Note: Descriptions are shown in the official language in which they were submitted.
FIELD OF THE INVENTION
~ his invention relates to use of a cooked,
specially-treated starch product as a paper wet-end
additi~e for the elimination of conventional surface
sizing.
DESCRIPTION OF PRIOR ART
Although a small amount of high grade paper is
manufactured from rag pulp, paper~is manufactured for the
most part from wood pulp. There are five different kinds
of wood pulp: mechanical pulp (ground wood), semi-chemical
pulp, sulfite pulp, sulfate or kraft pulp, and soda pulp.
The first is prepared by purely mechanical means, the second
by a combination of mechanical and chemical means, and the
other three by chemical means. The mechanical pulp contains
substantially all the wood except the bark and that lost
during storage and transportation. Semi-chemical pulps
are partially free of lignin. Chemical pulps, however,
are essentially cellulose, the unwanted lignin and other
non-cellulosic components of the wood having been dissolved
away by the cooking and bleaching treatment. Because of this,
chemical pulps are much superior to mechanical and semi-chemical
pulps for fine paper making. However, because of the special
processing reqùired, they are too expensive to serve as a
main source of fiber for the cheaper grades of paper such as
newsprint.
If the pulp fibers were the only constituents of
a paper sheet, the usefulness of the paper would be very
restricted because the sheet would be soft, have a yellowish
color, and could not be successfully written or printed upon
with ink. If the sheet were thin, it would be transparent
to matter printed upon the opposite siae. It is necessary,
then, to add other substituents, such as sizing or
coloring agents and fillers to the cellulosic fibers to
produce papers suited to its many uses.
Many papers, except the absorbent types, filter
papers and most packaging papers, must have a finely ground
filler added to them, the purpose of which is to occupy the
spaces between the fibers - thus giving a smooth surface, a
more brilliant whiteness, improved printability and
improved opacity. Sizing is added to the paper, other
than absorbent papers and filter paper, to impart resistance
to penetration by liquids. Common sizing agents added to
the pulp before it is formed into a sheet are wax emulsions
or soaps made by the saponification of rosin with alkali
The sizes are precipitated with alum. Such sizing as
described in this paragraph is known in the trade as
internal sizing.
The word sizing is also used in a second context
in the paper industry. This second use is known as surface
sizing. It differs from the internal sizing previously
described in that it is applied to the surface of the paper
where it cements the fibers to the body of the paper and
deposits a more or less continuous film on the paper surface.
Surface sizing is used to produ~ce a smooth hard surface which
will not catch a pen when the paper is written upon, will
not pick off if the paper is printed using tacky inks and
will not show feathering of the ink. An additional advan-tage
of a surface size is that oil resistance of the paper is
improved since the size tends to seal the pores of the
- 3 -
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paper. Surface sizing may be of greater importance than
internal sizing for certain types of papers such as writing
papers, printing papers and some wrapping papers. It is
important to surface size paper that is used in an o~fset
printing process since this prevents loosening of surface
fibers when the paper is moistened with water on the press.
A common way to add a surface size is to apply the
sizing agent to both sides of the paper when the paper is
passed through press rolls on the paper machine. These
rolls force the size into the paper and remove excess size
from the surface of the paper. Inherent in this process
are various mechanical problems which make it expensive to
apply and maintain a uniform layer of the size on the surface
of the paper. This mechanical equipment is costly, and
there is the added expense of evaporating the water
adaed to the paper with a dilute suspension of the size.
Commonly the size is a starch or a starch derivative
such as an hydro~yethyl starch.
Starch has long been used as an additive in the
manufacture of paper to strengthen the paper sheet. See
for example Whistler and Paschall, Starch: Chemistry and
Technology, Academic Press Inc., New York, N.Y. Vol. 2,
1967, Chapter VI. For this purpose, starch i~ added to
the pulp before the paper sheet is formed. The starch
is usually cookedin water before the mixture is added
to the pulp. This procedure produces much soluble material
which is not effectively retained in the paper sheet. An
~,'~..
