Effect of Plasticization on the Physical Properties of Polyvinyl Chloride Resin

R. M. Kessenikh, V. G. Sotnikov, V. G. Trippel, Yu. N. Shumilov,
Yu. G. Gruzdeva, A. P. Povelichenko

(Presented by the scientific seminar of the organic departments)

Polyvinyl chloride resin, PVC, is widely used in cable engineering. However, the molecular structure of PVC determines its rigidity, and therefore, in order to improve a number of properties (frost resistance, elasticity, etc.), plasticizers are introduced into the polymer [1, 2]. In the present work, the influence of low-molecular polar plasticizers—dioctyl phthalate and dibutyl sebacate—on the thermal and electrical properties of polyvinyl chloride resin is considered. The starting products are listed in Table 1.

Table 1

Samples of polyvinyl chloride without plasticizers and with plasticizers were prepared according to generally accepted technology. The pressing temperature was \(140^\circ\text{C}\) at a pressure of \(150\ \text{kg}/\text{cm}^2\). The plasticizer was introduced into the PVC resin in weight concentrations of 5, 10, 20, and 50%.

Experimental Data and Discussion

1. X-ray structural analysis was carried out on a URS-50I with copper radiation. As a result of the investigation, two halos were obtained for PVC at \(\Theta = 8^\circ\) and \(\Theta = 10^\circ\) (Table 2).

When plasticizers are added, these halos first become sharper and shift toward larger angles: for DOP—at 5 and 10%, and for DBS—at 10 and 20%. It may be assumed,

that in this case the structure becomes more ordered and denser (the short-range order becomes more definite and the intermolecular distances decrease). Apparently, this is due to improved conditions for chain orientation and to ordering of their mutual orientation. Upon introduction of 50% plasticizer

Table 2

both halos become less distinct than for pure PVC and shift slightly toward smaller angles. Probably, the plasticizer in larger quantities causes disordering of the structure and an increase in intermolecular distances (Fig. 1).

2. Viscosity of plasticizers. The temperature dependences of the viscosity of the plasticizers DOPh and DBS differ significantly from one another (Fig. 2). With increasing temperature, after the inflection point the viscosity decreases more slowly than before the inflection point.

Fig. 2. Dependence of viscosity on temperature; 1) DOPh; 2) DBS.

3. Effect of different concentrations of plasticizers on the temperature coefficient of linear expansion and the glass-transition temperature. Thermal expansion and glass-transition temperatures were determined over a cooling—heating cycle using an interferometer with a quartz dilatometer. The glass-transition temperatures were determined both for PVC resin and for PVC with different weight concentrations of plasticizers. From

Fig. 1. a) for DBS.

Fig. 1. b) for DOF.

Fig. 3. Dependence of the glass-transition temperature on the percentage content of plasticizers: o — dioctyl phthalate, x — dibutyl sebacate.

Fig. 4. Dependence of TK \(\alpha_p\) on the percentage content of plasticizers for the temperature interval \(20 \div 40^\circ\mathrm{C}\).

From the results presented (Fig. 3) it follows that the type of plasticizer has no effect on the glass-transition temperature \((T_{\mathrm{g}})\). In the range of low plasticizer concentrations, a sharp drop in \(T_{\mathrm{g}}\) is observed, while as the concentration increases the drop becomes more gradual. The temperature coefficients of linear expansion \((TKL\alpha)\) for all the compositions studied, PVC + DBS and PVC + DOP, are given for a temperature interval lying below the glass-transition temperatures. Naturally, above the glass-transition temperatures \(TKL\alpha_{\mathrm{p}}\) will have elevated values compared with those shown in Fig. 4.

We note that in the range of weight concentrations \(10 \div 20\%\), \(TKL\alpha_{\mathrm{p}}\) increases stepwise from \(60 \cdot 10^{-6}\) to \(230 \cdot 10^{-6}\ 1/^{\circ}\mathrm{C}\).

Fig. 5. Dependence \(\mathrm{tg}\,\delta = f(t^{\circ}\mathrm{C})\) at different frequencies for PVC + 50% DOP

1 — 500 Hz,
2 — 1000 Hz,
3 — \(10^{4}\) Hz,
4 — \(4.5 \cdot 10^{4}\) Hz.

In fact, already at a plasticizer content of 5% the glass-transition temperature decreases by 20°, the plastic compound loses rigidity, and thereby thermal expansion is facilitated. Thus, in comparison with pure polyvinyl chloride, in the PVC + 50% DBS and PVC + 50% DOP compositions the thermal coefficient increases approximately eightfold.

