Synthesis characterization and application of hexafunctional epoxy resin and comparison against commercial epoxy resin

Current study compares the various analytical results of hexafunctional epoxy resins based on bisphenol-A with conventional epoxy resins. Reaction of bisphenol-A, formaldehyde, and epichlorohydrin produces hexafunctional epoxy resin. The curing properties of commercial epoxy resin and hexafunctional epoxy resin were determined using a variety of hardeners, including diethylenetriamine, triethylenetetramine, phenalkamine, polyamido amines, and polyamides. The epoxy equivalent weight (EEW), hydrolyzable chlorine content, volatile content, Brookfield viscosity, weight average molecular weight, elemental analysis (C, H, N, O analysis), and Fourier transform infrared spectroscopy were used to characterize the hexafunctional resin (FT-IR). Jute and glass reinforced composites were also prepared by using hexafunctional epoxy resins and commercial epoxy resins. Mechanical properties (tensile strength, flexural strength, Izod impact strength, and Rockwell hardness), thermal properties, and chemical resistance were determined for each composite. Hexafunctional epoxy resin-based composites exhibited superior mechanical characteristics, thermal resistance, and chemical resistance properties than commercial epoxy resin-based composites.

(Tg) varied with functionality, whereas the strain energy release rate and the stress intensity factor varied insignificantly with functionality. 10 Epoxy-based fiber composites have become more common in automobile, electronic devices, and construction and aerospace industries. This is attributed to the attractive mechanical properties, dimensional stability, and corrosion resistance of the composites. [18][19][20][21] Recently, epoxy-based hybrid composites have been extensively used in many engineering and industrial applications. Load bearing engineering applications, superior adhesive properties, and mechanical strength are the key features of the composite material. In comparison with the synthetic fiber composites, natural fibers are characterized by their attractive price, low density, and lower abrasion. The energy consumption needed for production of synthetic fibers is much more than that needed for a similar quantity of natural fibers. 22 The synthetic fibers and natural fibers have a wide variation in diameter and length, which in turn affects the composite expected mechanical behavior. The variation in dimensions is contributed to fiber type, fiber maturity, harvesting time, and processing methods adopted for the extraction of fibers, which affect the diameter and stability of the fiber. Source, age, separating techniques, moisture content, and the history of fiber also play an important role in the filament and individual fiber properties. The implementation of natural fibers in composites is attractive for different industrial sectors like automobiles and construction. 23,24 The main aim of the present work is to synthesize bisphenol-A-based hexafunctional epoxy resin and determine different properties of resin such as epoxy equivalent weight, hydrolyzable chlorine content, elemental analysis, viscosity, rise in viscosity, volatile content, FT-IR, and weight average molecular weight. Curing kinetics of resin was analyzed by various hardeners. Mechanical, chemical, and thermal properties of jute and glass fiber reinforced composites were also studied. These various properties of resin and composites are compared with the commercial epoxy resin.

Raw materials
Solvents and chemicals used were of laboratory grade and purified prior to their use. Bisphenol-A and epichlorohydrine purchased from Sigma-Aldrich. Formaldehyde, diethylenetriamine, triethylenetetramine, methyl ethyl ketone, perchloric acid, acetic acid, tetraethylene ammonium bromide, and sodium hydroxide were purchased from S. D. Fine-Chem Ltd. Aromatic hardeners phenalkamine, polyamido amines, and polyamides were obtained from Admark Polycoats Pvt. Ltd, Vadodara. Carbon fiber was purchased from Composites Tomorrow, Vadodara, and commercial epoxy resin supplied by Atul Ltd, Atul, India.

Synthesis of hexafunctional epoxy resin based on bisphenol-A
Two liter three-neck flask equipped with a mechanical stirrer and condenser was placed into a thermostat heating unit. To this flask 1.0 mol of bisphenol-A and 4.0 mol of formaldehyde were added in the presence of alkaline medium. Stirring is maintained for 1.5 h at 55°C temperature. 3.0 mol of epichlorohydrin was added in a reaction flask and 65 mL (of 35% based on weight of bisphenol-A) of aqueous NaOH solution was added drop wise. After completion of addition, temperature was increased to 80°C and stirred reaction mixture for 3 h than 50 mL methyl ethyl ketone which was added as a solvent to maintain viscosity of resin. The resin first undergoes vacuum distillation for the removal of salt which is formed during the reaction and then distilled at 118°C temperature for the removal of excess water, methyl ethyl ketone, and epichlorohydrin. The resin that was cooled at room temperature has dark yellow color with no specific odor. Probable reaction is given in Figure 1.

