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The creep behavior of polymeric materials has been studied, with emphasis on dividing the total deformation into an elastic, viscous and retarded part via a 4-elements-model. It was found that the long-term creep behavior can be satisfactorily predicted based on creep experiments up to 1000h loading duration. An extrapolated Findley-approximation up to a predetermined criterion delivers data-points for long time intervals. The models developed allow a comprehensive description of the creep behavior. With that approach, the simulation of e.g. isochronous curves for arbitrary stresses and temperatures was possible with very good accordance to the real creep behavior.
The use of advanced lightweight materials to improve combat survivability has been of crucial interest to the U.S. Army for a number of years. The design, development, and performance testing of these advanced materials is critical for enabling Future Combat Systems and the Objective Force Warrior. Specifically, hybrid organic/inorganic polymer matrix nanocomposites show promise in providing many of the physical properties required (ie. lightweight structure, rugged abrasion resistance, high ballistic impact strength). However, as with any polymer system, these materials are susceptible to degradation over time when exposed to various environmental (i.e. sunlight, moisture, temperature) conditions. This structural degradation (1-4) will eventually comprise the original integrity of the materials’ desired properties. The focus of our research is to exploit nano-technology through incorporation of layered silicates for property enhancement.In this study, the impact of accelerated weathering upon newly developed polymethyl methacrylate-layered silicate nanocomposites materials was investigated. The silicate loading varied from 0 - 5 % by weight. A fluorescent ultraviolet (UV)/condensation weathering tester was selected for the exposure study. The materials were characterized by UV/VIS spectroscopy and FT-IR spectroscopy. The results reveal that the acrlyate linkages undergo a scission reaction upon UV exposure thereby compromising the original properties of the material. Furthermore, these scissions produce a yellowing of the polymer matrix which can inhibit its use where optical clarity in important.
We have used molecular dynamics to simulate the behavior of polymers in scratch tests. We start by creating a material on the computer consisting of chains with varying molecular mass, orientation and second phase concentration.Scratching is simulated by moving a force along the surface of the material. We measure the resulting deformation and we study penetration depth and recovery. We find that these depend on the local structure of the material as well as on the distribution of the second phase.Our simulations allow us to better understand scratching and scratch recovery. This helps in the creation of materials with improved tribological properties.
The impact fatigue properties of four kinds of glass-fiber or glass-bead reinforced polymers were studied through uni-axial and multi-axial fatigue. In performing fatigue tests, the authors paid special attention to the effect of interval times between loading and loading mode on the fatigue properties of those materials. It was found that the numbers of cycles to failure were strongly dependent on the duration of the interval time and loading mode. The failure mechanisms were investigated with acoustic velocity measurements and optical microscope (OM) observations. OM studies revealed that the fatigue life is strongly affected by the features of damage such as breakage of glass, micro-voiding, change in the orientation of glass fibers, and plastic deformation. The degree of damage localization also had a strong effect on the fatigue life. The difference in the damage development mechanisms were found to be caused by the difference in the elastic response of the specimens due to the different loading mode (uni-axial or multi-axial) and the interval time.
Current codes governing the design of fiber reinforced polymeric (FRP) components are generally based on ultimate strength testing. A large safety factor is then imposed for purposes of design. This approach ignores the onset of damage in the component. The onset of damage is related to the structural integrity of the component and its fatigue life.An experimental investigation has been conducted to determine the onset of damage in small coupon specimens with the acoustic emission technique. Damage was found to occur at different stages of loading for coupons made with different resin types and different fiber geometries.
Dynamic Mechanical Analysis (DMA) was used to characterize three polymers that display rubbery behavior around room temperature. Oscillatory modulus data were collected at zero degrees centigrade and above for these three rubbery materials: a thermoplastic olefin (TPO), an EPDM rubber and a silicone rubber. The in-phase modulus, out-of-phase modulus and tan ? data were synthesized into mastercurves using the principle of time-temperature superposition and compared. As expected, the TPO, the only non-cross-linked system, showed the most time dependence, while the silicone rubber was very nearly perfectly elastic, basically conforming to rubber elasticity theory. The EPDM data, while still superposable into a smooth mastercurve, showed evidence of an incomplete cure. Subsequent testing and analysis confirmed the incomplete cure. A cured sample of the EPDM polymer behaves in a fashion consistent with classical rubber elasticity theory. These DMA data are also shown to be useful for time-dependent, small-deformation stress and strain analyses.
