**2.3. Mechanical properties**

The molecular and supermolecular structure of a polymer determines its properties. When any variation occurs in the structure, it should manifest itself in some changes of the properties. Electron beam activates significant structural changes, namely, scission and crosslinking as well as oxidative degradation if irradiated in air. Each particular property of a polymer is unequally sensitive towards the structural modification; therefore, it is important for exami‐ nation to be focused exactly. Tensile properties, such as Young's modulus, strength at yield or break, elongation at yield or break, and flexural and impact parameters, reflect these variations well.

In general, when virgin PA is irradiated, the development of the tensile parameters follows the general framework that modulus and stress at yield are progressive, whereas stress at break and elongation are regressive. The scale of measured values is a question of the quality of the irradiated PA. The shape of stress-strain curves for the same PA remains similar irrespective of the absorbed dose [16].

The dimensional stability on load, the stiffness, is one important parameter for design engineers. In thermoplastics, the shape stability is affected by the crystallinity content to a great extent. Within the tensile properties, Young's modulus is the corresponding testing parameter for stiffness. As mentioned in Section 2.1, electron beam irradiation in the solid state results in crosslink formation primarily in the amorphous phase or at the crystal boundaries [3,4]. In addition, for thermoplastics, an observable increase in the strength parameters occurs when the gel content is at or more than 50 wt% [4]. Thus, unless the crystalline phase is substantially degraded, notable changes in the modulus will not occur as evident from the dependences of Young's modulus for virgin PA-6 and PA-6/GF composite (**Figure 8**).

In reality, regarding a large fluctuation in the standard deviation (brought on by GF dispersion within the PA matrix) for the composite, any variations of Young's modulus are statistically nonsignificant to be presented. Concerning virgin PA-6, the increase in the modulus is evident at 500 kGy dose when the gel content is more than 50%. In another study [11], a steady increase in the modulus of virgin PA-6 is measured within the dose range of 100 to 500 kGy. However, in this case, the gel point is measureable already at approximately 100 kGy and 50% growth

The behavior of PA-66 is similar to PA-6 in principle. Injection-molded PA-66 samples irradiated with 200 and 500 kGy doses show the decrease in initial *T*<sup>m</sup> regardless of the addition of TAC or the exposure at RT or 120°C. Nor does the water annealing affect this tendency, but the decrease for the samples with no TAC irradiated at RT is a little less when compared to the others. The same can be said about the crystallinity [15]. Thus, the dependences of the crystallinity on dose for PA-66 films [8] and for the injection-molded pieces are the same

Data on PA-12 melting characteristics are found for EPDM/PA-12/maleated EPDM blend after being irradiated with 25 to 100 kGy doses [22]. Melting temperature does not show any change and the crystallinity is more or less also the same. However, such a narrow dose range does

The molecular and supermolecular structure of a polymer determines its properties. When any variation occurs in the structure, it should manifest itself in some changes of the properties. Electron beam activates significant structural changes, namely, scission and crosslinking as well as oxidative degradation if irradiated in air. Each particular property of a polymer is unequally sensitive towards the structural modification; therefore, it is important for exami‐ nation to be focused exactly. Tensile properties, such as Young's modulus, strength at yield or break, elongation at yield or break, and flexural and impact parameters, reflect these variations

In general, when virgin PA is irradiated, the development of the tensile parameters follows the general framework that modulus and stress at yield are progressive, whereas stress at break and elongation are regressive. The scale of measured values is a question of the quality of the irradiated PA. The shape of stress-strain curves for the same PA remains similar irrespective

The dimensional stability on load, the stiffness, is one important parameter for design engineers. In thermoplastics, the shape stability is affected by the crystallinity content to a great extent. Within the tensile properties, Young's modulus is the corresponding testing parameter for stiffness. As mentioned in Section 2.1, electron beam irradiation in the solid state results in crosslink formation primarily in the amorphous phase or at the crystal boundaries [3,4]. In addition, for thermoplastics, an observable increase in the strength parameters occurs when the gel content is at or more than 50 wt% [4]. Thus, unless the crystalline phase is substantially degraded, notable changes in the modulus will not occur as evident from the dependences of

In reality, regarding a large fluctuation in the standard deviation (brought on by GF dispersion within the PA matrix) for the composite, any variations of Young's modulus are statistically nonsignificant to be presented. Concerning virgin PA-6, the increase in the modulus is evident at 500 kGy dose when the gel content is more than 50%. In another study [11], a steady increase in the modulus of virgin PA-6 is measured within the dose range of 100 to 500 kGy. However, in this case, the gel point is measureable already at approximately 100 kGy and 50% growth

essentially regardless of the mode of sample preparation.

