Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Expanding Polyethylene and Polypropylene Applications to HighEnergy Areas by Applying Polyolefin-Bonded Antioxidants T. C. Mike Chung* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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S Supporting Information *
ABSTRACT: Despite a broad range of commercial applications, polyolefins, including polyethylene (PE) and polypropylene (PP), are not recommended for applications that require long-term exposure to elevated temperatures, high electric fields, organic solvents, and combinations of those, due to both chemical and physical stability concerns. This paper discusses current antioxidants/stabilizers and crosslinking technologies and their shortcomings under extreme application conditions. The incompatibility of polar antioxidants in nonpolar and semicrystalline polyolefin prevents homogeneous distribution with adequate concentration required for protecting the polymer chain from thermal−oxidative degradation. A new approach for incorporating hindered phenol (HP) antioxidant groups in the polyolefin chain offers a specific HP concentration homogeneously distributed in the polyolefin matrix, which not only shows effective antioxidation protection but also in situ forms a polymer network via a coupling reaction between two deprotonated HP groups. Instead of weakening the mechanical strength, the resulting cross-linked PE-HP and PP-HP copolymers become stronger after high-energy exposure. Furthermore, PE-HP and PP-HP can serve as the antioxidant in PE and PP products, respectively. In a comparative endurance test at 140 °C in air, the well-formulated PP shows significant weight loss within 10 days. On the other hand, the new PP/PP-HP blend with similar HP content can last more than 2 years (1000
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Homogeneous mixing in xylene at 140 °C before evaporating the solvent. bSuspension mixing in acetone at ambient temperature overnight before evaporating the solvent.
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Figure 3. Isothermal TGA curves at 190 °C in air for the PP samples prepared by homogeneous mixing with (top) Irganox 1010 and (bottom) Irganox 1076 with (a) 1, (b) 0.5, (c) 0.3, and (d) 0.1 wt % antioxidant contents.
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MISCIBILITY OF ADDITIVES IN POLYOLEFIN To examine the miscibility between polar additives and polyolefins, we had carried out a systematical study using both computer simulation and experimental methods. As shown in Figure 5, molecular dynamics (MD) simulations73−76 were applied to examine the miscibility of Irganox 1010 antioxidant molecules in the PP matrix. Eight PP polymer chains, consisting of 1000 monomer repeating units in each chain, were placed in a 100 × 100 × 100 nm3 simulation box. Twenty Irganox 1010 molecules were then added into the same box in a random fashion. Because the formation of nanodroplets can take a considerable amount of time, a coarsegrained model was used to achieve a greater length and time scale within a reasonable time. The Martini model77 was used, as it has been proven to be successful in studies concerning additives in polymers.78,79 The simulations were run under the isobaric−isothermal NVT ensemble with the temperature and pressure of 298 K and 500 bar, respectively. Figure 5 compares the starting Irganox 1010/PP homogeneous mixture and a snapshot after 5000 ns simulation. Evidently, the Irganox 1010 antioxidant molecules were rapidly aggregated in forming separated domains in the PP matrix. The simulation results indicate the difficulty in maintaining a homogeneous distribution of HP antioxidant additives in the PP matrix. In parallel, we had also carried out an experimental study to examine the miscibility of HP antioxidants in PP products under similar conditions. Figure 6a shows a TEM micrograph of the PP/Irganox 1010 blend, containing 6 wt % of HP group content (like that in the MD simulation). This simple PP/HP blend clearly shows the phase separation of Irganox 1010 molecules in the PP matrix, forming many spherical particles with the sizes in the range of 100 kV/mm), a long life span (20−30 years), and a relatively high continuous working temperature (maximum operating temperature of up to 100 °C).80 The electric losses in the polymer film should be minimized (tan δ ∼ 2 × 10−4) and the films should be free from impurities and be mechanically strong to withstand the winding process. One main limitation for BOPP capacitor film is the maximum operating temperature of around 80 °C, restricting the reliable life span of the capacitor. The additives (antioxidants and processing aids) added into the PP resin after polymerization have a high impact on the performance of the resin in capacitor applications. Table 2 demonstrates the amount of Irganox Table 2. Dielectric Properties of BOPP (Thickness: 10 mm) and Irganox 1010 Blends
Figure 4. Oxidative-induction time (OIT) curve at 190 °C in air for the PP samples prepared by homogeneous mixing with (top) Irganox 1010 and (bottom) Irganox 1076 with (a) 1, (b) 0.5, (c) 0.3, and (d) 0.1 wt % antioxidant contents.
amount of antioxidant (wt %)
ε
0.5 1.0 2.0
2.25 2.27 2.30
tan δ (10 kHz) (×104) tan δ (100 kHz) (×104) 2.0 2.6 4.1
2.6 3.3 6.2
1010 antioxidant needed to be reduced to a minimum for current BOPP thin films to have low enough dielectric losses to perform satisfactorily in capacitors for AC applications. However, it also suffers from the reliability in long-term applications.
