WO2001085360A2

WO2001085360A2 - Coating process

Info

Publication number: WO2001085360A2
Application number: PCT/GB2001/002038
Authority: WO
Inventors: Mark P. Kearns; Antonio Marcilla Gomis
Original assignee: The Queen's University Of Belfast
Priority date: 2000-05-11
Filing date: 2001-05-10
Publication date: 2001-11-15
Also published as: AU2001256473A1; GB0011284D0; WO2001085360A3

Abstract

A process for coating a surface comprising the steps of admixing a plastic polymer in the form of a powder with a pigment to form a paint precursor; melting the paint precursor to form a liquid paint; and applying said liquid paint to the surface to be coated, is described. In coating plastics, the plastic material could be a thermoplastic material. Preferably, the powdered plastic of the paint precursor is the same polymer as the plastic material to be coated.The present invention is directed towards providing a cheaper and more versatile process for coating various surfaces including plastic materials. Apparatus for the process is also described.

Description

Coating Process

The present invention relates to a process for coating a material and in particular to a process for coating a surface, particularly but not exclusively for coating a plastic material .

Normal practice when colouring or protecting a plastic material is to introduce the pigment when forming the plastic such that the pigment particles lodge themselves interstitially between the polymer molecules giving the plastic material the colour of the pigment throughout.

This has the obvious disadvantages that an extra step has to be introduced during the production of the plastic, only one colour can be applied, and the process is irreversible. A large amount of pigment is also required to colour the whole plastic as the pigment is included during its make-up rather than simply coated on its surface. Furthermore, the pigment added during the manufacture of the plastic will alter its physical properties such as strength, which may be undesirable .

The present invention is directed towards overcoming these problems and in particular to providing a cheaper and more versatile process for coating various surfaces including plastic materials.

According to the present invention there is provided a process for coating a surface comprising the steps of:-

admixing a plastic polymer in the form of a powder with a pigment to form a paint precursor;

melting the paint precursor to form a liquid paint; and

applying said liquid paint to the surface to be coated.

Typically, the powdered plastic has a small particle size which can be obtained by any conventional process, for example, mechanical or cryogenic grinding. Preferably, the powdered plastic has a particle size with a diameter of less than 0.01 mm. This will produce a high quality surface finishing paint. A larger particle size could be used where an intermediate surface finishing paint is required. Many suitable polymers are useable in the present invention, such as polyethylene, polypropylene, nylon, etc. The paint precursor to be melted can be a dry blend of the powdered plastic and pigment, or a ground masterbatch with these constituents. Preferably, the paint precursor contains powdered plastic and pigment in a ratio range of between 4:1 and 19:1 respectively. More preferably the powdered plastic and pigment is in a ratio range of between : 1 and 9:1. However, the ratio of the powdered plastic to pigment chosen depends on the effect desired such as, the thickness of the paint layer, the covering power of the pigment and its effect on the concentration of the liquid paint and the physical strength of the coating required. Different coatings ranging from translucent to opaque may be obtained by changing the concentration and/or covering power of the pigment.

Preferably, the constituents of the paint precursor are thoroughly mixed prior to melting. This promotes uniform properties and colour throughout the paint precursor and helps prevent the formation of agglomerates. The constituents of the paint precursor can be mixed by fluidising the precursor in a conventional fluidising column. Optionally, the constituents of the paint precursor may be blended by other systems of mixing, for example, high shear dry blending, shaking, or dispensing (by weight or volume) the constituents in an entraining stream. Preferably, the constituents of the paint precursor are blended prior to fluidising the precursor in the fluidising column. This helps further reduce the formation of agglomerates. Preferably, the size of the precursor particles is determined prior to delivery of the precursor to the fluidising column. This can be done, for example, in an entraining system, by conventional sieving or other systems of controlling and classifying particle size. This promotes a uniform precursor particle size throughout the fluidising column preventing the accumulation of larger coarser particles in the fluidising column and helps reduce the formation of agglomerates.

The paint precursor is then melted to form a dispersion of liquid paint droplets. Preferably, this is done in a heat exchanger using hot air as the heating medium. Optionally, the heat exchanger also acts as a spraying device using the hot air as a propellant gas to apply the liquid paint to the surface to be coated.

Optionally, the paint precursor is heated prior to melting, for example by warming the air stream to the fluidising column.

Generally, the pigment of the paint precursor may be of any type. Preferably the pigment is compatible with the powdered plastic of the paint precursor. The type of pigment may also be chosen to provide a liquid paint with special effects, for example metallic or pearl effects.

