Studies on the loss of gloss of shellac and polyurethane finishes exposed to UV

11 Wood finishes protect the surface of wood from external agents, enhance its looks and improve its 12 gloss (luster). On constant external exposure, UV rays gradually degrade the film coating resulting 13 in loss of gloss. In this study, two commonly used finishes namely spirit shellac and polyurethane 14 finish were used to investigate the pattern of loss of gloss due to UV interference. Two coatings of 15 polyurethane (PU) and spirit shellac finish were applied on the surface of eucalyptus samples. The 16 gloss levels of these and unfinished samples were monitored for different times of exposure of them 17 to UV light. Gloss was measured at 60 0 gloss head using a Tri micro gloss meter regularly for 20 18 hours durations of UV exposure. Observations and analysis revealed that the natural gloss of 19 uncoated samples of eucalyptus were least affected on UV exposure with only 8.3%-10% loss in 20 gloss. The PU and shellac coated surfaces also showed very little reduction in gloss (6.9%-15.4%) 21 most of which happened in the first 40 hours of exposure. Thus both the finishes in this study were 22 found effective to a good extent in maintaining the gloss of the finished surface against exposure to 23 UV light.


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Wood is a widely used construction material since prehistoric times for both interior and 29 exterior applications because of its pleasant appearance, ease of working, availability and strength. 30 In spite of all these advantages, wood being a biological material is prone to degradation by 31 weathering agents, viz: moisture, light, abrasion. To minimize degradation of wood from 32 weathering agents several methods (surface coating, chemical modification and impregnation with 33 chemicals) can be adopted: among these, wood coating by application of wood finish on the wood 34 surface is the most commonly used method (Black andMraz 1974, Feist andHon 1984, Evans et al. 35 2002, Evans 2009). Wood finishes add beauty, provide protection to wood surface from weathering 36 agents viz: heat, light, temperature, moisture, wind, abrasion and provide a cleanable surface. It 37 adds beauty by enhancing its aesthetic value and improving its luster also known as gloss. Gloss is 38 an aspect of the visual perception of objects. It is an attribute of finished surfaces that gives them 39 shiny or lustrous metallic or matt appearances. Commonly used finishes on wood include spirit 40 shellac (natural polymer) and polyurethane (artificial polymer) finish. Polyurethane (PU) is a very 41 tough and durable finish. It is a slow drying finish and compared to other finishes it forms a harder, 42 tougher and more water proof film. Spirit shellac is a quick drying and transparent finish. Unlike 43 PU, it does not form a hard or water proof film. With time, wood products exposed to external 44 conditions often tend to lose the shine or gloss in a definite pattern due to interference of harmful 45 ultraviolet UV radiation reaching the earth's surface. UV rays degrade the film (coating) gradually 46 resulting in a reduction of gloss on surface. UV light causes photochemical degradation mainly in 47 lignin polymer in the cell wall. As the lignin is degraded, water leaches out degradation products are known to suffer a substantial loss of gloss after exposure to UV radiation (Wernsath, 1996). UV 53 light is reported to significantly degrade biological stains in wood species such as the red stain of 54 Acer negundo (Robinson et al 2013). 55 Against this background, a study was undertaken to assess the gloss reduction due to UV 56 exposure of PU and spirit shellac polishes on eucalyptus wood surface and compare the 57 performances of these finishes with respect to gloss reduction. Presently, eucalyptus is emerging as 58 a potential timber species as it is fast growing and possesses good strength. It is raised in plantations 59 and has effectively replaced the need of traditional timber species for furniture, veneer, pulp and 60 paper.

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For this study, seasoned and planed planks of eucalyptus were used. The planks were planed 64 both sides on a planer machine and were cut into samples of size 8" × 3" × ¾" using a circular saw.

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The cut samples were sanded on one side on a belt sander with 80 grit size to obtain smooth 66 surfaces. Further smoothening was done manually with a 120 grit size sand-paper.

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Nine samples were prepared and divided into three groups. Three samples each for PU 68 finish, spirit shellac and the remaining three were used as controls. The samples were numbered 69 accordingly. A commercial polyurethane finish was readily available in the laboratory. Shellac  drying time for polyurethane finish was 5-6 hours and that for spirit shellac was less than an hour.

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Gloss measurements in the study were carried out at 60 0 gloss head using a Tri micro gloss 76 meter. The gloss meter was calibrated each time before taking the reading. Gloss values on surfaces 77 of raw samples were also recorded. Several readings were taken on each surface and the mean 78 values were calculated.

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All the nine samples were placed in a UV chamber having a UV source (30 W, 254 nm) and 80 were exposed for a brief period of 3 minutes (0.05 hour) and gloss values were recorded. These 81 values were used as the initial gloss of the samples. The samples were then exposed to UV light in 82 the chamber and gloss values were measured after every 20 hours of exposure to understand the 83 gloss reduction pattern. Readings were taken up to 120 hours.  One-way ANOVA was adopted to test the significance in the differences of percent 89 reduction of gloss. Appropriate transformations were adopted for the percent reduction values 90 before running the ANOVA.  gradually as they continued to be exposed to the UV light. This reduction is slow for the very low unfinished wood surfaces exposed to weather/ UV are roughened by photo degradation and 114 undergoes subsequent erosion (Feist, 1988). 115 However, being organic in nature, the coatings that one applies to the wood surfaces are also 116 prone to this photo degradation causing slow erosion. On the other hand, they do protect the actual 117 wood surface from getting eroded through photo-degradation (Williams, 2010). The ability to 118 withstand the effect of UV exposure was studied through calculating the percent loss in gloss of  about sixty hours at a faster rate and thereafter the gloss reduction appears to be rather slow. To the UV degradation as far as the gloss is concerned happens mostly in the initial period of exposure.

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To understand the difference in loss of gloss behavior between the coatings, the data was 143 analysed for each exposure time. For any exposure time, there were no significant differences 144 between gloss reductions between the two finishes studied.

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The gloss of the unfinished eucalyptus surface was not affected by exposure to UV light.