Model tests for wear measurements in plastics processing

In plastics processing different mechanisms of wear are observed. In view of the complex nature of the tribological systems wear can be studied systematically by model tests only. These must be developed in a manner that they coincide directly with the practical process. The youngest DKI model test, the adhesive wear tester, is described and results for various metal pairings are presented.     

Mechanisms and kinetics of thermal decomposition of plastics from isothermal and dynamic measurements

The kinetics of decomposition of plastics are of interest from different points of view, i.e. evolution of harmful substances during fires or waste incineration, recovering of chemical raw materials from plastic refuses and designing of recycling procedures. To measure the formal kinetic parameters of the degradation of polymers isothermal and dynamic methods are applied in this work. Dynamic measurements are performed by combined thermogravimetry mass spectrometry (TG-MS), the isothermal measurements are carried out with a new closed loop-type reactor. To evaluate consistent kinetic data from isothermal and dynamic measurements, the energy balance for the sample in dynamic measurements has to be considered to obtain the true sample temperature and heating rate. Subject of this investigation is the exploitation of dynamic and isothermal methods for measuring and interpreting the kinetics of thermal decomposition of plastics. Results for commodity plastics polyethylene and poly(vinyl chloride) (PVC) are presented. The combined application of TG–MS, isothermal experiments in the closed loop-type reactor and DSC leads to new results for the decomposition kinetics of PVC. The dehydrochlorination mechanism at moderate temperature can be distinguished in an endothermal and exothermal part. The benzene formation is identified as a second order reaction. A great advantage of the isothermal method is, that changes in the mechanisms are detectable, i.e. changes in the apparent order of the reaction and the apparent activation energy. From that, new mechanistic aspects of the decomposition kinetics of polyethylene were obtained.     

Testing plastics for resistance to environmental stress cracking

Methods of test for measuring the resistance of plastics to environmental stress cracking are critically reviewed. Particularly consideration is given to the relative merits of constant strain and constant stress tests and the most suitable procedure for the generation of design data discussed. Note is taken of the position regarding national and international standardisation.     

Detection of stress whitening in plastics with the help of X-ray dark field imaging

The processing of thermoplastics can induce a wide range of defects such as stress whitening, cavitation and porosity, which can adversely affect the reliability of the final products. Hence, fast and effective non-destructive detection methods for such defects are highly important for quality assurance on production lines. In this paper, X-ray dark field imaging is presented as a new non-destructive testing method that allows the visualization of stress whitening or cavitation efficiently. The performance of the method is demonstrated for the case of an injection-moulded polyvinylidene fluoride part that exhibits stress whitening. Whereas the stress whitening could not be detected by conventional X-ray imaging, it was localized by an X-ray dark field image acquired within a few minutes. Once the precise location of the stress whitening was known, it was possible to verify the result by local micro X-ray computed tomography and by a micro section image.     

Quantitative surface-enhanced Raman measurements with embedded internal reference

Analytical applications of SERS are often more associated with qualitative than quantitative analysis, because of the difficulty in obtaining quantitative SERS results. In this paper we introduce a new strategy to quantitatively measure the SERS signals of analytes based on Au-core/Ag-shell nanoparticles with embedded 4-aminothiophenol as the internal reference. Successful detections of two analytes, Toluidine Blue O in aqueous solution (detection limit of 0.1 μM) and melamine in milk (detection limit of ∼5 μM), are demonstrated. The improvement in the linear fitting illustrates that the use of internal reference significantly improves the accuracy of the quantitative SERS measurements. The successful detection of melamine in milk illustrates the versatility of this detection scheme for a wide variety of analytes.     

Use of an internal reference in 13C chemical shift measurements

The use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in ¹³C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factor g². Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.     

Simultaneous measurements of stress and infrared dichroism on polymers

Summary A method for simultaneous measurements of stress and infrared dichroism as time-dependent behavior of polymer films was devised by using a double beam infrared spectrometer. The film sample held between clamps of a stretching device was placed just in front of the entrance slit of the spectrometer where the sample and reference beams came alternately. Two polarizers were used, one in the sample beam and the other in the reference beam. Thus the sample and reference beams were polarized to have the electric vectors parallel and perpendicular to the stretching direction of the sample, respectively. With this arrangement the spectrometer responded only to the difference in the transmittance of the two beams. Setting the spectrometer at one of the wavenumbers of the absorption band maxima, we could record continuously the change in its dichroism during mechanical treatments which gave rise to the molecular orientation in the sample. The stress was recorded automatically by means of a couple of strain gages pasted on the cantilever beam of the stretching device.By theoretical considerations, a simple relationship was found to exist between the quantity recorded on the spectrometer by this method and the orientation function of transition moment of a vibrational absorption band with respect to the stretching direction.The method was applied to the stress relaxation experiments of vulcanized natural rubber carried out at different elongations less than 600% and at the room temperature. Changes of infrared dichroism were measured for five absorption bands at 1664, 1380, 1361, 1129, and 844 cm^–1, of which the last one is a crystalline band. From the results of this study, it was concluded that the stress relaxation observed was ascribed mainly to the amorphous orientation rather than to the crystalline orientation, which was completed almost immediately after elongation.

