A simple way to predict molded-in stress
By Nancy Hermanson, Medical Market Technical Leader, The Dow Chemical Co., Midland, MI

Polycarbonate is desirable for use in medical device applications where clarity, heat resistance, and toughness are key requirements. But while the material has a good balance of physical properties, its chemical structure allows the penetration of certain types of solvents into the polymer matrix, causing localized swelling. This swelling reduces the glass transition temperature, creating a greater potential for crazing or cracking.

Scientists have long known that stressed polycarbonate has a much lower resistance to chemicals, and there are theoretical methods for predicting the stress level. But up until now, there was no easy way to determine it experimentally, meaning that parts would often require an annealing process--whether they needed it or not. However, researchers at Dow Chemical have now devised a simple test based on the solubility parameter, which takes into account the correlation between the critical strain for the onset of crazing or cracking in the polymer and the solvent-polymer interaction.

Studies have shown that when the solubility parameter of the solvent nearly matches that of the polymer, the polymer will probably be soluble or partially soluble in the solvent. As the difference between the solubility parameters increases, the polymer's resistance to failure under stress should improve.

In Dow's test, a combination of two solvents--ethyl acetate and hexane--are used in various ratios in order to vary the solubility parameter and observe the corresponding crazing of molded polycarbonate samples. The solubility parameter for ethyl acetate is 8.91 and for hexane is 7.27, while the solubility parameter for polycarbonate is 9.8. A stressed polycarbonate sample is exposed to a specific concentration of ethyl acetate in hexane. After one minute, the solvent is washed off and the part inspected for shiny, silver streaks or faint hairline cracks. Crazing in the main area of the part is more indicative of excessive stress, while slight crazing in the corners of the part is not.

If crazing is observed, the test is repeated with a new part and a lower ethyl acetate concentrated mixture. The part's critical strain and stress is determined as the lowest concentration at which crazing still occurs. The molded-in stress can then be determined from the accompanying chart, which plots critical strain of polycarbonate as a function of the concentration of ethyl acetate in hexane.

To speak with a Dow Chemical representative, call 1-800-441-4369.

Achieving long-life aluminum parts

Chris Jury, Vice President Luke Engineering & Mfg. Co., Wadsworth, OH

Engineers have long known that hardcoat anodizing of aluminum components increases wear and corrosion resistance properties. But its uneven microscopic structure also forms an excellent foundation for subsequent treatment with fluoropolymer resins, which are used to increase slip and release properties. Together, these two processes form a very hard, self-lubricated "synergistic" coating system having a wide variety of practical applications.

The most common lubricated hardcoats consist of a hard anodic coating and a dry solid-film lubricant. The underlying hardcoat structure provides excellent topcoat adhesion to the base, while the resin envelope provides enhanced corrosion and chemical resistance. In this two-step process, a resin containing either PTFE or MoS₂ is bonded to the surface of the hardcoat. Air drying or thermosetting (350 degrees F cure) resins are also available.

Taking the concept one step further, Luke Engineering developed the first PTFE-impregnated hardcoat, called Lukon 24. Performed in-line without unracking the hardcoated part, the impregnation process is simple, direct, and does not require a heating cycle. The loosely adherent PRFE retained within the hardcoat structure significantly reduces friction and wear; in one example, the wear life of a hardcoated hydraulic piston was increased by over 400%.

Lukon 24 is also an efficient anchor layer for FDA-approved Teflon(R). These fully fused (700F cure) topcoats, which are frequently used to provide molds and kitchenware with excellent release and non-stick properties, can be directly applied without primers or intermediate coats.

To speak with a Luke Engineering applications engineer, call (330) 335-1502 or fax (330) 336-6738.