INDUSTRIAL AND ENGINEERING CHEMISTRY
1288
sulted from thc gradual browning of the paper before tracing. light handling of the paper produced fingerprinting. SOLCTICIX 2. A modified formula permitted the chemicals tri be introduced on the paper in one dip and produced a papcr rvsistant to fingerprinting which darkened much less than did paper 1. A quantity of very successful papers has been produced by this formula, but the use of a warm impregnating solution introduced difficulties in quantity production. SOLUTIOS3. Studies of the behavior of benzidine papers revealed that, if the pH of the impregnating solution was lovercd, a paper was produced which darkened only slightly or not at all. .It the same time the permanence and sensitivity of the trace mere decreased. By placing the reagent on the paper in two dips and by lowering the pH of the second dip, it was found that a paper of superior stability could be produced without sacrificing sensitivity or permanence of trace, provided the pH of the second dip did not fall below 5.5. Efforts to reduce the brown discoloration by the addition of ant'ioxidants as suggested by Wagner (9) either aflected the st'ability or sensitivity adversely or had no effect. These papers darkened only to a light buff color after continuous storage at 40" C. There is no fading of the trace when the paper is allowed to dry out after tracing. SOLUTION 4. Papers made with o-t'olidine mere the most sensitive and the least stable. SOLUTION 5 . The p-anisidine papers were easier to prepare because the reagent dissolved in water. If the papers were used within two weeks of preparation, discoloration waa no problem. SOLUTION 6. Above 1.4 volts there was an electrolytic decomposition of the bromide, and the free bromine formed eosin dye, which gave a permanent trace. SOLUTION 7. The traces on paper impregnated with pyrogallol appeared on both sides of the paper. I n all previous examples t,he trace appeared only on the side touched by the anodic stylus. The authors' explanation was that, in addit,ion to anodic oxidation, there was a pH increase a t the cathode side of the paptir. which was followed by air oxidation of the pyrogallol.
.
Vol. 39, No. 10
SOLUTIOV 8. In the tellurium papers the stylus wa9 made c,Athode so that the trace consisted of reduced tellurium. There I\ ds no background discoloration; consequently an excellent contrast was obtained. I t was not found possible to increase t tic sensitivity by thr addition of further reagents. CONCLUSIONS
Several solutio~isfor making electrolytic recording paper auu [lie method of impregnation arc described. One of these, containing benzidine as the sensitizing ingredient, has been the subject of more intense study. Two recipes are given for using thip reagent to prepare a recorder paper of good sensitivity and st>a. hility. ACKNOWLEDGMENT
The authors wish to express their appreciation for the cooperation and assistance of the members of the Technical Department of thc E. €3. Eddy Company. LITERATURE CITED
( 1 ) Electrical Engineers' Handbook, ed. by Pender and DelMar Vol. 11, sect. 14, p. 18, New York, John Wiley & Sons, Inc. 1936.
Fagan, C. P., J . Sci. Instruments, 19, 184 (1942). Glas, Emil, Austrian Patent 135,822 (Dee. 11, 1933). (4) Hogan, Ressler. and Tribble, U. S. Patent 2,339!267 (Jan. L X
(2) (3)
1944).
(5) Schmidt, R., Ibkd., 1,918,199 (July 18, 1933). (6) Solomon, M., Ibid., 9,306,471 (Dec. 2 9 , 1943). (7) Talmey, P., I b i d . , 2,318,013 (April 25. 1942). ( 8 ) Zhid.,2,319,766 (May 18, 1 9 3 7 ) . (9! Wagner, E. R., U. S.Patent 2,358,839 (Sept. 26, 1944).
Kinetics of Cure of Resol Resins hIANUEL N. FINEhIAS' AND IRA E. PUDDINGTON iVational Research Laboratories, Ottawa, Canada
'rhe rate and extent of cross bonding in phenol-formaldehyde casting resins of varying composition are determined by measuring changes in their electrical resistance and density during the process of cure. The electrical measurements are apparently sensitive to changes in the internal molecular arrangement of these systems while the density determinations are mainly characteristic of their macro properties. The increased rate of cure due to accelerators and retarding effect of plasticizers are indicated experimentally by the results obtained. The procedures are useful for studying the kinetics of cure of these complex systems.
