Effect of γ-irradiation on the hydrolytic and thermal stability of micro- and nano-TiO₂ based urea–formaldehyde composites

Abstract

The hydrolytic stability and thermal behavior of organic–inorganic composites prepared by two-stage polymerization of urea–formaldehyde resin (UF) with micro- and nano-TiO₂ before and after irradiation has been investigated. Composites of urea–formaldehyde and particles of TiO₂ of different sizes were synthesized, namely: UF/micro-TiO₂ and UF/nano-TiO₂. The hydrolytic stability of the modified UF composites was determined by measuring the mass loss and liberated formaldehyde concentration of modified UF composites after acid hydrolysis. The studied modified UF composites have been irradiated (50 kGy) and the effect of γ-irradiation was evaluated on the basis of the percentage of liberated formaldehyde before and after irradiation. The minimum percentage (0.16%) of liberated formaldehyde was obtained in nano-TiO₂ modified UF resin after γ-irradiation which indicated a significant improvement in the hydrolytic stability compared with micro-TiO₂ modified UF resin (0.52%). The effect of γ-irradiation was evaluated also on the basis of the thermal behavior of the same modified UF composites before and after irradiation. The thermal behavior was studied by non-isothermal thermo-gravimetric analysis (TG), differential thermo-gravimetry (DTG) and differential thermal analysis (DTA) supported by data from attenuated total reflection infrared (ATR-IR) spectroscopy. DTG peaks of both composites are shifted to a higher temperature after irradiation, but UF/micro-TiO₂ after γ-irradiation shows less change in the thermal behavior than nano-TiO₂. In other words, UF/micro-TiO₂ shows better radiation stability. Gamma irradiation causes a minor effect on the ATR-IR spectra, specifically the decrease of the intensities of some bands.

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1. Introduction

Urea–formaldehyde (UF) resin belongs to the group of thermosetting aminoplastic resins produced by condensation reactions between its main components, urea and formaldehyde (FA). The reactions between urea and FA are very complex and by applying different reaction conditions and preparation procedures, a variety of condensed structures can be formed.

As a typical amino resin, the UF resin adhesive possesses some advantages, such as fast curing, good performance in the panel, water solubility and lower price.¹ In spite of some advantages UF resin adhesives also possess critical disadvantages: formaldehyde emission from the panels and poor water resistance. Furthermore, the hydrolytic degradation process and formaldehyde emission from some urea–formaldehyde (UF) bonded wood products has been recognized for a number of years as a potential source of indoor air pollution and health problems.^2–5

Formaldehyde is used mainly to produce resins used in particleboard products and as an intermediate in the synthesis of other chemicals. The emitted formaldehyde causes from a free formaldehyde present in UF resins after synthesis and mostly from hydrolysis of UF resins under acidic and moisture conditions.⁶ Exposure to formaldehyde may occur by breathing contaminated indoor air, tobacco smoke, or ambient urban air. Acute (short-term) and chronic (long-term) inhalation exposure to formaldehyde in humans can result in respiratory symptoms, and eye, nose, and throat irritation. Furthermore, the formaldehyde emission from the panels used for interior applications is known as one of the main factors causing sick building syndrome in an indoor environment.⁷ Therefore, the formaldehyde emission issue has been one of the most important aspects of UF resin research. Modification of UF resins is perhaps one of the best possibilities to create stabilized UF resins which could exceed toxicological and other limitations in their application. By adding the selected materials in certain quantities as fillers, catalysts and hardeners it is possible to get UF resins with improved physical and chemical properties.^1,8–10 In the polymer composites, different types of fillers are used for improving the thermal, mechanical as well as other properties. Among them, nanopowder is widely used as filler, primarily due to the surface area hence the chemical stability and the chemical reactivity of the material are all correlated with the particle size.¹¹ Different types of metal oxide nanoparticles such as SiO₂, TiO₂, ZnO are widely used for these purposes. These are nontoxic, stable, and highly thermostable inorganic fillers. Owing to all these properties, these are widely used in all types of materials like plastics, rubbers etc.

