Inhibited Autoxidation of Polypropylene - Advances in Chemistry (ACS

20 Inhibited Autoxidation of Polypropylene D A L E E .VANS I C K L E and D A V I D M . P O N D

Downloaded by UNIV OF PITTSBURGH on March 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0169.ch020

Research Laboratories, Tennessee Eastman Company, Division of Eastman Kodak Company, Kingsport, T N 37662

For di-tert-butyl peroxide-initiated oxidations in benzene solution at120°C,singly hindered phenols appear to have twice the chain-stopping power of the di-tert-butylphenols; e.g., a given molar quantity of 2-tert-butyl-4-methylpheno will give twice the inhibition period of 2,6-di-tert-butyl-4methylphenol. The stoichiometry of the polypropylene peroxy radicals reacting with 2,6-di-tert-butyl-4-alkylphenols is independent of their attachment in large molecules; all of the phenolic residues in these polyfunctional molecules were utilized in stopping oxidation chains. Although inhibition periods in polypropylene oxidations are proportional to the inhibitor concentration, the overall kinetics of the inhibited oxidations are complex. Attempts to demonstrate the supe riority of singly hindered phenols in oven-aging tests were only partially successful. "revaluation of antioxidants for polyolefin stabilization is done empirically by compounding resin with additive, subjecting the composition to elevated temperatures, and later measuring some extensive property such as flexibility or impact strength of the sample. Many properties of the inhibitor besides its reactivity toward peroxy radicals enter into its overall effectiveness; e.g., volatility, thermal stability during compound ing, and compatibility. For designing a superior antioxidant, it is desir able to separate these factors and to evaluate the chemical antioxidant activity of an inhibitor independently of the ancillary properties of the material. The phenomenon of inhibited oxidation of simple, low molec ular weight substrates [styrene ( 2 , 2 , 3 ) , tetrahydronaphthalene ( 4 , 5 ) , and cumene ( 6 ) ] has been investigated thoroughly, but extension of these studies to polymeric substrates has not been done analytically. A suitable set of conditions for the solution oxidation of l o w molecular weight crystalline polypropylene has been described ( 7 , 8 , 9 ) . 0-8412-0381-4/78/33-169-237$05.00/l © 1978 American Chemical Society Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


238

STABILIZATION A N D DEGRADATION O F P O L Y M E R S

The solution oxidation technique allows the study of polyolefin autoxi dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. T h e results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes w h i c h occur i n neat polymers where the effects of very high viscosity, partial crystalhnity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene i n solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-feri-butyl-4-methylphenol [butylated hydroxytoluene ( B H T ) ] has been made for comparison w i t h data obtained i n polypropy lene oxidations inhibited by a variety of other stabilizers w h i c h include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior i n the inhibited-solution oxidation of polypropy lene, and the application of this finding to stabilization technology was investigated briefly.

Experimental Materials. T h e polypropylene ( P P ) used was a l o w molecular weight crystalline material w h i c h has been described previously ( 8 ) . The polymer was extracted with hexane and acetone before use and dried at 0.1 torr on a steam bath. The extracted pellets had a number average molecular weight ( M « ) of ca. 29,000 and were completely soluble i n benzene at the 120°C reaction temperature. The 9,10-dihydroanthracene ( D H A ) from Aldrich Chemical Company was recrystallized from ethanol before use; mp 106°-107°C. Benzene, used directly, was M C B Chromatoquality; di-ter£-butyl peroxide (tert-Bu 0 ) 9 9 % from L u c i d o l , also was used directly. Azobis(2-methylpropionitrile) ( A B N ) was East man white label grade w h i c h was recrystallized from acetone-methanol. 2,6-Di-ierf-butyl-4-methylphenol Eastman Tenox B H T , food grade, appeared to be very pure by G L C analysis and was used directly. The other simple alkyl derivatives of phenol were obtained from the usual fine chemical sources (Eastman Organic Chemicals, Aldrich, Columbia, and Κ and K ) . A l l were distilled and/or recrystallized and the assigned structures were confirmed by N M R . Other commercial stabilizers were obtained from their manufacturers, recrystallized (usually from ethanol or ethanol-water), and submitted for N M R confirmation of the assigned structure and purity. . 2,4,6-Tris[3-ferf-butyl-4-(hydroxybenzyl)]mesitylene (III) was syn thesized starting from 2-ferf-butyl-6-chlorophenol, which i n turn was pre pared by the method of Kolka et al. (10). The 2-terf-Butyl-6-chlorophenol was alkylated with a ,a ,a -(trihydroxyhexamethyl)benzene ( A l d r i c h ) with the use of boron trifluoride etherate catalyst ( I I ) . A mixture of I and II was obtained, separated by fractional crystallization from methylcyclohexane. The structures of the two products were assigned on the 2

1

3

2

5

Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

20.

