Except for system cleanup, radiation- cured coatings generate essentially no VOCw Disadvantages of waterborne coatings include greater needs for surface cleaning and preparation, elimination of dirt from the work area, and temperature and humidity control.
B. RADIATION CURABLE COATINGS
Coatings that are cured through use of ultraviolet or electron beam exposure are referred to as radiation curable coatings. Generally these coatings contain very little or no organic solvent and will not dry upon exposure to the air. Instead they cure on exposure to ultraviolet (vv) or electron beam radiation. Essentially no VOCs are emitted during curing, which is a major advantage of these types of coatings. Other advantages include very rapid curing, very little energy input, and the ability to collect and reuse overspray. However, system cleanup does involve the use of solvents.
Radiation curable coatings are 100% reactive, i.e., there are no volatile solvent losses, and the liquid coating supplied to the substrate is totally converted into a solid cross-linked film. Since there are essentially no hazardous solvent emissions associated with the conversion process, radiation curable coatings are pollution free and are not energy intensive processes. Another advantage of radiation curable coatings is that in the curing process electrical or light energy is absorbed only by the coating and is not wasted in heating the substrate, as in the case with conventional thermally cured coating systems. This efficient use of energy allows radiation curable coatings to be applied and processed on heat sensitive substrates, resulting in finished products requiring relatively simple manufacturing operations. (28)
The coatings industry comprises the manufacture, sale, and use of clear and pigmented finishes which protect, decorate, and provide functional properties to a wide variety of surfaces and objects. The project line for this industry can be divided into three general categories: trade sales, industrial finishes, and special-purpose coatings. Trade sales coatings are formulated for normal environmental conditions and find general applications on new and existing residential or commercial building structures. Industrial finishes are usually formulated for original equipment manufacture (OEM) and can be applied to products as part of the manufacturing process. Special purpose coatings are designed for field applications, such as refinishing, or for extreme environmental stress conditions, such as high temperature and corrosion.
III-14
A generalized product line and product use description for this industry is shown in Table 4-1.
Radiation processing of coating materials is almost exclusively associated with the industrial finishing market area; major emphasis is on wood finishing, metal coatings or decoration, and paper or plastic film coatings, with limited use in wire and automotive applications. (47) Radiation curable coatings offer several advantages over conventional thermally converted solvent-based coatings systems. In conventional thermal coatings technology a polymer and reactive cross-linking oligomer is dissolved in a nonreactive diluent solvent. The ratio of solvent to polymer-cross-linking oligomer is usually 50 to 60% of the total coating system. This low viscosity liquid composition is then applied to a substrate and baked in a gas-fired oven which removes the solvent and sets the polymer cross-linking oligomer into a solid three- dimensional cross-lined network or finished coating. This process is energy intensive, since most of the thermal input energy goes to heat the substrate and remove the nonreactive diluent solvent. Additional thermal energy is also required to activate the cross-linking reaction of the polymer with the cross-linking oligomer in order to effect cure. In many cases the substrate is heat or moisture sensitive and a thermally cured coating operation causes shrinkage-warpage or dehumidification of the substrate which requires an additional manufacturing step to produce a usable finished product. The nonreactive diluent solvent used in conventional coatings technology usually is vented into the atmosphere (which causes pollution), burned, or recycled to make up part of the thermal energy of the oven used to cure the coating syste-
In radiation curable coatings a reactive polymer and coreactive cross-linking oligomer are dissolved in a completely coreactive diluent solvent. This mixture is applied in a similar manner as a conventional thermally cured coating but is cured ...
111-15
Table 4-r
RADIATION CURABLE COATINGS FOR WOOD FINISHING APPLICATIONS (30.36,50)
Coating Formulation Curinq Conditions ADDlication
Alkyds, polyesters, urea- Infrared oven 90 to Filler, base-coat formaldehyde, vinyl s , 120 sec cure times and top-coat acrylics, urethanes; 30- varnishes 65% solids (solvent or water based); clear poly- mer systems or containing. pigments
Acryl ated polyester resin, hexanediol diacrylate, vi nyl pyrrol i done, photoini ti ator
Same as above but add silica or titanium dioxide pigments
65 wt percent unsatu- rated polyester, 35 wt percent vinyl monomer: 2-ethyl hexyl acrylate or styrene acryl ic copolymers containing pendant vinyl unsaturation (unsaturation levels, 0.5-1.75 mol of double bonds per 1000 mol wt) and 35-45 wt percent of a vinyl monomer: 2- ethyl hexyl acrylate or styrene
Acryl i c monomers : acryl i c unsaturated epoxy ana acryl ic unsaturated poly- urethanes monomers: poly- functional vinyl intermediates
UV processor single lamp, cure time of 10 seconds
UV processor sing1 e lamp, cure time of 10-30 seconds
Cured with 300 keV electrons at 200 kGy/mi n (20 Mrad/mi n) cured with a total dosage o f 150 kGy (15 Mrad) electron beam;
El ect ron - curt ai n curing
C1 ear varnish
Filler or base- coat
Coatings for vinyl covered flat board stock
1. Ultraviolet Coatings
W curing uses high-intensity W light to initiate the free radical crosslinking of acrylate oligomers and prepolymers. W curable coatings are limited to applications where coatings of roughly 200 microns or less are useful. The use of W coatings is limited to clear lacquers or semitransparent applications. Some pigments, such as titanium dioxide, are strong absorbers of W light and their use retards curing. W
W coatings are very thin; they are limited to
coatings are used primarily on paper to achieve a high-gloss, transparent finish at curing times as fast as a fraction second. W coatings are also
nai surfaces. used on metal and plastics. W curing of coatings is a line-of-sight process and is limited to flat surfaces, though in some instances three- dimensional objects have been W cured.
Ultraviolet curing is a process in which ultraviolet (vv) light reacts with photosensitizers in the coating to create free radicals which initiate cross-linking to form a solid film. The main components of an ultraviolet curable coating are an ultraviolet-curable base polymer, diluent monomers, and ultraviolet photochemical initiators. Ultraviolet light for curing is effective on flat surfaces where the light reaches the surface vertically. When the ultraviolet light strikes a surface at an angle closer to the horizontal, the amount of absorbed light is too small for effective curing.
curing of finishes include: The advantages of W curing over conventional fossil energy-heated
0 Rapid drying speeds (seconds or less).
Reduction or elimination of organic solvents, thus eliminating air pollution and incineration problems.
