The present invention relates to a method for the preparation of particles comprising at least a polymer, wherein a melt comprising the said polymer is extruded by passing the melt through at least a die into a liquid solidifying medium having a temperature of below the melting temperature of the melt, the die being submersed in the said liquid solidifying medium, allowing the melt to solidify in the said liquid solidifying medium.

The invention further relates to particles obtainable by the said method, and to novel uses of such particles.

Background of the Invention

The method for the preparation of particles as described above is also known in the art as underwater granulation. According to this technique a polymer melt is extruded and passed through a die, which die is submersed in water. The water temperature may vary, usually between 20 and 100°C. As soon as the polymeric melt is passed through the die, the melt is cooled by the water and solidifies; the water has the function of solidifying medium.

A cutting device cuts the extruded material into pieces and the said pieces form particles during the solidification process. By cutting the melt, portions of the melt are individually solidified. In the art methods are known for cutting the melt that exits a die. For example cutter blades circling around the die opening are known for this purpose. Straight and angled cutter blades can be used and spacing between the cutter blades is important to the process. Because of the surface tension of the melt, the melt rounds up upon solidification in the solidifying medium. The melt can be passed through the die at a flow rate and wherein the cutting device is synchronized with the said flow rate such as to cut off equal portions of the passed melt. By synchronizing the cutting frequency with the flow rate of the melt through the die outlet opening it is possible to cut the melt at a predetermined length therewith controlling the shape of the particles to be formed upon solidification. By underwater granulation particles of high sphericity and roundness can be obtained.

A common problem of underwater granulation is the fact that the polymeric melt tends to solidify rather quick in the water, as the temperature difference between the water and the melting point of the melt is rather large.

It is commonly known that polymers do not have a definite melting point but a melting range, starting at the onset temperature, here and in the ISO 11357-1 1997 standard indicated by T _im , and ending at the end temperature indicated in the ISO 11357-1 1997 standard indicated by T _fm . The melting point T _pm is located between T _fm and T _pm , as indicated in the said standard. The term 'melting point' as used herein for polymers, refers to the peak melting temperature T _pm as determined according to the ISO standard 11357-1 :1997.

The temperature large temperature difference between that of the melting point of the melt and the water as solidifying medium brings also the risk of clogging of the die outlet openings of the granulation equipment, as the melt tends to solidify at the location of the die opening as the temperature at that location is too low to keep the melt in molten state. Further, optimal roundness and/or sphericity can not be obtained if the temperature difference between the solidifying medium and the melting point of the melt is too high; the particles solidify before the optimal thermodynamic shape (a sphere) could form. Another problem is that polymers having a high melting point cannot be used in underwater granulation, in view of the above temperature difference between the melting temperature of the melt and the temperature of the water. Only rather relatively large particles can be produced of polymers having a high melting temperature. For example, particles of polyetherimid (PEI) and of polyether ether ketone (PEEK) could not be produced with a particle size of below 3.0 mm. Smaller particles could only be obtained by mechanical processing of larger particles, e.g. by cracking the obtained particulate to smaller sizes and sieving to an envisaged particle size. However, such processes have a deteriorating effect on the roundness and sphericity of the particles.

In the art, the problem of clogging of the die outlet openings is known. There appeared to be some compensation possible by elevating the pressure or temperature of the melt. However it was found that using the conventional underwater granulation techniques at the required small diameters of the die outlet opening resulted, in most cases, in clogging of the die opening by solidified polymer.

Small polyamide beads are described in US2010/0285998. By heating the die with die openings of 0.8 - 1.2 mm to a temperature of 345 - 389°C, beads of a of 1.25 - 1.5 mm could be produced. However, the said publication is silent with regard to roundness and sphericity of the beads thus produced. Indeed, as water is used as solidifying medium, the above problem of the high temperature difference between solidifying medium and the melt still exists resulting in unround particles.

Up to now the granulation technique has been performed with water, not only having a high heat conductivity, but also a low atmospheric boiling temperature of 100°C. The underwater granulation technique was simply limited to this temperature. The temperature could theoretically be elevated slightly by using water under pressure, or by using an aqueous saturated salt solution. However, using pressure in the solidification process may lead to undesired risks, whereas the use of salt solutions implies a more intensive washing step when the particles are to be isolated. Furthermore, the elevation of the temperature is rather small (the atmospheric boiling point of a saturated NaCI solution is e.g. 108°C). Solutions in the art have therefore been sought in technical changes in the equipment, without contemplating about the composition of the solidifying medium.

