Epoxy resin (ER) was modified with four different epoxide compounds, 4,5-epoxy-4-methyl-pentane-2-on (EMP), 3-phenyl-1,2-epoxypropane (PhEP), 1-chloro-2,3-epoxy-5-(chloromethyl)-5-hexene (CEH), and a fatty acid glycidyl ester (FAGE), to improve its chemical and physical properties. The effects of the addition and amount of these modifiers on mechanical, thermal, and coating properties were investigated. Atomic force microscopy was used to observe the changes obtained with the modification. The influence of the modifying agents on the curing process was monitored through FTIR spectroscopy. The curing degrees of ER and modified ERs (M-ERs) were found to be over 91%. The results showed that tensile strength of ER improved till 30% (wt.) with addition of the modifier content. Modification with EMP and PhEP remarkably enhanced the thermal stability of ER to be highly resistant to the corrosive media.
Thermosets, such as epoxy resins (ERs), are important polymeric materials since they have been widely used as high performance materials such as adhesives, composite matrices, and anticorrosion coatings [
In the literature, a large number of studies have been conducted on the modification of highly cross-linked ERs to improve their impact resistance [
In our previous study, ER has been modified with biobased and styrene-based polymers. The results indicated that the modification process enhanced several properties of ER such as surface hardness, tensile strength, percentage elongation, and stress at maximum load of the blends [
The modifiers, 4,5-epoxy-4-methyl-pentane-2-on, 3-phenyl-1,2-epoxypropane, 1-chloro-2,3-epoxy-5-(chloromethyl)-5-hexene, and fatty acid glycidyl ester, were synthesized with the use of precursors mentioned in the following subsections. The details of the procedures were explained in these parts, too. A commercially available bisphenol-A type epoxy resin (NPEK 114, Konuray Chemical Co.) diluted with aliphatic C12–C14 glycidyl ether (ER) was used as the thermosetting matrix and it was cured with 30 wt.% Epamine PC17 (a cycloaliphatic polyamine, Konuray Chemical Co.). 2,4,6-Tris(dimethylaminomethyl)phenol (Sigma-Aldrich) was utilized as the epoxy embedding medium accelerator.
The EMP was synthesized through the oxidation of 4-methyl-3-pentene-2-on with hydrogen peroxide in basic medium. A solution of NaOH (2.0 mL, 20%) was added to the solution of 4-methyl-3-pentene-2-on (10.0 g) dissolved in diethyl ether (10.0 mL). Then, hydrogen peroxide (5.0 mL, 20%) was added dropwise to the mixture at room temperature. The mixture was stirred for 2 h and neutralized with a diluted HCl solution. Diethyl ether was removed using a rotary evaporator and EMP was obtained in 70% yield after vacuum distillation at 61-62°C/2 mmHg.
The PhEP was synthesized from the dehydrochlorination of 1-chloro-3-phenyl-2-propanol obtained through the alkylation of benzene with epichlorohydrin in the presence of AlCl3. Benzene (78.0 g) and AlCl3 (16.6 g) were placed into a three-necked flask equipped with a mechanical stirrer, thermometer, and dropping funnel. The mixture was cooled to 0°C and epichlorohydrin (11.6 g) was added dropwise to the mixture. After the mixture was stirred at 10–15°C for 10 min, 10% HCl solution was added to the mixture. Organic layer was separated, washed with water, and dried on Na2SO4. Most of the benzene was removed by a rotary evaporator, and then alkylation product, 1-chloro-3-phenyl-2-propanol, was obtained in 50% yield by vacuum distillation at 91°C/2 mmHg.
At the second stage, diethyl ether (150 mL) and KOH (56.1 g or 40.0 g of NaOH) were added in a three-necked flask equipped with a mechanical stirrer, thermometer, and dropping funnel. 1-Chloro-3-phenyl-2-propanol (34.0 g) was added dropwise to the mixture at 15°C. After the mixture was stirred for 2 h, it was dried on Na2SO4. Diethyl ether was removed with the use of a rotary evaporator, and then the PhEP was obtained in 80% yield after the vacuum distillation carried out at 59-60°C/2 mmHg.
