US 2961342 A
Description (OCR text may contain errors)
Nov, 22, 1960 R. B. SNYDER 2,961,342
METHODS OF MAKING GAS IMPERVIOUS SHEET MATERIAL Filed Aug. 15, 1957 /2 22 1 20 26 OVEN 32 K 3 22a L 200. 3 4 7 OVEN I81) 4 m Y OVEN 7 W 76 2 H6 3 F/G f4 26 50 5 3% Arm.
United METHODS OF MAKING GAS I1VIPERVIOUS SHEET MATERIAL Filed Aug. 15, 1957, Ser. No. 678,289
8 Claims. (Cl. 117-76) This invention relates to methods of making gas impervious sheet material. More particularly, it relates to such methods of making sheet material having a high degree of impermeability towater vapor and other light gases.
It has been discovered that the thickness of the insulation required in refrigerators and like'structures can be materially reduced by employing as the insulating material an insulating unit having a pillow-like construction and comprising an outer flexible hermetically sealed envelope containing a porous hat of glass fibers together with a gas having a low coeflicient of heat transfer such as dichloro-difluoromethane in sufiicient quantity to provide substantially atmospheric pressure in the envelope.
Although it has been possible heretofore to provide flexible sheet materials from which to form the envelope which are substantially completelyimpermeable to the dichlor'o-difluoromethane, the sheet materials available for this purpose have not possessed the same degree of impermeability to the lighter gases present in the atmosphere including water vapor as Well as the materials which exist only in the gaseous state at normal temperatures and pressures. As a consequence, there is over a period of time appreciable leakage of these light gases, including water vapor, into the envelope 'Which ta'ken with the lack of any leakage of the dichloro-difluoromethane from the envelopes produces an' appreciable increase in the total pressure within the envelope. This is objectionable not only because the increase inpressure increases the danger that the envelope will beruptured at high temperatures but also for the further reasons that the increase in pressure increases the coefficient of heat transfer of the envelope and the presence of the water vapor within the envelope leads to condensation and, as a consequence, to a further increase in the coefficient of heat transfer.
It is an object of this invention to provide improved methods of making multi-ply sheet material having a high degree of impermeability to light atmospheric gases including water vapor as well as to heavier gases such as dichloro-difluoromethane.
A further object is to provide improved methods of making multi-ply sheet material suitable for use in the forming of heat insulating envelopes as above described which comprises sheet materials which in combination have a high degree of impermeability to light atmospheric gases, including water vapor, as well as to heavier gases and in which one ply is coated directly on the other without any intervening bonding layer.
Still another object is to provide an improved method of, making such multi-ply sheet materials by coating one 'are accomplished by employing as a base 'a flexible sheet of material having a uniform thickness of the Patent order of 1 to' l /zmils and formed, for example, of cellophane to which has been applied a thin anch'oring coat of'vinylidene chloride copolymer such as the cellophane sheeting sold under the trademarks:' DuPont K202 or Olin OX. Other suitable base materials include vinylidene chloride copolymer films (such as those sold under the trademark Saran) vinylalcohol film, vinyl chloride film or a vinylidene chloride copolymer film. Also, the base material may be formed of the'polyesterof ethylene glycol and terephthalic acid, sold under the tradename Mylar, a high density polyethylene, etc. Further, it is very advantageous that the base sheet be of composite construction comprising a sheet of kraft paper seal coated with one of the organic resin materials mentioned, or a composite laminate of two of such resins. Of course, the resinous coating sealing the sheet ofkraft paper, or the exterior resin layer of the composite laminate, must be formed of a resinous material that is compatible with the applied vinylidene chloride copolymer layer and to which the applied vinylidene chloride copolymer layer may be directly adhered readily when applied directly thereto from a coating solution of the character described hereinafter. In the arrangement, wherein the base sheet is formed of kraft paper, it is recommended that the kraft paper be super-calendered and of a gauge of about 40#, and that the resinous coatings be of the character previously described;
At least one side, and preferably both sides, of the flexible base is coated directly with a layer of vinylidene chloride copolymei' such as the well-known copolymers of vinylidene chloride and vinyl chloride or vinylidene chloride and acrylonitrile also in a uniform thickness of the order of l to 1 /2 mils. These relatively thick coatings are obtained by coating with a solution of the copolymermaterial which gels substantially immediately upon application'so' that the coating shows substantially no tendency to flow. Coatings having a thickness of the order of 0.7 to 0.8 mil are readily applied in this Way in a single application so that two applications are suflicient to provide the desired final thickness, the first coat being dried completely before the second is applied.
