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An integrated assessment of hemp (Cannabis sativa L.) and flax (Linum usitatissimum L.) as sources of fiber for newsprint production

S. N. Lisson

CSIRO Davies Laboratory, University Drive, Private Mail Bag, Aitkenvale, Townsville, Queensland, Australia 4814.

        The primary objective of the study was to show whether fiber, of value as a reinforcing agent in newsprint production, could be produced economically from hemp and flax grown in Tasmania. This involved an integrated analysis of the whole potential industry, with studies into the key areas of crop production, pulp and paper manufacture and economic viability. Another objective of the study was to develop a computer model for simulating the growth, development and yield of hemp in response to climate, soil and management inputs. This would enable the results of this project to be extrapolated to other suitable production areas and hence assist in the initial assessment of cropping potential and the identification of optimum site and management conditions.
        Of the nine hemp cultivars that were assessed, ‘Kompolti’ and ‘Futura 77’ were the best performers. Producing crops in excess of 2 m high and yielding up to 1500 g/m² of oven-dry stem. The results from sowing-date trials suggest that September is the optimum month for sowing hemp in Tasmania. Later sowings resulted in a reduction of stem yield associated with shortening of the thermal time duration from sowing to flowering. Sowings prior to September appear to be limited by premature flowering in response to short daylengths. In an irrigation trial conducted in northwest Tasmania, significant stem yield differences were not apparent for irrigation regimes based on refill to field capacity, at deficits down to 120 mm. Maximum bark yield was obtained from regimes based on a 60 mm deficit or less (water consumption of 535 mm). The stem yields under rainfed conditions were subsequently below those of the irrigated treatments. Stem yield responded in a parabolic manner to plant densities ranging from 50 to 300 plants/m², with maximum yields at about 110 plants/m².
        Controlled environment studies were conducted into the response of pre-emergent development to temperature, and the flowering response of selected hemp cultivars to photoperiod. Parameters, constants and functions derived from these studies, the field trials and from selected references, were than used to develop a hemp simulation model. The model adequately predicted phenology, leaf area and biomass production for cv. Kompolti grown in north west Tasmania.
        The Australian newsprint industry currently uses a mixture of locally sourced eucalyptus, radiata pine and recycled paper pulps, blended with an imported kraft (chemical) pulp. The purpose of the kraft pulp is to reinforce the news-print. The primary aim of the pulping trials conducted in this study was to investigate the potential of using flax and hemp bark and whole stem pulps as alternative reinforcing agents in newsprint production. The existing cold caustic soda (CCS) and thermo-mechanical (TMP) processes were trialled with a view to harnessing the existing infrastructure and expertise. Cold caustic soda pulp made from the bark fraction formed paper of very high tear index, but with lower tensile index and tensile energy absorption than would be desirable from softwood kraft. This limitation might be overcome by using a higher proportion of non-wood pulp in the overall newsprint blend or through breeding improvements. The use of pulping (and pre-pulping) equipment more suited to non-woods may overcome handling difficulties associated with excessive fiber length.
        Pulps were also made from the core fraction to assess its suitability as a supplement to the short fiber component of the existing newsprint blend. Whilst potentially suited for use as a short fiber supplement in newsprint manufacture, the properties of the core pulps are not currently in demand within the industry.
        Interest from the newsprint industry in taking the financial risk of adopting hemp and flax based pulps as an alternative to kraft, would require that the total cost be some-what less than the imported option. Similarly, interest from primary producers requires that the gross returns from these crops are at least comparable with a range of crop alternatives. The minimum bark price (mill gate) that is likely to attract farmers would vary between hemp and flax, and between growing conditions. Irrigated hemp and flax crops grown in the more productive north-west area of the state would require a price in excess of about $650/t. These minimum bark prices are not attractive to the newsprint industry at present. Future viability will depend on a number of factors, including: fiber yield and quality (e.g. bark proportion in the stem and fiber tensile strength properties) improvements, elevated kraft pulp prices, and the establishment of strong markets for the stem core fraction and the seed of hemp and flax.