Investigation of the Effects of Titanate as Coupling Agent and Some Inorganic Nanoparticles as Fillers on Mechanical Properties and Morphology of Soft PVC

The effects of titanate as a coupling agent and some particulate nanoscale particles such as TiO2, CaCO3, and ZnO on thermal and mechanical properties of emulsion polyvinylchloride (E-PVC) were investigated by thermogravimetric analysis (TGA), and mechanical tests. In this research, it was found that, in the presence of nanoparticles of CaCO3, TiO2, and ZnO, the peak temperature of dehydrochlorination of E-PVC was shifted to higher temperatures, and the rate of mass loss was decreased. Also results of differential scanning calorimetry showed that the addition of nanoparticle of CaCO3, TiO2, and ZnO led to an increase in glass transition temperature. The impact strength, elastic modulus and toughness of the samples were enhanced after addition of 0–10 part of filer in hundred parts of resin (phr) nano-CaCO3, nano-TiO2, and nano-ZnO due to improvement of compatibility of the polymer and the nano-particles. Also UV and thermal stability of the samples were enhanced by means of the nanoparticles. It was found that, in the presence of titanate as coupling agent, content of additives that could be used in the composite of PVC shifts to higher amounts.


Introduction
Generally, PVC as a commodity plastic has been widely used in industrial fields such as outdoor applications, house siding panels, wastewater tubes, window profiles, and synthetic leather for many years, due to its good properties, such as nonflammability, low-cost, and formulating versatility.However, its low toughness and heat-softening temperature limit its application for some purposes [1,2].PVC is degraded thermally during processing and photochemically during applications with solar irradiation, UV light, and also by oxygen of atmosphere via radical mechanism.In contrast to some other polymeric materials, moisture is not an important factor to deteriorate its properties.Oxidation reactions are investigated by the growth of the IR absorption bonds for C=O around 1710 to 1720 cm −1 [3,4].
In recent years, nanocomposites based on PVC attracted much attention.Developing nanocomposites based upon polymers and nanoscale fillers has been an attractive approach to achieving good properties.Various nanoscale fillers, including silica, mica, calcium carbonates, titanium oxide, and some other nanoparticles have been reported to enhance mechanical and thermal properties of polymers, such as toughness, stiffness, impact strength, elongation percent, and modulus [5][6][7].
Nano-CaCO 3 is one of the most common spherical nanoscale fillers used in preparation of nano composite suspension PVC.A study on nano-CaCO 3 -polypropylene (PP) composites revealed the dramatic toughening effect of nano-CaCO 3 , but because of the nucleating effect of nano-CaCO 3 the yield strength of PP was slightly decreased [8,9].Nano-TiO 2 and nano-ZnO are common pigments that are used as fillers in preparation of nanocomposites.The roles of TiO 2 and ZnO as pigments in the degradation process have not been completely elucidated, and conclusions are contradictory.For many researchers there is a question: do TiO 2 and    ZnO act predominantly as physical UV absorbers and protect PVC, or do they act as photocatalyst for degradation of PVC?
The investigations of polymer scientists support the first view while application-technological experience is mixed.
To improve the overall quality of composites and treat the surface of fillers, coupling agents were used.The improvements are more apparent when composites are exposed to high humidity and high temperature.Coupling agents enhance adhesion between the filler and the polymer.They create strong bonds between their surfaces.Efficient coupling agents are silanes, and organotitanates, which are easily dispersed and present a high chemical resistance.
Organo titanate coupling agents create molecular bridges at the interface between inorganic fillers and polymer matrix.They overcome many of the limitations of incompatibility of polymers and inorganic fillers.The mechanism by which they couple different inorganic surfaces, which means that they are suitable not only for fillers having surface hydroxyl groups, but also for carbonates, carbon black, and other fillers that do not respond to silanes [10].They have the advantages of forming only a monomolecular layer.The absence of a multimolecular layer at the interface and the chemical structure of titanates, modify the surface energy of filler particles in such a way that the melt viscosities of polymers are lower than the other type of coupling agent [11].So the use of titanates is preferred.It has also been researched that the addition of titanates to nanocomposites, improved the mechanical properties of polymer [5,[11][12][13].Addition of montmorillonite in polyethylene (PE) and poly(ethylene terephthalate) (PET) in a twin-screw extruder caused a consideration improvement in compatibility and tensile strength of the composite [14,15].In this paper the effects of titanate as coupling agent and some nanoinorganic pigments and fillers on mechanical properties and morphology of the soft PVC composites were studied.

Materials and Sample Preparation.
Emulsion PVC (M w = 35000) was produced by European group of companies.Nano-CaCO 3 (NPCC 2-01, surface modifies with stearic acid) with particle size of 29 nm was produced by nanomaterial technology Pte.Chemical Co; Ltd; Singapore.Nano-TiO 2 (type HQ lab)-(P-38) with particle size of 80 nm was prepared by nano-photo-catalysts chemical company; China.Dioctyl trephthalate (DOP) was produced by Merck Company.Organo titanate coupling agent was industry grade product, ((2,2-bis(allyloxymethyl)butoxy) tris(((bis(octyloxy)phosphoryl) methyl)(hydroxy) phosphoryloxy) titanium).Nano-CaCO 3 , nano-TiO 2 , and nano-ZnO were dried at 80 • C for 24 h in a vacuum oven.PVC, DOP, and nanoparticles were mixed at a constant speed to give a homogenius PVC compound.ASTM D638 test method was used for tensile properties of plastics such as  PVC.Specimen dimension for thickness showed in this test method purposed type IV for specimen.

