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Preparation of starch-graft-polyacrylamide copolymers by reactive extrusion.

Graft copolymers of starch and polyacrylamide (PAAm) were prepared by reactive extrusion using a co-rotating twin-screw extruder and ammonium persulfate initiator. Feed rates were 109 g/min to 325 g/min (all components) at a moisture content of 50%, with screw speeds in the range 100 rpm to 300 rpm. Starch/acrylamide weight ratios ranged from 5:1 to 1.3:1. Conversions of acrylamide to PAAm were generally 80% or greater with residence times of 400 seconds or less. Conversion increased with feed rate, suggesting that reaction efficiency was proportional to the degree of fill in the extruder. Grafting efficiencies were in the range of 50% to 80%. PAAm molecular weight increased with increasing acrylamide content, consistent with free radical polymerization kinetics. Extrusion temperature had no significant impact on acrylamide conversion. Graft frequency, as measured by the number of anhydroglucose units per graft, was essentially constant over the starch: acrylamide ratio and temperature range studied. These results show that reactive extrusion offers the potential for rapid production of starch graft copolymers with unsaturated monomers.

INTRODUCTION

Starch graft copolymers (SGCs) have been extensively studied because of their relative ease of production and useful properties (1). SGCs have typically been prepared using batch slurry processes, which preserve granule structure in order to facilitate production. These processes often require reaction times of 2 hours or more, and use water/starch ratios (10:1 or greater) that generate large quantities of waste water. Properties of SGCs are often improved if the starch is gelatinized prior to grafting, but this step makes production more difficult.

Reactive extrusion allows combination of several chemical processes into a single continuous operation (2) and offers an alternative approach to preparation of SGCs. Various modified starches have been prepared by reactive extrusion, including esters (3, 4), oxidized starches (5), ionic starches (6-11), and starchpolyester graft copolymers (12). Relatively little data have been reported on the reactive extrusion of SGCs using unsaturated monomers. Using a co-rotating twin-screw extruder (30-mm diameter, L/D = 43:1) Carr et al. prepared starch graft copolymers with acrylonitrile (13) and acrylamide (14) using ceric ammonium nitrate (CAN) as the initiator. For starch/acrylonitrile ratios of 2:1 and 1:1, conversions ranged from 55% to 78% (13). For starch:acrylamide ratios of 4:1 to 1:1.5, monomer conversion decreased from about 35% (4:1 ratio) to about 10% (1:1.5 ratio). The maximum graft content achieved was approximately 15% at the highest monomer content (14). Feed rates were in the range of 300 g/min to 400 g/min at moisture contents of 55% to 70%. De Graaf and Janssen used a counter-rotating twin-screw extruder (50-mm diameter, L/D = 6) to prepare starch-g-polystyrene copolymers with styrene levels of approximately 10 wt% with conversion rates up to 95% (15). Outputs ranged from 1.8 kg/h to 2.4 kg/h. Depending on extrusion parameters, initiator type, and monomer feed, total monomer conversions ranged from 23% to 95%, while monomer conversion to graft copolymer ranged from 1.7% to 50%. The highest graft contents were obtained when maleic anhydride was incorporated as a comonomer.

Graft copolymers of starch and polyacrylamide (PAAm) prepared by batch processing (slurries with [approximately equal to] 20% solids or less) have been reported, using either irradiation (16-20) or chemical initiation (21-26) such as ceric ammonium nitrate or the redox couple ferrous ammonium sulfate/hydrogen peroxide. In general, initiation by irradiation gave conversions of 75% to approximately 100%, grafting efficiencies of approximately 80%, and PAAm molecular weights in the range 5 x [10.sup.5] to [10.sup.6], while chemical initiation gave lower conversions (65% or less), lower grafting efficiencies (10% to 50%), and lower PAAm molecular weights (8,000 to 200,000). Significant levels of ungrafted homopolymer were usually generated, and conversions were often low (1). Hebeish et al. reported conversions of 98% or greater using starch slurries (40% solids) and potassium permanganate-ascorbic acid initiator with grafting efficiencies of approximately 45% (26). They reported no PAAm molecular weight data.

