Natural weathering performance of exterior wood coatings on Pinus sylvestris and Pinus radiata in Germany and New Zealand.

The weathering of commercial coatings on pine sapwood in Germany and New Zealand was studied. The New Zealand site had a climatic index 50% more challenging. Significant differences in performance relating to coating, species, site, and exposure are presented. The coating and site had the most significant effects, while species had the least. A solvent-borne stain and a hybrid paint qualified for stable end-use situations in both countries. An acrylic paint performed better in Germany while an alkyd gave the poorest performance of the four coatings tested. More effective fungicidal coating properties are required in New Zealand to suppress mold growth. Areas of deficiency for internationalizing the European weathering standard EN 927 are discussed.

Keywords: Climatic index, exterior wood coatings, natural weathering, standards for testing, adhesion, appearance, durability, weatherability, solvent-based, water-based, acrylics, alkyds, wood

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The durability of exterior coatings on wood depends strongly on climatic factors. Coatings must provide sustainable protection for timber with no cracking, blistering, flaking or adhesion failure, minimal change in gloss and color, and minimal mold growth. Coating performance must not only negate the influence of weathering processes, but must fulfill market needs for a customer and environmentally friendly product.

Global exporting enhances the opportunities for wood processing and trading companies to deal in timber commodities and added-value timber products. This needs to be underpinned by the use of common international standards that harmonize national trading practices. The first such international standard in Europe (with respect to coatings), European standard EN 927, Parts 1, 2, 3 and 5, (1a-d) was designed to address the classification, testing, and performance characteristics for modern exterior wood coatings. The natural weathering test following EN 927-3:2000 is suitable for obtaining and interpreting results of wood coating performance within Europe. The results of natural weathering tests do not simply depend on the location, but also on the starting date and duration. Furthermore, the performance of an exterior wood coating may be related to the species or the substrate to which it has been applied. The standard procedure following EN 927-3:2000 prescribes European pine sapwood as the mandatory substrate for the test. While shortcomings and restrictions of the European standard are easily inferred, EN 927 is being developed further as an international standard, ISO/PRF 16053. (2)

Consequently, studies such as the present one, while run independently to standards development, can contribute useful data about coated wooden panels exposed in different climates and with different wood species as substrates. Such studies can also provide an indication of the breadth of applicability of any proposed international standard. As a part of a collaborative project between the Forest Research (FR), Rotorua, New Zealand, and the Fraunhofer-Institute for Wood Research, Wilhelm-Klauditz-Institute (WKI), Braunschweig, Germany, a weathering trial was run to compare the performance of wood coatings on pine sapwood. Four commercial coating systems, on panels of Pinus radiata and Pinus sylvestris, were weathered for 12 months on exterior racks following the procedures outlined in EN 927-3:2000.

MATERIAL AND METHODS

Timber Selection

The trial was based on the requirements of EN 927-3:2000, which describes the procedure for assessment of coating systems applied to pine sapwood substrates in order to evaluate their resistance to natural weathering. Pinus sylvestris is the mandatory test substrate and Pinus radiata was chosen as an optional substrate for the trial.

Timber was selected to be straight grained, free of all visible defects, and of growth rate between 2.5 and 8 annual rings per 10 mm for Pinus sylvestris, and 1 to 3 growth rings per 10 mm for the faster grown New Zealand Pinus radiata. Pine panels were selected to give a sapwood test surface on the convex side of the growth rings (bark side), with any heartwood confined to a zone not exceeding 10 mm in depth when measured from the rear face.

Preparation of 32 panels of both pine species (375 X 120 X 20 mm; longitudinal X tangential X radial) was done simultaneously at WKI for Pinus sylvestris and at FR for Pinus radiata. The Pinus radiata panels were then sent to WKI where they were conditioned at 20 [+ or -] 2[degrees]C and relative humidity of 65 [+ or -] 5% until constant weight was reached. (3) The panels were kept under the same conditions during drying of the coating systems. The panels were randomly assigned to the paint and site treatments.

