Color Inhibition of Phenolic Antioxidants in Ziegler-Natta Polyethylene. I. In-Situ Polymer Studies

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1 Color Inhibition of Phenolic Antioxidants in Ziegler-Natta Polyethylene. I. In-Situ Polymer Studies Norman S. Allen, 1 Christopher M. Liauw, 1 Aitor Reyes, 1 Michele Edge, 1 Brian Johnson, 2 Klaus Keck-Antoine 2 1 School of Biology, Chemistry and Health Science, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK 2 Chemtura, Toekomstlaan 13, B-2200 Herentals, Belgium Although the level of transition-metal catalyst residues in polyethylene (PE) has been drastically reduced over the years, they can still give rise to discoloration, particularly when associated with other additives such as antioxidants. This first of this series of papers screens a variety of candidate color suppressants featuring a range of functional groups, including alcohols, amine/ sulfur compounds, and acid-containing species. These candidate color suppressants were melt-blended into a Ziegler-Natta linear low-density PE in combination with 2,2 0 -isobutylidenebis(4,6-dimethylphenol) (a highly discoloring hindered bisphenol antioxidant) and zinc stearate antacid. Yellowness index measurements made after multiple extruder passes indicated that dipentaerythritol (DPE) and triisopropylamine (TIPA) gave good color inhibition and, in some cases, outperformed established phosphites. The DPE and TIPA were found (via melt flow rate measurement) not to affect melt stability, and hydroperoxide determination revealed that DPE had no peroxide decomposition activity. The latter results indicate that the color-suppression mechanism of DPE and TIPA is different from that associated with phosphites. J. VINYL ADDIT. TECHNOL., 15:12 19, ª 2009 Society of Plastics Engineers INTRODUCTION Color development in polymers upon aging or processing (aside from that due to basic oxidation products of the polymer) is often attributed to transformation products of phenolic antioxidants arising from reactions associated with the trapping of oxygen-centered radicals that would otherwise cause oxidation of the polymer. A number of such transformation products, notably conjugated and nonconjugated quinoidal compounds, have been evaluated for their absorption characteristics in the visible spectrum Correspondence to: Christopher M. Liauw; c.m.liauw@mmu. ac.uk DOI /vnl Published online in Wiley InterScience ( Ó 2009 Society of Plastics Engineers [1 6]. Quinone methides (QM), are the final and oxidation-stable transformation products of mono- and polycyclic phenolic antioxidants that are substituted in positions 2 or 4 to a hydroxyl group with methyl, methylene or methine groups. However, it is well-established that QM also exhibit a stabilizing effect when present in combination with phenolic antioxidants [7, 8] due to the formation of a stillbenequinone. Catalyst residues (usually chromium) in high-density polyethylene (HDPE) produced by using a Phillips metal oxide catalyst system also give rise to discoloration. Such metal oxide catalysts lead to a relatively high level of unsaturated species in the PE chains [9 11]. In contrast, Ziegler-Natta catalysts usually yield PE chains with a low level of unsaturation. It has been reported that catalyst residues are generally not bound to the polymer chain but nevertheless play an important role during processing, as they promote the formation of unsaturated species. Reaction between a phenolic antioxidant and a metal catalyst residue can result in the formation of chromophoric transformation products [11, 12], or in the case of titanium residues, colored titanium phenolates can be formed [13 16]. The rate of production of such products is dependent on both the concentration and the activity of the metallic catalyst residues present [13]. Formation of chromophoric products during melt-processing of polyolefins can be reduced by the use of phosphites or other trivalent phosphorus compounds in combination with hindered phenols. Here the phosphite protects the phenolic antioxidant via two main routes. It can act as a source of hydrogen atoms for the phenol, so that fewer phenolic transformation products will appear, or by decomposing the hydroperoxides responsible for oxidation of the phenolic additives. This approach for decreasing the production of chromophoric species is well-established [1, 17, 18] and continues to be popular. Phosphonites in combination with phenolic antioxidants in synergistic mixtures have also been reported as an option to improve discoloration [19]. JOURNAL OF VINYL & ADDITIVE TECHNOLOGY 2009

