TITLE: MEMBRANE POLYMER COMPOSITIONS
TECHNICAL FIELD
The invention relates to compositions suitable for use in forming membranes, in
particular for forming hollow fibre membranes for use in microfiltration. The invention
also relates to membranes prepared from such compositions, and to methods of their preparation.
BACKGROUND ART
The following discussion is not to be construed as an admission with regard to
the common general knowledge in Australia.
Synthetic membranes are used for a variety of applications including desalination, gas separation, filtration and dialysis. The properties of the membranes
vary depending on the morphology of the membrane i.e. properties such as symmetry,
pore shape and pore size and the polymeric material used to form the membrane.
Different membranes can be used for specific separation processes, including
microfiltration, ultrafiltration and reverse osmosis. Microfiltration and ultrafiltration are
pressure driven processes and are distinguished by the size of the particle or molecule
that the membrane is capable of retaining or passing. Microfiltration can remove very
fine colloidal particles in the micrometer and submicrometer range. As a general rule,
microfiltration can filter particles down to 0.1 μm, whereas ultrafiltration can retain
particles as small as 0.01 μm and smaller. Reverse Osmosis operates on an even smaller
scale.
As the size of the particles to be separated decreases, the pore size of the
membrane decreases and the pressure required to carry out the separation increases.
A large surface area is needed when a large filtrate flow is required. One known
technique to make filtration apparatus more compact is to form a membrane in the shape
of a hollow porous fibre. Modules of such fibres can be made with an extremely large
surface area per unit volume.
Microporous synthetic membranes are particularly suitable for use in hollow fibres and are produced by phase inversion. In this process, at least one polymer is
dissolved in an appropriate solvent and a suitable viscosity of the solution is achieved.
The polymer solution can be cast as a film or hollow fibre, and then immersed in
precipitation bath such as water. This causes separation of the homogeneous polymer
solution into a solid polymer and liquid solvent phase. The precipitated polymer forms a
porous structure containing a network of uniform pores. Production parameters that affect the membrane structure and properties include the polymer concentration, the
precipitation media and temperature and the amount of solvent and non-solvent in the
polymer solution. These factors can be varied to produce microporous membranes with
a large range of pore sizes (from less than 0.1 to 20μm), and altering chemical, thermal
and mechanical properties.
Microporous phase inversion membranes are particularly well suited to the application of removal of viruses and bacteria. Of all types of membranes, the hollow fibre contains the largest membrane area per unit volume.
Flat sheet membranes are prepared by bringing a polymer solution consisting of at least one polymer and solvent into contact with a coagulation bath. The solvent diffuses outwards into the coagulation bath and the precipitating solution will diffuse into the cast film. After a given period of time, the exchange of the non-solvent and solvent has proceeded such that the solution becomes thermodynamically unstable and demixing occurs. Finally, a flat sheet is obtained with an asymmetric or symmetric structure.
Hydrophobic surfaces are defined as "water hating" and hydrophilic surfaces as "water loving". Many of the polymers that porous membranes are made of are hydrophobic polymers. Water can be forced through a hydrophobic membrane by use of sufficient pressure, but the pressure needed is very high (150-300 psi), and a membrane may be damaged at such pressures and generally does not become wetted evenly. Hydrophobic microporous membranes are characterised by their excellent chemical resistance, biocompatibility, low swelling and good separation performance. Thus, when used in water filtration applications, hydrophobic membranes need to be hydrophilised or "wet out" to allow water permeation. Some hydrophilic materials are not suitable for microfiltration and ultrafiltration membranes that require mechanical strength and thermal stability since water molecules can play the role of plasticizers.
