Björn Terselius

Influence of pH and storage time on the sorption and solubility behaviour of three composite resin materials

OBJECTIVES The aim of this study was to determine whether pH and time has any influence on the sorption and solubility behaviour of composite resin materials stored in a buffer solution. METHODS Three hybrid composite restorative resin materials (Spectrum, Z-100 and ArtGlass) with different matrix structure and filler composition were studied. One hundred and twenty specimens of each material were produced according to ISO 4049. The materials were stored in McIlvains buffer solution at different pH (4, 6 and 8) at 1, 7, 60 and 180 days. pH measurements were carried out before and after completed storage. RESULTS Time had a significant influence on the sorption and solubility behaviour of the composite resin materials tested. One of the materials Z-100 showed a significant mass increase and/or decrease depending on the pH of the solution. The material Spectrum was significantly influenced by pH, resulting in different sorption depending on the pH of the solution. CONCLUSIONS Time of storage was important for the sorption and solubility behaviour of the composite resin materials tested. Comparison of solubility for one of the materials showed twice as high values in the present study as previously reported when distilled water was used as storage medium. pH in the solution seems to have an influence on the sorption and solubility behaviour of composite resin materials. The sensitivity of the sorption and solubility behaviour to time and pH of the materials tested seems to be related to the hydrophilicity of the matrix and the chemical composition of the filler.

Thermo oxidative stability of polyamide 6 films. I. Mechanical and chemical characterisation

Unstabilised and stabilised polyamide 6 films were subjected to long-term oven ageing in ambient atmosphere at temperatures in the range of 100–120°C. The strain at break of film strips showed a drop at a point of oxidation corresponding to a critical molar mass range of 15 000–19 000 g mol−1. Embrittlement occurred at a molar mass of 40–50% of the original value of 27 000 g mol−1. The efficiency of a phenolic antioxidant such as Irganox 1098 was poor, extending the drop in strain at break only by a factor of 2. All investigated polyamide films showed a high rate of oxygen uptake at the beginning of oxidation followed by a gradual decline. The oxidation was accompanied by an autoretardant accumulation of peroxides as well as of carbonyl groups consistent with the oxygen uptake data. This behaviour is different from that reported for polyolefins, which show a well-defined induction period followed by a sigmoidal increase in the oxygen uptake, also in the presence of a phenolic antioxidant. It is unlikely that the rate of oxidation solely depends on the concentration of peroxides since, throughout the oxidation, peroxides were accumulating, while the rate of oxygen uptake was decreasing.

Imaging chemiluminescence instrument for the study of heterogeneous oxidation effects in polymers

Abstract An imaging Chemiluminescence equipment used for studies of diffusion controlled oxidation effects in polymers is presented. The design involves an imaging photon counting system mounted on a cylindrical oven with a controlled atmosphere. The overshoot on heating is less than 2 °C and the temperature during isothermal conditions is maintained at ±0.4 °C, thus, a stable thermal environment is attained. To point out possible applications, the monitoring of oxidation profiles from aged nylon specimens and oxygen-barrier properties of an inorganic coating by chemical plasma deposition on a hydroxyl terminated polybutadiene substrate are presented. The results demonstrate the feasibility of studying diffusion controlled oxidation and also the possibility of the technique to estimate the function and durability of surface treatments on elastomers.

Thermal oxidation of hydroxyl-terminated polybutadiene rubber. I. Chemiluminescence studies

The chemiluminescence (CL) intensity from a hydroxyl-terminated polybutadiene (HTPB) rubber was measured during isothermal oxidation at temperatures ranging from 70 to 130°C. The CL isotherms were characterised by an approximately constant level of intensity followed by a rapid increase to a maximum intensity. The initial intensity as well as the maximum intensity levels of the different isotherms obeyed an Arrhenius expression, suggesting an invariable overall oxidation mechanism. Total luminous intensity (TLI) values in nitrogen atmosphere were obtained at selected times during the isothermal oxidation. The TLI data showed a good correlation with the CL isotherm, indicating that the time course of the isotherm can be explained in terms of peroxide build-up and decomposition. The build up of peroxides during oxidation was shown from FTIR data to be accompanied by a build-up of carbonyl groups. Above a certain TLI level, FTIR data showed a significant decrease in unsaturation, while swelling measurements showed an increase in the apparent crosslink density. This formation of a secondary network later accelerated, with gradual depletion of oxygen in the specimen, causing sever embrittlement of the rubber.