This paper presents data on the preparation of copolymers of starch and polyacrylamide using a co-rotating twin-screw extruder. Total feed rate, barrel temperature, and screw speed were the primary variables of interest. Conversion of monomer to polymer, graft content, grafting efficiency, and polyacrylamide molecular weight were determined.

EXPERIMENTAL

Materials

Starches were Pure Food Powder, an unmodified normal corn starch with approximately 23% amylose, and Waxy 7350, an unmodified corn starch with essentially no amylose (both provided by A.E. Staley, Decatur, IL). Prior to extrusion, starch was sifted through a U.S. standard 20 mesh screen to remove lumps. Moisture content of the starch was approximately 10%. Acrylamide (AAm), ammonium persulfate (APS), ethanol, and hydroquinone were obtained from Sigma Chemical Co. (St. Louis) and used as received. Deionized water was sparged with helium before use.

Extrusion Processing

Reactive extrusion was performed using a Werner & Pfleiderer ZSK30 co-rotating twin-screw extruder (Coperion Corporation, Ramsey, NJ). The barrel comprised 14 barrel sections, giving a length/diameter ratio of 44:1. The screw configuration is given in Table 1. Slotted mixing elements (TME and ZME) were used to enhance distributive mixing (27). TME (turbine mixing elements) are gear-type mixing elements, while ZME elements are single-flighted reverse (left-handed) elements with right-handed slots through the flight. Starch was fed into barrel section 1 of the extruder using a loss-in-weight feeder (Model 3000, AccuRate Inc., Whitewater, WI). The starch was sparged with nitrogen at 100 kPa for 48 hours prior to extrusion. During extrusion, the feed hopper, the nozzle and the feed throat were enclosed and purged with nitrogen.

Moisture content during extrusion was adjusted to 50% wt by injecting water (pH 8) into barrel section 2, simultaneously with the initiator solution (5 wt% ammonium persulfate, pH 8). A second initiator stream of equal flow rate was pumped into barrel section 8. Total APS content was 1 wt% based on starch solids, which corresponds to one sulfate radical per 70 anhydroglucose units (AGU). The monomer stream (50 wt% aqueous solution of acrylamide) was injected into barrel section 4 using a water-cooled injection nozzle. Liquids were pumped by triple-piston liquid metering pumps (Model BBB-4, Eldex Laboratories, Napa, CA).

The extruder feed throat (barrel section 1) was cooled with 5[degrees]C water. The remaining barrel sections consisted of 5 dual barrel section heating zones (2-3, 4-5, 6-7, 9-10, 11-12) and three single barrel section zones (8, 13, 14). Barrel temperatures for most extrusion runs were 80[degrees]C (barrel sections 2-3), 90[degrees]C (barrel sections 4-5, 6-7, 8), 110[degrees]C (barrel sections 9-10, 11-12), 100[degrees]C (barrel section 13), and 80[degrees]C (barrel section 14). Temperature effects were studied using flat profiles (same temperature for all barrel sections except the feed) at 50, 60, 70, 80, and 90[degrees]C. A die plate with 2 holes (4-mm diameter) was used. Melt temperature and pressure were measured at the die.

Approximately 25 grams of extrudate were taken at the die after extrusion parameters stabilized. The extrudate was dispersed in 200 ml of ethanol containing 0.5% hydroquinone in an explosion-proof Waring blender for 4 minutes to quench the reaction, followed by steeping overnight to remove unreacted monomer. Steeped extrudates were vacuum filtered and dried in a force air oven at 105[degrees]C for 1 hour. Polyacrylamide homopolymer was extracted by stirring 2.5 g of extrudate overnight in 200 ml of 30/70 ethanol/[H.sub.2]O (v/v) solvent. The insoluble fraction was separated by filtration and air dryed. The soluble fraction was collected after evaporation of the solvent.

