*3.2. Artificial Weathering*

### 3.2.1. Visual Analysis

The transparent and semi-transparent coatings had different performance during artificial exposure. The coatings A Acr/Alk, J Alk, and I Alk presented some type of chalking that occasionally could result in surface erosion. It is important to mention that some studies describe degradation of clear coating as cracking or flaking. The type of degradation found for the opaque coatings used in this study (A Acr/Alk and J Alk) were best described as chalking due to their powdery appearance (Figure 8).

Although the short-term exposure resulted in no major visual change on most of the treatments, coating I exhibited decrease in brightness with some degree of bleaching. Similar results were found on commercial coatings after 1000 h of accelerated weathering [43]. The long exposure of 1800 h resulted in slight chalking of coatings A and J and moderate chalking of coating I.

**Figure 8.** Surface change of selected tested samples. First and second rows correspond to before and after exposure, respectively. (**a**) After 360 h of exposure; and (**b**) after 1800 h of exposure.

#### 3.2.2. Color and Gloss Changes

The color changes of coated and uncoated CLT samples exposed for 360 h and 1800 h are summarized in Table 4. Although short-term exposure (360 h) showed discrete changes, there was statistical difference between treatments (α = 0.05). Overall, coating did not express great lightness degradation (Δ*L\**) in the first accelerated weathering test except for coating J (−4.8 units). The lowest value of Δ*L\** was reported for untreated samples (−10.4 units) that became darker after the test. This result was expected because wood chemical components, such as extractives, rapidly degrade with photo-radiation exposure leaving them darker [41].


**Table 4.** Color change values of artificial weathered CLT samples. Mean and (standard deviation).

Color of samples were not degraded during short-term exposure. The highest change for coated wood was found on coating I Alk (Δ*b\** = −3.5 units) and uncoated samples (Δ*b* = 7.3).

Samples coated with either coating I or J were less stable (Δ*E\**). The acrylic waterbased coatings C Alk/Acr and F Acr had better performance at the beginning of the test. The overall ranking associated with resistance to color change was: C Alk/Acr > F Acr > A Alk/Acr > I Alk > J Alk > Control.

The color changes after 1800 h of artificial weathering were statistically different among treatments (α = 0.05). Long-term exposure resulted in low resistance to darkening of coated and uncoated samples. The coatings A Alk/Acr, J Alk, and I Alk showed high sensitivity to light degradation (−12.9, −12.0, 11.0 respectively). This finding corroborates [44], which found effects of aging much earlier in alkyd coatings.

Overall, coatings did not show instability to changes in the Δ*a\** spectrum. The highest values were found for coatings A Alk/Acr and C (3.23 and 3.00, respectively). The major change in Δ*b\** was measured for coating I Alk (−12.6 units) followed by coating J Alk and control samples (−6.7 and 5.5 respectively). The higher color change after 1800 h of accelerated weathering may be related to the degradation of the protective coatings and the leaching of wood surface components (extractives and lignin). Coating F Acr was the most stable color treatment, which is consistent with the results of other research [44–46] that reported pigmented coatings to be more resistant to photo-degradation than clear coatings.

The gloss of coated and uncoated CLT significantly changed after artificial weathering exposure. Based on the initial surface luster of the samples, the oil-based coatings were affected more after exposure than water-based coatings for either exposure time (Table 5). Oil and alkyd finishes are less permeable and are more likely to decompose as time progresses [45,46]. If the coating is transparent, it is even more susceptible and sensitive to UV-degradation.


**Table 5.** Gloss change (Δ*G*) of coated and uncoated CLT after 360 h and 1800 h of artificial weathering (Standard deviation).

Similar results were found by [45], who reported gloss degradation on an oil-based coating after three weeks of artificial weathering exposure. The loss of gloss indicates that degradation is occurring due to non-chemical changes (surface wrinkling) or chemical changes located in the topmost portion of the coating [41].

#### **4. Conclusions**

Visual rankings and degrees of color change reported for samples exposed to outdoor weathering were highly consistent. In both site locations, coatings C Alk/Acr and F Acr were the most resistant. A reason for their superior performance is likely the inclusion of anti-microbial ingredients in their composition. Coatings A Alk/Acr and J Alk failing to protect the CLT surface coincided with increased mold growth, chalking, erosion, and color change over other treated samples. Gloss changed over time, specifically for coatings I Alk and C Alk/Acr, while other variations were not reported due to low values during initial exposure. Water uptake is influenced by substrate variations (defects, type of grain, earlywood/latewood, and end-joint) and climatic conditions. For these reasons, the effect of coatings on moisture content during exposure was not significant. Combinations of water, temperature, and solar radiation impacted coating performance. Even when the

wood surface is protected, variations in the CLT panels such as end-joint, cracks, and checks can facilitate water uptake that eventually will result in coating failure, delamination, and fungal attack.

Artificial weathering results were similar to the natural weathering. Coatings A Alk/Acr, I Alk, and J Alk had slight to moderate chalking after long-term exposure. These same coatings were the most sensitive to changes in lightness, color, and gloss. Therefore, an artificial weathering test of 1800 h or greater may screen potential durable coatings for CLT. However, it is important to consider that in artificial weathering, biological agents such as fungi and bacteria are not present. Once biological factors are added, the service life of coatings will be diminished.

**Author Contributions:** Conceptualization, M.N., K.M.O., and C.E.S.; methodology, C.E.S., G.d.S.B., M.N., and G.K.; formal analysis, G.d.S.B., D.J.V.L., and C.E.S.; investigation, G.d.S.B., C.E.S., G.K., and K.M.O. writing—original draft preparation, G.d.S.B.; writing—review and editing, C.E.S., D.J.V.L., M.N., K.M.O., and G.d.S.B.; supervision G.d.S.B., and C.E.S.; project administration C.E.S.; funding acquisition, M.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by USDA Forest Service, grant number FPL #16-JV-11111136-048. This publication is a contribution of the Forest and Wildlife Research Center, Mississippi State University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical restrictions.

**Acknowledgments:** The authors wish to acknowledge the support of U.S. Department of Agriculture (USDA), Forest Service. Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

