**3. Structural modification by using gamma irradiation**

As it is known that environmental problems caused by waste materials are in a constant growth and as a consequence different methods have been developed, some of them are consuming money and time. One novel alternative is to use ionizing radiation, such as gamma rays.

Gamma radiation has many advantages over other conventional methods such as chemical attack or thermal process. For example, initiation process is different; gamma particles only are necessary if that material is in contact with radioactive source, while in a chemical reaction, catalysts or additives are required; another important aspect is referred to the production of free radicals, when using chemicals these are produced through decomposition of the initiator in fragments, while in the case of irradiation process free radicals are produced by the absorption of energy of the polymer; moreover, with irradiation process the reaction can be controlled and be free from contamination. With respect to the temperature, gamma irradiation shows better behavior, because in the case of a chemical reaction often local overheating of the initiator is produced, while for irradiation no activation energy is found [11–13].

Applying gamma radiation for recycling polymers has increased its acceptation as a current technology due to the ecologic and economical features and mainly its capacity to modify physicochemical properties of the wastes without introducing any chemical initiators or the need to dissolve them [14]. In principle the molecular structure of materials can be modified by using gamma irradiation; this creates free radicals which will often chemically react in various ways, sometimes at slow reaction rates. The free radicals can recombine, forming the cross-links.

A competing process, called scissioning, occurs when polymers are irradiated. In this case, the polymer chains are broken and molecular mass decreases. The other process is called crosslinking, which depends on kind of polymer, and the number of cross-links can be controlled by the amount of irradiation dose. Scissioning and cross-linking occur at the same time where one may predominate over the other, depending upon the polymer and the dose. Both phenomena change the physical, chemical, and mechanical properties of polymer materials. In fact, more benefits can be obtained from recovered scrap polymer cross-linking by using gamma radiation [15, 16].

In the case of polyethylene terephthalate (PET), different opinions about radiation stability have been reported. Some authors report fair stability in the mechanical and physicochemical properties at high doses (900 kGy), with changes from cross-linking processes up to 35% from the starting values. Some authors have reported changes due to the chain scission process at low dose (from 0 to 10 kGy) while others have reported such events at a high dose (from 120 kGy to 5 MGy). The degradation mechanism for PET fibers or PET bulk is the same. No chemical degradation for PET fibers is found up to 200 kGy [17–20].

The recycling and reutilization of cross-linked elastomers are difficult due to their 3D formed network; nevertheless, it is necessary to find wise-strategies for reuse and to avoid ground contamination. The natural and synthetic rubbers such as styrene-butadiene-styrene (SBS) and styrene-butadiene-rubber (SBR) are the raw materials in the production of tires; the natural rubbers provide elastic properties while the synthetics provide thermal stability.

**3. Structural modification by using gamma irradiation**

166 Composites from Renewable and Sustainable Materials

As it is known that environmental problems caused by waste materials are in a constant growth and as a consequence different methods have been developed, some of them are consuming money and time. One novel alternative is to use ionizing radiation, such as gamma rays.

Gamma radiation has many advantages over other conventional methods such as chemical attack or thermal process. For example, initiation process is different; gamma particles only are necessary if that material is in contact with radioactive source, while in a chemical reaction, catalysts or additives are required; another important aspect is referred to the production of free radicals, when using chemicals these are produced through decomposition of the initiator in fragments, while in the case of irradiation process free radicals are produced by the absorption of energy of the polymer; moreover, with irradiation process the reaction can be controlled and be free from contamination. With respect to the temperature, gamma irradiation shows better behavior, because in the case of a chemical reaction often local overheating of the

Applying gamma radiation for recycling polymers has increased its acceptation as a current technology due to the ecologic and economical features and mainly its capacity to modify physicochemical properties of the wastes without introducing any chemical initiators or the need to dissolve them [14]. In principle the molecular structure of materials can be modified by using gamma irradiation; this creates free radicals which will often chemically react in various ways, sometimes at slow reaction rates. The free radicals can recombine, forming the

A competing process, called scissioning, occurs when polymers are irradiated. In this case, the polymer chains are broken and molecular mass decreases. The other process is called crosslinking, which depends on kind of polymer, and the number of cross-links can be controlled by the amount of irradiation dose. Scissioning and cross-linking occur at the same time where one may predominate over the other, depending upon the polymer and the dose. Both phenomena change the physical, chemical, and mechanical properties of polymer materials. In fact, more benefits can be obtained from recovered scrap polymer cross-linking by using

