THE ADAPTABILITY OF ~~~~ VJOODS TO THE kASUF'ACTURE 03'

my -FOFthED HARDBOARD*

by

R . G. Frashour

November 3, 1953

OREGON FOREST PRODUCTS LABORATORY

State Board of Forestry and School of Forestry, Oregon State College, Cooperating

Corvallia

9 A paper przsented before the Pacific Section of the Technical Association of the Pulp and Paper Industry, a t Longview, Washington.

THE ADAPTABILITY OF VESTEXY WOODS TO THE MAXIFACTURE OF CPIY-FCRE:fED HARD3OA.D

INTRODUCTION

The hardboard industry i n the Pacific Northwest has developed during the

past six years into a major industry of the region. Hardboard production i n

t h i s region now r iva l s t h a t of Sweden, although t h a t country's industry began

almost twenty years ear l ier .

Approxin~ately 25 per cent of the United States1 hardboard capacity is

i n the P!orthwest, and may be as high as 30 per cent by 1954. The estimated annual

capacity of the Nor th~es t is nor7 over 3bO million square f e e t (1/8-inch basis).

It is interest ing t o note tha t a t the present time the ent i re production

of hardboard is from s a ~ m i i l and veneer n i l 1 residues, and has imposed no

additional drain on the region's fores t resources.

PROCESSES

Methods of hardboard manufacture can be i l lu s t r a t ed most easi ly on the

basis of how the mat is formed. The mat can e i the r be formed from a water s lurry

o r by' f e l t i n g dry f iber ; hence the d e s i p t i o n s wet-formed and dry-f ormed. There

are vhriations i n each category which w i l l be explained l a t e r . A s early as 1925

hardboard vms being made commercially by t h e wet-formed method, but not u n t i l 1952

vms there a commercial plant in operation i n t h i s country using the dry-f ormed

process.

Wet-formed process

he continuous net-process is the oldest, and perhaps the most commonly

k n m , method. A flow sheet of a typical plant is shown in Figure 1.

SLAB

CHIP BIN

CYCLONE ADDITIVES WHITE WATER

CONSISTEICY PRdPORTIdNING REGULATOR BOX

REFINER

60AR.D-FORIJa.NG MACHINE

DEFIB?ZRATOR ....__- - - - . - - - - - - - I

M I X TANK

MACHINE CrnST

VACUUM CHARGING LOADEX MACHINE -

HOT PRESS

UALOtlDING CONXCNUOUS MACHINE HUlJlIDIFIEB

Figure 1. Continuous We t-f ormed Hardboard Process .

Slab mod or other wood residue i s f i r s t chipped and screened t o about

S/8-inch or 3/4-inch chip size. This s lab mod is usually bark-free, but not

always, as sat isfactory board can be made ( a t l eas t with some species) with

the bark included. The chips are then steamed, ei ther i n a cooker before passing

through a defiberator o r viithin the defiberator i t s e l f . Tiber is then blom or

otherwise conveyed t o a cyclone or similar uni t where water is added, bringing the

s lurry t o about 8 per cent consistency before going t o the refirier.

. From the ref iner the f ibe r i s pumped t o stock chests where rapid circulatio:

keeps the f ibers i n suspension. A consistency rekvlator reduces the stock t o

about 2 per cent consistency, and as the s lurry is pumped t o a mixing tank white

water from the ~ o u r d r i n i e r machine is added t o bring the f iber content t o about

1 per cent. Additivas, such as resins and sizes, usually are added t o the f iber

i n the nixing tank, although one company has made a sa t i s fac to r j hardboard without

the addition of chemicals.

The s lurry is pumped t o the head box of a Fourdrinier board-making machine

which i s equipped with necessary vacuum sections and pre-pess ro l l s . The formed

wet mat is cut in to 16-foot, or shorter, lengths by a flying cut-off saw and

conveyed t o a vacuum transfer mhich l i f t s the At mat and places it on a screen-

todped caul. The niat and caul then a r e transferred t o a press charger. Pressing

time i s dependent upon several factors such as thickness, temperature and additive

After pressing, the boards are discharged from the press, and the cauls and

screens are removed. The boards as they come from the press a re 2t close t o an

oven-dry condition and must be placed i n a humidifier t o bring the average moistur.

content up ' to t h a t which w i l l be encountered i n use.