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CURRENT CROSS-LINKING TECHNOLOGY FOR POLYOLEFIN As discussed, the limited physical stability (due to low softening temperature) is also a major concern in the application of both PE and PP thermoplastics in high-energy areas. Cross-linking between polymer chains is known to increase product mechanical strength,80−84 temperature stability, and resistance to solvents, creep, and stress cracking.85−89 Cross-linked polyethylene (x-PE) is used everywhere today in water piping and high-voltage electrical cables.90 They show 5 times the tensile and impact strength and 20 times the environmental stress crack resistance of the corresponding HDPE products. Although isotactic polypropylene (PP) has higher mechanical strength and a high melting point and shows superior heat resistance than PE, the crosslinked polypropylene (x-PP) is not currently practiced in the industry due to the lack of an effective cross-linking method. There are several methods that are commonly applied to form the cross-linked PE products, involving peroxides,91−98 UV irradiation,99−106 and silane groups.107−111 It is usually difficult in achieving a complete network formation due to various reasons, including free-radical-induced chain degradation (Scheme 1) in peroxide and UV methods and the broad
Figure 5. (a) Starting mixture with Irganox 1010 molecules homogeneously distributed in the PP matrix and (b) a snapshot of the mixture after 5000 ns. The Irganox 1010 antioxidants are shown in orange, and PP chains are shown in gray; the blue box demonstrates the simulations. (Reproduced with permission from ref 129.)
Figure 6. TEM micrographs of (a) the PP blend with 6 wt % of Irganox 1010 antioxidant and (b) PP-HP copolymer containing 6 wt % of antioxidant comonomer content. (Reproduced with permission from ref 129.) E
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 3. Synthesis Route for the Preparation of PE-HP and PP-HP Copolymers
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PREPARATION OF PE-HP AND PP-HP COPOLYMERS As discussed, it is essential to have antioxidant additives homogeneously distributed in the polyolefin matrix. Our approach is to chemically bond polar HP antioxidant groups in the polyolefin chains. Scheme 3 illustrates a facile chemical route that effectively prepares PE-HP and PP-HP copolymers with high molecular weight, controlled HP comonomer content, and high yield. The scheme involves two reaction steps, including the copolymerization reaction of α-olefin and silane-protected alcohol comonomer (undecenyloxytrimethylsilane) using metallocene catalyst systems. [(η5-C5Me4)SiMe2(η1-NCMe3)]TiCl2/MAO CGC-catalyst130 was used in ethylene copolymerization,131,132 and rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO iso-specific catalyst133 was applied in propylene copolymerization.134−136 After the copolymerization, the silane groups were deprotected by HCl/methanol washing during the polymer workup procedures. The resulting hydroxylated polyethylene (PE-OH) and hydroxylated polypropylene (PP-OH) copolymers were then engaged in the subsequent Steglich esterification137 of OH groups with 3,5bis(tert-butyl)-4-hydroxyphenylpropionic acid to form the corresponding PE-HP and PP-HP copolymers. Figures 7 and 8 compare the 1H NMR spectra of two pairs of PE-OH/PE-HP and PP-OH/PP-HP copolymers before and after esterification reactions, and both polymers contain about 1 mol % of functional comonomer units. In the PE case (Figure 7), a major chemical shift at 1.33 ppm corresponds to the methylene groups in the polyethylene chain. The triplet chemical shift at 3.65 ppm, corresponding to CH2−OH moieties in the PE-OH copolymer, completely disappeared with the appearance of three new triplet chemical shifts at 2.61, 2.89, and 4.10 ppm, corresponding to three methylene units located near ester and HP groups in the side chains of the resulting PE-HP copolymer. The 1:1 peak intensity ratio between the chemical shift at 4.10 ppm for CH2−O−CO and a singlet chemical shift at 7 ppm for two aromatic protons in the hindered phenol moiety clearly indicates the quantitative esterification to obtain a pure PE-HP product. The integrated intensity ratio of the chemical shifts between 1.33 and 4.10 ppm and the number of protons that both chemical shifts represent determines the concentration of HP groups in the PE-HP copolymer. On the other hand, three major chemical shifts at 0.95, 1.35, and 1.65 ppm (Figure 8) correspond to the methine, methylene, and methyl groups in the polypropylene chain. The triplet CH2−OH chemical shift at 3.65 ppm in the PP-OH sample also completely disappeared with the appearance of several new triplet chemical shifts at 2.61, 2.89, and 4.28 ppm, corresponding to three methylene units near ester group
