Method of forming a blocked cross-plied polymer film

A method is disclosed for forming a blocked cross-plied polymer film by extruding a polymer melt through a tubular rotary die to impart a molecular orientation of the polymer in the transverse (TD) direction during extrusion, expanding the film, and then blocking it by pressing opposing walls together to produce a film in which at lease two layers thereof have transverse molecular orientations which cross to form a balanced cross-plied film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for forming a blocked cross-plied 
polymer film. More particularly, it relates to a tubular extrusion and 
blocking method which produces a polymer film having a reduced machine 
direction (MD) orientation and an increased transverse direction (TD) 
orientation of the polymer molecules in two extruded layers, with the 
transverse direction orientation of the molecules of one layer of the film 
crossing the transverse direction orientation of the molecules of another 
layer of the film to produce a balanced cross-plied film. 
2. Discussion of the Prior Art 
Generally, when a single-layer or laminated plastic film is formed, for 
example of a high density polyethylene, it is extruded from a tubular 
extrusion die having stationary die lips. The extruded film is primarily 
unbalanced as the molecules of the polymer melt are principally oriented 
in a longitudinal direction of the film, commonly called the machine 
direction (MD). The molecular orientation in the transverse direction of 
the film is low. As a result, the resulting film has low strength to 
stresses applied in a direction deviating from the machine direction of 
the film. 
In order to improve the physical strength and characteristics of an 
extruded film, attempts have been made to reduce the unbalanced film 
orientation by achieving a greater transverse direction molecular 
orientation and a reduced machine direction molecular orientation in the 
extruded film so as to achieve a better balance of the two. 
Generally, transverse direction orientation of the molecules can be 
produced to some degree by blowing the film after extrusion thereby 
stretching it in a transverse direction. However, the limits to which a 
film can be expanded, the socalled blow up ratio (BUR), without breaking 
or becoming too thin severely limits the amount of transverse orientation 
which can be imparted during the expansion process. 
Attempts have been made to achieve a balanced cross plied structure in the 
wall of an extruded film by rotating an entire die during extrusion of a 
film. This technique is disclosed in U.S. Pat. No. 4,358,330. While this 
technique has some merits in producing a transverse molecular orientation 
which can be further enchanced during expansion of the film, the die and 
supporting apparatus which are required are complex and expensive to build 
and maintain. Moreover, since the molecular orientation imparted by die 
rotation takes place outside the die, this too limits the amount of 
transverse direction orientation which can be obtained, as any such 
orientation must be done before the film frost line is reached. 
Furthermore, excessive twisting of the thermoplastic melt will cause the 
tubular film to collapse, making it difficult to implement this type of 
orientation technique. 
It has also been attempted to bond separate film layers together each 
having different molecular orientation patterns in order to produce a 
resultant film of desired structural characteristics. This method does not 
use in-line techniques since the film layers are separately produced and 
processed and then bonded together. Since the process is not in-line, it 
requires additional processing and handling of the film layers which is 
costly and undesirable. 
SUMMARY OF THE INVENTION 
The present invention has been designed to overcome the foregoing problems. 
One object of the invention is to provide a method for forming a polymer 
film which has at least two layers in which the molecular directional 
orientation of one layer crosses that of the other layer to provide a 
cross-plied film structure having improved strength properties. 
Another object of the invention is to provide a method for producing a 
cross-plied film structure which is completely in line and which does not 
require out of line processing steps. 
These and other objects, features and advantages of the invention will be 
more readily perceived from the following detailed description of the 
invention which is provided in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a schematic representation of a rotary die 3 and, more 
particularly, an interior annular flow passage 41 defined by an inner die 
part 5 which is rotatable relative to an outer die wall 7. The annular 
extrusion passage 41 defined by the inner die part 5 and outer die wall 7 
terminates in an extrusion gap. The rotation of inner die part 5 imparts a 
molecular orientation to a film as it passes through the die and before it 
reaches the extrusion gap. The directional component of this molecular 
orientation is illustrated by arrows 2 and 4. Although for simplicity the 
directional components 2 and 4 are drawn above the portion of the die 
illustrated in FIG. 1, it should be appreciated that actual molecular 
orientation in the illustrated directions occurs as the melt flows through 
the annular extrusion passage 41. A tubular film extruded through rotary 
die 3 will have a TD/MC orientation which is more balanced than if the 
extrusion were effected through a completely stationary die. The TD/MD 
balance can be regulated by regulating the RPM of inner die part 5 and the 
speed of machine direction extrusion, which in turn controls the amount of 
transverse direction molecular orientation of the extruded melt. 
Additional TD molecular orientation can then be imparted during 
conventional blown expansion of the film by a gas pressure subsequent to 
extrusion. 
In the invention, a rotary die, as schematically represented by FIG. 1, is 
utilized to impart the desired transvere direction orientation to an 
extruded film. The extruded film is then blocked by pressing and adhering 
opposing walls of the film together to yield a cross-plied film. 