The present inventors have now surprisingly found that the granulation technique can be improved, avoiding clogging of the die outlet openings, and that polymeric particles can be obtained of polymers, that could not be produced by underwater granulation until the present invention was made. Also, polymers can be used that have a melting point T _pm that is significantly higher than the temperature of the solidifying medium. Further, the particle size of polymeric particles obtained can be much smaller by using the method according to the present invention, as compared to the particles, obtained by the common underwater granulation techniques. To this end, the invention is characterized in that the solidifying medium comprises a glycol. The present invention breaks the paradigm that under water granulation needs to be done with water as solidifying medium. By the invention the term underwater granulation is actually outdated, as now, alternative solidifying media comprising glycol are used according to the invention, which have a heat conductivity that is significantly lower than that of water. Nevertheless, the term 'underwater granulation', or 'underwater pelletizing' will be used herein as well, although the solidifying medium is different from water.

The present inventors have observed that because of the rather high heat conductivity of water, combined with the relatively low atmospheric boiling temperature thereof, is the main drawback of the above problems. A lower heat conductivity of glycol ensures a slower solidification of the melt in the solidifying medium, avoiding clogging at the die outlet opening, and allowing the particles to obtain the envisaged thermodynamically optimal form, i.e. a spheroid. By using water as solidifying medium, with its high heat conductivity and rather low atmospheric boiling point, a great amount of heat is absorbed by the solidifying medium from the melt as it passes the die outlet opening, therewith lowering the temperature of the melt within a short time period, resulting both in premature solidification and in unround particles, as well as in the risk that the melt solidifies as it is still at the die outlet opening, by direct cooling in contact with the solidifying medium or indirect cooling via the die-plate surface, resulting in clogging of the die outlet opening. In addition thereto, glycol has a higher boiling point than water, so that the solidifying medium can have a higher temperature than the boiling than water, so that a smaller temperature difference with the melt is possible, also resulting in a slower energy transfer from the melt to the solidifying medium.

By using a liquid solidifying medium comprising glycol, having a lower heat conductivity than water, the heat transfer between the solidifying medium and the melt occurs in a less fast manner, allowing the particles to settle in a more natural way. It was surprisingly found that small polymeric beads can be obtained that could not be produced before, or have improved roundness and sphericity as compared to particles of the art. The present invention therefore allows currently known polymers to be shaped in particles of a smaller size and narrower size-distribution, having improved roundness and sphericity.

Glycol is a compound belonging to the class of organic chemicals characterized by having separate two hydroxyl (-OH) groups, that contribute to high water solubility, hygroscopicity and reactivity with many organic compounds, on usually linear and aliphatic carbon chains. The general formula is C _n H2n(OH) ₂ or (CH ₂ )n(OH) ₂ . The wider meaning names include diols, dihydric alcohols, and dihydroxy alcohols. Ethylene glycol, HOCH2CH2OH, is the simplest member of the glycol family. Mono-, di- and triethylene glycols are the first three members of a homologous series of dihydroxy alcohols. They are colourless, essentially odourless stable liquids with low viscosities and high boiling points. Ethylene glycol is a somewhat viscous liquid and miscible with water; it has a boiling point of 198°C, and a flash point of 109°C.

Further, as clogging of the die outlet openings is reduced or prevented, production losses can be minimized. Additionally, the production capacity can be increased significantly, as the die openings remain operable without clogging, resulting in in larger production volumes.

The heat conductivity is to be measured at atmospheric pressure and at room temperature, i.e. 20°C and can be measured according to the ASTM D2717 standard, e.g. with the LabTemp30190 equipment of PSL Systemtechnik (Clausthal-Zellerfeld, Germany) according to the instructions of the manufacturer. The heat conductivity of water is e.g. 0.61 W/m/°C, whereas that of ethylene glycol is 0.26 W/m/°C. In the art, several disclosures, such as DE102006051452A1 , EP1083196A1 , EP0305862A1.US2010/0323047A1 and US2009/0121372A1 relate to underwater granulation, wherein, in addition to water, many organic liquids and gases, are mentioned as possible alternatives for the solidifying medium, however, without giving any possible advantage of using media, other than water. In contrast, without exception, all documents describe water as preferred solidifying medium and indeed, their examples only describe water as solidifying medium.

With the term 'liquid solidifying medium' is meant that the solidifying medium is in the liquid state when the melt is received therein from the die, and that the solidification of the melt indeed takes place in the liquid melt. The solidifying medium can be under pressure, but is preferably at atmospheric pressure. There is no pressure drop involved, such as e.g. described in US3003193, to effect solidification of the melt upon volatizing the solidifying medium.

The solidifying medium is preferably inert to the melt and the particles. With 'inert' is meant that the melt and the solidified particles do not dissolve substantially in the solidifying medium.

The term 'particle' comprises particles of any shape, such as spherical, spheroidal, elliptical, and right cylindrical shapes. Spherical shapes are preferred. The particle comprises at least one polymer, although polymer mixtures are also possible. It is however advantageous to use a single polymer as the polymer with the least qualities usually determines the properties of the particle.

Unless indicated otherwise, the term 'particle size' as used herein reflects the weight based particle size, which equals the diameter of the sphere that has same weight as a given particle. The weight based particle size is defined by the following formula: 2*(3*weight(particle)/4/pi/density(particle)) ^A (1/3).

