220
Ind. Eng. Chem.
Prod. Res. Dev., Vol. 18, No. 3, 1979
Hendriks, Ch. F, Van Beek, H. C. A., Heertjes, P. M. Ind. Eng. Cbern. Prod. Res. D e v . , preceding paper in this issue. Hendriks, Ch. F., Van Beek, H. C. A., Heertjes, P. M., Ind. Eng. Cbem. Prod. Res. Dev.. 17, 280 (1978). Nemecek,A. M., Herdriks; Ch. F., Van Beek, H.C. A., De Btuyn, M. A,, Kerdthoffs, E. J. H.,Ind. Eng. Cbem. Prod. Res. Dev., 17, 133 (1978). Powell, M. J. D., Comput. J . , 7 , 303 (1964). Smith, C. L., Pike, R. W.. Murriii, P. W., "Formulation and Optimization of
Mathematical Models",Intemahai Textbodc Company, Scranton, Pa.. 1970. Taylor, C. J. A., Marks, S..in "Solvents, Oils, Resins and Driers, Paint Technology Manuals II", Chapman and Hall, London, 1969.
Received f o r review April 24, 1978 Accepted March 6, 1979
GENERAL ARTICLES New Rubber Antioxidants and Fungicides Derived from Larrea Tridenfafa (Creosote Bush) Hector Belmares, Arnoldo Barrera, Ernest0 Castlllo, LUISF. Ramos, Fellx Hernandez, and Vlcente Hernandez Centro de Investigacion en Quimica Aplicada, Aldama Ote., 37 1 Saltillo, Coahuila, Mexico, and Centro de Investlgaciones Agricolas del Noreste, Matamoros, Coahuila, Mexico
The shrub of Larrea tridentata covers around 25% of the surface area of Mexico. The resinous material of the external leaf surface (about 16%, dry weight basis) is mostly phenolic in composition and exhibits antifungal activity for several fungi tested such as Rhizoctonb sobni, Phytium spp, Rhizopus nigrkans, Fusarium oxyspotum, VeHkillium dahlie (rss-4), and Verticillium dahlie (Inter-2). Field tests against the disease known as cotton rust caused by the fungus fuccinia cacabata have been started with promising results. Several resin derivatives were made such as the acetate ester, phenoxyacetic acid derivative, and a condensation product with formaldehyde. These derivatives also show antifungal activity. An additional application for the resin of Larrea as an antioxidant for natural rubbers (Guayule and Hevea rubbers) appears promising. Tests have been carried out for rubber gums and for vulcanized rubbers. Outdoor exposure studies and vulcanization time characteristics were also satisfactory.
Introduction In our programmed search for new, abundant, and renewable sources of industrial raw materials we have investigated the shrub called Larrea tridentata, also known as creosote bush. Larrea tridentata is an important component of the Chihuahua, Sonora, and Mojave deserts in North America and of the Monte desert in Argentina. It is the dominant plant in large areas of these deserts; it has good ecological adaptability to a wide set of extreme environments and is a renewable natural resource (Hunziker et al., 1977). Larrea tridentata extracts are known to contain more than a hundred chemical compounds that include waxes, volatile compounds, saponins, and phenolic compounds. The phenolic compounds comprise about 90% of the total extract (Mabry et al., 1977), with lignans and flavonoids listed as the main phenolic components without specifying their relative content ratio. The lignan called nordihydroguaiaretic acid or NDGA, (4,4'-(2,3-dimethyltetramethylene)dipyrocatechol), is one of the components of Larrea resin and has antimicrobial properties against salmonella, penicillium, M . pyogenes, S. cereuisiae, as