Effect of processing conditions on the cellular morphology of polyethylene hollow fiber foams for membrane applications

Abstract

A continuous method without any solvent is proposed to produce porous hollow fibers for membrane (HFM) applications. In this case, linear low-density polyethylene was combined with azodicarbonamide to produce samples via extrusion. In particular, the processing (chemical blowing agent content and temperature profile) and post-processing (stretching velocity) conditions were optimized to obtain a cellular structure having a high cell density and uniform cell size distribution. From the samples obtained, a complete set of characterization was performed (morphological, mechanical, physical, and gas transport). The results show that HFM having a higher cell density can increase gas permeability, especially for hydrogen. Overall, it is shown that low-cost polyolefins having a suitable cellular structure can be used for gas separation membranes.

Hollow Fiber Porous Nanocomposite Membranes Produced via Continuous Extrusion: Morphology and Gas Transport Properties

“…A large number of investigations focused on membrane production to improve the gas transport properties based on material selection, especially for commercial-scale applications. However, very few studies focused on the control of the morphological structure in relation with the membranes’ performances [5,6,7].…”

“…The use of leachable particles such as salts was shown to produce an open-cell structure based on polystyrene [9], polypropylene [10], and polyethylene [11]. Other solvent-free methods are stretching with or without particles [12] and direct foaming of semi-crystalline thermoplastics such as high-density polyethylene (HDPE) and polypropylene (PP) [7,13]. For stretching methods, the porosity is created by delamination between the matrix and local stress concentration points such as solid particles or crystalline zones, but thermal post-treatment is needed to stabilize the newly created microporous and crystalline structure to avoid shrinkage and warpage, which are the main drawbacks of such methods.…”

“…Thus, extrusion is very effective, fast, and economic to produce low-density foams [19,20]. The cellular structure can be produced using either chemical or physical blowing agents (PBAs), but chemical blowing agents (CBAs) are generally easier to handle with standard equipment compared to physical blowing agents [7,21].…”

“…The authors’ previous works showed how the morphological properties, such as cell size uniformity as well as cell density, can control the physical, mechanical and gas transport properties of hollow fiber polymer membranes [5,6,7,13]. Since these properties are tunable, this led to membrane performance enhancement via simultaneous improvement of both the surface area as well as gas transport properties [7,22]. Then, a melt processing method was optimized to produce hollow fiber foamed membranes (HFM) based on a microcellular blend of linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) to get a balance between foamability and mechanical strength.…”

“…), and post-processing (stretching) conditions were the main parameters to control the porous membrane structure. Direct foaming is interesting because it is solvent free, continuous, and fast to produce a porous structure in hollow fibers [5,7,13]. This configuration is the most interesting because of its higher active surface area per unit volume compared with flat membranes.…”