Sustainable Films and Coatings from Hemicelluloses: A Review

June 2008

Published by the American Chemical Society

Volume 9, Number 6

 Copyright 2008 by the American Chemical Society

Reviews Sustainable Films and Coatings from Hemicelluloses: A Review Natanya M. L. Hansen and David Plackett* Risø National Laboratory for Sustainable Energy, Technical University of Denmark, P. O. Box 49, DK-4000 Roskilde, Denmark Received January 17, 2008; Revised Manuscript Received March 5, 2008

This review summarizes the results of past research on films and coatings from hemicelluloses, biopolymers that are as yet relatively unexploited commercially. The targeted uses of hemicelluloses have primarily been packaging films and coatings for foodstuffs as well as biomedical applications. Oxygen permeability of hemicellulose films, an important characteristic for food packaging, was typically comparable to values found for other biopolymer films such as amylose and amylopectin. As expected, the modification of hemicelluloses to create more hydrophobic films reduced the water vapor permeability. However, modified hemicellulose coatings intended for food still exhibited water vapor permeabilities several magnitudes higher than those of other polymers currently used for this purpose. Research on hemicelluloses for biomedical applications has included biocompatible hydrogels and coatings and material surfaces with enhanced cell affinity. Numerous possibilities exist for chemically modifying hemicelluloses, and fundamental studies of films from modified hemicelluloses have identified other potential applications, including selective membranes.

Introduction Sustainable sources of materials to supply the needs of society in the coming decades are much needed at present as the world becomes increasingly aware of the limited nature of fossil fuels. In response to this situation, lignocellulosic biomass from trees, grasses, cereals, and other plants, has become the main focus of the developing biorefining industry.1 Plant materials are primarily made up of three main types of biopolymer: cellulose, lignin, and hemicellulose, and of these, cellulose and lignin have received by far the most attention in terms of material applications. While cellulose has a unique structure, the term hemicellulose is used to describe a number of noncrystalline hexose and pentose sugars. Traditionally, hemicellulose is defined as the alkali-soluble material after the removal of pectic substances from plant cell walls.2 Four main groups of hemicelluloses may be defined according to their primary structure: xyloglycans (xylans), mannoglycans (mannans), β-glucans, and xyloglucans.3 Subgroups are further defined within the main groups. In most cases, xylans consist of a β(1f4)-D-xylopyranose (Figure 1) backbone with side groups on the 2- or 3-position. Nonbranched homoxylans with β(1f3, 1f4) or β(1f3) * To whom correspondence should be addressed. E-mail: david.plackett@ risoe.dk.

glycosidic linkages occur in certain seaweeds. Heteroxylans include glucuronoxylans and arabinoxylans as well as structures with more complex substitution patterns often referred to simply as heteroxylans. Glucuronoxylans have a side chain on the 2-position of either R-D-glucuronic acid or its 4-O-methyl derivative, while arabinoxylans are substituted on position 2 and/or 3 with R-L-arabinofuranosyl residues. Two types of mannans exist, namely, galactomannans consisting of β(1f4) linked D-mannopyranoses and glucomannans comprised of D-mannopyranose and D-glucopyranose with β(1f4) linkages. Both types of mannoglycans have varying degrees of branching with D-galactopyranose residues in the 6-position of the mannose backbone. β-glucans have a Dglucopyranose backbone with mixed β linkages (1f3, 1f4) in different ratios and can form highly viscous solutions and gels.3 Xyloglucans have a backbone of β(1f4) linked D-glycopyranose residues with a distribution of D-xylopyranose in position 6. The distribution of the side chains divides the xyloglucans in two categories: one with two xylopyranose units followed by two glucopyranose units (termed XXGG) and one with three xylopyranose units followed by a single glucopyranose unit (termed XXXG). Additionally, a number of side chains occur

10.1021/bm800053z CCC: $40.75  2008 American Chemical Society Published on Web 05/06/2008

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Figure 1. Main constituents of hemicelluloses.

on xyloglucans, including R-L-arabinofuranose, which makes the characterization of this group of hemicelluloses especially challenging. Research on some of the hemicelluloses has been extensive. Konjac glucomannan derived from the tuber Amorphophallus konjac is an example. This hemicellulose has been utilized commercially for many years,4 but research has recently intensified because of the gel- and film-forming properties as well as biocompatibility and biodegradability of this polysaccharide. These properties have found use in numerous applications in areas such as drug delivery, cellular therapy, encapsulation, and emulsification.5 Another example of an extensively used hemicellulose is xyloglucan from tamarind seed, which is also utilized in drug delivery. The use of this and other polysaccharides for hydrogels in release formulations is the topic of a recent review by Coviello et al.6 Hemicellulose in the plant cell wall is bound to cellulose and lignin and detailed isolation procedures are required to separate these components from plant raw material. The composition of hemicellulose varies between different feedstocks, as well as between sources, depending on factors such as origin and growth stage. An example is a recent study of four rice varieties, which showed significant variations of the main components arabinose (5–23%), xylose (17–40%), and glucose (36–55%) between the different cultivars.7 A number of methods are used to obtain hemicelluloses from plant sources, including extraction with alkali, dimethyl sulfoxide, or methanol/water, as well as steam or microwave treatment.8 The composition of the extracted hemicellulose can be highly dependent on the isolation process and, for example, deacetylation, degradation, and contamination with lignin can occur. Work by Sun et al.9 illustrates the dependency of composition on the isolation procedure, as the pretreatment of wheat straw samples with various organic solvents before extraction resulted in markedly different hemicellulosic products (Table 1). This review is written in the context of ongoing biofuels research at Risø DTU10–12 and an interest in exploring higher value uses for hemicelluloses generated as a byproduct from bioethanol production. For this reason, a review of past research on material applications of plant-derived hemicelluose was considered appropriate so that potential new research directions might be identified. In the following, we review past research on the use of hemicelluloses to form films and coatings. Existing patents and filed patent applications in the field are also discussed. The formation of films from hemicellulose acetates was reported as early as 1949 by Smart and Whistler.13 Films and coatings from renewable materials have numerous potential applications in the food and medicinal industry including active food packaging, wound dressings, and drug capsules.14–19 The use of a plasticizer is often necessary to ensure flexibility and the most commonly used for hemicellulose films are sorbitol, glycerol and xylitol (Figure 2).