Coating Systems

Three opaque paints and a medium build reference stain (designated as ICP--Internal Comparison Product) were chosen for the trial (Table 1). Four panels were prepared for each coating system. The coating systems were applied by brush to the front face and the two edges of each panel with the back of the panel and both ends remaining uncoated. Twenty-four hours drying time was allowed between coats. The three test paint systems were applied at the manufacturer's recommended spreading rate. The spreading rate for the ICP corresponded to a total application of 150 g/[m.sup.2] wet film and a dry film thickness of 50 [+ or -] 10 [micro]m. When the test coatings and ICP had dried, the ends of the panels were sealed using two coats of a flexible, moisture-impermeable white paint (long oil alkyd), following EN 927-3:2000. After sealing, the panels were conditioned for six days in a controlled environment at 20[degrees]C and 65% relative humidity. The information on fungicidal content was as follows: alkyd (no fungicide); acrylic [0.5% fungicide Troysan Polyphase (Troy Chemicals)] and 0.1% bactericide Kathon LXE (Rohm and Haas); hybrid (fungicide information proprietary); and ICP [0.72% Biocide Preventol A5 (Bayer)].

Exterior Exposure

The exposure was carried out on racks inclined at an angle of 45[degrees] to the horizontal and facing south in Germany and north in New Zealand, in accordance to ISO 2810:1974. (4) For each coating system, three of the four panels were exposed for a period of 12 months from 28 November 1997 (see Discussion section for comments on the starting of the exposure period in Germany and New Zealand in different seasons). The unexposed fourth panel of each set was stored indoors, in darkness, at 20[degrees]C and 65% relative humidity over the exposure period.

Assessment of Exposed Panels

After 12 months of exposure, the panels were examined and rated for blistering while still on the exposure racks. The samples exposed in Rotorua were taken into the laboratory, stickered (filleted), and conditioned for six weeks at 20[degrees]C and 65% relative humidity before being dispatched to WKI. All assessments, except blistering, before and after exposure, were made at WKI by the same two independent, trained assessors. The mass of coatings applied, gloss, (5) and color (6) were determined before exposure. Gloss, color, and adhesion ratings were made after sponging the exposed panels with clean lukewarm water to remove surface deposits and atmospheric pollutants and were allowed to dry. The performance criteria assessed were blistering, (7) cracking, (8) flaking, (9) and adhesion failure, (10) with samples rated from 0 (no defects) up to 5 (dense defects). The following additional examinations were also carried out: mold growth, (9) (mold growth was also assessed on the uncoated back sides of the panels, and rupturing of coatings due to blue stain fungi was assessed separately), chalking, (11) color, gloss, and coating thickness. (12)

Climatic Indices

Climatic data for the exposure period in both Braunschweig and Rotorua were obtained from respective local meteorological agencies. Climate indices (CI), which were developed for the weathering of wood, were calculated using two models (C[I.sub.EU] (13,14) and C[I.sub.J] (15)):

C[I.sub.EU] = [I.sub.global]/20 + ([n.sub.r] * [R.sub.sum])/500 (1)

C[I.sub.J] = (0.165[T.sub.max] + 0.188RD + 0.029SH) * 12 (2)

where CI = Climate Index; [I.sub.global] = global radiation (kWh/[m.sup.2]) on panels tilted at 45[degrees]; [n.sub.r] = days of rainfall with more than 0.1 mm of precipitation; [R.sub.sum] = total precipitation (cm); [T.sub.max] = mean of monthly highest temperature ([degrees]C); SH = total sunshine (hr); and RD = number of rainy days of precipitation > 0 mm.

Performance Specifications of EN 927

The performance specifications of EN 927 are currently described in ENV 927-2:2000, according to their potential end-use categories: stable, semistable, or nonstable wooden parts. Four exterior assessment parameters (blistering, flaking, cracking, and adhesion), plus water absorption data, are used in defining the performance specification (Table 2). The results of the 12-month natural weathering exposure form the basis for classifying the test paints into one of the end-use categories.

Statistical Analyses

Analysis of variance (ANOVA) was used to test for treatment effects on the following seven variables assessed after 12 months: change in gloss, change in color, cracking, flaking, chalking, adhesion, and mold. First, a 4-factor ANOVA was used to test the effects of exposure (exposed and unexposed), site (FR and WKI), paint (four types), and species (Pinus radiata and Pinus sylvestris), and all interactions among these factors. A 3-factor ANOVA was then carried out on the exposed samples only [paint (4) X site (2) X species (2) X 3 replicates for a total of 48 observations]. A least significant difference test was used for pair-wise comparisons among the four paints.