V A N SICKLE A N D POND

Autoxidation of

239

Polypropyhne

C(CH ) 3

I,Ri

R2

=

R3

=

=

=

-OH

~OH; CI (CH ) Ç 3

Ri—Sy

j^—R2

R


C H r - l ^ J J - C H ,3 R

R3

2

3

= =

-CH OH

•OH

—CH2—

2

3

C(CH ): 3

III, R i

R = R;3 _ _ C H - / ^ ^ > - O H 2

2

basis of NMR spectra. I was dehalogenated by a sodium-liquid ammonia technique (12) to yield first a product containing 5 . 5 % chlorine and, with a repeated pass through the dehalogenation procedure, a product containing 2.76% chlorine. The NMR spectrum of this material indicated that it was 85 to 9 0 % III, the desired product. 2,4,6-Tris(3-ferf-butyl-4-hydroxy-6-methylbenzyl)mesitylene (IV) was obtained by alkylation of 2-ter^butyl-5-methylphenol with α ,» ,» (trihydroxyhexamethyl) benzene by the procedure indicated previously. From 0.020 mol of the phenol and 0.0048 mol of the tris(hydroxymethyl)benzene, 4.27 g of crude product was obtained from which 2.35 g of white powder, m p 220°-240°C, was isolated b y stirring with benzene. The NMR spectrum of the product was i n agreement w i t h the proposed structure, but it indicated that some loss of teri-butyl groups had occurred. Oxidation Procedure. The constant-volume/falling pressure oxida tion apparatus has been described previously ( 8 ) . Inhibitor solutions i n benzene were made u p i n volumetric flasks and a weighed portion was transferred to the reaction bulb which contained weighed quantities of polypropylene and initiator. Total volumes of reactant were usually 56-58 m L . The bulb was shaken i n the thermostat with oxygen at a total pressure of 5-6 atm. The vapor pressure of benzene at the reaction temperature of 120°C is 3 atm ( i n the absence of dissolved polypropy lene) so that the net partial pressure of oxygen i n the bulb void was 2-3 atm during the experiment. Time, oxygen reservoir temperature, and pressure readings were recorded at appropriate intervals, and the pres sure-time readings were corrected then (8) to a constant oxygen reservoir temperature. Oxygen consumption was plotted as a function of time, and the best lines were drawn to calculate inhibited (Rinh) and uninhibited (R ) rates. A typical result for polypropylene is shown i n Figure 1. T h e inhibition period, f , was taken as the time axis projection of the point of intersection of the two lines representing the inhibited and uninhibited rates. 1

0

inh


3

5

240


OXYGEN ABSORBED, MMOL [PP] - 4.0 M [BHT] = 3.05 χ 10" M [t-Bu O ] * 1.22 χ ΐ σ M 0

3

0

2


2

0

2

0

100 Figure 1.

Results and

200 300 TIME, MIN

400

500

Inhibited oxidation of polypropylene (PP)

Discussion

The results of the inhibited oxidation experiments with polypropyl ene are summarized i n Tables I and II. A few experiments where 9,10-dihydroanthracene was used as the substrate are summarized i n Table III. Mechanism of Antioxidant Action. The effect of antioxidant addi tives (AH) on an oxidizing hydrocarbon can be discussed profitably i n Table I.

Oxidation of Polypropylene (PP)

in Benzene Solution [tert-Bu 0 ] X JO , M 2

Run No. 1 2 3 4 5 6 7 8 9 10 11 12 13

[BHT] ° 0

χ

1(P,M

0.0 0.0 0.40 0.90 1.22 3.01 6.01 4.07 3.93 3.49 3.05 6.57 6.11

Initial concentrations. * Concentration in monomer units, mol wt =

[ΡΡ] ;·*Μ β

2.52 2.52 2.50 2.52" 2.48 2.55 2.52 0.0 0.55 1.24 4.00 2.52 2.51

2

2

1.23 1.27 1.19 1.22 1.02 1.24 1.21 1.27 1.40 i:21 1.22 3.58 7.10

β

42.1.


a

0

20.

