FIELD OF THE INVENTION

The invention is related to a method for producing pellets of a non- crystalline olefinic copolymer, said copolymer having a melt flow index greater than 2 and to free flowing pellets of a non-crystalline olefinic copolymer.

BACKGROUND OF THE INVENTION

Many elastomers are tacky or exhibit cold flow in their green or uncured state. As a consequence, these materials cannot be transported in bulk as free flowing pellets but must be shipped in bales. This practice requires that the ultimate elastomer processor must be equipped to cut up or mill the bales. The necessary equipment is generally large scale, expensive equipment. Additionally, the bales cannot be readily preblended with other materials. The necessity for baling results in high handling and shipping costs. In order to facilitate handling and processing of elastomers, it has been considered desirable to produce elastomer pellets. Generally, however, elastomer pellets exhibit "blocking" or cold flow characteristics which result in solidification into a solid mass after a short storage time, especially at elevated temperatures. Such describe blocking and cold flow behavior is especially exhibited by amorphous polymers, whereas polymers with a specific amount of crystallinity will be much less affected by such phenomena. For example the crystalline nature of polymers such as polyethylene, polypropylene or crystalline ethylene olefin copolymers and its derivatives results in a substantial physical stability of the pellets during regular transportation and storage conditions. In consequence, such crystalline polymer derivatives as for example described in US 4,927,888, US 3,932,368 and US 4,451 ,614 do not fall under the scope of the present invention.

Numerous attempts have been made to formulate elastomeric pellets which will remain free flowing until they are to be processed. Dusting the elastomeric pellets with inorganic materials, e.g., clay, talc, etc., has been found to extend the time over which the pellets are free flowing. Improved results have been achieved by dusting a coating with selected organic materials such as hydrocarbon waxes (British Pat. No. 901 ,664) or powdered polyethylenes and polypropylenes (British Pat. No. 928,120). However, because of the discontinuity of the dust coat, the coated pellets eventually flow together to form a solid mass. By blending the elastomer with a crystalline type polymer such as polyethylene, polypropylene, copolymers of ethylene and propylene, polyesters or polyamides, it has been possible to produce free flowing elastomer containing pellets. However, the elastomer content of the pellet must be less than about 65%. The product is, of course, not suitable for use in all elastomer applications. Such processes and products have been described in US 6,184,297 and US 6,077,906.

Another coating approach to the problem has been the coating of elastomer pellets with emulsions containing a tack free coating material. Coating is accomplished either by dipping pellets into the emulsion or spraying the emulsion onto the pellets. In either case the emulsion coating must be dried, and where the emulsion contains a solvent the solvent must be recovered. Drying and solvent recovery requirements result in increased costs.

Melt-coating methods for producing free-flowing elastomer pellets have also been suggested. According to U.S. Pat. No. 3,669,772 to Bishop, coating can be accomplished by using a die, similar to wire coating die, into which a strand of rubber to be coated is fed simultaneous with melt coating material. A continuous melt coated strand of rubber issues from the coextrusion die outlet, is cooled in a liquid cooling bath, and is subsequently pelletized. This melt-coating method not only adds significantly to rubber manufacturing costs, but has limitations from the standpoint of efficiently producing large quantities of coated pellets.

A rubber pellet composition comprising an elastomer and plastic insoluble in the elastomer can be caused to coat itself with a plastic skin by control of composition, extrusion conditions and die temperature. U.S. Pat. No. 4,622,193 to Hazelton describes an elastomer-plastic blend, extruded at a temperature above the melting point of the plastic, and pelletized as it exits from a die, the die having a temperature gradient across the die from inlet to outlet, the gradient being such that the die outlet temperature is substantially lower than the extrusion melt temperature (die inlet temperature), resulting in a pellet coated with a skin of plastic.

U.S. Pat. No. 3,927,166; of Nagao describes a method wherein between 1 and 30 wt% of surfactant is added to the polymer before extrusion.

A disadvantage of above-mentioned methods is that non-rubbery materials are mixed into the rubber, which may lead to blooming and, or deterioration of properties of the final product.

