Arthur W. Birley

Unsaturated polyesters based on terephthalic acid: 4. Sequence distribution of crosslinks in styrene cured resins

Abstract The sequence distribution of the styrene crosslinks in a cured polyester formed from propylene glycol, terephthalic acid and maleic anhydride has been studied by 13 C nuclear magnetic resonance and infrared spectroscopy. The n.m.r. method involved the degradation of the crosslinked polyester into a copolymer of styrene and fumaric acid and the interpretation of the peaks in the 13 C spectrum, which arise from the quaternary phenyl carbon atoms in sequences of styrene units, by considering the 13 C spectra of copolymer standards of styrene and diethyl fumarate. The crosslinking process was monitored directly by infra-red spectroscopy, and the distribution of sequence lengths of styrene units was then estimated by considering that crosslinking may be interpreted by a statistical treatment of copolymerization. Values of mol fractions of styrene units in sequences of monads, diads and triads determined by 13 C n.m.r. and infra-red spectroscopy were in fair agreement.

Appraisal of the current standards for stress relaxation measurements in compression for rubbers

Abstract The current British and American Standards for stress relaxation measurements in compression for rubbers are discussed in the light of results obtained with new, highly accurate equipment, which has been developed in the Institute of Polymer Technology. This work is part of a programme sponsored by the Engineering Research Station of British Gas, the Water Research Centre and the Victaulic Company PLC, and covers the development of equipment of satisfactory performance, which is suitable for commercial exploitation, and the investigation of meaningful tests for industrial use. The present paper reports work carried out to examine the test parameters which are required in the standards and, in particular, to question the specified values of some of them. Factors which have proved to be important include loading rate and temperature, whilst mechanical conditioning has been found to be less important in determining the pattern of stress relaxation. Above all, it is essential that the measurements are of sufficient accuracy, with a closely that the measurements are of sufficient accuracy, with a closely defined initial stress, for meaningful conclusions to be drawn.

The structure of propylene-ethylene sequential copolymers

Abstract Polypropylene is frequently modified by incorporating ethylene during polymerization or by blending with ethylene polymers or copolymers. Such ethylene-containing systems exhibit a very characteristic texture of small particles (globules) superposed on the spherulitic crystallinity of the polypropylene. The nature and occurrence of these particles have been investigated. The conclusion reached is that they contain ethylene polymers, both crystalline and amorphous, and that they can exist in a quiescent melt.

A new concept for stress relaxation measurements of rubbers in compression

Abstract 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.

Unsaturated polyesters based on terephthalic acid: 3. Characterization of poly(propylene terephthalate) prepolymer by gel permeation chromatography

Abstract Oligomers in prepolymers prepared by the polyesterification of terephthalic acid (T) with excess 1,2-propylene glycol (P) have been separated by gel permeation chromatography (g.p.c.). The assignment of chromatogram peaks to oligomers according to the structure P (TP) n where n is the number of 1,2-propylene terephthalate repeating units has been confirmed by a g.p.c. examination of bis(2-hyroxypropyl) terephthalate and by a 1 H nuclear magnetic resonance (n.m.r.) spectroscopic study of fractions isolated from a preparative separation. The infrared g.p.c. detector response has been interpreted quantitatively in order to deduce the concentration of each oligomer from the area of its chromatogram peak. Mol fraction distributions as a function of n have been determined for the prepolymer samples. Number average molecular weights have been calculated for the terephthalate-based components of the prepolymer and for all components including excess propylene glycol. These g.p.c. molecular weights are in excellent agreement with values previously reported in a study of prepolymers by 1 H n.m.r. spectroscopy. G.p.c. studies on prepolymers after reactions with a carbodiimide and diazomethane suggest a very minor quantity of carboxyl terminated species in the prepolymer samples.

