Acid-Catalyzed Synthesis of Foamed Materials from Renewable Sources

Article pubs.acs.org/IECR

Acid-Catalyzed Synthesis of Foamed Materials from Renewable Sources Ran Duan, Ismail Ibrahem, Håkan Edlund,* and Magnus Norgren* Fibre Science and Communication Network (FSCN), Mid Sweden University, SE-851 70 Sundsvall, Sweden ABSTRACT: In this study, lightweight biobased foamed materials were successfully synthesized by the modification of renewable polysaccharides, such as starch and microcrystalline cellulose. Low-cost and nontoxic organic acids were utilized as catalysts in the first-step esterification reaction of the synthesis. The effects of different reaction conditions on the water absorbency and weight loss of freeze-casted polysaccharide−citrate−chitosan foams are discussed. Physical properties, such as pore-size distributions and compressive stress−strain curves, of the foams were determined. The characterization results show that the amide bonds formed between the carboxylic acid groups of polysaccharide−citrate and the amino groups of chitosan are crucial to the foamed material’s performance.

1. INTRODUCTION During the past decade, the increasing efforts to replace petroleum-based polymers by renewable and more sustainable materials have led to a growing scientific interest in new strategies to shape and convert biobased materials into industrially applicable polymers.1,2 By chemical modification of biobased polymers such as cellulose, hemicelluloses, and starch, new biopolymeric materials can be designed and generated for further versatile and valuable applications. Light-weight materials, such as biopolymer-based solid foams, have a potentially wide range of applications. Depending on the foam composition, cell morphology, and physical properties, synthesized biobased foams can be divided into two different types: flexible and rigid foams. For example, flexible foams can be used in textile, water-absorption, transportation, furniture, and sports applications, and rigid foams can be used for weight reduction, building insulation, liquid absorption, packaging, buoyancy, metal adsorption, open-cell ceramic material manufacturing, and so on.1−5 In the synthesis of biobased foams, hemicelluloses have proven to be successful alternatives for the replacement of petroleum-based materials in foam production.6−12 For example, in 2010, Salam et al. reported a two-step synthesis of a starch−citrate−chitosan gel in an aqueous environment at 100 °C as an absorbent foam material.7 The first step involves the formation of a starch−citrate intermediate in which NaPH2O2 is used to accelerate the formation of the reactive intermediate citric anhydride.13 The reactive citric anhydride readily reacts with the hydroxyl groups of starch in an esterification reaction. The subsequent steps are filtration, washing with water, and drying in an oven. Thereafter, the dried starch−citrate intermediate is reacted with chitosan to form a starch−citrate−chitosan cross-linked product. Finally, freeze-casting of the starch−citrate−chitosan suspension produces the porous foamed materials. Later on, Salam et al. extended their work to a two-step synthesis of a hemicellulose− citrate−chitosan gel in an aqueous environment at 100 °C.8 Furthermore, Hafrén and Córdova14 demonstrated that simple organic acids could be successfully utilized as catalysts in the esterification of cellulose for further modification and © 2014 American Chemical Society

derivatization. However, to the best of our knowledge, the use of an inexpensive organic acid or an inorganic acid as a catalyst in the preparation of biobased foams has not been reported. Based on our research interest in development of low-cost, efficient methodologies for the preparation of foamed materials, we report herein an ecofriendly approach for the preparation of biobased foams from cellulose and starch that could be used in water absorption applications. The new conditions involve the use of a catalytic amount of an organic or inorganic acid to accelerate the esterification step and improve the formation of the first-step polysaccharide−citrate product. The synthesized polysaccharide derivative was cross-linked with chitosan in an aqueous medium to produce the porous foam materials.

