Interactions between carbon black and stabilisers in LDPE thermal oxidation

Polymer Degradation and Stability 72 (2001) 163±174

www.elsevier.nl/locate/polydegstab

Interactions between carbon black and stabilisers in LDPE thermal oxidation J.M. PenÄa a, N.S. Allen a,*, M. Edge a, C.M. Liauw a, B. Valange b a

Chemistry and Materials Department, Manchester Metropolitan University, Chester Street, Manchester M1 GD, UK b Cabot-corp, Rue Prevochamps 78, 4860 Pepinster, Belgium Received 6 August 2000; received in revised form 21 October 2000; accepted 20 November 2000

Abstract The interactions between two commercial hindered piperidine compounds, three commercial antioxidants, a secondary antioxidant and two types of furnace carbon blacks (with a di€erent surface area and surface chemistry) in the thermal oxidation of LDPE have been studied using the oxidation induction time test. During thermal degradation the interactions were variable, being both antagonistic and synergistic. Generally, for the primary antioxidants alone their thermal stabilising e€ects on the polymer were decisive and related to their chemical structure, with the main contribution being their synergism with HALS and a secondary antioxidant. Di€erences in the performance of the polymeric type of HALS were evidenced, as well as between the CB grades. Minor synergism and antagonism was found between HALS and CB depending on the chemical structure of the HALS. All the detected antagonisms were widely overcome by the synergistic interaction found in three- and four-additive formulations. Though the presence of CB (at the concentrations studied) was not a decisive factor in the LDPE thermal stability, its presence nevertheless showed a bene®cial e€ect. The nature of the CB plays some role in controlling its performance as a stabilising agent alone, as well as its interactions with HALS and antioxidants, via adsorption-desorption processes as well as other chemical interactions as was evidenced by adsorption studies via ¯ow microcalorimetry (FMC). # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon black; Synergism; Antagonism; Antioxidants; Stabilisers; Polyethylene

1. Introduction 1.1. Introduction to the thermal degradation of LDPE In a previous paper we examined in depth the factors involved in the photo stabilising performance of LDPE formulations containing carbon black (CB), hindered amine light stabilisers (HALS), primary antioxidants (AOX1) and secondary antioxidants (AOX2). The antioxidant e€ect of several carbon black grades was reported and related to the volatile content [1,2], which is related to the surface chemistry, as previously cited and [3±8]. An important antioxidant synergism was also reported in the presence of benzophenone [1]. In several separate papers we also reported an important adsorption capability of carbon black towards antioxidants and HALS [9±11], and this in¯uenced the photostability performances of LDPE formulations containing four additives * Corresponding author. Tel.: +44-161-247-6520; fax: +44-161247-1438. E-mail address: [email protected] (N.S. Allen).

(CB+HALS +AOX1+AOX2) [1]. The main factors involving such interactions were described in detail and were related to the surface chemistry of the blacks and the functionality and structure of the stabilisers. On the other hand, the thermal stabilisation activity of the CB can be summarised as follows: 1. Acts as a hydroperoxide decomposer. 2. Operates as a chain breaking donor/acceptor of free radical species. Increasing oxygen content (as volatile content or determined by XPS) results in better performance of the blacks as antioxidants [3] (by inhibiting any chain reaction caused by temperature, i.e. by acting as a catalyst for peroxide decomposition and as a free radical trap through quinone groups [30]). However, in some cases highly oxidised CB's can promote the thermal oxidation of polyole®ns and this is mainly attributed to the adsorption of antioxidants by the CB. In general, the CB eciency increases with increasing concentration when no detrimental e€ects are present and the described bene®cial e€ect for a speci®c grade. The

0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00016-7

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interactions of carbon black and other ®llers with the polymer and its additives can involve a complex inter-play of phenomena and these can be described as follows: 1. The surface area and pore volume. 2. The surface activity of the ®ller in¯uencing adsorption-desorption processes of the additives which in turn is dependent on: The functionality. The hydrophobicity of the ®ller. 3. The extent of any interaction between the polymer and the ®ller. 4. The thermal and photostabilisation properties of the ®ller. 5. The interactions of the ®ller with additives. On the other hand, physical mixtures of stabilizers are used with the aim of obtaining an overall synergistic e€ect (i.e. the observed e€ect of the combination is greater than that of simple addition of the e€ects of the individual stabilizers). However, both additive or antagonistic (i.e. lower than that of the simple addition) e€ects may also result as a consequence of using mixtures of stabilizers. Regarding the individual stabilising contribution, primary antioxidants (AOX1) are stabilisers of common use such as the hindered phenols, which are very ecient stabilisers against the degradation of polymers upon processing of the melt and during end-use such as exposure to long term thermal stress [12±14]. However, they are relatively ine€ective under photo-oxidative conditions, due to the fact that they are generally unstable to UV light and some of their oxidative transformation products. Generally, the mode of action and transformation products of phenolic antioxidants and phosphates are well established in the literature [15,16]. Other stabilisers are the hindered amine light stabilisers (HALS) and these are quite ecient thermal antioxidants as well as light stabilisers and their mode of action is known to be multifunctional in nature [17]. The overall high eciency of HALS as thermal and UVA stabilisers in polymers is attributed to the regeneration of nitroxyl radicals and the complementary role of the chain breaking donor/acceptor antioxidant mechanism. The aminoxyl radical is only formed during exposure of the stabilized polymer to heat and light, hence addition of a process stabilizer is needed to protect the polymer during processing. Nowadays, these compounds are gaining increasingly in importance as long-term thermal stabilisers, especially polymeric HALS, such as Chimassorb 9441 and Tinuvin 6221. The thermal stabilities (oven degradation) in HDPE o€ered by these polymeric HALS alone was also very good contrasting markedly with their performance in polypropylene [18]. The oven ageing temperatures and a restricted di€usion of the HALS was reported to be

