QPPI 0015

UV AND OTHER METAL-DECORATING PROCESSES Elgin D. Sallee*

Abstract Inks used i n meta2-decoration are not signif icant sources of emissions

since they contain essentia22y no v o k t i t e s . soZvent-borne surface coatings used in conjunction with metal-decoration, hmever, are important potentia2 sources of hydrocarbon emissions. ferred contro2 measure for such emissions i s r e f o m l a t i o n of the surface coating t o eZiminate, reduce, or a l t e r the vo la t i les content. Where t h i s i s not feasible, emissions are incinerated, using natura2 gas or LPG as auxiliary fue l . Incineration i s relegated t o l a s t resort action i n viev of the extremely cri t ica2 fue2 supp2y si tuation. savings features of solvent vapor incineration systems, as we22 as the environ- menta2 concems i n selection of chemicals used i n formulation of UV-curable and other sotvent-free or low-so2vent materials for metal coating and decorating.

The conventionu2 organic

The pre-

This paper describes fuel-

In discussions of the environmental concerns involved in any manufac- turing process, the first item of significance is the quantity of materials used. In attempting to total metal-decorating materials, confusion is encountered from the outset because of varying definitions of metal decorat- ing. ing any process in which a colorant is applied to the metal surface. This would include paint, enamels, inks, and even those varnishes, lacquers, or shellacs that impart color to the metal surface.

printed on the metal by means of an offset or lithograph press. Yet in instances such equipment is used to apply ink uniformly over the entire surface. Some people call this application surface coating rather than metal decorating. Ordinarily only a specialist in the business would be able to tell whether the dried, uniformly colored surface had been print

Many of those who are experienced in the business call metal decorat-

Others regard metal decorating as only those processes in which ink is

*Director of Environmental Sciences, Corporate Environmental Affairs American Can Company, Greenwich, Connecticut.

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w i t h an i n k o r whether an enamel o f the same c o l o r had been appl ied by a

r o l l e r coater machine. Many o f the ingredients o f inks and surface coatings are s imi la r .

are a lso close s i m i l a r i t i e s i n the environmental concerns and cont ro l meas- ures involved. Metal surface coat ing i s f requent ly considered a n c i l l a r y t o meta l p r i n t i n g ; both done i n the same manufacturing room, and sometimes on the same manufacturing l i n e w i t h both mater ia ls d r i e d simultaneously i n the same oven.

Accordingly, my comnents inc lude metal-decorating inks and coatings as separate items f o r those processes and mater ia ls now i n large-volume use, as we l l as some of those involved i n new technology.

The can-manufacturing indust ry i s by fa r t h e major user o f inks p r i n t e d

on metal. amounts t o less than 1 m i l l i o n pounds annually, whereas i n k f o r metal cans made i n t h i s country amounts t o about 15 m i l l i o n pounds per year. about 90 percent o f t h i s i n k i s p r i n t e d on metal sheets before the sheets are c u t t o s ize and formed i n t o cans; the remainder i s p r i n t e d on formed can parts.

ventional thermal ly cured type appl ied i n about equal proport ions by the dry of fset p r i n t i n g process and the wet l i t h o g r a p h i c process. of the same general type, and environmental aspects are the same for both p r i n t i n g processes.

drying process. moving i n t o the dry ing oven where they are picked up by supports ( c a l l e d wickets) on a continuously moving chain, which c a r r i e s them through the Oven w i t h exposure t o uni form temperature o f 330" t o 350" F fo r about 6 minutes. The lower p a r t of the oven (empty wicket chain r e t u r n sect ion) i s sealed o f f , except a t entrance and e x i t , from the upper p a r t through which the p r i n t e d sheets move. Uniform temperature i s maintained i n the oven

zones by the c i r c u l a t i o n of la rge volumes o f a i r heated by gas burners. A CQWaratively minor proport ion o f the c i r c u l a t e d a i r i s drawn from each oven zone, so there i s general a i r movement toward the oven exhaust stack. Fuel

combustion products as we l l as any mater ia ls v o l a t i l i z e d i n the oven are

There

To my knowledge usage i n indus t r ies other than can manufacture

Currently,

Inks are fo r the outside surfaces only. Current ly about 85 percent of the metal-sheet-pr int ing i n k i s the con-

These inks are

Figure 1 i s a schematic of the t y p i c a l conventional i n k app l i ca t ion and It depic ts the p r i n t i n g press w i t h the p r i n t e d sheets

exhaust& at this point. before they pick up the printed sheets; otherwise the quality of the printing is affected. Ink usage rates vary, of course, with the area of surface printed and thickness o f the ink film applied. Typically, it may be as little a s 0.1 lb/hr per color, or, in the case of a heavy white ink covering the entire sheet, as much as 35 lb/hr.

