MOR, IB, and water absorption of MDF panels made from 100 % black spruce juvenile wood were significantly superior to those of panels made from mature wood. However, LE of MDF panels made from 100 % juvenile wood was significantly higher than that of panels made from mature wood. The differences in MOE and TS between panels made from juvenile and mature wood were dependant on panel density. The MOE of Johnson-Neyman’s method suggests that MDF panels made from 100 % juvenile wood is not significantly different from that of panels made from the mixed furnish containing a high proportion of mature wood and a small proportion of juvenile wood if panel density drops in the region of 716-743 kg/m3. If panel density falls below 716 kg/m3 or rises above 743 kg/m3, difference in MOE between the two groups becomes significant. There is no significant difference in MOE between panels made from juvenile and mature wood within a panel density range of 642-748 kg/m3, however, if panel density is lower than 642 kg/m3or higher than 748 kg/m3, the difference between these two age groups becomes significant. Difference in TS of the panels between 1-20 and 21-40 age groups is not significant when panel density drops in the region of 427-679 kg/m3; however, if panel density is lower than 427 kg/m3, TS of panels from 1-20 age class is significantly larger than that of 21-40 age class; if panel density is higher than 679 kg/m3, TS of panels from 21-40 age class is significantly larger than that of panels from 1-20 age class. Johnson-Neyman’s method also suggests that there is no significant difference in TS between panels made from 1-20 and over 40 year-old fiber while panel density is in the region of 352-662 kg/m3, but if panel density is lower than 352 kg/m3, TS of panels from 1-20 age class is larger than that of panels from over 40 age class; if panel density is higher than 662 kg/m3, panel TS of over 40 age class is larger than that of 1-20 age class. Additionally, significant linear relationships between MOR, MOE, TS and panel density were found for the three age groups. For those young trees produced from intensive managed forest resource, as well as commercial thinnings from silvicultural practices of the kind done in Canada and the United States in recent years, conclusion can be made that a good use of those low quality resources is to manufacture fiber-based composite products. So the value of the resources can be increased, while the performance of the products is maximized. However, linear variation of the fiberboard panels made from juvenile wood must be controlled through effective technologies and modifications.
Bending properties of MDF panels made from black spruce top, mid, and butt logs were affected by low panel surface density. The low surface density caused reductions in panel MOR and MOE. Nevertheless, bending properties of the panels made from top, mid, and butt logs were still comparable since the differences in vertical density profile were fairly slight.
MOE and IB of MDF panels made from top and mid logs were significantly higher than those of panels made from butt logs; however, there were no significant differences in MOE and IB between panels made from top and mid logs. Water absorption of MDF panels made from top and mid logs was significantly smaller than that of panels from butt logs; again, the difference in water absorption between panels made from top and mid logs was not significant. The panels made from top logs yielded the highest MOR, and MOR of panels made from mid logs was significantly higher than that of panels from butt logs. The comparison of MOR between panels made from top and butt logs was dependant on panel density. Johnson-Neyman’s method suggests that there is no significant difference in MOR between panels made from top and butt logs when panel density drops from 519 kg/m3to 710 kg/m3. If panel density is higher than 710 kg/m3, the MOR of panels made from top logs is significantly higher than that of panels from butt logs. If panel density is lower than 519 kg/m3, the reverse is true. TS of panels made from top logs was significantly lower than that of panels from both mid and butt logs; and the panels made from butt logs swelled the most. There was no significant difference in LE among panels made from top, mid and butt logs. Linear relationships between MOR, MOE and panel density were found, however, the linear relationships between IB, LE, TS, or water absorption and panel density were not significant. It can be concluded that most of the properties of MDF panels made from top logs were superior to those of panels made from butt logs. This could be helpful when sorting black spruce logs for various uses. Butt logs can be sorted for lumber production because of their higher density, strength properties and product recovery. Forest residues such as tops may be assigned to fiberboard manufacturing. Thus, the value of the black spruce forest residue resource is added; and the high performance of the products is obtained.
