Morphology Evolution in Immiscible Polymer Blends during Compounding

Polymer researchers have had a long-standing interest in understanding the evolution of blend morphology when two (or more) incompatible homopolymers or copolymers are melt blended in mixing equipment. In industry, melt blending is conducted using either an internal (batch) mixer (e.g., a Banbury mixer or a Brabender mixer) or a continuous mixer (e.g., a twin-screw extruder or a Buss kneader). There are many factors that control the evolution of blend morphology during compounding, the five primary ones being (1) blend composition, (2) rheological properties (e.g., viscosity ratio) of the constituent components, (3) mixing temperature, which in turn affects the rheological properties of the constituent components, (4) the duration of mixing in a batch mixer or residence time in a continuous mixer, and (5) rotor speed in a batch mixer or screw speed in a continuous mixer (i.e., local shear rate or shear stress). When two immiscible polymers are compounded in mixing equipment, two types of blend morphology are often observed: dispersed morphology and co-continuous morphology. Numerous investigators have reported on blend morphology of immiscible polymers, and there are too many papers to cite them all here. Some investigators (Han 1976, 1981; Han and Kim 1975; Han and Yu 1972; Nelson et al. 1977; van Oene 1978) examined blend morphology to explain the seemingly very complicated rheological behavior of two-phase polymer blends, and others (Favis and Therrien 1991; He et al. 1997; Ho et al. 1990; Miles and Zurek 1988; Scott and Macosko 1995; Shih 1995; Sundararaj et al. 1992, 1996) investigated blend morphology as affected by processing conditions. Today, it is fairly well understood from experimental studies under what conditions a dispersed morphology or a co-continuous morphology may be formed, and whether a co-continuous morphology is stable, giving rise to an equilibrium morphology, or whether it is an unstable intermediate morphology that eventually is transformed into a dispersed morphology (Lee and Han 1999a, 1999b, 2000). Let us consider the morphology evolution in an immiscible blend consisting of two semicrystalline polymers, A and B, in a compounding machine, and let us assume that the melting point (Tm,A) of polymer A is lower than the melting point (Tm,B) of polymer B.

Pultrusion of Thermoset/Fiber Composites

Pultrusion of thermoset/fiber composites generally consists of pulling continuous rovings and/or continuous glass mats through a resin bath or impregnator and then into preforming fixtures, where the section is partially shaped and excess resin and/or air are removed. Finally, the preformed profiles are pulled through heated dies, where the section is cured continuously (Batch 1989; Meyer 1985; Price 1979; Richard 1986). The pultrusion process is one of the most cost-effective continuous processing techniques for producing thermoset composite materials. The laminating resin may be an unsaturated polyester resin, a vinyl ester resin, or an epoxy resin, but the majority of pultruded thermoset products currently use unsaturated polyester resins. The reason for this is that epoxy resins require high heat inputs and have relatively slow gelation, although some effort has been spent on development of new epoxy resin systems that can be pultruded at speeds comparable with unsaturated polyester resin systems (e.g., 0.6–0.9 m/min). Han and coworkers (Han et al. 1986, Han and Chin 1988) formulated and then solved numerically, via the finite difference method, a system of equations describing the cure kinetics of a thermoset resin and the heat transfer between the resin and the die wall, in order to model the pultrusion process for thermoset/fiber composites. Subsequently, other investigators (Batch and Macosko 1993; Chachad et al. 1995; Gorthala et al. 1994a, 1994b; Ma et al. 1986) reported similar studies. Experimental studies (Batch and Macosko 1993; Chachad et al. 1995; Ma et al. 1986; Price 1979; Price and Cupschalk 1984; Roux et al. 1998) on the pultrusion process for thermoset/fiber composites have also been reported. Some research groups (Aström and Pipes 1993; Larock et al. 1989; Ma and Chen 1991; Ruan and Liu 1994) have investigated the pultrusion process of fiber-reinforced thermoplastic polymers. While there are some similarities between the pultrusion of thermoset/fiber composites and fiber-reinforced thermoplastic polymers, the most important difference between the two lies in that the former involves chemical reactions during processing, whereas the latter does not.

