Note: Descriptions are shown in the official language in which they were submitted.
~ 21~9~0Q ~
-- 1 --
PROCESS FOR MARING MICRODENOMINATED CELLULOSE
- Michael K. Weibel
Richard S. Paul
This invention relates to microdenominated
cellulose and to a process for its preparation.
BAC~GROUND OF THE I~V~N110N
Since cellulose is the major structural
constituent of most plant matter, it is natural that
those interested in processing or refining such
materials refer to them as cellulosics. However this
general term connotes a multiplicity of meAn;ngs
whereby each is qualified by descriptors frequently
specific to the interest at hand. The commercial
applications of processed plant matter to produce a
refined cellulosic material are numerous and involve
use in many nonanalogous arts. For example, refined
celluloses are extensively used in paper and textile
applications. Refined cellulose is also used in
adhesives, food ingredients, industrial coatings and
various other diverse applications. For each end use,
the raw material, processing and final product(s)
comprise a technological field essentially unique to
itself.
In general, a wide variety of chemical,
thermal and mechanical transformations are known in
the art to refine, manipulate and modify cellulose for
numerous purposes. The following hierarchical
2S characterization has been devised to describe
previously known technology relating to structural
manipulation of refined cellulosic substances. This
characterization serves the additional purpose of
providing bases for distinction between the process of
the present invention and the prior art.
~39400
- 2 -
The molecular level or primary structure of
cellulose is the beta 1-4 glucan chain. All
celluloses share this level of structure and it is the
distinguishing difference between cellulose and other
complex polysaccharides. The natural chain length is
not known due to unavoidable modification and
degradation which occurs during the disassembly to
this level, but probably extends into the
polymerization regimes of many thousands of glucan
units. Transformations at this level of structure
involve forming and breaking chemical bonds.
Secondary structure is considered to be
submicroscopic strands formed from parallel, aligned
assemblages of glucan chains. This level of
organization is designated the microfibril.
Microfibrils are spontaneously formed from a plurality
of nascent glucan chains believed to be synthesized
simultaneously by a complex, motile, biosynthetic
organelle involved in the assembly of the primary
plant cell wall. The microfibril is of sufficient
size to be discernable with the electron microscope
and depending on the plant species ranges in its major
cross-sectional ~;men~ion from approximately 50 to 100
Angstroms. As with the beta-glucan chain, of which it
is composed, the length is indeterminant.
Non-covalent interactions, such as by hydrogen
bonding, stabilize secondary structure. Because the
interchain attraction is high, structural
transformation is probably rare unless preceded by
chemical modification of primary structure.
Tertiary structure is related to arrays and
associations of microfibrils into sheets and larger
stranded structures designated fibrils. The
distinguishing features at this level of structure are
sufficiently small that resolution is usually possible
only via the electron microscope. However, some
Zl:~9400
~ .
- 3 -
individual fibril assemblies are of sufficient gross
cross-sectional dimension (0.1 to 0.5 microns or
lO,000 to 50,000 Angstroms) to be discernible with the
light microscope. Structural deformation at this
level is largely mechanical and either organized
(disassembly/denomination) or random (indiscriminate
fracture/cleavage).
Lastly, quaternary structure deals with the
construct of tertiary elements-which form the primary
and secondary cell wall. This level of structure
.defines the physical ~;~ensions of the individual cell
and any gross structural speciallzation required ~or
physiological function of the differentiated cell.
Examples are libriform, tracheid and parenchymal cell
structure. Structural manipulation results from
indiscriminent comminution and is the most commonly
employed mechanical transformation practiced.
Conventional pulping of cellulosic materials
is primarily concerned with chemi-thermomechanical
processing of schlerenchymous or structural plant
tissue to achieve individually dispersed cells. The
result is a quaternary structure largely consisting of
cellulose derived from the primary and secondary cell
walls. Depending on the plant source and extent of
processing some heteropolysaccharides such as
hemicellulose (xylans, galacto~nn~n~, pectins, etc.)
may also be present. The important distinction of
pulping from other processing of celluloses is that an
anatomical destruction of intact plant tissue occurs.
This results in dispersed cellforms which represent a
minimal degree of quaternary and more basic structural
levels of manipulation. Some forms of cellulose, such
as cotton, are produced naturally in a dispersed state
and do not require pulping as a prerequisite.
