Long term accelerated influence on thermo-mechanical properties of glass/carbon fiber reinforced interpenetrating polymer network hybrid composites

Abstract

The prime importance of this work is that to compare the influence of hygrothermal analysis on the physical properties of different variant of interpenetrating polymer network (IPN) blend reinforced with E-Glass/Carbon and combination of both (hybrid) fibers. In this study, combinations of epoxy (EP)/polyurethane (PU), vinyl ester (VER)/polyurethane (PU), and epoxy (EP)/vinyl ester (VER) have been taken as the matrix material (IPN) to reinforce the glass, carbon, and combination of both fibers. Moreover, prepared specimens are subjected with boiling water immersion test by maintaining the temperature of 45°C, 55°C, and 65°C in order to thoroughly understand the influence of moisture absorption and temperature in their physical attribution as per ASTM standards. Besides, to better understand the thermal stability and compatibility, thermal-gravimetric (TGA) analysis and burn-off test were conducted as well. During this study, it was found that, combination of VER/PU possesses the high moisture absorption resistance amongst all variants (0.725% for 45°C, 0.854% for 55°C, 1.234 for 65°C). Similarly, epoxy/vinyl ester reinforced glass fiber IPN laminate (EVG) has shown notable TGA value as 418.6°C, as well burn-off test also shown that hybrid IPN composites have better wettability (less void presence) than all other laminates (EPGC-0.9%, VPGC-0.89%, EVGC-0.92%). Further, losses of physical strengths have been noticed on all specimens upon subjection on hygrothermal environment irrespective of IPN blend and fiber constituents.

Keywords

Interpenetrating polymer network moisture absorption thermal-gravimetric analysis burn-off test hygrothermal physical strengths

Introduction

The advance in materials technologies are completely drives or uplifts the mankind from Stone Age to bronze age and stick at with iron age. In present day most of the engineering sectors are kept occupied with various set of composite materials because of their unique characteristics of light weight, high strength, corrosion resistance, easy to shape depends upon requirement and robustness. The so called novel composite materials are not new to the human beings, it has been found as evidence by mankind for more than 3000 years. In ancient days, manhood has used rice straw with mud brick combination to build their houses. The word composite is arrived from the Latin word compositus, actually it meant that putting together the various like materials in the assumption to produce the newer material. In common practice, composite materials consist of more than two phases which are considered as the mechanically separable distinct components. Apart from that, composite materials are far superior than the individual constituents and most of the circumstances that proved as the unique in all kind of properties.^1–3

Generally, composites consist of two phases called as continuous and discontinuous, in that, normally the discontinuous phase appear as the harder and stronger one than the continuous phase, also it is called as the reinforcement, similarly continuous phase is named as the matrix material. Typically, composite materials have the widespread range of applications in all engineering sectors and domestic day to day applications. Conversely, it shows his footprint in building application, door and window fabrication, furniture manufacturing, electronic industries, wind mill blades and extending its wide spectrum applications in building most of the mechanical components also, in recent days all intricate shapes and products are completely manufactured with help of the composite materials. Besides that, the application of the composite materials in mechanical industries are kept increasing in past two decades in almost all the processing and product manufacturing industries by choosing the various kind of fiber and matrix material. Especially in the area of automobile part fabrication, most of the vital parts of the trucks are these days completely manufactured or replaced with help of the composite materials, even in the sports sector, tennis and squash rackets, bicycle frames, roller skates, javelins, arrow and bows are looked as the distinguished footnote of the composite materials.⁴

Further to this, normally fiber reinforced plastic (FRP) manufactures are often chose epoxy are their prime matrix material amongst all, even though the cost is comparatively high as against all prominently available resin material. Usually, epoxies are widely used in most of the applications such as coatings, adhesives, and FRP industries due to its prominent high strength and modulus, comparatively it gives less shrinkage and outstanding thermal stability when subjected with any environments. However, manufacturers felt considerable suffering towards using this resin as their base matrix material as it shows the marginal brittleness behaviorism and its unique cost factor. In the sequence of epoxy resins, there is another resin which is most commonly used in all of the marine application is vinyl ester (VER), it has good damping resistance, chemical resistance, and comparatively less cost than the epoxy (EP) resins. All set of above resins are shown marginally high modulus and resistance against all sort of environments but failed in particular aspects when comes towards the tear resistance.⁵

In the above context, there is another class of resins which gives high tear resistance when it is subjected with the end usage in all applications, it is called polyurethanes. It comes under thermoset polymer category, also which considered as the predominant substitute of epoxy and vinyl ester. Since it shows excellent chemical, water resistance and uniquely shows higher tear resistance and toughness characteristics, which is the reason why most of the manufactures are using this as floor coatings, plastic coatings and comprehensively used as the sealer material even though it is not shown enough value in modulus aspects. From all the resins materials, each and every matrix material has shown their distinctive characteristics in their profound areas but considerably fails in some environments. To overcome such as negative impact of each constituent, the concept of interpenetrating polymer networks (IPNs) have been originated in all divisions of mechanical industries. The common definition of IPNs is that, special class of polymers or alloys possessing two or more distinctive cross linked polymer networks have had entanglement with each other as novel blend material. While doing so, it exhibits substantial and notable property than that of the individual polymer materials.⁶

