Effect of Fillers on the Recovery of Rubber Foam: From Theory to Applications

The chemical compositions of the control NRF and NRF with fillers were analyzed by ATR–FTIR ( Figure 3 ). There is no significant difference in the functional groups of NRF [ 7 23 ]. There is almost no difference for the NRF with charcoal loading due to carbonization at high temperatures, which causes the charcoal powder to exhibit a hydrophobic nature [ 24 ]. However, there is a band growing at 1100 cmfor the NRF filled with silica. This band corresponds to the vibration absorption of the silane group (Si–O–C) [ 25 ], present in the rubber network, which usually exhibits within the ranges 800–850 and 1100–1200 cm

As presented in Figure 2 , the crosslinking density of the NRF samples increases with the presence of filler, which may be due to the additional carbon–sulfur linkages formed by the chemical reaction between the rubber and filler [ 22 ]. Another reason is that the amplification of the deformation of rubber chains in the NRF with filler loading is more than the control NRF. Fillers in NRF extend rubber chains due to the interaction of rubber chains at the filler surface, i.e., some rubber chains may be occluded in the voids of the filler, causing the extension of rubber chains and leading to an increased crosslinking density. However, at the same loading of vulcanizing chemicals, increasing filler loading causes fewer differences in the crosslinking density.

The density of the NRF samples increases with increasing filler loading, as presented in Figure 1 . This is due to increasing filler loading, which causes an increase in the mass of the NRF with filler. Kudori and Ismail [ 18 ] found that foam density increases as the filler size decreases. The smaller filler size hinders pore formation and increases the continuous matrix amount. In the present study, silica filler (nanometer) is smaller than charcoal (micrometer). Therefore, NRF with silica loading exhibits a higher density than NRF with charcoal loading at a given filler concentration. Increasing charcoal loading barely affects the density, which may be because charcoal acts as a nucleating agent during the process of foam growth [ 19 21 ].

As presented in Table 2 , increasing charcoal loading increases the average NRF pore size, decreasing the porosity and cell density. On the other hand, increasing silica loading decreases the average pore size, increasing the porosity and cell density. Although the density of the NRF increases with increasing filler loading, the cell density is more complicated. For charcoal loading, the density slightly increases or remains almost constant with the addition of more than 4 phr of charcoal. The cell density of the NRF with charcoal loading decreases and becomes almost constant with the addition of more than 4 phr of charcoal. Since charcoal can act as the nucleating agent, which can promote foam growth, excess charcoal may stop acting as a filler and become a nucleating agent, resulting in an almost constant density with a larger average pore size and smaller porosity and cell density. For silica loading, the density increases as increasing silica loading, indicating the decreasing of the average pore size while the cell density increases.

The morphological properties of NRF were investigated by SEM. The micrographs indicated that all types of NRF contain a cellular structure that exhibits an interconnected network of open cells, as presented in Figure 6 . Porosity analysis was determined by the ImageJ software by adjusting the threshold of the images. The white region corresponds to the pore shape, whereas the dark region corresponds to the open holes or pores ( Figure 6 ). The interconnected porosity of these NRFs is an important parameter that affects the mechanical properties [ 7 ]. This result can be explained by the cell density value. The cell density () is calculated as follows [ 26 ]:whereis the foam density,is the solid phase density (NR 0.93 g/cm), andis the average cell radius.

The compression set describes the elastic behavior of the NRF, which relates to the material’s recovery percentage. Figure 5 shows that increasing charcoal loading increases the compression set percentage and decreases the recovery percentage. As mentioned above, the addition of more than 2 phr of charcoal causes the foam to be sticky. Thus, when the NRF with more than 2 phr of charcoal loading is compressed at 75%height of its thickness for a long period (72 h), the ability to return to its original shape is decreased. On the contrary, increasing silica loading decreases the compression set percentage and increases the recovery percentage. Decreasing the compression set percentage indicates higher elasticity. Hence, the NRF with silica loading possesses higher elasticity than the NRF with charcoal loading. Microsized charcoal has been shown to behave like eggshell powder and rice husk powder in previous works [ 12 13 ], which decreased the percentage of NRF recovery when filler loading is increased and vice versa with NRF-filled nanosized silica. Therefore, we can propose the relationship between recoverability of NRF and filler concentration as the following polynomial equation:where %is the recoverability of the NRF with charcoal loading (%), %is the recoverability of the NRF with silica loading (%), [Ch] is the concentration of charcoal filler (phr), and [Si] is the concentration of silica filler (phr).

