Interesterification of starch with methyl palmitate

Interesterification of starch with methyl palmitate M. L. Rooney CSIRO, Division of Food Research, Food Research Laboratory, North Ryde, New South Wales, Australia (Received 19 December 1975) Thermoplastic polymers derived from natural products have been prepared by interesterifying starch with methyl palmitate. The degree of substitution (DS) of the esters has been found to be strongly dependent upon catalyst concentration and the ratio of methyl palmitate to starch, but is largely independent of temperature and starch concentration over the range studied. Replacement of methyl palmitate with methyl esters of shorter chain acids does not appear to affect the DS. These observations are interpreted from the results of studies on the effect of DS on the solubility parameter, the nature of the interchain bonding and the specific gravity of the polymers.

INTRODUCTION Most polymers isolated from natural products and subsequently modified retain the hydrophilic characteristic of the base polymer, whereas polysaccharide esters of long chain fatty acids have been shown to be more hydrophobic la. Recently several authors have proposed uses for such esters. Arnylose esters of a variety of fatty acids have some properties sought in dip coatings for foods 2, while starch esters can be used as reactive bases for polyurethane resins 3 or as an artificial skin 4. Starch esters may ultimately prove useful as low cost resins and replace some of those currently derived from petroleum. Fatty acid esters of starch are usually prepared in the laboratory using the acid chloride-pyridine method s. However, this method is of little commercial value because of the cost of acid chlorides 6. Direct esterification using the acid gives unsatifactory yields while the anhydrides are not readily available. Interesterification has been used extensively in the preparation of biodegradable detergents from sucrose and fatty acid methyl esters 6-s. Latetin et al. 9 reacted cellulose heterogeneously with methyl stearate in dimethylformamide to a maximum degree of substitution (DS) of 0.38 compared with a theoretical maximum of 3. The interesterification of starch with the methyl esters of higher fatty acids has not been reported, although it is of particular interest since starch, unlike cellulose, is soluble in such solvents as dimethylsulphoxide (DMSO). The present work was undertaken to determine the conditions under which starch could be interesterified with the methyl esters of long chain fatty acids, and to obtain data indicating the DS at which useful properties might be obtained. These studies include an examination of the effects of temperature, the concentrations of starch, methyl palmitate and potassium methoxide as well as the chain length of the fatty acid, on the DS of the resultant esters. The effects of DS on the specific gravity, solubility parameter and infra-red spectra of each of the esters were also determined. EXPERIMENTAL Materials Wheat starch (Fielders Ltd, Sydney) was defatted and dried to a moisture content of 2.1%. The methyl esters

of palmitic, lauric and octanoic acids were greater than 98% pure and had water and free fatty acid contents less than 0.05%. Methyl n-butanoate had water and free fatty acid contents of 0.38% and 0.17% respectively. Methanolic potassium methoxide solution, 1.5 M, was used as a catalyst. Dimethylsulphoxide (Ajax Chemicals, Sydney) was dried by vacuum evaporation of 15% of its volume 1°. Interesterification The method of interesterification was similar to that used with sucrose 7, although the starch and methyl ester solutions (dried to less than 0.02% water) were not mixed until the catalyst had been added to the starch solution and the methanol removed. The reaction was allowed to continue for 6 h after which the starch ester was precipitated with aqueous ethanol, extracted for 3 days both with boiling water and ethanol and then dried under vacuum at 80°C for 6 h. Analyses Water analyses were carried out using Karl Fischer reagent. Acyl contents of starch esters of palmitic and lauric acids were determined using the method of Berni et al. n for cellulose esters, except that after 48 h hydrolysis the starch formed was precipitated with methanol. Aliquots (0.5 #1) of the fatty acid salt solution were chromatographed on a Perkin-Elmer FII gas chromatograph with flame ionization detector and a column, 2 m x 3.2 mm o.d. packed with SE 30, 5% on Chromosorb W with a nitrogen carrier flow rate of 22 ml/min. The column and injection block temperatures were 238°C and 370°C respectively, the latter being necessary to pyrolyze the quaternary ammonium salts to methyl esters. At least two samples of each ester were hydrolysed for 48 h and a minimum of four aliquots of each were chromatographed. Acyl contents of octanoate and n-butanoate esters were determined by the alcoholic alkali method 12. Specific gravity The specific gravity of the polymers was measured sa by displacement of water and/or light petroleum ether (b.p. 100°-120°C). Water was used only for esters of DS greater than 0.6 while light petroleum ether was used for all esters of lower DS and for spot checking some esters of higher DS.

