A study of two aspects of medium density fibreboard manufacture
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis covers two aspects of MDF panel manufacture, In Part A of the thesis the development and application of a plant - based model of the blowline resin blending process used in the manufacture of Medium Density Fibreboard (MDF) is described, Experience quoted in the literature, operating experience in plants and data from specific plant trials have provided a number of insights into the process, but it was not until a calculation of the blowline steam flow became available that these disparate observations could be integrated into the comprehensive model of the blowline blending process that is described in this thesis, The steam flow in the blowline is calculated from a combined mass - energy balance over the refiner, allowing the steam velocity profile in the blowline to be established, In addition to the perspective that this data provides in understanding the various observations of the blowline blending process, it also shows a linkage between blowllne velocities and nozzle atomisation that prevents the use of high pressure atomising nozzles to create small resin droplets unless blowline velocities are high. It is concluded that this relationship is largely responsible for the higher resin contents that blowline resin addition requires when compared with MDF made using the dry blending process that is used for other wood - based composites. The refiner mass - energy balance shows that blowline conditions are sensitive to the refiner conditions, with relatively small changes in some parameters used to control the refiner operation changing the blowline steam flow significantly. This raises the possibility that changes in blowline conditions contribute to significant variability in the bonded fibre network that makes up MDF, Once the steam flow is known for a particular case, the blowline design can be optimised to provide the blending conditions that have been found to give a good blending result. This has been now been done in 25 MDF plants around the world; in 24 cases the blowline diameter was greater than the optimum for good blending. In the remaining case the diameter had been reduced, to the point where the choke flow limit was exceeded, leading to a fibre buildup on the dryer wall. The significance of resin droplet size, in terms of its effect on the strength properties of the panel and on the outcome of the blending process is discussed. The relationship between the mass of resin in each droplet and the mass of the fibre is significant in the blending process, particularly in the formation of resin spots. There are indications both from panel studies and from the resin performance in blowlines that have been optimised, that resin droplets should be smaller rather than larger. Resin atomisation, both in pressure and steam atomised nozzles and through interaction with the blowline steam flow is examined. This results in a wide range of resin droplet sizes, with the smallest from an effective atomising nozzle and the largest created by the interaction of the resin stream with a low steam velocity in the blowline. This wide range of conditions explains much of the variability in the blending outcome, ranging from good resin efficiency where resin droplets are small and poor resin efficiency where resin droplets are larger. The development of a steam - atomised resin injection nozzle is described. The nozzle is in commercial production with a number of units in service. Good resin efficiencies have been achieved, while the design has been successful in overcoming resin blocking problems that have occurred with other nozzle types. The blowline blending model has proved to be both robust and reliable. It was originally developed to reduce resin usage, with reductions of up to 25% of resin previously added having been achieved. In addition the model also provides solutions to a number of problems which have been shown to originate in the blowline. This thesis develops and extends the concepts used in the model to validate the approach and to provide insights into its further development. In Part B of the thesis the influence of temperature, moisture and fibre type on the stress response of a fibre mat to an applied strain was investigated to determine the impact of these variables on the development of the Vertical Density Profile (VDP) in MDF panels. There are a number of models of the in the literature that predict the development of VDP from the condition of the material in the press and the press conditions. While these models, developed for particleboard, claim to be sufficiently general to be extended to cover the case of MDF, there are features of the MDF profile that suggest that this might not be the case. This investigation was set up to determine the stress - strain response of MDF fibre under the conditions encountered in the hot pressing operation. A small circular press was designed so that it could be mounted in a compression testing machine, allowing the stress strain data to be recorded. The press was constructed so that heated and humidified air could be passed through the mat so that uniform temperature and moisture conditions could be established through the mat prior to the stress - strain measurements being made. The press was then closed to determine the stress - strain response of the fibre. After the press was closed it was held in that position and the relaxation response of the fibre mat measured under conditions where the compliance of the press imposed a small continuing strain on the mat as the relaxation of the fibre reduced its compressive resistance. The fibre response at moisture levels of 0% - 20% and at temperatures up to 100°C was measured although limitations of the conditioning limited the moisture content that could be reached at the higher temperature ranges. Two fibre types, prepared from high and low density P. Radiata wood, showed a very similar stress – strain response. The density stress response for the fibre mat appears to follow that for a foam compression model where the resistance to compression at constant temperature and moisture is proportional to the square of the density ratio. An extension to the cellular foam theory to the situation of a discrete fibre mat is proposed. The temperature and moisture effect on the compressive resistance of the fibre mat appears to follow established relationships for solid wood, although the inability to reach the high temperature - high moisture condition excludes this important area from the analysis. A modified stress relaxation response is characterised by an increase in density that, at constant moisture content, appears to be independent of the density of the mat, over the range encountered in the development of the density profile. The moisture effect on the stress relaxation response appears to be more significant than that due to temperature. It is concluded that temperature and moisture content gradients established early in the press are the most significant factors in determining the vertical density profile through the MDF panel. The squared relationship with density implies that these gradients are particularly important in the generation of the high density of the faces of the VDP. The stress relaxation effect provides a uniform increase in density through the mat, modified only by a moisture response which increases this effect where moisture contents are high. The data provide insights into the relative effects of temperature, moisture and density as these change in the pressing of MDF fibre, in a form that can be incorporated in a model of the pressing process and also used by MDF press operators to aid in their understanding of the processes that lead to generation of the VDP in commercial MDF operations.