GB2226318A - Non-attritive method for making polyester particles - Google Patents

Abstract

A non-attritive method for making fine particles of polyesters comprises heating a mixture of the polyester in a moderate solvent for the polyester to a temperature above the melting point of the polyester when in the moderate solvent and then cooling whereupon the particles recrystallise from the mixture. The particles comprise crystalline zones and amorphous zones containing imbibed solvent and may be used in coating, in coating compositions and in making porous particles.

Description

NON-ATTRITIVE METHOD FOR MAKING POLYESTER PARTICLES, PARTICLES OBTAINABLE BY THE METHOD AND THEIR USE This invention relates to a non-attritive method for making distinct fine particles of partially crystalline polyester polymers (including copolymers especially with isophthalic acid and rubbery polyesters, for example those incorporating chain segments composed of polyethers having low glass transition temperatures), to novel particles obtainable by the method and to the use of the particles, especially in coating processes and compositions.

Partially crystalline polyesters such as polyethylene terephthalate (PET) and polybutylene terephalthate (PBT) are well known thermoplastics.

They usually have a density of about 1.4 and 1.3g/cm3 respectively. A fuller description of the various types of polyester is given in Volume 18 of the third edition of Kirk-Othmer's "Encyclopaedia of Chemical Technology" published by John Wiley & Sons of New York in 1982 (see pages 549 to 574) the contents of which are herein incorporated by reference.

The commercial manufacture of polyesters produces pellets which usually have a maximum dimension of at least about 2mm. For some purposes such as powder coating or addition to coating compositions, the pellets need to be converted into distinct fine particles, that is to say particles having a maximum dimension of below 500cm. Hitherto this has been done by attritive methods such as grinding or milling. However attritive methods produce particles of an unpredictable shape which are therefore inconvenient to use in coating processes and compositions and the particles are also totally solid which is sometimes a disadvantage.

An object of this invention is to provide a non-attritive method for making distinct fine particles of partially crystalline polyester polymers. Another object is to provide novel fine particles of a less unpredictable shape which are less than totally solid and which are amongst other things especially suitable for use in coating processes and compositions. A further object is to provide a process for coating surfaces in which such novel particles are used.

Accordingly this invention provides a non-attritive method for making distinct particles of partially crystalline polyester wherein the method comprises a) heating a mixture comprising a moderate solvent for the polyester and at least 5 (preferably 10 to 60) wt% of the polyester (the percentage being based on the combined weights of the moderate solvent and the polyester) to a temperature above the crystalline melting point (Tm) of the polyester when in the mixture and preferably to a temperature in the range Tm + 10 C to Tm + 40 C and b) cooling the heated mixture under conditions such that solid/liquid phase separation occurs whereby distinct particles of polyester are produced.

Usually the particles have a maximum dimension of from 0.1 to lOOpm.

It is essential to use a moderate solvent for if a good solvent is used with the concentrations of polyester employed in the performance of this invention, then the polyester either fails to come out of solution or comes out either as a gel, agglomerate or as particles of unpredictable shape.

A "moderate solvent" is a solvent which depresses the crystalline melting point Tm of the polyester by not more than 60C when the polyester constitutes 20 wt % of a mixture of dry moderate solvent and dried polyester. A "dry solvent" contains less than 0.005 wt % of water and a "dried polyester" is a polyester which has been heated in a vacuum oven for 16h at 90"C. Crystalline melting point is determined by differential scanning calorimetry performed in turn on the polyester alone and on the above mixture. The mixture must be contained in a sealed capsule to prevent loss of the moderate solvent during heating.

More particularly, lOmg of polyester alone and lOmg of mixture are each in turn subjected to heating cycles performed under nitrogen in the calorimeter.

Each heating/cooling cycle comprises heating the sample under test (which may be polyester alone or a mixture) at a rate of 200C/min to cause crystalline melting which occurs at a temperature Tm, subsequently holding the sample for two minutes at a holding temperature Th which is above Tm, then cooling at a rate of 200C/min to cause recrystallisation which occurs at a temperature Tc and finally continuing cooling to a temperature of at least 100C below Tc. Tm and Tc are detected respectively as an endothermic trough and an exothermic peak in the graph of heat absorbed or evolved versus temperature. Each cycle is repeated to discover whether a consistent value for Tc can be obtained.If consistency is not obtained, another pair of heating cycles are performed using a slightly higher Th Further pairs of cycles with gradually increasing Th are performed until consistent values for Tc are achieved whereupon the pair of cycles which gave consistent values is repeated and the value for Tm obtained is defined to be the crystalline melting pointof the sample under tst.

