Thermoset Polyester Research Articles

Structural relaxation in a polyester thermoset as seen by thermally stimulated recovery

The thermally stimulated recovery technique (TSR), also known as thermally stimulated creep, is a low-frequency mechanical spectroscopic technique that is shown to enable the monitoring of the structural relaxation process in a polyester-based thermoset ( T g∼95°C). Thermal sampling experiments are able to isolate the contribution of a narrow window of relaxation times assigned to the strain of the material at different temperatures. Such experiments were performed after an ageing period at the same creep temperature. Three ageing temperatures and several ageing times were investigated. Characteristic parameters of the obtained TSR curves are observed to vary with T a and t a. Such variations are discussed on the basis of the general variation of the viscoelastic response upon structural relaxation, namely the shift of the distribution of relaxation times associated with the glass transition, towards longer times.

The mechanism of action of tin compounds as flame retardants and smoke suppressants for polyester thermosets

The mechanism of the flame retardance of inorganic tin additives has been investigated by examining their effects in three halogenated polyester thermosets, cross-linked with styrene. The techniques used have included oxygen indices, the nitrous oxide index, thermal analysis, pyrolysis-gas chromatography and residue analysis. The residue analysis was carried out on sequentially degraded samples in order to identify the temperature ranges at which volatilisation of both the metal and halogen occurred. Both condensed and vapour mechanisms appeared to be operating. The extent to which either mechanism was observed, depended on both the halogen present and the composition of the polyester. The nature of the interactions between the tin and halogenated polyesters is discussed along with evidence for the formation of the tin halides.

Molecular mobility in a thermoset as seen by TSR and DMA near Tg

The conformational mobility of a polyester thermoset in the glass transition region is studied by dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). All experiments were carried out in a commercial DMA apparatus, in the flexural mode. With this equipment thermally stimulated creep (TSCr) and thermal sampling (TS) experiments were also performed. Each TS curve was analysed according to a simple Voigt-Kelvin model where the retardation time follows the Arrhenius equation. Both the activation energy and the pre-exponential factor show the usual trends with temperature and present the well-known compensation phenomenon. The activation energy increases with temperature in the glassy state and even for lower temperatures the isolated processes are found to be highly cooperative. Two master curves for the storage modulus and the storage compliance were successfully constructed from the isothermal DMA experiments. The Vogel-Fulcher-Tamman-Hesse curve plot was directly obtained from the WLF parameters that were calculated without any non-linear fitting. The complementary character of the TSR and DMA techniques can be seen in an Arrhenius plot. The equivalent frequency of the TSR elementary processes increases with increasing temperature, with values around ∼10-2.8 Hz. Some results obtained by simulation were shown, allowing to conclude that this increase of frequency is a consequence of the increase of the activation energy with temperature in a system showing a compensation behaviour.

Composites Made from Lignocellulosics and Thermoset Polymers

Composites made from a stryrene/unsaturated polyester thermoset matrix and woodflours from different wood species have been prepared and tested. Pine (Pino Eliottis), eucaliptus (Eucaliptus Saligna) and marmelero (Ruprechia Laxiflora), a softwood and two semihard woods respectively, were selected for this study because of the availability and local abundance. The particles were used untreated and chemically modified with maleic anhydride (MAN). Thermogravimetric analysis and analytical techniques were used in the characterization of untreated and treated flours. Dispersion of the fibrous particles, as well as maximum filler concentration (accompanied by complete wetting of the wood fibers) was dependent on the treatment and on the wood species utilized. Bending and compression tests indicated some improvement in the performance of the composites, if the woodflour was previously esterified. Scanning electron microscopy (SEM) allowed to observe changes in the fracture surfaces due to MAN treatment of the fibers.

Fatigue behavior of continuous glass fiber composites: Effect of the matrix nature

AbstractThe effect of the matrix morphology on the fatigue behavior of a continuous glass fiber/polypropylene (GF/PP) composite system was studied by means of stress‐life and mode II cyclic delamination tests. The stress‐life behavior of a GF composite is considerably affected by the nature of the matrix. A two‐stage fatigue damage curve was observed in the composite made with a PP matrix, whereas a three‐stage curve was observed in the composite made with a thermoset polyester matrix. For a fatigue stress higher than 50% of the yield stress, the PP matrix composite showed a considerably longer fatigue life than the thermoset polyester matrix composite. Mode II cyclic delamination tests showed that the morphology itself of the PP matrix also played an important role. Higher fatigue delamination growth rates, at given strain energy release rates, and lower strain energy release rates at failure were obtained for a composite showing a coarse spherulitic morphology and well‐marked interspherulitic regions than for a composite showing a finer spherulitic morphology and less‐marked interspherulitic regions. While the fatigue mode of the composite with a coarse spherulitic morphology was interspherulitic, that of the composite with a finer spherulitic morphology was transpherulitic.

