Indian Journal of Chemical Technology Vol.8, September 2001, pp. 378-384

Synthesis, characterisation and evaluation of heated rubber seed oil and rubber seed oil-modified alkyd resins as binders in surface coatings

A I Aigbodion,•, C K S Pillaib, I 0 Bakare" & L E Yahayac

"End- Use Division. Rubber Research Institute of Nigeria, PMB I 049 Benin City, Nigeria

bPo lymer Section, Regional Research Laboratory,Thiruvanathapuram 695 019, India

<Department of Chemistry, University of Benin, Benin City, Nigeria

Received /8 September 2000; revised 29 May 2001; accepted 15 June 200/

Rubber seed o il (RSO) heated at 300 ± 5 °C for six hours and RSO-modified alkyd resins were evaluated as binders for surface coatings. During heating of rubber seed oil , the bulk viscosity varied exponentially wi th time. GPC analysis showed that RSO consists of a very high molecular weight fraction that is uncommon in vegetable oil in addition to free fatty acid,

mono- di-, tri-g lycerides and o ligomeric try g lycerides. Values o f M" and M w of 7393 and 13076 respectively were found for RSO. Heated rubber seed oil (HRSO) gave ., "of 475 and Ai w of 599: while RSO-modified alkyd samples of oi l contents

of 40% (I), 50% (11) and 60% (111) gave M" of 3234, 1379 and 3304 respectively; and M w of 6186, 2 14 7 and 8406 respectively. HRSO is narrowest in size distribution compared to RSO and all the alkyd samples as indicated by their polydispersity indices. RSO was found to be semi-<lrying as the film remained tacky after long period (about 48h) o f exposure. Heating enhanced its drying abi lity. The HRSO films exhibited reasonable pencil hardness and excellent resistance to ac id and salt solution but poor resistance to alkali solution compared to the alkyd samples. The drying ability and chemical resistance of HRSO and RSO-modified alkyd were greatly enhanced by modification with cashewnut shell liquid-formaldehyde resin ( I 0% wl w).

Exhaustible petroleum reserve and fluctuation in the

price of petroleum products have rekindled interest in

the search for, and development of alternative sources of raw material for domestic and industrial uses. The

use of biomass as renewable resource material with

particular reference to vegetable o ils in this regard, seems to be a viable alternative t·4 .

Rubber seed oil (RSO) has shown strong potential

in the manufacture of alkyd resin for use as binders in

surface coatings5-7; polymer processing8-12; and diesel

fuel substitute and/or extender13-15• Recently, the use

of RSO and its epoxidised derivative as processing aid for natural rubber has been reported 16• In

continued effort ai med at developing RSO as

industrial raw material, a study on the effect of heat

treatment on the characteristics of RSO, particularly,

its drying ability , as compared to the incorporation of

the oi l into polyester to y ield alkyd, with a view to

optimising the economic use of the oil in surface

coatings has been carried out. In the early stages of the surface coating industry,

*For Correspondence (E-mail: public @ fide lsemper.com;

Fax: 0023452 602362/250668)

vegetable oils used in surface coatings consisted

entirely of those oils with pronounced drying abil ity

like linseed 17• Then, the oils were seldom used in their natural form as they could not fulfil the standard

requirement for film properties like resistance to

chemicals and abrasion. Later, heat treatment of the

oil and their incorporation into polyester system to produce alkyds were evolved 18·19• Thus, semi-drying

oil such as soyb~an oil and even non-drying oils like

coconut oil are now frequently employed as binder in

surface coatings and other applications like printing

ink, after such treatment. Currently, heated vegetable o ils are being favoured for use as printing ink vehicle compared to petroleum- based vehicle20-22 . Despite the

suitability of RSO in the production of alkyd resins,

there has not been any report on heat treatment of

RSO and its possible application as binder in surface coatings. Since RSO is semi-drying, (i.e. a thin layer

of the o il is incapable of drying into a hard and durable fi lm no matter how long it is exposed), any

physical and chemi cal treatment that will enhance its

quality in th is respect will be of economic benefit. Thus, it is hoped that the findings from th is study will

expand the frontiers of application of RSO.

