Measurements of slip velocity and frictional heating in the capillary extrusion of linear‐low‐density polyethylene with a fluoropolymer processing aid

The slip velocity and frictional or slip heating of linear‐low density polyethylene with a fluoropolymer processing aid in capillary flow were measured by rheo‐particle image velocimetry and thermal imaging. The pure polymer did not show slip before the stick‐slip regime but exhibited strong slip when blended with the processing additive. However, for shear stresses beyond the stick‐slip regime, the pure polymer and the blend exhibited the same flow behavior with slip. The slip velocity increased with the shear stress at two different rates before and after the stick‐slip and the contribution of slip to the total flow rate exhibited a minimum. Significant rises in temperature were measured under slip and no slip conditions, being these much higher than the values predicted by the adiabatic flow assumption. Clear difference was made between viscous and frictional heating before the stick‐slip regime, even though they could not be distinguished from one another at higher stresses. Overall, in the presence of slip, frictional and viscous heating act synergistically producing higher temperature rises in the melt. Finally, in contrast to predictions by numerical simulations of viscous heating, measured velocity profiles did not evidence the heating effects in the shear stress range analyzed in this work. POLYM. ENG. SCI., 56:837–845, 2016. © 2016 Society of Plastics Engineers     

Stick—slip friction as a method of powder flow characterization

We have made use of a previously ignored quantity, stick—slip friction, in determining powder flow properties. Stick-slip friction can easily be measured with a Table Model Instron or similar device. The stick—slip friction of most powders can be measured with greater accuracy and precision than any type of angle of repose measurements. Stick—slip friction values for a set of plastic powders were related to the particle size, flow properties and tensile strength. While stick—slip friction may not be universally relatable to flow behavior, it does hold promise for more limited sample tests.     

Probing the Physics of Slip–Stick Friction using a Bowed String

Slip–stick vibration driven by friction is important in many applications, and to model it well enough to make reliable predictions requires detailed information about the underlying physical mechanisms of friction. To characterize the frictional behavior of an interface in the stick–slip regime requires measurements that themselves operate in the stick–slip regime. A novel methodology for measurements of this kind is presented, based on the excitation of a stretched string “bowed” with a rod that is coated with the friction material to be investigated. Measurements of the motion of the string allow the friction force and the velocity waveform at the contact point to be determined by inverse calculation. These friction results can be correlated with microscopic analysis of the wear track left in the coated surface. Results are presented using rosin as a friction material. These show that “sticking” involves some temperature-dependent shear flow in the friction material, and that the exact definition of the states of “sticking” and “slipping” is by no means clear-cut. Friction force during slipping shows complex behavior, not well correlated with variations in sliding speed, so that other state variables such as temperature near the interface must play a crucial role. A new constitutive model for rosin friction, based on the repeated formation and healing of unstable shear bands, is suggested.     

Effect of filler content and temperature on steady‐state shear flow of wood/high density polyethylene composites

Steady state shear flow of wood/high density polyethylene composites is investigated through capillary rheometry to gain better insight into rheology, extrudate distortions, and wall slip phenomena of wood/polymer composite melts. Effects of filler content and temperature on onset and end of stick‐slip transition, in terms of shear rate and shear stress, are also studied. Results show that shear rates at stick‐slip transition decrease while corresponding shear stresses increase with the addition of filler. Furthermore, temperature raises the shear rate and the shear stress at which the transition occurs. It is observed a log‐linear relationship in the plots of wall slip versus the shear stress, in particular, increasing the filler content and decreasing the temperature, these plots are shifted to higher shear stress, as a consequence of viscosity increment. Wall slip and filler content play a fundamental role in surface morphology; specifically, extrudates become smoother with increasing filler content and shear rate, whose increment always results in a rise of the wall slip. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Fire resistant optical cables with special plastic coated fibers

It is important to prevent transmission loss increase of optical fiber cables during a fire and fire fighting. One of the main reasons for the loss increase in a fire is the expansion and shrinkage of polymers used in optical fiber cables. The authors have developed a fire resistant optical fiber cable by studying the behavior of coating materials and coated fibers at high temperature. Polymer types investigated are thermoplastics, engineering plastics, and UV cured plastics. The fire resistant optical fibers are coated with a polymer which rapidly produces a carbonized layer by exposure to heat. The rapid carbonizing has the effect of reducing expansion and shrinkage. Besides the coating, the coated fiber is surrounded by carbon yarn to prevent breakage of the fiber. The fire resistant optical fiber cable passed the fire test at 840°C for 30 minutes and VTFT specified IEEE 383, without any breakage of the fibers. The optical loss of the fibers scarcely changed during the tests and after the tests. This paper presents the studies of coating materials and coated fibers at high temperature, the structure and properties of the coated fiber and characteristics of the fire resistant optical fiber cable.     

