Performance of polylactide against UV irradiation: Synergism of an organic UV absorber with micron and nano-sized TiO 2

Abstract

The main purpose of this study was to investigate mechanical and thermal performance of polylactide specimens against UV irradiation; first when only adding benzotriazole benzotriazole-based organic UV absorber (UVA), micro (200 nm) and nano (50 nm) sized titania (TiO₂) particles alone, and then to reveal possible synergism when they are added together. Compounds were prepared by twin-screw extruder melt mixing, while the 2 mm thick specimens were shaped by compression molding. Specimens were exposed to UV irradiation under fluorescent lamps (UVB-313) with 0.50 W/m² for the periods of 12 and 24 days. Changes in the performance of UV irradiated specimens were evaluated in terms of % weight loss, changes in color and chemical structure, including the decreases in the mechanical and thermal properties. Various tests and analysis revealed that synergistic benefits of using micro and nano TiO₂ particles together with benzotriazole-type UVA were not only due to the effective stiffening, strengthening and toughening actions of titania particles, but also due to their very significant “UV screening” actions absorbing the photons of the UV irradiation, thus decreasing the degree of the detrimental photodegradation reactions leading to chain scissions in their PLA matrix.

Keywords

Polylactide titania UV irradiation organic UV absorber benzotriazole

Introduction

Polymers derived from the renewable agricultural resources, called as “biopolymers” are recently being considered as a probable way out from the today’s petroleum-based traditional polymeric materials, because of the significant environmental concerns and the depletion of fossil resources. In this regard, poly(L-lactic acid), i.e. polylactide (PLA), among other available biopolymers, is becoming one of the most promising candidate replacement material due to its attractive mechanical properties and biodegradability/biocompatibility.

Nowadays, apart from its widespread applications in food packaging industry, use of PLA is becoming common in the fields of “agricultural mulch” films,¹ “biomedical devices” such as sutures, stents, drug carrier, orthopedic devices, scaffolds² and “automotive parts” such as pillar cover, door trim and front panel.³ In all these applications, investigation of the performance of PLA against UV irradiation becomes crucial, because agricultural mulch films and outer automotive components are exposed to UV irradiation of sunlight, while UV irradiation itself is used extensively for the periodic sterilization of many medical devices because of its simplicity, effectiveness and low cost of operation.^4–6

The energy of UV light between 200 nm (598 kJ/mole) and 400 nm (299 kJ/mole) is higher than the bond energies of C–O (358 kJ/mole), C–C (347 kJ/mole) and C–H (414 kJ/mole) in the molecular chain structure of PLA. It is known that UV absorption takes place especially by the low energy C–O bond in the ester linkage of the PLA structure which leads to bond breakage, i.e. “chain scission,” via Norrish I type photodegradation reaction.^5,7,8 It is also known that photons which could be absorbed by the C = O (745 kJ/mole) bond in the carbonyl group of the PLA ester backbone lead to excitations resulting in Norrish II type photodegradation reactions causing significant chain scission.^4,5,8–10 Many researches^4,8–15 revealed that this type of chain cleavage of course results in significant decreases in the molecular weight of the PLA, consequently deteriorated mechanical properties.

In the polymer industry, in order to decrease deteriorative influences of UV irradiation, there are many commercial organic and inorganic UV absorbers containing certain absorption bands in different wavelength regions below 400 nm. Examples of the commercially available organic UV absorbers are; benzotriazole, benzimidazole, benzophenone, triazine, oxanilide, salicylate and cinnamate types.^16,17 In this group, “benzotriazole-type” organic UV absorbers are known as one of the most effective one. In the literature, although there are certain numbers of studies revealing the effectiveness of this type of UV absorber for many polymeric materials including aromatic and aliphatic polyesters,^18–20 no work has been reported for the neat PLA structure. Therefore, one of the purposes of this study will be to indicate influence of benzotriazole-based organic UV absorber on the performance of PLA.

In terms of inorganic UV absorbers, n-type semiconductors are considered, especially rutile phase titania (TiO₂) particles due to its nontoxicity and chemical stability under UV irradiation.²¹ Rutile TiO₂ having energy band gap of 3.0 eV is capable of absorbing UV light at wavelengths less than 410 nm. Li et al.,²² Man et al.,²³ and Xiu et al.²⁴ indicated that use of both micron-sized (around 200 nm) and nano-sized (<100 nm) titania, if uniformly distributed, increases the resistance of the PLA structure against UV irradiation with very high rate of electron-hole recombination mechanism. These improvements were obtained for both sheet specimens and thick specimens.

Anatase crystal structure of TiO₂ having an energy band gap of 3.2 eV is also capable of absorbing UV light wavelength less than 384 nm. However, for this TiO₂, there is another possibility, i.e. having “photocatalytic” effect. Because in the anatase structure under UV irradiation, electron hole pairs not only recombine but they may also travel to the particle surface forming certain radicals such as O²⁻, HOO^•, OH^• with H₂O and O₂ present in the environment. As particles become smaller especially <10 nm due to the quantum size effect, the possibility of recombination decreases and the TiO₂ particles act as effective photocatalyst. Although the photocatalytic effects have certain usefulness in other sectors, it would accelerate the photodegradation reactions in the polymer structures. Li et al.,²² Buzarovska et al.,²⁵ Nakayama et al.,²⁶ Zhuang et al.²⁷ revealed that use of ≤21 nm sized anatase TiO₂ particles resulted in significant reductions in the molecular weight of the PLA with lowered mechanical properties. This detrimental photocatalytic effect was especially important in the film specimens;^25–27 however, in thick specimens,^23,24 since surfaces of the particles were covered with the matrix material, the possibility of the contact of the anatase particles with oxygen or water prevented, leading to no photocatalytic action.

