Utilization of Pectin Extracted Sugar Beet Pulp for ... - PubAg - USDA

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Journal of Biobased Materials and Bioenergy Vol. 6, 1–8, 2012

Utilization of Pectin Extracted Sugar Beet Pulp for Composite Application Bo Liu1 , Jinwen Zhang1 ∗ , Linshu Liu2 ∗ , and Arland T. Hotchkiss2 1

Materials Science and Engineering Program and Composite Materials and Engineering Center, Washington State University, Pullman, WA 99164, USA 2 Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA Sugar beet pulp (SBP) is the residue left after beet sugar extraction. SBP contains ∼25% pectin and is an important resource for pectin. However, sugar beet pectin does not have good gel-forming properties and complete extraction of pectin is not typically performed due to the low quality of the galacturonic acid-containing polysaccharide remaining following commercial pectin extraction. This study was to find out what level of residual pectin is needed for making SBP composites and the effects of pectin content on the properties of the resulting composites. In this work, the pectin extracted SBP (PE-SBP) was recombined with pectin to prepare SBP composites. The compounding of pectin and PE-SBP was conducted using a twin-screw extruder, and water and glycerol were used as plasticizers. Pectin was plasticized during extrusion and behaved like a binder or matrix for the pectin/PE-SBP compounds, depending on pectin content. Mechanical and thermal properties of the compounds were studied and the results showed that utilization of the PE-SBP as a resource for plastic materials is possible.

1. INTRODUCTION The increasing concerns of environmental impact and sustainability of synthetic polymers motivate researchers from academia and industry to obtain polymer materials using renewable resources. For the past two decades, natural polymers such as starch1–5 and soy protein6–15 have attracted great attention for plastic applications, in part owing to their abundant availability, biodegradability and compostability. Sugar beet pulp (SBP) is the residue of sugar extraction from beets and is an underutilized agricultural residue. Annually about 400 million metric tons of SBP are produced worldwide.16 17 SBP is composed of ca. 70–80% polysaccharides (cellulose, hemi-cellulose and pectin in an approximately 1:1:1 ratio) and other components including lignin, protein and residual sugar, etc. Usually SBP is used in animal feed. In recent years, SBP has received attention for polymer material applications. Rouilly et al. prepared SBP plastics using twin crew extrusion and investigated the effects ∗ Authors to whom correspondence should be addressed. Emails: [email protected], [email protected]

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of water content and specific mechanical energy input level on the properties of the resulting compounds.18–20 We recently detailed the effects of water and glycerol contents in the preformulated SBP on morphology, rheological, thermal and mechanical properties of the resulting SBP compounds prepared by twin screw extrusion.21 The compounding of SBP plastics was similar to the extrusion cooking of food in utilizing the water solubility and gelability of pectin in SBP. Water and glycerol also served as plasticizers in subsequent processing of the resulting SBP compounds.21 Dufresne et al. prepared cast films using pectin and the cellulose microfibrils separated from SBP.21 The results showed that pectin acted as a binder between cellulose microfibrils and improved the stress transfer in the composites. Mechanical high shear treatment was noted to improve the dispersion of cellulosic microfibrils in the composites. The results showed that pectin content had little influence on the mechanical properties of the composites. In order to improve the water resistance of SBP-based plastics and processibility, blending of SBP with other polymers have also been studied. Finkenstadt et al. studied the use of SBP as a filler in poly(lactic acid) (PLA)/SBP composites16 23 and investigated the mechanical properties of the materials. We investigated the use

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doi:10.1166/jbmb.2012.1206

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Utilization of Pectin Extracted SBP for Composite Application

