WO2004050994A1

WO2004050994A1 - A method for increasing the stability of fluorescent whitening agent

Info

Publication number: WO2004050994A1
Application number: PCT/SE2003/001867
Authority: WO
Inventors: Johan Lindgren; Roland Agnemo; Per Engstrand
Original assignee: Holmen Aktiebolag
Priority date: 2002-12-04
Filing date: 2003-12-02
Publication date: 2004-06-17
Also published as: SE524471C2; SE0203602D0; SE0203602L

Abstract

The present invention relates to a method for increasing the stability of fluorescent whitening agent after its preparation and during its use either on its own or mixed with other chemicals. The method is characterised in that the whitening agent is supplied with an organic acid containing aromatic group and/or can be esterified internally forming lactone, or the salt of the acid (additive A) and reducing chemical (additive B).

Description

A method for increasing the stability of fluorescent whitening agent

Technical field

The present invention relates to improving the stability of fluorescent whitening agent. Fluorescent whitening agent (FWA) used often to be known as optical brightening agent (OB A). This agent has the capability of absorbing ultraviolet light (with a wavelength of <380 nanometer) that is invisible to the human eye, and sending the light back in wavelengths that are visible to the human eye. The return of the light is pronounced within the wavelength interval 430-470 nanometer where the light has a blue tone. This occurrence is perceived by the man as if the brightness and/or the whiteness of the object in question to which FWA has been supplied, have/has increased. Fluorescent whitening agents comprise chemical compounds of organic type.

Usual ones are stilbene derivatives, i.e. chemical compounds that among others comprise two benzene rings. A more detailed description of these agents can be found in the material referred to and commented on further on in this document. For a thorough study of the chemical structure of various whitening agent's reference is made to textbooks and reference books.

A large number of application fields for FWA exists, such as additive in washing and cleaning agents, in textiles and in various plastic materials. FWA is also used in the pulp and paper industry. Considerable quantities are used particularly in the manufacture of paper and paperboard. The agent can either be mixed into the paper itself or applied on the surface of the paper in various ways. Background art

It has been found that FWA as a compound group are unstable, unstable meaning that the capability described above deteriorates in time, i.e. fades, possibly right down to zero. The most usual method of preparation is to dissolve the agent in a liquid and a suitable and normal liquid is water. Dilution of a water solution of whitening agent may, for instance, result in instability. When paper is being sized or coated FWA is often included in the size or coating agent applied on the paper. After application of the size (which has a very low content of solid substances) and/or the coating (which has a very high content of solid substances, e.g. 60-65%), the paper is allowed to dry so that the agent forms a thin, solid layer on the paper. It is known from the literature that the stability of the agent deteriorates and that it is even disintegrated by factors such as day-light (the ultraviolet radiation), heat and humidity.

An article in Textile Research Journal, 66 (7), pages 464-470, by H. Ikuno, M. Okuni, M. Komaki and T. Nakajima, entitled "Photofacing Behavior of Triazinyl amiostilbene Fluroescent Brightener in Cellophane" describes how increasing humidity has a decreasing effect on FWA's brightening ability when the agent is mixed into cellophane. It is taught that if sodium carbonate is mixed into the colour bath containing the agent during manufacture of the cellophane, this will reduce the instability of the agent in the finished product. It is also stated that cooking salt, i.e. sodium chloride, is generally used as additive when the agent is intended for treating (colouring) cellophane.

In an examination thesis from 1996, YTH-Graphic Technique, entitled "Optical brightening agent, effects of temperature and humidity" Britt-Marie Andersson investigates, amongst other things, the change in whiteness of coated samples of paperboard containing FWA, caused by increased temperature and changes in humidity. It was found that if the paperboard with its coating containing FWA was heated to temperatures of 140°C and above, both the brightness and the whiteness of the paperboard decreased noticeably. It is also found that the brightness- and whiteness-increasing ability of FWA did not return at room temperature, for instance, once the paperboard and thus also the FWA had been subjected to too high a temperature, such as 140° and above. Similar heat sensitivity of FWA is reported in the article "An Effect of

Optical Brightening Agent Degradation due to Heat and Moisture" by Nordstrom, J-E, P, Nordlund, J.P and Grδn J.P.L., TAPPI Proceedings, 1997 Coating Conference, pages 265- 280.