AIGBODION eta/. : RSO AND RSO MODIFIED ALKYD RESINS 379

Table 1-Recipe for the preparation of the alkyd samples.

Ingredients (moles)

Rubber seed oil Phthalic anhydride Glycerol Alkyd constant (k)

0.456 0.778 0.657 1.02

Table 2-Physico-chemical properties of rubber seed oil (RSO) and heated rubber seed oil (HRSO)

Properties RSO HRSO Colour

Specific gravity (0 C) Acid value (mg KOH/g) Free fatty acid (% as oleic acid) Saponification value (mg KOH/g) Iodine value (g 12/1 OOg) Bulk viscosity (Poise)

Experimental Procedure

Rubber seed oil (RSO)

dark

0.910 53.09 26.00

206.20 135.36 0.42

dark

0.946 4.16 2.09

181.41 21.34 152.00

RSO used in this study was obtained from M/s Kathivel and Bros., Virudhanagar, Tamil Nadu, India. RSO was analysed according to IUPAC standard method.23

RSO was heated in air at 300 ± 5 °C with constant stirring for six hours20. Samples were withdrawn periodically for bulk viscosity determination using Brookfield viscometer (Spindle No. 21 at 50rpm and at 30 oc ).

Preparation of the alkyds RSO-modified alkyds of 40% (1), 50% (II) and 60%

(III) oi l contents were prepared by alcoholysis method described elsewhere using the recipe in Table 124.

Infrared spectroscopy (/R) IR spectra of RSO and heated rubber seed oil

(HRSO) were obtained using Perkin-Elmer Spectrophotometer Model 882.

Gas Liquid Chromatography (GLC) The fatty acid profile of RSO was determined by

GLC (Hewlett Packard (HP) Series II , Model 5890 ) using its methyl ester derivatives prepared by AOCS method Ce 2-66.

Gel Permeation Chromatography (GPC) Molecular weight averages of RSO, HRSO and the

al kyd samples were estimated by GPC (Hewlett Packard I 081 B HPLC) with refractive index detector. Three GPC columns packed with J1 Styragel HT3

Alkyd samples

II III

0.575 0.702 0.652 0.531 0.307 0.328 1.02 1.02

15 0 I

0

l I I

100 ! I I i

il I

"' IJ VI

I ·c; .:; >-·~ ~50

I ,; .!!) ;/ >

I £!:: // :> CD /p

~ o/

?--" - 0 _...---

0 l 3 4 5 6 7 Time (h)

Fig. 1-Plots of bulk viscosity, IJ, versus time for heated rubber seed oil (at 300 ± 5 °C)Solid line = experimental data; Broken line = from equation of fit.

columns (dim. 300 mm x 75 mm) having porosity 100, 500 and 1,000 A (Waters Associates.) were arranged in series. Tetrahydrofuran of HPLC grade was used as the mobile phase.

Evaluation of the quality of HRSO and the alkyd samples

Performance characteristics of HRSO and the alkyds were evaluated by measurements of drying schedule (ASTM 01 640-69), viscosity (ASTM 0 1725-62), pencil hardness (ASTM 0 3363-75) and resistance of their films to acid, alkali , salt and water (ASTM 0 1308-57).

Results and Discussion

Analysis of RSO Physico-<:hemical properties of RSO and HRSO

are compared in Table 2. The colour of both products is dark while specific gravity of HRSO is slightly

380 INDIAN J. CHEM.TECHNOL., SEPTEMBER 2001

Fatty acid Saturated C16:o

Table 3-Fatty acid composition of rubber seed oil

(%)

CJS:O

Unsaturated CIS: I

C1s:2 C, s:3

Others

Palmitic acid Stearic acid

Oleic acid Linoleic acid Linolenic acid

higher than that of RSO. Heating resulted in a decrease in the level of acidity and saponification values of RSO. The level of unsaturation of HRSO (iodine value- 21) is also lower.

