**6.1 Effects of gamma sterilization in polymeric medical device**

Poly(vinyl chloride), PVC, is a polymer widely used for radiosterilizable food packaging and medical devices. However when the polymer systems are submitted to sterilization by gamma radiation (25 kGy dose), their molecular structures undergo modification mainly as a result of main chain scission and crosslinking effects. For PVC both processes coexist and either one may be predominant depending upon the conditions (temperature, environment, dose rate, etc.) under which irradiation is performed. The crosslinking and main scissions that take place during irradiation may lead to sharp changes in physical properties of the PVC (Vinhas et al, 2004).

During the interaction of gamma radiation with PVC, the reactions shown in Figure 7 can take place (Bacarro et al., 2003). This interaction gives rise to macroradicals deriving from C-Cl bond scission reactions (reaction I). The chlorine radical continues the reaction by way of a form center reaction in which HCl is formed and acts as a catalyst (reaction II). The A, B or C macroradicals recombine with each other forming networks due the restricted mobility of the macroradicals in the solid state (reaction III). It was reported, which crosslinking effect is predominant for PVC irradiated at lower doses (Silva et al, 2008). Oxidation reactions of macroradicals A, B or C (reaction IV), interaction of radical A with neighboring double bonds and other macroradicals from the impurities or from direct action of gamma radiation also can play an important role on crosslinking effect of PVC irradiated at lower radiation dose. However in presence of air the polymeric radicals A, B and C react with oxygen from air producing the peroxyl macroradical (reaction V). This radical formed can them undergo further reactions leading to main chain scission. This effect is predominant when the PVC molecule is irradiated at higher doses. Thus in the sterilization dose the commercial PVC undergoes the main chain scission (Ferreira et al, 2008).

Poly(methyl methacrylate), PMMA, also is used in manufacturing of medical supplies that can be sterilized by gamma irradiation at dose of 25 kGy and used in absorbed dose measurements in intense radiation fields. In general, polymer radicals are responsible for changes in the physical properties of PMMA**.** In particular, gamma irradiation of PMMA causes main scission and hydrogen abstraction from an α-methyl or methylene group. The extent of formation of each of the derivatives resulting from irradiation depends on the physical state of PMMA (Schnabel, 1981). The great majority of authors have reported that

additives used. The changes in mechanical properties may not be immediately apparent and there can be some time delay in their development. One visible side effect of irradiation sterilization is that many plastics will discolor or yellow as a result of the processing. Irradiated devices are completely safe to handle and can be released and used immediately

Many polymers are resistant to radiation at doses of up to around 25 kGy, the actual doses used will be higher than this to achieve sterilization, however complete sterilization and radiation damage of some magnitude will inevitably occur. The effect of radiation is cumulative and for items that must be repeatedly sterilized the total dosage can rise rapidly. For these items records need to be kept to insure that safe limits are not exceeded. Irradiation is very effective for fully packaged and sealed single-use items where only one

Poly(vinyl chloride), PVC, is a polymer widely used for radiosterilizable food packaging and medical devices. However when the polymer systems are submitted to sterilization by gamma radiation (25 kGy dose), their molecular structures undergo modification mainly as a result of main chain scission and crosslinking effects. For PVC both processes coexist and either one may be predominant depending upon the conditions (temperature, environment, dose rate, etc.) under which irradiation is performed. The crosslinking and main scissions that take place during irradiation may lead to sharp changes in physical properties of the

During the interaction of gamma radiation with PVC, the reactions shown in Figure 7 can take place (Bacarro et al., 2003). This interaction gives rise to macroradicals deriving from C-Cl bond scission reactions (reaction I). The chlorine radical continues the reaction by way of a form center reaction in which HCl is formed and acts as a catalyst (reaction II). The A, B or C macroradicals recombine with each other forming networks due the restricted mobility of the macroradicals in the solid state (reaction III). It was reported, which crosslinking effect is predominant for PVC irradiated at lower doses (Silva et al, 2008). Oxidation reactions of macroradicals A, B or C (reaction IV), interaction of radical A with neighboring double bonds and other macroradicals from the impurities or from direct action of gamma radiation also can play an important role on crosslinking effect of PVC irradiated at lower radiation dose. However in presence of air the polymeric radicals A, B and C react with oxygen from air producing the peroxyl macroradical (reaction V). This radical formed can them undergo further reactions leading to main chain scission. This effect is predominant when the PVC molecule is irradiated at higher doses. Thus in the sterilization dose the commercial PVC

