Description:FIELD OF THE INVENTION:
The present invention relates to the field of batteries and specifically, relates to a separator film as used in batteries. More specifically, the present invention relates to a process of preparing battery separator films from polyolefins by cast film extrusion and other methods.

BACKGROUND OF THE INVENTION:
Energy is an essential element in our daily life. The major demand for energy in modern society is growing rapidly. Among all energy storage systems, batteries are one of the most efficient devices. From small coin cells to mobile phone batteries, along with electric vehicle batteries, there is an increase in the number of stationary energy storage applications. The important components of a battery include electrodes, electrolyte, and separator. The separator is a porous polymer material used in batteries for the function of separating the cathode and the anode to prevent mechanical contact between them and a function of transmitting ions in the electrolytic solution. There are various prior known methods for preparing battery separator films and same are disclosed hereinbelow.

The EP3940840A1 document discloses nano-composite polymer separator with enhanced safety performance and preparation method thereof. The document discloses a method for manufacturing a porous membrane suitable for use as a separator of a lithium-ion battery. The document discloses a dry mixing process and describes the amount of the hydrophobic filler is from 5 % to 40 % by weight, preferably 10%-30 % by weight, based on the total weight of the polymer matrix and the filler. Use of polypropylene has also been described as “the polymer is preferably selected from polyolefin, polyamide (PA), polyethylene terephthalate (PET) and polyimide (PI). The polyolefin may be selected from polyethylene, polypropylene (PP), polyisobutylene (PIB), poly-1-butene (PB), copolymers of ethylene and propylene (EP), and copolymers of ethylene and alpha olefins. The polyethylene is preferably selected from high-density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE), especially HDPE, for example, HDPE F04660 commercially available from SABIC.

Further, the document discloses a master batch of HDPE and 20 wt.% hydrophobic fillers in Twin Screw Extruder followed by making into cast film, then biaxial stretched to make porous battery separator films. The hydrophobic filler used is Alumina, Alkyl silyl modified alumina, ZrO2, MgO and ZnO. Here the porosity is generated by biaxial stretching. It is a binary system of HDPE and filler.

The CN107749452B document discloses a battery, separator, battery pack, electronic apparatus, electric vehicle, power storage device and power system. The document discloses a substrate with the surface layer formed on at least one surface of the substrate and having a porosity higher than that of the substrate, wherein, the surface layer contains particles and resin material. The particles are selected from alumina, boehmite, yttrium oxide, titanium oxide, magnesium oxide, zirconium oxide, silicon oxide, zinc oxide, aluminum nitride, boron nitride, silicon nitride, titanium nitride, carbide At least one of silicon, boron carbide, barium titanate, strontium titanate, barium sulfate, porous aluminosilicate, layered silicate, Li2O4, Li3PO4, LiF, aluminum hydroxide, graphite, carbon nanotube, and diamond. The surface layer includes a first layer having protrusions and recesses, and a second layer formed between the first layer and the substrate, and the second layer has a lower layer than the first layer porosity, and the first layer is formed to have a thicker thickness than the thickness of the second layer.

Further, in the above document Polyethylene having a porosity of 30 – 40 % is used as substrate. A mixture of Alumina, Polyvinylidene Fluoride (PVdF) and 2-butanone (MEK) were mixed and the resin solution was applied uniformly to same thickness on both surfaces of the substrate and dried to remove MEK. This is a bilayer system.
The above document discloses use of polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), nylon as separator. The separators developed thus far, polyethylene (PE) and polypropylene (PP) porous membrane separators are the most dominant ones for commercial batteries, because of their superior properties such as cost-efficiency, good mechanical strength, pore structure and electrochemical stability.

However, the disclosed methods and the separator films have many drawbacks such as the process is complex, mostly uses a biaxial stretching which needs very bigger machines, and the separator films as disclosed in these prior art documents uses porous PE films along with a layer of fillers with lower mechanical properties such as tensile strength and puncture strength. Moreover, the disclosed films are bilayer / trilayers in nature with coating with fillers. Accordingly, there is a need for a process which is simple in execution and provides separator film with desired properties such as porosity, thickness, thermal shrinkage, melting temperature, tensile strength, puncture strength and pore diameter.