D
URING the past few years investigations of the kinetics of
polymerization and polycondensation have dealt mainlv with linear or partially branched high-polymer systems. For these the experimental procedure has usually involved viscosity or osmotic pressure determinations. However, experiments of this type are not feasible in the case of'highly crosslinked phenolics of the casting resin type where polycondensation continues even after the material has gelled and finally become insoluble. For cross-linked polymers, methods of determining degree of cure have usually been very arbitrary, including tests of hardness, tensile or impact strength, water absorption, surface resistivity, etc. One semiquantitative procedure involves grinding the solid polymer to a powder and extracting whatever soluble material 1
Present address, Stanford University, Calif.
may be present with a suitable solvent, usually acetone ( 2 ) . .I variation on this principle was reported by Barkhuff and Carsre11 (1). They studied the extent of cure of thermosetting resins by determining the time required for polished cured resin blocks to show a characteristic microscopic pattern when treated with acetone. By this method they were able to report the varying effects of accelerator concentration, resin composition, and temperature of reaction on the degree of cureof these materials. However they did not believe that the patterns thup obtained furnished any direct evidence as to the molecular structure of the cured resin. A comprehensive study of the hardening of cast phenolics, or resol resins, was reported by Medvedkov and Polyatskina (16) who investigated variations in F d n e s s , specific gravity, solubility, and coefficients of light absorption for resols curing a t 80" and 90" C. Each of these physical measurements for characterizing the degree of cure of a cast resin showed potentialities Fhich warranted further investigation. Fuoss and co-workers (7) demonstrated that a better understanding of the molecular arrangement in complicated highpolymer systems could be obtained by studying the properties of these materials in an oscillating electrical field. Variations i n dipole concentration and strength, relaxation time, and plastiriaer concentration gave variable dielectric constant and power factor curves with changing temperature and frequency. Though useful for linear polymers, this procedure too was soon shown to be inapplicable in the case of cross-linked materials such as vulcanized rubber (19) or cured phenolics (9). For the latter neither dielectric constant nor power factor is very sensitive to tempera-
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1947 ture or frequency. However, these studies vere made on the fully hardened or cur& resin, and none were attempted on the material at intermediate stages during the process of cure. X-ray examination of rherniosetting materials is also of little use ( 1 7 ) since amorphous patterns, caused by the presence of cross linkages, are a l ~ a y sobtained. Nevertheless, variations in these diffuse rings might make it possible to characterize wi,tain kinds of polymers and even to providr some indication as to the expent of cross bonding in them. Since the arbitrary procedures used at present for determining the degree of cure Jf cross-bonded materials are not exact, it was decided to investigate the possibilities of 8 procedure similar to that outlined h j - Iiienle and Rae(. (II), iu an attempt to establish the curing cycle and also, possibly, to elucidate w m e of t h e molecular changes
I
io6 In
50 10' z io4 V
f
5
Figure 2.
c
D E
IO'
W
cc lo'
5
15
IO
TIME
IN
20
25
DAYS
Figure 1. Change of Electrical Resistance w i t h Time of Curitig in Bath a t P O c A , resin alone; B , with 8% added amelerator) C, with 10% added plasticizer
1 0
5
Curve A B
1289
15 TIME IN
20
25
DAYS
Change of Electrical Resistance with Time of Curing
% Accelerator Added to Resin 11
10 9 8
Yone
pH of Sample 1 5
2.9 3.3 4.2 5 1
Weight, Grams
87.721 87.720 87,709 87.718 87.716
in these comples systems. Possible correlation between results so obtained and those found by accepted procedures xould make such work desirable. Iiien le and Itace ( 1 1 \ investigated the formation, gelation, and CUIY of alkyd resins by folloning changes in their electrical resistance with time and interpreted their data in terms of per cent esterification and also extent of cross bonding. Bincc, on curing, phenolFormaldehyde casting resins c.hange from liquids of low viscosity to gels and finall! tleconie solicis, it seemed reasonable t o sup~iosethat the [nobility of ions present in such systems would decrease progressively as the resin curcd. Consequently, the electrical resistance of the material should increase over ti wide range, as the viscosity increased, until the material. as a solid, became a good dielectric. This paper reports the changesin electricalresistance of cast resins of varying composition rrhen cured a t 80" C. in the presence of sccelerator and plasticizer.