A good solvent for PET such as orthochlorophenol depresses the melting point by well in excess of 60"C whereas moderate solvents such as dimethyl phthalate or diethyl phthalate depress the melting point by only 42"C and 26"C respectively. Dimethyl phthalate is a particularly preferred moderate solvent for use in the method of this invention. Other moderate solvents include diethyl phthalate, dibutyl phthalate or mixtures thereof and mixtures of the dimethyl esters of adipic, glutaric and succinic acids which have a boiling point in the range of 196 to 225"C at 1 bar. Mixtures of dibutyl phthalate and dioctyl phthalate are especially useful with PBT.

It is also important when performing the method of this invention that the mixture be heated to above the crystalline melting point of the polyester when in the mixture (Tm) for otherwise there will be obtained particles of unpredictable shape comprising some undissolved polyester and some re-crystallised polyester agglomerated into the undissolved polyester.Preferably the mixture should be heated at least to its clearing temperature, "Tc1,, The "clearing temperature" (TCl) of any chosen mixture comprising dry moderate solvent and dried polyester is the temperature at which the appearance of the mixture becomes clear to the unaided eye. TCl is determined by heating 2g of a chosen mixture until the polyester dissolves and the initially turbid solution obtained turns clear for a first time, then cooling the mixture to room temperature an finally re-heating the mixture until it turns clear for the second time. The temperature at which it turns clear for the second time is defined to be Tcl for that mixture. Heating to Tcl and above (preferably to 10 to 300C above TCl) leads to the formation of more uniform particle sizes. Uniformity of particle size is also enhanced by heating the mixture to a holding temperature Th which is above to for the polyamide when in the mixture (and preferably 10 to 40"C above) and holding the mixture at temperature Th for from 1 to 30 minutes although holding for 1 to 5 minutes is usually sufficient.

It has been found that some large pellets of polyesters available commercially can be inconveniently slow to dissolve. Where time saving is important, this problem can be alleviated by using a pre-heating and pre-cooling cycle as follows. The mixture is first pre-heated to a temperature about the melting point of the pure polyester, for example to 2500C. Such pre-heating causes a rapid dissolution of the polyester. The mixture is then pre-cooled to at least a temperature (for example 30 to 800C below the melting point of the pure polymer or lower) at which polyester re-solidifies from the mixture. Such re-solidification produces polyester in a form which dissolves quickly (usually within 2 minutes) on heating to TCl or above so producing a mixture consisting of a solution of polyester in moderate solvent which appears clear to the unaided eye.

In performing the process of this invention it is essential to employ conditions which cause solid/liquid phase separation to occur from the mixture when it is in its solution state for otherwise an agglomerated mass will be obtained. To achieve solid/liquid phase separation, it is necessary to use a moderate solvent, to use a mixture containing at least 5 wt% of polyester and to avoid shock cooling which usually means cooling at a rate of no faster than 3000C/min. The preferred cooling rates are from 10 C/min to 50 C/min. It is also preferred to stir the mixture during cooling.

Stirring promotes greater uniformity of particle size.

This invention also provides distinct fine particles of partially crystalline polyester wherein the particles comprise zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent. Generally the particles will comprise from 10 to 90 (preferably from 40 to 75) wt% of imbibed moderate solvent.

Usually large proportions of imbibed solvent and amorphous polymer are favoured by performing the method of this invention using high cooling rates (say 100 to 3000C/min). The particles are obtainable b the method of this invention as a slurry or paste in moderate solvent. Alternatively the particles can be obtained in a free flowable condition by removal of the solvent external of the particles by for example rinsing with a liquid miscible with the moderate solvent but which does not dissolve the polyester. Methanol, ethanol and aliphatic ketones are examples of such liquids.

The size of the particles can be adjusted by by varying the cooling rates employed in the method or by incorporating nucleating agents into the polyester. In general higher cooling rates and also nucleating agents favour smaller particles sizes.

