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reduction time was kept constant at 4 h, followed by filtration and thorough washing. Carbonyl content determination The carbonyl content of the oxidised fibres was determined by reaction with hydroxylamine hydrochloride. 6,10 The fibres were suspended in water and adjusted to pH 4, followed by dewater- ing to a gel-like consistency. Then, approximately 0.25 g (dry basis) of these fibres were stirred with 25 ml of 0.25 M hydroxy- lamine hydrochloride solution at pH 4 for at least 2 h before the fibres were separated from the solution by filtration using a pre-weighed filter paper. The exact mass of the fibres was then determined by oven drying of the filter paper and the car- bonyl amount was determined by titration of the filtrate back to pH 4 with 0.10 M sodium hydroxide. Two to three indepen- dent oxidations were performed at each oxidation time, and each reaction with hydroxylamine hydrochloride was per- formed in triplicate. X-ray di ff raction The crystallinity of the non-modified and modified materials was evaluated by collecting X-ray di ff raction (XRD) patterns using a PANalytical X ’ Pert PRO X-ray di ff raction system. Data were recorded in the reflection mode in the angular range of 5 – 50° (2 θ ) usingCuK α radiation (1.5418 Å). The XRD data were analysed by calculating the ratio between the di ff raction of the (002) lattice peak at about 22.5° and the minimum found between the (002) and (101) lattice peaks at about 18.5°, com- monly referred to as the crystallinity index, 11 and the crystallite width was estimated using the Scherrer formula on the (002) lattice peak, assuming a shape factor of 0.9. Paper preparation Handsheets with an approximate grammage of 150 g m − 2 were prepared using tap water in a Rapid Köthen sheet former (Paper Testing Instruments, Austria). The dewatering time ranged from 20 to 40 s. The sheets were dried at 93 °C under a reduced pressure of 95 kPa, first for 15 min between 400 mesh woven metal wires attached to regular sheet-former carrier boards, and then for 2 min between ordinary carrier boards. The sheets were then stored at 23 °C and 50% RH until further testing. Pressing Circular samples with a diameter of 40 mm were hot pressed between two bright annealed steel discs in a Fontijne TP400 press (Fontijne Grotnes, The Netherlands) for all further analy- sis, except for peel testing where rectangular (20 mm wide and 63 mm long) steel plates of the same area were used. The typical combination of pressure, temperature and time was 16 MPa, 150 °C and 2 min, but other combinations were also tested (see ESI † ). Thickness and density Thickness was determined, before and after pressing, as the average structural thickness according to the SCAN-P 88:01
are individualised, oxygen-barrier films with high ductility and formability can be fabricated. 7 However, since pure dialcohol cellulose is thermoplastic, 8,9 it is of interest to study whether a core – shell structure is su ffi cient to induce thermoplastic fea- tures in the material while retaining the good mechanical pro- perties of the native cellulose. The present study therefore aims at exploring a higher degree of derivatisation, and charac- terising the mechanical properties, thermoplastic behaviour and structure of papers and films made by a conventional papermaking technique, with the deliberate goal to produce high-performance materials from cellulose without the energy- consuming liberation of the fibres into CNFs, and to apply a simple form of heat processing, in this case hot pressing, to prepare highly transparent and strong films.
Experimental Fibres
Bleached softwood kraft fibres (K46) were supplied by SCA Forest Products (Östrand pulp mill, Timrå, Sweden). One part of the material was left unbeaten and one was mechanically beaten in a Voith mill to an energy input of 160 W h kg − 1 (about 30 SR). This increases the swelling of the fibres and makes them more flexible. Mechanical beating also produces small-particle material, so called fines. To ensure that only long macroscopic fibres were used during the modification, thereby easing the processing and data interpretation, the small-particle material was removed from both the non-beaten (3 – 4%) and the beaten (8 – 10%) fibres by filtration through a 200 mesh metal screen, using a Britt Dynamic Drainage Jar (Paper Research Materials, Seattle, USA). Chemicals Sodium( meta )periodate was provided by Alfa Aesar (98%), and sodium borohydride and hydroxylamine hydrochloride were supplied by Sigma-Aldrich. Other chemicals such as hydro- chloric acid, sodium hydroxide, isopropanol ( ≥ 99.8% purity) and sodium phosphate were all of analytical grade. Fibre modification Cellulose fibres were sequentially oxidised and reduced accord- ing to an earlier established protocol. 6 The fibres were partly oxidised to dialdehyde cellulose by adding 5.4 gram of period- ate per gram of fibre to a gently stirred beaker at a fibre con- centration of 4 g l − 1 . To limit the formation of radicals and unwanted side reactions, the reaction was performed in the dark. After the desired time of oxidation, 6, 12 or 24 h, the reaction was stopped by filtration and washing of the fibres. The fibres were then suspended to 4 g l − 1 and the dialdehyde cellulose formed was reduced to dialcohol cellulose by adding 0.5 g sodium borohydride per gram of fibres. To limit the pH increase to about pH 10 upon addition of sodium borohydride, monobasic sodium phosphate was added together with the borohydride in an amount corresponding to 0.01 M. The
Green Chem. , 2016, 18 , 3324 – 3333 | 3325
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