Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-11-04
2002-07-23
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S309000, C525S310000, C524S458000
Reexamination Certificate
active
06423783
ABSTRACT:
TECHNICAL FIELD
The present invention relates to modified polymers of the type including carbon/carbon double bonds. More particularly the invention relates to such polymers when modified by the polymerisation of at least one monomer of low water solubility within a pre-existing polymer latex to yield materials containing interlinked polymers.
BACKGROUND ART
Modification of rubbery polymer colloids containing double bonds is common, including for example post-polymerisation processing of styrene-butadiene rubber and of neoprene. Another example comprises modification of natural rubber latex. One of the major components in natural rubber latex is cis-1,4polyisoprene, of which certain properties are unmatched by most synthetic rubbers. In addition, the unsaturated backbone in natural rubber allows various types of chemical modification to be performed, to yield a wide range of rubber products found in today's market. Prior chemical modifications of the isolated polymer include hydrogenation, epoxidation, chlorination, grafting, vulcanising and other polymerisation processes. Modification of a rubber latex (ie., without prior isolation of the polymer) is more challenging. Typical of earlier studies are those carried out using natural rubber latex swollen with methyl methacrylate (MMA) and then polymerised. The resulting polymer consists in part of PMMA grafted onto natural rubber, commercially known as Hevea Plus, a product which has now been discontinued due to low levels of grafting and cracking on casting. Many attempts have been made by other investigators to graft oil-soluble vinyl monomers onto natural rubber latex using various types of initiators. However, the problems associated with their attempts range from secondary particle formation, types of grafting (ie. comb or T-shape) to non-uniform modification of all rubber chains. All current products are spatially non-uniform: eg., electron microscopy shows that the particles have spatially separate regions comprising pre-existing polyisoprene and new polymer.
DISCLOSURE OF INVENTION
The aim of this work is to modify rubber latices including those of natural rubber to produce novel materials in order to overcome some of the problems stated above. Experiments based on a mechanistic understanding of emulsion polymerisation were designed, which resulted in the use of monomers of very low water solubility, such as vinyl neo-decanoate. The inventors characterise the morphology and properties of the resulting materials as containing interlinked polymers. The term “interlinked” in this context is taken to mean physical and/or chemical linking between polymers.
The present invention consists in a process for the formation of a modified polymer of the type having unsaturated carbon/carbon bonds, the process including the steps of:
a) taking or making a colloidal dispersion of the polymer in an aqueous medium,
b) swelling the polymer in the colloidal particles with at least one polymerisable monomer having a solubility in the aqueous medium of less than 10
−3
molar, and
c) inducing a free radical polymerisation of the monomer(s) within the swollen colloidal particles of the polymer.
In another aspect the present invention consists of a material in the form of interlinked polymers produced by the method according to the present invention. In a still further aspect the present invention consists of a polymer of the type having unsaturated carbon/carbon bonds, wherein after polymerisinig an added monomer or monomers there is interlinked with the original polymer a new polymer or polymers formed in-situ within the original polymer from the added monomers.
The present invention is particularly applicable to the preparation of modified natural rubber latex. In this embodiment of the invention the natural rubber latex, which is already in the form of a colloidal suspension of polymer particles in water, is swollen with the monomer(s) and then polymerised. The present invention may with proper selection of reagents, however, be used to prepare modified polymers based on any polymer including unsaturated carbon/carbon bonds which may be formed into or exist as a colloidal suspension. Among the synthetic elastomers to which the present invention may be applied are isoprene rubbers, chloroprene rubbers including neoprene rubbers, polybutadiene rubbers, nitrile-butadiene rubbers, styrene-butadiene rubbers, polypentenamers, and ethylene-propylene-diene terpolymers. The unsaturated carbon/carbon bonds are usually double bonds in or pendant from an aliphatic chain but are not necessarily so.
The monomers for use in the present invention are characterised by their low solubility in aqueous media. They should have a solubility in water of less than 10
−3
M and preferably between 10
−4
M and 10
−9
M, more preferably between 10
−5
M and 10
−7
M. To be useful in carrying out the present invention they must be capable of swelling the polymer and also be capable of free-radical initiated polymerisation. The most preferred monomer for use in carrying out the present invention is vinyl neo-decanoate. Other monomers that are preferred are straight chain or branched chain alkyl or aryl vinyl esters having at least 6 carbon atoms, alkyl acrylates, alkyl methacrylates, vinyl acrylates and vinyl methacrylates. More preferred are vinyl 2-ethylhexanoate, vinyl 4-tert butyl benzoate, n-dodecyl acrylate, n-dodecyl methacrylate, 2-ethylhexyl acrylate, p-methyl styrene, vinyl toluene and 4-tert butyl styrene. The monomers are preferably formed into an emulsion with a suitable surfactant and added to the dispersion of the polymer or they are added to the dispersion and agitated. It is normal to allow the monomer(s) to sit in contact with the polymer for a period of from minutes to some hours in order for the monomer(s) to penetrate the polymer and swell it. The monomer(s) should for preference not be added in a greater amount than can be adsorbed by the polymer. Provided one or more of the low-solubility monomers described above are present, other monomers of any solubility may also be added to the reaction mixture and included in the polymers according to the present invention.
The term swelling in this context is taken to mean the entry of monomer(s) into the polymer particle and diffusion through the particle. There will usually be a concomitant physical increase in the size of the polymer particles.
Without limitation to the scope of the present invention the following explanation of the mechanism of the present invention is proposed. The process seeks to avoid the formation of secondary particles during the polymerisation process. It is thought that secondary nucleation results in the formation of new particles outside the polymer particles, and the subsequent imbibing of these new particles into pre-existing polymer particles is responsible for some or all of the spatial homogeneities observed in current products. The avoidance of secondary particle formation, including particles which may be later imbibed into pre-existing polymer particles, was carried out by choosing monomers of very low water solubility. This is because radicals formed from initiator are most likely to react with an individual monomer molecule, perhaps in the water phase to form a species that is subsequently free to polymerise anywhere in the particle. It is believed that if the polymer were present in the aqueous phase in a greater concentration there would be a reaction between the activated monomer molecule and one or more other monomer molecules resulting in secondary particle formation. The products of the present invention show a number of distinct glass transition temperatures. These temperatures often are very close to but not identical to the values that would be expected from the pure polymers. The fact that the glass transition temperatures have moved slightly suggests that in addition to the polymerisation reaction the activated monomer species may also enter into some grafting reactions with the polymer. During polymerisation some
Balic Robert
Gilbert Robert Goulston
Monteiro Michael John
Subramaniam Nadaraja
Asinovsky Olga
Nixon & Vanderhye
Seidleck James J.
University of Sydney
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