GB2417957A

GB2417957A - Method of forming a resin composition with additives

Info

Publication number: GB2417957A
Application number: GB0420157A
Authority: GB
Inventors: Sidney Thomas Harris
Original assignee: BIOCOTE Ltd; CHERRYHOLDER Ltd
Current assignee: HARRIS, SIDNEY T
Priority date: 2004-09-10
Filing date: 2004-09-10
Publication date: 2006-03-15
Also published as: GB0420157D0

Abstract

A method of forming a homogeneous composition comprising a thermosetting particulate base resin and a particulate additive, the method comprising heating the additive particles to a temperature above the glass transition temperature (Tg) of the resin and mixing the heated additive particles with unheated base resin whereby the surface of the base resin particles is softened to the extent that the additives can adhere thereto. A method of forming a homogeneous composition comprising a thermoplastic or thermosetting particulate base resin and a particulate additive, the method comprising temporarily bonding additive particles to base resin by electrostatic forces and heating the additive until the temperature of the surface of the base resin exceeds Tg thereof whereby the additives can adhere to the base resin. In one embodiment the base resin is a epoxy polyester and the additive is mica flakes or inorganic biocide particles.

Description

241 7957 Agent's Ret P5074GB

RESIN COMPOSITIONS WITH ADDITIVES

E.G. PIGMENTED POWDER COATING MATERIALS This invention relates to the manufacture of resin compositions with additives and more particularly to powder coating compositions.

The invention is of particular value in bonding metallic pigments to powder coating particles for subsequent application to substrates.

A number of methods described in the literature claim to be capable of "bonding" a minor proportion of an additive such as metallic pigment particles to a major proportion of the composition, i.e. to resin powder coating particles. The common approach is to heat the resin powder particles to a temperature close to the glass transition temperature thereof, at which point the particles convert from a crystalline structure to an amorphous form. This softening effect permits the metallic particles to "bond" to the surface of the powder particles. This process is invariably a batch process carried out in a specially designed mixing chamber, either as a single stage or multi-stage process. The amount of metallic pigment employed in coatings is generally in the range of about 2 to about 5% by total weight and it is necessary to heat the whole mix, the resin powder base plus the metal pigment. This is a time consuming and costly procedure.

The metallic pigment Is usually in the form of very thin metallic flakes which must be introduced into the heated powder base resin material and thoroughly dispersed using batch mixing machines These machines are large, costly to manufacture and install and expensive to run and maintain. t

It is an object of this invention to provide a continuous economic and versatile method of incorporating an additive in a resin composition.

The invention is based on the realisation that the desired composition may be made without the need to heat the bulk of the material.

In one aspect, the invention provides a method of forming a substantially homogeneous composition comprising a major proportion of a thermosetting particulate base resin composition and a minor proportion of a particulate additive, the method comprising: heating the additive particles to a temperature above the glass transition temperature of the base resin particles i') mixing the heated additive particles with unheated base resin particles whereby the surface of the base resin particles is softened to the extent that the particulate additive can adhere thereto.

The base resin particles will typically be at ambient temperature.

Preferably the mixing is carried out with a level of agitation selected to form the homogeneous composition without damaging the additive particles.

The additive particles may be uniform or randomly Irregular in shape depending upon the physical composition of the additive being processed. Particle size may range from sub micron to about 15 microns. They may be metallic, non-metallic, organic compounds or inorganic compounds. The invention is suitable for incorporating into powder coating formulations, additives such as metallic and micaceous pigments, coloured inorganic or organic pigments, biocides, fungicides, pesticides, insecticides, reaction accelerators, catalysts, flow modifiers, wetting agents, conductivity modifiers and the like. The invention is seen to particularly good effect when the additive is one which could be damaged by being subjected to normal processing methods. The present invention is suitable for use with particulate materials based on thermosetting resin such as polyesters; epoxies; acrylics; polyurethanes; mixed esters such as cellulose acetate butyrate; silicones; and combinations thereof, possessing glass transition temperatures in the range of about 45 C to about 65 C, and melt temperatures in the range of about 80 C to 1 25 C.

The invention is particularly useful in formulating resin powder coatings that require catalysts, accelerators and curing agents which could become highly reactive at the process temperatures employed in the hot melt extrusion stage of standard powder coating manufacture.

The products of this invention will find application in many areas, particularly of coating technology including coatings upon a wide range of substrates such as metals, plastics, paper, wood, wood composites; and the like.

Preferably the method comprises introducing the additive particles into a flow of the base resin particles at a temperature above the glass transition temperature of the base resin particles.

The residual heat of the additive particles locally softens the surface only, of the base resin particles allowing the additive particles to adhere or "bond" to the base resin particles. In this way, bonding has taken place without the need for the input of sufficient heat energy to raise the temperature of the total mix and the physical/chemical treatment to which the particles have been subjected is minimised.

