curing powder coatings with gas-fired equipment

7/30/2019 Curing Powder Coatings With Gas-Fired Equipment

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Eclipse Guide to

Curing Powder Coatings with Gas-Fired Equipment

SP-421

5/92

Introduction This paper has been prepared to familiarize you with powder coatings

an d the process of cur ing th em in gas-fired infrared and convection ov-

ens. It contains information and guidelines on the selection of heating

equipment and points out some of the pitfalls to be avoided in makingthose selections. In researching this paper, we interviewed the techni-

cal r epresentat ives of four major powder coat ing manufactur ers. Their

comments on ovens and combustion systems provided some valuable

insights into the process and its requirements.

What Are They are protective or decorative finishes applied to products without

the aid of solvents or carrier liquids. Most powders are primarily fine

particles of clear or pigmented plastic resins blended with agents to

promote desired surface textures, improve flowing characteristics or

speed curing times.

Most resins can be divided

into t wo cat egories: ther-

moplastic and thermoset-

ting. Thermoplastic resins,

the basis of the original

powder coatings, can be

repeatedly reheated and

softened. On the other

hand, thermosetting res-

ins undergo polymeriza-tion (cross-linking) when

they are heat ed, causing a

permanent molecular change, so they will not soften on reheating.

They are now the basis of most commercial powder coat finishes.

Table 1 lists some of the major families of powder coating resins.

History Powder coating originated in Germany in the mid-1950s. The original

process, which used thermoplastic resins, consisted of suspending pre-

heated objects in a fluidized bed of coating powder. The part tempera-

tu re was high enough to cause th e powder to soften and st ick t o its

surface, after which it was transferred to an oven to finish the fusingprocess.

With the advent of electrostatic spraying, new applications

opened up for powder coating. In this process, the dry powder

par t ic les ar e g iven a h igh vol tage e lect r os ta t ic cha rge as th ey

pass t h rough a sp ray gun . Th is char ge causes t hem t o be a t -

tr acted a nd cling to th e work piece, which is electr ically

Pow der Coat ings?

Table 1Types of Powder Coating Resins

Thermoplastic Thermosetting

Nylon Acrylic

Po lyvinyl Chlo rid e (PVC ) Ep oxy

Po lyest er Po lyester/ Ep oxy H yb rid

Polyethylene Polyester/ TGIC*

Po lyp ro pylene Po lyester/ Ureth an e

Polyvinlidene Fluoride (PVDF) Urethane

* Triglycidil isocyanurat e


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grounded. The work piece is t hen hea ted, causing the part icles to

soften , flow togeth er to form a un iform , non-porous coating, a nd fi-

na lly, to cur e, by cross-linkin g, int o a du ra ble finish.

Powder coatings have three advantages over solvent-based coat-

ings: firs t, becau se th ey cont ain no solvents, t hey can be cur ed more

quickly, which per mit s faster line speeds. Second, t hey ar e some-

what easier to apply without defects. But th eir th ird asset is becom-

ing the m ost importa nt they are en vironment ally more friendly.

Lacking solvent s, t hey em it no VOCs (volat ile orga nic compoun ds)

dur ing the curing process, and t his elimina tes t he n eed for fume in-

cinerat ors, solvent a dsorpt ion systems a nd worr ies about accidental

discha rges of vapors. It a lso redu ces concern s about th e fire an d ex-

plosion h aza rd of solvent va pors, alth ough th e fine powders still

ha ve to be ha ndled with r easona ble car e. The powders th emselves

can be recycled as long as car e is tak en t o avoid conta min at ion a nd

mixing of colors . Consequ ent ly, overspr ay a nd spilled powder can be

reu sed inst ead of ha ving to be disposed of as a ha zar dous wast e.

Wha t Is The The dr ying of solvent-based coat ings (Figure 1) consist s of heat ingth e work piece to evaporat e th e solvent . An a bun dan ce of oven

mak eup a ir is required to car ry off the solvent vapors a nd dilute

th em well below th eir min imum flam ma ble concent ra tion. A slow,

gent le temperat ur e rise is also needed to avoid har dening the su r-

face of the coat ing before t he solvent un dern eat h is dr iven off.

