Comparative studies of composites based on commercial epoxy resin with renewable resources-based tetra functional epoxy resin

Abstract

This research distinguishes itself by synthesizing a novel epoxy resin from three distinct renewable resources, moving beyond the conventional reliance on a single source found in many studies. The innovative epoxy resin was synthesized through a chemical reaction involving cardanol, resorcinol, furfural, and epichlorohydrin. Notably, resorcinol and a component of cashew nutshell liquid (CNSL) cardanol were incorporated as a partial substitute for up to 50% of the phenol in the synthesis process. This multifaceted approach not only enhances sustainability but also demonstrates the potential of eco-friendly materials in developing advanced composites while optimizing the resin performance characteristics. The modified tetrafunctional epoxy resin based composites were prepared from sustainable synthesized bio-based epoxy resin. This study compared the varied characterization data of cardanol-based tetra functional epoxy resin with conventional epoxy resin using Aromatic and aliphatic hardeners such as triethylenetetramine, phenalkamine, low viscous phenalkamine, and polyamide. The adhesive strength and curing properties of a modified epoxy resin with hardeners are examined in this study. Strong adhesive bonding of matrix with wood and metal substrates was demonstrated by lap shear strength (LSS). The Brookfield viscosity, volatile content, epoxy equivalent weight (EEW), hydrolyzable chlorine content, weight average molecular weight, and Fourier transform infrared spectroscopy were used to characterize the tetrafunctional modified epoxy resin. Thermal analysis using differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) showed a cure index and decomposition temperature of resin systems. Chemical resistance, and mechanical properties (Rockwell hardness, Izod impact strength, tensile strength, and flexural strength) were determined for every composite.

Keywords

Renewable resources composites curing agents cashew nutshell liquid epoxy resin

Introduction

The widespread use of epoxy resin in various industries, due to its versatility from exceptional durability, heat resistance, high-bonding strength, and dimensional stability, including aerospace and automotive, adhesive and coating, building materials, composites, and electronics, makes it suitable for a wide range of applications. Epoxy resin nanoparticle reinforced composites enhanced the mechanical, thermal, and adhesive properties for engineering applications.^1–4 However, the rapidly escalating global environmental crisis has cast a critical spotlight on these materials’ sustainability and environmental impact. The need to modify thermoset resins through integrating renewable resources and environmentally sustainable methodologies has become more than ever. The use of toxic chemicals in thermoset resin formulations has impacted both human health and the environment, so growing environmental concerns and rising prices of petrochemical products have encouraged academic and corporate researchers to focus on the synthesis of polymers using renewable resources. The production of chemical raw materials from renewable resources, such as furan-derived polymers and synthetic biofuels, has been researched for a long time.^5–9 In the field of thermoset resins, where phenolic compounds have long held a significant role, the search for sustainable alternatives has yielded interesting possibilities.¹⁰ Among these resources, two specific compounds, furfuraldehyde and cashew nutshell liquid (CNSL), have gained importance for their potential to replace conventional phenolic and aldehyde components in thermoset resins. In this context, cardanol, a significant constituent of CNSL, emerges as a promising alternative to phenol.^11–14

Modified epoxy, with its improved sustainability, can be employed in various industries. Their versatility ensures that sustainable resins can replace traditional ones in a wide range of applications. Reducing the overall environmental impact of these sectors.^15–19 These versatile materials have evolved beyond their traditional uses to meet the ever-growing demand for specialized, high-performance substances that can address the multifaceted requirements of modern industries.^20–23 The necessity for specific chemical functionality in epoxy resins enables customization to meet the particular demands of advanced applications.^24,25 The incorporation of additional functional groups into epoxy resins has the potential to influence their chemical and physical properties significantly.^26–28 This deliberate modification allows for some degree of customization that tailors these resins to meet specific industrial needs. Functional groups can be thoughtfully chosen to impart a range of properties, including enhanced mechanical strength, improved adhesion to various substrates, increased resilience to environmental influences, and unique thermal or electrical properties.^29–31

Epoxy resins can be crosslinked using particular curing agents to provide the appropriate characteristics for many different kinds of applications. Amine-based curing agents, like aliphatic and aromatic amines, are used in coatings adhesives because of their excellent mechanical strength and rapid curing.³² Anhydride curing more slowly, making them appropriate for advanced composites. Furthermore, epoxy frequently contains fillers in the form of nanoparticles like graphene, silica, and carbon nanotubes to enhance properties like hardness, heat conductivity, and abrasion resistance.^33,34 The cured epoxy matrix is substantially reinforced by these nanoparticles, which improves its performance in specific applications.^35,36 The curing of epoxidized resins in a transition from a low molecular weight liquid mixture to a strongly crosslinked network. Beyond the gel point, the process progresses to the production of a single infinite network, with significant increases in cross-link density, glass transition temperature (Tg), and final mechanical properties.³⁷ Farimah Tikhani et al. investigated the curing of epoxy composites utilizing three types of nanoparticles and observed that the glass transition temperature of unmodified epoxy resin is lower than that of the nanoparticle-containing epoxy system, due to the incomplete curing of the blank epoxy network in the absence of highly reactive functional groups.³⁸ The epoxy adhesive was used to make polymer and metal hybrid composites, which were then cured using varied quantities of silica and aluminum fillers. The mechanical properties of adhesive material were evaluated at various cure temperatures in order to determine the best cure temperature. The inclusion of filler improves the adhesive bond and lap shear strength.³⁹ Adhesive bonding has so many advantages over their joining techniques and is utilized in a wide range of industries, including the manufacturing of automobile and aircraft structures, its importance as a technology for combining structure is growing.^40–42 However, the primary disadvantages of epoxy adhesives are their high brittleness and poor crack resistance, which is why epoxy toughening is frequently required by the use of elastomers. Additionally, a variety of opposing factors, such as chemical, mechanical, and thermodynamic adhesion, affect how long an adhesive persists.⁴³ Multifunctional epoxy resin of bisphenol-c formaldehyde and hybrid composites with 4% sodium hydroxide-treated jute fiber were synthesized by Jignesh P. Patel et al. The results of the study demonstrated outstanding hydrolytic stability together with high mechanical and electrical stability.⁴⁴ B. Shivamurthy et al. modified the matrix by blending cashew nut shell liquid (CNSL) and they prepared jute fiber composites to improve mechanical properties.⁴⁵ G. U. Raju et al. investigated and prepared epoxy composites using groundnut shells with fillers in the epoxy matrix to improve the mechanical characteristics and thermal insulation.⁴⁶

Herein, the main aim of this present work is to replace the phenolic as well as aldehyde compounds with sustainable bio-based chemicals and also increase functionality and synthesize cardanol and resorcinol-based tetrafunctional epoxy resin (Figure 1). The synthesized modified epoxy resin determines properties such as the epoxy equivalent weight, hydrolyzable chlorine content, viscosity, rise in viscosity, volatile content, FT-IR, and weight average molecular weight of the resin. Various hardeners were used to study the kinetics of resin curing and adhesive bonding. Jute and glass fiber-reinforced composites were also studied in terms of mechanical, chemical, and thermal characteristics. These resin and composite properties are compared to the commercial epoxy resin.