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Zusammenfassung Eine Methode für gleichzeitige Messungen von Spannungs-und Infrarot-Dichroismus von Polymerenfolien in AbhÄngigkeit von der Zeit wurde unter Verwendung eines Zweistrahlen-Infrarotspektrometers entwickelt. Die Filmproben, zwischen Klemmen in einer Verstreckungsanordnung gehalten, sind am Ort des Eintrittsspaltes des Spektrometers justiert, an dem der die Probe durchlaufende und der Referenzstrahl alternierend eintreten. Zwei Polarisationen, eine im Probestrahl, die andere im Referenzstrahl, wurden verwendet. So sind der Probe-und Referenzstrahl mit dem elektrischen Vektor parallel bzw. senkrecht zur Streckrichtung der Probe polarisiert. Mit dieser Anordnung am Spektrometer wird nur die Differenz der DurchlÄssigkeit der Probe für beide Strahlen gemessen. Wenn man das Spektrometer auf die Wellenzahl eines Absorptionsmaximums der Probe einstellt, lÄ\t sich also kontinuierlich unmittelbar die Änderung des Dichroismus wÄhrend der mechanischen Verformung verfolgen, die aus molekularer Orientierung in der Probe resultiert. Die Spannung wurde automatisch mit Hilfe von Verlagerungsaufnehmern aufgezeichnet.Theoretische Betrachtungen lassen eine einfache Beziehung zwischen den Spektrometerkurven und der Orientierungsfunktion für die verschiedenen Absorptionsbanden in Bezug auf die Streckrichtung finden.Die Methode wurde auf Spannungsrelaxation in vulkanisiertem Naturkautschuk für verschiedene Strekkungen kleiner als 600% bei Raumtemperatur angewendet. Die Messungen der Änderung des InfrarotDichroismus fanden für die 5 Absorptionsbanden bei 1664, 1380, 1361, 1129 und 844 cm^–1 statt, wobei die letztere eine kristalline Bande ist. Aus den Ergebnissen wird geschlossen, da\ die beobachtete Spannungsrelaxation in der Hauptsache Orientierung im Amorphen und nicht einer Kristallit-Orientierung zuzuordnen ist. Letztere ist offensichtlich schon unmittelbar nach der Streckung fast vollstÄndig ausgebildet.

     

Simultaneous measurements of stress and infrared dichroism on polymers

Summary In order to obtain informations on the molecular nature and mechanism of rheological processes on polymers, simultaneous measurements of stress and infrared dichroism were made of polychloroprene (Neoprene Type AC) films during the course of continuous elongation at the constant rate 25%/min and of stress relaxation at 400% elongation. The time dependence of the infrared dichroism was obtained by measuring the intensity change at fixed wavenumbers of absorption band maxima on the differential polarized infrared spectra. Both the degree of crystallinity of sample films and the orientation function of transition moments were calculated from the results of the infrared dichroic measurements under the assumption of uniaxial orientation, which was confirmed to be the case by X-ray diffraction and birefringence measurements.In the continuous elongation experiments, it was found that the orientation functions for crystalline-sensitive bands showed maxima at 25% elongation and then decreased rapidly, changing their signs from positive to negative. There also appeared the yield point in the stress-strain curve and the beginning of sharp decrease of crystallinity at the same degree of elongation 25%. These facts were interpreted in terms of the orientation of the crystalline phase followed by the degradation of crystallites and drawing out of the molecular chain from the crystallites. In the stress relaxation experiments, moderate changes in the orientation functions were found for various characteristic absorption bands. Little difference was observed between changes in the orientation functions for the amorphous and crystalline-sensitive bands. This makes a contrast with the previous results for vulcanized natural rubber, where the orientation of the crystalline phase was completed almost immediately after elongation, while in the amorphous phase the molecular chains were oriented gradually during the course of stress relaxation.