Characterization

Nitrogen contents were measured using a Leco CHN2000 analyzer. Samples were analyzed in duplicate, using EDTA as a standard. Conversion, graft content, and grafting efficiency were calculated by the following equations:

Conversion (%) =[[100[N.sub.ext]/[[N.sub.f]]] (1)

Graft Content (%) = [[100[N.sub.f]]/19.72] (2)

Graft Efficiency (%) = [[100(1 - f)[N.sub.f]]/[(1 - f) [N.sub.l] + f[N.sub.s]]] (3)

where [N.sub.ext] is the nitrogen content (wt%) of the quenched extrudate, [N.sub.f] is the theoretical nitrogen content based on the feed rates, [N.sub.i] is the nitrogen content of the EtOH/[H.sub.2]O-insoluble fraction, [N.sub.s] is the nitrogen content of the EtOH/[H.sub.2]O-soluble fraction and f is the weight fraction soluble in EtOH/[H.sub.2]O. Nitrogen content of PAAm is 19.72 wt%. Grafting efficiency is based on the amount of monomer polymerized, and is equal to the ratio of insoluble PAAm to total PAAm.

Polyacrylamide was isolated from the quenched extrudates by acid hydrolysis of the starch, following the procedure of Reyes and Russell (21). Intrinsic viscosity of the isolated PAAm was measured in aqueous 1 M NaN[O.sub.3] at 30[degrees]C with Cannon-Ubbelohde tubes. Molecular weights were calculated using the MarkHouwink equation

[M.sub.w] = 3.73 * [10.sup.-4][[eta].sub.0.66] (4)

where [[eta]] is in dl/g (21).

For FTIR analysis, approximately 100 mg of sample was ground neat with a Wig-L-Bug for 240 seconds in a stainless steel vial with two ball-bearing pestles. The samples pulverized readily at room temperature. Approximately 1 mg of the resulting powder was stirred vigorously by hand with 300 mg of KBr and pressed into a pellet at 100,000 psi. This procedure was used to minimize the effects of water and KBr seen in the carbonyl region. The pellet was then analyzed by midrange transmission spectroscopy on a Nicolet Impact 410 FTIR spectrometer (64 scans).

X-ray diffraction data were obtained using a Philips PW 1830 generator table (Cu [K.sub.[alpha]]) with a PW 1820 goniometer. Step size was 0.05[degrees] (2[theta]) and the sampling rate was 4 seconds per step.

RESULTS AND DISCUSSION

Starch/Acrylamide Ratio 5:1

To determine the influence of feed rate and screw speed on grafting, a series of SGCs were prepared using normal corn starch with a starch/acrylamide ratio of 5:1. Total feed rates, including water, ranged from 109 g/min to 325 g/min. One set of conditions (163 g/min. 150 rpm) was replicated with three separate runs, while another (218 g/min, 100 rpm) was replicated twice. Other runs were performed once.

The results of these runs are summarized in Table 2. Monomer conversion is 75% or greater for all combinations of extrusion parameters. At feed rates of 163 g/min and greater, monomer conversion is essentially independent of screw speed or feed rate, and ranges from 84.9% to 92.8%. When the feed rate was 109 g/min, conversions ranged from 75.7% to 80.9%. Grafting efficiencies generally exceeded 70% at feed rates of 163 g/min or higher. As with conversion, the lowest feed rate (109 g/min) gave the lowest grafting efficiencies. The monomer conversions and grafting efficiencies in Table 2 exceed those reported by Carr (14) for reactive extrusion of starch and acrylamide at a 4:1 weight ratio ([approximately equal to] 30% conversion and [approximately equal to] 7% graft), as well as those reported for slurries with chemical initiators (21,23,24), and are comparable to values reported by Hebeish et al. (26). These results indicate that the persulfate initiator generates free radicals on the starch molecules from which polyacrylamide grafts grow. This is consistent with the ability of persulfate ions to oxidize secondary alcohols (28) and alkyl ethers (29) via free radical mechanisms. These results suggest that ammonium persulfate may be a more efficient initiator for acrylamide polymerization during reactive extrusion with starch than ceric ammonium nitrate (14).

The effect of initiator concentration was investigated using conditions of 109 g/min total feed and 100 rpm screw speed. The initiator was omitted in one run, and doubled (2 wt% based on starch) in another run. With no initiator present, the monomer conversion was 32%, graft content was 2.5%, and graft efficiency was 50%. Apparently grafting sites are created on the starch in the absence of initiator, probably through chain transfer with growing polyacrylamide chains or shear-induced generation of free radicals in starch by chain scission. When the initiator was doubled, the conversion was 81.5%, graft content was 9.6%, and graft efficiency was 66.2%. The conversion and graft content are not significantly different from the results obtained with 1 wt% initiator (see Table 2), while the graft efficiency is greater. Under these extrusion conditions, increasing the persulfate concentration improves grafting efficiency but may not be an effective means of increasing monomer conversion.