Important to the following discussion is the
distinction between disassembly and indiscriminate
`` 213g400
-- 4
fragmentation processes. In fragmentation the
localized energy excursion (by whatever means) is
sufficiently high and accumulates sufficiently rapidly
that an organized dissipation of internal energy by
the acquiring matrix is not effected. Here an intense
perturbation is applied and results in an
indiscriminate fracture or other ma~or disorganization
at translocations within a defined microdomain. In
the case of disassembly, on the other hand, the
acquired energy excursion is dissipated in a more
organized manner usually following a path of lowest
activation energy. For cellulose this appears to
involve segmentation along parallel fibril oriented
assemblies and possibly l~m; n~r sheet separation of
fibril arrays.
Mechanically beaten celluloses have long
been employed in the paper and packaging industry.
Chemi-thermomechanically refined wood pulps are
typically dispersed in hydrobeaters and then subjected
to wet refining in high speed disc mills. This level
of structural manipulation as presently practiced is
exclusively at the quaternary level. The objective of
such processing is to disperse aggregated fiber
bundles and increase available surface area for
contact during drying to increase dry strength.
Substantial size reduction and concomitant impairment
of dewatering are undesirable and circumscribe the
extent of processing. The measurement of the ease of
water drainage from a beaten pulp is termed Canadian
Standard Freeness and reflects the ease or rate of
interstitial water removal from the paper stock.
Finely ground or fragmented celluloses are
well known. These products are produced by mechanical
comminution or grinding of dried, refined cellulose.
They are employed largely as inert, non-mineral
fillers in processed foods and plastics The
Z139400
-
-- 5
manipulation is exclusively at the quaternary level of
structure. It is:achieved by application of a variety
of size reduction technologies, such as ball and bar
mills, high speed cutters, disc mills or other
techniques described in part in U.S. Patent No.
5,026,569. The practical limit of dry grinding is
restricted in part by the,thermal consequences of such
processing on cellulose and in part to the economics
of equipment wear and material-contamination of the
product. Micromilled cellulose (MMC) prepared in
aqueous or other liquid media as described in U.S.
Patent No. 4,761,203 avoids the thermal decomposition
associated with prolonged or intense dry grinding.
This technique allows particle size reduction into the
colloidal range (about 10 microns). It is believed to
operate by indiscriminate micro-fragmentation of
quaternary structure, without incurring the
fusion/thermal degrading effects characteristic of dry
grinding.
Microfibrillated cellulose (MFC), as
disclosed by Turbak et al (U. S. Patent No.
4,374,702), is principally a mechanical manipulation
of refined cellulose from wood pulp at the tertiary
level of structure. -The process employs high
pressure, impact discharge onto a solid surface of a
cellulosic dispersion in a liquid medium. This results
in a combination of direct energy transfer through
high, adiabatic shear gradients generated within the
impact domain and secondary effects of such shear (or
translational momentum,exchange) from solvent
cavitation to disassemble suspended cellulose
particles Depending on the extent of processing and
preconditioning of the raw material the structural
manipulation produces fibril ensembles of disassembled
quaternary structure These highly dispersed fibril
2~39~o~)
-
-- 6
structures impart unusual properties to the continuous
liquid phase in which they are prepared.
Microcrystalline cellulose (MCC), as
disclosed in U.S. Patent No. 3,023,104, exemplifies
structural manipulation which can occur at the
secondary level of structure. The process involves
selective acid hydrolysis of solvent accessible and
amorphous regions of secondary structure in refined
cellulose to produce relatively crystalline
microdomains that are resistant to further hydrolysis.
The dimension of the crystallite domains is on the
order of ten to thirty microns. If the never dried
crystallite is sheared, it disperses into parallel
clusters of microfibrils, reflecting periodic cleavage
along a fibril assembly. The microfibril crystallites
exhibit high surface area and readily reassociate on
drying into a hard, non-dispersible mass.
Furthermore, the production of rayon and
cellulose ethers such as cellulose gum (carboxymethyl
cellulose, CMC) involves manipulation at the primary
- level of structure. In the case of rayon the
modification is transient and reversible whereby the
reconstituted beta-glucan chain spontaneously
reassembles into semi-crystalline material that can be
spun into fibrils. Cellulose ethers represent a
deliberate, irreversible modification whereby the
individually formed beta-glucan chains are prevented
from reassembly due to the-chemical derivatization. A
li~ited variation of such derivatization is that of
powdered cellulose wherein the degree of substitution
is relatively low, to form e.g. forming carboxymethyl
or diethyl aminoethyl cellulose, CM cellulose and DEAE
cellulose, respectively. The latter materials are
useful as ion exchange media.
~39M~O
SU~IARY OF THE INVENl~ION
It is an ob~ect of this invention to provide
a relatively simple and inexpensive means for refining
fibrous cellulosic material into a dispersed tertiary
level of structure and thereby-achieve the desirable
properties attendant with such structural change. The
cellulosic fiber produced in this way is referred to
herein as "microdenominated cellulose (MDC)".
