Automotive plastics' challenge: 10-year paint durability.
Today's automotive coatings for plastics must travel an increasingly rough road. More sloping vehicle designs with more horizontal surfaces give increased visibility and uv exposure to plastic exterior and interior parts. And the car makers' goal of 10-year durability means that finishes for plastic parts must match the paint on steel body panels with regard to longterm gloss, color retention, and resistance to scratching, marring, and "environmental etch." Flexible plastics present a special challenge: The trick is to retain the durability of rigid coatings while being flexible enough to dissipate stress from impact.
As if those factors didn't make it tough enough to meet Detroit's 10-year challenge, the U.S. Environmental Protection Agency's push for lower VOC emissions throws yet another hurdle in the path. Formulators of solvent-based paints are working to keep ahead of the regulations and maintain their preferred position relative to the waterborne competition.
Still other issues include paint adhesion to TPOs and new developments in alternatives to conventional paints, including film laminates, powder coatings, and in-mold coatings.
TOWARD 'CLEANER' COATINGS
EPA mandates have prompted paint suppliers to look at a number of different approaches to lowering VOC content. Dr. Jack Hennessy, senior products program manager of DuPont Automotive, points out that many suppliers of painted parts to the auto makers - for example, molder-painters - are falling under more stringent environmental-emissions restrictions.
There are currently two main alternatives being pursued by paint suppliers to achieve the goal of lowering VOCs - "higher solids" solvent-based and waterborne coatings. Each has advantages and disadvantages. Advocates of the solvent-based approach say that higher solids content is not likely to require major retrofitting on the part of paint shops, unlike waterborne coatings. And EPA's latest VOC levels are achievable with solvent-based coatings in conjunction with solvent-abatement equipment, according to Dr. Rose Ryntz, technical specialist at Ford Motor Co.'s Plastics and Trim Products Div. in Dearborn, Mich.
Advocates of waterborne coatings, on the other hand, say that these finishes have better color and appearance, particularly when compared to higher solids solvent basecoats.
One way in which solvent-based coatings are achieving higher solids content is with reactive diluents, of which the new aldimine-based types that have emerged in the last two years look promising. Reactive diluents can help achieve coating solids levels as high as 60-70%, according to Ryntz.
Resins for higher solids two-component polyurethane fascia coatings are being developed by Bayer Automotive Coatings. The new systems reportedly use diluents to lower VOCs and to permit polishing without cryogenics.
Higher solids content in solvent-based coatings is also being pursued by DuPont Automotive with its Generation series of clearcoats. Its goal is to meet VOC regulations with little change in handling practices.
In basecoats, the move to higher solids has come up against aesthetic limitations. "Colors are muddier and metallics have less travel," says James Benefiel, technical director at BASF Industrial Coatings. "Solvent-borne is probably at its limit for basecoats."
A MOVE TO WATERBORNE?
Some paint suppliers see the appearance limitations of high-solids solvent-type basecoats as an argument in favor of waterborne systems. "Waterborne basecoats are not entirely driven by VOCs," says Dr. Jack Burgman, PPG Industries' technical manager of elastomeric coatings development. "A large driver for waterborne basecoats is the appearance issue. It's much easier to match a waterbase on a [steel] body with a waterbase on a plastic part."
James A. White, OEM business development manager at Akzo Nobel Coatings, seconds that view: "It seems that appearance requirements are the major reason some OEMs are recommending waterborne basecoat technologies. VOC savings have become a secondary reason. In fact, some paint shops are noticing that the tremendous cost of retrofitting or building a new facility may not justify the VOC savings." White says many shops are attempting to overcome the limitations of high-solids coatings. "The appearance requirements of the customer will play the deciding role."
