Alkaline hemp woody core pulping : impregnation characteristics, kinetic modelling and papermaking qualities

The aim of this thesis is to elucidate alkaline processing of hemp woody core, supporting the development and optimization of an efficient and non-polluting pulping process. This study has been a constituent of an integral programme to study fibre hemp.

It is known that the outer part of the fibre hemp stem can be used for textile and specialty paper purposes. The inner part consists of hemp woody core, which resembles hardwood and might be processed similarly for paper pulp. Literature data and prefeasibility studies show that alkaline processes can be used to produce hemp woody core pulp for papermaking.

Alkaline processes, based on sodium hydroxide (NaOH) are used for many wood and non-wood species. The most important process is the kraft process, but alternative NaOH based processes for pulp production have been investigated too. The currently developed alkaline process for high yield hardwood pulping may also be implemented for hemp woody core.

Pulp mixes for papermaking can vary, depending on the available fibre sources. The technological developments and the growing market outlet for hardwood fibres increase the possibilities to use hemp woody core for papermaking.

In chapter 2 swelling of hemp woody core chips after alkaline (peroxide) impregnation at 70 °C has been studied, as is practised in alkaline peroxide mechanical pulping (APMP) processes. Swelling of hemp woody core chips has been examined in relation to pulp yield and chemical composition of the chips after impregnation.

In contrast to wood chips, maximum swelling is attained at 70 °C without chemical treatment, possibly as the result of relatively high porosity of hemp woody core cell walls.

Alkaline swelling at 70 °C correlates with the xylan:lignin ratio. Swelling at ambient temperature shows some correlation with acidic group content.

Apparent pore size distributions have been examined, using 1H NMR spin-spin relaxation. Several apparent pore size distributions can be distinguished within each sample. Elevated temperature, NaOH and peroxide addition influence the apparent pore size distribution and the total pore volume in different ways.

Addition of peroxide results in remark-able increase of the apparent pore sizes. This emphasizes its importance, not only as bleaching chemical in the APMP process, but also promoting fibre flexibility.

Alkaline delignification of hemp woody core is studied in chapter 3. Shavings of hemp woody core were delignified isothermally at several temperatures with 1M NaOH in a flow-through reactor. From literature data and from experimental data reported in this chapter, it appears that the initial delignification stage is completed before customary reaction temperatures are reached. Consequently, modelling of alkaline delignification kinetics can be restricted to the bulk and residual delignification stages. This can be described with two simultaneous first order reactions: L/L0 = a2 exp (-k2t) + a3 exp (-k3t), with ki = Ai exp -Eai/(RT).

This equation has been solved integrally, calculating a2, a3, Ea2, Ea3, A2 and A3 for the best fit for the experimental data, resulting in an accurate description of the delignification reactions. This kinetic model has also been applied on literature data, supporting its validity for alkaline delignification kinetics in general.

This model is also used in chapter 4, to describe the kinetics of alkaline delignification in more detail, and to describe the degradation of xylan and cellulose. Shavings of hemp woody core were impregnated at room temperature with various NaOH concentra-tions (0.25-2.0M) and delignified isothermally at various reaction temperatures (150-180 °C) in a flow-through reactor.

Extraction and degradation of xylan from hemp woody core strongly depends on NaOH concentration. Consequently, to attain a certain lignin content, lower NaOH concentrations result in higher pulp yields. Extended pulping diminishes the differences in pulp yields, due to further xylan degradation.

The kinetics of lignin, xylan and cellulose degradation are modelled as a function of reaction time, temperature and NaOH concentration. The combined models resulted in a pulp yield model for hemp woody core, suitable for process optimization purposes. Degradation kinetics of perennial wood can be modelled similarly, which was illustrated using literature data on spruce and poplar.

In chapter 5 strength and surface properties of test sheets, produced from alkaline hemp woody core pulp were examined. The development of bulk and tear with beating are similar as found for straw pulp; maximum tear strength is attained without beating. Burst and tensile strength, scattering and opacity develop similarly as for hardwood pulps, with less mechanical energy needed. Tear strength is not affected by pulp yield or composition, whereas lower tensile and burst strength are found with decreased yield and lower xylan content.

As the paper strength and surface properties of hemp woody core pulp are comparable with those for hardwood and straw pulps, it is conceivable that similar amounts of alkaline hemp woody core pulp can be used in pulp mixes for printing paper grades.

The polymerization degree (DP) of hemp woody core pulps has been related to the paper strength properties, and modelled as function of pulping conditions and time.

The influence of NaOH concentration on depolymerization and cellulose degradation is much stronger than reported in literature for other pulps. This may be related to the low density of hemp woody core, preventing diffusion effects. Finally, crystallinity has been examined and related to cellulose degradation of alkaline hemp woody core pulp.

In chapter 6 the results and conclusions are discussed. It is emphasised that hemp woody core has a lower density than hardwood or softwood. It is suggested that this is the cause for maximal swelling without NaOH addition, the found xylan and yield losses with NaOH impregnation, and the strong dependency of cellulose degradation and depolymerisation on NaOH concentration.

In general, it was confirmed that hemp woody core can be delignified similarly as hardwood. The modelling techniques used have been proved to be generally applicable on degradation and depolymerisation kinetics, not only for hemp woody core, but also for hardwood and softwood species. The paper characteristics are comparable both to hardwood and to straw pulp characteristics.