important when the oxidation products of the HALS such as the nitroxyl/hydroxyl-amine products, which o€er good antioxidant properties, would be more stable at lower temperatures and the di€usion of the HALS relies on their compatibility with the polymer. The performance alone of Chimassorb 9441 was reported to be higher than that of Tinuvin 6221. As a result of the combinations of such stabilisers, interactions were reported between hindered piperidine compounds (HALS) and phenolic/phosphite antioxidants in the thermal and photochemical oxidation of polypropylene ®lm and high-density polyethylene (HDPE) [19± 22]. The role of the phosphorous antioxidants is mainly the control of melt viscosity, improving the colour control trough the suppression of AOX1 consumption [23]. Earlier works reported a synergistic behaviour in thermal degradation observed for the phosphite, Irgafos 1681, and other aliphatic phosphites with di€erent HALS compounds (secondary and tertiary HALS) at all molar ratios of the antioxidant combinations. A considerable e€ort was made towards the reduction of discolouration of stabilized polyole®ns using phosphite-HALS binary mixtures. Their mixture imparts a very good melt and longterm stability. A cumulative deactivation of ROOH chromophores was envisaged in the latter case [24]. On the other hand, an antagonistic e€ect was reported for such physical mixtures in polyole®ns during their photo-oxidation [25]. In all cases, the level of antagonism decreased with increasing the molar proportion of the phosphite in the mixtures. The antagonistic e€ect was related to the photosensitive nature of transformation products from the aryl phosphite (Irgafos 1681) and their adverse interactions with the HALS and their related products in formulations containing both stabilisers [25]. On this basis, the present investigation is concerned with a study of the e€ects and interactions on the thermal oxidative stability of LDPE in more complex systems containing as well as phosphites, HALS, and primary antioxidants, carbon black compounds as ®llers and colourants. In this approach, two commercial polymeric HALS, were combined with three di€erent primary antioxidants, one secondary antioxidant (an aryl phosphite) and two di€erent carbon blacks as ®llers. In addition, an in depth characterisation of the carbon blacks was performed, in order to determine what CB features are important in the interactions between CB and the stabilisers used in this study. 2. Experimental procedures 2.1. Materials The low-density polyethylene was a Escorene LD166 supplied by EXXON. The Primary antioxidants investigated were Irganox 10101, Irganox 10761, Cyanox

J.M. PenÄa et al. / Polymer Degradation and Stability 72 (2001) 163±174

17901 (Cytec) [12,18]. They are all sterically hindered phenols. Irgafos 1681 was the only secondary antioxidant investigated [12,18]. Further details are given in Table 1. The HALS investigated were Chimassorb 9441 and Tinuvin 6221 LD [12,18]. Hindered amine light stabilisers (HALS) are very e€ective and ecient stabilisers for polyole®ns by means of a mechanism of stabilisation that includes metal ion complexation, free radical scavenging and peroxide decomposition. Structures are given in the appendix. The furnace carbon blacks CB-A and CB-B were supplied by Cabot. Their features are given in Tables 2±4.

165

2.2. Mixing procedures A total of 72 mixes were prepared including as stabilisers commercial polymeric primary and secondary antioxidants, and as ®llers, two types of furnace carbon black have been chosen with di€erent surface areas and surface chemistry (Table 2). The ®llers and stabiliser concentrations used were: carbon black: from 0 to 0.83%; HALS, AOX1 and AOX2: from 0 to 0.083%. Masterbatches of 40% w/w were prepared for all the pigments using a Haake Rheomix 600. The cycle

Table 1 Commercial antioxidants and HALS investigated Trade name (supplier in parentheses)

Chemical structure

Irganox 10101 (Ciba) Irganox 10761 (Ciba) Cyanox 17901 (Cytec)

Tetrakis [methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]metane Octadecyl 3,5-di-tert-butyl-4-4-hydroxyhydrocinnamate 1,3,5- Tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)- 1,3,5-triazine-2,4,6 (1H,3H,5H)-trione Poly{6-[(1,1,3,3,-tetramethylbutyl)-imino]-1,3,5-triazine 2,4-diyl][2,2,6,6,-tetramethypiperidyl)-imino]-hexamethylene-[4-2,2,6,6,tetramethypiperidinyl)-imino} Butanedioic acid, polymer with 4-hydroxy-2,2,6,6-tetrametyl-1-piperidineethanol

Chimassorb 9441 (Ciba) Tinuvin 6221 LD (Ciba)

Molar mass 1178 531 699.94 3000±>2500 >2500

Table 2 Properties of carbon blacks studied CB type

CB-A CB-B a b

Surface area-N2a

Particle Size EM (nm)b 22 25

Stsa (m2 g 1)

Single point (m2 g 1)

86.8 79.3

106.6 79.5

Iodine number (mg g 1)

DBPA pellets (cm3/100 g)

119.6 79.2

96.6 101.6

Obtained by N2 BET measurements. Obtained by TEM analysis.

Table 3 Properties of carbon black studied CB Type

Tinting strength %

Volatile contenta %

PHa

Water contentb (wt. % CB)

CB-A CB-B

104.2 105.2

1.5 1

8.5 8.5

1.01 1.08

a b

Obtained by TEM analysis. Obtained by Karl Fisher measurements.

