I
CHARLES E. HUCKABA and J. C. GEDDES, Jr.' University of Florida, Gainesville, Fla.
Factors AfFecting
...
Preparation of Glass Paper Production of high quality glass paper requires optimization of both fiber length distribution and pH of the stock
INTEREST
IN PAPERS made from inorganic fibers has increased greatly in recent years. The unique properties of these papers as well as some of the factors involved in their preparation have been discussed by O'Leary and others (5-7). Some of the interrelationships among several groups of variables, which affect the preparation of paper, are indicated below. The present study elucidates further the effects of some of the variables in order to reduce the amount of trial-and-error experimentation presently required to make an inorganic paper having specific desired properties. In this exploratory investigation, all the paper sheets studied were produced in a sheet mold rather than on a continuous machine. Although the work was limited to paper made from 106 Type E glass Micro-Fibers, many of the basic considerations are generally applicable to the preparation of other inorganic papers.
Comparison of Glass and Cellulose Fibers Currently available industrial equipment for the preparation of paper stocks has been designed from the viewpoint and requirements of the production of organic (cellulose) paper. I n attempting to understand the action of this equipment on inorganic fibers, and in turn, the over-all effect on the properties of the finished paper, it is helpful to compare the basic structures of the cellulose a n d inorganic fiben. By knowing what the equipment has been designed to do to organic fibers, effect on inorganic fibers can, to some extent, be anticipated.
Background Literature on Cellulose Fibers vs. Glass Fibers Subject Basic structure of cellulose fibers Preparation of cellulose paper stock Basic properties of glass microfibers Preparation of glass paper stock
Ref. (9)
(1,11) (6,8) (6,7)
1 Present address, Ethyl Gorp., Baton Rouge, La.
Fundamental studies o f both the stock preparation a n d the sheet formation o f glass paper may lead to the postulation o f improved procedures a n d equipment. In further basic studies o f this type, the followi n g areas are worthy o f consideration: ,The mechanism and rate of the chemical action of the acid on the glass should be studied. It would be desirable to know the composition of the gelatinous material and the manner in which both it and the remaining unreacted portions of the fibers are affected by the hydrogen ion concentration and time of acid treatment. ,The electrical and colloidal properties of glass fibers should be further investigated. Studies of the zeta potential would be of great value in understanding ihe manner in which acid aids in dispersion of the stocks. This would also lead to the development of the best procedure for preparing a stock from which paper with certain desired properties could be produced. b In relation to the preceding item, the effect of other electrolytes on the properties of the stock and the paper made therefrom should be studied. At the present time, acid i s A study of the background literature listed in the table indicates that the main difference in the beating characteristics of organic fibers and glass fibers lies in the fact that the glass fibers are brittle and cannot be fibrillated. With glass fibers, the beater mereIy reduces the fiber length through crushing and cutting and promotes dispersion to form a stock of fairly uniform consistency. T h e fact that glass fibers do not need to be subjected to a beating-type action in order to produce fibrillation suggests that equipment other than the
used to aid in fiber dispersal during beating. However, results of the tests on the effect of time of acid treatment and hydrogen ion concentration indicate that a stronger sheet might be produced if some other electrolyte were used during the beating period. Then, prior to the sheet formation, acid could be added. If this procedure were followed, there would be no acid attack on the beater and storage tanks. The stock should not be affected as much by aging, and it i s possible that the stock could be stored without change until needed. ,The effect of stock aging, and its interrelationship with the above factors should be investigated. During aging the silicic acid micelles will have a tendency to grow and cause flocculation. The possible effect of this action upon the role of the fines in promoting tensile strength should be studied. traditional DulD beater may be better suited for the preparation of glass stocks. As the primary requirement in this case appears to be that of cutting or shearing, a colloid mill might be more appropriate for preparing paper stocks from glass fibers. Accordingly, a comparison was made between stocks using both a cycle beater and a colloid mill. .
