BE704514A - - Google Patents

Description

  

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   The invention relates to novel films characterized by an open cell structure and low bulk density, as well as to a process for preparing them from the crystalline polymers.



   It has been proposed to prepare films from
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 1111: 1 criotal polymers, eg polypropylene, by melt extrusion and cooling. Dandruff like this
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 present exceptional pl'opr: t 1 V'S such as high strength, stability and rigidity. The Applicant has found an entirely new type of film material which,
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 Other than those of the prior art, they have lower oppaTsnttJ3 densities than polymeric materials from deugtiell-, f111e! J are formed, usually at most 90%, of F ': rOft-rOI1CQ about 50 to 75% of the densities of the materials P "li; Vl' \: H,: Ul'1" tH3pondants.

   The dimensions of the 'passages to the voids or port. d; 1 open cells accessible from the outside of the film are less than 5000 angstroms, for example 150 Ii. 5000 sngstrCmH determined, from the point of view of porosity, by the penetratich of Mercury, a measure which also determines the volume of the voids ru 1. ' O! 3l) COD of the pores. 14 final crystallinity of these pellets is 40%, preferably at least 50% and more preferably 50 to 60%.



   Advantageously, the voids or spaces of the pores of the film of the present invention can be changed in a reversible manner as to the dimension, that is to say they can

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 to be enlarged in volumetric capacity and then reduced by expanding and then releasing the part of the film thus modified. If desired, such voids can be made to enlarge, diminish or disappear substantially, irreversibly. The importance of this characteristic will be readily apparent and the possibilities of using films according to the invention are consequently considerably increased.



   Consequently, the object of the invention is to prepare novel open-cell films characterized by relatively low bulk densities.



   A further object of the invention is to provide open-cell, dimensionally stable films exhibiting properties superior to those of known films.



   Another object of the invention is to provide films which have voids or pores which can be changed reversibly in volumetric capacity or else enlarged or reduced or made to disappear irreversibly.



     It is yet another object of the invention to provide a process for the manufacture of the elastic films described above.



   Other objects will become apparent during the detailed description below.



   Here the expression "Bulk density" means the weight per unit of total volume of the film, the total volume being determined by the amount of mercury diffusing into the film under high pressure. See R.O. QUYNN et Al in Journal of
Applied Polymer Science, Vol. 2, No. 6, pages 166-173. (1959).

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    @
In the specification the term open cell structure "means that most of the voids or pores of the structure within the geometrical boundaries of the film are accessible to the outer planar surfaces of the film.



   The novel films of the invention are forced from precursor films into a relatively crystalline film-forming polymer exhibiting elastic recovery at zero recovery time (defined hereinafter) at 25 ° C and 65% relative humidity. at least 50% when subjected to a standard elongation (extension) of 50% and preferably a zero time elastic recovery of at least 80% when subjected to a 50% elongation.



   It should be noted that although a standard elongation of 50% is used to identify the elastic properties of the precursor films of the invention, this elongation is merely exemplified. Films of this kind will of course generally exhibit elastic recoveries at elongations of less than 50% greater than those exhibited at 50% elongation and such films exhibit relatively high elastic recoveries at substantially higher elongations. at 50% for example elongations say 100%.



   The precursor films used according to the present invention differ from films formed from conventional elastomers. With conventional elastomers the elongation-tension behavior and in particular the stress-temperature relationship is governed by a mechanism of strain entropy (rubbery elasticity). The positive temperature coefficient of the retraction forces, i.e. the tension decreasing with decreasing temperature

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 and the total loss of elastic properties at the glass transition temperature are particular consequences of entropy-elasticity. The elasticity of the precursor films of the invention, on the other hand, is of a different nature.



  In qualitative thermodynamic tests with these precursors, the force increasing when the temperature decreases (negative temperature coefficient) can be interpreted as meaning that the elasticity of these materials is not governed by the effects of entropy but depends on an energy term. More significantly it has been found that the precursor films of the invention retain their stretching properties at temperatures at which entropy-elasticity could no longer be achieved. effective. Thus it can be concluded that the stretching mechanism of the precursor films is based on energy-elasticity and can be related to non-classical elastomers.



   The above characteristics necessary for the precursor films can be achieved using a critical combination of process variables and process steps. Broadly speaking, the necessary process includes forming ie extruding films under technical conditions followed by an annealing phase to impart the degree of elasticity. The necessary conditions of the process include; (1) a fast drawing speed, (2) high drawing ratio and (3) a low melt temperature, that is to say a temperature of the melt substantially closer to the temperature of the film-forming polymer than in the usual extrusion processes.



   The fast pulling speed can be achieved for example in an extrusion with a slot die by providing a take-up roll or spool for the extruded film.

