Evaluation of wood and aluminum stickers and their stains on heat treated spruce boards: influence on quality and cost

University essay from Luleå tekniska universitet/Skellefteå Campus/Wood Physics

Abstract: Wood as a natural resource material can be modified by applying different drying techniques in order to have end products with improved properties. In wood drying industries, each parameter that has effect on end product quality and cost is elaborated intensively. In this context, stickers which are used to enable heat and moist air circulate through timber layers that are put in kilns for drying process are in concern, too. It is due to the fact that stickers are one of the major cost parameters in wood drying industries: not only the cost issue, but stickers also have direct effect on heat treatment process generated in kiln chambers, the total volume of timber that can be put in kilns and end product quality.This thesis makes both an evaluation and comparison for wood and aluminum sticker types which were used in heat treatment of Norway spruce boards by ThermoWood process. It was in interest to use wood and aluminum stickers in one single wood stack and make investigations on them broadly. Therefore, the evaluation of stickers and sticker stains was conducted in four different dimensions which were referred as chemical, physical, statistical (multivariate analysis approach) and economical research aspects.In chemical aspect, formation of the sticker stains was tried to be understood by literature reviews and a simple acidity test was made on sticker stained zones of sample boards. It was inferred that the sticker stain formation was generated due to a combination of both physical and chemical phenomenon which take place during drying. In the acidity test, the pH values of sticker underneath zones were lower (more acidic) than the sticker adjacent zones. However, the differences between pH values were not significant and both zones had pH values around 4 units. In physical aspect, color measurements were made on sample boards in subsequently increasing planing depths in order to investigate the severity degree of sticker stains on heat treated wood surface. These colormeasurements were made in L' C' h color space and the sticker stains'severity degrees were successfully quantified by E'ab color difference values. The sufficient planing depth required to have a uniform looking heat treated surface without sticker stains was determined 1-2 mms. The other issues which might affect the sticker stain severity degree and planing process were discussed.In the statistical aspect, the data obtained from color measurements were used to make a multivariate analysis to investigate the variables which were influencing sticker stains and the relations between them. It was found that planing was the most influential factor on color variables.In the economical aspect, cost estimation and comparison were made for wood and aluminum stickers for short and long term utilization periods. The results indicated that aluminum stickers were costing less than wood stickers in long term utilization periods. However, it was also found out that the handling issues of stickers (installation-sticker removing and conveying systems) in sawmill operations was in great importance for a facility to make a decision for using a certain sticker type.