Hansen and Plackett

Hemicelluloses are hydrophilic in nature and films produced from these materials are generally hygroscopic, resulting in poor properties in environments with high humidity. As recently described in review papers, chemical modification of the polymer by either bulk or surface modification is a way to circumvent these problems.3,8,20,21 Hemicellulose, like cellulose, has an abundance of free hydroxyl groups distributed along the backbone and side chains and is, therefore, an ideal candidate for chemical functionalization. By forming hemicellulose derivatives through functionalizing available hydroxyl groups, properties such as crystallinity, solubility, and hydrophilicity may be modified. Researchers have explored these options using techniques such as esterification, etherification, or grafting methods. Hemicellulose esters have been formed through reactions with acid chlorides or anhydrides. For example, acetylation of hemicellulose can be performed under similar conditions as for cellulose with an expected increase in hydrophobicity. Sulfation has been used to enhance biological activity. Etherification reactions including carboxymethylation, alkylation, and benzylation have been explored, while cationic moieties have been introduced by various pathways. The free hydroxyl groups in hemicellulose have been used as initiator sites to perform polymerizations of a number of monomers to form grafts.8

Packaging The use of renewable resources for the production of food packaging in particular has recently received increased interest.22–24 Depending upon the application, low oxygen permeability as well as mechanical strength and flexibility can be important target properties for such packaging films. The ultimate goal of a study performed by Höije et al.25 was to produce oxygen barrier films or coatings. However, the main focus of the investigations was the influence of the pretreatment utilized in the isolation of hemicellulose from barley husks. Arabinoxylan was subjected to four different types of pretreatment before alkaline extraction: enzyme pretreatment, acidic pretreatment, or acidic pretreatment combined with either chlorite or organosolv (ethanol) delignification. The purpose of the prehydrolysis step was to remove proteins and delignification was shown to be essential to achieve this objective. The type of treatment had a great impact on the yield of arabinoxylan as well as the molecular weights of the final product (Mw ) 35000–45000 g mol-1). The four extracted arabinoxylans were soluble in hot water and could be cast as colored films. The chlorite delignification was deemed to be the most efficient (57% yield), and films cast from this arabinoxylan were studied extensively. The films were strong, with a stress at break of more than 50 MPa, elongation at break of 2.5%, and a Young’s modulus of 2900 MPa. Furthermore, the films were shown to be highly hygroscopic with a primarily amorphous structure. Oxygen barrier properties of glucuronoxylan from aspen wood have been examined by Gröndahl et al.26 Films from unmodified xylan were brittle (Tg ∼ 180 °C), and therefore, sorbitol and xylitol were used as plasticizers in concentrations of 20, 35, and 50 wt %. The glass transition of the plasticized films was determined by differential scanning calorimetry (DSC) to be below ambient temperature. All the films were semicrystalline (44–47%), regardless of plasticizer content. Tensile testing showed that the addition of 20 wt % plasticizer resulted in stress at break of more than 40 MPa; however, the elongation was only 2%. Increasing the plasticizer content resulted in a reduction of strength and concurrent increase in elongation.

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Table 1. Sugar Composition (Weight: % of Total Weight Hemicellulose) of Wheat Straw Samples Subjected to Different Organic Solvent Pretreatmentsa pretreatment acetic acid-H2O (80/20, v/v)

a

formic acid-acetic acid-H2O (20/60/20, v/v/v)

methanol-H2O (60/40, v/v)

ethanol-H2O (60/40, v/v)

arabinose xylose mannose galactose glucose uronic acids

3.0 80.3 0.4 1.1 15.1 5.5

5.3 74.3 1.8 3.6 14.9 5.2

16.5 67.7 1.9 4.1 9.8 6.6

15.9 67.8 1.0 3.9 11.8 6.3

Mw Mw/Mn

14650 6.0

13350 4.5

42710 8.4

44080 8.9

With 0.1% HCl as catalyst at 85 °C for 4 h. Post-treatment with 1.8% H2O2-0.18% NH2CN at 50 °C and pH 10 for 4 h.9

Figure 2. Commonly used plasticizers for the formation of films from hemicellulose.

Figure 3. Gas-phase fluorination of hydroxyl groups of arabinoxylan with trifluoroacetic anhydride.28

Sorbitol addition had the most pronounced effect on elongation. The storage modulus decreased with humidity, while increased plasticizer amount had the same effect. An oxygen permeability of 0.21 cm3 µm m-2 d-1 kPa-1 at 50% RH was reported for a film plasticized with 35 wt % sorbitol, which was identical to the value found for a poly(vinyl alcohol) (PVOH) film under the same conditions. PVOH is an excellent barrier material with good thermal and mechanical properties that, furthermore, is water soluble and biodegradable.27 Surface fluorination of arabinoxylan (AX) films was proposed by Gröndahl et al.28 as a method to produce hydrophobic films with a potential as food packaging. Arabinoxylan from barley husks was cast as films and then subjected to gas-phase fluorination with trifluoroacetic anhydride (Figure 3). The attachment of the fluorinated moiety was confirmed by FT-IR, while attenuated total reflectance IR (ATR-IR) verified the functionalization of the surface and not of the bulk. Static contact angle measurements demonstrated the change in character of the films, as the water contact angle increased from 30° for an untreated film to 70° for a film with a content of 7% fluorine. The hydrophobic modification furthermore resulted in a decrease in the equilibrium moisture content from 18 to 12%. The hemicellulose O-acetylgalactoglucomannan (AcGGM) originating from wood was studied by Hartman et al. as a potential oxygen barrier.29 AcGGM was obtained by concentration of process water from thermomechanical pulping. A water soluble product with Mw ∼ 10000 g mol-1 (PDI ∼ 1.3) was isolated by ultrafiltration. Films of 30–60 µm were cast with 21–25 wt % of one of the plasticizers glycerol, sorbitol, or xylitol. In addition, mixtures (2:1 on weight basis) of AcGGM with alginate (Mw ) 100000–200000 g mol-1) or carboxymethylcellulose (CMC) (Mw ) 100000–150000 g mol-1) were utilized for film formation. The plasticized films showed a decrease in storage modulus with increasing humidity, which was especially pronounced for the AcGGM film containing

glycerol (Table 2). The hybrid films consisting of polymer blends exhibited better mechanical behavior than the plasticized AcGGM films, retaining their mechanical properties at elevated humidity as well as generally having larger storage moduli. A blend film of alginate and AcGGM was plasticized with glycerol and demonstrated intermediate mechanical properties compared to the equivalent two-component films. The elongation of the films after DMA measurements mirrored the mechanical stability, as the highest elongation, 195%, was found for glycerol plasticized AcGGM, while the hybrid samples containing alginate or CMC gave elongations of 4 and 3%, respectively. Oxygen permeabilities were the lowest for the AcGGM-alginate and AcGGM-CMC films. The sorbitol-containing AcGGM film had an oxygen permeability of 2.0 cm3 µm m-2 d-1 kPa-1, which was markedly lower than the value of 4.4 cm3 µm m-2 d-1 kPa-1 found for the xylitol-plasticized film. It was not possible to perform measurements on the glycerol-plasticized film; however, the alginate-AcGGM film containing this component (17.5 wt %) had the highest oxygen permeability: 4.6 cm3 µm m-2 d-1 kPa-1. A second study of AcGGM was performed by Hartman et al.30 and an overview of the films formed in this work is given in Figure 4. AcGGM was mixed with alginate or CMC at a ratio of 7:3 (weight basis) and the blends were cast as films. The formed films were modified by either vapor-phase grafting with styrene or plasma treatment followed by styrene grafting to increase hydrophobicity. Benzylation of AcGGM was undertaken with benzyl chloride in alkaline solution (NaOH of varying concentration), which resulted in deacetylation of the ester groups, rendering them susceptible to benzylation (Figure 5). The benzylated AcGGM (BnGGM) formed films that were transparent, strong, and flexible. A blend of alginate and AcGGM was additionally benzylated and then cast as film. BnGGM was laminated on to an AcGGM-alginate film by dipping the latter in a solution of the former component. To our knowledge, lamination is a novel approach to forming multilayer films based on hemicellulose. The oxygen permeabilities of the BnGGM films were several magnitudes higher than those found for the blend films, as values of 130 and 559 cm3 µm m-2 d-1 kPa-1 were found at 50% RH (Table 3) and permeability increased with DS. The AcGGM-alginate and AcGGM-CMC blend films exhibited significantly lower oxygen transmission rates under the same conditions but were moisturesensitive at higher humidity (83% RH), while the BnGGM films retained their oxygen barrier properties. An excellent oxygen permeability value of 8 cm3 µm m-2 d-1 kPa-1 was measured for BnGGM-laminated AcGGM-alginate film. Comparison of an unmodified AcGGM film with a BnGGM film in DMA