SUMMARY OF THE INVENTION The invention is a method for producing free flowing pellets of an olefinic copolymer with a crystallinity at 23°C below 1 wt% measured by DSC according to ASTM E793 using HDPE as an external standard, said copolymer having a melt flow index measured according to ISO1 133 at 190 ⁰ C, 2160 g greater than 2 g/10min characterized in that the method consists in grafting from 0.5 to 5 wt% ethylenically unsaturated carboxylic acid material onto said copolymer backbone to form an acylated olefin copolymer, melt-extruding the resulting mixture and cooling, solidifying and pelletizing the resulting melt-extrudate, storing the resulting pellets for at least one week. In another aspect, the invention provides a free flowing pellet comprising an olefinic copolymer with a crystallinity at 23°C below 1 wt% measured by DSC according to ASTM E793 using HDPE as an external standard, said copolymer having a melt flow index measured according to ISO1133 at 190 ⁰ C, 2160 g greater than 2 g/10min, characterized in that the copolymer comprises from 0.5 to 5 wt% ethylenically unsaturated carboxylic acid material.

DETAILED DESCRIPTION

The invention is a method for producing free flowing pellets of a noncrystalline olefinic copolymer. The non-crystalline nature of the olefinic copolymer is defined by a crystallinity below 1 wt% measured by DSC according to ASTM E793 and using HDPE as an external standard. A crystallinity of less than 1 wt% is generally experienced by the absence of an endo- or exothermic crystallization in DSC measurements. In general and in the present application such materials are also addressed as amorphous or non-crystalline polymers. A typical category of non-crystalline olefinic polymer are ethylene- higher alpha-olefin copolymers, especially ethylene propylene copolymers that may consist of from about 30 to 60 wt% ethylene and from about 70 to 40 wt% propylene, cyclic or higher alpha-olefin. The preferred copolymers for practice of this invention are comprised of from 40 to 55 wt% ethylene and 60 to 45 wt% propylene. Hydrogenated random and block copolymers of a vinyl aromatic compound and a conjugated diene, or mixtures of conjugated dienes, are also suitable substrates for use in the present invention. Among these types of copolymers, hydrogenated random and block copolymers of isoprene-butadiene, styrene-isoprene or styrene-butadiene are preferred. - A -

The melt-index of the copolymers of the invention is the melt-index of the processed and pelletized copolymer compound. This viscosity is independent from the viscosity of the initial copolymer subjected to the melt processing since the described modification technology can lead to polymer degradation and cross-lining that will affect final viscosity.

The melt-index of the copolymer of the invention is measured in accordance with ISO1133 is more than 2 g/10 min (190 ⁰ C, 2160 g), preferably more than 4 g/10 min. Melt index is a common and simple to measure polymer characteristic, especially in production environment and is most often a leading and reliable quality control parameter. For the concerned amorphous copolymers correlation between melt index and more scientific methods such as molecular weight values determined by size exclusion chromatography are readily available.

An indicative number average molecular weight, Mn, that can be determined by gel permeation chromatography of the copolymer subject to the present invention is between 700 and 60,000 g/mol, preferably between about 3,000 and about 55,000 g/mol, more preferably between about 10,000 and about 50,000 g/mol. By the same means, a molecular weight distribution, M _w /M _n , of the polymer substrates of the present invention can be obtained that is less than 15, preferably 1.0 to 10.

The preferred olefinic copolymer substrate employed in the method of the present invention is derived from polymerizable C ₂ to C ₂ 3 olefins. Such copolymers are typically produced from ethylene, propylene, 1-butene, 2-butene, isobutene, cyclopentene, 1-hexene, 1-octene or norbornene.

Preferred polymers for use in the present invention are copolymers of ethylene and one or more C ₃ to C ₂₃ olefins. Copolymers of ethylene and propylene are most preferred. Other olefins suitable in place of propylene to form the copolymer or to be used in combination with ethylene and propylene to form a terpolymer include 1- butene, 1-pentene, cyclopentene, 1-hexene, 1-octene, norbornene, and styrene; also alpha, omega -diolefins such as 1 ,5-hexadiene, 1 ,6-heptadiene, 1 ,7-octadiene, etc., also branched chain alpha-olefins such as 4-methylbutene-1 , 5-methylpentene-1 and 6-methylheptene-1 and mixtures thereof.