Typical nucleating agents for polyesters include talc, sodium benzoate or the ionomeric copolymers of ethylene with minor amounts of carboxylate comonomers, for example those known as "Surlyn" A available from EI Dupont de Nemours Inc. The uniformity of particle size increases as the temperature to which the mixture is heated is increased up to Tm + 400C. No advantage is generally gained by using temperatures above Tm + 40"C and of course temperatures high enough to cause thermal degradation of the polyester should be avoided. Use of nucleating agents also improves uniformity of particle size.

The presence of a nucleating agent may cause a residual turbidity to persist at Tcl. However the skilled eye is able to distinguish between turbidity caused by the polyester and that caused by a nucleating agent and so determination of TCl and clearing are not unduly hindered.

The shape of the particles obtained varies with the concentration of polyester in the mixture. Lower concentrations favour flaky or rod-like particles whilst higher concentrations favour approximately spherical or oblate particles. Usually concentrations of at least 20 wt % should be used to obtain spherical or oblate particles with the best results being obtained with concentrations above 30 wt t.

It has been discovered that the presence of imbibed moderate solvent in the particles of this invention enhances their ability to flow and coalesce when heated so as to provide a continuous layer of polyester. Accordingly this invention also provides a process for coating a surface with polyester which process comprises a) applying a covering of particles (usually evenly) across the surface and b) heating the particles to cause them to coalesce wherein the particles employed comprise zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent.Preferably the particles are applied to the surface as a dispersion of usually 5 to 35 (especially 10 to 25) wt % of particles in either moderate solvent or a liquid which is not a solvent for the polyester, for example acetone or methyl ethyl ketone. Preferably the particles are heated to a temperature of from 200 to 2400C for a period of from 2 to 30 minutes. The surfaces may be metal, for example aluminium or stainless steel or non-metallic, for example glass. In particular the particles may be used to coat sheets, shaped articles such as cans for example in conventional powder coating operations. The especially the flaky or rod-like particles may be used to impregnate continuous rovings of a wide variety of fibres including glass and carbon fibres.If the impregnated fibres are heated to soften or melt the particles, they may be compressed to produce a composite which on cooling comprises fibre consolidated in polyester.

It also has been found that the particles according to this invention disperse well in coating compositions (for example paints and varnishes) based on organic solvents. Accordingly this invention provides a coating composition based on an organic solvent and comprising a binder of a type used in coating compositions and from 0.5 to 50 wt% of particles of polyester wherein the particles of polyester comprise zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent. Typical binders for coating compositions are described in the third edition of the book "Introduction to Paint Chemistry and Principles of Paint Technology" by G P A Turner and published by Chapman and Hall of London in 1988, the contents of which are herein incorporated by reference.The coating composition may also comprise pigments and extenders and other conventional ingredients described in the above book.

Particles made according to the method of this invention may be converted to porous particles by extraction of the imbibed moderate solvent.

Accordingly this invention provides a process for making porous particles of polyester wherein particles of polyester comprising zones of crystalline polyester and which particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent are immersed in a liquid extractant which is miscible with the moderate solvent but which is not a solvent for the crystalline polyester, until moderate solvent has been extracted into the extractant and then the extractant is washed from the particles. Particles having small pores can be obtained by using an extractant such as acetone, methyl ethyl ketone or petroleum ether which only extracts the moderate solvent.Porous particles may be used to increase the opacity of coatings obtained by applying the coating compositions or they may be applied to surfaces by conventional powder coating techniques or by electrostatic spraying.

The invention is further illustrated by the following Examples. In all cases the moderate solvent used had been dried to reduce its water content to not more than 0.005 wt % and the polyester had been dried by heating it in a vacuum (lm bar) oven for 16h at 900C.

EXAMPLES 1 TO 17 Making PET Particles: 10mug samples of a polyethylene terephalthate (PET) were each mixed with one of various moderate solvents as specified in Table 1 in amounts also specified in Table 1. The PET had an intrinsic viscosity of 0.64 to 0.66 in orthodichlorophenol at 25"C. Each mixture was heated at a rate of 200C/min to a holding temperature Th (see Table 1) which was above the crystalline melting point of the PET when in the mixture (T ) and also above the clearing m temperature TCl of the mixture so as to dissolve the PET so that the mixture existed as a solution. Tm for each mixture are also given in Table 1.The mixture was held at Th for 2 minutes and then cooled to room temperature again at a rate of 20cC/min. A solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical particles comprising crystalline zones and amorphous zones containing imbibed solvent. A typical dimension of the particles (as determined by scanning electron microscopy) is shown in Table 1.