POLYMER, 1976, Vol 17, July 555

Interesterification of starch with methyl palmitate: M. L. Rooney Table 1 Starch palmitates prepared* at various catalyst concentrations

KOCH 3 concentration (mol/ mol starch) DSt

Precipitation § Alkoxide

0.001 0.010 0.025 0.050 0.100 0.200

---+ + +

0.03 0.18 0.60 0.86 1.10 0.98

Ester + + + +

Yields (%) 67 65 90 88 93 94

* Reaction Conditions: Temperature 100°C; starch/DMSO ratio 9.5 X 10--3; methyl palmitate/starch ratio 3.0; time 6 h t The value 0.98 is not significantly different (P = 0.05) from values of 1.10 and 0.86 § Precipitate formed (+); no precipitate formed (--) Calculated from the weight of starch in esterified form compared with the original weight of starch

Infra-red Spectra Films less than 20 ~m thick for i.r. studies were prepared by means of a laboratory heat sealer and dried over phosphorus pentoxide. I.r. spectra were recorded using a JASCO IRA-1 spectrophotometer (Tosco, Sydney). RESULTS AND DISCUSSION

Starch concentration The mole ratio of starch* to DMSO was varied in four steps from 4.7 × 10 -3 to 2.8 × 10 -2 at IO0°C, while the mole ratios of catalyst and methyl palmitate to starch were held at 0.1 and 3.0 respectively. The DS of the resulting esters was independent (P = 0.05) of the starch concentration and so a starch to DMSO ratio of 0.5 x 10 -3, an experimentally convenient value, was chosen for subsequent studies. The reaction was also attempted using a dispersion of starch in pyridineSbut no starch ester could be isolated. Catalyst concentration The effects of raising the mole ratio of potassium methoxide to starch from 0.001 to 0.200 are shown in Table 1. The DS increased significantly (P = 0.05) with catalyst concentration up to a catalyst to starch mole ratio of 0.100. Based on the present data however, the DS value at the highest catalyst to starch ratio of 0.200 was not significantly different (P = 0.05) from the DS values at ratios of 0.050 and 0.100. The reaction occurred homogeneously or partly heterogeneously depending upon the catalyst to starch ratio (Table 1). Two factors may explain both this behaviour and the effect of catalyst to starch ratio on DS. These factors are the removal of catalyst by reaction with traces of water and the insolubility of highly esterified starch. The initial water content in each preparation was less than or equal to 0.02%, which is equivalent to one hundred times the catalyst concentration when the catalyst to starch ratio was 0.001. The very low DS value (0.03) obtained under these conditions was probably a result of catalyst destruction by water. The same probably applied to a decreasing degree as the catalyst to starch ratio was increased to 0.10, at which point the maximum possible water content equalled the catalyst concentration. * In t h i s r e p o r t a n h y d r o g l u c o s e , t h e s t a r c h r e p e a t u n i t , w a s u s e d as t h e b a s i s f o r c a l c u l a t i o n o f t h e n u m b e r o f m o l e s o f s t a r c h .