Figure 1.

Renewable resources based chemicals.

Materials and methods

Raw materials

Resorcinol, furfural, sodium hydroxide, acetone, and tetra ethyl ammonium bromide (TEAB) were purchased from Loba Chemie Pvt. Ltd. Epichlorohydrin, triethylenetetramine (TETA), methyl ethyl ketone, perchloric acid, and acetic acid were purchased from Sigma Aldrich Chemicals Pvt. Ltd. Cardanol, phenalkamine, low viscous phenalkamine and polyamides were obtained from Admark poly coats Pvt. Ltd, Vadodara. Commercial epoxy resin was supplied by Atul Pvt. Ltd. And jute and glass fiber were purchased from Composite Tomorrow, Vadodara.

Method

Synthesis of tetrafunctional epoxy resin based on cardanol and resorcinol

The modified epoxy resin was synthesized in an alkaline medium by using two different phenols. 1 mol of cardanol and 1 mol of resorcinol with 3 mol of furfuraldehyde were added in three neck round bottom flasks equipped with a condenser, heating mental and thermometer, after the complete addition stirred continued for 30 minutes at 40°C. maintain stirring, along with a 35% aqueous NaOH solution added dropwise. After the addition, the temperature was raised to 80°C, and the reaction mixture was stirred for 2 hours then cardanol and resorcinol-based phenolic resin modified by epoxidation using 4 mol epichlorohydrin while stirring with NaOH solution was added dropwise, after the addition, the reaction mixture was stirred 2 hours at 80°C before add 30 mL of methyl ethyl ketone solvent to keep the resin from becoming too viscous. The excess impurities, solvents, and byproducts were removed using vacuum distillation. The resin has a dark brown color and no odor after being cooled at room temperature. Figure 2 shows the probable reaction.

Figure 2.

Cardanol and resorcinol based tetrafunctional epoxy resin.

Cross-linking process of synthesised resin

To examine the distinct effects of four commonly used hardeners (triethylenetetramine, phenalkamine, low viscous phenalkamine, and polyamide) on curing behavior and material performance, the curing behavior of a tetrafunctional epoxy resin was studied. Each hardener was chosen based on its efficacy in epoxy systems. The exact epoxy to hardener ratio was determined based on the stoichiometric requirements for optimum crosslinking. The epoxy equivalent weight (EEW) of the epoxy and the hardener amine hydrogen equivalent weight (AHEW) can be used to calculate the appropriate hardener ratio required for a tetrafunctional epoxy resin. The curing process can be affected by an improper resin to hardener ratio, which can lead to insufficient or excessive cross-linking. Epoxy with too little hardener becomes tacky and sticky, decreasing its strength and ability to withstand chemicals and heat. However, too much hardener might result in a rigid, brittle structure that breaks easily.

The following formula has been employed for calculating the amount of hardener $P H R = 100 \times A H E W / E E W$ .⁴⁷

Preparation of fabricating reinforced composite of jute and glass fiber

A cardanol and resorcinol-based epoxy resin-jute fiber-triethylenetetramine (CRJT) composite was prepared as follows:

A resin-to-fabric ratio of 70:30 was used with a triethylenetetramine hardener that is measured by its amine hydrogen equivalent weight (AHEW). Resin and hardener mixture were applied alternately on 16 sheets of jute fiber of sizes 15 × 12 cm by hand layup technique. All sheets were layered between two Teflon sheets. These assembled sheets were then placed between plates of the compression machine for 2 hours of curing. After this curing period, a pressure of 50 psi was applied to the assembled sheets for 2 minutes (Figure 3).

Figure 3.

Preparation of fiber reinforced composites.

Afterward, the sheets were removed from the compression molding machine and allowed to cool to ambient temperature. All were tested for analytical and mechanical properties after the removal of the Teflon sheet. Composites of jute fiber with other hardeners were similarly prepared. Namely, cardanol and resorcinol based epoxy resin-jute fiber-phenalkamine (CRJP), cardanol and resorcinol based epoxy resin-jute fiber-low viscous phenalkamine (CRJPL), cardanol and resorcinol based epoxy resin-jute fiber-polyamides (CRJPA).

All glass fiber-reinforced composites were made using same technique as was previously mentioned, with the resin-to-fiber ratio being 60:40. Glass fiber-reinforced composites are cardanol and resorcinol-based epoxy resin-glass fiber-triethylenetetramine (CRGT), cardanol and resorcinol-based epoxy resin-glass fiber-phenalkamine (CRGP), cardanol and resorcinol-based epoxy resin-glass fiber-low viscous phenalkamine (CRGPL), cardanol and resorcinol-based epoxy resin-glass fiber-polyamides (CRGPA). Likewise, reinforced composites made of glass fiber and jute, using commercial epoxy resin, namely, epoxy resin-glass fiber-triethylenetetramine (EGT), epoxy resin-glass fiber-phenalkamine (EGP), epoxy resin-glass fiber-low viscous phenalkamine (EGPL), epoxy resin-glass fiber-poly amides (EGPA), epoxy resin-jute fiber-triethylenetetramine (EJT), epoxy resin-jute fiber-phenalkamine (EJP), epoxy resin-jute fiber-low viscous phenalkamine (EJPL), epoxy resin-jute fiber-poly amides (EJPA).

