**2. Electron beam irradiation of PAs**

The effect of electron beam on PAs depends on several factors, such as installed parameters of the accelerator, absorbed dose, rate of the dose, environment of irradiation, geometry of irradiated object, temperature, and postradiation treatment. Concerning the environment, air is used mostly and an inert atmosphere is less common but more thrifty towards the polymer. Under irradiation of polymers, free radicals are created. Subsequently, two main actions take place, that is, recombination of the radicals resulting in crosslinking and oxidation resulting in polymer degradation. Both processes run in parallel [2], competing mutually, and the results depend on concrete conditions.

What exactly occurs when a PA is irradiated with an electron beam? An interaction occurs primarily in hydrocarbon sequences (R). The complete action involves three basic steps: initiation (generation of free radicals), propagation, and termination. In general, the following scheme can describe such processes.

Initiation:

[ ] ( ) H O2 2 5 <sup>2</sup> <sup>5</sup> <sup>n</sup> n (CH ) CO N H HN CH CO - - ®- - - - é ù

adipic acid with hexamethylene diamine giving PA-66).

have brought a wider spectrum of properties or design variations.

mass.

250 Radiation Effects in Materials

The same result can be reached using the corresponding amino acid. However, the cleavage of water molecules is required, which is the characteristic for a polycondensation mechanism. Therefore, polycondensation is the second polyreaction used to prepare PAs. The most commonly applied reaction is the polycondensation of dicarboxylic acids with diamines (e.g.,

H N (CH ) NH HOOC (CH ) COOH <sup>2</sup> 26 2 2 4 2 6 24 n - - + - - ®- - - - - - - [ HN (CH ) NH OC (CH ) CO ] (2)

In such a case, the structure of the PA corresponds to the repetition of structure unit consisting of one of each monomer, so that they alternate in the chain unlike for a PA chain synthesized from a monomer as single starting reactant. Many demanding applications require a careful control of the synthesis as well as processing conditions considering the resulting molecular

Variation of the starting reactants enables a large scale of PAs differing in properties from hard and tough PA to soft and flexible to be acquired. Depending on the type, PAs absorb different amounts of moisture, which affect the mechanical as well as dimensional characteristics. In general, PAs are characterized by high rigidity, hardness, abrasion resistance, thermal stability, good sliding properties, stress cracking resistance, barrier properties against oxygen, smells, and oils. The disadvantages of PA types involve a weak stability in the presence of UV radiation, oxidizing agents, strong acids, and bases, high shrinkage in molten section, degra‐ dation in electrical and mechanical properties due to high moisture absorptivity, and high notch sensitivity. The various particular properties of PAs enable various PAs to be processed into various items. They are most frequently used in the textile industry, electrotechnics, and automotive industry as engineering plastics either virgin or composites with modifying fillers. Depending on the type of the product application, the manufacturers can adjust the properties of initial materials to some extent. Besides physical modification, involving admixture of proper additives, the chemical modification of PA can also be applied. According to PA type, a high melting temperature within 170°C up to 290°C occurs. Therefore, the homogenous admixture of intended agents into the melt is possible only for sufficiently thermally stable substances. When the chemical modification of PA requires the generation of radicals (e.g., grafting various functional groups or crosslinking initiated by organic peroxides), then this modification cannot be always carried out in melt due to a weak thermal stability of some substances participating in the process. In particular, peroxides will be thermally destroyed well before the PA is melted. This handicap can be overcome using a proper radiation technology enabling such a modifying process in the solid state. Presenting a technology of one step, the radiation technologies have been largely applied during the last decades and

ë û (1)

$$\text{R } \leadsto \text{ R } ^\ast \to \text{ R } ^\ast + \text{H } ^\ast \tag{3}$$

Propagation:

$$\text{R}^{\cdot} + \text{O}\_{2} \rightarrow \text{ROO}^{\cdot}\tag{4}$$

$$\text{ROO}^{\cdot} + \text{RH} \rightarrow \text{R}^{\cdot} + \text{ROOH} \tag{5}$$

$$\text{ROOH} \xrightarrow{h} \text{RO}^{\circ} + \text{HO}^{\circ} \tag{6}$$

$$\text{ROOR} \xrightarrow{\Lambda} \text{2RO}^\* \tag{7}$$

The generated radicals can take part in the destruction of the next hydrocarbon sequences, and hydroperoxides start the formation of new radicals and intermediates. In the presence of oxygen, the creation of carbonyl and aldehyde groups is significant as well.