Poly(methyl methacrylate), PMMA, also is used in manufacturing of medical supplies that can be sterilized by gamma irradiation at dose of 25 kGy and used in absorbed dose measurements in intense radiation fields. In general, polymer radicals are responsible for changes in the physical properties of PMMA**.** In particular, gamma irradiation of PMMA causes main scission and hydrogen abstraction from an α-methyl or methylene group. The extent of formation of each of the derivatives resulting from irradiation depends on the physical state of PMMA (Schnabel, 1981). The great majority of authors have reported that

**6.1 Effects of gamma sterilization in polymeric medical device** 

undergoes the main chain scission (Ferreira et al, 2008).

after sterilization.

radiation dose is required.

PVC (Vinhas et al, 2004).

scission results from a macroradical that itself is radiolysis product of a lateral bond as shown in the Figure 8 (reaction I ) (Guillet, 1985). The volatile products like HCOOCH3, CO, CO2, HCOCH3 and CH4, can be accounted for by the subsequent reactions of the carbomethoxy radical (B radical)**.** The formation of C radical is the basic reason for the radiation-induced degradation of PMMA. Under air atmosphere the C radical undergoes the chain oxidation process forming the peroxyl free radical (D). Once D radical is formed in PMMA, it can abstract hydrogen from PMMA chains to form hydroperoxide. The hydroperoxide decomposes slowly but steadily at room temperature to generate new oxidative products, which induce further degradation. In addition, it is believed that the free radical A, peroxyl radical (B) and the hydroperoxides are the main substances, which induce the changes in PMMA properties when it is gamma irradiated (Schnabel, 1981).

Fig. 7. Effects of gamma irradiation on PVC molecule

Polycarbonate (PC) fills an important niche as one of the most popular engineering resins in the medical device market. Bisphenol-A polycarbonate has been commercially available since the 1960s, and its use in medical devices dates from approximately that time. Possessing a broad range of physical properties that enable it to replace glass or metal in many products, polycarbonate offers an unusual combination of strength, rigidity, and toughness that helps prevent potentially life-threatening material failures. In addition, it provides glasslike clarity, a critical characteristic for clinical and diagnostic a setting in which visibility of tissues, blood, and other fluids is required because biocompatibility is essential for any material used in direct or indirect contact with patients (Freitag et al., 1988).

Sterilization by Gamma Irradiation 191

Ultrahigh molecular weight polyethylene (UHMWPE) possesses a unique structure and properties which have resulted in its having been the most widely used material for replacing damaged or diseased cartilage in total joint replacements for the last 35 year. UHMWPE is a linear (non-branching) semi-crystalline polymer which can be described as a two phase composite of crystalline and amorphous phases. The two resins of UHMWPE that are currently used in orthopaedics are GUR 1020 (3.5 million g /mol) and GUR 1050 (5.5–6 million g /mol). Orthopaedic components machined from UHMWPE are typically sterilized by irradiation with 25 kGy of 60Co gamma rays (Goldman et al, 1998). Such strong ionizing radiation is likely to have a detrimental effect upon the microstructure, such as entanglement density and tie molecules that give UHMWPE its needed properties for total joint replacement applications. The high-energy photons, such as gamma rays, can generate free radicals in polymers (P) through homolytic bond cleavage (reaction 1 in Figure 10). These radicals have been shown to have long lifetimes, especially those generated in the crystalline regions of the polymer where they can diffuse at low mobility into the amorphous regions of the polymer, and can therefore continue to undergo chemical reactions for many months and beyond. This time-dependent free-radical reaction mechanism poses serious concern for the radiation degradation of polymers, especially in the presence of oxygen as is observed in the reactions showed in Figure 10 (reactions 2 and 3), which has a high difusional mobility and is very reactive with the radicals. Hydroperoxides also are formed as the first product of oxidation and upon their decomposition free radicals are re-generated (reaction 4 in Figure 10). Every molecule of hydroperoxide produced subsequently undergoes radiolysis to generate an alkoxy radical which both provides new initiating radicals and at the same time produces carbonyl compounds (reaction 5 in Figure 10). Thus, the process is autocatalytic and can lead to the further formation ketones, alcohols, esters, and carboxylic acids in the polyethyelene chains. Therefore, as long as there is an oxygen source, the cycle can continue and the number of oxidation products will increase without any further irradiation (Schanbel, 1981). This process is known as post-irradiation aging and has been shown to occur in implants that were gamma sterilized in air and packaged in air-permeable packaging. Changes in physical, chemical and mechanical properties of UHMWPE as a consequence of oxidative degradation (Costa & P. Bracco, 2004). Property changes include an increase in percent crystallinity, an increase in density (an indirect measure of oxidation), an increase in elastic