Fabrication of jute and glass fiber-based composites
Bisphenol-A based hexafunctional resin-glass fiber-diethylenetriamine (BGD) composite as fabricated under: Fiber reinforced composite was prepared by using resin to fabric ratio of 60:40. Diethylenetriamine (DETA) is used as hardener and taken according to its amine hydrogen equivalent weight (AHEW). Required quantity of resin and DETA was mixed well and applied on 16 sheets of glass fiber with a size of 15 cm × 12 cm by hand lay-up technique. All sheets were stacked one over another in between two Teflon sheets. All sheets were kept between two plates of compression molding machine at 80°C temperature for 90 min for curing and finally 50 psi pressure was applied for 2 min. Prepared composite was cooled at room temperature and removed from two plates of compression molding machine. After removal of Teflon sheet, all dimensions of composite sheet were measured.

Epoxy equivalent weight (ASTM D 1652)
Accurate weight of the resin dissolved in tetraethylene ammonium bromide solution which was titrated against 0.5 N perchloric acid solution. Metrohm Autotitrator was used to carry out titration.

Viscosity measurement (ASTM D 789)
Brookfield viscometer model number RV digital viscometer was used to determine viscosity of the resin at 25°C temperature.

Volatile content (ASTM D 1259)
Accurate weight of the resin was poured in petri dish which was kept in an oven at 110°C temperature for 30°min. Volatile content was determined by percentage change in initial and final weight.

Hydrolyzable chlorine content (ASTM D 1726)
Two gram of the resin was dissolved in a mixture of 15 mL toluene and 25 mL alcoholic KOH solution. The reaction mixture was refluxed for 20 min at 300°C temperature. Blank reading was taken without the sample. Blank and sample were titrated against 0.5 N HCl solution. Metrohm Autotitrator was used to carry out titration.

Gel permeation chromatograph
The weight average molecular weight, ðMwÞ, of the resin was measured on Turbo Matrix-40 of Perkin-Elmer, USA.

Fourier transform infrared spectroscopy
Spectrum GX (Perkin-Elmer, USA) spectrophotometer is used to carried out FT-IR. The range of FT-IR is 10,000 to 370 cm À1 in presence of KBr pellet. The scanning speed of FT-IR was 0.2 cm/s and signal average over 20 scans was applied to measure the absorbance spectra.

C, H, N, O analysis (ASTM D 5373)
Carbon, hydrogen, and oxygen elements of bisphenol-A-based hexafunctional epoxy resin were measured using 2400 series II, Perkin-Elmer, USA.

Tensile strength (ASTM D 638)
Tensile strength measures material's ability to withstand the forces that tend to pull it apart and determines to what extent material stretches before breaking. Test was performed on UTM (universal testing machine) Shimadzu AG 100 at 100% strain rate and crosshead speed of 50 mm/min were maintained in test.

Flexural strength (ASTM D 790)
Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis. The stresses induced by the flexural load are a combination of compressive and tensile stresses. Flexural properties are reported and calculated in terms of the maximum stress and strain that occur at the outside surface of the test bar. Test was performed on UTM Shimadzu AG 100, at a crosshead speed of 1.2 mm/min.

Izod impact strength (ASTM D 256)
Izod impact test results are expressed in terms of kinetic energy consumed by the pendulum in order to break the specimen. The specimen used for the test was notched. Izod impact strength was measured on CEAST Izod tester.

Rockwell hardness (ASTM D 785)
The information from a hardness test can be used to provide critical material performance information and insight into the durability, strength, flexibility, and capabilities of a variety of component types from raw materials to the prepared specimens and finished goods. Digital Rockwell hardness tester with HRL indenter was used to measure the hardness value of the test specimen.
All mechanical properties were measured at room temperature. Three samples of each test were analyzed, and the average of the results was taken into account.

Thermogravimetric analysis
Resistance against weight loss at various temperatures of all composites was studied on Perkin-Elmer Pyris-1. The composite samples contain 5-10 mg scanned at temperature range of 50-1000°C at a heating rate of 10°C/min in nitrogen environment.