High-energy radiation is a well-known technique for modification of polymers.^12–15 Radiation stability–resistance is the ability of the polymer to retain irradiation despite initial chemical and physical properties, macrostructure and microstructure, i.e. to avoid crosslinking and/or degradation. However, little work concerning the effects of γ-irradiation on the hydrolytic stability and thermal properties of modified UF composites has been done.

The goal of this work was to examine the effect of γ-irradiation on hydrolytic and thermal stability of synthesized micro- and nano-TiO₂ modified UF composites. The hydrolytic stability of modified UF composites before and after irradiation was determined by measuring the mass loss and liberated formaldehyde concentration of modified UF resins after acid hydrolysis. The thermal behavior of modified UF composites (original and irradiated) was investigated using non-isothermal thermo-gravimetric analysis (TG), differential thermo-gravimetry (DTG) and differential thermal analysis (DTA) supported by data from ATR-IR spectroscopy.

2. Experimental

2.1. Materials

The following materials were employed in the study reported here: urea (NH₂)₂CO (Alkaloid-Skopje, FYR of Macedonia); 35% formaldehyde CH₂O (Unis-Goražde, Bosnia and Herzegovina); micro-TiO₂ and nano-TiO₂, (Sigma-Aldrich, USA). The average diameter, density and specific surface of the nano-TiO₂ were 21 nm, 4.26 g cm⁻³ and 36–65 m² g⁻¹, respectively. All the other materials and solvents used for analytical methods were of analytical grade.

2.2. Synthesis of modified UF composites

Two samples of modified urea–formaldehyde (UF) composites with formaldehyde to urea (F/U) ratio (0.8) with two different particle size of filler were synthesized using the same procedure. Synthesis procedure was as follows: 60 cm³ of distilled water and 0.1 mol of urea are mixed into reaction vessel with magnetic stirrer. Other components, 7.25 g TiO₂, 0.12 mol 35% formaldehyde and 0.6 cm³ of concentrated sulfuric acid were added into the reaction mixture according to following order. The pH value is lower than 6. Reaction mixture is mixed for 3 hours. 0.22 mol of sodium hydroxide dissolved in 6 cm³ of distilled water and added to reaction mixture before the stirring was done. The modified UF composites were cured at 110 °C for 2 h in a convective drying oven.

2.3. Determination of pH

The modified UF composites were ground into particles and 2 g of each resin was topped with 10 mL of distilled water and mixed. After 48 h, the mixture was stirred and the pH (three replications for each sample) was measured by pH meter. The same procedure was used for irradiated UF composite samples.

2.4. Determination of free formaldehyde

The percentage of free formaldehyde (FA) was determined by the sulfite method.¹⁶ 0.5 g of the grounded resin was mixed with 25 mL of distilled water. After adding 4–5 drops of thymolphthalein, mixture was carefully neutralized by titrating with 0.1 M sodium hydroxide. Then, 15 mL of 0.5 M sodium sulfate were added to the solution. The solution was stirred for 5 min; then, the mixture was slowly titrated with 0.1 M hydrochloric acid. The results were calculated. For each sample measurement was performed three times. To determine the blank, the solution without resin was determined according the same procedure and result was taking account in calculations.

2.5. UF composites hydrolysis

The modified UF composites were ground into particles and prepared by adding 0.5 g of the each resin into 250 mL beaker and 50 mL of 0.1 M HCl.⁹ Then, the mixture was hydrolyzed by continuously and vigorously stirring with a magnetic bar, at 50 °C for 90 min (tree replications for each sample). To determine the blank, 0.5 g of the same sample were extracted in 250 mL water for 90 min at room temperature.

2.5.1. Determination of mass loss. To determine the total hydrolysis mass loss (three replications for each sample), the suspension was filtrated (5893 blue ribbon filter) using a vacuum-filtration unit.⁹ The solid residue was washed and after drying (105 °C, 3 h), the sample residue was weighed. The mass loss was determined by weighing the difference of the weights before and after the hydrolysis.