1290
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 39, No. 10
iiected tht, ( Y ~ I I1 I) t tic) alterriating curriLnt hritigc. Seither resin iior accelerator had any*chrniicaa! effect o n the clean copper surface. Th(, (Y,II.were placed i n individual glass containers ant1 immersed i n an oil bath which was main,025 tained at a constant temperature of 80" (' I ! ( + 0.1 C ) . r' 0 RESISTAS(T1 3 m v ; h : . I n the early part of tht, investigation an altcrnating current ratio bridge. z ,020 {vas consructed, using a 1-kilocycle (oscillator;, > ratio arms of 1 kilohm each, and variable precmi$ion residorb and capacitors in parallel t o balancv ! i thc resistance and capacity of the unknown. z ,015 Measurements were made by substitution and imphones n'm u s d as detectors. Subsequently, LL N General Radio rapacitance bridge, Type 716-1'1 0 a0 @), was acquired which, a t 1 kilocycle, was dircw m reading in capacitancc and dissipation factor f o r Iz valurs of the latter Icss than 55cG. From di+ w .ipation factor data, the equivalent parallrhl ret.005 of the wsins could be readily calculatcd E ipation factor values greater than V an extrrnal-variablr precision resistor, having a z I I range from 0 to 1 megohm, was used to balancc, the crll. For maximum sensitivity the earphones w r e subsequently replaced by a cat tioil(, 2 4 6 8 IO 12 14 ray oscilloscope as t h TlME IN DAYS DILATO~IETERS. C1 F i g u r e 3. C h a n g e of Density w i t h Time of C u r i n g structed using mercury as the displacement liquid, and a horizontal capillary 1 mm. in dianlD,resin alonr; E. with 10% added plasticizeri c, B, and 4. with 8. 9, and 10% ,,f accelerator added, respectively eter and about 100 cm. lone: for measuring changes in volume as the &sin cured. h(.curately mighed amounts of the sample (about 20 grams) and mercury (about 180 grams \I vi't, intloducwl in the dilatomrters with the exclusion of any air Changes in t,he density of thew niatt:rialn on curing \vclre also p i i c k c t i . The dilatometers wrre kept in a constant temperature investigated and arr shown t o hear a rlose rescnihlanrc to t h ( , oil bath at 80" C. ( + 0.1'1, anti rhanges in the position of thc. resistance data. ineniar~isin thi, r*:tpillar,v \\-(srcs mc.asurrd to f 0.5 mm. From Recently tv-o E:uropean papers, inac ihkduringthe c ~ ) u r a i ~ o f l h i v readiiigs it \va* pohsihlc t o c~xlculati~ dcnsity chxnR(5- avc~llrarf~ly to + 0.001 gr:lnl per r r , this m r k , have been made available. One of them (14) reports comparisons of the electrical conduct,ivity of phcnolics n-ith viscosity and refractive indcx changes. I n t,he other (1.5) ,\Ian?HESIRS OF \IOLbH KATIO 1:2 , gold and Pet,zoldt follow the coursc of the condensation reaetioii Figure. I * 1 1 0 w tht. prt~liininai~y requ1t.e obtained ivith a (YJIIIin acid and alkaline media by studying changes in conductivity. iiitwial pht,iiol-foi,inHli~(,liy(it~ i w t ing rwin of approsimatc,l?- 1 : 2 The work reported here confirms their rcsults for curiiig in the molar ratio. Tht. initial rapid drop common to the t h r w (YII presence of an acid accelerator and extends the data over a ividcr Iyresc~ntst h e drop in resiataiice a:: t f i ( ~Yamples warmed up front range of accelerator concrlntratioii. Both of thew papers ixiphiii'oom tcanipc.raturc to th(8 tt~nipt~raturr~ of the bath. Thpn th(art. sized the potential importance of cbonductivity nicasuremcnts iii l o l l o ~ v c ~ad+low.ris(1in rf o v ~ ar period of about onv 01' twci helping to elucidate thr, reaction mrchanisni for these sj-etmis. i l a ) ~ . Thir is especiall. d in the cabe of the samplt. w ) i i APPARATUS tainiiia p1astickt.r. The samples then became