Scientific Research Journal
Utilization of epoxidized palm olein in epoxy coating formulations
R. M. Taharima, Dr. Junaidah Jaia,b, Najmiddin Yaakub, Dr. Maziyar Sabet and Dr. Norazah
Abdul Rahman
a
Faculty of Chemical Engineering
Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia
ABSTRACT
Epoxy coatings are highly in demand due to their excellent corrosion protective properties.
Current raw materials for epoxy coatings mainly originate from petroleum, a depleting, nonrenewable resource. Due to the versatility of oleochemistry, plant oils are potential sources for
alternative raw materials. This study aims to investigate the adhesive strength and anti-corrosive
properties of new epoxy coatings formulated using epoxidized palm olein (EPO). EPO has been
successfully produced commercially and EPO-DGEBA blends have been shown to be curable by
common epoxy curing agents. However, EPO has not been reported to be utilized in any coating
formulation. Thus, EPO-epoxy binders were prepared using EPO and diglycidyl ether of
bisphenol A (DGEBA). Titanium dioxide was added into the formulation for its opacity and
hiding properties. Curing was done using cycloaliphatic amine adduct. Two curing conditions
were selected; ambient and heat-curing. The effect of EPO content on the coating-substrate
adhesion was tested using standard test method for pull-off strength of coatings using portable
adhesion tester (ASTM D4541-09). EPO-epoxy coatings applied on mild steel were also
immersed in various corrosive media and then evaluated in terms of blistering and rust as per
Standard Practice for Evaluating Degree of Rusting on Painted Steel Surfaces (ASTM D610)
and Standard Test Method for Evaluating Degree of Blistering (ASTM D714), respectively. It
was found that increasing the level of EPO content higher than 10 wt. % adversely affects the
coating’s adhesion to mild steel. However, heat curing was found to be effective in overcoming
this negative effect. Visually, it was observed that EPO-epoxy coatings perform as EPO-free
coatings in preventing corrosion of mild steel in non-acidic corrosive environments.
Keywords: EPO, coating, anti-corrosive, epoxidized oil, bio-based
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Introduction
Coatings are the most widely used method used to protect metallic substrates against corrosion
[1]. There are three main constituents in any organic coating formulation, namely (a) binder, (b)
solvent, and (c) pigment. The main component contributing to a coating’s barrier properties is
the binder. A binder ‘holds’ the different components together by “forming a continuous
polymeric phase in which all other components can be incorporated” [2]. Binders are classified
according to their film-forming mechanisms. The most common processes involved in the
formation of a continuous solid film are solvent evaporation, oxidative crosslinking, chemical
curing or a combination of the processes. Epoxy coatings are chemically cured as crosslinking
takes place between compounds with epoxy moities with a curing agent. In general, epoxy curing
agents are hydrogen donors. There are three types of epoxy compounds, namely (a)
cycloaliphatic epoxy, (b) epoxidized oils and (c) glycidated resins. These compounds contain one
or more oxirane ring, which is also referred as epoxy ring (Figure 1).
Figure 1: General molecular structure of a compound with epoxy moieties
Most epoxy coatings today are based on diglycidyl ether of bisphenol A (DGEBA), a
glycidated resin produced from reacting bisphenol A (BPA) and Epichlorohydrin (ECH) [3].
Figure 2 illustrates the link between most epoxy coatings and petroleum. In order to reduce the
industry’s dependence on petroleum, researchers have been studying the possibility of using
epoxidized oils as potential raw materials in epoxy coatings [4-5].
Figure 2: Link of common epoxy resin to petroleum.
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Many researchers have reported positive findings from their studies on various epoxidized oils
such as epoxidized Annona squamosa oil [6], epoxidized linseed oil [7] and epoxidized soybean
oil [8]. In fact, coatings based on epoxidized soybean oil are already commercially available. It is
practical to note that the selection of plant oil is influenced by two factors; economy and
geography. The best option for our region is palm oil, based on its high yield and availability [910]. Globally, oil palm generates about 4–5 tonnes of oil/ha/year, about 10 times the yield of
soybean oil [11]. Currently, palm olein is used to produce a wide range of products such as
cooking oil, skincare products, biodiesel, and drilling mud.
Epoxidized plant oils maintain most of their original molecule structures, except that their
double bonds are replaced by oxirane rings. Epoxidation of plant oils have been successfully
carried out via two methods (a) in-situ chemical oxidation using peracid or (b) enzyme-catalyzed
epoxidation. Examples of plant oils that have been successfully epoxidized are linseed oil [12],
nahar seed (Messua ferra L.) oil [13], soybean oil, jatropha oil [14]and palm oil [15].