The m echa nism of cur ing powder coat ings is ent irely differen t (Fig-

ur e 2). When t hey ar e first exposed to heat , the resin pa rt icles begin

to soften. As th eir temper at ur e increases a nd t heir viscosity de-

crea ses, th ey flow int o one an oth er a nd in to th e microscopic irr egu-

lar ities of th e workpieces su rface, filling t he voids between th em-selves an d a dhering t o the pa rt . This process is called gelling. If th e

coat ing is a th ermoplast ic resin, it mu st t hen be removed from th e

heat to harden . With th ermosetting resins (Figure 3) th e tempera-

tu re is raised another 50 to 100F t o activat e th e cross-linkin g reac-

tions. As th ese rea ctions ta ke place, the coat ing becomes m ore r esis-

WARM AIR

Dried Finish(Mostly Pigment)

Pigment& Solvent WORKPIECE

Solvent EvaporatesInto Airstream

Figure 1Drying of Solvent-Based Coatings

Curing Proce ss?

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SofteningRange

CuringRange

SofteningBegins

CuringBegins

Curing Ended;Remove From Heat

PowderTemperature

Figure 3Curing Cycle of Thermosetting Powders

Damage To Finish

Time

Tempe

rature

PowderViscosity

WORKPIECE

Radiant and/or Convective Heat

Statically charged particles cling to the workpiece surface. The heat softens and melts

the particles, blending them together and into the surfaces irregularities, where they

cross-link and harden.

Figure 2Curing of Powder Coatings

ta nt to flow and fina lly ha rdens completely, even while it is still hot.

All this can t ake 20 m inut es or less, depending on t he coat ing form ula-

tion a nd t he pa rt its applied to. Some par ts can be cured in a s little asfive minut es.

Two of the most important quality at tr ibutes of any coat ing are its fin-

ish an d its r esistance to chipping and peeling. Both ar e enha nced by

giving th e powder finish a s mu ch t ime as possible in t he upper pa rt of

its gelling tempera tu re r an ge (when its viscosity is low) before cross-

linking sets in. This pr ovides ample t ime for t he coat ing to flow and de-

velop a smooth sur face and good a dhesion to th e base m at erial. Conse-

quen tly, the opt imum heat ing cycle consists of a rapid heat-up followed

by holding at a steady curing temperature. The curing (cross-linking)

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temperatu re range de-

pends on the type of

resin and additives in a

specific powder formula-

tion. As a general rule,

thermosetting resins

cure between 300 and

400F, with the exact

temperatu re range be-ing specified by the coat-

ing manufacturer. With-

in that range, the curing

speed increases with the

temperature (Figure 4).

For example, a coating

tha t cures in 20 minutes

at 340F might take only

half tha t t ime at 380F.

Curing Metho ds Becau se powder coat-

ings dont requ ire sol-

vent removal, user s

ha ve more lat itude in the ways th ey cur e them , and the speed of cur -

ing allows mu ch fast er pr ocessing of th e work .

Convection Ovens Man y powder coat lines were conver ted from liquid paints , so finishers

often continue to use their convection ovens. From a product quality

stan dpoint , convection ovens can do a fine job (in fact, the t echn ical

representa tives of some pa int companies prefer them), but th eyre fre-

quen tly criticized as too slow. This capa city sh ort fall is usua lly due t ospeeding up th e line to take advant age of powders fas ter curing cycle.

Becau se the oven opera tes a t a relat ively low ther mal head an d with

large am oun ts of makeup air, it simply can t pr ovide th e needed hea t-

ing intensit y. Although its sometimes possible to upgrade an oven

with a lar ger burner, modified air han dling system and higher operat -

ing tempera tu res, a more practical approach is to add an infra red

booster section (Figur e 5).

Two other potential problems are frequently mentioned when convec-

tion ovens a re considered for powder coatings powder blowoff an d

contamination.

