GB2226319A - Non-attritive method for making polyamide particles - Google Patents

Abstract

A non-attritive method for making fine particles of polyamides comprises heating a mixture of the polyamide in a moderate solvent for the polyamide to a temperature above the melting point of the polyamide when in the moderate solvent and then cooling whereupon the particles re-crystallise 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 POLYAMIDE 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 polyamide polymers (including copolymers and polyamides modified by the presence of additives for example rubbers or polyolefins to which carboxylic moieties may have been grafted), to novel particles obtainable by the method and to the use of the particles, especially in coating processes and compositions.

Partially crystalline polyamides are well known thermoplastics more commonly called nylons. They usually have a crystallinity of 30 to 50%. A fuller description of the various types of nylon 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 406 to 425) the contents of which are herein incorporated by reference. More recently nylon 4, 6 and so called partially crystalline aromatic nylons have become available. Aromatic nylons are polyamides comprising condensates of aromatic diamines such as 1,3-di(aminomethyl) benzene.

The commercial manufacture of polyamides 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 500;m. 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 polyamide polymers. Another object is to provide novel fine particles of a less unpredictable shape which are partially plasticised 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 polyamide wherein the method comprises a) heating a mixture comprising a moderate solvent for the polyamide and at least 5 (preferably 10 to 60) wt% of the polyamide (the percentage being based on the combined weights of the moderate solvent and the polyamide) to a temperature above the crystalline melting point (Tm) of the polyamide when in mixture and preferably to a temperature in the range Tm + 100C to Tm + 400C and b) cooling the heated mixture under conditions such that solid/liquid phase separation occurs whereby distinct particles of polyamide are produced.

Usually the particles have a number average maximum dimension of from 0.1 to 100 > m.

It is essential to use a moderate solvent for if a good solvent is used with the concentrations of polyamide employed in the performance of this invention, then the polyamide 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 polyamide by not more than 80"C when the polyamide constitutes 20 wt % of a mixture of dry moderate solvent and dried polyamide. A dry solvent" contains less than 0.05 wt % of water and a "dried polyamide" is a polyamide 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 polyamide 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, 10mug of polyamide alone and 10mug of mixture are each in turn subjected to cycles of heating and cooling performed under nitrogen in the calorimeter. Each heating/cooling cycle comprises heating the sample under test (which may be polyamide 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 20"C/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/cooling 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 point of the sample under test.A good solvent for nylon 6 such as phenol depresses the melting point by well in excess of 80"C whereas moderate solvents such as ethylene glycol, diethylene glycol, triethylene glycol or tetraethylene glycol depress the melting point by only 790C, 460C, 480C and 1"C respectively.

Tetraethylene glycol is a particularly preferred moderate solvent for use in the method of this invention. Other moderate solvents include benzyl alcohol, propylene glycol and glycerol.

It is also important when performing the method of this invention that the mixture be heated to above the melting point of the polyamide when in the mixture (Tm) for otherwise there will be obtained particles of unpredictable shape comprising some undissolved polyamide and some re-crystallised polyamide agglomerated onto the undissolved polyamide.Preferably the mixture should be heated at least to its clearing temperature "T ". The "clearing temperature" (TCl) of any chosen mixture comprising dry moderate solvent and dried polyamide 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 polyamide 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 the clearing temperature (Tcl) for that mixture. Heating to T 1 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 Tm 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 polyamides 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 above the melting point of the pure polyamide, for example to 250"C. Such pre-heating causes a rapid dissolution of the polyamide. The mixture is then pre-cooled to at least a temperature (for example 30 to 80"C below the melting point of the pure polymer or lower) so that polyamide re-solidifies from the mixture.Such re-solidification produces polyamide in a form which dissolves quickly (usually within 2 minutes) on heating to Tm or above so producing a mixture consisting of a solution of polyamide 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 polyamide and to avoid shock cooling which usually means cooling at a rate of no faster than 3000C/min. The preferred cooling rates are from 100C/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 polyamide wherein the particles comprise zones of crystalline polyamide and the particles are partially plasticised in that they also comprise zones of amorphous polyamide containing imbibed moderate solvent. Generally the particles will comprise from 10 to 90 (more usually 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 300"C/min). The particles are obtainable by the method of this invention as a slurry or paste in the 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 polyamide. Methanol ethanol and water are examples of such a 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 polyamide. In general higher cooling rates and also nucleating agents favour smaller particles sizes.

