Dynamic Mechanical Analysis and Creep-recovery behaviour of Polyvinyl Alcohol based cross-linked Biocomposite reinforced with Basalt fiber

Polyvinyl alcohol cold M.W. 850000–124000 with degree of hydrolysis 99% having viscosity (4% solution at 20 °C) 23–38 cP with purity greater than 99% was supplied by HPLC, India; Basalt fibers having diameter of 12–8 nm was purchased from NIKUNJ, India and Hydrochloric acid (HCl) M.W. 36.46 with acidimetric 35%–36% was purchased from MOLYCHEM, India.

To confirm the formation of cross-linking bonds Fourier transform infrared spectroscopy (FTIR) and water absorption test were performed. FTIR testing was performed by using ECO-ATR ALPHA, Bruker spectrometer with taking 24 scans at the resolution of 2 cm⁻¹. A spectra was recorded in transmittance mode varying from 600 cm⁻¹ to 4000 cm⁻¹. The morphology of the PVA based cross-linked composite reinforced with basalt fiber was characterized by scanning electron microscopy (SEM), using a JEOL JSM-6490LA microscope working in high vacuum mode, with an acceleration voltage of 5 kV. Water absorption test was performed to determine the water uptake of different composites by immersing the sample of size 1 cm × 2 cm into 100 ml distilled water for 24 h at room temperature. For water absorption test three sample were taken of each composition. Excess water from the sample surface was soaked through tissue paper before measuring the weight of the sample at the different time interval. Water absorption (WA) is calculated by the formula given below:

$$\begin{eqnarray}&&{\rm{WA}}\left( \% \right)=\displaystyle \frac{\left({{\rm{W}}}_{{\rm{2}}}-{{\rm{W}}}_{{\rm{1}}}\right)}{{{\rm{W}}}_{{\rm{1}}}}\times 100\end{eqnarray} \tag{ 1 }$$

Where, W₁ and W₂ are initial and after water uptake weight of the specimen, respectively.

Mechanical testing of the specimen was carried out as per ASTM D882-12 standard and five specimen of similar composition were used. Tensile specimen gauge length was taken 40 mm and testing was performed at speed of 2 mm min⁻¹ for all samples using universal testing machine AMT-SC-01521. Before performing the test, samples were dried into the oven at 70 °C for 1 h.

Thermo-gravimetric analysis (TGA) and derivative thermo-gravimetric (DTG) were performed to study the thermal stability of sample using STA7300, HITACHI. For that first, samples of weight about 3–6 mg were placed on the platinum crucible. The analysis was conducted for 25 °C to 500 °C at the heating ramp rate of 10 °C min⁻¹ under the nitrogen atmosphere at the flow rate of 200 ml min⁻¹. To determine the visco-elasticity response of composites, dynamic mechanical analysis (DMA) was performed on DMA7100, HITACHI under sinusoidal tensile mode. Sample size of 20 mm × 10 mm and temperature range from 25 °C to 180 °C with heating rate of 2 °C min⁻¹ were taken. Creep and recovery behavior of fabricated composites were performed under force-control module in tensile mode. Effect of temperature was studied by maintaining constant stress (4 MPa) for 15 min and recovery behavior was studied for 15 min after removal of stress. The effect of stress on creep behavior was studied at three different stress value (2, 4 and 8 MPa) for the constant temperature of 20 °C.