Tubular Film Blowing

Tubular film blowing has long been used to produce biaxially oriented films using such thermoplastic polymers as low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). Here, LDPE refers to a polymer that is synthesized by free-radical polymerization under high pressure (Fawcett et al. 1937). The discovery of linear low-density polyethylene (LLDPE) in the 1980s via the Unipol process (Beret et al. 1986; Jones et al. 1985), which uses a low-pressure gas-phase process, has led to additions to the family of tubular blown films during the past two decades. The discovery of metallocene catalysts (Stevens and Neithamer 1991; Welborn and Ewen 1994) in the 1990s further increased the number of LLDPEs that have been used to produce tubular blown films during the last decade. To distinguish LLDPE from LDPE, LLDPE is sometimes referred to as low-pressure low-density polyethylene (LP-LDPE) and LDPE is referred to as high-pressure low-density polyethylene (HP-LDPE) (see Chapter 6 of Volume 1). In this chapter, however, we use the terminologies LDPE and LLDPE. As described in Chapter 6 of Volume 1, LDPE has a high degree of long-chain branching, while LLDPE has short-chain branching with little or no longchain branching. However, the metallocene catalysts apparently allow one to produce LLDPEs having a wide range of side chains, including a certain degree of long-chain branching. The details of the synthetic procedures for producing such a variety of LLDPEs are closely guarded industrial secrets. Biaxially oriented film can be strong and tough in all directions in the plane of the film. As in fiber spinning, the polymer melt exiting from the die flows under a mechanical tension in the direction of flow. However, in the film blowing process, the tube of molten polymer is extended in both the transverse and the axial (machine) directions. Therefore, rheologically speaking, the film blowing process may be treated from the point of view of biaxial elongational flow, whereas the fiber spinning process may be treated from the point of view of uniaxial elongational flow.

Plasticating Single-Screw Extrusion

There are two types of extruder: (1) single-screw extruders and (2) twin-screw extruders. The single-screw extruder is one of the most important pieces of equipment in the processing of thermoplastic polymers. Accordingly, during the past three decades, many attempts have been made to analyze the performance of single-screw extruders using different degrees of mathematical sophistication (Cox and Fenner 1980; Donovan 1971; Edmondson and Fenner 1975; Elbirli et al. 1983, 1984; Halmos et al. 1978; Han et al. 1991a, 1991b, 1996; Lee and Han 1990; Lindt 1976; Lindt and Elbirli 1985; Shapiro et al. 1976; Tadmor 1966; Tadmor and Klein 1970; Tadmor et al. 1967). There are two types of single-screw extruders: (a) plasticating and (b) melt-conveying. The plasticating single-screw extruder conveys a solid polymer from the feed section to the melting section, where most of the melting (or softening) occurs, and then transports the melted or softened polymer to a shaping device (e.g., dies and molds). The meltconveying extruder does not include a melting section; it simply transports an already softened polymer to a shaping device (e.g., rubber extruder). Single-screw extruders are used for various purposes, such as melting and pumping, compounding with an additive(s) or filler, cooling and mixing, removing residual monomers or solvents in polymer (i.e., polymer devolatilization), and cross-linking reactions. Single-screw extruders are simple to operate and relatively inexpensive as compared with twin-screw extruders. However, there are situations where a single-screw extruder cannot function as effectively as a twin-screw extruder. In the design of plasticating single-screw extruders, one needs information on (1) the physical and thermal properties of polymers (e.g., friction coefficient between the solid polymer and barrel wall, thermal conductivity of polymer, specific heat as a function of temperature, melting point of polymer, and heat of fusion of polymer) and (2) rheological properties of polymers as functions of shear rate and temperature. Due to the complexity involved in the design of extruders, it is highly desirable for one to establish relationships between material variables and processing variables.

