Chapter 9

Effect of Epoxidation of Natural Rubber on the

Penraporation Separation of Acetondchlorinated Hydrocarbon Mixtures

Res~rlts of this chapter have been rommunicatedji,r publication in Journal of Membrane Science

S eparation of organic liquid mixtures by the use of pervaporation membrane

separation process has been gaining greater interest in recent years. since the

penraporation technique is considered to be an energy saving process. At the very

early stage of penraporation research it was recognised that this process possesses a

high potential for the separation of organic liquid mixture^.'^ Since then much

research has been carried out and in some cases pernaporation can

replace energy-intensive process like distillation. Pervaporation can be used to

separate any liquid mixture in all concentration ranges.'-" In practice, however, this

technique is employed for the separation of azeotropic mixtures, close boiling point

mixtures or isomers, and for the removal or recovery of trace substance^.'^-'^

Despite these studies no large-scale application for the organic-organic

mixture separation in the chemical industry can be found so far. This is mainly due

to the lack of good membranes for specific applications. Synthesis of new

polymers, modification of existing polymers and polymer blending are the methods

adopted for preparing efficient membranes for application in separation technology.

For the pervaporation of organic liquid mixtures, many new polymers and

copolymers were synthesised and te~ted.'~-'%ese polymers contained specific

groups which could preferentially interact with one component of a liquid mixture

resulting in improved permselectivity. The modification of polymers can be done

through a chemical reaction, radiation or plasma treatment or a combination of

these methods. In this way specific groups are introduced to the polymer bulkI9 or

orlly to the surface of polymer membranes.20

The main objective of this chapter is to elucidate the effect of epoxidation on

the pen~aporation performance of natural rubber membranes.

9.1 Results and discussion

9.1.1 Swelling behaviour of crosslinked membranes

Initially a piece of dry membrane was weighed and swollen in the solution

mixture of known composition for 48 h at room temperature. The swollen

membranes were taken out and wiped with tissue paper to remove adhering

solvents and then weighed. The difference between the weights gave the amount of

solvent absorbed by the polymer. The swelling ratio was calculated using the

equation

where Wd denotes weight of dry membrane and W, denotes weight of solvent

swollen membrane.

Generally an increase in the crosslink density of a membrane leads to less

solubility of a single component in a liquid mixture and less polymeric chain

mobility due to less free volume in the membranes. Therefore in the pervaporation

process, both the solubility and diffusivity of a permeant through the crosslinked

membrane tends to decrease. But the chemical properties of the polymeric chains

and the interaction between the solvent mixture and the polymer tend to affect the

solubility properties.

Figure 9.1 shows the variation in swelling degree with percent of

epoxidation for different compositions of acetone/dichloromethane.

Figure 9.1. Swelling degree of NR, ENR-25 and ENR-50 with feed concenlralion ofCH2C'12 in CH2Clz/ocetone mixture.

The swelling degree increases with percent of epoxidation. This may be

explained by the decrease in solubility parameter difference between polymer and

solvent with epoxidation. As the solubility parameter difference between polymer

and solvent decreases, the swelling degree increase^.^' The solubility parameter

difference between polymer and solvent is given in Table 9.1. It is also seen from

figure that as the hydrocarbon concentration in the feed increases the swelling

degree increases.

Further Figure 9.2 shows swelling degee of NR, ENR-25 and ENR-50 to

different chlorinated hydrocarbons. For all the three membranes the swelling degree

decreases from CCL to CHZCIZ which results from the decreased interaction of

these solvents with polymer as evident from the solubility parameter difference

between polymer and solvent.

Table 9.1. Solubility parameter difference between polymer and solvent (~m"$''.

6 €-CH 2C12/acetone:50/50 -CHCI 3/acetone:50/50

5 - ---CC!, /acetone:50/50

4 -

0 0 1 0 20 30 40 50

Epoxidaation level (76)

Figure 9.2. Swelling degree ofNR, ENR-25 and ENR-50 f i r 50/50 composilions c f d~ffirettr chlorinated hydrocarbons.

ENR-50

1.6

0.8

-0.6

ENR-25

2.4

1.6

0.4

Solvent

CHzClz

CHC13

CC14

NR

3.6

2.8

1.4

9.1.2 Pervaporation analysis

(U) Effect ofepom.datratron

The pervaporation properties are characterised by the flux, l , and selectivity

a,,. Flux was determined by measuring the weight of liquid collected in the cold

traps during a certain time at steady state condition. The pervaporation selectivity

a,, is defined as

where X and Y represent the concentration in the feed and in the permeate

respectively. Indices i and j refer to the more permeable component (chlorinated

hydrocarbon in this study) and the less permeable one (acetone), respectively.

Figure 9.3 shows the variation of separation factor for acet0ne:chlorinated

hydrocarbon (50:50) composition with epoxidation.

22 0 0 25 50

EPOXIDATION LEVEL (%)

Figure 9.3. Separation jacror j i ~ r 50/50 composirions c?( acrtorre/ch/orinalrd hydrocarbons with mol % epoxidation.

As can be seen, the separation factor increases with epoxidation. But it is

seen that the swelling degree increases with epoxidation. One would expect that the

high swelling degree will decrease the separation efficiency. However, the observed

increase in separation factor may be explained by the gel content in ENR. ENR

contains a highly crosslinked gel phase giving rise to a two phase morphology. The

SEM observations given in Figure 3.7 shows the two phase morphology for ENR.

