The performance of light stabilizers in accelerated and natural weathering

Polymer Degradation and

Stability SO (1995) 101-116 Elsevier Science Limited Printed in Northern Ireland

0141-3910(95)00134-4

0141-3910/95/$09.50

The performance of light stabilizers in accelerated and natural weathering F. Gugumus Ciba-Geigy AG, Base& Switzerland (Received

12 April 1995; accepted 1 June 1995)

The performance of different classes of light stabilizers is compared in polyolefins. The evolution of stabilization in the past quarter century is illustrated with data generated with polyolefin tapes, muhitilaments, fihns and plaques. The state-of-the-art, based on combinations of light stabilizers, mainly combinations of hindered amine light stabilizers (HALS) is discussed. The light stability data are generated either in various accelerated weathering devices or outdoors in various climates. Correlation between the various accelerated devices on the one hand and between accelerated and natural weathering data on the other hand was the subject of many publications. It is still one of the major problems encountered in assessing the durability of plastic materials for outdoor use. It is found that, even with adequate precautions ‘concerning the light source and its intensity as well as sample temperature, correlation between natural and artificial weathering is the exception rather than the rule. Therefore, another model for lifetime forecasting is proposed. It is based on the quantitative laws deduced for HALS performance in polyole6ns. They are, linear increase of performance with HALS concentration in PP and linear increase with the square root of HALS concentration in PE-I-ID. Then, the knowledge of the effect of one HALS concentration on outdoor weathering with a given polymer and sample form allows prediction of the lifetime with other HALS concentrations. As a consequence, the problem of lifetime forecasting is reduced to an analytical problem, i.e. determination of the stabilizer system.

PP is not to be exposed to light. It will be a stabilization against photothermal oxidation if UV light exposure is involved. The requirements for PE are similar to those for PP but the stabilizer levels are usually lower because PE is inherently less sensitive to oxidative attack than PP. Processing stabilization is usually achieved with combinations of high molecular mass phenolic antioxidants with phosphites or phosphonites. Long-term heat stabilization involves either high molecular mass phenolic antioxidants or high molecular mass or polymeric HALS. These aspects will not be developed here. For details the reader is referred to recent publications.‘4 In the following, the evolution of UV stabilization

1 INTRODUCTION Polypropylene is the most typical example of a major commercial plastic which would never have achieved any practical use without the development of adequate stabilizer systems. PP needs protection in every stage of its life cycle. It starts with storage, immediately after manufacturing. Then, a small amount of a phenolic antioxidant usually gives sufficient protection. It continues with processing where adequate stabilization is a prerequisite for minimizing degradation of PP in the molten state at temperatures between 200 and 300°C. It ends with a suitable stabilization for the application foreseen. This is a stabilization against thermal oxidation only, if 101

F. Gugumus

102

of polyolefins from the beginning trends will be shown.

to the actual

2 EXPERIMENTAL 2.1 Polymers and additives The polymers used were mainly unstabilized commercial resins stored in a refrigerated room to minimize oxidation. For some of the tests, polymers already containing a small amount of a phenolic antioxidant for storage stabilization were used. The most important characteristics of the polymers as well as the basic stabilization are indicated in the tables and figures. The antioxidants used for the basic stabilization were commercial products as identified in the Appendix. The antacid used was usually Ca stearate (MERCK). The light stabilizers were also commercial products, the structures are given in The concentrations of the the Appendix. additives are expressed in weight% unless otherwise stated. 2.2 Extrusion and injection molding The additives were dry-blended into an unstabilized or storage stabilized PP powder and extruder compounded at 220°C. PP multifilaments were melt-spun at 270°C with high melt index fiber grade material. The multifilaments consisted of 37 single filaments with a total denier of 130 and a draw ratio of 1:3.2 if not otherwise indicated in the tables and figures. PP tapes were manufactured by extruding a flat film at 260°C and drawing it to a ratio of 1:6. The polypropylene used for sample preparation was PP when not especially second generation identified as third generation PP. For PE-HD tapes the conditions were slightly different. Compounding was performed at 23O”C, film extrusion at 240°C and the draw ratio was 18.5. The thickness of the PP and PE-HD tapes was 50 mm unless otherwise stated. The 2 mm thick PP and PE-HD plaques were prepared by injection molding at 260°C. PE-LD was compounded at 200°C. Films were blown at 200°C and 2 mm thick dumbbells injection molded at 220°C. 2.3 Exposure The accelerated exposure devices used in the experiments include a Xenotest 150 and a

Xenotest 1200 as well as a Weather-Ometer WRC 600 and a Weather-Ometer Ci65. They were equipped exclusively with Xenon lamps. The Xenotest devices were used in the sample holder rotation mode. The black panel temperature of the artificial exposure devices was 53 f 2°C unless otherwise stated. The backing of the tapes and multifilaments in the accelerated exposure devices was white cardboard. Natural weathering was performed in Florida, the samples facing south with an angle of 45”C, in direct sunlight, without glass filter. The energy received is about 140 kLy/year (mean value). The samples were mounted without tension on transparent poly(methylmethacrylate) plates. 2.4 Testing Mechanical testing, i.e. determination of elongation, tensile strength, impact strength and tensile impact strength was in agreement with the procedures outlined in ASTM D638, D882 and D1822. The criteria used to test UV stability were usually the time on accelerated exposure received on natural (T,“) or the energy weathering (&) to 50% retained tensile strength for tapes and multifilaments, to 50% retained elongation for PE-LD films and plaques, to 50% retained impact strength for PP plaques and to 50% tensile impact strength for PE-HD plaques. Similarly, E,, and Echalkingstand for the energy received to retained impact strength of 15 kJ/m* and to the beginning of chalking for PP plaques, respectively. T,,,, is the time to 1.0 carbonyl absorbance.