2.5.2. Determination of liberated formaldehyde. Liberated FA concentration after the hydrolysis was determined by the sulfite method.^9,16 To quantify the amount of dissolved formaldehyde in the hydrolysis filtrate, a sample aliquot of 50 mL was taken from the suspension directly after hydrolysis. Filtered solutions were placed in 250 mL beaker, add ethyl alcohol to 100 mL, 4–5 drops of thymolphthalein and carefully neutralized by titrating with 0.1 M sodium hydroxide. Then, 15 mL of 0.5 M sodium sulfite were added to the solution. The solution was stirred for 5 min; then, the mixture was slowly titrated with 0.1 M hydrochloric acid. All measurements were performed at least as duplicates. The blank was determined according the same procedure and result was taking account in calculations.

2.6. γ-irradiation

Irradiations of prepared samples were performed in air in the Co-60 radiation sterilization unit at the Vinca Institute of Nuclear Sciences. The Radiation Unit of the Vinca Institute has been described in more detail elsewhere,¹⁷ the facility core is Co-60 gamma irradiator with wet storage working in batch mode (CEA, France). The samples were irradiated by gamma rays at room temperature with the dose rate of 10 kGy h⁻¹ and total absorbed dose of 50 kGy.

2.7. Thermal analysis

The thermal stability was investigated by non-isothermal thermogravimetry (TG, DTA) using a Setaram Setsys Evolution 1750 instrument (France). Samples (6 ± 0.2 mg) were placed in alumina crucibles. An empty alumina crucible was used as a reference. The samples were heated from 30 to 600 °C in a 20 cm³ min⁻¹ flow of argon atmosphere with a heating rate of 10 °C min⁻¹. The temperatures at maximum decomposition rate were determined from the peak maxima of the DTG curves.

2.8. ATR-IR spectroscopy

Spectra were collected at room temperature in a Thermo Nicolet 380 FTIR Spectrophotometer equipped with an overhead attenuated total reflection accessory (ATR). Sixty four scans were recorded for each spectrum in the spectral region between 4000 and 400 cm⁻¹ and nominal resolution of 4 cm⁻¹. Samples were applied on the surface of a diamond in a form of a powder and after measurement crystal were thoroughly washed. The cleaned crystal was carefully examined and checked with the background spectrum.

3. Results and discussion

In synthesis of modified UF composites we used two-step reaction of urea and formaldehyde which produces resins with a broad variety of both linear and branched chains.¹⁸ In order to get resins with good hydrolytic and thermal characteristics all modified resins used in this study were with low F/U mole ratio, 0.8. It was reported that UF resin with lower F/U mole ratio decreased methylol content and branching, leading to lower water sorption, greater inter chain bonding and had a many other side effects which resulted with increasing of hydrolytic stability of UF resins, itself.^19–21 Two fillers, micro- and nano-TiO₂ were used in order to improve hydrolytic and thermal stability of UF resins.

The idea was to investigate impact of γ-irradiation to hydrolytic and thermal stability of modified UF composites, because radiation could cause cross-linking and formation of network, but also degradation of polymers through scission reactions. Which one of this two opposed reactions would take dominance depends mostly of combination of polymer components, but also and many other factors.²²

The pH value of modified UF composites before and after irradiation was measured. It was found that pH values before and after γ-irradiation at room temperature with the dose rate of 10 kGy h⁻¹ and total absorbed dose of 50 kGy were decreased by 0.11 (UF/micro-TiO₂) and 0.23 (UF/nano-TiO₂) pH units, respectively (Fig. 1). The chain scission and crosslink's through radiation of the modified resins as complex systems certainly result in presence of new functional groups (such as carbonyl, carboxyl, ester and hydroxyl). They together contribute to the acidity of the resins.

Fig. 1 pH of modified UF composites before and after γ-irradiation.