Preparation of substrate for adhesive study

The preparation of the adhesive sample for shear strength is divided into two parts. The first part is the prepared adhesive of synthesized renewable resources based epoxy resin and commercial epoxy resin with different hardeners according to the required stoichiometric ratio. Synthesized epoxy resin and 2 mL methyl ethyl ketone solvent to maintain viscosity were added in a beaker, and the mixture was stirred for 2 minutes. Then add the required hardener and stir at 300 rpm for 10 minutes. Adhesive matrix namely, resorcinol and cardanol based modified epoxy resin with phenalkamine hardener (RP), with low viscous phenalkamine hardener (RPL), with polyamide (RPA), with triethylenetetramine (RT). As well as in commercial epoxy resin with phenalkamine hardener (CP), with low viscous phenalkamine (CPL), with polyamide (CPA), and with triethylenetetramine (CT) (Figure 4).

Figure 4.

Dimension of the prepared sample.

In the second part of the adhesive sample preparation shown in Figure 5, modified epoxy resin was used as a matrix material. Wood and aluminum panels were assembled according to ASTM specifications for adhesive testing. The wood and metal panels have dimensions of 101 mm in length and 25.4 mm in width. Apply an adhesive resin layer to overlap two panels with a cross-sectional area of 12.5 × 25.4. Prepared panels are clamped for 24 hours to ensure adhesion and then characterized mechanical properties. Before applying the adhesive layer to the metal panels, the surface of the panels was smoothed by 60 grit abrasive paper and cleaned with acetone to remove any polluted particles on the metal surface (Table 1).⁴⁸

Figure 5.

Prepared samples for adhesive study: Metal to metal, metal to wood, and wood to wood.

Table 1.

Dimension of adhesive matrix layer.

Hardener	Substrates	Dimension of synthesized epoxy adhesive layers (mm)		Dimension of commercial epoxy adhesive layers (mm)
Hardener	Substrates	Overlap length (mm)	Thickness of adhesive layers (mm)	Overlap length (mm)	Thickness of adhesive layers (mm)
Phenalkamine	Wood to wood	12.62 (±0.08)	0.36	12.73 (±0.03)	0.38
	Wood to metal	13.20 (±0.5)	0.32	12.66 (±0.04)	0.33
	Metal to metal	12.56 (±0.14)	0.41	12.64 (±0.06)	0.35
Low viscous Phenalkamine	Wood to wood	13.42 (±0.72)	0.44	12.71 (±0.01)	0.35
	Wood to metal	12.57 (±0.13)	0.54	12.75 (±0.05)	0.37
	Metal to metal	12.38 (±0.32)	0.49	12.59 (±0.11)	0.41
Polyamide	Wood to wood	12.60 (±0.1)	0.42	12.68 (±0.02)	0.43
	Wood to metal	12.65 (±0.05)	0.48	12.68 (±0.02)	0.36
	Metal to metal	12.57 (±0.13)	0.37	12.73 (±0.03)	0.40
TETA	Wood to wood	12.51 (±0.19)	0.33	12.64 (±0.06)	0.36
	Wood to metal	12.54 (±0.16)	0.35	12.66 (±0.04)	0.32
	Metal to metal	12.48 (±0.22)	0.33	12.70 (±0.00)	0.35

Characterization

Epoxy equivalent weight (ASTM D 1652)

The resin was accurately weighed and dissolved in tetraethylammonium bromide (TEAB) solution which reacts with a known excess of a strong acid. The unreacted acid was titrated with the standardized base solution. A Mettler toledo auto titrator was used to perform the titration.

Viscosity measurement (ASTM D 789)

The viscosity of the resin at 25°C was determined using a Brookfield viscometer model number RV digital viscometer.

Volatile content (ASTM D 1259)

The resin was accurately weighed and deposited into a petri dish, which was placed in a 110°C–130°C oven for 30 minutes. The percentage change in the initial and final weight was used to calculate the volatile content.

Gel time (ASTM D 2471-99)

Gel time and peak exothermic temperature of cardanol-based modified epoxy resin and commercial epoxy resin measured by Brookfield DV2 T instrument.

Hydolyzable chlorine content (ASTM D 1726)

A mixture of 15 mL toluene and 25 mL alcoholic KOH solution was used to dissolve two grams of the resin. At 290°C–300°C, the reaction mixture was refluxed for 20 minutes. A blank reading was obtained without a sample. Titration of the blank and sample was performed against a 0.5 N HCL solution, titration using a Mettler toledo auto titrator.

Gel permeation chromatography (GPC)

The weight average molecular weight (M $\bar{w}$ ) of the resin was measured on turbo matrix-40 of PerkinElmer U.S.A (Figure 6).

Figure 6.

GPC analysis of cardanol and resorcinol based modified epoxy resin.

Fourier transform infrared spectroscopy (FT-IR)

Spectrum GX (Perkin Elmer, U.S.A) spectrometer was used to carry out FT-IR. In the presence of a KBr pellet, the FT-IR range is 10,000 to 370 cm⁻¹. The FT-IR scanning speed was 0.2 cm/s, and the absorbance spectra were measured using a signal average over 20 scans.

Tensile strength (ASTM D 638)

Tensile strength is a measure of a polymer’s capacity to endure pulling stress. It is commonly determined by pulling a dumbbell-shaped specimen. The maximum value of tensile stress that a material can stand before under a constant load the test was carried out on a Shimadzu AG 100 UTM (universal testing machine) at 100 % strain rate with a crosshead speed of 50 mm/min maintained throughout.

Flexural strength (ASTM D 790)

The flexural strength of a material is a measure of its resistance to bonding or stiffness. The entire force is delivered in one direction during flexural testing three-point loading is produced by loading the nose is pushed onto the specimen at a steady pace of 2 mm/min on a conventional testing machine. The maximum stress and strain that occur at the test bar were reported and calculated. The experiment was carried out on a UTM Shimadzu AG 100.

Izod impact strength (ASTM D 256)

The impact energy absorbed in breaking a notched specimen is given as the original cross-sectional area of the specimen at the notch when the pendulum impacts the face containing the notch on the outside surface of the test bar. The CEAST Izod tester was used for the test.

Rockwell hardness (ASTM D 785)

The Rockwell test for polymer composites Using alternative indenter shapes and lower loads than metals. Due to the unique properties of polymer composites adjustments are required when using them. The test measures the depth of indentation made by an indenter under-regulated tension to assess hardness. The test specimen’s hardness value was determined using a digital Rockwell hardness tester and an HRL indenter.

