Field of the invention

The present invention is directed to a fluff pulp having improved absorption properties and higher network strength. The invention is also directed to a process for producing such fluff pulp and to the use thereof.

Background

Fluff pulp is commonly made from softwood pulp from sulphate pulping. Depending on the pulping method, the properties of fluff pulp differs. Raw materials for softwood pulp may be various softwood species, such as Scots pine, Norway spruce, Southern pine, Loblolly pine, Slash pine, Radiata pine, Douglas fir, Hemlock Cedar, White spruce, Lodgepole pine, Alpine fir or mixtures thereof.

Fluff pulp is used for example for producing absorption cores for hygiene products and in airlaid-nonwoven for various applications such as pre-formed absorbent cores, wipes, tabletop, food pads etc.

It would be desirable to be able to provide fluff pulp having improved defibration efficiency, lower knot content and lower fines content, particularly for products which incorporate a relatively large amount of fluff pulp. For thinner products it would be desirable with a more compressible fluff pulp. In addition, it is important to achieve adequate liquid management, i.e. uptake as well as distribution, in for example products comprising absorption cores. Summary of the invention

The present inventors have surprisingly found that by using a specific fraction of a fiber population, referred to herein as the reject fraction, containing longer and coarser fibres and less fines, a fluff pulp with improved defibration efficiency (less knots and less fines) can be obtained. This fluff pulp is suitable for traditional absorbent hygiene products using a large amount of fluff pulp in the absorbent core. Other advantages of such a fluff pulp are improved absorption properties and higher network strength.

The other fraction, the accept fraction, obtained in the fractionation according to the present invention contains shorter and slimmer fibres and can be used to produce a more compressible fluff pulp that is easier to densify and with improved liquid spreading properties. Such fluff pulp is suitable to be used in absorbent cores for thinner absorbent products.

By using the different fractions in a layering structure of air-laid nonwoven for use in absorbent hygiene products, it is possible to build a structure with for example the coarser fraction on the top, giving a fast acquisition time, and the thinner and more flexible fibre fraction in the bottom, enabling good spreading properties of the liquid utilizing the whole absorption core.

According to the present invention, a standard fluff pulp is fractionated, so that the average fibre length in one of the fractions, the reject fraction, the average fibre length increases from 2.03 to 2.50 mm, and the coarseness increases from 178 to 187 pg/m.

In the other fraction, the accept fraction, the average fibre length decreases from about 2 mm to about 1.7 mm, and the coarseness decreases from about 178 to about 165 pg/m. It was found that this fraction has increased compressibility. Fluff pulp prepared from this fraction has length weighted average fiber length in the range of from 1.6 to 1.8 mm and coarseness in the range of from 156 to 173 pg/m and absorption time, in the dry defibrated state, in the range of from 2 to 4 seconds according to the standard SCAN-C 33:80.

The present invention is thus directed to fluff pulp having length weighted average fiber length in the range of from 2.3 to 2.7 mm and coarseness in the range of from 180 to 194 pg/m and having fines content of less than 7% according to the standard ISO 16065 and having absorption time, in the dry defibrated state, in the range of from 2 to 4 seconds according to the standard SCAN-C 33:80.

The present invention is also directed to a method to produce said fluff pulp.

The present invention is also directed to the use of said fluff pulp. The fluff pulp is produced and delivered to customers in the form of rolls. The fluff pulp is first defibrated. The defibrated pulp is then used in the manufacture of for example absorption cores for hygiene products and in airlaid-nonwoven for various applications such as pre-formed absorbent cores, wipes, tabletop, food pads etc.

A further aspect of the present invention is the use of a combination of layers prepared from the different fractions in a layering structure of air-laid nonwoven for use in absorbent hygiene products. Thus, one aspect of the present invention is a product comprising at least one layer comprising or prepared from defibrated fluff pulp, wherein said fluff pulp is, prior to defibration, a fluff pulp having length weighted average fiber length in the range of from 2.3 to 2.7 mm and coarseness in the range of from 180 to 194 pg/m, and at least one layer comprising or prepared from defibrated fluff pulp, wherein said fluff pulp is, prior to defibration, a fluff pulp having length weighted average fiber length in the range of from 1.6 to 1.8 mm and coarseness in the range of from 156 to 173 pg/m. Description of the figures

Fig 1: Fiber length distribution of reference pulp and fractionated pulp (reject fraction, accept fraction).