The method comprising using electrostatic attraction to affect a temporary bond between the base resin particles and the additive particles, followed by radiant heating to soften the surface only of the base resin particles, thus allowing the additive particles to form a permanent bond with the base resin particles. In this way, bonding has taken place without the need for the input of sufficient heat energy to raise the temperature of the total mix and the physical/chemical treatment to which the particles have been subjected is minimised.

Preferably the method includes the preliminary step of bonding additive particles to base resin particles by electrostatic forces before the heating step.

In order that the invention may be well understood it will be described with reference to the accompanying diagrammatic drawing in which Figure 1 shows schematically a form of apparatus for residual heat bonding of additive particles to particles of thermosetting resin powder, and Figure 2 shows schematical apparatus for bonding particles electrostatically on a temporary basis.

The apparatus comprises a mixing chamber 10 containing a blending/mixing means 12 such as rotary driven screws or paddles or an air fluidising device.

The mixing chamber has a base resin feed device 13 located at the inlet end 1 1A. This feed is not heated and is used to introduce a measured flow of base resin particles into the mixing chamber. Alongside the resin base feed is an additive feed device 14 which contains a zone heater 15. This feed is used to introduce at a measured rate, additive which has been pre-heated to a temperature above the glass transition temperature of the base resin material, into the measured flow of base resin particles which are maintained at ambient temperature and fed from a feed device 13.

The zone heater 15 may be infra red (particularly NIR), induction, ultraviolet radiation, electron beam or the like.

In this way, when the additive particles come into contact with the resin base particles, the residual heat of the additive particles will be sufficient to soften the surface of the resin base particles (but not the bulk of the particles). The additive particles will adhere to the softened surface of the base resin particles and so create an effective bond with them.

There may be further feed devices (not shown) spaced at intervals along the mixing chamber for introducing the same or different heated additives.

The blending/mixing device 12 is selected to allow the heated flakes to tumble into the mainstream flow of base resin particles whilst maintaining a controlled degree of turbulence within the mixing chamber 10 thus ensuring good mixing of the particles whilst minimising any damaging treatment to which the flakes may be subjected during the bonding process. The bonded particles migrate to the outlet end 11 B of the mixing chamber under the influence of gravity where they are sieved to remove agglomerations and to achieve a desired particle size and fed into a collection chamber 16.

In one representative example, the base resin feed hopper was filled with epoxy- polyester thermosetting powder at ambient temperature. The glass transition temperature of this powder was determined to be 56 C. The additive feed hopper was filled with Mica flake as supplied by Merck Chemicals under the trade name Irodin 232 Royal Gold Satin and heated to a temperature of 76 C by means of an induction zone heater.

Epoxy-polyester powder was fed into the mixing chamber at the rate of 500 grammes per minute whilst, simultaneously, the pre-heated mica flake was fed into the mixing chamber at the rate of 50 grammes per minute.

An air fluidising device located in the bottom of the mixing chamber was used to create a controlled degree of turbulence within the mixing chamber and so ensure mixing of the mica particles and the epoxy polyester particles. The residual heat of the mica particles softened the surface of the epoxy polyester particles that they encountered and so bonded to them. The bonded particles migrated to the outlet end of the mixing chamber under the influence of gravity where they were sieved to remove agglomerations and to achieve a desired particle size and fed into a collection chamber.

By routine experiment the optimum mixing and blending settings can be ascertained for a range of additives and base resins by adjustments of the rates of feed, angle of inclination of the mixing chamber and the rate of air flow through the air fluidising bed.

Similarly, the optimum temperature to which particular additives should be heated can be ascertained by determination of the glass transition temperature of the base powder being processed, the heat transfer characteristics of the additive and the ambient temperature of the mixing chamber and contents.

In the embodiment of Figure 2, feed devices 21, 22 are present at the inlet end of an elongate inclined chamber 20, one to feed the base resin and the other to feed additive particles. Both are at ambient temperature. Each device comprises a powder spraying device or fluidising device. Each has an electrostatic charging facility, 21a, 22a, so that the particles fed to the chamber are charged either positively or negatively as required. A heater, such as a near infra red heater 23, is present at the outlet end of the chamber below which is a sieve and a collection device. In use, the charged base resin particles and the charged additive particles are fed into the chamber at a measured rate. The electrical attraction causes the additive particles to adhere to the resin base particles in a temporary fashion. These particles are passed to the heater 23 which is at a temperature to heat the surface of the resin particles above the glass transition temperature so that the surface (but not the bulk) of the resin particles softens causing the additive particles to bond to the resin particles as the temperature drops. The product particles are sieved at 24 and collected.