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 at a distance not exceeding 5 cm and preferably not exceeding 2.5 cm from the extrusion slot or orifice. At the same time the take-up roll must work at a high speed, that is a speed of at least 7.5 m / min of films measured on said roll since the speed of the film is different. actually at the extrusion slot (where the film is relatively thick) and the take-up roll (where the film is relatively thinner).



   Draft ratios suitable for preparing the precursors are as high as 1000: 1 and preferably in the range of 75: 1 to about 125: 1.



   The temperature of the melt for the extrusion of the precursors generally does not exceed 100 above the melting point of the polymer and in general the most suitable is between 10 and 40 ° C. above the melting point while the draw ratio of the extrusion i.e. the ratio of the linear speed of the take-up roller to the linear speed of the extrusion is at least 20: 1. The annealing phase is generally carried out at a temperature on the order of about 5 to 100 0 below the melting point of the polymer for a period of at least 10 seconds.



   The first of the two essential phases of the present invention is the pulling phase in which the film is pulled or stretched. This can be done continuously in conjunction with an extrusion machine or the film can be pulled in a separate operation. Regardless of the process followed, the extruded film must be pulled to achieve objects of the invention. By the expression “extruded film” is meant a film when it first solidifies on leaving the extrusion apparatus. We pull the film

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 extruded to impart to it the open-cell structure and to reduce the bulk density thereof.

   The ! The precise degree of draft is important to the invention because optimization of useful properties occurs for intermediate elongations rather than high rates of stretch.



   To obtain the optimum ratios of the characteristics mentioned above the extruded film should be drawn at a draft ratio of 1, 1a: 1 to 4: 1 and preferably 1.25: 1 to 2: 1, for example speed of the take-up roller relative to the speed of the supply roller. One can operate with a draft ratio higher than 4: 1 but usually these values will not be used as the improvement in open cell structure is only slight in most cases and in some cases. this operation under these conditions can be) detrimental to the structure.



   In accordance with the present invention, the film is preferably "cold drawn" to impart the desired void structure. In this operation the film is drawn at the above draft ratios at room temperature or at temperatures close to room temperature. The pulling can be carried out in any suitable manner using the known techniques in which one moves positively by one movement device to a second movement device operating at higher speed. The temperature of the film itself during printing is shown below as the printing temperature.



   The present invention is also directed to the printing of films at temperatures below ambient as well as at temperatures above these. However the

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 upper limit of the draft and critical temperature range and it is essential that this limit not be exceeded. This critical quality is illustrated by a graph in Figure 1 of the accompanying drawing. The graph shown shows the reduction in density expressed in g / cm3 on the ordinate of a typical polypropylene film with the draw or stretch temperature used expressed in degrees 0 on the abscissa.

   From the graph it is readily apparent that the noticeable reduction in the apparent density of the film can be achieved up to a temperature of about 93.3 ° C but when this temperature is exceeded there is a marked decrease in the reduction of film. the bulk density of the film. Surprisingly in this case no substantial reduction can be achieved when exceeding the upper limit of the stretching phase of about 11.1-0. Accordingly, preferably, the polypropylene films of the present invention are not stretched at a temperature exceeding 104.4 C, upper limit, for acetal films the temperature does not exceed 124 C, for polyethylene films the temperature does not exceed 104.4 C.



   When stretching the film at temperatures above room temperature, said film can be heated by any conventional means. D Preferably, the film is heated by means of heated moving rollers. However, the film can be heated. also the film by passing over a heated plate, in a heated liquid, hot gas or by means of the like.



   The second essential phase of the process of the invention is the phase of setting or hot annealing the film after it has been stretched while the film is in the stretched state. It has been found that heating the stretched film,

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 in the strained state, at a temperature of 90 ° C. or above, has a remarkable influence on the dimensional stability of open cells, and on the low density characteristics thereof.

   As a result, the stretched film is stabilized against excessive shrinkage, etc. heating to a temperature which is somewhat higher than the draw temperature while it is maintained under such tension that the film cannot retract freely or can retract only to a controlled extent, not exceeding for example 15% of the initial length.

   To ensure maximum relaxation of the stresses introduced into the films during previous work and thus impart the desired stability, the heat treatment is preferably carried out at a temperature in the range of 100 to.
160 C for polypropylene, from 100 to 160 0 for acetal polymers 100 to 135 0 for polyethylene, etc. The duration of the heat treatment should not be more than about 0.1 seconds and may be in the range of 0.5 seconds to about 30 minutes, preferably about 2 seconds to about 30 minutes.
15 minutes.

   The drawing operation and the heat treatment can be carried out successively or they can be combined in a single operation, by stretching the film on a metal surface, for example rolls. heated to the required temperature. As can be assumed from the above discussion, the bulk density of the resulting low density heat-treated film is significantly less than that of the precursor film.



   The resulting film, in the tension-free state, has a bulk density less than the density of the polymeric material from which it is formed, usually.