Chemical resistance test (ASTM D 543-87)
The chemical resistance of the composites was studied as per ASTM D 543-87 method. This method covers the chemical resistance of all composites for change in weight, dimensions, appearance, and strength properties by the action of different chemical reagents. Test samples have dimension of 1.5 cm × 1.5 cm immersed in containers in presence of 250 mL chemical reagents such as concentrated sulfuric acid (10% wt/wt), aqueous sodium hydroxide (10% wt/wt), sodium chloride (10% wt/ wt), methanol, and tetrahydrofuran. Water absorption of all composite test samples was studied. All samples were observed after exposure of 7 days in presence of chemical reagent. After 7 days, samples were washed by distilled water and dried by pressing them both sides with filter paper at room temperature. Before and after test cycle, all specimens were weighed in a precision electronic balance, and percentage weight loss/gain was measured. Percentage change in thickness was determined with the use of digital micrometer. In each case, two test specimens were used and their average values were reported as results.

Results and discussion
The epoxy equivalent weight of bisphenol-A-based hexafunctional epoxy resin was 537 gm and weight average molecular weight, ðMwÞ, observed was 3130 gm/mol. The results of epoxy equivalent weight and weight average molecular weight ðMwÞ confirmed functionality of synthesized epoxy resin was 5.83. The volatile content of the bisphenol-A-based hexafunctional epoxy resin was 0.98% while the commercial epoxy resin had 1.9% volatile content which clearly suggests that resin has lower amount of solvent impurity. Viscosity of bisphenol-A-based hexafunctional epoxy resin was found to be 20,625 cP (average of three measurements) determined by Brookfield viscometer while viscosity of commercial epoxy resin was 12,380 cP which was 1.8 times lower than bisphenol-A-based hexafunctional epoxy resin. The rise in viscosity of the resin was measured after putting the resin in an oven at 105°C temperature for 24 h to check the resin was useful for high temperature application. The rise in viscosity of the bisphenol-A based-hexafunctional epoxy resin was 1.2% while the commercial epoxy resin has 6.3% which clearly indicates that the bisphenol-A-based hexafunctional epoxy resin has better advantage than the commercial epoxy resin for high temperature application.
Hydrolyzable chlorine content of the bisphenol-A-based hexafunctional epoxy resin was 1.5% while the commercial epoxy resin has 3.6% which clearly indicates that reactivity of the bisphenol-A-based hexafunctional epoxy resin was higher than the commercial epoxy resin. C, H, N, O analyzer was used to determine percentage of carbon, hydrogen, and oxygen. Results of C, H, N, O gave the conformation of polymerization reaction. Calculated data and obtain data are shown in Table 1.
Gel time and peak exothermic temperature of commercial epoxy resin and bisphenol-A-based hexafunctional epoxy resin in presence of different hardeners are given in Table 2. For the bisphenol-A-based hexafunctional epoxy resin, diethylenetriamine required 54 min to convert the resin into gel form; meanwhile, polyamido amines work six times faster and converted the resin into gel form in 9 min. The commercial epoxy resin converted into gel form by various hardeners was almost four times slower than the bisphenol-A-based hexafunctional epoxy resin. Peak exothermic temperature of the bisphenol-A-based hexafunctional epoxy resin observed 1.5 times higher than the commercial epoxy resin for all hardener systems.
The FT-IR spectrum of the bisphenol-A-based hexafunctional epoxy resin and commercial epoxy resin is shown in Figure 2. In FT-IR spectra, peak at 3410.12 cm À1 is due to the stretching of -OH groups, but percentage transmission of this peak is lower than the commercial epoxy resin which indicates that more -OH groups were used in the reaction. The sharp peak at 2966.24 cm À1 indicated C-H stretching vibration. The peak at 1612.03 cm À1 indicates stretching of C=C of aromatic ring while peak at 1515.05 cm À1 indicates stretching of C-C of aromatic ring. C-O-C stretching peak was observed at 1268.32 cm À1 . The characteristic and strong peak of C-O-C of oxirane ring was observed at 846.23 cm À1 which confirmed the polymerization reaction.
Mechanical properties (tensile strength, flexural strength, Izod impact strength, and Rockwell hardness) data are given in Table 3. Tensile strength of glass reinforced composites ranges from 64.9 to 60.1 MPa. Increasing order of tensile strength of composites was BGPA> BGP> BGPD> BGD> BGT. Polymeric hardeners have higher molecular weight, so it provides additional strength to composites and so BGPA composite has higher tensile strength than BGT composite. Tensile strength of jute fiber-based composites ranges from 38.8 to 34.4 Mpa. Comparison of commercial epoxy resin-based jute and glass reinforced composite tensile strength shows decreases around 25%.
Flexural strength of composites found almost 2 to 2.5 times higher than their tensile strength results. BGPA composite has higher flexural strength at 139.2 MPa while BGT composite has lowest flexural strength at 131.9 MPa. Polymeric hardeners provide strength to composites due to their higher molecular weight, and their adhesion property was superior to aliphatic hardeners, so BGPA has higher flexural strength than BGD and BGT. Flexural strength of jute reinforced composites is 73.2-81.6 MPa. The range of flexural strength of jute and glass fiber composites based on commercial epoxy resin was almost 30% lower than jute and glass fiber reinforced composite based on bisphenol-A-based hexafunctional epoxy resin. Izod impact strength of all the composites was measured to study the resistance of specimen against the sudden impact force applied on it from the specific direction. Aliphatic hardener-based composites have lower Izod impact strength than aromatic hardener-based composites. BGT composite has 22.3% lower Izod impact strength than BGPA composite. Similar results were obtained for jute reinforced composites. Impact strength of all reinforced composites based on the commercial epoxy resin is around 15% lower than bisphenol-A-based hexafunctional epoxy resin-based composites.
Rockwell hardness was analyzed to check rigidity of composites. Increasing order of Rockwell hardness of glass reinforced composites was BGPA> BGP> BGPD> BGD> BGT. BGPA has higher Rockwell hardness which was almost 14% higher than BGT. Increasing order of Rockwell hardness of jute reinforced composites was BJPA> BJP> BJPD> BJD> BJT. Rockwell hardness of jute and glass fiber reinforced composites based on the commercial epoxy resin was 20% lower than composites of the bisphenol-A-based hexafunctional epoxy resin.
All mechanical properties results were taken three times and mean of them is shown in Table 3. All results of mechanical properties were statistically analyzed with data variance and are given in brackets of Table 3.
Chemical resistance of all composites was measured against six different chemical reagents. It was observed that the composites of the bisphenol-A-based hexafunctional epoxy resin had minor gain or loss of weight and therefore no erosion has taken place due to the exposure of chemical reagents. The results of chemical resistance are given in Table 4. All jute and glass fiber reinforced composites of bisphenol-A-based hexafunctional epoxy resin showed excellent chemical resistance and there was minor change in composite surface finishing against six different chemical reagents. In comparison with composites of commercial epoxy resin, weight loss was observed in all chemical reagents.    Thermogravimetric analysis, that is, weight loss as a function of temperature was done in order to understand the thermal stability of the particulate composites. Mass loss curves for all jute and glass fiber-based composites are shown in Figure 3 and Figure 4, respectively. Decomposition rate of the composites associated with various temperature intervals is calculated and shown in Figure 5 and Figure 6. Thermal kinetic parameters like initial system temperature (T 0 ), procedural decomposition temperature (PDT), and activation energy (Ea) were derived from TGA and DTGA curves and are tabulated in Table 5. Activation energy of the particulate composites was measured as per Broido's method. Thermal stability of glass reinforced composites was found in an order of BGPA>BGP>BGPD>BGD>BGT according to their Ea, and for jute reinforced   composites, the order was BJPA>BJP>BJPD>BJD>BJT. Composites of commercial epoxy resin-based composites have lower thermal resistance than composites of bisphenol-A-based hexafunctional epoxy resin.

Conclusion
Hexafunctional epoxy resin based on bisphenol-A that cures in the presence of a variety of curing agents is utilized as a matrix material in jute and glass fiber reinforced composites. An aromatic curing agent has higher capacity of cross-linking than an aliphatic curing agent. The low volatile percentage of resin suggests that it is suitable for applications requiring high temperature tolerance. Because the resin's viscosity is enough, a strong connection between the matrix and fiber interphase is formed. The mechanical properties and thermogravimetric studies of the composite indicate that they are suitable for high performance applications. Chemical resistance also contributes to the matrix-fiber phase bonding. In contrast to commercial epoxy resins, the result findings indicate that the bisphenol-A-based hexafunctional epoxy resins have better all-around characteristics.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.