2. MATERIALS AND METHODS 2.1. Materials. Microcrystalline cellulose (MCC) (index no. 14205) was purchased from Serva (Frölunda, Sweden) and used as received. Corn starch and medium-molecular-weight chitosan with a 75−85% degree of deacetylation (CAS registry no. 9012-76-4) were purchased from Sigma-Aldrich (Stockholm, Sweden) and used as received. The chitosan solution was prepared by dissolving chitosan in 1% acetic acid solution. The following reagent-grade chemicals were also used: sodium hypophosphite (SHP) (CAS registry no. 123333-67-5), citric acid (CA) (CAS registry no. 77-92-9), and sodium hydroxide (CAS registry no. 1310-73-2) were purchased from RiedeldeHaen AG (Seelze, Germany). Hydrochloric acid (CAS registry no. 7647-01-0) was purchased from VWR Scientific (Stockholm, Sweden). Acetic acid (index no. 607-002-00-6) and L-(+)-tartaric acid (index no. 932 A399904) were purchased from Merck AB (Solna, Sweden). Munktell filter paper (quantitative grade 5, 110-mm diameter) was obtained from Munktell Filter AB (Falun, Sweden). In all experiments, Milli-Q water was used. Received: Revised: Accepted: Published: 17597

March 19, 2014 October 2, 2014 October 23, 2014 October 23, 2014 dx.doi.org/10.1021/ie501169w | Ind. Eng. Chem. Res. 2014, 53, 17597−17603

Industrial & Engineering Chemistry Research

Article

Scheme 1. Stepwise Synthesis of Polysaccharide−Citrate−Chitosan Product 5

2.2. Synthesis of Polysaccharide−Citrate 3. Based on our previous work15 and the works of Salam et al.,7,8 we started our investigation toward producing biobased foams by utilizing different polysaccharides 2 in combination with citric acid 1 and chitosan 4. The synthesis starts with the esterification reaction of polysaccharides 2 and citric acid 1. The reaction was carried out in the presence of sodium hypophosphite (SHP) to form the first-step product, polysaccharide−citrate 3 (Scheme 1). Screening experiments revealed that the best results were obtained when 1.0 equiv of citric acid 1, 2.0 equiv of polysaccharide 2, and 0.2 equiv of SHP were utilized in the presence of a catalytic amount (2 wt %) of an organic acid. 2.3. Synthesis of Polysaccharide−Citrate−Chitosan 5 by Cross-Linking Reaction. In the next step of the synthesis, the dried polysaccharide−citrate 3 was further reacted in aqueous solution with chitosan 4 at 100 °C. For this reaction, 1.0 g of the dried isolated polysaccharide−citrate 3 was ground into a fine powder and dispersed in 100 mL of water. To this solution was added 100 mL of 1 wt % chitosan solution, and the mixture was refluxed in an oil bath at 100 °C for 3 h. Next, the resulting polysaccharide−citrate−chitosan suspension 5 was transferred into a plastic Petri dish and kept in a −20 °C freezer. 2.4. Foam Formation. The frozen samples were placed in a freeze-dryer at 0.02 hPa and −112 °C to remove ice crystals and form flexible dry foams.

measurements were performed with a DAT1100 (Fibro System AB) contact-angle analyzer on the foamed samples. Milli-Q water was used as the probe fluid. The time intervals for time periods 0−1 s and 1−5 s were 0.01 and 0.1 s, respectively. Compressive tests were carried out on an MTS 4/ML instrument with a 100 N load cell at 23 °C and 50% relative humidity (RH). The resolution of the pressure sensor was 0.1 N, and the speed of the crosshead was 5 mm/min. Each sample was preconditioned to the test atmosphere and had a surface area of 900 mm2. Because of surface roughness, a preloaded 0.05 N force was applied to the foam samples before initiation of the compression test in the direction vertical to the test surface. The materials were compressed to 50% strain, and the corresponding stress was determined. The samples were stored in a desiccator and placed in the test room in advance to reach equilibrium at the specified humidity and temperature. The carboxylic acid content in the dry polysaccharide−citrate 3 samples was determined by titration according to the method described by Mattisson and Legendre.16 Specifically, 0.1 g of dry polysaccharide−citrate 3 sample was transferred to a 75 mL beaker, and 20 mL of 0.025 mol/L sodium hydroxide (NaOH) solution was added. The suspension was stirred for 30 min with a magnetic stir bar at room temperature. Next, the excess NaOH was titrated to pH 4 with 0.048 mol/L hydrochloric acid (HCl). The reference employed, 20 mL of 0.025 mol/L NaOH solution, was titrated in parallel. The carboxyl group content was calculated according the equation

