Lignin: Historical, Biological, and Materials ... - ACS Publications

Chapter 17

Blends of Biodegradable Thermoplastics with Lignin Esters

Downloaded by UNIV OF GUELPH LIBRARY on June 12, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch017

Indrajit Ghosh, Rajesh K. Jain, and Wolfgang G. Glasser Biobased Materials and Recycling Center, Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0324

Thermoplastic blends of several biodegradable polymers with lignin and lignin esters were prepared by solvent casting and melt processing. Among the biodegradable thermoplastics were cellulose acetate butyrate (CAB), a starch-caprolactone copolymer/blend (SCC), and poly(hydroxybutyrate) (PHB). Lignin esters included the acetate, butyrate, hexanoate, and laurate of organosolv lignin, LA, L B , L H , and L L , respectively. Blend properties were analyzed by thermal, mechanical, and optical (transmission electron microscopy, TEM) analysis. The results indicate widely different levels of interaction between the two polymer constituents. Blends of L A and L B with C A B exhibited a high level of compatibility that was lost when the acyl substituent increased in size. The addition of unmodified lignin to PHB greatly retarded crystallization and produced blends with lower melting points. The same was true for SCC blends, which were found to crystallize and melt at lower temperatures if lignin was present. However, a significantly increased modulus at room temperature resulted with the addition of lignin, and this was attributed to increased crystallinity in the presence of lignin.

Biodegradable thermoplastics are making inroads into the polymer and materials market on account of their ease of disposal [1-4]. In general, biodegradable thermoplastics include starch and starch derivative-based formulations [5,6]; fermentation-produced polymers such as poly(hydroxy butyrate) (PHB), poly (hydroxy butyrate-co-valerate) (PHB/V), and polyactic acid (PLA) [7-9] and cellulose esters [10,11]. The engineering of specific target properties of polymeric

© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

331


332 materials has long taken advantage of the ability to mix polymers with different properties in solution-cast or melt-processed blend systems [12]. Additives of a wide variety are available for engineering the material properties of man-made polymers. However, fully biodegradable materials, consisting of biodegradable thermoplastics and biodegradable additives, have a much smaller variety of potential additives at their disposal. Qualified biodegradable additives include (again) fermentationproduced additives or those derived from the chemical modification of natural polymers [1]. Among the latter is lignin, the high modulus amorphous binder of woody plants [13]. Thermoplastic lignin and lignin derivatives are commercially or semi-commercially available [14], and they are candidates for biodegradable additives [15,16]. The qualifications of a polymeric additive to influence blend properties depend primarily on its ability to associate or develop a molecular interaction with the thermoplast in question. Secondary association-driven interactions between blend components are necessary to establish some degree of molecular compatibility [17]. While complete miscibility is virtually unachievable, phase compatibility on the nano-ievel has been observed in many systems. The degree of phase compatibility as opposed to macroscopic separation is usually established using thermal analysis or transmission electron microscopic techniques [18]. The objectives of this report deal with the determination of the effect of lignin and selected biodegradable lignin derivatives on the phase morphology and selected properties of blends of three biodegradable thermoplastics, cellulose acetate butyrate (CAB), poly(hydroxybutyrate) (PHB), and starch/ caprolactone copolymer/blend (SCC). Additional detail regarding blends of C A B with lignin derivatives have been given elsewhere [19], Materials and Methods Materials : C A B (CAB 381-20) was obtained from Eastman Chemical Company, Kingsport, Tennessee; PHB from Biopol (Marlborough Biopolymers), Cleveland, U K ; and starch-polycaprolactone blend (SCC) (tradename 'Envar') from Biomaterials Research Center, Michigan State University [20]. The approximate starch content of SCC is 30% by wt. and the polycaprolactone component has a M of 148,000 and a M of 473,000. Organosolv Lignin (L) was obtained from Aldrich Chemical Company, WI, U S A (Catalog #: 37,101-7). Lignin was esterified with acetic, butyric and hexanoic anhydrides, and lauryl chloride to yield the corresponding esters by adopting the esterification methodology described elsewhere [15]. Selected physical properties and molecular weight parameters of these polymers are summarized in Table I. n

w

Methods: Blends were prepared by either casting appropriate mixtures of the polymers from solutions or by melt-processing. For solvent casting, the blend solutions were prepared (~5 wt.%) using chloroform as solvent. The solutions were stirred for approximately 6 hours before they were cast into Teflon molds. The castings were kept at ambient temperature for 48 hours, allowing slow evaporation of the solvent by partially covering the molds. Extruded blends were prepared with a

