U.S. patent application number 09/875240 was filed with the patent office on 2003-03-20 for method for preparation of stabilized carboxylated cellulose.
Invention is credited to Jewell, Richard A., Komen, Joseph Lincoln, Weerawarna, S. Ananda.
Application Number | 20030051834 09/875240 |
Document ID | / |
Family ID | 25365434 |
Filed Date | 2003-03-20 |
United States Patent
Application |
20030051834 |
Kind Code |
A1 |
Weerawarna, S. Ananda ; et
al. |
March 20, 2003 |
Method for preparation of stabilized carboxylated cellulose
Abstract
The invention is directed to a method of making a heat and light
stable carboxylated cellulose fiber whose fiber strength and degree
of polymerization is not significantly sacrificed. The method
involves the use of a catalytic amount of a hindered cyclic
oxammonium salt as a primary oxidant and a peracid and halide salt
as a secondary oxidant in an aqueous environment. The oxammonium
compounds may be formed in situ from their corresponding amine,
hydroxylamine, and nitroxyl compounds. The oxidized cellulose is
then stabilized against D.P. loss and color reversion by further
treatment with an oxidant such as sodium chlorite, a chlorine
dioxide/hydrogen peroxide mixture, or a peracid under acidic
conditions. Alternatively it may be treated with a reducing agent
such as sodium borohydride. The method results in a high percentage
of carboxyl groups located at the fiber surface. The product is
especially useful as a papermaking fiber where it contributes
strength and has a higher attraction for cationic additives. The
product is also useful as an additive to recycled fiber to increase
strength. The method can be used to improve properties of either
virgin or recycled fiber. It does not require high
.alpha.-cellulose fiber but is suitable for regular market
pulps.
Inventors: |
Weerawarna, S. Ananda;
(Seattle, WA) ; Komen, Joseph Lincoln; (Bothell,
WA) ; Jewell, Richard A.; (Bellevue, WA) |
Correspondence
Address: |
Keith D. Gehr
Patent Department, CH 2J29
Weyerhaeuser Company
P. O. Box 9777
Federal Way
WA
98063-9777
US
|
Family ID: |
25365434 |
Appl. No.: |
09/875240 |
Filed: |
June 6, 2001 |
Current U.S.
Class: |
162/9 ;
162/157.6; 8/115.51; 8/116.1; 8/181; 8/196 |
Current CPC
Class: |
C08B 15/04 20130101;
D21H 11/20 20130101; D21C 9/002 20130101 |
Class at
Publication: |
162/9 ;
162/157.6; 8/116.1; 8/181; 8/196; 8/115.51 |
International
Class: |
D06M 013/322; D21C
009/00; D06M 013/244 |
Claims
1. A method of making a fibrous carboxylated cellulose which
comprises: oxidizing cellulose fiber by reacting it in an aqueous
suspension with a sufficient amount of a primary oxidant selected
from the group consisting of hindered heterocyclic oxammonium salts
in which the carbon atoms adjacent the oxammonium nitrogen lack
.alpha.-hydrogen substitution, the corresponding amines,
hydroxylamines, and nitroxides compounds of these oxammonium salts,
and mixtures thereof, and a secondary oxidant selected from
peracids and an alkali metal halide salt in a sufficient amount to
induce an increase in carboxyl substitution in the cellulose of at
least 2 meq/100 g; and protecting the carboxylated fibers against
degree of polymerization (D.P.) loss and color deterioration by
further treating them in aqueous suspension with a stabilizing
agent selected from the group consisting of oxidizing agents and
reducing agents in order to remove any cellulose substituents which
tend to cause molecular chain breakage.
2. The method of claim 1 in which the peracid is selected from the
group consisting of peroxymonosulfuric acid, peracetic acid, and
mixtures thereof
3. The method of claim 1 in which the alkali metal halide in the
secondary oxidant is present in an amount of about 0.1-10% by
weight based on cellulose.
4. The method of claim 3 in which the alkali metal halide is sodium
bromide.
5. The method of claim 1 in which the primary oxidant is present in
a range of 0.005-1.0% based on weight of cellulose present.
6. The method of claim 5 in which the primary oxidant is present in
the range of about 0.02-0.25% based on weight of cellulose
present
7. The method of claim 1 in which the peracid secondary oxidant is
present in the range of about 0.1-10% based on weight of cellulose
present.
8. The method of claim 6 in which the peracid secondary oxidant is
present in the range of 0.5-5% based on weight of cellulose
present.
9. The method of claim 1 in which the initial oxidizing reaction is
carried out at a pH between about 5.0-8.5.
10. The method of claim 9 in which the initial oxidizing reaction
is carried out at a pH between about 7.5-8.0.
11. The method of claim 1 in which the initial oxidation step is
carried out for a time between 1 minute and about 10 hours at a
temperature in the range of about 5.degree.-95.degree. C.
12. The method of claim 11 in which the initial oxidation step is
carried out for a time between 0.2-2.5 hours at a temperature in
the range of 20.degree.-80.degree. C.
13. The method of claim 1 which further comprises treating the
carboxylated cellulose fibers with a tertiary oxidizing agent to
stabilize the product by substantially converting any aldehyde
substituents to additional carboxyl groups.
14. The method of claim 13 which comprises further stabilizing the
carboxylated cellulose fibers after treatment with the tertiary
oxidizing agent by treatment with a reducing agent.
15. The method of claim 13 in which the tertiary oxidant is
selected from the group consisting of alkali metal chlorites, a
chlorine dioxide/hydrogen peroxide mixture, and peracids.
16. The method of claim 15 in which the tertiary oxidant is a
mixture of chlorine dioxide and hydrogen peroxide.
17. The method of claim 16 in which chlorine dioxide is present in
an amount of about 0.1-20% by weight and hydrogen peroxide is
present in an amount of about 0.01-10% by weight based on the
cellulose present.
18. The method of claim 17 in which chlorine dioxide is present in
an amount of about 0.3-1.0% by weight and hydrogen peroxide is
present in an amount of about 0.05-1.0% by weight based on the
cellulose present.
19. The method of claim 16 in which the oxidative stabilization
treatment is carried out under acidic conditions in the range of
about pH 0-5.
20. The method of claim 19 in which the oxidative stabilization
treatment is carried out under acidic conditions in the range of
about pH 2-3.
21. The method of claim 13 in which the tertiary oxidant is sodium
chlorite.
22. The method of claim 21 in which the sodium chlorite is present
during the stabilization reaction in a concentration of about
0.1-20% by weight of cellulose
23. The method of claim 22 in which the sodium chlorite is present
during the stabilization reaction in a concentration of about 1-9%
by weight of cellulose.
24. The method of claim 21 in which the oxidative stabilization
treatment is carried out under acidic conditions at a pH between
about 1.5-5.
25. The method of claim 24 in which the oxidative stabilization
treatment is carried out under acidic conditions at a pH between
about 2-4.
26. The method of claim 13 in which the tertiary oxidant is a
peracid.
27. The method of claim 26 in which the peracid is
peroxymonosulfuric acid.
28. The method of claim 26 in which the peracid is present in a
concentration of about 0.1-10% by weight of cellulose.
29. The method of claim 13 in which the tertiary oxidant is present
in the aqueous suspension during the stabilization reaction in a
molar ratio of 5-10 times the presumed aldehyde substitution on the
carboxylated cellulose.
30. The method of claim 13 in which the oxidation during the
stabilization reaction proceeds for a time between 5 minutes and 50
hours.