111-16
Significant reduction or elimination of fossil energy-heated drying ovens and incinerators.
e Increased production rates.
e More efficient use of polymeric coating materials because of less penetration of flowing material into substrates.
e Savings in space of application equipment.
The cost of a UV curing system will vary from $4,200 to $200,000 depending on the "ope of the operation (Chemical and Engineering News 1991).
The use of radiation curable coatings is a significant alternative for the reduction of energy consumption and compliance with air pollution legislation for this industry. The cost comparison and energy efficiency data for processing of a wood filler vehicle using W and infrared processing equipment is given in Table 5-1. The W processor and associated coating system result in lower product production costs with a substantial savings in energy over the thermal curing coating operation. The overall power consumption costs per square foot of product are approximately 0.1 for the W system, and 0.2 for the IR system. (123,124)
One of the major commercial successes in radiation processing over the thermal processing techniques for coatings and inks has been demonstrated by the can industry. In the U.S., Adolph Coors Company and American Can Company have been the leaders in utilization of W curable inks and overprint varnishes for beer and beverage can products. Coors has reported that 246% more energy is required to thermally cure conventional solvent or water-based inks or coatings than to cure ultraviolet sensitive inks and coatings. A typical can line (Figure 4-5) operating at 600 to 2,000 cans/min on a 7 hr/day, 250-day work year using a W dryer would save over 1 billion Btu a year with an approximate production rate of over 63 million beer cans annually. An estimate of energy savings and comparison of U V versus thermal drying of inks and coatings based on Coors experience is given in Table 5-2.
-
111-17
Table 5-1
UV AND I R PROCESSING COST AND ENERGY EFFICIENCY DATA (123)
Oven length ( f t )
Line speed (fpm)
Wood f i l l e r veh ic le
Nonvol a t i 1 e (%)
F i lm thickness (mi ls)
Coverage (wet) (sq f t / g a l )
Coats
Cure t ime (sec)
E x i t temp (OF)
coo 1
Cost o f f i l l e r ($/gal)
Cost per sq f t ( I # ) Per coat Total
Power (kw)
Power per sq ft (4)
Surface appearance
Hardness
Sandi n9
Typical UV
10-30
60- 150
Po 1 yes t e r
90- 100
2
700-800
1
10
100
No
5 .00-6.00
0.7-0.9 0.7-0.9
100
Less than 0.1
Ex (one coat)
Ex
Ex
Ex = excel lent , G = good, VG = very good
Typica l I R
90
60
Urea-A1 kyd
35-65
2
500
2
90
130
Yes
2.00-3.00
0.9-1- 3 1.8-2.6
2 50
App. 0.2 (two coats)
G (two coats)
G
VG
4
American Can Company has had similar energy savings experience (over 100 billion Btu saved in going from natural gas-fired ovens to W radiation processing equipment) with W curable inks and coatings in a system completely different in chemistry than that of the Coors coating. The Coors coating system is cured through a free radical mechanism; the American Can coating system is cured through a photoinduced cationic ring opening epoxy resin reaction mechanism. @3,54,124)
Table 5-3 summarizes data from a hypothetical cost model associated with the use of a thermal oven, an electron curtain processor, and a W curing chamber. (125) Each system is designed to process 180 million square feet of aluminum coil stock per year (1 mil thick coating) at line speeds of up to 150 feet per minute. In this model the hourly processing costs and cost per square foot of product produced for the conventional thermal coating/curing operation are approximately twice those of either radiation process. Coating material costs associated with this model are approximately 1.18 cents/square foot for a conventional solvent-based (35% solids) alkyd varnish. This is approximately equivalent to a 100% solids EB/UV varnish costing 1.17 cents to 1.19 cents/square foot.
ULTRAVIOLET (vv) LIGHT SOURCES
Ultraviolet (uv> radiation is the part of the. electromagnetic spectrum having wavelengths from 4 to 400 nanometers. The basic energy source for initiating reactions of UV responsive materials is the mercury vapor lamp. The major lamp systems in commercial use today are as follows:
Low mercury pressure (10” torr) germicidal lamps.
ID-18
Table 6-1
RADIATION PROCESSING END USE MARKETS AND PRODUCTS (E)
End Use Markets Products
Graphic Arts Inks (UV, EB) Decals (UV) Photopolymer plates (UV) Transfer letters Reproduction films (UV) Paper release coatings (EB)
Photopolymer plates (UV) Overprint coatings (UV, EB) Shrink films (EB) Closures (UV) Barrier coatings (UV, EB)
Tapes (UV, EB) .
Coated foils (UV, EB) Coated films (UV, EB)
Cosmetic cartons (UV) Liquor cartons (UV)
Bottles & bottle caps (UV)
Cans (UV)
Packaging Inks (UV, EB) Album jackets (UV, EB)
Labels (UV) cups (UV)
Consumer Magazi nes (UV) Furniture (UV) Catalogues (UV) Appliances (UV, EB) Book covers (UV) Lami nates (EB) Credit cards (UV) Name plates (UV) Decorative mirrors (UV) Flocked fabric (EB) Plaques (UV) Footwear (EB) Flooring (UV) Permanent press (EB)
Transportation Nameplates (UV, EB) Laminations (EB) Assembly parts (EB, UV) Replacement parts (UV, EB) Decorative finishes (UV, EB)
Conductive coatings (UV) Electrical insulation (EB)
Construction Panels (wood & particle Binders for abrasives (EB)
Flooring (UV) Electrical insulation (EB) Wal lpaper (UV)
El ectroni cs Printed circuit inks (UV) Electrical insulation (EB) - marking (UV) - etching (UV) - solder masks (UV) Photopolymer plates (UV) Encapsul ation/conformal Photopolymer masks (UV) coatings (UV, EB)
Fiber optics (UV) Magnetic tapes (EB)
board) (UV, EB) Lami nations (EB)
Coil coated stock (EB)
Photo resists (UV) Wire coatings (UV, EB) Conductive coatings (UV, EB)
Communication Speakers (UV) Dielectric coatings (EB) Electrical insulation (EB) Wire & coil bonding (EB)
Product usual ly prepared by u 1 traviol et (UV) or electron beam (EB) radiation processing conditions.
