CA1099882A - Process and apparatus for impregnating microporous films and the product produced thereby - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

There is disclosed a novel process for coating or impregnat-ing a film of a blend of a thermoplastic orientable polymer with an incompatible polymer and inorganic material/ or a polymer matrix having an inorganic filler followed by the selective stretching in a station provided with a set of grooved rollers to form an opaque, low density, porous film. The group pattern of the rolls are generally of a sinosoidal wave wherein the coated or impreg-nated film is stretched in a manner to affect uniform stretching between contact points of the material to produce a material of larger dimension in the direction of stretch. In accordance with a preferred embodiment, there is produced an interleaf for meat impregnated with an antibrowning agent, such as nicotinic acid.

Description

~9~

This invention relates to a novel process and apparatus for stretching a coated film of a synthetic therrnoplastic polymer or ~lend thereof, and more particularly to a novel process and apparatus for pro-ducing impregnated microporous films of a synthetic thermoplastic polymer by cold drawing such a oDated fiim.
Background OI the Invention - A microporous sheet or film is useful as a printing substrate, such as synthetic paper; as a substitute for leather, as a highly filbrillated sheet which can easily be ~hredded into fine fibrils to be used as substitutes for paper-making pulps, or as a filter material, such as battery spearators.
Porous ~Lper having sufficient wet-strength and Impregnated wi~ nicotinic acid, such as disclosed inU.S. Pat~ntNo. 2,491,646 is used as a meat-interleaf paper. In indu=try, meat-interleaf paper is stacked between freshly cut steaks to prevent forn~tion of dark brown color, and preserve the fresh red color meat during stc)rage.
e function of the porosity of the paper is to make small amounts of oxygen available to the meat pigment, myoglobin; oxygen is needed to retaln the myoglobin at the red state. The chemical additive, nicotinic acid, prolongs the time for which the red color is retained by a catalytic mechanism not entirely understood. Other chemicals are .

.
,,s~

~g~

Goudet rolls and lateral stretching by tenter frames, under tensions which approach the breaking strength of ~e base film often causés breaks and frequent interruptions of the process. Extrusion speeds are uneconomically slow: for instarlce, an acceptable draw rate of 200 cm/min in a single longitudinal draw step over Goudet rolls i'or a 90 wt % isotatic polyp,ropylene - l0 wt% polystyrene ~See Example l), would limit the extrusion rate (for a 3' linear die at a draw ratio of 2. 0 ~nd a film thickness of l00 micron) to 23. 2 lbthr.
In copending application Serial No. 261, 330 there is disclosed a method for fibrillating polyrner blends of incompatible polymers or filled polymers to form fibrillated or microporous structures by cold drawing at high tension such blends or polymers.
Such microporous structures are very difficult to impregnate with solvent9 or soluhons of chemicals because of the 9mall si~e of the pores. Surface tension and capillary action prevent the entry of solutions. ~hus, a microporous fllm made by the method of such application while usually permeable to vapors is not permeable to liquids. Permeability indicated complete open-cell pore structure of the fibrillated filrns.
Objects of ~e Invention It is an object of the pres`ent invention to provide a novel process and apparatus for stretching an impregnated film of a blend of synthetic thermoplashc orientable polymers.
Another object of the present invention i~ to provide a novel process and apparatus for stretching (cold draw) an impregnated film of a blend of synthetic thermoplastic orientable polymer thereof at economical production rates.
Still another object of the present invention is to prvvide a novel process and apparatus for sl;retching an impregnated film of synthetic thermoplastic orientable polymer and blends thereof to produce an impregnated, opa~ue, low density, microporous film or sheet.
A still further object of the present invention is to form novel impregnated mircroporous films of synthetic thermoplastic orientable polymers and blends thereof.
~ Tarious other objects and advantages of the pres~nt invention will become apparent from the following detailed description of an exemplary embodiment thereof with the novel features thereof being particularly pointed out in the appended claims, Summary d~ the Invention In accordance with the present invention, there is provided a novel process for coating or impregnating a film of a blend of thermoplastic orientable polymer with an incompatible second phase selected from the group consisting of an incompatible polymer and an inorganic material followed by the selective stretching in a station provided with a set of grooved rollers to form an opaque, low density, porous film. The groove pattern of the rolls is generally of a sino-soidal wave wherein the coated or impregnated film is stretched in a manner to affect uniform stretching between contact points oE the material to produce a material of larger dimension in the,direction of stretch.

.

