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Y. Cao et al.
oil. In boundary lubrication, ESBO shows the most excellent wear resistance [1].
After epoxidation, it is found that the oxidation stability of rapeseed oil is also
improved [37]. And biodegradation tests show that epoxidation does not affect its
biodegradability [33].
Both selective hydrogenation and epoxidation can improve the oxidation stability
of plant oils, but they cannot improve the low-temperature performance. So there is
a good idea to change the low-temperature performance and oxidation resistance at
the same time by improving the branching degree of plant oils.
Branching
Branched-chain fatty acid esters have good physical properties such as excellent
low-temperature performance and stronger hydrolysis stability due to the increase of
steric hindrance chain. Low pour point, good fluidity, high stability, and high flash
point make them widely used in the fields of lubricating oil, cosmetics, plastics, and
so on.
There are some effective methods of branching modification of plant oil. (1)
Esterification of hydroxy plant oil. By esterification of castor oil with C1 ~ C3
linear chain or branched chain anhydride, a mixture of monohydric, dihydroxy, and
trihydroxy castor oil ester will be produced with good lubricating performances.
The pour point can be reduced to - 40 °C, and the viscosity index is more than
135 [32]. (2) The direct branching modification of unsaturated fatty acid bonds
in plant oil. Unsaturated double bonds can be removed by using cyclopentadiene
and methylcyclopentadiene on the fatty acid chain, and hydrogen reacts with the
double bonds on the alkyl ring after addition reaction, the oxidation stability and
low-temperature fluidity of the products will be improved [19].
Transesterification or Esterification
The glyceryl group in plant oil has a strong tendency of hydrolysis and thermal
degradation. The structure of triglycerides can be replaced by transesterification to
yield polyol esters which have good oxidation stability and low-temperature performances [29]. There are two main ways to prepare polyol esters from plant oil: one is
to produce fatty acids from oil and then esterified with polyols [21]; another one is to
convert oil into methyl ester and then conduct transesterification with polyols [9, 10].
The corresponding polyol esters are prepared by transesterification of plant oil with
Trimethylol Propane (TMP), pentaerythritol, and 2,2-dimethyl-1,3-propanediol. The
transesterification products have good low-temperature fluidity, oxidation stability,
viscosity temperature, and lubricity [20].
Y. Cao et al.
oil. In boundary lubrication, ESBO shows the most excellent wear resistance [1].
After epoxidation, it is found that the oxidation stability of rapeseed oil is also
improved [37]. And biodegradation tests show that epoxidation does not affect its
biodegradability [33].
Both selective hydrogenation and epoxidation can improve the oxidation stability
of plant oils, but they cannot improve the low-temperature performance. So there is
a good idea to change the low-temperature performance and oxidation resistance at
the same time by improving the branching degree of plant oils.
Branching
Branched-chain fatty acid esters have good physical properties such as excellent
low-temperature performance and stronger hydrolysis stability due to the increase of
steric hindrance chain. Low pour point, good fluidity, high stability, and high flash
point make them widely used in the fields of lubricating oil, cosmetics, plastics, and
so on.
There are some effective methods of branching modification of plant oil. (1)
Esterification of hydroxy plant oil. By esterification of castor oil with C1 ~ C3
linear chain or branched chain anhydride, a mixture of monohydric, dihydroxy, and
trihydroxy castor oil ester will be produced with good lubricating performances.
The pour point can be reduced to - 40 °C, and the viscosity index is more than
135 [32]. (2) The direct branching modification of unsaturated fatty acid bonds
in plant oil. Unsaturated double bonds can be removed by using cyclopentadiene
and methylcyclopentadiene on the fatty acid chain, and hydrogen reacts with the
double bonds on the alkyl ring after addition reaction, the oxidation stability and
low-temperature fluidity of the products will be improved [19].
Transesterification or Esterification
The glyceryl group in plant oil has a strong tendency of hydrolysis and thermal
degradation. The structure of triglycerides can be replaced by transesterification to
yield polyol esters which have good oxidation stability and low-temperature performances [29]. There are two main ways to prepare polyol esters from plant oil: one is
to produce fatty acids from oil and then esterified with polyols [21]; another one is to
convert oil into methyl ester and then conduct transesterification with polyols [9, 10].
The corresponding polyol esters are prepared by transesterification of plant oil with
Trimethylol Propane (TMP), pentaerythritol, and 2,2-dimethyl-1,3-propanediol. The
transesterification products have good low-temperature fluidity, oxidation stability,
viscosity temperature, and lubricity [20].