An investigation of the utilisation of African locustbean
seed oil in the preparation of alkyd resins
A.I. Aigbodion
a,*, F.E. Okieimen
baRubber Research Institute of Nigeria,P.M.B.1049,Benin City,Nigeria bDepartment of Chemistry,Uni
6ersity of Benin,Benin City,Nigeria
Received 29 September 1999; received in revised form 13 March 2000; accepted 31 March 2000
Abstract
Preparation of alkyd resins using phthalic anhydride and glycerol modified with African locustbean seed oil was investigated. The oil was solvent extracted with petroleum spirit (60 – 80°C). The oil content of the seed ranged from 15 to 20%. Three grades of alkyds formulated to contain 27% (I), 50% (II) and 60% (III) oil were prepared using a recipe containing various amounts of phthalic anhydride, glycerol and African locustbean seed oil employing the alcoholysis method. The progress of reaction was monitored by determining the acid value of the aliquot of the reaction mixture at regular time intervals. The acid value decreased as the reaction progressed. The rate of decrease was higher at the initial stages of reaction and depended on the amount of reactants. Kinetic study showed that the initial stages of the reaction followed a second-order rate law, after which deviation was observed. The extent of reaction as determined at this region ranged from 57.9 to 63.5%, indicating an appreciable degree of conversion. The average degree of polymerisation at this point ranged between 2 and 3, and this suggests formation of low molecular weight species and branching at short intervals along the alkyd chains. The second-order rate constants for the linear
portion were 6.05×10−5 , 12.51×10−5 and 5.98×10−5 g (mg KOH)−1 min−1 for samples I, II and III,
respectively. The oil is essentially a semi-drying oil. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:African locustbean seed oil; Alkyd resins; Acid value; Iodine value; Extent of reaction; Average degree of polymerisation www.elsevier.com/locate/indcrop
1. Introduction
Alkyd resins are products derived from the polycondensation of a polybasic acid and a poly-hydric alcohol modified with monobasic fatty acid or its triglyceride oil (Kienle, 1949). They are
essential raw materials for the manufacture of different types of surface coatings, where they act as a binder. The type of oil used as the modifier is responsible to a large extent for the properties of the alkyd resin.
The demand for alkyd resin for use in the Nigerian surface coating industry has increased tremendously in recent times (Akinnawo, 1989). Consequently, there is increased demand for dry-ing oils required for the production of alkyd
* Corresponding author. Fax: +234-52-602362.
E-mail address: [email protected] (A.I. Aigbod-ion).
resins. Vegetable oils used as modifiers of alkyd resin for the purpose of imparting certain proper-ties, such as ability to air-dry, film hardness and durability, gloss and gloss retention, etc., include linseed oil, soybean oil, castor oil and tall oil (Killeffer, 1938). These drying oils, which are regarded as standards in the surface coating indus-try, are still largely imported.
As a result of the decline in foreign exchange earnings in recent times, importation of these oils could no longer be sustained. Therefore, there is the urgent need to investigate indigenous sources of drying oils for alkyd production. Preliminary inves-tigations revealed potential sources that could meet the country’s requirement of vegetable oil for alkyd production when properly developed (Adefarati, 1986; Nwankwo et al., 1986). The present study was conducted to determine the quality of African locustbean (Parkia filicoidea) Welw seed oil with respect to its utilisation as alkyd modifier, to investigate the chemistry of the preparation and to evaluate the quality of the alkyds made from it (particularly the drying schedule). This study is encouraged by the wide distribution of African locustbean plants throughout Nigeria.
2. Experimental
2.1. Materials
Laboratory-grade phthalic anhydride and glyc-erol from British Drug House (Poole, UK) were used in the preparation of the alkyds. The phthalic anhydride had an anhydride content of 99.00% and the purity of glycerol was 98.50%. African locust-bean seeds obtained from Uroe, Edo State, were
further dried and milled. The oil was solvent extracted by Soxhlet extraction using petroleum spirit (60 – 80°C) (Furniss et al., 1980).
2.2. Analysis of the oil
Properties of the African locustbean seed oil were determined using standard methods (British Stan-dard Institutions, 1976b).
2.3. Preparation of the alkyd resins
Three grades of alkyds formulated to have oil content of 27% (I), 50% (II) and 60% (III) were prepared with phthalic anhydride, glycerol and African locustbean seed oil (according to the recipe presented in Table 1) using the alcoholysis method (Oil & Colour Chemist’s Association of Australia, 1983). The reaction was carried out in a 2-l round-bottom flask fitted with a motorised stirrer, nitro-gen gas inlet tube, thermometer, and Dean and Stark apparatus carrying a water-cooled condenser (Kienle et al., 1939; Aigbodion, 1995). Xylene (15% v/w of total ingredients charged) was employed as the azeotropic solvent, and the reaction tempera-ture ranged from 230 to 250°C. Sampling was carried out at time intervals to determine the acid value of the reaction mixture. The drying schedule of the alkyd samples was determined according to ASTM D 1640-69.
