Emerging Polymeric Materials Based on Soy Protein

Chapter 17

Emerging Polymeric Materials Based on Soy Protein Thomas L. Krinski

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch017

Protein Technologies International, Checkerboard Square, St. Louis, MO 63164

This chapter will describe the chemical modification of native soy protein, a globular protein primarily used as a food source. A review of the past technology will describe the early industrial uses of soy protein and how the protein was modified to alter its function as an adhesive. Current and future directions will show how chemical modification and alteration of protein chain association can further enhance soy protein polymers function. The past unfavorable attribute of biodegradation of the soy polymer chain may now guide the future of soy polymer derivatives as the need increases for biodegradable polymers from renewable resources. Industrial Soy Protein of the Past Industrial soy protein used are primarily in the coating of paper and paperboard, with minor areas in water based inks and water based adhesives. The water based adhesives include bottle label adhesives, foil laminating adhesives, and cone/tube winding adhesives. Protein Composition and Isolation. The protein present in the soy bean is not a simple singular globulin but a mixed assortment of different types of protein fractions. These fractions consist of: Bowman-Birk Trypsin Inhibitor. A globular protein of 2 4M molecular weight that can form dimers and trimers due to high cystine content. Kunitz Trypsin Inhibitor. This is another low molecular weight protein (molecular weight of 21M) which possesses disulfide linkages. Hemaglutinin. A glycoprotein with a molecular weight of 100-110M without any disulfide crosslinks.

0097-6156/92/0476-0299$06.00A) © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

300

MATERIALS AND CHEMICALS F R O M BIOMASS

Lipoxygenase. An enzyme with a molecular weight of 102M which can be disassociated into two near equal fragments using guanidine hydrochloride.


7S Globulin. A large associated polymer of 180-210M molecular weight consisting of nine polypeptide chains. These peptide chains can be disassociated into 2S and 5S subunits at pH 2.0. At neutral and s l i g h t l y a l k a l i n e pH (pH of 7.6), the 7S can dimerize into the 9S f r a c t i o n . F u l l y disassociated, the polypeptide chains that make up the 7S have molecular weights i n the 20-25M range. 11S Globulin. A large 330-350M molecular weight associated polymer which e a s i l y disassociates into 7S size fragments. These fragments can likewise further disassociate into various smaller subunits capable of associating with one another t o form dimers and trimers. Urease. An enzyme having a sedimentation f r a c t i o n equivalent t o an 18S form (1). Soy proteins are i s o l a t e d by aqueous a l k a l i n e extraction of defatted soybeans. The native protein can be i s o l a t e d p r i o r t o any a d d i t i o n a l treatment by p r e c i p i t a t i o n at t h e i r average i s o e l e c t r i c point of pH 4.5 and separated from the soy sugars and s a l t s . The molecular weight d i s t r i b u t i o n by HPLC i n 6 Molar guanidine hydro chloride i s represented i n Figure 1. The native protein has an average molecular weight (Mw) of 192,000 daltons. Native soy protein i s d i f f i c u l t t o work with as an adhesive because the many d i f f e r e n t globulins disassociate and reassociate as the protein i s s o l u b i l i z e d on the a l k a l i n e side. Even though the molecular weight of the native protein i s quite high and i t has good adhesive q u a l i t i e s , the problem of s o l u t i o n rheology control usually discourages i t s use (2). Physico-chemical Treatment of Soy Protein (Caustic Treatment). Native soy protein has been modified using a c o n t r o l l e d a l k a l i n e heat treatment of the protein while i n s o l u t i o n . This processing u t i l i z e s three factors to control the protein reorganization: pH, temperature and time (3). Table 1 shows the primary events which occur during physicochemical treatment. Protein globulins slowly unfold with a minimum of backbone chain cleavage. As the chains unfold, they reorganize or reassociate, t h i s time by hydrophobic/hydrophilic regions, this exposes more p o t e n t i a l hydrophilic groups t o aqueous contact. A portion of the asparagine and glutamine residues are hydro lyzed t o the free acids. This increases the anionic character of the protein as seen i n the t i t r a t i o n curve of Figure 2. F i n a l l y , we also achieve some i n t e r n a l chemical c r o s s l i n k i n g through the formation of lysinoalanine (LAL). This occurs when the cysteine or serine residues undergo a β elimination reaction (Figure 3) i n the presence of elevated ph and temperature, forming a dehydroalanyl residue. This residue can then react with l y s i n e amine through a base catalysed reaction achieving p o t e n t i a l c r o s s l i n k i n g of two independent protein chains.


