Synthesis and Characterization of Chlorinated Rubber from Low

Chapter 20

Synthesis and Characterization of Chlorinated Rubber from Low-Molecular-Weight Guayule Rubber 1

Shelby F. Thames and Kareem Kaleem

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: July 13, 1990 | doi: 10.1021/bk-1990-0433.ch020

Polymer Science Department, University of Southern Mississippi, Hattiesburg, MS 39406-0076

The chlorination of low molecular weight natural rubber from Guayule (Parthenium Argentatum Grey) has been accomplished. The structure of the chlorinated product is consistent with that of chlorinated Hevea rubber. The use of Azo-bis-isobutyronitrile was as a catalyst resulted in increased chlorine content with a concomitant reduction in molecular weight, thereby allowing the preparation of lower viscosity grades of chlorinated rubber.

Recently, considerable attention has been given the development of a domestic source for natural rubber (NR).(l) Among the rubber bearing plants, Guayule (Parthenium Argentatum Grey) is known to provide good quality and high molecular weight NR, poly(cis-isoprene). The physical and mechanical properties of guayule NR are similar to that of Hevea (Malaysian rubber).(ls2) In contrast to Hevea, guayule rubber (GR) is isolated from the guayule shrub by a selective solvent extraction process and in addition to high molecular weight NR (Mn~ 1(f), other isolated by-products include low molecular weight rubber, (Mn~75,000), organic soluble resins, a water soluble resin fraction and bagasse. The value and quantity of the high molecular weight rubber alone is insufficient to offset the cost of the planting, cultivating, harvesting and the extraction process; thus coproduct commercialization is required if the production of GR is to be a viable commercial domestic industry. Hence, it is necessary to explore the opportunities offered by guayule coproducts in an effort to improve the economic outlook of GR production. Thus, investigators have evaluated the organic soluble resin fraction for a variety of applications errent address: 7831 Village Drive, Apt D, Cincinnati, OH 45242

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20. THAMES &KALEEM

Synthesis 6 ChanuterizationofChhrinated Rubber

including anti-termite activity (2) and adhesion modifiers for epoxy coatings.(4) Similarly, low molecular weight GR is of interest in that many of its properties are identical to that of high molecular weight rubber.(5) However, since mechanical properties are largely dependent on molecular weight, low molecular weight GR cannot be used for tire production and other conventional rubber products. In fact, there are reports that NR having molecular weight of lOP gm/mole are of no commercial value.Q) We have found, however, that low molecular weight GR rubber is an attractive feedstock for the synthesis of coatings grade chlorinated rubbers. For instance, the process of chlorination of NR typically involves two steps: mastication of high-molecular weight rubber to a reduced molecular weight and then chlorination. Since viscous solutions of NR pose manufacturing difficulties (such as gel formation and heat build up) the mastication process is necessary to reduce solution viscosity to a manageable level. The low molecular weight of naturally occurring GR is therefore an advantage as mastication is not required prior to chlorination.TTiis communication describes, for thefirsttime, the synthesis and characterization of coatings grade chlorinated rubber from low molecular weight GR. Experimental Materials and Methods. Guayule resin was supplied by Firestone Tire and Rubber Company, Akron, Ohio. Low molecular weight GR was isolated by treating the resin with 90% ethanolic solution in the following manner. Typically, 900 gm of 90% ethanol was added to 100 gm of guayule resin in a 1500 ml beaker. The low molecular weight rubber separates as a solid and the dark green supernatant solution was decanted from the low molecular weight rubber. The raw rubber was purified by dissolving in carbon tetrachloride (5% solution) followed by precipitation with the addition of 90% ethanolic solution. The purification was monitored by H NMR and C NMR. High purity, research grade chlorine gas purchased from Matheson Gas Products was used, as was research grade carbon tetrachloride, toluene, and ethanol. 1

1 3

Chlorination of Guavule Rubber. A dilute solution of guayule rubber (5%) in CC^ was added to a three neck flaskfittedwith a water condenser, gas dispersion tube and an adaptor. The gas dispersion tube was connected via Teflon tubes to a chlorine cylinder through two gas traps. The exit port of the condenser was connected via Teflon tubing to ice-cooled traps containing 2 Ν sodium hydroxide solution. The reaction flask was immersed in an oil bath for temperature control. Nitrogen was purged through the system for 10 minutes to insure complete removal of oxygen. A blanket of nitrogen was maintained over the reaction throughout the chlorination process. The solution was allowed to reflux at