Termination:

$$\mathrm{ROO}^{\circ} + \mathrm{H}^{\circ} \to \mathrm{ROOH} \tag{8}$$

$$\text{ROO}^{\cdot} + \text{R}^{\cdot} \rightarrow \text{ROOR} \tag{9}$$

In the case of PA-6, the splitting of H0 from ethylene group occurs most probably in the vicinity of the –NH–CO– sequence:

$$-\text{NH}-\left[\text{CO}-\left(\text{CH}\_{2}\right)\_{4}-\text{CH}\_{2}-\text{NH}\right]\_{\text{a}}-\leadsto-\text{NH}-\left[\text{CO}-\left(\text{CH}\_{2}\right)\_{4}-\text{C}^{\text{\*}}\text{H}-\text{NH}\right]\_{\text{a}}-+\text{H}^{\text{\*}}\tag{10}$$

However, abstraction of hydrogen can occur in any other place within the hydrocarbon sequence as well.

Propagation takes place according to the above-mentioned steps. Termination of the radicals can occur as follows:

a) Crosslinking:

$$\begin{aligned} \text{-2-NH}-[\text{CO}-(\text{CH}\_{2})\_{4}-\text{C}^{\cdot}\text{H}-\text{NH}]\_{\text{n}}-\rightarrow &-\text{NH}-[\text{CO}-(\text{CH}\_{2})\_{4}-\text{CH}-\text{NH}]\_{\text{n}}-\\ &| \text{ } &\text{ (11)}\\ -\text{NH}-[\text{CO}-(\text{CH}\_{2})\_{4}-\text{CH}-\text{NH}]\_{\text{n}}- \end{aligned} \tag{11}$$

b) Disproportionation:

$$\begin{aligned} \text{H} &- \text{NH} - \text{[CO} - \text{(CH}\_{2}\text{)}\_{4} - \text{C}^{\text{\textdegree}}\text{H} - \text{NH}\text{]}\_{\text{\textdegree}} - \rightarrow\\ \rightarrow & \text{NH} - \text{[CO} - \text{(CH}\_{2}\text{)}\_{5} - \text{NH}\text{]}\_{\text{\textdegree}} + - \text{NH} - \text{[CO} - \text{(CH}\_{2}\text{)}\_{4} - \text{CH}=\text{N}\text{]}\_{\text{\textdegree}-} \text{} \end{aligned} \tag{12}$$

c) Oxidation and following degradation:

$$-\text{NH}-[\text{CO}-(\text{CH}\_2)\_4-\text{C}^\text{I}\text{H}-\text{NH}]\_\text{a}-+\frac{1}{2}\text{O}\_2 \rightarrow -\text{NH}-[\text{CO}-(\text{CH}\_2)\_4-\text{CH}(\text{O}^\cdot)-\text{NH}]\_\text{a}-\tag{13}$$

$$\begin{aligned} \text{-NH-[CO-(CH\_2)\_4-CH(O}^\circ)-\text{NH}]\_\text{s} &\to \\ \rightarrow \text{-NH-[CO-(CH\_2)\_4-CH=O]\_\text{y}+\text{NH}\_2-[CO-(CH\_2)\_5-NH-]\_{(\text{(a)})}} \end{aligned} \tag{14}$$

Besides the above-mentioned irradiation conditions, the formation of intermediates and final products can be affected by the polymer structure, so that the products can be variable because of some side reactions.

#### **2.1. Variations in chemical structure**

ROOH RO HO <sup>D</sup>

ROOR 2RO D

oxygen, the creation of carbonyl and aldehyde groups is significant as well.

