Preparation And Properties Of Modified Low Molar Ratio Urea-formaldehyde Resin Adhesive For Moisture-proof Fiberboard

With the vigorous development of customized home,furniture and interior decoration industries,Chinese fiberboard industry has also developed rapidly.In order to ensure the environmental performance of fiberboard,low molar ratio(molar ratio of formaldehyde to urea,F/U)urea-formaldehyde resin is widely used in the production of fiberboard.However,the mechanical properties and moisture-proof properties of fiber board prepared by low molar ratio urea-formaldehyde resin are difficult to meet the requirements of high humidity environment such as bathroom,kitchen and floor.In this paper,based on the synthesis technology of low molar ratio urea-formaldehyde resin,the modified low molar ratio urea-formaldehyde resin adhesive for moisture-proof fiberboard was prepared by using melamine and aluminum hydroxide.The effect of the amount of melamine on the properties of urea-formaldehyde resin and its fiber board was studied.On this basis,in order to reduce the formaldehyde release of fiberboard,alumina hydroxide was introduced into the system of melamine modified urea-formaldehyde resin for joint modification,and the optimum process of preparing moisture-proof fiberboard by modified urea-formaldehyde resin was studied,which provided reference for its industrial production and application.The main research contents and results are as follows:(1)The influence of the amount of melamine on the physicochemical properties of urea-formaldehyde resin and the properties of the prepared fiberboard.In the alkaline addition stage of the synthesis process of urea-formaldehyde resin,different ratios of melamine(0,15%,20%and 25%)were added to the total weight of the resin.The experimental results showed that with the increase of the amount of melamine,the viscosity of urea-formaldehyde resin increased,the curing time shortened,and the content of free formaldehyde decreased.The mechanical properties of fiberboard were gradually improved,the expansion rate of 24 h water absorption thickness was gradually reduced,and the formaldehyde emission was first decreased and then increased.When the amount of melamine was 25%,the expansion rate of 24 h water absorption thickness of fiberboard was reduced from 12.73%to 3.69%.The optimal amount of melamine in modified low molar ratio urea formaldehyde resin for moisture-proof fiberboard was determined to be 25%.Melamine promoted the formation of branched chain structure in ureaformaldehyde resin,improved the cross-linking density of the resin after curing,blocked a large number of hydrophilic hydroxyl groups in the resin,and thus enhanced the mechanical properties and moisture-proof properties of the fiberboard.However,when the amount of melamine exceeds 15%,a large number of hydroxymethyl groups react with each other during curing,releasing free formaldehyde,resulting in an increase in the amount of formaldehyde released from the fiberboard.(2)Explore the best preparation technology of low molar ratio urea formaldehyde resin modified by aluminum hydroxide and melamine.On the basis of urea-formaldehyde resin modified with 25%melamine dosage,3-aminopropyl triethoxysilane(KH550)modified aluminum hydroxide was introduced to reduce the formaldehyde release of fiberboard.The results of single factor experiment show that the modified ureaformaldehyde resin has the best performance when the amount of KH550 is 2%of the mass of aluminum hydroxide,and the amount of KH550 modified aluminum hydroxide is 5%of the total mass of the adhesive.Under these conditions,the content of free formaldehyde of the modified urea-formaldehyde resin is 0.18%,the expansion rate of 24 h water absorption thickness of the fiber board prepared by the modified ureaformaldehyde resin is 3.19%,and the formaldehyde release is 7.42mg/100g.Compared with the modified urea formaldehyde resin with 25%melamine content,the 24 h water absorption thickness expansion rate decreased by 13.55%,and the formaldehyde release decreased by 12.46%.The modified aluminum hydroxide was embedded in the curing network of the resin through the chemical bonding reaction of the surface grafted amino group with formaldehyde and MUF resin molecules.The embedded aluminum hydroxide promotes the formation of branch structures in the resin,destroys the hydrogen bonds formed in the resin molecules,and improves the crosslinking density of the resin after curing.Moreover,aluminum hydroxide has the ability of acid buffering,reducing the hydrolysis rate of the resin,thus improving the moisture-proof performance of the fiberboard and reducing the formaldehyde release amount of the fiberboard.(3)The application of modified low molar ratio urea-formaldehyde resin in fiberboard.Orthogonal experiments were carried out with sizing amount,maximum hotpressing pressure,hot pressing temperature and hot-pressing time as factors.The results showed that sizing amount was the most important factor affecting the properties of fiberboard,followed by hot pressing time.The fiber board prepared by the process of 15%sizing volume,6MPa maximum hot-pressing pressure,175℃ hot pressing temperature and 210s hot pressing time has the best moisture resistance.The expansion rate of 24 h water absorption thickness is 3.07%,and the formaldehyde release is 7.94mg/100g.Compared with the performance and cost of adhesives for moisture-proof fiberboard on the market,it is found that the curing time of the adhesive prepared in this experiment is shortened by 140s,and the expansion rate of the water absorption thickness in 24 h is reduced by 24.75%.Under the condition that the physical and mechanical properties of the prepared fiberboard are consistent,the cost of producing fiberboard by the adhesive prepared in this experiment is lower.The low molar ratio urea-formaldehyde resin adhesive modified by aluninum hydroxide and melamine is feasible in the production of fiberboard.