A range of metallocene and Ziegler-Natta catalysed LLDPEs were prepared by injection moulding to determine the effects of density, molecular weight, MFI and polydispersity on their mechanical performance. Tensile results showed that hexene based mLLPDEs exhibited higher elongation to break while Young’s modulus of the materials was found to be more influenced by density. Impact results demonstrated that metallocene LLDPEs have superior impact strength at room temperature over conventional LLDPEs. Further analysis using differential scanning calorimetry and dynamic mechanical thermal analysis was performed to study the influence of the metallocene catalyst and co-monomer type on the properties of the materials.
Two commercial biodegradable polymers, polycaprolactone and polybutylene succinate, were used to study their processability in crosslinked foam processes. Benzoyl peroxide and t-butyl perbenzoate were used, respectively, to initiate crosslinking reactions. Zinc diacrylate was used to enhance the gel content of the crosslinked polymers. The change in melt strength of both polymer systems was assessed by measuring their dynamic mechanical properties. The effects of crosslinking agents and coagents on foam densities and gel contents are also reported.
This work deals with metallocene homogeneous and heterogeneous catalysts (Ph2C(Flu,Cp)ZrCl2 and Ph2C(Flu,Cp)ZrCl2/HM) to obtain syndiotactic polypropylene and propylene-1-hexene copolymers. The effects of polymerization temperature, comonomer concentration and different reaction solvents were studied.S-PP of 138/152°C melting temperature was obtained with these catalysts. Both homogeneous and heterogeneous systems showed the comonomer effect. By changing the solvent from toluene to hexane a decrease in activity and an increase in polymer melting temperature and crystallinity were observed.
This work investigates the characteristics and properties of syndiotactic polypropylene produced by Ph2C(Flu)(Cp)ZrCl2/MAO metallocene catalyst system. The obtained s-PP presented melt temperature around 120°C, being more amorphous as the polymerization temperature increased. From the data obtained through HAAKE mixture chambers we could carry out the rheological characterization of the polymer, which presented a non-Newtonian pseudoplastic behavior, as expected. The polymer also had a good mechanical performance compared to conventional i-PP.
In this work, the performance of the homogeneous catalyst system based on Ph2C(Flu,Cp)ZrCl2 was evaluated on ethylene copolymerization and 1-hexene. The influence of the support material was studied using acid mordenite HM zeolite and the characteristics of the produced polymers were also investigated. A study was performed to compare the influence of polymerization solvents as toluene, hexane and hexane/TIBA. An increase in activity was observed in relation to the comonomer addition for homogeneous and supported systems.
Polypropylene/clay nanocomposites modified with different levels of maleic anhydride grafted polypropylene (PPgMA) compatibilizers were compounded on a twin-screw extruder. The effect of PPgMA compatibilizers, including PB3150, PB3200, PB3000, and E43, were studied. The structure was investigated with X-Ray diffraction (XRD) and transmission electron microscopy (TEM). The relative complex viscosity curves also revealed a systematic trend with the extent of exfoliation and showed promise for quantifying the hybrid structure of the nanocomposites. Mechanical properties and thermal stability were determined by dynamical mechanical analysis (DMA) and thermogravimetric analysis (TGA), respectively. An optimum level of compatibilizers was found to yield the greatest improvement of composite properties. Though PPgMA with low molecular weight and high MA content could lead to good clay dispersion, it resulted in less improvement in both mechanical and thermal properties of PP/clay nanocomposites.
The desire to decrease unproductive time of injection moulding machines explains why the clamping systems are becoming increasingly more powerful.After significant studies, three families of clamping system supports are currently recognised : mechanical, hydraulic and magnetic supports. The choice of the appropriate production system remains to be made by taking into account the return on investment.Whereas the sales of the mechanical and hydraulic systems remain stable, the magnetic systems represent a remarkable evolution. In spite of its price, which is relatively high, the industrialists recommend this system because of its remarkable. However the prices, which remains a real barrier to its expansion, are falling thanks to an increase in sales.