The major disadvantage of PVA is its solubility in water which restricts its applications. It is known that PVA is obtained by hydrolysis of polyvinyl acetate in which replacement of acetate group (C₂H₃O₂) by hydroxyl group (O-H) occurs. This leads to formation of hydrogen bond between hydroxyl group of PVA and water molecules which result in the solubility of PVA. To overcome solubility and improve mechanical strength, cross-linking by Zimmer's Hydrogenesis (dehydration reaction) and esterification were proposed. Similar esterification reaction between PVA and citric acid also proposed by Brick et al [40]. It was mention that, the major factors which affected the cross-linking were concentration, time and temperature. In present investigation temperature of the reaction maintained constant at 70 °C and the effect of concentration and reaction time on cross-linking has been studied. In dehydration reaction, hydroxyl group (O-H) is replaced by chlorine (Cl) as shown in figure 1 which inhibits the solubility of PVA. Moreover in esterification reaction, two{-C-OH} bonds of same PVA chain (intermolecular) or different PVA chains (intramolecular) react with two HCl to form {-C-O-C-} bonds as shown in figure 2. This results in the interlocking of PVA chains which imparts high mechanical strength. When basalt fibers were introduced in PVA cross-linked matrix, hydrogen bonds are formed between Si of basalt fiber and hydroxyl group of PVA. This results in further decrease in the water absorption of the composites.

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Figure 4 represents the behavior of PVA based cross-linked composite reinforced with basalt fiber under tensile loading. In the present investigation characterization of composites has been carried out on the cross-linked PVA-HCl composite on the basis of tensile properties. In present investigation, the effect of time and normality of HCl on ultimate tensile strength (UTS) and elongation have been studied. It can be observed in figure 4 that the tensile strength of cross-linked PVA with 3.03 N HCl composite increased with increase in time. Whereas, for cross-linked composites with 3.49 N and 3.98 N HCl UTS first increases with time and then reduces. Moreover, UTS of cross-linked composites is almost similar after 24 h as HCl interacts very less with PVA in the liquid state. This is attributed to present of water molecules which result in the low amount of cross-linking. It is observed that with the increase in the normality of HCl, cross-linking increases which result in restriction of movement of PVA chain this lead in the increase of UTS and decreases elongation. With further increase in the normality of HCl, the depolarization of PVA chain occurs causes the decrease in UTS. The same phenomenon is occurring in case of time. It is also observed that when normality remains constant UTS first increases with increase in time and later decreases. From the above results, it has been found that PVA-3.49HCl shows highest UTS of 85 MPa after 48 h.

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Further enhancement of UTS has been achieved by fabricating composites reinforced with nano basalt fiber. It has been found that the UTS of composite increases to 152.5 MPa for 15 wt% content of basalt fiber which is 79.4% higher than PVA-3.49HCl composite and later reduces to 130.45 MPa at 20 wt% content of filer. It is known that basalt fiber consists of metal oxides of Al, Fe, Ca, Mg etc. Further, when it is added to HCl cross-linked risen/matrix it reacts with HCl, result in the replacement of H^- with metals ions of fiber [44–46]. Due to which amount of cross-linking decreases which results in the reduction of binding strength of matrix and hence at higher content UTS of composite decreases. The Modulus of toughness of basalt fiber cross-linked composite had been calculated through the area under stress-strain curve and value are represented in figure 4(e). It can be observed that the toughness decreases with increase in fiber content this can be rationalized in terms of drastic reduction in ductility of the composite.

Analysis of variance is used to investigate the effect of normality of HCl and time on mechanical properties (UTS and elongation) of the cross-linked composite. The result after ANOVA test is presented in 2. It is also noticed that time contributes substantially 70.44% on UTS followed by normality with a mere of 8.23% as in table 2.

Figure 5 represents the variation of water absorption of PVA based cross-linked films and cross-linked composites reinforced with basalt fiber. It is known that pristine PVA is completely soluble in water because of formation of hydrogen bond between the hydroxyl group and water molecules. As discussed before that the cross-linking results in the replacement of C-OH bond into C-Cl and intermolecular/intramolecular {C-O-C} bonds as shown in figure 1. This leads to decrease in the availability of hydroxyl group to form hydrogen bond which causes the restriction in the solubility. It can be noticed the maximum water absorption takes place within the 2 h of time and then remains nearly constant. Moreover, it is also observed that the PVA based cross-linked film absorbed around 128.6% of water whereas, water absorption decreases with increase in the content of basalt fiber. It is suggested that the water absorption of composite depends upon the uncross-linked hydroxyl group. When basalt fibers are introduced in PVA based cross-linked matrix as reinforce material the hydrogen bonds are formed between basalt fiber and hydroxyl group, which cause the further reduction in the availability of hydroxyl group to interact with water molecules. Moreover, with increase in the content of basalt fiber hydrogen bonds are formed between basalt fiber and hydroxyl group which results in the further reduction of water intake of composite as shown in figure 5.