31. The method of claim 30 in which the oxidation during the
stabilization reaction proceeds for a time between 10 minutes and 2
hours.
32. The method of claim 1 which further comprises treating the
carboxylated cellulose fibers with a reducing agent to stabilize
the product by substantially converting any aldehyde or ketone
carbonyl substituents to hydroxyl groups.
33. The method of claim 32 in which the reducing agent in the
aqueous suspension is a borohydride salt selected from the group
consisting of alkali metal borohydrides, cyanoborohydrides, and
mixtures thereof.
34. The method of claim 33 in which the reducing agent is present
in an amount of about 0.1-4.0% by weight of oxidized cellulose.
35. The method of claim 34 in which the reducing agent is present
in an amount of about 1.0-3.0% by weight of oxidized cellulose.
36. The method of claim 32 in which the reduction reaction proceeds
for a time between 10 minutes and 2 hours.
37. The method of claim 1 in which the cellulose fiber is selected
from the group consisting of bleached and unbleached kraft wood
pulps, prehydrolyzed kraft wood pulps, sulfite wood pulps and
mixtures thereof
38. The method of claim 36 in which the cellulose fiber is recycled
secondary fiber.
39. A method of making a fibrous carboxylated cellulose which
comprises: oxidizing cellulose fibers by reacting them in an
aqueous suspension with a sufficient amount of a primary oxidant
selected from the group consisting of nitroxides having the
composition 11wherein R.sub.1-R.sub.4 are one to four carbon alkyl
groups but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may
together be included in a five or six carbon alicyclic ring
structure, X is methylene, oxygen, sulfur, or alkylamino, and
R.sub.8 and R.sub.10 are one to five carbon alkyl groups and may
together be included in a five or six member ring structure, which,
in turn, may contain one to four substituent groups, the
corresponding oxammonium compounds, amines, and hydroxylamines of
these nitroxides, and mixtures thereof, and a sufficient amount of
a secondary oxidant comprising a peracid with an alkali metal
halide salt to induce an increase in carboxyl substitution in the
cellulose of at least 2 meq/100 g; and protecting the carboxylated
fibers against degree of polymerization (D.P.) loss by further
treating them in aqueous suspension with a stabilizing agent
selected from the group consisting of reducing agents and tertiary
oxidizing agents in order to remove any cellulose substituents
which tend to cause molecular chain breakage.
40. The method of claim 39 in which each X is oxygen, the oxygen
atoms being linked by a two to three carbon alkyl chain to form a
cyclic ketal substituent.
41. The method of claim 40 in which the nitroxide composition is
selected from the group consisting of the 1,2-ethanediol,
1,3-propanediol, 2,2-dimethyl-1,3-propanediol, and the glyceryl
ketals of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.
42. The method of claim 41 in which the nitroxide composition is
the 1,2-ethanediol ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy
free radical.
43. The method of claim 41 in which the nitroxide composition is
the 1,3-propanediol ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy
free radical.
44. The method of claim 41 in which the nitroxide composition is
the 2,2-dimethyl-1,3-propanediol ketal of
2,2,6,6-tetramethyl-4-piperidone-1-- oxy free radical.
45. The method of claim 41 in which the nitroxide composition is
the glyceryl ketal of 2,2,6,6-tetramethyl-4-piperidone-1-oxy free
radical.
46. The method of claim 39 in which the peracid is selected from
the group consisting of peroxymonosulfuric acid and peracetic
acid.
46. The method of claim 39 in which the peracid is
peroxymonosulfuric acid.
47. The method of claim 39 in which the peracid is peracetic acid.
Description
[0001] The present invention is a process for preparation of a heat
and light stable fibrous carboxylated cellulose suitable for
papermaking and related applications. The fibrous product of the
invention is one in which fiber strength and degree of
polymerization are not significantly sacrificed. The process is
particularly environmentally advantageous since no chlorine or
hypochlorite compounds are required.
BACKGROUND OF THE INVENTION
[0002] Cellulose is a carbohydrate consisting of a long chain of
glucose units, all .beta.-linked through the 1'-4 positions. Native
plant cellulose molecules may have upwards of 2200 anhydroglucose
units. The number of units is normally referred to as degree of
polymerization or simply D.P. Some loss of D.P. inevitably occurs
during purification. A D.P. approaching 2000 is usually found only
in purified cotton linters. Wood derived celluloses rarely exceed a
D.P. of about 1700. The structure of cellulose can be represented
as follows: 1
[0003] Chemical derivatives of cellulose have been commercially
important for almost a century and a half Nitrocellulose
plasticized with camphor was the first synthetic plastic and has
been in use since 1868. A number of cellulose ether and ester
derivatives are presently commercially available and find wide use
in many fields of commerce. Virtually all cellulose derivatives
take advantage of the reactivity of the three available hydroxyl
groups. Substitution at these groups can vary from very low; e.g.
about 0.01 to a maximum 3.0. Among important cellulose derivatives
are cellulose acetate, used in fibers and transparent films;
nitrocellulose, widely used in lacquers and gun powder; ethyl
cellulose, widely used in impact resistant tool handles; methyl
cellulose, hydroxyethyl, hydroxypropyl, and sodium carboxymethyl
cellulose, water soluble ethers widely used in detergents, as
thickeners in foodstuffs, and in papermaking.
[0004] Cellulose itself has been modified for various purposes.
Cellulose fibers are naturally anionic in nature as are many
papermaking additives. A cationic cellulose is described in Harding
et al. U.S. Pat. No. 4,505,775. This has greater affinity for
anionic papermaking additives such as fillers and pigments and is
particularly receptive to acid and anionic dyes. Jewell et al., in
U.S. Pat. No. 5,667,637, teach a low degree of substitution (D.S.)
carboxyethyl cellulose which, along with a cationic resin, improves
the wet to dry tensile and burst ratios when used as a papermaking
additive. Westland, in U.S. Pat. No. 5,755,828 describes a method
for increasing the strength of articles made from cross linked
cellulose fibers having free carboxylic acid groups obtained by
covalently coupling a polycarboxylic acid to the fibers.
[0005] For some purposes cellulose has been oxidized to make it
more anionic; e.g., to improve compatibility with cationic
papermaking additives and dyes. Various oxidation treatments have
been used. Various oxidation treatments have been used. U.S. Pat.
No. 3,575,177 to Briskin et al. describes a cellulose oxidized with
nitrogen dioxide useful as a tobacco substitute. The oxidized
material may then be treated with a borohydride to reduce
functional groups, such as aldehydes, causing off flavors. After
this reduction the product may be further treated with an oxidizing
agent such as hydrogen peroxide for further flavor improvement.
Other oxidation treatments use nitrogen dioxide and periodate
oxidation coupled with resin treatment of cotton fabrics for
improvement in crease recovery as suggested by R. T. Shet and A. M.
Yabani, Textile Research Journal Nov. 1981: 740-744. Earlier work
by K. V. Datye and G. M. Nabar, Textile Research Journal, July
1963: 500-510, describes oxidation by metaperiodates and dichromic
acid followed by treatment with chlorous acid for 72 hours or 0.05
M sodium borohydride for 24 hours. Copper number was greatly
reduced by borohydride treatment and less so by chlorous acid.
Carboxyl content was slightly reduced by borohydride and
significantly increased by chlorous acid. The products were
subsequently reacted with formaldehyde. P. Luner et al., Tappi
50(3): 117-120 (1967) oxidized southern pine kraft springwood and
summer wood fibers with potassium dichromate in oxalic acid.
