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The barrier and mechanical properties of biodegradable melt-mixed polyesteramide/octadecylamine-treated montmorillonite clay (filler) have been studied. Extruded films containing 5 and 13 wt% filler were prepared by melt-mixing. Samples compression molded after extrusion was also studied. Oxygen and water transmission rates were measured. X-ray diffraction was used to assess the periodic distance of the clay layers and transmission electron microscopy was used to assess the composite morphology. An increase of the periodic distance from 23.7 Å for pure filler to 32-36 Å for the processed composites was observed. This suggested that the collapsed stacks of clay particle of the first order X-ray reflection became intercalated upon extrusion. A decrease in the intensity with increasing rotation speed was observed, which suggested that higher shear rates promoted delamination, especially in composites with higher filler content. Transmission electron microscopy indicated that a sizable portion of the clay stacks were delaminated into smaller aggregates, containing generally one to three clay sheets. Density measurements indicated that shear-induced voids were formed in the nanocomposite, and these were, according to transmission electron microscopy, almost exclusively located between the clay sheets. The presence of voids limited the improvement in barrier properties with increasing filler content However, the very large improvement in stiffness and strength with filler content indicated that these properties were unaffected by these voids., INTRODUCTION Polymer nanocomposites have received considerable attention since the discovery that polymer properties may be greatly improved by the presence of nanosized particles. Since the introduction of the concept of [...]

Material samples of silicone rubber with known differences in their composition, i.e. different filler content and extra silicone oil added, have been aged at the Anneberg field station on the west coast of Sweden. ac or dc voltage supplied to cylindrical samples at stress levels of 50 or 100 V/mm. The work includes laboratory examination of material changes together with on-site, visual observations and leakage current measurements. Material samplings for the laboratory tests were made after 18 months of electrical aging, which went on for one more year in order to gather further information on the long-term electrical performance of the material. The dominant aging factor was the level of the applied stress, independent of ac or dc voltage. The dc stressed samples showed a higher leakage current and exhibited larger surface degradation compared with samples exposed to ac voltage. The material parameter, an addition of extra silicone oil, initially lead to an increase in adhesion of pollutants, whereas the overall performance was improved by the higher suppression of the leakage current related to oligomer diffusion. Samples with lower level of alumina trihydrate (ATH) exhibited a delayed onset of degradation, but once damaged they degraded more rapidly than the specimens with a higher ATH content. Infrared spectroscopy showed that the ATH was completely consumed at the eroded surface regions. The aging of the surfaces was further assessed by X-ray photoelectron spectroscopy (XES) and scanning electron microscopy (SEM). The low molar mass siloxanes present in the pollution layer were extracted and analyzed by size exclusion chromatography and gas chromatography-mass spectroscopy. The results indicated that the main degradation factor was thermal depolymerization activated by electrical discharges. Oxidative crosslinking of the silicone rubber, usually attributed to surface close corona discharges, appeared to have played a minor role.

Data for the diffusivity and solubility of methanol in blends of polyethersulfone (PES) and a liquid crystalline copolyester (poly(hydroxy-benzoic acid (73 mol%)-cohydroxy-naphthoic acid (27 mol° (LCP) are reported. Samples taken from injection molded and compression molded specimens over a wide composition range were studied by gravimetry (sorption and desorption), dielectric spectroscopy, differential scanning calorimetry and scanning electron microscopy. The sorption of methanol in PES led to the formation of an ordered phase, The sorption curves were S-shaped and they could be described by a diffusion model assuming a concentration-dependent diffusivity and time-dependent boundary conditions. The solubility of methanol in the blends was strictly proportional to the PES content. The zero-concentration-diffusivity decreased strongly with increasing LCP content and it was dependent on the morphology. Injection molding yielded samples with a fibrillar LCP phase with a greater continuity of the PES component and a high diffusivity (in relative terms). Compression molding led to samples with a more continuous LCP component even at low LCP contents and a low diffusivity., INTRODUCTION Thermotropic liquid crystalline polymers (LCP) consist of densely packed rigid polymer chains with a substantial degree of intermolecular interaction. These dense packing and the polarity of the commercially available [...]

