Introduction

Poly(vinyl chloride) (PVC) is a widely used thermoplastic polymer such as building and construction, packaging, automotive, electrical/electronic and healthcare. PVC nanocomposites with layered silicates have gained increased attention in recent years due to reinforcement effects induced by inorganic filler. Improved thermal resistance, enhanced stiffness and barrier properties, as well as flame retardation, have been reported. Layered silicates used for this purpose include mica, hectorite and sodium montmorillonite (Na+-MMT) [1]. Among these nanoscale additives, Na+-MMT plays an effective role as a reinforcement component in developing new polymer nanocomposites because of its abundance and eco-friendly nature.

Na+-MMT belongs to the dioctahedral smectite group and consists of silicate layers that are separated by van der Waals gaps, which are known as galleries or interlayers. The surface of each silicate layer contains negative electrical charges. The silicate layer is approximately 100–200 nm in length and 1 nm in thickness, and the spacing between stacked layers is 1 nm. Therefore, in order to reach the chemically electrical equilibrium, sodium ions (positively electrical charges) are located within silicate layers and this causes difficulty in dispersion of Na+-MMT in a polymer matrix.

However, one of the outstanding features of Na+-MMT is that the silicate layers can be expanded and even delaminated by exchanging Na+ with organic cationic molecules (ex. alkylammonium cations) under proper conditions [2]. The nanoscale silicate layers of organically modified MMT (O-MMT) can be more easily dispersed in the polymer matrix compared to Na+-MMT. Further, the in situ reinforcement phase can be formed on the molecular level during the blending process. This phenomenon is quite different from the one observed in conventional filled composites. Moreover, it has been found that the polymer/O-MMT nanocomposites can be prepared by conventional processing techniques, such as extrusion and injection methods [3, 4]. Because of the nanoscale effects, polymer/O-MMT nanocomposites often exhibit dramatic improvements in tensile strength, modulus, thermal resistance and gas permeability barrier properties with far less silicate content than those required in conventional filled polymer composites. Therefore, O-MMT is often used in the area of polymer nanocomposites [2].

In order to broaden PVC applications, PVC/cellulose whisker [5], PVC/nano-CaCO3 [6] and PVC/MMT [2, 7,8,9] nanocomposites were studied. For most polymer/MMT nanocomposite systems, the MMT is treated with organic ammonium salt before the intercalation of the polymer molecules in order to make the clay sufficiently organophilic and then to permit the insertion of polymer molecules between the silicate layers of MMT.

Although PVC/MMT nanocomposites were studied, researchers pay less attention on PVC/O-MMT nanocomposites containing various amounts of dioctyl phthalate (DOP). DOP is an external plasticizer for PVC. Plasticizers are an important class of low molecular weight and nonvolatile compounds that are widely used in polymer industries as additives. The primary role of such substances is to improve the flexibility and processing of polymers by lowering the second-order transition temperature and the glass transition temperature (Tg) [10]. Since DOP is a cheap and common external plasticizer for PVC and it is widely used to make soft and flexible PVC for the industrial applications like automobile manufacturing, building and construction material, cable, flooring and medical devices, DOP is chosen for this research.

In our previous reports [11,12,13,14,15,16], the influences of various additives on the morphology and fusion characteristics of PVC compounds were studied. In addition, fusion, thermal, mechanical and morphological characteristics of rigid PVC/Na+-MMT and PVC/O-MMT nanocomposites prepared by melt blending have also been revealed [17, 18]. It was found that the optimum amount of O-MMT was 3 phr (parts per hundred unit weights of resin) to attain proper thermal and mechanical properties in the resulted composites. Therefore, the amount of O-MMT is fixed as 3 phr in this study. Meanwhile, since flexible PVC products are as important as rigid PVC products, PVC/O-MMT/DOP nanocomposites containing various amounts of DOP are also studied in this paper. The effects of various amounts of DOP on the morphology and mechanical and thermal properties of PVC/O-MMT/DOP nanocomposites are examined.

Experimental

Preparation of PVC/O-MMT/DOP Nanocomposites

The materials used in this study were suspension PVC masterbatch powders without any plasticizers, containing 100 parts PVC resin particle (S60, Mw = 50,000; supplied by the Formosa Plastic Co., Taiwan), 3 parts O-MMT modified by dimethyldioctadecyl ammonium (DK3, purchased from Double Sun Trading Co., Ltd., Taiwan), 10 parts methacrylate-butadiene-styrene (MBS) (M50, impact modifier, supplied by the Formosa Plastic Co., Taiwan), 1.5 parts processing aid (PA820), 1.0 part PE wax, 1.0 part calcium stearate and 1.5 parts alkyl tin stabilizer (TM181FS). The PVC masterbatch powders were blended with various phr’s (0, 10, 20, 30, 40 and 50 phr) of DOP. All PVC/O-MMT/DOP nanocomposites were prepared in Haake torque rheometer (PolyLab 2000) equipped with an electrically heated mixing head and two non-interchangeable rotors. The processing temperature, rotor speed and blending time were set at 170 °C, 60 rpm and 5 min, respectively. The sample weight of each blending was controlled at about 62 g.

