The work described in this paper was not funded by the US. Environmental Rotmion Agency. The contents do not necessarily r d l m the views of the Agency and no official endorsement should be inferred

NOVEL ACRYLIC CURE POLYESTER POWDER COATING RESIN TECHNOLOGY

. Eric Dumain Reichhold Chemicals, Inc. 2400 Ellis Road Research Triangle Park, North Carolina 27709-3582

INTRODUCTION

increasingly stringent environmental regulations within the industrial finishing industry have caused coatings formulators to focus on the development of technologies that meet the dual requirements of environmental compliance and high performance. Powder coatings, as a result, are enjoying double digit growth as industrial finishers look for ways to meet these challenges.

Powder coatings have been projected to account for 15 ?A of the overall NorthAmerican industrial finishing market by the end of the decade. ’ As powder replaces traditional liquid paint technologies on an ever-increasing basis, tougher performance standards for powder coatings are being demanded. These new conditions have led to the development of novel powder coating resin technology that offers the formulator an excellent approach to meting the needs of today’s challenging coatings environment.

POWDER COATINGS BACKGROUND

Powder coatings, unlike conventional liquid paint technologies, contain no solvents (1 00 % solids), and arc virtually 100 % VOC h e . A major benefit is its’ high material utilization rate. OversFnyed powder can be reclaimed and, after mixing with virgin powder, can be reused. Other economic advantages of powder are the reduced costs of waste disposal and pollution control. (There is no liquid paint sludge, and spray booth air can be recycled to the plant facility.) Additionally, the choice of color is as varied as in liquid coatings, and special effect finishes are also possible through the use of various additives.

A typical powder coating consists of a thermosetting resin, crosslinker, flow and levelling agents, pigmentation, and inorganic fillers. These solid materials am premixed either in low cr high intensity mixing equipment prior to k i n g melt-mixed‘in a compounding extmder.2uring the extrusion process, the resinous mixture is heated above its’ melting point and made homogeneous through mechanical shear. Therefore, proper temperaturdshear relationiship is key to obtaining optimum dispersion of the coating’s components. The molten extrudate is usually cooled by the use of chill rolls, or a cooling belt, whereby it is converted into a crushable ribbon. This ribbon is then pulverized and classified to the desired particle size for application. For general metal applications, a 40 micron average particle size powder coating is common. Figure 1 shows a schematic representation of a continuous powder coating production pmess.

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I

”

i

FIGURE 1 Powder Manufacturing Process

Curative n Additives

premixer

I PI Filli 455 g & Pack ing

The powder can subsequently be applied to a grounded metal substrate with an electrostatic (corona charging) spray gun. Other application methods are also possible: tribocharging, whereby the powder particles receive a frictional charge as they pass through the spray apparatus, and fluidized bed dipping. Tribocharging is suitable for metal fabrications that have unusual geometries, and would therefore be vulnerable to Faraday cage effect. Fluidized bed dipping is utilized in applications such as pipe coatings that require film thicknesses in excess of.10 mils. The final step in the process is the curing of the powder coating. This can be accomplished by gas or electric convection ovens, infrared cure, or a combination of both convection.and radiation. During the cure cycle, the powder fuses, melts, and flows to form a continuous film. After a suitable cure cycle, the coating will have developed a highly crosslinked system, with the resultant physical properties.

Powder coating technologies can be broadly classified 8s either thermosetting or thermoplastic. Thermoplastic powders, the oldest powder coatings ttchno,logy, include materials such as nylon and polyvinyl chloride. Unlike the particle size mentioned above, the thermoplastics have a much larger average particle sue (> 100 microns), as it takes much more energy to grind them3 After the initial cure cycle they can be remelted if desired, and will solidify upon cooling. Thermosetting powders, conversely, cannot be remelted once they have undergone cure. The materials that are used for these powder coating vehicles (resin and crosslinker) are usually high molecular weight, crystalline polymers. They must have a carefU1 balance of rheological and reactive properties. Additionally, they must be low enough in Tg to have suitable flow properties, but at the same time, be stable under long-term storage conditions. The thermosetting variety are more widely used, comprising approximately 91 Yo of current U.S. powder consumption. 4

,

6-4 1

The thermosetting chemistries can be grouped as epoxies, polyesters, epoxy-polyester hybrids, and acrylics. As is the case with other coatings technologies, each powder chemistry has its’ inherent advantages and disadvantages. The oldest thermosetting powder technology involves the use of epoxy resins which are diglycidyl ethers of bisphenol A (DGEBA), crosslinked with amines, anhydrides, dicyandiamide, or linear phenoic curatives. While this system has excellent corrosion and chemical resistance, its’ exterior durability and overbake resistance is poor.