Thermogravimetric analysis

All composite’s resistance to weight loss at different temperatures was studied using the Perkin Elmer pyris-1. The 5–10 mg composite samples were scanned in a nitrogen temperature range of 50–1000°C with a heating rate of 10°C/min.

Chemical resistance test

The ASTM D 543-87 technique was used to study this technique covers the resistance of all composites to chemical reagents that alter their weight, size, and strength characteristics. Chemical reagents, such as methanol, tetra hydro furan, concentrated sulfuric acid (10% wt/wt), aqueous sodium hydroxide (10% wt/wt), and sodium chloride (10% wt/wt) were used in the chemical resistance test. There was also testing carried out on the composite’s water absorption. All test samples with dimensions of 1.5 × 1.5 cm were immersed in containers containing 250 mL chemical reagents for 7 days. After 7 days, the samples were rinsed with distilled water and dried at room temperature. All specimens were weighed in an accurate electronic balance before and after the test cycle, and the percentage weight loss/gain was calculated.

Differential scanning calorimeter (DSC)

Thermal study of neat epoxy and modified epoxy resin cured with different hardeners using differential scanning calorimeter (DSC-8000, Perkin Elmer, USA). The samples weighed 5 to 8 mg were heated in a sample holder presence of a nitrogen atmosphere between 32°C to 450°C for a 30°C/min heating rate.

Lap shear strength

Adhesive study of modified tetra functional epoxy resin and commercial epoxy resin using wood and metal substrate characterized by ASTM D1002 method. The Shimadzu AG 100 universal testing machine (UTM) was used for lap shear strength analysis. In the test using a crosshead speed of 50 mm/min and a strain rate of 100%. A polymer lap shear strength determines its adhesive bond ability to resist pulling stress.

Thickness of adhesive layer

The thickness of adhesive layers on wood and metal substrate is characterized using magnetic pull of gage according to the ASTM D7091 method. The adhesive layer thickness was measured at different points on the adhesive substrate.

Result and discussion

The cardanol and resorcinol-based modified epoxy resin was dark brown in appearance and the viscosity of the cardanol-modified epoxy resin was found to be 16,167 cP (mean of three measurements) using a Brookfield viscometer in comparison, the viscosity of the commercial epoxy resin was 12,685 cP which is 1.3 times lower. The rise in viscosity of the resin is checked to see whether it could be used at a high temperature. The rise in the viscosity of cardanol and resorcinol-based modified epoxy resin is tested after 24 hours in an oven at 105°C. The viscosity of cardanol and resorcinol-based modified epoxy resin raise by 2.3% whereas commercial epoxy resin raised by 6.1%, These properties make the synthesized cardanol and resorcinol based modified epoxy resin superior to commercial epoxy resin and particularly valuable for high-performance applications requiring durability and stability. The resin’s volatile content clearly shows that it has less solvent impurity than commercial epoxy resin. The volatile content of cardanol and resorcinol-based modified epoxy resin was 1.5% and commercial epoxy resin was 2.0%.

The synthesized cardanol and resorcinol-based modified epoxy resin functionality was 3.95 which was confirmed by the weight average molecular weight (M $\bar{w}$ ) and an epoxy equivalent weight results the cardanol and resorcinol based modified epoxy resin had an epoxy equivalent weight of 612 g/eq and an average molecular weight (M $\bar{w}$ ) of 2420 g/mole.

Peak exothermic temperature and gel time of cardanol and resorcinol-based modified epoxy resin and commercial epoxy resin checked in the presence of different hardeners were observed. The gel time and peak exothermic temperature of the cardanol and resorcinol-based modified epoxy resin were better than commercial epoxy resin shown in Table 2. For all hardeners systems the peak exothermic temperature of cardanol and resorcinol based modified epoxy resin was observed to be 1.3 times higher than commercial epoxy resin. Higher peak exothermic temperature shows that synthesized epoxy resin needs more energy to accomplish full crosslinking at this elevated temperature, resulting in a denser, stronger network structure.⁴⁹ A more stable and controlled crosslinking process is shown by higher exothermic peak temperature during the curing of the synthesized epoxy resin compared to commercial epoxy.

Table 2.

Gel time and peak exothermic temperature of cardanol and resorcinol based modified epoxy resin and commercial epoxy resin with different curing agents.

Hardeners name	Cardanol and resorcinol-based tetrafunctional epoxy resin		Commercial epoxy resin
Hardeners name	Peak exothermic temperature (°C)	Gel time (minutes)	Peak exothermic temperature (°C)	Gel time (minutes)
Triethylenetetramine	229	61	168	224.13
Phenalkamine	252	13.28	179	72.06
Phenalkamine low viscous	256	18.04	186	63.15
Polyamides	242	54.23	177	187.26

Cardanol and resorcinol-based modified epoxy resin has a hydrolyzable chlorine content of 0.74%, whereas commercial epoxy resin has a content of 0.44%. This shows that the reactivity of the synthesized tetrafunctional epoxy resin was comparable to that of commercial epoxy resin.

Figure 7 show the FT-IR spectra of cardanol and resorcinol-based modified epoxy resin and a commercial epoxy resin. The stretching of -OH groups causes a peak in the FT-IR spectra at 3402.58 cm⁻¹; however, the percentage transmission of this peak is lower than in commercial epoxy resin, indicating that more -OH groups have been consumed in the reaction. C-H stretching vibration is shown by the sharp peak at 2936.15 cm⁻¹. The aromatic ring C = C stretch is indicated by the peak at 1667.69 cm⁻¹, while the peak indicates the aromatic C-C at 1567.61 cm⁻¹. The polymerization reaction was confirmed by the strong peak C-O-C of the oxirane ring at 839.83 cm⁻¹.

Figure 7.

FT-IR of (A) commercial epoxy resin (B) cardanol and resorcinol based modified epoxy resin.

Mechanical and chemical properties of the glass and jute fiber reinforced composites

Tensile properties

The tensile strength of all cardanol and resorcinol-based epoxy resin glass fiber composites and commercial epoxy resin glass fiber composites are shown in Table 3. CRGP composite has higher tensile strength than CRGPL, CRGPA, and CRGT. Glass-reinforced composites have tensile strength between 64.3 MPa and 57.8 MPa jute fiber-based composites have strength between 39.2 MPa and 33.1 MPa. Tensile strength losses of around 28% are observed when compound to glass and jute-reinforced composites made of commercial epoxy resin.

