Opportunities for Hardwood Hemicellulose in Biodegradable Polymer

Chapter 7 Opportunities for Hardwood Hemicellulose in Biodegradable Polymer Blends 1

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Arthur J. Stipanovic , Jennifer S. Haghpanah, Thomas E. Amidon , Gary M. Scott , Vincent Barber , and Kunal Mishra 3

Downloaded by UNIV OF BATH on July 2, 2016 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch007

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Faculty of Chemistry, FacuIty of Chemistry-NSF Research Experience for Undergraduates, and Faculty of Paper Science and Engineering, State University of New York-College of Environmental Science and Forestry (SUNY-ESF), Syracuse, NY 13210 3

Hardwoods indigenous to the northeastern U.S., including birch, beech, maple and short-rotation woody crops such as willow, are relatively rich in the hemicellulose xylan. In this study, a combined biodelignification and hot water extraction procedure was employed to isolate polymeric xylan in its native acetylated state. To enhance the mechanical properties of xylan as a biodegradable material, solutions of xylan were mixed with solutions of commercially available cellulose esters followed by casting into solid films. In this fashion, it was possible to prepare acetylated xylan / cellulose triacetate "blends" with mechanical properties comparable to the cellulose triacetate itself up to 25 wt% xylan. Plasticizers were effective in increasing the strain to break for these materials but lowered the modulus at 1% strain.

© 2007 American Chemical Society

Argyropoulos; Materials, Chemicals, and Energy from Forest Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

107


108 As the world's petroleum supplies continue to dwindle, renewal biomass will become an increasing important feedstock for the manufacture of fuels, chemicals and advanced materials. A recent report issued jointly by the U.S. Departments of Agriculture and Energy, estimated that over one billion tons per year of agricultural and forest biomass will become available with the ultimate potential of providing one-third of the current U.S. consumption of transportation fuels (7). Since the cost of shipping and storing biomass is a critical factor in the economics of biobased industries, it is envisioned that "biorefineries" will be regional facilities that use locally harvested feedstocks (2). For the northeastern region of the U.S., forest hardwoods including birch, beech, maple and short-rotation woody crops such willow (J), are abundant sources of biomass. From a compositional perspective, hardwoods typically contain 40-55% cellulose, 20-35% hemicellulose and 18-30% lignin on a dry wt% basis (3,4). By comparison, softwoods contain a higher fraction of lignin (up to 35%) and lower level of hemicellulose. In addition, softwood hemicellulose contains a significant fraction of 2-3 different polymers, while hardwood hemicellulose tends to be predominately a single polymer, 4-0methylglucuronoxylan (xylan). The P-(l-4)-linked D-xylopyranosyl (xylose) backbone structure of hardwood xylan, shown in Figure 1, also contains about one 4-O-methyl-a-D-glucuronic acid residue and 7 acetyl ester substituents per 10 xylose monomers (6).

HO HOOH

^

OH

U

Figure 1: Idealized backbone structure of xylan (pyranose ring hydrogen atoms omittedfor clarity). Although the molecular weight of xylan is the subject of considerable debate because this polymer is typically associated with lignin (7), it degrades during chemical delignification processes (8), and it self-assembles in solution into aggregate structures (6), it is widely held that the degree of polymerization (DP) of xylan is in the range of 100-200 (6,8). Comparatively, native bacterial and plant celluloses were found to have DP's from 400-10,000 (9,10). Based on the relative abundance of hardwoods in the northeastern region of the U.S., efforts are being made at our institution and at others to fully utilize the hemicellulose fraction in fuels (xylose fermentation to ethanol), chemicals (microbial synthesis of propanediols), and biodegradable thermoplastic polymers (xylose biosynthesis into polyhydroxyalkanotes). In this study, we exploited a sequential biodelignification / hot water extraction process reported previously (77) to recover xylan from northeastern hardwoods (paper birch, sugar maple)


109 in an effort to characterize the material properties of films cast from these extracts. It was envisioned that these materials would be relatively inexpensive compared with other biodegradable polymers but due to the relatively low D P of xylan, we expected that the mechanical strength of these films would be limited. As a result, the objective of this work was to blend xylan with other commercially available polysaccharides and polyesters to determine i f these "hybrid" materials could be useful in packaging and other disposal consumer items.