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 95% interior, preferably 50-75% of the densities of the corresponding polymeric material.

   The dimensions of the passages between the voids or pores of the open cell structure accessible to the outer flat surface of the film are of the order of 5000 angstroms, for example 150 to 5000 angstroms, as is determined from the porosity point of view by mercury penetration, a measure which also determines void volumes or pore space. The final crystallinity of this skin. This is preferably at least 30% and more preferably 40% or even at least 50%, for example 50 to 100%.



   As indicated, the elastic precursor films used according to the invention consist of a polymer of a type capable of exhibiting a marked degree of crystallinity in contrast to the more usual or conventional elastic materials such as rubber which is essentially amorphous to the surface. 'non-extended or voltage-free state,
A characteristic group of polymers to which the invention can be applied include olefin polymers. example polypropylene, poly-3-methyl-butene-1, poly-4-methyl-pentene-1, polyethylene as well as copolymers of propylene, 3-methyl-butene-1, 4-methyl-pentene-1 or ethylene, the with each other or with minor amounts of other olefins.

   for example copolymers of propylene and ethylene $ copolymers of a major amount of 3-methyl-butene-1 and a minor amount of a straight chain n-alkene such as n-octene-1, n-hexé- ne-1, n-hexadecene-1, n-octadecene-1, or other relatively long chain alkenes, as well as copolymers of 3-methylpentene-1 and some of the n-alkenes mentioned above in connection with the 3-methyl-butene-1. These polymers in the form of films generally have a percentage of

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 crystallinity of at least 50%, for example 50 to 60%.



   Another group of polymers to which the invention can be applied are the high molecular weight polymeric acetals. Although acetal (or oxymethylene) homopolymers are contemplated, the preferred oxymethylene polymers are random oxymethylene copolymers, for example a polymer having repeating oxymethylene units i.e. -CH2O-, distributed with -OR- groups in the main chain, R representing a divalent radical comprising at least two carbon atoms directly linked to one another and placed in the chain between the two valences,

   any substituent on the radical R being inert, that is to say that it does not contain an interfering functional group and produces an undesirable reaction and in which a major proportion of the units - OR - is found or forms simple units attached to oxymethyl groups on each side. Preferred examples of polymers are the copolymers of trioxane and cyclic ethers having at least 2 adjacent carbon atoms, such as the copolymers described in US Pat. No. 3,027,352 to Walling et al. These polymers in the form of films have a crystallinity of at least 50%, for example 50 to 60%.



   As other crystalline polymers to which the invention can be applied, mention may be made of poly- (methylene sulfide), poly- (ethyl sulture), poly- (phenylene oxide), polyamides such as polyhexamethylene-adipamide ("Nylon 66 ") and polycaprolactan (" Nylon 6 ") and polyesters such as poly (ethylene glycol terephthalate).



   The types of apparatus which can be used for the formation of precursor film products according to the invention are well known. For example the film extruder

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 fitted with a shallow thread extrusion screw and a die as shown in Figure 2, which is incorporated herein by reference, is satisfactory. As seen on. FIG. 2 the resin is introduced into the hopper 1 of the extruder 2 which comprises a screw 3 and a sleeve equipped with heating elements 4, The resin is melted and transported by the screw in the die 5 from which it is extruded through a tent 6 in the form of a film 7 which is pulled by means of a take-up roller 8.

   Obviously more than one pickup roll can be used with different combinations.



   By using this type of apparatus a film can be extruded with a draw ratio of about 20: 1 to 1000: 1, preferably 75: 1 to 150: 1. The die opening or slot can be. of the order for example of 0.075 to 5 mm.

   Polypropylene can be extruded at a melting temperature of about 180-240 ° C., preferably 195-225 ° C., while acetal polymers, for example of the type described in US Pat. No. 3,027,352 already cited can be extruded at a melting temperature of about 185-235 ° C, preferably 195-21500 ° C. Polyethylene can be extruded at the extrusion temperature of about 175-225 ° C.



   The extrusion operation must be carried out with rapid cooling and stretching so as to obtain the maximum elasticity, this can be achieved by mounting the take-up roll relatively close to the extrusion tent, for example at a distance of the order 5 cm and preferably of the order of 2.5 cm, said roller rotating as quickly as possible without causing breakage. An air gap can be used within 2.5 cm of the slit. The take-up roller can be rotated for example at speeds of 6 - 300 m / min,

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 preferably 15 to 150 m / min.



   According to a variant for the formation of precursor films according to the invention, the film is blown by extrusion, using a hopper and an extruder which are essentially the same as in the case of the slot extruder of the film. figure 2. From the extruder the melt enters the die from where it is extruded through a round slot to form a tubing film having an initial diameter Said Air is introduced into the system and is distributed by means of an air ring or the like through an opening within said tubular film and has the effect of blowing the tubular film, so as to bring its diameter to the value D2.

