Polyurethanes stress cracking

Pacemaker Interfaces and Leads. Problems of existing pacemaker interfaces and pacemaker lead materials made from siUcones and standard polyurethanes are environmental stress cracking, rigidity, insulation properties, and size. 

Polyurethane elastomers are exceptionally tough, abrasion resistant, and resist attack by oil. The polyester types (AU) are susceptible to hydrolytic attack at above ambient temperatures, and certain polyester thermoplastic polyurethanes have been known to stress crack in cable jacketing applications when in contact with water at ambient temperatures this latter effect has sometimes, incorrectly, been ascribed to fungal attack. Polyether types (EU) are far more resistant to hydrolytic attack. Certain polyurethanes can be attacked by UV light, the resistance to this agency primarily being determined by the isocyanate used. 

Pellethane is an extrudable thermoplastic polyurethane based on MDI and a hydrophobic polyol was used in this application. For the most part, the application was successful, but evidence of stress cracking became apparent. The combination of the hydrolytic environment and the polymer under tension caused failures that led to current leakage and ultimate failure of the device. As a result, softer grades of Pellethane and alternative annealing procedures were adopted and reduced the problem dramatically. 

Cellulosics are normally solvent-cemented unless they are to be joined to another substrate. In these cases, conventional adhesive bonding is employed. Polyurethane, epoxy, and cyanoacrylate adhesives are commonly used to bond cellulosics. Surface treatment generally consists of solvent cleaning and abrasion. Cellulosics can be stress-cracked by uncured cyanoacrylate adhesives and some components of acrylic adhesives. A recommended surface cleaner is isopropyl alcohol. 

Hughes-Dillon K, Schroeder LW (1998) Stress cracking of polyurethanes by absorbed steroids. Polym Degradat Stabil 60(1) 11—20... 

Martin DJ, Warren LAP, Gunatillake PA, McCarthy SJ, Meijs GF, Schindhelm K (2001) New methods for the assessment of in vitro and in vivo stress cracking in biomedical polyurethanes. Biomaterials 22(9) 973-978... 

Polyurethane is a stiffer and firmer material than silicone introduced in 1977 for its better tear strength and low coefficient of friction. Due to documented cases of stress cracking discovered a few years after implantation, some processing changes were made, increasing performance satisfaction. Compared with silicone, it allows for a thinner lead body and when moist is slippery. This represents an advantage in cases in which two... 

Potential for environmental stress cracking (Fig. 1.16b), which is an oxidative condition that takes place on the surface of polyurethane leads (true for P80A)... 

Available as a dimethyl acetamide solution, this segmented, aromatic, polycarbonate-based TPE polyurethane was designed to mimic Ethicon Corporation s Biomer. The polymer is made from the addition of diphenylmethane 4,4 -diisocyanate to a polycarbonate diol followed by addition of a mixture of chain extenders and a molecular weight regulator. The polymer is believed to be resistant to environmental stress cracking such as that experienced by other polyurethanes coated onto pacemaker leads (Tables 4.4, 4.12, and 4.13). 

A polycarbonate TPE polyurethane that claims biostability is achieved through its replacement of virtually all ether or ester linkages with carbonate groups. The soft segment is composed of a polycarbonate diol formed by the condensation reaction of 1,6-hexanediol with ethylene carbonate. The polycarbonate diol is converted to a high molecular weight polyurethane by the reaction with 1,4-methylene bisphenyl diisocyanate (MDI) and 1,4-butanediol. It is reported to be resistant to environmental stress cracking as experienced with insulation on pacemaker lead wires. The polymer can be extruded, injection molded or compression-molded, and can be bonded with conventional urethane adhesives and solvents (Tables 4.4, 4.12, 4.13, and 4.14). 

Class 1 are the predominantly used class of polyurethanes and can be processed using traditional thermoplastic melt techniques. Class 2 represents a small specialist group used where higher temperature, lower compression set and environmental stress cracking resistance is required. 

With the exception of the polyolefins and other low surface energy plastics, they cope with almost all common engineering alloys and many plastics. The stress cracking of some plastics is a hazard for some versions of these adhesives. None of them bonds rubbers satisfactorily and polyurethane plastics may prove difficult. 

Ward R, Anderson J, McVenes R, Stokes K. In vivo biostability of polysiloxane polyether polyurethanes resistance to biologic oxidation and stress cracking. J Biomed Mater Res... 

Stokes, K., Chem, B. Planck, H., Egbers, G., Syre, 1. (Ed.) Environmental stress cracking in implanted polyurethanes. In Polyurethanes in biomedical engineering, Elsevier, Amsterdam, 1984... 

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