Effect of acetylation on pretreated wood flour of four different wood species, Boabab (Adansonia digitata), Mahoganny (Daniella oliveri), African locust bean (Parkia biglobosa) and Beech wood (Gmelina arborea), had been investigated. The first batch of wood species were acetylated using acetic anhydride while the second batch were acetylated with commercial vinegar. Both experiments were conducted in the presence of varying amount of CaCl2 as catalyst and at temperature of 120°C for 3 h. The success of acetylation was determined based on Weight Percent Gain for each sample treated with either chemicals used. FT-IR, a veritable tool was used for the analysis of both treated and untreated samples to further investigate the success of acetylation. The results showed the presence of important band such as carbonyl absorptions at 1743, 1744, 1746, 1731, 1718 and 1696 cm−1 as appeared separately in the spectra of acetylated samples, confirming esterification occurred. The purpose of this work was to investigate the applicability of vinegar for acetylation of lignocellulosic fibers. Blends/composites were prepared by solution casting and their kinetics investigated in distilled water. The results indicated they could be used in outdoor applications such as, decking and packaging.

1. Introduction

Composites have been described as materials composed of a hard material with discontinuous reinforcement that is embedded in a weaker, continuous matrix. Where the reinforcement matrix [1] maintains the position and orientation of the reinforcement. The constituents of the composites retain their individual, physical and chemical properties. Composite give a combination of qualities that are very different from the individual constituents that constituted the composite. Several reports on thermoplastic composites have been documented. Different types of modified and unmodified natural fibers such as wood fibers and flour, kenaf fibers, sago, rice starch, cornstarch, henequen fibers, and pineapple-leaf fibers, have been used as fillers in polymer matrices [1, 2].

Dimensional stability and strength of unmodified wood flour polyethylene composites was reported to have improved by increased in fiber loading(s). Unmodified starch have been used to produce composites of low density polyethylene (LDPE). Composites of unmodified starch have been reported to exhibit low mechanical properties, though with improved biodegradation. The introduction of ester groups unto starch surface, manipulate its properties and support the blending mechanism [3]. Wood-Plastic composites have been described as products form by combining treated or untreated wood in the form of fiber or particles and a thermoplastic polymer such as polyethylene, polyvinyl chloride, or polypropylene [4]. Wood-plastic composites have significant applications in various fields such as construction/engineering. For wood as a construction material, its use is impaired under certain degradative agents due to its hydrophilic nature, presence of cellulose, a food substrate for microorganisms. For this reason, ecofriendly methods to improve the performance of wood are been developed. All over the world, efforts has been made to move away from toxic to nontoxic means of wood modification.

Chemical modification of wood is a very good alternative to conventional wood preservation methods. Acetylation has received considerable attention more than any other chemical modification technique known [5]. This method not only protects wood/wood-based products against degradative agents, but also changes the hydrophilic nature of the wood components (cellulose, hemicellulose, and lignin) to hydrophobicity by increasing the acetyl contents in the polymers while decreasing their –OH groups. This, therefore, enhances the surface energy and interfacial interaction between acetylated wood-dust and the hydrophobic polymers. It has been reported that, for every acetyl gain, one –OH group is eliminated. Implying that there is a reduction in hydrogen bonding within the wood reactive polymers, this can increase the interfacial bonding of PE with the acetylated wood surface [6]. Since the surface qualities plays a significant role to satisfactory interfacial performance, we, therefore, acetylate four different wood flours of four wood species grown in Nigeria using acetic anhydride and as well investigate the feasibility to use commercially available vinegar for acetylation of cellulose in lignocellulosic materials because of its low cost, availability, and renewability. Acetylation of cellulose in wood flour was carried out in the presence of varying amounts of CaCl2 as a catalyst to manipulate its properties in order to support the blending mechanism. The objectives of this work were to investigate the feasibility of vinegar for acetylation, preparation of blends/composite, and investigation of their sorption kinetics in distilled water for application in industry.

2. Materials and Methods

Wood flour used was from four wood species Baobab (Adansonia digitata), mahogany (Daniella oliveri), African locust bean (Parkia biglobosa), and Beechwood (Gmelina arborea) collected from a saw mill located in the Niger state Nigeria. Reagents and chemicals used were obtained from May & Baker, and these include commercial vinegar, acetic anhydride, NaOH, ethanol, toluene, sulphuric acid, hypochlorite, and CaCl2. All reagents used were of analytical grade.

2.1. Soxhlet Extraction

To reduce the influence of wood extractives on the acetylation, 5 g of 2 mm size wood particles was extracted with a mixture of ethanol-toluene (2 : 1 v/v) for 3 h. After extraction, samples were rinsed with ethanol followed by hot water and then oven-dried at for 24 h to reach a constant weight. The extractible content was calculated as a percentage of oven-dried test samples.

2.2. Sample Pretreatment by Mercerization

Mercerization of the 2 g oven-dried extracted sample was mercerized using 20 mL of 10% NaOH solution. The mixture was placed on a shaker and shaked at 75 rpm for 1 h. Afterwards, the sample was washed with distilled water until it was neutral. The residue was oven-dried at for 1 h. The procedure was repeated for each wood flour sample.

