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Nanoparticle-Filled Thermoplastics and Thermoplastic Elastomer: Structure-Property Relationships
(2012)
The present work focuses on the structure-property relationships of
particulate-filled thermoplastics and thermoplastic elastomer (TPE). In this work
two thermoplastics and one TPE were used as polymer matrices, i.e. amorphous
bisphenol-A polycarbonate (PC), semi-crystalline isotactic polypropylene (iPP),
and a block copolymer poly(butylene terephthalate)-block-poly(tetramethylene
glycol) TPE(PBT-PTMG). For PC, a selected type of various Aerosil® nano-SiO2
types was used as filler to improve the thermal and mechanical properties by
maintaining the transparency of PC matrix. Different types of SiO2 and TiO2
nanoparticles with different surface polarity were used for iPP. The goal was to
examine the influence of surface polarity and chemical nature of nanoparticles on
the thermal, mechanical and morphological properties of iPP composites. For
TPE(PBT-PTMG), three TiO2 particles were used, i.e. one grade with hydroxyl
groups on the particle surface and the other two grades are surface-modified with
metal and metal oxides, respectively. The influence of primary size and dispersion
quality of TiO2 particles on the properties of TPE(PBT-PTMG)/TiO2 composites
were determined and discussed.
All polymer composites were produced by direct melt blending in a twin-screw
extruder via masterbatch technique. The dispersion of particles was examined by
using scanning electron microscopy (SEM) and micro-computerized tomography
(μCT). The thermal and crystalline properties of polymer composites were characterized by using thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The mechanical and thermomechanical properties
were determined by using mechanical tensile testing, compact tension and
Charpy impact as well as dynamic-mechanical thermal analysis (DMTA).
The SEM results show that the unpolar-surface modified nanoparticles are better
dispersed in polymer matrices as iPP than polar-surface nanoparticles, especially
in case of using Aeroxide® TiO2 nanoparticles. The Aeroxide® TiO2 nanoparticles
with a polar surface due to Ti-OH groups result in a very high degree of
agglomeration in both iPP and TPE matrices because of strong van der Waals
interactions among particles (hydrogen bonding). Compared to unmodified
Aeroxide® TiO2 nanoparticles, the other grades of surface modified TiO2 particles
are very homogenously dispersed in used iPP and TPE(PBT-PTMG). The
incorporation of SiO2 nanoparticles into bisphenol-A PC significantly increases
the mechanical properties of PC/SiO2 nanocomposites, particularly the resistance
against environmental stress crazing (ESC). However, the transparency of
PC/SiO2 nanocomposites decreases with increasing nanoparticle content and
size due to a mismatch of infractive indices of PC and SiO2 particles. The different
surface polarity of nanoparticles in iPP shows evident influence on properties of
iPP composites. Among iPP/SiO2 nanocomposites, the nanocomposite
containing SiO2 nanoparticles with a higher degree of hydrophobicity shows
improved fracture and impact toughness compared to the other iPP/SiO2
composites. The TPE(PBT-PTMG)/TiO2 composites show much better thermal and mechanical properties than neat TPE(PBT-PTMG) due to strong chemical
interactions between polymer matrix and TiO2 particles. In addition, better
dispersion quality of TiO2 particles in used TPE(PBT-PTMG) leads to dramatically
improved mechanical properties of TPE(PBT-PTMG)/TiO2 composites.
Induction welding is a technique for joining of thermoplastic composites. An alternating
electromagnetic field is used for contact-free and fast heating of the parts to be
welded. In case of a suitable reinforcement structure heat generation occurs directly
in the laminate with complete heating in thickness direction in the vicinity of the coil.
The resulting temperature field is influenced by the distance to the induction coil with
decreasing temperature for increasing distance. Consequently, the surface facing the
inductor yields the highest, the opposite surface the lowest temperature.
The temperature field described significantly complicates the welding process. Due to
complete heating the laminate has to be loaded with pressure in order to prevent delamination,
which requires the usage of complex and expensive welding tools. Additionally,
the temperature difference between the inductor and the opposite side may
be greater than the processing window, which is determined by the properties of the
matrix polymer.
The induction welding process is influenced by numerous parameters. Due to complexity
process development is mainly based on experimental studies. The investigation
of parameter influences and interactions is cumbersome and the measurement
of quality relevant parameters, especially in the bondline, is difficult. Process simulation
can reduce the effort of parameter studies and contribute to further analysis of
the induction welding process.