well as other pathogens and molds (Oliveto, 1972). Some other natural phenols act as phytoalexins, antimicrobial metabolites which accumulate in high concentrations in or around the damaged cells of the attacked plant. Among those phenols are phaseollin, hesperitine (McClure, 1975a), and diadzein (Birk et al., 1974). Experimental Section Resin recovery. Leaves and green twigs from Larrea were harvested on the road to Zacatecas at about 20 miles from Saltillo, Coah., Mexico. The intact material was extracted with the solvent (reagent grade) at its boiling point for about 6 min with the exception of water and 0019-7890/79/1218-0220$01.00/0
absolute ethanol which were used at room temperature. The solvent was evaporated under vacuum by means of a rotary evaporator. The resin obtained was kept overnight at 70 "C in a vacuum to free it from any traces of solvent until constant weight was obtained. The softening point of the extracted resin varies with the solvent. Thus, for chloroform it is 60 "C, for toluene 50 "C, and for methanol 65 "C. The batch extraction process has been adapted satisfactorily to the Pilot Plant and a continuous process is under study. Table I shows the yields of resin for different extraction solvents. The wax was isolated and identified by its NMR spectrum (Seigler,et al., 1974). The NDGA was obtained from Burdick and Jackson Laboratories, Inc., Muskegon, Mich. It was used as the standard for the vapor pressure osmometer and for the gas chromatography apparatus, as well as for the NMR studies (mp 188-190 "C). Among the solvents proposed for extraction of the resin to further isolate the NDGA are ethyl ether, sodium hydroxide, ethylene dichloride (Horn and Gisvold, 1945; Gisvold, 1945a, 1946,1947,1948),benzene (Gisvold 1945b), and ethanol (Duisberg et al., 1949). The extracted leaves have been found to be a good livestock feed (Adams, 1970). The phenolic portion of the resin extract (with diethyl ether) does not show significant chemical differences for the three races (diploid, tetraploid, and hexaploid) of Larrea tridentata (Mabry et al., 1977). Acetate Ester Derivative. In a 100-mL three-necked round-bottomed flask, equipped with a mechanical stirrer, reflux condenser, and external heating mantle, were placed 30.0 g (0.093 mol) of Larrea resin (polar fraction obtained by extraction of the leaves with absolute ethanol at room temperature after they had been previously extracted with chloroform), 11.2 g (0.109 mol) of acetic anhydride, and 30 mL of pyridine. The solution was refluxed, with 0 1979
American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 221
Table I. Solvent Comparison for Larrea Resin Extraction solub. param.: solvent n-hexane toluene chloroform dichloromethane acetone 100%ethanol 100%ethanol (after chloroform) water
H bondb
yield, %, dry wt basis
7.3 8.9 9.3 9.7 9.9 12.7 12.7
poor poor poor poor medium strong strong
0.8 (wax) 5 8 7 12 10 8
23.4
strong
,,,'/a
cm-3/a
0. Id
NDGA 9i
-OCH,/molC
0 9 6 34 24 26 35
0.7 0.6
NDe
NDe
0
1.0 1.3 0.6 0.8
At 25 "C (Barton, 1975). They include polar, hydrogen bond. and dispersion forces. As a comparison, natural rubber has a value of 8.1-8.5 c a l ' / ' ~ m - ~ / ~ -25 a t"C (Barton; 197%). Barton, (1575). Table VI shows hbw this.ratio was obtained. Organic fraction. e Not determined.
