Thursday, 20 October 2016

Chapter VI Multivariate Modeling of MDF Panel Properties in Relation to Wood Fiber Characteristics


Table des matières
Authors: Jun Li Shi, Bernard Riedl, and S. Y. Zhang
La relation entre les propriétés de panneaux de fibres de densité moyenne et les caractéristiques des bois et de la fibre a été étudiée dans ce travail. Les panneaux de fibres de densité moyenne ont été manufacturés en laboratoire à partir de différentes espèces et types de bois, qui étaient du bois d’épinette noire d’arbres agés de 0-20, 21-40, et plus de 40 ans, ainsi que les billes du haut, de la mi-hauteur, et du bas d’arbres d’épinettes noires, de trois clones de peuplier, de mélèze, et d’un mélange d’épinette, pin, et sapin. Les propriétés mécaniques des panneaux évaluées ont été le module de rupture (MOR), le module d’élasticité (MOE) en flexion, et la cohésion interne (IB), ainsi que la stabilité dimensionnelle évaluée par l’expansion linéaire (LE), le gonflement en épaisseur (TS), et l’absorption d’eau (WA), qui ont été analysées comme variables de réponse dans l’étude. Diverses caractéristiques du bois et de la fibre, telles que la densité du bois, son pH et sa capacité tampon basique, le pH de la fibre et sa capacité tampon basique, la longueur moyenne arithmétique des fibres, leur largeur, le pourcentage de fines des fibres, le pourcentages de fibres réparties dans les tamis de taille 3,240 mm2, 0,828 mm2, 0,281 mm2, 0,017 mm2, et sur le tamis 0,017 mm2, ont été mesurées, et ont été utilisés comme variables prédictrices. L’analyse de régression linéaire multiple a été utilisée afin de mettre en évidence les rapports fonctionnels entre les propriétés des panneaux et les caractéristiques des bois et de la fibre. Les résultats indiquent que le module de rupture est négativement lié au pourcentage arithmétique moyen de fibres fines dans la pulpe. Le module d’élasticité est affecté négativement par le pourcentage de petites particules (<taille 0,017 mm2) dans la pâte, et également lié au pH du bois et à d’autres caractéristiques du bois et de la fibre qui n’ont pas été mesurées et analysées. La cohésion interne dépend négativement du pourcentage arithmétique moyen de fibres fines, mais est amélioré par un contenu croissant de petites particules (<taille 0,017 mm2). Le pH de la fibre a eu un effet négatif sur le test de tension perpendiculaire. Les résultats indiquent également que la cohésion interne est liée à d’autres caractéristiques inconnues du bois et des fibres qui n’ont pas été incluses dans l’analyse. L’expansion linéaire du panneau s’est avérée être négativement liée à la densité du bois, et reliée à d’autres caractéristiques du bois et de la fibre qui n’ont pas été mesurées. Le gonflement en épaisseur a été négativement affecté par la longueur arithmétique moyenne des fibres. La largeur arithmétique moyenne des fibres a un effet négatif sur l’absorption d’eau des panneaux. Les valeurs des modules de rupture, d’élasticité et de test de tension perpendiculaire des panneaux sont liées au contenu de fibres fines dans la pâte indiquant un rôle significatif du processus de raffinage.
