**3.3. Mechanical properties**

The development of mechanical properties of PAs irradiated with γ-rays is, as usual, linked to changes in the molecular and supermolecular characteristics. Evaluating the behavior of PA-6 and composite PA-6/GF γ-irradiated in air [31] concludes that the Young's modulus of PA-6 is soft decreasing with rising irradiation dose, whereas, for the nonirradiated composite PA-6/GF, the values are almost identical with the material irradiated with the highest dose (**Figure 20**).

For both materials, the dependencies exhibit a shallow minimum around the gel point as a result of the superposition of the two opposite effects, namely, branching, chain scission, and cross-bond formation. These effects are more evident for PA-6 in comparison to the composite PA-6/GF. At first sight, it seems to be a misinterpretation. However, one has to take into consideration generally lower figures for PA-6 modulus and a small variation leads to a larger relative variation than in the case of the composite. Overall marginal variations of modulus are linked to a low gel content for PA-6 and no gel for PA-6/GF (Section 3.1) as well as small changes in crystallinity (Section 3.2). Because the level of crystallinity and crosslinking

**Figure 20** Variations of Young's modulus for virgin PA-6 and composite PA-6/GF γ-irradiated in air.

Waste PA finds exploitation in various material combinations. Hassan et al. [40] studied the effect of γ-irradiation on blends containing waste PA-6/PA-66 copolymer and ground rubber from tires with various ratios of these incompatible components. The blends irradiated in the range of 0 to 200 kGy give the melting temperature and crystallinity decreasing with increasing dose due to the crosslinking at interphase. The visible side shoulder in the endotherm for 100 kGy is missing in the 200 kGy endotherm and microphotographs show a relatively smooth fracture surface. Thermal stability measured by thermogravimetry is a little worse after irradiation. Montmorillonite clay is then added into the blend to formulate nanocomposite [41]. The composite after being γ-irradiated between 0 and 200 kGy obtains a markedly magnified thermal stability. When 12% montmorillonite is present in the mixture, the DSC data indicate the increase in the melting temperature with dose with reverse order of the onset in melting endotherm. The crystallinity is observed to be highest for the 100 kGy dose and the corresponding endotherm outlines multiplicity. The increase of montmorillonite portion to 18% leads to a decrease in the melting temperature. Also, the temperature onset falls. In this case, the highest crystallinity belongs to the nonirradiated composite and is followed by the 200 kGy dose. The dose of 100 kGy corresponds to the lowest crystallinity with the most structured melting endotherm. The multiplicity of the endotherm indicates a new element in the supermolecular structure as a consequence of γ-irradiation. Another composite consisting of the same polymer components PA-6/PA-6,6 copolymer and ground rubber but with added carbon black was examined by the same authors [42], applying the same doses of 0 to 200 kGy. As reported, the content of carbon black within 6% to 24% improves the thermal stability in both cases without and with γ-irradiation. The melting temperature and crystallinity of the composite with 12% carbon black decrease with rising dose slightly more when compared to 18%. The melting endotherm becomes smoother and the composite irradiated with 200 kGy presents a homogeneous fracture surface. Such studies are useful in optimizing a filler portion

regarding other required properties. Usually, some compromise is necessary.

The development of mechanical properties of PAs irradiated with γ-rays is, as usual, linked to changes in the molecular and supermolecular characteristics. Evaluating the behavior of PA-6 and composite PA-6/GF γ-irradiated in air [31] concludes that the Young's modulus of PA-6 is soft decreasing with rising irradiation dose, whereas, for the nonirradiated composite PA-6/GF, the values are almost identical with the material irradiated with the highest dose

For both materials, the dependencies exhibit a shallow minimum around the gel point as a result of the superposition of the two opposite effects, namely, branching, chain scission, and cross-bond formation. These effects are more evident for PA-6 in comparison to the composite PA-6/GF. At first sight, it seems to be a misinterpretation. However, one has to take into consideration generally lower figures for PA-6 modulus and a small variation leads to a larger relative variation than in the case of the composite. Overall marginal variations of modulus are linked to a low gel content for PA-6 and no gel for PA-6/GF (Section 3.1) as well as small changes in crystallinity (Section 3.2). Because the level of crystallinity and crosslinking

**3.3. Mechanical properties**

274 Radiation Effects in Materials

(**Figure 20**).

determines the modulus value, a significant variation of the modulus values should not be expected as demonstrated in **Figure 20**.