constant stirring, for 2.5 h. It was then cooled down to room temperature and poured with strong stirring into 2 L of ice water which contained 45 mL of concentrated hydrochloric acid. The precipitated resin was then dissolved in acetone and the solution was added with strong stirring to 3 liters of ice water which contained 30 mL of concentrated hydrochloric acid. The heterogeneous mixture was stirred for 10 min and then the organic fraction was extracted with toluene. The toluene layer was dried with anhydrous sodium sulfate and the solvent was evaporated under a vacuum. The residual resin was heated at 60 OC under vacuum for at least 30 h to constant weight, and thus any traces of solvent were removed. The yield of product was 85%; softening point, 50 OC. Phenoxyacetic Acid Derivative. In a 250-mL three-necked round-bottomed flask, equipped with a mechanical stirrer, reflux condenser, external heating mantle, and with nitrogen atmosphere, were placed 10.0 g (0.031 mol) of Larrea resin (polar fraction as in the previous case), 16.0 g (0.4 mol) of sodium hydroxide, 100 mL of butanol, 9.6 g (0.10 mol) of reagent grade monochloroacetic acid, and 30 mL of water. The solution was refluxed for 5 h under continuous stirring and then was cooled down. One gram of sodium dithionite and 30 mL of water were added and the solution was poured slowly and with strong stirring into an aqueous diluted solution of hydrochloric acid. The resinous precipitate was washed thoroughly with water and then dried under vacuum at 70 OC for 20 h in the presence of phosphorus pentoxide until constant weight was obtained. The yield of product was 85%. It is completely soluble in aqueous sodium bicarbonate while the parent resin of Larrea is not soluble; softening point, 80 "C. n-Butyl Ester of the Phenoxyacetic Acid Derivative. Three grams (0.0076 mol) of the phenoxyacetic derivative was dissolved in 6 mL (0.065 mol) of 1-butanol and then 0.03 g (0.00015 mol) of p-toluenesulfonic acid monohydrate was added. The solution was refluxed for 15 min and then the excess of 1-butanol was distilled off. The product was placed in a vacuum at 70 "C for 72 h to remove any traces of the 1-butanol. The product was insoluble in an aqueous solution of sodium bicarbonate; softening point, 30 "C. Polymerization of the Resin of Larrea with Formaldehyde. In a 250-mL three-necked round-bottomed flask, equipped with a mechanical stirrer, reflux condenser, and external heating mantle, were placed 10.0 g (0.028 mol) of Larrea resin (lipophilic fraction obtained by extraction of the leaves with chloroform at room temperature), 6.3 mL (0.086 mol) of an aqueous formaldehyde solution (38%), 20 mL of water, 1 mL of methanol, and 0.27 mL (38%) of concentrated hydrochloric acid. The reaction mixture was refluxed, with constant
stirring, for 2.3 h. During this time the resin (insoluble in the reaction medium) becomes more viscous. At the end of the specified time, the reaction mixture is cooled down to room temperature and the polymerized resin is washed thoroughly with water and then dissolved in 40 mL of isopropyl alcohol. The solution is poured slowly and with constant stirring into 2 L of ice water acidulated to pH 3 with hydrochloric acid. The light tan precipitate is filtered, washed with water until all the acid is removed, dried under vacuum at 70 OC for at least 48 h, and then carried to constant weight in the presence of phosphorus pentoxide to remove any traces of water; softening point for the dimer, 100 OC. Equipment and Test Methods. Gas-liquid chromatography (GLC) analyses were performed in a Varian Aerograph 2800 apparatus provided with flame ionization detector. The technique used is similar to the one reported by Birk (Birk et al., 1974) for flavonoids. GLC was performed on trimethylsilyl derivatives using a 6.3 mm 0.d. X 1.8 m stainless steel column packed with 5% OV-17 on 80-100 mesh Gas Chrom Q and operated at 280 "C with carrier gas (nitrogen) flow of 30 mL/min. Peak areas were determined as the product of peak height and the width at half height. Their relative retention times were referred to the position of the peak for the trimethylsilyl derivative of NDGA to which was assigned a value of 1.00. The recorded peaks did not show any shoulders. The proton NMR spectra were taken in a Varian EM-360 spectrometer with 0.5% tetramethylsilane as an internal reference and acetone-& as the solvent. Molecular weights were determined by vapor pressure osmometry in a Hewlett-Packard Model 302B apparatus. The calibration standard was NDGA and the solvent was ethanol for all samples. Each molecular weight involves six determinations at different concentrations from which the mean f standard deviation was calculated. The infrared spectra were taken in a Perkin-Elmer Model 297 infrared spectrophotometer by means of the KBr pellet technique and using polystyrene as the calibration standard, The oxidation exotherms for the samples of guayule gum were determined in a Dupont 990 differential scanning calorimeter (DSC) calibrated with indium and tin to ensure the accuracy of caloric data. Guayule rubber gum for the DSC studies was purified by several precipitations from tetrahydrofuran into methanol, then dissolved in tetrahydrofuran, and the antioxidant to be tested was added to this solution at a concentration of 1.25% by weight of rubber. Then, the solvent was evaporated carefully on a glass plate under vacuum and at room temperature for 48 h to obtain a thin film. The use of a single concentration to determine the antioxidant properties of a compound (or a mixture of them) for rubber is well justified by the literature (Wasson and Smith, 1953; Kotulak et al., 1970;
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
222
Table 11. Standard Formulation for Vulcanization or for Moonev Scorching Time Determination component
parts
natural rubber (Guayuleb or RSS1") zinc oxide stearic acid sulfur MBT (mercaptobenzothiazole)
100 5 2 3 0.5a 0.Bb 1 and 2 1 and 2
MBT
antioxidantog retarding agentb a
For vulcanization.