Handsheets made with the fibers showed improved wet strength
believed due to aldehyde groups. P. Luner et al., in Tappi 50(5):
227-230 (1967) expanded this earlier work and further oxidized some
of the pulps with chlorite or reduced them with sodium borohydride.
Handsheets from the pulps treated with the reducing agent showed
improved sheet properties over those not so treated. R. A. Young,
Wood and Fiber, 10(2): 112-119 (1978) describes oxidation primarily
by dichromate in oxalic acid to introduce aldehyde groups in
sulfite pulps for wet strength improvement in papers.
[0006] Brasey et al, in U.S. Pat. No. 4,100,341, describe oxidation
of cellulose with nitric acid. They note that the reaction was
specific at the C6 position and that secondary oxidation at the C2
and C3 positions was not detected. They further note that the
product was ". . . stable without the need for subsequent reduction
steps or the introduction of further reactants [e.g., aldehyde
groups] from which the oxidized cellulose has to be purged".
[0007] V. A. Shenai and A. S. Narkhede, Textile Dyer and Printer
May 20, 1987: 17-22 describe the accelerated reaction of
hypochlorite oxidation of cotton yarns in the presence of
physically deposited cobalt sulfide. The authors note that partial
oxidation has been studied for the past hundred years in
conjunction with efforts to prevent degradation during bleaching.
They also discuss in some detail the use of 0 1 M sodium
borohydride as a reducing agent following oxidation. The treatment
was described as a useful method of characterizing the types of
reducing groups as well as acidic groups formed during oxidation.
The borohydride treatment noticeably reduced copper number of the
oxidized cellulose. Copper number gives an estimate of the reducing
groups such as aldehydes present on the cellulose. Borohydride
treatment also reduced alkali solubility of the oxidized product
but this may have been related to an approximate 40% reduction in
carboxyl content of the samples.
[0008] R. Andersson et al. in Carbohydrate Research 206: 340-346
(1990) teach oxidation of cellulose with sodium nitrite in
orthophosphoric acid and describe nuclear magnetic resonance
elucidation of the reaction products.
[0009] An article by P. L. Anelli et al. in Journal of Organic
Chemistry 54: 2970-2972 (1989) appears to be one of the earlier
papers describing oxidation of hydroxyl compounds by oxammonium
salts. They employed a system of 2,2,6,6-tetramethyl-piperidinyloxy
free radical (TEMPO) with sodium hypochlorite and sodium bromide in
a two phase system to oxidize 1,4-butanediol and
1,5-pentanediol.
[0010] R. V. Casciani et al, in French Patent 2,674,528 (1992)
describe the use of sterically hindered N-oxides for oxidation of
polymeric substances, among them alkyl polyglucosides having
primary hydroxyl groups. A preferred oxidant was TEMPO although
many related nitroxides were suggested. Calcium hypochlorite was
present as a secondary oxidant.
[0011] N. J. Davis and S. L. Flitsch, Tetrahedron Letters 34(7):
1181-1184 (1993) describe the use and reaction mechanism of (TEMPO)
with sodium hypochlorite to achieve selective oxidation of primary
hydroxyl groups of monosaccharides. Following the Davis et al.
paper this route to carboxylation then began to be very actively
explored, particularly in the Netherlands and later in the United
States. A. E. J. de Nooy et al., in a short paper in Receuil des
Travaux Chimiqutes des Pays-Bas 113: 165-166 (1994), report similar
results using TEMPO and hypobromite for oxidation of primary
alcohol groups in potato starch and insulin. The following year,
these same authors in Carbohydrate Research 269: 89-98 (1995)
report highly selective oxidation of primary alcohol groups in
water soluble glucans using TEMPO and a hypochlorite/bromide
oxidant.
[0012] European Patent Application 574,666 to Kaufhold et al.
describes a group of nitroxyl compounds based on TEMPO substituted
at the 4-position. These are useful as oxidation catalysts using a
two phase system. Formation of carboxylated cellulose did not
appear to be contemplated.
[0013] PCT published patent application WO 95/07303 (Besemer et
al.) describes a method of oxidizing water soluble carbohydrates
having a primary alcohol group, using TEMPO, or a related
di-tertiary-alkyl nitroxide, with sodium hypochlorite and sodium
bromide. Cellulose is mentioned in passing in the background
although the examples are principally limited to starches. The
method is said to selectively oxidize the primary alcohol at C-6 to
carboxyl. None of the products studied were fibrous in nature.
[0014] A year following the above noted Besemer PCT publication,
the same authors, in Cellulose Derivatives, T. J. Heinze and W. G.
Glasser, eds., Ch. 5, pp 73-82 (1996), describe methods for
selective oxidation of cellulose to 2,3-dicarboxy cellulose and
6-carboxy cellulose using various oxidants. Among the oxidants used
were a periodate/chlorite/hydro- gen peroxide system, oxidation in
phosphoric acid with sodium nitrate/nitrite, and with TEMPO and a
hypochlorite/bromide primary oxidant. Results with the TEMPO system
were poorly reproduced and equivocal. The statement that ". . .
some of the material remains undissolved" was puzzling. In the case
of TEMPO oxidation of cellulose, little or none would have been
expected to go into water solution unless the cellulose was either
badly degraded and/or the carboxyl substitution was very high. The
homogeneous solution of cellulose in phosphoric acid used for the
sodium nitrate/sodium nitrite oxidation was later treated with
sodium borohydride to remove any carbonyl function present.
[0015] De Nooy et al. have published a very extensive review, both
of the literature and the chemistry of nitroxyls as oxidizers of
primary and secondary alcohols, in Synthesis: Journal of Synthetic
Organic Chemistry (10): 1153-1174 (1996).
[0016] Heeres et al., in PCT application WO 96/38484. discuss
oxidation of carbohydrate ethers useful as sequestering agents.
They use the TEMPO oxidation system described by the authors just
noted above to produce relatively highly substituted products,
including cellulose.
[0017] In WO 96/36621, Heeres et al. describe a method of
recovering TEMPO and its related compounds following their use as
an oxidation catalyst. An example is given of the oxidation of
starch followed by TEMPO recovery using azeotropic
distillation.
[0018] P.-S. Chang and J. F. Robyt, Journal of Carbohydrate
Chemistry 15(7): 819-830 (1996),describe oxidation often
polysaccharides including .alpha.-cellulose at 0.degree. C. and
25.degree. C. using TEMPO with sodium hypochlorite and sodium
bromide. Ethanol addition was used to quench the oxidation
reaction. The resulting oxidized .alpha.-cellulose had a water
solubility of 9.4%. The authors did not further describe the nature
of the .alpha.-cellulose. It is presumed to have been a so-called
dissolving pulp or cotton linter cellulose.
[0019] D. Barzyk et al., in Journal of pulp and paper Science
23(2): J59-J61 (1997) and in Transactions of the 11.sup.th
Fundamental Research Symposium, Vol. 2, 893-907 (1997), note that
carboxyl groups on cellulose fibers increase swelling and impact
flexibility, bonded area and strength. They designed experiments to
increase surface carboxylation of fibers. However, they ruled out
oxidation to avoid fiber degradation and chose to form
carboxymethyl cellulose in an isopropanol/methanol system.
[0020] Isogai, A. and Y. Kato, in Cellulose 5: 153-164 (1998)
describe treatment of several native, mercerized, and regenerated
celluloses with TEMPO to obtain water soluble and insoluble
polyglucuronic acids. They note that the water soluble products had
almost 100% carboxyl substitution at the C-6 site. They further
note that oxidation proceeds heterogeneously at the more accessible
regions on solid cellulose.