Testing tensile strength reveals that yield point can be observed only for PA-6, and the composite PA-6/GF exhibits brittle behavior without signs of yielding. A considerable decrease of yield stress from 71 to 51 MPa is observed already at the lowest dose of 50 kGy and then the curve leveled off up to the highest dose of 500 kGy. The tensile strength at break for PA-6 does not vary considerably with rising irradiation dose. This result is attributed to the gel content (Section 3.1), which contributes to retain the strength at break with no significant variation in the whole dose range compensating the strength decrease induced by degradative influence of γ-irradiation in air. In contrast to PA-6, all exposed PA-6/GF samples showed a little reduction in tensile strength at break. The reason is the different gel formation; whereas no gel was measured in the composite, a certain amount of gel was determined in PA-6 beyond gel point. Therefore, zero gel in PA-6/GF could not compensate the decrease in the strength due to the degradation of the polymer matrix.

Variations in elongation at yield for PA-6 as well as for the composite are negligible and this parameter for PA-6/GF is identical with the elongation at break. For PA-6, the starting elongation at break at ~147% increased to 240% already at 50 kGy without any change for the other doses. Such mild increase in the elongation at break is caused by the minor decrease of crystalline portion as well as the lamellae thinning as a result of oxidative degradation with the consequence of easier plastic deformation. In addition, at low gel content (maximum ~17%), shorter recombined chains act in the matrix as plasticizers and increase its deformability. The effect is not observed for the composite PA-6/GF, as it is overlapped by enhanced brittleness of the material due to the presence of the anisotropic GF.

Concerning irradiation in an inert argon atmosphere, a comparison of effect on the moduli is demonstrated in **Figure 21**.

**Figure 21.** Comparison of variations of Young's modulus for virgin PA-6 and composite PA-6/GF γ-irradiated by 500 kGy dose.

When PA-6 and PA-6/GF were irradiated in inert atmosphere, the relevant moduli showed lower values in comparison to the materials exposed to γ-irradiation in air. In consideration of the marginally changing results of Δ*H* in the first DSC run and so also the minor changes in crystallinity, the most probable reason consists of the lamellae thinning as a result of irradiation, demonstrated by the lowering in the melting temperature. It is known that the stiffness of the material may decrease if crystallites are smaller even when crystallinity does not change [4].

The reduction of tensile strength for both samples is more enhanced when irradiated in inert atmosphere. As displayed in **Figure 22**, the changes are rather small, except for yield, where the extent of changes is close to the values for Young's modulus.

**Figure 22.** Comparison of variations of tensile strength for virgin PA-6 and composite PA-6/GF γ-irradiated by 500 kGy dose.

The changes in elongation at break and at yield (in the latter case only for PA-6) are shown in **Figure 23**.

**Figure 23.** Comparison of variations of elongation for virgin PA-6 and composite PA-6/GF γ-irradiated by 500 kGy dose.

**Figure 21.** Comparison of variations of Young's modulus for virgin PA-6 and composite PA-6/GF γ-irradiated by 500

When PA-6 and PA-6/GF were irradiated in inert atmosphere, the relevant moduli showed lower values in comparison to the materials exposed to γ-irradiation in air. In consideration of the marginally changing results of Δ*H* in the first DSC run and so also the minor changes in crystallinity, the most probable reason consists of the lamellae thinning as a result of irradiation, demonstrated by the lowering in the melting temperature. It is known that the stiffness of the material may decrease if crystallites are smaller even when crystallinity does

The reduction of tensile strength for both samples is more enhanced when irradiated in inert atmosphere. As displayed in **Figure 22**, the changes are rather small, except for yield, where

**Figure 22.** Comparison of variations of tensile strength for virgin PA-6 and composite PA-6/GF γ-irradiated by 500

the extent of changes is close to the values for Young's modulus.

kGy dose.

276 Radiation Effects in Materials

not change [4].

kGy dose.

The elongation at yield for PA-6 irradiated with 500 kGy in inert atmosphere increased for up to quintuple value, whereas irradiation in air led to negligible change. In contrast, the elon‐ gation at break is higher after irradiation in air when compared to irradiation of PA-6 in inert atmosphere, and this is lower than for the unexposed sample. Irradiation of PA-6/GF in air led to the elongation at break with no change, whereas a little increase is observed when irradiated in inert atmosphere. This indicates some increase of the matrix deformability due to a low crosslinking level.

Concerning exposure in inert atmosphere, similar data are given for electrospun PA-66 fibers irradiated in nitrogen atmosphere with 20 and 50 kGy dose. Tensile stress lowers with dose, whereas an increase is observed after adding the crosslinking agent TAC [38]. The same is reported for Young's modulus.