2.3.1. Calculation of extent of reaction and a6erage degree of polymerisation
The extent of reaction,P, with respect to the acid value was calculated using the equation (Bobalek and Chiang, 1964; Bobalek et al., 1964):
P=(C0−Ct)/C0 (1)
whereC0andCtare the acid value at zero reaction
time and after time, t, respectively, while the average degree of polymerisation, DP, is given as:
DP=(1−P)−1 (2)
3. Results and discussion
Table 2 shows the properties of African locust-bean seed oil compared with those of linseed oil
Table 1
Recipe for the preparation of the alkyd samples
Alkyd sample Ingredients
I II III
Oil content (%) 27.00 50.00 60.00 25.00 39.00 28.00
Phthalic anhydride (%)
Table 2
Properties of African locustbean seed oil compared with those of linseed oil (British Standard Institutions, 1976a) and rubber seed oil (Aigbodion, 1994)
Linseed oil
Property African locustbean seed oil Rubber seed oil
0.931–0.936
Specific gravity at 28°C 0.970 0.921
Acid value (mg KOH g−1) 4.37 1.0 24.50
–
2.00 11.29
Free fatty acid (% as oleic acid)
175
Saponification value (mg KOH g−1) 176 192
188
104 155
Iodine value (Wijs) (g I2per 100 g)
(British Standard Institutions, 1976a) commonly employed in alkyd production, and rubber seed oil, an indigenous drying oil that has been found suitable in alkyd production (Aigbodion, 1994, 1995). The oil content is 15 – 20% by weight of the seed. The specific gravity of African locustbean seed oil is slightly higher than that of linseed oil or rubber seed oil. Its level of acidity (acid value 4)
is also slightly higher than that of linseed oil (acid value 1), but much lower than the acid value of
24 found for rubber seed oil. The saponification value of African locustbean seed oil is similar to that of linseed oil (Table 2). A property of vegetable oil that is perhaps the most crucial to its application in surface coating is the level of unsaturation as indicated by its iodine value. The iodine value of 104 for African locustbean seed oil
is lower than that of linseed and rubber seed oil. This implies that a thin layer of the locustbean seed oil is not capable of drying into a hard film by the process of autoxidation on exposure.
The large difference in the iodine values of the oils in Table 2 can better be appreciated by considering their component fatty acids. The component fatty acids of African locustbean seed oil (Oyenuga, 1968), linseed oil (British Standard Institutions, 1976a) and rubber seed oil (Aigbodion, 1994) are shown in Table 3. African locustbean seed oil is composed of: 46% saturated fatty acid, made up essentially of palmitic, stearic, arachidic and behenic acids; and about 54% unsaturated fatty acids, comprising mainly palmitoleic, oleic and linoleic acids. Linolenic acid containing three double bonds is absent. However,
Table 3
Component fatty acids of African locustbean seed oil compared with those of linseed oil (British Standard Institutions, 1976a) and rubber seed oil (Aigbodion, 1994)
African locustbean seed oil (%)
Fatty acid Linseed oil (%) Rubber seed oil (%)
Saturated fatty acid
–
Myristic acid C14:0 – 2.20
31.00
Palmitic acid C16:0 – 7.60
7.70
Stearic acid C18:0 – 10.70
4.20
Arachidic acid C20:0 – –
Behenic acid C22:0 3.10 – –
46.00 6–15
Total 20.50
Unsaturated fatty acid
2.70
Palmitoleic acid C16:1 – –
Oleic acid C18:1 8.80 13–19 20.00
42.50
Linoleic acid C18:2 15–31 36.00
Linolenic acid C18:3 – 44–54 23.50
79.50 54.00
linseed oil and rubber seed oil contain 44 – 54 and 23.50% linolenic acid, respectively. For the inves-tigated oil, the relatively lower degree of unsatura-tion is probably caused by the lack of linolenic acid (C18:3) in the fatty composition of the oil.
Since linolenic acid is highly prone to formation of chromophoric groups through conjugation (Payne, 1954), it is very likely that films that are produced from this oil will be less susceptible to development of off-colour (a degradation phe-nomenon commonly referred to as yellowing) when used in surface coatings.