Reaction Injection Molding

Reaction injection molding (RIM) is a thermoset processing operation during which the incoming feedstream(s) undergo cure reactions that give rise to a three-dimensional network structure (Becker 1979; Macosko 1989). Different from the operation of injection molding thermoplastic polymers presented in Chapter 8, in RIM operation the component(s) must cure rapidly (say, within 90 seconds) and a finished product is removed in 1−10 minutes, depending on the chemical systems, the part thickness, and the capabilities of the processing machine. The chief advantages of RIM over the injection molding of thermoplastic polymers are: (1) large parts can be produced at low energy consumption, (2) large parts with varying cross sections with or without inserts can be produced without the problem of sink marks, and (3) lightweight parts, owing to the microcellular structure, can be produced. However, the predominant industrial applications are in the automotive industry; for instance, in the production of automobile fascia. In the 1970s and 1980s, very intensive research activities were reported on a better understanding of the RIM operation. Thermosets must meet with some stringent requirements for RIM operation. These are: (1) viscosities must be fairly low at processing temperature, so that a rapid injection of the feedstreams can be realized; (2) the feedstreams must have sufficient compatibility for efficient mixing by the static impingement mixing technique; (3) cure reaction must be sufficiently fast, such that a finished product can be removed in a very short time after injection is completed; (4) a finished product must have sufficient stiffness and resiliency at elevated temperatures; and (5) a finished product must be released easily from the mold surface, etc. It is then clear that not many thermosets meet these requirements. It has been found that urethanes, with proper chemistry of the components, meet with the requirements. For this reason, urethanes have been the most widely used resin for RIM, although other thermosets (e.g., epoxy) have also been used to some extent.

Compatibilization of Two Immiscible Homopolymers

More often than not, the mechanical properties (e.g., impact and tensile properties) of immiscible polymer blends are very poor owing to the lack of adhesion between the constituent components, which originates from strong repulsive thermodynamic (segmental) interactions. Therefore, in the past, a great deal of effort (Barlow and Paul 1984; Fayt and Teyssie 1989; Fayt et al. 1981, 1987, 1989; Gupta and Purwar 1985; Ouhadi et al. 1986a; Park et al. 1992; Schwarz et al. 1988, 1989; Srinivasan and Gupta 1994; Traugott et al. 1983) has been made to improve the mechanical properties of two immiscible polymers by adding a third component (e.g., a block copolymer). In this chapter, we confine our attention primarily to the situations where a nonreactive third component is added to two immiscible homopolymers in order to improve their mechanical properties. A polymer blend consisting of two immiscible homopolymers (say, A and B) has a very narrow interface, as schematically shown in Figure 4.1, because they have strong repulsive segmental interactions giving rise to a positive value of the Flory–Huggins interaction parameter (χ), i.e., χAB > 0. Helfand and Tagami (1971, 1972) derived the following expression relating the interfacial thickness d of a pair of immiscible homopolymers of infinite molecular weight to χ: . . . d = 2b/(6χ)1/2 (4.1). . . where the Kuhn length b is assumed to be the same for both components. They also derived an expression for the interfacial tension γ between two immiscible homopolymers: . . . γ = (χ/6)1/2bρokBT . . . in terms of χ, where kB is the Boltzmann constant, T is the absolute temperature, and ρo is the reference density (the inverse of monomeric volume of a reference component). Equation (4.1) indicates that the interfacial thickness between two immiscible homopolymers will be larger when the extent of repulsive segmental interactions is less, and Eq. (4.2) indicates that the interfacial tension between two immiscible homopolymers will be lower when the extent of repulsive segmental interactions is less.

