^aIndustrial Agriculture Products Research Laboratory, Department of Chemistry, University of Benin, Benin City,
Nigeria.
^bEnd-use Division, Rubber Research Institute of Nigeria, Benin City, Nigeria
^ΦTo whom correspondence should be addressed: E-mail: esyikhuoria@yahoo.com

Code Number: pr04008

Abstract

Introduction

Alkyd resins are products of polycondensation reaction between polybasic acid and polyhydric alcohol modified with fatty acid or drying oil. Oil modified alkyd resins constitute a major group of resins used as binders in surface coatings. It is estimated that alkyd resins contribute about 70% to the conventional binders used in surface coatings today¹. The popularity of alkyd resins as vehicle for coatings is largely due to their unique properties such as film hardness, durability gloss and gloss retention, resistance to abrasion, etc. impacted on them through modification with drying oils². The oils that are mostly employed for alkyd resin synthesis are linseed, soybean, castor and tall oils^3,4,5 . These oils are largely imported to Nigeria for the formulation of coatings for metal cans used in packing of beverages, drugs, food, etc. However, drying oils are available locally, which have remained untapped. These include rubber seed oil, soybean oil, walnut oil and tobacco oil⁶. Rubber seed is obtained in high yield as a by-product of Hevea brasiliensis cultivated primarily for its latex. ^7,8, It is reported to be abundant in Nigeria and is found to contain 42% oil⁹. However, research has shown it to be a rich source of oil that is comparable in quality to dry oils commonly used in surface coatings^10,11,12 .

The industrial value of vegetable oil generally depends of its constituent fatty acids and the ease with which it can be modified or combined with other chemicals¹³ . Several physical and chemical modifications of the oils to enhance their initial quality have been evolved over the years¹⁴ . Examples of such modification techniques include acrylation¹⁵ , catalytic and thermocatalytic polymerization¹⁶ interesterification¹⁷, phosphorrylation and expoxidation¹⁸ . Chemical transformation of vegetable oil to fatty acid alkyl esters called transesterification or alcoholysis is one of the methods of modifying the quality of vegetable oils.

Raw vegetable oils are composed of glycerol, esters of fatty acids and various amounts of solubilised impurities such as pigments, vitamins, sterols, phospholipids etc, which may compromise the quality of the finished alkyd resins. Esters of vegetable oils would preclude some of the undesirable effects. This study reports on the in-situ alcoholysis of rubber seed oil (RSO), preparation and characterisation of alkyd resins the RSO and the methyl esters of rubber seed oil (MERSO) aimed at enhancing alkyl resins.

Materials and Methods

Materials

Rubber seeds were collected from the Rubber Research Institute of Nigeria, Iyanomon, Benin City. Laboratory grade phthalic anhydride (PA), glycerol (GLY) and xylene were obtained from BDH, England.

Preparation of rubber seed oil (RSO)

Rubber seeds were air dried in the oven at 50 ^oC for 72 hr and shelled to obtain the kernels which were then blended to obtain rubber seed meal. Rubber seed oil (RSO) was then extracted from the meal with nhexane using a soxhlet apparatus.

Preparation of methyl rubber seed oil (MERSO)

The methyl esters of RSO (MERSO) were prepared using the in-situ alcoholysis method⁴. In this method, the rubber seed meal was macerated with methanol for 30 min and the slurry obtained was magnetically stirred under reflux for 3 hr at room temperature (30 ^oC). At the end of the reaction, the mixture was filtered under vacuum, and the residue washed with 200 ml methanol and air-dried overnight at room temperature. The residue obtained was reextracted in a soxhlet apparatus with nhexane to obtain the oil fraction remaining in the residue. The percentage of the dissolved oil in methanol was calculated.

To the filtrate obtained from the in-situ alcoholysis was added 100 ml of water and the solution re-extracted with 50 ml n-hexane three times. The combined extracts were washed with water, dried over sodium sulphate and evaporated to give the esterified product. The ester content of the esterified RSO was determined by column chromatography¹⁹ .

Preparation of alkyd resins

Six different alkyds were prepared with RSO, MERSO, glycerol, phthalic anhydride using lead (II) oxide as catalyst using the formula in Table 1. The preparation was carried out in a three necked round bottom flask fitted with a motorised stirrer, a Dean-stark trap fitted with water-cooled condenser and a nitrogen inlet tube at a temperature of 230 – 250 ºC. Xylene was employed as the azeotropic solvent¹³ . Two stages were involved.

Stage I (Alcoholysis): At this stage, the monoglyceride was first prepared by reacting the glycerol with the methyl esters of rubber seed oil (MERSO) at a temperature of 230 to 250 ºC. Alcoholysis was completed when the solubility test in methanol was positive; one part of the reaction mass being soluble in three parts of methanol. The reaction mixture was cooled to 140 ºC.

Stage 2: This stage began with the addition of phthalic anhydride. The temperature was quickly raised to about 230 ºC and maintained at a range of 230 – 250 ºC. Condensed water of condensation was drained into a Dean-Stark trap. The reaction was monitored by periodic determination of the acid value of the mixture until acid value dropped to 10.

Physico-chemical characterisation of alkyd resin

The physico-chemical properties of the alkyd resin samples (colour, specific gravity, refractive index, acid value, free fatty acid, saponification value, iodine value and non volatile matter) were determined according to ASTM standard methods^20,21,22,23 .

Performance characteristics of the alkyd resin

The alkyd films were oven-baked at 100 ºC for 1 hr and the resistance of the alkyd films in different service media was determined using a standard method²⁴ . Pencil hardness of the dried films was also determined ²⁵ .

Results and Discussion

The results of the alcoholysis of RSO under various pretreatment conditions are shown in Table 2. Methyl esters yield from the maceration process was higher than the other pretreatment conditions. Thus the insitu alcoholysis involving maceration process was generally applied in the production of the methyl esters used in the preparation of the alkyds.

The chemical resistance of the alkyd resins to various solvent media are shown in Table 5. From the results, it can be seen that all the alkyd samples (I –VI) are unaffected by the salt medium. The RSO alkyd samples (I – III) have the same resistance to acid, base and salt, despite the difference in the oil length of the alkyds. However, sample II was less susceptible to water than other RSO alkyds. MERSO alkyds (IV – VI) have the same resistance to water, salt and acid, but sample IV is less resistant to basic solution than samples V and VI. In general, all the MERSO alkyds (IV – VI) have better resistance to the various solvent media than the RSO alkyd samples (I –III).