Molecular transport of aromatic solvents through microcomposites of natural rubber (NR), carboxylated styrene butadiene rubber (XSBR) and their blends

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1187–1194 www.elsevier.com/locate/compscitech

Molecular transport of aromatic solvents through microcomposites of natural rubber (NR), carboxylated styrene butadiene rubber (XSBR) and their blends Ranimol Stephen a, Kuruvilla Joseph b, Zachariah Oommen c, Sabu Thomas b

a,*

a School of Chemical Sciences, Mahatma Gandhi University, Kottayam 686 560, Kerala, India Post Graduate Department of Chemistry, St. Berchmans College, Changanacherry, Kerala, India c Department of Criminal Justice, Albany State University, Albany, GA 31705-2796, USA

Received 18 October 2005; received in revised form 10 May 2006; accepted 12 May 2006 Available online 10 July 2006

Abstract Transport properties of aromatic solvents such as benzene, toluene and p-xylene through micron sized fillers reinforced natural rubber, carboxylated styrene butadiene rubber and their 70/30 blend latex membranes were investigated. The effect of penetrant size, filler loading and temperature on the diffusion properties were studied. The dispersion of filler in the matrix were analysed from scanning electron micrographs. Due to the lack of polymer/filler interaction aggregation of filler was higher in NR. The filled samples showed reduced swelling rate owing to the tortuosity of the path. A dramatic decrease in diffusion coefficient of filled samples was observed. However, blend system exhibited unexpected diffusion behaviour because of the immiscibility of the two components. The activation energies for diffusion of the penetrant were computed from the Arrhenius plots. The activation energy needed for diffusion of penetrant was found to be higher for filled virgin polymers. The transport mechanism was investigated and found that all systems exhibited non-Fickian anomalous behaviour.  2006 Published by Elsevier Ltd. Keywords: Natural rubber; Carboxylated styrene butadiene rubber; Latex blends; Micron sized fillers; Diffusion

1. Introduction The examination of the rate of diffusion of small molecules through polymeric materials is relevant for many engineering applications. Nowadays, the polymer membranes are used as materials for cable coating, food packaging, electronic circuits, etc. [1]. Hence the transport properties of organic solvents and gases through polymers are of great technological importance [2,3]. Many researchers have studied the transport properties of polymeric membranes [4–10]. Blending of two polymers can be carried out to improve the transport properties of membranes. * Corresponding author. Tel.: +91 481 2730003/2597914; fax: +91 481 2731001/2731009. E-mail addresses: [email protected], [email protected] (S. Thomas).

0266-3538/$ - see front matter  2006 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2006.05.009

Thomas and co-workers [10–16] studied the transport properties of various blend systems using different organic solvents. A systematic study on the sorption behaviour of blends was first studied by Cates and White [17]. Since then, the literature has revealed a series of reported works on the transport behaviours of polymer blends from where it was found that it is possible to tailor desirable properties by the simple blending of polymers [18–20]. According to Hopfenberg and Paul [21] the study of diffusion, sorption and permeation in blend structure provides valuable information about the nature of blend. Aminabhavi and Phayde [22], studied the molecular transport of alkanes in PU/ PBMA interpenetrating polymer network system and aromatic liquids into tetrafluoroethylene/polypropylene system. The diffusion and transport properties of polymers were found to be strongly dependent on the additives and type of crosslinking. The study of absorption of various