The particles comprised about 50 wt% of imbibed solvent and were obtained as a paste consisting of particles and moderate solvent.

The pastes obtained could be converted to dry free flowing particles by rinsing with acetone.

Rinsing amounted to placing the paste on filter paper in a funnel and pouring acetone through for 30 seconds at room temperature.

TABLE 1

Eg Solvent Amount Tm Th Typical II JPET I I Particle in mix- C C Dimension ture, #m wt % I -I II I 1 dimethyl phthalate 10 192 230 2 1 21 " " 1 20 120112301 20 | 3| " " |30 |193 |230| 25 | | 4| " " | 40 121012301 25 | 5| " " | 50 121712301 25 1 | 6|diethyl phthalate 1 10 121412401 2 1 1 71 " " 20 12171 1 3 | | 8| " " | 30 |213| | 20 | | 9| " " | 40 12251 1 20 1101 " " | 50 12261 1 30 1 11 dibutyl phthalate 20 236 25 12 " " 30 232 15 1131 " " | I 40 12341 1 20 1 1141 " " 1 50 12381 1 20 1 115lmixture of 90% 1 10 12151 1 2 1 | diethyl phthalate I I I I I I mixed with 10 wt% of I I I I 1 Itetraisobutylene I I I I I Isold by BP Chemicals| I I I I Ltd as TIB 90 | | | | 16 " " 20 219 10 17 " " 30 217 15 EXAMPLES 18 TO 23 Making PBT Particles: 10g samples of a polybutylene terephthalate (PBT) pellets available as "Celanex" 2002-2 from the Celanese Company were ground and then each mixed with one of various moderate solvents as specified in Table 1 in amounts also specified in Table 1. Each mixture was heated to a temperature above the crystalline melting point of the PBT when in the mixture and to above the clearing temperature Tcl for the mixture so as to dissolve the PBT whereupon the mixture existed as a solution. The clearing temperature (TCl) for each mixture is given in Table 1. The mixture was held at this temperature for 2 minutes and then allowed to cool to room temperature.

A solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones TABLE 2

I I I I | I Example Solvent Amount TCl C Typical PBT Particle I I lin Mix- I IDimensionl I I Iture, I I m I I Iwt % I I I 1 1 I I | 18 |dimethyl phthalate | 20 | 211 | 18 1 19 | " " 1 40 1 230 1 12 1 20 Idiethyl phthalate 1 10 1 | 216 | 22 1 21 1 1 " | 20 1 230 1 13 22 dibutyl phthalate 20 231 10 23 " " 40 241 12 and amorphous zones containing imbibed solvent. A typical dimension of the particles is shown in Table 1.The particles comprised about 50 wt % of imbibed solvent and were obtained as a paste consisting of particles and moderate solvent.

The pastes obtained can be converted to dry free flowing particles by rinsing with acetone using the procedure employed in Examples 1 to 17.

EXAMPLES 24 TO 26 Making PBT Elastomer Particles: Quantities of PBT elastomer containing segments of polytetramethylene oxide available as "Hytrel" 7246 from EI Dupont de Nemours Inc were mixed with one of various moderate solvents as specified in Table 3 in amounts sufficient to provide a mixture containing 20 wt% "Hytrel". 10g of each mixture was heated to a temperature of 10"C above its clearing temperature to dissolve the "Hytrel" so that the mixture existed as a solution. The clearing temperature for each mixture is given in Table 3.

The mixture was held at this temperature for 2 minutes and then allowed to cool to room temperature again. A solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones TABLE 3

I I I I Example | Solvent |TCl C |Max.

I | I Particle I I I I Dimension| m 27 *dimethyl ester 90 20 I I mixture I I 1 28 Ipropylene carbonate 1 90 1 15 1 29 mixture of 90 wt% 1 90 1 20 1 diethyl phthalate | | | i Iwith 10 wt% T1B90 I I las used in Example I I 15 * Mixture of dimethyl esters of adipic, glutaric and succinic acids having a boiling point in the range 196 to 225 C.

and amorphous zones containing imbibed solvent. An estimate of the number average maximum dimension of the particles as determined by optical microscope is shown in Table 1. The particles comprised about 50 wt % of imbibed solvent and were obtained as a paste consisting of particles and moderate solvent.

The pastes obtained were suitable for conversion to dry free flowable particles by rinsing with acetone as in Examples 1 to 17.