turbid and finall> r)p:tquc'. Thc rtraight lin& near thc minima of the curvw indiResins having pheriol:formaldcPREPARATIOS OF S.mPLEs. (+at('tiit, pciints at which turbidity rvas observed for each sample hyde molar ratios of 1:0.8, 1: 1, 1: 1.2, 1: 1.5, and 1: 2 were prepared by condensing the proper amounts of liquefied phenol and Following this, t h e reiistanw incw rapidly with tinie ao t h c . formaldehyde with a small amount of sodium carbonate catalyst of curt' continut- anti finally reaches a constant valut. in a flask fitted with a reflux condenser. For examplc, for a i t t i ( , i i iuririg is i~iimplrte. I k p r a t experiments duplicated thew resin of ratio 1: 1, 478 grams of 887, phenol (Merck) \veri' coili.(,..ultc c l o w l ~ . The irrt.gu1aritic.u in measuring resistance in the densed with 300 grams of 37% formaldehyde (Merck, reagent neutral) with the addit,ion of 4-5 grams of sodium carbonate disriic~golinirang> ivere t.liniinated in the later experiments tjy rt*solved in 40 grams of water. The temperature was maintained 1)Iacirig th(3 hridge first used by the General Radio bridge. a t 90-95' C. for 2.5 hours with constant stirring. .4fter neutralThp threr curves of Figure I .show markedly the acceleratiiig izing the solution with lactic acid, it was dehydrated in a vacuum 1,ffrct of a ratalyst anti the retarding influence of the p l a s t i c k r oven a t 80" C. for 10-20 hours t o a refractive index reading of about 1.580 a t 25" C. The water content of the resins, checked o n the ratc of cure of the casting resin. The general shape of by the method of Feith (4),LYas constant a t about 14% by volume i'urvt' B arid t h e point a t which turbidity sets in agree closely with for all samples. These resins were then stored at low temperature thc r i 4 t s of llanegold and Petzoldt ( 1 5 ) ;who plotted conducto retard the process of cure, but were usually investigated in thv rivity against time for a casting resin (wring in the presence of a n following experiments within a week after preparation. In making up mixtures sf resin with accelerator or plasticizer. arid catalyst a t 90" C. weighed amounts were carefully stirred together, air bubbles T o rontirm thcse results with n i o r t ~accurate data, five sanipler: were removed by vacuum, and an accurately weighed amount ,)i' molar ratio 1:2 and of (qual weight, containing varying of the resulting solution was poured into the conductivity cellb amounts of accrlerator, were made up and placed in cells of and the dilatometers for the measurements. Glycerol was used as plasticizer, and the accelerator \vas a substituted sulfonic acid. identical size, as previously tiwcrihed. Figure 2 shows t h r varisCONDUCTIVITY CELLS. The cells in lvhich the resistance tion in electrical behavior otwrvrrl for these samples, as r i r l l NC: measurements were made were constructed of concentric copper t h(bir composition. tubing, '/I6 inch in wall thickness and 6 inches in length. The The initial slopes of these curves increase rapidly as the coIlcc'I1inner electrode, 3/4 inch in diameter, was shielded by the outer one, 11/2 inches in diameter, and supported in the center of the tration of accelerator is inrreased. The extreme case of a rwin latter by means of a Bakelite ring, 3 1 8 inch thick, cemented in rontaining a w r y high eoncnont,ration of accelerator is shown in the annular space at the bottom between the two cylinders. A wrve 9,where the sample hardeni. almost instantaneously and weighed amount of resin was poured into this annular space. reaches a resistance, valw tvhich rrmains constant ov(1r a proTwo copper leads, soldered t80the open ends of these tubrs, ron-
d
I
I
1
I
x
5
.
