Synthesis of Soy Polymers Using a "Green" Processing Method

Chapter 7

Synthesis of Soy Polymers Using a "Green" Processing Method Downloaded by STANFORD UNIV GREEN LIBR on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch007

Zengshe Liu and Sevim Z. Erhan Food and Industrial O i l Research, N C A U R , Agricultural Research Service, U.S. Department of Agriculture, 1815 North University Street, Peoria, IL 61604

Polymers with lower molecular weight (≤ 24, 908 g/mol), derived from soybean o i l were prepared in supercritical carbon dioxide (scCO ) medium using boron trifluoride diethyl etherate, BF •O(C H ) initiator. Influences o f polymerization temperature, initiator amount, and carbon dioxide pressure on the molecular weight were investigated. Results showed that the higher polymerization temperature favors polymers with relatively higher molecular weights. Larger amounts o f initiator also produced polymers with higher molecular weights. Higher pressure favored higher molecular weight polymers. 2

3

2

5 2

During the last few years, plant oils have attracted renewed attention as raw materials for the preparation o f polymeric materials, to replace the traditional petro-chemical based polymers, because o f their low production cost and biodegradability in some cases (7). Increasing social emphasis on issues concerning the environment, waste disposal, and the depletion o f non-renewable resources are the reason driving application o f bio-based products. U s i n g plant oils as raw materials in polymer synthesis is a good use o f green resources. Soybean is the second largest crop plant in the U . S . , accounting for about 2 8 % o f planted acreage, just behind corn, which accounts for about 30%, and ahead o f wheat, which accounts for about 2 3 % (2). About 3 billion bushels o f soybeans are grown annually in the U . S . The current market demand is about 2.9 b i l l i o n bushels. Thus, there is a need to develop new uses for surplus soybeans in order to prevent price depression due to oversupply. 70

U . S . government work. Published 2008 American Chemical Society.

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

71

Downloaded by STANFORD UNIV GREEN LIBR on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch007

Soybeans are comprised o f (w/w), 4 0 % protein, 3 0 % carbohydrates, and 2 0 % o i l (3, 4). Currently, about 9 5 % o f soy protein is used in feed and 4 % in food (for human consumption) applications. O n the other hand, about 9 4 % o f soybean o i l is used in food and only about 4 % in industrial applications. Soybean o i l is a triglyceride, which is a triester o f glycerol and three fatty acids. The main fatty acid composition o f soybean o i l is (w/w): linoleic (54), oleic (23), and linolenic (8), (5). These fatty acids contain 1, 2, and 3 double bonds, respectively, in their hydrocarbon chains. These double bonds or unsaturations are reactive sites and allow for the development o f soybean o i l for various applications. Plant oil-based polymers have been used for the production o f coatings, inks, plasticizers, lubricants and agrochemicals (6-12). In general, drying oils (these can polymerize in air to form a tough elastic film) are the most widely used oils in coatings, although the semi-drying oils (these partially harden when exposed to air, such as soybean o i l , corn o i l etc.) also find use in some applications, for example in printing inks. Polymerization o f semi-drying oils is difficult due to their lack o f active functional groups. L a r o c k et a\.(13-18) reported the direct conversion o f soybean o i l to useful solid polymers by cationic copolymerization o f soybean o i l with styrene and d i v i n y l benzene comonomers initiated by boron trifluoride diethyl etherate ( B F » O E t ) . The resulting polymers formed range from soft rubbers to hard plastics, depending on the reagents, stoichiometry and initiator used (13). These copolymers have been characterized by various techniques, including dynamic mechanical analysis ( D M A ) , thermogravimetric analysis ( T G A ) , differential scanning calorimetry ( D S C ) , scanning electron microscopy ( S E M ) and thermal mechanical analysis ( T M A ) . Scheme 1 shows the proposed process o f crosslinking o f natural oils, such as soybean o i l , fish o i l , corn o i l , linseed o i l etc. with styrene and divinylbenzene in the presence o f initiator (19). 3