It is known that using one kind of UV absorbers would be not an efficient way to prevent photodegradation reactions taking place in the polymer structures. A much better way is using mixtures of different kinds of UV absorbers. In the literature, there are many studies investigating the influences of using mixtures of two or more different organic UV absorbers/stabilizers,^{16,18,28–31} or using mixtures of organic and inorganic UV absorbers.^32–38 In this respect, there are limited number of studies using TiO₂ particles together with an organic UV absorber especially for polymeric coatings.^35–38

However, literature survey indicated that, to the best of our knowledge, no investigation was reported using benzotriazole type organic UV absorber together with TiO₂ particles for the PLA matrix. Therefore, the purpose of this study was, as the first time, to investigate mechanical and thermal performance of 2 mm thick PLA specimens against UV irradiation; first when only using benzotriazole-based UV absorber, micro (200 nm) and nano (50 nm) titania particles alone, and then to reveal possible synergism when they are used together.

Experimental

Materials used

Polylactide (PLA) matrix material used in this study was commercial L-lactic acid type polylactide with an extrusion grade (NaturePlast, PLE 001). According to the technical data sheet of this PLA, it melts between 145° and 155℃ and degrades in the range of 240°–250℃; its melt flow index is in the range of 2–8 g/10 min at 190℃ under 2.16 kg, along with a density of 1.25 g/cm³.

The commercial organic UV absorber (SONGSORB® 2340, Songwon Ind. Co., Ltd, South Korea) having a chemical formula of 2-[2-Hydroxy-3,5-di(1,1-dimethylbenzyl) phenyl]-2H-benzotriazole was supplied in powder form having minimum of 99% purity and 448 g/mol molecular weight. Its melting temperature is in the range of 137–141℃ with strong UV irradiation absorbing capability in the 300–400 nm region.

In this study, as the inorganic UV absorber, effects of two different size ranges of titania were used. The first one named as “micro-titania” was a commercial one (Sachtleben R 210, Germany) used in polymer masterbatch industry with 94% purity. Average particle size, d (50), of these TiO₂ particles determined by the particle size analyzer (Malvern Mastersizer 2000) was 200 nm. The second one named as “nano-titania” was laboratory grade nanopowders supplied from US Research Nanomaterials (USA) having a purity of 99.9%. SEM analyses (FEI Nova Nano 430) revealed that the average size range of these nanoparticles was around 50 nm. It was also determined by XRD diffractograms (Bruker D8 Advance A25) that both micro- and nano-sized TiO₂ particles were composed of 100% rutile phase.

Compounding and shaping

PLA matrix, organic UV absorber, macro- and nano-sized TiO₂ particles were all compounded via melt mixing and compression molding techniques with laboratory size equipment. Before melt mixing, powders of PLA, TiO₂ and organic UV absorber were dried in a vacuum oven at 60℃ for 15 h. Then, according to the composition, these powders were pre-mixed manually just before feeding into side feeder of the extruder. These mixtures were then melt compounded via laboratory size twin-screw extruder (Rondol Microlab 300, D = 10 and L/D = 20). The temperature profile used for this production from feeder to die was 115°–170°–180°–175°–145℃, and the screw speed used was 75 r/min all through the compounding stage, followed by air cooling and pelletizing of the continuous strands into 2–3 mm pellets using a four-blade cutter.

Before specimen shaping, pellets were allowed to re-dry for another 15 h at 60℃ in a vacuum oven. Specimens required for testing and analyses were melt-shaped via laboratory scale compression molder (MSE Press Series, LP_M2SH05, Turkey) in accordance to the related standards. Maximum loading capacity and heating plate size of the compression molder are 50 kN and 20 × 20 cm, respectively. Temperature of the steel molds and the applied compression load were kept at 160℃ and 25 kN throughout the shaping processes, respectively. In order to prevent mold sticking, first inner surfaces of the molds were sprayed with a commercial mold release agent. Then, thin sheets of PTFE were also inserted in between the upper and lower surfaces of the molds and heating plates. During compression molding, before application of full pressure for 5 min, a period of 6 min without pressure was applied in order to melt pellets of the compounds (placed in the molds) homogeneously. Finally, shaped specimens were carefully unmolded after sufficient time of cooling at room temperature.

In this study, in order to reveal the proposed synergism of the organic UV absorber (designated as UVA) with micron- and nano-sized titania (designated as m-TiO₂ and n-TiO₂), totally six different compositions were produced. Designations of these compositions were as follows: PLA, PLA/UVA, PLA/m-TiO₂, PLA/n-TiO₂, PLA/UVA/m-TiO₂ and PLA/UVA/n-TiO₂. In these compositions, the amount of each filler added was all 2 wt.%. Because in our former submitted study, under review, investigating the effects of the micro and nano titania contents on the mechanical performance of PLA, the optimum common amount determined for each particle was 2 wt.%. For comparative purposes, the amount of the UVA added was also 2 wt.%. Therefore, since the added amounts were all the same, 2 wt.% contents were not indicated in the designations of the specimens.

