FORM-2
THE PATENT ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(see section 10 and rule 13)
CAVITATED PROPYLENE POLYMER FILM THROUGH TUBULAR
QUENCH ROUTE
RELIANCE INDUSTRIES LIMITED
an Indian organization
of Reliance Industries Limited,
3rd Floor, Maker Chamber-IV
222, Nariman Point, Mumbi-400021,
Maharashtra, India.
Inventors
(1) Mathur Ajit; (2) Choudhary Manjeet;
(3) Kumar Ajay; (4) Kumar Prakash;
(5) Rao G S Srinivasa; (6) Dongre Tushar;
(7) Ahmad Zubair; (8) Basargekar Rajeev.
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

FIELD OF THE DISCLOSURE:
The present disclosure relates to a process for the preparation of a micro-voided
polypropylene film.
BACKGROUND:
Polypropylene is widely used as a flexible packaging material on account of its ease of processing in the form of thin film. Processing techniques like biaxial orientation, casting, and tubular quench are the most widely accepted techniques for manufacturing the polypropylene film. The propylene polymer exists in different crystalline forms specifically alpha, beta and gamma. Alpha and gamma crystalline form of polypropylene are thermodynamically more stable in comparison to the beta-crystalline form. The beta-crystalline form being relatively lesser stable tends to turn to the alpha crystalline form under mechanical /thermal stress. The density of the beta-crystalline form is lower than the alpha-crystalline form. Therefore transformation of the beta-crystalline form to the alpha-crystalline form leads to a volume contraction in the polypropylene crystalline lattice which in turn induces the formation of the voids/pores in the vicinity of the polypropylene crystalline lattice.
The formation of voids/pores adds new features to the polypropylene product such as lower density than normal PP products, toughness, low seal initiation

temperature, printability with low/no pretreatment, lower blocking, enhanced permeability, opacity and the like.
EXISTING KNOWLEDGE:
The details of the prior-art processes employed for inducing porosity in the polypropylene film will be more apparent from the discussion of the related prior-art.
United States Patent Application Number 2006024518 discloses a permeable film and its manufacturing methods. The permeable film includes a core layer containing polypropylene matrix, and first and second thermoplastic skin layers on either side of the core layer. The polypropylene matrix contained in the core layer of the permeable film is cavitated polypropylene. The cavity formation in the polypropylene matrix is induced by two component cavitation system. The first component of the cavitation system is a beta-nucleating agent, used to produce beta-crystalline form of polypropylene and the second component of the cavitation system is filler. A method of manufacturing a permeable film includes the steps of forming a melt containing a propylene polymer, a beta-nucleating agent and filler; cooling the melt to form a film layer; and stretching the film layer.
Jacoby Philip in his PCT Application WO2004094259 discloses an improved extruded polypropylene sheet containing a high level of beta-crystallinity and a

process for making such sheets. The polypropylene sheet contains at least one layer of a resinous polymer of propylene and an effective amount of beta-spherulites. The spherulites in the polypropylene sheet are produced by incorporating a beta-nucleating agent in the propylene polymer. The presence of the beta-spherulites in the propylene polymer facilitates the process of post-stretching the perforated sheet to produce a uniaxially or biaxially oriented mesh structure, and also broadens the temperature range over which this stretching can be performed. The extruded solidified polymer sheet having beta-spherulites is passed thorough a sheet flattening unit, a perforator, and various orientation units to obtain uni-axially or bi-axially oriented polymeric sheet having high strength and torsional rigidity characteristics.
United States Patent No. 7235203 discloses a successive biaxial orientation process for producing a polypropylene porous film. The process comprises the extrusion of the molten propylene resin compound containing a nucleating agent through a T-die. After extrusion, the extruded melt is cooled on a chill roll to obtain an un-stretched web sheet. The un-stretched polymer web sheet is then stretched in lengthwise direction and in widthwise direction to obtain a biaxially oriented porous polypropylene fdm. The longitudinally stretched sheet can be annealed under specific conditions after the longitudinal stretching and prior to the transverse stretching. The annealing further promotes the pore formation in the subsequent transverse stretching thereby improving properties of the porous film