Table 4 Surface elemental compositions of the CB-A and the CB-B CB sample

%C

%O

%S

%N

O/C

S/C

N/C

CB-A CB-B

99.44 98.48

0.55 0.96

0.02 0.54

0 0

0.0055 0.0097

0.0002 0.0055

0 0

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temperature was from 130 to 200 C during 7 min with increasing rotor speed in order to maintain constant torque during the last period of mixing. The masterbatches were then let down to a concentration of 0.83%.w/w of carbon black, adding the stabilisers at the chosen concentrations. 2.3. Oxidative induction time (OIT) analysis Oxidative induction time is a relative measure of a material's resistance to oxidative decomposition, which is determined by the thermoanalytical measurement of the time interval to onset of exothermic oxidation of a material at a speci®ed temperature in an oxygen atmosphere. Film samples were analysed by the standard OIT analyses according to the ``Standard Test Method for Copper-induced Oxidative Induction Time of Polyole®ns by Di€erential Scanning Calorimetry'' (ASTM D3895-92) [26]. The OIT analyses were performed at 200 C with a DSC (Mettler Toledo) with automatic sampling. The sample to be tested and the corresponding reference materials were heated at a constant rate in nitrogen. When the speci®ed temperature was reached, the atmosphere was changed to oxygen at the same ¯ow rate. The specimen was then held at constant temperature until the oxidative reaction was displayed on the thermal curve. The time interval from when the oxygen ¯ow is ®rst initiated to the oxidative reaction was referred to as the induction period. The end of the induction period was signalled by the abrupt increase in the specimen's evolved heat or temperature and may be observed by a di€erential scanning calorimeter (DSC). Thus, the oxidative induction time (OIT) was determined from the data recorded during the isothermal test. 2.4. Flow micro-calorimetry (FMC) The FMC used was a Microscal 3Vi, details of the downstream detector etc. are provided elsewhere [27]. For the most of additives, the carrier ¯uid was heptane (Aldrich HPLC grade) stored over freshly activated (350 C, 3 h) 4A molecular sieves. Chloroform was used with additives which were insoluble in heptane, e.g. Tinuvin 6221 because of its polar nature. The cell temperature

was 27 C ( 1 C). Probe concentrations of 0.03% w/v. The ¯ow rate was 3.30 cm3 h 1 and carbon black sample size was 67.5 mg ( 0.5 mg). Decahydronaphthalene was used as the non-adsorbing probe. Samples were left to equilibrate over night at a carrier ¯uid ¯ow rate of 0.033 cm3 h 1. The ¯ow rate was then increased to 3.3 cm3 h 1 and the system left to settle for ca. 1 h. An overview of the FMC technique is provided by Ashton [28]. 3. Results 3.1. Characterisation of the carbon blacks The characterisation of the black surface chemistry was carried out by X-ray photoelectron spectroscopy (XPS), FTIR, BET, Iodine adsorption and Karl Fisher measurements. Additional data were obtained from the Cabot North American Technical report [29]. The di€erences between the N2 Stsa speci®c surface and the N2 Multipoint speci®c surface can be related to the porosity of the carbon blacks. Therefore, according to Table 2 and similar types of carbon black studied in the bibliography [30] and [31], it is presumed that both blacks are non-porous. It should be noted that the results found rely exclusively on the chemical nature and structure of the blacks as no binder was present in any of the pigments studied. Both blacks displayed similar tinting strength and close particle size. The water content, volatile content and pH shown in Table 3, can be related to the hydrophilic character of the pigments, which implies greater oxygen surface functionality as shown in detail in the XPS analysis in Tables 4 and 5. CB-A appears to be the cleanest black, with no nitrogen and a very small quantity of sulphur (Table 4). According to the content of sulphur, the carbon blacks can be ranked as follows: CB-B (0.03% sulphur) > CB-A. CB-A is poorly oxidised as indicated by the lowest O content. CB-B contains the highest sulphur level. This carbon black is twice as ``oxidised'' as CB-A. This oxygen content is mainly found as hydroxyl and ethers on the CB surface.

Table 5 Detailed functional composition (ratio O/C, S/C) of of the CB-A and the CB-B Sample

O=C O-S Carboxylic aldehyde, ketone,

O-C Hydroxyl, ethers

O-C Phenol anhydride

H2O O2

S-C

SO2 Sulphones

SO4 Sulphates

C-N

NO2

Binding energy

531.5 eV

533.1 eV

534.5 eV

535.8 eV

163.8 eV

166.4 eV

168.8 eV

400 eV

405 eV

CB-A CB-B

0 0

0.0049 0.0082

0 0

0.0006 0.0006

0.0002 0.0043

0 0.0006

0 0.0003

0 0

0 0

J.M. PenÄa et al. / Polymer Degradation and Stability 72 (2001) 163±174

3.2. Carbon black/additive adsorption studies The heats of adsorption and desorption of stabilisers on CB were measured by the Flow Micro-calorimetry technique. The di€erences in the heats of adsorption and desorption provide a useful measure of the type of binding mechanism, i.e. chemical Ð versus physical Ð adsorption. The data in Table 6 for CB-A and CB-B show that the heat of desorption for the hindered piperidine stabiliser Chimassorb 944 is signi®cantly less than the heat of adsorption. The same applies to that of the hindered phenolic antioxidant Cyanox 17901, which indicates the presence of strong physical adsorption via hydrogen bonding or acid-base interactions with the CB functionalities determined by XPS analysis (Table 5). These interactions are associated on the CB surface through the amine and phenolic groups in their respective structures [9±11]. For the other phenolic antioxidants Irganox 10101 and Irganox 10761 the extents of adsorption are very low indicating that their structures have little anity for the CB surface because of the steric hindrance of the phenolic group. In the case of Irgafos 1681 its phosphite structure results in a weak interaction with the carbon black surface. The CB surface chemistry as well the speci®c area and porosity, control the absorption of the stabiliser. A more oxidised surface makes CB-B (Tables 4 and 5) more acidic, and hence adsorbs the amines and the phenolic compounds more strongly than CB-A. On the other hand CB-A can adsorb similar amounts per gram to CB-B, as its speci®c surface area is greater than CB-B, despite the less acidic surface of the former. Therefore, adsorption activity and amount adsorbed of Chimassorb 9441, Irganox 10101, Irganox 10761 and Irgafos 1681 for CB-B are higher than CB-A, except for the amount of Cyanox 17901 adsorbed when the order is reserved. The adsorption activity of the carbon black can e€ect the eciency of the stabiliser in a polyethylene matrix,

