3.1 Effect of ball milling time on the size of NaCl particles
The pore size of the finally obtained PTFE foam is directly dependent on the size of NaCl particles, due to using NaCl as the pore-forming agent in this experiment. And NaCl particles size is related to the ball milling time. The longer milling time leads to smaller particles. Therefore, it is necessary to investigate the effect of ball milling duration on the size of NaCl particles firstly. As shown in Fig. 2(a), when the ball milling time is 0 min (without milling), the raw NaCl (belongs to cubic crystal system) crystal exhibits a cubic structure due to its self-limitation property. The mean size of its crystal grains is 796.04 ± 296.87 µm. As shown in Fig. 2(b), when milled for 30 min, most of the NaCl grains are crushed becoming smaller ones, with irregular structures and uniform dispersion. The mean particle size is 33.22 ± 13.60 µm. Only a small number of NaCl grains are not completely crushed.
As shown in Fig. 2(c), when milled for 60 min, all the NaCl grains are thoroughly crushed into irregular shape, without noticeable larger particles. The mean size of grains significantly decreases to 16.45 ± 5.03 µm. It is found that the fine grains tend to agglomerate. It may be related to the increase in the electrostatic attraction of double electric layer on the surface, accompanied by the considerable increase of the specific surface area, when NaCl grains reach microscale level. The results reveal that as the ball milling duration increases, the particle size gradually decreases. And when milling time reaches 60 min, the microscale particle size distribution approaches a normal distribution, with a more uniform distribution. Thus, 60-min-milled NaCl material is used for subsequent preparation of PTFE foam.
3.2 Effect of PTFE content of raw composites on the foam structure
The structure of obtained foam is mainly dependent on the content of PTFE in the raw composites, which determines the mechanical and thermal property of PTFE foam as well as natures of hydrophobicity and lipophilicity. To explore the underlying relationship, the structures and kinetic process in the sintering process (at 340°C for 4 h) of PTFE foam obtained from composites with 50 wt%, 40 wt% and 30 wt% PTFE are discussed. Considering that the melting point of PTFE is 327°C and the temperature of decomposition is 370°C without liquid transition, the sintering temperature of 340°C is sufficient for the PTFE particles to be sintered into a monolithic bulk material. The morphology of NaCl particles shows few changes during the sintering process[13]. SEM images of normal and cross section of three samples are shown in Fig. 3.
For the foam sample made from composite with 50 wt% PTFE, PTFE particles express intense diffusion and migration behaviors at high temperature during the sintering process, which leads to aggregation of PTFE particles and forming of grain-like structures with few pores, leaving porous structure with larger pores in the surrounding regions. This kinetic process can be confirmed by the morphology, the black zone (representing grain-like structures) surrounded by the bright zone (representing porous structure), as shown in Fig. 3(a) and (d). The volume percentage of grain-like structures (pV) is estimated as 11.01%. For the foam sample made from composite with 40 wt% PTFE, the separations among PTFE particles are greatly increased due to the increment of NaCl particles, which makes PTFE particles disperse more sparsely. As a result, the migration and diffusion behaviors of PTFE particles suffer from massive inhibition due to the improved resistance. The size and volume proportion of grain-like structures become rapidly depressed, which reaches 180.12 µm and 6.66%, respectively, as shown in Fig. 3(b) and (e). The size and distribution of pores in the foam structure become more uniform. Compared with the foam sample developed from composite with 50 wt% PTFE, there are few obvious grain-like structures. For the foam sample made from composite with 30 wt% PTFE, the well dispersed foam structure with uniformly distributed pores occupies the main body of foam material, as shown in Fig. 3(c) and (f). Under pore-scale investigation, the mean size of individual pore is 16.48 µm, which corresponds to the particle size of NaCl porous agent (Fig. 2c). It reveals that NaCl particles only play a role of forming pores but not changing structure of solid PTFE. In addition, there are pore-connecting structures which even tend to merge into larger pores, resulting from the diffusion and migration of PTFE particles.
3.3 Effect of PTFE content on the tensile strength and thermal conductivity of PTFE foam
In general, there are some inevitable weak zones in pore structures, unavoidable leading to deterioration in mechanical property of foam materials, different from that of dense material with stable structures and mechanical property. Since most non-metal foams materials are used as functional materials, the mechanical property is not the major focus. However, actually if the foam material is too infirm, it is easy to break being used, leading to greatly reduction of service life as well as increase of removing difficulty. In order to evaluate the mechanical property of our prepared PTFE foams, the maximum tensile stress (σb) and tensile strain (ε) of samples are tested.
The derived stress-strain curves of PTFE foam materials are shown in Fig. 4. The value of mechanical parameters is listed in Table 1. It is revealed that for foam sample formed from composite with 50 wt% PTFE, the tensile strength (σb) is about 2 MPa, with an apparent density (ρa) of 1.04 g/cm3. For the one from composite with 40 wt% PTFE, σb is about 0.6 MPa, with ρa of 0.847 g/cm3. For the one from composite with 30 wt% PTFE, σb is about 0.2 MPa, with ρa of 0.683 g/cm3. The results indicate that the tensile strength of PTFE foam decreases rapidly with the decrease of material density, but their relationship is nonlinear. It is known that the tensile strength of dense PTFE is roughly between 25 ~ 35 MPa[14, 15]. The tensile strength of PTFE foam is 2 MPa, less than a tenth of that of dense PTFE, although the density only drops to half density of the dense material[14]. It is because that the pore structures in the foam material actually act as cracks, which easily induce break of material. However, compared with the current commercial foam plastics, such as PVC foam board and polyethylene foam board with tensile strength more than 0.15 MP[16, 17], our prepared PTFE foams still exhibit a better mechanical property, even for the one formed from mixtures with PTFE as low as 30 wt%. It is owing to the strong chemical bonds existing in PTFE material.
