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Saltzman, 8 . E. Anal. Chem. 1954, 2 6 , 1949-1955. Scott, W. W. Ind. Eng. Chem. 1924, 16, 74-82. Tagashira, Y.; Takagi, H.; Inagaki, K. Japanese Patent 75 106862, 1976: Chem. Abstr. 1976, 8 4 , 79359. Teletzke, G. H. Chem. Eng. frog. 1964, 6 0 , 33-38, Voorhoeve, R. J. H.; Remeika, J. P.; Freeland, P. E.: Matthias. B. T. Science
1972, 177, 353-354
Received for reuiew August 19, 1983 Revised manuscript received October 10, 1984 Accepted November 19, 1984
GENERAL ARTICLES Erasable Optical Disks for Data Storage: Principles and Applications M. Mansurlpur,' M. F. Ruane, and M. N. Horensteln College of Engineering, Boston University, Boston, Massachusetts 022 75
Erasable optical disk storage systems offering densities in excess of 10' bits/cm2 using thermomagnetic recording and magnetooptic readout are described and compared to conventional magnetic disks. The preparation and properties of amorphous rare earth-transition metal alloy thin films, which serve as the recording media, are also presented. The lack of a strong readout signal suggests the need for the enhancement of the signal-to-noise ratio (SNR). A multilayering scheme with improved SNR and superior thermal characteristics is discussed. Potential research and development issues are also indicated.
I. Introduction The use of optical techniques and materials for the storage of large bodies of data offers several advantages over the use of traditional magnetic storage media (White, 1980,1983; Bell, 1983). Compared to magnetic disk or tape recording, a typical optical storage system provides higher density of stored information as well as the means for data recording and retrieval without physical contact with the medium. This feasibility of remote readlwrite operation has significant consequences in terms of overall system reliability. Whereas a typical magnetic read/write head must be within a few tenths of a micron of a disk surface, optical heads can operate at a distance of a few millimeters from the medium, reducing the need for precise vertical alignment and highly controlled environments. In a typical system, the recording medium is embedded in a disk which rotates under a focused laser beam. The laser and the associated optics and optoelectronics constitute the read/write head which can move radially to address different tracks on the disk. The disk is either a plastic substrate on which the information is prerecorded in the form of surface moldings (read only disks), or an appropriate substrate on which the recording medium is deposited in the thin film form (write-once and erasable disks). The optical video disk and the compact digital audio disk are examples of the read only media. Ablative media with tellurium layers and bubble-forming media with gold-chromium layers are in the write-once category, while magnetooptic and phase-change media are in the
* Supported by National Science Foundation Grant No. ECS-8307928 and in part by the IBM Corporation.
erasable group (Bouwhuis and Braat, 1983; Bell and Bartolini, 1979; Mimura et al., 1978; Takenaga et al., 1983). In optical recording, higher densities can be achieved for two reasons. First, the small size of a focused spot allows individual bits to be placed close to each other, and second, ease with which high-speed, accurate tracking can be achieved allows tracks of data to be packed at high density. A bit density as high as lo8 bits/cm2, which is one or two orders of magnitude higher than the best achievable density in magnetic recording, can be achieved with red or near-infrared lasers in conjunction with a high-quality lens system that focuses the beam into a submicron spot. Optical disks can be coated with transparent dielectric layers which protect the medium from the environment but at the same time allow the passage of read/write beams. Since the beams can be focused under the coating layer, small dust particles, finger prints, and scratches that are detrimental to the operation of magnetic systems can be tolerated in optical systems. This feature allows optical disks to be removable, which is an advantage in many applications. While optical disk storage systems do offer several advantages, they also present problems not present in traditional magnetic storage systems. For one thing, highly accurate servo-mechanisms are needed to keep the laser beam focused on the target while the disk is rotating at high speed. The problem becomes more acute as short wavelength lasers, and the small spot sizes achievable with them, are used. Another problem is associated with the bulkiness of the optical readlwrite head, which houses the laser, detectors, optics, and the focusing and tracking servos. The limited speed with which the head can be