A warning is also issued that patchiness in the coated paperboard, caused by FWA, occurs if the paperboard is subjected to temperatures of 120°C and above. It is furthermore pointed out that increasing the relative humidity from 0% to 50% does not have such a great negative effect on FWA.

International (PCT) patent application WO 92/01115 describes a method of improving the whiteness in a cellulose material that contains FWA or to which FWA has been applied. The method comprises adding a water-soluble phosphonate that has 1 to 5 phosphonate groups, and/or a water-soluble carboxylate having 2 to 5 carboxylate groups, and no nitrogen atom in the chain, at a neutral or alkaline pH value. It is not claimed that these additives have a stabilizing effect on FWA, merely that the agent reinforces the whiteness-heightening effect in the cellulose material, e.g. paper and cotton, caused by the addition of FWA.

Disclosure of the invention Technical problem

As is evident from the above stated, fluorescent whitening agent is unstable from its preparation stage and onwards. Examples of the circumstances that affect the substance group's instability per se and as regards its brightness- and whiteness- heightening ability of the object, to which the substance is supplied and/or applied, have been provided above.

The solution The present invention contributes to solving this problem and relates to a method for increasing the stability of fluorescent whitening agent after its preparation and during its use either on its own or mixed with other chemicals, characterised in that the whitening agent is supplied with an organic acid containing aromatic group and/or can be esterified internally forming lactone, or the salt of the acid (additive A) and reducing chemical (additive B). A preferred method is for the additive A to consist of an organic acid containing aromatic group or the salt of the acid. Particularly preferred is for the additive A to consist of an organic acid or its salt with the general formula (acid form):

where Ri = -CHOHCH₂OH or -CHOHCOOH. Suitable substances within that group are ascorbic acid, arabo-ascorbic acid, saccaro-ascorbic acid and xylose-ascorbic acid or their salts. Ascorbic acid is extremely suitable, particularly the L-form of the acid or its salt. The L-form is also to be preferred in the case of the other acids and their salts apart from xylose-ascorbic acid and its salt where the D-form is preferred. The additive B may be any reducing agent at all. Examples are boron hydride, dithionite, hydrazine, thiourea dioxide, formadine sulphinic acid and hydrogen sulphite/sulphite. It is also possible to make use of catalytic hydration. Preferred reducing agents are those that are both effective and inexpensive. Particularly preferred are additives containing hydrogen sulphite and/or sulphite. As regards the quantities of agents A and B to be added, the quantity of additive A shall be in a mol ratio of 0.1-50 to 1 to the quantity of fluorescent whitening agent and the quantity of additive B shall be calculated in mol at least in the same quantity as the quantity of the additive A. The quantity of additive B may advantageously be seven times the quantity of additive A. It is already known to supplement the whitening agent and reinforce its effect with a so-called carrier. Several substances may act as carrier, one example being polyvinyl alcohol. This carrier may be added to the above described chemical mixture from the beginning or be brought into contact with the fluorescent whitening agent and the additive chemicals in accordance with the invention when the whitening agent is to be used.

After production of the fluorescent whitening agent this is delivered to the user either in solid form or in the form of a liquid solution. The agents A and B can be added in solid form to the whitening agent while the latter is in solid form. All three agents may, furthermore, be included in a liquid solution, preferably a water solution. It is also possible to delay the addition of agents A and B until just before or at the time of use of the whitening agent. When the whitening agent is to be used for the manufacture of paper/paperboard, for instance the agents may be added when the size and/or coating color is being prepared. In this connection it is also possible to add the additives A and B separately, e.g. after the size and/or coating color containing FWA is applied to the paper/paperboard.