As the evidence of the clinical consequences of oxidative degradation of UHMWPE total joint replacement components increased, the orthopaedic implant manufacturers began to

Fig. 9. Proposed mechanism of MDA formation by gamma irradiation of PU

modulus, and a decrease in elongation to failure.

Fig. 8. Radiolytic degradation of PMMA

The radiation-induced main chain scissions on PC occur in the carbonate groups, causing the evolution of carbon monoxide, carbon dioxide and hydrogen. The radiolysis of PC produces phenoxy and phenyl polymeric radicals that cause yellowness of the polymer. However, it has been reported in the literature that the crosslinking effect predominates at small doses, whereas at higher doses the main chain scission is more pronounced (Araujo et al, 1998).

Polyurethane (PU) is widely used in various medical devices because of its biocompatibility, and has some reports concerning its physicochemical stability and biological safety. However, among substances which were produced by degradation of PU, it was reported that a carcinogen, 4,4'-methylenedianiline (MDA), was produced from PU sterilized by gamma irradiation. On the other hand, a modified PU was produced and called thermosetting PU. In the case of thermosetting PU used in medical devices such as potting material in artificial dialysis devices, plasma separators, etc., the production of MDA upon sterilization showed a reverse tendency to non modified PU (Shintani, 1992). Their components and characteristics used in PU fabrication are much different, however their influences on the production of MDA by sterilization have not been sufficiently clarified.

As shown in Figure 9, it was suggested that the mechanism of MDA production might be the cleavage at urethane linkage successive to the terminalamino group, by radiation or hydrolysis (Shintani, 1992). Since more hydrophilic components were detected in the current experiment, we speculate the major cleavage portion will be at urethane linkage, thus producing MDA. The possibility of the cleavage at benzene-CH, linkage will not be significant due to no aniline or p-toluidine production.

The radiation-induced main chain scissions on PC occur in the carbonate groups, causing the evolution of carbon monoxide, carbon dioxide and hydrogen. The radiolysis of PC produces phenoxy and phenyl polymeric radicals that cause yellowness of the polymer. However, it has been reported in the literature that the crosslinking effect predominates at small doses, whereas at higher doses the main chain scission is more pronounced (Araujo et

Polyurethane (PU) is widely used in various medical devices because of its biocompatibility, and has some reports concerning its physicochemical stability and biological safety. However, among substances which were produced by degradation of PU, it was reported that a carcinogen, 4,4'-methylenedianiline (MDA), was produced from PU sterilized by gamma irradiation. On the other hand, a modified PU was produced and called thermosetting PU. In the case of thermosetting PU used in medical devices such as potting material in artificial dialysis devices, plasma separators, etc., the production of MDA upon sterilization showed a reverse tendency to non modified PU (Shintani, 1992). Their components and characteristics used in PU fabrication are much different, however their influences on the production of MDA by sterilization have not been sufficiently clarified.

As shown in Figure 9, it was suggested that the mechanism of MDA production might be the cleavage at urethane linkage successive to the terminalamino group, by radiation or hydrolysis (Shintani, 1992). Since more hydrophilic components were detected in the current experiment, we speculate the major cleavage portion will be at urethane linkage, thus producing MDA. The possibility of the cleavage at benzene-CH, linkage will not be

Fig. 8. Radiolytic degradation of PMMA

significant due to no aniline or p-toluidine production.

al, 1998).

