The release of free formaldehyde from urea-formaldehyde resin(UF)during production and use is the main drawback,and the production of UF with a low molar ratio(F/U)is an effective means to reduce the free formaldehyde,but has negative effect on its gluing strength.In this study,poly(diphenylmethane diisocyanate)(pMDI)was used to modify UF with low free formaldehyde content,and then the related properties were tested.The results showed that with the increase of pMDI dosage,the viscosity of UF rised,the curing time shortened,and the content of insoluble matter increased.When the addition amount of pMDI was 9%,the adhesive strength improved the most,and the dry adhesive strength was 1.17 MPa,and the wet adhesive strength was 1.15 MPa.When the addition amount of pMDI was 5%,the wet adhesive strength was 0.76 MPa,which reached the national standard GB/T 9846-2015 Plywood for General Use indoor Class II plywood requirements.Meanwhile,the cured UF formed a urethane structure and exhibited better heat resistance.
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