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solvents in polymers is very important from two points of view: first as a process which results in ageing and second as a tool that allows us to investigate the structure and composition of a material [23]. The diffusion coefficient of the polymers varying from one polymer to another. The diffusion process is a kinetic parameter depending on the free volume within the material, segmental mobility of polymer chains and the size of the penetrant molecule [24,25] Usually fillers are added to rubbers in order to cheapen the product and also for improving the properties. Carbon black is the most widely used reinforcing fillers in rubber industry. It is too complex to trace the reinforcement occurred in elastomers with the addition of filler. Mathai [26] studied the transport properties of various rubbers and blends in the presence of fillers. From this laboratory Thomas with other researchers [27,28] also studied the diffusion of organic solvents through fibre reinforced rubber composites. Recently, Dufresne and co-workers [29] studied the swelling behaviour of waxy maize starch nanocrystals reinforced natural rubber. They found that the solvent uptake of NR decreases upon the addition of starch nanocrystals. It was in the light of several diffusion studies that this present study was designed to investigate the transport of organic solvents through NR, XSBR and 70/30 NR/XSBR latex membranes containing micron sized fillers. It was also worth mentioning that the transport properties of NR/ SBR blends were previously studied from this laboratory [30].

latex was preserved by adding ammonia as primary preservative, low ammoniated (0.2%m/m ammonia content) system. The secondary preservatives used were 0.05%m/m tetramethyl thiuram disulphide (TMTD) and 0.02%m/m zinc oxide. Carboxylated styrene butadiene rubber latex having 47% dry rubber content was obtained from Apar Industries Ltd., Mumbai, India. The XSBR latex was stabilized by adding colloid stabilisers. To reduce the foaming tendency and the water-sensitivity of films the 0.5 pphm of colloid stabilisers were added to XSBR latex. The details are given in Table 1. The compounding ingredients such as vulcanising agents and accelerators were procured from M/s Bayer India Ltd., Mumbai. The particulate fillers used were of commercial grade. The composition of micron sized clay and silica are shown in Table 2.

2. Experimental 2.1. Materials Centrifuged NR latex with 60% dry rubber content was collected from Gaico rubbers Ltd., Kottayam, Kerala. NR

Table 2 Composition of micron sized fillers used Content

Weight (%)

Composition of clay SiO2 Al2O3 Fe2O3 (max.) TiO2 CaO (max.) MgO (max.) Na2O (max.) K2O (max.) Loss on ignition

45 38 0.5 0.55 0.06 0.07 0.25 0.1 14.5

Composition of silica SiO2 (dry material) Al2O3 Na2O Fe2O3 SO3 Drying loss

83–90 <0.3 0.6–2.5 <0.04 0.5–2.5 8–12

Table 1 Characteristics of latices Natural rubber (NR) latex

H2C H Supplied by Dry rubber content (DRC) (%) Total solid content (TSC) (%) Carboxylated styrene butadiene rubber (XSBR) latex (PLX-802)

C CH3 n

Gaico Rubbers Ltd., Kuravilangadu, Kottayam 60 61.25 H

OOC

Supplied by Dry rubber content (DRC) (%) Total solid content (TSC) (%) Styrene content (%)

CH2 C

H2 C

C

Apar Industries Ltd., Bombay, India 47 50.66 52

CH2

CH

CH

CH2

R. Stephen et al. / Composites Science and Technology 67 (2007) 1187–1194 Table 3 Formulation of mixes

3. Results and discussion 3.1. Swelling behaviour

N100sP

N70sP

N0sP

100

70

–

–

30

100

0.25

0.25

0.25

1.5 0.75 0.5 0.2

1.5 0.75 0.5 0.2

1.5 0.75 0.5 0.2

0, 2.5, 5, 7.5 0, 2.5, 5, 7.5

0, 2.5, 5, 7.5 0, 2.5, 5, 7.5

0, 2.5, 5, 7.5 0, 2.5, 5, 7.5

ZDC: zinc diethyl dithiocarbamate; ZMBT: zinc mercaptobenzothiazole.