UV irradiation of the specimens

Degree of the photodegradation in each six specimen compositions under UV irradiation was measured by using Q-LAB QUV/se model accelerated weathering test system without any moisture cycle. The specimens were exposed to UV light with fluorescent lamps (UVB-313, Q-Lab Corp., Cleveland, USA) providing irradiance in the range of 280–400 nm and having their peak emission at 310 nm. The light intensity was 0.50 W/m² (at 310 nm) and the distance between the UV lamps and the samples was 5 cm. During UV irradiation of the 2 mm thick flexural and fracture toughness test specimens mounted onto the plates of the testing unit, the temperature was set to 50℃. Effects of UV irradiation were investigated for the periods of 12 and 24 days. Therefore, specimen designations (PLA, PLA/UVA, PLA/m-TiO₂, PLA/n-TiO₂, PLA/UVA/m-TiO₂, PLA/UVA/n-TiO₂) were suffixed with 0 d, 12 d and 24 d where 0 d is used for the non-irradiated specimens.

Degree of the photodegradation in each specimen compositions after UV irradiation periods was evaluated by comparing the results of the following tests and analyses performed.

Measurement of weight losses

In order to reveal the possible mass losses, weight of the each specimen compositions was measured with a precision balance (Denver Instruments, SI-234) before (0 d) and after (24 d) UV irradiation. The % weight losses were calculated by using the following simple relation

% W_{Loss} = \frac{W_{0 d} - W_{24 d}}{W_{0 d}} \times 100

where W₀ _d is the initial weight, while W₂₄ _d is the weight of the specimens after UV irradiation. For each specimen composition, five weight measurements were done.

Analyses for color changes

For the possible color changes under UV irradiation, each specimen compositions were inspected both visually and quantitatively. Visual inspection was made by comparing the digital photographs of the exposed (24 d) and non-exposed (0 d) specimen surfaces.

The quantitative analyses for the color changes were conducted by using the diffused reflectance analysis (DRA) which determines the CIELAB color space parameters (L*, a*, b*) with the DRA unit of Agilent Cary 60 UV-Vis spectrophotometer. The measurements were performed in accordance with the CIE 1976 standards from the surfaces of five specimens for each composition, before (0 d) and after (24 d) UV irradiation.

Analyses for chemical structure changes

To be able to evaluate possible changes in the chemical structure of the PLA matrix, two different analyses were conducted: gel permeation chromatography (GPC) and Fourier transform-infrared spectroscopy (FT-IR). The decrease in the weight average molecular weight (M_w) of the neat PLA specimens was measured in tetrahydrofuran at 35℃ using a Malvern OMNISEC GPC system. The analysis was conducted with triple detection method by the use of refractive index, viscometer and light scattering detectors together with two columns (T6000M). The solutions were prepared via 40 h of magnetic stirring at concentrations above 2 g/L and filtered from 0.2 μ pore-sized membrane.

FT-IR spectroscopy was conducted in all specimen compositions via Bruker ALPHA IR spectrometer in order to investigate the possible alterations that can be observed in the chemical bonds of the specimens after 24 d UV irradiation period. Signals of at least 32 scans were averaged by the attenuated total reflectance (ATR) unit of the spectrometer with 4 cm⁻¹ resolution in the wavenumber range of 500 to 4000 cm⁻¹.

Testing for mechanical property changes

Effects of both 12 d and 24 d UV irradiation periods on the mechanical properties (strength, modulus, toughness) of all specimen compositions were evaluated with two different mechanical tests: flexural tests in terms of three-point bending and fracture toughness tests. These tests were carried out via 5 kN Instron 5565A universal testing system according to ISO 178 and ISO 13586 standards, respectively. The flexural test was applied to the specimens to measure their flexural strength (σ_Flex) and flexural modulus (E_Flex) values, while fracture toughness tests were carried out to determine K_IC and G_IC fracture toughness values of the specimens. For the fracture toughness tests, single-edge-notched-bending specimens were notched and pre-cracked with Ceast Notchvis system as defined in the standard ISO 13586. These mechanical tests were conducted at least for four specimens of each composition and the properties were determined as the average values including their standard deviations.

Analysis for fracture surface morphology changes

To observe the changes after 24 d UV irradiation period, morphological analyses were carried out under scanning electron microscope (SEM) (FEI Nova Nano 430) for the fracture toughness specimens whose fracture surfaces were sputtered with gold right after drawing a conductive path from silver paste. Secondary electron detector was used together with 5 kV accelerated voltage and 2 mm spot size in order to not damage specimen surfaces.

Analyses for thermal property changes

In order to compare transition temperatures, their enthalpies and thermal degradation temperatures of each specimen composition, after 24 d UV irradiation, differential scanning calorimetry (DSC) (SII X-DSC 700 Exstar) and thermogravimetric analyses (TGA) (SII TG/DTA 7300 Exstar) were conducted. The heating profile used for DSC was −80° to 220℃, whereas for TGA it was 30° to 550℃. For both analyses, the heating rate was 10℃/min under nitrogen gas flow.

Results and discussion

In the following sections, changes in the performance of UV irradiated neat PLA and its compounds containing micro- or nano-sized TiO₂ particles with and without organic UV absorber were evaluated in terms of % weight loss, changes in color and chemical structure, including the decreases in the mechanical and thermal properties.

Weight loss after UV irradiation

Degree of the weight loss in the mass of polymeric materials exposed to UV irradiation could be an indication of their stability or resistance against photodegradation. In the literature,^22,25–27 it was reported that if the PLA-based materials were especially in the form of thin sheets (e.g. 100–200 µm), and if they are exposed to high energy irradiations (e.g. 50–100 W/m²) for prolonged periods (e.g. 10–40 days), then percent weight loss could be as much as 15–40%.

