Increasing Polypropylene High Temperature Stability by Blending

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Increasing Polypropylene High Temperature Stability by Blending Polypropylene-Bonded Hindered Phenol Antioxidant Gang Zhang,† Changwoo Nam,† Linnea Petersson,‡ Joakim Jam ̈ beck,‡ Henrik Hillborg,‡ ,† and T. C. Mike Chung* †

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ ABB AB, Corporate Research, Forskargränd 7, 72178 Västerås, Sweden ABSTRACT: Currently, hindered phenol (HP) antioxidants mixed in PP products provide thermal-oxidative protection during PP melt processing (homogeneous mixing). However, there are concerns about their effectiveness during applications. This paper presents computer simulation and experimental results to demonstrate a facile phase separation of HP molecules in the PP matrix and investigates a new approach that can dramatically improve PP thermal-oxidative stability under elevated temperatures. This technology is centered on a new PP−HP copolymer containing a few comonomer units with HP moieties, homogeneously distributed along the polymer chain. Because of the cocrystallization between the PP and PP−HP copolymer, all HP antioxidant groups are homogeneously distributed in the PP matrix (amorphous domains). The resulting PP/PP−HP blends demonstrate a thermal-oxidative stability nearly proportional to the HP content. While commercial PP products (containing regular antioxidants and stabilizers) degrade within a few minutes at 210 °C in air, the PP/PP−HP blend, with the same concentration of HP groups, demonstrates nearly no detectable weight loss after 1000 h. In an ASTM endurance test under a targeted application temperature (140 °C in air), the commercial PP shows 1% weight loss within 10 days. On the other hand, the new PP/PP−HP (5/1) blend with the same HP content lasts for about 2 years under the same constant heating condition. Overall, the experiment results of the PP−HP antioxidant present the potential of expanding PP applications into a far higher temperature range (>140 °C) under thermal-oxidative environments.

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INTRODUCTION Isotactic polypropylene (PP) represents one-quarter of commercial plastics produced around the world, which touch everyday life.1,2 Despite its commercial success, special attention needs to be taken in applications that requires longterm exposure to high-energy conditions, such as elevated temperatures, UV radiation, and high electric fields,3−6 where high purity of the PP as well as a suitable type and amount of antioxidant are critical factors. Because of the presence of tertiary proton in every monomer unit, the PP polymer is liable to the free radical mediated chain degradation mechanism.7,8 As illustrated in Scheme 1, after removing the labile tertiary hydrogen (H*), the spontaneous oxygen oxidation of the polymeric C radical (I) occurs to form peroxy radical (II) and then hydroperoxide group (III). The subsequent decomposition of the hydroperoxide group follows with two possible degradation mechanisms of the polymer chain to create two pairs of PP chains with reduced polymer molecular weight. In addition to the formation of PP polymers with a terminal aldehyde (IV) or unsaturated group (VI), two new polymeric carbon radicals (V) and (VII) are also formed at the same time.9 In other words, this is a catalytic mechanism with a rapid degradation of the polymer chain.10 Thus, it is essential to have antioxidants that are able to donate hydrogen atoms (H*) and © XXXX American Chemical Society

are located adjacent to the polymer chain to neutralize polymeric radicals (I) and inhibit the oxidation−degradation cycles.11 It is a common practice in the polyolefin industry to introduce a small amount (100 °C). Figure 6 compares isothermal TGA curves at 190 °C in air

Figure 7. DSC curves of (a) PP−HP copolymer with 1 mol % HP content, (b) PP/PP−HP (5/1), (c) PP/PP−HP (10/1), (d) PP/PP− HP (20/1), (e) PP/PP−HP (50/1), (f) PP (capacitor grade), and (g) PP (general grade), at a heating rate of 10 °C/min under an O2 atmosphere.

