Structure and Properties of Composites Compression-Molded from

The mold was cooled to 50 °C with a fan at a rate of 10 °C min-1. ... Thermogravimetry analysis (TGA) of the specimen (about 5 mg) was performed ...
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Ind. Eng. Chem. Res. 2003, 42, 6786-6794

Structure and Properties of Composites Compression-Molded from Polyurethane Prepolymer and Various Soy Products Yun Chen,†,‡ Lina Zhang,*,† and Libo Du† Department of Chemistry, and Medical School, Wuhan University, Wuhan 430072, China

A series of protein composites was successfully prepared from 30 to 50 wt % polyurethane prepolymer (PUP) with soy dreg (SD), soy whole flour (SWF), and soy protein isolate (SPI), by a compression-molding process at 120 °C without addition of any plasticizer. The structure and properties of the sheets were characterized by Fourier transform infrared spectroscopy, wideangle X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, thermogravimetry analysis, dynamic mechanical analysis, tensile test, and biodegradability test. The results indicated that the -NCO groups in PUP reacted with -NH2, -NH-, and -OH groups in soy products to form a certain degree of grafting and cross-linking, showing a new glass transition temperature (Tg) at -32 to -25 °C, compared with raw materials. Moreover, the toughness, thermal stability, and water resistivity of the composite sheets significantly increased. By increasing PUP content, the elastomer materials blended PUP and soy protein could be obtained. The protein component in the soy products plays a role in enhancement of the adhesivity, processability, and biodegradability. In addition, with an increase of cellulose content in the system, the tensile strength and water resistivity of the composite sheets increased. The tensile strength, elongation at break, and water resistivity were 6.9 MPa, 100%, and 0.55 for SD-U50 sheet from SD with 50 wt % PUP, and 4.8 MPa, 140%, and 0.50 for SPI-U50 sheet from SPI with 50 wt % PUP, respectively. Therefore, composite materials could be prepared by controlling the content of PUP and changing the types of soy products to obtain desired properties. Introduction With an increase of worldwide environmental pollution caused by nonbiodegradable polymers, the research and development of biodegradable materials from renewable resources, including cellulose,1 starch,2 and protein3,4 have attracted much attention. Soy products such as soy oil, soy protein isolate (SPI), soy whole flour (SWF), and soy protein concentrated (SPC) have been considered as alternatives to petroleum polymer because of their abundant resources, low cost, and good biodegradability.5,6 Those soy products have been researched as environment-friendly materials in the fields of adhesives,7 plastics,8,9 textile fiber,10,11 and various binders.12 The use of soy proteins for plastics can be traced back to the 1930s and 1940s.13 The plastics made from soy products with water or glycerol as plasticizer exhibit moderate strength and good biodegradable performance,14 but they also show brittleness and water sensitivity.15 It has been reported by Gennadios16 and Sun et al.6,17 that SPI and SWF can be modified by physical and chemical treatments.18-20 In recent years, the blends of soy products with polyurethane such as incorporation of protein into polyurethane systems,21 and preparation of water-blown rigid or flexible polyurethane foams extended with soy products including SPI and SWF22-25 have been investigated. Sun et al. have reported that 2 wt % methylene diphenyl diisocyanate (MDI) plays a role in enhancement of compatibilization for the blends of soy protein isolate and poly* To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry. ‡ Medical School.