2

W o o l and coworkers developed soybean o i l resin based on acrylated epoxidized soybean o i l ( A E S O ) reacted with reagents to stiffen the polymer chain (20). Scheme 2 shows the oligomerization o f an A E S O with cyclohexane dicarboxylic acid (a) and with maleic acid (b), which introduces more doublebonds in the oligomers. They also reported using A E S O to make composites with natural fiber mats, o f flax, cellulose, pulp and hemp (21). The composites with natural fiber reinforcement o f about 10-50 w t % increased the flexural modulus between 1.5 and 6 G P a , depending on the nature o f the fiber mat. G u o et al. (22) reported the preparation o f polyols and polyurethanes by hydroformylation o f soybean o i l . They studied the physical properties and mechanical properties o f these polyurethanes. They also reported polyols synthesized by oxirane ring opening in epoxidized soybean o i l with hydrochloric acid, hydrobromic acid, methanol, and hydrogen, as shown in Scheme 3 (23). The brominated p o l y o l had 4.1 hydroxy groups, whereas the other three polyols had slightly lower functionality. Applications o f four polyols in


72 polyurethanes were investigated. In our previous articles (24-26), w e reported preparation o f e p o x i d i z e d soybean oil/epoxy based composites, reinforced w i t h carbon, glass, and mineral fibers or w i t h combinations o f fiber and clay, b y the extrusion solid freeform fabrication method. reviewed

polymers

from

natural

oils

M o r e recently, K u n d u et a l . (27)

and

discussed

the

synthesis

and

characterization o f new polymers from soybean, corn, tung, linseed and fish oils.

The effects o f different levels o f unsaturation i n the natural oils and

various catalysts

and comonomers

on the properties

o f copolymers were


considered. These polymers and polymeric composites demonstrate a variety o f properties,

ranging

from

elastomers

to

rigid

plastics.

Most

of

the

aforementioned w o r k s converted soybean oils to useful solid polymers w i t h other major components i n polymer matrix. focuses

on i m p r o v i n g the physical properties

In other words, the

research

o f solid thermoplastics

and

thermoset materials because, the triglyceride-based materials demonstrated l o w molecular weights and, light cross-linking, incapable o f displaying the necessary rigidity and strength required for structural applications.

'CH

CI \AMf&AA*^£f -

HC

I -(-CH-C^W:

L-

1 CH-)-

Scheme 1. The proposed

—^JH—CM )

( CH,

CH^

CH

0 process of cross-linking

CH

I

of natural oils with styrene

and diviny Ibenzene in the presence of modified initiator

(19).


73

1

0 >, ^ -"v"

\

OIL

•H

u tl


o \ J A

A

Co O

Oh

OH

U

OH

Scheme 2. The modification of acrylated epoxidized soybean oil (AESO) shown using cyclohexane dicarboxylic anhydrade or maleic anhydride. These AESOs were cured with styrene or other comonomers (20).



Scheme 3. Schematic representation of the ring-opening reactions ofESBO with various reactants (23).



75 Lubricants account for 1.2% o f the total petroleum use. Those lubricants from petroleum stock are toxic to the environment and difficult to dispose of. There is also increasing concern for environmental pollution from excessive mineral o i l use and disposal, especially in total loss lubrication, military applications, and in outdoor activities such as forestry, mining, railroads, dredging, fishing and agricultural hydraulic systems. Vegetable oils are potential substitutes for conventional mineral oil-based lubricating oils and synthetic esters. Vegetable oils are preferred over synthetic fluids because they are biodegradable and less expensive. O n the other hand, vegetable oils have poor oxidation stability, primarily due to the presence o f bis allylic protons. A l s o , l o w temperature studies have shown that most vegetable oils undergo cloudiness, precipitation, poor flow, and even solidification upon long-term exposure to cold temperature, w h i c h is in sharp contrast to mineral oil-based fluids. Before there w i l l be widespread use o f vegetable oils as the base fluid for environmentally friendly lubricants and hydraulic fluids, their oxidation and c o l d flow properties need to be improved. A useful approach is by the polymerization o f vegetable oils. A s mentioned above, polymerization o f semi-drying vegetable oils like soybean o i l is relatively difficult. Larock and coworkers (13) have reported the cationic homopolymerization o f soybean o i l using boron trifluoride diethyl etherate as the initiator and found the polymerization rate is l o w due to the relative high molecular weight and multiple chain structures o f soybean oils. Furthermore, soybean o i l has been found to be immiscible with the initiator employed. The copolymerization o f soybean o i l with styrene and norbornadiene or dicyclopentadiene, initiated by boron trifuloride diethyethyl etherate, resulted polymers with good mechanical properties and thermal stability. Supercritical carbon dioxide ( s c C 0 ) has been shown to be a promising alternative solvent medium for organic transformations and polymerization reactions. This stems from a list o f advantages ranging from solvent properties to practical, environmental, and economic considerations. Moreover, no residual solvent remains in the polymer product. Additionally carbon dioxide is inexpensive, readily available and nonflammable. In our laboratory, we have reported the direct conversion o f soybean oils in supercritical carbon dioxide to polymers with lower molecular weights (28). The resulting polymers could be used as lubricants and hydraulic fluids. The advantages o f these polymers are their availability from a renewable resource, their biodegradability and their "green" processing method. Here, we discuss the synthesis and characterization o f the resulting polymers from soybean oils. 2

