Article pubs.acs.org/IECR
Effect of Rubber−Filler Interaction on Transport of Aromatic Liquids through High Density Polyethylene/Ethylene Propylene Diene Terpolymer Rubber Blends Anil Kumar P. V.,† K. T. Varughese,‡ and Sabu Thomas*,§,⊥,¶ †
School of Technology and Applied Sciences, Mahatma Gandhi University, Pullarikunnu Campus, Malloossery P.O., Kottayam, Kerala, India 686 041 ‡ Central Power Research Institute, Bangalore, India 560 080 § Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarsini Hills P.O., Kottayam, Kerala, India 686 560 ⊥ Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selongor, Malaysia ¶ Center of Excellence for Polymer Materials and Technologies, Tehnoloski Park 24, 1000 Ljubljana, Slovenia ABSTRACT: The sorption and transport of four aromatic hydrocarbons through dynamically vulcanized high density polyethylene/ethylene propylene diene terpolymer rubber blends filled with carbon black and that filled with silica at the same loading have been investigated in the temperature range of 28−58 °C by an equilibrium swelling technique . Blends loaded with high abrasion furnace (HAF) black and those with silica of same loading have been used. The silica-incorporated blends sorbed a higher amount of aromatic solvents compared with the HAF filled blends. This has been explained in terms of the differences in the interaction between the filler particles and the blend components. The swelling coefficient, diffusion coefficient, and molar mass between cross-links have been computed to complement the experimental observations. filler incorporated into the polymer matrix. Reinforcing fillers such as silica and carbon black have a significant role in the transport properties of an elastomer and increases its mechanical durability and elastic properties and modifies the sorption and permeability to diffusants. The transport characteristics of the filler-modified elastomer systems depends on the filler size, surface area, state of aggregation, quantity of the filler and the interaction between the filler particles and the matrix. Compatible, inert fillers will take up the free volume within the polymer matrix and creates a tortuous path for the permeating molecules. Volume fraction of the filler, interaction of the filler with the matrix, shape, and orientation of the filler particles. etc. determine the degree of tortuosity. When the filler is incompatible with the polymer, voids tend to occur at the interface, which leads to an increase in free volume of the system and consequently, to an increase in permeability. Interaction of different fillers with the matrix can be understood from the studies of swelling and transport characteristics. Several researchers used the equilibrium swelling technique for finding out the interaction of different fillers/fibers with elastomers. For example, Manoj et al.18 reported the aromatic liquid transport through filled EPDM/ NBR blends. The highly reinforced filled systems showed reduced swelling rate because of the tortuosity of the path and the reduced transport area in the polymeric membrane. Stephen et al.19 investigated the interaction of nanostructured-layered silicates filled natural rubber with natural rubber,
1. INTRODUCTION The transport behavior of various organic solvents and gases through polymers is of great technological importance and it plays a vital role in a variety of barrier applications. The transport phenomenon is a complex process with a variety of industrial applications. A clear understanding of the permeation mechanism is required in order to succeed in the application of polymers as protective barriers in chemical and food industries, in membrane separation processes, in recovery or recycling of organic vapors from air streams, and in the separation of aqueous−organic or organic−organic mixtures by pervaporation. Many researchers have studied the transport behavior of polymeric membranes.1−7 Polymer blending has already been established as an effective means for constructively altering the transport properties of polymeric materials. The transport behavior in polymer blends was first reported by Cates and White.8 Later, a series of studies on the transport behaviors of polymer blends were reported and it was found that it is possible to tailor desirable properties by the simple blending of polymers.9−17 According to Hopfenberg and Paul [9] the study of diffusion, sorption and permeation in blend structure provides valuable information about the nature of blend. Minnath et al.10 studied the transport properties of substituted benzenes through a variety of membranes from thermoplastic polyurethane (TPU) and natural rubber (NR). Biju and coworkers reported the effects of blend ratio, compatibilization and dynamic vulcanization on permeation of gases through HDPE/EVA blends.11 The diffusion and transport properties of polymers were found to be strongly dependent on factors such as the nature of the polymer, nature of the penetrant, cross-link density, temperature, etc. Another important factor is the nature of © 2012 American Chemical Society
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carboxylated styrene butadiene rubber, and their blends by an equilibrium swelling method. George et al.20 investigated the effect of different types of fillers such as cork, silica, and carbon black on the transport of aromatic solvents in isotactic polypropylene/acrylonitrile-co-butadiene rubber blends. The present work was undertaken with a view of comparing the interaction of two fillers, namely, high-abrasion furnace black (HAF) and silica of same loading with high density polyethylene(HDPE)/ ethylene propylene diene terpolymer rubber(EPDM) blend membranes vulcanized by three different vulcanizing systems by equilibrium swelling method using four aromatic hydrocarbons such as benzene, toluene, paraxylene and mesitylene. Special emphasis was placed on the effect of filler loading, nature of fillers, blend-filler interaction, blend ratio, dynamic vulcanization, etc., on various transport properties such as diffusion coefficient, permeation coefficient, and activation energy for permeation and diffusion.