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Figure 6 shows the effect of cross-linking and basalt fiber as reinforcement material on thermal stability of PVA based cross-linked composites. It can be observed that pristine PVA experience weight loss in three temperature regions [39], i.e. 80 °C–150 °C, 230 °C–350 °C and 400 °C–450 °C. It can be attributed to moisture or dehydration of neat PVA in first range, the thermal degradation of PVA into acetone, aldehydes, ethanol etc as the major product and carbon dioxide and carbon monoxide as the major gasses [47] in second temperature range and the further degradation of second range products into carbons and hydrocarbons in third temperature range.

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Thermo-gram (figure 6(b)) of cross-linked composite shows that in second temperature range only 10% of weight loss occurs as compared to Pristine PVA where weight loss is 60%. Moreover cross-linking results in modification of PVA structure which causes the thermal degradation of cross-linked composite occurs only in one step as compared to PVA where it occurs in two steps as shown in figure 6(b). It could also be noticed that the thermal stability of PVA based cross-linked composites reinforced with basalt fiber is higher as compared to PVA. Thermo-grams of composites reinforced with basalt fiber show the shifting in the peak for the second temperature range from 230 °C–350 °C to 300 °C–450 °C as compared to Pristine PVA, this can be rationalized in terms of the formation of hydrogen bonding between basalt fiber and hydroxyl group Si^...OH.

Typical DMA thermo-grams of PVA cross-linked films and cross-linked composites loading with different basalt fiber content are represented in figure 7(a). It is observed that tan δ curve represent two peaks, first from 20 °C to 55 °C temperature range and second 120 °C to 150 °C. Similarly two peaks in tan δ were observed when PVA is cross-linked with citric acid in study of Brick et al [40]. The first peak is referred to beta relaxation, also known as beta transition (T_β) which represents the motion of side chains developed in composite due to cross-linking of main PVA chain during the esterification reaction as shown in figure 1. It is suggested that with increase in temperature material get warm up results in increase of free volume which facilitates the movement of side chains. The movement of the side chain is also favorable by the presence of moisture molecules which act as voids and on heating results in free volume. Moreover, HCl which act as the plasticizer also imparts free space by getting in between the PVA chain. The T_β transition might also be associated with glass transition temperature of small polyvinyl chloride chains which are formed by replacement of C–OH bond into C-Cl. The second peak which is of low intensity and wider than the first one is associated with the alpha transition which represents the movement of main PVA chain. With further increase in temperature free volume increased more which results in large scale movement of main PVA chain. The temperature at which it occurs is known as glass transition temperature (T_g). It is noticed that the intensity of this peak is very low due to the presence of cross-linking bond and basalt fiber which restricts the movement of PVA chain.

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Tanδ curve for basalt fiber reinforced cross-linked composites has lower value as compared to cross-inked composite which represents that the present of basalt fiber increases elasticity of composite. Storage modulus (E') of basalt fiber reinforced cross-linked composites is found to be higher than that of cross-linked composite which infers that mechanical strength of composite increases with basalt fiber loading. In T_g-transition region for cross-linked composite, value of storage modulus decreases drastically as compared to basalt fiber reinforced composite which concludes that presence of basalt fiber increases the mechanical strength of composite for higher temperature.

