The present invention is directed at a method for improving the adherence of a higher surface energy secondary coating to a lower surface energy coated substrate material to form a coated article. The present invention is ideally suited for improving the adherence of a polymeric bed liner coating to a low surface energy top coated truck bed substrate material, as will be described and illustrated below in
The method according to a preferred embodiment of the present invention comprises the steps of providing a coated substrate material having a low surface energy, applying a corona discharge or plasma treatment to the visible surface of the coated substrate material for a predetermined amount of time sufficient to increase the surface energy of the coated substrate material, and applying a liquid secondary coating onto the treated higher surface energy coated substrate material, wherein the liquid is cured or otherwise dried to form a adhered secondary coating on the treated substrate material. The surface energy of outermost primary coating layer of the coated substrate material, in the absence of the discharge treatment, is lower than the surface energy of the to-be applied secondary coating. The discharge treatment raises the surface energy of the outer surface of the outermost layer of the primary coating layer to a level higher than that of the subsequently applied secondary coating to increase the wettability, and hence the adherence, of the secondary coating to the outermost layer of the primary coating layers of the treated substrate material.
As the present invention will describe, the length of time for applying the discharge treatment to the outermost layer of the primary coating layers of the treated substrate material must be precisely controlled. If the discharge treatment is too short, the surface energy of the low energy outermost layer of the primary coating layers of the treated substrate material will not be raised to a level sufficient to maximize the adherence of the secondary coating to the outermost layer of the primary coating layers of the treated substrate material. If the discharge treatment is too long, the adherence of the secondary coating to the topmost layer of the primary coating will be substantially greater than the adherence of between any two of the layers of the primary coating material or between the bottommost layer of the primary coating material and the bare substrate material, which will result in cohesive failure occurring between the primary coating layers or occurring between the bottommost primary coating layer and the bare substrate material.
The bare substrate material may consist of a naturally occurring or synthetically-produced material, including for example a plastic material or the like, that have a relatively low surface energy and are typically non-polar in nature. It is to be understood that the substrate may comprise any suitable substrate, including, but not limited to, metal stampings, carbon graphite composites, fiberglass, polycarbonates, ABS (“acrylonitrile-butadiene-styrene”), and any other structural polymeric materials. The bare substrate material is then coated with one or more layers of a primary coating material that produces a low surface energy finish coated substrate, or treated substrate material. Examples of primary coating materials that may be used to form the primary coating layers include air-dried or temperature cured one-component or two-component polymeric coating materials.
The term “higher surface energy secondary coating” refers to a secondary coating material that is applied to the outermost primary coating layer of the lower energy coated substrate material. The surface energy of the secondary coating is measured relative to that of the surface energy of the outermost primary coating layer. When the secondary coating has a higher surface energy than the outermost primary coating layer to which it is applied, the secondary coating will tend to bead, like water on a waxed surface, rather than wet out on the outermost primary coating layer. Conversely, when the secondary coating has a lower surface energy than the outermost primary coating layer, the secondary coating will tend to wet onto the outermost primary coating layer more easily, which those of ordinary skill in the art have generally recognized as improving adhesion between the secondary coating and the outermost primary coating layer.
It is to be understood that the application step of applying the secondary coating material to the outermost primary coating layer may comprise any suitable application means such as, for example, spraying, dipping, brushing, and it may be desired to utilize suitable hoods, ventilation means, and/or standard paint style spray booths. It is to be further understood that any of the application means may be performed manually and/or automatically and/or robotically. Finally, the polymeric secondary material is then dried or cured to form a film of a desired thickness that adheres to the underlying outermost primary coating layer to form the coated article.
The corona discharge treatments described in the present application are conventional in nature and provided from conventional corona treating systems. All corona discharge treating systems have two components. The first component is the power source and the second component is the treater station. The power source generally consists of a high frequency generator and a high voltage output transformer. In very general terms, the purpose of the power source is to raise the incoming electricity (typically 50/60 Hz, 230/460 V) to a higher frequency (10-35 kHz) and higher voltage (10 kV). The power source is commonly referred to as a power supply or a generator. Typically, power supplies are rated in kilowatts (kW) and can range from 500 W to 30 kW, depending on the application. The treater stations have a high voltage electrode and a ground electrode. A solid dielectric (insulating) material is needed to cover one of the two electrodes in order to generate a corona atmosphere, as opposed to a “lightening bolt” charge. In general terms, treater stations are broadly classified as either covered rolls or bare roll treater stations. Covered roll stations have the dielectric covering on the ground roll and the high voltage electrode is bare metal. Bare roll stations have the dielectric covering on the high voltage electrode and the ground electrode is bare metal.