The en t i r e process usually is continuous, and requires only four o r f ive

men t o operate the production machinery.

A variat ion of the conventional net-process i s the "wet-batch" process.

This nethod is much the same as the continuous wet process; however, the mats are

formed individually i n a Ceckel box, rather than vdith a Fourdrinier machine.

To make a snooth-two-sides board, and stay within the l imi ts of practica-

b i l i ty , the f i b e r should have a moisture content before pressing of less than 10

per cant. A t t h i s lroisture content it is not necessary t o use a screen back t o

allow moisture t o leave the board in the form of steam during the pressing cycle.

A smooth-two-sides board can be made by the wet process i f the formed wet mat is

placed i n a continuous dr ier and dried t o a moistme content below 10 per cent

belore pressing. This metnod has been used, and i s generally called the

"wet-dry1' process.

Dq-formed processes

The semi-dry process, as shorn in Figure 2, has much the same equipment as

the w e t process un t i l t h e actual f e l t ing of the board mat. I n t h i s process the

wax or s i z e i s sometimes applied t o chips as they enter the cooker, before being

converted t o f iber . The cooking operation is usually done in a continuous steam

cooker (5-10 minutes a t 20-100 psi) . Steamed chips are fed t o a double-disc

rotating a t t r i t i o n m i l l and are defiberized without the addition of water.

Pick-up fans blow the f i b e r t o a mixer or blender where res ins are sprayed

onto the f iber . Fiber leaving the mixer is picked up i n a heated a i r stream which

delivers it t o t h e f e l t e r box at about 10 t o 40 per cent moisture content (oven-dr

basis), The f e l t e r box i s designed t o sif t the f i b e r onto a t ravel ing bel t , in a

manner similiar t o a heavy snowfall. A continuous mat is formed which varies from . . around 10 t o 12 inches i n thickness f o r a *-inch board, After leaving the f e l t e r

the f i b e r mat is t r ea ted i n much the same manner as i n the w e t process. It is pre

pressed, cut i n t o 16-foot, o r shorter, lengths, vacuum-transferred onto cauls and

loaded i n t o a multi-opening hot press. After leaving the press the two processes

are again identical.

; REJECT

PLA'E SURGE yARI ABLE-SPEED SCREW FUDER

WAX STi)RAGE

CAUL R!3TURN 1 I

Figure 2. Continuous Dry-formed Hardboard Process.

The semi-dry process has the advantages of greater recovary of f ibe r from a

given volume of chips, less pressing t ihe, a smooth-two-sides board i f so desired,

somewhat simpler and l e s s expensive machinery, and does not require huge volumes o:

water. This process does, however, require lnrger percentages of resin b i ~ d e r

t o obtain a given strength.

The dry-process method of hardboard manufacture i s the same as the semi-dry:

with the exception tha t the f ibe r is dried t o from 5 t o 10 per cent moisture

content before pressing. This process, l i ke t h e wet-dry process, does not require

. a screened baclc because of the low moisture content of the f i b e r as it enters the

press.

IhTESTIGATION OF SECONDARY SPECIES

The Laboratory has completed the f i r s t in a ser ies of studies on t h e

s u i t a b i l i t y of some of our western species of wood f o r the manufacture of semi-dry

process hardboard. Effor ts are being concentrated on so-called secondary species

because of the diminishing supplies of major sawtimber species i n some areas. The

semi-dry process was used i n t h i s study because of the apparent advantages of

lower cap i t a l investment, greatly reduced use of water and l o m r operating costs.

Also, since most present plants use the wet process, investigations of species

s u i t a b i l i t y f o r t h i s method are more ' l ikely t o be undertaken by operating companic

species t o be investigated are lodgepole pine, Sierra juniper, ponderosa

pine, western hemlock, white f i r and red alder. Of these, lodgepole pine has been

: studied (complete report available upon request), and juniper and hemlock are

being investigated at t h e present time. -An estimated plant cost analysis by the . -

: Laboratory f o r lodgepole pine semi-dry process hardboard w i l l be available soon.