Properties of medium density fiberboard (MDF) panels in relation to wood and fiber characteristics were investigated. Laboratory MDF panels were manufactured from raw fiber materials of black spruce ( Picea mariana (Mill.) BSP.), three hybrid poplar clones ( Populusspp.), two exotic larch ( Larix gmelinii and Larix sibirica ), and a mix of spruce, pine, and fir (S-P-F) wood chips. The panels were evaluated for modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), linear expansion (LE), thickness swell (TS), and water absorption (WA). These properties were analyzed as response variables. As predictor variables, various wood and fiber characteristics were measured including wood density, pH and base buffering capacity and fiber coarseness. Multiple linear regression analysis was performed to develop the functional relationships between panel properties (response variables) and wood fiber characteristics (predictor variables). Ten dummy variables were created and incorporated into the analysis to examine the effects of wood species or type on MDF panel properties. MOR was negatively related to arithmetic fine percentage. MOE was negatively affected by the percentage of small particles that can pass through the mesh with a size of 0.017 mm2 (mesh size smaller than 0.017 mm2) and wood pH. IB strength was negatively related to arithmetic fine percentage and fiber pH, but positively related to the percentage of small particles (mesh size smaller than 0.017 mm2). Wood density affected LE. TS was negatively affected by arithmetic mean fiber length. Arithmetic mean fiber width had a negative effect on panel WA. The presence of the dummy variables in MOE, IB, and LE models indicates that wood fiber characteristics other than those measured in this study affected panel MOE, IB, and LE significantly. The study indicates that refining process can play a significant role in manipulating MDF panel properties.
A number of factors influence the properties of medium density fiberboard (MDF) (Maloney 1993). Pressing schedule and platen temperature are critical since these factors can result in significant differences in the panel vertical density profile (VDP), which, to a large extent, determines panel physical and mechanical properties (Suchsland and Woodson 1974; Kelly 1977; Harless et al. 1987; Winistorfer et al. 1996; Wang et al. 2001; Wang et al. 2004). The VDP can be determined by the pressing schedule and the temperature of the platens used for panel consolidation (Andrews et al. 2001; Wang et al. 2001). The VDP is closely related to strength and physical properties of composite panels (Suchsland and Woodson 1974; Kelly 1977; Harless et al. 1987; Winistorfer et al. 1996; Wang et al. 2001; Wang et al. 2004). The shape of the VDP curve is of importance. High face density and low core density lead to excellent bending properties, but to relatively poor IB strength (Woodson 1976b). Characteristics of raw fiber materials and wood species used for MDF panel manufacturing are also important (Maloney 1993).
Wood density was found to be negatively related to most strength properties of MDF panels (Nelson 1973; Woodson 1976b). It is easier to make good bonded panels at a specific target panel density with low-density (LD) wood than with high-density (HD) wood due to higher compaction ratios (panel density divided by wood density) resulted from the former (Maloney 1993; Hsu 1997).
According to a study by Nelson (1973), strength properties and linear expansion (LE) of hardboards were positively correlated to pulp pH. Water absorption (WA) was also related to pulp pH; linear stability was positively related to fiber length. Yet, Nelson (1973) found that thickness stability was not related to any wood or fiber property studied. Therefore, fiber morphological and chemical properties, fiber composition and fiber strength are considered to be basic characteristics influencing fiberboard panel properties (Jones 1960; Johns et al. 1985; Myers 1987; Suchsland and Woodson 1990; Maloney 1993; Moss and Retulainen 1995; Hsu 1997; Groom et al. 1999).
The influence of fiber strength on fiberboard quality is dependant on panel density. It has little influence in the case of low density fiberboard (LDF), but its influence becomes significant in MDF. Fiber strength can be the predominant factor for high density fiberboard (HDF) (≥900 kg/m3) (Jones 1960). In LDF, failures usually occur in the bonds between fibers. In HDF panels, the contact between fibers is more intimate and thus failures occur within the fiber itself (Suchsland and Woodson 1990).
Fiber length is a critical factor determining the properties of paper (Dinwoodie 1965) and MDF (Suchsland and Woodson 1990). Longer fibers have the tendency to yield higher bulk density. Nelson (1973) found that low bulk density was associated with high panel strength. Fiber length and fiber orientation in the board are also correlated. Shorter fibers are more likely to deviate from the horizontal compared to longer fibers (Suchsland and Woodson 1990). Thus, shorter fibers could be disadvantageous since the contact area between fibers is diminished.