The process of the present invention can be used to paint any suitable surface to provide a temporary or permanent coating. Such surfaces include plastic, metal, glass, ceramic, etc. In one embodiment, the process can be used to coat metal such as the inside of a metal shell or mould, pre-moulding, to provide a coating attachable to the moulding material, e.g. as a colour scheme, to thicken an area of the moulding or to add any protective-like coating.

In coating plastics, the plastic material could be a thermoplastic material. Preferably, the powdered plastic of the paint precursor is the same polymer as the plastic material to be coated. Optionally, the powdered plastic of the paint precursor is a polymer which is compatible with the plastic material to be coated. . .

Preferably, the plastic material to be coated- is treated prior to being coated. Typically, , this includes heating the surface of the plastic material, preferably to a temperature which will melt the surface. This improves the adhesion of the liquid paint to the plastic material. In principal, any temperature below the melting point of the plastic material may be used, however, the limiting temperature and treatment time will be that which will assure no shape distortion of the plastic material. The treatment of the plastic material may further include flame, plasma, corona and/or chemical etching of the surface of the plastic material to be coated. These treatments of the surface of the plastic material to be coated are particularly useful when the powdered plastic of the paint precursor is not the same or is not compatible with the plastic material to be coated. The treatment of the plastic material prior to being coated improves the compatibility and adhesion of the liquid paint to the plastic material providing a more resistant coating.

Optionally, the liquid paint and surface such as a plastic material to be coated can be oppositely charged to improve the distribution of the liquid paint on the surface and promote its adherence thereto.

In another embodiment of the present invention, the present process is applied in a patterned manner, e.g. through a template or similar,, to provide a patterned coating.

In a further embodiment of the present invention, the coating is useable as an adhesive between the surface to which it is applied, and a second surface, e.g. two plastic parts or plates.

In a yet further embodiment of the present invention, the paint precursor includes one or more additives or filler materials such as a UN-adsorber, microwave, etc., glass microspheres, antibacterial agent, etc. Such substances are known in the art, and enhance the properties of the coating.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying Fig 1 which shows a schematic illustration of the process according to the present invention.

Initially, polyethylene (PE) polymer is ground into a powder with a particle size having a diameter of less than 0.01 mm. The powdered PE is then mixed in a feeder column 2 with a pigment in a ratio of 9:1 respectively. The mixture is subsequently shear dry blended to form a paint precursor 1.

In this embodiment, a polyethylene (PE) article 13 is to be coated and polyethylene (PE) was chosen to form the powdered plastic of the paint precursor 1 because of its compatibility therewith.

The paint precursor 1 is fed from the feeder column 2 through a sieve (not shown) to a fluidising column 3 by way of a paint precursor inlet pipe 4. The sieve controls and determines the precursor particle size fed to the fluidising column 3.

The fluidising column 3 further includes an air inlet pipe 5 at the base of the column 3, a perforated plate 6 positioned intermediate the base of the column 3 and the paint precursor inlet pipe 4 such that the outer perimeter of the plate 6 lies flush with the internal perimeter of the column 3, an air outlet pipe 7 at the top of the column 3, and a paint precursor delivery pipe 8. A system of valves (not shown) to control the flow of air and a filter is placed in the air outlet pipe 7. Air is fed through the pipe 5 and passes through the perforated plate 6, the paint precursor 1, and exits the outlet pipe 7. At a predetermined air- to-mass flow rate the paint precursor 1 in the column 3 takes on the properties of a fluid. This is commonly termed ^,λfluidisation" . In this way the pigment and powdered PE mix more thoroughly giving a paint precursor 1 with more uniform properties. Fluidising the paint precursor 1 also prevents the formation of agglomerates.

After the step of fluidisation, the paint precursor 1 is then delivered through the pipe 8 to a heater 9 where it comes into contact with a hot air stream passing through the heater 9 from a hot air inlet 10. This melts the precursor 1 to form a dispersion of liquid paint droplets 11. . .. : . . The liquid paint 11 so formed is sprayed from the outlet 12, using the hot air as a propulsion gas, onto the plastic article 13 to be coated.

The distance between the outlet 12 and the article 13 must be closely controlled to obtain optimum results. If the outlet 12 is held too far from the article 13 then the liquid paint 11 is exposed to the ambient air for too long, rapidly reducing the temperature of the liquid paint 11 and encouraging the onset of early solidification. If the outlet 12 is held too near the article 13, it may cause distortion of the article 13 due to the force and temperature of the liquid paint 11 being sprayed and the hot air.