Non-limiting examples of corona surface treater apparatus that may be used in treating the low surface energy substrate materials of the present invention include Corona Treater Model Nos. BD-20AC and BD-80, available from Electro-Technic Products, Inc. of Chicago, Ill.
Alternatively, a plasma discharge treatment may also be utilized. Like corona, plasma is the electrical ionization of a gas, but with plasma the gas is selected dependent on the material being treated and the application being performed. The plasma (glow) discharge creates a smooth, undifferentiated cloud of ionized gas with no visible electrical filaments. Unlike corona, plasma is created at much lower voltage levels and temperature.
One application that is ideally suited that utilizes the method of the present invention to improve the adherence of a two-component polymeric bed liner material 50 (i.e. the secondary coating) to a truck bed substrate 22 (coated or treated substrate material) for automobile truck bed applications. These truck bed liners 15 provide an additional protective liner material to the inside of the truck bed that allows the user's to transport and store materials while minimizing damage to the underlying substrate material that can result in corrosive damage over time.
An exemplary coated article, here a truck bed liner 15, is formed in accordance with the prior art as shown in
As further shown in
The two-component bed liner material 50 and dust coating material 52 are formed of a two component, VOC free pure polyurea elastomeric coating based on amine terminated polyether resins, amine chain extenders and aliphatic isocyanate prepolymers that are processed thru high-pressure plural component processing equipment. Collectively, the bed liner material 50 and dust coating material 52 may alternatively be referred to as the secondary coating 54, which, in their liquid state, have a higher surface energy than the untreated upper surface 44 of the topcoat layer 42. The bed liner material 50 and dust coating 52 have a short pot life, and substantially cure within seconds of application onto the topcoat layer 42.
Subsequent testing of the peel strength of the truck bed liner 15 after application of the secondary coating 54 to the primary coating layers 26 has revealed significant issues regarding the adherence of the bed liner material 50 to the top coated surface 42. Specifically, in 90-degree peel tests, secondary coating 54 is easily peeled away from the top coated layer 42 with a minimum of pressure. Applicants believe that two factors, both related to the wettability of the bed liner material 50 onto the upper surface 44 of the top coated surface 42, contributed to this phenomenon. First, the low surface energy of the upper surface 44 relative to the higher surface energy of the liquid bed liner material 50 minimizes the amount of wetting of the bed liner material 50 on the upper surface 44 prior to the bed liner material curing. Second, the extremely fast cure rate of the bed liner material 50 also contributes to the minimal amount of wetting of the bed liner material 50 on the upper surface 44 prior to complete cure.
The present invention, as shown in
As experiments below will illustrate, the length of time of the discharge treatment of the upper surface 44 of the top coated layer 42 is limited to a finite window of time in order to maximize the adherence between the secondary coating 54 and top coated layer 42, between all of the primary coating layers 26 relative to one another, and between the bottommost layer of the primary coating layers 26 (here the electrodeposition coating layer 34) and the bare substrate material 24. If the discharge treatment is too short, the surface energy of the upper surface 44 of the top coated material 42 will not be raised to a level sufficient to maximize the adherence of the bed liner material surface energy 50 to the top coated material 42. If the discharge treatment is too long, cohesive failure may occur between any two primary coating layers 26 or between the bottommost layer, here the electrodeposition coating layer 34, of the polymeric coating material 26 and the bare substrate material 24.
In Step 100, the primary coating layers 26 comprising the electrodeposition coating layer 34, primer layer 36, and topcoat layers 42 are applied by conventional means under conditions well known to those of ordinary skill in the automotive coatings industry and form no part in the inventive aspect of the present invention.
Next, in Step 110, the topcoat layer 42, after application, is cured or otherwise dried. The upper surface of the cured or dried topcoat layer 42 is characterized as a low surface energy layer 44.