231

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AGRICULTURAL AND SYNTHETIC POLYMERS

79P C with constant stirring via a magnetic stirrer. A slight excess of chlorine (10% mole excess) gas was bubbled through the solution with the liberated hydrogen chloride being trapped in the sodium hydroxide solution. The chlorinated rubber product was isolated as a white precipitate with addition of 90% ethanol. M IK spectra were recorded on a Mattson, Polaris spectrometer. Films were cast by the evaporation of a toluene solution of chlorinated rubber. The films were dried in a vacuum oven to insure removal of all solvent. H NMR and C NMR were obtained from a 300 MHz Brucker fourier transform spectrometer. The solutions (20% w/w) were prepared by dissolving the chlorinated rubber (CR) in CDCI3 and Q D for the C NMR spectra analysis with tetramethylsilane as an internal standard. Gel permeation chromatograms were generated from a Waters Associates, Inc. GPC equipped with a refractive index detector. The following operating conditions were employed: mobile phase, THF; flow rate; 1 ml/min., columns IOP, 10*, 500, 100 A . Sample concentrations were prepared at 0.2% (w/w); a 100 microliter aliquot was used for molecular weight analysis. Standard polystyrene samples (Polymer Laboratories, Inc.) were used to create a calibration curve. Thermal analyses were performed on a Dupont Model 9900 thermal analyzer under nitrogen atmosphere. A heating rate of lCPC/min. was employed for glass transition temperature (T ) determinations. Duplicated elemental analysis of CR samples were carried out by the MHW Laboratory, Phoenix, Az. 1

13

1 3


6

0

g

Results and Discussion. The synthesis of Chlorinated Rubber (CR) from low molecular weight Guayule Rubber (GR) is reported. Other investigators (5) have performed similar studies on the formation of chlorinated rubber from Hevea rubber. They found the empirical formula of the chlorinated product to be Q H Q C I ^ , indicating that chlorination involves more than one isoprene unit. Their products were soluble in organic solvents thereby lending additional support to the thesis that cyclization rather than crosslinking is the predominant reaction.(6) The chlorine content was found to be 65% and is consistent with a combination of substitution and addition followed by cyclization.(5) The concept of cyclized units along the polymer backbone have also been supported by FTIR and C NMR analysis.(7) The physical and mechanical properties of CR are often determined by its chlorine content and molecular weight. For instance, lower molecular weight CR is used for printing inks while higher molecular weight CR's are required for coatings applications. In our work we found that the chlorination of low molecular weight GR principally yielded coating grade CR. Since GR is contaminated with wax and other hydrocarbons it must be purified before chlorination. In our case purification was monitored by proton NMR spectroscopy (Figure 1). The peak assignments representing satisfactory purification are as follows: 13


20. THAMES & KALEEM

Synthesis & Characterization of Chlorinated Rubber 2

1.67 ppm (Cis double bond methyl protons) 2.00 ppm (methylene protons) 5.12 ppm (vinyl proton) The small peak at 1.25 ppm is representative of an impurity(ies) that was not removed during the extraction process. Further purification from an ethanolic solution reduces the impurity(ies) to an insignificant level. These peak positions are in excellent agreement with literature values for natural rubber (Hevea). The results of various experimental chlorinating conditions are summarized in Table I.


TABLE I. Experimental Conditions Used In The Synthesis Of Chlorinated Rubber Batch No. Amount of Guayule Rubber Used, g l

a

Ϋ

3°

3.200 5.00 5.00 a

Amount of Chlorinated Rubber Isolated, g 6.00 11.80 13.14

Cl%

59.81 60.00 63.67

Solvent; CQ,, 200 ml Solvent; ( Χ ζ , 100 ml Solvent; CQ,, 100 ml and 37.5 mg of AIBN

