Films, Orientation.pdf

FILMS, ORIENTATION

Introduction

Molecular orientation provides a wide range of improved properties for thermo-

plastic ﬁlms. Most often, molecular orientation results in signiﬁcantly higher

physical properties such as tensile strength and modulus. Orientation may im-

prove thermal properties of a ﬁlm by increasing the crystallinity. Optical prop-

erties are inﬂuenced by orientation. Films with high clarity to high opacity may

result from orientation. The polarization and reﬂective responses of ﬁlms are also

impacted by orientation. Enhanced electrical properties, such as increased dielec-

tric constants or piezoelectric properties, may result from orientation. Orientation

begins with a cast web suitable for stretching. This web may be stretched in one

direction, two directions sequentially, or two directions simultaneously.

Cast Web Considerations

Extrudate Uniformity. The quality of the cast web has a great inﬂuence

on subsequent orientation steps. While in the extruder, the polymeric melt needs

to become homogeneous. Thermal gradients, poor blending of additives or poly-

meric blends, and entrapped gases may result in imperfections in the cast web.

During orientation, these imperfections may lead to high stress concentrations

and fracture of the ﬁlm. Selection of the proper extrusion system, screw design,

temperature proﬁles, venting, and homogenizing equipment will improve the ori-

entation capability of the cast web.

Orientation of multilayer ﬁlms is a common practice. The various melt

streams either ﬂow together in a feedblock prior to entering the die or in a

559

560

FILMS, ORIENTATION

Vol. 2

From

Skin

Extruder

From

Core

Extruder

From

Skin 1

Extruder

From

Core

Extruder

From

Skin 2

Extruder

( a )

( b )

Fig. 1. ( a ) Feedblock and ( b ) die combining melt streams.

multicavity die (Fig. 1). As the layers ﬂow together, viscosity and speed matches

are critical. If the layers do not have a good match, interfacial instabilities occur.

These ﬂow effects produce minor to severe thickness variations in the extrudate.

These variations are typically very localized. During the orientation process, sig-

niﬁcant stress concentration may be generated around these thickness variations,

resulting in fracture of the ﬁlm.

Quench. Cooling the melt to solidify the viscous extrudate is an important

process parameter. Depending on the orientation process, the melt quenching pro-

cess may be very different. The two primary extrudate geometries are a ﬂat sheet

and a tube. Most often, heat needs to be removed from the extrudate as quickly

as possible. For semicrystalline polymers with a relatively slow crystallization ki-

netics, like PET, PEEK, or PPS, rapidly quenching the melt results in a molecular

amorphous state. For semicrystalline polymers with a relatively fast crystalliza-

tion kinetics, like polyethylene, polypropylene, or nylon 6, rapidly quenching the

melt allows for the growth of smaller spherulites.

The ﬂat sheet extrudate emerges from the die and falls on a rotating drum

or continuous belt (see Fig. 2). The rotating drum or chill wheel cools the melt

as quickly as possible. The internal ﬂow design of the chill wheel must pro-

vide a uniform cooling surface to the melt. If there are zones of different tem-

peratures, the extrudate will cool at different rates. This could lead to areas of

Vol. 2

FILMS, ORIENTATION

561

Die

Air Knife

Vacuum

Box

Vacuum Seals

Cast

Wheel

Cast

Wheel

( a )

( b )

Die

Ground

High Voltage

Wire

Cast

Wheel

( c )

Fig. 2. ( a ) Air knife quenching, ( b ) vacuum box quench, ( c ) electrostatic pinning, ( d ) nip

roll quench, and ( e ) water bath.

the cast web with different levels of crystallinity or spherulite size. The chill

wheel’s surface topography will impact the quench rate and smoothness of the ﬁnal

ﬁlm.

At high line speeds found in industrial applications, air entrapment between

the melt and chill wheel can greatly retard the cooling of the melt. Cooling the

melt at different rates in small zones may lead to nonuniform stretching and may

even result in fracture of the web during orientation. There are several methods

available to improve the heat transfer from the melt into the chill wheel. A slot

with high pressure air may be directed at the melt just after it hits the chill wheel.

This “air knife” process presses the melt against the chill wheel to increase the heat

562

FILMS, ORIENTATION

Vol. 2

Die

Air Knife

Nip Roll

Cooling Water

Cast

Wheel

Cast

Wheel

( d )

( e )

Fig. 2. (Continued )

transfer. The location, angle of incidence, and air ﬂow are the major parameters to

consider. Improper tuning of the air knife may result in a cast web with excessive

ﬂutter and resultant caliper variations. Uniform pressure or air ﬂow across the

entire melt curtain will lead to a uniform cast web.

