Polyester Films E-Book

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Beschreibung

This volume presents a comprehensive review of key aspects of polyester film technology, ranging from first principles to practical applications Bringing together world-class experts to review the state-of-the-art of key materials and processing elements of polyester film technology, Polyester Films covers a wide range of topics with direct utility to students, practitioners, business managers and researchers, in academia and industry. Topics covered in this volume include survey of optical and physical properties, microlayer coextrusion, polyester ionomers, polyester blends, biomedical applications and recycling . In particular, the text focuses on novel design and application of polyester films, such as those used in the production of flat panel displays, flexible electronics, and barrier films. The overriding objective of the book is to scope the multitude of options available to material and product designers in manipulating the properties of polyester films to meet specific performance and product criteria. These options include synthetic modifications (copolymerization), physical enhancements (blending), and process upgrades (tenter-frame changes, coextrusion, and coating). Edited by two highly qualified material scientists with extensive experience in academia and industry, Polyester Films covers topics such as: * Historical review of polyester film technology * Overview of physical performance and applications of key polyester films, especially PET and PEN * Synthetic options available for manipulating the structure and properties of polyesters, with special focus on polyester ionomers * Main blending options available to enhance the performance of commodity polyesters * Rheo-optical properties of polyester films and corresponding testing methodology * Micro-layer coextrusion technology as applied to modify the performance of polyester films * Bio-medical applications * Polyester recycling, with special focus on upcycling With an interdisciplinary approach covering the performance of real-life products and components, Polyester Films is an essential resource for researchers and engineers in academia and industry working in physics, material science, chemistry and process engineering. This volume should also be invaluable for graduate students and early-career researchers in similar fields.

Leseprobe

Polyester Films

Materials, Processes and Applications

Edited by

Miko Cakmak

Schools of Materials and Mechanical Engineering

Purdue University

West Lafayette, IN, USA

Jehuda Greener

J. Greener Consulting

Rochester, NY, USA

Copyright © 2023 by John Wiley & Sons Inc. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.

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Library of Congress Cataloging‐in‐Publication DataNames: Cakmak, Miko, author. | Greener, Jehuda, author. | John Wiley & Sons, publisher.Title: Polyester films : materials, processes and applications / Miko Cakmak, Jehuda Greener.Description: Hoboken, NJ : Wiley, 2023. | Includes index.Identifiers: LCCN 2023000322 (print) | LCCN 2023000323 (ebook) | ISBN 9781119535751 (cloth) | ISBN 9781119535799 (adobe pdf) | ISBN 9781119535744 (epub)Subjects: LCSH: Polyester films.Classification: LCC TP1180.P6 C35 2023 (print) | LCC TP1180.P6 (ebook) | DDC 668.4/225–dc23/eng/20230113LC record available at https://lccn.loc.gov/2023000322LC ebook record available at https://lccn.loc.gov/2023000323

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For Kaia, Gulgun, Emre, Karen, Helena, Maya, Eli and Ari

List of Contributors

Abdellah AjjiChemical Engineering DepartmentPolytechnique MontrealMontreal, Quebec, Canada

Miko CakmakSchools of Materials and Mechanical EngineeringPurdue UniversityWest Lafayette, IN, USA

Ebrahim Jalali DilPolyExpert Inc., Laval, Quebec, Canada

Abdullah Al FaysalMultifunctional CompositesManufacturing Laboratory (MCML)Department of Mechanical and Industrial Engineering, University of TorontoToronto, Ontario, Canada

Kazuki FukushimaDepartment of Chemistry and Biotechnology School of EngineeringThe University of TokyoTokyo, JapanJapan Science and Technology Agency (JST)Precursory Research for Embryonic Science and Technology (PRESTO)Saitama, Japan

Elena GabirondoPOLYMATUniversity of the Basque Country UPV/EHUDonostia‐San SebastiánSpain

Jehuda GreenerJ. Greener Consulting, Rochester, NY, USA

Coralie JehannoPOLYMATUniversity of the Basque Country UPV/EHUDonostia‐San SebastiánSpainPOLYKEYJoxe Mari Korta CenterDonostia‐San SebastianSpain