Typical nucleating agents for nylons include talc, fluorspar and those disclosed in British patent specification GB 1 465 046 the contents of which are herein incorporated by reference. The uniformity of particle size increases as the temperature to which the mixture is heated is increased towards Tm + 40 C.

No advantage is generally gained by using temperatures above Tm + 40 C and of course temperatures high enough to cause thermal degradation of the polyamide 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 polyamide and that caused by a nucleating agent and so the determination of TCl and clearing are not unduly hindered.

The shape of the particles obtained varies with the concentration of polyamide 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 %.

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 polyamide. Accordingly this invention also provides a process for coating a surface with polyamide 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 comprise zones of crystalline polyamide and the particles employed are partially plasticised in that they also comprise zones of amorphous polyamide 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 polyamide, for example acetone, methyl or ethyl alcohol or preferably water. Preferably the particles are heated to a temperature of from 200 to 240"C for a period of from 2 to 30 minutes. The surfaces may be metal for example aluminium, stainless steel or non-metallic, for example glass.

In particular the particles may be used to coat sheets and shaped articles such as cans for example in conventional powder coating operations. The particles (especially the flaky or rod-like particles) may also 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 polyamide.

It also has been found that the particles according to this invention disperse well in coating compositions (for example paints and varnishes) based on both water or organic solvents. Accordingly this invention provides a coating composition (which may be based on an organic solvent or water) and comprising a binder of a type used in coating compositions and (from for example 0.5 to 50 wt % of)particles of polyamide wherein the particles of polyamide comprise zones of crystalline polyamide and the particles are partially plasticised in that they also comprise zones of amorphous polyamide 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 polyamide wherein particles of polyamide comprising zones of crystalline polyamide and which particles are partially plasticised in that they also comprise zones of amorphous polyamide 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 polyamide 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 extractants such as acetone methyl or ethyl alcohol or water which only extract the moderate solvent. Porous particles may be used to increase the opacity of 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 over molecular sieves to reduce its water content to not more than 0.05 wt % and the polyamide had been dried by heating it in a vacuum (lm bar) for 16h at 90eC.

EXAMPLES 1 TO 12.

Making the Particles: 10 mg samples of a nylon 6 polyamide available as "Maranyl" B3 from Imperial Chemical Industries PLC were each mixed with one of various moderate solvents as specified in Table 1 in amounts also specified in Table 1. Each mixture was heated at a rate of 200cumin to a holding temperature Th (see Table 1) which was above the crystalline melting point of the polyamide when in the mixture (Tm) and above the clearing temperature Tcl of the mixture so as to dissolve the nylon so that the mixture existed as a solution. Tm for each mixture and a representative range of TCl are given in Table 1.The mixture was held at this Th for 2 minutes and then cooled to room temperature again at a rate of 20 C/min. A solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones TABLE 1

I I I I I Eg| Solvent |Amount |Tm |TCl|Th |Typical | |Nylon 6 ≈ | Particle in Mix- C C C Dimension ture, #m II Iwt % I I I I II I II II II I 1 ethyl glycol 20 141 - 220 25 2 " " 30 143 140 170 20 3 " " 40 146 - 180 25 4 diethylene glycol 20 174 - 195 25 5 " " 30 170 - 190 25 1 61 " " 1 40 1161119012001 40 | | 7|triethylene glycol | 20 |172|185|210| 30 | 8| " " | 30 1158118511851 30 1 9 " " 40 193 - 200 25 10 tetraethylene glycol 20 219 - 250 30 1111 " | 30 12171 - 12301 50 21 " " 187 - 245 35 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.

EXAMPLES 13 AND 14 Comparison of Rinsing and Extraction: These Examples illustrate how rinsing alone (Example 13) and rinsing plus extraction (Example 14) to remove the moderate solvent affect the ability of the particles to flow and coalesce.

A paste of nylon 6 made according to Example 7 was rinsed first in water and then in methanol 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 water through the particles for 1 minute followed by methanol for 0.5 minute.

The free flowable particles obtained above were divided into two equal portions and the first portion was subjected to an extraction treatment to remove imbibed solvent. Extraction was performed by refluxing the particles in methanol for 3h.