The lower pH contributes to the increase of stable –CH₂– group contained and also to the reduction of the free FA percentage. These results might be related to the molecular structure of cured UF resin.²³ Decreasing the content of formaldehyde in the formulation of such resins decreases the amount of terminal –CH₂OH groups, which are surely more reactive than the methylene ones. On the other hand, a lower F/U molar ratio allows producing a crystalline framework, which reasonably hinders the penetration of water in the bulk of the material, thus making the reactive moieties less available for hydrolysis.²⁴

3.1. Free formaldehyde and hydrolytic stability

One of the causes of the FA emission from wood-based panels is free formaldehyde present in UF resin after its synthesis. As reaction of formation of UF resin is reversible, the certain amount of free FA will exist in UF resin.²⁵ In the studies of many authors is has been pointed out that formaldehyde is mainly emitted from the following sources: residual formaldehyde in the resin; formaldehyde generated by condensation reactions between hydroxymethyl groups and formaldehyde released by hydrolytic degradation of weakly bonded structures is cured resin.²⁶ The percentage of free FA in pure UF resin (F/U ratio 0.8) was given in our previous work.¹

Fig. 2 shows measured percentage of free FA of micro-and nano-TiO₂ modified UF resins. Both samples show the same value before radiation and amounts of 0.06%. γ-irradiation lowered percentage of free FA found in nano-TiO₂ modified UF composite. Contrary, after irradiation UF/micro-TiO₂ sample showed relatively high free FA (0.23%). The reason could be additional interaction of hydroxyl groups present in surface of TiO₂ and as a result reduced interaction with the polymer matrix and reduce capability to scavenge FA. Another reason is degradation of cured resin due to irradiation and liberation of formaldehyde from dimethylene ether groups.

Fig. 2 Percentage of free formaldehyde from modified UF composites before and after γ-irradiation.

Hydrolysis of cured resin, i.e. splitting of ether bridges and terminal methylol groups, appears to contribute the most in subsequent formaldehyde release from urea resin bonded boards. The sensitivity to hydrolysis of a cured resin depends on its chemical nature and the degree of polymer cross linking. The hydrolytic reaction requires the presence of water and is accelerated by heat and acids. Hydrolytic stability of studied modified UF resins were examined in conditions of acid hydrolysis by determination of liberated FA and the mass loss. The studied UF resins with low mole ratio were generally linear structure;²⁷ therefore they had fewer number of methylol groups exposed to hydrolysis which could consequently improve the hydrolytic resistance.²¹

Content of liberated FA after acid hydrolysis is presented on Fig. 3. A sample with micro-TiO₂ particles showing lower content of released FA, before irradiation, than the sample with nano-TiO₂ particles. Probably, the methylol groups (–CH₂OH) of formaldehyde are covalently attached onto the surface of micro-TiO₂ via electrophilic reaction and retards the FA emission.²⁸ In the sample containing nano-TiO₂, the agglomeration of nano-particles is probably happened and many of the Ti–OH groups from the surface of the filler particles are “lost” for capturing FA. TiO₂ was a super hydrophilic material. Under high humidity and at room temperature, multi-layer molecule of water was adsorbed on the surface of TiO₂, leading to low resistance in wet air and insensitivity for formaldehyde. The water should be desorbed from the active sites on the surface. Then, the renewed surface of TiO₂ could be sensitive to formaldehyde.²⁹ After radiation all modified UF resins have lower percentage of liberated FA; hence, hydrolytic stability of studied UF resins was enhanced by irradiation. Upon exposure to irradiation, the energy absorbed by the polymeric material produces some active species such as radicals; thereby initiating various chemical reactions (chain scission, chain branching and/or cross linking). During these reactions, new active groups were formed ready to capture FA and improve the hydrolytic stability of irradiated resins compared to non-irradiated.

Fig. 3 Percentage of liberated formaldehyde from modified UF composites before and after γ-irradiation.

From Fig. 4 it is obvious that the results obtained from the gravimetric method and FA liberation correspond with each other. An irradiated micro-TiO₂ modified UF resin has smaller mass loss (40.78%) then irradiated nano-TiO₂ modified UF resin (59.22%). Bearing in mind all results it can be concluded that micro- and nano-TiO₂ actively participates in the reaction by utilizing formaldehyde for crosslinking, thereby consuming the excess formaldehyde to deplete the percentage of free formaldehyde in the resin samples. Low pH contributes to these modified resins to retain a potential for further crosslinking. γ-irradiation enhanced hydrolytic stability of both modified UF composites. An improvement in hydrolytic stability can be explained on the basis of formation of further crosslinking upon gamma radiation in addition to previous crosslinking through thermal treatment.