2 Metal stearate antacids can, in some cases, be used to prevent color formation in Ziegler-type polyolefins stabilized with hindered phenolic antioxidants. Indeed, acidic residues are known to promote dealkylation of phenolic antioxidants and therefore enhance the discoloration in the polymer [13]. Early studies demonstrated that the combination of titanium with phenolic antioxidants results in the formation of colored species [15, 20]. Some thiobisphenols have been reported to afford good color stability, and they perform as well as primary antioxidants, owing to the phenolic groups that they possess [21]. Good color stability in Ziegler-Natta HDPE has also been achieved by using a combination of glycerol, polyhydric alcohols, polyglycols, and polypentaerythritols, in combination with phenolic antioxidants [14, 22]. This result is thought to be due to deactivation of the metal via chelation by the hydroxyl groups of the polyhydroxyl species [23]. Sulfurcontaining organotin compounds having the general formula R n Sn(SR) 42n or R n Sn(SOR) 42n, where R represents an alkyl group, are also claimed to be good inhibitors of color formation [24]. Following on from this discussion, color inhibition in polyolefins is evidently an area that has been actively pursued for many years but is still not completely understood or resolved, particularly in the context of polyolefins produced using newer catalyst technology. In this first part of the study, candidate color suppressants have been screened, initially in a Ziegler-Natta linear low-density polyethylene (LLDPE) containing titanium-based catalyst residues. The candidate color suppressants (including polyfunctional alcohols, amine/sulfur compounds, and various carboxylic acids) have been selected partly on the basis of their ability to interact with metallic catalyst residues, thereby blocking interaction with phenolic/phosphate antioxidant transformation products. Taking this goal into consideration, Lowinox 22IB46 (L22IB46) (Chemtura) was selected as the most suitable phenolic antioxidant for this experiment because of its strong activity in protecting the polymer during melt processing, which reflects its role as a very efficient radical scavenger. Antioxidant L22IB46 also undergoes rapid discoloration during melt processing and is therefore an eminently suitable candidate for the study of chromophore formation. Owing to the strong radical-scavenging activity of L22IB46, there is no need to combine it with a phosphite or phosphonite to achieve high process stability; it can provide good stability when used alone. Comparative studies were undertaken with the best-performing candidate color suppressants in combination with other antioxidants, including vitamin E, Anox PP-18, Anox 20, and Lowinox 1790 (Chemtura, UK). The effect of a range of acid scavengers was also investigated together with a performance assessment of the best color suppression system without phenolic antioxidant. This study indicated whether or not the color suppression system possessed any stabilization activity alone. Hydroperoxide determinations were carried out to acquire insight into the mode of action of the candidate color suppressants; suppression of hydroperoxide formation may suggest a mechanism that is similar to that of phosphite secondary antioxidants. The best candidate color suppressants were then assessed in metallocene LLDPE, Phillips LLDPE, and a free-radical low-density polyethylene, in order to observe the respective effects of titanium in metallocene catalyst residues, chromium-based residues, and no metallic catalyst residue at all. EXPERIMENTAL Polymer Materials Several PE resins were used throughout this study to acquire insight into the effect of metallic residues from different polymerization catalyst systems on the production of colored degradation products. The PEs used are described