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positioned on the tracks in turn limits the minimum data access time. Despite the difficulties presented by these mechanical problems, optical data storage systems are commercially competitive with magnetic storage systems. Optical systems now on the market offer data access times in the 100-ms range, and the servo-mechanism problems have been solved, a t least for the range of bit densities now in use (Chen, 1984). The most ‘significant problem associated with the existing laser disk systems is their inability to allow for erasure and subsequent re-recording of data. While lack of repeated write/erase capability is actually an advantage in archival data storage, it is a major disadvantage in most other applications, particularly those involving working computer storage. Hence, a major thrust in much of the research on optical storage systems today centers around the materials and techniques that will make erasable media possible in commercial data storage systems. This paper is concerned with some of the specific issues of erasable optical storage. In particular, we discuss the magnetooptic properties of several amorphous rare earth-transition metal alloys which, since their discovery in 1973, have successfully been used in several prototype erasable optical systems. The system problems associated with thermomagnetic recording and magnetooptical readout of data on thin films of these materials are addressed in section 11. Section I11 discusses both the magnetic properties and preparation methods of the media. Section IV is concerned with multilayer structures and describes their advantages. Finally, in section V we give an overview of existing research in the field and suggest directions for new research. 11. Erasable Optical Storage Systems
(a) Thermomagnetic Recording and Erasure. The most likely choice for a recording medium in an erasable optical storage system is a thin amorphous film of some rare earth-transition metal (RE-TM) alloy deposited on a glass or plastic substrate (Chaudhari et al., 1973; Mimura et al., 1976; Urner-Wille et al., 1980; Togami et al., 1982; Connell et al., 1982). Most RE-TM alloys are ferrimagnetic materials in which the RE subnetwork exhibits a magnetization direction opposite that of the T M subnetwork. Such films also exhibit a strong uniaxial anisotropy in the direction perpendicular to the film plane and, as a result, the net magnetization has perpendicular orientation. The processes of thermomagnetic recording and erasure depend upon temperature-dependent changes in the coercivity of the media. At room temperature the RE-TM media, prepared in the proper range of composition, possess large coercivities and strong magnetic fields are therefore required for the reversal of the magnetization direction. However, if the temperature is increased, the coercivity drops significantly. As depicted in Figure 1, a “clean” medium to be recorded, with its magnetic orientation everywhere in the “zero” direction, is situated at the recording site within a dc magnetic field which points in the opposite -one” direction. A t room temperature, the strength of this field is below the value required for domain formation in the magnetic material. A laser beam focused at the site of a particular spot, however, can significantly raise the local temperature of the medium, allowing the dc field to overcome the medium’s coercivity, thereby creating a reverse-magnetized domain. When the laser beam is turned off, the temperature at the location of the spot returns to normal, but the domain persists. A magnetized domain thus reversed would represent a binary digit “one- in digital recording applications, while an un-
FOC”**Bd
Laser seam
t t‘ 4‘
4
Figure I. Thermomagnetic recording: a focused laser beam raises the local temperature of the medium and an externally applied field reverses the direction of magnetization in the heated region (a). Once the beam is turned off, the temperature returns to normal but the reversemagnetized domain persists (b).
reversed domain would represent a binary “zero”. The laser power required to properly heat the medium is well within the reach of available semiconductor diode lasers that operate in the A = 0.8-pm range. A t such wavelengths, diffraction-limited focused spots on the order of 1pm are achievable. Detailed thermal studies show that a focused 10-mW laser pulsed for 50 ns onto a relatively thin film (a few hundred A thick), can deliver enough energy to raise the spot temperature by several hundred degrees (Mansuripur et al., 1982a; Mansuripur and Connell, 1983a,b). A t these temperatures, domain reversal can be achieved by a magnetic field on the order of a few hundred oersteds, which is not of sufficient strength to cause domain reversal a t room temperature (Mansuripur et al., 1982d; Mansuripur and Connell, 1984). It is important to ensure that the highest temperature reached in the recording process does not exceed the crystallization temperature of the RE-TM material, which is typically in the range of 300 to 500 “C. Because the intensity distribution of a single-mode laser beam is usually Gaussian, the temperature at the center of the spot is much higher than the temperature a t the domain boundary. Hence, the peak temperature must be maintained below the crystallization temperature to avoid crystallization at the beam center. Note that to a first approximation the motion of the rotating disk need not be considered when analyzing the recording process. A 12-in. diameter disk rotating at loo0 rpm, for