Hybrid Processing of Thermoplastic Based Multimaterials

There is a growing interest in exploring novel material combinations and processing technology to manufacture complex 3D shaped polymer and composite structures. In the field of prosthetics and orthotics, this need is significant with additional challenges of providing patient specific customization to these devices in a cost-effective manner. Thermoplastic based materials(TP) including thermoplastic elastomers (TPE) and their composite counterparts have the potential to address these applications. The objectives of the research are to develop composite preforms that can conform to complex 3D structures, understand the processing phenomena in particular the fusion bonding mechanisms of different thermoplastic materials forming hard and soft interfaces and finally develop processing strategies that are suitable for the manufacture of complex 3D structures with these multimaterials. A novel TPE based composite preform consisting of continuous glass fibres was first manufactured by a continuous melt impregnation technique, extremely well suited for the studied TPE material. This process resulted in superior fibre wetting and impregnation characteristics for the composite preform. Furthermore, TPE reinforced by glass fibres provided preforms which can conform easily to 3D shapes owing to the flexible nature of the matrix. Then isothermal integration of polypropylene based glass fibre (iPPGF) composites with the TPE materials was studied and the bond strengths of bilayer specimens under different processing conditions were determined. The bonding temperature and pressure were tuned to promote the best fusion bonding conditions which was verified from peel tests and microscopic characterization of the interfaces. Further improvements in bond strength and fusion bonding time was achieved by non-isothermal fusion bonding techniques namely overmolding and 3D printing. The processing windows were then obtained for these two processes under a wide range of fusion bonding conditions and for various TPE and TP based materials. The presence of a continuous plasticized iPP phase in the TPE was determined to play an important role in the bonding through the establishment of intimate contact and interdiffusion mechanisms during the molding process. The influence of the interface temperature and bonding pressure was shown to govern the wetting and intimate contact of the interfaces and to compensate shrinkage respectively. The established processing conditions permitted tailoring of the bond strengths for TPE-iPP and TPE-iPPGF interfaces. These results can be directly applied for high volume manufacture of complex 3D shaped composite parts. The effects of the TPE composition (fillers, reinforcements, TP content, surface roughnessâŠ) and of bonding temperature, pressure and time, were thus better understood under the non-isothermal bonding conditions encountered in the additive processes. The results of the FDM printing can be combined with other low pressure processing techniques such as thermoforming to manufacture hybrid multimaterials 3D structures. The research is applied in developing a hybrid processing technology which proposes the combination of thermoplastic-based multimaterials and different processing techniques that are suitable for the manufacture of the next generation of composite applications in particular for customized affordable prosthetic limbs.

Morphology and interfacial strength of nonisothermally fusion bonded hard and soft thermoplastics

Pierre-Etienne Bourban, Rajasundar Chandran, Jan-Anders Månson

Nonisothermally fusion bonded butt joints were prepared by overmolding thermoplastic elastomers (TPEs) onto isotactic polypropylene (iPP) inserts in order to investigate the effect of processing conditions on the bond strength and interfacial microstructure. The mold temperature (Tm) was the most important factor for bond strength, as determined from interfacial mechanical tests. Extensive melting and recrystallization took place at the surface of the iPP insert at high Tm, promoted by migration of plasticizer from the TPE, whereas the original structure of the iPP remained intact at low Tm. Bond strengths of at least 50% of the cohesive strength of the TPE were nevertheless obtained at low Tm, suggesting intimate contact between the TPE melt and the iPP surface to be sufficient to provide useful adhesive bond strengths in these materials. The influence of pressure was less marked than the Tm, high pressures not being necessary to achieve intimate contact for the bonding times of about 5 s used here. However, the combination of a low bonding pressure with a high Tm typically led to poor quality bonds in thick specimens owing to uncompensated shrinkage during solidification, and voiding at the interface and in the melt zone of the iPP insert.

2018

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

EPFL2006

Novel structured adsorber and oxidation catalysts for the removal of VOCs in low concentration