241

Autoxidation of Polypropylene


terms of the following equations, where RH is the substrate molecule and I is an initiator. Where the substrate is polypropylene, R is assumed to be [ C H 2 - ( C H ) C ] . 2

3

I

ki - » 21·

2

R = 2ek [I ] i

1

(1)

2

k -> I0 · 2


I · [or R· from Equation 3] + 0

2

(or R 0 · )

2

A- [from Equation 6] + 0

k/ 2

k. '

2

Α0 ·

(2)

(2')

2

2

k R0 - + RH -> R 0 H + R3

2

(3)

2

k R0 · + R * 2

4

nonradical products

k 2R0 - -> nonradical products + 0

(4)

5

2

(5)

2

k R0 - + A H ?± R 0 H + Ak. e

2

(6)

2

e

at 1 2 0 ° C Inhibited by 2,6-Di-ferf-butyl-4-methylphenol ( B H T ) Rin» X 10*, Mfmin' ) 1

—

0.84 0.70 0.40 0.47 0.43 0.17 0.37 0.29 0.43 0.73 (1.1) 2.7

ti„ft (min) 500* 230 240 204 210 600 196 195 235 1 r > 500' 0

«

2.1 2.3 3.0 1.5 1.8 2.5 3.1 1.8 /

t 0.7'

No indication of acceleration to an R of 2-2.8M mhr within 500 min; strong orange color developed. *No obvious inhibition periods; slow acceleration of oxidation rate to constant value after ^ 100 min. Initial (and constant) rate. β

Q

1

9


244

STABILIZATION A N D DEGRADATION


Table III.

β

OF

POLYMERS

Inhibited Oxidations of

Run No.

Inhibitor

1 2 3 4 5

None 2,6-Di-fer£-butyl-4-methylphenol ( B H T ) 2,6-Di-ieri-butyl-4-methylphenol ( B H T ) Pentaerythritol tetrakis[3-[3,5-di-£er£-butyl4- (hydroxy phenyl) propionate] 4,4'-Methylenebis (2,6-di-ieri-butylphenol)

6 7 8

2- ieri-Buty 1-4-methy lphenol 2-£er£-Butyl-4-methylphenol 2-£er£-Butyl-4-methylphenol

Solution 60°C, [DHA] = 0.63M, [ABN] = 0.014M. 0

0

k R0 - +Α· 2

7

- » nonradical products

(7)

k A· + R H -> A H + R k 8

(8)

9

Α· + A -

- » nonradical products

(9)

kio AH + 0

2

-> Α· + Η 0 ·

(10)

2

Where feri-butyl peroxide is the initiator, Equation 1 takes two steps: kla tert-Bu 0 - » 2 tert-BuOR* == 2 fc [Bu 0 ] (la) tert-BuO- + R H - » i e r i - B u O H + R (lb) 2

lo

2

2

2

Use of tert-butyl peroxide also introduces a possible competing reaction for Equation 6: k ' 6

+ AH

tert-BuO-

tert-BuOR

+ Α·

(6')

F o r A H to bè an efficient inhibitor, it is necessary that k [ A H ] > > k [ R H ] and that k [ R 0 - ] > > k [ R H ] and k . [ R 0 H ] . Although usually not discussed, it is also necessary that equihbrium (Equation 2') lie on the left. Another and possibly final requirement is that chains not be started by a direct o x y g e n - A H reaction (Equation 10). 6

3

7

2

8

6

2


20.

V A N SICKLE

245


A N D P O N D

9,10-Dihydroanthracene in Benzene

β

[Inhibitor] ο Χ 10 , M


Rinn Χ ΙΟ , M (min )

Unh (min)

3

0.0 1.90 3.69 0.646 (2.58) 1.63 (3.26) 1.00 1.86 2.97

4

~ 0 175 ~ 400 270

1

Ro Χ 10 , M 4

1.4 1.0 1.8

26 13.9 13 10.5

320

1.3

9.7

94 ~250 ~365

3.0 2.2 1.4

17.4 12.5 10.4

—

(min ) 1

Rate Laws. W h e n the conditions just given are met, the rate of consumption of oxygen (R h) during the inhibition period, t , has been given by H o w a r d and Ingold (4,5) as in

inh

ek!k [ R H ] [I ] 1, [ΓΑΤΤ1 k AH] 3

•^inh^

2

2

\> lL

6

where e is the efficiency factor for the production of radicals from the initiator I [ = 0.60 for 2,2'-azobis[2-methylproprionitrile ( A B N ) ] . This equation was developed for application to inhibited oxidations of tetrahydronaphthalene where kinetic chain lengths are significantly above unity and termination by Equations 4 and 5 is ignored. A s pointed out by H o w a r d and Ingold (4,5), the kinetic form is independent of which termination (Equation 7 or 9) is important as long as transfer reaction k_ is negligible. I n our case, where initiation was by tert-butyl peroxide and Reaction 6' was considered, the usual steady-state treatment gives the following equation for the inhibited rate of oxidation: 2