Medium pressure (1 to 2 atmospheres) mercury vapor lamps having electrode configurations for operation.
0 Medium pressure mercury vapor lamps activated by microwave energy radiation and thus do not require electrodes (electrodeless lamp operation developed by Fusion Systems Corporation).
Flash lamps or pulsed xenon gas arcs.
Metal doped or hybrid xenodmercury vapor lamp systems.
Description of the power supplies, typical lamp configurations, and design considerations for low mercury pressure, medium mercury pressure, and flash lamps are given in Figures 2-11 and 2-12. General operating characteristics for several of these lamp systems are listed in Table 2-3. (7)
The two major lamp systems having commercial radiation processing significance today are the conventional electrode and electrodeless (Fusion Systems Corporation) medium pressure mercury arcs. A direct operational characteristic comparison between each lamp is given in Table 2-4. (8) Either W lamp system is housed in a reflector and must be cooled with air or water to promote efficient lamp operation and a reasonable life expectancy. A typical linear array of electrode lamps, electrodeless lamp system, reflectors, and methods of cooling is shown in Figure 2-13. Further developments in cooling, gas inert blanketing, filtering of unwanted excess infrared radiation (which is always present in the output spectra of a mercury lamp), and novel lamp housing or equipment have been pioneered by Union Carbide Corporation. A complete review of light sources used in photo-processing application is given in references 3 and 7. Examples of typical commercial installations for W radiation processing equipment are described in Table 2-5. (9-13)
ID-19
Thermionic Cathode
Starting Switch
Ballast A-C Supply
Power Supply Circuit for a Low Pressure Mercury Arc
Capacitor
Transformer Power Supply for a Medium Pressure Arc
A
Transformer
R e C t i f i e r
Spark Gap U
Power Supply for a Flash Lamp
Figure 2-11. UV/Vis Light Source Power Supplies (1)
input Power-
Cap Connector
Molybdenum Ribbon Connection Tungsten Wire
Quartz Mounting Stub I
Diagram of a Medium Pressure Mercury Lamp
6 mm ID
Electrodeless Lamp Bulb Activated by Mlcrowave Energy (Fusion Systems Corp.)
8 mm ID
Air fC\ (5) H20 H20 Air (5)
(1) UV Source: Linear Electrode Lamp (200/300 w/in) (2) Power Supply: 1.5 KV AC > 90% Efficiency (3) Reflector (Parabolic or Elliptical)
Power Main
I-
,
(4) Energy Profile (5) Cooling (Air, H20) (6) Housing: Radiation Containment (7) Conveyor Bed
J + + -00
t t Blower -/
Ref lector
Negative Positive Air Cooling Air Cooling
Power Supplies
Controller
Radiator (10'' long 16" tall 4" -wide) - Exhaust
Power Cable
Reflector
Electrodeless Lamp Curing System (Fusion Systems)
Figure 2-13. Commercial UV Processor Un i t s (8)
Table 2-3
UV LAMP OPERATING CHARACTERISTICS (8) -
Microwave Low Pressure Medium Pressure Energized
Mercury Di scharqe Mercury Mercury
coo 1 High High
Flash Xenon
Moderate (Water Cooled)
.l to 10 Mega- Watts Peak Power
.6 - 30 Inches Linear, Ci rcu 1 ar , He1 ical
High
Lamp Temp.
Lamp Power 1 - 10 Watts/In. 100 - 400 Watts/In. 300 Watts/In. -
Arc Lengths 10 - 75 Inches 1-1/2 - 77 Inches 10 Inches
Bulb Shapes Li near , Linear , Curved Linear Ci rcu 7 ar
Re 1 at ive System Costs
Low Moderate High
Input Power Lamp Warranty
1 - 10 Watts/In. 110 - 440 Watts/In. 550 Watts/In. 17,500 Hours 1,000 Hours 3,000 Hours
-- 1,000 Hours
Major Output Wave1 engths
Spectral Variations
("1 254
None
365, 436, 546, 365, 636, 580 546, 580
4 50
Limited Moderate Extensive
Spectral Efficiency
Radiant Efficiency
Excel 1 ent Good Very Good Poor
Poor Very Good , Good Fair
Overall E f f i CI enty
Practical Limits
Fair Good Good Poor
Low Efficiency Low Intensity None Limited Sizes
Total System Cost $4,000 $200 Under $2,000 $3 to 7,000
Table 2-5
UV CURING EQUIPMENT ( 9 ) -
UV Curing Device Descriptions
Small (15" maximum); 1 or 2 lamps
Medium (18" and up); multiple lamp systems
Large multichamber UV drying ovens (80" wide to 60' long with as many as 12 rows of lamps)
UV lamps mounted over belts or rollers so that the substrate is cured and stacked
38" wide in-press sheet curing UV systems; narrow and wide continuous web UV curing systems--flex0 and letterpress assemblies (UV lamps can be used 80" in length)
Multiple lamps housing assemblies
End Use Applications
Laboratory and in-plant ink, coating and adhesive testing.
Production curing of electronic components; erasing of EPROM computer chips and curing of areas with point source lamps.
Heavy substrates, multiple lamp systems for curing glass, metal and wood; for curing in PC boards and photoresist systems for the electronics industry and multiple lamp sys- tems for paper and plastics (screen, letter- press offset or flexoprinted).
Floor tile, electronics, special textured coatings, abrasion resistant coatings and combinations of U V - I R for the graphic arts industry on paper, board or glass substrates.
Sheet paper or board stock; screen printing and circuit board manufacture.
Graphics art i ndustry--sheetf ed mu1 t icolor presses, wet-on-wet printing o f UV clear coatings over UV or solvent-based net off- set inks--tag and label products and wall- paper or linoleum substrates.
UV curing of three-dimensional objects such as cups, lids, wire, opt ica l fibers, tiibzs, PC boards containing attached components (compound coatings), partial and fully assembled furniture, tabletops, and she1 vi ng.
2. Electron-Beam Curable Coatings
Electron beam (EB) curable coatings offer an advantage over W curable coatings in that thicker and more opaque coatings can be developed on a workpiece. EB curables utilize a monomer/prepolymer or a monomer/slightly polymerized mixture that is low in viscosity but contains no organic solvents. The coating cures (polymerizes) when exposed to a focused electron beam. Curing times are instantaneous, and almost no heat is generated in the item being coated. For high-gloss applications, time must be allotted after application to allow the coating or ink to flow out and level, before entering the instantaneous cure step.