October 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ioiiged period. This behavior will be discussed niorc fully later. All of th(, nther curves shol7 final re4stance values of about 5 megohms. Curve D rises rapidly t,o this value after ahout 35 days. Khile measuring the '.hang(, in resistance anti dii.sipation factor of thew tiiatcrials, capacity changes wr(1 alPo carefully noted and generally were found t o follon- the same pattern -1plot of effective capacity in riiicroIiiicrofarads againsl :ime of cure, t , could bc approximated by nieaiis of ail *qiiatioiiof t h e form, C'rt,
= Cm
+ Alt
ir-hcre C, is the capacity oi r h t , completely cured resin. a n d -1is a constant. How-
w ' r , the capacitance read:iigs ntire not so accurate ab.
I291
10'
v, IO6
2
I 0
Z 10'
-
w
?Q
io4
53 W
CL: IO'
I0 '
rt~rcsting in denoting variatiniis with time of cure oi :rithchangcsiriconipoi;itioi~. T o coinpare the electrical iitthavior of these material. 5 IO 15 20 25 $3-ith changcs in density ;I.< TIME IN DAYS i h c a , v rurf,, some of thesi Figure 1. (:hangr i r i Electrical Resistance w i t h Time of Curing for R e s i n s of Variable w i i p k s \yere investigated Phenol-Formaldeh? de \Iolar Ratio in dilatonietcrs with results 1. repin of ratio 1:l.z: 11. 1 : l . z ; C,1 : l : D and E , 1:0.8 at pH of 4 and 8 , respectitely LLS zliown in Figurr 3. ('urve I ) represents the don. iiicr(Lase in dcnsitJ- as tht UISCLSSIOV OF 1 : Z MOLAR RATIO ItESISS resin alone cures. The slopc ( i t r h r . ( l t ~ i i . . i t ~r~i i t ~ r oi. t . \ . c ~ i ~It-* \ \ l l t j i l IOf; plasticizer is added to thtx rt+iii ( r u r v ~A. ' ) , Tlic. initial c I o ~ ) I , igt' of currttut through these samples niay iiic~ascjnrapidly as increasing amounts (8, 9, a n d loc; j of a?to ticpend upon the frec mobility of any ions which are present i n t.c~It~rstor are added to t h r reGin. The, hchavioi. of rui'vt~.I i. -4s thc. sample currs, cross bonds are set up and the. eFpccially interesting since it &plays a marked dccwast. in tlcriof thr: casting resin, or resol, is soon replaced by a .ity follon-ing the initial rapid incrcww. Curves 1, B , C, and I ) three-dimensiunal solid structure, or resite. I n such a structure oi Figurcx 3 represent thr dcnsity hiahavior of tht. samples who>(* the mobility of the ion> will be restricted b - steric factors and will qslc*ctricalbehavior is de.sritwtl ill Figure 2 by curve. R. f', 11, a n ~ l clecwase continuously as the number of cross bonds in the system /ity changta- 01' $1>aliiincrease. Thus the ratc and t)stent of crow bonding, which repreill(, mntaining llmc accelerator was attemptrd, but the matt'rial sents the rate and degrcv of curing of these matci,ials, can be comcured too rapidly to give accurate results in tht, clilatoinc~tc~r. pured by measuring the changes in the r.lecatrica1 resistauct, of tht, Qualitatively, at least, density measur~:ments a g i w nith electriw n p l e s as they cure. cal data in describing variations in the rate of cure of thrst. Hearing this mechanism in mind, the effcct of added plasticizcr inaterials with varying amounts of ac.crllrrator and plasticizer. ilia>-be explained as follow: I n the case of linear polymers, plasTo ensure that the electrical contacts in the cylindrical cell did ticizers act, as lubricants between neighboring chains and thus not, deteriorate as a result of shrinkage of the specinirln during prevent them from coming too close to one anot1it.r. Similarly wring, a sample electrical determination was made using two in the curing of a t,hree-dimensional polymer, thi. function of the probe electrodes immersed in the rrsin. 