Experimental Section Materials Soybean o i l (SO-5) was purchased from Purdue Farms Inc., Refined O i l D i v i s i o n , (Salisbury, M D ) . B o r o n trifluoride diethyl etherate, B F * 0 ( C H ) , purified and redistilled, from A l d r i c h Chemical Inc. (Milwaukee, W I ) was used 3


2

5

2

76 as received. Carbon dioxide (>99.8%) was obtained from Linde Gas L L C . (Independence, O H ) . Sodium bicarbonate, and magnesium sulfate were purchased from A l d r i c h Chemical Inc. (Milwaukee, W I ) . Tetrahydrofuran ( T H F , A . C . S . grade) was obtained from A l d r i c h Chemical Inc. (Milwaukee, W I ) .

Analysis


Gel Permeation

Chromatography

(GPC)

The original soybean o i l ( S B O ) , the processed soybean o i l under similar conditions without catalyst ( S B O control), and polymers o f soybean o i l ( P S B O ) were dissolved in T H F . M o l e c u l a r weights and molecular weight distribution were measured by G P C with a differential refractive index detector using T H F as an eluent. The flow rate was 1.00 m L / m i n at 40 °C. The injection volume was 100 u L . Linear polystyrene standards (Polymer Laboratories ( P L ) , M n = 580100K, M w / M n = 1) were used for calibration o f molecular weights o f a l l polymers o f P S B O . 2 P L gel 3 urn mixed E columns (300 m m * 7.5 mm) in series were used to resolve the samples.

Nuclear Magnetic Resonance !

(NMR)

1 3

H and C N M R spectra for S B O and P S B O samples were recorded using a Bruker A V - 5 0 0 spectrometer (Bruker, Rheinstetten, Germany) at an observing frequency o f 500.13 and 125.77 M H z respectively on a 5 m m dual probe. F o r H and C experiments, sample solutions were prepared in deuterated chloroform ( C D C 1 , 99.8% D , A l d r i c h , M i l w a u k e e , W I ) in 15 and 3 0 % v/v concentrations respectively. Proton N M R spectra were obtained on 16 scans at a delay time o f Is. The integration values in H spectra were referenced to 4.00 between the ranges o f 4.1-4.44 ppm. l

1 3

3

!

Viscosity A Brookfield Engineering Rheometer, M o d e l D V - I I I fitted with a Cone/Plate spindle C P - 4 0 was used to measure the viscosity o f the polymer samples. The C P - 4 0 was chosen for its relatively small sample volume requirements (0.5 ml). The sample was measured and delivered with a syringe, allowed to equilibrate at temperature for 15 minutes and then rotational force was applied. Data was collected using Brookfield Engineering software


77 Rheocalic V 2 . 4 . The Bingham mathematic model was used to determine viscosity. The B i n g h a m equation is: t = t + \iy. Where t is the shear stress applied to the material, y is the shear strain rate (also called the strain gradient), t is the y i e l d stress and \i is the plastic viscosity. 0

0


Polymerization Procedure in s c C 0 2 Polymerization reactions were carried out in a 600 m L high pressure reactor from Parr Instrument C o . ( M o l i n e , I L ) . The schematic diagram o f the experimental set-up used for polymerization o f soybean o i l is depicted in Scheme 4. The reactor was attached to an Isco M o d e l 2 6 0 D high pressure syringe pump used to charge the reactor with C 0 . In a typical experiment, 100 g o f S B O was added to the reactor, which was then sealed. N 2 was purged into the reactor for 5 minutes. C 0 was pumped in until the desired pressure was reached. A controller (Parr 4843) was used to control temperature. Once the reactor was brought to the appropriate temperature and pressure, B F » 0 ( C H ) was charged into the reactor by manual injection using a Rheodyne Injector. Then C 0 was pumped in to clean initiator supplier line. After reaction o f 2 hr, 2 m L o f e t h a n o l / H 0 (1:1) was added into reactor to deactivate catalyst. Polymer product was dissolved in 100 m L hexane and washed sequentially with H 0 and 5% aqueous sodium bicarbonate. Hexane solution was dried over sodium sulfate, filtered, and evaporated under reduced pressure. A b o u t 100 g o f polymer was obtained. 2

