Influence of EPS bead fusion on pattern degradation and casting formation in the lost foam process

The effects of bead fusion in the pattern on foam degradation and on casting formation have been studied. Injection molded ASTM D638 tensile specimens have been used to develop a microscopic technique to quantify the extent of bead fusion in the pattern. The tensile properties of the polymer have been correlated with the measured degree of bead fusion. A variety of experiments have been conducted to highlight the effects of fusion on foam degradation. The flow behavior of a molten aluminum alloy has been studied in patterns with various levels of fusion. The results indicate that the degree of fusion has a significant effect on bead collapse and viscous residue formation in the polymer. The mold fill times generally increase with increasing bead fusion in the pattern. The temperature at the metal front drops more rapidly as the bead fusion increases.     

A study of the effect of molecular weight on the tensile strength of ultra-high modulus polyethylenes

In this paper the influence of molecular weight and molecular weight distribution on the tensile strength (tenacity) of melt spun and drawn linear polyethylene are investigated with the aim of outlining the requirements for a high strength fibre. The tenacity was investigated over the molecular weight average ¯M _w range 60×10³ to 330×10³ with polydispersities ¯M _w/¯M _n ranging from 1.1 to 13.3. It was found that both molecular weight and its distribution affected tensile strength. The drawing conditions were also found to be important, a high draw temperature and a high draw ratio being needed for a high strength, high modulus fibre. By using a polymer of high ¯M _w and low polydispersity, and drawing at the optimum conditions, strengths of 1.65 GPa and moduli of 85 GPa have been achieved for test temperatures of –55° C.     

Influence of molecular weight on the mechanical performance of a thermoplastic glassy polyimide

Mechanical testing of an advanced thermoplastic polyimide (LaRC^TM-SI) with known variations in molecular weight was performed over a range of temperatures below the glass transition temperature. The physical characterization, elastic properties and notched tensile strength were all determined as a function of molecular weight and test temperature. It was shown that notched tensile strength is a strong function of both temperature and molecular weight, whereas stiffness is only a strong function of temperature. A critical molecular weight (M _c) was observed to occur at a weight-average molecular weight (M _w) of 22000 g/mol below which, the notched tensile strength decreases rapidly. This critical molecular weight transition is temperature-independent. Furthermore, inelastic analysis showed that low molecular weight materials tended to fail in a brittle manner, whereas high molecular weight materials exhibited ductile failure. The microstructural images supported these findings.     

Crystalline state extrusion of high -density polyethylene as a function of die entrance angle

High-density polyethylene (HDPE) of two different average molecular weights, ¯M _w, has been solid-state extruded in an Instron capillary rheometer through brass capillary dies of several entrance angles from 10 to 180°. Extrusion rates increase substantially at smaller angles whereas the maximum achievable extrusion draw ratio, (R _E), is realized at larger angles. At a constant R _E, maxima are observed in plots of linear expansion coefficient, Young's modulus, tensile strength and elongation at break against die entrance angle. These maxima occur at angles of 20 to 30° for the lower molecular weight HDPE (¯M _w = 59 000). For the higher molecular weight HDPE (¯M _w = 92 000) maxima were at angles of 14 to 20°.     

Effect of the molecular structure of phenolic novolac precursor resins on the properties of phenolic fibers

A series of phenolic resins with different weight-average molecular weights (M_w) and ortho/para (O/P) ratios were prepared. The effect of the phenolic precursor resin structure on the structure and properties of the resulting phenolic fibers was investigated. The structures of the resins and fibers were characterized by nuclear magnetic resonance spectroscopy, gel permeation chromatography, melt rheometry, dynamic mechanical analysis, and thermogravimetric analysis. The results show that the O/P ratio, unsubstituted ortho and para carbon ratio (O_u/P_u), and M_w of the phenolic resins play an important role in determining the properties of the phenolic fibers. The tensile strength of the phenolic fibers increases with increasing novolac precursor O_u/P_u ratios, corresponding to low O/P ratios, at comparable resin M_w values. Also, the tensile strength of the phenolic fibers increases with increasing novolac M_w values at comparable O/P ratios. Phenolic fibers with high tensile strength and good flame resistance characteristics were generated from a phenolic precursor resin, possessing a high weight-average molecular weight and a low O/P value.     

