CA1208631A

CA1208631A - Redispersible microfibrillated cellulose

Info

Publication number: CA1208631A
Application number: CA000449964A
Authority: CA
Inventors: Franklin W. Herrick
Original assignee: ITT Industries Inc
Current assignee: ITT Inc
Priority date: 1983-03-28
Filing date: 1984-03-20
Publication date: 1986-07-29
Also published as: NO165932C; DE3484688D1; US4481076A; NO165932B; NO840717L; MX161160A; IN160347B; FI841220A0; EP0120471A2; EP0120471B1; FI79725B; EP0120471A3; FI79725C; FI841220L; ATE64420T1; JPS59189141A

Abstract

Abstract of the Disclosure Redispersible microfibrillated cellulose is prepared by the addition to a liquid dispersion of the microfibrillated cellulose, an additive compound capable of substantially inhibiting hydrogen bonding between the cellulose fibrils. The microfibrillated cellulose, upon drying, is characterized by having a viscosity when redispersed in water of at least 50% of the viscosity of an equivalent concentration of the original dispersion.

Description

F. ~/V. Herrick 22 This invention relates to redisperible microfibrillated cellulose and to a process of preparing microf ibrillated cellulose which may be dried and redispersed.
Microfibrillated cellulose is a natural cellulose in which the cellulose fibers have been opened up and unravelled to form fibrils and microfibrils by repeated passage through a homogenizer. Microfibrillated cellulose is characteri7ed by very high water retention values, a high degree of chemical accessibility and the ability to form stable gels in water or other polor solvents.
Its preparation and properties are more fully disclosed in U.S. patent 4~374,702 and a variety of uses are shown in U.S. patents 4,341,807 ancl 4,378,381.
A dispersion of microfibrillated cellulose in water is a gel having pseudoplastic or thixotropic viscosity properties. On drying, however, the properties of microfibrillated cellulose are severely modified. Its dispersibility, hydration and viscosity properties are lost or substantially reduced, depending on the severity of drying. Microfibrillated cellulose has many end uses, such as in foods, cosmetics, and medicinal products in which it would be advantageous to use microfibrillated cellulose formulations that can be dried and redispersed without loss of viscosity or other properties. In other uses, it would be advantageous to prevent hornification and physical disruption that has been found to occur upon drying.
The ability to be rehydrated, after drying or dehydration, is a desirable goal for many hydrated materials, both cellulosic and noncellulosic. This is a particularly desirable goal for many foods and medicines which, once dehydrated, are difficult or impossible to rehydrate. In the case of conventlonal cellulosic pulps, drying is known i3~
F. W. Herrick - 22 to reduce the chemical reactivity and water absorbency of the pulps. It is also known in wond pulp technology that certain additives can be used to reduce the inter-fiber bonding that occurs on drying. Debonding agents have been added to pulps before drying to reduce the energy required to defiber pulp sheets; i.e., separate the dry fibers for use, for example, in fluffed pulps. Such debonding agents are generally cationic surfactants such as fatty acid quarternary amines which func~ion at low percentage additions. See, for example, Svensk Papperstidning, Kolmodin et al, Ns. 12, pgs. 73-78, 1981 and U.S. patent 4,144,122. However, the surface of microfibrillated cellulose is enormously greater than that of ordinary wood pulp fibers; e.g., on the orcler of a thousand times greater, and thus the problems of hornification and physical disruption ondrying are order of magnitude more severe with microfibril!ated cellulose.
It is a major object of this invention to provide dry microfibrillated cellulose which is substantially unchanged when dried and which may be rehydrated and redispersed in water to a viscosity essentially equivalent to that of undried microfibrillated cellulose.
It is an additional object of the present invention to provlde a convenient and economical process for avoiding irreversible changes that occur in microfibrillated cellulose upon drying.
The foregoing and other objects of the invention are achieved by a process comprising microfibrillating cellulose while suspended in a liquid medium, there~ore drying the suspension of microfibrilla~ed while there is present in said suspension a compound capable of substantially inhibiting hydrogen bonding between the fibril~ in the cellulose. The product of the invention is dry microfibrillated cellulose containing in admixture a compound capable of substantially inhibiting hydrogen bonding between the cellulosic fibrils in the cellulose characterized by having a viscosity when redispersed in water of at least fifty percent of the viscosity of an equivalent concentration o~ said microfibrillated cellulose dispersed in water prior to drying.