In this present work, a series of compositions like epoxy/vinyl ester (EP/VER), vinyl ester/polyurethane (VER/PU), and epoxy/polyurethane (EP/PU) matrix (IPNs) material have been prepared as the blend with some proportions (70:30 wt.%) with the common reinforcement of E-Glass/Carbon fibers. Moreover, to completely understand the hygrothermal behavior of the prepared specimens, test pieces are kept in the various temperatures such as 45°C, 55°C, and 65°C, and their behaviors with respect to various temperatures towards thermal-gravimetric analysis (TGA), burn-off test, tensile, flexural, ILSS and impact tests have been carried out as per ASTM standards in ambient temperature. Disparately to compare the hygrothermal nature of the IPN laminates, test specimen results are compared with dry specimens and with each others.⁷

Materials and methods

Materials used in specimen preparation

The chosen PU prepolymer (viscosity index value 1500–2500 cps) and their corresponding curing agent was purchased from cross link technologies, Hyderabad, India.

In the same way, vinyl ester (VER) was purchased which is having the specific gravity level of 1.06–1.058 and their matching viscosity index value was 450–650 cps, and their corresponding catalyst, accelerator and was also received from vasavi bala resins, Chennai, India. Epoxy resin—LY 1564 and their subsequent hardener was received from javanthi fibers, Chennai. All the resins materials and corresponding hardeners and their composition and proportions were mentioned in the Table 1. At last E-Glass and Carbon fibers were purchased from raj fibers, Chennai, India, as per the technical specifications given in the Table 2.

Table 1.

Composition and proportion of the matrix and hardener.

Name of the resin	Main component	Hardener
Epoxy	Diglycidyl ether of bisphenol A (DGEBA)	Dimethylaminomethyl (DMP-30) (TMDA): 7%
Vinyl ester	Unsaturated polyester	MEKP—methyl ethyl ketone peroxide (catalyst: 2%)
		Cobalt naphthenate (Initiator: 1.5%)
		Dimethylaniline (Promotor: 1%)
PU—prepolymer	Toluene diisocynate—Shore hardness CG-60A	MOCA (4,4^′-Methylenebis (2-chloroaniline: 7%)

Table 2.

Technical specification of the E-Glass/Carbon fabrics.

Fiber	Density (g/cm³)	Tensile strength (GPa)	Young’s modulus (GPa)	Elongation (%)	Coefficient of thermal expansion (10⁻⁷/ºC)	Poison’s ratio	Ref
E-glass	2.58	3.445	72.3	4.8	54	0.2	8
Carbon	1.85	4.225	240	1.7	21	0.3	9

All the resin and fiber materials were used as such received from the suppliers.

Preparation of EP/PU, VER/PU, and EP/VER IPN blends

All the chosen resins were heated separately (prepolymer) up to 60°C in order to reduce their viscosity index value to enable the proper miscibility among the blends. Subsequently, their hardeners were added with appropriate weight ratio specified by the supplier. Similarly, all the blends were kept in the vacuum, before it is being used for fabrication purpose.¹⁰ All the compositions and proportions used in the particular work was guide lined as per the percentage weight ratio specified in the Table 3. Nevertheless, the proportions mentioned in the Table 3 were inclusive of hardener, promoter, and catalyst of respective resins.

Table 3.

Interpenetrating polymer network—Blend with reinforcement combinations.

Notations	IPN—Blending combination & weight ratio	Fiber reinforcement
EPG	Epoxy (70%) + polyurethane (30%)	Glass
EPC	Epoxy (70%) + polyurethane (30%)	Carbon
EPGC	Epoxy (70%) + polyurethane (30%)	Glass/Carbon
VPG	Vinyl ester (70%) + polyurethane (30%)	Glass
VPC	Vinyl ester (70%) + polyurethane (30%)	Carbon
VPGC	Vinyl ester (70%) + polyurethane (30%)	Glass/Carbon
EVG	Epoxy (70%) + vinyl ester (30%)	Glass
EVC	Epoxy (70%) + vinyl ester (30%)	Carbon
EVGC	Epoxy (70%) + vinyl ester (30%)	Glass/Carbon

Fabrication methodology

As combinations, nine different types of IPN laminates were fabricated through the hand lay technique. Initially, the combinations of IPN blends were prepared and kept separately. Likewise, the fibers were cut with the dimensions of 300 mm x 300 mm as in the pre-processing stage. The poly vinyl glychol (PVA) was chosen as the releasing agent, it was pasted over the flatten surface. The prepared blends with exact weight ratio have been used as the matrix material and coated over the flatten surface. Following this, the pre-cut fibers were rolled over the wet surface simultaneously. Equally, six set of fibers were kept over one another until to get the final size of 3 mm.¹¹ As soon as the fabrication process was got over, all set of IPN laminates were kept in atmospheric temperature (24 h) in order to get complete polymerization between the blends. Furthermore, laminates were placed in hot air over for the period of 2 h to ensure the complete polymeric condensation process. Successively, laminates were cut with help of the diamond saw cutter as per the ASTM standards mentioned in the Table 4.