Currently, poly(tetrafluoroethylene) (PTFE), Polyethylene (PE), Polypropylene (PP) and poly(vinylidene fluoride) (PVDF) are the most popular and available hydrophobic membrane materials. Poly(vinylidene fluoride) (PVDF) is a semi- crystalline polymer containing a crystalline phase and an amorphous phase. The crystalline phase provides good thermal stability whilst the amorphous phase adds some flexibility to the membrane. PVDF exhibits a number of desirable characteristics for membrane applications, including thermal resistance, reasonable chemical resistance (to a range of corrosive chemicals, including sodium hypochlorite), and weather (UV) resistance. While PVDF has to date proven to be the most desirable material from a range of materials suitable for microporous membranes, the search continues for membrane materials which will provide better chemical stability and performance while retaining the desired physical properties required to allow the membranes to be formed and worked in an appropriate manner. In particular, a membrane is required which has a superior resistance (compared to
PVDF) to more aggressive chemical species, in particular, oxidising agents such as sodium hypochlorite and to conditions of high pH i.e. resistance to caustic solutions. DESCRIPTION OF THE INVENTION
According to a first aspect the invention provides the use of polymer suitable for forming into an ultrafiltration or microfiltration membrane, said polymer being a terpolymer of tetrafluoroethylene (TFE), PVDF and hexafluoropropylene monomers. Preferably, the polymer includes from 20-65% PVDF monomer, from 10-20% hexafluoropropylene monomer and 30-70 % TFE.
More preferably, the polymer includes from 30-50% PVDF monomer, from 15-
20% hexafluoropropylene, and from 30-55% TFE. Even more preferably, the polymer includes from 35-40% PVDF and 17-20% HFP and 40-48% TFE.
Most preferably, the polymer is a terpolymer of 44.6% tetrafluoroethylene (TFE)
monomers, 36.5% PVDF monomers, and 18.9% hexafluoropropylene monomers.
Unless otherwise indicated, all percentages are expressed as weight percentages.
According to a second aspect the invention provides an ultrafiltration and/or microfiltration membrane formed from a terpolymer including TFE monomers, PVDF
monomer and hexafluoropropylene monomer. Preferably the monomer composition is
approximately 44.6% tetrafluoroethylene (TFE) monomer, 36.5% PVDF monomer and
18.9% hexafluoropropylene monomer.
The membranes of the second aspect have an improved chemical stability to
oxidising agents and caustic soda relative to a membrane formed from PVDF alone.
According to a third aspect the invention provides a method of manufacturing a
microfiltration or ultrafiltration membrane including the step of casting a membrane
from a composition including a terpolymer of 44.6% tetrafluoroethylene (TFE)
monomer, 36.5% PVDF monomer and 18.9% hexafluoropropylene monomer.
Preferably, the membrane is in the form of a hollow fibre, cast by the TIPS
procedure, or more preferably by the DIPS procedure.
Most preferably, the polymer used is THV 220G, obtained from Dyneon® (3M) as
a solvent soluble fluoropolymer. The polymer is a combination of approximately 44.6%
tetrafluoroethylene (TFE) monomer, 36.5% PVDF monomer, and 18.9% hexafluoropropylene monomer.
According to a fourth aspect, the invention provides a method of forming a polymeric ultrafiltration or microfiltration membrane including the steps of: preparing a leachant resistant membrane dope; incorporating a leachable pore forming agent into the dope; casting a membrane; and leaching said leachable pore forming agent from said membrane with said leachant.
Preferably, the leachant resistant membrane polymer includes a terpolymer of TFE, PVDF and hexafluoropropylene. More preferably, the polymer includes 44.6% tetrafluoroethylene (TFE) monomers, 36.5% PVDF monomers, and 18.9% hexafluoropropylene monomers.
Preferably, the leachable pore forming agent is silica, and the leachant is a caustic solution, but the pore forming agent may for preference be any inorganic solid with an average particle size less than 1 micron while the leachant may be any material/solution that leaches the said pore forming agent from the membrane.
According to fifth aspect, the invention provides a method of improving the structure of a polymeric ultrafiltration or microfiltration membrane by the addition of a nucleating agent to a membrane dope. Preferably the nucleating agent is added in catalytic amounts and most preferably it is TiO2, however, any insoluble/inert (unleachable) inorganic solid with an average particle size less than 1 micron may be used.