2.2. Preparation of samples Prevulcanisation of latices was carried out by heating the compounded latex at 70 C for 2 h using water bath. The compounding ingredients were added in accordance with the recipe given in Table 3. Films were prepared by casting on a glass plate and dried at ambient temperature. In the sample coding, N stands for NR; the subscripts 100, 70 and 0 represent the weight percentage of NR. The codes sP stands for sulphur prevulcanisation, C and Si indicate clay and silica respectively. The subscript numbers 2.5, 5 and 7.5 indicate the weight percentage of fillers used. 2.3. Diffusion experiments Circular samples of 2 cm diameter were cut from polymer sheets by means of a standard die. The thickness and initial weight of the samples were taken. The samples were kept in diffusion bottles at constant temperature in an air oven. The samples were periodically removed from the bottles, the adhering solvent removed by using tissue paper and the samples were weighed on an electronic balance. The samples were then immediately replaced into the diffusion bottle. The experimental procedure was continued until the equilibrium swelling was attained. The solvent uptake (Qt (%)) of the samples was computed using the equation, Qt ð%Þ ¼

Ms MMs

Mp

 100

ð1Þ

The diffusion curves for the microcomposite of NR latex film in benzene are given in Fig. 1. The curves are plotted as the fractional uptake of penetrant against t1/2/h, where h is the thickness of the sample. This type of plots is referred as reduced sorption curves [22], because the sample thickness is included in the abscissa. Micron sized clay filled NR exhibit reduced swelling rate than silica filled system. At the initial stage, the swelling rate is very high. This is because of the large concentration gradient and the polymer sample is under severe solvent stress. Finally, it reaches at equilibrium solvent uptake, where the concentration gradient is zero. As compared to clay the increased swelling in silica filled sample is due to the filler- filler interaction and subsequently the filler aggregates in the polymer matrix. The precipitated silica possessing many hydroxyl groups on its surface and as a result the intermolecular hydrogen bonds between hydroxyl groups on the surface of silica are very strong, it can aggregate tightly. It is further clear from the scanning electron micrographs of clay and silica filled NR. It is shown in Figs. 2(a–b). Aggregation of filler in NR system could be seen in SEM. Fig. 3 is the benzene uptake (%) vs. t1/2/h graphs of micron sized fillers reinforced XSBR latex film. As a result of the high cohesive energy density of XSBR it shows reduced absorption as compared to NR. It can be seen that, upon the addition of filler the swelling rate decreases. This can be ascribed in terms of the polar nature of XSBR. The polar–polar interaction will take place between the polymer and the filler. It will reduce the agglomeration of filler. Scanning electron micrographs of microcomposite of XSBR latex film is displayed in Figs. 4(a–b). More uniform distribution of filler can be found in XSBR matrix.

5

4

N100sP N100sPC2.5

Qt (%)

60% Centrifuged natural rubber (NR) latex 47% Carboxylated styrene butadiene rubber (XSBR) latex 10% Potassium hydroxide solution 50% Sulphur dispersion 50% ZDC dispersion 50% ZMBT dispersion 50% Zinc oxide dispersion 50% Clay dispersion 20% Silica dispersion

1189

N100sPSi2.5

3

where Ms is the mass of solvent sorbed, MMs is the molar mass of solvent and Mp is the mass of polymer. The experiments were carried out using benzene, toluene and p-xylene.

2

2.4. Scanning electron microscopic analysis

0

1

0

The samples were cryogenically fractured and analysed for morphology of blend system using scanning electron microscope, JEOL JSM-840A.

20

40 1/2

60

80

100

1/2

t /h (min /mm) Fig. 1. Qt vs. t1/2/h plots of microcomposites (2.5 phr) of NR membranes in benzene at 30 C.

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Fig. 2. Scanning electron micrograph of filled NR. (a) Clay and (b) silica.

