Polyester Films.pdf

30 POLYESTER FILMS

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POLYESTER FILMS

Introduction

78 ◦ C) was developed by ICI (1) in Europe and DuPont (2) in

the United States in the 19850s with DuPont introducing the ﬁrst commercial ﬁlm

line in the late 1950s. There was a slow increase in the number of ﬁlm manufac-

turers through the 1960s and 1970s, and production increased in the 1980s and

1990s, but in the late 1990s onwards was a major consolidation in the industry

with DuPont buying the ICI Films Business and then forming a joint venture

with Teijin, Toray acquiring Rhone Poulenc and Chiel, and joint venturing with

Saehan, and Mitsubishi acquiring Hoechst. Global capacity of PET ﬁlm in 2002

was 1,550,000 ton. There are now over 50 producers of PET ﬁlm worldwide, many

in the rapidly expanding Chinese market. DuPont Teijin Films and Toray Saehan

Inc. are the largest, with declared capacities of about 290,000 and 280,000 ton

respectively. Mitsubishi and SKC form the “second tier” with approximately half

the capacity of the top two. The next is Kolon (Korea) with about half the capacity

of Mitsubishi and SKC.

255 ◦ C; T g =

The ﬁrst patent covering poly(ethylene naphthalate) (PEN); 2 (I m =

263 ◦ C;

120 ◦ C) was ﬁled in 1948 by Cook and co-workers (3) not long after the dis-

covery of PET. However, it was not until the 1970s that the dimethyl ester of 2,6-

naphthalene dicarboxylate (2,6-NDC) became available in sufﬁcient quantities for

the ﬁrst PEN ﬁlms to be produced on a semitechnical scale. Several manufactur-

ers explored this area, with the ﬁrst PEN ﬁlm being produced in the early 1970s.

However, the raw material continued to be very scarce and costly, and the result-

ing small scale of ﬁlm production led to an extremely expensive product compared

with PET. This proved to be uneconomic for most applications and there was conse-

quently little commercialization of PEN ﬁlm until high value speciality videotapes

were found to beneﬁt from the use of PEN ﬁlm in Japan during the 1980s. As a

result of the promise of larger scale and more economic raw material supply, plus

greater interest from the end market, PEN ﬁlms were launched commercially in

the early 1990s. Since then investment in a world-scale 2,6-NDC production facil-

ity by Amoco Chemical Co. (now BP) has signiﬁcantly aided the economics of PEN

ﬁlm production (see P OLY ( ETHYLENE NAPHTHALATE ) (PEN)). Sales of PEN ﬁlm are

currently of the order of several thousand tonnes and DuPont Teijin Films is the

leading producer with its Teonex brand range of ﬁlms.

Biaxially drawn polyester ﬁlm based on poly(ethylene terephthalate) (PET); 1

( T m =

T g =

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POLYESTER FILMS 31

Fig. 1. A Typical ﬁlm manufacturing process.

The Film Process

Biaxially oriented PET and PEN ﬁlms are exclusively produced by a stenter pro-

cess where commonly the amorphous cast ﬁlm is drawn in the machine direc-

tion (MD) by passing it over heated rollers and then fed into a stenter frame

to achieve a draw in the transverse direction (TD) (see F ILMS ,M ANUFACTURE ;

F ILMS ,O RIENTATION ). A schematic of the process is shown in Figure 1.

Normally the sequence of steps is as described above (MD–TD), but the pro-

cess can be reversed (TD–MD) (4–6); and even a simultaneous stenter process (7)

whereby the clips are not interconnected and stretching can therefore be carried

out by accelerating the clips in the MD within the diverging TD draw section, has

been commercialized. Using this basic process ﬁlm with thicknesses from 0.6 to

350

(1) Polymer preparation and Handling

(2) Extrusion and casting

(3) Drawing and heat setting

(4) Slitting, Winding, and Recovery

The ﬁlm process and in particular the morphology developed during process-

ing has been described in more detail elsewhere (8,9).

Polymer Preparation and Handling. Polymer can be extruder-fed to the

drawing process or it can be directly fed from a continuous polymerizer (CP), but in

both cases the virgin polymer tends to be of a number-average molecular weight

of about 20,000 although higher and lower molecular weights are ﬁlmed. With

the extruder-fed ﬁlm lines the polymer handling involves blending and drying.

This is a consequence of the ﬁlm process never being 100% material efﬁcient and

virgin polymer is therefore blended with polymer reclaimed from the ﬁlm process.

Drying is essential in closed (single-screw) extrusion systems as the polyesters are

susceptible to hydrolysis, resulting in a reduction in molecular weight, but less

commonly, processes have evolved based on vented (twin-screw) extruders where

m can be prepared.