A second method of pressing the melt against the chill wheel is through

electrostatic forces (1). Passing a high voltage through a wire suspended just

above the melt produces charged particles which are attracted to the ground,

ie, the chill wheel. Electrostatic pinning may work better for some materials or

thinner cast webs. Deposits on the wire over time will reduce its effectiveness.

Continually feeding new wire will help eliminate this source of variability in the

process.

A third approach to improve the heat transfer to the chill wheel is to re-

move the air between the melt and chill wheel. By applying a vacuum between

the die and melt, a more intimate contact between the melt and chill wheel oc-

curs. The vacuum box needs to have seals on the edge of the die in order to

produce a strong enough vacuum. Uniformity of the vacuum across the melt

curtain will result in a more uniformly quenched cast web. Too high a vacuum

will result in the melt curtain being pulled backwards. This could result in the

melt developing scratches in it by being dragged across the lip of the die. Ide-

ally, the vacuum should be adjusted to have the melt drop straight away from

the die.

A fourth method of assuring good contact to the chill wheel is to mechanically

nip the melt against the chill will. This nip roll may also have to be cooled. The

surface of the nip usually needs to have some ﬂexibility in it. The cast web may

not be perfectly ﬂat and a nip roll will need to press on the entire surface of the

melt. High temperature rubber sleeves on the nip roll are most often used, but

these may result in producing a rough surface.

The chill wheel may also be partially submerged in a bath to provide ad-

ditional cooling from the second surface of the extrudate. The ﬂow in a water

Vol. 2

FILMS, ORIENTATION

563

bath must be adjusted with care. Continually circulating the water will increase

the effectiveness of the quench. High pressure water jets can help keep pres-

sure on the web against the chill wheel. These jets can also deform the surface

of the cast web. If they are at the wrong angle, the web could be pulled away

from the chill wheel, reducing the quench rate. If this technique is used, the

chill wheel needs to be dried off by the time it rotates to the point where melt

is again placed on it. If it is too wet, steam bubbles may form creating serious

nonuniformities in the cast web. A nip roll or air knife has been used to accom-

plish the drying of the roll. Combination of both may be required in high speed

operations.

When extruding a tube through an annular die there are two kinds of orien-

tation processes available, blown ﬁlm process and tubular ﬁlm process. Blown ﬁlm

conducts the orientation in the melt state. The tube is rapidly pulled away from

the die by a nip at the top of a tower. Air is pumped through the annular die to

inﬂate the tube and to provide additional cooling. The molecular orientation pro-

duced in blown ﬁlm is quite low compared to solid-state orientation. The molecules

are above their melt and have very fast relaxation times. One often refers to the

frost line in a blown ﬁlm process. This is the point where the melt crystallizes.

The polymeric web goes through a clear to hazy transition at this point. Further

molecular orientation in the web in this stage of the process typically does not

occur.

In the tubular ﬁlm process, or double bubble process, the extruded tube en-

compasses a cooled mandrel and is pulled away by a nip (Fig. 3). The mandrel

should not impart scratches in the tube. This ﬁrst “bubble” is usually quenched

as rapidly as possible for the same reasons in ﬂat ﬁlm. Controlling the air pres-

sure inside the ﬁrst bubble is another handle used to determine the quench rate.

A water bath on the outside of the tube provides additional cooling of the extru-

date. Water ﬂow around the tube is very important. Too great an impingement

against the melt may result in surface defects. A tube quenched in this manner

will allow subsequent orientation to occur in the solid state at signiﬁcantly lower

temperatures, resulting in higher molecular orientation.

Melt-State Orientation—Blown Film

The blown ﬁlm process, as shown in Figure 4, orients the molecules while they

are in the melt state. Inﬂating the melt bubble provides the orientation. The air

pressure in the bubble is maintained to achieve a certain “blow up ratio,” the ratio

of bubble diameter to the die diameter. The strain rates are relatively low and

relaxation times are very fast. The subsequent molecular orientation obtained via

this process falls between the high levels of orientation obtained by stretching

in the solid state and very low levels obtained in standard casting operations.

The orientation occurs during elongational ﬂow of the melt. Although elonga-

tional ﬂow is much more effective at orientation than shear ﬂow, this process has

limitations.

There are many methods to increase the amount of orientation frozen

in during the blown ﬁlm process. Systems have been developed to approach

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: