Note: Descriptions are shown in the official language in which they were submitted.
11 I
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-IP+ 1 PROCESS FOR COMPRESSION ROLLING OF POLY~ERIC FIL~iS
2 This invention relates to films formed from sheets of
3 polymeric materials. More particularly, it relates to the pro-
4 duction of polymer films by the continuous cold rolling of
5 thermoplastic polymer in sheet form.
6 Films made from polymer materials, and particularly
7 from synthetic organic thermoplastic polymers such as polyethylene
8 and polypropylene, have found widespread utility in such diverse
9 fie~ds as packaging, construction, magnetic tape recording and
10 photography. However, there has been a long-felt need among film
11 processors and users for polymer films having improved physical
12 properties such as strength, stiffness and clarity.
13 Various methods have been developed in the past for
14 enhancing the physical properties of already-formed (e.g~, cast,
15 extruded or skived) films. For example, the film may be fed into
16 a quenching bath immediately after having been formed by melt-
17 extrusion or casting. In addition, films may be stretched in one
18 or more directions or calendered at temperatures above the soft-
19 ening point or range of the polymeric material by means of heated
20 rollers. However, these post-formation procedures for improving
....
21 the characteristics of polymer film~ have drawbacks which limit
- 22 their usefulness in many cases. Thus, stretching methods tend to
enlarge any pin holes or voids which may be present in the poly-
24 mer film as originally prepared, thereby decreasing the moi~ture-
25 barrier properties of the film and diminishing its usefulness for
26 many packaging purposes. Moreover, calendering often fails to
, 27 achieve the degree of property enhancement desired, particularly
:,.. . . . :
.,: .:: - , . : ::
6~i
with regard to film clarity and color uniformity.
Another method which has been investigated with a view
toward the processing of pre-formed polymer films, and to which
the present invention is directed, involves the compression roll-
ing of thermoplastic sheet material which in effect extends and
orients the polymer molecules within the latter. Previous
efforts toward developing processes for the compression rolling
of plastic are described in Williams et al., SPE Journal 17,
42-48 (1971) and in U.S. 3,504,075, U.S. 3,194,863, U.S.
3,083,410, and Re. 27,404. These methods generally involve
"full fluid rolling" (i.e., rolling with the use of a layer of ~
lubricant between the film and roller surfaces). In this tech- ~ ;
nique, the surfaces of the plastic sheet material and the
rollers at the "nip" (i.e., the point at which compression
actually takes place) are separated by the lubricant which
forms a "hydrodynamic wedge" between the rollers and the sheet
material in front of the nip as the material passes between
the rollers.
In hydrodynamic or full fluid lubrication the surfaces
in relative motion (i.e., the work roll surfaces and polymeric
sheet material) are separated at all times by a continuous
uninterrupted fluid lubricant layer so that at no time is there
actual physical contact between opposing surfaces. In practice,
however, it is often impossible or disadvantageous to maintain
a continuous plastic film rolling operation under hydrodynamic
lubrication conditions. Thus, hydrodynamic lubrication is
limited by the adverse effect of the applied loads or pressures.
; An increase in the applied load, a frequent requirement for ~r
;; achieving a desired degree of reduction in the polymeric film
with resultant improvements in the film clarity and physical
: properties, requires a compensating increase in the viscosity
of the fluid lubricant and/or an increase in the rolling speed.
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~;~
Inasmuch as increments of applied load may require proportion-
ately much larger increases in the fluid viscosity and/or roll-
ing speed, the compression rolling of polymeric films under
conditions of hydrodynamic lubrication imposes serious practical
limitations in successfully compression rolling under high roll
pressures. Thus, increasing the rolling speed causes the gen-
eration of unwanted heat as a result of the additional work
done on the fluid film. On the other hand, for a given rolling
speed and applied laod, there is only one optimum value of the
lubricant viscosity under hydrodynamic conditions.
The foregoing problems are solved according to the
present invention by the cold compression rolling (i.e., rolling
at ambient temperature) of pre-formed polymer sheet or film
material between rollers which exert a pressure on the sheet at
the "nip" or roll contact area which is sufficient to effect a
substantial reduction, between about 5 and 95 percent, in the
thickness of the sheet in a single pass. A fluid can be used in
the manner described hereinbelow to facilitate the passage of
the sheet between the nip of the rollers. This fluid, which
is not a "lubricant" in the normal sense of the word, can be
applied to either the polymer sheet material directly or placed
on the rollers so as to transfer it to the material as it passes
between the rollers, or both.