Lodgepole Pine

Lodgepole pine stands i n Ore3on include some 620 million fbm of saw-timber

Since only about 1/5 of tile t rees i n mature stands reach saw-timber size, the

t o t a l volume would be several times t h i s amount. Lucli of t h i s timber is

ccncentrated i n Klamath, Deschutes and Lake counties.

Preparation of material

Four-foot bol ts of lodgepole pine were chipped and screened t o obtain uni-

form 5/8-inch chips. The chips were then cook3d for 1/2 hour under 25 pounds

steam pl'cssure (a p re lh ina ry investigation showed that f ibe r from steamed chips

produced stronger boards than did f i b e r from unsteamed chips). After steaming,

the chips were ground i n a 2binch double-disk a t t r i t i o n m i l l a t a p la te se t t ing

of 25 m i l s . The ground f ib? r was then allowed t o dry prior t o formation and

pressing of the h d i v i d u a l mats.

Preparation of the boards

The calculated percentages of additives (resins and waxes) were added as a

spray while the f i b e r was being agitated in a propeller-type mixer. The prepared

f i b e r was weighed, and fe l t ed in to 12- by 12-inch mats. These mats then were

pressed i n a 24- by 24-inch steam-heated press a t ei ther 320 deg F or 400 deg F

with a ten-mimte press cycle a t an ini t ia l pressure of 1,000 psi. The i n i t i a l

pressure was maint.rrined f o r bS seconds, a f t e r which time the pressure was releasec

t o 100 p s i f o r the remainder of the press cycle. The pressed boards then were cut

in to test samples and placed i n a constant humidity room u n t i l tested.

Testin2 the Soards

A l l the boards were tes ted f o r modulus of rupture, water absorption, specif

gravity and moisture content values.

The modulus of ru2ture t e s t s were performed i n accordance with the Federal

HardSoard Specification ILL-F-311, except tha t the length of the sample was 11

inches (instead of 12 inches) m d the breaking 'loads werz read t o the nearest 0.5

pound instead of the nearest f ive pounds.

After the modulus of rupture t e s t s , a 3- by 3-inch water-absorption sample

was cut from each broken bending suaple. The samples vere immersed for 24 hours

under 1 inch of wafer maintained a t 70 deg F.

Specific gravity values viere obtained by mercury immersion of 1- by 3-inch

coupons cut from the bendling samples.

knalysis & discussion - of resul t s

I n order t o detect important differences i n board properties caused by

various treatments, the t e s t data .;rere subject& t o the 'comnon s t a t h t i c a l method

of analysis of vzriance. Only . t e s t . values f o r modulus .of rupture, per cent water

absorption and speci f ic gravity were analyzed because of time limitations.

The work presented represents the r e su l t s of tes t ing 14 screen-backed

boards, one-quarter inch i n thickness.

From an analysis of the f i r s t phase of t h i s study it was found t h a t by the

addition of 2-1/2 per cent of phenolic res in a sat isfactory board with regards t o

modulus of ru9ture could be made. Water-absorption values w2re well above

acceptable limits and because of th is , the remainder of the study was an attempt

t o improve water resistance. The improvement of water resistance was accomplishe

by using various s i zes i n conjunction with a 50-50 mixture of lN sulp!luric acid

and lN a l u m solution f o r pH control ( 4 . e . O pH).

The ef fec t of pH control on water resistance of the boards is shown in

~ a b l e s ' - 1 and 2. The analysis of water-ahorption values revealed tha t pH control

appreciably increased t h e water resistance of al l boards, regardless of the

additives or press temperatures used. Table 2 shows t%at tine additive conbinatic

affected the degree of improvement caused by pH control. The increase i n press

Table 1. Average Test Values Obtained from Lodgepole Pine Hardboard, a t Two Temperatures; Y'lith and Mthout pH Control.

