Environ. Sci. Technol. 2009, 43, 2563–2568
Reactive Processing of Recycled Polycarbonate/Acrylonitrile Butadiene Styrene WOO-HYUK JUNG,† YEON-SIL CHOI,‡ JUNG-MIN MOON,‡ N A T H A N T O R T O R R E L A , §,| CHARLES L. BEATTY,§ AND J A N G - O O L E E * ,‡ Nano Materials R&D, Thermolon Korea, Ltd., 16th Floor, Cangen Tower, 6-ga 12, Jungang-dong, Jung-gu, Busan 614-714, Korea, Department of Polymer Science and Engineering, Pusan National University, 30 Jangjeon-dong Geumjeong-gu, Busan 609-735, Korea, and Materials Science and Engineering Department, University of Florida, P.O. Box 116400, Gainesville, Florida 32611
Received November 30, 2008. Revised manuscript received February 4, 2009. Accepted February 11, 2009.
Cellular phone housings were ground to make original particulates using a knife mill. Foams and adhesives with a lighter density than water were removed from ground mixtures using a sink-float process in water; ground metals, button rubbers, and wires were separated from desired materials by using a sink float process in salt. All housing materials, consisting of seven thermoplastics included in cellular phone housings, showed better tensile properties than pure housing materials made of polycarbonate/acrylonitrile butadiene styrene, but they only had about half of the impact strength. In contrast, the low impact strength for all housing materials was improved by adding 25 wt % polyethylene elastomer and/or 2.4 wt % ground epoxy circuit boards for batch mixing. Impact strengths, tensile strengths, and the energy absorption ability of all housing materials were improved by adding 5.4 wt % glycidyl methacrylate for twin screw extrusion.
Introduction General solid waste, accumulated extensively each year from households, industrial plants, and municipalities, has caused a great dilemma to the world for a long time. This situation has become more aggravated as societies and the waste sources have expanded. As household incomes increase, personal consumption and industrial production increase, which causes an increase in solid waste. Disposal techniques for solid waste were devised, as industrial and municipal interest grew, which stimulated the original sources generating the solid waste to develop ways to deal with it in an economical and environmentally sound manner. Incineration, composting, and recycling have been the means of disposing of domestic, trade, office, industrial, garden, street, park, and building refuse through the separation of wastes * Corresponding author phone: +82-51-510-2404; fax: +82-51513-7720; e-mail:
[email protected]. † Thermolon Korea, Ltd. ‡ Pusan National University. § University of Florida. | Present adderss: Kohler Company, Mail Stop 063, 444 Highland Dr., Kohler, WI 53044. 10.1021/es803355t CCC: $40.75
Published on Web 03/06/2009
2009 American Chemical Society
(1). Plastic solid waste has potentially been reused by the separation of undesirable materials and thermoplastics. Plastic product waste is pretreated by size reduction methods, which generally contains external materials such as metals, wires, thermosets, or glasses. Sorting techniques to separate undesired materials from desired ones are considered important techniques for plastic recycling (2). For instance, plastic products that are recycled include carpets (3), communication cables, beverage bottles (4), office goods, automotive parts (5-7), rubber (8), and electronic appliances. Specifically, electronic products commonly have various foreign materials, where specially designed size reduction machines and sorting techniques are needed for their granulation and separation. One of the most interesting materials to obtain from electronic appliances is polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) alloys, which combine the advantages of PC with those of ABS. The outstanding properties of PC/ABS alloys include its comparatively high impact strength, high distortion temperature under load, high ignition resistance, good ultraviolet and color stability, and excellent tensile and flexural strength (5, 9). Reactive modifications of the impact strength of recycled PC/ABS or PC/ABS alloys were carried out by impact modifiers such as poly(methyl methacrylate-butadienestyrene) (5), polymaleic anhydride-g-polypropylene, epoxy (2 phr) (10), and polymaleic anhydride-g-ABS. A major component of the recycled cellular phone housing in this work was the PC/ABS alloy, which was reactively modified with GMA by using a batch mixer or twin screw reactive extruder. The incompatibility and hydrophobicity of polyethylene (PE), added as an impact modifier, were improved by reactive processing with glycidyl methacrylate (GMA). GMA was chosen as the reactive species for this work because the reactivity and hydrophobicity of GMA were lower and higher, respectively, than those of epoxy, but the compatibility of GMA with all of the housing was better than that of epoxy. GMA had already been used for blending poly(ethylene-1octene) with high density polyethylene and copolymers of 3,5-dimethylphenyl methacrylate and GMA (11), as previously studied (12). Sink-float processes, using various separating liquids, were studied for plastic recycling because sink-float methods using water for the analysis of minerals were reported in 1941 (13). A stabilized heavy medium of sodium or lithium salt such as sodium