AmineQuinone Polymers as Binders for Metal-Particle-Tape Formulation

Chapter 31

Amine—Quinone Polymers as Binders for Metal-Particle-Tape Formulation

Downloaded by UNIV OF LIVERPOOL on November 27, 2015 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch031

Effect of Polymer Structure on Dispersion Quality and Corrosion Protection Antony P. Chacko, Russell I. Webb, and David E. Nikles Department of Chemistry and Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487-0336

The effect of polymer structure for amine-quinone polyurethanes on the metal particle dispersion and corrosion protection in metal particle tape was studied. The polyurethanes were block copolymers containing the monomers: 2,5-bis(N-2-hydroxyethyl-N-methylamino)-1,4-benzoquinone (AQM-1), polytetrahydrofuran diol (M = 650), and 2,4-tolylene diisocyanate. Three amine-quinone polyurethanes containing 20, 30 or 40 weight percent A Q M - 1 were investigated. These polymers had a two-phase microstructure consisting of a discrete crystalline hard segment phase dispersed in a continous amorphous phase. Adsorption isotherms from THF solution showed that the polymer having 40% A Q M - 1 had the highest affinity for the iron particle surface. Dynamic rheological measurements on the coating formulations showed that storage modulus was lowest for the polymer containing 20% AQM-1. Magnetic remanence data were interpreted to show that the polymer containing 20 % AQM-1 had the best magnetic dispersion. The polymer containing 40% AQM-1 had the best corrosion protection. These observations were interpreted within a film formation model where the had segment partitioned onto the iron particle surface, providing a crystalline barrier against attack by oxygen or moisture. n

As the information storage industry continuously endeavors to increase data storage capacity, metal particle magnetic recording tape is the leading candidate for high density storage media. Metal particle (MP) tape consists of fine iron particles dispersed in a polymeric binder on a polymer substrate. The iron particles have the highest saturation magnetization and can be made with coercivities exceeding 2000 Oe, making them the best commercially available magnetic particles (7, 2). Iron is inherently susceptible to corrosion and this has led to concerns about the reliability of M P tape for data archiving (5). The iron particle corrosion problem has been largely solved by the use of ceramic coatings

0097-6156/96/0648-0525$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


526

FILM FORMATION IN WATERBORNE COATINGS

on the particle (4, 5). However, different iron pigments can give M P tape that display vastly different rates of corrosion (6). Recent data show vastly different rates of corrosion for commercial D-2 tapes from two different manufacturers (7). M P tape is made by a continuous web coating process where a magnetic ink, consisting of iron particles dispersed in a mixture of organic solvents, is applied to a polyester base film. The ink also contains binder polymers, lubricants, and other additives that facilitate tape handling. The signal output performance of the tape depends on the quality of the magnetic dispersion (8). Magnetic particles have strong tendency to form aggregates because of their strong magnetic attraction forces. High performance media requires consistent and uniform magnetic characteristics over the entire media surface which can be only achieved by a well dispersed suspension of the magnetic particles. The ideal dispersion consists of individual iron particles coated with a binder polymer. The binder polymer magnetically separates the iron particles to minimize magnetic interactions. Magnetic interactions between the particles lead to noise on the tape. However, to maximize the signal level the volume fraction of magnetic particles should be maximized. This leads to a tradeoff between increasing the volume fraction of pigment and minimizing the interparticle magnetic interactions. A typical tape formulation contains 30 volume percent pigment. Magnetic tape coating formulations are made by dispersing iron particles in a mixture of organic solvents. In the dispersion process the particles are subjected to high shear in a mill, which breaks up aggregates and allows a wetting binder to adsorb on the particle surface. The ideal magnetic ink would contain individual magnetic particles coated with the wetting polymer, suspended in the solvent. The use of improved dispersion techniques (high shear mixing and milling equipment) and new functionalized wetting polymers (thermoplastic polyurethanes or polyvinylchlorides) have enhanced dispersion quality (9). However, the high magnetization ( o = 125 to 145 emu/g) and high specific surface area (45 to 60 m /g) make these iron particles difficult to disperse. Our interest in the study of long term performance of metal particle tape has led to the exploration of means to enhance the stability of M P tape. Erhan et al synthesized quinone amine polymers which was reported to have high affinity for surface of iron and displaced moisture from the surface (10, 11). We have synthesized polyurethanes, containing a 2,4-diamino-1,4-benzoquinone functional group, which has been shown to inhibit corrosion of commercial iron pigments in M P tape (12-14). These polymers were also used to replace the ceramic coatings, used in commercial particles, giving iron particles with good corrosion resistance and better magnetic properties (15). These demonstrations inspired us to do a systematic study of structure-property-processing relations of these polymers on their application in M P tape coating formulations. The amine-quinone polyurethanes have a two phase microstructure, consisting of a crystalline hard segment dispersed in an amorphous, soft segment continuous phase (13). We speculate that the amine-quinone functional group strongly adsorbs to the particle surface where the hard segment crystllizes around the particles. This provides a barrier against attack by oxygen or moisture, thus preventing corrosion. The high affinity of the amine-quinone polymers for the iron particle surface suggests that these polymers may be useful for dispersing the particles. In this report we examine the use of amine-quinone polyurethanes to disperse iron particles and make a connection between the rheology of the magnetic inks and the quality of the magnetic dispersion in tapes made from the inks. s