3 LIGHT STABILIZATION SECTIONS

OF THIN

Light stability of polyolefins has been improved successively in the past decades as more and more powerful light stabilizers become available. At first mainly UV absorbers were available. The most important classes, the benzophenone and the benzotriazole type UV absorbers are still used today. The stability imparted by UV absorbers was improved when Ni stabilizers such as Ni-1 were developed. With the development of the benzoate type light stabilizers such as Bz-1 another improvement was achieved, especially for PP. However, before the benzoates were used

The performance of light stabilizers in accelerated and natural weathering

commercially on a large scale, a new class of light stabilizers was introduced 20 years ago, the Hindered Amine Light Stabilizers (HALS).’ At the beginning, low molecular mass HALS only were available. HALS-1 is the most typical and still most used representative of this class of compounds. To circumvent some disadvantages associated with low molecular mass stabilizers, e.g. high migration rate and moderate resistance to extraction, polymeric HALS were developed. HALS-2 and HALS-3 were the first polymeric HALS available commercially. In the last few years many new hindered amine type compounds have been developed. Some of these compounds show advantages in very specific applications. However, the UV protection conferred by these new compounds is usally of the same order as that of the compounds already available. It is noteworthy, that no fundamental new class of compounds showing an efficiency comparable to that of the hindered amines has been commercialized so far. This explains readily why the most important developments in UV stabilization of polyolefins are based on combinations of UV stabilizers. 3.1 PP tapes The data of Table 1 illustrate the performance in PP tapes of the various light stabilizer classes mentioned above. Thus, on natural weathering in Florida, the benzotriazole UV absorber UVA-1, the benzophenone UV absorber UVA4 and the nickel stabilizer Ni-1 show comparable efficiency. The benzoate Bz-1 is significantly better than the above stabilizers, confirming that a marked improvement of UV stability had been achieved with the development of this stabilizer class. Table 1. Iduence of UV stabilizer type on light ?+thility of PP tapes UV stabilization Control 0.5% UVA-1 0.5% Bz-1 0.5% Ni-1 0.5% UVA-4 0.05% HALS-1 0.10% HALS-1 0.10% HALS3 0.20% HALS3

103

However, the low molecular weight HALS, HALS-1, yields an even better light stability than Bz-1, although it is used at a concentration ten times lower (0.05% vs 0.5%!). The superiority of polymeric HALS such as HALS3 is less pronounced than that of the low molecular mass HALS-1. Nevertheless, it is still considerable. The plot of Fig. 1 shows comparable performance for HALS-1 and the corresponding N-methylated HALS4 in third generation PP on artificial exposure. The polymeric HALS3 and the non-polymeric, high molecular mass HALS-5 also show comparable performance but on a somewhat lower level than HALS-1 and HALS4. Figure 1 shows that, within experimental error, there is a linear increase of performance with stabilizer concentration. This extends to third generation PP the previous findings with second generation PP. 6,7In addition, the results in Fig. 1 confirm once more the usual equivalency between secondary HALS and the corresponding N-methylated derivatives.6 The performance of combinations of various UV stabilizers with HALS-1 on artificial exposure is presented in Table 2. The data show quite clearly that the benzotriazole type UV absorber UVA-1 and the nickel stabilizer Ni-2 do not contribute to the UV stability already conferred by HALS-1. Only the benzoate Bz-1 gives a significant improvement of UV stability when added on top of HALS-1. However, this contribution of Bz-1 is significantly below that of the same amount of HALS-1. Thus, the combination of two UV stabilizers from two distinct classes of UV stabilizers does not yield any synergistic effect. The combination of two UV stabilizers from which no synergistic effect could be expected, a priori, notably two -

Es,, (kLy) Florida 30 65 140 65 70 205 320 205 60

PP homopolymer (PP-1) + 0.05% AO-2 + 0.05% P-l + 0.05% Ca stearate. Florida exposure started July 1986.

oiws,

ml4us.a

r-

r-

Fig. 1. Influence of HALS concentration and structure on UV stability of PP tapes. PP-2 (third generation) + 0.1% Ca stearate + 0.05% AO-1 + 0.05% Pl.

104

F. Gugumus

Table 2. Contribution of UV stabilizers to the performance of HALS-1 in PP tapes

UV stabilization

T’,, (h) Xenotest 150 380 4500 6800 4450 5600 4800 780 1600

Control 0.25 % HALS-1 0.50% HALS-1 0.25% HALS-1 + 0.25% UVA-1 0.25% HALS-1 + 0.25% Bz-1 0.25% HALS-1 + 0.25% Ni-2 0.50% UVA-1 0.50% Bz-1

30 pm

PP homopolymer (PP-3) + 0.1% AO-2. Tapes: thick. Xenotest 150: black panel temperature 40 & 2°C.

HALS, gives the most astonishing results. The use of combinations of low molecular mass HALS with polymeric or non-polymeric high molecular mass HALS led to a significant improvement in polyolefin stabilization. Data obtained with a combination of the low molecular mass HALS-1 with the polymeric HALS3 on natural weathering are presented in Table 3. It can be seen that, overall, the light stability increases with the relative proportion of HALS-1 in the HALS-l/HALS-3 blend. However, there is synergism in the whole composition range. These data were obtained with second generation PP. The first results of artificial exposure of a third generation PP with HALS-1, HALS-3 and their 1:l combination are presented in Fig. 2. For comparison purposes the corresponding data obtained with a second generation PP are also plotted in Fig. 2. It can be seen that the performance of the combination as well as that of the two components increases linearly with the concentration. This confirms for third generation PP, at least on artificial exposure, what had already been found with second generation PP. The results obtained with the combination Table 3. Performance of HALS-l/HAL!&3 binations in PP tapes

HALS (0.2%) None HALS-1 HALS-1 /HALS3 3: 1 HALS-1 /HALS-3 1:l HALS-l/HAL?&3 1:3 HALS3

com-

E,,, (kLy) Florida 30 460 440 370 305 190

PP homopolymer (PP-4) + 0.05% AO-2 + 0.05% P-l + 0.1% Ca stearate. Florida exposure started in March 1982.

Fig. 2. Influence

of HALS concentration on UV stability of PP tapes manufactured with second and third generation polymer. PP+ 0.1% Ca stearate + 0.05% AO-1 + 0.05% P-l.

HALS-l/HALS-3 are especially interesting because they involve two secondary hindered amines. Thus, the main difference between the two compounds resides in the backbone structure and, of course, in the molecular mass. Similar effects can be obtained combining the low molecular mass HALS-1 with the polymeric HALS-2. With this combination, besides the difference in molecular mass, the substitution of the tetramethyl-piperidine nitrogen atom is also different. The HALS combinations envisaged so far, i.e. HALS-l/HALS-2 and HALS-l/HALS-3, involve a low molecular mass HALS and a polymeric HALS. It is even more unexpected that the combination of two polymeric HALS such as HALS-2 and HALS-3 also yields synergistic effects in PP tapes. Thus, the data in Table 4 show the superiority of the 1:l combination HALS-2/HALS-3 over both compounds at the same concentration. The synergistic effect also looks straightforward in Fig. 3 which shows the influence of the composition of the combinations for the total HALS concentrations tested. In summary, with PP tapes, the results obtained so far suggest a linear increase of Table 4. Performance of polymeric PP tapes

HALS (0.2%) ~~None HALS-2 HALS3 HALS-2/HALS-3

HALS

in

ES, (kLy) Florida

1:l

36 155 235 260

PP homopolymer (PP-5) + 0.05% AO-2 + 0.05% P-l + 0.1% Ca stearate. Florida exposure started in August 1988.