Powder blowoff refers to the dislodging of stat ically-cha rged powder by

th e ovens air str eam or, more often , by the a ir curtain a t t he ovens en-

tr ance. A poll of coat ing manufactu rers tech reps suggests that th e

problem ma y be overst at edthey sa id that blowoff isnt u sua lly a con-

cern as long as th e oven is properly balanced an d its a ir curtain is set

correctly. Exceptionally thick coatings (6 mils and up) are at greater

risk becau se the electr osta tic cha rge gets weak er a s th e coat ing thick-

ness increases, but coat ings this th ick ar e rar e. If th ere is an infrared

Max.

Min.

M in. M ax.

Curing Temperature

CuringTimeRequir

ed

Figure 4Characteristic Time/Temperature Curing

Curve for Thermosetting Powders

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booster section at the beginning of the oven, blowoff is not a problem, be-

cause the coat ing has already begun to melt and adhere by the time it en-

ters the convection section.

Contamination can be a more serious problem, primarily because cleanli-

ness is more critical in powder coat ing opera tions. Overspray and spills

dont harden and st ick to sur facesthey are loose par ticles, free to be

blown or carried around the plant along with stray dust and dirt . Without

scrupu lous housekeeping, they all can enter the ovens recirculating or

combust ion a irstream and wind up imbedded in the finish.

There are also occasional complaints of discolora tion and deglossing due to

exposure to combustion products. Whites or light colors may take on a yel-

lowish cast . If lower curing temperatures or changes in the paint formula-

tion dont fix the problem, indirect firing of the oven will. In fact, indirect-

fired ovens are becoming popular in operations where color consistency

and matching are important.

These ovens use the best of both worldsa high intensity IR section to ob-

tain a fast temperature rise, quickly melting the coating, followed by a con-

vection section t o finish curing at uniform temperatures without overheat-

ing. Many of these ovens are simply convection units with IR booster sec-

tions added to keep up with faster line speeds. However, the concept has

become a standard with many oven companies because it offers a good

combina tion of speed and quality in less floor space than a st ra ight convec-

tion oven.

Some combina tion ovens are essentially convection ovens with rows of In-

frared burners running through them. The theory behind this concept is

that the IR burners provide a continuous heat boost a long the length of the

oven. The shortcoming in this approach is that mixing convection and ra-

diation heating systems in a common chamber can compromise the best

qualities of both . Unless the IR burner heads are carefully located with re-

spect to product flow, they can promote temperature non-uniformity the

convection system was supposed to avoid.

Convection SectionInfrared

Booster Section

Figure 5Convection Oven with Infrared Booster Section

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Combina t ion Infrared/

Convect ion Ovens


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The flip side of the problem is that if enough IR burners are installed to

make a significant increase in line speed, the convection heating system

contributes little to the process. From a h eat t ransfer and temperature con-

trol standpoint, it makes more sense to concentrate the IR heating at t he

entrance of the oven and a llow the rest of the oven to operate as a straight

convection unit.

Stra ight Infrar ed Ovens Ovens employing only IR heating, whether gas or electric, are gaining

popular ity as manufacturers continue t o switch to powder coat ings. They

provide the intense, rapid heat-up favored for powder coatings and canhandle a given production ra te in a fraction of the floor space taken by a

convection oven. However, because IR operates at such a high temperature

head, it must be carefully controlled to avoid product damage, and because

it heats only those surfaces it sees, its sometimes difficult to use with com-

plex or h ighly three-dimensional product shapes (Figure 6). The key consid-

eration is whether all coated surfaces will be exposed to radiation. Unless

conduction through the part can be counted on to carry heat to coated areas

out of IRs line of sight, convection or a combination oven may be a better

choice.

In addition to product configurat ion, the conveying system should be con-sidered when selecting a heating system. If the conveyor allows parts to

Radiant Surface

Radiant Surface

Thin, flat and hollow shap es lend them selves w ell to infrared heating.