Detailed description

As used herein, the term, "fluff” means defibrated, i.e. fiberized or shredded “fluff pulp”.

For the tests below, the fluff pulp should be conditioned at 23°C, 50% RH at least 4 h before analysis.

As used herein, the term “absorption time” refers to the absorption properties of fluff pulp according to the standard SCAN-C 33:80, which involves the determination of the time consumed to completely saturate a standardized test specimen with absorbed liquid. A 3g fluff sample, 50 mm in diameter, is formed in a forming unit. The sample is placed in a sample holder and a load of 2.8 kPa is applied. Tap water (23°C) is applied underneath and the time required for the water to reach the top of the sample is recorded (average of 5 replicates).

As used herein, the term “compressibility” refers to the density of a defibrated fluff pulp sample, wherein the sample has been prepared at a specified pressure measured after different pressures (70 kPa or 1380 kPa). This is done as follows: 3g of defibrated pulp is compressed in a tube with a cross sectional area of 19.625 cm ² in an Instron with a speed of 600 mm/min. The density of the compressed sample is calculated after measuring the thickness after 2 minutes waiting time and determining the weight of the compressed sample.

As used herein, the “knot content” of the fluff pulp refers to the knot content measured by using Alpine Air Jet with screen DIN ISO 3310 1.4 mm. A sample of fluff is agitated for 10 minutes above a wire using 4000 kPa negative pressure. The knots remaining on the wire are weighed and reported as a percentage of the total sample weight (average of two replicates). The knot content of the fluff pulp according to the present invention is preferably less than 13%, more preferably less than 11%, more preferably less than 10%.

As used herein, the term “network strength” refers to the force required to break the fiber network of a fluff sample. The network strength of the fluff pulp according to the present invention is preferably in the range of from 6.5 to 8.0 N. The network strength is determined as follows:

A 1g fluff sample, 50 mm in diameter, is formed in a forming unit. A 1.1 kg load is placed on the sample and remove after 2 min. The fluff sample is placed in a tube with a smaller tube inside holding the edge of the fluff sample in place. The tubes with the fluff sample is then placed in Instron Universal testing machine. The edge of the samples is under load by applying a load on the inner tube holding the edge while a piston penetrating with a constant rate through the fluff sample and the maximum force achieved (to break the fibre network) is equal to the network strength.

As used herein, the term “defibration energy” refers to the energy required for defibration of the fluff pulp. The defibration energy for the fluff pulp according to the present invention can be determined as follows: stripes of fluff pulp, 50 mm width, are weighted and then defibrated in a laboratory Hammer mill HM- DW50 at an infeed speed of 80 mm/s and a mill speed of 3300 rpm. The defibration energy, kJ/kg is then calculated.

The fines content can be determined according to the standard ISO16065. The fines content of the fluff pulp according to the present invention is less than 7%, preferably less than 6%, more preferably less than 5%, determined using a Metso PulpExpert.

The fluff pulp according to the present invention can be produced by the steps of a) preparing sulphate pulp; b) subjecting the pulp to fractionation; c) collecting a pulp fraction having length weighted average fiber length in the range of from 2.3 to 2.7 mm; d) drying the pulp to obtain fluff pulp, having a moisture content of less than 10 weight-%, in the form of a roll.

Fluff pulp fibers may be prepared by one or more known or suitable digestion, refining, and/or bleaching operations such as, for example, known mechanical, thermomechanical, chemical and/or semichemical pulping and/or other well-known pulping processes. The pulp is preferably sulphate pulp (Kraft pulp). The cellulosic fibers of the fluff pulp according to the present invention preferably originate from softwood such as spruce and/or pine. Preferably, the sulphate pulp originates to at least 90% from softwood, less than 10% hardwood has been used in the preparation of the sulphate pulp. Thus, the sulphate pulp mainly originates from softwood but may contain traces of hardwood. Preferably, from 65 wt-% to 90 wt-% of the fibers are fibers from Scots pine and from 10 wt-% to 35 wt-% of the fibers are fibers from Norway spruce.