.
improvement in starch cooking was disclosed in U.S. Patent
2,805,~66, issued September 10, 1957, wherein the starch
slurry was heated in a steam inject:ion cooker. This permitted
control of the heating so that the ma~ority of the starch
granules were swollen but not ruptured. However, the
temperature range over which the granules of starch swell
ana gelatinize is large. Even in this process only a
portion of the granules can be obtained in the desired
swollen state. Some starch granules may be still unswollen
and hence useless as an adhesive while others are solubilized
and not retained in the starch. Furthermore, many of the
swollen starch granules so prepared have a tendency to
disintegrate and form more solubles when their slurries
in water are subjected to vigorous agitation.
One method to obtain a starch whose swollen
granules do not disintegrate during agitation was disclosed
in U.S. Patent 2,113,034, issued April 5, 1938. This was
accomplished by reaction o~ starch with formaldehyde. The
product is resistant to dispersion in hot water, and
requires treatment with alkali and vigorous beating to
render the starch suitable as a paper pulp additive.
As a result of these treatment requirements and because
the starch is only partially retained in the paper sheet,
the product has never found acceptance in the paper
manufacturing industry.
A second method to obtain a starch whose swollen
granules would not disintegrate during agitation was dis-
closed in U.S. Patent 2,328,537, issued September 7, 1943.
This was accomplished by reacting the starch with certain
antimony or phosphorous chlorides or oxychlorides. The
~r
' - 5 ~
patent suggested that the products might be useful in the
manufacture of paper. Again, these products have never
found acceptance in the paper manufacturing industry,
because the products described show limited swelling in
hot water and are only partially retained in the paper sheet.
One method which partially overcomes these dif-
ficulties is disclosed in copending U.S. Application, Serial
No. 7~0,614, filed March 23, 1977. This method employs a
specially-treated starch whose granules swell to a desired
degree in cold water. This modification eliminates the
need for specially-controlled heating of the starch as
required in the prior art but it still necessitates pre-
treatment of the starch under carefully controlled
conditions.
SUMMARY OF~THE INVENTION
It would therefore be an advance in the art if
an additive were discovered which could be incorporated
into the paper pulp prior to sheet formation that would
impart improved surface properties to the paper. This would
avoid the necessity for a separate surface sizin~ step with
the attendant addition of moisture and the necessity of
further drying of the product. Moreover, if such surface
properties could be imparted without impartin~ detrimental
side effects, and loss of desirable properties, the additive
in question would Eind ready acceptance in the art. If on
, ~ .
. . ~ ., .
the other hand, properties of the finally formed paper article,
such as dry strength and tensile strength were improved by means
of the additive, this would be an added economical benefit
to be gained through use of an additive of this type.
It is therefore an object of the invention
to provide a new and improved method for imparting the
characteristics of surface sizing to manufactured paper
by the addition of a specific additive to the pulp during
the paper processing.
:
A further object is to provide an agent of this
type to improve surface properties without interfering
with the other additives and substances used in the makeup
and manufacture of paper and without showing adverse effects
on the chemical and physical characteristics of the
finished sheet.
Another object of the invention is to provide
an additive for improvlng surface properties which is
retained inand not washed out o the paper in the sheet
forming process.
An important object of the invention is to provide
an additive for improving surface properties of manufactured
papel- which will operate on a wide variety of paper stocks,
is safe to handle and will impart to the finished sheet
desirable characteristics which have not heretofore been
available when prior art attempts have been made to make
paper with the additional separate step of applying a
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y~J' : :
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coating of surface size. A specific object of this invention
is to provide a method of improving surface properties
of manufactured paper by the addition of a specific additive
to the wet-end which also imparts to the finished paper
article improved dry strength.
A still further object of this invention is to
provide a starch additive for paper that is readily prepared,
whose granules swell in hot water to the desired degree
without the need to carefully control the heating conditions,
and whose swollen granules do not disintegrate when subjected
to vigorous agitation.
This invention relates to an improved process
for the manufacture of paper which comprises the steps of:
(a~ adding an aqueous suspension of a
cooked, cross-linked starch having
a cooked swollen volume (hereafter
referred to as CSV~ of from about
4 mlJg to about 25 ml/g to an
aqueous suspension of cellulosic
pulp;
(b) adding a polyelectrolyte to the
starch-pulp mixture; and
(c) forming paper from said mixture of
pulp, starch and polyelectrolyte.