Karnav KanugaMedtronic Inc.,Dublin, Ireland

Patrick C. LeeMultifunctional CompositesManufacturing Laboratory (MCML)Department of Mechanical and Industrial EngineeringUniversity of TorontoToronto, Ontario, Canada

Dennis J. MassaMassa Science and Technology ConsultingPittsford, NY, USA

Ion OlazabalPOLYMATUniversity of the Basque Country UPV/EHUDonostia‐San SebastiánSpain

Amir SaffarProAmpac Inc.,Terrebonne, Quebec, Canada

Haritz SardonPOLYMAT, University of the Basque Country UPV/EHUDonostia‐San SebastiánSpain

Troy SuMultifunctional CompositesManufacturing Laboratory (MCML)Department of Mechanical and Industrial EngineeringUniversity of TorontoToronto, Ontario, Canada

Andy H. TsouNational Taiwan UniversityHouston, TX, USA

Robert A. WeissDepartment of Chemical EngineeringUniversity of ConnecticutStorrs, CT, USA

Baris YalcinGlobal Packaging R&DThe Coca‐Cola CompanyAtlanta, GA, USA

Jianxiang ZhaoMultifunctional CompositesManufacturing Laboratory (MCML)Department of Mechanical and Industrial Engineering, University of TorontoToronto, Ontario, Canada

Preface

Polyester film serves as a foundation, literally and figuratively, to many important technologies that define this technological age. In fact, many advances in the digital and information revolution over the past four decades owe a great deal to developments in several key material technologies such as polyester films, which comprise and enable many critical components in various flat panel displays, photovoltaics, microelectronics systems, and biomedical applications among others. Polyester films often serve as either substrates (bottom or carrier layers) or superstrates (top or protective layers) in a particular two‐dimensional film structure in many of these application categories and are the materials of choice owing to a combination of remarkable physical and mechanical properties and a relatively low cost. These materials are especially well suited for roll‐to‐roll operations, which provide significant cost benefits and operational flexibility in the manufacture of many critical film components in the various application areas.

Through contributions of recognized experts, this volume presents the state of the art of many aspects of polyester film technology covering materials, processing, and applications and identifies areas with potential opportunities for further progress. The overriding objective of this manuscript is to identify the multitude of options (tool‐kit) available to material and product designers in manipulating the properties of polyester films to meet specific performance and product requirements. These include synthetic modifications (copolymerization), physical enhancements (blending), and process upgrades (tenter‐frame changes, coextrusion, and coating).

A brief historical review of polyester film technology and overview of properties and applications of key polyester films, PET and PEN, is presented in Chapter 1. The vast synthetic options available for manipulating the structure and properties of polyesters are surveyed in Chapter 2, with a special focus on polyester ionomers given in Chapter 4. Chapter 3 identifies the main blending options available to improve the properties of commodity polyesters. The unique rheo‐optical properties and testing methodology of polyester films are discussed in Chapter 5. Chapter 6 presents the micro‐layer coextrusion technology, which provides a unique and not fully explored methodology for preparing multi‐layered films. Heat‐setting is one of the critical steps in the tentering process commonly used for manufacturing polyester films. Key aspects of this step are reviewed in Chapter 7. The use of polyester films in biomedical applications is surveyed in Chapter 8, and finally, the important topic of polyester recycling is covered in Chapter 9 with a special focus on upcycling.

The editors would like to thank all the contributors to this volume for their excellent contributions, patience, and understanding through the difficult Covid‐19 pandemic while this book project was undertaken.

Miko Cakmak and Jehuda Greener

1Polyester Films: An Overview

Jehuda Greener

J. Greener Consulting, Rochester, NY, USA

1.1 Introduction

Many advances in the digital and information revolution over the past four decades owe a great deal to developments in several key material technologies. A case in point is polyester films, which comprise and enable many critical components in various flat panel displays (FPDs) [1–3], photovoltaics [4, 5], microelectronics systems [6], and biomedical applications [7], among others. Polyester films often serve as either substrates (bottom or carrier layers) or superstrates (top or protective layers) [8] in a particular two‐dimensional film structure in many of these application categories and are the materials of choice owing to a combination of remarkable physical and mechanical properties and a relatively low cost. These materials are especially well suited for roll‐to‐roll (R2R) operations [9], which provide significant cost benefits and operational flexibility in the manufacture of many critical film components in the various application areas.

