1.793); W, 246.0 eV, (9.87); Mol 270.0 eV (9.82); P, 189 eV (1.18). A Hitachi S-800scanning electron microscope was used to observe the surface of the films. The samples were prepared for analysis by the deposition of a 200-A layer of gold on the surface. The samples were mounted at 45O to the incident beam. The thermogravimetric measure(4) (a) Dim, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. 5th International Congress on Non-Impact Printing Technologies, San Diego, CA, 1989. (b) Dim, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. 63rd Colloid and Surface Science Symposium, ACS, Seattle, WA, 1989. ( 5 ) Scofield, J. H. J.Electron Spectrosc. 1976,8, 129.
0743-7463/92/2408-1857$03.00/0 0 1992 American Chemical Society
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I I
Diameter (um)
Temperature, Degree C Figure 1. TGA of styrene-butyl methacrylate containing 1.35 pmol/g dodecatungstophosphate and 8% (by wt) carbon.
Figure 2. SEM of milled particle of the blend containing styrenebutyl methacrylate, 0.68 pmol/g dodecatungstophosphate and 8% (by wt) carbon. ments were carried out under a nitrogen atmosphere using a 5 deg/min heating rate.
Results and Discussion The powders used in this study were prepared by melt mixing at 160-180 "C for approximately20 min. The resin and the salts in the blend should survive the mixing conditions since they are stable to heating up to 300-400 "C as seenin Figure 1with the thermal gravimetric analysis (TGA) for the blend containing [Cl8H37NH3+]3[P04(W03)123-]. The 8%residue remaining after 500 "C is consistent with the carbon black content in the sample. Although not shown, the TGA for the blend containing [C~~H~~NH~+]~[PO ~ ( is Mvirtually O O ~ ) superimpos123-] able. This result is consistent with previous reports indicating that the dodecametallophosphate salts are stable up to 500 oC.6 The milled particles are irregularly shaped and have 8-10 pm average diameters as can be seen in the scanningelectron micrographs shown in Figure 2 for the blend containing [C1&I37NH3+]3[P04(W03)&]. All the powders had similar particle size distributions resembling the distribution shown in Figure 3. Since these materials are highly insulating,only the ions in the surfaceregion of the particles are expected to directly (6) Yong, W.J.; et al. Thermochim. Acta 1987,111,325.
Figure 3. Particle size distribution for the polymeric powder containing 1.35 pmol/g dodecatungstophosphate and 8% (by wt) carbon.
affect the charge. X P S was used to analyze the surface compositions of the particles. The ion content was determined from the N 1s signal for the stearylammonium cation and the Mo 3d or W 4d signals for the corresponding dodecametallophosphate anion. The signal for P in the anion was not detected. The values found for C and 0 agree with the values calculated considering only the salt and resin and disregarding the carbon pigment. This agreement implies that the carbon pigment is not exposed on the surface of the particles. The salt content on the surface is different for each salt blend. The P04(W03)123-anion content on the surface is close to the amount in the formulation. However, the stearylammonium content is ca. 60 times in excess of the salt stoichiometry. The P04(M003)12~anion content on the surface is ca. 2 times greater than the bulk, and the stearylammonium content is again ca. 60 times the salt stoichiometry. With these metallophosphate salts it is only the stearylammoniumcation (or stearylamine)which is heavily concentrated