AU2014203757A1 - Method for treating the interior surfaces of a plastic pail - Google Patents

Abstract

The present invention relates to a method for treating the interior surfaces and/or lid of a plastic pail for the purpose of reducing the incidence of paint contamination arising from the lack of adhesion of paint skins to those surfaces and lids. The method includes the step of exposing at least part of the surface to be treated to a plasma until the surface tension of the exposed part is at a level of at least 38 dynes/cm. (16 00 r0 [-76

Description

AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention title: Method for treating the interior surfaces of a plastic pail The following statement is a full description of this invention, including the best method of performing it known to us: 1 Method for treating the interior surfaces of a plastic pail Field of the invention [0001] The present invention relates generally to methods for treating the interior surfaces and lids of a plastic pail, and to plastic pails and lids having surfaces treated thereby. More particularly, the present invention relates to methods for treating the interior surfaces and lids of plastic pails for the purpose of reducing the incidence of paint contamination arising from the lack of adhesion of paint skins to those surfaces and lids. Background of the invention [0002] In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date: part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned. [0003] Paint stored in unopened pails has been observed to dry and form into slivers known as 'skins'. When paint is stored in metallic pails, paint skinning is a relatively harmless phenomenon, due to a tendency of the skins to adhere to the interior surfaces of the pail and underside of the lid. The tendency of paint skins to adhere to metallic pails arises from the inherent properties of the metallic pail itself, as well as from the presence of coatings that are often found on the interior surfaces and lids of metallic pails. Being adhered to the interior surface and lid, the skins do not present a contamination risk to the paint stored in the pail. [0004] However, plastic paint pails (made primarily from polypropylene and high density polyethylene) are now commonplace, due to their lower cost of manufacture and beneficial physical properties such as rust and dent-resistance. Unlike metallic surfaces, paint skins do not have a natural tendency to adhere to plastic surfaces. Therefore, once formed in the paint, the skins are at risk of falling into and contaminating the paint stored in the pail. [0005] The two broad approaches to addressing the problem of paint skinning in plastic pails are coatings and specialised polymer formulations. One approach to coating 2 involves spraying the interior surfaces and lid of the pail with a solution in order to promote paint skin adhesion. However, such an approach is far from ideal, due to the increased costs of manufacture associated with the incorporation of the solution into the manufacturing process. [0006] Another approach, also involving coating, is described in Australian patent no. 760094, which teaches applying a 5 micron thick, two part epoxy resin-based coating to the interior surfaces and lid of a polypropylene pail, again with the aim of promoting paint-skin adhesion. Although the patent reports encouraging results in terms of paint-skin adhesion, the approach suffers the same disadvantage as solution spraying in terms of increased costs of manufacture. [0007] Australian patent specification nos. 2011254082 and 2006326859 describe the use of halogenated polymers as anti-skinning surface coatings. The described coatings have a surface tension of at least 38 dynes/cm, which equates to the coating having paint skin-adhesion properties. [0008] The alternative approach to the paint skinning problem (i.e specialised formulations) is described in Australian patent specification no. 2008201253. The formulation is a polymer composition including at least one polyolefin and a humectant. The humectant is described as imparting a relatively higher surface energy and/or increased wettability to the formulation, which in turn provides the surface of the formulation with paint skin-adhering properties. [0009] The present invention aims to provide an alternative approach to the problem of paint-skin contamination in plastic pails that does not involve surface coatings or specialised polymer formulations. Summary of the invention [0010] According to a first aspect of the present invention there is provided a method for treating the interior surfaces and/or lid of a plastic pail, the method including exposing at least part of the surface to be treated to a plasma until the surface tension of the exposed part is at least 38 dynes/cm. [0011] An advantage of the present invention is that it allows the manufacture of plastic pails (including polypropylene pails) that do not unduly suffer from paint-skin contamination, but do not require the incorporation of surface coatings. The present 3 invention also realises cost savings from the elimination of a consumable (in the form of surface coatings) from the plastic pail manufacturing process. [0012] Plasma treatment of the interior surfaces of the pail and/or lid to raise the surface energy to a level above 38 dynes/cm has been found to impart the exposed surface with paint skin-adhering properties. Although plasma treatment of the outside surfaces of flexible plastics (for example polyethylene bread bags) is known to facilitate the adhesion of printing inks to the plastic's surface, the present invention involves treating the inside surfaces of rigid plastics so as to increase the surface tension to a point that allows the adhesion of a paint skin and subsequent skin. [0013] Preferably, the plasma is applied to the exposed surface at approximately or exactly atmospheric pressure. [0014] Typically, the surface is exposed to the plasma for a period of approximately five seconds or less. The presently preferred treatment time is between 4 and 4.5 seconds. [0015] Optimally, the surface tension of the exposed part is initially raised to around 60 to 70 dynes/cm. [0016] According to another aspect of the present invention there is provided a method for packaging paint, comprising the steps of: obtaining a plastic pail and/or plastic lid; exposing the interior surfaces of the pail and/or the lid to a plasma until the surface tension of the exposed parts is at a level of at least 38 dynes/cm; and introducing paint into the pail. [0017] According to another aspect of the present invention there is provided a plastic pail and/or lid that has undergone the treatment method according to the first aspect of the invention.

