Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-12-31
2004-04-06
Griffin, Steven P. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S159000, C162S090000, C162S051000, C162S181200
Reexamination Certificate
active
06716310
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to methods for making cellulosic paper products, and, more particularly, to methods for reducing or eliminating malodor released from a cellulosic base sheet upon re-wetting.
BACKGROUND OF THE INVENTION
Commercial paper products such as hand towels are manufactured from cellulosic base sheets. A cellulosic base sheet is a paper product in its raw form prior to undergoing post-treatment such as calendaring and embossing. In general, cellulosic base sheets are made by preparing an aqueous suspension of papermaking fibers and depositing the suspension onto a sheet-forming fabric to form a wet web, which is then dewatered and dried to produce a base sheet suitable for finishing.
Wet web base sheets are commonly dried by through-air drying, which comprises removing water from a wet web by passing hot air through the web. More specifically, through-air drying typically comprises transferring a partially dewatered wet-laid web from a sheet-forming fabric to a coarse, highly permeable through-drying fabric. The wet web is then retained on the through-drying fabric while heated air is passed through the web until it is dry. One process for through-drying base sheets is the Un-Creped Through Air Dried (UCTAD) process, as described, for example, in U.S. Pat. No. 6,149,767, which is hereby incorporated by reference. In the UCTAD process, a wet base sheet is partially dewatered and through-air dried by passing hot air through the wet sheet as it runs over a through-drying fabric on a drum roll.
Based upon consumer complaints, it was observed that a strong, burnt popcorn odor was often emitted from hand towels when the towels were wetted. Upon investigation, this problem of malodor was found to be present in cellulosic base sheets which had been through-air dried at relatively high air temperatures including, for example, sheets dried by the UCTAD process. It was hypothesized that over-drying or over-heating of the base sheets was leading to the malodor problem upon re-wetting. By operating the through-air drying process at lower temperatures and slightly longer residence times, the malodor problem can be largely eliminated. However, lower operating temperatures and longer residence times adversely affect the overall productivity of the base sheet manufacturing process. Therefore, a need exists for a process which can eliminate malodor in through-dried cellulosic base sheets wherein higher drying temperatures and shorter residence times can be used to increase product throughput and productivity.
SUMMARY OF THE INVENTION
Among the several objects of the present invention, therefore, is the provision of a process for making a cellulosic paper product from a wet-laid web; the provision of such a process wherein the paper products exhibit a reduced malodor upon re-wetting; the provision of such a process wherein the wet-laid web can be through-air dried at higher temperatures and shorter residence times; the provision of such a process wherein productivity and throughput are increased; and the provision of such a process which is relatively inexpensive and easy to implement.
Briefly, therefore, the present invention is directed to a process for manufacturing a cellulosic paper product. The process comprises forming an aqueous suspension of papermaking fibers; depositing the aqueous suspension onto a sheet-forming fabric to form a wet web; and dewatering and drying the wet web. The process is further characterized in that a borate compound is introduced into the aqueous suspension of papermaking fibers, the borate compound having the formula:
wherein R
1
, R
2
and R
3
are independently selected from the group consisting of hydrogen and a saturated or unsaturated, substituted or unsubstituted, branched or straight chain hydrocarbyl moiety having from 1 to about 20 carbon atoms and x, y and z are integers ≧0 such that x+y+z=3.
In one preferred embodiment, the process of the present invention comprises forming an aqueous suspension of papermaking fibers and introducing boric acid into the aqueous suspension. The aqueous suspension is deposited onto a sheet-forming fabric to form a wet web after the introduction of boric acid into the aqueous suspension and the wet web is dried by passing heated air through the wet web.
The present invention is also directed to cellulosic paper products exhibiting a reduced malodor upon re-wetting. The cellulosic paper product is produced by a process comprising forming an aqueous suspension of papermaking fibers; depositing the aqueous suspension onto a sheet-forming fabric to form a wet web; and dewatering and drying the wet web. The process is further characterized in that a borate compound is introduced into the aqueous suspension of papermaking fibers, the borate compound having the formula:
wherein R
1
, R
2
and R
3
are independently selected from the group consisting of hydrogen and a saturated or unsaturated, substituted or unsubstituted, branched or straight chain hydrocarbyl moiety having from 1 to about 20 carbon atoms and x, y and z are integers ≧0 such that x+y+z=3.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, it has been discovered that a cellulosic base sheet having a reduced malodor upon re-wetting can be produced by introducing a borate compound into an aqueous suspension of the cellulosic papermaking fibers from which the base sheet is formed. The wet-laid base sheets formed from such aqueous suspensions can be dried at higher temperatures and shortened residence times while significantly reducing malodor produced upon re-wetting of the base sheets.
As part of the present invention, possible reaction mechanisms in the base sheet production process which may be contributing to the presence of odorous compounds in cellulosic base sheets have been investigated. Without being held to a particular theory, it is believed that malodor in base sheets dried at high temperatures is caused by acid-catalyzed reactions which form volatile organic compounds or odor precursors during drying. It is believed that these odorous compounds are formed within a cellulosic base sheet during drying and bound within the sheet until the moment that the sheet is re-wetted. The combination of acid in the sheet and the addition of water upon re-wetting cleaves the odorous compounds from the sheet and releases the compounds into the environment. In particular, experience to date suggests that a large number of the odor-causing compounds released from re-wetted base sheets can be characterized as medium chain aliphatic aldehydes (e.g., octanal, nonanal, decanal) and/or furans (e.g., furfural, furfuryl alcohol, hydroxymethyl furfural). Thus, it is believed that the presence of volatile aldehyde compounds and/or furan compounds, either alone or in combination, may be responsible for the base sheet malodor. These odor-causing compounds may be produced during high temperature drying of the wet web by any conventional means including Yankee dryers and through-air dryers, but are particularly problematic in through-dried base sheets, perhaps due to the highly oxidative environment and unique mass transfer phenomena provided by the air stream passing through the web.
Aldehyde Hypothesis
Experience to date with analyzing re-wetted base sheets, as described, for example, in Example 1 below, indicates that a substantial component of the malodor released from through-dried cellulosic base sheets upon re-wetting comprises medium-chain, aliphatic aldehydes having from about 6 to about 10 carbon atoms. Without being bound by a particular theory, it is believed that the aldehydes are formed within the base sheet by the oxidation of fatty acids present in the aqueous suspension of papermaking fibers. For example, during chlorine dioxide bleaching, which is conducted under acidic conditions at a pH of about 3.5, fatty acids p
Spence Tameka
Sun Tong
Tanner James J.
Griffin Steven P.
Halpern Mark
Kimberly--Clark Worldwide, Inc.
Senniger Powers Leavitt & Roedel
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