The film which is extruded by the FIG. 1 die is shown in greater detail in 
FIG. 5, wherein opposing walls of the film have transverse direction 
orientations crossing one another. 
FIG. 4 illustrates the overall extrusion and blocking performed in 
accordance with the teachings of the invention. The film extruded through 
rotary die 3 is expanded by a conventional pressurized gas blowing 
technique and is then blocked by means of a blocking station 9 which 
contains a pair of nip rolls 8 for pressing opposing walls of the tubular 
extrusion together. If the walls of the extruded polymer have sufficient 
adhesion characteristics, the resulting film from the blocking station has 
two layers which are integrally connected, as shown in FIG. 6, with each 
layer having an orientation pattern in the transverse direction which 
crosses that of the other, as clearly illustrated in FIG. 5. 
The blocked film has a unified cross-plied structure which provides an 
improved static property to the film while also sufficiently improving its 
high speed tear properties resulting in resistance to puncture and tears. 
This can be varied by adjusting the adhesion quality between the walls and 
by the relative rpm of the rotating member. The overall film has an 
enhanced toughness, strength and puncture resistance over conventional 
tubular extruded films, and can be formed into high strength bags by 
overlapping two layers of the blocked cross-plied film and heat sealing 
them together. 
If the extruded polymer does not have sufficient self-adhesion to allow 
blocking to occur, the extrusion through die 3 may be of a two or more 
layer laminate with the inner layer of the extruded tubular film being an 
adhesive layer so that the nip rolls of the blocking station 9 press the 
adhesion layers together to form the resultant blocked cross-plied film 
structure. FIG. 7 illustrates a blocked structure having an inner layer 89 
formed of an adhesive material. Typical adhesive materials which may be 
used include ethylvinylacetate, ethyleneproplyene rubber, polybutadiene 
and Surlyn (T.M. Dupont Chem. Co.). Typical materials which can be used as 
the outer extruded layers include polyethylene, polyproplyene and 
polystyrene. 
FIG. 8 illustrates a six layer film which may be produced in which the 
inner two layers are formed of an adhesive material which bonds to itself 
and to a surrounding thermoplastic layer. 
A suitable die which can be used to create the tubular extrusion which is 
blocked to form the FIG. 8 film is shown in FIGS. 2 and 3 of the 
application. 
This die includes structures for forming three layers of polymer, which may 
be similar or dissimilar, in an annular extrusion passage 41 so that the 
extruded film has a layered wall structure. If all polymer melts are the 
same, the extrusion would effectively be one uniform layer of the same 
polymer. However, the die illustrated in FIG. 2 could also be used so that 
an inner melt layer is adhesive to facilitate later blocking of the 
extruded and expanded film. 
The construction of the die illustrated in FIGS. 2 and 3 will now be 
described. 
The die includes an outer die body 15 having an interior peripheral wall 65 
which defines one side of an annular flow passage 41. The other side of 
annular flow passage 41 is formed by an outer peripheral surface 67 of a 
rotary wall 45. The annular flow passage 41 terminates at a die orifice 
formed by an inner die lip 11 and an outer die lip 13 respectively 
provided at the rotary wall 45 and outer die body 15. 
A thermoplastic polymer melt is introduced into the annular flow passage 41 
by a plurality of annular melt inlet passages 59a, 59b and 59c. These 
annular melt inlet passages are formed in a distribution plate 35 and a 
melt seal/distribution block 37 and are respectively connected to melt 
inlet orifices 69a, 69b and 69c. 
The polymer melt flows into the annular flow passage 41 from the annular 
melt inlet passages 59a, 59b and 59c through respective groups of holes 
61a, 61b and 61c provided in the melt seal/distribution block 37. These 
holes, which have openings into annular flow passage 41 equally spaced in 
each group, are shown in greater detail in FIG. 2. Each group of holes, 
e.g. 61a, is on a fixed common radius from the die axis. The different 
groups of holes 61a, 61b and 61c are each on a different radius, as shown 
in FIG. 2. In addition, the holes of one group are shifted in a 
circumferential direction, i.e. radially offset, relative to the holes of 
another group, so that respective holes 60a, 60b, 60c from all three 
groups align on line 62, as illustrated in FIG. 3. 
The arrangement of the groups 61a, 61b and 61c of holes in the melt 
distribution block 37 causes polymer melts respectively introduced at 
inlet orifices 69a, 69b and 69c to be layered in the annular flow passage 
41 to thus form a layered co-extrusion of the melts. The manner in which 
this layering is achieved, and the manner in which it is affected by die 
rotation, will be described in greater detail below. 
Distribution plate 35 includes a bearing 29 which provides thrust support 
and radial location of a rotary wall input shaft 17. 
