CA1133328A - Method for coating a tubular casing - Google Patents

Abstract

12,087 METHOD FOR COATING A TUBULAR CASING The method comprises supporting an inflated casing between secured ends so as to establish a state of tension while exposing the inflated casing to a cloud of electrostatically charged particles of a predetermined resinous powder material of predetermined size, subjecting the coated casing to a temperature sufficient to sinter the coating in less than 5 minutes and cooling the sintered coating.

Description

~333z~ 12,087 This invention relates to composite casings snd more particularly to a method of forming a pin-hole free coating of resinous polymer upon a tubular cellulosic casing .
Tubular regenerated ceIlulose and fibrous casings are used in the food industry for processing food products such as meats, sausage, turkey, etc. A fibrous casing is composed of regenerated cellulose reinforced with a cellulosic fiber in the form of a paper preferably a long fiber hemp paper. The food product is stuffed into the casing and processed in situ. The casing serves as a container during processing of tne food product and ~s a protective wrapping for the finished product. Since there are so many differences in recipes for making processed food products, such as s~usages and so many diffexent modes of processing the different products, it is difficult to provide a casing w;~ich is accep~able for ; all uses. There are also several casing applications where low moisture vapor transmission with or without low oxygen transmission are extremely important. Accord-ingly, it is desirable to coat fibrous and cellulose casings with a polymer resin particul~rly to satisy gas and vapor permeability requirements.
Heretofore composite casings have been formed by covering a conventionally extruded tubular cellulose or - fibrous casing with a coating of a preferred res~n composi-eion prepared from a solution or dispersion. The resin composition is applied by a procedure of dipping, spraying, .. . . . . . . .. . . . . ... . ... ... .. . .... .. . .. ... . . . . .... . ...

~ 12,087 slugging, gravure coating, or doctoring the solution or dispersion directly onto a surface of the tubular cell-ulose or fibrous casing.
In the conventional solution and emulsion coat-ing processes, heat must be applied gradually to the coated rasing t~ vaporize the solvent and at an adequate temperature to effect sintering. Rapid drying of the casing may result in entrapment of solvent or water between the casing and the coating leading to "pin-holes" and/or blisters in the coating. A "pin-hole" free coating is defined for purposes of the present invention as a continu-ous film essentially free from voids. The drying rate is therefore a limiting factor controlling the length of time required to sinter the coating i.e., flow and coalesce to form a continuous, tenaciously adherent coat-ing on the casing surface. In addition, the sintering temperature cannot be too high as this could cause desic~
cation. Accordingly, the drying operation must be care-fully controlled and monitored, since it is a principal factor in establishing the operating speed and it plays an important role in applying a uniform and continuous coating thickness.
- Applicant has discovered, in accordance with the present invention, a new method of forming a rela-tively thick pin-hole free coating of a polymer resin upon a tubular cellulose or fibrous casing which eliminates the drying step in conventional solution and emulsion coating process~ thereby providing increased flexibility over the time of exposure to heat, operating speed and sintering temperature. In particular the sintering r 3 . . , . . , . .. , .. , . .. . . , . . . . , , ~ . . . .. . ..

12 ,087 time may be substant~ ally reduced relative eo the time required in conventional processes.
The present process also provides control over the uniformity in coating thickness. Thickness varia-tions of less than about + 30% from the measuret average thickness have been readily attained with the process of the present invention whereas prior art variations extend to about 80-100% from average.
The process of applicant's invention for cozting the exterior surface of a casing comprises: inflating the tubular cellulose or fibrous casing; securing the in-flated casing from opposite ends thereof such that the inflated casing is held in a state of tension; exposing the casing to a cloud of electrostatically charged part-icles of a resinous polymer material having an average particle size less than 125 microns for a period of time sufficient to form a surface deposit of such polymer material around the casing periphery; subjecting the coated casing to a temperature sufficient to sinter said coating in a time period of less than about 5 minutes; and cooling the sintered coating.
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The process of applicant's invention for coating the interior surface of an inflated tubular cellulose or ~ fibrous casing comprises: securing the inflated casing from '~ opposite ends thereof such that the inflated casing is held in a state of tension; introducing a slug of miCrOn~
ized resin powder into the interior of such ;:

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l ~ n~7 inflated casing; generating an electrostatic field external of said casing and adjacent said slug of resin powder so as t3 form a deposit of said powder on the inside surface o the casing; advancing the casing at a prede~ermined rate past a sinteringstation; subjecting the coa~ed casing within said sintering seation to a temperature sufficient to sinter said coating about the interior of said casing; and cooling the sintered coating.
It is accordingly, the principal object of the present invention to provide a method for forming a rela-tively thick pin-hole free coating of a resinous ` material upon the surface of an extruded tubular cellulose or fibrous casing.
Additional objects and advantages of the pres-ent invention will become apparent from the following descriptions when read in conjunction with the accompany-ing drawings of which:
, Figure 1 is a schematic representation of ~n exemplary system for carrying out the method of the invention for coating the exterior surface of a casing;
Figure 2 is a schematic representation of the system of the present invention for coating the interior of a casing; and Figure 3 is a cross section of the casing taken along line 3.3 of Figure 2.
,. The cellulose or fibrous casing iO, as identi;
fied in Figure 1 of the drawing, is a conventional casing of tubular geometry made by any conventional extrusion process upon which a pri~er operating as a suitable ad-hesion promoter has been applied.

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12, 087 PrLmer materials which h~ve been found compatible with t~e process of the present invention include eh~ foll-owing compositions: polyhydroxylated alkoxy alkyl mel~mine complexes~ triazine amine formaldehyde complexes, ethylene imine type compound, &nd the condensa~ion product of a polyamide with epichlorohydrin or a polyamine-polyamide with, epichlorohydrin or a polysmine with e~ichlorohydrin, The extruded tubular casing 1~ preferably with a primed surface,is thereafter flaetened snd wound onto 8 feed roll 12 whereu~on. if desired, it may be stored before ini~iating the coating method of the present invention Coating of the tubular casing 10 is accom~lished by passing the casing 10 fro~ the feed roll 12 throug~ a coating and sintering operation at a conerolled sDeed to a pic'.c up roll ~ 16 as hereinafter described. A section 20~ representing a predetermined length of tubular casing 10. is controllably inflated to a predetermined pressure by introducing ~ir into the casing and trapping the air between the two ends 22 and `
24 of the section ~0. The end 22 is squeezed between a pair of nip rolls 26 and 28 whereas ~he end 24 of the section 20 is squeezed between a pair of nip rolls 30 and 32 respectivel~.
Although the casing may be held in either a horizontal or a vertical plane during the coating operation it is preferred to have the casing aligned in a vertical , plane. If the coating is applied to a casing aligned in a horizon~al plane, the ca~ing may sag since the casing can not be supported un~il the resinous powder is anchored , !

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1~33~28 12,087 permanently from the sintering process. When the casing - is permitted to sag it becomes more difficult to apply a uniform coating and/or to assure uniform sintering. This problem of sagging is further aggravated at increased coating speed since the sintering time must remain constant, thereby increasing the length of un-supported casing. This length of unsupported casing is avoided or minimized by passing the casing 10 through a preheater 25 before exposing the section 20 to the elec-trostatic cloud 33. The preheater 25 should be located upstream of the coating chamber 31 either preceding or following the inflated end 22 of section 20 of the casing 10. Preheating the casing increases the degree of ad-herence between the electrostatically coated powder particle and the casing surface as will become more evi-dent hereafter in connection with the discussion of the coating operation.
The pair of nip rolls 30 and 32 is spaced at a predetermined distance above the pair of nip rolls 26 and ; 20 28 in a common substantially vertical plane so that section 20 is held in the preferred substantially verti-cal position during the sequence of operations for coating and sintering the section 20. It is also preferred to -~ maintain the section 20 under at least some tension during treatment by a differential nip roll operating speed.
The inflated and preferably preheated section 20 is advanced at the co~trolled speed through a coating chamber 31 in which the exposed section Ls subjected to a , 7 : . . .. . . . .. .. . . . .. . . . . . . .. . . ... .

~33~3~28 12,087 cloud 33 of electrostatically charged resinous polymeric particles for forming a coating of such particles about the periphery of the section 20. Preheating of the section 20 enhances adhesion of the coated particles by initiating sintering within the chamber 31.
The cloud 33 of electrostatically charged particles may be established by use of an electrostatic spray gun 34 as exemplified in the drawing or by means of an electrostatic fluidized bed. In each case an electrostatic field is established in which the resin particles are charged and prcpelled to form the electro-static cloud. Upon dispersement,the electrostatic cloud is attracted to the tubular casing 10 which is maintained at ground potential.
In utilizing the electrostatic spray technique, it is preferred that at least two conventional electro-static spray guns 34, 36 be employet on opposite sides o the tubular section 20 during the coating operation with one of the guns preferably elevated relative to the other. The guns are used to charge and propel the powdered resin particles which form the electrostatic cloud 33.
The particle size of the polymeric material has been found to be a critical parameter in the spray coating process. A particle size range of less than 125 microns but preferably between 20-80 microns was found necessary to form a uniform relatively thick deposit of particles.