SUMMARY OF THE INVENTION:
The present disclosure relates to a process for the preparation of battery separator films by compounding different porogens with polyolefins by various mixing procedures followed by the cast film extrusion method and other methods.

The process includes steps of preparing a blend consisting of polyolefins, porogens, nano inorganic fillers, and compatibilizers, wherein the blend is a dry blend and after preparing the blend, the blend is mixed and extruded to produce extrudates or granules.

The blend is mixed in a high-speed mixer, microcompounder or a torque rheomixer and extruded though a single screw extruder, a twin-screw extruder. The blend is mixed and extruded at a temperature profile of 160°C to 220 °C, and a screw RPM (rotation per minute) of 30-60.

Wherein, the polyolefins are selected from polypropylene, polyethylene, or a combination thereof. Wherein, the porogens are selected from thermoplastic starch, sodium chloride, paraffin oil, polyethylene glycol or a combination thereof. Wherein, the nano inorganic fillers are selected from Nano SiO2, Nano Al2O3 or a combination thereof. Wherein, the compatibilizers are selected from PP-g-MA, PP-g-TMPTA or a combination thereof.

Then cast film extrusion of the blend to produce a cast film, wherein, the cast film extrusion process is performed with a single screw extruder having a coat hanger die and a temperature of hot rollers is from 90oC–120oC with stretching. Then cutting the cast film into a required shape cast film, wherein the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film.

Stretching the required shape cast film in a uniaxial direction to obtain a stretched cast film, the stretching is done in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen atmosphere and a temperature of 120oC-130oC.

Extracting the porogens from the stretched cast film to obtain a porous battery separator films, wherein extracting is performed by using a solvent and the solvent is selected from water, hexane, acetone and a combination thereof. Drying the battery separator film for a time period of 18-28 hours at a temperature of 40 °C-80°C.

BRIEF DESCRIPTION OF THE DRAWING:
The detailed description below will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative.

It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1: illustrates a schematic process flow diagram of the process as disclosed herein.

DESCRIPTION OF THE INVENTION:
The present disclosure is related to the process for the preparation of battery separator films by compounding different porogens with polyolefins by various mixing procedures followed by the cast film extrusion method. The porogens are selected from thermoplastic starch, sodium chloride, paraffin oil, polyethylene glycol or a combination thereof.

The present disclosure is related to a process for the preparation of battery separator films by using a ternary composite system of polymer, porogens (thermoplastic starch, sodium chloride, paraffin oil etc), nano inorganic fillers and compatibilizers such as PP-g-MA, or PP-g-TMPTA. All three materials are dry blended, and melt mixed by different processing techniques as given below.
(a) Single step process of making cast films: In this process, the above ternary composite blend is extruded in a single screw extruder by varying temperature from 160-220°C, RPM of 30 -60 into cast films by uniaxial stretching of about 3 – 10%. This is followed by high temperature stretching ranging from 50 to 300% in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen and a temperature of 120oC-130oC. Then these films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, and a combination thereof.
(b) Two-step process to make cast films: Melt blending of ternary composite blend in a micro-compounder at a temperature of 160-220°C, RPM - 30 into granules followed by Cast film extrusion with a stretching of 3 – 10%. This is followed by high temperature stretching from 50 – 300% in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen atmosphere and a temperature of 120oC-130oC Then these films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, acetone and a combination thereof.
(c) Two-step process to make cast films: Melt blending of ternary composite blend in a Torque rheomixer at a temperature of 160°C, RPM 30 – 60 for 5-10 min into lumps which were cut into small flakes. These flakes were extruded by Cast film extrusion with a stretching of 5-10%. This is followed by high temperature stretching from 50 – 300% in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen atmosphere and a temperature of 120oC-130oC. Then these films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, acetone and a combination thereof.
(d) Separator Films of UHMWPE: Dry blending of UHMWPE with Porogen (Thermoplastic Starch / Paraffin oil) and fillers were done in high-speed mixer at room temperature followed by compression moulding into films. These films were extracted with a solvents as above to generate required porosity, wherein the solvent is selected from water, hexane, acetone and a combination thereof.