such as porosity, air permeability, and thickness uniformity. The longitudinal stretching of the un-stretched web sheet involves adjusting the neck-in-ratio in the rangeof25%to55%.
European Patent Application No. 865914 discloses a polyolefin film comprising a polypropylene-based resin layer containing micro-voids. The micro-voided polypropylene-based resin layer is formed by orienting the beta-form of polypropylene, and at least one olefin copolymer outer layer thereon. The polyolefin film is bi-axially stretched 4.5 times in the extrusion direction at 90 °C and 8 times in the transverse direction at 136 °C to give 55.0 urn thick film showing density of 0.58g/cm3, optical density of 0.85 and gloss (20 °) of 30.8.
European Patent Application No. 0865912 discloses a bi-axially oriented polyolefin film having a thickness of at least 50 um. The polyolefin film comprises a micro-voided polypropylene based resin layer. The micro-voided polypropylene resin based layer is formed by stretching a web containing the beta-form of the polypropylene at a stretch ratio of at least 15:1. The micro-voided polypropylene film is produced by extruding or co-extruding the polypropylene melt though a slot die to form a polymer web, which is cooled and thereafter sequentially bi-axially stretched 3.5 times in the extrusion direction and 6 times in the transverse direction to give 95 um thick film.

European Patent Application No. 0865911 discloses a polyolefin film comprising a base layer of micro-voided polypropylene resin, and a heat seal layer on the base layer. The base layer containing micro-voided polypropylene resin is formed by stretching a polypropylene web containing the beta form of the polypropylene resin by 4.5 times in the extrusion direction and by 8 times in the transverse direction at 144°. The biaxial stretching of the polypropylene film provides 54.0 urn thick micro-voided polypropylene film having density of 0.59g/cm3 and heat seal threshold of 122°. The polypropylene web is produced by extruding the polypropylene and beta-nucleating agent in an extruder to form the polymer melt, extruding the polymer melt through a slot die to form a polymer web. The obtained polymer web is cooled and thereafter stretched bi-axially to form a micro-voided polypropylene film.
Fujiyama M. in his publication International Polymer Processing (11(2), 159-166, 1996) describes the structure and properties of blown films produced from polymer melt containing a propylene polymer and beta-nucleating agent. The polymer blown films are cooled by water-cooling or air-cooling methods. The effect of cooling conditions in crystallization was also studied by Fujiyama. Polypropylene containing beta-nucleating agent does not form beta-phase but rather the smectic or alpha-phase at low cooling temperature. However, compared with polypropylene without beta-nucleating agent, polypropylene forms the alpha-phase at lower cooling temperature and the beta-phase at higher cooling

temperature. Polypropylene containing beta-nucleating agent forms only smectic phase in the water-cooled inflation but form the beta-phase in the air-cooled inflation, only if the content of the beta-phase nucleating agent is high. The film properties are not found to be affected by the addition of the beta-nucleating agent. However, the dart impact strength increases for the air-cooled films containing the beta-phase.
As evidenced from the forgoing discussion of the related prior-art, the hitherto known processes for formation of the micro-voided polypropylene film essentially involve at least two steps; the first step is crystallization step and second step is stretching. Typically, in a first process step, the crystallization of the polypropylene melt is carried out to produce a polypropylene-web or polypropylene pre-film containing the beta-crystalline form of polypropylene. Subsequently, the polypropylene pre-film or polypropylene-web so formed is stretched (uni-axial or bi-axial) under controlled thermal condition to transform beta-crystalline form to the alpha-crystalline form. The transformation of the crystalline phases leaves fine voided structures on the final polymer product.
Apart from above described polymorph transformation processes, some other processes have also been disclosed in the related prior-art. For example, formation of the beta-spehrulites in the polymer melt and extraction of the beta-spherulites

leaving void/pores in the polymer lattice, inclusion of the leach-able additives in the polymer melt, and inclusion of the inorganic filler in the polymer formulation.
The transformation of the beta-crystalline form to the alpha-crystalline form leaving micro-voids in the polymer film is the most widely accepted technique. For transformation of the crystalline phases, the beta-crystalline phase should be present in an amount sufficient to introduce porosity in the polypropylene lattice. In order to improve the proportion of the beta-crystalline phase in the propylene polymer at the thermodynamically stable conditions and to ensure sufficient porosity in the micro-voided polypropylene film, a suitable nucleating agent is incorporated in the propylene polymer. The formation of the micro-voided polypropylene film is usually carried out in step-wise manner i.e. formation of the polypropylene pre-film comprising beta-crystalline form of polypropylene and subsequent transformation of the beta-crystalline form to the alpha-crystalline form by stretching the polypropylene pre-film under thermal conditions.
These conventional processes are usually allied with numerous disadvantages which include: less efficiency, processing complexity, large capital expenditure and the like.