Fig. 9. Proposed mechanism of MDA formation by gamma irradiation of PU

Ultrahigh molecular weight polyethylene (UHMWPE) possesses a unique structure and properties which have resulted in its having been the most widely used material for replacing damaged or diseased cartilage in total joint replacements for the last 35 year. UHMWPE is a linear (non-branching) semi-crystalline polymer which can be described as a two phase composite of crystalline and amorphous phases. The two resins of UHMWPE that are currently used in orthopaedics are GUR 1020 (3.5 million g /mol) and GUR 1050 (5.5–6 million g /mol). Orthopaedic components machined from UHMWPE are typically sterilized by irradiation with 25 kGy of 60Co gamma rays (Goldman et al, 1998). Such strong ionizing radiation is likely to have a detrimental effect upon the microstructure, such as entanglement density and tie molecules that give UHMWPE its needed properties for total joint replacement applications. The high-energy photons, such as gamma rays, can generate free radicals in polymers (P) through homolytic bond cleavage (reaction 1 in Figure 10). These radicals have been shown to have long lifetimes, especially those generated in the crystalline regions of the polymer where they can diffuse at low mobility into the amorphous regions of the polymer, and can therefore continue to undergo chemical reactions for many months and beyond. This time-dependent free-radical reaction mechanism poses serious concern for the radiation degradation of polymers, especially in the presence of oxygen as is observed in the reactions showed in Figure 10 (reactions 2 and 3), which has a high difusional mobility and is very reactive with the radicals. Hydroperoxides also are formed as the first product of oxidation and upon their decomposition free radicals are re-generated (reaction 4 in Figure 10). Every molecule of hydroperoxide produced subsequently undergoes radiolysis to generate an alkoxy radical which both provides new initiating radicals and at the same time produces carbonyl compounds (reaction 5 in Figure 10). Thus, the process is autocatalytic and can lead to the further formation ketones, alcohols, esters, and carboxylic acids in the polyethyelene chains. Therefore, as long as there is an oxygen source, the cycle can continue and the number of oxidation products will increase without any further irradiation (Schanbel, 1981). This process is known as post-irradiation aging and has been shown to occur in implants that were gamma sterilized in air and packaged in air-permeable packaging. Changes in physical, chemical and mechanical properties of UHMWPE as a consequence of oxidative degradation (Costa & P. Bracco, 2004). Property changes include an increase in percent crystallinity, an increase in density (an indirect measure of oxidation), an increase in elastic modulus, and a decrease in elongation to failure.

As the evidence of the clinical consequences of oxidative degradation of UHMWPE total joint replacement components increased, the orthopaedic implant manufacturers began to

Sterilization by Gamma Irradiation 193

with few peaks at the carbonyl position. The integrated absorption of the C=O band centered about 1720 cm-1 has been assumed to give a quantitative evaluation of the radiation induced oxidation. Since the PE is a polyolefin the carbonyl group is obtained in reaction 5 of the scheme in Figure 10. Oxidation tends to start at tertiary carbon atoms because the free radicals formed here are more stable and longer lasting, making them more susceptible to attack by oxygen. The carbonyl group can be further oxidized to break the chain, this weakens the material by lowering its molecular weight, and cracks start to grow in the

Fig. 11. FT-IR spectrum of PP exposed to gamma sterilization in air

The changes in the PP molecule by gamma sterilization are associated with the changes in crystallinity and morphology of the polymer. The correlations between the changes in both morphology and crystallinity with other properties during irradiation are important to explain the mechanism that lead to crystallinity change. Some studies investigated the response of PP to γ-radiation and relate the crystallinity and morphological changes to corresponding changes in other properties such as mechanical properties, viscosity, melting temperature, etc. Kushal et al. (1995) relate the drop in the melting temperature, viscosity and mechanical properties versus the increases in crystallinity during γ-irradiation to the breakdown of crystallites with a concomitant formation of smaller

The extent of chain scission and crosslinking of PP is dependent on the γ-irradiation dose but not the initial starting morphology (Zhang and Cameron 1999). Using WAXD (Wide angle X-ray diffraction) and DSC (Differential scanning calorimetry) techniques, Alariqi et al. (2006) found change in the degree of crystallinity, which caused by γ-irradiation, depends on the γ-irradiation dose ( see Table 6) and Kostoski and Stojanovic (1995) found the increase in crystallinity of oriented isotactic polypropylene with low absorbed doses of γ-radiation, up to 200 kGy. They have also found that the peak melting temperature decreased with absorbed dose. The results were explained in terms of the scission of the tie molecules followed by the growth of new thin crystal lamellae, as well as to the fact that

regions affected.