Compression Molding of Thermoset/Fiber Composites

Glass-fiber-reinforced thermoset composites have long been used by the plastics industry. Two primary reasons for using glass fibers as reinforcement of thermosets are: (1) to improve the mechanical/physical properties (e.g., tensile modulus, dimensional stability, fatigue endurance, deformation under load, hardness, or abrasion resistance) of the thermosets, and (2) to reduce the cost of production by replacing expensive resins with inexpensive glass fibers. In place of metals, the automotive industry uses glassfiber- reinforced unsaturated polyester composites. One reason for this substitution is that the weight per unit volume of composite materials is quite low compared with that of metals. This has allowed for considerable reductions in the fuel consumption of automobiles. Another reason is that composite materials are less expensive than metals. The unsaturated polyester premix molding compounds in commercial use are supplied as sheet molding compound (SMC), bulk molding compound (BMC), or thick molding compound (TMC) (Bruins 1976; Parkyn et al. 1967). These molding compounds can be molded in standard compression or transfer molds. The basic challenge in molding unsaturated polyester premix compounds is to get a uniform layer of glass reinforcement in place in the die cavity while the resin fills the cavity and reaches its gel stage during cure. Temperature, mold closing speed, pressure, and cure time are all functions of the design of the part being produced. The flow of the mixture through the gate(s) can result in variations in strength across the part due to fiber orientation during the flow. The precise end-use properties depend on the fiber orientation, fiber distribution, and fiber content in the premix compounds, which are greatly influenced by the processing conditions. Since the mechanical properties of the molded articles depend strongly upon the orientation of the glass fibers, it is important to understand how to control fiber orientation during molding. Unsaturated polyester accounts for the greater part of all thermosets used in glass-fiber-reinforced plastics. Glass-fiber-reinforced unsaturated polyesters offer the advantages of a balance of good mechanical, chemical, and electrical properties. Depending upon the application, a number of additives are employed to provide specific products or end-use properties.

Foam Extrusion

There are two processes used in the production of thermoplastic foams, namely, foam extrusion and structural foam injection molding (Benning 1969; Frisch and Saunders 1973). Foam extrusion, in which either chemical or physical blowing agents are used, is the focus of this chapter. Investigations of foam extrusion have dealt with the type and choice of process equipment (Collins and Brown 1973; Knau and Collins 1974; Senn and Shenefiel 1971; Wacehter 1970), the effect of die design (Fehn 1967; Han and Ma 1983b), the effect of blowing agents on foaming characteristics (Burt 1978, 1979; Han and Ma 1983b; Hansen 1962; Ma and Han 1983), and relationships between the foam density, cell geometry, and mechanical properties (Croft 1964; Kanakkanatt 1973; Mehta and Colombo 1976; Meinecke and Clark 1973). Chemical blowing agents are generally low-molecular-weight organic compounds, which decompose at and above a critical temperature and thereby release a gas (or gases), for example, nitrogen, carbon dioxide, or carbon monoxide. Examples of physical blowing agents include nitrogen, carbon dioxide, fluorocarbons (e.g., trichlorofluoromethane, dichlorodifluoromethane, and dichlorotetrafluoroethane), pentane, etc. They are introduced as a component of the polymer charge or under pressure into the molten polymer in the barrel of the extruder. It is extremely important to control the formation and growth of gas bubbles in order to produce foams of uniform quality (i.e., uniform cell structure). The fundamental questions one may ask in thermoplastic foam processing are: (1) What is the optimal concentration of blowing agent in order to have the minimum number of open cells and thus the best achievable mechanical property? (2) How many bubbles will be nucleated at the instant of nucleation? (3) What is the critical pressure at which bubbles nucleate in a molten polymer? (4) What are the processing–property relationships in foam extrusion and structural foam injection molding? Understandably, the answers to such questions depend, among many factors, on: (1) the solubility of the blowing agent in a molten polymer, (2) the diffusivity of the blowing agent in a molten polymer, (3) the concentration of the blowing agent in the mixture with a molten polymer, (4) the chemical structure of the polymers, (5) the initial pressure of the system, and (6) the equilibrium (or initial) temperature of the system.

