Interactive Behavior in Polymer Degradation - American Chemical

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Interactive Behavior in Polymer Degradation Downloaded via UNIV OF ROCHESTER on August 1, 2018 at 06:46:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Mathew C. Celina, Roger L . Clough, and Gary D. Jones Department of Organic Materials, Sandia National Laboratories, Albuquerque, NM 87185-1411

A novel dual stage chemiluminescence detection system has been applied to study remote interaction effects occurring during polymer degradation. There has long been speculation that infectious agents and reactions transferring initiators or antioxidants are important aspects in polymer aging. Evidence is presented that in an oxidizing environment a degrading polymer (i.e. PP) is capable of infecting a different polymer (i.e. polybutadiene) over a relatively large distance. Similarly, traces of thermally sensitive peroxides in the vicinity of PP are found to induce degradation remotely. These observations document cross-infectious phenomena. Likewise, inhibitive volatiles from stabilized elastomers were shown to retard a degradation process remotely. Such interactive phenomena are important to better understand polymer interactions, fundamental degradation processes and long-term aging effects of multiple materials in a single environment.

© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

38 Background and Experimental Approach There is some evidence that the aging of polymers may involve heterogeneous processes and that degradation can be initiated via impurities, catalysts or other chemical reagents (1-4). It has also long been recognized that the thermal degradation of polymers under oxidative conditions as described by the complex auto-oxidation scheme first established in the 1940-50's (5-7) involves the participation of peroxides and other oxidized species resulting from oxidation. Importantly, for the degradation of polypropylene (PP) samples composed of individual reactor particles, the induction time and thus degradation timing of the weakest particles was found to control the lifetime of the collective sample of this material (2). Such observations resulted in the suggestion of a heterogeneous model (1,2) allowing for localized reactions, active intermediates and propagation of degradation reactions throughout the material (3,4,8) with the notion that infectious volatile could carry the degradation from particle to particle via the gas phase (2). Similar infectious phenomena for materials in 'nature' have been described for the 'cross-talk' ripening of fruit via ethylene transfer (9-11), or the tin disease (tin pest) (12). A suitable experimental method to study such degradation phenomena in polymers is the sensitive technique of chemiluminescence (CL) (1,13), with the broad application of CL as applied to polyolefin degradation having recently been reviewed (14). A l l thermal oxidation reactions of hydrocarbon based polymers are accompanied by the emission of visible photons, with the intensity being an indicator of the activity of the degradation process (1,2,14). CL is an ideal tool to analyze in-situ polymer degradation reactions (1,2) as it provides information on the degradation process as a function of time and temperature. For this study a novel highly sensitive dual-stage CL instrument was developed and utilized to study the synergistic interaction of two different polymeric materials in a thermo-oxidative environment, as well as transfer processes of volatile initiators (peroxides) or inhibitors (antioxidants). As shown in the schematic setup in Figure 1, the CL instrument was designed to incorporate two individually temperature-controlled hot stages and a highly sensitive single photon counting photomultiplier tube (PMT) (15). A large diameter PMT is capable of collecting the simultaneous CL signal from both stages. The left and right hot stage are separated by approximately 25 mm. The initiating polymer, as well as peroxides or antioxidants, were placed on the left stage with a carrier gas (oxygen flow ranging from 25 to 250 cc/min) directing any gaseous volatiles, infectious or inhibitive agents towards the right sample stage. To allow for infectious phenomena between two different polymers to be studied, the experiments required a fast and actively degrading polymer, and a receiving one with a slower response. Polypropylene (PP) was chosen as the particularly reactive polymer and was used as an unstabilized reactor powder


