Conductive Coatings 91-5A new class of low-dimensional materials — for example, polymeric metallophthalo-cyanines with such metals as aluminum, chromium, tin, and gallium at the center o
Trang 2Conductive Coatings 91-5
A new class of low-dimensional materials — for example, polymeric metallophthalo-cyanines with such metals as aluminum, chromium, tin, and gallium at the center of the ring — has been synthesized
Various transition metal ions have been coordinated with conjugated ligands to synthesize polymers with metal ions within the main chain.36 Ligands such as tetrathiosquarate, tetrathionaphthalene, tetrathi-afulvalene, and tetrathiooxalate have been used The tetrathiooxalate complexed with nickel ions gave linear polynickel tetrathiooxalate oligomers with conductivities as high as 20 (Ω⋅cm)–1
A completely different approach to depositing conducting organometallic coatings involves the use of
a low-pressure plasma (LPP) environment The LPP environment may be used to deposit a polymeric
coatings by LPP/organometallic monomers were iron, tin, mercury, tantalum, lead bismuth, and metal coatings by the LPP posttreatment were gold, platinum, palladium, silver, and lead The generation of metal surface coatings from certain organometallic coatings can be also achieved by thermal means
formed on a heat-sensitive substrate without the use of elevated temperatures The process also permits formation of adhering gold and platinum coatings otherwise difficult to deposit on plastic substrates None of the conducting organometallic coatings or their deposition processes have gone beyond the research stage However, the conducting organometallic coatings effort is very new compared with the other types mentioned before
91.3 Commercially Available Conductive Coatings
Whereas the metallized plastics effort is a multimillion-dollar industry, the commercial application of conductive polymer coatings as a paint, lacquer, ink, adhesive, or a solution of some kind forms a very small industry indeed Below are summarized several typical products available commercially
A proprietary aluminum containing paint, AG 9680, manufactured by A.I Technology, Inc., Princeton, New Jersey, is claimed to approach the shielding effectiveness of 70 to 75 dB, an effectiveness similar to that of pure silver
A silver-filled silica matrix elastomer, Aremco-Shield 615, has been developed by Aremco Products, Inc., Ossining, New York This material has been formulated into a conductive paint that can be applied
by either brush or spray; it cures at room temperature and bonds to metals, glass, and plastics
N
N N N
N
N N N
N
N N N
N N M
X X
N N M
X
N N M
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Trang 391-6 Coatings Technology Handbook, Third Edition
A silver lacquer (Eccocoat 2) and an elastomeric, silver-filled, conductive coating (Eccocoat CC-40) have been developed by Emerson and Cuming, Canton, Massachusetts The lacquers and the elas-tomer coatings may be applied by dipping, spraying, silk screening, roll coating, or brushing In most
the air-dry method Oven curing will give improved conductivity
A rather extensive series of conductive coatings under the trade name Evershield has been developed
by the E/M Corp of West Lafayette, Indiana The series of products consists of a graphite-filled acrylic resin system, EC-G-102, intended mainly for applications for electrostatic charge dissipation; a high performance nonoxidizing copper-filled acrylic resin system, EC-C-301, is suitable for spray gun appli-cation The coating has an attenuation performance of 50 to 70 dB at 10 to 1000 MHz A popular nickel-filled acrylic resin system, EC-N-501, easily paintable and with superior adhesion characteristics for a wide variety of plastic substrates, is also available It has an attenuation performance of 50 to 60 dB at
30 to 1000 MHz
91.4 Applications
91.4.1 Shielding from Electromagnetic Interference
The advent of the FCC Docket 20780, which regulates electromagnetic emissions from computing and communication devices used in industrial and residential locations, has really provided a stimulus for the industry to come up with cost-effective methods for limiting the level of electromagnetic interference (EMI) To meet the set standards, the manufacturers have adopted a variety of methods for controlling EMI These methods have ranged from redesigned basic circuitry to incorporation of conductive shielding