It was shown that clonal variation had a significant effect on the flexural properties and internal bond strength of MDF panels made from hybrid poplar; however, panel dimensional stability was not significantly different between hybrids. Specifically, MOR of MDF panels made from clone 915311 was significantly higher than those of panels from clones 915303 and 915313; however, there was no significant difference in MOR between panels made from clones 915303 and 915313. MOE of MDF panels made from clone 915311 was the highest and was significantly different from those of panels from clone 915303 and 915313; MOE of panels made from clone 915303 was the smallest and was significantly lower than that of panels made from clone 915313. MDF panels made from either clone 915303 or 915311 were superior to those panels made from clone 915313 in IB strength; while there was no significant difference in IB between panels made from clone 915303 and 915311. No significant differences were found in LE, TS, and water absorption among those panels made from the three poplar hybrids. The information derived from this study implies that it is likely to improve panel flexural properties and internal bond strength through tree breeding.
Using the panels made from S-P-F wood chips as a control sample, it is found that the two individual exotic larch species Larix gmelinii and Larix sibirica aged between 10 to 15 years old are feasible for MDF panel manufacturing. The panels made from larch had favorable IB strength compared to those panels made from S-P-F. IB of larch panels was 0.78 MPa, and met the requirement of ANSI grade 140. MOR values for the panels made from larch and S-P-F were 18.6 MPa and 23.1 MPa, respectively, and both met the requirement of ANSI Grade 120. MOE of larch panels was 1276 MPa, which did not meet the ANSI minimum requirement. MOE of the panels made from S-P-F wood chips met the requirement of ANSI Grade 120. Bending properties of larch panels were lower than those of panels made from S-P-F, but those properties can be improved by adopting an optimal hot-pressing scheme. Compared to the panels made from S-P-F, the large LE of larch panels appears to be a problem. Modifications should be taken into consideration in order to enhance larch panels in linear stability. TS and WA of panels made from larch and SPF were below the maximum levels specified in the ANSI/AHA A 135.4-1995 standard for basic hardboard.
A significant effect of wood fiber characteristics on MDF panel properties was found. Modulus of rupture was negatively affected by the arithmetic fine fiber percentage.Modulus of elasticity was negatively related to the percentage of small particles (mesh size smaller than 0.017 mm2), and also related to wood pH. Internal bond strength was negatively dependant on arithmetic fine fiber percentage and fiber pH, but positively related to the percentage of small particles (mesh size smaller than 0.017 mm2). Linear expansion was negatively influenced by wood density, while TS was negatively related to arithmetic mean fiber length. Arithmetic mean fiber width had a negative effect on WA. The presence of the dummy variables in MOE, IB, and LE models suggests that wood fiber characteristics other than those measured in this study had significant effects and should be analyzed in order to correlate them with MDF panel properties. As for different raw materials, the refining parameters should be set differently and appropriately in order to produce high performance fibers with less fine fiber content.
Characteristics of raw material had an impact on performance of MDF panels. To improve panel flexural properties and IB strength, there are two major approaches. One approach is to better understand the effects of refining parameters on the performance of fibers and properly adjust those parameters to produce favorable fibers; the other is to manufacture panels using optimal pressing parameters. The relationship between refining parameters and performance of produced fibers is not clear for the time being. As stated previously, refining parameters should be adjusted according to species, and how refining parameters such as refining energy, gap between the two plates, retention time, and the properties of raw material fed into the refiner (e.g. moisture content and dimension of wood chips) so far affect fiber performance needs to be thoroughly studied. The influence of hot-pressing parameters (e.g. closure time, pressing time, opening time, and temperature) has been well studied. However, this subject may be investigated in terms of raw material type used since different raw material results in different mat structure (for example, bulk density), which can affect panel density profile considerably.
MDF panel manufacturing is a complex process; there are a number of factors influencing the properties and performance of the products. With regard to the refining process, the following should be taken into account. What is the performance of the fibers produced from refining in terms of different raw material? In other words, which type of wood generates fibers with more fines or small particles, wood with lower density and thinner cell wall or with higher density and thicker cell wall? Refining process and hot-pressing can be concluded as two critical processes in composite manufacture. Before initiating plans for refining and hot-pressing, the characteristics of raw material need to be well known.
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