crystalline entities.

study and then to employ alternative sterilization methods such as gamma radiation sterilization in inert environment (e.g. argon, nitrogen, vacuum) packaging as a means to minimize oxidation during shelf aging. For UHMWPE, cross-linking dominates when the polymer is irradiated in nitrogen, while chain scission dominates when the material is irradiated in air. This is due to the fact that oxygen is extremely reactive with the free radicals produced by irradiation, forming peroxides which can break down and lead to further radical production, so that the total number of free radicals generated and the total extent of chain scission, are greatly increased. It should be noted that, without some additional manufacturing step to extinguish any remaining entrapped free radicals, oxidation will occur upon exposure to oxygen (such as during in vivo use). For gamma sterilization in an inert environment to be successful, it must be combined with barrier packaging to prevent access of atmospheric oxygen to the UHMWPE during shelf storage. Thus, barrier packaging is expected to effectively reduce the risk of oxidative degradation of UHMWPE during shelf storage (Rimnac & Kurtz, 2005)

Fig. 10. Polyolefins oxidation caused by gamma sterilization

Polypropylene (PP) is one of the most widely used plastics for packaging applications. Polypropylene is one of the most popular polymers in the manufacturing of medical disposables, since it exhibits high transparency, good mechanical properties, low cost and chemical inertness over other polymers. In a continuously increasing part of this market, especially in the pharmaceutical area, but also in food packaging and especially in the manufacturing of syringes, security lenses, surgical clothing, etc. Medical instruments employed in the diagnosis or treatment of a patient, especially those that can penetrate the protective, barrier of the skin, must be completely exempt of germs.

Changes in polymer properties were observed when PP medical devices are sterilized by gamma irradiation undergoing oxidative degradation if sterilized in air. Oxidation of PP is usually relatively easy to detect owing to the strong absorption by the carbonyl group in the FT-IR spectrum as is showed in Figure 11. Polypropylene has a relatively simple spectrum

study and then to employ alternative sterilization methods such as gamma radiation sterilization in inert environment (e.g. argon, nitrogen, vacuum) packaging as a means to minimize oxidation during shelf aging. For UHMWPE, cross-linking dominates when the polymer is irradiated in nitrogen, while chain scission dominates when the material is irradiated in air. This is due to the fact that oxygen is extremely reactive with the free radicals produced by irradiation, forming peroxides which can break down and lead to further radical production, so that the total number of free radicals generated and the total extent of chain scission, are greatly increased. It should be noted that, without some additional manufacturing step to extinguish any remaining entrapped free radicals, oxidation will occur upon exposure to oxygen (such as during in vivo use). For gamma sterilization in an inert environment to be successful, it must be combined with barrier packaging to prevent access of atmospheric oxygen to the UHMWPE during shelf storage. Thus, barrier packaging is expected to effectively reduce the risk of oxidative degradation of

UHMWPE during shelf storage (Rimnac & Kurtz, 2005)

Fig. 10. Polyolefins oxidation caused by gamma sterilization

protective, barrier of the skin, must be completely exempt of germs.

Polypropylene (PP) is one of the most widely used plastics for packaging applications. Polypropylene is one of the most popular polymers in the manufacturing of medical disposables, since it exhibits high transparency, good mechanical properties, low cost and chemical inertness over other polymers. In a continuously increasing part of this market, especially in the pharmaceutical area, but also in food packaging and especially in the manufacturing of syringes, security lenses, surgical clothing, etc. Medical instruments employed in the diagnosis or treatment of a patient, especially those that can penetrate the

Changes in polymer properties were observed when PP medical devices are sterilized by gamma irradiation undergoing oxidative degradation if sterilized in air. Oxidation of PP is usually relatively easy to detect owing to the strong absorption by the carbonyl group in the FT-IR spectrum as is showed in Figure 11. Polypropylene has a relatively simple spectrum with few peaks at the carbonyl position. The integrated absorption of the C=O band centered about 1720 cm-1 has been assumed to give a quantitative evaluation of the radiation induced oxidation. Since the PE is a polyolefin the carbonyl group is obtained in reaction 5 of the scheme in Figure 10. Oxidation tends to start at tertiary carbon atoms because the free radicals formed here are more stable and longer lasting, making them more susceptible to attack by oxygen. The carbonyl group can be further oxidized to break the chain, this weakens the material by lowering its molecular weight, and cracks start to grow in the regions affected.