The following table is a suitable set of parameters : -

Table 1

To aid proper coating of the article 13 , its surface temperature may be raised. This helps adhesion of the liquid paint 11 to the surface of the article 13.

It will be appreciated that any suitable gas such as nitrogen may be used instead of air during the fluidising and melting steps. This in particular would prevent fire risks which may be experienced during the melting of the paint precursor 1.

It will also be appreciated that the pigment and powdered PE may be control fed to the fluidising column 3 through different pipes to control, adjust or affect the paint precursor 1. It will further be appreciated that the air fed to the fluidising column may be pre-heated to increase the temperature of the paint precursor 1 prior to melting. Having the melting process as a two step process instead of one is more efficient.

The liquid paint 11 and the article 13 to be coated may also be oppositely charged to improve the distribution of the liquid paint 11 on the article 13 and promote its adherence thereto.

, The process of the present invention can be used to coat any plastic material. However, it is best if the powdered plastic of the paint precursor 1 is the - same polymer as the plastic material to be coated. Optionally, the powdered plastic of the paint precursor 1 is a polymer which is compatible with the plastic material to be coated.- ' • •

It will be appreciated that alternative systems and methods other than a fluidising column may be used to -mix and blend the constituents of the precursor 1. Other methods which may be employed include, for example, high shear dry blending, shaking, or dispensing (by weight or volume) the • constituents in an entraining stream. However, any system assuring an approximately constant flow which avoids the formation of agglomerates can be used.

Modifications may be made to the foregoing within the scope of the present invention.

Claims

1. A process for coating a surface comprising the steps of: -

melting the paint precursor to form a liquid paint; and

applying said liquid paint to the surface to be coated.

2. A process as claimed in Claim 1 wherein the , powdered plastic has a particle size with a diameter of less than 0.01mm.

3. A process as claimed in Claim 1 or Claim 2 wherein paint precursor contains powdered plastic and pigment in a ratio range of between 4:1 and 19:1 respectively.

.

4. A process as claimed in Claim 3 wherein the powdered plastic and pigment is in a ratio range of between 4:1 and 9:1.

5. A process as claimed in any one of the preceding Claims wherein the plastic polymer and pigment are mixed by fluidising.

6. A process as claimed in any one of the preceding Claims wherein the plastic polymer and pigment are blended prior to admixing to form the precursor.

7. A process as claimed in Claim 5 or Claim 6 wherein the size of the precursor particles is determined prior to fluidising.

8. A process as claimed in any one of the preceding Claims wherein the paint precursor is melted in a heat exchanger.

9. A process as claimed in Claim 8 wherein the heat exchanger also acts as a spraying device to apply the liquid paint to the surface to be coated.

10. A process as claimed in any one of the preceding Claims wherein the paint precursor is heated prior to melting.

11. A process as claimed in any one of the preceding Claims wherein the surface is a plastic material.

12. A process as claimed in Claim 11 wherein the plastic material is a thermoplastic material.

13. A process as claimed in Claim 11 or Claim 12 wherein the powdered plastic of the paint precursor is the same polymer as the plastic material to be coated.

14. A process as claimed in any one of the preceding Claims wherein the pigment is compatible with the powdered plastic of the paint precursor.

15. A process as claimed in any one of the preceding Claims wherein the surface to be coated is treated prior to being coated.

16. A process as claimed in Claim 15 wherein the surface is heated to a temperature which will melt the surface.

17. A process as claimed in Claim 15 or Claim 16 wherein the surface is flame, plasma, corona and/or chemical etched.

18. A process as claimed in any one of the preceding Claims wherein the liquid paint and surface to be coated are oppositely charged.

19. A process as claimed in any one of the preceding Claims wherein the plastic polymer is polyethylene, polypropylene or nylon.

20. A process as claimed in any one of the preceding Claims wherein the paint precursor includes one or more additives or filler materials.

21. A plastic material whenever coated by a process as claimed in any one of Claims 1-20.

22. Apparatus for coating a surface comprising means to admix a plastic polymer in the form of a powder with a pigment to form a paint precursor, means to melt the paint precursor to form a liquid paint, and means to apply said liquid paint to the surface to be coated.

23. Apparatus as claimed in Claim 22 wherein the means for admixing is a fluidising column.

24. Apparatus as claimed in Claim 22 or Claim 23 . wherein the means for melting is a heat exchanger.

25. Apparatus as claimed in any one of Claims 22 to 24 wherein the means for applying the liquid paint is a spraying device.

26. Apparatus as claimed in Claim 24 or Claim 25 wherein hot air is used.