Comparative study of the performance of acetylated bamboo with different catalysts

Abstract

The catalytic acetylation of bamboo (Phyllostachys pubescens) was compared with acetylation using concentrated sulfuric acid, acetic acid, potassium acetate, and noncatalytic acetylation at 120 °C for 3 h. The weight percentage gain, dimensional stability, color difference, and wettability of bamboo after the acetylation was comprehensively measured. Also, the chemical and thermal properties of the resultant bamboo were characterized by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis. The results showed that the potassium acetate-catalyzed acetylation of bamboo greatly accelerated the reaction degree, had little effect on color change, extraordinarily decreased wettability, and had little irregular impact on the dimensional stability. The catalytic activity was followed by potassium acetate, sulfuric acid, noncatalytic acid, and acetic acid. The FTIR analysis showed that the functional groups in the acetylated bamboo were mainly affected by different catalysts. The thermal stability of acetylated bamboo was higher than the untreated bamboo. In particular, potassium acetate-catalytic acetylation greatly reacted with -OH groups and increased thermal decomposition.

Full Article

Comparative Study of the Performance of Acetylated Bamboo with Different Catalysts

Saisai Huang, Zhongqing Ma,* Yujing Nie,* Fengzhu Lu, and Lingfei Ma *

The catalytic acetylation of bamboo (Phyllostachys pubescens) was compared with acetylation using concentrated sulfuric acid, acetic acid, potassium acetate, and noncatalytic acetylation at 120 °C for 3 h. The weight percentage gain, dimensional stability, color difference, and wettability of bamboo after the acetylation was comprehensively measured. Also, the chemical and thermal properties of the resultant bamboo were characterized by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) analysis. The results showed that the potassium acetate-catalyzed acetylation of bamboo greatly accelerated the reaction degree, had little effect on color change, extraordinarily decreased wettability, and had little irregular impact on the dimensional stability. The catalytic activity was followed by potassium acetate, sulfuric acid, noncatalytic acid, and acetic acid. The FTIR analysis showed that the functional groups in the acetylated bamboo were mainly affected by different catalysts. The thermal stability of acetylated bamboo was higher than the untreated bamboo. In particular, potassium acetate-catalytic acetylation greatly reacted with -OH groups and increased thermal decomposition.

Keywords: Bamboo; Acetylation; Catalyst; Dimensional stability; Color; Wettability

Contact information: School of Engineering, Zhejiang Provincial Collaborative Innovation Center for Bamboo Resources and High-Efficiency Utilization, Zhejiang A & F University, Lin’an district, Hangzhou City, 311300 Zhejiang Province, China;

* Corresponding authors: mazq@zafu.edu.cn; nieyujing@zafu.edu.cn; malingfei@zafu.edu.cn

INTRODUCTION

With the implementation of natural forest protection policy, the application of bamboo has become an important way to fill the gap between wood supply and demand (Chen et al. 2018a;Zhang et al. 2018). Compared to woody material, bamboo has the advantage of a shorter growth period after harvesting, stronger physical and mechanical properties in the longitudinal direction, and higher rigidity and wear resistance. Therefore, bamboo has been widely used for household utilities, as a composite material, decoration material, or temporary construction material. However, bamboo material suffers from poor dimensional stability and is susceptible to fungi and insects because of the abundant free hydroxyl groups in the cellulose and hemicellulose of bamboo (Ma and Ma 1997; Yang et al. 2008; Chen et al. 2018b). Currently, several methods are being carried out to improve the dimensional stability of bamboo, including mechanical, physical, and chemical methods. By the chemical method, several chemicals have been used, such as anhydrides, acid chlorides, ketene carboxylic acids, isocyanates, formaldehyde, acetaldehyde, and epoxides (Tarkow et al. 1950; Karlson and Svalbe 1977; Rowell 1975; 1982; Kumar 1994; Hon 1996; Hill et al. 2000; Hill 2007).