Table 3.

Mechanical properties of jute and glass fiber composites based on cardanol and resorcinol based epoxy resin and commercial epoxy resin.

Sr No.	Sample code	Rockwell hardness	Impact strength (J/cm)	Tensile strength (MPa)	Flexural strength (MPa)
1	CRGT	47.5 (0.9)	3.56 (0.4)	57.8 (1.2)	142.2 (2.1)
2	CRGP	56.0 (1.1)	5.03 (0.5)	64.3 (1.4)	149.0 (2.1)
3	CRGPL	53.1 (1.0)	4.99 (0.4)	62.3 (1.3)	146.07 (1.9)
4	CRGPA	49.3 (0.9)	4.37 (0.4)	61.9 (1.5)	144.1 (1.7)
5	CRJT	29.7 (0.9)	2.77 (0.3)	33.1 (1.2)	74.03 (1.5)
6	CRJP	34.1 (1.2)	3.89 (0.3)	39.2 (1.3)	83.3 (1.8)
7	CRJPL	32.8 (1.3)	2.97 (0.2)	37.4 (1.3)	81.0 (1.5)
8	CRJPA	30.6 (1.4)	3.11 (0.2)	35.3 (1.2)	76.7 (1.7)
9	EGT	27.1 (0.9)	3.24 (0.3)	43.1 (1.2)	97.5 (1.8)
10	EGP	33.5 (1.2)	4.09 (0.4)	47.4 (1.3)	102.3 (1.9)
11	EGPL	29.9 (1.0)	3.67 (0.3)	45.7 (1.3)	100.1 (2.0)
12	EGPA	28.4 (0.9)	3.42 (0.3)	46.8 (1.5)	99.0 (1.9)
13	EJT	16.6 (0.9)	1.92 (0.3)	24.5 (1.3)	50.3 (1.8)
14	EJP	24.7 (1.3)	2.27 (0.4)	28.4 (1.4)	57 02 (1.7)
15	EJPL	21.1 (1.2)	2.19 (0.4)	26.3 (1.4)	54.7 (1.7)
16	EJPA	20.9 (0.9)	2.08 (0.3)	27.0 (1.3)	53.6 (1.6)

Three-point flexural properties

The flexural strength of composites was observed to be nearly 2 to 2.5 times greater than their tensile strength. A chain of amines in a CNSL-based phenalkamine hardeners provides high mechanical characteristics to the resin by enhancing crosslinking density. So CRGP composite has a higher flexural strength of 149.0 MPa whereas CRGT composite has the lowest flexural strength of 102.3 MPa. Jute fiber reinforced composites had a flexural strength of 57.02 – 83.3 MPa. Flexural strength ranges of jute and glass fiber reinforced composites based on commercial epoxy resin were around 32% lower than flexural strength ranges of jute and glass fiber reinforced composites based on cardanol and resorcinol based modified epoxy resin.

Izod impact properties

The Izod impact strength of all samples of glass and jute-reinforced composites made from synthesized modified epoxy resin was tested to study the sudden force applied and the energy required to break them. Compared to the CRGP composite, the CRGT composite has an Izod impact strength that is 29.22% lower because compared to aromatic hardeners based on aliphatic hardeners have a lower Izod impact strength. Jute fiber-reinforced composites had similar outcomes. Cardanol and resorcinol-based modified epoxy resin composites impact strength of all reinforced composites based on commercial epoxy resin is around 18% lower.

Hardness properties

Rockwell hardness was measured to study the surface rigidity of prepared composites. Rockwell hardness values of cardanol and resorcinol-based modified epoxy resin glass and jute fiber reinforced composites are shown in Table 3. The hardness value of commercial epoxy resin-based composites was lower than that of synthesized modified tetra functional epoxy resin. Glass fiber-reinforced composites, increasing Rockwell hardness was CRGP>CRGPL>CRGPA>CRGT. CRGP has a Rockwell hardness that is nearly 15% higher than CRGT. The jute fiber reinforced composites, increasing order of Rockwell hardness was CRJP>CRJPL>CRJPA>CRJT.

The distinctive molecular structure of tetrafunctional epoxy resin signifies their ability to have excellent mechanical characteristics than standard epoxy resin with amine based hardeners also higher crosslink density of tetrafunctional epoxy resin improves their ability to effectively distribute mechanical loads, which adds to their resilience to tensile and flexural stresses.

Table 3 shows the average of the three measurements made for each of the mechanical properties and all mechanical properties results are showing brackets based on statistical analysis using data variance.

Differential scanning calorimetry (DSC)

The thermal characteristics of cardanol and resorcinol based modified epoxy resin containing four different hardeners and neat epoxy were studied using differential scanning calorimetry (DSC) analysis shown in Figures 8 and 9. Two aliphatic and two aromatic hardeners are among the four samples, demonstrating distinct thermal characteristics. The analysis indicates that synthesized epoxy with aromatic hardeners have a higher glass transition temperature (Tg) Than those with aliphatic hardeners, as shown in Table 4, indicating greater thermal mobility in the cured resin network. Modified epoxy resin with aromatic hardeners shows superior thermal stability. This is probably because of their stiff aromatic structure, which increases resistance to thermal degradation. The decomposition peak of cured modified epoxy resin using four hardeners was found between 250°C and 350°C, and the decomposition peak of neat epoxy was found between 200°C and 250°C indicating the initial stage of thermal decomposition. These results indicate the variations in thermal stability across the modified epoxy with four hardeners and neat epoxy, showing the importance of hardener selection balance to produce the desired performance characteristics in epoxy systems.

Figure 8.

DSC analysis of neat epoxy and modified epoxy resin with different four hardeners.

Figure 9.

Thermal decomposition of neat epoxy and modified epoxy resin with hardeners.

Table 4.

Curing behavior of cardanol and resorcinol based modified epoxy resin.

