Bayblend® FR 3010 - Casing Material for a Heart Sound Cardiotocograph
March 15, 2006
LEVERKUSEN -- The new Avalon FM30 cardiotocograph (CTG) from Philips Medizin Systeme Boblingen GmbH, Germany, needs to function properly at all times and even in hectic circumstances to reliably monitor the health of expectant mothers and their babies. It monitors the heartbeat of the fetus and the intensity of labor pains in the delivery room and in the surgeries of doctors in general practice, for instance. As well as other parameters, the mother's blood pressure and oxygen saturation can also be measured. A wide range of demands are placed on the compact unit's casing. These concern safety in terms of fire-resistance and mechanical and thermal durability, ensuring the unit functions perfectly. This therefore called for a casing material with a wide range of outstanding properties that could also be processed efficiently using the injection molding process. The material ultimately chosen was Bayblend® FR 3010, a flame-retardant blend of polycarbonate (PC) and acrylonitrile-butadiene-styrene copolymer (ABS) from Bayer MaterialScience AG.
"We were particularly pleased that our material fulfills Philips's extremely rigorous requirements in terms of its resistance to a broad range of common cleaning agents and disinfectants," explained Thomas Eckel from Bayer MaterialScience, who is currently developing new flame-retardant Bayblend® versions. High resistance to these cleaning agents and disinfectants is necessary because regular contact with them during the casing's service life can cause stress cracking, fracture, and color changes. To test the chemical resistance of Bayblend® FR 3010 as realistically as possible, Philips provided a wide range of standard disinfectants and cleaning agents. As part of its technical customer support service, Bayer MaterialScience used these for ESC (environmental stress cracking) tests in a bent strip testing procedure complying with EN ISO 4599. This involved clamping test pieces of a specified size over metal templates with defined outer fiber strains of 0.4, 0.8, 1.6, and 2.4 percent and leaving them for seven days at room temperature in the relevant medium. The Bayblend test pieces were then examined for fractures and visually identifiable changes in the surface, such as crazing and fine cracks. The selected seven-day exposure period results in an extrapolated service life for the unit of around ten years with over 10,000 cleaning processes.
The heart sound cardiotocograph is used in a whole host of countries throughout the world under widely varying conditions and is frequently subjected to severe loads on a daily basis in hospitals. Philips therefore conducted a series of application-related standard tests - environmental tests - to obtain data for various simulations. These included drop and impact tests, thermocycling tests, and function checks with reduced air pressure to simulate heights.
Heat resistance in continuous operation
The cardiotocograph has been approved for an ambient temperature of up to 40 °C, which is required for many medical devices. On top of this, the manufacturer demands an extra margin of safety of 10 to 15 °C as an internal quality standard. Heat dissipation resulting from power loss can cause the inside of the casing to heat up by a further 25 °C. Overall, the casing material thus needs to be able to cope with long-term temperatures of approx. 80 °C. "The high levels of heat resistance of Bayblend® FR 3010 mean it can deal with these stresses. Its relative thermal index (RTI) to UL 746 B is between 85 and 95 °C, depending on the load and wall thickness. This index is a measure of the maximum thermal stress in long-term use. The high Vicat temperature (B 120) of 108 °C ensures that short-term temperature peaks aren't a problem for the material either," said Burkhard Thuermer, who works in technical product support for PC/ABS blends at Bayer MaterialScience.
The flame retardance package of Bayblend® FR 3010 is free of antimony, chlorine and bromine. This ensures compliance with the two new EU directives on the disposal of plastics from electrical and electronic equipment: the WEEE (Waste Electrical and Electronic Equipment) and the RoHS (Restriction of the Use of Hazardous Substances in Electrical and Electronic Equipment). Bayblend® FR 3010 also satisfies the demands of major ecolabels such as "TCO 05" and the "Blue Angel", which have recently been tightened up. It also meets the requirements of fire protection standard UL 94 V (Underwriter Laboratories), achieving the best possible classification V-0 for 1.5-mm wall thicknesses. The outstanding fire resistance of Bayblend® FR 3010 exceeds the requirements of the domestic appliance standard IEC 60335-1, which was recently reinforced. The glow wire flammability index (GWFI) in line with IEC 60695-2-12 has a value of 960 °C for test piece thicknesses of 1.0 to 3.0 mm. The glow wire ignitability temperature (GWIT) in line with IEC 60695-2-13 is 850 °C for the same wall thickness range.
Long tool service life
During processing, outgassing of volatile substances from Bayblend® FR 3010 is extremely limited. Indeed, the potential for outgassing is at the same low level as non-reinforced PC/ABS blends without flame retardance. "If our material is used correctly, plate-out is all but avoided. The risk of damage to molds due to this effect is virtually eliminated. This means longer periods between cleaning cycles, which pays off in higher productivity," explained Thuermer.
In selecting materials, it was also important that all lettering such as logos, labels, and equipment data could be applied to the finished unit using a computer-controlled laser printing process. "Bayblend® FR 3010 opens up a wide processing window, leading in turn to process reliability. Good legibility and high abrasion resistance of the printed symbols are notable features," commented Thuermer.
The PC/ABS blend is not the only material from the broadly-based Bayer MaterialScience product portfolio that is used in the cardiotocograph. The casing of the transducer, which is placed on the mother's stomach to measure contractions during labor, is made of Makrolon® 2858, a polycarbonate approved for medical applications. One of the reasons for its use was the ease with which it can be sterilized using high-energy radiation, for example. The transducer's elastic measuring membrane is manufactured with the thermoplastic polyurethane Desmopan®. The material feels soft and warm and is thus particularly kind to the skin. It is also resistant to frequent disinfection.