Inorganic tin compounds as flame retardants and smoke suppressants for polyester thermosets

The fire retardant action of inorganic tin additives in polyester resin thermosets cross-linked with styrene has been investigated. Thermal methods were used to analyse the degradation and oxidation index and smoke measurements were also carried out on the resins with and without tin additives. The interaction between the tin compounds and the halogens introduced into the polymer structure using halogen-containing reactive monomers affects the degradation, fire retardant and smoke suppressant properties. The changes are described and the nature of the fire retardant action is discussed.

Pyrolysis–gas-chromatographic study of a series of polyester thermosets

Thirteen different unsaturated polyester prepolymers have been synthesised in an attempt to establish a relationship between the structures of the commonly used polyesters and the pyrolysis products of their cross-linked polymers. All the polyesters contained maleic anhydride units as the unsaturated sites and all were cured using styrene as the cross-linking monomer. The variation in structure involved either the glycol component or the aromatic acid. The cross-linked polyesters were pyrolysed at 600°C and the products separated and identified using gas chromatography and gas chromatography–mass spectrometry. Many products were identified as arising from either the styrene cross-links or the polyester chains. The presence of cinnamate esters confirmed that degradation occurred across both distinct parts of the polymer network. The identity of some other products, particularly the phthalate esters, suggested that more than one degradation mechanism was occurring.

Oak sawdust and hazelnut shells as fillers for a polyester thermoset

Composites made from a styrene cross linked unsaturated polyester resin and vegetable particles (oak sawdust and milled hazelnut shells) were prepared. The influence of the particle's chemical treatment on the final properties of the materials was investigated. The fillers were modified by a simple alkaline treatment or by further addition of acrylic acid to the hydroxyl groups of the lignocellulosic materials. The modification changed the chemistry of the particles as determined by infrared spectroscopy. Three-point bending tests were performed on the composites filled with the different sawdust batches and with untreated milled hazelnut shells. The modulus was maximum for the composite made with alkaline treated sawdust. However, the ultimate strength as well as the displacement were best if the fibers were acrylic acid treated. Composites made with milled shells showed the lowest ultimate properties.

In situ Monitoring of the Cure Process of Fiberglass/Polyester Composites

The theoretical relationship between the extent of cure and the temperature obtained in thermoset polyester resins is introduced. The curing behavior of resins in the presence of reinforcing phases was also investigated using a computer-assisted thermal analysis system. The outcome of the study was that the relationship between the cure time and temperature readings on the specimen can be used to assess the extent of cure.

Thermal Tranesterification of Bis(hydroxymethyl)propane-1,3-diyl Units in (Poly(ethyl terephthalate) and Poly(butylene terephthalate): A Route to Thermoset Polyesters

Copolymerization of 5,5-bis(hydroxymethyl)-2-phenyl-1,3-dioxane with dimethyl terephthalate and either ethylene glycol or 1,4-butanediol affords polyesters with pendant ketals. Acid-promoted debenzalation provides 2,2-bis(hydroxymethyl)propane-1,3-diyl structural units. Thermal transesterification to afford rigid, adhesive thermosets is controlled by the composition of the copolymer and curing conditions (temperature and time).

Thermoelastic and Creep Analysis of a Reinforced Plastic Interrupter Shell Assembly

In the present paper an interrupter shell assembly that forms an essential part of electrical switch gears is considered for creep analysis and subjected to thermal loads in addition to static leads actuated by circuit breakers. This exercise forms a part of the design evaluation of a new product. The shell is made out of a high strength fiberglass reinforced thermoset polyester sheet molding compound. A conventional finite element formulation based on the classical theory of semicoupled thermoelasticity is employed. The creep phenomenon is considered as an initial strain problem and a time hardening law is followed for creep analysis in the secondary zone. The structure is discretized by bilinear thick and thin flat shell isoparametric elements. Results show the contrasting deformation patterns, creep and thermomechanical stresses with the strains for two temperature cases. Creep characteristics of the shell are depicted for the entire service life of the component.