Like the W coatings, the electron-beam process is good for line-of- sight applications only, and is limited to flat surfaces. The EB process is relatively new and expensive. It has seen limited use in high-volume printing applications, and is also used on wood and plastic substrates. It also is used to finish automotive hubcaps and wheel rims.
$1 million mark (Chemical Engineering News 1991). A basic system costs $6OO,ooO, most systems installed approach the
HIGH ENERGY ELECTRON RADIATION PROCESSING EQUIPMENT
Electron-beam processors are used commercially to cross-link polymers, insulations, and wire-cable coverings, taking advantage of their ability to penetrate very thick coatings. The basic components or subsystems that make up a high-energy electron processor are a power supply (DC or RF), a source of electrons (e.g., a heated wire filament or ribbon), a beam acceleration system, a vacuum chamber (lo4 to lod torr), output windows, and shielding or housing required to contain the X-rays generated by the high-energy electrons impinging on the surface enclosing the electron accelerator (Figure 2-14). (2) Accelerators can be classified according to their rated terminal voltage power supply requirements which range in value form 0.25-30 MeV. The LINAC accelerator and several electrostatic accelerator systems (Van de Graaf, Pelletron, Laddertron) require very high terminal voltage power supply operating potentials (minimum 4 MeV for the LINAC; between 0.3 to 30
EB coatings are thicker than W coatings; generally they too are limited to flat surfaces.
m-20
MeV for electrostatic accelerators). However, these devices are not normally used for ...
The EB process is relatively new, and installed systems are expensive.
W-VISIBLE LIGHT PROCESSING CHEMISTRY
The treatment of polymeric ultraviolet (vv) or visible light radiation falls into two classes: 100% reactive liquid systems (coatings, inks, adhesives) and photosensitive preformed polymer structures. In this context there are five characteristics of W and visible-light energy irradiation or photocuring of liquid photopolymer systems. (33)
A stable light source is required, capable of producing W and visible wavelengths of light, Le., near and far W, 200-400 nm to visible, 400-700 nm, with sufficient power or intensity to be commercially feasible. (7)
A photoinitiator is required, capable of absorbing W and visible- light radiation at appropriate wavelengths of energy as emitted from the light source (Table 3-2). (34)
Active free radicals or acid intermediates must be produced through the action of light absorption by the photochemically active photoinitiator. The free radicals initiate polymerization of unsaturated monomers, oligomers, and polymers; the photochemically liberated acid intermediates initiate cationic or
m-21
Table 3-2
PHOTOINITIATORS USED IN ULTRAVIOLET RADIATION CURABLE POLYMERIC MATERIALS (34)
Free Radical Photoinitiators
0 Alkyl ethers o f benzoin
0 Benzil ketals
0 Acetophenone derivatives
0 Ketone-amine combinations
0 Halogenated conpounds
Cationic Photoini tiators
e Diazonium salts of Lewis acids
0 Aryl iodonium salts o f Lewis acids
e Aryl sulfonium salts o f Lewis acids
ring opening polymerization reactions of epoxy functional monomers, oligomers, and polymers.
Unsaturated, high boiling acrylic or methacrylic monomers, oligomers, cross-linking agents, and low molecular weight polymers comprise the fluid, low viscosity, light-curable coating system and are analogous to the coating materials used in thermal curing processes. Low molecular weight and high molecular weight epoxy resins (cationic curing mechanism) would also be formulated in a similar manner as the unsaturated materials (free radical curing mechanisms) (Table 3-3). (33)
Free radical initiation or cationic ring opening reactions of the reactive liquid system and propagation into a fully cured, cross- linked solid coating or film. (35)
The mechanism of cationic curing and free radical curing is outlined as follows.
UV-Vis photoinitiator (PI) .-> PI. 1 ight energy free radical intermediate
-x PI. + I ,+ mu1 t i functional unsaturated monomers and polymers
three dimensional network structure
m-22
Table 3-3
MATERIALS USED IN RADIATION (UV AND EB) CURABLE COATINGS (33)
- Free Radical Curinq Mechanisms
0 Single-functional vinyl monomers 2-ethylhexyl acrylate, styrene, N-vinyl- pyrrol idinone, vinyltoluene, lauryl methacrylate
0 Multifunctional vinyl monomers 1 -6-hexanediol di acryl ate, tetraethyl ene glycol diacrylate, trimethylolpropane triacryl ate, pentaerythri to1 tri acryl ate
0 Unsaturated polymers mal ei c-fumari c acid unsaturated polyesters, acryl ic copolymers containing pendant vinyl unsaturation, epoxy acrylates, polyurethane acrylates
Cationic Curins Mechanisms
0 Single-functional monomers
0 Mu1 ti functional epoxide monomers
vinyl methyl ether, lauryl epoxide
diepoxides or triepoxides phenolic and polyhydroxy a1 coho1 compounds
3. Process Description in Wood Finishing Operations
Wet System
Filler coating
Base coat
Grain print ink or lamination with wood veneer
Top coat
The wood finishing industry can be roughly divided into wood furniture and fixture finishes (three-dimensional coating processes) and wood or composition board (particle board) flat stock finishes for use in the manufacture of decorative panels or furniture.
The traditional method of finishing these products is through forced hot air or infrared drying oven thermal treatments of volatile solvent or water-based coating materials. At the present time the major interest of non-IR radiation processing (UV and EB 100% reactive coating systems) is in flat stock materials; a future trend is modification of these systems for three-dimensional coating applications. (48-52)
Two systems have been developed for finishing of flat stock for furniture or paneling. The requirements for these coating/ink/adhesive systems are as follows:
Dry System
Filler coating
Lamination adhesive
Decorative paper, polystyrene (PST) or polyvinyl chloride (PVC) film
Top coat
Wet finishing involves direct application and curing of liquid sealers, varnishes, and paints; dry finishing involves application of a prefinished decorative paper or piastic film to the board surface. j52j A pictorial diagram for each system and the individual coating functions is shown in Figure 4-2. A typical radiation processing wood finishing line is described in Figures 4-3 and 4-5. (50) A summary of the coating chemistry associated with thermal or IR, W E B polymer materials for the wood finishing industry is described in Table 4-2.