The objective of this work is the development of a process variant of induction welding
preventing complete heating of the laminate in thickness direction. For optimal
welding the bondline has to reach the welding temperature whereas the other domains
should remain below the melting temperature of the matrix polymer.
For control of the temperature distribution localized cooling by an impinging jet of
compressed air was implemented. The effect was assessed by static heating experiments
with carbon fiber reinforced polyetheretherketone (CF/PEEK) and polyphenylenesulfide
(CF/PPS).
The application of localized cooling could influence the temperature distribution in
thickness direction of the laminate, according to the specifications of the welding
process. The temperature maximum was shifted from the inductor to the opposite side. This enables heating of the laminate to welding temperature in the bondline and
concurrently preventing melting and effects connected to this on the outer surface.
Inductive heating and the process variant with localized cooling were implemented in
three-dimensional finite-element process models. For that purpose, the finiteelement-
software Comsol Multiphysics 4.1 was used for the development of fully
coupled electromagnetic-thermal models which have been validated experimentally.
A sensitivity analysis for determination of different processing parameters of inductive
heating was conducted. The coil current, field frequency, and heat capacity were
identified as significant parameters. The cooling effect of the impinging jets was estimated
by appropriate convection coefficients.
For transfer of the developed process variant to the continuous induction welding
process, a process model was created. It represents a single overlap joint with continuous
feed. With the help of process modeling a parameter set for welding of
CF/PEEK was determined and used for joining of specimens. In doing so, the desired
temperature field was achieved and melting of the outer layers could be prevented.
Unidirectional (UD) composites are the most competitive materials for the production
of high-end structures. Their field of application spreads from the aerospace up to
automotive and general industry sector. Typical examples of components made of
unidirectional reinforced composite materials are rocket motor cases, drive shafts or
pressure vessels for hydrogen storage. The filament winding technology, the pultrusion
process and the tape placement are processes suitable for the manufacturing
using UD semi-finished products. The demand for parts made of UD composites is
constantly increasing over the last years. A key feature for the success of this technology
is the improvement of the manufacturing procedure.
Impregnation is one of the most important steps in the manufacturing process. During
this step the dry continuous fibers are combined with the liquid matrix in order to create
a fully impregnated semi-finished product. The properties of the impregnated roving
have a major effect on the laminate quality, and the efficient processing of the
liquid matrix has a big influence on the manufacturing costs.
The present work is related to the development of a new method for the impregnation
of carbon fiber rovings with thermoset resin. The developed impregnation unit consists
of a sinusoidal cavity without any moving parts. The unit in combination with an
automated resin mixing-dosing system allows complete wet-out of the fibers, precise
calibration of the resin fraction, and stable processing conditions.
The thesis focuses on the modeling of the impregnation process. Mathematical expressions
for the fiber compaction, the gradual increase of the roving tension, the
static pressure, the capillarity inside the filaments of the roving, and the fiber permeation
are presented, discussed, and experimentally verified. These expressions were
implemented in a modeling algorithm. The model takes into account all the relevant
material and process parameters. An experimental set-up based on the filament
winding process was used for the validation of the model. Trials under different conditions
have been performed. The results proved that the model can accurately simulate
the impregnation process. The good impregnation degree of the wound samples
confirmed the efficiency of the developed impregnation unit. A techno economical
analysis has proved that the developed system will result to the reduction of the
manufacturing costs and to the increase of the productivity.
Thermoplastic polymer-polymer composites consist of a polymeric matrix and a
polymeric reinforcement. The combination of these materials offers outstanding
mechanical properties at lower weight than standard fiber reinforced materials.
Furthermore, when both polymeric components originate from the same family or,
ideally, from the same polymer, their sustainability degree is higher than standard
fiber reinforced composites.
A challenge of polymer-polymer composites is the subsequent processing of their
semi-finished materials by heating techniques. Since the fibers are made of meltable
thermoplastic, the reinforcing fiber structure might be lost during the heating process.
Hence, the mechanical properties of an overheated polymer-polymer composite
would decline, and finally, they would be even lower than the neat matrix. A decrease
of process temperature to manage the heating challenge is not reasonable since the
cycle time would be increased at the same time. Therefore, this work pursues the
adaption of a fast and selective heating method on the use with polymer-polymer
composites. Inductively activatable particles, so-called susceptors, were distributed in
the matrix to evoke a local heating in the matrix when being exposed to an
alternating magnetic field. In this way, the energy input to the fibers is limited.