example, causes a maximum spot displacement of only 0.8 pm during the 50-11s laser pulse width, which is less than the diameter of the spot itself. (b) Magnetooptic Readout. Once the data have been recorded, they may be retrieved by one of several magnetooptic effects. All methods make use of the interaction of polarized light with the magnetization of the RE-TM alloy to interrogate the direction of magnetization at a specific location on the disk (Treves, 1967; Hunt, 1969; Chen et al., 1973; Cheng et al., 1982). Most systems in use today utilize the polar Kerr effect, which lends itself easily to practical data retrieval systems. If a linearly polarized beam of light (of much weaker intensity than that used in the recording process) is incident on an RE-TM film, the reflected beam will have both a parallel and a perpendicular component relative to the polarization of the incident beam, as defined by amplitude reflectivities rll and rl, respectively. While rl, is independent of the state of magnetization of the film, the sign of rL (+ or -) corresponds to the direction of magnetiza-
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the pattern of magnetization. The signal from the detectors is weak since polar Kerr angles are typically a few tenths of a degree. This signal is accompanied by noise from several sources, including noise due to the quantum nature of photodetection (shot noise), thermal noise in the electronic circuitry (Johnson noise), and noise due to the surface roughness and nonuniformity of the media (media noise). Before the discovery of the RE-TM alloys, media noise tended to be the dominant noise source in optical disk systems (Chen et al., 1973). Polycrystalline MnBi-based alloys, for example, once considered promising media, were discarded, partly due to the excessive amounts of noise generated at the grain boundaries. Amorphous RE-TM alloys, on the other hand, contribute insignificant amounts of media noise (Connell et al., 1983). Likewise, Johnson noise can be substantially reduced if photomultiplier tubes or avalanche photodiodes are used in place of regular photodetectors. Hence, in present systems shot noise is the dominant source of noise during the readout process. The shot noise variance in the system of Figure 2 is given by (Mansuripur et al., 1982b,c)
'7
-Magnetic
To Focussing and Tracking Control System
Film
r$dBeam Splitter
,
I
t
i
Photodiode-(!
1 Photodiode
Readout Signal
t
cr2 =
Differential Amplifier
Figure 2. Schematic diagram of a differential detection readout system.
tion. Thus, the magnetic direction of a particular spot can be determined by monitoring the polarization of the reflected beam. Note that although rl,and r L are in general complex numbers (i.e., the reflected beam is elliptically polarized), it is usually only the relative phase of rl that is of interest; hence r and r , can be treated as real numbers. A typical data readout system for determining the phase of r L is depicted in Figure 2. The interrogating laser beam is first collimated and linearly polarized, and, after passing through the beam splitter, is focused on the magnetic layer of the disk. Note that the effect of the beam splitter on the incident beam is not important here, but the incident beam must pass through the splitter if the reflected beam is to be separated out for analysis. The beam splitter thus separates the reflected beam and directs it toward the detection arm of the system. Here the polarizing beam splitter (PBS) divides the energy of the beam between the two photodetectors according to the sign of rl. If Po is the useful incident power, the power arriving at the PBS will be This power is split between the photodetectors according to 1 -Po(rll+ ri)* 2 and (3)
The output signal of the differential amplifier is then proportional to
2rlPor11rL
(4)
where 7 is the responsivity of the photodiodes. The sign of the signal thus reflects that of r L , which is in turn determined by the direction of magnetization under the focused spot. Therefore, as the disk rotates under the beam, the output of the differential amplifier reproduces
2rleP,,B(rIl2+ r L 2 )
(5)
where B is the signal bandwidth and e is the electronic charge. The signal-to-noise ratio (SNR) in logarithmic units (dB) is thus given by
2rlPorp2r12 SNR = 10 log
e(rl12+ r L 2 ) B
(6)
For typical parameter values of Po = 1 mW, rl,= 0.8, ri = 0.003, B = lo7 Hz, and 17 = 0.5 A/W, eq 5 gives a signal-to-noise ratio equal to 37.5 dB. In practice the SNR obtained is somewhat lower because of (i) other sources of noise, (ii) overlap of the reading beam spot with neighboring domains, (iii) lack of perfect focusing and tracking, and (iv) variation of domain size from site to site. In any event, the SNR obtained is often marginal for many applications. In digital data systems, for example, a minimum SNR of 20dB is required, while analog applications (such as frequency-modulated video disk recording) require an SNR of about 50 dB. For high performance systems, it is thus necessary to improve the signal-to-noise ratio by utilizing multilayer disk structures. We will return to this issue after we have discussed the material properties in the next section. 