Rheo-infrared spectroscopy was used to study the development of orientation of molten narrow molar mass fractions of poly(ethylene oxide) [molar masses between 18,000 and 120,000 g/mol] during non-Newtonian shear flow at shear rates between 2 and 270 [s.sup.-1] and temperatures between 75 and 100 ° C. The steady state degree of orientation [expressed as the Hermans orientation function (f.sub.ss)] reached a saturation level with increasing shear rate; [f.sub.ss] increased with increasing molar mass (M) according to [f.sub.ss] = [C.sub.1] - [C.sub.2]/M ([C.sub.1] and [C.sub.2] are coefficients; the latter depended on shear rate and temperature). The coefficient [C.sub.1] ([f.sub.ss]) for a polymer with infinite molar mass took a universal value close to 0.05 for the temperatures and shear rates used). Under large shear stresses, the relationship between stress and orientation deviated markedly from linearity. The time to establish a steady state level of orientation was proportional to [M.sup.1/2]. The recovery of the isotropic state after the cessation of shear could initially be described by a simple exponential relaxation law: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], where [Τ.sub.Ρ] is the relaxation time. The latter showed a weak molar mass dependence according to [Τ.sub.r] ∞ [M.sup.0.6] and an Arrhenius temperature dependence with an activation energy of ~60 kJ/mol. The relaxation of the shear stress after the cessation of shear was more rapid than the recovery of the isotropic state., INTRODUCTION When a molten polymer is exposed to a stress field, it changes its chain conformation from the 'chaotic' entangled isotropic state to become oriented along the direction of deformation [...]

A method is presented for assessment of lifetime of polyethylene pipes undergoing thermal oxidation (Stage III failure) based on oxidation induction time (OIT) data obtained by DSC. The method requires pressure testing for limited time periods, sampling from unexposed and exposed pipes, the establishment of a linear relationship between the OIT and the antioxidant concentration, and the taking of OIT profiles through the pipe wall. The method is not applicable to pipes with improper dispersion of the antioxidant. A mathematical procedure based on a model for the migration and chemical consumption of the antioxidant system is used to extrapolate short-term OIT profiles to obtain the OfT profiles after extended exposure. The lifetime prediction is also based on the empirical establishment of a criterion, a critical OIT value, for the onset of thermal oxidation of the polymer (Regime C). The total lifetime is then obtained under the assumption that the time period of Regime C constitutes 10% of the total lifetime. The pressure testing time required to obtain adequate data for modeling is only a fraction (20% to 70%) of the failure times of the pipes studied. The average difference between the actual and predicted times was 15% for the MDPE pipes studied., INTRODUCTION Lifetime predictions for pipes are typically based on data obtained by long-term hydrostatic pressure testing at several elevated temperatures. The procedure is schematically described in Figures 1 and 2. [...]

The development of orientation in molten narrow molar mass fractions of polystyrene during shear flow in the non-Newtonian shear rate range (11-24 [s.sup.-1]) at temperatures between 225 and 275 ° C has been assessed by measurement of the infrared dichroism. Deviation from linearity between shear stress and orientation was found at high shear stresses in the steady state region. A sudden change in the shear rate caused a change in the shear stress that was one to two orders of magnitude faster than the response in the Hermans orientation function (f). The steady-state level of the Hermans orientation function ([f.sub.ss]) was dependent on temperature, shear rate and molar mass (M): [f.sub.ss] = [C.sub.1] - [C.sub.2]/M (where [C.sub.1] and [C.sub.2] are constants). The time to reach steady state on the inception of shear was proportional to [M.sup.1/2]. The recovery of the isotropic state after the cessation of shear followed initially a simple exponential law: f [varies] [e.sup.-t/Τ], where t is the time and Τ is the relaxation time. The relaxation time showed only a weak molar mass dependence (Τ [varies] [M.sub.0.8]), indicating that the decay of orientation was initially governed by processes other than reptation., INTRODUCTION Sheared molten polymers of sufficiently high molar mass undergo orientation of the molecules along the shear direction. It is known that 'internal' factors such as the structure of the [...]

Films based on a wide range of thermoplastic and XLPE (crosslinked polyethylene) including compounds containing aromatic voltage stabilizers (fluoranthene, naphthalene, pyrene, oterphenyl, vinylnaphthalene and anthracene) were exposed to external PD (partial discharge) in dry air at 298 K. Very low molar mass PE, [Mathematical Expression Omitted] g/mol, exhibited a very low resistance towards PD. It is believed that the low concentration of interlamellar tie chains and the pronounced mechanical brittleness caused early void formation induced by the mechanical action of the plasma. PE with a higher molar mass showed a PD life that was essentially independent of molar mass, degree of crosslinking, crystallinity and crystal thickness. The added voltage stabilizers had no effect on the PD lifetime.