Synthesis of Nano-ZnO
Particles.About 20 gr of zinc sulfate dehydrate was dissolved in minimum amount of distilled water, and the solution volume increased to 50 mL.Then NaOH solution (4 molar) was added by dropping funnel until a white precipitate was prepared which was then converted to a colloidal solution.Addition of NaOH was continued to pH 7. The solution was then mixed for 12 h.The precipitate was filtered and washed with distilled water several times.The resulted solid was dried in oven at 100 • C for 3 h and finally was heated at 400 • C for 2 h to afford nano-ZnO particles.

Sample Preparation.
In a 200 mL baker, PVC (10 gr), DOP (5 gr), and different amounts of fillers according to Table 1 were mixed completely for 5 minutes by using mechanical stirrer to gave a homogeneous paste.

Film Preparation.
The homogeneous paste was molded on an aluminum sheet for 0.3 mm in thickness and cured at 180 • C for 10 min in a vacuum oven, followed by cooling to room temperature [8].

Characterization
2.2.1.Methods and Instrument.Surfaces of composites for determining dispersion of nanoparticle in PVC matrix were observed by scanning electron microscopy (SEM) with a JEM 1600 EX apparatus running at an acceleration voltage of 80 kV.X-Ray diffraction was performed to determine the diameters of nanoparticles by X-Ray Diffractometer, Bruker, D8ADVANCE.
Tensile tests were performed at room temperature at a crosshead speed of 50 mm/min using an Instron tensile tester from Santam Co. (SMT-5); BONSHIN, Model: DBBP-500 according to ASTM D 638M.The test specimen shall conform to the dimensions showed in this standard test method.Type IV specimen was used for testing nonrigid plastics with a thickness of 4 mm or less.Thermogravimetric analysis was performed by Metler TG analyzer (TG-50) to determine the thermal stability of polymer composites.

Morphology Observation.
To investigate the effects of CaCO 3 , TiO 2 and ZnO, and also their nanoparticles on mechanical properties and morphology of PVC, different  samples containing different amounts of the fillers were prepared (Table 1).The surface morphology of the prepared samples were investigated by Scanning Electron Microscopy (SEM), and the results were shown in Figures 1, 2, and 3.As shown in these figures, dispersion of nanoparticle is better in PVC films.It is also observed that in the presence of titanate as coupling agent, a nanoparticle homogenously dispersed in PVC matrix indicating good compatibility of nanofiller and polymer.
The XRD micrograph of synthesized ZnO nanoparticles shows the nanometric size of particles in Figure 4.It was obtained from this micrograph (by using Scherrer equation) that the size of nanoparticles of ZnO used in PVC is 21 nanometer.

Tensile Properties.
Mechanical properties of the prepared composites and nanocomposites were investigated, and the results were shown in Tables 2, 3, and 4. It could be obtained from the tables that the elongation at break of both composites and nanocomposites for all fillers was decreased with increase of the filler contents, but it can be seen that the rate of loss is more in composite than in the related nano composite.It has also been obtained from the tables that elastic modulus of the nanocomposites could be increased with increase of the nanoparticle content.But the value of modulus in nanocomposites is lower in contrast with PVC/CaCO 3 composites.It can be concluded from this result that, beside the inherent properties of polymer matrix, the dispersion state of the fillers also affects on mechanical properties such as elastic modulus of composites.Table 4 shows mechanical properties of the nanocomposites in the presence of titanate as coupling agent.It can be obtained that addition of titanate to PVC compound led to an increase in elongation percent.Also the content of filler that could be used increased to 40 phr for nano-CaCO 3 and 20 phr for nano-TiO 2 and nano-ZnO.A thermal analysis curve related to PVC/nano-CaCO 3 (sample number nC 5 ) was shown in Figure 6.For some other samples, the thermal analysis data were recorded, and the results are collected in Table 5.

UV Studies.
FTIR spectrum of the reference sample showed an absorption for carbonyl group which was related to DOP (Figure 7).In order to investigate the UV stabilization of the samples, first a reference sample was treated with UV light for 45 days.FTIR spectra of this sample after treating with UV light (Figure 8) showed an increase in intensity of carbonyl absorption which was related to PVC degradation with UV light.In other samples with nanofillers, the intensity of this peak decreased which was confirmed the UV stability of these fillers.As a typical example, FTIR spectrum of sample code (nZ 5 ) after treating 45 days with UV light was shown in (Figure 9).

Conclusion
PVC composites and nanocomposites were prepared by PVC plastisol and mixing process at room temperature.Nanoparticles are well dispersed in the polymer matrix by titanate as a coupling agent.In this process addition of tiatnate acts as a compatibilizer and cause increases of tensile strength and modulus of the samples while the content of nanoparticle was less than 10 phr.The impact strength and elastic modulus of E-PVC could be increased by addition of nanoparticles, and higher toughness could be also achieved from these composites.

Figure 9 :
Figure 9: FTIR spectrum of sample code (nZ 5 ) after treating 45 days with UV light.

Table 2 :
Mechanical properties of prepared composites.

Table 3 :
Mechanical properties of prepared nanocomposites.

Table 4 :
Mechanical properties of prepared nanocomposites in the presence of titanate coupling agent.

Table 5 :
Thermal analysis data of composites obtained by TG/DTG.