U. S. DEPARTMENT OF COMMERCE

DANIEL C. ROPER, Secretary NATIONAL BUREAU OF STANDARDS

LYMAN J. BRIGGS, Director

NATIONAL BUREAU OF STANDARDS MISCELLANEOUS PUBLICATION M123

PRODUCTION OF PRESSBOARD

FROM CORNSTALKS

By

BAKER WINGFIELD, T. R. NAFFZIGER, E. R. WHITTEMORE,

C. B. OVERMAN, O. R. SWEENEY, and S. F. ACREE

ISSUED AUGUST 4, 1936

UNITED STATES

GOVERNMENT PRINTING OFFICE

WASHINGTON : 1936

For sale by the Superintendent of Documents, Washington, D. C. Price 5 cents

M123

PRODUCTION OF PRESSBOARD FROM CORNSTALKS

By Baker Wingfield, T. R. Naffziger, E. R. Whittemore, C. B. Overman,

0. R. Sweeney, and S. F. Acree

ABSTRACT

A process for the production of pressboard from cornstalks, consisting in stalk preparation, pulping, forming, pressing and drying under pressure, and sizing is outlined. Stalk preparation by reduction to short lengths and removal of wire and dirt facilitated handling and reduced wear on the machines.

The experimental results showed that boards could be produced with a range

of the modulus of rupture from 2,040 to 7,500 lb/in.2 by varying the degree of cooking given the stalks from to 3 hr at 140° C, in water, followed by a twostage mechanical refining treatment. The two-stage milling was found to be

advantageous in producing a uniform pulp.

The pulp was formed into wet mats, which were dried at about 150° C under

a pressure of about 500 lb/in2

. It was found that excess water could be expressed

mechanically until only about 60 percent remained in the wet mat, with about

20-percent loss in strength of the resulting board. This permitted the hot presses

to be brought up to pressure more rapidly and probably reduced their steam

consumption. An electrical-resistance method was used for determining the minimum drying time of pressboards.

Moisture resistance was obtained by adding emulsions of paraffin or rosin before the mats were formed. However, better sizing could be obtained by treating the finished board with the size dissolved in a suitable solvent. CONTENTS

Page

I. Introduction 2

II. Equipment 3

III. Experimental manufacture of board 3

1. Preparation of stalks 3

2. Pulping 4

3. Forming 5

4. Pressing and drying 5

(a) Initial and final thickness 6

(b) Initial moisture 6

(c) Drying 7

(1) Pressure and temperature 7

(2) Drying time and measurements 8

(3) Mechanical aids for the escape of steam 8

5. Sizing for water resistance 8

(a) Beater or tank sizing 8

(b) Wet-mat sizing 9

(c) Surface sizing 9

(d) Solvent sizing 9 IV. Summary and conclusions 10 70005—36 1
2 Miscellaneous Publications, National Bureau of Starodards

I. INTRODUCTION

A series of investigations was started in 1927 by the National Bureau

of Standards on the utilization of agricultural wastes. Some of these

studies 1 2 3 4 5 were carried out in cooperation with the Engineering

Experiment Station at Iowa State College, Ames, Iowa. The use of cornstalks as a raw material for insulation board has been shown to be

feasible by the results obtained from extensive studies carried on by

the Bureau in cooperation with the Experiment Station and by the

establishment of a commercial plant. This, however, did not provide an outlet for all the available raw material, and pressboard seemed to promise a further means of using cornstalks.

Pressed boardlike materials have come into use in recent years for bookbinding; electrical insulation; panels for walls, furniture, automobiles, airplanes, and an interior finish for buildings ; forms for concrete; floor covering; etc. They are made commercially from wood

chips, sulfite wood-pulp screenings, bagasse, etc., in sheets % to % in. in thickness and generally 4 ft in width and 8 ft in length. The

boards have one smooth surface and one which shows the imprint of

the screen used to facilitate the escape of steam during drying.

The first work on pressboard from cornstalks was started in the

chemical engineering laboratories at Iowa State College in 1921,6 and

in 1926 a board of greater density than insulating board was obtained

by pressing the wet mat in a steam-heated Irydraulic press. This

board had a clean, smooth surface produced by a highly polished sheet

of metal in contact with it during the drying process. In 1924 Cast- ner 7 was granted the first United States patent covering the production of a good grade of pressed fiber board free from any binding material. In 1926 Sutherland 8 patented a pressed fiber board to which

had been added a suitable binder. Two of the more recent patents

are those of Mason 9 covering a grainless, hard woodboard, and

Sweeney 10 embracing a pressed board from cornstalk pulp or like fibrous material. Conflicting interests in this field have led to patent

litigations.