Acetylation treatment has the advantage of hypotoxicity, a fast reaction rate, and a simple reaction system, such that it has proven to be one of the most important chemical methods. During the process of acetylation treatment, the hydroxyl groups are converted into acetyl groups by the esterification reaction, which results in lower equilibrium water content and significantly improved dimensional stability (Ohkoshi and Kato 1997). Brelid et al. (2000) reported that acetylation using a limited amount of acetic anhydride in a liquid phase could greatly improve resistance to tunneling bacteria and decay fungi. Furthermore, Ohkoshi (2002) stated that the color of acetylated softwood changed less than that of the untreated softwood after up to 1000 h of exposure to light. Although these previous studies obviously improved the dimensional stability and resistance to biodegradation, the conventional acetylation treatment had some problems with the accessibility of the reagent to the cell wall, which would greatly decrease the reaction rate of acetylation. Meanwhile, a series of complicated separation procedures to recover the chemicals was required after acetylation treatment (Li et al. 2007). Therefore, a certain extent of modification of acetylation, such as the addition of catalysts, is essential.

Rowell (2006) reported that the most commonly used catalysts in the acetylation treatment were weakly alkaline chemicals or neutral chemicals. Because these series of chemicals could promote the swelling efficiency of the cell wall matrix structure, the reagents to form more acetyl groups could be penetrated in the cell wall. Former studies compared the performance of the acetylation wood by different catalysts, such as zinc chloride, potassium acetate, sodium acetate, acetic acid, magnesium persulfate, and trifluoroacetic acid. Obataya and Minato (2008) indicated that the potassium acetate-catalyzed acetylation of wood could significantly improve the reaction rate with minimal influence on the mechanical properties. Yang et al. (2008) showed the acetylated bamboo pulp, catalyzed by sulfuric acid and a solvent with acetic acid, had higher acetylation activity at a low temperature. Li et al. (2009) stated that the acetylation of wood catalyzed by magnesium chloride showed very little or no effect on the color change or the mechanical properties of the wood. Based on the review of the above references, relatively little information was found to compare the performance of acetylated bamboo with the catalyst of sulfuric acid, acetic acid, and potassium acetate.

The key objective of this work was to investigate the difference in performance of the acetylated bamboo with three types of catalyst, i.e., concentrated sulfuric acid, glacial acetic acid, and potassium acetate. The weight percent gain, dimension stability, color and wettability, functional groups, and thermal properties of acetylated bamboo were tested.

EXPERIMENTAL

Materials

Bamboo was collected in the mountains of the Lin’an district, Hangzhou, Zhejiang province, China. The bamboo was cut into chips of certain dimensions, and the outer and inner skin of bamboo was removed. The sizes of bamboo chips were mainly divided into two sets: one set with dimensions of 50 mm × 20 mm × 5 mm (L × R × T) for the color change and wettability tests, and another set with dimensions of 20 mm × 20 mm ×5 mm (L × R × T) for the dimensional stability testing. Each set had 6 replicates for the test.

The chemicals used in this experiment were analytically pure. Acetic anhydride [(CH₃CO)₂O] was bought from the Yonghua Chemical Reagent Plant (Jiangsu, China), concentrated sulfuric acid [H₂SO₄] and potassium acetate [CH₃COOK] were bought from the Sinopharm Chemical Reagent Plant (Shanghai, China), and glacial acetic acid [CH₃COOH] was bought from the Tianjin Yongda Chemical Regent Company (Tianjin, China).

Experiment of bamboo acetylation

Bamboo acetylation was performed in a vacuum container. The schematic diagram of this container is shown in Fig. 1. This container was made of stainless steel. The rated capacity, pressure, and temperature was 50 L, 1.5 MPa, and 220 °C, respectively.

