Preparation of Acetylated Wood With Solvent-Free Gas Phase Method

Preparation of acetylated wood with solvent-free gas phase method

XIAOLI GU*^1,2, XU MA^1,2, LIXIAN LI¹,

(1 Nanjing Forestry University, College of Chemical Engineering, Nanjing CHINA

2 State Key Laboratory of Pulp and Paper Engineering , South China University of Technology , Guangzhou CHINA)

ID:	309281
Password:	753404

 

KEYWORDS: poplar; gas phase method; wood acetylation; acetic anhydride;

  For wood acetylation as chemical modification, the agent of acetic acid reacting with molecules of wood on hydroxyl, hydrophobic acetyl replaced the hydrophilic hydroxyl. Acetylated wood have advantages such as uniform density, smooth surface, dimensional stability, strong corrosion resistance, low heat ductility and decreasing the toxicity, etc.

Wood powder and sawdust acetylation were reported in the 20th century (Guan 2007, Jianzhang et al. 2002), but the research of real wood acetylation reports began in 1946. Then the United States, Japan and other countries of the world enterprises, universities and research institute successively started research the wood acetylation technology and commercial application.

Current research found low acetylation agent efficiency, e.g., about 85% of anhydride not participating in the reaction (Kazuya et al. 2004, Yoriko et al. 2005, Eiichi et al. 2009, Mohamed et al. 2008). Based on the actual problem, we successfully took the advantages of gas phase acetylation with solvent-free process.

Poplar wood (size 20 mm x 20 mm x 10 mm), produced in Jiangsu, with 105 °æ drying for 24 h was applied as substrate in the experiments. A DHG-9146 type electro-thermal constant temperature drying oven and a Nicolet FT-IR infrared spectrometer were used as characterization instruments.Pieces of wood samples with 105 °æ drying for 24 h before the experiment. Acetylation processing temperature was set for 90-175°æ and the processing time was set for 0-10 h.

m_(t) was the experimental mass at each monitoring time, and m₀ was the initial dry mass.

Figure 1A showed the effects of the reaction temperature and reaction time on the WPG of treated samples. The WPG increased with the increment of reaction temperature and reaction time. The acetylated wood measured 6.69% WPG at 1h and 14.92% WPG at 2h when the reaction temperature was 140°æ. Then the acetylated wood showed a small weight gain with the increase of temperature and the WPG was 18.79% at 4h. At 140°æ, the WPGs of the acetylated wood increased quickly with the reaction period from 1h to 2h.

At 150°æ,the WPG was only 0.29% more than that at 140°æ when the reaction time for 2h. At 150°æ, the acetylated wood was measured 19.73% WPG at 4h,which was 0.94% more than that at 140°æ in the same reaction time. Therefore, the improvement of the reaction temperature, the effect of promoting initial stage of acetylation reaction was obvious.

At 160°æ the WPG of the acetylated wood increased quickly than that at 150°æ, e.g., the WPG was 18.87% at 2h. The acetylated wood showed a small weight gain after reacting for 3h. The WPG of two periods from 3h to 4h were 0.05%, 0.11% respectively. It was shown that the reaction time was not the prominent factor for the increase of acetylated rate.

The WPG of the acetylated wood increased obviously in the reaction temperature reached 170°æ above. At 170°æ, as the reaction time increased from 2h to 4h, the WPG of acetylated wood increased from 21.82% to 24.07%. At 175°æ, the WPG of acetylated wood increased from 21.92% to 26.09% at the same conditons. Thus it can be seen, after reaction temperature 170°æ above, increasing the temperature leaded to the WPG increase apparently, which was related to a tendency to the carbonization of the acetylated sample as the temperature increased.

In order to show the superiority of gas phase method, we carried on liquid acetylation experiments for comparison. The experimental results (figure 1B) showed that the WPG reached the maximum value at 8h in liquid phase and the acetylated wood showed a small decrease trend when continual increasing the temperature.

At 110°æ, the WPG was 8.3% at 2h in liquid phase, which was 0.3% less than that at 170°æ for 0.5h in gas phase. At 120°æ, the WPG was 17.91% at 8h in liquid phase, which had a certain increase in small degree as compared with that at 110°æ. At 120°æ, as the reaction time increased from 4h to 8h, the WPG of acetylated wood increased from 12.36% to 17.91%.

When the reaction temperature reached 130°æ,the increase of WPG was obvious as the reaction time increased. The WPG was 22.63% at 8h. Under the conditions of 130°æ, the excretion of massive acetic acid as the by-products of acetylation promoted the increase of WPG in some degree. In addition, we also found that the most obvious time of WPG increased was from 4h to 8h during liquid phase acetylation.

Fig 1A weight percent gain of gas phase acetylation

Fig1B weight percent gain of liquid phase acetylation

 

The absorption peaks of related groups in acetylation are: -OH absorption peak at 3400 cm^-1, saturated esters carbonyl absorption peak at 1742 cm^-1, -CH of acetate, absorption peak at 1384 cm^-1, C-O of acetyl absorption peak at 1248 cm^-1. The absorption peaks degree of related groups of acetylated wood were strengthened obviously as shown in figure 2A. And integration of FTIR data of several related absorption peaks resulted was also shown in figure 2B:

Fig 2A  Comprehensive comparison of different treatment acetylation samples

Fig2B  Integral value of related groups absorption peaks

Note: A) the substrate sample; B) the acetylated wood with liquid phase acetylation at 90°æ for 8h; C) the acetylated wood with liquid phase acetylation at 130°æ for 8h; D) the acetylated wood with gas phase acetylation at 140°æ for 4h; E) the acetylated wood with gas phase acetylation at 175°æ for 4h.

The hydroxyl absorption peak at 3400 cm^-1 had largely reduced after acetylation as shown in Fig2B, the amount of reduced hydroxyl was above 60%. Because of some water content in the wood, the amount of reduced hydroxyl can't reach zero. As compared with the substrate sample, we can see the acetylation significantly occur from several of acetyl characteristic peaks , especially the absorption peak of saturated esters carbonyl at 1742 cm^-1 increased obviously, average amount up to 300 times or so, e.g., the maximum group can reach 373.76 times. Meanwhile, the average amount of the absorption peak of -CH in the acetate at 1384 cm^-1 increased up to 158 times and the average amount of the absorption peaks of acetyl C-O at 1248 cm^-1 up to about 50 times.

In conclusion, With the pure acetic anhydride as acetylation agent for the poplar wood acetylation process, the influence of the processing temperature (140-175°æ) and the processing time (2-4h) on the acetylation yield was investigated. Experimental results showed that as the temperature and the reaction time increase, the acetylation yield increase gradually. The acetylation yield was above 26% with the temperature of 175°æ for 4 hours. The acetylation rate is 2.27%/h in liquid phase acetylation and the acetylation ratio of gas phase is 7.21%/h, which was 3.18 times of the former method. Therefore, the gas phase method is a high yield of acetylation method with the advantage of pure acetic anhydride as reagent, not adding other solvents and short reaction period.

REFERENCES

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