[0021] Isogai, in Cellulose Communications 5(3): 136-141 (1998)
describes preparation of water soluble oxidized cellulose products
using mercerized or regenerated celluloses as starting materials in
a TEMPO oxidation system. Using native celluloses or bleached wood
pulp he was unable to obtain a water soluble material since he
achieved only low amounts of conversion. He further notes the
beneficial properties of the latter materials as papermaking
additives.
[0022] Kitaoka et al., in a preprint of a short 1998 paper for
Sen'i Gakukai (Society of Studies of Fiber) speak of their work in
the surface modification of fibers using a TEMPO mediated oxidation
system. They were concerned with the receptivity of alum-based
sizing compounds.
[0023] PCT application WO 99/23117 (Viikari et al.) teaches
oxidation using TEMPO in combination with the enzyme laccase or
other enzymes along with air or oxygen as the effective oxidizing
agents of cellulose fibers, including kraft pine pulps.
[0024] Kitaoka, T., A., A. Isogai, and F. Onabe, in Nordic Pulp and
Paper Research Journal, 14(4): 279-284 (1999), describe the
treatment of bleached hardwood kraft pulp using TEMPO oxidation.
Increasing amounts of carboxyl content gave some improvement in dry
tensile index, Young's modulus and brightness, with decreases in
elongation at breaking point and opacity. Other strength properties
were unaffected. Retention of PAE-type wet strength resins was
somewhat increased. The products described did not have any
stabilization treatment after the TEMPO oxidation.
[0025] Van der Lugt et al., in WO 99/57158, describe the use of
peracids in the presence of TEMPO or another di-tertiary alkyl
nitroxyl for oxidation of primary alcohols in carbohydrates. They
claim their process to be useful for producing uronic acids and for
introducing aldehyde groups that are suitable for crosslinking and
derivitization. Among their examples are a series of oxidations of
starch at pH ranges from 5-10 using a system including TEMPO,
sodium bromide, EDTA, and peracetic acid. Carboxyl substitution was
relatively high in all cases, ranging from 26-91% depending on
reaction pH.
[0026] Besemer et al. in PCT published application WO 00/50388
teach oxidation of various carbohydrate materials in which the
primary hydroxyls are converted to aldehyde groups. The system uses
TEMPO or related nitroxyl compounds in the presence of a transition
metal using oxygen or hydrogen peroxide.
[0027] Jaschinski et al. In PCT published application WO 00/50462
teach oxidation of TEMPO oxidized bleached wood pulps to introduce
carboxyl and aldehyde groups at the C6 position. The pulp is
preferably refined before oxidation. One process variation uses low
pH reaction conditions without a halogen compound present. The
TEMPO is regenerated by ozone or another oxidizer, preferably in a
separate step. In particular, the outer surface of the fibers are
said to be modified. The products were found to be useful for
papermaking applications.
[0028] Jetten et al. in related PCT applications WO 00/50463 and WO
00/50621 teach TEMPO oxidation of cellulose along with an enzyme or
complexes of a transition metal. A preferred complexing agent is a
polyamine with at least three amino groups separated by two or more
carbon atoms. Manganese, iron, cobalt, and copper are preferred
transition metals. Although aldehyde substitution at C6 seems to be
preferred, the primary products can be further oxidized to carboxyl
groups by oxidizers such as chlorites or hydrogen peroxide.
[0029] TEMPO catalyzed oxidation of primary alcohols of various
organic compounds is reported in U.S. Pat. Nos. 6,031,101 to Devine
et al. and 6,127,573 to Li et al. The oxidation system is a
buffered two phase system employing TEMPO, sodium chlorite, and
sodium hypochlorite. The above investigators are joined by others
in a corresponding paper to Zhao et al., Journal of Organic
Chemistry 64: 2564-2566 (1999). Similarly, Einhorn et al., Journal
of Organic Chemistry 61: 7452-7454 (1996) describe TEMPO used with
N-chlorosuccinimide in a two phase system for oxidation of primary
alcohols to aldehydes.
[0030] I. M. Ganiev et al in Journal of Physical Organic Chemistry
14: 38-42 (2001) describe a complex of chlorine dioxide with TEMPO
and its conversion into oxammonium salt. Specific applications of
the synthesis product were not noted.
[0031] Isogai, in Japanese Kokai 2001-4959A, describes treating
cellulose fiber using a TEMPO/ hypochlorite oxidation system to
achieve low levels of surface carboxyl substitution. The treated
fiber has good additive retention properties without loss of
strength when used in papermaking applications.
[0032] Published European Patent Applications 1,077,221; 1,027,285;
and 1,077,286 to Cimecloglu et al. respectively describe a
polysaccharide paper strength additive, a paper product, and a
modified cellulose pulp in which aldehyde substitution has been
introduced using a TEMPO/hypochlorite system.
[0033] Published PCT application WO 01/29309 to Jewell et al.
describes a cellulose fiber carboxylated using TEMPO or its related
compounds which is stabilized against color or D.P. degradation by
then use of a reducing or additional oxidizing step to eliminate
aldehyde or ketone substitution introduced during the primary
oxidation.
[0034] None of the previous workers have described a stabilized
fibrous carboxylated cellulose that can be made and used in
conventional papermill equipment, using environmentally friendly
chemicals, with no requirement for chlorine or hypochlorites.
SUMMARY OF THE INVNETION
[0035] The present invention is directed to a method for
preparation of a fibrous carboxylated cellulose product using a
hindered cyclic oxammonium salt as a primary oxidant. This may be
generated in situ by the oxidation of a corresponding amine,
hydroxylamine, or nitroxide. The method does not require an alkali
metal or alkaline earth hypochlorite compound as a secondary
oxidant to regenerate the nitroxide. Instead, a peracid salt has
been discovered to serve this function. An alkali metal halide,
preferably an alkali metal bromide, is used in conjunction with the
peracid to promote the nitroxide regeneration. The initially
oxidized product is then treated, preferably with a tertiary
oxidant or, alternatively, with a reducing agent, to convert any
unstable substituent groups into carboxyl or hydroxyl groups.
[0036] In the discussion and claims that follow, the terms
nitroxide, oxammonium salt, amine, or hydroxylamine of a
corresponding hindered heterocyclic amine compound should be
considered as full equivalents. The oxammonium salt is the
catalytically active form but this is an intermediate compound that
is formed from a nitroxide, continuously used to become a
hydroxylamine, and then regenerated, presumably back through the
nitroxide. The secondary oxidant will convert the amine form to the
free radical nitroxide compound. Unless otherwise specified, the
term "nitroxide" will normally be used hereafter in accordance with
the most common usage in the related literature.
[0037] A chemically purified fibrous cellulose market pulp is the
basic material for the process. This may be, but is not limited to,
bleached or unbleached sulfite, kraft, or prehydrolyzed kraft
hardwood or softwood pulps or mixtures of hardwood and softwood
pulps. While included within the broad scope of the invention,
so-called high alpha cellulose or chemical pulps; i.e., those with
an .alpha.-cellulose content greater than about 92%, are not
generally preferred as raw materials.
[0038] The suitability of lower cost market pulps is a significant
advantage of the process. Market pulps are used for many products
such as fine papers, diaper fluff, paper towels and tissues, etc.