111. Preparation Techniques and Properties of the RE-TM Alloys In the design and implementation of any optical data storage system, it is important to consider the properties of the recording media, and in particular the methods by which they can be prepared. In the case of RE-TM alloys, thin films can be deposited on glass or plastic substrates by either an evaporation or a sputtering process (Chaudhari et al., 1973; Heiman et al., 1976; Mimura et al., 1978; Gangulee and Taylor, 1978; Urner-Wille et al., 1980). Sputtering, however, appears to be the more useful technique in that it can produce films with perpendicular anisotropy from a wide range of materials and compositions. Sputtering also facilitates control of the media characteristics. The composition of the film can be controlled by varying the geometry of a mosaic target made up of the various component alloy materials. A base pressure in the sputtering chamber of at least lo4 torr is generally required for achieving clean and uniform films. The substrate must be water-cooled to avoid crystallization of the media, and the sputtered films
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 83 +SiO,
i) i) Intermediate Layer (2.1000 i)
Overcoating Layer (-1000
--Magneto-optic +SiO,
--Aluminum
i
+Glass
I
Material (-200
Reflecting Layer (2.500 A)
or Plastic Substrate (2.1 mm)
I
Figure 4. Quadrilayer magnetooptical disk structure. Specified materials and thicknesses are typical of an optimum design. 0
TCO
TC
T ( O K1
Figure 3. Typical magnetization vs. temperature curves for an RE-TM alloy. MREand MTMare the corresponding subnetwork magnetizations; both vectors are perpendicular to the film surface but are oppositely oriented. M,is the net saturation magnetization which is the vector s u m of the subnetwork magnetizations. T, is the compensation point and T,is the Curie point temperature.
must be overcoated to prevent oxidation upon exposure to air. Application of a negative bias voltage to the substrate during deposition has been found to increase the crystallization temperature of the media, which is a desirable feature in a practical system (Togami, 1982). Proper negative bias also produces more stable films. It is suspected that the preferential resputtering of the rare earth ions under the effect of negative bias is responsible for these phenomena. To date, several alloy combinations have been considered for erasable recording applications, including GdCo, GdFe, TbFe, DyFe, GdTbFe, and GdTbCo. The magnetic order in these alloys is ferrimagnetic with the magnetization of the T M subnetwork oppositely oriented to that of the RE subnetwork. As Figure 3 shows, the subnetwork magnetizations of a typical alloy decrease uniformly with temperature until they vanish at the Curie point (TJ.The net magnetization, however, being the difference of the subnetwork magnetizations, first drops to zero at a lower temperature, called the compensation point (T,),and rises again before vanishing at T,.Around the compensation temperature, the magnetization is small, and therefore large magnetic fields are required for revesing its direction. In magnetic nomenclature, it is said that the coercivity is high around the compensation point. By varying the relative RE-TM composition, the compensation point can be adjusted within a range of temperatures. For example, GdalCo7g,Gd25Fe75,and Tb21Fe,9 all have a compensation temperature of T, = 300 K. This choice of T,, helps make the recorded data immune to alteration from stray magnetic fields and, by reducing the demagnetizing effect, prevents spontaneous breakdown of the medium’s magnetization into striped domains. The fact that the net magnetization is close to zero when T,,is near room temperature does not prevent the optical reading of data since, at wavelengths used in readout, the light mainly interads with the TM subnetwork alone. The magnetic electrons of the RE subnetwork are in the 4 f shell (which is well shielded by the 5 s and 5 p shells) and are therefore inaccessible by the red or near-infrared photons. The existence of perpendicular anisotropy in amorphous films of the RE-TM alloys is one of the most significant characteristics of these media as far as optical recording is concerned. It allows perpendicular (as opposed to inplane) recording, and helps to reduce the domain wall thickness. For high-density storage and large readout signal-to-noise ratios, both perpendicular domains and narrow domain walls are essential. The source of anisot-
ropy in the media, however, is not yet well understood (Chaudhari and Cronemeyer, 1976; Takagi et al., 1979). It is suggested that a small inhomogeneity in the atomic distribution in the form of pair ordering gives rise to the anisotropy through magnetic dipole-dipole interactions (Mizoguchi and Cargill, 1979). At the same time, non-S state rare earth ions such as Tb3+ and Dy3+ are believed to couple strongly to the “crystal” electric field and create random axis anisotropy (Rhyne et al., 1974; Harris et al., 1973). A combination of these effects is probably responsible for the observed anisotropy, but more experimental evidence is needed to support these ideas.
