Polymer Blends: Overview of Industrially Significant Blends

Polymer Blends: Overview of Industrially Significant Blends

Polymer Blends: Overview of Industrially Significant Blends Since the mid-twentieth century, blending of dissimilar polymers has been a major path to ...

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Polymer Blends: Overview of Industrially Significant Blends Since the mid-twentieth century, blending of dissimilar polymers has been a major path to tailor materials with new properties in industry (Paul and Newman 1978, Utracki 1989, Paul and Bucknall 2000). Especially in the area of engineering thermoplastic materials, this approach has created a significant number of large-volume products, like polyphenyleneether\high impact polystyrene blends (PPE\HIPS), polycarbonate\styrenic blends (PC\ABS), polycarbonate\polybutyleneterephthalate blends (PC\PBT), polyamide\polyphenylene ether alloys (PA\PPE), and polyamide\styrenic alloys (PA\ABS). The commercial success of these materials is mainly related to their combinations of properties that enable their use in a multitude of applications. Therefore, the sales of these polymer blends still increase with higher growth rates than the sales of most engineering thermoplastic materials (Warth and Wittmann 1999). This article summarizes the major features of polymer blending as well as the properties of engineering polymer blends that have reached broad acceptance.

1. Polymer Blending Contrary to the mixing of low molecular weight species, the mixing of two dissimilar polymers usually leads to a mixture with a phase-separated structure. The reason for this behavior can be explained by looking at the thermodynamics of polymer mixtures. The thermodynamics of polymer–polymer mixtures can be treated by a simple lattice model. This treatment illustrates the small contribution of the mixing entropy to the free energy of mixing in polymer–polymer mixtures (see Polymer Mixtures, Thermodynamics of). Therefore, the enthalpy of mixing is mainly responsible for the miscibility of polymer mixtures. The miscibility behavior of different polymers is subsequently related to interactions between the components, which could be of different nature, for example dipole–dipole interactions, ionic interactions, or hydrogen bonding (Olabisi et al. 1979). Therefore, polymer blends can be distinguished by their phase behavior. The most important methods for judging the phase behavior of polymer mixtures are thermal analysis by differential scanning calorimetry (DSC), or microscopic methods. The whole number of polymer blends can be divided in four groups (Table 1). In miscible blends, the chain segments of the different polymers are miscible on a molecular level. These blends show only a single glass transition temperature (Tg), which mainly depends on the composition of the blend. Polyphenyleneether\poly-

styrene-blends are considered to be the most important example for miscible blends. Systems that are either partially miscible or completely immiscible but offer attractive mechanical performance are often designated as compatible polymer blends. These blends usually have two glass transition temperatures, which may slightly deviate from the Tg of the blend components. The deviation of the glass transition temperatures from the Tg of the blend components might be different, and depends on the partial miscibility of each component in the other. On a microscopic scale, these polymer blends have a phase-separated structure (morphology) which could be of different nature, depending on the composition of the blends (Fig. 1). Usually the major component forms the matrix phase (Fig. 1(a) and 1(c), wherein particles of the minor phase are dispersed. In the area of a 1:1 mixture, a bicontinuous morphology can also be observed (Fig. 1(b)). The size of the dispersed particles is related to the interfacial tension and the viscosity ratio between the matrix and the dispersed phase. Usually the particle size of the dispersed phase in compatible blends is in the range of 1 µm to 3 µm. Another important feature of compatible blends is the high interfacial strength in these systems, which is mainly responsible for the attractive properties of these products. The most popular system belonging to this group is PC\ABS. The largest group is the immiscible blends, having a completely phase-separated structure. Therefore, the glass transition temperatures of the components in the blends are exactly the same as for the pure components. Immiscible polymer blends form the same kind of structures as discussed for the compatible blends, but the size of the domains is usually larger (5–10 µm). The interfacial strength in the immiscible blends is quite low, leading to adhesive failure and poor mechanical properties. The morphology of immiscible blends can be modified by the addition of compatibilizers, which act like emulsifiers in oil\water mixtures. In polymer blends, the compatibilizers are usually copolymers (block or graft copolymers) consisting of different segments which are miscible with the respective components of the blend. During the melt mixing procedure the compatibilizer acts as polymeric surfactant (see Block Copolymers and Polymer\Polymer Interfacial Tension), therefore the interfacial tension between the immiscible polymers is reduced, which leads to a significant particle size reduction of the dispersed phase. During the melt mixing process of immiscible polymers, the particle size of the dispersed phase first decreases as a consequence of the applied shear forces. At a certain particle size the tendency for coalescence of the particles increases as a consequence of the increasing number of particles, therefore the average particle size grows again with mixing time (Fig. 2(a)). Since the surface of the dispersed particles is covered 1