The coating solution is preferably formulated to be readily flowable only when it is heated and then maintained at elevatedtemperature'during application. Such a solution may be prepared,'for example, by dissolving the vinylidene chloride copolymer resin in a suitable volatile solvent and then diluting the resulting mixture with a less expensive volatile diluent which is usually not a solvent for the resin or polymer. For example, when using a vinylidene chloride and'acrylonitrile copolymer such as that sold under the trademark or"des ignation Dow Resin" F242, 25 parts of this resin is preferably dissolved .in 25 parts of a solvent such 'as tetrahydrofuran which. has been heated to elevated temperature, and the resulting solution is then diluted with only suflicient diluent, such as methyl ethyl ketone or the solvent sold under the trademark Cellanese Solvent 601 (ordinarily about 50 parts) to provide a: flowable solution at the temperature of operation. In general,
the coating solution preferably comprises a major pro 7 portion of a'volatile vehicle for the resin, and the vehicle in turn comprises a volatile solvent for the resin, together with a proportion of a volatile diluent, to make the solu-' tion readily flowable Whenheated;
Suitable apparatus for making the film or sheet prod the accompanying drawing which specification and in which: I
Figure 1 is a diagrammatic elevational view of ;suit-' able apparatus for making the sheet material;
Figure 2 is a view in cross section on the line 22 of Figure 1;
Figure 3 is a view in cross section on the line 33 of Figure 1;
Figure 4 is a view in cross section on the line 4--4 of Figure l; and
Figure 5 is a view in cross section on the line 55 in Figure 1.
Referring first to Figure 1, it will be seen that there is provided a supply roll of the base sheet material, such as cellophane coated on both faces with a thin layer of vinylidene chloride copolymer as above, suitably mounted to permit a continuous web 12 to be unwound from the roll. From supply roll 10 the web 12 is drawn over a rotatable roll 14 forming part of a knife coater designated generally 16 and including a hopper 18 mounted above the roll 14 and provided with a knife edge 20 below the outlet 22 of the hopper. A body 24 of coating solution such as is described above is placed in the hopper 18 and the knife edge 20 is spaced above the roll 14 to meter the coating applied to the web 12 through the outlet 22 to the proper thickness to form a layer 26 having a thickness when dried of approximately 0.7 mil. The solution 24 is preferably heated to a. temperature of about 130 to 140 F. before it is filled into hopper 18 and is also preferably maintained at a temperature in the same range while it is in the hopper by suitable heating means (not shown).
The web 12, after passing the knife edge 20 and receiving the layer 26 of vinylidene chloride copolymer resin solution, is drawn through an oven 28 provided with a source (not shown) of infrared heat. The oven 28 is constructed and arranged in a known manner completely to dry the layer 26.
From the oven 28 the web 12 is drawn around a pair of rotatable guide rolls 30 and 32 rotating in the directions indicated by the respective arrows shown thereon, after which it passes through a second coating device 16a identical with that described above. The like parts of this second coating device are numbered in the drawing with the same numbers applied to the parts of the device first described but followed by the letter a and function in the same way.
As the web 12 passes the knife edge 20a, a second layer 34 of the vinylidene chloride copolymer resin solution is applied thereto over the layer 26 to provide the desired final coating thickness on that face of the web 12 and is dried completely as the web 12 is then drawn,
through the oven 28a which is identical with the oven 28.
Two more rotatable guide rolls 36 and 38 direct the web 12 from oven 28a to a third coating device 16b and then through a third oven 28!). Still another pair of rotatable guide rolls 40 and 42 finally guide the web 12 past a fourth coating device 16c and through a final oven 280. Each of the coaters 16b and 160 is also identical with the coater 16 and corresponding parts bear the same number with a further identifying letter. Similarly, ovens 28b and 280 are identical with oven 28 above described.
As may be seen the coater 16b applies a third layer 44 of the vinylidene chloride copolymer resin solution to the face of the web 12 opposite to that on which layers 26 and 34 Were applied and coater 16c deposits a fourth layer 46 on top of the layer 44.
