Diffusion and Solubility of Hindered Amine Light Stabilizers in

29

Diffusion and Solubility of Hindered

Amine Light Stabilizers in Polyolefins

Influence on Stabilization Efficiency and

Downloaded by UNIV OF SYDNEY on September 4, 2014 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch029

Implications for Polymer-Bound Stabilizers

1,2

3

3

Ján Malík , Alexander Hrivík , and Dam Q. Tuan 1

VUCHT a.s., 836 03 Bratislava, Slovakia

3

Slovak Technical University, Faculty of Chemical Technology, Bratislava,

Slovakia Results of diffusion and solubility light stabilizers

(HALS)

measurements

in polyethylene

sented. These physical parameters

of hindered

and polypropylene

amine

are pre-

are correlated with the efficiency of

stabilizers. Moisan's empirical relationship for phenolic antioxidants in low-density polyethylene ship supports

also applies to HALS

the idea of polymer-bound

efficiency. This relation-

stabilizers;

therefore,

some

results and ideas in this field are also presented.

H I N D E R E D

A M I N E L I G H T STABILIZERS ( H A L S ) were introduced to industry

relatively recently b u t soon they took the leading role i n light stabilization o f p o l y m e r s , e s p e c i a l l y p o l y o l e f i n s . S i n c e t h e i r i n t r o d u c t i o n , n o n e w class o f l i g h t stabilizers has a p p e a r e d that surpasses t h e l e v e l o f p o l y o l e f i n stabilization of fered b y H A L S . C o n s e q u e n t l y , H A L S has b e e n o n e o f t h e most actively investigated clas ses o f p o l y m e r a d d i t i v e s i n r e c e n t y e a r s . T h e n u m b e r chemical mechanisms

of H A L S

o f papers dealing with

i n p o l y m e r stabilization is indicative o f t h e i r

importance. M o s t authors considered oxidation products

of H A L S

(especially nitroxyl

radicals) to b e t h e k e y to their excellent light stabilization performance. recent works s h o w e d that the H A L S

Some

m e c h a n i s m is q u i t e c o m p l e x . I n a d d i t i o n

to the various transformation products, the charge transfer c o m p l e x o f the par ent amine w i t h p o l y m e r c a n play a n important role i n p o l y m e r stabilization ( J 5). 2

Current address: Clariant Huningue S. Α., Polymer Additives, BP 149, F-68331 Huningue Cedex, France.

0065-2393/96/0249-0455$12.00/0 © 1996 A m e r i c a n C h e m i c a l Society

In Polymer Durability; Clough, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


456

POLYMER DURABILITY

It is well known that the degradation of a stabilized polymer is accom panied by a loss of most of the effective stabilizer. Obviously this loss can be caused by chemical consumption of the stabilizer in stabilization reactions, but it may also be caused by physical loss of the stabilizer by such processes as evaporation, leaching, and blooming. Contrary to the great amount of published work devoted to stabilization mechanisms of H A L S , practically no data are available concerning the physical behavior of H A L S in polymers. The published works on physical loss problems (6-8) do not include the data for hindered piperidine stabilizers. In our earlier works (9, 10), we presented some results on diffusion and solubility measurements of H A L S in polyolefins. In this chapter we present a summary on the diffusion and solubility measurements. W e also attempt to correlate these parameters with the measured efficiency of the stabilizers. This correlation lead to the experiments with polymer-bound stabilizers, and the results and suggestions in this field are also discussed.

Diffusion and Solubility of HALS Low-Density Polyethylene. Diffusion coefficients (D) and solubil ities (S) of H A L S i n polyolefins were measured by the dynamic method de scribed by Moisan (8). The method was modified slighdy to avoid formation of air bubbles between individual films of polymer (9). After the diffusion experiment the individual films were peeled off from the stack and were ex tracted in CC1 for 24 h. Each extract was analyzed for H A L S content. From the concentration profile in the stack of films, D and S were computed. For this computation a mathematical approximation of the appropriate solution of Fick's second law was used (9). In the experiments where solubility values exceeded 7-8% (observed with Dastib 845 [see Chart I] in low-density poly ethylene [ L D P E ] at higher temperatures), a deviation of the theoretical curve from the experimental data was observed. The reason for this deviation could be a concentration dependence of diffusion at these relatively high concen trations of diffusant. Each diffusion experiment was performed at least twice at the same tem perature. The experimental error of a single D estimation was ± 2 0 % . The structures of stabilizers are shown in Chart I. The results obtained in D and S measurements are presented in the form of parameters of Arrhenius equations for D and S. Table I shows the results for L D P E . With Diacetam 5 (see Chart I), we were not able to measure the penetration of the stabilizer into the stack of polymer films at room temper ature. The unmeasurable penetration was caused by very low solubility of this stabilizer in L D P E matrix. With oligomeric Chimassorb 944 (see Chart I), no measurable penetration occurred after 10 months at room temperature. Also, experiments with this stabilizer at elevated temperatures were not successful. 4


M A L I K ET AL.