I
Stock Preparation in a Cycle Beater A Noble and Wood cycle beater having a nominal 5-pound capacity was employed in these studies. The perforVOL. 52, NO. 7
m
JULY 1960
569
I
1
PULP FIBER PROPERTIES Fiber type & chemical composition Chemical properties Physical properties Fiber diameter and length distribution Physical make-up (aggregation) Tensile strength Flexibility
SHEET FORMATION VARIABLES
~
Action of equipment Stock homogeneity
I
,
STOCK PROPERTIES Consistency (grams/liter) Fiber length distribution PH Freeness Colloidal & related properties I
t
I
STOCK PREPARATION VARIABLES Action of equipment Adjustment of equipment Time duration of each treatment Consistency (grams/liter) PH Temperature Aging Degree of preparation of pulp fibers before feeding to equipment
i
l
I
1 PROPERTIES OF THE SHEET Basis weight uniformity Visual appearance Thickness and compressibility Tensile strength
1
I
These Interrelationships among the Variables Determine the Quality of Paper Sheets mance of this equipment in the preparation of glass stocks was evaluated with respect to the beater roll clearance and the time of beating. As O'Leary ( 5 ) had obtained his best results a t a p H of 3.0, this value was used throughout all of the tests with the cycle beater. The remaining stock preparation variables shown above were held constant as indicated in the experimental procedure given below. Also, as the same type of glass fibers was used throughout this work. all of the pulp fiber properties listed should have remained fairly constant. Thirteen gallons of water were added to the beater, and the beater was turned on to provide circulation. The water was heated to 85' F. with live steam, and the p H was adjusted to 3.0 with approximately 200 ml. of 8 to 1 hydrochloric acid. Five hundred grams of 106 Type E glass Micro-Fibers, as received, were added by submerging and hand separating the fibers into small wads about one inch in size. Eight minutes were taken for this addition, and 2 minutes more were taken for additional mixing and for readjusting the pH to 3.0. The beater roll was then lowered to the beating position, the timer started, and samples were taken at the desired time intervals. Usually a l'jz-quart sample was taken. A p H of approximately 3.0 was maintained throughout the entire beating cycle by adding slowly dilute hydrochloric acid as needed. The resulting stock was evaluated with respect both to consistencygrams solids per liter of stock-and fiber
570
INDUSTRIAL AND ENGINEERING
classification. Although freeness is a widely used test for cellulose stocks, there is evidence (9, 70) that this test is not suitable for comparing stocks produced b\ different types of equipment. Classification provides a more reliable measure of beating action (2). The standard procedure for performing classification tests with the Bauer5 l c S e t t Classifier (Model S o . 203) as outlined in their operational procedure pamphlet was followed, except that a p H of approximately 3.0 was maintained. A stock sample equivalent to 5 grams of fiber solids was used. This was the largest sample size from lvhich reproducible results could be obtained. Screens of +28, +48, +loo, and +200 mesh divided the fibers in the stock into fractions better than any
CHEMISTRY
Table I.
of the other available screen sizes. All of the input not entrapped on any of these screens was considered to be -200 mesh fines, A wash water flow rate of 3 gallons per minute for 20 minutes !vas employed. Using a separatory funnel, 8 to 1 hydrochloric acid was added dropwise to the water stream entering the first classifying compartment to maintain a p H of approximately 3.0 throughout the run. The ultimate proof of the suitability of a stock lies in the quality of the paper which can be produced. However, as previously discussed, certain sheet formation variables tend to become interrelated so that the finished sheet reflects some sort of composite of the effects of stock properties and sheet formation variables. Thus, the prop-
Reproducibility of Experimental Measurements and Procedures KO.of Samples Measured
Deviation, Av. %
Precision of Measurements on Stock Consistency (g./l.) Classification : high % fines low % fines
3 3
1.7 1.6
3
3.9
Precision of Measurements on Single Paper Sheets 10-18 Basis weight 4 Tensile strength
<
for instance, a n attempt is made to relate beater roll clearance and beating time to tensile strength of the finished sheets. These data were obtained under
Previous considerations led to the conclusion thax equipment which subjects the fibers to a very severe cutting action might produce a more suitable stock for the preparation of glass sheets. Therefore, the use of a colloid mill for the preparation of inorganic stocks was investigated. ,4 Charlotte Colloid *Mill (Model SD-1) manufactured by Chemicolloid Laboratories handled glass fibers satisfactorily provided that the design of the rotor and stator was modified slightly to prevent clogging of the fibers. As in the case with the batch-type cycle beater, the colloid mill can be adjusted with respect to clearance. However, as the mill is a continuous device, increased time of action can be obtained only by recycle. In many cases, stock which had been prepared in the cycle beater was adjusted to a p H of 3.0 and passed through the mill. \$'hen starting the preparation directly with the glass fibers, thr pads had to be cut into 1-inch cubes before being fed to the colloid mill. For small scale studies, an Osterizer (Model l o ) , which produces a high shearing action somewhat similar to that in a colloid mill, was employed. I n this case, the desired amount of fibers. consistent throughout a series of runs, but not exceeding 5 grams, was hand dispersed in about 24 ounces of water. After the p H was adjusted, the Osterizer was turned on and mixing was continued for the desired time of operation. The stocks prepared in the colloid mill and Osterizer were subjected to the same evaluation procedure as used for the cycle beater stocks. Typical results obtained with the high shearing equipment are shown in Table I1 where the analyses of two of the 25-minute Osterizer stocks are listed. As expected, the high shearing action produces a large amount of -200 mesh fines. Because of the significant differences in fiber fraction make-up between these stocks VOL. 52,. NO. 7
JULY 1960
571
Table 111.