17. KRINSKI

6M GUANIDINE HYDROCHLORIDE pH 10 TSK-G5000PWxl

%/Min.


301

Emerging Polymeric Materials Based on Soy Protein

1000

10,000

100,000

1,000,000

10,000,000

Molecular Weight Figure 1. Native protein Mw distribution.

Table I. Caustic Treatment (pH>10.5, 40°C OH

ASPARTIC ACID

D

ASPARAGINE

R Ο

L I

GLUTAMINE

2

-CH C0 Η 2

}

2

11.33 -CH

Ο Κ C^

2

NH

2

- C H C H C0 H

GLUTAMIC ACID

Ρ Η

SD I E OROVP

CYSTEINE

H Y

MOLE%

2

}

2

18.11

2

•°

- C H C H C^ 2

2

NH

HISTIDINE

2.19

LYSINE

5.50

-CH

2

^

2

ΗΝ Ν

C

- C H C H C H C H NH 2

2

2

2

Η ARGININE

5.56

- CH CH CH NC*'

SERINE

6.34

• C H OH

2

2

2

2

NH

2

NH


2

306

MATERIALS AND

CHEMICALS F R O M BIOMASS


replace them with additional acid groups (Figure 9 & 10). The increased anionic character i s evident i n the t i t r a t i o n curves of Figure 11 where d i and t r i c a r b o x y l i c anhydrides were used to modify the soy protein. This type of modification i n coatings tends to decrease the v i s c o s i t y of soy protein even further (Hercules Rheogram i n Figure 12). Increased anionic charge on the protein further reduces the associative nature of the protein and increases i t s dispersant nature. A s h i f t to lower average molecular weight (133,000 daltons - Figure 13) occurs due to increased processing time and increased d i s s o c i a t i o n of the soy globulins. Crosslinking Reactions. Soy proteins can be mildly crosslinked through the use of various simple or complex reagents. Using epichlorohydrin (Figure 14), the soy protein molecular weight and solution v i s c o s i t y can be increased by varying the amount of reagent used. A more complex monomer such as methyl acrylamidoglycolate methyl ester (MAGME) which has the a b i l i t y to c r o s s l i n k both during i n i t i a l reaction and l a t e r during application (Figure 15) can be used (8). Under basic conditions, the primary reaction with the protein can occur either at the double bond or at the activated ester. At elevated temperatures, the proton on the amide group i s a c i d i c enough to encourage further c r o s s l i n k i n g at the amide nitrogen. This second reaction could occur during the soy polymers use i n the coating-drying-calendering of paper. Base Catalysed Reactions. Due to the a l k a l i n e processing conditions, nucleophilic addition reactions are excellent f i t s for modification of soy protein and can be t a i l o r e d even to s e l e c t i v e amino acid residues, water soluble monomers such as acrylamide (Figure 16) and a c r y l o n i t r i l e provide excellent r e a c t i v i t y with lysine amine groups and sulfhydryl of cysteine. Hydroxyacrylates can also be used to modify any free amine groups and add additional hydroxyl residues to the protein (9). In paper coating applications, t h i s allows the protein to hydrate more thus adding water holding to the paper coating as i t i s applied. The r u n n a b i l i t y of the coating as i t ' s applied to the paper i s improved. Exotic monomers, such as g l y c i d y l oxypropyl trimethoxy s i l a n e , can add a s i l a n e ester to the protein amines (10). This modification greatly increases the v i s c o s i t y of the protein with clay pigments or causes t o t a l d e s t a b i l i z a t i o n of the clay suspension. Interpolymer Technology. When soy polymers are used i n the coating of paper, a styrene butadiene latex i s also used. The increased dispersant action and lower v i s c o s i t y of the anionic modified soy protein has been used i n the preparation of various l a t i c e s (Figure 17). Monomers can be emulsified by using the soy polymer as the only surfactant. These miscelles can then be polymerized i n the presence of the soy polymer r e s u l t i n g i n a soy latex interpolymer whose core i s s t a b i l i z e d by a "surfactant s h e l l " of soy protein. Figures 18 and 19 show the p a r t i c l e s i z e analysis of such a styrene butadiene latex with an average s i z e of 0.2 micron. This i s


