The study investigated the effect of the action of air with increased humidity contents and cyclic changes in humidity on the properties of OSB/4 boards glued in the centre layer with PMDI resin. The research results show that OSB/4 boards used for the purpose of the study are characterized by very high values of the modulus of rupture and modulus of elasticity. The long-term action of air with the humidity of 90%, as well as cyclic changes in humidity result in only a slight decrease in these values. The boards meet the requirements of the PN-EN 300 standard in terms of internal bond after the boil test. However, their water-resistance measured with this parameter does not comply with the standard requirements after 9-week seasoning in the air with the humidity of 90% and after three cycles of the action of the air with changeable humidity. The long-term action of air with both increased and changeable humidity results in a durable deformation in the thickness of OSB/4 boards reaching 8%.

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