sorption experiments. The samples were then immersed in solvents (15−20 mL) in closed diffusion bottles kept at constant temperature in a thermostatically controlled heating oven. Periodically, the samples were removed from the bottles and weighed immediately using an electronic balance that measured reproducibility within ±0.0001 g. They were then placed back into the test bottles. The process was continued until equilibrium swelling was achieved. The experiments were conducted at 28, 38, 48, and 58 °C. The time taken for each weighing was kept within 20−30 s to minimize the error due to solvent evaporation. The results of the diffusion experiments were expressed as moles of solvent uptake by 100 g of polymer sample, Qt mol %.
2. EXPERIMENTAL SECTION Materials. High density polyethylene (HDPE-Relene, M60 200) of density 932 kg m−3 and melt flow index 20 g/10 min (at 230 °C/2.16 kg) was obtained from Reliance Industries Ltd. Hazira Gujarat, India. EPDM with an E/P ratio of 62/38 and a diene content of 3.92% supplied by Herdillia Unimers, New Mumbai was used. The fillers used were HAF (high abrasion furnace black) and silica (Ultrasil VN3), supplied by RuboChem Indistries, Mumbai, India. The solvents benzene, toluene, paraxylene, and mesitylene were of reagent grade (99% pure) and were double distilled before use to ensure purity. All other chemicals were reagent grade. Sulfur, dicumyl peroxide (DCP), and a mixture of sulfur and DCP (mixed) were used as cross-linking agents for the blends. In the case of the peroxide system, we have used only 1 phr of the DCP, as we found that the addition of more DCP caused problems in moulding. To study the effect of fillers, 10, 20, and 30 phr of the filler (HAF or silica) is added in filled systems. The formulation of the dynamic vulcanized blends is shown in Table 1. The blends were prepared in a Brabender plasticorder by
The effects of blend ratio, type of fillers, filler concentration, penetrants, dynamic vulcanization, and temperature on the diffusion and transport of four aromatic hydrocarbons were studied. Differential Scanning Calorimetry. We have employed the commonly used procedures to measure the crystallization behavior and melting properties of polyethylene in the presence and absence of EPDM. The crystallization behavior of the blends was determined using a Mettler 820 DSC Thermal Analyzer. The first heating was done from room temperature to 200 °C at a rate of 40 °C per minute followed by isothermal heating for 3 min and first cooling and second heating were performed at 10 °C per minute in nitrogen atmosphere. The percentage crystallization was estimated from the normalized enthalpy of fusion (ΔHf) using the following equation
⎛ mass of solvent sorbed ⎜ molar mass of solvent Q t mol% = ⎜ ⎜ mass of polymer ⎝
(
⎛ ΔHf ⎞ X% = ⎜ ⎟100 ⎝ ΔHf100 ⎠
a
sulfur
mixed
peroxide (DCP)
100 5 2 0.2 0.1 0.05
100 5 2 0.1 0.1 0.05 0.5 variablea
100
variablea
(1)
(2)
3. RESULTS AND DISCUSSION Effect of Blending and Filler Reinforcement. Figure 1 shows the comparison of mole percent toluene uptake by pure EPDM, 50/50 HDPE/EPDM, and 50/50/30 HDPE/EPDM/ HAF blends cross-linked by DCP. From the figure, it is clearly seen that pure EPDM with flexible chains which easily adjust with the solvent ingression, shows the highest solvent uptake. Their blends showed an intermediate behavior. The blending of semicrystalline HDPE with EPDM introduces rigid chains in the matrix, which reduces the solvent sensitivity of EPDM. In the blends the crystalline HDPE phase makes a tortuous