Tensile Strength of Sheets Increases as Average Fiber Length in Stock Decreases
Run
No.
$28
1
100
2
-28
... ... ...
30
20 10 0 17.92
(1)
22.36 (J)
23.84
29.08
(K)
(L)
TENSILE STRENGTH, OULCES/INCH
Figure 2. Comparing A-E-/, 6-F-J, C-G-K, and D-H-I, respectively, shows that as the percentage of mill stockhigh in fines-increases, the sheet tensile strength also increases
EZB - 2 0 0 fiber fraction [=I - 100 to +200fiber
fraction - 4 8 to $ 100 fiber fraction l i - 2 8 to $ 4 8 fiber fraction N4-28 fiber fraction
and those prepared in the cycle beater, the possibility of relating fiber fraction distribution to the tensile strength of the finished sheet was explored. Relationship between Fiber Fraction Distribution a n d Tensile Strength
Sheets were prepared using the fiber fractions obtained by classification with no -200 mesh fines present. The desired weight of each fraction was added to the sheet mold and redispersed with the perforated stirrer. The p H was adjusted to 3.0 and the fibers were subjected to acid treatment for 15 minutes before the sheet was formed. The tensile strength values of the sheets prepared from the classified fiber fractions are shown in Table 111. This series of tests corresponds to some extent to part of the work reported by O’Leary (7). However, when he prepared sheets composed of various screen fractions, he did not treat the stock with acid, and, therefore, it is not possible to make a direct comparison of O’Leary’s data with the results of this investigation. To investigate further the effect of fines upon tensile strength, two additional
572
... ... ...
...
...
100
... ...
100
...
30 25 20
40 25
+
+
...
10
40
+
Fiber Fraction Percentages 48 -48 100 -100 200
100
20
10
25
25 40
30
series of runs were made. I n the first set of runs, groups of sheets were prepared by mixing fibers of one classification fraction a t a time with stock containing a high percentage of fines prepared in the colloid mill. The results of these runs are presented in Figure 2. As the percentage of mill stock (and thus the percentage of -200 mesh fines) increased the tensile strength of the sheet increased with the single exception of sheet H . This trend may be observed by comparing A , E, and I; B, F, and J; and remaining runs. In these studies, the fiber fraction analyses were made on the pulp stocks and not on the finished sheets. Although there were no tests made to determine the degree of retention of fines during the sheet formation, such losses were observed to be small, and the classification fractions give a good picture of the actual sheet make-up. In the next series of runs, a stock was prepared in the Osterizer resulting in the fiber fraction distribution listed as run B in Table 11. This stock contained no fibers of the +28 fraction length but was high in - 200 mesh fines. By having available this stock and also a supply of the various individual fiber fractions, the fines could be varied from 0 to 68.3%. Figure 3 shows the tensile strength of these sheets as a function of the fiber length distribution of the stocks. Again it is clearly evident that as the
-200
...
... ... ... ... ... . I .