The result of GLC analysis of RSO is given m Table 3. It shows that RSO consists essentially of 18.9% saturated fatty acids comprising mainly of palmitic (10.2%) and stearic (8.7%); and 80.5% unsaturated fatty acid comprising mainly oleic (24.6%), linoleic (39.6%) and linolenic (16.3%). RSO is lower in linolenic acid (triene) content than linseed oi l (52%) (ASTM 02245 - 72). This implies that a thin layer of RSO is incapable of drying into a hard and durable film by the process of autoxidation on exposure. However, its drying ability may be enhanced by physical or chemical treatment such as heating and modification with phenolic resin20"25

Heat treatment of RSO Fig. I shows that during the heating of RSO change

in viscosity is negligible at the initial stages of heating followed by a sharp rise in viscosity after about 5h of heating. A similar trend in variation of viscosity has been reported during the heating of safflower and rapeseed oil under similar conditions26. The initial flat portion of this plot is referred to as the induction period due to the presence of antioxidant precursors (tocopherols) in vegetable oil, which inhibit thermal degradation of oil27 . It has been proposed that during the early stage of heating of vegetable oils, the oil molecule undergoes different kinds of reactions such as the isomerisation of unconjugated systems into conjugated systems and decomposition into lower molecular weight products like hydroxyl, carboxyl, carbonyl compounds, and esters which have a greater

. 28 Th" tendency to recombme to form new products . IS

initial induction period is immediately followed by rapid interaction of the decomposition products to form new ones including cyclic products with

"' u c _g ... 0 VI

. 0 q:

10.2 8.7

Total 18.9

24.6 39.6 16.3

Total 80.5 0.6

I~

b

__. _ I 1 1 I I J,__J__.__._I__~\ _.___.__.___ ___ . _

3600 . 2800 ' 1700 1200 700 Wavenumber ( cm -1)

Fig. 2-IR spectra of (a) rubber seed oil and (b) heated rubber seed oil (at 300 °C for 6h)

propensity to increase the bulk viscosit/ 9• Thus, the sharp increase in viscosity after 5h of heating, (Fig. I )

indicated by upward curvature of this plot could be attributed to the presence of products containing polar groups and possibly commencement of recombination of decomposition products leading to formati on of new molecules. IR spectrum of HRSO (Fig. 2b) show strong hydroxyl absorption band at 3482 em·' in addition to the carbonyl absorption band at around 1750cm·'.

For proper understanding of the variation of viscosity during the heating of RSO, the bulk viscosity data obtained in this study were fitted into an exponential model equation thus:

... (I )

where 11 is bu lk viscosity, tis time of heating, a and b arc constants determined by data fi tting. Using the

AIGBODION et al. : RSO AND RSO MODIFIED ALKYD RESINS 381

method of the least squares, the values of a and b were obtained as 0 .11 poise and 1.13 h-1 respectively with correlation coefficient of 0.986 indicating good correlation at 0.1% confidence level. Therefore, the expression for the fit of data is given as:

'7=0.lleu3t (2)

This high correlation coefficient indicates that during heating of RSO viscosity does not increase linearly with time of heating. In other words, there is optimum time of heating for obtaining a product (HRSO) of desired quality. The curve for the data fitting corresponding to the model equation is shown as a dotted line in Fig.1.

d

(1) { 21 lfl c 0 0.. Ill (1) L

.... 0 -u (1) a -<ll 0 c

d

( 1 ' . . . . l

8 10 12 14 16 18 Retention time (min)

Fig. 3 - GPC chromatograms of ( I) rubber seed oi l and (2) heated rubber seed oil (at 300 °C for 6h).