on the particle surface and must be a consequence of its surface-active properties. The details of this ion separation process were not pursued. The salt may separate into the two ions, stearylammonium cation and metallophosphate anion (which retains 2, 1, or no stearylammoniums). If moisture is involved to stabilize the separated charges, the surface may contain stearylammonium hydroxide. Alternatively,stearylammonium may transfer a proton back to the metallophosphate anion and separate as a neutral stearylamine (the reverse of eq 1). In this case the surface would contain stearylamine. The enhanced surfacecontent of the ions has been observed ~ ~ ~ containing Et4N with other molecular ~ a l t s .Powders toluenesulfonate and 4-ethyl-N-methylpyridiniumtoluenesulfonate,show a surfacesalt (both ions) content which is 4-6 times greater than the amountsin the f ~ r m u l a t i o n . ~ ~ ~ The salts do influence the contact charge as seen by the Q/M values listed in Table 11. The powder develops a negative charge when activated against each of the three beads; however, only with the epoxy-coated beads does the charge scalewith the saltcontent. With the bare beads there is very little variation in the charge with the [C1~H37NH3+]3[P04(W03)123-] content, and with [CISH37NH3+]3[P04(Mo03)123-] the charge actually decreases to near zero at the highest salt content used. The negative charge is a complete contrast with the positive charge observed with the salts containing the BQN+ cation (no labile protons). This comparison is made directly with the P04(W03)12* salts in Table 11. The value in the table for the B a N + salt is from a report by Suzuki;2we have, however, repeated the measurement with this salt in our S-BMA/carbon composition and confirm the positive
Charge Control Additives in Toner Powders
Langmuir, Vol. 8, No. 7, 1992 1859
Table I. Powder Surface Compositions from XPS, Atom % 2.70 fimol/g [C~~H~~NHS+]S[POI(WOS)~~'I in Powder C 0 N W P anal. 91.85 7.40 0.71 0.05 ndo calcdb 93.26 6.68 0.011 0.046 0.004 3.80 pmoVg [C~~~~NH~+Is[PO~(Mo0~)~~9-1 in Powder C 0 N Mo P anal. 91.47 7.24 1.03 0.17 nd calcdb 93.11 6.78 0.016 0.066 0.005 33.2 pmollg E t N OTs in Powder C 0 N 93.0 6.9 0.24 93.23 6.77 0.044
anal.
calcdb
S 0.28 0.44
Styrene-co-Butyl Methacrylate Copolymer C 0 N 93.18 6.82 0.0 93.43 6.57 0.0
S
0.0 calcdb 0.0 None detected, below detection limit. Calculated from bulk anal.
composition.
Table 11. Charging Results with Three Different Beads [salt], pmollg
0.68 1.35 2.70 0.95 1.90 3.80 3.21
powder charge, pC1g (bead wed) spherical irregular coated [ C ~ ~ H ~ ~ N H ~ + I ~ [ PinO Powder I(WO~)~~~] -12.5 -10.3 0.3 -12.5 -10.9 -8.7 -13.9 -9.1 -10.4 [C~~H~~NH~+]~[POI(MOOS)~~~-I in Powder -9.04 -8.7 2.8 -5.07 -8.0 -9.4 +1.01 -3.2 -11.8 [NB~+I~[POI(WO~)~Z"I* +24
Table 111. Charging and XPS Results for Bare Irregular Beads QIM,WCIg atom % (XPS) Before Contact with Powder C 28.80 0 62.92 N 0.11 W, Mo nda P Fe, Mn 8.17 2.70 pmollg [ C ~ ~ H ~ ~ N H ~ + I ~ [ P O I in ( WPowder O~)~~~-I C 52.65 0 41.04 0.23 (-9.1)b N 0.24 W 0.02 P nd 5.80 Fe, Mn 3.80 pmoUg [Cl~37NHs+13[PO4(Mo03)1~9-] in Powder C 47.00 0.08 (-3.2) 0 37.85 N 1.20 Mo 0.44 P nd 13.5 Fe, Mn None detected. Corresponding QIM value for powder.
charge on the p ~ w d e r .The ~ negative charge parallels the results with other inorganic salts, e.g., T-37, the 2:l chromium(II1) complex azo dye available from Hodagaya Chemical, Inc.8 The cation in T-37 is NH4+. In S-BMA ~
(7) Jagannathan, R.;Seymour, M. Unpublished results.