in Table 1. Additives In addition to Lowinox L22IB46 (which was chosen for its poor color development characteristics combined with good melt stabilization performance), several other phenolic antioxidants (Table 2) were evaluated with the best candidate color suppression system. Zinc stearate (ZnSt) was selected as an acid scavenger (at 500 ppm) for the color suppressant evaluation studies, as its use is widespread. However, in a separate study, calcium stearate (CaSt), zinc oxide (ZnO), and hydrotalcite (DHT 4A from Kyowa) were also investigated. The performance of the candidate color suppressants (Table 3) was assessed relative to that of the well-established organophosphite peroxide decomposers Alkanox 240 (A240) and Alkanox P24 (AP24) (both from Chemtura). TABLE 1. Details of polyethylenes investigated. PE type Supplier Trade name and grade Catalyst system Residue metal present Melt flow rate a (dg min 21 ) LLDPE Innovene Innovex LL 0209 AA Ziegler-Natta Ti 0.9 LLDPE Repsol Development product Metallocene Ti 1.3 LLDPE Total Development product Phillips metal oxide Cr 0.5 LDPE Exxon ExxonMobil LD 100MED Free radical None 2.0 a 1908C/2.16 kg. DOI /vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY

3 TABLE 2. Phenolic antioxidants investigated (all from Chemtura, UK). Trade name Chemical name Code Molar mass Lowinox 22IB46 2,2 0 -Isobutylidenebis(4,6-dimethylphenol) L22IB Anox PP18 Octadecyl 3-(3 0,5 0 -di-t-butyl-4 0 -hydroxyphenyl)propionate APP Vitamin E a-tocopherol VitaE 430 Anox 20 Tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane An Premixing of Polymer With Additives Premixes of formulations (1.3 kg total mass) were weighed out and then blended by using a Stephan high speed mixer at 3000 rpm for 150 s. The mixer chamber was cooled throughout the process to minimize heat buildup. Melt Blending The mixtures of polymer powder and additives were melt-blended under nitrogen by using a Brabender PLE 651 dynanometer drive unit fitted with the PL2000 singlescrew extruder attachment. The screw diameter was 19 mm; the L:D ratio was 44:1; and the compression ratio was 4. The temperature profile used is the first entry in Table 4. After the compounding step, the pellets were subjected to a further five extruder passes in air by using the second temperature profile in Table 4. For performance assessment of the best candidate color suppressants in polyethylenes produced via different catalyst systems, a Thermo-Prism TSE24HC co-rotating twin-screw extruder (28:1 L:D ratio) was used for melt blending/melt degradation under the same conditions described in Table 4, though in this case the melt blending step (pass 0) was carried out in air to replicate commercial conditions. 1608C in a 50 T clamp force steam-heated press. The mold preheating time was 5 min, after which the formulation was added to the mold and pressed for 2 min. The mold was then transferred to a water-cooled press where pressure was applied for 3 min. Hydroperoxide Determination The concentration of hydroperoxides in the polyethylene pellets was measured after the third extruder pass by using the well-known iodometric method [25] described as follows. Into a round-bottom flask (100 ml), PE granules ( g) were placed together with 2-propanol (19 ml), glacial acetic acid (1 ml) and sodium iodide (0.1 g). The sample and blank solutions (the latter without polymer) were heated under reflux for 30 min. The solutions were then cooled in an ice bath, and the UV absorbance due to the I 3 ion was measured at 420 nm by using a Perkin-Elmer Lambda 7 UV spectrometer. It should be noted that the wavelength for maximum absorbance of the I 3 ion is actually 375 nm. However, polymer additives and associated polymer and additive degradation products can also absorb at this wavelength. Therefore, to avoid these artifacts, the absorbance of I 3 was measured at 420 nm. A calibration curve was constructed by using a series of cumene hydroperoxide solutions in 2-propanol. Compression Molding The pelletized formulations were compression-molded into ca. 100-lm thick films between aluminum plates at TABLE 3. Candidate color suppressants (all supplied by Aldrich apart from DLTDP, LL77 supplied by Chemtura, and C3346 supplied by