1. Effect of plasticization on the magnitude of the tangent of the dielectric loss angle. Dielectric losses \((\mathrm{tg}\,\delta)\) were measured in the range of audio frequencies \((500\ \text{Hz} \div 1.5 \cdot 10^{4}\ \text{Hz})\) and in the temperature interval \(-80 \div +80^{\circ}\mathrm{C}\).

Fig. 5 gives a typical curve \(\mathrm{tg}\,\delta = f(t^{\circ}\mathrm{C})\) at different frequencies for PVC + 50% DOP. On the curves obtained we did not discover—

of dipole-radical losses (d.r.l.) has been established. It may be supposed that, upon plasticization, the small magnitude of the maximum of d.r. losses merges with the smoothly decreasing branch of the curve \(\operatorname{tg}\delta = f(t^\circ\mathrm{C})\). From this same curve it is evident that an increase in frequency causes \(\operatorname{tg}\delta\) to increase in the region of elevated temperatures.

Plasticization of polyvinyl chloride causes a shift of the maximum of dipole-elastic (Figs. 6 and 7) losses toward lower temperatures with increasing concentration of DOP and DBS. It should also be noted that the introduction of a larger amount of plasticizer not only shifts the maximum \(\operatorname{tg}\delta\) into the region of lower temperatures and makes it diffuse, but also decreases its magnitude. This fact may be explained by a decrease in the viscosity of the system and, consequently, by a decrease in intermolecular forces, as a result of which the energy required for orientation of the segments decreases. From a comparison of the curves in Figs. 6 and 7 it follows that the shift of \(\operatorname{tg}\delta_{\max}\) toward lower temperatures for PVC plasticized with DBS is greater than in the case of DOP, which is evidently explained by the properties of the plasticizers themselves. Indeed, in Fig. 2 we see that the viscosity of DBS over the temperature range always remains lower than that of DOP; consequently, for the PVC + DBS composition thermal segmental motion is possible at lower temperatures. At the same time it should be noted that the magnitudes of the maxima themselves remain comparable, despite the fact that in absolute value

Fig. 6. Dependence
\(\operatorname{tg}\delta = f(t^\circ\mathrm{C})\) at a frequency of 500 cps:
1 — PVC + 50% DOP,
2 — PVC + 20% DOP,
3 — PVC + 10% DOP,
4 — PVC + 5% DOP,
5 — PVC.

Fig. 7. Dependence
\(\operatorname{tg}\delta = f(t^\circ\mathrm{C})\) at a frequency of 500 cps:
1 — PVC + 50% DBS,
2 — PVC + 20% DBS,
3 — PVC + 10% DBS,
4 — PVC + 5% DBS,
5 — PVC.

\(\operatorname{tg}\delta\) of these plasticizers changes significantly with increasing temperature [3]. This once again confirms the possibility of changing the energy of intermolecular interaction of the system, upon introduction of a plasticizer, in either direction depending on the characteristics of the polymer and plasticizers [4].

Figure 8 gives the dependence of \(\operatorname{tg}\delta_{\max}\) as a function of the percentage content of plasticizers at \(f = 500\) cps. From the curve shown it is evident that the influence of the plasticizers DOP and DBS on the maximum value of \(\operatorname{tg}\delta\) is identical and, as was already indicated above, with increasing concent-

…as the concentration of plasticizers increases, the maximum value of the dielectric-loss tangent decreases. Accordingly, an increase in the glass-transition temperature causes an increase in the maximum value

Fig. 8. Dependence of \(\operatorname{tg}\delta_{\max}\) on the percentage content of the plasticizers DOP and DBS at a frequency of 500 Hz.

of the dielectric-loss tangent, since in this case segmental thermal motion is hindered (Fig. 9).

Fig. 9. Dependence of \(\operatorname{tg}\delta_{\max}\) on the glass-transition temperature.

Table 3

Note: \(t = 20^\circ\text{C}\).

Fig. 10. Specific resistance as a function of the percentage content of plasticizers at a temperature of 28.5°C:
1 — PVC + DOP;
2 — PVC + DBS.

Fig. 11. Specific resistance as a function of the percentage content of plasticizers:
1 — PVC + DOP;
2 — PVC + DBS.