According to a sixth aspect, the invention provides an elastic polymeric ultrafiltration or microfiltration membrane having an asymmetric cross section defining a large-pore face and a small-pore face; said membrane having a higher flux at a given
pressure from said large-pore face to said small-pore face than from said small-pore face
to said large-pore face.
Preferably the elastic membrane is formed from the preferred membrane forming
mixtures of the preceding aspects, and may also be formed using the addition of
leachable pore forming agents and/or nucleating agents.
The invention will now be described with particular reference to specific examples.
It will be appreciated, however, that the inventive concept disclosed therein is not limited
to these specific examples
BEST MODE FOR CARRYING OUT THE INVENTION
MEMBRANE FORMATION
DIPS PROCEDURE
THV 220G, obtained from Dyneon® Corp (3M) was dissolved in N-
methylpyrrolidone (NMP) at approximately 20 wt%. A flat sheet membrane was cast
from this solution and precipitated in water at 60°C before being examined using
scanning electron microscopy (SEM).
A standard DIPS process was employed as follows: Polymer solutions were mixed
and heated to around 50°C and pumped (spun) through a die into a 5 metre water-filled
quench (or solidification) bath at 65°C. Non-solvent (lumen) consisting of 20% NMP,
10% water and 70% polyethylene glycol (PEG200) was fed through the inside of the die
to form the lumen. The hollow fibre was then spun into the quench bath and solidified, before being run out of the bath over driven rollers onto a winder situated in a secondary
water bath at room temperature to complete the quench and washing of the fibre.
The membrane structure was reasonable although a skin was found on the surface
of the membrane that prevented exposure of surface pores.
The caustic resistance of the membrane was tested by placing a sample of the flat
sheet into 5 wt% caustic solution and comparing the appearance with a control of PVDF
membrane cast by the TIPS process.
Both samples were thoroughly wet out with alcohol prior to immersion in the
caustic solution. The THV samples become transparent upon complete wetting. The results of the caustic immersion test are shown in table 1.
Table 1 shows the results of the caustic resistance tests. The results indicate that
while the membranes are not impervious to caustic, as would be the case for a material
like Teflon, they show extremely limited degradation for an extended period of time in a
comparatively strong caustic solution. All subsequent exposures to 5% solutions have
shown the same result, that a slight yellowing occurs upon immediate contact with the
solution but no further degradation (either visually or affecting the membrane properties)
occurs.
In addition to colour changes, the stiffness of both the PVDF and the THV samples
were examined. The PVDF membrane had lost a marked amount of flexibility and was
quite brittle, while by contrast, the THV sample appeared to be relatively unaffected.
The results strongly suggest that no detrimental modification of the polymer
membranes takes place as a result of such caustic immersion.
TABLE 1
MODIFICATION OF MEMBRANE HYDROPHOBICITY HYDROPHILICITY
Those skilled in the art will appreciate the desirability of preparing membranes that are hydrophilic in character. For instance, as described earlier hydrophilic membranes are simpler to operate than hydrophobic membranes as they do not require an additional wetting step.
It was established in the present case that THV 220G is compatible with Lutonal A25 (Polyvinylethylether) at concentrations of around 2%. Lutonal A25 makes the DIPS membranes of the present application less hydrophobic.
Other than modifying hydrophobicity, the addition of Lutonal A25 appeared to make little difference in the physical structure of the membrane, apart from opening the
membrane structure slightly. However membranes prepared with or without Lutonal are
still acceptable in terms of their structure.
The addition of Lutonal A25 reduced the mixing time of the dopes quite
dramatically.
Other elements of the DIPS process have also been investigated in conjunction
with the use of THV 220G as a membrane polymer. It was found that non solvents can
be used in a dope mix such as the addition of 5% glycerine triacetate (GTA) into the
mixture without undue detrimental effects.
LEACHABLE DOPANTS
In order to produce membranes without a dense surface skin and having a more
hydrophilic nature, silica was added to the dope with the intention of leaching the silica
out of the matrix by the use of a caustic solution.
A hydrophilic silica Aerosil 200 and a hydrophobic silica Aerosil R972 were tested
separately as additives to the THV 220G membrane mixture. The dopes were cast into
flat sheet membranes, and were quenched in hot water at 60°C as described previously.