In a very attractive embodiment, the glycol of the solidifying medium is chosen from ethylene glycol, propylene glycol or a mixture thereof. Propylene glycol has an atmospheric boiling point of 188°C, and ethylene glycol has an atmospheric boiling point of 197°C, and can be used in any mixture with water. Ethylene glycol is therefore very versatile for use as solidifying medium, and polymers having a high boiling point, such as PEI or PEEK can be subjected to underwater granulation resulting in small particles of high sphericity and roundness using ethylene glycol, optionally in admixture with water, as solidifying medium. Therefore, in a very attractive embodiment, ethylene glycol is used in or as solidifying medium.

Preferably, the liquid solidifying medium comprises a mixture of water and glycol. Although the solidifying medium may contain other components apart from glycol, it is preferred that the solidifying medium is composed of glycol, or of a mixture of glycol and water. 'Glycol' can be any mixture of glycols as defined herein. As mixture of water and glycol, a mixture of ethylene glycol and water is preferred.

Glycols can self ignite at their respective flash point. For security reasons, the working temperature is preferably lower than the lowest flash point of the glycols in the solidifying medium. For example, when the solidifying medium comprises ethylene glycol, the working temperature is preferably below 109°C. In a very attractive embodiment, the water content of the liquid solidifying medium is chosen such, that the atmospheric boiling point of the liquid solidifying medium is below the flash point of the glycol. For example, a preferred mixture of 10 v/v% water and 90 v/v% ethylene glycol as solidifying medium has an atmospheric boiling point of below 109°, avoiding any self ignition risk of the glycol in the solidifying medium when working at atmospheric pressure. As safety measurement, the solidifying medium will start to boil before self ignition can take place.

In a preferred embodiment, the heat conductivity of the solidifying medium is below 0.55 W/m/K (also W/m/°C), preferably below 0.50 W/m/K, more preferably below 0.40 W/m/K, most preferably below 0.30 W/m/K. When the heat conductivity is as low as indicated, particles having improved roundness and sphericity can be obtained.

Preferably the temperature of the solidifying medium is at least 70°C, more preferably at least 80°C and even more preferably at least 90°C, still even more preferably at least 100°C. Already at a relatively low relatively low temperature of e.g. 90°C particles of a small particle size and of improved roundness and stability can be obtained. It is also possible to work at temperatures of the solidifying medium of 120°C, or even above 140°C, if sufficient security measurements are taken to avoid e.g. self ignition of the glycol. The low heat conductivity of the solidifying medium enables slow solidification, therewith allowing the particles to take their thermodynamically optimal form, i.e. a spheroid, and reduces clogging of the die outlet openings. A higher temperature is preferably to be chosen when the melt has a higher melting temperature, otherwise the risk of clogging may again occur, and the particles may solidify too quickly to arrive at their optimal shape. At higher temperatures, polymers having a higher melting point can be used for the 'underwater' granulation techniques of the present invention. A totally new range of polymeric particulate materials can therewith be provided, that are produced by under water granulation, without the need for further processing such as cracking the obtained particles to smaller sizes and sieving to an envisaged particle size.

In another attractive embodiment, the temperature of the solidifying medium is at most or below 250°C, preferably at most or below 200°C, even more preferably at most 150°C, and most preferably at most 125°C. For reasons as explained above, the temperature of the solidifying medium is preferably below the flash point of the glycol present in the solidifying medium. The temperature can be limited for reasons of safety, and by limitations of the equipment, or to fulfil local safety regulations.

Preferably, the solidifying medium has an atmospheric boiling point of more than 100°C, preferably of between 100°C - 120°C, most preferably 105°C - 1 10°C. A high boiling point allows the solidifying medium to be used at higher temperatures which may be required for polymeric melts having a relatively high melting temperature.

In another preferred embodiment, the liquid solidifying medium is held at a pressure, and the melt is allowed to solidify in the said solidifying medium at the said pressure. The said pressure is preferably the atmospheric pressure. This means that when the melt passed the die and enters the solidifying medium, the melt solidifies in the said solidifying medium without any pressure change being necessary. As the melt solidifies in the liquid solidifying medium, no volatising of the solidifying medium has to take place, e.g. by a drop in pressure, to effect solidification of the melt.

In order to allow small particles to be formed, the outlet opening of the die preferably has a diameter of 4.0 mm or less, preferably 2.0 or less, more preferably 1.2 mm or less, even more preferably 0.8 mm or less, most preferably 0.75 mm or less. Attractively, the outlet opening of the die has a diameter of 0.3 mm or more, preferably 0.4 mm or more, more preferably 0.5 mm or more, most preferably above 0.6 mm or more. Preferred ranges in this respect are 0.3 - 4.0 mm, more preferably 1.0 - 2.0 mm.