3. MATERIAL CHARACTERIZATION FT-IR spectra of the furnished materials were recorded on a Nicolet 6700 spectrophotometer (Thermo Scientific). All spectra were obtained in 128 scans with a resolution of 8 cm−1 at 400−4000 cm−1. Specific surface areas (SSAs) were determined by low-temperature (77 K) nitrogen adsorption on a Micromeritics ASAP-2400 instrument. Dynamic contact angle

carboxylic acid content =

(VR − VS)c HCl × 100 sample weight

(1)

where VR is the volume of HCl consumed by the reference (20 mL of 0.025 mol/L NaOH solution), VS is the volume of HCl consumed by the sample, and cHCl is the concentration of HCl. 17598

dx.doi.org/10.1021/ie501169w | Ind. Eng. Chem. Res. 2014, 53, 17597−17603


Article

higher carboxylic acid group contents than for the reaction without an organic acid as a catalyst (Table 1, entry 1). On the other hand, stronger acids such as phosphoric acid (pKa = 2.15) and oxalic acid (pKa = 1.25) were less efficient and delivered the polysaccharide−citrate 3 with a lower carboxylic acid content and a lower isolated yield. However, in the case of starch as the polysaccharide, we were able to measure the isolated yields of polysaccharide−citrate 3 because of the solubility of the unreacted starch in water during the workup step, namely, washing with water. After filtration and drying, the isolated starch−citrate 3 was analyzed by FT-IR spectroscopy. On the other hand, cellulose as the polysaccharide provided cellulose−citrate 3 in quantitative isolated yield. Cellulose is insoluble in water; thus, we obtained a mixture of unreacted cellulose and cellulose−citrate 3. FT-IR analysis of the furnished polysaccharide−citrate 3 revealed a clear absorbance peak at 1718 cm−1 from the ester group, in agreement with the perviously reported results and FT-IR analysis of Salam et al.,7,8 indicating a successful esterification reaction. In addition, additives such as LiOH and NaOH that have been used successfully in the dissolution and derivation of cellulose17,18 failed in improving the esterification reaction (Table 1, entries 8 and 9). Additional acid additives, such as iron(II) chloride tetrahydrate (FeCl2· 4H2O), iron(III) chloride hexahydrate (FeCl3·6H2O), and phosphoric acid H3PO4, exhibited low catalytic activities in promoting the esterification reaction in comparison to tartaric acid and malonic acid in terms of the formation of the desired polysaccharide−citrate 3, the isolated amount, and the carboxylic acid content. Interestingly, the use of tartaric acid as a catalyst in the catalytic amount of 2 wt % in the absence of sodium hypophosphite (SHP) resulted in a successful esterification reaction with a high isolated amount of starch−citrate 3 (70% isolated yield), but with a longer reaction time: 16 h at 115 °C. Thus, tartaric acid as a catalyst is sufficient for catalyzing the esterification reaction of polysaccharide with citric acid. 4.2. Carboxylic Acid Content of the Intermediate Polysaccharide−Citrate 3. The carboxylic acid group contents of the polysaccharide−citrate 3 intermediates produced using different additives were determined by titration and are listed in Figure 1. The data depicted in Figure 1

The water interaction was investigated by water absorption and weight loss experiments. In this case, a 0.1 g sample of predried polysaccharide−citrate−chitosan foam 5 (WS) was placed in a preweighed tea bag (WT). The sample was immersed in a beaker filled with water, and after 1 h, the tea bag was removed from the water and weighed (Wwet) until the excess water had been removed by gravity. Next, the tea bag including the sample was dried in an oven at 105 °C overnight and weighed again (Wd). The water absorption (water-tosample weight ratio) and sample weight loss after water absorption were calculated according to the equations W − WwT − WS water‐to‐sample weight ratio = wet WS (2) ⎛ W − WT ⎞ sample weight loss = ⎜1 − d ⎟ × 100 WS ⎠ ⎝

(3)

where WwT is the weight of the wet tea bag.

4. RESULTS AND DISCUSSION 4.1. Optimization of the Esterification Reaction. Esterification can take place without addition of catalysts because of the weak acidity of carboxylic acids themselves. However, the reaction is extremely slow and requires several days to reach equilibrium under normal reaction conditions. Acids such as H2SO4, HCl, and HI have been employed successfully to catalyze the esterification reaction. When polysaccharides are involved, the modification requires milder acids to avoid hydrolysis. As indicated in Table 1, we initially investigated the ability of different organic and inorganic acids to catalyze and improve Table 1. Initial Examination and Screening NaPH2O2 (20 wt %) additive (2 wt %)

citric acid + polysaccharide ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ polysaccharide−citrate 1

2

entry polysaccharide 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a

starch starch starch starch starch starch starch starch starch cellulose cellulose cellulose cellulose cellulose cellulose cellulose

H2O,6 h,115 ° C

3

additive

polysaccharide− citrate

yield (%)

none tartaric acid malonic acid oxalic acid phosphoric acid FeCl2·4H2O FeCl3·6H2O LiOH NaOH none tartaric acid malonic acid oxalic acid phosphoric acid FeCl2·4H2O FeCl3·6H2O

3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3k 3l 3m 3n 3o 3p

38 98 68 60 41 27 25 10 10 nda nda nda nda nda nda nda

Not determined.

the esterification reaction of citric acid 1 and polysaccharide 2. Organic acids such as tartaric acid and malonic acid, with pKa values of 2.89 and 2.83, respectively (Table 1, entries 2 and 3), exhibited good catalytic activities and furnished the ester polysaccharide−citrate 3 in higher isolated amounts and with

Figure 1. Carboxylic acid contents of polysaccharide−citrate 3. 17599



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4.3. Foam Morphology. The foamed materials that were produced from starch−citrate−chitosan and microcrystalline cellulose−citrate−chitosan were of low density. In Figure 3, scanning electron microscopy (SEM) images of the produced foams show continuous irregular pore structures

indicate that the organic acid additives tartaric and malonic acids, with suitable and moderate pKa values, improved the synthesis by successfully providing the desired ester products 3b,3c and 3k,3l with increased carboxylic acid contents. The results presented in Figure 1 indicate higher carboxylic acid contents in starch−citrate 3 products than in MCC−citrate 3 products. We believe that this is due to the solubility of the unreacted starch in water in the work-up step, leading to more pure starch−citrate 3 products. On the other hand, MCC is insoluble in water, thus resulting in a mixture of unreacted MCC and MCC−citrate 3 products after the work-up step, which show lower carboxylic acid contents compared to the starch−citrate 3 products. As shown in Figure 1, the stronger acids, such as phosphoric acid and oxalic acid, were less successful and provided ester products 3d,3e and 3m,3n with poor isolated amounts and low carboxylic acid contents. Acids with pKa values lower than 2.5 can hydrolyze polysaccharides, which is a disadvantage to the esterification reaction. In addition, we measured the improvement in the yield of the isolated starch−citrate 3b−3d when organic acids were used as additives (Table 1, entries 2−4). The higher the carboxylic acid content in the intermediate polysaccharide−citrate 3, the more improved the reaction with chitosan 4, and the more successful the amide bond formation in the final network structure polysaccharide−citrate−chitosan 5. Furthermore, depending on the nature of the additives, the appearance and color of the solid foams were white to light yellow/brown. The chemical structures of pure polysaccharide−citrate 3, chitosan 4, and polysaccharide−citrate−chitosan foam 5 were analyzed by FT-IR spectroscopy. As shown in Figure 2, the peak at 1720 cm−1 for polysaccharide−citrate 3 is

Figure 3. SEM images of (left) starch−citrate−chitosan and (right) MCC−citrate−chitosan.