TEM examination

Ultrathin sections (about 80 nm) were cut from the rectangular blocks of the PVC/O-MMT/DOP nanocomposites using an ultramicrotome (RMC MT-X, RMC Instruments Corp., USA). The dispersion of O-MMT in the nanocomposites was directly observed by TEM (JEM-1230, JEOL Ltd., Japan) at an acceleration voltage of 100 kV.

TGA analysis

A thermogravimetric analyzer (TGA; PerkinElmer, model: TGA 7, USA) was used to analyze thermal degradation of the PVC/O-MMT/DOP nanocomposites in the temperature range of 50–800 °C with 10 °C/min heating rate under nitrogen atmosphere. The thermal degradation onset temperature and the thermal degradation weight loss of PVC/O-MMT/DOP nanocomposites were recorded and analyzed.

DSC analysis

To examine the thermal properties of the PVC/O-MMT/DOP nanocomposites, differential scanning calorimeter (DSC; PerkinElmer, model: DSC 7, USA) was used with heating rate of 10 °C/min under nitrogen atmosphere. The sample was two-stage heated. In order to release the internal stress of the sample, it was heated from 50 to 100 °C and then cooled to room temperature. Then, the sample was reheated from 50 to 200 °C with a heating rate of 10 °C/min. The Tg of PVC/O-MMT/DOP nanocomposites was recorded and analyzed.

Mechanical properties

Tensile strength, yield strength and elongation at break were measured by a universal testing machine (Model HT-3102, Hong-Ta Instruments Com., Taiwan) at a crosshead speed of 20 mm/min. Test specimens were prepared to conform to ASTM D638 specifications. Six different specimens were tested from each nanocomposite. The six testing results were averaged and then reported.

Results and discussion

Figure 1a shows the TEM micrograph of PVC/O-MMT/0phrDOP nanocomposite. It shows a small portion of intercalated structures in the nanocomposite, in which the thickness of stacked layers is 4–10 nm and this corresponds to the size of 2–5 stacked layers. Moreover, some individual silicate layers are also observed in the PVC/O-MMT/0phrDOP nanocomposite. Thus, it can be concluded that both intercalated and exfoliated structures of O-MMT coexist in the PVC/O-MMT/0phrDOP nanocomposite. Figure 1b is the TEM micrograph of PVC/O-MMT/30phrDOP nanocomposite. It also shows that intercalated and exfoliated structures of O-MMT coexist in the PVC/O-MMT/30phrDOP nanocomposite. The result is similar to that of the PVC/O-MMT/0phrDOP nanocomposite. Therefore, this implies that the addition of DOP has no obvious influence on the dispersion of O-MMT in the PVC/O-MMT/DOP nanocomposite.

Fig. 1
figure 1

TEM images of PVC/O-MMT/DOP nanocomposites containing: a 0 phr DOP, b 30 phr DOP

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Figure 2 shows TGA curves of the PVC/O-MMT nanocomposites containing 0, 10, 30 and 50 phr DOP, respectively. Three weight-loss stages with respect to the three thermal degradation stages are observed in all TGA curves. For PVC/O-MMT/0phrDOP nanocomposite (refer to Fig. 2a), the first weight-loss stage occurred in the range of 260–280 °C due to dehydrochlorination of PVC molecules and formation of a polyene structure. Below 225 °C, no marked change in the sample mass is observed. The second weight-loss stage appeared in the range of 450–470 °C due to the thermal degradation of carbon chain of PVC, which produces flammable volatiles [19,20,21,22]. The third weight-loss stage appeared in the range of 550–600 °C due to the thermal degradation of a polyene structure formed during first thermal degradation stage. In this study, we focus on the first weight-loss stage and illustrate the effect of DOP loading on the Tonset (the first thermal degradation onset temperature) and ΔY (the first thermal degradation weight loss) of PVC/O-MMT/DOP nanocomposite.

