Potential market gain for Canadian softwood plywood in residential construction could arise from the emerging Chinese market to build massive numbers of affordable apartments and the upcoming rebuilding effort in Japan following the earthquake and tsunami disaster. Compared to the main Chinese species (poplar), common BC species, such as Douglas-fir, spruce and hem-fir, have competitive advantages in the aspects of log diameter, wood properties and veneer quality and processing productivity. For non-residential construction, Canadian plywood concrete forms also offer competitive advantages over Chinese overlaid poplar counterparts due to their higher stiffness and strength. However, the production cost has to be kept to below US$ 500/m3 for a profit margin. Further, three-ply and four-ply Canadian softwood plywood panels are ideally suited for the base materials of multi-layer composite floor, which currently is gaining momentum in China and other countries.
A sizeable increase in industrial and remodelling market is anticipated for the Canadian plywood industry. This will be mainly driven by a number of specialty plywood products, such as container floor and pallet, light truck, utility vehicle, trailer and camper manufacturing. However, these products are not commonly manufactured by larger commodity manufacturers in Canada. China is currently the largest global supplier of container floors, most of which are made from imported plywood, bamboo and poplar veneer. To meet their stringent requirements and gain a market share, Canadian plywood industry should take appropriate actions in adjusting veneer thickness, veneer grade, veneer treatment, and panel lay-up.
Japan has developed customized products such as oversized plywood for wall applications, and termite/mould resistant plywood for above ground and ground-contact applications. China has developed numerous new value-added veneer products for niche markets. Such products include marine plywood, sound reducing plywood, non-slip plywood, metal faced plywood, curved plywood and medium density fiberboard (MDF) or particleboard (PB)-faced plywood.
In order to stay competitive in the global market, Canadian plywood industry needs to:
remove the trade constraints between softwood plywood and hardwood plywood,
remove in-plant manufacturing barriers to deal with both softwood and hardwood processing,
diversify products for both appearance and structural based applications, and
develop new value-added products for niche markets.
This study suggests the following opportunities for Canadian plywood producers to
incorporate naturally decay-resistant species such as cedar as surface veneer and/or perform veneer or glueline treatment to make marine and exterior plywood for improved durability,
characterize veneer properties from the changing resource for better utilization,
peel some thinner and higher quality veneer for making specialty plywood,
conduct stress grading in combination with visual grading to maximize value recovery from the available resource,
increase the flexibility of panel lay-up for domestic/overseas markets and various applications,
develop mixed species plywood by mixing available hardwood species such as birch, maple, alder, aspen veneer (as overlay materials) with softwood plywood to achieve better appearance and higher performance,
develop new structural composite lumber (SCL) products such as veneer strand lumber (VSL) from low quality logs, particularly beetle-killed, and random veneer or waste veneer,
develop new drying, pressing and adhesive technologies for processing high moisture veneer, particularly hem-fir and spruce, to improve productivity and bond quality and reduce panel delamination,
develop light weight and strong hybrid plywood panels for furniture applications, by adding MDF or PB on the face of plywood,
develop hybrid plywood for floor applications to reduce thickness swell and increase dimensional stability and stiffness,
develop hybrid cross-laminated timber (CLT) panels from lumber, plywood and laminated veneer lumber (LVL) for low- and mid-rise residential and non-residential applications, and
develop a series of new product standards for specialty plywood.
A market research study for each product opportunity is recommended to develop a solid business case for each.
Within the limits of this study, the results indicate that it is quite possible to develop new Engineered Structural Lumber products from MPB wood and to maximize its value for uses in traditional and next generation wood buildings. New product and processing technologies have to be developed first to convert severely dried and checked MPB wood into competitive structural lumber products. Further research and development, particularly in stranding technology for dry logs is recommended.
In this study, several types of oriented strandboard (OSB) panels were compared including those prepared using a novel method involving the application of plastic film directly to the OSB faces by hot-pressing. Different edge seal methods were also evaluated. A flooring simulation test involving both water spray and drying cycles was used to evaluate the dimensional stability performance of the OSB panels. In total, three tests were carried-out to evaluate the swelling properties of OSB panels with different edge seal and face plastic cover combinations.
Results showed that when plastic coated panels were installed in the flooring simulation set-up and then sealed around the edges with tape, the edge and inside edge thickness swelling could be significantly reduced compared to control panels. Also, the panels with a plastic covering on both faces warped less and recovered better after air-drying compared to panels with plastic film on the top face only.
