Superabsorbing fibers and films and processes for preparing...

Details Superabsorbing fibers and films and processes for preparing... Superabsorbing fibers and films and processes for preparing...

1999-11-03

2001-09-25

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S183000, C521S184000, C521S189000, C528S322000, C528S328000, C528S363000, C528S480000, C528S482000

Reexamination Certificate

active

06294588

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to superabsorbing polymers based on L-aspartic acid in synthetic fiber and film forms and to process(es) for preparing such fibers and films.
Polysuccinimide (PSI) is prepared by thermal polycondensation of L-aspartic acid which can then be base-hydrolyzed to polyaspartate salt which has many industrial uses such as lubricant in metalworking fluids. Crosslinking PSI before or after hydrolysis renders the hydrolyzed salt superabsorbent in that it can absorb many times its weight of liquid such as water. This capability of absorbing significant quantities of fluids, including body exudates and aqueous compositions of all kinds, creates another important class of application for these polymers in products such as diapers, sanitary napkins, incontinence products, towels, tissues and the like. These superabsorbing polymers are in the prior art as typically disclosed in U.S. Pat. No. 5,461,085 (Nagatomo et al); U.S. Pat. No. 5,525,703 (Kalota) and U.S. Pat. No. 5,612,384 (Ross et al). Though articles of superabsorbing polymers derived from L-aspartic acid are recognized in this art as desirable, to date they are only disclosed in unshaped, particulate form as recovered from the hydrolysis step forming the salt. Note all the examples of the patents referenced above. Continued absence in the art of these polymers in fiber or film form is likely due to the important conflicting requirements of being sufficiently plastically extensible to permit drawing fiber or forming film while being crosslinked and hydrolyzed to create superabsorbency.
In view of the noted applications, it would be highly desirable to provide biodegradable superabsorbent polymers derived from L-aspartic acid in fiber or film form to facilitate formation into products such as diapers and the like.
The superabsorbent fibers and films of the present invention provide a solution to many problems encountered with unshaped, particulate superabsorbent polymers derived from L-aspartic acid. For example, the superabsorbent fibers of the invention have the advantages of (1) avoiding the problem of migration by having the ability to entangle with fluff pulp fibers, (2) being compatible with other fibers resulting in greater flexibility in applications and fabrication, and (3) having a large surface area resulting in a faster absorption rate. For example, the superabsorbent films of the invention have the advantage of avoiding the problem of migration by having the ability to remain stationary within an article containing the film.
SUMMARY OF THE INVENTION
Now, significant developments have been made in producing superabsorbent polymer fibers and films derived from L-aspartic acid.
Accordingly, a principal object of this invention is to produce synthetic, superabsorbent fibers and films of L-aspartic acid derivatives.
Another object is to provide a method for producing such fibers or films.
A further object is to chemically modify PSI to render it capable of drawing into fiber or forming film.
Yet another object is to provide an intermediate precursor for production of fibers or films based on PSI prepared from L-aspartic acid.
Other objects will in part be obvious and will in part appear from the following detailed description.
These and other objects are accomplished by the following multiple aspects of the invention.
i) A fiber- or film-forming plastic composition must be sufficiently plastically extensible to permit forming into film or drawing into a filament-like shape without fracture. It is difficult to draw fibers or form films from crosslinked and hydrolyzed PSI which has not been further modified. This problem is remedied by one aspect of the invention by forming an uncrosslinked, non-hydrolyzed, internally plasticized poly(imide-co-amide) intermediate precursor which is useful in later forming such superabsorbing polymeric fibers or films. The intermediate is prepared by reacting a regulated amount of about 1 to about 20% of succinimide groups of the PSI with a minor, internally plasticizing amount of one or more monoamine compounds, such as diethanolamine. The partial amidation with the monoamine compound softens the polymer and facilitates subsequent drawing into fiber form or forming into film. For example, the poly(imide-co-amide) intermediate produced using diethanolamine as the monoamine compound has repeating structural units represented by formula (1)
and repeating structural units represented by formula (2)
the mole fraction of repeating structural units represented by the formula (1) being about 0.01 to about 0.20.
ii) The internally plasticized poly(imide-co-amide) intermediate is subsequently hydrolyzed to convert essentially all of the succinimide groups into aspartate groups. A portion of the aspartate groups in the polyamide produced as a result of the hydrolysis are then partially acidified to produce a partially acidified, hydrolyzed, internally plasticized polysuccinimide composition.
iii) Since fiber- or film-forming polymer cannot be effectively drawn or formed when crosslinked, another aspect of the invention incorporates crosslinking agent into the partially acidified, hydrolyzed, internally plasticized polysuccinimide composition before shaping fiber or film from the crosslinkable but uncrosslinked intermediate composition. Crosslinking aspartate groups of the polymer occurs after fiber or film formation in an after-treating or curing step by subjecting the fibers or film to crosslinking conditions to render them capable of superabsorbing. Premature crosslinking is minimized or avoided by incorporating heat reactive crosslinking agents into the composition at low temperature, i.e. from or about 0° C. to or about 25° C.
iv) Products of the process differ from those previously known in that the superabsorbing polymer is importantly in fiber or film form. The crosslinked fibers or film are of polyamide containing at least three divalent or polyvalent moieties randomly distributed along the polymer chain having the following formulas:
where A represents hydrogen, alkali metal cation, ammonium, quaternary ammonium or mixtures thereof, R represents a divalent or polyvalent crosslinker moiety, x, y and z represent mole fractions of the moieties in the polyamide and are respectively about 0.01 to 0.20; about 0.40 to 0.90 and about 0.01 to 0.50 wherein the sum of x, y and z is 1.0, and n is an integer from 0 to 4. R
1
and R
2
are substituents on the monoamine compound used for the internal plasticization of PSI and can be the same or different. Optionally, the superabsorbent polymer contains minor amounts of unreacted succinimide repeating units, i.e. repeating unit disclosed in formula (2) above, and unreacted acidified aspartate repeating units, i.e. repeating unit disclosed in formula (4) herein. As used herein, a minor amount of succinimide repeating units or acidified aspartate repeating units is an amount up to that amount which has a detrimental effect on the absorbency properties of the superabsorbent fiber or film.
v) In a specific aspect of the invention, a process is provided for preparing superabsorbing fibers or film which comprises the following steps in the recited sequence:
i) condensation polymerizing L-aspartic acid to form polysuccinimide (PSI) having a weight average molecular weight of at least about 20,000 Daltons;
ii) reacting about 1 to about 20% of the succinimide groups of the PSI with one or more monoamine compounds to form an internally plasticized poly(imide-co-amide) intermediate;
iii) hydrolyzing essentially all of the succinimide groups of the poly(imide-co-amide) to form an internally plasticized polyamide;
iv) acidifying about 1 to 50% of the aspartate groups of the polyamide of iii);
v) admixing crosslinker with the partially acidified polyamide of iv) under non-crosslinking conditions to form a crosslinkable, uncrosslinked, partially acidified, hydrolyzed, internally plasticized PSI composition;
vi) drawing fibers or forming film from the composition of v); and
vii) subjecting the fibers or film to crosslinking

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