(8) Niimura, I.; Motohaehi, K.; Yamaga, H.; Akuzawa, N. US Patent 4,624,907, November, 25,1986, Hodagaya Chemical Co.
containing 8%carbon pigment, the presence of 1%(w/w) T-37, ca. 15 pmollg, charges the toner to -20 pC/g,9 and the analog salt with the stearylammonium cation also charges the powder to -28 pC/g.l0 The charge distribution in the powder/bead mixture was approximated by removing the powder stepwise from the mixture using a series of increasing air pressures. In this procedure, the air pressure applied to the mixture was increased incrementally from 2 to 62 psi and AQIhM was measured after each increment. The results for the powder containing 2.7 pmol/g [ C I E H ~ ~ N H ~ + I ~ [ P O ~ (WO3)1&] are in line with the results from the total blowoff measurements (charged against coated irregular beads). The incremental charges are negative and most of the particles have a AQIAM in the range -4 to -8 pC/g. A few particles had -11 pCIg (ca. 5%) and others -16 pC/g (ca. 3%1. The powder containing 3.8 pmol/g [Cl&I37NH3+]3[P04(M003)12~-1showed positive and negative charged particles. The beads recovered from the powder/bead mixture after rolling and blow-off of the powder were also analyzed by XPS. Two powders were used; one contained 2.7 pmoll and the other 3.8 pmol/g g [CI~H~~NH~+]~EPO~(WO~)I~~-I [CUH~~NH~+I~[PO~(M TheObeads ~ ~ ) ~which ~ ~ I .were not contacted with powder unfortunately had a small N signal which interfered somewhat with the analysis. The beads contacted with the powder containing [C1&37HN3+13[P04(W03)12*] showed signals for N (0.24) and W (0.02). Even after adjusting the size of the N signal by simply subtracting the signal observed on the unused beads, the N signal is still ca. 5 times greater than the W signal. Thus, the preferential transfer of the stearylammonium cation to the beads must contribute to the charge. The 0.23 pCIg charge on the beads corresponds to a surface charge (ion) density of 1.5 X 1O1O charges (ions)/cm2 (assuming a spherical geometry) and to a 0.02 % surface coverage since a monolayer of ions with a ca. 100 A2 footprint has 1014ions/cm2. This surface concentration is an order of magnitude less than the amount of N found on the beads and indicates that there is excessive ion transfer during the contacts. Finally, we cannot dismiss the occurrence of some concurrent contamination of the beads by fragments of the blend which shear off the particles during the rolling process. Such contamination of the beads is consistent with the higher C and lower 0 signal. The beads contacted with the powder ' containing [C~~H~~NH~+l~[P0~(Mo0~)~~3-l showed larger N and Mo signals, which is reasonable considering that the salt is accumulated on the surface of these powders. Again more cation than anion was found on the beads. Thus, with both salts the preferred transfer of the cation to the bead surface combined with the correspondence in the sign of the cation and the charge strongly supports the importance of ion transfer in charging. The retention of the anion then gives the powder the negative charge. The higher content of both ions on the beads may be the cause of the low charge.3~'~ These results parallel those with the styrene-methylvinylpyridinium toluenesulfonate ionomers where the beads charge negative and the mobile toluenesulfonate is clearly seen on the beads? In the ionomer, the methylpyridinium cation is "anchored" to the copolymer and does not transfer. Likewise, in the contact charging experiments with cetylpyridinium bromide, extensive transfer of both ions was found." However, the (9) Gutierrez. A.: Dim. A.: Baird. B. Lammuir 1991. 7. 1923. (10)Gutierrez, A.; Dik, A. Unpublished;esults. (11)Mizes, H. A,; Conwell, E. M.; Salamida, D. P. Appl. Phys. Lett. 1990,56, 1597. I
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1860 Langmuir, Vol. 8, No. 7, 1992
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Figure 4. SEM of bare irregular beads imaged with low-energy secondary electrons (a) and high-energy backscattered electrons (b), after contact and removal of the powder.