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Table 2. Properties of O-Acetylgalactoglucomannan (AcGGM) and Blend Films29 material

plasticizer

AcGGM AcGGM AcGGM AcGGM-CMCb AcGGM-alginate AcGGM-alginate

glycerol sorbitol xylitol

a

Could not be obtained.

glycerol b

storage modulus 30% RH (MPa)

storage modulus 60% RH (MPa)

2.8 2.8 3.0 3.1 3.2 3.4

1.8 2.6 2.4 3.0 3.2 3.0

O2 permeability (cm3 µm m-2 d-1 kPa-1) a

2.0 4.4 1.3 0.6 4.6

elongation after DMA (%) 195 17 a

3 4 17

CMC: carboxymethylcellulose.

Figure 4. Overview of the utilization of AcGGM for the formation of films.30 The emphasized boxes each represent a produced film and, in some cases, the label of the film is indicated in italics.

Figure 5. Benzylation of AcGGM performed by Hartman et al.30

measurements demonstrated that while the storage modulus was lower for the BnGGM sample at 20% RH (2.5 MPa vs 3.5 MPa) this film retained its mechanical properties at increased humidity (up to 70% RH), which was not the case for the AcGGM film. Static contact angle measurements on all the films showed that the hydrophobicity was not altered to a great extent by the chemical modifications, as water contact angles ranged from 45 to 78° (Table 3) and a contact angle of 63° was measured for untreated AcGGM. There was a difference in terms of water droplet adsorption time as, unlike the other films, water droplets were retained on the BnGGM film surface even after 10 min. The unmodified films dissolved rapidly when immersed in water, whereas only minor weight losses (16–28%) were registered for the BnGGM films after a prolonged period of immersion. The films with surface-grafted styrene were also water soluble; however, dissolution was much slower than for the unmodified films. Oxygen permeability data for a number of biobased polymers as well as more traditional packaging materials are shown in Table 4. Although it can be difficult to compare permeability data obtained by various research groups using different

equipment, procedures, and units in which to express permeability results, the data in Table 4 give an indication of the oxygen permeability of the materials. Comparing the oxygen barrier properties of materials derived from hemicellulose with the other materials, the former are seen to be very promising as new materials in the field. Films from amylose and amylopectin obtained from starch have good barrier properties with permeability values in the same range as the studied hemicellulose films (i.e., 7 and 14 cm3 µm m-2 d-1 kPa-1, respectively31). Chitosan, poly(vinyl alcohol) PVOH, and ethylene vinyl alcohol (EVOH) films have excellent oxygen barrier properties with values below 0.5 cm3 µm m-2 d-1 kPa-1 in all cases, which was matched by the xylan film produced by Gröndahl et al.26 Comparable but slightly higher oxygen permeabilities were found by Hartman et al.29 for an AcGGM film as well as for blend films of AcGGM with CMC and alginate (with and without grafted styrene). Research in the area of oxygen barrier materials from hemicellulose is ongoing and seems to hold great promise for future practical applications.

Coatings Edible coatings for foodstuffs are often necessary to prolong shelf life and maintain important properties such as texture, taste and mouthfeel. The uptake of moisture is often a crucial factor in the rate of degradation of a given product, therefore coatings acting as moisture barriers are of the utmost interest in the food industry. Two parameters are defined, whereby the barrier properties of a film or coating may be assessed: the water vapor transmission rate and the water vapor permeability. Both parameters are determined by gravimetrically measuring the



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Table 3. Properties of O-Acetylgalactoglucomannan (AcGGM) and Blend Films Subjected to Different Treatments30 film abbreviation

G30A BnGGM1

a

O2 permeability 50% RH (cm3 µm m-2 d-1 kPa-1)

material

treatment

AcGGM AcGGM-CMCa AcGGM-alginate AcGGM-alginate AcGGM AcGGM G30A film G30A film

none none none benzylated benzylated benzylated laminated with BnGGM1 solution vapor phase grafting with styrene

CMC: carboxymethylcellulose.

b

Not measured. c Not obtained.

d

O2 permeability 83% RH (cm3 µm m-2 d-1 kPa-1)

b

b

1.28 0.55

c

b

b

130 559

170 546 8d

b

1.75

contact angle (°) 63 54 78 45 70 57 63 72

c

c

Measured at 73% RH.

Table 4. Oxygen Permeabilities of Packaging Material Measured at 23 °C, 50% RH material

plasticizer

AcGGMa AcGGM-alginate (7:3) AcGGM-alginate (7:3) vapor phase grafted with styrene AcGGM-CMCa amylopectin amylose chitosan ethylene vinyl alcohol (EVOH) low-density polyethylene (LDPE) poly(lactic acid) (PLA) poly(hydroxyalkanoate (PHA) poly(vinyl alcohol) (PVOH) whey protein xylan

sorbitol (21 wt %)

glycerol (40 wt %) glycerol (40 wt %) glycerol (25–50 wt %)

glycerol (25–60 wt %) sorbitol (35 wt %)

O2 permeability (cm3 µm m-2 d-1 kPa-1)

reference

2.0 0.6 1.8

Hartman et al.29 Hartman et al.29 Hartman et al.29

1.3 14 7 0.1–0.4b 0.3b,c 7900b,c 160b,c 150b,c 0.21 50–325c 0.21

Hartman et al.29 Rindlav-Westling et al.31 Rindlav-Westling et al.31 Butler et al.32 van Tuil et al.33 van Tuil et al.33 van Tuil et al.33 van Tuil et al.33 Gröndahl et al.26 Sothornvit et al.34 Gröndahl et al.26

a AcGGM: O-acetylgalactoglucomannan; CMC: carboxymethylcellulose. b Values recalculated from the original paper. c Values estimated from diagrams in the original paper.

transport of water through a film specimen. The water vapor transmission rate, WVTR is defined as

WVTR )

∆m ∆t × A

where ∆m is the change in weight, ∆t is the elapsed time, and A is the area of the tested film. The water vapor permeability (WVP) is defined as

WVP )