Epoxidation of palm olein can be represented by the epoxidation of oleic acid (Fig. 3).
Figure 3: Epoxidation of oleic acid.
Epoxidized palm olein (EPO) is now commercially available and utilized in the production of
polyurethane foam and polyester [16]. Theoretically, an oxirane ring readily undergoes reactions
with other compounds due to the high degree of strain within it [17]. Consequently, different
methods can be used to generate a polymeric material using EPO. Wan Rosli et al. studied the
curing of EPO-cycloaliphatic diepoxide system using UV radiation and reported evidence of the
ring opening, indicating that chemical curing took place [18]. This study also provided the early
indication that EPO affects the resulting material’s tensile strength and hardness. Similar results
were achieved in a study of biodegradable polymers using epoxidized soybean oil-modified resin
[8]. However, instead of UV-curing, this study used N-Benxylpyrazinium hexafluoroantimonate
(BPH), a thermally latent initiator. Theoretically, epoxy resins cure through nucleophilic
addition, during which oxirane ring opens and protons are donated by the curing agent, forming
covalent bonds between the macromolecules [19]. Based on the presence of oxirane rings in EPO
and commercial epoxy resins, as well as the knowledge that commercial liquid epoxy resins cure
through the oxirane ring opening [20], researchers also studied the reaction of EPO with
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common epoxy curing agents. Tan and Chow reported that an anhydride curing agent could be
used to cure epoxy blends containing DGEBA, cycloaliphatic epoxide, epoxy novolac resin and
EPO whereas Sarwono et al. utilized m-xylylenediamine to cure an epoxy blend consisting of
EPO and DGEBA [21-22]. Although these studies reported significant changes in the mechanical
properties of the epoxy system, they are instrumental in showing EPO’s capability to react with
common epoxy curing agents such as anhydrides, amines and polyamides.
Curing of bio-based materials is mostly done at elevated curing temperatures [6, 13, 21-23].
Nonetheless, ambient-curing would require less energy and therefore offer more economical and
practical means to coating application. A study using camphor oil showed that despite the longer
curing time, atmospheric curing produced coatings with good corrosion resistance [24].
In general, epoxy binder formulations may use liquid resins such as DGEBA, novolac resin
and phenolic resin. In this study, binders were prepared using EPO and DGEBA at several
predetermined ratios. The aim of this study is to study the effect of EPO content on epoxy
coatings’ adhesive strength and anti-corrosive properties.
There have been various theories on adhesion. It has been postulated that coating-metal
adhesion can be attributed to mechanical bond, chemical bond or both. Another theory is based
on the presence of a weak boundary layer near the coating-substrate interface [25]. According to
this theory, coating fractures occur in this layer, which lies within the coating. This theory
suggests that adhesive strengths within the coating affect the coating-substrate adhesion.
Consequently, fracture may occur at the interface (adhesive failure) or near the interface
(cohesive failure). Adhesion between coating and metallic substrates has also been theorized to
be a result of Van der Waals interactions between functional groups in the coating and the metal
ions.
In this experiment, all plates were cleaned and prepared using the same method. Although
there may be inevitable variations in the substrate’s degree of cleanliness and surface roughness,
it can be assumed that the degree of mechanical bond between all coatings and substrate is
approximately the same for all coatings. Therefore, any measured differences in coatingsubstrate adhesion was expected to be due to the varying degree of (a) adhesive forces within the
coating network, or (b) chemical bonds between substrate and coating, or (c) Van der Waals
interactions between substrate and coating.
Previous studies on other epoxidized oils also studied adhesion properties, but used standard
methods such as ASTM 3359, which is suitable for coatings of thickness less than 125 µm [6,
26-27]. For thicker coatings, the standard testing methods are as per ASTM 4541-09. This study
reports the average pull-off strength of the coatings listed in Table 1.
As seen from previous studies, anti-corrosive properties can be evaluated by immersing
coating plates into corrosive media for a fixed duration [12, 24, 26, 28]. It was reported that
EPO-epoxy systems are susceptible to water uptake due to the presence of hydroxyl groups that
are formed during crosslinking process [22]. On the other hand, EPO was known to be
hydrophobic. Therefore, it was expected that higher levels of EPO would lead to lower water
absorption rate and better coating performance in water. 0.5M NaCl solution was used to
represent a marine environment while 0.1M H2SO4 and 0.5M NaOH solution were used to
demonstrate the coating’s performance in the opposite ends of the pH range. Data on immersion
tests of amine-cured EPO-epoxy systems in these media are currently not available. This study
reports the final condition of the test plates in terms of degree of rust and blistering.
