Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REDISPERSIBLE MICRODENOMINATED CELLULOSE
- Michael K. Weibel
This invention relates to redispersible
microdenominated cellulose and to the production of
microdenominated cellulose which may be dried and
redispersed.
S B~C~GROUND OF THE lNv~Nl~IoN
Microdenominated cellulose (MDC) is produced
from fibrous cellulosic material that has been
extensively refined and converted into a dispersed
- - tertiary level of structure, thereby achieving certain
lo desirable properties attendant upon such structural
change.
MDC is prepared 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.
The production of M~C can be efficiently
- carried out using standard refining equipment, e.g. a
double disk refiner, operated in a way differing from
2s 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, the same equipment is
used to achieve the opposite effect in preparing MDC
i.e., a high degree of disintegration of the fiber
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structure, which results in a cellulose product having
very high surface area and high water absorbency. The
degree of disintégration is sufficiently severe that,
as refining continues beyond that level normally used
S for paper manufacture (a 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 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
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The product of the invention, MDC, is
characterized by a settled volume greater than about
S0~ after twenty--four hours, as based on 1~ by weight-
aqueous suspension, and water retention greater than
S about 500%. Procedures for determining the settled
volume and water retention values of MDC are described
in detail below. Details regarding the preparation of
microdenominated cellulose (MDC) are set forth in our
copending Canadian Patent Application No. 2,139,400,
entitled "Process for Making Microdenominated
Cellulose" in the names of Michael K. Weibel and
Richard S. Paul, which is commonly owned and filed
December 30, 1994. Because the extent
to which it is refined, MDC has a highly fibrillated
structure resulting in a very high surface area and
-the ability to form stable gels. Many of the uses
contemplated for MDC in food, pharmaceutical,
cosmetics, and the like are best served by providing
MDC as a-dried product that can be readily rehydrated
and redispersed, whereby it exhibits properties
approximating those of never dried-MDC, i.e. MDC as
discharged from the refiner or prior to drying.
Unfortunately, this favorable structure and the
desirable properties associated therewith are lost or
greatly diminished upon drying the material. This
- occurs as a result of a partially irreversible
collapse of the structure of the cellulose fibers due
to shrinkage forces exerted during drying
(hornification). The favorable dispersibility,
hydration and viscosity properties of MDC are lost or
substantially modified depending on the severity of
drying
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A number.of techniques have been developed
heretofore to avoid or substantially lessen the
adverse effects of drying on cellulose They include,
among others, the use.of additives, solvent
S replacement of water and modified drying techniques.
The latter two approaches are .described in U.S. Patent
No. 3,023,.104. Water can be displaced by a water
miscible organic compound of low molecular weight such
as methanol, ethanol, propanol, etc. followed by
evaporation of the displacement fluid. Modified
~ drying methods include spray drying i.n vacuum or air
up to 100 to lOS degrees Centigrade, freeze drying and
drum drying.
Most additives are intended to prevent
drying stress or hornification.by inhibiting hydrogen
bonding of the cellulose fibrils. As disclosed in
U.S. Patent No. 4,481,076, the additive forms hydrogen
bonds or complexes with the cellulose fibrils and
prevents them from bonding to each other during
drying; thus, the cellulose fibrils remain readily
accessible to water and.-easily rehydrated. To perform
this function the additive must be capable of
substantially inhibiting.the hydrogen bonding between
the fibrils of the cellulose.. Among the compounds
found as useful additives are polyhydroxy compounds
including particularly carbohydrates or carbohydrate-
like compounds. These additive compounds must be used
in substantial quantities, generally at least one half
of the dried weight of the microfibrillated cellulose
and preferably at least equal to microfibrillated
cellulose weight in order to achieve the desired
effect
SUMMARY OF THE INVENTION
This invention provides dry microdenominated
3S cellulose that can be
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readily redispersed in water and exhibit properties
after redispersion that are essentially equivalent to
those of never dried microdenominated cellulose.
.
The foregoing is realized by a process of
microdenominating cellulose in liquid suspension,
drying the suspension of microdenominated-cellulose in
the presence of at least one dispersion agent which is
~ believed to function by reducing or preventing bonding
between the fibrils of cellul~-,se. The resultant
product is a composition comprising the dry,
microdenominated cellulose and the dispersion agent in
an amount effective to impart to the cellulose a
viscosity, when redispersed in water, that is at least
fifty per cent of the viscosity of the equivalent
concentration of never dried microdenominated
cellulose.