Saturated polyester resins followed, and a host of curing agents are commercially available for these polymers. These polyesters are generally classified as carboxyl fuctional or hydroxyl functional.

The carboxy polyesters can be cured with epoxy functional resins. These systems are referred to as polyester-epoxy hybrids, and are characterized by very good film smoothness and mechanical properties. They also exhibit the problems associated with straight epoxy systems, that is, fair to poor exterior durability. One system that does offer excellent exterior durability features a trifunctional curative- triglycidylisocyanurate, or TGIC. However, recently published toxicity data has raised some health concerns for personnel. using this material. ‘Generally, an overall advantage of the epoxykarboxy crosslinking mechanism is that it proceeds by addition, hence no volatiles are evolved during the cure cycle.

Hydroxyl functional polyester resins can be crosslinked with several different hardeners, the most widely used being blocked isophoronediisocyanate (IPDI). Epsilom caprolactam is typically the blocking agent used in the IPDI prcpolymers. Coatings made with these materials are referred to as . polyurethanes and exhibit good film smoothness, exterior durability, storage stability, and mechanical properties; Disadvantages of polyurethanes include elevated bake temperature requirements (> 350 F), and the evolution of small amounts of volatiles during cure. A portion of the blocking agent sublimes during the cure process. In inadequately vented ovens, it appears as a bluish haze, and is perceived to be a health-hazard. The effectiveness of infrared curing lanips can also be decreased as the x -caprolactam accumulates on the heating surfaces. As more powder coatings end-users utilize IR curing, solutions to this problem will need to be developed. Possibly as a proactive measure, a growing number of urethane users are requesting “ -caprolactam-free” coatings, without sacrificing the performance benefits of smoothness and flexibility normally associated with this technology. One such alternative will be discussed in this paper.

Another major class of powder coatings arc the acrylics. They can be hydroxy, carboxy, or glycidyl functional. The crosslinking of these resins occurs much in the same fashion as their polyester analogues. The OH and COOH acrylics can be cured with the same materials as the polyesters. However, they have not seen the same acceptance as the polyesters due to poorer mechanical properties and high cost vs. the polyesters.

The glycidyl functional acrylics have seen recent growth because of their excellent exterior durability, relatively low cure temperatures (300 F), and film smoothness. Acrylics based upon glycidyl methacrylate (GMA) crosslinked with linear dibasic acids such as dodecanedioic acid (DDDA) have been found to be the highest performing thermoset powder coating in terns of exterior durability and acid rain resistance. This technology does have a few disadvantages, however. Gh4A acrylics usually have lower mechanical properties such as flexibility and impact resistance than polyesters. Additionally, ‘formulated GMA acrylic/DDDA powder coatings (or “pure” acrylics) typically exhibit incompatibility with other powder coating chemistries. A polyester coating contaminated with less than 1 % of a Gh4.A acrylic coating will Cause massive film defects (i.t. cratering). It is not the acrylic component that

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-

induces the incompatibility, but rather the use of siloxane flow modifiers within the acrylic powder coating that give the undesired result. The observed cratering is the end result of the large surface tension gradient between the polyester and acrylic films. -

Researchers at Reichhold Chemicals, Inc. and its’ parent company, Dainippon Ink and Chemicals (DIC) of Japan, have sought to develop unique resins for the powder coatings industry that would help overcome some of the above mentioned problems and disadvantages, while advancing the state-of-the-art in powder coating technology.

Two examples of our progress here include the development of high performance coatings for precoated metal (PCM) and development of powders with extremely low gloss that are highly reproducible. The new resins should also find utility in general metals applications that require a high level of performance.

