A reliable artificial weathering test for wood coatings: accelerated weathering plays an important part in the development of new finishing systems. A study coordinated by CTBA, involving four wood coatings manufacturers and ten research institutes, has singled out a test that shows promise.

Artificial weathering plays an important role in the development of improved coating systems by providing a means of obtaining performance assessments in much shorter time than it is possible using natural weathering. However, there exists at present no agreed artificial weathering test applicable to wood substrates.

At the end of 1996 the European Community has approved an extensive European research project called "Arwood." This project involved ten wood research institutes (CTBA, BRE, WIG, VTT, TRAETEK, EMPA, CC, NIT and RUG) and four industrial partners (Tikkurila Oy, Gori-Dyrup, ICI Paints and Cecil) between 1996 and 2000. The project was co-ordinated by CTBA.

The principal aim of the project was to develop an artificial weathering method to assess the durability of exterior wood coatings in a short period of time and consequently to speed up the industrial development of new types of coatings systems required to meet increasingly demanding environmental regulations.

The development of the test itself had to comply with a particular economic criterion--to wit, the use of a device that is accessible to small and medium-sized companies. This is why the studies have been directed at cycles using fluorescent lights which have, moreover, turned out to be effective in other industrial sectors (steel, plastics).

The work of this project had to be the basis for a new CEN standard within the frame of EN 927 "Paints and varnishes-Coating materials and coating systems for exterior wood" describing an artificial weathering test for exterior wood coatings. This standard should complement several existing standards which are addressing the artificial exposure with fluorescent UV, but which are not specific regarding the exposure on a wood substrate. (1,2,3)

Within this research project several natural and artificial round-robin exposure trials with different coating systems and wood substrates were carried out. This paper mainly reports the findings of two subsequent series of tests which were especially designed to develop and test an "optimized" exposure cycle for the fluorescent UV devices for the artificial weathering of exterior wood coating systems.

Materials and Methods

Substrates: Exposure tests were carried out on coated wood panels following the specifications of the natural exposure test as described in prEN 927-3 (4). Test panels consisted of planed defect-free pine (Pinus silvestris) sapwood with a growth ring orientation to the exposed face of zero to 45[degrees]. Panel sizes were adapted to fit the device specific sample holders: in the evaluation test series the sample size was 300x75x15mm. Some laboratories used samples with a 150x75x15 mm panel size to accommodate a higher number of samples in one device and the same test run; for the "optimized" test series a slightly thicker sample with an uniform size of 150x74x18 mm was used.

Three replicate samples per coating system were tested in each laboratory and a set of unexposed samples was kept as reference controls. To ensure "identical" sample material the sample preparation was done in each test series centrally by one of the participating laboratories.

Coatings systems: The greater part of this research project was performed with one species: Scots pine (Silvestris pine) and six different coatings systems: two white paints--one solvent-based paint and one water-based one; two high-built wood stains--one solvent-based (the ICP which is used in the EN927 part 3: natural weathering test) and one water-based; one low-built solvent wood stain; and uncoated wood to have a control of the weathering of the wood itself.

Uncoated samples were also included to study the weathering effects on bare wood. For the sake of simplicity the uncoated samples are referred to as coating system F in Table 1 and in further text. Table 1 gives an overview of the selected coatings. For each series of tests coating application was done by brushing. Deviating from the natural exposure test in prEN 927-3 the back side of the panels were coated.

Weathering device: Seven weathering devices of the fluorescent UV condensation type were available in the two test series, each modified with a spraying system.

Most of the devices were fitted with a programmable microprocessor to control the exposure cycle. Several devices were equipped with an automatic irradiance control while others had to rotate and change the lamps according to the manufacturers recommendations. Samples were also rotated periodically within the chamber to ensure a uniform exposure.

Performance assessments: The performance assessments were done according to prEN 927-3 including cracking, blistering, flaking, chalking, gloss and color measurements.