Table 1 gives general information on the composition of conventional, heat-ured metal-printing inks. Note that the extent of vehicle or resin in individual formulations may vary widely dependent on the extent of color- ant and extender (the extender is primarily for dilution of the colorant). Some inks require no extender, dryer, or solvent. All inks printed on metal are comonly termed as free of volatiles. any free solvent is driven off in the oven along with any volatiles released in the resin polymerization. At most, however, the extent of volatiles from the latter amounts to 3 percent of the resin. the ink is less than 1 percent, with a maximum of 3 percent.

The returning empty wickets must be heated just

This is not absolutely true, since

Typical volatiles content o f

Accordingly, emission problems involved in application of conventional thermally cured ink for metal are practically nonexistent. Ordinarily no emission-control devices are required for the oven exhaust if the process involves use of ink only.

However, in the major proportion of printing on metal (in my company about 70 percent) the printing is followed by a tandem roller coating of solvent-borne varnish. over the wet ink, as shown in figure 2. oven-dried simultaneously. problems in the use of metal-printing ink would be essentially nil.

ink is the conventional heat-cured type. Essentially all of the remain is the ultraviolet-cured type. Use of UV ink has no marked advantage ( disadvantage) over conventional inks with respect to emission control, neither contains a significant extent o f volatiles. Use of UV ink wi ventional tandem roller coating of overprint varnish, as shown in figu still rewires use o f an oven for the varnish. This process is ordin the first phase in the development of technology for use of UV curabl' materials. This arY-ngement does have a distinct advantage in savin fuel gas in those s~tdations where the ink must be dry before varnis

In most instances this overprint varnish is applied 7 Both the ink and the varnish are

Were it not for the varnish, emission control

I mentioned previously that currently about 85 percent of metal-prin

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applied. With inks that must be dry before varnishing, the figure-3 arrangement eliminates one pass of the metal sheets through the gas- heated oven. for gas-heated oven operation.

on metal involves obvious emission control problems. Measures to counteract these include reformulation of the coating to eliminate or reduce extent of solvent content, or change to preferable solvents, such as less photochemi- cally reactive types, or incineration o f the oven exhaust. emission control devices, based on adsorption, absorption, condensation, scrubbing, chemical oxidation, electrostatic precipitation, or a combination of these principles, have been evaluated thoroughly and found inadequate or impractical except in rare instances. hecessary, incineration is almost invariably the only dependable control technology. eration i s relegated to last resort action. Where there is no practical alternative to incineration, it is essential to do it with minimum fuel consumption.

equipment is similar to that for roller coating o f varnish illustrated pre- viously, except that the oven for drying enamels and lacquers is usually larger to accommodate greater usage rates of material. vapor emission rates from a print-varnish oven are 40-80 lb/hr, whereas a

modern coater oven may emit as much as 200 lb/hr. Figure 5 depicts a roller coater unit with "straight-through" incinera-

tion of the oven exhaust air. To reduce fuel consumption, various arrangements of catalyst systems,

heat exchangers, and heat-recovery systems, or a combination of these, are used. Figure 6 is a schematic for a catalytic incineration system showing typical temperature measurements.

Figure 7 traces the route of oven exhaust through a heat exchanger and a thermal incinerator and typical temperatures involved. Figure 8 shows an arrangement of heat exchanger, burner, and catalyst unit, and figure 9 the Same kind o f arrangement but with the addition of heat recovery.

BTU's required for UV curing are about one-tenth of those

Application of varnish or of any organic solvent-borne roller coating

Other types of

Where an emission control device is

Particularly because of the fuel supply situation, use of incin-

Figure 4 is a schematic of a typical sheet-coating operation. Basic

Typical solvent

This consumes vast quantities of fuel gas.

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burners for the oven zones and wicket preheat section.

Under present circumstances it is preferable to change to use of coating materials that obviate need for emission control devices. This, however, much easier said than done. Successful reformulation of coating materials used by can manufacturers is a tremendous task, particularly for those coatings used for can linings.