Therefore, there is felt a need to provide a process wherein method steps of film formation and film processing are integrated so as to overcome the shortcomings associated with the hitherto reported processes.
OBJECTS:
Some of the objects of the present invention are as follows:
It is an object of the present invention to provide a process for the preparation of a
micro-voided polypropylene film.
Another object of the present invention is to provide an efficient and economical process for the preparation of a micro-voided polypropylene film.
Yet another object of the present invention is to provide a micro-voided polypropylene film with desirable physical properties.
These and other objects of the invention are to a great extent dealt with the invention herein after disclosed".

SUMMARY
In accordance with the present invention, there is provided a single step process for the preparation of a micro-voided polymer film in a tubular quench assembly comprising an extruder with a hopper that is connected to a die, film blowing, draw down and winding assembly; said process comprising the following steps:
(a) extruding a polypropylene melt comprising a beta-nucleating agent and propylene polymer through an annular die to form a tubular film;
(b) concurrently crystallizing and stretching the tubular film in a molten or semi-molten state on exiting from the die to obtain a thin and micro-voided polypropylene containing polymer film; wherein said crystallizing and stretching is carried out under pre-set processing parameters which include at least one parameter selected from the group consisting of melt temperature, gap between die to cooling ring, cooling water temperature, extrudate tubular film temperature, bubble to die diameter ratio, extrudate output ratio and nip-roll speed.
Typically, the polypropylene melt is prepared by melt blending the propylene polymer and beta-nucleating agent in an extruder at a temperature above the melting point of the propylene polymer.

The beta-nucleating agent as used in the process of the present invention includes at least one selected from the group consisting of an alkali metal salt of a diacid, an aryl amide derivative, a dye, and other hetro-cyclic compounds.
Typically, the alkali metal salt of a diacid is selected from the group consisting of
calcium pimelate, calcium suberate, calcium phthalate, calcium terephthalate. The
aryl amide derivatives include N,N'-dicyclohexyl-2,6-naphthalene dicaboxamide,
N.N'-dicyclohexylterephthalamide, N,N'-dicylcohexanecarbonyl-p-
phenyleneamine, 1,3,5-benzenetrisamides having substituent like 2,3-Dimethylcyclohexyl. Dye such as quinacridone is also one of the beta nucleating agents. Other hetrocyclic compounds that can be used as the beta nucleating agents include triphenol ditriazine, triphenodithiazine aluminium salt of quinizarin sulphonic acid.
Typically, the amounts of the beta-nucleating agent and the propylene polymer used for preparing the polypropylene melt are about 0.05 to 0.25% and about 99 to 99.75% respectively.
Typically, the polymer melt is extruded in vertically downward manner.
Typically, the propylene polymer is a homopolymer of propylene.

Typically, the gap between die to cooling ring is set in the range varying from 290 mm to 555 mm; preferably 550 mm.
Typically, the cooling water temperature is maintained in the range varying between 13.3 °C to 48 °C; preferably 45 °C.
Typically, the temperature of the extruded tubular film at the exit of the die during said concurrent process steps of crystallization and stretching is maintained in the range varying between 175 °C to 190 °C.
Typically, the nip speed of the nip roller varies between 5.8 to 20 rpm; preferably 16 rpm.
Typically, the bubble to die diameter ratio is adjusted in the range varying between 1.2:lto 2.5:1; preferably 1.5:1 to 2.0:1.
Typically, the film is stretched uni-axially.
Typically, the film is stretched bi-axially.
In accordance with the present invention there is also provided a thin and micro-voided polypropylene film characterized by the surface pore-size varying

between 5 micron to 100 microns, and in the core between 0.15 to 0.80 microns, film thickness varying between 0.02 to 0.055 mm and density varying between 0.5225 g/cc to 0.8490g/cc, prepared in accordance with the process of the present invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 illustrates a set-up for the tubular quench process for the formation of a micro-voided polypropylene film, in accordance with the present invention; comprises an hopper (1), an extruder (2), a die (3), an air cooling ring (4), a water cooling ring (5), nip-rollers (6) and a winder (7).
Figure 2(a), 2(b) and 2(c) illustrate Scanning Electron Micrographs of the micro-voided propylene polymer film, in accordance with the present invention; and
Figure 3(a) and 3(b) illustrate Scanning Electron Micrographs of cryogenic microtomed propylene polymer film, in accordance with the present invention;

