Methods for reducing fluorescence in paper-containing samples

Details Methods for reducing fluorescence in paper-containing samples Methods for reducing fluorescence in paper-containing samples

1999-07-30

2001-09-25

Nguyen, Dean T. (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Waste paper or textile waste

C162S006000, C162S008000, C162S065000, C162S072000, C162S078000

Reexamination Certificate

active

06294047

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for reducing fluorescence in paper-containing samples.
BACKGROUND OF THE INVENTION
An important aspect in the manufacture of paper products is the whiteness of the paper. Traditionally, one means of increasing the whiteness of paper has been through the use of fluorescent whitening agents (FWAs). FWAs absorb light in the near ultraviolet (UV) and emit this light in the visible spectrum, thereby increasing the amount of visible light reflected from the paper, and thus increasing the apparent whiteness of the paper.
The presence of FWAs in manufactured paper, however, is limited by federal regulations, particularly in paper products destined to contact food products. Federal Regulation 21 CFR §176.260 prohibits the presence of any harmful or deleterious materials that can migrate to food products from recycled papers used for food packaging.
Recycled paper is frequently used in the manufacture of new paper products, which poses several potential problems. First, recycled paper often contains FWAs, which, as discussed above, are forbidden in products destined for food-contact. Second, recycled paper arises from many different sources and the whiteness of the final manufactured product will vary dramatically depending on the amount of FWAs present in the starting materials. Third, for optimal performance of a paper mill, processing recycled paper requires the amount of fluorescence in the pulp going to the mill to be constant. Therefore, the recycling of paper products which have been previously treated with FWAs cannot be undertaken without an effective method for destroying the FWAs (Dubreuil, Progress in Paper Recycling, August, 1995, 98-108).
The principal FWAs used in the paper industry are diaminostilbenes which have the general structure represented in Formula I:
One common FWA is Tinopal SFP, which has the structure of Formula I wherein R
1
is NH
2
and R
2
is NH(CH
2
CH
2
OH). Other potential R
1
substituents include, but are not limited to, N(CH
3
)(CH
2
CH
2
SO
3
H), and OCH
3
. Other potential R
2
substituents include, but are not limited to, N(CH
2
CH
2
OH)
2
, NH(C
6
H
4
SO
3
H), and N(CH
3
)(CH
2
CH
2
SO
3
H). Other FWAs are known and have the general structure as shown in Formula II, below.
Several techniques are known for the removal of FWAs from recycled paper. For example, oxidizing agents, such as chlorine, destroy the stilbene dyes which make up most FWAs, rendering them nonfluorescent. The destruction of FWAs can be assessed by monitoring the fluorescence of a sample. As the FWAs are destroyed, the sample's fluorescence decreases. Increasingly stringent environmental regulation of organic halides imposes limits on such use of chlorine in the removal of FWAs from paper-containing samples.
Other non-chlorine reagents have been evaluated for their potential use in the destruction of FWAs, including sodium hydrosulfite, formadine sulfinic acid, and hydrogen peroxide; however, each of these had the reverse effect of actually increasing the measured fluorescence of the samples. Ozone has also been used as an agent to destroy FWAs. Although ozone does reduce the fluorescence of test samples, unacceptably high levels of ozone are required to reduce fluorescence. Ozone also has the undesirable effect of degrading cellulose, resulting in a paper product with decreased strength properties. These disadvantages, coupled with the high cost of ozone, make the use of ozone impracticable.
There is, therefore, a need for methods of reducing fluorescence in paper-containing samples that do not suffer from the drawbacks of existing methods. There is a further need for methods of reducing fluorescence in paper-containing samples that contain FWAs. There is also a need for improved methods of destroying FWAs in paper-containing samples.
SUMMARY OF THE INVENTION
The present invention is directed to novel methods for reducing fluorescence in paper-containing samples. According to the methods of the present invention, a paper-containing sample is combined with an oxidoreductase, a mediator, and oxygen to form a mixture, which mixture is then optionally incubated, resulting in reduction of fluorescence of the paper-containing sample.
The methods of the present invention may also be used for destroying FWAs in a paper-containing sample.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention arises from the surprising discovery that the amount of fluorescence in a paper-containing sample may be reduced by mixing the paper-containing sample with an oxidoreductase, a mediator, oxygen, and optionally heat. The present inventors have discovered that FWAs present in a paper-containing sample may be destroyed by using this method.
As used herein, the term “fluorescence,” also referred to as the fluorescent component of a sample, is expressed in terms of the number of brightness units (
0
ISO) emitted by a sample. Fluorescence is the radiation emitted as the result of the absorption of incident radiant energy of different wavelengths. When irradiated by UV, the fluorescent radiation is often visible. The higher the fluorescent component, the more fluorescence is being emitted by the sample.
As used herein, the term “reduce fluorescence,” and variations thereof, are used herein synonymously with inhibit, reduce, suppress, decrease, diminish, and lower fluorescence. The present invention includes methods that substantially inhibit fluorescence. Substantial inhibition of fluorescence refers to a reduction of fluorescence from about 1% to about 100% compared to the untreated paper-containing sample comprising FWAs. Preferably, fluorescence is inhibited up to about 50%, more preferably up to about 75%, and even more preferably up to about 100% (i.e. such that the fluorescence of the treated paper-containing sample is indistinguishable from background).
As used herein, the term “background” refers to the level of fluorescence emitted by a paper-containing sample containing no FWAs. A typical paper-containing sample comprising FWAs may have a fluorescent component of about 5. Following treatment of such a paper-containing sample by the method of this invention, the fluorescence of the paper-containing sample may drop to about 1. Thus, treatment has resulted in a reduction of fluorescence in the paper-containing sample.
The fluorescent component of a paper-containing sample is obtained by measuring the brightness for a specified blue light measured under standardized conditions, with and without UV illumination, on an instrument designed and calibrated for this purpose. Brightness without UV illumination is subtracted from brightness with UV illumination, yielding the fluorescent component.
0
ISO brightness is one of the standardized reflectivity techniques used for pulp, paper, or paperboard.
As used herein, the term “paper-containing sample” refers to a sample which contains paper. As used herein, the term “paper” refers to all kinds of sheets of fiber formed on a support screen from a water suspension and includes sheets formed by other methods. Paper may be recycled or non-recycled, and may comprise pulp from both natural and artificial sources.
As used herein, the term “recycled” refers to paper and/or paper by-products which have previously been used. “Recycled” may refer to samples containing only previously used paper and/or paper by-products as well as to samples containing previously used paper and/or paper by-products mixed with non-recycled paper. In a preferred embodiment, the “paper-containing sample” comprises recycled paper and FWAs.
As used herein, the term “pulp” refers to mechanically ground or chemically digested materials, especially wood. Paper pulps are composed of fibers and are often prepared from wood, cotton, or grasses using chemical or mechanical processes. Synthetic pulps are also known to those of skill in the art.
As used herein, the term “fluorescent whitening agent” (FWA) refers to any material which increases the brightness of a sample. “Fluorescent whitening agents” include optical brig

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