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of polymeric diphenylmethane diisocyanate as a coupling agent to improve the compatibilization between the SBP filler and PLA matrix.24 Furthermore, we recently studied use of SBP as a plastic-like component in blending with poly (butylene adipate-co-terepthalate)/SBP blends and the resulting blends demonstrated significantly higher mechanical and thermal properties than the blend prepared by processing SBP as a filler.25 Pectin is a valuable natural polymer and widely used in food products. While pectin is ∼25% of the dry mass of SBP, the high degree of acetylation, feruloylation, rhamnogalacturonan I branching and rhamnogalacturonan II crosslinking limits its use as a gel-forming food ingredient.26 However, sugar beet pectin is an oil-in-water emulsifier with food and non-food applications of this activity under current investigation.26 The complete removal of pectin from SBP is a challenging effort and the galacturonic acidrich polysaccharide remaining after commercial pectin extraction has low quality for current pectin applications. Additionally, hemi-cellulosic polysaccharides and cellulosic microfibrils also respectively make ∼25% of the dry mass of SBP. Developing viable utilization of the residual hemicellulose and cellulose ingredients either separately or together for value-added products may economically benefit the process of pectin isolation from SBP. Sugar beet cellulose with glucuronoxylan present formed liquid crystals,27 28 while the cellulose consisted of nanofiber “whiskers” once the hemicellulose was removed.29 In this work, pectin and pectin extracted SBP (PE-SBP) in different weight ratios were recombined and compounded using a twin-screw extruder. The PE-SBP was obtained by commercial extraction of pectin from the native SBP. Glycerol and water were used as plasticizers during compounding. Mechanical, thermal properties of the resulting compounds were examined. The main objective of this study is to evaluate the content of residual pectin needed in the PESBP for it to be melt compounded for composite material applications.

sealed plastic bags and then equilibrated at room temperature for 8 h. The formulated pectin/PE-SBP mixture was then compounded to obtain thermoplastic-like SBP (TSBP) using a co-rotating twin-screw extruder (Leistritz ZSE-18) equipped with a volumetric feeder. The diameter of the screw was 18 mm with a length-to-diameter ratio (L/D) of 40. The extruder has a feeding throat zone cooled by running tap water, seven individual heating zones and an adapter/die section. The zone temperatures were set from 70 C (zone 1) to 90 C (die) and the screw speed was 100 rpm. The compound is openly discharged without a die amounted at the end of the barrel. The extrudate was collected directly in a plastic bag. Table I gives the formulations of compounds.

2. EXPERIMENTAL DETAILS

2.4. Scanning Electron Microscopy (SEM)

2.1. Materials

To observe the effect of extrusion on the morphology of PE-SBP, the pectin/PE-SBP compounds were extracted in distilled water at 90 C for 10 h. The precipitated solid residues were washed several times and dried by freeze

PE-SBP and pectin extracted from SBP were provided by CP Kelco AsP (Lille Skensved, Denmark). The PE-SBP and pectin received were in powder forms and contained ∼6% and 8% moisture, respectively. PE-SBP used in this study was the leftover of pectin extraction from the original SBP with approximately 80% pectin removed. All materials and chemicals were used as received. 2.2. Preparation of Pectin/PE-SBP Prior to extrusion compounding, PE-SBP, pectin, glycerol and water were mixed using a kitchen mixer, stored in 2

2.3. Preparation of Pectin/PE-SBP Sheets The extruded compound without drying was used directly for sheet preparation. Sheets of ca. 1-mm in thickness were prepared using a mini hot press machine (Carver). Retaining a certain level of water is the basis for the compression molding process. The temperature of the press was set at 90 C and molding pressure at 20 MPa. The preheating time and the pressing time were ca. 5 and 10 min, respectively. The sheet was cooled down to 50 C before removed from the mold. The hot pressed sheets were cut into standard tensile (ASTM D638, Type I) test specimens. Tensile testing was conducted on a screw-driven universal testing machine (Instron 4466) equipped with a 10 kN electronic load cell and mechanical grips. All tests were performed at a crosshead speed of 5 mm/min and the strain was measured using a 25-mm extensometer (MTS634.12E-24) and data acquired by computer. The procedure was carried out according to the ASTM standard, and five replicates were tested for each sample to get an average value. All specimens were conditioned for two weeks at 23 C and 50% RH. The thickness of the specimen was measured prior to testing.

Table I.

Formulations of pectin/PE-SBP composites.

Sample (pectin/PE-SBP) Control Pectin-5 Pectin-15 Pectin-25 Pectin-35 a

PE-SBPa (phr)

Pectina (phr)

Glycerol (phr)

Waterb (phr)

75 95 85 75 65

25 5 15 25 35

0 5 15 25 35

60 60 60 60 60

Dry weight, b Including the moisture in SBP and pectin.