In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

333 Table I : Some Physical Properties Preparation Synthesis Method CAB Anhydride PHB Fermentation SCC Extrusion Lignin (L) Precipitation LA A A ; PPT-W LB AA^PPT-BC LH AA'iPPT-BC LL AC ; PPT-E 1

2

3

3


4

2

3

4

5

5

of Biodegradable Polymers and Lignin M Physical Tg xlO' Form (°C) 134 69.6 Powder 0.8 Powder 148.0 -60 Pellets 0.82 107 Powder 1.55 92 Powder 2.31 Powder 52 2.65 Powder 30 13.4 2 Tar n

3

Additives M xlO 127.6 473.0 3.14 5.89 7.73 9.44 33.2 w

3

Acid anhydride, A A Precipitation in water, PPT-W Precipitation in bicarbonate solution, PPT-BC Acid Chloride, A C Extraction with chloroform/acetone 3/2, Ε

Custom Scientific Instruments Mini-Max injection molder with the extrusion temperatures in the range of 200-220°C for C A B blends, 165-190°C for PHB blends and 175-190°C for SCC blends, and a residence time of no more than 2 mins to avoid degradation. The extruded samples for testing included dog-bone and rectangular specimens. A l l samples were dried under vacuum at 30°C for 4 hours and stored in air-tight plastic bags in a vacuum desiccator till tests were performed. Analytical methods included thermal analysis by differential scanning calorimetry (DSC) using a Perkin-Elmer Model DSC-4 interfaced to a thermal analysis data station; and dynamic mechanical thermal analysis (DMTA) with a Polymer Laboratories Ltd. dynamic mechanical thermal analyzer in the single or dual cantilever bending mode, or in the shear mode depending on sample geometry and sample consistency. The spectra for D M T A were collected at a heating rate of 4°C min" at a frequency of 1 Hz. Transmission electron microscopy (TEM) was performed using a JEOL JEM-100CX-II electron microscope operated at an accelerating voltage of 80 kV. The mechanical properties (modulus, strength and ultimate strain) of the blends were determined on a Miniature Materials Tester (Minimat model # SM9-06) by Polymer Laboratories, Loughborough, England. Tests were conducted at room temperature with a 1000 Ν load beam using strain rates of 5 mm/min. 1

Results and Discussion 1. Cellulose Acetate Butyrate (CAB) Blends : Blends of C A B with lignin esters were examined by DSC, D M T A and T E M . Results by DSC revealed single glass transition temperatures for all polymer blends


334 with L A (Fig.l) and L B (not shown). Glass transitions of C A B shifted progressively to lower temperatures as L A and L B content rose. Single glass transitions were also observed by D M T A (Fig.2), where single and sharp tan δ-transitions were observed at increasingly lower temperatures following the addition of L A and L B . When L H was blended with C A B instead of L A or L B , however, a low temperature shoulder appeared in the DMTA-thermograph (tan δ-transition) at >20% LH-content that was indicative of macroscopic phase separation. This shoulder was even more apparent in the loss modulus (E and G ) thermogram of the C A B / L H blends, and this clearly suggests the separation of polymeric phases on a scale of 0.5 to 1 μπι. The onset of blend softening on a temperature scale, however, revealed a significant dependence on lignin content for all blends with lignin esters. By examining the impact of lignin esters on the shift in glass transition temperature using the well-known Fox-equation [21],