This study has shown that when panels are first coated with plastic by hot-pressing and then sealed around the edges with tape after installation, thickness swell and warping can be minimized. This method is especially promising for providing sufficient protection of OSB when used in flooring applications which are exposed to short-term exposure to wetness due to weather exposure.
In this study, we conducted mill tests to: 1) measure the distributions of veneer clipping width and moisture content (MC) for different green veneer sorts; and 2) evaluate the tangential veneer shrinkage at different MC levels for common veneer species. We also performed lab tests to evaluate the effect of veneer species, log location, veneer thickness, sapwood and heartwood, drying conditions (temperature and humidity) and final veneer moisture (MC) on veneer width shrinkage.
Based on the mill measurements, although a positive trend exists between veneer clipping width and green veneer MC, the correlation is generally weak for common softwood species such as SPF and Douglas-fir veneer. This result contradicts the prevailing concept of veneer MC clipping. The primary reason could be attributed to the inaccurate measurement of green veneer MC with the current industrial MC sensors.
Based on both mill and lab testing results, a good correlation exists between veneer width shrinkage and final veneer MC. However, the correlation between veneer width shrinkage and green veneer MC is weak or at most fair for common softwood species. Also within each veneer sort, this correlation is very poor. These results bring up an issue as to what strategy to use to perform green veneer clipping.
Based on both mill and lab measurements, a shrinkage of 1.5 ~ 3% in veneer width occurs when final average veneer MC is still above fiber saturation point (FSP). This contradicts the well-established theory that shrinkage of wood only starts to occur when its MC drops below the FSP. The main reason is probably due to the MC gradient through veneer thickness, and MC variation across veneer width. Once the MC of the wood cells of the veneer surface drops down to FSP, it starts to shrink. However, due to the constraint caused by the wood cells of the inner part of veneer with MC above the FSP, the internal stresses develop and shrinkage decreases.
Veneer width shrinkage varies among species and logs, and also within the log, namely, from sapwood to heartwood. It is further affected by drying conditions, veneer thickness and final veneer MC. At fast or conventional drying conditions such as higher temperature and faster airflow, veneer drying rate increases, the veneer MC gradient through the thickness becomes significant. In this manner, constraints, resulting from uneven shrinkage and internal stress, occur between the outer and inner part of the veneer. As a result, veneer width shrinkage is reduced compared to slow or air drying conditions. For common softwood species such as SPF and Douglas-fir, the effect of drying temperature on tangential veneer shrinkage is more pronounced with heartwood veneer than sapwood veneer. In other words, at a higher drying temperature, shrinkage of heartwood veneer is reduced more than that of sapwood veneer. Meanwhile, the effect of humidity on veneer width shrinkage is negligible.
At the same conventional drying condition, for Douglas-fir sapwood veneer, thicker veneer (1/8”) shrinks more than thinner veneer (1/10”); In contrast, for Douglas-fir heartwood veneer, thicker veneer (1/8”) shrinks less than thinner veneer (1/10”). The shrinkage of thicker veneer (1/8”) is more considerably affected by the drying conditions compared to that of thinner veneer (1/10”). The difference in shrinkage between sapwood and heartwood veneer is smaller for thinner veneer (1/10”) than for thicker veneer (1/8”).
It is important to improve drying productivity since this process is the bottleneck in plywood production. To address this issue, we evaluated the veneer moisture content (MC) distribution and the accuracy of radio frequency (RF) sensors both in the laboratory and in mill trials in this study.
We analyzed the distribution of green veneer moisture content (MC) and density from sapwood to heartwood for lodgepole pine logs via veneer peeling in the Forintek’s composites pilot plant. The results show that the MC distribution appears to be a dual-peak pattern both for heartwood and for sapwood. The position of the first peak is more consistent whereas the second peak varies more among logs. This information is useful for determining the proper number of green veneer sorts and the optimum cut-off MC level for each sort.
Readings of the radio frequency (RF) sensor are affected by veneer species, veneer density, veneer thickness, temperature, green veneer MC, grain angle, and distance between the sensor source and veneer surface. Of these variables, veneer density, veneer species and the distance are found to be the three main factors affecting the readings of the sensor. Based on the measurement results from different species, we conclude that the RF sensor is only suitable for measuring green veneer MC below 80%.
