The Influence of Furnish Moisture Content and Press Closure Rate on the Formation of the Vertical Density Profile in Oriented Strandboard

Authored by:  Chris Andrews, Paul Winistorfer and Richard Bennett

Source:  Andrews, C. K., P.M. Winistorfer and R.M. Bennett. 2001. The influence of furnish moisture content and press closure rate on the formation of the vertical density profile in oriented strandboard. Forest Products Journal. 51(5):32-39.

Abstract:  The vertical density profile of wood composite panels is defined as the change in density through the panel thickness.  The density profile results from the influence of processing parameters used in panel manufacture.  A study was completed to examine the relationship of the formation of the vertical density profile in oriented strandboard as influenced by furnish moisture content (MC) and press closure rate.  Twenty-seven, 40-pcf, 1/2-inch-thick southern pine, laboratory strandboards were manufactured with three furnish MC treatments (3%, 6%, and 9%) and three press closure rate treatments (20, 40, and 60 sec.) in a completely randomized split-plot design with three replications.  Applications of phenol-formaldehyde liquid resin (3%) and emulsion wax (0.5%) were held constant.  The density profile was measured for all treatments.  For subsequent analysis, the density profile was divided into five zones and the minimum and maximum density and their respective locations within each zone were recorded.  Zones 1 and 5 were considered face zones, 2 and 4 were intermediate zones, and 3 was considered the core zone.  The data were analyzed to examine the effects of furnish MC and press closure rate on density formation within individual zones of the vertical density profile.  Furnish MC strongly influenced the density in all zones and the density locations in the intermediate zones of the vertical density profile.  Press closure rate strongly influenced the density in the face zones and the density locations in the face and intermediate zones of the vertical density profile.  The density locations in the core zone were unaffected by furnish MC of press closure rate.  Significant interactions between furnish MC and press closure rate were present.

Reduction of Residual Stresses in Medium Density Fibreboard
Part 1. Taguchi Analysis

Authored by:  Jeroen van Houts, Debes Bhattacharyya and Krishnan Jayaraman

Source:  van Houts, J., D Bhattacharyya and K Jayaraman. 2001. Reduction of residual stresses in medium density  fibreboard. Part 2. Effects on thickness swell and other properties. Holzforschung. 55(1):67-72.

Abstract:  This paper demonstrates how the Taguchi method of experimental design can be utilised to investigate methods for relieving the residual stresses present in medium density fibreboard (MDF). Panels have been subjected to heat, moisture and pressure, and after equilibration to room conditions, the changes in residual stresses through various layers have been measured using the dissection method. The application of heat and/or moisture has reduced the magnitude of residual stresses while generally the application of pressure has no effect on these stresses. The subsequent paper in this series uses Taguchi analysis to investigate how other board properties such as thickness swell, internal bond strength, surface layer tensile modulus and surface layer tensile strength are affected by the different treatment methods.

Keywords:  Taguchi analysis, residual stress, medium density fibreboard, wood fibre.

Reduction of Residual Stresses in Medium Density Fibreboard
Part 2. Effects on Thickness Swell and Other Properties

Authored by:  Jeroen van Houts, Debes Bhattacharyya and Krishnan Jayaraman

Source:  van Houts, J., D Bhattacharyya and K Jayaraman. 2001. Reduction of residual stresses in medium density  fibreboard. Part 2. Effects on thickness swell and other properties. Holzforschung. 55(1):73-81.

Abstract:  The Taguchi method of experimental design has been utilised to investigate various treatments for relieving the residual stresses present in medium density fibreboard (MDF). These treatments involved subjecting panels to different combinations of heat, moisture and pressure. This paper reports on the Taguchi analysis of the internal bond strength, surface layer tensile modulus, surface layer tensile strength and thickness swell of the treated specimens. These properties were measured to indicate whether the treatments had any effect on panel strength and dimensional stability. A strong correlation between residual stresses and thickness swell has been identified. When the change in residual stress through the outer layers of a panel is almost completely removed, a reduction in thickness swell of approximately 20% for a 24 hour water soak is observed.

Keywords:  residual stress, medium density fibreboard, wood fibre, thickness swell, internal bond strength, tensile modulus, tensile strength.

Flake Compression Behavior in a Resinless Mat as Related to Dimensional Stability

Authored by:  Siqun Wang and Paul Winistorfer

Source:  Wang, S., and P.M. Winistorfer. 2001. Flake compression behavior in a resinless mat as related to dimensional stability. Wood Science and Technology (in press).