111 this case shrinkagt: plasticizer must be to prevent the linear chains from approaching could only improve the coiltact;;. The results observed wer(' one another and hence retard the formation of cross bonds. analogous to those obtained whrri the cylindrical type of crll prt.Thercfore, the mobi1it.y of the ions will not be restricted quite so viously described was employed. much, and the electrical resistance will increase inuch less rapidly To investigate briefly the effect, of teniperature 011 the t,lectrical than if no plasticizer were present. Furthermort:, the final resistbehavior of these resins, two identical samples were cured a t 80' ance value attained by a fully c u r d resite cont:iiriing plasticizer and 90" C., respectively. The resistance curves obtained werca appears to be less than that of an unplasticized sample. essentially similarto that shown by curve A , Figure 1, although I n contrast with the effect of a plasticizer, the addition of an the slope of the resin curing at the higher temperature increased accelerator must activate the molecules in a way that speeds up much more rapidly during the first few days of cure. Since t h r the process of cross bonding. Figure 2 shows that all the Curves reaction is exothermic, curing with accelerator a t 90" C. or ahovt. (except A ) eventually become constant a t abou: the same value results in boiling inside the resin and very rapid hardening. of resistance. Therrforr it niay he deduced that, although the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1292
.04
.03
*
8
-02
.04
.OI
.03
K
5 z
.02
>
t
2 W
.GI
0
LL
0
2 .04
0
z
3 8 .03 W
z
.02
-01
0 0
4
8
12 TIME IN
16 DAYS
20 .
24
Figure 5 . Change of Density with Time of Curing for Resins of Variable Phenol-Formaldehyde Ratio with Added Accelerator arid Plasticizer H . E. and I I . resins alone; A , D. and G , with added arrrlerator:
and J . with added plasticizer
< F. ~
differing initial pH values of the systtciri IiiarI,ctllj. atYect tlie rate of cure of t,hese materials, they do not appreciahly'alter the final degree of cure. Therefore, the molecules must be able to orient theniselves t o permit the formation of the greatest number of cross bonds possible for the system. This ptocws of orientation evidently takes place gradually as more and more of thr crosc bonds are formed, and thus permit's complete curing. For sample -1,however, \There the concentration of accclerator is high, the format,ion of a rigid three-dimensional molecular struct,ure is probably so rapidly accomplished that the possibility of optimum orientation of the molecules to give complete cross bonding may be eliminated. Hence t,he electrical resistance remains approximately constant at a value much lower that that of the other samples. Thus it appears t,hat a factor which increases the,ratc of cure may not necessarily give rise to a higher degree of cure, as is commonly supposed (1). The behavior of sample A is almost duplicatcd tiy the sample containing 105% accelerator (curve B, Figurc 2). I n this case, although the rapid formation of a rigid structure temporarily retards cross bonding after the initial cure, the system must still be sufficient,ly plastic to permit further orientation of the molecules to take place and thus make further cross bonding possible. It has been generally accepted in industry that casting resins increase in density as they cure. One,may regard this phenome-
Vol. 39, No. IC