EXAMPLES 30 TO 32 Use of Nucleated PET to obtain Finer Particles: 10g samples of a PET containing a proprietry nucleating agent and available as "Melinar" TIOOX from Imperial Chemical Industries PLC were each mixed with a moderate solvent which was dimethyl phthalate in amounts specified in Table 1. Each mixture was heated to a temperature which was above the crystalline melting point (Tm) for the PET when in the mixture and was 100C above the clearing temperature of the PET/solvent mixture so as to dissolve the PET whereupon the mixture existed as a solution. The clearing temperature for each mixture is given in Table 1. The mixture was held at this temperature for 2 minutes and then allowed to cool to room temperature.A solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones TABLE 4

I I I I |Example|Amount |TCl C | I lNucl. I of I IPET IPET/ I | | in Mix= |solvent| Iture, I mix I I Iwt % I I I I I I 30 10 190 31 15 190 32 20 195 and amorphous zones containing imbibed solvent.The particles comprised about 50 wt % of imbibed solvent and were obtained as a paste consisting of particles and moderate solvent. In all cases the particles had a typical particle dimension of below Claim as determined by optical microscope.

EXAMPLES 33 TO 36 Comparison of Rinsing and Extraction: These Examples illustrate how rinsing particles of PET or PBT alone (Examples 33 and 34 and rinsing followed by extraction to remove the moderate solvent Examples 35 and 36) affect the ability of the PET or PBT particles to flow and coalesce.

A paste of PET (Examples 33 and 35) made according to Example 7 and a paste of PBT made according to Example 21 were each rinsed twice in 40/60 petroleum ether to remove the moderate solvent which was external of the particles so as to produce particles which after drying were free flowable.

Rinsing amounted to placing the paste on filter paper in a funnel and pouring 40/60 petroleum ether through the particles for 30 seconds allowing the ether to drain from the particles and then again pouring ether through the particles for 30 seconds.

The free flowing particles of PET and PBT were each divided into two equal portions and one portion of each was subjected to an extraction treatment to remove imbibed solvent. Extraction was performed by ref fluxing the particles in 40/60 petroleum ether for 3h.

The rinsed only portions (Examples 33 and 35) and the rinsed and extracted portions (Examples 34 and 36) were each mixed with dry "Isopar" L to produce pasty dispersions suitable for use in coating. "Isopar L" is a mixture of isoparaffins having a boiling point of from 190 to 210 C and available from Esso Chemicals Ltd. Each dispersion was applied to flat smooth sheets of aluminium and glass using a grooved doctor bar comprising parallel grooves 300m deep space 4.5mm apart so that the applied dispersion was in the form of a plurality of ridges extending across the sheet. The sheets were placed in an oven and heated to 260"C for 3 minutes and then allowed to cool to room temperature.

It was found that Examples 33 and 35 (rinsed only) produced continuous smooth glossy coatings of PET or PBT whereas the PET or PBT of Examples 34 and 36 remained substantially in the ridges and did not flow to produce a continuous covering. Clearly the presence of imbibed moderate solvent in the particles enhances their usefulness as melt-coating materials.

EXAMPLES 37, 38 AND COMPARATIVE EXAMPLE A Quantitative Assessment of Flowability of PBT Particles: The flowability of rinsed only PBT particles made according to Example 35, of rinsed and extracted PBT particles made according to Example 36 and conventional ground PBT obtainable as "Valox" from G E Plastics Inc was assessed as follows: A quantity of each type of particle was compressed into a self-supporting cylinder by ramming the particles into an open-ended glass tube of an internal diameter which give the cylinder widths shown in Table 5. Equal weights of each particles were used to create a cylinder 2.8mm high. The cylinder of particles was pushed out of the glass tube and heated to a temperature Tx (as shown in Table 5) at which the particles were seen to melt.

They were held at that temperature for 3 minutes.

