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Contribution to the study of mechanism of stabilizing action of esters of phosphorous acid.

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Die Angewandte Makrornolekulare Chemie 9 (1969) 129-135 ( N r . 111)
From the Research Institute of Macromolecular Chemistry, Brno, CSSR
Contribution to the Study of Mechanism of
Stabilizing Action of Esters of Phosphorous Acid
R Y ~ A Vand
(Eingegangen am 20. Mai 1969)
The rate of the reaction of a series of phosphites with cyclohexene hydroperoxide
and cyclohexene peroxy radicals was studied. The results of the measurements
were compared with the stabilization efficiency of the same compounds in isotactic
and in atactic polypropylene at 160°C and 120°C respectively and during the
course of atmospheric ageing. It was found the reverse relation between the
stabilization efficiency and the reaction constants of the reaction of phosphites
with hydroperoxides. The main stabilization reaction of the phosphorous stabilizers
seems to be the reaction of phosphites or of their decomposition products with
radicals. The possibility of splitting off of free phenol from phosphites as a result
of the reaction of the esters with hydroperoxide groups was checked.
Die Reaktionsgeschwindigkeit einer Reihe von Phosphiten rnit Cyclohexenylhydroperoxid und mit Peroxy-Radikalen des Cyclohexens wurde untersucht. Die
Ergebnisse wurden mit der Stabilisierungswirksamkeit derselben Verbindungen
an isotaktischem Polypropylen bei 120°C und 160°C und wiihrend der atmospharischen Alterung verglichen. Es wurde festgestellt, daD die Stabilisierungswirksamkeit den Reaktionskonstanten der Phosphite mit den Hydroperoxiden
umgekehrt proportional ist. Die wichtigste Stabilisationsreaktion der Phosphitstabilisatoren ist die Reaktion der Phosphite bzw. der aus ihnen entstehenden
Stoffe rnit den Radikalen. Die Moglichkeit der Bildung von freien Phenolen als
Folge der Reaktion der Phosphite mit Hydroperoxidgruppen wurde uberpriift.
1. Introduction
The esters of phosphorous acid represent one of the important groups of
stabilizing additives used in polyoleiines and rubber. In spite of the fact that
D. RYE~AvJ~.
and Z. SLAMA
these compounds have been used for many years, the mechanism of their
action remains still questionable.
While the older assumption of the stabilizing action of phosphites (PX,)
is defined by the reaction 1,
+ PX3 = ROH + OPX3
new theories postulated recently suppose the reaction of phosphites ( Q ) with
radicals. For instance LEV IN^ has proposed the following reaction scheme :
RO' + Q
+ QO
+ ROQ' -+ROR + QO.
On the other hand other authors2 have supposed that free phenol is released
as a result of the ester hydrolysis in the first place and that this compound is
the real stabilizing agent reacting with radicals.
The aim of the present work was t o bring further experimental results which
might have solved this problem. The rate of the reaction of phosphites with
hydroperoxide and in other series of experiments also with radicals ROi was
studied in model systems. The stabilizing efficiency of the same esters on
polypropylene was measured.
2. Experimental
Phosclere T 310, Triisodecyl phosphite, Pure Chemicals Ltd., England;
Phosclere T 210, Diisodecyl-phenylphosphite, ibid;
Phosclere T 36, Triphenyl phosphite, ibid;
Phosclere T 2 15, Dinonylphenyl-phenylphosphite, ibid; Tri-(4-nonylphenyl)phosphite, Research Institute of Macromolecular Chemistry (RIMCH),
Czechoslovakia ;
Tri-[4-(l'-phenyl)ethylphenyl]phosphite, Chemicals Works of J. DIMITROV,
Azobisisobutyronitrile, RIMCH,Czechoslovakia;
Cyclohexene hydroperoxide, RIMCH,Czechoslovakia ;
Austria, 2. isotactic, prepared with the
Polypropylene: 1. isotactic, DANUBIA,
catalytic system TiCl~-Al(Et)3in RIMCH,3. atactic, obtained by the propan
extraction of polymer prepared in RIMCH,Czechoslovakia.