The foregoing object is achieved by
repeatedly passing a liquid suspension of fibrous
cellulose through a zone of high shear, which is
defined by two opposed surfaces, with one of the
surfaces rotating relative to the other, under
conditions and for a length of time sufficient to
render the suspension substantially stable and to
impart to the suspension a Canadian Standard Freeness
that shows consistent increase with repeated passage
of the cellulose suspension through the zone of high
shear.
It has now been discovered that
microdenominated cellulose can be produced using
standard refining equipment, e.g. a double disk
refiner, operated in a way differing from the
conventional use of this equipment in refining pulp
for paper manufacture. Whereas paper manufacture
calls for minimum damage to the fiber during refining
and a Canadian Standard Freeness consistent with good
drainage of water from the pulp, it will be apparent
from the following disclosure that use of the same
- equipment may be employed to achieve the opposite
effect, i.e., a high degree of disintegration of the
fiber structure, which results in a cellulose product
having very high surface area and high water
absorbency. The degree of disintegration is
sufficiently severe that, as refining continues beyond
that level normally used for paper manufacture (a
~3~oo
-- 8
Canadian Standard Freeness value approximating 100), a
reversal of the Canadian Standard Freeness values
occurs. The reason for this reversal is that the
dispersed fiber becomes sufficiently microdenominated
S that gradually greater amounts:of fiber begin to pass
through the perforated plate of the Canadian Standard
Freeness tester with water, thus leading to a
progressive increase in the measured value as refining
continues. Continuation of refining ultimately results
in essentially all of the refined fiber readily
passing through the perforated plate with water. At
this stage of processing, the measured Canadian
Standard Freeness value is typical of that for
unimpeded passage of water through the perforated
plate of the test unit.
Whereas a single stage, and at most two
stages are used for conventional refiner processing in
paper manufacture, the process of this invention
requires multiple passages of the pulp through the
zone of high shear, which may typically involve ten to
forty passages.
In paper manufacture beating or refining
increases the area of contact between dispersed fibers
by increasing the surface area through dispersion and
fibrillation. MDC manufacture applies and extends
such processing to a much greater degree. It is
believed that the extent of refinement needed to
achieve this high degree of fibrillation leads to a
concomitant disassembly of tertiary structure, and
perhaps even secondary structure. The result is an
ultrastructurally dispersed form of cellulose with
very high surface area.
The process for preparing MDC is more
closely associated with disassembly than it is with
3S the indiscriminate fragmentation used in mechanical
comminution or grinding of dried, refined cellulose or
39~00
g
micromilling of cellulose in liquid media. It is also
quite different from the approaches noted above based
on chemical hydrolysis or chemical modification. The
product, MDC, is most nearly like microfibrillated
cellulose, MFC, produced by high pressure impact
discharge of a cellulosic dispersion in a liquid
medium onto a solid surface, as disclosed by Turbek et
al. However, contrary to the teachings in the Turbek
et al patent, to the effect that beating and refining
as practiced in the paper industry are relatively
inefficient processes since large amounts of energy
are expended to gain relatively minor amounts of fiber
opening and fibrillation, the opposite appears to be
true based on the research leading up to this
invention, as will be explained below.
The MDC product of the invention has very
high surface area, consisting essentially of
thread-like structures (most of which are not
discernable with the light microscope). These
represent longitudinally oriented clusters of
microfibrils with attendant, protuberant
ultrastructure emanating from their surfaces. These
structures form entangling and interacting networks
which lead to a unique form of microscopic
compartmentalization for ~ixtures of discontinuous
materials in water or other continuous phase systems.
Such behavior results in the formation of interesting
viscoelastic characteristics such as gel structure,
mouthfeel, textural quality and other properties
highly desired in foods, pharmaceutical and cosmetics
products.
Specifically, it has now been found that MDC
generates organoleptic response in comestibles ranging
from little or no response to a creamy mouthfeel.
This attribute is believed to be a result of the
degree to which the submicron dimensions of the
~39~oo
- 10 -
diameter of the dominant threadlike component has been
reduced. This property is highly desirable and a
sought after textural quality in food ingredients.
A second desirably quality of MDC in
comestibles is the ability of the threadlike
structures to entangle and form particle gel networks
which structure or compartmentalize the continuous gel
networks which structure or compartmentalize the
continuous phase or medium into which they are
dispersed. The properties of such particle gel
structures are typically a function of the degree to
which the MDC has been processed but are typically
formed at concentrations in the 0.5~ to 1.0%
weight/volume range in the domains of the suspending
medium. These gel like networks are very effective in
providing spatial stabilization of suspended or co-
dispersed materials which would otherwise either
settle or buoyantly cream over time according to their
specific gravity relative to that of the continuous
phase.