6

p U i n h

R [RH] x i y j L-'-^-'-J J k [RH] + k [AH] χ 2k [AH] x

-

k (k

lb

l b

3

6

r

[RH] - k ' [AH]) + k 6

6

l b

J

(12)

where Ri = 2 k i [ t e r £ - B u 0 ] = the rate of initiation. Where Reaction & is omitted from the scheme ( k ' = 0 ) , Equation 12 reduces to Equation 13, a

2

2

6


246

STABILIZATION AND DEGRADATION

p

-fit ^3 [ R H ] 2k [AH]

K i n h

K

POLYMERS

p

n

i

« v

( 1 3 )

w h i c h is identical to Equation 11 except for the additional term, R i . B H T Inhibition of Polypropylene Oxidation at 120°C. The results of applying the measured, inhibited oxidation rates listed i n Table I to Equation 13 are described here. W e can confirm that the inhibited oxidation rate (Rinh) shows a first-order dependence on initiation rate ( R i ) up to a value of 6.5 χ 1 0 M m i n " for R i . Figure 2 shows the results of plotting R against [ t e r f - B u 0 ] . The initiation rate corre sponding to the ter£-Bu 0 concentrations can be computed from E q u a 5

1

i n h


, +

e

OF

2

2

RjnhxKP.MMIN'

0

1

2

2

1

2

3

4 5 [t-Bu 0 l x10 ,M

6

7

8

2

2

Figure 2.

2

Inhibited-rate dependence on initiator concentration

tion l a where k has the value 4.7 χ 1 0 m i n at 120°C. Uninhibited oxidations of polypropylene (8) are ~ 1/2-order dependent on R i , but a plot of R vs. [ f e r f - B u 0 ] shows definite curvature. BHT-inhibited polypropylene oxidations may be inverse-order de pendent on B H T concentration as suggested by Equation 13, but zeroorder dependence on inhibitor concentration is not rigorously excluded. The appropriate plot is shown as the lower line of Figure 3. Experiments at sufficiently low B H T concentration to distinguish zero-order from inverse-order dependence were not practical since inhibition times would be too short to accurately estimate R . According to Equation 13 the 4

i a

i n h

2

2

- 1

1/2

i n n


20.

247



Rj h χ 10 , M MIN" 4 4

1

n

[DIHYDROANTHRACENE] ο =0.63 [ABNJo -1.4x10' M 2

60°C 2-t BUTYL-4-METHYLPHENOL :

[PP] o*2.5 M [t-Bu O ] -1.2x10" M 2


2

2

0

120°C BHT

1

1/ΠΝΗ] x10 ,M 3

Figure 3.

2

3

M

Inhibited-rate dependence on inhibitor concentration

intercept of the line of Figure 3 should be equal to Ri. T h e apparent value of the intercept is 0.4 X l O ^ M m i n ' , whereas the value of Ri at 1.2 χ 10" M tert-Bu 0 is calculated from Equation l a to be 0.1 X 1 0 " M m i n . Uncertainty i n the value of the intercept does not seem to be the cause of the discrepancy. 1

2

2

4

2

-1

Rjnh ° 'MMIN 3 x 1

4

1

[BHT] »3x10- M [t-Bu 0 J »1.2x10" M 3

0

2

2 μ

2

2

0

120 C e

Figure 4.

Inhibited-rate dépendance

on polymer concentration

American Chemical Society Library .

.

ι -I

~. .

1155 16îh St. N. W.

Allara and Hawkins; Stabilization and Degradation of Polymers D. Society: C. 20036 Advances in Chemistry;Washington, American Chemical Washington, DC, 1978.

