Handbook of Plastics Technologies Part 6 docx

9.17 FIGURE 9.11 ABS capacity expansions by region and year (× 1000 Mton). Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.18 CHAPTER 9 duced: a styrene-acrylonitrile (SAN) continuous phase and a discrete grafted polybutadi- ene one. The rubber particles are finely dispersed in the rigid phase. Thus, a situation is created in which copolymerizing acrylonitrile and styrene in the presence of PBD results in an amorphous molding polymer that is much better suited to automotive applications than the polystyrene homopolymer. Although the addition of the more polar monomer brings about improvements in modulus, equally important is a step improvement in resis- tance to impact damage. Figure 9.12 shows transmission electron microscopy images of two distinct morphologies present in commercial ABS resins. The darker domains are droplets of grafted polybutadiene that have been stained by osmium tetraoxide. One can see that the rubber particles are very different in terms of average diameter and appear- ance. Particles with diameters on the order of the wavelength of light can be made with the emulsion process, whereas larger particles with occluded matrix resin is very typical of mass. In mass ABS, control of the particle size and particle density allows for a broad range of gloss. The key function of the dispersed, grafted PBD phase is to dissipate energy in the case of an impact event. There are two types of energy absorption found in ABS: crazing and shear banding. In the case of crazing, rubber particles can dissipate energy by initiating and terminating this type of microcracking. The initial step in the dissipation process is the deformation of the rubber particles to the point of void formation. This void formation, in turn, initiates additional crazes that are terminated at neighboring rubber particles. Crazes characteristically have high levels of surface area in the form of fibrils spanning the craze direction. This is the dominant mechanism for lower AN containing ABS resins. In higher AN containing ABS copolymers, the dominant mechanism becomes shear, yielding evi- denced by the appearance of banding. Submicron-size particles are reported to facilitate this energy dissipation mode. The composition of the ABS has a very large bearing on the final performance of the resin. Figure 9.13 illustrates the relationship between composition and structure in ABS. Polymer scientists carefully balance the proportions of these monomers when designing ABS resins for particular applications. Table 9.8 shows properties typical of some com- mercially available classes of ABS. These material properties are suitable for injection molding automotive parts. ABS copolymers can be used in a wide variety of applications ranging from plated ex- terior grills to molded-in-color glove box doors. However, advances in vehicle aerody- namics have resulted in much more sunlight exposure for interior parts and an overall increase in maximum temperatures. As a rubber modified amorphous resin, the glass tran- sition temperature (T g ) of the rigid phase and the phase volume of the dispersed elastomer have a very large influence on its modulus response to temperature. There are numerous approaches to extending the upper service temperatures of ABS. These include alloying with higher-heat polymers, reinforcing with fibrous fillers, and terpolymerization. This group of chemically modified resins is commonly referred to as high-heat ABS (HHABS). FIGURE 9.12 ABS phase development schematic. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.19 Terpolymerization is a popular method of raising the heat resistance of ABS, and these resins account for a large portion of the volume used in automotive applications. That is, an additional monomer is added to the polymerization. These monomers can act in two ways when incorporated in the rigid phase: chain stiffening and/or modification of the co- hesive energy density. Table 9.9 lists the T g of some common styrenic polymers. You can see that the influence of comonomer AN is a modest 0.3 to 0.4ºC/percent. The three most commonly used monomers that increase the T g of ABS are substituted styrenes, maleim- ides, and maleic anhydrides (MAs). The majority of HHABS uses α-methylstyrene (αMS) or N-phenylmaleimide (NPMI) for enhancing the T g . The efficiency with which these monomers raise the T g depend on the chemistry employed. The most common monomer added to HHABS is αMS. Reaction kinetics change significantly with the introduction of TABLE 9.8 Typical Properties of ABS Resins for Automotive Applications Property Test method (ASTM) High- flow mass General- purpose mass High- impact mass General- purpose emulsion Specific gravity D-792 1.05 1.04 1.03 1.04 Coefficient of linear thermal expansion (cm/cm, °C) D-696 7.6E-05 9.3E-05 9.3E-05 8.8E-05 Notched Izod impact strength (23°C, J/m) D-256 160 310 553 203 Flexural modulus (MPa) D-790 2170 2070 1980 2620 Tensile yield strength (MPa) D-638 39 42.2 37 43 Ultimate elongation (%) D-638 60 25 30 40 Melt flow rate (230°C, 3.8 kg) D-1238 6.5 2.5 0.9 8 Heat deflection temperature (unannealed, °C) D-648 80 83 82 79 FIGURE 9.13 Monomer contribution to ABS resin performance. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.20 CHAPTER 9 this substituted styrene, requiring that the emulsion process be used for higher levels of monomer incorporation. Maleic anhydride (MAH) is an excellent monomer for modifying styrenics for heat resistance; however, it can’t be used in the polymerization of ABS be- cause of its tendency to induce cross-linking of the rigid phase. This can be avoided by blending a styrene-MAH copolymer with ABS in a compounding step. These two poly- mers are highly compatible. Finally, NPMI has recently found its way into HHABS in both the emulsion and mass process. This monomer is especially reactive with styrene and compatible with multiple processes. 9.2.2.7 PC/ABS Blends. Polymer blends have an important role in bringing property options for automotive applications that can’t be reached with a single material. Unfortu- nately, due to unfavorable thermodynamics, only a few examples of miscible blends have found their way into commercial use in this industry. In contrast, many compatible (or par- tially miscible) blends have reached commercial importance and can be found in key ap- plications. Numerous examples are impact modified resins: ABS, TPO, PVC, IM-PC, and so forth. However, as a binary mixture of resins approaches equal volume fraction in the blend, the requirements for compatibility at the phase interfaces increases dramatically and excludes the majority of combinations. One such blend having properties that make it unique and useful in automotive applications is PC/ABS. The concept of blending PC and ABS dates back to 1964 with U.S. patent no. 3130177, Borg Warner’s grandfather patent in the area. Additional inventions have ensued: a patent was granted to Teijin in 1974 that covers impact strength and rubber location in PC/ABS/MBS blends (U.S. patent no. 3582394). U.S. patent no. 3880783 was issued in 1975 to Bayer, which demonstrates that polycarbonate and ABS (emulsion polymer) blends give high gloss and good impact. U.S. patent no. 4098734 was issued in 1978 to Monsanto, showing that improved properties are obtained for PC/ABS alloys when bimodal rubber particles are employed. Based on its versatility, this blend is considered a mainstay engineering material for applications such as automotive instrument panels, body panels, and wheel covers. Although ABS and PC are extremely useful amorphous resins in their own right (see the ABS and PC sections), they have some limitations that can be solved by their blends. Although PC has exceptional clarity, toughness, and heat resistance, it is notch sensitive and more difficult to process than ABS resins. Similarly, ABS is a tough material, very processable, and adheres to paint and foams well. Due to exceptional compatibility be- tween the phases, alloys of these two resins result in a resin that has a unique combination of their properties. PC/ABS blends are noted for having high heat properties, stiffness, and toughness with less notch sensitivity, improved processability, and versatile surface char- acteristics. TABLE 9.9 Properties of Some HHABS Resins, Medium and High Heat Range Heat range Grade Supplier Technology MFR (g/10 min, 220°C, 10 kg) Charpy impact (kJ/m 2 ) Vicat (°C) Medium heat MAGNUM ™ 3325MT Dow Chemical Co. Mass, low residuals 10 18 101 BDT 5510 GE Plastics αMS, mass 15 13 100 High heat MAGNUM ™ 3416SC Dow Chemical Co. NPMI, mass 6.5 18 108 Ronfolin HX-10 BASF Corp. αMS, emulsion 3.5 12 110 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.21 Since there are at least two major, immiscible phases in the PC/ABS blends, it is not surprising that careful attention has to be paid to developing an optimum morphology dur- ing processing. Figure 9.14 shows the morphology developed in a commercial grade of PC/ABS. Note that the darkened phase is ABS surrounded by PC. Thus, scales as diverse as molecules through to phase morphology, and part scale millimeters to meters, contrib- ute to useful parts. Numerous studies have been done to understand the role that