In Step 120, a corona discharge or plasma discharge is applied to the upper surface 44 of the top coat layer 42 for a period of time sufficient to raise the surface energy to a high surface energy upper surface 45 that is above the surface energy of the yet to be applied secondary coating 54. The length of time is limited to a period of time sufficient to increase the surface energy of the upper surface 44 above that of the bedliner material 50 yet insufficient to otherwise cause cohesive failure between layers 26 or between the electrodeposition layer 34 and the bare substrate material 24. Experimentation as to the amount of time necessary to achieve this desired result is application dependent, as one of ordinary skill recognizes, depending upon the composition and surface energy of the material comprising the top coat layer 42 and further upon the surface energy of the yet to be applied the bed liner material 50, and further yet upon the surface energy of each of the respective layers 26 and bare substrate material 24,
For a bed liner material 50 applied to a top coated layer 42 as described in
in Step 130, the application of the bed liner material 50 and dust coating 52 are applied to the higher energy upper surface 45 of the top coat layer 42 by conventional spraying techniques and form no part of the inventive aspect of the present invention.
Experimentation, as discussed in Examples 1-4 below, has shown that a bed liner material 50 introduced to the topcoat layer 42 achieved significant improvement in adherence to the underlying topcoat layer 42 when treated with a corona discharge treatment for a predetermined limited amount of time. However, as the length of corona discharge treatment increased past a predetermined amount of time, the performance benefits gained were lost, as the panels experienced cohesive paint failure as described below.
A series of electrogalvanized steel panels were prepared and evaluated with coatings and bed liners designed to substantially match the exemplary secondary coating 54 coupled to a truck bed substrate 22 as illustrated in
For this experiment, a series of electogalvanized panels were first coated with a layer of Dupont Cormax 6 electrocoat (ACT Material Code Cormax 6 EP) applied using conventional electrodeposition techniques and thicknesses commonly found in automotive truck beds. The electrocoat was cured to the panels in an oven at standard baking conditions. Next, a layer of Dupont black primer (ACT Material Code 554DN082) was applied to electrocoated panels, again using conventional application techniques and at conventional film thicknesses Finally, a layer of Dupont Black Waterborne Basecoat (ACT Material Code 686DN027(S40343)) is applied to the black primer layer at conventional film thicknesses. A layer of Dupont Gen V Clearcoat (ACT Material Code RK8073 (RKA01199)) was applied wet-on-wet to the basecoat layer at conventional film thicknesses. A first portion of the top coated panels were cured in an oven at 285 degrees Fahrenheit for 17 minutes (standard bake), while the rest of the panels were cured in an oven at 325 degrees Fahrenheit for 17 minutes (overbake). The surface energy of top coated surface of each of the panels was measured using Dyne pens and confirmed to be around 6 dynes/cm.
Next, a portion of the panels (both standard and overbaked) were treated with an Electro-Technic Products Laboratory Corona Treater (BD-20AC) with an output voltage of between about 10,000 to 48,000 and a frequency of between about 4-5 MHz. This device provides continuous corona discharge for a minimal amount of time sufficient to raise the surface energy from an initial level of about 6 dynes/cm to a level at or exceeding 40 dynes/cm.
The other portion of the standard bake and overbaked panels remained untreated.
Finally, a layer of bed liner material and dustcoat was spray applied using standard spray equipment at between 140 and 165 degrees Fahrenheit, and more preferably 165 degrees Fahrenheit, to each of the panels at substantially uniform cured film thicknesses of between about 40 and 55 mils. Preferably, this is done at a mix pressure between 1800 and 2500 psi, and more preferably around 2000 psi, and at a coating thickness of about 15 mils per pass. The bed liner material consists of a two component, VOC free pure polyurea elastomeric coating based on amine terminated polyether resins, amine chain extenders and aliphatic isocyanate prepolymers. The bed liner material ambiently cures at the application temperature within a matter of minutes.
Each of the panels was then evaluated for peel adhesion using a 90-degree peel test. The 90-degree peel test is a test wherein a mandrel or similar device is utilized to try to peel the layers of a coating off a panel. Weights are added to the mandrel device to apply more force on the coating until the coating peels from the substrate. More weight, in terms of pli (pounds per linear inch), indicate better adhesion. This testing confirmed that peel adhesion between the bed liner layer and topcoat layer went from about 2 pli for panels not treated with the corona discharge to greater than 25 pli for panels treated with the corona discharge treatment. This confirmed that the corona discharge treatment improved adhesion between the bed liner layer and topcoat layer.