caprolactone.17 It is worth noting that regenerated cellulose films coated with interpenetrating polymer network (IPN) coating, which were synthesized from castor-oil based polyurethane (PU) with chitosan or nitrocellulose, can be degraded by microorganisms in natural soil to release CO2, H2O, glucose, and aromatic ether decomposed from PU in the IPN coating, suggesting that PU/natural polymer materials could be biodegraded.26 Therefore, it is interesting to prepare composite materials with a certain biodegradability from blends of soy products and other polyurethane with polypropylene glycol in place of castor oil. Soy dreg (SD) is one of the cheapest soy byproducts from soy protein, whose price in China is 1/10 that of SPI. It mainly contains cellulose fiber, soy protein, polysaccharide, and their compounds with many active groups including -NH2, -COOH, and -OH.27 The active groups in soy dreg may react with isocyanate to give new materials. The structure and properties of the materials obtained by blending soy protein with polyurethane greatly depend on the protein component.24 However, effects of species of soy products and different content of polyurethane prepolymer on blending and reacting in a compression-molding process have been scarcely reported. The objective of this work was to compression-mold blends of SD or SPI with polyurethane prepolymer (PUP) to produce water-resistant sheets. The structure, thermal and mechanical properties, and biodegradability were investigated. To clarify the effect of different components such as cellulose, soy protein, and polysaccharide in SD, the sheets blended from SPI, SWF, and cellulose (CEL), respectively, with PUP were prepared, and their structure and properties were compared.

10.1021/ie0301381 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/15/2003

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Experimental Section Materials and Preparation of Composite Sheets. SD, SWF, and SPI were purchased from Hubei Yunmeng Protein Technology Co. in China. SD and SWF were milled through an 80-mesh sieve, and then treated with acetone and vacuum-dried for 24 h at 60 °C. The main compositions in SD were determined to be about 77% cellulose, 12% protein, and 11% polysaccharide.27 Cellulose powder (DP ) 300) was a gift from Guangzhou Chemistry Institute, Chinese Academy of Sciences. 2,4Toluene diisocyanate (TDI) supplied by Shanghai Chemical Reagent Co. was vacuum-dried at 80 °C for 2 h before use. Polypropylene glycol (PPG; Mw ) 1000) from Nanjing Chemical Factory was vacuum-dried at 105 °C for 5 h. The mixture of TDI and PPG (2:1, by equivalent weight ratio) was heated to 80 °C with mechanical stirring in a three-necked flask for 2.5 h to obtain polyurethane prepolymer (PUP), which was cooled to room temperature and stored in a desiccator. This polyurethane prepolymer is a viscous colorless liquid at room temperature, and could not form a membrane when sealed in a desiccator containing P2O5 for at least 2 months. When it was exposed to air for more than 3 days, a transparent membrane from PUP formed due to the reaction between -NCO groups in PUP and moisture contained in the air, which was coded as PUPM. PUP was mixed with SD and pestled in a mortar for 30 min, then extruded by a single screw extruder (PolyDrive with Rheomex R252, ThermoHaake, Germany; diameter 19.1 mm; length/diameter 25:1). The screw rotation speed was 12 rpm, and the temperature profile along the extruder barrel was 90, 100, and 110 °C (from feed zone to exit). The blends exited from the extruder were immediately compression-molded by a lab-built compression-molding device.28 The procedure was the following: 5 g of the material was placed into a mold and covered with a polished stainless steel plate at both sides. The temperature of molding was controlled to be 120 °C, and the pressure was quickly increased from 0.5 to 20 MPa for 1 min, and then retained for 7 min. The mold was cooled to 50 °C with a fan at a rate of 10 °C min-1. The sheet was released from the mold, and stored in a desiccator containing desiccant. By changing the weight percent of PUP such as 30, 40, and 50%, a series of sheets from PUP and SD were prepared and coded as SD-U30, SD-U40, and SDU50, respectively. The SPI-U, SWF-U, and CEL-U sheets were prepared with SPI, SWF, and CEL, respectively, instead of SD, by the same extrusion and compression-molding process. Characterization of Structure. Fourier transform infrared (FTIR) spectra of the specimen were recorded on a Fourier transform infrared spectrometer (PerkinElmer Co., Boston, MA) in a range of wavenumbers from 4000 to 400 cm-1. The specimens were cut into powders and vacuum-dried for 24 h, and then mixed with KBr to laminate. The morphology was observed in a scanning electron microscope (SEM, S-570, Hitachi, Japan). The samples were frozen in liquid nitrogen and fractured immediately, and then the fracture surfaces were coated with gold for SEM observation. Wide-angle X-ray diffraction patterns (WAXD) were carried out on an X-ray diffractor (XRD-6000, Shimadzu, Japan) with Cu KR radiation (λ ) 15.405 nm) at 40 kV