Advantages

The examples described later on in this specification show that the additives A and B can assist in stabilizing, i.e. to an increased extent maintaining, the positive effect of fluorescent whitening agent on the brightness/whiteness of various objects to which the whitening agent is added. It is inevitable that this effect will fade in time and the circumstances have already been described that particularly contribute to the declining effect or, in other words, the destabilization. Use of the invention produces a stabilizing effect both in the fluorescent whitening agent per se, in the form of a water solution, for instance, and when the whitening agent is applied in or on an object.

The additives A and B are not particularly expensive and neither, to our knowledge, do they have any negative influence on the environment.

Description of the drawings

Figure 1 shows fluorescence spectra of water solutions containing whitening agent with a specific, higher chemical concentration. Figure 2 shows fluorescence spectra of water solutions containing whitening agent with a specific, lower chemical concentration.

Best embodiment

The invention is described in the following in more detail in certain respects, and experiments performed in laboratories are presented at the same time. Example 1

Water solutions of a fluorescent whitening agent of type TINOPAL^® ABP-Z Fluessig from Ciba AG were prepared in a laboratory.

The structural formula for this chemical is as shown below:

Two water solutions of whitening agent were prepared, one with 0.1 per cent by weight of whitening agent and the other with 0.3 per cent by weight of whitening agent. When preparing whitening agent solutions as such and/or when the whitening agent is included in a mixture with other chemicals such as a size or a coating color, it is normal also to use a so-called carrier. This may consist of polyvinyl alcohol (PVA). Said water solutions were divided into three batches. The first batch was intact, i.e. a zero sample. PVA was added to the second batch (comparison sample) and, as well as PVA, ascorbic acid of pro analysis quality -plus sodium sulphite of pro analysis quality - was also added (preparation in accordance with the invention), h the case with a whitening agent concentration of 0.1 per cent by weight (%), the addition of PVA was 0.4 per cent by weight in the two batches concerned and the addition of ascorbic acid 0.1 per cent by weight and of sodium sulphite 0.5 per cent by weight in the third batch. In the case with a whitening agent concentration of 0.3 per cent by weight (%), the corresponding additions were 1.2, 0.3 and 1.5%. When preparing the two third batches the chemicals were mixed in the following order: 1) = polyvinyl alcohol, 2) = ascorbic acid, 3) = sodium sulphite and 4) = FWA. Deionized water (Maxima Ultra Pure Water) was used for the solution and dilution before the water solutions were combined. To protect the FWA from being subjected to light exposure the solutions were mixed in a volumetric flask clad in aluminium foil. For the same reason the FWA solution was mixed in last.

These solutions were measured using a fluorescent spectrometer of type SPEX FLUOROLOG 1680 0.22 m DOUBLE SPECTROMETER with relevant software DATAMAX for Windows (version 2.20). The samples were exited using light with a wavelength 350 nanometer and fluorescence from the samples was measured in the wavelength interval from 380 nanometer to 650 nanometer.

The various sample solutions or batches were then subjected to illumination with light having a wavelength of 365 nanometer from a high-pressure mercury lamp with water-cooled filters for said mercury line or wavelength for a certain period of time. The illumination effect at said wavelength of 365 nanometer was approximately 240 mW/cm². This illumination corresponds to 40 times normal sunlight in the wavelengths where FWA absorbs light. After a treatment time of 15 minutes the batches were measured in the manner described above. As mentioned earlier, there are several reasons for fluorescent whitening agent becoming destabilized. One reason is that the agent is illuminated by ultraviolet light which, as is known, is included in daylight. In this example we have chosen the described illumination as destabilization method.

Figures 1 and 2 show the results measured. The light intensity is stated in cps (counts per second) along the y-axis and the wavelength is stated in nanometers along the x-axis.

The curves in Figure 1 relate to the stronger chemical solution with a concentration of 0.3% of the fluorescent whitening agent, while the curves in Figure 2 relate to the weaker chemical solution with a concentration of 0.1% of the fluorescent whitening agent.

The same curve designations are used in the two figures. The unbroken curve is the zero sample, i.e. that with only the fluorescent whitening. The curve with long dashes is the batch to which PNA was added and which was not subjected to destabilizing light treatment and the curve with short dashes represents the same batch after having been treated with light for 15 minutes. The curve with alternating dashes and dots represents the batch in accordance with the invention, i.e. containing ascorbic acid plus reducing agent in the form of sodium sulphite as well as PNA and, finally, the dotted curve represents the same batch after being treated with light for 15 minutes.

