1. Field of the Invention
This invention relates to a method for minimizing the accumulation of static electricity charges on fibers separated from pulp lap sheets in a dry-forming operation.
2. Description of the Prior Art
Dry-formed fibrous webs including wood pulp fibers are well known in the prior art. In forming these webs it is common practice to separate, or comb fibers from dense pulp lap sheets to form a loosely compacted fibrous assemblage. For example, it is common practice to form a loosely compacted batt of 100% wood pulp fibers for use as the absorbent component in sanitary napkins and disposable diapers. Also, in forming air-laid webs including wood pulp fibers blended with longer reinforcing fibers it is common practice to initially form a loosely compacted feed mat of wood pulp fibers from pulp lap sheets prior to the blending operation. A representative method in which wood pulp fibers are blended with longer reinforcing fibers to form a fibrous web is disclosed in U.S. Pat. No. 3,862,472, issued to Norton et al, and assigned to Scott Paper Company.
Apparatus commonly employed for separating fibers from pulp lap sheets and forming a fibrous assemblage therefrom include a fiberizing device, such as a rotating fiberizing roll, for performing the fiber separating function. Such apparatus also include a fiber conveying duct through which the separated fibers are directed and a forming wire upon which the fibers are deposited after they have passed through the conveying duct. The fibers are deposited on the forming wire in the form of a web or mat, and thereafter, the web or mat may be directed to subsequent processing operations, as desired.
The build up of static charges on individual wood pulp fibers commonly results from the combing of said fibers from the dense pulp lap sheets. These static charges cause the fibers to build up on the walls of the fiber conveying duct to obstruct the flow of fibers from the fiberizing roll to the forming wire. This results in high, undesirable basis weight variations in the fibrous assemblage formed on the forming wire. In fact, excessive fiber build up in the conveying duct can actually block the entire flow of fibers to completely "choke" the apparatus. When the fiber build up on the walls of the conveying duct becomes excessive, the apparatus must be shut down to permit it to be cleaned out. Obviously, the necessity of shutting down equipment in a commercial processing operation is highly undesirable.
In order to achieve high production speeds in processes requiring the separation of fibers from pulp lap sheets it is necessary to provide a high mass flow of fibers through the fiber separating equipment. In order to achieve this high mass flow it is common practice to direct multiple pulp lap sheets, in the form of a stack, into engagement with a fiberizing roll of the equipment. It is highly desirable to achieve this high mass flow in a manner which minimizes the accumulation of static charges on the wood pulp fibers.
U.S. Pat. No. 3,268,954, issued to Joa, indicates one prior art approach for attempting to minimize the accumulation of electrostatic charges on individual fibers separated from multiple pulp lap sheets in a dry-fiberizing process. Joa suggests that air which is humidified heavily with water or other liquid can be directed into the processing area in which individual fibers are separated from the multiple sheets of pulp lap. Accordingly, Joa suggests minimizing the accumulation of static charges on the fibers by attempting to treat virtually all of said fibers after they have been separated from the pulp lap sheets. This manner of treating the fibers is not very effective in minimizing the accumulation of static charges. In order to be effective the liquid in the air must be effectively transferred to the fibers. However, the reaction time required to effectively transfer the liquid from the air to the fibers is longer than the dwell time of the fibers in the fiber-separating region of the device. Also, heavy humidification of the air, as taught by Joa, requires the use of undesirably large quantities of liquid.
Other prior art approaches are known for treating virtually all of the fibers of pulp lap sheets to attempt to minimize the accumulation of static charges on said fibers when they are separated, or combed from said sheets. For example, it is known to humidify the ambient air in a storage facility in order to permit moisture to uniformly penetrate rolls of pulp lap sheets. In order to humidify the ambient air in a storage facility expensive humidification systems are required. These systems undesirably increase the capital costs associated with dry-forming processes. Moreover, in some instances the generation of static charges is so severe that humidification of the fibers with water, by itself, will not effectively minimize the accumulation of such charges. In other instances an excessively high degree of humidification is required to minimize the accumulation of static charges on the fibers, and such a high degree of humidification can cause other problems, such as the formation of fiber clumps. These clumps are often directed through the entire process, and wind up in the finished web product to provide an undesirable appearance. In summary, humidification, by itself, represents a high cost approach to minimizing the accumulation of static charges, with, in many cases, limited effectiveness.
It is also known to treat wood pulp fibers with softeners and debonders in the pulping process employed to form pulp lap. For these additives to be effective in minimizing the accumulation of static charges on the individual fibers during a subsequent fiberizing operation it is necessary to include such a large quantity of the additives as to make the process uneconomical.
In several prior art fiberizing processes only a single sheet of fibrous material is separated into its individual fibers. It has been suggested in such processes to apply an antistatic agent to the single fibrous sheet to minimize the accumulation of static charges on the individualized fibers. Representative disclosures of this type of antistatic treatment can be found in U.S. Pat. Nos. 3,591,450 (Murphy et al); 3,881,222 (Roberson); 3,894,314 (Nayfa) and 2,440,399 (Hill). None of the above disclosures are in any way directed to a reliable and efficient process for minimizing the accumulation of static charges on wood pulp fibers resulting from the separation of individual fibers from a stack containing several pulp lap sheets. It is to such a unique process that the present invention is directed.