The CEH was prepared by the dehydrochlorination reaction, which was a condensation product from 3-chloro-2-methylpropene and 2,3-dichloropropanal catalysed by AlCl3. 3-Chloro-2-methylpropene (18.0 g), AlCl3 (3.0 g), and benzene (40 mL) were placed into a three-necked flask equipped with a mechanical stirrer, thermometer, and dropping funnel. After 2,3-dichloropropanal (2.6 g) was added dropwise to the mixture, it was stirred for 2 h at room temperature. The mixture was neutralized by washing with water. Organic layer was treated with a solution of NaOH (40%, wt.) in diethyl ether (20 mL). Dehydrochlorination step was completed after stirring the mixture for 3 h. Then, the mixture was reneutralized by washing with water and dried on Na2SO4. Vacuum distillation of crude product at 64–66°C/4 mmHg gave the CEH in 70% yield.
The waste of sunflower oil as fatty acid (FA) potassium salt was supplied from Zade Chemical Industry, Konya, Turkey. The FAGE was obtained by esterification of a fatty acid (FA) potassium salt with epichlorohydrin in alkaline medium. The FA waste (10.0 g) and benzene (10 mL) were placed into a flask (50 mL). A solution of KOH (4 mL, 40%) was added to the mixture followed by dropwise addition of epichlorohydrin (3.0 g) within 20–30 min at 40°C. After the completion of the addition, temperature was increased to 70–80°C and the mixture was refluxed for 5 h. The FAGE was obtained by distillation under reduced pressure.
The synthesized epoxide compounds were mixed with epoxy matrix in weight percentages of 10, 20, 30, 40, and 50%. The mixtures were stirred at 1200 rpm for an hour using a Heidolph RZR1 stirrer. Afterwards, Epamine PC17 hardener (30 wt.%) and epoxy accelerator (1 wt.%) were added and the final mixture was transferred into the mould. Samples were prepared in stainless steel moulds according to ASTM D638 standard. The samples were cured for 24 h at 40°C followed by a postcuring process for 48 h at 120°C.
In order to determine the percentage of epoxy group, 0.5 g of sample and 25 mL of a solvent mixture (60 mL of acetone + 1.5 mL of concentrated HCl) were placed into a 250 mL flask. The mixture was stirred for 2 h at room temperature. The epoxy groups were cleaved by the addition of excess HCI. The remaining HCI was titrated with 0.1 N KOH and its amount was calculated. The percentage of epoxy group was calculated using
The FTIR spectra of the synthesized epoxide compounds and M-ERs were taken with the use of Bruker-Platinum ATR-vertex 70.
The morphology of M-ERs was examined with a Solver P47H atomic force microscope (AFM, NT-MTD, Moscow, Russia) operating in tapping mode in air at room temperature. Diamond-like carbon (DLC) coated NSG01 DLC silicon cantilevers (from NT-MTD) with a 2 nm tip apex curvature were used at the resonance frequency of 150 kHz. The Nova 914 software package was used to control the system and for the analysis of the AFM images.
A Shore Durometer TH 210 tester was used for measuring the hardness of the samples. The resistances to stretch properties were determined by Stretch and Pressing Equipment TST-Mares/TS-mxe.
Thermal analysis experiment was carried out using a NETZCH-Geratebau GmbH model thermogravimetric analyser. The samples were heated under nitrogen atmosphere from 50°C to 800°C at a heating rate of 10°C
The percentage of water sorption was determined by a gravimetric method. The M-ERs were stored in deionized water at room temperature for 30 days. To explore the temperature effect on water sorption, the same procedure was carried out at 45°C. The samples were daily taken and weighed after drying the surface. Water sorption of the coatings was calculated using
The adhesion property of M-ER coatings was determined by the “lattice notch method.” The adhesion capability of the M-ERs was determined using a metal (cold rolled carbon steel) that contains C (0.2%), Mn (1.5%), Si (0.1%), P (0.01%), and S (0.008%) with the dimensions of 50 mm
According to the “lattice notch method,” a thin epoxy layer was divided into small squares (1 mm
The metal surfaces coated with M-ER films (100–120
Figure
FTIR bands appear in the spectra of EMP, PhEP, CEH, and FAGE.