17. KRINSKI

307


THEORETICAL IONIC STATES OF AMINO ACID CHAINS AS A FUNCTION OF THE pH.

DI^OCIATION

S I D E

1.0 ι BH*

° S

0.8

-COOH


0.6 e x . 0.4 0.2

Figure 7. Ionization of side residues.

OHProt-NH + (CH CO) 0 — Prot - NH - C - CH + CH COO " 2

3

3

2

3

Ο Figure 8. Anhydride reaction with amine groups: monocarboxylic anhydride.

Prot-NH + C H - C ^ 2

2

ι y CH - C '

Qu-

• Prot - NH - C - CH - CH - C '

2

2

o

H

2

x

o-

o Figure 9. Anhydride reaction with amine groups: dicarboxylic anhydride.

Figure 10. Anhydride reaction with amine groups: tricarboxylic anhydride.


308

MATERIALS AND CHEMICALS FROM BIOMASS

A C I D - B A S E B A C K TITRATION O F S O Y PROTEINS

lg PRODUCT IN 50g SOLUTION +5 mis. 0.5N NaOH


pH 14,

ol

0

1

2

3

4

5 6 Mis. 0.5N HCI

7

8 9

10

Figure 11. Protein titration curve.

COATING RHEOLOGY OF ANIONIC Shear Rate

S O Y POLYMER

Shear Stress Figure 12. Soy polymer rheogram.


309


17. KRINSKI

6 M GUANIDINE HYDROCHLORIDE pH 10 TSK-G5000PWxl

%/Min.


2 -Anionic -Hydrolyzed * Native

1000

10,000

100,000 1,000,000 M o l e c u l a r Weight

10,000,000

Figure 13. Protein Mw distribution.

NaOH C H - C H - C H C I + Prot - N H 2

2

H C - C H - C H - H N - P r o t + NaCI

2

2

2

V

^ O 2.

P r o t - N H - C H - C H - C H + Prot-NH Prot-NH-CH -CH-CH -X-Prot \ / OH Ο + Prot-SH OH 2

2

2

2

-

2

1

+ Prot-OH Figure 14. Crosslinking reactions using epichlorohydrin.

METHYL ACRYLAMIDOGLYCOLATE METHYL ESTER CH H C = 2

c'

3

OCH

I

3

C-NHCH-C-OCH3 n

A

ο

11

O

• I

Β

Figure 15. Crosslinking reactions using MAGME.


310


UNSATURATED C A R B O N Y L S C A N REACT WITH UNPROTONATED AMINES NH

pH 9.0

2

NH

2

C H = C H - C = 0 +Prot- N H — • Prot - NH - C H - C H - C = Ο 2

2

2

2


ARCYLAMIDE Figure 16. Base catalyzed additions (nucleophilic).