Av. Tensile Strength, Ounces/Inch 1.20 3.00 2.88 3.48
2.24 3.00 4. 08
percentage of fines increased the tensile strength of the sheet also increased. These results make possible a fairly plausible interpretation of the contribution of fines to tensile strength of the finished glass sheet. Tensile strength is apparently derived from both intermeshing of fibers and chemical bonding due to the adhesive action of the gelatinous silicic acid during sheet formation. Fines, in the presence of acid, are believed to increase the extent to which both of these factors can contribute to the sheet tensile strength. I n the absence of fines, the tensile strength always seems to be low-e.g., compare Table 111 with Figure 2. As the glass fibers are essentially cylindrical rods, the contact area at each junction is very small, so that even though the fibers have adhered at the junctions, very little strength is derived from a single junction. -4s the fiber length decreases, the sheet becomes more dense with a corresponding increase in the number of fiber junctions. In other words, there is more bonding area as the fiber length decreases. Once the union a t the fiber junction is broken, the cylindrical fibers tend to slide apart with very little tensile strength being derived from intermeshing. I n the case of a stock containing a medium amount of fines-such as is produced in the cycle beater-it is believed that during sheet formation
70
n 50-
n
U
TENSILE STRENGTH, OUICES/INCH
Figure 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Sheet tensile strength increases with increase in amount of fines in stock
- 2 0 0 fiber
fraction fiber fraction - 4 8 to + l o 0 fiber fraction - 2 8 to + 4 8 fiber fraction 4-28 fiber fraction
[=1
B
- 100 to f 2 0 0
GLASS P A P E R P R E P A R A T I O N most of the fines are caught at the fiber junctions and increase the contact area and, in turn, the strength of the union. T h e fibers will also be roughened by fines adhering to them, and intermeshing will thereby become more effective in enhancing the tensile strength of the sheet. When subjected to a load, a sheet will pull apart a t the weakest point. A long fiber, with sufficient strength added to the contact points with other fibers because of the action of the fines, will tend to distribute any pulling force over a larger area than would be possible with a shorter fiber. Also uneven breakage a t the large fiber unions would produce some additional roughening. T h e shorter fibers in the stock would be more rigid and would intermesh to a lesser degree than the long fibers. With stocks having this medium range of fines, O'Leary ( 5 ) conducted his tests and came to the conclusion that in order to produce strong sheets, long fibers were needed along with a sufficient amount of fines to bind the fibers together. In the presence of a very high percentage of fines, as found in stock prepared in a blender or colloid mill, chemical bonding is believed to occur to such a n extent that the advantage of long fibers is largely overcome. With shorter fibers, the sheet becomes more dense, and the fines tend t o fill the interstices to a greater degree than would be possible with a less dense sheet formed by using long fibers. Although the foregoing tests demonstrate a distinct effect of fines upon tensile strength, further examination of the data indicates that there must also be other significant factors involved. For instance, comparison of Figures 2 and 3 shows that in these runs there is no direct relationship between the magnitude of the tensile strength and the fiber fraction distriburion alone. Whereas within each of these series of runs there is an unmis40
r
m
36-
Y
$
e
12
0
1
2
3
4
5
6
7
8
9 1 0
PH
Figure 4. Variation in sheet tensile strength with the pH of the stock during a 15-minute acid treatment period
:f 5 E
\
w
36 32
0
2
28
I 1
1
0
2
4
6
8 1 0
15
30
TIME, MIN.
Figure 5. ment
Variation in tensile strength with length of time
of stock acid treat-
pH of 3.0 0 Experimental values X Value from Figure 4
takable trend of tensile strength with percentage of fines, there appears to have been some other factor which was different in the two sets of runs that exerted a significant effect upon tensile strength. As discussed below, additional information obtained in subsequent tests concerning the acid treatment provides a possible explanation of this situation.
