Preparation and ageing-resistant properties of polyester composites modified with functional nanoscale additives
© Guo et al.; licensee Springer. 2014
Received: 14 January 2014
Accepted: 15 April 2014
Published: 7 May 2014
This study investigated ageing-resistant properties of carboxyl-terminated polyester (polyethylene glycol terephthalate) composites modified with nanoscale titanium dioxide particles (nano-TiO2). The nano-TiO2 was pretreated by a dry coating method, with aluminate coupling agent as a functional grafting additive. The agglomeration resistance was evaluated, which exhibited significant improvement for the modified nanoparticles. Then, the effects of the modified nano-TiO2 on the crosslinking and ageing-resistant properties of the composites were studied. With a real-time Fourier transform infrared (FT-IR) measurement, the nano-TiO2 displayed promoting effect on the crosslinking of polyester resin with triglycidyl isocyanurate (TGIC) as crosslinking agent. Moreover, the gloss retention, colour aberration and the surface morphologies of the composites during accelerated UV ageing (1500 hours) were investigated. The results demonstrated much less degree of ageing degradation for the nanocomposites, indicating an important role of the nano-TiO2 in improving the ageing-resistant properties of synthetic polymer composites.
Ultra-violet (UV) radiation is a cytotoxic waveband of solar radiation reaching the Earth's surface . Exposure to UV radiation from the sun is associated with different harmful effects for the organisms and synthetic polymer materials [2, 3]. DNA is obviously one of the key targets for UV-induced damage in a variety of organisms which is traditionally attributed to the direct absorption of UV photons by nucleic acids and protein [1, 4]. And the exposure of polymers to UV radiation may produce degradation discoloration and/or brittle fracture . This is due to the UV irradiation-induced chemical reactions such as chain scission, crosslinking, oxidation or bond cleavage in the polymers [6–8]. All these damages may be undesirable due to their adverse impacts on the safety of organisms and the period of use of polymers. So, many organic and inorganic filters have been used to absorb and scatter UV radiation [3, 9].
Titanium dioxide (TiO2), which can be either amorphous or crystalline , is used extensively in numerous applications, such as bone tissue engineering , bactericidal agents  and cosmetics . The light absorption properties of anatase and rutile TiO2 are excellent since their absorption (approximately 400 nm) falls between the visible and UV regions . Especially, ultrafine rutile TiO2 particles (< 100 nm) were used as a functional nanoscale additive because of its potential for the wide range (both UVB and UVA regions) of UV-ray shielding by their absorption, scattering and reflecting properties . Once TiO2 is exposed to UV radiation, an electron is promoted from the valence band to the unoccupied conduction band, creating excitons . Rayleigh's theory implies that shorter wavelengths of light are more efficiently scattered by smaller particles . However, the smaller size leads to higher values of surface area which presents high surface energy and activity, so the nanoparticles tend to form agglomeration [18–20].
Particle aggregates in composite materials would decrease adhesion between nanoparticles and polymeric materials, which will result in an early failure at the interface and thus increase the susceptibility to physical and mechanical failure [21, 22]. To achieve proper dispersion of nanoparticles in polymer matrix and to yield a better compatibility between the nanoparticles and polymeric materials, several groups have attempted to prevent the aggregation by modifying the surface groups of nano-TiO2 with different reagents including the silane coupling agent [23, 24], the hydrolysis-condensation reactions (sol-gel method)  and in situ bulk polymerization [25, 26]. Several polymers have been mixed with nano-TiO2 successfully including polystyrene (PS) , polythiophene (PTh) , poly(methyl methacrylate) (PMMA) , etc. Polyester resin has been widely studied as they possess many advantages including good mechanical properties, transparency, remarkable durability and flexibility [29, 30]. However, the widespread use of polyester resin is blocked in many high-performance applications because of its limited ageing-resistant properties. Many methods have been used to improve the ageing-resistant properties of the polyester resin, such as synthesizing and modification of the resin, selection of curing system and curing agent in powder coatings and composited with suitable functional additives [31–34]. Nevertheless, to the best of our knowledge, it is still highly desirable to develop more industrial available processes for the surface modification of nano-TiO2, preparation of polyester/nano-TiO2, and their ageing-resistant properties.
In this investigation, we pretreated the nano-TiO2 particles and prepared the polyester/nano-TiO2 composites by melt-blend extrusion method. The aluminate coupling agent was employed as a functional grafting agent to realize a surface modification of the nano-TiO2. The particle size distribution, hydrophilic angle, UV reflection characteristic of the nano-TiO2, and its dispersion state in the polyester were detected. Moreover, the effect of nano-TiO2 on the gloss retention, colour aberration and morphology of the composites was investigated during the UV ageing. The dry modification method for the nano-TiO2 and its application as functional nanoscale additive are highly available for the widespread applications of polyester resin/TiO2 composites and would provide considerable insights into the protection of natural and synthetic carbohydrate polymers from the UV irradiation.
