Epoxy powder coating systems deliver the best chemical and corrosion resistance.. 89.4.2 Pigments and Fillers Most pigments and fillers used in liquid coatings are suitable for use in po
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89.3.2.3 Polyester–Non-TGIC ( β-Hydroxyalkylamide, Tetramethoxymethyl Glycoluril)
As we discussed earlier, TGIC was found to have mutagenic properties While some controversy stillremains as to its hazardous nature, many coatings companies and regulatory bodies have taken a con-servative stance and limited its use There have been regulations enacted that require warnings to beplaced on labels of coatings that contain TGIC Coatings companies have also turned to curing agentsthat do not have these types of hazards associated with them
The two types that are used most often are the β-hydroxyalkylamides and tetramethoxymethyl coluril The hydroxy-amides are tetrafunctional, which makes them highly reactive at curing tempera-tures Their major drawback is that they release water from the reaction that must escape the curing film.This requires further formulation to insure defect-free films The glycoluril is also tetrafunctional andreleases a VOC, methanol, as part of the reaction process
gly-89.3.3 Acrylic Systems
There are two primary acrylic systems: those based on hydroxy-functional acrylic resins and those usingepoxy- or glycidyl-functional polymers Carboxy-functional materials have been produced However,they have not made much progress into the industry
89.3.3.1 Acrylic–Isocyanate (Acrylic–Urethane)
Acrylic–urethanes are formed in exactly the same way as their polyester counterparts The acrylic resinsare linear instead of aromatic However, they use hydroxy-functionality and blocked-isocyanates to formthe urethane bonds
89.3.3.2 Acrylic–Diacid (Glycidyl–Acrylic)
Epoxy- (glycidyl-) functional acrylic resins can be compared to the hybrid systems discussed earlier Theyare generally reacted with dicarboxylic acids or anhydrides The most common cross-linker is 1,12-duo-decanoic acid (1,12-dodecane dioic acid)
CH2
N
CH OH R1 C
CH2 CH
R1 OH O
CH2
N CH
C O
OH +
O C C R3
CH2CH
R1 O
O
C C R3 "
C OH O C O
OH
O
C C R3 "
C OHO
C O
OH
CH2
N
CH O R1 C
CH2CH
R1 O
O C
O R2
O H +
Trang 3Thermoset Powder Coatings 89-11
There are two primary functions for any protective coating They are chemical protection and exteriordurability As with many coating properties, these two tend to be in opposition The best systems forchemical protection are usually the poorest for exterior durability
Epoxy powder coating systems deliver the best chemical and corrosion resistance However, they havethe least effective exterior durability The double bonds in the aromatic rings are easily broken by theultraviolet (UV) light from the sun Glossy finishes will go flat with as little as 6 months of exposure,with film degradation following soon thereafter
Urethane systems are also very good for chemical resistance, and they have fairly good exteriordurability, as well Hybrids offer good chemical resistance, however, the polyester component makes themless effective They are also poor in relation to exterior exposure due to the epoxy portion of the cross-link network
Acrylic and TGIC powder coatings provide the best exterior durability Some systems can survive for up
to 20 years of exposure They offer fair to good chemical resistance Urethane systems seem to be the bestcompromise between chemical and exposure properties As mentioned, they are very good in both areas
89.4.2 Pigments and Fillers
Most pigments and fillers used in liquid coatings are suitable for use in powder coatings.11 There are only
a few special requirements for use They must be sufficiently heat stable so they withstand the heat ofextrusion and curing without degradation or color change Normally, the heat of extrusion is 125°C orless for a minute or two The heat of cure is usually 160 to 200°C for 10 to 20 min
Second, they must be insoluble and nonreactive in the resin system Blooming and color shift are themost common results from pigments that are partially soluble or reactive with the binder Some epoxysystem curing agents are especially susceptible to reaction with pigments
89.4.3 Additives12
89.4.3.1 Flow and Leveling
Flow and leveling agents are designed to minimize surface defects such as craters, pinholes, and orangepeel The mechanism of their function alters the surface tension and rheology of the coating The likelihood
of a smooth defect-free film is improved by reducing one (or both) of these properties in a coating Most flow and leveling agents are liquids Many are blended with inert inorganic materials to offerthem in a conveniently solid form The chemistries of these are usually polyacrylates or polysiloxanes Afew new flow agents are available, however, in solid organic form
89.4.3.2 Debubbling (Degassing)
The most common debubbling agent is benzoin (2-hydroxy-1,2-diphenyl ethanone) It is used to keepthe surface of a curing film open long enough to allow for entrained air and evolved gasses to escape.Trapped air and gas bubbles are cause for premature failure of coating films, because they make thecoating brittle The one drawback to the use of benzoin is its tendency to cause yellowing in lighter colors