Table 4.

Interpenetrating polymer network laminate specimen geometry as per ASTM standards.

ASTM standards	Size of the specimen (Length x breadth x thickness in “mm”)
ASTM D 3039—tensile	250 x 25 x 3.2
ASTM D 790—flexural	127 x 12.7 x 3.2
ASTM D 256—Impact	63 x 12.8 x 3.2
ASTM D 570—Moisture absorption	ø50.4 x 3.2
ASTM D 2344—ILSS	18 × 6 × 3

Experimental procedure

Thermal-gravimetric analysis

Thermal-gravimetric analysis (TGA) was conducted on all nine set of IPN laminates in order to find out their thermal stability to compare with each other such a way to predict the laminate behavior in hygrothermal environments. The test was conducted on shimadzu DTG-50 instrument under proper nitrogen atmosphere (40 mL/min). IPN laminate samples with the volume of 7–8 mg of each was heated from the temperature of 30°C–530°C with steady rate of 10°C/min.¹²

Burn-off test

The main aim behind that of conducting the burn-off test was that, to determine the exact fiber and resin constituents in the final product. To carry over this analysis, the specimen size of 25 x 25 x 3 mm was kept in the thoroughly cleaned crucible, following this crucibles were placed in hot muffle furnace by maintaining the temperature of 525± 15°C for exact 6 h.

Moisture absorption characterization

Initially, the boiling water absorption test was conducted as per procedure mentioned in ASTM D 570 standard. All the test pre-cut specimens mentioned in the Table 4 were aged by immersing the specimens upon maintaining the various temperatures of 45°C, 55°C, and 65°C, for the period of 12 months. After completing accelerated aging tests for the specific interval of time, specimens were taken out from the hot water bath tub and dried in the ambient temperature for the period of two days (dry specimen). Also, the specimens had been placed in the water tub in such a way to avoid the moisture penetration on the flat surface of the specimens without any obstructing. Moreover, hot water bath tub was used to accommodate all the specimens completely as shown in the Figure 1.¹³

The specimens’ initial weight and final weight were measured with the accuracy of ±2 mg with the electronic weighing machine and their equivalent percentage weight gain was calculated based on the equation (1).

\frac{(W w - W d)}{W d}

(1)

Where, W_w—Specimens weight at wet condition; W_d—Specimens weight at dry condition.

Mechanical analysis

As specifications stated in the Table 4, the pre-cut specimens were fixed in the upper and lower jaws and tensile load was applied at the feed rate of 2 mm/min till the specimen fractures. As similar as that of tensile strength analysis, flexural strength analysis as well, the specimens were kept on the respective fixtures and vertical load was applied correspondingly till the specimen’s fractures by maintaining the feed rate of 2 mm/min. Similarly, while doing ILSS analysis also, same feed was applied till the specimen’s fractures. While doing the impact analysis test, the work piece kept in the vertical direction in the kit holder sub sequentially pendulum was allowed to fall over the work piece and their corresponding load was noted from the graduated scale.¹⁴ All tests were conducted at ambient temperature and their respective evidences were mentioned in the Figure 2.

Results and discussion

Thermo-gravimetric analysis

The thermal resistance of the various proportionate of IPNs were measured by using thermo-gravimetric analysis (TGA). The weight loss against the temperature of different combinations of E-Glass/Carbon fiber reinforced hybrid IPN laminate’s trends were shown. Similarly the Table 5 illustrates the distinctive temperatures of all the IPN laminates.

Table 5.

Volume fraction and thermal-gravimetric of the constituents in experimented interpenetrating polymer network laminates.