RESULTS

Climatic Data

The climatic data in the trial year, for Braunschweig and Rotorua, are summarized in Table 3. Rotorua received 50% more sunlight and twice the rainfall. There were out of phase maxima in global radiation due to summer seasons in different hemispheres. Rainfall in Braunschweig was uniform throughout the year, while in Rotorua winter rainfall was very high. The values of the two climatic indices for Rotorua both exceeded those of Braunschweig by more than 50% (C[I.sub.EU] = 113 and 73; C[I.sub.J] = 127 and 85, respectively), thereby confirming that climatic conditions are harsher in Rotorua than in Braunschweig.

Dry Film Thickness

Dry film thickness (Table 4) was highest for the alkyd, with an average of 147 [micro]m over all the panels and lowest for the ICP stain, which averaged 54 [micro]m. The acrylic and the hybrid coating systems averaged 81 and 89 [micro]m, respectively.

Changes in Measured Weathering Parameters

None of the panels exhibited any blistering after 12 months of exposure, while the average change over all the samples for adhesion, flaking, chalking, and color change was less than 0.60 (Table 5). The largest average values were for gloss change (up to -54 units), cracking (rating of 3.7), and mold growth (rating of 4.0), and usually occurred for the Rotorua weathered samples. Mold growth was most prolific on the alkyd, acrylic, and hybrid coatings weathered in Rotorua. The hybrid and the ICP coatings weathered in Braunschweig were not affected by mold growth; there was also a low rating for mold growth on the acrylic coating in Braunschweig. Mold on the uncoated backside of the panels (data not presented) was severe for all coating systems on both substrate species when panels were exposed in Rotorua; there was less, but still substantial, mold growth on the corresponding panels exposed in Braunschweig. Blue stain fungi, growing from the backside through to the coated side, resulted in rupture of the alkyd and acrylic coatings. (16) The Pinus radiata samples at Rotorua were again worse and the Pinus radiata samples at Braunschweig were less affected. The extent of the blue stain fungi defect was comparable to the mold rating.

There was a general trend of loss of gloss on natural exposure, especially for the alkyd and the ICP coatings weathered in Rotorua. By measuring color change ([DELTA]E), relative to a white ceramic disk, the positive color change of the alkyd coating reflects a dulling of the white coating and the negative [DELTA]E for the ICP represents a bleaching. The dulling of the alkyd was higher in Braunschweig, while the bleaching of the ICP was stronger in Rotorua.

Statistical Analyses

Changes in gloss and color were approximately normally distributed and of homogeneous variance, and thus satisfied the ANOVA assumptions. The other variables, being subjective ratings, were not normally distributed, and did not fulfill ANOVA assumptions. Despite this, because ANOVA is known to be a robust procedure, (17) the results for these variables were considered to be reasonably reliable.

The 4-factor ANOVA was performed primarily to confirm differences between the exposed and unexposed samples. Statistically, this comparison showed that all variables were highly significant (p = 0.01) except for flaking, which was significant (p = 0.05), and adhesion, which was nonsignificant.

The 3-factor ANOVA was on the exposed samples only. When paint was the treatment factor there was significant difference for all weathering assessment parameters. A least significant difference test, significant at the 5% level, was therefore applied (Table 6a). In the great majority of cases the trend was for the rating to be increasing in the order ICP-Hybrid-Acrylic-Alkyd. When site was the treatment factor, there was a significance difference in the mean ratings for all weathering assessment parameters, except for flaking, and the rating was always greater for Rotorua samples. When species was the treatment factor, in all cases except color change, none of the weathering parameters were significantly different.

The interaction of the paint and site treatment factors for the hybrid and ICP coatings showed no differences for cracking, flaking, and adhesion (Table 6b). There was more adhesion failure of the alkyd and cracking of the acrylic at Rotorua, but more flaking of the alkyd in Braunschweig. For gloss, color, chalking, and mold, the alkyd and ICP showed significantly higher ratings in the Rotorua samples with the exception of higher color change in the alkyd at Braunschweig, while there was no difference for the hybrid (except for higher mold at Rotorua).

Of the 28 possible interactions of the paint and species treatment factors, there were only three significant differences (Table 6b): the alkyd on Pinus radiata cracked more, the acrylic on Pinus sylvestris flaked more, and the alkyd on Pinus sylvestris darkened more.