A third desirable quality which now has been
discovered for use of MDC in comestibles is the
ability to bind and control the mobility of water and
other fluids within the regions of dense, dispersed
microfibrils characteristic of MDC particle gel
networks. This water and other polar or even
dispersed lipid phases are effectively immobilized by
MDC. Control of moisture and lipid migration in
comestibles is likewise a highly sought after property
of ingredients for use in food products.
The product of the invention, MDC, is
characterized by a settled volume greater than about
50~ after twenty-four hours, as based on 1~ by weight
aqueous suspension, and water retention greater than
about 350~. Procedures for determining the settled
"~
volume and water retention values for MDC are
described in détail below.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photomicrograph of wheat fiber
described in EXAMPLE 1 below before refining shown at
a magnification of 100 times.
FIG. 2 is a photomicrograph of the aforesaid
wheat fiber after refining shown at a magnification of
100 times.
FIG. 3 is a photomicrograph of the aforesaid
wheat fiber before refining shown at a magnification
of 250 times.
FIG. 4 is a photomicrograph of the aforesaid
wheat fiber after refining shown at a magnification of
250 times.
FIG. 5 is a photomicrograph of softwood
fiber described in EXAMPLE 2 below before refining
shown at a magnification of 100 times.
FIG. 6 is a photomicrograph of the aforesaid
softwood fiber after refining shown at a magnification
of 100 times.
FIG. 7 is a photomi~oy~aph of the aforesaid
softwood fiber before refining shown at a
magnification of 250 times.
FIG. 8 is a photomicrograph of the aforesaid
softwood fiber after refining shown at a magnification
of 250 times.
FIG. 9 is a photomicrograph of oat fiber
described in EXAMPLE 3 below before refining shown at
a magnification of 100 times.
EIG. 10 is a photomicrograph of the
aforesaid oat fiber after refining shown at a
magnification of 100 times.
? : ` `
s4~0
- 12 -
FIG. 11 is a photomicrograph of the
aforesaid oat fiber before refining shown at a
magnification of 250 times.
FIG. 12 is a photomicrograph of the
aforesaid oat fiber after refining shown at a
magnification of 250 times.
DETAILED DESCRIPTION OF THE lNV~NllON
In accordance with the present invention,
microd~no~;n~ted cellulose is produced from cellulosic
material by repeatedly passing the material in an
aqueous suspension through a zone of high shear,-
defined by two opposed surfaces, one of which is
caused to rotate relative to the other. According to
a preferred embodiment, the cellulose suspension is
passed through a double disk refiner of the type
typically used in the processing of wood pulps for
paper manufacture. Whereas processing with such
equipment in conventional paper applications is
limited so that the degree of refining achieved
corresponds to Canadian Stan~rd Freeness (CSF) values
of about 100 or greater, the present invention calls
for a degree of ref;nem~nt whereby the CSF value is
reduced toward zero and then progresses through
freeness values, approaching and ultimately exceeding
the values for never-processed pulp in aqueous
suspension.
Examples of the use of this invention are
set forth below for softwood pulp, white wheat fiber -
and oat fiber. Other types of cellulosic fibrous
material can also be processed in accordance with the
present invention. However, long-fibered materials
such as the softwood pulp and wheat fiber appear to be
better suited to this approach than short-fibered
material.
. - ~
2139400
- 13 -
The starting material for the process is
conveniently prepared by beating cellulosic sheet
material in a hydrobeater in the presence of a
suitable liquid, which disintegrates the sheet
material and uniformly disperses the fibers in the
liquid.
The exact amount of refining time required
to produce MDC depends on the characteristics of the
starting material e.g. the fiber length, the
temperature of refining and the solids concentration
in the pulp. The length of processing is also
influenced by the parameters of the shear zone in
which the cellulose suspension is processed. In the
case of a double disk refiner, these parameters
lS include the amount of back pressure exerted on the
cellulose suspension as it is subjected to shear
. stress during refining, the refiner plate surface
configuration, the space between confronting refiner
plates, refiner plate diameter and plate peripheral
speed. Efficiency is enhanced by operation at high
pulp solids concentration, an elevated back pressure
on the pulp during refining, elevated pulp
temperatures coupled with m~ m temperature control,
adjustment of the gap between confronting refiner
plates by keying on a pre-selected value of amperage
to the refiner motor and a refiner plate configuration
and peripheral speed that promotes "rubbing" or
fraying rather than cutting. Although refining
proceeds most efficiently as the solids concentration
in the pulp is increased, however, there is a limit to
how high the solids concentration can be and still
have the pulp flow through the system. A short-
fibered material like oat can be concentrated to
almost twice the solids concentration possible with
softwood and wheat, both long-fibered materials.
