WO1990002613A1

WO1990002613A1 - A method of forming a film for paint

Info

Publication number: WO1990002613A1
Application number: PCT/AU1989/000373
Authority: WO
Inventors: Anthony Michael Cooke; Walter Franciscus Mannot
Original assignee: James Hardie and Coy Pty Ltd
Current assignee: James Hardie and Coy Pty Ltd
Priority date: 1988-09-05
Filing date: 1989-09-05
Publication date: 1990-03-22
Anticipated expiration: 1991-03-05

Abstract

A method of forming a film of a water containing paint on a temperature sensitive substrate. A film of paint is applied to the surface of the substrate and subsequently the surface is irradiated with microwave radiation to heat the film of paint without overheating the substrate. In a preferred form at least the surface of the substrate is dried to a moisture content of less than about 7 % prior to application of the paint film.

Description

"A METHOD OF FORMING A FILM FOR PAINT" TECHNICAL FIELD

This invention relates to a method of forming a film of paint on temperature sensitive substrates. More particularly the invention provides for the curing of paint systems which hitherto have required heating processes in which the temperature of the substrates to which the paint is applied would be unacceptably elevated. The invention has application to all paints containing water and has particular application to zinc silicate paint systems and emulsion paint systems.

BACKGROUND ART

Zinc silicate paints based on sodium silicate have been used for many years to produce durable decorative coatings on a variety of substrates. When used on compressed asbestos cement substrates these paints provide durable coatings able to withstand external exposure for thirty years or more without significant change in appearance. Moreover, zinc silicate paint systems are quite inexpensive compared to other paint systems capable of giving similar performance.

However, it has been hitherto not being possible to apply zinc silicate paints to the cellulose reinforced cement sheets that have replaced asbestos cement. This is because the method of curing zinc silicate paints required the heating of the painted article to about 350°F for approximately 20 minutes. It has been found that cellulose reinforced cement products cannot withstand this heating.

Emulsion paints are commonly cured by air drying or by accelerated drying with radiant heat or hot air. These paints consist of an emulsion of resin particles, pigment particles, surface active agents and other components in water. The paints lose their water when drying and the resin and pigment particles come together to form a film. The properties of the cured paint film are influenced primarily by the molecular weight of the resins in the film and their chemistry. In general films of high molecular weight will be harder and stronger than those of low molecular weight. Each particular resin or paint system has a minimum film forming temperature (MFT) below which film formation will not take place. Film formation, or coalescence as it is known, may be assisted by the incorporation of high molecular weight solvents into the paint formation which have the effect of reducing the MFT. This enables the film forming ability of the paint to be altered to suit the conditions under which the paint film is to be applied. Use of coalescent solvents is more usual for resins of high molecular weight however it can have the effect of reducing the hardness of the final film if the coalescent solvent remains in the film after curing. Slow loss of this solvent usually occurs over a period of time after the painted article has been exposed to the atmosphere and the paint film is said to have cured out. Painted products which have not been exposed to the atmosphere may retain the coalescent solvents for an extended period of time and the paint coatings may remain soft. In the case of stacks of flat painted sheets the coating may remain susceptible to damage due to the abrasion of the sheets one against the other.

If the paints have been dried in air where water is removed by natural evaporation and the film formed spontaneously such films must have an MFT less than the atmospheric temperature. To obtain a hard coating it is usually necessary to incorporate large amounts of coalescent solvent into such a coating. It is possible to obtain a similar result using a hard resin with a high MFT if the curing can be achieved at an elevated temperature. These resins can be fully cured at the elevated temperature and after cooling they are fully hardened thus making the painted article serviceable immediately. As with zinc silicate paint systems however, the disadvantage of curing at an elevated temperature is that the entire article to be painted must be heated to the required curing temperature. This is often not possible due to the temperature sensitive nature of some substrates. Additionally, after curing of the paint cooling can be required which results in a substantial waste of energy.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a method of forming a film of a water containing paint on a temperature sensitive substrate which will overcome, or at least ameliorate, one of the foregoing disadvantages.

Accordingly, this invention consists in a method of form «ing a film of a water containing paint on a temperature sensitive substrate comprising the steps of applying a film of paint to the surface of the substrate and subsequently irradiating the surface with microwave radiation to heat the film of paint without overheating of the substrate.