These pulps generally have about 86-88% .alpha.-cellulose and
12-14% hemicellulose whereas the high .alpha.-cellulose chemical or
dissolving pulps have about 92-98% .alpha.-cellulose. By stable is
meant minimum D.P. loss in alkaline environments, and very low self
cross linking and color reversion. The method of the invention is
particularly advantageous for treating secondary (or recycled)
fibers. Bond strength of the sheeted carboxylated fibers is
significantly improved over untreated recycled fiber.
[0039] The "cellulose" used with the present invention is
preferably a wood based cellulose market pulp below 90%
.alpha.-cellulose, generally having about 86-88% .alpha.-cellulose
and a hemicellulose content of about 12%.
[0040] The process of the invention will lead to a product having
an increase in carboxyl substitution over the starting material of
at least about 2 meq/100 g, preferably at least about 5 meq/100 g.
Carboxylation occurs predominantly at the hydroxyl group on C-6 of
the anhydroglucose units to yield uronic acids. Carboxyl levels up
to about 35-40 meq/100 g can be produced in a one step process.
Substitution may be increased to considerably higher levels by
multistage addition of the oxidants.
[0041] The cellulose fiber in an aqueous slurry or suspension is
first oxidized by addition of a primary oxidizer comprising a
cyclic oxammonium salt. This may be conveniently formed in situ
from a corresponding amine, hydroxylamine or nitroxyl compound
which lacks any .alpha.-hydrogen substitution on either of the
carbon atoms adjacent the nitroxyl nitrogen atom. Substitution on
these carbon atoms is preferably one or two carbon alkyl groups.
For sake of convenience in the following description it will be
assumed, unless otherwise noted, that a nitroxide is used as the
primary oxidant and that term should be understood to include all
of the percursors of the corresponding nitroxide or its oxammonium
salt.
[0042] Nitroxides having both five and six membered rings have been
found to be satisfactory. Both five and six membered rings may have
either a methylene group or a heterocyclic atom selected from
nitrogen, sulfur or oxygen at the four position in the ring, and
both rings may have one or two substituent groups at this
location.
[0043] A large group of nitroxide compounds have been found to be
suitable. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO)
is among the exemplary nitroxides found useful. Another suitable
product linked in a mirror image relationship to TEMPO is
2,2,2'2',6,6,6',6'-octa- methyl-4,4'-bipiperidinyl-1,1'-dioxy
di-free radical (BI-TEMPO). Similarly,
2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-oxy free radical;
2,2,6,6-tetramethyl-4-methoxypiperidinyl-1-oxy free radical; and
2,2,6,6-tetramethyl-4-benzyloxypiperidinyl-1-oxy free radical;
2,2,6,6-tetramethyl-4-aminopiperidinyl-1-oxy free radical;
2,2,6,6-tetramethyl-4-acetylaminopiperidinyl-1-oxy free radical;
2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical and ketals of
this compound are examples of compounds with substitution at the 4
position of TEMPO that have been found to be very satisfactory
oxidants. Among the nitroxides with a second hetero atom in the
ring at the four position (relative to the nitrogen atom),
3,3,5,5-tetramethylmorpholine-1-oxy free radical (TEMMO) is very
useful.
[0044] The nitroxides are not limited to those with saturated
rings. One compound anticipated to be a very effective oxidant is
3,4-dehydro-2,2,6,6-tetramethyl-piperidinyl-1-oxy free radical.
[0045] Six membered ring compounds with double substitution at the
four position have been especially useful because of their relative
ease of synthesis and lower cost. Exemplary among these are the
1,2-ethanediol, 1,3-propanediol, 2,2-dimethyl-1,3-propanediol,
(1,3-neopentyldiol), and glyceryl cyclic ketals of
2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical.
[0046] Among the five membered ring products,
2,2,5,5-tetramethyl-pyrrolid- inyl-1-oxy free radical has been
found to be very effective.
[0047] The above named compounds should only be considered as
exemplary among the many representatives of the nitroxides suitable
for use with the invention and those named are not intended to be
limiting in any way.
[0048] It is also considered to be within the scope of the
invention to form the nitroxides in situ by oxidation of the
corresponding amines or hydroxylamines of any of the nitroxide free
radical products. Oxammonium salts of the nitroxides are produced
by oxidation of the corresponding nitroxide, hydroxylamine, or
amine. These oxammonium salts are known to oxidize primary alcohols
to aldehydes and aldehydes to carboxyl groups. While the nitroxide
is consumed and converted to an oxammonium salt then to a
hydroxylamine during the oxidation reaction, it is continuously
regenerated by the presence of a secondary oxidant. Basic
peroxymonosulfuric acid is a preferred secondary oxidant. Since the
nitroxide is not irreversibly consumed in the oxidation reaction
only a catalytic amount is required. During the course of the
reaction it is the secondary oxidant which will be depleted. The
amount of nitroxide required is in the range of about 0.005% to
1.0% by weight based on cellulose present, preferably about
0.02-0.25%. The nitroxide is known to preferentially oxidize the
primary hydroxyl located on C-6 of the anhydroglucose moiety of
cellulose. It can be assumed that a similar oxidation will occur at
primary alcohol groups on hemicellulose.
[0049] While the free radical form of the selected nitroxide may be
used, it is often preferable to begin with the corresponding amine
and form the nitroxide and oxammonium salt in situ. Among the many
possible amino compounds useful as starting materials can be
mentioned 2,2,6,6-tetramethylpiperidine,
2,2,6,6-tetramethyl-4-piperidone (triacetone amine), ketals
prepared by reacting triacetone amine with 1,2-ethanediol,
1,3-propanediol, glycerol, diglycerol, polyglycerol, and alkyl or
carboxyl substituted forms of the above diols and polyols.
[0050] The peracid used as a secondary oxidant may be any
peralkanoic acid such as peracetic acid or perpropionic acid,
substituted alkanoic acids such as peroxytrifluoroacetic acid,
substituted aromatic peracids such as perbenzoic acid or
m-chloroperbenzoic acid, or an inorganic peracid such as
peroxymonosulfuric acid, or salts of the above peracids.
Peroxymonosulfuric acid (Caro's acid) is a preferred compound.
[0051] The usual procedure is to slurry the cellulose fiber in a
suitable amount of peracid solution at about pH 5 to 8.5,
preferably about 7.5-8.0. The chosen peracid is present in an
amount of about 0.1-10% by weight of cellulose, preferably 0.5-5%
by weight. To this is added a catalytic amount (0.005-1.0% by
weight of cellulose, preferably about 0.02-0.25%) of the nitroxide
compound or one of its percursors along with 0.1-10.0% of the
alkali metal halide. This is added to the pulp slurry and allowed
to react for from 1 minute to 10 hours, preferably about 0.2 to 2.5
hours, at a temperature from about 5.degree.-95.degree. C. more
preferably about 20.degree.-80.degree. C.
[0052] Following oxidation, the cellulose is normally washed to
remove any residual chemicals and may then be further processed. To
achieve maximum stability and D.P. retention, the oxidized product
is reslurried in water for treatment with a stabilizing agent to
convert any substituent groups, such as aldehydes or ketones, to
hydroxyl or carboxyl groups. The stabilizing agent may either be
another oxidizing agent or a reducing agent. Unstabilized oxidized
pulps have objectionable color reversion and may self crosslink
upon drying, thereby reducing their ability to redisperse and form
strong bonds when used in sheeted cellulose products.