Coating End-Use Categories

Coating end-use category assignments were made for the weathered coatings at both sites by comparing the ratings for blistering, cracking, flaking, and adhesion against the performance criteria as specified in ENV 927-2:2000 (Table 2a). The assignment of end-use categories in the current work was made without water absorption data. The end-use category of the alkyd on Pinus radiata was ungraded when exposed at either site and was nonstable on Pinus sylvestris (Table 2b). The largest contributor to the low performance end-use classifications of the alkyd paint was cracking, with lesser contributions from adhesion failure and flaking (Tables 5 and 6b). The acrylic was highly variable with each end-use category being represented depending on site and species. Cracking and flaking observed on the pine samples weathered in Rotorua resulted in an ungraded classification on Pinus radiata and nonstable category for Pinus sylvestris. In Braunschweig, the slight cracking resulted in a stable end-use category on Pinus radiata and semistable on Pinus sylvestris. Both the colored opaque coatings, the hybrid and the ICP, gained a stable end-use rating for both sites and substrates.

DISCUSSION

Two pine species were used in this study. In New Zealand, Pinus radiata is the dominant plantation species (18) and has one of the fastest growths among the different pine species worldwide. In the European Union, Pinus sylvestris is one of the most important species for timber production and manufacturing. (19)

Climatic Effects

The C[I.sub.EU] parameter (13,14) shows that global radiation and precipitation are the two most influential climatic factors for correlating the performance of coatings on wood in different climates. This climatic index was developed within the scope of a European Union round robin test in nine countries following early versions of EN 927-3. The other climatic index, C[I.sub.J], was postulated in a Japanese round robin (15) which used uncoated wood veneers (did not follow EN 927-3 protocols). Uncoated veneers were exposed vertically and monthly weight loss was monitored over a year. Because of statistical uncertainty, the usefulness of both climatic indices is limited, but they do provide a trend relating coating performance on wood with climatic conditions. The interaction of climate and the weathering of wood coatings is complex and the results of modelling cannot be applied generally. (13,14) In the EU weathering study, it was found that the climatic indices of the sites were too similar and it was recommended that future studies involve more climatically challenging sites. The impact of climate on paint performance in the current work is shown by the frequency of significant outcomes in ANOVA for both site and paint/site interactions. The site effect is primarily a reflection of the significantly higher mean global radiation and precipitation in Rotorua compared with Braunschweig. Other factors having an unknown effect include: when test sites are in different hemispheres (resulting in six-month phase differences in concurrent test periods) and the variations, within and between years, in any given climate (20) (typical or atypical test periods could affect the end-use category assigned on the basis of a weathering test).

The former point may be significant--starting the exposure late spring/early summer in Rotorua compared with winter in Braunschweig introduces a different ordering of temperature, moisture, and insolation stresses to the coating and the wooden substrate. This in turn can initiate different weathering reactions which potentially could affect weathering assessment ratings (e.g., high humidity or rainfall in the first two months may affect mold development differently than samples receiving high humidity or moisture in the last two months of exposure). However, starting exposures in different hemispheres at the same time is practical from other points of view: paint and timber manufacturing companies want results as quickly as possible; preparing samples for simultaneous exposure avoids a six-month storage period for one set of samples before exposure; and in practice painters will apply coatings at various times over the whole year. The variation in climatic data over a long period will also not be captured in a one-year exposure, during which the weather is unlikely to replicate the long-term average.

The propensity of the solventborne high-build alkyd to crack may reflect the tendency of alkyd binders to become brittle after weathering due to their increase in glass transition temperatures with time of exposure. (21) For the acrylic paint system, the higher cracking at the Rotorua site demonstrates an influence of climatic conditions at the two sites. It is likely that the higher rainfall at Rotorua has leached additives such as plasticizers and coalescent agents resulting in a sufficiently raised glass transition temperature that found expression in cracking as the wooden substrate moved. The relative absence of cracking for the hybrid paint and the ICP suggests better flexibility of these paints.

The weathering parameters mold, gloss, color, and chalking are not used in determining the end-use categories, but are part of EN 927-3:2000. The highly significant interactions between paint and site for mold (Table 6b) reflects the higher rainfall in Rotorua compared with Braunschweig. This contrasts with the results of a previous study, (14) which found a negative correlation between mold growth and rainfall. An average wood moisture content of over 20% is generally thought to be necessary for fungal growth. (22) It has been reported (23) that some soft-rot fungi can grow on and in wood at moisture levels lower than 20% if above normal ambient temperatures exist. Another study (24) reported that mold and stain fungi can grow on wood at lower moisture contents, but not elevated temperatures, more than decay and soft-rot fungi, because they probably access moisture which wood surfaces had absorbed from humid environments. No apparent relationship between wood moisture content and the ability of the sapstain Sphaeropsis sapinea to colonize Pinus radiata has been reported. (25) The importance of relative humidity and temperature for the development of mold fungi on pine and spruce sapwood has been highlighted. (26)