This paper presents oxidation induction time (OIT) data for samples taken from a range of polyethylene hot-water pipes before and after hydrostatic pressure testing with internal stagnant water and external air. Linear relationships between OIT and antioxidant concentration were established for the antioxidants/polymers used. A model assuming Fickian diffusion of the antioxidant to the surrounding media and chemical consumption of the antioxidant (Regime B model) was applied to the OIT-profile data. The Regime B model was successfully applied to OIT-profile data except for in the case of a pipe with a substantial scatter in the OIT data, indicative of compositional heterogeneity. The choice of initial conditions for the modeling was critical. The use of initial conditions based on insufficiently exposed pipes led to an overestimate of the diffusivities caused by the combined action of Regimes A (internal precipitation) and B loss mechanisms. The time period associated with Regime A constituted up to 25% of the lifetime for the pipes studied. Antioxidants with sterically accessible polar groups showed a higher melting point and greater interaction with dissolved water and carbon black. Obtained radial dependences of the antioxidant diffusivities (D) indicate that the water concentration in the polymer influenced D primarily through cluster formation involving water and antioxidant molecules and by competition between water and antioxidant molecules about adsorption sites on carbon black particles and to a much lesser extent by plasticization. Antioxidant concentration profiles calculated for the failure time interval in pressure testing appeared in the same concentration range, and fracture initiation occurs in the regions in the pipe wall first reaching depletion of the antioxidant system., INTRODUCTION This paper is a continuation of a series of papers (1-8) dealing with the phenomenology and the underlying mechanisms of the failure of polyolefin hot-water pipes. Yielding, craze fracture, [...]

The resistance to slow crack growth and to internal over-pressure of pipes based on a commercial polyethylene gas pipe grade (BPE; 0.6 mol% butyl branches and ) blended with both an injection molding grade linear polyethylene (LPE; [Mathematical Expression Omitted]) and 0.5 ± 0.1 mm glass spheres was studied. The blends and the pure polymer components were characterized by density measurements, differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy. The fracture properties of extruded pipes were measured using hydrostatic pressure testing and notched uniaxial testing on samples cut from the pipe wall. The polymer components were macroscopically uniformly blended and differential scanning calorimetry indicated the occurrence of partial co-crystallization between the branched and linear components. Transmission electron microscopy showed molar mass segregation on a 100 nm level. Notched uniaxial testing showed that the slow crack growth resistance of pure BPE was considerably higher than that of the LPE/BPE blend with 30% LPE. Fractography indicated that the fracture-initiating particles were larger in pipes failing after shorter period of time in the hydrostatic pressure testing. The lifetimes of hydrostatic pressure tested pipes based on the BPE grade containing glass spheres were similar to those of pipes based on the LPE/BPE blend with 30% LPE., INTRODUCTION Failures by slow crack growth, as observed in polyethylene (PE) pipes at relatively low stresses, is believed to occur through the disentanglement of tie chains (1, 2). The average [...]

The extensive initial loss of efficiency of antioxidant [4,4′-thiobis(3-methyl-6-tertbutylphenol)], during hydrostatic pressure testing of medium density polyethylene pipes at 95 and 105 ° C has been studied. Samples from the pressure-tested pipes were subsequently investigated by scanning electron microscopy and energy dispersive spectroscopy. Antioxidant-containing particles were found in the pipe wall. The particles had evolved during the early stages of exposure and consisted of antioxidant that had phase separated and precipitated from the polymer matrix because the initial antioxidant concentration was greater than the solubility limit at the pressure testing temperatures. Relative antioxidant concentration profiles over the pipe wall, as measured by calorimetric oxidation induction time analysis, showed losses of > 80% of the initial effective antioxidant content during the first 1000 h of pressure testing. A model assuming diffusion of antioxidant into precipitates was developed. Values for the diffusivity of the antioxidant in different parts of the pipe wall were obtained by fitting a previously developed model to long-term antioxidant concentration profiles. Using these values, and by using the distance between two adjacent precipitates as an adjustable parameter, it was possible to fit the newly developed model to the experimental short-term antioxidant concentration profiles., INTRODUCTION This paper is a continuation of a series of papers (1-7) dealing with the deterioration and failure of polyolefin hot-water pipes. In this application there is a competition between [...]