As regards the results achieved it can be ascertained that the carrier, in this case PNA, shifts the spectrum towards wavelengths (e.g. 430 nm) where the monomer form of the fluorescent whitening agent fluoresces, which is preferable, especially from the whiteness aspect. Many chemicals and/or materials may be used as carrier. The cellulose in pulp fibres is one example. However, cellulose is not as effective as PNA, for instance. As regards samples or batches 2 and 3, in both Figure 1 and Figure 2 the batches should in the first place be compared with each other, i.e. the sample before it was subjected to destabilization and after 15 minutes of light radiation. In Figure 1 the highest fluorescence, measured at wavelength 430 nanometer, for instance, is achieved when only PNA had been added to the fluorescent whitening agent, without radiation (the curve with the long dashes). After 15 minutes of radiation (curve with short dashes) the light intensity has dropped drastically, more specifically 30% at the wavelength 430 nanometer. The reverse is the case in the case of the sample or batch in accordance with the invention. After radiation of the sample for 15 minutes (curve with dots) the light intensity at wavelength 430 nanometer has increased by 30% in comparison with the unradiated sample (dot-dash curve).

In practice the light intensity of the fluorescent whitening agent (mixed with carrier, for instance) at the moment when it is supplied to or applied on an object, e.g. paperboard in the form of a web, is of no interest. It is when the object, e.g. paperboard, is to be used that is of interest and it is the brightness/whiteness of the object at this point in time that matters. If a comparison is in any case made between batch 2 and batch 3 after 15 minutes of light radiation at a whitening agent concentration of 0.3%, i.e. in accordance with Figure 1, it is found that the light intensity at a wavelength of 430 nanometer, for instance, is somewhat higher for the batch prepared in accordance with the invention than for the batch containing only whitening agent and carrier. Looking at wavelengths above 430 nanometer, the advantage for the batch prepared in accordance with the invention is striking. Similar results are achieved with a weaker whitening agent solution (0.1 % whitening agent), as can be seen from Figure 2. However, it can be ascertained that all the curves for batches 2 and 3 are much closer to each other in this Figure than is the case for corresponding curves in Figure 1.

Example 2

The following experiment was performed in the laboratory of a paperboard mill. Solid paperboard of standard quality having a basis weight of 240 g/m² and pre-coated twice with a standard coating color in amounts of 10 and 6 g m², respectively, was selected at the mill. No pulp fibres containing lignin was used in any of the different layers in a number of five. Only bleached sulphate pulp was used in the various pulp stocks which included both softwood pulp and hardwood pulp. All the pulp batches had a brightness in excess of 88% ISO.

After having been cut to A4 size, samples of the above-mentioned paperboard were coated with 13 g/m² of two coating colors in a bench coater at the laboratory. The first color, a standard coating color, contained the following ingredients:

70 parts calcium carbonate in the form of milled marble (of type Hydrocarb) 30 parts clay (of type Kaofine) 16 parts latex

0.8 parts polyvinyl alcohol 0.4 parts thickener (carboxymethyl cellulose)

0.2 parts setting agent in the form of ammonium zirconium carbonate (of type Bacote) 0.2 parts FWA (of type Blankophor^® P Fluessig)

The formula for the fluorescent whitening agent and its chemical structure are described in Example 1.

In absolute weight the color contained 1 gram of the agent calculated on 500 gram pigment, i.e. calcium carbonate plus clay.