[0053] Alkali metal chlorites are one class of oxidizing agents
used as stabilizers, sodium chlorite being preferred because of the
cost factor. Other compounds that may serve equally well as
oxidizers are permanganates, chromic acid, bromine, and silver
oxide. A combination of chlorine dioxide and hydrogen peroxide is
also an excellent oxidizer. Peracids under acidic conditions are
also very useful as stabilizing agents.
[0054] Stabilization using sodium chlorite may be carried out at a
pH in the range of about 0-5, preferably 2-4, at temperatures
between about 10.degree.-110.degree. C., preferably about
20.degree.-95.degree. C. for times from about 0.5 minutes to 50
hours, preferably about 10 minutes to 2 hours. One factor that
favors oxidants as opposed to reducing agents is that aldehyde
groups on the oxidized cellulose are converted to additional
carboxyl groups, thus resulting in a more highly carboxylated
product. These stabilizing oxidizers are referred to as "tertiary
oxidizers" to distinguish them from the nitroxide/peracid
primary/secondary oxidizers. The tertiary oxidizer is used in a
molar ratio of about 1.0-15 times the presumed aldehyde content of
the oxidized cellulose, preferably about 5-10 times. In a more
convenient way of measuring the required tertiary oxidizer needed,
the preferred sodium chlorite usage should fall within about
0.1-20% by weight of cellulose, preferably about 1-9% by weight,
the chlorite being calculated on a 100% active material basis.
[0055] When stabilizing with a ClO.sub.2 and H.sub.2O.sub.2
mixture, the concentration of ClO.sub.2 present should be in a
range of about 0.1-20% by weight of cellulose, preferably about
0.3-1.0%, and concentration of H.sub.2O.sub.2 should fall within
the range of about 0.01-10% by weight of cellulose, preferably
0.05-1.0%. Time will generally fall within the range of 0.5 minutes
to 50 hours, preferably about 10 minutes to 2 hours and temperature
within the range of about 10.degree.-110.degree. C., preferably
about 30.degree.-95.degree. C. The pH of the system is preferably
about 2-3 but may be in the range of 0-5.
[0056] Peracids may be used as oxidative stabilizers under both
acidic and alkaline conditions, generally within a pH range of
about 2-7.5. A peracid concentration of about 0.1-10% by weight of
cellulose present is satisfactory.
[0057] A preferred reducing agent is an alkali metal borohydride.
Sodium borohydride (NaBH.sub.4) is preferred from the standpoint of
cost and availability. However, other borohydrides such as
LiBH.sub.4, or alkali metal cyanoborohydrides such as NaBH.sub.3CN
are also suitable. NaBH4 may be mixed with LiCl to form a very
useful reducing agent. When NaBH.sub.4 is used for reduction, it
should be present in an amount between about 0.1 and 10.0 g/L. A
more preferred amount would be about 0.25-5 g/L and a most
preferred amount from about 0.5-2.0 g/L. Based on cellulose the
amount of reducing agent should be in the range of about 0.1% to 4%
by weight, preferably about 1-3%. Reduction may be carried out at
room or higher temperature for a time between 10 minutes and 10
hours, preferably about 30 minutes to 2 hours.
[0058] After stabilization is completed, the cellulose is again
washed and may be dried if desired. Alternatively, the carboxyl
substituents may be converted to other cationic forms beside
hydrogen or sodium; e.g., calcium, magnesium, or ammonium.
[0059] One particular advantage of the process is that all
reactions are carried out in an aqueous medium to yield a product
in which the carboxylation is primarily located on the fiber
surface. This conveys highly advantageous properties for
papermaking. The product of the invention will have at least about
20% of the total carboxyl content on the fiber surface. Untreated
fiber will typically have no more than a few milliequivalents of
total carboxyl substitution and, of this, no more than about 10%
will be located on the fiber surface.
[0060] The carboxylated fiber of the invention is highly
advantageous as a papermaking furnish, either by itself or in
conjunction with conventional fiber. It may be used in amounts from
0.5-100% of the papermaking furnish. The carboxylated fiber is
especially useful in admixture with recycled fiber to add strength.
The method can be used to improve properties of either virgin or
recycled fiber. The increased number of anionic sites on the fiber
should serve to ionically hold significantly larger amounts of
cationic papermaking additives than untreated fiber. These
additives may be wet strength resins, sizing chemical emulsions,
filler and pigment retention aids, charged filler particles, dyes
and the like. Carboxylated pulps do not hornify (or irreversibly
collapse) as much on drying and are a superior material when
recycled. They swell more on rewetting, take less energy to refine,
and give higher sheet strength.
[0061] It is thus an object of the invention to provide a method of
making a cellulose fiber having enhanced carboxyl content using an
aqueous reaction medium.
[0062] It is also an object to provide a method for making a
carboxylated cellulose fiber that does not employ chlorine or
hypochlorite compounds.
[0063] It is another object to provide a process for making a
carboxylated cellulose fiber that can be carried out in equipment
commonly found in pulp or paper mills.
[0064] It is a further object to provide a cellulose fiber having
an enhanced carboxyl content at the fiber surface.
[0065] It is yet an object to provide a carboxylated cellulose
fiber that is stable against D.P. loss in alkaline
environments.
[0066] It is an object to provide a stable cellulose fiber of
enhanced carboxyl content with a D.P. of at least 850 measured as a
sodium salt or 700 when measured in the free acid form.
[0067] It is still an object to provide a cellulose fiber having a
high ionic attraction to cationic papermaking additives.
[0068] It is an additional object to provide cellulose pulp and
paper products containing the carboxyl enhanced fiber.
[0069] These and many other objects will become readily apparent
upon reading the following detailed description taken in
conjunction with the drawings
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0070] Abundant laboratory data indicates that a nitroxide
catalyzed cellulose oxidation predominantly occurs at the primary
hydroxyl group on C-6 of the anhydro-glucose moiety. In contrast to
some of the other routes to oxidized cellulose, only very minor
reaction has been observed to occur at the secondary hydroxyl
groups at the C-2 and C-3 locations. Using TEMPO as an example of
one useful nitroxide, the mechanism to formation of a carboxyl
group at the C-6 location proceeds through an intermediate aldehyde
stage. 2
[0071] The TEMPO is not irreversibly consumed in the reaction but
is continuously regenerated. It is converted by the secondary
oxidant into the oxammonium (or nitrosonium) ion which is the
actual oxidant. During oxidation the nitrosonium ion is reduced to
the hydroxylamine from which TEMPO is again formed. Thus, it is
secondary oxidant which is actually consumed. TEMPO may be
reclaimed or recycled from the aqueous system. The reaction is
postulated to be as follows: 3
[0072] As was noted earlier, formation of TEMPO in situ by
oxidation of the corresponding hydroxylamine or amine is considered
to be within the scope of the invention.
[0073] The resulting oxidized cellulose product will have a mixture
of carboxyl and aldehyde substitution. Aldehyde substituents on
cellulose are known to cause degeneration over time and under
certain environmental conditions. In addition, minor quantities of
ketone carbonyls may be formed at the C-2 and C-3 positions of the
anhydroglucose units and these will also lead to degradation.
Marked D.P., fiber strength loss, crosslinking, and yellowing are
among the problems encountered. For these reasons, we have found it
very desirable to oxidize aldehyde substituents to carboxyl groups,
or reduce then to hydroxyl groups, to ensure stability of the
product.