The correlation between readings of the sensor and green veneer MC varies from species to species. In general, this correlation could be improved using an exponential or a power equation. The readings of mill sensor are more inconsistent than those of the lab sensor for the heart sort veneer. The lab sensor underestimates MC for the heart sort veneer and overestimates MC for the sap sort veneer. However, the mill sensor overestimates in both positive and negative ways for each sort. To improve the accuracy of the readings, the sensor needs to be calibrated based on the species and veneer thickness, and the distance between the sensor source and veneer surface has to be kept as small as possible. However, this is not practical for the spruce, pine and subalpine fir (SPF) group since these species are not separated on a commercial basis.
We conducted three mill visits to: 1) measure the distribution of green veneer MC for different veneer sorts, and 2) assess the accuracy of current green veneer sorting. The results demonstrate that the accuracy of moisture sorting differs among mills and species, and the species mix like SPF generates larger MC variation within each sort. In general, the heart veneer sort is well done, but there is a significant overlapping between light-sap and sap veneer sorts. This indicates that the current industrial RF sensors cannot ideally sort higher MC veneer. The results also show that there is potential for more accurate green veneer sorting which would result in a 5% increase in drying productivity. This improvement would generate more than $1 million in annual savings per mill.
This work provides scientific support for, and confirms, what most mills already use as rough and dirty rules of thumb as best practices for manufacturing plywood: i.e. dry veneer should be pressed when its temperature is 100°F or less; average veneer moisture can be 4 %; assembly times should not exceed 20 minutes; and glue spreads should be approximately 32 lbs. per M ft2 SGL.
In addition, this report used the data generated to formulate multivariate statistical models that could be used to develop or enhance existing in-mill process control software, and/or quality procedures at member operations.
This report documents the results of an extensive investigation of plywood dryout and delamination. The study included laboratory and mill tests of key manufacturing variables used in the production of phenol formaldehyde (PF) bonded plywood. Relationships between key variables and plywood quality were used to develop a statistical equation to quantify the effect of veneer moisture content, temperature, assembly time and glue spread rate on wood failure percentage. Testing methods using vacuum/pressure boil-dry-boil, and 6-cycle soak were used and a new multi-step pressing schedule was examined. The following are the main findings:
Veneer with a low moisture content (MC) level is more likely to create glueline dryout than high MC sheets when PF resin is used. Although veneer with a high MC level could minimize the occurrence of dryout, PF gluing systems accept a maximum allowable veneer MC (peak moisture) range of 6 to 8%.
Sheets having temperatures over 100°F are strongly correlated with dryout problems.
An excessively long assembly time approaching 20+ minutes could significantly affect bonding, especially when veneer or ambient temperatures are high.
Increasing glue spread rate can be used to minimize dryout caused by high veneer temperature and low veneer MC; however, a higher glue cost per M ft2 is incurred.
Flexure tests cannot be relied upon to detect bond quality as bending strength is heavily influenced by surface panel properties.
During the mill study, it was learned that variations within each of the above mentioned controllable factors could not be avoided in a mill situation. Good manufacturing process control can ensure that all variables stay within ideal ranges and occurrences of dryout are minimized. The statistical models developed during this project could, possibly, be used to develop or enhance process control software.
A multiple-step hot-pressing schedule, capable of improving plywood bonding properties with or without sacrificing volume recovery, was developed to minimize bond problems caused by dryout. Existing mill presses may be able to implement this approach seamlessly or after a few minor adjustments have been made. Some mills may have to peel thicker veneer to compensate for increased veneer compression associated with multi pressing and in doing so sacrifice log recovery. Pressing schedules illustrated in this study indicate that press production is not sacrificed as single or multi-step pressing time is identical for most thicknesses.
Laboratory tests showed that wood failure percentage figures from a glueline shear test, a standard method for evaluating bond quality, are useful glue dry-out indicators for softwood plywood.
An attempt to develop a tack strength test that could assist in evaluating dryout was unsuccessful as excessive variation was present within recorded data.
An extensive investigation of plywood dryout and delamination was conducted in this study. It included laboratory tests of key variables for urea formaldehyde (UF) bonded plywood. A statistical equation was developed to quantify the effect of veneer moisture content, temperature, assembly time, glue spread rate, and amount of catalyst on wood failure percentage. The following are the main findings:
Veneer with a low moisture content (MC) level is more likely to create glueline dryout than high MC sheets when UF resin is utilized. Although veneer with a high MC level could minimize the occurrence of dryout, maximum allowable veneer MC (peak MC) is limited by other factors. UF gluing systems can employ a maximum MC range of 10 to 12%, which is much higher than the one that applies to PF resin (6 to 8%).