After leaving the oven 280, the web 12 with the dried layers of vinylidene chloride copolymer resin thereon is wound about a rotatable roll 48 which is driven in the direction indicated by the arrow shown thereon and serves to pull the web through the four coaters and the four ovens. When a suflicient thickness of the coated web is wound around the roll 48, it is removed and packaged or stored as desired.
As indicated in Figure 2 of the drawing, the web of cellophane has a thickness of 0.0013 inch, or 1.3 mils, and has applied to it in the first coater through the appropriate setting of the knife edge 20 a layer of the vinylidene chloride copolymer resin solution which has a thickness when dried of approximately 0.0007 inch, or 0.7 mil. In the second coater, the knife edge 20a is set to provide a coating of similar thickness so that the sheet as shown in Figure 3 prior to passing through the coater 16b consists of the base layer 12 and a composite layer 50 of the vinylidene chloride copolymer resin formed by the merger on drying of the two layers 26 and 34 having a dried thickness of approximately 0.0014 inch, or 1.4 mils, and adhering to the base 12. Similarly, layers 44 and 46 are applied in the same thickness and merge on drying to form a composite layer 52 of the copolymer as shown in Figures 4 and 5.
It is believed that the operation of the apparatus illustrated diagrammatically in Figure 1 will be readily apparent. A supply of coating solution, such as is described above, is placed in the containers 18, 18a, 18b and 18c and a roll of cellophane as described is provided as the supply roll 10. A web 12 unrolled from this roll is threaded through the coaters and the ovens and secured to the windup roll 48. The means for driving the roll 48 is then put into operation and as the web 12 is pulled through the coaters and ovens the desired layers of the vinylidene chloride copolymer resin are applied to the base and dried in the manner above described.
The invention is not limited, however, to the particular coating mechanism which has been described. Any desired type of roll coater or knife coater or other coating device which is capable of applying a coating of the coating solution in the desired thickness to a. base such as the base 12 may be employed for the purpose of this invention. Any desired means for drying the applied layer of coating solution, i.e., evaporating volatile solvents, may be employed for the purpose of this invention although it is preferred to employ infrared heating as described. It is also preferable to gradually increase the heat as the web moves through the oven preferably reaching a temperature of the order of 300 F. with the combination of materials used as an illustration since this provides more efficient drying of the coating. Four ovens are shown in the drawings for convenience of illustration but it should be understood that it is possible by the provision of suitable guide rolls to direct the web through the same oven more than once for the purpose of drying a plurality of coatings which are applied to give the desired final thickness.
It will also be obvious that instead of employing the windup roll 48 to draw the web 12 through the coaters and ovens any other desired means may be employed for this same purpose. For example, a pair of cooperating rolls, at least one of which is driven, may be positioned so as to grip the web after the final coating has dried and to provide the desired pull thereon. In fact, such an arrangement has the advantage that the problem of variation in lineal speed of the web resulting from variations in the diameter of the windup roll 48 is there by avoided.
Although heating means for the containers 18, 18a, 18b and 18c are not shown in the drawing, it is preferable and advantageous to heat the coating solution to keep it warm as it is applied. This prevents gelling, reduces the viscosity of the coating solution, and also eliminates condensation of water vapor during the coating operation. High temperature also increases the solvating tendency of the diluent preferably employed in the coating solu tion and makes the coating solution more stable. Heating the coating solution also makes it possibleto employ a more concentrated solution which gels rapidly after it is applied so the tendency of the applied coat to flow is minimized.
The preferred coating solution which, as indicated above, includes both a solvent for the resin and a diluent, preferably has a concentration of resin as high as possible to permit coating of 50% by weight. It is within the scope of the invention to employ both higher and lower concentrations of resin. For example, when concentrations as high as 30% to 50% are employed, such heavy solutions should be maintained under closely controlled elevated temperature in order to provide a desirable viscosity without excessive evaporation.
As indicated above, the invention contemplates the application to one of the bases described of any desired vinylidene chloride copolymer resin. Furthermore, it is within the scope of the invention to employ any desired solvents and diluents and to vary their relative proportion so long as the resulting coating solution has the proper characteristics to permit it to be coated on to a base as described.