Additive treatment

Control

Y e - a t u r e

8680 38.2

6630 18.8

5640 15 -3

5830 17.5

6170 22.7

6390 43 -4

4730 25.9

320U F

I

400° No pH Control

h~odulus liat e r of Absorption

Rupture

No pH Contol Modulus Water

of Absorption Rupture

P s i

es in (2-1/2%) I 8500 63 a 4

-

F pH Control

iviodulus hater of Absorptior

Rupture

pH Control Modulus Water

of Absorption Rupture

7280 51.8 1 8620 54.5 I

Per cent --

esin+1=1/2%ofVIIaxA

Resin + 1-1/2% of Wax B i I

pesin + 1-112% of Wax C I

Ps i - Per cent -- P s i - Per cent -- Psi - Per cent --

6820 32.8 1 6430 47.7 23*6 1 8130

59b0 I u 2 0 57.0 1 6350 53.2 1 7160

4180 72.2 3090 257 05

6950 59 03

fiesin + Rosin A 41.4

2900 251.5 3590 91.5

5790 21.5 41.0

5230 27.4 j 7590 35.5

/,sin + ,sin B 1 zz:: 61.9 542 0 4 5 3 1 7340 53.6 I resin + Rosin i 6610 73.5 I 4020 44.3 I 7b50 54.8

I .

f

Additive Decrease i n water treatment absorption percentages

kTax B - Wax emulsion (non-precipitating type) 38 m3

- Sol id t.ype added t o chips

pax A - Wax emulsion (precipi ta t ing type)

pes in - Phenolic

I osin A - Powder

os in B - Pas te

Control 12.6

temperature from 320 deg F to 400 deg F decreased water absorption fo r some, but

not a l l , of the additive treatments.

In summary, the following relationships were established from the analysis

of xater-absorption values:

a. Control of pH i s an important factor i n the,improvement of

water resistance regardless of additive treatment or press

temperature used i n t h i s study.

b: An increase of press temperature may or may not improve

water resistance depending on the part icular additives

concerned.

Ea'odulus of Rupture

A l l of t h e major variables had a significant general e f fec t on the modulus

of rupture values of the boards. The addition of supplementary additives reduced

the strength of boards compared t o those made with res in only (Table 1).

In general, pH control lowered the strength of the boards. Increased press

temperature generally improved the modulus of rupture.

Conclusions ~econnnendatj ons

. , Lodgepole pine chips which had been steaned prior t o grinding produced

. a f ibz r vdlich made stronger boards than did f i b e r ground from raw chips.

Furthermore, the steamed f iber appeared t o make more e f f i c i en t use of a wax

emulsion added t o improve water resistance.

From the water-resistance study on s t e a e d f iber , it was apparer-t t ha t . .

pH control of the fiber-additive mixture had a defini tely beneficial effect on

water absorption, but usually reduced the strength of the boards, depending on

the additives used (Table 1).

An increase of press temperature from 320 deg F t o 400 deg F generally

improved board strength, but any increase i n water resistance was dependent on

the additives used.

Although resu l t s of laboratory-scale experiments cannot be d i rec t ly

t ranslated in to expected resul t s from a commercial-size operation without

fur ther study, the resul t s presented here show tha t lodgepole pine can be a

promising new source of raw material f o r dry-formed hardboard.

A more complete report t i t l e d "The Sui tabi l i ty of Lodgepole Pine fo r

Dry-formed Hardboard", can be obtained by writing f o r Report L-3, Oregon Forest

Products Laboratory, Corvallis, Oregon.

Western Hemlock

.Procedure f o r t h i s work was based on resul t s of the lkgepo le pine study.

The combination of additives which produced the most sat isfactory hardboard from

lodgepole pine was used as a s t a r t ing point f o r t h i s study. Results were

compared on the basis of what was known of lodgepcle pine Douglas-fir

fiberboard.. A summary of the hemlock t e s t data is shown i n Table 3. . -

Boards containing. 2-1/2 per cent of res in and 1-1/2 per cent of a vrax

emulsion pressed a t 400 deg F with controlled pH m e t the proposed commercial and

federa l specifications f o r l/l-inch standard smooth-one-side hardboard.

Strength values of hemlock were found t o be superior to those of both lodgepole

pine and Douglas-fir boards. Boards pressed with the same additives without pH

control a t 4OO deg F or with pH control a t 320 deg F came very close t o meeting

the standards, considering the small s ize of t h e water-absorption samples.

S ier ra Juniper

It was suggested t h a t juniper chips from which extractives had been remwed

mi&t be a sui table material f o r the manufacture of h.ardboard. A s ,a resul t , a

short-term investigation of juniper f i b e r f o r hzrdboard making was in i t ia ted .

