Rochester, NY--When you make racks for test tubes, the last thing you want to hear is customers complaining about formaldehyde escaping from the racks during high-heat sterilization. However, that's exactly what happened to Nalge Nunc Int'l (NNI), one of the world's largest research lab suppliers.
"Fumes from the original materials used to make the test tube racks had become a problem for some of our customers, especially those autoclaving multiple racks," says Sharad Rajguru, senior plastics engineer at NNI. "We had to find a solution."
NNI found that solution in Carilon(reg) polymers, a family of aliphatic polyketones from Shell Chemicals (Houston). The original material, a glass-filled acetal homopolymer, had been used since the early 1980s. When exposed to high heat, the polymer's fumes were reported by lab workers to cause watery eyes and sinus irritation. "We had looked for an alternative material for many years," Rajguru explains. "In fact, we had already evaluated about 100 materials when the project was assigned to me. Because of our stringent requirements, no material had yet been identified to replace acetal."
Rajguru was tipped off to Carilon polymers' desirable qualities by a sales representative from LNP Engineering Plastics (Exton, PA), a provider for NNI's glass compounding. About the same time, an NNI executive had seen the materials profiled at a trade show.
"Rajguru called us and explained the situation, and we decided to put our polymers to the test," says Bob Pilotti, Carilon polymers' market development manager. "We knew we had the temperature resistance and impact strength required, and since our materials are made with carbon monoxide and alpha olefins, there should not be the emission problem experienced with acetal."
NNI formally began evaluating Carilon polymers in mid 1997. The material NNI desired had to:
- Tolerate the medical sterilization processes commonly done in an autoclave device.
- Be heavier than water and not warp or deform in immersion tests.
- Be colorable in order to meet medical lab color-coding needs.
- Withstand NNI's impact test requirements.
NNI's test tube rack consists of many knit lines where the intricate material flow paths come together, which made the impact test a technical challenge for other materials," explains John Kelley, Carilon polymers' research engineer. "Our materials have a unique balance of high impact strength and flexural modulus, allowing them to absorb impact--even for parts with potential knit-line problems."
In addition, the resins resist hydrolysis, a defining factor in their ability to maintain performance during immersion tests. And they process easily on most conventional injection-molding equipment.
Not only did the NNI tests confirm that Carilon polymers worked with the company's existing production equipment, but the material offered several key benefits:
- A lower specific gravity than acetal, decreasing material requirements.
- Reduced production set-up time, coupled with a change in the injection-molding pressure, that resulted in increased capacity and decreased production costs.
- Lack of odors when processing, which eliminated the need for expensive exhaust systems.
- Ability to accept general-purpose colorants instead of more expensive custom coloring agents.
Actually, Carilon polymers' unfilled grade met all the production requirements, including strength and impact resistance. However, NNI decided to use a 15% glass-filled grade. The added rigidity in the compound gave the test tube racks the same "feel" as the former glass-filled acetal racks.
NNI began production of the test tube racks last fall. "Because of Carilon polymers' combination of benefits, the finished part actually cost less than the acetal version," notes Michelle Londa, Carilon polymers' marketing manager-Americas. As a result, NNI is considering the use of the material for several other lab products.
POLYKETONE KEYS TEST TUBE RACK DESIGN
CARILON POLYMERS DUPONT
GRADE DPR1115-1000 570
% GLASS FIBER BY WEIGHT 15 20
MELT POINT (DEG F) 428 347
SPECIFIC GRAVITY 1.34 1.56
TENSILE STRENGTH (PSI) 12,500 8,500
ELONGATION @ BREAK 6.0 12.0
FLEXURAL MODULUS (PSI) 550,000 730,000
NOTCHED IZOD IMPACT @
-40F (FT-LB/INCH) 1.2 0.5
DEFLECTION TEMP @
264 PSI (DEG F) 425 316
Source: Shell Chemicals