Sample code	T_onset (°C)	T_endset (°C)	ΔT	Tg (°C)	ΔH_∞ (J/G)	ΔT*	ΔH*	CI	Curing behavior
Neat epoxy	63.61	75.92	12.31	69.78	1.32	-	-	-	-
RP	82.47	95.19	12.72	88.80	4.28	1.03	3.24	3.34	GOOD
RPL	78.23	91.41	13.26	82.47	6.85	1.07	1.06	1.72	GOOD
RPA	54.80	89.10	34.30	71.98	8.82	2.78	6.68	8.58	GOOD
RT	52.80	79.57	26.77	52.80	8.65	2.10	6.57	13.83	GOOD

The curing behavior of neat epoxy (used as the reference sample) and cardanol and resorcinol based modified epoxy systems cured with four hardeners was evaluated by calculating the cure index. This index was derived using the onset temperature (T_onset) and endset temperature (T_endset) of the samples, the temperature range of the curing process (ΔT_s) compared to the reference temperature range of neat epoxy (ΔT_ref), and the heat released during curing (ΔH_s) relative to the enthalpy of neat epoxy (ΔH_ref). The curing behavior, whether good or poor, is directly influenced by the cure index (CI), as it reflects the curing efficiency and crosslinking density of the system. The calculated values are presented in Table 4.⁵⁰

Scanning electron microscope

The scanning electron microscope study of cardanol and resorcinol based modified epoxy resin composites with various hardeners indicated significant variations in the bonding between the fibers and matrix. Figures 10 and 11 show glass fiber composites showed a significantly stronger connection with the matrix than jute fiber composites. Glass fiber composites demonstrated superior mechanical characteristics than jute fiber composites. Furthermore, phenalkamine showed the strongest connection with the resin, whereas triethylenetetramine displayed the least bonding. In glass, and jute fiber-based composites, the order of increasing adhesion between resin and hardener was Phenalkamine > polyamide >Triethylenetetramine. Due to their higher molecular weight and superior adhesion properties than aliphatic hardeners, polymeric hardeners give composites strength.

Figure 10.

SEM images of cardanol and resorcinol based modified epoxy resin and glass fiber reinforced composites with hardeners (G1) Phenalkamine based composite (G2) Polyamide based composite (G3) Triethylenetetramine based composite.

Figure 11.

SEM images of cardanol and resorcinol based modified epoxy resin and jute fiber reinforced composites with hardeners (J1) Phenalkamine based composite (J2) Polyamide based composite (J3) Triethylenetetramine based composite.

Chemical resistance properties

Table 5 shows the chemical resistance results. When subjected to six distinct chemical reagents (H₂O, NaOH, NaCl, H₂SO, Methanol, THF), the cardanol and resorcinol-based modified epoxy resin reinforced composites made of jute and glass fiber demonstrated exceptional chemical resistance with only a slight alternation in the surface of the composite. All composites showed weight loss made of commercial epoxy resin. The following equation was used to determine the percentage of weight loss or gain:

Weight loss % = \frac{I n i t i a l W e i g h t - F i n a l W e i g h t}{I n i t i a l W e i g h t} \times 100

Table 5.

Chemical resistance properties of jute and glass fiber composites based on cardanol and resorcinol based modified epoxy resin and commercial epoxy resin.

Sr. No.	Sample code	Water		10% NaOH		10% NaCl		10% H₂SO₄		Methanol		THF
Sr. No.	Sample code	CW%	CT%	CW%	CT%	CW%	CT%	CW%	CT%	CW%	CT%	CW%	CT%
1	CRGT	NC	NC	NC	NC	0.4	0.1	NC	NC	NC	NC	1.8	1.2
2	CRGP	NC	NC	NC	NC	0.7	0.1	NC	NC	NC	NC	1.2	1.4
3	CRGPL	NC	NC	NC	NC	0.6	0.3	NC	NC	NC	NC	1.1	1.0
4	CRGPA	NC	NC	NC	NC	0.3	0.1	NC	NC	NC	NC	1.7	1.1
5	CRJT	0.5	0.2	1.7	0.6	1.4	0.3	2.3	0.7	1.4	0.4	2.4	1.0
6	CRJP	0.8	0.3	1.2	0.8	0.7	0.4	1.8	0.6	0.8	0.8	1.8	1.3
7	CRJPL	0.6	0.1	1.1	0.8	0.9	0.4	1.6	0.7	1.0	0.6	3.0	1.6
8	CRJPA	1.1	0.4	1.6	0.7	1.1	0.4	1.9	0.8	1.2	0.4	2.1	0.9
9	EGT	NC	NC	0.5	0.3	0.9	0.3	0.9	0.6	0.9	0.5	2.8	1.2
10	EGP	NC	NC	0.1	0.0	0.3	0.5	0.4	0.8	0.5	0.6	1.9	1.0
11	EGPL	NC	NC	0.3	0.2	0.5	0.3	0.6	0.4	0.7	0.3	2.1	1.3
12	EGPA	NC	NC	0.6	0.3	0.6	0.2	0.7	0.3	0.6	0.4	2.4	1.1
13	EJT	1.3	0.6	2.0	1.1	1.5	0.6	2.1	1.0	2.1	1.1	3.1	1.4
14	EJP	0.8	0.4	1.1	0.8	0.9	1.2	1.6	1.1	1.1	0.7	2.3	1.7
15	EJPL	1.0	0.3	1.3	0.5	1.0	0.7	1.8	0.9	1.3	0.4	2.8	1.2
16	EJPA	1.2	0.6	1.5	0.9	1.2	0.5	1.9	0.9	1.8	0.9	3.0	1.4

NC: no change; CW%: change in weight; CT%: change in thickness.

Thermal properties

Thermogravimetric analysis, or weight loss as a function of temperature, was performed to better understand the thermal stability of the particle composites. Figures 12 and 13 show mass loss curves for all jute and glass fiber-based composites. The thermogravimetric analysis (TGA) of synthesized modified epoxy resin with jute and glass fiber composites using four hardeners (Triethylenetetramine, Phenalkamine, Low viscous Phenalkamine, and Polyamide) showed distinct thermal behavior. Among the four hardeners given, the results of the composites with the best and lowest hardener’s thermal stability are as follows:

Figure 12.

Mass loss curves of glass fiber composites of (a) cardanol and resorcinol based modified epoxy resin (b) commercial epoxy resin.

Figure 13.

Mass loss curves of jute fiber composites of (a) cardanol and resorcinol based modified epoxy resin (b) commercial epoxy resin.