The data in Table 2 show that screw speed has little effect on conversion or grafting efficiency at feed rates of 163 g/min and greater. The fact that the lowest conversion levels are observed with the lowest feed rate suggests that the degree of fill in the extruder barrel is an important parameter in reactive extrusion of starch and acrylamide. This is consistent with the effect observed by De Graaf and Janssen for reactive extrusion of starch with low levels of styrene in a short counter-rotating twin screw extruder (15). Residence time was not a factor in determining the degree of conversion. At the feed rate/screw speed combination of 109 g/min and 100 rpm, the measured residence time was 420 seconds, determined using a CuS[O.sub.4] tracer (data not shown). Since residence time in co-rotating twin-screw extruders decreases as screw speed or feed rate increase (30), this figure represents the maximum expected residence time for this study. The independence of conversion on residence time implies that the polymerization kinetics of acrylamide in the presence of persulfate during extrusion is sufficiently rapid to be complete in less than [approximately equal to] 400 seconds.

The operating characteristics of co-rotating twinscrew extruders comparable to the one in this work have been modeled using the specific throughput as a parameter (30). Specific throughput is defined as Q/N, where Q is the throughput (mass or volume per unit time) and N is the screw speed. It was found that conditions of high Q/N led to higher drag flow and increased degree of fill. Figure 1 shows a plot of monomer conversion against specific feed rate Q/N for the data in Table 2. Although there is some scatter in the data, conversion increases with specific feed rate. Figure 1 suggests that the critical parameter in determining monomer conversion is degree of fill, not residence time, for the graft polymerization of starch with acrylamide during extrusion.

[FIGURE 1 OMITTED]

Grafting efficiencies were determined after extraction of the quenched extrudates using aqueous ethanol (30/70 EtOH/[H.sub.2]O v/v), which is a solvent for polyacrylamide but not starch. The soluble fractions shown in Table 2 range from 11.6% to 18.8%. Given the standard deviations shown, these differences are probably not significant. Some carbohydrate is soluble in the EtOH/[H.sub.2]O, since the PAAm contents of the soluble fractions range from 22.6% to 36.8%. The soluble carbohydrate could be either low molecular weight degradation fragments, or chemically bonded to the soluble PAAm in the form of graft copolymer. When the starch was extruded with persulfate and no monomer, 13.6% of the extrudate was soluble in EtOH/[H.sub.2]O. All the starch-g-PAAm materials in Table 2 gave turbid solutions in EtOH/[H.sub.2]O after filtering off the insoluble solids, except the samples extruded at 300 rpm.

Molecular weights of PAAm formed during extrusion ranged from 34,300 to 213,100; no attempts were made to separately measure [M.sub.W] for the soluble and grafted fractions. No clear dependence on feed rate at constant screw speed is observed (compare 100 rpm or 300 rpm data). Higher screw speeds at constant feed rate generally yielded lower molecular weights (compare 109 g/min and 325 g/min data). The PAAm molecular weights in Table 2 are comparable to or greater than those reported for batch processes when chemical initiation was used (21,23,24) but less than those obtained when irradiation initiation was used (16,17).

Starch/Acrylamide Ratio 1.3:1 to 3:1

Starch-g-polyacrylamide materials with higher levels of acrylamide were also extruded. The starch feed rate was fixed at 67.5 g/min (dry basis) with a screw speed of 150 rpm. Although the data in Table 2 indicate that higher feed rates could be used, these conditions were selected to reduce the amount of acrylamide handled. Since the acrylamide monomer was fed as a 50 wt% solution, the acrylamide content was increased by increasing the pump rate, keeping the total moisture content at 50%. Therefore the total feed rate increased as the acrylamide content increased. The lowest total feed rate was 163 g/min and the highest was 240 g/min.

Data obtained with normal corn starch are given in Table 3. Conversions of monomer to polymer are greater than 80% in all cases. Conversion is independent of feed rate; at the highest acrylamide content, though, the conversion is slightly lower. Grafting efficiencies are all approximately 75%, again indicating that the persulfate initiator generates free radicals on the starch molecules. As expected, the graft content increases with acrylamide feed. The highest graft content prepared was 31.6%. The monomer conversion and graft content in Table 3 are significantly greater than values obtained by Carr et al. at comparable starch/acrylamide ratios (20-25% conversion and 10-15% graft content at starch/acrylamide ratios of 2:1 and 1:1, respectively) (14).

FTIR spectra of the extrudates from Table 3 after quenching and removal of residual monomer are shown in Fig. 2. Two absorption peaks are seen in the 1500 c[m.sup.-1] to 1800 c[m.sup.-1] range, which increase as the acrylamide content increases. The peak centered around 1676 c[m.sup.-1] corresponds to the amide I carbonyl, while the peak centered around 1616 c[m.sup.-1] corresponds to amide II (hydrogen-bonded) carbonyl. The increase in carbonyl absorption, compared to starch absorption in the 1000 c[m.sup.-1] to 1200 c[m.sup.-1] range, correlates well with the nitrogen content measured for these samples and is consistent with the presence of grafted PAAm.

Extraction of the normal corn starch samples with EtOH/[H.sub.2]O yields roughly the same level of solubles seen with the starch/acrylamide ratio of 5:1. The amount of soluble PAAm increases as the acrylamide content increases. All the materials in Table 3 gave turbid supernatants after the insoluble fractions were removed by filtration.

Data for graft copolymers prepared with waxy maize starch, which has essentially no amylose, are shown in Table 4. As with the normal corn starch, monomer conversions are high, exceeding 90% in all cases except the highest acrylamide content. Graft contents are somewhat lower than those observed with normal corn starch, while graft efficiencies are considerably lower, ranging from 56.5% to 76.6%. At lower starch: acrylamide ratios, the waxy maize starch materials have higher EtOH/[H.sub.2]O solubles, reflecting the higher solubility of extruded waxy maize starch compared to normal corn starch (31), while the amount of soluble PAAm is comparable for the two starches. Supernatants after extraction were clear for the starch/acrylamide feed ratios of 5:1 and 3:1, and turbid for the higher ratios.

Tables 3 and 4 show that the PAAm molecular weight increases as the monomer content increases, regardless of starch type. Since the data in Table 2 suggest that feed rate has little or no effect on molecular weight, it appears that these increases are related to the monomer concentration. According to standard free radical polymerization kinetics, chain length is proportional to the ratio of the monomer concentration to the square root of the initiator concentration (32). Since the initiator concentration was fixed at 1 wt% of the starch, this ratio increases with monomer feed. As shown in Fig. 3, the PAAm molecular weight is proportional to this ratio for both normal corn and waxy corn starches.

[FIGURE 2 OMITTED]

Correlation of Extrusion Parameters With Polymerization

Figure 4 illustrates the dependence of die pressure with total feed rate for the samples given in Tables 2-4. As expected, the die pressure increases with increasing feed rate. Comparison of normal corn starch samples at equivalent feed rate but different acrylamide content shows that the die pressure is greater for materials with higher acrylamide content. This increase is consistent with the formation of PAAm during extrusion. Further evidence of the correlation between polymerization and die pressure is given by the run with no initiator in which the die pressure was 0.33 ([+ or -]0.02) MPa compared to 0.77 ([+ or -]0.03) with initiator present (total feed rate = 109 g/min). In addition, the specific mechanical energy with no initiator present was 116 ([+ or -]5) J/g compared to 226 ([+ or -]11) J/g with initiator. The die pressures measured for the waxy maize starch extrudates are consistently lower than those seen with the normal corn starch. The lower pressures are apparently due to differences in the rheological properties of the waxy maize starch. The effect, if any, of melt rheology on monomer conversion is not clear.

Specific mechanical energy values ranged from [approximately equal to] 180 J/g to [approximately equal to] 600 J/g. Monomer conversion was essentially independent of SME over this range. Although the lowest conversion levels were observed at the high end of the SME values, it seems that the degree of fill is a more important factor. Lower feed rates and higher screw speeds increase SME and reduce fill, and these are also the conditions that gave the lowest conversions seen in Table 2.

[FIGURE 3 OMITTED]

Effect of Extrusion Temperature

A series of starch-g-PAAm materials were prepared at different extrusion temperatures using normal corn starch. Flat temperature profiles were used for 50, 60, 70, 80, and 90[degrees]C. The same profile as in Tables 2-4 was used, and one where the maximum temperature was reduced from 110[degrees]C to 100[degrees]C. Feed rate was 163 g/min at 50% moisture content with a screw speed of 150 rpm. The starch/acrylamide ratio was 3:1.

The results are shown in Table 5. Acrylamide conversions are 80% or greater, with grafting efficiencies between 70% and 80%. Over the temperature range from 60[degrees]C to 100[degrees]C, the PAAm molecular weight increases with temperature. In contrast, molecular weight typically decreases with increasing polymerization temperature in free radical polymerizations (33). The PAAm molecular weight at 110[degrees]C is comparable to the values given in Tables 2 and 3. The nature of the molecular weight change with temperature is not clear, but may be related to the rheology of the melt in the extruder. The EtOH/[H.sub.2]O solubles appear to decrease slightly at the lower extrusion temperatures, although the change may not be significant except at the extremes of the range. The fraction of PAAm in the solubles changes little with temperature, except at 50[degrees]C.

[FIGURE 4 OMITTED]

X-ray diffraction data for several extrudates are shown in Fig. 5. As indicated by the scattering peaks between [approximately equal to] 18[degrees] and [approximately equal to] 23[degrees] (2[theta]), residual crystallinity is present after extrusion at 50[degrees]C and absent above 60[degrees]C. At extrusion temperatures above 80[degrees]C, a sharp scattering peak appears at 2[theta] [approximately equal to] 20[degrees], which is due to the presence of helical inclusion complexes between amylose and lipid material in the starch. The presence of residual crystallinity at the lower extrusion temperatures may affect the accessibility of starch to initiator and monomer, and also affect the rheology of the mixture in the extruder. While it is not clear what effect residual crystallinity has on the graft polymerization process, it is apparent that extrusion conditions above 50[degrees]C to 60[degrees]C are needed to completely disrupt the native granule structure.

Graft Frequency

Graft frequency is a measure of the average number of anhydroglucose units (AGU) between graft points. The average number of AGUs between grafts ([N.sub.AGU]) may be estimated as

[N.sub.AGU] = M(1-g)/162g (5)

where M is the PAAm number average molecular weight, g is the graft content (weight fraction), and 162 is the AGU molecular weight. M was estimated to be one-half the weight average molecular weight measured by intrinsic viscosity. Values calculated for [N.sub.AGU] using this relation are given in Tables 2-5. Given the AGU/initiator ratio of 70, it appears that the grafting efficiency of the persulfate radicals is on the order of a few percent.

In all cases, [N.sub.AGU] exceeds 1000, and in most cases is greater than 2000. At a fixed starch:acrylamide ratio (Table 2), it appears that increasing screw speed decreases [N.sub.AGU]; samples extruded at higher speeds have more grafts of lower average molecular weight. It also appears that increasing the feed rate at constant screw speed gives the same effect, although there are some discrepancies in this trend. When normal corn starch is extruded with increasing levels of monomer, [N.sub.AGU] is approximately constant, while the graft molecular weight increases (Table 3). The same general trend is observed with the waxy maize starch extrudates (Table 4), although [N.sub.AGU] for the lowest monomer content is greater than the other values, reflecting the lower graft content. [N.sub.AGU] is not significantly affected by changes in extrusion temperature, as seen in Table 5. The data in Tables 2 through 5 suggest that feed rate and screw speed are more important than temperature and starch:acrylamide ratio in determining graft frequency in reactive extrusion of the starch-acrylamide persulfate system.