As indicated above, polymers can be used for the underwater granulation technique of the present invention having a high melting point. Therefore, in an attractive embodiment the melting temperature T _pm of the polymer is at least 120°C, preferably at least 150°C, more preferably at least 180°C.

Preferably, the temperature of the melt is 180 - 400°C. The temperature of the melt also has an impact on the temperature of the die. A higher temperature of the melt assists in holding the temperature of the die at the required level. Further, the processability at higher temperatures is improved, as the viscosity of the melt will be lower. A too high temperature will however result in degradation of the polymer in the melt. The temperature of the melt is preferably 220 - 400°C, more preferably 260-400°C.

As the present invention provides for the first time alternative solidifying media comprising glycol, the temperature difference between the melting temperature T _pm of the melt and the temperature of the solidifying medium can be chosen to be rather small, enabling to form a polymeric particulate from polymers having a high melting temperature. To this end, the temperature difference between the solidifying medium and the melting point T _pm of the polymer is 150°C or less, preferably 100°C or less, more preferably 80°C or less. By choosing the temperature of the solidifying medium accordingly, the temperature difference between the melt and the solidifying medium is large enough to allow a good solidification of the melt, without allowing the melt to cool off too much at the location of the die outlet opening, therewith avoiding clogging of the said outlet opening.

The particles that are formed by the method of the present invention preferably have a particle size of at most 4.0 mm, preferably at most 3.0 mm, more preferably at most 2.5 mm, even more preferably at most 2.0 mm, still more preferably at most 1.3 mm, even still more preferably at most 1.0 mm, and most preferably at most 0.8 mm.. Attractively the particles have a particle size of at least 0.3 mm, more preferably at least 0.5 mm, most preferably at least 0.6 mm. Preferred ranges are defined by a combination of any of the above upper and lower limits. Especially preferred are ranges of 0.3 - 3.0 mm, more preferably 0.5 -1 ,3 mm, even more preferably 0.6 - 1.0 mm, most preferably 0.6 - 0.8 mm.

Herein, the term 'the polymer' also encompasses an optionally cross linked polymer, an optionally cross linked copolymer, an optionally cross linked terpolymer, an optionally cross linked block(co) polymer or any mixtures thereof unless indicated otherwise. The particle can also advantageously comprise one or more filler materials, e.g. to adjust the specific weight of the particle, to reinforce it, or to reduce costs of the particle material.

Many polymers are known that are suitable to be used as polymer for the polymer comprising particle of the present invention. The polymer in the melt preferably comprises an optionally cross-linked polymer, an optionally cross-linked copolymer, an optionally cross-linked terpolymer, an optionally cross-linked block(co) polymer or a mixture thereof.

The polymer is preferably thermoplastic in order to minimize abrasion in the fractures. The importance of the thermoplastic nature of proppants is e.g. explained in US7,322,41 1 , herein incorporated by reference.

The polymer is preferably non-expandable and the melt is preferably free of blowing agent. This means that the polymer is not significantly expanded and that the specific weight, or specific gravity, of the particles is about equal to that of the melt. It is the aim of the invention to produce small particles. Expansion of the particles will result in undesired growth of the volume of the particles.

The polymer is preferably chosen from the group consisting of polyolefines, polystyrenes, acrylonitrile-butadiene-styrene polymers, aromatic or aliphatic partially crystallized polyesters, polycarbonates, polyamides, polymethylmethacrylate, polyacetals, polyphenyleneoxides, polyphenylesulfides, polyetheretherketons, polyetherketonketons, polysulphones, polyarylates or a mixture thereof.

As indicated above, the polymer preferably comprises a thermoplastic polymer. Examples of suitable thermoplastic polymers include, but are not limited to, polyamides, polyacetals, polyesters (including aromatic polyesters and aliphatic polyesters), liquid crystalline polyesters, polyolefins, polycarbonates, acrylonitrile-butadiene-styrene polymers (ABS), poly(phenylene-oxide)s, polyphenylenesulfide)s, polymethylmethacrylate, polysulphones, polyarylates, polyetheretherketones (PEEK), polyetherketoneketones (PEKK), polystyrenes, and syndiotactic polystyrenes. Preferred thermoplastic polymers include polyamides and polyesters.

Polyamides may be homopolymers, copolymers, terpolymers, or higher order polymers. Blends of two or more polyamides may be used. Suitable polyamides can be condensation products of dicarboxylic acids or their derivatives and diamines and/or aminocarboxylic acids, and/or ring-opening polymerization products of lactams. Suitable dicarboxylic acids include, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, isophthalic acid and terephthalic acid. Suitable diamines include tetramethylenediamine, hexamethylenediamine, octarnethylenediamine, nonamethyienediamine, dodecamethylene- diamine, 2-methylpentamethylenediamine, 2-methyloctamethylenediamine, trimethylhexa- methylenediamine, bis(p-aminocyclohexyl)methane, m-xylylenediamine and p-xylylene- diamine. A suitable aminocarboxylic acid is 1 1-aminododecanoic acid. Suitable lactams include caprolactam and laurolactam.