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~333~8 12,087 In addition, it was found that certain electro-static spray parameters such as spraying distanc`e, powder flow rate and spray time must be maintained within pre-determined ranges to achieve a relatively thick and evenly distributed deposit around the tu~ular section 20. The spraying distance or distance between the outlet nozzle of each of the spray guns 34 and 36 respectively should be maintained between about 6-9 inches from the tubular section 20. The powder flow rate should be held preferably between 2-5 grams per second from each spray gun 34 and 36 respectively. The spray time is determined by the rate of travel of the section through the electro-- static spray chamber. The rate of travel may then be - varied to establish the desired thickness of deposit.
When the coating thickness was under about .5 mils, pin-holes were observed in the finished coating.
In addition to the preferred vertical disposi-tion of the casing 20 in the electrostatic chamber, and the selection of spray parameters and particle size range, it is necessary that the section 20 of tubular casing be in-flated to a pressure which maintains the casing fully inflated, and preferably between 10-S0 inches of water9 during both the coating and sintering sequence. The in-flation of the tubing, particularly within the preferred range, not only assists in assuring an even distribution of particles but prevents shriveling of the casing due to loss of moisture during the relatively fast sintering operation.

~333Z8 12,087 The preheating of the casing is also important in that sintering may actually be initiated for promoting adhesion berween the electrostatic par~icles and the casing within the coating chamber.
Sintering of the electrostatically coated casing occurs upon passage of the casing through a stack of radiant heaters 40 for a period of less than 5 minutes and preferably less than 3 minutes at a suitable temp-erature of, for exa~ple, 400F to effect sintering. The sintering period can be reduced to under thirty seconds at a higher sintering temperature of about 510F.
Cooling of the sintered coated section of the tubula casing is preferred before passage through the nip rolls 30 and 32. A preferred method of cooling is to use an air ring 42 for passing ambient air at a controlled flow ' rate about the sintered coating. The section 20 is pro-gressively being renewed with the coated casing at the end 24 being reflattened and wound up on the take-up roll 16 while the uncoated casing upstream of end 22 is being advanced through nip rolls 26 and 28 until the entire tubing is coated with a continuous pin-hole free coating about its exterior surface.
Resinous polymers suitable for use in coating the casing of the present invention include "polyolefins", ionomers, polyamides, polyesters, polyacrylonitriles, "vinyl polymers" and epoxy resins. By polyolefins we mean polymers such as polyethylene, ethylene acrylic acid and ethylene viny~ acetate. By vinyl polymers we mean poly-v~nyl chlor~de, polyvinylidene chloride and the copolymers .. . .. .. .. .. . . . .. . .

11333~

of vinylidene chloride. As used herein the term polymer includes homopolymers, copolymers, terpolymers, block copolymers and the like. Examples of polyvinylidene -chloride copolym~rs include vinylidene chloride poly-~erized with such materials as vinyl acetate; vinyl chloride; alkyl acrylate or methacrylate such as methyl, ethyl, propyl, butyl, isobutyl; acrylonitrile; meth-acrylonitrile; styrene; and the like or mixture of two or more of these compounds. ..
The resins uset as coatings may include suitable plasticizers, stabilizers, slip and anti-blocking agents, pigments and other additives which are well known in the art.
The polyvinylidene chloride resin (PVDC) compo-sition includes more than 50% vinylidene chloride and prefera~ly between 70-95% vinylidene chloride. The following Table shows the spraying conditions for pin-hole free coatings with a PVDC resin coating composition and a polyethylene coating composition.

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11333~8 12087 C
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Figure 2 is an illustration of the preferred procedure for establishing the coat~ng on the interior side of the inflated section 20 of casing 10. ~or sim-plicity of explanation the same reference numbers have been used to identify corresponding elements between Figure 1 and Figure 2~
The flat casing 10, internally coated with a primer, is held in tension between the two sets of nip rolls 26, 28 and 30 and 32 respectively in the same manner ag explained heretofore with respect to Figure 1. A
slug of micronized resin powder 48 is in-troduced ~nto the casing 10 within the inflated section 20.
The powder coating composition is equivalent to tha~
taught earlier for coating the exterior of the casing 10.
An electrostatic field can be established by several methods using for example a high voltage AC or DC source or by means of a corona discharge. Figures 2 and 3 show one technique for imposing a high voltage using for pur-pose of illustration a pair of annular electrodes 50 and i - 20 52. The electrodes 50 and 52 surround the outer surface of the inflated section of casing 20 at a location pre-ferably in the vicinity of the top of the column of resin powder 48 and are electrically connected to a high volt-age generator 54. An electrostatic field of desired strength is generated about the column for electrosta-., .