The control NRF and NRF with fillers were compressed up to 75% ( Figure 4 ). The compression strength at maximum compression shows that the NRF with silica loading has higher compression strength than the NRF with charcoal loading at a given filler concentration. There are two different regions in the compression stress–strain curves of foam materials: elasticity at the low-strain region and solidity at the high-strain region [ 7 ]. Increasing filler loading increases the solidity or stiffness of the NRF at high strain, where the foam cells with each other, leading to the immediate increase of compression stress. There is also the stress-induced crystallization of rubber chains that affects the increase of compression stress at high strain. The addition of more than 2 phr of charcoal causes the foam to be sticky, explaining the unfavorable interaction within the foam structure. Although the crosslinking density of the NRF with various silica loadings is almost identical, the compression strength is significantly different. The better interaction within the foam structure is due to the smaller silica filler size, which has more specific surface areas compared to the charcoal filler. We found a linear relationship between compression strength and filler loading in both types of fillers. This relationship depends on the rubber–filler interaction, presented as:whereis the compression strength of the NRF with charcoal loading (kPa),is the compression strength of the NRF with silica loading (kPa), [Ch] is the concentration of charcoal filler (phr), and [Si] is the concentration of the silica filler (phr).

3.3. Thermodynamic Aspects

28,30,

F   =   ( ∂ U ∂ L ) − T ( ∂ S ∂ L )   =   F u   +   F s

(11)

F u   =   ( ∂ U ∂ L )    

(12)

F s   =   − T ( ∂ S ∂ L )    

(13)

F

is the compression force causing changes in the length of NRF (

L

),

U

is the internal energy of NRF,

T

is the temperature used in the experiment, and

S

is the entropy of NRF. When plotting the compression force graph as a function of the conducted temperature, the interception at 0 K is equal to

F

u, and the slope multiplied by the temperature is equal to

F

s.

Thermodynamic studies of the deformation process in uncrosslinked rubbers have already been discussed [ 27 29 ]. Most of the works showed the results of the temperature dependence of the stress in the extended state. In the present work, the mechanical compression properties of the crosslinked NRF samples are remarkable, especially for the %compression set and %recovery; thus, it is interesting to investigate the thermodynamic aspects. From the perspective of thermodynamics, the elasticity of rubber attributes to the changes in the conformations of rubber molecules from the unstrained molecules to the strained molecules. Such changes are related to the changes of internal energy and entropy associated with the deformation process as the following relationship [ 27 31 ]:whereis the compression force causing changes in the length of NRF (),is the internal energy of NRF,is the temperature used in the experiment, andis the entropy of NRF. When plotting the compression force graph as a function of the conducted temperature, the interception at 0 K is equal to, and the slope multiplied by the temperature is equal to

To investigate the relationships between force (compression mode) and temperature, the NRF samples were compressed up to 20%strain, 30%strain, 40%strain, 50%strain, 60%strain, and 70%strain in the temperature controller chamber (at 298.15, 308.15, 318.15, 328.15, and 338.15 K). The relationships between compression force and temperature of the control NRF, NRF with 8 phr of charcoal, and NRF with 8 phr of silica are presented in Figure 7 Figure 8 and Figure 9 , respectively. The graphs of the other samples are presented in Figures S1–S6 . The results reveal that the compression force to the sample increases with increasing %strain. At a given strain, the compression force decreases with increasing temperature. Moreover, the slope decreases at a higher strain due to a decrease of the rubber chains’ degree of freedom in the NRF, which is unfavorable for entropy.

Figure 7,

F

) and relative internal energy contributing to the compression force (

F

u/

F

) at 298.15 K can be calculated as indicated in

F

u and

F

increase with increasing compression strain, whereas the values of

F

u/

F

decrease. Since the internal energy (

U

) is varied by the compression force (

F

), the internal energy increases with increasing compression force. The entropy (

S

) can be varied by the length of the NRF, indicating the degree of freedom of rubber chains during the compression process. The compression causes a reduction in the length of the NRF, leading to a decrease in the rubber chains’ degree of freedom. Thus, the entropy of compressed NRF is also reduced.Figure 8 and Figure 9 show the values of the compression force () and relative internal energy contributing to the compression force () at 298.15 K can be calculated as indicated in Table 3 . The results of the other samples are presented in Table S1 . The values ofandincrease with increasing compression strain, whereas the values ofdecrease. Since the internal energy () is varied by the compression force (), the internal energy increases with increasing compression force. The entropy () can be varied by the length of the NRF, indicating the degree of freedom of rubber chains during the compression process. The compression causes a reduction in the length of the NRF, leading to a decrease in the rubber chains’ degree of freedom. Thus, the entropy of compressed NRF is also reduced.

