Mixture models were studied in an effort to predict the microwave frequency permittivities of unidirectional-fiber-reinforced thermoplastic-matrix composite materials as a function of fiber volume fraction, fiber orientation relative to the electric field, and temperature. The permittivities of the constituent fiber and plastic materials were measured using a resonant cavity perturbation technique at 9.4 GHz and 2.45 GHz. The permittivities of the composite specimens were measured using a reflection cavity technique at 9.4 GHz and 2.45 GHz. Simple" rule of -mixtures II models that use the fiber and plastic permittivities have been found to approximate the complex dielectric properties of the composite for varied fiber volume fractions. The permittivities of oriented composites were successfully modeled at 9.4 GHz using a tensor rotation procedure. Composite permittivities were modeled with temperature up to the glass transition temperature of the thermoplastic matrix. Good agreement was found between the mixture model and experimental results for permittivity as a function of temperature at 9.4 GHz. / Master of Science

A two-dimensional axisymmetric finite-element model developed to simulate the microwave processing of polymers and polymer-matrix composites in a cylindrical resonant cavity was improved. The model consists of two submodels: the electromagnetic submodel and the heat transfer submodel. These two models are coupled together by the heat generation term arising due to the microwave energy. A single finiteelement program was written to implement the two submodels. The heat generation term arising due to exothermic chemical reactions was added to the heat conduction equation. The model can now handle thermosetting resins as well as amorphous thermoplastic polymers. The governing equations for the electromagnetic submodel are the complex, time-harmonic Maxwell's equations. Since an axisymmetric model was developed, the material needs to be axisymmetric and centered in the cavity. The material can have anisotropic conductivity and permittivity. A separate eigenvalue code was developed to compute the resonant frequency for given cavity dimensions. This eigenvalue code can account for non-homogenous material properties. The heat transfer model is governed by the unsteady heat conduction equation with the addition of heat generation terms accounting for exothermic reactions and microwave energy. All three types of heating: microwave only, convection only, and combined microwave and convection heating can be simulated by the electromagnetic and the heat transfer models. Several test cases were run to validate the programs. The results of the eigenvalue code were compared to those published in the literature. Simple test cases for which analytical expressions are available were run to verify the electromagnetic and heat transfer submodels. Excellent agreement was obtained in all of the comparisons. Once the programs were validated, several simulations were done to study microwave processing and/or convective heating of polymers and polymer-matrix composites. The materials considered were nylon 66, S-glass/polycarbonate composite, and S2-glass/epoxy composite. To study the advantages and disadvantages of microwave processing over conventional processing, comparisons were'made between the simulations of the two processes. / Master of Science