Notations	Vf (%)	Vm (%)	Vv (%)	Temperature 5% loss (Tg) (°C)
EPG	46.12 $\pm$ 0.43	52.24 $\pm$ 0.62	1.64 $\pm$ 0.71	396.7
EPC	45.54 $\pm$ 0.62	52.92 $\pm$ 0.71	1.54 $\pm$ 0.64	382.6
EPGC	46.34 $\pm$ 0.73	52.76 $\pm$ 0.64	0.9 $\pm$ 0.23	387.6
VPG	45.61 $\pm$ 0.82	52.68 $\pm$ 0.62	1.71 $\pm$ 0.65	412.5
VPC	46.42 $\pm$ 0.12	51.94 $\pm$ 0.41	1.64 $\pm$ 0.41	395.6
VPGC	45.72 $\pm$ 0.23	53.39 $\pm$ 0.91	0.89 $\pm$ 0.54	401.5
EVG	46.18 $\pm$ 0.42	52.14 $\pm$ 0.65	1.68 $\pm$ 0.43	418.6
EVC	46.42 $\pm$ 0.54	51.99 $\pm$ 0.23	1.59 $\pm$ 0.12	401.5
EVGC	45.62 $\pm$ 0.62	53.46 $\pm$ 0.46	0.92 $\pm$ 0.57	408.3

From the Figure 3, it was clearly found that, EVG specimens had shown higher thermal stability among all other specimens, their corresponding value was 418.6°C. The main reason behind that of this particular value was that, reinforcement of glass fiber into the IPN matrix as glass fiber possesses the inherent property of heat resistance, firstly it absorbs (insulation) the heat energy to the maximum possible way and subsequently transfers to the IPNs. Whereas the similar combination of epoxy and vinyl ester reinforcement of carbon fiber possess the value of 401.5°C. The reduction in the value shows that higher heat conductive capacity of the carbon fiber, it transfers the heat as such received without any delay, and disparately transfers the same to the matrix, so it holds lesser value than that of the glass fiber reinforcement. Furthermore, the hybrid EVGC retains the property of both glass and carbon fiber reinforcement; it shows the value of about (EVGC) 408.3°C. In the same way, the vinyl ester and polyurethane combination IPNs had showed the value of 412.5°C. Though the vinyl ester seems as the fire retarded matrix, because of the presence of hard segment (hydroxyl group presence) incorporation of the polyurethane (PU) significantly drops the heat resistance capability of the IPNs to marginal level as it holds soft segment presence. The soft segment of PU distorts even before the hard segment of vinyl ester distorts, thus the way it immediately dilutes the permanent entanglement between the constituent matrixes in substantial level. Moreover, the obtained values showed the marginal increase in the heat resistance value towards the glass fiber reinforcement and subsequent negative surge in carbon fiber reinforcement and cumulative of both on hybrid (glass & carbon) IPN laminates as shown in the values in the Table 5. The obtained values were 412.5°C, 395.6°C, and 401.5°C for the combinations of VPG, VPC, and VPGC, respectively. Likewise, polyurethane loaded epoxy IPN laminate had shown the higher value for the reinforcement for glass fiber reinforcement as it shows the better insulation property, their subsequent obtained value was 396.7°C. Also, the EPC shows the marginally negative surge in the table by giving the value as 382.6°C and their corresponding hybrid (EPGC) had showed the value of 387.6°C. From the above study, it was clearly understood that, the addition of PU into the IPNs decreases the heat resistivity of the laminate to larger level. In the contrary, the glass fiber reinforcement holds and possesses good heat resistivity than the carbon fiber reinforcement.¹⁵

Burn-off test analysis

During this analysis, the measured volume fractions of all nine specimens were tabulated in the Table 5. It was interesting to note that, the volume fractions found in all the nine samples are practically equal with each other. Whereas, it was seen that, there was slight decline in value which was predominantly found in all IPN hybrid laminate. From the results it was seen that, the IPNs with hybrid have had better wettability property with the fibers and also creates better interfacial adhesion, thus the way it avoids the void presence in the IPN laminate in significant level. Hence it can be concluded that, the better interfacial adhesion (without void) enhances the IPNs laminate strength to marginal level.

Characteristics of IPN laminate specimens in boiling water

The Figure 4(a)–(c) illustrates the moisture absorption characteristics of various proportionate of E-Glass/Carbon fiber reinforced IPN laminates with different time interval upon the subsequent temperatures of 45°C, 55°C, and 65°C. From the Figure 4(a) it was visibly seen that the EPG specimens had absorbed maximum amount of water uptake during the initial day of immersion with respect to temperature. All the specimens irrespective of temperature had started to absorb more amount of water, this way of absorption seems that, the entanglement mechanism between the polymers loosens due to the elasticity nature of polyurethane and concurrently the fiber reinforcement loosens its interfacial strength between the IPN and fiber, so that creates more volume of tiny spaces between the fiber and matrix and absorbs more amount of water. The above impressive effect evidences from the values that √4.4 days it absorbs 0.505%, 0.68%, 0.978% for 45°C, 55°C, and 65°C, respectively. Additionally, after having taken the maximum water uptake its absorption capacity saturates and start absorbs the water gradually not as such during the trend had happened during the initial timings. Then the surge of trend was nearly halted or saturated, during the √19.1 days it absorbs the value of 0.982%, 1.156%, and 1.454% for 45°C, 55°C, and 65°C, respectively.