167

when the level of stabiliser adsorption on the CB reaches 1±3% w/w, it is that all stabiliser added to a commercial CB/p6 formulation is adsorbed by the CB. This is obviously dependent on the type and structure of the stabilisers and CB. 3.3. Oxidation induction time (OIT) study on low density polyethylene ®lled with carbon black, antioxidants and HALS (standard data analysis) The melt thermal degradation of LDPE formulations has been studied by oxidation induction time (OIT) at 200 C. Oxidative induction time is a qualitative assessment of the level (or degree) of stabilisation of the material tested. This test is typically used as a quality-control measure to monitor the stabilisation level in formulated resin as received prior to extrusion. Thus, the performances of the antioxidants, HALS and CBs and their combinations in the di€erent formulations are represented in Table 7, as well as their additive stabilising e€ect and the synergism or antagonism. Each value corresponds to the time interval from when the oxygen ¯ow is ®rst initiated to the onset of the oxidative reaction measured by DSC. The DSC analyses yield a thermal curve as displayed in Fig. 1. The data reported in Table 7 shows that during the DSC test the stability of the polymer ®lms only containing one additive, all the three primary antioxidants were superior to HALS and the phosphite Irgafos 1681 (AOX2), following the order: Cyanox 17901 …40:4 min† > Irganox 10101 …32:2† > Irganox 10761 …16:5†: This trend does not correspond neither with the oven ageing of HDPE, in which the antioxidant performance was related with thermal additive stabilities followed the order Irganox 10101>Irganox 10761>Cyanox 17901 [19], neither with the photostabilisation performance described in our previous work [1] which was Irganox

Table 6 Adsorption data from ¯ow microcalorimetry via heptane solvent Stabiliser (adsorbate)

CB (adsorbent)

C 1790 AOXI

I 1010 I 1076

AOX2

HALS

I 168 C 944 T 622

CB-A CB-B CB-A CB-B CB-A CB-B CB-A CB-B CB-A CB-B CB-A CB-B

Adsorption

Desorption

Energy ( mJ m 2)

Amount % w/w of CB

Amount % w/w of CB

Energy (+mJ m 2)

29.07 31.51 0.28 1.41 1.99 1.95 0.25 0.94 2.60 15.76 0.12 0.24

6.06 5.69 0.51 0.56 0.33 0.48 0.20 0.22 4.03 3.63 0.2 0.25

1.76 2.53 0.28 0.54 0.25 0.22 0.04 0.03 0.32 0.23 0.2 0.28

8.43 9.24 0.00 0.65 1.58 1.75 0.09 0.25 1.40 5.06 0.03 0.05

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J.M. PenÄa et al. / Polymer Degradation and Stability 72 (2001) 163±174

Table 7 Formulations and the corresponding oxidation induction times (OIT) compared with the additive e€ects of each stabiliser and CB when present Mix no.

Stability ranking OIT 200 C

Additive e€ect (b)

Time OIT 200 Ca (a)

Formulation % w/w each (a±b)

(min) 01 LDPE 02 CB-A 03 CB-B 04 I1010 05 I1076 06 Cy1790 07 C944 08 I168 09 T622 10 I1010+CB-A 11 I1076+CB-A 12 Cy1790+CB-A 13 C944+CB-A 14 I168+CB-A 15 T622+CB-A 16 I1010+CB-B 17 I1076+CB-B 18 Cy1790+CB-B 19 C944+CB-B 20 I168+CB-B 21 T622+CB-B 22 I10+T622+CB-A 23 I1010+C944+CB-A 24 I1076+T622+CB-A 25 I1076+C944+CB-A 26 Cy1790+T622+CB-A 27 Cy1790+C944+CB-A 28 I1010+T622+CB-B 29 I1010+c944+CB-B 30 I1076+T622+CB-B 31 I1076+C944+CB-B 32 Cy1790+T622+CB-B 33 Cy1790+C944+CB-B 34 I1010+I168+T622+CB-A 35 I1010+I168+C944+CB-A 36 I1076+I168+T622+CB-A 37 I1076+I168+C944+CB-A 38 Cy1790+I168+T622+CB-A 39 Cy1790+I168+C944+CB-A 40 I1010+I168+T622+CB-B 41 I1010+I168+C944+CB-B 42 I1076+I168+T622+CB-B 43 I1076+I168+C944+CB-B 44 Cy1790+I168+T622+CB-B 45 Cy1790+I168+C994+CB-B 46 I1010+I168+T622 47 I1010+I168+C944 48 I1076+I168+T622 49 I1076+I168+C944 50 Cy1790+I168+T622 51 Cy1790+I168+C994 52 I1010+T622 53 I1010+C944 54 I1076+T622 55 I1076+C944 56 Cy1790+T622 57 Cy1790+C944 58 I1010+I168 59 I1010+I168+CB-A 60 I1010+I168+CB-B