Coextrusion

Coextruded products were first commercialized in the 1950s by the fiber industry, which produced conjugate fibers (Sisson and Morhead 1953; Hicks et al. 1960, 1967). Subsequently, in the 1960s and 1970s, the plastics industry developed coextrusion processes to produce multilayer films and sheets by extruding two or more polymers. Schrenk and coworkers (Schrenk 1974; Schrenk and Alfrey 1973; Schrenk et al. 1963) pioneered the concept of a coextrusion die system. However, there are a number of technological problems that must be understood in order to achieve successful coextrusion operations. In the 1970s, a number of research groups devoted their efforts to a better understanding of the fundamental problems associated with the coextrusion processes; namely, (1) interface deformation (i.e., encapsulation of one component by another component) during coextrusion (Everage 1975; Han 1973, 1975; Khan and Han 1976; Lee and White 1974; MacLean 1973; Southern and Ballman 1973, 1975; White and Lee 1975) and (2) interfacial instability during coextrusion (Han and Shetty 1978a; Khan and Han 1977; Schrenk et al. 1978). Those efforts are summarized in two monographs by Han (1976, 1981). Since then, further efforts have been made to investigate interface deformation during coextrusion via finite element analysis (Karagiannis et al. 1990; Matsunaga et al. 1998; Mavridis et al. 1987; Mitsoulis 1988; Mitsoulis and Heng 1987; Puissant et al. 1994) and to investigate interfacial instability, both experimentally (Han et al. 1984; Wilson and Khomami 1992, 1993) and theoretically (Anturkar et al. 1990; Khomami 1990; Su and Khomami 1992). Coextruded sheet for thermoformed high-barrier containers has become an important business sector for food and beverage packages for meats, baby food, beer, carbonated soft drinks, etc. Such packaging requires improved barrier protection to extend the shelf life of such products in thermoformed barrier containers. It should be mentioned that the coextruded sheets or films are of little commercial value unless the component polymers adhere together. This means that the component polymers must be compatible,1 at temperatures ranging from service temperature to the melt processing temperature in order to have good adhesion between the layers in the coextruded films or sheets.

Injection Molding

Injection molding is one of the oldest polymer processing operations used to produce goods from thermoplastic polymers. Today, almost all commercial injection molding machines have a reciprocating single screw for softening (or melting) under heat a thermoplastic polymer, and polymer melt is then injected into an empty mold cavity, as schematically shown in Figure 8.1. In the injection molding operation, the mold is first closed and then a predetermined amount of polymer melt from the screw section is injected into an empty mold cavity. Pressure is maintained for some time after the mold cavity has been filled to permit the build-up of adequate pressure in the mold cavity. Cooling water is circulated through channels in the mold so as to keep the mold cavity walls at a temperature usually between room temperature and the softening (or melting) temperature of the polymer. Thus, the hot polymer begins to cool as it enters the mold cavity. When it is cooled to a state of sufficient rigidity, the mold is opened and the part is removed. Some of the important variables in the operation of an injection molding machine are: (1) pressure applied by the screw, (2) temperature profile of the screw section, (3) mold temperature, (4) the screw forward time, (5) the mold closed time, and (6) the mold open time. Relationships between these variables are very complicated. In general, one would like to know the pressure, temperature, and density of the polymer in the mold cavity as functions of time during and after the mold is filled. In principle, these quantities can be calculated, via a mathematical model, during the entire period of mold filling and subsequent cooling when information on the geometry of the mold cavity, the rheological properties of the polymer, the temperature at which the polymer enters the mold cavity, and the mold temperature is available. However, in practice it is not easy to develop a rigorous theory because of the geometrically complex shapes of mold cavities, the complex nature of mold filling patterns (i.e., jetting) at normal injection speeds of industrial practice, and the highly viscoelastic nature of polymer melts, which varies with temperature, pressure, and injection rate (i.e., shear rate in the runner).