Process for the preparation of partially hydrogenated...

Details Process for the preparation of partially hydrogenated... Process for the preparation of partially hydrogenated...

1998-08-13

2001-10-23

Wilson, Donald R. (Department: 1713)

Optics: measuring and testing

By dispersed light spectroscopy

With raman type light scattering

C525S329300, C525S338000, C525S339000

Reexamination Certificate

active

06307624

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for the preparation of partially hydrogenated acrylonitrile-butadiene rubber (HNBR) in a pressurised reactor by hydrogenation of acrylonitrile-butadiene rubber (NBR) by means of homogeneous or heterogeneous catalysis with the application of Raman spectroscopy.
2. Description of the Prior Art
The partial hydrogenation of the C—C— double bonds in acrylonitrile-butadiene rubbers (NBR) results in a special rubber, the hydrogenated nitrile rubber (HNBR).
In the hydrogenation processes carried out industrially at present, the NBR solutions are hydrogenated with hydrogen in an agitated pressurised autoclave in batches in the presence of a homogeneous or heterogeneous catalyst. The concentration of polymer in the solution to be hydrogenated is about 15 wt. %. In “Ullmann's Encyclopedia of Industrial Chemistry” dated 1993 [1], the homogeneous and heterogeneous catalysts used for the hydrogenation and the reaction conditions for the hydrogenation are described.
In the homogeneous hydrogenation both the catalyst and the substrate used for the hydrogenation are in solution. Chlorinated aromatic hydrocarbons such as, for example, chlorobenzene, are used as solvent. Rhodium-phosphine complexes or ruthenium-phosphine complexes are preferably used as catalysts. Depending upon the catalyst chosen and its concentration, the reaction temperatures are within the range of 100 to 150° C. The reaction pressure, which is determined substantially by the hydrogen partial pressure, can vary from a few up to about 190 bar.
In the heterogeneous hydrogenation of NBR, palladium catalysts on, for example, carbon, calcium carbonate or silicon dioxide are preferably used, and the catalysts are dispersed in the dissolved substrate. The reaction is generally carried out in ketones as solvent at a temperature of about 50° C. and at a pressure of about 50 bar.
Whereas sulfur or sulfur donors can be used for the vulcanisation of partially hydrogenated HNBR, the use of peroxide or high-energy beams is necessary for curing in the case of the completely hydrogenated product. Because of their good elongation at break and tear strength, commercially the partially hydrogenated HNBR types are preferred to the completely hydrogenated products.
A considerable problem, in particular in the production of the partially hydrogenated HNBR products, is the exact and reproducible establishment of the required degree of hydrogenation. It is known that the C—C— double bonds of the 1,2-vinyl-configured butadiene units in NBR are hydrogenated very rapidly, followed by the 1,4-cis configured units. The 1,4-trans configured butadiene units are hydrogenated comparatively slowly. The NBR products used for the hydrogenation are distinguished by a predominant proportion of the 1,4-trans configured double bonds.
The progress of the hydrogenation can be found by determination of the hydrogen absorption or, more precisely, by infrared spectroscopic (IR) analysis of samples withdrawn from the reactor. An appropriate method of IR analysis is described in ASTM D 5670-95. The disadvantage of this procedure is that as a rule about 20 to 30 minutes elapse before the analytical results are available. During this period the reaction can already have continued over and beyond the desired end point. As the hydrogenation procedure requires expensive, pressure-resistant reactors, the economic efficiency of the entire process is also substantially dependent on the space-time yield. The economic efficiency of the hydrogenation process can be decidedly improved by increasing the product throughput while at the same time ensuring product quality according to specification.
NIR (near-infrared) spectroscopy is frequently used for the purpose of process control. As suitable optical fibres for NIR technology are available, the relevant NIR spectrometer can even be set up at a relatively great distance from the reactor. However, the disadvantage of NMR technology is that the fundamental vibrations of the IR spectrum are not measured, but rather the overtone and combination vibrations which are as a rule superimposed. Provided that the hydrogenation always proceeds under the identical conditions (temperature, polymer concentration, pressure), the degree of hydrogenation can be determined by means of chemometric methods. As technical processes always vary within a certain bandwidth, a reliable determination of the required reaction variables is not feasible.
The object was accordingly to find a new process which renders possible the establishment of a required degree of hydrogenation of HNBR with at the same time an improved spacetime yield.
SUMMARY OF THE INVENTION
This invention accordingly provides a process for the preparation of partially hydrogenated acrylonitrile-butadiene rubber in a pressurised reactor by hydrogenation of acrylonitrile-butadiene rubber by means of homogeneous or heterogeneous catalysis, which is characterised in that the reactor contents are rendered inert before commencement of the hydrogenation, the Raman spectra of the reactor contents are recorded at short time intervals and the actual degree of hydrogenation of the product is determined from the intensities of the Raman emission lines and, on attainment of the required degree of hydrogenation, the reaction is arrested by suitable means.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The Raman effect of polymers is described in the literature (see, for example, P. J. Hendra, C. H. Jones and G. Warnes: Fourier Transform Raman Spectroscopy, Techniques and Chemical Applications, Ellis Horwood, Chichester (UK) (1991)). The emitted Raman signal, which is excited by an intensive light source, is measured. As a rule laser light of defined wavelength &lgr;
0
is used in order to excite the Raman effect. The lasers conventionally used are neodymium-yttrium-aluminium-garnet (Nd:YAG) lasers, which emit at a wavelength of 1.06 &mgr;m, helium-neon (&lgr;=633 nm) lasers, argon ion (&lgr;=488, 515 nm) lasers or semiconductor lasers (various wavelengths). The use of the Nd:YAG laser has the advantage over the helium-neon laser that, owing to the longer exciting wavelength, interfering fluorescence of organic materials is produced less strongly.
As a rule the “Stokes lines (&lgr;
i
>&lgr;
0
)” of the Raman spectrum are used for the analysis (&lgr;
i
: wavelength of the Raman signal). Particularly at a more elevated temperature, the “anti-Stokes lines (&lgr;
i
<&lgr;
0
)” can also be of significance for analysing the Raman spectra.
The Raman spectra can be measured by means of dispersive spectrometers available on the market and described by C. Henry in Analytical Chemistry News & Features, May (1997) 309A or by means of Fourier Transform (FT)-Raman spectrometers, which produce the Raman spectrum with an interferometer. In the case of dispersive spectrometers CCD detectors can be used, while InGaAs detectors or germanium detectors cooled with liquid nitrogen are suitable for FT-Raman equipment. In the process according to the invention it is preferable to use FT-Raman spectrometers with excitation of the Raman radiation by an Nd:YAG laser, because the excitation of interfering fluorescence is thereby largely avoided.
Suitable devices according to the invention for carrying out the Raman spectroscopy in a pressurised reactor are inspection glasses, preferably an inspection glass directly on the reactor, which renders possible the introduction of the exciting radiation into the reactor and the exit of the Raman radiation out of the reactor. Industrial safety requirements can be complied with particularly easily with the use of inspection glasses, as these can be purchased as standard structural components for the visual control of the contents of pressurised reactors.
Materials suitable for use as inspection glasses are, for example, borosilicate glasses, quartz or sapphire, which show no absorption or fluorescence in the range of the exciting, wavelength &lgr;
0
including the rel

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