III-23
WET FINISHING
Gluing on Filled Board
1
Panel of Decorative
Wood
DRY FINISHING
Wood Lamination Clear
Coat TOP
Tinting I With Color Wood Veneer
Preparation of Continuous Sheets
TI Embossing Printing a Impregnation
Figure 4-3. Wood F in i sh ing Operations ( 5 0 )
I ' I
2 3 3 3
Varnlshlng of both faces: Speed - 15 to 30 mlmln
Total length of the llne - 60 to 65 m.
1 - Tinting of the wood (solvent
2 - Thermal oven 3 - Sanding and vacuum cleanlng
AI , A2 - Appllcatlon with roller-
UV - 2 to 4 UV lamps of mln 80 Wlcm (both faces)
A3 - Appllcatlon with roller-coater of 15 glms on one face only
base)
coater of 15 g/m*
F i g u r e 4-4 . Varn ish ing and UV C u r i n g / F i n i s h i n g L i n e f o r F l a t Stock Wood Products 1 (50)
1 - Sandlng and vacuum cleaning 2 - Application of pigmented flller
with reverse roll coater (25 to 150 glm2)
3 - Electron beam Irradiator 4 - Application of finishing paint
with curtain coater (80 to 120 glmz)
5 - Concrete shleldlng 6 - lneti gas unit
Figure 4-5. Electron Beam Curing L i n e f o r Wood Products (50)
The advantage of using W E B radiation processing technologies over IR or conventional thermal cure systems is that the resultant product can be more readily manufactured; in some cases a superior product can only be achieved through the use of low-temperature high- energy curing methods. In conventional thermal curing coating systems the board is also heated and subsequently dried out, which requires cooling and sometimes rehumidification before shipping to a furniture manufacturer. With W E B curing the board is finished essentially at ambient temperature without a major loss of moisture content. Hence, the substrate can be processed and shipped immediately to the furniture manufacturer. Another advantage to low thermal energy coating curing processes is their ability to finish heat sensitive substrates such as plastics, paper, or polyvinyl chloride (PVC) vinyl films. At the present time there are approximately 100 U.S. wood finishing or manufacturing companies using W light curable coatings and only two or three U.S. companies using EB processing equipment for manufacture of high performance low pressure laminate finished wood products. The reasons for this division in radiation processing utilization are product performance constraints and economics. Infrared curing or other forms of thermal curing of coatings are still widely practiced by the wood industry. However, W processing can offer several major advantages which will be discussed in the cost comparisons section of this report. In cases where only a very special product is produced, such as heavily pigmented panels or low pressure laminates, the initial high cost of EB processor equipment can be justified. A typical EB wood finishing line is shown in Figure 4-5 and EB versus thermal cure finished product performance comparisons are shown in Table 4-3. (50,52)
In the wood finishing industry (flat stock) the consumption of UV radiation curable coating systems for 1974-75 was 6,803 metric tons or 15 million pounds of material. This converts to approximately 1.1 million gallons of coating, assuming a density of 8 pounds/gallon for clear finishes (10% of the market or 1.5 million lbs = 200,000 gallons) and a density of 15 poundslgallon for filler coating (90% of the market or 13.5 million pounds = 900,OOO gallons). (124) In 1981 the consumption of UV cured coatings for this industry was 10 million pounds or 0.73 million gallons. (142) The recent annual decrease in volume of millions of gallons shipped for this industry (Figure 6-3) indicates that this market will have a relatively low 1990 final shipment value. (47,135) At
-
III-24
the present time UV radiation curable coatings are only 5 to 9% of the prefinished board shipment value (1 to 2% of the total wood finishing market) and in this application, IR coatings represent a major threat to UV-radiation processing systems. Electron beam (EB) coatings are not considered to be a major product for this market because the required coatings are thin enough or the pigment type and levels are such that UV and IR processing techniques can cure the finish for acceptable product performance levels. The future for UV radiation processing is in the area of three-dimensional finishing operations.
111-25
C. HIGH SOLIDS COATINGS
High solids coatings can be substituted for the normally used solvent-based coatings with little effort and expense. Techniques are described in various recent publications, including the Hazardous Waste Minimization Handbook (1989), Finishing Handbook and Directory (1991), Metal Finishing Guidebook and Directory (1991), and the New York State Waste Reduction Guidance Manual (1989).
1. General Information
- High solids coatings contain less solvent than standard solvent-based coatings. They can be used as substitutes for normally used solvent- based coatings with little quality penalty or increased expense. The finished coat obtained from high solids coatings is comparable to typical solvent-based coatings. The primary advantage of high solids coatings is significantly reduced VOC emissions. Up to 50 percent reduction is possible, depending on the finishing system you are currently using. High solids coatings may also decrease toxic chemical emissions; however, you should check the chemical content of the finish you are currently using against the contents of the high solids coating that you plan to use, to be sure that there is a significant toxic chemical difference.
High-solids coatings reduce solvent emissions. The basic ingredient in an organic coating is the binder or resin, which is a film-forming organic organic polymer with glassy, plastic, or rubbery properties in the dried state. There are two categories of high solids resins: two- component ambient temperature cured and single-component heat converted. High-solids coatings can be used to reduce solvent emissions in a variety of industrial coating processes (Ross 1989).
~
2. Equipment Requirements
The equipment commonly used for applying solvent-based coatings can be used to apply high solids coatings. This equipment includes:
0 air-atomized sprayers airless sprayers
0 airless electrostatic sprayers 0 electrostatic bells and discs
III-26
high-volume, low-pressure (HVLP) sprayers.
High solids coatings are easily adapted and
stitutes for common solvent- based paints.
p~actical Sub-
Use of high solids coatings will reduce VOC emissions and may reduce toxic chemical emissions and wastes.
High Solids Coat- ings may be applied by a variety of methods. Paint heaters are needed to reduce the viscosity of many high solids coatings.
Although high solids coatings are very similar to the solvent-based coatings you may be using, there are some differences. Usually they contain polyester or acrylic resin varieties with special low molecular weight oligomers instead of polymers and copolymers. This results in lower viscosity. However, even with these special additives high solids coatings may require the use of heated spraying systems. The use of heaters is the only significant processing change likely to be required in the changeover to high solids coating. Some high solids coatings that are available contain enough solvent to be used without heating.