Fig. 11. FT-IR spectrum of PP exposed to gamma sterilization in air

The changes in the PP molecule by gamma sterilization are associated with the changes in crystallinity and morphology of the polymer. The correlations between the changes in both morphology and crystallinity with other properties during irradiation are important to explain the mechanism that lead to crystallinity change. Some studies investigated the response of PP to γ-radiation and relate the crystallinity and morphological changes to corresponding changes in other properties such as mechanical properties, viscosity, melting temperature, etc. Kushal et al. (1995) relate the drop in the melting temperature, viscosity and mechanical properties versus the increases in crystallinity during γ-irradiation to the breakdown of crystallites with a concomitant formation of smaller crystalline entities.

The extent of chain scission and crosslinking of PP is dependent on the γ-irradiation dose but not the initial starting morphology (Zhang and Cameron 1999). Using WAXD (Wide angle X-ray diffraction) and DSC (Differential scanning calorimetry) techniques, Alariqi et al. (2006) found change in the degree of crystallinity, which caused by γ-irradiation, depends on the γ-irradiation dose ( see Table 6) and Kostoski and Stojanovic (1995) found the increase in crystallinity of oriented isotactic polypropylene with low absorbed doses of γ-radiation, up to 200 kGy. They have also found that the peak melting temperature decreased with absorbed dose. The results were explained in terms of the scission of the tie molecules followed by the growth of new thin crystal lamellae, as well as to the fact that

Sterilization by Gamma Irradiation 195

Fig. 12. Effects of sterilization by gamma irradiation in silicone molecule

flourishing fraction of that thermo-oxidative- and UV stabilizers.

either by preventing chain initiation, and/or stopping chain propagation.

**6.2 Action of stabilizers in polymeric medical device exposed to gamma sterilization**  With the development of space science, the stability of polymeric materials against radiation has been drawing the attention of scientists. Polymers which contain aromatic groups are well known to have relatively good radiation stability, but are also very expensive. The practical solution of these protection tasks are connected to specific chemical agents, well engineered polymer additives, elaborated mainly for the stabilization of general purpose polymers. The radiation stabilizers, called "antirads" represent only a modest, but

The reason behind the parallel technical development of conventional and radiation stabilizers is related to the fact, that the UV degradation and thermo-oxidative degradation as well as radiation degradation of polymers are all similar chain reactions. As such, these processes consist of several steps of: chain initiation, chain propagation, chain branching and chain termination. The scheme according to which these reactions proceed on a H containing polymer chain P is seen in Figure 6. In spite of the differences in fine details the task is similar in all the three main (thermooxidative, UV and radiation) degradation processes, namely to control and/or diminish the danger of deterioration of properties

Additives may promote radiolytic stabilization on properties of polymers thought two primary mechanisms: a) scavenging of excited-state energy (quenching), and b) scavenging of paramagnetic species (free radicals, secondary electrons). Also the incorporation of additives, plasticizing type, act as "mobilizer" on polymer chains. Additives and stabilizers

irradiation produces defects in the polymer structure which decrease its thermal stability. However, the number of chain scission increased with decreasing the dose rate. From, lowering molecular weight, increased chain scissions, increased crystallinity, it can be understood that the rise in crystallinity is due to re-crystallization of shorter chains which are produced by the chain scission of tie molecules forming new perfect crystallites leading to an increase in crystallinity. On the other hand, the decrease in crystallinity was attributed to the formation of crosslinking. Krestev et al. (1986) have found that part of monoclinic αphase of PP is converted into triclinic γ-phase during gamma irradiation. It was reported that the formation of γ-phase was not due to the crystallization of low molecular fraction but to the high internal pressure caused by the crosslinking.