The pH and buffering capacity of the fibers are important parameters for resination. If urea-formaldehyde (UF) resin is used, higher fiber pH and lower buffering capacity are desirable (Nelson 1973; Johns and Niazi 1980; Johns et al. 1985; Maloney 1993; Hsu 1997). A too low pH may eventually cause pre-cure during the initial phase of hot-pressing, while buffering capacity influences the cure rate of UF resin.
Fiber composition refers to the percentages of fines, fiber bundles, whole and broken fibers of the pulp (Myers 1987). Low percentages of fiber bundles and fines are desirable as the former destroy the uniformity of fiber distribution and the latter consume more resin (Woodson 1976a; Moss and Retulainen 1995; Groom et al. 1999; Barnes 2002).
A high level of refining energy spent on wood chips, flakes, or shavings may lead to reduction in fiber length and production of debris (Clark 1978; Myers, 1987; Kure and Dahlqvist 1998; Kure et al. 1999). Steam time and pressure, disc diameter, plate pattern, loading rate, gap between the grinding plates, and rotation speed of the plates also affect fiber composition and size distribution.
The previous reviews show that the impact of the various parameters on panel properties is complex. The objective of this study was to investigate the relationships between MDF panel properties and fiber characteristics. MDF panels were fabricated from eleven raw fiber materials. Fiber samples were taken from the eleven materials for wood characterization. To eliminate the influence of the processing parameters, a constant hot-pressing program was used for the manufacturing of the panels. 
Sixty black spruce ( Picea mariana (Mill.) BSP.) trees were collected in July 1999 in Réserve Ashuapmushuan (Québec, Canada) from a second-growth natural stand. The trees of 70-80 years of age were bucked to 8-foot-long butt, middle and top logs. The debarked butt logs were cut into 1-foot-long sections. On the cross section of each 1-foot-long section, growth rings were counted and marked with three different zones according to cambial age: 1-20 year (juvenile wood), 21-40 year (mostly mature wood), and over 40 year (mature wood). The transition age from juvenile to mature wood occurs in 20-25 year (Alteyrac 2005). A band saw was used to separate the three types of wood along the marked lines from the short sections. Moreover, top, middle, and butt logs were collected to make three samples. The logs were debarked and chipped (a portable chipper was used). Thus, there were six samples from black spruce: 1) 1-20 year old wood, 2) 21-40 year old wood, 3) over 40 year old wood, 4) top logs, 5) middle logs, and 6) butt logs.
Hybrid poplar ( Populus spp.) material came from a clonal trial established by the Forest Research Branch of the Québec Ministry of Natural Resources in St-Ours, Southern Québec, Canada in 1993. The poplar trees were planted at a 1.5×3.5 m spacing in this site that is a part of the Champlain marine deposit with rich salty-clay soil (40 % clay). A systematic thinning was carried out in the spring of 1996 and the thinning led to a spacing of 2.5×3.0 m (Zhang et al. 2003). Four trees from each of hybrid poplar ( P. maximowiczii and P. balsamifera ) clones 915303, 915311, and 915313 were harvested from this site in December 2002 at the age of 10 years.
Larch material was a mix of two individual species ( Larix gmelinii and Larix sibirica ) with an approximate proportion of 4:1. The trees were collected from an exotic larch plantation grown in Northern Ontario, Canada through a commercial thinning. The trees were 10-15 years old, and thus were composted of juvenile wood. In total, seventy sample trees were collected and transported to Forintek Canada Corp. for this study as well as a lumber study.
As MDF mills in eastern Canada usually rely on mixed chips of black spruce, jack pine and balsam fir (S-P-F), a mixed sample of the S-P-F wood chips was collected as a reference from a composite panel mill in Québec, Canada. 