Cytec). Class of compound Chemical name Code Molar mass Polyfunctional alcohols Dipentaerythritol DPE 254 Glycerol monostearate GMS 358 Pentaerythritol Pent E 136 2,5-t-butylhydroquinone 25DTHQ Triisopropanolamine TIPA 191 Poly(ethylene oxide) PEG Amine and sulfur Lowilite 77 LL77 compounds Cyasorb 3346 C3346 Lowinox DLTDP DLTDP 515 Acid-functional compounds Adipic acid 1,6HDA 146 Citric acid CA 192 Benzene phosphinic acid BPA 142 Yellowness Index Measurements After each extrusion pass, the yellowness index (YI) of each sample of pellets was measured by using a reflectance spectrophotometer (Gretag Macbeth Color Eye 2180). Results of duplicate samples were averaged. The accuracy/reproducibility of this method has been estimated as 60.7%. Melt Flow Rate Melt flow rate (MFR) measurements were carried out by using a Ceast MFR Number 1 semiautomatic melt TABLE 4. Extrusion temperature profiles. Operation Zone 1 Zone 2 Zone 3 Zone 4 Pass 0 (melt blending in N 2 ), 8C Passes 1 5 (melt degradation in air), 8C JOURNAL OF VINYL & ADDITIVE TECHNOLOGY 2009 DOI /vnl

4 FIG. 1. Yellowness index versus number of extruder passes for OHfunctional candidate color suppressants (all added at 250 ppm) in Z- LLDPE containing 250 ppm of L22IB46 and 250 ppm of ZnSt: ~ DPE, 3 GMS, & 25DTHQ, ^ PentE, þ TIPA. Control samples are * ZnSt only, l ZnSt þ L22IB46 (see Table 3 for codes). flow rate instrument with the barrel temperature set to 1908C. For samples obtained after the compounding extrusion pass (pass 0) and the fifth extruder pass (pass 5), MFR was determined by using the 21.6-kg and 10-kg masses to obtain MFR 21 kg and MFR 10 kg, respectively. These two figures were then used to determine the melt flow rate ratio (MFRR), which is a measure of the degradation that has occurred during melt processing. The MFRR was calculated by using Eq. 1. h i MFRR ¼ h 1 MFR 10kg 1 MFR 21:6kg Pass 5 1 MFR 10kg 1 MFR 21:6kg RESULTS AND DISCUSSION i Pass 0 Polyfunctional Alcohols Different polyfunctional alcohols were dry-blended into Ziegler-Natta LLDPE (Z-LLDPE) with 500 ppm of ZnSt, 250 ppm of Lowinox 22IB46, and the candidate color suppressant. The latter was added at levels of 125 ppm (only in the case of DPE), 250 ppm, and 500 ppm (in order to monitor concentration dependency), and the mixtures were then melt-blended under nitrogen and subjected to five extrusion passes in air. Samples were collected during extrusion passes 0, 1, 3, and 5, and their yellowness index was measured. It was noticed that the effect of these candidates at 500 ppm was very similar to that at 250 ppm. Therefore, data obtained at 250 ppm only were plotted (Fig. 1; see Table 3 for codes). It is clearly observed that GMS has no color protection activity at all. The 25DTHQ has poor color protection activity at the 0th and 1st pass and marginal color protection activity at passes 3 and 5. The PentE, DPE, and TIPA all show very good color protection at the 0th and 1st pass, though by (1) the 3rd and 5th passes, the performance of PentE falls below that of DPE and TIPA. The latter two candidates performed best of all; in fact, DPE gave good results even at 125 ppm (see Fig. 2). Increasing the DPE level beyond 125 ppm did not always bring about an improvement in performance, particularly at the 5th extruder pass. The 250-ppm DPE level appeared to be a sensible dosage for more moderate levels of melt processing (i. e., passes 1 and 3). The PentE and TIPA may actually operate via the same mechanism in the absence of oxygen, as the initial color is maintained. It may be considered that the discoloration produced at this stage is mainly associated with the metal catalyst residues. Amine and Sulfur Compounds A range of amine and sulfur compounds was evaluated at 250 and 500 ppm by using the same multiple extrusion approach. All the amine and sulfur-based candidates showed no color suppression activity at all, apart from Lowilite 77, Lowinox DLTDP, and Cyasorb UV3346 showing some color suppression activity during the compounding step (pass 0), which was carried out under nitrogen. This observation was somewhat surprising in view of the good activity reported for Irgastab FS 042 [26]. Additives With Acid Groups Using the same base formulation of phenolic