Sheets having temperatures over 100°F are strongly correlated with dryout problems.
An excessively long assembly time could significantly affect bonding, especially when veneer or ambient temperatures are high.
Increasing glue spread rate can be used to minimize dryout caused by high veneer temperature and low veneer MC.
In UF gluing systems, catalyst level heavily influences bonding and the extent to which dryout occurs.
A new green veneer moisture measurement method was developed based on the principle of light transmission. Compared to current radio frequency (RF) moisture measurement, the new method shows improved accuracy in green veneer MC detection for regular softwood veneer. As well, an off-line portable veneer testing system was developed based on the light transmission, which was successfully used in the mill trials to evaluate the accuracy of current green veneer moisture sorting. Meanwhile, a laboratory dry veneer moisture measuring system was successfully modified to measure the variation of dry veneer MC. Further a pilot-scale veneer dryer was upgraded to simulate the mill drying conditions. These developments ensure us to promptly transfer the technologies to the industry.
The effect of drying temperature, air flow velocity and air humidity on dry veneer final MC was investigated. It was found that drying at high temperature was faster than at the low temperature. The difference in final MC between low temperature drying and high temperature drying was greater when the initial MC was higher. However, the variation of final MC was smaller with low temperature drying compared to high temperature drying. The effect of air velocity was more apparent when the initial veneer MC was higher. When the initial veneer MC was below 70-80%, the effect of air velocity on veneer drying was very small, which may indicate that for heart veneer drying, the velocity was not a dominant factor whereas for sap and light-sap veneer drying, the air velocity played a significant role. In general, the effect of air humidity on veneer drying was drying temperature dependent. At low drying temperature, low humidity helped veneer drying. In contrast, at a drying temperature higher than 150 °C, the effect of air humidity on veneer drying rate was not significant. However, to improve the veneer drying quality and material recovery, high humidity could be used to reduce veneer brittleness for easy handling.
During stacking period, the moisture spread from the wet area to the dry area. The variation of MC between and within veneer decreased with the stacking time before reaching an equilibrium state. The equilibrium MC depended on the initial MC of veneer and stacking conditions. Compared to lower temperature stacking, the higher temperature stacking accelerated the moisture spread. Therefore, the allowable maximum MC of wet spot on veneer before stacking could be determined based on target dry veneer MC, ambient temperature (season) and stacking time.
A hot stacking model was developed to simulate the change of veneer MC during hot stacking. The prediction results agreed well with the experimental results. This model will be incorporated into the existing VDry models to simulate the effect of post-drying on final veneer MC.
It is recommended that the light green veneer moisture scanner should be further developed for industrial applications. The VDry model is a useful tool which should be applied for mill customization and optimization.
This project was placed top priority because veneer drying is the bottleneck and the biggest item for energy consumption in plywood mills. During the past three years, the Forintek veneer drying manual was first upgraded. Our research capabilities in drying were significantly improved by the establishment of a mini-dryer and the installation of pilot scale veneer dryer. While the former allows for in-situ monitoring of moisture content change in veneer during drying, the latter can be used to simulate industrial dryers such as jet dryers or longitudinal dryers. Both are capable of testing the effects of such parameters as drying temperature, humidity and air flow. Based on the improved understanding from the experimental tests and theoretical analyses, initial computer models (VDRY-L and VDRY-J) have been developed to simulate the drying processes for longitudinal dryers and jet dryers, respectively.
The combination of laboratory studies and computer simulation led to effective mill studies to evaluate the existing drying technologies, to optimize the existing dryers without capital cost and develop future opportunities.
The key findings from these studies were:
Lab tests showed that temperature and airflow rates are dominant factors affecting drying rate for the whole drying process. While veneer drying increases as drying temperature and airflow rates are increased, higher temperatures and airflow rates both have a greater influence on drying rate at the early drying stages compared to the final stages.
Lab tests also showed that veneer can be dried at high rates under high humidity and temperature conditions. A combination of high temperature and humidity is a good drying mode to save energy and increase output.
