The influence of the pore structure of PVDF foam sheet on its mechanical and functional properties

Polyvinylidene fluoride (PVDF) is a thermoplastic fluoroplastic with excellent chemical stability, weather resistance and mechanical properties. PVDF foam sheet prepared by foaming process has broad application prospects in aerospace, new energy, environmental protection and other fields due to its unique porous structure. As an important feature of PVDF foam sheet, pore structure plays a decisive role in its mechanical and functional properties. In-depth study of the relationship between pore structure and material properties will help to achieve precise control of PVDF foam sheet performance and meet the needs of different application scenarios.

1. Pore structure parameters of PVDF foam sheet
Porosity
Porosity refers to the percentage of pore volume in PVDF foam sheet to the total volume, which directly reflects the density of the material. The size of the porosity mainly depends on the foaming process conditions, such as the type and dosage of the foaming agent, the foaming temperature and pressure. A higher porosity means that the material contains more gas, which reduces the material density and weight.

Pore ​​size
Pore size is another important parameter to describe the pore structure of PVDF foam sheets. Depending on the pore size, the pores can be divided into micropores (pore size less than 2nm), mesopores (pore size between 2-50nm) and macropores (pore size greater than 50nm). In PVDF foam sheets, the pore size is usually at the micron level, and its distribution range has a significant impact on the material properties. Uniform pore size distribution is conducive to improving the consistency of material properties, while a wide pore size distribution may lead to non-uniform material properties.

Pore connectivity
Pore connectivity refers to the degree of interconnection between pores in PVDF foam sheets. The connected pore structure is conducive to the transmission of gas or liquid inside the material, while the closed pore structure is more conducive to the performance of the material such as heat insulation and sound insulation. Pore connectivity is mainly determined by the bubble growth and merging behavior during the foaming process, and is closely related to factors such as the decomposition rate of the foaming agent and the melt viscosity.

2. Effect of pore structure on mechanical properties
Compression strength
The compression strength of PVDF foam sheet is closely related to porosity. As the porosity increases, the load-bearing structure inside the material decreases, resulting in a decrease in compression strength. This is because the presence of pores makes the material more likely to deform and collapse during compression. At the same time, the pore size also affects the compression strength. Smaller pores can provide more uniform stress distribution, which helps to improve the compression strength of the material; while larger pores are more likely to become stress concentration points, reducing the compression strength of the material. In addition, pore connectivity also has a certain effect on compression strength. In the compression process of the connected pore structure, gas is easy to escape, making the material more likely to deform, thereby reducing the compression strength; while the closed pore structure can better resist compression deformation, which is conducive to maintaining a higher compression strength.

Bending strength
Bending strength reflects the ability of PVDF foam sheet to resist bending deformation. Similar to compression strength, an increase in porosity will lead to a decrease in bending strength. During bending, the outer layer of the material is subjected to tensile stress, the inner layer is subjected to compressive stress, and the presence of pores weakens the overall load-bearing capacity of the material. Pore size and pore connectivity also affect bending strength. Smaller and evenly distributed pores can effectively disperse bending stress and improve the bending strength of the material; while the interconnected pore structure may make the material more prone to interlayer separation during bending and reduce the bending strength.

Elastic modulus
The elastic modulus is an indicator of the material's ability to resist elastic deformation. The elastic modulus of PVDF foam sheet decreases with the increase of porosity. This is because the existence of pores reduces the effective load-bearing phase inside the material and makes the material more prone to elastic deformation. The effect of pore size on elastic modulus is relatively complex. Generally speaking, smaller pores help to improve the elastic modulus of the material because smaller pores can better limit the deformation inside the material; and the effect of pore connectivity on elastic modulus is mainly reflected in the effect of gas on material deformation. The gas in the interconnected pore structure will have a certain buffering effect when the material is deformed, resulting in a decrease in elastic modulus.

3. Effect of pore structure on functional properties
Adsorption performance
The pore structure of PVDF foam sheet has an important influence on its adsorption performance. Larger porosity and rich pore structure provide a larger specific surface area, which is conducive to the adsorption of more substances. In addition, the matching degree between pore size and the size of adsorbate molecules is also crucial. When the pore size is close to the molecular size of the adsorbate, a strong adsorption effect will occur, that is, capillary condensation. For PVDF foam sheets with connected pore structures, gas or liquid can diffuse into the material more easily, thereby improving the adsorption efficiency; while closed pore structures may limit the entry of adsorbates and reduce adsorption performance.

Thermal insulation performance
Thermal insulation performance is one of the important functional characteristics of PVDF foam sheets. The pore structure plays a key role in the thermal insulation process. The closed pore structure can effectively prevent the conduction of heat because the thermal conductivity of gas is much lower than that of solid materials. Therefore, higher porosity and good pore closure help improve the thermal insulation performance of PVDF foam sheets. The pore size also has a certain effect on the thermal insulation performance. Smaller pores can reduce the convective heat transfer of gas and further improve the thermal insulation effect. However, when the pore size is too small, it may cause the specific surface area of ​​the material to be too large, increase the heat conduction of the solid part, but it is not conducive to thermal insulation.

Acoustic performance
The pore structure of PVDF foam sheets determines its acoustic performance. Materials with higher porosity have better sound absorption performance because the pores can provide more space for sound waves to propagate inside the material, increase the interaction between sound waves and materials, and thus consume more sound energy. The pore size and pore connectivity affect the propagation path and energy loss of sound waves inside the material. The connected pore structure is conducive to the in-depth propagation and energy dissipation of sound waves, and the appropriate pore size can resonate with sound waves of different frequencies, further improving the sound absorption effect.

The pore structure of PVDF foam sheet, including parameters such as porosity, pore size and pore connectivity, has a significant impact on its mechanical and functional properties. In terms of mechanical properties, an increase in porosity usually reduces the compressive strength, flexural strength and elastic modulus of the material; pore size and pore connectivity also play an important role in mechanical properties by affecting stress distribution and material deformation behavior. In terms of functional properties, the pore structure determines the adsorption, thermal insulation and acoustic properties of the material. A suitable pore structure can improve the performance of the material in adsorption, thermal insulation and sound absorption. Therefore, in practical applications, the pore structure of PVDF foam sheets can be optimized by regulating the foaming process to meet the requirements of material mechanics and functional properties in different fields. In the future, as the research on PVDF foam sheets continues to deepen, the relationship between its pore structure and performance will be more accurately revealed, providing stronger support for the innovative design and application expansion of materials.