CERTAIN PROPERTIES OF PAPREG AFFECTED 

BY WET-PRESS PRESSURE ON 

BASE PAPER 

July 1943 

This Report is One of a Series 

Issued in Cooperation with the 

ARMY AIR FORCES 

and the 

BUREAU OF AERONAUTICS 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1575 

UNITED STATES DEPARTMENT OF AGRICULTURE 

F O R E S T S E R V I C E 

F O R E S T P R O D U C T S L A B O R A T O R Y 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
CERTAIN PROPERTIES OF PAPREG AFFECTED 

1 BY WET-PRESS PRESSURE ON MITSCHERLICH BASE PAPER 

By 

G. E. MACKIN, Associate Industrial Specialist 

R. J. SEIDL, Assistant Chemical Engineer 

P. K. BAIRD, Senior Chemist 

Introduction 

Preliminary tests previously reported by the Forest Products Laboratory 2 indicated that some properties of papreg were influenced by the amount 

of wet-press pressure applied to base paper during its fabrication on the 

paper machine. This report presents more conclusive data on the effects of 

using Mitscherlich type sulfite papers produced under five wet-press pressures 

on certain properties of the laminated plastic, Variations in wet-press 

pressure affect the density, porosity, thickness, and other properties of 

paper, and these in turn individually affect the properties of papreg. The 

results reported here are based on a direct correlation between wet-press 

pressure and the properties of papreg. 

Experimental Material 

Paper making.--The preparation of papreg for this investigation included the production of papers processed under various wet-press pressures followed by a uniform impregnating and laminating procedure. A commercial 

Mitscherlich sulfite pulp, meeting the requirements for this purpose, was 

used for the experiments, All runs were made from the same lot of pulp and 

all paper machine variables, except the wet-press pressure, were held as constant as possible throughout the series. The pressure used in each run was 

applied to all of the three wet presses. The pressure per lineal inch 

applied to the wet web of the paper was approximately 6.5 times that indicated on the gage. The gage pressure indicating the pneumatic force applied 

to the lever system of the presses was used for convenience in reporting the 

operating conditions. 

1

This mimeograph is one of a series of' progress reports prepared by the Forest 

Products Laboratory to further the Nation's war effort. Results here reported are preliminary and may be revised as additional data become available. 

2 Restricted Mimeograph No. 1395, "High-strength Laminated Paper Plastics for 

Aircraft." 

Mimeo. No. 1575 -1-
The pressures applied for each of the runs and the physical properties 

of the papers produced are given in table 1. If stock is delivered to the 

wire at a constant rate, the thickness, porosity, density, and strength of the 

paper can be altered by the pressure applied at the wet presses. Increasing 

the wet-press pressure over the range used in this series caused a marked 

increase in tensile strength, porosity value, and density and a decrease in 

thickness. The combined effect of these changes, as measured by the wetpress pressure, was reflected in the final properties obtained in the papreg. 

Impregnating, laminating, and testing--Bakelite res in No l BV-16303 

was used to impregnate these papers. The resin content was held at 36.5 

percent + 0.5 percent and the volatile content at 4.5 percent + 0.1 percent 

throughout the series. Plastic test panels were made by parallel laminating 

the various impregnated papers at both 75 pounds per square inch and 250 

pounds per square inch for 12 minutes at 325° F. Test specimens were prepared 

conditioned, and tested in tension, compression, and bending, and for specific 

gravity, and water absorption, according to the "Proposed Federal Specification 

for Organic Plastics, General Specification (Methods of Physical Tests), 

July 7, 1942.'' 

Discussion of Results 

Specific gravity.--The specific gravity of papreg molded at 75 pounds 

per square inch increased from 1.19 to about 1.36 as the wet-press pressure 

applied to the paper was increased through the range of 2.5 to 25.0 pounds 

gage pressure. At 250 pounds per square inch molding pressure, the specific 

gravity of the papreg increased from 1.37 to about 1.41 under the same wetpress prossure Conditions (table 2 and fig. 1) and appeared to be approaching 

a maximum constant value. 