It is a feature of the present invention that the com-
pression rolling operation is carried out under conditions of
lubrication known as semi-boundary (semi-fluid, non-hydrodynamic)
and boundary (dry, non-fluid) lubrication. The latter condition
includes, as the extreme case, dry rolling. In every form,
however, the process of the invention is characterized by actual
surface-to-surface contact between the rollers and the plastic
sheet material. The primary purpose of any liquid which may be
used is to serve either as a coolant and temperature regulator
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~$62~
or as a means of establishing and maintaining frictional con-
tact between the film and roll surfaces rather than as a true
hydrodynamic lubricant. The compression rolling of plastics
under these conditions of lubrication has been found to elimin-
ate serious disadvantages characteristic of compression rolling
under conditions of hydrodynamic lubrication and, additionally,
results in important processing benefits.
It is a further feature of the present invention that
the "film rewind tension", i.e., the tension which the film
rewind rollers exert on the film emerging from the mill or work
rolls, is kept as high as possible without exceeding the elastic
limit or yield strength of the polymer film. The optimum rewind
tension for a particular sample of polymer film can be deter-
mined empirically by plotting yield strength versus degree of ;~
film thickness reduction. It has been discovered that by
; operating under the aforesaid rewind tension, an unexpected
enhancement in physical properties such as yield strength,
resistance to water penetration and film clarity are realized.
In this connection, the process of the invention can be con-
ducted at rewind tensions which are slightly below the vicinity
of the elastic limit of the film provided that the concomitant
loss in the aforesai~ properties or the decrease in the degree
of film reduction can be tolerated for the particular use for
which the film is intended. It is also important to control
the adjustment of the film unwind tension concurrent with the
rewind tension in order to ensure a proper rate of feed to the
work rolls.
The work rolls used in the process of the present invent-
ion are preferably precision flat profile work rolls (no "crown")
and in this regard they differ from conventional metal rolling
work rolls which usually have a deliberate convex "crown" where-
in the diameter of the roll increases slightly from end-to-center.
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~ ~`r;~
In some cases, it may be desirable in the practice of the
present invention to employ work rolls having a concave crown,
wherein the diameter of the roll decreases slightly from end-to-
center.
In contrast to the work rolls, the idler rolls are ad-
vantageously provided with a very slight convex crown to pre-
vent film wrinkling; the degree of crown depending on the
width of the film being rolled. In addition, the idler rolls,
with the exception of the rewinds, should have as high a sur-
face finish as possible which has been found to further de-
crease the tendency of the polymer film to develop wrinkling.
One way of achieving a high surface finish on the idler rolls
is to apply a coating of fluorinated polymeric olefin (e.g.,
"*Teflon"). An alternative technique suitable for use in
the present invention is the use of crowned "herringbone"
idlers.
Without wishing to be bound to theory, it is believed
that the success of the present invention is due in part at
least to the behavior of the polymer film and whatever fluid
may be present between the rollers. In this connection, it
is helpful to consider the essential nature of the three
-~ primary types of lubrication, namely, hydrodynamic, semi-
:
~ boundary, and boundary lubrication. In so doing, reference
`-~ is made to FIGS. 1 and 2 of the drawings wherein the qualita-
.:
tive relationships of the major tribological parameters of
lubricant viscosity (Z), rolling speed (N) and applied load
or pressures between the rollers (P~ are shown. FIG. 1 is a
plot of the dimensionless parameter, ZN/P, versus the co-
efficient of friction, ~. FIG. 2 depicts the corresponding
variation of ZN/P with lubricant film thickness, h. The three
factors of ZN/P, coefficient of friction, and lubricant film
thickness are related to and determine the three basic types
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*Trade Mark for polytetrafluoroethylene
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of lubrication, namely, hydrodynamic, semi-boundary, and
boundary.
Hydrodynamic lubrication occurs when the values of
lubricant viscosity (Z), rolling speed (N), and pressure be-
tween the rollers (P) are such as to form a fluid film which
generates sufficient pressure to separate the roller surfaces
from the surfaces of the sheet of material passing therebetween.
Referring the FIG. 2, it can be seen that hydrodynamic
or full fluid lubrication becomes operative when the value of
ZN/P is sufficiently large so as to produce a fluid film of
maximum thickness. These are also the conditions under which -~
the coefficient of friction is at a minimum value, as shown
in FIG. 1. Further increases in the value of ZN/P have no
further beneficial effects in terms of increased lubricant
film thickness. Instead, the thickness of the lubricant film
remains approximately constant while the coefficient of friction
continues to increase. Thus, it can be seen that one of the
practical disadvantages of compression rolling under conditions
of hydrodynamic lubrication is that there is only one optimum ~-
value of the N parameter. On the other hand, as the value
of ZN/P decreases, the coefficient of friction (~) is no longer
a linear function of ZN/P but rather, begins to increase as
- the fluid film thickness (h) decreases. As ZN/P continues to
decrease, we enter into a range in which the lubricating con-
ditions are defined as semi-fluid or semi-boundary lubrication.