and 30 mA with a scan rate of 10° min-1. The diffraction angle ranged from 4 to 40°. Thermal Analysis. Differential scanning calorimetry (DSC) was measured on a DSC-204 apparatus (Netzsch Co., Germany) under a nitrogen atmosphere at a rate of 10 °C min-1 from -150 to 250 °C. Prior to the test, the specimens were heated from room temperature to 100 °C to remove moisture, and then cooled to -150 °C. Thermogravimetry analysis (TGA) of the specimen (about 5 mg) was performed by a thermobalance (PRT2, Beijing Optical Instruments Factory, China) under a nitrogen atmosphere from 25 to 600 °C at a heating rate of 10 °C min-1. Dynamic mechanical thermal analysis (DMTA) was carried out on a DMTA-V (Rheometric Scientific Co.) at a frequency of 1 Hz. The specimens were heated from room temperature to 100 °C to remove moisture, and then cooled to -80 °C. The test temperature ranged from -80 to 220 °C with a heating rate of 5 °C min-1. Mechanical Properties Test. The tensile strength (σb) and elongation at break (b) of the sheets were tested on a universal testing machine (CMT6503, Shenzhen SANS Test Machine Co. Ltd., China) according to ISO6239-1986 (E) with a tensile rate of 50 mm min-1. Test of Water Resistivity and Water Absorption. To study the water resistivity, the sheets were immersed into water at 25 °C for 1 h, and then the tensile strength in wet state (σb(wet)) of the sheets was measured under the same conditions mentioned above. Water resistivity (Rσ) of the sheets was evaluated from the tensile strength in dry state (σb(dry)) and σb(wet) by the following equation:

Rσ ) σb(wet)/σb(dry)

(1)

Water absorption was determined according to the ASTM standard D570-81 with minor modification. The samples were dried in an oven at 50 °C for 24 h. Subsequently, they were cooled in a desiccator for a few minutes, weighed, and submerged in distilled water at 25 °C for 26 h. The sheets were removed and the surface moisture was wiped off with a paper towel, and then were weighed again. After the sheet was removed from water, the container was placed in an oven for 24 h at 50 °C to evaporate the water. The residuals were the water-soluble contents. The weight gain of the sheet plus the weight of the water-soluble residuals was counted as the total absorbed water. Water absorption (Ab) was calculated as follows:

Ab ) [( W1 - W0 + W2 )/W0 ] × 100 %

(2)

where W0 and W1 are the weight of the sheets before and after being submerged in water, and W2 is the weight of the water-soluble residual. Biodegradation Test. Natural soil was used as the environment for the biodegradability test.26,29 The dried specimens were enclosed separately in nylon mesh and buried about 15 cm beneath the soil on Wuhan University campus. After a designated time, the degraded specimens were taken out one after another, rinsed carefully with water, and dried in a vacuum at 50 °C for 24 h, and weighed. Weight loss (Wloss) of the degraded sheets in soil was calculated as follows:

Wloss ) (Wb - Wa )/Wb

(3)