molecular weight and composition distributions in the silane method. Furthermore, all of these cross-linking chemistries used to prepare x-PE networks are not suitable for forming the corresponding x-PP network. So far, there are no commercial x-PP products, due to the prompt degradation of the PP backbone under free-radical conditions112−115 and the catalyst poison by a silane functional group during PP copolymerization. There were many attempts to minimize the free-radical chain scission by grafting multifunctional monomers116,117 or moisture cross-linkable silane monomers.118−120 However, the resulting x-PP products only showed minor improvement, and the gel contents (insoluble fraction after solvent extraction) were generally below 80%.121−124 Both grafting and the chain scission are parallel-competing, which is slightly influenced by the reaction condition (i.e., temperature, the amounts of peroxide, and cross-linking agents). Despite the potential importance, especially for applications under extreme environments (high fields and high temperatures), it is still a technological challenge to develop a new chemical method to prepare the x-PP network. Scientifically, it would be most interesting to develop new nonfree-radical cross-linking chemistry for achieving a completed x-PE or x-PP network with high purity. In the past few years, we have been investigating a new research approach with the objective to simultaneously overcome both chemical and physical stability limitations in polyolefin, especially expanding PE and PP applications to high-energy (temperature, irradiation, electric field, etc.) areas.125−129 The research focused on PE-HP and PP-HP copolymers, containing a specific concentration of pendant HP antioxidant groups, with high molecular weight and narrow molecular weight and composition distributions. We had developed a facile chemical route to prepare these copolymers with a homogeneous distribution of HP antioxidants located along the polymer chain, which show a dramatic increase of HP antioxidant activity and prevent polyolefin from thermal− oxidative degradation under high-energy (temperature, irradiation, electric field, etc.) conditions. In addition, the polyolefinbonded HP groups also engage in a coupling reaction to in situ form a completely cross-linked polymer network to strengthen polyolefin physical stability. Furthermore, the PE-HP and PPHP copolymers can also serve as the compatibilizer in commercial polyolefin products containing regular antioxidant/stabilizer additives. They are very efficient to disperse regular antioxidant molecules in polyolefin matrix and show good stability. This paper will review all key experimental findings and discuss the advantages of new PE-HP and PP-HP antioxidants for expanding PE and PP products to high-energy areas. F
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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Table 3 summarizes the material characterization data for three PE-OH and PE-HP copolymer sets, with 1.1, 2.4, and 5.4 mol % of functional comonomer concentrations and two PPOH and PP-HP copolymer sets with 1 and 2 mol % of functional comonomer concentrations, respectively. DSC results show that both melting temperature (Tm) and heat of fusion (ΔH) systematically decrease with the increase of the functional group content but while nearly independent of the structure of the functional group, consistent with the Flory theory138,139 predicting the comonomer units restricted in the amorphous regions. Under the equilibrium condition, the consecutive ethylene or propylene units in the semicrystalline PE or PP copolymer run from one side of lamellae to the other, which governs the thickness of lamellae and melting temperature. Overall, we have established an effective chemical route (Scheme 3) to prepare PE-HP and PP-HP copolymers with well-controlled molecular structures. The experimental results indicate the homogeneous distribution of HP groups along PEHP and PP-HP copolymer chains. With the chemically bonded HP groups and homogeneous distribution along the polyolefin chain, it is very interesting to observe their distribution in the polyolefin matrix. Figure 5b shows a TEM micrograph of PP-HP-1.0 (run D-2) sample, which is compared with that of the simple PP/Irganox 1010 blend with a similar antioxidant content shown in Figure 5a. The uniform morphology of the PP-HP sample, without any detectable phase separation, completely contrasts with that of a simple PP/Irganox 1010 blend. The experimental results strongly support the homogeneous HP antioxidant group distribution in the PP chain and matrix, which is an essential element in our strategy to provide the effective antioxidant activities and prevent the polyolefin chain from thermal− oxidative degradation.
