AU685328B2

AU685328B2 - Powder coating recycling process

Info

Publication number: AU685328B2
Application number: AU38983/95A
Authority: AU
Inventors: Michael Cosgrove; Mark Joseph Doolin; Brett Andrew Kerry; Mark Leeder; James McKie Smith
Original assignee: ICI Australia Operations Pty Ltd
Current assignee: Orica Australia Pty Ltd
Priority date: 1994-11-24
Filing date: 1995-11-23
Publication date: 1998-01-15
Anticipated expiration: 2015-11-23
Also published as: AU3898395A

Description

POWDER COATING RECYCLING PROCESS TECHNICAL FIELD OF THE INVENTION

This invention relates to powdered film-forming polymer for use as powder coatings for industrial and architectural application. More particularly, it relates to a process for recovering powdered polymer waste and the recycling of the powdered polymer waste into useful coating compositions. BACKGROUND OF THE INVENTION

Powdered polymer coatings are extensively used, especially for industrial applications. Typical powder coating compositions are epoxies, polyesters and acrylics and they may be thermoplastic or thermosetting. The coatings are usually applied using electrostatic spray gun delivery to minimise overspray and waste. The powder particles, usually pigmented, are typically delivered from a spray gun under pressure with a high-voltage, for example 50-90 kV, low-amperage charge. For most applications the particle size of the powder is in the range 10 to 100 μm with the preferred range being 20-40 μm. This application process coats an electrically grounded metal object with a covering of powder. Because of the electrical insulating characteristics of the deposited powder the total coating thickness that is electrostatically attracted is limited. The coating process is completed by fusion of the particles to form a continuous film by heating in an oven causing melting of the film forming polymer. Usually a crosslinking reaction takes place after melting to form a thermoset film that is hard, tough and resistant to chemicals and solvents.

Powder that is not deposited on the object is usually recovered by an extraction system. Recycling of recovered powder by immediate further spray application or by blending with new stocks of powder is often unsuccessful because of the different size characteristics of the recovered powder. Generally, deposition efficiency is a function of particle size of the powder and non-deposited powder tends to be smaller in particle size. The applicator's inability to recycle non-deposited powder means that not only is typically 5-10% of the product wasted, but environmentally acceptable disposal of the waste powder is an added cost burden. Recycling of the recovered powder is commonly used. However, even with recycling there is a small percentage of powder that is not useable and must be disposed. Powder coatings are usually manufactured by a process of melt-mixing a uniform blend of predominantly solid raw materials in an extruder using heat to melt the resin. In most processes there is an optional premixing stage where all the ingredients are high speed dispersed to achieve a general mixing of the materials to form the uniform blend. Mechanical mixing of the melt results in dispersion of the pigment and other additives in the molten resin. The melt mix is then discharged, cooled and broken into flakes, pellets or granules typically 10 to 20 mm in size. These flakes, pellets or granules are then mechanically ground to provide the required particle size of the powder. This grinding stage involves impacting the flakes, pellets or granules with grinding elements in a suitable mill such as a pin mill or high-speed hammer mill. The mill is usually equipped with an internal or external vane-type classifier which separates any powder particles greater than the desired size range and these are then recycled through the mill. The powder is swept from the grinding mill by a stream of air and separated using a cyclone. Recovery rates of 95 to 98% can be achieved. The residual 2-5% of the powder is usually of smaller particle size and is rejected as waste. Not only does this waste represent an effective increase in formulation cost but disposal of the waste is becoming an environmental concern and added expense.

Simple reprocessing of the fine particle waste in the manufacturing process by adding the waste to the melt mixing stage in the manufacturing process is unsatisfactory as the fine particles tend to stick to and build up on the moving components of the axial extruder usually used for the mixing.

There are thus two sources of waste powder in the powder coating process, first the fines generated during the grinding process and second the overspray particles that are not deposited on the target substrate by the applicator and which are too small for satisfactory recycling.

Various solutions have been proposed to this powder waste problem. These are discussed in the publication "Powder Coatings Bulletin" June 1993 Volume 16 Number 2 edited by S.T. Harris. However, none of these are very satisfactory. In summary, these prior art processes tend to be either inappropriate for thermoset powder and/or are not generally cost effective.

In addition to the processes discussed in the above paper, there are a number of general processes for the size enlargement of small particles which offer promise of a solution to the powder waste problem. Size enlargement is defined as a process whereby small particles are gathered into larger permanent masses in which the original particles can still be identified. However, of the various available size enlargement processes available, it has been found surprisingly that only the pressure compaction process described in this specification provides satisfactory results for the recycling of powder coatings wastes.

Processes that lead to localised heating whereby melting and premature crosslinking of thermoset powders can take place need to be avoided as premature crosslinking of the material in the processing stage will ultimately lead to processing difficulties and/or very poor paint properties.

An alternative candidate method for size enlargement of fine particles is agglomeration by tumbling. However, in this case a liquid binder is usually required to be added to the fine powder and agglomerates are formed by tumbling, vibrating, stacking or paddle mixing. Addition of a binder to powder coatings is undesirable as it is likely to effect film properties such as gloss. Furthermore, powder coatings tend to be sensitive to small quantities of additives and even very low levels of certain additives may lead to imperfections in film appearance. A process of agglomeration without the addition of binder is theoretically possible by heating the powder such that the particle surfaces become tacky. However, in practice, this process would not be viable as control of surface melting would be too difficult to regulate. Furthermore, as discussed above any melting process is potentially disastrous for thermoset powders. Processes involving moulding presses are a further possibility for size enlargement of fine particles. However, for powder coatings, particularly thermoset powder coatings heat from these processes could result in a premature of the crosslinking reaction.