Once the membranes had been cast, a portion thereof was leached in a 5% aqueous
caustic solution at room temperature for 14 hours. Without wishing to be bound by
theory, it is believed that the silica reacts with caustic to make the membrane hydrophilic as discussed below. Also, the leaching using caustic soda provides a membrane of good open structure. A number of membranes containing silica were cast. The results are
shown in Table 2.
TABLE 2
Table 2 demonstrates that the silica is required in reasonably high concentrations to make the membranes hydrophilic. It also shows the trend of increasing viscosity with increasing silica content.
After the membranes were cast, and prior to leaching, the membranes were examined using scanning electron microscopy. The structures were generally extremely
promising with the surface of the sheets completely open and totally free of any skin.
The cross-sectional appearance was more like a conglomerate of precipitated particles, rather than a true honeycomb like structure.
The best form of the silica appeared to be the hydrophobic Aerosil R972, although
both forms of silica produced a hydrophilic membrane with a highly porous structure.
Subsequently placing the sample in caustic soda to leach the silica provided a
dramatic opening up in the membrane structure even further. The result of the leaching
was a change in the cross-section from the abovementioned conglomerate-like structure to the more traditional lace or sponge-like formation.
The optimal dope for forming a DIPS polymer appears to be from a mixture of
72% NMP, 20% THV, 6% silica and 2% Lutonal. This provides a hydrophilic
membrane from a dope possessing a viscosity in the range that can be easily pumped.
A number of hollow fibre membranes were prepared from the above dope. The
wetting characteristics were as desired and the membrane structure showed an extremely
open surface. While 6% silica was used in the present invention, it will be appreciated
that the quantity can vary significantly without departing from the present inventive concept.
Fibres incorporating silica with thicker walls were prepared and the current
properties of the fibre membranes were examined. The fibre was then subject to
leaching with a 5% caustic solution at room temperature for 18 hours.
It can be seen that leaching the membrane changes the permeability and bubble
points significantly without altering the desirable physical properties of the membrane. The leaching of the silica from the membranes has a positive effect upon permeability.
Thus, before leaching, the membrane had very few pores and extremely low flows.
After leaching, however, the situation is reversed and there are a multitude of pores and a
high flux.
A long leaching time is not necessarily required and can be incorporated in the
production process as a post-treatment of the final modular product. The leaching
process can be carried out at any time, however there is an advantage to postponing the
leaching process as long as possible, since any damage to the surface of the fibres during
handling can be overcome by leaching which physically increases the porosity of the
membrane. Existing PVDF membrane surfaces can be damaged irreconcilably during
production, resulting in a decrease in permeability and flux of the fibres.
SEM analysis of the membranes showed a high degree of asymmetry. Asymmetry
is defined as a gradual increase in pore size throughout the membrane cross-section, such
that the pores at one surface of the hollow fibre are larger than the other. In this case, the
pore size increase was seen from the outer surface where the pores were smallest (and a
quite dense surface layer was present) to the inner surface where the pores were
significantly larger than those on the outer surface.
Preparation of the fibres was run at 65°C rather than 50°C as in a typical DIPS
process. Increasing the quench bath temperature by 10-15°C dramatically affects the
surface structure. The higher temperature gives a much more open surface. The use of
the higher temperatures therefore accordingly means it is feasible to increase the polymer
concentrations and possibly the silica concentration if it is desired to bolster the existing
membrane and increase the mechanical strength.
Further it has been found that a more particular mixing procedure contributes to
the success of forming a membrane of high permeability. Mixing constituents together
in a random manner does not produce such a good result as following a more stringent
procedure whereby the Aerosil R972 is dissolved in the total quantity of NMP and this
solution is allowed to degas. The polymer pellets are mixed with the liquid Lutonal A25
to coat the pellets. When these two procedures are complete, the two mixtures are
combined. The advantage of this appears to be that the silica is dispersed effectively and
does not clump (which can lead to macrovoids) and also, the pellets do not clump (which
has the effect of increasing mixing time and consistency of the dope) since they are
coated with a sufficient quantity of Lutonal A25 for a sufficient time to allow them to
dissolve individually.