The Role of Acid Treatment
A series of experiments was conducted to study the sensitivity of the effect of acid treatment during stock preparation upon the tensile strength of hand sheets. Exactly 3.84 grams of glass fibers were hand separated into roughly '/Z-inch wads and placed in the Osterizer. Then 24 ounces of water, adjusted to the desired pH, were added. The mixture was agitated for exactly 1 minute, with p H adjustments to within zk0.05 unit every 15 seconds. After this agitation period, the pulp stock was kept in the Osterizer for the desired period of time, with acid being added as necessary to maintain the desired pH. This mixture was then added to the sheet mold which contained a prescribed amount of water, also adjusted to the desired pH, and the sheet was prepared by the normal procedure. Figure 4 shows the variation in the tensile strength with the p H of the stock for a 15 minute acid treatment period. The tensile strength increased as the p H decreased. There is some evidence (72) that there may be a transition point between a p H of 3 and 4 in which the charge on the fibers may change from positive to negative. If so, then a more correct representation may be that indicated by the broken line in Figure 4. Xevertheless, the important result is that much stronger paper was produced a t
p H values less than 3, the p H value used throughout most of the present investigation and indicated by previous workers to be the optimum value. As the basis weight for all the sheets remained essentially constant, there is no appreciable change in the dissolution of fibers over the p H range of 1 to 9. Figure 5 shows the effect of the length of time of acid treatment of the stock on the tensile strength of the sheets. The p H was maintained a t 3.0 for all runs. A broken line has been drawn to represent the suspected extrapolation. The initial point a t about 11.4 ounces per inch was chosen as this is the value obtained for sheets prepared from stock produced with tap water-i.e., zero time a t pH of 3.0 The fragmentary data given in Figures 4 and 5 show that stock held in a strongly acid condition for a short period of time prior to the formation of the sheet will give the strongest paper. The sheet having a tensile strength of 44 ounces per inch is by far the strongest glass sheet ever produced in this laboratory. Postulation of the mechanisms giving rise to the results depicted in Figures 4 and 5 is not an easy matter. Morey ( 4 ) describes the complex nature of the interaction between water and glass, and a n equally or even more complex situation would be expected in the action of acid on glass. Although no definite conclusions regarding the underlylng mechanisms can be drawn with confidence from the data a t hand, the following hypotheses are given in the hope of stimulating further studies. Use of acid in the preparation of the stock apparently exerts a multiple effect upon the tensile strength of glass paper. Through the conversion of the glass to silica gel, the tensile strength of a sheet of glass paper depends upon both the VOL. 52, NO. 7
JULY 1960
573
bonding together of individual fibers and the uniformity of the sheet as well as upon the strength of the individual fibers. Thus, the use of acid tends to enhance the tensile strength of a sheet by aiding in the dispersion of fibers and thereby increasing the uniformity of the stock and through the creation of the silicic acid bonding agent. I n the latter case, swelling due to acid action and to subsequent hydration of the gel increases the bonding area of both fibers and particles and further enhances the tensile strength of the sheet. However, the strength of the individual fibers decreases as acid action continues. The net result of these various factors upon the tensile strength of the sheet is illustrated in Figure 5. At low reaction times the strengthening effects due to dispersion, bonding, and swelling predominate over the weakening of the individual fibers. However, after approximately 4 minutes, in this case, the latter factor begins to overshadow the combined effects of the others, and the net result is a decrease in tensile strength. As seen in Figure 4,the curve showing the effect of p H on sheet tensile strength does not go through a maximum. The higher acid concentrations in the region above a p H of 3.0 apparently induce a sufficient degree of swelling and uniformity of dispersion to overcome the effect of the weakening of the fibers. Likewise, the apparent transition as depicted by the dashed line may indicate a transformation of surface conditions on the fibers which could exert a significant effect upon tensile strength. Data, except at a p H of 3.0, are available only for the 15-minute reaction period, and thus, the generality of these results must be established through additional experimentation. Although further work will be required before a completely adequate interpretation of these results can be given, the acid treatment must be subjected to much closer control than had been generally recognized. This factor may well account for at least some of the differences found in tensile strength measurements for sheets prepared from earlier supposedly identical runs in which only a moderate amount of care was taken to maintain the pH roughly a t 3.0 for a given length of time. Effect of Sheet Formation Variables
A limited number of exploratory tests were made to determine possible effects upon tensile strength of the pressure and drying conditions used in the formation of the sheets. T o determine if the rate of drying of the sheet affected the tensile strength, sheets were prepared by air drying a t room temperature without supplying additional outside heat. These sheets had the same tensile strength as sheets
574
32-
O!
0
.
250
,
500 PRESSURE.
750
1000
ps~.