In 1927 Hartford and Richardson, staff members of the National

Bureau of Standards stationed at Iowa State College, started to investigate the methods and equipment which would be most suitable

for the production of pressboard from cornstalks.

The general procedure for the conversion of cornstalks into press- board, detailed in the following sections, consists in cutting the stalks

into short lengths, reducing this material to fibers in suitable machines

(in some cases preceded by a digestion with hot water or with chemicals), and forming the fibers into mats from a water suspension. The

fiber mats are then cold-pressed to reduce the quantity of water and

dried in a hot press.

iO. R. Sweeney and W. E. Emley. Manufacture of Insulation Board from Cornstalks. Misc. Pub. BS MH2 (1930).

2 O. E. Hartford. Manufacture and Properties of a Cellulosic Product (Maizolith) from Cornstalks and

Corncobs. Misc. Pub. BS M108 (1930).

3 O. R. Sweeney, C. E. Hartford, R. W. Richardson, and E. R. Whittemore. Experimental Studies on

the Production of Insulating Board from Cornstalks. Iowa State College Eng. Experiment Station Bui.

102 (1931).

* E. R. Whittemore, C B. Overman, and Baker Wingfield. Separation of Cornstalks into Long Fiber,

Pith, and Fines. NBS Misc. Pub. M148 (1935).

6 E. R. Whittemore, C. B. Overman, and Baker Wingfield. Semicommercial drying of pressboard by Measuring Its Electrical Resistance. Ind. Eng. Chem. 27, 831 (1935).

6 See footnote 3.

1 U. S. Pat. 1499291, June 24, 1924.

« U. S. Pat. 1598260, August 31, 1926.

» U. S. Pat. 1663505, March 20, 1928. w U. S. Pat. 1772502, August 12, 1930.
NBS Miscellaneous Publication Ml 23

Figure 1.

Split-platen press and loading device. A, B, Platen sections; C, wire screen; D, screen rolls; E, steel plate; F, loading rack.

Production of Pressboard from Cornstalks 3

II. EQUIPMENT

The equipment consisted of a shredder, cookers, rod mill, washer,

jordan, suction mold, and forming machine, which have been described

in a previous article, 11 and, in addition, an attrition mill and hydraulic

presses. The attrition mill consisted essentially of two 24-in. steel disks

mounted face to face on separate shafts and enclosed within a steel housing. When operating, the disks rotated in opposite directions

at approximately 1,800 rpm. The degree of disintegration of the

stalks was controlled by increasing or decreasing the compression of

springs acting on one of the rotating disks, which made the faces

of the disks rub with greater or less pressure on the material passing

between them. Hydraulic presses, with heated platens, were used

for pressing and drying the mats. The split-platen press with a

loading device, designed 12 particularly for this work, is shown in

figure 1. This press was the standard type with four steel platens

cored with steam ducts. Steam was admitted through hinged inlet pipes. Each platen was built in two horizontal 30- by 39-in. sections,

A and B, with a % in. space between them. An individual continuous

screen belt passed over the top of section A and returned through the

space between sections A and B. Section A of each platen was

drilled with &-in. holes on 2-in. centers. Section B of each platen was faced with a polished stainless-steel plate E. The continuous

screen belts C, were of stainless steel, spiral-weave wire supported on

guide rolls D, which were mounted on the front and back of every

platen. Each continuous screen belt was driven from the front roll and synchronized with a continuous canvas belt of a corresponding

shelf on the loading rack F.

III. EXPERIMENTAL MANUFACTURE OF BOARD

1. PREPARATION OF STALKS

After breaking open the bales, the stalks were reduced to lengths of 3 or 4 in. in an ensilage cutter. The material was then ready for the pulping operation.

The reduction of the stalks to short lengths increased the cooker

charge and aided in the handling of the rod mill feed. Baled stalks

generally contained a considerable amount of dirt. In the procedure used in the experimental work, this material was largely

removed by washing in a continuous, rotary washer after the rod

milling. Other results have shown that the stalks can be washed

cleaner by four to five batch washes in a mixing tank following the

cutting operation. In the commercial manufacture of insulating

board from cornstalks, two additional machines have been found

advantageous: a bale breaker to disintegrate frozen, mud-cake bales

and to aid in feeding the cutter; and a magnetic separator to remove

baling wire and scrap iron and thus increase the life of the cutter and

refining machines.