All panels were conditioned in a chamber at 22 oC and 65 % relative humidity (RH) for four weeks until they reached equilibrium moisture content. For each type of panel, nine static bending (338×75 mm), thirty IB (50×50 mm), six LE (305×76 mm), and six TS and WA (152×152 mm) specimens were prepared. The procedures and methods described in ASTM D 1037-99 (2001) and ANSI A 208.2-2002 (2002) were followed for the determination of modulus of elasticity (MOE), modulus of rupture (MOR), IB, LE, TS and WA. The IB specimens were sanded to remove 1.5 mm from both surfaces before they were glued to blocks. X-ray density profiles were performed on the panel specimens to verify that the flatter profiles were achieved. Panel properties are listed in Table 6-2.
ASTM (D 2395-02) method B (2004) was followed for wood density determination. Wood chips collected from each sample were oven-dried at 103 ± 2 oC until constant weight was achieved. The oven-dry weight of the wood chips was taken using a scale. Then the oven-dried chips were soaked in cold water until saturated. The volume of the chips was determined by volume displacement. Density of the wood chips was based on oven-dry weight of wood chips divided by green volume. A large number of wood chips was measured for each sample to ensure that the value was representative. Basic wood density of the eleven samples is listed in Table 6-3.
Wood base buffering capacity was calculated by multiplying the ml of NaOH solution required to raise the starting pH of the wood extract to pH 7 by the normality of the solution (Johns and Niazi 1980). First, wood chips from each sample were ground to powder. Then, 25 g of oven-dried wood powder was added to 200 ml of distilled water for refluxing (20 min). The mixture was filtered through a filter paper using a vacuum and washed several times. The extract was diluted to 500 ml and cooled down before titrating. 50 ml of extract solution was pipetted into a 150 ml beaker and the pH was measured. The extract solution was then titrated to pH 7 with nominal 0.025 N NaOH and a titration curve in 0.1 ml steps was created. The means of four measurements are presented in Table 6-3.
For fiber pH and base buffering capacity, 10 g oven-dried fiber was refluxed and the same procedure as described above was applied. Table 6-3 presents the average of four measurements.
Fiber quality was evaluated using a HiRes Fiber Quality Analyzer (FQA) at the ‘Centre specialisé en pates et papiers’ (Université du Québec à Trois-Riviéres, Canada). Fiber quality parameters evaluated for each sample include fiber coarseness, arithmetic mean length, length weighted mean length, arithmetic mean width, arithmetic fine percentage and length weighted fine percentage are given in Table 6-3.
The Bauer-McNett Classifier was used to determine fiber size distribution according to the procedures described in Tappi T 233 cm-95 (1995). Four screens of mesh size 3.240 mm2, 0.828 mm2, 0.281 mm2, and 0.017 mm2 were used. 10 g (oven-dry weight) of fibers were poured into the first compartment. The water flow rate was 11.355 L/min. A 20 min motor running period was employed. The fiber pads were collected and dried at 105 oC to a constant weight. The measurement was replicated three times for each sample and the values were averaged (Table 6-3).
Based on an assumption that the density profiles of all panels were flat and equal, multiple linear regression was performed to model the properties of MDF panels made from different fiber samples in relation to various wood and fiber characteristics. All the predictor and response variables are presented in Table 6-4. Ten dummy variables z1, z2, …, z10 were created and the eleven wood samples were identified by combining the values of these variables (Table 6-1). There were twenty seven predictor variables in the wood fiber characteristic matrix including the ten dummy variables (Table 6-4). Panel bending properties (MOR and MOE), IB strength and dimensional stability (LE, TS, and WA) were response variables and incorporated into the panel property matrix (Table 6-4). All variables were examined for normal distribution, and the results show that the data sets of wood density, wood pH, base buffering capacity of wood chips, length weighted mean fiber length, arithmetic mean fiber width, and percentage of fibers passed through the mesh (0.281 mm2) but retained on the mesh (0.017 mm2) were not normally distributed (Table 6-4). So the data sets of these variables were transformed until they met normal distribution requirement. The transformation function and resulting variables are shown in Table 6-4. The transformed forms together with other variables in the wood fiber characteristic matrix were analyzed as predictor variables for multiple linear regression.