Figure 6. Tensile strength of hand sheets decreases rapidly with increase in pressure
X A
Run P-2, stock beaten 18 minutes Run P-1, stock beaten 10 minutes 0 Run P-3, sheet pressed dry
prepared normally by drying in a photographic drier; hence, the tensile strength is not affected by either the drying rate or by the drying temperatures encountered on the sheet drier. To determine if a possible increase in acid concentration as a result of evaporation during the drying period affected the properties of the finished sheet, several sheets were subjected to ammonia after the sheet had been formed and pressed. The tensile strength of these sheets appeared to be unaffected even for the sheets that had a very strong ammonia odor before drying. Several sheets were water washed immediately after formation while they were on the hand mold to determine if washing away the acid had any effect. These sheets had only a slightly lower tensile strength than normal, and this could be caused by imperfections produced by mechanical aciion during the washing. For several sheets, prior to the sheet lav down, the stock in the sheet mold \vas made basic with concentrated aqueous ammonia solution (pH values of about 8.5). The tensile strength of these sheets was considerably lower than normal. Finally, sheets were prepared from a stock that had been neutralized in the sheet mold with dilute aqueous ammonia solution (pH about 7.0) and were found to have a very low tensile strength. The above results indicate that the sheet obtains the portion of the tensile strength which may be attributed to chemical action during the sheet formation. Once the sheet has been prepared, the drying rate, temperature, and pH of the remaining entrained water, before and during drying, apparently do not appreciably affect the tensile strength. In order to explore the possible effect of pressure on the sheets, two
INDUSTRIAL AND ENGINEERING CHEMISTRY
series of tests were performed in which identical sheets, before drying, were subjected to different pressures. Run P-1 was carried out by subjecting identically formed sheets to different pressures while the sheets were still wet. A stock beaten for 10 minutes was used in the preparation of these sheets. Run P-2 was a rerun of P-1 using a different stock prepared by beating for 18 minutes and thus containing more fines. A third run, P-3, was performed in which each sheet was dried after being couched from the screen, and then subjected to a series of increasing pressures. After each pressing the tensile strength of each sheet was determined, and the results of these tests are shown in Figure 6 . The observed reduction in tensile strength resulted from breaking and crushing of fibers. A crushing noise was heard during the pressing of the hand sheets and can also be heard during the calendering of machine-made glass paper. Although pressing enhances the visual appearance of the paper, it may decrease markedly the strength of the sheet. Acknowledgment
The authors wish to express their appreciation to the Diamond Ordnance Fuze Laboratories for the support of the work reported in this paper. The many helpful suggestions of R . D. Walker, Jr., and W. J. Nolan of the University of Florida are also gratefully acknowledged. literature Cited
(1) Clark, J. d’A., Paper Trade Journal 97, NO. 26, 25-31 (1933). (2) Clark, J. d’A., T a p p i 38, No. 11, 702 ( 1955). (3) Hagglund, E., “Chemistry of Wood,” UP. 41-76, Academic Press, New York,
ihi.
(4) Morey, G. W., “Properties of Glass,” 2nd ed., pp. 101-5, Reinhold, New York, 1954. (5) O’Leary, M. J., Hobbs, R. B., Missimer, J. K., Erving, J. J., Ta@i 37, NO. 10, 446-50 (1954). (6) O’Leary, M. J., Hubbard, D., J. Research N a t l . Bur. Standards 5 5 , pp. 1-9 (Research Paper 2599) (1955). (7) O’Leary, M. J., Scribner, B. W., Missimer, J. K., Erving, J. J., T a p p i 35, NO. 7, 289-93 (1952). ( 8 ) Schulmeyer, S. W., T a p p i 39, No. 9,
217A-20A (1956). (9) Steinschneider, M., and others, PapierFabr. 34, No. 23, 180 (1936). (IO) Steinschneider. M., Grund, E., PapierFabr. 36,No. 1, l(1938). (11) Stephenson, J. N., ed., “Pulp and Paper Manufacture,” Vol. 11, pp. 186264, McGraw-Hill, New York, 1951. (12) Weiser, H. B., “Inorganic Colloid Chemistry,” Vol. XI, p. 226, Wiley, New York, 1935. RECEIVED for review March 26, 1959 ACCEPTED March 14, 1960 This paper is based on results of an M.S. thesis by J. C . Geddes, Jr., Graduate School, University of Florida, Gainesville, Fla., February 1958.