Figure 7. 1H NMR spectra of (a) PE-OH random copolymer with a 1.1 mol % OH content and (b) the corresponding PE-HP copolymer. (Reproduced with permission from ref 128.)
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PERFORMANCE OF PE-HP AND PP-HP COPOLYMERS It is very interesting to examine the resulting PE-HP and PPHP thermal−oxidative stabilities. Figure 9 compares isothermal TGA curves at 190 °C under air atmosphere between PE-HP1.1 (run A-2) or PP-HP-1.0 (run D-2) copolymers and their corresponding commercial HDPE or PP polymers, respectively. This experimental condition is well beyond the recommended PE and PP application temperatures. Figure 9 (top) shows the commercial HDPE (containing regular antioxidants) with an initially small weight gain, indicating some oxidation reaction with oxygen, before losing its weight due to polymer chain degradation. On the other hand, the PEHP-1.1 copolymer shows a completely flat line up to 1000 min, without any indication of weight gain or loss during the entire measurement. The comparative results clearly show the great thermal−oxidative stability of this PE-HP-1.1 copolymer. The pendent HP group effectively offers hydrogen atom to the polymeric C* radical (I) (Scheme 1) and completely stops the PE chain degradation process. Similar results were also observed in the PP case. In Figure 9 (bottom), the generalgrade PP shows a rapid decomposition after heating the sample at 190 °C in air for about 50 min. The capacitor-grade PP shows slightly better endurance, about 300 min at 190 °C. Based on the control experimental results shown in Figure 3 (measured under the same TGA condition), we can estimate the antioxidant content in the commercial general-grade PP with >0.3 wt % Irganox 1076 antioxidant and in the commercial capacitor-grade PP with 97% recovered insoluble fraction. A small amount of x-PP product may be lost during the sample handling. It is very interesting to directly monitor the HP group activity in PE-HP and PP-HP copolymers during the oxygeninduced thermal cross-linking reaction. UV−visible spectroscopy was used to observe the conversion of the incorporated HP groups into the conjugated bis-quinonemethide (x-linker) moieties, as illustrated in Scheme 4. Figure 13 shows UV− visible spectra of PE-HP-1.1 and PP-HP-1.0 copolymers before and after heating at various temperatures in air for 24 h. Both cases exhibit similar results. Two absorption peaks at λ = 275 nm (P1) and λ = 310 nm (P2) correspond to a hindered phenol J
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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property after long-term exposure in high-energy conditions. Figure 15 compares the strain−stress curves of the PP-HP-1.0
Figure 15. Strain−stress curves of PP-HP-1.0 copolymer (a) before and (b) after heating at 140 °C in air for 1 week. (Reproduced with permission from ref 126.)
copolymer before and after thermo-oxidative treatment at 140 °C in air for 1 week. Before the heating treatment, PP-HP-1.0 film exhibits tensile strength (21.5 MPa), Young’s modulus (0.7 GPa), and elongation to break (480%). Under 140 °C in air condition, most commercial PP materials lose antioxidants and stabilizers in the early stages of the heating process, and the polymer chains are degraded by oxygen with the reduction of mechanical properties. On the other hand, the PP-HP-1.0 copolymer shows a slight increase in tensile strength and modulus and some reduction in its elongation to break from 470 to 350%. The mechanical property is consistent with the formation of an x-PP network after the oxidation/coupling reaction of HP moieties.
Figure 14. Strain−stress curves of (top) PE-HP-1.1 and (bottom) PE-HP-2.4 films after exposing in air at various temperatures for 24 h. (Reproduced with permission from ref 128.)