Polymer Blends: Overview of Industrially Significant Blends Table 1 Different groups of polymer blends. Number of glass temperatures Tg

Interfacial adhesion

Important systems

Miscible

1

no interface

Compatible

2

high

Immiscible

2

low

Alloy

2

high

PPE\PS, PVC\PBT, PEO\PAA PC\PSAN, PBT\PSAN, PES\PC, PC\PBT PS\PB, PA\SAN, PA\PPE, PA\PP, PS\PE PPE\PA, PA\ABS, (PC\PBT)

Blend type

(a)

(b)

(c)

Figure 1 Schematic representation of possible morphologies in immiscible polymer blends. (a) Polymer B (dark) dispersed in the polymer A matrix. (b) Polymer A and polymer B are co-continuous. (c) Polymer A dispersed in the polymer B matrix.

by the compatibilizer in polymer alloys (Fig. 2(b)), the coalescence rate of the dispersed particles is tremendously reduced, keeping the morphology of the material stable during the later stage of mixing or during subsequent processing steps (Scott and Macosko 1992) (see ReactiŠe Polymer Processing: Interfacial Aspects). In the bulk state, the compatibilizers at the interface provide a strengthening of the interface which is an important issue for the toughness of materials (see Block Copolymers at Interfaces, Mechanical Strength of). Compatibilizers can either be added as a third component or created during the compounding step (Guegan et al. 1994). Since the preparation of welldefined block or graft copolymers is limited to monomers which can be polymerized by anionic or cationic processes, this approach only works for a limited number of blend systems. The in situ preparation of graft or block copolymers during the processing step is a quite versatile technology, often called reactive blending (Xanthos 1992) (see ReactiŠe Polymer Processing: Fundamentals of REX, Block Copolymers and Compatibilization : ReactiŠely Formed). A prerequisite for this technology is of course the availability of polymers with functional groups. For this reason, reactive blending is mainly used to compatibilize blends of polymers like polyamides or polyesters, where reactive endgroups and reactive bonds in the chain allow the synthesis of the respective block or graft copolymers. Technical exam2

ples are of course PA\PPE-alloys or PA\ABS-alloys. Subsequently, examples for interesting technical polymer blends and alloys will be discussed. 2. Important Engineering Polymer Blends and Alloys 2.1 PPE\HIPS Blends In 1956 the synthesis of PPE was developed (Hay 1962). Although PPE had interesting properties like a high glass transition temperature and low flammability, its poor processing behavior limited its technical use. Since 1960 it was known that PPE and polystyrene form homogeneous polymer mixtures. By the addition of polystyrene it was possible to improve the processability of PPE and use these mixtures as thermoplastic materials (Boldebuck 1962). Already in 1965, PPE\PS and PPE\HIPS blends were commercialized by General Electric Plastics (Noryl). All PPE\PS mixtures have a single glass transition temperature which depends on the blend composition. This behavior allows the adjustment of the heat distortion temperature of PPE\HIPS blends by the simple variation of the blend composition. Therefore, one of the major disadvantages of polystyrene, the low heat distortion temperature of approximately 90 mC, can be significantly improved by the addition of PPE. Actually, some of the available PPE\PS blends offer a

Polymer Blends: Overview of Industrially Significant Blends (a)

Particle size

(b)

1. with compatibilizer 2. without compatibilizer

2.