In this study, before and after 24 days of UV irradiation, at least four fracture toughness test specimens of all compositions were weighted by a precision balance. The decreasing trend in the average % weight loss values with standard deviations is represented in Figure 1. It was seen that since the specimens used in this study were 2 mm thick bulk specimens, not thin sheets of micron thicknesses, %wt. loss values are all rather very low, being less than 1 wt.%. However, the values indicate the trends observed in each specimen composition. It was seen that the highest %wt. loss occurs in the neat PLA specimen. When it is filled with UVA, micro and nano TiO₂, then %wt. loss values decreases substantially, because of their UV absorbing capabilities. Moreover, Figure 1 revealed that the least %wt. loss takes place when UVA was added together with micro and nano TiO₂ particles. This could be interpreted that UVA has synergistic UV absorbing action when used together with titania particles.

Color change after UV irradiation

In the polymer industry, from consumers’ point of view, change in the color of the polymeric components can be very problematic, which can take place when these components are exposed to UV irradiation during their outdoor use under sunlight or sterilization under UV lamps. Therefore, in this study, possible color changes in each specimen composition after (24 d) UV irradiation period were evaluated first visually by the photographic images (Figure 2) and then also quantitatively by the CEILAB color space parameters (L*, a*, b*, ΔE*) via diffused reflectance analyses (DRA) as given in Table 1.

Table 1.

Changes in the CIELAB color space parameters (L*, a*, b*) and total color change (ΔE*) values of the specimens after (24d) UV irradiation.

Specimens	L*	a*	b*	ΔE*
PLA-0d	30.00 ± 1.020	−0.24 ± 0.054	−0.72 ± 0.026
PLA-24d	32.72 ± 1.147	−0.27 ± 0.040	−1.59 ± 0.143	2.91 ± 1.028
PLA/UVA-0d	28.71 ± 0.888	−0.61 ± 0.091	−0.10 ± 0.076
PLA/UVA-24d	33.13 ± 0.715	−0.53 ± 0.055	−0.60 ± 0.222	4.46 ± 0.313
PLA/m-TiO₂-0d	95.97 ± 0.085	−0.85 ± 0.040	2.63 ± 0.138
PLA/m-TiO₂-24d	96.56 ± 0.108	−0.78 ± 0.010	2.07 ± 0.094	0.82 ± 0.140
PLA/n-TiO₂-0d	96.21 ± 0.086	−0.97 ± 0.037	2.67 ± 0.20
PLA/n-TiO₂-24d	96.74 ± 0.134	−0.80 ± 0.025	1.85 ± 0.032	0.99 ± 0.093
PLA/UVA/m-TiO₂-0d	96.08 ± 0.302	−1.08 ± 0.024	3.11 ± 0.11
PLA/UVA/m-TiO₂-24d	96.57 ± 0.05	−1.49 ± 0.064	4.60 ± 0.166	1.62 ± 0.045
PLA/UVA/n-TiO₂-0d	96.17 ± 0.150	−1.09 ± 0.029	3.05 ± 0.048
PLA/UVA/n-TiO₂-24d	96.08 ± 0.054	−1.43 ± 0.025	4.23 ± 0.049	1.24 ± 0.052

It is seen in Figure 2 that the neat PLA specimen before (0 d) irradiation appears very transparent, because due to the very slow melt crystallization rate, PLA had no ability to crystallize during the high cooling rate of the compression molding process used for shaping of the specimens. Therefore, all neat PLA specimens had very high level of amorphous transparent structure. During UV irradiation tests, the temperature in the unit was 50℃, which was insufficient for the start of cold crystallization of PLA. Therefore, even after 24 days, the amorphous transparent structure of neat PLA specimen remained almost the same. Similarly, Table 1 shows that during UV irradiation of the neat PLA, there was slight changes in the CIELAB color space parameters (L*, a*, b*), in which the total color change (ΔE*) value was only around 3. By the addition of 2 wt.% UVA into the PLA matrix, it was observed that the PLA/UVA specimen appeared with a very slight yellowish tone, again with a very high level of amorphous transparent structure. After 24 days of UV irradiation, color space parameters (L*, a*, b*) again changed slightly, and this time the total color change (ΔE*) value was around 4.5.

Due to the very well-known perfect whiteness of titania particles, Figure 2 indicated that, with the addition of only 2 wt.% micro or nano TiO₂ particles, the specimens appeared opaque and very white both before (0 d) and after (24 d) UV irradiation. Similarly, the quantitative color parameters in Table 1 revealed that the values of L* parameter which represents “whiteness” for these compositions were all above 96, while L* values for the neat PLA and PLA/UVA specimens were around 30. After irradiation, ΔE* total color change value for the m-TiO₂ and n-TiO₂-filled compositions was extremely low, being less than 1.

When TiO₂ particles were added together with UVA, no significant color changes were observed; L* value being again more than 96, and ΔE* value becoming slightly above 1. Therefore, it can be stated that use of micro and nano titania particles either alone or together with a benzotriazole type organic UV absorber could be very effective to keep white color of the PLA products under prolonged UV irradiation conditions.

Chemical structure change after UV irradiation

It is known that the most significant deteriorative effects of UV irradiation influencing all mechanical properties of PLA would be in the form of molecular weight (M_w) which decreases via chain scission reactions. Therefore, in order to determine the level of molecular weight reduction of PLA matrix after UV irradiation, gel permeation chromatography (GPC) analysis was used. GPC analysis revealed that the weight average molecular weight (M_w) of unirradiated (0 d) neat PLA matrix being 105.80 × 10³g/mol reduced down to 86.98 × 10³g/mol after 24 days of UV irradiation. As will be evaluated in the next section, this level of reduction in the M_w of PLA matrix led to substantial decreases in the mechanical properties of all specimen compositions.