dihydrogendodecawolframmate, Na6(H2W12O4), was used for recycling electrical scraps with a density ranging from 1.01 to 2.9 g/cm3 (14). Near-critical and supercritical fluids composed of carbon dioxide and/or sulfur hexafluoride have been produced to sort thermoplastic waste mixtures with a density of 0.80-1.60 g/cm3 at room temperature (15). Reactive twin screw extruders were used for reactive injection molding because of the ease in controlling the reaction temperature (16). Comparison of mechanical properties between nonreactive and reactive polyalloys for reactive batch mixing and twin screw reactive extrusion was performed via three routes. Reactive blending for polypropylene/ polycaprolatam (80/20) was carried out at three different places, where the first trial was to perform functionalization and reactive compatibilization simultaneously on the feed. The second trial was to perform functionalization, reactive compatibilization, and devolatilization in sequence. The third trial was to separate functionalization and reactive compatibilization by devolatilization (17). In this work, we used the first trial to functionalize partially compatible polyalloys. VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Profile of a twin screw extruder.
FIGURE 2. FTIR sprecta of two mixtures compounded by a twin screw extruder. FIGURE 4. Tensile properties of the all housing/PE/epoxy (1/1/ 0.4) mixture, according to GMA content, compounded by a batch mixer.
FIGURE 3. FTIR spectra for all housing/PE (1/1) mixtures, according to GMA content, compounded by a batch mixer.
Experimental Procedures Materials. The Motorola cellular phone housing of i1000 plus cellular phones was ground by a knife mill (Cairo Mfg., Inc., φ3 mm screen), and desired materials were separated using sink-float processes in water and salt. Ground mixtures consisted of 15.2 wt % metals, 1.9 wt % foams, 1.4 wt % rubbers, and 81.4 wt % thermoplastics (termed all housing) with a major component of PC/ABS alloys (about 62 wt %). The foam particles and metal parts were removed from the mixture by floating and precipitation of materials using a sink-float process in water, respectively. Residual wires, rubbers, and metals were separated from the thermoplastic mixtures using a sink-float process in a 21 wt % salt solution because the density of common rubbers was 1.07 g/cm3, which is similar to the density of a 20 wt % salt solution. GMA (Aldrich Chemical Co.), a reactive species, was purified to remove inhibitors of 50 ppm p-methoxy phenol using column chromatography, as described by H. Tang (18). Recycling Scheme. All metal parts, including aluminum electromagnetic interference (EMI) plates, batteries, and 2564
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ferrite antennae, were removed using disassembly and sink-float process steps. Solder and metals, attached on printed circuit boards, were separated using electrostatic separation after they were finely ground with a hammer mill (Hosokawa Co., pulverizing mill). Thermoplastics, recovered from the cellular phones, were processed with a reactive process and could be potentially used for automotive, office, and home supplies. The sink-float processes effectively separated PC/ABS materials from the mixture. Thermoset particles were recovered using a density difference by the sink-float process and then ground to be very fine powders using the hammer mill. The ground solder and metal parts were separated from desired epoxy powders by electrostatic separation using the roll method. Fine thermoset powders were compounded with all housing materials and PE for use in the applications of decking for boats and machine shop floors. Reactive Compounding. All housing, PE, and/or ground epoxy powders were compounded at 170 °C for 2 h using a batch mixer (Beken blade blender). GMA was added by continuously floating and dropping through a condenser during compounding. A specially designed high-pressure pump (Zenith pump) was used for reactive processing, with a corotating and intermeshing twin screw extruder (φ30 mm, APV Chemical Machinery). GMA, mixed with 2 wt % dicumyl peroxide, was fed at 5.4 g/min from a Zenith pump directly to a feed hopper, while the feeder transferred the ground housing and circuit boards and/or PE to the feed hopper at a 18.3 g/min feeding rate. The profile of the extruder used for simultaneous functionalization and reactive compatibilization is shown in Figure 1. The screw speed was set to 100 rpm at 275-281 °C, and heating and cooling were controlled by a cooling unit filled with coolant. The comminuted particles of circuit boards were ground with the hammer mill again and separated with a set of testing sieves using a
TABLE 1. Mechanical Properties of Reactive Polyalloys and Polymer Mixtures Compounded by a Batch Mixer all housing/PE/epoxy (223.6 µm)a GMA (wt %) impact strength (J/m) rtc LNd modulus (MPa) stress at break (MPa) stress at yield (MPa) strain at break (%) strain at yield (%) energy to break (Nmm) energy to yield (Nmm)
0
14
18
22
0
17
21
25
292 117 239 4.2 4.2 4.0 4.0 582 582
220 103 242 1.3 1.4 3.3 1.2 173 78
192 124 297 1.1 1.1 2.3 1.5 105 28
181 105 367 2.1 2.1 1.2 1.2 69 67
482 101 188 4.2 4.2 4.5 4.5 530 530
273 91 216 1.3 1.4 1.8 1.5 65 51
316 73 296 1.7 1.8 1.4 1.2 73 47
220 64 171 2.0 2.0 3.0 3.0 181 181
a Compounded at a ratio of 1/1/0.4. liquid nitrogen (LN).