For Mooney scorching time.
Table 111. Formulation for White Samples of Rubber for Outdoor Exposure Studies component
parts
natural rubber (RSS1) stearic acid zinc oxide titanium dioxide sulfur MBT (mercaDtobenzothiazole1 antioxidant iested
100 2 40 25 3 1 1
Spacht et al., 1962; Kitchen et al., 1950). The sample (10.0 mg) was placed in the apparatus cell in an oxygen atmosphere and heated from room temperature to about 280 "C, which is beyond the main oxidation exotherm temperature of the sample. Three to five replicates were run for each sample and the beginning of the oxidation exotherm temperature was determined. From these values, the average temperature and its standard deviation were calculated for the pure guayule rubber gum and for each one of the samples containing the antioxidant being tested. Then the temperature difference between the pure guayule rubber gum and each formulation was calculated. The greater the difference, the more effective was the antioxidant. For the air-oven test, the rubber samples were vulcanized at 140 "C for 20 min and then aged in an air oven at 100 "C for 48 h following the ASTM-D-573-67 method. The tension testing of vulcanized rubber was done in an Instron Tensile Tester Model 1122 in accordance with ASTM Method D412. The Mooney scorching time tests (time in minutes taken for a sample to increase five Mooney units above the minimum viscosity) were run in a Monsanto Mooney viscometer at 140 "C. Table I1 shows the formulations used for the Mooney scorching time determinations and for the vulcanization of the rubber samples for aging studies. Finally, the formulation shown in Table I11 was used for the outdoor exposure studies of vulcanized white samples of rubber. The vulcanization was carried out at 140 "C for 20 min. The samples were subjected to outdoor exposure for two months in Saltillo, Coahuila, Mexico, in the spring of 1976. The rubber plates (16 X 8 cm) were initially covered with aluminum foil and every two weeks a fresh area (4 X 8 cm) of the plate was uncovered for outdoor exposure. This allows, on the same sample, a gradation of discolorations corresponding to the different exposure times. The color scale for comparison was made by preparing vulcanized white rubber samples to which successively were added 1,2,3,4,and 5% of ferric oxide powder by weight of rubber in the formulation of Table 111. The numbers for color intensity were respectively 1 to 5, corresponding to the respective percentages of ferric oxide. The fungi Rhizoctonia solani (ATCC 18184) and Rhizopus nigricans (ATCC 24795) were obtained from the American Type Culture Collection, Rockville, Md., while
Table IV. Typical Gas-Liquid Chromatogram for the T w o Important Fractions of Larrea Resin. Relative Retention Times of the Trimethylsilyl Derivatives rel. ret. timesa
lipophilic fraction area ratio, %
0.44 0.47 0.51 0.61 0.71 0.85 1.00 1.07 1.15 1.64
10.2 17.2 6.0 10.8 3.2 6.0 (NDGA) 31.5 15.0
...
