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DESAUNATION

ELSEVIER Desalination 163 (2004)207-214 w ww.el sevlea'.eonffl o~ale/d e ~ i

Siloxane-urethane membranes for removal of volatile organic solvents by pervaporation

Wojciech Czerwifiski a, Barbara Ostrowska-Gumkowska a, Janusz Kozakiewicz b, Wojciech " .a, Kujawskl , Andrzej WarszawskP

aFaculty of Chemistry, Nicolaus Copernicus University, ul. Gagarina 7, 87-100 Torud Poland Tel. +48 (56) 611-4315; Fox +48 (56) 654-2477; email: kujawski@chem.uni.torun.pl bIndustrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warszawa, Poland

Received 17 July 2003; accepted 3 September 2003

Abstract

Aqueous dispersions of siloxane-urethane copolymers (polysiloxaneurethanes) containing 0-49% of siloxane moieties were synthesized by the modified prepolymer-ionomer method. The properties of the dispersions were determined. Dense membranes were prepared fi'om those dispersions by crosslinking with a multifunctional aziridine derivative. The structure of the obtained membranes was investigated by FTIIL solid state 13C NMR, 29Si NMR and DSC. Prepared membranes were tested in vacuum pervaporation of the following water-organic mixtures: water-tert- butyl methyl ether (1.8% wt. MTBE) and water-butyl acetate (0.25 % wt. BuAc) at a temperature of 313 K, It was found that pervaporation properties of the polysiloxaneurethane membranes depended on the content of siloxane moieties. The best separation and transport properties were observed for the membrane containing 49% of siloxane groups. For this membrane separation factor ct was equal to 750 and 1370 for water-MTBE and water-BuAc mixtures, respectively. The ratio of organic to water partial molar fluxes (Jorg./Jwater) w a s 2.8 and 0.5, respectively. This membrane was highly hydrophobic, thus the measured flux of pure water was about half of that for the membrane with a lower content of siloxane moieties.

Keywords: Polysiloxane-urethanes; Membranes; Membrane structure; Pervaporation; Volatile organic solvents

1. In troduct ion

Pervaporation is an energy-saving membrane technique used to separate liquid mixtures. This

* Corresponding author.

process can be used for the dehydration of organic solvents, separation of binary organic- organic mixtures, extraction o f organics from aqueous solutions and/or recovery o f aroma compounds in the food and cosmetic industry [1 ].

Presented at the PERA4EA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hunga~, Poland and Slovakia), September 7-11, 2003, Tatranskd Matliare, Slovakia.

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved PII: S0011-9164(04)00106-7

208 W. Czerwifisla" et al. /Desalination 163 (2004) 207-214

Removal of organics from aqueous solutions is of particular interest for the water recycling process and treatment of waste water. Dense membranes prepared from various rubbery poly- mers such as polybutadiene, polyether copoly- mers or polydimethylsiloxane are used for the pervaporative removal of volatile organic com- pounds from water [2-7].

Siloxane-urethane copolymers, also called polysiloxaneurethanes, combine the properties of polysiloxanes (heat resistance, weather stability, low-temperature flexibility, low surface tension) with those of polyurethanes (high mechanical strength, abrasion resistance). They have recently attracted much attention as coatings for various applications (e.g., waterproofing, biocompati- bility, antifouling), binders and heat-resistant layers [8]. Siloxane-urethane copolymers can be also regarded as interesting membrane material.

In this study siloxane-urethane copolymers containing soft segments were synthesized in the form of aqueous dispersions. Three kinds of soft segments - - polysiloxane (low- and high-mole- cular) and polyether - - were incorporated into a macromolecule. Polyether (PTMG 2000) was chosen to partially replace the expensive poly- siloxane diol by its cheaper substitute. The objective of this work was also to investigate the possibility of using dispersions for the prepa- ration of dense pervaporation membranes. The transport and separation properties of these membranes were tested in pervaporation of water-organic (methyl tert-butyl ether, and butyl acetate) mixtures.

Table 1 Molar ratios of polysiloxanediols (Tegomer H-Si) to polytetrahydrofurane (PTMG 2000)

Dispersion Molar ratios

Tegomer H-Si PTMG 200

A -- Only polyether moieties

B 0.O7 a 1 C 0.22 a 1 D 0.07 b 1 E 0.58 a 1 F 1.15 ~ 1 G 2.42 a 1 H 9.28 ~ 1 I Only siloxane --

moieties K Only siloxane - -

moieties

aTegomer H-Si, Mw = 900; bTegomer H-Si, M w = 2800.

mixture ofdiols. Polysiloxanediols (Tegomer H- Si 2111,Mw = 900 and Tegomer H-Si 2311, Mw = 2800, Goldschrnidt) and polytetrahydrofurane (PTMG 2000, BASF) were used at various molar ratios to synthesize polymers with different contents ofsiloxane and ether moieties (Table 1).