b
Compounded at a ratio of 1/1.
TABLE 2. Mechanical Properties of Polymer Mixtures Compounded by a Twin Screw Extruder polyblends GMA (wt %) impact strength (J/m) notched at rtb modulus (MPa) breaking stress (MPa) stress at yield (MPa) strain at break (%) strain at yield (%) energy to break (Nmm) energy to yield (Nmm)
all housing/PEb
all housing/PE/epoxy/GMA mixturesa 0
5.4
295
237
149 1.7 2.7 5.8 2.9 362 160
457 3.6 5.0 3.9 1.5 439 150
a Compounded at a ratio of 1/1/0.05. temperature (rt).
b
Fractured at room
gyro sifter as filler. The mean diameter of the ground circuit boards was 37 µm, measured with a particle size distribution analyzer (Coulter LS 230). Characterization. Izod impact test specimens were molded at 260 °C via compression molding. Notched impact strengths for both unreactive and reactive polyalloys were measured according to ASTM D256 using a 2 or 10 ft-lb/in. weight pendulum. Notching was performed using a TMI notching cutter, and specimen size was 63 mm × 12.8 mm × 12.6 mm and 8.8 mm in ligament length (specimen width minus the notch depth). Tensile strength test specimens were also molded at 260 °C using compression molding according to ASTM D638, and the tensile strength test was performed using a MTS model 1122 instrument at a crosshead speed of 5 mm/min with a load cell of 20 lb. Fourier transform infrared (FTIR) spectra were obtained with an Ar-Ne laser with a 20 mL of liquid nitrogen flow rate by using a Nicolet MR-740 FTIR spectrometer. The fracture surfaces of polyblends, polyalloys, and polymer mixtures, produced from impact test specimens at a liquid nitrogen temperature, were observed using a Jeol JSM-6400 scanning electron microscope (SEM). The fracture surfaces, bonded to an Al plate, were coated with a thin layer of Au-Pd formed by vapor evaporation to minimize electrostatic charging in the SEM. Dynamic mechanical analysis (DMS) was carried out at a fixed frequency of 1 Hz and a heating rate of 2 °C/min using a DMS 110 Seiko instrument. The specimens were molded using compression molding to a size of 10 mm × 3 mm × 50 mm, with a scanning temperature ranging from -100 to 170 °C.
Results and Discussion Grafting Reaction of GMA. Dicumyl peroxide was thermally and homolytically dissociated at 120-140 °C. Its half-life at
c
Fractured at room temperature (rt).