Hindered Amine Light Stabilizers


29.

Chart I. Structural formulas for HALS.


457

458

POLYMER DURABILITY


Only one stabilizer (Diamine 6, see Chart I) exhibited first-order transition (melting) within the measured temperature range. With this stabilizer we ob served a change in the slope of the Arrhenius plot for S, where the difference between change in enthalpy (ΔΗ) over melting point (mp) and ΔΗ under mp was higher than the heat of fusion of this stabilizer (71.7 kj/mol vs. 42.9 kj/ mol fusion, respectively). No obvious change in the slope of the Arrhenius plot for D was found (the calculated difference was within the span of exper imental error). P o l y p r o p y l e n e . Results obtained with polypropylene (PP) are pre sented in Table II. Again, no measurable diffusion of Chimassorb 944 was observed. For Diamine 6 the same effect of melting on D and S was observed with P P as with L D P E — n o evident change in activation energy of diffusion E and relatively great change in S. The difference in ΔΗ (87.6 kj/mol) over and under mp is again much bigger than the heat of fusion. In comparison to the other measured stabilizers, ΔΗ values for Diamine 6 under mp are relatively high in P P and L D P E . d

Table I. Diffusion and Solubility of HALS in L D P E log D

HALS

mp

D-845 T-770 Diac-5 D-6 D-6

28-31 81-83 134-137 61-63 —

57.2 83.1 38.3 75.5

1.55 5.3 -1.6 4.37

—

D

Τ

ΔΗ

log S

2.0 0.74 1.6 1.5

30-75 25-75 50-80 30-75 >mp

18.2 43.2 30.6 96.2 24.5

7.54 10.55 7.21 19.8 8.4

0

—

—

S

0

39,600 3700 180 17,760 —

NOTE: D-845 is Diastib 845, T-770 is Tinuvin 770, Diac-5 is Diacetam 5, and D-6 is Diamine 6 (see Chart I): mp is melting point (°C), E is activation energy of diffusion (kj/mole), D is preexponential parameter of Arrhenius equation for diffusion, D is diffusion coefficient at 50 °C (ΧΙΟ" cm/s), Τ is temperature (°C), AH is heat of solution (kj/mole), S is preexponential parameter of Arrhenius equation for solubility, and S is solubility at 50 °C (ppm). 0

d

8

2

0

Table II. Diffusion and Solubility of HALS in PP HALS

mp

D-845 T-770 Diac-5 D-6 D-6

28-31 81-83 134H37 61-63 —

log D

D

Τ

ΔΗ

log S

109 89.5 79.9 114.7

8.2 4.9 3.3 9.1

3.8 2.7 2.5 3.6

—

—

—

60-90 55-80 60-90 55-80 >mp

14.7 69.7 17.7 147.1 59.5

8.65 14.5 5.81 27 13.4

0

0

S 18,780 1800 890 1670 —

NOTE: D-845 is Dastib 845, T-770 is Tinuvin 770, Diac-5 is Diacetam 5, and D-6 is Diamine 6 {see Chart I): mp is melting point (°C), E is activation energy of diffusion (kj/mole), D is preexponential parameter of Arrhenius equation for diffusion, D is diffusion coefficient at 50 °C (X10~ cm/s), Τ is temperature (°C), ΔΗ is heat of solution (kj/mole), S is preexponential parameter of Arrhenius equation for solubility, S is solubility at 50 °C (ppm). d

10

0

2

0


29.

M A L I K ET A L .