(See FIG. 1). In this region, lubrication is neither hydro-
dynamic nor is it boundary lubrication; rather, it involves
elements of both types of lubrication. With a still further
decr2ase in ZN/P, the region of boundary lubrication is attained.
30 In this region of lubrication, a continuous fluid film no longer
exists. The frictional and load bearing capabilities of the
lubricant under conditions of boundary lubrication are now
--6--
primarily functions of the properties of the solid surfaces
involved, including the surfaces of the polymeric films, the ~ -
work rolls, and the lubricant itself which is interposed be-
tween these surfaces.
Thus, it is possible to utilize higher applied work
roll loads when rolling under semi-boundary and/or boundary
lubrication conditions than is practicable when compression
rolling under the hydrodynamic conditions taught in the prior
art. In particular, as the ZN/P conditions operative with the
use of fluid lubricants approaches the area of semi-boundary
lubrication, such lubricants become increasingly ineffective
and even inoperative. The only remedy if hydrodynamic lub-
rication is to prevail is that of increasing the value of
ZN/P by increasing the viscosity of the inert fluid, increas-
ing the rolling speeds, by decreasing applied loads on the
work rolls. These measures are counter-productive in practice,
particularly when it is necessary and desirable to conduct
compression rolling under conditions of high work roll load-
ings, and/or take advantage of heat control properties of low
viscosity fluids.
, The unique compxession rolling technique of the present
invention, which is outside the scope of hydrodynamic lubricat-
ion, provides substantially increased flexibility in the choice -:
and application of the operating parameters and in the produc-
- tion of better quality films. A major advantage of the invention
is that it permits utilization of some of the beneficial charact-
eristics of hydrodynamic lubrication without the attendant dis-
advantages, while also providing the superior virtues of semi-
., ,
~ boundary and boundary lubrication. In this regard, a salient
30 consideration is that of the desirability of physical contact
between the roll surfaces and the polymeric film surfaces, such
contact being impossible in hydrodynamic lubrication. The
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ability to compression roll satisfactorily with solid-to-solid
contact between the work rolls and the polymeric film improves
the smoothness and related optical properties of the polymeric
film surfaces. The flow of the polymeric film between the
work rolls is also more effectively controlled in the absence
of a hydrodynamic film and in such cases the polymer film
surface itself provides the necessary "lubrication".
In this connection, it has been found that the advant-
ages of the invention are only realized when the circumferent- ;~
ial speed of the work rolls is essentially equal to the linear
speed of the plastic material passing therebetween. More ~
particularly, it is important that the work roll circumferent- ~ -
ial speed he equal to the entering linear speed of the poly-
meric sheet material plus an incremental amount resulting from
the reduction in gage as the film exits from the work rolls. ;
This is due to the fact that as the polymeric material passes
through the "neutral" roll contact area, the speed of the ~`
,i
polymeric film exiting from that area will increase by an
amount equivalent to the lengthening of the film by virtue of
the reduction in film thickness and by an incremental amount
: due to the phenomenon of forward extrusion. In order to
realize this state of affairs, it is necessary to prevent
any slippage between the surfaces of the work rolls and poly-
mer film, since work roll speeds which are either excessive or
.,i
significantly less than the other mill operating parameters,
-~ will greatly increase the tendency toward breakage of the
-,
polymeric film in the work roll-film contact area. The avoid-
ance of such slippage and the degree of reduction per pass can
be enhanced by selecting a work roll surface having a coeffic-
ient of friction appropriate to that of the polymer being rolled.Thus, conventional alloy tool steel work roll surfaces can be
used to roll plastics of average coefficient of friction; work
~ ~, .
,. -i
rolls coated with a low-friction material such as a fluorin-
ated polymeric olefin (e.g., "Teflon") can be used for roll-
ing of polymer film having a high coefficient of friction;
hard rubber-clad work rolls whose coeffecients of friction
are relatively high can be advantageously employed to roll
more slippery films such as those made from polyolefins.
Relatively high coefficient of friction work roll surfaces
can also be achieved by the use of highly polished chrome
plated, nickel plated or any conventional alloy steel work
roll which can take and retain a high polish, the degree of
polish required to achieve a desired coefficient of friction
being determinable on a case-by-case basis. In addition, so-
called "non-lubricant" or "anti-lubricant" fluids such as
aqueous solutions of inorganic silicates can be used in lieu
of increasing the coefficient of friction of the work rolls.