1. Mixing time

Figure 2 (a) Particle size reduction as a function of mixing time during melt mixing of immiscible polymers. (b) Schematic representation of a dispersed particle covered by graft copolymers in a polymer alloy.

heat distortion temperature of up to 190 mC. In order to improve the ductility and solvent resistance of PPE\PS blends, usually HIPS is used as a blend component in commercial products. The further addition of styrene-butadiene block copolymers (SB, SBS) or hydrogenated block copolymers (styrene-hydrogenated butadiene styrene; SEBS) is another tool to improve the toughness of these materials. The major advantage of PPE, its low flammability, is still maintained in PPE\HIPS blends, therefore just the addition of 5–10 wt.% of phosphate esters gives formulations which fulfill the Underwriter Laboratories’ burning tests (UL 94), without addition of halogenated additives. As completely amorphous materials, PPE\HIPS blends also offer good dimensional stability which allows their use up to temperatures near the glass transition temperature without big dimensional changes. Other advantages of PPE\HIPS blends are their low moisture uptake and good hydrolytical stability. Due to this interesting combination of properties, PPE\HIPS blends are excellent materials for a broad range of applications. The main fields of application are any kind of housings in the electrical industry, automotive interiors (dashboard, radiator grills), and machinery (pump parts, etc.). The overall consumption of PPE\HIPS blends already exceeded 300 000 t yr−" in 1998 (Warth and Wittmann 1999).

2.2 PC\Styrenic Blends Since polycarbonate and poly(styrene-co-acrylonitrile) (PSAN) are compatible, the mechanical properties of appropriate rubber-toughened blends are

excellent and exceed some of the properties of the blend components to a certain extend (Bottenbruch 1993). PC\ABS blends were first commercialized (Cycloy) in 1967. Compared to polycarbonate, PC\ABS blends offer improved processability, chemical resistance, and toughness. Alternatively, the heat distortion temperature, as well as the toughness of the ABS, are substantially improved by the addition of polycarbonate (see Polycarbonate, Polystyrene and Styrene Copolymers). The phase behavior of PC\PSAN blends is still somewhat controversial. By DSC measurements, usually a small increase of the Tg of the PSAN phase and a small reduction of the PC glass transition temperature are observed. In some publications (Keitz et al. 1984, Mendelson 1985) this behavior is counted as a partial miscibility of both polymers, whereas other publications discuss the influence of low molecular PC and PSAN (Guest and Daly 1989, Callaghan et al. 1993). This phase behavior is also influenced to a certain extent by the AN-contents of the PSAN. The best phase adhesion between PSAN and PC is observed with PSAN having an AN content of about 25 wt.% (Keitz et al. 1984). Due to the phase-separated structure of PC\PSAN blends there is no linear relationship between the heat distortion temperature and the composition of the materials. The heat distortion temperature of the blend is determined by the nature of the matrix phase. If the PC content exceeds 55–60 wt.%, PC forms the matrix phase wherein the ABS particles are dispersed. Most commercial products have this kind of morphology, which is also responsible for the high impact strength of these blends. Like PPE\HIPS blends, PC\ABS blends offer low flammability, which makes them valuable materials 3

Polymer Blends: Overview of Industrially Significant Blends for applications in the electrical industry. In order to pass the UL-94 burning test, phosphate esters (triphenylphosphate, resorcinoldiphenyldiphosphate) have to be applied with loadings up to 15 wt.%. Since these compounds are miscible with PC as well as ABS, the Tg of both phases are significantly suppressed. This causes a significant drop of the heat distortion temperature. For certain applications, flame retardant PC\ABS with higher heat distortion temperatures (HDT B  125 mC) are necessary. Such products can be obtained by the addition of halogenated flame retardants. As halogenated flame retardants, brominated polycarbonates or polyhydroxyethers are used. Due to the high ductility of PC\ABS blends, these materials are widely used for automotive applications like interiors, mirror housings, panels, and spoilers. One drawback of PC\ABS blends is the low UV and heat resistance, caused by the presence of polybutadiene rubber. Therefore, other kinds of PC\ styrenics blends with more stable rubbers (acrylate rubber: ASA, EPDM rubber: AES) are available. Compared to PC\ABS blends these blends (PC\ASA, PC\AES) offer improved weatherability.

