Polyamide-coated particles and method for their manufacture

Polyamides with, among other interesting properties, excellent mechanical properties, good vis-à-vis inertia of many chemical agents and excellent adhesion to many materials, it has tried to coat particles of various materials a layer of polyamide to equip an envelope which will serve either to protect, or, bind with other materials by melting and cooling.

Unfortunately, achieving such a coating is not easy.

One can, for example, mixing in an extruder substrate particles with molten polyamide, extruding strands of this mixture, cooled, cut into pellets and then reducing these powder granules by cryogenic grinding.

But such a method has many inconveniences among which we mention the following:<ul><li>- For the polyamide substrate mixture has a good thermoplastic behavior during his time in extruder, it must have a high content of polyamide which results in a thick coating, and, moreover, irregular final particles,</li><li>- During grinding, many particles are exposed,</li><li>- The particles do not melt front nor react with the polyamide during the passage in an extruder, this limit choice.</li></ul>

Just as it was proposed to coat copper son of a thin polyamide layer by passing these son in a hot solution of polyamide in a subsequent drying and cresol (US Patent 4,216,263), could knead particles to be coated in such solutions and dry - but, cresols (which are almost the only effective solvents for polyamides) are dangerous and difficult to eliminate body - in addition, the particles would stick together.

To remedy this, the French patent 2,258,263 proposes to mix the particles with solutions in methyl alcohol or other solvents (easy to handle and readily removable subsequently by drying) not polyamides (which would be insoluble) but copolyamides.

US Patent 4,065,519 teaches a similar type of mixing; the particles to be coated are mixed with either homopolymers or copolymers with polyamide in a suitable solvent.

The process is simple - unfortunately, the coated particles are obtained copolyamides that do not have the same properties as the pure polyamides - (greater water absorption, lower chemical resistance etc ...) that can be inconvenient for some applications - in addition, the coating is not regular.

The process which is the object of the present invention solves the problem without having the disadvantages mentioned above. It allows to obtain particles coated with an even layer of polyamide which had never been obtained otherwise.

This process consists essentially in:<ul><li>- Putting the substrate particles which it is desired be coated suspended in a reaction medium capable of generating a polyamide or copolyamide by anionic polymerization in solution, that is to say a medium mainly comprising a lactam, a solvent for this lactam and an anionic catalyst</li><li>- Initiate the anionic polymerization by the addition of an activator</li><li>- Isolating, once the polymerization is complete, the substrate particles on which came deposit the polyamide as and when it is formed.</li></ul>

Such reactions were disclosed in French No. 1,521,130 and 1,601,194, and DE OS 1942046.

All lactam polyamide generator may be used in the invention; include especially those who have acquired industrial importance: caprolactam, oenantholactam the caprylla ctame and lauryllactam.

Can also be used a mixture of two or more lactams, which leads to a coating of the substrate by a layer of copolyamide.

The solvent used has a boiling point between 80 and 200 ° C; this may be an aliphatic hydrocarbon, the paraffinic, for example, or alicyclic or aromatic (such as xylene or toluene), halogenated or not.

It is essential that this solvent:<ul><li>- Dissolve the lactam but not the substrate</li><li>- Reacting with any of the compounds used in the process: catalyst, activator, lactam substrate.</li></ul>

The catalyst may be sodium or a compound thereof such as sodium hydride or sodium methoxide.

The enhancer may be a lactam-N-carboxya- Nylide, isocyanate, carbo di imide, a cyanimide, an acyl-lactam compound, a triazine, a urea, a N-substituted imide, ester, etc .. .

Can, optionally, introducing regularize the polymerization reaction medium in a N, N'-alkylenebisamide.

It should be introduced into the environment no particles other than the substrate which it is desired to encapsulate.

The substrate can be very diverse in nature:<ul><li>- Mineral (glass beads, glass fibers, feldspar grains, calcium carbonate, magnesia, iron powder etc ...)</li><li>- Or, organic (carbon fibers, polymer grains such as polyvinyl chloride, polybutadiene, polyethylene, cross-linked polystyrene, cellulose derivatives ... carbon black etc ...).</li></ul>

The substrate does not need to be regularly shaped, its surface can be smooth or solid asperities.

No special density is required as long as we manage to keep it suspended in the reaction medium with agitation which is maintained there; thus can be coated, without difficulties, copper powders or yet very dense nickel.