It is an outstanding advantage of the present invention that it provides for the first time a composite film or sheet consisting of a flexible base (formed of cellophane or other suitable material) having firmly secured thereto on one or both faces a relatively thick layer of a vinylidene chloride copolymer resin material thereby providing a composite sheet material which has a high degree of impermeability, both to the light atmospheric gases such as oxygen and nitrogen, and also to water vapor. The film of vinylidene chloride-acrylonitrile copolymer is most advantageous, as it is much more impervious (about ten times) than a corresponding film of vinyl chloride-vinylidene chloride copolymer. The invention has the further advantage that it provides for the first time a method by which such a composite sheet may be made.
Such a sheet has perhaps its highest utility when it is employed to form the outer flexible hermetically sealed bag or envelope for a heat insulating unit such as is described above. Such envelopes and their formation, as well as their use for that purpose in the void defined in associated hollow structure, are described in a copending application of Thomas Steve Simms, et al., Serial No. 504,248, filed April 27, 1955, which points out that these heat insulating units when used in refrigerator constructions, for example, may be subject to a normal operating temperature range extending from about 30 F. to about +l50 F.
As indicated in that application, an envelope of the type described may be formed by taking a portion of a flexible composite sheet, such as is described in the present application, of the required area and configuration and folding it about a deformable porous mass of solid poor heat conducting material, such as a porous bat of glass fibres, with the vinylidene chloride copolymer layer forming the inner wall, after which the opposed edges are sealed together around the three sides which remain open, with a suction tube of a suitable thermoplastic material extending through between the sealed together edges at any desired point. As set forth in the above copending application, the container or bag thus formed, in which the solid poor heat conducting material is confined so that it generally fills out the volume thereof, is evacuated through the aforesaid tube and the air so withdrawn is then replaced with a gas such as dichlorodifluoromethane having a thermal conductivity or coefiicient of heat transfer lower than that of air, after which the thermoplastic tube is heat sealed to complete the closure of the envelope. The charge of gas is such as to complete the fill of the bag and thoroughly permeate through the porous mass. It has been found that such an envelope is not only eflfective to retain the charge of low thermal conductivity gas therein, thus providing a very effective heat insulator, but at the same time the imperviousness of the envelope, both as to water vapor and other light atmospheric gases, minimizes the leakage of such gases into the interior of the envelope so that its initial heat insulating quality is maintained to a high degree over long periods of time and under adverse conditions.
It is apparent that many widely different embodiments of this invention may be made without departing from the spirit and scope thereof and is, therefore, not intended to be limited except as indicated in the appended claims.
What is claimed is:
l. The method of making a composite sheet comprising applying to one surface of a flexible base sheet a uniform first layer of a solution of vinylidene chloride copolymer resin in a volatile solvent, which solution gels substantially immediately upon application, said solution being applied in said first layer in an amount suflicient to provide a first coating having a uniform thickness of the order of 0.5 to 0.75 mil when dry, drying said applied first layer, applying to said dried first coating a uniform second layer of said solution to provide a second coating having a similar thickness when dry, and drying said applied second layer.
2. The method set forth in claim 1, wherein said base sheet is formed of an organic resin.
3. The method set forth in claim 1, wherein said base sheet is formed of cellophane having an initial relatively thin anchoring coating of vinylidene chloride copolymer resin on the surface thereof to which said first layer is applied.
4. The method set forth in claim 1, wherein said base sheet is formed of kraft paper seal-coated with an organic resin over which said first layer is applied.
5. The method set forth in claim 1, wherein said solution is at an elevated temperature when said layers are applied therefrom.
6. The method set forth in claim 1, wherein said solution comprises a major proportion of a volatile organic vehicle for said resin, and said vehicle comprises a volatile organic solvent for said resin and a volatile organic diluent for said solvent.
7. The method set forth in claim 1, wherein said layers are dried by exposure to infrared heat.
8. The method set forth in claim 1, wherein said solution comprises an organic solvent for said resin and an organic diluent for said solvent, and said solution is at an elevated temperature when said layers are applied therefrom.
References Cited in the file of this patent UNITED STATES PATENTS 1,969,621 Munters Aug. 7, 1934 2,067,015 Munters Jan. 5, 1937 2,386,700 Manchester Oct. 9, 1945 2,618,575 Oswin Nov. 18, 1952 2,704,732 Copeman et al. Mar. 22, 1955 2,779,066 Gaugler et a1 Jan. 29, 1957 2,819,984 Ackerman Ian. 14, 1958 2,824,025 McIntyre Feb. 18, 1958