Beyond classical theory: Predicting the free energy barrier of bubble nucleation in polymer foaming

Chemistry, Processing, Properties, and Applications of Rubber Foams

With the ever-increasing development in science and technology, as well as social awareness, more requirements are imposed on the production and property of all materials, especially polymeric foams. In particular, rubber foams, compared to thermoplastic foams in general, have higher flexibility, resistance to abrasion, energy absorption capabilities, strength-to-weight ratio and tensile strength leading to their widespread use in several applications such as thermal insulation, energy absorption, pressure sensors, absorbents, etc. To control the rubber foams microstructure leading to excellent physical and mechanical properties, two types of parameters play important roles. The first category is related to formulation including the rubber (type and grade), as well as the type and content of accelerators, fillers, and foaming agents. The second category is associated to processing parameters such as the processing method (injection, extrusion, compression, etc.), as well as different conditions related to foaming (temperature, pressure and number of stage) and curing (temperature, time and precuring time). This review presents the different parameters involved and discusses their effect on the morphological, physical, and mechanical properties of rubber foams. Although several studies have been published on rubber foams, very few papers reviewed the subject and compared the results available. In this review, the most recent works on rubber foams have been collected to provide a general overview on different types of rubber foams from their preparation to their final application. Detailed information on formulation, curing and foaming chemistry, production methods, morphology, properties, and applications is presented and discussed.

Insight into the Influence of Properties of Poly(Ethylene-co-octene) with Different Chain Structures on Their Cell Morphology and Dimensional Stability Foamed by Supercritical CO2

Poly(ethylene-co-octene) (POE) elastomers with different copolymer compositions and molecular weight exhibit quite distinctive foaming behaviors and dimensional stability using supercritical carbon dioxide (CO₂) as a blowing agent. As the octene content decreases from 16.54% to 4.48% with constant melting index of 1, both the melting point and crystallinity of POE increase, due to the increase in fraction of ethylene homo-polymerization segment. the foaming window of POE moves to a narrow higher temperature zone from 20–50 °C to 90–110 °C under 11 Mpa CO₂ pressure, and CO₂ solubility as well as CO₂ desorption rate decrease, so that the average cell diameter becomes larger. POE foams with higher octene content have more serious shrinkage problem due to lower compression modulus, weaker crystal structure and higher CO₂ permeability. As POE molecular weight increases at similar octene content, there is little effect on crystallization and CO₂ diffusion behavior, the foaming window becomes wider and cell density increases, mainly owing to higher polymer melt strength, the volume shrinkage ratio of their foams is less than 20% because of similar higher polymer modulus. In addition, when the initiate expansion ratio is over 17 times, POE foams with longer and thinner cell wall structures are more prone to shrinkage and recovery during aging process, due to more bending deformation and less compression deformation.

Nucleation stage of multicomponent bubbles of gases dissolved in a decompressed liquid

A new kinetic analysis of degassing and swelling of a decompressed liquid solution with several dissolved gases has been presented. The analysis has been performed for the nucleation stage of formation and growth of supercritical gas bubbles in a closed system under conditions of a limited availability of the dissolved species. The nucleation stage is an important stage of degassing, on which a certain size distribution of gas bubbles is formed, being the starting point for further growth. This stage starts with the appearance of supercritical gas bubbles and is widely completed when the nucleation rate of supercritical gas bubbles diminishes by a decimal order. Neglecting the role of the Laplace pressure in large supercritical bubbles, we were able to introduce the concept of total gas supersaturation and to develop a theory of this stage for liquid solutions with arbitrary number and any values of supersaturations and solubilities of the dissolved gases. First, we have considered slowly growing bubbles within the mean-field approach assuming a stationary diffusion of gases to bubbles at moderate total gas supersaturation. In the case of large total gas supersaturation, we have built a description of fast growing bubbles on the basis of the extended excluded volume approach with nonstationary nonuniform diffusion shells around the bubbles and mean-field mixing of the concentration of gases at the external boundaries of the shells. A main novel feature of the developed theory is its ability to predict the kinetic behavior of the whole ensemble of bubbles with different sizes under changes in the initial gas composition in the liquid solution at its fast decompression. It has been shown that the effects of nonstationary diffusion may be very significant in the growth of multicomponent bubbles and, in particular, are responsible for a significant swelling of a decompressed liquid solution. Distribution of supercritical bubbles in sizes as a function of concentrations of solute gases at any moment of the nucleation stage, the duration of the nucleation stage, and the swelling ratio at the end of the nucleation stage have been determined.

