 |
Description  |
|
|
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention is directed to a process for producing paper using
water-insensitive starch fibers to replace all or part of the cellulosic
or other pulp conventionally employed, and to the paper produced thereby.
The invention also relates to a novel method for the production of certain
specialty papers, as well as to methods for the incorporation of
functional additives into paper during the production thereof and for
binding fibers in non-woven webs.
II. Brief Description of the Prior Art
Various natural fibers (other than cellulose) as well as a variety of
synthetic fibers have been employed in making paper, however, all these
replacements have failed to provide a commercially acceptable substitute
for cellulose due to their cost, poor bonding properties, chemical
incompatibilities, difficulty in handling in papermaking systems, etc.
While it has also been suggested to use starch fibers in various aspects
of the papermaking process, commercial attempts to use such fibers have
not resulted in any degree of success and paper is still being
manufactured almost completely from wood-based cellulosic ingredients--the
supply of which is being rapidly depleted.
It is apparent that the aqueous systems normally employed in the paper
making operations require pulp fibers possessing sufficient
water-insensitivity that they can be used in all aspects of the
manufacturing process throughout a relatively wide pH range without losing
their integrity. In this regard, the few references which suggest the
replacement of starch fibers for cellulose fibers (e.g. U.S. Pat. No.
1,682,293) require chemical modification of the starch in order to
radically change its naturally occurring properties prior to forming the
fiber so as to provide the degree of water-insensitivity required in the
papermaking process. Alternatively, other references (e.g., U.S. Pat. No.
2,570,449) require that the papermaking process itself be modified as by
replacing the conventionally employed aqueous system with an alcohol
solvent in which the starch fibers are not soluble. It will be recognized
that the use of such techniques is both impractical and uneconomical when
employed on a commercial basis.
As another aspect of the papermaking operation, it is often necessary to
incorporate additives into the pulp in order to achieve specific end
properties. Thus, additives such as pigments, latices, synthetic
microspheres, fire retardants, dyes, perfumes, etc. are often employed in
the manufacture of paper. The efficient retention of these additives at
the wet end of a paper machine presents difficulty to the manufacturer
since that portion which is not retained creates not only an economic
loss, but also a significant pollution problem if it becomes part of the
plant effluent. Furthermore, such additives are also added via coating or
saturation processes commonly known in the art. These processes usually
require that excess heating energy be consumed to re-dry the paper after
coating. Moreover, in some instances the coating systems are required to
be solvent based which then creates extreme capital expense and requires
regulation to recover volatile materials.
It is therefore an object of the present invention to provide a
commercially viable process for the use of starch fibers as a partial or
complete replacement for cellulose in conventional papermaking operations.
It is also an object to provide a process which efficiently enables the
retention and incorporation of additives into paper during the manufacture
thereof.
It is a further object to provide a process which enables water-insoluble
additives to be introduced into the paper as fiber encapsulated additives.
Another object is to provide ordinary and improved specialty papers
according to such process.
A further object of the invention is to provide an efficient and economical
process for binding synthetic and/or natural fibers in non-woven web form.
These and other related objects will be apparent from the description which
follows.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been found that the above
and related objects are attained when water-insensitive starch fibers,
produced by the precipitation of a colloidal dispersion of starch in a
coagulating salt solution, are employed as partial or complete
replacements for cellulose and similar fibers in conventional paper and
paperboard manufacturing operations. The fibers may be used to extend the
pulp, as a means for incorporating additives into the paper product, as
binder for the fibers in non-woven webs or for any combination thereof.
As used herein, the term "paper and paperboard" includes sheet-like masses
and molded products made from fibrous cellulosic materials as well as such
fibrous materials as may be derived from synthetics (such as polyamide,
polyester, rayon and polyacrylic resin), mineral fibers (such as asbestos
and glass), and the like.
As used herein, the expression "conventional papermaking operation" means
the process of introducing an aqueous slurry of wood cellulose fibers
(which have been beaten or refined to achieve a level of fiber hydration
and to which a variety of functional additives can be added) onto a screen
or similar device in such a manner that the water is removed, thereby
forming a sheet of the consolidated fibers which, upon pressing and
drying, can be processed into dry roll or sheet form. Also included within
the scope of this expression are the conventional processes for the
production of wet and dry-laid non-wovens.