Dummy variables were created as follows:
z1 = 1if subject is in black spruce 1-20 age zone
  1. otherwise
z2 = 1if subject is in black spruce 21-40 age zone
  1. otherwise
z3 = 1if subject is in black spruce over 40 age zone
  1. otherwise
z4 = 1if subject is in black spruce top logs
  1. otherwise
z5 = 1if subject is in black spruce mid logs
  1. otherwise
z6 = 1if subject is in black spruce butt logs
  1. otherwise
z7 = 1if subject is in hybrid poplar clone 915303
  1. otherwise
z8 = 1if subject is in hybrid poplar clone 915311
  1. otherwise
z9 = 1if subject is in hybrid poplar clone 915313
  1. otherwise
z10 = 1if subject is in the two individual larch species
0 otherwise
Then the eleven wood species or types can be identified by the values of the dummy variables z1, z2, …, z10.
z1 = 1, z2, z3, …, z10 = 0: black spruce 1-20 year-old wood
z2 = 1, z1, z3, …, z10 = 0: black spruce 21-40 year-old wood
z3 = 1, z1, z2, …, z10 = 0: black spruce over 40 year-old wood
z4 = 1, z1, z2, …, z10 = 0: black spruce top logs
z5 = 1, z1, z2, …, z10 = 0: black spruce mid logs
z6 = 1, z1, z2, …, z10 = 0: black spruce butt logs
z7 = 1, z1, z2, …, z10 = 0: hybrid poplar clone 915303
z8 = 1, z1, z2, …, z10 = 0: hybrid poplar clone 915311
z9 = 1, z1, z2, …, z10 = 0: hybrid poplar clone 915313
z10 = 1, z1, z2, …, z9 = 0: two individual larch species
z1, z2, …, z10 = 0: S-P-F wood chips
The multiple linear model for the relationships between response and predictor variables took the form:
Y = b0 + bix1~17 + bjz1~10 + bkx1~17z1~10 + … + ε
where: Y is the response variable, MOR, MOE, IB, LE, TS, or WA;
x1, x2, …, x17, and z1, z2, …, z10 are the predictor variables;
b0 is the intercept, bi, bj, and bk are the regression coefficients (i = 1, …, 17; j = 18, …, 27; k = 28, …, 197);
ε is the error term.
Statistical Analysis System (SAS) (1990) software was used for data analysis. Since there were a number of predictor variables, the multivariate model candidate selection was based on two criteria, stepwise selection and model R square. Thus, preliminary predictor variables selected by the stepwise procedure were listed. These predictor variables were then screened by model R square selection criterion.
The predictor variables entered in the model might be highly correlated (multicolinearity), which can result in unstable estimates and high standard errors (Draper 1980). To avoid this, the selected predictor variables were checked for multicolinearity using syntax “MODEL response variable=predictor variable/VIF COLLIN”. The calculation of the variance inflation factor (VIF) is to regress a specific predictor variable on other predictor variables in order to check multicolinearity of that variable. Generally, if the VIF value is higher than 10, attention should be paid to this variable (Belsley et al. 1980).
There were nine variables that entered into MOE model by the stepwise procedure (Table 6-5). The percentage of fine fibers that can pass through the screen with mesh size of 0.017 mm2 (<0.017 mm2) is the first variable entered to the model and it can explain 46.8 % of the variation in MOE. With only the variable, percentage of fine fibers (<0.017 mm2) in the model, it has a R square of 0.468. Adding variables tangent wood pH and z5 into the model, the model R square was improved to 0.850. This means that the three variables, percentage of fine fibers (<0.017 mm2), tangent wood pH, and z5 can account for 85.0 % of the variation in MOE. Thus, these three variables were selected to describe MOE and multiple linear regression was performed using the three variables as predictor inputs. The values of VIF calculated for the three selected predictor variables were fairly small compared with the multicolinearity cut-off value of 10. This indicates little correlation between the three predictor variables. Hence, the MOE model can be given by the following equation based on the parameter estimates (Table 6-6).