As well as silica, the leaching process allows for the introduction of other
functionalities into the membrane, such as introducing hydrolysable esters to produce
groups for anchoring functional species to membranes.
Surprisingly, it has also been found that the membrane remains hydrophilic after
leaching. Again, without wishing to be bound by theory, the silica particles have a size
in the order of nanometres so consequently the silica disperses homogeneously
throughout the polymer solution. When the polymer is precipitated in the spinning process, there is a degree of encapsulation of the SiO2 particles within the polymer
matrix. Some of the particles (or the conglomerates formed by several silica particles) are wholly encapsulated by the precipitating polymer, some are completely free of any
adhesion to the polymer (i.e. they lie in the pores of the polymer matrix) and some of the
particles are partially encapsulated by the polymer so that a proportion of the particle is
exposed to the 'pore' or to fluid transfer.
When contacted with caustic, it is believed that these particles will be destroyed
from the accessible side, leaving that part of the particle in touch with the polymer
matrix remaimng. The remainder of the silica particle adheres to the polymer matrix by hydrophobic interaction and/or mechanical anchoring. The inside of the particle wall is
hydrophilic because it consists of OH groups attached to silica. Because the silica is
connected to hydrophobic groups on the other side, it cannot be further dissolved.
Thus, when the membranes are treated with caustic solution, the free
unencapsulated SiO2 reacts to form soluble sodium silicates, while the semi-exposed
particles undergo a partial reaction to form a water-loving surface (bearing in mind that
given the opportunity, such particles would have dissolved fully). It is believed that the
pores in the polymer matrix formed during the phase inversion stage yet filled with SiO2
particles are cleaned out during leaching, giving a very open, hydrophilic membrane.
NUCLEATING AGENTS
TiO2 (titania) was also added to the membrane at a variety of concentrations. TiO2 has been added to membrane forming mixtures previously as a filler to provide
abrasion resistance or to act as a nucleating agent, to increase the rate of fibre
solidification.
However, surprisingly in the present case, it was found that the addition of TiO2 in concentrations below that used for reinforcement of membranes, a high degree of
asymmetry was introduced into the membranes. In particular, this was as a result of the
formation of a dense outer layer. Without wishing to be bound by theory, the applicant believes that the TiO2 particles provide a site for phase inversion or precipitation to
begin. In hollow fibre membranes prepared by the DIPS process, the high number of fast
solidification sites at which precipitation occurs means that the pores formed near the
membrane surface are smaller, fewer and further between.
The use of too much titania can cause a dense outer layer on the membrane to
restrict permeability. Further, as the titania disperses very well throughout the dope, only
of the order of a catalytic amount is required. For example, only about 0.1-0.2 wt%
titania need be incorporated into the membrane, although as much as 3% can be used
depending on the desired effect.
A dope formulation giving good results is 20 wt% THV 220G, 6 wt% Aerosil
R972, 2 wt% Lutonal A25, 0.2% TiO2 and 71.8% N-methylpyrrolidone.
A dope having the above formulation was mixed and cast according to the DIPS
method. They were then leached in 5% caustic soda solution for approximately 24 hours
and then soaked in glycerol. Soaking fibres in glycerol or the like is a highly desirable
step, since the material is relatively flexible and will allow pores to collapse. The results
for the TiO2 trial fibres are given as Table 3.
TABLE 3
Table 3 lists the properties of the membranes made which incorporate a small
proportion of TiO2. The most apparent property to note is the high permeability of the
membrane.