In accordance with a preferred embodiment of the present invention, there is provided a process for coating or impregnating a blend of thermoplastic orientable polymer with an incompatible second phase selected from the group consisting of an incompatible polyrner and an inorganic material followed by the stretching of such a coated or impregnated film in a first and second station wherein the first and second stations are provided with sets of rolls having grooves parallel and perpendicular, respectively~ to the axis of each set of rolls The film of synthetic material is stretched in a rnanner to fibrillate such film to produce an opaque low density porous sheet, an ir~pregnated microporous sheet or film of thermoplastic rnaterial In a particularly preferred embodiment, a plurality of stations are arranged in a preselect ma~ner, as determined by product requirernents, e. g. a multiplicity of sets of rollers having parallel grooves, perpendicular grooves, alternating parallel and perpendicular grooves, etc.
~ he impregnated microporous films produced by the process and apparatus exhibit useful properties as specialty products depending on the impregnating chemicals, as more clearly hereinafter demonstrated .
Brief Description of the Drawings The invention will be more clearly understood by reference to~the following detailed description of an e~emplary embodiment ~hereo~ ~n conjJmction with Ihe accornpanying drawings wherein:

8~

Figure 1 is a schematic side ele~ational view of a portion of the apparatus and process of the present invention;
Figure 2 is a schernatic side elevational view of the remaining portion of the apparatus and process of the present invention;
Figure 3 is an enlarged view of the film entering the rolls;
Figure 4 graphically illustrates a sinosoidal curve; and Figure 5 is a top view of the second station Eollowed by another first station.
Detailed Description of ~e Invention Drive and support assemblies, timing; and safety circ~its and the like known and used by those skilled in the art have been omitted in the interest of clarity.
Referring to Figure 1, illustrating the front end of the process and apparatus of ~e present invention, there is provided a suppl~ roll 10 on which is mounted a film 12 of a blend of a thermo-plastic orientable polymer with an incompatible second phase selected from the group consisting of an incompatible polymer or an inorganic material. The film 12 is coursed about rollers 13 through a coating or impregnating liquid or solution, generally indicated as "S", contained in a vessel 15. ~he coated or impregnated film 12 is then coursed between a nip 14 ot' a pair of rolls 16 having a plurality o~ tips 18 forming grooves 20 ~ parallel to the axis of the rolls 16, as seen in Figure 1.
The film 1~ is maintained against the lower grooved roll 16 by a pair of press rolls 22 to ensure that the velocil~y Vl of the film 12 is substantially identical to the surfa~e velocity ~1 of the grooved rolls 16. The grooves 20 of the rolls 16 are intermeshed like gears, as known to those skilled .

1~9B8~

in the art. As the film 12 enters the nip 14, the film 12 assumes the shape of the grooves 20 and is stretched (See Figure 3) by a Eactor determined by the length of the sinus wave "1" (See ~igure 4) of the groove divided by the distance "w" between contact points of each respective groove tip, since the film 12 is prevented from slipping by the press rolls 22 to prevent the introduction of more material, as is more commonly practiced in the corrugating art.
The draw ratio (l/w) is calculated by the following equation:
l/w =l/n~ ~'l~s~x dx where, a = ~d/w; and d = groove depth. Thts, for d/w ratios of 1.0, 0.75 and 0.5 the draw ratios are 2 35; 2.0 and:l.,6, respectively.
The longitudinal draw rateiis defined by the following equation: ;
draw rate = V2 ~ Vl whe re Vl = film veloc ity ente ring rolls; and V2 = film velocity leaving rolls .
The Actual Draw Rate (ADR) for longitudinal or lateral stretching is calculated by the following equation:
ADR = (draw ratio - l)V

4 d/w JI~Ta~tT~
whe re-, d = groove de pth;
w =distance between tips;
l = length of sinosoidal wave;
the draw ratio = l/w;

8~

V is the velocity of the film entering the nip of the rolle rs; and R is the radius of the rollers.
The r`oller spced can be calculated as follows:
V - ADE~ 4 d/a J R/d - 1/4 draw ratio - 1 Thus, if the critical ADR for a composition operating at about 80% of breaking tension is 100 cm/min, and d/w =i 1, draw ratio is 2. 25, R is 10 cm. and d = 0. 3 cm., then Vl = ADR 18. 41 =
1841 cm/mm., which is 18. 41 times faster than permissible with Goudet rolls. For a 6 inch wide film die making 4 mil. film, an extrusion rate of 565 lbs/hr. can be obtained vice 30.4 lbs/ hr.
The film 24 îafter passage through the nip 14 of the rolls 16 is pulled away by a pair of tension rollers 26 having a surface velocity V2 greater than the surface velocity of the rollers 16, but not greater than a factor of the draw ratio affected in the - nip 14 of the rollers 16. In a^cordance with the present invention, the length of the fllm is therefore increased by this factor.
It is noted that the film does not undergo narrowing while being longitudinally stretched or extended, as is the case with conventional roller systems. It is apparent to one skilled in ~e art that the film may sequentially pass through a plurality of pairs of grooved rollers 16 to further stretch lengthwise the film 24 prior to lateral stretching as more fully hereinafter described.