In the current work, rupturing of coatings due to blue stain fungi (Figure 1) occurred only on the alkyd and the acrylic paints, and it occurred in both Rotorua and Braunschweig. Mold occurred on all the coatings in Rotorua but only on the alkyd and acrylic in Braunschweig. These data suggest that the fungicidal efficacy of the hybrid and ICP is adequate for controlling mold in Braunschweig, but in the wetter Rotorua environment it is insufficient, thereby enabling mold colonization. The acrylic had some mold fungicidal activity in Braunschweig. The alkyd, without any fungicide, had a mold rating of 2.7 in Braunschweig, lower than in Rotorua, reflecting less fruitful conditions for growth. The absence of blue stain rupturing on the hybrid--and ICP-coated boards suggests that the fungicidal levels were effective against the sapstain fungi. Moisture contents of the panels were not determined. Thus, it is possible that the panels had different moisture levels and this may have also had an effect on blue stain rupturing. The comments made for the prevalence of mold in the alkyd and acrylic also apply to blue stain coating rupture in the alkyd and acrylic.

There were significantly different gloss and color changes as well as chalking between the two sites (Table 6a). The paint/site interaction effect for the hybrid paint was not significant for these parameters, but the reductions in gloss and color changes for the alkyd and the ICP were highly significant (Table 6b). Higher gloss loss corresponded with an increase in chalking, which probably reflects the stronger insolation and rainfall in Rotorua.

The relatively small significance of paint and species interaction may indicate a basic similarity between the fast and slow growing wood species of the same genus. If different genera or if a softwood-hardwood comparison were made, then the paint and species interaction may be significant.

Implications for International Standards for the Weathering of Wood Coatings

It was concluded that the use of a climatic index to transfer the weathering test data between sites was not practical for international standards. (14) That report also concluded that the use of an ICP offers a better prospect for success. Differences in quality of wood, in the weather, and site conditions can be recognized and allowed for in the test method by comparing the test with an ICP. For example, the calculated climatic indices (CI) for the ICP can be used (following ENV 927-2:2000) to categorize the Rotorua climate as medium and the Braunschweig climate as mild. The EN 927 standard protocols followed in this study form an excellent framework from which to build, since they embody the traditions of the European experiences in the weathering of wood coatings in one standard. It is pertinent to consider some of the results and implications of the present study on the development of international standards relating climatic regions to wood coating weathering data.

* Not backsealing boards makes the test harsher than if the boards were backsealed. However, as was seen, this creates an avenue for the development of blue sapstain fungi colonizing through the board and rupturing some coatings, thereby destroying the important physical barrier to the weathering elements. The use of a sealer coating or fungicidal treatment to the backside of the board may be a necessary consideration.

* A closely related issue is the fungicidal level present in the coatings or the wood which may be depleted or become less effective with rainfall. The microbial populations at different sites may have a considerable impact on the weathering effect on boards.

* The water absorption test requires constant weight to be attained. However, some samples will not achieve this requirement. For example, in New Zealand samples are frequently treated with light organic solvent preservatives using sequential vacuum-pressure cycles. These samples retain solvent for extended periods of time with solvent flash-off decreasing exponentially with time. The preservative practices of different countries will need to be recognized.

* The physical-thermal properties of coatings, such as glass transition temperatures, can also find different expressions in cracking intensity in different climatic conditions.

* The effect of the starting date of the trial involving different hemispheres introduces seasons in a different sequence. The effect of when high insolation or rainfall impacts the samples could be significant. Accelerated chalking, gloss and color change, and change in glass transition temperatures may result. Overlaid with this is the variation in weathering patterns resulting in sizeable differences between single year and multiyear averages in climate data, which will change the climatic index of a site.

* The substrate was found to be an insignificant factor in this study involving two pine species. The mandatory use of Pinus sylvestris may relax somewhat, yet the effect of other species with different wood properties may be significant. The practical issues of provision of one mandatory single species over sites in different continents or locations will need to be resolved.

* In a similar vein, the provision of the ICP with a consistent composition across the world is an important facet, since the ICP appears to be the sole means available with the potential for transference of weathering data between regions of different climates.

* Consistency of test panel assessment is another factor requiring consideration. A method to achieve this is an important element when countries with diverse cultures, traditions, and practices are involved. Some form of training for the test and reporting would be advantageous.