Thus, in one aspect the present invention provides a feasible, efficient
and economical process for extending existing raw material sources.
Further, it allows the paper manufacturer a far greater degree of
flexibility in his operations: he is able to obtain starch fibers in dry
or wet-slab form and store them for subsequent use or he may incorporate
the starch fiber manufacturing process into his plant as an integrated
step in his plping and/or papermaking operations.
Moreover, the present invention offers the manufacturer a new means for
incorporating additives into paper products with increased retention and
consequently less economic loss and fewer pollution problems. As
previously discussed, it is common practice in the manufacture of paper to
introduce additives in conjunction with the fibers used in the pulp. Such
additives are incorporated in order to achieve specific paper properties
other than what is contributed by the fiber itself. Such additives include
materials which function as pigments (titanium dioxide, for example) as
well as other materials introduced into paper to achieve such properties
as improved brightness, opacity, smoothness, ink receptivity, fire
retardance, water resistance, increased bulk, etc. As an additional
embodiment of the present invention, it has been found that when starch
fibers are produced so as to contain various functional additives, and
such fibers are then utilized in the aqueous paper making process,
retention of the additives is greatly increased when compared with that
achieved using current methods. In addition to the increased retention, a
further advantage of the addition of additives in this manner is the fact
that there is no necessity for relying upon the sensitive charge balance
relationship between the cellulose fiber additive and the flocculant
(e.g., alum) or other retention aids. Indeed, it is unnecessary to use a
flocculant or retention aid with the starch fibers used in the present
invention.
It has also been found that non-woven webs can be produced in wet or
dry-laid form in accordance with the present invention wherein starch
fibers are incorporated within the web to serve as binders therefor. The
starch fibers may be retained in the final web or, if the base fiber
employed in the web is non-combustible, may be removed, depending upon the
desired end use.
Specifically, the present invention is directed to an improvement in a
process for manufacturing paper and paperboard comprising the steps of
introducing an aqueous slurry of a fibrous pulp material onto a screen in
such a manner that the water is removed thereby forming a sheet of
consolidated fibers which, upon pressing and drying, yields the final
paper product. The improvement comprises the step of replacing from 1 to
100% by weight of the pulp with water-insensitive starch fibers of 10 to
500 microns in diameter produced by extruding a thread-like stream of a
colloidal dispersion of the starch, at 5 to 40% by weight solids, into a
moving coagulating bath comprising an aqueous solution of a coagulating
salt selected from the group consisting of ammonium sulfate, ammonium
sulfamate, mono-basic ammonium phosphate, di-basic ammonium phosphate and
mixtures thereof, the solution containing the coagulating salt in an
amount at least sufficient to coagulate the starch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention may readily be adapted to be used on
any conventional paper making equipment using the procedures commonly used
in the specific plant, with the only difference residing in the
substitution of starch fibers for all or part of the cellulose pulp.
The starch fibers employed may be produced using a number of variations,
the only requirement being that the waterinsensitive fibers have a
diameter of 10 to 500 microns and that they be precipitated by the
extrusion of a thread-like stream of a colloidal dispersion of starch at
5-40%, by weight solids, into a suitable moving coagulating salt solution.
Fibers may be employed which are produced from any naturally occurring or
fractionated starch. Thus, corn starch, waxy maize, rice, tapioca, wheat,
potato, high amylose corn starch, commercial amylose powder, etc. may be
employed with naturally occurring corn starch, tapioca and waxy maize
being preferred due to their economy and availability.
The concentration of the starch solids in the dispersion should be about 5
to 40% by weight. While higher concentrations of starch solids may be
used, the resulting dispersions become very viscous and special equipment
is required to handle them. The particular concentrations employed in the
dispersions will however, affect the properties of the final fiber and the
desired end use. As an example, starch fibers prepared from 5% solids
dispersions have been found to be particularly useful in the production of
glassine or greaseproof papers while starch fibers prepared from 15%
solids dispersions have been found better suited for use in more porous
papers such as filter paper.