WVTR × L ∆p

where L is the film thickness and ∆p is the difference in vapor pressure on the two sides of the film. In the following, a number of studies on hemicellulose for edible food coatings are discussed. β-glucan extracts (75–79%) from hulled barley, hull-less barley, and oats were tested as water vapor barriers by Tejinder.35 These hemicelluloses were plasticized with glycerol (30 or 40 wt %) and cast on plexiglass plates to yield potentially edible films. The films were opaque, with opacity values depending on the concentration of the cast solution (2 or 4 w/v %). The tensile strength was also dependent on solution concentration, with higher values found with higher concentration (approximately a 2-fold increase). β-glucan source also influenced film tensile strength. Values for films cast from 4 w/v % solutions were expressed in Newtons (N), with the highest value found for oats (13 N) and the lowest for hulled barley (2.3 N). Water vapor permeability (WVP) of films also increased with an increasing solution concentration. A WVP of 4.5–6.1 g mm m-2 h-1 Pa-1 was found for films cast from 4 w/v % solutions, while WVP values of 2.5–3.2 g mm m-2 h-1 Pa-1 were found for the films cast from less-concentrated solutions. Water vapor transmission rates were similar for all films and in the range 0.47–0.60 g m-2 h-1. The β-glucan-

based films from oats were highly water soluble, whereas the films of β-glucans from barley were less soluble, with the hullless barley samples being intact after immersion in water for 24 h. Skendi et al.36 tested water soluble β-glucans from different types of oats as potential materials for edible films. Molecular weights of 27000–85000 g mol-1 were obtained for products isolated by aqueous extraction, and these molecular weights were reduced to 18000–78000 g mol-1 after acidic treatment. Intrinsic viscosities increased with molecular weights with values of 4.9-6.4 dL g-1 calculated for samples ranging from 27000 to 85000 g mol-1 (different work-up methods). Rheological measurements showed that β-glucan solutions were viscoelastic fluids and exhibited shear thinning behavior. The β-glucans adopted gel properties in oscillatory measurements, behaving as an elastic gel network, with the gelation time increasing with increasing molecular weight (43 h for 78000 g mol-1). Tensile strengths of 20-80 MPa were observed for films cast from a sample of Mw ) 71000 g mol-1, where an increase in moisture and sorbitol (15 wt %) resulted in increased elongation and reduced tensile strength. For the above-mentioned films as well as films from a sample of Mw ) 18000 g mol-1 elongations in the range 1–22% and Young’s modulus of up to 6000 MPa were observed. Corn hull arabinoxylan (Mw ) 50600 g mol-1) was the main component of edible films produced by Zhang and Whistler.37 Films plasticized with propylene glycol, glycerol, or sorbitol (0–22 wt%) were cast on glass plates to thicknesses of 22–32 µm. Arabinoxylan films plasticized with propylene glycol were brittle and the properties were almost independent of plasticizer content with elastic moduli of 1290–1314 MPa, tensile strengths ranging from 53 to 61 MPa and elongations between 6 and 8%, which was reported to be similar to results for cellulose films

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containing this plasticizer. The properties varied much more with plasticizer content for the glycerol- and sorbitol-plasticized films with tensile strengths of 10–47 MPa and 20–48 MPa, respectively, while elongations ranged from 6 to 12% and 6 to 9%, respectively. The best moisture barrier properties were found for sorbitol-plasticized films with WVP ) 0.23 × 10-10 g m-1 Pa1- s-1 (13% sorbitol), while values of 0.31 × 10-10 and 0.36 × 10-10 g m-1 Pa1- s-1 were found when using propylene glycol (16%) and glycerol (5%), respectively, as plasticizers. A high content of glycerol (>10%) resulted in an increase in permeability due to the hygroscopic nature of the plasticizer. The reduction of moisture loss from grapes was tested with these films as coatings. The films were stable, smooth, and transparent and demonstrated good moisture barrier properties over a period of seven days with a reduction of weight loss from 82 to 59%. Furthermore, the coatings proved more efficient as moisture barriers than coatings of corn amylopectin and carboxymethyl cellulose films produced for comparison. Xylans from birchwood, grass, and corncob were used by Kayserilioglu et al.38 as additives in wheat gluten to form potentially edible composite films. The birchwood xylan was commercially available, whereas the grass and corncob xylans were isolated by alkaline extraction. Weight fractions of 0-40% birchwood xylan were mixed with wheat gluten and cast under acidic and alkaline conditions (pH of 4 and 11, respectively) and films were dried at ambient and elevated temperature (80 °C). Tensile testing showed tensile strength was not significantly dependent on composition but that values were 2–3 times higher for films cast at pH ) 11 than at pH ) 4, which could be due to the more uniform films formed during the casting process at high pH. In addition, drying at room temperature seemed to give the best tensile properties. The opposite effect was seen for the elastic modulus and the elongation, which both increased with lower pH and higher drying temperature. Films with 20 wt % xylan from grass or corn cob were cast from alkaline solution at ambient temperature and compared to the analogous birchwood xylan-containing film. A tensile strength of approximately 8 MPa was measured for the latter film (close to the value found for pure gluten), while considerably lower values were found for films from grass or corn cob (1.5–3 MPa). Corn cob xylan-containing films were the most stretchable with E ∼ 10 MPa and elongation of 600% (approximately 50% for the other two). In addition, corn cob xylan films were found to be the most homogeneous by SEM analysis. The xylan-gluten films did not lose their integrity in aqueous solution after 24 h and solubility was seen to depend on the drying conditions of the film as well as the source of xylan. Water vapor transfer rates were slightly lower for films cast at elevated temperature than for those cast at ambient temperature. However, values were of the same order of magnitude ranging from 6 × 102 to 8 × 102 g m-2 per 24 h. Numerous approaches to producing edible water vapor barriers have been attempted by a collaboration based in France.39–43 In all cases, arabinoxylan was extracted from maize bran and plasticized with glycerol (15 or 20 wt %). The first modifications entailed mixing the hemicellulose/glycerol (15 wt %) with palmitic acid (C16), oleic acid (C18), triolein, or a hydrogenated palm oil (HPKO).39 Emulsification of the lipids was ensured with the addition of glycerol monostearate and treatment with a homogenizer. Films of 65–100 µm thickness were cast from the emulsified lipids in AX-glycerol mixtures, as well as the plasticized AX. The films based on emulsified lipids were more opaque than the parent AX film. WVP of the homogeneous AX film was 1.8 × 10-10 g m-1 Pa1- s-1, while

Hansen and Plackett

Figure 6. Water vapor transmission rates (WVTR) and water vapor permeabilities (WVP) of films of arabinoxylan (AX), arabinoxylan containing hydrogenated palm kernel oil (AX-HPKO), palmitic acid (AX-C16), oleic acid (AX-C18), and triolein (AX-triolein), cellophane, and low-density polyethylene (LDPE).39