Functional group | Bands, cm−1 | |||
---|---|---|---|---|
For EMP | For PhEP | For CEH | For FAGE | |
Epoxide | 1250; 915; 828 | 1250; 913; 830 | 1250; 913; 875; 830 | 913; 830 |
Fatty acid -(CH2) |
— | — | — | 720 |
Ester C-O-C | — | — | — | 1166 |
C=O ester | — | — | — | 1750 |
R2C=CH2 | — | — | 1655 | — |
Aliphatic C-H plane in bending (for CH2) | — | 1400 | 1450 | 1450 |
Aliphatic -CH grubu | — | 1375 | 1375 | 1375 |
Alkane C-H stretching | 2975 | 3000 | 3000 | 2833; 2950 |
Aliphatic C-H plane off bending (for CH2) | 1100 | 1100 | 1056 | 1100 |
Adjacent to the carbonyl CH2 | 1410 | — | — | — |
Aromatic C=C | — | 1600 | — | — |
Aromatic C-C stretching | — | 1500 | — | — |
Monosubstituted benzene | — | 770 | — | — |
Aromatic C-H plane in bending (for monosubstituted benzene) | — | 1030 | — | — |
Ketone C=O stretching | 1722 | — | — | — |
C-Cl | — | — | 745 | — |
Chemical formula of synthesized compounds.
FTIR spectra of synthesized epoxide compounds.
The end groups (epoxy, hydroxide, glycol, chlorine, and bisphenol-A) were determined by classical chemical methods [
Epoxide numbers.
Modifier | Epoxide number (%) |
---|---|
EMP | 13.48 |
PhEP | 8.43 |
CEH | 7.30 |
FAGE | 10.50 |
Most of the mechanical properties of the ERs are closely related to the chemical composition of the material. The liquid aliphatic polyamines such as polyethylene polyamines (PEPAs) are among the first curing agents used with epoxies. Curing of epoxy matrices with these agents can be successfully carried out even at room temperature. Compared to aliphatic amines, cycloaliphatic amines produce cured resins having improved thermal resistance and toughness. They can provide an excellent balance of the properties such as fast curing, low viscosity, low toxicity, good adhesion to damp concrete, and excellent colour stability. However, they are more expensive than other types of curing agents [
Almost all ERs were converted into solid, infusible, and insoluble three-dimensional thermoset networks by the curing process with the use of appropriate cross-linkers. Optimum performance properties can be obtained by cross-linking using the appropriate ERs and cross-linkers, often called hardeners or curing agents. Besides influencing the viscosity and reactivity of the formulation, curing agents also determine the types of chemical bonds formed and the degree of cross-linking that will occur. These affect the chemical resistance, electrical properties, mechanical properties, and heat resistance of the cured thermosets [
Curing degrees of ER and M-ERs were determined by FTIR spectra. Epoxy group showed typical bands at the regions of 830, 913, and 1250 cm−1 [
As seen from Figures
FTIR spectra of uncured (a) and cured (b) ER.
FTIR spectra of cured M-ERs.
Table
Curing degrees of the neat ER and M-ERs with 30 wt.% of modifiers.
Sample | Curing degree (%) |
---|---|
Neat ER | 91.70 |
Modified ER with EMP | 99.20 |
Modified ER with PhEP | 92.89 |
Modified ER with CEH | 92.85 |
Modified ER with FAGE | 93.11 |
Atomic force microscopy is one of the most important microscopic techniques and used in the analyses of the surfaces of polymers at nanometer scale. The additional advantage of using AFM is that it can give distinguished surface topography and surface heterogeneity. The representative 3D surface morphologies of ER and M-ERs are given in Figures
AFM image of neat ER.
AFM images of M-ERs with (a) EMP, (b) PhEP, (c) CEH, and (d) FAGE.
The strength and toughness of ERs below the glass transition temperature (
The results on the tensile strength and hardness are given in Table
Effect of modifier type and amount on mechanical properties.