d
Fractured at
145 °C is 16.8 min (19), but its half-life can be as short as several seconds at the die temperature of 280 °C. Polymerization of GMA was initiated by radicals of the peroxides. Thermally initiated chain ends were propagated by adding more GMA, which produced actively pendent glycidyl groups. Even though the pendent glycidyl groups had a relatively weak reactivity, many of them could participate in ring opening reactions caused by tertiary carbon atoms in the polymer chains of all housing materials to attain grafting reactions. Termination reactions were carried out by recombination or disproportionation of propagating radicals. The ring opening of the glycidyl groups produced hydroxyl groups compounded by a twin screw extruder (Figure 2). The hydroxyl groups’ vibration absorption was observed at 3260 cm-1, which is a weak peak for the mixture of all housing/ PE/epoxy (1/1/0.05) that was added to 5.4 wt % GMA. The ring-opening reactions for batch mixing were also confirmed for the all housing/PE (1/1) mixtures, with various GMA contents, compounded at 170 °C for 2 h. The peaks of -OH, corresponding to the resultants of the ring-opening reactions of the glycidyl groups, were not observed for the all housing/ PE (1/1) mixture with no GMA, but when the GMA was added, the peak appeared around 3500 cm-1 and increased gradually as the GMA content increased (Figure 3). Mechanical Properties of Reactive Compounding. Adding GMA significantly affected the mechanical properties of the mixtures. Stress-strain curves for the reactive polymer mixtures of all housing/PE/epoxy (1/1/0.4), with a different content of GMA, are shown in Figure 4a. As the GMA content increased, the yield stress, the strain at break and at yield, and the energy to break and to yield decreased, but the tensile modulus increased. This indicated that GMA had the same tendency as the filler, increasing the modulus and decreasing impact toughness 22 (Figure 4b and Table 1). As the GMA content increased, the tensile strength at yield decreased and then increased slightly, resulting from the reactive materials hardened by grafting onto other polymer chains. Tensile stress at yield decreased and then increased slightly, as observed for reactive mixtures of all housing/PE/epoxy. Mechanical behaviors for the reactive polyalloy of the all housing/PE (1/1) mixtures, compounded by a batch mixer according to the GMA content, are also shown in Table 1. Tensile strength at break and at yield and energy to break and to yield also decreased and then increased as the GMA increased. This is because poly(glycidyl methacrylate) [poly(GMA)] acted as an impurity at low GMA content, but the interfacial bonding increased at high GMA content. The polymerization of GMA slightly increased the impact strengths for the batch mixing (Table 1). The mechanical properties of the polymer mixtures, compounded by a twin screw extruder, are listed in Table 2. Adding GMA greatly influenced the tensile strength. When VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. SEM images of the fracture surfaces of the polyalloys compounded by a batch mixer at 170 °C for 2 h.
FIGURE 5. SEM images of the fracture surfaces of the all housing/PE mixtures with various PE content. GMA of 5.4 wt % was added to a mixture of all housing/ PE/epoxy (1/1/0.05), the impact strength decreased, but the modulus and breaking and yield stress increased significantly, which showed opposite results compared to those of the all housing/PE/epoxy mixtures compounded by a batch mixer (Table 1). This could be explained by GMA, reacting with the surface of the epoxy powders, producing rigidly covalent bonds between the surfaces of the epoxy powders and matrix. When moving through the die, the melt was oriented to a 2566
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machine direction, extended by mechanical strengths, and then cooled in water. The melt orientation during the extrusion potentially increased the tenacity of the polymer mixture, reducing the impact strength, even though some orientation memory of the extruded polymer mixture could possibly be removed during compression molding. Influence of Reactions on a Fracture Surface. Fracture surfaces of all housings, compounded by a twin screw extruder according to the PE content, were obtained by impact tests and observed by SEM (Figure 5). The particle sizes of PE increased when the PE contents increased, and the PE particles were well-dispersed through the whole matrix, contributing to the high impact strength. The impact modification of PE was similar to that of the rubber particles dispersed into a rigid matrix, and a significant improvement in the impact strength was observed at 12.5 and 25 wt % PE (Figure 5a,b). The highest impact strength at 25 wt % PE was ascribed to the well-dispersed PE phases with relatively regular sizes and the continuality of all housing phases. Weakening the interaction between the PE particles and matrix created flexible surfaces, resulting in weak mechanical properties at 50 wt % PE. Obviously, too much added PE phase broke the continuality of the all housing matrix and became a new continuous phase at 50 wt % PE, showing weak adhesion to the all housing matrix (Figure 5c). Fracture surfaces at the liquid nitrogen temperature of the all housing/ PE (1/1) mixtures and the all housing/PE/GMA (1/1/0.4) mixtures (referring to 17 wt % GMA), compounded by a batch mixer, were observed by SEM. The PE particles were seen as
FIGURE 8. Variation of E′and Tg of the all housing/PE mixtures.