...
polar fraction area ratio, % 2.6b
...
34.6 11.2 6.9
...
34.9 (NDGA) 5.7
...
4.1
a All retention times relative to the trimethylsilyl deriPeaks of less percentage are not listed. vative of NDGA.
Fusarium oxysporum, Phytium spp, and Verticillium dahlie (rss-4 and Inter-2 strains for this latter one) were obtained from the Phytopathology Department of the Centro de Investigaciones Agricolas del Noreste (CIANE) in Matamoros, Coahuila, Mexico. The fungi were grown on potato dextrose agar (PDA) according to a reported technique (Birk et al., 1974) in petri plates (85 mm in diameter) with 15 mL of growth medium. Disks (7 mm in diameter) of agar with mycelium were cut from the margins of 48-h old colonies with the aid of a sterile cork borer and transferred to petri plates containing PDA supplemented with various amounts of resin of Larrea or its derivatives. Mycelial growth was allowed to proceed at 28 "C until the mycelium in the control reached the margin of the petri plate. Each experiment was carried out in four replicates, and mycelial growth was determined as the fraction of the area covered by the mycelium and expressed as percent growth. Results and Discussion Extraction Studies. Table I shows a solvent comparison for Larrea resin extraction. The solubility parameters are taken from a review (Barton, 1975) and included the forces of dispersion, polar forces, and hydrogen bond forces. The percent of NDGA in the extracted resin was determined by GLC. For the methoxyl groups, the NMR region between 3.3 and 3.9 ppm (Me4Si as standard) was assigned to them (Dyer, 1965). From Table I we see that the ratio (-OCH,/mole) is inversely proportional to the NDGA content of the resin and that the ratio (-OCH,/mol) goes through a maximum as the solubility parameter of the solvent increases. From these results the chloroform extract (which we call the lipophilic fraction) and the absolute ethanol extract (which we call the polar fraction) were chosen for further testing. The latter extract was obtained from the same batch of leaves that had been extracted previously with chloroform to get a combined yield of resin of 16% (dry weight basis). For natural phenols such as flavonoids and flavones the literature reports the same direct relationship between lipophilicity and methoxyl content (McClure, 1975b). For one year (1976), the variation of the ratio (-OCH,/ mol) was 1.2 to 1.4 for the lipophilic fraction and 0.6 to 0.8 for the polar fraction and the variation of NDGA content was 6 to 10% for the former 30 to 40% for the latter, These results agree well with related constant phenolic pattern studies made before (Mabry et al., 1977). Table IV shows the results of a routine gas chromatogram for the lipophilic and polar fractions of the Larrea resin. For an antioxidant a good compatibility with the substrate (rubber for this case) is a very important factor for an acceptable performance (Kotulak et al., 1970). Since
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
223
Table VI. Nuclear Magnetic Resonance Spectrum of the Lipophilic Fraction of the Larrea Resin (Me,Si as Standard)
Ho HO
~
How$
Guaiaretic
h % ? i k y d r o g u a id retic Acid)
Acid
C HO , HO
'qoc,,
9
OH
HO HO
Pa r t i d I l y De,methylated Dihyd r o g u a i a r e t i c Acid
D ihy d r og u a id r e t i c Ac id
Figure 1. Lignans from Larrea tridentata. Table V. Nuclear Magentic Resonance Spectrum of NDGA (Me$ as Standard) position, ppm 0.75 1.2-2.8 6.0-7.0 7.4
no. of area hydroratioa gensb
multiplicity complex
1.00
6
1.27
singlet
--
--
1.2-2.8 3.3-3.9
complex complex
1.00 0.66
6 3.6
6.0-7.3 7.3-8.3
complex broad
1.00 0.53
6 3.2
OH
3 ' - D e m e t h o x y i s o g uaiac in
Nor i s og u d id c i n
position, PPm 0.75
~~~~
no. of hydro- area multiplicity gens ratiou doublet, J = 7 cps complex
6
1.00
6
1.00
complex broad singlet
6 4
1.00 0.66
hydrogen type 2 aliphatic methyls -CH,- and > CHaromatic phenolic 0-H
Normalizing the two aliphatic methyls to 1.00.