Specifically, the present disclosure is related to the process for preparation of battery separator films from polyolefins through a cast film extrusion. The process includes steps of preparing a blend of polyolefins, porogens, nano inorganic fillers, and compatibilizers then cast film extrusion of the blend to produce a cast film. After that cutting the cast film into a required shape cast film and stretching the required shape cast film in a uniaxial direction to obtain a stretched cast film. Then extracting the porogens from the stretched cast film to obtain a porous battery separator film, wherein extracting is performed by using a solvent. Thereafter drying the battery separator film for a time period of 18-28 hours at a temperature of 40 °C-80°C.

In the present process as disclosed the polyolefins are selected from polypropylene, polyethylene, or a combination thereof. The porogens are selected from thermoplastic starch, sodium chloride, paraffin oil, polyethylene glycol or a combination thereof. The nano inorganic fillers are selected from Nano SiO2, Nano Al2O3 or a combination thereof. The compatibilizers are selected from PP-g-MA, PP-g-TMPTA or a combination thereof.

The blend is a dry blend and after preparing the blend, the blend is mixed and extruded to produce extrudates or granules. The blend is mixed in a micro compounder at temperature of 160-220°C, RPM - 30, or a torque rheomixer with a temperature of 160°C, RPM 30 – 60 for 5-10 minutes and extruded though a single screw extruder, a twin-screw extruder. The blend is mixed and extruded at a temperature of 160°C to 220 °C, and a screw RPM (rotation per minute) of 30-60.

In the present process as disclosed, the cast film extrusion process is performed with a single screw extruder having a coat hanger die and a temperature of hot rollers is from 90oC–120oC. In the present process as disclosed, the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film.

In the present process as disclosed, the stretching is done from 50 – 300% in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen and a temperature of 120oC-130oC.

In the present process as disclosed, the solvent is selected from water, hexane, acetone and a combination thereof. In the present process as disclosed, drying the battery separator film for a time period of 18-28 hours at a temperature of 40°C-80°C.

Materials:
Polypropylene: Homopolymer, with MI – 12 g/ 10 min & 3 g/ 10 min
Ultra-High molecular weight Polyethylene, Thermoplastic tapioca starch:
Sodium chloride,
Paraffin oil,
Nano aluminium oxide,
Nano silicon dioxide.

Testing of Polyolefin Battery Separator Films

Mechanical properties such as tensile strength and puncture strength are measured as per the test methods defined below.

Tensile properties such as tensile strength is measured using a universal testing machine (Model 50 ST, Tinius Olsen, UK), according to ASTM D-882 (gauge length of 250 mm) at a crosshead speed of 50 mm/min. A minimum of five specimens were tested for each reported value.

Puncture strength is measured using a universal testing machine (Model 50 ST, Tinius Olsen, UK), according to ASTM D-5748 at a crosshead speed of 100 mm/min. A minimum of five specimens were tested for each reported value.

The battery separator film as produced by the process disclosed herein has higher tensile strength and puncture strength compared to commercial separators films and prestine PP, wherein the tensile strength is 5-32 MPa and puncture strength is 280-1838 gf.

In an embodiment, the battery separator film as produced by the process disclosed herein has tensile strength 17-18 MPa and puncture strength is 509-510 gf.
In an embodiment, the battery separator film as produced by the process disclosed herein has tensile strength 10-19 MPa and puncture strength is 1222-1835 gf.

In an embodiment, the battery separator film as produced by the process disclosed herein has tensile strength 10-32 MPa and puncture strength is 1224-1838 gf.

Thermal properties are measured by using a Differential scanning calorimetry (DSC Discovery 2500, TA Instruments Ltd, USA) to determine the melting behavior of the prepared battery separator films as per ASTM D-3418.

Contact angle measurement is done by using a contact angle meter (KRUSS Advance) to measure the wettability of separator with electrolyte as per ASTM D-5946

Thermal shrinkage of the battery separator film samples is determined by using Lab think thermal shrinkage tester in accordance of ASM D-2732-03 sample size 100 by 100 mm.

The battery separator film as produced by the process disclosed herein has lower contact angle and lower thermal shrinkage compared to commercial separators films and prestine PP, wherein the contact angle is 26-42 degree and thermal shrinkage is zero percent at 90 degree Celsius/ 1 hour.