DEVELOPMENT OF NOVEL CURING MECHANISMS

A novel acrylic cure technology has been developed that features the crosslinking of carboxyl functional sites on polyester resins with glycidyl methacrylate (GMA) acrylic curing agents. This addition chemistry, referred to as an acrylic/polyester hybrid, has inherent advantages over the conventional carboxy/glycidyl powder technologies described previously in that it combines good exterior durability with lower associated health risks. This curing mechanism produces uniform, high- gloss finishes with film smoothness compkble to that of a standard TGIC-cured coating. At the same time, it can be considered an alternative to -caprolactam-containing urethane powder coatings. The curing mechanism is shown in Figure 2.

ACRYLIC CURE POLYESTER

FIGURE 2

Carboxyl Polyester

I GMA ACRYLIC I U.S. Patent 4,499,239

4 b o

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A second curing mechanism similarly involves the utilization of GMA acrylic curing agents with a polyester resin that has bifunctionality, that is, there are both carboxyl and hydroxyl groups available for crosslinking. An isocyanate is used here to react with the OH groups of the polyester. However, the required amount is much less than the standard polyurtthane, so the associated volatile level is lower. Figure 3 shows the curing mechanism forthis system. Again, coatings based on this approach show good film smoothness, mechanical properties, and very good exterior durability.

MULTI-CURE ACRYLICPOLYESTER

Patent Pending FIGURE 3

Bifunctional Polyester

\ OH

NCO

L-+- GMA ACRYLIC

A third approach also uses the bifunctional polyester. As before the carboxyl groups of the polyester react with the oxirane groups of the acrylic curing agent, and the hydroxyVisocyanate reaction takes place. However, the GMA acrylic hardener in this case also has OH groups, and these can also react with the NCO curative. This curing mechanism is referred to as a multi-crosslinking system. It has a high crosslink density and exhibits the benefits of urethane and acrylic chemistries. Although slightly more blocked isocyanate is used in this system, it is still less than would be found in a conventional polyurethane powder coating. Formulated coatings based on these materials exhibit excellent smoothness, flexibility, chemical and stain resistance, as well as good exterior durability. With this balance of properties, the bifunctional polyester/acrylic has the attributes necessary to satisfy most coating service environments. The reaction diagram for this system is shown in Figure 4. Formulations for the three acrylic/polyester systems arc listed in Table 1, with their corresponding cured physical properties listed in Table 2. It should also be noted that these acryIic/polyester curing mechanisms do not ‘cause the contamination problems associated with “pure” GMA acrylic powder coatings.

. -.I

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MULTI-CROSSLINKING ACRYLIC CURE POLYESTER

FIGURE 4

/ Bifunctional Polyester U.S. Patent 4 8 18,79 1

\

r^ OH

/” e3 GMA ACRYLIC

I OH

NCO

BLOCKED NCO

NCO

PRE-COATED METAL FINISHING

Pre-coated metal (PCM), or blank, finishing technology has been established commercially in Southeast Asia and Europe for several years, and is now being developed in North America. The concept is similar to liquid coil coating, in that it involves the coating of flat steel stock. However, powder coating of blanks gives edge coverage usually not associated with liquid coil coating.

Blanks are cut to size, holes punched in the blanks, and then the piece is powder coated. After the coating has cured, the blank is formed into the shape of a refrigerator, laundry machine, or file cabinet, for example. Since the blanks are subjected to large amounts of bending, (also called post- forming) excellent flexibilty is a prerequisite for the applied powder coating. Additionally, the appliances will ultimately be exposed to severe operating conditions, (washing machines, for instance) and the powder coatings applied to these products must have superior resistance to detergents and other chemicals that can stain or damage the finished product. For these reasons, the above mentioned multi- crosslinking technology (Figure 4) is an excellent approach to this new coating process.

A unique feature of blank lines is that they typically use infrared cure technology. Gas convection curing dominates the general metal finishing markets, but infrared cure is preferred for the PCM process because of its’ efficient mode of heat transfer. Unlike convection curing, which cures the coating by transferring heat from the substrate to the film, infrared energy is absorbed directly by the coating. Also, the IR energy must “see” the material that it is targeted for. Therefore, the simple -geometry of a PCM application line is well-suited for IR cure. With this arrangement, it has been found that the blocking agent in urethane coatings is not suitable for IR ovens, and hence a new challenge for the PCM market hasoccurred. That is, PCM-grade coatings are most desirable when they are -

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caprolactam-free. The commercially available internally-blocked ( -caprolactam-free) IPDl crosslinker, because of its’ higher melt viscosity and slower reactivity, generally gives powder coatings with poorer film appearance and lower mechanical flexibility. Multi-crosslinking powder coatings formulated with this -caprolactam-free isocyanate exhibit only minor decreases in film appearance while still retaining their post-formability. Table 3 shows a physical property comparison of multi- crosslinking power coatings with -caprolactam blocked IPDI and -caprolactamafree IPDI.