**2.3. Mechanical properties**

260 Radiation Effects in Materials

of the absorbed dose [16].

well.

not allow the estimation of further development under higher doses.

Young's modulus for virgin PA-6 and PA-6/GF composite (**Figure 8**).

**Figure 8.** Variation of Young's modulus with absorbed electron beam dose for virgin PA-6 and PA-6/GF (30%) compo‐ site [7]. With permission of Elsevier.

in the gel content can be estimated at approximately 150 kGy. The relative increase in the modulus at a 500 kGy dose is approximately the same (28% over the initial rate) as the abovementioned result. When the crosslinking agent is added to the polymer to increase the gel formation, the limit of 50% gel is achieved earlier. Therefore, the higher modulus is observed when compared to the virgin polymer [11], as the crystalline phase, being not yet impaired, is supported by the sufficient gel portion.

The modulus and yield tensile strength of injection-molded PA-66 irradiated in the range of 0 to 500 kGy are found to increase over the unexposed samples with dose but displays a maximum at 200 kGy. Similar results are obtained in the elongation development; however, beyond 200 kGy, the elongation is reduced below the initial value [15] due to crosslinking.

The influence of the crosslinking agent can also be demonstrated on tensile strength and elongation of PA-6 exposed to irradiation within the range of 0 to 100 kGy. These characteristics are kept the same [6], because this dose range does not involve the gel point yet. However, the addition of the crosslinking agent increases the tensile strength and a decrease in the elongation immediately from 40 kGy, indicating that the corresponding gel point is lower than 40 kGy in this case. The higher the crosslinking agent concentration, the greater the effect is found.

The importance of the selection of the crosslinking agent is reflected by the experiment where PA-66 is doped with 1% TAC, TAIC, or TMPTMA when irradiated with doses up to 600 kGy [12] in air. The tensile strength for virgin PA-66 and PA-66 with TAC exhibits a similar pattern of initial rise up to 200 kGy followed by a gradual reduction up to 600 kGy. In the case of PA with TMPTMA, the tensile strength reduces continually. In the case of TAIC, in contrast, the strength rises throughout the whole range of irradiation. The elongation of PA-66 virgin as well as PA-66 with TAC or TMPTMA reduces displaying a sharp decrease at 100 kGy for the PA containing the agents and a more moderate decrease for virgin PA-66 up to 300 kGy. Beyond these doses, the corresponding values change negligibly. The elongation of PA-66 with TAIC follows a completely different course. It gradually drops over the entire range of dosage and the resulting elongation is the highest when compared to others (**Figure 9**). This possibly indicates that, despite having the highest gel content, the network is less dense, with TAIC saving more elasticity compared to others.

**Figure 9.** Relative variation of elongation of PA-66 with and without crosslinking agents with dose of e-beam. Adapted from Pramanik et al. [12].

Concerning the modulus (**Figure 10**), this increases beyond 200 kGy sharply for virgin PA-66 in compliance with the above-mentioned finding that at least 50% gel is needed to contribute towards the modulus increase [4].

**Figure 10** Relative variation of tensile modulus of PA-66 with and without crosslinking agents with dose of e-beam. Adapted from Pramanik et al. [12].

The addition of the crosslinking agents increases the modulus over the virgin PA-66 in the order of TAIC>TAC>TMPTMA, although the dependences follow individual courses. The crosslinking efficacy of the agents in PA-66 is of the same order as well.

The effect of another agent, glycidyl methacrylate (GMA), is similar [21]. PA-6 doped and then irradiated with a dose in the range of 0 to 200 kGy in nitrogen to condensate mutually shows a modulus at 100 and 200 kGy, which is 36% and 45%, respectively, above the unexposed virgin PA-6. At the same time, the corresponding tensile strength is determined to be approximately 15% over the virgin PA-6.

indicates that, despite having the highest gel content, the network is less dense, with TAIC

**Figure 9.** Relative variation of elongation of PA-66 with and without crosslinking agents with dose of e-beam. Adapted

Concerning the modulus (**Figure 10**), this increases beyond 200 kGy sharply for virgin PA-66 in compliance with the above-mentioned finding that at least 50% gel is needed to contribute

**Figure 10** Relative variation of tensile modulus of PA-66 with and without crosslinking agents with dose of e-beam.

The addition of the crosslinking agents increases the modulus over the virgin PA-66 in the order of TAIC>TAC>TMPTMA, although the dependences follow individual courses. The

The effect of another agent, glycidyl methacrylate (GMA), is similar [21]. PA-6 doped and then irradiated with a dose in the range of 0 to 200 kGy in nitrogen to condensate mutually shows

crosslinking efficacy of the agents in PA-66 is of the same order as well.

saving more elasticity compared to others.

from Pramanik et al. [12].

