Mechanical Properties Of Poplar Wood Modified With N-hydroxymethyl Resin/Alcohols

Chemical modification of wood improves various wood properties such as dimensional stability,resistance against fungal decay and weathering.However,an issue is that modification also causes a reduction in certain mechanical properties,especially the impact strength.Such reduction can considerably limit its application as structural material.Therefore,this study attempts to abate the negative effects of modification with N-methylol resin on the mechanical properties of wood,especially on the impact strength.In this study,both the ethylene glycol(EG)and propylene glycol(PG)were jointly used as the plasticizer-like agents in the 1,3-dimethylol-4,5-dihydroxy-ethyleneurea(DMDHEU)to modify the poplar wood.The effects of addition of EG or PG on the mechanical properties of resulting wood were systematically evaluated.The investigation includes optimizing the formulation of treating solution and treating parameters such as concentrations of DMDHEU/EG or PG,catalysts and pH value.The evaluation on the modifying effects include weight percent gain(WPG)of wood,leaching ratio(LR)of chemical,water absorption(WA)and equlibrium moisture content(EMC)of wood;the effects on the mechanical properties were emphatically analyzed,mainly focusing on the impact strength,modulus of elasticity(MOE),modulus of rupture(MOR),compression strength and hardness.Both the distribution of chemical in the wood microstructure and the fractured morphological features were observed using a scaning electronic microscope(SEM);Fourier transform infrared spectroscopy(FTIR)and dynamic mechanical analyst(DMA)were used to analyze the reaction modes between wood and chemicals and the dynamic mechanical properties,respectively.The results obtained through the study are presented as follows:1)Wood obtained a WPG ranging from 20 to 70%as they were treated with increasing concentration of DMDHEU/alcohols solutions.Use of magnesium chloride as catalyst facilitated to chemically fix the chemicals in the wood structure.The equlibrium moisture content of wood was reduced due to treatments.Incorporation of EG or PG jointly with DMDHEU into the wood caused a reduction in the leaching ratio(5-16%)as compared to that without EG/PG At pH 5,use of mixture catalysts containing both citric acid and tartaric acid resulted in a highest WPG and lowest LR of poplar wood treated with DMDHEU/alcohol solution.2)The MOE,compression and hardness strength of DMDHEU treaed wood increased but the MOR and impact strength reduced 14-29%and 9-35%,respectively,as compared to the untreated controls.The impact strength of 20%DMDHEU/EG and 30%DMDHEU/PG treated wood was of 13.04%and 17.39%greater than that of untreated wood,respectively.Treatment with citric acid and tartaric acid as the mixing catalyst caused a lower impact strength than the untreated controls but a greater one than that treated with magnesium chloride as the catalyst.The MOE,MOR and compression strength were also improved compared to the system catalyzed with magnesium chloride.The MOE of wood samples treated with DMDHEU/EG and DMDHEU/PG in the solutions at pH 5 increased 25%and 38%,respectively Compared to the untreated controls,the compression strength increased up to 68%in the longitudinal direction,while the MOR reduced 4%.3)The impact fractured surface of untreated wood were uneven,irregular,jagged and complex showing toughness fracture.The fractures of DMDHEU/magnesium chloride treated samples were,however,even and smooth;the breakage line is straight.This indicates embrittlement of wood due to modification.Compared to the samples treated with DMDHEU/magnesium chloride,the impact fractured surfaces of poplar wood treated with DMDHEU/EG or DMDHEU/PG were more irregular;the tearing fibers appeared on the surface and some of microfibrils were pulled out of the cell wall.This suggests that the samples treated with DMDHEU/alcohol presents a more tough feature than the DMDHEU/magnesium chloride treated samples.4)For the water washed wood treated with DMDHEU/alcohol,the wavenumbers at 1730 cm-1,1400-1550 cm-1,and 1200 cm-1 can be assigned to the carbonyl(C=O),acylamino(-CH2-N-C=O),ether linkage(C-O-C)and sub-methyl or methyl(-CH2-/-CH3),respectively.All of these bands were enhanced due to DMDHEU/alcohol treatments.This indicates that the chemicals chemically react and fix in the wood structure.The formation of ether bond may be attributed to the reaction of N-methylol groups with hydroxyl groups of wood or alcohols.The enhancement at 1730 cm-1、1400-1550 cm-1 can be attributed to the introduction of carbonyl and two acylaminos in the DMDHEU molecule.5)The results of dynamic mechanical analysis(DMA)showed that the storage mudulus(E’)of wood increased with the temperature;the E’ of wood treated with DMDHEU/alcohol were greater than that of untreated controls.This may be due to crosslinking reaction in the wood,forming rigid network sturcutures.In addition,lower equlibrium moisture content may also lead to proper increase in the rigidity of the treated wood.From-25 ° C to room temperature,there was an increasing trend of loss modulus(E")and loss factor(tan δ).Water molecules exist with ice crystals under 0℃,the viberation of water molecules is weak.The E" of DMDHEU/alcohol samples increased with temperature in the range of 20-100℃;while tan δ was stable.This is due to increased molecule vibration of wood and water.The E" and tan δ decreased apparently at the temperature ranging from 100 to 120 C.The reason may be due to the evaporation of water molecules in the wood.Degradation of lignin of DMDHEU/alcohol treated samples may explain the increase of E and tan δ at 120℃ or above.Summarily,DMDHEU treatment catalyzed with magnesium chloride can cause considerable embrittlement of wood.Incoporation of low molecule weight alcohols in the DMDHEU solution can improve the toughness of the resulting wood.