At β-relaxation and α-transition polymer absorbs energy in degradation of polymer chain molecules under the action of load, which is referred as activation energy and determined by Arrhenius equation [48] given below:

$$\begin{eqnarray}&&In\left(F\right)=In\left(A\right)-\displaystyle \frac{Ea}{RT}\end{eqnarray} \tag{ 2 }$$

Where F represents the frequency in Hz, Ea is the activation energy (kJ/mol), A is pre-exponential factor, R is the gas constant (kJ/mol-K) and T is the temperature (K). The value of activation energy is presented in table 3 at T_β and T_α is obtained from the slope of curve between In(F) and 1/T graph as shown in figure 7(b). From the result it has been found that the total activation energy (i.e. combining the Ea value for T_β and T_α transition) of composites increases continuously with increase in fiber loading and maximum value is obtained at 15% fiber content.

Creep analysis of polyvinyl alcohol based composite has been shown in figure 8. There are mainly three stages in creep behavior that include instantaneous deformation also known as elastic strain (ε_e), visco-elastic strain (ε_v) and viscous strain (ε_∞). In recovery phase, on removal of stress there is instantaneous recovery of strain which represents elastic nature of composite. In addition there is time dependent recovery which represents visco-elastic behavior which is followed by permanent deformation depicting the viscous nature of composite. The constant stress is applied to the sample directly for 15 min and recovery behavior is studied for 15 min after removal of stress. To characterize the creep data Burgers model [49–51] is been used, it consists of Maxwell and Kelvin-Voigt element in series combination.

$$\begin{eqnarray}&&\in \left(t\right)={\in }_{e}+{\in }_{v}+{\in }_{\infty }\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\in \left(t\right)=\displaystyle \frac{{\sigma }_{0}}{{E}_{M}}+\displaystyle \frac{{\sigma }_{0}}{{E}_{K}}\left(1-{e}^{-t/\tfrac{{\eta }_{K}}{{E}_{K}}}\right)+\displaystyle \frac{{\sigma }_{0}t}{{\eta }_{M}}\end{eqnarray} \tag{ 4 }$$

Where $\in (t)$ is creep strain, E₁ and E₂ represent Maxwell and Kelvin spring elastic modulus, and η₁ and η₂ represents viscosities of Maxwell and Kelvin dashpots. Time required to generate 63.2% of deformation in Kelvin unit known as retardation time $\tau \left(\tau =\tfrac{{\eta }_{2}}{{E}_{2}}\right).$ The value of above parameters has been calculated and presented in table 4.

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Figures 8(a)–(c) shows the effect of stress (2 MPa, 4 MPa and 8 MPa) on the creep and recovery behavior of PVA based composite at 20 °C temperature. Results show that composites have varying response for different stress level. It can be observed that creep deformation is accelerated with increase in stress level. Moreover, cross-linked and reinforced composites resist creep deformation at each level of stress which results in the higher stiffness to creep deformation as compared to neat PVA. This is attributed to the formation of dense structure in composites as shown in figure 1 which causes the restriction in the movement of PVA chain. As it is also noticed that the creep deformation is further reduced for basalt fiber reinforced composite due to the restriction of the main chain. The response of basalt reinforced composite under stress depends upon the interfacial fiber/matrix bonding. The low creep deformation can be attributed to the higher interfacial adhesion between PVA and basalt fiber which also results in the better mechanical properties than PVA. Unrecovered creep deformation of cross-linked and reinforced composite is found to be lower than PVA which shows the higher elastic nature of cross-linked and reinforced composite. Effect of temperature on creep and recovery behavior has also been studied as shown in figures 8(d)–(e). It is noticed that with increase in temperature stiffness of composite is decreased which results in higher instantaneous deformation and creep strain. With increase in temperature, creep strains of PVA increases extensively whereas, cross-linking largely reduces creep deformation followed by reinforcement with basalt fiber which shows less temperature dependence of these composites. During unloading unrecovered creep strain of PVA is higher as compared to cross-linked and reinforced composite which shows the higher viscous behavior of PVA as compared to cross-linked and reinforced composite.