The development of solvent uptake of microcomposite of 70/30 NR/XSBR is presented in Fig. 5. It is observed that filled blend exhibit higher swelling rate than pristine

N0sP

4

N0sPC2.5 N0sPSi2.5

Qt (%)

3

2

1

0 0

20

40

60 1/2

80

100

120

140

1/2

t /h (min /mm) Fig. 3. Qt vs. t1/2/h plots of microcomposites (2.5 phr) of XSBR latex membranes in benzene at 30 C.

polymer. The unexpected diffusion behaviour of 70/30 NR/XSBR blend can be explained in terms of the immiscibility of two phases. As a result of the difference in polarity, the two components are thermodynamically immiscible. The affinity of two phases towards filler is different, subsequently non-uniform migration of filler arises. This is the reason behind the unusual behaviour of 70/30 NR/XSBR blend system. Aggregation of filler can be seen in the scanning electron micrographs of blend system (Figs. 6(a–b)). The reduced sorption of micron sized fillers reinforced NR and XSBR can be explained in terms of the tortuosity of the path and the reduced area of transport in the presence of fillers. The change in equilibrium uptake as a function of weight percentage of filler is given in Fig. 7. As the filler concentration increases the solvent uptake at equilibrium decreases in the case of NR and XSBR. It can be seen that, for XSBR it decreases constantly with filler loading. This can be explained in terms of the polymer/filler interaction due to the polarity of XSBR. However, in NR it seems to be level off at higher loading owing to its poor rubber/ filler interaction. The uneven distribution of filler in the two phases of 70/30 NR/XSBR results in rather complex behaviour.

Fig. 4. Scanning electron micrograph of filled XSBR. (a) Clay and (b) silica.

R. Stephen et al. / Composites Science and Technology 67 (2007) 1187–1194

5

N70sP

4

N70sPC2.5

Qt (%)

N70sPSi2.5 3

2

1

0 0

10

20

30 1/2

40

50

60

1/2

t /h (min /mm) Fig. 5. Qt vs. t1/2/h plots of microcomposites (2.5 phr) of 70/30 NR/XSBR latex membranes in benzene at 30 C.

1191

By rearranging this equation, the diffusion coefficient can be calculated using the equation [32],  2 hh ð4Þ D¼p 4Q1 where h is the slope of the initial portion of the plot of Qt vs. t1/2 and Q1 is the equilibrium mole percentage uptake. Fig. 8 demonstrates the change in diffusion coefficient of benzene as a function of weight percentage of filler. The diffusion coefficient decreases dramatically upon the addition of filler. The XSBR system exhibits lower diffusivity value that indicates higher polymer/filler interaction. It is also due to the restricted chain mobility of XSBR because of its high cohesive energy density. The NR chain segments are more flexible and it exhibits higher diffusivity values as compared to XSBR. But for 70/30 NR/XSBR blend system the diffusivity first decreases upon the addition of filler while it shows remarkable increase at higher loading. The

5.0

ð2Þ where t is the time, h is the initial thickness of the sample, D is the diffusion coefficient and n is an integer. From this equation it is understand that a plot of Qt vs. t1/2 is linear at short time and D can be calculated from the initial slope. The equation for short time limiting is [31],  1=2 Qt 4 D ¼ t1=2 ð3Þ Q1 h p

N100sPC N0sPC N70sPC

4.8

Equilibrium benzene uptake (%)

The process of diffusion is a kinetic parameter related to the penetrant size and to the polymer segment mobility. The diffusion coefficient of a polymeric sample immersed in an infinite amount of solvent can be calculated using the equation [31], "   nX  # Qt 8 ¼1 1 2 2 t ¼1 exp Dð2n þ 1Þ p 2 p2 n¼0 ð2n þ 1Þ2 Q1 h

4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 0

1

2

3

4

5

6

7

8

Weight % of filler (phr) Fig. 7. Equilibrium benzene uptake of microcomposite of NR, XSBR and 70/30 NR/XSBR as a function of weight percentage of filler at 30 C.

Fig. 6. Scanning electron micrograph of filled 70/30 NR/XSBR. (a) Clay and (b) silica.

