DENSITIES OF CELLULOSE ESTERS C i R L J. 3LiL31, LEO R. GESUYG, AND JOHN
v.
Eastmclri h - o d d Company, Rochester,
FLECKEYSTEIN
4.1..
Densities of cellulose. cellulose acetates, acetate propionates, and acetate butyrate- h a l e been measured b! the 4rchimedec a n d flotation methods. T h e j h a l e alco been calculated from unit molecular weights and unit molecular lolumes based on the densities of regenerated cellulose and the various acids. Obserted and calculated results agree n i t h i n two units in the second decimal place. Regenerated cellulose samples were found to h a l e a density of about 1.52. The densities of other cellulose sample6 varied, depending upon their origin and degree of purit?. Obserted cellulose ester densities taried with coniposition reaching talues a s low as about 1.28 for the triacetate arid
1.16 for the tributvrate. The Archimedes method is preferred for film samples, but t h e flotation method is useful for pow-ders and fibrous samples. In t h e measurement of densities it is necessary- to select immersion media which do riot dissolve or sw-ell the sample. IVater or aqueous cadniium sulfate solutions were used for esters of high propiori>-Ior butyr!-I content, and carbon tetrachloride or mixtures of carbon tetrachloride and benzene or tetrabromoethane and xylene were used for celluloL ce acetates and niixed esters of l o w ~ r o p i o n yor l butyryl content. Calculated approximate values are also presented for simple fatty acid esters from formate to stearate.
HE density of a nianufacturcd articlt' is of practical iniportanccs and iiiterest because most raw materials are purchased by weight, whereas the amount requiwd for a finished object depiirids on its volume. Materials of lower density thus have a dvantage on a volunie basis over materials of higher density. I t is therefore desirable to knon. the densities of commercially available cellulose esters Ivheii c~onsitieringpossible uses for these materials and when selecting tht. twst material for a particular use. In determining the dimit?- of a cc~llul tive saniplc must be obtained and, if 11 form suitatile for t h r density nieasurt~nient. Thus, it may bt, c o w verted to a film or a filament or prrssed into a block, or, if none of these i$ practicalilc, it may lie pondered. Care must be taktsn t(J reinovt' c~)nipletelyfrom the saniplt~all riLsidual solvents, nioibture, and air; tilt: 1attc.r may btt c*orwiderablrin the cas' of OWders. Thtj m c ~ ~usrful t methods for determining the density of cellulost. csti'i's require immeraioir of the samples in a liquid. Since cellulose esters dissolve or sw,butyratr, a w t a t e propiomto, arid acetate butyratr.
111s
METHODS OF MEASUREMEKT
T l i t . gcmeral principles and rsperimrntal details of thv measurenit'nt of the densities of solidb art' discussed in standard reference b o o h . ;iuch as that of Rrilly and Rae ( 1 7 ) and of Keissberger (20). Sis of the most important methods used are the volumetric nictthod, use of dilatonietei,, use of volumenometer, use of pycnouietc~r,niethod of .Ii,c-hinicLdt,s,and flotation nitathod. T h e lattcsi, t h r w methods are widtb1L uscd becausc of the simplicity of the apparatus and its grnr,ral availability. The .irchimedc,s and py(*nometermethods arc us.cd I)!. thta .Inierican Society for Testing .\Iatt~ikilsEm dctt,i,miri;itil)riof s p t 4 i r gravity of plastic?; ( 1 ) .
METHODOF A R C H T V E L E ~'This . has been found t o he suitable for use with cellulose ester films and filaments. I t is less satisfactory for powders than is the flotation method. The Archimedes method involves weighing a body in two media of knon.n densities, usually air and a liquid such as water 01'carbon tetrachloride. The density of the body can then be calculated from the differ('rice iri weights as f o l l o w : T..