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Zusammenfassung Um Informationen über die molekulare Natur und den Mechanismus rheologischer Prozesse von Polymeren zu erhalten, wurden gleichzeitige Messungen der Spannung und des Infrarot-Dichroismus an Polychloropren-Filmen (Typ AC-Neopren) während der kontinuierlichen Verstreckung bei konstanter Verstreckungsgeschwindigkeit von 25%/min und während der Spannungsrelaxation bei 400% Verlängerung durchgeführt. Die Zeitabhängigkeit des Infrarot-Dichroismus wurde aus der Intensitätsänderung im festen Wellenzahlbandmaximum eines Kristallisationsbandes mit einem differenz-polarisierten Infrarot-Spektrographen festgestellt. Beide, der Kristallanteil der Probe und die Orientierungsfunktion für die Übergangsmomente lassen sich aus den Resultaten unter Annahme einachsiger Orientierung auswerten. Letztere, die einachsige Orientierung, wurde mit Röntgenbeugung und Doppelbrechung sichergestellt.In den Experimenten mit kontinuierlicher Verlängerung zeigten die Orientierungsfunktionen der Kristallempfindlichen Banden bei 25% Verlängerung ein Maximum. Danach sanken sie rasch unter Wechsel ihres Vorzeichens von positiv nach negativ ab. Der Fließpunkt in der Spannungs-Dehnungs-Kurve und der Beginn des scharfen Abfalls der Kristallinität zeigen sich vom gleichen Verlängerungsgrad 25% ab.Diese Tatsachen werden aufgrund der Orientierung der kristallinen Phase erklärt, gefolgt von einem Abbau der Kristallite und einem Herausziehen der Molekülketten aus den Kristalliten. In den Spannungsrelaxationsversuchen wurden gewisse Änderungen der Orientierungsfunktionen für die verschiedenen charakteristischen Absorptionsbanden gefunden. Es ergaben sich geringe Differenzen in der Änderung der Orientierungsfunktionen für die amorphen und kristallempfindlichen Banden. Das steht im Gegensatz zu früheren Resultaten an vulkanisiertem Naturkautschuk. Bei letzteren war die Orientierung der Kristallphase beinahe unmittelbar nach der Dehnung vollständig, während in der amorphen Phase die molekularen Ketten nach und nach im Laufe der Spannungsrelaxation orientiert wurden.

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With 8 figures in 9 details and 1 table     

Bayesian estimation of the internal structure of proteins from single-molecule measurements

In single-molecule protein experiments, the observable variables are restricted within a small fraction of the entire degrees of freedom. Therefore, to investigate the physical nature of proteins in detail, we always need to estimate the hidden internal structure referring only to the accessible degrees of freedom. We formulate this problem on the basis of Bayesian inference, which can be applied to various complex systems. In the ideal case, we find that in general the framework actually works. Although careful numerical studies confirm that our method outperforms the conventional method by up to two orders of magnitude, we find a striking phenomenon: a loss-of-precision transition occurs abruptly when the design of the observation system is inappropriate. The basic features of the proposed method are illustrated using a simple but nontrivial model.     

Yield stress and viscoelastic properties of high internal phase ratio emulsions

 The rheology of high internal phase ratio oil-in-water emulsions was investigated using a controlled-stress rheometer. The dispersed-phase (oil) concentration was varied from 71.24 to 89.61% by volume. Three different types of rheological experiments were conducted for each emulsion, namely: steady shear, oscillatory shear, and creep/recovery experiments. All the emulsions investigated in this study possess a yield stress. The yield-stress values obtained from different rheological experiments for the same emulsion show good agreement with each other. The yield-stress value increases exponentially with an increase in the dispersed-phase concentration. The yield-stress data of this study can be described quite well with the Princen and Kiss equation for high internal phase ratio emulsions provided that the thickness of the interdroplet films is taken into account. For any given emulsion, the storage modulus, measured in the linear viscoelastic region, is found to be constant, independent of the frequency, indicating a solid-like behaviour. The value of the storage modulus increases with an increase in the dispersed-phase concentration. The storage modulus data are interpreted in terms of the Princen and Kiss equation. Received: 23 October 1998 Accepted in revised form: 18 February 1999     

A new concept for stress relaxation measurements of rubbers in compression

Current relaxation measurements and associated equipment are not reliable enough to carry out prolonged tests, especially at high temperatures and in liquid environments; the difficulties encountered in such measurements are discussed. The objectives of the present sponsored programme were to examine the best available equipment and experimental techniques and to develop a reliable and reproducible method for measuring stress relaxation in compression which could form a basis for a standard test procedure. In the course of this work, a method of evaluating the available equipment was developed and this is described.     