The particular starch employed must be used in the form of a colloidal
dispersion. For the purposes of this invention, the term "colloidal
dispersion" means dispersion of starch which is substantially free of
granules and which exhibits, on standing at the temperature at which it is
to be used, little evidence of gelation or precipitation. This state of
dispersion may be obtained using a variety of techniques depending upon
the particular starch base employed, the desired end use and the equipment
available.
When native starches that are very high in amylopectin content, such as
waxy maize, are employed, a suitable colloidal dispersion may be prepared
merely by thoroughly cooking the starch in water with no chemical
additives or modifications required. In most cases where starches which
contain less than about 95% amylopectin are employed, it will be desirable
to chemically derivatize or modify the starch to ensure its colloidal
dispersion before adding it to the aqueous system. The derivatization or
modification is carried out to an extent which will insure the production
of the desired colloidal dispersion without affecting the ability of the
starch to subsequently precipitate. Alternatively, if there is no
objection to the presence of caustic in the system, the latter starches
may be dispersed in aqueous sodium hydroxide, potassium hydroxide or other
common alkali. As further alternatives, the starch bases may also be
dispersed in a minor amount of an organic solvent such as
dimethylsulfoxide and then added to water, or the starch base may be
dispersed in conjunction with chemical additives such as urea and/or
paraformaldehyde. In the cases where causticizing is employed, the amount
of alkali used must be sufficient to adequately disperse the starch.
Typical amounts of alkali used when sodium hydroxide is emplyed are from
15 to 40%, by weight, based on the weight of the starch.
In preparing the starch dispersion, the starch is added to the dispersing
medium and vigorously agitated until a state of colloidal dispersion is
achieved. In the case of dilute dispersions of starch (i.e. about 5-10%
starch solids by weight), this will require about 45 minutes, with longer
periods and/or moderate heat required for more concentrated starch
dispersions or for certain chemically modified starch bases.
Most of the starch dispersions, including those of waxy maize and most of
the chemically modified starches, may be cooled to room temperature prior
to introduction into the coagulating bath. In the case of a few of the
less chemically modified starches, it will be preferred to employ the
dispersions at approximately the elevated temperatures at which they are
prepared so as to maintain the necessary colloidal dispersion and to
insure efficient fiber production.
The coagulating bath used in preparing the starch fibers employed in the
present invention comprises an aqueous solution containing specific
ammonium salts selected from the group consisting of ammonium sulfate,
ammonium sulfamate, mono- and dibasic ammonium phosphate and mixtures
thereof. It is also possible to combine the above-mentioned functional
salts with other compatible salts which will form a starch precipitate so
as to obtain satisfactory coagulation and a fibrous product. Suitable
salts for this purpose include ammonium persulfate, ammonium carbonate,
ammonium bromide, ammonium bisulfite, ammonium nitrite ammonium nitrate,
ammonium bicarbonate, ammonium oxalate, sodium and potassium chloride,
sodium and potassium sulfate, among others. Generally, no advantage is
seen in using these additional salts since the ammonium sulfate, sulfamate
or phosphate salts must still be present in their respective minimum
amount in order to effect coagulation. The only instances where the
presence of substantial amounts of other salts may be desirable is in the
use of the recycled coagulation bath wherein salts are present which have
been generated in situ, as will be discussed hereinbelow.
The minimum concentration of the salt required to effect coagulation as
well as the preferred salt or salt blend will vary depending upon the
particular starch base employed. For example, in the case of waxy maize
starch, it is necessary for ammonium sulfate to be present in amounts of
at least 35%, by weight of the total solution, ammonium sulfamate 72%
(saturation), di-basic ammonium phosphate 37% and mono-basic ammonium
phosphate 40%. In the case of corn starch or similar starches containing
about 64-80% amylopectin, lower concentrations of salt may be used with
ammonium sulfate required in amounts of 20%, ammonium sulfamate 50%,
mono-basic ammonium phosphate 25% and di-basic ammonium phosphate 30%. In
the case of hybrid corn starches containing less than about 50%
amylopectin, ammonium sulfate must be present in amounts of at least 15%,
ammonium sulfamate 40%, di-basic ammonium phosphate 25% and mono-basic
ammonium phosphate 20%.
