C. D. Papaspyrides

The effect on the mechanical properties of carbon/epoxy composites of polyamide coatings on the fibers

Abstract An experimental investigation has been carried out to study the effect of polyamide coatings on the mechanical properties of unidirectional carbon/epoxy composites. Carbon fibers were coated with polyamide 6,6 by means of interfacial polyamidation and solution dip coating in a laboratory-scale unit developed for this purpose. The amount of coating deposited on the fibers was determined by thermogravimetric analysis (TGA); the morphology and microscale homogeneity were examined by scanning electron microscopy (SEM). Transverse tensile tests and short-beam shear tests were performed in order to assess the effect of the coating on the adhesion of the fiber to the matrix. Composites containing solution-coated fibers exhibited improved tensile and ILSS performance over those of uncoated-fiber composites, although interfacial coating induced deterioration in performance. The double-cantilever beam (DCB) test was used to investigate the effect of the two different polyamide fiber-surface coatings on mode I interlaminar fracture toughness. Initiation fracture toughness (G ICinit ) increases for both polyamide coatings. Moreover, interfacial coating causes extensive fiber bridging leading to much increased propagation values. The mechanical results are attributed to the different nature of the two polyamide coatings and to differences in the microstructure of the resulting composites.

The effect of hygrothermal history on water sorption and interlaminar shear strength of glass/polyester composites with different interfacial strength

Abstract Glass fabric/polyester composites of varying interfacial strengths have been subjected to water absorption cycles. Modification of the interfacial strength was achieved through different surface treatments of the fabrics. In particular, composites containing clean glass fabrics, fabrics treated with a silane coupling agent and fabrics coated with a polydimethylsiloxane elastomer have been studied. Specimens were immersed in water for a short period of time, dried and subjected to three more reabsorption steps. Measurements of the interlaminar shear strength throughout the sorption–desorption cycles allowed estimation of the interfacial contribution to the absorption behavior.

Polyamide coated Kevlar™ fabric in epoxy resin: mechanical properties and moisture absorption studies

Abstract An experimental investigation was carried out to study the effect of polyamide coatings on the mechanical properties and moisture absorption behavior of epoxy/Kevlar™ composites. Kevlar™ fabrics were coated with polyamide 6,6 by in situ interfacial polymerization coating and solution dip-coating. Short beam shear tests were performed in order to assess the effect of coating on the adhesion of the fiber to the matrix. Both interfacial and solution coated specimens exhibited worse interlaminar shear strength (ILSS) over uncoated fabrics. Furthermore, double cantilever beam tests were also performed to investigate the effect of the coating on the Mode I fracture toughness. Initiation fracture toughness increases for the solution coated fabrics whereas interfacial coating induced deterioration in performance. Finally, water absorption tests were carried out. Weight gain data versus time of immersion were collected at different temperatures and the water uptake at equilibrium ( M ∞ ) and the diffusion coefficient ( D ) were calculated. It seems that there is a direct relationship between the ILSS and the diffusion results. The polyamide coating facilitates debonding of the fibers from the matrix and delamination, allowing more water uptake.

Functional barriers: Properties and evaluation

Functional barriers are multilayer structures deemed to prevent migration of some chemicals released by food-contact materials into food. In the area of plastics packaging, different migration behaviours of mono- and multilayer structures are assessed in terms of lag time and of their influence of the solubility of the migrants in food simulants. Whereas barriers to oxygen or to aromas must prevent the diffusion of these compounds under conditions of use, a functional barrier must also be efficient under processing conditions, to prevent diffusion of substances when the polymer layers are in contact at high (processing) temperatures. Diffusion in melted polymers at high temperatures is much slower for glassy polymers, than in polymers that are rubbery at ambient temperature. To evaluate the behaviour of functional barriers under conditions of use, a set of reference diffusion coefficients in the 40-60°C range were determined for 14 polymers. Conditions for accelerated migration tests are proposed based on worst-case activation energy in the 40-60°C range. For simulation of migration, numerical models are available. The rules derived from the models can be used both by industry (to optimize a material in terms of migration) or by risk assessors. Differences in migration behaviour between mono- and multilayer materials are discussed.