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F. W. Herrick 22 The mechanism by which an addi~ive yields a redispersible micro-fibrillated cellulose is believed to be related to the prevention of drying stress or hornification of cellulose by hydrogen bonding. The additive forms hydrogen bonds or co~plexes with the cellulose fibrils and prevents them from bonding to each other, thus forming a tight difficulty hydratable product. In the presence of additive, the cellulose fibrils remain accessible to wa~er and are easily rehydrated to form high-viscosity dispersions. The additive should accordingly be a compound capable of substantially inhibiting hydrogen bonding between ~he cellulosic fibrils in the cellulose. It should also be one which does not react with the cellulose, nor have substantial volatility, at the drying temperature. A wide nunber of organic and inorganic additive compounds, both liquid and solid, have been tested and certain of the compounds have been found to possess the characteristics required to yield redispersible microfibrillaeed cellulose. It has further been found that the additive compounds must be used in substantial quantities, generally at lesst one half the dried weight of the microfibrillated cellulose and preferably at least equal ~o ~he microfibrillated cellulose weight. Among the most useful additives are polyhydroxy compounds including particularly carbohydrate or carbohydrate related compounds, other than cellulose itself, such as glycols, sugars, carbohydrate gums, starches, oligo- and polysaccharides, seaweed (marine plant) extracts and derivatives of ehe carbohydrate and glycol related compounds. By derivatives herein is meant substituted or other firs~ stage reaction products of carbohydrates or glycols which retain their polyhydroxy functionality and their carbohydrate or glycol characteristics. Useful glycols include e~hylene, propylene, dipropylene and butylene glycol, glycerin and low molecular weight glycol polymers such as the~ polyglycols and such glycol derivatives as triethanolamine. Useful sugars include Lhe common 5 and 6 carbon sugars such as glucosel dextrose, mannose and 3~L

F. W. Herrick 22 galactose and disaccharides such as sucrose and lactose; sugar alcohols such as mannitol and so~bitol; such carbohydrate derivatives as the bisulfite adducts of the common sugars such as sodium mannose bisulfite and sodium glucose bisulfite; sugar acids such as aldonic acids, S saccharic and saccharinic acids and uronic acids; and the very broad glycoside group of acetal derivatives of sugass such as methyl glucoside. Certain foods containing large proportions of sugars, pectins or plant gums are also useful such as fruit and vegetable pulps and non-fat dry milk. Cther useful carbohydraCe derivatives are the carboxymethyl and hydroxyethyl starches, carboxymethyl and hydroxyethyl cellulose and methyl and ethyl cellulose. A very effective and economical polyhydroxy additive is sucrose, a disaccharide that is easily dried as a nonhydrated complex with microfibrillated cellulose.
In addieion to the polyhydroxy compounds, the alkali metal (e.g., lS sodium, potassium) salts of borates, polybora~es, phosphates and polyphosphates are also useful, although not as effective as the polyhydroxy compounds. In addition, certain aprotic solvents such as dimethylsulfoxide or a dialkylacylamide such as dimethylacetamide and dimethlyformamide are also effective additives~ These aprotic solvents, are components of solvent systems for cellulose (see for exsmple U.S.
patents 4,076,933 and 4,302,2523. The inorganic salts and ap~otic solvents are believed to form complexes with cellulose or hydroxyl gruups and thus prevent hydrogen bonding.
In general, low molecular weight compounds are the best additives.
At approximately equal levels of microfibrillated cellulose and additi~e, low molecular weight compounds do not affect viscosity characteristics of the dispersion. Higher molecular weight additives, such as carboxymethyl cellulose or hydroxyethyl cellulose, increase viscosity in proportion to their concentration and molecular weight; however, such mixtures redisperse very nicely, indicating that molecular size does not prevent 8~

