Preparation and application of superhydrophobic polytetrafluoroethylene materials and micro/nanoindentation

A method for preparing superhydrophobic polytetrafluoroethylene materials by micro- and nanoimprinting is discussed. Surfaces with superhydrophobic properties were prepared by designing and imprinting micro- and nano-structures on polytetrafluoroethylene materials. The experiments on weather resistance and durability revealed that the microstructure of screens of different mesh sizes was processed onto the surface of PTFE material by micro-nano thermal imprinting to make it hydrophobic and oleophobic, and retained the original excellent properties of corrosion resistance and low surface attachment, etc. The material processed by the new method has a wide range of application prospects in various fields.


Introduction
Polytetrafluoroethylene (PTFE), with a melting point of 327°C and a white waxy color, is a thermoplastic.At temperatures above its melting point, it will appear transparent.PTFE has a wide range of applications in many fields such as defense, energy, medical, construction, household goods, etc., and is of great research value.Due to its strong internal C-F atomic bonds, PTFE has a very high melt viscosity and is able to have microstructures processed on its surface by micro-nano thermal embossing.
Ordinary PTFE has poor weathering resistance, however, processing PTFE materials by the micronano thermal embossing process can make them hydrophobic while maintaining their original excellent properties.After this treatment, the material can still keep its original excellent performance unchanged.
Superhydrophobic materials can effectively reduce the corrosion efficiency of metals and prolong the service life of materials by virtue of their excellent anti-corrosion properties, thus reducing costs.Liu et al. prepared a superhydrophobic magnesium alloy surface by electrochemical process, which successfully protected the internal metal from corrosion.This superhydrophobicity also means that the material is somewhat oleophobic, and the rolling down of the droplets can take away some of the impurities, making the material extremely self-cleaning.Ma et al. prepared clothes made of copolymer fibers by means of electrospinning, which is not only comfortable and soft but also has a strong selfcleaning ability.Jia et al. prepared a superhydrophobic coating by spraying carbon black nanoparticles, PDMS, which was also used to protect the internal metal from corrosion, PFDTES mixture, using epoxy resin as binder.The coating achieved a product contact angle of 161° and a rolling angle of 1.4°, which extended the water freezing time on the surface of the material from 30 to 120 seconds.However, most of the current methods for preparing superhydrophobic surfaces use chemical methods, which are costly and have low production efficiency.Meanwhile, the weather resistance of the products when facing complex environments has yet to be strengthened.Therefore, how to prepare superhydrophobic materials with high weathering resistance in large quantities, quickly and at low cost is still a worthy research problem [1][2][3][4].

Superhydrophobic properties analysis
Superhydrophobic surfaces refer to material surfaces with a static contact angle θ e greater than 150° and a rolling angle α lower than 10°.The static contact angle θ e and rolling angle α are common parameters to describe the surface infiltration of materials.
Surface infiltration refers to the nature of a liquid replacing an otherwise incompatible gas or liquid on a solid surface to form a new and stable solid-liquid mixing interface.The contact Angle is the Angle formed by the tangent line of the gas-gas interface and the solid-liquid interface at the junction of the solid-liquid-gas interface.When the contact angle θ e is not less than 90°, it means that the liquid cannot infiltrate the solid surface, and when the contact angle θ e is less than 90°, the liquid can infiltrate the solid surface [5].
When the solid surface is an absolutely ideal surface, the relationship between the solid-liquid-gas three-phase interface tension and the contact angle θ e can be expressed by Young Equation, as shown in Formula (1): Where γ sv, γ sl, and γ 1v are the interfacial tension between solid-gas, solid-liquid, and liquid-gas, respectively.
As can be seen from the above formula, the lower the surface energy of the solid material is, the greater the contact angle of the droplet is.However, the material has a surface roughness, so the actual contact area is larger than the ideal contact area.The roughness factor can be introduced in the formula [6], as shown in Formula (2): Where w  is the contact angle of the droplet on the roughness surface; r is the roughness factor, the ratio of the actual surface area to the geometric projected area.
As can be seen from Formula (2), when the material is hydrophobic, θ w will increase as r increases, that is, when the hydrophilic surface increases its roughness, the hydrophobic capacity of the surface will be enhanced.
When the surface roughness of the material is large, the air forms an air cushion in the groove.When the Wenzel model fails, the surface contact state is converted to the Cassie model [7].In this model, the residual gas in the solid surface groove forms a new solid-liquid-gas composite interface with the droplet and the solid surface, as described in Formula (3): When the static contact angle is greater than 150°, the rolling angle is higher than 10°; if the static contact angle is greater than 150°, the 10° is lower.Because the surface tension of oil droplets is generally smaller than water, the superhydrophobic materials are generally oil-hydrophobic.In the face of the polluted environment, the self-cleaning ability is strong.Because of its excellent nature, it plays an important role in preventing corrosion, ice, pollution, resistance, and other aspects.PTFE materials prepared with micro and nanoscale structures by direct replication method and plasma treatment method have stronger anti-ice attachment properties than general hydrophobic materials [8].
At present, the main ways to prepare superhydrophobic surfaces are sol-gel, phase separation, anodic oxidation, chemical etching, self-assembly, and so on.These methods are basically chemical methods, which are difficult to use in industrial production due to the complex process and low production efficiency.