The increase in width of the cylinder was measured and is also shown in Table 5 from which it will be seen that the rinsed only particles flowed considerably better than the others whilst the rinsed and extracted particles flowed marginally less well than the ground particles: TABLE 5

I I I I I I |Eg|Particle |Initial |Final |% Increase| Tx | I 1 Type CylinderlCylinderl I I width mm width mm C 37 Rinsed only 3.2 4.8 50 220 38 Rinsed & BR< Extracted 3.0 3.7 23 245 A Ground 2.7 3.0 11 245 EXAMPLE 39 Paint Formulation:: A copolymer consisting of 82 wt% polyethylene terephthalate and 18 wt% isophthalic acid and having an intrinsic viscosity in orthochlorophenol at 250C of 0.63 to 0.65 was mixed with the dimethyl ester mixture used in Example 27 to produce a mixture containing 20 wt% of the copolymer. The copolymer was then heated to 180 C (which is above Tm for the mixture) and then allowed to cool to room temperature. During cooling, solid/liquid phase separation and re-crystallisation occurred resulting in the formation of a paste comprising approximately spherical partially plasticised particles comprising crystalline zones and amorphous zones containing imbibed solvent.

The paste was then dispersed in further dimethyl ester solvent and diluted with methyl ethyl ketone dispersion containing 10 wt% of particles. The dispersion was painted onto the surface of a smooth flat aluminium and then heated to 2350C for two minutes, then to 2200C for two minutes and then to 2000C for two minutes. A smooth glossy continuous coating was formed which adhered well to the aluminium sheet.

The invention has been performed with blends of polyester and other polymers. In particular polycarbonates and polyvinylidene flourides have been shown to increase the toughness of coatings obtained according to this invention.

Claims (19)

1. A non-attritive method for making distinct particles of partially crystalline polyester wherein the method comprises a) heating a mixture comprising a moderate solvent for the polyester and at least 5 wt % of the polyester (the percentage being based on the combined weights of the moderate solvent and the polyester) to a temperature above the melting point (Tm) of the polyester when in the mixture and b) cooling the heated mixture under conditions such that solid/liquid phase separation occurs whereby distinct particles of polyester are produced.

2. A method according to Claim 1 wherein the mixture comprises from 10 to 60 wt % of the polyester.

3. A method according to Claim 1 or Claim 2 wherein the moderate solvent is chosen from dimethyl, diethyl or dibutyl phthalate or mixtures thereof.

4. A method according to Claim 1 or Claim 2 wherein the moderate solvent is a mixture of dimethyl esters of adipic, glutaric and succunic acids which mixture has a boiling point of from 196 to 225 CC at 1 bar.

5. A method according to any one of Claims 1 to 4 wherein the mixture is cooled at a rate of less than 300"C/min.

6. A method according to any one of Claims 1 to 5 wherein the mixture is cooled at a rate of 10 to 50 C/min.

7. A method according to any one of Claims 1 to 5 wherein the mixture is cooled at a rate of 100 to 3000C/min.

8. A method according to any one of the preceding Claims wherein the mixture is heated to a temperature of from Tm + 10 to Tm + 40 C

9. a method according to any one of Claims 1 to 7 wherein the mixture is heated to a temperature above its clearing temperature, Tcl.

10. A method according to Claim 9 wherein the mixture is head to a temperature of from TCl + 10 to TCl + 300C.

11. A method according to any one of the preceding Claims wherein the mixture is heated to a holding temperature (Th) which is above Tm and the mixture is held at temperature Th for from 1 to 30 minutes before cooling occurs.

12. A method according to any one of the preceding Claims wherein the particles obtained are converted to a free flowing condition by removal of the moderate solvent external of the particles.

13. Distinct fine particles of partially crystalline polyester wherein the particles comprise zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyamide containing imbibed moderate solvent.

14. Particles according to Claim 13 wherein the particles comprises from 40 to 75 wt % of imbibed solvent.

15. Particles according to Claim 13 or Claim 14 wherein the particles have a number average maximum diameter of from 0.1 to 100cm.

16. A process for coating a surface, which process comprises a) applying a covering of particles across the surface and b) heating the particles to cause them to coalesce wherein the particles comprise zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent.

17. A process according to Claim 16 wherein the particles are applied to the surface as a dispersion comprising from 5 to 35 wt % of particles in liquid.

18. A coating composition based on an organic solvent and comprising a binder of a type used in coating compositions and wherein the particles of polyester comprise zones of crystalline polyester and are the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent.

19. A process for making porous particles of polyester wherein particles of polyester comprising zones of crystalline polyester and the particles are partially plasticised in that they also comprise zones of amorphous polyester containing imbibed moderate solvent, are immersed in a liquid extractant which is miscible with the moderate solvent but which is not a solvent for the crystalline polyester until moderate solvent has been extracted into the extract ant and then the extractant washed from the particles.