Mechanical recycling of postused high-density polyethylene crates using the restabilization technique. I. Influence of reprocessing

A remelting–restabilization technique was applied for the recycling of postused, yellow-pigmented high-density polyethylene (HDPE) bottle crates. Multiple extrusion cycles procedure, at different reprocessing temperatures, was performed for monitoring the processing stability of the restabilized and nonrestabilized material as reference. In addition, mechanical properties measurements were carried out, to study further the effect of restabilization on the performance of the recycled material. Finally, repigmentation was used to investigate the role of new pigments on the final recycled product. The results illustrate that restabilization of postused crates leads to careful reprocessing without severe degradation, which is an essential prerequisite for reuse in the original application.

Recycling of post-used PE packaging film using the restabilization technique

Abstract The remelting–restabilization technique was applied for the recycling of post-used, polyethylene packaging film. A multiple extrusion cycles procedure was performed at different reprocessing temperatures for monitoring the processing stability of the restabilized material. Furthermore, at different film blowing conditions mechanical properties were evaluated, to study the effect of restabilization on the mechanical performance of the recycled material. The results illustrate that the restabilization of PE film is very impressive in limiting the degradation effect, which is an essential prerequisite for the re-use in the original application, i.e. for closed-loop recycling.

Solid-state polyamidation of nylon salts

Abstract Solid-state polyamidation (SSP) processes may proceed through intermediates of lower melting point, resulting in a distinct solid-melt transition (SMT). In this study an attempt is made to correlate the phenomenon with structural characteristics of the starting material. Nylon salts formed from aliphatic diamines and aliphatic diacids were considered forst and, surprisingly, the same transitional behaviour was eventually detected independent of salt nature. However, when research was oriented to better organized salt structures, to effect increased resistance to hydration, the picture changed: ethylenediammonium fumarate provides no evidence for transition even at a reaction temperature closely approaching its decomposition point.

Recycling of polypropylene‐based composites

The potential of recycling polypropylene-based composites by means of a dissolution process was investigated. The composites were dissolved in an appropriate solvent, and subsequently the polymer solution, containing the fibers in suspension, was filtrated for recovery of the reinforcing agent and of the polymer matrix. Different amounts of hot solvent were employed for washing during filtration, to vary the polymer content remaining on the fibers. The recycled fibers were incorporated in the same polymer matrix, but of virgin quality. The results on the tensile and impact performance of these second generation composites were compared to those of the base material containing unused fibers. It becomes evident that a significant increase of the tensile modulus and strength occurs as the polymer phase deposited on the fibers increases. However, it appears that this interphase alteration has a negative effect on the Izod impact strength. The overall behavior is discussed in terms of fiber dispersion aspects and fiber/matrix adhesion.

Solid-state polyamidation of dodecamethylenediammonium adipate

Abstract The solid-state polyamidation of nylon salts comprises an attractive research field. The low sublimation rate of dodecamethylenediamine ensures salt stability and accurate relationships between amine end-group analysis and extent of the reaction. This accuracy is further enhanced by studying the solid-state polyamidation of dodecamethylenediammonium adipate (DMA), a nylon salt with a low melting point. Reaction rate was found to be influenced drastically by temperature, but the temperature range to effect true solid-state reaction is actually limited. Even at temperatures significantly below the melting point of DMA, a transition of the process, from the solid to the melt state, was clearly observed. Kinetic data obtained were tested against well known solid-state and melt reaction models. Nevertheless, none of them can exclusively describe the polyamidation studied, possibly because the true reaction model should take into consideration the process leading to the solid-melt transition observed.

Recycling of polypropylene by the dissolution/reprecipitation technique: I. A model study

Abstract The dissolution/reprecipitation technique is proposed here for the recycling of polypropylene (PP). It comprises dissolution of the plastic in an appropriate solvent, reprecipitation by using a non-solvent, thorough washing of the material obtained and drying. Furthermore, the solvent mixtures involved are separated by fractional distillation for reuse. In this paper model experiments on virgin material, either in the form of pellets or of commercial pipes for hot water piping applications, are presented. Xylene/acetone proved to be a particularly effective solvent/non-solvent system. Further investigation was focused on the effect of sample history through successive recycling cycles. The recycled grades were evaluated in terms of the following properties: melt flow index (MFI), crystallinity, mechanical performance in tensile and impact mode and grain size analysis. The properties of the recycled grades proved to have no substantial difference from the starting material.