phenolic-type compounds are the largest amount of material of the total extract of Larrea (Mabry et al., 1977), we can assume as a first approximation that the larger the methoxyl content of the extracted resin the larger would be the lipophilicity of the latter. Characterization of the Larrea and Its Derivatives. It has been reported that lignans and flavonoids are the two main types of phenolic compounds present in the Larrea resin. Their chemical structures have been elucidated although their weight ratio was not reported (Mabry et al., 1977). Figure 1 shows the chemical structures of the reported lignans. For our case the lipophilic and the polar fractions of Larrea resin are soluble in aqueous sodium hydroxide but insoluble in water or aqueous sodium bicarbonate. Their infrared spectra have a strong and broad band centered at 3400 cm-' (0-H stretching vibration for phenols) and a band at 3025 cm-' (aromatic C-H stretching vibration) (Dyer, 1965). The lipophilic fraction has a molecular weight (vapor pressure osmometry) of 352 f 30 and the polar fraction of 324 f 25. The lipophilic fraction contains 10% or less of wax which is insoluble in aqueous sodium hydroxide and presents an ester infrared absorption band at 1735 cm-'. This wax was identified by its NMR spectrum (Seigler et al., 1974). Tables V and VI show that there is a close analogy between the NMR spectra of NDGA and the two fractions of Larrea resin. Therefore, based on the data presented, we conclude that both fractions of Larrea resin are composed mainly of lignans although the presence of relatively small amounts of flavonoids in both fractions is not discounted. Additionally, it has been shown that lignans are the major components of the lipophilic fraction (Fernandez et al., 1978). Similar procedure for the polar fraction is in progress. Table VI1 shows the derivatives prepared from the two selected fractions of Larrea resin. The acetate ester derivative shows strong infrared bands at 1750 cm-' and at
hydrogen type 2 aliphatic methyls wax ( methylenes)c -CH,- and > CH1.2 aromatic methoxylsd aromatic phenolic ( -OH)"sf
AsNormalizing the aliphatic methyl area to 1.00. suming an average of two methyl groups/mol. Seigler et al. (1974). Essentially none for the polar fraction. d 0.6 methoxyl groups for the polar fraction. Position for methoxyl groups assigned after Dyer (1965). e Active hydrogen exchanged by deuterium (by deuterium oxide treatment). f About 4.0 for the polar fraction.
Table VII. Derivatives of the Resin of Larrea resin tv-oe
derivatives
lipophilic fraction polar fraction
formaldehyde-condensation polymers acetate -OC( = O)CH, oxyacetic acid -OCH,COOH
Table VIII. Lipophilic Fraction of Larrea Resin and Its Formaldehyde-Condensation Dimer. Main Physical and N M R Data as ComDared with NDGA lipophilic fraction dimerf NDGA molecular weight 352 r 30" -OCH,/molu 1.3 aromatic hydrogens ( A H p 6.0 3.2 active hydrogensd methylene bridgesup 0 total substitution (TS)' 4.5 10.5 AH + TS
668k 50 2.6 8.6 6.2 0.98 10.8 19.4
302 0 6 4 0 4 10
Introduced by reaction By NMR spectroscopy, The sum of the measured amounts with formaldehyde, of methoxyl groups, active hydrogens, and methylene They were exchanged by deuterium (by deubridges, terium oxide treatment). e It contains about 6% wax with a molecular weight (Mabry et al., 1977) of approximately 750. f This particular sample is identified as the dimer throughout the rest of the paper. Its m-olecular weight is g Signal centhe number average molecular weight (Mn). tered at 3.70 ppm. In comparison, methylene signal of diphenylmethane occurs at 3.92 ppm.