459

The relative error of E estimation amounted to ± 10%, and the relative error of AH was higher ( ± 2 0 % ) . Measurements with Diamine 6 were the exception, because the relative error of E estimation in P P was ±24 % and the relative error of AH in P P and L D P E reached 35%. Later investigations showed that Diamine 6 is not a very stable compound. In the presence of water (even exposed to air moisture for an extended time), Diamine 6 forms several crystaUine modifications with different heats of fusion and different mps. Therefore, the presented data for Diamine 6 should be regarded with this fact in mind. However, the observed discontinuities in the Arrhenius plots of S of Diamine 6 are outside the error. The discontinuities in Arrhenius plots for D and S of phenolic antioxidants in L D P E at their mps were observed by Moisan (8). Al-Malaika et al. (II) also reported discontinuities for D of several antioxidants in L D P E . However, explanations of this phenomenon were different. Whereas Moisan (8) attrib uted the discontinuity in E to differences in the physical form of the additives, Al-Malaika et al. (II) suggested that the discontinuity is more likely due to morphological changes of the polymer. In this connection, Billingham (12) pointed out that diffusion of an ad ditive is a molecular-level process that is determined by an interaction be tween polymer and an isolated diffusant molecule/Therefore, no physical reason exists for the change in E at the mp of diffusant. Billingham (12) also suggested that the change in E results from morphological changes on an nealing of the polymer during experiments. As we already mentioned, no ob vious change in the slope of Arrhenius plot for D was observed with Diamine 6 in L D P E and in PP. A change in Arrhenius plot for S of an additive in a polymer is a different phenomenon. When a crystaUine additive was used as a source of diffusing molecules (12), a change in the AH was observed that corresponded to the heat of fusion of the used additives within experimental error. We used (9) pure additive as a source of diffusing molecules and observed a marked change in the slope at the mp of Diamine 6 in L D P E and PP. However, it is difficult to explain the change in the Arrhenius S plot that was reported by Moisan (8), who used a polymer with 2-10% additive as the source of the diffusing molecules. The source was prepared by processing at 150 °C, a temperature that is higher than the mps of the additives. We are not aware of any reports of a well-characterized crystallization of antioxidants inside polyolefins, and so the antioxidants after the processing were probably presented in a form of supersaturated solution. Hence a change in Arrhenius S plot connected to the heat of fusion of crystalline additive at its mp can hardly be expected. d


d

d

d

d

S o l u b i l i t y i n M o d e l L i q u i d s . The D coefficient values of all lowmolecular stabilizers in L D P E and PP obtained in our measurements (Tables I and II) were comparable, but the S values differed greatly. Therefore, the S results obtained from dynamic measurements were compared with S values


460

POLYMER DURABILITY

of the stabilizers in organic liquids (see Table III). The relative ranking and ratios of S in model liquids coincide very well with the results obtained in the polymers. Surprisingly the S of oligomeric Chimassorb 944 in the nonpolar liquids was one of the best. Therefore, the reason for nonmeasurable pene tration of the oligomer into the stack of polymer films was the size of stabil izer's molecules.


Permeation Experiments and Extrapolation of Oligomer Diffusion Parameters Because the experiments with a stack of polymer films did not enable us to obtain the D parameters of oligomeric Chimassorb 944, we tried to measure the permeation of this stabilizer through swollen L D P E film (JO). Two types of experimental arrangements were used (Figure 1) and both offered com parable results. In the first type, a steel chamber was divided into two parts that were separated by L D P E film. One part of the chamber contained con centrated solution of Chimassorb 944 in CC1 and the other part contained pure CC1 . In the second type, the concentrated solution of the stabilizer was thermowelded into an L D P E bag, and the bag was then placed into a vessel with pure CC1 . The experiments were done at room temperature (at higher temperatures C C l dissolves polyethylene films). The amount of penetrated stabilizer for a given experimental time (Q ) was measured in the solvent by UV-spectroscopy of the H A L S complex with iodine (9). The obtained values were used for computation of D from an appropriate form of Fiek's law, which included an intercept on the time axis known as "time-lag": 4

4

4

4

t

Q = -DA(c t

2

- cJOAftt - d /6D] 2

where A is the area of the film, I is film thickness, and t is time. The concen trations c and c are given as: 2

1

c = (Qo ~ QÙ/V2 2

c = x

Q/V,

where Q is the original amount of H A L S in the solution, and V and V represent volume of H A L S solution and volume of pure solvent, respectively. Experimental data generated by measurements with Chimassorb 944 were not properly fitted by the theoretical curve. The experimental dependence started to settie down after permeation of only about 3% Q dissolved in the concentrated solution. Apparently, only a small part of the oligomer was able to penetrate the swollen L D P E film. Therefore, in our next calculations only a fraction of the original amount of stabilizer presented in concentrated so lution was used. With 4% Q the experimental data fit very well with the 2

0

0

0


x


29.

M A L I K ET AL.