antioxidant (L22IB46) and ZnSt, the YI values obtained after multiple extruder passes in the presence of the acid-functional candidate color suppressants are shown in Fig. 3. In this case, no color suppression was observed during the compounding extrusion step under nitrogen (pass 0), though a degree of color suppression was observed when melt mixing in air during passes 1 5. Citric acid and benzene phosphinic acid conferred good color stability to the resin, especially during the first 3 passes. However, FIG. 2. Effect of DPE level (In Z-LLDPE containing 250 ppm of L22IB46 and 250 ppm of ZnSt) on yellowness index after ^ Pass 0, & Pass 1, ~ Pass 3, and * Pass 5. DOI /vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY

5 FIG. 3. Yellowness index versus number of extruder passes for carboxylic-acid functional candidate color suppressants (all added at 250 ppm) in Z-LLDPE containing 250 ppm of L22IB46 and 250 ppm of ZnSt: ~ Adipic acid, 3 Citric acid, & Benzene phosphinic acid, ^ Glycolic acid. Control samples are * ZnSt only, l ZnSt þ L22IB46. FIG. 4. Yellowness index versus number of extruder passes for assassment of acid scavengers as candidate color suppressants (all added at 250 ppm) in Z-LLDPE containing 250 ppm of L22IB46: ~ CaSt, 3 ZnO, & Hydrotalcite. Control samples are * ZnSt only, l ZnSt þ L22IB46. adipic acid and glycolic acid gave a higher YI than the base formulation after passes 0 and 1, although by pass 3, the YI reached a limiting value below that of the base formulation. At pass 5, the performance of all the acid-functional candidates was similar. The color suppression mechanism of the acidic-functional candidates will be explored in the second paper of this series; experimental data suggest that quinone methide formation is inhibited. Acid Scavengers Acid scavengers are mainly added to Ziegler PE in order to remove acidic residues arising from the catalyst system, and at the same time they improve the melt rheology of the resin by acting as lubricants. The color suppression activity of different acid scavengers, in the presence of L22IB46, has been examined by using the same multiple pass extrusion approach (see Fig. 4). Note that the alternative acid scavengers replaced the ZnSt. Owing to its lower molar mass and different structure, ZnO gives good performance because it neutralizes more acid than CaSt at the same mass loading. However, ZnO adds about 10% more opacity relative to CaSt. This physical effect may lead to apparently improved color stability [27]. Hydrotalcite shows greater color suppression activity than the metal stearates. Also, CaSt offers better performance than ZnSt. Therefore, further optimization of color stability can be achieved by the careful selection of acid scavenger. The mode of action of the latter and other additives is not well-understood and is explored in the second paper of this series, in which a solution-based model-compound approach, together with second-derivative UV/Vis spectroscopy, is used to investigate interactions between titanium ions and candidate color suppressants. Color Suppression of DPE With Other Antioxidants Preliminary studies indicated that some phenolic antioxidants (AOs) induce more discoloration than others because of variations in the structure of the transformation products [27]. The next section explores the performance of the best candidate color suppressant (DPE) in combination with a range of phenolic AOs and ZnSt. The chosen phenolic antioxidants are Lowinox 22IB46 (very effective melt stabilizer but very discoloring); vitamin E, which is used at low dosage for melt stabilization; and Anox PP18, which can be considered a historic additive that was effective during the earlier years of polyolefin stabilization chemistry. Their performance in isolation and in combination with dipentaerythritol is shown in Fig. 5. It is important to point out that there was no improvement in color stability when DPE was combined with Anox PP18. However, it is also important to note that the processing stability of Anox PP18 was very poor (the FIG. 5. Yellowness index versus number of extruder passes for comparison of color suppression performance of DPE with: ^^ L22IB46, ~ ~ VitaE, and *l APP18 (all at 250 ppm) in Z-LLDPE. Open symbols denote zero DPE, and black symbols denote DPE added at 250 ppm. The base formulation (BF) consists of Z-LLDPE and ZnSt (250 ppm) only. See Table 3 for codes. 16 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY 2009 DOI /vnl