;/

1 _ Tg

;/

Wj

=

W2

+ T i g

(1) T 2 G

where T is the glass transition temperature in degrees Κ for polymer 1, 2, of the blend; and w is the weight fraction of component 1 and 2 in the blend, a quantitative measure of compatibility can be established (Fig. 3a). Whereas L A and L B are in apparently full compliance with copolymer behavior as required by equation (1), C A B / L H blends reveal incompatibility at lignin derivative contents in excess of 10% (Fig. 3b). This suggests that, whereas L A and L B form compatible blends over the entire mixing range, L H is incompatible with C A B on a scale in excess of ca. 0.1 μπι [18]. A comparison of transmission electron micrographs with resolution on the nano-scale reveals phase separation between the cellulose and lignin ester components on different levels (Fig. 4). Solvent-cast blends exhibit included (discontinuous) phases that vary with lignin ester content (Figs. 4a and 4b), and these are always larger than those observed by the corresponding melt blending technique (Fig. 4c). This is expected because phase dimensions are a consequence of chemical intermolecular affinity (i.e. thermodynamic effects) as well as the time available for different polymers to separate from each other (i.e., kinetic effect). Since polymers have a virtually unlimited amount of time to phase separate during solvent casting, and they are near their equilibrium condition at all times, the included polymer phases are always larger during solvent-casting than during melt blending (Fig. 4a vs. 4c). A comparison between 20% C A B / L A and L H blends (Fig. 4d vs. 4c) reveals dimensional differences favoring C A B / L A blends. Since the latter display an included phase on the scale of 10 to 30 nm (Fig. 4d), whereas they are 30 to 100 nm for the corresponding LH-blends (Fig. 4c), the results are seen to agree with the thermal analysis experiments. The superior phase compatibility of the L A and L B blends of C A B as compared to L H and L L , find further support in the mechanical stress vs. strain experiments. Whereas all blends reveal a decrease in tensile strength, strain and modulus with lignin ester content rising to >20% lignin derivative content, C A B blends with L A and L B reveal increases in tensile properties and in modulus with g



335

Temperature (°C) Figure 1.

DSC thermograms of solvent (CHCb) cast samples of CAB and L A Numbers on each curve denote L A content (wt.%) in the blend. These traces arefromthe second heating scan (after quenchingfrommelt at a rate of 300°C/min). (Reproduced with permissionfromref.19, copyright 1998 by John Wiley & Sons, Inc.)



336

Figure 2.

Tan δ vs. temperature curves for blends of C A B and L A obtained from D M T A experiments at 1 Hz. (Adopted from ref.19)



337

160

ο DSC (2nd Scan)

140

•

120

DMTA

I 100 ^ '—V

y „ GO

H

8060 40

'v 20-

(b) 0^ 0

1 10

20

30

40

50

60

70

80

90

100

LH Content (wt. %) Figure 3.

Fox equation fit for blends of C A B and (a) L A and L B ; (b) L H . (Adopted from ref. 19)



338



Figure 4.

Transmission electron micrographs of C A B / L ester blends with (a) solvent-cast, 20% L H ; (b) solvent-cast, 50% L F ; (c) melt-blended, 20% L H ; (d) melt-blended, 20% L A by weight (inagnification at 10,000 X ) . (Reproduced with permission from ref 19, copyright 1998 by John Wiley & Sons, Inc.)


340 20%. Although lignin esters are effective plasticizers of C A B by lowering T , they contribute to an increase in modulus at lignin contents below 20%. g


2. Poly(hydroxybutyrate) (PHB) Blends: PHB is a thermoplastic polymer with significant propensity for crystallization [9]. One of its biggest handicaps constraining commercial utilization is its rapid loss of stability when heated above T . The effect of lignin on PHB must, therefore, be examined in terms of the impact on melting and crystallization behavior. When neat PHB is cooled from the melt at 10°C min" , a significant exothermic crystallization peak is observed at 81°C (Fig. 6). When heated again, the corresponding endotherm at 175°C indicates melting. Cooling the sample for a second time (at 10°C min" ), a broader crystallization event is observed compared to the first time reflecting partial thermal degradation and loss of molecular uniformity. Reheating and recooling the pre-melted PHB at different heating/cooling rates produces T and T -events that vary in uniformity and location on the temperature scale (Fig. 6). The presence of (unmodified) lignin in a solvent-cast PHB/L blend inhibits or retards crystallization at any cooling rate and at lignin contents of 10 and 20% (Fig. 7). Glass transition temperatures are observed at around 5°C for P H B , and this undergoes spontaneous crystallization when heated to >50°C at any heating rate and any lignin content (Fig. 7). Similar results were observed for DSC curves of PHB and L B blends (not shown). The observed T values for the PHB phase reveal shifts towards the glass transitions of L (or LB) indicating some interaction between PHB and L or L B (Fig.8a). The normalized A H values for PHB/LB are higher than those for PHB/L blends (Fig.8b). This might be an indication of greater interaction between PHB and L as compared to PHB and L B . On the other hand, the T values for PHB/LB blends are observed to shift towards lower temperatures (Table II). These results suggest that PHB and L (or LB) have a high degree of compatibility, and that lignin inhibits or retards PHB-crystallization. m