integrity (melt-down) and fails to obtain a meaningful strain− stress curve. The starting PE-HP-1.1 copolymer exhibits tensile strength (22.0 MPa), Young’s modulus (0.86 GPa), and elongation to break (204%). This copolymer is somewhat softer than the HDPE homopolymer, due to the comonomer units that slightly reduce the overall crystallinity. However, after heating at 150 °C for 24 h in air, the PE-HP-1.1 copolymer shows no significant change (slight increase) in tensile strength (22.2 MPa) and Young’s modulus (0.92 GPa) and some reduction in the elongation to break (135%). Under the same condition, the HDPE films melt and lose the film structure. Further increasing the temperature to 190 °C in air, the trend continues to increase PE-HP-1.1 tensile strength and modulus and decrease the elongation to break. In the PE-HP-2.4 sample, higher HP content further reduces its crystallinity and melting temperature and shows lower initial tensile strength (11.7 MPa) and modulus (0.15 GPa) but higher elongation to break (380%). After heating at 150 °C for 24 h in air, both tensile strength and Young’s modulus in the elastic region remain the same, with only a slight reduction in the elongation to break (365%). This polymer film shows a strain-hardening phenomenon, continuously increasing the tensile strength with strain ratio and the significant increase in its toughness. The polymer film reaches its fracture point with maximum stress near 30 MPa. All mechanical properties are consistent with the formation of the cross-linked structure in the x-PE network. As discussed, PP is most prone to the oxidative−thermal degradation and is difficult in forming x-PP network. It is a scientific challenge to maintain the desirable mechanical
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ENDURANCE TESTS Polymer aging and endurance time under certain operational conditions are essential information in considering polymer applications. As discussed, it is highly desirable to broaden the polyolefin application temperature and maintain constant performance for a long period of time. With the experimental results shown in the newly developed PE-HP and PP-HP copolymers, it is very curious to know the new operational window and endurance time at an operational temperature of >100 °C, which is important in many high-energy applications but not attainable in current PE and PP products. Thus, the ASTM 1877-17 method142 was employed to determine polyolefin endurance time at elevated temperatures in air. The failure is defined at a 1 wt % polymer weight loss. This method involves a TGA measurement with various heating rates. For comparison, the high-quality commercial PE and PP samples were also examined side by side as the control runs. Based on the resulting TGA curves, Figure 16 plots log (heating rate) vs heating temperature (1/T) for capacitorgrade PP (top) and the PP-HP-1.0 copolymer (bottom) under various specific polymer weight loss (conversion) conditions. The slope of each line was used to calculate the activation energy (Ea) of each polymer weight loss (conversion during the polymer chain oxidation and degradation reaction), using the equation in the 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 14, β = heating rate (K/min), T = temperature (K) at K
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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temperature, following the ASTM 1877 equation Tf = Ea/ (2.303 R[log tf − log{Ea/(R β)} + a]), wherein a = approximation integral, tf = estimated lifetime, and Tf = failure temperature for a given value of conversion. Figure 17 plots the
Figure 16. Log heating rate constant vs inverse temperature kinetics plot for various conversions (weight loss) of (a) capacitor-grade PP and (b) PP-HP-1.0. (Reproduced with permission from ref 126.)
Figure 17. Estimated lifetime of (a) capacitor-grade PP and (b) PPHP-1.0 based on a 1 wt % polymer weight loss under various temperature conditions. (Reproduced with permission from ref 126.)
constant conversion, gas constant R = 8.314 J/(mol K), and b = 0.457/K on the first iteration. Table 4 summarizes the activation energy (Ea) for both capacitor-grade PP and PP-HP-1.0 copolymer with various
estimated endurance (lifetime) vs application temperature in air for both commercial capacitor-grade PP and PP-HP-1 with a 1% polymer weight loss, assuming that this weight loss level is acceptable in the application (without significantly reducing mechanical properties). The PP-HP-1.0 copolymer shows much higher endurance than all commercial PP products in the entire elevated temperature range. The perpendicular line in Figure 17 indicates that the material lifetime for capacitor-grade PP is about 1 day under 160 °C in air. On the other hand, the lifetime of PP-HP-1.0, under the same condition, is near 104 days (28 years). The 4-order increase in endurance is astonishing, which clearly reinforces the idea of PP-bonded HP groups with a combination of chemical and physical protections, not only preventing the PP chain from thermooxidative degradation but also forming a cross-linking (3D) network structure. The experimental results raise the strong possibility that this new PP-HP polymer may address the concern around thermal and oxidative stabilities (aging issue) of PP products operated at an elevated temperature or highenergy (radiation and electric field) conditions.
