Degradation of Polyurethanes for Cardiovascular Applications - Advances in Biomaterials Science and Biomedical Applications - page 71

The thermal degradation of polyurethanes allows determination of the proper conditions for
manipulating and processing them and for obtaining high-performance products that are
stable and free of undesirable by-products; if not processed properly, commonly by extru‐
sion or by injection moulding, the PU's would generate toxic products to the human body,
which is very critical in biomedical applications [Gomes Lage et al. 2001].

It is well known that polyurethanes are not thermal stable polymers and that the onset deg‐
radation temperature of the urethane bond depends on the type of isocyanate and alcohol
used. It is a general rule that the more easily formed polyurethanes are less stable, i.e. more
easily dissociated when compared with more difficulty formed ones [Petrovic et al. 1994].
Petrovic reported that the degradation temperature for these materials ranged from 120°C to
250°C depending on their structure [Petrovic et al. 1994]; however, literature reports proc‐
essing temperatures closer to 180°C [Guignot 2002].

Polyurethanes are thermally degraded through three basic mechanisms. First, by urethane
bond dissociation into its starting components (isocyanate and alcohol); secondly, by break‐
ing the urethane bond with formation of primary amines, carbon dioxide and olefins; and
finally, splitting the urethane bond into secondary amine and carbon dioxide [Petrovic et al.
1994; Cervantes-Uc et al. 2009].

5. Degradation mechanism

The nature of PU chemistry is central to understand why some PUs undergo faster degrada‐
tion than others (Santerre et al., 2005). However, the degradation mechanism of polyur‐
ethanes depends on not only the PU chemistry structure but also the degradation
environment, i.e. in the presence of water, acidic, alkaline or oxidative conditions, or in the
presence of enzymes. Generally, the characterization of the by-products during the degrada‐
tion of the polyurethane is the key to understand the mechanisms of degradation. Identifica‐
tion of degradation products is an important issue but of equal interest is the eventual
toxicity of the degradation products. If the biomaterial degrades, either spontaneously or
due to biological activity, components can leach into surrounding tissues and cause an in‐
flammatory response if not easily metabolized by natural pathways. Therefore, it is compul‐
sory to identify the major species produced at different stages of degradation and the
kinetics of their formation (Azevedo et al., 2005).

Accelerated degradation has been used to determinate stability of non degradable polyur‐
ethanes (Gunatillake, 1992) but it can be used to provide valuable information about degra‐
dation mechanism of resorbable polyurethanes. In this context, both soluble products and
solid residues can be studied with different analytical techniques and tests to determine
their composition.

The main techniques used to evaluate the degradation of biomaterials can be divided into
surface analysis (infrared spectroscopy, X-ray photoelectron spectroscopy, contact angle
measurements), which are more appropriated to monitor the changes occurring in the first