A number of new advances have entered the market in an attempt to match the efficiency of benzoinDK4036_book.fm Page 11 Monday, April 25, 2005 12:18 PM
Trang 489-12 Coatings Technology Handbook, Third Edition
89.4.3.3 UV Inhibitors
Various UV light inhibitors are available to aid coating resistance to degradation by the sun’s rays Themost common are hindered amines, phosphites, sulfates, and phenolics Most will have some positiveeffect on any coating’s UV resistance However, each system will require testing to determine the bestcombination of inhibitors Some systems, like epoxies and hybrids, will not develop any substantial UVresistance due to their aromatic nature
89.4.3.4 Catalysts
Catalysts or accelerators are used to reduce the reaction time or curing temperature of the resin andcross-linker They allow for faster production time by shortening the gel or “set” time of the thermosettingcoating Energy can be conserved, because full cure may be attained at lower oven temperatures Themost common catalysts are thiazoles (used in polyesters), phosphines and ammonium halides (used inepoxies), and thiocarbamates (used in urethanes)
2 P G Clements Patent GB 643,691; 9/27/50, Schori Metallizing Process, Ltd
3 E Gemmer, Patent DE 933,019; 9/15/55, Knapsack-Griesheim AG
4 E Gemmer, Patent U.S 2,844.489; 7/22/58, Knapsack-Griesheim AG
5 Patent GB 915,575
6 E P Miller, “Electrostatic finishing methods,” paper presented at the Annual Meeting of theNational Paint, Varnish, & Lacquer Association, Colorado Springs, CO, September 12, 1963
7 D A Bates, The Science of Powder Coatings, Vol 1 London: Scholium Intl., 1990
8 Statistics from Powder Coatings Institute, Alexandria, VA
9 Rohm and Haas Brochure 82F2, Primid XL-552 — A Novel Crosslinker for Powder Coatings, 1990
10 Pulverizing Machinery (product brochure), Mikropul Corp., Summit, NJ
11 R Campbell and R Kumar, “Organic pigments for powder coatings,” Am Paint & Coat J., April
22 (1991)
12 Josef H Jilek, Powder Coatings (monograph) Blue Bell, PA: Federation of Societies for CoatingsTechnology, 1991
Type Typical Applications
Epoxy Shelving, transformer cases, primers, bathroom fixtures, refrigerator racks, sweepers, sewing machines, power
tools, room air conditioners, office furniture, instrument cases, garden tools, kitchen furniture, fire extinguishers, toys, refrigerator liners, dryer drums, microwave ovens, mixers and blenders, fertilizers spreaders, screening, oil filters, automobile springs, hospital equipment, bus seat frames, business machines, glass bottles Hybrids Tool boxes, farm equipment, electrical control boxes, hot water heaters, hot water radiators, primer/surfacers,
grain storage bins, transformer covers, 01.1 filters and air cleaners, air conditioner housings, fire extinguishers, toys, screening wire, power tools, shelving, office furniture
Urethane Fluorescent light fixtures, steel and aluminum wheels, patio furniture, playground equipment, fence fittings,
chrome wheels and trim, garden tractors, range side panels and broiler, ornamental iron, air conditioner cabinets, restaurant furniture supports, transformer cases
TGIC Irrigation pipe and fixtures, outdoor furniture, air conditioning units, steel and aluminum, wheels, wire fencing,
fence poles and fittings, farm equipment, aluminum extrusions, transformers Acrylic Range side panels, refrigerator cabinets and doors, washing machine parts, dishwasher exterior, aluminum
extrusions, microwave ovens, garden tractors, automotive trim coating
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Trang 590 Peelable Medical
Coatings
90.1 Introduction 90-190.2 Cold-Seal Coatings 90-290.3 Heat-Seal Coatings 90-2
90.1 Introduction
Prepackaged sterile medical devices and supplies became necessary in the late 1960s with the growth inprepaid health insurance programs Insurers required that health care providers itemize the cost of allthe supplies used during a procedure This led to the rapid growth in the development of disposable orsingle-use devices These were packaged in paper/plastic pouches, trays, or containers and then sterilized
At the time of use, the package was torn open to expose the sterile device When the package was torn,the device was showered with particulates and bacteria that caused a great deal of concern At that time,the medical device industry was not regulated by the U.S Food and Drug Administration (FDA)
In 1968, the E I DuPont Company introduced a polyethylene, paperlike material (Tyvek®) that hadmost of the properties needed for packaging medical devices Tyvek is a unique material that meets almostall of the critical requirements for medical packaging It is a good bacterial filter — very porous, water-resistant, and puncture- and tear-resistant Being made from polyethylene, it is stable during both ethyleneoxide gas and radiation sterilization It does not stand up well during steam sterilization, so only a fewadhesives have been developed for this purpose
With an acceptable packaging material available, peelable coatings were developed to seal Tyvek toplastic films or thermoformed trays to form pouches and trays that could be peeled open at the point