~9~

:E~eferring now to Figure 2, the longitudinally stretched film 24 from ~e first station is introdlced into a nip 26 formed by a pair of rolls 30 having a plurality of tips 32 forming grooves 3~ parallel to the circumference of the rolls 30 in a second station of the apparatus.
The film 24 is caused to be coursed into the nip 28 by a pair of press rolls 36 which holds the film 24 against the lower roll 30 to thereby prevent the film 24 from narrowing prior to introduction. Once in the nip 28, the film 24 assumes the shape of the groove pattern (~ee Figure 3) and becomes laterally stretched by a factor of the draw ratio determlned in a mannex similar to the draw ratio discussed with:. Eeference . to F.igure: 1.
The crimp pattern is flattened out by stretchi~;;g the sheet 3 7 laterally by means o:E tenter clamps or curved Mount E~ope rolls, generally indicated as 39, such as known and used by one skilled in the art.
Inthe second station, i. e lateral stretching, the sheet 3.7 is woun~l up atabout the same velocity as the feed velocity with the product being collected on a roll 40. For best results, the longitudinal and lateral stretching steps are repeated alternately through multiple passes each having a relatively low draw ratio, until the total permissible draw ratio is reached. The number of longitudinal and lateral passes, as well as the e~{tent of the stepwise draw ratios, can be chosen so that a final film is obtained wi.th the desired properties.
Figure 5 illustrates the film 37 being further coursed into another set - , ~

8~;~

of rolls 42 having grooves 44 parallel to the axis for further longitudinal stretching, which could be subse~uently followed by another lateral stretching.
As hereinabove indicated, microporous Eilters have many industrial uses, such as bacteria or enzyme filters, or battery separators, etc. Besides wetting agents, other chemicals such as insecticides, fungicides, deodorants, desinfectants, drugs, flame retardants, etc. can be impregnated into such a microporous film for later release during use.
~xamples of the Invention Operation of the process and apparatus is described in the following e~amples which are intended to be merely illustrative and -the invention is not to be regarded as limited thereto.
Example I
A 0 004" thick film (4'k4") comprised of 85 wt % polypropylene and 15 w~o clay coated with a solution of a wetting agent (10 wt% poly-ethylene o2{ide in methanol~ is introduced at room temperature through a pair of grooved rolls (as shown in Figure 1). The grooves have an approximate sir~3oidal shape and are 3 mm. deep and 3mm. apa~ and produce a draw ratio of-about 2 When the film is stretched to conform wi~h the shape of the grooves, 8 groove tips simultaneously engage the film. The film is introduced into the nip of the intermeshing grooved rolls rotating at 60 RPM to produce a feed velocity Vl of 1914 cm. /min.
and is woui~d at 3828 cm. /min. The actual film draw rate is 120 cm./ min. The filrn is passed through twice in each direction and stretched to a dimension of 6. 5 " x 6. 5 ll and having a thickness of 0.0025 ". The solvent is evaporated and the resulting film sample tested for water permeability. The film sample is cut to a circlé to fit a 3"
Buchnerfunnel. A suction of 20 mm. vacuum is applied to the funnel with 10 ml. of water added to the funnel. Water is filtered in 12 min.
through the porous film.
Example II
The procedure of Example I is repeated without the impreg-nation step and produced a sample stretched to a dimension of 6. 5"
x 6. 5 " and having a thickness of e . 004 inch - - calculated porosity of 39 %. The film is similarly tested for water permeability in a manner substantially similar to that of ~E~xample I, however, water did not penetrate through the film in a period of 24 hours although exhibitingairpermeable properties.
Exam~le III
A sample prepared in accordance with Example II is boiled in a wetting agent solution for one hour to affect coating of the pores by the wetting agent and to cause water to penetrate through the porous film. The solvent is dried off and the film tested for filtration of water as described above. Water would not pass through the film indicating that the wetting agent solution merely coated the pores near the surface of the film, but did not penetrate deeply into film, when applied after the stretching process.
Example I~
Three stre~ched sheets of porous film of the film of Example I
are stretched to test the effect o~ retaining the red color of freshly cut meat. One sheet (A) is not coated. A second sheet (B) is coated with a 2% solution of nicotinic acid in water, with excess solution being wiped off the surface to avoid the presence of unabsorbed chemical. A third sheet (C) is coated at room temperature with a nicotinic acid solution prior to stretching. Freshly cut cut beef is stacked on top of each other, with the film sheets interleafed between the meat surfaces. The meat surfaces in contact with film A and B
turn dark within 2 hours of storage in a refrigerator at 30F. The meat surface in contact with film C retained its fresh red color for 24 hours and is only slightly brown after 48 hours.
Example V
Films B and C of Example IV is extracted with 50% water/50'10 acetone to determine the amount of nicotinic acid ~extracted (infrared spectroscopy of the extract and related to the film weight):
Sample B -- 0. 05% nicotinic acid Sample C -- 1. 02% nicotinic acid While the present invention has been described with reference to the passage of a film through a first longitudinal stretching station and thence a lateral stretching station, it is apparent that such :
stations may be altered with the film being first introduced into a lateral stretching station. Further, the film may be subjected to a plurality ~f longitudinally orientated stretching rolls. It will also be appreciated that the grooves need not be exactly parallel or per-pendicular as long as the grooves intermesh.