Fig. 1. Schematic of vacuum container for bamboo acetylation

The experimental scheme of bamboo acetylation using different catalysts is shown in Fig. 2. This experiment was divided into five groups, and twelve samples were used in each group. Each experiment was done three times. Before acetylated treatment, each sample was oven-dried at 105 °C for 5 h. The control sample (CN groups) was oven-dried at 105 °C for 5 h without any acetylated treatment.

For the sulfuric acid as a catalyst (SA groups), the samples were first vacuumed at 0.098 MPa for 30 min in the container. Then, a limited amount of glacial acetic acid was added into the container at room temperature and kept for 2 h under atmosphere pressure. Next, the acetic acid was removed at the outlet from the bottom of the container. Afterwards, a solution containing acetic anhydride (98 parts) and concentrated sulfuric acid (2 parts) was added into the container. Then, the container was heated to 120 °C under atmosphere pressure and kept for 3 h.

For the acetic acid as a catalyst (AA groups), the samples were first vacuumed for 30 min (ca. 0.098 MPa) in the container. Then, the samples were soaked in a solution that contained 7 wt% acetic acid and 93 wt% acetic anhydride for 2 h under atmosphere pressure. Next, the container was heated to 120 °C and held for 3 h.

For potassium acetate as a catalyst (PA groups), these bamboo chips were primarily soaked in the solution of 15 wt% potassium acetate (KAc) under atmosphere pressure for 12 h, then air-dried for another 12 h. After being oven-dried for 5 h, these samples were placed into the container and vacuumed for 30 min (ca. 0.098 MPa). Then, the acetic anhydride was introduced to the container, and the bamboo samples were submersed for 2 h at room temperature. Afterwards, the container was heated to 120 °C and kept for 3 h.

For the no catalyst (NC groups), these samples were first immersed into acetic anhydride under vacuum conditions (ca. 0.098 MPa) for 30 min. In the next phrase, the bamboo samples were soaked in acetic anhydride under atmosphere pressure for 2 h. After that, the container was heated to 120 °C and kept for 3 h.

After acetylation, the cooling device was turned on, and the container was cooled to 50 °C. The acetylated bamboo specimens were leached with running water for 24 h to remove the unreacted acetic anhydride or by-product as acetic acid. Then, the acetylated bamboo was vacuum-dried at 0.098 MPa until constant weight was achieved. The drying method was 60 °C for 48 h, 80 °C for 72 h, and 105 °C for 48 h (Wu et al. 2018).

Fig. 2. Experimental scheme of bamboo acetylation using different catalysts

Evaluation of Catalytic Acetylated Bamboo

Weight percentage gain

Prior to the experiments, both the treated and untreated bamboo samples were dried at 105 °C for 5 h to a constant weight, and then the weight was measured. When all of the chemical treatments were finished, the weights of the acetylated bamboo samples were measured again. The weight percentage gain (WPG) was calculated according to Eq. 1,

WPG (%) = (M₁ – M₀ ) / M₁ × 100 (1)

where WPG is the rate of weight gain (%), M₀ is the initial dry weight of the sample (g), and M₁ is the weight of treated sample (g).

Dimensional stability

Dimensional stability of the acetylated bamboo was characterized by water absorption and the volume swelling efficiency. The water absorption and the volume swelling efficiency were evaluated according to Chinese National Standard GB/T 1931 (2009) and GB/T 1934.2 (2009). The samples were first submersed into water at 25 °C for 72 h, and the increasing rate of volume and weight were calculated. Then, the samples were oven-dried at 60 °C for 2 h, 80 °C for 2 h, and 103 °C for 4 h to a constant weight, and the decreasing rate of volume and weight were obtained. The steps of water-soaking and oven-drying were repeated for three times. The water absorption and the volume swelling rate were calculated using Eqs. 2 and 3,

WAR (%) = (W_n – W_i) / W_i × 100 (2)

VSE (%) = (V_n – V_i) / V_i × 100 (3)

where WAR is the water absorption rate (%), W_i is the initial weight of the sample (g), W_n is the weight of the sample after being immersed in water (g), VSE is the volume swelling efficiency (%), V_i is the original volume of the sample (m³), and V_n is the volume of the sample after being immersed (m³).