BASF's Benefiel believes the gradual move of steel body colors to waterborne will eventually force plastic parts to be painted with waterborne, too. According to Steve Fisher, director of marketing for PPG's Automotive Trims Group, "Waterbase has great styling and aesthetic appeal, but it's cost-prohibitive." He adds that PPG has issued a challenge to suppliers to develop more affordable equipment with better transfer efficiencies for waterbase coatings.
Movement to waterborne clearcoats may be farther off than it is for basecoats, say some paint suppliers. One reason is the OEMs' demands for improved etch resistance and weatherability. According to Jim Carlisle, director of automotive marketing at Morton Industrial Coatings, waterborne clearcoats will probably fall short of meeting Detroit's 10-year goal. "Waterborne clearcoats overall won't have the same durability that you can get with a solvent-borne clear," he states.
In addition to the question of waterborne vs. solvent-based paints, there is the issue of one-component (1K) melamine-based coatings versus two-component (2K) polyurethane coatings. In exterior basecoats and clearcoats, 1K systems have traditionally dominated in North America while 2K coatings have been more widely used in Europe. An advantage of 2K systems is good etch resistance. Given the U.S. auto industry's 10-year durability goals, several paint suppliers expect 2K urethane coatings to gain predominance in exterior clearcoat applications such as fascias. BASF is testing a new 2K clearcoat for flexible plastics that is expected to become available next year.
On the other hand 1K paints have the advantage of easier application because there is no need to control mix ratios. This is particularly an issue where mix ratios vary from color to color.
DuPont sees a trend toward 1K basecoats, and is working toward a 1K clearcoat with improved etch and mar resistance. Development of a flexible 1K system with improved etch resistance and durability is also a goal at PPG.
Although 2K coatings usually require lower bake temperatures than 1K coatings, all Big Three auto producers have gone to higher bakes for both types of coatings to achieve better adhesion. This trend could be reversed if improved pretreatment technology succeeds in enhancing adhesion at lower bake temperatures.
TRENDS IN INTERIOR COATINGS
The trend toward waterborne coatings appears to be on better footing for interior applications. According to Akzo Nobel's White, "A large VOC disparity exists between water- and solvent-borne interior coatings. Consequently, a paint shop can spray substantially more parts with a waterborne coating within its VOC limits." He adds, "Because painters of interior components are generally smaller and use less overall volume of coatings than exterior painters, they are less likely to invest in solvent-incineration equipment."
So-called "soft-feel" coatings, which are gaining wider use in auto interiors, are also making the switch from solvent to water. According to Gary Baker, market-development manager for Dexter Coatings, waterborne soft-feel coatings are not quite as soft as comparable solvent paints but should equal them in durability. White of Akzo Nobel also expects these coatings to move toward waterborne in the future.
Suppliers say 2K PUR soft-feel coatings offer resistance to uv fading and degradation, superior scratch and mar resistance, and sound-deadening properties. Soft-feel coatings are replacing lacquer coatings and vinyl coverings on instrument panels. They are also especially suited to coating airbag covers as a means of preventing shattering when the bag deploys.
MORE PAINTABLE TPOs
Two different approaches are being taken to improve the paintability of TPOs, according to Dr. Satchit Srinivasan, research manager of engineered polyolefins at D&S Plastics International. One is the development of materials that can be directly painted without requiring any surface modification or adhesion promoter. The other aims at improving the painted-part performance of TPOs that still require pre-treatment.
D&S Plastics introduced the first commercial "directly paintable" TPO, called Ontex, in the mid-1980s. That material had limited success in the automotive market. By incorporating polarity into an inert substrate, Ontex achieved excellent painted performance, says Srinivasan. But the material had a limited property range and restricted coatings options that precluded widespread usage, he says.
Himont (now Montell Polyolefins) also developed a directly paintable TPO back in the late 1980s. It looked good in the lab and in initial field trials, but was shelved after more extensive customer trials. According to Steve Dwyer, Montell technology director of Advanced Materials product and applications development, the greatest challenge to developing a directly paintable TPO is achieving consistent paintability under a wide range of processing conditions. Another challenge is paint transfer efficiency, since elimination of the adhesion promoter reduces the conductivity of the TPO part.
