Laminated high strength cellulose structures

Laminated high strength cellulose structures Introduction Formaldehyde containing adhesive systems are commonly used in wood based board production due to simple and well-known processing routes, low cost of raw materials and manufacturing. Environmental aspects and consciousness of consumers have increased the research and interest towards alternative systems and even biomaterial originating glues. Wood based cellulose is the largest cellulose source on earth, and it has been used in paper and board manufacturing dating back for centuries. Cellulose nanofibrils (CNF) are one of the most interesting nanoscale biomaterial available from nature. Cellulose nanofibrils have different characteristics depending on the fibre origin and processing method. CNF can be liberated from various plant fibers using mechanical forces, chemical treatment, enzymes or combinations of these. The most typical mechanical methods include homogenization, microfluidization, microgrinding or cryocrushing. After fibrillation the width of micro/nanofibrils is typically between 5 to 20 nm and length from hundreds of nanometers to several micrometers, even exceeding 5 µm. The aspect ratio may therefore exceed 250. The aspect ratio contributes to high strength of nanocellulose network structure and possible to composite materials containing nanofiber networks.1-3 Nanocellulose is typically in the form of water suspension and shows gelation at concentrations as low as 1 or 2%. Normally cellulose nanosuspensions have been used to make films by filtering the suspension using vacuum to obtain a wet gel followed by evaporation of water.4-6 Fibrils are mechanically tangled and as the water evaporates capillary forces attract individual nanofibers together followed by secondary attraction forces, including hydrogen bonding. Films and structures made from CNF in general have comparable tensile strength to aluminium although the surface appearance is paper-like.Particle board and MDF production adhesives based on natural resources have no significant commercial importance. Main research interest have concentrated on soy-based adhesives as well as lignin and tannin based adhesives. The main adhesive systems used in the European wood-based panels industry today are UF, MUF, PF and PMDI adhesives7. Alternative for bio-based adhesive and traditional boards could use all-cellulose structure based on combining carton board using special type of nanocellulose as glue. The approach is able to produce structures having superior bending strength compared to chip board and MDF. Even slightly higher bending strength compared to softwood veneer can be reached. Nanocellulose fibrils are impregnated into carton board structure during manufacturing to produce a hybrid-structure consisting of cellulose fibres and nanoscale fibres within the board structure. Impregnated nanocellulose suspension softens the carton board enabling surface patterning of the structure. This study focused on a novel manufacturing technology to produce solid structures from carton board and nanocellulose. Structures may find use as alternative for domestic dividing walls instead of gypsum and chip board, office separation walls being light and sound absorbing, furniture frames. Surface finishing could take advantage of embossed patters, printed pictures or regular painting. Nanocellulose used was made using a novel manufacturing technology8 to produce nanocellulose suspension showing much higher solids content, up to 10%, compared to typical nanocellulose water suspensions of 1 or 2%.1 The high solid nanocellulose suspension was used to coat individual carton board layers followed by stacking of layers, wet pressing and drying.Experimental sectionPreparation of high consistency CNFBleached softwood pulp from a Finnish pulp mill (MetsäFibre, Äänekoski, Finland) was used as the raw material for producing CMF at high consistency (HefCel). The enzymatic treatment was carried out at a consistency of 25% (dw) for 6h at 70oC, pH 5 using a two shaft sigma mixer (Jaygo Incorporated, NJ, USA) running at 25 rpm. The pulp batch size was 300g (dw). After the treatment enzyme activity was stopped by increasing temperature of the mixer to 90oC for 30min. The fibrillated material was diluted with deionised water, filtered and washed thoroughly with deionised water. Finally, the fibrillated material was dewatered to a consistency of 18% by filtration. Yield of the fibrillated material was 90%. The material was stored at +4oC until used.Preparation of CNF used as glue Fibrillated CNF material was diluted with deionized water to a concentration of 100 g/L. The fibril suspension was dispersed for 60 minutes using Diaf-mixer at 3000 rpm. After 60 minutes a sorbitol plasticizer, D-sorbitol CAS 50-7-4 (30% from amount of dry fibrils) was added and dispersion continued for 30 minutes. When preparing the all-cellulose structure nanocellulose was applied to individual carton board layers having thickness of approximately 500 microns. Applied wet layer of nanocellulose for single side of each carton board used in the study was approximately 500 microns. To produce structures used in strength measurements having 12 mm final thickness after pressing, 24 individual carton board sheets were stacked. Stacking was followed by wet pressing for 10 minutes. Drying was done in heat cabinet at 105 oC for at least 12h under 10 kg weight. After drying structures were cooled in ambient conditions under weight to prevent curling caused by re-absorbing air moisture Results and discussionAn up-scalable method to produce novel structures combining nanocellulose and cellulose was developed. To demonstrate the method interior design elements were produced. Bending strength properties surpassed the reference materials. Surface of structures was finished using patters, 3D-forms and pictures. All material components in these novel cellulose structures are derived from wood and are bio-based. Thus structures are sustainable omitting no hazardous components and hence completely recyclable. At the end of life structures are bio-degradable and compostable. CMF produced in high consistency can be used as glue once diluted to ~ 100 g/L consistency. The viscosity at this consistency level is comparable to the viscosity of homogenized and microfluidized nanocellulose at a concentration of 0.5%. The difference in viscosity is due to lower aspect ratio of high consistency CMF. High solids make it possible to add the CMF on carton boards without excess water. During wet pressing stage normally only water containing very little amount of nanocellulose is existing the stacked structure. Drying time was not evaluated during this study but a time long enough (12 hours) to evaporate all water in the cellulose structures was applied. By incorporating proper industrial scale methods the drying time should be suitable for production scale implementations. Cross-section microscopic images showed at least partial saturation of nanocellulose into stacked carton boards. The coated surfaces were recognizable in the structure as well. The carton boards acted as a carrier for the nanocellulose and enhanced the ductility of the whole structure while nanocellulose provided the strong intermediate layers. No limit for the amount of stacked layers was identified in the study. However since drying water evaporation during lab scale study was occurring under weights water evaporation was occurring only from the sides. Therefore water evaporation is much slower compared to condebelt type of drying or drying happening between moving wires. Up to one hundred individual layers were stacked on top of each other resulting in 50 mm thick pile when dried nanocellulose experiences high shrinkage forces. Sorbitol plasticizer was added to nanocellulose to decrease shrinkage forces. When working without plasticizer the structures were more easily twisted after drying. Finished laminated structures were finished using woodworking tools such as drilling, milling and sawing. The surface finishing quality was close to MDF. Following finishing to correct size some structures were printed using ink-jet printing. Printing quality and properties were comparable to paper. Conclusions Novel, up-scalable method to produce structures combining nanocellulose and cellulose was developed. To demonstrate the method interior design elements were produced. The strength properties surpassed the reference materials. Surface of structures was finished using patters, 3D-forms and pictures. Laminated structures produced combining may provide interior architecture and building a new eco-friendly option that is bio-based and fully recyclable. Technology will be developed further to provide enhanced water tolerance for structures and even higher bending strength.