Reducing thickness swell is the most critical and challenging problem facing oriented strand board (OSB) manufacturers. This is especially important for new end applications for OSB including sub-flooring, web stock and concrete forming.
To address the thickness swell problem for OSB, the present report discusses the fundamentals of thickness swell in Part 1 describing in detail the dimensional stability of wood strands under the interactions of heat, moisture pressure and time, develops a statistical model in Part 2 and finally in Part 3, develops a new practical patented radio-frequency method to reduce thickness swell in OSB.
It is recommended that the statistical models generated from the present study should be combined with some of the earlier work by Hsu (1994) and Sean (1997) to form a comprehensive computer software for OSB manufacturers.
The objectives for this project are to investigate cellular behaviour of wood strands under the interaction of heat, moisture, pressure and time; to improve the basic understanding of OSB dimensional stability by linking the cellular behaviour of wood and OSB hot pressing processes; and based on the improved understanding, to develop the best strategies to reduce OSB thickness swell.
In early 2003, there were no established methods for the measurement of wax distribution in OSB and only recently had mills expressed concerns about how wax is distributed and its effect on panel properties, especially thickness swell and product consistency. To address these questions, a two-year project “Measurement of Wax Distribution on OSB” was initiated in April, 2003, as part of Forintek’s National Research Program (NRP).
In the first year, two technologies were selected for testing wax measurement, optical image analysis and near infra-red (NIR) spectroscopy. An online NIR (near infrared) sensor commercially available from Moisture Systems Corporation (Spectra-Quad 3000) was tested at the Alberta Research Council (ARC) for wax measurement of furnish samples blended at levels of 0, 0.5, 1.0, 1.5 and 2%, with both emulsified (e-wax) and slack waxes. Results showed very good correlations (R² > 0.9) between sensor measurements and wax level for both slack and e-wax. However, this method is limited because it only measures the relative level of wax application and not the evenness of wax dispersion on strands.
An optical, image analysis method for measuring wax distribution was also developed and evaluated. This is an off-line, benchtop system using a digital camera and lens to capture and analyze magnified images of wax dispersion on strand surfaces. To enable detection and measurement, a ultra-violet (UV) tracer is added to the emulsified or slack wax before spray application to the furnish. Furnish samples are then scanned by the imaging system using a UV light to fluoresce the wax/tracer coverage. Unlike the NIR system, this method is capable of measuring the relative evenness of wax coverage and spot size distributions.
In the second year of the project, pilot plant tests were carried-out at ARC to determine how different wax distributions affected panel properties and whether changes in distribution could be measured using the newly developed image analysis method. Good and poor wax distributions were produced by changing blending parameters to simulate variable mill conditions. Results showed that the poor blends produced panels with significantly higher thickness swell (TS) and water absorption (WA) compared to the better wax blends. For slack wax, WA and TS were 40% and 26% higher respectively for the poor wax blends. For e-wax, WA and TS were 26% and 13% higher respectively for the poor blends. Meanwhile, image analysis measurements showed that the wax coverage distribution of the poor blends had much higher COV values (coefficient of variation; calculated as standard deviation/average) compared to the good blends, for both wax types.
Finally, a mill trial was carried-out using the image analysis system to assess the coverage distributions for both e-wax and slack waxes. Results showed that both wax types were relatively easy to visualize and measure. Measurements also showed that wax coverage COV values were relatively high and similar to the poor pilot plant blends. This trial showed that the image analysis system was useful in gauging the mill’s wax distribution performance, and it’s implications to panel properties, by referencing to earlier pilot plant data.
In summary, this project developed practical methods for measuring wax distribution that can be used by OSB mills, and also showed that wax distribution should be monitored because it significantly influences panel properties, especially thickness swell and water absorption.
This report summarizes the existing knowledge on building movement related to wood-frame construction. This knowledge includes fundamental causes and characteristics of wood shrinkage, instantaneous and time-dependent deformations under load, major wood-based materials used for construction and their shrinkage characteristics, movement amounts in publications based on limited field measurement, and movement estimations by construction practitioners based on their experience with wood-frame construction. Movement analysis and calculations were also demonstrated by focusing on wood shrinkage based on common engineering design assumptions, using six-storey platform buildings as examples. The report then provides engineering solutions for key building locations where differential movement could occur, based on the literature review as well as a small-scale survey of the construction industry.
The report emphasizes the importance of comprehensive analysis during design and construction to accommodate differential movement. Most building materials move when subjected to loading or when environmental conditions change. It is always good practice to detail buildings so that they can accommodate a certain range of movement, whether due to structural loading, moisture or temperature changes. For wood-frame buildings, movement can be reduced by specifying materials with lower shrinkage rates, such as engineered wood products and drier lumber. However, this may add considerable costs to building projects, especially when specifications have to be met through customized orders. Producing lumber with a lower moisture content adds significant costs, given the additional energy consumption, lumber degrade and sorting requirements during kiln drying. Specifying materials with lower moisture content at time of delivery to job site does not guarantee that wood will not get wet during construction, and excessive shrinkage could still be caused by excessively long time of exposure to rain during construction. On the other hand, effective drying can occur during the period between lumber delivery and lumber closed into building assemblies. Appropriate measures should be taken to ensure lumber protection against wetting, protected panel fabrication on site, good construction sequence to facilitate air drying, and supplementary heating before closing in to improve wood drying.
This report also provides recommendations for future work, including field measurement of movement and construction sequencing optimization, in order to provide better information for the design and construction of wood buildings, five- and six-storey platform frame buildings in particular.