DETAIL DESCRIPTION
The present invention in one aspect provides a single step process for the preparation of a micro-voided propylene polymer film, wherein formation of the beta-crystalline form of polypropylene and its transformation to the alpha-crystalline form are carried out concurrently. The transformation of the beta-crystalline form to the alpha-crystalline form leaves micro-voids in the vicinity of the propylene polymer film.
The single step process for the preparation of a micro-voided propylene polymer film in accordance with the present invention is typically carried out in a tubular quench set-up as shown in Figure 1 of the accompanying drawings.
The single step process in accordance with the present invention typically comprises the vertically downward extrusion of the propylene polymer melt to form an extruded tubular film, and concurrently crystallizing and stretching the extruded tubular film under pre-set process parameters to obtain a micro-voided polypropylene film.
In accordance with the present invention, there is provided a single step process for the preparation of a micro-voided propylene polymer film; said process comprises the steps of extruding a polypropylene melt comprising the beta-nucleating agent through an annular die to form a tubular film; concurrently

crystallizing and stretching the tubular film in molten and semi-molten state on exiting from the die to obtain a thin and micro-voided propylene polymer film.
The concurrent process of crystallization and stretching in accordance with the process of the present invention is carried out at pre-set process parameters which include at least one parameter selected from the group consisting of melt temperature, gap between die to cooling ring, cooling water temperature, tubular film temperature, nip roll speed, bubble to die diameter ratio, and extrudate output ratio.
The polypropylene melt as employed in the process of the present invention is prepared by mixing and blending the propylene polymer and other raw materials in an extruder.
The propylene polymer and other raw materials in a pre-determined weight ratio arc mixed to form a propylene polymer mixture. The raw materials that are mixed with the propylene polymer typically include a beta-nucleating agent, and other additives.
The propylene polymer as employed in the process of the present invention for the purpose of producing a micro-voided polypropylene film is preferably a homopolymer of propylene.

The additives other than beta-nucleating agent used in the process of the present invention typically include standard stabilizers, and lubricants selected from the group consisting of primary and secondary antioxidants as well as metal stearates and combinations thereof. The additives as employed in the process of the present invention are compatible with the beta-nucleating agent and do not obstruct the activity of the beta-nucleating agents used for the purpose of the present invention.
The propylene polymer and other raw material are admixed. The amount and selection of the beta-nucleating agent mixed with the propylene polymer depend on the desired crystalline morphology of the micro-voided polypropylene film. The amount of the beta-nucleating agent mixed with the propylene polymer varies and depends on the type of the beta-nucleating agent used for the purpose of the present invention.
The beta-nucleating agent as used in the process of the present invention includes
at least one selected from the group consisting of an alkali metal salt of a diacid,
an aryl amide derivative, a dye, and other hetro-cyclic compounds. Typically, the
alkali metal salt of a diacid is selected from the group consisting of calcium
pimelate, calcium suberate, calcium phthalate, calcium terephthalate. The aryl
amide derivatives include N,N'-dicycIohexyI-2,6-naphthalene dicaboxamide,
N,N'-dicyclohexylterephthalamide, N,N'-dicylcohexanecarbonyl-p-

phenyleneamine, 1,3,5-benzenetrisamides having substituenl like 2,3-Dimethylcyclohexyl. Dye such as quinacridone is also one of the beta nucleating agents. Other hetrocyclic compounds that can be used as the beta nucleating agents include triphenol ditriazine, triphenodithiazine aluminium salt of quinizarin sulphonic acid.
Typically, the amounts of the beta-nucleating agent and propylene polymers used for preparing the polypropylene melt are from 0.05 to 0.25% and from 99 to 99.75% respectively.
In accordance with one of the embodiments of the present invention, the beta-nucleating agent in powder or granular form is admixed with propylene polymer in powder and granular form along with other additives to form a homogenized propylene polymer mixture.
In accordance with another embodiment of the present invention, the dispersion of the beta-nucleating agent in a suitable dispersion medium is dispersed on propylene polymer powder or granules to impregnate the same.
After mixing the propylene polymer, beta-nucleating agent and other additives, the obtained propylene polymer mixture is fed to the extruder through a hopper. The typical extruder attached to one end through a hopper and to a distal end through a

die typically mixes and melts the polymer mixture to obtain a polymer melt in homogenized condition.
The temperature of the extruder in accordance with the process of the present invention is typically maintained at temperature higher than the melting point of the polypropylene i.e. > 200 °C. The extruder typically melts the ingredients of the propylene polymer mixture to provide a homogenized propylene polymer melt. The extruder then pumps out the propylene polymer melt to an annular die. The temperature of the annular die is also maintained above the melting temperature of the propylene polymer.
The obtained propylene polymer melt containing the beta-nucleating agent is subsequently extruded through an annular die in a vertically downward manner to form a tubular film of the propylene polymer melt containing the beta-nucleating agent.
The extruded tubular film is then blown out through the annular die. The pre-set process parameters during the concurrent process of crystallization and stretching are adapted to control porosity and pore-size of the micro-voided propylene polymer film. The controlled porosity and pore-size of the micro-voided polypropylene film governs the crystalline morphology of the micro-voided polypropylene film.