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Utilization of Pectin Extracted SBP for Composite Application (b)

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(a)

Fig. 1. SEM micrographs of SBP powder and cryo-fractured surfaces of pectin/PE-SBP compounds. (a) and (b) PE-SBP powder, (c) pectin-5, (d) pectin-15, (e) pectin-25, and (f) pectin-35.

drying. Morphology of the residue was examined using SEM (FEI Quanta 200 F). Morphology of pectin/PE-SBP composites was also examined using SEM. The cryofractured surfaces and the tensile fracture surfaces were examined. All samples were sputter coated with gold prior to examination.

(DMA) (TA Q-800) DMA specimens with dimensions of 30 × 10 × 1 mm3 were cut from hot pressed samples and tested using a single-cantilever fixture at a frequency of 1 Hz. All tests were conducted at a strain of 0.03% using a 3 C/min temperature ramp from −80 to 120 C.

2.5. Dynamic Mechanical Properties

2.6. Thermogravimetric Analysis (TGA)

Dynamic mechanical properties of PE-SBP/pectin composites were studied by dynamic mechanical analysis

TGA was performed on a SDT Q600 thermogravimetric analyzer (TA instruments). The test samples were scanned


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from 35 to 600 C at a heating rate of 10 C/min under continuous nitrogen flow (100 mL/min). The weight loss under 180 C was taken as the moisture content of the sample.

3. RESULTS AND DISCUSSION

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3.1. Formation of Pectin/PE-SBP Plastics Figures 1(a) and (b) shows the micrographs of the PE-SBP powder. The particles looked irregular in shape, which was resulted from the mechanical breakage of SBP before pectin extraction. At higher magnification, SBP appeared porous, which was attributed to the cellular structure of the beet plant. The cell wall was broken during the mechanical milling but still kept its original morphology to a certain degree. Our previous study21 showed that the native SBP from sugar extraction could be used for plastics when SBP was sufficiently plasticized by water and glycerol. In this study pectin was added back to the PE-SBP. In the presence of sufficient water pectin gelated under heating and was subsequently plasticized by water and glycerol under extrusion. The plasticized pectin functioned like a binder for the PE-SBP at low concentrations and a thermoplastic-like matrix at high concentrations. This was identified by examining the cryo-fracture surfaces as shown in Figures 1(c) to (f). At the 5 and 15% levels, pectin was hardly recognizable in the pectin/SBP compounds. At the 25% level, pectin became noticeable, and at the 35% level pectin looked apparently like a matrix polymer in the compound. Like the native SBP in the thermoplastic SBP prepared by compounding in the presence of water and/or glycerol,20 the PE-SBP in the pectin/PESBP composites experienced a similar deformation during compounding as showed in the micrographs of hot water extracted samples (Fig. 2). Hot water extraction caused little change to the morphology of PE-SBP particles (Fig. 2(a)) compared to that of the unextracted PE-SBP particles (Fig. 1(b)). In contrast, the hot water (a)

Fig. 2.

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Liu et al. Table II. Effect of pectin content on tensile properties of pectin/PESBP composites. Plasticized Pectin content (%) Pectin-5 Pectin-15 Controla Pectin-25 Pectin-35 SBP-20b

Strength (MPa)

Strain (%)

Modulus (MPa)

21.5 ± 1.2 15.1 ± 0.9 28.8 ± 2.9 10.6 ± 0.6 7.0 ± 0.1 9.2 ± 0.3

1.4 ± 0.2 7.0 ± 0.8 0.7 ± 0.1 11.2 ± 1.3 16.0 ± 1.7 4.1 ± 0.7

3046 ± 73 1203 ± 130 4593 ± 486 624 ± 45 257 ± 24 744 ± 104

a Containg 25% pectin but without glycerol (Table I), b The data was adopted from the sample W30G20 in Ref. [24].

extracted pectin/PE-SBP indicated that PE-SBP particles were greatly disrupted through compounding, appearing in much smaller pieces (Fig. 2(b)). This result suggests the added pectin in this study had the similar effect as the pectin in the native SBP during compounding.20 3.2. Tensile Properties Water and glycerol were used as plasticizers for pectin during compounding and sheet preparation. After conditioning, most water evaporated and glycerol remained in the end products. In this study, glycerol was considered to mainly remain in the pectin phase that was evidenced by TGA. As a plasticizer, glycerol reduced the interactions between molecules of pectin and increased its flexibility, ductility and processibility. For the samples with 25% pectin (control and pectin-25) (Table II), addition of glycerol increased strain at break greatly but decreased strength and modulus due to the plasticizing effect of glycerol on pectin. This result was quite similar to that of the native SBP plastic having a similar composition.21 Figure 3 shows the stress-strain curves of pectin/PESBP compounds with different plasticized pectin contents. With 5% pectin, the pectin/PE-SBP compound exhibited a brittle behavior, showing a strain at break of ∼1%. As the content plasticized pectin increased, the strain (b)

SEM micrographs of hot water extracted PE-SBP (a) and pectin/PE-SBP (pectin-25) (b).