" O. R. Sweenev, C. E. Hartford, R. W. Richardson and E. R. Whittemore. Experimental Studies on the Production of Insulating Board from Cornstalks. Iowa State College Eng. Experiment Station

Bui. 102 (1931). « Designed by 0. R. Sweeney, C. M. McDowell, and T. R. Nafiziger.
4 Miscellaneous Publications, National Bureau oj Standards

2. PULPING

Two methods of pulping were investigated: (1) the prepared stalks were cooked and then refined mechanically; and (2) they were pulped

by mechanical means alone.

Cooked pulps were prepared by charging cut stalks and water into

the cooker. The cooker was brought up to temperature and maintained for a definite time. The charge was then drained and fed,

together with water, into the rod mill. The discharge from this mill passed into a continuous rotary-screen washer where it was washed with additional water. The washed pulp and water were

introduced into a mixing tank and pumped to the attrition mill or the steep-angled jordan refiner. After the refining, the pulp was

sized in the beater or in mixing tanks.

A series of pressboards ranging from weak boards of low density

to strong ones of high density was produced by varying the degree

of cooking. Table 1 shows the range of the physical properties of the boards obtained when the stalks were pulped mechanically and

also by increasingly severe cooks followed by mechanical refining.

Only three conditions have been tabulated and they represent those

most suitable for the production of boards of low, medium, and

high density and strength. Chemical cooks, such as with lime, soda

ash, and 2 to 4 percent of caustic soda, produce boards somewhat

denser and stronger, but this is probably too costly a procedure for the small increase in strength.

In either process the mechanical refining is an important step.

The machines adaptable to the close control of fiber quality, length,

and softness, are not suited to the direct, single treatment of raw

or lightly cooked stalks. It is better to partially reduce the material

in one and finish the refining in another machine. The refining

operation separates the stalks into crude fiber bundles, and cuts and softens them.

A number of machines were investigated to determine their suit- ability as primary refiners. Such machines must be able to handle

the cut stalks and the beater, the swing-hammer mill, or the rod mill

met this requirement. The rod mill produced a good pulp at a

higher rate than the other two machines and combined continuous

operation with low power consumption. It was selected as the most

suitable for the initial refining. The output and the power con- sumption of the rod mill were investigated for different types of

pulp. In treating uncooked and pressure cooked stalks, the power

requirement per ton decreased from 168 kwhr in the former case

to 113 kwhr in the latter and the output increased from 0.10 to 0.15

ton per hour.

Table 1.Yields and physical properties of unsized pressboard obtained under

different cooking conditions

Cooking treatment

Yield on original stalks i Range of physical properties

After cooking

After rodmilling and

washing

Density Modulus of rupture Moisture a

Vi hr

bsorption *

24 hr None

4 hr at 82 to 100° C

3hrat 140° O %

100

75 to 87

68 to 85 %

74 to 80

62 to 68 54 to 48

lb/fts 39 to 48

50 to 58 52 to 65

lb/in.* 2,040 to 4,000

3,200 to 4,400

5,000 to 7,500 %

11 to 26

11 to 24

11 to 15 %

79 to 110

70 to 84

5S to SO

i Oven-dry basis. 1 Increase in weight after they had been immersed in water for the time indicated.
Production oj Pressboard from Cornstalks 5

Of the machines suitable for the second-stage refining, the attrition

mills produced pulps which were properly rubbed and brushed, and

also exerted the slight cutting action desired. The disk type of mill

described in the equipment section and the steep-angled jordan gave

suitable pulps, consumed less power than the other types and gave

higher outputs. A comparison of power requirement and rate of

refining was made between the two machines, but because of the

difference in size definite conclusions could not be drawn as to their

relative efficiencv.

3. FORMING

For some test boards the mats were formed in a stationary mold,

but were generally made on a continuous board-forming machine. 13 This was a modified papermaking machine with a head box, illus- trated in figure 2. The prepared pulp, at about 3-percent consistency, was pumped into the rear section of the head box. The force of the

discharge was broken by the baffles and the forward speed was gradually decreased by the successively wider openings between them.

The mat formed on the wire passed between press rolls and left the

OVSRFLOr/

Figure 2.

Forming machine head box, showing baffle arrangement.

machine with a moisture content of approximately 75 percent. The

continuous sheet was cut into 2.5 to 3 ft lengths. The maximum

thickness of mat that could be formed on the machine without

crushing 14 in the press rolls was approximately 3 in. This varied

slightly according to the "slowness" of the stock.