Some (10) samples of paperboard were coated with this color and were then allowed to air-dry in the laboratory at room temperature. The same number of samples of paperboard were coated with a color having the above composition and provided with two additional chemicals in accordance with the invention, namely 3 gram ascorbic acid and 17 gram sodium sulphite (Na SO₃) calculated on 1 gram FWA, which can be termed the second mixture. The paperboard samples were then allowed to air-dry in the laboratory at room temperature. The air-dried paperboard samples were exposed to light in a test chamber of type Suntest XLS+ for one day. The light was generated by a Xenon lamp, whose light resembles that of the sun. However, certain light wavelengths had to be filtered off. This is done using filters of glass that allow through ultraviolet light (UN). During the experiments a filter was used that filters out light with a wavelength less than 320 nanometer, this light resembling that in a display window. The output of the Xenon lamp was 600 W. Exposure in the test chamber for one hour is equivalent to approximately 3.5 days in daylight.

Some interruptions in the exposure of the paperboard samples were made in order to measure their optical properties. The properties measured were brightness, whiteness and wavelength-dissolved reflectance. The measurements were performed using Elrepho SF 450 from Datacolor International. The measurements were also carried out on the paperboard samples before they were inserted into the test chamber.

Table 2 below records the D65 brightness and whiteness values in accordance with CIE, for each series of paperboard samples. CTE, which stands for "Commission Internationale de l'Eclairage", shows the spectral energy distribution for a number of different standard lights. A corresponds to light from an ordinary light-bulb, C corresponds to daylight at the rear of a sunlight object and D65 is equivalent to C but with a somewhat increased ultraviolet component.

Table 2

It can be seen that the added chemicals have a possible initial (see the values for 0 hours) positive effect on the brightness and whiteness, hi the case of brightness the difference is only 0.2 units, which might be within the error margin. However, in the case of whiteness the increase is 1.9 units.

In the case of the paperboard samples containing layers with unstabilized FWA, one hour of exposure to light resulted in 5.6 unit decrease in brightness and all of 14 units decrease in whiteness. This constitutes a dramatic decrease, particularly in the whiteness. Equivalent decreases in the case of the paperboard samples having layers of FWA stabilized in accordance with the invention were 2.5 and 6.7 units, respectively.

After 24 hours' exposure of the paperboard samples to light the brightness and whiteness of the samples with a layer containing FWA stabilized in accordance with the invention had decreased by 6.3 and 18.1 units, respectively, whereas the equivalent decreases in the paperboard samples with layers containing unstabilized FWA were 8.4 and 22.1 units, respectively. In other words, the paperboard samples where the method according to the invention was utilized had a brightness over two units higher and a whiteness six units higher than the reference samples.

Example 3

Once again solid paperboard of standard quality was taken from the mill. More detailed information concerning the paperboard is provided in Example 2. However, in this case the paperboard was not pre-coated. After having been cut to A4 size the paperboard samples were coated in said bench coater in the laboratory with both the standard coating color as described in Example 2, and the same color with the addition of the additive chemicals in accordance with the invention in four different additive levels. These different levels were 0.5 g ascorbic acid plus 2.5 g sodium sulphite, 1 g ascorbic acid plus 5 g sodium sulphite, 2 g ascorbic acid plus 10 g sodium sulphite, and 4 g ascorbic acid plus 20 g sodium sulphite, calculated per gram FWA.

The paperboard samples were handled and exposed to light in the same way as described above. However, the exposure time was extended to 48 hours. Furthermore, measurement of the brightness in accordance with CIE D65 was replaced by measurement in accordance with ISO. One series of paperboard samples was then placed in a heating own with the temperature set at 80°C in order to discover how increased temperature affects the process of decreasing the brightness and whiteness in time. In this case also the exposure time was 48 hours.

Table 3 below shows how the brightness and whiteness of the various paperboard samples altered depending on the exposure time and Table 4 shows how these parameters changed depending on the time the paperboard samples remained in a temperature of 80°C. In these tables the paperboard samples with a coating layer of only FWA are designated reference samples. Furthermore, the addition of ascorbic acid is stated in grams per 500 g pigment in the coating color.