The conversion of polymer into ﬁlm falls into four basic stages:

32 POLYESTER FILMS

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moisture is removed just after melting. In the drying stage, polymer is crystallized

ﬁrst to avoid the chip sintering during drying and then dried for several hours

at 160–180 ◦ C to reduce the moisture level to 10–30 ppm. Close-coupled CP ﬁlm

lines do not have this stage and melt is pumped directly through ﬁltration to

the die.

Extrusion and Casting. The blended and dried polymer is next melt-

extruded through a slot die. There is usually melt ﬁltration before the die to

remove degraded polymer, gels, catalyst residues, and pipe deposits. The extru-

sion system is typically designed to deliver stable output up to about 2.6 ton/h

over a wide range of operating conditions and throughputs. Exceptionally, higher

outputs are possible, up to about 3.5 ton/h on thick ﬁlm lines, using complex tan-

dem or parallel extrusion systems (10). More recently twin-screw extruders have

been introduced on some ﬁlm lines to widen the operating window and to provide

capital-efﬁcient high throughputs. These are able to cope with a wider range of

molecular weights, improve the mixing, and have the advantage of extruding at

lower melt temperatures. Other combinations such as tandem single screws with

melt pumps are also used to give a stable output. Parallel extrusion systems are

also commonly used for high output but these present the problem of ensuring

homogeneous melt stream blending. Whichever extrusion system is employed, its

purpose is to transport a consistent ﬂow of polymer melt to the ﬂat ﬁlm die of a

stenter process.

The die which can be center- or end-fed converts the melt from a circular cross

section to a uniformly thick melt curtain of the required width. The thickness of

the ﬁlm is continuously measured across the web after the stenter process, giving

a thickness or gauge proﬁle. This proﬁle data is used to make ﬁne adjustments to

ﬂow proﬁle at the die either through thermoviscous heating or by actuation of me-

chanical bolts (which physically modulate the die gap proﬁle to achieve uniform

ﬁlm thickness proﬁle). Combinations of thermoviscous and mechanical modula-

tion are also employed in some cases.

The purpose of the casting is to produce a continuous uniformly thick ﬁlm of

noncrystalline polymer with no surface blemishes and this is achieved by drawing

down the melt curtain onto a casting drum. The polymer melt from the extrusion

system will normally be between 280 and 310 ◦ C so as to minimize crystallization,

which would increase ﬁlm haze and brittleness and possibly cause a ﬁlm breakage

later in the ﬁlming process (9). To ensure this the molten ﬁlm is cooled as quickly as

possible below its glass-transition temperature by cooling the casting drum using

recirculated water which passes through a heat exchanger to control its tempera-

ture between typically 10 and 15 ◦ C. Thin ﬁlm can be satisfactorily cooled using a

single drum, normally of size 600–900 mm in diameter, but for thicker ﬁlms where

the insulating properties of the ﬁlm prevent cooling through to the air (nondrum)

contacting side of the melt, a second drum is used to provide additional cooling.

As the casting drum rotates, air is drawn into the gap between the ﬁlm of

melt and the drum, affecting the contact of the two surfaces and the effectiveness

of the cooling. This is avoided by electrostatically charging the ﬁlm surface by

using a pinning wire or blade electrode stretched across the drum just below the

die face (11–13). This creates an electrostatic ﬁeld around the wire or blade which

induces a charge on the melt curtain surface. Since the drum is earthed the charge

forces the melt curtain onto its surface.

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POLYESTER FILMS 33

The surface of the casting drum must be of a very high standard so as to

avoid imprinting any patterning or “graininess” onto the cast ﬁlm. The surface

must also be hard in order to avoid damage and be resistant to corrosion so that

no pitting occurs. Therefore a drum that is hard chrome plated and highly polished

is usually favored. During operation the casting drum must be free of vibration

and rotate smoothly so as to minimize any source of variation in thickness in the

MD of the ﬁlm.

Drawing and Heat Setting. The cast ﬁlm initially passes through a pre-

heat zone, where the temperature of the cast ﬁlm is raised by passage over a series

of heated contact rolls until a point, usually about 15 ◦ C above its glass transition,

T g , where the material can be readily stretched.

The forward draw stage physically stretches the heated polyester ﬁlm be-

tween two nip roll systems with a surface speed differential and is designed to

improve its tensile properties in the MD. Stretch ratios of around 3.5:1 are em-

ployed, and the stresses in the structure caused by this step align the molecular

chain segments in the direction of the stress and thereby raise its tensile modulus

and strength by a factor of about 3.