with platelike solid pore walls. In general, the macropores in starch−citrate−chitosan foams are larger than those in MCC− citrate−chitosan foam. Furthermore, to describe the micro- and mesopore structures, the surface areas and pore-size distributions were determined by nitrogen adsorption−desorption isotherms at 77 K. However, because of the low Brunauer−Emmett−Teller (BET) surface area (1.5−3.5 m2/g) measured from the foam samples, the data were used only to provide a trend. In Figure 4, the nitrogen isotherms of samples prepared were of type IIb

Figure 2. FT-IR spectra of the products of different reaction steps: (a) starch-based foam, (b) MCC-based foam, (c) starch−citrate ester, (d) MCC−citrate ester, and (e) chitosan.

Figure 4. Nitrogen adsorption−desorption isotherms of starch-based foam.

assigned to the carbonyl stretching of ester bonds formed between citric acid 1 and polysaccharide 2.19−21 From further analysis of the FT-IR spectra of polysaccharide−citrate− chitosan foam 5, we observed peaks at 1635 and 1538 cm−1 assigned to amide I and amide II, respectively.22 The peak observed at 1708 cm−1 confirms the amide bond formation in polysaccharide−citrate−chitosan 5 through reactions of the amine groups of chitosan 4 and the carboxylic acid groups of polysaccharide−citrate 3, in agreement with previously reported work.7,8,23

with a hysteresis loop, typical for platelike structures with nonrigid slit-shaped pores.24 The H3 type of hysteresis might due to wide capillaries having narrow openings and an interstice between the parallel plates.25 In Figure 5, the two types of foams produced from starch and MCC can be characterized by a Barrett−Joyner−Halenda (BJH) pore-size distribution sharply peaked in the mesoporous range between 20 and 27 17600



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the ester and amide covalent bonds play an important role in the water interaction performances of foams. Thus, a higher carboxylic acid group content in the intermediate product polysaccharide−citrate 3 leads to a higher and more efficient formation of covalent amide bonds in polysaccharide−citrate− chitosan, which is stable in aqueous medium. The possibility of amide bond formation can be predicted from the free carboxylic acid content in the polysaccharide− citrate intermediate 3. As shown in Figure 7, starch-based foams

Figure 7. Water absorption and sample weight loss of starch−citrate− chitosan samples 5a−5g.

produced from intermediates 3 with lower contents of carboxylic acid groups showed poor water absorption performance (lower water-to-sample weight ratio). For example, foam 5c produced from starch−citrate 3c showed higher stability in water than foam 5d, which was produced from 3d. Starch− citrate 3c has a higher carboxylic acid content than starch− citrate 3d (Figure 1), which results in improved and increased amide bond formation in the final foam 5. The higher the degree of amide bond formation, the higher the stability of the material in aqueous medium. The weight loss data presented in Figure 7 indicate that a portion of the material was dissolved and discarded with the water during the experiment. In the case of microcrystalline cellulose-based foams, the results in Figure 8 indicate a trend similar to that observed in Figure 7. The foamed materials 5k−5m produced from MCC− citrate 3k−3m (Table 1, entries 11−13) with higher carboxylic

Figure 5. BJH pore-size distributions for (a) starch-based foam and (b) MCC-based foam.

nm. The t-slopes shown in Figure 6 for the studied materials indicate very little or no microporosity in the samples.

Figure 6. t-slopes of MCC-based foam.

The densities of foamed materials 5 were found to be strongly related to the solid-to-liquid ratio in synthesis step 2 and slightly related to the starting materials.7,8,26 In this study, the bulk density of most of the produced foams was around 15 kg/m3. By changing the process parameters, bulk densities in the range of 10−25 kg/m3 were obtained. Based on the bulk density of the foams, it can be calculated that the total pore volume occupied 99% of the materials’ apparent volume. 4.4. Water Interaction of Polysaccharide−Citrate− Chitosan Foam 5. Previous investigations7,15 showed that

Figure 8. Water absorption and sample weight loss of MCC−citrate− chitosan samples 5j−5p. 17601



