Such a plastic pellet is disclosed in WO 01/00714.

Said publication discloses polymer particles covered with a layer in which the additive, for instance an antioxidant, a lubricant or a colorant, and a polyester-based binder are present. The ratio between the amounts of binder and additive in said document in general lies in the range between 1: 1 and 0.75 : 1. With such high loads the layer binder which screens off from the environment the additive particles closest to the external surface is rather thin for a relatively large amount of those particles. As a result this layer is worn through during storage and in particular during pneumatic transport which is frequently used for transport and supply of plastic pellets, so that part of the additive gets into the transport system and fouls it, especially if the additive is a colorant or a pigment.

This problem can be solved by increasing the amount of binder relative to the amount of additive, so that the additive is diluted. However, in order to affect the properties of the plastic in the pellet as little as possible, it is desirable to restrict the amount of binder.

The aim of the invention now is to provide a pellet as indicated in the opening lines hereof in which a high ratio of additive to binder is acceptable and which is more resistant to mechanical effects, in particular during pneumatic transport.

This aim is achieved according to the invention in that the additive content at the outside of the covered pellet is lower than in a more inwardly located part of the layers.

Thus, the content in the most outwardly located part of the layers can be chosen so low that also in case of wearing off of that exterior part no significant amount of additive is released, while the content in the more inwardly located parts of the layers can be chosen very high. In this way, with an unchanged amount of binder relative to the amount of additive, a pellet is yet obtained which when worn will release less additive. Due to the presence of the more outwardly located part with a relatively small amount of additive the concentration of additive in the more inwardly located layers can be chosen very high (and so the binder content accordingly lower).

From US 4,517, 246 it is known to coat a polyolefin reactor powder by

treating it with a mixture of a polyethylene and a filler material above the melting point of the polyethylene. Particles coated with a layer consisting of the filler bound by the polyethylene are obtained : In claim 7 in complete isolation and without any support in the description or any disclosure of possible effects or advantages particles having a two-layer coating are claimed. The inner layer contains a filler and the outer layer is said to mainly contain the binder. This document will not lead the skilled person to coat thermoplastic pellets, not being reactor powder material, with a composition of a binder and an additive, not being a filler, in such a way that the outer parts of the coating layer contains less additive than the more inner parts of the layer in order to make the coated pellets able to withstand pneumatic transport without abrasion of additive containing material.

The plastic in the pellet according to the invention is a thermoplastic polymer. Examples of such polymers are polyolefins and, due to their polar nature more preferred, polyesters, polyamides, polyethers, polycarbonate, acrylonitrile- butadiene-styrene polymers, polyamides and polyacetals. In the current processing and shaping techniques for thermoplastic polymers, the polymer formed as a reactor product is molten and pelletized. In many cases in this step customary additives are mixed in that are required regardless of the further application of the pellets. Examples of these are certain stabilizers, fillers and reinforcing components. Examples of other additives that usually will not be mixed in in the total pellet production but at later stages for different applications are colorants, lubricants, blowing agents, pigments, dyes, reactive dyes, antioxidants, specific thermostabilizers and UV stabilizers, antistatic agents, antiblocking agents, release agents, and flame-retardants. These other additives, of which fillers and reinforcing materials are excluded, can be present in the additives-containing layer in the pellet according to the invention.

The invention thus enables easy diversification of the additive package in polymer batches.

The additives can be present as such as a layer around the coatings, but preferably the additives-containing layer contains a binder in which the additives are dispersed and which causes them to be bonded more firmly to the pellets. Suitable binders are those which can be wetted by or dissolved or dispersed, for instance as a latex, in a dispersing agent in which the relevant additive can also be dispersed or dissolved. The mixture of the three components, i. e. dispersing agent, binder and additive, has then been applied onto the surface of the pellet by means of the techniques to be discussed in the following. After removal, for instance by evaporation,

of the dispersing agent a layer of additive formed by the binder remains behind on the outside of the pellet. This process can have been repeated a few times, so that several additive-containing layers with or without binder can be present on the pellet.

Suitable binders are those which can be stably dispersed or dissolved in a suitable dispersing agent. Suitable dispersing agents for said binders are known per se and can be selected by the person skilled in the art on the basis of their compatibility and acceptability in relation with the material of the pellet and the envisaged application. Preferably thermoplastic binders are used, for instance polyvinyl alcohol, polyvinyl acetate, polyurethanes, epoxies, polyvinyl pyrrolidone, alkoxylated polyvinyl pyrrolidone, polyethers, polyvinyl caprolactam, polyoxazolines, polyacrylates and other polycarboxylates, polyolefins, polystyrene, polyamides, polyesters and copolymers of these. The melting point of the binder is chosen in relation with the melting point of the pellet polymer such that the binder will be molten at the temperature where the polymer will be processed by melt forming. The melting point thus may be below or above the melting point of the pellet polymer, the latter being preferred because this promotes a fine dispersion of the binder containing the additives in the polymer The thickness of the layers can vary within wide limits, for instance between 1 Slm and 1 mm, for instance in dependence on the amount of additive that has to be applied and the allowable amount of that material, which is unlike the pellet itself. Suitable values are at least 1,2, 5,10, 20 or even 50 tm and at most 250,200 or even 150 ptm. This minimizes the share of binder in the pellet thus minimizing any possible side-effect of the presence of the binder on the properties of the pellet material.