Rheological properties of internal viscosity models with stress symmetry

Internal viscosity models (IVM) for dilute-solution polymer dynamics differ in how they define the deformational force F ^d which includes φ, the IV coefficient, and in how they treat polymer rotational velocity Ω. Here, the handling of angular momentum is shown to be crucial. A torque balance in simple shear flow at shear rate G leads to stress symmetry and specification of Ω(G) which differs greatly from the conventional Ω = G/2. This determines the G dependence of viscosity η and normal stress coefficient ζ. There are also implications of a transition in rotational behavior as φ approaches a critical value. Predictions of η(G), ζ(G), and η^*(ω) are presented for two versions of F^d : one derived recently by the authors and one being most commonly used at present. Limiting cases for high and low φ, and for high and low G and ω, are discussed. Some differences exist between predictions of the two F^d models, but these are surprisingly minor.     

Sinusoidal normal stress measurements with the Weissenberg rheogoniometer

In measuring sinusoidal normal stress difference of concentrated polymer solutions in a Weissenberg rheogoniometer, the resistance to motion of the upper platen due to the sample between the cone and plate should be taken into account. Correction terms are derived. The theoretical predictions of Lodge and others concerning the relationship between oscillatory normal stress difference and oscillatory shear stress are found to agree with experimental data if these corrections are applied.     

Corrosion activity of carbon plastics and protection of metal power structures in contact with carbon plastics

Methods for assessing the corrosion activity of carbon plastics against metal materials and development of complex anticorrosive protection of load-bearing units of structures including carbon plastics in contact with metal materials are considered. Data on the electrochemically measured corrosion rates in carbon plastic-metal contact pairs and on the efficiency of separating layers (sealants, glass fabrics) are presented. Recommendations on corrosion protection of carbon plastic-metal joints are given.     

Liquefaction of plastics

A plastic liquefaction process which features a combination of thermal and catalytic cracking has been developed. The process can charge polyethylene, poly(propylene) or polystyrene and produces relatively low pour and highly aromatic liquid at a yield of about 85 %. Based on laboratory development a 500 ton per year prototype unit has been constructed and operated successfully both for testing and further development. An additional 1000 ton per year prototype unit is in the design phase and is scheduled for operation in 1992. The salient aspects of the process are described.     

Quantitative measurements of oxidative stress in mouse skin induced by X-ray irradiation

To find efficient methods to evaluate oxidative stress in mouse skin caused by X-ray irradiation, several markers and methodologies were examined. Hairless mice were irradiated with 50 Gy X-rays and skin homogenates or skin strips were prepared. Lipid peroxidation was measured using the skin homogenate as the level of thiobarbituric acid reactive substances. The level of lipid peroxidation increased with time after irradiation and was twice that of the control at 78 h. ESR spectra of skin strips showed a clear signal for the ascorbyl radical, which increased with time after irradiation in a manner similar to that of lipid peroxidation. To measure levels of glutathione (GSH) and its oxidized forms (GSSG) simultaneously, two HPLC methods, sample derivatization with 1-fluoro-2,4-dinitrobenzene and detection with a UV detector (method A) and no derivatization and detection with an electrochemical detector (method B), were compared and the latter was found to be better. No significant change was observed within 24 h after irradiation in the levels of GSH and GSSG measured by method B. The GSH/GSSG ratio may be a less sensitive parameter for the evaluation of acute oxidative stress caused by X-ray irradiation in the skin. Monitoring the ascorbyl radical seems to be a good way to evaluate oxidative stress in skin in vivo.     

Comparing flat and micro-patterned surfaces: Gas permeation and tensile stress measurements

Micro-patterning is a suitable method to produce structured membranes that display increased flux compared to flat membranes. In this work we studied the permeation of four different gases (nitrogen, helium, oxygen and carbon dioxide) through Kraton™ polymer (SBS) membranes. It is possible to cast a micro-patterned membrane with 25 μm high and 30 μm wide lines that has a thickness of 5 μm at its thinnest point. Using this micro-pattern, the experimental diffusive gas flux was increased up to 59% compared to non-patterned membranes with the same polymer volume. Finite element simulations confirm this enhancement. Selectivities are similar for both flat and micro-patterned membranes and in accordance with literature. Tensile stress measurements confirm that the micro-patterned membranes yield only limited loss in mechanical strength. Although only one material and geometry is explored here, this principle is generally applicable to all diffusion-driven processes.