In the second stage of the stenter oven, the edges of the ﬁlm web are clipped

and led along diverging rails that cause the material to be stretched at tempera-

tures above 100 ◦ C (135 ◦ C for PEN), for the second occasion, by a factor of between

3 and 4. The object of this step, the sideways draw, is to develop the properties of

the ﬁlm in the TD via orientation at the molecular level, to a point where they

balance or approximately balance those measured in the MD (9). The process

tends to align molecular chain segments not already aligned in the MD and to

realign some MD-oriented crystallites toward the TD. The ﬁlm at this stage is

anisotropic.

The ﬁnal stage in the stenter oven is designed to develop a crystalline mor-

phology in the ﬁlm which retains the improved mechanical properties from the

drawing stages and which is more stable over time and at elevated temperature.

The heat set or crystallization stage of the process comprises three or more

regions of the stenter oven, each with independent temperature control and the

capability to adjust the lateral dimension of the web. Thus ﬁlm can be treated to

a range of thermal and strain programs to optimize its ﬁnal properties. Temper-

atures of the ﬁlm can exceed 230 ◦ C and although residence time may be only a

few seconds this is sufﬁcient for density changes equivalent to a rise of 30–40%

in crystallinity to occur. On the same timescale, the noncrystalline regions of the

ﬁlm can exhibit signiﬁcant molecular relaxation.

Unless all physical anisotropy can be removed from the noncrystalline frac-

tion of biaxial PET ﬁlm, the product will undergo residual shrinkage at elevated

temperature. By managing both the ﬁlm temperature and a relaxation of strain,

achieved by a small convergence of the stenter rails (known as toe-in) during the

heat set stage, it is possible to achieve considerable control of this ﬁlm property (9).

Slitting, Winding, and Recovery. The ﬁlm in and close to the clips is very

thick and cannot be wound into ﬁlm. This is slit off as the ﬁlm exits the stenter

and reclaimed for reprocessing into ﬁlm. It is combined with scrap ﬁlm and is

either cut up into ﬂake and compacted into particulate form or is reextruded and

formed into pellets. This reclaimed polymer either is fed back in with the virgin

polymer at the start of the ﬁlm process or is fed into the CP process.

34 POLYESTER FILMS

Vol. 11

The edge trimmed ﬁlm is then wound up. Rolls of ﬁlm can be produced either

on the stenter, or alternatively they can be slit down off line to the width and

lengths required.

Surface and Bulk Properties Control

Film Properties. The ﬁlm process described produces rolls of PET and

PEN ﬁlms that have the properties required for a standard PET or PEN ﬁlm,

ie high mechanical strength, good ﬂexibility, excellent visual properties, ﬂat and

dimensionally stable, and available in a range of thicknesses. The difference in

chemical structures of PET and PEN is shown by 1 and 2 . The substitution of

the phenyl ring of PET by the naphthalene double ring of PEN has very little

effect on the melting point ( T m ), which increases by only a few degree Celsius.

However, there is a signiﬁcant effect on the glass-transition temperature ( T g ),

which increases from 78 ◦ C for PET to 120 ◦ C for PEN. The result of this is that

although the good thermal properties of PET and PEN ﬁlms enable them to retain

physical, chemical, and electrical properties over a wide temperature range, PEN

has signiﬁcantly improved thermal resistance relative to PET. This is particularly

noticeable with regard to PEN’s higher continuous use temperature (14) Table 1.

The typical properties listed in Tables 1–5 are from DuPont Teijin Films

Teonex PEN ﬁlm datasheet and are for illustrative purposes only and are not

intended to be used as design data.

Table 1. Thermal Properties of PET and PEN

Teonex ® PEN ﬁlm PET ﬁlm standard

Sample thickness,

Q51-25

Grad-25

Test method

Melting point, ◦ C

269

—

258

— DSC

Glass-transition temperature, ◦ C

121

—

Shrinkage (150 ◦ C, 30 min), %

0.4

—

1.5

— JIS C-2318

(modiﬁed

to TDF)

0.0

—

0.2

—

Shrinkage (200 ◦ C, 10 min), %

2.0

—

4.0

— Ditto

1.0

—

1.5

—

Coefﬁcient of thermal expansion

(10 − 6 /RH%), –

—

— TDF method

Coefﬁcient of hydrolic expansion

(10 − 6 /RH%), –

—

— Ditto

Continuous use temperature, ◦ C

Mechanical

160 (

≥

25)

— 105 (all)

— UL 746B

Electrical

180 (

≥

25)

— 105 (all)

—

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