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where Wb and Wa are the weights of the dried sheets before and after being buried in soil, respectively. Results and Discussion Structure and Morphology. FTIR spectra of the raw materials and the sheets with 50 wt % PUP are shown in Figure 1. The absorption of -NCO groups at 2273 cm-1 in IR spectra of PUP was not observed in the resulting composite sheets, indicating the absence of free -NCO groups. Comparing with the broad absorption of -OH stretching vibration in the range from about 3300 to 3350 cm-1 for the raw materials, the peaks for the sheets narrowed and shifted to higher than 3400 cm-1, implying the contribution of -NH stretching vibration30 from PUP. It was noted that the new peaks at 1726 cm-1 (stretching vibration of urethane carbonyl groups),31,32 1637 cm-1 (Amide I), and 1543 cm-1 (Amide II) occurred in CEL-U50, displaying the formation of urethane bonds in the sheets. It has been reported that the -OH groups in glucose unit of cellobiose33 and in cellulose acetate34 can react with -NCO groups to form urethane bonds. During the formation of urethane bonds for CEL-U50 sheet, the -OH groups from cellulose or moisture contained in air could be reacted with -NCO groups. Ryabov et al.25 have proved that the -OH groups from glucose unit of starch reacted much faster than that from moisture in air with -NCO groups under 90 °C in air for 1 h. Furthermore, in our experiment, the time of being exposed in air for PUP with cellulose was very short, so that the reaction of -NCO groups with moisture could be neglected in this case. Therefore, the absence of absorption for -NCO groups and the occurrence of absorption for urethane bonds in IR spectra of CELU50 indicated a success of reaction between PUP and cellulose, suggesting that -NCO groups in PUP reacted with -NH2, -NH-, and -OH groups in SD, SWF, or SPI to form urea and urethane bonds, resulting in the differences in IR spectra of the sheets from that of the raw materials. A certain degree of chemical crosslinking and graft between the protein products and PUP in the composite materials, similar to graft-IPN from polyurethane and nitro-lignin, could not be avoided in this case.35 SEM images of the fracture-surfaces for CEL-U, SDU, SWF-U, and SPI-U sheets are shown in Figure 2. Because the reactions between PUP and the raw materials in the extrusion process are not homogeneous, the multiphases with different components could be formed and were observed in the fracture surfaces, especially when the PUP content was 30 wt %. With an increase of PUP content, the cross-section of the sheet became relatively homogeneous, indicating that the content of -NCO groups in the system plays an important role in improving the compatibility among the components in the composite system. Figure 3 shows the X-ray diffraction patterns for raw materials and their compression-molded sheets with 50 wt % PUP content. The dotted lines in the X-ray diffraction patterns are the simulated curves given by computer, and the degree of crystallinities for CELL, SD, SWF, and SPI powers are calculated with a peak separation method to be about 56, 28, 28, and 33%, respectively, while that for sheets CELL-U50, SD-U50, SWF-U50, and SPI-U50 decreased to be 42, 21, 21, and 23%, respectively. The shape, position, and relative strength of the diffraction peaks for the sheets (espe-

Figure 1. FTIR spectra of the raw materials and the sheets with 50 wt % PUP content.

cially for SD-U50, SWF-U50, and SPI-U50 sheets) were lower than those for the corresponding raw materials, implying the reaction of PUP with SD, SWF, or SPI. Thermal Properties. DSC thermograms and corresponding data for the raw material and sheets are shown in Figure 4 and listed in Table 1, respectively. The soy products and the composite sheets exhibited

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Figure 2. SEM photographs of cross-sections for the sheets. Table 1. DSC and DMTA Data for the Raw Materials and the Sheetsa DSC sample cellulose CEL-U50 SD SD-U30 SD-U40 SD-U50 SPI SPI-U50

a

DMTA

Tg(°C) (midpt)

∆Cp (J g-1 K-1)

Tdn (°C) (midpt)

Tm (°C) (midpt)

-27.6

0.21

153.9

-31.8 -31 -31.5

0.16 0.11 0.21

-24.8

0.23

* 133.7 177.4 171 * 132.3 *

148 150 152.1 146.3

sample CEL-U30 CEL-U40 CEL-U50 SD-U30 SD-U40 SD-U50 SPI-U30 SPI-U40 SPI-U50 PUP-M

Tg (°C) (midpt)

Tdc (°C) (midpt)

