Advances in High-Perforknce Powder Coatings for Automotive Exterior Trim by Champ Bowden, Dawn Ostrander, and Susan 111. Miller

Ferro Corp., Cleveland

A s consumers’ demands have spurred auto- makers to design better vehicles with longer useful lives OEMs have looked to suppliers to

develop materials with improved performance prop- erties. To achieve their goal of producing a “Ten- Year Vehicle,” manufacturers seek components that meet increased, stringent weatherability require- ments while offering improved appearance, greater color capabilities, and increased durability. Materi- als also must be cost effective and environmentally friendly.

OEMs have specified powder coatings for exterior trim parts for the past two decades due to their superior toughness compared to liquid paint and the elimination of environmental concerns. Since styling concepts have incorporated greater use of black trim on body pillars and window frames, the UV-light durability of trim coatings has had to increase to approach that of the body paint.

Changes in OEM performance specifications for exterior trim are driving developments in powder coating chemistries. One North American auto- maker now requires polyesters to pass two-year Florida exposure tests and acrylics to pass five-year Florida tests. Previously approved polyester resin systems are “grandfathered” for current programs but will not meet the higher specifications for the future.

Other domestic OEMs are slowly incorporating increased weatherability requirements in their trim specifications and have added high-performance powder coatings to their lists of approved materials. And while European OEMs don’t use the system of specifications and approved products common in North America, many are asking suppliers to guar- antee a higher level of W durability for trim coat- ings.

POWDER COATING OPTIONS

Four basic resin systems currently are available for powder coating automotive trim parts: polyesters, glycidal methacrylate acrylics (GMAs), hydroxyl functional acrylics (HFAs), and super-durable poly- esters.

Polyesters were originally used for these applica-

tions because of their color and gloss range, smooth appearance, and abrasion resistance. Although their weatherability will not meet the new higher require- ments they remain in wide use for current products.

GMAs offer W durability comparable to that of liquid body paints but have not enjoyed widespread use due to incompatibility with polyester powders. Severe cratering results unless extreme caution is used in changeovers.

HFAs are newer resin systems developed to meet the higher performance requirements for weather- ability and mechanical properties. While technically in the acrylic family their formulations do not be- have like GMAs and do not present the severe con- tamination problems in changeovers with polyes- ters.

Super-durable polyesters also are newer resin sys- tems. Commonly used to coat architectural products in Europe they are superior to standard polyesters in weatherability, but some appearance issues limit their utility in automotive trim applications. Again, the term “polyester” is a bit of a misnomer; although the description is chemically accurate these systems actu- ally are blends of distinct powders, which do not be- have like typical polyesters and are not completely compatible with them.

Table I provides an overview comparing the per- formance properties of these four resin systems.

WEATHERABILITY

The greatest increase in OEM performance require- ments has been in W durability. GM& offer the best performance in this area with the ability to retain gloss after more than five years of exposure to direct sunlight; however, in addition to their incom- patibility with existing polyester systems, their cost is relatively high, they have less gloss range capa- bility, and their scratch and mar resistance is mar- ginal.

HFAs, which offer superior mechanical properties at a moderate cost, perform almost as well GMAs in Florida exposure tests (Fig. 1). Accelerated weath- ering testing using the Xenon Arc Weatherometer demonstrates HFAs’ clear superiority over standard polyesters and super-durable polyesters (Fig. 2).

North American automotive OEM specifications

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Table I. Performance Comparison Overview

Polyester GMA HFA Super-Durable Polyester

l-2 years weatherability 5+ years weatherability 5 years weatherability 5 years weatherabiity Good scratch and mar Marginal scratch and mar Excellent scratch and mar Good scratch and mar

resistance resistance resistance resistance Superb corrosion resistance Superb corrosion resistance Superb corrosion resistance Superb corrosion resistance Compatible with existing Incompatible with existing Moderate compatibility Moderate compatibility

trim products trim products with existing trim with existing trim products products

Complete gloss range Less gloss range capability Complete gloss range Less gloss range capability capabilities capabilities

Lower cost Higher cost Medium cost Medium cost Medium hardness Medium hardness Harder coating Medium hardness Smooth, nongrainy Smooth, nongrainy Smooth, nongrainy Grainy appearance at low

appearance appearance appearance gloss

call for either Florida or Xenon Arc Weatherometer test data. Another accelerated test method, Equato- rial Mount with Mirrors for Acceleration with Water (EMMAQUA), sometimes is used for evaluation by European OEMs. This method, which is performed outdoors using natural sunlight, demonstrates rela- tive performance similar to that shown by the Xenon Arc (Fig. 3).

MECHANICAL PERFORMANCE

When discussing mechanical performance of coat- ings for automotive trim parts the relevant charac- teristics are hardness and mar resistance due to the usage faced by door handles, window frames, and luggage racks. Impact resistance-traditionally in- cluded when considering mechanical properties of coatings for general applications-is not a factor in automotive OEM specifications for exterior trim.

Some trade-offs exist between weatherability and mechanical performance, as Figure 4 illustrates. GMAs, which are the best in terms of W durability, are comparatively poor in mechanical performance. The inverse is true of polyesters.

The newer resin systems have been engineered to

Figure 1. Florida durability (5” South- black box exposure).

bridge these gaps. Super-durable polyesters have better weatherability than standard polyesters and greater scratch and mar resistance, hardness, and flexibility than GMAs.