LOW-GLOSS POWDER COATINGS

Interest in low-gloss powder coatings continues to increase, particularly in the area of exterior durable coatings. Reproducible low-gloss coatings are well-established in the epoxy and epoxy-polyester hybrid coating technology segments. However, for exterior applications, these chemistries are not suitable because of their tendency to easily chalk and yellow.

Previously, methods for producing exterior durable powders relied upon the use of flattening agents such as silica, or by dry-blending polyester powders of differing reactivities together. Such techniques have frequently resulted in an inconsistent matte appearance, or have been dependent upon carefully controlled extrusion and curing conditions. The dry-blending method has also been shown to be quite labor intensive and impractical when carried out in a production environment.

The novel acrylic cure powder coating resin technology is well-suited for this coatings challenge. Both the carboxy polyester/GMA acrylic and bifunctional polyesterlGMA acrylic/NCO (multi-cure) systems can be produced in a full range of gloss, from matte to high gloss, with excellent smoothness and mechanical properties. In addition, these systems have excellent gloss stability over a wide range of cure cycles. By varying the ratio of the GMA acrylic hardeners- Fine-Clad@ A-249-A and A-229-30-A for the bifunctional polyester type, Fine-Clad@A-tWA and A-229-30-A for the carboxy polyester type- a consistent and predictable gloss level is achieved. Tables 4 and 5 display powder coating formulations and physical property test data for the multi-cure systems in a full range of gloss. Table 6 illustrates the linear relationship between GMA acrylic hardener ratios and 60’ gloss in these prepared powder coatings. Table 7 displays the gloss stability of a formulated matte finish (Formulation I -Table 4) for a timehemperatwe cure matrix. This gloss stability is especially critical for end-users that paint substrates of varying mass on the same finishing line, where actual metal temperatures may widely fluctuate.

CONCLUSION

Industrial finishing is changing rapidly. Today’s strict environmental laws pose significant challenges for coatings end-users. Powder coatings have demonstrated their capabilities as environmentally sound alternatives to the traditional solvent-borne coatings technology. The novel acrylic cure polyester approach presented here allows the industrial finisher to apply superior coatings, while significantly reducing waste and VOC-type pollution. The acrylic/polyester approach provides state-of-the-art performance in the new pre-coated meWblank, low-gloss, and exterior durable polyester arenas.

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REFERENCES 1. Bocchi, G. Powder Coatings- Markets and Applications, Powder Coatings Institute, Alexandria, Virginia, 1993. 41 pp. 2. Glossary of Powder Coating T e n s . Powder Q&ting. 57, 1994. 3. Q’Donnell, G. Recent Developments in Thermoplastic Powder Coatings. Paper presented at 1993 13th International Conference, International Centre for Coatings Technology, Brussels, Belgium. November 15-1 7, 1993. 4. Skeist, Incorporated. POWDER COATMGS 11 A Multiple-Client Study. Skeist, Incorporated, Whippany, New Jersey, September 1990. 5. Bate, D.A. Low Gloss Curing Derivatives. la : The Science of Powder Coatings Chemistry, Formulation and Application. Vol. 1. London, United Kingdom, 1991. p.75.

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TABLE 1: ACRYLICPOLYESTER FORMULATIONS

DESCRIPTION

Formula No.

FINE-CLAD@ "8400 Carboxy Polyester (I)

FINE-CLAD@ A-2.39-J Bifunct.Polyester (I )

FINE-CLAD@ A-239-X Bifunct.Poiyester (1)

FINE-CLAD@ A-22.9-30-A GMA Acrylic (1)

FINE-CLAD@ A-244-A GMA Acrylic (1)

Huls B-I 530 lPDl (E-caprolactam blocked)

Actiron DBT (2)

Modaflow 2000 (3) Benzoin

Ti02

(1) Reichhold Chemicals, Inc.