   A second source of air is provided as well as means for directing this air around the extruded film so as to cool the latter quickly and efficiently. After a short run where the film cools and completely hardens, it is wound up on a feeder roll.



   Using the blown sleeve process, the stretch ratio is preferably between 20: 1 and 200: 1, the slit opening is 0.5mm to 5mm, the ratio D2 / D1 is 0.5 to 6.0 and the reception speed is 9 to 210 m / min.



  The temperature of the melt may be within the range stated above for extrusion with a straight slot die.



   As previously indicated, the extruded precursor film can be subjected to an initial annealing in order to develop the desired elasticity. In general, this first annealing is carried out at a temperature of the order of 5 to 100 ° C. below the crystalline melting point of the polymer for a period of at least 5 seconds. Thus for polypropylene the preferred annealing temperature is about 100 to 155 ° C while

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 for the copolymers. of oxymethylene (acetal) the preferred annealing temperature is about 110 to 165 C.



   One method of performing the annealing operation is to use hot rollers (not shown) in which case the annealing time is generally on the order of 5 to 90 seconds or more. Another method is to place the film in a tension-free state in an oven (not shown) at the desired temperature, in which case the residence time is preferably on the order of 30 seconds at one haul.



   According to the invention, the extruded and optionally annealed film 7 passes over a roll 8 and between suitable stretching devices. As shown in Figure 2 this can be achieved by passing between rollers 9a and 9b and rollers 10a and 10b, the rollers 10a and 10b rotating at higher peripheral speeds than those of the rollers 9a and 9b so as to achieve stretching. the film in a predetermined proportion. For example, the rollers 9a and 9b can be made to rotate with a peripheral speed of about 3 to 330 meters / minute and the rollers 10a and 10b at a peripheral speed of 3.5 to 400 meters / minute to communicate the e- desired stretching of the film as the stretching operation proceeds.

   The stretched film 11 then passes from the rolls 9a, 9b, 10a and 10b through a heater 12 and over a deflection roll 13 to wind onto the spool 14. The peripheral speed of the roll 13 can be lower, the same or higher. to that of rollers 10a and 10b so that the film passing through heater 12 cannot retract or can retract to a controlled extent. The rollers 8, 9a, 9b, 10a and 13 can be driven by means of continuous speed variators to adjust the draw or stretch ratios. The feed speed of the

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 film to the heater and the reheating times are adjusted so that the desired objectives are obtained at the chosen stretching and stabilization temperature.



   The heater 12 may be heated as indicated in any suitable manner, for example by means of a hot plate, heated liquid or heated gas, etc. These can obviously be replaced by any equivalent heating means.



   Thanks to the invention, it is possible to obtain a polypropylene film having, at 25 ° C. and in a relative humidity of 65%, an elastic recovery, for an elongation of 50%, from 50 to 90%, a tensile strength of 700 to 2500 kg / cm2 and an elongation at break of 200 to 350%, a modulus of 7,000 to 35,000 kg / cm2 (all previous figures do not refer to the direction of stretch during manufacture).

   In addition, polypropylene films generally exhibit other properties in the following range! an opacity ranging from 25% to 100%, a permeability at 02 from 100,000 to 150,000 and a permeability to N2 from 100,000 to 150,000, a permeability to CO2 from 15,000 to 30,000, the permeability units for these three gases being expressed in cm3 / 24 hours / m2 / atmosphere. The other properties are void volumes from 0.565 to 0.075 cm3 / g, density from 0.60 to 0.85 g / cm3, and a pore size from 150 to 5000 angstroms, the values of the last three properties being determined. by mercury porosimetry.



   The films prepared according to the invention of oxymethylene copolymers (such as those described in US Pat. No. 3,027,352) generally exhibit properties in the direction of stretch during manufacture within the following ranges to 25. C and for 65% relative humidity: elastic recovery for an elongation of 50%: from 50 to 90%

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 tensile strength of 700,500 kg / cm2, elongation at break of 50 to 150%, modulus of 7000 to 25000.kg/cm2.

   In addition, such films exhibit an opacity ranging from 20% to full opacity, a density of 0.95 to 1.3 g / cm3, a void volume of 0.343 to 0.061 and a dimension of. pores of.



  150 to 5000 angstroms, the values of the last three properties being determined by mercury porosimetry. The above properties are given by way of illustration of a typical product and should not be construed as limiting the invention.



   The invention is further illustrated by the examples below.



   Example
The film-forming polymer of this example is a copolymer having a melt index of 2.5, derived from trioxane with 2% by weight of ethylene oxide polymer of the type described in United States Patent No. 3,027-352 already cited, having undergone a treatment so as to eliminate unstable groups as described in US Pat. No. 3,229,623, and suitably stabilized.