Fig. 2
figure 2

TGA curves of PVC/O-MMT/DOP nanocomposites containing: (a) 0, (b) 10, (c) 30 and (d) 50 phr DOP, respectively

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The TGA curve of PVC/O-MMT/10phrDOP nanocomposite (Fig. 2b) also shows three marked weight-loss stages. The first weight-loss stage occurred in the range of 250–270 °C due to desorption of DOP and dehydrochlorination of PVC molecules. Below 210 °C, negligible change in the sample mass is observed. If loading of DOP is increased, the Tonset is significantly decreased (Fig. 2c, d). Figure 3 shows the effect of DOP loading on the Tonset of the PVC/O-MMT/DOP nanocomposite. The result indicates that the Tonset of PVC/O-MMT/DOP nanocomposite is decreased as the amount of DOP is increased. For PVC/O-MMT/0phrDOP nanocomposite, Tonset is 280 °C; however, the Tonset of PVC/O-MMT/50phrDOP nanocomposite dropped to 242 °C. This is because the thermal degradation temperature of DOP is lower than that of PVC. Therefore, the Tonset of PVC/O-MMT/DOP nanocomposite is decreased with increasing the DOP loading.

Fig. 3
figure 3

Effect of the amount of DOP on the Tonset of PVC/O-MMT/DOP nanocomposites. Note: Tonset stands for the first thermal degradation onset temperature of PVC/O-MMT/DOP nanocomposite measured by TGA

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On the other hand, the ΔY of PVC/O-MMT/DOP nanocomposite is increased with increasing the DOP loading (refer to Fig. 4). As mentioned above, this phenomenon occurs because the thermal degradation temperature of DOP is lower than that of PVC. Therefore, desorption amount of DOP is increased with increasing the DOP loading.

Fig. 4
figure 4

Effect of the amounts of DOP used on the first thermal degradation weight loss of PVC/O-MMT/DOP nanocomposites measured by TGA

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Generally, the main function of a plasticizer, such as DOP, is to reduce the modulus of a PVC compound at the application temperature by lowering its glass transition temperature (Tg). Figure 5 shows the DSC curves of PVC/O-MMT/DOP nanocomposites containing various amounts of DOP. As shown in Fig. 5, the glass transition of the plasticized PVC/O-MMT nanocomposite is observed over a wider range of temperatures than the unplasticized PVC/O-MMT nanocomposite. In addition, increasing the loading of DOP can induce the glass transition shifting from the high-modulus (glassy) plateau region to the low-modulus (rubbery) plateau region. Thus, it implies that the addition of DOP can significantly decrease the Tg of the PVC/O-MMT/DOP nanocomposites [23].

Fig. 5
figure 5

DSC curves of PVC/O-MMT/DOP nanocomposites containing: (a) 0, (b) 10, (c) 20, (d) 30, (e) 40 and (f) 50 phrs DOP

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The effect of DOP loading on the Tg of the PVC/O-MMT/DOP nanocomposite is illustrated in Fig. 6. The result shows that the Tg of PVC/O-MMT nanocomposite is significantly decreased with an increase in DOP loading. This is due to the remarkable compatibility of DOP with PVC. Meanwhile, the Tg of DOP (= − 55 °C) is markedly lower than that of PVC (= 83 °C). Therefore, the Tg of PVC/O-MMT/DOP nanocomposite can be significantly decreased with increasing the DOP loading. For the unplasticized PVC/O-MMT nanocomposite, its Tg is around 80 °C; however, the Tg of PVC/O-MMT/50phrDOP nanocomposite is around 0 °C.

Fig. 6
figure 6

Effect of the amounts of DOP used on the Tg of PVC/O-MMT/DOP nanocomposites

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Experimental Tg of the PVC/O-MMT/DOP nanocomposites is fitted by adopting the Fox equation:

$$\frac{1}{{T_{\text{g}} }} = \frac{{w_{1} }}{{T_{{{\text{g}}1}} }} + \frac{{w_{2} }}{{T_{{{\text{g}}2}} }}$$

where Tg is the glass transition temperature of the PVC/O-MMT/DOP nanocomposite. Tg1 and Tg2 are the glass transition temperatures of the unplasticized PVC/O-MMT nanocomposite and DOP, respectively. The w1 and w2 are the mass fractions of the unplasticized PVC/O-MMT nanocomposite and DOP, respectively. The results obtained via using the Fox equation comply with the experimental results which are shown in Fig. 7.