F

u/

F

values of uncrosslinked rubber in the extension mode are typically in the range of 0.1–0.2 [

F

u/

F

values of the crosslinked NRF in the compression mode are in the range of 0.6–0.8, which are approximately three times higher than those of the literature review. The difference in

F

u/

F

values could come from the material structures (uncrosslinked rubber vs. crosslinked rubbers) and the test methods (extension mode vs. compression mode). The NRFs with fillers have higher

F

u/

F

values than the control NRF at a given strain level, possibly explained by the interaction of rubber and filler, which promotes changes of entropy in the deformation process. The slope direction of the

F

u/

F

values of the NRF with charcoal and NRF with silica are different (

F

u/

F

values. However, the NRFs with silica loading possess a similar degree of freedom of rubber chains at different compression limits. This indicates that the stability of the degree of freedom of rubber chains at different compression strains or compression limits (λ) is related to the high mechanical property of the NRF with silica loading. Although the compression strength of the NRF with filler increased with the increment of filler, the ratio of

F

u/

F

indicates that the addition of filler affects the mechanical properties in the aspect of thermodynamics.

Thevalues of uncrosslinked rubber in the extension mode are typically in the range of 0.1–0.2 [ 27 ]. In this work, thevalues of the crosslinked NRF in the compression mode are in the range of 0.6–0.8, which are approximately three times higher than those of the literature review. The difference invalues could come from the material structures (uncrosslinked rubber vs. crosslinked rubbers) and the test methods (extension mode vs. compression mode). The NRFs with fillers have highervalues than the control NRF at a given strain level, possibly explained by the interaction of rubber and filler, which promotes changes of entropy in the deformation process. The slope direction of thevalues of the NRF with charcoal and NRF with silica are different ( Figure 10 ). This is due to the control NRF and NRF with charcoal loading possess different degrees of freedom of rubber chains at different compression limits, and lower compression limit leads to lowervalues. However, the NRFs with silica loading possess a similar degree of freedom of rubber chains at different compression limits. This indicates that the stability of the degree of freedom of rubber chains at different compression strains or compression limits (λ) is related to the high mechanical property of the NRF with silica loading. Although the compression strength of the NRF with filler increased with the increment of filler, the ratio ofindicates that the addition of filler affects the mechanical properties in the aspect of thermodynamics.

G

) and entropy (Δ

S

) in the NRF with fillers compared to the control NRF. These thermodynamic parameters can be calculated by the Flory–Huggins equation and statistical theory of rubber elasticity as follows [

∆ G =   R T [ ln ( 1 − V r ) + V r + V r 2 χ ]

(14)

    ∆ S =   − ∆ G T    

(15)

R

is the gas constant (8.3145 J/mol·K), and

T

is the test temperature (300.15 K).

We also investigated the change in Gibbs free energy (Δ) and entropy (Δ) in the NRF with fillers compared to the control NRF. These thermodynamic parameters can be calculated by the Flory–Huggins equation and statistical theory of rubber elasticity as follows [ 17 32 ]:whereis the gas constant (8.3145 J/mol·K), andis the test temperature (300.15 K).

G

and Δ

S

are shown in

V

r) with fillers is higher than the control NRF. The swelling behavior of the NRF with various filler loadings decreases with increasing filler loading, indicating that the filler enhances the rubber swelling resistance against the penetration of the solvent. Since the filler is the hard phase, which is impermeable to solvent molecules, there must be a higher interaction between phases and more rubber chains attached to the filler surface. Hence, the swelling ability of NR is reduced while increasing the volume fraction of rubber [

From the perspective of the crosslinking density, Δand Δare shown in Table 4 . The volume fraction of rubber () with fillers is higher than the control NRF. The swelling behavior of the NRF with various filler loadings decreases with increasing filler loading, indicating that the filler enhances the rubber swelling resistance against the penetration of the solvent. Since the filler is the hard phase, which is impermeable to solvent molecules, there must be a higher interaction between phases and more rubber chains attached to the filler surface. Hence, the swelling ability of NR is reduced while increasing the volume fraction of rubber [ 17 ].