Abstract:  The purpose of this study was to evaluate compression and swelling characteristics of individual furnish elements sampled through the thickness of lab panels pressed without resin. Commercial southern pine OSB furnish was used to press resin-less mats so individual flakes could be removed from the panel after pressing and evaluated for compression behavior. 19 flake sets, each set consisting of 15 southern pine flakes with 0.65% wax, were marked and measured for thickness and mass. One set of marked flakes was randomly distributed in one layer of a mat which consisted of 19 total layers; each of the 19 layers had 15 marked flakes randomly distributed in the layer. After hot pressing each marked flake was removed from the mat. After achieving equilibrium at 35%, 65% and 98% relative humidity, each flake was again remeasured for thickness and mass. Experimental results include flake compaction ratio and its distribution through the mat thickness; flake thickness swelling under different RH environments; compaction ratio-thickness swelling relationship; and individual flake compaction ratio and thickness swelling variations. Comparison is made to adsorption, desorption behavior of pressed flakes. Flakes from surface layers exhibited compression of 25 to 37%, about double that of flakes in core layers. As expected, flakes from surface layers showed much greater thickness swell than core flakes and the response was accentuated with higher EMC conditions.

Step-Closing Pressing of Medium Density Fiberboard
Part 1: Influence on the Vertical Density Profile

Authored by:  Siqun Wang, Paul Winistorfer, Tim Young and Chris Helton

Source:  Wang, S., P.M. Winistorfer, T.M. Young, and C. Helton. 2001. Step-closing pressing of medium density fiberboard. Part 1: Influence on the vertical density profile. Holz als Roh-und Werkstoff. 59(1/2):19-26.

Abstract:  Medium density fiberboard (MDF) is one of the most rapidly growing composite board products available in the marketplace. A key product attribute of MDF is the density profile through the panel thickness. A superior MDF panel for laminating, gluing and finishing should have an unbalanced density profile, in which the face density is considerably higher than the core density. The homogeneous core of MDF makes it especially suitable for embossing, moulding, and general machining. A uniform density through the panel thickness results in better fastening properties.

A step-closure schedule to change the density profile of lab-made MDF panels is presented.MDF mats were pressed using eight different step-closure schedules. Our in-press density monitoring system was used during pressing to provide additional information about the formation of the density profile. The effects of step-closure on the density profile formation are discussed.

The step-closure schedules significantly changed the traditional shape of the vertical density profile of the laboratory made MDF panels. The step-schedules resulted in multiple densification peaks rather that the traditional two peaks associated with conventional pressing. The step-schedules resulted in increased core density, which was greater than the face density. Comparison is made to traditional MDF closing schedules and OSB step-closing schedules.

Step-Closing Pressing of Medium Density Fiberboard
Part 2: Influences on the Panel Performance and Layer Characteristics

Authored by:  Siqun Wang, Paul Winistorfer, Tim Young and Chris Helton

Source:  Wang, S., P.M. Winistorfer, T.M. Young, and C. Helton. 2001. Step-closing pressing of medium density fiberboard. Part 2: Influences on the panel performance and layer characteristics. Holz als Roh-und Werkstoff (accepted and in-press).

Abstract:  The previous paper in this series discussed the step-closure pressing schedule to change the density profile of lab-made MDF panels. The purpose of this research was to investigate the effect of the step-closure schedule on the end-product performance, especially on the layer thickness swell and its relationship with layer density. The results of this work showed that step-closure schedules resulted in improved internal bond strength of the tested specimens. The step closing schedule significantly increased the core density and decreased the face density, resulting in lower bending properties. The results showed that greater core density did not result in higher internal bond strength. The severer unsteady phase in a mat resulted in the poorer quality of the bond formation. The layer thickness swell after three water exposure times is significantly and positively related to layer density in all panels studied. The correlation coefficient between the actual layer thickness swell and layer density increased as water exposure time increased. The greater thickness swell in the surface layers of MDF suggests that efforts to improve dimensional stability of MDF should be focused on stabilizing the high-density surface layers.

Determination of Layer Thickness Swell of MDF and OSB by Optical Technique

Authored by:  Siqun Wang and Paul Winistorfer

Source:  Wang, S., and P.M. Winistorfer. 2001. Determination of layer thickness swell of MDF and OSB by optical technique. Forest Products Journal (in press).

