Nihat Ali Isitman

Mass attenuation coefficients, effective atomic numbers and effective electron densities for some polymers

In this study, the total mass attenuation coefficients (μ(m)) for some homo- and hetero-chain polymers, namely polyamide-6 (PA-6), poly-methyl methacrylate (PMMA), low-density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) were measured at 59.5, 511, 661.6, 1173.2, 1274.5 and 1332.5 keV photon energies. The samples were separately irradiated with (241)Am, (22)Na, (137)Cs and (60)Co (638 kBq) radioactive gamma sources. The measurements were made by performing transmission experiments with a 2″×2″ NaI(Tl) scintillation detector having an energy resolution of 7 % at 662 keV gamma ray from the decay of (137)Cs. The effective atomic numbers (Z(eff)) and the effective electron densities (N(eff)) were determined experimentally and theoretically using the obtained μ(m) values for the investigated samples. Furthermore, Z(eff) and N(eff) of each polymer were computed for total photon interaction cross-sections using theoretical data over a wide energy region from 1 keV to 10 MeV. The experimental values of the selected polymers were found to be in good agreement with the theoretical values.

Interfacial Interactions and Flammability of Flame-Retarded and Short Fiber-Reinforced Polyamides

Interfacial properties, crystallinity and flammability of short fiber reinforced and flame retarded polyamide 6 and polyamide 66 compounds are investigated, emphasizing the influence of flame retardant fillers on the resistance of fiber/matrix interface to shear. Interfacial shear strengths are derived through a micromechanical approach by determining the tensile properties and residual fiber length distributions. Validated by fracture morphologies, interfacial strengths are found to be governed by filler – induced apparent crystallinities and fractional occurrence of polyamide polymorphs, obtained via peak deconvolution of X-Ray diffraction patterns. Although flame retardant additives based on Br/Sb synergism are found to impart excellent flammability reductions regarding oxygen index and UL94 classifications (V-0 rating), degree of crystallinity; thus, interfacial properties are deteriorated due to lowered thermal expansion and increased cooling rates. Red phosphorus as a flame retardant also induces a UL94 V-0 and significant reduction in flammability together with the facts that crystallinity is not altered and a strong fiber/matrix interface is maintained. Use of melamine cyanurate in an unreinforced polyamide improves the limiting oxygen index considerably; however, the UL94 rating remains unchanged as V-2 as a consequence of increased level of melt dripping. Melamine cyanurate additionally increases the degree of crystallinity through promotion of heterogeneous nucleation.

Effect of partial substitution of aluminum hydroxide with colemanite in fire retarded low-density polyethylene

Metal hydroxides have long been considered as abundant and low-cost fillers for the development of halogen-free flame-retarded polyolefins. However, large filler loading levels are required in the matrix resin to achieve satisfactory fire retardant performance, which results in deteriorated processing characteristics and poor mechanical properties of compounds. Therefore, this study was aimed at improving the fire retardancy of low-density polyethylene and decreasing the total filler loading in the resin using combinations of aluminum hydroxide and a natural hydrated calcium borate, namely, colemanite mineral. Flame retardancy was studied by the mass loss calorimeter analysis, limiting oxygen index measurements, and UL94 classification. Peak heat release rate and total heat evolved were lowered by around 20% and 15%, respectively, with partial replacement of aluminum hydroxide by colemanite, which facilitated the use of lower total filler loading in the resin. Depending on colemanite content and total filler loading, limiting oxygen index was increased by 1%–4% while maintaining the UL94 V-0 rating. Fire retardant effect of colemanite used in conjunction with aluminum hydroxide was attributed to better protective character of fire residues and more effective fuel trapping in the condensed phase. Lower elastic modulus and higher ductility was obtained due to decreased total filler loading in the matrix.

Interactions at fiber/matrix interface in short fiber reinforced amorphous thermoplastic composites modified with micro- and nano-fillers

This study aims at systematically extracting fiber/matrix interfacial strength in short-glass fiber-reinforced polymer composites using an experimental micromechanics approach which employs mechanical properties and residual fiber length distributions to derive the apparent interfacial shear strength. We started from neat high-impact polystyrene matrix short-glass fiber-reinforced composites (HIPS/GF) with varying fiber loading and proceeded toward HIPS/GF hybrid composites containing micro- and nano-fillers where complex fiber/matrix interfacial interactions exist. It was found that apparent interfacial shear strength does not vary with fiber content, while the presence of fillers with different length-scales alters fiber/matrix interactions depending on their influence on physical properties of the polymer matrix, particularly in the vicinity of reinforcing fiber surfaces.

Halogen-free Flame Retardants that Outperform Halogenated Counterparts in Glass Fiber Reinforced Polyamides

Flammability, fire performance, and thermal stability of short glass fiber reinforced polyamide-6 and polyamide-66 containing halogenated and halogen-free flame retardants (FRs) were compared. Flammabilities were assessed by limiting oxygen index tests and UL94 classifications. Fire behavior was evaluated by mass loss cone calorimetry, a bench-scale tool, to assess fire performance of materials. Halogen-free, phosphorus-based FRs were shown to perform superior to halogenated counterparts on the basis of important fire properties, peak heat release rate, time to ignition, and fire growth index. Moreover, thermal stabilities were maintained at an acceptable level as a clear advantage of halogen-free FRs.

Continuum Micro-mechanics of Estimating Interfacial Shear Strength in Short Fiber Composites

A new continuum approach to micro-mechanics of short fiber composites yielded two separate methods of estimating the apparent interfacial shear strength and fiber orientation efficiency. The methods exploit the compilation of the effects of fiber length distribution and interfacial shear strength on strengthening efficiency into a function of strain. The In-Built Method derives a unique combination of apparent interfacial shear strength and fiber orientation efficiency being able to reproduce the experimental stress–strain curve of a short fiber reinforced composite with a very low residual standard deviation. The Boundary Method accomplishes rapid interfacial shear strength screening in materials selection by constructing and utilizing the proposed selection chart.