Fig. 7
figure 7

DSC-determined values of Tg of PVC/O-MMT/DOP nanocomposites (filled square) as a function of the weight fractions of DOP. The solid curve represents Tg values predicted by the Fox equation

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Figure 8 shows the effect of DOP loading on the Young’s modulus of the PVC/O-MMT/DOP nanocomposite. It is observed that the Young’s modulus of the PVC/O-MMT/DOP nanocomposite is decreased with an increase in DOP loading. According to the definition of ASTM, the polymeric material is classified as rigid if its Young’s modulus is above 700 MPa (i.e., unplasticized PVC). For semirigid polymeric materials, its Young’s modulus is between 70 and 700 MPa. If the Young’s modulus of a polymeric material is below 70 MPa, it is classified as soft materials [24]. By adopting this classification principle, the unplasticized PVC/O-MMT nanocomposite is a rigid product since its Young’s modulus is 726 MPa. With increasing the loading of DOP from 0 to 20 phr, the Young’s modulus of PVC/O-MMT/DOP nanocomposite is gradually decreased (Fig. 8). PVC/O-MMT/10phrDOP (695 MPa) and PVC/O-MMT/20phrDOP (584 MPa) nanocomposites are in the category of semirigid products since their Young’s modulus is between 70 and 700 MPa. However, even PVC/O-MMT/10phrDOP nanocomposite with Young’s modulus of 695 MPa is classified as semirigid product, its property is actually very close to rigid materials due to low loading of DOP in its the PVC compound. In addition, PVC/O-MMT/30phrDOP nanocomposite can be also classified as a semirigid product since its Young’s modulus is 246 MPa. Meanwhile, the Young’s modulus of PVC/O-MMT/30phrDOP nanocomposite is only about one-third of that for unplasticized PVC/O-MMT nanocomposite. With further increase in DOP loading, the nanocomposite becomes softer and softer. For example, Young’s modulus of PVC/O-MMT/40phrDOP is around 27 MPa. Meanwhile, for PVC/O-MMT/50phrDOP, it is around 12 MPa. These results illustrate that DOP is a good plasticizer for PVC compounds to decrease its rigidity. It can be noticed that the Young’s modulus of PVC/O-MMT/40phrDOP is only about one-tenth of that for PVC/O-MMT/30phrDOP. Therefore, results indicate that the percolation threshold concentration for the plasticization of PVC/O-MMT nanocomposite, in order to be claimed as semirigid, is 30 phr DOP.

Fig. 8
figure 8

Effect of the amount of DOP on the Young’s modulus of PVC/O-MMT/DOP nanocomposites

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Figure 9 shows the effect of DOP loading on the tensile strength of the PVC/O-MMT/DOP nanocomposites. It shows that the tensile strength of the PVC/O-MMT/DOP nanocomposite is decreased with an increase in DOP loading. For instance, the tensile strength of the unplasticized PVC/O-MMT nanocomposite is 90.4 MPa, while that of PVC/O-MMT/50phrDOP nanocomposite drops to 28.8 MPa. Figure 10 shows the effect of DOP concentration on elongation at break of the PVC/O-MMT nanocomposites. As shown in Fig. 10, the elongation at break of the PVC/O-MMT/DOP nanocomposite is increased with increasing the DOP loading. The result reveals that DOP is a good plasticizer for the PVC compound to improve its flexibility. For instance, the elongation at break of unplasticized PVC/O-MMT nanocomposite is only 39.0%. However, the elongation at break of PVC/O-MMT/50phrDOP nanocomposite can be extended up to 251.5%.

Fig. 9
figure 9

Effect of the amount of DOP on the tensile strength of PVC/O-MMT/DOP nanocomposites

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Fig. 10
figure 10

Effect of the amounts of DOP on the elongation at break of PVC/O-MMT/DOP nanocomposites

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Conclusions

The influences of various amounts of DOP on morphology and thermal and mechanical properties of the PVC/O-MMT nanocomposite are studied in this paper. TEM results show that intercalated and exfoliated structures of O-MMT coexist in the PVC/O-MMT nanocomposites containing various amounts of DOP. It shows that the addition of DOP has no obvious effect on the dispersion of O-MMT in the PVC/O-MMT/DOP nanocomposites. The Young’s modulus, tensile strength, Tonset, and Tg of PVC/O-MMT/DOP nanocomposites are significantly decreased with increasing the DOP loading. On the other hand, elongations at break and ΔY of PVC/O-MMT/DOP nanocomposites are increased with increasing the DOP loading. The Tgs of the PVC/O-MMT/DOP nanocomposites comply with the Fox equation in this study. With increase in DOP loading, the PVC/O-MMT/DOP nanocomposites change from rigid (PVC/O-MMT/0phrDOP nanocomposite) to semirigid (PVC/O-MMT/10phrDOP, PVC/O-MMT/20phrDOP and PVC/O-MMT/30phrDOP nanocomposites) and gradually into flexible product (PVC/O-MMT/40phrDOP and PVC/O-MMT/50phrDOP nanocomposites).