Radiation resistance polypropylene

The launch of new resins being rare, progress on materials intended for medical applications is by way of formulation, alloys and modifications of existing resins. Thus, new radiation-stable polypropylene grades are available for the manufacture of cups, boxes, baskets, mixers, etc. that are radiation sterilized. And polyesters with better environmental stress cracking resistance are extending their potenti in the area of medical testing. 

The various materials (often polymers) have to be chosen such that they are sufficiently radiation resistant to withstand the sterilization dose. Under the dose conditions, needed for sterilization some polymers such as polyethylene, polystyrene, poly(ethylene terephthalate), polyurethanes, and epoxides maybe stable, whereas others such as polypropylene, poly(methyl methacrylate), polyvinyl chloride (PVC) may not. When the radiation stability of the plastic... 

The experimental investigations of the mechanical properties of various organic materials after low temperature irradiation are very few, although many studies have been made after room temperature irradiation. Van de Voorde studied the influence of 77 K and 20 K irradiations on mechanical properties and reported that polyimides and aromatic-based epoxies have a good radiation resistance. The present studies show that polypropylene, polycarbonate, and Mylar have low radiation resistance due to chain fracture, and these materials are too brittle to permit mechanical testing. 

Gamma radiation, frequently used for sterilization in diverse medical applications, and for crosslinking of pol3aners, may cause important modifications in both structure and properties of polypropylene (PP) and related copolymers. It was foimd that the radiation resistance of polymers strongly depends on their amorphous or crystalline structure. 

Though not completely resistant to UV radiation, being a linear aliphatic polyester, PHA has been reported to have better resistance to UV radiation than polypropylene. Due to this property, a PHA (reported as a PHA-I6001 from Metabolix with a crystallinity of 25%) was blended with PVC (a UV sensitive polymer) to provide this latter higher UV resistance without promoting biodegradation. PHA, being miscible with PVC, did not lead to phase... 

Polypropylene is intrinsically less resistant to ultraviolet radiation. Although polypropylene shows powdering rather than grass surface grazing after sunlight exposure, minute surface cracks are visible under the microscope. Under certain conditions the microcracks can act as points of stress concentration. This can reduce the impact strength of the component. 

Syndiotactic polypropylene resins allow the production of small flexible containers with very good y-radiation resistance and a degree of transparency never achieved before with any polypropylene. Moreover, the final objects transmit a feeling by contact that has been qualified as soft and warm. Only some highly expensive specialty and engineered polymers can achieve the same combination of properties, but require a significantly longer cycle time. 

Figure 5.132 Radiation resistance ofSOfjm thick polypropylene films irradiation in a vacuum with 5000 kGy/h with 1 MeV electrons from a van de Graaf generator and irradiation in air at...

Jia, H., Wang, H., Chen, W. The combination effect of hindered amine light stabilizers with UV absorbers on the radiation resistance of polypropylene. Radiat. Phys. Chem. 76, 1179-1188 (2007)... 

The most common material used is cellophane, which is a cellulose film, which acts as a membrane and is capable of resisting zinc penetration. The cycle life of cells utilizing this material is severely limited due to the hydrolysis of the cellophane in alkaline solution. Various methods have been tried to stabilize cellulose materials, such as chemical treatment and radiation grafting to other polymers, but none have, as of now proved economically feasible. The most successful zinc migration barrier material yet developed for the nickel—zinc battery is Celgard microporous polypropylene film. It is inherently hydrophobic so it is typically treated with a wetting agent for aqueous applications. 

In addition to, or instead of, polystyrene and oils, polymers such as polypropylene, polyethylene, or ethylene-vinyl acetate copolymer can be blended with these block copolymers. Blends with S-B-S or (S-B) -X block polymers usually show greatly improved ozone resistance (S-EB-S already has excellent ozone resistance). In addition, these blends have some solvent resistance. In certain cases, some oils that are stable to UV radiation reduce the stability of the blends however, the effects can be minimized by the use of UV stabilizers and absorptive or reflective pigments (e.g., carbon black or titanium dioxide). 

---