Table 1
Performance parameters of PTFE foam materials
pm PTFE | pV (%) | ρa (g/cm3) | LM (N) | σb (MPa) | ε (%) | θ (°) | λa (W/m·k) | s (MJ/m3·K) | a (mm2/s) |
50% | 11.01 | 1.04 | 321.87 | 2 | 56.91 | 133 | 0.1186 | 0.91230 | 0.13 |
40% | 6.66 | 0.847 | 100.56 | 0.6 | 25.68 | 131 | 0.0884 | 0.58900 | 0.15 |
30% | 5.21 | 0.683 | 39.64 | 0.2 | 21.18 | 137 | 0.0690 | 0.40580 | 0.17 |
One of the main applications of PTFE foam is used as thermal insulation material. As shown in Fig. 5, the apparent thermal conductivity (λa) of PTFE foam decreases with the decrease of mass percentage of PTFE in the raw composite. This is because that the thermal conductivity is related with the porosity of foam material. More pores mean more spaces in foam structures occupied by air, the thermal conductivity of which is lower than that of PTFE rigid skeleton. Thus, λa of PTFE foam manifests a positive correlation with the PTFE content and apparent density. Similarly, the volumetric specific heat of PTFE foam (s) is also positively related to the PTFE mass percentage and apparent density, due to the lower specific heat of air compared to the one of solid PTFE. The thermal conductivity of bulk PTFE foam could be further depressed via minishing pore size and raising porosity. In comparison with the heat conductivity of conventional thermal insulation materials (polyurethane foam of 0.02 W/m·K[18], rock wool board of 0.040 W/m·K[19], ceramic foam plate of 0.10 W/m·K[20]), the thermal insulation performance of PTFE is moderate. Considering the quite wide working temperature range (-180 ~ 260 ºC[21]) of PTFE, PTFE foam material is advantageous in ability of maintaining stable mechanical flexibility and strength with harmless to human body when employed in harsh environment. It is a potential thermal insulation material for spacesuit production in space environment.
3.4 Excellent hydrophobic and lipophilic properties of PTFE foams
Another major application of PTFE foam materials is in oil-water separation. Dense PTFE is inherently highly hydrophobic, with water contact angle of 110° [22], widely used as non-stick coating and lubricating material. The hydrophobicity of PTFE can be improved by increasing the surface roughness when made into porous structure. The average measured contact angle (θ) of foam samples is listed in Table 1. θ is about 133°, 131° and 137°, corresponding to samples from mixture with 50 wt%, 40 wt%, 30 wt% PTFE, respectively. The measured value shows minute changes for multi detect sites on the sample surface, which may be related to the roughness of detected location, as shown in Fig. 6. All the value of three samples is lower than that of superhydrophobic case (> 150° [23, 24]). However, the contact angle exhibits a little increase when the content of PTFE decreases, which is due to that the surface becomes rougher with the growing of quantity of pore structures.
The size of PFTE and NaCl particles after milling in this experiment is just at micro scale, which can be continually reduced, providing opportunity for enhancing the hydrophobicity of PTFE foam. Considering the comprehensive cost and cost performance, our proposed bulk PTFE foam can satisfy the actual hydrophobic needs, comparable to the using effect of some superhydrophobic materials[25].
One of the most attractive features of PTFE foam is that it is able to be quite hydrophobic and lipophilic simultaneously. To study the lipophilic property of PTFE foam material, an oil absorption experiment is carried out. 5 mL of water (stained sky blue by being mixed with copper sulfate) and 5 mL of oily CCl4 (light yellow) are mixed (see Fig. 7a). The mixed solution is added drop by drop to the foam sample from composite with 30 wt% PTFE, placed obliquely in a glass dish (see Fig. 7b). It is experimentally investigated that, during the oil absorption experiment, water rolls off naturally on the surface of PTFE foam, while CCl4 solution is absorbed into the pores of foam sample. The volume of collected liquid water in the residual solution approaches 5 mL (Fig. 7c).
The separation efficiency of foam sample is estimated as more than 90%. The strong hydrophobic and lipophilic property of bulk PTFE foam is greatly potential in oil/water separation application. As is known, water resource is facing serious pollution, with human exploitation of the ocean and lakes. The most urgent of these is the various oil pollution generated by leaking offshore drilling rigs, surface vehicles and human discharges to the ocean, which may cause serious ecological disasters and habitat damage. Beneficial from the unique chemical inertness (resistant to various acidic and alkaline environments) and long service life (aging life up to 15 years[3]), bulk PTFE foam shows huge application potentials in the field of removal of oils from water surface.