F. W. Herrick 22 hydrogen bonding with microfibrillated cellulose on drying which in turn prevents microfibrillated cellulose from bonding with itself.
In the aforementioned U.S. patents 4,341,807 and 4,378,381 covering uses of microfibrillated cellulose, reference is made to the preparation of microfibrillated cellulose with mixtures of glycerin and water and to the addition of certain hydrophilic polymers to assist the process of microfibrillation of the cellulose. There is also disclosed ~he addition of such food additives as sucrose to certain of the microfibrillated cellulsse formulations~ However, there is no disclosure of drying cellulose with these additives nor i3 there recognition in this patent and application that these additives are capable of preventing irreversible ~odification of the microfibr;lla~ed cellulose when dried.
The amount of the addi~ive required to yield redispersible microfibrillated cellulose will vary considerably depending on which additive is used, the degree of microfibrillation of the cellulose, the extent to which the microfibrillated cellulose is subsequently dried and the severity of the drying process. Generally, however, when used as an additive to enhance redispersion, the amount will vary from as little as one half to as high as twice the weight of the cellulose. If the addi~ive is itself intended as the major component of the product, it may be used in amounts considerably exceeding even twice the weight of the cellulose. The additive may be mixed or dissolved in the microfibril-lated cellulose slurry or it may be added to ~he liquid suspension of fibrous cellulose prior to microfibrillation in a homogenizer. An advantage of mixing the addi~ive with the fibrous cellulose prior to microfibrillation is that it would reduce the ~ost of homogenization if the process is based on the use of dry wood pulp raw materials.
Cellulosic pulps that have not previously been dried, so-called never-dried pulps, are more responsive to ho genization than dry pulp 3~
F. W. i lerrick - 2Z

stocks. Drying the pulp with the additive present prevents a type of drying stress that occurs and is equivalent to a number of passes o~ homogenization. Thus, forexample, instead of a 10 pass homogeniza~ion process on standard dry pulp without additive to produce a high-viscosity micro~ibrillated cellulose, a 5 pass process on clry additive-treated pulp will obta;n similar viscosity characteristics.

The fibrous cellulose or micro~ibrillated cellulose may be dried by any one of a number of drying techni~ues wQll known in the art. Drying at from 25 to 105 C under arnbient or ~orced-dra~t conditions and both freeze and spray drying have been carried out experimentally. Room temperature drying is not effective for many additives because water is held as a hydrate. A 50-70C
drying ternperature is the most practical and corresponds to temperatur0s used in drying many food products with which microfibrillated cellulose finds great utility.
Micro~ibrillated cellulose is normally prepared as a liquid dispersion or suspension containing less than about 10% cellulose by weight and usually from about 1-6%, the specific concentration depending on whether cu~ or uncut pulp stocks or whether laboratory or commercial size homogenizers are used in microfibrillated cellulose preparation. Except as otherwise herein set ~orth, the preparation of microfibrillated cellulose is as set ~orth in the aforesaid U.S.
patent 4,374,702. As there set forth, the process involves passing a liquid suspension o~ fibrous cellulose through a small diameter orifice in which the suspension is subjected to a pressure drop o~ at least 3000 psi and a high velocity shearing action ~ollowed by a high velocity decelerating impact and repeatin~ the passage of the suspension through the orifice until the cellulose suspension becomes substantially stable. The ~Z~63~