• Use of metal enclosures
• Metallic foil tapes
• Metal coatings on plastic enclosures
• Conductive paints on plastic enclosures
• Conductive plastic enclosures
• Flexible laminates with metal foil
The shielding effectiveness of a homogeneous medium, such as a conductive coating, is related to the propagation of the electromagnetic field through the coating The shielding effectiveness is directly related
to the electronic and magnetic properties of the coating; therefore, for best shielding effectiveness, materials with both high relative magnetic permeability and high electrical conductivity are necessary Thus, it has been found that the various metals and alloys form the following “series” in decreasing order
of effectiveness:
Ag > Cu > Au > Al > Zn > brass > Ni > Sn > steel > stainless steel Currently, the most cost-effective and the most problem-free materials for shielding are claimed to be nickel-filled acrylic or polyurethane conductive paints.46
91.4.2 The Stealth
One of the more glamorous applications of conducting coatings has been in the “stealth” technology
be said about its materials technology The so-called stealth materials can provide a minimal radar profile for military aircraft and naval vessels This profile is achieved primarily by a combination of geometric design and materials properties The active components consist of several classes of materials: carbon fiber composites, radar-absorbing coating, ferrite layers, and interference layers in the form of certain
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Until the Pentagon revealed the top-secret Stealth (Figure 91.1) fighter on November 10, 1988, little could
Trang 491-8 Coatings Technology Handbook, Third Edition
References
1 T J Miranda, in Applications of Polymers R B Seymour and H F Mark, Eds New York: Plenum
Press, 1988
2 H Narcus, Trans Eelctrochem Soc., 88, 371 (1945).
3 J I Kroschwitz, Ed.-in-Chief, Encyclopedia of Polymer Science and Engineering, Vol 9 New York:
Wiley, 1985
4 C C Ku and R Liepins, Electrical Properties of Polymers, Chemical Principles New York: Hanser,
1987
5 G J Shawham and B R Chuba, in “Materials — Pathway to the future,” SAMPE, 33, 1617 (1988).
6 Y Ikenaga, T Kanoe, T Okada, and Y Suzuki, Ausz Eur Patentanmeld I, 3(12), 485 (1987).
7 P S C Ho, P O Hahn, H Lefakis, and G W Rubloff, Ausz Eur Patentanmeld I, 2(29), 1367 (1986).
8 S John and N V Shanmugam, Met Finish., 84(3), 51 (1986).
9 R Liepins, Camille Dreyfus Laboratory Annual Report, Research Triangle Institute, Research Triangle Park, NC, December 31, 1970, and unpublished results after 1970
10 J Fredrich, I Loeschcke, and J Gahde, Acta Polym., 37(11–12), 687 (1986).
11 L J Krause, “Electroless metal plating of plastics,” U.S Patent 4,600,656 (July 15, 1986)
12 R Cassat, “Metallizing electrically insulating plastic articles,” U.S Patent 4,590,115 (May 20, 1986)
13 R Cassat and M Alliot-Lugaz, “Metallization of electrically insulating flexible polymeric films,” U.S Patent 4,564,424 (January 14, 1986)
14 D E Davenport, in Conductive Polymers R B Seymour, Ed New York: Plenum Press, 1984, p 39.
15 J E McCaskie, in Modern Plastics Encyclopedia 1985–1986, 62, No 10A J Aranoff, Ed New York:
McGraw-Hill, 1985, p 381
16 V Krause, Kunsts Plast (Munich), 78(6), 17 (1988).
17 R A Baldwin, A J Gould, B J Green, and S J Wake, British Patent 2,169,925A (July 23, 1986)
Symposium, April 7–10, 1986, p 1583.
19 J C Cooper, Ru Panayappan, and R C Steele, in Proceedings of the 1984 IEEE National Symposium
on Electromagnetic Compatibility, April 24–26, 1984, San Antonio, TX, p 233.
20 R Liepins, B Jorgensen, A Nyitray, S F Wentworth, D M Sutherlin, S E Tunney, and J K Stille,
Synth Met., 15, 249 (1986).
21 A Wirsen, “Electroactive polymer materials.” National Technical Information Service Publication PB86-185444, January 1986
22 W Werner, M Monkenbusch, and G Wegner, Makromol Chem., Rapid Commun., 5, 157 (1984).
23 H Naarmann and N Theophilous, Synth Met., 22, 1 (1987).
24 T Yamamoto, K Sanechika, and A Yamamoto, J Polym Sci., Polym Lett Ed., 18, 9 (1980).
25 S Hotta, T Hosaka, and W Shimotsuma, Synth Met., 6, 317 (1983).
26 S T Wellinghoff, Z Deng, J Reed, and J Racchini, Polym Prepr., 25(2), 238 (1984).
27 A G MacDiarmid, J C Chiang, M Halpern, W S Huang, S L Mu, N L D Somasiri, W Wu,
and S I Yaniger, Mol Cryst Liquid Cryst., 121, 173 (1985).