The rinsed only portion (Example 13), and the rinsed and extracted portion (Example 14) were each mixed with just sufficient water to produce pasty dispersions suitable for use in coating. Each dispersion was applied to a flat smooth sheet of aluminium using a grooved doctor bar comprising parallel grooves 300m deep spaced 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 290"C for 3 minutes and then allowed to cool to room temperature.

It was found that the second portion (Example 16, rinsed only) produced a continuous smooth a glossy coating of nylon whereas the nylon of the first portion (Example 17) 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 15 TO 18 Other Nylons: 10g of various nylons as specified in Table 2 were each mixed with triethylene glycol to produce mixtures containing 20 wt% of the polyamide or 30 wt% in the case of the nucleated nylon.

Each mixture was pre-heated to 2500C and allowed to pre-cool to room temperature to convert the nylons to a form which could be relied on to dissolve quickly and then they were heated to about 5"C above the crystalline melting point of the nylon when in the mixture or in the case of the nucleated nylon, to about 40"C above that melting point. The mixtures were then allowed to cool to room temperature.

Cooling caused solid/liquid phase separation and re-crystallisation resulting in the formation of fine, distinct approximately spherical partially plasticised particles comprising crystalline zones and amorphous zones containing imbibed moderate solvent. The particles comprised about 50 wt% of imbibed solvent and had typical dimensions (as determined by optical microscope) as shown in Table 2.

TABLE 2

Example Nylon Type *RV Typical I I I Particle Size m 15 nylon 6,6 44 to 50 30 1 16 ICopolymer of 97 wt% Nylon 4 to 501 30 16,6 with 3 wt% Nylon 6 l l I I | I I 17 |Colpolymer of 90 wt% Nylon|44 to 50|25 to 30| | |6, 6 with 10 wt% Nylon 6 1 1 18 Nucleated Nylon 66 44 to 50 30 * RV is Relative Viscosity of the nylon determined at 250C on a solution consisting of llg of the nylon in 100ml of an aqueous solution of formic acid consisting of 90 wt% formic acid and 10 wt% water.

EXAMPLES 19 AND 20 Coating Other Surfaces: A paste obtained according to Example 16 was spread across the flat surfaces of a sheet of stainless steel (Example 19) and glass (Example 20) using the doctoring bar employed in Examples 13 and 14. The sheets were then heated in an oven to 220 C for a period of 5 mins. On removal from the oven, the particles were found to have coalesced into a smooth continuous layer of nylon which were tough and glossy and bonded firmly to the surface of the sheet.

Claims (21)

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

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

3. A method according to Claim 1 or Claim 2 wherein the moderate solvent is chosen from mono, di, tri or tetra ethylene glycol or mixtures thereof.

4. A method according to any one of Claims 1 to 3 wherein the mixture is cooled at a rate of less than 300"C/min.

5. A method according to any one of Claims 1 to 4 wherein the mixture is cooled at a rate of 10 to 50 C/min.

6. A method according to any one of Claims 1 to 4 wherein the mixture is cooled at a rate of 100 to 300"C/min.

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

8. A method according to any one of Claims 1 to 6 wherein the mixture is heated to at least its clearing temperature (TCl).

9. A method according to Claim 8 wherein the mixture is heated to a temeprature of from TCl + 10 to Tcl + 30or.

10. 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.

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

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

13. Particles according to Claim 12 wherein the particles comprises from 40 to 75 wt % of imbibed solvent.

14. Particles according to Claim 12 or Claim 13 wherein the particles have a number average maximum diameter of from 0.1 to 100;m.

15. 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 employed comprise zones of crystalline polyamide and the particles are partially plasticised in that they also comprise zones of amorphous polyamide containing imbibed moderate solvent.

16. A process according to Claim 15 wherein the particles are applied to the surface as a dispersion comprising from 5 to 35 wt % of particles in liquid.

17. A process according to Claim 16 wherein the liquid is water.

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

19. A coating composition according to Claim 18 wherein the solvent is water.

20. A coating composition according to Claim 18 wherein the solvent is an organic solvent.

21. A process for making porous particles of polyamide wherein particles of polyamide comprising zones of crystalline polyamide and the particles are partially plasticised in that they also comprise zones of amorphous polyamide 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 polyamide until moderate solvent has been extracted into the extractant and then the extractant washed from the particles.