F. W. Herrick 22 resulting microfibrillated cellulose product is generally characterized as having a water retention value of over 280%, a settling volume after 60 minutes in a 0.5% by weight susp_nsion in water of greater than 60%
and a ra~e of degradation increase by hydrolysis at 60C in one molar hydrochloric acid at least twice as great as cellulose beaten to a Canadian Standard Freeness value of 50.
The dried microfibrillated cellulose product of the present invention is characterized herein in terms of its ability to recover at least 50% of its initial viscosity in water. Visco~ity is used in this characterization because it is an accurate measure of the ability of a carbohydrate material to form a hydrated structure. Without the additive of the inven~ion, microfibrillated cellulose recovers from as little as

2% to a maximum of 20% of its original viscosity after it is dried, again depending on the severity of drying. In the preferred prac~ice of the invention, this recovery is over 75% and in many cases the recovery approaches nearly 100% of the original viscosity.
The following examples illustrate ~he practice of the invention.
Unless otherwise indicated, all parts and percentages are by weight.

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F. W. Herrick 22 Example_l In this example, a 20.8 liter volume of 2% microfibrillated ~-- -cellulose was prepared from a southern pine sulfite pulp. The pulp was dry cut to reduce the pulp fiber length to 0.7 mm. A small commercial Gaulin homogenizer was used at 55 mPa (megaPascals, 8000 psi) pressure.
A 12 ~ volume of 0.8% microfibrillated cellulose (96 g cellulose), from a previous run was used as a suspending agent for an additional 320 g dry-basis pulp (340 g as is, 6% moisture) and 8760 ml of deionized water. The dilute microfibrillated cellulose slurry (12 ~) was placed in a homogenizer reservoir and operation of both the homogenizer and feed pumps was beg~m. Pressure was adjusted to 55 mPa (8000 psi3. The dry-cut pulp (340 g as is) was ~lurried in 4 ~ of water, mixed with the remaining water (4760 ml) and dilute ~icrofibrillated cellulose in several portions, all of which were added back to the system from the top of the reservoir. This operation required about 20 minutes and temperature rose to 70C. At this point timing was begun so that all of the input pulp fibers received 10 passes through the homogenizer.
Process temperature was controlled in the range of 75 to 85C by applying cooling water to the jacketed recirculating lines. Initially it was assumed that pumping rate through the homogenizer was 5 ~/min.
This pumping rate was confirmed by measurement at about 2, 5 and 8 passes and total homogenizing time adjusted accordingly. In this example, total homogenizing time for 10 passes was 42 minutes. The recovered final product volume was 16 ~, containing 2.02% solids. Holdup in the apparatus was 5 ~, which could be mostly recovered by dilu~ion and displacement with water, for use as a suspending agent in subsequent runs. The product had the following viscosity properties at 22C, measured with a Fann Model 39 recording ~iscometer:

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F. W. H~rrick 22 _ g _ Shear Rate, sec Viscosity, mPa s Following the processing of a cellulose slurry and che desired number of (volume) passes through the homogenizer, the microfibrillated cellulose product was cooled to room temperature and stored in a suitable closed container. Before testing or sampling for analyses all products were carefully remixed or shaken7 The solids content of microfibrillated cellulose products was determined by drying 10 to 20 g samples for 18 hours a~ 80~C in a forced draft oven, followed by two hours at 105C.

A simple test to measure the effectiveness of the additive was used. The test involved mixing 400 g of 2% microfibrillated cellulose (8 g microfibrillated cellulose solids) with 8 g dry basis additive. The viscosity of this mixture was recorded~ Dry films were then prepared because the drying rate and stress of films could be controlled with relatively small samples. The films were then eut into small squares and redispersed at 2% microfibrillated cellulose solids in water. Unless otherwise indicated, this test, as more specifically set forth in Example 2, was used in all of ~he examples.
Example 2 Various proportions of sucrose were added to 2% dispersions of microfibrillated cellulose prepared as set forth in Example 1. Dry films were then prepared fro~ the microfibrilla~ed cellulose/sucrose dispersions by placing about 90 g of the microfibrillated cellulose product on a polished chrome-plated steel sheet (25 x 36 cm). A
stainless steel bar, adjusted to a height of 2.5 mm above the shect, was used to sp~ead the material into a rectangular ~hape of 16 x 22 cm. This ~2~63~