Fig. 4 Percentage of mass loss from modified UF composites before and after γ-irradiation.

3.2. Thermal analysis

During manufacture of UF resin, the final reaction products between urea and formaldehyde can range from the simple monomethylolurea to very complicate cross-linked structures. During resin curing, a three-dimensional network structure is built up. The formation of linear condensation products in cure process begins at lower temperature, if the resin contains greater amount of reactive methylol groups. Depending on different synthesis conditions and technology, the generally used two-step reaction of urea and formaldehyde produces resins with a broad variety of both linear and branched chains.¹

TG is one of the methods of thermal analyses which can be used to measure the mass change, thermal decomposition, and thermal stability of composite materials. The thermal stability of a material is defined as the specific temperature or temperature–time limit in which the material can be used without excessive mass loss.³⁰

The thermal behavior of unirradiated and irradiated composites based on UF resins occurs in three and four main stages (Fig. 5 and 6). The mass loss at different temperatures is summarized in Table 1. The rate of the thermal decomposition reaction before and after irradiation, shows more than one maximum rate with temperature is increasing. This behavior indicated that thermal decomposition of these resin passed through multiple stages, depending on the state of decomposition and not on the components.^31,32

Fig. 5 TG curves unirradiated and irradiated modified UF composites.

Fig. 6 DTG curves unirradiated and irradiated modified UF composites.

Table 1 DTG, DTA data of peak values, and total mass loss for unirradiated and irradiated composites based on UF resin

Samples	Dose of γ-irradiation (kGy)	Temperature intervals (^oC)	DTG peak values (^oC)	Mass loss (%)	Total mass loss (%)	DTA endothermic peak values (^oC)
a Overlapping peaks.
UF/micro-TiO2	0	30–95	67.9	9.3	54.8	71.9
		117–297.7	218.9^a	27.8^a		264.9
			265.0^a
		323.2–350.3	335.8	14.4
	50	30–96.5	68.5	11.1	57.6	72.8
		187.9–302.5	217.3^a	28.6^a		269.3
			269.1^a
		331.1–357.4	339.8	14.2
						441.1
UF/nano-TiO2	0	30–110.1	81.4	10.2	55.2	82.5
		213.4–370.9	259.5^a	39.3^a		260.4
			334.4^a
	50	30–110.9	83.8	18.1	64.5	85.2
		207.9–355.9	260.4^a	41.3^a		261.4
			331.9^a
		437–561.2	507.0	3.7		456.1

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From the TG, DTG and DTA curves shown in Fig. 5–7, the first mass loss below 200 °C can be attributed to the water evaporation and the slow free formaldehyde emission, accompanied by large endothermic peak before 100 °C (Fig. 7). The minimum of the large endothermic peak is attributed to water evaporation that occurs to lower temperatures for UF/micro-TiO₂ than UF/nano-TiO₂ composite.

Fig. 7 DTA curves unirradiated and irradiated modified UF composites.

The first-step degradation occurs in the temperature region around 30–111 °C for all samples; and with a DTG peaks observed around 68–81 °C and 69–84 °C for unirradiated and irradiated modified UF composites, respectively, indicating water and formaldehyde evaporation. The mass loss is range to 9.3 and 10.2% for unirradiated modified UF composites with UF/micro-TiO₂ and UF/nano-TiO₂, indicating water and formaldehyde evaporation; and 11.1 and 18.1% for irradiated modified UF composites. The large endothermic peaks (Fig. 7) with minimum at 71.9 and at 82.5 °C for unirradiated and at 72.8 and at 85.2 °C for irradiated modified UF composites, respectively, indicating water evaporation from the initial waters resin.