Experimental Methods The hardwood biodelignification process and hot-water hemicellulose extraction procedures used in this study to isolate xylan were reported previously (11). Typically, for sugar maple chips, 60% of the original xylan present in the original ground wood can be recovered by this treatment. Solid xylan was recovered by precipitation in acetone / ethanol. A water-soluble birch xylan sample in dry powdered form, originally isolated in the laboratory of Professor T.E. Timell (Professor Emeritus, - S U N Y - E S F , circa 1960's) by dilute K O H extraction was also studied (12). *H-NMR results on this sample indicated a degree of acetylation of 1.06 acetate groups per xylose residue. Xylan valerate was synthesized in the Timell laboratory (150°C) and the polymer is glassy under normal application conditions. Differential Scanning Calorimetry was employed to determine the T of naturally acetylated xylan extracts and blends of xylan with cellulose triacetate (CTA), cellulose acetate butyrate (CAB) and P H B - H V as summarized in Table II. A DSC thermogram where heat flow is monitored with increasing temperature is provided in Figure 3 for C T A , birch xylan and a 50:50 wt:wt blend of C T A and birch xylan. Table II illustrates that C T A , birch xylan and maple xylan each exhibit a single T although differences are observed between the two xylan sources. Irvin has shown (32) that xylan from the hardwood Eucalyptus regnans exhibited T values in the range of 160-180°C for dry samples while moisture served as a plasticizer lowering T to almost 45°C at 20% water content. It is possible that the T differences seen for the maple and birch xylan samples arise from variations in acetyl content, associated lignin or small differences in moisture content that might occur between sample preparation and DSC analysis. g

g

g

g

g

g

g


113 Table II. Thermal Properties of Xylan Filins g

1. Cellulose Triacetate (CTA)

DMF

195

2. Birch Xylan (Acetate)

DMF

151 (Weak)

Blend of 1 and 2

Film(l)

164 (Weak)

3. Maple Xylan (Acetate)

DMSO

128 (Weak)

4. C T A

DMF Film(l)

163 (Broad)

5. P H B - H V Bacterial Polyester

DMF

-7

6. Birch Xylan (Acetate)

DMF

151

Film(l)

2 T g ' s : - 4 , 139

Blend of 3 and 4


T (°C)byDSC

Solvent

Polymer

Blend of S and 6

195

(1) A solid film cast from the solvent shown

When two solid polymers are mixed, a true "thermodynamic" blend is reflected in a single T instead of two T ' s associated with each individual component (33). In general, the free energy of mixing for two polymers (AG,™ = AHrnix - Τ ASrnix ) is negative only if the enthalpy of mixing ( Δ Η ^ ) is negative because the entropy of mixing (AS^ ) for two high molecular weight species is usually not positive (34). In the case of xylan blends with cellulose esters, a negative Δ Η ^ could result from hydrogen bonding or the association of hydrophobic ester side chains. D S C results shown in Figure 3 (summarized in Table II) provide some evidence that C T A and acetylated birch xylan form a homogenous blend since a single T is observed although this thermal feature is relatively weak and broad. Clear evidence is provided that xylan and a thermoplastic bacterial polyester are not thermodynamically compatible since two T 's are observed (Table II). A n alternative approach was also attempted to obtain "compatible" blends in which ester derivatives of cellulose and xylan were mixed in solution followed by film casting. In this case, commercially available C A B was blended with a hardwood xylan valerate ( X V ) previously prepared at our institution (6). In general, long hydrocarbon side chains on polymers reduce T and enhance film extensibility. D S C results, shown in Table III, illustrate that lower T ' s were generally observed for C A B and X V compared to C T A and naturally acetylated xylans. However, a blend of these derivatives exhibited two T ' s indicating that true thermodynamic mixing had not occurred. A blend of C T A with X V also displayed two T values characteristic of a two domain, non-homogenous solidstate morphology. It is interesting to note that the two T ' s observed for the X V g

g

g

g

g

g

g

g

g


Argyropoulos; Materials, Chemicals, and Energy from Forest Biomass ACS Symposium Series; American Chemical Society: Washington, DC, 2007. ^ Η ' Η ( Ι

|

M i lt μ

au

1

* »h

g

233

T of C T A

i i Iι

• CTA

240

F/gwre 5. DSC of CTA, birch xylan, and a 50:50 wt:wt blend of CTA and birch xylan cast from DMF.

Η

g

J, T of " B l e n d "


115 Table III. Thermal Properties of Polysaccharide Ester Films Solvent

1. Cellulose Acetate Butyrate (CAB)

THF

105

2. Xylan Valerate ( X V )

THF

10-20

Film

2 Tg's: 8,105

DMF

-7

Film

2 Tg's: -22, 1

Film

2 Tg's:-4, 195

Blend of 1 and 2 3. P H B - H V Blend of 2 and 3 4. C T A + X V Downloaded by UNIV OF BATH on July 2, 2016 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch007

T (°C)byDSC

Polymer

g

Table IV. Tensile Properties of Xylan Blends Stress at 1% Strain (MPa, 40°C) 23.2

Strain at Break (%) 1.8

Brittle, Friable

-

90:10 C T A : M X

23.8

2.2

75:25 C T A : M X

21.0

1.2

65:35 C T A : M X

15.9

1.3

50:50 C T A : M X

2.9

1.6

CAB XV

7.6 5.2

1.7 3.5

50:50 C A B : X V

4.7

5.3

Sample Cellulose Triacetate (CTA) Maple Xylan ( M X )

blend with P H B - H V are observed at lower temperatures than the "parent" polymers. This could be a reflection of some domain compatibility or a cooperative "plasticizing" effect induced by the short valerate side chains present on each polymer.