71 70 69 44 55 36 60 72 62 42 53 33 56 68 64 47 52 43 57 67 61 34 10 51 27 18 5 26 14 48 46 23 6 13 7 41 24 8 1 15 4 30 37 9 2 16 21 39 50 11 3 32 31 49 45 29 12 28 25 17

0.8+/ 1.1+/ 1.3+/ 32.2+/ 16.5+/ 40.4+/ 6.8+/ 0.6+/ 6.1+/ 35.1+/ 18.1+/ 43.5+/ 13.7+/ 3.7+/ 4.8+/ 28.5+/ 19.8+/ 34+/ 13.3+/ 3.9+/ 6.1+/ 41.8+/ 89.1+/ 21.2+/ 55.5+/ 65.6+/ 99.5+/ 56.7+/ 79.6+/ 26.4+/ 29.3+/ 61.5+/ 94+/ 81.1+/ 91.5+/ 35.8+/ 61.3+/ 90.7+/ 146+/ 76.8+/ 104.5+/ 47.9+/ 40.3+/ 89.9+/ 124.5+/ 75.6+/ 65+/ 39.9+/ 24.7+/ 86.9+/ 106.2+/ 43.9+/ 44.9+/ 25.1+/ 30+/ 49.3+/ 82+/ 54.5+/ 59.5+/ 75.3+/

0.1 0.1 0.1 0.2 0.2 0.2 0.1 0.05 0.1 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.4 0.2 0.2 0.4 0.5 0.4 0.4 0.2 0.2 0.4 0.5 0.5 0.5 0.2 0.4 0.5 0.5 0.5 0.5 0.2 0.2 0.4 0.5 0.4 0.4 0.2 0.2 0.5 0.5 0.4 0.4 0.2 0.2 0.4 0.5 0.4 0.4 0.4

0.83

0.083

CB

AOX1

CB-A CB-B

33 18 42 8 2 7 34 18 42 8 2 7 39 40 24 24 48 48 40 40 24 25 48 49 40 41 24 25 48 49 40 41 25 25 48 49 39 40 23 24 47 48 38 39 23 23 47 47 33 34 34

2 1 2 6 2 2 5 2 8 5 2 1 2 49 3 31 18 51 17 39 3 5 14 46 41 51 12 36 43 97 37 64 23 15 42 75 37 25 17 1 40 58 6 6 3 7 3 35 22 26 41

CB-A CB-A CB-A CB-A CB-A CB-A CB-B CB-B CB-B CB-B CB-B CB-B CB-A CB-A CB-A CB-A CB-A CB-A CB-B CB-B CB-B CB-B CB-B CB-B CB-A CB-A CB-A CB-A CB-A CB-A CB-B CB-B CB-B CB-B CB-B CB-B

CB-A CB-B

I 1010 I1076 C 1790

I1010 I1076 C 1790

I1010 I1076 C 1790

I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1076 I1076 C 1790 C 1790 I1010 I1010 I1010

HALS

C 944 T 622

C 944 T 622

C 944 T 622 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944 T 622 C 944

AOX2

I 168

I 168

I 168

I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168

I 168 I 168 I 168

(continued on next page)

J.M. PenÄa et al. / Polymer Degradation and Stability 72 (2001) 163±174

169

Table 7 (continued) Mix no.

Stability ranking OIT 200 C

Additive e€ect (b)

Time OIT 200 Ca (a)

Formulation % w/w each (a±b)

(min) 61 Cy1790+I168 62 Cy1790+I168+CB-A 63 Cy1790+I168+CB-B 64 C944+I168 65 C944+I168+CB-A 66 C944+I168+CB-B 67 T622+I168 68 T622+I168+CB-A 69 T622+I168+CB-B 70 I1076+I168 71 I1076+I168+CB-A 72 I1076+I168+CB-B a

22 20 19 58 54 59 66 65 63 40 38 35

64.8+/ 65.3+/ 65.3+/ 9.7+/ 17.2+/ 9.5+/ 4+/ 4.1+/ 5+/ 39+/ 40.2+/ 41.2+/

0.4 0.4 0.4 0.1 0.2 0.1 0.05 0.05 0.05 0.2 0.2 0.2

41 42 42 7 9 9 7 8 8 17 18 18

24 23 23 2 9 1 3 4 3 22 22 23

0.83

0.083

CB

AOX1

CB-A CB-B CB-A CB-B CB-A CB-B CB-A CB-B

HALS

C 1790 C 1790 C 1790 C 944 C 944 C 944 T 622 T 622 T 622 I1076 I1076 I1076

AOX2 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168 I 168

OIT data are from triplicate samples.

Fig. 1. Oxidation induction time (OIT) for formulation from 14 to 18, containing CB plus antioxidants or HALS.

10761 (722 h)>Irganox 10101 (622 h)>Cyanox 17901 (596 h). Less hindrance on the phenolic group can explain the higher eciency of Cyanox 17901 in comparison of the more hindered Irganox 10101 and Irganox 10761 [1]. The polymeric HALS being less ecient than the AOX1 follows the order: Chimassorb 9441 (6.8 min)> Tinuvin 6221 (6.1 min). This di€erence can be related to the type of piperidine and the compatibility of the HALS according their molecular structure [1]. The presence of Irgafos 1681 alone showed a small antagonistic e€ect on the LDPE stability. The CB presence alone at a concentration of 0.83% w/w resulted in a very low stabilising e€ect compared with the presence of antioxidants or HALS. Small dif-

ferences in stabilising performance were found between CB-A and CB-B with the CB-B containing formulations being the most stable in comparison with the corresponding formulations containing CB-A. Because of the feature similarities such as particle size, tinting strength and so on, described in Tables 2 and 3, the di€erent stabilisation performance between both blacks cannot be explained. However, as was reported in a previous paper [1,2], the di€erent behaviour of CB-A and CB-B was explained by a higher content of oxygen (0.0097 O/ C ratio) on the latter, as hydroxyl groups and sulphur (0.0055 S/C ratio) determined by XPS analysis (Tables 4 and 5). Thus, according to the surface chemistry, CB-B showed a better stabilising eciency than CB-A, which is consistent with the higher antioxidant activity of the