Unsaturated polyesters based on terephthalic acid: 1. Proton magnetic resonance spectrum of poly(propylene terephthalate) prepolymer

Abstract Peaks in the proton magnetic resonance spectrum of the prepolymer prepared by the polyesterification of terephthalic acid with excess propylene glycol are assigned to the methyl, methylene and methine protons in propylene glycol units which may exist in four different environments. The assignments are confirmed by removing the excess propylene glycol from the prepolymer, by benzoylating the prepolymer, and by obtaining spectra for isopropanol, isopropyl benzoate, propylene glycol 1,2-dibenzoate, propylene glycol, and bis(2-hydroxypropyl) terephthalate.

Unsaturated polyesters based on terephthalic acid: 2. Quantitative characterization of prepolymers by proton magnetic resonance spectroscopy

Abstract Integrated proton magnetic resonance spectra have been obtained for poly(propylene terephthalate) prepolymers prepared by the polyesterification of terephthalic acid with excess propylene glycol. From the peaks for the phenyl hydrogens and the methyl hydrogens, the molecular weight of the terephthalate-based components of the prepolymer is determined. The procedure may be extended to the determination of the molecular weight of all components, including excess propylene glycol. Methods for determining the concentration of excess propylene glycol and the quantity of propylene glycol lost during polyesterification are also presented.

Electrostatic charge occurrence, significance and measurement

Abstract Polymeric materials often have a resistance to earth in excess of 106ω. This condition results in the ability to accumulate and store electrostatic charge over significant periods of time. Once an electrostatic charge has been gained (either by tribocharging or corona discharge techniques) it reduces at a rate depending on the resistivity of the material, and on the level of charge remaining on the material. The dependence of charge decay rate on charge magnitude is described by the relaxation equation; however, charge decay values are often quoted (for a fixed time period). Such values can be ambiguous and often lead to erroneous conclusions; a new ‘performance rating’ system is introduced here, giving an accurate and easily interpreted indication of charge decay rate and charge levels involved. Measurements of surface charge are obtained by electrostatic fieldmeters, one of which is linked to a data-logger for automatic data collection for manipulation and presentation of results. This procedure enables accurate assessment of antistatic systems and atmospheric effects (such as relative humidity).

Studies of recovery : a new dimension in understanding the viscoelasticity of elastomers

Abstract Wide differences in reversibility of stress and strain may be anticipated depending upon the type and nature of relaxation processes in an elastomer. The compound can undergo viscoelastic deformation or deterioration over a period of time due to temperature and a variety of service conditions. In order to study these differences, relaxation and recovery behaviour of various compounds are compared. Compression stress relaxation data at various temperature conditions, which involves both physical and chemical processes of relaxation, can be difficult to interpret without separating the one from the other. Recovery, which is a measure of the extent to which a specimen regains its original dimensions after release from compression, under the same environmental conditions as applied during the compression stage, can provide a technique to assess the contributions due to the different processes in relaxation.

Stress relaxation of rubbers in compression: the modulus enhancement factor

Abstract Measurements of the stress relaxation of rubbers in compression have been made routinely for many years. Hitherto, however, data for the tangent modulus at a compressive strain level of 25% for various times under load have not been generally available. The ratio of the modulus after compression to the initial tangent modulus at 25% strain defines the modulus enhancement factor, MEF. With the development of new equipment in the Institute of Polymer Technology, this situation has now changed, and numerous reliable data for stress relaxation, SR, and MEF are becoming available. MEF must offer further information about the network, but the interpretation of the MEF results lags behind our capability of generating them. The effects of the important variables, time and temperature, on SR and MEF have been investigated; additionally antioxidant content, filler content, mechanical conditioning, water immersion and exposure in nitrogen have been included in the programme. A possible connection between MEF and hysteresis has been explored. Some measurements of the swelling in toluene of natural rubber compounds, and of the effect of prolonged compression on the swelling, have given unexpected results, especially that at high temperature, 100°C, the course of relaxation and the final dimensions after compression and swelling are the same for air and nitrogen environments.