Waste and Biomass Valorization
Fig. 4 Testsheets made out of SR-MDF (A) without additional beating steam treated at severi- ties of 2.5, 2.8, 3.1, 3.4, 3.7 and 4.0 (from left to right)
SR-MDF (B). The explanation might be differences in the share of hardwood and softwood in the waste MDF samples concluded from the chemical composition. While it was possible to produce fibers with comparable compression strength to recycled pulp from the sample SR-MDF (A), the paper strengths achievable from sample SR-MDF (B) were lacking. From these findings it can be concluded that the composition of the waste MDF raw material does not only influence the chemical interactions during the steam- ing process [18], but also the papermaking quality of the resulting pulp. Consequentially, intimate knowledge of the waste MDF composition is needed for process optimization and will likely play a key role in successful valorization. Additionally, the surface of MDF is often laminated using thermoplastic polymers which tend to melt in high moisture and high temperature environment such as the steam treat- ment presented in this study. Thus, efficient sorting will be of substantial importance. Although the tensile and tear strength of the test sheets produced from the MDF (A) samples were not as high as the ones produced from the recycled pulp utilized in industry, a comparable compressive strength was determined. This is the one of the most important properties for application in corrugated board packaging. Using the information provided in this study as a basis, optimization of the processes might further improve the attainable paper strengths. Besides the attainable paper strengths, the final price of the steam treated
Besides the shade of brown, the severity of the steam treat- ment also influenced the brightness of the test sheets, with higher severity treatments leading to a lower brightness value, as presented in Fig. 5. A slight increase in measured brightness following more intense beating can be observed for both MDF samples, irrespective of treatment severity. The brightness values for SR-MDF (B) and for SR-MDF (A) do not differ considerable, ranging from around 10% for high severity treatments up to around 27% for low severity treatments. The darkening and the loss of brightness of steam treated fibers has been reported before [76, 77] and can be attributed to the creation of chromophoric groups in the lignin follow- ing condensation and oxidation reactions [80]. However, the low brightness and dark brown hue of the waste MDF fibers should not be detrimental to the application in packaging material if used in the central layers of linerboard or in the flute, as their optical properties play a subordinated role.
Conclusion
Distinct differences in response to the refining treatment between the two steam treated waste MDF samples were found. An increase in treatment severity had a positive effect on the paper making quality of the SR-MDF (A) fib- ers, while no such influence could be found for the fibers of
Fig. 5 Influence of treatment severity and beating duration on the brightness of test sheets made from SR-MDF (A) ( a ) and SR-MDF (B) ( b )
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Beating degree [°SR] 0 10203040506070 5
Beating degree [°SR] 0 10203040506070 5
( a
b )
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(
Log R 0 = 2.5
Log R 0 = 2.8
Log R 0 = 3.1
Log R 0 = 3.4
Log R 0 = 3.7
Log R 0 = 4.0
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