Electronics Bioplastics
Electronics Bioplastics for IT-applications by Joe Kuczynski* and Dylan Boday^ Systems Technology Group IBM Corporation *Rochester, Minnesota, USA ^Tuscon, Arizona, USA The past decade has witnessed explosive growth in bioplastics with new product announcements occurring on a weekly basis. Biobased compositions based on starch or resins such as polylactic acid (PLA) have achieved significant penetration in the non-durable goods market. However, stringent product requirements have slowed the adoption of biobased plastics for the electronics industry. Within the electronics industry, hardware designs have focused on miniaturization and weight reduction. Since engineering thermoplastics can be injection molded into complex shapes at very thin wall thicknesses, they have rapidly become the material of choice for complex enclosures. Moreover, as traditional petroleum-based thermoplastics can be rendered ignition resistant, the demand for flame retardant thermoplastics has experienced steady growth. The American Chemical Industry estimates that 725,000 tonnes of thermoplastics were sold to the electrical/electronics industry in 2010. Coupled with the fact that plastics represent the largest volume component of electronic scrap, a significant opportunity exists to drive the industry toward a more sustainable design point. A typical product offering within the information technology marketplace is a server. A server is a complex hardware device composed of numerous components that typically includes a printed circuit board, daughter cards, processors, a power supply, hard drives, network connections, and the associated cabling required to interconnect various servers. The entire system must meet various industry standards such as those specifying permissible radiated emission levels, flammability classification, and noise levels. To comply with these requirements, electromagnetic compatibility gaskets, ignition-resistant thermoplastic housings, and acoustic foam are strategically designed into the server. Although numerous potential applications exist where biobased alternatives can displace petroleum-based materials, acoustic foams and thermoplastic covers are considered to be the two most easily addressed applications. Acoustic foam Acoustic foam is typically an open cell, polyurethane foam synthesized via the reaction of an isocyanate with a polyol. Acoustic foam is fabricated to a targeted density and pore count (0.0320 g/cm³ and 27 pores/cm, respectively, being the most common). Although there is presently no renewable source for the isocyanate, either soy bean or castor bean oil may be used as a sustainable source for the polyol. A direct substitution of biobased polyols for petroleumbased polyols is not presently possible as the physical properties of the resulting foam produced from biopolyols have been determined to be inferior (from IBM’s point of view). Consequently, commercially available acoustic foams contain less than 20 wt% biobased polyol. For acoustic foam applications, where the primary material property of interest is the sound absorption coefficient, theoretically greater bio-polyol content is possible. However, such foams have yet to be commercialized. Nevertheless, two biobased acoustic foams have been qualified for use in servers based on functional evaluation in a reverberation room (Fig. 1). At frequencies below 1000 Hz, which tend to be the most problematic to attenuate in server computers, the biobased polyurethane foams outperform their petroleum-based counterparts. Moreover, both of these bio-based foams meet the flammability requirements (UL 94 HBF) in all thicknesses 36 bioplastics MAGAZINE [06/12] Vol. 7
Electronics Figure 1. The frequency dependence of the absorption coefficient of acoustic foams (courtesy M. Nobile, IBM Poughkeepsie). Figure 2. Physical property comparison of petroleum-based PC/ABS and PLA/PC blends. 1.2 Absorption Coefficients of Acoustic Foams Tensile Stress @ yield 120% 100% 80% 60% 40% 20% HDT B @ 0.45 MPa Tensile elongation @ yield Absorption Coefficient, 0.8 0.6 0.4 0.2 Petroleum-based foam Bio-based foam; Vendor A Bio-based foam; Vendor B Notched Izod Impact Tensile elongation @ break 0 50 80 125 200 315 500 800 1250 2000 3150 5000 63 100 160 250 400 630 1000 1600 2500 4000 Flexural modulus Flexural stress @ 5% Strain Tensile modulus One-Third Octave-Band Center Frequency, Hz 30 wt% PLA Blend 40 wt% PLA Blend PC/ABS of interest (generally 2.5-6 cm) and do so without the use of brominated flame retardants, some of which are prohibited by various regulations in the global market. Furthermore, the flame retardants used are non-halogenated, an important feature as the current trend in the electronics industry is migration away from such materials. Finally, both of these commercially available foams are essentially cost neutral, an extremely important consideration in driving these materials into products. Consequently, IBM has been shipping product incorporating the biobased acoustic foam since the fourth quarter of last year. Electronic enclosures Due to its excellent combination of physical properties, polycarbonate/acrylonitrile-butadiene- styrene (PC/ABS) resin has been the material of choice for electronic enclosures. The ability to mold complex geometries in very thin wall cross sections (down to 1.5 mm), coupled with creep resistance and a high flexural modulus required for latches and snap fits, has garnered PC/ABS the majority of share in the IT equipment market. In addition to these properties, server computers must meet stringent flammability requirements (UL 94 V0 classification at the minimum wall thickness of the part). Although various bio-based thermoplastics may be envisaged to replace PC/ABS blends, those based on polylactic acid (PLA) hold the greatest promise. However, since the homopolymer of PLA is very brittle (Notched Izod impact strength of 26 J/m compared to 747 J/m for a typical PC/ABS blend), it must be toughened. In addition, PLA has proven more difficult than PC/ABS to render ignition resistant. In blends with Polycarbonate (PC) where the PLA content exceeds 20 wt%, it has been found that straight compounding of PLA with traditional nonhalogenated flame retardants resulted in blends with inferior properties. However, specialty compounders have successfully addressed these issues and have developed PLA blends at 20-40 wt% loading levels that compete favorably with flame retardant PC/ABS with respect to physical properties (Fig. 2). It can be seen that the physical properties of the PLA blends are within 80% of the PC/ABS benchmark material with the exception of the room temperature notched Izod impact strength. Impact strength decreases dramatically as the PLA content is increased from 30 wt% to 40 wt%, but functional part testing at the system level demonstrated that this reduction in impact strength is not a concern. Enclosure covers for server products that are currently installed in the field were molded from both of the PLA blends. The renewable resins passed all technical qualifications required for use in IT hardware. A major concern associated with the use of PLA blends is cost. However, it is projected that the cost of PC/ABS will continue to rise at 3-5%/year whereas the price for PLA blends should decrease as both demand and volume increase. Joint development efforts with material suppliers and plastic compounders will result in higher PLA concentrations within blends with acceptable physical properties. Although this effort is currently focused on plastic enclosures and housings, numerous other applications exist where renewable materials may displace petroleum based materials. www.ibm.com bioplastics MAGAZINE [06/12] Vol. 7 37