2

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

31. CHACKO ET AL.

527

Amine-Quinone Polymers as Binders

Experimental


Materials. The amine quinone polyurethanes were synthesized by melt polymerization from 2,5-bis(N-2-hya"oxyethyl-N-methylamino)- 1,4-benzoquinone (AQM-1), Terathane 650 (polytetrahydrofuran), and tolylene diisocyanate. The hard segment content was increased by increasing the AQM-1 content from 20 weight percent for AQPU-13 to 30 weight percent for AQPU-16 to 40 weight percent for AQPU-15. 4-N,N-Dimemylarninopyridine was used as polymerization catalyst. The synthesis of amine quinone monomer was described elsewhere (13, 14).

Amine-Quinone Monomer (AQM-1) Two different commercial polyvinylchloride wetting binders, UCARMAG-536 (Union Carbide) and MR-110 (Nippon Xenon), and a commercial polyurethane, CA-271 (Morton International), were included for comparison. The magnetic pigments were a commercial ceramic coated iron having a coercivity of 1500 Oe, saturation magnetization of 122 emu/g and a specific surface area of 49 m /g. The average particle diameter was 200 nm. The particles were shown by electron microprobe to have the approximate composition; Fe 60 mole percent, A l 2.5 mole percent, 0.3 mole percent Si, Ο 38 mole percent. This was consistent with iron particles protected from oxidation with an aluminosilcate ceramic coating. 2

Adsorption experiments. Adsorption experiments were carried out by adding a known amount of iron particles to a 10 mL solution containing a known concentration the polymer in tetrahydrofuran. The resulting mixture was shaken to ensure complete suspension of metal particles and to ensure intimate contact between metal particles and polymers. The suspension was allowed to equilibrate for a week, the particles allowed to settle, and the supernatant was filtered through a 2 micron pore-size syringe filter to completely remove the particles. The polymer concentrations were determined by measuring the absorbance of the supernatant polymer solution. From the concentration before and after adsorption, the amount of polymer adsorbed in mg polymer/g pigment was calculated. Magnetic dispersion and coatings. The metal particle dispersions were made by milling a mixture of a the iron particles with a polymer solution in cyclohexanone with 2 mm steel beads using a reciprocating paint shaker for 6 hours. The formulation was simplified by excluding other ingredients in a typical commercial M P tape formulation such as lubricants, antistatic agents, dispersants and abrasives. The particle loading was fixed at 75 weight percent (of the total solid content) and a 20 weight percent solid content was used. For example, a typical formulation contained 7.5 g pigment, 2.5 g polymer and 40 g cyclohexanone. After milling, a portion of the dispersion was removed for Theological study and the remaining was used for coating. M P tape coatings were made by casting the dispersion onto a polyester web using a hand draw-down knife coater. The coating


528


was simultaneously drawn through a magnetic field to longitudinally orient the iron particles. The coatings were dried in an oven at 50°C for 24 hours.