Figure 6. Isothermal TGA curve comparison at 190 °C in air between commercial PP and several PP/PP−HP blends.

between Borclean PP and a series of PP/PP−HP blends with the weight ratio from 1/1 to 500/1, using the same PP−HP with 1 mol % HP content. The well-formulated capacitor grade Borclean PP begins its sharp weight loss after heating the sample at 190 °C for about 220 min, while the general grade PP decomposed around 50 min under the same condition.30 All PP/PP−HP blends exhibit much improved stability. The onset time of weight loss is proportional to the PP−HP content; the higher the PP−HP in the blend, the longer degradation onset time and slower the weight loss. Five PP/PP−HP blend samples (with 50/1, 20/1, 10/1, 5/1, and 1/1 wt ratios) demonstrate no detectable weight loss within 1000 min. It is interesting to note that the PP/PP−HP (10/1) blend contains about 0.7 wt % of HP group content, similar to the HP antioxidant concentration in many commercial PP products. However, this PP/PP−HP (10/1) blend sample exhibits no weight loss, clearly indicating the importance of the homogeneous distribution of HP antioxidant groups throughout the entire PP matrix. By further reducing the PP−HP content in the blend to 100/1, 200/1, and 500/1 weight ratios, representing exceptional diluted HP concentrations of 0.07, 0.035, and 0.014 wt % in the PP matrix, respectively, the onset degradation time systematically decreases to 750, 550, and 350 min. They are still longer than that of Borclean PP, which is known to have high dielectric breakdown strength in capacitors. Evidently, the HP antioxidant group with a homogeneous distribution in the PP matrix is highly effective in protecting PP from thermal-oxidative degradation, by donating hydrogen atoms (H*) to neutralize the polymeric radicals (I) and inhibit

onset temperature (OOT) between two commercial PP polymers (general and capacitor grades) and several PP/PP− HP blends as well as the associated PP−HP copolymer with 1 mol % HP content. Following the ASTM E 2009-08 measurement procedure, the samples were heated at a heating rate of 10 °C/min under an O2 atmosphere. In all DSC curves (Figure 7), the endothermal peak for two PP polymers and the PP/PP−HP blends show a rather constant melting temperature in the range of 160−163 °C, with the exception of the PP−HP copolymer at 144 °C. However, the onset of the exothermal peak (OOT) is very different, which indicates the starting point of material oxidative degradation.52 Borclean PP, with a combination of wellformulated antioxidants and stabilizers for dielectric capacitor applications under high electric field conditions, shows its OOT at 237 °C, which is about 17 °C higher than the general-grade PP at 220 °C. All PP/PP−HP blends, with 50/1, 20/1, 10/1, and 5/1 wt ratios, demonstrate higher oxygen oxidation temperatures at 241, 244, 245, and 251 °C, respectively. The corresponding PP−HP exhibit the OOT at 265 °C. Evidently, the oxygen onset temperature for thermal-oxidative degradation systematically increases with the increase of PP−HP content in the blend. Another strong indication of HP antioxidant groups in the PP/PP−HP blends are homogeneously distributed in the PP matrix. Oxidative induction time (OIT), measured under constant elevated temperature and oxygen atmosphere, is another F

DOI: 10.1021/acs.macromol.7b02720 Macromolecules XXXX, XXX, XXX−XXX

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that the thermal-oxidative reaction does not cause immediate polymer chain degradation with sharp weight loss. In addition to the HP antioxidant activities retarding polymer chain degradation (Scheme 1), the PP−HP antioxidant, known to form cross-linked network upon oxidation, may provide unique PP cross-linking activity and delay the weight loss. Figure 9 shows gel content of four PP/PP−HP blends (20/1, 10/1, 5/1, and 1/1), using the same PP−HP copolymer with 1

common ASTM method using DSC to determine the stabilization efficacy of antioxidants.26 Following ASTM D3895-14 procedures, the samples were heated to 190 °C under N2 for 5 min. The gas was then changed to O2 at a flow rate of 50 mL/min.37 This changeover point to O2 is the zero time of the experiment. The heat flow to the sample was then monitored, associated with the endothermal heat for polymer chain degradation. Figure 8 compares OIT curves between a

Figure 9. Comparison of gel content after heat treatment at 190 °C in air for 24 h and the starting PP−HP content in several PP/PP−HP blends. Figure 8. Oxidative-induction time for (a) Daploy PP, (b) Borclean PP, (c) PP/PP−HP: 200/1, (d) PP/PP−HP: 100/1, (e) PP/PP−HP: 50/1, (f) PP/PP−HP: 20/1, (g) PP/PP−HP: 10/1, and (h) PP−HP.