In the second step the NCO terminated pre- polymer containing carboxylic groups was neutralized with amine. The third step consisted of dilution of the prepolymer-ionomer with N- methylpyrrolidone and its subsequent emulsify- ing in water and crosslinking with polyamine. Polyamine was used as a crosslinker at the stage of the dispersion formation.

2. Experimental

2.1. Synthesis

Synthesis of aqueous polymer dispersion was carried out by a three-step prepolymer-ionomer method [9]. The first step was the polyaddition of isophorone isocyanate (Hills), dimethylolpro- pionic acid (Aldrich Chemical) and diol or

2.2. Characteristics of dispersions

The solids content of dispersions was deter- mined by drying ca. 0.5 g of dispersion at 393 ± 2 K over 1.5 h. pH of dispersions were deter- mined using pH indicator papers (2-10 pH range). Viscosity of dispersions was determined at 298 K using a Hrppler rheoviscometer. Mechanical stability of dispersions was deter-

W. Czerwifisla" et al./ Desalina~'on 163 (2004) 207-214 209

mined by rotating ca. 30 g of dispersion in a Universal 16/16R centrifuge (Hettieh Zentri- fugen) at 4000 rpm. Dispersion stability was considered as "very good" (+) if no phase separation or precipitate formation was found after 90 rain of rotation.

2. 3. Membrane preparation

Membranes were prepared by casting a mixture composed of an aqueous polysiloxane- urethane dispersion and a crosslinking agent [3% by weight of 25% aqueous solution oftrimethyl- olpropane tris(2-methyl- 1-aziridinepropionate) supplied by Aldrich] onto a glass plate and drying at 298 K for 48 h. After rinsing with distilled water, the membranes were conditioned in air at room temperature.

2.4. Characteristics o f membranes

The FT-IR spectra of the annealed under vacuum (378 K, 8 h) and then pressed mem- branes (200 kg.cm -2) were recorded on the FT-IR Spectrum 2000 spectrometer (Perkin Elmer) in the frequency range of 400-4000 cm -1. Solid- state a3C and 295i NMR spectra were recorded on Brucker AMX-300 equipment using a TOSS pulse sequence (4500 Hz). 295i NMR spectros- copy was performed to identify any possible changes in the chemical structure of the silicone component as well as to check if the ratios of components taken to the basic reaction are the same as in the products. Differential scanning calorimetry (DSC) measurements were per- formed using a PL DSC calorimeter (Polymer Laboratories). The procedure consisted of quenching, initial heating, cooling and reheating cycles.

2. 5. Pervaporation

Pervaporation experiments were carried out in a laboratory-scale pervaporation system consist-

FEED

'rhermosmea feed tank

Ree[rculati0n pump

i [ Per~,aporation

cell

, , [MEMBRANE

j VACUUM

Fig. 1. Scheme of the pervaporation set-up.

ing of a temperature-controlled feed tank, pervaporation cell, recirculation and vacuum pumps and permeate traps (Fig. 1). Feed solution circulated over the membrane. Water-tert-butyl methyl ether (1.8% wt. MTBE) and water-butyl acetate (0.25% wt. BuAc) mixtures were used as the feeds. The active membrane area was 22 cm 2. The permeate was collected in cold traps cooled by liquid nitrogen.

During experiments the upstream pressure was maintained at atmospheric pressure while the downstream pressure was kept below 1 mbar by using a vacuum pump. The permeate flux was determined by weight, whereas the feed and permeate compositions were determined by using a Varian 3300 gas chromatograph. Additionally, the permeate flux of pure water through the membranes was determined. Pervaporation experiments were nm at 313 K.

3. Results and discussion

3.1. Characteristics o f dispersions

The properties of the polysiloxaneurethane dispersions prepared as described above are presented in Table 2. All dispersions contained approximately 31-35 % per weight of solids. Fast gelation occurred in dispersions containing 19- 34% of siloxane moieties. Therefore, solids content, viscosity and pH value of these

210 W. CzerwifisM et aL / Desalination 163 (2004) 207-214

Table 2

Characteristics o f aqueous polysiloxaneurethane dispersions prepared in the study

Dispersion Content o f specific moieties in Solids Viscosity,

dispersion solids, % content, % mPa.s pH Mechanical

stability

A 0 6.20 1.50 34.00 220 7.7-8.0 +

B 1.46 5.88 1.50 34.94 360 7.7-8.0 +

C 4.11 9.25 2.33 33.83 320 7.7-8.0 +

D 5.30 5.77 1.56 33.93 240 7.7-8.0 +

E 9.30 9.58 2.69 34.45 460 7.7-8.0 +

F 19.24 7.33 2.85 34.40 - - - - -

G 20.76 11.74 2.77 . . . .

