Paper TF-TuM10
Modification of Nonwoven Polymer Fiber Structures by Atomic Layer Deposition

Tuesday, October 21, 2008, 11:00 am, Room 302

Nonwoven polymer fiber structures are receiving much recent attention due to their potential industrial, medical, and military applications. We report the modification of nonwoven fiber structures by the deposition of thin metal oxide films via ALD. Fiber substrates coated to date include melt blown polypropylene micro and nanofibers, electrospun PVC nanofibers and nylon hydroentangled microfibers. Our specific focus is to characterize the effect of film thickness and/or coating material the resulting mechanical properties of coated fiber structures Initial experiments involved the deposition of Al2O3 by trimethylaluminum and water at temperatures from 50 to 100C. The gas pulse and purge times used resulted in film growth rates of 1.2A/cycle on planar silicon independent of temperature. Substrates for ALD coating consisted of fiber mats approximately 1cm x 3cm x 0.05cm. After ALD, some samples were calcined in air at ~850°C for 30 min to remove the polymer substrate to allow SEM analysis of the deposited oxide layer. SEM images show good macroscopic uniformity of the ALD coating over the substrate surface area. Other samples that were not heat treated were analyzed under tensile testing to observe the stress-strain behavior as a function of ALD process conditions and film thickness. Specifically, melt-blown polypropylene nonwoven fiber mats were tested using a microstrain analyzer to determine elastic modulus, yield stress and material toughness. Coating the polypropylene with Al2O3 ALD at 50°C or 100°C lead to an increase in the elastic modulus that scaled linearly with the number of ALD cycles, resulting in an increase of ~50% after 200 ALD cycles (~25nm). In addition, when the ALD was performed at 50C, the samples showed an increase in yield stresses with some improvement in material toughness. However, when processed at 100C, the coated polypropylene structures showed a decrease in yield stress and toughness as compared to untreated substrates, consistent with some degradation of the starting material properties at elevated temperature. Strengthening of the fiber structure by the deposition of a thin metal-oxide film may allow coated nonwovens to be used in applications where the traditional low-strength of nonwoven structures prevents the required level of performance.