Waste and Biomass Valorization
The different behavior of the two samples might be explainable with a differing amount of softwood and hard- wood fibers in the sample. Due to the carbohydrate com- positions (Table 1), the fiber dimensions (Table 2), and the high amount of acetic acid found in the liquid phase after steaming of sample SR-MDF (A) [18], a higher content of hardwood in sample SR-MDF (A) was concluded. However, generally a high amount of softwood fibers in the pulp cor- relates positive with the compression and tensile strength of the produced paper due to the higher fiber length [58, 75], and thus higher paper strengths could be expected for sample SR-MDF (B). One possible explanation for the low influence of the steaming treatment on the fiber and paper properties of SR-MDF (B) could be a less intense auto- hydrolysis of the main components in the steam treatment due to the lower amount of acetyl groups in softwood. The reduced autohydrolysis might lead to a lower flexibility of the fibers of SR-MDF (B) in comparison to SR-MDF (A). Another difference between softwoods and hardwoods is the reaction of lignin constituents under steam treatment. Guaiacyl lignin, which makes up the majority of the lignin found in softwood, is prone to condensation reactions [78, 79], and a high degree of lignin repolymerization might decrease the flexibility of the fibers. The potential effect of lignin repolymerization, might be overcome by the addition of condensation inhibitors such as 2-naphtol, which might be advantageous. Another aspect to look into is the addition of mineral acids in the steaming of waste MDF samples with a high share of softwood to compensate for the lower amount of acetyl-groups. The effect of raw material composition and process conditions of the MDF production on the subsequent recycling deserves further attention.
respectively. The CI measured from the two recycled pulps gathered from industrial manufacturers of corrugated board production for comparison presented in Table 5 is found to be similar, ranging from 12.4 to 27.4 N m g −1 . The CI of the SR-MDF (A) samples measured in this study range from roughly 10 N m g −1 at low beating degrees up to 27 N m g −1 at high beating degrees in combination with a high severity treatment, showing similar compression strength to recycled pulp currently found in industrial use. At a beating degree of roughly 35 °SR for example, a CI of 17.3 N m g −1 was determined for the SR-MDF (A) sample treated with a sever- ity of 3.4, falling between the CI of recycled pulp #1 with 12.4 N m g −1 and recycled pulp #2 with 21.2 N m g − 1. The tensile and tear strength achievable with the SR-MDF samples are low in comparison to the paper strengths of the two recycled pulps. The steaming of the wood in the TMP process of the MDF production is done at temperatures ranging from 160 to 180 °C for a duration of 3 to 6 min [1]. This steaming leads to a softening of the lignin as the glass transition point is exceeded and an ensuing coating of the fiber surface with lignin, which hinders the defibrilla- tion in the subsequent refining [55]. However, steam treat- ment of higher severities are reported to lead to softening of the fibers and an increased flexibility [76, 77], which can be advantageous in the subsequent refining steps. Distinct differences in the influence of the steam treatment severity on the achievable paper strengths between the two steam treated waste MDF samples were observable. For the SR- MDF (A) sample, a higher treatment severity had a positive effect on the paper strength, leading to higher paper strength at the same beating degree in most cases. The SR-MDF (B) sample saw no influence of treatment severity on the result- ing paper strength, reflecting the unchanged fiber morphol- ogy described in Sect. 3.1. While at a low severity of 2.5 the achievable paper strength of the two SR-MDF samples does not differ greatly, a substantial higher paper strength could be achieved using the SR-MDF (A) sample treated at high severities. As the two sample sets underwent the exact same treatment and evaluation processes, the differences in response to the steam treatment must be caused by differ- ences in the two MDF sample sets.
Optical Properties
The severity of the steaming treatment shows a direct influ- ence on the coloration of the fibrous material and the pro- duced test sheets as presented in Fig. 4. At higher severi- ties, a distinctively darker shade of brown can be observed with the eye than at lower severities, while no differences between SR-MDF (A) and SR-MDF (B) were discernible.
Table 5 Paper strength properties of recycled pulp gathered from two different
Beating duration Beating degree Compression-Index Tensile-Index Tear-Index (min) (°SR) (Nm/g) (Nm/g) (mN*m 2 /g)
industrial corrugated paperboard producers
Recycled pulp #1 0
37.0 53.5 61.5 18.0 37.5 54.5 61.5
12.4 17.0 18.3 12.8 21.2 25.2 27.4
19.6 28.5 30.6 14.1 34.2 46.5 50.9
5.8 5.2 4.5 5.5 8.1 6.2 5.7
2 4
Recycled pulp #2 0
2 8
10
1 3
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