6 TABLE 5. Melt flow rate ratios (calculated by using Eq. 1) showing the effects of DPE and TIPA without AO. Formulation MFRR Base formulation (BF) a 1.52 BF þ L22IB BF þ TIPA 1.22 BF þ DPE 1.40 a The base formulation contains Ziegler-Natta LLDPE (Innovene Innovex LL 0209 AA) and ZnSt (500 ppm) only. MFRR was 1.2, whereas that of 22IB46 was 1.0). For the sake of consistency with the other antioxidants, the loading of vitamin E used in this study was rather higher than the usual 150 ppm. The high melt stabilization performance of vitamin E was reflected in a good MFRR value (1.0). The reduction in color development arising from addition of DPE was excellent, considering that vitamin E converts into strongly chromophoric species. Therefore, the data generated thus far indicate that DPE shows strong potential as a coadditive for color retention in the presence of phenolic antioxidants that are known to form chromophoric transformation products. The color suppression mechanism is likely to be associated with the complexation of catalyst residues by DPE. The latter aspect will be verified via a model-compound approach in the next paper in the series. Stabilizing Effect of the Best Color Suppressants in Absence of Antioxidant The DPE and TIPA additives had thus far shown the best color suppression performance. Therefore, the next step was to establish whether or not these additives had a stabilizing effect when used alone, i. e., without the Lowinox 22IB46 primary phenolic antioxidant. The usual multiple-pass extrusions were followed by the determination of YI and MFRR. Neither DPE nor TIPA alone led to a significant increase in YI, relative to the base formulation of LDPE and ZnSt: after five extruder passes, the YI values of the base formulation (BF) only, BF þ DPE, and BF þ TIPA were 20.13, 0.09, and 20.02, respectively. The MFRR values are given in Table 5; predictably, the base formulation underwent significant melt degradation, and the BF þ L22IB46 formulation showed sensibly zero melt degradation. Interestingly, the BF þ TIPA formulation showed a degree of melt stabilization which was similar to that of a poorly performing AO (i. e., Anox 18). The BF þ DPE formulation showed more limited melt stabilization. The data for BF þ TIPA are significant, as they imply that the level of primary phenolic AO could be reduced, a change which might lead to further improvements in color stability. Hydroperoxide Content It was important to establish whether or not the color suppression associated with DPE was related to peroxide decomposition. Therefore, hydroperoxide levels were determined after the 0th and 3rd extruder passes (Table 6). It is evident that DPE addition does not give rise to any reduction in hydroperoxide level relative to L22IB46; in fact, after the 3rd extruder pass, the hydroperoxide level with DPE was even higher than that of the BF. However, the YI of the BF þ DPE formulation after the 3rd extruder pass was This result clearly indicates that DPE suppresses chromophore formation via a mechanism entirely different from the peroxide decomposition route associated with phosphite and phosphonite secondary AOs. As noted above, the next paper in the series will explore the mechanism of color suppression via a solution-based model-compound approach. Effect of Polymerization Catalyst System on Color Suppression In view of the achievement of good color suppression in Z-LLDPE, the next logical step was to assess the performance of the best candidate color suppressants (DPE and TIPA) in polyethylenes produced by using different catalyst systems. The performance of DPE and TIPA was compared alongside the established phosphites Alkanox 240 and Alkanox P24, both with and without calcium stearate antacid. Two other candidates were also included, as they will feature in the second part of this series of papers. They were 6000 molar mass poly (ethylene glycol) (PEG6000) and phenyl phosphinic acid (PPA). The initial melt blending and degradation of these