IV. Multilayer Structures As mentioned in section 11, the lack of a strong perpendicular component of reflected polarization (rL)from the RE-TM alloy films results in an inadequate signalto-noise ratio for most applications. Hence, additional techniques for improving the SNR are required. By incorporating the magnetic medium in a multilayer structure such as shown in Figure 4, this goal can be achieved (Mansuripur et al., 1982b,c). If the multilayer film is designed such that the magnetooptic signal generated in the first path constructively interferes with the signal generated in the second path (after reflection from the aluminum layer), the reflected signal will be enhanced, leading to an increase in the readout signal-to-nosise ratio. Experiments and calculations have shown that multilayering can improve the signal-to-noise ratio by as much as 8 dB. The use of a multilayer structure can provide benefits in the recording process as well. If the multilayer is designed to be antireflective, a more efficient use of the recording laser power can be made. The power normally reflected from a bare film (typically around 40%) is absorbed in a multilayer film and converted to heat a t the recording spot location. In situations where the available laser power is small or the sensitivity of the recording medium is poor, efficient use of power is an important consideration. Similarly, it is even possible to control the thermal properties of the structure by controlling the thickness of the reflecting layer (which is also a heat sink) and the intermediate layer (which determines the extent of thermal coupling between the two metallic layers). The diffusion of heat in the radial direction, for example, can be substantially reduced if the aluminum layer is brought close to the magnetic film (Mansuripur and Connel, 1983b). In situations where plenty of laser power is available, the heat sink effect allows the readout operation to be performed at a higher power level, without risk of altering any of the recorded magnetic domains. An enhancement of signalto-noise ratio can thus be achieved. It is fortunate that the benefits of multilayers in both the recording and readout processes can be obtained simultaneously. The structure that achieves maximum
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signal-to-noise ratio in readout is also entireflective; hence the same structure can be tuned to the recording process for optimum use of laser power and improved temperature distribution a t the spot location.
V. Concluding Remarks Erasable optical storage technology is an attractive alternative to conventional magnetic storage systems. It offers, among other advantages, large capacity, robustness, and high data rates. Amorphous rare earth-transition metal alloys are at present the likely candidates for the recording medium. In preliminary studies, the media have retained both structure and magnetic properties, as well as integrity of recorded domains over extended periods of time (Togami et al., 1982; Cheng et al., 1982). A comprehensive test of long-term stability, however, has yet to be reported. An understanding of the magnetization, anisotropy, crystallization, thermal behavior, domain formation, and domain stability in these media is of utmost importance. Also important is progress in the area of substrate production. Clean, uniform, and stable substrates are required if high-density optical recording is to become commercially viable. In the long run, short wavelength lasers in the blue and ultraviolet region must be considered for very high density storage. Focusing and tracking servos that can perform reliably at these short wavelengths must be developed, and the ability of the media to support smaller domains must be examined. All in all, the emerging technology of optical storage is as fascinating as it is promising. While potential applications in commercial, industrial, and military arenas promise a vast market for the optical recording products, the engineering and scientific problems associated with the development of this technology will continue to challenge a broad range of specialists in the years to come.