FIGURE 7. SEM images of the fracture surfaces of the polyalloys compounded by a twin screw extruder at 280 °C die temperature. the spheres dispersed in the matrix of all housing (Figure 6a). When the GMA was added, the PE sphere sizes became smaller, and more brittle fracture occurred, resulting in weaker mechanical properties, except for a modulus higher than that of the nonreactive alloy (Table 1). The size of the PE spheres (about 10-12.8 µm) decreased by approximately 4-5 times (about 1.4-2.8 µm) for a batch mixer. Poly(GMA) was obviously compatible with the PC/ABS matrix to enlarge the matrix phase (Figure 6b). The PE spheres ranging from 4 to 20 µm in diameter for the all housing/PE/epoxy mixtures (Figure 7a) almost disappeared and were compatible with the matrix when the GMA was added, allowing strong adhesion onto the surface of the glass fibers (Figure 7b). The polyalloys of the all housing/PE/epoxy/GMA mixtures showed stronger tensile properties than those of its nonreactive mixtures (Table 2), and a small amount of the epoxy particles and GMA improved the modulus and tensile stresses for the extrusion, as previously reported (12). Processing Conditions Determined from the Variation of Tg. The variation of the glass-transition temperature (Tg) values for the ground all housing materials was observed using the DMS according to the PE content. The Tg values of pure PE (100 wt % PE) and all housing materials (0 wt % PE) were measured to be -43 and 123 °C, respectively (Figure 8a). In the spectra for the all housing materials, the Tg value of ABS was not observed, which means that the ABS content of PC/ABS for the all housing materials was so small as to be miscible (9). The Tg values of the all housing/PE mixtures moved into higher temperatures as the PE content decreased. The plot of the storage modulus (E′) versus temperature for
polymer alloys with 12.5 and 25 wt % PE showed typical separation patterns for Tg values commonly observed in incompatible phases. A high Tg value, corresponding to the Tg value of the PC-rich phase, appeared at 117 °C, which formed a composition range confined by low and high Tg values, called an “optimum PE range” (Figure 8b). The optimum PE range was in the area surrounded by Tg values versus PE contents, where desired compounding compositions were obtained to provide high mechanical properties. The PC phases were not observed from 50 wt % PE and above. Effect of Reaction on the Dynamic Mechanical Properties. Dynamic mechanical spectra of nonreactive and reactive materials compounded by the twin screw reactive extruder at a temperature range from 255 to 280 °C from the hopper to the die are shown in Figure 9a. The all housing material has a single Tg value at 127 °C, and because no ABS peak was observed around 100 °C, the amount of ABS was assumed to be relatively very small or partially compatible to PC (9). When PE was added to the all housing mixture, the Tg value was separated into the Tg value of ABS and PC (Figure 9b). The Tg value of PE in the all housing/PE (1/1) mixture appeared at -28 °C, which decreased further to -33 °C, when the ground epoxy powder was added to the all housing/PE mixture. When GMA was added to the all housing/PE/epoxy (1/1/0.05) mixture at 5.4 wt %, the Tg value of PE increased to -5 °C, but the Tg of PC/ABS decreased to 102 °C. The addition of GMA also decreased the difference between the two Tg values of PE and PC/ABS (Figure 9a,b). It is clear that the incompatibility between the components decreased for the reactive polymer mixture of all housing/PE/epoxy/GMA by adding GMA. The front covers of cell phone housings were ground using a knife mill, and the mixtures were separated into the desired and undesired materials for compounding using the sink-float process in water and salt solutions. The reactive compounding with PE or epoxy powders was carried out by a batch mixer and twin screw extruder. It was observed from FTIR that ring-opening reactions of GMA were performed on polymer alloys and mixtures. The addition reaction of VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Literature Cited
FIGURE 9. Dynamic mechanical analyzer (DMA) of the polymer alloys and mixtures. GMA decreased the impact strength and strains but significantly increased the breaking and yield stress and energy to break for the extrusion. It was confirmed from DMS that the incompatibility of the all housing/PE/epoxy (1/1/0.05) mixtures was obviously improved by the addition of GMA.
Acknowledgments Financial support from the Motorola Foundation and the Florida Center for Solid and Hazardous Waste is greatly appreciated and acknowledged. It is a pleasure to have a foundation and a state agency fund this recycling research effort that benefits industry and the public. We are also grateful to the Major Analytical Instrumentation Center and Engineering Research Center in Materials Science and Engineering Department at the University of Florida for providing instruments and technical advice.
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ES803355T
