Effect of different types of water soaking and re-drying on mechanical properties of glass fiber-epoxy composites

Abstract Fiber-reinforced polymer (FRP) composites undergo different environmental conditions in the designed life span. An investigation on aging of FRPs in water helps to enhance the material’s durability. This article aims to explore the effect of different types of water soaking (viz., tap, sea, and rain water) and re-drying conditions on the tensile and flexural strengths of glass fiber-epoxy composites (GFEC). The prepared specimens are placed in the sea, tap, and rain water for 10 weeks. Some specimens are re-dried at 50°C for 5 weeks. The water uptake (%) of seawater aged specimens decreases compared to tap and rain water-aged specimens at the end of 10 weeks. Water soaking of composites has reduced the tensile and flexural strengths of GFEC by 14 to 17%. The strength of water-soaked composite specimens is partly retrieved in specimens that have been re-dried. More than 90% of the original value is retrieved for the strength of GFECs (as-made specimens). SEM analysis of the tensile specimen fracture surface reveals the causes of the specimen failure as well as the impact of water soaking, followed with re-drying.


Introduction
The demand for exceptional performing and lightweight composites to substitute conventional metals is growing in several applications.It can be predicted that composite materials will not only be used to manufacture the new components but also to repair the old ones.Fiber-reinforced polymer (FRP) composites are a new class of materials that are being utilised and experimented extensively in various fields in place of conventional materials."Glass fiber-epoxy composites" (GFECs) are generally utilised in a wide variety of applications, for instance, car body, insulation wall, overpasses, ship hulls, and domestic applications (Shelly et al., 2018;Singh et al., 2013).
However, environmental factors and other conditions have adverse impacts on the quality of composite.Once subjected to the working environment, composite material absorbs the moisture, which declines the matrix-controlled properties (Frigione & Lettieri, 2018).Therefore, an investigation on the aging behaviour of FRPs under water immersion is helpful in enhancing the material's durability.Zhong et al. (2019) investigated the effect of water soaking on the tensile strength of the GFEC prepregs.The specimens are kept at 80°C in tap water for 38 days.It is observed from the results that the rate of moisture absorption is higher at the initial period which diminished later.The tensile strength of the water-soaked specimen has reduced by 28%, in comparison to the as-made samples.Kattaguri et al. (2020) did his research on analyzing the impact of seawater aging on the flexural strength of GFECs.The specimens are seawater aged for 150 days at room temperature, maintained below 25°C.It is observed from the water uptake percentage vs.aging time graph that initially the curve is linear.As the aging time increases, water uptake becomes slow; after 90 days, the curve becomes almost flat, indicating saturation of water uptake.Flexural tests are conducted on both as-made and aged specimens (at intervals of 30 days up to 150 days).Results show the trend of decrement in flexural strength with aging time.The flexural strength decreases due to the epoxy chain split caused by hydrolysis induced by water.
Although many studies have already been conducted on this current issue, many questions remain unanswered and most researchers have claimed that moisture sorption has a detrimental effect on the FRP properties (Kini et al., 2018;Krauklis et al., 2019;Najafi et al., 2017;Rafiq et al., 2014).The observed results imply that the three components of the composite-the fibers, the matrix, and the fiber and matrix interface-are subject to a number of intricate transformations.Moisture diffusion into the composite causes physical and chemical aging, viz., plasticization of matrix (Saidulu & Manzoor Hussain, 2018), hydrolysis (Shettar et al., 2018), swelling (Glaskova-Kuzmina et al., 2019), interface de-bonding (Chani et al., 2019), formation of crack.Obradović et al. (2021) investigated the effect of water immersion and re-drying effect on the tensile strength of Kolon/epoxy composites.The specimens are immersed for 72 days, followed with re-drying.The tensile strength of the majority of the specimens exposed to water immersion and then re-drying are not significantly changed; however, they have somewhat lower values compared to as-made specimens.Guo et al. (2021) reported the findings of effect of water immersion and re-drying on the properties of carbon/glass fiber reinforced epoxy hybrid composite rods.It is concluded that, the water molecules that are present in the hybrid rod included both bound and free water, and a linear connection is found between the two types of molecules.The reversible effect of resin plasticization and the irreversible effect of interface de-bonding caused by the ingression and diffusion of water molecules results in the linear deterioration of properties with water content.
The current work explores the influence of different types of water soaking, viz., tap, sea, and rain water, on the mechanical properties of the GFECs.Also, the present study addresses and compares the as-made, water soaking, and re-drying conditions on tensile and flexural strengths of GFECs.Also, fractured surface of tensile test specimens are studied to learn the reasons for the failure of specimens.The impacts of different types of water-soaked and re-dried environments on the properties of GFECs are not well documented in the literature.This appears to be significant in extending the array of applications for the FRPs, especially in the automobile, maritime, civil, and domestic areas.This research intends to determine if re-drying of the water-soaked composites allows the composite properties to be recovered.Also, the work aims to establish the relationship between tensile strength and fracture surfaces using SEM.