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3046 to 257 MPa, respectively. On the other hand, strain at break increased from 1.4 to 16.0%. As the content of plasticized pectin increased, the tensile fracture of the compounds changed from brittle to ductile (Fig. 4). The change of fracture behavior reflected the effects of plasticized pectin in the samples. The pectin/PESBP samples showed brittle fracture of the material at low pectin contents (Figs. 4(a) and (b)). At higher pectin contents (25% and 35%), the strain at break improved greatly due to the flexibilizing effect of glycerol to pectin. As a result, plastic deformation was clearly seen on the fracture surfaces in Figures 4(c) and (d).

25 5%

Strength (MPa)

20

15% 15 25% 10

35%

5

0 0

5

10 Strain (%)

15

20

Fig. 3. Effect of plasticized pectin content on tensile behavior of pectin/PE-SBP composites.

at break gradually increased while tensile strength and modulus continuously decreased. Tensile properties of the pectin/PE-SBP compounds are summarized in Table II. When pectin content increased from 5 to 35%, tensile strength and modulus decreased from 21.5 to 7.0 MPa and

3.3. Dynamic Mechanical Properties Figure 5 shows the effects of plasticized pectin content on storage modulus (E ), loss modulus (E ) and damping (tan ) of pectin/PE-SBP composites as functions of temperature. E decreased with temperature ranging from −80 to 120 C (Fig. 5(a)). This is a common phenomenon for typical thermoplastic composites. E of the sample control (25% pectin without glycerol) exhibited a larger value than that of the sample with 25% plasticized pectin within most of the experiment temperature range.

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Fig. 4.

SEM micrographs of the tensile fracture surfaces of pectin/PE-SBP composites. (a) pectin-5, (b) pectin-15, (c) pectin-25, and (d) pectin-35.


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However, as the temperature decreased to ca. −30 C or lower, E of the latter was much higher than E of the former. This phenomenon was probably due to the antiplasticizing effect of glycerol at the temperature lower than −30 C. A similar anti-plasticizing effect was also reported in other natural polymer-based composites.6 30 31 At very low temperatures, the water–glycerol mixture (a)

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crystallizes and thus the plasticizers, water and glycerol, stiffened the pectin/PE-SBP composites.6 30 31 Since the pectin was greatly plasticized by glycerol and became soft, E of pectin/PE-SBP composites decreased with increasing plasticized pectin content. There was a relaxation peak in each E curve (Fig. 5(b)). For the sample containing 25% pectin (control), the peak height was low and the peak temperature was high. This result indicates that pectin without glycerol behaved like a rigid plastic. With addition of glycerol, the peak temperature shifted to lower temperature and the height increased greatly, suggesting that glycerol behaved as a plasticizer for pectin and improved the molecular movement ability of pectin greatly. As the plasticized pectin content increased, the peak height in Figure 5(b) increased. Pectin and glycerol in a ratio of 1:1 were added to PE-SBP prior to compounding. However, like water, some glycerol also migrated into PE-SBP that is mainly composed of hydrophilic components such as hemi-cellulose, cellulose and proteinaceous matter. Because glycerol would mainly remain in the water soluble pectin, the actual pectin/glycerol weight ratio in the pectin phase of the resulting composites varied with pectin loading level. At low pectin contents, relatively more glycerol migrated to the PE-SBP phase. Therefore, the relaxation peak of pectin appeared at higher temperatures. As the plasticized pectin content increased, relatively less glycerol migrated to the PE-SBP phase as it was saturated by glycerol, yielding relaxation peak at lower temperatures. The temperature dependence of tan is shown in Figure 5(c). In general, tan increased with increasing temperature. Three peaks were observed in most of the curves. The peak at ∼70 C was attributed to the glass transition of cellulose or hemi-cellulose in PE-SBP. The glass transition temperature (Tg of neat dry pectin was ∼37 C.32 The Tg of pectin was hardly recognized in the curve of the sample control, because the relatively low concentration of the pectin and low damping of the unplasticized pectin. The peak at ∼20 C was the Tg of plasticized pectin which was evidenced in all samples containing glycerol plasticized pectin. As the plasticized pectin content increased, another relaxation appeared in the low temperature (∼50 C). This relaxation was also noted in our previous study21 and in other pectin composites.3 Similarly, Chen and Zhang reported two glass transitions in glycerol plasticized soy protein.13 In that study, neat soy protein plastics with 25–40% glycerol displayed a two-phase structure, i.e., protein-rich and glycerol-rich domains and two corresponding Tg s. In the pectin/PE-SBP composites, a similar two-phase structure might exist due to the two relaxation temperatures. 3.4. Thermogravimetric Analysis (TGA)