Literature Cited Bell, A. E. Proc. S P I E Conf. 1983, 382, 2. Bell, A. E.: Bartoiini, R. A. Appl. Phys. Lett. 1979, 34,275. Bonwhuis, G.; Braat, J. J. M. I n "Applled Optlcs and Optical Engineering"; Academic Press: New York, 1983; Vol. 9. Chaudhari, P.; Cronemeyer, D. C. Proc. AIP Conf. 1976, 29, 113. Chaudhari, P.; Cuomo, J. J.; Gambino, R. J. Appl. Phys. Leff. 1973, 22,337. Chen, D. Laser Focusl€lectro-Opt. 1984, 2 0 , 42. Chen, D.; Otto, G. N.; Schmit, F. M. I€€€ Trans. Mag. 1973, 9 ,66. Cheng, D.; Treves. D.; Chen, T. Proc. SPI€ Conf. 1982, 329,223. Conneli. G. A. N.; Allen, R.; Mansuripur, M. J . Appl. Phys. 1982. 53,7759. Connell, G. A. N.; Treves, D.; Alien, R.; Mansuripur, M. Appl. Phys. Leff. 1983, 42, 742. Gangulee, A.; Taylor. R. C. J . Appl. Phys. 1978, 49, 1762. Harris, R.; Plischke, M.; Zuckermann, M. J. Phys. Rev. Lett. 1973, 31, 160. Heiman, N.; Lee, K.; Potter. R. I . ; Kirkpatrick, S. J . Appl. Phys. 1976, 47, 2634. Hunt, R. P. I€€€ Trans. Mag. 1969, 5 , 700. Mansuripur, M.; Connell, G . A. N. Appl. Opt. 1983a, 22,666. Mansuripur. M.; Connell, G. A. N. J . Appl. Phys. 1983b, 54,4794. Mansuripur, M.; Connell, G. A. N. J . Appl. Phys. 1984, 55,3049. Mansuripur, M.; Connell, G. A. N.; Goodman, J. W. Appl. Opt. l982a, 2 1 , 1106. Mansuripur, M.; Connell, G. A. N.; Goodman, J. W. J . Appl. Phys. l982b, 53,4485. Mansuripur, M.; Connell, G. A . N.; Goodman, J. W. Proc. S P E Conf. 1982c, 329.215, Mansuripur, M.; Connell, G. A. N.; Treves, D. I€€€ Trans. Mag. 19826, 18, 1241. Mimura. Y.; Imamura, N.; Kobayashi, T. Iff€ Trans. Mag. 1976, 12, 779. Mimura, Y.; Imamura, N.; Kobayashi, T.; Okada. A,; Kushiro, Y. J , Appl. Phys. 1970, 49, 1208. Mizoguchi, T.; Cargill, G. S. J . Appl. Phys. 1979, 50,3570. Rhyne, J. J.; Schelleng, J. H.; Koon, N. C. Phys. Rev. 8 . 1974, 1 0 , 4672. Takagi, H.; Tsunashima, S.; Ushiyama, S.; Fujii, T. J . Appl. Phys. 1979, 50, 1642. Takenaga. M., et al. Proc. SPIE Conf. 1983, 420, 173. Togami, Y. I€€€ Trans. Mag. 1982, 18, 1233. Togami, Y.; Kabayashi, K.; Kajiura, M.; Sato, K.; Teranishi, K. Proc. S P E Conf. 1982, 329,208. Treves, D. J . Appl. Phys. 1967, 38, 1192. Urner-Wille, M.; Hansen, P.; Witter, K. I€€€ Trans. Mag. 1980, 16, 1188. White, R. M. Sci. Am. 1980, 243, 138. White, R . M. Iff€ Spectrum 1983. 20,32.
Received for review August 14, 1984 Accepted September 28, 1984
Wollastonite Extenders in Anticorrosive Alkyd Metal Primers Cllve H. Hare* and Michael G. Fernald Clive H. Hare, Inc., Holbrook, Massachusetts 02343
Fifteen extenders, including eight silane- and titanate-treated wollastonite materials, have been evaluated for their contribution to the corrosion and blistering resistance of a long oil alkyd primer over a range of PVCs. Studies conducted in 5 % salt spray show that wollastonite in either a treated or untreated form improves performance, although judicious selection of treatment type provides maximized performance levels. Factors affecting performance appear to be the particle size of the pigment and the type of treatment as well as PVC. Key to performance appears to be the matching of functionality of treatment to that of the vehicle system employed, with silane-treated pigments giving better overall results than titanate-treated pigments. These data coincide with the findings of a similar earlier study with epoxy primers. Optimized PVC:CPVC ratios (calculated) are predictable (0.65-0.85: I), with better performance extenders giving wider latitude in PVC placement. Evaluations of the effect of increasing wollastonite loading as a percentage of the total extender loading at a given PVC show advantages from such increase within the PVC:CPVC ratio range that good inhibitive performance might be expected.
Introduction In studies by Hare and Wright (1983) and Hare (1984) evaluating the effect of functional extender pigments on the corrosion and blistering resistance of polyamide- and polyamine-cured nontoxic metal primer systems based on epoxy resins of low, medium, and high molecular weight, it was found that calcium silicate in the form of wollas-
tonite consistently gave systems which outperformed similar primers based on other extender pigments. The studies went on to show that when wollastonite was surface-treated with an epoxy-terminated silane, its value as a functional extender was significantly enhanced and primer systems displaying yet further improved corrosion and blistering resistances were secured.
0196-4321/85/1224-0084$01.50/0 1985 American Chemical Society