Self-reinforcement and hydrolytic degradation of amorphous lactic acid based poly(ester-amide), and of its composite with sol-gel derived fibers

The self-reinforcing and hydrolytic degradation of an amorphous poly(ester-amide) (PEA) based on lactic acid have been studied and compared with those of poly-L-lactide (PLLA). The studied PEA-rods were self-reinforced (SR) by solid-state die drawing resulting double shear strength. The hydrolytic degradation of PEA was studied during exposure to phosphate buffered saline at pH 7.4 and at 37 °C for 18 weeks. The degradation and mechanical properties of PEA were also followed in a self-reinforced composite structure consisting of PEA and sol-gel derived SiO₂-fibers (SGF, 8 wt %). The hydrolytic degradation of the SR-PEA-rods with and without SG-fibers was significantly faster than that of SR-PLLA-rods. The weight average molecular weight (M _w) of PEA decreased by 90% from the initial M _w during the first 6 weeks in hydrolysis, when the M _w of the PLLA decreased by 10%.     

Bending of square aluminium extrusions with aluminium foam filler

Summary An experimental programme consisting of 24 tests was carried out to study the three-point-bending behavior of square AA6060 aluminium extrusions filled with aluminium foam under quasi-static loading conditions. The outer cross section width and span of the beams were kept constant at 80 mm and 800 mm, respectively. The main parameters investigated were the foam density, the extrusion wall strength and the extrusion wall thickness. The experiments showed that the foam filler significantly altered the local deformation pattern of the beams. Simple design formulae were developed in order to predict the load bearing capacity of the foam filled beams.Notation d load cell displacement - total beam rotation - ₀₂ total beam rotation at extrusion flange yielding (non-filled beam) - ₁, ₂ beam end rotations - b outer component cross section width - h component wall thickness - L total beam span length - s central load section of beam - A _e extrusion cross sectional area - A _f foam cross sectional area - P load cell force - M total cross section moment - M _e extrusion moment - M ₀₂ maximum elastic moment capacity of extrusion - M _pl perfect plastic moment capacity of extrusion - M _f foam core moment - M _sf maximum foam core moment - M _s total initial peak moment - M _sa total moment capacity after foam failure - M _u ultimate extrusion moment capacity - M _se calculated moment resistance of extrusion at foam failure - stress, engineering - ₀₂ extrusion wall stress at 0.2% plastic strain - _U extrusion wall ultimate stress - ₀ extrusion wall characteristic stress, 0.5(₀₂+ _U ) - _f foam plateau stress - _s foam tensile failure stress - strain, engineering - ₀₂ extrusion strain corresponding to yield stress ₀₂ - _u extrusion strain corresponding to ultimate stress _U - _s foam tensile failure strain - beam curvature - ₀₂ beam curvature at extrusion flange yielding - _u beam curvature at extrusion ultimate stress - _s beam curvature at foam tensile failure strain - f extrusion section shape factor - k elastic bending stiffness ratio between foam core and extrusion - E extrusion Young's modulus - E _t extrusion tangential modulus - n extrusion material hardening constant - _f foam density - _i foam batch density,i=1, 2, 3 - x, y, z coordinate reference system (x-beam axis) - x _c distance from beam end (change in formula for curvature)     

Observation of different photo-degradation behaviour in two similar polypropylenes

The photodegradation properties of injection moulded bars 3 mm thick made from two nominally similar toughened polypropylene compounds (PP and MPP) have been found to differ remarkably in some aspects. In both of them the plot of the mass average molecular mass (M _w) versus distance from the exposed surface displayed a steep sigmoidal shape with very low values near the surface and values close to the undegraded value near the bar centre. With PP the steepest part of the M _w versus depth plot remained at almost the same position irrespective of exposure time for tests carried out for periods up to 64 weeks whereas with MPP the steepest part of the plot shifted progressively in from the exposed surface as exposure increased. The tensile strength fell rapidly with exposure time in both PP and MPP but with PP partial recovery was observed whereas with MPP the fall in strength appeared to be monotonic with exposure time.     