D&S, Montell, and other suppliers continue trying to develop a universally acceptable, directly paintable TPO. Dwyer estimates that success may be three to five years away. Cost of such a grade will likely be 20-30% more than a typical TPO, he believes.
A recent developmental TPO from Ferro Corp. is said to be directly paintable with some urethane and acrylic paints, though it is not suitable for iso-cyanate crosslinking systems, says Ferro technical marketing manager Paula Phipps. Potential uses include bumpers and exterior and interior trim.
PROGRESS IN PRETREATMENT
For the moment, at least, it appears that TPO substrates will continue to need some form of surface pretreatment to improve adhesion, and there are several major developments in this area.
Adhesion promoters based on chlorinated poly-olefins (CPOs) may have 80-85% VOCs. DuPont is bringing to market a "zero-VOC" liquid adhesion promoter of proprietary composition, which Hennessy estimates will be available in six to 12 months.
Waterborne adhesion promoters are now being marketed by several paint suppliers as a way to lower VOCs. According to Carlisle of Morton Industrial Coatings, "Solvent-borne adhesion promoters are up around 6.5 lb/gal VOC, and waterbornes are down to well under 2 lb/gal. I see the trend moving even lower than that."
A zero-VOC water-based adhesion promoter is available from A-Line Products. Approved by General Motors and Chrysler, AL-1120 is offered in conductive black or gray and non-conductive white or clear. It can be applied with conventional spray equipment.
Other alternatives for promoting adhesion without VOCs include plasma, flame, and corona treating (PT, Feb. '93, p. 58). Some coatings suppliers think that flame treatment, which is widely used in Europe, will gain a foothold in the U.S. as European car makers like BMW and Daimler Benz open assembly plants here.
One drawback of corona-discharge and flame treatment, notes Ryntz of Ford, is that there is no way to determine how effectively a plastic surface has been treated except by in-depth laboratory analysis. For corona treatments, a new method for determining surface wettability has been developed by Tantec Corp. The Contact Angle Meter is said to improve the precision of testing and eliminate the subjectivity associated with traditional dyne solution testing.
Although cold gas plasma is established in Europe, it has yet to be put into large-scale commercial use by U.S. auto makers (PT, Oct. '92, p. 64). One paint supplier that has tested plasma treating on TPOs is Red Spot Paint & Varnish. According to TPO program manager Russ Pierce, plasma treatment can provide a wider operating window than chemical adhesion promoters. In Red Spot's tests, plasma-treated TPO parts that were painted and cured at 190 F performed well in car-wash tests for which a 250 F paint bake is usually recommended. "The fact that we can pass the test at a lower temperature shows us that plasma can be a better pretreatment than adhesion promoters," he concludes.
On the other side of the coin, plasma treatment has the disadvantage of being a batch process. Yet Pierce notes that progress is being made to increase process speeds. "Once they get the process worked out, plasma will definitely be an option. It's inexpensive to run, does a very good job, and gives you a very big window." Plasma could also help drive down bake temperatures, he adds.
One new technology makes possible low-VOC spraying of both topcoats and adhesion promoters. First introduced in 1990 by Union Carbide, the Unicarb process uses a compressed gas (usually C[O.sub.2]) in the form of a "super-critical fluid" (SCF) to replace most of the solvent in conventional and high-solids coatings (PT, March '91, p. 29). This process offers up to 80% VOC reduction and improved transfer efficiency, according to John Argyropoulos, Automotive Group technology manager.
Pierce of Red Spot, which has licensed the SCF process, says better transfer efficiency is achieved because of reduced overspray due to narrow droplet-size distribution. Pierce also reports being able to nearly triple the solids content of Red Spot's adhesion promoter while achieving good flow and leveling.