;~ As a desirable option, the film emerging from the work
- rolls can be subjected to lateral tension, e.g., by use of a -
"tenter frame", in order to improve the properties of the film
in this direction, since the work rolls ordinarily contribute r
; 20 to the properties of the rolled film primarily in the direction
:;
in which the film travels. The use of a tenter frame in the
practice of the present invention is an attractive feature com-
pared to conventional compression rolling of polymeric materials
since a rolled film in which the physical properties are en- ;~
hanced in the lateral as well as in the direction of rolling
has greater applicability in a wider range of uses than a film
with only unidirectional improvement in properties.
It is also possible to use the cold compression rolling
~ .
process of the invention to produce polymeric netting from
plastic sheet netting material. An unexpected advantage which
is realized through this approach is the superior physical
properties of the product, both laterally and transversely,
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which presumably result from the fact that the elements or
"fibers" in the netting are oriented at about a 45 degree
angle to the direction of rolling. Cold compression rolled
netting produced according to the present invention is useful,
for example, in making sacks for fruits or vegetables.
In rolling pre-formed polymer film to achieve a re-
duction in thickness according to the present invention, it
is desirable to employ a starting polymer material which meets
fairly precise control of gage dimensions, both from front- ~ -
to-back and from side-to-side. In order to realize this, it
may be desirable to "pre-condition" the starting film prior
to cold compression rolling, with a light reduction rolling or
conditioning pass using heated rolls such that the polymer,
e.g., polyethylene or polypropylene, is subjected to a temp~
.
erature of between about 150-250F. and preferably about 200F.
- The achievement of semi-boundary or boundary lubricat-
ion conditions in the cold rolling of plastic sheet material
according to the present invention can be achieved in practice
by virtue of the fact that the specific nature of the lubricant
20 does not affect the operating characteristics of a full fluid
cold rolling process. Only when the conditions of semi-boundary
and boundary lubrication are achieved d~ the properties of the
; lubricant affect the performance of the operation. Therefore,
a change in the composition of the lubricant during a cold
rolling process will serve as an indicator of whether or not
the process conditions of the present invention have been
realized. Thus, the incorporation of so-called "oilyness agents"
or "antiwear agents" (e.g., long-chain fatty acid salts) into
a lubricant under semi-boundary or boundary lubrication con-
ditions will cause the lubricant's coefficient of friction to
drop, thus necessitating a decrease in the film rewind tension.
This phenomenon is not observed when operating under conditions
--10--
of full-fluid lubrication.
The conversion of a given full fluid (hydrodynamic)
plastic cold rolling process to the semi-boundary or boundary
lubrication method of the invention is conveniently brought
about by increasing the unit load on the rollers, decreasing
the linear speed of the plastic sheet material through the
rollers, decreasing the diameters of the rollers, or in-
creasing the rewind tension on the sheet emerging from between
the rollers. Under conditions of boundary lubrication, the
surfaces of the work rolls and the rolled plastic film emerg-
ing from the roll nip are dry to the touch even when the
operation is accompanied by the use of a fluid coolant or "non-
lubricant". In contrast, a layer of fluid is clearly dis-
cernible to the touch on the aforesaid surfaces when the rolling
.
is conducted under full fluid lubrication.
~ The use of non-inert fluids and materials which possess
; desirable properties as lubricants under conditions of semi-
boundary lubrication is illustrated in FIG. 3 wherein it can
be seen that the incorporation of additives such as long chain
polar compounds into the fluid permits extension of the effect
of hydrodynamic lubrication into the semi-boundary lubrication
area even though the film thickness has now become thinner than
that associated with full hydrodynamic lubrication.
Examples on non-inert fluids and materials which possess
desirable properties as lubricants under conditions of semi- ~
boundary lubrication suitable for use in the present invention ;
are natural fats including vegetable, animal and marine com-
pounds, long chain fatty acids, alcohols, amines, amides, poly-
ethylene gylcols, esters of these and of various acids and
alcohols, and the like. When used as such, they act as hydro-
dynamic fluids in the same fashion as any inert fluid of
equivalent viscosity properties, but additionally, are effective
--11--
~2e6
lubricants under semi-boundary conditions.