2.3 PC\PBT\MBS Blends PC\PBT blends are a combination of an amorphous (PC) and a semicrystalline (PBT) material. The excellent toughness and dimensional stability of PC is combined with the good melt flow properties and the chemical resistance of PBT. For the improvement in toughness, especially at low temperatures, commercial PC\PBT blends contain core shell-type rubbers with a butadiene\styrene core and a methylmethacrylate shell (MBS rubber). These rubbers are usually exclusively distributed in the PC phase. Different results for the phase behavior of PC\PBT blends can be found in the literature. Depending on the source of the materials used and the blending conditions, PC\PBT blends are considered either partially miscible or immiscible. Since PBT usually contains traces of metal catalysts from the polycondensation process, transesterification reactions between the two polymers lead to the formation of polyestercarbonates (Devaux et al. 1982), which act as compatibilizers. For this reason, PC\PBT mixtures can also be considered as polymer alloys. Subsequently, a major issue for the compounding and processing of PC\PBT\MBS blends is the control of the transesterification reaction. Without rigorous control, this reaction proceeds in the melt until completely amorphous copolyestercarbonates are formed. These copolyestercarbonates are quite tough but suffer from a very low heat distortion temperature. The control of the transesterification reaction can be done by the addition of phosphorous compounds (phosphates, phosphites, or phosphonates), which deactivate the metal compounds to an appropriate 4

extent, allowing the compounding and processing of PC\PBT\MBS alloys. Most of the commercial products have a bi-continuous morphology, which in this system is essential for an optimum combination of dimensional stability, low temperature impact toughness, and chemical resistance. These properties are important as well for the main applications of PC\PBT\MBS blends in the automotive industry, where this material is extensively used for large exterior parts like fenders, body panels, and spoilers. Other fields of applications are large injectionmolded parts for machinery and appliances. 2.4 Polyamide Alloys (a) PA\PPE alloys. PPE alloys offer a unique combination of high heat distortion temperature, solvent resistance, good melt flow, and low temperature ductility. These properties allow the use of PA\PPE alloys for injection-molded body panels, which can be painted by the usual painting techniques used in the automotive industry (online painting). Since PA and PPE are completely immiscible, several strategies to compatibilize this system were developed. The most striking one is the grafting of unsaturated functional monomers to the PPE backbone in the melt (Ueno and Maruyama 1982). During the extrusion procedure, the amino endgroups of polyamide can react with functionalized PPE to form graft copolymers in situ (Fig. 2). These copolymers stay at the interface and reduce the interfacial tension between the immiscible polymers, leading to the formation of submicron PPE particles in the PA 6,6 matrix. In order to boost the toughness of the system, these alloys furthermore contain hydrogenated styreneethylene-butylene-styrene (SEBS) block copolymers. Due to the presence of the styrene blocks, these impact modifiers are located exclusively in the dispersed PPE particles. In the commercial products, the size of the dispersed particles (PPE\SEBS) are below 1 µm to achieve an appropriate toughness level. As a consequence of the high Tg of the PPE phase, these alloys have a high heat distortion temperature of up to 190 mC, which allows their use for online painted panels in the automotive industry. (b) Styrenics\polyamide alloys. The combination of polyamide 6 and, preferably, ABS in a polymer alloy gives materials with excellent ductility, high chemical resistance, and good flow behavior. To overcome the immiscibility of PA 6 and PSAN, terpolymers of styrene, acrylonitrile, and maleic anhydride (MAH) are added. During the compounding step, the amino endgroups of PA 6 react with the anhydride units of the terpolymer and form graft copolymers consisting of PA 6 grafts and the terpolymer as backbone. Since