The cooling of the extruded tubular film in accordance with the process of the present invention is typically carried out by using an air-cooling ring and a cooling water ring around the extruded tubular film of the propylene polymer melt.
The cooling of the extruded tubular film of polypropylene melt containing the beta-nucleating agent induces the formation of the beta-crystalline form of polypropylene. The employment of air and cooling water ring further provides dimensional stability to the tubular film (refer to Figure 1 of the accompanying drawings).
The cooling of the extruded tubular film in accordance with the process of the present invention is typically carried out under controlled process parameters to control the rate of crystallization. The rate at which the extruded tubular film is cooled plays a decisive role in the formation of the beta-crystalline form of polypropylene. The desired amount of the beta-crystalline form of polypropylene is obtained by controlling the rate of crystallization. Furthermore, the growth of the beta-crystalline form of polypropylene is also controlled by crystallization rate of the extruded tubular film of polypropylene melt.
Typically, the rate of crystallization of the extruded tubular film is controlled by adjusting the process parameters that include at least one selected from the group

consisting of melt temperature, gap between die to cooling ring, tubular film temperature, and cooling water temperature.
During the extensive research, the inventors of the present invention have
identified determinates that control the extent, nature and duration of the
crystallization.
One of the determinants is melt temperature of propylene polymer melt. Typically,
the melt temperature of the polypropylene melt containing the beta-nucleating
agent is maintained above the melting point of the polypropylene i.e. > 200 °C.
Another determinant is an extruded tubular film temperature. The cooling of the extruded tubular film by using air and water cooling ring reduces the temperature of the extruded tubular film of polypropylene. The reduction in the temperature of the extruded tubular film of propylene polymer melt induces the formation of the beta-crystalline form of polypropylene. The rate with which the temperature of the extruded tubular film is reduced governs the amount and size of the beta-crystalline form of polypropylene.
Typically, the temperature of the extruded tubular film of polypropylene melt containing the beta-nucleating agent is maintained in the range varying between 175 °C to 210 °C; preferably between 175 °C to 190 °C at the exit of the die.

Preferably, the temperature of the extruded tubular film of polypropylene melt near the air -ring is maintained in the range varying between 175 °C to 195 °C.
Preferably, the temperature of the extruded tubular film near cooling water ring is maintained in the range varying between 175 °C to 80 °C (refer to figure 1 of the accompanying drawings).
The gradual decrease in the temperature of the extruded tubular film of polypropylene melt in accordance with the process of the present invention is achieved by adjusting the gap between die and cooling ring.
Typically, the gap between die and cooling ring is maintained in the range varying from 290 mm to 555 mm; preferably 550 mm. It is surprisingly found out by the inventors of the present invention that if the gap between die to cooling ring is doubled, then desired rate of crystallization is achieved.
Still another determinant in accordance with the process of the present invention is
cooling water temperature.
Typically, the cooling water temperature is maintained in the range varying
between 13.3UC to 48° C; preferably 45° C.
Generally, in the hitherto reported tubular quench processes, the crystalline growth
of polypropylene is quenched by cooling the tubular film by chilled water during

the formation of the film, which results in a film having low haze and high optical clarity. Nevertheless, the controlled crystallization rate in accordance with the process of the present invention provides a micro-voided polypropylene film with high opacity, high porosity and good mechanical properties wherein the crystalline formation of the propylene polymer takes place at controlled crystallization rate instead of quenching the crystallite growth with chilling water.
The inventors of the present invention have surprisingly found out that on increasing the process-parameters from more to extreme during the crystallization of the extruded tubular film, the rate of crystallization slows down. The slow crystallization rate promotes the growth of the beta-crystal line form of polypropylene. The presence of the alpha and the beta-crystalline form of polypropylene in the crystallized tubular film prepared in accordance with the process of the present invention is confirmed by differential scanning calorimetery and x-ray diffraction techniques.
The crystallized tubular film comprising beta-crystalline form of polypropylene is simultaneously stretched to transform the beta-crystalline form of polypropylene to the alpha-crystalline form. The transformation of the beta-crystalline form to the alpha-crystalline form leaves micro-voids in the vicinity of the polypropylene fiJm.