Mill case studies showed that the sorting of green veneer prior to drying was poorly done. Lab tests showed that a probable reason for this was due to the inaccuracy of RF sensors used for measuring moisture content of green veneers. The RF accuracy significantly dropped when veneer moisture content exceeded 30%.
Mill case studies showed that a sensitivity analysis of drying parameters is a very useful method for determining effective measures for optimizing dryer performance.
Mill case studies showed that by using higher temperatures and humidity levels in the early drying stages and lower temperatures in the final stages, veneer production can be increased and drying energy can be reduced.
An impact analysis showed that mills can potentially save up to 10% in energy costs and increase production by 10% by optimizing dryer performance.
It is recommended that mills use the results and methodologies from this project as guidelines for optimizing dryer performance. Further research should be undertaken to improve green veneer moisture sorting. The current computer simulation models of drying should be calibrated and used as a quality control tool for determining optimum dryer parameter settings.
On March 31, 2003, Forintek Canada Corp completed the project “Optimization of Veneer Drying Processes”. Forest Industry Investment and Forintek members funded this one-year project. The purpose was to develop practical methodology to transfer new Forintek knowledge about veneer drying and quantify the benefits of optimizing existing dryers. Currently in BC there are 15 softwood plywood mills producing approximately 1.7 billion square feet of plywood (3/8” basis) per year and employ over 2800 people.
In plywood production, veneer drying is one of the key manufacturing processes. Currently, dryers create a bottleneck in the mill, restricting plywood production. Dryers are expensive to purchase and operate. The results show that by optimizing existing dryer operations the potential for increased production and efficiency savings is significant. This provides BC manufacturers with the opportunity to reduce costs without investing capital for costly new equipment or modifications.
At the beginning of the project, five BC mills were visited to identify and quantify key variables affecting dryer operation. Discussions with mill personnel were important for determining specific limitations that could be made to the drying process for optimization and to understand the main problems and potential for improvement from a mill perspective.
At Forintek’s laboratory in Vancouver, small-scale drying tests were conducted to further develop the fundamental knowledge of commercial drying. For this purpose, a laboratory dryer was specially designed to simulate industrial drying processes. Testing focused on determining the drying rate relationships involving drying air temperature, airflow speed and humidity. Results showed consistently that the best set-up for maximizing drying rate was by operating the dryer at high temperature with high humidity and airflow speed. The lab study also showed that these conditions are most importantly applied at the early stage of drying. Near the end of the drying process, the effects of high temperature were diminished, suggesting that temperature can be reduced at this stage to economize on energy consumption without significantly affecting drying rate. As well, the lower drying temperature near the end of the drying process reduces the risk of surface inactivation (loss of bonding sites) in the veneer. Additional testing at Forintek was conducted to determine the accuracy of radio frequency moisture sensors currently used for sorting green veneer prior to drying. Results showed that moisture content measurement error increased significantly with moisture content above 30%.
Case studies were conducted at two BC mills to quantify the relationships of key drying parameters and to test methods for optimization. In both mills, prior to drying, measurements of green veneer showed that sorting was very inefficient with a large proportion of veneers incorrectly directed into the heart, light-sap and sap bins. Dryer control modifications demonstrated that dryer temperatures could be increased by the same order of magnitude as in the laboratory tests by restricting damper openings to raise humidity. This not only increased veneer feed speed, but also reduced steam consumption used to heat the dryer. At one mill, changes to temperature and humidity conditions resulted in an increase of the veneer feed speed from 9.3 to 10.8 ft/min. for light sap veneer, representing an increase of 16% in productivity through the dryer. At the same time, it was estimated that 10% of the annual energy use for the dryer could be saved based on reduced steam consumption.
Based on the case study results, it was conservatively estimated that the 15 plywood mills currently operating in BC could potentially increase plywood production by 5% or 85 million square feet per year. At today’s prices, this translates into approximately $31 million per year by implementing, at no cost, the methodology presented here to improve dryer performance. In addition, results also showed that dryer energy consumption could be potentially reduced by 10% A practical 5% reduction would amount to a savings of $3.0 million for the BC plywood industry. No additional implementation cost was required to achieve this energy savings. In addition, Forintek estimated that better green veneer sorting by mills could significantly reduce the moisture content variability of dry veneer, further improving dryer productivity. However, this would require more accurate moisture sensing technology than is now commercially implemented. It is recommended that new technology be developed.