Abstract:  A nondestructive optical technique was developed to determine thickness swell of discrete layers within intact samples of wood composites. Layer thickness swell of commercial MDF and OSB are presented. Layer swell within the sample is important in understanding the swell phenomena of wood composites. Results from standard specimens show edge layer thickness swell after 2, 8, and 24-hour water soak and include measures of precision and variation for this technique. Measurement variation decreased as water exposure time increased and was less than 3.4% after 24-hour water exposure. The contribution of high-density surface layers to overall MDF thickness swell were 95.76%, 75.5% and 61.77% after 2-hour, 8-hour, 24-hour water exposure, respectively. The contribution of high-density surface layers to overall OSB thickness swell were 74.36%, 64.39% and 57.3% after 2-hour, 8-hour, 24-hour water exposure, respectively. Thickness swell for both products was dominated by the high density surface layers throughout the 24 hour soak cycle. However, dense surface layers contributed more to the overall swell measurement during the early period of the soak cycle compared to the swell measurement at the completion of the 24 hour soak cycle. The relative contribution of the core layers to overall thickness swell increased with length of exposure period. This optical technique is recommended for the measurement and study of in-situ layer swell properties for all wood composite panel materials.

Determination of Residual Stresses in Medium Density Fibreboard

Authored by:  Jeroen van Houts, Debes Bhattacharyya and Krishnan Jayaraman

Source:  van Houts, J., D Bhattacharyya and K Jayaraman. 2000. Determination of residual stresses in medium density fibreboard. Holzforschung. 54(2), 176-182.

Abstract:  Due to the moisture and temperature gradients developed during hot pressing of medium density fibreboard (MDF), residual stresses occur within the board as it equilibrates to room conditions. It would be extremely useful to measure these residual stresses and to determine their effects on board properties such as moduli of elasticity and rupture in bending, internal bond strength and dimensional stability. In this article two methods, namely dissection and hole drilling, have been adapted to measure residual internal stress distributions in six different samples of industry produced MDF. The dissection method involves cutting several pieces of MDF perpendicular to the thickness direction at different depths. The residual stresses released by the dissection can be determined by measuring the curvatures of cut pieces and knowing their elastic moduli. The hole drilling method, on the other hand, involves mounting three strain gauges on the surface of a piece of MDF and drilling a hole to release residual stresses in close proximity. The released stresses are manifested as strains in the forms of which can be measured in three directions on the surface of the board.

A theoretical model for predicting residual stresses involving various parameters has been developed and an excellent agreement with the experimental results from both the dissection and hole drilling methods has been achieved. Linear moisture expansion coefficient appears to have the greatest influence on residual stress. When compared against each other, the residual stresses measured by the hole drilling method show some shortcomings towards the centre of the board. While all six of the MDF boards exhibited similar trends in their residual stress distributions, significant differences were identified in the magnitudes of residual stress measured. Finally, some preliminary results linking the residual stress with the thickness swell of the samples and their surface densities have been presented.

Keywords:  residual stress, medium density fibreboard, wood fibre, hole drilling method, dissection method, thickness swell, surface density.

Consolidation of OSB Mats Under Theoretical Laboratory Pressing and Simulated Industrial Pressing

Authored by:  Siqun Wang and Paul Winistorfer

Source:  Wang, S., and P.M. Winistorfer. 2000. Consolidation of OSB mats under theoretical laboratory pressing and simulated industrial pressing. Wood And Fiber Science. 32(4):527-538.

Abstract:  To achieve a more fundamental understanding of material behavior during the pressing process, a radiation-based system for measuring density of wood composite mats during consolidation is used to build in-situ cross-sectional density distributions of flakeboard mats with pressing time. The fundamentals of densification within flakeboard mats during hot and cold pressing are discussed in this paper. The pressing schedules included theoretical laboratory pressing schedules and a schedule simulating industrial pressing. All tests were conducted at either ambient or 204^oC temperature. The results include stress relaxation of flakeboard mats during cold and hot pressing, stress-strain behavior, in-situ density-strain behavior and in-situ cross-sectional density distributions of flakeboard mats with pressing time. Results of laboratory studies indicate that the stress relaxation during hot pressing after the press reached final position was much quicker than during cold pressing. The observed stress-strain responses of flakeboard mats in hot pressing and cold pressing were similar, characterized by a long stress plateau followed by a rapid increase in stress and an immediate fall-down after press reached final position. The process to simulate the industry operation resulted in another stress plateau. The stress-strain responses of flakeboard mats were characterized by a long stress plateau followed by a rapid increase in stress, and an additional high stress plateau followed by an immediate fall-down after the press reaches final position. There was no clear indication that the maximum gas pressure attained is affected by press closing time.