39 material with typical samples of 10 mg. For the polymer interaction experiments unstabilized HTPB (cured hydroxyl-terminated polybutadiene) (16) was selected and used as thin films of approximately 5 mg cut from a 2 x 6 mm strip sample. Due to the fact that the CL emission from PP at 150°C is considerably higher and would swamp any signal originating from HTPB at lower temperatures, the PP sample holder was covered with Al-foil. This allowed only approximately 1% of the total signal to reach the PMT, which was still sufficiently intense to clearly identify the PP degradation peak and its relative timing features. For the benzoyl peroxide (BPO) or azoisobutyronitrile (AIBN) experiments, commercial materials (Aldrich 99%) were dissolved and diluted in toluene, with small quantities deposited on the PP or in samples pans using a standard micro liter syringe and subsequent solvent evaporation. affects

Figure 1. Schematic of instrumental setup with photon emission and detection from two hot stages allowing infectious and inhibitous phenomena to be explored.

Infectious Cross-talk between Polymers A range of experiments were conducted to explore if a degrading polymer (i.e. PP) is capable of initiating thermal degradation in a different polymer (HTPB). As an example of PP infecting HTPB, Figure 2 shows the signal of


40 two individual samples, the PP at 150°C displaying the individual peak degradation intensity at ~lh and the HTPB at 50°C showing a main peak at -120 hours. Included in Figure 2 is the predicted combined CL signal obtained by simply adding the PP and HTPB signals, which should be observed in an experiment of two individual samples if no interaction processes were present. Included in Figure 2 is also the actual CL trace resulting from an experiment where both samples are simultaneously placed on their respective hot stage. It is clear that the normally slower HTPB sample has degraded considerably faster in the presence of the initiating PP on the left sample stage, as the HTPB peak was observed after only -33 hours, or -28 % of its normal peak position. Additional experiments were conducted at different temperatures with similar results. It is also noteworthy to mention that in a similar experiment conducted with the PP degrading at the lower temperature of 130°C, the HTPB also showed a shorter degradation peak, demonstrating that infectious volatiles also originated from the lower temperature PP. The shift in the HTPB's responses are best presented in an Arrhenius diagram (Figure 3) of the HTPB t ^ data versus inverse temperature. The longer aging times correspond to the individual material's behavior, whereas the shorter times relate to the peak position time in the presence of the initiating PP (at 150°C) in combined experiments. The shift towards shorter degradation times is significant and apparent for all temperatures investigated. Even for an HTPB exposed at 40°C a peak time of -300 hours would be predicted based on linear Arrhenius extrapolation, however, in the presence of the initiating PP a significantly shortened peak is observed after only 100 hours.

Remote Action of Peroxides The above experiments clearly show that some organic volatiles must have infectious properties and act as pro-degradants that are carried across from the faster degrading polymer. A likely candidate for such infectious agents are peroxides or fragments thereof that are key intermediates in thermal polymer degradation. To test the hypothesis that even traces of peroxides would be sufficiently reactive and could act as remote initiators, a few simple overview experiments were conducted. Benzoyl peroxide (BPO) is an organic peroxide that degrades rapidly at temperatures above 100°C (i.e. 1 hour half life time at 92°C). Figure 4 shows the degradation of a PP sample at 100°C with a small amount of this peroxide directly deposited on the PP. Even traces are capable of accelerating the degradation. The effect could still be detected for 0.01 μg BPO on a 10 mg sample (equals lppm) (17). Larger amounts of lOOppm and above accelerate the degradation significantly. Similar results were also observed for the thermal degradation of PP at 110°C and 120°C. As expected the peroxide will decompose quickly producingfreeradicals and initiating oxidation of the PP.


41

Material A PP 150°C

A_PP 150°C individual B_HTPB 60°C individual Predicted A + Β individual Combined experiment

Material Β HTPB 60°C

200 3

Time [10 s] Figure 2. Individual and combined degradation of PP (150°C) and HTPB (50°C) samples monitored by chemiluminescence.

Figure 3. Arrhenius plot of the t^ times for HTPB at different temperatures showing the faster degradation when infected by the degrading PP at 150°C.