   The polymer described above was subjected to melt extrusion at 375 ° C through a 20 mm slot as shown in Figure 2 at a shear rate of about 46 reciprocal seconds using an extruder. 25mm with a Shallow Thread Extrusion Screw $. The length to diameter ratio of the extruder barrel is 24: 1. The extrudate is stretched very quickly at a draw ratio of.
270 1, it is contacted with a rotatable molding roll maintained at 142.8 ° C and about 6mm from the die slot.



   The film produced in this way is rolled up, it presents

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 the following properties:
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<tb> thickness <SEP> 0.012 <SEP> mm
<tb>
<tb> recovery <SEP> for <SEP> a <SEP> lengthening
<tb>
<tb> of <SEP> 50 <SEP>% <SEP> 46 <SEP>%
<tb>
 
The polymer is then subjected to annealing in an oven in an unstretched state at 145 ° C. for 16 hours.

   At the end of the annealing period, it is removed from the oven, allowed to cool and the film has been found to have the following properties:
 EMI17.2
 
<tb> Thickness <SEP> 0.012 <SEP> mm
<tb>
<tb>
<tb> Recovery <SEP> for <SEP> a <SEP> lengthening
<tb>
<tb>
<tb> from <SEP> 50 <SEP>% <SEP> to <SEP> 25 C <SEP> 92 <SEP>%
<tb>
<tb>
<tb> Density <SEP> determined <SEP> by <SEP> porosimetry
<tb>
<tb>
<tb> at <SEP> mercury <SEP> 1.40 <SEP> g / cm3
<tb>
 
The film was stretched at room temperature to 69% of its original length, held stretched with metal tweezers, and exposed to heat in a 150 ° turn for 2 minutes.

   At the end of this period, it is removed from the oven, it is left to cool and it is found to have the following properties! Volume of voids determined by porosimetry at
 EMI17.3
 
<tb> mercury <SEP> 0.18 <SEP> cm3 / g
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Density <SEP> determined <SEP> by <SEP> porosimetry <SEP> at
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> mercury <SEP> 1.12 <SEP> g / cm3
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> <SEP> dimensions of <SEP> pores <SEP> determined <SEP> by <SEP> porosi-
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> metry <SEP> at <SEP> mercury <SEP> 100 <SEP> - <SEP> 900 <SEP> A
<tb>
    Example II This example illustrates for which This example illustrates one of the uses for which the films of the present invention can be used,

   in this case as a microporous membrane for the desalination of saline solution.

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   The polymer was melt extruded as described in Example 1 and annealed in an oven at 145 ° C. for 16 hours. At the end of this period, the film is removed from the oven and left to cool. A 91% film (n * 1) was stretched at room temperature and heated in a strained oven at 142 ° C for 8 minutes. The film N 2 is not subjected to anything after the first annealing operation.



   Part 1 is placed in a supported cell and it is brought into contact on one side with a salt solution under pressure (0.5% by weight of NaCl under 42 kg / cm2). The solution flowed through the film due to the external pressure gradient and recorded a value of 170 l / d / m2. The salt concentration of the solution which was forced through the membrane was reduced to 0.17% by weight. The film is turned over, that is, the other side is brought into initial contact with the pressurized salt solution. The flux and salt content are 72.8 l / d / m2 and 0.27% by weight.



   The above process is repeated with part no. 2. No flow is obtained with either side facing upstream.



   Example III
Crystalline polypropylene from India 4.0% melt% density 0.905 g / cm3 at 193 ° C. was melt extruded through a 20 mm slit fiber using a 45 mm extruder with a dc screw. 'shallow thread extrusion. The ratio of the length to the diameter of the extruder barrel is 24: 1. The extrudate is stretched at a draw ratio of 90: 1, contacted with a rotatable molding roll maintained at 85 ° C and

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 6 mm from the die slot.

   The film produced in this way has the following properties:
 EMI19.1
 
<tb> Thickness <SEP> 0.025 <SEP> mm
<tb>
<tb>
<tb>
<tb> Elastic <SEP> recovery <SEP> to <SEP> from
<tb>
<tb>
<tb>
<tb> of a <SEP> lengthening <SEP> of <SEP> 50 <SEP>% <SEP> 49 <SEP>%
<tb>
 
The resulting film was divided into two pieces which were subjected to annealing in an oven at 130 ° C. for 15 minutes. At the end of this period, remove from the oven and leave to cool. The film No. 1 was stretched 190 at room temperature and subjected to hot fixing in the extended state for 15 minutes at 130 C. The part No. 2 was not subjected to any treatment after the first rework operation. - cooked.

   Mercury porosity analysis (infusion) of the two samples gives the following results:
 EMI19.2
 Sample N 1 Sample: llon N 2
 EMI19.3
 
<tb> Volume penetration
<tb>
<tb> (cm3 / g) <SEP> 0.15 <SEP> 0.01
<tb>
 
 EMI19.4
 Density (8m / cr3) ô, 79 0.89
 EMI19.5
 
<tb>
<tb> Dimension <SEP> of <SEP> pores <SEP> 100-1000 <SEP> A <SEP> -------
<tb>
   Example 17
The polymer described in Example III was extruded in the molten state except that the draw ratio was 180: 1.