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Experimental
Material
EPO was procured from Malaysian Palm Oil Board (MPOB), Bangi, Selangor. The pH value and
acid value of the EPO is 5-6 and 0.31, respectively. DGEBA-based liquid epoxy resin (DER
331), cycloaliphatic amine adduct and titanium dioxide were supplied by Suka Chemicals Sdn.
Bhd., Selangor.
Method
The functional groups of EPO were characterized using Fourier Transform Infrared Ray (FTIR)
Spectrometer (Perkin Elmer, USA). FTIR spectra from the wavelength of 4000-450 cm-1 were
recorded as the infrared radiation transmitted through the EPO sample.
EPO and DER331 were measured to a predetermined ratio and then mechanically mixed at
approximately 300 rpm until no phase separation was observed. New mild steel plates of
dimensions 150mm x 150mm x 3mm and 30mm x 80mm x 3mm were initially cleaned to Sa 2.5
standard1 using sandblasting method to remove all millscale. In the laboratory, the plates were
abraded using sandpaper No. 120 and wiped with acetone. Coatings were applied in two layers,
using short-hair rollers. Plates for adhesion test (150mm x 150mm x 3mm) were coated on one
side only whereas plates for immersion test (30mm x 80mm x 3mm) were coated on both sides.
Heat curing was carried out using an electric oven preheated to 180oC. Plates were heated for 10
minutes before they were removed from the oven. Ambient curing was carried out for 7 days.
Dry film thickness (DFT) was measured by Quanix 8500 at the end of the curing duration.
The average thickness was found to be 190 µm. Therefore, ASTM 4541-09 was adopted for
evaluating the adhesive strength of the coatings. Table 1 lists the formulations prepared in this
experiment.
Adhesion tests were done using Elcometer 108, using standard testing method as per ASTM
D454 (Test Method C). Adhesive strength was measured as the greatest perpendicular force that
the coating can bear before a plug is detached.
For chemical tests, plates were placed vertically in 250 ml-glass beakers, each containing
distilled water, 0.5M NaCl, 0.1M H2SO4 and 0.5M NaOH. Test plates were taken out of the
beakers after 288 hours and wiped. Evaluations of test plates were immediately done according
to ASTM 1654-08 Procedure B (Standard Practice for Evaluation of Painted or Coated
Specimens Subjected to Corrosive Environments), ASTM D 610-08 (Standard Test Method for
Evaluation Degree of Rusting on Painted Steel Surfaces) and ASTM D 714-02 (Standard Test
Method for Evaluation Degree of Blistering of Paints).
1
International Standard ISO 8501-1(1998)
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Table 1: Summary of Formulations
Formulation
EPO0A
EPO5A
EPO10A
EPO15A
EPO20A
EPO25A
EPO30A
EPO0B
EPO5B
EPOB
EPO15B
EPO20B
EPO25B
EPO30B
EPO (wt. %)
0
5
10
15
20
25
30
0
5
10
15
20
25
30
DER331 (wt. %)
100
95
90
85
80
75
70
100
95
90
85
80
75
70
Curing condition
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Ambient
Heat
Heat
Heat
Heat
Heat
Heat
Heat
Results and discussion
FTIR spectrum of EPO (Figure 4) showed that the molecular structure of EPO consists of –CH
bond (2925 and 2855 cm-1), -C=O bond (1745 cm-1), -C-O bond (1161 and 1113 cm-1), -CH3
bond (1466 and 1378 cm-1), -OH bond (3464 cm-1) and epoxide group (723 cm-1). Therefore, the
presence of oxirane rings or epoxy rings in EPO was confirmed.
Figure 4: FTIR Spectrum of EPO
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Ambient-cured formulations
The average results of the pull-off adhesion tests conducted on ambient-cured coatings are
depicted in Figure 5. It is evident from the chart that coating-substrate adhesion was affected by
the level of EPO in the coating formulations.