The performance

of light stabilizers in accelerated and natural weathering

4aa

Fig. 3. Influence of the composition of combinations of HALS-2 and HALS3 on W stability of PP tapes. PP-5 + 0.1% Ca stearate + 0.05% AO-1 + 0.05% P-l.

performance with HALS concentration, as long as the latter is not too high. This law is of broad validity and has been checked with low molecular mass HALS as well as with high molecular mass HALS or polymeric HALS. Furthermore, combinations of low molecular mass HALS with high molecular mass HALS/polymeric HALS and combinations of two polymeric HALS also show the linear dependence of performance on total HALS concentration. Most of the data available so far were generated with second generation PP. However the first results of artificial exposure with third generation PP show similar dependence on HALS concentration. The lifetime of PP tapes can be expressed by the following relation: T50 = c = (ti)o + b (HALS), where (ti)o is the lifetime in the absence of HALS, i.e. that of the base stabilization, (HALS), the concentration of HALS and b is a proportionality factor.

105

many times.3~9~10The same holds for the performance of HALS in PP multifilaments on artificial exposure. It was found to increase often linearly with HALS concentration.3*G10 This is the same dependency as that found with PP tapes as discussed above. However, some data with PP multifdaments show linear increase of the performance with the square root of the HALS concentration.6,7 This was attributed to variations in polymer quality combined with the fact that the stabilizer concentrations used with PP multifilaments are usually significantly higher than the concentrations in PP tapes. The data in Fig. 4 confirm this difference in behavior for HALS3 on outdoor weathering. However, they also give a hint to a possible explanation of this variation. As a matter of fact, with the second generation PP, there is a linear increase of light stability with HALS-3 concentration. With the third generation PP, the performance increases proportionally to the square root of the HALS3 concentration but on a higher level than that observed with the second generation polymer. Although there are not enough data, so far, it looks as if the low amount of catalyst residues in third generation PP would not only lead to a higher level of UV stability but also to a linear increase of the UV stability with the square root rather than directly with the HALS concentration. As already shown for PP tapes, combinations of HALS can also be advantageous in PP multifilaments. However, it is stated once more that, because low molecular mass HALS may be lost on various thermal treatments of the PP multifilaments, mainly polymeric or other high

3.2 PP multifilaments The light stabilizers used for PP multifilaments are overall the same as those used for PP tapes. Thus, HALS are state-of-the-art in PP multifilaments as much as in PP tapes. However, with PP multifilaments the choice of HALS is usually more restricted than with PP tapes. This is because some applications of PP multililaments involve more or less drastic thermal treatments. This can lead to more or less pronounced loss of low molecular mass HALS by migration or extraction. Data illustrating this behavior of low molecular mass HALS have been published

Feg. 4. Influence of HALS concentration and PP type on light stability of PP multifilaments. Base stabilization: 0.1% Ca stearate + 0.05% AO-3 + 0.05% PE wax + 0.1% P-l + 0.25% titanium dioxide (coated rutile). Florida exposure, started July 1991.

F. Gugumus

106

molecular mass HALS are used for the stabilization of PP multifilaments. Hence, the use of combinations of low molecular mass HALS with polymeric HALS is not generally recommended. However, under special conditions the performance of such combinations can be outstanding.3 Nevertheless, the combinations of polymeric HALS such as HALS-2 and HALS-3 are more generally applicable. As with PP tapes, synergistic effects can be observed (Fig. 5). Not only are combinations of the tertiary HALS-2 with the secondary HALS-3 interesting,’ combinations of two tertiary HALS such as HALS-2 and HALS-5 may also yield improved performance (Table 5). An additional advantage of such a combination is the reduced discoloration under special conditions (‘gas fading’). 3.3 PE-HD tapes The superiority of HALS over the other light stabilizer classes is as pronounced in PE-HD tapes as in PP tapes or PP mu1tifi1aments.“~‘~‘”The performance of the high molecular mass HALS2, HALS-3 and HALS-5 is shown in Fig. 6. It can be seen that the lifetime reached with the different HALS is proportional to the square root of the HALS concentration. It can be expressed by the following relation: r,, = ti = b (HALS)~ where (HALS), stands for the initial HALS concentration and b is a proportionality factor. This dependence has been found many times with PE-HD tapes, both on artificial and natural exposure.3,6,7 Of course, it is quite different from that found for PP tapes when linear increase with

0,

HIL62

5:1

2:1

1:1

1:2

1:5

Table 5. Combinations of HALS in PP multitilaments UV stabilization Control 0.6% HALS-2 0.6% HALS-5 0.3% HALS-2 + 0.3% HALS-5

T,,, (h) Xenotest 1200 250 3800 4300 4850

PP fiber grade (PP-9) + 0.05% AO-3 + 0.05% P-l + 0.1% Ca stearate + 0.25% titanium dioxide (coated rutile). Multifilaments without thermal treatment.

HALS concentration seems mostly valid. The difference of behavior of HALS in PP and in PE-HD is explained by fundamentally different photoinitiation photooxidation, especially mechanisms.‘j.’ It was shown above (see Table 2) that the addition of a benzotriazole type UV absorber (UVA-1) to HALS-1 did not contribute to the light stability of PP tapes. The data in Table 6 clearly show a significant detrimental effect of the benzophenone type UV absorber UVA-4 on the light stability conferred by HALS-1, both on artificial and natural weathering. The combination of the low molecular mass HALS-1 with the polymeric HALS-3 does not show such detrimental effects. However, the data of natural weathering do not point to any advantage of the combination over the component HALS-1 and HALS-3 used alone (Table 7). Combinations of the polymeric HALS-2 and HALS-3 showed additive effects only on artificial exposure.” This is being checked on natural weathering. As a preliminary conclusion, it can be stated that combinations of the polymeric HALS-2 and HALS-3 are much less advantageous for UV stabilization of PE-HD tapes than for PP tapes. Therefore, HALS-3 or its

I “ALS-3

Fig. 5. Influence of the composition of combinations of HALS-2 and HALS3 on UV stability of PP multifilaments. PP fiber grade (PP-8) + 0.1% Ca stearate + 0.05% AO-3 + 0.05% P-l + 0.25% TiO, (coated rutile). Total HALS concentration: 0.3 %.

