POTENTIALITIES OF PAPER-I3ASE LAMINATE S
AS COMPARED WITH OTHER LAMINATE S
February 1944
Q
No. 8145 2
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVIC, rE
+ FOREST PRODUCTS LABORATOR Y
,~
Madison, Wisconsi n
4
In Cooperation with the University of Wisconsin
POTENTIALITIES OF PAPER-BASE LAMINATE S
AS COMPARED WITH OTHER LAMINAT S1
Ft
!By .
ALFIED J . STAIN, Principal Chemis t
Plastics are becoming increasingly important for the fabrication o f
a vast variety of articles ranging from the most insignificant gadget t o . '
moderately stressed and even highly stressed parts of airplanes, Prior to .
the war, plastic uses and.production increased largely because of thei r
corrosion resistance, electrical properties, appearance, and ease of fabri cation, Low fabrication costs have helped to put many plastic articles on .
the counters of the five- and ten-cent stores .
Since the beginning of the war, more emphasis has been placed on th e
strength properties of plastics, because of the increasing substitutton o f
plastics for metals . Although most of the military applications are stil l
nonstructural or semistructural, thef
t,
a definite trend to use plastic s
more and more in strength"requiri .erays . These new requirements have bee n
met better by laminates, in both panel and molded form, than by other form s
of plastics . As a result, laminate°have increased in importance and pro s
duction at a remarkable rate, Paper`4e a .laminates are high in importanc e
among laminates, Although the amount of paper going into paper-base plas tics is only .a fraction of a percent of the, total production it is signifi cant that the amount going into plastics increased several fold during th e
past year .
It . shouldbe of importance to analyze the reasons for the increae d
use of paper-base laminates, and compare their properties with those o f
other laminates .
By far the most important laminate from a production and utilizatio n
suandpoint is plywood .- Plywood'is not generally classed as a plastic
'
laminate because only relatively small amounts of glue or resin are use d
for assembling , the plies, and because the glue or resin is almost entirel y
concentrated at the glue line .. The term plastic airplane has, however, bee n
applied quite .frequently during. the last few years when referring to one o f
the molded plywood airplanes . Resin treated laminated woods in both th e
uncompressed and compressed forms, however, should be truly considered as
laminated plasticsx
.
1Presented at the Annual Meeting of the Technical Association of tine Pulp &
Paper Industry, Hotel Commodor e * New York, N . Y ., Feb . 14-17, ]944
published in the Paper Trade Journal, Vol . 118, No . 21, May 25, 1 9
Mimeo . R1452
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Up to the present time, fabric-base laminates have perhaps mor e
varied military applications than have paper-base laminates . Asbestos-bas e
laminates, in the forms of both asbestos paper and fabrics, have found onl y
special applications . Plastic laminates made from glass fabric, and a combination fabric with glass in one direction and merely enough cotton thread
in the other to hold the glass .fiber in place have recently created con siderable interest because of their excellent strength properties . Thei r
use so far, however, is quite limited . The glass laminates have been made
with both phenolic resins and the newer contact resins, such as Columbi a
CR39, Laminae, and Plaskon 900 .
%
Cost s
Table I gives a comparison between the costs of the different bas e
materials, the average amount of phenolic resin used in making the lami ates, and the cost of the raw materials per pound of laminate . It is o f
interest that paper is considerably cheaper than most of the base material s
and is as cheap as the cheapest species of veneer . Further, paper require s
less resin than any of the other laminates with the exception of wood . The
face that paper is as cheap as veneer on a weight basis may be surprisin g
to many because of the relatively simple processing which veneer undergoes .
This is primarily because veneer must be cut from large, quality logs tha t
are both expensive to procure and expensive to ship and handle . 'Even thoug h
wood pulp represents only about a 4 5 . percent yield of the original wood ,
the veneer yield of quality material is rarely much better than this .' It
is thus obvious that paper-bas e , laminates have an appreciable advantage ove r
all other laminates with the exception of wood on a cost basis per uni t
..~ weight of raw materials . The slight advantage of wood, however, can b e
. expected to decrease with time as quality logs for cutting veneer becom e
more difficult to obtain .