Nucleating agents are very beneficial additives in polypropylene because by promoting crystallization they improve molding productivity, enhance thermal properties, achieve simultaneously high impact and strength and provide improved clarity for special applications. Uniform dispersion of the nucleating particles throughout the polymer matrix is critical in achieving the desired performance. Additives fed as powders are intuitively thought to provide the best dispersion, but suffer from handling difficulties, including loss of the additives in the feeding / transport system. Designing a non dusting blend (NDB) form that can feed as a pellet but at the same time will disperse uniformly in polypropylene during extrusion is a challenging task. In this paper we report several NDB forms that achieve dispersion equivalent to the powdered blends and compare with systems where such dispersion could not be achieved. The influence of appropriate co-additives, as well as selection of the optimum NDB manufacturing process are discussed. Crystallization temperature and haze measurements were used to quantify the quality and extent of the dispersion.
Thermotropic liquid crystalline polymer (TLCP) / poly(ethylene 2,6-naphthalate) (PEN) / poly(ethylene terephthalate) (PET) ternary blends were prepared by melt blending, and were melt spun to fibers to improve fiber performance and processability. The mechanical properties of ternary blend fibers could be significantly improved by annealing. This result was attributed to the development of more ordered crystal structures and the formation of more perfect crystallites. Multiple melting behaviors observed in the annealed ternary blend fibers depended on annealing temperature and time, which were caused by the different lamellae thickness distribution.
Silica nano-particle filled PEN composites were melt-blended to improve mechanical and physical properties, and processability. Tensile modulus and strength were improved adding silica nano-particles to the PEN. The melt viscosity of the nano-composites during the process was decreased by adding small amount of the fumed silica, and hence the processability of those was improved. Dispersity and compatibility of silica nanoparticle filled PEN composites will be investigated near future. Non-isothermal crystallization kinetics and spherulite growth rate were analyzed by Ozawa equation and image analysis, and the calculated Avrami exponents revealed three dimensional growth of crystallite, and the crystallization rate constants were increased with increasing silica contents. Fumed silica may act as a nucleating agent in the PEN matrix, and the same results were obtained by calculating nucleating activity.
Diglycidyl ether of bisphenol-A (DGEBA) based epoxy resin was reinforced by natural and organically modified montmorillonites, namely Cloisite Na+ and Cloisite 30 B. The process involved mechanical and ultrasonic mixing of the epoxy resin with clay particles and diffusion of the resin into the space between the silicate layers. SEM analysis indicated that as the clay loading increased, the particle size of the clay agglomerates increased. X-ray analysis showed that in nanocomposites with 3-weight % Cloisite 30B, the d-spacing increased from 1.83 nm to 3.82 nm. Adding 0.5 weight % organically modified clay improved the impact strength of the neat epoxy resin by 137.1 %.
Polypropylene/Montmorillonite nanocomposites were prepared by melt compounding with different concentrations of the compatibilizer, which is a maleic anhydride grafted polypropylene, using a reactive twin-screw extruder. They were characterized using tensile testing, DSC, XRD and TEM. It was found that beyond a certain concentration of the compatibilizer, the mechanical properties deteriorated. This was explained on the basis of shear rate-viscosity relationships and the nature of interfaces governing the nanocomposites.
Nanocomposites using PMMA and Laponite were prepared by melt processing in a reactive twin-screw extruder. The clay particles were dispersed in an aqueous medium before being mixed with PMMA. Polymer nanocomposites were prepared at different solids (clay) loading and samples were characterized using wide-angle x-ray diffraction, TEM, as well as stress-strain measurements, DSC and Nanoindentation. One of the nanocomposites was almost as transparent as pure PMMA and exhibited better mechanical properties than pure PMMA. Results will be presented and discussed.
Nanocomposites using Polycarbonate and Ni- Zn based ferrite nanoparticles were prepared using a reactive twin-screw extruder. They were found to be quite transparent and were characterized using tensile testing, XRD, DSC and TEM. Their mechanical behavior was found to be more brittle than pure polycarbonate. Also, the viscosity of the nanocomposite was significantly lower than the original polymer. This was explained on the basis of the nature of the material used to coat the nanoparticles, and the melt processing conditions employed for preparing the nanocomposites. It was also found that if the nanoparticles were not coated, they agglomerated and hence, the nanocomposites were no longer transparent.
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