| Particleboard is a promised product due to the large quantity requirement of wood in the word. Now the main problem of Chinese particleboard is that free formaldehyde emission exceeds the standard value seriously. According to the new criteria, GB18580-2001, formaldehyde emitted from El graded composite shouldn't be more than 9mg/100g. Chinese composite industry is standing a severe test.This article contributes to Low F-emission particleboard study, which is closely related with practice. The author developed a unique method to decrease F-emission sharply. The best result could be 7.7mg/100g, meeting El requirement. In addition, excellent physical and mechanical properties are also ensuredIn research, modern analysis equipment are used to help study. Author explains the bonding mechanism between UF, MDI and wood, and finds the cause of L-emission and high physical properties on molecular level. MDI acts as an coupling agent by which wood is chemically linked with UF. This is why UF could cure in nonacid condition.Based on properties test results and DSC analysis, we think acid curing agent affects mixing way. When UF : MDI=6 : 1, properties in order are listed as follow:Spraying mixed UF-MDI without NH4C1 > Separately spraying MDI and UF with NH4C1 > Separately spraying MDI and UF without NH4C1 > Spraying mixed UF-MDI with NH4C1But when UF: MDI=9: 1, NH4C1 is necessary to be added. Considering stability mixed adhesive should be sprayed on particles within 1 h.Through comparison UF: MDI=6: 1 is considered a suitable ratio and water repellent is still needed adding. Particleboard in this proportion is qualified to standard, thickness swelling and anti boiling is sharply improved compare with UF particleboard.Properties are closely related with particleboard density in polynomial. The lowest density shouldn't be smaller than 0.63g/cm3 in order to ensure qualified properties.Pat moisture content (MC) doesn't affect properties between 10 - 14%, but have adverse affect when MC up to 17%. In addition, blister apparent in hot pressing. F-emission increases with MC increases. Suitable MC shouldn't be more than 14% as UF particleboard.Properties are best when hot pressed for 4.5min at 175癈. Though 3min is beneficial to production, curing is not enough; 6min is too long deducing particleboard deterioration. Both 3min and 6min result in increased F-emission.Comparing with UF particleboard, F-emission of mixed particleboard decreases sharply. Prolonged pressing time could reduce F-emission, but shouldn't be too long.When UF: MDI=6:1, F-emission meets El standard under 175癈temperature and pressed for4.5min, then F-emission is 7.7mg/100g.Supported by multiple analysis methods such as DSCU NMR^ DMA fP FT-IR, we found bonding mechanism between UF, MDI and wood. UF affects wood molecule movement a little, while MDI affects largely.In mixed adhesive, MDI acted as a couple agent because it reacts with UF and wood and links UF with wood by chemical bonding. Stable -COOR and -CH2- increase and -C-O-C- -NH2, -CH2OH decrease due to MDI participation. So properties are improved and F-emission decreases.MDI begins to react with water after UF reacted with MDI for 30癈, and rate is slower than latter, so water couldn't blunt MDI.In short, when UF: MDI=6: 1, hot press under 175癈 for 4.5min, the particleboard E-emission qualified El requirement and antihygroscopicity is better than UF. Utilizing UF-MDI mixed adhesive for El particleboard is an effective method and the research have theoretical meaning based on discussing bonding mechanism.
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