Elastomers based on polydimethylsiloxane (PDMS) are used as insulating material in outdoor electrical power applications. It is believed that migration of small molecule PDMS species plays an important role in the recovery of hydrophobicity of oxidized or polluted PDMS elastomer surfaces. This paper reports data on diffusivity and solubility of low molar mass PDMS liquids in PDMS rubbers [Mathematical Expression Omitted] obtained by sorption measurements. It was found that the diffusivity (D) of linear PDMS liquids was approximately independent of the concentration of penetrant and that in the molar mass range [Mathematical Expression Omitted] it decreased with molar mass [Mathematical Expression Omitted] of the diffusing liquid according to [Mathematical Expression Omitted]. Theory and previous data for other oligomers and elastomers predict that D is proportional to [M.sup.-1]. Linear PDMS liquids of lower molar mass exhibited a stronger molar mass dependence. The diffusivity of a given PDMS liquid increased with increasing elastomer crosslink density. The activation energy of the diffusivity was constant at 15.5 ± 2 kJ/mol for linear PDMS liquids of [Mathematical Expression Omitted] larger than 1000 g/[mol.sup.-1] with only a negligible influence of network density and filler content. The activation energy of the lowest molar mass penetrant was considerably lower, 6 to 7 kJ/mol. The solubility increased markedly with decreasing molar mass of the penetrant and with decreasing elastomer crosslink density., INTRODUCTION Silicone elastomers based on polydimethylsiloxane (PDMS) are currently replacing porcelain and glass insulations used in outdoor electrical power applications due to their favorable surface properties, low density, and good [...]

The microstructure and resistance to slow crack growth of two commercial polyethylene pipe materials were studied. Differential scanning calorimetry, small-angle X-ray scattering, and transmission electron microscopy were used to reveal the crystallite thickness and width distributions and the size of the lamellar stacks. The resistance to slow crack growth was assessed by uniaxial constant loading of notched specimens and by hydrostatic pressure testing of notched and unnotched pipes. The high-density material contained roof-lamellae, suggestive of a segregation of low molar mass species. Notched uniaxial testing revealed large differences in slow crack growth-resistance between the two PEs despite the fact that the average tie-chain concentration was similar. Hence, low-molar mass segregation, which was found to be higher for the high-density material, definitely decreases the resistance to slow crack growth. Notched uniaxial testing was a sensitive method for ranking these PEs according to their resistance to slow crack growth, and 15 times faster than that achieved in conventional unnotched pressure testing. Failure time extrapolations from higher temperatures to 20 ° C were made, using a multiple linear regression method (SEM-Q1), the Arrhenius equation, and universal shift-functions to investigate their applicability. The extrapolations resulted in longer life times compared with experimental data, regardless of the method used. The SEM-Q1 method (lower-confidence-limit data) gave the best fit to the 20 ° C experimental data followed by the Arrhenius equation., INTRODUCTION The lifetime of plastic pipes is controlled by material, environment, and loading factors. The creep rupture curve of polyethylene (PE) pipes may be divided into three parts, hereafter denoted [...]

A rotational parallel plate rheometer that enables simultaneous measurement of the transient or steady-state rheological properties and infrared dichroism was designed and constructed to study orientation in molten polymers. Measurements can be carried out at shear rates between 0.05 and 300 [s.sup.-1] and at temperatures between 20 and 300 ° C. Both shear stress and axial normal force together with dichroism are continuously measured during shear flow. Infrared dichroism data for polypropylene showed excellent agreement with data obtained with a FTIR-instrument. The Hermans orientation function for molten poly(dimethyl siloxane) at steady state showed a strong shear-rate dependence in the region 0.1-20 [s.sup.-1]. Rheological data for molten poly(dimethyl siloxane) agrees with data obtained from a conventional rheometer., INTRODUCTION We describe the design and the operation of a novel rotational rheometer that includes a system for the measurement of infrared dichroism. We demonstrate that reliable rheological data can [...]

A program was developed that yields the number of chain scission and crosslinking events representing the best fit of a simulated molar mass distribution to a molar mass distribution determined by size exclusion chromatography. The program was applied to molar mass distributions of samples taken from pressure-tested pipes of an unstabilized medium-density polyethylene. Good fits were obtained after various exposure times at different temperatures and at different positions in the pipe wall. The ratio of the number of chain scission to crosslinking events generally decreased with increasing exposure time and with decreasing accessibility to oxygen in the pipe wall. Correction for long-chain branching effects led to significant changes in the optimum chain scission and crosslinking values obtained. The broad and asymmetric shape of the molar mass distributions, typical of degraded material from originally stabilized medium-density pipes, was shown to arise from a combination of a uniformly degraded and a nondegraded component. The program was also successfully applied to the molar mass distribution of a sample taken from a degraded commercial polybutylene-1 pipe., INTRODUCTION This paper is a continuation of a series of papers (1-5) dealing with the aging of polyolefin hot water pipes. There is a competition between mechanical phenomena, such as [...]