It will be recognized that alkali salts are generated in the coagulating
bath when causticized starch dispersions are employed, with satisfactory
production of the desired starch fibers continuing until the level of the
generated salt is relatively high. The generated salt tolerance level
above which production of the fibers becomes inefficient will vary
depending upon such factors as the specific salt employed, the total salt
solids employed, the starch solid concentration in the dispersion, the
amount of amylopectin in the starch base, etc. Once this salt tolerance
level is determined, a steady-state system may be achieved at this maximum
level (or less) by the periodic addition of ammonium sulfate on a
continuous basis. As an example, when sodium hydroxide is used as a
dispersing medium and the starch mixture is extruded into an ammonium
sulfate coagulating bath, sodium sulfate is generated. In this case, it
has been found that production of corn starch fibers (13% solids
dispersion) will continue at a satisfactory level until a maximum of about
70 parts sodium sulfate per 30 parts ammonium sulfate (44% solids
solution) is present in the bath. Above this level of sodium sulfate,
production of the starch fibers becomes less efficient and the resulting
fibers tend to lose their individual integrity. However, by adding a small
amount of an inorganic acid to the initial coagulating bath or to the bath
during formation of the fibers, the level of the generated salt in the
system may be appreciably raised before production of the fibers is
seriously affected. Thus, using the example discussed previously, the
addition of as little as 3 parts of sulfuric acid per hundred parts of the
initially charged coagulating bath salt results in a tolerance level of 90
parts sodium sulfate per 10 parts ammonium sulfate thereby increasing the
longevity of the coagulating bath.
It is apparent that the salt solution used in the fiber forming process may
be recycled and used again once the fibers have been removed. It is,
however, important that the salt concentration be maintained, especially
where the salt is being depleted through a chemical reaction involving the
starch dispersion as it is introduced. In this regard, the starch
dispersions which do not contain caustic present little difficulty in
recycling other than that the solids content of the salt be maintained.
However, in those cases where causticized starch dispersions are employed,
chemical reactions with the coagulating solution will occur. For example,
if ammonium sulfate is used, the reaction results in the formation of
ammonium gas and sodium sulfate. The recycling of such a system can be
extended by recovering the ammonia in an acid scrubber and returning it to
the system as ammonium sulfate. The generated sodium sulfate can be used
in the coagulating bath as part of the salt blend until the tolerance
levels discussed previously are attained or can be used as a raw material
in other aspects of the pulp or papermaking operation, e.g. as a source of
"salt cake" in the production of Kraft pulp.
Starch fibers can be produced at any temperature at which the starch
dispersion can be handled. Generally, the coagulation bath is maintained
at about room temperature (20.degree. C.) during production of the fibers,
however, temperatures as high as about 70.degree. C. may be used. These
higher temperatures may be desired under certain conditions since they
increase the solubility of the salt in the coagulating bath resulting in
more concentrated solutions. Thus, when it is desired to produce waxy
maize fibers using mono-basic ammonium phosphate as coagulant, it is
desirable to increase the temperature of the bath so as to obtain a
concentration of salt of approximately 40% (saturation level for the
mono-basic ammonium phosphate at 20.degree. C. is 28%).
In preparing the starch fibers used in the invention, the starch dispersion
is introduced continuously or by drops in the form of a thread-like stream
into the moving coagulating salt solution. This introduction may be
accomplished either from above or below the salt solution using any
conventional techniques. Thus, the dispersion may be extruded through an
apparatus containing at least one aperture, such as a spinnerette, a
syringe or a biuret feed tube. Alternatively, the dispersion may be
discharged under pressure from a pipe or tube containing a plurality of
apertures into a surrounding enclosed area, e.g. a concentric pipe,
containing the moving coagulating solution. Various adaptations of the
above and related techniques may be used and the fibers may be thus
produced using either batch or continuous operations.