At best, all solvent vapor incineration systems consume some fuel gas

In most instances, particularly in can-manufacturing plants, it i s not possible to make practical use o f heat discharged from such systems for other than the litho-coating process itself. circumstances . Most sol vent vapor incinerator systems are add-on instal 1 a- tions made long after the plant was constructed. Possibly for a new plant, beneficial use of heat discharged from such control devices can be designed into the plant for space heating or for processes other than heating the 1 i tho-print or coater oven itself.

discharge air i s ducted to the various heating zones of the oven, replacing

This, of course, depends on

Figure 10 depicts a system used extensively in my company. Incinerator

Table 2 lists the most common ingredients of conventional solvent-borne can coatings in decreasing order of volume usage for the respective types o f ingredients. extent of such usage is comparatively small, their total combination amounting to less than 10 percent of the respective total. such coatings contain no drying oils. Lubricants most commonly used are silicones or wax. the resins listed may be used in an individual formulation. may contain only one solvent, such as mineral spirits; others may contain as many as 10 different solvents.

The effort to reformulate to low-solvent or no-solvent coatings i s c plicated by the wide variety of coating material end uses and resultant physical requirements. Well over 600 different coating material formulatf are used in the can-manufacturing industry. than 300 formulations of solvent-borne coatings. plants use as many as 90 different formulations annually.

summarized in table 3. This list is an oversimplification of the conc

Several others of each type ingredient are actually used, but

Except for varnishes, most

Various combinations of The majority contain no additives.

Some coatings

My company routinely uses Some of our individual

The important criteria for all coatings used in can manufacture are,

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involved. A formidable amount of investigative work goes into the successful development of each new coating material. despite long experience and knowledge of the chemical and physical properties of the various individual ingredients, ordinarily compounds and tests a great many formulations before deciding on one considered sufficiently promising t o warrant submitting samples to the user. manufacturer then subjects the new material to a gamut of testing with emphasis on freedom from difficulties and hazards in application and use of the material and assurance that the material will do the job intended with respect t o maintaining quality of end-use product. The latter includes packing the customer's product in the cans and examining it after varying storage and abuse conditions.

Mere toxic properties of the ingredients and formulated coating are not already known, this must be evaluated carefully. relatively nontoxic to can-plant employees, plant neighbors, can packers, and t o anyone involved in the product-distribution, product-consumer, and container-disposal and -recycling system. animals) must be made available for toxic effects through ingestion, inhala- tion, and skin absorption, as well as tests for eye irritation and determina- tion of whether the material is a primary skin irritant. Such information is required on individual ingredients as well as the compounded formulation. Where screening-test information indicates potential for adverse effects, further detail is required, such as potential chronic toxicity information and hazard-control detail.

ingredients and the formulated material, with respect to protection of our Plant employees and control of emissions. Fire hazard and any nuisance or Physical hazard to employees or facilities are also important considerations,

Stability of the material is important. It must not change during transportation and storage at our plant. @ncountered in use of the material in can-manufacturing processes, or ad- verse effects might be encountered by our customers or ultimate consumers.

The formulating laboratory,

The user or can

Toxicity of the coating material, of course, is of primary importance.

The materials must be

Screening information (tests on

Closely allied to toxicity detail is knowledge of the volatility of the

Otherwise, troubles might be

Applicability of a new formulation is extremely important. It must be f significant

-

prob 1 ems in aPP lication to the metal surface, drying the

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coated film, and fabrication of the coated metal into containers at high production rates. The dried film must not be affected during storage and transportation and must cause no difficulties to the customer in filling, sealing, and processing the filled containers.

required shelf life for the canned product and preserve its quality. No detectable portion o f the coating material can leach or migrate into the canned product.

Administration sanction, is used in a coating formulation for lining of cans for edible products, then lengthy extraction and animal-feeding tests are required. The usual time required for these is 2 to 4 years at costs now exceeding $500,000 for each new resin system.

The standard procedure for evaluation of a new coating material sub- mitted to the can manufacturer involves laboratory examination followed by plant trial use, fabrication of about 1,dOO cans, filling them with the customer's product, and periodic examination after storage. passes all requirements, then larger quantities are fabricated, packed, examined, etc. Usually, progression in three stages to the million-can

The prime requisite of the container is its integrity; it must provide

Flavor, odor, and texture of the product must not be affected. If a new resin system, one that does not have prior Food and Drug

If the material

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quantity is required before customer approval for use of the new coating formulation in commercial quantity is obtained.