Analytical grade (LACHEMA,
Czechoslovakia) solvents and chemicals were used.
Stabilizing Action of Esters of Phosphorous Acid
A p p a r a t u s f o r f o l l o w i n g t h e r e a c t i o n of p h o s p h i t e s w i t h c y c l o h e x ene h ydroperoxide
A jacketed reaction vessel equipped with a reflux condenser and an argon inlet,
placed on an electromagnetic stirrer, was used for the measurements. The argon
inlet served also for dosing the reaction components and for sample withdrawing.
The samples of the reaction mixture were diluted by thirty-fold excess of solvent
and cooled immediately to stop the reaction.
A n a l y t i c a l d e b e r m i n a t i o n of c y c l o h e x e n e h y d r o p e r o x i d e
A photometric method modified by BOEEK~
was used. The method is based on
the oxidation of Fe++ --f Fe+++ by hydroperoxide and the unreacted Fe++ is
determined photometrically as a Fe++-phenantrolinecomplex.
The oxidation of cyclohexene was followed in a WARBURGapparatus (Labor,
The induction period determination
The induction period was defined as the time required for 1 g of polymer to
absorb 2 ml of oxygen (0,4y0 of tertiary C-H bonds of polypropylen is oxidized).
The detailed description of the apparatus used was given previously4.
P r e l i m i n a r y d e t e r m i n a t i o n of s t a b i l i t y i n t h e c o u r s e of a t m o speric ageing
The strips of the size 6 x 50 x 0,5mm were fixed on an aluminium plate declined
in an 45" angle and directed to south. A suitable place near Brno (Czechoslovakia)
was chosen for sample exposing. In the time intervals the strips were checked by
bending them by 180". The number of sunshine hours causing that five strips out
of ten were broken when bent characterized the stability of the sample and this was
called the induction period.
T h e d e t e r m i n a t i o n of p h e n o l b y gas c h r o m a t o g r a p h y
A PYE104 chromatograph equipped with the 5' column packed with 10% of
polyethylene glycoladipate adsorbed on Cellit (100-120 mesh) was used for the
measurements. The carrier gas (argon) flow rate was 50ml/min, the column
temperature 125°C.
3. Results and Discussion
3.1 The reaction of phosphites with hydroperoxih
The rate constants of the reaction of various phosphites with cyclohexene
hydroperoxide are given in Table 1. The highest rate was found for trialkyl
phosphite. When alkyls were replaced by phenyls and phenyls by alkylaryl
groups the reaction rate drops in this series. The same dependence was found
D. R Y ~ A Vand
in the previous work when following the reaction of phosphites with tert.but ylhydroperoxide5.
Table 1. The rate constants of the reaction of phosphites with cyclohexene hydroperoxide.
triisodecyl phosphite
triphenyl phosphite
tri-(4-nonylphenyl) phosphite
1 0 3..see)
3.2 The reaction of phmphites with radicals RO;
The rate of oxidation of cyclohexene (using azobisisobutyronitrile - AIBN as an initiator) in the presence of various phosphites was followed. The concentration of AIBN was chosen so high that the effect of hydroperoxides,
formed in the course of the reaction, on the initiation rate was negligible.
Under these conditions decrease of the rate of the oxygen absorption in the
presence of various phosphites could only be due t o reaction of RO; radicals
with the phosphite or with its reaction products.
The results of the measurements which were made a t a temperature of45"C
are given in Table 2. Triisodecyl phosphite seems t o be almost ineffective;
Table 2. The rate of the cyclohexene oxidation. Conc. of AIBN: 2 10-2 mol/l;
cone. of phosphite: 2 * 10-3 mol/l; cone. of cyclohexene: 50%; temperature: 45OC; v = initial rate of oxidation.
triisodecyl phosphite
diisodecyl phenyl phosphite
triphenyl phosphite
109. v
(mol/l . sec)
2 660
2 400
2 030
Stabilizing Action of Esters of Phosphorous Acid
tri-[a-(1'-pheny1)ethylphenyll phosphite exhibits the highest retardation effect.