The ethylene-olefin copolymers may contain minor amounts of other olefinic monomers such as conjugated or nonconjugated dienes. The polymerization reaction used to form the ethylene-olefin copolymer substrate is generally carried out in the presence of a conventional Ziegler-Natta or metallocene catalyst system. The polymerization medium is not specific and can include solution, slurry, or gas phase processes, as known to those skilled in the art. When solution polymerization is employed, the solvent may be any suitable inert hydrocarbon solvent that is liquid under reaction conditions for polymerization of olefins; examples of satisfactory hydrocarbon solvents include straight chain paraffins having from 5 to 8 carbon atoms, with hexane being preferred. Aromatic hydrocarbons, preferably aromatic hydrocarbon having a single benzene nucleus, such as benzene, toluene and the like; and saturated cyclic hydrocarbons having boiling point ranges approximating those of the straight chain paraffinic hydrocarbons and aromatic hydrocarbons described above, are particularly suitable. The solvent selected may be a mixture of one or more of the foregoing hydrocarbons. When slurry polymerization is employed, the liquid phase for polymerization is preferably liquid propylene. It is desirable that the polymerization medium be free of substances that will interfere with the catalyst components.

A first step of the method of the invention consists in grafting from 0.5 to 5 wt% ethylenically unsaturated carboxylic acid material onto said copolymer backbone to form an acylated olefin copolymer. These carboxylic reactants which are suitable for grafting onto the ethylene copolymer contain at least one ethylenic bond and at least one, preferably two, carboxylic acid or its anhydride groups, or a polar group which is convertible into said carboxyl groups by oxidation or hydrolysis. Preferably, the carboxylic reactants are selected from the group consisting of acrylic, methacrylic, cinnamic, crotonic, and maleic, fumaric, and itaconic reactants of the general formula: o

Y' C C X'

O wherein R is an alkyl group having from 0-4 carbon atoms, X and X' are the same or different and are independently selected from the group consisting of -OH, -O- hydrocarbyl, -0-M ⁺ wherein M ⁺ represents one equivalent of metal, ammonium or amine cation, -NH ₂ , -Cl, -Br, and together X and X' can be -O- so as to form the anhydride, and Y and Y' are the same or different and are independently selected from the group consisting of hydrogen, branched or straight chain alkyls having 1-12 carbon atoms, a halogen atom, or an organo anhydride, ketone, or heterocyclic group having 2-12 carbon atoms. Ordinarily, the maleic or fumaric reactants will be maleic acid, fumaric acid, maleic anhydride, or a mixture of two or more of these. Maleic anhydride is generally preferred due to its commercial availability and ease of reaction. The carboxylic reactant is grafted onto the prescribed polymer backbone in an amount of from about 0.5 to about 5 wt% of carboxylic reactant per 100 wt% of the polymer backbone, preferably, at least 1 wt%. More preferably, at least 2 wt%.

The grafting reaction to form the acylated olefin copolymers is generally carried out with the aid of a free-radical initiator either in solution or in bulk, as in an extruder or intensive mixing device. When the polymerization is carried out in hexane solution, it is economically convenient to carry out the grafting reaction in hexane as described in U.S. Pat. Nos. 4,340,689, 4,670,515 and 4,948,842. The resulting polymer intermediate is characterized by having carboxylic acid acylating functionality randomly within its structure.

In the bulk process for forming the acylated olefin copolymers, the olefin copolymer is fed to rubber or plastic processing equipment such as an extruder, intensive mixer or masticator, heated to a temperature of 150 to 400 ^° C. and the ethylenically unsaturated carboxylic acid reagent and free-radical initiator are separately co-fed to the molten polymer to effect grafting. The reaction is carried out optionally with mixing conditions to effect shearing and grafting of the ethylene copolymers according to U.S. Pat. No. 5,075,383, incorporated herein by reference. The processing equipment might be purged with nitrogen to prevent oxidation of the polymer and to aid in venting unreacted reagents and byproducts of the grafting reaction. The residence time in the processing equipment is sufficient to provide for the desired degree of acylation and polymer degradation and to allow for purification of the acylated copolymer via venting.

The free-radical initiators which may be used to graft the ethylenically unsaturated carboxylic acid material to the polymer backbone include peroxides, hydroperoxides, peresters, and also azo compounds and decompose thermally within the grafting temperature range to provide free radicals. Representatives of these free- radical initiators are azobutyronitrile, dicumyl peroxide, 2,5-dimethylhexane-2,5-bis- tertiarybutyl peroxide and 2,5-dimethyl-hex-3-yne-2,5-bis-tertiary-butyl peroxide, bis- tertiary-butyl peroxide, 3,3,5,7,7-pentamethyl 1 ,2,4-trioxepane and 3,6,9-triethyl-3,6,9- trimethyl-1 ,4,7-triperoxonane. The initiator is used in an amount of between about 0.005% and about 1% by weight based on the weight of the reaction mixture. Other methods known in the art for effecting reaction of olefin copolymers with ethylenically unsaturated carboxylic reagents such as halogenation reactions, thermal or "ene" reactions or mixtures thereof can be used instead of the free-radical grafting process. Such reactions are conveniently carried out in mineral oil or bulk by heating the reactants at temperatures of 250 to 400 ⁰ C. under an inert atmosphere to avoid the generation of free radicals and oxidation byproducts.