CONCLUSIONS

It has been demonstrated that graft copolymers of starch and polyacrylamide can be prepared by continuous reactive extrusion, with high acrylamide to PAAm conversions and high grafting efficiencies, using ammonium persulfate as an initiator. Monomer conversion appeared to be independent of residence time. At a fixed starch:acrylamide ratio of 5:1, conversion to PAAm increased with increasing specific feed rate (Q/N). Higher screw speeds yielded lower PAAm molecular weights. PAAm molecular weights increased as the acrylamide content increased in a manner consistent with free radical polymerization kinetics, whereas the expected decrease with increasing extrusion temperature was not observed. Some degree of control over graft frequency and molecular weight can be obtained through extrusion parameters.

[FIGURE 5 OMITTED]
Table 1. Screw Configuration for Reactive Extrusion of Starch and
Acrylamide.

 Element Type Cumulative Length (mm)

42/42 (4x) 168
28/28 (2x) 224
KB 45/5/45 269
KB 45/5/28 297
KB 45/5/14 311
42/42 353
28/28 381
KB 45/5/28 409
KB 90/5/28 437
28/28 465
TME 14/14 479
TME-N 14/14 493
TME 14/14 LH 507
TME 14/14 521
TME 14/14 N 535
TME 14/14 LH 549
KB 45/5/14 LH 563
28/28 591
TME 14/14 605
TME-N 14/14 619
TME 14/14 LH 633
TME 14/14 647
TME 14/14 N 661
TME 14/14 LH 675
KB 45/5/14 LH 689
42/42 731
28/28 759
KB 45/5/14 773
KB 45/5/14 LH 787
28/28 815
TME 14/14 829
TME 14/14 N 843
TME 14/14 LH 857
KB 45/5/14 LH 871
28/28 (2x) 927
ZME 20/20 (6x) 1047
KB 45/5/14 LH 1061
28/28 1089
ZME 20/20 (6x) 1209
KB 45/5/14 LH 1223
28/28 1251
20/20 (4x) 1331

KB = Kneading Block, TME = Turbine Mixing Element, ZME = Special Mixing
Element, LH = Left Handed, N = Neutral. For KB elements with designation
A/B/C, A is the stagger angle between lobes, B is the number of
lobes, and C is the element length in mm

Table 2. Properties of Starch-Graft-Polyacrylamides. Starch/Acrylamide
Feed Ratio = 5:1.

Feed rate/screw Conversion (%) Graft content Graft
speed (g/min; (%) efficiency (%)
rpm) (a)

109/100 80.9 8.6 52.9
109/200 77.4 8.4 54.7
109/300 75.7 9.6 58.0
163/150 88.9 (4.5) (b) 12.8 (1.8) (b) 70.1 (6.3) (b)
163/300 85.5 12.8 73.1
218/100 86.1 (1.0) (c) 11.8 (1.9) (c) 69.1 (5.1) (c)
218/200 84.9 10.7 66.2
218/300 84.6 13.2 74.3
325/125 92.8 14.3 77.0
325/150 86.7 14.0 80.5
325/300 87.0 12.6 74.0

Feed rate/screw PAAm [M.sub.w] (g/mol Solubles (%) % PAAm (in
speed (g/min; x [10.sup.-3]) solubles)
rpm) (a)

109/100 112.3 17.2 36.8
109/200 87.7 18.8 30.1
109/300 34.3 17.6 32.6
163/150 222.5 (38.5) (b) 12.1 (5.2) (b) 34.7 (8.9) (b)
163/300 131.8 18.0 21.5
218/100 112.7 (11.7) (c) 14.4 (2.8) (c) 32.1 (9.7) (c)
218/200 45.7 12.4 38.6
218/300 118.1 16.8 22.6
325/125 154.5 13.2 28.1
325/150 77.1 11.6 25.9
325/300 53.2 14.8 25.5

Feed rate/screw AGU/graft
speed (g/min;
rpm) (a)

109/100 3680
109/200 2950
109/300 1120
163/150 3800 (820) (b)
163/300 2770
218/100 2600 (210) (c)
218/200 1180
218/300 2400
325/125 2860
325/150 1460
325/300 1140

a) All components; moisture content = 50%.
b) Average of 3 separate trials; standard deviation in parentheses.
c) Average of 2 separate trials; standard deviation in parentheses.

Table 3. Properties of Starch-g-Polyacrylamide Copolymers From Normal
Starch.