compositions was carried out by using the twin-screw extruder, with the initial melt blending step being carried out in air. Ziegler-Natta LLDPE This grade of Z-LLDPE is the same as that used in the previously described studies and is included here because of the different melt blending/degradation conditions employed for this part of the study. Without CaSt, the two phosphites Alkanox 240 and Alkanox P-24 led to the highest YI recorded (Fig. 6a) after the 3rd extruder pass, and after the 2nd and 1st extruder passes, the YI was similar to that with L22IB46 alone. Addition of CaSt antacid to A240 and AP24 significantly improved the performance. Despite this improvement, DPE and TIPA performed best of all. The PEG6000 had no effect, even TABLE 6. Formulation Hydroperoxide data comparing L22IB46 and DPE. Hydroperoxide concentration (10 6 mol kg 21 ) Pass 0 (melt blending under N 2 ) Pass 3 (in air) Z-LLDPE only Z-LLDPE þ L22IB Z-LLDPE þ DPE DOI /vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY

7 though it has an abundance of OH groups. The PPA gave performance comparable to that of DPE and TIPA. Metallocene LLDPE This specific grade of metallocene LLDPE (m-lldpe) was chosen because of the presence of titanium catalyst residues in order to observe whether it had any similarities with the Z-LLDPE (Fig. 6b). In this case, PPA was totally ineffective, while DPE and TIPA only achieved performance similar to that of Alkanox P-24, which showed the best performance in this PE. It should be noted that Alkanox P-24 has been reported to decompose into different structures, one of which is pentaerythritol, and this decomposition may explain its excellent color suppression activity [28, 29]. Phillips LLDPE Phillips LLDPE (Ph-LLDPE) was chosen because of the presence of chromium catalyst residues. Surprisingly, here (Fig. 7a) TIPA did not perform as well as it did in the other PEs. In Ph-LLDPE, color suppression activity was improved by Alkanox P-24 and especially by DPE, results which once again show the significant suppression of color development. Free Radical LDPE The main reason for examining the yellowing in LDPE was that this polymer is generally free of metallic residues. Therefore, it would be possible to assess whether FIG. 7. Color stabilization performance of candidate color supp ressants relative to established phosphites in (a) Ph-LLDPE and (b) LDPE. The white, shaded and black bars denote extruder passes 1, 2, and 3 respectively. the potential color suppressants found so far were just as efficient when there were no metallic residues in the matrix. Figure 7 (b) shows the results obtained. It is noted here that all the systems gave a degree of color inhibition, with the PPA being the least effective. Both phosphites were effective and paralleled the effects of the DPE and TIPA. FIG. 6. Color stabilization performance of candidate color supp ressants relative to established phosphates in (a) Z-LLDPE and (b) m-lldpe. The white, shaded and black bars denote extruder passes 1, 2, and 3 respectively. CONCLUSIONS The best candidate color suppressants in a Ziegler- Natta (Z-LLDPE) (Innovene Innovex LL 0209 AA) containing the hindered phenol 2,2 0 -isobutylidenebis(4,6- dimethylphenol) (shown to form highly chromophoric transformation products) and the antacid zinc stearate were triisopropanolamine, dipentaerythritol, and pentaerythritol. The performance of the latter two candidates was at a level such that they can be considered as viable alternatives to the established phosphites (such as Alkanox P-24). They actually outperformed the latter additive in the Z-LLDPE and were at least as effective as the phosphites in metallocene LLDPE, Phillips LLDPE, and LDPE. The TIPA and DPE also suppressed color development in the presence of Vitamin E. Unlike phosphites, DPE does not suppress hydroperoxide formation, an observation that implies a rather different color suppression mechanism. The latter will be explored in Part 2 of this series of papers, in which a study of the chelation of titanium ions by DPE and TIPA is undertaken by using UV/ vis and infrared spectroscopy. 18 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY 2009 DOI /vnl

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