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former. Thus, a higher oxygen content, mainly as hydroxyl groups will have a role as chain terminating and peroxide decomposing agent, acting as well as act as a reservoir of hydrogen as proposed by [6,7]. The e€ects on polymer stability during melt thermal degradation of formulations containing two or more additives are variable, being both antagonistic and synergistic depending on the additive mixture under study. Examples are the important synergistic performance between HALS (Chimassorb 9441 and Tinuvin 6221) and primary antioxidants. This can be in¯uenced by the nature of the polymer matrix as reported in other studies [19]. As expected, the primary antioxidants are synergistic with the HALS, with Cyanox 17901 exhibiting the strongest synergism with Chimassorb 9441, followed by Irganox 10101 and Irganox 10761 (Table 7) although the last two showed minor synergism and therefore, in this case, the overall stability relies on the additive e€ect rather on the synergism. On the other hand, important synergism (close or higher than the additive e€ect) was found between the AOX1 and the AOX2 Irgafos 1681 resulting in performances as good as the combinations of the HALS with AOX1. Combinations of HALS with AOX2 resulted in minor synergy or antagonism as in the case of Tinuvin 6221. Thus, the performance of two additive formulations (AOX1 or AOX2+HALS) together with their corresponding OIT data and the synergism can be ranked as follows for their interaction and additive times:

because of its better stabilising e€ect. An interesting feature is that the antioxidants also showed both small antagonistic or synergistic e€ect in most of the cases and these could be partially related to the adsorption by the CB [9,10]. The data in Table 6 shows that the more adsorbent CB-B exhibits antagonism with the less hindered phenol Cyanox 17901 and Irganox 10101. CB-A, in turn, exhibited low synergism with all the three AOX1, and is the less adsorbent CB. As observed, this is coincident with the acidic character of the black and therefore with its volatile content or surface functionality, which a€ects the adsorption capabilities of the CB. Thus, CB-B being more acidic than CB-A will adsorb amines more strongly through both an acid-base reaction or/and H bonding. This con®rms previous observations reported by several authors [5]. In addition, some decomposition of the antioxidant by the CB can occur through a catalysed oxidation with CB surface acidic groups as was reported to happen with secondary amines [32]. The antagonism with the more oxidised CB-B can be attributed to the quinone type oxygen of the black, which is a stronger hydrogen acceptor than the chain propagating radical hence the phenolic antioxidant is consumed. The resulting stabilising eciency of two additive formulations (AOX1+ CB) reported in Table 7 can be ranked as follows: …12 Cy1790 ‡ CB-A† …43:5; 2 min† >

…57 Cy1790 ‡ C944† …82 min; 35 min† > …61Cy1790‡

…10I1010 ‡ CB

I168† …64:8; 24 min† > …58I1010 ‡ I168† …54:5; 22 min† > …56Cy1790 ‡ T622† …49:3; 3 min† > …53I1010 ‡ C944†

…18Cy1790 ‡ CB B† …34; 8 min† > …16I1010 ‡ CB-B† …28:5; 5 min† >

…44:9; 6 min† > …52I1010 ‡ T622† …43:9; 6 min† > …70I1076 ‡ I168† …39; 22 min† > …55I1076 ‡ C944†

…17I1076 ‡ CB-B† …19:8; 2 min† > …11I1076 ‡ CB A† …18:1; 1 min†:

…30; 7 min† > …54I1076 ‡ T622† …25:1; 3 min† > …64C944 ‡ I168† …9:7; 2 min† > …67T622 ‡ I168†

In general, the stability of binary formulations of HALS with carbon black were less stable than those with AOX1. As for the AOX1, the existence of antagonistic e€ects in formulations containing only Tinuvin 6221 and carbon black resulted in lower stability than the additive e€ect of each one alone as displayed in Table 7. However, in the case of Chimassorb 9441, both CB-B and CB-A showed weak synergism (5 and 6 min respectively), and this e€ect could be due to its stronger antioxidant character compared with that of CB-A. Such antagonism could also be related to the higher adsorption activity of CB-B towards Chimassorb 9441 than CB-A, as reported in Table 6. The adsorption of the HALS, as well antioxidants by the CB can result in a concentration decrease of stabiliser in the polymer matrix resulting in a loss of stabilisation eciency. Therefore, the adsorption capabilities of the carbon black can play a role in the eciency of the formulation by controlling the concentration of stabilisers in the polymer. Hence, the reason why Tinuvin 6221 showed a similar antagonistic e€ect with CB-B, and CB-A independently of

…4; 3 min†: Again the same trend for AOX1 is observed, and the less hindrance on the phenolic group can explain the higher eciency and synergistic behaviour of Cyanox 17901 in comparison with Irganox 10101 and Irganox 10761. The structure of the HALS also a€ects the interaction with the antioxidants. For instance, Chimassorb 9441 with a labile H in the piperidine group can interact with Cyanox 17901 and the Irgafos 1681 more selectively than Tinuvin 6221 [1], possibly via an acid-base interaction. The types of interaction in melt degradation for binary formulations in OIT analysis are very di€erent to that reported in our earlier work [1] dealing with photodegradation studies. In such work an antagonist e€ect was reported between HALS and AOX1 and was found to be more important with Chimassorb 9441 than Tinuvin 6221, even though formulations containing Chimassorb 9441 are more stable

