Hybridisation of pulp fibres with LDH for applications in composites and lightweight fibrous foam

The research around the pulp and paper (P&P) sector has changed dramatically during the past 15 years. P&P industry is particularly keen to find new solutions for the raw-materials they provide as well as explore novel process technologies. This thesis is a step towards engineering of hybrid materials from pulp fibres and synthetic hydrotalcite (LDH). The purpose was to gain knowledge from the different in situ synthesis techniques of LDH with pulp fibres, properties of the prepared hybrid materials and how the hybridised fibres behave in different applications, specifically in a composite and a lightweight fibrous foam.The hybrid material prepared in this work consisted of either a sulphate pulp (Kraft) or a thermomechanical pulp (TMP), and synthetic LDH particles from hydrated magnesium and aluminium salts. Hybridisation was conducted by three different in situ synthesis routes that followed either the preparation of a high super saturated (hss) or low super saturated (lss) aqueous pulp suspension, or, by the aid of urea hydrolysis (Uhyd). It was noted that the hybridisation of hydrogen peroxide bleached TMP pulp of spruce (Picea abies) was successful and that the functionalisation of the hybridised fibres with sodium dodecyl sulphate (SDS) was possible if Mg(NO3)2 and Al(NO3)3 were applied as LDH precursors. The apparent contact angle (θ) of a sessile water droplet reached 135° upon functionalisation. It is noteworthy that the LDH particle size on fibre surface varied from 70 nm up to 5 μm depending on the synthesis route. Hybridisation of Kraft fibres by urea hydrolysis produced mineralised fibres that expressed lower compliance than the other two synthesis routes. Absorption studies with methylene blue and metanil yellow probe molecules showed that the lss route retained most of the fibres’ original cationic capacity, but also provided the highest capacity for anions suggesting an ampholytic character of the hybrid fibres. The relatively short aliphatic hydrocarbon chain of SDS on LDH surface was not able to improve the coupling of fibres and polymer matrix in a polypropylene composite. The bottleneck was in particle cohesion. In the lightweight fibrous foam, the in situ synthesis of LDH was engineered to include both micron and nano-sized particles by applying lss and Uhyd synthesis routes. The pulp contained approximately 34% w/w of LDH. Under combustion the amount of CO2 and soot and the peak heat release rate (PHRR) were reduced significantly. The in situ synthesised LDH particles shielded the fibres from external heat by reducing liberation of volatile gases. Effective charring was observed on the surface of LDH nanoparticles.Synthetic LDH appears as a promising platform to functionalise fibre surfaces. It equips pulp fibres giving them an ampholytic character and reduces liberated heat under forced burning. LDH is also able to mineralise Kraft fibres if the synthesis route is correctly chosen. These characteristics can be further exploited in diverse applications where fibres require ability to absorb both cationic and anionic substances. On the other hand, flammability issues are important whenever the fibres are used in-house applications. Pulp fibre industry can readily apply LDH synthesis into the existing processes and take advantage of the proposed hybridisation. LDH may also be useful in more general terms in fibre technology. The key issue is that LDH formation does not require complex chemistry.