H 29.63 12.59 3.23 . . . .

I 33.53 12.62 3.17 - - - - - -

K 49.05 7,24 2.30 31.44 76 7.7-8.0 +

H2C~'-~/CH ~ ~ + ;N,+ - - - - - I

c o o - H coocl c Nrl co -c _-cH_c-or

Fig. 2. Scheme of erosslinking of the polysiloxaneurethane membrane material with a multifunetional aziridine derivative.

dispersions were not tested. The other dispersions were homogeneous and mechanically stable. The pH value of the dispersions was 7.8-8.0 and the viscosity ranged from 76 to 460 mPa's.

3. 2. Characteristics o f the membranes

Membranes were prepared using the cross- linking agent containing aziridine moieties. The reaction of carbonyl group with aziridine was reported as a useful way of crosslinking poly- urethane dispersions [10]. The crosslinking scheme of the membrane polymer is presented in Fig. 2.

It is worth noting that membranes were coated by a hydrophobic layer due to the surface phase separation. The presence of a polysiloxane segment-rich layer of a few nanometers thickness on the surface was confirmed by ESCA studies of films made from dispersions. It was much more distinct for dispersions prepared from the mixtures of Tegomers and PTHF.

In FT-IR spectra of all the membranes, the absorption bands appeared at 3298-3318 cm -1 (urethane N-H stretch) and 1720-1740 cm -1 (urethane C=O stretch) confirming the formation of urethane linkage. The absence of absorption band at around 3500 cm -1 indicated that the OH groups present in diols used as starting materials disappeared. Two absorption bands in the region 1200-1136 cm -a (Si-O-Si stretch) and the absorp- tion band at around 1100 c m -1 ( C - O - C stretch) suggested an incorporation of polysiloxane and polyether segments into the polymer chain.

29Si NMR spectra of the polysiloxane-based polymers contained a very intensive band at about -24 ppm, attributed to the silicon atom linked with methyl groups (Fig. 3). The band of low intensity at about 5 ppm attributed to the silicon atom bounded with terminal methylene groups appeared in the spectra of polymers containing low-molecular polysiloxane segments. In the 13C NMR spectra of polymers containing polyether segments the band at 25.6 ppm attri-

W. Czerwi~h" et al. / Desalination 163 (2004) 207-214 211

OO

t

ppm 20 0 -20 -40 -60

Fig, 3. 29Si N-MR spectrum of the polysiloxane-based polymer.

buted to the methylene groups showed the highest intensity (Fig. 4). The very intensive resonance line at -0.16 ppm attributed to the carbon atom linked with the silicon atom was present in the spectrum of the polymer containing the highest content of siloxane moieties (the K dispersion, Table 1). The presence of urethane linkage was proved by the band at 156 ppm.

In the DSC thermograms of the membranes the second-order transitions were observed both in low (167-182 K and 201-206 K) and high (336-349 K) temperatures, being characteristic of the glass transition temperatures of the soft and hard segments, respectively. It suggested the phase separation of soft and hard segments and a domain morphology of the membranes. The first- order transition in the intermediate temperature region corresponding to the melting process of the PTMG crystallites (T~ = 310.4 K) was found only in the D membrane containing high- molecular siloxane segments.

3. 3. P ervaporatJon

The results of pervaporation experiments are presented in terms of the normalized permeate

Table 3 Normalized flux of water

Membrane Content of Normalized flux, siloxane moieties, % g pm m -2 h -1

A 0 3735

B 1.46 3657

C 4.11 3713

D 5.30 3800

E 9.3 3624

K 49.O5 1880

fluxes Jand the separation factor c~:

j = m l At (1)

e~,,~.,~. = (c~ / c ~ ) v (2)

where m is the mass of the permeate, A the membrane area, l the membrane thickness, t the time, and Coe, cwato r are the organics and water concentration, respectively. The superscripts P and F indicate permeate and feed solutions, respectively.

The flux of water through membranes with low content of siloxane moieties was practically constant and equal to 3700±100 g p m m -2 h -1 (Table 3). Only in the case of the K membrane with a high content of siloxane moieties did the water flux decrease to about 1900 g jxrn m -2 h -1 . Taking into account the results obtained for membranes prepared from styrene/isoprene- siloxane copolymers [11], it can be assumed that the hard segments (i.e., urethane moieties) form a continuous mierophase with the soft (siloxane and/or ether) segments dispersed in it. The urethane domains are more permeable for water than the siloxane and/or ether domains. The siloxane domains prevail in the K membrane (i.e., with high content of siloxane moieties) making it much more hydrophobic.