461

Figure 1. Experimental arrangements of CCl solvent (1), saturated solution of stabilizer in CCl (2), and LPDE membrane (3) for permeation measurements. Part A: steel chamber. Part B: LDPE bag. 4

4

Table III. Solubility of H A L S in Model Liquids HALS

Heptane

Dastib 845 Tinuvin 770 Diacetam 5 Diamine 6 Chimassorb 944

49.20 5.80 0.08 30 53.70

Parafin Oil

PPOil 1.20 0.70 0.04

23.20 2.10 0.16 4.5 14.50

—

3.00

N O T E : For structures of HALS, see Chart I. All values are in weight percent. SOURCE: Reproduced with permission from reference 9. Copy right 1992 Elsevier Science.

theoretical curve ( F i g u r e 2). (The dotted line i n the F i g u r e 2 corresponds to the real value o f Q .) T h e calculated values for D o f 4 % o f the oligomer ranged f r o m 2.1 Χ 1 0 " to 2.6 Χ Ι Ο " cm /s. 0

1 0

10

2

T h e same experiments were done w i t h low-molecular D a s t i b 845. T h e experimental data fit w e l l w i t h the theoretical curve for the whole amount o f stabilizer (100% ρ ) , and D ranged f r o m 1.6 Χ 1 0 " to 2.49 Χ 1 0 " cm /s. T h e D value obtained from the measurement i n a stack was 7 X 10"" cm /s, w h i c h means that D i n swollen L D P E was 2 - 3 times faster. ο

9

9

2

10

2

I n the next step an attempt for mathematical extrapolation o f D p a r a meters o f the oligomer was done. T h e extrapolation was based o n three equa tions (10):



462

POLYMER

DURABILITY

3500 time [h]

Figure 2. Permeation of Chimassorb 944 showing experimental data (solid boxes) calculated line for actual amount of Chimassorb 944 (broken line), and calculated line for Q = 4% of the actual amount of Chimassorb 944. 0

• • •

Arrhenius equation for D empirical Auerbach dependence of D on molecular weight of diffu sant dependence of the logarithm of preexponential factor of Arrhenius equation on E , known also as "compensation effect equation" d

Mutual substitution of these three equations leads to the dependence of E on molecular weight of diffusant (M ):

d

w

E

d

= Α' + Β» X In M

w

For the computation of the constants A' and B' in the extrapolation equa tions, data from Table I were used. The D parameters of the oligomer were calculated, and results are presented in Table IV. Obviously, only molecules with one structural unit can diffuse through L D P E matrix at a comparable rate to that of the low-molecular weight stabilizers. The D values of molecules with two and more structural units are too small to be comparable with the rate of diffusion of low-molecular weight stabilizers. The computed D for one structural unit is about one-half the value obtained from permeation experi ments; a similar difference was found for low-molecular weight Dastib 845. O n the basis of the obtained and extrapolated results, we assumed that the oligomer should contain approximately 4% low-molecular weight fraction cor-


29.

M A L I K ET AL.

463



responding to molecules with one structural unit. This assumption was later confirmed by gel permeation chromatographic measurement of molecular weight distribution. As seen in Figure 3, the fraction with molecular weight under 1000 is really about 5%. According to the previously stated results, approximately 95% of the mol ecules of the highly effective oligomerie stabilizer are translationally immobile in the polymer matrix. Therefore, the common explanation of the decreased efficiency of oligomerie stabilizers with increased molecular weight as a result of reduced stabilizer mobility is questionable.

Table IV. Extrapolated Diffusion Parameters of Chimassorb 944 i n L D P E No. units 1 2 3 4 5 6

M

E

w

600 1200 1800 2400 3000 3600

In D

d

X 101.1 X 10" 7.8 X 10" 1.1 X io2.5 X 10" 6.8 X 10" is activation energy of

24.1 58.1 78.1 92.2 103.2 112.1

115.5 210.8 266.6 306.2 336.9 362.0

D

0

1.2

-10 -12

-14 -14

-15

-16

NOTE: M is molecular weight, £ diffusion (kj/mole), D is preexponential parameter of Arrhenius equation for diffusion, and D is diffusion coefficient at 23 °C (cmVs). w

d

0

Figure 3. Molecular weight distribution of Chimassorb 944.



464

POLYMER DURABILITY

Research data from Ciba-Geigy (13) obtained with a polymerized acrylic derivative of H A L S implied that the stabilizer gives the maximum perform ance at molecular weight —2700. However, Minagawa (14) reported the max imum in H A L S efficiency at a much lower molecular weight (—600), whereas the thickness of tested P P specimens was the same (50 μηι). Hrdlovic and Chmela (15, 16) showed that stabilization efficiency of a copolymer of acrylate H A L S i n P P decreases exponentially with increasing molecular weight of the copolymer. They also (17) documented that copolymers of acrylate H A L S with monomers containing long alkyl chains and having molecular weight

Diffusion and Solubility of Hindered Amine Light Stabilizers in

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