1600
Pull-off Adhesion (psi)
1400
1200
1000
800
600
400
200
0
0
5
10
15
20
25
30
EPO wt. %
Figure 5: Adhesive strength of ambient-cured EPO-epoxy coatings as a function of EPO content
Initially, adhesive strength increased with the introduction of EPO. This pattern was expected
due to EPO’s flexible aliphatic chains, which would introduce more flexibility to the network
structure. As a result, adding EPO would decrease the internal stress of the system. However,
when EPO level was increased from 10 to 15 wt. %, a significant drop was observed in the pulloff adhesion test result. The measured pull-off adhesion continued to decline as EPO level was
gradually increased by 5% from 15 to 30 wt. %. A similar pattern was observed in a study using
epoxy blends containing DGEBA and epoxidized soybean oil [8], in which adhesive strength
within the blends was reported to initially increase, then peak at ESO content of 40 wt. % and
gradually decreased with further increase of ESO. The observed decline in Figure 5 may have
been caused by the significant reduction of the number of epoxy rings, aromatic rings and
aliphatic hydroxyl groups in the binder due to reducing amount of DGEBA [8]. Epoxy rings are
involved in crosslinking process with cycloaliphatic amine adduct while aromatic rings and
hydroxyl groups are involved in Van der Waals interactions at the coating-substrate interface.
Consequently, these functional groups are known to enhance adhesion between dissimilar
materials [25]. Therefore, as DGEBA level was reduced, the net result is decreasing adhesive
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strength within the blends. The type of fractures produced from the pull-off adhesion test is
indicative of the difference in fracture mechanism. The fractures on the test plates coated with
EPO5A, EPO10A and EPO 30A are depicted in Figure 6. Through visual examination, it was
observed that the fracture on EPO5A is mostly caused by coating-substrate (adhesive) failure
whereas the fracture on EPO30A is mostly caused by coating-coating (cohesive) failure. Fracture
of EPO10A is most likely a combination of both types of failure.
Figure 6: Fractures caused by adhesion test on (a) EPO5A, (b) EPO10A and (c) EPO30A.
Table 2: Evaluation of degree of rusting and blistering of mild steel coated with ambient-cured
EPO- epoxy coatings
Test solution
Formulation
Distilled water
0.5M NaCl
0.5M NaOH
0.1M H2SO4
(288 h)
(288 h)
(288 h)
(288 h)
Rust
Blister
Rust
Blister
Rust
Blister
Rust
Blister
10
10
7S
10
10
10
4S
2 (few)
EPO0A
10
10
7S
10
10
10
4S
1 (few)
EPO10A
10
10
8S
10
10
4 (few)
0
c.da
EPO20A
10
10
9S
10
10
2 (few)
0
c.da
EPO30A
Rust grade rating is 0-10 (10 = no corrosion); S = spot corrosion
Blister size rating is 10-0 (10 = no blistering, 0 = largest). Frequency of blisters designated as
dense, medium dense, medium or few .
a
c.d: coating delaminated
The corrosion resistance behavior of the ambient-cured formulations is given in Table 2. In
general, neither blister nor rust was detected on all plates immersed in distilled water (Figure 8).
In contrary to a finding by Sarwono et.al, water absorption was not severe enough to cause the
formation of water-filled blisters on any of the test plates immersed in distilled water [22].
Test specimens immersed in 0.5M NaCl solution were also free from blisters (Figure 9).
However, very little rust was detected on the test plates; it was limited to sharp corners and
edges, where coating thickness was most likely to be the lowest due to the geometry. The
absence of blistering and rusting on the immersed area indicates the coatings’ good wet adhesion
in the test solutions. In other words, the coatings stayed attached to the substrate, providing a
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barrier between the test solution and the substrate. From visual inspection, no defects were
observed in the coating as EPO level was increased from 0 to 30 wt. %.
Test plates immersed in 0.5M NaOH solution were also free from rust (Figure 10). However,
only EPO0A and EPO10A test plates were free from blisters. A few blisters were discovered on
specimens coated with EPO20A and EPO30A, indicating the early signs of adhesion loss. In
addition, the same specimens were found to be covered with a layer of transparent gel, which
was found to have the consistency of soap. Thicker gel was discovered when this experiment was
repeated using 1M NaOH and EPO30A. The presence of unreacted oil in thermoset polymers
oils has been previously reported [29]. In this case, it was probable that unreacted EPO from
within the coating reacted with the sodium and hydroxyl ions from the NaOH solution, forming a
soap solution as in the case of oleic acid in water [30-31]. FTIR analysis of the gel confirms the
presence of _______ group in the NaOH solution at the end of the experiment (Figure 7). It is
postulated that upon immersion, free EPO left the network structure thus creating voids within
the coating matrix. As a result, test solution could permeate through the coating with less
resistance and eventually created blisters. Visually, it could be observed that increasing EPO
mildly affected the coating performance in NaOH solution.