The first commercial installation of the SCF process for applying adhesion promoters is at Ford's Milan, Mich., plant, where it is pretreating bumpers, fascias, and other selected parts, according to Ryntz. Carbide's Argyropoulos says another commercial application for adhesion promoters should come soon.
In automotive topcoats, the SCF process has been used commercially for about 18 months at Textron Automotive's Farmington, N.H., plant to apply flexible clearcoats on exterior trim pieces and grilles.
PPG, another SCF licensee, is close to commercializing the technology for applying SMC primers, says Steve Fisher. "We can get substantial reduction of VOC and improved appearance, flow, and leveling over the SMC, and we can give system cost reductions," he says. Fisher adds that PPG is also interested in evaluating the process for solvent-based clearcoats.
Still another low-VOC alternative is powder coatings. Although most paint suppliers and OEMs do not view them as viable for coating thermoplastics in the near future, at least one supplier of these coatings sees potential in finishing thermosets. Lee Winters, manager of automotive powder coatings at Ferro, says powder coatings have overcome past limitations on smoothness and cure temperatures that have constrained their use on plastics.
He says the company's acrylic powder coatings are now being evaluated for thermosets. He expects to see the first commercial applications in 1996. He says the powder coatings are applicable for primers or clearcoats on exterior, Class A, above-the-beltline parts with high uv-durability requirements. Low-end cure temperatures for these coatings are presently around 300 F.
While successful in SMC compression molding and urethane RIM, in-mold coatings have made a number of false starts in injection molding over the years. If successful, this method would eliminate a secondary operation and keep VOCs in the mold. Powder in-mold coatings for thermoset injection molding have been tried by Ferro and DSM in the U.S. and Europe but have not caught on. A coinjected PP coating approach was shown by Battenfeld and Evode Powder Coatings of the U.K. at the Interplas '93 show in England (PT, Dec. '93, p. 15), but the project is now reportedly on hold because of technical problems with the coating material.
A 2K urethane in-mold coating for thermoplastic injection molding has been talked about since 1986 but has never been commercialized. Sherwin-Williams developed the material and Krauss-Maffei adapted a small RIM metering system to mount under the injection press (PT, May '92, p. 23).
Although Sherwin-Williams declines to comment on the status of the project in the U.S., Richard Hogue, general sales manager of reaction process machinery at Krauss-Maffei, says the program is still alive in Europe, where it has been adapted to RIM molding rather than thermoplastics. A prototype unit at K-M in Munich is being used by OEMs to test run urethane parts such as rock guards, bumper covers, fill-in parts, and wheel covers. Krauss-Maffei may show sample parts at K'95 in Dusseldorf next month.
'DRY' FILM LAMINATES
At least two firms are marketing formable dry paint films as a finishing approach that eliminates VOCs, paint-adhesion problems, and the high capital cost of paint lines. Avery-Dennison's Automotive Div. recently commercialized new Avloy film laminates for exterior parts (PT, June '94, p. 25). The company supplies plastic films with already cured coatings. The films can be thermoformed and placed in an injection mold for a variant of insert molding.
The newest version of Avloy is color-matched body colors for 1996 car models. Potential applications include body side moldings.
A second new automotive paint-replacement film technology has been introduced by 3M's Automotive Design Systems Div. This product needs no preforming step. Rather, it is formed in the mold during the injection cycle. Designed for use with TPO substrates, the 10-mil-thick film is comprised of fluoropolymer clearcoat and colorcoat layers and a 6-mil TPO backing. The film is placed inside the mold, where it is held in place by a framing system, and the resin is injected behind it. Use with substrates other than TPO would require an adhesive layer.
3M is initially focusing on exterior applications, including body side cladding and D pillars, according to marketing development manager Angela Dreis. A preformed product for bumper fascias has also been developed. In the future, 3M may apply the product to injection-compression molding.
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|Author:||De Gaspari, John|
|Date:||Sep 1, 1995|
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