The mechanism of roughness-induced CO2 microbubble nucleation in polypropylene foaming

Within the framework of classical density functional theory, the thermodynamic driving forces for CO₂ microbubble nucleation have been quantitatively evaluated in the foaming of polypropylene containing amorphous and crystalline structures. After the addition of fluorinated polyhedral oligomeric silsesquioxane particles into the polypropylene matrix, we construct different composite surfaces with nanoscale roughness for bubble nucleation. Meanwhile, as the dissolved CO₂ molecules increase, the corresponding CO₂/PP binary melts can be formulated in the systems. Due to the roughness effect coupled with the weak interactions of particle-PP, PP chains in the binary melts are depleted from the surfaces, leading to a significant enhancement of osmotic pressure in depletion regions. During the foaming process, a large number of dissolved CO₂ molecules are squeezed into the regions, thus local supersaturations are dramatically improved, and the energy barriers for bubble nucleation are dramatically reduced. Moreover, when the nanocomposite surfaces display ordered nanoscale patterns, the energy barriers can be further reduced to their respective minimum values, and the bubble number densities reach their maximum. Accordingly, the bubble number densities can be enhanced by 4 or 5 orders of magnitude for bubbles nucleated on the crystalline or amorphous PP nanocomposite surface. In contrast, when the foaming pressure is increased from 15 to 20 MPa, the elevated bubble number density in the foaming PP matrix is less than one order of magnitude. As a result, the enhancement of local supersaturation induced by the controlled nanoscale roughness is much more effective than that of bulk supersaturation given by high pressure.

Stabilization and Antifouling of Polymer Films on a Planar Surface by CO2 Pressurization

In this article, we study the dewetting phenomenon of a polymer and carbon dioxide (CO₂) mixture on a planar surface by combining density functional theory and the string method. It is found that dewetting is a first-order discontinuous phase transition. When the pressure is lower than the completely dewetting pressure (P_d), CO₂ stabilizes the polymer films. The density fluctuation of the polymer decreases significantly with the inclusion of CO₂. When the pressure is above P_d, the polymer film is depleted far away from the surface, leaving a thick layer of pure CO₂ in the region near the surface. P_d is proportional to the surface energy strength. The CO₂ molecules enhance the density fluctuation of the polymer during the dewetting process. The polymer-rich phase at the triple point dewets to a CO₂-rich vapor film, as the CO₂-rich liquid film near the surface is metastable. These results have promising application in the industry of fabricating polymer films and antifouling polymers on attractive surfaces.

Numerical Selection of the Parameters in Producing Microcellular Polymethyl Methacrylate with Supercritical CO2

A systematized calculation of foaming parameters in producing microcellular polymethyl methacrylate (PMMA) with supercritical carbon dioxide (SCCO2) was proposed. The calculated optimal results were saturation temperature = 313 K, pressure = 25 MPa and time = 15 h, which were achieved by the Chow model, classical nucleation theory and Fick diffusion law, respectively. Additionally, the Center-Composite Design (CCD) statistical approach in Response Surface Method (RSM) was employed to evaluate the validity of the calculated parameters. The error between the optimal parameters predicted by CCD statistical approach and the calculated ones was acceptable, demonstrating the accuracy of the calculated parameters.

Insight into the adsorption mechanism of benzene in HY zeolites: the effect of loading

An interesting two-stage adsorption mechanism, defined as “ideal adsorption” and “insertion adsorption”, was first proposed for the benzene/HY system by Metropolic Monte Carlo simulations at loadings below and above an “inflection point”.