Figure 7
Immersion tests in 0.1M H2SO4 resulted in serious damage to EPO-epoxy coatings at all levels
of EPO (Figure 11). Blisters appeared on the first day of immersion, signaling immediate loss of
coating-substrate adhesion. It is theorized that hydrogen ions permeated through pinholes on the
coatings and initiated a set of electrochemical reactions at the surface as suggested by equation
[1] & [2], where metal dissolved and hydrogen was evoluted. Basically, acid corrosion took
place under the coatings and due to the formation of hydrogen at the corrosion sites, the blisters
grew in size and exacerbated the situation by increasing the area of corrosion. Eventually, the
coatings delaminated from the substrate.
Cathodic reaction:
Anodic reaction:
2H+ + 2e- → H2
Fe → Fe2+ + 2e-
[1]
[2]
Figure 8: (Left-right) Uncoated plate, EPO0A, EPO10A, EPO20A, EPO30A after 288 hours
immersion in distilled water.
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Figure 9: (Left-right) Uncoated plate, EPO0A, EPO10A, EPO20A, EPO30A after 288 hours
immersion in 0.5M NaCl.
Figure 10: (Left-right) Uncoated plate, EPO0A, EPO10A, EPO20A, EPO30A after 288 hours
immersion in 0.5M NaOH.
Figure 11: (Left-right) Uncoated plate, EPO0A, EPO10A, EPO20A, EPO30A after 288 hours
immersion in 0.1M H2SO4.
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Heat-cured formulations
The adhesive strengths of the different formulations are shown in Figure 12. A significant
difference was observed in the results when compared to that of ambient-cured formulations. In
this case, increasing EPO content from 0 to 30 wt. % did not seem to cause any significant effect
on the coatings’ adhesive strength.
Pull-off adhesion (psi)
1400
1200
1000
800
600
400
200
0
0
5
10
15
20
25
30
EPO wt. %
Figure 12: Adhesive strength of heat-cured EPO-epoxy coatings as a function of EPO content
Table 3: Evaluation of degree of rusting and blistering of mild steel coated with heat-cured EPOepoxy coatings
Test solution
Distilled water
0.5M NaCl
0.5M NaOH
0.1M H2SO4
(288 h)
(288 h)
(288 h)
(288 h)
Rust
Blister
Rust
Blister
Rust
Blister
Rust
Blister
10
10
9S
10
10
10
5S
1 (few)
EPO0B
10
10
9S
10
10
10
0
c.da
EPO10B
10
10
9S
10
10
10
0
c.da
EPO20B
10
10
9S
10
10
10
0
c.da
EPO30B
Rust grade rating is 0-10 (10 = no corrosion); S = spot corrosion
Blister size rating is 10-0 (10 = no blistering, 0 = largest). Frequency of blisters designated as
dense, medium dense, medium or few.
a
c.d: coating delaminated
Formulation
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The corrosion resistance behavior of the heat-cured formulations is given in Table 3.
Generally, no rusting or blisters were observed on test plates immersed in distilled water, 0.5M
NaCl and 0.5M NaOH solutions. Rust spots were detected on specimens 0.5M NaCl solution, but
they were limited to spot corrosion which could have been caused by the presence of
contaminants on between substrate and coating. Under immersion condition, good anti-corrosive
performance is related to excellent wet adhesion. Therefore, it is evident that heat-cured
formulations possess excellent wet adhesion in distilled water, 0.5M NaCl solution and 0.5M
NaOH solution.
In H2SO4, the performance of heat-cured formulations is similar to that of ambient-cured
formulations. In general, the coatings showed poor anti-corrosive properties in this acidic
medium.
Conclusion
Coating formulations including EPO and epoxy resin based on DGEBA were proven to produce
coating films that are suitable for protecting mild steel from corrosion. However, the level of
EPO affected the adhesive strength of the coatings. It was observed that heat-curing could
improve this property for EPO levels less than 40 wt. %. It was also found that the increasing
level of EPO did not reduce the anti-corrosion properties of the EPO-epoxy coatings in distilled
water and 0.5M NaCl solution. However, based on the experiment with NaOH solutions, it was
observed that the EPO-epoxy’s anti-corrosive performance was better for heat-cured
formulations. In general, it was concluded that EPO-epoxy coatings are unsuitable for protecting
mild steel in acidic environments.
Acknowledgement
The authors express their grateful acknowledgement for the technical support from the Research
Management Institute (RMI), Universiti Teknologi MARA and the financial support from the
Fundamental Research Grant Scheme (FRGS) from the Ministry of Higher Education, Malaysia.
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