Fig. 6. Influence of HALS structure and concentration UV stability of PE-HD tapes. PE-HD-1 + 0.1% stearate + 0.05% AO-4.

on Ca

The performance of light stabilizers in accelerated and natural weathering Table 6. Performance

of HALS/UV absorber combinations in PE-HD tapes

UV stabilization

G,, (h) WeatherOmeter WRC 600

E,, (kLy) Florida

Control 0.05% HALS-1 0.05% HALS-1 + 0.05% UVA-2 0.10% HALS-1 0.10% HALS-1 + 0.10% UVA-2 0.20% HALS-1

1170 7200 5800 9600 7900 15950

105 235

*Figure deduced by interpolation. PE-HD-2 AO-1 + 0.1% Ca stearate. Florida exposure started in October 1981.

combinations with the low molecular mass HALS-1 remain the stabilizers of choice for PE-HD tapes. 3.4 PE-LD

107

films

Combinations of UV stabilizers have been used for a long time for the stabilization of PE-LD films used for agricultural purposes, e.g. in greenhouse covers. The combination of the nickel stabilizer Ni-1 with the benzophenone type UV absorber UVA4 is the most typical example in this respect. Hence, it was straightforward, as soon as the polymeric HALS had been commercialized and used for stabilization of PE-LD, to combine them with benzophenone type UV absorbers. It should be mentioned that, when used alone in PE-LD, HALS perform better than nickel stabilizers which are more efficient than W absorbers. The performance of combinations of the polymeric HALS-2 with the benzophenone type UV absorber WA-4 is shown in Table 8 as a function of film thickness. It can be seen in Table 8 that, the contribution of UVA4 to the light stability conferred by HALS-2 is already Table 7. Performance of HALS-l/HALS3 tions in PE-HD tapes

combina-

UV stabilization

ES,, (kLy) Florida

Control 0.10% HALS-1 0.05% HALS-1 + 0.05% HALS3 0.10% HALS3 0.20% HALS-1 0.10% HALS-1 + 0.10% HALS3 0.20% HALS3

140 550 550 600 760 700 700

PE-HD3 (Ti catalyst, d = 0.945) + 0.05% AO-1 + 0.1% Ca stearate. Florida exposure started in March 1982.

25;($0*) 255 370 (Ti catalyst, d = 0.950) + 0.05%

significant for a film thickness of 100 m. It becomes very pronounced when the thickness reaches 200 m. These synergistic effects become even more important when the HALS are combined with benzotriazole type UV absorbers. It is especially pronounced with HALS-3 as shown in Table 9. The unexpected superiority of UVA3 over UVA4 is attributed to the fact that UVA3 is protecting the polymer according to mechanisms which are complementary to the protection mechanisms of HALS, i.e. absorption of UV radiation and deactivation of the excited states of carbonyl groups.‘l The combinations of HALS with UV absorbers also show high performance in PE-LD copolymer (EVA). In Table 10 it can be seen that the combination HALS-3/UVA-4 is also synergistic whereas in PE-LD homopolymer it was not (see Table 9). With PE-LD films containing china clay to absorb IR radiation (‘thermic films’) the behavior of the benzophenone type W absorber in combination with HALS is again different. Thus, it is found that combining UVA-4 with HALS-2 yields an increase in W stability but that this increase is less pronounced than that caused by an additional quantitiy of HALS-2. In Table 8. Light stability of PE-LD blown films UV stabilization

& 50j.~m film

Control 0.15% HALS-2 0.15% HALS-2 + 0.15% UVA4 0.30% HALS-2 0.30% HALS-2 + 0.30% UVA4 0.60% HALS-2

27 105 105 155 140 180

(kLy) Florida 1001*m 2OOpm film film 31 120 190 165 255 210

34 170 295 215 420 330

PE-LD homopolymer + 0.03% AO-1. Florida exposure, without backing, started in May 1980.

F. Gugumus

108 Table 9. Light stability of PE-LD

blown ftbns (200 pm)

Table 11. Light

stability

E,,, (kLy) Florida

UV stabilization

of PE-LD (200 ccm)

UV stabilization

0.15% 0.30% 0.15% 0.15% 0.30% 0.30%

HALS3 HALS-3 HALS3 HALS-3 HALS-3 HALS3

+ + + +

0.15% 0.15% 0.30% 0.30%

Without backing

On aluminum

400 500 465 540 625 725

310 450 365 475 475 625

UVA4 UVA-3 UVA-4 UVA3

PE-LD homopolymer + 0.03% AO-1. Florida exposure started in May 1981.

combination with HALS-3, the effect of UVA-4 is negligible or even negative (Table 11). However, the combination of HALS-3 with a benzotriazole type UV absorber shows an interesting performance, similar to that observed in the absence of china clay (Table 11). The combination of the polymeric HALS-2 with the polymeric HALS-3 also shows synergistic effects with kaolin containing PE-LD films. This is shown in Fig. 7.

4 LIGHT STABILIZATION SECTIONS

OF THICK

Since absorption of UV radiation according to the Beer-Lambert law is one of the main mechanisms of stabilization by UV absorbers, it that UV absorbers will can be expected contribute increasingly to light stability when sample thickness is increased. However, this contribution of UV absorbers manifests itself mainly by a better protection of the deep layers of the polymer. Hence, it will result in a better protection of the polymer properties depending mainly on these deep layers, i.e. some mechanical properties. It is obvious, that sample thickness has

blown

+ 0.15% UVA4 + 0.15% UVA3 + 0.30% UVA-4 + 0.30% UVA3

films

E,, (kLy) Florida Without backing

Control 0.15% HALS-3 0.15% HALS-3 0.15% HALS-3 0.30% HALS3 0.30% HALS3 0.30% HALS3 0.60% HALS3

thermic

22 150 210 275 270 245 500 500

On aluminum 19 135 145 235 235 180 395 350

PE-LD homooolvmer + 5% china clay + 0.03% AO-1. Florida exposire $tarted in July 1986..

no direct effect on protection of the superficial characteristics layers and of the polymer depending upon these superficial layers such as gloss, chalking, surface roughness, etc. Sample thickness can have a positive effect on surface protection in an indirect way if the stabilizers, protected from UV light in the deep lying layers, are able to migrate to the surface layers. Of mechanisms of UV course, the protection absorbers not related to UV absorption, e.g. quenching of excited states, are also efficient in the superficial layers of the polymer samples. 4.1 PP thick sections The general considerations exposed above are illustrated in Table 12 summarizing data obtained in 2 mm thick injection molded PP plaques. UV stability was evaluated according to two different criteria. They involve, on the one hand, impact strength (E,,) as representative for mechanical properties. On the other hand, they involve chalking (Echalking)as an indication for the state of the surface of the polymer samples. The data in

Table 10. Light stability of EVA blown films (200 pm)

UV stabilization

Control 0 .15% HALS-3 0.15% HALS3 + 0.15% UVA-4 0.30% HALS3 0.30% HALS3 + 0.30% UVA4 0.60% HALS3

E,,, (kLy) Florida Without backing

On aluminum

50 380 580 460 670 560

40 350 580 450 580 560

EVA copolymer (14% VA) + 0.03% AO-1. Florida exposure started in May 1981.