Fabrication of superhydrophobic fiber coatings by DC‐biased AC‐electrospinning

Mesh‐like fiber mats of polystyrene (PS) were deposited using DC‐biased AC‐electrospinning. Superhydrophobic surfaces with water contact angles greater than 150° and gas fraction values of up to 97% were obtained. Rheological study was conducted on these fiber surfaces and showed a decrease in shear stress when compared with a noncoated surface (no slip), making them excellent candidates for applications requiring the reduction of skin‐friction drag in submerged surfaces. We have also shown that addition of a second, low‐surface energy polymer to a solution of PS can be used to control the fiber internal porosity depending on the concentration of the second polymer. Contact‐angle measurements on mats consisting of porous and nonporous fibers have been used to evaluate the role of the larger spaces between the fibers and the pores on individual fibers on superhydrophobicity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Experimental study of the melt fracture behavior of filled high‐density polyethylene melts

In this work, the melt fracture behavior of microfilled polymer melts based on a high‐density polyethylene (HDPE) was investigated by means of a capillary rheometer, which operated at constant piston velocity. The microfilled melts examined had the same filler content (10 vol%), but differed for the type of filler (glass beads, discontinuous glass fibers, and talc). The results demonstrated that the presence of rigid fillers influences the melt fracture behavior of the filled melts in a way that is dependent on the type of filler dispersed in the HDPE melt. Opposite effects were induced by lamellar particles of talc and by glass fillers (either beads or fibers): the former promoted flow stability, whereas the latter fostered the occurrence of instabilities of “stick‐slip” type. The effects induced by the presence of the glass fillers on the oscillating flow that takes place when “stick‐slip” instabilities occur were also analyzed and discussed. POLYM. ENG. SCI., 54:364–377, 2014. © 2013 Society of Plastics Engineers     

Sharkskin and oscillating melt fracture: Why in slit and capillary dies and not in annular dies?

The sharkskin and stick‐slip polymer extrusion instabilities are studied primarily as functions of the type of die geometry. Experimental observations concerning the flow curves, the critical wall shear stress for the onset of the instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions are presented for linear low‐density polyethylenes. It is found that sharkskin and stick‐slip instabilities are present in the capillary and slit extrusion. However, annular extrusion stick‐slip and sharkskin are absent at high ratios of the inside‐to‐outside diameter of the annular die. This observation also explains the absence of these phenomena in other polymer processing operations such as film blowing. These phenomena are explained in terms of the surface‐to‐volume ratio of the extrudates, that is, if this ratio is high, sharkskin and stick‐slip are absent. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Role of internal additives in the friction and wear of carbon‐fiber‐reinforced polyimide

Polyimide composites should function in sliding contacts under high temperatures, but the interference of carbon fibers with sliding mechanisms is difficult to predict: they often increase the coefficients of friction and act abrasively but show lubricating properties under other conditions. The friction and wear behavior of thermoplastic polyimides reinforced with short carbon fibers and filled with solid internal lubricant (polytetrafluoroethylene) or silicon oil was investigated in this study with a reciprocating cylinder‐on‐plate tester under 50 N at 0.3 m/s with steel counterfaces that were heated at 23–260°C. We concluded that polytetrafluoroethylene additives effectively reduced the coefficients of friction over the entire temperature range, especially under thermally controlled sliding conditions at 120°C, whereas the internal silicon oil increased the coefficients of friction. The wear rates of the fiber‐reinforced polyimide significantly decreased with respect to those of the thermoplastic polyimide, whereas additional fillers slightly increased the wear rates. We further analyzed the role of internal additives by considering the deformation and maximum polymer surface temperature during sliding. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Rheological evaluation of metallocene polyethylenes with processing aids by multi‐wave oscillations

During the die flow of metallocene polyethylenes, flow instabilities may occur. Namely, wall slip, “sharkskin,” and stick‐slip (pressure oscillations) and gross fracture may be obtained depending on the volume flow rate and die geometry. It was reported that fluoroelastomers and boron nitride powders with hexagonal crystal structure can be used as suitable processing aids in melt extrusion processes. Fluoroelastomers at low concentrations act as die lubricants and may eliminate flow instabilities such as surface and stick‐slip melt fracture. On the other hand, specific boron nitride powders may not only eliminate surface and stick‐slip melt fracture, but also postpone gross melt fracture to higher volume flow rates. In this paper, a way for quantitative differentiation of the influence of polymer processing additives on rheological behavior is shown. Standard material functions show no clear‐cut differences. However, using multi‐wave oscillations with higher strain amplitudes make a quantitative assessment possible. Polym. Eng. Sci. 44:2047–2051, 2004. © 2004 Society of Plastics Engineers.     