b c

The radical initiator, Azo-bis-isobutyronitrile (AIBN) was used in catalytic amounts to determine its efficacy, if any, in affecting changes in the chlorine content of the products. Recent studies of rubber chlorination have reported that AIBN can be used to increase the rate and amount of chlorination. In our case, use of purified and partially purified GR provided essentially identical levels of chlorination at 60%. The lower yield obtained (6.00 gm) in the first instance is attributed to the presence of impurities in the starting materials. However, purified GR (Figure 2), provided for a significant increase in product yield. The use of AIBN increased the chlorine content to a value approaching the theoretical value for fully chlorinated rubber (64.7%). While AIBN was instrumental in increasing the degree of chlorination its mechanistic role is not fully understood. The FTIR spectra of Figure 3 comparing a commercial grade rubber (Alloprene CR-20 from ICI) with guayule CR shows the two materials to be essentially identical. Absorption bands characteristic of CR appear near 780 cm" and 736 cm" and represent the secondary C-Cl and the CHg rocking 1

1


234



Τ—•—I ' I 9.0 8.0 7.0

1

I 6.0

1

I—•—T"T—I • I —Γ" — 5.0 4.0 3.0 2.0 1.0 0.0 1

1

1

Figure 1: HNMR of Purified Guayule Natural Rubber

ι ι

160

140

120

100

80

60

40

20

PPM

Figure 2:

1 3

C NMR Spectrum of Guayule Rubber in CDCLj


20. THAMES &KALEEM

Synthesis & CharacUrizatwnof Chlorinated Rubber 23

W«v«l*ngth (μιη) Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: July 13, 1990 | doi: 10.1021/bk-1990-0433.ch020

3

4

5

Β

15

τ—ι—ι—rH—ι—ι—rH—ι—I—ι—ι—ι—Η—ι—τ-Η

4000

3600

3200

2600

2400

2000

1600 1200 800

Wav6numb6rs

Figure 3: FTIR Spectrum of Chlorinated Rubber (a) Guayule CR (b) Commercial CR


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AGRICULTURAL A N D SYNTHETIC POLYMERS

1

frequency, (7) respectively, while the absorbance bands at 2939 cm " , 1440 cm and 1260 cm" are due to the C-H stretching and bending absorptions, respectively. The weak absorbance near 1630 cm" indicates residual unsaturation. The major C NMR chemical shifts of guayule CR are shown in Table II; the spectra are displayed in Figure 4. 1

1

1 3

13

TABLE II. Characteristics of C NMR Spectra of Guayule CR And Commercial Grade CR


Chemical Shifts (PPM) Guayule Rubber

Literature

Assignment

21.5, 28.7, 34.7, 37.6

21.5, 28, 34.7, 37.4, 21.5, 38, 37.4

CH^CH,

45.4, 48.01

45.4, 48

-CH, CI

62.25, 64.37

62-64

= CHC1

75.1, 77.18

74 - 77

= CCI

To identify the unsaturated carbons, spectra were taken in C D C I 3 solvent, and peak assignments were based on spectra obtained for chlorinated hydrocarbons of known structure.(Sdû) The values for guayule CR chemical shifts are in excellent agreement with the literature values reported for commercial grade chlorinated rubber.Q) Makani and coworkers (2) concluded that the broad peaks observed in C NMR spectra are a result of a variety of structures present in CR making its chemistry complicated. The peaks at 74 ppm and 77 ppm are due to quaternary carbons linked to a single chlorine atom. The CHC1 group appears at 63 and 64 ppm. Gel permeation chromatograms of the various guayule CR products are shown in Figures 5 and 6, with a chromatogram of commercial grade Alloprene CR-20 (CR-20) chlorinated rubber being included for comparative purposes. The guayule CR and commercial grade CR-20 (10) are of the same molecular weight range, and molecular weight distributions (Figure 5) with the exception that the commercial CR-20 exhibited a low molecular weight shoulder. Since CR-20 is used primarily in the coatings industry and especially in traffic paints and marine coating these data suggest a similar use for guayule CR. Guayule CR seems particularly suited for this purpose as it forms continuous, transparentfilmsfrom toluene solutions (20% solution). In contrast, guayule CR, obtained in the presence of AIBN, was found to have a lower molecular weight than that prepared without AIBN. Its molecular weight corresponds to that of commercial grade Alloprene CR-5 (10) (CR-5)


THAMES & KALEEM

Synthesis & Characterizatwn of Chforinated Rubber 2


20.