mol % HP content. They were heated in air at 190 °C for 24 h, similar to the isothermal conditions used in previous experiments but with a long time. It is interesting to note that commercial PP homopolymers lost the weight entirely while the PP/PP−HP blends (20/1, 10/1, 5/1, and 1/1) demonstrate no weight loss under this aggressive heattreatment condition (Figure 6). All heat-treated PP/PP−HP blend samples were subjected to Soxhlet extraction by refluxing xylene for 2 h to remove the soluble polymer chains. The gel content is the insoluble fraction. The PP/PP−HP (20/1) blend sample shows only 5% gel content, which is the same percentage of PP−HP in the blend, indicating the complete degradation of the PP homopolymer in the blend. However, the PP/PP−HP blends, with 10/1, 5/1, and 1/1 weight ratios, showed 20%, 32%, and 68% gel contents (higher than PP−HP contents), respectively. The experimental results clearly show some portion of the PP homopolymer also involved in the cross-linking reaction. The insoluble fraction contributed from the PP homopolymer is 10%, 12%, and 18%, systematically increasing with the increase of PP−HP content in the PP/PP− HP blends. Evidently, the PP−HP copolymer not only forms a cross-linking network by itself but also helps some portion of the PP homopolymer engage in the free radical coupling reaction (Scheme 2) during this aggressive thermal-oxidation condition. Considering the concentration of HP antioxidant groups (0.1, 0.2, and 0.5 mol %) in these PP/PP−HP blends, both PP and PP−HP polymer chains must be homogeneously intertwined in the amorphous domains. Endurance under Elevated Operation Temperatures. Polymer aging and endurance time under certain operational conditions are essential information in considering polymer applications. The ASTM 1877 method is commonly used to determine polymer endurance time at elevated temperatures in air.39 The failure is defined at 1 wt % polymer weight loss. This method involves a TGA measurement (Figure 10) with various heating rates. For comparison in this study, we investigated two PP/PP−HP blends (5/1 and 20/1) side-by-side, respectively.

general-grade PP (Daploy), a capacitor-grade PP (Borclean), the PP−HP copolymer with 1 mol % HP groups, and several PP/PP−HP blends with a series of weight ratios (200:1, 100:1, 50:1, 20:1, and 10:1). General-grade PP (Daploy) shows an OIT at about 15 min, while the well-formulated capacitor-grade PP (Borclean) exhibits the OIT at around 60 min. Under the same condition, the PP−HP copolymer with 1 mol % HP groups shows a complete flat OIT line up to 500 min, without any detectable onset heat flow to the sample. All blends samples demonstrated longer OIT at 190 °C than two commercial PP polymers. The PP/PP−HP (10/1), PP/PP−HP (20/1), PP/PP−HP (50/1), PP/PP−HP (100/1), and PP/PP−HP (200/1) demonstrated OIT at 230, 180, 110, 90, and 80, respectively. Considering the same linear hindered phenol moiety in both PP−HP and commercial PP polymers, the HP groups bonded to the PP chain offer much higher antioxidant performance. Overall, the OIT results are consistent with all TGA and DSC results, indicating the importance of HP group homogeneous distribution in the PP matrix, in which all HP groups can participate in antioxidant activities to protect PP chains (Scheme 1). Gel Content. It is interesting to compare TGA (Figure 6) and DSC (Figure 8) isothermal results in oxygen at 190 °C for the same polymer sample. Only the PP−HP copolymer shows no detectable weight loss and heat flow in both measurements, indicating excellent thermal-oxidative stability under the testing condition. All other PP samples, containing either commercial HP antioxidants or a PP-HP copolymer antioxidant, show the DSC endothermal heat flow (indicative of the thermal-oxidative reaction) within 300 min. However, the TGA curves show only a few PP/PP−HP blends (weight ratio >100/1) with weight loss within 1000 min. Every sample shows significantly shorter onset time for an oxidation reaction than weight loss. It is clear G

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Figure 10. TGA curves under various heating rates for (A) PP/PP− HP (5/1) and (B) PP/PP−HP (20:1).

Figure 11. Log heating rate constant vs inverse temperature kinetics plot for various conversions (weight loss) of (A) PP/PP−HP (5/1) and (B) PP/PP−HP (20/1).