Fig. 7. Influence

of the composition of combinations of HALS-2 and HALS-3 on UV stability of PE-LD films (200 pm). PE-LD homopolymer + 5% china clay + 0.03% AO-1. Florida exposure on aluminum backing started July 1986. Total HALS concentration: (0) 0.3%; (*) 0.6%.

The performance

of light stabilizers in accelerated and natural weathering

Table 12. Performance 3 HALS-3/W

abso~I~rs combinations in PP plaques (2 mm)

UV stabilization Control 0.05% HALS3 0.05% HALS3 0.05% HALS3 0.10% HALS3 0.10% HALS-3 0.10% HALS3 0.20% HALS3

&

(kLy)

&.alking&Ly)

14 90 130 140 135 205 190 215

+ 0.05% UVA-2 + 0.05% UVA-4 + 0.10% UVA-2 + 0.10% UVA-2

40 100 130 130 130 190 190 310

PP homopolymer (PP-10) + 0.05% AO-2 + 0.05% 0.1% Ca stearate. Florida exposure started in July 1980.

P-l +

Table 12 are limited to the polymeric HALS-3 and its combinations with the benzotriazole and benzophenone type UV absorbers UVA-2 and UVA-4. It can be seen in Table 12 that both UV absorbers contribute significantly to the preservation of the mechanical properties. This contribution is of the same magnitude as the contribution of an additional, equal quantity of HALS3. The contribution of the UV absorbers to protection of the surface layers, i.e. to inhibition of chalking is less pronounced. This is concentration especially so if the HALS3 reaches 0.1% and more. Then, the contribution of the UV absorbers is below that obtained with the same additional amount of HALS3. The low molecular mass HALS-1 and the polymeric HALS-2 show overall behavior similar to that of HALS3. However, the stability level on the HALS. In this reached depends connection, it is mentioned that the protection conferred by HALS-2 to the mechanical proTable 13. Performance of combinations of HALS-2 with W absorixxs in PP plfques (2 mm). Influence of pig ments on impact strength UV stabilization

E15: (kLy) Florida 0.5% Titanium dioxide

0.5% Phthalocyanine blue

+ 0.05 % UVA-

:: 110

16 115 125

26 240 260

+ 0.05% UVA-

150

125

230

+ 0.10% UVA-

60 240

230 185

265 315

+ 0.10% UVA-

260

185

315

75

470

390

Unpigmented

Controi 0.05% HALS-2 0.05 % HALS-2 2 0.05% HALS-2 4 0.10% HALS-2 0.10% HALS-2 2 0.10% HALS-2 2 0.20% HALS-2

PP homopolymer (PP-10) + 0.05% AO-2 + 0.05% P-l + 0.1% Ca stearate.Plorida exposure started in July 1980.

109

perties is rather limited in unpigmented samples. For this reason it is recommended to use HALS-2 in combination with a UV absorber in the absence of adequate pigments. Table 13 shows the influence of pigments on retention of mechanical properties conferred by HALS-2 and its combinations with UV absorbers. It can be seen that in the absence of any pigment, the UV absorbers, UVA-2 and UVA4, contribute significantly to the preservation of the mechanical properties. However, in the presence of the pigments titanium dioxide or phthalocyanine blue, the contribution of the UV absorbers to retention of mechanical properties can practically be neglected. In any case this contribution is always inferior, often significantly inferior, to that of the same amount of HALS-2. The comparison of the results obtained with HALS-2 as the only UV stabilizer, in the samples containing titanium dioxide, with the results obtained with the combinations HALS-2/UV absorbers in the unpigmented plaques is especially instructive. It shows that the role of the UV absorber in the unpigmented plaques has been taken over quantitatively by the pigment so that addition of a UV absorber becomes of no use. The effect of phthalocyanine blue is even more pronounced than that of titanium dioxide, especially with low stabilizer concentrations. It should be mentioned that these observations are valid for retention of mechanical properties only. If chalking is the test criterion chosen, it is with phthalocyanine blue that the results are worst. The data discussed so far have been generated with PP homopolymer. The combination of UV absorbers with HALS yields analogous results with PP copolymers. Nevertheless, sometimes significant differences can be observed. However, this goes beyond the scope of this paper. Combinations of HALS with UV absorbers are used preferentially in thick sections. They present significant advantages in unpigmented samples. However, the combinations of low molecular mass HALS with polymeric HALS also yield synergistic effects in thick polymer samples. This is shown in Table 14 for combinations of HALS-1 with HALS3. It can be seen that the 1:l combination is practically equivalent to the low molecular mass HALS-1 used alone. This is not only so if retention of impact strength is taken into account, it is also valid for protection from chalking. Figure 8 shows the influence of the concentration on the performance of different combina-

F. Gugumus

110 Table 14. Performance of combinations HALS3 in PP plaques (2 mm) UV stabilization 0.1% HALS Control HALS3 HALS3/HALS-1 HALS-1

1:l

Table 15. Light stability of talc filled PP plaques (2 mm)

HALS-l/

UV stabilization

Es, (kLy)

&,a,k,ng (kLy)

28 175 280 300

80 160 250 250

Control 0.4% HALS-1 0.2% HALS-1 + 0.2% HALS-3

PP homopolymer (PP-11) + 0.05% AO-2 + 0.05% P1 + 0.1% Ca stearate. Florida exposure started in June 1985.

tions HALS-l/HALS-3. It can be seen that for the combinations as well as for the constituents, UV stability increases linearly with HALS concentration. Hence, the law already deduced with PP tapes is also valid with PP thick sections. HALS combinations are also useful with filled PP. Table 15 shows the performance of a HALS-l/HALS-3 combination in talc filled PP in comparison with HALS-1 used alone at the same concentration. It can be seen that the combination is more efficient than HALS-1 although the test criteria shown pertain essentially to surface properties. The performance of combinations of two high molecular mass HALS such as HALS-2 and HALS-3 is currently under investigation. 4.2 PE-HD thick sections For UV stabilization of PE-HD thick sections, the same stabilizers are used, in principle, as for PP thick sections. They are essentially HALS, low molecular mass HALS, and polymeric HALS, and UV absorbers. Data obtained with the polymeric HALS-2 and HALS-3 in Ziegler type PE-HD are presented in Table 16. It can be

Incident energy (kLy) to: Loss of gloss

Chalking

20 150 175

20 175 200

PP homopolymer (PP-12) + 0.1% AO-2 + 0.3% S-l + 0.1% Ca stearate. 40% talc, additives in weight percent relative to PP. Florida exposure started in April 1984.