Rheological measurements. Rheological measurements were carried out on a Carri-Med model CS-100 controlled stress rheometer with a 6 cm parallel plate fixture. Constant shear stress measurements were made using the steady mode and sinusoidal oscillations were done using the dynamic mode. The gap height used was 1 mm which was much greater than diameter of the iron particle (200 nm). The samples were directly taken from the milling stage for the measurements and a small shear was applied manually before loading the sample for rheological test. Similar preshear, loading technique and rest time was used for all samples and the reproducibility was generally good. Magnetic measurements. The magnetic hysteresis loops and isothermal and dc remanence magnetization curves were measured on a Digital Measurement Systems model 880 vibrating sample magnetometer. The remanence curves were analyzed using a technique described by Huang et al. (16) to obtain values of the interparticle interaction parameter α introduced by Che and Bertram (77). We have used the interparticle interaction field parameter as a measure of the dispersion state of the particles (18). In an ideal magnetic dispersion with no agglomerates the value of α would be zero. M P tape samples show negative values for a, indicative of demagnetizing interparticle interactions. The closer α is to zero , the better the magnetic dispersion. Corrosion experiments. Test pieces ( 6 mm diameter ) were punched from the hand drawn magnetic tape samples and subjected to accelerated aging conditions. The level of iron corrosion was determined by obtaining values for the saturation magnetization (as) from magnetic hysteriesis curves. When iron corrodes it forms non magnetic oxide, thus a decrease in as is a direct measure of the amount of iron corrosion. Test pieces were exposed to accelerated aging environment, either soaked in pH 2 aqueous buffer or placed in a temperature/humidity chamber at 80°C and 80% relative humidity. Periodically the samples were removed , a

s

measured then returned for further exposure. The value of as after exposure were divided by the initial values of as giving the relative saturation magnetization. Results and Discussion Adsorption isotherms. Figure 1 shows adsorption isotherms for adsorption of AQPU-13, AQPU-16, and AQPU-15 onto metal particles from tetrahydrofuran at 25°C. Plots of mg polymer adsorbed per g of particle obeyed the Langmuir relation,

equation 1, where A is the Langmuir capacity , Β is Langmuir affinity, and C is the concentration of polymer solution. Values for the Langmuir affinity, Table I, for the three polymers were similar, however the Langumuir capacity increased with increasing AQM-1 content. AQPU-15, having the highest AQM-1 content, had the highest affinity.


31.

CHACKOETAL.

200

529

Amine—Quinone Polymers as Binders

r

150


ο 100

h

50

h

AQPU-13 • - - AQPU-16 - A - - AQPU-15

Polymer Concentration (weight percent) Figure 1. Adsorption isotherms for adsorption of amine-quinone polymers onto iron particles from THF solution.

Polymer AQPU-13 AQPU-15 AQPU-16

AQM-l content 20% 40% 30%

Capacity (A) 140±2 157±3 142±3

Affinity (B) 2.59±0.23 2.96±0.31 3.00±0.40

Rheology of metal particle dispersions. Rheological measurements were made on the metal particle dispersions in order to compare the different amine-quinone polymers with the commercial polymers. In Figure 2 is a plot of viscosity as a function of shear rate for metal particle dispersions made with amine-quinone polyurethanes, a commercial poly(vinylchloride) wetting binder or a commercial polyurethane. A l l amine-quinone polyurethanes showed similar viscosity at high shear rate. AQPU-13 had the lowest viscosity at low shear rates, suggesting a lower yield stress. The presence of a yield stress and shear-thinning, suggests the break-down of a network structure in the coating fluid. Viscoelastic measurements were carried out to assess the magnitude of elastic contribution resulting from structure formation. The storage modulus G'(co) was a measure of the elasticity in the system, while the loss modulus G"(co) was a measure of the viscous response, where ω is the angular frequency at which the measurements are carried out. In Figure 3 is a plot of G' as a function of ω for the different metal particle dispersions. In all cases G' increased with increasing frequency. A Q P U 13, containing the lowest AQM-1 content, showed lowest storage modulus while AQPU-15 showed the highest storage modulus. Higher values of G ' for a magnetic dispersion suggests an increase in the network structure. The network


530



• AQPU-13 • AQPU-16 • AQPU-15 - - • - - PVC Copolymer-A - - • - - Polyurethane

2

10"

10

1

10°

1

10

10

2

10

3

10

4

Shear Rate (1/s) Figure 2. Plot of viscosity as a function of shear rate for the metal particle dispersions.

Figure 3. Plot of storage modulus, G', as a function of frequency for the metal particle dispersions.


31.

CHACKO ET AL.