In this experiment, the panels were top coated and cured as described above in Experiment 1. Next, the panels (both standard and overbaked) were treated with an Electro-Technic Products Laboratory Corona Treater (BD-20AC) with an output voltage of between about 10,000 to 48,000 and a frequency of between about 4-5 MHz for a minimal amount of time sufficient to raise the surface energy from an initial level of about 6 dynes/cm to a level at or exceeding 40 dynes/cm.
Finally, a layer of bed liner material and dustcoat as described in Experiment 1 was applied to each of the panels at various film thicknesses, bed liner to dustcoat flash times, and booth temperature/humidities. A summary of the various experimental parameters is detailed in Table 1:
Each of the panels described in Table 1 was then evaluated for peel adhesion using a 90-degree peel test. This testing confirmed that peel adhesion between the bed liner layer and topcoat layer was greater than 25 pli for all of the panels. This experiment, in conjunction with Figure, helps to confirm that the corona discharge treatment improved adhesion between the bed liner and topcoat layer, regardless of the processing parameters of the subsequently applied bed liner material.
In this experiment, the panels were top coated and cured as described above in Experiment 1. Next, the panels (both standard and overbaked) were treated with an Electro-Technic Products Laboratory Corona Treater (BD-20AC) with an output voltage of between about 10,000 to 48,000 and a frequency of between about 4-5 MHz for a minimal amount of time sufficient to raise the surface energy from an initial level of about 6 dynes/cm to a level at or exceeding 40 dynes/cm.
Next, a bed liner coating having a varying ratio of the components (polyurea/amine to isocyanate) of the bed liner material was introduced to the treated top coated panels. Moreover, some panels were subjected to bed liner coating repair, wherein a portion of the bed liner coating was removed and resprayed to a consistent thickness Each of the panels was then evaluated for peel adhesion using a 90-degree peel test. The results were summarized in Tables II and III.
This testing confirmed that peel adhesion between the bed liner layer and topcoat layer was greater than 25 pli for all of the panels, regardless of the ratio of polyurea/amine to isocyanate in the coating. This experiment, in conjunction with Experiment 1, helps to confirm that the corona discharge treatment improved adhesion between the bed liner and topcoat layer, regardless of the processing parameters of the underlying layers, the ratio of components in the subsequently applied bed liner material, or repairs made post application of the bedliner.
A series of electrogalvanized steel panels were prepared and evaluated with coatings and bed liners as described above in Experiment 1, with the only exception being the amount of corona discharge applied to the upper surface of the topcoat prior to introduction of the bed liner material.
In this experiment, a portion of the panels was treated for 0, 1, 2, and 4 minutes, respectively, with the corona discharge coater. Next, each of the panels was then evaluated for peel adhesion using a 90-degree peel test. The results are shown in Table IV:
The results confirmed first that the surface energy improved and maintained higher surface energy after corona discharge treatment. The results also showed that peel adhesion for untreated panels was about 2 pli, and the failure occurred between the bed liner and top coated layer. Moreover, peel adhesion for panels discharge treated at 1 and 2 minutes exceeded 25 pli, with failure occurring between the bed liner material and the topcoat layer. Panels treated for 4 minutes showed a drop off in their peel strength, with cohesive failure occurring not between the bed liner and topcoat layer, but instead between the topcoated layer or primer layer and the electrocoat layer (i.e. cohesive failure of the paint layer).
Thus, Experiment 4 confirms that the length of time for applying the discharge treatment to the upper surface of the top coated layer must be precisely controlled. If the discharge treatment is too short, the surface energy of the low energy substrate material will not be raised to a level sufficient to maximize the adherence of the secondary coating to the top coated layer. If the discharge treatment is too long, the adherence of any underlying coating to the substrate (here, the bedliner, the topcoat, primer and/or electrocoated layer to the steel substrate) can be compromised.
While the present invention has been proven with respect to adhering a bed liner material to a topcoat for a truck bed application, it is anticipated that the present invention may be utilized to improve the adherence of any secondary coating to an untreated low energy substrate material (bare or coated). Thus, the present is not limited to its preferred application. While particular embodiments of the invention have been shown and described, numerous variations or alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
The present invention claims priority from U.S. Provision Application Ser. No. 60/806,577, filed Jul. 5, 2006, entitled “Method For Increasing Surface Energy of Low Energy Substrate Utilizing A Limited Length Corona Or Plasma Discharge Treatment To Improve Adherence Of A Subsequently Applied Secondary Coating Thereto,”
Number | Date | Country | |
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60806577 | Jul 2006 | US |