Preferred aliphatic polyamides include polyamide 6, polyamide 6,6, polyamide 4,6, polyamide 6,9, polyamide 6,10, polyamide 6, 12, polyamide 10, 10, polyamide 1 1 and polyamide 12. Preferred semi-aromatic polyamides include poly(m-xylylene-adipamide) (polyamide MXD.6), poly(dodecamethyleneterephthalamide) (polyamide 12,Τ) _Ϊ poly(decamethyleneterephthalamide) (polyamide 10,T), poly(nonamethylene- terephthalamide) (polyamide 9,T), the polyamide of hexamethyleneterephthalamide and hexamethylene-adipamide (polyamide 6,T/6,6); the polyamide of hexamethyleneterephthalamide and 2-methylpentarnethyleneterephthalarnide (polyamide 6,T/D,T); the polyamide of hexamethylene-isophthalamide and hexamethylene-adipamide (polyamide 6,1/6,6); the polyamide of hexamethyleneterephthalamide, hexamethylene-isophthalamide, and hexamethylene-adipamide (polyamide 6.T/6, 1/6,6) and copolymers and mixtures of these polymers.

Examples of suitable aliphatic polyamides include polyamide 6/6 copolymer, polyamide 6,6/6,8 copolymer, polyamide 6,6/6, 10 copolymer, polyamide 6,6/6, 12 copolymer, polyamide 6,6/10 copolymer, polyamide 6,6/12 copolymer, polyamide 6/6,8 copolymer, polyamide 6/6, 10 copolymer, polyamide 6/6,12 copolymer, polyamide 6/10 copolymer, polyamide 6/12 copolymer, polyamide 6/6,6/6, 10 terpolymer, polyamide 6/6,6/6,9 terpolymer, polyamide 6/6,6/1 1 terpolymer, polyamide 6/6,6/12 terpolymer, polyamide 6/6, 10/11 terpolymer, polyamide 6/6,10/12 terpolymer and polyamide 6/6,6/PACM (bis-p- {aminocyclohexyl)methane)terpolymer.

The polymer preferably comprises polyethylene terephtalate, preferably cross- linked, the polyethylene terephtalate preferably being recycled polyethylene terephtalate, most preferably cross-linked recycled polyethylene terephtalate. Polyetylene terephtalate, in particular cross-linked polyethylene terephtalate has appeared to be a very suitable polymer to be used as proppant, i.e. having the desired properties as discussed above. In a very attractive embodiment the polyethylene terephtalate is recycled polyethylene terephtalate, preferably cross-linked.

In another attractive embodiment, the polymer comprises polycarbonate, PET,

PEI or PEEK.

Preferably the polymer melt comprises a filler. The filler can be used to reinforce the particles, and also to modulate the specific weight of the particles and reduce cost. The filler should be capable of reinforcing the polymer, while also reducing the potential for crush as exemplified below.

The filler is preferably chosen from the group of sand, carbon black, graphite, mica, silica, silicon carbide, alumina, quartz, nanotubes, coconut, walnut, natural fibre, glass fibre, glass beads, hollow glass spheres, glass powder, glass fibres, ceramics, grits, clays (e.g., kaolin), staurolite (including staurolite sand) and wollastonite or a combination thereof. The fillers may be in a variety of forms such as ground particles, flakes, needle-like particles and the like. The size and form of the particles should be selected such that they may easily be incorporated into the polymeric carrier and allow for the formation of particles having the desired size.

The fillers preferably have a Mohs hardness of at least about 3, or more preferably of at least about 5, or yet more preferably of at least about 6, or still more preferably of at least about 7.

The fillers may optionally be pre-treated with one or more compatibilising and/or coupling agents that facilitate adhesion to or other compatibility with the polymer.

Compatibilising and/or coupling agents may also be added to the filler and polymer mixture prior to or during melt blending to form the particles. The compatibilising and/or coupling agents may be used in about 0.01 to about 1 weight percent when they are added prior to or during melt blending. Examples of coupling agents suitable for use with sand or glass are silane coupling agents such as gamma-aminopropyltriethoxysilane (silane A-1100).

Preferably the particles according to the invention comprise about 20 to about

100 weight percent of polymer and about 80 to about 0 weight percent filler, wherein the weight percentages are based on the total weight of the particles. In a particular embodiment of the invention the polymer comprises 100% polymer. In another embodiment the particles according to the invention comprise about 20 to about 80 weight percent of polymer and about 80 to about 20 weight percent filler.

The melts for the particles according to the invention that comprise one or more fillers can be formed by melt blending the fillers with the polymers. Any melt blending technique known in the art may be used. For example the component materials may be mixed using a melt-mixer such as a single- or twin-screw extruder, blender, kneader, roller,

Banbury mixer, etc.