42 As a follow-up experiment, small amounts of the peroxide were placed on the left sample stage and any remote influence on the PP sample located on the right stage (i.e. 10 μg) was evaluated. An example is included in Figure 4 for the degradation of a PP sample at 100°C. It is clear that the PP sample was affected by the presence of the decomposing BPO via volatile fragments, since the degradation is considerably faster than without the BPO. The small amount of peroxide can clearly infect the PP sample on the right stage located 25 mm away. Similar experiments were also conducted with AIBN, a non peroxide-based radical initiator with similar thermal decomposition features. AIBN was also found to be capable of leading to remote initiation of PP. These experiments clearly demonstrate that small amounts of initiators can act as infectious agents and can be carried successfully via the gas phase. Whether the infectious species in this case are un-decomposed initiator, its degradation products or a combination thereof is presently unclear. As discussed above, it is well known that PP degradation involves the evolution of gaseous species that can be infectious and similar evidence was demonstrated for an EPDM material (2,18). The potential for peroxidic species to be involved and the very small amounts that can lead to infectious behavior, even when physically separated from the sample, is demonstrated here.

Remote Action of Antioxidants Of further interest was to identify if a common phenolic antioxidant would be volatile enough to affect the degradation of unstabilized PP in a similar remote fashion as observed for the remote activity of radical initiators. A small amount (50 μg) of Vanox MBPC (2,2'-methylene-bis 4-methyl-6-f-butylphenol), an antioxidant with a molecular weight of 340.5 g/mol and melting point of 125°C, was deposited via a suitable toluene solution onto a DSC sample pan. Figure 5 shows the CL monitored degradation of an individual PP sample at 110°C (10 mg) and the corresponding CL signal when the PP sample is placed on the right stage in the presence of 50 μg AO on the left stage (also at 110°C). The PP degradation process is clearly retarded by traces of the AO or possibly any of its thermal decomposition products that would have been transferred onto the PP sample. This is intriguing since the AO due to its high melting point and molecular weight would not be regarded as a particularly volatile substance. Furthermore, the PP consists of powder particles with large surface areas, which would make it difficult for traces of antioxidant to uniformly inhibit any degradation process. However, even traces (i.e. 1 μg) of AO deposited directly on the PP were found to result in a low-level stabilization effect (i.e. 0.01%). This would certainly support the observation of remote stabilization. Similar transfer of antioxidative species leads to successful inhibition of the PP sample also at higher temperature, as well as for BHT, a slightly more volatile antioxidant.


43

3

Time [10 s] Figure 4. PP (Wmg) degradation at 100°C and initiation by small amounts of benzoyl peroxide deposited either on the sample or independently on the left stage.

It was further explored if a stabilized elastomer sample with dissolved AO in the matrix could give off antioxidant in sufficient quantities to measurably inhibit the degradation of the PP. A 5 mg sample of stabilized HTPB with 1% Vanox MBPC was placed on the left stage and a 2.5 mg PP sample on the right stage. For such experiments no significant effect on the PP was observed, which suggests that insufficient quantities of the AO were transferred across or are too diluted by the time they reach the PP. For example, a 5mg sample of HTPB contains -50 μg of antioxidant, but the available quantities for desorption (surface effects) and transfer will be considerably lower. The antioxidant will be mostly retained in the polymer matrix and its limited volatilization will lead to relatively low quantities in the gas phase, which might be very diluted at the right sample location in the current experimental setup. It will also be an issue of volatility versus affinity (adherence) to either the elastomer or the PP. To explore if cross-inhibitive effects could nevertheless occur between different polymers, the two polymer samples were positioned closer together but not touching, thereby increasing the local volatile concentrations by placing them in one sample pan (-7 mm diameter). Figure 6 shows the results for such a combined experiment of PP and HTPB at 140°C. It is obvious that the degradation of the PP material in this experiment is now considerably inhibited due to sufficient antioxidative species being transferred from the HTPB. The W of the PP is shifted from about 2.1 h to 7.9 h. No infectious effect of the PP