Thesis (MScEng)--Stellenbosch University, 2013. / AFRIKAANSE OPSOMMING: Saamgestelde materiale het baie gewilde materiale in die lugvaart- en motor industrië geword as gevolg van die gewigsbesparende voordele wat dit inhou. Kostes en ander verwerkingsprobleme het tradisioneel die wydverspreide gebruik van spesifiek termoplasties-versterkte vesels in hierdie areas verhinder. Baie van die vervaardigingsprobleme (spesifiek lang siklusse) is aangespreek met die aanvang van termoplastiese matriks materiaal soos Polyphenolien Sulfied (PPS). Hierdie materiaal voldoen ook aan die lugvaart-industrie se brand-, rook- en giftigheidstandaarde. Termoplastiese saamgestelde materiale kan byvoorbeeld gevind word op komponente in vliegtuie se binneruimtes en ook die voorste rand van die vlerke. Hierdie komponente is hoogs vatbaar vir impakskade. Die hoë sterkte en styfheid tot gewig verhoudings van saamgestelde materiale laat toe vir dun materiaal dwarssnitte. Komponente is dus kwesbaar vir uit-vlakkige impak beladings. Saamgestelde materiale kan ook intern deur hierdie beladings beskadig word en kan nie met die blote oog waargeneem kan word nie. Dit is dus nodig om die skade weens hierdie beladings tydens normale gebruik akkuraat te voorspel. Verder sal dit nuttig wees om die struktuur se gedrag te bepaal in toepassings waar byvoorbeeld passasier veiligheid krities is, soos op vliegtuig ruglenings tydens noodlandings. In hierdie studie is die potensiële vervaardigingsvoordele van termoplastiese saamgestelde materiale gedemonstreer. Daarbenewens is 'n uit-vlakkige impak deur 'n sagte liggaam herbou in 'n laboratorium omgewing. Die primêre doelwit van hierdie studie was om die impak numeries te modelleer. Vervaardigingsvoordele van `n vesel versterkte termoplastiese laminaat is gedemonstreer deur die vervaardiging van 'n konkawe, agt laag laminaat uit 'n vooraf gekonsolideerde geweefde doek. Die totale verwerkingstyd van die plat laminaat na 'n konkawe laminaat was minder as vyf minute. 'n Eenvoudige plat laminaat en 'n konkawe laminaat is onderwerp aan 'n lae snelheid impak deur 'n sagte projektiel. Die impak is gemodelleer deur die evaluering van drie modelleringsmetodes vir die saamgestelde paneel. Die evalueringskriteria het o.a. ingesluit of laminaat se volle gedrag suksesvol gemodelleer kon word met behulp van slegs 2D dop elemente. Die reaksie van die saamgestelde paneel en gepaardgaande faling is met wisselende vlakke van sukses deur die drie geëvalueerde modelle voorspel. Die faling van tussen-laminêre bindings (verwys na as delaminasie) kon nie deur enige van die modelle voorspel word nie. Twee van die modelle het egter in-vlak faling met redelike akkuraatheid voorspel. / ENGLISH ABSTRACT: Due to weight saving advantages composite materials have become a highly popular material in the aerospace and automotive industries. Traditionally processing difficulties and costs have been a barrier to widespread composite material use in these industries. With the advent of thermoplastic matrix materials such as Polyphenoline Sulphide (PPS) the processing difficulties (especially long cycle times) experienced with traditional thermosetting resins can be addressed while maintaining aerospace Fire-Smoke and Toxicity (FST) approval. Thermoplastic composites can for example be found on aircraft interior components and leading edges of the wings. These areas are highly susceptible to impact damage. The high strength- and stiffness to weight ratios of composites allows for thin material cross sections. This leaves the components vulnerable to out-of-plane impact loads. Composite materials may also be damaged internally by these loads, leaving the damage undetectable through visual inspections. It may therefore be necessary to predict the amount of damage a component would sustain during normal operation. Additionally, it would be useful to predict structural response of these materials in applications where passenger safety is crucial, such as aircraft seat backrests during emergency landings. In this study the potential processing benefits of thermoplastic composite materials were demonstrated. Additionally an out-of-plane impact from a soft bodied projectile was reconstructed in a laboratory environment. The primary objective was to numerically model the impact event. Processing benefits of thermoplastics were demonstrated by producing a single curvature eight layered laminate from a pre-consolidated woven sheet. The total processing time from flat panel to a single curvature panel was below five minutes. A simple flat laminate and a single curvature laminate were subjected to a low velocity drop weight impact load from a soft bodied projectile. These impact events were modelled by evaluating three modelling methods for the composite panel structural response and damage evolution. Part of the evaluation criteria included whether laminate failure could be modelled successfully using only 2D shell elements. The response of the composite panel and accompanying failure were predicted with varying levels of success by the three evaluated models. The failure of interlaminar bonds (referred to as delamination) could not be predicted by either model. However two of the models predicted in-plane failure with reasonable accuracy.

