Coating processes – Foraminous product produced
Reexamination Certificate
2000-01-19
2001-11-06
Cameron, Erma (Department: 1762)
Coating processes
Foraminous product produced
C427S331000, C427S336000, C427S337000, C427S372200
Reexamination Certificate
active
06312760
ABSTRACT:
The present invention relates to improved coatings particularly, but by no means exclusively to surface coatings wherein one or more oils or oil soluble substances are confined and protected indefinitely until they are released under pre-determined circumstances to perform their designated function.
The entrapment of oils or oil soluble substances (especially perfumes and coloured dye precursors) in microcapsules and their subsequent coating onto paper and other surfaces is well known in the art. Microcapsules of this type comprise individual droplets of oil or oil soluble substances (of size ranging from sub-micrometer to tens of millimeters in diameter) around which polymer walls have been formed by one of a number of chemical processess. Usually such microcapsules are prepared as an aqueous suspension which is then capable, with the addition of suitable modifying reagents, of being sprayed or printed onto paper and other surfaces. The object in so doing is usually to prevent the evaporation of volatile substances (for example, perfumes) or the degradation or chemical reaction of oil soluble species (for example, colourless dye precursors) until the microcapsules are broken by the application of shear forces by scratching or scraping the coated surface with the consequent release of their contents. Such coatings find major uses, for example, in the forms of “scratch and sniff” perfume coatings or NCR (No Carbon Required) paper.
However, such coatings and the use of microcapsules which form them suffer from a number of disadvantages.
Firstly, the process by which microcapsules are formed is a lengthy and uncertain one in which control over temperature, pH and the absence of any form of contamination is essential. The formation of microcapsules, for example, by complex coacervation from gelatine and an anionic complexing species such as gum acacia takes many hours and demands very close control of pH, temperature and cooling rate. Similarly, the formation of microcapsule walls from aminoplast resins, such as malamin-formaldehyde or urea-formaldehyde takes at least eight hours during which precise control over all controllable parameters needs to be effected. Moreover, the effectiveness and completeness of any individual encapsulation process and therefore the quality of the microcapsules so formed depends largely on the chemical nature of the oil and/or oil soluble substances being encapsulated.
A further disadvantage of microcapsulation is that the thickness and therefore the strength of the microcapsule wall is variable and is not easily controllable and varies with the nature of the oil or oil-soluble substances being encapsulated. Thus microcapsules made by the same process but from different oils may have widely differing strengths and resistance to breakage during the printing process and during subsequent storage and use.
A yet further disadvantage of microencapsulation is the limited number of chemical processes and the limited number and type of polymeric wall materials which are available to form them. The choice as to the properties of the wall materials is consequently limited with regard to their flexibility, tensile strength, permeability, chemical inertness, mammalian toxicity and other properties including solubility and melting point (if any). In addition, some of the chemicals commonly used in the wall forming process are themselves highly irritating and may themselves be toxic such, for example, as the use or release of formaldehyde (a potential carcinogen) during the manufacture of aminoplast resin walls. Moreover, the remaining traces of formalin in the resulting microcapsule suspension are virtually impossible to eliminate to below acceptable levels for uses of microcapsules and requires special precautions to be taken during the manufacturing process.
A further disadvantage of microcapsules which are used in surface coatings is that the microcapsule walls have a limited deformability, that is, they can only be deformed to a limited extent during the surface coating process (typically a printing process) before they will rupture and prematurely release their contents. The extent of their ability to deform when squeezed, for example, between nip rollers on a printing press set with a gap smaller than the average diameter of the microcapsules, depends partly upon the tensile properties of the polymer wall, its thickness and on the size of the microcapsules being squeezed.
Other methods for coating paper and like surfaces with mobile oils are known but these are generally inferior to coating with microcapsules since they do not effectively trap and protect the oils from evaporation or degradation during manufacture and subsequent storage prior to use. For example, perfumes may be sprayed or otherwise coated onto paper surfaces in order to give paper products a pleasant smell—as for instance, with perfumed drawer liners wherein the coating is sprayed on perfume and not a microencapsuled perfume. Such products have a limited shelf life (because of the premature evaporation of the perfume) and the outer packaging of the product is usually the only (and relatively ineffective) barrier to loss of perfume or other volatile substances during storage.
The present invention addresses the above described problems and disadvantages, and provides a surface coating containing entrapped oils or oil soluble substances which can be manufactured quickly, efficiently and reproducibly wherein a polymer film protecting the oils droplets is of a controllable thickness and strength which is largely independent of the nature of the entrapped oil droplets.
Furthermore, the invention provides a coating wherein the entrapped oils can be caused to be released predictably by making use of known properties of a resistance to abrasion, chemical inertness, opacity, solubility, melting point and other chemical and physical properties.
Furthermore, the invention provides a means of surface coating capable of releasing oils or oil soluble materials without the need to use toxic or potentially toxic materials.
According to a first aspect of the invention there is provided a method for coating the surface of a substrate comprises the steps of:
contacting the surface with a dispersion of a film forming polymer, said dispersion containing droplets of a suspended biliquid foam or emulsions; and
allowing the dispersion to dry so as to coat the surface with a coating comprising the droplets trapped within a film of said polymer.
In this way, a surface coating is provided which can exhibit the above described advantages. Furthermore the range and number of soluble or suspendible film forming polymers is very great compared to the numbers of polymers available, for example, to form the walls of microcapsules and the variety of useful properties available with the present invention compared to coatings containing microcapsules is, consequently, very much greater and wider.
Preferably, a biliquid foam is used. Biliquid foams are systems bearing some resemblance to gas foams, which exhibit some different and, in the context of the present invention, more preferable properties to those exhibited by emulsions, including the ability to be suspended in aqueous gel systems. Biliquid foam are described in the following literature articles by Sebba: “Biliquid foams”, J. Colloid and Interface Science, 40 (1972) 468-474; and “The Behaviour of Minute Oil Droplets Encapsulated in a Water Film”, Colloid Polymer Sciences, 257 (1979) 392-396. Neither of these articles suggest that biliquid foams might be used in a polymeric surface coating system. Biliquid foams can be made quickly and efficiently from non-polar materials, such as oils and oil soluble substances, and a hydrogen bonded material (typically water but including alcohols and glycols), provided that both phases are liquid during manufacture.
Preferably, an aqueous dispersion of the polymer is used. Water has the advantages of being economical, environmentally friendly and permitting a wide ranging choice of compatible polymers.
The biliquid foam or emulsi
Cameron Erma
Disperse Limited
Renner , Otto, Boisselle & Sklar, LLP
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