DETAILED DESCRIPTION OF THE INVENTION --
The starting.material for producing MDC 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
include-the amount of back pressure exerted on the
cellulose suspension as it is subjected to shear
stress during refining, the refiner plate surface
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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 max~mum 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 "rub~ing" 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.
Preferred operating-conditions for
preparation of MDC in a double disk refiner are as
follows: fiber length of about 50 to 3000 microns, or
greater; refining temperature of about 60~F to about
200~F; 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 remaining 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 typical 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 Standard Freeness value as measured using
test equipment and procedures contained in TAPPI 227
"Freeness of Pulp" J Casey, Pulp and Paper (1980).
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Freeness has been shown to be related to the-surface
conditions and the swelling of fiber which influences
drainage. As réfining 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
- 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 ~pon-standing for
a reasonable period of time.
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
3S weight of cellulose (dry weight basis) in a known
amount of water, e.g. in a graduated cylinder. After
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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. A characteristic of MDC is that a 1~ by
weight aqueous suspension has a settled volume greater
than 50~ after twenty-four hours.
W~ter retention is another parameter for
characterizing MDC. Water retention values are
determined ~y employing a pressure filtration
apparatus (Baroid Model 301 for iow 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
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)
i ~3~399
g
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 Broo~field Viscometer
model ~V-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
viscosity of approx.imately 6,000 and the oat fiber a
viscosity of approximately 1,300 at the same
measurement conditions as for the softwood f-iber. 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
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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.
According to a preferred embodiment, two
substances have been found to produce a synergistic
effect when used in combination as a dispersion agent
in the practice of this invention. The substances are
maltodextrin and carboxymethylcellulose ~CMC), which
need only be added in modest quantities relative to
the weight of the MDC. The preferred amount of the
maltodextrin is about one-half to one and one-half the
weight of the MDC while the preferred amount of the
CMC is about 5% to about 15% the weight of the MDC.
Maltodextrins are short chain
oligiosaccharides reduced by the controlled hydrolysis
of starch. The degree of polymerization (DP) of
maltodextrins is typically less than thirty and higher
than ~ive. Commercial maltodextrins are excellent
film ~ormers and display low viscosity aqueous
solutions at relatively high solids levels, typically
10 to 30%. They are readily available in food grades
at reasonable cost.
CMC is a random ether substituted
homopolymer of glucose produced by reaction of
alkaline cellulose with chloroacetic acid. It also is
food approved and readily available at relatively low
cost. CMC has long been employed as a dispersant for
cellulose slurries in the pulp and paper industry. It
has been used as a drying additive for redispersion of
microcrystalline cellulose and in drying of other,
refined high surface area cellulose products. CMC
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(and other random substituted cellulose ethers) arebelieved to have..regions of low to no substitution
which have relatively high affinity for certain
surface orientation of their particulate counterpart,
the unsubstituted beta-glucan chain ensemble
constituting refined cellulose. In the case of CMC
for which each carboxymethyl substituent bears a
stationary negative charge at a pH greater than 3.5,
the binding of this substituted oligiosaccharide to a
cellulosic surface would impart substantial stationary
charge and negative zeta potential. Although not
intending to be bound by any particular mechanism of
operation, it is believed.that such surface potential
tends to retard collapse of structure on drying and
interfere with interparticle hydrogen binding as well
as to enhance dispersement of particle structure in a
continuous, polar phase such as water.
Although the mechanism by which maltodextrin
and CMC act synergistically to keep the fibrils from
bonding firmly together during drying has yet to be
fully elucidated, it is believed to involve
cooperative interaction, of sorts, between the two
substances. The maltodextrin is believed to provide a
glass-like matrix that encases the cellulose fibrils
while the CMC apparently binds to the fibril surfaces
sufficiently to retard collapse to the point that
enough water is removed to solidify the entrapping
matrix. The two substances in combination are very
effective in prohibiting irreversible collapse of MDC
during drying, thus allowing rapid dehydration and
dispersement to occur and preserving to a substantial
extent the favorable properties exhlbited by never
dried MDC.
The dispersion agent may optionally include
lecithin in an amount from 0.1%-to about 10% ~ased on
the weight of the MDC.
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The following examples are provided to
describe in further detail the preparation of MDC in
accordance with t:he 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 Com~any, 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 examination indicated
a largely heterogeneous population of thin needle-like
sclerchyma cells ranging in major/minor dimensions of
500 to 1000 / lO 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 tQ 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
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critical dimensions of these bars and grooves are thebar 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
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 refiner disc extending to 13
and 1/4 inches 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 approximately 250 gallons per minute
through the refiner and back to the holding tank.
