Covalent Bonding of Carboxylated Cellulose Fiber Webs

Abstract

Methods are provided for creating covalent bonding of webs by combining cellulosic fibers having a carboxyl content approximately greater than 7 meq/100 g with one or more crosslinking agents. In a first step, a carboxyl group is placed onto a fiber. In an embodiment, the fiber is then reacted with an oxazoline-functional polymer which has been combined with a polycarboxylate compound. Heat is applied to the treated web, and this enables formation of a cross-linked bridge in the form of a covalent bond. In an embodiment, the covalent bonding of the carboxylated cellulose pulp webs utilizes oxazoline-functional polymers and polyacrylic acid. The oxazoline polymer in combination with polyacrylic acid should form a network polymer with covalent bonds to the cellulose carboxyl groups. The non-woven web may be strengthhened by covalent bonding, thereby improving overall wet/dry strength of the final product.

Description

FIELD OF THE INVENTION

The present invention generally relates to methods for providing covalent bonds on cellulose fiber webs.

BACKGROUND OF THE INVENTION

Cellulose fibers are generally held together by hydrogen bonds. The average energy of a hydrogen bond is 1-5 Kcal. The strength of a paper product is typically related to the strength of the hydrogen bonding. Often times when attempts are made to strengthen the bonding of fibers, other properties are compromised, such as bulk, stiffness, etc. In some cases, increasing bond strength can increase the overall cost of the product, which is undesirable.

Thus, a need exists for a method for increasing bond strength between cellulose fibers without compromising properties of the final product.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention are described in detail below with reference to the following drawings.

FIG. 1 is a diagram of a system for forming covalent bonds in an embodiment of the present invention; and

FIG. 2 is a representation of Epocros polymers in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for creating covalent bonding of webs by combining cellulosic fibers having a carboxyl content approximately greater than 7 meq/100 g with one or more crosslinking agents. In a first step, a carboxyl group is placed onto a fiber. In an embodiment, the fiber is then reacted with an oxazoline-functional polymer which has been combined with a polycarboxylate compound. Heat is applied to the treated web, and this enables formation of a cross-linked bridge in the form of a covalent bond. In an embodiment, the covalent bonding of the carboxylated cellulose pulp webs utilizes oxazoline-functional polymers and polyacrylic acid. The oxazoline polymer in combination with polyacrylic acid should form a network polymer with covalent bonds to the cellulose carboxyl groups. The non-woven web may be strengthened by covalent bonding, thereby improving overall wet/dry strength of the final product.

FIG. 2 illustrates a general class of polymers that have been functionalized with an oxazoline group.

Conventional papermaking fiber may be utilized and a furnish for the same may refer to papermaking fibers made from any species, including hardwoods and softwoods, and to fibers that may have had a debonder applied to them but that are not otherwise chemically treated following the pulping/bleaching process or off-line post pulping/bleaching & drying process. The cellulose fiber may be obtained from any source, including cotton, hemp, grasses, cane, husks, cornstalks or other suitable source. In an embodiment, the cellulose fiber is chemical wood pulp.

The oxazoline-functional polymers may be, for example, any polymer containing an oxazoline containing moiety on the side chain. In place of oxazoline containing polymers, one can use a polyfunctional compound capable of reacting to carboxyl groups (e.g. polyols, polyepoxides, etc.).

The polycarboxylate compound may be, for example, a polymer or oligomer containing multiple carboxyl groups.

The crosslinking agent can include a catalyst to accelerate the bonding reaction between the crosslinking agent and the cellulose molecule, but most crosslinking agents do not require a catalyst. Suitable catalysts include acidic salts which can be useful when urea-based crosslinking substances are used. Such salts include ammonium chloride, ammonium sulfate, aluminum chloride, magnesium chloride, or mixtures of these or other similar compounds. Alkali metal salts of phosphorus containing acids may also be used.

The crosslinking agent typically is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of cellulose fiber. The polycarboxylate compound is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of cellulose fiber.

The cellulosic fibers may have been treated with a debonding agent prior to treatment with the crosslinking agent. Debonding agents tend to minimize interfiber bonds and allow the fibers to separated from each other more easily. The debonding agent may be cationic, non-ionic or anionic. Cationic debonding agents appear to be superior to non-ionic or anionic debonding agents. The debonding agent typically is added to cellulose fiber stock.

Suitable cationic debonding agents include quaternary ammonium salts. These salts typically have one or two lower alkyl substituents and one or two substituents that are or contain fatty, relatively long chain hydrocarbon. Non-ionic debonding agents typically comprise reaction products of fatty-aliphatic alcohols, fatty-alkyl phenols and fatty-aromatic and aliphatic acids that are reacted with ethylene oxide, propylene oxide or mixtures of these two materials.