248


The upper line of Figure 3 shows that for a simple substrate, dihydroanthracene, R shows a strong dependence on inhibitor con centration. The agreement of the value of the intercept w i t h the computed Ri (~ 0.1 X 1 0 M min" ) is again poor. Figure 4 shows that there is no first-order dependence of R h on polypropylene concentration as predicted b y Equation 13. T h e actual dependence of the inhibited rate on substrate concentration is essentially zero, and useful kinetic chain lengths (KCL/s) cannot be attained b y increasing polypropylene concentration u p to 4.0M. Uninhibited (8) oxidations of polypropylene are ~ 0.7-order dependent on substrate, and computed KCL/s range upwards to 40. Computed KCL/s i n this case, i.e., Rinh/Ri, range from one to four for the conditions shown i n the Figure 4 legend. The failure to regularly increase the K C L of the inhibited oxidation b y increasing the substrate concentration discourages application of more complex (13) rate laws to the polymer system. A correct kinetic analysis (14) w i l l have to make use of a theory of concentrated polymer solutions where conformational and interpene trating network effects greatly alter the simple statistical basis upon which descriptive kinetics is based. Inhibition Periods of Polypropylene Oxidations. Although the k i netics of B H T - i n h i b i t e d polypropylene oxidations may be complex, the consumption of the inhibitor seems to follow simple stoichiometry, at least up to 3 X 10" M B H T . I n Figure 5, the t values for polypropylene oxidations [ P P ] — 2.5M, [terf-Bu20 ]o — 1.2 X 1 0 M are plotted against the initial B H T concentrations (triangles). The heavy line close to these inh

_4

1


in

3

0

inh

2

2

tta, MIN

Figure 5.

Inhibition of polypropylene (PP) oxidation


20.


INHIBITION MIN

249


TIME,

700

600

[PP] = 2.5 M [t-Bu 0a] = 1.2 χ ΙΟ" M

h-

2-t-BUTYL-4-METHYLPHEN0L

500 I Downloaded by UNIV OF PITTSBURGH on March 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0169.ch020

2

a

400 U

300 I

L

200

100 I

1

2

CONCN OF

P H E

3

4 10 .

{ j g L I C MOIETY

s

Figure 6. Inhibition of polypropylene (PP) oxidation. 1 = Dioctadecyl 3,5-di-tert-butyl-4-(hydroxybenzyl)phosphonate, 2 = 2,6-Bis(2octadecyl)-4-methylphenol, 3 = 2,6-Di-teTt-butylphenol, 4 = Pentaerythritol tetrakis[3-[3,5-di-ter±-butyl-4-(hydroxyphenyl)]propoinate], 5 = 2-tert-Butyl-4,6-dimethylphenol, 6 = 2-teit-Butylphenol, 7 = 2,6-Diisopropylphenol, 8 = 1,3,5-Tris[3,5di-tert-butyl^-Oiydroxybenzyl^^^'trihydroxytriazine, 9 = l,3,5-Trimethyl-2,4,6tris[3,5-di-tert-butyl-4-(hydroxybenzyl)]benzene 10 = Methyl 3-[3,5-di-tert-butyl4-(hydroxyphenyl)]propionate, 11 = Monoethyl 3,5-di-teTt-butyl-4-(hydroxybenzyl)phosphonate, nickel salt, 12 = 2-tert-Butyl-5-methylphenol, 13 = 4,4'-Methylenebis(2,6-di-tert-butylphenol) 14 = 2-tert-Butyl-6-meihylphenol, 15 = "Alkylated" 2-teTt-butyl-5-methylphenol, 16 = 2-tert-Butyl-4-ethylphenol, 17 = Methyl 3-[3teit-butyl-4-(hydroxyphenyl)]propionate, 18 = 2,4-Di-tert-butylphenol. 9

>

points represents the total number of radicals generated from decompo sition of the tert-butyl peroxide initiator as a function of time and was computed from the relationship