the lipid-containing films exhibited significantly lower values in the range of 1.2-1.5 × 10-10 g m-1 Pa1- s-1 (Figure 6). Similar behavior was observed for the water vapor transfer rate (WVTR) of AX-based films, where values of 2.7-3.5 × 10-3 g m-2 s-1 were found for the AX films with emulsified lipids compared to 3.9 × 10-3 g m-2 s-1 for the nontreated film. The lowest value for WVTR was found for the AX-HPKO film, while AX-triolein and AX-C18 performed best regarding permeability. Values of WVP were 3–100 times larger than for commercial cellophane and LDPE films investigated for comparison. Contact angles of water droplets on the film surfaces showed large differences in character, with measurements ranging from 39 to 94° (71° for AX), indicating dissimilarity in hydrophobicity. Tensile strength and elastic modulus were both reduced with the introduction of the lipids. The tensile strength of the AX film was 27 MPa but only 6–9 MPa for the mixed films. Similarly, the elastic modulus was reduced from 72 MPa to 26–59 MPa. Elongation was increased with the addition of triolein and HPKO, whereas it was reduced with palmitic (C16) and oleic (C18) acid. HPKO was the most promising lipid and studies of the AX films with emulsified HPKO were therefore continued.40 Four sucroesters with hydrophilic–lipophilic balance (HLB) values varying from 2 to 15 were utilized as emulsifying agents to obtain different globule sizes of the lipid so that the influence of this parameter on barrier properties could be assessed. Two or more sizes of lipid globules were present in all the emulsions as seen by laser light-scattering granulometry. The smaller globules were 0.5-0.9 µm in size, while the sizes of the larger ones were dependent on the nature of the emulsifying agent. The emulsion stability was governed by the concentration of the sucrose ester and to a lesser extent by the solubility of this compound in the aqueous phase. No direct correlation between globule diameter and water vapor permeability was found, as the former depends on the nature of the emulsifying agent. The most lipophilic sucroester SP10 (HBL ) 2) gave the best emulsion stability during drying and also the lowest WVP of 9.5 × 10-11 g m-1 Pa1- s-1, which was almost identical to the value found for an HPKO-containing film without sucrose ester and very close to 1.0 × 10-10 g m-1 Pa1- s-1 found for a hydroxypropylmethylcellulose (HPMC) film prepared in parallel to the AX films. This indicated that barrier properties may be improved by (1) stabilizing the emulsion during drying, (2) ensuring an even distribution of the lipid globules, or (3) by destabilization of the emulsion by aggregation (i.e., formation of a bilayer film structure with HPKO at the surface). Some WVP values found for the AX films are shown in Figure 7 in which the values measured for HPMC as well as for com-


Figure 7. Water vapor permeability values found for arabinoxylan films plasticized with glycerol, with and without added hydrogenated palm kernel oil (HPKO).40 Films produced by the same method from hydroxypropylmethylcellulose (HPMC) were prepared for comparison. Commercial cellophane and low-density polyethylene (LDPE) were also included in the study.

mercially obtained cellophane and low-density polyethylene films are also given. Studies on the influence of drying temperature of AX-based films were undertaken with the two sucrose esters SP10 and SP70 (HLB ) 15).41 Temperatures of 30, 40, and 80 °C were chosen as these are below, slightly above and much higher than the melting point of HPKO, respectively. Emulsion stability was seen to decrease with drying temperature resulting in the formation of bilayer films when drying at 80 °C. Increase in drying temperature also gave a slight decrease in permeability as well as an increase in static water contact angle. The presence of the sucrose esters gave rise to increased water absorption rates compared to the films containing plasticizer (glycerol) and lipid (HPKO), which was especially pronounced for the hydrophilic SP70. The mechanical properties were found to deteriorate with the addition of the lipid, realized by the reduction of both elongation and strength at break. Tensile strength at break was 0.8-3.0 times less than that determined for homogeneous AX film. Strength at break for the latter film was approximately 22 and 16 MPa at drying temperatures of 40 and 80 °C, respectively. Using a drying temperature of 80 °C reduced the strength at break and increased elongation compared to drying at 40 °C. Peroval et al.42 attempted a different approach with the AX films by grafting the monomers stearyl acrylate (SA) and stearyl methacrylate (SM) on to the films by electron beam treatment. Three distinct processes were carried out: (A) cold plasma treatment of glycerol-plasticized AX films followed by impregnation with monomer and grafting by electron beam, (B) emulsified films (i.e., films from blends of monomer, glycerol, glycerol monostearate, and AX) were electron-beam treated, (C) pretreatment of AX films with electron beam followed by impregnation with monomer and grafting by electron beam. Treatment A was also performed with HPKO as monomer, resulting in potentially edible films. Comparison of the films showed that the monomer in all cases induced increased hydrophobicity with static water contact angles increasing from 71° up to a maximum of 123° in the case of an SM-grafted film. Water permeability was decreased with the introduction of the monomers and was furthermore dependent on the grafting process. For SA and SM, treatment A was the least effective in reducing permeability. WVTR and WVP of SA-grafted films were lower or equivalent to values found for SM-grafted films in all cases. The lowest values found were for SA monomer,


1499

which had undergone treatment C and these were WVTR ) 1.5 × 10-3 g m-2 s-1 and WVP ) 6.8 × 10-11 g m-1 Pa1s-1. The plasma treatment followed by monomer impregnation and grafting (treatment A mentioned above) was also undertaken with omega-3 fatty acids from fish as well as linseed oil.43 Grafting levels of 5.1-6.4% were obtained with the oils. Contact angle measurements showed an increase for two of the fish oils and a decrease for the third, while linseed oil did not influence this parameter. The change in static contact angle over time varied for the oils indicating different modifications (surface vs bulk) when grafting with the various oils. Water vapor barrier properties were improved with grafting and most significantly with linseed oil, where WVTR of 2.9 × 10-3 g m-2 s-1 and WVP of 1.1 × 10-10 g m-1 Pa1- s-1 were found. AX was plasticized with 20 wt % glycerol (as opposed to 15 wt % used in the previous experiments) giving values of 4.5 × 10-3 g m-2 s-1 and 2.1 × 10-10 g m-1 Pa1- s-1 for WVTR and WVP, respectively, for the untreated film. Xylan, isolated from cotton stalks containing residual lignin, was used by Goksu et al.44 to form films intended for food packaging applications. To our knowledge, this is the only example in the literature of the formation of coherent films from xylan without the use of an additive or plasticizer. Complete lignin removal from xylan was shown to be detrimental to the film-forming properties, resulting in cracked films. Commercially obtained birchwood xylan was used as model compound to determine the lower limit of lignin needed to generate films. By adding lignin to birchwood xylan solutions a concentration of 1% (w/w, lignin/xylan) was assessed to be necessary for the formation of films. Solutions of 8, 10, 12, and 14% (w/w) cotton stalk xylan containing lignin in the aforementioned amount were cast to form films with thicknesses of 0.29–0.38 mm. Film thickness, tensile strength, strain at break, and elastic modulus generally increased with the concentration of the casting solution (Table 5). Values of WVTR were normalized to eliminate effects from varying film thickness, and these values decreased significantly from 917 (g m-2 day-1)/mm thickness for the 8% solution film to 574 (g m-2 day-1)/mm thickness for the 14% solution film. Films plasticized with glycerol (2% w/w) were furthermore prepared from a 10% xylan solution. These films were observed by SEM to have rougher surfaces than the nonplasticized films. The authors suggested that plasticizer addition could increase the free volume within the film and enhance water evaporation, leading to decreases in film thickness. WVTR was increased slightly by the addition of plasticizer, and tensile strength was reduced considerably from 1.34 to 0.76 MPa. All the generated films were water soluble (98–99% solubility). Past research in producing barrier films from hemicellulose (Table 6) demonstrates the difficulties of modifying a hygroscopic material to form a moisture barrier; however, modified hemicelluloses showed major improvements in the barrier properties compared to the parent compounds. Table 7 shows the water vapor barrier properties of several commercial polymers as well as a number of biomaterials. WVP and WVTR values of LDPE, polyethylene terephthalate (PET), PS, and PLA are several orders of magnitude lower than the corresponding values for the different hemicellulose films (Table 6). Corn hull arabinoxylan films37 exhibited slightly better barrier properties than cellophane. Films from cotton stalk xylan44 as well as from maize bran arabinoxylan with various hydrophobic modifications39–43 had comparable barrier properties to those of cellophane and HPMC. All the hemicellulose films targeted for coatings were superior to amylose and amylopectin films in