Figure 11 shows the plots of log (heating rate) vs heating temperature (1/T) under various specific polymer weight loss (conversion) conditions, based on the TGA curves in Figure 10. The slope of each line was used to calculated the activation energy (Ea) of each polymer weight loss (conversion during the polymer chain oxidation and degradation reaction), using the equation in ASTM 1877 test Ea = −(R/b)Δ log β/Δ(1/T), wherein Δ log β/Δ(1/T) = slope of the line obtained in Figure 10, β = heating rate (K/min), T = temperature (K) at constant conversion, gas constant R = 8.314 J/(mol·K), and b = 0.457/K on the first iteration. Table 2 summarizes the activation energy (Ea) for both of the two PP/PP−HP (5/1 and 20/1) blends with various polymer weight loss conditions. With the activation energy (Ea), we can estimate the material endurance time under a specific value of conversion and failure temperature, following the ASTM 1877 equation Tf = Ea/ {2.303R[log tf − log Ea/(Rβ) + a]}, wherein a = approximation integral, tf = estimated lifetime, and Tf = failure temperature for a given value of conversion. Figure 12 plots the estimated endurance (lifetime) vs application temperature in air for both of the two blends, PP/PP−HP (5/1) and PP/PP−HP (20/1) with 1 wt % polymer weight loss, assuming this weight loss level is acceptable in the application. Meanwhile, we added two more previous test results around commercial Borclean PP and PP− HP with 1 mol % hindered phenol groups.30 It is clear that the blends shows higher endurance than the commercial Borclean PP in the entire elevated temperature range. The perpendicular line in Figure 12 indicates that the material lifetime for Borclean PP is about 10 days under constant heat at 140 °C in air. On the other hand, under the same condition, the lifetime of PP/PP−HP (5/1) blend is about 2 years. The PP/PP−HP (5/1) blend, having a typical HP antioxidant content in many

Table 2. Activation Energy under Various Polymer Weight Loss (Conversion) for (a) PP (Borclean) and (b) PPHP-1 activation energy Ea (kJ/mol) conv (%)

PP

PP−HP

PP/PP−HP (5/1)

PP/PP−HP (20/1)

1.0 2.5 5.0 10.0 15.0 20.0

126.8 117.6 104.7 97.0 93.6 91.7

209.1 185.1 163.5 145.8 150.8 171.8

168.2 137.3 114.0 98.2 85.9 79.5

145.1 131.3 120.7 113.2 112.2 109.2

commercial PP products, shows a significantly higher endurance time, which clearly presents the advantage of using PP−HP as the antioxidant in the PP matrix.

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CONCLUSION We have applied both computer simulation and an experimental study to show the facile phase separation of polar hindered phenol (HP) antioxidant molecules in the nonpolar semicrystalline PP matrix. The results explain the limitations of commercial HP additives, which provide good antioxidative protection during PP melt processes but not in the PP applications. In this paper, we demonstrate a new PP polymer-bounded HP antioxidant (PP−HP copolymer) that forms cocrystallization with the PP homopolymer and homogeneously distributes HP antioxidant groups in the entire PP product. We carried out a systematical study to examine the thermal-oxidative stability of PP/PP−HP polymer blends, involving TEM, DSC, and TGA techniques and several H

DOI: 10.1021/acs.macromol.7b02720 Macromolecules XXXX, XXX, XXX−XXX

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Figure 12. Estimated lifetime of (a) Borclean PP, (b) PP/PP−HP (5/ 1) blend, (c) PP/PP−HP (20/1) blend, and (d) PP−HP with 1 wt % polymer weight loss under various temperature conditions.

standard ASTM methods. All experimental results clearly show that the PP−HP copolymer containing 1 mol % HP concentration can effectively protect PP from thermal-oxidative degradation. This new PP−HP antioxidant not only donates hydrogen atoms to stop the PP thermo-oxidative decomposition cycle but also cross-links PP polymer chains to prevent polymer weight loss. With the combination of chemical and physical stability, the PP applications can be extended to a much higher temperature range (>140 °C). In addition, the experimental results may also provide the guidance of a future antioxidant (additive) design in other polymer systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.C.M.C.). ORCID

T. C. Mike Chung: 0000-0002-3123-944X Author Contributions

G.Z. and C.N. contributed equally to this work and are co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this work through a grant from ABB Company. REFERENCES

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