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The precipitation of starch ester gel at catalyst to starch ratios of 0.025 or greater may be understood by reference to Figure la which shows the effect of DS on the Hildebrand solubility parameter (8) obtained from measured specific gravities 14. Esters of solubility parameter less than about 11 should be insoluble in DMSO Is (solubility parameter 12.93) since a difference between the solubility parameters of solute and solvent of less than 1.7 to 2 is a critical requirement for solubility 16. This difference rule is probably modified by the specific reaction between DMSO 17 and carbohydrates and hydrogen bonding within the polymer 16. The lowest DS ester which was precipitated (Table 1) had a solubility parameter of 10 which is 2.93 less than that of the solvent. The fact that the maximum DS obtained was 1.10 instead of 3.0 leads to the suggestion that as the DS increased the solubility parameter decreased, and in the presence of DMSO, the gel became less accessible to dissolved catalyst and methyl palmitate. The same two factors which affected the DS also affected the yield. Removal of catalyst from solution, by means of formation of starch alkoxide immobilized on gel molecules, prevented further reaction. In this way dissolved starch did not come into contact with the catalyst and the reaction stopped. A second catalyst addition after gel precipitation might have resulted in higher yields. At low catalyst concentrations the catalyst removal by water would also have reduced yields. Potassium carbonate was also used as a catalyst but gave results similar to those obtained using the methoxide.

Reagent ratio The reverse reaction of methanot with starch palmitate was inhibited by distillation of the methanol formed and by increases in the ratio of methyl palmitate to starch as

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Interesterification o f starch with m e t h y l palmitate: M. L. Rooney Table 2

Starch palmitates prepared* at various reagent ratios

Methyl palmitate/ starch (mol/mol) 1 3

Yield~

DS

(%)

0.59 1.10

75 93

82

5

1.48

10

1.52

71

15

1.54

67

* Reaction Condit=ons: Temperature 100°C;catalyst/starch ratio 0.10; starch/DMSO ratio 9.5 X 10--3; time 6 h Calculated from the weight of starch in esterified form compared with the original weight of starch

Table 3 Degree of substitution of starch palmitates prepared at various temperatures Tern pe ratu re (°C)

OS

80

0.71

90

0.90

1O0 110

1.08 0.86

shown in Table 2. The DS increased significantly (P = 0.01) as the reagent ratio was increased to a value of 10. Based on the present data, the DS value at the highest reagent ratio of 15 was not significantly different (P = 0.01) from the DS values at ratios of 5 and 10. This form of relationship is similar to those reported for cellulose stearate 9 and sucrose palmitate s formation.

Temperature The effect of increasing the reaction temperature, from 80° to 110°C, on the DS of starch palmitate is shown in Table 3. This temperature ranged from the boiling point of DMSO at the vacuum used to the temperature above which starch degradation may become serious. All DS values were significantly different (P = 0.05) except those for 90 ° and 110°C. The differences between DS values for 100 ° and 110°C on the one hand and 80 ° and 100°C on the other are significant at P = 0.01. This very small effect of temperature on DS largely supports the findings of Latetin et al. 9 for cellulose esterification. Length o f the fatty acid The methyl esters of lauric, octanoic and butanoic acids were reacted with starch to establish whether or not the length of the fatty acid determined the maximum DS attainable at 100°C. The mole ratios of catalyst to starch, methyl palmitate to starch and starch to DMSO were 0.10, 9.5 x 10 -3 and 5.0 respectively. Methanol was removed from the methyl butanoate reaction by means of a stream of nitrogen since the low boiling point of the methyl butanoate precluded use of refluxing under vacuum. The DS values of the esters of octanoic, lauric and palmitic acids formed under these conditions were 1.53, 1.47 and 1.47 respectively. Such small differences could be attributed to similar relationships between solubility and DS for each of the fatty acid esters. The highest DS obtained for starch butanoate was 0.38 and this is attributed to the inadequate removal of methanol by the nitrogen stream. This apparent independence of DS on length of the acid, at least for acids from C8 to C16, is in agreement with the

results of Gros and Feuge 2 for esterification using acid chlorides.