Termination:

252 Radiation Effects in Materials

In the case of PA-6, the splitting of H0

of the –NH–CO– sequence:

sequence as well.

a) Crosslinking:

can occur as follows:

b) Disproportionation:

The generated radicals can take part in the destruction of the next hydrocarbon sequences, and hydroperoxides start the formation of new radicals and intermediates. In the presence of

( ) 2 2 4 4 ( ) <sup>2</sup> <sup>n</sup> <sup>n</sup> NH CO CH CH NH NH CO CH C H NH H ° ° - - - - - - - - - - - -+ é ù é ù ë û <sup>Î</sup> ë û (10)

However, abstraction of hydrogen can occur in any other place within the hydrocarbon

Propagation takes place according to the above-mentioned steps. Termination of the radicals

2 NH [CO (CH ) C H NH] NH [CO (CH ) CH NH]

NH [CO (CH ) NH] NH [CO (CH ) CH N]

2 4 n

°

2 NH [CO (CH ) C H NH]

c) Oxidation and following degradation:


° - - - - - - ®- - - - - -

2 4 n 2 4 n

25 x 2 4 n x

® - - - -+- - - - = - (12)

® +o o (6)

® <sup>o</sup> (7)

ROO + ® H ROOH o o (8)

ROO + ® R ROOR o o (9)

from ethylene group occurs most probably in the vicinity

2 4 n


(11)


NH [CO (CH ) CH NH]


In general, the modification of semicrystalline polymers by energetic radiation in the solid phase below the melting temperature of the crystallites is characterized by changes proceeding preferentially or, in many cases, almost exclusively in the amorphous phase [3,4]. Within the pre-crosslinking phase, there is the branching in the polymer [5] as revealed by the solution viscosity measurement for PA-6 [6,7] and glass fiber-reinforced PA-6 (PA-6/GF) with 30% GF [7], indicating an increase in the molecular weight due to the recombination of macroradicals at the polymer chain centers (**Figure 1**). Consequently also, viscosity increases in normal cases.

**Figure 1.** Dependence of viscosity number for virgin PA-6 and PA-6 in PA-6/GF on electron beam absorbed dose. Adapted from Porubská et al. [7].

A measurable portion of insoluble gel occurs at approximately 200 kGy (gel point) when the irradiation is conducted in air (**Figure 2**).

**Figure 2.** Dependence of gel content on absorbed electron beam dose for virgin PA-6 and PA-6/GF (30%) composite [7]. With permission of Elsevier.

This gel point is confirmed in several studies [7–10], although a low gel content for PA-6 was measured under the 150 kGy dose as well [11,12]. Some specific results in experiments with electron beam irradiation of PA-6 and PA-6,6 tyre cords and calendered fabrics are published by Aytaç et al. [13] when, contrariwise, some decrease in limiting viscosity number with increasing dose was observed within doses of 0 to 75 kGy. The measured data are explained by sufficient time for oxygen to diffuse into the samples during the foregoing calendering as well as by a low dose rate (25 Gy/pass). Under those conditions, radiolytic and oxidative degradation occur. The following strength testing confirmed the objectivity of the results.


**Table 1.** Radiochemical *G*s scission yield and *G*c crosslinking yield of PA-6 irradiated at different temperatures [10]. With permission of Elsevier.

Besides dose, temperature at the irradiation site also plays a role. As study [10] demonstrated that the crosslinking effect grows with increasing dose and temperature. Corresponding experiments with PA-6 film irradiated over a range of 15 to 1200 kGy (and at a dose rate of 4.48 kGy/min) were carried out at different temperatures from room temperature (RT) to 80°C involving the glass transition temperature (*T*g) of approximately 50°C. Although the PA-6 gel point for all samples was observed more or less at 200 kGy, the gel content increased with temperature and the highest value (75%) was observed for a temperature of 80°C and dose of 800 kGy. The crosslinking rates of PA-6 irradiated above *T*g are higher than those samples irradiated at temperatures below *T*g. The increasing tendency in crosslinking with increasing temperature is attributed to the enhanced mobility of the PA-6 molecules above *T*g; therefore, the probability of radical recombination is higher. The radiochemical yields of crosslinking and degradation determined according to the Charlesby-Pinner equation [14] are given in **Table 1**.

The ratio *p0/q0* [(ration of main chain fractures to chain units)/(units crosslinked to chain units)] for PA-6 decreases with the increase in radiation temperature of the samples. This is a consequence of higher mobility of the chains and the recombination of radicals generated by the main chain scission as well as by the release of hydrogen radicals due to electron beam energy [Eq. (3)], indicating that crosslinking is the predominant process at temperatures above *T*g. In contrast, the scission prevails at RT, below *T*<sup>g</sup> [10,11]. Compared to the *G*s/*G*<sup>c</sup> values reported for PA-66, the scission dominates more in PA-66 than in PA-6 [8,9,15,16]. This demonstrates that a small difference in PA structure can play a role.