As used herein the term "temperature sensitive substrate" is intended to refer to substrates of the kind unable to withstand the stoving conditions normally associated with the curing of zinc silicate paint systems and emulsion paint systems, typically above about 120°C. It is estimated that in the method according to this invention the bulk temperature of the substrate, that is the temperature of the substantial portion of the substrate, does not exceed about 120°C.

Preferably, the method further comprises the step of drying at least the surface of the substrate to a moisture content less than about 7% prior to the application of the paint film. This drying is preferably achieved by the application of microwave radiation. It is also preferred that the method comprises the further step of heating the surface of the substrate to a temperature of about 60°C prior to the application of the paint film. It is believed that there is an additional advantage associated with initial drying because when the paint contacts the surface a thin cured layer is initially formed on the surface of the substrate by the migration of moisture into the drier surface of the sheet material. It is further believed that this process may assist in preventing the subsequent bubbling of the paint during application of the microwave radiation.

It will be apparent that in the case of cellulose reinforced cement sheet the substrate has a moisture level of around 18% when it is manufactured. This can reduce to as much as 6-7% in equilibrium during storage depending upon the ambient conditions. The degree of drying required in the case of cellulose reinforced cement sheets can thus vary.

It is preferred that the applied energy density during irradiation of the painted surface is from 2.5 to

2

6.6 J/mm .

The method of this invention has been found to provide for the rapid curing of both the zinc silicate paints and emulsion paint systems on temperature sensitive substrates that would be damaged by the stoving conditions normally required for curing. It is believed that this is achieved because the paint films typically contain considerably more water than the substrate and are therefore heated more efficiently by the microwave energy. It is also possible that the chemical reactiojns involved during curing are favourably affected by the microwave irradiation. It will be apparent that the method of this invention also has the advantage that the paint films can be cured very rapidly in an on-line apparatus incorporating a microwave generator that necessity having large expensive drying ovens.

BEST MODES FOR CARRY OUT THE INVENTION

The following examples serve, by way of example only, to further illustrate the invention. Example 1

Zinc silicate paint of the formulation given in Table 1 was mixed and ground for 3 minutes in a Waring Blender until its fineness of grind was measured to 6 on a Heguan gauge. The viscosity of the formulations was then adjusted to 0.4 poise and it was allowed to stand for 60 minutes. TABLE 1; Gun Metal Zinc Silicate Paint

Zinc Oxide 100 Titanium Dioxide 40

Iron Oxide (Marigold Yellow) 0.3

Iron Oxide (Yellow) 2.0

Iron Oxide (Black) 0.34

Ground Silica 15

Sodium Silicate Solution 300

Water 150

The formulation was applied to compressed cellulose reinforced sheet preheated to 60°C. It was cured for 12 minutes in a microwave oven of 680W power input at 2450MHz fitted with a rotating table. Satisfactory performance with regard to appearance was achieved, however the boil resistance was barely satisfactory and the acid extraction result was not satisfactory. Example 2

The formulation of Example 1 was modified with up to 10% by weight of a 10% solution of ZnSO 4 in water to the weight of sodium silicate solution. It was found that above 4% by weight of ZnSO solution, the paint gelled prematurely. However, below this amount, the paint applied and cured as in Example 1 had a boil resistance which was adequate and an extraction resistance which was improved but not satisfactory. Example 3

A formulation was prepared and cured as described in Example 2 but catalysed by 5% cadmium sulphate as a 10% solution in water. Satisfactory boil resistance was achieved with a satisfactory extraction resistance. Sam le 4

The formulation of Example 1 was prepared, applied and cured with the addition of an ion exchanging calcium bentonite. Extraction resistance was unaltered but boil resistance was significantly improved. Example 6 Continuously Cured Paints

Fibre-cement sheets were predried to a surface moisture content of less than about 7% in a microwave oven having at a power level of 20 Kw at a frequency of

2450 MHz. The oven included a belt conveyor to transport the sheets through the microwave radiation. Zinc silicate paint of the formulation given in Table 1 was applied at an approximate dry film thickness of 40μm and the sheets were cured by exposing them to various levels of power in the same continuous microwave oven.

