The Effect of Explosive Treatment on the Structure and Thermomechanical Properties of Filled Polytetrafluoroethylene

Introduction

The increasing requirements of industry, including the oil and gas sector, concerning the strength and heat resistance of components can be met by using heat-resistant polymers. One of the new approaches to creating superstructural plastics is the use of polymer–polymer systems based on thermoplastics such as polytetrafluoroethylene, polyimides, aromatic polyamides, polyarylates, and polyesters. The combination of these polymers in composites makes it possible to create materials of low density with increased heat resistance (up to 300 °C), chemical resistance, and weather resistance, specific strength, and antifriction properties [1,2].

The level of the service properties of composites is ensured primarily by the intensity of adhesion interaction of components, increase in which is possible by using mechanical, chemical (treatment with surface-active substances), or high-energy treatment (shock treatment, radiation exposure, ultraviolet, ionising, laser, and ultrasonic irradiation, etc.) [3–11]. A promising method for producing filled polymer composites (PCMs) is explosive treatment (ET), which makes it possible to combine processes of activation and pressing of powder composite mixtures. The use of explosives by exciting high-velocity shock waves (SWs) in solids ensures simultaneously high pressures (up to 10 GPa) and temperatures (up to 1000 °C), which causes activation of the components of the powder mix, a change in both physical and strength characteristics, and the emergence of new properties [8–11].

The present work is devoted to an investigation of the effect of different ET conditions on the intensity of structural transformations affecting the thermomechanical properties of composites of polytetrafluoroethylene (PTFE) with rigid-chain, glassy polyarylate (PA) (polyoxybenzoyl).

Investigation Procedure

In the work, PCMs based on PTFE (GOST 10007–80) filled with 30 vol% PA (GOST 25288–82) obtained by the ET of a powder composite mix were investigated. The ET conditions were varied by creating different shock pulse configurations [plane loading with a sliding shock front (SF) and an annular SF in a cylindrical ampoule] [12]. The intensity of loading was controlled by using different types of explosive substance, which made it possible, by changing the detonation rate of the explosive from 1800 to 3800 m/s, to achieve pressures of 0.9–4.6 GPa. In an analysis of the results of SW loading, a shock-front pressure, P, calculated using a computer program, was used [12]. The obtained specimens comprised solid pressings, and in the case of loading with a sliding SW with an ET pressure of 0.9, 2.8, and 4.6 GPa they had the form of 50 × 100 mm sheets of 3.0 mm thickness.

In the case of ampoule ET, the SWs converge in the central part of the pressing, where a sudden increase in ET pressure to 1.5 GPa occurs, in contrast to the peripheral part, where the pressure does not exceed 0.6 GPa [12]. The collision of SWs at the centre of the treated material leads to the most intense impact of the powder particles and their deformation, friction, and heating up, and the material in the central part of the solid cylindrical pressing of 14 mm diameter and 120 mm height acquires a darker colour than at the periphery.

Thermomechanical investigations were conducted on a Hyperion 402 F1/F3 thermomechanical analyser by measuring the depth of penetration of the indenter (1.0 mm diameter) into the investigated specimen measuring 5 × 5 mm, of 2 mm height, with a load of 1 N and a heating rate of 5 °C/min. The loading of plane specimens was conducted along the direction of shock compression, and, in view of the sharp difference in the structure of the centre and periphery of the cylindrical pressing, specimens from different zones of the pressing were accordingly investigated by loading along the ampoule axis. The relative deformation, e, was defined as the ratio of the depth of penetration to the initial height of the specimen. The temperatures of softening (glass transition, melting) and flow were determined from the characteristic infections of the thermomechanical curves (TMCs). The given TMCs were obtained by approximation of experimental data for three identical PCM specimens.

The microstructures of the PCMs were investigated on an Olympus BX-61 optical microscope in reflected light. The morphology of the specimens was studied on a high-resolution Versa 3D electron scanning microscope.

Results and Discussion

The conducted investigations showed that, irrespective of the intensity of SW treatment, the shape of the TMCs was identical ( Figure 1 ). After ET with a sliding SW, irrespective of the ET pressure up to temperatures below 303–319 °C, close to the melting of crystallites of both PA (335 °C) and PTFE (327 °C) [13], low values of deformations are observed (up to 1.1–1.7%), determined by the highly crystalline structure of the rubbery (high-elastic) PTFE and the rigid-chain PA ( Table 1 ). Further increase in temperature above the softening point, t_s, leads to a rapid increase in the relative deformation (4.4–6.8%), which is due to weakening of intermolecular interaction as a result of the polymers passing through the melting points of the crystalline phase, and PA passing also into the rubbery state (320 °C). Subsequent sudden increase in deformations above the temperature of flow, t_fl (376–384 °C), is due to viscous flow of the polymer composite.

