The relationships between the material parameters, i.e., the fiber fineness, porosity, areal density, layering sequence, and airflow resistivity with the normalincidence sound absorption coefficient of nonwoven composites consisting of three layers have been studied. The monofiber or multifiber needle-punched nonwovens included poly(lactic acid) (PLA), polypropylene (PP), glass fiber, and hemp fibers. Air flow resistivity was statistically modeled and was found to increase with decreasing fiber size and nonwoven porosity. The former models developed for glass fiber mats in the literature were found to be inconsistent with the air flow resistance of the nonwovens reported below. The effects of the layering sequence on air flow resistivity and sound absorption were obtained. It was found that when the layer including reinforcement fibers, i.e., hemp or glass fiber, faced the air flow/sound source, the air flow resistance and the absorption coefficient were higher than the case when the layer including reinforcement fibers was farthest from the air flow/sound source. The difference was more pronounced if there was a greater difference between the resistivity values of the constituent layers of the nonwoven composite. Sound absorption coefficient was statistically modeled in terms of air flow resistivity and frequency.

A method to determine tortuosity in a fibrous porous medium is proposed. A new approach for sample preparation and testing has been followed to establish a relationship between air permeability and fiberweb thickness which formed the basis for the determination of tortuosity in fibrous porous media. An empirical relationship between tortuosity and fiberweb structural properties including porosity, fiber diameter and fiberweb thickness has been proposed unlike the models in the literature which have expressed tortuosity as a function of porosity only. Transverse air flow through a fibrous porous media increasingly becomes less tortuous with increasing porosity, with the value of tortuosity approaching 1 at upper limits of porosity. Tortuosity also decreased with increase in fiber diameter whereas increase in fiberweb thickness resulted in the increase in tortuosity within the range of fiberweb thickness tested.

The average effective thermal conductivity Keff is measured for forty-eight multilayered needle-punched nonwoven samples. Samples are produced using glass and ceramic fibers layered in several different constructions and punched with needles with varying numbers of barbs. The thermal conductivities are determined at steady state, using a Holometrix guard hot plate at an average applied temperature of 455°C. Statistical results show an ability to predict effective thermal conductivity with greater than 88% accuracy. Important parameters of the model include fabric weight, thickness, porosity, and structure, along with the applied temperature. Results also show that the nine-barbed structure with the highest ceramic content has the greatest potential for thermal insulation at elevated temperatures.

A theoretical equation of the combined thermal conductive, convective, and radiative heat flow through heterogeneous multilayer fibrous materials is presented. Samples whose properties are analyzed by this equation were constructed from glass and ceramic webs and used in an earlier work to experimentally determine their thermal conductivities. In that experimental work, overall effective thermal conductivities were determined using a guarded hot plate instrument with temperatures ranging from 430 to 480°C. In the theoretical equation presented here, thermal convective heat flow is ignored because of fabric structural conditions, and the conduction component of the overall conductivity is determined by Fricke's equation. Furthermore, the results of Fricke's equation and the overall effective thermal conductivity are used to estimate the radiative thermal conductivity of the samples.Heat transfer through porous materials can be attributed to the simultaneous operation of three mechanisms: solid-to-solid conduction, gas conduction, and radiation [3, 10, 13, 17]. Viskanta [ 18] stated that due to interactions, these three mechanisms, strictly speaking, are not separable. In some special instances, however, treating total, heat transfer as the sum of the three independent contributions can be a reasonable approximation. As a general principle, separation of conduction and radiation is not valid. If the distance between the two surfaces is small and if they are separated by a transparent medium, only then is it a tolerably good approximation to ignore the interaction of conduction and radiation [18]. Radiation leaving a surface and passing through the material may pass through the voids in the porous materials, be transmitted through the particles, be absorbed by the particles and subsequently re-emitted, or be scattered by the particles [ 18].Heat transfer by simultaneous conduction and radiation in thermal radiation absorbing, emitting, and scattering materials was investigated theoretically by Viskanta [18]. He considered a one-dimensional system consisting of two diffuse, nonblack, isothermal, parallel plates separated by a finite distance. The space between the two plates was filled with an isotropically scattering material, and the problem was formulated exactly in terms of integro-differential and integral equations. The results both defined and illustrated several mechanisms of radiant energy transfer and showed how one mode of heat transfer influenced the other.The &dquo;state of the art&dquo; for heat transfer in materials of this type was reported by Wechsler and Glaser [20]. When radiation is coupled with the other modes of heat transfer, the energy equation, which is normally a differential equation, becomes a nonlinear integro-differential equation. This comes about because the radiative contribution to the total energy flux is due, in part, to the geometric configuration of the system and reflection [ 19]. There are no general solutions available for integrodifferential equations, but a few attempts ...

Nonwovens have been increasingly used in car interiors for noise reduction. Most of these nonwovens are subjected to thermal treatments to give the nonwovens their final three-dimensional forms. Therefore, it became crucial to investigate the effects of thermal treatment on sound absorption characteristics of nonwovens. In this study, the effects of the material and treatment parameters on airflow resistivity and normalincidence sound absorption coefficient of thermally treated three-layered nonwoven composites have been studied. The material parameters included fiber size and porosity. The treatment factors included the temperature and duration. The thermally treated three-layered nonwoven composites are classified into three types based on the material content and fiber blend. Sandwich structures consisting of polylactide/hemp/polylactide and polypropylene/glassfiber/polypropylene layers were called LHL and PGP, respectively. The sample which consisted of three layers of an intimate blend of polypropylene-glassfiber was named as PGI. Both temperature and duration of thermal treatment have been found to affect air flow resistivity and sound absorption. An increase in air flow resistivity and a decrease in sound absorption have been detected with heat treatment. A similarity has been observed between the thermal behaviors of PGP and PGI, which included the same thermoplastic polymer fiber. Variation in air flow resistivity of sandwich structure nonwoven composites increased with the increase in temperature, which was not observed in the intimate blend ones. The air flow resistivity of heat-treated nonwovens followed a steeper trend compared to unheated nonwovens per change in material parameters. In terms of treatment parameters, the difference