Excimer Laser Cutting: Experimental Characterization and 3D Numerical Modelling for Polyester Resins

Excimer laser has been proposed in these years as a new technology for precision cutting of plastic materials, due to the photo ablative mechanism and to the low thermal damage. Previous experiments, done by the authors, on the cutting of keviar reinforced composites with epoxy matrix, using excimer laser, highlighted very high qualities not attainable with other technologies. In the present work the attention was focused on thermoset polyester resin, because of the extensive use of this material as matrix for plastic composites. The methodology proposed in this work allows to simulate in three dimensions the laser cutting shape and size by means of a photo-ablation approach, that takes also into account the thermal effects. An experimental characterisation of the resin was performed to determine the threshold fluence (energy surface density) and the other material parameters needed to apply a photo-ablation model. The numerical simulations, compared with the experimental cuts, gave results in good accordance in terms of shape and depth of cut.

An investigation into the uv breakdown of thermoset polyester coatings using impedance spectroscopy

The breakdown of powder polyester coatings when exposed to UV b radiation (270–390 nm, peak ~313 nm) has been explored by monitoring changes in their ion transport properties using impedance spectroscopy. The properties of the coatings were measured immediately after formation and also after artificial weathering (with UV radiation, temperature cycling and high humidity). Impedance spectroscopy demonstrated that one manifestation of weathering was the development of an increased level of porosity in the films which could be measured quantitatively. The mechanism of breakdown of the coatings has been shown to be different for clear and pigmented systems, in that the clear coats showed very little change in porosity after initial detection whereas the pigmented systems failed by catastrophic cracking, probably right through the coating. After weathering, the pigmented films consist of a thin brittle surface layer over a thicker more elastic layer, whereas for the clear coatings there is a gradual decrease in brittleness with coating depth. The results from impedance spectroscopy were supported by SEM and gloss loss measurements.

Compositional analysis of samples of thermoset polyester and migration of ethylbenzene and styrene from thermoset polyester into pork during cooking.

The levels of ethylbenzene, styrene, benzene and benzaldehyde in samples of thermoset polyester were determined by dynamic headspace/gas chromatography. Styrene was present at levels from 50 to 1400 mg kg-1, ethylbenzene up to 25 mg kg-1 and benzaldehyde up to 180 mg kg-1. Benzene levels were usually less than 1 mg kg-1, but one article contained 19 mg kg-1. Levels of styrene dimers and trimers were estimated using size exclusion chromatography followed by gas chromatographic (GC) analysis and were present at approximately 100 mg kg-1 and 700 mg kg-1 respectively. The principal identified constituents of solvent extracts were stearic and palmitic acids, present at a combined level of nearly 1% by weight of the plastic. Styrene was shown to be produced by thermal depolymerization at temperatures of 175 degrees C and above. The migration of ethylbenzene and styrene into belly pork cooked in thermoset polyester dishes for 1.5 h at 175 degrees C was measured by Likens-Nickerson extraction of the cooked meat and GC/MS analysis of the extracts. Migration ranged from 6 to 2400 micrograms kg-1 for styrene and from < 6 to 34 micrograms kg-1 for ethylbenzene.

Identification of potential migrants from samples of dual-ovenable plastics.

The analysis of samples of dual-ovenable cookware articles has identified chemicals with the potential to migrate into food. Samples were analysed by dynamic headspace analysis at 200 degrees C using a Tenax TA trap cooled in ice, and/or solvent extraction, followed by gas chromatography/mass spectrometry. Plastics analysed were crystalline poly(ethyleneterephthlalate) (cPET), thermoset polyester, poly(ethersulphone) and poly (4-methyl-pent-1-ene) (TPX). The greatest number and quantity of chemicals with the potential to migrate were found in thermoset polyester.

Fracture of glass/polypropylene laminates: influence of cooling rate after moulding

This paper presents results from mechanical tests on glass fibre-reinforced polypropylene laminates. Materials were manufactured in a press and the average rate of cooling from the forming temperature after pressing was varied from 0.23 to 55°C min −1. The cooling rate was found to affect the matrix structure and to strongly influence fracture behaviour. Faster cooling resulted in improved Mode I and Mode II fracture energies, and smaller delaminated areas after impact. Results are compared with those obtained on a glass fibre-reinforced thermoset polyester composite.

Migration of styrene monomer from thermoset polyester cookware into foods during high temperature applications.