Table 1:Formulation

Zinc Oxide (white seal) 30

Zinc Oxide (submicron) 70

Titanium Dioxide 20

Black Iron Oxide 22

Sodium Silicate 300 Cadmium Sulphate (10% w/w) 15

Water 85 The resulting painted product was tested for adequacy of cure by means of alkali extraction, boil resistance and stack breakdown tests with the results given in Table 2.

Table 2:Results of Testing

Microwave Extraction Boil Mark Stack Breakdown Substrate Energy Value-HCl Rating Rating Condition J/mm² ml .01N (1-4) (1-5)

5.3 2.1 4 3 Satisfactory

9.7 1.5 4 2

11.6 1.6 2-3

18.8 1.7 1-2 1 Charred

Example 7 Variation of substrate

Samples of plain uncompressed, patterned uncompressed and compressed fibre cement sheet were coated with the same paint formulation as above. The sheets were then cured at various power levels as described in Example 6 and tested as before. The results outlined in Table 3 were obtained. Example 8

Fibre cement sheets were surface dried to less than about 7% moisture in a continuous microwave oven using microwave energy of 2450 MHz frequency. A pigmented emulsion paint without coalescent solvents was applied to fibre cement sheet by air assisted spray at a dry film thickness of approximately 30um. Sheets were separately cured in the same continuous microwave drying oven at the rates of 2.5, 3.3, 5.0 and 6.6 Jmm of

microwave enrgy. Control sheets were also coated with paint and either air dried or subjected to a radiant cure at 60°C for 10 minutes. The painted and cured sheets were then tested using standard paint testing methods to give the following results.

Curing Cross Hatch Blocking Boil Paint Condition Adh'n % Resistance Resistance Appearance

Softened Good

Softened Good Softened Good

Softened Good

The Cross-Hatch Adhesion refers to the result of an adhesion test where the paint is scribed with a grid of 25 squareseach with a side of 5mm, a piece of adhesive tape is applied over the scribed area and then removed. The amount of the paint remaining after the test is estimated and reported. It will be seen that there is apparently an optimum microwave curing energy input rate which will result in the greatest adhesion.

The blocking resistance refers to the ability of painted sheets to resist sticking to the backs of adjoining sheets when stacked (and forming a solid block) . No significant change in the blocking resistance was observed with this system with any particular curing regime. This however is more a reflection of the paint resin system rather than the curing system.

Boil resistance is determined by subjecting the painted article to boiling water for a specified time, allowing the article to cool and then observing any changes to the cooled paint film. In this case the microwave curing did not affect boil resistance compared to the air or radiant cured films.

Claims

1. A method of forming a film of a water containing paint on a temperature sensitive substrate comprising the steps of applying a film of paint to the surface of the substrate and subsequently irradiating the surface with microwave radiation to heat the film of paint without overheating of the substrate.

2. A method as claimed in claim 1 further comprising the step of drying at least the surface of said substrate to a moisture content of less than about 7% prior to the application of said paint film.

3. A method as claimed in claim 2 wherein the drying is achieved by the application of microwave radiation.

4. A method as claimed in any one of claims 1 to 3 further comprising the step of heating the surface of said substrate to a temperature of about 60°C prior to the application of said paint film.

5. A method as claimed in any one of claims 1 to 4 wherein the bulk temperature of the substrate does not exceed about 120°C.

6. A method as claimed in any one of claims 1 to 5 wherein the applied energy density during said subsequent irradiating of the surface with microwave radiation is from 2.5 to 6.6 J/mm².

7. A method as claimed in any one of claims 1 to 6 wherein the frequency of the microwave radiation is about 2450 MHz.

8. A method as claimed in any one of claims 1 to 7 wherein said paint film thickness is about 40 m.

9. A method as claimed in any one of claims 1 to 8 wherein said paint is cured by the evaporation of water.

10. A method as claimed in claim 9 wherein said paint is a zinc silicate paint system.

11. A method as claimed in claim 9 wherein said paint is an emulsion paint system.

12. A method as claimed in any one of claims 1 to 11 wherein said substrate is a fibre reinforced cement shee .

13. A method as claimed in any one of claims 1 to 11 wherein said substrate is a compressed fibre reinforced cement sheet.

14. A method of forming a film of paint containing water on a temperature sensistive substrate substantially as herein described with reference to any one of Examples 1 to 8 herein.