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Hansen and Plackett

Table 5. Water Vapor Transfer Rate (WVTR) and Strength Properties of Films Based on Cotton Stalk Xylan44 xylan concentration (% w/w)

thickness (mm)

tensile strength (MPa)

strain at break (%)

elastic modulus (MPa)

normalized WVTR ([g m-2 day-1]/mm thickness)

8 10 12 14 10, 2% (w/w) glycerol

0.29 0.32 0.37 0.38 0.30

1.08 1.34 1.27 1.39 0.76

45.6 48.7 52.5 56.8 88.9

0.11 0.37 0.44 0.49 0.08

917 832 699 574 905

terms of WVP values. Although improvements in barrier properties can be significant for modified hemicelluloses, these films are not yet capable of competing with commercial films from nonrenewable sources such as PET, LDPE, and PS in terms of water vapor barrier properties.

Biomedical Applications Considerable research has been done in the area of hemicelluloses for biomedical purposes due to the biocompatibility, biodegradability, and high stability of these compounds. As previously mentioned, a number of hemicelluloses are already widely used in the pharmaceutical industry,5,6 while others are as yet relatively unexploited. The formation of hydrogels using xylan extracted from birch wood has been studied by Gabrielii and Gatenholm.46 Films with thicknesses of 50 µm were cast with 0–35 wt % of chitosan under acidic conditions. Self-supporting films were formed with a chitosan content of 10 wt %, while hydrogels were obtained with 30 wt % chitosan. Dynamic mechanical measurements indicated the presence of two phases, substantiated by atomic force microscopy (AFM), in which the observed fibrils were believed to be the chitosan phase. Water uptake increased with chitosan content, most markedly between 15 and 30 wt %. Gabrielii et al.47 generated hydrogels utilizing hemicellulose from aspen wood. Xylan (Mw ) 73100 g mol-1, Mn ) 48000 g mol-1) with 12% branching (determined by two-dimensional NMR) was isolated by alkaline extraction. Self-supporting films were formed from mixtures of xylan with chitosan (10–100 wt %). Highly swellable hydrogels were formed with the addition of up to 20 wt % chitosan, while water-soluble films were formed at higher chitosan content. Pure xylan yielded brittle aggregates and was unable to form free-standing films. Crystallinity was reduced from 37 to 0% by increasing the chitosan amount from 0 to 100%, as seen by wide-angle X-ray spectroscopy. β-(1f3)(1f6)-glucan from Shiitake mushrooms was coated/ grafted onto the surface of poly(D,L-lactic-co-glycolic acid) film to enhance the cell affinity of the surface.48 The coating process was performed with and without plasma pretreatment, with the former procedure yielding the best results in terms of human dermal fibroblast cell proliferation on the surface. These films were envisioned to regenerate skin tissue effectively. Magnetite microparticles intended for oral intake and applied as magnetic markers for monitoring gastrointestinal motility were coated with xylan from corn cobs.49 The purpose of the polymer coating was to reduce dissolution of the particles in the patient’s stomach, ensuring passage into the colon. Comparison of coated and uncoated microparticles showed a difference in both size and size distribution. The coated spherical microparticles exhibited a more narrow, monomodal distribution with larger particle size (5-fold increase) than the uncoated particles. The superparamagnetic properties of the magnetite microparticles were found to be unaltered by the xylan coating.

Dissolution of magnetite was decreased significantly with the coating, as 28.5% of the uncoated particles were dissolved after 120 min in acidic media, whereas only 2.3% of the content was lost from the coated particles under the same conditions. As progress in the pharmaceutical industry leads to increased demands on materials for specific applications, more specialized drugs and methods of drug delivery will be necessary to fulfill requirements. A number of hemicelluloses such as guar gum (a galactomannan) are already commonly used in this field,50 but other less investigated hemicelluloses may find application in the future.

Fundamental Studies on Hemicellulose Films While many studies on hemicellulose films and coatings have described well-defined commercial targets, there are also a number of research papers that are focused on more fundamental research topics. This is especially characteristic of some of the work on chemically modified hemicelluloses, in which functionalization itself can appear to be the objective. In some cases, films have been cast from the resulting products and the properties of these have been assessed. Esterification was used by Moine et al.51 for the generation of hydrophobic films from beechwood xylan and maize bran heteroxylan. The beechwood xylan was identified as 4-Omethylglucuronoxylan, while the heteroxylan consisted of a mixture of xylose, arabinose, galactose, glucuronic acid, and its 4-O-methyl derivative. Lauroyl chloride was used to introduce esters on the hydroxyl groups of the xylans by a microwave-activated reaction in N,N-dimethylacetamide/LiCl catalyzed by dimethylaminopyridine. The degree of substitution (DS) was 1.3, as determined by 1H NMR for xylan, with a similar value of 1.2 found for the heteroxylan. FT-IR was used to confirm the substitution of the hemicelluloses. Elastic moduli of 366 and 157 MPa and stress at break of 15 and 6 MPa were found for dodecyl-grafted xylan and heteroxylan, respectively. Both the functionalized xylans showed elastic deformation up to 80 °C with a constant storage modulus observed in dynamic mechanical testing, which was also used to determine Tgs of 133 and 132 °C. Maize bran heteroxylan was used by Fredon et al.52 to produce hydrophobic films. The heteroxylan (Mw ∼ 28000 g mol-1) consisted of arabinose, xylose, galactose, glucuronic acid, and glucose monomer units. In addition to these hemicellulose components, some residual starch was also detected. Modification entailed oxidation with sodium periodate followed by reductive amination with dodecylamine and sodium cyanoborohydride. The result was ring opening and incorporation of dodecylamine on the remaining hemicellulose backbone (Figure 8). Thus, modified heteroxylans were obtained with DS ) 0.6–1.1, which were subsequently cast as films. Tgs were assessed to be between -30 and 0 °C, rendering plastic films at ambient temperature. DS influenced the mechanical properties, as elastic moduli of 118 and 87 MPa were found for substituted

maize bran

maize bran

maize bran

maize bran

maize bran

cotton stalk

arabinoxylan

arabinoxylan

arabinoxylan

arabinoxylan

arabinoxylan

xylan

a

b

25

25

glycerol (20 wt %) glycerol (20 wt %)


none

glycerol (2 wt %)

c

20

25

25


glycerol (15 wt %)

25

22

30 23

glycerol, propylene glycol, sorbitol (0–20 wt %) none glycerol (15 wt %)

glycerol (30/40 wt %) glycerol (2 wt %)

additives

7.72–13.70

17.7–20.5 9.31–13.82

20.5 6.8–17.2

17.7 10.9–19.1

0.76 1.08–1.39

2.52–3.08a

26.5

3.14a

3.92–4.45

4.45 1.50–3.77

3.92 2.86–3.89

53.8 6.4–8.8

2.71–3.52

4.7 11.8–15.2

0.91–13.02b 1–9c

tensile strength (MPa)