Preferably the polymer has a VICAT softening point, expressed as the so- called VICAT softening point, according to ISO standard 306, of between 50 - 200°C, more preferably between 70 - 160°C. A softening point in the said ranges is advantageous, as the particles remain rigid at higher temperatures, although some deformation may take place.

Another embodiment of the invention comprises the use of flow-aids during extrusion and/or solidification of the particles in order to facilitate the formation of small spherical and round particles. These flow-aids can be any known melt viscosity reducing agent such as wax, liquid or solid flame retardant, organic solvent, hydrocarbons in general etc. These flow-aids may optionally be removed from the particles once they are solidified in whole or in part in a subsequent process step by known extraction agents such as water, organic alcohols such as glycols or any appropriate extraction medium. The extraction can also take place during solidification.

Preferably the portions of the melt after passing through the die and being cut by the cutting device have a length that corresponds with 0.7 to 5 times, in particular to 3 times, more in particular to 1.3 times the diameter of the outlet opening of the die. More preferably, said length corresponds with the said diameter, in order to produce spherical particles.

In a very attractive embodiment the melt is passed through a die plate having a plurality of dies. In this way, many particles can be produced in parallel simultaneously. A suitable die plate is e.g. described in US7,776,244. Therein the die plate is used for the preparation of expandable polystyrene. However particles according to the present invention are preferably not expandable. Preferably the outlet openings of the dies of the plate are equal, enabling the preparation of particles of equal size.

The die preferably has an inlet having an inlet opening having a diameter being larger than that of the outlet opening. The outlet comprises a number of individual outlet channels having a channel diameter. The outlet channel preferably has an inlet diameter that is at least equal to the outlet diameter. The die further preferably comprises a die area located upstream of the channels having a diameter of at least the diameter of the outlet opening. The channel has preferably a total length of at least two times the diameter of the outlet diameter, preferably 6-4 times. These features are further in detail described in US7,776,224, herein incorporated by reference and it has been found that these features contribute to an equal flow distribution over the number of outlet channels and thereby to a uniform particle diameter. The pressure upstream of the die opening may be up to 250 bar, preferably between 60 and 200 bar, more preferably between 90 and 150 bar. As described above, the pressure may be of influence to the clogging of the outlet opening of the die. A higher pressure may give less clogging.

Attractively, the particles are recovered from the solidifying medium. Many state of the art methods can be used to recover the particles, e.g. by filtration, centrifugation or by treatment in a cyclone. In another embodiment the particles can be dried and sieved. Preferably, the particles are dried after recovery, i.e. by exposure to a stream of air or an inert gas. In case water was used as solidifying medium, the particles are preferably dried by air. In case an organic liquid was used as solidifying medium it can be contemplated to wash the recovered particles with water or any other liquid first. However the recovered particles can also be dried without being washed. In that case, an air stream can be used to dry the particles. It can also be advantageous to use an inert gas such as nitrogen for drying. By washing with water, the temperature of the particles may drop to an undesirably low temperature. The temperature is preferably kept as high as possible in order to provide for a cost effective drying process. Said drying can e.g. take place in a vertical falling bed, wherein the particles flow downward, and wherein the drying medium such as air or nitrogen is blown in counter current upward direction. The crystallisation and drying step can preferably be efficiently combined into one unit operation.

If desired, e.g. when PET is used the particles can attractively be cured by incubation of the particles at or above the crystallisation temperature T _pc thereof. Such a curing step, or crystallisation step, provides additional hardness to the particles. This curing or crystallisation can e.g. take place in a residence time crystalliser.

The crystallisation temperature is explained in figure 1 below. Although a separate curing step can be performed, the said curing can attractively take place during the solidifying process in the solidifying medium. In that case, the temperature of the solidifying medium must be chosen to be above the said T _pc of the particles. The curing step can also attractively be combined with the drying step. In that case, the temperature of the drying medium should be above the said T _pc of the particles. Or the curing step can be performed by use of the latent heat of the particles or by external heating or by a combination thereof.

The invention also relates to a particle, obtainable by the method according to the invention, having a diameter of 1.0 mm or less, preferably of 0.8 mm or less, more preferably of 0.7 or less. In another embodiment, the particles preferably have a particle size of between 0.3 - 0.9 mm, most preferably between 0.4 - 0.8 mm. Said particle sizes can be conveniently obtained by the choice of the diameter of the die outlet opening and of the length of the cut melt as indicated above.

Further, the invention relates to particles comprising at least a polymer, said particles having a material density of 1.0 to 3.5 g/cm ³ , and a particle size of at most 3.0 mm. Other preferred particle sizes and ranges thereof are described above. Such particles could not be made before the present invention was made. The term 'density' herein is meant to be the material density unless otherwise specified. The term 'specific gravity' as used herein is also referring to material density.