-- 8 --
~ ~L~
Another aspect of this invention is a wet-end
paper additive composition which comprises:
(a) an aqueous suspension of a cooked,
cross-linked starch having a CSV of
from about ~ ml/g to about 25 ml/g;
and
(b) a polyelectrolyte.
The present invention also is directed to an
improved process for the manufacture of paper which
comprises spraying an aqueous suspension of a cooked/
cross-linked starch having a CSV of from about 4 ml/g
to about 25 ml/g onto the wet web in the paper-forming
process and drying such web.
DETAILED DESC~IPTION OE THE INVENTION
In accordance withthis invention, it has been
discovered that certain cross-linked starches which show a
specific degree of swelling in hot water impart excellent
surface properties to manufactured paper when they are added
to the pulp in the paper-making process. The paper so obtained
compares very favorably with surface-sized papers manufactured
by the expensive surface sizing process. Furthermore, the
process gives a finished paper with improved dry strength.
The incorporation of the cross-linked starches of this process
does not interfere with the other additives and substances
used in the makeup of the manufacture of paper. There are no
adverse effects on the chemical and physical characteristics
of the finished sheet.
_ g _
, ~... . . : - . ,. : -
~J~
The starch reactant itself that may be used in
practicing the invention may be derived from any vegetable
source such as for example corn, wheat, potato, tapioca,
rice, sago and grain sorghum. It may be either waxy or
nonwaxy. The term stareh is used broadly herein and
eneompasses unmoaified starch and tailings, and as well,
stareh that has been modified somewhat by treatments with
acids, alkalies, enzymes or oxidizing or derivatizing agents.
f the starting stareh is modified or derivatized in some
~ manner, it nevertheless is useful as long as the product
is still in the granular form and still contains hydroxyl
groups capable of reacting with the cross-linking reagents.
The cross-linking agents used in this invention
may be any of the well-known organie or inorganie poly-
funetional reagents which can react with two or more starch
hydroxyl groups~ Examples of such reagents that are well
known in the art are phosphorous oxychloride, sodium
trimetaphosphate, epiehlorohydrin and acrolein.
The cross-linked starches of this invention may
be prepared by the general procedure disclosed in U.S. Patent
2,328,537, issued September 7, 1943. It is neeessary to
eontrol the eross linking earefully to obtain a suitable
produet. Suitable produets for this invention are those
which when eooked in water gi~e a CSV of from about 4 to
about 25 ml/g, preferably about 7 to about 20 ml/g
CSV is measured as follows:
-- 10 --
.~
6~38
To 10.00 g dry basis of starch in a 600-ml
stainless steel beaker is added 190.0 g of distilled
water. Cover with a watch glass having a center hole
for a stirring shaft. Stir at 500 rpm for 18 min. in
a boilin~ water bath. Cool to 28c by stirring in
a cold water bath. Add distilled water to exactly
replace the water lost by evaporation, transfer to a
250 ml centrifuge bottle and centrifuge at 2000 rpm
for 10 min. Mark the level of the paste in the bottle.
To determine solubles, a weighed aliquot of the
supernatant liquid is evaporated on a steam bath.
Then the residue is dried for 4 hours at 120C in
a vacuum oven and weighed. The per cent solubles
is calculated as follows:
Solubles = l~eight of Residue x 1900
Weight of Aliquot
The volume the paste occupied in -the centrifuge bottle
is measured in milliliters. The CSV is calclulated as
follows:
Paste volume 'in ml
CSV = Insoluble Weight in g
[Where Insoluble Weight = 10.00(1 _ ~ InlOolubles)]
If no layer of paste separates after centrifugation,
the test is repeated using a 5-g sample of starch
with corresponding adjustment in the calculations.
-- 11 --
!~
~ ~4~t~ 8
It has been found that products with a suitable
CSV are obtained when starch is treated with from about
100 to about 2000 micromoles of POC13 per mole of starch.
(A mole of starch is defined as 162 g of starch on a dry
basis, the weight of one anhydroglucose unit of the
starch.) Suitable a~ounts of other cross-linking agents
are readily determined by experimentation.