238


Figure 5: Gel Permeation Chromatograms of Chlorinated Rubber (a) Guayule CR, CI ~ 60% (b) Commercial CR-20, CI ~ 64-65%



500

1500

TIME(S)

2000

2500

Figure 6: Gel permeation Chromatograms of Chlorinated Rubber (a) Guayule CR prepared with AIBN, Ecess Chlorine (b) Guayule CR prepared with AIBN (CI - 63.7%) (c) Commercial grade CR-5 (CI - 6465%)

1000



240

(Figure 6), which is used primarily in the printing ink industry. It is clear therefore that the use of AIBN results in a slight increase in the chlorine content yet with a concomitant and significant reduction in molecular weight. Films prepared from CR-5 and AIBN derived guayule CR were found to be brittle and difficult to remove from the steel panels. These results indicate that guayule low molecular weight NR can be successfully chlorinated in a fashion similar if not identical to that of Hevea natural rubber or synthetic poly cis-isoprene. The DSC spectrum of guayule CR with 60% chlorine content (Table I, batch 1), is shown in Figure 7. The glass transition temperature (TV) of guayule CR is approximately lOSPC, while the Τ ' s are 12&C and 128? C for CR-5 and CR-20 respectively. The lower T values for guayule CR may be due to traces of waxy materials which can act as a plasticizer and reduce the glass transition temperature. Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: July 13, 1990 | doi: 10.1021/bk-1990-0433.ch020

g

0.4 τ

1

I

0.2

-0.8

Α

0

ι

1

50

1

1

100

1

ι ι 150

1

200

1

1

1

250

Temperature (°C) Figure 7: DSC of Guayule Chlorinated Rubber Conclusions The low molecular weight GR can be successfully chlorinated to obtain coating grade CR. C NMR and elemental analysis of the chlorinated products confirm that its chemical structure and composition is similar to that of commercial grade chlorinated rubber. The use of AIBN during chlorination significantly reduces the molecular weight and in turn allows the preparation of lower viscosity grade chlorinated rubbers. 1 3


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Synthesis & Characterization of Chlorinated Rubber 2

Acknowledgments Financial support from the United States Department of Agriculture Grant No. 89-COOP-1-4218 is gratefully acknowledged. We are particularly thankful to Mr. George Donovan and Dr. s Richard Wheaton and Daniel Kugler for their encouragement and support.


Literature Cited 1. Hammond, B. L.; Polhamus, L. G. Research on Guayule (Parthenium Argentatum Grey): 1942-1959, U.S.D.A, Agr. Res. Ser. Tech. Bull. No. 1327, Washington, D. C., 1965, 143. 2. Backhaus, R. Α.; Nakayama, F. S. Rubber Chemistry and Technology, 1986, 61, 78-85. 3. Bultman, J. D.; Gilbertson, R. L.; Amburgey, T. L.; Bailey, C. A. "Guayule resin - a new wood preservative." In Annual Meeting of the Wood Preservers' Association, Minneapolis, MN, (1988). 4. Thames, S. F.; Kaleem, K. Communication to "Biomass," 1989. 5. Bloomfield, G. F. J. Chem. Soc., 1943, 289. 6. Rubber Chemistry. J. A. Brydson, Ed.: Applied Science publisher Ltd, London, (1978), pp 178-179. 7. Makani, S.; Brigodiot, M.; Maréchal, Ε.; Dawans, F.; Durand, J. P., Journal of Applied Polymer Science, 1984, 29, 4081-4089. 8. Torosyan, Κ. Α., Voskanyan, E. S., Mkryan, G. M., and Karapetyan, N. G. arm. Khim. Zh., 1973, 26, 413. (C.A. 79, 93181d); 871 (1973) (C.A. 80, 97016x); Hlevca, B., Gheorghe, G. Rom. Pat. 67,006 (C.A. 95, 170767j (1981). 9. Velichko, F. K.; Chukovskaya, E. G.; Dostovalova, V. Il; Kuzmina, Ν. Α.; Freidlina, R. Kh. Org. Mag. Res., 1975, 7. 10. Commercial Grade CR-20, CR-5 were obtained from Polyvinyl Chemical Co. (ICI Resins US), Wilmington, MA. RECEIVED

January 18, 1990


Synthesis and Characterization of Chlorinated Rubber from Low

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