In the case of polyethylene terephthalate (PET), different opinions about radiation stability have been reported. Some authors report fair stability in the mechanical and physicochemical properties at high doses (900 kGy), with changes from cross-linking processes up to 35% from the starting values. Some authors have reported changes due to the chain scission process at low dose (from 0 to 10 kGy) while others have reported such events at a high dose (from 120 kGy to 5 MGy). The degradation mechanism for PET fibers or PET bulk is the same. No

The recycling and reutilization of cross-linked elastomers are difficult due to their 3D formed network; nevertheless, it is necessary to find wise-strategies for reuse and to avoid ground contamination. The natural and synthetic rubbers such as styrene-butadiene-styrene (SBS) and

chemical degradation for PET fibers is found up to 200 kGy [17–20].

initiator is produced, while for irradiation no activation energy is found [11–13].

cross-links.

gamma radiation [15, 16].

In the case of elastomers (such as tire rubber), gamma radiation causes morphological deterioration and chemical changes, including accelerated oxidation [21]. Physicochemical properties of blends of rubber stocks and virgin or recycled elastomers are improved after irradiating with gamma particles. For example, rubber stocks blended with recycled and irradiated butyl crumb show shortened vulcanization period and antitearing properties. Moreover, improvement on the plasticity of crumb rubber, as well as great moldability of virgin rubber and recycled crumb blends, when they are irradiated at 70 kGy.

Vulcanization of chlorine butyl rubbers by using gamma radiation decreases the tensile strength and elongation-at-break up to 25 kGy, but after this dose, stability of such properties is observed, up to 200 kGy. Moreover, thermal stability is reduced through the degradation and scission of molecular chains [22]. Other study is based on the effects of gamma irradiation in polydimethylsiloxane rubber foams and their relationship with mechanical properties and chemical structure, which are measured by compression strength, infrared attenuated total reflectance (ATR) spectroscopy and X-ray-induced photoelectron spectroscopy (XPS). The results show a higher cross-linking of polymer chains when increasing the irradiation dose, thus foams became harder [22].

By using gamma radiation, ground tire rubber (GTR) and recycled high-density polyethylene (HDPE) blends can be functionalized through higher interaction between elastomer and acrylamide functional groups, allowing improvement of their mechanical properties for doses from 25 to 50 kGy. Elongation-at-break and Charpy impact strength of the blends are significantly increased due to the presence of GTR; moreover, blends' Young's modulus values are only slightly decreased due to the radiation-induced cross-linking of the HDPE matrix [22, 23].

The use of gamma radiation as a mechanism for reaction initiation and accelerator of the polymerization of a monomer in a ceramic matrix can bring considerable advantages. One of the most important objectives is to obtain higher adhesion between fibers and the matrix. In the case of the Tetra Pak components, the first investigations focused on the influence of gamma radiation on lignocellulose materials, in terms of increasing the solubility of insoluble highpolymerized sugars such as cellulose. Application of gamma irradiation on cellulose results in decrease in molecular weight and crystallinity, as well as formation of oxidation products, because cellulose is a predominantly chain-scissioning polymer. After irradiation, changes in the main chain of the cellulose are observed, where radicals provoke random cleavage of glycoside bonds, as well as splitting of carbon-bonded hydrogen and dehydrogenation reactions. Another studied parameters are the degree of polymerization (DP) and specific gravity. Such changes are beneficial for manufacturing products such as medical grade cellulose.

As it is known, the cross-linking reaction is affected by the initial degree of crystallinity, crystal size distribution, and molecular weight. In general terms, crystallinity increases and reaches a maximum at certain irradiation dose, but it decreases on further increase of irradiation dose. Microfibrils are composed of cellulose crystals and amorphous zones, in which more penetration of chemicals is observed. Such zones have different appearances such as cracks and irregular morphological shapes.

Tetra Pak panel boards (TPPBs) show decrease in the mass up to 200°C which is related to the evaporation of physical water. In general, thermal degradation of paper is located between 200°C and 400°C, particularly two decomposition peaks are observed. The first one at 300°C due to hemicellulose and the second at 360°C due to thermal degradation of α-cellulose. For higher temperature from 400°C to 461°C, degradation of remaining paper and LDPE is considered. After thermal process can be found two kinds of residues, char and aluminum foil [24].

In the case of irradiated polyester resin some physicochemical properties are affected, for example, when increasing the dose a better thermal stability is obtained at low temperatures, because its glass transition temperature increases. But at high temperatures, the decomposition temperature is unaffected. After analyzing both thermal and mechanical properties a relationship is observed. Moreover, a typical behavior is observed: improvement of the compressive strength depends on the increment of the irradiation dose [23].