seen that, in this test series, the polymeric HALS-2 shows only fair performance, if used alone in unpigmented samples. However, if HALS-2 is used in combination with the benzotriazole type UV absorber UVA-2, it becomes much more efficient and comes closer to HALS-3 than when the UV absorber is absent. In titanium dioxide pigmented samples, the contribution of the UV absorber to light stability is small for both HALS-2 and HALS-3. Furthermore, the performance of HALS-2 is comparable to that of HALS-3 in the presence and in the absence of the UV absorber. In unpigmented and titanium dioxide pigmented samples the performance of the low molecular mass HALS-1 is close to that of the polymeric HALS-3 (data for HALS-1 not shown in Table 16). The data presented in Table 16 cannot be considered as representative for all PE-HD thick sections. As a matter of fact, preliminary data obtained with another PE-HD batch (Table 17) show an advantage for HALS-2 over HALS-3 even in the absence of any UV absorber or pigment. The combination HALS-2/HALS-3 looks better than both components used separately. It is not yet possible to find out the Table 16. Light stability of PE-HD plaques (2 mm) UV stabilization

Es0 (kLy) Florida Unpigmented

0.5% Titanium dioxide

5:

/

Fig. 8. Influence of HALS concentration and composition of the combination of HALS3 and HALS-1 on UV stability of 2 mm PP plaques. PP-11 + 0.05% AO-2 + 0.05% P-l + 0.1% Ca stearate. Florida exposure started June 1985.

Control 0.05 % HALS-2 0.05% HALS-2 + 0.05% UVA-2 0.10% HALS-2 0.05% HALS-3 0.05% HALS-3 + 0.05% UVA-2 0.10% HALS-3

40 70 330 135 320 470 320

60 550 580 550 600 670 680

PE-HD-4 (Ti catalyst, d =0.960) +0.03% stearate. Pigment: coated rutile. Florida exposure started in March 1981.

AO-1 +0.2%

Ca

The performance of light stabilizers in accelerated and natural weathering Table 17. Performance of I&US-2/HALS-3 in PE-HD plrques (2 mm) UV stabilization

Control 0.1% HALS-2 0.1% HALS3 0.1% HALS-2/HALS-3

con&nations

Retained tensile impact strength after 700 (kLy) of outdoor exposure 0% 82% 73% 84%

1:l

characteristics of the PE-HD responsible for these differences, although the catalyst residues and their more or less careful deactivation may account for them. Tentatively, it is proposed to systematically envisage combinations of HALS2/HALS-3 for stabilization of PE-HD thick sections because according to the results available so far they should always show excellent performance. This is being checked. 4.3 PE-LD and PE-LLD thick sections For UV stabilization of PE-LD (and PE-LLD) thick sections, polymeric or high molecular mass HALS only and UV absorbers can be used. Numerous stabilizers used in PP and PE-HD cannot be used in PE-LD and PE-LLD because they are not compatible. This is especially so for the low molecular mass HAM-1 and similar compounds. It can be seen in Table 18 that the polymeric HAD-2 confers excellent light stability to unpigmented PE-LD 2 mm plaques even in the absence of any UV absorber. This is also an indication that catalyst residues may be involved in the poor performance of HALS-2 in some unpigmented PE-HDs. The benzophenone type UV absorber UVA-4 also shows good Table 18. Light stabMy of PE-LD plaques (2 mm) E,, (kLy) Florida Unpigmented

Control 0.1% UVA-4 0.2% UVA-4 0.1% HALS-2 0.1% HALS-2 + 0.1% UVA4 0.2% HALS-2

95 265 335 700 970 835

PE-LD homopolymer + 0.03% AO-1. Pigment: coated rutile. Florida exposure started in November 1981.

efficiency in PE-LD. The combination HALS2/UVA-4 yields quite pronounced synergism. In the presence of titanium dioxide, UVA4 no longer contributes to UV stability. However, HALS-2 shows an excellent performance.

5 LIFETIME PREDICTION

PE-HD-5 (Ti catalyst, d = O.!XiO)+ 0.03% AO-1 + 0.1% Ca stearate, unpigmented. Florida exposure started in June 1988.

UV stabilization

111

0.5% titanium dioxide 360 330 320 >1600 >1600 >1600

The prime goal of most of the studies involving exposure of plastics materials to UV light is to assess the stability or durability of the material under well-defined, if possible standardized conditions. From the knowledge obtained this way it is expected to be able to estimate or calculate the lifetime of the manufactured articles in use. The durability is, usually, first assessed on artificial exposure. From the results it is thought to be able to predict the lifetime in natural weathering. This already poses the problem of correlation between both exposure modes. 5.1 Cordation exposure

between artifidpl ad

natural

The dficulties associated with correlation between various artificial exposure tests on the one hand and various outdoor weathering tests on the other hand have been treated in some length previously (see Ref. 12 and references cited therein). In the same study, fair correlation was found between various outdoor exposure sites for PE-LD blown films. Thus, data obtained in tropical climates, e.g. Florida, can be used for the prediction of the lifetime in a Mediterranean or temperate climate. In this respect, natural weathering in a tropical or desert climate can be envisaged as a kind of accelerated outdoor weathering with good correlation to less drastic climates. The use of sun-concentration devices is another form of accelerated outdoor weathering whose usefulness needs still to be demonstrated. Outdoor weathering accelerated by artificial light exposure during night did not come up to the initial expectations of an acceleration factor of 6.13 However, the main problem remains that of correlation between various artificial exposure tests and natural weathering. The main results from previous studiesl’ led to the conclusion that there may be no correlation between an artificial exposure test and natural weathering for trivial reasons. The most obvious

F. Gugumus

112

in this respect concerns the presence of short-wavelength radiation in the artificial exposure device, i.e. radiation below the 295 nm cut-off of sunlight on earth. However, this aspect can easily be taken care of by choosing an adequate light source and/or by using the correct filters. So far, the best correlation was found by using filtered xenon arcs. Too high a temperature during artificial exposure is another trivial reason precluding satisfactory correlation with natural weathering. The high temperature can be a direct consequence of the heat generated by the light source. This can be eliminated or at least reduced considerably by ventilation and/or cooling. The high temperature may also be caused by the infra-red content of the light emitted. Then, the light source must be chosen accordingly. Otherwise, the temperature will depend not only on the chemical structure of the polymer but also on the fillers. Of course, it also depends heavily on the pigments used. For the ‘correlation’ data presented below, the trivial factors discussed above were taken into account as far as they were known. For the sake of brevity, the comparisons will be limited to PP tapes. It was already shown previouslyl’ that correlation between artificial exposure in a xenon arc equipped exposure device and natural weathering can be different for low molecular mass HALS and high molecular mass HALS. This is visualized in Fig. 9. It can be seen, that there is linear correlation for polymeric or high molecular mass HALS. However, the low molecular mass HALS perform better on outdoor weathering than would be expected from the linear relation valid for polymeric HALS. In another exposure series, only high molecular