_, î
2~394~
- 14 -
Preferred operating conditions for
preparation of MDO in a double disk refiner are as
follows: fiber length of about 50 to 3000 microns, or
greater; refining temperature of about 60F to about
200F; a solids concentration of about 2 to about 10
by weight of the cellulose suspension; and back
pressure of about 10 to about 40 psi.
The re~; n; ng parameters, including plate
configurations, spacing between adjacent plates, plate
diameter and peripheral plate speed will depend on the
particular model of refiner selected to process the
MDC. A t-~pical run employing a Black Clawson 28-inch
Twin Hydradisc refiner is exemplified below.
A primary indicator used to monitor the
extent of refining of the cellulosic material is the
Canadian St~n~rd Freeness value as measured using
test equipment and procedures contained in TAPPI 227
"Freeness of Pulp" J. Casey, Pulp and Paper (1980).
Freeness has been shown to be related to the surface
conditions and the swelling of fiber which influences
drainage. As refining continues beyond levels normally
practiced in conventional paper making, the dimensions
of the resulting structures become sufficiently small
such that a reversal of freeness values occurs, i..e.
increasing-rather than diminishing values of freeness
as refining continues. This anomalous rise of
freeness is referred to herein as "false freeness".,
Once the reversal occurs and refining continues
thereafter, the measured freeness value increases
until a maximum value of approximately 800 is reached.
At this point the refined material has been rendered
sufficiently supple and fine (dimensionally small)
that it readily passes through the perforations of the
perforated plate of the tester along with the water.
In other words, the suspension behaves as though it
were fiber-free water of the same total volume as the
2J ~ 9 ~0 0
- 15 -
fiber-containing sample being measured. This is the
limiting condition for obtaining meaningful data from
freeness measurements. As the cellulose suspension
achieves this desired level of freeness, it becomes
substantially stable, which is intended to mean that
there is no visible segregation of the continuous
phase from the disperse phase, even upon standing for
a reasonable period of time.
F.~m; n~tion of photomicrographs of fiber
samples provide insight as to the degree of
fibrillation that is achieved by refining, with
reference to the starting material. FIG. 1 shows that
the length of the fiber prior to refining is in most
- cases at least 1000 microns and the fiber width is one
to two microns. Shown in FIG. 2 is the wheat fiber
structure resulting from the refining process
described in EXAMPLE 1, below, at a magnification of
100 times. FIG. 3 and FIG. 4 show the wheat fiber at
a magnification of 250 times and reveal detail
regarding the refined fiber in FIG. 4. It is apparent
from FIG.2 and FIG. 4 that the refined fiber is highly
disassembled. There is no evidence of the original
quaternary structure shown in the fiber before
refining. It has been disintegrated by the extended
period of refining and replaced by a network of
fibrils of vastly increased surface area. These
fibrils as viewed in the light microscope, appear as
very long threadlike strands of extremely small -
diameter for those that can be seen.
FIG. 5 shows the fiber structure of softwood
fiber at a magnification of 100 times before refining
and reveals a somewhat longer length (1000 to 3000
microns long) and greater width (two to four microns
wide) than the wheat fiber described above. FIG. 6
and FIG. 8 show fiber structure for the refined
~` 2'1;~940~ i 3
- 16 -
softwood that appears to be quite similar to that of
the wheat fiber sample discussed above.
E~m; n~tion of photomicrographs of oat fiber
samples provide insight as to the influence of fiber
length of the starting material on the degree of
fibrillation achieYed by refining as compared to the
longer fiber starting materials. FIG. 9 shows the oat
fiber prior to refining to be between 500 and 1000
microns in length and two to four microns in width.
FIG. 10 and FIG. 12 show that refined oat fiber
structure undergoes disassembly but not to the degree
of the long fibered wheat and softwood samples. There
is some evidence of the original quaternary structure
shown in the fiber before refining. A smaller
percentage of the structure of oat fiber has been
converted to a network of fibrils. This has resulted
in less surface area being created than occurs when
long fibered materials are refined.
As will be appreciated from the foregoing
description, MDC is the result of disassembly of
cellulose structure via essentially physical
manipulation. As such, MDC is distinguishable from
cellulosic products produced by chemical
transformation. No appreciable chemical change of the
cellulose starting material occurs during the refining
process described herein.