2

3

2

5

2

2

2

2

Results and Discussion Effect of Catalyst Amount T o investigate the effect o f initiator amount on the molecular weight o f the P S B O , amount o f B F « O E t was varied from 0.0 g to 2.5 g, while keeping the monomer amount, S B O , at 100 g, and keeping other reaction conditions constant. Entry 1 in Table I is the soybean o i l control without initiator catalyst, it shows that weight average molecular weight d i d not change after 2 hours under reaction conditions. Because soybean o i l and S B O control have the same G P C profile. The results o f catalyst concentration effect are presented in Table I. Results show that as the B F » O E t amount increased, the weight average molecular weight also increased. The weight average molecular weight changed from 1,447 to 24,908 g/mol. F r o m G P C profiles (Figure 1) it is o f note that between the soybean o i l (control) peak (1,097) and high molecular weight peak o f P S B O (19,521), there are only two peaks at (1,638) and (2,649), 3

2

3

2



(Reproduced

CO, CvlindcrSyringe Pump

\

600 cc Reactor

C O

Society.)

Temperature Controlled Heater

1

Scheme 4. The schematic diagram of the experimental reactor set-up. with permission from reference 28. Copyright 2007 American Chemical

x

_Catalyst Injection Loop

©

MagneDrive Stirrer


^1 00

79 Table I. Effect of Catalyst Amount on the Molecular Weight of P S B O

SBO Entry


:ontrol) 1

p (bar)

T CC)

Reaction Time (hr)

Cat. Amount (g)

Viscosity (mPa.s)

(g/mol)

0

46.7

1,103

1.0 1.5

108 222 722

1,447 5,648 10,480

1,909

24,908

100

110

120

2

2

100

110

120

2

3 4

100

110 110 110

120 120

2 2 2

5

100 100

120

2.0 2.5

Mw

corresponding to a dimer and a trimer o f soybean o i l molecule, respectively. Figure 1 shows the G P C profile o f soybean o i l control and P S B O . It seems that formation o f dimers and trimers from soybean o i l is the slow step o f polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean o i l molecule itself, a large molecule with low activity, due to m i d chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. W e propose further investigation on the possible combination o f two polymerization mechanisms for the polymerization o f soybean o i l in s c C 0 . Kinetic study o f soybean o i l polymerization currently is carried out in our laboratory. 2

l

Figure 2 shows H N M R spectra o f S B O and P S B O . The signals at 5.40 ppm are characteristic for olefinic hydrogens and signal at 5.1-5.3 p p m represents the methine proton o f - C H - C H - C H - the glycerine backbone. The signals at 4.0-4.4 ppm are from the methylene protons o f - C H - C H - C H - the glycerin backbone. The peak at 2.80 ppm corresponds to the protons i n the C H groups between two carbon-carbon double bonds. The signals at 2.10 ppm are a methylene proton C H adjacent to carbon-carbon double bonds. It can be seen from Figure 2 that the peaks at 5.1-5.4 ppm, 2.80 ppm, and 2.1 ppm o f P S B O are greatly decreased compare to S B O . This observation indicated that the polymerization o f S B O occurred, and the number carbon-carbon double bonds were reduced. 2

2

2

2

2

2

1 3

Figure 3 shows C N M R spectra for S B O and P S B O . The peaks at 127132 p p m are due to olefinic carbons. It is also observed that peaks at 127-132 ppm are decreased after polymerization. [

Figure 4 shows H N M R spectra o f S B O and P S B O at different initiator concentrations ranging from 1.0 g to 2.5g. Figure 5 shows C N M R spectra o f P S B O at initiator concentrations mentioned above. The intensities o f olefins and associated peaks in H N M R spectra (5.1-5.3ppm, 2.80 ppm, and 2.1 ppm) 1 3

!



80

Figure 1. Overlays ofSBO (control) and PSBO obtainedfrom the RI detector of the GPC (Reproduced wjth permission from reference 28. Copyright 2007 American Chemical Society.)