mass HALS were involved. The plots in Fig. 10 show linear correlation between artificial exposure in a Xenotest 1200 and natural weathering in Florida, as expected. However, the linear correlation depends on the nature of the high molecular mass HALS. Thus, there is linear correlation for HALS-2 and HALS-5 on the one hand, and for HALS3 and the combinations HALS-2/HALS-3 on the other. This suggests that high molecular mass tertiary HALS such as HALS-2 and HALS5 respond differently to a change from artificial to natural exposure than polymeric secondary HALS such as HALS-3. It is noteworthy, that the combinations of the tertiary HALS-2 with the secondary HALS-3 behave like the latter in this respect. These results give a strong hint to fundamentally different behavior of high molecular mass tertiary HALS compared to high molecular mass secondary HALS concerning the prediction of lifetime on outdoor exposure on the basis of accelerated exposure data. There was no such difference between the low molecular mass secondary HALS-1 and the corresponding N-methyl derivative HALS-4.8 It should be mentioned, that the pronounced superiority of low molecular mass HALS over high molecular mass HALS on outdoor exposure has been attributed to diffusion of the low molecular mass stabilizer to the superficial layers of the tapes during the dark periods.l* Since the surface is especially prone to photooxidation, replacement of the UV stabilizer consumed during the light period would indeed lead to an enhancement of UV stability. There is another factor affecting correlation between artificial and natural weathering, namely oxygen diffusion. Thus, on artificial weathering, the initiation rate

i

0

so

Fig. 9. Comparison

(June

rm

mu

2c!o

Ei?kwmz

3w

of Xenotest 1200 and Florida exposure 1980) of PP tapes (50 pm). PP-lO+ 0.1% Ca stearate + 0.05% AO-2 = 0.05% P-l.

Fig. 10. Comparison of natural weathering with accelerated exposure of PP tapes. Influence of HALS structure. PP-5 + 0.1% Ca stearate + 0.05% AO-2 + 0.05% P-l.

The performance of light stabilizers in accelerated and natural weathering

is more or less pronounced and continuous. This can lead to more or less pronounced oxygen depletion in the samples and, therefore, to different reaction sequences. On outdoor exposure, even if sometimes, e.g. in noon sunlight, oxygen is consumed at a faster rate than it can be replenished by diffusion into the samples, the initial level of oxygen will be reached again during the dark period. Hence, photooxidation will dominate during natural weathering, whereas photolysis reactions may be very important on artificial exposure. Poor stabilizer compatibility in the polymer under test can also significantly affect correlation between artificial and natural exposure. This is visualized in Fig. 11. It can be seen that UV stability on natural weathering can be markedly lower than that expected from artificial exposure. This is observed essentially with stabilizers identified also by other means as insufIiciently compatible. There may be no correlation between artiticial and natural weathering for other fundamental reasons. One such effect was found with PE-LD fihns.14 On natural exposure and with some artificial exposure devices, photooxidation involves preferentially the superficial layers of the films and to some extent the bulk. With some accelerated exposure devices, oxidation of the surface layers becomes negligible in comparison with oxidation of the bulk. This has to be attributed to the fact that, with the devices envisaged, the chain reaction occurring in the superficial layers and involving oxidation products becomes insignificant because the rate of initiation and, therefore, the rate of termination are too high for any chains to develop.14 The discussions, so far, involved unpigmented polymer samples. The problem of correlation

113

between artificial and natural exposure becomes even more complex with pigmented samples. This is shown in Fig. 12. It can be seen, that there is not the slightest hint to any generally valid correlation. This is especially annoying because only white pigments were used for the experiments. It can be expected that it will be even more complex if the colors and/or the chemical structures of the pigments are also varying! From the discussions above, it can already be concluded that artificial exposure can sometimes give an idea of W stability of plastics materials. However, because valid extrapolation to natural weathering is limited to very specific examples, artificial exposure cannot be used for lifetime prediction. The more accelerated exposure becomes, the less reliable it is in this respect. As a rule, it should be used mainly for quality control. 5.2 Propo@ prediction

for an altedve

Metime

In view of the poor reliability of prediction based on various accelerated test methods, alternative methods must be sought. It is quite clear that, natural weathering is not a valuable alternative. As a matter of fact, even if tropical climates are used as a standard, the duration is usually much too long to be practical. The data and discussions in Sections 3 and 4 above suggest an alternative possibility for lifetime prediction. Thus, it was found that, with a given sample form and testing, e.g. PP tapes on mechanical testing, the lifetime (rl;o or Em) is determined essentially by the polymer quality and the stabilization system. It had already been shown previously,l’ that outdoor weathering of stabilized PE-LD blown films is quite re-

0

am

am

mm

8m

TllttllWUOlWT~

Fig. 11.Comparison of Weather-Ometer

and Florida exposure of PP tapes (50 pm) stabilized with experimental HALS. PP-13 + 0.1% Ca stearate + 0.05% AO-2 + 0.05% P-l. Florida exposure started July 1984.

*cmad 1 az!A~~+Ila*

~L1xiW.H . w%-*alo

+MwAu DM-rbL)

0 &auIwM*am

12. Comparison of Xenotest 1200 and Florida exposure of PP tapes. PP-1 + 0.1% Ca stearate + 0.05% AO-2 + 0.05% P-l. Florida exposure started July 1986.

Fig.

114

F. Gugumus Table 19. Data reproducibility

November 1977

0.3% HALS-2 + 0.3 % UVA-4

blown 6lms (200 pm)

E,,, (kLy) Florida exposure started in:

UVstabihzation

Control 0.6% HALS-2

OIInatural weathering of PE-LD

70 350

March 1979 42 355 290* 360

September 1979

May 1980

70

35

320

330* 335*

March 1981

July 1984

42 375**

34 290***

350*

310***

April 1989 33 375***

Results marked * originate from a different batch of film from those marked ** or ***. Base stabilization: O.&% AO-1. Exposure without backing.

