Materials and methods Procedure

2. Materials and methods

Ground wheat straw was obtained from Natu- ral Fiber Board Inc. Minneapolis, KS. The com- mercial wheat straw had been ground to a particle size range of 1.9 cm to dust with the nodes and residue grain removed. Reagent grade acetic an- hydride was obtained from Aldrich Chemical Company Inc. Milwaukee, WI. A diphenyl- methyane diisocyanate resin binder, Rubinate 1840, was obtained from ICI Polyurethanes Geismar, LA. 2 . 1 . Acetylation process A process flow diagram of the acetylation sys- tem used in this research is presented in Fig. 1. Air was passed through drierite to remove mois- ture, the dried air then flowed through a flow meter, which regulated and measured the flow rate. The dry air then was bubbled through acetic anhydride AA in two saturation bottles in series that were housed in a constant temperature oven. The AA saturated air then flowed into the top of a 2-l glass reactor vessel also in the oven that contained ground wheat straw. Finally, the air stream with chemical residues exited the bottom of the reactor vessel, and then the oven and was neutralized by passing through a scrubber con- taining an aqueous sodium hydroxide solution. 2 . 2 . Experimental design Response surface methodology with a central composite design CCD was used in this study. The four variables were reaction time, reaction temperature, initial moisture content of the straw, and air flow rate, and the response was the extent of acetylation as determined by add-on weight calculated on a dry basis. The levels of each variable entered into the central composite design matrix are listed in Table 1. The straw weight gain response was analyzed with Statistical Analysis System software SAS, 1992, and the RS-reg function was used to develop a model equation. The standard deviations SD of each dependent variable at the centrepoint of the CCD was re- ported as the SD for all data analysis.

3. Procedure

The moisture content MC of the straw was adjusted to the desired level two days before acetylation, and the straw was sealed in a plastic bag. For the experiment, the reactor vessel was filled with 150 g dry weight basis of MC-ad- justed straw. The vessel and the saturation bottles containing the AA were placed in the preheated oven. The air flow was set at a given rate and continued for the reaction time and then the saturation bottles were removed from the oven, Fig. 1. Process flow diagram of the straw acetylation system. Table 1 Process variables levels of the response surface central composite design Process variables Levels − 1.5 − 1 + 1 + 1.5 5 10 Moisture content 15 2.5 17.5 Oven temperature °C 80 72.5 95 110 117.5 1 2 0.5 3 Reaction time h 3.5 250 Air flow rate ccmin 500 1000 1500 1750 and the air flow passed directly through the reac- tor vessel. The oven temperature, then was dropped to 50°C, and the air flow rate was set to 500 ccmin to remove excess reagent and by- product and air flow was continued for 16 – 19 h. Sheen 1992 did preliminary studies on a 100 kgday pilot-scale acetylation process and found that a flow of air at 50°C through the fiber was the most efficient method to remove the excess reagents. The reaction vessel was removed from the oven, and the straw was collected and sealed in a plastic bag. Preliminary tests had shown that all the moisture was removed during acetylation. The straw moisture content then was adjusted to 7. The bag then was sealed, shaken, and allowed to equilibrate overnight. 3 . 1 . Strawboard preparation Rubinate 1840 binder 5 was mixed into the acetylated straw with a paddle mixer Hobart model N50. The resinated straw was pressed into boards using a 15.2 × 15.2 cm mold and a hot press Carver model 3889 auto C. A 15.2 × 15.2 cm × : 0.64 cm board was produced. The press conditions were 2.68 MPa 389 psi and 176.7°C 350°F for 3 min. One treatment procedure was performed per day. A total of six untreated con- trol strawboards were made with the same press conditions and reported as control samples having 0.0 weight gain. Because the compressibility of the acetylated straw was influenced by the degree of acetylation, the board samples had a range of thicknesses 0.6 – 0.9 cm. Samples were sanded both sides to a uniform thickness with a table belt sander. The 15.2 × 15.2 cm boards were sanded to 0.54 9 0.05 cm in thickness then a 15.2 × 4.4 cm sample was cut off the board. This sample was used in the humidity cycle test. The remainder of the sample was sanded to 0.49 9 0.02 cm in thickness and then cut into two 15.2 × 4.4 cm samples for test- ing of mechanical properties. It was assumed that boards with the initial thickness of 0.6 – 0.9 cm would have a relatively uniform cross-sectional density profile, and that sanding would not sig- nificantly influence board properties. 3 . 2 . Acetyl weight gain The amount of acetyl groups added to the straw during reaction was estimated by the weight gain during treatment, on a dry weight basis using Eq. 1, AC = AT − BT − WS × MC WS − WS − MC 1 where AC = weight gain, AT = weight of straw plus reactor vessel after treatment, BT = weight of straw plus reactor vessel before treatment, WS = weight of straw in reactor, and MC = moisture content of straw. The MC of the treated straw was assumed to be zero; therefore, all the weight gain during the treatment, calculated on a dry straw basis, was caused by acetylation. 3 . 3 . Equilibrium moisture content The equilibrium moisture content EMC of each sample was determined at 65 and 90 RH. The samples were stored for 3 weeks at 65 RH or 90 RH at 27°C, then were oven-dried for 2 h at 130°C, and EMC was calculated. Each point was the average of two 4.4 × 4.4 cm samples. 3 . 4 . Dimensional stability A humidity cycle test was used to determine the dimensional stability of the board samples. The relative humidity was cycled between 1 week at 90 and 1 week at 30 RH at 27°C. The sample length and thickness were measured by calipers, and the linear expansion and thickness swell were determined at each humidity transition for six cycles. Each value of percent linear expansion and thickness swell were the average of two measured points on one board specimen 15.2 × 4.4 × 0.54 cm. 3 . 5 . Mechanical properties Method D1037-93 ASTM, 1995 was fol- lowed for a 3-point flex test using an Instron universal testing machine with a crosshead speed of 5 mmmin and a 101.6 mm span. Modulus of rupture MOR and modulus of elasticity MOE then were calculated for each sample using equa- tions given in this method. Each value of MOR and MOE were the average of two board speci- mens 15.2 × 4.4 × 0.49 cm. 3 . 6 . Board density The samples were preconditioned at 65 RH and 25°C for 1 week prior to measurement. The board density of each sample was obtained by measuring the average thickness, width, and length with calipers to calculate board volume, and then dividing the mass of the sample board by the volume. The reported density is the average of the two board specimens from the mechanical properties test prior to testing 3 . 7 . Scanning electron microscopy The cross section of an individual piece of a straw was obtained by putting a wheat straw sample in a plastic drinking straw and filling it with ethanol. The ends of the drinking straw were clamped, and then the entire straw was dropped into liquid nitrogen. The frozen drinking straw then was cut with a razor blade, which produced a sharp cut normal to the direction of the straw fibers. This procedure was done with samples of untreated wheat straw and acetylated wheat straw sample treated to a 19 weight gain. Strawboard samples about one cm 3 made from the same two straw treatments were cleaned with distilled water in an ultrasonic cleaner, and then dried. All sam- ples were viewed with an E-Tech Auto Scan scan- ning electron microscope and micrographs were taken.

4. Results and discussion