Materials and specimen preparation
Epoxy resin (Lapox −12) and hardener (K-6) are purchased from "Atul Polymers" and have a 10:1 mixing ratio.Lapox-12 is a non-modified liquid epoxy with a medium viscosity (9 K-12 K mPa.s).K-6 is a low viscosity liquid hardener, which cures at room temperature.Epoxy and hardener are easily combined at room temperature, and the resulting product has a little pot life and quick curing.The 360 GSM bi-directional E-glass fiber reel is purchased from "Yuje Enterprises, Bengaluru" and is used as reinforcement.
GFECs are made using hand lay-up method and compression molding.The glass fiber is kept at 50 weight percentage.The laminates have a 300 mm × 300 mm × 3 mm size.The bi-lateral tolerance of 1% is retained.The laminates are compressed manually with a stopper in order to provide a constant thickness.All laminated composites have a 3 mm thickness marinade.Cast composites' thickness is not homogeneous prior to compression moulding due to the hand lay-up process.Therefore, in order to achieve uniform thickness, an additional procedure such as compression molding is required.The cured laminate (Figure 1 (a)) is then cut into GFEC specimens (Figure 1 (b)) with the necessary dimensions.

Water-soaking test
The prepared specimens are placed in the sea, tap, and rain water for 10 weeks at room temperature.In order to determine the difference in weight and the percentage of water absorption, the samples are weighed once in a week for 10 weeks as per ASTM D570-98 standard, using a digital weighing device (Model: Jee-Lit GJE Precision Balance GJEJW600 with least count of 1 mg).Some of the specimens are re-dried in an oven at 50°C for 5 weeks.
The following equation is used to calculate the percentage of water uptake: where Wws-Water-soaked specimen weight; Wam-As-made weight of specimen.

Testing
Following tests are carried out on as-made, water-soaked, and re-dried specimens.Five specimens of each condition are subjected to each tests.

Tensile test
The ultimate tensile strength is determined following the ASTM D3039 standard.Tensile test specimens are cut to the dimension 250 mm × 25 mm × 3 mm.The speed of the test is maintained at 2 mm/min using the "ZWICK-ROELL Z020, LOADCELL 20 kN".

Flexural test
The flexural strength is determined by the three-point bending test following ASTM D7264.Flexural test specimens are cut with a dimension 128 mm × 13 mm × 3 mm.The specimens are tested at 1 mm/min speed using the "ZWICK-ROELL Z020, LOADCELL 20 kN".During testing, a 20:1 ratio for span: thickness is retained.

SEM analysis
A "Scanning Electron Microscope (ZEISS, Model: EVO18)" is used to take the SEM images of the specimen's fractured surface.To fit in the specimen holder of the microscope, samples must be sliced to the appropriate sizes.For efficient imaging, the sample surface must be conductive.Utilizing a small sputter coater ("Model: SC7620") machine, a tiny layer of conductive material is applied to the specimen's surface for 10 minutes.The gold-palladium (at the ratio of 80:20) sputtering object is used by the sputter coater device.