HFAs are not included in Figure 4 because their combination of characteristics has largely eliminated the trade-offs. Their weatherability approaches that of GMAs yet they offer excellent scratch and mar resis- tance and are harder coatings than polyesters, super- durable polyesters, or GMAs.

COMPATlBlLlTY

GMAs are well known for causing severe cratering when trace amounts contaminate polyesters com- monly used to coat trim parts. HFAs do not cause the same problem, for, although they are classified as acrylics, they are made from entirely different components with different chemical properties. Their behavior is more similar to polyesters than to GMA acrylics.

A changeover from a polyester powder to an HFA powder is no different than an ordinary color change. The only cleanup required is to run the polyester powder out of the spray module and the

- -Standard Polyester

O- 0 500 1000 1500 2000 2500 3000 3iOO 4000 d

Xenon Arc Weatherometer (Kl/m2)

Figure 2. Xenon arc weatherometer (SAE J1960 Cycle).

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Initial 280 klJ/mZ 560 tiJ/m2 840 MJ/m2

EMMAQUA (MJ/m2)

280 MJ/m2 EMMAQUA - 1 Year Florida Exposure

Figure 3. EMMAQUA durability.

hopper. In a changeover from an HFA to a polyester the spray module walls should be wiped and the hopper completely cleaned. Hoses and filters also should be changed, which is a recommended main- tenance routine during any cleanup.

Likewise, super-durable polyesters do not have the same chemical composition as standard polyesters. They are blends of dissimilar powders and require the same cleaning procedures for changeovers as do HFAs. In fact so-called standard polyesters are not all com- patible with one another. Cleaning is recommended for any powder change.

A recent design of experiment undertook a major compatibility study of standard polyesters, HFAs, and super-durable polyesters. The study found vir- tually no difference in compatibility with standard polyesters between HFAs and super-durable polyes- ters. At low levels of contamination (less than 10%) both powders create some appearance issues such as rough texture, pin holing, or dry spray. Neither, however, cause the massive cratering that occur with GMA contamination.

APPEARANCE

OEM specifications for exterior trim call for a smooth appearance at gloss levels ranging from 10 to 90. Tolerances are very tight, particularly at the low and very low gloss levels.

Super-durable polyesters can achieve a smooth ap- pearance only in a high-gloss finish. These powders can be formulated with typical methodologies. Low- gloss products, however, must be manufactured by a three-step process that involves production of two dis- tinct powders, which then are blended. This results in a grainy, speckled appearance. Because there is vari- ability in each manufacturing step it also is difficult for these powders to hold to the tight tolerance require- ments for low-gloss coatings.

MECHANICAL (Mar resistance. Hardness)

Less

Moderate

Good

WEATHEBABlLll’Y (UV durability)

Good

Moderate

Less

Figure 4. Relative performance characteristics.

Standard polyesters provide the required smooth- ness and gloss levels in a range of colors but cannot meet the more stringent weatherability specifications.

HFAs, which meet the five-year weatherability re- quirement, offer a smooth appearance at all gloss levels (see Fig. 5). OEM-approved products are available in a variety of colors at gloss levels ranging from 10 to 90.

GMAs provide superior weatherability and a smooth appearance but have less gloss range capability.

COSTS AND PERFORMANCE

GMAs also have the highest cost of the four chem- istries. This, combined with fewer available glosses, marginal scratch and mar resistance, and compati- bility issues, has limited their usage in automotive trim applications.

Polyesters have the lowest relative cost and meet mechanical performance requirements as well as most current weatherabilty specifications. They are likely to remain in wide use until competitive pres- sures push other automakers to match the new five-year W durability standard.

Super-durable polyesters cost more than stan- dard polyesters but less than GMAs. They meet five-year weatherability requirements and have good scratch and mar resistance but have appear- ance problems at lower gloss levels. Super-durable polyesters are used in Europe but more commonly as architectural coatings where smoothness is not as critical as in automotive exterior trim applica- tions.

The cost of HFAs is now comparable to that of super-durable polyesters. Although previous costs were somewhat higher, market adjustments of raw material prices for HFAs have put costs of both newer resin systems on par. Due to their five-year weatherability, excellent scratch and mar resistance, and smooth appearance at all required gloss levels, they have entered the market in the U.S. and Europe and performed well. (See Fig. 6).

May 1999 17

Figure 5. HFA powder coatings provide five-year weatherability in the full range of OEM-specified gloss levels for automotive exterior trim parts.

As automakers continue to strive for production of a true ten-year vehicle, performance require- ments for trim parts coatings will increase. Today’s standard polyesters will disappear from approved products lists. HFAs are well positioned to replace them as standard coatings for automotive exterior trim applications.

Figure 6. Excellent scratch and mar resistance makes HFAs a good choice for bumpers and they are available in a variety of colors and glosses.

BIOGRAPHIES

Champ Bowden is Market Manager, Automotive Powder Coatings, based in Troy, Mich.

Dawn Ostrander is Product Manager, Automotive Powder Coatings, based in Troy, Mich.

Susan M. Miller is Technical Manager, Automo- tive Powder Coatings, based in Cleveland.

All are part of the Industrial Coatings Group of Ferro Corp. NW

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