2.3. Chemical Modification
2.3.1. Acetylation

Acetylation was carried out in batches. The first batch was conducted using acetic anhydride while the second batch was carried out using commercial vinegar as acetylating agents in the presence of varying amounts of CaCl2 as a catalyst. 2 g of oven-dried samples of wood species oven-dried at with a constant weight () were treated with the acetylating chemicals each with 0.5 and 1 g CaCl2 catalyst at during 3 h refluxing. After modification, the residues were thoroughly washed with distilled water until neutral and then oven-dried for 3 h. The extent of acetylation was calculated as weight percent gains (WPGs) based on the differences in oven-dried weight of the samples before and after modification () and after modification () according to the equation

2.3.2. Blending Procedures

The dissolution of high-density polyethylene (packaged water sachets) follows the method described by [7]. In brief, 1 g pieces of nonprinted portion were placed in a reaction flask and 30 mL of toluene was added. The contents was placed on a hot plate at , and a thermometer was inserted. The sachets swelled at and then rapidly dissolved at to give a clear solution. Blending of high-density polyethylene and acetylated wood flour is thus described. Into the clear solution of high-density polyethylene sachets was introduced 0.5/1.0 g of each vinegar/acetic anhydride treated or untreated wood flour while stirring for 5 min and then spread on clean metal plates to form a film. The thin films formed were peeled off the metal plates to afford acetylated wood flour/polyethylene blends [2].

2.4. Water Absorption Test of Acetylated and Untreated Blends

Three sets of specimens were prepared. Acetylated blends and untreated blends were subjected to water absorption test. The blends were soaked for 1, 2, and 16 h, respectively. After each immersion, the blends were removed and wiped off using filter paper and then weighed and weights were noted, using the formula where = initial weight of blend before immersion in water and = weight of blend after immersion in water.

2.5. Fourier Transform Infrared Spectroscopy Analysis

The properties of acetylated and untreated samples were characterized using FT-IR, Perkin-ELMER-8400S Spectrophotometer in the range 4000  to 400 . Samples were run using the KBr pellet technique at the National Research Institute for Chemical Technology (NARICT), Zaria, Kaduna-Nigeria.

3. Results and Discussion

The IR spectra of acetic anhydride or vinegar treated wood flour showing the major absorption bands are shown in Tables 1 and 2. The following strong absorption bands as reflected separately on the IR spectra of vinegar treated wood flour are characteristic absorptions of bonded –OH, and these include 3978–3339, 3477–3375, 3426–3360, 3907–3372, 3429–3387, and 3874–3111  [2, 8, 9]. The following peak intensities at 3029, 3104, 3345, 3602, 3352, 3852, and 3938  as reflected separately on the IR spectra of acetic anhydride treated wood flour are characteristic absorptions of bonded –OH group stretching vibrations in cellulose and hemicelluloses. Acetic anhydride or vinegar treated wood flour showed reductions in –OH absorption bands. The –OH absorption bands of untreated wood flour were predominantly detected at 4008, 4055, 4107, 4290, 4394, 4413, 4567, 4570, 4685, and 4696 . The intensity of the –OH absorption bands in the treated samples decreased. This decrease in the intensity of –OH band is an indication that the hydroxyl group contents in wood flour were reduced during reaction. Indicating that some level of acetylation had taken place [2, 9, 10]. The presence of –OH absorption in vinegar/acetic anhydride treated wood flour has been attributed to hydroxyl functionalities that are not accessible to chemical reagents [11]. The band at 1746–1731  as reflected on acetic anhydride treated wood flour is characteristic absorption of carbonyl (C=O ester) stretching vibration of acetate group in cellulose and uronic ester in hemicelluloses. This band showed evidence of acetylation [9, 1215]. The peak absorption increased in acetic anhydride treated samples. Indicating high level of acetyl gain as shown in Table 3. While vinegar treated samples showed this band at 1718  and 1696 . The low level of acetylation could be attributed to low-percent acetyl content in vinegar and shorter reaction time adopted for acetylation. Since vinegar treated Baobab sample gave 11.61% (WPG) during four-hour reaction unlike the other samples who gave less than 11.61% during 3 h reaction as reflected in Table 3. This implies that longer reaction times are required for acetylation using vinegar. 1696  carbonyl (C=O) absorption peak has been reported in ketene treated wood cellulose [2]. The peak here has been attributed to the low penetration of acetyl, consequently leading to lower percent gains. The absorption bands at 2723  and 2820  observed in vinegar treated samples have been assigned to C–H stretching vibrations of –CH3 and –CH2 groups in cellulose and hemicelluloses [2, 15]. The presence of absorption peaks at both 2938–2091  reflected on acetic anhydride and 2928–2904  in vinegar treated samples has been assigned to asymmetric stretching vibration of aliphatic –CH3 group which is evidence of acetylation [2, 8, 12, 16]. The small bands at 1628–1618 and 1629–1624  in acetic anhydride or vinegar treated samples are assigned to adsorbed water, β-glycosidic linkages (ether –C–O–C–) in the cellulose chain or sugar units or could be due to aromatic ring (C=C) vibrations in lignin (syringyl) [2, 912]. The absorption bands at 1527–1509  are characteristic to absorption of aromatic (C=C) stretching vibrations caused by lignin (guaiacyl) [2, 812]. This suggests the presence of lignin [9, 11]. The bands at 1439–1428  are due to C–H deformations and bending vibrations of –CH2 (methylene) in cellulose and hemicelluloses [2, 14, 15, 17, 18]. These bands were not found in acetic anhydride treated wood flour. The reason could be that longer reaction time adopted led to product (cellulose/hemicelluloses) degradation and subsequently their extraction by acetylating chemical occurred and was probably washed alongside with the byproducts. An important band which showed that some level of acetylation was achieved by the two reagents used for acetylation was observed at 1330–1373  for acetic anhydride and 1340–1348 for vinegar. This important band was attributed to aliphatic C–H deformation/bending vibration of –CH3 in the acetyl groups and this is evidence of the formation of ester bond due to acetylation in cellulose and hemicelluloses [2, 9, 10, 13]. This absorption peak was missing in the control samples. A prominent absorption band which further gave evidence of acetylation appeared at 1285–1242  for acetic anhydride while 1254–1247  peak absorption was detected for vinegar as alternative acetylation agent. This band has been assigned for the stretching of C–O and the deformation of C=O in the acetate bond formed during acetylation in xylan and lignin [8, 9, 13, 15]. The absence of absorption band at 1800–1760  in all the spectra of acetic anhydride treated samples was an indication that the acetylated products were free of unreacted acetic anhydride, implying that the reagent was used up during the reaction, while the absence of absorption peak at 1700  in all spectra is evidence of the absence of unreacted carboxylic acid in vinegar treated samples and as byproducts in acetic anhydride treated wood flour, respectively. The bands at 1044–1042  are assigned to C–O stretching vibrations in cellulose, hemicelluloses, and that of primary alcohol [2, 9, 10, 12]. The medium band at 1052  ascribed to C–O stretching vibration of the acetyl group in acetate [2, 12, 14] is evidence of acetylation.

