Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2000-06-07
2001-08-28
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C525S250000, C525S271000
Reexamination Certificate
active
06281327
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention related to the production of haze-free block copolymers, in particular block copolymers of at least one hydrocarbon conjugated diene, such as butadiene, and at least one monovinylarene, such as styrene, prepared via anionic polymcrisation using an alkali metal initiator.
DESCRIPTION OF THE PRIOR ART
It is well known in the art to decolorise polymers of the vinyl aromatic-conjugated diene block copolymer type by using a mineral acid or a mono- or polycarboxylic acid to avoid the deep yellowish color generated during polymerisation and processing. This treatment results in transparent but yellowish polymers. It is important for the polymer appearance, and particularly for the use of the polymers with foodshuffs, to make this yellowish coloration disappear and to obtain totally transparent, haze-free and colorless copolymers. A number of methods are known for obtaining such transparent, haze-free colorless copolymers by treating solutions of such copolymers with a decolorising agent.
It is known in the art that vinyl aromatic-conjugated diene block copolymers, such as styrene-butadiene-styrene (SBS) block copolymers, may be prepared by anionic polymerisation employing an alkali metal initiator. In such a process, alkali metal derivatives, primarily alkali metalalcoholates, are required to be neutralized prior to recovery of the polymer with an acid capable of converting those derivatives into neutral and inert alkali metal salts of the corresponding acids. Typically the alkali metal employed is lithium and typical reaction scheme is as follows:
R—OLi+R′—COOH→R-O-H÷R′—COOLi
where R is a polymer, typically an SBS block copolymer, a styrene butadiene rubber, a butadiene rubber or a polybutadiene, and R′ is a saturated aliphatic group.
reaction with phenolic moieties present in an antioxidant system provided for protecting the block copolymers against thermoxidative attack, which would otherwise promote the formation of strongly colored by-products. Such reaction by such alkali metal species therefore would decrease or suppress the resistance to thermoxidative attack of the transparent colorless block copolymers;
reaction with ester (R—O—COR′) groups of some antioxidants, leading to chain cleavage and producing volatile residues;
reaction with the functional groups of fillters, pigments, colorants, and ultraviolet-protecting additives, thereby reducing their efficiency.
EP-A-0084795 and its corresponding US-A-4,403,074 disclose the production of clear, haze-free, colorless impact-resistant resinous copolymers, in particular SBS block copolymers, in which alkaline lithium derivatives comprising the coupled polymer-lithium entity, are terminated with water and linear saturated aliphatic dicarboxylic acids selected from C
2
, C
3
and C
5
-C
14
acids. While it is disclosed in those documents that the use of such a wide range of linear saturated aliphatic dicarboxylic acids produces haze-free polymers, the process disclosed in those prior specification can produce polymers which suffer significant reduction in transparency and significant increase in haze when the polymer is subjected to a humid environment.
US-A-4,877,863 disclose the production of vinyl aromatic-conjugated diene copolymers in which the copolymers are decolorised by treating the copolymer with a thiocarboxylic acid which may be a monoacid or a diacid. The use of such thiocarboxylic acids as disclosed in that document can also lead to degradation in the transparency and haze of the polymer when the polymer is subjected to a high humidity environment
EP-A-0358283 discloses a process for the removal of metal catalyst residues from water insolubel polymer solutions by treating the solutions with a dicarboxylic acid to form insoluble metal precipitates.
DE-A-1911886 discloses a process for metallising an unsaturated polymer.
It is also known to employ carbon dioxide or propionic acid as a decolorising agent to neutralise the alkaline lithium residues. The us of carbon dioxide or propionic acid also leads to such polymer degrading with regard to their transparency and haze-free properties when the polymers are required to withstand a highly humid environment.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a vinyl aromatic-conjugated diene block copolymer, the process comprising solution polymerising at least one vinly aromatic hydrocarbon and a conjugated diene in the presence of a catalyst based on an alkali metal to produce copolymer chains which end with an alkali metal and treating the copolymer chains in solution with an alkyl dicarboxylic acid having at least 2 carbon atoms and the acid being in the form of a slutty in an organic liquid with the acid being granulated and having an average granule size of less than 50 microns.
The copolymer may have some level of transparency resulting from a styrene content of from 15 to 95wt. %. Preferably, the vinly-aromatic-conjugated diene block copolymer is transparent, haze-free and colorless and comprises a styrene-butadiene-styrene block copolymer having at least 70wt % styrene content.
In accordance with the invention, the dicarboxylic acid decolorising agent is added to the block copolymer is solution as a slurry. Preferably, the slurry comprises finely ground particles of the dicarboxylic acid, preferably having an average particle size of from 5 to 50 microns, in an organic liquid, such as an alkane, for example hexane. A particularly preferred organic liquid comprises hexane as a mixture of cyclohexene and n-hexane in a ration of 85wt % cyclohexene/15wt % n-hexane. In such a hexane mixture, azelatic, sebacic and dodecanoic acids have a concentration, determined at room temperature of 0.34, less than 0.002, and 0.015wt % respectively.
The present inventors have found that the use of a slurry of finely ground dicarboxylic acid in an organic liquid significantly accelerates the neutralization of the alkaline lithium derivative. US-A-4,403,074 and EP-A-0084795 disclose that the aliphatic dicarboxylic acid can be added in the neutralization step as an aqueous solution or dispersion, or can be admixed with a dry solvent, preferably cyclohexene, when added to the polymer cement or concentrated polymer cement. However, the present inventors have discovered surprisingly that in order to incorporate such diacids via industrially feasible economical techniques, the diacid must be incorporated as a slurry of the finely ground particles in an organic liquid with the particles having an average particle size of less than 50 microns. This provides the advantage of significantly reducing the neutralization time and ensuring that the polymer when recovered has the required optical and mechanical properties. If the average particle size is less that 5 microns, then health and safety problems may be encountered in an industrial environment, particularly with regard to the hazard of inhalation of the particles.
Preferably, the diacid granules are comminuted, for example by micronising in known manner, so as to have an average particle diameter of less than 50 microns, most preferably less than 25 microns, and still more preferably less than 10 microns. The use of sebacic acid granules down to an average particle size of 23 microns, for example, reduces the neutralization time to 10 minutes as compared to neutralization times of up to around 6 hours for “as received” industrially available sebacic acid granules having an average granule size of around 138 microns. When the sebacic acid granules are comminuted in size to down to around 8 to 10 microns average size, the neutralization time may be reduced yet further to around 1 minute.
It is a further aim of the present invention to provide for the production of transparent, haze-free, colorless block copolymers having an enhanced resistance to degradation of the optical properties, in particular the transparency and haze-free properties, when the polymers are subjected to a highly humid environment.
Accordingly, preferab
Fina Research S.A.
Teskin Fred
Wheelington Jim D.
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