TECHNICAL FIELD

This disclosure relates to methods of and systems for forming modified fiber, in particular intrafiber crosslinked cellulose fibers, from pulp sheets and/or fragments of pulp sheets.

BACKGROUND

Traditionally, cellulose fibers from southern pine and other softwood species are used in absorbent products, in large part because the morphology of these fibers provides good absorbent performance. Compared to hardwood fibers, southern pine and other softwood fibers tend to be longer (e.g., having a length weighted fiber length of about 2.5 mm) and more coarse (e.g., having a coarseness greater than about 20 mg/100 m), and form low density pads with sufficient void volume to hold several times their weight in liquid. Hardwood fibers, on the other hand, are known for their performance in paper applications where shorter fiber length (e.g., about 1 mm) and lower coarseness (e.g., less than about 20 mg/100 m) provide a dense structure and smooth paper surface.

Crosslinked cellulose fibers are usually produced by applying a crosslinking agent to a dried sheet or roll of conventional softwood pulp fibers, generally at a dilute concentration to ensure chemical impregnation of the sheet, followed by wet fiberization in a hammermill to generate treated, individualized cellulose fibers. These fibers are then dried, such as in a flash drier, and cured, such as in an oven. The resulting fibers exhibit intrafiber crosslinking in which the cellulose molecules within a cellulose fiber are crosslinked. Intrafiber crosslinking generally imparts twist and curl to the cellulose fiber, and also imparts bulk to the fiber, properties that are advantageous in some absorbent products.

One drawback of this method is the high capital cost of the production process, as well as high energy costs due to drying the fiber prior to curing. Another drawback is that wet hammermilling can generate fiber and chemical buildup under usual mill conditions of heat and high airflow. Additionally, wet hammermilling produces undesirable features such as knots, which are unfiberized fiber clumps or pieces of the original pulp sheet. Generally, as production speeds increase, the level of knots also increases as the hammermilling efficiency is reduced.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

Various embodiments of methods of forming crosslinked cellulose products, as well as crosslinked cellulose products formed therefrom, are disclosed herein. The products may include, for example, individual crosslinked cellulose fibers, as well as mats, pads, sheets, webs, and the like generally made from individual crosslinked cellulose fibers.

In one aspect, the present disclosure provides methods of forming crosslinked cellulose products that include mixing a crosslinking agent with cellulose fiber mat fragments formed of hydrogen-bonded cellulose fibers having a high solids content—that is, a solids content of at least about 45% and up to about 95%. The crosslinking agent is added in an amount suitable to effect a desired level of crosslinking in the cellulose fibers based on the solids content of the mat fragments. In some methods, the mixing is sufficient to achieve individualizing (fluffing) the cellulose fibers while forming a substantially homogenous mixture of fibers and crosslinking agent. In some methods, the mixing is performed at ambient conditions. In some methods, the solids content of the mixture (of the crosslinking agent and the mat fragments) is set to be about 40-60%, such as by adding the crosslinking agent at a concentration that will achieve such a mixture solids content when mixed with the mat fragments. This may involve diluting or concentrating the crosslinking agent prior to mixing it with the mat fragments. The methods further include drying the resulting mixture (also referred to in terms of what it consists of—that is, chemically treated individual fibers) to 85-100% solids, then curing the dried chemically treated individual fibers to crosslink the fibers. Some methods further include, prior to mixing, preparing the mat fragments by fragmenting—that is, shredding, cutting, dicing, or otherwise breaking into pieces—a cellulose fiber mat or sheet, such as a pulp sheet. These mats or sheets may be provided in bale, wet lap, or roll form. In some cases, a mat or sheet may be moistened, such as to soften it, prior to or during fragmenting. Some examples of moistening agents include water, crosslinking agent, a catalyst solution, other liquid based additives, or various combinations thereof.

In one particular, non-limiting example of such a method, cellulose fiber mat fragments having a high solids content are formed by shredding, cutting or dicing a cellulose pulp sheet, followed by mixing a polyacrylic acid crosslinking agent with the mat fragments in an amount to achieve a chemical on pulp level of about 2-14%, wherein said crosslinking agent is mixed with the fiber fragments at ambient conditions. The target solids content of the mixture is about 50-60%, and is set by adding the crosslinking agent at a concentration suitable to achieve the target mixture solids content and the desired chemical dosage. During mixing, the mat fragments are individualized into discrete cellulose fibers in the mixer. The resulting chemically treated individual fibers are then dried and cured as above.

In another aspect, the present disclosure provides embodiments of a system for forming crosslinked cellulose products, which include a mixer configured to form, from cellulose fiber mat fragments formed of hydrogen-bonded cellulose fibers and having a high solids content of about 45-95% and a crosslinking agent, a substantially homogenous mixture of non-crosslinked, individualized cellulose fibers and crosslinking agent, at ambient conditions. This mixture is also referred to as chemically treated individual fibers. The system further includes, downstream of the mixer, a dryer configured to dry the substantially homogenous mixture to a consistency of 85-100% without curing the crosslinking agent; and a curing unit coupled to the dryer that is configured to cure the crosslinking agent, thereby forming dried and cured crosslinked cellulose fibers.

In yet another aspect, the crosslinking agent can be added to the pulp sheet prior to generating individual cellulose fibers by means described herein and other methods known in the art. More particularly, the crosslinking agent can be added to the pulp sheet or mat prior to the formation of the mat fragments, or after the formation of the mat fragments. Addition prior to fragmenting is possible by means such as coating, spraying, dipping, etc. Crosslinking agent can be added subsequent to fragmenting, for example, by spraying prior to mixing in the mixing unit. If wet lap is used as the starting cellulose mat, it is also possible to add the crosslinking agent during the wet lap process such that the crosslinking agent is present in the wet lap mat, for example in the targeted dosage.

In another aspect, the present disclosure provides intrafiber crosslinked cellulose pulp fibers having a chemical on pulp level of about 2-14% and an AFAQ capacity of at least 16.0 g/g. In some embodiments, the cellulose fibers are, or include, hardwood cellulose pulp fibers, such as eucalyptus cellulose pulp fibers or mixtures of fibers.

The concepts, features, methods, and component configurations briefly described above are clarified with reference to the accompanying drawing and detailed description below.