Carboxyl-terminated polyester resin (polyethylene glycol terephthalate) was purchased from Cytec Surface Specialties Inc., Woodland Park, NJ, USA, with an acid value of 33 mg KOH/g and a curing temperature of 190°C. Triglycidyl isocyanurate (TGIC) was used as curing agent and also purchased from Cytec Surface Specialties Inc. Rutile nano-TiO2 was purchased from Panzhihua Iron & Steel Research Institute in China, with grain size of 30 to 50 nm. Aluminate coupling agent was purchased from Chongqing Jiashitai Chemical Co. (Chongqing, China).
Surface modification of nano-TiO2
The nano-TiO2 particles were modified with 1.5 wt.% aluminate coupling agent (based on the nano-TiO2 particles content). Firstly, the nano-TiO2 particles were put into a high-speed mixer (Dachen Machinery Manufacturing Co., Beijing, China, SHR-10A) and pre-mixed with a rotate speed of 2,000 rpm at 130°C. The collisions of the powder with stirring blade resulted in a high impaction and dispersion. Some powders were brought out for the other characterizations in this work. Then, 1.5 wt.% of aluminate coupling agent was added into the powder, and the mixtures were stirred further for 20 min. Subsequently, the mixtures were centrifuged and washed with fresh ethanol to remove the coupling agent adsorbed physically on the surface of nano-TiO2 particles. Finally, the modified particles were dried at 60°C for 2 h.
Preparation of polyester/nano-TiO2 composites
We prepared the polyester/nano-TiO2 composite with different amounts of modified nano-TiO2. Firstly, four groups of the polyester resin 93 g and TGIC 7 g were blended physically and labelled as samples 1, 2, 3 and 4, respectively. Then, the modified nano-TiO2 with the amount of 0.5, 1.0, 1.5, and 2.0 wt.% based on the polyester resin content were added into the samples, respectively. The raw materials were mixed (at 90°C for 5 min) with a rotating speed of 2,000 rpm. During the mixing, the raw materials were melted and then extruded in a twin screw extruder. The extrudate was milled and sieved into particle with size less than 100 μm for further measurements.
The surface functional groups of nano-TiO2 were analyzed by Fourier transform infrared (FT-IR) spectrometer (Bruker, Tensor 27, Madison, WI, USA) with a detection resolution of 4 cm-1. The samples were acquired by compacting sheet of nano-TiO2/potassium bromide powder mixture (1:100 in mass) and then drying at 110°C for 5 min. The crystalline structure of the nano-TiO2 was detected by X-ray diffraction (XRD) (X'Pert, Philips, Amsterdam, The Netherlands) using a 4-kW monochromatic Cu Kα (λ = 0.15406 nm) radiation source.
The nano-TiO2 powder was pressed to be compact sheet, and then the surface modification effect of the samples was evaluated by measuring the hydrophilicity. An automatic contact angle analyzer (DSA 100, Kruss, Hamburg, Germany) was employed.
The nano-TiO2 powder was dispersed in ethanol with a viscosity of 0.5 mPa · S. Then, the particle size and size distribution of the nano-TiO2 powder was analyzed by Dynamic light scattering spectrum (DLS) (ZS-90, Malvern, Grovewood Road, Malvern, UK).
The dispersion of nano-TiO2 in the composites was investigated by field emission scanning electron microscopy (FE-SEM) (FEI, Inspect F, Hillsboro, OR, USA). Nano-TiO2 with 1.5 wt.% addition amount was added to prepare the composite powder, which was then cured in a PTFE mould at 190°C for 15 min and formed the sheets with thickness of 3 mm. Then, the sheets underwent brittle fracture in liquid nitrogen atmosphere, followed by gold sputter coated on the fracture sections. The FE-SEM was carried out with an accelerating voltage of 20 kV.
The reflection characteristics of the nano-TiO2 before and after surface modification were measured by ultraviolet-visible spectrophotometer (UV-vis) with a wavelength range from 190 to 700 nm.
The UV ageing resistance of the samples was carried out under the light-exposure conditions that simulate the requirements for real outdoor applications. A UV accelerated ageing chamber was equipped with fluorescent lamps emitting in the spectral region from 280 to 370 nm, of which the maximum irradiation peak occurs around 313 nm. The samples were placed for 1500 h in the chamber, and the time-dependent gloss retention and colour aberration of the samples across the ageing was measured.