Color measurements

The lightness, redness, and yellowness of the selected points were measured with a colorimeter (DC-P3 Automatic Color Difference Meter, Beijing Xingguang Color Measuring Instrument Co., Ltd., Beijing, China) before and after acetylation treatment according to the CIELAB color standard system (L*, a* and b*). The differences in parameters ∆L*, ∆a*, ∆b* and the total color differences ∆E* were calculated using Eq. 4,

(4)

where ΔE* is the total color difference, L* is the lightness, and a* and b* are the color coordinates under any test condition. The ∆ value represents the differences between the initial and final parameters of the specimens after the modification.

Wettability

The contact angle (θ) of bamboo was measured by the drop shape method at 20 °C in a static drop contact angle and interfacial tension tester (OCA 200, DataPhysics, Filderstadt, Germany). A drop of distilled water (3 mL) was laid upon a bamboo strip using a micro syringe and photographed by the trendsetting 10-fold zoom lens and the high-performance camera with USB 3.0 interface. The contact angle of the droplet with the solid surface was recorded at 0.5 s. At least 10 measurements were made at different inner skin and outer cuticle positions on each group of bamboo specimens.

Fourier transform infrared spectroscopy (FTIR) analysis

The FTIR spectra of the sample were tested by a Nicolet 6700 (Shimadzu Corporation, Kyoto, Japan) spectrometer. First the modified and untreated bamboo specimens were ground into powder separately and passed through a 200-mesh screen. After the samples were oven-dried at 105 °C, 32 scans were accumulated for each spectrum at a resolution of 4 cm⁻¹ in the region from 4000 cm⁻¹ to 500 cm⁻¹ using KBr pellets (1 mg power of samples/100 mg KBr).

Thermogravimetric (TG) analysis

The thermal stability of bamboo was tested using a Thermogravimetric analyzer (TG-209, NETZSCH, Bavaria, Germany). Approximately 5 mg of bamboo powder was placed in an Al₂O₃crucible. The carrier gas and protective gas were high-purity nitrogen with a flow rate of 20 mL/min and 20 mL/min, respectively. The temperature was set from 30 °C to 600 °C with a heating rate of 10 °C /min.

RESULTS AND DISCUSSION

Figure 3 shows the differences in the weight percentage gain of acetylated bamboo with different catalysts. The results showed that the PA groups exhibited the highest value of WPG (19.59%) followed by SA (16.89%), AA (14.18%), and NC (17.44%). These results indicated that a higher content of acetic anhydride penetrated the bamboo cell walls when the KAc catalyst was used. This result was in line with what was reported by Obataya and Minato (2009). They reported that the reactivity of cellulose was powerfully improved by the presence of KAc, thus effectively accelerating the acetylation of wood. Moreover, water-soluble KAc can be easily introduced into the bamboo cell wall. This process can easily accelerate the penetration of acetic anhydride into fibers.

In the presence of acetic acid dissolved into acetic anhydride, the acetylated bamboo did not experience a remarkable WPG in comparison with the other catalysts. This may have been due to the use of 7 wt% acetic acid, which, like a solvent, reduced the reaction rate because of dilution of the modifier. It can be explained that acetic acid used in AA group played the role as a solvent to reduce the reaction rate. And less acetic anhydride could be reacted with hydroxyl in the bamboo. This outcome was similar to the findings of Yao (2012), who indicated that the bamboo strips also obtained a 13.6% WPG by the catalyst of 8% acetic acid at 130 °C.