Juniper chips, screened t o a uniform S/8-inch size, were f iber ized hot

i n the Bauer m i l l a f t e r steam d i s t i l l a t i o n for volat i le o i l recovery was

completed ( 5 t o 8 hours a t e i ther 267 deg F or 3QO deg F). It was discovered

that a wider s l a t e clearance was required t o produce a f i b e r comparable with

f i b e r from other species. Three different plate sett ings were used during the

grinding of the juniper chips, each batch being segregated.

Boards of juniper f i b e r were made i n the sane manner as previously with

lo$~epo le pine and hedock boards. The same combinations of additives t h a t

proved most successful with lodgepole pine vrere used with juniper. i90 pH

control vms attempted on t h i s f iber . The boards vrere tes ted i n an ident ica l

manner t o those of lodgepole pine and hemlock (Table 4).

A l l , of the .juniper boards regardless of pressing temperature used were

characterized by a, glossy, uniformly colored surface on the smooth side. Because

of the extended steaming times, somewflat more water solubles than previously

experienced with other species were d.eposited on the screen back. I n commercial

operation, it would be desirable t o brush off t h i s ~ a t e r i a l because it i s s t icky

t o the touch.

A comparison of the modulus of rupture values revealed t h a t the juniper

boards were somewhat stronger than those from lodgepole pine, but weaker than

hemlock boards, when pressed under the s m e conditions. The water resistance d

juniper boards was superior t o tha t of e i the r lodgepole pine or hemlock boards.

There were no s ignif icant differences ' betrieen grinds, although three p la te

spacings were used i n order t o wind f i b e r similar t o tha t from lodgepole pine . -

and n~,s+vern hemlock.

Increased press temperatures generaqv caused an increase i n bending

s t r e n s h and water resistance i n juniper boards, as with boards from the

other two s ~ e c i e s studied.

Table 3. S ~ ~ ~ ' s l a r y of Results from Testing of Hardboards from Western Hemlock Fiber.

PH of Water Specif ic PH Additives Rupture absorption* gravity a t pre s s iq

psi Per cent -- No 2$% Resin 7,770 57.5 0.96 7.9 Yes 2 9 Resin 7,580 47-7 0.97 3.8

No 2&% Resin 8,930 43.3 1.00 7.9 Yes 2g% Resin 7,610 25.9 1.01 3.7

32 0 No 2$% Resin + 1$$ W a x 6,800 24.0 0. 98 7.9 320 Yes 2 s Resin + 12% W a x 8,380 18.5 1 04 4.2

No 228 Resin + 1% W a x 9,UO 18.4 1.03 7 .9 Yes 2&.Resin + 1& Wax 0,510 15.0 1.02 4.0

* least signif icant difference i n water absorption values: 3.5

Table b. Summary of Results from Testing of Hardboards from Sier ra Juniper FiSer .

Press Grindinq run temperature Additives Property la Zb 3G

Degrees F

320 None (Control)

400 None (Control)

Modulus of rupture; - ps i Plater absorption; per cent -- Specif ic gravity pH

Modulus of rupturej p s i - Water absorption, per cent -- Specific gravity pH

Mociulus of rupture; - p s i Water absorption; per -- cent Specif ic gravity

Modulus of rupture; psi Water absorption; per cent - Specif ic gravity PH

a At t r i t ion xi11 pla te clearance: 23 mils. At t r i t ion m i l l p la t e clearance: 35 mils. At t r i t ion m i l l p la te clearance: bO m i l s .

The inclusion of r e s in and wax signif icant ly increased the bending strength

of the boards regardless of the f i b e r or press temperatur= used. In addition,

res in and wax effected a greater strength increase where the boards were

pressed a t the lower press temperature (320 deg F).

The juniper boards a l so were noticeably more uniform i n specific gravity

than bo- from other species pressed under the same conditions. This demonstratc

tha t t h i s f i b e r possesses superior flaw characteristics.

' .In conclusion, t h i s preliminary evaluation shows c lear ly tha t extracted

juniper chips could be a promising source of raw material f o r hardboard. A more

detailed study i s now underway on the use of juniper f o r hardboard.