3.2. Epoxidation and subsequent esterification of PKME for
biolubricant production
The result of the effect of temperature and time at different times and
temperatures, on the relative fractional conversion of the palm kernel
methyl ester sample to oxirane is presented Fig. 9. The conditions used
for this study were H2O2-ethylenic
unsaturation molar ratio of 1.5:1, 2.5 wt%
H2SO4 catalyst conc., and 1200 rpm
stirring speed.
Temperature and time effects on the rate of epoxidation of the palm
kernel methyl ester sample, was determined at 35, 45, 55, 65 and 75 °C.
The result indicates that the relative fractional conversion to oxirane,
increased directly with reaction time during the early stages of the
reaction as could be seen in Fig. 9. However, it later started to
decrease with additional increase in time of the reaction. This
abnormality associated with additional increase in time was due to the
commencement of oxirane ring opening [30-31]. Also, it was found
that temperature increase favored peracetic acid formation. Hence, the
resulting implication was not limited to accelerated epoxidation, but
also in enhanced hydrolysis rate; that is oxirane ring opening of the
product [32].
Furthermore, Fig. 9 shows the times necessary to achieve maximum
relative conversion at different temperatures. As could be seen in the
Figure, the time necessary to attain maximum relative conversion were 8,
7.9, 6, 6, and 4 hr., at temperatures of 35, 45, 55, 65 and 75 °C,
respectively. From the Fig. 9, it was observed that smaller rates were
noticed for the reactions at lower temperatures of 35 and 45 °C. Though,
more stable oxirane rings were obtained at these temperatures, compared
to those of higher temperatures 55, 65 and 75 °C, which showed higher
rate, but more unstable oxirane ring, leading to higher degradation of
the epoxide. It is important to state that at the higher temperatures of
55, 65 and 75 °C; the relative conversion of the PKME sample to oxirane
attained maximum values in 6, 6 and 4 hr., respectively. Nevertheless,
it was noticed that there was decrease in the relative conversion to
oxirane with additional increases in time. For instance, at temperature
of 75 °C, the observed decrease in the relative conversion to oxirane
was very clear since it began after 4 hr. In the same way, at
temperatures of 55 and 65 °C, their relative conversion started after 6
hr. Nevertheless, at 35 and 45 °C, more stable rings were obtained for
the sample. This is because there was no decrease in the relative
conversion to oxirane within the studied time intervals (see Fig 9).
Similar results were also obtained by Agu et al. [2], Goud et al.
[32] and Cai et al. [33] for the epoxidations ofTerminalia catappa L. methyl ester, mahua and soyabean oils,
respectively. The highest fractional conversion to oxirane obatined was
0.87, at 75 °C in 4 hr. as shown in Fig 9.
[CHART]
Fig. 9: Effects of temperature and time on relative fractional
conversion to oxirane for the epoxidation of palm kernel methyl ester
Physicochemical properties of PKO and PKO biolubricants
The physicochemical properties of the PKO and the biolubricants produced
by transesterification with trimethylolopropane (TMP) and
epoxidation-esterification methods are presented in Table 1. The
obtained oil yield of palm kernel was 49.82 %. This value was slightly
higher than the 47.5 % and 49.2 % yields reported for palm kernel by
Ibiam and Anosike [8] and Hossain et al. [34]. However, this
value was lower than the 49.9 % yield, reported by Zaidul et al.
[35]. This difference in the PKO yields could be attributed to the
extraction method used [36].
A number of studies have shown that vegetable oils have short comings
with respect to its use as biolubricant without modification of its
structure [37-38]. These limitations include thermal, oxidative and
hydrolytic instability as well as inadequate low temperature fluidity as
a result of their high pour points [1]. These limitations are as a
result of the glycerol moiety which is a major constituent in vegetable
oil. Most of these problems are significantly reduced by chemical
modification of the vegetable oil, in this case PKO. Hence, the reason
for the modification of the PKO using the methods of transesterification
with trimethylolopropane (TMP) [1,17] and epoxidation-esterification
[37-40].
The synthesis of PKO biolubricant using two stage transesterifaction
methods causes the removal of the β-hydrogen atom from the oil
structure. Hence, provides an ester with high oxidative and thermal
stability [41]. In this two stage transesterification process, the
unstable hydrogen that is bared by the glycerol molecule is substituted
by a more stable TMP. At the first stage of the transesterification
reaction, the glycerol molecule is removed, while in the second stage, a
more effective TMP molecule, replaces the glycerol. Thus,
trimethylolpropane triester (TMPTE) that has more superior properties
and performance is produced [1].
Similarly, the synthesis of PKO biolubricant was also achieved using the
three step synthesis of the methyl ester that involves epoxidation, ring
opening and esterification steps. The epoxidation process helps in the
removal of unsaturation in the methyl ester by converting them into
epoxy-groups that improves the oxidative stability. It is a known fact
that the existence of double bonds in vegetable oils chains accelerates
oxidative degradation. On the other hand, there are poor low temperature
properties in the oil, leading to solidification at lower temperatures
like 0 °C. These short comings, limit their application at low operating
temperature, especially as biolubricants for automotive and industrial
purposes [40]. Hence, suitable method to improve the low temperature
flow properties is to attach branching sites at the epoxy carbons. This
leads to the need for the ring opening step of this synthesis approach.
This was achieved by careful carrying out esterification ring opening
reaction using acid anhydride and boron trifluoro diethyletherate
catalyst. The obtained branched methyl ester was used as precursors for
the synthesis of modified triester-derivatives by acetylation with
octanoyl chloride.
Conventional lubricant, the petroleum lubricant sample was analyzed
using similar parameters as those determined for the biolubricant. This
was to evaluate the properties nearness of the biolubricant samples to
the conventional lubricant. Table 1 shows the physicochemical properties
of the PKO, palm kernel biolubricat (PKBLT) synthesized
by transesterification of trimethylolpropane (TMP), palm kernel
biolubricat (PKBLE) synthesized by
epoxidation-esterification of the methyl ester and the conventional
petroleum lubricant.
Generally, it could be seen from the table that the biolubricants
samples were more viscous and have higher weight than the petroleum
lubricant counterpart. Hence, these properties give rise to the
advantages of better mechanical load and thermal resistance of
biolubricants over mineral lubricant [1]. Furthermore, the viscosity
indices and pour points values of the studied samples were similar to
the petroleum lubricant sample. The flash point values of the
synthesized biolubricant samples were higher than the PKO sample. This
is an indication that the synthesized samples had enhanced thermal
resistance as a result of the transesterification with
trimethylolpropane (TMP) and epoxidation-esterification reactions. The
flash point values of the synthesized biolubricant samples were close to
that of petroleum lubricant. Thus, this is an indication of their
greater thermal stability.
Table 1: Physicochemical properties of PKO and modified PKO
biolubricants