Water uptake
As-made specimen and different water-soaked specimens after 70 days of water soaking are shown in Figure 2. The effect of immersion time (number of days) in sea, tap, and rain water on water uptake (%) of GFEC is shown in Figure 3.It clearly shows that in all conditions, water uptake increases as the duration of immersion increases.Water uptake is quite fast in the beginning, though, the water uptake rate declines over time.In all cases, composite shows the usual water uptake polymer behavior owing to the "no moisture-absorption property" of glass fiber.
The water absorption characteristics of GFRP are being thoroughly studied using two different methodologies.One of these is the free-volume method, where water diffuses into the polymer  and stays in the material's free volume (Quino et al., 2018).The space that is not occupied by molecules is known as the free volume.Additionally, immersing in water may cause fiber-related mechanisms to move water molecules in the composite.These mechanisms consist of the diffusion of water molecules through the matrix or the diffusion of water molecules along the fibermatrix interface.
The second method analyses the relationship between the polymer and the penetrant and is known as polymer-water affinity or hydrophilicity (Bal & Saha, 2017).The structure of the polymer has a significant impact on how water diffuses into the polymer used in the matrix.The hydrophilicity of the polymer is predicted using the number of H 2 -bonding sites created in the polymer as a result of interactions with water molecules.As most glassy polymers, like epoxy, include a hydroxyl group, they are hydrophilic by nature and will absorb water molecules.
The water uptake (%) of sea water-aged specimens decreases compared to tap and rain wateraged specimens at the end of 10 weeks.Since seawater also includes a variety of minerals, including salt, which are absorbed by the samples at the same time as moisture.In contrast, in tap and rain water, comparatively fewer contaminants exist.Although the initial rate of weight increase in seawater is higher, with time, the pores over the surface get clogged as more salt and other minerals build up on the pore's gate, which may prevent moisture absorption.As a result, the specimen is prevented from absorbing more water infiltration due to an osmotic pressure created by the differential in salt particle concentration between the specimen and seawater (Chakraverty et al., 2015;Hu et al., 2014).

Tensile strength
From Figure 4, it can be observed that the tensile strength of GFEC decreases under different water soaking conditions compared to as-made conditions.The tensile strength should be assessed to enhance the service life and design of materials used in various applications since the strength of composite materials impacts their service life.Water-soaking conditions have reduced the tensile strength of GFEC by 14 to 17%.Water soaking can lower the tensile strength of the GFEC by plasticizing the matrix and reducing the interfacial bonding of the epoxy and glass fiber.The degradation of the interface brought on by the interaction of water molecules with the polymer matrix has a negative impact on the mechanical properties.The main causes of the reduced mechanical properties include hydrolysis, interface de-bonding, matrix swelling, physical damage to the interface, and interface degradation (Wang et al., 2015).
The maximum tensile strength reduction is under seawater soaking condition, i.e., 195 MPa as compared to other conditions viz., 240, 200, and 203 MPa under as-made, tap water, and rain water conditions, respectively.This is in agreement with the study results of Sugiman et al. (2019), on salt water and distilled water durability of glass fiber-polyester composites.At the end of 30 days of aging, the tensile strength of salt water-aged specimens is lower than that of distilled water-aged specimens.

SEM analysis
The composites' fractured surface is examined using a scanning electron microscope.Tenisle test specimen's fractured surface SEM images of as-made (dry/unaged), tap water, seawater, and rain water environments are shown in Figures 5-8.The specimens failed due to pull-out fiber, matrix rupture, matrix-fiber de-bonding, and fiber breaking when subjected to tensile load.The fiber and matrix have a strong bond in the as-made (dry/unaged) condition (Figure 5), and fiber pull-out is found to be the primary cause of specimen failure.A significant number of matrix residues are revealed surrounding the glass fibers, which shows that the glass fibers and matrix resin have a strong bond.
Failure of the specimens under water-immersed conditions (Figures 6-8) occurs due to matrix degradation, including cracking or rupture of matrix, and fiber-matrix interface degradation.Figure 6 shows that after a prolonged immersion in tap water, the pulled out fibers are not having traces of matrix owing to the deterioration of the fiber-matrix interface.Under sea water-immersed conditions, engrossed moisture damages the matrix (Figure 7) and fibermatrix interface, leading to cracks' initiation and propagation.Along with de-bonding at the fiber-matrix interface, water can also cause the epoxy resin to swell and result in plasticization.The fibers and epoxy bonding are weakened, and small bits of epoxy are spotted on the surface.During rain water soaking, water molecules fill the microscopic spaces, which causes additional voids and fractures to form, ultimately deteriorating the interface (Figure 8).Also, specimens show extensive detaching of fibers (fiber imprints) from the surrounding matrix after failure.The aforementioned causes are what cause composite materials to lose strength when submerged in water.The 3-point bending test exhibits the material's response to simultaneous tensile and compressive loadings for flexural strength.Figure 9 illustrates the flexural strength of GFEC under various soaking conditions.Five specimens are examined for each condition to prevent instrumentation mistakes, and mean flexural strength data are reported in the present work.The flexural strength of GFEC is highest in as-made (dry/unaged) specimen (360 MPa).Water-soaking conditions have reduced the flexural strength of GFEC to 308, 297, and 312 MPa under tap, sea, and rain water conditions, respectively.The primary mechanism for water uptake is diffusion, and the polarity of polymer chains facilitates some absorption.Polymer chains are forced apart as a result of water molecules occupying spaces between them and penetrating into the spaces between the chains.The polymer chains generally become more mobile due to the plasticizing effect of water molecules, which lowers the material's flexural strength (Jesthi & Nayak, 2019).Additionally, moisture-induced matrix swelling has been implicated in the reduction of the polymer matrix's stiffness (Nayak et al., 2016).