Apart from the reduction in the M_w of PLA matrix, in order to observe other changes in the chemical structure of each specimen, ATR-FTIR analyses were also conducted. Their IR spectra before (0 d) and after (24 d) UV irradiation are compared in Figure 3(a) and (b). The first spectrum in Figure 3(a) belongs to unirradiated (0 d) neat PLA specimen indicating the well-known typical five main absorption bands of the PLA structure. Starting from the low energy bands these are: (i) C–H stretching vibrations of R₃C–H stretching at 2995, 2944, 2925 and 2853 cm⁻¹; (ii) C = O stretching mode vibrations of the carbonyl group at 1747 cm⁻¹; (iii) CH₃ vibrations in two different modes as the antisymmetric CH₃ bending mode at 1453 cm⁻¹ and C–CH₃ deformation mode at 1382 and 1360 cm⁻¹; (iv) C–O stretching vibrations in between 1300 and 1000 cm⁻¹ in three different modes as (O=)C–O stretching of ester group at 1180 cm⁻¹ and 1264 cm⁻¹, antisymmetric C–O–C stretching mode at 1079 cm⁻¹ and 1127 cm⁻¹, and C–C–O stretching mode at 1041 cm⁻¹; and (v) C–C stretching mode at 955 and 867 cm⁻¹. All these characteristic IR bands observed in this study were very well matching with the ones reported in the literature.³⁹

In the literature^4,9,25 it has been indicated that upon absorption of a photon during UV irradiation, photo degradation mechanisms known as “photolysis” and “photo-oxidation” leading to “chain scission,” i.e. breakage of the PLA backbone structure especially takes place in the C–O and C–C bonds of the ester linkage, resulting in the decreased IR intensity of these bands.

Figure 3(a) and (b) shows that, for all specimen compositions, compared to their unirradiated (0 d) spectrum, IR intensities of not only C–O and C–C bonds, but also other typical bands of the UV irradiated (24 d) specimens were decreased. Of course, it was very clear that the most significant decrease in the IR intensity of the bonds occurred in the neat PLA specimen. In the (24 d) irradiated specimens having either UVA, micro or nanoTiO₂ particles, the decrease in the intensity of IR bands was much lower. Moreover, it was observed that, due to the synergism, when UVA was added together with micro or nano TiO₂, the decrease in the IR intensities was almost negligible.

Apart from decreases in the intensity of IR bands, it has been also discussed^22,24,26 that, upon UV irradiation, there might be formation of new C = C bands (around 1600 cm⁻¹) and O–H bands (around 3500 cm⁻¹). The formation of the first new band is believed to be due to the excitation of the C = O ester carbonyl during the main photodegradation reaction known as Norrish II type photocleavage. The second new band that might appear would be due to the formation of hydroperoxides at the terminals of the new broken PLA chains. In this study, closer examination of the IR spectrums of all 24 d irradiated neat and UVA, micro/nano TiO₂-filled PLA specimens revealed that there were tiny broad peaks for the new C = C bands at 1636, 1646 and 1654 cm⁻¹; and for the new –O–H bands at 3629, 3649 and 3676 cm⁻¹ (Figure 3(a) and (b)). However, for the specimens having UVA and micro/nano TiO₂ together, these tiny new peaks were not observed.

McNeill and Leiper⁷ and Ikada⁸ indicated that, apart from new peak formations, UV irradiation might also lead to shifting of the certain IR peaks of the typical PLA bands to lower wavenumbers. In this study, upon (24 d) UV irradiation of the neat PLA specimen, the (O=)C–O and C–C–O stretching bands shifted from 1180 cm⁻¹ to 1179 cm⁻¹, 1264 cm⁻¹ to 1263 cm⁻¹; and from 1042 cm⁻¹ to 1040 cm⁻¹, respectively, while the C = O carbonyl stretching peaks shifted from 1747 cm⁻¹ to 1744 cm⁻¹. However, for the specimens containing UVA, micro/nano TiO₂ particles, alone or together, these shifts were not observed.

Therefore, it can be pointed out that, compared to neat PLA, due to the efficient photon absorbing abilities of UVA and TiO₂ particles, the change in the chemical structure of PLA matrix, i.e. the degree of “chain scission” in these filled compositions was rather lower.

Mechanical property changes after UV irradiation

In this study, in order to determine degree of the reductions in the mechanical properties of the UV irradiated specimens, two types of tests were conducted. The first one, three-point bending, was used to compare “Flexure Strength, σ_Flex” and “Flexure Modulus, E_Flex” values, while the second one was used to compare “K_IC and G_IC Fracture Toughness” values. It should be pointed out that, since during UV irradiation of the specimens, the temperature of the exposure unit was 50℃; all unirradiated specimens were first dried in a vacuum oven at 50℃ in order to make proper comparison.

Results of the first test for the 0 d and 24 d are given in Figure 4 in the form of “flexural stress-strain curves”, while the σ_Flex and E_Flex data determined for 0 d, 12 d and 24 d UV irradiation periods are tabulated in Table 2.

Table 2.

Changes in the flexural modulus (E_Flex), flexural strength (σ_Flex) and fracture toughness (K_IC and G_IC) values of the specimens before (0 d) and after UV irradiation periods of 12 d and 24 d.