Molecular weight characterisation of RSO, HRSO and the alkyd samples

GPC chromatograms of RSO and HRSO are shown in Fig. 3. Both chromatograms consist of four peaks each with the fourth peak (d) more prominent in the chromatogram of HRSO. It can be seen in Fig. 3 that as peaks a and b decreased peaks c and d increased upon heating of RSO. Comparison of Fig. 3( 1) with chromatograms of a series of vegetable oils investigated by Husain et aP0 and White & Wang31

reveals that the presence of the fourth peak (d) in the chromatogram of RSO is uncommon. Rationalisation of these peaks shows that peak a represents low molecular weight fraction in RSO consisting a mixture of free fatty acids and monoglycerides; peak b corresponds to diglycerides; and peak c, a mixture of triglycerides and oligomeric glycerides. Peak d corresponding to a fraction with rather high average molecular weight (>38801) is suspected to originate from polyisoprene molecules of latex as the tocotrienol distribution in rubber latex has been analysed to be the same as that found in RS032. Also, small amount of polyisoprene (<1 % wlw) has been reportedly been isolated from the sludge obtained from the degumming of RS033.

The relative peak percent and number average molecular weights of the different fractions are presented in Table 4 . The percent free fatty acid (-20) of RSO obtained by GPC is in reasonable agreement with the value of 26 obtained from chemical analysis (Table 2). Similarly, the equivalent weight (- 272) of RSO estimated from the saponification value (Equivalent weight = 5611 0/saponification value)34 also compares favourably well with the value of 249 from GPC.

The number average molecular weights (Mn) and weight average molecular weight ( M w) and polydispersity (M wiM n) determined for RSO, HRSO

and the alkyds are shown in Table 5. The value of M n

for RSO and HRSO are 7393 and 475 respectively ; while - w values are 13076 and 599 for RSO and HRSO respectively. This result shows that the average molecular weight of RSO is larger than that

Table 4 - Peak per cent and average molecular weights of the different fractions of rubber seed oil obtained from GPC.

Peak Ave. mol. Wt. (%)

a 249 20 b 1460 25 c 9066 -45 d >3880 1 < I

382 INDIAN J. CHEM.TECHNOL., SEPTEMBER 2001

Table 5-Number average (M 0 ) and weight average molecular weights (M w) of rubber

seed oil (RSO), heated rubber seed oil (HRSO) and the alkyd samples.

Mol. Wt. Averages Polydispersity

Mn Mw (M wi"M .) RSO 7393 13076 1.77 HRSO 475 599 1.26 Alkyd samples I 3234 6186 1.91 II 1379 2147 1.56 III 3304 8406 2.54

Table 6-Properties of heated rubber seed oil (HRSO) as binders for surface coatings.

Properties HRSO

Specific gravity (at 30 °C) 0.940 Viscosity (Poise)

4.89 60% solution (w/v) Solid content(%) 60.36 Drying time

150 Surface dry (min) Dry through (hr) 24 Pencil hardness Scratch F Gouge 2H

of HRSO. The polydisperity indices are respectively 1.77 and 1.26 for RSO and HRSO indicating that HRSO is narrower in size distribution than RSO and therefore, more homogeneous.

On the other hand, values of M n for the alkyds are 3234, 1379 and 3304 for samples I, II and Ill

respectively; and values of Mw are 6186, 2147 and 8406 respectively. Their molecular weight distribution (MWD) as indicated by the polydispersity indices show that sample II with a value of 1.56 is most homogeneous and sample III with a value of 2.54 is the least homogeneous.

Properties of HRSO and the alkyds Properties of HRSO and the alkyd samples as

related to surface coatings are given in Table 6. The specific gravity ranges from 0 .940 for HRSO to 0 .966 for sample Ill. Viscosity at 3:2 dilution in a hydrocarbon solvent for HRSO is 4.89 poise; and 3.11, 7.46 and 2.54 poise for samples I, II and Ill respectively. Their drying ability which is critical to their use as binder shows that while a thin layer of RSO remained tacky after long period of exposure (about 48 h), thin layer of HRSO dried to a hard solid film. Thus, RSO can be classified as a semi-drying oi l and that heating improves its drying ability. Alkyd sample II exhibited the fastest rate of drying. The pencil hardness of the films of HRSO and alkyd

Alkyd samples

II III

0.956 0.944 0.966

3.11 9.46 2.54

69.21 74.03 64.76

150 120 210

24 12 24

H HB 28 3H F HB

samples I- III are for scratch and gouge: F/2B, H/3H, HB/F and 2B/HB respectively.