Polyesters are polymers produced via a polycondensation reaction from diols and diacids or other related precursors (e.g. diesters), and as such, cover a vast material space with a wide range of molecular structures and physical properties [10]. However, only a small fraction of the known polyesters are good film formers suitable for high‐end film applications. The best‐known and most common film polyester is poly(ethylene terephthalate) (PET), and somewhat less known but equally important is poly(ethylene‐2,6‐naphthalene dicarboxylate) (PEN), see Figure 1.1. These polyesters and many variations thereof are transparent, semi‐crystalline, possess relatively high glass transition temperatures (Tgs) and are used in a variety of film products spanning a wide range of applications.

Figure 1.1 Molecular structures of PET and PEN.

We note that the PET film market is a relatively small segment of the global PET resin market, which is dominated by the textile (fiber) industry and by packaging applications, driven mainly by growth in the beverage sector [11, 12]. Yet, as noted above, this segment of the market is critical to many mature and emerging technologies.

The physical and chemical properties of these and related film products can be manipulated and enhanced through a variety of well‐established material design strategies. PET, a polycondensation product of ethylene glycol (EG) and terephthalic acid (TA) or dimethyl terephthalate (DMT), can be modified by changing the comonomer composition to impart certain desired properties. For example, by substituting some of the TA with an ionic diacid or diester such as (sodiosulfo) isophthalate (SIP), it is possible to increase the hydrophilicity of the base polymer while still retaining its film‐forming ability [13, 14]. Similarly, by substituting some of the EG with a poly(ethylene glycol) (PEG) segment the polymer is “softened” and its Tg can be suppressed by controlling the length and substitution level of the PEG moiety [15]. By contrast, by fully substituting TA with the bulkier naphthalate diacid moiety, to form PEN, the Tg and stiffness of the polyester film are significantly enhanced, as will be discussed in Section 1.3. Many other comonomers can be used to tweak certain properties of the base polyester films to achieve a desired performance [10]. However, a synthetic modification of the base polyester is often not feasible from an economic or operational standpoint.

Another common approach to boosting and manipulating the properties of polyester films is to blend the base polyester with another miscible polymer possessing some desirable attributes. For example, blending PET with fully miscible polyether‐imide (PEI) can raise the Tg of the blended resin relative to PET and enhance its overall thermal stability [16]. Along these lines, addition of various immiscible fillers, solid, liquid, or gas, at a certain loading level to create a polyester composite can also enhance or modify certain bulk or surface properties of the polyester film [16]. In order for the polyester composite to retain its essential film‐forming ability, the polyester phase must be the major (“continuous”) phase of the composite material, while the added filler must be well dispersed and confined to the minor (discrete) phase of the composite. If the characteristic domain size of the minor component is less than the wavelength of visible light, then the composite material system is defined as a nanocomposite, which carries some implications for the physical and optical properties of the corresponding material.