In an embodiment, the battery separator film as produced by the process disclosed herein has contact angle 30-37 degree and thermal shrinkage is zero percent at 90 degree Celsius/ 1 hour.

In an embodiment, the battery separator film as produced by the process disclosed herein has contact angle 26-33 degree and thermal shrinkage is zero percent at 90 degree Celsius/ 1 hour.

In an embodiment, the battery separator film as produced by the process disclosed herein has contact angle 33-36 degree and thermal shrinkage is zero percent at 90 degree Celsius/ 1 hour.

FE SEM (JSM 7800) is used to measure the pore diameter of the film.

Example 1
Polypropylene (MFI - 3) was blended with Thermoplastic Starch, (TPS) Nano Aluminum oxide (Al2O3) and extruded into films by cast film extrusion with a temperature profile of 160 °C to 220 °C, screw RPM 30-60 and by variation of temperature and stretching from 3- 10 % in a uniaxial direction. The cast film extruder is a single screw extruder having a coat hanger die and the temperature of hot rollers varied from 90oC–120oC. The cast films produced were cut into required shapes, wherein the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film. In the present example, the required shape cast film is rectangle shape pieces (L x W of 8 x 3 cm). The required shape cast film is further stretched in the range of 50 – 300% in an environmental chamber of a universal testing machine (UTM) under nitrogen at 120oC – 130oC with strain of > 50 % from the fixed position in a uniaxial direction. Then the porogens (starch) present in separator films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, and a combination thereof. Specifically, the porogens (starch) present in separator films were extracted by using hot water and dried for 24 hours at 60 °C. All these compositions are given in Table 1.

In 2nd series of Experiments, starch was replaced with (a) sodium chloride (b) Paraffin oil and made cast films as above and only representative samples given here.

In 3rd series of experiments, nano silicon dioxide was used in place of nano aluminum oxide and produced films for battery separator as above. All these battery separator films were evaluated for their thermal properties, mechanical properties, wettable characteristics, porosity, and battery performance characteristics.
Table 1: Polypropylene /Thermoplastic Starch / Alumina compositions
Sr. No. PP / TPS / Al2O3 Compositions Polypropylene
(gm) TPS
(gm) Nano Al2O3 (gm) Nano SiO2 (gm)
1 Polypropylene 100 0 0 0
2 RT10A1 89 10 1 0
3 RT10S1 89 10 0 1
Table 2: Properties of PP / TPS / Alumina Separator Films made by Direct Cast Film Extrusion
Properties Commercial
Separator (Reference) Polypropylene RT10A1 RT10S1
Thickness (µm) 21 24 48 34
Thermal shrinkage (90 degree Celsius/ 1 hour) % 4 0 0 0
Melting temperature (degree Celsius) 168 162 164 163
Tensile strength (MPa) 6 15 17 18
Puncture strength (gf) 806 417 510 509
Pore diameter (nm) 49-123 NA 130- 180 45- 135
Contact angle with the electrolyte ( °) 42 31 37 30
Open circuit Voltage (V) 3 2.5 2.7 2.8

Example 2
Polypropylene granules were dry blended with Thermoplastic Starch (TPS) and Nano Aluminum oxide (Al2O3) The blend was compounded in a micro compounder at a temperature profile of 160 °C to 220 °C, screw RPM of 30, and extruded into granules. These granules were further extruded into battery separator films by using the cast film extruder by variation of stretching from 3.5 to 10 % in uniaxial direction. The cast film extruder is a single screw extruder having a coat hanger die and the temperature of hot rollers varied from 90oC–120oC. The films were made into required size and shape, and further stretched. The cast films produced were cut into required shapes, wherein the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film. In the present example, the required shape cast film is rectangle shape pieces (L x W of 8 x 3 cm). The required shape cast film is further stretched from 50 to 300 % in an environmental chamber of a universal testing machine (UTM) under nitrogen at 120oC – 130oC with strain of > 50 % from the fixed position in a uniaxial direction. Then the porogens (TPS) present in separator films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, acetone and a combination thereof. Specifically, the porogens (TPS) present in separator films were extracted by using hot water and dried for 24 hours at 60 °C. All these compositions are given in Table 3.
In 2nd series of Experiments, starch was replaced with (a) sodium chloride (b) Paraffin oil and made cast films as above.
In 3rd series of experiments, nano silicon dioxide was used in place of nano aluminum oxide and battery separator films are produced as in Example 1. All these separator films were evaluated for their thermal properties, mechanical properties, wettable characteristics, porosity and battery performance characteristics.

