The Effect of Rheological Additives on the Flow of Polysulphone Melts

Abstract

The rheological characteristics of a melt of home-produced polysulphone of grade PSF-190 with processing additives of different nature – silsesquioxane, fluorine-containing, and polymethylsiloxane additives – were investigated. The flow curves of PSF-190 with additives in different quantities in a wide range of temperatures, shear velocities, and shear stresses were obtained. It was shown that the functional properties of the additives depend considerably on their thermodynamic compatibility with PSF-190 at high temperatures (290–320°C), their solubility, and their content. It was established that the introduction of 2.5 wt% silsesquioxane additive lowers the viscosity of PSF-190 most effectively (by a factor of 2.5).

Keywords

thermoplastics polysulphones rheological characteristics rheological additives silsesquioxane polymethylsiloxane

Polysulphones are amorphous thermoplastic materials of structural designation with high thermophysical, dielectric, and physicomechanical properties, but they have high melt viscosity, which makes it difficult to process the material into articles of complex configuration. Increase in temperature lowers the melt viscosity, but there is then a danger of thermooxidative degradation of the polymer and consequently a deterioration in the service characteristics and the appearance of articles. Therefore, to lower the viscosity, processing additives of different nature – lubricants, etc. – are used.

In the present article, rheological data and flow curves are given for home-produced polysulphone of grade PSF-190 with different contents of processing additives of different chemical nature in a wide range of shear stresses, shear velocities, and processing temperatures.

Materials

The investigation was conducted on home-produced polysulphone of grade PSF-190 (Institute of Plastics) and on processing additives of different chemical nature and molecular chain structure – oligomeric silsesquioxane (OSS), polymethylsiloxane (PMS), and a fluorine-containing additive (FA) – which possess different thermodynamic compatibility (solubility) with polysulphone at processing temperatures (290–320°C).

Methods

The rheological additives were introduced into PSF-190 on an MP-2015 twin-screw extruder (Baker Perkins, UK) with a temperature in the barrel working zone of 290–292°C, and in the head of 275–280°C, and with a screw speed of 35–40 rpm. The material was predried at a temperature of ~145°C for 4 h under vacuum to a residual moisture content of no more than 0.05%.

The flow curves were taken on an LCR-7000 capillary viscometer (Dynisco, USA) in a wide range of shear velocities and shear stresses in an isothermal regime at different temperatures (290–320°C). The activation energy of viscous flow was calculated for the home-produced polysulphone and for composites containing processing additives. The effect of sliding of the polysulphone melt during flow was assessed by a known procedure [2] that makes it possible to assess the effect of sliding at the wall of the capillary during flow of the polymer at different shear velocities, shear stresses, and temperatures, and also to determine the main parameters of sliding of the polymer and assess the effectiveness of the influence of processing additives on the process of flow [3].

Results and Discussion

Viscosity and flow curves are among the main characteristics for the processing of polymer melts.

Figures 1 to 3 present flow curves for PSF-190 and composites with processing additives OSS, PMS, and FA in the range of processing temperatures.

Figure 1

The flow curves for PSF-190 (1–3) and for composites based on PSF-190 with a modifying fluorine-containing additive (4, 5) in a quantity of 0.1 wt% (4) and 1.0 wt% (5) at different temperatures: 1, 4, 5 – 290°C; 2, 4, 5 – 300°C; 3, 4, 5 – 320°C. Vertical axis: lg γ; Horizontal axis: lg τ

From the data given in Figures 1 and Figures 2 it follows that the introduction of rheological additives PMS and FA in different concentrations into the polymer matrix of PSF-190 has practically no significant effect on melt flow in the temperature range 290–320°C. Here, the melt viscosity hardly changes, and the sliding effect that is typical when incompatible lubricants are introduced and the components separate during flow is not observed. This is probably due to the fact that, at high temperatures (290–320°C), the dissolution of small quantities (0.1–1.0 wt%) of organofluorine and polymethylsiloxane additives in the polysulphone occurs.

Figure 3 presents the flow curves for PSF-190 polysulphone modified with an oligomeric silsesquioxane additive (OSS).

Figure 2

The flow curves for PSF-190 (1–3) and for composites based on PSF-190 with a modifying silsesquioxane additive (4, 5) in a quantity of 0.1 wt% (4) and 1.0 wt% (5) at different temperatures: 1, 4, 5 – 290°C; 2, 4, 5 – 300°C; 3, 4, 5 – 320°C. Vertical axis: lg γ; Horizontal axis: lg τ

From Figure 3 it can be seen that the greatest reduction in melt viscosity of PSF-190 is observed when 2.5 wt% OSS is introduced. Subsequent increase in the content of this additive to 5 wt% leads to a reduction in flow of the composites. Similar results were obtained in Jones et al. [4], in which it was shown that increase in the silsesquioxane concentration to 10 wt% does not lead to any reduction in the melt viscosity of polysulphone.