Unlike the conventional processes, the simultaneous stretching of the crystallized tubular film in accordance with the process of the present invention is typically carried out at the stage wherein the crystallized tubular film containing the beta-crystalline form of polypropylene is present in molten and semi-molten state. The single step process in accordance with the present invention typically comprises the simultaneous stretching of the tubular film at the forming stage of the beta-crystalline form of polypropylene.
Apart from the crystallization rate, the stretching rate also controls porosity and pore-size of the micro-voided polypropylene film. The stretching rate of the extruded and crystallized tubular film in accordance with the process of the present invention is controlled by adapting various process-parameters.
The stretching of the crystallized and extruded tubular film in accordance with the process of present invention at the forming stage of the beta-crystalline form of polypropylene is carried out under controlled process parameters that include at least one selected from the group consisting of bubble to die diameter ratio, nip speed vs extrudate output ratio i.e. control blow up ratio and nip speed.
Typically, the stretching of the extruded tubular film at the forming stage of the beta-crystalline form of polypropylene in accordance with the process of the present invention is biaxial stretching. The biaxial stretching comprises the

stretching in longitudinal direction (machine direction) and in transverse direction (perpendicular to the machine direction).
The stretching in machine direction in accordance with the process of the present invention is typically carried out by controlled blown up ratio and nip roll speed. The ratio of the extrudate output vs nip roll speed is maintained to facilitate the stretching in machine direction. The lower extrudate output and higher nip speed increases the stretching of the tubular film in machine direction.
Typically, the ratio of the extrudate output and nip roll speed is adjusted to achieve machine direction stretch wherein the nip roll speed is maintained from 5.8 to 20 rpm.
The stretching of the crystallized tubular film in transverse direction in accordance with the process of the present invention is carried out by adjusting the bubble to die diameter ratio.
Typically, the bubble to die diameter ratio is adjusted in the range varying between 1.2:lto 2.5:1; preferably 1.5:1 to 2.0:1,
The stretching of the extruded tubular film of polypropylene melt in accordance with the process of the present invention in machine direction as well as in

transverse direction at the forming stage of the beta-crystalline form induces the porosity in the extruded tubular propylene polymer film, as large fraction of the beta-crystalline form polypropylene gets converted to the alpha-crystalline form. This transformation of the crystalline phases leaves micro-voids on the surface and in the core of the propylene polymer film. The inventor of the present invention have further found out that on increasing the stretching conditions from more to extreme i.e. at higher nip speed and low extrudate output ratio, the polypropylene film with large void-size is obtained.
The controlled crystallization and stretching rate in accordance with the process of the present invention controls the porosity and pore-size of the micro-voided film. The pore size of the micro-voided propylene polymer film increases as the conditions of the film formation shifts from more to extreme. The slower rate of cooling and higher stretching rate favors the formation of micro-voided polypropylene film having high porosity and large void-size. At higher rate of stretching, the pores present on the surface of the micro-voided polypropylene film fuse to form large cavities or voids.
Typically, the pore-size at the surface of the micro-voided polypropylene film in accordance with the process of the present invention varies between 5 urn to lOOµm.

Tne size of the voids in the core of the micro-voided polypropylene film are about 6 to 700 times smaller than the size of the voids present on the surface of the micro-voided polypropylene film. Typically, the size of the voids in the core of the micro-voided polypropylene film varies in the range of from 0.15 to 0.80 microns.
The controlled stretching rate in machine as well as in transverse direction by adjusting the propylene polymer extrudate out put ratio, nip roll speed and bubble to diameter ratio further aids in achieving the desired thickness of the micro-voided polypropylene film. The stretching of the extruded tubular film at the forming stage of the beta-crystalline form of polypropylene in accordance with the process of the present invention provides a thin micro-voided polypropylene film.
Typically, the thickness of the micro-voided propylene polymer film varies between 0.02 mm to 0.55 mm.
In accordance with another aspect of the present invention there is also provided a thin and micro-voided polypropylene film characterized by the surface pore-size varying between 5 micron to 100 microns, pore size of the voids in the core varying between 0.15 to 0.80 microns, film thickness varying between 0.02 to 0.055 mm and density varying between 0.5225 g/cc to 0.8490g/cc, prepared in accordance with the process of the present invention.