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citrate 3 leads to higher and more efficient amide covalent bond formation in the next step of the synthesis.

acid group contents (Figure 1) showed better results in terms of water absorption and weight loss than foams produced from MCC−citrate with lower carboxylic acid contents (5l vs 5n, Figure 8). The results depicted in Figures 7 and 8 confirm that the formation of amide bonds in the polysaccharide−citrate− chitosan foams 5 have a strong effect on the material performance in water. During the dynamic contact angle measurements, the probe drops were absorbed by starch- and MCC-based foams within 0.3 s. This indicates that the samples had a much higher water absorption rate compared with most commercial superabsorbents. 4.5. Mechanical Properties of Polysaccharide−Citrate−Chitosan Foam 5. Foams that were produced from starch−citrate−chitosan and microcrystalline cellulose−citrate−chitosan were of low density, elastic and compressible. The compressive stress−strain curves of the foam samples produced from starch and MCC conditioned at 50% RH and 23 °C are presented in Figure 9. As shown in Figure 9, the

5. CONCLUSIONS We have successfully produced low-density, elastic, and compressible polysaccharide-based foams by freeze-casting of polysaccharide−citrate−chitosan 5 solutions that were synthesized from polysaccharide 2 and chitosan 4. The FT-IR spectra of polysaccharide−citrate−chitosan foams 5 produced by different methods a were found to be re identical. The use of an organic acid as a catalyst efficiently improved the esterification reaction, and the isolated yield of intermediate product polysaccharide−citrate 3 increased by approximately 4 times. The stiffness of the foam material was found to have a strong relationship to the liquid/solid ratio and the content of amide bonds in the foam material. Low-temperature oven heating with the addition of a catalytic amount of organic acid, followed by oil-bath heating for cross-linking with chitosan 4, proved to be most efficient in producing low density, elastic, compressible foams with very low weight loss. The high water uptake and fast absorption rate reflected in the water interaction measurements indicated the potential to use this kind of foams in the water absorption applications to complement the low absorption rate of superabsorbent. In our ongoing research, we are working on locating more parameters to improve the wet stability of biobased foams. Further on, we are investigating approaches toward new methods for more low-cost and practical foam production. We will also investigate the effect of salinity on the absorption efficiency.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +46 60 148706. E-mail: [email protected]. *Tel.: +46 60 148877. Fax: +46 60 148820. E-mail: Magnus. [email protected]. Notes

Figure 9. Compressive stress−strain curves of (A) MCC-based foam, (B) starch-based foam, and (C) concentrated starch-based foam.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Bo Rydin Foundation for Scientific Research, the Administrative Board of the County of Västernorrland, and the EU structural Funds Objective 2 is acknowledged. We also thank the SCA R&D center (Sundsvall, Sweden) for their collaborative contribution.

curves display the same trend, the typical characteristics of elasto-plastic foams.2 The stress−strain curves of the foam samples produced either by the addition of catalytic amount of an organic acid or without, showed similar elastic behavior. Thus, the acid catalyst shows no effect on the mechanical property of the foams (curves A and B, Figure 9). Furthermore, starch-based foam produced by freeze-casting with a lower liquid/solid ratio (higher concentration, curve C, Figure 9), display higher stress at the same strain, compared to starch foams with a higher liquid/solid ratio (curve B, Figure 9). These results are in agreement with the fact that the porosity and pore morphology of the foamed materials are depending on the solidification conditions and the amount of water crystals totally removed by freeze-casting.26 In addition, the results in Figure 9 indicated that the starch-based foam clearly display higher stress than MCC-based foam at the same strain. The higher amide bonds presence in the foamed material is believed to be responsible for the higher stress performance of starch-based foam. As mentioned before, the starch−citrate intermediate 3 showed higher carboxylic acid group content than the MCC−citrate intermediate 3, a higher carboxylic acid group content in the intermediate product polysaccharide−

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REFERENCES

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Acid-Catalyzed Synthesis of Foamed Materials from Renewable Sources

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