Several other parameters or properties, in
addition to Canadian Standard Freeness, serve to
characterize MDC.
A parameter useful in the characterization
and description of MDC is the settled volume of
aqueous dispersions of differing solids content after
twenty-four hours of settling. The settled volume of
a sample of MDC is determined by dispersing a known
weight of cellulose (dry weight basis) in a known
amount of water, e g in a graduated cylinder. After
~ :~9400
- 17 -
a prescribed settling time, the volume of the bed of
suspended cellulose is measured with reference to the
total volume of the continuous aqueous phase. The
settled volume is expressed as a percentage of the bed
volume to the total volume. From this data the solids
concentration in an aqueous dispersion that results in
a settled volume that is fifty percent of the original
volume can be determined and used to characterize the
product. The results of such measurements are shown
in Table 1. Ultrastructural parameters are also
important in this characterization. The very long
fibril softwood has the lowest solids concentration
for 50~ settled volume at 0.18~. The intermediate
wheat fiber is next at 0.23~ and oat fiber with a very
short fibril is highest at 0.87~. A characteristic of
MDC is that a 1~ by weight aqueous suspension has a
settled volume greater than 50~ after twenty-four
hours.
TABLE 1
Example Fibrous False Value Viscosity at S0~ Settled ~ Water
Number Material of CSF (ml) 1.5 Wt.~ (cp~ VolumeWt.~Retention
1 Wheat 780 5,860 0.23 1,005
2 Softwood 730 7,850 0.18 1,110
3 Oat 810 1,300 0.76 569
Water retention is another parameter for
characterizing MDC. Water retention values are
determined by employing a pressure filtration
apparatus ~Baroid Model 301 for low pressure fluid
loss control measurements, N. L. Baroid Corporation,
Houston, TX) routinely used to evaluate drilling fluid
properties. A 100 gram aliquot of a nominal 4 to 8~
w/w aqueous dispersion of cellulose is loaded into the
filter cell chamber, the cell chamber is capped and
subjected to 30 psig pressure from a regulated
f~ 3
21~9~0
- - 18 -
nitrogen source. The water discharged from the
filtration cell chamber is collected and pressure
continued for thirty seconds after observation of the
first gas discharge. The nitrogen source is then
turned off and collection of discharged water
continued for one minute or until the gas discharge
ceases, whichever event occurs first. Basically the
technique employs pneumatic, pressure filtration to
remove interstitial water from the particulate phase.
The expressed volume of water is recorded
along with the weight of wet cake. The wet cake is
then dried for sixteen hours at 95 degrees Centigrade
or until a constant weight is recorded. The water
retention value is computed as the ratio of (wet cake
weight minus the dry cake weight) to (dry cake weight)
times 100. This technique provides a good estimate of
the capillary and absorptive retention of water by the
cellulose solids by removing the interstitial water
from the cake solids. The procedure is quick (5 to 10
minutes) and highly reproducible. The water retention
value of MDC is characteristically at least 350~, and
preferably at least 500~.
Viscosity may also be used as a
characterizing property of MDC. Apparent viscosities
of an aqueous dispersions of 1.5 ~ w/w MDC solids
samples were determined with a Brookfield Viscometer
model DV-III using spindle SC4-16 with the small cell
adapter at a number of-shear conditions (5 through 100
RPM). The samples were pre-dispersed by high speed
mixing for three minutes at 10,000 RPM with a rotor
stator type mixer (Omni International, model 1000).
The viscosities measured for final refined product
(MDC) of the three examples are shown in Table 1. The
softwood fiber product exhibited a viscosity of
approximately 8,000 centipoise at a spindle speed of
100 RPM. The white wheat fiber product had a
~ r~
;~ 9400
~ . -- 19
viscosity of approximately 6,000 and the oat fiber a
viscosity of approximately 1,300 at the same
measurement conditions as for the softwood fiber. It
appears the wide range in the measured viscosities is
primarily due to the differences in fibril length and
other ultrastructural characteristics of the starting
materials.
It should be understood that the above
viscosity measurements on MDC dispersions are made on
a heterogeneous mixture (an interacting particle
ensemble suspended in a fluid medium). Viscosity
measurement is normally applied to homogenous systems.
Because of the heterogeneous nature of the mixture a
certain degree of mechanical distortion occurs in the
mixture around the rotating spindle used to determine
shear stress forces-within the mixture. Consequently
shear/shear stress measurements are time and history
dependent. As such the measurement is not a true
viscosity in the conventional sense but rather
provides a reproducible measurement that has been
found useful for characterizing the degree of
microdenomination and in describing the implementation
of this invention.