3.1. Water Absorption Kinetics of Acetylated and Control Wood Flour Samples

Three sets of specimens were prepared, as shown in Table 4. Blends prepared from acetylated wood flour were flexible, brittle, and very smooth and absorbed less water compared to untreated blends which were coarse with rough surface. The modification of fiber surface really helps to manipulate its properties and supports the blending mechanism as was observed on blends made from treated wood flour. It has been established that when the accessible hydroxyl groups in the cell wall polymers have been substituted by acetyl group, reduction in water and moisture sorption are observed, and this depends on the level of acetylation too. The results also showed that water absorption of blends is dependent on fibers in the matrix. It is expected that the sorption rate for 16 h soaking duration should give about 16 times the results obtained for 1/2 h soaking periods. However, this was not observed, indicating that the fibers in the polymer matrix have reached their saturation point and thereby cannot absorb much water molecules. Acetylated blends reached the fiber saturation point faster compared to untreated blends as indicated by the percent water uptake/sorption rate in Table 4. This is attributed to the modification of the hydrophilic fiber surface by acetyl moieties and subsequent hydrophobicity improvement of fiber surface which allowed for proper interfacial bonding with polyethylene.

3.2. Results of the Formation of Blends

Blends obtained from the incorporation of acetylated wood flour with polyethylene were flexible and had smooth surface (Figures 1(a) and 1(b)). These blends were easily peeled off the film forming plate while those obtained from untreated wood flour were porous, coarse, and difficult to peel off from the film forming plate. For this reason, blends had rough surface (Figure 1(c)). Poor blend formation exhibited by untreated wood flour could be attributed to facial differences between the two materials—hydrophilic biopolymers in wood flour and the hydrophobic polyethylene, causing very poor interfacial interaction between the materials [2]. Treated wood flour resulted in the formation of flexible blends with smooth surface after incorporation and solution casting on metal plates. The properties exhibited by these blends was an indication that acetylation using vinegar and acetic anhydride successfully transformed accessible hydrophilic hydroxyl surfaces of the biopolymers in wood flour to hydrophobic surface (acetyls) which causes proper interfacial interaction with polyethylene as reported by [2, 19, 20].

4. Conclusion

In this work we have been able to carry out acetylation of cellulose in wood flour using vinegar successfully. Results of the kinetic studies of blends indicated that treated blends had lower water sorption than untreated blends, implying that treated blends had lower hygroscopic surface with a high hydrophobic energy surface that facilitated its interaction with polyethylene during the blending process, while untreated blends had high water sorption, due to the presence of accessible hydrophilic hydroxyl groups in cellulose and other biopolymers in wood which allowed for strong formation of hydrogen bonds with water molecules. Treated wood flour blends could be very useful in outdoor applications such as decking and packaging.