High solids coatings usually require oven drying. Some compositions that contain slightly higher solvent contents are air dryable; however, the trade-off is between the use of drying ovens versus greater solvent con tent.
3. Advantages and Disadvantages of High-Solids Coatings
Aside from protecting the environmental, substituting high solids coatings for the common solvent-based coatings yields the following advantages:
Cost savings through reduction in wasted solvent
0 Fewer gallons of coating are needed, saving on storage space, materials handling, and possibly shipping costs
Energy savings in baking oven make-up air.
Some disadvantages of using high solids coatings include the following:
0 High solids coatings show a high viscosity change with temperature, requiring that a reasonably uniform temperature be maintained. Fail-safe heaters should be used.
High solids films stay sticky until heated in the oven. Pickup of airbome particles cause a rough finish. Good air quality main- tenance and control of air movement in the spray booth and the flash-off zone are important.
At one time it appeared that high solids coatings would become the “standard” replacement for solvent-based coatings. However, develop- ments in water-based coatings and solvent free powder coating have led -
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to highly acceptable systems that are even more environmentally accept- able than high solids coatings. Even so, high solids coatings have estab- lished a significant market niche (see Figure II-2).
Oven drying might be required.
Principal advan- tages o€ high solids coatings are evident in lower cost and lower energy consumption.
Disadvantages of high solids coatings include high vis- cosity and sticky surface until cured.
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ID-29
I c I
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I
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F. REFERENCES
Literature references cited in this section are listed below. Additional sources of information are compiled in Section VI, Bibliography.
Bocci, Greg. 1991. Reprint from “Products Finishing,” Gardner Publications, Inc.
Chemical and Engineering News. 1991. “Higher Paint Sales Brighten Profits Outlook,” October 14, 1991, pp. 29-56.
“Finishing Handbook and Directory.” 1991. Sawell Publications Ltd., by the Publisher of Product Finishing.
“Hazardous Waste Minimization Handbook.” 1989. Lewis Publishers, Inc., Thomas E. Higgins.
“Hazardous Waste Minimization - -Industrial Overviews” No date. Published by the Air and Waste Management Association. Harry M. Freeman, editor.
Industrial Extension Service, School of Engineering, North Carolina State University. 1984. “Managing and Recycling Solvents - North Carolina Practices, Facilities, and Regulations.” December 1984. pp. 46- 47.
Levinson, S. B. 1988. “Application of Paints and Coatings.” Federation Series on Coatings Technology, Federation of Societies for Coatings Technology, August 1988.
“Metal Finishing Guidebook and Directory Issue ’91”. 1991. 59th Guidebook and Directory Issue 1991. Metals and Plastics Publications, Inc. Volume 89, No. lA, Mid January, 1991.
* III-30
New York State Waste Reduction Guidance Manual. 1989. Prepared by ICF Technology Incorporated for the New York State Department of Environmental Conservation. March 1989.
USEPA Guide to Application of Clean Technologies for Replacement Coating Materials, unpublished draft. Review draft dated January 10, 1992.
III-3 1
SECTION IV MINIMIZING WASTE THROUGH IMPROVED OPERATIONS
Aside from use of more environmentally safe coatings and more efficient means of applying these coatings, there are other techniques available to minimize waste losses and VOC emissions. Below some of Good housekeeping
and practices can also the major areas of note are addressed. "ize waste and VOC emissions.
A. DIRECT TRANSFER OF COATING TO THE GUNS
Direct transfer of finish is a technique described in the recent literature, especially the Finishing Handbook and Directory, 1991. In transferring coatings from storage to the guns, waste and emissions can be minimized by using a paint circulation system or direct transfer. A direct transfer system offers economy, safety, productivity improvement, and better finish quality. It is likely that direct transfer can be econom- ically justified if as little as 30 to 40 gallons per week of a single finish is used in your shop. The savings generated by installing direct transfer will pay for installation costs in about a year. The savings are generated by:
Direct transfer 0
saves operating costs, time, and
well as minimizing waste and solvent releases.
cleanup costs as 0
0
Buying in bulk quantities (larger discounts)
Elimination of the need for filling a can from the drum
Elimination of spilling, evaporation, and loss of skimmings on the side of the drum
Elimination of labor needed to fill pressure pots, gravity containers, or cups
Elimination of the time required to collect finishes from the paint storage area
Elimination of time spent in adjusting the coating to the correct viscosity (or adjusting for incorrect viscosity)
Elimination of container cleaning at the end of the shift.
Direct transfer does not merely increase productivity, it provides a method of attaining consistent quality in the coating reaching the guns. There are three general types of coating transfer systems:
IV-1
Deadend
0 Simple flow
Fully recirculating.
The dead-end system is not a circulating system; it merely pushes finish out to the point of use and there is no return line. The dead-end system is useful for coatings that are completely stable and where there is no possibility of settlement. It is not suitable for pigmented coating materials.
The most widely used direct transfer system is the simple flow and return type. The return line is taken from the farthest point of use and returned to the storage tank. The continual circulation prevents settling and is suitable for most pigmented coating materials.
the hose of the spray gun. A fully recirculating system is used only when high settlement rates are encountered, for example, in the use of acrylic lacquers by the automotive industry.
Usually nonpigmented material is pumped directly from the shipping drum. Pigmented materials may be pumped directly from the shipping drum if a specially built drum cover with built-in pump and agitator is utilized. If spraying viscosity adjustment is necessary, it is preferable to use a mix tank that has facilities for adding solvent.
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In a fully recirculating transfer system the finish is circulating even in
Iv-2
B. HEATEDSPRAY
Heated spray is a technique described in the recent literature, especially the Finishing Handbook and Directory (1991) and Levinson (1988). The use of heated finish for spraying lowers waste and emissions in several ways:
0 Heated coating has lower viscosity, and permits the use of less solvent; therefore, you pay less for solvent and there are less VOC emissions and less toxics to be concerned about.
Heating the paint also lowers waste and VOC emissions.
