Note: Descriptions are shown in the official language in which they were submitted.
~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,
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~ 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
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12 ,087
33Z8
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.
J
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
;:
.. . . . ..... .
11333Z8
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.
S
s
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11333Z8
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
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~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
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12 087
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
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J~333Z8
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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-
.,
.
13
~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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' 14