Starch/acrylamide Conversion Graft content Grafting
 ratio (%) (%) efficiency (%)

 5:1 (163) (a) 89.2 13.7 75.7
 3:1 (180) 85.2 18.9 77.3
 2.1:1 (200) 90.4 25.8 79.0
 1.6:1 (220) 87.9 29.0 79.2
 1.3:1 (240) 82.7 31.6 77.3

Starch/acrylamide PAAm [M.sub.W] Solubles % PAAm
 ratio (g/mol x [10.sup.-3] (%) (in solubles)

 5:1 (163) (a) 178.2 11.6 33.5
 3:1 (180) 275.0 12.0 40.8
 2.1:1 (200) 348.5 10.8 56.6
 1.6:1 (220) 322.0 10.4 65.8
 1.3:1 (240) 527.0 12.4 65.7

Starch/acrylamide AGU/graft
 ratio

 5:1 (163) (a) 3460
 3:1 (180) 3640
 2.1:1 (200) 3090
 1.6:1 (220) 2430
 1.3:1 (240) 3520

Moisture content = 50%; screw speed = 150 rpm; starch feed rate = 67.5
g/min (dry basis).

(a) Total feed rate in g/min for all components in parentheses.

Table 4. Properties of Starch-g-Polyacrylamide Copolymers From Waxy
Maize Starch.

Starch/acrylamide Conversion Graft content Grafting
 ratio (%) (%) efficiency (%)

 5:1 (163) (a) 94.9 10.7 65.6
 3:1 (180) 93.1 16.5 76.6
 2.1:1 (200) 94.4 20.8 56.5
 1.6:1 (220) 98.2 26.1 59.3
 1.3:1 (240) 83.1 26.2 60.6

Starch/acrylamide PAAm [M.sub.w] Solubles % PAAm
 ratio (g/mol x [10.sup.-3] (%) (in solubles)

 5:1 (163) (a) 241.5 12.0 41.1
 3:1 (180) 278.0 8.0 58.0
 2.1:1 (200) 227.4 19.6 65.7
 1.6:1 (220) 314.1 20.8 68.3
 1.3:1 (240) 358.3 19.6 69.8

Starch/acrylamide AGU/graft
 ratio

 5:1 (163) (a) 6220
 3:1 (180) 4340
 2.1:1 (200) 2670
 1.6:1 (220) 2740
 1.3:1 (240) 3120

Moisture content = 50%; screw speed = 150 rpm; starch feed rate = 67.5
g/min (dry basis).

(a) Total feed rate in g/min for all components in parentheses.

Table 5. Effect of Extrusion Temperature on Graft Copolymerization of
Starch and Acrylamide.

 Extrusion Conversion Graft Graft PAAm [M.sub.w] (g/mol
temperature (%) content efficiency x [10.sup.-3])
([degrees]C) (%) (%)

 110 80.6 18.3 71.4 228.3
 100 104.1 24.1 77.2 404.1
 90 96.7 23.3 80.3 336.7
 80 80.9 20.2 78.9 262.0
 70 92.8 21.2 73.9 244.6
 60 88.7 21.1 74.3 208.9
 50 91.7 20.4 74.2 303.0

 Extrusion Solubles % PAAm AGU/graft
temperature (%) (in.solubles)
([degrees]C)

 110 16.0 38.3 3150
 100 12.8 48.4 3930
 90 11.2 45.3 3420
 80 12.5 38.3 3200
 70 13.6 47.6 2810
 60 12.8 49.6 2410
 50 10.4 61.2 3650

Feed rate = 180 g/min; moisture content = 50%.
Starch/acrylamide ratio = 3:1.
Screw speed = 150 rpm.


ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of B. K. Jasberg, R. P. Westhoff, G. D. Grose, R. L. Haig, J. Salch, and T. Hathway.

* Corresponding author: willetjl@ncaur.usda.gov

Names are necessary to report factually on available data: however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.

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J. L. WILLETT* and V. L. FINKENSTADT

Plant Polymer Research Unit U.S. Department of Agriculture, Agricultural Research Service National Center for Agricultural Utilization Research Peoria, Illinois
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