The additive content relative to the total of additive and binder in the part of the additive-containing layer not exposed to the environment as a rule lies between 3,5, 10 or even 25% and 50,75, 90 or even 97%. The maximum is for instance determined by the amount of additive with which the binder can still actually form a layer which does not come loose wholly or partially from the pellet. This may occur for instance when the layer becomes too brittle or easily flakes owing to a too low binder content.

In the pellet according to the invention the above-described difference in concentration can be obtained in various manners. Thus, for instance, the additive content can decline evenly from the inside to the outside, but it can also

change in steps, for instance due to an additive-free outer layer on top of the additive- containing layers being present. An outer layer is a layer which is present on the last- applied of the additive-containing layers and which is thus exposed to the environment.

An outer layer is considered to be free of additive if on average between 0 and at most 3% of an additive is present in it. If any additive is present in the outer layer, it is preferably present in a more inwardly located part of the outer layer or the amount of additive that is present shows a declining gradient from the outer surface towards the outer additive-containing layer. In this way it is achieved that the amount of additive close to the outer surface of the outer layer, which outer surface is exposed to environmental effects, is restricted to a minimum. Release of additive into the environment as a result of wearing off of the outer layer is thus minimized. This embodiment is one of the preferred embodiments. This is because in this case the additive containing layer or layers need not as such be wear resistant at all, as the wear resistance can be provided by the protective additive-free outer layer. This outer layer can consequently be thin as it contains no or hardly any additive, which could be released into the environment when the outer layer wears off.

The outer layer can consist of any material, which is capable of forming a layer on the last-applied additive-containing layer, but it preferably consists of a film-forming polymer. More preferably the outer layer substantially, for instance to a degree of at least 70%, consists of a wear resistant material offering a high resistance to abrasion.

The thickness of the outer layer can vary within wide limits, for instance between 1,2, 5,10, 25,50 or even 100 ptm and 100,250 or even 500 um, but depending on the wear resistance of the material and the load which the pellets must be able to resist, it is chosen such that when the anticipated load is exerted the outer layer will not disappear completely, so that the last-applied additive-containing layer will not appear at the surface of the pellet. Above this required minimum the thickness of the outer layer is chosen as low as possible in order to restrict to a minimum the amount of foreign material in the objects to be manufactured from the pellets. It is of advantage to choose the material of the outer layer such that its presence has no adverse effects on the properties of the plastic. As there is no or hardly any additive present in the outer layer there is more freedom in the choice of the material for the outer layer than in the choice of the binder for the additive-containing layer.

If there is no separate outer layer the additive content of the part of the outer layer which is exposed to the environment and consequently to wear is

chosen such, also taking into account the wear resistance of the binder, that during the envisaged processing of the pellets the amount of additive wearing off remains within acceptable limits. The more wear resistant the binder is, the higher the additive content of the outer layer part can be. If only one layer is applied the additive or additives content of that layer preferably declines at least on average from the inside to the outside. Thus the greater part of the additive is present in the more inwardly located part of the additive-containing layer, which part is screened off from the environment by the outer part of the layer, and the release of additive can be minimized.

It is also possible to have multiple additive-containing layers on the pellet, optionally each with different additives and/or with different binders, giving the opportunity to fine-tune the binder additive combinations and to choose as the binder in more inward layers binders having lower wear resistance. The content levels in the more inwardly located layers can then be chosen arbitrarily, so high as well, provided that the outer layer meets the requirements in terms of additive content and the distribution thereof, so that the release of additive due to wear remains within acceptable limits.

The additive-containing layers can be applied on the pellet by the processes known per se for the application of a coating on an object, for instance by spraying a solution or a dispersion of the additive, for instance via a nozzle or by means of a propellant gas.

In addition the additive can be applied onto the pellet by means of the following novel process, which also comes within the scope of the present invention.

The invention thus also relates to a process for the production of a plastic pellet as described in the foregoing, comprising the application of one or more layers containing at least one additive by contacting the pellet with a composition which contains the additive and is sprayed by means of a spraying system.

Such a process is known from a publication by P. J. Palermo in the Conference Proceedings SPE, Polymer Modifiers & Additives Division, pages 131- 139. This publication discloses the possibility of maintaining a fluid bed of plastic pellets in a fluid bed chamber and spraying the pellets with a dispersion of the additive to be applied with a binder in a liquid as dispersing agent.