F. W. Herrick 22 uniform layer of the microfibrillated cellulose product was then dried in a forced-draft oven at 60C for about 2 hours. The resulting dry film of about 0.04 to 0.08 mm thickness, depending on the additive content, was stored in a plastic film envelope until used in tests. Film moisture content under these conditions was about 5%.
In both this and the following examples, films prepared as set forth above, were cut into 2 x 2 cm pieces and added to water to produce a 2%
microfibrillated cellulose dispersion. For e~ample, a 50/S0 micro-fibrillated cellulose/sucrose dispersion was prepared by adding a 4.2 g sample (4 g dry basis) of microfibrillated cellulose/sucrose to 95.8 g water. The 100 g sample was stirred with an electric counter-rotating mixer for 10 minutes at moderate speed and at room temperature.
The viscosity was mea~ured with a recording viscometer at room temperature (22 to 24~C). The viscosity at a shear rate of 1000 sec was used in comparing the characteristics of the micro-fibrillated cellulose dispersions. Table I compares the ViSC05ity of the dispersions before and after drying of various proportions of sucrose additive at various drying temperature.

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F. W. Herrick 22 TABLE I
Viscosity Original Viscosity ~
Dispersion DryingRedispersion 5Sample MFC/SucrosemPa-s,1000_sec 1 C~rPa s, 1000 sec 1 100/0 (control) 22fi 60 38 2 16.5/83.5 236 60 282 . ~ . _ _

3 66.7/33.3 22~ 60 199

4 50.0/50.0 224 25 22 8 10 5 66.7/33.3 201 60 154 6 50.0/50.0 176 6~) 185 _ .

Table I shows that a 50/50 (wt/wt) MFC/Sucrose mixture can be dried at 60C without reducing the viscosity of the redispersed product (at 25C dryirlg was unduly lengthened~; sucrose at ratios over 50/50 yield a completely redispersible high viscosity MFC; ratios of ~ucrose of less than 50/50 yield a redispersible MFC but wieh some loss in visosity.
Optimum levels of sucrose range from about 33% to as much as 200% by weight of the cellulose.
Exampl e 3 A series of additional comparative tests were conducted as in Example 2 but using glycerin, rather than sucrose 8S the additive. All samples were dried at 60C. Although anhydrous glycerin does not lose weight in 2 hours at 60C, some loss of glycerin was evidently caused by the presence of water during drying for ~hese tests. The results are shown in Table II~

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F. W. Herrick 22 TABLE II

Viscosity Original Viscosity ---Dispersion Redispersion

5 SampleMFC/GlycerinmPa-s,1000 _ec 1mPa s, 1000 sec~

2 28.5/71.5 222 *
3 16.5/83.5 219 *

1 10 5 66.7/33.3 226 188 ~ 7 66.7/33.3 226 201 . . .
8 66.7/33.3 193 119 9 50/50 1~9 191 _ . . . _ . .. . . . .
* MFC redispersed but the films remain~d ~acky because of excess glycerine.

As in the case of sucrose, Table II shows that 50/50 mixtures of MFC/glycerin can be dried wi~h little effect on vis~osity. At levels above about 70X~ it becomes difficult to dry the dispersions. At ?0 glycerin levels below about 33%, some viscosity loss occurs. Optimum levels of glycerin range from about 40 to 60% of ~he MFC weight. These results may again be compared with Sa~ple 1, the control 9 in Table I.
Example 4 The tests of Examples 2 and 3 were repeated using ethylene glycol and propylene glycol as the additives. Again drying of ~he redispersed mixtures was at 60C. The resulte are shown in Table III.