Energy input for refining MDC in the manner
described herein ranges from about 0.5 to about 2.5
kilowatt-hours per pound of MDC (dry weight basis) and
associated refining times vary from two to eight hours
depending on the cellulosic material being processed.
This is significantly lower than the energy
requirements for microfibrillated cellulose as
reported by Turbak et al in U.S. Patent No. 4,483,743.
Based on five to ten passes of a 1~ MFC solids aqueous
dispersion through an 80~ efficient-homogenizer at
8,000 psig. the energy requirement ranges from 4.4 to
8 7 kilowatt-hours per pound of MFC.
21~944~
. .
- 20 -
The following examples are provided to
describe in further detail the preparation of MDC in
accordance with the present invention. These examples
are intended to illustrate and not to limit the
invention.
EXAMPLE
Never dried white wheat fiber was mixed with
2,190 gallons of water in a hydrobeater (Black Clawson
Model 4-SD-4 with Driver No. 45) to make up a pulp of
4.5~ w/w solids. The white wheat fiber used in this
example is a commercially available refined fiber
product derived from bleached wheat chaff obtained
from Watson Foods Company, West Haven CT. The white
wheat product was obtained as a nominal 40~ w/w
nonvolatile solids fiber mat. The product was stated
to be 98~ total dietary fiber by the Prosky method.
The particle size by microscopic ex~m;n~tion indicated
a largely heterogeneous population of thin needle-like
sclerchyma cells ranging in major/minor dimensions of
500 to 1000 / 10 to 20 microns with few interspersed
parenchyma cells of 200/50 microns.
After beating the pulp for twenty minutes at
room temperature it was transferred to a water
jacketed holding tank to be repeatedly passed through
a Black Clawson Twin Hydradisc refiner. The refiner
of this example is a twenty-eight inch diameter double
disc unit powered by a 250 horsepower motor. The
refiner plates mounted on the discs are made of
sharloy (a nickel hardened steel). The refiner plates
were not equipped with dams. The faces of the
particular refiner plates used in this refiner
consists of alternate bars and grooves oriented so
that bars of the adjacent refiner plates (one static
and the other revolving) move relative to one another
with a scissoring action occurring as the bars of each
confronting plate move past one another. The three
2~ n ~1
- 21 -
critical dimenslons of these bars and grooves are the
bar width, channel' width and channel depth. For this
particular unit, they were, respectively, 2/16 of an
inch, 4/16 of an inch and 3/16 of an inch (expressed
S as 2,4,3 by Black Clawson's convention).
The refiner plates on the revolving disc
move at 713 revolutions per minute. Based on the
outer periphery of the re'finer disc extending to 13
and 1/4 inch from the centerline of the drive shaft,
this corresponds to peripheral speed of about 4,900
feet per minute. The pulp was continuously circulated
at a rate of apprsximately 250 gallons per minute
through the refiner and back to the holding tank.
Passage of the cellulose suspension through the
lS refiner occurs so as to have equal flow on each side
of the revolving disc.
One disc of the refiner is fixed while the
other is sliding. This allows the distance between
adjacent discs to be adjusted. In the full open
position (typical of startup or shutdown), discs are
one and three-quarters inch apart. During refining,
the discs are of the order of one to two thousands of
an inch apart. Rather than adjust the gap between
discs to a specific spacing, the value of the amperage
to the motor driving the refiner is used to establish
spacing. The procedure upon startup is to move the
discs from the full open position to a closer position
where the amperage reading increases until it reaches
310 amps. At this point, maximum power is being
delivered from the motor. Once this point is reached,
the back pressure on the refiner is increased by
closing down the valve on the line returning pulp from
the refiner to the holding tank. The back pressure is
normally raised from an initial value of about 14 psig
to a final value of about 35 psig. As the back
pressure is increased without adjustment of the
2 139~00
- 22 -
sliding disc location, the amperage drawn by the motor
decreases to about 260 amps. With the back pressure
at 35 psig, the sliding disc is ad~usted to bring the
discs closer together until the desired 310 amps are
drawn by the motor. Once this is done, there is no
further adjustment of the sliding disc unless the
motor amperage drops significantly. This may occur as
refining proceeds if certain properties of the pulp
change significantly. In that event, the sliding disc
is moved to reduce the gap between the discs until
either the desired amperage is once again achieved, or
the discs begin to squeal. S~uealing is to be avoided
as it is indicative of excessive disc wear and leads
to high refiner plate replacement costs.