Fabrication Factor s
Fabrication of the various laminates involves impregnation, drying ,
$ cutting, assembling, and pressing . Paper normally used in laminates can
be impregnated and dried more rapidly per wilt length than the-other lami nating materials, but a greater'length must be impregnated and dried pe r
unit weight .- Impregnating costs hence should not vary appreciably betwee n
the different,-laminates ; assembly costs may be slightly greater for, th e
paper than for the other laminates because more sheets are involved pe r
unit weight . If this is done mechanically, however, the difference shoul d
not be great .
The various laminates are pressed at quite different pressures .
Plywood assembly pressures using hot-Trees phenolic glues range, i n
general, from 150 to 250 pounds per equal's inch, the lower pressuresbein g
R1452*
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used for the softwoods ' (conifers) , such as Douglas-fir, ' and the highe r
pressures being used'for'the hardwoods (broad-leaved woods), such as birch .
Impreg laminates can be made at the sa. a piessires . High-density cothpre g
requires pressures - of_1,000 to - 1,200 pounds per square inch when nonadvanced water-soluble resins are used,'and 2-,500 to 3,000'pounds per squar e
inch when advanced spirit-soluble resins : are used .. , With the newer lo w
pressure phenolic resins, paper-base laminates can'be made with practically'
maximum' specific gravity (1 .35 to 1 .40) and strength properties under pres s
sures as low as 100 pounde . per square'inch . Fabric-base laminates and' '
-' lignin laminates, however, require conaiderably higher pressures to obtai n
maximum specific gravity and to avoid delamination . In general, fabric phenolic laminates and lignin-plastic laminates are made at pressures o f
1,000 to 2,000 pounds per square inch . The same is true for'asbestos-bas e
laminates . Glass fabric is laminated with phenolic resins at pressures o f
100 pounds per square inch or less . It is laminated with the new contactpretsure resins under practically negligible pressures. Ability to lower ,
the laminating pressure is not of great importance in making flat panels, .
but it is ' ofv extreme- importance in molding . '
'Comparison of Various Fiber Base s
' The specific gravity ; water absorption, and strength properties o f
laminates made with different fiber bans are given in Table II, togethe r
with the corresponding values-for aluminum alloy 24ST•used extensively-in '
airplane construction . The paper-base ; cotton-fabric, lignin-sheet, an d
compreg laminates when compressed to practically the ultimate compr'ession, '
all have about the same specific gravity __about half that of aluminum .
The glass- . and asbestos-base laminates have somewhat higher specifi c
gravity values than do the - other fiber-base laminates, due to the heavie r
filler .
No systematic .differences exist between the water-absorption values .
Differences in the degree of precure .of-the resin and its distributio n
throughout the structure cause a greater difference in water absorptio n
than do differences in the fiber base .
Table II shows that paper=base laminates compare favorably with th e
best of the fiber-bae laminates in practically all specific strengt h
properties The properties given hi-column 1 for Mitscherlich hig h
strength paper represent minimum values that can be readily attained ,
whereas values in`the other,columns b the-averages for specimens tested :
Values 20 percent greater than those given in column 1 are frequently
obtained . 'Tile only specific strength property for this paper-base laminate
that is exceeded by the cotton-base laminates is ultimate compression .
Where the inpact strength is not of vital importance, the compressive-yiel d
strength can = be'increased by - about 60 percent at'the expense of toughness ,
by impreg .ating with a-nonarldanced-water=-'soluble--resin- instead of the conventional somewhat Advanced spirit-soluble resin . This will also lower the
water absorption to about 1 percent .
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'
P
It has been rumored that glass-fabric laminates with strength prop erties considerably higher than -those given in the table have been attained .
The high cost and, at present, small production facilities, however, see m
to limit the use of this type of laminate to very special purposes, Th e
greatest virtue of glass Laminates is in their extremely high impact - .
strength .