Table 7 shows the resistance of HRSO and alkyd films in different service media. HRSO and the alkyd films are unaffected by acid (10% H2S04) and salt ( 10% NaCI) solutions while they are slightly affected by water with HRSO film more resistant to water than the alkyds. All the coatings exhibited poor resistance to alkali (10% KOH) solution. This poor alkali resistance may be explained on the basis that these coatings are composed essentially of ester linkages, which are susceptible to hydrolysis, by alkali . From these results, it can be deduced that HRSO and the alkyds could be used as binders in coatings where alkali resistance is not the main requirement.

Effect of blending RSO and its alkyds with cashewnut shell liquid-formaldehyde resin

To further establish the potential of HRSO and RSO-modified alkyds as binder, they were blended with cashewnut shell liquid-formaldehyde (CNSL-F) resin by heating at 230 °C for one hour. The products obtained were diluted with xylene (3:2) followed by the addition of metallic driers and they were examined for their drying schedule (ASTM D 1640-69) and chemical resistance (ASTM D 1308-57 ). Cashewnut shell liquid is a renewable resource material, which has been identified as a versatile raw

AIGBODION et al. : RSO AND RSO MODIFIED ALKYD RESINS 383

Table 7-Chemical resistance of the alkyds and HRSO films

Type of Coating Medium Distilled Alkali Acid Salt

water (N/10 KOH) (N/10 H2S04) (5% w/v NaCI) Alkyd samples I 4 6 II 5 3 Ill 5 6 HRSO 2 6

No effect= 1, Whitening= 2, Slight shrinkage= 3, Little blisters= 4, Film softened= 5, and Film removed= 6

Table 8-Effect of modification with Cashew nut sheilliquid-formaldehyde (CSNL-F) resin on the drying ability of the alkyds, HRSOand RSO.

Type of Coating Drying Time

Set to Touch Surface Dry Dry through (min) (min) (h)

Alkyd Samples I 25 80 >5 II 20 60 <4 lii 35 85 >5 HRSO 40 85 4 RSO >60 120 >5

Table 9- Effect of modification with Cashew nut shell liquid-formaldehyde (CSNL-F) resin on the chemical resistance of the alkyds, HRSO and RSO films.

Type of Coating

Alkyd samples I II Ill HRSO RSO

Distilled water

I 5

No effect =1, Whitening 2, Slight shrinkage= 3,

Alkali (N/1 0 KOH)

I I 6 6 6

Medium

Acid (Nil 0 H2S04)

Salt (5% w/v NaCI)

Little blisters= 4, Film softened= 5, and Film removed= 6

material for a number applications including surface coatings35. The results obtained are presented in Tables 8 & 9. It is obvious from Table 8 that modification with CNSL-F resin significantly improves the drying ability of HRSO, the alkyds and even that of the crude RSO. Similarly, modification with CNSL-F resin increases the chemical resistance of HRSO and the alkyds films (Table 9). The poor resistance of alkyd sample III after modification with CNSL-F resin compared to others may be due to its high oil content (60%). Although modification with CNSL-F resin improved the drying ability of RSO, its resistance to alkali and water is poor.

Conclusion It can be deduced from the results that heating

enhances the drying ability of RSO, which is naturally

a semi-drying oil. The films of HRSO exhibited less pencil hardness than the films of the alkyd samples. HRSO and the alkyd films also exhibited excellent resistance to acid and salt solutions. Modification with CNSL-F resin improves the drying ability and chemical resistance of HRSO and the alkyds.

Acknowledgement Thanks are due to Dr. G. V. Nair, Director,

Regional Research Laboratory, Thiruvananthapuram, India for providing the laboratory facilities for this work. The financial support extended to Dr. A. I. Aigbodion by Council of Scientific and Industrial Research (CSIR), Government of India and Third World Academy of Sciences (TWAS), Italy is gratefully acknowledged.

384 INDIAN J. CHEM.TECHNOL., SEPTEMBER 2001

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