Table 4. Activation Energy Under Various Polymer Weight Losses (Conversion) for (a) Capacitor-Grade PP and (b) PP-HP-1 Copolymer activation energy Ea (KJ/mol) conversion (%)
capacitor-grade PP
PP-HP-1.0
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
polymer weight loss conditions. In all side-by-side weight loss comparisons, the PP-HP-1.0 polymer shows significantly higher activation energy than capacitor-grade PP, indicating that PP-HP-1.0 is far more resistant to oxygen oxidation and polymer chain degradation. In the details, it is interesting to note that the different trend in activation energy vs polymer weight loss (Table 4) occurs between two cases. Capacitorgrade PP shows an initial Ea value of about 126.8 KJ/mol and then systematically reduces its activation energy and increases chain degradation activities with the increase of polymer weight loss, which may be associated with lower polymer molecular weight, lower crystallinity, and higher chain mobility. On the other hand, the initial activation energy (Ea = 209.1 KJ/mol) of PP-HP-1.0 is significantly higher than that of capacitor-grade PP. In addition, the initial reduction of activation energy of PP-HP-1.0 is recovered at a higher conversion level, which may be related to the in situ formation of cross-linkers in the resulting x-PP network. With the activation energy (Ea), we can estimate the material endurance time under a specific value of conversion and failure
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POLYOLEFIN BLENDS We further investigated the PP/PP-HP blends by mixing the PP-HP copolymer with the PP homopolymer. The research objective is to use PP-HP as an alternative (more effective) antioxidant additive for PP products. Scientifically, it is very curious to know the minimum amount of antioxidant moieties homogeneously distributed in the PP matrix to observe the effective antioxidant activities. To achieve homogeneous HP distribution in this polymer blend, it is essential to have the cocrystallization between the PP homopolymer and PP-HP copolymer. Figure 18 shows a DSC study, starting with a simple PP/PP-HP-1.0 mix (1:1 wt ratio) at ambient temperature. The first heating cycle shows two Tm endotherms, with the main peak at 163 °C for PP and a broad peak centered at 137 °C for the PP-HP-1.0 copolymer. In the following cooling L
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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Figure 18. DSC curves of the PP/PP-HP-1.0 (1:1 wt ratio) powder mixture during the (a) first and (b) subsequent heating−cooling cycles. (Reproduced with permission from ref 129.)
Figure 20. Isothermal TGA curve comparison at 190 °C in air between commercial capacitor-grade PP and several PP/PP-HP-1.0 blends. (Reproduced with permission from ref 129.)
cycle, there are two exothermic crystallization peaks at 98 and 110 °C, respectively, indicating some individual crystallization processes in forming both PP and PP-HP-1.0 crystalline domains. Keeping the molten state at 230 °C for a slightly longer time to allow for interpolymer chain diffusion, there is only a single endothermic Tm peak at 158 °C and an exothermic crystallization peak at 109 °C in the subsequent heating−cooling cycles. Both melting and crystallization peaks in this PP/PP-HP-1.0 blend are located between those of individual PP and PP-HP-1.0 peaks in the first heating− cooling cycle, respectively. The DCS results clearly indicate the cocrystallization phenomenon between the PP homopolymer and PP-HP copolymer. Due to high melting temperature and viscosity, the PP-HP-1.0 copolymer requires a long interchain diffusion time (without agitation) to mix with the PP homopolymer. The miscibility of PP/PP-HP blends was also evidenced by TEM micrographs. Figure 19 shows two typical TEM
commonly recommended PP application temperature range (1000 >1000 >1000 >1000 >1000 750 550 350 >1000 645 383 12 318 143 61 2
>1000 >1000 230 180 110 90 80 >1000 132 81 5 87 37 27 2
Homogeneous mixing in xylene at 140 °C before evaporating the solvent.
a
than two commercial PP polymers. The PP/PP-HP-1.0 (10:1), PP/PP-HP-1.0 (20:1), PP/PP-HP-1.0 (50:1), PP/PP-HP-1.0 (100:1), and PP/PP-HP-1.0 (200:1) demonstrated OIT at 230, 180, 110, 90, and 80, respectively. Considering the same linearly hindered phenol moiety in both PP-HP-1.0 and commercial PP polymers, the HP groups bonded to the PP chain offer a much higher antioxidant performance. Overall, the OIT results are consistent with the TGA results, indicating the homogeneous mixing in PP/PP-HP blends and the importance of HP group homogeneous distribution in the PP matrix. It is very interesting to compare the PP/PP-HP blend (Figures 20 and 21) and the corresponding commercial general-grade PP and capacitor-grade PP (Figures 3 and 4) with a similar HP antioxidant content, under the same TGA and DSC measurement conditions. Table 5 summarizes the initial weight loss time and OIT time at 190 °C in air for all three comparative sets of PP samples containing Irganox 1010, Irganox 1076, and PP-HP antioxidants, respectively. Since the Irganox 1010 antioxidant contains four active HP groups in each molecule and both PP-HP and Irganox 1076 antioxidants have the monomeric HP moiety, a