Specimens	σ_Flex (MPa)	E_Flex (GPa)	K_IC (MPa√m)	G_IC (kJ/m²)
PLA-0d	117.23 ± 1.75	3.77 ± 0.01	3.57 ± 0.06	5.88 ± 0.54
PLA-12d	112.67 ± 0.29	3.58 ± 0.27	3.38 ± 0.06	4.84 ± 0.19
PLA-24d	99.44 ± 0.84	3.38 ± 0.04	2.92 ± 0.18	3.79 ± 0.39
PLA/UVA-0d	114.71 ± 0.83	3.74 ± 0.13	3.64 ± 0.13	6.31 ± 1.27
PLA/UVA-12d	113.27 ± 0.24	3.71 ± 0.01	3.58 ± 0.02	6.10 ± 0.18
PLA/UVA-24d	105.85 ± 0.82	3.49 ± 0.01	3.31 ± 0.06	4.78 ± 0.09
PLA/m-TiO₂-0d	118.83 ± 2.31	3.97 ± 0.06	3.76 ± 0.10	7.05 ± 0.13
PLA/m-TiO₂-12d	113.91 ± 4.33	3.84 ± 0.18	3.71 ± 0.14	6.66 ± 0.52
PLA/m-TiO₂-24d	107.66 ± 0.06	3.68 ± 0.02	3.43 ± 0.04	5.43 ± 0.11
PLA/n-TiO₂-0d	123.25 ± 3.70	4.09 ± 0.18	3.85 ± 0.12	7.42 ± 0.76
PLA/n-TiO₂-12d	115.14 ± 1.16	3.86 ± 0.04	3.73 ± 0.04	6.79 ± 0.58
PLA/n-TiO₂-24d	108.73 ± 0.19	3.72 ± 0.05	3.59 ± 0.02	5.81 ± 0.71
PLA/UVA/m-TiO₂-0d	118.01 ± 0.71	3.93 ± 0.15	3.92 ± 0.33	7.33 ± 1.18
PLA/UVA/m-TiO₂-12d	116.74 ± 2.08	3.90 ± 0.04	3.73 ± 0.08	6.69 ± 0.25
PLA/UVA/m-TiO₂-24d	110.73 ± 2.28	3.72 ± 0.10	3.56 ± 0.04	5.46 ± 0.091
PLA/UVA/n-TiO₂-0d	120.06 ± 6.24	4.05 ± 0.27	3.94 ± 0.30	7.59 ± 1.66
PLA/UVA/n-TiO₂-12d	118.36 ± 1.50	3.97 ± 0.05	3.83 ± 0.18	7.18 ± 0.78
PLA/UVA/n-TiO₂-24d	115.52 ± 4.13	3.90 ± 0.23	3.68 ± 0.09	6.12 ± 0.34

Figure 4 and Table 2 revealed that the flexural stress–strain curves of all specimens were affected from UV irradiation leading to certain decreases in the values of both flexural strength and modulus (σ_Flex and E_Flex). It was clear that the most significant deterioration in the strength and modulus values was for the neat PLA specimen. The reduction in the σ_Flex and E_Flex values of this specimen after 24 d irradiation was 15% and 10%, respectively, because of the photodegradation reactions leading to “chain scissions” and the consequent decreases in the molecular weight of PLA structure.

However, due to the UV absorbing actions of both TiO₂ particles and UVA molecules, decreasing the level of chain scissions in their PLA matrix, these reductions were lower in the specimens of PLA/UVA, PLA/m-TiO₂ and PLA/n-TiO₂. When these fillers were added together, then the lowest level of reductions was obtained. For example, for the PLA/UVA/n-TiO₂ specimen, the reductions in the σ_Flex and E_Flex values after 24 d irradiation were both only 3%.

Benefits in the strength and elastic modulus values of the specimens filled with micro and nano TiO₂ particles were especially due to their inorganic rutile phase structure having very high chemical stability during UV irradiation. Consequently, these TiO₂-filled specimens exhibited higher σ_Flex and E_Flex values not only before (0 d) but also after (24 d) UV irradiation. Because these compositions kept their function on the well-known composite strengthening and stiffening mechanisms of “load transfer from the matrix” and “decreased mobility of the matrix chains” also during UV irradiation.

The possible reason of the synergistic behavior when UVA was used together with TiO₂ particles could be due to the higher efficiency during absorption of the UV photons and then transformation of the energy of these photons into less harmful heat by more amount of electron hole recombination.

Changes in the fracture toughness, i.e. ability of the structure to hinder crack initiation and propagation leading to fracture, of the specimens were determined in terms of “Critical Stress Intensity Factor (K_IC)” and “Critical Strain Energy Release Rate (G_IC)” values as given in Table 2.

Just like in the case of σ_Flex and E_Flex, Table 2 shows that the reductions in the fracture toughness values K_IC and G_IC in the 24 d irradiated neat PLA specimen were significant being as much as 18% and 36%, respectively. Similarly, these reductions were again much lower in the specimens having UVA and TiO₂ particles either alone or together. For instance, in the specimen of PLA/UVA/n-TiO₂, the reductions in the K_IC and G_IC values after 24 d irradiation were 7% and 19%, respectively.

The contribution of the organic UVA molecules and the inorganic TiO₂ particles to the fracture toughness values of the PLA matrix before and after UV irradiation could be discussed in two manners. First of all, since they have high chemical stability under UV irradiation, they could keep their function in the composite toughening mechanisms. For instance, UVA might act as plasticizing agent for the “shear banding or plastic deformation” mechanism, while micro and nano TiO₂ particles keep their action in the mechanisms of “crack deflection,” “crack bowing,” “particle debonding” and “particle pull-out”. Secondly, their very efficient UV-absorbing actions decreasing the level of photodegradation (i.e. chain scission) in their PLA matrix would result in lower amount of crack initiation and propagation.