producible in tropical climates such as Florida. It is shown again in Table 19. It can be seen that the lifetime of the control films varies considerably with the time of year when exposure was started. However, with well-stabilized films, there is much less influence of the exposure starting time. There is good reproducibility as soon as the total lifetime is greater than one year. Of course, PE-LD is a typical example where polymer quality is rather constant because of the absence of catalyst residues. Then, it is only the stabilization system which determines the lifetime of 200 pm thick films. The problem seems much more complicated with PP, PE-HD and PELLD where varying amounts of catalyst residues can exert a significant influence on polymer lifetime. However, the progress in catalyst technology leads to a considerable reduction of the level of titanium catalyst residues. First results with tapes prepared with third-generation PP point to much better reproducibility of the lifetime reached with a given stabilization. Thus, it can be expected, that in the future, polymer quality may be much more constant and will no longer be a decisive variable factor. Then, the main factor determining the lifetime will be the stabilization system. It was shown above that, stabilization with the most used stabilizers for polyolefins, HALS, follows well-defined experimental laws. Thus, in PP, there is usually a performance increasing linearly with HALS concentration if it is not too high. In PE-HD, performance is proportional to the square root of HALS concentration. Therefore, the knowledge of the outdoor lifetime with one HALS concentration permits us to calculate the lifetime expected with another concentration by use of the above mentioned laws. As a consequence, the problem of lifetime prediction is reduced to an analytical problem, i.e. determination of the

amount of HALS in the samples. More generally, it is reduced to the knowledge of the stabilization system. The discussion so far, concerned mainly the influence of polymer quality and stabilizer system. It is obvious, that other factors determining UV stability must also be taken into account. One of them is sample thickness. The effect of thickness was shown with PE-LD films in Table 8. The pigment or pigment system is another important factor determining the lifetime of plastics materials. Its effect on UV stability must be studied as systematically as the effect of the light stabilizers. Finally, the test criterion must be related strongly to the most important plastics characteristics determining failure in actual use. The result of these calculations or estimations will be an ‘ideal’ outdoor weathering lifetime. It should be close to that which would be obtained by extrapolation of artificial exposure results to natural weathering if a good correlation was present. Of course, this could be checked with a few specific examples where correlation is satisfactory. In practice, the real use conditions of the plastic articles will determine the true lifetime on outdoor weathering. Of course, this cannot be estimated accurately in advance. Thus, it must be taken into account by some correction factor corresponding to a mean value deduced from actual field experience.

6 CONCLUSIONS Combinations of stabilizers very often yield optimum UV protection of polyolefins. Thus, combinations of HALS with UV absorbers give excellent results, mainly in thick cross-sections. However, even in relatively thin PE-LD films

The performance

of light stabilizers in accelerated and natural weathering

(200 pm), there can be pronounced synergism between HALS and UV absorbers. In the presence of pigments such as titanium dioxide or phthalocyanine blue, the contribution of the UV absorbers is usually too small to be of any use. The effect can even become antagonistic. It is noteworthy, that with some organic pigments, especially yellow and red pigments, it is recommended to add a UV absorber not only to protect the polymer but, even more so, to protect the pigment. Combinations of low molecular mass HALS with high molecular mass or polymeric HALS can also yield pronounced synergistic effects. More important, this effect does not depend on sample thickness and can be observed with thin as well as with thick sections. The synergistic effect observed by combining two polymeric HALS such as HALS-2 and HALS-3 is even more astonishing than that observed between low molecular mass and high molecular mass HALS. By using such combinations, it is possible to achieve simultaneously good UV stability and indirect food contact approval. Forecasting the lifetime of plastic articles on natural weathering is particularly difficult. Exposure in tropical climates can be used for prediction in other climates, of course, after calibration. However, usually it is attempted to relate an exposure time in an artificial exposure device to some time or energy on natural weathering. With due precautions concerning the characteristics of the exposure apparatuswavelength of light intensity, emitted, temperature-it is possible to find some linear relationship between data from natural and artificial exposure. However, such a correlation is usually valid only for the systems with which it has been established. Therefore, it is of very restricted predictive value. Since this approach to lifetime forecasting is not very promising, another method is proposed. It is based on the quantitative laws determined experimentally for HALS performance in polyolefins. Then, the knowledge of the performance with one concentration of a given stabilizer system in a given substrate and substrate form, allows us to forecast the performance with different concentrations. The problem of forecasting is reduced to an analytical problem, i.e. determination of the stabilizer system. This is usually done in a relatively short time, even if compared with exposure in highly accelerated devices.

115

REFERENCES 1. Gugumus,

F., Oxidation inhibition in plastics, in Inhibition of Oxidation Processes in Organic Materials, Vol. 1, Chap. 4, ed. P. Klemchuk & J. Pospisil. CRC Press, Boca Raton, FL, 1989, pp. 61-172. 2.Gugumus, F., ‘Antioxidants’ in Plastics Additives, 4th edn, ed. R. Gaechter & H. Mueller. Hanser Publishers, Munich, 1993, pp. l-104. 3.Gugumus, F., Polym. Degrad. Stab., 44 (1994) 273. 4.Gugumus, F., Polym. Degrad. Stab., 44 (1994) 299 5.Gugumus, F., 4e Conference Europeemre des Plastiques et des Caoutchoucs, Paris, 1974, Kunstst. Plast., 22 (1975) 11; Caout. P&t., 558 (1976) 67. Gugunus, F., Angew. Makromol. Chem., 1761177 (1990) 241. Gugumus, F., Polym. Degrad. Stab., 40 (1993) 167. Gugumus, F., Angew. Makromol. Chem., 190 (1991) 111. Gugumus, F., Photooxidation of polymers and its inhibition, in Inhibition of Oxidation Processes in Organic Materials, Vol. 2, Chap. 2, ed. P. KIemchuk & J. Pospisil. CRC Press, Boca Raton, FL, 1989, pp. 29-162. 10.Gugumus, F., In Plastics Additives, 4th edn, ed. R. Gaechter & H. Mueller. Hanser Publishers, Munich, 1993, pp. 129-270. 11. Gugumus, F., Polym. Degrad. Stab., 39 (1993) 117. 12. Gugumus, F., The use of accelerated tests in the evaluation of antioxidants and light stabilizers, in Developments in Polymer Stabilisation-8, Chap. 6, ed. G. Scott. Elsevier Applied Science Publishers, Barking, 1987, pp. 239-89. 13. Gueugnaut, D., Lappai, G., Vautherin, P., Rousselot, D. & Mayo, M., Evaluation du Pro&de de Vieillissement Nature1 Acc&re ‘NATACC’, Applique sur Tubes en polyethylene de Moyenne Densite Utilises pour la Distribution du Gaz. Presented at the lO&mes JoumCes d’Etudes sur le VieiIIissement des Polymbres, Bandol, France, September 23-24, 1993. 14. Gugumus, F., Angew. Makromol. Chem., 182 (1990) 85.

APPENDIX Appndii shudure

HALS-2

TrsdeMme

TINWIN

622

116

F. Gugumus

UVA

TINUVIN 326

UVA

TINUVIN 328

UVA.

3HIMASSORS 81

AO-1 I-1

:YASORS I

A02

-12

!-I

-P-Y+

AO-3

RGANOX 1425

lx

P-l

RGAFOS 168

3

0

1’-

-

o \/

‘INUVIN 12 ERR0 AM

OH

IA-1

1:

4z

w\ -

N/N \ /

‘INUVIN 32

IRGANOX PS So2