G

, which decreases with increasing filler and is a favorable spontaneous system. This is due to the restriction in the ability of the rubber chain motion in the presence of filler, resulting in a decrease in the Gibbs free energy, which can be attributed to good compatibility between polymer and filler [

S

increases with increasing filler loading, which is favorable in thermodynamics. This result is in good agreement with the result of the

F

u/

F

value.Table 4 shows that all samples present a negative Δ, which decreases with increasing filler and is a favorable spontaneous system. This is due to the restriction in the ability of the rubber chain motion in the presence of filler, resulting in a decrease in the Gibbs free energy, which can be attributed to good compatibility between polymer and filler [ 17 32 ]. Δincreases with increasing filler loading, which is favorable in thermodynamics. This result is in good agreement with the result of thevalue.

E’

) and tan δ results of the NRF with filler loading were determined by temperature sweep using dynamic mechanical analysis or DMA. The results are presented in

Based on the dynamic mechanical properties of the sample, the storage modulus () and tan δ results of the NRF with filler loading were determined by temperature sweep using dynamic mechanical analysis or DMA. The results are presented in Figure 11 . The addition of filler, both charcoal and silica, affects the storage modulus and tan δ.

V

f), which causes a higher stress relaxation rate of rubber molecules where they require more time to unload the applied force [

S

) of the NRF with filler loading is more pronounced compared to the control NRF due to the stress relaxation rate of rubber chains from the amplification of chain deformation between the rubber and filler interaction, as shown in Equation (16).

∆ S =   ∆ V f · ∆ S t T

(16)

S

is the change of entropy, ∆

V

f is the change of volume fraction of filler, ∆

S

t is the change of stress relaxation rate of rubber molecules, and

T

is the temperature.

The DMA results presented in Table 5 reveal that the storage modulus in both the glassy state and rubbery state of the NRF with filler loading is higher than the control NRF. The addition of filler decreases the free volume within the foam, which causes more rigidity, resulting in a higher storage modulus in the glassy state [ 33 ]. The storage modulus in the rubbery state of the control NRF is lower than the NRF with filler loading. This is due to the effect of the filler on the relaxation time of rubber chains. Increasing filler loading increases the volume fraction of filler (∆), which causes a higher stress relaxation rate of rubber molecules where they require more time to unload the applied force [ 17 33 ]. This affects the degree of freedom of the rubber molecules to be more pronounced, i.e., when there is a greater number of interactions between the rubber chains and filler, the stress relaxation rate is increased, resulting in an increase in entropy [ 33 34 ]. Therefore, we can propose a model of the control NRF ( Figure 12 a) compared to the NRF with filler loading ( Figure 12 b). The thermodynamic meaning of this work can be explained as follows: the change in entropy (∆) of the NRF with filler loading is more pronounced compared to the control NRF due to the stress relaxation rate of rubber chains from the amplification of chain deformation between the rubber and filler interaction, as shown in Equation (16).where ∆is the change of entropy, ∆is the change of volume fraction of filler, ∆t is the change of stress relaxation rate of rubber molecules, andis the temperature.

T

g value or peak of tan δ of the NRF with various fillers. The addition of filler causes this value to shift toward higher temperatures when compared to the control NRF. The shift of the

T

g value toward the higher temperatures indicates ionic and hydrogen bonding interactions between the rubber chains and filler [

t

A, resulting in lower activation enthalpy (∆

H

a) than the control NRF.Table 5 also presents thevalue or peak of tan δ of the NRF with various fillers. The addition of filler causes this value to shift toward higher temperatures when compared to the control NRF. The shift of thevalue toward the higher temperatures indicates ionic and hydrogen bonding interactions between the rubber chains and filler [ 17 ]. However, the nonpolar charcoal might not disperse well in the concentrated natural latex or agglomerate and, instead, form filler–filler networks within the foam. This may cause a synergy effect where the filler–filler networks might defeat the movement of the free chains of rubber. Therefore, the addition of charcoal affects higher hysteresis with increasing tan δ max and, resulting in lower activation enthalpy (∆) than the control NRF.

H

a in this work are in the same order as in the works of Sadeghi Ghari and Jalali-Arani [

At the same time, NRF is well-reinforced with silica. The rubber chains within the NRF with silica are hindered to freely move, and there are interactions between rubber–filler within the NRF. Thus, it has a higher activation enthalpy than the control NRF. The tan δ max of the NRF with filler has a higher value than control NRF refers to more dissipation energy of the NRF in the existence of filler. The values of ∆in this work are in the same order as in the works of Sadeghi Ghari and Jalali-Arani [ 17 ].