Modifier (wt.%) | Elongation at break (%) | Tensile strength (MPa) |
|
Hardness (Shore D) |
---|---|---|---|---|
For neat ER | ||||
— | 0.487 | 41.75 | 5.59 | 75.5 |
|
||||
For M-ER with EMP | ||||
10 | 0.542 | 59 | 6.50 | 83.9 |
20 | 0.545 | 58 | 5.53 | 81.0 |
30 | 0.556 | 56 | 4.97 | 81.5 |
40 | 0.559 | 56 | 4.80 | 73.0 |
|
||||
For M-ER with PhEP | ||||
10 | 0.351 | 48 | 5.07 | 81.5 |
20 | 0.445 | 44 | 5.33 | 80.7 |
30 | 0.451 | 47 | 6.04 | 80.3 |
40 | 0.464 | 30 | 4.07 | 77.2 |
|
||||
For M-ER with CEH | ||||
10 | 0.477 | 48 | 5.52 | 86.6 |
20 | 0.522 | 56 | 5.93 | 86.7 |
30 | 0.527 | 55 | 4.23 | 86.2 |
40 | 0.541 | 53 | 4.83 | 86.1 |
|
||||
For M-ER with FAGE | ||||
10 | 0.532 | 51 | 4.37 | 79.7 |
20 | 0.540 | 49 | 4.93 | 80.2 |
30 | 0.527 | 50 | 5.73 | 78.2 |
40 | 0.536 | 39 | 4.83 | 72 |
Tensile behaviour of neat ER and M-ERs.
Tensile strengths of M-ERs modified up to 30% were observed to be higher than that of ER. The highest tensile strength was achieved with ER-EMP (with 10% modifier content), which has a carbonyl group. The second highest was obtained with ER-FAGE that has alkene groups and the highest number of carbon atoms. In our previous study, we reported that all epoxy blends with various polymer types show higher values of tensile strength and elongation at break than pure epoxy matrix due to the polar side-chain groups in the polymers used. Moreover, extent of these properties increased with the increase in the amounts of the polymers. Best results were obtained with oligo(ethers-esters) [
Brittle materials such as ceramic do not undergo plastic deformation and break under low tension [
Unlike metals, there is no correlation between hardness and tensile strength for elastomers. The tensile strength of an elastomer may increase or decrease as hardness value increases depending on the nature of the constituent components [
In many materials, an applied stress (up to a certain limit) is directly proportional to a resulting strain, and a graph relating these two quantities yields a straight line. The linear-elastic region is below the yield point. If a yield point is not easily identified on the stress-strain plot, it is defined to be 0–0.2% of strain and is defined as the region of strain in which no yielding (permanent deformation) occurs [
Thermal analysis is an important analytical method to understand the structure-property relationships and thermal stability of the polymers. The nature of the substituents plays an important role in thermal stability. Polystyrenes (PS) modified with various aldehydes were reported to be more stable to thermal destruction than unmodified PS, depending on structure of active polyfunctional groups bound to the aromatic ring [
In the present study, effect of the structure of modifying agent on the thermal properties of ER was investigated by using TGA analysis in the temperature range of 50–800°C and the results are given in Tables
TGA data of neat and M-ERs.
Modifier |
IDT |
SDT |
|
|
|
---|---|---|---|---|---|
Neat ER | |||||
— | 100 | 250 | 173 | 262 | 358 |
|
|||||
M-ER | |||||
EMP | 100 | 261 | 195 | 270 | 376 |
PhEP | 125 | 316 | 200 | 271 | 383 |
CEH | 95 | 210 | 145 | 230 | 360 |
FAGE | 100 | 241 | 166 | 260 | 364 |
Effect of modifier type on thermal properties of M-ERs.
Modifier (30 wt.%) | Loss of weight (%) | Residual weight (%) | ||||||
---|---|---|---|---|---|---|---|---|
200°C | 300°C | 350°C | 400°C | 450°C | FSD |
SSD |
800°C | |
Neat ER | ||||||||
— | 7.5 | 15 | 33 | 67 | 77 | 5.26 | 79.94 | 9 |
|
||||||||
Modified ERs | ||||||||
EMP | 6 | 12 | 34 | 64 | 73 | 7.78 | 79.03 | 14 |
PhEP | 5 | 11.5 | 27 | 55 | 70 | 9.16 | 78.07 | 17.5 |
CEH | 8 | 22 | 45 | 69 | 81.5 | 7.18 | 82.26 | 15 |
FAGE | 5.2 | 17.5 | 41 | 66.5 | 77.5 | 4.59 | 82.10 | 12 |
TGA curves of neat ER
The residual char percentages of 12%, 14%, 15%, and 17.5% were observed in the thermal decomposition (
The LOI values calculated using (
The versatile properties of ERs make them valuable, particularly in human daily life and specific applications. In particular, industrial maintenance coating applications constitute the largest epoxy coating market globally. The main function of these coatings in applications is the protection of the metal and concrete structures from aggressive environmental conditions and for their long term use. Resins based on bisphenol-A and bisphenol-F epoxies are commonly used. Likewise aliphatic polyamines, aliphatic and aromatic amines, ketimines, phenalkamines, amidoamine, and polyamide resins can be used as curing agents for ER. Special modifiers and curing agents should be used to produce specific properties [
The water sorption properties of ER and M-ERs (with 30% (wt.) modifier content) were tested in deionized water for 30 days at room temperature. Preliminary studies showed that this is a sufficient time for reaching the equilibrium for all types of M-ERs studied. In addition, the effect of temperature on water sorption was investigated at 45°C. Figure
Water sorption of neat ER and M-ERs: (a) at room temperature; (b) at 45°C; (c) desorption at 45°C.
The equilibrium water content of ER was measured as 0.81%, which is consistent with the reports in the literature [
The order of the water sorption percentages of M-ERs having different functional groups was ER-EMP > ER-CEH > ER-FAGE > ER-PhEP. This order is also in agreement with the tensile strength results. At the end of the 30 days, the highest water sorption was observed to be with the M-ER having ketone group (ER-EMP) as 4.22%. On the other hand, the most promising result was achieved with ER-PhEP as its water content was 0.83% which is almost equal to that of ER (Figure
To understand and improve the performance of ER at harsh environmental conditions, it is essential to investigate the influence of temperature on the water equilibrium over 20–25°C. As seen from Figure
Water desorption properties were also examined at 45°C. As seen from Figure
The adhesion and chemical resistance properties of bisphenol-A type ERs are well known [
Adhesion and corrosion properties of M-ERs.
Modifier | Adhesion (%) | Corrosion resistance | |||
---|---|---|---|---|---|
5% NaCl | 5% NaOH | 5% HCl | Air | ||
EMP | 100 | ++ | ++ | ++ | ++ |
PhEP | 100 | ++ | ++ | ++ | ++ |
CEH | 100 | ++ | +− | +− | ++ |
FAGE | 100 | ++ | +− | +− | ++ |
++: high resistance, +−: medium resistance.
Polarizing microscope images (mag. 2.52x) of M-ERs with (a) EMP, (b) PhEP, (c) CEH, and (d) FAGE.
Corrosion properties were examined by keeping the samples in 5% solutions of NaCl, NaOH, and HCl for 15 days (open air). After the completion of soaking, physical appearances of the M-ERs were tested and no change was observed in the samples. Similarly, coatings displayed a considerable resistance to salt water. Liu et al. demonstrated that increasing the cross-linking density may improve the corrosion resistance of coatings [
In the present study, ER was modified using four different epoxide compounds to improve mechanical, thermal, and coating properties. It was observed that tensile strengths of the M-ERs with 30% (wt.) modifier content were higher than that of ER. Similarly, hardness values of the M-ERs were also higher compared to the neat ER. The M-ERs displayed broad stress-strain regions with respect to the ER, which means that the modification enhanced the plasticity of neat ER. Thermal stability of ER was also improved with the modifiers PhEP and EMP, while CEH and FAGE had negligible effects on this property. The best water sorption percentage was achieved with ER-PhEP (0.83%), which was almost equal to that obtained with neat ER (0.81%). The water sorption percentage of the other M-ERs varied between 1.34% and 4.22%. Water desorption percentages decreased in the order of ER-EMP ≈ ER-PhEP > ER-FAGE > ER-CEH > ER. The M-ER coatings displayed a considerable resistance to acidic, basic, and salt water. The ER-EMP and ER-PhEP were more resistant to acid and alkaline medium compared to the ER and other M-ERs. As a result, significant enhancements were obtained in the properties of ER with the modification.
The authors declare that there is no conflict of interests regarding the publication of this paper.