Notched impact strength of PAs is often unfavorable property. An examination of this aspect was demonstrated by Charpy notched impact test being carried out for both PA-6 and PA-6/GF irradiated within 0 to 500 kGy in air [31]. The dependence of the impact strength on dose reveals a different behavior. The PA-6 curve shows a maximum at 50 kGy. The corre‐ sponding impact strength is 80% higher compared to the initial value. This can be attributed to the perturbation of initial physical nodes in amorphous phase and branching, which facilitates more elastic dissipation of impact energy and supports a deceleration of crack propagation. Higher doses form a network, although not a dense one, but it supports the absorbing impact because of its elasticity. Whereas oxidation-degradation occurs simultane‐ ously, the result represents a superposition of the positive contribution of elasticity and a negative oxidation-degradation under irradiation. At 500 kGy, the final impact strength for PA-6 is 1.5 times higher than the starting value. However, an opposite trend is observed for the PA-6/GF composite. In this case, the impact strength decreases mildly with rising dose up to 82% of the initial value linking the zero gel observed (**Figure 14**) and more or less constant crystallinity (Figures **17** and **18**). As evidenced by SEM photos, although easier GF dewetting in nonirradiated composite may act as an obstacle for crack propagation at impact, after irradiation, higher GF adhesion makes the material more compact and less resistant to impact failure.

Blends containing waste PA-6/PA-66 copolymer and ground rubber from tires irradiated within 0 to 200 kGy in air show lower values in tensile strength and elongation compared to the unexposed sample, whereas elastic modulus is changed little. All the variations are a function of the component ratio [40]. When montmorillonite clay is involved in the blend, formulating a nanocomposite, a decrease in tensile strength, and an elongation at break is observed for each dose in comparison to the starting material. However, the courses of both parameters are a function of the montmorillonite portion, indicating an optimum for 12% clay. Concerning modulus, a decrease is measured regardless of dose, but the differences between the compositions are not large [41]. The same doses of 0 to 200 kGy were applied on the mixture of the above-mentioned polymer components PA-6/PA-66 copolymer and ground rubber with the addition of carbon black [42]. The tensile strength of nonirradiated sample is observed to be higher than the irradiated samples, except the dose of 50 kGy being above the initial material. However, irradiation led to a decrease in elongation for all doses compared to that unexposed. For the latter case, the addition of carbon black increased elongation significantly; for others, there was a slower increase. In both tensile characteristics, the differences between doses 100 and 200 kGy are marginal, whereas the variations are more profound for the dose of 50 kGy. The dependences of the parameters on carbon black portion within 6% to 24% are almost flat.

γ-Rays are applied in the field of flame retardation of polymers, too. The recent study of Sonnier et al. [43] demonstrates for blend PP/PA-6/crosslinking agent that, depending on the type of retardation, the best flame-retarded blend before irradiation can become the worst one after irradiation at a higher dose. In this case, a dose up to 100 kGy was used. Testing of the dependence of Young's modulus on temperature provided results similar for all blends at RT for whatever the dose, and these decreased slowly when the temperature increased. Above 150°C, the decrease was faster. Heat distortion of the blends accelerated with rising dose, whereas the distortion was not observed for nonirradiated blend. It is concluded that, if heat shielding effect is applied to provide the flame retardation, the top protective layer can be disrupted and heat release rate will increase to a considerable extent. In such case, the barrier layer is not capable to prevent the subsequent transfer of heat.

#### **3.4. Sterilization of PA materials by γ-radiation**

Low doses of γ-radiation are often used for the sterilization of food packaging. Polymers used to package food intended for irradiation must receive relevant approvals. Evaluation of ebeam, γ- and X-ray treatment on the chemistry and safety of polymers used with prepackaged irradiated foods showed that the three forms of irradiation have virtually indistinguishable effects on polymers irradiated in vacuum [44]. However, γ-irradiation in air results in facilitated damage due to slow dose rate providing enough time to oxidative degradation. It is accepted, in general, that the foods in contact with irradiated polymeric materials should not be endangered by radiolytic products with adverse impact on health.

to 82% of the initial value linking the zero gel observed (**Figure 14**) and more or less constant crystallinity (Figures **17** and **18**). As evidenced by SEM photos, although easier GF dewetting in nonirradiated composite may act as an obstacle for crack propagation at impact, after irradiation, higher GF adhesion makes the material more compact and less resistant to impact