Similarly, EPC specimens had showed the maximum amount of water uptake during the initial periods like √1.1 days, there was sudden uptake of water up to 57.920% of the total volume of absorption during the entire study, then the progress of water uptake subsides or saturates and at last reaches to the level of 0.891%, 1.125%, and 1.452% for 45°C, 55°C, and 65°C, respectively. From the above two tests absorbed from the EPG and EPC, the EPG specimens had absorbed more amount of water with respect to the modified temperature. From the results observed from EPGC, the acquired results had shown the mixture response of glass and carbon fiber reinforcement, since it consists of both glass and carbon fiber as equal volume, the result showed from that study also shown, mixed response of both EPG and EPC. As the evidence, the obtained final values at the saturation levels were 0.935%, 1.1405%, and 1.453% for 45°C, 55°C, and 65°C, respectively.¹⁶

From the Figure 5(a)–(c) it was clearly seen that, during the initial days, specimens had shown the sudden uptake of moisture content up to the level of √1.12 days, then the curve had not shown the sudden surge, rather it shows the gradual and consistent curve of water uptake. The reason behind this exact mechanism was that, basically vinyl ester resin has had a fewer open spots in its long chain molecular, actually this property have made the resin more resistant against water uptake (hydrolysis) and simultaneously leads to osmotic blistering. Moreover, apart from the above characteristics, vinyl ester shrinks very less during the curing and avoids the stress cracking thus the way it prevents the water uptake through the micro cracks. Apart from that, the cross bonding mechanism of the long chain molecular also makes the vinyl ester as the very less moisture sensitive matrix material. Due to the above reasons, the vinyl ester incorporation with the IPNs avoids the water uptake of the laminate considerably. The VPG specimens had shown the value of 0.725%, 0.854%, and 1.234% for 45°C, 55°C, and 65°C, respectively. Even though the obtained values were comparatively less than the other two set of IPN combinations, but still during the accelerated aging it loses its interlaminar shear strength with the fiber and start absorbs more amount of water, that too also when the incorporation of polyurethane taken place considerably with the IPNs, the soft segment presence of PU abruptly disturbs the physical entanglement mechanism between the IPNs and reduces its physical strength and evidences as the maximum amount of water uptake when the specimens subjected during accelerated aging period. The same trend of graphs also observed for VPC specimens also, the observed values were 0.813%, 1.125%, and 1.412% for 45°C, 55°C, and 65°C, respectively. The hybrid VPGC laminates had shown mixed response of VPG and VPC, as such values obtained for its counterpart EPGC, but the values arrived for the hybrid combination of VPGC was 0.769%, 0.989%, and 1.323% for 45°C, 55°C, and 65°C, respectively. The obtained values of VPGC was nearly 24% lesser than the value of EPGC, this was because of the presence of long chain molecules of the vinyl ester. The combination of epoxy and vinyl ester reinforcement with E-Glass fiber (IPN Laminate) had shown the value of 1.225%, 1.435%, and 1.625% for 45°C, 55°C, and 65°C, respectively. The arrived values of this combination had shown much higher water uptake than all other IPN combinations. Though, the vinyl ester was complete resistant against the water absorption characteristics, still the combination of epoxy and vinyl ester blend had shown the higher level of absorption value, because the constituent epoxy resin possesses the epoxide aromatic rings which leads the hydrophilic nature and concurrently induces much absorption rate. The obtained values were proof for the above entanglement mechanism between the epoxy and vinyl ester.¹⁷

From the Figure 6(a)–(c), the obtained values of the EVC were 1.125%, 1.528%, and 1.785% for 45°C, 55°C, and 65°C, respectively. The obtained values of the EPC was quite lesser than the value of EPG as surface of the carbon fiber provides better interfacial adhesion between the IPNs and fiber, that’s why it prevents the water uptake to significant level. The obtained values of EVCG were 1.175%, 1.4815%, and 1.705% for 45°C, 55°C, and 65°C, respectively. The arrived values showed the mixed response of both EVG and EVC, also it was found that upon doing the accelerated aging test on the combinations of EPG, EPC, EPCG, VPG, VPC, VPGC, EVG, EVC, and EVCG the combinations of the epoxy and vinyl ester possess less moisture resistance property against all set of combinations, similarly the vinyl ester and polyurethane combination of IPN had showed better water resistance property towards the accelerated aging test.¹⁸

Effect of hygrothermal analysis on physical behavior of IPN composites

Effect of accelerated aging on tensile strength behavior of IPN laminate

The specimens had kept in the accelerated aging (45°C, 55°C & 65°C) environment for the period of 365 days and their corresponding tensile strength behavior was tested and showed as the bar chart in Figure 7.