Rotating the part may improve radiation coverage.

Radiant Surface

Radiant Surface

Complex, highly three-dimensional shapes are difficult to heat uniformly

with infrared. The shaded areas of the shapes show where undercuring is likely

to occur. Beware of overheating of surfaces closer to the burners.

Figure 6Suitable Product Configuration for Effective IR Heating

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ha ng in random ori-

entat ions, th e quality

of th e finish ma y be

equally ra ndom.

Table 2 shows some

of th e points t ha t

ha ve to be considered

in selecting a h eat ing

system for a powdercoating line and how

the different heating

methods stack up in that regard.

In sum ma ry, convection ovens offer flexibility an d low risk of overheat -

ing, but at th e expense of size, speed and en ergy efficiency. Radiat ion

(infra -red) ovens deliver high pr oduction ra tes in a sma ll space an d

with low energy consum ption, but th eyre not as flexible an d requ ire

close cont rol to avoid overheat ing th e product. Combina tion ovens, a s

expected, r epresent a compr omise between t he t wo.

Keep in m ind tha t if th e product mix includes an y solvent -based coat -ings, the finisher will need a traditional convection oven, although he

can a dd a booster IR section for t he powder coat jobs. This booster m ay

also help speed up the line when curing solvent-based coatings, but

use carethe booster section must be enclosed and provided with

proper exhau st to keep solvents from escaping int o the workspace, and

its heat input must be carefully controlled to avoid finish defects re-

sulting from too-ra pid hea ting.

Ther e are differen ces of opinion whether its necessar y to hea t t he en-

tire workpiece to the curing temperature to get good adhesion of a

powder finish. In a convection oven, it s u na voidable; all port ions of theworkpiece an d finish will come t o the sa me t empera tu re because of th e

way theyre h eated.

With IR heat ing, th is isnt n ecessar ily tru e. The ra diation st rikes th e

coat ing first, an d th e only heat r eaching the workpiece under neat h is

residua l radiat ion pa ssing thr ough t he coat ing and condu ction from

th e coat ing or other locat ions on t he workpiece. If the workpiece is

th in, such as a sheet m eta l part , its probably safe to assu me it will be

within a few degrees of th e coat ing tem pera tu re. If, however, its thick

or h as a complex shape, uniform temper at ur es are less likely.

Pr ocess data sheets issued by coat ings manu factur ers usu ally relatecuring times to substra te temperat ures, and th is suggests th at the en-

tire workpiece mu st be heat ed. Among coat ing tech reps sur veyed,

th ere seemed to be genera l agreement t ha t a s long as th e surface of

th e piece in cont act with th e coat ing was up t o temper at ur e, th at was

sufficient. Bonding a nd adh esion ar e affected only by the t empera tu re

of the su rface in direct cont act with th e coat ing.

In short , if th e work piece is th in and well-exposed to the source of ra-

diation, size the bur ners to heat everyth ing, because th at s wha t

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Table 2Comparison of Heating Methods

Parameters

Straight

Convection

Straight

Radiation

Floor space required High Low

Energy consumption High Low

Heating speed Low High

Product configuration Any Simple only

Product mix (shapes & weights) Many Limited

Susceptibility to overheating Low High

IR Burner Sizing:

To Heat the Part or Not?

Guidelines for

Select ing

a Heat ing Method


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is most likely to happen. On t he other ha nd, if the par t is complex,

hea vy or poorly exposed to th e ra diat ion, like a cast wh eel, its en tir e

ma ss wont r each the curing temper at ur e, and input can be scaled

down somewhat . Exactly how mu ch, u nfort un at ely, is a judgment

call. If in doubt, go heavy on t he in put an d pr ovide a cont rol system

with good turndown capability. Multiple control zones, allowing

some bur ners to be tur ned off if necessar y, may be the best ap-

proach.

Factors Affecting These are no different for powder coating than for any other product, but

now is a good time to review them.