~133328 12,087 tically charging the powder 48 adjacent the electrodes 50 and 52 through ind~ctlon. The powder h8 will be electrost~tically attracted ~o the casing 20 to form a surface deposit wh~ch forms a uniform coating upon pass-ing the coated cas~ng through the sintering station 40 as explained heretofore in connection with Figure 1.

It may also be desirable to preheat the casing 10 using a preheater 25 to increase the degree of adherence between the electrostatically coated particles and the casing surface during the coating step.

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Claims (15)

12,087 WHAT IS CLAIMED IS:

1. A process for forming a pin-hole free coating of a resinous polymeric material on the exterior surface of an extruded tubular cellulose or fibrous casing to form a continuous, tenaciously adherent coating on said casing surface comprising the steps of:
inflating said casing;
securing the inflated casing from opposite ends such that the inflated casing is held in a state of tension;
exposing said inflated casing to a cloud of electrostatically charged particles of said polymer material having an average particle size of less than 125 microns for a period of time sufficient to form a surface deposit of said material around the casing periphery with a coating thickness equal to at least about .5 mils;
subjecting said coated casing to a temperature sufficient to sinter said coating; and cooling said sintered coating.

2. A process as defined in claim 1 further comprising the step of preheating the casing prior to said exposure step.

12,087

3. A process as defined in claim 2 wherein said casing is inflated to an internal pressure of between 5-50 inches of water.

4. A process as defined in claim 3 wherein said casing is exposed to a cloud of electrostatically charged particles of said polymer material having an average particle size range of between about 20-80 microns.

5. A process as defined in claim 4 wherein said resinous material is a polymer selected from the group consisting of polyolefins, ionomers, polyamides, polyesters, polyacrylonitriles, vinyl polymers and epoxy resins.

6. A process as defined in claim 5 wherein said resinous material is bonded to said casing through a primer material selected from the class consisting of polyhydroxylated alkoxy alkyl melamine complexes, triazine amine formaldehyde complexes, ethylene imine type compound, and the condensation product of a polyamide with epichloro-hydrin or a polyamine-polyamide with epichlorohydrin or a polyamine with epichlorohydrin.

7. A process as defined in claim 6 wherein said resinous material comprises a composition containing at least 50% vinylidene chloride.

8. A process as defined in claim 7 wherein said opposite ends of said inflated casing are aligned in a substantially vertical plane.

12,087

9. A process for forming a pin-hole free coating of a resinous material on the interior surface of an inflated tubular cellulose or fibrous casing to form a continuous, tenaciously adherent coating on said casing surface comprising the steps of:
securing the inflated casing between opposite ends such that the inflated casing is held in a state of tension;
introducing a predetermined measure of a resin powder into the interior of said inflated casing at a predetermined location between said opposite ends;
generating an electrostatic field external of said casing and adjacent said resin powder so as to form a deposit of said powder on the inside surface of the casing;
advancing the casing at a predetermined rate past a sintering station;
subjecting the coated casing within said sintering station to a temperature sufficient to sinter said coating; and cooling the sintered coating.

10. A process as defined in claim 9 further comprising the step of preheating the casing upstream of said predetermined location.

12,087

11. A process as defined in claim 10 wherein said resinous material is a polymer selected from the group consisting of polyolefins, ionomers, polyamides, polyesters, polyacrylonitriles, vinyl polymers and epoxy resins.

12. A process as defined in claim 11 wherein said resinous material comprises a composition containing at least 50% vinylidene chloride.

13. A process as defined in claim 12 wherein said opposite ends of said inflated casing are aligned in a substantially vertical plane.

14. A process as defined in claim 13 wherein said resinous material is bonded to said casing through a primer material selected from the class consisting of:
polyhydroxylated alkoxy alkyl melamine complexes, triazine amine formaldehyde complexes, ethylene imine type compound, and the condensation product of a polyamide with epichloro-hydrin or a polyarnine-polyamide with epichlorohydrin or a polyamine with epichlorohydrin.

15. A composite tubular cellulosic casing comprising a tubular cellulosic layer and a resinous polymer layer bonded to said tubular layer wherein said resinous polymer layer has a pin-hole free coating thick-ness of at least about .5 mils and a surface uniformity which varies less than about ? 30% from its measured average thickness formed by the process of:

12,087 inflating said casing;
securing the inflated casing from opposite ends such that the inflated casing is held in a state of tension;
exposing said inflated casing to a cloud of electrostatically charged particles of said polymer material having an average particle size of less than 125 microns for a period of time sufficient to form a surface deposit of said material around the casing periphery of said section with a coating thickness equal to at least about .5 mils;
subjecting said coated casing to a temperature sufficient to sinter said coating; and cooling said sintered coating.