better comparison shall be based on the active HP group content (wt %), not the PP/ antioxidant wt ratio. Comparing the PP/PP-HP (D-4) sample with the PP/Irganox 1010 (A-4) sample and PP/Irganox 1076 (B-3) sample, with the HP group content in the range of 0.3− 0.4 wt %, the PP/PP-HP sample shows significantly better thermal/oxidative stability at 190 °C in air. Evidently, the HP antioxidant group with a homogeneous distribution in the PP matrix is essential for the long-term protection of the PP chain from thermal−oxidative degradation, by donating hydrogen atoms (H*) to neutralize the polymeric radicals (I) and inhibit (or slow down) the oxidation−degradation cycles illustrated in Scheme 2. In addition, the PP-HP antioxidant can also offer a cross-linking network that may further delay the polymer weight loss under thermal/oxidative environments. We further estimate the endurance time of PP/PP-HP-1.0 blends under certain application conditions. A series of TGA measurements were measured with various heating rates. The slope of the plots between log (heating rate) vs heating temperature (1/T) under various specific polymer weight
losses from TGA curves was used to calculate the activation energy (Ea) of each polymer weight loss, using the equation in the ASTM 1877 test. The activation energies (Ea) for two PP/ PP-HP-1.0 blends with 5:1 and 20:1 weight ratios with a 1% weight loss are 168.2 and 145.1 KJ/mol, respectively. They are positioned between two individual polymers, with 126.8 for PP and 209.1 KJ/mol for PP-HP-1.0, respectively. The Ea value is basically proportional to the HP antioxidant content in the PP/PP-HP-1.0 blends. The activation energy (Ea) is then used to estimate the material endurance time under a 1% polymer weight loss at the specific failure temperature. Figure 22 plots the application lifetime vs application temperature in air for PP/PP-HP-1.0 (5:1) and PP/PP-HP-1.0
Figure 22. Estimated lifetime of (a) PP-HP-1.0, (b) PP/PP-HP-1.0 blend (5:1), (c) PP/PP-HP blend (20:1), and (d) capacitor-grade PP under various temperature conditions. (Reproduced with permission from ref 129.)
(20:1) with a 1% polymer weight loss, compared with those of commercial capacitor-grade PP and PP-HP-1.0 (shown in Figure 17). The blends show endurance higher than the commercial capacitor-grade PP but lower than pure PP-HP-1.0 in the entire elevated temperature range. With the homogeneous distribution of HP groups, the PP/PP-HP-1.0 blend stability is proportional to the HP concentration. The perpendicular line in Figure 22 indicates that the material N
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
1.0 copolymer. Both PP/PP-HP blends exhibit losses in the same range as the capacitor-grade PP, the level in which the blends realistically can be used in industrial capacitor applications. Figure 24 compares the dielectric constant and dielectric loss of Borclean PP and several PP/PP-HP-1.0 blends containing 2−20 wt % PP-HP-1.0 copolymers, before and after thermal aging at 130 °C in a forced air oven. All samples exhibit an expected dielectric constant ε ∼ 2.1 at 50 and 80 °C. The thermal aging seems to have no significant effect on the dielectric constant for all samples, except for a slight decrease in the neat PP and a mild increase in most of PP/PP-HP blends. There is a slight but clear trend in the dielectric loss of PP/PP-HP blends, which decreases after thermal aging. This trend is further enhanced at 80 °C, reaching the same dielectric loss level of capacitor-grade PP (Borclean). As discussed, the extremely low dielectric loss in PP is a major advantage of using PP films in energy storage applications. Considering neat PP showing no detectable dielectric loss change with thermal aging, the reduction in the PP/PP-HP blend may be related to the cross-linking of the PP-HP copolymer, which reduces the segmental mobility of the HP side groups at higher temperatures. Overall, the dielectric results indicate that the addition of the PP-HP copolymer (antioxidant) in PP does not result in a significant increase in the dielectric losses. Moreover, it decreases further with time at an elevated temperature. Another property essential to the energy storage applications is electric breakdown strength. It is important to understand the effect of the PP-HP antioxidant on the breakdown strength of PP products. Since the PP-HP copolymer can offer a network structure, it is especially interesting to know the effect under elevated temperatures. The short-term DC breakdown strength of the materials was measured in Midel oil at 100 °C with a ramp rate of 0.5 kV/s, using compressed films with thickness (70−90 μm). Figure 25 shows Weibull plots of DC breakdown strength for neat PP (Borclean) and PP/PP-HP-1.0 (5:1) blend before and after aging at 130 °C for 1 week. A
lifetime for capacitor-grade 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-1.0 (5:1) blend is about 2 years. The PP/PP-HP-1.0 (5:1) blend, having typical HP antioxidant content in many commercial PP products, shows a significantly higher endurance time, which clearly presents the advantage of using PP-HP as the antioxidant additive in the PP matrix and the potential to extend PP product applications to higher-temperature areas.