1200 cm-' characteristic of esters and a strong and broad band at 3400 cm-' due to the 0-H stretching vibration of phenols (Dyer, 1965). The phenoxyacetic acid derivative is soluble in an aqueous solution of sodium bicarbonate but insoluble in water as should be expected for a carboxylic acid derivative. Its infrared spectrum shows a broad band at 2600 cm-' and a strong band at 1740 cm-', both characteristic of carboxylic acids (Dyer, 1965). An additional strong and broad band at 3400 cm-' characteristic of phenols is also present in its IR spectrum. Tables VI11 and IX show the main NMR data and molecular weights for the lipophilic and polar fractions of Larrea resin and for its derivatives. All data are in agreement with the structural assignments expected for these derivatives. Antioxidant Properties of Larrea Resin and Its Derivatives. Table X shows the results (DSC) of the antioxidant properties of the lipophilic and polar fractions
224
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table IX. Polar Fraction and Its Derivatives. Main Physical and NMR Data _
_
_
_
~
~
polar fraction
oxy acetic acid deriv.
butyl ester of oxyacetic acid derivaa
3 2 4 i 25
387 f 40 1.5
4 3 2 f 50
6.0 0.6
6.0 0.6
4.0 4.6 10.6
3.5e 4.1 10.1
molecular weight -OCH,COOH/molb 0C0CH / mol aromatic hydrogens (AH)b -OCH,/molb -OCH,COO(CH ),CH,/molb active hydrogent pc total substitution (TS)d AH + TS
... ...
420
...
...
?I
35
...
...
1.7 6.1 0.6
6.0 0.6 1.0 2.5 4.1 10.1
...
*..
acetate ester
...
2.2 4.5 10.6
Parent acid had 1.0 oxyacetic acid units/mole. By NMR spectroscopy. Exchanged by deuterium (by deuterium oxide treatment). As in Table VI11 it is the sum of the measured amounts of substituent groups. e 1.5 active hydrogens belong t o the carboxyl group of the oxyacetic acid substituent; therefore there are two phenolic -OH groups remaining. Table X. Larrea Resin and Its Derivatives, Antioxidant Properties in Guayule Rubber Gum by Differential Scanning Calorimetry (DSC) ~
guayule rubber without antioxidant, "C lipophilic fraction polar fraction acetate ester oxyacetic acid derivative
guayule rubber with 1.25% antiooxidant, C
~~
diff, "C
154i 3
207
150i 3
204 e 2
54
146 f 2 159k 1
200 i 2 188 f 2
54i 3 29 3
f
2
Table XI. Comparison of the Antioxidant Properties (DSC) of the Lipophilic Fraction of Larrea Resin and Several Commercial Antioxidant8 in Guayule Rubber Gum
53 f 4 f
4
+_
of Larrea resin as well as the acetate ester and phenoxyacetic acid derivatives found in guayule rubber gums. It can be seen from Table X that the acetate-ester and phenoxyacetic acid derivatives (both synthesized from the polar fraction of Larrea resin) differ significantly in their antioxidant properties, the acetate ester being a better one. From Table IX we can see that both derivatives have almost the same molecular weight, about the same proportion per mole of substituent groups (methoxyl and acetate ester or methoxyl and oxyacetic acid), and nearly the same proportion per mole of phenolic hydroxyls. However, they must differ widely in pK, because the reported pKa for phenols is about 10 and for phenoxyacetic acids is around 3.1, both at 25 "C (Merck, 1968). Since the only significant difference between the derivatives is their respective pK,, this difference must bring about a difference in their rubber compatibilities and therefore in their antioxidant