The experimental series revealed the induction particle heating effect to be mainly
related to susceptor material, susceptor fraction, susceptor distribution as well as
magnetic field strength, coupling distance, and heating time. A proper heating was
achieved with ferromagnetic particles at a filler content of only 5 wt-% in HDPE as
well as with its respective polymer fiber reinforced composites. The study included
the analysis of susceptor impact on mechanical and thermal matrix properties as well
as a degradation evaluation. The susceptors were identified to have only a marginal
impact on matrix properties. Furthermore, a semi-empiric simulation of the particle
induction heating was applied, which served for the investigation of intrinsic melting
processes.
The achieved results, the experimental as well as the analytic study, were
successfully adapted to a thermoforming process with a polymer-polymer material,
which had been preheated by means of particle induction.
An isogeometric Reissner-Mindlin shell derived from the continuum theory is presented. The geometry is described by NURBS surfaces. The kinematic description of the employed shell theory requires the interpolation of the director vector and of a local basis system. Hence, the definition of nodal basis systems at the control points is necessary for the proposed formulation. The control points are in general not located on the shell reference surface and thus, several choices for the nodal values are possible. The proposed new method uses the higher continuity of the geometrical description to calculate nodal basis system and director vectors which lead to geometrical exact interpolated values thereof. Thus, the initial director vector coincides with the normal vector even for the coarsest mesh. In addition to that a more accurate interpolation of the current director and its variation is proposed. Instead of the interpolation of nodal director vectors the new approach interpolates nodal rotations. Account is taken for the discrepancy between interpolated basis systems and the individual nodal basis systems with an additional transformation. The exact evaluation of the initial director vector along with the interpolation of the nodal rotations lead to a shell formulation which yields precise results even for coarse meshes. The convergence behavior is shown to be correct for k-refinement allowing the use of coarse meshes with high orders of NURBS basis functions. This is potentially advantageous for applications with high numerical effort per integration point. The geometrically nonlinear formulation accounts for large rotations. The consistent tangent matrix is derived. Various standard benchmark examples show the superior accuracy of the presented shell formulation. A new benchmark designed to test the convergence behavior for free form surfaces is presented. Despite the higher numerical effort per integration point the improved accuracy yields considerable savings in computation cost for a predefined error bound.
The development of autonomous vehicle systems demands the increased usage of software based control mechanisms. Generally, this leads to very complex systems, whose proper functioning has to be ensured. In our work we aim at investigating and assessing the potential effects of software issues on the safety, reliability and availability of complex embedded autonomous systems. One of the key aspects of the research concerns the mapping of functional descriptions in form of integrated behavior-based control networks to State-Event Fault Tree models.
Recently, a new Quicksort variant due to Yaroslavskiy was chosen as standard sorting
method for Oracle's Java 7 runtime library. The decision for the change was based on
empirical studies showing that on average, the new algorithm is faster than the formerly
used classic Quicksort. Surprisingly, the improvement was achieved by using a dual pivot
approach — an idea that was considered not promising by several theoretical studies in the
past. In this thesis, I try to find the reason for this unexpected success.
My focus is on the precise and detailed average case analysis, aiming at the flavor of
Knuth's series “The Art of Computer Programming”. In particular, I go beyond abstract
measures like counting key comparisons, and try to understand the efficiency of the
algorithms at different levels of abstraction. Whenever possible, precise expected values are
preferred to asymptotic approximations. This rigor ensures that (a) the sorting methods
discussed here are actually usable in practice and (b) that the analysis results contribute to
a sound comparison of the Quicksort variants.
Capital budgeting or investment decisions have an essential influence on companies’ performance. Instead of a rational choice, capital budgeting might be regarded as a process of reality construction. Research suggests that decision makers have only limited control over their own cognitive biases in this construction process. It is in this perspective that this paper intends to answer the following research question: What are behavioral determinants for a successful capital-budgeting decision process? The authors identify and discuss three behavioral success factors (reflective prudence, critical communication and outcome independence) for five stages of the capital budgeting process against the backdrop of the findings of the managerial and organizational cognition theory and cognitive psychology.
In this study, two outstanding subgroups of organic-inorganic hybrid materials have been investigated. The first part covers the design, synthesis, characterization and application of seven novel Metal Organic Frameworks (MOFs) containing functionalized biphenyl dicarboxylates as linkers. In the second part, the surface modification of the metal oxides ZrO2, TiO2 and Al2O3 using phosphonate derivates is reported.