Mechanical Properties of Xylan Films and Blends From an applications perspective, the tensile properties of xylan films and blends are critically important in defining the commercial viability of these potentially biodegradable materials. Typically, unplasticized or un-blended hardwood xylan cast into films is extremely brittle and cannot withstand any



116 mechanical deformation while C T A , widely used in film, fiber and coating applications, exhibits a relatively high modulus but modest % strain at break when uniaxially deformed. Results provided in Table IV illustrate that blending maple xylan films with C T A , at concentrations of xylan ranging from 10-25%, can provide tensile properties very similar to C T A alone. A t higher concentrations of xylan (35-50%) a significant deterioration is observed in stress at 1% strain while % strain at break remains comparable. (Note: Young's Modulus at low deformation = Stress at 1% Strain / 0.01). Table IV also contains results for the "higher" esters C A B and X V . These materials show a lower modulus at 1% strain compared to C T A but X V appears to exhibit an improved strain to break value. The 50:50 mixture of C T A and X V showed very good elongational properties. The xylan film tensile properties reported above can be compared to recent work by the Gatenholm group who has demonstrated that the open chain form sugars xylitol and sorbitol are effective plasticizers for aspen wood (hardwood) xylan providing tensile stress values at 1% strain in the range 5-30 M P a (30). Further, at high levels of plasticizer (35-50 wt%), up to 12% strain to break was observed by these authors. As a result of these findings, we evaluated the effect of triethylcitrate (TEC), shown to be good plasticizer for C T A , for C T A / xylan blends (35). Data are given in Table V .

Table V. Effect of Plasticizer on the Mechanical Properties of Xylan Blends Sample Cellulose Triacetate (CTA) C T A + 20% T E C

Stress at 1% Strain (MPa, 40°C) 23.2 14.2

Strain at Break (%) 1.8 4.1

C T A / M X Blends + 20% TEC 90:10 C T A : M X

-

-

18.3

4.8

75:25 C T A : M X

16.9

1.4

For C T A alone, T E C has the effect of decreasing modulus compared to unplasticized C T A while significantly enhancing the degree to which the film can be stretched before breaking. Similar results were observed for a 90:10 blend of C T A : M X . For both plasticized and unplasticized C T A : M X , the 75:25 blend showed good tensile stress at 1% strain but a low strain at break in both cases.


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Conclusions As biorefineries evolve as regional sites for the production of fuels, chemicals and polymeric materials, locally available biomass feedstocks will be exploited as sources cellulose, hemicellulose and lignin. In the northeastern U.S., forest and plantation grown hardwoods represent an attractive and abundant source of biomass, comparatively rich in the hemicellulose xylan. Prior work at our institution has shown that biodelignification is an "enabling" technology which significantly increases the yield of polymeric xylan potentially extracted from hardwoods. Despite its relatively low molecular weight and brittle solidstate morphology, this study has shown that blends of xylan with other biobased polymers can provide potentially useful film materials. Specific results are summarized below: •

Biodelignification followed by hot water xylan extraction at 140°C can yield up to 60% of the hemicellulose originally present in the biomass feedstock largely in polymeric form as evidenced by SEC.

•

In the absence of plasticizers, hardwood xylan forms very brittle films with essentially no capacity for deformation under load.

•

Mixtures of xylan with C T A may form polymeric blends based on D S C results that show a single broad T . g

•

These C T A blends show good mechanical properties at 10-25% xylan in C T A , above which modulus and extensibility deteriorate.

•

The addition of triethylcitrate plasticizer reduces the modulus of C T A and xylan blend films but enhances the % strain to break.

Future studies will be focused on: (1) the characterization of xylan molecular weight after biodelignification and hot water extraction, (2) quantification of residual lignin and its effect on film properties, (3) preparation and characterization of synthetically acetylated xylan blends with C T A with and without T E C plasticizer, (4) evaluation of a broader spectrum of commercially available cellulose derivatives as blend components with xylan and its derivatives and, (5) the thermal characteristics of xylan blends leading to a more fundamental understanding of mixing thermodynamics establishing a more rational approach for the selection or synthesis of other polymers for optimized blending compatibility with xylan.


118

Acknowledgements


Support of this research was provided by the U S D A Mclntire-Stennis Cooperative Forestry Research Program and a US Department of Interior - U S Forest Service grant to S U N Y - E S F . Ms. Haghpanah was supported by a National Science Foundation - Research Experience for Undergraduates (REU) program grant during the summer of 2005. Consultation and facilities were also provided by the S U N Y Cellulose Research Institute and the Empire State Paper Research Institute (ESPRI), both located at SUNY-ESF.

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Opportunities for Hardwood Hemicellulose in Biodegradable Polymer

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