Keywords:  Densification, consolidation, density profile, compression, in-situ measurement, flakeboard, pressing, radiation, moisture, bonding, resin, unsteady state.

Fundamentals of Vertical Density Profile Formation in Wood Composites
Part 1. In-Situ Density Measurement of the Consolidation Process

Authored by:  Paul Winistorfer, William Moschler, Siqun Wang, Esteavo DePaula, and B.L. Bledsoe

Source:  Winistorfer, P.M, W. W. Moschler, S. Wang, E. DePaula, and B.L. Bledsoe. 2000. Fundamentals of vertical density profile formation in wood composites. Part 1. In-situ density measurement of the consolidation process. Wood And Fiber Science. 32(2):209-219.

Abstract:  We have designed a radiation-based system for measuring density of wood composite mats during consolidation. The system is installed on a laboratory hot press and has been used to study consolidation of MDF and OSB mats. Measuring density of the wood mat during consolidation is a key parameter for understanding subsequent product performance. The in-situ measuring system provides for density measurement at three horizontal planes in the wood mat, at positions of 25%, 50% and 75% of the mat thickness at any time during the press cycle. The system incorporates three cesium¹³⁷ sources and electronic detection equipment, collimated to move in concert with the up-acting press platen. Radiation count data taken through the mat during pressing is converted to density after pressing. Press position and time are simultaneously recorded with the count data. Moisture migration during hot pressing resulted in significant density changes as measured by the in-press radiation-based system. Clearly established in all laboratory pressing studies is the indication that the vertical density profile of wood composite panels is formed from a combination of actions that occur both during consolidation and also after the press has reached final position; measurements recorded in the press show that mat densification continues after the press has reached final position. A description of the radiation system and data from elementary pressing examples is presented, along with experimental results of the effects of moisture migration in the mat on measured density during pressing.

Keywords:  Density profile, in-situ measurement, pressing, radiation, consolidation, wood composite, MDF, OSB, moisture migration.

Fundamentals of Vertical Density Profile Formation in Wood Composites
Part 2. Methodology of Vertical Density Formation Under Dynamic Condition

Authored by:  Siqun Wang and Paul Winistorfer

Source:  Wang, S., and P.M. Winistorfer. 2000. Fundamentals of vertical density profile formation in wood composites. Part 2. Methodology of vertical density formation under dynamic condition. Wood And Fiber Science. 32(2):220-238.

Abstract:  The vertical density profile or density distribution through the panel thickness has been identified as one of the important panel characteristics that correlates well with strength and physical properties of wood-based composite panels. We have studied the fundamentals of oriented strandboard (OSB) vertical density profile formation during hot pressing.  Experimental results are from the in-situ density measuring system installed on our laboratory hot press. Results indicate that the vertical density profile of OSB is formed from a combination of actions that occur both during consolidation and also after the press has reached final position (i.e., thickness). We propose a methodology to describe the formation of the density profile into two periods and five stages. The consolidation period is the time of consolidation until the press reaches final position and contains 2 stages. The adjusting period is the time after press has reached final position and continues until the culmination of the cycle.  The adjusting period contains three stages. The resulting density profile is influenced by both periods and all five stages. The vertical density profile results from the combined effects of many process variables, but basically occurs from the effects of furnish moisture conditions, mat structure and the pressing environment. During pressing the mat is always in an unsteady state and internal mat temperature, moisture content distribution, vapor pressure, layer density and compaction stress are all related to the pressing operation. The unsteady state of the mat during the early stages of pressing may result in poor bonding strength development throughout the mat.

Keywords:  density profile, in-situ measurement, oriented strandboard, pressing, radiation, consolidation, moisture, bonding, resin, unsteady state, compression.

Effect of Species and its Distribution on the Layer Characteristics of OSB

Authored by:  Siqun Wang and Paul Winistorfer

Source:  Wang, S., and P.M. Winistorfer. 2000. Effect of species and its distribution on the layer characteristics of OSB. Forest Products Journal 50(4):37-44.

