Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-04-13
2002-06-04
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S403000
Reexamination Certificate
active
06398861
ABSTRACT:
The present invention relates to a process for preparing low- or non-dusting, substantially non-volatile highly light reflective metal pigment compositions by a ball milling process using water as the major milling fluid.
The preparation of metal flake pigments is well documented in the patent literature. For example, U.S. Pat. No. 3901688 describes a wet ball milling process in which metal powder or chopped foil is milled with an organic liquid such as mineral spirits and a small amount of a lubricant such as stearic or oleic acid. The metal flakes so produced are separated, for example by wet sieving to provide the desired particle size distribution and thereafter brought to a paste-like consistency of, typically, 55-80% weight metal content. In this conventional process the lubricant is required to avoid cold welding of the metal flakes under the action of the grinding media. The purpose of the organic liquid is to reduce the viscosity of the system, enabling efficient conversion of the starting powder to flakes.
Metal flakes may also be prepared in the complete absence of solvent by a dry ball milling process, such as that described in U.S. Pat. No. 4115107.
A further method of preparation of metal flake is by electrodeposition of a thin film of metal onto an inert support, followed by removal and fragmentation into flakes. The product is generally provided either as dry flake or as a dispersion of the metal flakes in solvent.
The aforementioned methods for preparing metal flake pigments suffer from a number of disadvantages. For example, dry milling processes are now little used, due to the explosive and contaminating properties of the dry flake products. Though the paste form, in which the metal flake particles are damped by solvent, is inherently safer and easier to handle, it is not without problems. The 20-45% by weight solvent portion of metal pigment pastes may be acceptable in certain coating applications such as automotive paints, but in others, especially printing inks, such solvents typically slow down the drying rate and may cause odour in the final printed film, due to retention of minute concentrations of these typically high boiling point solvents. This is especially disadvantageous in printed films on packaging intended for food contact. The presence of organic solvent in other application areas, such as plastics, can also be very undesirable. The solvent vaporises during processing, causing bubbles and surface blemishes in the pigmented plastic article.
Flake formation by the electrodeposition process can give very bright flakes of narrow particle size distribution, but it is a very costly process, unsuited to large scale production. This is because the equipment required to maintain the hight vacuum environment in which the metal is vaporised is very expensive to construct and operate.
It is also found that the storage stability of the metal pigment pastes themselves is finite, due to the tendency of the organic solvent component to evaporate, leading to aggregation of the metal flake particles. This is especially true for pastes in which a more highly volatile solvent has been employed, perhaps to ensure the compatibility of the metal pigment paste in a particular coating system. Once aggregated, redispersion is difficult. In derived coatings, hiding power, or opacity, and also gloss, may be greatly reduced.
Increasingly too, environmental concern about organic solvent is leading to legislation which encourages a lower use of volatile solvents in industrial processes. The costs associated with solvent purchase, storage and recovery are increasing. Containment equipment, required to meet increasingly severe legislative constraints on the emission of solvent to the atmosphere is expensive. As a result replacement of organic solvents by water is a strategic goal in many industries.
Recent changes in health and safety classifications require a number of high boiling point petroleum derived hydrocarbon solvents traditionally used in metal pigment manufacture to be designated carcinogenic. This increases the pressure for their elimination from metal flake pigment manufacturing processes.
The need for a stable, safe and easily handled product metal pigment form, free of the disadvantages of both dry flakes and solvent containing pastes has been met by the products of European Patent 0134676. There is described therein a process for the preparation of a solid low- or non-dusting, metal pigment composition which comprises forming a coherent paste of an organic binder medium, an organic liquid vehicle and metal pigment, in powder or flake form, the paste being formed by mixing a first component comprising organic binder medium and a second component comprising metal pigment, with either or both of the first and second components comprising organic liquid vehicle and the paste containing from 3-45% of the organic binder medium based on the weight of the metal pigment, and either sub-dividing the coherent paste-into particles and removing substantially all organic liquid vehicle from the particles, or removing substantially all the organic liquid vehicle from the coherent paste and sub-diving the resulting mass into particles, at least 98% by weight of the resulting particles being retained on a sieve having a 150 &mgr;m aperture and each containing a plurality of metal pigment particles dispersed in a matrix of organic binder medium. The so-called “granule” products of this process can be prepared using organic binder media compatible with the end application. Thus, for example, synthetic aldehyde and ketone resins could be employed for ink applications and polyolefin derivatives for the mass pigmentation of thermoplastics.
Though the products of European Patent 0134676 are substantially free of organic solvent, the process itself still involves processing of solvent. It is thus still subject to the aforementioned legislative pressures. There is therefore a clear need for a process for preparing an easily handled, dust free, metal pigment product, which does not employ organic solvents.
It is possible to prepare a dust free, metal pigment product without the use of organic solvents if the metal pigment is prepared by dry milling and thereafter incorporated into a suitable organic carrier material by mixing at a temperature above the melting point of the organic carrier. Once a homogeneous mixture has been attained, the mass is cooled, often with extrusion into a more easily handled form, such as granules or pellets. Such processes were once widely employed for the preparation of plastics masterbatch. The need to use dry metal pigments, with their attendant disadvantages, and the difficulty of wetting such pigments satisfactorily into the carrier resin, have contributed to the decline in this type of process. Although the problem of metal flake dusting may be reduced if the metal powder precursor is milled in the presence of the carrier resin, methods traditionally used to separate wanted from unwanted particle size fraction, such as dilution with solvent and screening, are then no longer operable.
A more satisfactory processing liquid is water. It is readily available, inexpensive and nonflammable. However, a difficulty arises in the case of the more reactive metal pigments, especially aluminum, which is the most widely used of such pigments. Aluminum metal may react with water to form aluminum oxide and hydrogen gas, according to the equation:
2Al+6H
2
O−>2Al(OH)
3
+3H
2
Generation of hydrogen is dangerous, because it is highly explosive. Sealed containers of aqueous metal pigment pastes may become pressurised and explode. In addition, the pigmentary properties of the metal flake are destroyed.
Many attempts have been made to stabilise aluminum against this reaction. Methods employed fall into several categories, for example resin encapsulation or chemical treatment. As an example of the former, there may be mentioned U.S. Pat. No. 4213886. Prominent types of chemical treatment are silica coating (see, for example, U.S. Pat. No. 2885366), treat
Green Anthony
Ratner & Prestia
Silberline Limited
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