Fig. 3. The weight percentage gain of acetylated bamboo using different catalysts

Chen et al. (2011) showed that the catalysis of sulfuric acid had greatly activated the hydroxyl groups on the cellulose and lignin, which meant their reaction degree was high. This differs slightly from the findings presented in this study. Here it is reasonable that sulfuric acid used in SA group would deposit on the surface of rayon fibers which retarded the penetration of acetic anhydride into the fibers. Therefore, this would reduce the reaction rate. In contrast, at 120 °C without the presence of a catalyst, acetic anhydride reacted with bamboo and showed higher reactivity. Jebrane et al. (2011) indicated that acetylated wood can obtain high WPG without a catalyst. The present study showed a similar result.

Dimensional Stability Analysis

The dimensional stability of acetylated bamboo was evaluated by the WAR and the VSE. Figure 4 shows the water absorption rate and the volume swelling efficiency under different catalysts. The results showed that the acetylated bamboo (SA, AA, PA, and NC) had a higher water resistance capacity and lower VSE than the untreated bamboo (CN). The results revealed that the acetylation treatment decreased the hydrophilicity in bamboo as the number of hydroxyl groups were reduced and the acetyl groups in bamboo were inflated (Rowell 1975).

Fig. 4. The WAR (a) and the VSE (b) of control sample and acetylated bamboo using different catalysts

As shown in the Fig. 3, the WPG values of different acetylated bamboo were PA (, P<0.05), NC (, P<0.05), SA (, P<0.05), AA (, P<0.05), respectively. No remarkable difference was observed in the WAR among the three different catalysts. Although the KAc-catalyzed acetylation of bamboo had the highest WPG, its WAR value was the lowest. This suggested that KAc had a negligible effect on the water absorption of the acetylated bamboo. This finding was consistent with that of Obataya and Minato (2008), who stated the effects of uncatalyzed and KAc-catalyzed acetylation were not significantly different at the same WPG value.

However, it was obvious that the KAc-catalyzed specimens had a slightly lower VSE value compared to the other acetylated specimens. Such partial acetylation may have resulted in easy access to the inner surface of the cell wall for KAc, while the cell wall was bulked by acetyl groups due to the efficiency of the KAc catalyst.

It was apparent that the SA groups exhibited lower WAR and VSE values than other acetylated bamboo (AA and NC). Yang et al. (2008) stated that concentrated sulfuric acid showed remarkable activation in cellulose with the solution of glacial acid and thus increased the degree of acetylated substitution. This finding supported the work of Yang et al. (2008). The AA groups showed higher values of WAR and VSE in this work. Previous studies have demonstrated that acetic acid, as a weak acid, was unable to activate hydroxyl groups in the bamboo, which causes low WPG in acetylation. However, the NC group caused irregularly low dimensional stability with high WPG. It has been suggested that acetylated wood, without the use of a catalyst, has a high resistance because of the covalent bonds being formed between wood and anhydride (Li et al. 2000). This did not appear to be the case when compared to other catalytic acetylation.

Through three soaking-drying tests, the values of all groups decreased remarkably after the first sopping because the extractives were solved in water during the sopping process. The second and third tests saw the values of bamboo specimens steadily decline, which might have been related to the breakage of the water sensitive weak bonds and the swelling of unmodified bamboo cell walls. More esterification may have been required to obtain a higher resistance.

Color Analysis

Figure 5 shows the color variation of acetylated bamboo under different catalysts. The vertical 0 represents the color of the untreated bamboo. Some published studies have indicated that acetylated wood undergoes a slight color change. Compared with the untreated samples, group AA had a slight color change, because the brightness (∆L*) and red index (∆a*) showed little change. The noncatalytic treatment resulted in more dramatic color changes in all of the samples, mainly a noticeable decrease in ∆L* and a noticeable increase in ∆a*. This showed that NC groups tended to turn darker and redder after treatment. The green index (∆b*) of KAc catalytic bamboo specimens increased the most, which led the treated bamboo to become yellow. Meanwhile, the degree to which ∆L* decreased was similar to the increase of ∆a*. The sulfuric acid-catalyzed acetylation of bamboo had a decrease in ∆L* and an increase in ∆a* and ∆b*, which resulted in an increase in color difference (∆E*).