The pulp used according to the present invention is fractionated. Preferably, the fractionation is carried out on wet pulp in a pressure screen. Preferably, the pressure screen has a hole diameter in the range of from 1.0 mm to 1.4 mm, preferably in the range of from 1.1 mm to 1.3 mm, such as 1.2 mm. Preferably, the fractionation is carried out on wet pulp in one step. The inject pulp is pumped through the screen and split into one finer fraction and one coarser fraction. The flow rate is typically about 1000 l/min. A pulp consistency of 10 g/l thus gives a mass flow of 10 kg/min. The mass reject ratio in the screen is selected to give the desired fiber length in the coarser fraction.

The pulp may be diluted to a desired consistency, such as between 3 and 15 g/l, such as between 3 and 6 g/l, before fractionation. The pulp is washed, either before or after fractionation. The washing is typically carried out using water and may be carried out in one or more steps and may involve filtration. As part of the washing or immediately after the washing step, the pH may be adjusted. Preferably, the pH of the washed pulp should in the range of from 5 to 8, such as from 5 to 7. The washed pulp preferably contains less than 0.5 weight-% (based on the solids) extractives, more preferably less than 0.2 weight-% extractives, even more preferably less than 0.1 weight-% extractives and most preferably less than 0.05 weight-% extractives. In this context, extractives are for examples resins and fatty acids.

The washed pulp is dried on a drying machine using methods known in the art. The dry pulp is formed into rolls after drying. The dried material has a moisture content of less than 10 weight-% (ISO 287), preferably less than 9%, such as in the range of from 6 to 9 weight-%.

In fiber defibration, the fluff pulp or dried web may be fed into one or more mill, such as hammermills, saw mills, knife mills, and/or pin mills, or any combination thereof. They may have a series of series of saws, knives, or pins, or small hammers, for example, that rotate at high speed separating the pulp into individual loose fibers.

The fluff (i.e. the defibrated form of the fluff pulp) according to the present invention can be used in articles and products wherein fluff is typically used. Thus, the fluff can be used in absorption cores in baby diapers, incontinence products and feminine hygiene products. The fluff is also the main component in airlaid nonwoven.

One embodiment of the present invention relates to a core, comprising the fluff and one or more superabsorbent polymer (SAP). In the course of making the core, the fluff may be combined with one or more superabsorbent polymers (SAP). SAPs are known in the absorbent product art. Nonlimiting examples of SAPs include starch-acrylonitrile copolymer, hydrolyzed starchacrylonitrile copolymer, acrylic acid (co)polymer, acrylamide (co)polymer, polyvinyl alcohol, polyacrylate/polyacrylamide copolymers, polyacrylic acid (co)polymers, sodium polyacrylate, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymer, polyethylene oxide, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile, salts thereof, and combinations thereof.

The amount of SAP in the core may suitably range from 1 to 95% by weight of the core.

Thus, the fluff pulp according to the present invention can, following defibration, for example be used in absorption cores and in airlaid nonwovens. It can be used as or in a liquid absorption material such as in absorbents in diapers, such as baby diapers as well as incontinence products. It can also be used for example in napkins and tissues. Such products can be produced using methods known in the art. The products typically comprise one or more further components, such as superabsorbents, polymers, wetting agents, viscose/rayon/lyocell fibers, bicomponent fibre, cotton fiber, other fibers, standard latex binder, specialty latex binder etc.

Examples

Example 1

A standard fluff pulp of Scots pine and Norway spruce in the proportion 80/20 was fractionated in a pressure screen with hole of diameter 1 .2 mm.

The average fibre length in one of the fractions (reject fraction) increased from 2.03 to 2.50 mm and the coarseness increased from 178 to 187 pg/m. The fibre length distribution curves are seen in Fig 1. The average fiber width of the reject fraction was 28.7 pm.

The average fibre length in the other fraction (accept fraction) decreased from 2.03 to 1.68 mm, and the coarseness decreased from 178 to 165 pg/m. The fibre length distribution curves are seen in Fig 1. The average fiber width decreased from 27.4 to 26.2 pm.

The improved properties compared with the standard material are found in the table below.

Table 1. Properties of reference and fractionated pulp

Table 2. Methods used for the analysis In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be affected without departing from the spirit and scope of the invention.