Main mass loss happened in the second stage. In this stage degradation occurs in the temperature region around 117–371 °C for all samples. The second and third degradation regions are overlapped for all samples (Fig. 6), and they indicate simultaneously occurring two processes of polymer degradation with a total mass loss of 27.8 and 39.3% for unirradiated modified UF composites and 28.6 and 41.3% for irradiated modified UF composites, respectively. The second stage corresponds to the main decomposition process and proceeds at a high rate. Degradation of cured resin composite begins with the liberation of formaldehyde from dimethylene ether groups. This kind of destruction can be regarded as post curing of resin composite, as released formaldehyde participates in further reaction, finally giving more stable methylene group. The type of bond between the UF molecules depends on the reaction conditions: low temperatures and only slightly acidic pH favor the formation of methylene ether bridges (–CH₂–O–CH₂–), while higher temperatures and lower pH led to the formation of stable methylene (–CH₂–) bonds. Degradation of cured resins begins with release of FA from dimethylene ether groups¹ and the maximum degradation rate happens when the stable methylene ether linkages deconstruct.

In the DTA measurements (Fig. 7) the endothermic peak with minimum at 264.9, 260.5 for unirradiated UF resin composites and at 269.3, 261.4 °C for irradiated UF resin composites, respectively, is attributed to the degradation of methylene ether bridges into methylene bridges and crosslinking reactions in the resins network¹⁸ and also is attributed to loss of water linked to the polymeric chains.³³ This kind of degradation process can be regarded as post curing of resin, as released formaldehyde participates in further reaction, finally giving more stable methylene group.

The third degradation step occurs in the temperature region around 320–360 °C for unirradiated and irradiated UF/micro-TiO₂ composite, respectively. The mass loss at third-step degradation is 14.4 and 14.2% for unirradiated and irradiated modified UF resins with micro-TiO₂, respectively. For irradiated UF/nano-TiO₂ composite four-step degradation starts at 437 °C and ends at 561 °C, the percentage of mass loss 3.7% could correspond to the decomposition of titania-bonded groups such as –OH for irradiated UF/nano-TiO₂ composite.^34,35 This proved the small endothermic peak with minimum at 456.1 °C (Table 1).

These results show that the thermal stability of the both modified UF composites increases with increase of dose γ-irradiation.

The UF/micro-TiO₂ composite showed lower values for total mass loss (54.8 and 57.6%) than UF/nano–TiO₂ composite (55.2 and 64.5%) before and after irradiation, respectively.

3.3. ATR-IR spectroscopy

ATR-IR spectra of modified UF composites (original and irradiated samples) are shown in Fig. 8. Shape of band in ATR-IR spectra of the modified UF resin samples depends on pH values. When the pH reaction decreased the characteristic absorption bands of the modified UF resins became sharper and move to lower values of wave number. The assignments of characteristic IR bands to various modified UF resins are summarized in Table 2.

Fig. 8 FTIR spectra unirradiated and irradiated UF composites.

Table 2 Important IR-characteristic bands observed for unirradiated and irradiated modified UF composites^a

Modified UF composites Wavenumbers (cm^−1)				Assignment
UF/micro-TiO2		UF/nano-TiO2
0 kGy	50 kGy	0 kGy	50 kGy
a ν-stretching vibrations, δ-bending vibration in plane, γ-bending vibration out to plane.
3325	3329	3323	3323	ν(NH) in 2^o-amine
3034	3038	3031	3035	ν(CH) of CH2
2963	2963	2958	2958	ν(CH) of CH2 of ether, CH2OH and N–CH2
1627	1631	1628	1628	ν(C=O) in –CONH2 (amide I) and δ(OH) in water
1544	1561	1559	1563	δ(NH) in NH–CO in 2^o-amine (amide II)
1435	1440	1439	1435	γ(CH2) mode of the methylene (–N–CH2–N), –CH mode in CH2O; N–CH2–N; δ(CH2)
1382	1382	1379	1383	δ(CH) in CH2/CH2OH/N–CH2–N; δ(CH2)
1244	1244	1246	1246	νas(C–C–O) asym. stretch
1136	1136	1135	1135	νas(N–CH2–N), ν(C–O–C) of ether linkage, ν(C–O) of –CH2OH; γ(N–H) in 1° and 2° amines
1095	1095	1097	1092
1036	1036	1037	1037
999	999	1002	998