= density of the body, grams/cc. Jvhere d TV, = w i g h t of the body in air, grams TTm = weight of the body i n liquid (oftori negative in sign 1, grams D = density of inimersion liquid at temperature at which measurtxment is made, gramsjcc.
For very accurate work it may be desirable t o apply corrections for the buoyancy of air and for surface tension ( 1 7 ) .
1499
INDUSTRIAL A N D ENGINEERING
1500
CHEMISTRY
VoI. 39, No. 11
c ---o Formate
,ate
-1yristate -Stearate
U.Y
I
0
Figure 1 .
I
I
IO
20
,
,
I
30
40
1
I
l
60 Per Cent Acyl
50
,
1
70
,
I
80
I
l
,
90
I(
Calculated Demitiee of iliphatic h i d Ester6 of Cellulow
may be taken a5 an indimtian of undesirable SH elling or change. I n this way a measure can be coniw~ientlyobtained of the suitability of the liquid as an immersion rru~liumfor that particular sample. Carbon tetrachloride is B witabIe immem-on liquid for cellulosc acetate and c d l u k ~ s eacetate pi opionates and acetate butyrates containing up t o 2 5 5 higher acyl. Wit6 esters containing Iarger aniuunts of higher acyl, xater vith a.14.255 of 3, commercial wetting agent has been used sat&fa( torilv. The denvty of the immersion liqud, at the ternpcratuie of the experiment muat be hAz been found convenient t o prepar tcniperatuie graph when one medium ficquently. Filaments ale measured as described, except ~i compact nacl is prepaied and fastened t o the n i l e and sinker The evacuation step is e-.entia1 in this case. Ponders inav be meavurrtl in 3, qimilar n-ay uqing a suitable small container made oi hne scieen n hich is suspended from the 1 uiii Caic ihould be taken t h a t the pond c o : t i w inc8.h than the screen. The accuracto pooror in these caqe5 than 751th film becauw of diffic.ultiv\ cawed by the large mi faces in& volved arid possible 1 0 s ~of ponder through t h P 1x1
11
~ I , O T - \ T I O S ~ I E T H O DThis .
For the niessureinent of films a suitable sainplc free i'roln ail, bubbles is cut into as large a shape as can be manipulated in the glass cylinder used for this purpose, and two holes are punched in the film for hooking it' t o a suspending wire from the balance itriii and for att'aching a small sinker or plumb. The filiii is t h c i i dried thoroughly to remove moisture and residual coating soivents, and it's weight in air is measured in a closed weighing b o t t k . The film is hooked onto the suspending wire from the arm of ail analytical balance, the plumb is hooked i n the lo\vcr hole in t lit, film, they are placed in the cylinder, immersion liquid i: sddcci to the required depth, and the weight is then determined as quickly as possible. The film is then removed, and the weight of the n.iw and plumb in the liquid is determined. The liquid level should bv adjusted if necessary so the same amount of wire is inimersid ab in the determination. The difference betneen these tlvo w i g h t s is the w i g h t of the sample in the liquid, and this value is ncgstive in sign Tvhen heavy immersion liquids arc used. When the sample is known t o be porous and xhen greater a?curacy is desired, t,his procedure i film, and suspending wire in the cylinder with a stopper carrying a and a connection t o a vacuum lir oughly evacuated to remove as much trapped and absorhrd air as possible, and the immersion liquid is added through tlic tirol)ping funnel to cover the sample before air is adniitted and the pressure rest>ored. The suspending wire is attached t o the h l ance arm, the liquid level is adjusted t o the proper point, and thcb weighing is made as before. This method is particularly advantageous because the tiiiie of immersion can be kept short. .iEtcr the measurement the w i g h t can be checked from t i i w t o time, and a significant drift in weight
1.52 c Calculoted
1.48
.-
o Obserged
1.44
x
n
is easily applicable
to small objects including films, filaments, alitl. Pairs of iiirrt liquids, one of Ion-cr and one of highrr haii thc sample, arc chosori. Thr folloning pairs l i a v ~ I ) c ~ i iuwd: concentrated cadmiurn sulfate solution and water;
rvnceiitratetl calcium chloride solution and water; tetrabromoethxiic arid xylene: carbon tctrachIoride a n d bclnzt.ne. Thv bwt practice is to placc a small amount of the saniplr) o r 1 thr bottom of an empty cylinder; evacuate as described for the
.~rchinirdesmethod, add a suitable mixture whose density proxiiii:iti,s that of the sample t o cover thc saniplc,, release IIUIII. and adjust the niisture so t,hP sample neither floats nor sinks but remains suspended. Sufficient time must 1x1allo\vcxl for fibrous samples t o come t o equilibrium with the suspending iiictlium. Films and granules prewnt no problem, b u t fincly tiivicitd pondrrs are difficult t o cvacuatr, and results tr~ntlto be low, The density of the suspcnding medium is then detprniined. at tiir same tempvmtui'i> using a pwnometer or other suitable means. The obwrvcd density is also rhr density of the sainplr. Organic liquid pairs, such as carbon t lees than about 25$ propionyl o r tnrtyryl
tectrd, slid close temperature control d u r
difficult.
