Phthalate esters such as diethylhexl-phthalate (DEHP) are the most widely used plasticizers for flexible poly-vinyl chloride (PVC) medical compounds and have been for 50 years, but issues concerning their possible effects on human health have caused some device manufacturers to consider compounds containing alternative plasticizers. When considering alternatives, it is possible to base decisions on one factor that favors a particular alternative plasticizer, rather than conduct a comparative study that takes all important considerations into account, due to the considerable pressure to find an alternative.
There are a number of alternative plasticizers on the market, and while Teknor Apex is best known as a manufacturer of medical-grade PVC compounds, the company also produces some of these plasticizers. Two popular alternatives are trioctyl trimellliate (TOTM), a trimellitate ester which Teknor Apex manufactures and sells primarily to the wire and cable industry due to its low volatility; and dioctyl terephthalate (DOTP), a terephthalate which Teknor Apex does not produce. The company uses both plasticizer types in producing medical-grade, flexible PVC compounds, and as part of a program to support customers in weighing the pros and cons of available alternative plasticizers, Teknor Apex has determined the DOTP provides a better balance of properties than TOTM for many medical device applications.
TOTM is a monomeric plasticizer whose high molecular weight makes it less mobile than DEHP and many alternative plasticizers. Although it has a chemical designation of a terephthalate, DOTP is classified as a non-ortho-phthalate, different from DEHP in terms of animal toxicology and metaboilization.
The high molecular weight of TOTM offers an advantage over DOTP due to the phenomenon of stress cracking in connectors or other rigid components that interface with flexible PVC components such as tubing. The stress cracking is caused by the migration of plasticizer to the interface with the rigid component, and it is most pronounced in the case or amorphous rigid materials like polycarbonate. Crazing, also known as stress cracking, that weakens the component happens more slowly with TOTM than with DOTP.
Although DOTP may cause more crazing than TOTM, it resists crazing better than other alternative plasticizers and even better than DEHP. TOTM is also outperformed in this regard by polymeric plasticizers. Over the years, device manufacturers have avoided serious crazing issues with DEHP through appropriate design and other measures, and it is now possible to minimize the issue of stress cracking by using specially formulated, stress-crack resistant rigid PVC compounds in place of polycarbonate for connectors.
In all respects, except for the issue of crazing, DOTP appears for be the preferable candidate for replacing DEHP. Although DOTP is relatively new to the global marketplace, it already outsells TOTM by a ratio of 12:2. Of all the plasticizers that have emerged as alternatives to DEHP, DOTP has the largest market share.
Peter M. Galland is the industry manager for regulated compounds at Teknor Apex Co.