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It appears that wood-based panels are the area which has received the greatest atten- tion for carbon storage calculations [11,113]. Wood-based panels are increasingly drawing on Category B and C material, with some use of Category A material when it is available. The residence time for some panel products in the home is shorter than structural timber; however, if plywood and OSB are used in structural elements, such as flooring cassettes or wall panels, they may remain for the life of the building, like the softwood joists [11]. On the other hand, particleboard and MDF are predominantly used in fitted kitchens and other furniture, which have a shorter retention period. It is estimated that the average household replaces their fitted kitchen every 15 years [113]. For some items, such as flat pack wardrobes, the product life is estimated as being considerably shorter, possibly as low as 5 years [114]. Spear [11] proposed simple estimates of durations and combinations for the residence time of wood-based panels to combine into recycling scenarios. This led to a potential residence time of 150 ± 40 years if a structural panel such as OSB is recycled into structural-grade particleboard. A combined lifespan of 125 ± 27 years was suggested for a joinery panel recycled into structural wood panels or a structural panel being recycled into a joinery panel such as MDF. Shorter examples involved panels used in other sectors, including shop fitting and packaging, as the total pool of wood-based panels serves a broad range of markets. Such suggestions rely on continually emerging service life data, but it is clear that enhancing the options for recycling post-consumer particleboard and MDF products will provide an additional life prior to final disposal. Brunet-Navarro et al. demonstrated that carbon storage increases linearly for extending the product life, but exponentially if the recycling rate is increased [115]. Enhancing the options for the re-use of MDF into novel products such as nanocellulose would also lead to a storage benefit [105]. Couret et al. [113] simulated the potential volumes of MDF waste available for nanocellulose production, based on 2012 MDF waste wood levels. The development of new markets to take MDF into alternative products would extend the number of options for recycling a challenging material. If the current recovery of TMP fibres from MDF for use in insulation products for buildings gains popularity, then the expected service life of the recycled insulation will be decades. This would enhance the wood products pool for carbon storage through the large quantity required per dwelling as well as the long duration of product use. 7.3. Lower Carbon Footprint Utilising recycled timber for fillers or particleboard manufacturing significantly re- duces carbon emissions and global warming potential (GWP). The use of recycled wood requires less energy to process into panels than raw timber, as the material is drier [1,12]. Products, such as door cores and particleboard made from recycled wood, can have a lower carbon footprint than its fresh wood counterpart [73,116,117]. Forster et al. showed that Scope 1–3 emissions were substantially reduced in scenarios where the cascading of wood was employed, compared to business as usual [12]. 8. Conclusions While some product groups that use recycled wood have changed little over the past two decades in the UK, the market has matured considerably. The strict grading categories selected early in the evolution of recycled wood usage in wood-based panels in the UK have led to continued and substantial advances in the segregation and cleaning technologies. However, there is still scope for further innovation, such as the cleaning of the chemically treated wood, which is currently assigned to Category D (destined for controlled disposal to minimise the risk to the environment). As competition for the resource intensifies, there
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