Effect of Plasticizer Concentration on the Electrical Conductivity of Polyvinyl Chloride

A study of the effect of the plasticizers DOP and DBS on the value of the specific volume resistivity (Fig. 10) shows a sharp decrease in resistivity with increasing plasticizer concentration; moreover, the DBS + PVC curve lies considerably lower than the curve for DOP + PVC (when the plasticizer content is above 10%). The fact that at a temperature of 28°C, \(\rho_v\) for compositions containing from 5 to 10% DBS is somewhat higher than for the composition containing DOP can probably be explained by the different temperature dependence of the viscosity of these plasticizers. This conclusion is confirmed by the fact that at a temperature of 80°C, \(\rho_v\) for the PVC + DBS composition, at any percentage content of plasticizer, is lower than for PVC + DOP (Fig. 11). From these same figures it is evident that the decrease in \(\rho_v\) begins already at a small percentage content of plasticizers. However, the difference in the absolute values of the specific volume resistivity between PVC + DOP plasticate and PVC + DBS plasticate up to a concentration of 10% at a temperature of 20°C is practically small (Table 3).

Conclusions

The results of the study of the effect of the weight concentration of polar plasticizers on polyvinyl chloride resin make it possible to draw the following conclusions:

1. Plasticization affects the maximum of dipole-elastic losses; moreover, the introduction of a larger amount of plasticizers not only shifts the maximum into the region of lower temperatures, but also reduces its magnitude, which can be explained by a decrease in the viscosity of the system and, consequently, by a decrease in intermolecular forces, as a result of which the energy required for orientation of the segments is reduced.

2. The shift of the maximum of \(\operatorname{tg}\delta_{\max}\) toward lower temperatures is greater for PVC plasticized with DBS than in the case of PVC plasticized with DOP, which is evidently explained by the different viscosity of the plasticizers.

3. The absolute values of the maxima of the tangent of the dielectric loss angle remain comparable for PVC plasticized with dibutyl sebacate and dioctyl phthalate, despite the fact that the values of \(\operatorname{tg}\delta\) for these plasticizers differ considerably from one another with increasing temperature.

4. A noticeable effect of plasticizers at \(t = 20^\circ\mathrm{C}\) on the value of the specific volume resistivity of plasticized PVC is observed when the plasticizer content is 20% or more. The greatest effect in this case is exerted by the plasticizer dibutyl sebacate, which lowers the value of the specific volume resistivity by three orders of magnitude compared with pure polyvinyl chloride resin; and at a 50% content of the DBS plasticizer, the specific resistivity decreases by five orders of magnitude. At elevated temperatures the effect of the plasticizers is intensified still further.

5. An increase in the percentage content of plasticizer lowers the glass-transition temperature, shifting it into the region of lower temperatures. Already at a 5% content of plasticizers (DOP and DBS), a shift of the glass-transition temperature by 20°C relative to the glass-transition temperature of PVC is observed. A further increase in the concentration of plasticizers causes a shift of the glass-transition temperatures toward lower temperatures, and at a 50% content of plasticizers the glass-transition temperature becomes equal to 4°C [5].

1. The plasticizers DOP and DBS have practically the same effect on the thermal expansion and glass-transition temperature of PVC plasticate, beginning with a content of 5% and up to and including 50%. The small deviations observed in this case are characteristic of the plasticizer with the lower viscosity (DBS).

2. X-ray diffraction analysis has established that the introduction of plasticizers into polyvinyl chloride resin (up to 20%) brings about a more ordered structure, since the conditions for orientation of the chains and ordering of their mutual orientation are improved. A high content of plasticizers (50%) causes disordering of the structure and an increase in intermolecular distances. The observed stepwise change in thermal expansion at weight concentrations from 10 to 20% plasticizer content corresponds to the more ordered state of the polymer established by X-ray diffraction analysis. Comparison of these results does not explain the cause of the stepwise change in the thermal expansion temperature of PVC plasticate.

REFERENCES

1. G. P. Mikhailov, B. I. Sazhin. High-Molecular Dielectrics. Zhurnal “Uspekhi khimii”, vol. 29, no. 7, 1960.

2. R. M. Fuoss, J. Amer. Chem. Soc. 63 (1949), 369.

3. Van—Gauta. Vestnik elektropromyshlennosti, No. 3, 1960.

4. V. A. Kargin and G. A. Slonimskii. A Brief Outline of the Physicochemistry of Polymers, 1961.

5. V. A. Kargin, G. A. Slonimskii, and L. Z. Rogovin. Kolloidnyi zhurnal XIX, No. 2, p. 142, 1957.