9.7–60.7

7.5–10.0a 6.9–9.3a

WVTR (10-3 g m-2 s-1)

2.3–4.3

68–169a

WVP (10-11 g m-1 Pa1- s-1)

Tensile strength given in N (as in the original paper). Values estimated from diagrams in the original paper.

emulsified films are generated with globules of stearic or palmitic acid, triolein, palm oil none grafting with fatty acids from fish oils on the formed film by plasma and electron beam treatment none grafting with stearyl (meth) acrylate on the formed film by plasma and/or electron beam treatment none mixing with sucroesters as emulsifiers and hydrogenated palm oil as hydrophobic phase mixing with sucroesters as emulsifiers and hydrogenated palm oil as hydrophobic phase residual lignin 1% (w/w lignin/xylan)

none mixing with wheat gluten (major component) none

modification

Values recalculated from the original paper.

β-glucan xylan

arabinoxylan

source

barley, oat birch, grass corncob corn hull

hemicellulose

test temperature (°C)

Table 6. Water Vapor Permeability (WVP), Water Vapor Transfer Rate (WVTR), and Strength Properties of Edible Hemicellulose Films

0.11–0.49

0.08

72.4

1316 25.84–59.2

365–1320

10–200c

elastic modulus (MPa)

reference

Goksu et al.44

Phan The et al.41

Phan The et al.40

Peroval et al.42

Peroval et al.43

Peroval et al.39

Zhang et al.37

Tejinder35 Kayserilioglu et al.38

Sustainable Films and Coatings from Hemicelluloses Biomacromolecules, Vol. 9, No. 6, 2008 1501

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Hansen and Plackett

Table 7. Water Vapor Permeability (WVP) and Water Vapor Transfer Rate (WVTR) of Selected Polymer Films material

test temperature (°C)

WVP (10-11 g m-1 Pa1- s-1)

amylose amylopectin cellophane low-density polyethylene (LDPE) hydroxypropylmethyl-cellulose (HPMC) polyethylene terephthalate (PET) polystyrene (PS), oriented poly(lactic acid) (PLA), oriented

23 23 25 25 25 37.8 37.8 37.8

119a 144a 6.9 0.19 10.0 0.28 0.42 1.34

a

WVTR (10-3 g m-2 s-1)

6.2 0.13 0.04a 0.06a 0.18a

reference Rindlav-Westling et al.31 Rindlav-Westling et al.31 Peroval et al.39 Peroval et al.39 Peroval et al.40 Auras et al.45 Auras et al.45 Auras et al.45

Values recalculated from the original paper.

Figure 8. Oxidation followed by reductive amination of vicinal hydroxyl groups of arabinofuranose to yield dodecylamine functionalized heteroxylan.52

Figure 10. Generation of cationic xylan derivatives by etherification with 2,3-epoxypropyltrimethylammonium chloride.54

Figure 11. Conversion of xylan to hydroxypropyl xylan (HPX) and further to acetoxypropyl xylan (APX).56

Figure 9. Esterification of xylan with furan-2-carboxylic acid assisted by N,N′-dicarbonyldiimidazole.53

xylans with DS of 0.7 and 1.1, respectively, while the corresponding elongations at break were 8 and 22% for these films. The casting method affected the film properties as seen by comparison of solution-cast and tape-cast films (DS ) 1.1). The tape-cast film exhibited increased elasticity with E of 57 MPa and an elongation of 38%. Hesse et al.53 synthesized furan-2-carboxylic esters of a number of polysaccharides targeted as potential macroporous membrane materials. 4-O-Methylglucuronoxylan from birch wood (Mw ) 13000 g mol-1) was among the chosen polysaccharides in addition to cellulose, curdlan, dextran, and starch. Esterification was performed with furan-2-carboxylic acid in the presence of N,N′-dicarbonyldiimidazole to yield photocrosslinkable polymers (Figure 9). Xylans with DS of 0.09 to 0.86 were obtained, of which the most substituted was soluble in DMSO and could be cast as self-supporting films. Atomic force microscopy was used to determine surface roughness, and this was found to be dependent on the backbone structure, with the most branched structures yielding the roughest films: Ra ∼ 150 nm and Rmax ∼ 1500 nm were determined for the xylan furoate. Pore sizes of 250-750 nm were found for this film by scanning electron microscopy (SEM). The pore size differed from the glass-film contact side to the film-solvent contact side, which was also the case for the other polysaccharide esters. Schwikal et al.54 functionalized 4-O-methylglucuroxylan from birch wood with 2,3-epoxypropyltrimethylammonium chloride to yield cationic xylan derivatives (Figure 10). The reaction was performed in aqueous NaOH and DS of 0.2-1.6 was obtained by varying base concentration, reaction temperature, and molar

ratio of the reactants. Turbidity tests indicated that the substituted xylans were water soluble, with a DS > 0.4. Films with thicknesses of 50 and 100 µm had pore sizes ranging from 250 to 1500 nm, depending on the concentration of the cast solution and the thickness of the final film. Thus, porous films with permanent positively charged ammonium groups as well as carboxylic acid groups were generated. Buchanan et al.55 produced composite films of esters of arabinoxylan and cellulose. Two hemicellulose fractions were isolated from corn fiber: hemicellulose A (Mw < 25000 g mol-1, water soluble at pH > 10) and hemicellulose B (Mw > 500000 g mol-1, water soluble). Acetate, propionate, and butyrate arabinoxylan esters were synthesized from the corresponding anhydrides. Pure hemicellulose A could not be substituted, whereas pure hemicellulose B and mixtures of the two could be modified with DS ) 1.6–2.3. Onset of degradation for the three esters was found to be above 225 °C by thermogravimetric analysis (TGA) with the highest thermal stability for the butyrate ester. Esterification reduced Tg of the xylan and this was most pronounced with butyrate functionalization, resulting in a reduction from 198 to 61 °C. Commercial cellulose acetate (CA), (DSA ) 2.47) was mixed with arabinoxylan acetate in solution and used to cast optically clear films with compositiondependent Tgs, as seen by DSC. The blends were comprised of one or two phases depending on the type and concentration of solvent. Storage experiments indicated that phase separation also took place over time (8 months). In contrast, mixtures of arabinoxylan butyrate and cellulose acetate butyrate (DSA ) 1.03, DSB ) 1.72) exhibited macroscopic phase separation and formed opaque films under all casting conditions. Jain et al.56 functionalized xylan from barley husks with propylene oxide to yield hydroxypropyl xylan (HPX), which in turn was acetylated with acetic anhydride to produce acetoxypropyl xylan (APX) (Figure 11). HPX formed tough transparent films, which were water soluble at low DS (0.2–0.5) and exhibited tensile strengths of 35–45 MPa. In contrast, films


cast from APX were soluble in a number of organic solvents. No Tg was observed for the unsubstituted xylan, while an increase in DS of both HPX and APX clearly had a diminishing effect on Tg due to internal plasticization. Both types of substituted xylan were used as thermoplastic additives in polystyrene (0–20%) revealing a decline in dynamic shear viscosity with increasing xylan content. APX was furthermore injection molded with polystyrene and studied by dynamic thermomechanical analysis (DTMA), whereby the plasticizing properties of the additive were demonstrated. A number of functional groups have successfully been introduced on the hydroxyl groups of various hemicelluloses by esterification, etherification and amination. The resulting products have altered properties compared to the parent compounds in terms of strength, elasticity, hydrophobicity and thermoplastic properties. The films from the presented work are potentially relevant in a number of applications such as barrier and membrane materials.