262 Radiation Effects in Materials

towards the modulus increase [4].

Adapted from Pramanik et al. [12].

Radiation technology is used in an effort to improve the deficient compatibility of different polymers in blends or composites and to model the resulting properties to a certain extent due to the binding of each other through free radicals generated in the components. PA-6 and linear low-density polyethylene (LLDPE) are typical immiscible polymers. The morphologic examination of the mixture of the PA-6/LLDPE/GMA exposed to electron beam under nitrogen atmosphere within the range of 5 to 200 kGy displays a reduced diameter of the dispersion particles and an increase in the interfacial adhesion. The elongation at break of the blend irradiated at 100 kGy is approximately four times higher than that of the virgin PA-6. This parameter is strongly reduced when the blend is irradiated at 200 kGy, which is supposed to be the crosslinking of LLDPE [20]. The tensile strength and modulus increase with rising irradiation dose nonlinearly.

Electron beam irradiation of multilayer film LDPE/PA-6/LDPE with doses up to 125 kGy in nitrogen results in the increase of the tensile strength and the decrease of the elongation [18].The growth is more rapid up to 50 kGy and then is more moderate. In contrast, the elongation decreases presenting a mirror curve to the strength. This phenomenon is explained by the formation of carbonyl groups in LDPE in the case of the low doses, which facilitate the miscibility of the LDPE with PA-6. On the contrary, at high doses, the presence of crosslinked LDPE would most likely introduce microregions of immiscibility with the PA-6.

The preparation of a thermoplastic elastomer from an immiscible blend consisting of the EPDM, maleated EPDM, and PA-12 is another demonstration of the utility of electron beam irradiation [22]. The irradiation was conducted up to 100 kGy in nitrogen so that PA-12 chain scission occurred generating free radicals but no crosslinking and led to a mutual link of the components and from disastrous mechanical properties to desired ones. The increase is quoted for Young's modulus, tensile strength, and elongation; in addition, the recyclability of the thermoplastic elastomer is gained for three cycles at least.

Waste polymers can be used again when they are combined with other appropriate compo‐ nents. Therefore, waste PA copolymer PA-66/PA-6 blended with acrylonitrile butadiene rubber and subjected to electron beam irradiation shows tensile strength and elongation, depending on dose and composition, at levels between the parameters of those individual components. The obtained compatibility matches with scanning electron microscopy (SEM) images [26].

A composite consisting of ethylene-vinyl acetate copolymer (EVA) flame retarded by a combination of cellulose acetate butyrate microencapsulated ammonium polyphosphate, PA-6, and TAIC reveals a drastic increase in tensile strength by 62% over the initial value at 160 kGy. Beyond this dose, the tensile strength falls, whereas the elongation at break decreases continually from the beginning [27].

Adhesive joints in composite items often involve PAs. Polycarbonate (PC) sheet covered with PA-6 being irradiated with dose of 43 to 432 kGy [28] displays variation in the fracture stress

and elasticity of the joint as dependence on dose differing from the neat components. Whereas the fracture stress for the composite increases up to approximately 120 kGy and levels beyond this, the strain falls with rising dose up to 220 kGy and then increases. Some optimum in the characteristics could be found according to topical design requirements.

Young's modulus of composite (PP/PA-6+talc) with or without a compatibilizer or crosslinking agent TAIC, applying a dose of 0 to 200 kGy [25], gives the dependence on dose with compo‐ sition. Whereas the modulus of the mixture containing all components accounts for only 73% (PP/PA-6+talc) before irradiation, it exceeds the other mixtures after 200 kGy irradiation. Qualitatively, the same tendency is observed for tensile strength, too. These results indicate that, with a combination of suitable composition and electron irradiation, materials could be designed with the desired properties.