Fig. 5. Storage modulus (a), loss modulus (b) and tan (c) of pectin/PE-SBP composites.

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The thermal stability of the composites was studied using TGA. The weight loss under 180 C was considered to be J. Biobased Mater. Bioenergy 6, 1–8, 2012

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Table III. TGA results of pectin/PE-SBP composites. Pectin Moisture (%) % 60 117 115 124 131

5 15 25 35 a

a

Peak 1 Rate 1 Peak 2 Rate 2 Peak 3 Rate 3 ( C) (%/ C) ( C) (%/ C) ( C) (%/ C) 156.6 149.3 175.6 169.1 170.0

0.099 0.227 0.197 0.189 0.165

247.9 239.3 232.2 229.1 227.8

0405 042 0483 0598 0732

343.2 342.3 342.1 341.3 340.0

0.620 0.565 0.453 0.381 0.313

The weight loss below 180 o C in TGA test was assumed to be moisture.

due to moisture evaporation. The measured moisture contents are listed in Table III. It is obvious that the moisture contents of pectin/PE-SBP composites were higher than that of PE-SBP powder. However, as the added pectin varied from 5 to 35%, the moisture content changed slightly. Above 200 C, there was an obvious trend that higher pectin content led to larger weight loss (Fig. 6(a)) at the end of TGA experiment. This result indicated that pectin was more susceptible to thermal degradation than cellulose. In Figure 6(a), it was clear that there were several segmental drops in the curves. To better discern the weight loss, the first-order differential was used to analyze the rate of weight loss under different temperatures (Fig. 6(b)). Three peaks were noted for each curve. The temperatures (a)

4. CONCLUSIONS The water soluble and adhesive characteristics of pectin were utilized to turn PE-SBP into composite materials. Pectin was plasticized by sufficient water and glycerol during compounding and behaved like a binder (or adhesive) at low concentrations and a matrix at high concentrations in the resulting pectin/PE-SBP composites, respectively. Because most water evaporated after conditioning, glycerol was the major plasticizer in the end composites. Tensile strength and modulus decreased significantly with the presence of glycerol in the final products while the tensile strain at break increased. DMA results showed that Tg decreased with increasing plasticized pectin content in the composites. Glycerol exhibited an anti-plasticization effect in the low temperature range. The results from this study suggest that the SBP residues from pectin extraction can be turned to composite materials if it still contains a certain level of residual pectin. Acknowledgment: The authors are grateful for financial support from the Agricultural Research Service of the United States Department of Agriculture, specific cooperative project #58-1935-9-965.

References

Fig. 6. Effect of pectin content on (a) thermal weight loss of PE-SBP composites (b) thermal weight loss rate of PE-SBP composites.


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(b)

and the heights of the peaks are summarized in Table III. The peak temperature of decomposition of neat pectin was ∼239 C (curve is not shown). The first peak was associated with the evaporation of the bound water, the second peak was assigned to the decomposition of hemicellulose and/or pectin and the third peak the decomposition of cellulose.33 With addition of the plasticized pectin, the peak temperature of the second peak shifted slightly to lower temperatures but the height increased. This result was probably due to the lower decomposition temperature of the plasticized pectin and faster decomposition rate. Furthermore, the evaporation of glycerol could not be recognized separately because it occurred in the same temperature range. The presence of glycerol in the composites also affected the decomposition temperature of cellulose but not as much as it to the that of pectin. This result indicated that the plasticized pectin was compatible but not miscible with cellulose. With increasing pectin content, the decomposition rate of cellulose decreased due to the decrease of cellulose content in the composites.


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