Patent Literature In this section we review filed patent applications and existing patents on the formation of films and coatings from hemicelluloses. References are only cited if the given examples specifically include hemicellulose, as numerous sources include hemicellulose in the patent claims while solely focusing experimental work on cellulose. Results given in the examples are included here to the extent that they are considered relevant for this review. Films. Crosslinking was used to ensure water insolubility of hemicelluloses in a Japanese patent application from 1992.57 Chemical crosslinking agents such as acid chlorides were employed to form highly swellable gels. Transparent films with tensile strengths of approximately 500 kg/cm2 were produced with oxygen permeability of 1.7–1.8 cc/(cm2 24 h atm.) for 50 µm thick films. The formation of water soluble films from hardwood hemicelluloses is claimed in a patent from 1974, where the reaction of the hemicellulose (DP < 400) in alkaline medium with epichlorohydrin, and either an alkanolamine or glycerol lead to the formation of transparent, colorless, and relatively strong films.58 Laminated films consisting of at least two layers have recently been patented in Japan,59 where one layer is comprised of a thermoplastic film, more particularly polyester, while the other consists of carbon nanotubes and hemicellulose. The latter component is preferably extracted from Abelmoschus monihot or Hydrangea paniculata obtaining polymers with DP of 100 to 10000. The carbon nanotubes were uniformly dispersed in the hemicellulose films giving conducting and mechanically strong films. Arabinoxylan from Sorgo was utilized in a French application60 for the fabrication of water-soluble films (solubility above 200 g/L). The films were swellable, with a water uptake between 10 and 25% and had mechanical yield strengths of 15-40 MPa. Oxygen barrier films were produced from xylans (Mw ) 15000 and 34000 g mol-1) in a recent Swedish patent application.61 The films were plasticized with xylitol, sorbitol, or water, and blends of xylan with polyvinyl alcohol or cellulose were also generated. Oxygen permeabilities of 0.18–1.10 cm3 µm m-2 d-1 kPa-1 were obtained for the films. Stress at break decreased with plasticizer content (3–39 MPa), while strain at break increased up to approximately 10% (50% plasticizer). The addition of cellulose to xylan increased stress at break to 103


1503

MPa. Successful coating of a polystyrene film was furthermore obtained with a coating thickness of 1 µm. Heteroxylans, more specifically arabinoxylan, were used to manufacture films for the formation of hard or soft capsules intended for use in the pharmaceutical, phytotherapeutic, and food sectors.62 Viscosity of the AX-solutions showed better stability at high stresses than gelatine; however, the obtained values were also slightly lower. The overall gelling procedure was faster with the AX-based capsules than with gelatine or hydroxypropyl cellulose. Glycerol content was varied to form hard or soft capsules (2.4 or 25 wt %, respectively). Flat films were also produced with thicknesses of 100–130 µm and rupture strengths of 31–62 N. Coatings. Hemicellulose from grain hulls was used as material for seed coatings in a very recent patent application.63 Seeds of corn and soybean were successfully covered with hemicellulose by spraying, which did not hinder germination of the seeds. The coating could contain additives such as insecticides, herbicides, and so on, thereby acting as a barrier for species that are detrimental to seed growth. Heteroxylans extracted from maize (Mw ) 100000–250000 g mol-1) were used in a French patent application64 to form edible films for encapsulation of flavoring or other components in foodstuffs. The performance of the films was comparable to gum Arabic in the sugar coating of almonds in terms of stability, texture, taste, and appearance. Seeds of carrots, cabbage, and salad (chicory) were coated with the heteroxylan films, and while the initial germination rate was reduced, the final level and speed of growth was equivalent to uncoated seeds and superior to commercial cellulose-based seed coatings. Similar results were found when incorporating insecticides or fungicides in the films. From the examples in the patent literature, it can be seen that many different applications of hemicellulose have already been investigated. It is, however, also obvious from the recent dates of many of the applications that this is an area in which there is still ongoing work, and all the possibilities have certainly not been exploited. We will probably see many other applications of hemicelluloses in both the food and the pharmaceutical industry in the future.

Summary Cellulose, lignin, and hemicellulose derived from trees or agricultural crop residues form the basis for the present biorefinery concept that is now much discussed in the context of the rapidly developing biofuels industry. Although there is a long history of research and development on new chemicals and materials from cellulose and lignin, hemicellulose has remained relatively unexplored in this sense until quite recently. The material applications for hemicellulose that have been identified include packaging films, food coatings, and biomedical uses. Food packaging material derived from sustainable resources is now receiving considerable interest, not least because of growing public awareness of environmental issues and the advantages in product marketing. Low oxygen permeability can be a key requirement for food packaging materials in addition to good mechanical properties in terms of high strength and flexibility and hemicellulose-derived packaging materials have been produced with oxygen permeability values that compare favorably with those of other biopolymers such as amylose, amylopectin and chitosan as well as PVOH and EVOH. There are also a number of recent patent applications on oxygen barriers from hemicellulose.

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Moisture barriers in the form of edible coatings have been produced with moderate success from hemicellulose. The main obstacle in this field is the hygroscopic nature of the initial material resulting in poor moisture barrier properties. Increased hydrophobicity may be achieved by either chemical modification (e.g., grafting) or addition of hydrophobic compounds (mixing or emulsification). In some cases, WVTR and WVP have been improved significantly, but the performance has generally not been comparable with that observed for nonedible commercial films such as LDPE. The patent literature discloses the use of heteroxylans for coatings for seeds and candy and these were claimed to be superior to cellulose-based coatings. Some hemicelluloses such as konjac glucomannan and guar gum have been used for many years in the pharmaceutical industry, while others have not yet been employed commercially. The formation of hydrogels from xylan in combination with chitosan has been demonstrated and these gels might be suitable for biomedical purposes. Ingested nanoparticles coated with xylan have been shown to be capable of withstanding acidic media ensuring passage to the colon of a patient, where the nanoparticles might be used as magnetic markers. Although considerable research on hemicelluloses for biomedical purposes has been performed, the increasing need for specialized medicine and forms of customized drug delivery may open up new applications. A significant amount of research has been aimed at modifying hemicellulose chemically to alter the characteristics in comparison to the parent compounds. Modifications have led to stronger, more elastic, and more hydrophobic or thermoplastic products, and the resulting films may find a variety of potential applications such as in selective membrane materials. Although there has been extensive work on films and coatings from hemicelluloses and derivatives, there are still many potential pathways that have yet to be investigated, and therefore, it can be argued that application of hemicelluloses in fields such as packaging and biomedicine still holds considerable promise for the future.

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Sustainable Films and Coatings from Hemicelluloses: A Review

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