Changes in the acid value with an increasing time of reaction for the different alkyd samples are shown in Fig. 1. These plots show that as the reaction progressed, the acid value decreased. The decrease in acid value is more rapid during the early stages of the reaction than the later stages of reaction. This pattern of changes in acid value during polycondensation reaction has been ex-plained on the basis of the different reactivities of primary and secondary hydroxyl groups of glyc-erol (Goldsmith, 1948). Since a primary hydroxyl group reacts faster than a secondary hydroxyl group, it is believed that the rapid decrease in acid
value at the early stages of reaction corresponds to the period when primary hydroxyl groups re-act, while the later stage represents the period when secondary hydroxyl groups react. The de-crease in acid value is most rapid for sample II and the least for sample III. Thus, the rate of decrease in acid value can be said to depend on the amount of oil used in the formulation. It has been reported that the period when change in acid value during polycondensation reaction is less rapid probably indicates the beginning of forma-tion of three-dimensional network as a result of crosslinking of alkyd chains (Aigbodion and Ok-ieimen, 1996). Thus, it can be inferred from these results that during the later stage of the reaction when secondary hydroxyl groups react, there is probably crosslinking of alkyd chains, resulting in increased viscosity of the reaction medium.
Monofunctional and polyfunctional condensa-tion reaccondensa-tions are considered to be equivalent since they are essentially reactions between func-tional groups, and their rate can be expressed as a second-order rate law (Flory, 1946):
(1−P)−1=C
0kt+1 (3)
whereC0is the initial concentration of reactant,t
is time of reaction, Pis extent of reaction, and k
is the rate constant. In this study, acid value is substituted for concentration. According to Eq. (3), plots of (1−P)−1
versus time should be linear if k is constant throughout the reaction. However, plots of (1−P)−1 versus time for the
three samples of alkyds studied here are not linear throughout the reaction (Fig. 2). The first portion of these plots is linear up to a certain point. Thereafter, it deviates from linearity. A similar trend was found for alkyds modified with rubber seed oil (Aigbodion and Okieimen, 1996). The first linear portion is considered to represent time of formation of linear molecules, and the second non-linear portion represents period of crosslink-ing of the alkyd chains.
The extent of reaction and the average degree of polymerisation calculated for the alkyd samples at the point of deviation from linearity are given in Table 4. These results show that the extent of reaction at this region vary from 58.0% for
Fig. 2. Plots of (1−P)−1 versus time for the alkyd samples.
obtained for reaction between phthalic anhydride and glycerol (Carothers, 1936), they indicate a significant degree of conversion. The relatively low average degree of polymerisation tends to suggest the presence of low molecular weight spe-cies at this point and branching at relatively short intervals along the alkyd chains.
Values of the second-order rate constant, k, calculated from the linear portions of Fig. 2 are given in Table 5. Sample II gave the largest value of 12.51×10−5g (mg KOH)−1 min−1, while the
values for samples I and III are 6.05×10−5 and
5.98×10−5
g (mg KOH)−1
min−1
, respectively. This indicates that the rate of alkyd production depends on the ratio of reactants used in the formulation. Based on the result of this study, alkyds of 50% oil content would best be prepared with the oil investigated.
The drying schedule of the alkyds was deter-mined since this is the most crucial requirement for their application as binder in air-drying coat-ings. The results obtained showed that the alkyd films remained tacky after long exposure (48
h). This observation is attributable to the absence of linolenic acid in the oil. Consequently, African locustbean seed oil may be classified as semi-dry-ing. Thus, the alkyds investigated in this study are recommended for blending with other fast-drying film-forming resins such as nitro-cellulose and phenol-formaldehyde, where they may be used to impart flexibility and gloss to the film, and also to reduce production cost.
4. Conclusion
A preliminary investigation into the utilisation of African locustbean seed oil as alkyd modifier has been carried out. Results obtained show that the total saturated fatty acids is more than total unsaturated fatty acids, and this accounts for the lower iodine value recorded in the investigated oil, compared with linseed and rubber seed oils. Alkyds prepared from locustbean seed oil may be suitable for blending with fast-drying resins to impart gloss and flexibility, and to reduce cost. Kinetic study showed that initial reactions follow
Table 4
Extent of reaction and average degree of polymerisation at the onset of gelation
Parameter Alkyd samples
II III I
60 120
90 Time (min)
58.6 63.5 57.9 Extent of reaction (%)
2.4
Average degree of 2.7 2.4
polymerisation
Table 5
Values of the second-order rate constant,k, calculated from the linear portion of plots of (1−P)−1 versus time for the
alkyd samples
Alkyd sample k×105(g (mg KOH)−1min−1)
I 6.05
II 12.51
III 5.98
a second-order rate law with a rate constant of the order of 10−5 g (mg KOH)−1 min−1. The
esterification reaction reached an appreciable level before deviation from the second-order rate law.
Acknowledgements
The authors express their profound gratitude to Dr A.B. Fasina, formerly Acting Director, Rub-ber Research Institute of Nigeria, for his support and encouragement, and V.O. Aiwekhoe for typ-ing the manuscript.
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