4. PRESSING AND DRYING

The mat sections from the forming machine, with 20-niesh screens on both sides, were introduced into an unheated, hydraulic press. The pressure was brought up slowly to about 400 lb/in. 2 and held for

4 or 5 min. The pressure was released and the mat, with one side

covered by a 20-mesh screen and the other by a polished metal plate, was placed in a heated hydraulic press, pressure applied, and the board

allowed to dry.

13 O. R. Sweeney, C. E. Hartford, R. W. Richardson, and E. R. Whittemore, Experimental Studies on

the Production of Insulating Board from Cornstalks. Iowa State College Eng. Experiment Station Bui.

102 (1931).

11 "Crushing" is a term applied to the phenomenon of broken mat formation which occurs when water

is expressed too rapidly by roll or flat presses.
6 Miscellaneous Publications, National Bureau of Standards

The important factors in the pressing and drying of pressboard were found to be initial moisture, pressure, temperature, thickness of the board, type of pulp, and the mesh and weave of the screen. These

factors were investigated separately, as far as possible, to determine

their relative effects on the time of drying and the strengths of the

boards.

(a) INITIAL AND FINAL THICKNESS

It was found that the relation of the thickness of the initial mat to that of the final board has no direct bearing on the time required to dry the board nor on the strength of the final product. The initial thickness is of importance, however, when the working space is taken

into consideration in designing the press. The data in tables 2 and 3 show that the ratio of the initial to the final thickness is fairly constant

for wide ranges of thickness, moisture content, and different types of

pulp. The results shown in table 2 were obtained by pressing two or more wet mats together to form single boards and those listed in table 3 were obtained from single mats.

Table 2.

Effects of thickness on compression during drying mechanical-nevjs

pulp 1

Number of mats J Total

original thickness Final

thickness Decrease in

original thickness

2

in. 2\i 2%

4

5

in.0.28

.39 .58

.73

%

86.8

85.8 85.5 85.4

34

5

1 Mechanical-news pulp consists of about 82 percent of mechanical cornstalk pulp and 18 percent rA pulped

newspapers. The boards prepared from this pulp correspond in their physical properties to boards prepared from cornstalk pulp cooked under atmospheric pressure. The addition of pulped newspapers to mechanical

cornstalk pulp reduces the dusting properties of the boards.

3 The equipment available limited the thickness of a single mat to that which would produce pressboard

of 0.2 in. final thickness. Table 3.

Effects of moisture content on compression during drying

[Basic pulp types]

Pulp type

Initial moisture

Decrease in thickness

Mechanical %

73.5 S3.0 82.5

56.0

80.0

73.0

%

85.8 84.2

83.8

82.8

85.9

85.6

Do

Mechanical-news _ Do

Cooked - Do „ „

(b) INITIAL MOISTURE

A series of mats was prepared from mechanical pulps which con- tained differing percentages of water. Those having above 54 percent

of moisture had been subjected to different pressures to express the

water, and those having less than 54 percent were prepared by pressing

followed by treatments in a hot-air drier. The results of these experi-
Production of Pressboardjrom Cornstalks 7

ments, table 4, indicated a decrease in modulus of rupture with a decrease in initial moisture.

Table 4.

The effect of initial moisture on the modulus of rupture of mechanical

pulp boards

Original moisture

70.5.

63.5.

60.0. 54.0.

50.0. 46.5.

37.0.

34.5. 31.0.

0.0.. 0.0..

» No pressure was applied to this board in the hot press. The other boards in this study were dried in the hot press under a pressure of about 200 lb/in2

. A smooth curve, drawn through a plot of the modulus versus per- centage initial moisture, indicated that, in its upper portion, the

percentage decrease in moisture was approximately equal to the per- centage decrease in modulus. Therefore, to facilitate the handling

and storing of the wet mats from the forming machine, they were

pressed for 4 to 5 min. between two wire screens at 400 to 500

lb/in2, in an unheated hydraulic press. This reduced their initial moisture content from approximately 75 to 60 percent. This initial pressing also made it possible to bring the mats up to pressure more

rapidly in the hot press without increasing the danger of crushing

them. The use of cold-pressing in commercial practice would depend on whether the saving in operating time and steam consumption in

the hot presses offset the cost of cold-pressing and loss in strength.

