10 Values of these parameters for a few representative chemicals are shown in Table I. Where d, p, and h are the dispersive, polar, and hydrogen bonding components of the total solubility parameter. Hansen proposed that the solubility parameter was composed of contributions from the three major types of cohesive forces, namely dispersive, polar, and hydrogen bonding, so that
Where CED is cohesive energy density, Δ E vap is the heat of vaporization, and V m is molar volume. The solubility parameter,, is defined as They also showed that absorption of the liquid by the polymer tends to be correlated by the solubility parameters of the liquid and polymer. Perhaps the best-known work of this kind is that of Kambour et al., who demonstrated-in studies on polycarbonate, 6 poly(phenylene oxide), 7 polysulfone, 8 and polystyrene 9-that the absorption of the solvent and concomitant reduction in the polymer's glass-transition temperature can be correlated with a propensity for stress cracking. 3–5 Numerous studies have linked the ability of a solvent to swell a plastic with its ability to craze the plastic. (Lipid solutions and isopropanol fall into this category.) It is the general consensus in the literature that the majority of stress-crack failures experienced by end-users results from this category of chemical.
This article, however, will emphasize chemicals that cause stress cracking simply by swelling or wetting the polymer. 1 The reduction in polymer molecular weight from the hydrolysis can lead to crazing and eventual catastrophic failure-a mechanism that has been identified as the probable cause for stress-crack failures in one-piece carbonated soft-drink containers.
An example of the latter would be the caustic or aqueous sodium hydroxide that can hydrolyze poly(ethylene terephthalate) (PET). Our strategy will be to examine, in some detail, the roles of the three critical components of environmental stress cracking: the chemical environment, the plastic, and the tensile stress.Ĭhemicals that cause environmental stress cracking can be divided into those that swell or wet the polymer and those that chemically react with the polymer. The goal of this article is to provide a framework for evaluating the suitability of a plastic for medical uses in terms of its stress-crack behavior. The proper selection of a medical plastic requires a thorough analysis and interpretation of the phenomenon of environmental stress cracking.
Products like medical luers can be susceptible to crazing when under stress and exposed to aggressive chemicals. In these applications, chemicals such as isopropanol and lipid solutions can initiate crazes-microcracks bridged by polymer fibrils-in the plastic and seriously compromise its mechanical integrity. The potential for environmental stress cracking is of paramount concern when plastics are used in medical device components such as luers and stopcocks. If the plastic is under stress, it may also experience environmental stress cracking, which can be defined as the crazing and cracking that may occur when a plastic under a tensile stress is exposed to aggressive chemicals. These can include weight gain if the plastic absorbs the chemical, weight loss if the plastic is degraded by the chemical or if the chemical extracts low-molecular-weight components of the plastic, dissolution if the chemical is a good solvent, or other changes such as variations in opacity or color. streng ingegryp يَتَّخِذ إجراءات صارِمَـه действам решително aplicar medidas duras podniknout rázná opatření das Durchgreifen slå ned på παίρνω μέτρα εναντίον castigar severamente, tomar medidas severas (contra) rangeid meetmeid võtma, otsustavalt (millegi vastu) tegutsema سختگیری بیشتری کردن prendre des mesures énergétiques sévir contre לִנקוֹט בְּצֵעָדִים חֲמוּרִים गैर कानूनी कार्य को रोकने के लिए की गई कार्रवाही pritisnuti szigorúan fellép vki v.When a plastic is exposed to a chemical environment, the material may undergo numerous changes.
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