Table 3

Exposure Brightness, ISO Whiteness, CIE time in hours Quantity of ascorbic acid Quantity of ascorbic acid

Ref 0.5 g 1 g 2 g 4 g Ref. 0.5 g 1 g 2 g 4 g

0 89.7 90.5 90.9 90.4 90.4 105.0 107.6 108.5 106.0 105.8

1 86.2 88.8 89.4 89.7 89.8 97.6 99.4 100.7 103.2 102.3

2 87.9 88.4 89.0 89.3 89.3 95.6 97.1 98.1 100.4 99.4

4 87.6 88.2 88.5 89.1 88.6 93.4 95.0 95.5 98.5 95.5

6 - 88.4 - 88.5 - 94.1 - 94.9

8 87.4 87.7 - 88.6 - 91.4 92.3 - 95.8 -

15 - 87.7 - 87.7 - 89.8 - 90.5

16 87.7 87.9 87.4 88.7 87.3 89.7 90.3 87.5 93.1 87.7

24 87.5 87.7 - 88.4 - 88.1 88.9 - 91.2 -

48 87.3 87.3 - 87.9 - 86.1 86.0 - 87.8 - Table 4

Storage Brightness, ISO Whiteness, CIE time in hours Quantity of ascorbic acid Quantity of ascorbic acid

Ref 0.5 g i g 2 g 4 g Ref. 0.5 g i g 2 g 4 g

0 89.9 90.4 90.5 90.4 90.3 105.8 107.1 107.6 105.1 105.4

1 89.8 90.3 90.3 90.2 90.2 104.9 106.3 106.6 104.4 104.9

2 89.6 90.2 90.1 90.1 90.0 104.4 105.8 106.1 104.1 104.4

4 89.7 90.2 90.3 90.2 90.2 104.5 106.0 106.4 104.3 104.9

8 89.4 89.9 90.0 90.0 90.1 103.6 105.2 105.7 103.8 104.3

16 89.9 90.7 90.8 90.8 90.7 104.3 106.5 107.2 105.2 105.8

24 89.7 90.5 90.7 90.8 90.7 103.7 106.1 107.1 105.0 105.6

48 89.3 90.3 90.3 90.5 90.5 102.3 105.2 105.8 104.3 105.3

A comparison of the results in Tables 3 and 4 shows that the exposure to light has a more destructive effect on the fluorescent whitening agent than an increase in temperature to 80°C.

In these experiments also, the addition of ascorbic acid and sodium sulphite stabilizes the fluorescent whitening agent, although the effect in these experiments is not as pronounced as in the experiments in accordance with Example 2. It cannot be unequivocally determined in these experiments that the addition of a certain quantity of these substances gives an optimal effect. However, it would appear that the addition of 1 to 2 gram ascorbic acid per gram FWA is sufficient. The quantity of reducing agent shall be at least the same as the quantity of organic acid, e.g. ascorbic acid, added, and suitably seven times this quantity.

Claims

1. A method for increasing the stability of fluorescent whitening agent after its preparation and during its use either on its own or mixed with other chemicals, characterised in that the whitening agent is supplied with an organic acid containing aromatic group and/or can be estenfied internally forming lactone, or the salt of the acid (additive A) and reducing chemical (additive B).

2. A method as claimed in claim 1, characterised in that the additive A consists of an organic acid containing aromatic group or the salt of the acid.

3. A method as claimed in claiml , characterised in that the additive A consists of an organic acid or its salt with the general formula (acid form):

where R_x = -CHOHCH₂OH or -CHOHCOOH

4. A method as claimed in claim 3, characterised in that the additive A consists of ascorbic acid or its salt.

5. A method as claimed in claims 1-4, characterised in that the additive B consists of a reducing agent which is effective and inexpensive.

6. A method as claimed in claim 5,characterised in that the additive B consists of a chemical containing hydrogen sulphite and/or sulphite.

7. A method as claimed in claims 1-6, characterised in that the additive A is supplied in a quantity in a mol ratio of 0.1-50 to 1 to the fluorescent whitening agent and the additive B is supplied in a quantity calculated in mol that is at least the same as the quantity of the additive A calculated in mol.

8. A method as claimed in claims 1-7, char act er is ed i n that a carrier is also included or is brought into contact with the chemical mixture.

9. A method as claimed in claims 1-8, char acteris ed in that the fluorescent whitening agent and the additives A and B are included in a size and/or coating color that is applied to the surface of paper/paperboard.