28 J C Chiang and A G MacDiarmid, Synth Met., 13, 183 (1986).
29 W S Huang, A G MacDiarmid, and A P Epstein, J Chem Soc Chem Commun., 1784 (1987).
30 R B Bjorklund and B Liedberg, J Chem Soc Chem Commun., 1293 (1986).
31 S P Armes and B Vincent, J Chem Soc Chem Commun., 289 (1987).
32 S P Armes, J F Miller, and B Vincent, J Colloid Interface Sci., 118(2), 410 (1987).
33 S P Armes, M Aldissi, and R D Taylor, “Non aqueous polypyrrole colloids,” LA-UR-88-3855,
submitted to J Chem Soc Chem Commun.
34 P M Kuznesof, R S Nohr, K J Wynne, and M E Kenney, J Macromol Sci Chem., A16(1), 299
(1981)
35 T J Marks, Science, 227, 881 (1985).
36 J R Reynolds, J C W Chien, F E Karasz, and C P Lillya, Polym Prepr., 25(2), 242 (1984).
DK4036_book.fm Page 8 Monday, April 25, 2005 12:18 PM
Trang 5Conductive Coatings 91-9
37 R Liepins and K Sakaoku, J Appl Polym Sci., 16, 2633 (1972).
38 E Kny, L L Levenson, W J James, and R A Auerbach, J Phys Chem., 84, 1635 (1980).
39 G Smolinsky and J H Heiss, Org Coat Plast Chem., 28, 537 (1968).
40 R K Sadhir and W J James, in Polymers in Electronics T Davidson, Ed ACS Symposium Series
No 242 Washington, DC: American Chemical Society, 1984
41 R Liepins, M Campbell, J S Clements, J Hammond, and R J Fries, J Vac Sci Technol., 18(3),
1218 (1981)
42 E Kny, L L Levenson, W J James, and R A Auerbach, Thin Solid Films, 85, 23 (1981).
43 R Liepins, “Method of forming graded polymeric coatings on films,” U.S Patent 4,390,567 (June
28, 1983)
44 R Liepins, “Method of forming metallic coatings on polymer substrates,” U.S Patent 4,464,416 (August 7, 1984)
45 R W Simpson, Jr., in Proceedings of the 1984 IEEE National Symposium on Electromagnetic Com-patibility, April 24−26, 1984, San Antonio, TX, 1984, p 267
46 D Staggs, in Proceedings of the 1984 IEEE National Symposium on Electromagnetic Compatibility,
April 24–26, 1984, San Antonio, TX, 1984, p 43
47 B Bridge, M J Folkes, and H Jahankhani, in Inst Phys Conf Ser No 89, Session 8, p 307, 1987.
48 T A Hoppenheimer, High Technology, December, 58 (1986).
49 R H Baughman, R L Elsenbaumer, Z Igbal, G G Miller, and H Eckhardt, in Electronic Properties
of Conjugated Polymers H Kuzmany, M Mehring, and S Roth, Eds New York: Springer-Verlag,
1987, p 432
50 K J DeGraffenreid, in Proceedings of the 1985 IEEE International Symposium on Electromagnetic Compatibility, April 24–26, 1984, San Antonio, TX, 1985, p 273.
51 Emerson and Cuming, Technical Bulletin 4-2-14, Canton, MA
52 M Gazard, J C Dubois, M Champagne, F Garnier, and G Tourillon, J Phys Paris Colloq., C3,
537 (1983)
53 F Garnier, G Tourillon, M Gazard, and J C Dubois, J Electroanal Chem., 148, 299 (1983).
54 W J Miller, in Modern Plastics Encyclopedia 1985−1986, 62, No 10A J Aranoff, Ed New York:
McGraw-Hill, 1985, p 380
55 H E Coonce and G E Macro, in Proceedings of the 1985 IEEE International Symposium on Electromagnetic Compatibility, April 24–26, 1984, San Antonio, TX, 1985, p 257.