This thesis focuses on designing thermoplastic composites with high mechanical properties and a low processing temperature. Thermoplastic composites, which are used in this work, are composed of thermoplastic polyurethane (TPU) matrices and plain woven E-glass fabrics (GFs). TPUs were synthesised with large quantities of hard segments (HS), including 70% and 90%wt HS. The GF-TPU composites manufactured in this study have a melting point of around 175oC. As such, 180oC represents the processing temperature, which was used to produce GF-TPU composites. The influences of HS content and annealing treatment at 80oC on the thermal, dynamic mechanical and mechanical properties of TPU samples and GF-TPU composites with 25% fibre volume fraction (Vf) have been investigated. The highest crystallinity, storage modulus, Tg, yield strength, tensile strength and tensile modulus of all the TPU samples are seen in the TPU/90 samples annealed for 4 days. The TPU/90 samples display higher tensile properties than the TPU/70 and polypropylene (PP) samples, while the PP samples show the greatest elongation at break point. Furthermore, the tensile properties of the TPU/70 and TPU/90 samples are much higher than those of commercial TPUs. As such, annealed GF-TPU/90 composites with 25% Vf present the greatest dynamic mechanical, flexural, and tensile properties. GF-TPU/90 composites with 25% Vf show higher flexural strength than GF-PP composites or GF-polyamide 6 (PA6) composites with the same Vf. The effects of fibre surface treatments on the mechanical properties of GF and GF-TPU/70 composites with 25% Vf have also been studied in this investigation. GF treated with burn-off treatment is found to exhibit the lowest tensile properties. The interfacial adhesion between GF treated by NaOH for 0.5hrs and a TPU/70 matrix is greater than between GF treated by acetone for 5hrs and a TPU/70 matrix. Silanised GF presents greater tensile properties than desized GF. Thus, enhanced interfacial adhesion and tensile, flexural, ILSS and GIC properties are observed in the silanised GF-TPU/70 composites than in the desized GF-TPU/70 composites. GF-TPU/70 composites based on GFs treated by NaOH for 0.5hrs then sized with 0.15%wt. aminosilane display the greatest interfacial adhesion, flexural properties, ILSS and GIC, damage tolerance and impact-damage resistance. Conversely, the lowest interfacial adhesion, GIC, damage tolerance and impact-damage resistance are seen in the GF-PP composites based on 25% Vf as-received GF. There is a significant increase in the tensile and flexural properties of GF-TPU/90 composites with increasing the Vf from 25% to 50%. Moreover, the flexural strength of GF-TPU/90 composites with 50% Vf is not only higher than that of GF-EP composites or GF-vinyl ester composites with normalised 50% Vf, but is also much higher than that of GF-PP composites with 50% Vf. Despite this result, GF-TPU/90 composites with 50% Vf show the lowest fracture toughness, impact-damage resistance and damage tolerance, which are improved by adding 25% and 50%wt. of TPU/70 to the TPU/90 matrix. GF-TPU/90 composites based on a modified matrix have higher GIC, GIIC, impact-damage resistance and damage tolerance than GF-TPU/90 composites based on an unmodified matrix. The GIC, GIIC, impact-damage resistance and damage tolerance of GF-TPU/90 composites based on a modified matrix increase with increasing the percentage of TPU/70. Hence, the highest GIC, GIIC, impact-damage resistance and damage tolerance are seen in the GF-TPU/90 composites based on a modified matrix with 50%wt. of TPU/70.

Composites are widely used in an increasing number of applications in diverse fields. However, most traditional composite materials are difficult to recycle. Because of their enhanced recyclability, thermoplastic single-polymer composites (SPCs), i.e., composites with fiber and matrix made from the same thermoplastic polymer, have attracted much attention in the recent years. High-performance polymer fibers in combination with same polymer matrices would lead to a fully recyclable single polymer composite that has major ecological advantages. However, because a single polymer is involved in the composite, thermoplastic SPCs manufacturing presents a unique set of technical problems, and different approaches from those in standard composites manufacturing are frequently needed. Two specific issues in SPCs manufacturing are how to produce distinct forms of the same polymer and how to consolidate them. So far, most investigations have been reported on a single-component hot compaction method and two-component molecular methods. However, in these methods, either the processing window is too narrow or some impure materials are introduced into the system. The key issue in thermoplastic SPCs processing is how to melt-process the matrix without significantly annealing or even melting the fiber. To overcome the above drawbacks in existing SPCs processing, particularly to widen the SPCs processing temperature window and to purify the SPCs, a novel SPCs manufacturing process utilizing the characteristics of slowly crystallizing polymers was developed and investigated. Highly oriented and highly crystalline fibers made of a slowly crystallizing polymer are mixed with the amorphous form of the same polymer and then consolidated together under heat and pressure. In this dissertation research, two slowly crystallizing polymers, poly(ethylene terephthalate) (PET) and poly(lactic acid) (PLA), were used as model systems for SPCs processing.. To study the deformation and failure mechanisms of PET and PLA SPCs, the SPCs were characterized using tensile test, tearing test, impact test, SEM, optical microscopy, and other methods. The change of crystallinity and orientation of the material forms during SPCs processing were characterized by DSC and XRD. The effects of major process conditions on the performance of the SPCs were studied. It was found that the processing temperature played a profound role in affecting the fiber-matrix bonding property. The compression molded SPCs exhibited enhanced mechanical properties. For the PET SPCs with 45% by weight fiber content the tensile strength is four folds of that of non-reinforced PET. After reinforcement, the tearing strength of the PLA SPCs is almost an order higher than that of the non-reinforced PLA. The fusion bonding behavior of two crystallizable amorphous PET sheets was also studied. Several characterization methods including SEM, TEM and polarized microscopy (either on etched or on non-etched samples) were used to observe interfacial bonding morphology of the crystallizable amorphous PET sheets. For a bonded sample, a layer of transcrystals with a thickness of 1-2 Ým was found right at the interface. A secondary but much larger zone with a distinct morphology was observed outside the transcrystal layer. With increase of the heating time, the width of the whole interfacial region decreases. The interfacial morphology was found to significantly affect the interfacial bonding quality. The testing results further indicated that high bonding temperature with an appropriate holding time promotes interfacial bonding of two crystallizable amorphous PET.