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The ATR-IR spectra of the UF/micro–TiO₂ and UF/nano–TiO₂ show a strong absorption band at 3324 and 3323 cm⁻¹. This band is sharp and typical hydrogen bond between N–H and –OH. The sharpness of these bands were indicated a reduction in the extent of hydrogen bonded interaction which is expected as the structure becomes more cross-linked due to methylenization reaction.³⁶ A pair of weak absorption peaks at approximately 3000 cm⁻¹ (Table 2) was attributed to the symmetrical –C–H stretching vibrations –CH₂– group of ether, –CH₂OH and N–CH₂– groups. A strong absorption band multiple and some overlapped bands is observed around 1627 cm⁻¹ in all spectra of modified UF composites, which may be assigned to the –C=O stretching vibration (amide-I) of –CONH₂ group and amide II, as well as the N–H scissors of amide I. The bands around 3320 cm⁻¹ and 1630 cm⁻¹ are due to the bending vibration of –O–H bond of chemisorbed water on the surface of the TiO₂.^37,38

The strong and overlapped bands at 1544–1560 cm⁻¹ are attributed to N–H bending vibrations of amide II. The cross-linking between two methylol groups provides ether linkages (–CH₂–O–CH₂–) to which –NH is attached to both sides. The weak multiple peaks at ∼1440 cm⁻¹ may be attributed to –C–H bending vibrations of –CH₂O and –CH₂–N group, while the small peaks at area of 1380–1440 cm⁻¹ can be assigned to stretching –C–N vibrations of amide I and II. The weak absorption band, around 1380 cm⁻¹ for all polymer samples may be ascribed to –C–H bending mode in –CH₂/–CH₂OH/N–CH₂–N. The medium absorption band in the region of 1100–1135 cm⁻¹ assigned to asymmetric stretching vibrations of –N–CH₂–N– and ν(C–O–C) of ether linkage.^39,40 These peaks (–N–CH₂–N– and –C–O–C–) show the cross-linking in the resin via N atoms. Moreover, –C–O–C– peaks in the both resins show polyether product.⁴¹

IR spectra of irradiated modified UF composites show almost identical wave numbers and decreases of the intensity of peaks compare to unirradiated samples. Fig. 8 shows that the intensity of the carboxyl group at 1627 cm⁻¹ decrease with increasing the irradiation dose. The aliphatic stretch band at 2960 cm⁻¹ and –C–O band at 1090 cm⁻¹ decrease as the irradiation dose increases due to the formation of hydrogen bonds with the hydroxyl groups on the TiO₂ surface, also.

4. Conclusion

In this work the effect of γ-irradiation on hydrolytic stability and thermal behavior of synthesized micro- and nano-TiO₂ modified UF composites were examined. Based on the results obtained the following conclusions were established:

1. The pH values of all modified UF composites were decreased after γ-irradiation and hydrolytic stability of micro- and nano-TiO₂ modified composites determined through liberated FA was enhanced. The modified UF composite with micro-TiO₂ after irradiation has higher mass loss and percentage of free FA, but there is a significant improvement in a percentage of liberated FA after acid hydrolysis.

2. TGA investigations indicate the DTG peaks of both UF composites are shifted to a higher temperature after γ-irradiation. After irradiation, UF composite with micro-TiO₂ shows minor changes in the thermal behavior compared with UF composite with nano-TiO₂, which indicates that the UF/micro-TiO₂ composite has more resistant to the effect of γ-irradiation than UF/nano-TiO₂ composite. The total mass loss for UF/micro-TiO₂ composite is less than that UF/nano-TiO₂ composite.

3. Gamma irradiation causes minor effect on the ATR-FTIR spectra of both modified UF resin specifically the decrease of intensities of some bands such carboxyl group.

Acknowledgements

Financial support for this study was granted by the Ministry of Science and Technological Development of the Republic of Serbia (Projects Numbers 45022 and 45020).

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