1

1

c

m

g

m

m

3. Starch-Caprolactone Copolymers/Blends (SCC) : Melt processable S C C undergoes thermal transitions and softening events that are dictated by the presence of the polycaprolactone component. The introduction of unmodified lignin or lignin ester derivative by solvent-casting or melt-processing methodology causes the crystallization and melting points of the P C L component to decline in relation to lignin content (Fig. 9). Melting behavior thereby mirrors crystallization behavior (Fig. 9a and 9b). When analyzing the overall energetics of the crystallization and melting phenomena (Table III), it is apparent that different



341

Ο

10 20 30 40 Lignin Ester Content (wt.%)

0

10

20

30

40

50

50

Lignin Ester Content (wt.%)

Figure S.

Tensile stress (a) and modulus (b) vs. lignin ester content for meltblended CAB/lignin ester blends: (O) C A B / L A , ( • ) C A B / L B , ( O ) C A B / L H , (Δ) C A B / L L . (Adopted from réf. 19)



342

Cool 2.5°C min'

PHB (melt processed)

1

-10

20

50

80

110

140

170

200


Effect of processing and crystallization times on P H B . The cooling traces arefromthe first cooling scans from melt and the heating traces arefromthe second heating scan. (Some curves have been expanded on the y-axis for greater clarity; y-axis has arbitrary scale).



343

-10

20

50

80

110

140

170

200


DSC thermograms of melt blended samples of PHB and L. The cooling traces arefromthe first cooling scans from melt and the heating traces arefromthe second heating scan.



344

L or LB Content (wt %)

Figure 8.

Glass transition temperatures (Tg) (a) and normalized heat-of-fusion values (ΔΗ™) (b) for PHB and L or L B blends: (O) PHB/L, ( • ) PHB/LB.


345 Table II : Thermal Characteristics of Blends of PHB. Normalized

T*

ΔΗο*

âU

àHc

âHm

(°C)

(°C)

(°C)

J/gm

J/gm

J/gmof PHB

J/gmof PHB

0.5

173.1 177.8

81.5 78.1

-61.3 -63.3

87.6 92.8

-61.3 -63.3

87.6 92.8

PHB/L Blends 90/10 80/20 70/30

7.6 10.0 11.6

177 175 175

69* 74* 81*

-70.3* -62.7* -51.6*

71.1 64.2 53.1

-78.1 -78.4 -73.7

79.0 80.3 75.9

PHB/LB Blends 90/10 80/20 70/30

4.3 2.4 1.8

170 161 156

63* 75* 80*

-73.7* -64.6* -50.8*

75.4 66.7 52.7

-81.9 -80.8 -72.6

83.8 83.4 75.3

Preparation


PHB (native) PHB (melt processed)

T

g

T

m

c

m

* The reported crystallization temperatures (T ) and heat of crystallization (ΔΗς) values for the blends are the peak temperatures of the crystallization endotherms observed in the second heating scans (usually reported as cold crystallization) at a scanning rate of 10°C/min. No crystallization was observed during the cooling scans at a scanning rate of 10°C/min for any of the blend samples. The T s reported for PHB (without lignin component) are from the crystallization endotherms observed during the first cooling scans from melt at a cooling rate of 10°C/min. c

c

lignin components vary in their effect on P C L morphology. Whereas the presence of unmodified lignin in melt-processed blends enhances the crystallization of PCL, L B appears to have the opposite effect. The latter is more pronounced when lignin is introduced by solvent-casting than by melt-processing (Table ΙΠ). These results are consistent with earlier observations [16] that suggest that high-T unmodified lignin may serve as nucleating agent for P C L , thereby enhancing crystallization and consequently also melting, and low-T lignin esters show a significant compatibility with P C L enhancing an amorphous component and reducing crystallization and fusion. The impact of lignin and lignin ester content on the stress vs. strain properties of SCC blends mirrors the changes in morphology (Fig. 10). Dramatic increases in tensile stress, by 300%, following the addition of lignin at the 10%-level suggest greatly enhanced nucleation of PCL. Presumably smaller crystallites and greater overall crystallinity (at 10% L-content) contribute to significant strength gains that are not realized if (a) greater amounts of L or (b) lower-T lignin derivatives (LB) are used for blending. g

g

g



346

Ο

10

20

30

40

50

Lignin Component in Blend (wt.%) Figure 9.