56 H Munstedt, in Electronic Properties of Polymers and Related Compounds H Kuzmany, M Mehring,
and S Roth, Eds New York: Springer-Verlag, 1985, p 8
57 M E Gross, A Appelbaum, and P K Gallagher, J Appl Phys., 61(4), 1628, (1987).
58 R Liepins, B S Jorgensen, and L Z Liepins, “Process for introducing electrical conductivity into high-temperature polymeric materials” (submitted for patent)
59 T Hioki, S Noda, M Kakeno, A Itoh, K Yamada, and J Kawamoto, in Proceedings of the Inter-national Ion Engineering Congress, September 12–16, 1983 Kyoto, Japan, 1984, p 1779.
60 A Auerbach, Appl Phys Lett., 47(7), 669 (1985).
61 A Auerbach, J Electrochem Soc., 132(6), 1437 (1985).
62 J Y Lee, H Tanaka, H Takezoe, A Fukuda, and E Kuze, J Appl Polym Sci., 29, 795 (1984).
63 T Cacouris, G Scelsi, R Scarmozzino, R M Osgood, Jr., and R R Krchnavek, Meter Res Soc Proc., 101, 43 (1988).
64 J E Bouree and J E Flicstein, Mater Res Soc Proc., 101, 55 (1988).
65 A Gupta and R Jagannathan, Mater Res Soc Proc., 101, 95 (1988).
66 L Baufay and M E Gross, Mater Res Soc Proc., 101, 89 (1988).
67 A M Lyons, C W Wilkins, Jr., and F T Mendenhall, Mater Res Soc Proc., 101, 67 (1988).
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Trang 692 Silicone Release
Coatings
92.1 Introduction 92-1 92.2 Thermal-Cured Silicone Release Agents 92-2 92.3 Radiation-Curable Silicone Release Agents 92-7 92.4 The Future 92-9 References 92-9
92.1 Introduction
Silicone release coatings are vitally important to the tag and label industry, which could not exist in its present form without reliable release agents Silicones possess unique physical and chemical properties that make this class of substances ideal for the purpose of releasing pressure-sensitive adhesives Silicone release agents worth $130 to $150 million were sold worldwide in 1988, contributing to products with
a total value that exceeds $3 billion
The term “silicones” as commonly used refers to linear (two-dimensional) polydimethylsiloxanes, which may be structurally depicted as follows:
where x is an integer greater than 1 Silicon is tetrafunctional, so an infinite number of silicone polymers may be devised with different organic groups replacing methyl, or with three-dimensional resin structures wherein silicon atoms are incorporated in the polymer structure via three or four —Si–O— linkages Since, however, the low surface tension, nonpolarity, chemical inertness, and low surface energy respon-sible for the outstanding release characteristics of silicone coatings all derive from the linear dimethyl-silicone structure, this discussion focuses on linear polymers
Silicone coatings that release pressure-sensitive adhesives have been in use for some 35 years The chemistry and applications of silicone release coatings have undergone remarkable change during this time, with the pace of development accelerating in recent years In the face of increasingly sophisticated and demanding requirements, silicones remain the only proven means of providing pressure-sensitive adhesive release for the tag and label industry
The liner most often used is paper, usually a machine-calendered (i.e., supercalendered kraft), clay-coated, or glassine paper designed to minimize penetration during coating and curing of silicone Good
CH3
CH3 SiO
x
Richard P Eckberg
General Electric Company
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The laminate structure normally used by the label industry is illustrated in Figure 92.1
Trang 7Silicone Release Coatings 92-3
50 SiH is a very reactive chemical species that readily condenses with silanol (SiOH) groups, forming extremely stable siloxane bonds and liberating hydrogen in the process:
≡SiOH + ≡SiH→≡SiOSi≡ + H2
Many different catalysts accelerate or initiate this condensation reaction; metal soaps and driers such
as dibutyltin acetate are the most efficient and economical, and are therefore in general use
Condensation cure systems are applied as solutions in organic solvents (toluene or heptane, or mixtures thereof), or as oil-in-water emulsions, because in the absence of a dispersing medium, a catalyzed mixture of a silanol-stopped silicone plus polymethyl-hydrogen-siloxane cross-linker sets up
to an insoluble cross-linked gel in a few minutes at room temperature There is no known means of retarding the condensation reaction sufficiently at room temperature to permit solvent-free coating without rendering the composition uncurable at oven temperatures Solvent (or water in the case of emulsions) therefore acts as a bath life extender through the dilution effect, while also permitting easy, convenient coating of the silicone material
Although use of solvents or water mandates high oven temperature and solvent recovery, and entails fire or explosion risk, such materials are readily coated via simple techniques such as direct gravure, reverse roll, metering rod, and doctor blade Coating out of a solvent vehicle also gives the silicone supplier wide latitude in silanol molecular weight; such dispersion products as General Electric SS-4191 consist
of approximately 30 wt% solutions of high molecular weight silanol gums (MW in aromatic solvents) Even at 70% solvent, these products as furnished have viscosities exceeding 10,000 cps, requiring further dilution to about 5 wt% silicone solids with more solvent to render them coatable The cross-linker is normally packaged in the silanol solution; catalyst is added to the fully diluted bath at time of use Controlling silanol molecular weight is a proven means of controlling the release characteristics of the cured condensation-cross-linked coating Long chains of polydimethyl-siloxane between cross-linking sites provide a rubbery, elastomeric coating; shorted intercross-link intervals lead to higher cross-link density and a harder, more resin-like coating The rubbery coatings provide tight (high) release, which displays a marked dependence on delamination speed in comparison to the low (easy) release independent
of stripping speed obtained from highly cross-linked silicone films Accordingly, silicone suppliers offer several different molecular weight silanol-based dispersion products, permitting the end user to obtain
a desired range of release The relationship between silanol chain length and nominal release level is Addition cure silicones resemble condensation cure silicones in some respects: both types of system rely on thermally accelerated cross-linking reactions between polymethyl-hydrogen siloxane cross-linker molecules and a separate reactive dimethylsiloxane polymer Addition cure processes utilize catalyzed reaction of unsaturated organic groups attached to otherwise unreactive dimethylsilicones with SiH groups present on the cross-linker Polymers in use are vinyl-functional silicones, the general structure
of which may be represented as follows:
The curing reaction is an addition to the SiH group across the olefin double bond, also known as a hydrosilation process:
CH3
CH3
CH3
CH3
SiO
CH3
CH3
Si — CH
X
CH
CH3 SiO
Y
CH — SiO
H2C
CH2
CH2
X + Y = 50 to >4000; Y ≥ 0
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graphically shown in Figure 92.2
Trang 8Silicone Release Coatings 92-5
Addition cure silicone release agents are available as solvent-free, low viscosity vinyl silicone fluids, as solvent-dispersed vinyl silicone gums (analogous to most silanol-based condensation cure products), and
as emulsions In each case, sufficient vinyl functionality is built into the linear silicone molecules to promote formation of highly cross-linked resinous cured coatings These products therefore provide uniformly low (premium) release from most pressure-sensitive adhesives Controlled release is not obtained by varying molecular weight of these vinyl silicone polymers (unlike the silanol case), and thus
a different approach has been taken by silicone suppliers to combine the advantages of addition cure chemistry (particularly solventless packages) with a controllable range of release
Studies of cured dimethylsilicone release coatings by electron spectroscopy for chemical analysis have confirmed that the surface is much more organic than would be predicted from the stoichiometry of the (CH3)2SiO polymer unit.16 An adhesive in a laminate construction is therefore largely in contact with unreactive, bulky, freely rotating methyl groups; more polar —Si—O—Si— polymer backbones
cross-linked nonelastomeric silicone coatings, it follows that breaking up this nonpolar, featureless “methyl landscape” by inclusion of materials that alter the polarity of the silicone should alter the release of the coating This is, in fact, accomplished by adding vinyl-functional silicone resins to the basic linear vinyl silicone polymers.17 “Resin” is here defined as nonlinear silicone structures bearing high concentrations of
functionally Solventless high release additives are therefore mixtures of vinyl silicone resins with vinyl silicone fluids Because these resins are normally friable solids when isolated, their blends with vinyl
INCREASING TEMPERATURE
IDEAL CURE
UNCURED REGION
CURED REGION
OBSERVED CURE
O
O Si
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trate beneath the coating surface A depiction of this postulated structure is offered in Figure 92.4
Trang 992-8 Coatings Technology Handbook, Third Edition
radiation cure of silicones requires silicone polymers incorporating radiation-sensitive organofunctional groups, as illustrated below:
In addition, UV cure normally requires high concentrations of photosensitizers or photoinitiators, such as benzophenone, benzoin ethers, and cationic-type “onium” salts The presence of polar organo-functional moieties plus photocatalysts in radiation-cured release agents causes significant performance differences between radiation-cured and conventional thermally cured silicones Nonetheless, industry demands for low (or “zero”) temperature processing of silicone coatings to permit use of thermally sensitive films and to prevent demoisturization of papers have prompted considerable efforts by major silicone suppliers to develop, then improve, radiation-curable products Radiation-curable silicones are now available from several sources
are in commercial use at a small number of coating facilities equipped to perform EB cure These materials have an important performance drawback inherent in free radical acrylate cross-linking chemistry: because cure is subject to severe inhibition by atmospheric oxygen, efficient inerting (<500 ppm O2) of
EB or UV cure chambers with nitrogen is essential for fast, complete cure to occur While nitrogen blanketing is not impossible, inerting adds complexity and cost to the coating operation
Cure chemistry pioneered by W R Grace & Company overcomes oxygen inhibition problems that interfere with radiation cure of acrylates Mercapto-olefin addition is initiated by UV light in the presence
of suitable photosensitizers, or by EB radiation The reaction is analogous to hydrosilation:
The chemistry has been extended to release coatings by development of mercaptoalkyl-functional
market acceptance of products based on this technology has been slowed by their objectional odor (skunk fragrance is derived from mercaptans) and by the tendency of unreacted mercaptan residues in the cured coatings to chemically react and bond (via addition) to free acrylate usually present in cross-linkable acrylic pressure-sensitive adhesives The same issues affect hybrid acrylic silicone-mercaptosilicone
Certain “onium” (sulfonium and iodonium) salts are known to be capable of initiating photopoly-merization of epoxides28 and vinyl ethers.29 Epoxy-functional silicones are readily prepared,30 so
performance advantage inherent in epoxy silicone-iodonium salt photocurable systems results from the non-free-radical nature of this cross-linking This particular cure mechanism is not subject to oxygen inhibition, making UV-curable epoxy silicone based release agents particularly well suited to wide web converting operations, as nitrogen blanketing is not needed
The epoxy silicone UV cure system has been shown to combine exceptionally fast UV cure response with premium, stable release versus cross-linkable acrylic, styrene-butadiene rubber, and hot-melt adhe-sives.16 As with other radiation-curable silicone release systems,34 however, controlled release additives capable of providing a broad, predictable range of release for the UV epoxy silicone coatings have remained elusive Another problem associated with these cationic cure silicone materials is substrate-dependent performance Excellent cure, anchorage, and release are obtained when corona-treated films
CH3
CH3 SiO
m
CH3
SiO
X = mercaptan, methacrylate, acrylate, epoxy, vinyl ether
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Trang 1092-10 Coatings Technology Handbook, Third Edition
Con-ference, San Francisco, 1988
21 J D Nordstrom et al., U.S Patents 3,577,256; 3,650,813
22 G Koerner et al., U.S Patent 4,306,050
23 R Viventi, U.S Patent 3,8166,282
24 J Bokerman et al., U.S Patent 4,052,059
25 J A Colquhoun, U.S Patent 4,070,525
26 R P Eckberg et al., U.S Patent 4,558,147
27 F Hockemeyer et al., U.S Patent 4,571,349
28 J V Crivello et al., J Polym Sci., 17, 977, 1047 (1979)
29 J V Crivello et al., in Radcure IV Proceedings, Chicago, 1982
30 F D Mendecine, U.S Patent 4,046,930
31 R P Eckberg et al., U.S Patent 4,279,717
32 R P Eckberg et al., U.S Patent 4,421,904
33 R P Eckberg et al., U.S Patent 4,547,431
35 G R Homan et al., U.S Patent 4,525,566
36 T J Drahnak, U.S Patent 4,510,094
37 R P Eckberg, U.S Patent 4,670,531
38 J V Crivello et al., U.S Patent 4,617,238
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