Otherwise, it would be better to continue with conventional

New formulations can be rejected for failure to measure up to standa The track record for new formulations

Obviously, availability of the new coating formulation at practical COS is essential. time-proved materials with provision o f the control devices for the emission

for any of the foregoing criteria. attests to the difficulties.involved. of 3 of 100 candidate materials submitted by coating-material suppliers eventually reach the commercial use stage.

Appreciable success is being attained in reformulating to water-bo varnish and various other surface coatings for metals. these contain no organic solvent, but in most of those now in comrica the volatile portion consists of 20 percent organic solvent and 80 per water. With some materials prohibitive troubles are encountered if th

In my company's experience an ave

A relative few 0

organic so lvent po r t i on o f the v o l a t i l e s i s lower than 30 percent. commonly used organic solvents i n water-borne coatings are ce l losolves and bu ty l alcohol.

Comparatively amazing success has been rea l i zed i n development o f UV

curable inks. There i s good promise t h a t t h i s w i l l continue and t h a t t h i s experience w i l l enhance success i n development o f UV curable surface coatings f o r metal.

(coatings and inks) now showing considerable promise. var ia t ions , o f course, i n choice o f these components and concentrations invo lved i n i nd i v idua l formulations. i s being made w i t h ac ry la te systems ( f r e e r a d i c a l cure systems), bu t s i g n i f - i c a n t progress i s a lso being rea l i zed w i t h ca t i on i c , two-package epoxy

O f prime importance i n the development of UV mater ia ls i s t he requirement t h a t v o l a t i l i t y o f the mater ia l be c lose ly l i m i t e d . be con t ro l l ed by spec i f i ca t ion . It i s essent ia l f o r p ro tec t ion of can p lan t employees as we l l as prevention o f emissions t o t he extent t h a t no emission con t ro l devices are required.

Figure 11 i l l u s t r a t e s UV i n k app l i ca t i on by tandem presses, each f o l - lowed by a UV d ry ing system. two co lo rs o f inks.

which requi res no emission con t ro l device f o r t he oven exhaust a i r . The t y p i c a l UV cur ing u n i t has two exhaust ducts handling a t o t a l o f

about 3,000 CFM. This i s f o r exhaust of a i r f o r coo l ing o f t he UV lamps, the ozone they generate, and any monomers o r oligomers v o l a t i l i z e d dur ing the UV cure. As mentioned, extent of the l a t t e r i s l i m i t e d by spec i f i ca t ion . My company r e j e c t s UV-curable mater ia l formulations i f the v o l a t i l e s r e s u l t i n emissions greater than 0.5 l b /h r . atmosphere var ies w i t h power o f t he lamps, bu t never exceeds 1 ppm i n the exhaust a i r or 0.02 lb /h r .

Even though hydrocarbon emission problems are resolved through use o f uv inks and water-borne coatings, f u e l gas consumed by the dry ing oven f o r the l a t t e r , o r fo r h igh so l i ds content solvent-borne coatings, remains as a

c r i t i c a l problem. Accordingly, progress i s being made i n development o f UV

The most

Table 4 l i s t s the kinds o f ingredients i n UV metal-decorating mater ia ls

There are many

A t t he moment, apparently most progress

. formulat ions.

This must

I n most such arrangements each press may apply These are followed by a p p l i c a t i o n of water-borne varnish

Ozone evolved and exhausted t o the

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r o J l e r coatings. used, and the gas-heated oven i s el iminated.

f a b r i c a t i o n i n t o cans, technology developments i n decoration and coating o f formed cans are keeping pace. We expect continued r a p i d growth o f UV

mater ia ls i n the metal-decorating indust ry , w i t h major replacement o f thermal ly cured mater ia ls i n on l y a few years. c u r r e n t l y has 17 metal-decorating l i n e s equipped w i t h UV u n i t s i n commerical use. E ight more w i l l be using UV mater ia ls by mid-1976. By 1980 we a n t i c i - pate t h a t a l l of our metal p r i n t i n g and ha l f o f our metal coat ing w i l l be w i t h UV mater ia ls .

tex t . )