more mobile bromide ion transferred in excess of the cation and determined the negative charge on the second surface. Although >97% of the weight of the powder is removed from the beads during blow-off, some of the fines are retained in the crevices of the beads and this is shown in Figure 4. The figure shows scanning electron micrographs (SEMs) of the identical bead site measured by using the high-energy backscattered electrons and low-energy secondary electrons. This technique distinguishes between the high-density metal-bead and the low density polymer particles. The low-energy micrograph shows many small particles trapped in a bead crevice. The high-energy micrograph of the same site showsthat these small particles are not part of the bead but are powder fines which are invisible to the high-energy backscattered electrons. A key question is how the salt affects the contact charge on the resin particle. Does the powder acquire a negative charge because it transfers a positive ion from the salt to the beads during contact or does it accept an electron? Electron transfer could be possible since these materials have relatively low reduction potentials (close to the calomel electrodepotential). For example, potassium dodecatungstosilicate and octadecatungstophosphate are reduced reversibly in water (E" at 0.1 to 0.4 V vs SCE) and in DMF (E" at -0.6 to -0.8 V vs SCE).12 On the other hand, they are quite stable to oxidation. These salts are good electron acceptors and bad donors; therefore, if electron transfer is the basis of the charge, these salts are expected to acquire a negative charge. Electron transfer does not explain the change in the sign of the charge with the structure of accompanying cation, nor is electron transfer likely when the beads are coated with an inert epoxy resin. Alternatively, charging may result from the transfer of ions from the salt. However the amounts of ions from the salt observed on the beads are minimal and could be largely the result of general material transfer and not of a selective ion transfer process. Proton transfer13J4to the beads is also a reasonable mechanism for chargingespeciallywith free acids. Organic sulfonic acids15are known to induce a negative charge to (12) Keita, B.; et al. J. Electrochem. SOC.1988,135, 87. (13) Birkett, K. L.;Gregory, P. Dyes Pigm. 1986, 7,341. (14) Macholdt, H.-T.; Sieber, A. Dyes Pigm. 1988,9, 119. (15) Gruber, R. J. SID 87 Digeat, p 272.
the resin. For example, 58 pmol/g toluenesulfonic acid produced a charge of -7 pC/g and 30.7 pmol/g dodecylbenzenesulfonic acid produced a charge of -11 pC/g when added to the same resin-carbon pigment mixture.1° The protic acids of the metallophosphates have not been used in charging studies. Crystals of the protic acids, H3P04(WO&, and H3P04(Mo03)12 have high proton conductivities.16 This suggests high proton mobility and a good likelihood of proton transfer from the protic acid to the second surface leaving behind a negative charge. This is an important consideration should trace amounts of the protic acid exist in equilibrium with the salt being used. Another source of protons is the dissociation of surface moisture, where in the contact, protons are preferentially transferred to the oxide surface of the bare beads or the oxygen atoms of the epoxy resin on the coated beads. Proton transfer involving surface moisture has been invoked to explain contact charging.17-19
Conclusions Polymer powders containing [Cl~H37NH3+]3 [Pod(WO3)12~-1[C18H37NH3+13[P04(Mo03)123-charge l negative against both bare metal and epoxy-coated beads. Surface analysis of the powders indicates that there is an accumulation of the stearylammonium cation on the surface. Surface analysis of the beads after contact and removal from the powder indicates that there is material transfer. With both salt/polymer blends the stearylammonium cation transfers in excess of the anion and this ion imbalance must contribute to the resulting contact charge. However, the importance of surface moisture and proton transfer in the charging process is always a consideration.
Acknowledgment. We thank Jose Vazquez and Dolores Miller for their assistance with several aspects of this study. (16) (a) Nakamura, 0.;Ogino, I.; Kodama, T.Advances in Hydrogen Energy; Pergamon: New York, 1981; Vol. 2. (b) Nakamura, 0.; Ogino, I.; Kodama, T. Hydrogen Energy Frog. 1 , 119. (17) Folan, L. M.; Arnold, S.; O'Keeffe, T.R.; Spock, D. E.; Schein, L. B.; Dim, A. F. J . Electrostatics 1990,25,155. (18) Mataui, N.; Oka, K.; Inaba, U. The 6th International Congress on Advances in Non-Impact Printing Technologies, 1990. (19) Wollmann, D.; Dim, A. SPIE/SPSE Conference on Electronic Imaging Science and Technology, San Jose, CA, 1991.