All of the aforementioned material design approaches are potentially useful so long as the modified polyester resin is processable using suitable film processing methodologies and existing film‐making infrastructure. In fact, one of the most common and generally highly effective approaches to boosting or modifying the properties of polyester films is through judicious film processing methods. The physical and mechanical properties of melt‐cast PET or PEN films are generally inferior and do not meet the requirements of most film applications. A significant boost in properties and overall physical performance is achieved by converting the polyester resin into film using the so‐called tenter‐frame (tentering) process [17, 18] commonly used for the manufacture of polyester films. This process, depicted schematically in Figure 1.2, generally comprises five main steps: (1) extrusion, (2) melt casting, (3) machine direction (MD) stretching, (4) transverse direction (TD) stretching, and (5) heat‐setting (constrained high‐temperature annealing). The melt‐cast film following Step 2 is largely amorphous and structureless and, as noted above, has inferior mechanical properties. The stretching steps applied within the tenter‐frame (Steps 3 and 4 above), as shown in Figure 1.2, induce the formation of a desired crystalline morphology and biaxial molecular alignment along both principal in‐plane directions, while the heat‐setting step helps refine and modify the crystalline microstructure of the biaxially oriented film, thereby enhancing its dimensional stability and mechanical properties [19–21]. Further improvement in dimensional stability often requires an additional annealing step, called heat relaxation [22], whereby the biaxially oriented and heat‐set film is exposed to high temperatures (Tg < T < Tm) under low tension. Heat relaxation is often done offline (i.e. outside the tenter‐frame machine) because of the low conveyance tension requirements. By judicious choice of the tenter‐frame process conditions, especially casting temperature, stretch ratios, stretch temperatures, stretching rates, and heat‐set temperature, the properties and overall quality and uniformity of the film can be manipulated and optimized for its intended application. It is noted that the order of the stretching steps as listed above, although most common in conventional film‐making machines, can be reversed or both stretching steps can be applied simultaneously (“simul‐stretching”), which would require changes in machine configuration and process conditions [23]. Simul‐stretching is especially useful when a small machine size is desirable. But fundamentally the film must be biaxially oriented to insure uniform and balanced mechanical and physical properties in the plane of the film, so that the exact sequence of the stretching steps is not inherently important. The applied stretch ratios are selected to maximize molecular orientation and mechanical properties and attain a desirable thickness uniformity based on the so‐called ConsidereConstruction[24], which is a strain‐hardening condition.1 Typically, optimal stretch ratios for both PET and PEN vary in the range of 3–4X with the actual value depending mainly on the stretch temperature, Ts, the stretch rate (related to line speed and dimensions of the stretching stations), and the target thickness of the film product. Ts usually falls in the range {Tg, Tcc}, where Tcc is the so‐called cold‐crystallization temperature of the processed resin. The choice of the optimal heat‐set temperature, THS, depends on several process and product considerations discussed in detail elsewhere [21]. If the base polymer (PET, PEN, or other similar polyesters) is modified by copolymerization or by blending, the optimal process conditions must be adjusted accordingly, assuming that a viable process window exists.

Figure 1.2 Schematic of the tenter‐frame process.

The tenter‐frame process requires specialized equipment described in detail by Tobita et al. [17]. This process is especially well suited for relatively slow crystallizing polymers such as PET and PEN and is designed to insure that the crystalline morphology develops in a controlled and measured way to produce a film with the desired “microstructure” and properties as well as acceptable thickness and structural uniformity. Thus, any of the changes in polymer composition noted above (comonomer type and substitution level, polymer blend composition, filler loading, etc.) must take into consideration the corresponding impact on film processability in the tenter‐frame process. If a given material change adversely affects film processability and cost, then the corresponding approach may be deemed ineffective and/or impractical.

Film properties can also be enhanced by coating the polyester film product with a variety of functional and ancillary layers that provide a desired functionality and effective control of the surface properties of the film [25]. A coating step may be incorporated within the tenter‐frame process before or after the stretching and heat‐setting steps, or it can be added offline, i.e. in a separate R2R line. The types of surface properties that can be imparted by coating include adhesion [26], electric charge dissipation (anti‐static control) [27], barrier [28], friction coefficient, scratch resistance, color, reflectivity, planarity, roughness, light management (to be discussed in Section 1.4), etc.

Polyester films can also be functionalized and their bulk properties modified via coextrusion [29, 30]. Addition of layer(s) to the base polyester film by coextrusion provides wide latitude in boosting certain surface and bulk properties that are otherwise not attainable with a single extruded polyester layer or with a coated layer. Designing an effective coextruded film structure, however, requires that the selected coextruded polymers can withstand the tenter‐frame process conditions and that they adhere well to the base polyester layer while providing the desired functionality. The main advantages of coextrusion over coating are the ability to add relatively thick functional layers anywhere within the film structure (i.e. not only on the external surfaces as in the case of coatings) and the relative ease and operational flexibility in adding a coextrusion capability to an existing tenter‐frame machine. Of special note here is the micro‐layer coextrusion technology, a subset of film coextrusion, developed originally at Dow Chemical in the 1960s. This process methodology, discussed in detail in Chapter 6 of this volume, makes it possible to create a multilayered structure that offers considerable design flexibility in manipulating the physical and mechanical properties of the film through a wide variety of process and material options [31].

In the next section, we provide a brief historical overview of polyester film technology since its inception in the mid‐twentieth century. Some important properties of polyester films are surveyed in Section 1.3 and contrasted with those of other high‐performance polymer film types, highlighting some key advantages of polyester films. Although these films are used in a host of application areas, as noted above, we review one particular area – optical films for FPDs – in Section 1.4 to illustrate the great utility of polyester films in this critically important emerging technology space. Finally, a brief summary of this important materials technology is given in Section 1.5.