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F. W. Herrick 22 TABLE III

Viscosity Original Vi5 cosity `-Dispersion Redispersion Sa~nple MFC/Glycol mPa s,1000 sec~l mPa-s 1000 sec~
~ _ _ ?
Ethylene glycol*
1- 66.7-r33.3 ~~~ 224 71 Propylene glycol*
3 66.7/33.3 191 180 * Ethylene glycol, and to a lesser extent, propylene glycol were subject to volatile loss in drying in the presence of water. Thus, results at higher glycol levels did not give meaningful comparative viscosity levels.

These glycols thus yield dispersible dry MFC at 50/50 and higher ~FC/glycol levels. Ethylene glycol loss on drying at 60C made it less effective at the lower levels.
Example 5 Comparativ~ tests were carried out with sugars other than sucrose and with sugar derivatives. All samples were at 50/50 M~C/additive levels to 2% MFC di~persions. The results are shown in Table IV.

F. W. Herrick 22 - 14~

TABLE IV

Viscosi~y Original Viscosity --Dispersion Redispersion 5Sample Additive_a s~_000 sec 1mPa~s, 1000 sec~
.
1 dextrose (glucose) 256 250 2 galactose180 205 3 sodium 195 244 glucoheptonaee 4 sorbitol 195 191 mannitol 250 187

6 xylose 183 209

7 methyl-c-D 176 218 glucoside 15 All of the sugar and sugar derivative additives of Ta~le IV were effective for producing exceptionaily smooth redispersions of dried MFC
at the 50/50 MFC/additive levels. The variation in original dispersion viscosity was largely the result of the use of different 2% MFC
preparations. Reference should again be made eo the viscosity of the 2%
MFC control dispersion, Sample 1 of Table I, for comparative redispersibility results without any additive.

The following tests were carried out with 2~ MFC dispersions with a varie~y of additives, including starch, a glycol, and inorganic salts.
All additives were at the 50/50 MFC/additive level. Results are set forth in Table V.

F. W. Herrick 22 - TABLE V

Viscosity Original Viscosity Dispersion Redispersion 5 Sample AdditivemPa-s71000 sec 1mPa~s, 1000 sec~
1 none (control)226 38 2 soluble starch213 160 (potato) 3 dipropylene 187 277 glycol 4 trisodium 230 139 phosphate disodium hydrogen 217 143 phosphate 6 sodium 238 135 perborate Starch was a particularly effective additive. Even though the viscosity of the redispersion was reduced somewhat to 160 mPa 5, the redispersion was quite smooth and produced a good stable gel. Certain alkali metal salts of phosphates and borates are partially effective. Dipropylene glycol appears to react with MFC.
Example 7 Various food products were used as additives to 2~ MFC dispersions, all at 50/50 MFC/additive levels and films were dried at 60C. Results are set forth below.

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F. ~. Herrick 22 TABLE VI

Viscosity Original Vissosity Dispersion Redispersion Sample Additive mPa~s,1000 sec~l mPa-s, 1000 sec~
1 non-fat dry milk 197 201 2 apple cooked pulp 160* 158 3 carrot cooked pulp 150* 146 4 dextrin 189 152 * These viscosity values are estimates extrapolated from the viscosity of the base 2% MFC mixturesO

Non-fat dry ~ilk contains the disaccharide lactose while apple and carrot contain pestin on the one hand and gums and sugars on the other, respectively. Dextrin is a low molecular weight hydrolyzed starch. All four of these additives were effective in varying degrees to produce redispersions of the MFC.

Various cellulose der;ved or natural gums or seaweed extrac~s were added to 2% MFC dispersion, all at 50/50 MFC/additive levels and films were dried at 60~C. Results are set forth in Table VII.

i31 F. WO Herrick 22 TABLE VII

Viscosity Original Viscosity Dispersion Redispersion SampleAdditive mPa s,lO00 sec l mPa-s, 1000 sec~

1 pectin 197 180 2quar gum 172 258 3gum arabic 197 191 4 agar 250 133 (seaweed extract) 5sodlum c~rboxymethyl416 496 cellulose (medium viscosity) 6*hydroxyethyl 156 148 cellul 05 e (high viscosity) . _ * This sample was at total 2% solids - 1% MFG, 1% HEC.