Crystallization temperatures (T ) (a) and melting temperatures (T ) (b) vs. weight percent of lignin component for SCC blends. The data points arefromthe crystallization exotherms (a) or melting endotherms (b) of the DSC curves in thefirstcooling cycle (a) or second heating cycle (b). (O) SCC/LB (solvent cast), (•) SCC/LB (melt processed), (Δ) SCC/L (melt processed). c


m

347

Table III: Crystallization and Melting Data of SCC-Blends.


Preparation

(J/gm)

Normalized*

Normalized*

(J/gm PCL)

(J/gm PCL)

Normalized*

Xc(%)

(J/gm)

SCC (original) SCC ( C H C I 3 cast) SCC (Melt Extruded)

-49 -49 -48

58 57 54

-69 -69 -69

81 81 78

60 60 58

SCC/L Blends 95/5 90/10 80/20 70/30

-49 -48 -40 -36

54 56 43 38

-74 -77 -71 -73

81 89 77 78

60 66 57 58

-43 -40 -33 -6

48 42 40 26

-68 -71 -68 -16

76 75 81 76

56 56 60 56

-39 -38 -31 -9

43 42 38 26

-62 -68 -63 -26

69 75 77 74

50 56 57 55

SCC/LB (Melt Blended)

90/10 80/20 70/30 SCC/LB Cast)

(CHCI3

90/10 80/20 70/30

*

(,)

AHc, AHf and X represent the heat of crystallization, heat of fusion and degree of crystallinity in the sample, respectively. The crystallization data are from the first cooling scans from the melt, by DSC; and the melting data are from the subsequent (second) heating scans. AHc or A H multiplied by (content of P C L in blend)" . Calculation of Xc is based on âH°f «135 J/gm for pure crystalline PCL. c

1

f

i 2 )

This suggests that, while clearly immiscible, L in particular has much to contribute to SCC by virtue of interacting with the P C L component. L and L B have opposite effects on crystallization: whereas L enhances the crystallization of P C L (at 10% L-content), L B depresses T , T and AHc probably by virtue of favorably interacting with PCL. Gains in tensile stress of 80-300% are observed when 10-20% of L or L B are added to SCC. c

m

Conclusions Lignin and lignin esters show a significant degree of interaction with all thermoplastic biodegradable polymers studied. The degree of compatibility varies



25

Ο -! Ο

,

.

200

400

.—

1

600

Tensile Strain ε (%) re 10. Tensile stress vs. strain curves for SCL/L and SCL/LB blends.


349 with lignin ester type; with lignin and lignin derivative content; with thermoplastic polymer type; and with method of blending (i.e., solvent casting or melt processing). The degree of compatibility is consistently revealed by thermal analysis, including DSC and D M T A . Degree of phase compatibility is also revealed quantitatively by TEM.


Acknowledgment The authors wish to express gratitude to the Eastman Chemical Company for supplying C A B , and to Professor Ramani Narayan, Michigan State University, for providing the thermoplastic starch derivative (Envar). Thanks is also given to Professor H . Marand, Department of Chemistry, Dr. Mike McLeod, Department of Chemical Engineering, and Professor Charles E. Frazier, Department of Wood Science and Forest Products, all Virginia Tech, for valuable advice and assistance as part of this study. This publication represents Part 16 of the series entitled "Multiphase Materials with Lignin." Earlier publications have appeared in the Journal of Wood Chemistry and Technology 14, 119 (1994), Polymer 35,1977 (1994), Journal of Applied Polymer Science 51,563 (1994), Macromolecules 27, 5 (1994), and elsewhere.

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2.

3.

4. 5.

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7. 8.

9.

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nd

14. 15. 16. 17. 18. 19. 20. 21.


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