The modified epoxy resin glass fiber composite with triethylenetetramine hardener (CRGT) experienced an initial weight loss of 1.96% at 100°C, and a weight loss at 200°C is 4.702%. Between 200°C and 400°C, the composite undergoes significant degradation with a weight reduction of 14.349%, including the degradation of the composite. The TGA analysis of commercial epoxy resin with triethylenetetramine hardener glass fiber composite (EGT) weight loss is 6.328% at 100°C, and weight loss at 200°C is 10.503%. Between 200°C and 400°C composite maintain continued mass loss up to 970°C.

The thermogravimetric analysis (TGA) of the modified epoxy resin glass fiber composite with phenalkamine hardener (CRGP) shows improved thermal stability. The composite exhibits an initial weight loss of 1.62% at 100°C, followed by a moderate weight loss of 4.168% at 200°C. Between 200°C and 400°C, the composite undergoes a more gradual degradation with a weight reduction of 9.273%, indicating enhanced resistance to thermal breakdown compared to other hardener systems. In contrast, the commercial epoxy resin glass fiber composite with phenalkamine hardener (EGP) shows an initial weight loss of 3.456% at 100°C and 9.657% at 200°C. From 200°C to 400°C, the commercial resin composite undergoes a sharper weight loss of 19.021%, indicating more rapid degradation. After 400°C, both composites continue to lose mass, but the modified resin system demonstrates more stable performance up to 970°C. Phenalkamine based composite shows superior thermal resistance compared to other hardener systems in both synthesized and commercial epoxy resin, but the comparison in epoxy resin synthesized from renewable resources based glass fiber composite is superior thermal stability compared to commercial epoxy resin.

The TGA analysis of synthesized modified epoxy resin jute fiber composite with triethylenetetramine (CRJT) and phenalkamine hardener (CRJP) is depicted in Figure 13. Initially, up to 100°C, both hardener systems composite exhibited minimal weight loss, respectively 2.975%, and 3735%, By 200°C the mass loss increased to 8.367% and 7.495%. While in the temperature range of 200°C to 400°C, a significant weight loss in both hardener composites of approximately 38% was observed, indicating thermal degradation and volatilization of epoxy matrix and jute fiber. Beyond the 400°C the composite showed a steady mass loss, leaving the final. The commercial epoxy resin jute fiber composite with triethylenetetramine and phenalkamine hardeners (EJT and EJP) exhibits varying thermal degradation behaviors. For EJT, an initial weight loss of 4.22% occurs at 100°C, increasing to 15.47% at 200°C. Between 200°C and 400°C, the composite undergoes a further mass reduction of approximately, 22.319%, highlighting a faster degradation rate compared to glass fiber composites. For EJP, the initial weight loss is slightly lower at 3.89% at 100°C but rises to 11.56% at 200°C. FROM 200°C to 400°C, EJP loses about 18.44% of its mass. Beyond 400°C, both jute fiber composites (EJT and EJP) continue to lose mass steady up to 970°C, demonstrating lower thermal stability compared to their glass fiber counterparts.

The decomposition rate of the composites correlated to different temperature ranges was determined and shown in Figures 14 and 15. The particle composites’ activation energy was determined by using Broido’s method. The TGA and DTGA curves were used for determining thermal kinetic parameters, which are shown in Table 6 and include initial system temperature (To), procedural decomposition temperature (PDT), and activation energy (Ea). Using Broido’s method, the particle composites’ activation energy was determined. Thermal stability for glass-reinforced composites has been shown to be CRGP>CRGPL>CRGPA>CRGT and for jute-reinforced composites CRJP>CRJPL>CRPA>CRJT based on its Ea. The highest thermal stability of CRGP and the lowest is CRGT in glass fiber composites. As well as in jute fiber composites highest thermal stability is CRJP and the lowest is CRJT.

Figure 14.

DTGA curves of glass fiber reinforced composites (a) cardanol and resorcinol based modified epoxy resin (b) commercial epoxy resin.

Figure 15.

DTGA curves of jute fiber reinforced composites (a) cardanol and resorcinol based modified epoxy resin (b) commercial epoxy resin.

Table 6.

Thermal kinetics parameter of cardanol and resorcinol based epoxy resin and commercial epoxy resin.

Sample code	T_o (°C)	PDT (°C)	E_a (J/mole)
CRGT	30	264	8.34
CRGP	30	249	10.53
CRGPL	30	240	10.68
CRGPA	30	285	9.51
CRJT	30	245	7.24
CRJP	30	258	6.52
CRJPL	30	251	8.02
CRJPA	30	277	6.48
EGT	30	237	6.33
EGP	30	235	6.77
EGPL	30	239	6.60
EGPA	30	224	6.04
EJT	30	217	6.032
EJP	30	268	5.85
EJPL	30	218	4.92
EJPA	30	281	6.08

Shear strength

The shear strength of adhesive joints for metal and wood panels with cardanol and resorcinol based modified epoxy resin and commercial epoxy resin is characterized by using different hardeners, and its results are shown in Figures 16 and 17. The shear strength of modified epoxy resin with all hardeners in metal substrate ranges from 5.36 N/mm² to 5.53 N/mm², while commercial epoxy resin adhesive samples range between 4.63 N/mm² and 4.47 N/mm². Synthesised modified epoxy with wood-to-wood substrate range from 5.23 N/mm² to 5.34 N/mm², and commercial epoxy resin with wood-to-wood substrate range is 4.31 N/mm² to 4.4 N/mm². The cardanol and resorcinol based modified epoxy resin provides stronger adhesion than commercial epoxy due to its functionality, which creates a denser cross-linked network, and its optimized viscosity, which improves bonding and durability. This increased adhesive strength varies with the substrate, with the highest strength in metal-to-metal bonding, moderate strength in wood-to-wood, and the lowest in metal-to-wood, demonstrating the resin’s tailored compatibility with each material.

Figure 16.

Lap shear strength of commercial epoxy resin.

Figure 17.

Lap shear strength of modified epoxy.