The final amount of the gel in polymer is limited. Generally, in virgin PA, the gel portion grows dramatically beyond the gel point and, in a certain phase, becomes stabilized. The reason is that a balance between the scission and recombination occurs, as the increasing viscosity in the matrix due to crosslinks breaks the mobility of the radicals to recombine. Although the gel content remains the same, the decrease of the swelling [17] or molecular weight between crossbonds [10] with increasing doses indicates that the network becomes denser [7] as seen in **Table 2**.

**Figure 2.** Dependence of gel content on absorbed electron beam dose for virgin PA-6 and PA-6/GF (30%) composite [7].

This gel point is confirmed in several studies [7–10], although a low gel content for PA-6 was measured under the 150 kGy dose as well [11,12]. Some specific results in experiments with electron beam irradiation of PA-6 and PA-6,6 tyre cords and calendered fabrics are published by Aytaç et al. [13] when, contrariwise, some decrease in limiting viscosity number with increasing dose was observed within doses of 0 to 75 kGy. The measured data are explained by sufficient time for oxygen to diffuse into the samples during the foregoing calendering as well as by a low dose rate (25 Gy/pass). Under those conditions, radiolytic and oxidative degradation occur. The following strength testing confirmed the objectivity of the results.

**Irradiation temperature** *p***0/***q***<sup>0</sup>** *G***<sup>s</sup>** *G***<sup>c</sup>** *G***s/***G***<sup>c</sup>** RT 0.63 0.49 0.39 1.3 50°C 0.60 0.61 0.50 1.2 70°C 0.49 0.46 0.50 0.91 80°C 0.29 0.23 0.40 0.58

**Table 1.** Radiochemical *G*s scission yield and *G*c crosslinking yield of PA-6 irradiated at different temperatures [10].

Besides dose, temperature at the irradiation site also plays a role. As study [10] demonstrated that the crosslinking effect grows with increasing dose and temperature. Corresponding experiments with PA-6 film irradiated over a range of 15 to 1200 kGy (and at a dose rate of 4.48 kGy/min) were carried out at different temperatures from room temperature (RT) to 80°C involving the glass transition temperature (*T*g) of approximately 50°C. Although the PA-6 gel point for all samples was observed more or less at 200 kGy, the gel content increased with temperature and the highest value (75%) was observed for a temperature of 80°C and dose of

With permission of Elsevier.

254 Radiation Effects in Materials

With permission of Elsevier.


**Table 2.** Average molecular weight between crosslinks (Mc) in PA-6 samples irradiated at different temperatures and doses [10]. With permission of Elsevier.

Irradiation experiments were also conducted in an inert (nitrogen) atmosphere and also with the annealing what led to lower chain scission and increased crosslinking reaction [18].

The crosslinking agents for polymers, including PAs, are molecules that contain two or more double bonds per molecule, such as triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), and trimethylolpropane trimethacrylate (TMPTMA). The energy of electron beam easily cleaves them into radicals formatting cross-bonds. Then, the crosslinking agent can increase the crosslinking rate and shift the gel point to a much lower absorbed dose [6] because of the higher efficiency of free radical production. The presence of a filler (e.g., GF reinforcement) in the PA matrix can also modify the crosslinking rate, even if contrarily [7] in comparison to the crosslinking agents. The filler increases the viscosity of the matrix, which leads to a retardation of the cross-bond formation when compared to virgin PA due to slower macroradical recom‐ bination, leaving more time for disproportionation.

Concerning gel formation, the application of electron beam irradiation on incompatible polymeric blends or layers in laminates to generate free radicals and the following interaction, the grafting, between the components to improve their compatibility, can vary from case to case. Gel point shifts depending on the character of the polymeric components as well as possible crosslinking agent addition are shown in several works involving PA-6 [19–21]. Rare data on electron beam effect on gel creation in PA-12 can be found for ethylene propylene diene monomer (EPDM)/PA-12/maleated EPDM blend and these do not show any PA-in origin gel up to 200 kGy [22].

It is worth mentioning that crosslinking becomes easier as the number of methylene groups between the amide groups increases [23,24]. This finding is based on the gel measurement on PA-6, PA-610, and PA-12 irradiated under the same conditions. This conclusion is under‐ standable because, as mentioned above, the initiation starts through a hydrocarbon sequence. The longer the hydrocarbon sequence, the more probable the hydrogen abstraction occurs.