of use without compromising the sterility of the device
There are five basic types of adhesives used to seal Tyvek to plastic surfaces to form sterile packages.These are cold seal, lacquer-based heat seal, water-based heat seal, hot-melt-based heat seal and low-density polyethylene All of these adhesives are very difficult to formulate and apply so that they meetall of the requirements of the medical device manufacturers
Some of the requirements these adhesives must meet are as follows:
• Must peel cleanly without generating particles
• Have a peel strength over 1 lb per inch of width and less than 3 lb per inch of width regardless
of the peel angle
• Be very stable both before and after sterilization (shelf life requirements can be as long as 10 years)
• Meet the U.S Pharmacopoeia requirements for medical device plastics
Donald A Reinke
Oliver Products Company (Retired)
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Trang 6Peelable Medical Coatings 90-3
integrity has been maintained A clear track through the seal area will cause the package to be rejected,because there is a path for bacteria to enter the package
Formulating a coating that is nontoxic and strong, with good hot tack and good adhesion to a variety
of plastic surfaces, and that is able to peel with a controlled peel strength, is a difficult task
Several methods have been used to achieve peelability The use of primer coats between the adhesiveand the substrate is the one most often chosen; this creates a parting layer between the adhesive and thesubstrate Release coating on the substrate surfaces are also used when paper is used instead of Tyvek.Most people in the industry feel that the best method is to have an adhesive with a controlled cohesivestrength When the package is peeled open, the adhesive will split with a controlled force that is belowthe delamination strength of the substrate By using this method, the surface fibers of the substrate arenot raised or broken
Cohesive failure of the adhesive can be achieved by formulating an adhesive that has two phases: one
a strong adhesive to hold the package together and the other a weak friable material that breaks up thestructure of the first phase By varying the percentage of the two phases, the cohesive strength of thecoating can be controlled within narrow limits
The nature of peelable medical packaging materials is currently undergoing a change There is a drive
to reduce the cost of medical devices, causing a shift to paper and away from Tyvek Also, environmentalconcerns are pressuring medical device companies to use some method other than ethylene oxide gassterilization The requirement for a peelable package, however, remains strong
The medical device industry is now under the control of the FDA, which requires complete validation
of processes and materials Formulating and validating a new adhesive will generally take from 1 to 3years For this reason, packagers of medical devices are very reluctant to change suppliers They do nothave the engineering staff they had in the earlier years when the industry first started To change anadhesive and develop the documentation required by the FDA is very expensive Most companies havealternate suppliers they can use if they have problems
Further information on medical device packaging can be found in the proceedings of the TechnicalAssociation of the Pulp and Paper Industry’s Coatings and Laminations conferences during the yearsfrom 1980 through 1987
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Trang 791 Conductive Coatings
91.1 Introduction 91-191.2
91.4 Applications 91-6
91.5 New Developments 91-7References 91-8
91.1 Introduction
In 1986, sales in the coatings industry exceeded $10 billion, and production approached a billion gallons.1The breakdown of sales was $4.1 billion for architectural coatings, $3.5 billion for industrial coatings,and $2.4 billion for specialty coatings Conductive coatings — a minuscule part of these trade sales —have been used both as industrial coatings and as specialty coatings Regulations of the Federal Com-munications Commission (FCC), in Docket No 20780, which regulates electromagnetic emissions fromcomputing devices, have provided a strong impetus for the commercial development of conductivepolymeric materials (including coatings and paints) Since October 1, 1983, it has been necessary for anycomputing device that generated signals or pulses in excess of 10 kHz to comply with the emissionstandards set forth in the docket Although conductive polymeric coatings have made inroads in areaswhere metallic coatings previously were used, progress has been slow
A product related to conductive coatings is metallized plastic The most important commercial cesses for metallizing plastics are electroless plating, metal spraying, sputtering, and vacuum metallizing.The first commercial plating of plastics was recorded in 1905.2 Metallizing of plastics occurred duringWorld War II, and large-scale production started in the early 1960s All these processes are now multi-million-dollar industries Large quantities of plastics are metallized each year, with automotive itemsmaking up more than 60% of the market on a plated area basis.3