Further research in this field is open and not all efforts lead to desired results. For example, experiments with composites of monomer casting PA-6 containing 2% nanofillers as particle carbon or silicon carbide or carbon shortcut fibers with no crosslinking agent give only 6% increase in tensile strength and Young's modulus when irradiated by electron beam with 20 kGy dose [29].

The weakness of PAs as semicrystalline materials is quantified for a low impact strength or toughness. After virgin PA-6 is electron irradiated, a considerable deterioration of these parameters is found. The Izod impact strength falls sharply with the rising dose, and after absorbing 600 kGy, the impact strength retains 31% of the initial level only. The addition of the crosslinking agents changes this uniformly decreasing behavior. According to the agent type, the dependence of Izod impact strength on dose is of variable character, reaching a maximum at 400 kGy followed by a sharp decline beyond this dose [11]. However, the net result is a decrease when compared to the nonirradiated sample. A rather different situation occurs for PA-66 [12]. The Izod impact strength of virgin PA-66 is reduced at 100 kGy to 68% of the initial value and then displays a leveling effect up to 300 kGy, and beyond this dose, a sharp decrease is followed by no significant change at higher doses. Again, the crosslinking agents have a variable influence and result in a dependence including smaller fluctuations in comparison to the virgin PA-66. An observation of the fracture surface leads to the finding that both virgin and crosslinker-doped PA-66 irradiated up to 300 kGy show ductile failure, whereas the materials irradiated with doses of 400 to 600 kGy indicate brittle failure. One of the several reasons why electron irradiation lowers the impact strength may be a certain obstruction in the dissipation of the impact energy when the initial amorphous phase is progressively crosslinked and restricted in the chain motion and relaxation. The contribution of oxidative degradation becomes more evident in the reduction at higher doses.

The flexural characteristics for PA-6 reveal an analogous behavior with the corresponding tensile strength and tensile modulus (i.e., they increase with dose) [11]. The increase in the flexural modulus at 500 kGy is 13% over the initial level and the flexural strength 20%, respectively. The addition of effective crosslinking agents TAIC and TAC raises these values. In the case of PA-66, the flexural modulus increases up to 400 kGy by 40% and then it falls sharply, remaining still at an increase of 20% over the initial level [12]. The same crosslinking agents as in the PA-6 increase the flexural modulus, almost copying the behavior of virgin PA-66 at a higher level.

The irradiation of composite (PP/PA-6+talc) with 0 to 200 kGy doses leads to an increase in flexural strength by 18%; when the composite contains also the compatibilizer and crosslinking agent TAIC, the increase is higher by 137%. Both corresponding flexural moduli are increased by 24% and 200%, respectively [25].

It can be concluded that, besides irradiation conditions, the mechanical properties in blends of PA with other polymers or modifiers are dependent on the character of all components and the interactions between them induced by the irradiation.

#### **2.4. Thermal resistance**

and elasticity of the joint as dependence on dose differing from the neat components. Whereas the fracture stress for the composite increases up to approximately 120 kGy and levels beyond this, the strain falls with rising dose up to 220 kGy and then increases. Some optimum in the

Young's modulus of composite (PP/PA-6+talc) with or without a compatibilizer or crosslinking agent TAIC, applying a dose of 0 to 200 kGy [25], gives the dependence on dose with compo‐ sition. Whereas the modulus of the mixture containing all components accounts for only 73% (PP/PA-6+talc) before irradiation, it exceeds the other mixtures after 200 kGy irradiation. Qualitatively, the same tendency is observed for tensile strength, too. These results indicate that, with a combination of suitable composition and electron irradiation, materials could be

Further research in this field is open and not all efforts lead to desired results. For example, experiments with composites of monomer casting PA-6 containing 2% nanofillers as particle carbon or silicon carbide or carbon shortcut fibers with no crosslinking agent give only 6% increase in tensile strength and Young's modulus when irradiated by electron beam with 20