(c) DRYING

(1) Pressure and temperature.—The reiatiocship between the

strength of the board and the pressure applied to it during the drying

process is given for two different types of pulp in table 5. The data

indicate that maximum strength results at about 500 lb/in2.

Density Modulus of rupture

lb/ft»48.8 48.8

46.9 49.4

46.3

43.2

45.0 43.2

45.0 37.5

30.7

12.5

lb/in*.835

3, 230

3, 235

3, 365

2, 610 2,240 2,540

1,970

1, 650

1, 620

705

» 220

Table 5.

Effect of pressure on the strength and density of boards from cooked and

mechanical pulps

Type of pulp Drying

pressure Density Modulusof rupture

f 795

706 818

530

441 353

lb/ft*64.1 67.4

64.8

65.5

61.3

57.1

lb/in2.6,030

7, 050

6, 830 7,230

7, 500

6, 270

f 700

J 500 300

I 100

64.0

61.7

55.9 45.4

3, 340

3,590

3,000

2,040
8 Miscellaneous Publications, National Bureau oj Standards

The temperature used for drying pressboards throughout this

work was about 150° C or 50 to 55 lb gage on the steam presses. Some experiments were conducted on temperature effects but the

evidence was not conclusive. In some cases, it was found that the

modulus was decreased slightly in boards dried at 160 to 165° C

and, in a few cases, the surfaces of the boards were scorched slightly

by overdrying. In the case of wet-mat sizing, the boards dried at 150° C had improved moisture resistance. (See table 7.) (2) Drying time and measurements.—The determination of the drying time for pressboards was found to be necessary for two reasons:

first, that slightly underdried boards were blistered by the expansion

of entrapped steam or moisture when the pressure was released ; and

second, that overdrying consumed steam and mill space. Overdrying

the boards for 30 min seemed to cause no perceptible improvement or

deterioration on the resulting product.

Two general methods for the determination of the dry point were

investigated. One was the compilation of an actual time curve for boards of like pulp types and varying thicknesses. The other was

the end-point method. 15 The first method proved to be unsatisfactory, as the data showed

wide variations in the drying time. Of the end-point methods, the

measurement of electrical resistance of the board was best.

(3) Mechanical aids for the escape of steam.—The use of mechanical

aids to permit the rapid escape of steam was necessary for economical

dr}dng. After experimenting with various types of screens and per- forated plates, it was found that a 20-mesh screen against one face

of the board gave the best results. Coarse-mesh screens reduced the

drying time but increased the roughness and dusting of the board

surface. Screens on both surfaces of the boards greatly reduced the

drying time, but roughened both sides. Spiral-weave wire, while more serviceable as a conveyor, imparted very rough surfaces to the

boards.

5. SIZING FOR WATER RESISTANCE

Inasmuch as untreated pressboards made from cornstalk fibers absorb moisture rapidly, and as a result warp and lose strength

rapidly, some kind of sizing is necessary to obtain moisture resist- ance. This may be accomplished by beater sizing, wet-mat sizing,

surface sizing, or by solvent sizing, and these methods were studied.

(a) BEATER OR TANK SIZING The sizing material was added to a water suspension of the fibers in a beater or tank and precipitated with a suitable chemical after

thorough mixing. A number of different sizing materials were used

and the best results obtained are shown in table 6. The best water

resistance, without excessive loss of strength, was obtained with the

commercial paraffin emulsion P-l, although comparable results were obtained with rosin sizing by the method commonly employed

in the beater sizing of paper. The boards sized with asphalts of

different melting points showed comparable water resistance but

lost about 50 percent of their original strength.

i*E. R. Whittemore, C. B. Overman, and Baker Wingfield. Semicommercial drying of pressboard by measuring its electrical resistance. Ind. Eng. Chem. 27, 831 (1935).
Production oj Pressboard from Cornstalks 9

Table 6.

Beater sizing

oizing material Amount

of size Emulsifying

agent

Precipi- tating agent

(alum)

Strength

as percent

of original modulus

Water absorption 1 h hi- 24 hr

Rosin 4.5

3.0

S.O

2.0

3.0

3.0

3.0

3.0

%

7.0 4.0

5.0 4.0

4.0

4.0

3.0

4.0

%

72

11357

79

re

5.5 15.7 5.4

4.4 6.9

6.

1

8.2

6.5

%

31.8 59.0

19.6 23.1 25.0

32.1 23.9

21.1

Sodium alginate. Paraffin 2 P-l 3 Asphalt 2 ' Rosin-T

Asphalt 3» Rosin-T..