A drawback of the known process is that after the application of one or more additive-containing layers on a batch of pellets, at least the spraying mechanism and the supply lines are fouled with the said dispersion. This means that the spraying mechanism has to be cleaned whenever it is necessary to switch to

application of other additives. This affects the degree of utilization of the installation and moreover after each cleaning operation a quantity of contaminated cleaning liquid remains.

The aim of the invention is to provide a process as described above, but in which said drawback has wholly or largely been eliminated.

This aim is achieved according to the invention in that after application of the additive an additive-free outer layer is applied by spraying a composition that has a cleaning effect on the spraying system.

By this measure it is achieved that the additive remaining in the spraying mechanism is entrained by the liquid and by spraying deposited onto the plastic pellets, instead of having to be carried off as a contaminated liquid. This depositing is fully acceptable because on the pellets a layer containing the additive involved had already been applied anyway as the last additive-containing layer.

A supplementary advantage is that in this way the cleaning process involves simultaneous application of an outer layer, so that a plastic pellet as described above can be produced. The part of this outer layer which has been applied first and so adjacent to the last additive-containing layer still contains an amount of the additive from that additive-containing layer. The additive content further declines from there towards the outside as the spraying mechanism is gradually cleaned further.

Depending on the amount of the liquid that is sprayed very little additive or none at all will be present yet at the surface. In order to obtain an outer layer which does not contain any additive the cleaning can be carried out in the absence of the plastic pellets and after that the outer layer is applied in a cleaned installation.

The plastic pellets that can be provided with an additive-containing layer in accordance with the invention consist of the plastics as described in the foregoing. The process is suitable for application of the additives mentioned in the foregoing. Besides the additive the sprayed composition also contains a dispersing agent in which the additive has been dispersed and more preferably also a binder which is dispersible and preferably soluble in the dispersing agent. Suitable binders are those which have already been mentioned in the foregoing. Preferably reactive binders are used. A reactive binder is understood to be a compound having a relatively low molecular weight so as to be easily processable in the coating process and containing reactive groups, optionally blocked, which after the pellet having been coated can react to higher molecular weight compounds, providing good mechanical properties.

Examples of reactive groups are isocyanate-, epoxy-and amine groups but also

other reactive groups known in the art can be applied.

Contacting the pellet with the additive-containing composition can take place in a manner known to the person skilled in the art, in particular by spraying the composition, which contains a dispersing agent in which a preferably dissolved or otherwise finely divided binder is present and in which the additive is dispersed, onto or across the pellets. The pellets are preferably maintained in a fluid state in a manner known per se in a fluid bed. The composition can be sprayed in such a manner that it is entrained in a sprayed state by the gas flow that maintains the fluid bed, but it can also be sprayed sideways or from above into the space in which the fluid bed is maintained.

By'spraying'is understood here finely dividing the composition in the form of droplets or a mist, for instance by pumping it through a nozzle or by means of a propellant gas.

The techniques known per se for this purpose can be applied in the process according to the invention.

When the pellets have been provided with the required additive- containing layers an additive-free outer layer is applied.'Additive-free'should be read here as described in the foregoing. The outer layer is applied by spraying a film-forming composition which has a cleaning effect on the spraying system. Such a composition consists for instance of a film-forming material dissolved or dispersed in a liquid. It is also possible to use for this purpose for instance the binder material with which the underlying additive-containing layer or layers was or were applied. A composition is referred to as'film-forming'here if after application of a layer of it onto a surface a solid uninterrupted layer can be formed on it by for instance drying, hardening or evaporation of liquid. It is allowable for the resulting layer to be porous or not completely uninterrupted, provided that the underlying layers are screened off from the kind of external mechanical effects that occur during transport and storage of the pellets.

The film-forming composition has a cleaning effect on the spraying system. This implies that the liquid used in the spraying system must be able to entrain the additive that has remained behind in the spraying system and if necessary to detach it from a substrate. Suitable liquids therefore are those liquids that are suitable as a dispersing agent or solvent for the binder, as described in the foregoing.

The amount of cleaning composition applied can be chosen in dependence on the required thickness of the outer layer. If a binder is present the desired layer thickness can be obtained due to appropriately chosen combinations of the concentration of the binder in the liquid and the amount of the composition applied.

If no separate additive-free outer layer is applied, an outer layer

section with a lower additive content can be applied by decreasing gradually or in steps the additive content of the composition of additive, binder and dispersing agent to be applied, for instance by connecting to an adjustable mixing tap a supply line for the said composition as well as a supply line for an additive-free mixture of binder and dispersing agent and by causing the amount of the additive-free mixture to increase in time, with a corresponding decrease of the amount of the additive-containing composition.