(2) Dibutyl Tin Dilaurate- Synthron, Inc.

(3) Monsanto, Inc. I

"ACRYLIC CURE

1 80.0

- -

20.0 - -

- I .5 0.5

43.0

"MULTI- "MULTI-X- CURE

2

76.0 -

15.0 - 9.0

0.6 1.5

0.5 43.0

LINKING"

3 - -

72.5 -

11.0

13.5

0.3

I .5 0.5

43.0

6-4 8 I

c

TABLE 2 : PHYSICAL PROPERTIES FOR ACRYLIC/CURE POLYESTERS

DESCRIPTION

FORMULA NO.

Baking Schedule, min.?F

Film Thickness,mils

Gloss,60°/200

"b value

PC1 Flow Rating (1-1 0) 10=Best

PenGI Hardness

Gardner Impact:F/R (5107, IN.-LBS.

Mar Resistance

Crosshatch Adhesion

Solvent Cure Test-PC1 # 8 B- (1)

Inclined Plate Flow (PC1 # 7), mm

Gel time (PC1 #6), seconds

100 % Overbake Resist., Gloss 6Oo/2O0

100 % Overbake Resist., "b" value

Delta b

Xenon Arc Weatheremeter Exposure: % Gloss Retention after 500 hours .

'ACRYLIC CURE"

1

101400 F

2.5

9 1/66

0.35

3

H

40120

Excellent

100%

patlfull cure

50 mm

30-35 sec.

90/63

0.65

0.3

65%

(1) Powder Coating Institute Test ## 8B: 100 Double Rubs with MEK I

'MULTI-CURE" 'MULTI-X-LINKING"

2 '

101400 ' F

2.1

91/62

-0.14

4

H

60160

Good

100%

part./full cure

85 mm

65-70 sec.

88/50

.

0.16

0.3

75%

3

101400 F

2.4

93/82

0.7

6

2H

120/120

Excellent

100%

Full cure

100 mm

45-50 sec.

92ff 5

1.4

0.7

50%

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TABLE 3: MULTI-CROSSLINKING POWDER COATINGS: COMPARISON OF -CAPROWCI'AMtontaining and - CAPROLACTAM-free coatings

FINE-CLAD @ A-239-X Bifunct.Polyester (t) FINE-CLAD @ A-244-A GMA Acrylic (1) HUls B-1530 IPDl (E-caprolactam blocked) Huls BF-1540 (E-caprolactam-free) Actiron DBT (2) Modaflow 2000 (3) Benzoin TiOz

(1) Reichhold Chemicals, Inc. (2) Dibutyl Tin Dilaurate- Synthron, Inc. (3) Monsanto, Inc.

Baking Schedule, m i n h Film Thickness,mils Gloss,60°/200 "b" value PC1 Flow Rating (1-1 0) Pencil Hardness Gardner Impact:F/R (5/8"), IN.-LBS. Mar Resistance Crosshatch Adhesion Solvent Cure Test-PC1 # 8 A- (4) Inclined Plate Flow (PC1 # 7), mm Gel time (PC1 #6), seconds 100 % Overbake Resist., Gloss 60°/200 100 % Overbake Resist., "b" value Delta b Flexibility: T-Bend (4) Powder Coating Institute Test # 88: loo double rubs with MEK

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A 72.5 11.0 13.5

- 0.3 1 .o 0.5

43.0

10/400 F

2.4 93/82

0.7 6 2H

12011 20 Excellent

100% Full cure 100 mm

45-50 sec. 92/75

* 1.4 0.7 2T

B 72.5 11.0

- 14.4 1.5 1.5 0.5

43.0

10/400 F

2.3 91/58

0.3 5-6 2H

60160 Fair

100% partial cure

99 mm 95-1 00 sec.