The weakness of PAs as semicrystalline materials is quantified for a low impact strength or toughness. After virgin PA-6 is electron irradiated, a considerable deterioration of these parameters is found. The Izod impact strength falls sharply with the rising dose, and after absorbing 600 kGy, the impact strength retains 31% of the initial level only. The addition of the crosslinking agents changes this uniformly decreasing behavior. According to the agent type, the dependence of Izod impact strength on dose is of variable character, reaching a maximum at 400 kGy followed by a sharp decline beyond this dose [11]. However, the net result is a decrease when compared to the nonirradiated sample. A rather different situation occurs for PA-66 [12]. The Izod impact strength of virgin PA-66 is reduced at 100 kGy to 68% of the initial value and then displays a leveling effect up to 300 kGy, and beyond this dose, a sharp decrease is followed by no significant change at higher doses. Again, the crosslinking agents have a variable influence and result in a dependence including smaller fluctuations in comparison to the virgin PA-66. An observation of the fracture surface leads to the finding that both virgin and crosslinker-doped PA-66 irradiated up to 300 kGy show ductile failure, whereas the materials irradiated with doses of 400 to 600 kGy indicate brittle failure. One of the several reasons why electron irradiation lowers the impact strength may be a certain obstruction in the dissipation of the impact energy when the initial amorphous phase is progressively crosslinked and restricted in the chain motion and relaxation. The contribution

of oxidative degradation becomes more evident in the reduction at higher doses.

The flexural characteristics for PA-6 reveal an analogous behavior with the corresponding tensile strength and tensile modulus (i.e., they increase with dose) [11]. The increase in the flexural modulus at 500 kGy is 13% over the initial level and the flexural strength 20%, respectively. The addition of effective crosslinking agents TAIC and TAC raises these values. In the case of PA-66, the flexural modulus increases up to 400 kGy by 40% and then it falls sharply, remaining still at an increase of 20% over the initial level [12]. The same crosslinking

characteristics could be found according to topical design requirements.

designed with the desired properties.

kGy dose [29].

264 Radiation Effects in Materials

The thermal resistance of polymers is usually measured by heat deflection temperature (HDT) and Vicat softening temperature. There are other specific tests adjusted to the factual require‐ ments of manufacturers. However, they are used by narrow groups of design engineers and in the framework of quality testing.

The HDT (or heat distortion temperature) is the temperature at which a polymer sample deforms under a specified load. It might be expected that parameter HDT will increase with rising dose. In fact, for PA-6 irradiated within 0 to 500 kGy, it is found that there is a progressive growth up by 9°C at 500 kGy. All the related values appear near 50°C, the *T*g of PA-6 [7]. The HDT increase with increasing dose reflects the progressive restriction of the chain mobility in the amorphous phase as a consequence of the network structure formation and the lower deformability (**Figure 11**).

**Figure 11.** Variation of HDT with absorbed electron beam dose for virgin PA-6 and PA-6/GF (30%) composite. Adapt‐ ed from Porubská et al. [7].

A different situation is in GF-reinforced PA-6. The presence of GF restrains the segmental motion of the polymer chains, and therefore, the initial HDT (190°C) observed for the compo‐ sitePA-6/GF is much higher than virgin PA-6 (47°C). Unlike the virgin PA, the HDT values for PA-6/GF decreases in response to irradiation, with a decrease of 6°C at the highest dose [7]. Analogous results can be assumed for other composites as well.

How much crosslinking can affect HDT for PA-6 is seen from the electron beam irradiation of PA-6 containing 2% TAC. Whereas the HDT before exposure is 120°C, after absorbing 80 kGy dose, this figure increases up to 170°C [6] due to ~96% gel content (**Figure 12**).

**Figure 12.** Variation of HDT with absorbed dose for PA-6 at 2% TAC level. Adapted from Dadbin et al. [6].

The irradiation of composite (PP/PA-6+talc) in the range of 0 to 200 kGy displays the rise in HDT depending on the dose from 63°C for the nonirradiated sample to 72°C for that irradiated with 200 kGy. However, when compatibilizer and crosslinking agent TAIC is incorporated in the composite, the HDT gives value of 84°C [25].

The Vicat softening temperature (or Vicat hardness) is taken as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 mm2 cross-section. There is not much information on the effect of electron irradiation on the Vicat softening temperature for PAs in the scientific sources. Unlike the HDT, the Vicat temperature for virgin PA-6 (186°C) reaches a maximum (190°C) at the lowest dose of 50 kGy and then decreases with increasing dose nearly returning to its original level (187°C) [7]. The mentioned increase is possibly related to the release of physical entanglements due to the supplied energy from the electron radiation. That enables the involvement of some released segments in amorphous phase into the crystalline phase and its enlargement. A moderate increase in the melting enthalpy/crystallinity at 50 kGy dose corresponds to this. The decrease in the Vicat temperature at a higher dose is assigned to the progressive disruption of the surface structure and the thinning of the lamellae what is obvious also from the lower melting onset (**Figure 3**). The softening temperature for PA-6/GF composite is less pronounced, with a total decrease of only 2°C (**Figure 6**). This observation corresponds to the different crystallinities of both materials (PA-6/GF > PA-6).