Passage of the cellulose suspension through the
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
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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 adjusted 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 tha-t 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. Squealing 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
250 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 following examples illustrate the
teachings of the invention.
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EXAMPLE 2
A typical high alpha-cellulose content
cellulose from wheat was refined to a Canadian
Standard Freeness of 790 according to the procedure
described in Example 1, above, mixed with dispersion
agent and dried in a 25 square foot Buflovak double
drum dryer (two 24-inch diameter, 24-inch width
drums). The resultant mixture on a dry basis
consisted of 56~ w/w MDC, 39~ w/w maltodextrin
obtained from Staley, Decatur, IA ~Lodex-15~, 4~ w/w
carboxymethylcellulose type HP-5HS obtained from
Dai-~chikoyyo Seiyaku Company Ltd., Japan and 1~ w/w
soy lecithin obtained from Cargill, Decatur IL.- This
mixture was then fed as an aqueous dispersion of 4.55
total solids at a rate of 414 pounds per hour to a
double drum dryer to produce a dried product of 93.3%
solids. The drums revolving at five revolutions per
minute were heated by 100 psig steam. The nip
thickness on both drums was set at 0.01 inches. Dried
product was removed as a thin continuous film from the
drums and subsequently ground to flake and powdered
products.
Both the flake and powdered products were
redispersed-in water and the viscosity measured for
comparison with the original MDC. All viscosities
were measured in an aqueous dispersion at 1.5% w/w MDC
solids and the additive concentrations noted above in
a Brookfield Viscometer model DV-III with the small
cell adapter using spindle SC4-16 at shear stresses
imposed by a range of rotational speeds from S to 100
revolutions per minute (rpm). The viscosity of the
aqueous dispersion of MDC with added dispersion agent
before drying was 5,520 centipoise at 5 rpm. The 5
rpm viscosities of the dried flake and powdered
products after drying and aqueous redispersion at the
same solids content as the original dispersion with a
- lG -' '
Hamilton Beach Scovill Mixer model 936 ZSA at 2 minute
(medium speed) were 3,410 and 3,095 centipoise,
respectively. The complete viscosity/shear profiles
for these three aqueous dispersions are shown in Table
1. The viscosity of the same powdered product after
high shear redispersion for three minutes at 10,00
0 rpm with a Omni Digi-system rotostator type mixer
(20 mm. generator) was 3,265 centipoise.
TABLE 1
VISCOSITIES OF 1.5% W/W MDC AQUEOUS DISPERSrO~S (cp~
spindle RPM 5 10 20 SO 100 100 50 20 lO S
Before Dryi~g 5520 2990 1940 1010 730 760 1350 2500 3600 5220
From Flake 3410 2190 1590 850 S90 600 920 1760 2570 3570
From Powder 3095 1770 1390 880 S90 590 900 1790- 2530 3450
EX~PLE 3
Never dried wheat fiber MDC refined to a
false value of Canadian Standard Freeness of 780,
according to the procedure described in Example 1,
above, was mixed with dispersion agent and dried-on a
two foot wide by 100 foot long belt dryer. The
resultant mixture on a dry basis consisted of 64.1
w/w MDC, 32.0% w/w maltodextrin, 3.2~ w/w
carboxymethylcellulose (CMC) and 0.7~ w/w lecithin.
The feed at a total solids content of 7.25~ in an
aqueous dispersion was fed at a rate of 917 pounds per
hour to the dryer and dried to a solids content of 90%
solids. The belt speed was 59 feet per minute and the
nip thickness at the applicator bar was set at 0.026
inches. A sixty foot length of the belt was heated by
50 psig. steam. Dried product was removed as a thin
continuous film and subsequently ground to flake and
powdered products
Film, flake and powdered products were
redispersed in water and the viscosity measured for
comparison with the original MDC as in EXAMPLE l The
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5 rpm viscosity of the original MDC was 5,640
centipoise. The-5 rpm viscosities of the film, flake
and powdered products after redispersion with a
Hamilton Beach Mixer were 4,320, 3,775 and 3,410
centipoise, respectively. The viscosity/shear
profiles for these four aqueous dispersions are set
forth in Table 2.
~ABLE 2
VISCOSITIES OF 1.5% W/W MDC AQUEOUS DISPERSIONS (cp)
10 Spindle RPM 5 10 20 50 l00 100 50 20 10 5
Before Drying 5640 - 580
From Film 4320 605
From Flake 377S 595
From Powder 3410 S80
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.
