(221a) 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)

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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