1.2 Historical Overview

The first synthetic polyester ever made is attributed to the work of Berzelius in 1847, who reacted tartaric acid with glycerol [32]. But the earliest synthetic polyester produced via a controlled step‐growth polymerization reaction was prepared by Carothers in his pioneering work on synthetic polymers in the late 1920s [33]. The earliest patent on biaxially stretched PET was filed by Swallow and Baird [34] of ICI in 1950, followed by a patent filed by Scarlett [35] of DuPont in 1958 on the biaxial stretching (tenter‐frame) process, which was further refined and extended by C. J. Heffelfinger [36] in 1966. A drawing from Scarlett’s patent shown in Figure 1.3 represents one of the earliest renditions of the tenter‐frame process. One of the earliest applications of PET film was as a film base (substrate) for X‐ray films. Consumer photographic films (at the time, one of the largest polymer film markets) have been in use with some type of a solvent‐cast cellulosic film base (cellulose nitrate or cellulose triacetate) since the late nineteenth century. Early attempts in the 1950s to apply PET film as a film base to replace cellulosics in consumer photographic roll films were unsuccessful, primarily because of the high strength (toughness) and the tendency of PET‐based films to retain high curl during photographic processing, making it difficult to finish and process using conventional photo‐processing equipment. However, PET film base was found to be much better suited for X‐ray film applications, mainly because of its low moisture sensitivity, high‐dimensional stability, and high stiffness, especially compared to cellulosic films. With the development of biaxially oriented PET film technology at DuPont in the late 1950s, all X‐ray film products migrated to PET film base, followed soon after by motion picture print films and various graphic arts film products. Since then PET films have been produced at a wide range of gauges (1–400 μm) serving many industries and covering many diverse application areas [37] including imaging, packaging, electronics, display, and biomedical. Although PEN was discovered and patented around the same time as PET [38], it was somewhat slower in making inroads into the polyester film industry despite clear advantages over PET in thermal stability and stiffness (see Section 1.3). The slow adaptation of PEN had been mostly due to the high cost and scarcity of one of its precursors, 2,6‐naphthalene dicarboxylate (2,6‐NDC). PEN production started to gain traction in the 1990s with the growing demand for specialty consumer photographic films (APS format) and the growing markets in the display and microelectronics sectors where the unique attributes of PEN films (very high stiffness and high‐temperature stability. See Section 1.3) are especially attractive.

Figure 1.3 Drawing of the tenter‐frame process taken from Scarlett’s 1958 patent.

Source: Scarlett [35]/Arthur C Scarlett/Public Domain.

1.3 Physical Properties

As noted in Section 1.1, there are many types of polyester films in use today in many segments of industry, covering a wide variety of applications and gauges and possessing a range of properties, but in this brief survey, we will focus on the two key polyester film types, PET and PEN, and contrast these films with other high‐performance polymer films used in several high‐end application areas. Several important physical properties of PET and PEN films are listed in Table 1.1 and compared to those of polycarbonate (PC), polyethersulfone (PES), polycyclo‐olefin (polynorbornene, PCO), polyarylate (PAR), and polyimide (PI) films, all of which are considered high‐performance films. All the materials listed are commercially available film products. Although the relevant properties and optimal property ranges depend on the particular application category and (film making) process conditions as discussed in Section 1.1, the properties listed are fairly representative of the corresponding materials and are especially important in various high‐end applications such as display, microelectronics, and photovoltaics. Of the films listed, only PET and PEN are semi‐crystalline. All the other film materials in this table are high‐Tg amorphous polymers used mostly in niche applications. High‐temperature stability is especially important in various microelectronic and photovoltaic applications that involve high‐temperature device fabrication and printing steps with tight registration tolerances. Although the Tgs of PET and PEN are generally lower, these films can be fabricated and used at temperatures above their corresponding Tgs because of their respective crystalline networks that boost the rigidity, stiffness, and dimensional stability of the film well above Tg. In fact, if properly heat‐set and heat stabilized, these films can be fabricated and used at temperatures up to 150 and 200 °C for PET and PEN, respectively. (By film fabrication we mean the steps taken to convert the film product into a functional component or device in display, microelectronics, photovoltaics, etc., as opposed to the film‐making process or film processing, i.e. the process of converting the raw polymer resin into the film.) The lower Tgs of PET and PEN also facilitate conversion of the corresponding polymer resins into film via the tenter‐frame melt process discussed briefly in Section 1.1. Film processing of some of the high Tg amorphous resins listed in Table 1.1 requires a very high‐temperature melt‐casting process or solvent casting process, both of which are costly and undesirable from operational and environmental considerations.