Conclusion

A tetrafunctional epoxy resin was developed using renewable resources derived from phenols and aldehyde. The resin, modified with cardanol and resorcinol, underwent detailed characterization to examine its properties. Various curing agents were employed to study their effects on the resin performance. This approach emphasizes the use of bio-based materials while enabling the resin to achieve enhanced mechanical and thermal properties, offering a sustainable option for high-performance applications. It was used as a matrix material in the preparation of jute and glass fiber composites. Compared to aliphatic curing agents, aromatic curing agents are more capable of creating cross-links. The tetrafunctional resin low volatility and optimal viscosity promoted strong adhesion, resulting in a void-free bond between the matrix and fibers, and ultimately improving the mechanical properties. Additionally, the renewable resin has superior curing performance, with a higher cure index and enhanced cross linking density. With greater functionality compared to commercial epoxy resin, this bio-based resin is well-suited for engineering applications, offering increased strength, durability, and reliability, making it a strong contender for use in challenging industrial applications. Cardanol and resorcinol based modified epoxy resin composites demonstrate outstanding chemical resistance, which significantly improves the material’s lifespan. Thermogravimetric analysis at elevated temperatures revealed that the modified resin composites with jute and glass fibers exhibited lower mass loss and high residual content compared to commercial epoxy resin composites. Table 7 summarizing previous studies on thermoset resin with various fiber reinforcements, along with their mechanical and thermal properties, has been included to compare the performance. The findings clearly show that the synthesized renewable resources-based modified epoxy resin achieves superior mechanical strength and thermal stability compared to conventional and various modified epoxy resins. These results, combined with the superior mechanical properties, make the modified bio-based resin well-suited for high-performance applications. Overall, the findings suggest that the renewable resources-based tetrafunctional epoxy resin provides enhanced all-around properties compared to commercial epoxy resin (Tables 8 and 9).

Table 7.

Previous published research data of epoxy resin and its various fiber composites with thermal and mechanical properties.

Composition	Properties	References
Epoxy resin with jute fiber	Tensile strength: 12.46 N/mm² Flexural strength: 44.71 N/mm²	[51]
Epoxy resin with glass fiber	Tensile strength: 5 MPa–15 MPa	[52]
Cardanol-based phenolic resin and epoxy resin with jute fiber	Tensile strength: 9 MPa–20 MPa	[53]
Cardanol and epoxy resin with bagasse fiber	Tensile strength: 18 MPa–24 MPa Hardness: 26–43 HRB	[54]
Bisphenol-C mixed epoxy phenolic resin with jute/carbon/ glass hybrid composites	Tensile strength: 10 MPa–21 MPa Flexural strength: 17 MPa–24 MPa	[55]
Epoxy resin with jute and glass hybrid composite	Tensile strength: 29.52 MPa–56.68 MPa Flexural strength: 28.18 MPa–28.81 MPa Thermal stability: At 400 C 63.54% weight loss	[56]
Phenol-formaldehyde resin with jute fiber	Tensile strength: 27.21 MPa–33.07 MPa	[57]
CNSL-formaldehyde resin (CF), CNSL polymerized resin (CP) with jute fiber	(CF) tensile strength: 40.83 MPa (CF) flexural strength: 59.98 MPa (CP) tensile strength: 38.12 MPa (CP) flexural strength: 49.73 MPa	[58]
Phenol-formaldehyde resin, CNSL-phenol formaldehyde resin with jute fiber	Thermal stability of phenol formaldehyde resin at 600°C weight % is 25.48 Thermal stability of CNSL-phenol formaldehyde resin at 600°C weight % is 25.55	[58]
Phenol-formaldehyde resin with glass fibers	Tensile strength: 7 MPa–42 MPa Flexural strength: 10 MPa–73 MPa	[59]
Novolac phenol-formaldehyde resin (NPF), resol phenol-formaldehyde resin (RPF), NPF/ RPF blend with glass fiber	(NPF) flexural strength: 124.18 MPa (RPF) flexural strength: 117.23 MPa (Blend) flexural strength: 134.92 MPa	[60]
Phenol furfural resin (PC1-PC3), phenol -formaldehyde resin (PC4) with sisal fiber	(PC1-PC3) impact strength: 53 J/m–136 J/m (PC4) impact strength: 182 J/m	[61]
Epoxy resin (LY556) with banana fiber	Tensile strength: 25.2 MPa–30.4 MPa Flexural strength: 42.2 MPA–56.3 MPa Hardness: 54–68 Thermal stability: At 450°C 80% weight loss	[62]
Epoxy resin with sisal fiber	Tensile strength: 34 MPa Flexural strength: 23 MPa	[63]
Cardanol resin with coconut shell	Tensile strength: 29 MPa Flexural strength: 46 MPa	[64]
Phenol-formaldehyde with banana fiber	Tensile strength: 28 MPa Flexural strength: 50 MPa	[65]

Table 8.

Comparative properties of cardanol and resorcinol based modified epoxy resin and commercial epoxy resin.

Sr no.	Properties of resin	Modified epoxy resin	Commercial epoxy resin
1	Viscosity (cP)	12,685	16,167
2	EEW (gm/eq)	612	600
3	Hydrolysable chlorine content (%)	0.74	0.44
4	Volatile content (%)	1.5	2.0
6	Gel time (minutes)	13.28°C–61°C	63.15°C–124.13°C
7	Adhesive bonding	High	Low

Table 9.

Comparative properties of cardanol and resorcinol based modified epoxy resin and commercial epoxy resin fiber reinforced composites.

Sr no.	Properties of fiber reinforced composites	Renewable resources modified epoxy resin fiber reinforced composites	Commercial epoxy resin fiber reinforced composites
1	Mechanical properties	High	Low
2	Thermal stability	High	Low
5	Procedural Decomposition Temperature (PDT) (°C)	240–285	217–281
6	Chemical resistance	Superior	Moderate
7	Activation energy for decomposition (J/mole)	6.48–10.68	4.92–6.77

Footnotes

Acknowledgements

The author would like to thanks: - Institute of Science and Technology for Advance Studies and Research (ISTAR), CVM University, Vallabh Vidyanagar, voluntarily contributed to this study.

Author contributions

The article was written through contributions of all authors. Dr Mahendrasinh Raj and Lata Raj provided the idea and the guidance and revised this paper. Meet Patel and Mitali Yadav contributed in a literature review, collection of data, structuring of the manuscript, paraphrasing, and removing plagiarism. The data interpretation and adhesive properties were carried out by Meet Patel and Pragnesh Rathva. All authors have given approval to the final version of the manuscript.

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.

Ethical statement

ORCID iD

Meet Patel

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