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7

2

Diffusion coeffficient of benzene x10 (cm /s)

14

N100sPC N0sPC N70sPC

12 10

The reinforcement of fillers is predicted by using Kraus equation [34]. The equation is   f ð6Þ V ro =V rf ¼ 1  m 1f where Vrf is the volume fraction of rubber in the solventswollen filled sample and is given by,

8

ðdfwÞ qp

6

V rf ¼

4

where d is the deswollen weight, f is the volume fraction of the filler, w is the initial weight of the sample, qp is the density of the polymer, qs is the density of the solvent, and As is the amount of solvent absorbed Vro is

2 0 0

1

2

3

4

5

6

7

8

d qp

Weight % of filler (phr)

higher diffusivity value of unfilled and filled blend system is due to the immiscibility of the system. The activation energy for diffusion as well as permeation can be calculated using the equation [33], log D ¼ log D0 

ED 2:303RT

ð5Þ

where D is the diffusion coefficient of penetrants through polymer, D0 is the constant, T is the absolute temperature, ED is the activation energy for diffusion and R is the universal gas constant. The slopes of the plot of log D vs. 1/T gives the value of ED. From these plots the activation energy needed for diffusion of penetrants through polymer is calculated and is shown in Table 4. The activation energy of diffusion of benzene through micron sized fillers reinforced latex membrane is found to be higher for clay and silica filled XSBR, clay filled NR and silica filled 70/30 NR/XSBR blend system. This can be ascribed in terms of the reinforcement of the polymer upon the addition of fillers due to the enhancement in polymer/ filler interaction.

Table 4 ED values of microcomposite of (2.5 phr) NR, XSBR and 70/30 NR/ XSBR membranes for the transport of benzene Sample

ED (kJ/mol)

N100sP N100sPC2.5 N100sPSi2.5 N0sP N0sPC2.5 N0sPSi2.5 N70sP N70sPC2.5 N70sPSi2.5

15 33 11 50 63 71 43 30 33

V ro ¼

d qp

ð8Þ

þ Aqss

A plot of Vro/Vrf as a function of (f/1  f) should give a straight line with slope m. The value of m is a direct measure of the reinforcing ability of the filler used. According to this theory, reinforcing fillers have negative slope, indicate better polymer/filler interaction. Kraus plots of micro filled NR, XSBR and 70/30 NR/ XSBR are given in Figs. 9 and 10. It can be seen that the curves are deviated in different degrees to the downward and upward direction indicating poor reinforcement effect due to the aggregation of filler. However, from the plot it is clear that the linear relation in accordance with the Kraus plot is not obeyed in these systems. This deviation from linearity is due to the aggregation of filler in the rubber matrix. As a result some portion of the filler is not available in the matrix to resist the solvent penetration. Consequently, the Vrf value decreases than expected

N100PC 1.3

N0PC N70PC

1.2

Vro/Vrf

Fig. 8. Diffusion coefficient of transport of benzene through microcomposites of NR, XSBR and 70/30 NR/XSBR as a function of weight percentage of filler at 30 C.

ð7Þ

d  fw þ Aqss qp

1.1

1.0

0.9

0.8 0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

f/(1-f) Fig. 9. Kraus plot of microcomposites of (clay filled) NR, XSBR and 70/ 30 NR/XSBR latex films.

R. Stephen et al. / Composites Science and Technology 67 (2007) 1187–1194

1.4

Table 5 n and k values of microcomposites of (2.5 phr) NR, XSBR and 70/30 NR/ XSBR latex films for the transport of benzene

N100sPSi N0sPSi

1.3

Sample

N70sPSi

N100sP N100sPC2.5 N100sPSi2.5 N0sP N0sPC2.5 N0sPSi2.5 N70sP N70sPC2.5 N70sPSi2.5

1.2

Vr0/Vrf

1193

1.1

1.0

Benzene n

K · 102 (g/gminn)

0.69 0.51 0.81 0.78 0.76 0.96 0.62 0.81 0.74

4.57 2.16 2.30 1.85 1.82 1.78 4.06 2.17 1.82

0.9 0.00

0.02

0.04

0.06

uptake (%) values decrease with increase in molecular weight of the penetrant molecule.