Table 1.1 Selected properties of high‐performance films.

Source: MacDonald [37, 39].

Property\Polymer

PET

PEN

PC

PES

PAR

PCO

PI

T

g

(°C)

78

120

150

220

340

330

>350

T

m

(°C)

255

263

na

na

na

na

na

CLTE ppm/°C

a

15

13

60–70

54

53

74

30–60

% Transmission

b

>85

85

>90

90

90

91.6

Yellow

Water absorption (%)

0.14

0.14

0.4

1.4

0.40

0.03

1.8

Young’s modulus (GPa)

5.3

6.1

1.7

2.2

2.9

1.9

2.5

Tensile strength (MPa)

225

275

na

83

100

50

231

WVTR (g/(m

2

24 h))

c

21.3

6.7

OTR (cm

3

/(m

2

24 h atm))

d

55

21

a Linear thermal expansion coefficient over −55–85 °C.

b Light transmission over 400–700 nm.

c Water vapor transmission rate.

d Oxygen transmission rate.

Other results in Table 1.1 show, with a few exceptions, significant advantages for PET and PEN films in dimensional stability (lowest thermal expansion coefficient, CLTE, and water absorption), stiffness (highest Young’s modulus), and toughness/tear resistance (highest tensile strength). Of the two polyester films, PEN is clearly stiffer, stronger, and has a wider operating temperature range owing mainly to its higher Tg, but, of course, these advantages must be weighed against a significantly higher price point relative to PET. PEN also has significantly better moisture and oxygen barrier properties than PET [37], which is critically important in some display, microelectronic and photovoltaic applications. It must be noted, though, that this improvement in barrier performance for PEN film is well below the “super barrier” performance required in some display applications (e.g., OLED display), which is usually attained by vacuum deposition of multiple inorganic–organic barrier layers on a PET substrate [40, 41].

Despite the semi‐crystalline nature of the polyester films listed in Table 1.1, both PET and PEN films are optically clear and highly transmissive. The high optical transmission is possible for these semi‐crystalline films because the characteristic sizes of the crystalline domains generated in the biaxially oriented polyester films by the tenter‐frame process are smaller than the wavelength of visible light. We note, however, that both the PET and PEN films as well as other polyester films are highly optically anisotropic because of the high molecular order induced by the tenter‐frame process. This optical anisotropy is typically manifested by high out‐of‐plane birefringence. The in‐plane birefringence for both of these films, however, is expected to be low if the biaxial stretching process is well balanced, i.e. if the MD and TD stretching steps induce similar levels of molecular alignment in both principal in‐plane directions. This point will be discussed in more detail in Chapter 5 of this volume.

We note that while the properties listed in Table 1.1 are fairly representative of the respective polymers, many physical properties of polyester films as well as other polymer film types are not absolute but are rather dependent on the conditions applied during the film‐making process. It was noted in Section 1.1 that the dimensional stability of polyester films could be significantly enhanced through an added heat relaxation (stabilization) step. The mechanical properties and thermal stability can likewise be improved and optimized through changes in the stretching and heat‐setting conditions in the tenter‐frame process. To illustrate this point we examine the effect of heat‐set temperature on the dimensional stability of PET and PEN films as expressed by thermo‐mechanical analysis (TMA) scans shown in Figure 1.4[21]. These results demonstrate the dramatic effect of heat‐set temperature in suppressing the shrinkage of the film sample by shifting the onset of shrinkage up to temperatures well above the corresponding Tgs, close to the applied heat‐set temperature for both film types, thus extending their corresponding temperature fabrication and application ranges. These results also underscore the considerable leverage we have over the physical performance of the film sample through judicious film processing. Though the effect of heat‐set temperature on dimensional stability, as illustrated in Figure 1.4, is clearly evident, other variations in process conditions can have a more subtle effect on the physical and mechanical performances such that the properties listed in Table 1.1 would depend to some extent on film grade, thickness, and vendor, and the values shown should be considered only as a rough guide.