Amine—Quinone Polymers as Binders

531


structure arising from increased magnetic attractions between the particles in the dispersion and therefore a lower quality of dispersion. At every frequency the dispersion containing UCARMAG-536 had the lowest value of G' and therefore the best dispersion. The dispersion containing the commercial polyurethane had the highest value of G' and was therefore the worst dispersion. For the aminequinone polyurethane the dispersion containing AQPU-13 had the lowest value of G \ while AQPU-15 had the highest the value of G*. Therefore AQPU-13 gave the best dispersion and the quality of the dispersion approached that provided by the commercial wetting binder UCARMAG-536. Dispersion Quality in Magnetic Coatings. A major source of media noise is attributed to the interparticle interactions which arises from the magnetic particle aggregates or agglomerates (79). These particle interactions have been analyzed through the use of magnetic remanance curves and using a model by Che and Bertram (77). The interparticle interaction parameter, a, extracted from magnetic remanance curves measured on the magnetic coatings prepared from various dispersions are tabulated in Table II. For an ideal dispersion where particles are separated, the interparticle interaction fields are negligible and values of α are expected to be zero. As the particles come into contact due to insufficient steric repulsion, the interaction field increases and the alpha deviates from zero. The higher the absolute value of a, the poorer the dispersion quality. As expected, the values of α for all the amine-quinone polymers were negative, indicating that the interparticle interaction were demagnetizing in nature. AQPU-13 had the lowest absolute of a, suggesting that it had the best magnetic dispersion. AQPU-15 had the highest absolute values of a, suggesting that this dispersion was the worst. The remanence measurements on the tape correlated well with the rheological measurements on the coating fluids. By both measures, AQPU-13 gave the best dispersion, and the quality of the dispersion decreased in the order AQPU-16 > AQPU-15. Table II. Interparticle interaction parameter obtained from magnetic remanence measurements Binder Polymer AQPU-13 AQPU-16 AQPU-15

a

Inn -0.128 -0.136

The quality of dispersion generally dependent on the amount of polymer adsorption and conformation of the adsorbed layer on particle surfaces. Although AQPU-15 had the highest adsorption, AQPU-13 found to have better dispersion from bom rheological measurements on dispersion and magnetic measurements on the tape. This may be attributed to the differences in the conformation of adsorbed polymer chains on the metal particle surface. Because of the affinity of aminequinone functional group to iron surfaces, the hard segments containing this


532



monomer adsorbed strongly to the metal particle surface. The adsorption of these polymers was be interpreted in terms of segregated train-tail conformation in which hard segment preferentially anchors to the particle surface and the excluded soft segment blocks extend away from the surface as tails. Longer tails from the longer soft segment chains in AQPU-13 were more suitable for sterically blocking the magnetic attraction between the particles in the dipsersion. Corrosion Protection. In a comparative accelerated corrosion study, the M P tape coatings were exposed to pH 2 buffer and the amount of corrosion, as measured by the relative saturation magnetization was determined as a function of exposure time, Figure 4. The M P tape made with commercial polyvinyl chloride binders completely corroded after about 2 hr exposure to pH 2 buffer. There was no loss in saturation magnetization for the tape made with amine quinone polymers even after 16 hour exposure. Among the amine quinone polyurethanes AQPU-15 showed the best corrosion protection while AQPU-13 the least. The higher affinity of AQPU-15 for the metal particle surface leads to better coverage by aminequinone functional group, providing a better protective barrier than AQPU-13. This trend was also seen in samples exposed to 80°C and 80% relative humidity, Figure 5, where AQPU-15 had the best corrosion protection. Transmission electron microscope studies on these amine-quinone polyurethanes suggest a higher hard segment segregation and hard segment crystallite size with AQPU-15 (Nikles, D.E.; Chacko, A.P.; Webb, R. I. The University of Alabama, unpublished data). The hard segment may be crystallizing around particle surface and blocking moisture absorption, which leads to reduced corrosion. The corrosion inhibition of steel by quinone monomers and oligomers has been recently reported (20-21). The affinity of the amine-quinone functional group to iron may arise from the interaction between this electron rich molecule and the vacant d-orbitals in iron. The nature of the interaction between amine quinone polymer and the iron surface is currently under investigation . Conclusions The composition of amine-quinone polyurethane has a strong influence on the performance of the magnetic coatings made with these polymers. The amount of polymers adsorbed on the particle surface increases with amine-quinone monomer content. The polymer with highest amine-quinone content gave a better corrosion protection. The dispersion quality is governed by the nature of conformation of the adsorbed polymer chains. The polymer having the lowest amine-quinone content, and therefore the highest soft segment content gave the best dispersions. There was a good correlation between the elasticity of the magnetic ink, as measured by G', and the quality of the magnetic dispersion in tape made from these inks, as measured by the interparticle interaction parameter, a. Acknowledgments We thank Dr. Peter C. Clark, Department of Mineral Engineering, for the use of his rheometer. This project was supported in part by the Department of Defence Advanced Research Project under a grant administered by the National Storage Industry Consortium.