-26.9 -18.7 -8.4

201.7 203.9 203. 6

-11.4 -8.9 -11.6 -4.3 -1.9 -11.8

Tdn, Denaturation temperature; Tm, melting transition temperature; *, superposition of Tdn and Tm; Tdc, decomposition temperature.

endothermic peaks in the range of 130 to 177 °C in DSC thermograms due to the thermal denaturation temperature (Tdn) of soy protein component. The Tdn of protein for SD-U30 and SD-U40 sheets occurred at 171 and 177 °C, respectively, while the Tdn of SD-U50 and SPIU50 sheets were at 152 and 146 °C, respectively, implying the effects of PUP content on the Tdn of protein composition in the composite sheets. The Tdn of protein in the composite sheets is higher than that in SD raw material (134 °C), suggesting that the formation of urethane or urea bonds enhanced the thermal stability. This temperature agreed with the thermally stable

temperature of urethane (180 °C) and urea bonds (200 °C) in soy flour/polyurethane foam systems reported by Hsieh et al.23,24 After introduction of -NCO groups, the sheets exhibited Tg (midpoint) at -25 to -32 °C, owning to the glass transition temperature of PUP segment grafted or cross-linked onto cellulose or soy protein molecules. Usually, it was difficult to prepare compression-molded sheets from cellulose or SD without addition of plasticizers such as water and glycerol. In this work, the sheets having good toughness were molded from cellulose and SD with PUP, implying that PUP reacted with cellulose and SD to form new materials

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Figure 3. X-ray diffraction patterns for raw materials and the sheets with 50 wt % PUP content.

with chemical cross-linking and grafting structure. In addition, there was no glass transition and denaturation transition for cellulose DSC thermogram in the range of testing temperature, so the broad peak at 154 °C for CEL-U50 was assigned to the melting transition temperature (Tm) of PU composition,36 namely the softening temperature in the system. The broad peaks at 152 °C and 146 °C for SD-U50 and SPI-U50 sheets were due to the superposition of Tdn of protein and Tm of PUP composition. Thermal degradation curves of the raw materials and the sheets are shown in Figure 5. There were three distinct stages of decomposition in the curves. The sheets and raw materials decomposed slowly in the first stage from room temperature to 180 °C. In the second stage, the sheets decomposed quickly in the temperature range of 180 to 400 °C with a weight loss of 60 to 70%. In the former two stages, the thermal stability of SD-U and SPI-U sheets was higher than that of the raw materials, in good agreement with the result of DSC. In the third stage, the weight loss of the sheets was larger than that of corresponding raw materials and increased with an increase of PUP content, owing to PU degradation. The DMTA spectra of the sheets are shown in Figure 6, and the corresponding data are summarized in Table

Figure 4. DSC thermograms of SD, SPI, cellulose, and SD-U, SPI-U, and CEL-U sheets.

1. The PUP-M membrane could be used as a control to determine the Tg of the soft segments, which occurred at about -12 °C. The mechanical loss factor (tan δ) at -2 to -10 °C in the sheets with 50 wt % PUP was

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Figure 5. TGA curves of SD, SD-U30, SD-U40, SD-U50, SPI, and SPI-U50.

assigned to the Tg of the PU composition. The Tg obtained from DMTA was about 20 °C higher than that measured by DSC. A different mechanism between DSC and DMTA yielding a large difference in the Tg values is practically related to the viscoelastic behavior of the PUP phase, and this effect is translated into a large difference in the relaxation times of polymer molecules, particularly in the temperature range near Tg. It was noted that there were two tan δ peaks for the sheets with less than 40 wt % PUP, resulting from the multi-