Heated spraying at lower Viscosity lowers the air pressure required to propel the coating (this is true for both air spray and airless spray). The use of lower pressures yields less overspray and gives higher transfer efficiencies. The net result is that less coating is needed to do the same job, and VOCs and toxics are minimized.
The use of lower pressure will also lower your power (electricity) costs, especially in multigun plants.
1. Effective Applications
Heated paint may Table XY-1 illustrates the effect of heating on coating viscosity, pres- sure energy, and overspray for both conventional air spray and airless spray conditions. Heated coating may also be used for electrostatic
be used with air spray, airless, or electrostatic equipment. applications. If high solids coatings are being used, heating of the coating
may be a requirement to lower viscosity for proper application. Field tests directly comparing cold air spraying with heated coating sprayed at lower pressures have shown that coating costs can be cut by 25 percent or more. No advantage, however, is gained from heating waterborne Heating offers no coatings or spirit stains. advantage for
waterborne or spirit stains. A variety of methods is available for heating coatings for spraying. If
you wish to consider a heated coating system, you should contact both your coatings supplier and your equipment supplier to determine the system most suitable for your production setup. A partial list of suppliers is presented in Section V if you should need other sources of information.
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Table IV-l. The effect of preheating on coating viscosity.
Pressure EneFgyb
atGun Req. @si) 0
Overspray
@) Req. = required; psi = pounds per 4. in.; HP = horsepower.
50
80 I 9 I Considerable
6 Moderate
1500 I 1 I Slight
500 I 0.3 I Negligible
Source: Levinson 1988.
A7-4
2. Advantages and Disadvantages of Heated Spray Systems
Preheated spraying results in a number of possible advantages; many yield higher production rates and lower VOC and toxic release problems. These advantages include:
Elimination of the thinning operation.
Lowering of coating costs.
About 25 percent increased coverage.
Increased coating film thickness per coat.
Faster drying of applied coating.
Spraying at lower ambient temperatures and higher humidity conditions (eliminates the need for temperature and humidity control for unheated spraying when weather conditions are poor).
Improved coverage of porous and rough surfaces, smoother coating film and improved adhesion.
Disadvantages of heated spray are few:
More cumbersome equipment.
Need for additional capital investment.
Iv-5
C. REUSE OR RECYCLING OF WASTE SOLVENTS
Reclaiming solvent can reduce waste management and raw material costs.
Spent cleaning solvent can be reused after the paint residue settles out.
Spent cleaning solvents can be distilled and reused.
Reuse and recycling techniques are described in the recent literature, especially in “Small Solvent Recovery Systems” (North Carolina Department of Environment, Health and Natural Resources 1987) and “Managing and Recycling Solvents” (Kohl et al. 1984).
1. Techniques for Reusing Waste Solvents
Cleanup solvents may be reused many times. The cleanup solvent can simply be left to settle in the spent solvent container. After an appropriate length of time, the coating residue will settle on the bottom of the container and the clear, “cleaner” solvent from the top can be reused for cleaning. During the settling period the container should be properly covered to prevent solvent evaporation and/or possible combustion. If more than one cleanup solvent is used, of course, the spent solvents should be properly segregated to permit efficient reuse.
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2. Techniques for Recycling Waste Solvents
Solvents used in cleaning sprayer lines and guns can constitute a sig- nificant hazardous waste stream. These spent solvents must be disposed of properly or sent to a recycling operator. The best solution for han- dling spent solvents is in-house recycling. Commercially available stills capable of handling 0.5 to 100 gallons per hour can be purchased. The smaller units are self-contained, off-the-shelf items that can be readily installed. The usual utility requirements are electrical power and cooling water; some stills may require compressed air. The cost of purchasing a recycling still can be offset by savings on hazardous waste disposal costs and lower purchase needs for solvent.
Nitrocellulose is explosive when dry. Consequently, distillation is not recommended if nitrocellulose is present in the solvent.
To take full advantage of a still, you should use only one cleaning solvent. If more than one is used, it is usually best to keep them separate. Mixed spent solvents may be amenable to distillation if they have boiling points that are similar; however, you must then be sure that the distilled mixture is suitable for reuse in your cleaning procedures. For solvents with boiling mints below 200 F a simole still can be used. If
IV-6
the solvent you are using has a boiling point above 200 F, then a vacuum still is required. Vacuum stills are slightly more complicated to run and are more expensive.
a sample of your waste so that you can evaluate the quality of the distilled product in your cleaning process. Also the trial distillation can be used to evaluate the percentage of spent solvent that can be recovered. This will allow you to estimate the savings you might achieve through lower fresh solvent costs.
Use of a still will eliminate most of your waste solvent disposal
Before purchasing a still, it is appropriate to have the supplier distill
Still bottoms costs; however, the still bottoms will usually be hazardous waste that must be disposed of properly. The cost of waste disposal may be avoided
usually are a hazardous waste.
Y if you can find a use for the bottoms. To determine if your still bottoms may be used in this fashion, it is necessary to work with your coatings supplier to determine your options.
Iv-7
D. OTHER OPERATING PRACTICES FOR WASTE MINIMIZATION
Inventory control, standardization of coatings, better scheduling, and
The recent literature, especially “Waste Reduction Options: Painting” (Georgia Tech Research Institute 1991) and the New York State Waste Reduction Manual (1989), decribes practices that can help
proper operator traintng also can
toxic releases and reduce costs.
minimize coating waste. These include inventory control, standardization of coatings, improved scheduling, equipment maintenance, and proper training of spray system operators.
waste and
L Inventory Control
An inventory control and tracking system should be utilized to minimize sludge generated by finishes left unused in storage. In some cases finish is left unused because workers are unaware that it is in stock. In other cases, finishes are overordered. An inventory control and tracking system will also help you consolidate finish usage so that purchases of finishes can be made in bulk. On the other hand, you should only order as much finish as is needed, even if it means foregoing discounts available on larger lots. Remember that the waste disposal costs must be considered, once the shelf life is exceeded.
minimize waste. These include: A number of storage and transfer practices should be practiced to
Checking drums for leaks.
Storing drums near areas where they are used to reduce leaks and spills during transport.
0 Using spigots or pumps to dispense new materials, and using funnels to transfer waste materials; these practices will avoid leaks and spills.
instaiiing tight-fitting iids and spigots to reduce evaporation.