Table 6. Effect os gamma sterilization on crystallinity of Polypropylene

Polyisoprene, especially in the form of natural rubber latex, is widely used in prophylactic medical disposables, such as gloves and condoms, and found to be an effective barrier. Because of its unsaturation, natural rubber and many other elastomers will slightly crosslink when exposed to radiation sterilization conditions. Such crosslinking will not detract from the overall extensibility or elongation of these rubber devices. Natural rubber formulations, as well as formulations based on other elastomers, can also be used as gasketing materials in devices. Although isobutylene is well known to scission when exposed to radiation, a halogenated copolymer of isobutylene and isoprene, commonly brominated butyl rubber (BIIR), can be formulated to exhibit radiation response when used in the tyre industry. Having been previously crosslinked with a zinc oxide system, BIIR can withstand the radiation exposure required for sterilization. Such elastomeric materials form the sealed caps on injectable drugs, being able to reseal themselves after having been penetrated by the needle of a syringe.

Silicone rubber is widely used in medical applications, where sterilized is an essential requirement for all medical tools and devices that contact the body or bodily fluid and medical components must be sterilized frequently by gamma irradiation. Gamma radiation is known to induce changes in the molecular architecture of silicone rubber, resulting in an increase in molecular weight and a decrease in elasticity. This effect is also observed in samples previously subjected to post-cure treatments. Radicals are generated by chain scission and/or methyl or hydrogen abstraction (see Figure 12) and are subsequently terminated via oxidation reactions or coupled to form longer chain branches. Although these two mechanisms compete against each other, crosslinking reactions dominate in silicone materials; higher dosages of gamma radiation and longer treatment cycles have been shown to result in higher crosslink densities (Traeger & Castonguar, 1966). An increase in polymer-filler interfacial interactions through crosslinking reactions is also observed.

irradiation produces defects in the polymer structure which decrease its thermal stability. However, the number of chain scission increased with decreasing the dose rate. From, lowering molecular weight, increased chain scissions, increased crystallinity, it can be understood that the rise in crystallinity is due to re-crystallization of shorter chains which are produced by the chain scission of tie molecules forming new perfect crystallites leading to an increase in crystallinity. On the other hand, the decrease in crystallinity was attributed to the formation of crosslinking. Krestev et al. (1986) have found that part of monoclinic αphase of PP is converted into triclinic γ-phase during gamma irradiation. It was reported that the formation of γ-phase was not due to the crystallization of low molecular fraction but

> 0 38.5 36.3 10 48.0 42.9 25 33.2 32.6

Polyisoprene, especially in the form of natural rubber latex, is widely used in prophylactic medical disposables, such as gloves and condoms, and found to be an effective barrier. Because of its unsaturation, natural rubber and many other elastomers will slightly crosslink when exposed to radiation sterilization conditions. Such crosslinking will not detract from the overall extensibility or elongation of these rubber devices. Natural rubber formulations, as well as formulations based on other elastomers, can also be used as gasketing materials in devices. Although isobutylene is well known to scission when exposed to radiation, a halogenated copolymer of isobutylene and isoprene, commonly brominated butyl rubber (BIIR), can be formulated to exhibit radiation response when used in the tyre industry. Having been previously crosslinked with a zinc oxide system, BIIR can withstand the radiation exposure required for sterilization. Such elastomeric materials form the sealed caps on injectable drugs, being able to reseal themselves after having been penetrated by the

Silicone rubber is widely used in medical applications, where sterilized is an essential requirement for all medical tools and devices that contact the body or bodily fluid and medical components must be sterilized frequently by gamma irradiation. Gamma radiation is known to induce changes in the molecular architecture of silicone rubber, resulting in an increase in molecular weight and a decrease in elasticity. This effect is also observed in samples previously subjected to post-cure treatments. Radicals are generated by chain scission and/or methyl or hydrogen abstraction (see Figure 12) and are subsequently terminated via oxidation reactions or coupled to form longer chain branches. Although these two mechanisms compete against each other, crosslinking reactions dominate in silicone materials; higher dosages of gamma radiation and longer treatment cycles have been shown to result in higher crosslink densities (Traeger & Castonguar, 1966). An increase in polymer-filler interfacial interactions through

**Degree of crystallinity (%) WAXD DSC** 

to the high internal pressure caused by the crosslinking.

Table 6. Effect os gamma sterilization on crystallinity of Polypropylene

**Irradiation dose (kGy)** 

needle of a syringe.

crosslinking reactions is also observed.

Fig. 12. Effects of sterilization by gamma irradiation in silicone molecule
