Chemistry Charts the Course in Coatings

By Tim McDonough
Thursday, July 10, 2014

Sherwin-Williams’ WB Acrolon 100, developed through waterborne technology, delivers aesthetics and protection in one topcoat. It all adds up to extended maintenance and recoat cycles.

Inland river crews have a lot on their plate. When your job is to be a deckhand, you don’t want to spend much time being a painter. There’s never a good time to be a painter. However, keeping vessels free of corrosion and looking good is part of the job. So crews must attend to painting the freeboards, superstructures and decks, usually on the go. Solvent-based urethane topcoats have been the standard choice of port engineers and fleet managers for combating the effects of harsh environmental exposures to tugs, barges and towboats. But it is far from the only choice.
Inland operators can now take advantage of the latest advances in waterborne coatings to achieve both good looks and long-lasting performance. We can now achieve high functionality when it comes to environmental exposures with an ultra-durable, high gloss acrylic polyurethane with performance properties far above those typically associated with premium-quality solvent-based urethanes.
The inherent deficiencies of early waterbornes were long ago overcome, and expectations go well beyond just decorative attributes. In fact, Sherwin-Williams’ WB Acrolon 100 offers performance across the widest spectrum of performance criteria. It provides the gloss retention of a fluoropolymer or urethane with a clear coat, the color retention of a polysiloxane, and abrasion resistance and hardness demonstrably superior to any topcoat on the market today. Comparing this waterborne product to a traditional solvent-based urethane, in accord with various test methods, tells the story. 

Test Methods Benchmark Performance
Sherwin-Williams subjected this innovative technology to a number of standard test methods to assess performance in the key areas of gloss and color retention. To evaluate the material’s durability in the face of exceptionally long lifecycle expectations, Q-TRAC testing was employed. This speeds up the weathering process while concentrating natural sunlight using Fresnel lenses to magnify the solar radiation. The concentrator uses photo-receptor cells to maintain alignment with the sun and ten mirrors to reflect the sunlight onto the specimens. In this way, it is possible to simulate 63 years of UV radiation exposure in a single year.
A second form of testing, QUV-A accelerated weathering (per ASTM D4587, Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings), captures only one small bandwidth of radiation, so it does not replicate the sun, but it does focus on the most damaging ultraviolet rays. Here sample racks are placed in a chamber, with the sunlight simulated by fluorescent UV lamps. For evaluating gloss retention, this testing gives good rank order correlation to natural sunlight.
ASTM D1014, Standard Practice for Conducting Exterior Exposure Tests of Paints and Coatings on Metal Substrates, is a third, unaccelerated test method that is particularly useful for evaluating color retention. Here, samples are placed on inclined racks oriented at the sun at an angle ensuring exposure to the full spectrum of solar radiation, from infrared to ultraviolet, over a two-year period. The gold standard venue for this test is Florida where temperatures, UV intensity and humidity, all needed for maximum degradation, are at their peak.

Color, Gloss Test Results 
Gloss retention – Measured in percentage gloss, or luster, retained after six months, compared to the original sample, Q-TRAC testing resulted in limited loss of luster for the innovative product, with 98.2 percent retention. In the QUV-A testing, it was necessary to log 13,000 hours of exposure before a 50 percent loss of luster was measured.
Color retention – The human eye is believed able to distinguish a color shift only greater than 1 E, depending on hue, saturation and value of the color.
Q-TRAC testing of the acrylic polyurethane showed a shift of only 0.33 E, which is a superior measurement of its ability to withstand premature or excessive lightening of the original color.  In the Florida test, a shift of 0.52 E was recorded.     
Using standard ASTM test methods for abrasion resistance and pencil hardness, the performance comparisons to traditional results are also instructive. To measure the ability of a coating to withstand or resist wear, ASTM D4060 (Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser) utilizes a CS-17 wheel with a 1 KG load and runs for 1,000 cycles. Results are reported in mg loss. The breakthrough technology recorded a 25 mg loss; standard aliphatic urethanes typically experience losses from 40-90 mg.
In measuring a coating’s pencil hardness, or the ability to resist scratching, permanent deformation or damage, ASTM D3363 (Standard Test Method for Film Hardness by Pencil Test) determines the grade of the hardest pencil that won’t mar the coating. Pencil grades range from 9H (hardest) to 9B (softest).
WB Acrolon 100 measured at 3H, while most aliphatic urethanes record a pencil hardness of H to HB, the middle part of the range.
The combination of these performance metrics adds up to a waterborne finish coat that stands up to the most aggressive marine environments. The deck is one of the most expensive parts of the vessel when it comes to painting, and taking it out of commission for any length of time makes it difficult to get other work done.
That alone should be reason to consider new technologies that deliver significantly extended maintenance and recoat cycles.
But it goes beyond practicality. When a vessel lives in the port, a good-looking, well maintained exterior reflects well not only on the structure itself but also on the crew and owner in the eyes of a prospective client, buyer, or class surveyor. Advances in coatings chemistry make it feasible for owners to achieve with one product what has only been elusive before.



(As published in the July 2014 edition of Marine News - http://magazines.marinelink.com/Magazines/MaritimeNews)
 

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