4 Brief description of the drawings [0018] The invention will now be further explained and illustrated by reference to the accompanying drawing which is a schematic diagram of the treatment method according to a preferred embodiment of the present invention. Detailed description of the drawings [0019] As known to those skilled in the art, plasma can be described as an ionized gas, or as an electrically neutral medium of positive and negative particles. In this context, 'ionised' includes the presence of free electrons which are not bound to an atom or molecule but are free to travel with their nucleus. Plasma is considered to be the fourth state of matter in addition to solid, liquid and gas. [0020] The present invention involves utilising plasma at atmospheric pressure (also known as 'open air plasma') to modify the surface tension of the interior surfaces of a polypropylene pail and lid. As described in greater detail below, a beam of plasma is directed at the surfaces of the pail and lid in order to effect the modification (ie. increase) of the surface tension. [0021] Referring to Figure 1, a plasma generation system 10 is diagrammatically illustrated. As would be understood by those skilled in the art, one mode of plasma generation involves passing purified air received from a source 11 through an air line 14. A high voltage generator 12 is located adjacent to air line 14 and is attached to a plasma generating unit 18 by way of high tension cabling 16. Air line 14 is also fluidly connected to plasma generating unit 18. [0022] Plasma generating unit 18 also includes an interior arc (not shown), a flange 15 and a pair of circumferentially placed and diametrically opposed nozzle jets 20 that project outwardly from flange 15. [0023] Purified air molecules that arrive at generating unit 18 from air line 14 are ionized into a plasma stream. The plasma stream is generated at exactly (or very close to exactly) atmospheric pressure. This allows for a larger overall plasma density in comparison with other processes such as the corona process, flame impingement (both also sometimes known as 'burning') or low-pressure plasma. In addition, atmospheric pressure plasma enhances the rate and degree to which the plasma-energised molecules are incorporated onto a material's surface.

5 [0024] The present invention is not limited to any particular means of generating a plasma stream. [0025] The plasma stream at atmospheric pressure exits plasma generation unit 18 through nozzle jets 20. Nozzle jets 20 each include one or more controllable orifices (not shown) through which the plasma stream is delivered. The volume of plasma delivered through the orifices can be suitably controlled in a manner that is known to those skilled in the art, such as by adjusting the voltage across the high voltage arc and other fine tuning. [0026] The discharge section of the plasma generating unit 18 includes a movable shaft 19 for adjusting the vertical position of nozzle jets 20. Nozzle jets 20 are also movable in radially inwards and outwards directions from flange 15. This allows the horizontal positioning of each nozzle jet 20 to be adjusted. In this way, both the horizontal and vertical distances between nozzle jets 20 and the surface being treated can be easily and precisely controlled. [0027] In addition, each nozzle jet 20 is rotatable about a vertical axis passing through the centre of shaft 19. [0028] Polypropylene pails 22 with a generally circular cross section that are suitable for packaging paint, are located on a conveyer 24. Each pail is first directed to a location underneath plasma generation unit 18. Flange 15 is then lowered, or pail 24 is raised, so that nozzle jets 20 are in position in the interior of pail 22. Nozzle jets 20 can be suitably extended or retracted if necessary. When in position in this way, nozzle jets 20 are located approximately 5 mm to 10 mm from the interior surface of the pail. [0029] The nozzle orifices are immediately opened and the nozzle jets 20 are rotated at an angular velocity of approximately 100 rpm to 150 rpm. During rotation of flange 15, the plasma stream exits the orifices and contacts the interior surfaces of pail 22. Plasma is thus directed onto the inner surface of pail 22 in this manner for approximately 5 seconds. [0030] When the plasma stream comes into contact with the polypropylene surface, energy is transferred from the plasma onto the surface to effect an increase in the material's surface tension. To impart suitable paint skin-adhering properties to the polypropylene surface, the surface tension must be raised from a starting point of around 29 dynes/cm to at least 38 dynes/cm, and optimally to around 60 to 70 dynes/cm. A surface tension of 60 to 70 dynes/cm for polypropylene can be achieved with five seconds or less of treatment with atmospheric pressure plasma.