Starches with the CSV specified above give,
when cooked in water, swollen granules particularly
useful in the paper-making process. Use of these
cross-linked starches obviates the need for carefully
controlled heating of the starch slurries as was required
in the prior art processes. Furthermore, it has been
found that the swollen granules prepared from the
cross-linked starches with the specifid CSV have little or no
tendency to disintegrate when subjected to vigorous
agitation.
The starch is prepared for addition to the paper
pulp by cooking a suspension in water. Cooking may be
accomplished in a batch cooker or a jet cooker. The cross-
linked starch, properly cooked in water, may be added to
the paper furnish at various points in the operation where
there is efficient mixing. A suitable point of addition ;
is at the suction side of the fan pump.
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f ~,.
The starches of the present process are effective
when used at levels of about 1~ to about 20% by weight of
dry solids in the mixture of starch and pulp. The pre-
ferred use level is from about 5% to about 10% by weight
of dry solids in the mixture. It has been discovered that
such cross-linked starches are well retained and that when
used at the preferred use levels less than about 20% of the
starch added is lost from the paper in the sheet-forming
process if a polyelectrolyte is added to the mixture
following the addition of the starch. The surprising
finding is that such polyelectrolytes, which have been
used previously for the retention of mineral pigment in the
paper, are effective in promoting the retention of the ~
cross-linked starches. Such polyelectrolytes may be ~-
used at a level of about 0.01% to about 1%, preferably
at a level of about 0.025% to about 0.1% by weight of
the dry weight of furnish. The polyelectrolyte is added
to the pulp at a point in the process after the cross-
linked starch has been added.
The polyelectrolyte employed in the process of
this invention may be those employed by paper makers as
flocculating agents or as aids to improve the retention
of pigments in the paper. They may be either anionic or
cationic. Examples of those which are suitable are the
acrylamide-based copolymers sold by the Hercules Corporation,
Wilmington, Delaware, under the Reten trademark; those sold -~
by the Nalco Chemical Company, Oak Brook, Illinois, under
the Nalco trademark; and those sold by the Betz Laboratories,
Trevose, Pennsylvania, under the Betz trademark.
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~. .
B8
The best polyelectrolyte for use in a particular
paper-making sys-tem is determined by the following method:
In a Britt Dynamic Drainage Jar, distributed by
Paper Research Materials Co., 770 James Str.,
Apt. 1206, Syracuse, N.Y. 13203, is placed
500 ml of prepared furnish. The mixture is
agitated at 700-800 rpm for 15 seconds before
the required amount of polyelectrolyte is
introduced. The mixture is agitated for an
additional lO seconds before the clamp is opened
and 100 ml of filtrate is collected~ The filtrate,
diluted if necessary, is placed in an electro-
photometer and per cent transmission is determined
at 425 m~. The polyelectrolyte which gives the
highest transmission in this test is the best
polyelectrolyte for use in the particular system
tested.
In a separate embodiment of this invention,
it has been discovered that the starches of this invention
may be sprayed onto the wet web in the paper-making
process. Papers produced by this method exhibit
excellent surface properties comparable with those of
surface-sized papers. This process also avoids the
necessity for a separate surface sizing step with the
attendant addition of moisture and the necessity of
further drying of the product.
The following examples further illustrate
the practice of this invention. It should be understood,
however, that the invention is not limited to the examples
given below.
l~ - 14
8~
EXAMPLE I
Cross-linked starches were prepared using 400,
800 and 1600 micromoles of phosphorous oxychloride per
mole of commercial corn starch. Paper handsheets were
prepared incorporating these cross-linked starches by
the following procedure. The starch was cooked by
heating a stirred 5% slurry in water for 15 minutes at
95-100C. The starch slurry was added to the paper pulp
prepared from a 50/50 bleached hardwood/softwood kraft
pulp. The pulp also contained alum in the ratio of 2%
by weight of the dried pulp. There was added 0.1%, based
on total solids weight, of a polyelectrolyte, Reten 421.
The pH of the pulp slurry was adjusted to 4.5 with sulfuric
acid before the starch was added. The dilute pulp slurry
containing approximately 3% by weight solids was used to
make paper handsheets using a Williams sheet machine for
a series of 4 sheets formed continuously, pressed and dried.