0 Lifting drums with powered equipment or hand trucks to prevent damage or puncture.
IV-8
2. Standardization of Finishes
Customer requirements should be reviewed with thought of possible standardization. If you have two customers using a somewhat similar color and in the same type of finish, you may be able to get one to change to another color. Or you may get both to change to a common color. This can lead to less leftover finish and less waste. Additionally, it may allow you to order in larger quantities at a greater discount. (Another benefit could be less gun and line cleanup, if the customer’s jobs can be scheduled sequentially.)
3. Improved Scheduling
Scheduling of spray jobs from light to dark colors can reduce wastes generated in equipment cleanup. Scheduling to permit the end-of-day cleanup to coincide with the end of a job can eliminate a cleanup procedure and eliminate the waste from one cleanup.
4. Proper Training of Personnel
Proper spray techniques must be practiced by the spray gun operator. Training videos are available (some are referenced in Section VII of this manual). Obviously the use of techniques that minimize overspray and control the spray overlap, etc., can minimize finish waste and should be monitored closely.
--.
5. Cost Estimate for Using a Closed System for Equipment Cleaning
Iv-9
6. Guidelines for Solvent Emission Controls for Wood Finishing Operations
Add-on solvent emission controls that are not recommended include thermal or catalytic incineration, carbon absorption, and gasfliquid extraction (U.S. EPA 1980). The reasons for this are
the high capital cost the high air flow rates and relatively low concentrations of solvent vapors that result in poor efficiency and cost-effectiveness the fire hazard associated with combustion of dried nitrocellulose overspray.
0
U.S. EPA 1980. Reducing Emissions from the Wood Furniture Industry with Waterborne Coatings, EPA 600/2-80-160, July, pp. 49 - 55.
Iv-10
7. Case Studies
4 from PIES
Iv-11
T
i
r
E. REFERENCES
Literature references cited in this section are listed below. Additional sources of information are compiled in Section VI, Bibliography.
“Finishing Handbook and Directory.” 1991. Sawell Publications Ltd., by the Publisher of Product Finishing.
Georgia Tech Research Institute. 1991. “Waste Reduction Options: Painting.” Guide No. 5, Spring 1991.
Kohl, Jerome; Moses, Philip; and Triplett, Brook. 1984. “Managing and Recycling Solvents.” North Carolina Practices, Facilities and Regulations. December 1984.
Levinson, S. B. 1988. “Application of Paints and Coatings.” Federation Series on Coatings Technology, Federation of Societies for Coatings Technology, August 1988.
New York State Waste Reduction Guidance Manual. 1989. Prepared by ICF Technology Incorporated for the New York State Department of Environmental Conservation. March 1989.
North Carolina Department of Environment, Health and Natural Resources. 1987. “Small Solvent Recovery Systems.” Pollution Prevention Tips, Pollution Prevention Program, North Carolina Department of Environment, Health and Natural Resources. March 1987.
Iv-12
MANUFACTURERS OF RADIATION PROCESSING EQUIPMENT AND MATERIAL SUPPLIERS
INFRARED SYSTEMS
Soneko, Inc. 87 E l i zabe th Avenue Somerset, NJ 08873 (201 1 873-2217
ULTRAVIOLET SYSTEMS
Canrad-Hanovia, Inc. 100 Chestnut S t ree t Newark, NJ 07105 (201) 589-4300 ~ 2 0 8
Fusion UV Curing Systems D i v i s i o n of Fus i o n Sys terns Corporat ion 7600 Standish Place Rockv i l l e , MD 20855 ( 301) 251-0300
Union Carbide Corporat ion L inde D i v i s i o n 5705 W . Minnesota Ind ianapo l i s , I N 46421 ( 317 214-1200
HIGH ENERGY ELECTRON SYSTEMS
Energy Sciences I n t e r n a t i o n a l D i v i s i o n o f Energy Sciences Incorporated 109, Rue de Lyon 1211 Geneva 13 Swi tzer land 22-45-88-21
PHOTOlNITIATORS
Aceto Chemical Co., Inc. 126-02 Northern Boulevard Flushing, NY 11368 (718) 898-2300
C i ba-Gei gy Corporat i on 3 Sky l i ne Dr ive Hawthorne, NY 10532 (914) 347-4700
MONOMERS
ARCO Spec ia l t y Chemicals D i v i s i o n o f ARCO Chemical Company Westtown Road a t W. Chester Pike West Chester, PA 19380 (215 ) 692-8400
Cel anese Chemical Company 1250 W. Mockingbird Dal las, TX 75247 (214) 689-4000
Diamond Shamrock Chemicals Co. D i v i s i o n o f Diamond Shamrock Corp. 350 M t . Kemble Avenue Morristown, NJ 07960-1931 (201) 267-1000
Morton Chemical D i v i s i o n o f Morton Thiokol, Inc. 101 Carnegie Center Pr inceton, NJ 08540 ( 609 ) 396-4001
RPC Indus t r i es 3210 Investment Boulevard Hayward, CA 94545 (415) 785-8040
OLIGOMERS
ARCO Spec ia l ty Chemicals Division o f ARCO Chemical Company Westtown Road a t W . Chester Pike West Chester, PA 19380 (215 1 692-8400
Diamond Shamrock Chemicals Co. Division o f Diamond Shamrock Corporati on 350 M t . Kemble Avenue Morristown, NJ 07960-1931 (201) 267-1000
Lord Corporation Indus t r ia l Coating D i v i s i o n 2000 West Grandview Boulevard Er ie , PA 16514 (814) 868-3611
Morton Chemical Division o f Morton Thiokol, Inc. 101 Carnegie Center Princeton, NJ 08540 ( 609) 396-4001
POLYMERS
ARCO S p e c i a l t y Chemicals Division of ARCO Chemical Company Westtown Road a t W. Chester Pike West Chester, PAa 19380 ( 215 ) 692-8400
Cel anese Chemical Company 1250 W. Mockingbird Dal l a s , TXx 75247 (214 ) 689-4000
Lord Corporation I ndus t r i a1 Coati n g s D i v i s i on 2000 West Grandview Boulevard E r i e , PAa 16514 (814 ) 868-3611
Morton Chemical Division o f Morton Thiokol, Inc. 101 Carnegie Center Pr inceton, NJ 08540 ( 609 ) 396-4001
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