Firstly three functionalized MOF structures; ZnBrBPDC, ZnNO2BPDC and ZnNH2BPDC were synthesised using 4,4´-biphenyldicarboxylic acid derivatives with different functional groups (-Br, -NO2, -NH2) Powder X-ray diffraction (PXRD) measurements indicated that the synthesised MOFs posses the interpenetrated IRMOF-9 structure with a cubic topology, which was also confirmed with single crystal X-ray measurements. The chemical structure of the MOF materials was further proved by solid state NMR and IR measurements. N2 adsorption measurements showed Type I isotherms for all three structures with large surface areas. TGA measurements of the evacuated samples were in good agreement with the elemental analysis data. The results proved that their thermal stability is between 325 °C - 450 °C.
Adsorption properties of these MOF structures were tested using light alkanes (CH4, C2H6, C3H8, and n-C4H10) at three different temperatures. For all adsorbents, the maximum uptakes were observed at 273 K. When the temperature was increased, the amount of the adsorbed gas decreased. All three MOFs showed strong affinities for n-butane. The lowest uptakes were observed for CH4.
The effect of functional groups on the IRMOF series was also examined by synthesizing amide functionalized biphenyl linkers. For this purpose, four different linkers containing amides with different alkyl chains (C1-C4) were synthesized and used for the synthesis of four new MOF structures ZnAcBPDC, ZnPrBPDC, ZnBuBPDC and ZnPeBPDC.
PXRD measurements of ZnAcBPDC indicated that the structure contains two different phases. PXRD patterns of ZnPrBPDC, ZnBuBPDC and ZnPeBPDC revealed non-interpenetrated structures which were further proved by single crystal X-ray measurements. The chemical structure of the MOF materials was further confirmed by X-ray spectoscopy, solid state NMR and IR measurements.
N2 adsorption measurements of the MOF structures were carried out using different activation methods. For all four MOFs, Type I isotherms were obtained. ZnAcBPDC showed the highest BET surface area. ZnAcBPDC and ZnBuBPDC were tested for their alkane, alkene and CO2 adsorption capacities.
In the second part of the work, the surface modification of three different metal oxides, ZrO2, TiO2 and Al2O3 was performed. For this purpose firstly three different fluorescent phosphonate derivatives containing thiophene units were synthesized from their halo derivatives in a four step synthesis and then used as coupling molecules for the surface modification. Nine different surfaces were obtained (38@TiO2, 39@TiO2, 40@TiO2, 38@Al2O3, 39@Al2O3, 40@Al2O3, 38@ZrO2, 39@ZrO2, 40@ZrO2).
All three modified metal oxide surfaces were characterized using elemental analysis, solid state NMR and IR spectroscopy. The BET surface areas of the materials were determined by N2 adsorption measurements. TGA was used to determine the stability of the surfaces. Maximum loadings were obtained for ZrO2 surfaces.
Due to the strong luminescence of the coupling molecules, the modified surfaces were checked for their light emission. All ZrO2 and Al2O3 surfaces showed fluorescence with exception of 40@Al2O3. On the other hand, for the modified TiO2 surfaces, no fluorescence could be observed.
By natural or man-made disasters, the evacuation of a whole region or city may become necessary. Apart from private traffic, the evacuation from collection points to secure shelters outside the endangered region will be realized by a bus fleet made available by emergency relief. The arising Bus Evacuation Problem (BEP) is a vehicle scheduling problem, in which a given number of evacuees needs to be transported from a set of collection points to a set of capacitated shelters, minimizing the total evacuation time, i.e., the time needed until the last person is brought to safety.
In this paper we consider an extended version of the BEP, the Robust Bus Evacuation Problem (RBEP), in which the exact numbers of evacuees are not known, but may stem from a set of probable scenarios. However, after a given reckoning time, this uncertainty is eliminated and planners are given exact figures. The problem is to decide for each bus, if it is better to send it right away -- using uncertain numbers of evacuees -- or to wait until the numbers become known.
We present a mixed-integer linear programming formulation for the RBEP and discuss solution approaches; in particular, we present a tabu search framework for finding heuristic solutions of acceptable quality within short computation time. In computational experiments using both randomly generated instances and the real-world scenario of evacuating the city of Kaiserslautern, we compare our solution approaches.