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Polyester 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 Considere Construction [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...