Further, the invention relates to particles comprising at least a polymer, said particles having a material density of 1.0 - 3.5 g/cm ³ , preferably of 1.0 - 2.5 g/cm ³ , more preferably of 1.0 - 1.5 g/cm ³ . In case the particles according to the invention should stay in aqueous suspension, the preferably have a density between 1.05 and 1.35 g/cm ³ and most preferably between 1.05 and 1.10 g/cm ³ . This is advantageous, as the particles have less tendency to settle, and can also be pumped by mechanical pumps without the risk of clogging the pump.

Preferably, the particles obtainable by the method according to the invention have a particle size of at most 3.0 mm, preferably at most 2.5 mm, more preferably at most 2.0 mm, even more preferably at most 1.3 mm, still more preferably at most 1.0 mm, most preferably at most 0.8 mm. Said particles preferably have a particle size of at least 0.3 mm, preferably at least 0.5 mm and most preferably at least 0.6 mm These sizes can be conveniently be obtained, e.g. by choosing a diameter of the die outlet opening to be equal to or somewhat smaller than the envisaged particle size, and to synchronize the cutting device with the flow rate of the melt passing the die outlet opening accordingly, preferably to allow a length of the cut melt to be 0.7 to 1.3 times the said diameter. By solidification, the cut melt solidifies into a spherical particle of optimal roundness and sphericity.

In a further preferred embodiment the particles according to the invention has a roundness, as well as a sphericity of 0.7 - 1 , preferably of 0.8 - 1 according API 19C. Such roundness and also the sphericity can be obtained by underwater granulation techniques according to the present invention.

The particle size can also be expressed as pellet weight, i.e. the weight of exactly 100 particles. The particles of the invention preferably have a pellet weight of 1.5 g or less.

The invention will now be further explained by the following figures and examples, wherein:

Figure 1 is a graph, showing a dsc curve according to ISO 1 1357-1 1997, Figure 2A-H show pellets of several samples as prepared according to the invention,

Figure 3 shows a reference table taken from the standard API 19C for the determination of particle roundness and sphericity. Referring to figure 1 , which is taken from ISO 11357-1 1997, to which is expressly referred to herein, polymers may display specific behaviours called "glass transition", "cristallisation" and "melting" as shown in the figure by 'g', 'c' and 'm', respectively. These transitions are characterised by the energy required for these transitions and thereby become measurable as described in IS01 1357. Of relevance is T _pm , the peak indicating the melting point of the polymer, although polymers have a melting range starting at T _im and ending at T _tm . T _pm , T _im , and T _tm , can easily be determined following the instructions of the IS011357 protocol.

In figures 2A-H pellets of samples RPI061 , RPI 190, RPI081 , RPI186, RPI 11 1 ,

RPI 196, RPI 141 and RPI238 are shown respectively. For all samples, except for PRI 141 and PRI238 (figures 2G and H, respectively), 10 Particles are encircled that were used to assess roundness and sphericity according to standard API 19C, see also the below examples. Each figure 2A-I also shows a reference table that is magnified in figure 3.

Figure 3 shows a reference table, used in standard API19C to determine the roundness and sphericity.

Examples Example 1

Preparation of pellets

Starting materials

Polymers

High impact polystyrene ( HIPS): polystyrene impact 6540 obtained from Total Petrochemicals 64170 Lacq, France.

Polycarbonate (PC18) : Xantar 18 UR , Mitsubishi Engineering- Plastics Corporation 40549 Dusseldorf, Germany.

Polycarbonate (PC25) : Xantar 25 SR FD , Mitsubishi Engineering- Plastics Corporation 40549 Dusseldorf, Germany.

Polyethyleneterephthalate (PET): polyclear 1101 , Invista B.Bigles CH-6301

Zug, Switzerland.

Recycled polyethylenetherephthalate (rPET): CorePET FR80 grun, Pet Recycling, Arnhem, Netherlands. Ethylene glycol

Mono Ethylen Glycol (MEG) Art. Nr. 93.50.2682, Wittig Umweltchemie GmbH, Grafschaft bei Bad Neuenahr-Ahrw, Germany One skilled in the art will notice that sample RPI1 11 was produced using a polycarbonate with a low melt viscosity ( MVR 23) and that sample RPI196 was produced with a polycarbonate with a high melt viscosity (MVR 5). This further emphasizes the effectiveness of the present invention.

A polymer melt of starting materials according the formulations as given in table 1 was added via standard loss-in-weight feeders to the throat of a 6 barrel long extruder (Berstorff ZE60, Germany) a twin screw length of 2400 mm equipped with a standard vacuum set-up. At the outlet of the extruder, a screen pack changer with 630/315 sieve (K-SWE-121/RS, BKG, Germany) was installed to free the polymer melt from any solid particles followed by a 70/70 high pressure melt pump (BKG, Germany) to generate the required die plate pressure, as given in table 1. The polymer melt was passed through a multiple die-plate with a die-opening arrangement according to figure 1 B of US7,776,244, the outlet opening having a diameter of 0.5 and 0.75 mm, respectively, as shown in the table above and cut to size by a cutter hub on a pelletizer (type AH2000, BKG, Germany). The solidifying medium was water, ethylene glycol, or a 90: 10 v/v% mixture ethylene glycol:water, having a temperature as indicated in table 1. The process parameters for each formulation can be found in table 1.

