It is desired in various industries to use rheology modifiers to achieve thickening, flow control, water retention and other properties in aqueous systems. A number of rheology modifiers are commercially available, such as sodium carboxymethylcellulose (CMC), guar gum, sodium alginate, hydroxyethylcellulose (HEC), alkali-soluble lattices, starch or starch derivatives, and the like.
Coated paper manufacturers have traditionally used one or more of the above rheology modifiers as additives for paper coating compositions in order to attain key application properties. Paper coatings are typically composed of a pigment, such as kaolin clay and calcium carbonate, binders, such as starch and styrene butadiene latex, water, various specialty additives, and at least one of the above mentioned rheology modifiers
It has been determined in prior industrial practice that the above rheology modifiers are often interchangeable when employed in the application of paper coating thickening, where the coating has been applied by traditional methods such as blade metering. For general use, CMC is established as one of the most prevalent rheology modifiers for traditional blade metered paper-coating application.
In recent years, a new method of applying aqueous pigment containing paper coatings onto the surface of paper has been devised that is known as “pigmented metering size press” or sometimes “film transfer coating”. In this method of coating paper, a wet coating is first metered onto a roller that is typically composed of stainless steel or synthetic substrate. The wet coating film layer is then transferred by contact to a moving web of paper. Typically this operation is performed on both sides of the paper simultaneously.
Unfortunately, in the pigmented metering size press application of paper coating to a moving paper web, it is often observed that an aerosol mist of the wet paper coating is formed as the paper web leaves the nip of the adjacent rollers. It is, thus, generally known in the paper industry that “misting” of pigmented paper coatings is a severe problem that is often observed when these are applied by means of metering size press equipment. Misting refers to the tendency of pigmented coatings to form an aerosol on the exit side of the film press during application to the paper web. This problematic coating mist can deposit on surfaces in the mill, and is a severe cleanup problem, as well as depleting expensive coating in a wasteful manner. Because of the prevalence of this problem, the large-scale industrial use of pigmented metering size press has been slow to develop commercially. The producers wanting to use this technology has had to develop coating formulations that are very closely formulated with equipment operations that are strictly controlled in order to minimize the adverse misting effect. The use of most rheology modifiers in pigmented metering size press application is ineffective for reducing misting under many conditions.
Since, In the past, adjustments of equipment, processing conditions, and/or coating color formulations have been made in an attempt to improve runnability and reduce misting. Hence, a need exists in the paper industry to develop a pigmented metering size press coating process to minimize the above mentioned problems.