Advanced Materials for Additive Manufacturing: Architected Polymer Foams via Direct Bubble Writing

The polymer foam material refers to a microporous material based on a polymer (plastic, rubber, elastomer, or natural polymer material) with numerous bubbles inside, and can also be regarded as a composite material using gas as a filler. At present, polymer foam materials are mainly prepared by directly expanding and foaming the resin with a foaming agent. How to accurately control the micro-cell shape, the appearance and macro-foam structure are still a difficult problem.

In order to solve some problems in the process of preparing polymer foam by direct expansion foaming method, Professor Jennifer A. Lewis and co-workers describe the fabrication of architected polymer foams by "direct bubble writing". In this process, bubbles are ejected into the air, deposited onto a substrate, and then photopolymerized with UV light, and open-and-closed-cell foams with locally graded densities can be printed into 3D objects such as 3D lattices, shells, and out-of-plane pillars. 

The researchers designed a special nozzle with a double-layer structure inside and outside: the inner tube deposits a polymer precursor solution containing polymer monomers, initiators and surfactants, the outer tube supplies gas, and the two are mixed at the nozzle mouth to form a writeable bubble "ink". A single bubble drops from the nozzle onto the substrate to accumulate. After the polymerization is initiated by ultraviolet light, the bubbles are bonded to form a macro-foam block. According to the theoretical and experimental results, the ink dropping flow rate and gas pressure are the key parameters for preparing different microscopic cell morphologies.

Compared with traditional foam preparation methods, direct writing has several advantages. The most important benefit is the controlled cell morphology and distribution. Precisely formulating cell morphology and preparing foam with excellent performance is the biggest innovation of the direct writing foaming method. The cell morphology mainly includes three elements: open/closed cell structure, pore size, and distribution. The researcher controls the structure of the open and closed cells by the type of gas: when using oxygen with polymerization inhibition as a gas, it can stop or delay the polymerization of monomers that contact the oxygen surface and have a depth of about 40µm, so that the cell wall becomes thin and becomes an open-cell foam. When inert nitrogen is used as the gas source, polymerization normally occurs, and the cell walls are thick, forming closed-cell foam. The pore size and distribution are adjusted by air pressure. The cell diameter obtained at lower air pressure is approximately 0.5 mm, and the distribution is very uniform; when the air pressure is increased, the obtained cell pore diameter is between 0.3-0.7 mm, and the distribution is also non-uniform.

Open and closed-cell foam preparation process

Another advantage is the preparation of gradient foam with variable mechanical properties. During the continuous 3D writing process, the process parameters are adjusted intermittently to change the density and modulus of the local foam, so that the resulting foam exhibits a gradient in shape, thickness, and distribution. Gradient Foams with variable mechanical properties are difficult to produce by conventional foaming methods.

The elastic foam materials are mainly used for the production of protective helmets, personalized orthoses, high-performance shoes, and other various application products.

This research results have shown that the direct writing method can accurately control the micro-cell morphology and macro-foam structure, program the mechanical properties of the foam, and can also easily give new functions to the foam. 3D printing is capable to make uniform and gradient foams, which will revolutionize the foam industry and produce products that are more comfortable, safer, lighter and can be personalized.

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