This clearly shows that the retardation efficiency of the series of phosphites
increases in the reverse order when compared with rate of hydroperoxide
3.3 Stabilization efficiency of phosphorous acid esters i n polypropylene
Four typical esters were selected for these measurements. The efficiency of
the stabilization of isotactic polypropylene was evaluated by the induction
period determined a t 160°C. For the atactic polymer the dependence of the
induction period on phosphite concentration a t 120"C was employed for the
evaluation of the stabilization efficiency. The results are summerized in Table
3 and in Fig. 1.
IP (hours)
1 O C
Fig. 1. Dependence of the induction period on the phosphite concentration in
atectic polypropylene at 120°C. IP = induction periode in minutes,
C = phosphite concentration in mmol/kg PP, 1 = tri-[4-(1'-phenyl)ethyl
phenyl] phosphite, 2 = tri-(4-nonylphenyl) phosphite, 3 = triphenyl
phosphite, 4 = triisodecyl phosphite.
I n addition t o the thermostability measurements the efficiency of two
esters (trilauryl and tri-(4-nonylphenyl) phosphite) was checked in the atmospheric ageing. The results are given in Table 4.
D. R Y ~ A Vand
Table 4. The efficiency of the phosphorous acid esters in the course of atmospheric ageing. Isotactic polypropylene prepared in our Institute.
Phosphite concentration: 5 . 10-3 mol/kg PP. Induction periods in
sunshine hours.
(sunshine hours)
trilauryl phosphite
tri-(4-nonylphenyl) phosphite
The experimental results obtained from all stability measurements (on
isotactic polypropylene a t 160"C, on atactic polypropylene a t 120"C, on
isotactic a t atmospheric ageing) show that the retardation efficiency of the
series of phosphites increases in the reverse order when compared with rate
of hydroperoxide decomposition but the efficiency parallels the reaction rate
of ROi radicals with phosphites. On the basis of these results it may be concluded that the reaction of phosphite with the hydroperoxides is not the main
stabilization reaction of these compounds. The most important factor determing the stabilization activity is the rate of the reaction of phosphites or of
their reaction products with radicals. Detailed elucidation of this mechanism
would be possible on the ground of further studies.
Within the limits of the present work we tried t o prove the assumption that
the peroxy radicals do not react with the phosphorous acid esters directly but
that they react with the free phenol which is released from the ester owing to
its reaction with hydroperoxide.
3.4 The influence of hydroperoxide on the splitting off of the phenols from the
The following reaction was suggested :
+ ROOH -+
The hypothesis defined by the reaction 3 seemed t o be very attractive because
it may have explained both stabilization functions of the phosphites. The
following experiments were made to check this theory. The mixture of tert.butylhydroperoxide and triphenyl phosphite in benzene (concentration of
both compounds was 5 * 10-2 mol/l) and a solution of triphenyl phosphite
without hydroperoxide was prepared. Both solutions were separately stirred
Stabilizing Action of Esters of Phosphorous Acid
for one hour a t room temperature. During this time the concentration of tert.butylhydroperoxide dropped t o 52 yo of its origin value. The concentration of
the free phenol was then determinated in both solutions using the gas chromatographic method. The same value, i. e. 3.10-3 mol/l of free phenol was found
in both solutions. That means that the hydroperoxide groups do not have any
influence on the releasing of the phenol from triphenyl phosphite.
P. I. LEVIN,Zhur. Fiz. Khim. 38 (1964) 672.
M. S. CHLOPJANKINA,Neftekhimiya, 5 (1965) 49.
P. B O ~ E EChem.
PrGmysl 17 (1967) 439.
D. RYBAvG,Polymer [London] 8 (1967) 449.
D. RY~AV;,Chem. PrGmysll8 (1968) 20.
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acid, phosphorous, stud, mechanism, esters, action, contributions, stabilizing
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