properties. This conclusion is in agreement with related work (Kotulak et al., 1970). Table XI has a comparison of the antioxidant properties (DSC) of the lipophilic fraction of Larrea resin vs. several commercial antioxidants in guayule rubber gum. The CAO-14 and BHT are nonstaining phenolic antioxidants; the AgeRite D is semi-staining and the DPPD is staining (DuPont, 1973). It is concluded from Table XI that the lipophilic fraction of Larrea resin is at an equal level with the commercial nonstaining antioxidants tested for guayule rubber as far as it can be established by DSC. Table XI1 shows the results of the staining characteristics of the lipophilic fraction of Larrea resin. The formulation used is given in Table 111. I t is concluded that the lipophilic fraction of Larrea resin is a nonstaining antioxidant for natural rubbers. A very desirable practical property of an antioxidant is that it should not change significantly the vulcanization rates of rubber formulations. Therefore, tests of Mooney scorching time were performed with the formulation
no antioxidant lipophilic fraction CAO-14 (2,2-methylenebis(4methyl-6-tert-butylphenol)) AgeRite D (polymer of 2,2,4trimethyl-l,2-dihydroquinoline) DPPD (N,N'-diphenyl-pphenylenediamine) BHT (2,6-di-tert-butyl-4methylphenol) NDGA (nordihydroguaiaretic acid)
temp, "C
diff, "C
154f 3 207 i 2 209t 1
53 f 4 55f 3
205f 1
51i3
231il
77f3
156 i 1
2 f 3
206 f 2
52
f
4
Mention of a product by us does not imply approval or recommendation of it. Table XII. Relative Degree of Discoloration of Vulcanized White Rubber (Hevea RSS1) Due to Outdoor Exposure. Spring 1976, Saltillo, Coahuila, Mexico sample control lipophilic fraction dimer acetate ester derivative toluene extract
degree of discoloration ( 2 months outdoor exposure) 4 3 4 5 5
Commercial Antioxidants BHT 2 (nonstaining) CAO-6 4 (nonstaining) CAO-14 4 (nonstaining) 4 (nonstaining) Wingstay L AgeRite Geltrol 4 (nonstaining) much larger than 5 AgeRite Resin D (semistaining) black in 8 h (standing) DPPD Table XIII. Mooney Scorching Time for Larrea Resin and Several Retarding Agents in Guayule Rubber parts per hundred scorch time, retarding agent of rubber min none lipophilic fraction lipophilic fraction salicylic acid salicylic acid maleic anhydride maleic anhydride phthalic anhydride phthalic anhydride
0 1
2 1
2 1
2 1
2
3.0 i 3.8 t 3.8 * 8.5 i 9.5 t 8.0 f 10.0 * 5.0 * 8.0 f
0.2 0.2 0.2 0.5 0.5 0.5 0.5 0.5 0.5
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table XIV. Influence of t h e Molecular Weight on the Antioxidant Properties (DSC) of Formaldehyde Condensation Polymers in Guayule Rubber Gums
sample 1iDoDhilic - fraction I-AB-178-1 I-AB-180-3 I-AB-188-1
mol wt
Guayule Guayule rubber rubber without with 1.25% antiantioxidant, oxidant, "C "C
352
30
154 i 3
207
2
53 + 4
498 i 30 668 i 50 701+ 5
150 i 2 156 i 2 159f 2
200 * 2 195 + 3 1875 2
50 * 3 39 f 4 28f 3
f
i
diff, "C
Table XV. Tensile Retention of Nautral Hevea Rubber (RSS1) with Several Antioxidants after Aging at 100 OC for 48 h (ASTM-D-537-67) % tensile retention
antioxidant
2phr'
none BHT CAO-14 lipophilic fraction condensation polymerb
43 61 69 43
1phru Ophr'
42 41 60 62 42
Antioxidant content in parts per hundred of rubber. Molecular weight of 701 f 5.
a
Table XVI. Resin of Larrea and Its Derivatives. Antifungal Activity (as Percent Growth) against Rhizoctonia solan i OXY