microphases in the system. The tan δ peaks of the sheets became narrow and shifted to high temperature with an increase of PUP content, owing to enhancement of more grafting and cross-linking reactions between PUP and CEL, SD, SWF, or SPI. It also indicated that PUP could increase the compatibility of the multiphases in the composite materials. This conclusion was supported by the results from SEM. Mechanical Properties. Figure 7 shows the stressstrain curves of SD-U sheets in dry and wet state. The toughness of the specimen could be estimated by the area under the stress-strain curve.17 The area under the stress-strain curve of the composite sheets increased with an increase of PUP content, indicating the toughness of the sheets increased when PUP content increased. The effects of PUP content on the tensile strength, elongation at break, and Young’s modulus (E) of the sheets are shown in Figures 8, 9, and 10, respectively. The σb and b of the PUP-M were measured to be about 6.9 MPa and 1100%, respectively. It exhibited a certain tensile strength and very high elongation at break, implying that PUP composition greatly contributed to the toughness of the composites. With an increase of PUP content, the σb of CEL-U and SD-U sheets increased, whereas that of SWF-U and SPI-U decreased. When the PUP content was 40 or 50 wt %, the σb of SD-U sheets was the highest among the sheets with the same PUP content. As mentioned above, SD raw material contained relatively more cellulose and a certain amount of protein, which led to the reinforcing function of cellulose cooperating with the adhesivity of protein. On the other hand, the b of the sheets all increased significantly, but the Young’s modulus decreased greatly, with an increase of PUP content. The b value of the SD-U50 and SPI-U50 sheets with 50 wt % PUP was 5 and 14 times that for SD-U30 and SPIU30, respectively. The SD-U30 and SPI-U30 sheets exhibited a yield-point that represents plastic characteristic. However, the SD-U50 and SPI-U50 sheets with higher PUP content have no yield-point, suggesting a rubber-like property, resulting from PUP plasticization and certain cross-linking of PUP with SD or SPI. The sheets SD-U50, SWF-U50, and SPI-U50 exhibited elastomer behavior. Such a wide change of mechanical properties of the sheets provided a way to prepare

Figure 6. Dependence of tan δ on temperature for CEL-U, SD-U, and SPI-U sheets.

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Figure 7. Stress-strain curves of SD-U and SPI-U sheets in dry state (left) and wet state (right).

Figure 8. Dependence of tensile strength (σb) on PUP content for the sheets.

Figure 9. Dependence of elongation at break (b) on PUP content for the sheets.

different materials for various applications by changing the PUP content and soy products category. Comparing with the sheets in dry state, the tensile strength of the sheets in wet state (σb(wet)) decreased slightly, while the b(wet) increased significantly. For example, the b(wet) values of SWF-U50 and SPI-U50 sheets increased from

Figure 10. Dependence of Young’s modulus (E) on PUP content for the sheets.

35% for SWF-U30 to 334% and from 8% for SPI-U30 to 727%, respectively. The great enhancement of elongation at break for wet sheets is due to the obvious plasticization of water to protein-containing molecular chains.13 Water resistivity of the sheets is shown in Figure 11. One of the major shortcomings of soy protein materials is their water sensitivity.17,37 The Rσ values of sheets molded from pure raw material SD and SPI, respectively, with 30 wt % glycerol as the control samples were measured to be less than 0.2.37 Thus, the Rσ values of the composite sheets were higher than that without addition of PUP. This can be explained by the fact that the urethane and urea bonds between PUP and cellulose, SD, or SPI led to the elimination of some hydrophilic groups such as -OH and -NH2, and the introduction of PUP added a large amount of hydrophobic groups such as -CH3 and benzyl ring. Moreover, the water resistivity of CEL-U and SD-U sheets was higher than that of others, suggesting that relatively more cellulose in the sheets could enhance the water resistivity. This result was further proved by the water absorption of the sheets dipped into water for 26 h shown in Figure 12. It has been reported that the water resistivities of the molded SPI sheets with plasticizer are in the range from about 200 to 600%.14 With an increase of PUP or cellulose content, water absorption of the