path to the transport of solvent through the amorphous regions in the blends. The effect of reinforcement of HDPE/EPDM matrix, which results in improved barrier properties, is evident from the sorption curves of the blends loaded with HAF black. Effect of Blend Ratio. Figure 2 shows the effect of blend composition on the solvent uptake behavior of EPDM, HDPE, and HDPE/EPDM blends, reinforced with HAF and silica, at 30-phr loading, vulcanized by sulfur at 28 °C. The solvent used was xylene. It is clear from the figure that the liquid uptake tendency decreases with an increase in HDPE content in the blends. Upon blending EPDM with increasing content of semicrystalline HDPE, the volume fraction of the crystalline
vulcanization systems ingredients
⎟⎟ ⎠
where ΔHf100 is the enthalpy of fusion at 100% crystalline HDPE, which is taken as 290 J/g.
Table 1. Formulation of the Mixes (phr)
polymer ZnO stearic acid sulfur TMTD MBTS DCP carbon black or silica
) ⎞⎟100
1 variablea
phr = 10, 20, and 30.
melt mixing at 150 °C. Samples for transport studies were prepared by compression molding of the sample in a hydraulic press into 2 mm thick sheets at 160 °C. Solvent Sorption Experiments. The HDPE/EPDM blend samples for diffusion experiments were punched out in a circular shape of diameter 1.9 cm from tensile sheets (15 × 15 × 0.2 cm3) and were dried in a vacuum desiccator over anhydrous CaCl2 at room temperature for about 24−28 h. The original weight and thickness were measured before the 6698
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with 20 phr of carbon black (HAF) and silica, vulcanized with DCP in benzene are given in Figure 3. The experiments were
Figure 1. Sorption curves of pure EPDM, 50/50 HDPE/EPDM, and 50/50/30 HDPE/EPDM/HAF in toluene at 28 °C. Figure 3. Effect of different fillers on the mol % benzene uptake of HDPE/EPDM blends vulcanized by DCP.
done at 28 °C. It can be seen from the figure that the loading of HDPE/EPDM blend samples with carbon black as well as silica reduces the Qt values. Moreover, the carbon black incorporated sample takes up a lesser amount of benzene compared to the silica-filled one. The same trend is shown in other solvents such as toluene, xylene, and mesitylene too. Even though the particle size of silica is lower (18 nm) than that of HAF black (29 nm), the carbon back addition showed less solvent uptake, and we found that when we compared silica and carbon black, other parameters are more important than the particle size. The lower Qt values exhibited by the carbon black-filled blends compared to the silica-filled one is attributed to the different surface chemistry of these fillers. The carbon black surface is almost nonpolar while the surface of silica is polar due to the covering layer of polar silanol groups. Hence we can expect that the nonpolar carbon black will form stronger contacts with the likewise nonpolar polymer than will the silica. On the other hand, the silica particles will prefer each other as interaction partners, especially as solubility forces are strengthened by specific hydrogen bonding. In otherwords, the carbon black vulcanizates are predominatly characterized by filler−polymer interaction, while in silica compounds, the filler−filler interaction is predominant. The strong filler−polymer interaction restricts the mobility of the individual polymer chains, and this leads to low sorption bebaviour. An attempt has been made to apply the Kraus equation21 to find the extent of reinforcement of different fillers in the blend matrix.
Figure 2. . Equilibrium xylene uptake by different HDPE/EPDM blends reinforced with HAF and silica and vulcanized by sulfur.
content increases. This will result in lower penetrant ingression into the matrix, which regularly decreases with increase in the weight percent of HDPE in the blends. The crystalline regions of HDPE put up stiffer resistance to the penetrant molecules, leading to a lower extent of solvent uptake. The percentage of crystallinity values of different HDPE/EPDM blends are given in Table 2. Effect of Different Fillers. The sorption curves of 30/70 HDPE/EPDM blends, 30/70 HDPE/EPDM blends reinforced
⎡ f ⎤ Vr0 = 1 − m⎢ ⎥ Vrf ⎣1 − f ⎦
Table 2. Crystallinity of HDPE/EPDM Blends (from DSC Data) sample
normalized % crystallinity
H100 H70 H50 H30 H0
58 57 55 47
(3)
where Vr0 and Vrf are the volume fraction of the swollen polymer blend in the fully swollen unfilled sample and in the fully swollen filled sample, respectively. f is the volume fraction of the filler and the slope m will be a direct measure of the reinforcing capacity of the filler in the matrix. The reinforcing effect of the filler can be obtained by plotting Vro/Vrf against f/(1 − f). According to Kraus theory, the curves 6699
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increases the reinforcement and reduces the solvent uptake. This is in agreement with the sorption behavior in different concentrations of fillers shown in Figure 4. Effect of Penetrants. The effect of penetrants on the sorption and diffusion of four aromatic hydrocarbons through 30/70/30 HDPE/EPDM/silica systems vulcanized by DCP is shown in Figure 6. There is a systematic trend in the sorption
with higher negative slope indicate a better reinforcing effect. The slopes of silica- and HAF-filled samples have been found to be −1.28 and −1.52, respectively. It is observed that the negative value of the slope increases in the order silica < HAF indicating the degree of reinforcement in the same order. The order of reinforcement is complementary to the observed solvent uptake trend for the two filler loaded systems. Effect of Filler Concentration. Figure 4 shows the sorption curves of 30/70 HDPE/EPDM blends reinforced
Figure 6. Mol% uptake of 30/70/30 HDPE/EPDM/silica systems vulcanized with DCP in different solvents at 28 °C.
behavior of liquids of different molecular mass. With an increase in molecular mass of the solvent molecules, there is a decrease in the values of Qt. Benzene shows the highest value of Qt, and mesitylene shows the minimum, among the solvents used in the work. Toluene and xylene take intermediate positions. The same trend is observed with HAF filled samples also. This can be explained on the basis of free volume theory,19 according to which the diffusion rate of a molecule depends primarily on the ease with which the polymer chain segments exchange their positions with penetrant molecules. The ease of exchange becomes less, as the penetrant size increases, particularly in the case of filled matrices, and this leads to a decrease in the values of diffusion coefficient. Effect of Dynamic Vulcanization. Figure 7 shows the sorption curves of 30/70 HDPE/EPDM blends, loaded with 10 phr of HAF and cross-linked by three different vulcanizing systems, namely, sulfur, peroxide(DCP), and a mixed system consisting of sulfur and peroxide, with mesitylene as the probe molecule. It can be seen that the liquid sorption behavior decreases in the order sulfur > mixed > DCP. A representative model for the changes accompanying the dynamic vulcanization is shown in Figure 8. Before vulcanization, the rubber particles are coarsely distributed in the plastic phase. With sulfur vulcanization, there is only a marginal reduction in the phase dimensions, but with mixed and peroxide vulcanization, an increased extent of cross-linking and particle breakdown occur, depending on the effectiveness of the vulcanization system. The difference in the transport behavior through the matrices with different vulcanizing systems is attributed to the types of crosslinks formed during vulcanization and also to the difference in the distribution of cross-links. The nature of the different networks possible during vulcanization is given in Figure 9. Sulfur vulcanization leads to the formation of flexible S−S linkages between the polymer chains, and easily rearranges
Figure 4. Effect of filler loading on the equilibrium xylene uptake of the filled HDPE/EPDM blends vulcanized with DCP.
with HAF and silica for different filler loadings vulcanized by DCP at 28 °C in xylene. The solvent uptake decreases in the order 10phr > 20 phr > 30 phr for all the samples. It implies the reduction in free volume or microvoids due to better filler reinforcement. An increase in filler concentration also restricts the macromolecular chain mobility resulting in a tortuous path for the diffusion of penetrants. At low filler loading, usually a dispersion gel is formed as in Figure 5 (i.e., relatively large
Figure 5. Schematic diagram illustrating the concept of multiple segmental absorption: (a) single attachment, (b) multiple attachments, (c) interparticle attachment.
distances between the filler particles). In this case, the rubber chains are attached to one filler particle due to a single attachment (Figure 5a) or multiple attachment of chain segments of one rubber molecule (Figure 5b). With an increase in the concentration of the filler, a coherent gel can be formed (Figure 5c). Here, intraparticle attachment predominates, wherein rubber molecules connect two or more filler particles. So the formation of coherent gel at high filler loading 6700
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Figure 7. Effect of cross-link systems on the mol % mesitylene uptake of 30/70 HDPE/EPDM reinforced with 10 phr HAF at 28 °C.
Figure 9. Structure of the various cross-links formed during the vulcanization: (a) DCP system, (b) sulfur system, (c) mixed system.
Table 3. Values of Mc
under solvent stress to permit the penetrants to permeate relatively easily. The peroxide vulcanization produces rigid C−C linkages and in mixed systems, intermediate sorption behavior is observed, as it contains both C−C and S−S linkages. These observations can also be explained using cross-link density and molar mass between the cross-links for the different vulcanizing systems. The molar mass between cross-links (Mc) of the network polymer chain was calculated using the Flory− Rehner relation22 −ρP VS(φ1/3 − φ /2) ln(1 − φ) + φ + χφ 2
HAF (30 phr)
silica (30 phr)
0/100 30/70 50/50 70/30 100/0
1916 1814 1312 588 316
2170 2048 1436 782 404
table, it can be seen that the Mc values decrease with increase in the HDPE content in the blends, for a given filler in accordance with the solvent uptake behavior. The semicrystalline HDPE phase induces physical cross-links in addition to the permanent chemical cross-links, formed during vulcanization, in the matrix. The values for the fillers used are in the order silica > HAF for a given blend ratio. As the value of Mc increases, the available free volume between the adjacent cross-links increases. Hence more solvents can be accommodated between the cross-links within the matrix. This result also supports the observed highest solvent uptake in silica loaded samples and the lowest in HAF systems. Effect of Temperature. To study the dependence of diffusion behavior of HDPE/EPDM/Silica and HDPE/EPDM/ HAF systems, the experiments were conducted at 28, 38, 48, and 58 °C. The sorption behavior of 30/70/30 HDPE/EPDM/ HAF blends vulcanized by DCP in toluene at different temperatures are given in Figure 10. There is an increase in the rate of diffusion and the maximum solvent uptake with an increase in temperature. This is due to the increased segmental mobility and free volume within the matrix at higher temperature. The kinetic energy of the penetrants also increases with increasing temperature. The disruptions of the crystalline regions in HDPE in the matrix at higher temperature can also contribute to the enhanced segmental motions and the subsequent higher solvent uptake.
Figure 8. Schematic model illustrating the type of vulcanizing system on the morphology of the resulting blend.