[0074] The following groups of nitroxy compounds are known to be
effective primary oxidants: 4
[0075] in which R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be
included in a five or six carbon alicyclic ring structure; X is
sulfur or oxygen; and R.sub.5 is hydrogen, C.sub.1-C.sub.12 alkyl,
benzyl, 2-dioxanyl, a dialkyl ether, an alkyl polyether, or a
hydroxyalkyl, and X with R.sub.5 being absent may be hydrogen or a
mirror image moiety to form a bipiperidinyl nitroxide. Specific
compounds in this group known to be very effective are
2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO);
2,2,2',2',6,6,6',6'-octamethyl-4,4'-bipiperidinyl-1,1'-dioxy
di-free radical (BI-TEMPO);
2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-ox- y free radical
(4-hydroxy-TEMPO); 2,2,6,6-tetramethyl-4-methoxypiperidinyl- -1-oxy
free radical (4-methoxy-TEMPO); and
2,2,6,6-tetramethyl-4-benzyloxy- piperidinyl-1-oxy free radical
(4-benzyloxy-TEMPO). 5
[0076] in which R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be
included in a five or six carbon alicyclic ring structure; R.sub.6
is hydrogen or C.sub.1- C.sub.5 alkyl; R.sub.7 is hydrogen,
C.sub.1-C.sub.8 g alkyl, phenyl, carbamoyl, alkyl carbamoyl, phenyl
carbamoyl, or C.sub.1-C.sub.8 acyl. Exemplary of this group is
2,2,6,6-tetramethyl-4-aminopiperidinyl-1- -oxy free radical
(4-amino-TEMPO); and 2,2,6,6-tetramethyl-4-acetylaminopi-
peridinyl-1-oxy free radical (4-acetylamino-TEMPO). 6
[0077] in which R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be
included in a five or six carbon alicyclic ring structure; and X is
oxygen, sulfur, NH, N-alkyl, NOH, or NOR.sub.8 where R.sub.8 is
lower alkyl. An example might be
2,2,6,6-tetramethyl-4-oxopiperidinyl-1-oxy free radical
(2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical). 7
[0078] wherein R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may be linked
into a five or six carbon alicyclic ring structure; X is oxygen,
sulfur, -alkyl amino, or acyl amino. An example is
3,3,5,5-tetramethylmorpholine-4-oxy free radical. In this case the
oxygen atom takes precedence for numbering but the dimethyl
substituted carbons remain adjacent the nitroxide moiety. 8
[0079] wherein R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may be linked
into a five or six carbon alicyclic ring structure. An example of a
suitable compound is
3,4-dehydro-2,2,6,6,-tetramethylpiperidinyl-1-oxy free radical.
9
[0080] wherein R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be
included in a five or six carbon alicyclic ring structure ; X is
methylene, oxygen, sulfur, or alkylamino; and R.sub.8 and R.sub.10
are one to five carbon alkyl groups and may together be included in
a five or six member ring structure, which, in turn may have a one
to four alkyl or hydroxy alkyl substitutients. Examples include the
1,2-ethanediol, 1,3 -propanediol, 2,2-dimethyl-1,3-propanediol (1,3
-neopentyldiol), and glyceryl cyclic ketals of
2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical. These
compounds are especially preferred primary oxidants because of
their effectiveness, lower cost, ease of synthesis, and suitable
water solubility. 10
[0081] in which R.sub.1-R.sub.4 are one to four carbon alkyl groups
but R.sub.1 with R.sub.2 and R.sub.3 with R.sub.4 may together be
included in a five or six carbon alicyclic ring structure; and X
may be methylene, sulfur, oxygen, -NH, or NR.sub.11, in which
R.sub.11 is a lower alkyl. An example of these five member ring
compounds is 2,2,5,5-tetramethylpyrroli- dinyl-1-oxy free
radical.
[0082] Where the term "lower alkyl" is used it should be understood
to mean an aliphatic straight or branched chain alkyl moiety having
from one to four carbon atoms.
[0083] Among the preferred peracids are peroxymonosulfuric acid and
peracetic acid in concentrations of 0.5% to 10% based on cellulose.
Peroxymonosulfuric acid (Caro's acid) has been found to be
particularly useful.
[0084] It should again be emphasized that in all cases the primary
catalyst may be used as an amine, a hydroxylamine, or in the
nitroxyl form to generate the active oxammonium salt.
EXAMPLE 1
Carboxylation of Cellulose using TEMPO and Caro's Acid
[0085] A solution of Caro's acid was formed by adding with stirring
30.0 g of potassium persulfate (K.sub.2S.sub.2O.sub.8) to 45.0 g of
concentrated sulfuric acid. The mixture was allowed to react for
about 25 minutes. The reaction product so formed was stirred into a
beaker containing 200 mL water and 300 g of ice. This was
neutralized with NaHCO.sub.3 to pH 7. A catalyst solution was
formed by dissolving 100 mg TEMPO in 50 mL water containing 2.0 g
NaBr.
[0086] A cellulose pulp slurry was formed by dispersing 26.9 g
(25.0 g O.D.) of the kraft pulp in 450 mL of water buffered to pH
8.5 by a NaHCO.sub.3/Na.sub.2CO.sub.3 mixture. The cellulose was a
southern pine bleached kraft market pulp obtained from a
Weyerhaeuser Co. North Carolina mill and designated as NB 416. Half
of the neutralized Caro's acid solution was added and the pH
adjusted to 8.5 with Na.sub.2CO.sub.3 solution. Then the aqueous
solution of TEMPO and NaBr was added and mixed well into the
cellulose slurry. The liquid turned an orange color. Oxidation was
allowed to proceed for 15 minutes at 25.degree. C. Then the
remaining half of the Caro's acid solution was added and oxidation
allowed to proceed an additional 30 minutes at 25.degree. C. The
oxidized pulp sample was drained and washed well with deionized
water. A small sample was retained for analysis but the bulk of the
material was taken to the next stage for stabilization.
[0087] The wet oxidized pulp (92 g total, 25 g O.D.) was dispersed
in a Na.sub.2HPO.sub.4/citric acid buffer solution at pH 3.5. This
contained 3 g of NaHPO.sub.4 and 5 g of citric acid in 937 mL
water. To this dispersion was added 6.0 g of 30% H.sub.2O.sub.2 and
6.0 g of NaClO.sub.2. Temperature was 25.degree. C. After 24 hours
the pH was raised to 9.5 with an aqueous solution of
Na.sub.2CO.sub.3. Then the material was drained and again washed
with deionized water.
[0088] The unstabilized material had a carboxyl content of 42.4
meq/100 g whereas the stabilized sample had a carboxyl content of
47.6 meq/100 g.
EXAMPLE 2
Effect on Cellulose D.P. after Caro's Acid Oxidation
[0089] An oxidized cellulose sample was prepared in similar manner
to that of Example 1 except that the pulp used was a never dried
sample of northern mixed conifer bleached kraft furnish obtained
from a Weyerhaeuser Company Grand Prairie, Alberta mill. The Caro's
acid was prepared from K.sub.2S.sub.2O.sub.8 and 98% sulfuric acid
and diluted with deionized water to give 60 mL of a 0.28% solution.
This was further diluted with 60 mL of deionized water and adjusted
to pH 7.5 with NaHCO.sub.3. The oxidation catalyst was prepared by
dissolving 0.012 g of the 1,3-propanediol ketal of
2,2,6,6-tetramethyl-4-piperidone-1-oxyl in the Caro's acid
solution. Then 51 g (12.5 g O.D.) was suspended in the basic Caro's
acid solution and finally 0.25 g of NaBr was added and mixed well.