In this section finally, in order to reveal benefits of using UVA- and TiO₂-filled compositions rather than using neat PLA, all the mechanical properties of these filled specimens before (0 d) and after (24 d) UV irradiation were compared as given in Figure 5. Note that benefits of using these filled compositions compared to PLA-0 d and PLA-24 d specimens are shown in terms of “% Increase” in each mechanical property at 0 d and 24 d.

Figure 5 revealed that in terms of all mechanical properties, the use of micro or nano TiO₂ particles alone or together with a benzotriazole-type UVA in PLA matrix would be quite beneficial not only under normal conditions but also under UV irradiation conditions. % Increases data given in Figure 5 indicated that if TiO₂ particles were used together with UVA, then the benefits would be much higher. For instance, for the specimen of PLA/UVA/n-TiO₂-24 d, the % Increases in σ_Flex and E_Flex values were 16% and 15%, respectively, while in K_IC and G_IC values, % Increases were as much as 26% and 61%, respectively.

It could be once more stated that synergistic benefits of using micro and nano TiO₂ particles together with benzotriazole-type UVA were not only due to the effective stiffening, strengthening and toughening actions of titania particles but also due to their very significant “UV screening” actions absorbing the photons of the UV irradiation, thus decreasing the degree of the detrimental photodegradation reactions in their PLA matrix. This behavior was also reported for certain polymeric coating materials in the literature.^36,37

Therefore, it could be concluded that since PLA/UVA/n-TiO₂ specimen had very close or higher mechanical properties even after 24 days of UV irradiation compared to unirradiated neat PLA specimen, use of this composition could be considered as an alternative PLA-based material for automobile and biomedical applications having durability requirement especially in terms of UV resistance.

Fracture surface morphology changes after UV irradiation

In order to observe changes in the fracture surface morphology, SEM analysis was conducted on the fracture surfaces of the fracture toughness test specimens of all compositions before (0 d) and after 24 days of UV irradiation as given in Figure 6(a) and (b). All the images on the left hand side of Figure 6 belong to the unirradiated (0 d) specimens. In this group, it is seen that the fracture surfaces of neat PLA and only organic UVA filled PLA are rather very smooth and flat, representing the well-known inherently brittle character of PLA structure. When 2 wt.% micro or nano TiO₂ particles were added, it was observed that these titania particles were all rather homogeneously distributed in the PLA matrix, and the smooth fracture surface morphology did not change much.

On the other hand, in the images of the right hand side of Figure 8 belonging to the 24d UV irradiated specimens, it was seen that all the images were obscured, with very tortuous and rough fracture surface morphology. It was even not possible to recognize the TiO₂ particles in the matrix. Due to the photodegradation reactions leading to significant deteriorative chemical changes in the PLA matrix of the specimens, certain numbers of main and secondary cracks and cleavages were also observed in the fracture surfaces of all irradiated specimens.

Thermal property changes after UV irradiation

In order to evaluate changes in the thermal properties of the UV irradiated specimens, two different thermal analyses were conducted; differential scanning calorimetry (DSC) analyses and thermogravimetric analyses (TGA).

First, heating DSC thermograms of all specimens before (0 d) and after (24 d) UV irradiation are given in Figure 7, while the values of the glass transition (T_g), cold crystallization (T_c) and melting (T_m) temperatures together with the enthalpies of melting (ΔH_m) and crystallization (ΔH_c) including the percent crystallinity (Xc) determined from these curves are tabulated in Table 3. The relation used in the calculation of the percent crystallinity is given below

X_{c} = \frac{Δ H_{m} - Δ H_{c}}{w_{PLA} \cdot Δ H_{m}^{\circ}} \cdot 100

where w_PLA is the weight fraction of the PLA matrix and ΔH_m° is the melting enthalpy of 100% crystalline PLA given as 93 J/g in the literature.⁴⁰

Table 3.

Changes in the transition temperatures (T_g, T_c, T_m), enthalpies (ΔH_m, ΔH_c) and crystallinity percent (X_c) of the specimens after (24 d) UV irradiation.

Specimens	T_g (℃)	T_c (℃)	T_m (℃)	ΔH_m (J/g)	ΔH_c (J/g)	X_C (%)
PLA-0d	60.1	124.6	150.2	17.2	13.7	3.76
PLA-24d	55.7	122.7	149.3	5.69	6.58	0.96
PLA/UVA-0d	58.4	124.1	149.8	10.2	14.1	4.30
PLA/UVA-24d	57.0	122.9	149.3	3.11	4.15	1.14
PLA/m-TiO₂-0d	60.1	118.5	149.8	27.3	22.3	5.49
PLA/m-TiO₂-24d	56.0	118.1	149.4	6.98	7.95	1.06
PLA/n-TiO₂-0d	60.2	118.2	149.1	24.5	16.5	8.69
PLA/n-TiO₂-24d	57.2	117.8	148.7	7.80	9.63	2.01
PLA/UVA/m-TiO₂-0d	59.2	122.6	148.9	4.85	3.19	1.86
PLA/UVA/m-TiO₂-24d	58.1	120.6	148.7	5.24	3.91	1.49
PLA/UVA/n-TiO₂-0d	59.0	121.5	149.5	7.74	6.00	1.95
PLA/UVA/n-TiO₂-24d	58.2	119.5	149.0	8.07	6.53	1.72

Figure 7 and Table 3 indicated that there were important changes in only two parameters; T_g and X_c. Due to the deteriorative actions of the photodegradation reactions on the chemical structure of PLA matrices, glass transition temperature T_g of the PLA specimen decreased by 5℃ after 24 d UV irradiation. The decrease in the T_g values of the specimens, filled with UVA, micro and nano TiO₂ particles, was all less than 5℃.