The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

The rate of initiation and growth of cracks in linear high-density polyethylene with different molecular weights was observed in single-edge-notched tensile specimens under plane strain condition as a function of applied stress, notch depth and temperature. The initial rates of crack initiation all have the form of C ^m a ₀ ⁿ exp (–Q/RT) or AK ^pexp (–Q/RT) where = stress, a ₀ = notch depth and K= stress intensity factor. For the different molecular weights, m, n, P and Q are almost the same where m=5, n=2, P=4.7 and Q=115 kJ mol^–1, but the constants C and A varied as (¯M _w–¯M _c)^–1 where ¯M_c is a limiting molecular weight for sudden fracture. A molecular model based on tie-molecules has been used to explain the dependence on ¯M _w. The effect of ¯M _w on the fast-fracture strength at low temperature and the relationship to tie-molecules have also been investigated. Quantitative relationships between the concentration of tie-molecules and the fracture behaviour have been obtained.     

A novel shortened electrospun nanofiber modified with a ‘concentrated’ polymer brush

Abstract

We report the fabrication of shortened electrospun polymer fibers with a well-defined concentrated polymer brush. We first prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate, with number-average molecular weight M_n=105 200 and weight-average molecular weight M_w=296 700 (M_w/M_n=2.82). The fibers had a diameter of 593±74 nm and contained initiating sites for surface-initiated atom transfer radical polymerization (SI-ATRP). Then, SI-ATRP of hydrophilic styrene sodium sulfonate (SSNa) was carried out in the presence of a free initiator and the hydrophobic fibers. Gel permeation chromatography confirmed that M_n and M_w/M_n values were almost the same for free polymers and graft polymers. M_n agreed well with the theoretical prediction, and M_w/M_n was relatively low (<1.3) in all the examined cases, indicating that this polymerization proceeded in a living manner. Using the values of the graft amount measured by Fourier transform infrared spectroscopy, the surface area, and M_n, we calculated the graft density σ as 0.22 chains nm^?2. This value was nearly equal to the density obtained on silicon wafers (σ=0.24 chains nm^?2), which is categorized into the concentrated brush regime. Finally, we mechanically cut the fibers with a concentrated poly(SSNa) brush by a homogenizer. With increasing cutting time, the fiber length became shorter and more homogenous (11±17 μm after 3 h). The shortened fibers exhibited excellent water dispersibility owing to the hydrophilic poly(SSNa) brush layer.     

Fracture and microstructure of open cell aluminum foam

SEM and EDS measurements were used to scrutinize the microstructure of Duocel open cell 6101 aluminum foam in relation to its fracture properties. In-situ SEM tensile tests on the open cell aluminum foam were performed to investigate the different fracture modes of struts and Aramis/Digital Image Correlation software was used to map the strain in individual struts. Observations during tensile tests showed that the microstructure of the struts has a great influence on the fracture behaviour of the foam. In particular AlFeSi-precipitates, which are due to the casting of the 6101 aluminum alloy, and the morphology of the foam alters the fracture mode of the struts in the foam from transgranular to intergranular. Less energy is needed for intergranular fracture of struts and the strain to failure of the foam is decreased due to weak individual struts.     

Aluminum integral foam castings with microcellular cores by nano-functionalization

The goal of the present work is the refinement of the pore morphology of aluminum integral foam castings. Integral foam molding, a modified high pressure die casting process, is used where a mixture of melt and blowing agent particles (magnesium hydride, MgH₂) is injected at high velocity into a permanent steel mold. At the mold surface, decomposition of the blowing agent and pore formation is suppressed due to the high solidification rate whereas solidification of the core is much slower allowing blowing agent decomposition, pore nucleation, and growth. Blowing agent particles not only act as gas suppliers but also represent pore nuclei. Thus, microcellular foam cores can be attained by increasing the number of MgH₂ particles. But increasing the number of powder particles by powder milling strongly decreases the flowability and strong particle agglomeration as a result of the increasing cohesive forces leads to inhomogeneous foams. Flowability of the powder can be restored by coating it with SiO₂-nano-particles resulting in a homogeneous microcellular foam morphology.     