Natural rubber has got a smooth surface morphology. But with increasing

epoxidation the gel content increases giving rise to two phases, a lightly crosslinked

sol phase and a highly crosslinked gel phase. With increasing epoxidation level, gel

content increases. This leads to a more compact network structure, resulting in less

chain mobility. Therefore the separation factor increases even though the swelling

degree increases. Another factor which may be contributing to the increased

separation efficiency is the polar nature of ENR network and the higher hydrogen

bonding efficiency of acetone compared to chlorinated hydrocarbon. As the level of

epoxidation increases, the polarity increases and consequently extent of hydrogen

bonding with acetone and ENR increases resulting in improved separation with

increasing level of epoxidation. The 1R spectrum of ENR swollen with acetone

given in Figure 9.4 shows a peak at 1690 cm-'.

Figure 9.4. IR spectrum of'ENR swollen in acetone

The characteristic peak of -C=O stretching in acetone is expected in the range

1730-1705 cm-'. The shift in this peak to 1690 cm-' results fiom the hydrogen

bonding of acetone with E N R

Figure 9.5 shows the permeation rate for 5050 compositions. The flux has

been found to decrease even though the swelling degree increases with epoxidation.

Figure 9.5. Pervaporatiotl j lux for 50/50 compositions (4 acetone/chlorinated hydrocarbons with mol % eprxidation.

I t could be considered that the kinetic effect would affect the total flux. The

predominant mechanism of transport for liquid-separating membranes involves the

dissolution and subsequent diffusion of molecules in a non-porous or dense

membrane. When a penetrant i diffises through a membrane, the flux J, is the

product of the concentration C, and the linear velocity v, of the penetrant inside the

membrane. The velocity is the product of the mobility, B, and driving force. In the

case of pernaporation, the driving force is a gradient in the chemical potential

across the membrane. i.e., -dp,ldx. Therefore the following relation expresses the

flux in pervaporation process.22

According to this relation the flux should decrease as the mobility B, of the species

decreases. Clearly the highly crosslinked gel phase affects the mobility of the

permeating species through the membrane. A schematic model representing this

phenomena is g i ~ e n in Figure 9.6. The effect of gel fraction on the permeability of

the penetrants is clear from this model. The gel fraction offers a more tortuous path

for the penetrants thereby reducing the permeability through ENR matrix.

FEED FEED FEED

@ e t @ ef' e@

PERMEATE PERMEATE PERMEATE

Figure 9.6. Schematic model representing the permeation of solvent molecules through NR, ENR-25 and ENR-50.

(5) Effect of feed composition

As discussed earlier, the permeation of molecules through a non-porous.

polymer membrane is generally described by solution-diffusion mechanism in a

sequence of three steps, viz., sorption, diffusion and evaporation. According to this

model the pennselective properties of pervaporation membranes are determined by

solubility and diffusivity of the permeating components in the membrane. Since

generally both sorption and diffusion phenomena are dependent on the composition

of the liquid mixture, the permeation characteristics of membranes are usually

strongly influenced by the feed composition.

The effect of the feed composition on the flux and selectivity was

investigated over the range 75:25:, 5050 , and 25:75 of chlorinated hydrocarbon:

acetone mixture. Figure 9.7 represents the effect of the feed composition on the

pewaporation fluxes.

Epoxidation level (%)

Figure 9.7. E ~ C C I ( f f i i ~ d com/~o.si/ior~ O I I J U X with mol % r/)oxido/iori.

The flux decreases with decrease in hydrocarbon concentration in the feed.

According to the equation (9.3) flux of a mixture component in the feed decreases

because the activity in the permeate side is kept constant by a continuous

evacuation. The observed fluxes are in agreement with this.

Figure 9.8 shows the selectivity with different feed compositions. As the

concentration of acetone in the feed increases the selectivity increases. From

Figure 9.1 it is seen that the swelling degree decreases with increase in acetone

concentration. As acetone concentration in the feed decreases. the swelling degree

increases which increases the chain flexibility and free volume of the polymer.

Consequently the other component (acetone) may also pass through the membranes

resulting in decreased selectivity. As solvent swelling decreases. the plasticization

effect is less leading to a preferential permeation of the chlorinated hydrocarbon

molecules resulting in increased selectivity.

0 1 l 0 10 20 30 40 50

Epoxidation level (%)

Figure 9.8. Efect offeed composition on se/ecfivity u'ith m01 96 epoxidurion.

(c) Effeect ufpenetrunt size

The eRect of penetrant s i ~ e on pervaporation fluxes is given in Figure 9.5. It

is seen that as the penetrant size increases, the flux decreases. But the swelling

degree increases with increase in molecular size. Usually an increase in swelling

degree should increase the flux. But as discussed earlier, the permeation of

molecules is described by solution-diffusion mechanism when sorption, diffusion

and evaporation occur in a sequence of three steps. The sorption depends on

interaction of the solvent with the polymer. As the interaction increases, sorption

increases, which should contribute to the overall flux. But the obsened decrease in

flux may be due to the decrease in permeation coefficient as can be seen From the

Figure 9.9. Clearly the permeability through the polymer decreases with increase in

penetrant size. Thus the permeability decrease in the order CHzC12 > CHClr

This leads to decreased flux values for CC& compared toCHCl3 and CHzClz

01 10 0 10 20 30 40 50

Epoxidation level (%)

Figure 9.9. EffQct of molecular size on permeation coefficient and sorption coefJicien f .

Selecti~ity has been found to decrease with increased penetrant size. The

increased plasticization and the consequent increase in free xolume with increased

solvent swelling may be the contributing factor for decreased selectivity. As the

swelling increases, the comparati\ely smaller acetone molecules also can ditiuse

through the polymer.

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