Figure 3

The flow curves for PSF-190 (1) and for PSF-190 modified with rheological additive OSS in a concentration of 1 wt% (2), 2.5 wt% (3), and 5 wt% (4) at temperatures of (a) 290°C and (b) 320°C. Vertical axes: lg γ; Horizontal axes: lg τ

From the flow curves it can be seen that, with increase in temperature to 320°C, the effect of rheological additive OSS in a concentration of 2.5 wt% on the melt viscosity of PSF-190 decreases. This seems to be due to its dissolution when the temperature is raised to 320°C in PSF-190.

The activation energy of viscous flow for PSF-190 with OSS in a quantity of 2.5 wt% is reduced by a factor of 1.5 by comparison with polysulphone (from 85 to 61 kJ/mol).

Figure 4 presents the dependence of the melt viscosity of PSF-190 with OSS on the shear velocity.

In Figure 4 it is shown that, for PSF-190 polysulphone with the optimum content of rheological additive OSS (2.5 wt%), in the region of low shear velocities (45–200 s^–1), the maximum reduction in viscosity by a factor of ~2–3 (from 6000 to 2000 Pa s) is realised. Further increase in the shear velocity (to over 200 s^–1) leads to a reduction in the effect of the rheological additive on the melt flow of polysulphone. This is probably due to the increase in solubility of OSS in PSF-190 at high shear velocities, and here the melt viscosity increases.

Figure 4

The dependence of viscosity at 290°C for PSF-190 (1) and for composites with different contents of processing additive (2 – 1.0 wt% OSS; 3 – 2.5 wt% OSS; 4 – 5.0 wt% OSS) on shear velocity. Vertical axis: lg η; Horizontal axis: lg γ

Figure 5 gives the dependences of the viscosity of composites based on PSF-190 on the OSS content at a constant shear velocity (630 s^–1).

From Figure 5 it can be seen that, when OSS is introduced into PSF-190 polysulphone in a quantity of 2.5 wt%, there is a sharp reduction in viscosity by a factor of ~2.5 (from 2000 to 850 Pa s) and an improvement in its processability. Further increase in the OSS content to 5 wt% leads to an increase in viscosity to 1600 Pa s. The increase in viscosity when 5–10 wt% OSS is introduced [4] is probably due to the fact that, at an OSS content of 2.5 wt%, the solubility limit of OSS in PSF-190 at 290°C has already been reached. It is known [5] that, on phase separation, in the region lying between the binodal and spinodal of the phase state diagram of the binary system, a stable microemulsion of OSS is formed in the polymer. Microemulsions of this type possess extremal properties, including the effect of a sharp reduction in viscosity.

Figure 5

The dependence of the viscosity at 290°C of PSF-190 on the content of oligomeric silsesquioxane additive. Vertical axis: η, Pa s; Horizontal axis: OSS content, wt%

From the presented data, the following main conclusions can be drawn:

•

It has been shown that the chemical nature of the processing additive and its content have a significant influence on the thermodynamic compatibility with PSF-190 in the region of high processing temperatures (290–320°C). It has been established that the most effective rheological additive is oligomeric silsesquioxane.

•

The optimum content of OSS in PSF-190 (2.5 wt%) has been established. With this OSS content, the melt viscosity of the polymer decreases by a factor of ~2.5–3 (from 2000 to 850 Pa s), depending on the temperature and the shear velocities and stresses.

•

A polymer composite with high flow and improved processability has been developed, based on home-produced PSF-190 with a new rheological additive.

References

Zweifel

, Plastics Additives Handbook, 5th edition. Hanser, Munich (2001).

Simonov-Emel'yanov

I.D.

, Assessment of the Effectiveness and Selection of Rheological Additives for the Processing of Polymeric Materials. MITKhT, Moscow, 130 pp. (2013).

Simonov-Emel'yanov

I.D.

, Methods for Determining the Processing Properties of Fillers and Polymeric Materials. MITKhT, Moscow, p. 75 (2014).

Jones

P.J.

, Polyhedral oligomeric silsesquioxane–polyphenylsulfone nanocomposites: investigation of the melt-flow enhancement, thermal behavior, and mechanical properties. J. Appl. Polym. Sci., 121: 2945–2956 (2011).

Kuleznev

V.N.

, Polymer Blends. Khimiya, Moscow, 279 pp. (1979).