Ind. Eng. Chem. Res., Vol. 42, No. 26, 2003 6793 Table 2. Theoretical Average Numbers of Grafting PUP (Ngc) Per Cellulose Molecular Chain in CEL-U Sheets

Figure 11. Dependence of water resistivity (Rσ) on PUP content for the sheets dipped into water for 1 h at 25 °C.

sample

WPUP/WCEL

Ngc

CEL-U30 CEL-U40 CEL-U50

30/70 40/60 50/50

15 24 36

U40 sheet buried in soil. The sheet quickly degraded in the first 10 days and then the degradation rate of the sheets reduced obviously. Interestingly, the Wloss values of CEL-U40 and CEL-U50 sheets buried in soil for 30 days were about 8 and 20%, respectively, suggesting the cross-linking between PUP and cellulose greatly reduced the degradation rate of cellulose. However, the Wloss of SD-U40 and SD-U50 sheets buried in soil for 30 days were more than 40 and 50%, respectively, indicating a certain amount of protein in the sheets significantly prompted the degradation process. Interactions between PUP and Raw Materials. Jane et al.9 reported that the extrusion processing of soy protein plastic sheets containing 10 parts of glycerol or less was very difficult. However, in this work, cellulose, SD, SWF, and SPI could be compressionmolded, after reacting with PUP, without plasticizers such as water and glycerol. One of the main reasons is the reaction of -NCO groups with -NH2, -NH-, and -OH groups to form new polyurethane-like materials; another is the internal plasticization of flexible PUP segments. When PUP was compression-molded with cellulose, the chemical reaction between -NCO groups of PUP and -OH groups of cellulose resulted in urethane bonds, grafting PUP onto cellulose chains. The mole ratio of -NCO and -OH groups in this system could be calculated theoretically as follows:

-NCO/-OH ) (162 N WPUP)/(3 WCEL) Figure 12. Dependence of water absorption (Ab) on PUP content for the sheets dipped into water for 26 h at 25 °C.

(4)

where N is the mole numbers of -NCO groups in a gram of PUP, in mol g-1; WPUP and WCEL are the weight of PUP and cellulose in the mixture, respectively, in g. N is 1.48 × 10-3 mol g-1, and the degree of polymerization (DP) of cellulose molecule is 300. So, the number of -OH groups in every cellulose molecule is about 900 and the average numbers of grafting PUP chains (Ngc) onto per cellulose chain can be calculated to be:

Ngc ) 36 WPUP/WCEL

(5)

The theoretical Ngc values with different WPUP/WCEL are listed in Table 2. With an increase of WPUP/WCEL, the cellulose molecular chains have more chances to link with PUP chains and to be plasticized by flexible PUP chains, resulting in an enhancement of elongation at break and water resistivity. Therefore, the cellulose component in SD was not only a reinforcer, but also mainly provided -OH groups to react with -NCO groups. The protein component reacted with PUP to increase the adhesivity and processability of the system. Figure 13. Dependence of weight loss (Wloss) on degradation time (t) for SD-U40 sheet buried in soil.

sheets decreased. In view of the results, the water resistivity of the SD-U50 sheet (0.55) was the highest and the water absorption (52%) was the lowest in the sheets molded from PUP and soy products, indicating a cooperation of PUP, cellulose, and protein. Biodegradability. Figure 13 shows the dependence of weight loss (Wloss) on degradation time for the SD-

Conclusions A series of sheets from soy dreg, soy whole flour, and soy protein isolate, respectively, with polyurethane prepolymer was successfully prepared by a compressionmolding method without addition of plasticizers. The composite sheets possessed better processability, mechanical and thermal properties, and water resistivity than the sheets molded from SD or SPI plasticized by