MOE = 2.51×103 – 409 _200_ + 45.6 tanwood_pH + 479 z5
The MOE model indicates that very small particles that can pass through a 200 mesh screen had a negative effect on panel MOE. MOE was also positively related to tangent wood pH. According to the feature of the tangent function, wood pH would affect MOE positively if wood pH fell in the ranges of 1/2π – π, π – 3/2π, 3/2π – 2π or 2π – 5/2π. π, 3/2π, 2π, and 5/2π are critical values for wood pH in determining its effect on MOE. If wood pH is equal to π or 2π, there is no effect of wood pH on MOE; if wood pH approaches to the values of 3/2π or 5/2π, the effect of wood pH on MOE becomes instantly great. In addition, the presence of the dummy variable zindicates that an unknown wood fiber characteristic significantly influenced panel MOE and it belongs to black spruce mid logs. 
Table 6-5 lists the predictor variables that entered in and then were removed from the IB model by the stepwise procedure. There were nine variables left in the model. Arithmetic fine percentage can explain 43.9 % of the total variation; together with the percentage of fine fibers (<0.017 mm2), z7, and fiber pH, the model accounts for 91.7 % variability (Table 6-5). Adding the other variables listed in Table 6-5 such as z1, z9, z6, fiber base buffering capacity, and z3 into the model did not significantly improve the R square. Thus, the relationship between IB and wood fiber characteristics can be described by the four variables, namely, arithmetic fine percentage, percentage of fine fibers (<0.017 mm2), z7, and fiber pH. The estimated parameters for IB model are shown in Table 6-6. A model for IB strength can be written as follows.
IB = 3.48 – 0.0509 arith_fine + 0.405 _200_ + 0.304 z7 – 0.383 fiber_pH
Internal bond strength is linearly and negatively related to arithmetic fine fiber percentage. However, it is positively related to the percentage of very small particles (<0.017 mm2) of the pulp. Moreover, IB is negatively affected by fiber pH. This seems to be contradictory with the previous findings (Johns and Niazi 1980; Johns et al. 1985; Maloney 1993; Hsu 1997). The negative correlation between IB and fiber pH might be attributed to the relatively low temperature for panel consolidation used in this study. The dummy variable z7 appearing in the model implies that other unknown wood fiber characteristics belonging to hybrid poplar clone 915303 influenced IB strength significantly. 
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Table 6-1 Refining parameters, fiber moisture content and creation of dummy variables.
Table 6-2 Panel bending properties, internal bond strength and dimensional stability.
Table 6-3 Measured wood and fiber characteristics.
Table 6-4 Predictor/response variables and transformed predictor variables.
Table 6-5 Summary of stepwise model selection for MOR, MOE, IB, LE, TS, and WA.
Table 6-6 Multiple linear regression model parameter estimates for MOR, MOE, IB, LE, TS, and WA.
Refining speed, cooker pressure, retention time and temperature were 2500 rpm, 7.5 bar, 3 min and 160 oC for all types of material.
* Moisture content after resin and wax blending. 
Values in parenthesis represent standard deviations.
LE was determined by the linear variation with changing RH from 50 % to 80 % as the percentage change in length based on the length measured at 50 % RH. TS and WA were obtained by calculating the variation in thickness and weight following 24 h water soaking divided by the thickness and weight measured on the specimens equilibrated at 22 oC and 65 %.
Values in parentheses represent standard deviations.
Numbers in columns ‘>3.240 mm2’, ‘0.828-3.240 mm2’, ‘0.281-0.828 mm2’, and ‘0.017-0.281 mm2’ were the percentages of fibers retained on the screens with mesh sizes of 3.240 mm2, 0.828 mm2, 0.281 mm2, and 0.017 mm2. Numbers in column ‘<0.017 mm2’ were the percentages of fibers passed through the screen with mesh size of 0.017 mm2.

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