Cell structure and compressive behavior of an aluminum foam

The plastic collapse strength, energy absorption and elastic modulus of a closed cell aluminum foam are studied in relation to cell structures. The density, node size and the cell wall thickness of the aluminum foams decrease with increasing cell size. The failure of the foam cells under compressive load progresses successively from the top or/and bottom to the mid-layer of the compression specimens, and no initial rupture of the foam cells is observed in the mid-height of the foam samples. When foam density increases from 0.11 to 0.22 g/cm ³, the plastic collapse strength rises from 0.20 to 1.29 MPa, while the elastic modulus of the closed cell aluminum foam increases from 0.70 to 1.17 GPa. In contrast, the energy absorption of the foams decreases rapidly with increasing cell size. When cell size increases from 4.7 to 10.1 mm, the energy absorption drops from over unity to 0.3 J/cm ³. The normalized Yong’s modulus of the closed cell aluminum foam is E^*/E_s = 0.208 (ρ^*/ρ_s), while the normalized strength of the foams, σ */σ_s is expressed by σ */σ_s = c ⋅ ρ */ρ_s where c is a density-dependent parameter. Furthermore, the plastic collapse strength and energy absorption ability of the closed cell aluminum foams are significantly improved by reducing cell size of the aluminum foams having the same density.     

Effect of whisker aspect ratio on the density and fracture toughness of SiC whisker-reinforced Si3N4

Controlling the suspension properties prior to slip casting optimizes the homogeneity, density and fracture toughness of silicon carbide whisker reinforced silicon nitride (SiC_w/Si₃N₄). Further improvements in the mechanical properties are realized by combining ball milling with ultrasonic dispersion of the composite suspension. Ball milling reduces the SiCw aspect ratio from 25 to 15 which in turn increases the dispersion of the whiskers within the suspension, resulting in increases in the green and sintered density, along with the fracture toughness. In a binderless process, 20 volume% reduced aspect ratio (r = 15) SiC_w/Si₃N₄ can be densified to 95% theoretical density by pressureless sintering using 8% Y₂O₃ and 2% Al₂O₃ by weight as sintering aids. These composites had measured values of fracture toughness from 9–10.5 MPa · m^1/2, representing an average increase of approximately 30% over the fracture toughness for monolithic Si₃N₄ processed under identical conditions.     

Stiff and strong polyethylene with shish kebab morphology by continuous melt extrusion

In a previous work, it was shown that highly oriented fibres with 10 GPa modulus could be obtained by continuous single-stage melt extrusion of a medium molecular weight polyethylene to which 3% ultra-high molecular weight (M _w ∼ 3 to 5 × 10⁶) material had been added by solution blending. It was demonstrated that a special interlocking shish kebab structure was responsible for the favourable mechanical properties. In the present work, we succeeded in achieving the same effect from an unblended polyethylene by choosing starting materials with an inherently suitable molecular weight distribution. Both the high and low molecular weight tails of the distribution are very influential: the high tail contributes to the formation of extended-chain fibrils (which constitute the backbones of the shish kebabs), while the low tail affects melt extrudability and strength. Melt strength is important because unusually high tensile stresses are required during wind-up. The wind-up stress was measured and found to be an order of magnitude greater than that encountered in conventional melt spinning — where no shish kebabs are formed. The implications of the above findings for polymer processing, crystal morphology and melt rheology are discussed.     

Effect of composition on phase morphology and mechanical properties of PP/PA66 in situ composites via extrusion-drawing-injection method

The reinforced and toughened PP/PA66 in situ composites were prepared via extrusion-drawing-injection method. The relationship among composition, phase morphology and mechanical properties, together with their functional mechanism, was investigated. The results show that in the range of PA66 weight fraction (f _w) from 0 to 20% and under the experimental processing conditions, the main changes in phase morphology of the composites with f _w are that the number of in situ formed PA66 microfibers and remained PA66 particles increases with f _w whereas the fiber transverse size and its dispersity decrease till f _w = 15% and then increase. This can be attributed to the combined effect of break-up, coalescence and deformation of the PA66 phase in the PP phase in the course of extrusion and drawing. The tensile strength of composites has a maximum value at f _w of 15% and Young's modulus increases with f _w up to a plateau level, while impact strength continuously rises with f _w, an effect which can be ascribed to the distinct dependence of these properties on the phase-morphological factors mentioned above.     