31. CHACKO ET AL.

533

Amine-Quinone Polymers as Binders

Figure 4. Plot of relative saturation magnetization as a function of time exposed to pH 2 aqueous buffer.

1.2

r

0.6 0.4

h

-B -Θ-

0.2

0

5

AQPU-13 AQPU-16 AQPU-15 UCARMAG-536 MR-110

10 15 20 Exposure Time (days)

25

30

Figure 5. Plot of relative saturation magnetization as a function of time exposed to 80°C and 80% relative humidity.


534


Literature Cited 1. 2. 3. 4. 5. 6.


7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Richter, H. J. IEEE Trans. Magnetics 1993, 29(5), 2185-2201. Bate, G. J.Magn. Magn. Mat. 1991, 100, 413-424. Speliotis, D. IEEE Trans. Magnetics 1990, 26(1), 124-126. Yamamoto, Y.; Sumiya, K.; Miyake, Α.; Kishimoto, M.; Taniguchi, T. IEEE Trans. Magnetics 1990, 26(5), 2098-2100. Okazaki, Y.; Hara, K.; Kawashima, T.; Sato, Α.; Hirano, T. IEEE Trans. Magnetics 1992, 28(5), 2365-2367. Mathur, M . C. Α.; Hudson, G. F.; Hackett, L. D. IEEE Tran. Magnetics, 1992, 28(5), 2362-2364. Parker, M . R.; Venkataram, S.; DeSmet, D. IEEE Trans. Magnetics, 1992, 28(5), 2368-2370. O'Grady, K.; Gilson, R.G.; Hobby, P. C. J.Magn. Magn. Mat. 1991, 95, 341355. Kim, K. J.; Glasgow, P. D.; Kolycheck, E. G. J. Magn. Magn. Mat. 1993, 120, 87-93. Kaleem, K.; Chertok, F.; Erhan, S. Prog. Org. Coatings, 1987, 15, 63-71. Nithianandam, V. S.; Kaleem, K.; Chertok, F.; Erhan, S. J. Appl. Polym. Sci 1991, 42, 2893-2897. Nikles , D.E.; Liang. J. IEEE Trans. Magnetics. 1993, 29(6), 3649-3651. Nikles, D.E.; Cain, J. L.; Chacko, A.P.; and Webb; R. I.; Liang, J.-L., Belmore. K. J. Poly. Sci. Part A, Polym. Chem. 1995, 33, 2881-2886. Nikles, D.E.; Cain, J.; Chacko, A.P.; Webb, R. I.;. Liang, J.; Belmore. K. Polymer Materilas Encyclopedia , CRC Press., in press. Nikles, D.E.; Cain, J. L.; Chacko, A. P.; Webb, R. I. IEEE Trans. Magnetics 1994, 30(6), 4068-4070. Huang, P.; Harrell, J. W.; Parker, M . R. IEEE Trans. Magnetics . 1994, 30(6), 4002-4004. Che, X.; Bertram, Η. N. J.Magn. Magn. Mat. 1992, 116, 121. Cheng, S.; Fan, H.; Harrell, J. W.; Lane, A . M . ; Nikles, D. E. IEEE Transactions on Magnetics 1994, 30(6), 4071-4073. Clarke, M . D.; Bissell, P. R.; Chantrell, R.W.; Gilson, R. J.Magn. Magn. Mat. 1991, 95, 17-26. Slavcheva, E; Sokolava, E.; Raicheva, S. J. Electroanal. Chem. 1993, 360, 271-282. Muralidharan, S.; Phani, K.L.N.; Pitchumani, S.; Ravichandran, S.; Iyer, S. V.K. J.Electrochem.Soc. 1995, 142(5), 1478-1483.


AmineQuinone Polymers as Binders for Metal-Particle-Tape Formulation

Recommend Documents