#### **2.5. Water absorption**

Generally, PAs tend to absorb water due to the nonbinding interactions of water molecules with polar groups in the matrix. The water present in PA acts as a plasticizer. The water absorption affects the dimensional stability and mechanical properties of the polymers, whereas humidity in PA matrix can support hydrolytic destruction and can also be a source of oxygen when PAs are processed or irradiated accelerating the polymer oxidative degrada‐ tion. Therefore, water absorptivity is one of the crucial parameters affecting the PA properties. That is why the comparison of some characteristic values for various PAs requires a standard conditioning (temperature, humidity, and time) before the PA is tested.

How much crosslinking can affect HDT for PA-6 is seen from the electron beam irradiation of PA-6 containing 2% TAC. Whereas the HDT before exposure is 120°C, after absorbing 80 kGy

dose, this figure increases up to 170°C [6] due to ~96% gel content (**Figure 12**).

**Figure 12.** Variation of HDT with absorbed dose for PA-6 at 2% TAC level. Adapted from Dadbin et al. [6].

specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 mm2

the composite, the HDT gives value of 84°C [25].

(PA-6/GF > PA-6).

266 Radiation Effects in Materials

**2.5. Water absorption**

The irradiation of composite (PP/PA-6+talc) in the range of 0 to 200 kGy displays the rise in HDT depending on the dose from 63°C for the nonirradiated sample to 72°C for that irradiated with 200 kGy. However, when compatibilizer and crosslinking agent TAIC is incorporated in

The Vicat softening temperature (or Vicat hardness) is taken as the temperature at which the

There is not much information on the effect of electron irradiation on the Vicat softening temperature for PAs in the scientific sources. Unlike the HDT, the Vicat temperature for virgin PA-6 (186°C) reaches a maximum (190°C) at the lowest dose of 50 kGy and then decreases with increasing dose nearly returning to its original level (187°C) [7]. The mentioned increase is possibly related to the release of physical entanglements due to the supplied energy from the electron radiation. That enables the involvement of some released segments in amorphous phase into the crystalline phase and its enlargement. A moderate increase in the melting enthalpy/crystallinity at 50 kGy dose corresponds to this. The decrease in the Vicat temperature at a higher dose is assigned to the progressive disruption of the surface structure and the thinning of the lamellae what is obvious also from the lower melting onset (**Figure 3**). The softening temperature for PA-6/GF composite is less pronounced, with a total decrease of only 2°C (**Figure 6**). This observation corresponds to the different crystallinities of both materials

Generally, PAs tend to absorb water due to the nonbinding interactions of water molecules with polar groups in the matrix. The water present in PA acts as a plasticizer. The water absorption affects the dimensional stability and mechanical properties of the polymers,

cross-section.

The entry of water molecules into a PA matrix is controlled by diffusion. The penetration of the matrix by water molecules requires enough space inside the matrix for the translation motion of water molecules. The space is formed by vacancies due to the rotating movement of segments of macromolecule chains. The fewer the restrictions of motion in the polymer segments, the more vacancies are created. Because the crosslinking shortens the chain sequen‐ ces between the cross-points and thus restricts the segmental rotation, water molecule mobility in the polymer should be more difficult suppressing water diffusion. This supposition is confirmed empirically. Water absorption in PA-6 irradiated in range of 0 to 600 kGy decreases throughout all doses from 2.3% to 1.7% at the highest dose [11]. As expected, the decline is greater with the incorporation of the crosslinking agents TAIC or TAC (**Figure 13**).

**Figure 13.** Variation of water absorption for PA-6 without and with 1% crosslinking agents with dose of e-beam radia‐ tion [11]. With permission of Elsevier.

Because the concentration of the effective crosslinking agent affects the gel content, it is expected to influence water absorption in the PA correspondingly. Such an example is shown in PA-6 doped with 1%, 2%, and 3% TAC [6]. The measured data confirmed this supposition and the largest decrease at 80 kGy is read for PA-6 with 3%>2%>1% TAC. The decline of water absorption compared to virgin PA-6 irradiated within 0 to 200 kGy occurs also after adding approximately 3% GMA under identical conditions [21]. Similarly, injection-molded PA-66 reveals a lowering of water absorption when irradiated in the range of 0 to 500 kGy, but the PA-66 dipped in TAC solution before the irradiation shows a further decrease in this parameter [15].