All of the above additives were effective. The simple low molecular weight gums (samples 1-4) were best in that the MFC~additive viscosity was not affected. The polymer gums (samples 5 and 6) have their own viscosity superimposed on that of MFC.
Ex ~
Various organic compounds were added to 2% MFC dispersions at 50/50 MFC/additive levels and films were dried at 60C. Results are set forth in Table VIII.

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F. W. Herrick 22 TABLE VIII

ViscQsi~y Criginal Viscosity Dispersion Redispersion Sample Additive _ mPa s,1000 se~ l mPa~s~ 1000 sec~
1 dimethylacetamide172 208 2 dimethylsulfoxide171 289 3 triethanol amine 185 258 The aprotic solvents, dimethylacetamide and dimethylsulfoxide, were effective, as was triethanol amine, to produce excellent MFC
redispersions by forming complexes with the cellulose.
Lxample 10 In this example, quaternary ammonium compoundx of the type disclosed in U.S. patent 4,144,122 were used as the additive. In all samples, the additive was a fatty acid quaternary a~ine sold under the trademark Berocell 584. The amount of the quaternary compounds was varied from sli~htly less than 0.2% by weight of the MFC eo an amount equal to the MFC weight. The result~ were as follows:

TABLE IX
._ Viscosity Original Viscosity Dispersion ~edispersion MFC/Quaternarv Amine mPa s,1000 sec 1 mPa-s, 1000 sec~
-- . , 1 99.8/0.2 203 45 These results indicate that only the 50/50 MFC/quarternary amine produced a redispersible MFC product. In practice, a unts of quaternary amine should be a minimum of about 75~ by weigh~ of the MFC.

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F. W. Herrick 22 Example 11 In this example, in place of drying films of the MFC samples, the original 27~ MFC dispersions, with and without additive, were spray dried using a laboratory spray dryer having an inlet temperature adjusted to 5 200C. Material was pumped to a high speed turbine at 50 ml/minute.
Outlet temp~rature was 67C. Viscosities were measured, as in the previous examples, before and after drying. Results are shown in Table X.

.
TABIF X

Viscosity 10Original ` Viscosity Dispersion Redispersion Sample MFC/Additive mP s,1000 sec~l mPa-s, 1000 sec~

no additive sucrose 266.7/33.3 195 70 3 ~0l50 195 199 glycer in 466.7/33.3 193 9 50l50 lg3 103 propylene glycol 66607/33.3 193 4 Th~ control sample (1) of spray dried MFC with no additive had virtually 25 no viscosity at all indicating more severe drying than oc urs wieh film at 60C. The sucrose additive at 50/50 level was the most effective additive in this example. The glycerin test at 66.7/33.3 and both of the propylene glycol ~ests had severe addit*e losses through volatility in the dryer.

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F. W. Herrick 22 The additive may be mixed or dissolved in the microfibrillated cellulose or it may be added to cellulosic pulp, which preferably has not previously been dried, prior to microfibrillation. Previous work has established that never-dried pulps were more responsive to homogenization than dry pulp stocks. Drying ~he pulp with the addi~ive present is equivalent to about 5 passes through the ho genizer. This is illustrated in the following example.
Example 12 ~-- .
Samples of a bleached sulfite pulp which were at a 30~ consistency and had not previously been dried were treated with a dry additive at either 67/33 or 50/50 levels of MFC/additive, or in the cRse of the control, with no additive~ The pulp and additive were thoroughly mixed, dried at 60C and dry cut to 0.7 mm ~iber length. Slurries containing 2% of the cellulosic pulp were then microfibrillated as in Example 1 by passing through the homogenizer from 2 to 10 times. Films of the MFC
~ere then prepared and dried at 60C as in the previous exa~ples.
Table XI shows the viscosity of the original MFC dispersions after various numbers of passes through the homogenizer. It also shows the viscosity levels after drying and redispersing the 10 pass MFC samples.