A gate-type mixer in the holding tank
continuously mixed the contents during refining. A
back pressure of 34 pounds per square inch was
maintained in the return line from the refiner outlet
to the holding tank. The recycle operation continued
for approximately six hours during which the Canadian
Standard Freeness of the pulp changed from an initial
value of 190 to a final "false" value of 780 ml.
During refining the temperature of the pulp
increased from an initial value of 64 to a final value
of 190 degrees Fahrenheit. The amperage drawn by the
2S0 horsepower motor of the refiner varied from 310
initially to 290 amperes at completion of refining.
Energy input to the refiner was approximately 1.2
kilowatt-hours per pound of refined fiber processed
(dry weight basis). The resulting product is
characterized in TABLE 1.
EXANPLE 2
Dry, softwood fiber used in this example was
obtained from Stora Forest Industries Ltd., Port
Hawkesbury, Nova Scotia, Canada as a bleached sulfite
2~:~940~
_ - 23 -
pulp. It was derived from softwood species (balsam fir
and black and white spruce). The bleaching sequence
was reported to be (D70 +-D70) E (DE) D. The ash
(TAPPI 211 and 85) is 0.6~ and the CSF 660. The
dispersed individual fibers appeared to be 20 to 25
microns in diameter and ranged from one to three mm.
in length with the average fiber 25 microns by 2 mm.
Sheets of dry, softwood fiber (Storafite
04-620972) were mixed with 2,080 gallons of water in
the same hydrobeater as used in EXAMPLE 1 to make up a
pulp of 3.7~ solids. After beating the pulp for
twenty minutes at room temperature it was transferred
to the holding tank to be repeatedly passed through
the same Black Clawson refiner as used in EXAMPLE 1.
Pulp was circulated at a rate of approximately 250
gallons per minute through the refiner and back to the
holding tank. A gate-type mixer continuously mixed the
contents of the holding tank during refining. A back
pressure of 34 pounds per square inch was maintained
in the return line from the refiner outlet to the
holding tank. The recycle operation continued for
approximately six hours during which the Canadian
Standard Freeness of the pulp changed from an initial
value of 620 to a final "false" value of 730 ml.
During refining the temperature of the pulp
increased from an initial value of 64 to a final value
of 144 degrees Fahrenheit. The amperage drawn by the
250 horsepower motor of the refiner varied from 310
initially to 290 amperes at completion of refining.
Energy input to the refiner was approximately 2.4
kilowatt-hours per pound of refined fiber processed
(dry weight basis).
EXAMPLE 3
Dry oat fiber (Williamson Type 9780) was
mixed with 1,055 gallons of water directly into the
2~9400
- 24 -
holding tank for the refiner to make up a pulp of
7.86~ solids. The'-dry oat fiber used in this example
, is a commercially available refined,fiber product
derived from bleached oat hulls (from Opta Food
S Ingredients, Inc. in Cambridge MA). The product,
identified as Better Basics TM type 780, is stated to
be 98~ total dietary fiber by the Prosky method. It
was obtained as a dry, light tan colored powder that
was readily hydrated in the refiner tank prior to
refining. The particle size was such that 98~ on a
weight basis passed through a 50 mesh screen using-an
Alpine Airjet Sieve. Microscopic ~m; n~tion
indicated particles consisted largely of
heterogeneous dispersed fiber cells with major/minor
dimensions of 100 to 600 / 10 to 40 microns. In
contrast to wheat and softwood oat represents a
relatively short fiber structure.
The already finely divided state of the oat
fiber made it possible to eliminate the hydrobeater
step. The pulp was refined in the same Black Clawson
unit as used in the two previous examples. Pulp was
circulated at a rate of approximately 250 gallons per
minute through the refiner and back to the holding
tank. A gate-type mixer in the holding tank
continuously mixed the contents during refining. Back
pressure maintained in the return line from the
refiner outlet to the holding tank varied from 34 to
31 pounds per square inch gauge. The recycle
operation continued for two hours and forty minutes
during which the Canadian Standard Freeness of the
pulp changed from an initial value of 310 to a final
"false" value of 810 ml.
During refining the temperature of the pulp
increased from an initial value of 6S to a final value
of 168 degrees Fahrenheit. The amperage drawn by the
250 horsepower motor of the refiner varied from 310
2139400 ~
- 25 -
initially to 260 amperes at completion of refining.
Energy input to the refiner was approximately 0.5
kilowatt-hours per pound of refined fiber processed
(dry weight basis).
While certain preferred embodiments of the
present invention have been described and examplified
above, it is not intended to limit the invention to
such embodiments, but various modifications may be
made thereto, without departing from the scope and
spirit of the present invention as set forth in the
following claims.