Impact strength is the poorest property of paper-base laminates .
In this respect paper-base laminate is superior only to the lignin laminat e
as shown in Table II, Other forms of lignin laminates, however, are als o
superior to the paper-base laminate in this property . Improving the impac t
strength of paper-base laminates without seriously impairing other proper ties is the biggest research job now confronting paper and resin chemist s
interested i n . laminates . This seems to' involve making a more ductil e
product without too serious a loss in modulus of elasticity .
On a specific3 strength basis the properties of aluminum . alloy 24ST
are, in general, not much better than the fiber laminates . The superiorit y
of aluminum rests largely in its compressive-strength properties and in it s
impact strength ,
Ordinary plywood ranks highon a specific strength basis with the
other laminates except in ultimate compression and excels in specifi c
flexural properties because of the low specific gravity .
Most of the other physical properties of the highly compressed lami nated plastics are quite similar . The electrical properties depend more o n
the absence of electrolytes from the filler and resin, the degree o f
precure of the resin, and the distribution of resin throughout the structur e
of the laminate than on the choice of the laminate base . Neither th e
coefficients of thermal expansion nor the thermal conductivity values o f
the different highly compressed laminates differ appreciably .' The burning
rate of the laminates with *inorganic bases, such as the glass and'asbestos, is nil . The burning rate for all the laminates with an organic filler i s
slow ; in fact, they are self-extinguishing . The resistance to continue d
heating is also higher for the laminates with inorganic fillers . Glass fabric laminates, impregnated with phenolic resin, will stand continue d
heating at 48 00 F. ; asbestos laminates will stand 25 00 to 30 00 F,, whil e
paper and cotton-fabric laminates will stand only a temperature of 212° t o
250° F . When heat resistance Is sought-, the laminates with inorgani c
fillers should be used .
.It appears, from the analysis of the properties of laminates mad e
with different fiber bases, that paper-base plastics deserve the importan t
place they are finding among laminates . Although they may have t o
relinquish some of their present war-time uses to metals, their excellen t
properties and relatively low cost will assure them many future industria l
and structural uses .' Further reductions in fabricating costs should assur e
. them a place in house construction and furnishings . Even at present price s
they appear promising for facing of the better grades of plywood . - Becaus e
of the better dimensional stability and freedom from face checking o f
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plywood faced . with"a paper-base laminate, this material should fin d
extensive outdoor use .
Hydrolyzed-wood laminates and combined lignin-and-paper laminate s
also show great promise as competitive building materials . Although
their properties are, in general, somewhat inferior to paper-base phenoli c
laminates, their potentially lower cost will mak e. them desirable for man y
uses . ,Plastic laminates made from paper mill products thus appear t o
have real future in store for them . This future, howevery will be' attaine d
only by continual research and improvemen t - and c h_eaptening of products ,
,for those sponsoring other forms of laminate and the metals will exten d
themselves to the'utmost to place their products ahead of laminatespr o ducea. from wood and wood products in all industrial,2 applications .
.• r
.
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Table 1 .---Comparative material cost of different types of phenolicresin laminates
Resin4
:
Tota l
: base
content of material cost of laminat e
material : laminate
:
per pound
: per pound :
:
:
:
Percen t
Cost of
Base material
Douglas-fir veneer3
Yellow birch veneer3
:
: $0 .0 7
413
$0 .10
523
.11
523
.15
.1 2
:
06
35
Mitscherlich sulfite paper . . . :
.0 7
35
.14
Cotton fabric (coarse weave) . :
.3 0
45
.28
Cotton fabric (fine weave) :
.40
45
Asbestos paper :
.30
50
Kraft paper
Asbestos fabric
Glass fabric
:
:
.80
:
50
2 .00
:
50
:
.13
A
0n the basis of the total weight .
0n the basis of the phenolic-resin costing $0 .25 per pound (100
percent solids) .
Select, rotary cut, 1/16-inch .
4Pl ;rwo od .
5Impreg or compreg .
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