Mc =
HDPE/EPDM
(4)
where, ρp is the density of the polymer, VS is the molar volume of the solvent, φ is the volume fraction of the polymer in the fully swollen state, and χ is the polymer−penetrant interaction parameter determined as suggested by different researchers.23,24 The estimated values of Mc are given in Table 3. From the 6701
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higher compared with that of the filled ones. This has been attributed to the strong adsorption of polymer segments on to the filler surface, which leads to a decreased mobility of the macromolecular chains. The HAF filled samples show lower swelling coefficient values. Diffusion and Permeation Coefficients. The kinetic parameter, diffusion coefficient D for the different systems, under investigation was estimated by25
⎡ hθ ⎤2 D = π⎢ ⎥ ⎣ 4Q ∞ ⎦
(6)
where h is the sample thickness, θ is the slope of the sorption curve before attainment of 50% of equilibrium uptake, and Q∞ is the equilibrium sorption value. The values of D are given in Table 5. It is clear from the table that the values are higher for silica-filled samples in a given penetrant compared to the corresponding black-loaded ones. From the dependence of D on the number of carbon atoms of the penetrants, as shown in Figure 11, it is found that D varies inversely with increase in the
Figure 10. Mol % toluene uptake of 30/70/30 HDPE/EPDM/HAF systems vulcanized by DCP at different temperatures.
Swelling Coefficient. To assess the extent of the swelling behavior of the blends reinforced with HAF and silica and vulcanized by DCP, the swelling coefficients (α) were evaluated by the following equation13 w − w1 −1 α= 2 x ρs w1 (5) where w1 and w2 are the weights of the sample before swelling and at equilibrium swelling, and ρs is the density of the solvent. The swelling coefficient values of the different unfilled and filled blends in toluene are given in Table 4. It has been found that Table 4. Values of Swelling Coefficient HDPE/EPDM
unfilled
HAF(30 phr)
silica (30 phr)
0/100 30/70 50/50 70/30 100/0
2.84 2.08 1.88 1.46 1.24
1.58 1.42 1.34 1.12 1.04
1.74 1.52 1.44 1.28 1.16
Figure 11. Dependence of diffusivity (D) on the number of carbon atoms of aromatic hydrocarbons in 50/50 HDPE/EPDM blends.
number of carbon atoms. An inverse relationship was observed with D and molar volume. Variation of D with filler loading is shown in Figure 12. Diffusivity decreased with increased filler loading. These results are complementary to the observed solvent uptake tendency.
the swelling coefficient value decreases with increase in HDPE content in the blends. This clearly indicates the increase in crystallinity of blend membranes with increase in HDPE content. The swelling coefficient values of unfilled samples are
Table 5. Values of Diffusion and Permeation Coefficients of DCP Vulcanized Samples at 28 °C D × 106 (cm2/sec) sample HDPE/EPDM/filler 0/100/30 0/100/30 30/70/30 30/70/30 50/50/30 50/50/30 70/30/30 70/30/30 100/0/30 100/0/30
silica HAF silica HAF silica HAF silica HAF silica HAF
P × 106 (cm2/sec)
benzene
toluene
xylene
mesitylene
benzene
toluene
xylene
mesitylene
8.98 7.64 8.12 7.16 7.46 6.30 5.32 4.52 1.64 1.26
7.82 6.88 7.21 6.58 6.86 5.74 3.80 2.74 1.28 1.10
6.49 5.92 6.16 5.24 5.22 4.98 3.58 2.06 1.10 0.64
5.64 4.86 5.04 4.52 4.24 4.12 2.70 1.82 0.80 0.42
4.68 3.18 3.86 2.66 3.56 2.12 3.02 1.84 1.34 1.04
4.15 2.89 3.32 2.28 2.92 1.84 2.42 1.32 1.12 0.82
3.18 2.42 2.51 1.84 2.18 1.63 1.98 1.08 0.88 0.68
2.74 1.96 2.18 1.56 1.72 1.28 1.34 0.90 0.74 0.56
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Table 6. Van’t Hoff’s ParametersEntropy, Enthalpy, and Free Energy (Penetrant, Toluene) thermodynamic parameters sample HDPE/EPDM/filler 0/100/30 0/100/30 30/70/30 30/70/30 50/50/30 50/50/30 70/30/30 70/30/30 100/0/30 100/0/30
The permeation process through any matrix is a combination of sorption and diffusion and hence the permeation coefficient depends on sorption coefficient and diffusion coefficient. The permeation coefficient P for all the systems under investigation was computed as26 (7)