The Burne r Head Must See What I t Heats . Radiation t ra vels

only in str aight lines, so if par t of th e work load isn t in t he bur ner s

line of sight, it will not receive direct radiation. It may, however,

be heated by radiation reflected from other surfaces or conduc-

tion from hotter

portions of the

same piece. If in

doubt , visua lize th eworkpiece as being

illumina ted by flash-

lights locat ed wher e

th e burn ers will be.

If portions of the

workpiece will be in

the shadows, they

will not see direct

radiat ion from t he

burner heads.Radiat ion Scat-

ters . Although ra -

diat ion t ra vels in str aight lines, it doesnt a ll tr avel in pa ra llel

lines. The beam of ra diant energy leaving th e burn er h ead

spreads a s it goes away from t he bur ner, mu ch like the beam of

a sea rchlight (Figur e 7). The a ngle at which it sprea ds varies

with th e configura tion of th e burn er, becau se th e burn er h ead is

a focusin g device, sam e as th e reflector in t he light. If th e beam

an gle is na rr ow, the r adiat ion r emains highly concent ra ted, even

at consider able dista nces from th e burn er, but it covers a limitedar ea. Wide beam a ngles provide rapidly increa sing ar ea cover-

age, but th e inten sity of th e ra diat ion quickly decrea ses with in-

crea sing distan ce from th e burn er (Figur e 8). From an a pplica-

tion sta ndpoint, this mean s tha t a solid arra y of burn er hea ds

isnt r equired t o produce a un iform pat ter n of ra diat ionth e

overlapping beam s will provide it. The far th er t he workpiece is

from th e burn er h eads, the more uniform the ra diation will be,

alt hough it will also be less concentr at ed. The optimu m

Figure 7Radiant Energy Spread From An IR Burner

IR Burner Placement

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burner-to-product spacing is one that will heat the product evenly without

a lot of radiation escaping past the edges of the product.

In most IR ovens, the work is continuously moving past the burn ers. This

also tends to even out the heat flow to various portions of the work, but its

still a good idea to stagger or zig-zag burner head patterns to get uniform

coverage, especially on large pieces.

Product surface characteristics are probably the bigge st variable

in radiation transfer, and, unfortunately, theyre the least understood.

Its not tha t difficult t o determine the radiating power and pat tern of aburner head, but that s only half the picture. No surface will absorb a ll the

radiation that str ikes it; some is reflected away. The absorpt ivity of a sur -

face (its ability to accept radiant energy) is mainly a function of its textur e,

although color and temperat ure also play a part. Highly polished metal

surfaces may absorb less than 10% of the radiat ion that strikes them,

while a rough, oxidized or sandblasted surface could absorb up to 90%.

Theres no reliable way to calculate absorptivitieswhat little data that

exists has been determined by experimentat ion, and very litt le work has

been done on powder coatings applied to metal surfaces. In any case, they

will vary with the coat ing thickness and the size of the powder granules.

On the basis of data for other materials, however, absorptivities probably

average from 0.4 to 0.6 (40 to 60% of the radiation absorbed). In a ll likeli-

hood, the absorptivity is initially higher, but decreases as the coating melts

and presents a smoother, more reflective surface to the incoming radiation.

Gas vs. Electric IR: As radiant heat ing of powder coatings has gained popularity, many of the

old gas vs. electr ic debates have resurfaced. One which has gone on since

the earliest days of IR drying of solvent-based pa ints is tha t because colors

absorb radiation at different wavelengths, the IR source should be able to

be tuned to match the paint . It seems to have its origin in the observation

Is the Ability to

Tune Radiation

an Asset?

Greater Areaof Coverage

I.R.Burner

Target

Wide Beam Angle

I.R.Burner

MoreConcentrated

Radiation

Target

Narrow Beam Angle

Figure 8Wide vs. Narrow Radiation Beam Angles

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that some colors (reds are frequently mentioned) dont dry as readily as

others, like black. The spectra l absorpt ion characteristics of different colors

have been credited (or blamed) for the difference, on the theory that the

wavelengths put out by the IR emitter werent properly matched to the ab-

sorption frequencies of the pigment. The ability to control the emitter tem-

perature, and therefore, the frequencies emitted, is said to produce more ef-

fective heat ing. The electr ic IR manufacturers have made par ticularly ef-

fective use of this argument.