Experimental materials and sample preparation
The materials used in the experiments are mainly 304 stainless steel sieves of 300 mesh, 600 mesh, 800 mesh, 1200 mesh, and 1500 mesh; 0.2 mm, 0.3 mm, and 0.5 mm PTFE membranes; anhydrous ethanol.The correspondence between sieve mesh and aperture size is shown in Table 1 Through the analysis, it was found that on the 0.3 mm processed PTFE membrane, the water beads were full of ellipsoidal, slightly tilted and then rolled down, with better hydrophobic properties, and the material had a rough, uniform surface.Therefore, the thickness of 0.3 mm PTFE membrane was finally chosen as the experimental material.

Embodiment of hydrophobic properties and the effect of screen mesh size on contact angle
The higher the mesh number of the sieve mesh is, the smaller its pore size is, and the larger the surface roughness of the processed material is, which enhances the hydrophobic properties of the material.At the same time, changing the mesh size of the sieve mesh does not affect the diameter of the wire used to weave the sieve mesh, so the processed microgroove structure remains almost unchanged.However, as the aperture of the screen mesh decreases, the denseness of the grooves increases significantly, allowing more gas phase to be mixed into the solid-liquid phase, which in turn enhances the hydrophobic properties of the material.In summary, in order to obtain the best superhydrophobic PTFE material, a 1500 mesh screen should be selected for processing.

Influence of the temperature of heat preservation on the surface microstructure and hydrophobicity of the material
The contact angle increases and then decreases slowly with the increase of heating temperature.From the contact angle theory, the larger the contact angle is, the better the hydrophobicity of the measured material surface is.At the heating temperature of 180°C, the contact angle for the maximum value of 148°, when the hydrophobicity of the processed material is the best.The heating temperature and holding temperature should be selected as 180°.

Effect of holding pressure on the surface microstructure and hydrophobicity of the material
The experimentally selected heating temperature is the heat deformation temperature of PTFE.When the material has softened to the point where microstructures can be machined, appropriate pressure is required to fully and uniformly replicate the microstructures on the mold surface to the surface of the material to be processed.The maximum pressure that can be exerted by the laboratory machine is about ICAMIM-2023 Journal of Physics: Conference Series 2720 (2024) 012015 IOP Publishing doi:10.1088/1742-6596/2720/1/0120154 5 MPa, so 1 MPa, 2 MPa, 4 MPa, and 5 MPa were selected.A 1500 mesh screen was chosen, the heating temperature was 180°, and the holding time was 120 s [9,10].