Polymer Blends: Overview of Industrially Significant Blends

Figure 3 TEM image of an ABS\PA alloy containing 6 wt.% of poly(styrene-co-acrylonitrile-co-maleic anhydride) stained with OsO and phosphotungstic acid (magnification: i10 000). %

the amount of MAH in the terpolymer is in the area of 1 wt.%, the terpolymer is completely miscible with the PSAN phase of the ABS. The optimum performance in terms of ductility is obtained if a terpolymer with 1 wt.% of MAH is used to create the compatibilizer (Lavengood et al. 1986). Another prerequisite for the desired combination of toughness and chemical resistance is the bi-continuous morphology of the blends. The transmission electron microscopy (TEM) image depicted in Fig. 3 shows dark stained rubber particles in a gray-appearing PSAN phase and the co-continuous dark gray stained PA 6 phase. To reach this morphology, the amount of PA 6 has to be in the range of 38–60%. These properties allow the use of ABS\PA 6 alloys for automotive interior applications as well as for bumpers. 3. Concluding Remarks Due to a number of significant advantages, the development of new polymer blends and alloys is still a vital process in industrial and academic laboratories. Since the developments of polymer blends usually

starts from well-known polymers, materials with interesting properties can be obtained with comparatively low investment for product development. If already existing product lines, usually twin screw extruders, are available, flexible production at a competitive cost level is already possible for smaller quantities. Additionally, the possibilities of tuning properties by changing the composition gives the blending technology a high versatility for further developments. Bibliography Boldebuck E M 1962 Recording medium and polysiloxane and the resin mixture thereof. US Pat. 3 063 872 Bottenbruch L 1993 Technische Polymer-Blends. KunststoffHandbuch 3\2. Hanser, Mu$ nchen, Germany Callaghan T A, Takakuwa K, Paul D R, Padwa A 1993 Polycarbonate-SAN copolymer interaction. Polymer 34, 3796–808 Devaux J, Godard P, Mercier J P 1982 Bisphenol A polycarbonate-poly(butylene terephthalate) transesterification. IV. Kinetics and mechanism of the exchange reaction. J. Polym. Sci., Polym. Phys. 20, 1901–7 Guegan P, Macosko C W, Ishizone T, Hirao A, Nakahma S

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Polymer Blends: Overview of Industrially Significant Blends 1994 Kinetics of chain coupling at melt interfaces. Macromolecules 27, 4993–7 Guest M J, Daly J H 1989 The use of glass transition data for characterizing polycarbonate-based blends. Eur. Polym. J. 25, 985–8 Hay A S 1962 Polymerization by oxidative coupling II. Oxidation of 2,6-disubstituted phenols. J. Polym. Sci. 58, 581–91 Keitz J D, Barlow J W, Paul D R 1984 Polycarbonate blends with styrene\acrylonitrile copolymers. J. Appl. Polym. Sci. 29, 3131–45 Lavengood R E, Harris A F, Padwa A R 1986 Rubber modified nylon composition. Eur. Pat. 202 214 Mendelson R A 1985 Miscibility and deformation behavior in some thermoplastic polymer blends containing poly(styreneco-acrylonitrile). J. Polym. Sci., Polym. Phys. 23, 1975–95 Olabisi O, Robeson L M, Shaw M T 1979 Polymer–Polymer

Miscibility. Academic Press, New York Paul D R, Bucknall C B 2000 Polymer Blends. Wiley, New York Paul D R, Newman S 1978 Polymer Blends. Academic Press, New York Scott C E, Macosko C W 1992 Model experiments for the interfacial reaction between polymers during reactive polymer blending. J. Polym. Sci., B 32, 205–13 Ueno K, Maruyama T 1982 Resin compositions. US Pat. 4 315 086 Utracki L A 1989 Polymer Alloys and Blends. Hanser, Munich, Germany Warth H, Wittmann D 1999 Polymer blends. Kunststoffe 10/89, 124–9 Xanthos M 1992 ReactiŠe Blending. Hanser, Munich, Germany

M. Weber

Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 7197–7202 6