Strain dependent energy dissipation in multi-scale carbon fiber composites containing carbon nanofibers

This study investigates strain dependent energy dissipation characteristics in carbon nanofiber (CNF) reinforced carbon fiber epoxy composites (multi-scale composites) by characterizing their viscoelastic properties and vibrational damping response. The air damping effect on the energy dissipation characteristics is also examined. The viscoelastic properties of epoxy containing two weight fractions (3 and 5 wt%) of added CNFs were characterized using dynamic mechanical analysis. Carbon fiber layers were then infiltrated with the two epoxy resins containing the CNFs to form multi-scale composites. A strain dependent loss factor behavior of the multi-scale composites was observed in the dynamic cyclic testing due to CNF’s stick–slip friction, showing a 53% increase in loss factor for the composites containing 5 wt% CNFs. The beam vibration test results also indicated an improvement in loss factor for the multi-scale composite beams relative to those without the CNF addition in the first two resonant frequencies. The multi-scale composite beams exhibit an increase in loss factor, up to 43%, at high amplitude excitation, while a reduction in loss factor was seen at low amplitude. These observed strain dependent damping characteristics seem to result from both the stick–slip friction and the air damping effect.     

Flax‐fabric‐reinforced arylated soy protein composites: Brittle‐matrix behavior

Biocomposites were successfully prepared by the reinforcement of soy protein isolate (SPI) with different weight fractions of woven flax fabric. The flax‐fabric‐reinforced SPI‐based composites were then arylated with 2,2‐diphenyl‐2‐hydroxyethanoic acid (DPHEAc) for 4 h to obtain arylated biocomposites. A new method was proposed to determine the amount of carbon dioxide evolved during the arylation of the soy protein in the presence of DPHEAc. Characterizations of the arylated and nonarylated biocomposites were done by Fourier transform infrared spectroscopy, thermogravimetric analysis, and dynamic mechanical thermal analysis. The results indicate that the arylated soy‐protein‐based composites exhibited mechanical behavior like brittle‐matrix composites, which differentiated them from nonarylated soy‐protein‐based composites, which showed mechanical behavior similar to polymer–matrix composites. In the arylated composites, there was clear evidence of a stick–slip mechanism, which perhaps dominated and, therefore, prevented easy deformation of the reinforced film. Scanning electron microscopy studies revealed cracks in the arylated soy protein composites when they were subjected to tensile tests. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Properties of kenaf fiber/polylactic acid biocomposites plasticized with polyethylene glycol

Biocomposites of kenaf fiber (KF) and polylactic acid (PLA) were prepared by an internal mixer and compression molding. PLA was plasticized with polyethylene glycol (PEG) (10 wt%) and evaluated as the polymer matrix (p‐PLA). Fiber loadings were varied between 0 and 40 wt%. The tensile, dynamic mechanical, and morphological properties and water absorption behavior of these composites were studied. Reinforcing effect of KF was observed when fiber loading exceeded 10 wt% despite of the inferior fiber‐matrix adhesion observed via scanning electron microscopy (SEM). Un‐plasticized PLA/KF composite exhibited higher tensile properties than its plasticized counterpart. Fiber breakage and heavily coated short pulled‐out of fibers were observed from the SEM micrographs of the composite. The presence of PEG might have disturbed the fiber‐matrix interaction between KF and PLA in the plasticized composites. Addition of PEG slightly improved the un‐notched impact strength of the composites. Dynamic mechanical analysis showed that the storage and loss moduli of p‐PLA/KF composites increased with the increase in fiber loading due to increasing restrictions to mobility of the polymer molecules. The tan delta of the composites in contrast showed an opposite trend. p‐PLA and p‐PLA/KF composites exhibited non‐Fickian behavior of water absorption. SEM examination revealed microcracks on p‐PLA and p‐PLA/KF surfaces. POLYM. COMPOS., 31:1213–1222, 2010. © 2009 Society of Plastics Engineers     

Using an electronic microbalance technique to study the stick–slip behavior of lubricated polypropylene fibers

By use of an electronic microbalance, fiber/fiber friction measurements were made on cleaned and lubricated polypropylene (PP) filaments. The filaments were coated with a 0.14‐μm‐thick layer of one of the five lubricants: two different hydroxylated oils (hydroxylated oleate and hydroxylated stearate) and three different ethoxylated surfactants [(i) ethoxylated 10 OE dioleate; (ii) ethoxylated 15 OE dioleate; and (iii) ethoxylated 20 OE trioleate]. Despite the thick layer of lubricant applied, stick–slip persisted. Theoretical considerations of experimental conditions (load, speed, and viscosity) show that the pressure is very high at the contact point, and this would induce film thinning and thus stick–slips arising from phase transitions. Statistical evaluations and atomic force microscopy images show that contacts between PP surfaces take place in presence of lubricants because surface asperity heights are larger than the lubricant film thickness. Oils or surfactants having similar surface tensions give different interfiber cohesion. Some explanations correlating the lubricant chemical structure and its spatial conformation, as well as its capacity to form intermolecular bonds and associative organization, to interfiber friction are given. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 645–654, 2003     

