Cure conditions, rubber study

A rather comprehensive study on the relation between curing conditions and product properties of epoxy acrylates, diluted with a reactive diluent and a vinyl-terminated acrylonitrile/butadiene rubber has been carried out by Small et al. 

This paper discusses an attempt to prepare and characterize a standardized crosslinked rubber. By careful control of preparations and cure conditions, it should be possible to obtain specimens whose properties are reproducible from batch to batch. Distribution of such materials to those doing research on the viscoelastic behavior of cross-linked polymeric systems will stimulate a rapid advance in our understanding of such systems. Moreover, even in cases where research studies are not intended, they can serve as useful materials for apparatus calibration or for round robin tests. 

The value of Tg is important, since rigidity decreases in the glass transition region, Tg is closely related to the structure and crosslinking density of the cured resin. The structure of the cured resin can be derived from the initial starting materials, the epoxy prepolymer and hardener, and reaction conditions. However, the structures of many cured resins is still unclear which prevents to establish the structure-mechanical properties relationships. Further studies are needed. Furthermore, the commercial epoxy formulations may contain several components and also diluents, plasticizers, liquid rubbers, etc. which makes a prediction of Tg and mechanical properties even more difficult. 

Early works [24,25] concerned with the study of the profiles of temperature and state of cure developed through the rubber mass, after it was extracted from the mold and allowed to cool down in motionless air at room tanperature, already showed that an increase in the state of cure could be expected during the cooling period. Moreover, an improvement in physical properties such as the swelling in toluene and mechanic properties measured either under static or dynamic conditions [16] has been found. 

The process consists of a heat transfer through the rubber compounds and the cure in each of these compounds. The problem is studied in the case of two sheets of rubber cured together under the same conditions. 

Two NR samples (cured and uncured) were studied. In all studies, the samples were stretched to 500% elongation. The Fourier-transform Raman spectrum of NR is presented as a function of time of cold soaking at -25C and of strain with respect to laser polarisation. Under both sets of conditions, changes occur in the spectra that can be attributed to crystallisation. Difference spectra showing only those bands due to crystallisation (i.e. spectra of crystalline NR) are presented, which allows the crystallisation process to be discussed with respect to the conditions under which crystallites are formed. A combination of Fourier-transform Raman and Fourier-transform IR depolarisation spectra was used to deduce preliminary assignments for some of the vibrational bands of natural rubber. 40 refs. 

Vulcometry started as a research tool to study vulcanization. It was then used to control uniformity of rubber mixed in the factory. Also programmed temperature-profile vulcometry has been used to develop recipes for industrial use. The cure temperature-time profile of an industrial mold can be imposed on the curing cavity of the cure meter. The test sample can then be vulcanized in the cure meter under the same conditions as those encountered in the factory. Both the extent of cure and temperature can be simultaneously displayed as functions of time. 

Because of the limits of industrial equipment and cost constraints, curing is done at a constant temperature for a period of time. This can be done both to initially cure the material or to post-cure it. (The kinetic models discussed in the next section also require data collected imder isothermal conditions.) It is also how rubber samples are cross-linked, how initiated reactions are run, and how bulk polymerizations are performed. Industrially, continuous processes, as opposed to batch, often require an isothermal approach. UV light and other forms of nonthermal initiation also use isothermal studies for examining the cure at a constant temperature. 

If a study of vulcanization chemistry is to become part of a routine technological investigation it is necessary for a technique to be developed that will give very rapidly information on the distribution of rubber-sulphur reaction products for a specified set of vulcanization conditions (e.g. sulphur level, accelerator level, cure time, cure temperature). A most useful contribution in this direction has been made by Lautenschlaeger (1977). The model compound 2-methyl-2-pentene was heated with typical curing systems in Pyrex tubes from 10 to 100 minutes over a range of temperatures from 100 C to 150 C. The reaction products were then subjected to gas... 

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