Edgewise compressive strength. --The maximum edgewise compressive strength. of papreg molded at 75 pounds per - square inch increased from about 16,000 to 

about 20,700 pounds per square inch as the wet-press pressure was increased 

(fig. 2). Papreg molded at 250 pounds per square inch increased in compressive 

strength over tho same range of wet-press pressures, but tho degree of improvement was not appreciable. This effect can be explained by the fact that the 

specific gravity of papreg made From paper processed under high wet-press pressure and molded at 75 pounds per square inch (fig. 1) approached that usually 

obtained with paper subjected to ordinary low wet-press pressure (2 to 5 pounds 

gage pressure) and molded at the higher pressure of 250 pounds per square inch. 

Modulus of rupture.--The modulus of rupture of papreg molded at 75 

pounds per square inch increased from about 27,600 to about 37,200 pounds 

per square inch as the wet-press pressure was increased over the range used. 

At 250 pounds per square inch laminating pressure, the modulus of rupture increased from about 36,000 to about 39,500 pounds per square inch (fig. 3). 

These results show that by using paper made under high wet-press pressure 

papreg can be molded at 75 pounds per square inch having a bonding strength 

equal to that obtained at 250 pounds per square inch molding pressure using 

ordinary paper. 
Ultimate tensile strength.--The ultimate tensile strength of papreg 

molded at 75 pounds per square inch increased from about 32,600 to about 

41,000 pounds per square inch as the wet-press pressure was increased. As 

in the case of compression and bending, the increase in the tensile strength 

Of papreg molded at 250 pounds per square inch was not as large (fig. 4.). 

Moduli of elasticity.--The moduli of elasticity in compression, bending, 

and tension, of papreg molded at 75 pounds per square inch and 250 pounds per 

square inch increased-with an increase in -net-press pressure. The modulus of 

elasticity in conpression (fig. 5) increased from about 2,760,000 to about 

3,035,000 pounds per square inch in papreg molded at 75 pounds per square 

inch though the range of wet-press pressures used. In bending, the increase 

was from about 2,400,000 to about 2,900,000 through the same range of wetpress pressure. In tension, this value increased from about 2,860,000 to 

3,200,000 in papreg molded at 75 pounds per square inch. Papreg molded at 

250 pounds per square inch showed slight but definite increases in moduli of 

elasticity over the range of wet-press pressures used. 

Water absorption.--The water absorption of papreg molded at 75 pounds 

per square inch decreased from 7.3 percent to about 4.5 percent as the wetpress pressure was increased in the range of papers used, but papreg molded 

at 250 pounds per square inch did not change appreciably in water absorption 

(fig. 6). 

Conclusions 

Under the conditions described in this report certain mechanical and 

physical properties of papreg were definitely affected by changes in such 

sheet properties as density, thickness, porosity, and strength, all of 

which were varied by changes in the wet-press pressure used in making the 

laminating paper. 

The following effects on the papreg were observed when the wet-press 

pressure used on the paper machine was raised f'rom 2.5 to 25.0 pounds gage 

pressure: When papreg was molded at 75 pounds per square inch, specific 

gravity was increased 14 percent, edgewise compressive strength was raised 

30 percent, the ultimate tensile strength was raised 26 percent, modulus of 

rupture was raised 35 percent, the average of the throe moduli of elasticity 

was raised 13 percent, and the total water absorption was lowered 36 percent. 

When papreg was molded at 250 pounds per square inch, the specific gravity 

was raised 3 percent, the edgewise compressive strength was raised 8 percent, 

the modulus of rupture was raised 10 percent, the ultimate tensile strength 

was raised 9 percent, the average of the three moduli of elasticity was 

raised 5 percent, and the total water absorption was not affected. 

The effects of variations in wet-press pressure in paper making are 

evidently much more pronounced in papreg molded at 75 pounds per square inch 

than in that molded at 250 pounds per square inch. The advantage of using 

higher wet-press pressures appears to be chiefly the ability to obtain 

higher strengths at the lower laminating pressures. 

Mimeo. No. 1575 -3-
Table 1.--Properties of five papers producted under variations in wet-press 

pressure 
Table 2.--Effects of wet-press pressure applied to paper as reflected in 

papreg properties. 

1 Laminates cured for 12 minutes at 325° F. Strength tests made parallel to fiber 

direction (grain). All tests made according to "Proposed Federal Specification, 

for Organic Plastics; General Specification (Methods of Physical Tests), 

July 7, 1942." 

Mimeo. No. 1575