Increase in ET pressure from 0.9 to 4.6 GPa led to a monotonic decrease in t_s from 319 to 303 °C and in t_fl from 384 to 376 °C, and to an increase in the relative deformation by a factor of 1.5 (Figure 1, Table 1). Similar results were obtained with the activation of PTFE by a plane SW with a pressure of 2.1 GPa [14], where the deformations were more significant and reached 60%. The differences in the behaviour of materials during heating were governed by the presence in the investigated composite of rigid PA, which checks the deformation of PTFE, and by the smaller effect of sliding-shock-wave ET on the structure of the polymers [9,10]. The obtained results indicate the intensification of phase transformations during heating and a reduction in viscosity of the investigated composite with increase in ET pressure as a result of the occurring structural changes, which is borne out by microstructural investigations ( Figure 2 ). It was revealed that, at a low ET pressure (0.9 GPa), the polymer particles retain their initial shape, and their deformation occurs quasi-statically ( Figure 2a ). At a pressure of 2.8 GPa, repacking of mobile powder particles (structural deformation), the crumpling of their contacts, and contact deformation occur ( Figure 2b ). After ET at the highest pressure P = 4.6, as a result of the intense explosive effect, significant plastic deformation of PTFE particles is observed along the SF, as well as crushing of glassy PA during rearrangement into a denser packing under the action of high pressure ( Figure 2c ).

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Table 1

The characteristic temperatures and deformations of PCMs of PTFE + 30% PA after explosive treatment with a sliding SW

ET pressure, GPa	Deformation (%) at different temperatures				ts, °C	tfl, °C
	300 °C	325 °C	350 °C	375 °C
0.9	1.3	2.8	4.4	4.7	319	384
2.8	1.3	4.9	5.3	5.4	314	378
4.6	1.7	6.8	6.8	7.9	303	376

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Figure 1

Thermomechanical curves of PCMs of PTFE + 30% PA after ET at different pressures: 1 – 0.9 GPa; 2 – 2.8 GPa; 3 – 4.6 GPa. Vertical axis: ɛ, % Horizontal axis: t, °C Key to fg.: 1. t_s; 2. t_fl

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Figure 2

The microstructures of PCMs of 70% PTFE + 30% PA after ET at different pressures: (a) 0.9 GPa; (b) 2.8 GPa; (c) 4.6 GPa; dark areas – PTFE; light areas – PA

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Figure 3

The microstructures of PCMs of PTFE + 30% PA over the radius (a) and along the axis (b) of the ampoule: I – periphery; II – central zone; dark areas – PTFE; light areas – PA

Investigation of the microstructure of pressings produced in an ampoule ( Figure 3 ) showed a sharp change in the mechanism of structure formation. Intense deformation of polymer particles is observed, and also crushing not only of the glassy PA but also of the rubbery PTFE, which is due to its mechanical glass transition under the intense shock effect. On electron micrographs, the arising bands of shear both in PTFE ( Figure 4a ) and in PA ( Figure 4b ) can be seen. The greatest changes occur in the central part of the pressing, with the formation of a continuous reinforcing phase of glassy PA and the orientation of the structure along the direction of the SF ( Figure 3b ) as a result of the convergence of SWs, which causes an intense localised plastic deformation of the structural constituents, with their activation, and the formation of a metastable structure, as observed during ET of metal–polymer composites [8].

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Figure 4

Electron scanning micrographs of PCMs of PTFE + 30% PA: (a) PTFE; (b) PA

The change in nature of the structure affects the properties of the materials. Thermomechanical analysis (TMA) showed (Figure 5, Table 2) that ampoule ET, by comparison with plane loading with a sliding SF (Figure 1, Table 1), leads to an increase in deformations during heating from 1.3–7.9% to 2.8–20.6%, to a 30–48% reduction in the softening points, and to a 11–30 °C increase in the temperatures of viscous flow of the material. The lower t_s values both of the material of the peripheral part (271 °C) and of the material of the central part (273 °C), by comparison with material obtained by ET with a sliding SW (depending on the ET pressure: 303, 314, and 319 °C), are due to reduction in the thermal stability of the crystalline part of the polymers as a result of the intense shock effect, and possibly also to the emergence of fractions of lower molecular weight, which was observed in an investigation of Forum material [14]. Here, for specimens of the central zone, the deformations during heating increase by a factor of 2.5 (from 7.7 to 20.6%) by comparison with the periphery, which is caused by the formation of the most oriented metastable structure in this zone. Increase in t_fl, with a maximum for the material of the central zone (406 °C), indicates intensification, during ampoule ET, of intermolecular interaction, the formation of intermolecular bonds, similar to the crosslinks in ultrahigh-molecular-weight polyethylene under SW action [15], and also the breakdown of macromolecules with their partial recombination and chemical interaction of PTFE with PA, which was observed during the ET of composites of PTFE with Cu [11].

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Table 2

The characteristic temperatures and deformations of PCMs of PTFE + 30% PA after ampoule ET

Zone of pressing	Deformation (%) at different temperatures				ts, °C	tfl, °C
	300 °C	330 °C	360 °C	380 °C
Periphery	2.8	4.3	5.8	7.7	271	395
Centre	9.8	14.4	17.4	20.6	273	406

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Figure 5

The thermomechanical curves of PCMS of PTFE + 30% PA after ampoule ET: 1 – peripheral zone; 2 – central zone. Vertical axis: ɛ, % Horizontal axis: t, °C Key to fg.: 1. t_s; 2. t_fl

Conclusions