Examples of debonding agents may be found in Hervey et al U.S. Pat. Nos. 3,395,708 and 3,544,862, Emanuelsson et al U.S. Pat. No. 4,144,122, Forssblad et al U.S. Pat. No. 3,677,886, Osborne III U.S. Pat. No. 4,351,699, Hellston et al U.S. Pat. No. 4,476,323 and Laursen U.S. Pat. No. 4,303,471 all of which are in their entirety incorporated herein by reference. A suitable debonding agent is Berocell 584 from Berol Chemicals, Incorporated of Metairie, La. It may be used at a level of 0.25% weight of debonder to weight of fiber. Again, a debonding agent may not be required.

In FIG. 1, a conveyor 12 transports a cellulosic mat 14 into a treatment zone 16 where an applicator 18 applies a crosslinking agent onto the mat 14. Typically, chemicals are applied optionally to both sides of the mat. The mat 14 is then conveyed into a dryer 20 followed by a flow through oven 22 to cure the crosslinking agent.

The treated pads have low density and good stiffness. The pads can be cut easily using a sharp knife. The material is absorbent and strong even when wet.

The present invention may be better understood by way of the following examples. It should be understood that, in the following examples, to produce the desired carboxyl groups in meq/100 g for experimentation, processes described in U.S. Pat. Nos. 6,379,494; 6,352,348 and 6,919,447 were utilized.

EXAMPLE 1

Ratios on Epocros WS500 and Polyacrylic Acid

Fluff pulp modified to have a carboxyl content of 21 meq/100 g was used to make a 6 inch airlaid pad at 125 gsm. The carboxylated pulp can be in either a neutralized form or in a fully protonated (acid) form. The pads were sprayed with 10 gm of a solution of oxazoline functionalized polyacrylate (Epocros WS500) manufactured by Nippon Shokubai and polyacrylic acid (MW˜3500) from Rohm&Haas, to yield the required level of Epocros and polyacrylic acid shown in the table below. The Epocros level was varied from 3% to 7% based on fiber weight and the polyacrylic acid from 1% to 5% based on fiber weight. The control pads contained only polyacrylic acid. The pads were dried and cured in a convection oven at 120° C. for 10 minutes. The pads were then tested for wet and dry tensile strength using an Instron testing device/system with a vertical pull. For the wet tensile, the pads were sprayed with 10 gm of deionized water, let stand for 10 minutes, then tested.

From Table 1 it can be seen that there is a substantial increase in the dry tensile index with higher strength values for increasing polymer content. The data also show that higher strength values are obtained from the acid form of the carboxylated pulp. Similar results are shown in Table 2 for the wet tensile index.

TABLE 1
Dry tensile Index, Nm/g
	Percent Epocros	Percent Polyacrylic Acid
	WS500	1%	3%	5%
Tensile Index for acidic form of pulp
	Control	0.83	1.20	2.02
	3%	1.17	3.62	2.38
	5%	1.12	5.70	3.56
	7%	1.53	5.08	4.18
Tensile Index for neutralized form of pulp
	Control	0.83	1.20	2.02
	3%	2.65	3.62	2.29
	5%	4.10	3.20	4.40
	7%	3.28	3.74	5.15

TABLE 2
Wet Tensile Index, Nm/g
	Percent Epocros	Percent Polyacrylic Acid
	WS500	1%	3%	5%
Tensile Index for acidic form of pulp
	Control	0.17	0.35	0.49
	3%	0.33	1.01	0.65
	5%	0.32	1.21	1.18
	7%	0.41	1.82	1.24
Tensile Index for neutralized form of pulp
	Control	0.17	0.35	0.49
	3%	0.39	0.50	0.37
	5%	0.89	0.71	0.69
	7%	0.54	0.67	0.75

EXAMPLE 2

Effect of Increasing Carboxyl Content in Pulp

Fluff pulp modified to have a carboxyl content from 3 to 35 meq/100 g was used to make a 6 inch airlaid pad at 125 gsm. The carboxylated pulp can be in either a neutralized form or in a fully protonated (acid) form. The pads were sprayed with 10 gm of a solution of Epocros WS500 and polyacrylic acid (MW˜3500) from Rohm&Haas, to yield the required level of Epocros and polyacrylic acid(PAA) shown in the table below. The Epocros level was varied from 3% to 7% based on fiber weight and the polyacrylic acid was held at 3% based on fiber weight. The pads were dried and cured in a convection oven at 120° C. for 10 minutes. The pads were then tested for wet and dry tensile strength using an Instron testing device/system with a vertical pull. For the wet tensile, the pads were sprayed with 10 gm of deionized water, let stand for 10 minutes, then tested.

From Table 3 it can be seen that there is a substantial increase in the dry tensile index with an increase in the Epocros content, and there is an optimum carboxyl level. It is also apparent that the acid form of the carboxylated pulp is more reactive, yielding higher tensile strengths. Similar results are shown in Table 4 for the wet tensile index.