Table 3: Polypropylene /Thermoplastic Starch / Alumina compositions made in Microcompounder

Sr. No. PP / TPS / Al2O3 / SiO2 compositions Polypropylene
(gm) TPS
(gm) Nano Al2O3
(gm) Nano SiO2
(gm)
1 Polypropylene 100 0 0 0
1 RT10A1B2 89 10 1 0
2 RT10 S1 B2 89 10 0 1
Table 4: Properties of PP / TPS / Alumina Separator Films made by two step process in Microcompounder and Cast Film Extrusion

Properties Commercial separator (Reference) Polypropylene RT10A1B2 RT10S1B2
Thickness (µm) 21 24 68 121
Thermal shrinkage (90 degree Celsius/ 1 hour) % 4 0 0 0
Melting temperature (degree Celsius) 168 162 164 163
Tensile strength (MPa) 5.6 15.2 19 10
Puncture strength (gf) 806 417 1222 1835
Pore diameter (nm) 49-123 NA 132-176 46-133
Contact angle with the electrolyte (°) 42 31 33 26
Open circuit Voltage (V) 3 2.5 2.7 2.8

Example 3
Polypropylene granules were dry blended with Thermoplastic Starch (TPS) and Nano aluminum oxide as given in Table 5. The blend was compounded in a torque mixer at a temperature of 160 °C, Screw RPM 30 – 60 and time 5 to 10 minute into lumps. These lumps were cut into small pieces using a cutting mill and extruded into battery separator films by cast film extrusion by variation of stretching from 3 to 10%. The cast film extruder is a single screw extruder having a coat hanger die and the temperature of hot rollers varied from 90oC–120oC. The battery separator cast film produced was made into the required size, shape, and stretched. The cast films produced were cut into required shapes, wherein the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film. In the present example, the required shape cast film is rectangle shape pieces (L x W of 8 x 3 cm). The required shape cast film is further stretched from 50 to 300% in an environmental chamber of a universal testing machine (UTM) under nitrogen atmosphere at 120oC with strain of > 50 % from the fixed position in a uniaxial direction. Then the porogens (starch) present in separator films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, and a combination thereof. Specifically, the porogens (starch) present in separator films were extracted by using hot water and dried for 24 hours at 60 °C.

In a similar way, PP grafted maleic anhydride (PP-g-MA) was used with Nano Aluminum oxide and prepared PP battery separator films as above. The samples were tested for battery performance, mechanical properties, thermal properties, wettability, Pore structure analysis. The PP-g-MA acted as compatibilizer between PP and Aluminium Oxide and produced better films with higher properties as given in Table 6.

Table 5: Polypropylene /Thermoplastic Starch / Alumina / PP-g-MA compositions made in Torque Mixer
Sr. No. PP / TPS / Al2O3 / PP-g-MA Compositions Polypropylene
(gm) TPS
(gm) Nano Al2O3
(gm) PP-g-MA
(gm)
1 Polypropylene 100 0 0 0
2 RT10A1B3 89 10 1 0
3 RT10 A1 M5 B3 84 10 1 5

Table 6: Properties of Polypropylene /Thermoplastic Starch / Alumina / PP-g-MA films made by two step process in Torque Mixer and Cast Film Extrusion