composi- tion of the neat polymers, phase volumes, compatibilization, and processing play in the ultimate performance of this blend. The thermodynamics of polymer blends, although semiquantitative, play a key role in understanding phase morphology. For the ABS composition, Callaghan et al. 8 determined that an optimum AN level exists at about 25 percent. This gives the highest level of phase adhesion and compatibility. Morphology development at the micron scale must also be op- timized so as to get the most desirable toughness. Figure 9.15 shows the response of im- pact resistance to the weight percent of PC in the blend. Clearly, toughening is optimum near the composition at which the ABS phase first becomes co-continuous (or 65/35 per- cent by weight PC/ABS). Stability of both the morphology and the components must be considered in this sys- tem. Although it can undergo oxidative chain scission under very extreme temperatures, mass ABS is known to be robust during molding processes. Polycarbonate, with its active carbonyls, is known to undergo hydrolysis and can be attacked by basic impurities result- ing in molecular weight losses. Figure 9.16 shows the effect of time and temperature on the molecular weight of the PC portion of the blend. Note that, if PC molecular weight falls below 20,000 amu, then embrittlement of that phase occurs. The rate of degradation is a function of some the common impurities and even moisture. Depending on the level of these agents, degradation can be changed dramatically. Plastics producers understand the FIGURE 9.14 Transmission electron micrograph of a PC/ABS blend (phases light to dark: SAN, PC, polyb- utadiene). Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.22 CHAPTER 9 need to start with PC and ABS with a minimum of impurities to have molecular weight control through compounding and processing into parts. Impurities in ABS are known to be strongly dictated by the polymerization process employed. The process to manufacture emulsion ABS involves substances that are detrimental at trace levels (surfactants, flow aids, heat stabilizers, and so on) compared to mass ABS. In addition, the mass process ex- poses the polymer to less heat history during its manufacture than emulsion. Likewise, im- purities in polycarbonate or PC blends, particularly those capable of an alkaline reaction, can reduce its resistance to moisture. Stability of the melt is especially significant as resins suppliers push to minimize viscosities during molding via lowering the feedstock molecu- lar weights. High-flow PC/ABS resins must start with high-purity feedstocks to assure that molecular weight attrition doesn’t lead to brittle parts. Table 9.10 shows the properties typical of commercial PC/ABS resins. Four perfor- mance types are listed: general-purpose, high-flow, blow molding and low-gloss grades. Some important characteristics of PC/ABS can be learned from this table. For example, these PC/ABS resins are all engineered with low-temperature ductility and a robust modu- lus. The rheology is modified to meet the forming applications. 9.2.2.8 LGF PP and ABS. Estimates are that about 30 percent of the 2 million Mton of e-glass fiber consumed globally for polymer reinforcement is used in thermoplastic com- posites. Glass-filled thermoplastic composites have been growing at a very healthy pace of 15 to 20 percent per year, largely fueled by automotive applications. Two key reasons that glass-reinforced thermoplastics are becoming so important are their recyclability and com- patibility with the injection-molding process. One of the most revolutionary technologies to come to the forefront is the long glass fiber-reinforced (LGF) resins. Specifically, the use of long glass fiber reinforcement in polypropylenes has allowed the use of a lower-cost FIGURE 9.15 PC/ABS impact resistance parallel and perpendicular to flow vs. blend composition. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.23 TABLE 9.10 Properties of Some Typical Automotive Grades of PC/ABS Properties ISO test method Unit General- purpose Easy- processing Blow molding Low- gloss Specific gravity 1183 B; 1987 g/ml 1.13 1.14 1.13 1.13 Tensile modulus 527-2; 1993 MPa 2500 2390 2350 2500 Tensile yield strength 527-2; 1993 MPa 53 55 54 51 Elongation at break 527-2; 1993 % 125 110 115 120 Flexural modulus 178; 1993 MPa 2375 2375 2400 2300 Notched Izod impact strength (23°) C (–30°) C (–40°) C 180-4A; 1993 180-4A; 1993 180-4A; 1993 180-4A; 1993 kJ/m 2 kJ/m 2 kJ/m 2 kJ/m 2 — 50 36 24 — 46 34 25 — 53 25 20 — 52 32 24 DTUL (0.45 MPa), unannealed 75A; 1987 ° C 131 130 132 128 DTUL (1.8 MPa), unannealed 75A; 1987 ° C 109 110 109 106 Melt flow rate (MFR), 260°C 3.8 kg 1133; 1991 g/10 min 7 18 4 4.5 FIGURE 9.16 Effect of molding time and temperature on PC molecular weights in PC/ABS blends. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.24 CHAPTER 9 polymer to be used in structural, engineered applications. LGF-PP is especially interest- ing, since the glass fiber’s enhanced reinforcing abilities and the cost/performance of PP make it a very economical engineering material. This material is quickly gaining in impor- tance, as evidenced by annual growth rates of >35 percent over the last four years. Long glass fiber PP derives its unique properties by artfully combining a low-density, semicrystalline PP resin with compatibilized e-glass fibers in such a way as to preserve the filler aspect ratio. All references to fiber length will be for the molded part, not the starting fiber length. It has long been understood that this is a key to maximizing the potential of glass fiber reinforcement of PP; however, the combination of material and transformation science has just now gelled to make this a popular commercial option. Composites theory can be used to describe the effect of fillers on modulus as a function of aspect ratio. * (Note that this predicts modulus in the direction of fiber orientation.) These relationships can be seen in Fig. 9.17. Another important property improvement seen in LGF-PP is impact re- sistance. Figure 9.18 compares the falling dart impact (FDI) strength and tensile properties of long and short glass-reinforced PP. Clearly, an impressive level of toughening is achieved with long glass fiber reinforcement. Material scientists attribute this drastic im- provement † in impact energy management to a mechanism whereby energy is dissipated due to slippage at the long glass/PP interface. * Fiber length/diameter ratio. † Relative to short glass-reinforced PP. FIGURE 9.17 Effect of filler volume percent on modulus improvement. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.25 FIGURE 9.18 Comparison of falling dart impact and tensile properties of GF-PP. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.26 CHAPTER 9 Another important element to attaining optimum properties via LGF-PP is the addition of a compatibilizing, grafted additive. Because of the chemical mismatch at the interface of PP and e-glass fibers, without additional treatments, there is the potential for very poor en- ergy transfer between fiber and matrix. Numerous microscopic studies have verified poor adhesion of glass fibers to PP and motivated the development of grafted polypropylenes to improve the adhesion. The addition of maleic anhydride grafted PP (MA-g-PP) has been shown to improve the adhesion between the two major phases. Figure 9.19 compares the relative adhesion of a fracture surface with and without modification by MA-g-PP. Smooth fibers indicate very poor adhesion and stress transfer, whereas a roughened glass surface is indicative of cohesive failure at the glass-PP interface. Better interfacial bonding has been shown to be a strong contributor to modulus in these types of composites. LGF-PP properties are very much dependent on the conversion process used to incor- porate the fibers and form the parts. Thus, representative properties should be reported for each of the major processes and glass levels. Table 9.11 gives a summary of properties from both direct compression and pellet injection. Three glass fiber levels were chosen: 20, 30, and 40 percent. PP can be modified with long glass fibers and formed into articles in a number of ways. However, two very distinct methods are used to incorporate PP and long glass fibers: di- rect and pellet processes. In the direct process, glass roving is fed to a portion of the form- ing process, and the fiber filled melt is transferred to either an injection or compression molding process. Alternatively, if existing injection molding capital must be used, then molders have the option of purchasing a material from the pellet process. In the direct process, glass roving is added to a stage of the process where shear can be carefully controlled and fiber lengths are maximized. Two major direct processes are com- mercially practiced: injection and compression. The direct injection process uses a cou- pled process whereby a compounding extruder is coupled to an injection molding press. Typically, the extruder is operated continuously and fills an accumulating tank that, in turn, feeds the molding machine. In this case, the extruder can clearly be seen mounted on the top of the press. Direct compression is also linked to the forming process, and glass modified, molten “buns” are transferred to a compression press. Pellet processes are practiced by suppliers who may also do more conventional mold- ing. The original process, begun by Fiberfill Co. in the 1950s, consisted of simply pulling glass rovings through wire coating dies. This incomplete wetting of the glass fibrils re- FIGURE 9.19 Scanning electron micrograph of PP-glass fracture surfaces. The sample on the left had no compatibilizer, sample on right had MA-g-PP added. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2006 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS [...]... designed to bolster impact performance of the part TABLE 9.12 Properties of Direct Compression LGF-PP with Impact Modifier Added On-Line (WO 2004/035295) 1 Composition PP POE Glass Color SA Properties FM, MPa FS, MPa TS, MPa TE , % IDI @ 3 mm, J IDI @ 4 mm, J IDI @ 5 mm, J 2 3 4 67 0 30 2 1 65 2 30 2 1 63 4 30 2 1 61 6 30 2 1 4 760 97 52 1.4 15 18 29 5330 113 55 1 .6 14 20 33 5000 115 53 1.4 14 22 33 4820... influence the quality of the product and output of the process include impregnation chamber design, die geometry, line speed, number of strands, and extruder output Line speed must be balance against the desired degree of fiber wet-out with minimal free fibers Long glass fiber PP offers a unique engineering option of automotive engineers in place of metal or high-performance engineering thermoplastics (ETPs)... (MPa, 2 mm/min) Izod impact @ 23°C (kJ/m2) 20 1.03 96 2.7 5.0 140 30 1.12 120 2 .6 6.7 170 40 1.22 78 .6 2 5.7 1 46. 9 15.3 30 1.12 54.5 1.5 5.0 108.3 15.1 sulted in poor dispersion and suboptimum part properties Although this process was ultimately abandoned, development began again in the 1980s, fueled by the rapid growth of advanced composites As a result of their intense efforts, pultrusion processes to... SA == glass/PP-compatibilizer 9.3 REFERENCES 1 Market Analysis of Thermoplastics Elastomers, Robert Eller Associates, Inc., September 2000 2 N Kawamura., et al., Super Olefin Polymer for Material Consolidation of Automotive Interior Plastic Parts, SAE Technical Paper 960 2 96, 19 96 3 A Y Coran and R Patel, Rubber-Thermoplastic Compositions Part I EPDM-Polypropylene Thermoplastic Vulcanizates, Rubber Chem... 300,135,1898 6 J E Jansen, U.S patent no 2, 468 ,982, 1949 7 G M Kissinger and Nicholas P Wynn, U.S patent no 5, 362 ,400, 1994 8 T A Callaghan, K Takakuwa, D.R Paul and A.R Padwa, Polymer 34, 37 96, 1993 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 20 06 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given.. .PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.27 PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS TABLE 9.11 Example Properties of LGF-PP at Three Levels of Glass Reinforcement Pellet Direct Process Injection Injection Compression Compression Long glass fiber, % Specific gravity... given at the website PLASTICS AND ELASTOMERS: AUTOMOTIVE APPLICATIONS 9.28 CHAPTER 9 FIGURE 9.20 Automotive applications for LGF-PP demonstrates that, by adding a specific levels of elastomer impact modifier (i.e., ethylene/ octene copolymer) directly to the process, impact strengths can be greatly increased This masterbatch approach could also bring other functionality to the part, such as weather... for the life of the vehicle Although various methods for impact modifying LGF-PPs are possible, a novel approach using masterbatches was recently reported by Richardson et al (WO 2004/035295 A1) She Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 20 06 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given... absorption Numerous applications for LGP-PP have been developed The most popular applications in automotive are: instrument panels, underbody shields and front end carriers Figure 9.20 shows the relative share of each application As LGF-PPs find their way in to new applications, it is only natural that additional performance requirements are requested by automotive molders One such development is for exterior, . larger particles with occluded matrix resin is very typical of mass. In mass ABS, control of the particle size and particle density allows for a broad range of gloss. The key function of the. 1.04 Coefficient of linear thermal expansion (cm/cm, °C) D -69 6 7.6E-05 9.3E-05 9.3E-05 8.8E-05 Notched Izod impact strength (23°C, J/m) D-2 56 160 310 553 203 Flexural modulus (MPa) D-790 2170 2070 1980 262 0 Tensile. Padwa, Polymer 34, 37 96, 1993. TABLE 9.12 Properties of Direct Compression LGF-PP with Impact Modifier Added On-Line (WO 2004/035295) 1234 Composition PP 67 65 63 61 POE 02 46 Glass 30 30 30 30 Color