HIGH POLYMER CONCENTRATIONS
Attempts at making polymer concentrations above 20 wt% were attempted. Doing
so however caused alternative problems mainly based around a dramatic increase in
viscosity. Once the polymer portion rises to above 25 wt%, viscosity becomes too high
to pump in conventional pumps. However, high polymer concentrations were seen to
correlate with an increase in the mechanical strength of the membrane. Optimal results
of workability and strength were achieved with the hollow fibre having a polymer
concentration of 22%. The best was seen to be 22 wt% THV 220G, 6 wt% Aerosil
R972, 2% Lutonal A25 and 70% N-methylpyrrolidone. Concentrations as high as 30
wt% polymer did produce a feasible membrane. The high polymer concentration
membranes were leached in a 5% caustic solution for 24 hours and then soaked in
glycerol. The results are shown in Table 4. A point of note is that the increase in
polymer concentration or the addition of TiO2 does not appear to improve the bubble
point or burst pressure of the fibres in any way. The mechanical strength of the fibre
appears to be mainly a function of wall thickness and lumen diameter.
TABLE 4
Table 4 lists the properties of the membrane made using 22% polymer (without
TiO2). Comparing the results to Table 3, the membrane exhibit very similar
characteristics with the exception that Table 3 indicates possibly a higher
permeability/flux for titania containing membranes.
PHYSICAL PROPERTIES OF MEMBRANES
The bubble point measurements in Tables 3 and 4 do not give an entirely accurate
determination of the bubble point, the pore size or molecular weight cut off of the
membrane because the membranes are somewhat rubbery and flexible so that under
pressure the membrane expands and hence the pores stretch like a rubber band. It has
been observed that the fibres increase in size slightly under a backwash pressure of as
low as 100 Kilopascals.
This behaviour is apparently due to the high elastic nature of the polymer which
also gives extremely high break tension described in Table 4. This elastic behaviour
would adequately describe the apparently low bubble point recorded for the membrane, since as the membrane is stretched by the pressure applied, the pores would be stretching proportional to the overall size increase of the fibre. This property is extremely valuable
for cleaning a membrane, since the pores may be opened up by the application of a liquid
backwash and any material fouling the pores may be easily dislodged and flushed away.
The elastic behaviour also indicates that the membrane (and hence the pores) may recover up to 100% of such a deformation, thus the pores would return to their original size.
To demonstrate this characteristic behaviour, the permeability and fluxes of the fibres were measured. Permeability and flux are typically measured with a filtration direction (direction of the filtrate flow relative to the membrane surfaces) outside-in with the filtrate collected from the inside of the hollow fibre. To prove that the pore structure is increasing in size, the flow was reversed so that the filtration direction was inside-out, with filtrate emerging on the outer side of the fibre. Table 5 shows the results of these "outside-in" and "inside-out" tests
TABLE 5
Table 5 and Figure 1 show that the flux for inside-out flow increases as the pressure increases, while the outside-in flow remains almost completely constant. This indicates that the pressure applied from the inside is expanding the pores to allow far
higher flows. This elasticity described is one of the most desirable properties of the membranes discussed. POTTING
As a result of this one of the desirable features of the membranes according to the present invention is their ability to be potted directly into epoxy. PVDF membranes require a more flexible potting material such as polyurethane to prevent damage to the fibres. PVDF fibres can break with relative ease if the fibers are potted in a potting material which lacks any flexibility. If there is no flexibility in the potting material there can be breakage of the fiber at the point where the fiber enters the pot. By contrast, the membranes of the invention can be potted into epoxy potting material and the fibers will not be significantly damaged during use. In fact, the membranes of the present invention can be stretched to the normal break extension of the fibre when pulled parallel to the pot
surface i.e. 90° to the potted direction.
The comparison of the properties of the THV membranes of the present application and PVDF prepared with the DIPS process are shown in Table 6. TABLE 6
Table 6 gives a comparison between THV membranes manufactured using the
DIPS process and the best (to date) PVDF membranes manufactured using the DIPS
process. The main differences are the spontaneous wetting of the THV membrane and
also the high clean water permeability, both of which are lacking in current PVDF
membranes. The other difference lies in comparing the stiffness of the membranes,
which is directly attributable to the polymers used to produce the membrane.
It would be appreciated by those skilled in the art that while the invention has been
described with particular reference to one embodiment, many variations are possible
without deviating from the inventive concept disclosed herein.