In a second step of the invention the acylated olefin copolymer is extruded, cooled, solidified and pelletized.

A typical but not limiting way to perform the peptization under the scope of the invention is to process the acylated melt through a pressure building zone in an extruder to force the polymer through a die at a typical temperature between 100 and 300 ⁰ C to be cut by an underwater pelletizer. Other exit media such as a turbulator, a strand bath or a gear pump may be used. In applications in which rapid cooling of the product is desired, an underwater pelletizer is preferred. Temperature of the cooling water will be typically maintained between 10 and 60 ⁰ C. More preferred the temperature will be between 15 and 35°C.

The pelletize material will have a defined residence time in the cooling medium to allow sufficient heat exchange between the cooling medium and the polymeric material. Considering pellet size and temperatures as well as temperature and dimensions of the cooling system, a typical residence time will be between 15 seconds and 5 minutes before being separated and dried from the cooling water by cyclone and drier technology. Longer residence times will require increased dimensioning of the cooling system with the benefit of an improved cooling result or the possibility to work with a higher temperature of the cooling medium. Shorter cooling times will require higher turbulent flows and reduced temperature of the cooling medium to achieve acceptable final pellet temperature with the benefit of a smaller cooling system.

The resulting pellets are stored for at least one week. The storage can take place at room temperature in bags, bales or boxes. Alternatively a conditioned environment consisting of heating or cooling to specific temperatures and maintaining specific humidity levels can be applied.

In one embodiment, the pellets are stored in bales for at least one week, and subsequently de-lumped by a mechanical force into free flowing pellets. A typical de-lumping process consists of a grinder large enough to accept feeding of entire bales of the original material with sufficient air flow to remove heat generated by the grinding process. The pellets are recovered after pneumatic transport via a cyclone and repackaged in bags or the original boxes. Dusting agents can be applied before, during or after the grinding process.

In another embodiment, the conditioned pellets are dusted with a partitioning agent either during de-lumping of the bales or thereafter in a suitable dusting equipment. This process might be extended with other mechanical treatments such as a continuous or batch operated tumbling device to allow homogeneous distribution of the dusting agent. Many kinds of partitioning agents, i.e. dusting agents, can be applied. Commercial calcium stearate, aluminum silicate or magnesium silicate powders are examples of such a partitioning agent. Specific additives designed for partitioning and characterized by their high surface area such as fumed silicas are other examples of suited partitioning agents. The dusting agent may be a single dusting agent or a combination of more than one type of dusting agents. The selection of dusting agent types, to a large extent, depends on the final application by the users, and can be selected by those of ordinary skill in the art. Typical partitioning agents are metal salts of organic aliphatic acids like calcium stearate powder, talc, calcium carbonate, clay, and crystalline polyolefin powders, like low or high density polyethylene, polypropylene, ethylene-vinylacetate copolymers as well as amorphous synthetic compounds such as fumed silica or furnace black. To make a free flowing pellet of the current invention, only a minimum amount of dusting is necessary to avoid pellets bounding together. The level of dusting agent required will be a further function of the specific surface of the particulate rubber and additive. If calcium stearate dust is used on EP(D)M pellets, the level of dusting may be from a lower level of 0.01 , or 0.05 wt% to an upper level of 0.2, or 0.5, or 2 wt%. Addition of the dusting agent can be performed on a continuous basis before, during or after grinding. Optionally the dusting agent can be added batch wise, under the condition of allowing substantial residence time and back mixing for a homogeneous distribution of the additive throughout the pellets. The dusting device can be selected from a number of commercial dusting devices, including, for example, Kason Vibratory Conveyor, Hirshel mixer, Rotary Tumbler, roller mixer, or any vibratory or screw conveyors. The dusting device is selected to achieve uniform distribution and coating of dust on pellets. Also, the particle sizes of the dusts could have an impact on how much dusting agent will be used. Typical dust size varies from 1 to 100 microns. However, dust particle sizes could be as large as 500 to 1000 microns. Generally, the finer the dust particle, the lower the dust level needed to avoid that pellets stick together.