* ENV 927-2:2000 represents an excellent guideline. The choice of four weathering parameters as performance criteria, along with water absorption measurements, is simple. The purpose of ENV 927-2:2000 is a standardized approach to accumulate data for the selected important weathering parameters. An important purpose of ENV 927-2:2000 or its successor will likely be to facilitate standardized practices to assist trade in coated wooden products over wide-ranging climatic, national, and political regions with an expectation of demonstrable and reproducible weathering performance. Given the importance of marketing products internationally, a case exists for mold assessment to be part of the successor of ENV 927-2:2000.

[FIGURE 1 OMITTED]

* Will accelerated weathering testing devices be featured in new international weathering standards? Their accuracy in reproducing conditions for testing anywhere in the world in a short time frame is a strong factor in their favor. This may give valuable information about the effect of insolation and rainfall on changes in the physiochemical properties of coatings. The natural weathering testing, however, must ultimately retain its central position since the biotic factor is critical in testing.

CONCLUSION

This study contributes to the understanding of the issues that are relevant to the development of an international weathering standard for wood coatings. It has illustrated the benefit of incorporating sites from outside the European Union in the development of climatic index models, since climatic diversity is essential in the study of variance in weathering data. The results of the least significant difference test in the present study showed significant interaction effects of paint and site, as well as no significant effect of the two pine species used. The strong site effect was explainable in terms of the climatic indices of the two sites, where the CIEU values were 113 (Rotorua) and 73 (Braunschweig).

Table 1 -- Number of Coating Layers Applied and a General Description of
the Wood Coating Systems Used in the Trial

                  Primer  Topcoat
Coating System    Layers  Layers   Coating Properties

Alkyd paint       0       3        Solventborne; gloss; white; high
                                   build
Acrylic paint     1       2        Waterborne; semigloss; white; high
                                   build
Hybrid (acrylic-  0       3        Solventborne; semigloss; brown; high
polyurethane)                      build
paint
Alkyd stain       0       3        Solventborne; gloss; red-brown;
                                   build

Table 2a -- Performance Criteria and End-Use Category as Described in
ENV 927-2:2000

Assessment
Characteristics        Stable             Semistable         Nonstable

Blistering (a)             0.3                0.7             1.0
Cracking (a)               0.7                1.7             3.0
Flaking (a)                0.3                0.7             1.3
Adhesion failure (a)       0.7                1.7             2.7
Maximum individual         2                  3               4
results (b)
Maximum sum value (c)      6                 12              18
Water absorption       < 175 g/[m.sup.2]  < 250 g/[m.sup.2]  No limit
value (d)

(a) These values are the maximum allowed, for the arithmetic mean, (to
one decimal place) of the three replicates from the 12-month exposure
trial.
(b) The single highest individual result allowed for any panel in any
category.
(c) The sum of the 12 (4X3) individual results.
(d) EN 927-5:2000.

Table 2b -- End-Use Category of Coatings in Trial Based on Table 2a
Criteria (a)

Coating  FR, R     WKI, R    FR, S      WKI, S

Alkyd    Ungraded  Ungraded  Nonstable  Nonstable
Acrylic  Ungraded  Stable    Nonstable  Semistable
Hybrid   Stable    Stable    Stable     Stable
ICP      Stable    Stable    Stable     Stable

(a) Water absorption values were not determined.

Table 3 -- Climatic Data for Braunschweig (Germany) and Rotorua (New
Zealand) for the Year November 1997 to October 1998

Climatic Data                                Braunschweig  Rotorua

Rainfall (mm)                                 697          1346
Sunshine (hr)                                1302          2063
Mean daytime temp ([degrees]C)                  9.3          13.7
Mean highest daytime temp ([degrees]C)         13.3          18.2
Mean lowest daytime temp ([degrees]C)           6.4           9.1
Mean daily global irradiance (MJ/[m.sup.2])     9.0          14.0
Mean daily relative humidity (%)               83.6          77.4

Table 4 -- Dry Film Thickness ([micro]m) Measured on Nonexposed Boards

Coating  FR, R  WKI, R  FR, S  WKI, S  Mean  Std Dev

Alkyd    114    153     160    161     147   22
Acrylic   82     70      96     75      81   11
Hybrid    93     63     115     86      89   21
ICP       58     44      69     44      54   12
Mean      87     83     110     92      --   --

Table 5 -- Averaged (n = 3) Weathering Data (a) After 12 Months of
Exposure

                                            Adhesion  Gloss   Color
Paint    Site  Wood (b)  Cracking  Flaking  Failure   Change  Change (c)