G 0 and G x are the gloss of the sample without ageing and that of the sample aged with several hours, which were measured by a gloss spectrometer (WGG-66, Shanghai precision optical instrument Co., Ltd., Shanghai, China).
where L x , a x and b x are the lightness, redness-greeness and yellowness-blueness, respectively. These parameters of the samples before and after ageing were measured by a colour spectrometer (CR-10, Minolta Co., Osaka, Japan).
The surface morphology and roughness of the composites before and after ageing were studied by Atomic force microscopy (AFM) (Nanoscope Multimode APM, Vecco Instrument, Plainview, NY, USA) with a tapping mode under ambient condition.
Results and discussion
Figure 3c,d compared the dispersion homogeneity of nano-TiO2 with 1.5 wt.% in the polymeric matrix. The unmodified nano-TiO2 agglomerated obviously, and the particle size was about 350 nm. It is resulted from limited compatibility of the unmodified nano-TiO2 with hydrophilic (Figure 3c). Nevertheless, Figure 3d exhibits fewer agglomerates of modified nano-TiO2 in the sample. Although the dispersion of nanoparticles is also limited due to the melt-blend extrusion, the size of the modified nano-TiO2 is uniform of about 100 nm. This is in accordance with the DLS result. Here, we could see significantly improved dispersion of nano-TiO2 particle in the polyester matrix, which further illustrates the importance of the surface modification process.
Generally, the absorption band around 910 cm-1 was assigned to monitor the epoxy equivalent conversion (the C-O-C bond of epoxy groups) [45, 46]. Figure 5b,c shows the FT-IR spectrum of the composites with unmodified and modified nano-TiO2, respectively. The decreased intensity of the absorption band could be attributed to the ring-opening of epoxy groups induced by the reaction between hydroxyl of COOH and epoxy groups during the crosslinking. In contrast to the sample with unmodified nano-TiO2, the sample with modified nano-TiO2 exhibits larger decreasing amplitude of the absorbance. Particularly, the absorbance at 908 cm-1 as a function of temperature for the two systems were plotted in Figure 5d, demonstrating a faster decreasing tendency of the absorbance at this band for the polyester/modified nano-TiO2 composite. It suggests a promoting effect of modified nano-TiO2 on the crosslinking reaction.
As a supplement, the colour aberration (determined by the value of ΔE) of the samples during the accelerated ageing was also observed. Figure 6b shows significant decrease in the colour aberration of the samples with modified nano-TiO2. This is due to lower degradation occurred in the polyester/nano-TiO2 composites. In this case, the nano-TiO2 plays a role in shielding UV radiation by absorption and scattering. After 1500-h ageing, the ΔE of the sample modified with 2.0 wt.% nano-TiO2 is 2.15, with reduction of 27.6% compared to a 2.97 ΔE of the sample without nano-TiO2. Coinciding with the results of gloss retention, the colour aberration of the sample decreases with the concentration of nano-TiO2.
Mean value of surface roughness parameters (Ra) and root-mean-square (RMS) height of the samples
Polyester without nano-TiO2
Polyester/2.0 wt% nano-TiO2 composite
The nano-TiO2 was modified with aluminate coupling agent by a dry coating method. The FT-IR, contact angle and DLS measurements demonstrated a linkage of organic functional groups to the nano-TiO2, resulting in improved agglomeration resistance. Then, the modified nano-TiO2 was employed as a functional additive to prepare the polyester/nano-TiO2 composites by melt-blend extrusion method. With a real-time FT-IR study, the nano-TiO2 exhibited a promoting effect on the crosslinking reaction of polyester with TGIC. Furthermore, the gloss retention, colour aberration and the surface morphologies of the composites during an accelerated UV ageing were investigated. The results indicated that the nanocomposites exhibited much less degree of ageing degradation, due to a strong UV shielding ability of the nano-TiO2. Particularly, the polyester/nano-TiO2 presented an improvement of 42.5% in the gloss retention and a reduction of 27.6% in the colour aberration after 1500 h UV ageing. This work proposed a dry modification method for the nano-TiO2 and its application as functional nanoscale additive, which are highly available for the widespread applications of polyester resin/TiO2 composites, and would provide considerable insights into the protection of natural and synthetic carbohydrate polymers from the UV irradiation.
This work was financially supported by the National 863 Project (2003AA32X230), National S&T Major Project (2011ZX09102-001-10 and 2013ZX09301304-007), Science & Technology Support Programm of Sichuan Province (2013FZ0076) and Younger Fund of the Ministry of Education (10XJCZH005). And we would like to show our great thanks to Wang Hui (Analytical & Testing Center, Sichuan University) due to her great help in SEM observation.
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