In accordance with either embodiment, the aqueous salt coagulating solution
should be moving when the starch dispersion is introduced and the
directionality of the two flows can also be utilized in controlling fiber
lengths and diameters or widths. Thus, if the salt solution is moving in a
direction generally concurrent with the flow of the starch dispersion,
rounder fiber lengths are formed; if the starch dispersion is introduced
at an angle of about 90.degree. to the flow of the salt solution,
relatively flatter fibers are formed. Generally aperatures of 10 to 500
microns in diameter are preferred, in order to produce fibers of the size
required herein. Thus, the starch fibers used in the present invention
have diameters (widths) of 10 to 500 microns and will generally have
lengths of from about 0.1 to 3.0 mm. if they are to be used as cellulose
pulp replacements in paper. For non-woven application, fibers of longer
lengths may be employed.
It will be recognized that the length, cross-sectional size and
configuration of the resultant fibers are dependent upon a number of
interrelated parameters in addition to those described hereinabove. Thus,
the viscosity, the solids content of the starch dispersion, as well as the
particular components used in the coagulating solution and/or starch
dispersion and the relative flow viscosities thereof are additional
factors which can be used in conjunction with the parameters discussed
previously in order to control the dimensions of the resultant fiber.
This and similar coagulating processes producing starch fibers useful
herein are described in our co-pending patent application Ser. No. 670,342
filed on even date herewith, now U.S. Pat. No. 4,139,699 the disclosure of
which is included herein by reference, as well as in U.S. Pat. No.
2,902,336. Various modifications of the processes may also be employed as
long as the final fiber possesses sufficient water insensitivity to be
employed in the papermaking operation.
The resulting aqueous slurry or suspension of starch fibers may be used
directly by introducing it into the pulp stream thereby enabling
production of fibers and paper web "inline" in the paper manufacturing
plant. If this embodiment is to be used, it is generally preferred to
first wash the fibers free of coagulating salt prior to introducing the
slurry into the paper manufacturing operation. Alternatively, the fibers
may be recovered in the dry state by collecting from water on a screen or
similar device. It is then preferably to reslurry the fibers into a
non-aqueous solvent such as methanol, ethanol, isopropanol, acetone or the
like in which the fibers are not soluble. The fibers are then recovered,
as by filtration, from the solvent and dried. Other methods such as
centrifuging, flash-drying or spray-drying may also be used to remove the
water. Once dried, the fibers may be re-introduced into an aqueous medium
and will exhibit excellent re-dispersibility maintaining their discrete,
discontinuous structure. Alternatively, the fibers may be recovered from
the slurry, as by filtration, washed and placed in water at levels of up
to about 50% solids and formed into "wet-slabs" for subsequent use.
It is also to be noted that the starch employed may be chemically treated
to vary the properties of the fiber produced or to help effect formation
of the colloidal dispersion. Alternatively, the starch fibers may be
treated after formation in order to produce certain functional
characteristics. Thus, the starch may be chemically treated, as by
aminoethylation, in order to provide rapid dispersibility of the starch in
the dispersion, which treatment will also result in the production of a
fiber which possesses a cationic charge when employed in an aqueous
medium. Similarly, a starch may be used which is modified to contain
anionic groups so as to be stable in a dispersion and which, after
regeneration, will produce a fiber having anionic properties. The fibers
may also be modified after their formation in order to achieve specific
functional properties. Thus, improved anionic functionality might be
obtained by bleaching the fibers after precipitation as long as the
conditions are not so severe as to destroy the fibers. The properties of
the fibers may also be controlled by using blends of modified and
unmodified starches or by the addition of other functional materials, such
as polyacrylic acid, to obtain the specifically desired properties.
As one of the advantages of the method of the present invention there is
provided a means to improve paper products in a variety of manners due to
the properties which are either inherent in or which may be imparted to
the starch fiber itself. As an example of such improved properties, we may
consider the production of such diverse specialty papers as glassine paper
and filter paper which require special treatment when conventionally
produced.
Glassine paper is made from pulps in which the quality of the fiber permits
a high degree of hydration. It is the mechanical treatment of the pulp
while suspended in water that causes the distinctive greaseproof
properties. The fibers are fibrillated and swollen to an almost gelatinous
condition. When paper is made from hydrated fibers, a dense non-porous
sheet is formed on the wire. The resultant sheet is resistant to the
penetration of greases and oils because it is composed of nearly
continuous well hydrated cellulose. To get the cellulose in this well
hydrated form requires a considerable amount of energy. Glassine
manufacturers must subject their stock to refining for extended periods of
time or increase the number of refiners through which the stock must pass.