Infra-red spectra The effect of DS on the hydrogen bonding in starch esters was studied by measurement of i.r. spectra. Esters of DS 0.6 or greater gave quite distinct spectra with many resolved bands, while the spectrum of an ester of DS 0.19 was quite similar to that of starch except for the carbonyl absorption. At DS values less than 0.19, polymer films were not prepared due to the failure of the polymers to melt. The principal features of these spectra are the profile and wavelength of maximum absorption of the hydroxyl fundamental around 3400 cm -1, as shown in Figure 2 for esters of DS 0.19, 0.64 and 1.54. This band is markedly influenced by DS, being (a) totally unresolved from that of the CH stretch at a DS of 0.19; (b) an asymmetrical band of maximum absorbance centering at 3340 cm -1 at a DS of 0.64 and (c) a more symmetrical band with maximum absorbance at 3460 cm - t at a DS of 1.54. The result of partial acetylation of an ester of DS 1.52 is shown in Figure 2. The hydroxyl absorption in the solution spectrum of an ester of DS 1.54 (0.02 M in carbon tetrachloride) was a composite of bands at 3460 cm -1 and 3550 cm - ] . The latter band disappeared as the concentration was increased, until a saturated solution gave a spectrum similar to that of the solid polymer. The spectral results may be interpreted as follows. As the DS increases from 0.19 to 0.54 the predominant form of hydrogen bonding changes from polymeric to dimeric ~s. Acetylation provides a free environment for some hydroxyls and allows mainly dimer formation for the rest. Solution in

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v (crn-') Absorption frequency of hydroxyl fundamental: A, DS = 0.19; B, DS = 0.64; C, DS = 1.54; D, DS = 1.52 then acetylated

Figure 2

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Interesterification of starch with methyl palmitate: M. L. Rooney carbon tetrachloride causes disruption of interchain hydrogen bonds leading to an increased proportion of dimers on dilution. Polymeric hydrogen bonding would probably involve hydroxyls on several chains or segments whereas dimers can be formed between hydroxyls at C2 and C3 on adjacent anhydroglucose units. The frequencies of maximum absorbance (v) of the hydroxyl fundamental in the spectra of all of the esters prepared have been fitted to the equation: v= 3238 + 135 xDS with a standard error of estimate of 14.6 and a correlation coefficient of 0.94. The scatter of the results is attributed to the flatness of the absorption band as would be expected when several closely spaced bands overlap. Thus the frequency of maximum absorbance of this band provides a measure of the DS of the ester. There is also a sharp band near 720 cm -1 which is probably due to the methylene rocking vibration of palmitate chains. The intensity of this band increases markedly with increasing D S and may also offer some scope for quantitative analysis of these and related esters. Starch octanoate o f D S 1.53 and the laurate o f D S 1.47 both had their hydroxyl fundamental maximum at 3405 cm -1, which is more than two standard errors lower than the frequencies for the corresponding palmitates. This would be expected with shorter chain fatty acid esters which would interfere less with hydrogen bonding.

may also have limiting specific gravity values independent o f DS over a range similar to that for the palmitates. Since the specific gravity of the palmitate esters appears to be fairly constant over the D S range from 1 to 2 approximately, it is possible that some of the valuable properties reported 2 for the high D S esters might be found in esters of much lower DS also. This suggestion is supported by the nature of the solubility parameter versus D S curve in Figure la. ACKNOWLEDGEMENTS The author wishes to thank Mr E. G. Davis and Dr R. V. Holland for their helpful advice; Mr D. J. Best for carrying out statistical analyses and Mr A. J. Shorter for his valuable technical assistance. REFERENCES 1 2 3 4 5 6 7

Specific gravity F~gure l b shows the effect o f D S on the specific gravity (SG) at 23°C of starch palmitates. The broken curve was calculated using the linear combination of specific gravities of starch and palmitic acid. The disruption of optimal packing of molecules in both of the free components is reflected in the lower specific gravity values of the esters over the range studied. This is consistent with the i.r. results which indicate disruption of interchain hydrogen bonding. The apparent approach of the experimental results to a limiting SG of around 1.01 is supported by the reported value 2 of 1.022 (at 30°C) for amylose palmitate of DS 2.02. The octanoate o f DS 1.53 has a specific gravity of 1.115 compared with 1.119 reported 2 for amylose octanoate of DS 2.25. Similarly the starch laurate o f D S 1.47 has a specific gravity of 1.056 compared with 1.059 reported 2 for amylose laurate o f D S 2.49. It appears that these esters

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POLYMER, 1976, Vol 17, July

8 9 10 11 12 13 14 15 16 17 18

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