88/49

1.4 1.1

5-6T

TABLE 4: POWDER COATING FORMULATlONS FROM MATTE TO HlGH GLOSS

Formulas I-VI: Bifunctional PolyesterlGMA Acrylic/ IPDI;

FINE-CLAD *A-239-X Bifunct.Polyester (1) FINE-CLAD" A-229-30-A GMA Acrylic (1) FINE-CLAD"A-249-A GMA Acrylic (1) FINE-CLAD" "8400 Carboxy Polyester (1) FINE-CLAD A-244-A GMA Acrylic (1)

Dodecanedioic Acid (2) .

Actiron DBT (3) C ,, Z imidazole (4) Modaflow 2000 ( 5 ) Benzoin

HUIS 8-1530 IPDl

TiOl

1

64.5 0.0

24.0 - -

12.0 1.5 0.2 0.2 0.5 0.5

43.0

ll

64.5 8.0 12.0 -

- 16.0 1.5 0.4 0.2 0.5 0.5

43.0

lu 141 Parts by weight 64.5 64.5 10.0 12.0 10.0 8.0

- - -

16.0 16.0 1.5 1.5 0.4 0.4 0.2 0.2 0.5 0.5 0.5 0.5

43.0 43.0

Y

64.5 14.0 6.0 - -

16.0 1.5 0.4 0.2 0.5 0.5

43.0

- VI

64.5 16.0 4.0

- 16.0 1.5 0.4 0.2 0.5 0.5

43.0

(1) Reichhold Chemicals, Inc. (2) DuPont Plastics (3) Dibutyl Tin Dilaurate- Synthron, Inc. (4) Air Products, Inc. (5) Monsanto, Inc.

PREMIX: Each batch blended in Henschel mixer for 30 seconds @ 2000 RPM. m: Werner & Pneiderer ZSK-30 twin-screw extruder: 250 RPM screw speed Feed rate = 60 Ibs./hr., B m l temperatures: Zone 1 = 90 C, Zone 2 = 80 C.

Extrudate passed through Strand chill rolls and ground using a Brinkmann ZM-1 centrifugal mill @ 15,000 RPM. Powder classified through a US. 200 mesh screen (75 microns) and sprayed onto cold rolled steel with a Nordson Versa-Spray electrostatic spray gun.

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TABLES :AcwliclPolvcster Powder Coating Evaluations: MATE TO HIGH GLOSS

Formulas I-VI: Bifunctional PolyesterlGMA Acrylic/ IPDl

Formula No. Baking Schedule, min.PF Film Thickness,mils Gloss,6Oof2O0 "b value PC1 Flow Rating (1-1 0) Pencil Hardness Gardner ImpactFIR (5/8"), IN.-LBS. Mar Resistance Crosshatch Adhesion Solvent Cure Test (PC1 # 8 6) Inclined Plate Flow (PC1 # 7), mm Gel time (PC1 #6), seconds 100 % Overbake Resist., Gloss 6O0/2O0 100 % Overbake Resist., "b value Delta b

I I! J!! IY Y 1 0/400°F 10/4OO0F 1 0/4OO0F 1 0/4OO0F 1 0/4OO0F 2.5 mils

1on 0.7 6 H

160/160 good 100%

full cure 40 mm

30-40 sec. 10/2 0.9 0.2

2.3 mils w3 2.3 4 H

160/160 good 100%

full cure 51 mm

3540 sec. 24/4 2.75 0.45

2.0 mils 4018 1.7 4 H

16011 60 good 100%

pahcure 56 mm

3540 sec. 4015 2.1 0.4

2.6 mils 5511 3 3.0 4 H

16011 60 good 100%

part.cure 54 mm

45-50 sec. 56/12 3.5 0.5

2.0 mils tot22 1 .o 5 H

100/100 good 100%

part.cure 59 mm

45-50 s e t 67/22

1.9 0.9

Yl 1 0/4OO0F 2.0 mils 86/42 2.9 5 H

100140 good 100%

part.cure 59 mm

45-50 sec. 85/46

3.3 0.4

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90

80

70

60

50

40

30

TABLE 6:Matte to High Gloss AcryliclPolyester . Powder Coatings

1 - 60 Degree Gloss 1 I

I

I I - I

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6 p o GLOSS I O MINUTES @ 180 C 9.1

20 MINUTES @ 180 C 8.0

30 MINUTES @ 180 C 7.5

10 MINUTES @ 200 C 8.6

20 MINUTES @ 200 C 8.1

30 MINUTES @ 200 C 9.5

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