The additive particles are fed into the mixing chamber 20 at a measured rate via one of the particle feeding devices 21, 22 and electro statically charged. Simultaneously the resin base particles are fed into the mixing chamber 20 at a measured rate via a second particle feeding device and these particles are electro statically charged with the opposite polarity to the additive particles. The blending/mixing device maintains a controlled degree of turbulence within the mixing chamber thus ensuring good mixing of the particles whilst minimising any damaging treatment to which the flakes may be subjected during the bonding process. In this way, the mutual attraction of the oppositely polarised particles will cause the additive particles to form a temporary bond with the base resin particles.

The temporarily bonded particles migrate to the outlet end of the mixing chamber under the force of gravity where permanent bonding is achieved by passing the electro statically bonded particles through the heat source 4 which may be infra red (particularly NIR), induction, ultraviolet radiation, electron beam or the like.

The heat source will momentarily cause the surface of the base resin particles (but not the bulk of the particles) to be heated to a temperature above the glass transition temperature of the base resin, typically so that the surface of the base resin (but not the bulk of the base resin) softens allowing the additive particles to adhere or "bond" to the base resin particles.

The now permanently bonded particles are sieved to remove any agglomeration and to achieve a desired particle size and fed into a collection chamber.

This aspect of the invention is further explained by way of example: ln one representative example, polyester thermosetting powder was fed into the mixing chamber at the rate of 500 grammes per minute by means of a corona powder spray device and positively charged. Simultaneously, inorganic biocide particles as supplied by Clarient under the trade name JMAC Composite PG, was fed into the mixing chamber at a measured rate of 25 grammes per minutes by means of a second corona spray device and negatively charged. An air fluidising device located in the bottom of the mixing chamber was used to create a controlled degree of turbulence within the mixing chamber so ensuring good mixing of the biocide particles and the polyester particles As the oppositely polarised particles migrated through the mixing chamber under the influence of gravity, positively charged polyester particles were attracted to negatively charged biocide particles and formed temporary bonds one with the other.

The electro statically bonded particles migrated to the outlet end of the mixing chamber under the influence of gravity, where they were passed through a near infra red heater.

The bonded particles were subjected to high intensity short duration heat energy so that the surfaces of the polyester particles were heated to a temperature of 76 C this being 20 C above the pre-determined glass transition temperature of the polyester resin. As the surfaces of the polyester resin particles softened and became tacky, the biocide particles which were temporarily attached to the resin particles by electrostatic attraction, adhered to the polyester resin particles and so formed a permanent bond.

The bonded particles passed from the NIR heater to a sieve to remove agglomerations and to achieve a desired particle size. The graded, permanently bonded particles were fed into a collection chamber. By routine experiment the optimum mixing and blending settings can be ascertained for a range of additives and base resins by adjustments of the rates of feed, angle of inclination of the mixing chamber and the rate of air flow through the air fluidising bed. Similarly the optimum time and intensity of the heat energy to which particular electro statically bonded particles should be subjected can be determined by reference to the glass transition temperature of the base powder being processed. The apparatus of Figure 2 and the method described can be employed for thermoplastic base materials.

The benefits of this invention over current bonding technologies include: i) provides a continuous low cost manufacturing process.

ii) produces high quality metallic effect powders comparable to wet paint coatings.

iii) enables the incorporation of catalysts into rapid thermal cure powder coating systems at relatively low temperatures.

iv) enables the efficient production of tinted powders from pre-made base powders.

v) significantly enhances ability to control powder resistivity.

v') significantly enhances the control of the gloss and aspect characteristics of powder coatings.

vii) enables low cost incorporation of a wide range of additives into premade base powders.

Claims

1. A method of forming a substantially homogeneous composition comprising a major proportion of a thermosetting particulate base resin composition and a minor proportion of a particulate additive, the method comprising: i) heating the additive particles to a temperature above the glass transition temperature of the base resin particles ii) mixing the heated additive particles with unheated base resin particles whereby the surface of the base resin particles is softened to the extent that the particulate additives can adhere thereto.

2. A method according to Claim 1, wherein the mixing is carried out with a level of agitation selected to form the homogeneous composition without damaging the additive particles.

3. A method according to Claim 1 or 2, wherein the heated additive particles comprise mica flakes or inorganic biocide particles.

4. A method according to any preceding Claim, including the preliminary step of bonding additive particles to base resin particles by electrostatic forces before the heating step.

5. A method of forming a substantially homogeneous composition comprising a mayor proportion of a thermosetting or thermoplastic particulate base resin . composition and a minor proportion of a particulate additive, the method comprising: temporarily bonding additive particles to base resin particles by electrostatic forces; and heating the additive particles until the temperature of the surface of the base resin particles exceeds the glass transition temperature thereof whereby the surface of the base resin particles is softened to the extent that the additive particles will permanently adhere thereto.