Thermo-oxidative stability of polyamide 6 films: III. Isothermal microcalorimetry

Isothermal microcalorimetry (MC) was used to study the thermo-oxidative stability of polyamide 6 film in the temperature range of 100–130°C. The heatflow was found to vary for ageing times beyond the embrittlement of the film samples. The heatflow curve was characterised by an initial peak and a sigmoidal increase through a maximum. Polyamide 6 stabilised with the phenolic antioxidant Irganox 1098 shows a similar heatflow time curve although shifted towards lower heatflow levels and longer times. A maximum in the heatflow curve was found to correspond to an accumulated heat energy in the range of 15–20 J g−1, whereas a drop in strain at break occurred in the range of 45–55 J g−1. Microcalorimetry was compared to oxygen uptake and chemiluminescence measurements. All of these techniques respond to oxidation but exhibit distinctly different rate time curves.

Thermo-oxidative stability of polyamide 6 films: II. Chemiluminescence techniques

Chemiluminescence (CL) and imaging chemiluminescence (ICL) were used to study the thermo-oxidative stability of polyamide 6 in the temperature range of 100‐140C. The CL intensity time curves (CL curves) for both unstabilised and stabilised polyamide 6 film indicated an initial period at a high intensity level of varying shape followed by a basically sigmoidal increase and a subsequent maximum. This maximum was found to occur at a time corresponding to a drop in strain at break. Polyamide 6, stabilised with the phenolic antioxidant Irganox 1098, showed a similar CL curve although shifted towards lower intensity levels and longer times. The ICL study revealed essentially homogeneous oxidation. Both the ICL study and the CL studies clearly indicated oxidation from the beginning of ageing, also in the presence of the phenolic antioxidant. The CL intensity is not directly related to the rate of oxygen uptake. Instead, the CL intensity in oxygen seems to be related to the content of peroxides. However, other species causing luminescence cannot be excluded. # 2000 Elsevier Science Ltd. All rights reserved.

Multi-cell imaging chemiluminescence to map heterogeneous degradation of polypropylene plaques

The heterogeneity of the degradation of stabilized polypropylene (PP) plaques was studied using a recently designed multi-cell imaging chemiluminescence instrument. Mapping of the degradation of unaged PP plaques showed randomly distributed oxidation induction times (OIT), indicating random initiation presumably at polymerization catalyst residues. Contrary to this, hot-air oven aged PP plaques showed position related degradation. Zero OIT values were found at the edges, indicating complete oxidation, whereas the OIT increased towards the centre of the plaque. This non-random OIT distribution can be explained by a difference in evaporation rate of the stabilizer at different positions in the plaque.

Oxidation of unstabilised polypropylene particles as studied by microcalorimetry and chemiluminescence techniques

Microcalorimetry (MC) and chemiluminescence (CL) measurements have been used to study the oxidation of unstabilised polypropylene (PP) particles. The oxidation profiles obtained by the two techniques were found to be similar although a time shift between the profiles was observed, particularly at low temperatures. The similar shape suggests a close relation between the rate of the overall oxidation process as measured by MC and the rate of the CL emitting reactions. MC revealed that the energy evolved up to the maximum of the MC oxidation profile increased significantly above 110 °C indicating a change in the overall oxidation process, possibly associated with a more pronounced secondary oxidation. The results indicate that above 110 °C the maximum of an oxidation profiles (as measured either by MC or CL techniques) is not suitable for evaluating the temperature dependency of oxidation. FTIR was used to follow the formation of oxidation products. It was found that the oxidation profiles obtained by either MC or CL measurements did not reflect the carbonyl build-up but more likely the hydroperoxide build-up.

Thermal oxidation of hydroxyl-terminated polybutadiene rubber II. Oxidation depth profiles studied by imaging chemiluminescence

The chemiluminescence (CL) intensity during isothermal oxidation in the surface-layer and in the core from hydroxyl-terminated polybutadiene rubber was measured by an imaging CL instrument. Skin-core oxidation was observed and further indicated by infrared spectroscopy in terms of a carbonyl index and in the relative absorption of unsaturation. The oxidation depth in pre-aged samples, in terms of CL depth profiles, was evaluated from imaging chemiluminescence measurements in nitrogen atmosphere. The oxidation depth in pre-aged samples was in agreement with the depth of the CL emission during in situ oxidation. Samples subjected to a sinusoidal dynamic-mechanical compression during the oxidation indicated a prolonged oxidation in the surface layer.

New multi-cell imaging chemiluminescence (ICL) instrument to evaluate the stability of polymers

Developments of position-sensitive photon detectors and charged coupled device (CCD) cameras, have made it possible to measure thermo-oxidative degradation of polymers in real-time with high precision (imaging chemiluminescence, ICL). The main problems in using this technique to determine the stability of multiple samples are the appearance of temperature gradients and the possibility of infection of neighbouring specimens, which can lead to a decrease of their stability. These problems are overcome with the apparatus presented. With this new apparatus the obtained chemiluminescence (CL) intensity-time curves of 23 unstabilised polypropylene PP powder samples overlapped after normalisation, indicating a constant temperature and an effective gas flow distribution in the cells of the specimen chamber. It is also shown that the sensitivity of the apparatus is very high. Thus, the stability of a single PP particle of less than 10 μg could be determined.