Improving agricultural waste pulps via self-blending concept with potential use in moulded pulp packaging

Highlights

• Pulps from banana stem, pineapple leaf, and rice straw vary in quality.

• Superior pulp fibres tend to have high cellulose content and slenderness ratio.

• Pulp blending concept is highly efficient for enhancing quality of inferior pulps.

• Self-blending of agricultural waste pulps shows great potential in packaging use.

Abstract

Agricultural wastes have been considered as an alternative sustainable resource for pulp production. In this work, banana stem (B), pineapple leaf (P) and rice straw (R) were used as raw materials for pulp extraction. The obtained pulps were moulded into sheets and their mechanical properties were evaluated. The sheets prepared from P and R pulps showed promising performance for moulded pulp packaging use. Having high cellulose content, slenderness ratio, and flexibility, these pulps were superior with a high degree of fibre bonds. Conversely, sheets prepared from B pulp had sub-standard properties. To improve the properties of B pulp, it was blended with either P or R pulp. Both tensile strength and Young’s modulus of the blended B/P and B/R sheets were improved significantly, around 63–167 % and 55–117 %, respectively. SEM images revealed that the long and flexible P and R fibres were well-entangled with the B fibres, confirming a strong network of the blended pulp sheets. A positive deviation from the linear additivity of the blended sheet’s strength was also observed. Mixing 30 % of P or R pulp with B upgraded both blended sheets to the acceptable range, showing a tensile index of approximately 44–45 Nm/g. These results indicated that pulp blending was highly efficient for enhancing properties of sub-standard pulp. Furthermore, this could possibly enable the use of all agricultural wastes as alternative raw materials for pulp and paper industries.

Introduction

The consumption of pulp and paper continues to increase and is predicted to reach 600 million tons by the end of 2020 [1]. The pulp and paper industries have been criticized for their negative impact on wood resources. Utilization of agricultural residues as alternative raw materials has recently attracted more attention, because they can be used to produce wood-free pulp and paper with acceptable properties [2]. Thailand is a top agricultural producing country with approximately 66 million tons of agricultural residues per year much of which still remain unutilized [3]. Agricultural wastes (AWs) such as bagasse, corn stalk, pineapple leaf, rice straw, and wheat straw contain high percentages of cellulose content (up to 82 %) and thus are considered valuable for pulp production [4,5]. Instead, AWs are usually burned. Pulping these AWs requires less chemicals, time and power consumption due to their lower lignin content compared to wood [6,7].

Soda and Kraft processes have often been reported for non-wood or AWs pulping [8]. The properties of the pulps or cellulosic fibres extracted from AWs vary widely depending on fibres’ dimension, structure, and chemical composition. Several of these fibres (e.g. extracted from oil palm, sugarcane, pineapple, and rice straw) are quite strong and show promise for use in the paper and packaging industry. However, some of them (e.g. from banana and corn stalk) are of inferior quality and likely to result in sub-standard products [9].

Typically, in order to improve weak recycled fibres or optimise the final product performance, virgin wood fibres are used to blend in at different ratios during recycling of pulp and paper [10,11]. Blending of AW fibres (e.g. from bagasse, wheat straw, and rice straw) with recycled fibres has been attempted in order to improve properties of the final product [[11], [12], [13]]. Moreover, AW fibres have also been mixed with virgin wood pulps in an effort to relieve shortage of both hardwood and softwood plant species [[14], [15], [16], [17]]. Previous research has demonstrated a significant increase in the strength of paper samples mixed with the AW fibres owing to their good fibre characteristics and ability to conform around other fibres and create a high degree of fibre-fibre bonds [[18], [19], [20], [21]]. These results suggested that AW pulps can be used as a beneficial substitute in recycled fibres, as well as, in virgin wood fibres for making paper and related products.

Pulps from AWs, mainly bagasse and bamboo are already broadly used for production of paper-based food packaging and moulded pulp containers [22,23]. Pulps from straws have also been shown to be good alternative raw materials. Moulded flat trays of blended pulps of wheat straw (60−80 wt%) and Kraft fibre (20−40 wt%) have been reported to have satisfactory tensile properties [24]. Blended hand sheets made of pineapple leaf fibre (40 wt%) and bagasse pulp showed an increase in both tensile and tear strength properties [4,10,25]. Some AW fibres, however, showed a relatively low ability to enhance the properties of waste or recycled paper, for instance, the pulps extracted from banana residues [26,27]. Still, some studies have reported that the mechanical performance, such as bursting and tear indices of the inferior banana stem fibre hand sheet could be improved when blended with 20−40 wt% of hardwood, wheat straw, or bagasse pulps, all of which have longer length fibres [[28], [29], [30]].

In this study, three AWs including banana stem (B), pineapple leaf (P), and rice straw (R) were chosen as raw materials for pulp extraction by using a soda-anthraquinone (AQ) process. The fibre characteristics and chemical composition of each pulp was examined. The pulps were partially dewatered, hot-pressed into moulded sheets, and tested to evaluate their mechanical properties. One sub-standard pulp was identified and, therefore, pulp blending with superior pulps was applied in order to increase fibre-fibre bonding and upgrade the quality of the inferior pulp. The effect of blending composition and ratio on the moulded sheet properties was studied in order to assess and ensure the degree of improvement of an inferior pulp towards acceptable values. In addition, the mechanical performance of the B, P, and R moulded pulp samples and their blends was evaluated and compared to the available commercial moulded pulp food containers.