Figure 1.4 TMA scans for (a) PET and (b) PEN films: % change in sample dimension (along MD) vs. temperature: Effect of heat‐set temperature. (NHS: non‐heat‐set sample.)

Source: Greener et al. [21]/with permission from John Wiley & Sons.

The viscoelastic properties of solid polymer films also play an important role in controlling their dimensional stability, and in particular, their propensity to acquire curl (curvature or non‐planarity) in R2R and other rolling operations [42, 43]. The property most relevant in this regard is the time‐dependent relaxation modulus, E(t); generally the faster the film relaxes when subjected to a constant strain, the higher its propensity to acquire curl in rolling operations. Figure 1.5 compares the relaxation moduli of several commercial film samples, including PET, cellulose acetate (CA), polypropylene (PP), and nylon (MXD). Of the films shown, PET and MXD are the slowest to relax, thus these films are likely to curl less in R2R operations. This is critically important in applications with tight planarity specifications. The relaxation properties of PET or PEN films can be modified by physical aging in a way that the relaxation time is increased (relaxation process is slowed down) with more extensive aging [44]. Thus, long storage times at high temperatures (<Tg) (annealing storage) can moderate somewhat the propensity of the film to acquire curl and thereby improve the dimensional integrity of the film product.

Figure 1.5 Tensile relaxation moduli at 21 °C for various film types: (•) cellulose acetate, (*) PET, (Δ) MXD nylon, (x) polypropylene.

Source: Greener et al. [43]/with permission from John Wiley & Sons.

1.4 Light Management Films

One key application of polyester films since the late 1990s is as substrates or superstrates for various light management films (LMFs) used in flat panel displays (FPDs) [1–3]. FPDs are now ubiquitous and are all around us. All FPD types, but especially liquid crystal (LC) displays (LCDs), require effective management of light to increase brightness, enhance light efficiency (~increase battery life), and improve overall image quality. A variety of optical films are typically used in different LCDs, with the particular composition and type of films and the LCD structure depending on the end use and market segment served by the display (e.g. TV, tablet, laptop, computer monitor, and smartphone). In portable displays (e.g. smart phones, laptops, tablets, etc.), light efficiency (~battery life) is paramount, while in large stationary displays (e.g. TVs and desktop computer monitors) image quality, brightness, and long‐term stability are most critical.

A typical LCD module is shown in Figure 1.6, and a schematic cross‐section of a display module is depicted in Figure 1.7. The display usually consists of two main assemblies: the LCD panel (LC assembly) and the backlight unit (BLU assembly). The LC assembly comprises the LC cell, where the actual pixelized image is formed, as well as a number of optical films. The LC cell is bounded by two crossed polarizers, each of which is sandwiched between two CTA (cellulose triacetate)2 protective films. Additional films usually found in between the polarizers and the LC cell in the LCD panel are compensation films used to extend the viewing angle and improve the color gamut of the display.

Figure 1.6 Typical structure of an LCD module.

Source: Kobayashi et al. [2]/with permission from John Wiley & Sons.

Figure 1.7 Schematic cross‐section of a typical LCD module (not to scale).

The BLU assembly contains the light source (usually fluorescent lights (CCFLs) or light‐emitting diodes (LEDs), distributed along the edges of the display) and a variety of LMFs whose main functions are to increase brightness, improve brightness uniformity, and enhance the overall light efficiency of the display. The most common LMFs in the BLU assembly, shown schematically in Figure 1.7, are light guide plates (LGPs), reflector films, brightness films (BEF®), diffuser films, reflective polarizers (DBEF®), and quantum dot (QD) films, although other LMF types have also been used.

The LGP is used to redirect light from the edge light source, usually LEDs placed along the edges of the LGP, distribute it across the area of the display, and direct it as uniformly as possible toward the LC assembly and the viewer. The LGP typically comprises a relatively thick sheet of PMMA or similar acrylic polymers, or it can have wedge‐like configuration [45]. A white reflector film placed at the bottom of the LGP helps redirect incident light by reflection from the edge light source toward the LC assembly and recycle back light reflected from the reflective polarizer.