A further assessment called "general appearance" was introduced. General appearance is the human eye's impression of the overall condition of the tested coating system. It was rated on a visual scale from zero (no change) to five (severe changes) in comparison, to the unexposed reference samples. Assessments during the exposure were done by each laboratory ("on-site assessments") while the final assessments were also repeated centrally by one operator in one of the participating laboratories ("common assessments").

In the following graphs time-series data always represent on-site assessments containing some possible operator bias. Final condition data are based on a common assessment and reflect differences due to exposure effects.

Evaluation Test Series

The testing procedures were based on existing standards (1,2,3) and the experience of the different laboratories.

In this evaluation test series, seven exposure cycles were established and tested as listed in Table 2. Cycles one to three are variations of exposure cycles from earlier development work on the fluorescent UV devices of some partners using both condensation and water spray. The main focus of these three cycles is on different timing options. Cycles four and five are based on recommendations in existing standards using only UV irradiation and condensation. Only the type of UV lamps is different (UVA-340nm and UVA-351nm). Cycles six and seven are using different treatment (soaking, freezing) to increase exposure stresses.

Fig. 1 shows the general appearance after 2,000 hours of exposure for the seven tested cycles. As expected the opaque coating systems A and B show only small to moderate changes while the most severe changes are observed for the uncoated wood (F). The acrylic stain D clearly performs better than the alkyd stain C and even slightly better than the two opaque coatings A and B.

[FIGURE 1 OMITTED]

Considerable differences between the different exposure cycles are observed and there is no clear ranking over all coating systems. An example of the appearance of the samples at the end of the seven exposure cycles is presented in Photo 1 for coating system E and in Photo 2 for coating system F. Only limited cracking (up to a rating of two) was observed after 2,000 hours for all the exposure cycles with the film forming coatings (system A to D). No cracking at all was present in any of the samples with the acrylic paint A. There was however a rapid crack development with the low build stain (system E) and the uncoated wood and Toshiba America Electronic Components Inc. (San Jose, Calif.) have jointly developed and will separately market a 512-Mbit (64-Mbyte) flash memory device in a 7 x 10-mm ball grid array package.

The Mobile DiskOnChip G3 flash disk will enable smaller portable devices with more features while reducing the bill of materials for a portable system by $10 to $14, said Chuck Schouw, M-Systems' president and chief executive officer. Toshiba plans to introduce its version of the device within a few weeks.

The new memory consists of Toshiba's 0.13-micron multilevel-cell (MLC) NAND flash, a controller, an execute-in-place boot block and the x2 flash-management software technology that M-Systems developed with Toshiba and introduced last October.

Schouw said the x2 technology makes it possible to use standard MLC NAND flash devices for both code and data storage while offering twice the binary NAND capacity or four times the NOR capacity of devices with a similar footprint.

The M-Systems software supports data storage applications such as video or MP3 downloads and is said to secure operating-system code, registry files and other vital information reliably. It also enables a reduction of 40 percent or more in silicon size, provides error detection and correction based on a combination of algorithms, and features direct memory access, parallel multiplane operation, a configurable burst mode and fast boot time. A Sequential Access File Translation Layer helps to overcome MLC NAND access sequence limitations and reduced partial-page programming.

A cell phone with the new chip can store in 1.5 seconds a movie trailer that would take 30.5 seconds with competing solutions, the companies said. "In addition to a better experience for a user, that reduction eliminates a significant drain on a cell phone battery," Schouw noted.

Because the chip has access to an application programming interface to the NAND flash, it can shut down dc/dc converters and reach a "deep power-down mode" quickly. "Discrete flash components don't have access to the 'guts' of flash memory, but our chip takes a systems approach, with a controller and boot block in addition to flash storage," Schouw said.

Stephen Marlow, executive vice president at Toshiba America Electronic Components, said the new memory device offers capabilities between those of NOR and NAND, with code execution and ease of integration comparable to NOR, and write speed, read speed, file storage capability and standby power close to NAND. "It benefits applications that have space constraints but nevertheless require large storage capacity," he said.

M-Systems Inc., [phone]

www.m-sys.com

Toshiba America Electronic

Components, [phone]

www.toshiba.com/taec

http://www.eet.com

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