· · · · · · ·

f^fvN

I taê M t

· · · · · · ·

i

· Protein Soin

··«·

•

-

{λ···\

^S!t\ •

JOV' Monomers

Emulsified Monomer The Interpolymer is prepared by the free radical polymerization of styrene a n d butadiene in the presence of modified soy protein. The monomers are first emulsified by the protein. These emulsified particles are spherical in shape with the hydrophillic soy protein covering the surface a n d the hydrophobic monomers buried in the interior. The soy protein is polymerization. Figure 17. Interpolymer preparation.

£3KM

^BSr Polymerized Interpolymer

SOY INTER POLYMER PARTICLE SIZE ANALYSIS Volume %

Particle Diameter (um) Figure 18. Interpolymer particle size.


17.

KRINSKI

311



S O Y INTER POLYMER PARTICLE SIZE ANALYSIS Number %

1

5 10 Particle Diameter (um)

50 100

500 1000

Figure 19. Interpolymer particle size.

comparable to commercial l a t i c e s yet, when used i n the coating of paper and paperboard, possesses the advantages of both soy polymer and S/B latex (11). Through the use of chemical modification, we have brought the old caustic treatment technology of soy protein out of the dark ages. Combining the caustic treatment step with chemical modification has enabled us to change s u b s t a n t i a l l y the f u n c t i o n a l i t y of the protein polymer by modifying a small number of amino acid residues. Using these "tools", we can a l t e r the r e a c t i v i t y / a s s o c i a t i v i t y of the globular protein of the native soy with i t s e l f , a l t e r i t s molecular weight and generally provide a more uniform protein. Future Soy Polymers Previously, most industries had considered soy proteins s t r i c t l y as adhesives (12). However, t h i s attitude has dramatically changed as the chemistry of the soy protein has been modified. Modern soy polymers are now being used as paper pigment structuring agents and flow modifiers. Their amphoteric nature has shown they can be used as protective c o l l o i d s and even surfactants i n the s t a b i l i z a t i o n of latices. The use of chemical modification to change soy protein function i s s t i l l i n i t s infancy. As we learn how to use more reagents to modify the protein i n conjunction with other treatments which can control protein reorganization, we can further s p e c i a l i z e the f u n c t i o n a l i t y of the finished soy polymer. In doing so, we can expand the use of soy polymers into other i n d u s t r i e s . Use of soy polymers i n aqueous based inks has increased due to the environmental concerns of hazardous fumes from solvent based inks. As environmental concerns of biodegradability, waste


312


disposal, and sewer effluent treatment become issues, new soypolymers could be offered as solutions. Creation of new soy interpolymers, solution viscosity modifiers, flocculating agents and ingredients for biodegradable plastics, are potential avenues for future soy polymer technology. References 1. Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch017

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Smith, Α. Κ., and Circle, S. J . , Soybeans - Chemistry and Technology; 1978, Vol. 1; Proteins, pp. 114-128. Coco, C. E . , Krinski, T. L . , Tappi Coating Binders Seminar, 1986. Ishino, K, Okamoto, S., "Molecular Interaction in Alkali Denatured Soybean Proteins"; Cereal Chemistry 52, 9, 1975, p 19. Coco, C. E . , Preprints Tappi Coating Conference, 1987. IBID. Means, G. E . , Feeney, R.E., Chemical Modifications of Proteins, 1971, p 14. Coco, C. E . , Krinski, T. L . , Graham, P. Μ., U.S. Patent #4474694. Steinmetz, A. L . , Krinski, T. L . , U.S. Patent #4554337, 11/19/85. Krinski, T. L . , Steinmetz, A. L . , U.S. Patent #4687826, 8/18/887. Krinski, T. L . , Steinmetz, A. L . , U.S. Patent #4713116, 12/15/87. Coco, C. E . , Preprints Tappi Coating Conference, 1987, pp. 133-140. Strauss, R. W., Protein Binders in Paper and Paperboard Coating; Tappi; Monograph No. 36.

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Emerging Polymeric Materials Based on Soy Protein

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