Samples of RPI186 and RPI196 were produced by ethylene glycol as solidifying medium at a temperature indicated of 123°C and 121 °C respectively. This solidifying medium has a lower heat conductivity than water (water: 0.61 W/m/°C whereas the conductivity of ethylene glycol is 0.26 W/m/°C). Therefore the relatively cold solidifying medium flowing along the die-plate is causing a reduced temperature loss of the melt at the die outlet openings of the die plate. The skilled person is capable to adapt the equipment if necessary, i.e. by adapting the chamber wherein the solidifying medium is accommodated. This enables the production of smaller pellets using common pelletizing equipment. From the table, it can be seen that the pellet weight, being an indication for particle size, is 17% less for HIPS when produced with glycol as solidifying medium (RPI 190) as compared to Table 1 : Starting materials and production parameters

Solidified in water (Comparative examples).

Solidified in a solidifying medium of ethylene glycol.

Solidified in a solidifying medium of 90 v/v% ethylene glycol - 10 v/v% water water (RPI061), For PC the said reduction is 32% (RPI 196 andRPI 1 11) and for PET the reduction is 56% (RPI 186 and RPI081). A comparison of samples RPI 141 and RPI238 show that the use of recycled PET (RPI238) results in particles of better roundness and sphericity as compared those, prepared from virgin PET (RPI 141). Importantly, by using a solidifying medium of the invention, smaller particles can be obtained, while using larger die holes. This is a surprising effect, resulting in even less clogging of the die holes. E.g. with sample RP1141 particles of 0.72mm are obtained with a die opening of 0.5 mm, whereas with samples RPI238, particles of 0.67 mm are obtained using a die opening of 0.65 mm. If the same die openings are used (RPI081 vs RP1186, 0.5 mm), smaller particles of better sphericity are obtained.

Determination of the theoretical particle size

From the extruder throughput, the density of the material, the number of die outlet openings, the cutting frequency of the cutter hub (to be determined by the number of blades on the cutter hub and the speed of the cutter hub) one skilled in the art can easily calculate the expected particle size. In the comparative example of RPI061 , the throughput was 300 kg/h, the number of die outlet openings was 450 and the melt passing each die outlet opening was cut by 20 knives, rotating at 3636 rpm (rotations per minute). The density of the material was 1060 kg/m ³ . The volume of the pellets can be calculated by the formula throughput per second per die opening divided by the density of the polymer melt, divided by the cutting frequency of the cutter hub per second. In this example, the particle volume is therefore 300/3600/450/1060/20/(3636/60)* 10 ⁹ =0.144 mm ³ . This makes an expected particle size of about 0.65 mm diameter, as the shape is assumed to be spherical (volume = Tr/6*d ³ , i.e. 3.14/6*0.65 ³ =0.144 mm ³ ).

Determination of the actual particle size.

The above expected particle size was compared with the actual particle size. The latter was obtained by weighing 100 particles (herein also 'pellets') on a Sartorius A200S micro-scale from Sartorius GmbH, Gottingen Germany. From the said weight, one skilled in the art can now calculate the actual average pellet volume and size through the known density from the formulation. An actual pellet weight above the theoretical particle size is an indication of die-holes being clogged, as the same volume of material is passing through a lower number of open die openings, resulting in a proportionally higher volume per open die opening. In this example the weight of 100 pellets is 0.0234 gram, or a volume per pellet of 0.0234*10/1.06=0.22 mm ³ so the number of open holes is 450 *0.144/0.22=294. The acceptable limit for weight of 100 pellets can be calculated upfront from the maximum allowable pellet size that is specified. Production was halted approaching this limit to free the frozen die-holes.

Separation of the pellets

The water/pellet slurry was separated in a standard centrifugal dryer. The remaining hot solidifying medium that passes through the centrifugal dryer 0.3 mm sieve was treated in a BKG Optigon filter drum (BKG, Germany) to remove the undersized pellets and to recuperate the solidifying medium that was recycled back to the pelletizer. The undersize pellets were either wasted, or recycled into the extruder. The hot and dried pellets coming of the centrifugal dryer were subsequently sieved to remove oversized material before packaging. The oversized material was wasted, or recycled into the extruder.

Determination of pellet roundness and sphericity

10 individual pellets of different samples RPI061 , RPI 190, RPI081 , RPI 186, RPI 1 11 and RPI196 as encircled in figures 2A-F, respectively, were visually examined for roundness and sphericity and valued in accordance with the standard API 19C, shown in figures 2A-F. The average sphericity of comparative example RPI081 was e.g. determined to be 0.5, whereas that of example RPI 186 was determined to be 0.9 and the average roundness to be 0.9, based on the data as presented in figure 2.