Parameter Commercial separator Polypropylene RT10A1B3 RT10A1M5B3
Thickness (µm) 21 24 38 42
Thermal shrinkage (90 degree Celsius/ 1 hour) % 4 0 0 0
Melting temperature (degree Celsius) 168 162 162 163
Tensile strength (MPa) 5.6 15.2 32 10
Puncture strength (gf) 806 417 1224 1838
Pore diameter (nm) 49-123 - 101-128 67-173
Contact angle with the electrolyte (°) 42 31 36 33
Open circuit Voltage (V) 3 2.5 2.6 2.7
Example 4
Polypropylene battery separator films were made by two-step process (1) Dry blending polypropylene with other porogens like NaCl, TPS with nano aluminum oxide and extruded in a twin-screw extruder (TSE) and extruded into granules with temperature profile of 160 °C to 220 °C screw RPM 30 – 100.The TSE is having 10 zones with L/D ratio of 40 and screw diameter 18 mm. These granules were further extruded into battery separator films by using the above cast film extruder by variation of stretching from 3 to 10% in uniaxial direction and the films were made into the required size, shape, further stretched, and the porogens were extracted using hot water (NaCl), Thermoplastic starch (hot water) and dried as mentioned in the above examples.
For example, the cast films produced were cut into required shapes, wherein the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film. In the present example, the required shape cast film is rectangle shape pieces (L x W of 8 x 3 cm). The required shape cast film is further stretched from 50 – 300% in an environmental chamber of a universal testing machine (UTM) under nitrogen atmosphere at 120oC – 130oC with strain of > 50 % from the fixed position in a uniaxial direction. Then the porogens (starch) present in separator films were extracted with a solvent to generate required porosity, wherein the solvent is selected from water, hexane, and a combination thereof. Specifically, the porogens (starch) present in separator films were extracted by using hot water and dried for 24 hours at 60 °C.
All these battery separators films were evaluated for their thermal properties, mechanical properties, wettable characteristics, porosity and battery performance characteristics.

Table 7: Polypropylene /Thermoplastic Starch / Alumina / NaCl Compositions made in Twin Screw Extruder
Sr. No. PP / TPS /Al2O3 / NaCl Compositions Polypropylene
(gm) TPS (gm) Nano Al2O3
(gm) NaCl
(gm)
1 Polypropylene 100 0 0 0
2 RT10A5 85 10 5 0
3 RN10A1 89 0 1 10
Table 8: Properties of Polypropylene /Thermoplastic Starch / Alumina / NaCl separator films made in Twin Screw Extruder and Cast Film Extrusion

Parameter Commercial separator Polypropylene RT10A5 RN10A1
Thickness (µm) 21 24 38 42
Thermal shrinkage (90 degree Celsius/ 1 hour) % 4 0 0 0
Melting temperature (degree Celsius) 168 162 163 164
Tensile strength (MPa) 6 15 5 10
Puncture strength (gf) 806 417 298 280
Pore diameter (nm) 49-123 - 150- 320 37-123
Contact angle with the electrolyte (°) 42 31 31 27
Open circuit Voltage (V) 3 2.5 2.6 2.7

Example 5
Polyethylene (UHMWPE) Battery separator films were made by a two-step process (1) By dry blending UHMWPE separately with porogen paraffin oil and mixed with fumed silica in a high-speed mixer. These compositions were made in to separator films by compression molding. The films were extracted using hexane (paraffin oil) to generate porosity and the films were dried as in Example 1.

Table 9: Polypropylene /Paraffin Oil / Fumed Silica / Compositions and Compression Molding into Films

Sample name UHMWPE
(gm) Paraffin oil (gm) Fumed silica (gm) Compression Molding Pressure
(bar) Molding Temperature (°C)
UHMWPE 10 0 0 150 190
UHM/PO 5 % 9.5 0.5 0 150 190
UHM/PO 10 % 9 1 0 150 190
UHM/Silica/ paraffin oil 3 2 5 150 190

Accordingly, the above disclosed examples for preparation of battery separator films are simple and do not require complex process steps. Further, the example and process as disclosed hereinabove requires initial mixing in a single screw extruder, a micro-compounder, a Torque mixer, a twin-screw extruder followed by cast film extrusion. Such two-stage processes provide better homogenization. Further, by using different porogens and uniaxial stretching in an environmental chamber of UTM provides better porosity. However, in the prior known methods biaxial stretching is done to generate porosity as compared to using porogens and uniaxial stretching to generate porosity. Further, the example and process as disclosed herein uses ternary composite system of polymers (PP or UHMWPE) with porogens and nano inorganic fillers instead of binary system as mentioned in the prior known methods.