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glycerol. The contents of PUP, cellulose, and protein in the composite materials have great effect on the structure and properties of the compression-molded sheets. The -NCO groups in PUP reacted with -NH2, -NH-, and -OH groups in the soy products and cellulose to form urea and urethane bonds, resulting in enhancement of compatibility, flexibility, toughness, and water resistivity. The existence of cellulose and protein in soy products is available to enhance chemical and physical interactions between PUP and raw materials, leading to an improvement of thermal and mechanical properties, and water resistivity of the sheets. Cellulose plays an important role in enhancing the water resistivity of the sheets molded from soy dreg and PUP, due to the reactions between -OH and -NCO groups to form urethane bonds. Moreover, protein mainly contributed to increasing the adhesivity, ductibility, processability, and biodegradability. It is possible to obtain different materials, from plastic to elastomer, by changing the PUP content and soy product category. Acknowledgment This work was supported by a Major Grant of the National Natural Science Foundation of China (59933070), Major Grant of Science and Technology Project from Hubei Province, and the Key Laboratory of Cellulose and Lignocellulosic Chemistry of the Chinese Academy of Sciences. Literature Cited (1) Zhang, L.; Zhou, Q. Effects of molecular weight of nitrocellulose on structure and properties of polyurethane/nitrocellulose IPNs. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1623. (2) van Soest, J. J. G.; Benes, K.; de Wit, D. The influence of starch molecular mass on the properties of extruded thermoplastic starch. Polymer 1996, 37 (16), 3543. (3) Otaigbe, J. U.; Goel, H.; Babcock, T.; Jane, J. Processability and properties of biodegradable plastics made from agricultural biopolymers. J. Elast. Plast. 1999, 31, 56. (4) Cuq, B.; Gontard, N.; Guilbert, S. Thermoplastic properties of fish myofibrillar proteins: application to biopackaging fabrication. Polymer 1997, 38, 2399. (5) Paetau, I.; Chen, C.; Jane, J. Biodegradable plastic made from soybean products. 1. Effect of preparation and processing on mechanical properties and water absorption. Ind. Eng. Chem. Res. 1994, 33, 1821. (6) Zhong, Z. K.; Sun, X. S. Thermal and mechanical properties and water absorption of sodium dodecyl sulfate-modified soy protein (11S). J. Appl. Polym. Sci. 2001, 81, 166. (7) Hettiarachchy, N. S.; Kalapathy, U.; Myers, D. J. Alkalimodified soy protein with improved adhesive and hydrophobic properties. J. Am. Oil Chem. Soc. 1995, 72 (12), 1461. (8) Schilling, C. H.; Babcock, T.; Wang, S.; Jane, J. Mechanical properties of biodegradable soy-protein plastics. J. Mater. Res. 1995, 10, 2197. (9) Zhang, J.; Mungara, P.; Jane, J. Mechanical and thermal properties of extruded soy protein. Polymer 2001, 42, 2569. (10) Huang, H. C.; Hammond, E. G.; Reitmeier, C. A.; Myers, D. J. Properties of fibers produced from soy protein isolate by extrusion and wet-spinning. J. Am. Oil Chem. Soc. 1995, 72 (12), 1453. (11) Zhang, Y,; Ghasemzadeh, S.; Kotliar, A. M.; Kumar, S.; Presnell, S.; Williams, L. D. Fibers from soybean protein and poly(vinyl alcohol). J. Appl. Polym. Sci. 1999, 71, 11. (12) Mungara, P.; Zhang, J.; Jane, J. Extrusion processing of soy protein-based foam. Polym. Prepr. 1998, 39 (2), 148. (13) Sue, H. J.; Wang, S.; Jane, J. Morphology and mechanical behaviour of engineering soy plastics. Polymer 1997, 38 (20), 5035. (14) Wang, S.; Sue, H.; Jane, J. Effects of polyhydric alcohols on the mechanical properties of soy protein plastics. J. Macro. Sci. - Pure Appl. Chem. 1996, 33 (5), 557.

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Received for review February 14, 2003 Revised manuscript received July 24, 2003 Accepted July 24, 2003 IE0301381