Weatherability enhancement experiments
Before hot embossing, anhydrous ethanol mixed with silica microspheres is uniformly applied to the screen mesh as a mold, and after all the anhydrous ethanol has evaporated, the mold is placed on the PTFE sample to be processed for hot embossing.While the screen microstructure was processed on the surface of the material, the silica microspheres were transferred to the surface of the PTFE film with the aid of the screen.The temperature of the hot embossing was selected to be 180°C, the holding time was 120 s, the applied pressure was 5 MPa, and the screen mesh was selected to be 1500 mesh, 1200 mesh, and 300 mesh.After the hot embossing was completed, the mold was removed while it was still hot.A drop of water was applied to the surface of the product with a rubber-tipped burette and its morphology was observed.Then the contact angle was measured and the results are shown in Figure 1: (a) 300 mesh (b) 1200 mesh (c) 1500 mesh Figure 1.Contact angle of the sample after adding silica microspheres processing.According to the results of the contact angle measurement, it can be seen that after adding silica microspheres, the hydrophobicity of the samples processed through different screen mesh is improved: the contact angle of the samples processed after 300 mesh is 135°; the contact angle of the samples processed after 1200 mesh is 146°; the contact angle of the samples processed after 1500 mesh is 154°, which can satisfy the requirements of superhydrophobicity.Next, the 1500 mesh samples were selected for friction, folding, and static tests, and then their contact angles were measured.The results are shown in Figure 2:  As shown in Figure 2, the surface of the material has a groove-like microstructure processed with a screen as a mold.Attached to the surface of the groove structure are silica microspheres attached to the screen before processing, which are transferred to the surface of the material by thermal imprinting, and they form a new secondary microstructure with the screen indentation.The silica balls in the process of embossing will inevitably be processed on the surface of the material with more tiny indentations.These new secondary microstructures on the material hydrophobicity are critical to the improvement of the hydrophobicity of the material.At the same time after a long period of time after the static, the surface of the groove structure of the material is destroyed, but the secondary microstructures will not be altered or affected by the less.These microstructures contribute to the maintenance of the material's hydrophobicity, as shown in Figure 3.In the direct hot embossed product, after durability and long-lasting tests, the contact angle decreased from 148° to 115°, 109°, and 95°, respectively; and after adding silica microspheres and then after testing the contact angle, it decreased from 154° to 148°, 143°, and 134°, respectively.According to the contact angle measurements of the samples before and after rubbing, folding, and resting, compared with the contact angle measurements of the samples thermally embossed after the addition of silica microspheres before and after the durability and permanence tests, it can be found that the reduction of the contact angle of the samples decreased substantially.The hydrophobicity angle of the samples processed by adding silica microspheres is significantly larger than that of the samples processed by adding silica microspheres, indicating that the samples processed by adding silica microspheres are still able to maintain a better hydrophobicity after friction, folding, and resting, and the durability and endurance are significantly stronger than that of the products made by direct hot embossing.

Results and discussion
After weathering and durability tests, the hydrophobicity decreases dramatically.It was found by scanning electron microscopy that the microstructure of the groove surface of the sample after friction and folding appeared to be smoothed and filled, which caused the microstructure of the material surface to be destroyed, the roughness to be reduced, the proportion of meteorology at the solid-liquid interface to be reduced, and the hydrophobicity to be reduced.Therefore, on the basis of previous hot embossing, using anhydrous ethanol mixed with silica microspheres evenly coated on the screen.After drying this as a mold, under the conditions of a heating temperature of 180℃, holding pressure of 5 MPa and holding time of 120 s, 300 mesh, 1200 mesh, and 1500 mesh were used for hot embossing again.The contact angle measurements of the improved experimental results showed that the contact angle of all the samples was greatly improved, with the contact angle of the samples made through 1500 mesh reaching 154°, which meets the requirements for superhydrophobicity.The decrease in contact angle of these samples after durability and permanence tests was significantly lower than that of the samples processed without the addition of silica microspheres, indicating that the addition of silica microspheres greatly enhanced the weatherability and permanence of the processed materials.
(a) Contact angle after friction; (b) Contact angle after folding; (C) Contact angle after resting.

Figure 2 .
Figure 2. Contact angle measurement after durability and permanence test.As shown in Figure2, the surface of the material has a groove-like microstructure processed with a screen as a mold.Attached to the surface of the groove structure are silica microspheres attached to the screen before processing, which are transferred to the surface of the material by thermal imprinting, and they form a new secondary microstructure with the screen indentation.The silica balls in the process of embossing will inevitably be processed on the surface of the material with more tiny indentations.These new secondary microstructures on the material hydrophobicity are critical to the improvement of the

Figure 3 .
Figure 3.The surface of the screen with SiO 2 microspheres attached and PTFE film under electron microscope.In the direct hot embossed product, after durability and long-lasting tests, the contact angle decreased from 148° to 115°, 109°, and 95°, respectively; and after adding silica microspheres and then after testing the contact angle, it decreased from 154° to 148°, 143°, and 134°, respectively.According to the contact angle measurements of the samples before and after rubbing, folding, and resting, compared with the contact angle measurements of the samples thermally embossed after the addition of silica microspheres before and after the durability and permanence tests, it can be found that the reduction of the contact angle of the samples decreased substantially.The hydrophobicity angle of the samples processed by adding silica microspheres is significantly larger than that of the samples processed by adding silica microspheres, indicating that the samples processed by adding silica microspheres are still able to maintain a better hydrophobicity after friction, folding, and resting, and the durability and endurance are significantly stronger than that of the products made by direct hot embossing.

Table 1 .
: Correlation between mesh number and pore size