A† …35:1; 2 min† >

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the black can be related to the chemical structure of the tetramethyl-piperidine group. This stabiliser does not possess reactive labile hydrogen, which results in a lower reactivity and therefore in a di€erent type of adsorption or interaction with the carbon black compared with Chimassorb 9441. In addition to the adsorption process, deactivation of the amine can occur through a reaction with CB oxygen containing groups as was reported in the case of diphenyldiamines in rubbers [33]. This will be dependent on the surface chemistry of the CB and on the chemical structure of the HALS. Under photodegradation conditions, the hydroxyl and phenolic groups onto the CB surface can form H bonds with the amines resulting in sterically hindered proton transfer reactions decreasing the activity of the HALS. However, in the melt degradation (200 C) such an interaction is not feasible and instead some chemisorption or catalysed reactions could be present between the carbon black and the stabilisers. This e€ect can also be related to the adsorption process, where it was reported in FMC studies [9±11] that the main anchorage point of the HALS was precisely the amine and piperidine groups. …13 C944 ‡ CB-A† …13:7; 6 min† > …19C944 ‡ CB B† …13:3; 5 min† > …21T622 ‡ CB B† …6:1; 1 min† > …15T622 ‡ CB-A† …4:8; 2 min†: On the other hand, no signi®cant di€erences and minor synergism was found to occur between the carbon black and the secondary antioxidants (phosphite). The stability of these formulations is lower than those containing CB and HALS being ranked as follows: (20 I168+CB-B) (3.9, 2 min)>(14 I168+CB-A) (3.7, 2 min). In general, the synergism described between two additive formulations was also found to occur in three and four additive formulations, each component contributing to synergism between pairs of additives present to the overall stability of the formulations. Therefore, in all cases increasing stability was achieved by the addition of a further additive to the mixes to obtain the subsequent three and four additive formulations. For three and additive formulations the highest stabilities were obtained in presence of: 1. an AOX1 (preferably Cyanox 17901, followed by Irganox 10101), 2. an AOX2 (Irgafos 1681) when it gives the strongest synergism with AOX1, 3. a HALS (preferably Chimassorb 9441 when it gives synergism with AOX1 and AOX2). Although the low synergism or antagonism exhibited by CB in two-additive formulations (formulations 10 to 21 inclusive (Table 7) its presence in three-additive formulations results in comparable stability to similar formulations no containing CB. In several cases, the

171

performance of CB is superior to the Irgafos 1681 with the same HALS and AOX1, i.e. formulations 23 and 29 compared with formulation 47 (Table 7). This case can be compared with the CB containing formulations 22 and 28 (both with Tinuvin 6221 instead of Chimassorb 9441) and 46 (no CB). In the latter case, formulation 46 exhibited a higher stability, and hence Irgafos 1681 in formulation 46 imparts more stability than CB-A and CB-B, because of the presence of Tinuvin 6221 by comparison with the former case. This implies that the interactions involved in formulations of more than two additives are increasingly complex. On the other hand, addition of CB can result in similar stability in a three additive formulation, when CB replaces HALS (Tinuvin 6221), i.e. formulation 60 compared with 46 and 47. Thereby, a synergism of three factors seems to be involved when the CB is present which cannot be explained by the contributed e€ect of two additive interactions described above. The synergism obtained with the CB-A and CBB is variable depending on the other two additives involved in the formulation, though CB-A showed higher performance in most of the cases. In general, the performances of CB-A are superior to that of CB-B in three additive formulations. The stability for three additives formulations can be ranked with the corresponding OIT and synergism as follows: …51 Cy1790 ‡ I168 ‡ C944† …106:2; 58† > …27Cy1790‡ C944 ‡ CB-A† …99:5; 51† > …33Cy1790 ‡ C944‡ CB B† …94; 46† > …23I1010 ‡ C944 ‡ CB A† …89:1; 49† > …50Cy1790 ‡ I168 ‡ T622† …86:9; 40† > …29I1010 ‡ C944 ‡ CB B† …79:6; 39† > …46I1010‡ I168 ‡ T622† …75:6; 37† > …60I1010 ‡ I168 ‡ CB

B†

…75:3; 41† > …26Cy1790 ‡ T622 ‡ CB A† …65:6; 18† > …62Cy1790 ‡ I168 ‡ CB A† …65:3; 23† > …63Cy1790‡ I168 ‡ CB B† …65:3; 23† > …47I1010 ‡ I168 ‡ C944† …65; 25† > …32Cy1790 ‡ T622 ‡ CB B† …61:5; 14† > …59I1010 ‡ I168 ‡ CB

A† …59:5; 26† > …28I1010‡

T622 ‡ CB B† …56:7; 17† > …25I1076 ‡ C944‡ CB A† …55:5; 31† > …22I10 ‡ T622 ‡ CB A† …41:8; 2† > …72I1076 ‡ I168 ‡ CB B† …41:2; 23† > …71I1076 ‡ I168 ‡ CB A† …40:2; 22† > …48I1076 ‡I168 ‡ T622† …39:9; 17† > …31I1076 ‡ C944 ‡ CB …29:3; 5† > …30I1076 ‡ T622 ‡ CB B† …26:4; 3† > …49I1076 ‡ I168 ‡ C944† …24:7; 1† > …24I1076 ‡ T622 ‡CB A†…21:2; 3† > …65C944 ‡ I168 ‡ CB-A† …17:2; 9† > …66C944 ‡ I168 ‡ CB B† …9:5; 1† > …69T622 ‡ I168 ‡ CB I168 ‡ CB-A† …4:1; 4†:

B† …5; 3† > …68T622‡

B†

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In most of the three-additive formulations synergism was observed, but antagonism was also observed in some formulations. Thus, as a result of antagonism in twoadditive formulations between Tinuvin 6221 and both CB's and the aryl phosphite Irgafos 1681 (formulations15, 21 and 69 respectively) the three-additive formulations 67, 69 and 24 showed the corresponding antagonism when no synergism was present to overcome it. In general, in four-additive formulations the addition of a primary antioxidant to the three additive formulations (HALS+secondary antioxidant+CB), or a HALS to the corresponding three additive formulations (primary+secondary antioxidant+CB) results in an important increase in stability performance. This is a result of the synergism between the primary antioxidants with the AOX2 (Irgafos 1681) (i.e. mixes 58, 61 and 70 in Table 7) and the HALS (i.e. mixes 52±57 in Table 7) and the three factor interactions described above when CB is present. Therefore, in most cases, four-additive formulations are more stable than their corresponding three additive formulations. One of the exceptions is the lower stability of formulation 36 (four-additive system containing CB-A) compared with 48. In turn, formulation 42 (four-additive system containing CB-B) is more stable than 36 and 48. This points to the e€ect of the CB type on the whole stability of the formulation depending on the rest of stabilisers involved. It can be summarised, that the highest stability is achieved by formulations containing four additives, when its addition does not result in antagonism reducing the eciency of the mix. Because the stability relies on the whole synergistic e€ect by the formulation as well as on the individual contribution of each stabiliser, the formulation has to deal with the interactions rather than with the separate e€ects of each additive. Thus, the relative contributions of the synergistic and additive e€ects are in¯uenced by the formulation. The presence of a primary antioxidant, a secondary antioxidant and a HALS is a mandatory condition to attain high OIT stabilities. In three additive formulations, a CB (CB-A better than CB-B) can be used instead of Irgafos 1681 (AOX2) attaining similar levels of stability as is the case for formulations 27 and 33 compared with the more stable formulation 51. The use of a phenolic antioxidant is rather more ecient than the use of HALS, but this relies on both the type of HALS or antioxidant. As HALS, Chimassorb 9441 was more e€ective than Tinuvin 6221 LD1, and as antioxidants Cyanox 17901 or Irganox 10101 depending on the rest of additives. The use of CB in the formulations as a fourth additive appears to be of practical value in the melt stability of LDPE. The CB grade is also important where it can a€ect markedly the stability of the formulation. In three and four additive formulations, CB-A was found to be in general the more e€ective stabiliser. However, depending on the type of stabiliser CB-B can be more

e€ective than CB-A, for instance in the presence of Irganox 10101 (formulations 60 compared with 59, Table 7). The ranking of stability of the 8 best formulations can be ranked with the synergistic e€ect in brackets as follows: …39 Cy1790 ‡ I168 ‡ C944 ‡ CB-A† …146; 97† > …45Cy1790 ‡ I168 ‡ C994 ‡ CB B† …124:5; 75† > …51Cy1790 ‡ I168 ‡ C994† …106:2; 58† > …41I1010 ‡ I168 ‡ C944 ‡ CB B† …104:5; 64† > …27Cy1790 ‡ C944 ‡ CB A† …99:5; 51† > …33Cy1790 ‡ C944 ‡ CB

B† …94; 46† >

…35I1010 ‡ I168 ‡ C944 ‡ CB A† …91:5; 51† > …38Cy1790 ‡ I168 ‡ T622 ‡ CB A† …90:7; 43† > …44Cy1790 ‡ I168 ‡ T622 ‡ CB

B† …89:9; 42†:

Finally, it has been shown that this standard analysis of the performance reported in Table 7, allows a partial picture of the interactions of the additives in three and four additive formulations, because of the system complexity. Therefore, a more adequate method of analysis is needed to achieve a more in depth understanding of the interactions in such complex systems. 4. Conclusions The information gathered through the data analysis gave some clues of the mechanism involved in the thermal stability of the LDPE in the melt and these may be summarised as follows. 1. The stability performance during OIT of a formulation relies on the individual contribution of each stabiliser as well as the respective interactions with others. 2. The interactions are variable, being both synergistic (mainly) and antagonistic (few cases). 3. Generally for the primary antioxidants alone their thermal stabilising e€ects on the polymer are decisive and appear to be related to the stability of the additives. 4. In the presence of a hindered piperidine light stabiliser no such correlations exist between polymer and antioxidant stability. 5. In most of the cases synergistic interactions were found between HALS and the antioxidants. 6. Di€erences in the performance of the polymeric type of HALS were evidenced, as well as between the CB grades. The HALS containing labile H (Chimassorb 9441) was more ecient than the Tinuvin 6221 (containing no such moiety). 7. Small antagonistic e€ects were found between two-additive formulations.

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8. Such antagonism is widely overcome by a powerful interaction of three-additive formulations primary+secondary antioxidants+HALS. 9. Small synergism and antagonism was found between one HALS and carbon black depending on the chemical structure of the HALS. 10. Finally, the presence of CB (at the concentrations studied) was not a decisive factor in the LDPE thermal melt stability, However, the nature of the CB appears to have some role in controlling their performance as a stabilising agent alone, as well as their interactions with light stabilisers and antioxidants, via adsorption-desorption processes as well as other chemical/physical interactions. Acknowledgements The authors thank the Cabot Corporation for partial ®nancial support and the materials for this programme of work. Appendix. Molecular structures of stabilisers

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