Waste and Biomass Valorization
80% of the produced corrugated board is manufactured with recycling fibers [43]. As a result of limitations in available forest-based materials [44] and growing environmental awareness of the population, the corrugated board industry is utilizing recycling fibers to an increasing degree, which, from an environmental point of view, can be considered a benefit due to reduced emission of CO 2 [45]. However, the recycling process causes a deterioration of the mechani- cal properties of the fibers, which can lead to a strength loss of up to 30% of its original strength [46]. The passage of the fibers through the recycling chain (which includes repeated drying, printing, converting, storing, deinking and re-refining) leads to hornification, a reduction of cell wall porosity, fiber shortening, loss of flexibility, re-adhesion of fibrils onto the fiber surface, micro-indentations and -com- pressions, loss of degree of polymerization, self-sizing, loss of hemicelluloses on the fibers surface and the accumulation of contaminants, all of which can have a negative impact on the paper strength [46–51]. One potential way of mitigating the effect of the deteriorating quality of the recycled fibers available is the introduction of new fibers into the production process, such as wheat straw pulp [52, 53]. Thus, to converge the afore mentioned challenges of the large quantities of waste MDF to be disposed of, the high demand of fibrous material for the packaging industry, and the deteriorating quality of the recycled pulp, this study aims to assess the viability of utilizing steam refined fibers from post-consumer MDF in recycled paper packaging as filler or reinforcement material. Currently, most MDF in European countries is produced from softwood. As a shift in forest management is leading to a reduction of softwood stands and an increase in hardwood stands in Europe [54], the result- ing shortage of softwood and oversupply of hardwood will likely push the MDF manufacturers to increase the amount of hardwood utilized. To take this development into account, a waste MDF sample set containing a high amount of hard- wood and one containing a high amount of softwood were used and compared to each other and to recycled fibers used in the industry for manufacturing of corrugated board. The waste MDF samples were treated at six different severity grades, ranging from 2.5 to 4.0. In a previous publication the effect of steam refining on the reactions of chemical com- ponents was reported [18]. In the present paper the effect of steaming severity and refining intensity on fiber morphol- ogy and strength properties will be evaluated. As the gap between the rotating blades and the inner wall of the steam refining reactor is large (10 to 20 mm) in comparison to typical fiber dimensions (0.6 to 4.4 mm length and 10 to 50 μm diameter [55]), the steam treated waste MDF fibers are present in small fiber bundles [18]. A secondary refining and beating was performed on the steam refined fibers to separate the fiber bundles and improve the fiber properties for papermaking. In refining and beating, flocs and fibers
are deformed in the presence of water by two metallic sur- faces moving in relation to each other, causing compressive and shear forces in the pulp. The main positive effects on the fibers are an external and internal fibrillation as well as the formation of fines, leading to an increase of surface area available for fiber to fiber bonding [56–58]. The result- ing pulp was characterized using an automated fiber length analyzer and used for test sheet preparation. The tensile-, compression-, and tear-strength, as well as brightness of the test sheets were measured and the relationship between the fiber morphology and paper strength evaluated.
Material and Methods
Raw Material and Their Preparation
Two chipped, clean waste MDF samples without lamination supplied by École supérieure du bois (Nantes, France) were used in this study. The chipped waste MDF featured a length of roughly 10 to 50 mm, a width of 15 mm and a height of around 10 mm. Batches of 300 g (dry matter) waste MDF chips were steam treated in a 10-l reactor with a diameter of 22 cm and a length of 25 cm (Martin Busch & Sohn GmbH, Germany) at severities ranging from 2.5 to 4. The severity (log R 0 ) of the steam treatments was calculated according to Eq. (1). In the last 30 s of the treatment, the fibers were defibrated within the steam filled reactor by rotation of a built-in four bladed system at a speed of 1455 rpm.
( T − 100 )
(1)
log R
0 = log (
t × e
14,75 )
with T: steam temperature in °C, t: steaming duration in min. At the end of the treatment duration, the steam was released through a valve in about 90 s. Subsequently, the fiber fraction was separated from the extract by filtration through a sieve bag and dewatered for 10 min at 2800 rpm in a spin dryer (Thomas Centri 776 SEK, Thomas, Germany). The fiber fraction was passed through a 12 Ǝ Sprout-Bauer laboratory refiner (Andritz, Graz, Austria) three times. In the first pass, the gap distance was adjusted to 0.5 mm and the consistency of the pulp was adjusted to 4%. In the following two passes, the gap distances were reduced to 0.2 mm and the consistency decreased continuously due to the addition of rinsing water to a final consistency of around 2%. The fib- ers were separated from the rinsing water using above men- tioned procedure. The complete process is depicted in Fig. 1. In Table 1 the chemical composition of the two waste MDF samples and the fibers after steaming are presented. Detailed information regarding the determination of the chemical composition of the samples, as well as a dis- cussion on the process reactions is given in a previous
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