212 W. CzerwifisM et aL / Desalination 163 (2004) 207-214

ii ' 1

. . . . . : 6o . . . . . . . . . . . . . . . . . . . . . . . . . s b . . . . . . . . . b "

Fig. 4. ~3C NMR spectrum of polymer with polyether segments.

In contact with water-organic mixtures all investigated membranes were selective toward an organic component, but to a different extent, depending on the content of siloxane moieties in the membrane material (Figs. 5 and 6, Tables 4 and 5). The lowest selectivity was found for the membrane without siloxane (Jo~/Jw~t~ = 0.08 and 0.11, ct = 23 and 293 for water-MTBE and water- BuAc mixtures, respectively). The selectivity of the membranes slightly increased with incorpora- tion of siloxane moieties into the membrane polymer. Unfortunately, F-I dispersions (i.e., containing 19-33% of siloxane) were unstable and it was impossible to estimate the threshold value in pervapomtion. The percolation concept is often used to the description of transport of the permeates through membranes with different rigid and flexible segments [ 12,13]. It was shown by Kerres et al. [14], Roizard et al. [15] and Kujawski et al. [11] that the percolation thresh-

old for the pervaporation of water-organic systems through the membranes with different rigid and flexible segments occurred usually for membranes containing 35-40% of the flexible component. In fact, the K membrane containing 49% ofsiloxane moieties was the most selective (Jorg/Jwater = 2.8 and 0.5, ct = 750 and 1370 for water-MTBE and water-BuAc mixtures, respect- ively), so we can assume also that in the investi- gated siloxane-urethane membranes the percola- tion threshold should occur in the same region of the siloxane content.

Comparing pervaporation results with those obtained for commercial membranes with poly- dimethylsiloxane layer O.e., Pervap 1060 and Pervap 1070, manufactured by Sulzer Chemtech Membrane Systems), it can be stated that the K membrane was even more selective than the commercial ones [11].

W. Czerwifisla" et al. / Desalination 163 (2004) 207-214 213

3,0~

2.5-

2,0-

1.5-

1.0-

0.5-

0,0.

[] BuAe-water

[ ] MTBE-water ,i

0,0 1,5 4,1 5,3 9.3 S!loxane moieties content [wt%]

49.1

Fig. 5. Ratio of molar partial fluxes Jorg/Jwat~r vs. siloxane moieties content.

1400

t200

1000

80C

6O0

400

200 '

O( 0

• MTBE-water

• BuAc-water

I, i

Depe 10 20 30 40 50

$11oxane moieties content [wt,%]

Fig. 6. Separation factor ~ vs. siloxane moieties content,

Table 4 Normalized partial fluxes of components in pervapo- ration ofwater-tert-butyl methyl ether (1.8% vet. MTBE) mixture

Table 5 Normalized partial fluxes of components in per- vaporation of water-butyl acetate (0.25% wt. BuAc) mixture

Membrane Content of Normalized flux, siloxane g g m -~ h -1 moieties, %

MTBE Water

Membrane Content of Normalized flux, siloxane g gm m -2 h "I moieties, %

BuAc Water

A 0 3,152 7,625 B 1.46 6,929 8,417 C 4.11 5,249 6,383 D 5.30 5,071 5,494 E 9.30 9,097 6,483 K 49.05 38215 2,785

A 0 4,045 5,515 B 1.46 7,703 5,951 C 4.11 5,279 6,452 D 5.30 3,344 4,508 E 9.30 4,378 4,452 K 49.05 7,847 2,290

Generally, fluxes of both separated compo- nents increased with the increase of siloxane moieties content in the membrane material (Tables 4 and 5). It should be noted, however, that the partial flux of water during transport of water-organic mixtures exceeded water flux in the single-component system (Table 3). This phenomenon can be explained by a plasticization of the polymer membrane material by an organic component of the separated mixture [16]. As a result, transport of water through the plasticized membrane proceeded easier and an augmentation in water flux was observed.

The partial fluxes of organic compound and water through the D membrane were below those through the B and C membranes, despite the higher content of siloxane moieties in the D membrane (Tables 4 and 5). On the other hand, water flux through the D membrane was similar to that through other membranes with low content of siloxane moieties (Table 3). Taking into account the DSC results, it was stated that the crystallinity of the D membrane could cause more rigid structure of this membrane and then could limit the plasticization by an organic component.

214 W. Czerwifisla" et al. / Desalination 163 (2004) 207-214

Acknowledgement

This work was financially supported by the grant from the State Committee for Scientific Research (KBN) for Project No. 7 T08E 067 20.

The authors wish to thank Anita Koncka- Foland, M.Sc., for assistance in the synthesis and the characterization o f dispersions.

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