Alkyd    FR    S         2.7       0.0      1.0       -54      2.6
Alkyd    FR    R         3.3       0.0      1.7       -49      1.7
Alkyd    WKI   S         2.0       1.3      0.0       -16      7.1
Alkyd    WKI   R         3.7       1.3      0.0       -17      4.9
Acrylic  FR    S         3.0       1.3      0.0         1     -0.2
Acrylic  FR    R         3.3       0.0      0.0         4     -0.6
Acrylic  WKI   S         1.3       0.3      0.0         9      0.5
Acrylic  WKI   R         0.0       0.3      0.0        11     -0.2
Hybrid   FR    S         0.0       0.0      0.0       -19     -1.6
Hybrid   FR    R         0.0       0.0      0.0       -16     -2.1
Hybrid   WKI   S         0.0       0.0      0.0       -12     -1.8
Hybrid   WKI   R         0.3       0.0      0.0       -17     -2.1
ICP      FR    S         0.3       0.0      0.0       -26     -7.1
ICP      FR    R         0.0       0.0      0.0       -30     -6.8
ICP      WKI   S         0.0       0.0      0.0         4     -1.8
ICP      WKI   R         0.0       0.0      0.0        -3     -1.9

                               Ruptures:
Paint    Site  Chalking  Mold  Blue Stain

Alkyd    FR    1.0       4.0   2.3
Alkyd    FR    1.0       4.0   3.3
Alkyd    WKI   0.7       2.7   3.0
Alkyd    WKI   0.7       2.7   1.5
Acrylic  FR    1.0       3.7   3.0
Acrylic  FR    1.0       4.0   3.7
Acrylic  WKI   0.0       1.7   1.5
Acrylic  WKI   0.0       1.3   1.5
Hybrid   FR    0.0       4.0   0.0
Hybrid   FR    0.0       3.7   0.0
Hybrid   WKI   0.0       0.0   0.0
Hybrid   WKI   0.3       0.0   0.0
ICP      FR    1.0       1.7   0.0
ICP      FR    1.0       1.3   0.0
ICP      WKI   0.0       0.0   0.0
ICP      WKI   0.0       0.0   0.0

(a) Weathering data obtained from assessment of exposed panels.
(b) S = Pinus sylvestris and R = Pinus radiata.
(c) Color change is change in [DELTA]E.

Table 6a -- Means and Least Significant Difference (LSD) Test from
3-Factor ANOVA of Exposed Samples

                                         Adhesion     Gloss
Factor      Cracking         Flaking     Failure      Change

Paint
Alkyd       2.92      a (a)  0.67     a  0.67      a  -33.8   a
Acrylic     1.92      b      0.50     a  0.00      b    6.34  c
Hybrid      0.08      c      0.00     b  0.00      b  -16.2   b
ICP         0.08      c      0.00     b  0.00      b  -13.5   b

Site
FR          1.58      a      0.17     a  0.33      a  -23.5   a
WKI         0.92      b      0.42     a  0.00      b   -5.01  b

Species
Radiata     1.33      a      0.21     a  0.21      a  -14.4   a
Sylvestris  1.17      a      0.38     a  0.13      a  -14.1   a

            Color
Factor      Change     Chalking     Mold

Paint
Alkyd        4.06   d  0.83      a  3.33  a
Acrylic     -0.10   c  0.50      b  2.67  b
Hybrid      -1.92   b  0.08      c  1.92  c
ICP         -4.39   a  0.50      b  0.75  d

Site
FR          -1.77   a  0.75      a  3.29  a
WKI          0.60   b  0.21      b  1.04  b

Species
Radiata     -0.90   a  0.50      a  2.13  a
Sylvestris  -0.28   b  0.46      a  2.21  a

(a) Means for each parameter followed by the same letter are not
significantly different (p = 0.05; LSD test).