Once the stock is hydrated and introduced on the wire it drains very
slowly. As a result, machine speeds are limited to between 150-500 fpm
depending somewhat upon the basis weight of the paper. The stock
temperature may be elevated with steam to accelerate water removal on the
wire. Attempts by glassine manufacturers to use cationic polyelectrolytes
for improving drainage has met only limited success. The flocculation of
the fibers may improve drainage but this disruption in formation can cause
pinholes which reduce oil and greaseproof properties of the product.
We have now found that when starch fibers are combined with cellulose
fibers which have been beaten to a degree less than would be required in
conventional glassine manufacture, the resultant mixture has a
significantly higher freeness and will drain at lower temperatures in
about one-third the time usually required at the elevated temperatures
presently used, with higher wet mat solids after pressing and improved
drying efficiency relative to the conventional glassine stock. Moreover,
the resultant sheet properties of this novel paper exhibits greater
internal strength (Z-directional strength), improved oil holdout
properties and greater resistance to the passage of air relative to
conventional glassine paper. It is apparent that the reduction of the
cellulose refining requirements can result in significant energy savings
since the fiber mix need not be elevated in temperature to achieve
acceptable water removal rates as is common practice in conventional
glassine manufacture.
Starch fibers may also be employed to provide a more porous sheet which is
a property that can be desirable in such papers as filter or saturating
grades. In prior art methods, reduced refining of cellulose has been found
to aid the development of this property, but does so only at the expense
of weaker web strength. The incorporation of starch fibers according to
the present invention, in conjunction with the cellulose, can result in a
more porous sheet structure while maintaining, and often improving, the
required strength properties.
As a further feature of the invention it is possible to incorporate certain
hydrocolloids in the dispersing medium and to extrude the hydrocolloids
together with the starch in order to produce a starch-hydrocolloid fiber
which may be used in the papermaking process of the present invention. In
order to achieve this fiber composition, it is only necessary that the
hydrocolloid (in minor amounts, i.e. less than 50% by total solids
weight), together with the starch portion, be placed in a state of
colloidal dispersion prior to contact with the coagulating bath. Thus, in
the case of water-dispersible hydrocolloids such as polyvinyl alcohol,
carboxymethylcellulose, hydroxyethylcellulose, etc., it is only necessary
to add the hydrocolloid to the water in which the starch is dispersed. In
the case of other hydrocolloids, such as casein, it will be necessary to
causticize the dispersion in order to form the colloidal dispersion
required.
As an alternative embodiment of the present invention, water-insoluble
additives may be uniformly admixed throughout the starch dispersion and
subsequently encapsulated within the resultant starch fiber. Thus,
water-insoluble additives, including pigments, metallic powders, latices,
oils, plasticizers, microspheres (glass beads, foamed silica or other low
density materials either in blown or unblown form), etc., may be
encapsulated within the starch fibers of the invention. In a similar
manner, water-insoluble synthetic polymers or latices, such as polyvinyl
acetate, polyacrylonitrile, polystyrene, etc., may be incorporated within
the fiber. It will also be noted that the density of the starch fibers may
be varied by incorporating air or other gases in the starch dispersion
prior to passing it into the coagulating bath.
It is to be further noted that certain water-soluble solid additives may
also be co-extruded with the starch fibers. In such cases, the additive
will be dissolved in the aqueous starch dispersion and the coagulating
bath which is employed in forming the starch fibers will be adjusted by
the addition of a sufficient quantity of a compatible salt capable of
precipitating the additive. As an example, a commercial rosin size can be
added to the starch dispersion and extruded into a coagulating bath
containing the functional starch-coagulating salt together with sufficient
aluminum sulfate to precipitate the rosin, thereby forming a
co-precipitated starch-aluminum rosinate fiber.