In addition to fluid, it has been discovered that cer-
tain solids are likewise effective in the compression rolling
of polymeric plastic films. In the absence of any other fluid,
water can be used in conjunction with these solids for purposes
of heat control. Examples of suitable solids found to be use-
ful are polytetrafluorethylene (Teflon), polyamides, polycarbon-
ates, polyacrylates and methacrylates. Solid films of colloid-
al graphite, colloidal molybdenum sulfide as such or pre-applied
to the work roll surfaces with suitable bonding agents are also
~ .
effective under certain desirable operating conditions. It
has been further discovered that the combined use of fluids such ;
.. ~
as the long chain polar compounds with non-polar fluids is also ~
' effective in the practice of the present invention. - -
~' The following examples are intended to illustrate, ~- -
" ~ .
' without limitation, the cold rolling process of the present
invention and the advantages thereof.
Example I
`' Compression Rolling: Semi-Boundary Lubrication
'' 20 A roll of high density polyethylene film (density = 0.9
. ~,, .
' to 0.99) 23.25 inches wide and 0.016 inch thick is mounted on
... .
an unwind spool at the entry side of a 4-hi cold xolling mill.
The roll diameters are 9 inches and the face width of each roll
is 27 inches. The work rolls are provided with a chrome-nickel
alloy finish and have a precision flat profile (no crown). The
unwind spool is equipped with a brake or clutch whereby the
polymeric film can be fed to the work rolls under a wide range
of extensive (as opposed to compressive) stresses across the
entire width of the film.
The film is threaded through the work roll and taken up
on a rewind spool. The rewind spool is adapted to enable the
film winding speed to be varied in relation to the peripheral
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~9t~2~
speed of the work rolls which permits the film exiting from the
work rolls to be sub~ected to a range of uniform extensive
stresses across the full width of the film.
The take-up spool is activated and the gears of the
work rolls are engaged to a speed of 125 rpm. The polymeric
film in the contact areas of the two work rolls is subjected
to increasing vertical pressures exerted through the work roll
screw-down elements. The unwind and rewind tensions on the
film are simultaneously adjusted to produce a compression -
rolled polymeric film of the desired thickness having greater
flatness (i.e., uniform gauge across the width of the film),
optimum clarity and optimum physical properties. The film
entering the work rolls is flooded on both the top and bottom
:: . .
sides with water for purposes of cooling.
Under the foregoing conditions, the exit gauge of the
film is 0.004 inch, representing a single-pass reduction in ;~
- gauge of 75 percent (i.e., reduction to 25 percent of the
entry gauge).
Example II
~,
Compression Rolling: Boundary (Dry) lubrication
The procedure in the preceding example is repeated ex-
.
cept that instead of flood cooling, the work rolls are pre-
` conditioned in the following manner.
The work roll surfaces are thoroughly degreased with
the aid of an organic solvent such as naphtha, methylethyl -~
ketone, toluene, benzene and the like. The work rolls are then ``
vapor blasted by either conventional wet or dry blasting tech-
niques using as the preferred grit aluminum oxide particles of
Tyler mesh size in the 150 to 200 range. The vapor blasting is
conducted so as to produce a surface finish in the range of 20
to 30 microinches. Finally, the work roll surfaces are coated
with a dispersion of a 1:4 to 4:1 blend of finely divided MoS2
(submicron to not more than 10 micron particle size) and micro-
`s? -13-
- ~
62~
ni~zed graphite in a phenolic thermoplastic resin binder. ~ -
This coating is preferably applied by spraying, e.g., with an
artist's air brush or commercial spray nozzel in 2 or 3 passes
to produce a coating having a uniform thickness of between
0.0002 and 0.0005 inch. The applied coating is then air cured
until the surface is dry to the touch or, preferably, by
exposure to infrared or other heating means at a temperature
of between 200 and 250F. for a period of time of between 15
and 30 minutes.
The compression rolling is carried out without the use
of any flood cooling fluid. The polymer film and/or work roll
^~ surfaces are sprayed only as needed with a fine spray of water
for the purpose of controlling the heat generated by the friction
between the film and the work roll surfaces.
Example III
Compression Rolling: Boundary (Dry) Lubrication
i~ The procedure in the preceding example is repeated ex-
cept that the preconditioning of the work roll surfaces i5
~` carried out in the following manner to provide a dry, pre-
~- 20 lubricated surface on the work rolls.
After degreasing and grit blasting of the work roll
surfaces, the latter are sprayed with an extremely find
;~ dispersion of TFE fluorocarbon in an inorganic binder and then
cured. A suitable commercial formulation is *Molykote 523
manufactured by Dow Corning.
The foregoing examples are presented for the purpose
of illustrating the process of the present invention. It is
understood that changes and variations can be made therein with-
out departing from the scope of the invention as defined in the
following claims.
~ *Trade Mark for molybdenum disulfide lubricant