  The film prepared in this way has the following properties:
 EMI19.6
 
<tb> Thickness <SEP> 0.012 <SEP> mm
<tb>
<tb> Recovery <SEP> for <SEP> a <SEP> lengthening <SEP> of
<tb>
 
 EMI19.7
 50% at 250C 48%
A sample of this film was annealed in a stress-free oven at 125 ° C for one hour, removed from the oven, and left to cool.

   We find

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 that it has the following properties
Recovery from lengthening
50% at 25 ° C. The samples of both films, annealed and unannealed, were stretched at room temperature in two sets of different elongation and heat setting in the molten state for 5 minutes at 125 ° C. Both abandoned. samples on cooling and it is found that they exhibit a bulk density and void volume hereinafter measured by mercury porosimetry.



   Apparent density a. g / cm3
 EMI20.1
 
<tb> Annealing <SEP> before <SEP> None <SEP> annealing
<tb>
<tb> stretching <SEP> before <SEP> stretching
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Percentage <SEP> of elongation
<tb>
<tb>
<tb> 50 <SEP> 0.804 <SEP> 0.853
<tb>
<tb>
<tb> 100 <SEP> 0.784 <SEP>, <SEP> 0.834
<tb>
<tb>
<tb> 200 <SEP> 0.755 <SEP> 0.831
<tb>
<tb>
<tb> 300 <SEP> 0.734 <SEP> 0.829
<tb>
   Pore Volume cm3 / g
 EMI20.2
 
<tb> Annealing <SEP> before <SEP> No <SEP> annealing
<tb>
<tb> stretching <SEP> before <SEP> stretching
<tb>
<tb>
<tb> Percentage <SEP> of elongation
<tb>
<tb>
<tb> 50 <SEP> 0.144 <SEP> '<SEP> 0.070
<tb>
<tb>
<tb> 100 <SEP> 0.174 <SEP> ";

   <SEP> 0.087
<tb>
<tb> 200 <SEP> 0.224 <SEP> @ <SEP> 0.094
<tb>
<tb> 3 <SEP> 00 <SEP> 0.263 <SEP> @ <SEP> 0.096
<tb>
 
The comparative values for bulk density and pore volume illustrate the beneficial aspects due to the annealing operation prior to the stretching operation of the invention.



    Example
The polymer described in Example III is subjected to extrusion according to the procedure of this example except that the ratio

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 The stretch was 180: 1. The film thus produced was subjected to oven annealing as in Example IV and allowed to cool.

   It has been found that it exhibits the following properties:
 EMI21.1
 
<tb> Recovery <SEP> for <SEP> a <SEP> lengthening <SEP> of
<tb>
 
 EMI21.2
 50 9L at 25 C 90 fa
 EMI21.3
 
<tb> Permeability <SEP> to <SEP> oxygen <SEP> 2700 <SEP> cm3 / 24h / m2 /
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> atm / <SEP> '
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Permeability <SEP> to <SEP> nitrogen <SEP> 600 <SEP> cm3 / 24h / m2 / atm
<tb>
 
A sample of the film thus produced was stretched so as to obtain an elongation of 50% at room temperature, subjected to hot fixing in the extended state (under tension) for 5 minutes at 125 ° C.

   At the end of this period, the film was removed from the oven, allowed to cool and was found to exhibit the following properties:
 EMI21.4
 
<tb> Permeability <SEP> to <SEP> oxygen <SEP> greater <SEP> to
<tb>
<tb> 120000 <SEP> cm3 / 24h / m2 / atm
<tb>
<tb> -Permeability <SEP> to. <SEP> nitrogen <SEP> 118000 <SEP> "<SEP>" <SEP> "<SEP>"
<tb>
 
Example VI
The polymer of Example III was subjected to melt extrusion, annealed on heated rolls as follows: the roll of film produced was slit continuously between two points and collected in two rolls of a single thickness, thanks to an industrial cutting and winding device at high speed.