Another reduction observed for all UV irradiated specimens was the amount of the percent crystallinity X_c in their PLA matrices. This could be interpreted that all those deteriorative photodegradation reactions were occurred not only in the amorphous regions of the PLA matrices, but also in the crystalline regions of the matrix. That behavior was also discussed in detail by Tsuji et al.⁴ for the PLA matrix.

As the second thermal analysis, thermogravimetric (TG) curves of all specimen compositions before (0 d) and after (24 d) UV irradiation were obtained as given in Figure 8. Then, the thermal degradation temperatures of T_5%, T_10%, T_25% representing the mass loss temperatures at 5, 10, 25 wt.%; T_max representing the temperature at maximum mass loss; and %residue of each specimen determined at 550℃ are tabulated in Table 4.

Table 4.

Changes in the thermal degradation temperatures (T_5%, T_10%, T_25%) of the specimens at 5, 10 and 25 wt.% mass losses and the maximum mass loss temperature (T_max); and the %Residue at 550℃ after (24d) UV irradiation.

Specimens	T_5% (℃)	T_10% (℃)	T₂₅ _% (℃)	T_max (℃)	%Residue at 550℃
PLA-0d	332.2	342.2	353.4	362.4	0.16
PLA-24d	323.5	335.4	349.8	361.2	0.06
PLA/UVA-0d	330.3	340.2	352.4	363.3	0.10
PLA/UVA-24d	325.5	337.0	350.3	361.4	0.01
PLA/m-TiO₂-0d	332.7	340.2	344.3	355.0	2.29
PLA/m-TiO₂-24d	321.8	332.3	343.8	351.4	2.18
PLA/n-TiO₂-0d	338.6	346.2	355.5	363.6	2.35
PLA/n-TiO₂-24d	321.4	335.7	350.1	361.8	2.20
PLA/UVA/m-TiO₂-0d	326.8	336.5	348.9	359.8	2.11
PLA/UVA/m-TiO₂-24d	324.1	335.0	347.7	358.8	2.06
PLA/UVA/n-TiO₂-0d	334.0	342.7	354.1	364.3	2.22
PLA/UVA/n-TiO₂-24d	331.4	340.9	353.1	363.7	2.09

Figure 8 and Table 4 revealed that because of the deteriorations in the chemical structure of the PLA matrices upon 24 d UV irradiation, there were certain reductions in all thermal degradation temperatures of all specimen compositions. It can be deduced from Table 4 that the most significant reduction occurred in the onset degradation temperatures of T_5%, which is being as much as by 10℃ for the neat PLA specimen. That reduction was gradually decreased in the specimens filled with UVA, micro or nanoTiO₂ particles. It was seen that when benzotriazole-type UVA and TiO₂ particles were added together, i.e. the synergistic compositions, the reduction in the values of T_5% was only 2–3℃.

Table 4 also indicated that for the specimens containing micro and nano TiO₂ particles, the values of the %Residue at 550℃ were all around 2 wt.% before (0 d) and after (24 d) UV irradiation. This could be interpreted that photodegradation reactions resulted in no significant changes in the structure of these inorganic TiO₂ particles.

Conclusions

Visual photographic images and quantitative CEILAB color space parameters indicated that use of micro and nano titania particles either alone or together with a benzotriazole type organic UV absorber (UVA) could be very effective to keep white color of the PLA products under prolonged UV irradiation conditions.

GPC analysis revealed that the weight average molecular weight (M_w) of unirradiated (0 d) neat PLA matrix reduced from 105.80 × 10³ down to 86.98 × 10³g/mol after 24 days of UV irradiation via chain scission actions of the photodegradation reactions, leading to consequent reductions in the mechanical properties of the specimens.

Three-point bending tests showed that after 24 days of UV irradiation, the most significant deterioration in the σ_Flex strength and E_Flex modulus values was for the neat PLA specimen being 15% and 10%, respectively. When UVA, m-TiO₂ or n-TiO₂ were added alone or together, then much lower reductions were obtained, e.g. being only 3% reduction for the PLA/UVA/n-TiO₂ specimen.

Similarly, fracture toughness tests indicated that K_IC and G_IC values of the neat PLA specimen were reduced as much as 18% and 36%, while for the synergistic specimen composition PLA/UVA/n-TiO₂ these reductions after 24 days of UV irradiation were 7% and 19%, respectively.

Synergistic benefits of using micro and nano TiO₂ particles together with benzotriazole-type UVA were not only due to the effective stiffening, strengthening and toughening actions of titania particles but also due to their very significant “UV screening” actions absorbing the photons of the UV irradiation, thus decreasing the degree of the detrimental photodegradation reactions in their PLA matrix.

DSC and TGA analyses indicated that PLA specimens filled with UVA, micro and nano TiO₂ particles would be also beneficial in terms of keeping the glass transition temperature T_g and the onset degradation temperature T_5% of the PLA matrices after UV irradiation.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors gratefully acknowledge the financial support of the Middle East Technical University (METU), Scientific Research Fund (BAP) – Project Grant No: GAP-308-2018-2671.

ORCID iD

Cevdet Kaynak

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