non as resulting from the fact that, with the setting up of cros> bids, the molecules become oriented more closely together in >pace a i d occupy a smaller volume. Thus an explanation is h n d for the behavior of curves B , C, D,and E of Figure 3: Tht plasticizer retards the rate of formation of cross bonds and thl'wcelerator speeds that rate. Curve '4, however, represents ar, inciiiialy n-hicli is more difficult to explain. \Itdvcdkov arid Polyatskina ( 1 6 ) ,who obtained a curve siniilar t o il ~ v h c nplotting the change in density of an alkaline-cata~ I y z e dresin with time, suggest that, after the sample has attained i t < masimuni viicosity and hardened, no further changes in the ?.sti:inal surface of tlic rwin are possible. Thus any further con! r:irtion which accompani(:s polycondensation must result in the (orniatiori of cavities inside the rebite. IIomvcr, the'formatior. of iiiteval pores gives ri.v to a reduction in density. Hence they ~ 7 ~ ~ i ~ rthat, l u d i Ivhenei-er ~ the hai,drning process is sufficiently rapid to give the resol a solid exterior relatively early in the curing ) J r o ( ' c s , any further curing rThich takes place within the solid structure must result in a nc:r decrease in density due to the for!iiation $ i f internal purL',q. Invrqtigations with an ultramicro- . ' Y I } J I . and :I ricphelometer, reported by the Russian authors, rew i l r d as many ae 100,000 p ~ i r ~p+ ( cc. ~ of resin st the end of !.:irdrnirig. I ion.r,ver, t11c formarion of H h a r d external surface should pre-iiniatJly preclude the possiblit- of detecting, by dilatometric !iiimi.., :lit, formation of internal pore?. Hence, although thesr : I ! J ~ ~may s indeed be present, riwy Laodd not be the cause of the mohservcd decrease in dcn>ity. -1 possible explanation for this j ~ h e n ~ ~ i n ~mal~ i i ohe n that, under, certain conditions, the resite,.:sude some of the Ion-molecular-~~cigllt products of condensatior, ~ u c has n.uter, and henre an app?rerit increase in total volume ii oliserved in the dilat,omcters. Soriiially these condensation iiroducts are di..persed colloidally ilirougliout the body of the wain arid givr rise to a turbid or opayuc sample ( 3 ) . Feith ( 5 ' wccritly wnfirnicd the depcndence of opacity on the R-ater conrent of tlic rt.sols and showed t!iat these become translucent a? plasticizcr concentration is increased. Where plasticizer is present to take up some of the water produced during condensation. it has l m n shon-n (16) that no reduction in density is obtained. rthelrss, most resites, whether plasticized or not, exude n.att.r oil curing. The samples here investigated in the closed ~lilatomet,crslost exuded products ranging from 0.lG to 2.0y0 by weight. I n the open cells in \i-hich the resistance measurements m r c made, the loss TTas somewhat higher, probably due to evapoIsition. This loss of m-ater does not account for the increase iu wsixtnrice obscrved since other polymeric systems, such as styrcnc (recently reported, 6 ) . exhihit similar behavior although no water is present. R E S I S S OF MOLAR RATIO LESS THAN 1:2
'
'l'he phenol-formaldehyde molar ratio of 1:2 is close t o the opti. mum for the formation of a completely cross-bonded three-dimcnsional solid. It is also possible to obtain three-dimensional solide by curing rcsols of molar ratio between 1:l and 1 :2. Homver. under acid conditions, resols of molar ratio 1:0.8 or less yield. instead, liuear polymers of the resitol type. According to existing theories ( 1 3 ) , one would suppose that in the latter case no cross bonds are set up between the linear polymers, and hence the three-dimensional structure is not obtained. Assuming that electrical resistance can be used to indicate ex1 ent of cross bonding, electrical measurements were made on resins of molar ratio less than 1:2. Results of these measurements confirm the usefulness of this procedure and even ext'end its applicability. Figure 4 s h o m the variation in electrical behavior on curing five samples of varying phenol-formaldehyde molar ratio without accelerators or plasticizers. Resols represented by curves A , B , and C were all acidic at a pH of 4.2, 4.6. and 4.1, respectively. Thev may be compared with rurve E.