   Tightly wound, single rollers and continuously passed around three metal rollers maintained at 120 0, the total contact time on the hot rollers is 37 seconds. When the film separates from the heated metal rolls and is wound onto the spool, it exhibits the following properties:

   
 EMI21.5
 
<tb> Thickness <SEP> 0.019 <SEP> mm
<tb>
<tb> Recovery <SEP> to <SEP> from <SEP> of an <SEP> lengthening
<tb>
<tb> from <SEP> 50 <SEP>% <SEP> to <SEP> 25 <SEP> 86 <SEP>%
<tb>
 

 <Desc / Clms Page number 22>

 
 EMI22.1
 
<tb> Resistance <SEP> in <SEP> traction <SEP> (direction
<tb>
<tb> stretching <SEP> to <SEP> the <SEP> manufacturing <SEP> in <SEP> kg / cm2) <SEP> 476
<tb>
<tb> Module <SEP> in <SEP> kg / cm2 <SEP> (direction <SEP> of stretching <SEP> to
<tb>
<tb> the <SEP> manufacture) <SEP> 21000
<tb>
<tb>
<tb> Elongation <SEP> at <SEP> the <SEP> break <SEP> in <SEP>% <SEP> 1 <SEP> 330
<tb>
 
300 meters of film prepared in this way are continuously stretched to give it an elongation of 50% by passing it over four rotating metal rollers of 30 cm in diameter,

     the last two rotating at an angular speed 1 times 1/2 greater than the angular speed of the first two. The film is partially stabilized in the extended state by the action of a heating lamp placed between the third and fourth rollers. As the film leaves the fourth roll, it is received under tension and not given a chance to relax. The tightly wound film thus produced was subjected to heat stabilization by placing it in an oven at 125 ° C. for 1 and 1/2 hours.

   At the end of this period, the tightly wound roll of film is removed from the oven and the following properties are determined.
 EMI22.2
 
<tb> Resistance <SEP> in <SEP> traction <SEP> (direction <SEP> of stretching
<tb>
<tb>
<tb> to <SEP> the <SEP> manufacturing) <SEP> in <SEP> kg / cm2 <SEP> 735
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Elongation <SEP> (direction <SEP> of stretching <SEP> to <SEP> the
<tb>
<tb>
<tb>
<tb>
<tb> manufacturing) <SEP> in <SEP>% <SEP> 275
<tb>
<tb>
<tb>
<tb>
<tb> Module <SEP> 7000
<tb>
<tb>
<tb>
<tb>
<tb> Elastic <SEP> recovery <SEP> to <SEP> 25 C <SEP> for <SEP> a
<tb>
<tb>
<tb>
<tb>
<tb> elongation <SEP> of <SEP> 50 <SEP>% <SEP> (direction <SEP> of stretching <SEP> to
<tb>
<tb>
<tb>
<tb>
<tb> the <SEP> manufacture)

   <SEP> 84 <SEP>%
<tb>
<tb>
<tb>
<tb>
<tb> Apparent <SEP> density <SEP> determined <SEP> by <SEP> porosimetry
<tb>
<tb>
<tb>
<tb> at <SEP> mercury <SEP> in <SEP> g / cm3 <SEP> 0.755
<tb>
 

 <Desc / Clms Page number 23>

 
 EMI23.1
 
<tb> Volume <SEP> of <SEP> pores <SEP> determines <SEP> by <SEP> porosimetry
<tb>
<tb>
<tb>
<tb> at <SEP> mercury <SEP> cm3 / g <SEP> 0.228
<tb>
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> Dimensions <SEP> of <SEP> pores <SEP> determined <SEP> by <SEP> poro-
<tb>
<tb>
<tb> simetry <SEP> at <SEP> mercury <SEP> 100-1000A
<tb>
   Example VII
A sample of polyethylene having a density of 0.960 and a melt index of 0.7 was subjected to melt extrusion through a die with a thin annular opening of 1 mm and 100 mm in diameter.

   The hot tubular film thus formed is expanded 1.5 times by internal air pressure and cooled by a current of air impinging on the film from a ring of air placed around and beyond. above the die. Extrusion was performed using an extruder with a length to barrel diameter ratio of 20: 1 and a shallow thread extrusion screw.

   The extruded product is stretched very quickly to achieve a draw ratio of 100: 1 and passed between a series of rollers which flatten the tube and maintain the internal air pressure. of 30 m / min it is found that the film has the following properties:
 EMI23.2
 
<tb> Thickness <SEP> 0.012 <SEP> mm
<tb>
<tb>
<tb>
<tb> Recovery <SEP> to <SEP> from <SEP> of a <SEP> lengthening <SEP> of
<tb>
<tb>
<tb>
<tb>
<tb>
<tb> 50 <SEP>% <SEP> 45 <SEP>%
<tb>
 
The film was then subjected to oven annealing under tension at 115 ° C. for 16 hours.



   After removal from the oven, the film is left to cool, it is stretched by 50% at room temperature. Hot set in an oven, in the molten state, under tension, for 5 minutes at 120 ° C. The film treated in this manner was found to exhibit the following properties as measured by mercury porosimetry.