Blends containing waste PA-6/PA-66 copolymer and ground rubber from tires irradiated within 0 to 200 kGy in air show lower values in tensile strength and elongation compared to the unexposed sample, whereas elastic modulus is changed little. All the variations are a function of the component ratio [40]. When montmorillonite clay is involved in the blend, formulating a nanocomposite, a decrease in tensile strength, and an elongation at break is observed for each dose in comparison to the starting material. However, the courses of both parameters are a function of the montmorillonite portion, indicating an optimum for 12% clay. Concerning modulus, a decrease is measured regardless of dose, but the differences between the compositions are not large [41]. The same doses of 0 to 200 kGy were applied on the mixture of the above-mentioned polymer components PA-6/PA-66 copolymer and ground rubber with the addition of carbon black [42]. The tensile strength of nonirradiated sample is observed to be higher than the irradiated samples, except the dose of 50 kGy being above the initial material. However, irradiation led to a decrease in elongation for all doses compared to that unexposed. For the latter case, the addition of carbon black increased elongation significantly; for others, there was a slower increase. In both tensile characteristics, the differences between doses 100 and 200 kGy are marginal, whereas the variations are more profound for the dose of 50 kGy. The dependences of the parameters on carbon black portion within 6% to 24% are

γ-Rays are applied in the field of flame retardation of polymers, too. The recent study of Sonnier et al. [43] demonstrates for blend PP/PA-6/crosslinking agent that, depending on the type of retardation, the best flame-retarded blend before irradiation can become the worst one after irradiation at a higher dose. In this case, a dose up to 100 kGy was used. Testing of the dependence of Young's modulus on temperature provided results similar for all blends at RT for whatever the dose, and these decreased slowly when the temperature increased. Above 150°C, the decrease was faster. Heat distortion of the blends accelerated with rising dose, whereas the distortion was not observed for nonirradiated blend. It is concluded that, if heat shielding effect is applied to provide the flame retardation, the top protective layer can be disrupted and heat release rate will increase to a considerable extent. In such case, the barrier

Low doses of γ-radiation are often used for the sterilization of food packaging. Polymers used to package food intended for irradiation must receive relevant approvals. Evaluation of ebeam, γ- and X-ray treatment on the chemistry and safety of polymers used with prepackaged irradiated foods showed that the three forms of irradiation have virtually indistinguishable effects on polymers irradiated in vacuum [44]. However, γ-irradiation in air results in

layer is not capable to prevent the subsequent transfer of heat.

**3.4. Sterilization of PA materials by γ-radiation**

failure.

278 Radiation Effects in Materials

almost flat.

PAs are governed by excellent barrier performance. That is why several works are devoted to the issue of food packaging involving PAs. An examination of various plastic multilayer PA-6 films, used for meat and cheese, after being irradiated (up to 12 kGy) reveals that the release of ε-caprolactam from exposed PA-6 is of a much higher extent compared to that of nonirra‐ diated samples, indicating chain scission [45]. Félix et al. [46] conducted a migration assay at 40°C for 10 days focusing on the effect of γ-irradiation with 12 kGy dose on ε-caprolactam migration from multilayer PA-6 films into food simulants. The results revealed that the irradiation caused almost no changes in ε-caprolactam levels, with the exception of olive oil, which showed an increase in the caprolactam level. However, all the tested films were within the legislation and did not exceed limits for ε-caprolactam migration. Park et al. [47] reported that 5 kGy γ-irradiation significantly increased the formation of ε-caprolactam in PA-6 from 70.76 to 164.10 ppm. The formation of ε-caprolactam ranged between 122 and 164 ppm in the dose range of 5 to 200 kGy.

Barrier five-layer food packaging films, consisting of two outer PA-6 layers (~15%) and a middle LDPE layer (50%), after being irradiated with a larger range of doses of 5 to 60 kGy were analyzed focusing on volatile and nonvolatile radiolytic products and sensory changes [48]. The data show that a large number of radiolytic products are produced such as hydro‐ carbons, alcohols, carbonyl compounds, and carboxylic acid but also amide type of products. These substances are detected even at the lower doses of 5 and 10 kGy. Most of the substances are assumed to come from LDPE because that is used also as recycled. The type and concen‐ tration of radiolytic products increase progressively with the absorbed dose. In addition, irradiation dose appears to influence the sensory properties of table water in contact with the films being classified according to stricter requirements.

In another study [49], the authors analyzed 13 different multilayer polymeric materials for food used before and after their exposure to γ-radiation regarding the profile of volatile compounds released from the polymeric materials. Thermosealed bags of different materials were filled with either air or nitrogen to evaluate the oxygen influence. One third of the samples were analyzed without irradiation, whereas the rest were irradiated at 15 and 25 kGy. Half of the samples were processed just after preparation and the other half was stored for 8 months at RT before analysis. Significant differences between nonirradiated and irradiated bags were found. Sixty to 80 compounds were released and identified per sample. Independent of the filling gas, the results of nonirradiated materials were almost identical. In contrast, the chromatographic profile and the odor of irradiated bags filled with nitrogen were completely different from those filled with air. The migration of compounds from irradiated materials to the vapor phase was much lower than the limits established in the relevant EU Commission Regulation.