F. W. Herrick 22 TABLE XI

Viscosity Original Viscosity Dispersion Redispersion ~ MFC/AdditivemPa s91000 sec 1mPa-s, 1000 sec~

no additive 5 pass 70 10 pass 144 20-30 2 sucrose 66/335 pass 164 10 pass 219 131 50/50 2 pass 8Z
5 pass 201 10 pass 258 244 3 glycerin 77/235 pass 152 10 pass 217 127*

81/195 pass 135 10 p~ss 254 78*
-* The loss of glycerin on drying reduced the MFC/glycerin weigh~
ratio from 67/33 to 77/23 and from 75/25 to 81/19.

The above table indicates that the additive may be used to reduce homogenization energy by 50% or more. That is, both the 67/33 and 50/50 MFC/sucrose samples prepared by 5 passes through ~he ho genizer had higher original viscosity than the 100% MFC prepared by 10 passes ~hrough the ho genizer. Moreover, the Table Xl results also show ehat by mixing the additive with the cellulose prior to homogenization, ~he stresses introduced by drying before ho genization are essentially elimina~ed while the dispersibility of the cellulose, after homogenization, is substantially maintained.

Claims

CLAIMS:

1. Dry microfibrillated cellulose containing in admixture a compound capable of substantially inhibiting hydrogen bonding between the cellulosic fibrils in the cellulose characterized by having a viscosity when redispersed in water of at least 50% of the viscosity of an equivalent concentration of said microfibrillated cellulose dispersed in water prior to drying.
2. The dry microfibrillated cellulose of claim 1 having a viscosity when redispersed in water of at least 75% of the viscosity of an equivalent concentration of said microfibrillated cellulose dispersed in water prior to drying.
3. The dry microfibrillated cellulose of claim 1 in which the compound is present as an additive at levels of at least one half the weight of the cellulose.
4. The dry microfibrillated cellulose of claim I in which the compound is a polyhydroxy compound.
5. The dry microfibrillated cellulose of claim 4 in which the compound is a polyhydroxy compound selected from the group consisting of a carbohydrate, a glycol and derivatives thereof.
6. The dry microfibrillated cellulose of claim 5 in which the compound is a carbohydrate.
7. The dry microfibrillated cellulose of claim 6 in which the carbohydrate is sucrose in an amount of about one half to two times the weight of the cellulose.8. The dry microfibrillated cellulose of claim 5 in which the polyhydroxy compound is a glycol selected from the group consisting of ethylene, propylene, dipropylene and butylene glycol.

9. The dry microfibrillated cellulose of claim 1 in which the compound in an alkali metal salt of a borate, plyborate, phosphate or polyphosphate.
10. The dry microfibrillated cellulose of claim 1 in which the compound in an aprotic solvent.
11. A process of preparing dry redispersible microfibrillated cellulose comprising microfibrillating cellulose while suspended in a liquid medium therefore a drying the suspension of microfibrillated cellulose while there is present in said suspension a compound capable of substantially inhibiting hydrogen bonding between the fibrils in the cellulose.
12. the process of claim 11 in which the compound is selected from the group consisting of a polyhydroxy compound, an alkali metal salt and an aprotic solvent and quaternary amine.
13. The process of claim 12 in which the polyhydroxy compound is selected from the group consisting of carbohydrate, a glycon and derivatives thereof.
14. The process of claim 11 in which compound is added to said liquid suspensionprior to microfibrillation of the cellulose.
15. The process of claim 11 in which the additive is added to said suspension ofmicrofibrillated cellulose after microfibrillation of the cellulose.
16. The process of claim 11 in which the suspension is dried at a temperature ofform 50-70°C.
17. The process of claim 13 is which the polyhydroxy compound is a carbonhydrate.
18. The process of claim 17 in which the carbohydrate is a sugar.
19. The process of claim 18 in which the sugar is sucrose in an amount of at least one half the weight of cellulose.