Tensile strength
Figure 10 shows that the tensile strength of the re-dried GFEC specimens is higher for all three water-soaking conditioned specimens.Due to the removal of the majority of the water from the polymer network and the low impact of plasticization, the tensile strength of water-soaked specimens is partially recovered in specimens that have been re-dried.More than 90% of the original value is retrieved for the tensile strength of GFECs (as-made specimens).This recovery of the tensile strength may be due to the reversible effects of the water saoking, viz., swelling and plasticizing effect on the matrix and the fiber-matrix interface.However, the unrecovered portion of the tensile strength can be endorsed to the perpetual damage forced by water (Quino et al., 2020).
The tensile strengths of re-dried specimens (i.e.,220,215,and 217 MPa) are still less than that of as-made specimens (i.e., 240 MPa).A similar finding is reported by Zhong et al. (2019).The study includes immersing carbon-fiber epoxy composites into the water at 80 °C and re-dried in an oven at 45°C.The tensile strength of carbon-fiber epoxy composites is said to be restored to 95.75% of its original value after re-drying.while not allowing for total water removal.These microcracks formed as a result of the re-dried conditioned specimens' reduced fracture toughness relative to the as-made specimens.In comparison to as-made specimens, this could be the cause of the specimens' inadequate recovery of tensile strength.

Flexural strength
As shown in Figure 12, after re-drying (i.e., removing the moisture absorbed), the flexural strength GFECs (i.e., 335, 329, and 339 MPa) are higher than that of the water-soaked specimens (i.e., 308, 297, and 312 MPa under tap, sea, and rain water conditions, respectively) and lesser than as-made specimens (360 MPa).After re-drying, more than 90% of flexural strength is recovered as compared to as-made specimens.This supports the idea that water absorption permanently reduces the strength of the fiber-matrix interface.The results of the experiment therefore lend credence to the idea that potential fibre and moisture-induced interface deterioration causes irreparable harm inside the GFECs, which in turn diminishes the strength of the composite.As the matrix properties contribute significantly to the flexural strength of GFEC composites, it is judicious to assume that under re-dried conditions, the adverse changes in the matrix, interface, and fiber-matrix debonding are primarily accountable for the irreparable degradation (Tamrakar et al., 2021).

Conclusions
This study aims to establish how different types of water-soaking conditions and re-drying influence the properties of GFECs.Mechanical properties, viz., tensile and flexural strength, are determined.The three different water-soaking (i.e., tap, sea, and rain water) and subsequent re-drying effects on the mechanical properties of GFECs are investigated.The following conclusions are drawn.
(1) The water uptake (%) of seawater aged GFEC specimens decreased compared to tap and rain water-aged specimens at the end of 10 weeks.Although the initial rate of weight in seawater is higher, with time, the pores over the surface get clogged as more salt and other minerals build up on the pore gate, which may prevent moisture absorption.
(2) Tensile and flexural strengths of GFEC tend to decrease in all three water-soaking conditions; however, the decrease of both strengths in saltwater is severer than in tap and rain water.
(3) Re-drying of GFECs cannot eliminate all the water absorbed and allows only a partial recovery of the original mechanical properties of the composites.
(4) The reasons for the specimen failure under tensile load are identified by SEM analysis of the fracture surface.The SEM images display shear leaps, matrix rupture, and fiber pull-out.
(5) The applications of GFEC composites could thus be expanded in the automobile, marine, civil, and domestic sectors based on the findings and discussions.
(6) The effect of re-drying on the composite properties before the saturation level needs to be investigated further.Furthermore, it needs to be determined if there is a temperature or moisture content threshold at which degradation becomes irreversible.

Figure 2 .
Figure 2. As-made specimen and different water-soaked specimens.

Figure 4 .
Figure 4. Tensile strength of composite under different water soaking conditions.

Fiber
Figure Fracture surface SEM image of tap water-soaked specimens.
Figure Fracture surface SEM image of rain water-soaked specimens.
Figure 10.Tensile strength of composite under different water soaking and re-dried conditions.