Flame-Retardant Cellulose Fibre/Fibril Based Materials via Layer-by-Layer Technique

Sammanfattning: According to an analysis conducted by the Swedish Chemicals Inspectorate in 2006, the approximate numbers of fire injuries per year in Sweden are 100 deaths, 700 major and 700 minor injuries.1 Observations also show that there has been an increase in the number of house fires during recent years. One possible explanation can be the increased use of plastics in the building industry and in furniture. The advantages of easy processing, light weight and low cost make plastic materials most prevalent in the market.  However, plastics behave significantly differently from natural materials in the case of fire. Polymeric materials, including rigid polyurethane foams (PU) which are widely used in the building industry due to their insulating properties, are highly flammable and they release heat at a very high rate. In addition, polymeric materials release more harmful smoke, toxic gases and combustion products than natural materials. A house fire typically starts with the ignition of a combustible material. Flames then spread to nearby materials and shortly thereafter the heat radiation generated reaches a point where the contents of the room suddenly and simultaneously ignite. This stage is called a flash over. After this stage, the fire is fully developed and it continues until everything is consumed. The higher rate of heat and smoke production from plastic materials reduces the time to flash over and hence the time to escape from a fire. The traditional flame-retardant treatments are based mainly on halogenated compounds which are classified as gas phase flame-retardants. The halogenated flame-retardants are under severe investigation due to their adverse effect on health and on the environment since they release toxic gases during combustion and they may leach out and accumulate in the food chain.2-3 The restrictions due to growing environmental concerns have been a driving force to develop alternative flame-retardants by using natural and renewable resources. In recent years, the layer-by-layer (LbL) technique has been used as a simple and versatile surface engineering technique to construct functional nanocoatings through the sequential adsorption of polyelectrolytes and charged nanoparticles in an effort to impart flame-retardant characteristics by inhibiting the combustion cycle.4-5 This thesis presents the physical modification of cellulose fibre/fibril based materials as a means of improving flame-retardant properties.In the first part of work described in this thesis, the adsorption of polyelectrolyte multilayers onto pulp fibres was investigated as a way to impart flame-retardant characteristics to paper-based materials. It was found that intumescent nanocoatings consisting of nitrogen and phosphorus containing polyelectrolytes such as chitosan (CH) and poly(vinylphosphonic acid) (PVPA) were able to significantly improve the thermal stability and flame-retardant properties of sheets made of LbL-treated fibres, and were able to self-extinguish the flame in the horizontal flame test (HFT). High magnification images revealed that this improvement in flame-retardancy was due to the formation of a coherent char layer on the fibres (Paper I).6 In addition to imparting flame-retardancy by the LbL-coating of polyethylenimine (PEI) and sodium hexametaphosphate (SHMP), it was also possible to improve the mechanical properties of the paper material with this treatment (Paper III).7In the second part of the work, wet-stable porous cellulose fibril-based aerogels were developed by freeze-drying and used as a template for the build-up of intumescent nano-brick wall assemblies. The formation of multilayers of CH, PVPA and montmorillonite clay (MMT) was investigated as a function of solution concentration, and it was found that five quadlayers (QL) of CH/PVPA/CH/MMT treated aerogels using 5 g/L solutions of the respective components were able to self-extinguish the flame in HFT and that they showed no ignition under the heat flux of 35 kW/m2 used in cone calorimetry (Paper II).8 In a different application, a novel low density, porous, wet-stable cellulose fibre network was developed using chemically modified cellulose fibres by solvent exchange from water to acetone followed by drying at room temperature. The fibre networks (FN) were modified using the LbL technique to construct a flame-retardant nanocoating consisting of CH, SHMP, and inorganic particles (i.e., MMT, sepiolite (SEP), and colloidal silica (SNP)). The influence of the shape of the nanoparticles on flame-retardancy was investigated and it was found that plate-like and rod-like clays with a high aspect ratio showed self-extinguishing behaviour in HFT. A 5 QL of CH/SHMP/CH/SEP reduced the peak heat release rate and total smoke release by 47% and 43%, respectively, with an addition of only ~8 wt% to FN (Paper IV).Finally, non-crystalline cellulose gel beads were used as a substrate for the LbL assembly of CH and SHMP in model studies aimed at identifying the molecular mechanisms responsible for the fire-retardant properties of the LbL structures. The beads were formed by precipitating the dissolved cellulose-rich fibres according to an earlier described procedure,9 and it was shown that these smooth cellulose beads can be utilized as a model substrate to study the influence of LbL chemistry and nanostructure on flame-retardancy. These new types of model systems thus constitute a new important tool for clarifying the mechanism behind flame-retardant nanocoating systems (Paper V).