The invention further relates to a free flowing pellet comprising a noncrystalline olefinic copolymer, said copolymer having a melt flow index greater than 2, wherein the copolymer comprises from 0.5 to 5 wt% ethylenically unsaturated carboxylic acid material grafted onto said copolymer backbone.

The pellet according to the invention preferably comprises a copolymer with a glass transition temperature of less than 20 ⁰ C as determined by standard DSC analysis. The pellet of the invention preferably comprises a copolymer with a maximum gel level of 0.5 wt% determined via solubilization and filtration from a solution. For the example of a maleic anhydride modified ethylene propylene copolymer THF is used at room temperature.

A storage friability test has been developed consisting of a 10 cm diameter and 10 cm high aluminum cylinder that can be removed sideway by opening along the vertical axis. The cylinder is filled with the granulate and subjected to storage conditions by applying a weight representing single bag or entire pallet stacking height for a defined time and temperature. This test simulates the flow behavior of the bottom fraction of material stored in individual boxes or bags, stacked up to 1.5 meter respectively. When the constraint is released, the time required for the pellets to crumble is monitored. If after one hour no crumbling occurred, the pellet made cylinder is manipulated with increasing force. Failure of the flow behavior consists of a cylinder of material that can be lifted by hand without disintegration.

Example I

Bales of rubber (Keltan 3200A, a product of DSM Elastomers, 49 wt% ethylene and 51 wt% of propylene with a weight average molecular weight of 180 kg/mol), were stripped of film wrap and fed via an hopper to a melt dosing extruder and transported to an extruder arrangement by a melt pump at a feed rate of 50 kg/h. According to the setup, excess moisture was allowed to leave the melt to the atmosphere, subsequently, molten maleic anhydride (80 ⁰ C) was fed to the extruder at a feed rate of 2.5% of the rubber throughput. Di-tert-butyl peroxide (Akzo Nobel, Trigonox B) as a 30 wt% solution in mineral oil was fed at a combined rate of 0.4% of the rubber throughput. Screw speed was 250 rpm to reach a reaction melt temperature of 201 °C. Degassing of unreacted product was done via a vent zone at a vacuum of 200 mbar. Final compression of the melt in the extruder head gave a final melt temperature of 298°C. The rubber melt exiting the extruder was fed to an under water pelletizer system including a pelletizing head with a star knife assembly and water recirculation system with heat exchanger. The pellets were cooled for an average of 1 minute in the 18°C water stream of the under water peptization system before being collected at a pellet temperature of 27°C.

The obtained maleic anhydride grafted rubber was a clear light yellow rubber with a melt flow index (MFI) of 4.9 g/10 min (190 ⁰ C, 216O g), a gel level of 0.05 wt% and a maleic anhydride functional level measured by IR method of 1.93 wt% (conversion 77%).

The freshly recovered material shows a tendency to agglomerate such that the pelletized product is recovered as bales. This is in line with the polymer analysis, which does not suggest that a stable pellet is possible at all. However after storage of at least one week of the material followed by de-lumping by mechanical force by means of a rotating knife rubber grinder, the generated pellets were free flowing. The so obtained free flowing material was subjected to the storage friability test by simulating compressing of the pellets by a pallet height (1.5 meter) of its own weight at 35°C. Upon release of the constraint, the compressed pellets crumbled readily upon touch. Alternatively the ground material was dusted with 0.5 wt% of Aerosil

200 before the storage friability testing. Upon release of the constraint, the pellets crumbled immediately.

The result is that though the material is still fully amorphous it can be reprocessed as a free flowing product.

Comparative Experiment A

This experiment was carried out under identical condition compared to example 1 with the difference that no maleic anhydride and peroxide was dosed. The obtained pelletized rubber was a clear nearly colorless rubber with a melt flow index (MFI) of 2.3 g/10 min (190 ⁰ C, 216O g), a gel level of less that 0.01 (detection limit) and a maleic anhydride functional level measured by IR method of less than 0.05 wt% (detection limit).

The freshly recovered material shows an extreme tendency to agglomerate such that the pelletized product is recovered as solid bales. After storage of at least one week of the material de-lumping by mechanical force proved to be impractical due to cold flow and stickiness of the polymeric mass.