The key is to be solid, insoluble in the reaction medium and inert vis-à-vis the environment; in particular, it should make no trace of water.

Thus when it is desired to coat glass beads or glass fibers which are mostly already coated with a sizing composition, it is essential to ensure through analysis or preliminary tests that this sizing does detrimental to the reaction.

As discussed in Example 5 below, may be coated particle materials that are not completely inert vis-a-vis the catalyst. It is sufficient to introduce the particles into the medium when it will no longer contain catalyst, that is to say after the catalyst will have reacted with the lactam to transform it into lactamate.

In the examples we cite below examples are intended to illustrate the invention without limiting it, the tests were carried out in a reactor with a capacity of 20 liters, fitted with a paddle stirrer, a jacket in which circulates heating oil, with a draining system at the bottom and an introduction airlock reagents flushed with dry nitrogen.

An azeotropic vacuum distillation device eliminates all traces of water from the reaction medium.

The solvent used is a paraffinic hydrocarbon fraction with a boiling range is between 130 and 160 ° C.

The molecular weight of the polyamide layer deposited is determined by measuring the inherent viscosity, at 25 ° C, a solution of 0.5 g of this layer in 100 g of m-cresol.

example 1

Coating of solid glass beads

The balls used marketed by the Company MICROBEADS AG under the reference PF 11 R have a density of 2.45 and an average diameter of 26 microns.

Their surface has been treated with a coupling agent type "Silane" which, as has been verified by a preliminary test, is inert with respect to the reaction medium and thus not harmful to the filing of polyamide.

successively introduced into the reactor under nitrogen at room temperature and with gentle stirring:<ul><li>- 10 liters of solvent,</li><li>- 1 500 grams of glass beads PF 11 R</li><li>- 1 500 grams of caprolactam.</li></ul>

The temperature of the bath at 110 ° C, which was maintained for 60 minutes at a stirring speed of 360 revolutions / minute so that any caprolactam dissolves, distills 100 cc of solvent to remove azeotropically water that may be present, returns to atmospheric pressure, introduced 28.67 grams of sodium hydride at 50% purity, door to 130 ° C, and the stirring speed is always maintained at 360 rev / minute, introduced in 4 hours at linear speed, a solution of 82 grams of stearyl isocyanate in 1.9 liters of solvent.

Once this injection, the temperature was maintained 130 ° C for 30 minutes, cooled to 100 ° C, stop agitation, drain the reactor contents into a filter to separate the solvent, dried at 90 ° C and empty the powder obtained by the processes 1 500 cc of a 3% aqueous phosphoric acid solution so as to neutralize the alkaline residues from the catalyst and dried again.

As shown in the attached picture taken with an optical microscope, the powder is composed of introduced micro glass beads, coated with a very uniform layer of very regular and 3 to 5 microns thick polyamide 6.

Furthermore, the inherent viscosity of the polyamide layer was found to be 0.82.

Example 1 Bis

Returning to the example 1, but introducing 300 g of glass beads and 1500 g of caprolactam, resulting in the implementation of 20 grams of sodium hydride and 82 cc of stearyl isocyanate.

The ratio polyamide / glass beads being higher than in the preceding example, the glass beads are embedded at the end of a thicker layer of polyamide: 6 to 8 microns instead of 3 to 5. The viscosity of this layer has the value 0.71.

example 2

Coating of hollow glass spheres

The beads used are hollow glass spheres sold by the Company MICROBEADS AG under the reference GL, not sized, average diameter 47 microns and density of 0.35.

The procedure is as in Example 1 above but with<ul><li>- 300 grams of these hollow spheres</li><li>- 2700 grams of caprolactam</li><li>- 25.2 g of sodium hydride at 50%</li><li>- 122.94 grams of stearyl isocyanate.</li></ul>

The balls that floated to the surface of the bath it disperse as soon as it establishes stirring at 360 revolutions / minute.

Is obtained, upon operation, microbeads coated with a very uniform and even, thick 6 microns, polyamide 6 having an inherent viscosity equal to 0.75.

Example 2 Bis

The hollow balls have this time, a diameter of 70 microns. The operating conditions are the same with the difference that one uses 22.9 grams of hydride.

The final 6 polyamide layer has a thickness of 6 microns and a viscosity of 0.80.

example 3

Coating glass fibers

Same procedure as for the previous examples but by charging the autoclave with 500 grams of glass fiber supplied by Vetrotex Saint-Gobain under the name Vetrotex EC 10 N 99 of diameter approximately equal to 10 microns and length between 30 and 250 microns.

