J . Chem. Eng. Data 1999, 44, 649-655
649
Absolu te Viscosity a n d Den sity of Tr isu bstitu ted P h osp h or ic Ester s
S. Ka n n a n * a n d K. Kish or e†
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560 012, India
This paper presents measurements on the absolute viscosity (η) and density (F) of trisubstituted phosphoric
esters which are useful in understanding their flow mechanism necessary for accessing their role as
plasticizers. The effect of chain length and branching has been examined on the η and F trends. From η
data, by using the Vogel-Tammann-Fulchur (VTF) equation, the VTF temperature (T
which also represents the ideal glass transition temperature. T is related to the flexibility of the molecules.
It is observed that T initially decreases with molecular weight, reaches a minimum, and increases
thereafter. The initial decrease in T has been attributed to the enhanced flexibility of the phosphate
o
) has been obtained
o
o
o
esters. Reversal of flexibility with relative molar mass beyond 400 is due to the gentle collision of the
arms of the trisubstituted phosphoric esters. This has been further corroborated from the molar mass
exponent as exhibited in the η-molar mass plot. The isomeric effect on η has also been investigated in
tricresyl phosphates, hitherto for the first time. The ortho isomer has highest η among the isomers. The
para isomer was found to have lowest T
o
and hence highest flexibility compared to the ortho and meta
isomers.
In tr od u ction
Ta ble 1. Str u ctu r e a n d Molecu la r Weigh t of Tr ia lk yl
P h osp h a tes, P (O)(O-(CH2)x-CH3)3
Despite the commercial importance of phosphate esters
as plasticizers and lubricants, their flow behavior has not
been studied in detail. An understanding of the flow
behavior would help in the elucidation of the molecular
mechanism of plasticization and hence would facilititate
proper selection and design of plasticizers. The flow
behavior in liquids is generally described from Eyring’s free
volume model in terms of free energy of activation for the
name
triphosphate)
molecular
formula
molecular
weight
(
abbrev
1
2
3
4
5
6
7
8
9
0
methyl
ethyl
tmp
tep
tpp
isop
nbp
isob
isoam
thp
C3H9O4P
140
182
224
224
266
266
308
350
519
603
C6H15O4P
C9H21O4P
C9H21O4P
C12H27O4P
C12H27O4P
n-propyl
isopropyl (B)a
n-butyl
isobutyl (B)
isoamyl (B)
n-hexyl
n-decyl
n-dodecyl
viscous flow (∆G#
) which is obtained directly from absolute
C15H33O4P
n
C18H34O4P
C30H63O4P
C36H75O4P
viscosity (η) and density (F) data at a given temperature.
In this paper, we present an investigation on the η and F
of a homologous series of phosphate ester, namely trialkyl
phosphate esters, as a function of temperature. Tricresyl
phosphate is the most widely used fire retardant plasti-
cizer, but the isomeric effect in their plasticizing or flow
properties has not been studied. We are also examining
here the effect of isomers on the η and F values of tricresyl
phosphates. These data on the above long-chain esters have
not been reported hitherto.
tdp
tdodp
1
a B, Branch in alkyl arms.
Ta ble 2. Str u ctu r e a n d Molecu la r Weigh t of Tr ia r om a tic
P h osp h a tes, P (O)(OR)3
name
triphosphate)
molecular
formula
molecular
weight
(
abbrev
1
2
3
o-cresyl
m-cresyl
p-cresyl
o-tcp
m-tcp
p-tcp
C21H21O4P
C21H21O4P
C21H21O4P
368
368
368
Ma ter ia ls
POCl
3
, alcohols, tertiary amines, and solvents, of ana-
Aliph a tic Ester s. All the esters were prepared by the
lytical grade, were procured from SD Fine Chemicals,
Bombay. They were purified by distillation using standard
procedures (1). The phosphate esters, synthesized in the
present investigation, are given in Tables 1 and 2.
condensation of POCl and alcohols, (as given below) in ice-
cold conditions using pyridine as the acid quencher (2). The
liquid esters were purified by fractional distillation, while
the solid esters were purified by recrystallization.
3
P r ep a r a tion of Ester s
The pentavalent trisubstituted phosphoric esters (PO-
3
(OR) ) have been synthesized and classified into aliphatic
and aromatic based on the nature of the R group.
Ar om a tic Ester s. These esters were synthesized by
interfacial condensation using tetra-n-butylammonium
bromide (TBAB) as the phase transfer catalyst (3). The
chloroform solution of POCl was added dropwise to the
3
aqueous solution of sodium salt of aromatic alcohols in ice-
cold conditions. The compounds were isolated from the
*
Present address: Laboratoire de Chimie des Polymeres Organique
(LCPO), Ecole Nationale Superiere Chimie et Physique de Bordeaux
ENSCPB), Avenue Pey-Berland, BP 108, 33402, Talance Cedex,
(
France. Telephone: 33 5 56 84 27 34. Fax: 33 5 56 84 84 87. E-mail:
kannan@iagp.enscpb.u-bordeaux.fr.
†
This article is dedicated to the memory of Prof. K. Kishore.
1
0.1021/je980276a CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/16/1999