Aspahlt 4 2... SA-T

Pitch 2 S-T

1 All water absorptions in tables 6, 7, and 8 are given as weight percent,

i Emulsions prepared in the laboratory containing 25 percent of size: Asphalt 2 mp 127° F. Asphalt 3 mp 147° F. Asphalt 4 mp 243° F. Rosin Papermakers' rosin. S_ Soap. SA Stearic acid. T Triethanolamine.

3 Commercial emulsion containing 38 percent of paraffin.

(b) WET-MAT SIZING Unsized wet mats were cold-pressed to about 65-percent of moisture,

covered with sizing material, and then dried under pressure at differ- ent temperatures. Table 7 lists the results obtained with paraffin

oil, paraffin, and halogenated naphthalene. Paraffin at 150° C, as in

the beater-sizing tests, gave the best moistureproofing combined

with low loss in strength.

Table 7.

Wet-mat sizing

Sizing material

Paraffin Do

Do.

Paraffin oil.. Do.

Halogenated naphthalene

Press temperature

150 140

130

150

130

150

140

Density

lb/ft 3 63.7

63.1 61.3 68.9

60.2

62.9

62.8

Strength

as percent

of original modulus

75.2

70.0

64.5

59.1 48.4

89.4 90.0

Water absorption ^ hr

4.8 6.5

6.5 9.2

8.6 39.2

24 lure, 21.6

33.8 35.9

36.8

28.5

83.9 89.8

(c) SURFACE SIZING Only one surface-sizing experiment was made. Unsized, finished

boards were covered with bakelite varnish and dried in a heated

press. The boards thus treated had a moisture absorption of about

11 and 22 percent for % and 24 hr, respectively, and showed an

increase of about 10 percent in the modulus of rupture over the

original boards. The bakelite imparted a smooth, glossy finish to the surface of the boards and greatly improved their appearance.

(d) SOLVENT SIZING The sizing material was dissolved in suitable solvents and the

unsized, finished boards were impregnated with the solutions. The

solvents were removed by drying at 105° C for 30 min.
10 Miscellaneous Publications, National Bureau of Staridards

The data in table 8 show that boards impregnated with a solution

of paraffin in carbon tetrachloride and dried were more moistureresistant than when sized by any other method and were comparable

with the best commercial products. There was no apparent loss in strength in the boards from the sizing operation.

Table 8.

Solvent sizing

Sizing material

Size re- tained

in the board

Solvent

Solvent

retained

in the board

Water absorption Hhr M hr

Paraffin.... %

3.6

8.4

1.0

2.1

or700.9

7.5

.9 %

8.3

4.4

11.1 14.5

%

12.6 M6.0

13.4 25.6 Do CCL._._

Halogenated naphthalene Gasoline Do. CC1< .3

» 4-hr absorption is equal to approximately Yi of the 24-hr absorption value,

k 24-hr absorption value. IV. SUMMARY AND CONCLUSIONS

1. A process for producing pressboard from cornstalks is outlined

which consists in stalk preparation, pulping, forming, pressing and

drying under pressure, and sizing.

2. The reduction of the stalks to short lengths in a cutter increased

the output of the cookers and aided in the feeding of the pulping

machines.

3. The removal of dirt, wire, etc., from the stalks is advantageous

in lengthening the life of the pulping machines and improving the

board.

4. A wide range of board strengths was obtained from cornstalk

pulps: the highest strength from those cooked under pressure; medium

strength from those cooked at atmospheric pressure; and low strength

from those prepared mechanically.

5. A two-stage mechanical refining in separate machines appears

to be better than single-stage refining in the preparation of uniform

pulps.

6. It is possible to reduce by pressure the moisture content of the

wet mats by 20 percent. This lowers the board strength by the same

amount, but it should reduce the steam consumption in the drying

process.

7. Boards from similar pulps were of maximum strength when dried

under pressures of approximately 500 lb/in2

. 8. A drying temperature of approximately 150° C is recommended.

9. The drying time for pressboards varied, in general, with the

thickness of the final product ; however, to prevent losses from blister- ing and waste of time in overdrying, a more dependable criterion than

thickness should be employed.

10. The electrical-resistance method for determining the drying

time is recommended.

11. The most effective of the moisture-proofing methods investigated was that of solvent sizing of the finished board, but it was comparatively expensive. Beater or tank sizing with paraffin or rosin

gave boards which absorbed approximately 20 to 30 percent of water

when immersed for 24 hr. Washington, May 12, 1936.

O