It was evidently seen that, throughout all set of combination of IPN laminate, dry specimens had shown significant upsurge in all proportions irrespective of reinforcement. The accelerated aged specimen with respect of various temperature had shown negative surge based on the temperature gradient. The EPG specimens had shown tensile strength value as 565.15 MPa, 535.24 MPa, 505.72 MPa, and 487.54 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. Similarly, the carbon fiber reinforcement had shown strength as almost double the fold of the EPG, from the graph it could be seen clearly. Moreover, the obtained value on EPC shows that, reinforcement leads the significant role in up surge of the tensile strength. Besides that, EPGC shows the mixed response of both the fiber reinforcement of EPG and EPC, since it possesses the equal volume of both the glass and carbon fiber. In addition, VPG specimens also had showed slightly below the value of the EPG, this was mainly because of the strength of the matrix of vinyl ester. The VPG specimens had shown the tensile values as 434.78 MPa, 412.24 MPa, 389.12 MPa, and 364.12 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. Further, Vinyl ester based IPN laminate also lowers its strength when the specimens were subjected with the accelerated hygrothermal aging environment. The fiber reinforcement plays a substantial role in absorbing the tensile strength to the maximum level. Likewise, the EV based specimens had showed the noteworthy increment in tensile strength value as compared with all set of specimens. Since epoxy holds the basic back bone structure with six aromatic groups which consists of 6 carbons with benzene rings which normally takes three times higher the load of vinyl ester and polyurethane, apart from that as vinyl ester possess the hard segment presence, both the combinations of epoxy and vinyl ester takes maximum tensile load before it fractures with respect of fiber reinforcement and temperature. From the tensile analysis it was evidently observed that, as much as temperature increases on the specimens, it loses its strength. Similarly, fiber reinforcement also plays the major role in absorbing the tensile strength with respect of temperature.¹⁹

Effect of accelerated aging on flexural strength behavior of IPN laminate

The Figure 8 shows the E-Glass and Carbon fiber reinforced epoxy, vinyl ester, and polyurethane IPN laminates flexural strength subjected with various set of accelerated hygrothermal aging tests.

During the test, it was observed that, dry specimens had shown upper values in terms of flexural strength as compared with all set of specimens subjected with the hygrothermal analysis. To confirmation that, the EPG specimens had shown the stress value as 385.45 MPa, 354.56 MPa, 315.41 MPa, and 285.46 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. All the values obtained in tensile and flexural strength analysis exhibited the decreasing trend in stress which was the influence of direct impact on the hygrothermal environment. As much as temperature increases on water bath, the strength value of the specimens abruptly decreases. The reason behind that of this effect was, during the temperature raise the matrix material loses its geometrical integrity and partially disturbs the interfacial strength between the fiber and matrix material, thus the way it reduces the stress transfer across the specimens and this leads to loss of interfacial strength and simulates early fracture on the specimens. All the specimens across the tests had shown the same kind of trend throughout the experimental analysis. Whereas another important finding was that, strength of the fiber was directly influences in increasing the strength of the laminate to the higher level. To substantiation that, the carbon fibers known for his high strength to stiffness and mostly recommended for most of the aerospace construction purposes had showed tremendous load bearing capacity flexural analysis as well as irrespective of IPN blends. It showed commendable load bearing capacity in all hygrothermal environments. As such observed in the declining trend of hygrothermal effect on EPG specimens, like EPC, EPGC, VPG, VPC, VPGC, EVG, EVC, and EVGC specimens as well showed the negative trend towards the hygrothermal environment. Finally, results reveal that, as much as temperature gradient increases in water bath it adversely affects the specimens flexural strength irrespective of fiber reinforcement.²⁰

Effect of accelerated aging on (ILSS) Interlaminar shear strength behavior of IPN laminate

The Figure 9 shows the interlaminar shear strength behavior of E-Glass/Carbon fiber reinforced interpenetrating polymer blend (EP, VP, and EV) laminates.

From the graphical representation of ILSS, it was very clearly seen that, EPG specimens had shown the value as 28.51 MPa, 26.11 MPa, 22.19 MPa, and 20.01 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. Whereas the carbon fiber reinforced IPN laminate had shown the (EPC) values as 52.74 MPa, 45.69 MPa, 42.161 MPa, and 37.92 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. This upsurge in the values of carbon fiber reinforced IPN laminate had shown as the evidence and influence of carbon fiber into the IPN laminate. The carbon fiber shows predominant level of surge towards the reinforcement irrespective of IPN blends. Moreover as such as glass fiber reinforcement, the carbon fiber as well loses its interfacial shear strength when the specimens subjected with the accelerated hygrothermal analysis. Likewise, the EPGC had shown the mixed response of both the EPG and EPC specimens, as the proof, obtained values were 40.62 MPa, 35.90 MPa, 32.17 MPa, and 28.96 MPa for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. As such values obtained for the EP (epoxy and polyurethane) other blends also had showed the similar kind of trend like, carbon fiber reinforced specimens had showed better ILSS property against the glass fiber reinforcement. Further to this, from the obtained values, the IPN blend also plays the significant role in increasing the laminate’s ILSS strength in marginal level. Of the three blend tested with different fiber reinforcement (glass/carbon and hybrid), the epoxy and vinyl ester combinations had showed higher level of ILSS values as compared with all set of remaining two blends. This raise of the value shows that the matrix material plays the substantial role in increasing the ILSS level to maximum level. It seems that, the combination of epoxy and vinyl ester creates better interfacial adhesion between the fiber materials and proves as the better ILSS property. In the same way, the hygrothermal analysis as well considerably affects the ILSS property to the marginal level.^21–22