total radicals = 2 [£er£-Bu 02] [l — exp (— k t) ] 2

k

l a

0

1&

= 4.7 Χ 10" min" at 120°C 4

1



250

STABILIZATION

A N D D E G R A D A T I O N

O F

P O L Y M E R S

The coincidence of the line and the experimental points is very good although there is some indication that significant departure intrudes at the highest B H T concentrations; the last point, corresponding to 0.006M B H T concentration, falls well below the line. Reaction 6' may compete significantly with Reaction l b under these conditions. Most other inhibitors, as long as they contain the basic 2,6-di-fertfbutylphenol structure, can be included i n this correlation. The circles plotted i n Figure 6 represent data for most of the phenols tested besides B H T and 2-ferf-butyl-4-methylphenol. The lower Une appears to be a reasonable correlation for both the monofunctional phenols as well as commercial stabilizers when the actual molarity of the phenolic groups is plotted. The full utilization of all of the phenolic groups on the polyfunctional inhibitors is somewhat surprising, but is consistent w i t h other aspects of this investigation as described later. The most interesting data pertain to the effectiveness of singly hindered phenols as antioxidants. The top line plotted i n Figure 5 correlates data for oxidations inhibited by 2-ter£-butyl-4-methylphenol. Some other 2-ter£-butyl-4-alkyrphenols are included along the top line of Figure 6. Clearly, a given concentration of the singly hindered phenol gives a significantly greater inhibition period, actually by a factor of two. Exceptions to this classification include o-ter£-butylphenol, w h i c h seems to have the same inhibiting stoichiometry as the di-terf-butylphenols. 2-ierf-Butyl-5-methylphenol appears also to correlate best w i t h doubly hindered phenols. Figures 5 and 6 show that two polypropylene peroxy radicals are scavenged by the singly hindered phenols, whereas only one is scavenged by the di-ierf-butylphenols. For most inhibitors acting on the low molecular weight substrate cumene, a stoichiometry of two radicals consumed per inhibitor molecule has been found (15). Apparently for polypropylene substrate, 2-ferf-butyl-4-methylphenol more nearly ap proaches this ideal. Thus, for B H T - i n h i b i t e d oxidations Reaction 9 applies, whereas w i t h singly hindered phenols, Reaction 7 is appropriate. Reaction 7 along w i t h 2:1 stoichiometry is commonly assumed for neat polymer systems so that the number of radicals generated during an oxidation can be determined (16,17). In the absence of analytical data for the products formed from the antioxidants, we can only speculate on the reason for the observed stoichiometry. However, a reviewer has suggested that an explanation may lie i n the instability of peroxy radical-phenoxy radical products at 120°C (18) and that the only effective terminations are those that occur between two phenoxy radicals. F o r the singly hindered phenoxy radical product, there is the possibility of terminating two additional peroxy radicals as follows.


20.



251


F o r the doubly hindered phenoxy radical products, there are no active hydrogens to be scavenged by the peroxy radicals; further, disproportionation of the dimer back to the parent phenol and a quinone methide must be assumed to be slow (19).

Table IV.

Oven Aging of Polypropylene in the Presence of Antioxidants Oven Life, Hr for 5-Mil Film Samples

Run No.

, , r, 7 ,· Polypropylene Formulations : Unstabilized, Crystalline Polypropylene + the Following:

160° C

140°C

1 2 3 4 5 6 7

Control (no stabilizer) + III,0.1wt% + I V (0.1%) V(0.1%) + I I I (0.1%) + D L T P D " (0.3?o) + I V (0.1%) + D L T D P (0.3%) + V (0.1%) + D L T D P (0.3%)

1 15 15 20 90 40 70

1 340 200 300 600 650 230

n

β

+

Dilauryl 3,3'-thiodipropionate.



252

STABILIZATION

A N DD E G R A D A T I O N

O F

P O L Y M E R S

Application to Oven-Aging Tests. The results of the preceding section suggest that a superior polyolefin antioxidant could be obtained by keeping a l l phenolic moieties i n the molecule singly hindered with one teri-butyl group. T w o high molecular weight antioxidants whose structures were assigned as III and I V were synthesized and evaluated in oven-aging tests on polypropylene. These materials were compared w i t h the similar commercial stabilizer V that contains the 2,6,-di-ieributylphenol moiety. In oven-aging tests at 140°C on thin films w i t h the synergist (20) dilauryl 3,3'-thiodipropionate added, stabilizers III and I V appeared to be superior to V (see Table I V ) . Under other conditions, the results were inconclusive. Conclusion This investigation has shown that most antioxidants based on the 2,6-di-ter£-butyl-4-alkylphenol moiety behave similarly as inhibitors i n the solution oxidation of polypropylene. Although the kinetics of oxidations are complex, the length of the inhibition period is proportional to the phenolic group concentration. W i t h 2-£erf-butyl-4-alkylphenols, the apparent stoichiometry for reaction with polypropylene peroxy radicals is increased from one to two.

Literature Cited

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Inhibited Autoxidation of Polypropylene - Advances in Chemistry (ACS

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