October 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 2, which represents the behavior of a resin of ratio 1:2. The slight dip observed in curve E of Figure 2 after 10 days becomes progressively more marked in curves A and B of Figure 4 and, finally, for a low ratio of 1 : 1 results in a straight line, curve C. Resols of even loner molar ratio represented by curves D and E do not show any appreciable rise in resistance on curing. Curve E is below that of curve D on Figure 4 because the alkaline sample Fhich it represents was prepared by diluting the acidic one ivith an alkaline solution. As a result the alkaline resin had a lonet viscosity and showed the higher initial conductivity on the graph. These samples were also investigated for density changes in the presence of accelerator and plasticizer. The results obtained are show^ in Figure 5. The two resols of ratio 1:0.8,not represented in the density graph, behaved in a similar manner although the changes were smaller in extent; the alkaline resol showing ii slkhtly grmter density change t,han tlic acidic sample. DISCUSSION OF LOW-AIOLAR-RATIO RESINS
Fibare 4 seems to indicate that the curing proteas may take p1ai.i. in a t least txvo stages. During the first 10 days the behavior iz the same for all resins of ratio between 1: 1 and 1 : 2 involving, apparently, the setting up of a certain number of cross bond.ara-siibstitutrd phenol, ivill have few cross linkages n-hcn cured & I , hencr, will resenible partially cured resols in structure Ilesols of ratio 1:O.S or less will hare fern, if any, cross bonds and, hrrice, ni:tkc possible the plienomcnon known as postforming which has reccritly found wide application in industry. The procedures outlined herc have been extended to studies of the rate and degrce of cure of i,esorcinol-formaldehy~~e resins arid unsaturatcd polyesters (Si, and the data thus obtained may also be interpreted in terms of the rate and extent of cro$s bonding i n 'liehe thrrmoetting r c h . . I Ct i V 0 U LE DGJIEXT
The authors wish to express thcir thanks to the Hull Iruri and Steel Plastics Laboratory for niaking available the commercial cast resin, used in the pwliminary experinimts, and thc arcelrr%tor. LII'ERATURE CITEI) 1; HarkhufT.
R.h.,Jr.. arid Cnrswell. T. S.. Isn. E s a . CHEM..36
461-6 (1944). 2 1 Barron, H., Brit. Plastics, 16, 400-4 (1944). j Draper, C. R., Paint .llaiauf., 11, 179-64, 19s (1941; ) Feith, F.,Kunststofle, 34, 71-6 (1914). JI. Ibid..34. 127-32 11944). (6j Fineman, hf. N.,~ and Puddington. I. E., C a n . J . Rcsearch B25, 101-7 (1947). 7 ) F u o ~ s R. , M., et a l . . J . Am. L ' h t m . Soc., 67, 1566-70 (1945). 8 ) General Radio Co., Catalogur K, 3rd etl., pp. 66-7. Cai:ilJridge Mass.. 1914. 9 1 Ilartshorri. L. SIemon. ?;. J. L.. aiid Ilushton. E.. J . I n s t . Elec E T I Q ~, .83, S 4 7 4 - i 7 (1938). 10) Houniiik, R , Tiam.Faradall Soc , 32, 122. 131 (1938' 11' Kienle', I% H and R a m H H Trans. A m Electrochem Soc. 65, 87-107 (1934). ' 1 2 ) Iioslov. 1'. SI., T p ! t d y Sessii A k a d . .Yai~k.O r g . Khim., 1939, 91-7. I 13) Len-in, d., and Itolitschek, P., Bril. Plastics. 17, 316-22 (1945) (14) Lomakin, B. --I.,and Guseva, V, I., flastichesiiie .IIQSQJ,2. 251-96 (1937). :15j Manegold, E., and Petzoldt, W., Kolioid-Z., 95,59-60 (1941). 16) Medvedkov, E. S., and Polyatskina, B. M.,Prom. Ora. Khim (U.S.S.R.), 7, 232-6 (1940). '17) Penn, IT.S., Plastics (London), 10, 41 (1946) (18) Redfarn, C. A , , Brit.Plastics, 13,481-2 (1932). 119) Scott, A. H., McPherson, A. T., and Curtia. H. L., J . Research .',-all. Bur. Standards. 11. 173-209 (1933). (20) StKger, H., Siegfried, I+., and Sanger, R., Sch wciz. Arch. angeus Wiss. Z L . Tech., 7 , 130, 1.53 (1941).
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