Effect of accelerated aging on impact strength behavior of IPN laminate

The Figure 10 illustrates the combinations of various blends reinforced with glass/carbon fiber and hybrid IPN laminates impact strength comparison as bar chart.

During the impact analysis, it was found that, the EPG specimens showed the impact strength value as 16.54 kJ/m², 15.11 kJ/m², 14.23 kJ/m², and 13.27 kJ/m² for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. Though the polyurethane matrix material was proven as the moderate damping materials against all set of constitutes, in this particular study, when it combined with other matrix material as blend, it partially losses its impact characteristics to momentous level. Although some of the other physical properties showed the declining (tensile, flexural, and ILSS) trend towards the incorporation of polyurethane, it increases the impact strength value to the predominant level by the slight negative agreement with other properties. Like other property showing the negative surge towards the hygrothermal environment, during the impact analysis as well, the hygrothermal environment proactively affects the impact strength to the substantial level. Another interesting finding was that, among all set of IPN blend combination, the vinyl ester and polyurethane combinations had showed the better impact resistance. To proof for that, the VPC combination shows the values as 33.57 kJ/m², 31.16 kJ/m², 30.59 kJ/m², and 28.88 kJ/m² for DS (Dry specimen), 45°C, 55°C, and 65°C, respectively. The exact reason behind of higher impact resistance of this particular specimen was that, the vinyl ester possesses the side chain molecules to absorb the higher amount of energy, similarly the polyurethane also possesses the equivalent damping resistance as equal to the vinyl ester. Hence the combination of both vinyl ester and polyurethane exhibits higher impact resistance characteristics as compared with all set of IPN blends in the study.^23–24

Conclusion

The IPN blends with combination of epoxy, vinyl ester, and polyurethane have been used as the matrix material in this experimental analysis, along with the reinforcement of glass, carbon, and combination of both fibers.

1. It was found that, TGA, VPG specimens had shown better widened gap and their respective losses on the matrix have taken place on 412.5°C, along the way the EVG specimens had also shown their respective TGA value as 418.6°C. This higher in TGA value exhibits the fire retardant property of the vinyl ester resin.

2. Also, the burn-off test reveals that, the hybrid IPN laminates have had fewer residues when compared with other values, this phenomenal effect shows that the void presence in the hybrid composites considerably lessens due to the better interfacial adhesion and wettability property between the fiber and matrix materials. The EPGC shows 0.9 $\pm$ 0.23, similarly VPGC shows 0.89 $\pm$ 0.54 and EVGC exhibits 0.92 $\pm$ 0.57.

3. Of the hygrothermal studies, the polyurethane based IPN specimens have shown lesser moisture absorption characteristics out of all specimens. To confirm that, EPGC shows 0.9365%, 1.14%, and 1.45% for 45°C, 55°C, and 65°C, respectively. Another notable finding was that, the vinyl ester and polyurethane based IPNs also showed remarkable performance in this study. The obtained values are (VPGC) 0.769%, 0.985%, and 1.323% for 45°C, 55°C, and 65°C, respectively.

4. In the course of all the hygrothermal analysis, carbon fiber reinforced IPN laminates exhibit superior values than other combinations; this replicates the extraordinary load bearing capacity of the carbon fiber. In addition to that, the polyurethane specimen shortly loosens its strength when it was subjected in hygrothermal environment, which shows that, soft segment (SS) presence of PU loosens its elasticity drastically and adversely leads to early failure on all physical tests.

Footnotes

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.

ORCID iD

G Suresh

References

Qin Sun

Zhang

Wadsworth

, et al. Processing and property study of cotton-surfaced nonwovens. Textile Res J 2000; 70(5): 449–453. DOI: 10.1177/004051750007000511.

Zhang

. Carbon fibers from oriented polyethylene precursors. J Thermoplastic Compos Mater 1993; 6(1): 38–48. DOI: 10.1177/089270579300600104.

Zhang

Sun

Beard

, et al. Innovative polytrimethylene terephthalate (Ptt) polymers for technical nonwovens. J Ind Textiles 2002; 31(3): 159–178. DOI: 10.1106/152808302025393.