The bubble stability of polypropylene melt in accordance with the process of the present invention is retained while adjusting the process parameters to control the rate of crystallization and stretching. The polypropylene film thickness also remains uniform around the circumference of the bubble and .no wrinkles are formed while flattening the bubble at the nip. The processing of the micro-voided propylene polymer film is found to be stable despite the increase in die to cooling ring gap by 2 times and cooling water temperature from 13.3 °C to 48 °C.
The micro-voided polypropylene film obtained by the process of the present invention is glossy, opaque to translucent and no blockiness is obtained. On lowering the rate of crystallization and increasing the stretching rate of the extruded tubular film in accordance with the process of the present invention increases the opacity and percentage haze of the micro-voided polypropylene film.
The micro-voided polypropylene film produced in accordance with the process of the present invention also exhibit good mechanical strength. Tensile yield stress of the micro-voided polypropylene film produced in accordance with the process of the present invention increases to about 40 % at room temperature.. The production of the micro-voided polypropylene film by incorporating the beta-nucleating agent increases the folding resistance of the polypropylene film. The

film smoothness also drops to half as compared to the un-nucleated polypropylene film.
The micro-voided polypropylene film produced in accordance with the process of the present invention finds variety of applications in its produced form or through modifications in medical, sports apparels, battery separators, house-wraps, pressure sensitive labels, filters and the like.
The present invention will now be further described with reference to the following examples which are to be regarded solely as illustrative and not as restricting the scope of the present invention.
Example 1:
The master batch of beta-nucleating agent, aryl amide compound, (Melting Point: >340°C, white crystalline powder, bulk density: 0.54g/cc) having particles size of 1,6 microns was prepared using 300 grams of powder of propylene homopolymer (MFI 8-12dg/min).
A through mixing of additives was ensured to obtain homogeneous polymer mixture. The polymer mixture is extruded twice in sequence using Haake single screw extruder maintaining melt temperature above 200 °C. The extruded strands

were passed through cooling water trough and palletized in the form of 3 mm granules and dried.
The master-batch was mixed to polypropylene granules of MFI 10 dg/min such that 1000 ppm level of beta-nucleator is attained in the polymer mass. This mixture was used for film extrusion by tubular quench process keeping the following process conditions. The diameter and lip gap of the tubular die was 200 mm and 1.8 mm, respectively. The temperature profile of extruder and die was adjusted such that the required melt temperature is attained while maintaining the screw speed and blow up ratio as 40-60 rpm and 1.5: 1, respectively. The film extrusion was found smooth with good bubble stability in all the conditions i.e. temperature, cooling rate, stretch while varying the film thickness to desired level i.e. 0.002 to 0.055 mm (200 to 550 microns). Table-1

Experiment No. 1 2 3
Beta nucleating agent, ppm Nil 1000 1000
Melt temperature, °C 175 195 195
Gap: die to cooling ring 290 550 550
Cooling water temperature, °C 13.3 33 45
Nip speed 5.8 5.8 16
Film Thickness, mm 0.05 0.055 0.02
Pore size range, micron 5-40 50-95
The obtained micro-voided propylene polymer film is characterized by scanning electron micrograph (SEM). Figure 2 of the accompanying drawings shows the SEM images of polypropylene film prepared under the experimental conditions

given in Table-1. No formation of voids is observed in un-nucleated sample. Pore formation is observed in the samples containing beta-nucleating agent and pore size increases as the conditions of film formation shifts from more to extreme conditions i.e. higher melt temperature, increased die-cooling ring gap, increased cooling water temperature and nip speed.
Example 2:
The Scanning Electron Micrograph of the propylene polymer film (Microtomed under cryogenic conditions) prepared under the conditions as given in Table~2. It is found that voids in the core of the film are much smaller 0.15 to 0.80 micron (150-800 nm) than the surface. Table-2

Experiment No. 1 4
Beta Nucleator, ppm nil 1000
Melt temperature, °C 175 195
Gap: die -cooling ring 290 550
Cooling water temperature, °C 13.3 45
Nip speed 5.8 5.8
Film thickness, mm 0.05 0.05
Example 3:
The density of the polypropylene film samples prepared under different processing conditions was measured by weight of the sample (1 foot square sample) and film thickness. The results are provided in Table-3. The decrease in the density of the polypropylene film is indicator of increase in porosity of the film. The data

indicates that porosity of the film increase under the conditions that favors the lowering of cooling rate and high stretching of the polypropylene extrudate as listed in Table-3. Table-3

Experiment No. 5 2 6 3
Beta Nucleator, ppm nil 1000 1000 1000
Melt temperature, °C 175 195 195 195
Gap: die -cooling ring 290 550 550 550
Cooling water temperature, °C 13.3 33 33 45
Nip speed 10 5.8 10 16
Film thickness, mm 0.03 0.055 0.03 0.02
Film Density, g/cc 0.8490 0.7501 0.7395 0.5225
Example 4:
The haze value of the micro-voided propylene polymer film prepared under different conditions was measured. The percentage haze of the propylene polymer film increases by lowering the cooling rate and increasing the stretching of the polypropylene extrudate. The details of the experiments related to examples 4 are listed in Table-4. TabIe-4

