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Institute
Polymer materials that regulate the relative humidity in their environment are relevant for applications in the packaging and building sectors. By integration of salts in polymer structures, such materials are able to absorb and desorb high amounts of water vapor. In this study, films of polylactic acid and polypropylene with dispersed calcium chloride (2 and 4 wt %) were produced and biaxially stretched to induce the formation of cavities. The resulting cavities in these films account up to 10 vol % and are able to contain emerging calcium chloride solution formed by water vapor absorption. These films absorb reversibly up to 15 wt % water vapor at 75% relative humidity at 23 °C. This absorption behavior is described by effective diffusion and effective sorption coefficients. Using a simple model, the effective water vapor diffusion coefficient of these films can be estimated from the permeation coefficient of the polymer and the sorption coefficient of the absorber.
This study aims to investigate the effect of the material thickness and build orientation on the mass transfer of low molecular weight substances through polyamide 12 (PA12) structures produced by laser sintering (LS).
Design/methodology/approach
Disc-shaped PA12 sheets having a nominal thickness ranging from 700 to 2,000 µm were built in horizontal, vertical and diagonal orientations and their permeation properties to oxygen and water vapor were measured. The structural properties of the sheets were examined by X-ray micro-computed tomography, differential scanning calorimetry and polarized light microscopy.
Findings
All the LS sheets that were investigated had water vapor and oxygen permeation coefficients that are in the range of those of PA12 produced by traditional manufacturing technologies. Despite significant differences in the porosity characteristics, the permeation properties of sheets built in different orientations were similar. The pores seem to have no measurable effect on the mass transfer rates in the sheets, and the transport processes seem to predominantly follow the rules of a regular solution-diffusion mechanism. The results showed a non-significant trend toward thickness-dependent permeation coefficients, which agrees with the observed differences in the crystal structures of the sheets.
Practical implications
The results are an important basis for the qualification of LS technology for direct manufacturing in applications requiring special barrier performance.
Originality/value
This study provides new information on mechanisms of mass transport through LS PA12 and the effect of the material thickness and build orientation. Furthermore, the results enhance understanding of the structural properties of thin polymeric sheets produced by LS.
Porosity distribution in laser-sintered polymeric thin sheets as revealed by X-ray micro tomography
(2019)
Laser Sintering (LS) is an additive manufacturing technology that is progressively used to manufacture functional products for end-use applications. However, LS technology provides limited ability to produce thin-walled structures that feature high material quality. A key parameter determining the quality of LS polymeric parts is porosity. This article presents a new approach towards clarifying the porosity characteristics of thin-walled structures produced by LS. Therefore, thin sheets of different thickness built in different orientations within the building chamber are examined by means of X-ray micro tomography. The porosity analysis includes the overall porosity, pore number density, pore size distribution and porosity distribution along the thickness of the thin sheets. The results show that the porosity characteristics strongly depend on the thickness and the build orientation of the thin sheets. This investigation helps to estimate the quality of three dimensional hollow bodies produced by LS and moreover, contributes to a better understanding of the pore formation in LS processes.
Laser sintering of polymers is widely used for the production of individual products and small-batch series. However, the qualification of laser-sintered polymeric components for new application fields, e.g. in the food and packaging industry, is still limited due to missing knowledge on physicochemical material properties. This work investigates the mass transfer of low molecular weight substances through laser-sintered polyamide 12 sheets in comparison to an extruded polyamide 12 film. Analysis of structural material properties reveals significant differences between both materials depending on the production processes. Despite their apparent porosity, laser-sintered sheets show lower permeation coefficients for water vapor and oxygen compared to extruded films. This might be related to the higher crystallinity of the laser-sintered vs. the extruded material, arising from the slow cooling rate of the polymer in the laser sintering process.
This research brings knowledge of the different permeation behavior of laser-sintered and extruded polyamide 12 in relation to the structural properties.
The laser sintering of polymers is an additive manufacturing technology that is becoming increasingly established in the industrial environment. This study investigated the thickness‐ dependent permeation properties of laser‐sintered (LS) polymers as required to design and produce components with a special barrier performance to gaseous substances. Helium and oxygen permeation experiments were carried out on quenched and standard LS polyamide 12 (PA12) sheets generated with two, four, six, and eight layers at a constant powder layer thickness of 100 μm. The structural properties of the sheets were examined by differential scanning calorimetry, light microscopy, and X‐ray micro‐computed tomography. A reduction in thickness resulted in higher diffusion coefficients for both types of LS sheets. An explanation could be the large volume fraction of poorly sintered powder particles adhering to the surfaces and incomplete melting and low consolidation of the polymer at small thicknesses. The thickness‐dependency of the solubility coefficients was the opposite, especially for the standard LS sheets, which might be related to the larger pore volume in thicker sheets. As both effects compensated for each other, nearly constant permeation coefficients for all thicknesses were observed. The results provide further insights into different material characteristics of thin LS PA12 structures and offer new information on factors relevant to their solution and diffusion behavior.