 <Desc / Clms Page number 24>

 
 EMI24.1
 
<tb>



  Apparent density <SEP> <SEP> 0.785 <SEP> g / cm3
<tb>
<tb>
<tb> @ <SEP> Volume <SEP> of <SEP> pores <SEP> 0.25 <SEP> cm3 / g
<tb>
<tb> @ <SEP> @ <SEP>. <SEP> @
<tb>
<tb> Dimensions <SEP> of <SEP> pores <SEP> 100 <SEP> - <SEP> 20000 <SEP> A
<tb>
 
The recovery values given above re. have the recovery determined as follows at 25 C and a relative humidity of 65%,
A 15 mm wide sample was placed in an Instron machine for testing tensile properties with the jaw gap being 5 cm. The sample is stretched at a rate of 2.5 cm / min until an elongation of 50% is obtained.



   The sample is held at this stretch for one minute and then released at the same rate as stretched. In the unloaded state,,, the help of the Instron apparatus is recorded. The elastic recovery is calculated as @ following: Total length recovery elastic final distance = when extended between the jaws x 100 length added when the film is extended
The other properties mentioned were determined according to the Standard ASTM method as follows:

   
Tensile strength ASTM D882 Method A - sample 15 mm wide
Elongation at break ASTM D882 Method A - 15 mm wide sample
Modulus ASTM D882 Method A - sample 2.5 cm wide
Vapor permeability ASTM 9663T Method B - Process B wet Permeability to O2, N2, CO2 ASTM D1434-63 Cell method. ', @ Dow except that due to the excessively high gas permeability two films are laminated together with a coating from the Company "Interchemical" "NB 483C" which has a

 <Desc / Clms Page number 25>

 thickness of 0.0025 mm.



  Density determined by the penetration of mercury. See Textile
Research Journal volume 33 .. p.21 et seq. 1963 by R.G,
QUYNN Clarity ASTM D1003 process A, see fig. 2.



    @
The values of the percentage of crystallinity given above are determined using the method described in an article by R.G. QUYNN et al. Journal of Applied Polymer Science, Vol.2, No.5, p. 166-173,1959.



   .



   The elastic films of the invention can be used as printable packaging, microporous membranes for dialysis operations, and the like.



   It is understood that the detailed description above is given merely for illustrative purposes and that modifications and variations can be made without departing from the scope of the present invention.



   CLAIMS.



   1. A film of a polymer, said film being characterized by an open cell structure and a bulk density lower than the actual density of said polymer, when said film has been stretched.

 

Claims (1)

2. - The film of claim 1, in which the dimension of the voids formed in said open cell structure is less than 5000 angtroms. 3- Film according to claim 2, characterized in that the volumetric capacity of said voids can be changed reversibly. @ <Desc / Clms Page number 26> 4, - Film according to claim 1, characterized in that the polymer has a crystallinity of at least 40%. 5.- Film according to claim 4, characterized in that the polymer is' polypropylene. 6.- Film according to claim 4, characterized in that the polymer is an oxymethylene polymer. 7.- Film according to claim 4, characterized in that the polymer is polyethylene. 8. A film of dimensionally stable polymeric material, said film having an open cell structure comprising pores of less than 5000 A and characterized by an energy-elasticity-based stretching mechanism. 9.- Process for preparing a film from a polymer with a crystallinity of at least 40% and an elastic recovery, for an elongation of 50%, of at least 60% at 25 C, characterized in that stretching said film and annealing the film under tension. 10. A method according to claim 9, characterized in that the stretching is carried out with a stretching ratio of between 1.10: 1 and 4: 1. 11. A method according to claim 9, characterized in that the polymer is propylene and the drawing is carried out at an internal temperature of 104.4 Cà. 12, - Process according to claim 9, characterized in that the polymer is an oxymethylene polymer and the stretching is carried out at a temperature below 107.2 C. 13. A method according to claim 9, characterized in that the polymer is polyethylene and the stretching is carried out at a temperature below 93.3 C. <Desc / Clms Page number 27> 14.- A method according to claim 9, characterized in that the annealing is carried out at a temperature ranging from 90 C to the melting point of the polymer, 15.- Method of manufacturing a film from a polymer, said film having an apparent density of not more than about 90% of the density of the polymer, which process comprises extruding in film form the molten polymer, the latter having a solid state crystallinity of at least 40 %, at a melt temperature exceeding the crystalline melting point of the polymer by at most 100 0, receiving the resulting film at a polymer draw ratio of at least 20: 1 and annealing the resulting film at a temperature in the range of 5 to 1000 below the crystalline melting point of the polymer for a period of at least 5 seconds and sufficient to develop recovery in the film. plastic of at least 70% of the elongation of 50% at 25 C, stretching the film at a stretch ratio of 1.25: 1 to 4: 1, and hot-setting of the film thus stretched. 16. A method according to claim 15, characterized in that the polymer is extruded in the form of a tubular film with air pressure inside which swells the tubular film to a greater diameter, the ratio of the diameter of the film blown to the diameter of the film at the exit of the extruder being of the order of 0.5 to 6.0 with a draw ratio of between 20: 1 and 100: 1 n @