Experiment No. 1 5 2 4 7 6 8 3 9
Beta Nucleator, ppm nil nil 1000 1000 1000 1000 1000 1000 1000
Melt temperature, °C 175 175 195 195 175 195 195 195 195
Gap: die-cooling ring 290 290 550 550 290 550 550 550 290
Cooling water temperature, °C 13.3 13.3 33 45 13.3 33 45 45 13.3
Nip speed 5.8 10 5.8 5.8 5.8 10 10 16 10
Film thickness, mm 0.05 0.03 0.055 0.05 0.055 0.03 0.03 0.02 0.03
Haze/thickness,
%/mm 44 56 1363 1536 918 2883 2463 4485 1500

TECHNICAL ADVANTAGES:
The technical advantages of the present invention lies in providing a process for the preparation of a micro-voided polypropylene film that involve:
1. Single step process;
2. highly efficient and allied with low capital expenditure;
3. thin micro-voided polypropylene film with high opacity and good mechanical strength.
"Whenever a range of values is specified, a value up to 10 % below and above the lowest and highest numerical value respectively, of the specified range, is included in the scope of the invention".
While considerable emphasis has been placed herein on the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other changes in the preferred embodiments as well as other embodiments of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the forgoing descriptive matter to be implemented merely as illustrative of the invention and not as limitation.

We claim:
1. A single step process for the preparation of a micro-voided polymer film in a tubular quench assembly that comprises an extruder, a die, an air cooling ring, a water cooling ring and nip-rollers; said process comprising the following steps:
(a) extruding a polypropylene melt comprising a beta-nucleating agent and propylene polymer through an annular die to form a tubular film;
(b) concurrently crystallizing and stretching the tubular film in a molten or semi-molten state on exiting from the die to obtain a thin and micro-voided polypropylene containing polymer film; wherein said crystallizing and stretching is carried out under pre-set processing parameters which include at least one parameter selected from the group consisting of melt temperature, gap between die to cooling ring, cooling water temperature, extrudate tubular film temperature, bubble to die diameter ratio, extrudate output ratio and nip-roll speed.
2. The process as claimed in claim 1, wherein the polypropylene melt is prepared by melt blending the propylene polymer and beta-nucleating agent in an extruder at a temperature above the melting point of the propylene polymer.

3. The process as claimed in claim 1, wherein the beta-nucleating agent is at
least one selected from the group consisting of alkali metal salts of a diacid ;
and aryl amide derivatives.
4. The process as claimed in claim 3, wherein the alkali metal salt of a diacid is at least one selected from the group consisting of calcium pimelate, calcium suberate, calcium phthalate and calcium terephthalate.
5. The process as claimed in claim 3, wherein the aryl derivative is at least one selected from the group consisting of N,N'-dicyclohexyl-2,6-naphthalene dicaboxamide, N,N'-dicyclohexylterephthalamide, N,N'-dicylcohexanecarbonyl-p-phenyleneamine, 1,3,5-benzenetrisamides having substituent like 2,3-Dimethylcyclohexyl.
6. The process as claimed in claim 1, wherein the amounts of the beta-nucleating agent and the propylene polymer used for preparing the polypropylene melt are about 0.05 to 0.25% and from 99.00 to 99.75% respectively.
7. The process as claimed in claim 1, wherein the polymer melt is extruded in vertically downward manner.

8. The process as claimed in claim 1, wherein propylene polymer is a homopolymer of propylene.
9. The process as claimed in claim 1, wherein the gap between die to cooling ring is set in the range varying from 290 mm to 555 mm.
10. The process as claimed in claim 1, wherein the bubble to die diameter ratio is in the range varying from 1.2:1 to 2.5:1.
1 i. The process as claimed in claim 1, wherein the cooling water temperature is maintained in the range varying between 13.3 °C to 48 °C.
12. The process as claimed in claim 1, wherein temperature of the extruded tubular film during said concurrent process steps of crystallization and stretching is maintained in the range varying between 190 °C to 80 °C.
13. The process as claimed in claim 1, wherein the nip speed of the nip roller varies between 5.8 to 20 rpm.
14. The process as claimed in claimed in claim 1, wherein the film is stretched uni-axially.

15. The process as claimed in claimed in claim 1, wherein the film is stretched bi-axially.
16. A thin and micro-voided propylene film characterized by pore-size of the void on surface ranging between 5 urn to 100|im., pore size in the core ranging between 0.15 micron to 0.80 micron, film thickness of 0.02 to 0.055 mm and density within the range from 0.5225 g/cc to 0.8490 g/cc, prepared in accordance with the process of claim 1.