The water-insolubility of the starch fibers of the present invention can be
further enhanced by the incorporation of conventional cross-linking
agents, such as urea-formaldehyde, glyoxal, urea-melamine-formaldehyde,
Kymene (registered tradename of Hercules, Inc., Wilmington, Delaware),
etc. These crosslinking agents may be incorporated into the starch
dispersion prior to extrusion or may be post-added to the starch fiber.
Generally, any additives employed will be used in amounts less than about
50% by weight of the total solids, however, certain additives including
clay and pigments may be incorporated at levels up to about 80% by weight.
It will be realized that the specific additive selected for incorporation,
as well as the amount employed in any of the above-described embodiments,
will depend upon what properties are desired in the final fiber. Thus,
pigmented fibers show improved opacity and may be incorporated by
conventional methods into the fibrous web with overall improved pigment
retention relative to that obtained by merely adding pigment to a paper
stock system. Fire retardant properties may be conveyed to a substrate by
incorporating polyvinyl chloride powder and antimony trioxide or other
fire retardant chemicals within the starch fiber. Starch fibers containing
microspheres may be incorporated into paper webs at high levels of
retention. The retention of such spheres enables the production of sheets
of high bulk and low weight as compared with cellulose sheets of
comparable weight. In conventional sheets containing microspheres, the
presence of the microspheres between the fibers has a debonding effect on
the fibers and this results in a sheet of low strength. In contrast, the
sheets of the present invention possess excellent strength properties as
the spheres are encapsulated within the starch fibers so that the
debonding effect on the spheres is minimized. The density of the starch
fibers, and resultant paper, may also be varied by the incorporation of
air or other gases in the starch dispersion prior to passage into the
coagulating bath.
Furthermore, by using additive encapsulating fibers it will be possible,
not only to provide a novel process of incorporating additives in paper,
but also to produce novel effects in the paper itself. As an example,
there are papermaking machines that produce a final web which is
constructed of individual layers compressed together. Such equipment may
be described as cylinder machines or Fourdriniers with a second down-line
headbox or with multiple headboxes. Machines of this type normally use
lower quality fibers for the inner plies and quality pulp as the top
liner. By utilizing a pigmented starch fiber in the top line, production
of paper web having the surface properties of coated board is possible. In
essence a coated board would be produced via a wet-end application process
due to the high concentration of starch and pigment at the substrate
surface. Alternatively, special decorative or construction paper could be
manufactured having different colored sides. Dyed fiber could be prepared
at various colors and fed to two different headboxes. Such twocolored
sided paper is prepared today but requires the use of surface applications
during processing.
One of the advantages of the use of water-insoluble synthetic polymers
encapsulated within the starch fiber is that it permits a high retention
in paper and paper-like webs of synthetic fibers (such as rayon, acrylic,
polyester, nylon or polypropylene). Most of these fibers carry very low
surface charge and therefore their retention in commonly used latex binder
systems, which rely upon precipitation and fiber deposition techniques,
are poor. Such poor retention can result in low binder efficiency and
problems with foam, sticking and accumulation of polymer in the system.
The resin encapsulating starch fiber insures efficient retention and
provides the desired end sheet properties.
An additional feature of the present invention is that the starch fibers
may also be employed in the production of dry laid nonwovens of synthetic
fibers. In such applications, a web is produced using air as the medium
for depositing the fibers on a moving wire. Since the synthetic fibers are
not hydrated, bonding is inhibited and relatively weak and soft structures
are produced. Thus, in order to provide integrity to the web, it is
necessary to spray a binder on its surface. In accordance with the present
invention, it is possible to blend dry starch fibers with the synthetic
fibers. Such a method would be particularly advantageous in the area of
disposable nonwovens wherein the biodegradable properties of the starch
fiber would be superior to those obtained with the presently employed
synthetic fiber binders. As binders those fibers particularly high in
amylopectin content are preferred. It is to be noted that the starch fiber
may be retained in the final non-woven web or removed therefrom if
desired. If the starch fiber is to be removed, as for example, from a
ceramic web, exposure to ashing conditions sufficient to burn off the
starch fibers provides a suitable means for removal thereof.
The starch fibers, filled or unfilled, may be successfully used alone in
the formation of an | | |