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions ([M.sub.w] = 2500 and 76000), which were either quenched in air from the melt or isothermally crystallized at 123 [degrees] C. Although the morphology and initial mechanical properties of the materials tested were significantly different, the vented tree growth characteristics were similar for all of them. This is in disagreement with the electromechanical models of water treeing, which correlate water tree growth with the fracture toughness of the material. Time to breakdown distributions were also similar for both LDPE and the binary LPE blends, which indicates that, regardless of the initial material morphology and the actual structure of water trees, the length of water trees is one of the controlling factors in insulation failure. The visible light image of water trees in LPE blends did not disappear upon drying as it usually does in LDPE and crosslinked polyethylene insulation.

An extrusion-grade medium-density polyethylene was fractionated by size exclusion chromatography and selective solvent extraction. The fractions were studied by infrared spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Infrared spectroscopy showed that chain branching was concentrated to the low molar mass material. Transmission electron microscopy showed the presence of roof-shaped crystals in the low molar mass samples and curved S- or C-shaped lamellae in the high molar mass samples. These features could be used for identification of different molar mass species in the unfractionated sample., INTRODUCTION The development of polyethylenes of greater fracture toughness has occurred stepwise or through different 'generations.' The first generation of high-density polyethylenes (HDPE) used in gas pipes installed before 1975 [...]

The microstructure and the thermal and mechanical properties of injection molde samples of different blends of Vectra (LCP) and poly(butylene terephthalate) (PBT) have been studied. Differential scanning calorimetry and hot-stage polarized light microscopy showed that the crystallization of PBT was unaffecte by the presence of LCP. X-ray diffraction showed that the PBT component was always unoriented in the injection molded samples. Blends with less than 28 vol LCP exhibited the same stiffness and the same coefficient of linear thermal expansion as PBT. Blends containing more than 38 vol% LCP contained an oriented LCP phase and had a stiffness in accordance with the upper-bound composite equation. The coefficients of linear thermal expansion for these blends were close to that of pure LCP., INTRODUCTION During recent years, blends of thermotropic liquid crystalline polymers (LCP) and isotropic polymers have received considerable attention (1-12). A parallel paper (13) presents a detailed summary of the earlier [...]

Pipes of an unstabilized medium-density polyethylene have been pressure tested with internal stagnant water and moderately circulating air as the external medium at temperatures ranging from 70 to 105 ° C and changes in molecular structure and crystallinity have been studied. The stage III (fractur induced by thermal oxidation) life of the unstabilized polyethylene pipes was less than 12% of the life of the corresponding stabilized polyethylene pipes. Infrared spectroscopy and size exclusion chromatography showed an earlier and more extensive increase in the quantities of oxidation end-products and a more pronounced decrease in molar mass of the outer-wall-material than of the inner-wall-material of the pipe. Mass crystallinity, measured by differential scanning calorimetry, increased on an average by a quantity corresponding to 45 methylene groups per chain scission event. The life of the unstabilized pipe wa divided into an induction period during which no detectable thermal oxidation occurred and a subsequent polymer degradation period. The induction period exhibited an Arrhenius-temperature-dependence with an activation energy of 75 k [mol.sup.-1]., INTRODUCTION This paper is a continuation of a series of papers (1-4) dealing with aging processes involved in the deterioration of polyolefin hot-water pipes. There is a competition between a [...]

Injection molded samples of binary blends of Vectra (LCP) and the three amorphous polymers polyethersulfone (PES), polycarbonate (PC), and aromatic poly(ester carbonate) (APEC) have been subjected to morphological and rheological characterization, and coefficients of linear thermal expansion and Young's moduli have been determined. The Young's modulus of the PES/LCP blends exhibited a near lower-bound behavior that could be predicted by the one-adjustable-parameter equations of Halpin-Tsai (Ξ = 0.18) and Takayanaga (b = 0.23), whereas the coefficients of linear thermal expansion followed the Takayanaga equation with a value of b = 0.50. The chain orientation of the LCP component was essentially constant in all PES/LCP blends with a Herman's orientation parameter of 0.39 ± 0.03. Transesterification reactions led to randomization of the constituents of the PC/LCP and APEC/LCP blends. The effect was more pronounced in the PC/LCP blends. The introduction of the LCP into the PC/LCP blends led to no reduction in melt viscosity and no self-reinforcement. APEC/LCP exhibited self-reinforcement in blends with a content greater than 27 vol% LCP, and especially the blend with 67 vol% LCP. Th self-reinforcement was caused by the presence of an oriented LCP phase, confirmed by X-ray diffraction, and by improved interfacial bonding, presumably resulting from the transesterification reactions occurring at the phase boundaries., INTRODUCTION Blends of thermotropic liquid crystalline polymers (LCP) and isotropic polymers have received considerable attention during recent years (1-24), because of the attractive signatures of these blends: low melt viscosity, [...]