To this glass fiber was added 2700 grams of caprolactam and 25.2 g of sodium hydride at 50% and then polymerized by the addition of 122.94 grams of stearyl isocyanate.

Once completed, are obtained coated glass fibers over their entire length and ends with a layer of polyamide 6 5 microns thick.

example 4

coating particles of oxide of ground magnetic iron, of diameter between 10 and 30 microns.

Into a 5 liter reactor,<ul><li>- 2,840 liters of solvent and then, successively,</li><li>- 80 g of lauryllactam,</li><li>- 1069 g of iron oxide</li><li>- 1 g of N, N'-ethylenebisstearamide.</li></ul>

It sets the agitation at 3501 / min. and the temperature of the bath at 110 ° C.

After azeotropic distillation of 500 cc. of solvent under a vacuum of 200 torr, are introduced under nitrogen 3.0 g of sodium hydride at 80% purity. Maintained for 30 minutes at 110 ° C, cooled to 100 ° C and at this temperature, conducts the injection of 8.5 g of stearyl isocyanate in 2 hours. After the injection, the temperature is then raised to 110 ° C for 1 hour.

After cooling, decanting and drying, a homogeneous powder with a particle size between 12 and 35 microns.

All the oxide grains have been coated with a layer of polyamide 12, an inherent viscosity equal to 0.81.

example 5

PVC particles of coating (polyvinyl chloride)

This is of PVC particles obtained by the process called "suspended", perfectly dried before their implementation, and average diameter of 20 microns.

To coat polyamide 12, again using the reactor which was used in Examples 1 to 3.

Having found by preliminary experiments, that these particles were not completely inert with respect to sodium hydride, it is introduced after the reaction of the hydride with the lactam to give the lactamate sodium.

The procedure is the following: having loaded the reactor:<ul><li>- 10 liters of solvent,</li><li>- 88.3 g of N, N'-ethylenebisstearamide</li><li>- 2,850 g of lauryllactam,</li></ul> carried out the dissolution at 105 ° C with stirring, distilled 100 cc of solvent, introduced 13.88 g of sodium hydride at 50% purity, are introduced under nitrogen 150 g of PVC and, maintaining the process temperature at 105 ° C and stirring at 360 revolutions / minute, introduced in 4 hours, a solution of 89.3 g of stearyl isocyanate in 2 liters of solvent.

After washing and drying as in Example 1, we finally obtain a powder consisting of PVC grains coated with a perfectly uniform layer of polyamide 12, an inherent viscosity equal to 0.62. PVC beans which initially were not spherical, it became - their average diameter increased from 20-26 microns.

Example 5 Bis

By working analogously, but without the use of N, N'-ethylenebisstearamide and with 300 g of PVC, 2700 lauryllactam, 9.86 hydride and 94 isocyanate, PVC grains obtained coated 40 microns in diameter - viscosity of the polyamide layer 12 was 1.77.

The uses of the substrates thus coated are varied:<ul><li>1. The solid glass beads of Example 1 and the glass fibers of Example 3 can be used to strengthen composite materials. During the manufacture of these materials, it is sufficient to bring them to a temperature slightly above the melting point of the polyamide component and the coating to cool to create good bonding between the balls or glass fibers and the materials with which they are in contact.</li><li>2. With the coated iron oxide powder of polyamide of Example 4, can easily be made permanent magnets of desired shape by compressing this powder in a mold, laying the piece few minutes at 185 ° C so as to obtain complete melting of the polyamide and allowed to cool.</li></ul>

In this connection, an original application that is also the subject of the present invention is the use of metal powder coated polyamide in lieu of pure polyamide powders in the production of flexible composite materials claimed by demand of French patent No. 84.05627 in the name of the applicant, the content is added to the present description.

Thus, powders of copper of 3 to 5 microns in diameter, 13 microns after enro bage by a polyamide layer 12, nickel powders of 12 microns diameter, 25 microns after coating, may come up to 35 percent by weight in the manufacture of composite flexible mentioned above, allowing after processing thereof in plates by assembly, melting and cooling, to obtain essentially materials consisting of polyamide, glass fiber and fat metal evenly distributed which gives them interesting electrical or electromagnetic properties.

3. The polyamide PVC powders coated with 12 of Examples 5 and 5a may be used in cosmetics to advantageously replace talc.