Table 6b -- Means and Statistical Significance from Interaction of
Factors in 3-Factor ANOVA of Exposed Samples

                                                     Adhesion
Factor   Interaction  Cracking          Flaking      Failure

Paint    Site
Alkyd    FR           3.00      NS (a)  0.00     **  1.33      **
         WKI          2.83              1.33         0.00
Acrylic  FR           3.17      **      0.67     NS  0.00      NS
         WKI          0.67              0.33         0.00
Hybrid   FR           0.00      NS      0.00     NS  0.00      NS
         WKI          0.17              0.00         0.00
ICP      FR           0.17      NS      0.00     NS  0.00      NS
         WKI          0.00              0.00         0.00

Paint    Species
Alkyd    Radiata      3.50      *       0.67     NS  0.83      NS
         Sylvestris   2.33              0.67         0.50
Acrylic  Radiata      1.67      NS      0.17     *   0.00      NS
         Sylvestris   2.17              0.83         0.00
Hybrid   Radiata      0.17      NS      0.00     NS  0.00      NS
         Sylvestris   0.00              0.00         0.00
ICP      Radiata      0.00      NS      0.00     NS  0.00      NS
         Sylvestris   0.17              0.00         0.00

                      Gloss       Color
Factor   Interaction  Change      Change      Chalking      Mold

Paint    Site
Alkyd    FR           -51.36  **   2.15   **  1.00      *   4.00  **
         WKI          -16.22       5.97       0.67          2.67
Acrylic  FR             2.76  *   -0.38   NS  1.00      **  3.83  **
         WKI            9.93       0.17       0.00          1.50
Hybrid   FR           -17.76  NS  -1.89   NS  0.00      NS  3.83  **
         WKI          -14.53      -1.95       0.17          0.00
ICP      FR           -27.71  **  -6.96   **  1.00      **  1.50  **
         WKI            0.78      -1.81       0.00          0.00

Paint    Species
Alkyd    Radiata      -32.61  NS   3.28   **  0.83      NS  3.33  NS
         Sylvestris   -34.96       4.84       0.83          3.33
Acrylic  Radiata        7.62  NS  -0.39   NS  0.50      NS  2.67  NS
         Sylvestris     5.06       0.19       0.50          2.67
Hybrid   Radiata      -16.56  NS  -2.14   NS  0.17      NS  1.83  NS
         Sylvestris   -15.73      -1.69       0.00          2.00
ICP      Radiata      -16.16  NS  -4.33   NS  0.50      NS  0.67  NS
         Sylvestris   -10.77      -4.45       0.50          0.83

(a) ** = Significant at the 1% level; * = Significant at the 5% level;
NS = Not significant at 5% level.

ACKNOWLEDGMENTS

Mark Kimberley, Forest Research, Rotorua, New Zealand, is thanked for doing the statistical analyses. Seed funding to initiate collaboration between the authors, provided through the New Zealand-German Science and Technology Cooperative Agreement, is acknowledged.

References

(1) EN 927. "Paints and Varnishes--Coating Materials and Coating Systems for Exterior Wood," (a) Part 1. Classification and Selection, EN 927-1:1996 (September); (b) Part 2. Performance Specification, ENV 927-2:2000 (August); (c) Part 3. Natural Weathering, Test EN 927-3:2000 (July); and (d) Part 5. Assessment of the Liquid Water Permeability, EN 927-5:2000 (July).

(2) ISO/PRF 16053. "Paints and Varnishes--Coating Materials and Coating Systems for Exterior Wood," natural weathering test (in development).

(3) ISO 554:1976. "Standard Atmospheres for Conditioning and/or Testing--Specifications."

(4) ISO 2810:1974. "Paints and Varnishes--Notes for Guidance on the Conduct of Natural Weathering Tests."

(5) ISO 2813:1994. "Paints and Varnishes--Determination of Specular Gloss of Non-Metallic Paint Films at 20 Degrees, 60 Degrees and 85 Degrees."

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(8) ISO 4628-4:2003. "Paints and Varnishes--Evaluation of Degradation of Coatings--Designation of Quantity and Size of Defects, and of Intensity of Uniform Changes in Appearance--Part 4: Assessment of Degree of Cracking."

(9) ISO 4628-5:2003. "Paints and Varnishes--Evaluation of Degradation of Coatings--Designation of Quantity and Size of Defects, and of Intensity of Uniform Changes in Appearance--Part 5: Assessment of Degree of Flaking."

(10) ISO 2409:1992. "Paints and Varnishes--Cross-Cut Test."

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Bernard S.W. Dawson** -- New Zealand Forest Research Institute*

Adelheid Gottgens and Guido Hora -- Wilhelm-Klauditz-Institute ([dagger])

* Private Bag 3020, Rotorua, New Zealand.

[dagger] Fraunhofer-Institute for Wood Research, Bienroder Weg 54E, Braunschweig, D-38108, Germany.

** Author to whom correspondence should be addressed. Email: [e-mail].