0.08

f/(1-f) Fig. 10. Kraus plot of microcomposites of (silica filled) NR, XSBR and 70/30 NR/XSBR latex films.

resulting in the increase in the ratio Vro/Vrf, because Vro is constant. Another reason for the occurrence of deviation is due to the interaction between the filler and the curing agents. Similar, observations have been reported [35] in ternary blends reinforced by fillers such as carbon black and silica. The effect of penetrant size on the swelling behaviour of rubber has been analysed using benzene, toluene and xylene. As the penetrant size increases the solvent uptake of the polymer decreases. This is because of the bulky groups will reduce the migration rate. Fig. 11 is the equilibrium solvent uptake vs. molecular weight of micron sized fillers reinforced NR, XSBR and 70/30 NR/XSBR latex membranes. It is observed that the equilibrium solvent

4.8

Equilibrium solvent uptake (%)

4.4 4.0 3.6 3.2

N100sP N100sPC N100sPSi N0sP N0sPC N0sPSi N70sP N70sPC N70sPSi

2.8 2.4 2.0 75

80

85

90

3.2. Transport mechanism The transport properties of polymeric membranes can be followed by using the empirical equation [36,37],   Qt log ¼ log k þ n log t ð9Þ Q1 The slope of the plot log Qt/Q1 vs. log t gives the value of n, indicating the mechanism of transport and its y-intercept is the value of k, depends upon the structural significance of polymer as well as its interaction with the solvent. According to the n values obtained from the above equation, three basic modes of transport are distinguished. If n = 1/2 the diffusion mechanism is Fickian [31], in that case the rate of diffusion of permeant molecules is much less than the polymer segment mobility. If n = 1, the mechanism is non-Fickian [31], this may be considered in systems in which permeant diffusion rates are much faster than polymer relaxation process. If n lies between 1/2 < n < 1 the diffusion mechanism is non-Fickian and is anomalous [31], it occurs when the permeant mobility and polymer segment relaxation rates are similar. Table 5 presents the n and k values in benzene of NR, XSBR and 70/30 NR/XSBR latex membranes reinforced with micron sized fillers. From the n values it can be understood that all systems exhibit non-Fickian, anomalous behaviour. The value of k implies the structural characteristics of the polymer and gives an idea about the nature of the interaction between polymer and the solvent. The k values of filled systems are lower that indicate lower polymer/ solvent interaction. 4. Conclusions

95

100

105

110

Molecular weight of solvent Fig. 11. Equilibrium solvent uptake vs. molecular weight of solvents of microcomposite of NR, XSBR and 70/30 NR/XSBR in benzene, toluene and xylene at 30 C.

The molecular transport properties of microcomposites of natural rubber; carboxylated styrene butadiene rubber and their 70/30 blends were investigated using solvents such as benzene, toluene and p-xylene. Microcomposites exhibited reduced swelling rate except for 70/30 NR/

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XSBR blend system owing to the tortuosity of path and the reduced transport area in polymeric membrane. The unexpected behaviour of blend system was due to the immiscibility of the two components, which led to the uneven distribution of filler in the two phases. It was found that the diffusion coefficient of filled samples decreased as a function of increase in weight percentage of filler. However, blend system showed increase in diffusivity values at higher concentration of filler. The activation energy needed for the diffusion of penetrant molecule was found to be higher than pristine polymer. The aggregation of fillers in NR and 70/30 NR/XSBR blend was observed from SEM.

[15]

[16]

[17]

[18]

[19]

Acknowledgement One of the authors Ms. Ranimol Stephen is thankful to Council of Scientific and Industrial Research (CSIR), New Delhi for providing the Senior Research Fellowship.

[20] [21] [22]

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