A variety of different styrene monomers (styrene (S), p-fluorostyrene (p-FS), pentafluorostyrene (PFS), p-chlorostyrene (p-CIS), p-bromostyrene (p-BrS), p-methylstyrene (p-MS), p-tert-butylstyrene (p-tBS), p-methoxystyrene (p-MOS), p-ethoxystyrene (p-EOS), and p-propoxystyrene (p-POS)) have been polymerized by radical and cationic mechanisms. Copolymers of S/PFS, S/p-CIS, S/2-vinyl-naphthalene (2-VN), and S/p-phenylstyrene (p-PhS) were prepared by radical polymerization. All polymers were fully amorphous and transparent and exhibited only one glass transition temperature in the temperature range 77 to 141 degrees C. Thin films of the polymers were subjected to external electrical partial discharges (PD). Typically, 20 PD experiments were conducted for each polymer and the data for the time to breakdown were adapted to the two-parameter Weibull distribution function. The resistance towards PD of the various polymers was found to depend strongly on the monomeric structure. Complementary PD experiments on a series of narrow molecular mass polystyrenes confirmed that the minor variation in molecular mass between the different styrene polymers is of no importance for the PD resistance. The PD resistance of the homopolymers decreased in the following order: PS approximately equal to P(p-MS) > P(p-tBS) approximately equal to P(p-BrS) > P(p-POS) > P(p-CIS) approximately equal to P(p-MOS) > P(p-EOS) approximately equal to P(p-FS) > P(PFS). The time to breakdown for copolymers of S and PFS decreased monotonously with increasing PFS content. Styrene copolymers with low molar contents of 2-VN and p-PhS exhibited a higher resistance towards PD than PS. The results are discussed with reference to various properties of the studied polymers, such as segmental mobility, ionization potential, resonance stabilization, and reactivity of the radicals formed.

Pipes of isotactic polybutene-1 (PB) have been exposed to internal water and external air in pressure tests at 105 degrees C. The pipes exposed to different internal pressures exhibited different failure mechanisms, referred to as stages I, II, and III. Samples taken from these pipes have been analyzed by infrared (IR) spectroscopy, wide-angle X-ray scattering (WAXS), size exclusion chromatography (SEC), and differential scanning calorimetry (DSC). IR spectroscopy and WAXS confirmed that thermal oxidation of the amorphous part of the polymer accompanied the onset of stage III. Thermal oxidation led to extensive molar mass reduction and to a significant increase in mass crystallinity and melting peak temperature. Extensive and visible degradation in pipes failing according to stage III was confined to so-called 'oxidation spots.' The latter were first formed at the outer wall and propagated inwards, finally constituting approximately 50% of the wall thickness. The antioxidant concentration profiles obtained by DSC were always symmetrical and were successfully adapted to a previously developed model. It was shown that migration of the antioxidant was the dominant loss mechanism, that the diffusion coefficient was constant through the pipe wall, and that the evaporative loss to the external air was marginally greater than the loss to the internal water phase. Chemical consumption of the antioxidant was found to be negligible.

Commercial carbon-black filled medium density polyethylene pipes were tested for failure with static water as the internal agent and circulating air as the external agent at temperatures of 80 to 105 C. Results show that the pipes display Stage I failure at high circumferential stresses and Stage III failure at low levels. Mass average molar mass degradation rate was 10% and oxidation involved only the amorphous state. In addition, antioxidant concentration decreased rapidly at the beginning and was double at the pipewall center than at the near-inner and outer-wall of the concealed pipe.

A mathematical model derived from available antioxidant concentration profiles is applied to a commercial grade medium density polyethylene pipe material in hot-water applications. The model shows antioxidant concentration as a function of time and temperature subject to variable parameters such as extraction, diffusion and evaporation rates and chemical reaction of the antioxidant. Results show that the antioxidant loss is greater for water/water(internal/external) exposure. Nonuniform distribution of antioxidant causes differences between the model and experimental profiles.