The mixture was placed in a polyethylene bag and heated in a water
bath for 15 minutes at 60.degree. C. The fiber was filtered off and
washed well in deionized water. A small sample was retained for
analysis and the bulk of the material taken to the next stage for
stabilization.
[0090] The wet oxidized pulp prepared above was dispersed in 250 mL
of a Na.sub.2HPO.sub.4/citric acid buffer solution at pH 3.5. To
this was added 1.5 g NaClO.sub.2 and 1.5 g 30% H.sub.2O.sub.2 The
mixture was again placed in a polyethylene bag and heated in the
60.degree. C. water bath for 30 minutes. The pH was then raised to
9.5 with an aqueous solution of Na.sub.2CO.sub.3. The resulting
product was then again filtered off and washed with deionized
water.
[0091] Carboxyl content of the unstabilized sample was 5.8 meq/100
g and 8.8 meq/100 g for the stabilized product. D.P. of the
stabilized material was 1479. D.P. of the original untreated pulp
was about 1700.
EXAMPLE 3
Stabilization of Oxidized Cellulose using Caro's Acid Under Acidic
Conditions
[0092] An additional sample of the never dried Alberta pulp was
oxidized using
7,7,9,9-tetramethyl-1,4-dioxa-8-azaspiro[4.5]decane-2-methanol
rather than TEMPO. This material is also designated as the glyceryl
ketal of triacetoneamine. A 145 mg portion of the amine was
dissolved in 250 g of 0.28% basic Caro's acid solution at pH 7.5. A
slurry of 102 g never dried Grand Prairie kraft pulp (25 g O.D.)
was then dispersed in the solution. The mixture was placed in a
plastic bag and 500 mg NaBr was added and dispersed throughout the
mixture. The bag was sealed and placed in a water bath at
60.degree. C. for 30 minutes. The oxidized cellulose was drained
and thoroughly washed with deionized water. A small portion was
retained for analysis and the remainder divided into two parts.
[0093] A first 30 g portion of the oxidized cellulose (8.0 g O.D.)
was dispersed in 500 mL of Na.sub.2HPO.sub.4/citric acid buffer
solution at pH 3.5 for stabilization. Then 3.0 g NaClO.sub.2 and
3.0 g 30% H.sub.2O.sub.2 were added and mixed well. The mixture,
contained in a polyethylene bag, was placed in a water bath at
60.degree. C. for 30 minutes. The pH was then raised to 9.5 with
addition of Na.sub.2CO.sub.3. Then the sample was drained and
washed with deionized water.
[0094] The second 30 g (8.0 g O.D.) portion of oxidized cellulose
was dispersed in 250 g of 0.28% Caro's acid solution at pH 6.5.
This was contained in a sealed polyethylene bag and placed in a
60.degree. C. water bath for 30 minutes. After stabilization the pH
was raised to 9.5 with Na.sub.2CO.sub.3 and the product drained and
thoroughly washed.
[0095] Prior to stabilization the carboxyl content was 5.4 meq/100
g. Following stabilization by the first method using NaClO.sub.2
and H.sub.2O.sub.2 carboxyl content was measured as 11.0 meq/100 g
and D.P. was 1183. Using the second stabilization method employing
Caro's acid the carboxyl content was 10.8 meq/100 g and D.P. was
993.
EXAMPLE 4
1,3-Propanediol Ketal of Triacetone Amine Used to Generate its
Oxammonium Salt In Situ
[0096] A Caro's acid stock solution was prepared using 200 g of 98%
H.sub.2SO.sub.4 and 40 g of 70% H.sub.2O.sub.2. An 0.80 g portion
of this was added to 100 g of deionized water and the pH raised to
7.5 with Na.sub.2CO.sub.3. The concentration of Caro's acid was
0.28% and of H.sub.2O.sub.2 0.02% by weight. Into this solution was
dispersed 51 g (12.5 g O.D.) of the never dried Alberta pulp of
Example 2. A catalyst solution was made by dissolving 0.0048 g of
the 1,3-propanediol ketal of triacetoneamine and 0.250 g of NaBr in
50 g of a solution brought to pH 7.5 with NaHCO.sub.3. This was
added to the cellulose slurry in Caro's acid solution and the
mixture was placed in a polyethylene bag and immersed in a
60.degree. C. water bath for 15 minutes. After the initial
oxidation the pulp was drained and washed and a small sample taken
for analysis.
[0097] The oxidized cellulose was then dispersed in 500 mL of a
Na.sub.2HPO.sub.4/citric acid buffer solution at pH 3.5 for
stabilization. To this slurry was added 3.0 g of sodium chlorite
and 3.0 g of 30% H.sub.2O.sub.2. The slurry was again placed in a
polyethylene bag immersed in the 60.degree. C. water bath. After 15
minutes the pH was raised to 9.5 with an aqueous solution of
Na.sub.2CO.sub.3. The fiber was again drained and washed with
deionized water.
[0098] Carboxyl content of the unstabilized material was measured
as 5.7 meq/100 g and 8.8 meq/100 g for the stabilized material.
EXAMPLE 5
Effect of Stabilization on Brightness Reversion of Oxidized
Pulps
[0099] A 100 g batch of carboxylated cellulose was prepared by
using 2,2,6,6-tetramethylpiperidine to form the primary oxidant. A
first portion of the oxidized material was washed and treated with
a solution of about 2 g/L Na.sub.2CO.sub.3 for about 5 minutes at a
pH between 9-10. The unstabilized product was then washed with
deionized water but left undried. The second portion was stabilized
using a NaClO.sub.2/H.sub.2O.sub.2, mixture at about pH 3 as
described above. The stabilized product was drained and washed,
treated with basic water at pH.about.10, and again washed.
[0100] Analyses of the original and two treated samples gave the
following results:
1 Sample D.P. Carboxyl, meq/100 g Untreated 1650 .+-. 100 4.0 .+-.
0.5 Unstabilized 650* 13.7 .+-. 0.5 Stabilized 1390 .+-. 60 21.6
.+-. 0.1 *DP results of unstabilized materials are unreliable due
to degradation in the alkaline cuene solvent.
[0101] Handsheets were then made of the above three samples for
study of color reversion after accelerated aging. These were dried
overnight at room temperature and 50% R.H. Brightness was measured
before and after samples were heated in an oven at 105.degree. C.
for 1 hour. Heated samples were reconditioned for at least 30
minutes at 50% R.H. Results are as follows:
2 Initial Oven-aged Brightnes ISO Bright- ISO Bright- Reversion,
Sample pH ness, % ness, % % Control 5 89.84 .+-. 0.13 88.37 .+-.
0.12 1.48 Control* 5 90.13 .+-. 0.07 88.61 .+-. 0.13 1.52
Unstabilized Unadjusted 91.43 .+-. 0.16 78.85 .+-. 0.28 12.59
Unstabilized 5 91.93 .+-. 0.08 87.38.+-. 4.55 Stabilized Unadjusted
92.68 .+-. 0.09 90.74 .+-. 0.12 1.94 Stabilized 5 92.89 .+-. 0.14
91.31 .+-. 0.12 1.57 *Base washed before testing
[0102] The superior brightness retention of the stabilized samples
is immediately evident from the above test results.
[0103] It will be evident to those skilled in the art that many
reaction conditions and many hindered nitroxide compounds that have
not been exemplified will be satisfactory for use with peracids as
secondary oxidants. It is the intention of the inventors that these
variations be included within the scope of the invention if
encompassed within the following claims.
* * * * *