Figure 4.1.3: Cellulose-water mixture just mixed (left) and aged for 24 h (right). When they are initially mixed, a suspension of cel- lulose particles in water is formed, then after 24 h, a translucent gel, as the cellulose has been fully hydrated.
Some problems with de-watering and in- homogeneity in the extrusion mix were ob- served during extrusion. This is partly re- lated to the kinetics of the water absorption of cellulose. It has been shown that the hy- dration of hydroxyethylcellulose is gradual process with a timescale of hours - it will absorb 5.5 times its own mass in water, swelling considerably, over a period of 10 h, forming a viscous gel [11]. Therefore, it is important to allow the cellulose time to hydrate and come to an equilibrium with the water when preparing an extrusion mix- ture. It was hypothesised that the best extrusion mixture might be produced from cellulose that had sufficient water to form a gel, but no more water, as this would pre- vent de-watering during extrusion. In order to determine what the minimum cellulose- water ratio was to form a gel, a series of
compositions of water and cellulose were examined. Table 4.1.2 shows the different com- positions and results obtained after 24 h. Some compositions formed a gel, proving the necessity of ageing the extrusion mix before extrusion in order to allow the water to hy- drate the cellulose and come to equilibrium with it. Fig. 4.1.3 compares a cellulose-water mixture that has just been mixed, with one aged for 24 h. Previous compositions that had been extruded immediately after being mixed had shown signs of de-watering, as the water had not yet been absorbed by the cellulose.
cellulose water water:cellulose observations (g) (g) (mass ratio) (after 24 h) 0.427 0.244 0.57:1 3/4 of cellulose still dry
0.407 0.297 0.73:1 3/4 of cellulose wet
0.409 0.412 1.01:1 3/4 cellulose wet
0.408 0.506 1.24:1 nearly all cellulose wet 0.423 0.652 1.54:1 1/2 wet cellulose,1/2 gel
0.427 0.786 1.84:1 cloudy gel formed 0.407 0.800 1.97:1 cloudy gel formed 0.409 0.900 2.20:1 cloudy gel formed 0.408 0.986 2.42:1 cloudy gel formed 0.423 1.304 3.08:1 cloudy gel formed
Table 4.1.2: Cellulose-water absorption experiment. All compositions were aged for 24 hr. The last five compositions were made up from the first five, by adding more water. The total water added is shown in the table.
Above a mass ratio of 1.84:1 (water:cellulose), there is enough water to form a gel with the cellulose, and at 1.53:1, there is insufficient water. Assuming that the optimum amount of water in an extrusion recipe is that which just hydrates the binder, the best water:cellulose ratio for an extrusion recipe is between 1.54:1 and 1.84:1. This was tested by preparing extrusion mixes with varying water:cellulose ratios, and ageing them overnight.
LSM cell. water water:cell. sintering shrinkage water water molecules: (g) (g) (g) (mass ratio) (longitudinal, %) (vol.%) cell. unit 5.138 0.511 1.376 2.69:1 22 45.8 8.5-10.6 5.348 0.511 1.148 2.25:1 23 40.9 7.1-8.9 5.558 0.511 0.942 1.84:1 17 35.7 5.8-7.3 5.768 0.511 0.840 1.64:1 17 32.7 5.2-6.5 Table 4.1.3: Variation of the cellulose:water mass ratio in extrusion mixes, with longitudinal tube shrinkage during sintering at 1200 ◦C for 1 h. Cell. = 2-hydroxyethylcellulose. The
possible range of the the number of water molecules per repeating unit of the cellulose is also shown. The range depends on the degree of substitution of the cellulose hydroxyl groups, which is assumed to be between 1 and 3 hydroxyethyl groups per repeating unit, giving the range shown here.
Table 4.1.3 shows the different compositions, and fig. 4.1.4 shows the tubes produced by extruding them. It can be seen that the best tubes are produced with a water:cellulose ratio of 1.64:1, i.e. just enough water to hydrate the cellulose and form a gel, but no more. As the water content increases, there is more free water in the mixture not bound to the cellulose, so the stiffness decreases, and de-watering may occur.
4.1. EXTRUSION WITH HANDHELD RAM EXTRUDER 129
Figure 4.1.4: LSM tubes extruded from the compositions in table 4.1.3. The water: cel- lulose mass ratio is shown in the picture on each tube. As the water content reduces, the tubes are smoother, and have less holes in the surface.
made by substituting hydroxyethyl groups for some of the hydroxide groups on the cellulose.
Figure 4.1.5: 2-hydroxyethylcellulose repeat- ing unit. 2-hydroxyethyl cellulose is a derivat- ive of cellulose made by substituting hydroxyl groups with hydroxyethyl groups.
Fig. 4.1.5 shows the repeating monomer in 2-hydroxyethylcellulose, which has a vary- ing molecular mass according to the de- gree of substitution by the hydroxyethyl group. Assuming that between 1 and 3 of the hydroxyl groups are substituted, the range of the molecular mass of the 2- hydroxyethylcellulose repeating monomer may be calculated. If a water: cellu- lose mass ratio of 1.64:1 is also assumed, then it can be determined that there are between 5.2 and 6.5 water molecules per 2- hydroxyethylcellulose repeating monomer. As there are three substituent groups on
each monomer, either -OH or -O-CH2CH2OH, this means there are about 2 water mo-
lecules per OH group. It is suggested that this water is tightly bound to these groups, and any further water is more loosely bound, which reduces the viscosity of the cellulose-water mixture. A study of a mixture of hydroxyethyl cellulose and carboxymethyl cellulose used DSC (differential scanning calorimetry) and solid state NMR to study the position of water after hydration. DSC measurements looked at the proportion of non-freezable water, i.e. the water that is tightly bound to the functional groups on the cellulose. It found that there were 5.8 water molecules per repeating unit of the cellulose, which form the inner solvation shell. NMR measurements indicate that there are 6-9 water molecules bound tightly to each repeating unit of the polymer, and that if more water was added, it is bound loosely [12]. These values are in good agreement with the results obtained from the cellulose-water absorption experiments. Table 4.1.3 shows the calculated number of water molecules per repeating unit of the cellulose, assuming that each repeating unit is substituted with between 1 and 3 hydroxyethyl groups. It was observed from fig. 4.1.4 that
the tube with a water:cellulose ratio of 2.69 lengthened under its own weight as it came out the extruder, to such an extent that gaps appeared in the tube. This indicates that at a water:cellulose ratio of 2.69, there is excess water in the mixture, that is not bound. The range of water molecules per cellulose repeating unit for this composition is 8.5-10.6. As some of this water is loosely bound, the true number of tightly bound water molecules per repeating unit must be less than this range. Therefore, appropriate cellulose-water ratios in extrusion mixes may be predicted by considering the ratio of hydroxyl groups in the cellulose, to the number of water molecules.