44 on the HTPB is apparent (identical peak position of the HTPB). A similar experiment was repeated using stabilized PBAN containing an amine antioxidant. The inhibition effect in the PP is similar as observed for the HTPB experiment demonstrating again that sufficient quantities of AO causing inhibition were transferred (19). Such experiments demonstrate that antioxidants or their decomposition products with antioxidative properties can also be successfully transferred from a stabilized polymer and result in remote inhibitive effects in a neighboring polymer. The magnitude of this effect will depend on relative volatility, expected to be primarily controlled by its molecular weight, and concentration in the gas phase, as well as AO solubility and affinity to the polymer. It has long been speculated that such inhibitive remote interaction may be a potential problem in closed environments (closed aging ovens) where polymers age jointly. Larger amounts of stabilized polymers are expected to give off large enough quantities of antioxidants that could easily enhance the intrinsic stability of less stable polymers in their proximity.

Conclusions A novel chemiluminescence technique has been introduced and applied to study potential interaction processes of polymer materials when they degrade. It was demonstrated that in an oxidizing environment a degrading polymer A (i.e. PP) is capable of infecting a different polymer Β (i.e. polybutadiene, HTPB) over a relatively large distance. This suggests that materials degradation of completely different polymers can involve infectious processes, as discussed previously for the degradation of individual polymer samples. In the presence of the degrading material A, the thermal degradation of polymer Β is observed over a significantly shorter time period. Infectious intermediate volatiles from material A are able to initiate and shorten the degradation processes in material B. Considering that the initiating polymer is a sample of only lOmg, that the materials are separated by -25 mm, and that a significant carrier gas flow is applied, all imply that an extremely reactive infectious volatile must be transmitted and that the receiving polymer must be susceptible to traces of such species. This observation is perhaps not unexpected, but has not been experimentally documented before. Understanding such infectious behavior is important for predicting polymer materials interactions, material degradation processes and long-term aging effects in combined material exposures. Long term efforts will require the identification of infectious agents. Perhaps even solidfragments(nanoparticles) of the degrading polymer carrying peroxides, a reactive macromolecular species, could act as an initiator. As expected, low quantities of a temperature sensitive peroxide in direct contact with PP were shown to be an extremely efficient initiator.


45

Figure 5. CL monitored degradation of PP (110°C) and inhibited by volatile antioxidant transferredfrom the left sample stage (50fig Vanox MBPC).

Figure 6. Simulated degradation of individual 4.6mg HTPB and 2.5mg PP samples, a combined experiment (left and right sample stage) and a single sample pan experiment showing the retarded peak of PP when in the vicinity of HTPB (all experiments conducted at 140°C).


46 Similarly, the activity of only trace amounts (micrograms) of peroxide in the vicinity of a receiving PP sample was sufficient to remotely initiate the degradation process. For inhibitive interaction process, traces of common antioxidants or their decomposition products are sufficiently volatile to be carried between polymer samples and can lead to remote inhibition. There are some limits of this effect when the amounts of transferred AO from a stabilized elastomer are too low to result in significant retardation of the degradation in the receiving polymer. This is in contrast with the very low amounts of peroxidic species (micrograms) found capable of inducing remote initiation (17). However, when the separation is reduced between the polymers, remote inhibition indicative of cross-talk between the materials could be observed. This demonstrates that some antioxidants (or their degradation products) are sufficiently volatile to be transferred, but that critical concentrations are required to inhibit degradation in the receiving polymer. In summary, these experiments have demonstrated that polymers can interact remotely, that small amounts of peroxides can lead to remote initiation, and also that remote inhibitive behavior via transfer of antioxidant between materials that age jointly is a distinct possibility. Variations in their individual stability due to such remote interactions may be observed in closed environments where polymers age jointly.

Acknowledgements Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC0494AL85000. The technical assistance of Ana B. Trujillo is gratefully acknowledged.

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Interactive Behavior in Polymer Degradation - American Chemical

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