Blank sheets were prepared using the same pulp-alum
composition without the addition of any starch. Bursting
strength (Mullen points), critical wax pick and K & N
ink holdout tests were measured by TAPPI (Technical
Association of the Pulp and Paper Industry, 360 ~exington
Avenue, New York, New York 10017) Methods T403ts-63,
T459su-65, UM413, respectively. The data are summarized
in Table I.
This Example shows that the addition of a
cooked, cross-linked starch to paper pulp improves the
dry strength, wax pick and ink holdout of the paper~
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; "
8~3
TABLE I
PAPER HANDSHEET PROPERTIES
10% CROSS-LINKED STARCH - 90% MIXED BLEACHED KRAFT P~LP
Mullen
Bursting K & N
Strength, Critical Ink
Starch a~(pts) Wax Pick b) Holdout c)
_
40Q micromole POC13
XL Corn 18.5 16 55.1
10 800 micromole POCl
XL Corn 3 13.0 10 57.0
1600 micromole POC13
XL Corn 11.5 9 58.5
400 micromole POC13
XL Waxy Corn 18.5 16 48.3
Blank Sheets
(Control~ 10.0 7 59.3
a) XL = Cross-Linked
b) Average of determinations on at least two sheets.
c~ Average of three determinations.
- 16 -
,
EXAMP`LE II
.
The procedure of Example I was repeated using the cross-
linking agent at a level chosen to give starch with a CSV in the
preferred range. The cross-linking agents used were epichloro-
hydrin, sodium trimetaphosphate, and phosphorous oxychloride. The
starches were used at the 10% level as in Example I. Results are
given in Table II.
This Example shows that various cross-linking agents are
equally effective in producing cross-linked starches which improve
the properties of the paper.
- 17 -
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~ . .
. .
TABLE II
PAPER HANDSHEET PROPERT].ES
Mullen
Starch - Bursting
Cross-Linking CSV Strength Critical
Reagent (ml/g~ (pts) Wax Pick a)
_
None -- 11 8
Phosphorous :
10Oxychloride 14 22 16
Sodium
Trimeta-
phosphate 14 22 16
Epichlorohydrin 11 22 16
a~ Average o~ determinations on at least two sheets.
- 18 -
~ r
. . .
EXAMPLE III
Tests were run on a pilot paper machine using a
pulp furnish that was 50% bleached hardwood kra~t and 50%
bleached softwood kraft mix~ure beaten to a 450 ml Canadian
standard freeness. Fortified rosin size (1%) and alum (2%)
were added to the pulp in the beater. Starch was cooked
in water using batches o~ 9.1 kg starch and 143 kg water.
The cooked starch and pulp were mixed together in the machine
chest at a 2% by dry weight of furnish consistency. The paper
machine was run at a speed of 27.~ m/min using 72.6 kg/hr
of pulp to make 66.6 g/m basis weight paper. A 0.75%
aqueous solution of Reten 421, an anionic polyelectrolyte, was
added at a standpipe just prior to the fan pump. Addition
was made at such a rate as to give a concentration of the
polyelectrolyte of 0.05% by dry weight of the total solids.
All paper was calendered three nips on the paper machine
calender stack.
A surface-sized control paper was made using an 0.07
degree of substitution, 80-fluidity hydroxyethyl starch,
applied as a surface size. This was cooked at 15% commercial
moisture basis solids and diluted to 10% solids for size press
application. There was 4~ pickup of this starch on the paper.
Results of the experiments are given in Table III.
The IGT Printability Test was performed according to
TAPPI Method T499su-64. The porosity and smoothness tests
were performed according to TAPPI Methods T460m-49 and 479sm-48,
respectively, and employed the Gurley-Hill S-P-S Tester,
manufactured by Gurley Testing Instruments, Troy, New York.
The amount of starch retained in the paper is determined as
follows:
-- 1 9
~',`A
Prepare CaC12 solution by dissolving 540 g of
CaC12 2H2O in water and dilute to 1 liter. Adjust the
specific gravity to 1.30 at 16 with water. Adjust the
pH to 1.8 with glacial acetic acid.