Table II. Effect of Reaction Temperature on the Molecular Weight of PSBO

SBO Entry 1

100

2 3 4

100 100 100

100 100

3 4 5

100 100 100

6

100 100

7

p (bar) 55 69 76 93 100 110 121

T (°Q 120 120 120 120 120 120 120

Time (hr)

Cat. Amount

Viscosity (mPa.s)

Mw (g/mol)

84

1,536 2,863 4,152

2

2.0 2.0

2 2 2

2.0 2.0 2.0

440 458

2 2

2.0

722

6,228 6,699 10,480

2.0

N/A

11,349

2

N/A 276

suggested soybean o i l polymer formation. The resulting polymers with molecular weights ranging from 1,384 to 22,814 g/mol were observed. The effects o f temperature, pressure and catalyst concentration were evaluated experimentally. Overall, this polymerization o f soybean o i l in the s c C 0 medium offers potential for several key advances, including (i) the ability to produce liquid soy polymers that could be used as lubricants and hydraulic fluids, and (ii) its potential as a "green" processing method. 2

References 1. 2. 3.

Kaplan, D . L . Biopolymers from Renewable Resources; New York: Springer, 1998. Naeve, S. L.; Orf, J. H. Quality of the United States Soybean Crop; U S Soybean C r o p Quality Survey, US Soyean Board: 2006. L i u , K. Soybeans Chemistry, Technology, Utilization; Aspen Publisher Inc.: Geithersberg, MD, 1999.

4. 5.

B o c k i s h , M. Fats and Oils Handbook; A O C S : Champaign, I L 1998. Lawate, S. S.; L a l , K.; Huang, C . Vegetable oils - Structure and Performance, in Tribology Data Handbook;. Booser, E . R., E d . ; CRC Press: N e w Y o r k , 1997; pp 103-116. 6. Cunningham, A.; Y a p p , A. U . S . Patent 3,827,993, 1974. 7. Bussell, G . W . U . S . Patent 3,855,163, 1974. 8. H o d a k o w s k i , L. E.; Osborn, C. L.; Harris, E . B . U . S . Patent 4,119,640, 1975. 9. Trecker, D. J.; Borden, G. W.; Smith, O . W . U . S . Patent 3,979,270, 1976. 10. Trecker, D . J.; Borden, G. W.; Smith, O . W . U . S . Patent 3,931,075, 1976. 11. Salunkhe, D. K.; Chavan, J. K.; Adsule, R. N.; K a d a m , S. S. World



87

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

Oilseeds: Chemistry, Technology and Utilization; Van Nostrand Reinhold: New York, 1992. Force, C. G.; Starr, F. S. U.S. Patent 4,740,367, 1988. L i , F.; Hanson, M. V.; Larock, R. C. Polymer 2001, 42, 1567-79. L i , F.; Larock, R. C. J. Appl. Polym. Sci. 2001, 80, 658-70. L i , F.; Larock, R. C. J. Polym. Sci. Β Polym. Phys. 2000, 38, 2721-38. L i , F.; Larock, R. C. J. Polym. Sci. Β Polym. Phys. 2001, 39, 60-77. L i , F.; Larock, R. C. Polym. Adv. Technol. 2002,13,436-49. L i , F.; Larock, R. C. J. Appl. Polym. Sci. 2002, 84, 1533-43. L i , F.; Hasjim, J.; Larock, R. C. J. Appl. Polym. Sci. 2003, 90, 1830-1838. Khot, S. N.; La Scala, J. J.; Can, E.; Morye, S. S.; Williams, G. I.; Palmese, G. R.; Kusefoglu, S.; Wool, R. P. J. Appl. Polym. Sci. 2001, 82, 703-723. O'Donnell, Α.; Dweib, Μ. Α.; Wool, R. P. Comp. Sci. Technol. 2004, 64, 1135-1145. Guo, Α.; Demydov, D.; Zhang, W.; Petrovic, Z. S. J. Polym. Environ. 2002, 10, 49-52. Guo, Α.; Cho, Y.; Petrovic, Z. S. J. Polym. Sci. A Polym. Chem. 2000, 38, 3900-3910. Liu, Z . S.; Erhan, S. Z.; X u , J.; Calvert, P. D. J. Appl. Polym. Sci. 2002, 859, 2100-2107. Liu, Z. S.; Erhan, S. Z.; Xu, J.; Calvert, P. D. J. Appl. Polym. Sci. 2004, 93, 356-363. Liu, Z. S.; Erhan, S. Z.; Xu, J.; Calvert, P. D. J. Amer. Oil Chem. Soc. 2004, 81, 605-610. Sharma, V . ; Kundu, P. P. Progr. Polym. Sci. 2006, 31, 983-1008. Liu, Z . S.; Sharma, Β. K . ; Erhan, S. Z . Biomacromolecules 2007, 8, 233239.


Synthesis of Soy Polymers Using a "Green" Processing Method

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