~09~882 It will be appreciated that the grooved roll drawing permits multiple simultaneous draw necks which allow for further actual speed where draw tension is high. At high draw tension (low tem-perature), the fibrillation phenomenon occurs which is highly desirable for porous films. Additionally, the grooved roll drawing permits a partial draw (draw below the natural draw ratio) in multiple stages thereby further reducing the actual draw rate and increasing the produclion rate. Still further defects in the base film, i. e gels, holes, etc. are carried through the grooved roll drawing with no interruption in the process as distinguished from drawing in conventional Goudet and tenter frame drawing wherein such defects usually result in breaks and the necessity for subsequent shutdown.
While the invention has been described in connection with an e~emplary embodiment thereof, it will be understood that many modifications will be apparent to those of ordinary skill in the art and that this application is intended to cover any adaptations or variations thereof. Therefore, it is manifestly intended that this invention be only limited by the claims and the equivalents thereof.

Claims (12)

I CLAIM:

1. An improved process for longitudinally stretching incremen-tal portions of a film comprised of a blend selected from the group of a polymer matrix having an inorganic filler or of a thermoplastic orientable polymer with an incompatible second phase selected from the group consisting of an incompatible polymer or inorganic material, which comprises .
a) passing said film through a solution of an impregnating chemical;
b) introducing said film of step (a) into a nip of interdigitating rollers having grooves substantially parallel to the axis of said rollers;
c) controlling the velocity of introduction of said film into said nip to assume and maintain the velocity substantially identical to the surface velocity of said rollers thereby to longitudinally stretch incremental portions of said film and d) collecting the thus formed film.

2. The process as defined in Claim I wherein the withdrawal velocity of step (c) is not greater than a factor of the draw ratio of said nip.

3. The process as defined in Claim 1 wherein said impregnating chemical is a solution of nicotinic acid.

4. The process as defined in Claim 3 wherein said resulting film is cut into sheets.

5. An improved process for bi-axially stretching incremental portions of a film comprised of a blend selected from group of a polymer matrix having an inorganic filler or of thermoplastic orien-table polymer with an incompatible second phase selected from the group consisting of an incompatible polymer or inorganic material, which comprises:

a) passing said film through a solution of an impregnating chemical;
b) introducing said film into a nip of interdigitating rollers having grooves parallel to the axis of said rollers;
c) controlling the velocity of introduction of said film into said nip to assume and maintain the velocity substantially identical to the surface velocity of said rollers thereby to longitudinally stretch incremental portions of said film d) withdrawing said film from said rollers at a velocity greater than the rotational velocity of said rollers;
e) introducing said film into a nip of interdigitating rollers having grooves substantially perpendicular to the axis of said rollers;
f) controlling the velocity of introduction of said web into aid nip to assume and maintain the velocity substantially identical to the surface velocity of said rollers thereby to laterally stretch incremental portions of said web;
g) withdrawing said web from said rollers at a velocity substantially corresponding to the velocity of introduction; and h) collecting the thus stretched film.

6. The process as defined in Claim 5 wherein the steps (b) to (g) are repeated prior to step (h).

7. The process as defined in Claim 6 wherein steps (e) to (g) are affected prior to step (b) to (d).

8. The process as defined in Claim 7 wherein said resulting film is cut into sheets.

9. The process as defined in Claim 1 wherein said impregnating chemical is a solution of nicotinic acid.

10. An interleaf for meat which comprises a stretched blend selected from the group of a polymer matrix having an inorganic filler or of a thermoplastic orientable polymer with an incompatible second phase selected from the group consisting of an incompatible polymer or inorganic material, wherein said material is impregnated with an antibrowning agent prior to stretching.

11. The product as defined in Claim 10 wherein said material is bi-axially stretched.

12. The product as defined in Claim 10 wherein said anti-browning agent is nicotinic acid.