6 [0031] The preferred surface tension for the polypropylene surface is approximately 60 to 65 dynes/cm. The achieved surface tension is to some extent dictated by the requirement of prolonging the benefit of the treatment (i.e paint skin-adhering properties) for a desired period of time. In this regard, it is observed that the surface tension of treated surfaces can be expected to drop around 10% in the first 24 hours after treatment and then to decay more slowly over the subsequent weeks and months. An initial treated surface with a surface tension of 60 dyne/cm is expected to maintain a surface tension of around 45 to 50 dynes/cm at eight months after treatment, which equates to a surface with paint-skin adhering properties. [0032] Subsequent to and immediately after plasma treatment, the plasma stream is turned off and flange 15 is raised out of the interior of pail 22. Treated pails are delivered by the conveyer 24 to an outlet chute (not shown), whereupon the next pail is moved into position underneath generation unit 18, ready for treatment. [0033] The underside surface of lids (not shown) can be treated with a plasma stream in a similar manner as described above in relation to pails 22. The surface tension of the lids is raised to the same level as that of the pails, in order to impart the same paint skin-adhering properties to the inside surface of the lid. [0034] The treatment method described above an be integrated into known processes for packaging paint. For example, conveyor 24 can be connected to an existing paint filling process. In this way, treated paint pails can be delivered to a paint filling and lidding station to complete the packaging operation. [0035] The word 'comprising' and forms of the word 'comprising' as used in this description do not limit the invention claimed to exclude any variants or additions. [0036] Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.

Claims (11)

1. A method for treating the interior surfaces and/or lid of a plastic pail, the method including the step of exposing at least part of the surface to be treated to a plasma until the surface tension of the exposed part is at a level of at least 38 dynes/cm.

2. A method according to claim 1, wherein the plasma is applied to the exposed surface at approximately or exactly atmospheric pressure.

3. A method according to claim 1 or claim 2, wherein the surface is exposed to the plasma for a period of approximately five seconds or less.

4. A method according to any one of claims I to 3, wherein the surface is exposed to the plasma for a period of between 4 and 4.5 seconds.

5. A method according to any one of claims 1 to 4, wherein the surface tension of the exposed part is initially raised to around 60 to 70 dynes/cm.

6 A method for packaging paint, the method comprising the steps of: obtaining a plastic pail and/or plastic lid; exposing the interior surfaces of the pail and/or the lid to a plasma until the surface tension of the exposed parts is at a level of at least 38 dynes/cm; and introducing paint into the pail.

7 A method according to claim 6, wherein the plasma is applied to the exposed surface at approximately or exactly atmospheric pressure.

8. A method according to claim 6 or claim 7, wherein the surface is exposed to the plasma for a period of approximately five seconds or less.

9. A method according to any one of claims 6 to 8, wherein the surface is exposed to the plasma for a period of between 4 and 4.5 seconds

10. A method according to any one of claims 6 to 9, wherein the surface tension of the exposed part is initially raised to around 60 to 70 dynes/cm. 8

11. A plastic pail and/or lid that has undergone the treatment method according to any one of claims I to 5.