Kim Martin Nikolajsen

One possibility for the removal of volatile organic compounds (VOCs) is catalytic oxidation. For the removal of VOCs in low concentration (≲ 3000 ppmv for propane) it is not possible to sustain the required oxidation temperature over the catalyst without external heating. An approach to overcome this problem is to concentrate the VOC by adsorption, in a "two-step adsorption-incineration" process. This is an unsteady state process in which the VOC laden effluent gas is cleaned by passing it through an adsorber. Once the adsorber is saturated the VOC is desorbed by heating and purging. The concentrated VOC from the desorber can then be oxidized over the catalyst, producing enough heat to sustain the oxidation temperature. Conventional adsorbents have some drawbacks such as tailing during desorption due to the large particle size, high pressure drop and hot-spot formation. The objective of this work was to develop, characterize and test the adsorbent and catalyst, with low resistance to mass transfer resulting from thin films of active material, for the two-step adsorption-incineration process. Both the adsorbent and the oxidation catalysts were supported on sintered metal fibers (SMF). This is a novel, structured support offering several advantages over conventional randomly packed beds: Low pressure drop, due to the high porosity of the SMF, which is important where large flow rates and/or high quantities of gas must be treated. High thermal conductivity, due to the metallic nature, leading to smaller temperature gradients in the fixed-bed. A very high geometric surface area, due to the small fiber size. On the order of 200,000 m2/m3 with a fiber diameter of 20 µm. Furthermore, a mathematical model was developed to show the feasibility of the coupling of the two steps in the process. The adsorber was made from a thin, homogeneous film of MFI-type zeolite covering the fibers of the SMF. The film was grown by the seed-film method by hydrothermal treatment. A 3 µm film was obtained with 10 wt.% zeolite on the fiber after 24 h synthesis at 125 °C. This material had a specific surface area of 30 m2/g. The adsorption equilibrium of propane was described well by the Langmuir isotherm and the heat of adsorption was found to be ΔH0ads = - 43.1 kJ/mol. Isothermal breakthrough curves were measured as a function of temperature and film thickness. A mathematical model, comprised of a tanks-in-series model with a linear driving force (LDF) mass transfer description, successfully described the breakthrough behavior. The overall mass transfer was found to be solely due to diffusion in the zeolite film. However, the thin films show very low resistance to mass transfer leading to low internal concentration gradients and efficient utilization of the adsorbent. The pressure drop was measured and compared to that of a randomly packed fixed-bed of spheres. The equivalent diameter for a constant volumetric flow rate and superficial cross sectional area was dp = 180 µm, for a fiber diameter of df = 26 µm at 10 wt.% zeolite loading. The oxidation catalysts were all supported on SMF and cobalt oxide was used as the active phase. Three different groups of catalysts were developed. The support was modified by coating the fibers either with a thin film of fine powder (alumina or catalyst powder) or with a MFI-type zeolite film identical to the adsorbent. The third group was made from cobalt oxide impregnated directly on the oxidized fibers. The latter type of catalyst was found to be the most active during the screening experiments. The base support composition was varied (stainless steel (SS), FeCrAlloy and inconel) to investigate the support effect on the kinetics. The stainless steel and FeCrAlloy supports had similar activities, surpassing that of the inconel supported catalyst, which might be attributed to a Co3O4 surface enriched with iron from the support. Between the 1.1 wt.%Co3O4/SMFSS and 1.5 wt.%Co3O4/SMFFeCrAlloy catalyst the FeCrAlloy based had the highest normalized activity at low temperature (< 310 °C), low propane mole fraction (0.13 %) and high oxygen mole fraction (16.7 %). The chemical kinetics were determined from a novel and efficient experimental design. The apparent activation energy for this catalyst was found to be 87.5 ± 2.6 kJ/mol, with reaction orders in propane of 0.38 ± 0.04 and in oxygen of 0.30 ± 0.08. The catalysts were tested up to 350 °C. Mass transfer limitations were absent and the kinetics was described well with the power rate law as compared to the Mars-van Krevelen model. Catalysts based on Co3O4, supported directly on oxidized SMF, are very active catalysts, simple to prepare, withstand deactivation due to hot-spot formation and show great potential in comparison to the industrial reference catalyst (copper and manganese oxide supported on alumina). The adiabatic desorption process was investigated by mathematical modeling. Simply purging the adsorption bed with the hot exhaust gas from the reactor cannot result in the concentration of the VOC due to the high heat capacity of the fixed-bed. Hence, it will be necessary to use either a high heat carrier like steam, with the adverse effects of drying and separation of water and VOC, or an approach in which the desorber is heated prior to purging the bed, changing the adsorption equilibrium in favor of high gas concentrations. For the latter method, the minimum theoretical propane mole fraction for an autothermal process was found to be 350 ppmv to sustain a catalytic incinerator at 250 °C. Finally, it can be concluded that for the first time zeolite films on SMF were used as structured adsorbents, leading to very low internal mass transfer and increased process efficiency. Furthermore, the advantages of the SMF can be exploited in a catalytic oxidizer, which can be combined with the adsorber to annihilate VOCs in low concentrations. A novel method for the deposition of fine powder catalyst on SMF was also developed for the first time.