Polypropylene, radiation stability

Polypropylene, stabilized irradiated. Radiation-stabilized grades,... 

An alternative approach to uaiproving radiation stability has been through coupling the desirable processing properties of polypropylene with the radiation stability of polyethylene to form copolymers (polyallomers). Block polymers in which the chains comprise polymerized segments of each of the monomers are as amenable to processing as polypropylene while having better clarity and radiation stability. 

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... 

Crystallinity within the limits 37-70% has no effect on radiation stability of polypropylene (Fig. 3.12). 

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)... 

Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [98], In contrast, photostabilization of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2-hydroxy-propoxy) benzophenone using gamma radiation [ 147], In this case, the grafted compound acting as a U V stabilizer is chemically bound to the backbone chain of the polymer and its evaporation from the surface can be avoided. 

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. 

Of prime importance to the continued growth of polypropylene are the stabilizer systems which must be used to protect the resin during processing, and during exposure of finished products to various environmental and use conditions. The weak tertiary carbon—hydrogen bonds in polypropylene make it particularly susceptible to degradation caused by heat, oxidation, process shearing, and ultraviolet radiation (24). 

Sewell et al. (73) evaluated the stability of sodium nitroprusside (0.5 and 1.67 mg/ mL in 5% dextrose) injection packaged in polypropylene syringes and administered by a syringe pump system. When exposed to artificial radiation and window-glass-filtered daylight, the time to 10% decomposition, was about four hours. After 24 hours of exposure, the level of free cyanide exceeded 2mg/L. 

Irradiation of polymers with y- or electron beams is an attractive alternative to chemical sterilization because of its speed, ease of control, and the absence of residue. Radiation treatment of polypropylene, however, also initiates chemical changes which lead ultimately to embrittlement. These changes in physical properties may not become apparent until some time after the treatment. The ability of antioxidants to prevent radiation damage does not always follow the trends observed in thermal oxidation, which has stimulated efforts to develop new stabilizers or optimized combinations of existing ones. 

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). 

Both yellowing and embrittlement of polypropylene are dose dependent but are significant at or around 25 kGy (the customary compendial dose for sterilizing medical devices). The extent of radiation-induced discoloration can be relieved by use of phosphite stabilizers. Stability against embrittlement may be improved by adding antioxidants, but these may in turn create problems such as surface blooms. ... 

Many research groups have been working on radiation processing of polymer blends, to stabilize the phases and improve performance. The polymers that have been used in the radiolytic studies of blends include polyethylene (PE), polypropylene (PP), ethylene propylene rubber (EPR), polyvinyl chloride (PVC), polystyrene (PS), and polymethylmethacrylate (PMMA). 

As all polyolefins, polyethylene is sensitive to UV radiation, although less than polypropylene. For outdoor use polyethylene needs special stabilization against UV light. The light stabilizers for polyethylene are in principle the same as for polypropylene. On accelerated weathering, HALS show much better performance in HDPE tapes than UV absorbers, despite the latter being used in much higher concentrations. The comparison between HALS is, however, in favor of the polymeric HALS-III, which has the same performance when added at a concentration of 0.05% as HALS-I and HALS-II at 0.1%. 

Portnoy RC, et al. Superior stabilization of polypropylene for radiation sterilized medical devices and packages. ANTEC 2004, conference proceedings. Society of Plastics Engineers 2004. 

Perera, R., Albano, C., Gonzalez, J., Silva, R, Ichazo, M., The effect of gamma radiation on the properties of polypropylene blends with styrene-butadiene-st5n ene copolymers. Polymer Degradation and Stability 2004,85,741-750. 

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