Effects of the combination of solid lubricants and short carbon fibers on the sliding performance of poly(ether imide) matrix composites

Solid lubricants, that is, graphite flakes and poly(tetrafluoroethylene) powders, were incorporated with short carbon fibers into a poly(ether imide) matrix to improve the tribological performance. Wear tests were performed with a polymer pin against a mild steel counterpart at a constant sliding speed of 1 m/s under various temperatures and contact pressures. Composites filled with equilibrium contents of solid lubricants and short carbon fibers, that is, 10 vol % of each filler, exhibited the lowest wear rate and friction coefficient. The relatively lower concentration of solid lubricants adversely affected the wear resistance, whereas the friction coefficient did not vary significantly in comparison with the friction coefficient of the composites filled with only short carbon fibers. The improved tribological behavior was attributed to more continuous and effective friction films formed on the material pairs during sliding. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1428–1434, 2004     

Multiple percolation in a carbon‐filled polymer composites via foaming

This article describes an investigation into the effects of foaming on the electrical conductivity for a carbon‐filled cyclic olefin copolymer (COC) composite incorporating both chopped carbon fibers (cCF) and carbon black (CB). Foamed and solid samples were injection molded and then analyzed for cell size, fiber length, fiber orientation, and electrical conductivity. Foamed samples exhibited higher electrical conductivity in the through‐plane direction for materials containing only CB or composites containing both filler types, and reduced electrical conductivity in the cCF‐filled composites. The increased electrical property gained by foaming was attributed to multiple percolation with CB aggregates forming more effective conductive clusters and networks in the continuous polymer phase during growth of the gas domains. A mechanism for the phenomenon was proposed based on these experimental observations. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Stability of high‐bandwidth graded‐index polymer optical fiber

The properties of poly(methyl methacrylate) (PMMA)‐based graded‐index polymer optical fiber (GI POF), including the thermal stability, thermal humidity, and mechanical properties, were studied for polymer optical fiber research and applications. The glass‐transition temperature of the fiber core was 103°C in the presence of the dopant, which was close to that of the PMMA matrix without the dopant. A special refractive‐index profile derived from the distribution of the dopant was stable at 60°C. Moreover, GI POF exhibited good mechanical properties. The excellent performance indicated that GI POF could be applied not only for indoor use but also for outdoor use. However, PMMA‐based GI POF exhibited poor hot‐water/humidity resistance. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2330–2334, 2004     

Optical transparency,thermal resistance,intermolecular interaction,and mechanical properties of poly(styrene‐butadiene‐styrene) copolymer‐based thermoplastic elastomers

The optical transparency, thermal resistance, intermolecular interaction, and mechanical properties of poly(styrene‐block‐butadiene‐block‐styrene) (SBS), which were modified by blending with crystalline polypropylene (PP) or amorphous polystyrene (PS), were analyzed. The dynamic mechanical test indicated that the PP exhibited an intermolecular interaction with SBS and PS was compatible with SBS. The optical properties indicated that the direction of the light was changed due to the difference between the refractive indices of SBS and the added modifiers. Additionally, refraction and reflection occurred at the interface, reducing the transparency of SBS. The thermal resistance of SBS clearly improved upon modification by the addition of crystalline PP polymer. The thermal treatment increased the tensile strength and the elongation at breakage of modified SBS by reducing the internal stress, which was generated during the blending process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The effects of hot pressing on the swelling behavior of P(MMA‐co‐NVP) hydrogel discs

Compression of dehydrated P(MMA‐co‐NVP) hydrogel at the glass transition temperature (161°C) results in discs which exhibit orthotropic expansion when hydrated in water. It has been found that the expansion is nonuniform, with an outer annulus expanding more rapidly than the inner core. This behavior has been investigated by comparing the crosspolarized light microscopy, swelling response, and hardness tests for discs pressed with a lower friction (PTFE) interface and those pressed with a higher friction interface (aluminum). Only in the latter case was a clear boundary observed. The outer 4 mm of a 13 mm disc showed increased radial chain alignment as compared to the inner core, as well as a lower average hardness value (81.7HK compared to 97.7HK). These results were not observed in samples pressed with a PTFE interface. This behavior has subsequently been modeled using axisymmetric loading and assuming Von Mises Criteria, showing that the change in behavior can be explained by the presence of a stick–slip boundary formed during hot pressing. POLYM. ENG. SCI., 55:1290–1295, 2015. © 2015 Society of Plastics Engineers