There is only one problem with this argumentit isnt so. One of the rea-

sons many solvent-based red paint s are slower to cure is tha t they contain

less pigment and more solvent. Why? Red pigments ar e among the most

expensive, so paint manufacturers try to use as litt le as possible, depend-

ing on high solvent contents and, possibly, other additives to achieve uni-

form color coverage. Either way, it means more liquids to evaporate and

slower curing times. Conversely, colors like black are so effective at cover-

ing that they can use lower solvent contents and can be applied in thinner

coats.

The same logic applies to powder coatings, particularly with respect tocoat ing th ickness. All of th e tech reps int erviewed said they could not

at tr ibute differen ces in powder cur ing speed to color.

It is not difficult to unders ta nd if you look a t F igure 9, which shows the

ra diant energy distr ibution vs. wavelength for var ious emitt er t em-

pera tu res. So little of th e total r adiat ion falls into the visible spectr um,

20

15

10

51 2 3 4 5 6 7 8 90

0.4

Microns

0.7

Microns

This band represents visible w avelengths; it is the only po rtion of the

total area under the above curves which can be affected by the

pigment co lor.

Black

Body

Radiation,

Btu/hr./

sq.

ft.

in

1000s

Wavelengths, M icrons

2200F Source

1800F Source

1500F Source

Figure 9Radiant Energy Distribution vs. Wa velength


11/121 1

regardless of temperature or receiver color, that its effect on total heat

transfer is negligible. Most of the real work is taking place to the r ight of

the 0.7 micron line, where the infra-red spectrum begins. The real issues to

consider in gas vs. electric radiant heating are system costs, energy costs,

controllability, maintainability and ease of opera tion. A well-designed gas

system with modern controls can match or beat an electric system in a ll

these areas.

Summary Powder coatings are the fastest-growing segment of the product finishingindustr y. This growth is being driven by their freedom from VOC emis-

sions, their r educed volume of hazardous wastes to dispose of, and their

ability to be cured more quickly than liquid finishes. This presents a major

opportunity for oven and burner manufacturers to upgrade customers cur-

ing lines. Radian t, convection and combina tion ovens a re a ll feasible ways

to cure powder finishes; the best choice will depend on factors such as prod-

uct mix, configuration and temperat ure sensitivity.

References Eclipse Guide to Infra-Red Heating, Publicat ion SP-193, Eclipse Com-bustion, Rockford, IL, 1992.

Powder Coating, Douglas Richart , Pr oducts Finishing 1989 Directory,

October 1988.

Curing methods and oven designs that save energy with reduced air

movement, Bruno Ezerski and Bob Stoffel, Powder Coating, Decem-

ber, 1991.

Convection and infraredwhat to consider in making a choice, H.

Robert Stroman and Barbara J. Unruh, Powder Coating, December,

1 9 9 1 .

High-intensity infrared and convection curinga comparison, Dale A.

Hillman, Powder Coating, June, 1991.

Special considerations for designing a powder coating line, Sherrill A.

Stoner , Powder Coat ing, J un e, 1992.

OEM Coating Trends: Yesterday, Today, Tomorrow, Patrick A.

Hammond, Finishing 91, Vol. 7, No. 3, Society of Manufacturing Engi-

neers, Third Quarter 1991.

Getting the right formulationwhat every end user needs to know,Steven L. Kiefer, Powder Coating, June, 1991.

Handbook of Chemistry and Physics, 40th Edition, Chas. D. Hodgman,

ed., Chemical Rubber Publishing Co., Cleveland, OH, 1959.

Air Pollution Engineering Manual, Anthony J. Buonicore & Wayne T.

Davis, eds., Air & Waste Management Association, New York, NY,

1 9 9 2 .


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curing powder coatings with gas-fired equipment

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