Migration of styrene from thermoset polyester cookware into foods has been studied during normal cooking applications and for the fatty food simulant olive oil under high temperature test conditions. The analysis of foods was by automated headspace gas chromatography/mass spectrometry (GC/MS) in the selected ion mode. Cyclohexane was added to the slurried foods to improve headspace partition and deuterated-styrene was the internal standard. To achieve adequate sensitivity with olive oil, styrene was extracted and concentrated by a steam distillation method and then determined by GC/MS. Styrene in foods cooked in thermoset polyester articles was in the range < 5 to 5 micrograms/kg and 5 to 30 micrograms/kg where the polyester contained 9 and 380 mg/kg residual monomer respectively. Testing for 2 h at 175 degrees C into olive oil resulted in significantly higher migration of styrene than seen for other foods, although there was a marked decrease in migration on repeat-use. Testing of the complete articles as sealed systems filled with olive oil led to higher migration than testing by total immersion of plaques. Of the test methods used with olive oil, testing as filled articles with no lidding gave migration results closest to (but generally higher than) those seen for real foods.

Reactive rubbers as toughening agents for thermoset polyester resins. Molecular analysis by FTIR and fracture behavior of the resulting blends

AbstractA reactive rubber obtained by isocyanation of hydroxyl‐terminated polybutadiene was used as toughening agent for an unsaturated polyester resin. Both the isocyanation and the successive reaction between the modified rubber and the polyester were investigated by Fourier‐transform infrared spectroscopy (FTIR). Fracture measurements at high and low strain rate were carried out on the cured materials to test their toughness. The failure mechanisms were established by morphological analysis of fractured surfaces, performed by scanning electron microscopy. © 1993 John Wiley &amp; Sons, Inc.

Effectiveness of silane treatment on absorbable microfibers.

Preliminary experiments suggest pretreatment of absorbable crystalline, calcium-sodium-metaphosphate (CSM) microfibers with trimethoxy-based silane coupling agents yields a polysiloxane film barrier which protects the fiber surface from early dissolution due to water. Compared to thermoplastic poly(L-lactic acid) (PLLA) composites reinforced with untreated fibers, PLLA composites reinforced with a variety of silane pretreated microfibers showed increased mechanical properties, suggesting improved adhesion was achieved at the fiber/matrix interface. Unfortunately, the silane pretreated CSM/PLLA composite showed no increase in wet strength retention after short-term in vitro exposure. This may be due to plasticization from water entering the composite at areas of incomplete fiber wetting by the highly viscous molten PLLA. However, when a nonabsorbable, low viscosity unsaturated polyester thermoset resin was reinforced with methacryloxy-silane pretreated CSM microfibers, there was no decrease in flexural strength and less than a 10% decrease in flexural modulus after 7 days exposure to 7.4 pH Tris-buffered saline at 37 degrees C.

Migration testing of plastics and microwave‐active materials for high‐temperature food‐use applications

Temperatures have been measured using a fluoro-optic probe at the food/container or food/packaging interfaces as appropriate, for a range of foods heated in either a microwave or a conventional oven. Reheating ready-prepared foods packaged in plastics pouches, trays or dishes in the microwave oven, according to the manufacturers' instructions, resulted in temperatures in the range 61-121 degrees C. Microwave-active materials (susceptors) in contact with ready-prepared foods frequently reached local spot temperatures above 200 degrees C. For foods cooked in a microwave oven according to published recipes, temperatures from 91 degrees C to 200 degrees C were recorded, whilst similar temperatures (92-194 degrees C) were attained in a conventional oven, but over longer periods of time. These measurements form the basis for examining compliance with specific and overall migration limits for plastics materials. The testing conditions proposed depend on the intended use of the plastic - for microwave oven use for aqueous foods, for all lidding materials, and for reheating of foods, testing would only be required with aqueous simulants for 1 h at 100 degrees C; for unspecified microwave oven use, testing with olive oil would be required for 30 min at 150 degrees C; and for unspecified use in a conventional oven testing with olive oil would be required for 2 h at 175 degrees C. For microwave-active materials, it is proposed that testing is carried out in the microwave oven using a novel semi-solid simulant comprising olive oil and water absorbed onto an inert support of diatomaceous earth. The testing in this instance is carried out with the simulant instead of food in a package and heating in the microwave oven at 600 W for 4 min for every 100 g of simulant employed. There is an option in every case to test for migration using real foods rather than simulants if it can be demonstrated that results using simulants are unrepresentative of those for foods. The proposed testing conditions were validated as being realistic by measurement of the specific migration of various components from different plastics into foods under actual conditions of use and comparing with migration into simulants. Migration of plasticizers from PVC and VC/VDC copolymer films was monitored for both microwave reheating and cooking of foods. Total oligomer concentrations were measured from poly(ethylene terephthalate) (PET) trays, and volatile aromatics from thermoset polyester trays, using both types of container in microwave and conventional ovens.(ABSTRACT TRUNCATED AT 400 WORDS)