Diffuser films are used to distribute light as evenly as possible across the display area as well as hide cosmetic defects. Typically light diffusion is achieved by coating scattering beads on a PET substrate (surface diffuser), but other diffuser film types (e.g. volume diffusers) can also be used. Quite often more than one diffuser film is used in a BLU assembly.

The brightness films are sometimes called prism films because they comprise a linear prismatic structure made of photolithographed acrylic layer deposited on a PET substrate (Figures 1.8 and 1.9) [46]. The main function of these films is to collimate the light coming from the LGP toward the viewer. Often two crossed‐brightness films are used in order to achieve uniform collimation in both principal directions. Common names for brightness films are prism films or BEF® (brightness enhancement film), but BEF is trademarked by 3M, which pioneered the technology. The effectiveness of the collimation function depends on the prism geometry and refractive index of the photolithographed prism material (usually a high‐index clear acrylic). We note that a similar prismatic structure used in brightness films can also be produced by a suitable hot embossing process of a transparent base film or other methods.

Figure 1.8 Schematic cross‐section of a prism film (not to scale).

Figure 1.9 Typical structure of a prism film.

Source: Hanzawa [46]/with permission from John Wiley & Sons.

Because of the absorptive nature of the polarizers used in typical LC displays, such displays are inherently light‐inefficient, i.e. only light polarized along the polarization plane of the bottom polarizer can pass through, while most of the light directed from the BLU toward the bottom polarizer will be absorbed (“wasted”). One way to mitigate this inherent problem is to add a reflective polarizer just below the bottom polarizer of the LC assembly (see Figure 1.7) with the planes of polarization of the reflective polarizer and bottom polarizer being fully aligned [46]. A well‐designed reflective polarizer can substantially enhance the brightness and overall light efficiency of the display by passing light only along one polarization direction, which is aligned with the plane of polarization of the bottom (absorptive) polarizer in the LC assembly. The rest of the light is reflected back to the BLU assembly and recycled. This allows a more efficient use of light by minimizing absorption losses imparted by the absorptive bottom polarizer, thus producing a considerably higher brightness and improving battery life in the case of mobile devices. The most common type of a reflective polarizer is based on the concept of giant birefringent optics in multilayered polymer films [47]. The multilayered film structure in such a component can be manufactured, for example, by the micro‐layer coextrusion film technology pioneered by Dow [31] with a typical embodiment comprising alternating layers of uniaxially oriented PMMA and PEN designed to yield the desired reflective polarization effect. The original reflective polarizer film technology was developed by 3M under the trade name DBEF® (Double Brightness Enhancement Film).

Some LC displays with LED backlights (mostly TV’s) use QD films placed on top of the LGP to achieve a significantly larger color gamut and hence better image quality [48]. Such films typically comprise a QD film layer sandwiched between two PET‐based barrier films.

As is clear from the above, a variety of polyester films play a key role in various components of LC displays as well as other display types. In most embodiments, these films have a passive function serving mostly as substrates or superstrates for various functional layers. However, in some cases, e.g. reflective polarizers, the polyester layer is actively involved in the optical function of the film.

1.5 Summary

Polyester film technology is an important component of many established and emerging technologies, including display, microelectronics, photovoltaics, and biomedical. The most prominent examples of film polyesters are PET and PEN, but other synthetic polyester material combinations, e.g. various copolyesters, miscible polyester blends, or polyester composites, can also be used in this space. The main attraction of polyester films is a unique combination of low cost and outstanding physical and mechanical properties, some of which can be attained by a judicious film processing methodology. Indeed, through optimization and control of the tenter‐frame process, most commonly used for the manufacturing of biaxially oriented polyester films, it is possible to adjust and optimize the properties of the biaxially stretched film for its intended application. In particular, changes in stretch ratio, stretch temperature, and heat‐set temperature can have a profound impact on the microstructure and crystalline morphology of the stretched film and thereby on its overall physical performance. The bulk and surface properties of biaxially stretched polyester films can also be modified and adjusted by copolymerization, miscible and immiscible blending with various value‐added components, coextrusion, and coating.

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