pro-There are various reasons for metallizing plastics In the automotive industry, metallized plasticcombines the consumer appeal of metal with light weight Electroless copper metallization is an indis-pensable part of the modern electronics industry Printed circuit boards use electroless copper to coatnonconductive plastic surfaces to define the circuit patterns Zinc arc and flame-spray techniques provideelectromagnetic interference shielding on many plastics The plastics that account for most of the sub-strates metallized are acrylonitrile-butadiene-styrene (ABS), polypropylene, polyphenylene oxide,epoxies, phenolics, polyimides, and polyesters The commercial process for metallization of plastics meritsseparate discussion and is not further considered in this chapter
Polymers (coatings) with conductivities greater than 1(Ωcm)–1 are defined as conductive polymers(also metallically conducting plastics, synmetals).4 Unfortunately, the literature is not clear-cut, and often,materials that are semiconductors with conductivities less than 1(Ωcm)–1 are also called “conducting.”
Raimond Liepins
Los Alamos National Laboratory
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Types of Conductive Coatings 91-2
Miscellaneous
Trang 8Conductive 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
Com-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 intohigh-temperature polymeric materials” (submitted for patent)
59 T Hioki, S Noda, M Kakeno, A Itoh, K Yamada, and J Kawamoto, in Proceedings of the national Ion Engineering Congress, September 12–16, 1983 Kyoto, Japan, 1984, p 1779.
Inter-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 992 Silicone Release
Coatings
92.1 Introduction 92-1
92.4 The Future 92-9References 92-9
92.1 Introduction
Silicone release coatings are vitally important to the tag and label industry, which could not exist in itspresent form without reliable release agents Silicones possess unique physical and chemical propertiesthat make this class of substances ideal for the purpose of releasing pressure-sensitive adhesives Siliconerelease 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 polymersmay be devised with different organic groups replacing methyl, or with three-dimensional resin structureswherein 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 Thechemistry and applications of silicone release coatings have undergone remarkable change during thistime, with the pace of development accelerating in recent years In the face of increasingly sophisticatedand demanding requirements, silicones remain the only proven means of providing pressure-sensitiveadhesive release for the tag and label industry
The liner most often used is paper, usually a machine-calendered (i.e., supercalendered kraft), coated, or glassine paper designed to minimize penetration during coating and curing of silicone Good
clay-CH3
CH3SiO
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 10Silicone Release Coatings 92-3
where n can vary from about 50 to more than 4000, while m is much less than n; m normally is 10 to
50 SiH is a very reactive chemical species that readily condenses with silanol (SiOH) groups, formingextremely stable siloxane bonds and liberating hydrogen in the process:
≡SiOH + ≡SiH→≡SiOSi≡ + H2Many 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, ormixtures thereof), or as oil-in-water emulsions, because in the absence of a dispersing medium, acatalyzed 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 ofretarding the condensation reaction sufficiently at room temperature to permit solvent-free coatingwithout rendering the composition uncurable at oven temperatures Solvent (or water in the case ofemulsions) 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 entailsfire 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 supplierwide 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 furtherdilution to about 5 wt% silicone solids with more solvent to render them coatable The cross-linker isnormally 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 thecured condensation-cross-linked coating Long chains of polydimethyl-siloxane between cross-linkingsites provide a rubbery, elastomeric coating; shorted intercross-link intervals lead to higher cross-linkdensity and a harder, more resin-like coating The rubbery coatings provide tight (high) release, whichdisplays 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 offerseveral 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 isAddition cure silicones resemble condensation cure silicones in some respects: both types of systemrely on thermally accelerated cross-linking reactions between polymethyl-hydrogen siloxane cross-linkermolecules and a separate reactive dimethylsiloxane polymer Addition cure processes utilize catalyzedreaction of unsaturated organic groups attached to otherwise unreactive dimethylsilicones with SiHgroups 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 ahydrosilation process:
DK4036_C092.fm Page 3 Thursday, May 12, 2005 9:55 AM
graphically shown in Figure 92.2