TABLE 3
Dry Tensile Index, Nm/g
Percent
Epocros - PAA	Carboxyl content of pulp, meq/100 g
on pulp	3	12	21	35
	Tensile Index for acid form
3-3	1.13	1.59	3.62	1.62
5.3	1.45	1.60	5.70	2.15
7.3	2.14	1.82	5.08	4.01
	Tensile Index for neutralized form
3-3	1.22	1.75	2.93	2.83
5-3	1.45	2.21	3.20	2.87
7-3	2.16	2.97	3.74	3.97

TABLE 4
Wet Tensile Index, Nm/g
Percent
Epocros - PAA	Carboxyl content of pulp, meq/100 g
on pulp	3	12	21	35
	Tensile Index for acid form
3-3	0.25	0.35	1.01	0.48
5.3	0.30	0.45	1.21	0.54
7.3	0.39	0.49	1.82	0.81
	Tensile Index for neutralized form
3-3	0.25	0.34	0.50	0.48
5-3	0.29	0.48	0.71	0.54
7-3	0.38	0.65	0.67	0.87

EXAMPLE 3

Ratios on Epocros WS500 and Polymaleic Acid

Fluff pulp modified to have a carboxyl content of 21 meq/100 g was used to make a 6 inch airlaid pad at 125 gsm. The carboxylated pulp can be in either a neutralized form in a fully protonated (acid) form. The pads were sprayed with 10 gm of a solution of Epocros WS500 and polymaleic acid (MW˜3500) from Rohm&Haas, to yield the required level of Epocros and polymaleic acid shown in the table below. The Epocros level was varied from 3% to 7% based on fiber weight and the polymaleic acid from 1% to 5% based on fiber weight. The control pads contained only polymaleic acid. The pads were dried and cured in a convection oven at 120° C. for 10 minutes. The pads were then tested for wet and dry tensile strength using an Instron testing device/system with a vertical pull. For the wet tensile, the pads were sprayed with 10 gm of deionized water, let stand for 10 minutes, then tested.

From Table 5 it can be seen that there is a substantial increase in the dry tensile index with higher strength values for increasing polymer content. The data also show that higher strength values are obtained from the acid form of the carboxylated pulp. Similar results are shown in Table 6 for the wet tensile index.

TABLE 5
Dry tensile Index, Nm/g
	Percent Epocros	Percent Polyacrylic Acid
	WS500	1%	3%	5%
Tensile Index for acidic form of pulp
	Control	0.53	1.20	1.35
	3%	0.66	3.65	1.95
	5%	1.56	3.9	2.64
	7%	2.37	3.67	3.04
Tensile Index for neutralized form of pulp
	Control	0.53	1.20	1.35
	3%	2.507	1.64	1.84
	5%	2.04	2.26	1.75
	7%	2.50	2.66	3.19

TABLE 6
Wet Tensile Index, Nm/g
	Percent Epocros	Percent Polyacrylic Acid
	WS500	1%	3%	5%
Tensile Index for acidic form of pulp
	Control	0.15	0.32	0.40
	3%	0.21	0.69	0.49
	5%	0.47	1.23	0.74
	7%	0.86	1.24	1.13
Tensile Index for neutralized form of pulp
	Control	0.15	0.32	0.40
	3%	0.32	0.31	0.34
	5%	0.48	0.47	0.45
	7%	0.62	0.61	0.72

The Epocros is described as an oxazoline functionalized polymer. The particular polymer backbone used in the example here is a polyacrylate co-polymer. Other heating methods beyond those listed above are contemplated which will accelerate the reaction. These methods are known by those skilled in the art. The temperature range for heating may be approximately 60 degrees Celsius to 150 degrees Celsius. Curing for the process may occur via heat and/or pressure.

While the embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the embodiments. Instead, the invention should be determined entirely by reference to the claims that follow.

Claims

1. A method of covalent bonding of a cellulosic web, the method comprising the steps of:

forming a web from highly carboxylated cellulose fibers;

applying a cross-linking agent to the highly carboxylated cellulose fibers to create a treated web; and

curing the treated web.

2. The method of claim 1 wherein the treated web is cured under heat.

3. The method of claim 1 wherein the treated web is cured under pressure.

4. The method of claim 1 wherein a carboxyl content of the fibers is approximately greater than 7 meq/100 grams.

5. The method of claim 1 wherein the crosslinking agent is a mixture of oxazoline functionalized polymer and polycarboxylate compound.

6. The method of claim 1 wherein the crosslinking agent is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of highly carboxylated cellulose fibers.

7. The method of claim 5 wherein the polycarboxylate compound is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of highly carboxylated cellulose fibers.

8. A cellulosic web comprising:

a highly carboxylated cellulose fiber;

a functionalized polymer; and

a polycarboxylate material;

wherein the functionalized polymer and the polycarboxylate material form a cross-linking agent and further wherein a covalent bond is formed between the highly carboxylated cellulose fiber and the cross-linking agent.

9. The cellulosic web of claim 8 wherein the functionalized polymer is an oxazoline-containing polymer.

10. The cellulosic web of claim 8 wherein the functionalized polymer is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of highly carboxylated cellulose fibers.

11. The cellulosic web of claim 8 wherein the polycarboxylate material is applied in an amount ranging from about 8 kg to about 100 kg chemical per ton of highly carboxylated cellulose fibers.

12. The cellulosic web of claim 8 wherein a carboxyl content of the highly carboxylated cellulose fibers is approximately greater than 7 meq/100 grams.