where D is the diffusion coefficient and S is the sorption coefficient,which is the ratio of the mass of the penetrant molecule at equilibrium swelling to the mass of the polymer sample. The calculated values of P is also given in Table 5. It is observed that the P values of the studied blend/solvent systems follow the same trend as that of diffusion coefficient, in terms of blend ratio, fillers, and penetrant size. Thermodynamic Parameters. From the amount of liquid sorbed by a given mass of the polymer, the equilibrium sorption constant Ks for the samples at different temperature has been computed as KS =
no. of moles of solvent sorbed at equilibrium mass of the polymer
■
(8)
From the values of Ks, enthalpy ΔH and entropy ΔS of sorption has been determined using the Van’t Hoff relation7 log K s =
ΔS ΔH − 2.303R 2.303RT
−ΔG (KJ/mol)
16.78 15.32 14.82 13.74 12.44 9.84 8.62 7.68 6.68 5.44
1.72 1.22 1.48 0.98 1.36 0.82 1.12 0.71 0.78 0.54
4.14 3.86 3.64 2.78 2.58 2.37 1.84 1.18 0.96 0.56
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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The computed values of ΔH and ΔS are given in Table 6. The ΔH values are negative for both carbon black and silica-filled systems. The values also exhibit a regular increase from benzene to mesitylene for a given sample. The ΔS values also show a regular trend which decreases with increase in the HDPE content. From ΔH and ΔS, the free energy of sorption process has been obtained using the relation ΔG = ΔH − T ΔS
−ΔH (KJ/mol)
4. CONCLUSIONS The barrier properties of HDPE/EPDM blends reinforced with carbon black and silica have been studied using four aromatic hydrocarbons, namely, benzene, toluene, paraxylene, and mesitylene as penetrants with special reference to the effects of blend ratio, dynamic vulcanization, and penetrant size. The regular reduction in solvent uptake by the blends with an increase in HDPE content in the majority of the blends has been attributed to the semicrystalline nature of the HDPE. The silica-filled samples have been found to sorb a higher amount of aromatic solvents compared with the HAF filled ones. The samples cross-linked by the DCP showed the lowest equilibrium uptake in all penetrants compared to the samples with sulfur and mixed vulcanization modes for a given type of filler. This has been explained in terms of the differences in the nature and distribution of cross-links in the network. The extent of filler reinforcement has been evaluated by using the Kraus equation, which has been complementary to the solvent uptake. The swelling coefficient, diffusion coefficient, and molar mass between cross-links have been computed to complement the observations during the swelling studies. The thermodynamics of the system has also been studied by the calculation of enthalpy, entropy, and free energy.
Figure 12. Variation of diffusivity of 50/50 HDPE/EPDM blends with filler loading.
P = DS
silica HAF silica HAF silica HAF silica HAF silica HAF
ΔS (J/mol/K)
ACKNOWLEDGMENTS The authors acknowledge the financial support from (i) the Ministry of Higher Education, Science and Technology of the Republic of Slovenia through the Contract No. 3211-10000057 (Center of Excellence Polymer Materials and Technologies) and (ii) Nanomission, DST, India.
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REFERENCES
(1) Bhattacharya, M.; Biswas, S.; Bhowmick, A. K. Permeation characteristics and modeling of barrier properties of multifunctional rubber nanocomposites. Polymer 2011, 52, 1562−1576. (2) Chen, G. Q.; Scholes, C. A.; Qiao, G. G.; Kentish, S. E. Water vapor permeation in polyimide membranes. J. Membr. Sci. 2011, 379, 479−487.
The ΔG values are given in Table 6. The value of ΔG becomes more negative with an increase in the percentage of HDPE in the blend. These values indicate the increase in tortuosity of diffusion process through the blends with an increase in the percentage of HDPE in the blends. 6703
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dx.doi.org/10.1021/ie202408s | Ind. Eng. Chem. Res. 2012, 51, 6697−6704