Further, the example and process as disclosed herein above provides battery separator films wherein single layer is used as a battery separator. However, the prior known methods provide porous polyethylene (PE) film and above that a layer of resin solution (Alumina / PVdF / MEK) is added. These films of the prior known methods are bilayer in nature with coating, which add extra process steps, time as well increase cost due to additional coating materials.

The process as disclosed herein have many advantages such as it provides a novel method for making separator from polyolefins like polypropylene and polyethylene with different porogens (thermoplastic tapioca starch, sodium chloride, paraffin oil, polyethylene glycol), nano inorganic fillers (nano SiO2 / nano Al2O3) and compatibilizers such as PP-g-MA or PP-g-TMPTA. Further, process as disclosed herein provides novel formulations made by dry blending followed by melt mixing by different processing techniques and then extruded into granules/flakes. Then the extruded granules/flakes made into battery separator films by cast film extruder of single screw extruder, compression molding, etc. Further, the battery separator films as prepared show less thermal shrinkage, higher mechanical properties such as tensile strength, puncture strength compared to commercial separators. , Claims:1. A process for preparation of battery separator films from polyolefins through a cast film extrusion, wherein the process comprises steps of:
preparing a blend of polyolefins, porogens, nano inorganic fillers, compatibilizers;
cast film extrusion of the blend to produce a cast film;
cutting the cast film into a required shape cast film and stretching the required shape cast film in a uniaxial direction to obtain a stretched cast film;
extracting the porogens from the stretched cast film to obtain a porous battery separator film, wherein extracting is performed by using a solvent; and
drying the battery separator film for a time period of 18-28 hours at a temperature of 40 °C-80°C.

2. The process as claimed in claim 1, wherein, the polyolefins are selected from polypropylene, polyethylene, or a combination thereof.

3. The process as claimed in claim 1, wherein, the porogens are selected from thermoplastic starch, sodium chloride, paraffin oil, polyethylene glycol or a combination thereof.

4. The process as claimed in claim 1, wherein, the nano inorganic fillers are selected from Nano SiO2, Nano Al2O3, Fumed silica or a combination thereof.

5. The process as claimed in claim 1, wherein, the compatibilizers are selected from Polypropylene grafted Maleic anhydride (PP-g-MA), Polypropylene grafted Trimethylolpropane triacrylate (PP-g-TMPTA) or a combination thereof.

6. The process as claimed in claim 1, wherein, the blend is a dry blend and after preparing the blend, the blend is mixed and extruded to produce extrudates or granules.

7. The process as claimed in claim 1, wherein, the blend is mixed in a micro compounder, or a torque rheomixer, and extruded though a single screw extruder, a twin-screw extruder.

8. The process as claimed in claim 1, wherein, the blend is mixed and extruded at a temperature of 160°C to 220 °C, and a screw RPM (rotation per minute) of 30-60.

9. The process as claimed in claim 1, wherein, the cast film extrusion process is performed with a single screw extruder having a coat hanger die and a temperature of hot rollers is from 90oC–120oC.

10. The process as claimed in claim 1, wherein, the required shape cast film is selected from a two-dimensional shape cast film, or a three-dimensional shape cast film.

11. The process as claimed in claim 1, wherein, stretching of 50 – 300% is done in an environmental chamber of a Universal Testing Machine (UTM) with a strain of >50 % from a fixed position in a uniaxial direction, wherein the environmental chamber has nitrogen and a temperature of 120oC-130oC.

12. The process as claimed in claim 1, wherein, the solvent is selected from water, hexane, acetone or a combination thereof.

13. The process as claimed in claim 1, wherein, drying the battery separator film for a time period of 20-26 hours at a temperature of 50 °C-70°C.

14. The process as claimed in claim 1, wherein the battery separator film has lower contact angle and lower thermal shrinkage, wherein the contact angle is 26-42 degree and thermal shrinkage is zero percent at 90 degree Celsius/ 1 hour.

15. The process as claimed in claim 1, wherein the battery separator film has higher tensile strength and puncture strength, wherein the tensile strength is 5-32 MPa and puncture strength is 280-1838 gf.