OTHERJ k T I i U T W . The pycnoiiieter methurl ni:xy urtd for p o ~ ~ d ~films, r s . and filanients. The samples of films aiiti filmients ~ into are cut into pieces small enough t o I J iiisertd the pycnom6ter. Thc same precautions as to pi'cpctration and drying t h e sample, removal of air b y cvacuatioii, choice of immersion mcdiuin, con1 rill or measurement of tfmperature, etc., apply as i n the other methods. In this case the time of immer-ion is grrater than for t h e Archimedes method. Tht! volumetric method is usually unsuitable hcause the mcmurrment of gross volume is too inaccurate, and no account is taken of the voIume of pores and voids.
1.40
PREPARATIOX O F SAXIPLE 1.36 1.32
1.28 Per C e n t A c e t y l
Figure 2 . Densities of Cellulose .icetates Showillp variation of Calculated atid Obserred \-slues with Acetyl Content
Tlii>iii(s:wurcrneiit of tlic tit,iisity of a 1 1 trhjcLcat inof eri'or arid special prr. \T'hcn the density of a c t ~ l l u l o ~c+tc.r.--for c~ rsamplc, a cnmnlercial samof 4 0 . 4 7 acetyl conteriti-; t o hi> ine:rsured, sonic ne~r difficulties are atltltd. ],:ither the m n p l e must be powdercd and R iiwiisureiuwt m:itle xhich involves the difficul-
November 1947
I N D U S T R I A L A N D E N G I N E ~ R I N GC H E M I S T R Y
1501
a , by the following equation (3'1:
7.8Bn \- - -2 .- 102.4 - n
(1)
.
1Iolecular weight,
162
+ 42.Ya
(2)
Thc molccular volunle of the cellulose portio^^ the ester, 106.6, is found by dividing thc unit iiiolecular n-eight, 162, by the derisity of ccllulose hydrate, 1.52. The atomic volumes of hydrogen rtnd osygcn are takcri t o be 5.5 and 7.8, rcspectively, and the volume of hydroxyl is 13.3 (8).The volume of the cellulose portion I J ~ 'the cbster is t h u s 106.6 - 5.5.LTa. The volume of the acetyl group is tlie 1110lwulitr weight of acetic acid, 60.05, dividctl by its doiisity, 1.049, minus the volume of the hydroxyl group, 13.3, n-hich gives a value of 43.0 for the group CH,CO. The moliwdar volumi, of cacllu01'
lose acetate is thus,
+ 43.9S0 = 106.6 + 3 8 . 4 s - (3) 162 + 42V. Den-ity of cellulose acetate -- (4) 106.6 + 38.4S.
5
106.6 - 5.5.\-a
=
A
P Figure 3.
I'or csamplr, c