The die further includes the rotary wall input shaft 17 in which is formed 
a gas passage 19 which extends throughout the entire axial length of the 
die. Gas passage 19 is used to blow and expand an extruded polymer film, 
as well known in the art. 
A sprocket 21 is attached to the rotary wall input shaft 17 so that the 
former drives the latter in rotation. A suitable driving source (not 
shown) is coupled to sprocket 21 by means of a driving chain. 
A bearing retainer 27 is provided which supports both the bearing 29 and 
the sprocket 21. Driving movement of shaft 17 by rotation of sprocket 21 
in turn causes rotation of rotary wall 45. 
The stationary melt seal/distribution block 37 which surrounds shaft 17 is 
connected with the distribution plate 35. The melt seal/distribution block 
37 has a cylindrical upper portion which has on its outer circumferential 
periphery a screw thread 39 forming flight channels of an extruder-type 
seal. The other part of the extruder type seal is formed by the inner 
peripheral surface 73 of the rotary wall 45. The screw threads 39 and wall 
73 are arranged such that rotation of rotary wall 45 by drive shaft 17 
causes an extruder effect which forces any polymer melt tending to escape 
from the annular flow passage 41 through a gap 55 existing between the 
bottom of rotary wall 45 and top of melt seal/distribution block 37 back 
into the annular flow passage 41. The extruder-type seal is highly 
effective in preventing loss of polymer melt even when it is under 
considerably high pressure. 
A die orifice adjustment ring 47 is provided which is fixed to the outer 
die body 15 and and which is adjustable in position to properly set the 
width existing between the inner die lip 11 and outer die lip 13 about the 
entire die orifice. 
As noted, the holes which are provided in the melt seal/distribution block 
37 open into the annular flow passage 41 in the manner illustrated in FIG. 
2. Each group of holes is respectively fed from one of the annular melt 
inlet passages 59a, 59b and 59c which are connected to respective melt 
inlet orifices 69a, 69b or 69c. As a result, different polymer streams 
respectfully emanate from each of the groups of holes 61a, 61b and 61c. 
This causes a layering of the polymer streams in the annular flow passage 
41. If rotation is imparted to rotary wall 45, the respective polymer 
streams will be uniformly distributed in flow passage 41 circumferentially 
of the die, but will form individual layers within annular flow passage 
41. As a result, an extruded polymer film is produced having a number of 
layers of uniformly distributed melt corresponding to the number of melt 
streams introduced into annular flow passage 41. In the die illustrated in 
FIG. 1, three such flow streams will be present; however, it should be 
appreciated by those skilled in the art that the number of flow streams 
(hole groups and annular flow paths) may be reduced or increased depending 
on the layering effect desired in the extruded film. 
It is found that even a moderate degree of rotation of rotary wall input 
shaft 17, e.g., approximately 2 RPM, is sufficient to produce a uniform 
layering of the polymer streams in the extruded film. 
Because a uniform layering of the melt streams is produced upon rotation of 
the rotary wall 45, the layer ratios or thickness of the extruded polymer 
streams can be controlled solely by the flow rates of the polymer streams 
through the melt inlet orifices 69a, 69b and 69c. Additional, complex, 
internal die structures are not required to regulate layer thickness or 
distribute a melt circumferentially. 
The melt pressure in gap 55 which serves to load bearing 29 also has a 
tendency to cause melt to be squeezed out of the die and into the space 
between the stationary support member 49 and the inner peripheral surface 
73 of rotary wall 45. If high melt pressures are involved, this would be a 
difficult leakage path to seal. To seal this path, an extruder-type seal 
is employed with the screw threads 39 provided on the outer peripheral 
surface of melt seal/distribution block 37 cooperating with the rotating 
inner peripheral surface 73 of the rotary wall 45. The inner peripheral 
surface, in effect, acts as the barrel of an extruder during rotation 
forcing any melt in the area between the stationary block 37 and movable 
wall 45 back through gap 55 and toward the annular flow passage 41. 
As is apparent from the foregoing description, when the FIG. 2 die is used 
with wall 45 rotating and the resulting extrusion is expanded and then 
blocked by nip rolls 8, a blocked cross-plied polymer film is produced in 
an in-line operation, without requiring any further processing steps. The 
method is simple and may be used with presently existing blowing and 
blocking structures without requiring a considerable increase in equipment 
cost or expense. The resulting film produced by the method has improved 
structural characteristics which are not obtainable with a usual tubular 
extrusion of a polymer melt. Moreover, a lower blow-up ratio (BUR) can be 
used to impart a desired transverse direction molecular direction 
orientation to the film due to the molecular orientation produced during 
extrusion. 
While a preferred embodiment of a method of forming a blocked cross-plied 
polymer film has been described, it should be apparent that many 
modifications can be made to the invention without disparting from the 
spirit and scope thereof. Accordingly, the invention is not limited by the 
foregoing description, but is only limited by the scope of the claims 
appended hereto.