Distillation of (meth) acryloxy-bearing alkoxysilane

Details Distillation of (meth) acryloxy-bearing alkoxysilane Distillation of (meth) acryloxy-bearing alkoxysilane

1999-06-18

2002-01-01

Manoharan, Virginia (Department: 1764)

Distillation: processes, separatory

Addition of material to distilland to inhibit or prevent...

For inhibiting or preventing a polymerization reaction

C203S021000, C203S040000, C203S072000, C203S073000, C203S089000, C203S091000, C203S098000, C556S479000

Reexamination Certificate

active

06334935

ABSTRACT:

This invention relates to a commercially advantageous method for distilling an acryloxy or methacryloxy-bearing alkoxysilane.
BACKGROUND OF THE INVENTION
Acryloxy or methacryloxy-bearing alkoxysilanes are widely used in the industry as silane coupling agents, novel polymerizable monomers and the like since they possess a polymerizable functional group in the form of an acryloxy or methacryloxy group in the structure.
Commonly employed for the industrial mass-scale purification of the acryloxy or methacryloxy-bearing alkoxysilane is a method of isolating and purifying the alkoxysilane under high temperature/long time distillation conditions, known as long thermal history distillation conditions, using a distillation device having a reboiler combined with a multi-stage distillation column.
The product obtained by this purifying method, however, suffers from the quality problem that the product in a container for storage gradually becomes white turbid if the product has a chance of contact with air as can occur during manipulations such as opening and closing of a cap of the container and transfer from the container. This phenomenon is herein referred to as an open-air whitening phenomenon because the product experiences prominent whitening when it is open to air. The (meth)acryloxy-bearing alkoxysilane products, though they have been purified, often receive unexpected claims on their quality because of their visual changes caused by the open-air whitening phenomenon.
The open-air whitening phenomenon can be prevented by secondary countermeasures such as by treating with adsorbents such as active carbon and silica gel or fully purging with nitrogen at the end of manipulation for air-blocked storage. These countermeasures are not only unsatisfactory solutions, but also impose more burdens because the treatment or operation following purification becomes very complicated. No essential solutions have been available to the open-air whitening phenomenon. Therefore, it has long been desired to establish an effective solution to the open-air whitening phenomenon.
Further, since (meth)acryloxy-bearing alkoxysilanes are thermally unstable compounds having self-polymerization capability, the above-described purifying method with a long thermal history has a probability that polymerization will occur during the purifying process, which can cause clogging of the distillation column or thickening or gelation of the liquid in the reboiler.
For the purpose of avoiding these problems, a variety of polymerization inhibitors have been developed. None of the polymerization inhibitors have a fully satisfactory function. When the polymerization inhibitors are blended in (meth)acryloxy-bearing alkoxysilane products, some cause quality problems undesirable in their applications like coloring and a decline of polymerization capability, and some are toxic or hazardous. For this and other reasons, the polymerization inhibitors do not achieve a satisfactory improvement. There is a desire to have a short thermal history method of purifying (meth)acryloxy-bearing alkoxysilanes while essentially suppressing the self-polymerization capability thereof.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for isolating, by distillation, a (meth)acryloxy-bearing alkoxysilane of high purity in a commercially advantageous manner while suppressing the occurrence of self-polymerization during the process and minimizing the potential open-air whitening phenomenon of the product upon exposure to air.
The inventor has found that by subjecting a reaction solution containing an acryloxy or methacryloxy-bearing alkoxysilane represented by the general formula (1):
wherein R is hydrogen or methyl, R
1
and R
2
each are alkyl of 1 to 4 carbon atoms, and n is an integer of 0 to 2, to thin-layer distillation at a temperature up to 160° C. and a vacuum of up to 15 mmHg, the acryloxy or methacryloxy-bearing alkoxysilane can be purified to a high purity while suppressing the occurrence of self-polymerization during the process and minimizing the potential open-air whitening phenomenon upon exposure to air.
The development process of open-air whitening was investigated. Namely, minute quantities of impurities in a (meth)acryloxy-bearing alkoxysilane product were separated by gas chromatography and identified by mass spectroscopy, thereby examining the behavior of minute impurities before and after the open-air whitening phenomenon. As a result, it was found that a causative substance (A) represented by the following formula (2):
wherein R
2
is as defined above forms in a minor amount as a by-product during distillation under long thermal history conditions and is left admixed in the product isolated by distillation. Upon contact with air of the product containing causative substance (A), the causative substance undergoes selective hydrolytic condensation reaction with moisture in the air to form fine gel-like fractions, which brings about the open-air whitening phenomenon. As the thermal history during distillation becomes greater (higher temperature and/or longer time), the amount of causative substance (A) by-product increases, with which the degree of whitening increases.
Paying attention to the above-described causative substance (A), the inventor investigated how to suppress the formation of this substance. Surprisingly, it has been found that by effecting distillation under the controlled conditions in a thin-layer distillation device, causative substance (A) is not produced at all and there is obtained a product which will be free from open-air whitening. The controlled distillation is also effective for substantially avoiding self-polymerization. Therefore, the alkoxysilane can be purified to a high purity through a simple process without a need for cumbersome post-treatment. Additionally, continuous distillation becomes possible, with an improvement in hourly productivity.
The invention provides a method for isolating and purifying an acryloxy or methacryloxy-bearing alkoxysilane of formula (1), comprising the step of distilling a reaction solution containing the acryloxy or methacryloxy-bearing alkoxysilane in a thin-layer distillation device at a temperature of up to 160° C. and a vacuum of up to 15 mmHg.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The method for distilling an isolating an acryloxy or methacryloxy-bearing alkoxysilane according to the invention starts with a reaction solution containing the alkoxysilane. The (meth)acryloxy-bearing alkoxysilane is represented by formula (1):
R is hydrogen or a methyl group, each of R
1
and R
2
is an alkyl group of 1 to 4 carbon atoms, such as methyl, ethyl, propyl or butyl, and n is an integer of 0 to 2.
Illustrative examples of the (meth)acryloxy-bearing alkoxysilane of formula (1) include
3-acryloxypropyltrimethoxysilane,
3-acryloxypropyltrimethoxysilane,
3-methacryloxypropyltrimethoxysilane,
3-methacryloxypropyltriethoxysilane,
3-acryloxypropylmethyldimethoxysilane,
3-acryloxypropylmethyldiethoxysilane,
3-methacryloxypropylmethyldimethoxysilane, and
3-methacryloxypropylmethyldiethoxysilane.
With respect to the reaction solution containing the (meth)acryloxy-bearing alkoxysilane of formula (1), its synthesis process is not critical. There may be used any of reaction solutions which are synthesized by various conventional processes and optionally concentrated. Preferred is a reaction solution in which the concentration of the (meth)acryloxy-bearing alkoxysilane of formula (1) is at least 70% by weight and the concentration of impurities having a lower boiling point than the alkoxysilane is not more than about 1% by weight. The reaction solution may be once distilled before it is subjected to distillation according to the invention.
The thin-layer distillation device used herein may be a device of commonly known specification having an agitation drive section of spreading the reaction solution in thin layer form and a heating and evaporation/condensation section of heating the reaction solution thin layer under vacuum for evaporation and condens

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