With these the f igure-12 schematic o f process equipment i s

Although my comments have been on decoration o f metal sheets before

My company, f o r example,

(On the fo l l ow ing pages are the 4 tab les and 12 f i gu res c i t e d i n the

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Table 1. Conventional (heat-cured) meta l -decorating i n k -

Component Percent 30 t o 95 Low-vi scos i ty res i n

(e.g., o i l -base alkyd, epoxy es ter )

Pigment o r co lorant 5 t o 60 (e.g., cobal t blue, i r o n oxide, ear th colors, organic dye coprec ip i ta ted w i t h aluminum hydroxide, t i t an ium dioxide)

Extender (e.g. aluminum hydroxide)

Dryer (e. g. , cobal t naphthenate, manganese naphthma t e )

0 t o 50

0 t o 0..2

Solvent 0 t o 1.0 (e.g., h igh-bo i l ing mineral s p i r i t s )

Table 2. Conventional solvent-borne r o l l e r coatings f o r metal sheets

Res i ns Sol vents Vinyl A1 i p h a t i c petroleum hydrocarbons EPOXY Methyl i sobuty l ketone A1 kyd Methyl e thy l ketone Phenol i c Glycol ethers & e ther esters Pol ybutadiene Xylene 01 eores i nous Butyl acetate Polyester Butyl alcohol A c r y l i c Ami no

Addi t ives Drying o i l s Ti tanium d iox ide Linseed Aluminum powder Cottonseed Zinc oxide Soya Lubr icant Coconut

Saf f 1 ower

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Tab1 e 3. Criteria for metal -decorating material - Toxicity Stabil i ty Integrity Vol ati 1 i ty Applicability Availability and cost

Table 4. Ultraviolet-curable metal-decorating materials

Component Percent Vehicle 40 to 90

(e.g., bisphenol-A epoxy diglycidyl ether, trimethanolpropane triacrylate, urethane, 2-package epoxy systems)

Photoinitiator or photosensitizer (e.g., bis henol, Michler's ketone, Lewis acid P

, . Accelerator 1 to 6 (e.g., methyl diethanolamine, other I"

tertiary ami nes) Stabilizer

(e. g . , benzoquinone) 0 to 1

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Additives 0 to 5 (e.g., wax, silica gel)

NOTE.-The above materials are typical o f those used in UV varnish. They are also typical o f those used in UV inks containing up to 50 percent pigment.

PRINT

EXHAUST STACK

OVEN

EXHAUST mACK

PR I NT VARNISH COATER

OVEN

Figure 2. Meta7 sheet decorating u n i t using conventiona7 i n k and so7vent-horne varnish ro77er coated over the wet i n k .

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PRINT VARNISH COATER

EXHAUST STACK

Figure 3. Metal decorat ing u n i t using u l t r a v i o l e t curable i n k and solvent-borne ove rp r in t varn i s h .

-8- COATER

Figure 4. Metal sheet r o l l e r - c o a t i n g w i t h solvent-borne enamel , lacquer, s i z i n g o r varnish.

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EXHAUST STA - a -

1,250" - 1,40O0F

-& COATER

Figure 5 . Rol ler -coater u n i t w i t h thermal i nc ine ra to r for oven exhaust.

EXHAUST I LL I STACK BURNER CHAMBER

Figure 6. Coater u n i t w i t h c a t a l y t i c i n c i nera t i o n system.

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OUT

EXH

EX

300"

AUST IN

:HAUS1 IN

f

u u BURNER CHAMBER

HEAT EXCHANGER

Figure 7. Thermal i n c i n e r a t o r w i t h heat exchanger.

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1.400"

EXHAUST OUT

CATA LVSf 1 1 1 I

EXHAUST IN

BURNER CHAMBER

HEAT EXCHANGER

1111 HEAT RECOVERY

Figure 9 . Catalytic incinerator with heat exchanger and heat recovery.

P, 0

INCINERATOR 1150°-13000 F L !

I

' i INCINERATOR DISCHARGE AIR DUCTED TO

i OVEN ZONES, REPLACING ZONE BURNERS If

1 c PREHEATER ' EXHAUST !

Figure 10. Thermal incinerator w i t h heat recovery.

PRINT

u v

PRINT

EX HA UST I I STACK

I

VARNISH COATER

OVEN

u v

VARNISH COATER

Figure 72. Tandem UV p r i n t i n g followed by UV varnish.

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