It is an object of this invention to provide a process of recycling fine powder particles. SUMMARY OF INVENTION

Accordingly this invention provides in one form a process for recycling powder coating composition comprising pressure compacting the powder coating into friable tablets and combining these tablets with an initial blend of polymer in an extrusion powder coating process wherein the original particles can still be substantially identified in the tablets.

The term friable is used to mean the tablets are easily fractured and in practise it is usual for the tablets to be able to be broken by ordinary finger pressure.

Preferably the pressure compaction is effected using a tabletting process.

Preferably the tablets are 5-25 mm in diameter.

Preferably the powder is thermosettable.

Best results are achieved with tablets of size about 25 mm. As the diameter of the tablets is reduced it becomes more difficult to introduce the fine powder satisfactorily and then manufacture the tablets. Furthermore, the process is more expensive with smaller tablets as the capacity of equipment is usually less. Similarly, if larger tablets are prepared much higher forces are required to be used to achieve the same level of compaction. Typically, the tablets are 5-10 mm in thickness but thinner tablets are acceptable though more expensive to manufacture. The shape of the tablets is unimportant to the working of the present invention. However, machinery designed to manufacture disc shaped tablets is more readily available and cost effective. Pressures used to achieve the desired level of compaction are preferably 1 to 1.5 tonnes per 500 mm² which calculates at 330 - 500 kilograms/cm². If the pressure used is substantially less than 300 kilo/cm² excessive dust in the handling of the tablets can arise. If the pressure used is too great localised heating may arise leading to unacceptable results. The simplest tabletting equipment involves the filling of a cylindrical die with powder and compaction of the powder with moveable pistons. Other tabletting means as for example having opposed roller surfaces with mating recesses are feasible but in practice are not as efficient as conventional tabletting equipment such as the Manesty tabletting machine. Furthermore, as powder coatings contain extenders which may be considerably more abrasive than the excipients commonly used for pharmaceutical or veterinary tablets more hard wearing parts of tabletting machines may be required. In a preferred form the process includes a variable speed screw feed arrangement whereby a constant feed rate of powder to the tabletting machine may be conveniently achieved. The invention will be further described by reference to a preferred embodiment described in the following example were parts are parts by weight.

EXAMPLE

This example describes the preparation of tablets according to the present invention.

Epoxy powder coating composition

Epoxy resin (Bisphenol A) and hardener (amine based) 58%

Extender (Barytes) 27%

Pigment (titanium dioxide) 15% Fine particle size powder of the above composition and with particle size 2-15 μm was introduced into a Manesty tabletting machine. A pressure of 400 kg/cm² was applied and disc shaped tablets of 20 mm in diameter and 10 mm in thickness were obtained. These tablets were essentially non dusting and found to be suitable for reprocessing when reblended with starting materials in a new batch of epoxy powder coating. This machine was operated at a rate of 800 kg/hour. Powder coating product prepared using tablets according to the present invention had application and film characteristics indistinguishable from that of a standard batch of powder coatings when the tablets were blended 5% with standard material before the high speed dispersion first stage. It will be appreciated that as well as using the fine particle waste from the grinding process in preparing powder, recovered powder from overspray of the application process can also be used. Provided only small amounts of powder, 5- 10%, are recycled according to the process of this invention colour segregation will generally be unnecessary, provided light colours are used for light colour batches. However, care needs to be exercised so that incompatible polymer types are not mixed.

A further advantage of the present recycling process is that as it provides a convenient cost effective method of recycling the manufactured wastes it enables narrower particle size product to be produced. Without the availability of effective recycling processes the particle size range of powders used in practise is a compromise including minimising the amount of waste from undersized particles consistent with achieving a satisfactory throughput and manufactured cost. This compromise may lead to paint properties which whilst generally satisfactory are not optimum. However, by using the present process as a convenient method for recycling particles it is possible to narrow the particle size specification of product thereby enabling improved paint properties to be achieved.

Claims

The claims of the invention are as follows:

1. A process for recycling powder coating composition comprising pressure compacting the powder coating into friable tablets and combing these tablets with an initial blend including film forming polymer in an extrusion powder coating process wherein the original particles in the tablets can still be substantially identified.

2. A process according to Claim 1 wherein the pressure compaction is effected using a tabletting process.

3. A process according to either Claim 1 or 2 wherein the tabletting pressure is in the range of 300 - 500 kilograms/cm².

4. A process according to any one of Claims 1 to 3 wherein the particle size of the recycled powder particles is less than 20μm.

5. A process according to any one of Claims 1 to 4 wherein the tablets are 20 - 30 mm in diameter.

6. A process according to any one of Claims 1 to 5 wherein the pressure compaction is effected in a Manesty tabletting machine.

7. A process according to any one of Claims 2 to 6 which process further uses a variable speed screw apparatus for providing a constant feed rate of recycled powder to a reservoir.

8. A process according to any one of Claims 1 to 7 wherein the powder is a thermosettable powder.

9. Powder coating prepared by a process as described in any one of Claims 1 to 7.