Prepare a UO2(C2~3o2)2 solution by dissolving
lOg of UO2(C2H3O2)2 2H2O in 80 ml water and 20 ml glacial
acetic acid. Do not heat above 60C. Dilute with 100 ml
of the CaC12 solution.
Weigh accurately a 2-g sample of paper cut into
pieces approximately 1 cm square. Macerate in 10 ml
water in a 250--ml beaker. Add 60 ml CaC12 solution and boil
vigorously for 30 minutes with occasional stirring. Add
water as needed to maintain a constant liquid level. Trans-
fer quantitatively the cool mixture into a 100 ml Kohlrausch
flask containing 10 ml o UO2(C2H3O2)2 solution- Dilute
to volume with CaC12 solution. Filter by gravity through
an 18.5-cm retentive paper into a dry flask, discarding the
first portion of filtrate. Determine the optical rotation
of the sample in a 2-dcm polarimeter tube.
Degrees Angular Rotation x 100 x 100
% Starch = 2 dcm x 203 x Sample Weight (g)
Where: 203 = specific rotation of pure corn starch
This example shows that the cooked cross-linked
starches of this invention when added to the paper pulp
give paper with properties as good as or better than those
of surface sized paper. They also show that the starches
of this invention as well retained within the paper when ~;
they are added to the pllp with a polyelectrolyte.
: ,~
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- 20 -
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~ ~868B
EXAMPLE I~
Handsheets were prepared using 50% bleached hardwood
kraft and 50% bleached softwood kraft pulp. A POC13 cross-
linked starch with a CSV of 14 was added at a level of 7%
by dry weight o~ tho starch pulp mixture. The furnish also
contained 1% by weight dry basis of rosin and 2% by weight
dry basis of alum plus a small amount of a polyelectrolyte
added in dilute solution. Starch retention and handsheet
properties are given in Table IV.
These results show that both anionic and cationic
polyelectrolytes increase the retention in paper of the
starches of this invention. Paper prepared with these
additives has superior surface properties as measured by
critical wax pick to those of paper prepared without these
additives.
~: :
.
-22-
~8~8~
TABLE IV
HANDSHEET PROPERTIES AND STARCH RETENTION
USING POLYELF:CT:ROLYTES
Polyelectrolyte _ Mullen
% in Starch Bursting
Furnish - Retained Strength Critical
N ~ Charge (Dry 8asis) (%) _(pts) Wax Pick~)
None - 60 22.2 14
Reten 421Anionic 0.05 89 21.2 18
Betz 1215LAnionic 0.10 - 85 20.1 18
Betz 1225LAnionic 0.10 83 20.7 16
~alco 625Anionic 0.10 86 21.7 18
Nalco 7PD095 Cationic 0.10 71 22.8 16
Bet~ 1260Cationic 0.10 80 23.8 18
~lank Sheets (No Starch) 15.2 9
a) Average of determinations on at le~st t~ sheets.
~ '
.
--23--
8~
EXAMPLE V
Paper handsheets were prepared from a pulp
furnish containing 50% bleached hardwood kraft and 50%
bleached softwood kraft. A 0.25% dispersion of starch
in water was sprayed on the wet handsheet web while it
was still on the wire. Sufficient material was sprayed
on the paper to give a total addition of 2% starch by
weight on a dry solids basis. Properties of the dried
sheets are given in Table V.
These results show that the cross-linked starch
of this invention, when sprayed on a paper web, imparts
superior surface properties to the finished paper as
measured by the critical wax pick.
~: :
-24_
B~8~
TABLE V
SPRAYING OF STARCHES ONTO PAPER HANDSHEETS
Mullen
Bursting
Dispersion StrengthCritical
Starch Method (pts)Wax PickA)
POC13 Cross-LinkedCook~d at 100C13.8 14
(CSV = 14)
80-Fluidity Cooked at 100C 11.0 5
Hydroxyethyl
80-Fluidity Slurried 12.2 10
Hydroxyethyl
Unm~dified CornSlurried 13.a 8
None - 10.2 3
a) Average of determinations on at least two sheets.
-25-