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ELECTRIC PROPERTY With long endurance time under elevated temperature conditions, it is interesting to know the electric properties of PP/PP-HP blends, particularly the effect of PP-HP to the PP low dielectric loss and high breakdown strength. They are two key advantages of using PP polymer in many electric energy storage devices. Figure 23 compares dielectric loss vs frequency
Figure 23. Dielectric loss vs frequency for (●) PP Borclean and two PP/PP-HP-1.0 blends, (■) PP/PP-HP-1.0 (20:1 wt ratio), and (◆) PP/PP-HP-1.0 (50:1 wt ratio). The dielectric measurements were performed at 50 °C.
between a control capacitor-grade PP (Borclean) sample and two PP/PP-HP-1.0 blends containing a 2 and 5 wt % PP-HP-
Figure 24. Dielectric constant (top) and dielectric loss (bottom) of neat PP (Borclean) and PP/PP-HP blends before (filled circles) and after (unfilled circles) thermal aging at 130 °C in a forced air oven. The dielectric measurements were performed at 50 and 80 °C. O
DOI: 10.1021/acs.macromol.9b00855 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
under various operational conditions. The current practice of using antioxidant and stabilizer additives (polar molecules) in polyolefin (nonpolar and semicrystalline matrix) is insufficient, although they are effective during melt-processing (with molten state and mechanical mixing) but are ineffective during the application due to facile phase separation and diffusion (loss) on polymer surfaces. Our new approach, the direct incorporation of HP antioxidant groups to the polyolefin chain, offers an effective solution. Based on all experimental results from TEM, DSC, TGA, and several standard ASTM tests, the homogeneously distributed HP groups in the PE-HP or PP-HP matrix with a constant concentration can actively engage in an oxygen-induced oxidation/coupling reaction mechanism (Scheme 4). This not only protects polyolefin chains from degradation but also forms a complete cross-linked polymer network. In other words, instead of weakening mechanical strength in most commercial polyolefin products, PE-HP and PP-HP copolymers become stronger materials upon exposure to high-temperature oxidative conditions. Furthermore, the PE-HP and PP-HP copolymers can serve as polymeric antioxidants/compatibilizers in PE and PP products, respectively. Due to cocrystallization, the resulting PE/PE-HP and PP/PP-HP blends also exhibit a uniform morphology with a homogeneous distribution of HP antioxidant groups in the entire polyolefin material. The experimental results demonstrate highly effective antioxidative activities to stop (or slow down) polyolefin thermo-oxidative decomposition. Evidently, we have discovered an approach that can expand polyolefin applications to much higher-temperature ranges. Recently, we have also been investigating other high-energy conditions (high electric fields and UV radiation). Some experimental results have shown that biaxially oriented PP-HP dielectric films can offer a significantly higher breakdown strength than current biaxially oriented PP (BOPP) dielectric films, especially under high operational conditions, which is a very important concern in polymer film capacitors for energy storage. Overall, the experimental results may also provide a new approach and guidance in designing antioxidants, stabilizers, and other additives for all polymer systems.
Figure 25. Weibull plots for DC breakdown strength comparison between neat PP (Borclean) and PP/PP-HP blend (5:1 wt ratio) before and after aging at 130 °C for 1 week. The breakdown measurements were performed at 100 °C.
Weibull cumulative distribution function was fitted to the dielectric breakdown strength. The PP/PP-HP-1 (5:1) film (thickness: 85 μm) exhibits a similar or slightly higher average breakdown strength than the neat PP reference (thickness: 74 μm). Since the breakdown strength increases with the decrease of film thickness, the average breakdown strength of the blend sample was underestimated. More importantly, the breakdown strength of PP/PP-HP-1.0 (5:1) film shows a significant increase after heating at 130 °C in air for 1 week. The crosslinking network formed upon heating the PP/PP-HP film in air has a significant effect on the breakdown strength. It should be noted that the breakdown strength values are also strongly dependent on the quality of the polymer films.143,144 To investigate the breakdown characteristics further, it is ideal to prepare biaxially oriented films with a minimum of defects and homogeneous thickness (