Abstract:  Southern pine and aspen are widely utilized species for OSB production in the North America. In general, aspen with a relatively low specific gravity and uniform cell structure arrangement, is considered more suitable for composite panel manufacture than is Southern pine.   Aspen OSB typically is lower in density and has better dimensional stability when compared to pine OSB.

The purpose of this study was to investigate the effects of species use and distribution within OSB panels on the formation of the vertical density profile, resulting layer thickness swelling, and end-product layer characteristics. Five species configurations were used in this study (by weight): all pine, all aspen, 50%/50% mixture, 25% aspen faces/50% pine core, and 25% pine faces/50% aspen core. Results show that the shape of the vertical density profile was significantly affected by species and species distribution.  Resulting shapes of the vertical density profile were described as steep or gradual, referring to the densification of the face layers relative to the core.  A steep profile has high density surface layers relative to the core and a gradual profile exhibits less difference between the face and core layers.  The face density region of the density profile was further described as either a narrow density peak or a wide density peak, referring to the degree of density change within the face itself.  The all aspen panel had a steep density profile with a very narrow density peak. The 25% pine faces/50% aspen core panel had a more gradual density profile with a wide density peak.  There was strong relationship between layer density and layer thickness swelling. Physical and strength properties for all species combinations of panel types are reported.

Hot-Pressing of Oriented Strandboard by Step-Closure

Authored by:  Siqun Wang, Paul Winistorfer, William Moschler and Chris Helton

Source:  Wang, S., P.M. Winistorfer, W. W. Moschler, and C. Helton. 2000. Hot-pressing of oriented strandboard by step-closure. Forest Products Journal 50(3):28-34.

Abstract:  The vertical density profile (VDP) is an important panel property that describes the change in density through the panel thickness. The panel industry has effectively used press closure rate to manipulate the VDP to alter product performance.  Using a slow press closure rate to manipulate the density profile has some obvious limitations related to resin precure, density profile symmetry and total press cycle time. Traditional press schedules consist of a single closing step until the press reaches final board thickness or position.  In this research, OSB mats were pressed using both two and three step closing schedules.  Mats were pressed to 110%, 105% or 95% of target panel thickness and held at that position for times ranging from 20-120 seconds before closing to final thickness.  For the three step schedules, the mat was compressed to an additional intermediate position before final closure. The step closure schedules significantly changed the traditional shape of the vertical density profile of the laboratory-made OSB panels.  The step schedules resulted in multiple surface densification peaks of the VDP rather than the traditional two density peaks of the VDP associated with conventional pressing. Step pressing schedules are one processing method that can be used to alter the densification process in the panel and subsequently influence panel physical properties. Results are unique in that both in-situ density profiles and density profiles measured after pressing are shown for different step-closure conditions and a comparison is made to traditional closing schedules.  The physical properties for panels produced from each step-closure schedule are also shown.  The step-closure procedure is one method that can be used to balance the structure of the panel density within the mat and in the resulting panel.

Mechanical Properties of Natural Fibre-Reinforced Thermoplastic Composites

Authored by:  Jeroen van Houts, Debes Bhattacharyya and Krishnan Jayaraman

Source:  van Houts, J., D Bhattacharyya and K Jayaraman. 1997. Mechanical properties of natural fibre-reinforced thermoplastic composites. Proceedings of an International Conference on Advanced Materials Development & Performance, eds. W G Ferguson and W Gao, Auckland, New Zealand. 477-482.

Abstract:  The renewability of natural fibres and the recyclability of thermoplastic polymers provide an attractive eco-friendly quality to the resulting composite materials. Three types of natural fibre-reinforced thermoplastic composites - wood fibre/polypropylene, sisal fibre/polypropylene and wood fibre/waste stream plastics - have been investigated in this study. Radiata pine high temperature mechanical pulp fibre mats have been laid up with polypropylene powder or fibre and pressed into composite sheets in a platen press. Random sisal mats were placed between sheets of polypropylene and consolidated using the vacuum-bag method. Four types of plastics from the waste stream have been melt blended with Radiata pine fibres and then injection moulded to form test specimens. Tensile and flexural properties of the natural fibre-reinforced composite materials have been determined as a function of fibre content, matrix, interface and processing parameters.

Keywords:  natural fibres, thermoplastics, composite materials, recycling, medium density fibres, sisal, mechanical properties.