Azhar

Zhang

, et al. Fabrication and mechanical properties of flaxseed fiber bundle-reinforced polybutylene succinate composites. J Ind Textiles 2020; 50(1): 98–113. DOI: 10.1177/1528083718821876.

Suresh

Jayakumari

. Finite element analysis of IPN reinforced woven fabric composite. Revista Materia 2017; 22(4): 1–6.

Suresh

Jayakumari

. Evaluating the mechanical properties of E-Glass fiber/carbon fiber reinforced interpenetrating polymer networks. Polímeros 2015; 25(1): 49–57.

Suresh

Jayakumari

. Analyzing the mechanical behavior of E-Glass fibre reinforced interpenetrating polymer network composite pipe. J Composite Materials 2016; 50: 3053–3061.

Sathishkumar

Satheeshkumar

Jesuarockiam

. Glass fiber-reinforced polymer composites - A review. J Reinforced Plastics Composites 2014; 33: 1258–1275. DOI: 10.1177/0731684414530790.

Xiong

Vaziri

, et al. Mechanical behavior of carbon fiber composite lattice core sandwich panels fabricated by laser cutting. Acta Materialia 2012; 60: 5322–5334.

10.

Suresh

Loganathan

Sekar

, et al. Influence of water absorption on glass fibre reinforced IPN composite pipes. Polímeros 2019; 29(3): 1–8.

11.

Suresh

Sekaran

Akash

, et al. Evaluation of mechanical behavior of carbon fiber reinforced nanoclay filled IPN matrix composite. Mater Res Express 2019; 6(12): 125311. DOI: 10.1088/2053-1591/ab54ec.

12.

Lima Sobrinho

Ferreira

Bastian

. The effects of water absorption on an ester vinyl resin system. Mat Res 2009; 12(3): 353–361. DOI: 10.1590/S1516-14392009000300017.

13.

Wang

. Interpenetrating polymer networks of polyurethane and graft vinyl ester resin: polyurethane formed with toluene diisocyanate. Eur Polym J 2000; 36(4): 735–742. DOI: 10.1016/S0014-3057(99)00113-5.

14.

Ganesh

Sumesh

Palani

, et al. Investigation on the mechanical properties of ramie/kenaf fibers under various parameters using GRA and TOPSIS methods. Polym Composites 2022; 43(1): 130–143.

15.

Guoqiang

Pang

Helams

, et al. Stiffness degradation of FRP strengthened RC beams subjected to hygrothermal and aging attacks. J Compos Mater 2001; 36(7): 795–812. DOI: 10.1177/0021998302036007614.

16.

Eslami

Honarbakhsh-Raouf

Eslami

. Effects of moisture absorption on degradation of E-glass fiber reinforced vinyl ester composite pipes and modelling of transient moisture diffusion using finite element analysis. Corrosion Science 2015; 90(1): 168–175. DOI: 10.1016/j.corsci.2014.10.009.

17.

Qin

Cai

. Damping properties and morphology of polyurethane/vinyl ester resin interpenetrating polymer network. Mater Chem Phys 2004; 85(3): 402–409, DOI: 10.1016/j.matchemphys.2004.01.019.

18.

Tagliavia

Porfiri

Gupta

. Influence of moisture absorption on flexural properties of syntactic foams. Composites Part B: Eng 2012; 43(2): 115–123, DOI: 10.1016/j.compositesb.2011.06.016.

19.

Feih

Mathys

, et al. Influence of water content on failure of phenolic composites in fire. Polym Degrad Stab 2008; 93(2): 376–382. DOI: 10.1016/j.polymdegradstab.2007.11.027.

20.

Yao

Ziegmann

. Water absorption behavior and its influence on properties of GRP Pipe. J Compos Mater 2007; 41(8): 993–1008. DOI: 10.1177/0021998306067265.

21.

Kamangar

Shokrieh

. A novel model for quantification of the moisture absorption of polymeric laminated composites. Polym Test 2020; 91(1): 106772. DOI: 10.1016/j.polymertesting.2020.106772.

22.

Ashik

RameshSharma

Guptha

VLJ

. Investigation of moisture absorption and mechanical properties of natural/glass fiber reinforced polymer hybrid composites. Mater Today: Proc 2018; 5(1): 3000–3007, DOI: 10.1016/j.matpr.2018.01.099.

23.

Kumar Singh

Khan

Kumar Mishra

, et al. Fabrication and evaluation of mechanical properties of polymer matrix composite using nano fibers as reinforcement. Mater Today: Proc 2021; 46(2): 1376–1383, DOI: 10.1016/j.matpr.2021.02.488.

24.

Srinivasan

Suresh

Ramu

, et al. Effect of water absorption of the mechanical behavior of banana fiber reinforced. IPN Natural Composites 2021; 45(2): 1334–1337, DOI: 10.1016/j.matpr.2020.06.024.