Comparative study of size and distribution of lamellar thicknesses and long periods in polyethylene with a shish-kebab structure

This study contains a combined application of three different techniques for the study of injection moulded polyethylene (PE), showing an oriented shish-kebab structure: small angle X-ray scattering (SAXS), low frequency Raman spectroscopy (LAM) and transmission electron microscopy (TEM), A series of linear PEs and molecular weights in the range 51000–478000 has been investigated and two injection temperatures have been used (T _m=144 and 210 °C). SAXS patterns from the highly oriented regions show the presence of either one axial long period (L ₁) or two (L ₁ and L ₂) depending on molecular weight (¯M _w) and T _m. It is shown that L ₁ and L ₂ increase with ¯M _w up to a given critical molecular weight ¯M _c. Above ¯M _c, L ₁ and L ₂ remain constant. Raman results qualitatively confirm the existence of two separate distributions of straight-length chain segments for those samples having molecular weights above the critical value. Shorter segments are shown to be more abundant than the longer ones. In the lowest molecular weight sample, results from SAXS, TEM and Raman spectroscopy seem to be consistent with each other, although in some cases a tilted molecular arrangement within the lamellae has to be invoked. On the other hand, in case of the highest molecular weight sample, the length of the short straight-chain segments derived from Raman spectroscopy agrees well with the double periodicity obtained from SAXS. On the contrary, long periods measured from TEM only correspond to the shorter SAXS periodicity. This result is discussed by assuming the occurrence of crystalline bridges among adjacent lamellae.     

Fatigue mechanisms in poly (methyl methacrylate) at threshold: effects of molecular weight and mean stress

Fatigue tests were conducted on three linear poly (methyl methacrylate) (PMMA) resins having weight average molecular weights (M _w) of 82 000, 205 000 and 390000 and on a fourth, cross-linked sample (M _c=3337 g mol^–1). Fatigue threshold test conditions included a constant load ratio (R ^c=0.1) and a constant maximum stress intensity level (K _max ^c =0.52 MPa m^1/2). TheR ^c=0.1 test results demonstrated that fatigue resistance increased with increasingM _w, and that the cross-linked sample possessed a higher fatigue threshold than the linear Iow-M_w material. However, the K _max ^c test results revealed the opposite trend, with fatigue resistance decreasing with increasingM _w and chemical crosslinking. The marked change in relative fatigue resistance of the PMMA resins investigated under high mean stress conditions is believed to be a consequence of the competition between two molecular deformation mechanisms: chain scission and chain slippage. The presumed shift in operative mechanism as a function of theR level is reflected in differences noted on the fracture surfaces of the PMMA resins studied. Discontinuous growth band formation, which is indicative of large amounts of chain slippage, is favoured by lowM _w and lowR ratios, but disappears in association with high-M _w and highR-ratio test conditions.     

Synthesis and properties of urethane foams having a N3P3 ring compound

To prepare the flexible and non-flammable urethane foam, the product formed by the reaction of 1,1-diamino 3,3,5,5-tetraphenoxycyclotriphosphazene, N₃P₃ (NH₂)₂ (OC₆H₅)₄, and TDI-80 consisted of 80% 2,4-tolylenediisocyanate and 20% 2,6-tolylenediisocyanate was carried out with polyol(PPG-BM: (oxypropyrene)triol;M _w=3000, hydroxyl number 56) and water in the presence of various concentrations of amine (triethylenediamine) and tin (dibutyltin dilaurate) catalysts. The foam without shrinkage was obtained with 0.1 to 0.2g amine and 0.3 g tin catalysts. It had a limiting oxygen index value of 21 and the tensile strength and elongation of the foam were about 0.075 MPa and 140%, respectively. However, theT _g of the foam estimated from tan δ was −24°C and this was almost the same value as that of the foam without N₃P₃ ring compounds.     

Diffusion of styrene-acrylonitrile copolymers

Tracer diffusion coefficient, D ^*, and the Newtonian zero shear-rate viscosity, ₀, were measured for a high molecular weight random copolymer (SAN) of styrene and acrylonitrile. As predicted by the reptation model, D ^*was in agreement with the molecular-weight scaling of M _w ^–2 (M _w is the weight average molecular weight of the chain being probed) and was independent of matrix molecular weight. ₀ showed a molecular-weight scaling of M _W ^3.4 . While the temperature dependence of these two relaxation processes could be well explained by the free-volume model up to 252 °C, they showed a discrepancy in activation enthalpy, E _a, of about 20%, with a larger value for diffusion.