Thesis (Ph.D.)--Mississippi State University. Department of Chemistry. / Title from title screen. Includes bibliographical references.

Thesis (M.Eng.(Chemical Engineering))--University of Pretoria, 2006. / Summary in English. Date on t.p. of paper copy different (2007). Includes bibliographical references.

Polystyrene (PS) nanocomposite foams were prepared using supercritical fluid (SCF) CO2 as a solvent and blowing agent. PS was first in-situ polymerized with a range of concentrations of montmorillonite layered silicate (MLS). The polymerized samples were then compression molded into 1 to 2mm thick laminates. The laminates were foamed in a batch supercritical CO2 process at various temperatures and pressures from 60°-85°C and 7.6-12MPa. The resulting foams were analyzed by scanning electron microscopy to determine effect of MLS on cellular morphology. Differential scanning calorimetry was used to determine the impact of nanocomposite microstructure on glass transition of the foamed polymer. X-ray diffraction spectra suggested that the PS/MLS composite had an intercalated structure at both the 1% and 3% mixtures, and that the intercalation may be enhanced by the foaming process.

The objective of this thesis is to investigate the effects of adding natural fiber (kenaf fiber, retted kenaf fiber, and sugarcane fiber) into polymer materials. The effects are obtained by considering three main parts. 1. Performance in thermoplastic composites. The effect of fiber retting on polymer composite crystallization and mechanical performance was investigated. PHBV/PBAT in 80/20 blend ratio was modified using 5% by weight kenaf fiber. Dynamic mechanical analysis of the composites was done to investigate the glass transition and the modulus at sub-ambient and ambient temperatures. ESEM was conducted to analyze fiber topography which revealed smoother surfaces on the pectinase retted fibers. 2. Performance in thermoset composites. The effect of the incorporation of natural fibers of kenaf and of sugarcane combined with the polyester resin matrix is investigated. A comparison of mechanical properties of kenaf polyester composite, sugarcane polyester composite and pure polyester in tensile, bending, dynamic mechanical thermal analysis (DMA) and moisture test on performance is measured.. 3. Performance in sandwich composites. The comparison of the performance characteristics and mechanical properties of natural fiber composites panels with soft and rigid foam cores are evaluated. A thorough test of the mechanical behavior of composites sandwich materials in tensile, bending and DCB is presented here.

The high strain rate fiber direction tensile response of Ultra High Molecular Weight Polyethylene (UHMWPE) composites is of interest in applications where impact damage may occur. This response varies substantially with strain rate. However, physical testing of these composites is difficult at strain rates above 10^-1/s. A Monte Carlo simulation of composite tensile strength is constructed to estimate the tensile behavior of these composites. Load redistribution in the vicinity of fiber breaks varies according to fiber and matrix properties, which are in turn strain rate dependent. The distribution of fiber strengths is obtained from single fiber tests at strain rates ranging from 10^-4/s to 10^-1/s and shifted using the time-Temperature Superposition Principle (tTSP) to strain rates of 10^-4/s to 10^6/s. Other fiber properties are obtained from the same tests, but are assumed to be deterministic. Matrix properties are also assumed to be deterministic and are obtained from mechanical testing of neat matrix material samples. Simulation results are compared to experimental data for unidirectional lamina at strain rates up to 10^-1/s. Above 10^-1/s, simulation results are compared to experimental data shifted using tTSP. Similarly, through-thickness shear response of UHMWPE composites is of interest to support computational modeling of impact damage. In this study, punch shear testing of UHMWPE composites is conducted to determine shear properties. Two test fixtures, one allowing, and one preventing backplane curvature are used in conjunction with finite element modeling to investigate the stress state under punch shear loading and the resulting shear strength of the composite. / Ph. D.