Thermoset materials characterization by thermal desorption or pyrolysis based gas chromatography-mass spectrometry methods

Journal Pre-proof Thermoset materials characterization by thermal desorption or pyrolysis based gas chromatography-mass spectrometry methods C. Garrett Campbell, Dominik Jordon Astorga, Mathew Celina PII:

S0141-3910(19)30360-X

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.109032

Reference:

PDST 109032

To appear in:

Polymer Degradation and Stability

Received Date: 12 September 2019 Revised Date:

8 November 2019

Accepted Date: 18 November 2019

Please cite this article as: Campbell CG, Astorga DJ, Celina M, Thermoset materials characterization by thermal desorption or pyrolysis based gas chromatography-mass spectrometry methods, Polymer Degradation and Stability (2019), doi: https://doi.org/10.1016/j.polymdegradstab.2019.109032. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Probing Thermosets via Fragmentation Products Volatile Pyrolysis Products Analyzed by TED-GC-MS are Representative of Resin and Curatives

Pyrolysis (TGA, T >300°C)

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22

44

Elution Time (min) Elution Time (min)

66

NETWORK STRUCTURE AND AGING STATE IDENTIFICATION

Thermoset Materials Characterization by Thermal Desorption or Pyrolysis Based Gas Chromatography-Mass Spectrometry Methods C. Garrett Campbell, Dominik Jordon Astorga, Mathew Celina Sandia National Laboratories, PO Box 5800, MS 1411, Department 1853, Albuquerque, NM 87185-1411

Abstract: Thermoset materials characterization is often limited to solid state analytical techniques such as IR, NMR, DSC, TGA and mechanical testing. Alternatively, their off-gassing behavior can also be evaluated using GC based techniques such as TD-GC-MS, allowing this method to be applied to thermoset materials analyses such as identification, aging characterization, and formulation optimization. As an overview, common thermoset materials were evaluated by analyzing their gaseous degradation products via TGA-based pyrolysis and subsequent TD-GCMS for the identification of representative volatile signatures. It is thereby possible to distinguish different classes of phenolic materials or cured epoxy resins, as well as their amine or anhydride curatives. Additionally, this method enabled quantification of a volatile fragment (bisphenol A, BPA) which is associated with oxidation of epoxy/amine thermoset materials. The amount of evolved BPA increased linearly with aging time and this trend exhibits linear Arrhenius behavior over the temperature range (80-125°C) studied, in agreement with oxidation sensitivies based on oxygen consumption data. Further, TD-GC-MS was used to explore how off-gassing of residual anhydride curative from an epoxy/anhydride material depends on formulation stoichiometry. Even in formulations that theoretically contain enough epoxy to consume all anhydride (1:1 stoichiometry), residual anhydride remained due to imperfect final cure state and could evolve from the material. For such materials, a slightly epoxy-rich formulation is required to ensure that the material contains no residual unreacted anhydride. Analysis of volatiles generated by thermal exposure is an attractive characterization approach enabling compositional analysis as well as complementary diagnostics for materials degradation. Key words: Polymer Analysis/Characterization; Thermal Desorption Mass Spectrometry; Thermoset Composition; Volatiles from Thermosets; Degradation Signatures Corresponding author: Mathew Celina, Sandia National Laboratories PO Box 5800, MS 1411, Organic Materials Science Dept. 1853 Albuquerque, NM 87185 Ph: 505-8453551, fax: 505-8449781, Email: [email protected]

1

1.

Introduction

Gas chromatography-mass spectrometry (GC-MS) is a powerful analytical tool for the separation, quantification, and identification of volatile compounds. Traditional GC-MS, however, is limited to substances which can be easily vaporized, hence insoluble solid materials and polymers cannot be analyzed by these methods. Various sample preparation/collection techniques have therefore been developed to analyze off-gassing behavior of such materials including solid phase micro-extraction (SPME) [1], pyrolysis (PY) [2], and thermal desorption (TD) [3]. Pyrolysis is typically the only method in which the sample can be loaded into a thermal exposure apparatus connected directly to the GC-MS. The sample is pyrolyzed at high temperatures and degradation products are cryo-focused prior to injection onto the column. In contrast, SPME is typically used as a headspace or solution analysis technique [4]. A solid sample is heated in a sealed container, and the volatiles produced are captured on a sorbent which is then placed into the GC inlet where those volatiles are desorbed directly onto the GC column. Thermal desorption can be regarded as a compromise between these two techniques. Similarly to SPME, typical TD sample preparation involves the collection of volatiles on a sorbent, though the techniques for this collection are much more varied (stir bar sorption [5], headspace extraction [6,7], or thermogravimetric sorption [8-10]). This volatile-containing sorbent is then placed in a desorption chamber that is heated to evolve the trapped volatiles which are then cryo-trapped prior to injection in the same manner as described above for pyrolysis. In addition to this traditional TD sample preparation solid samples can be directly loaded into the desorption chamber allowing for direct desorption of trapped volatiles or gentle pyrolysis (~200-300°C) depending on the need for additional decomposition [11]. It should be noted here that the difference between PY-GC-MS and direct desorption TD-GC-MS is the absence of degradation in the latter. The instrumentation for PY and TD is similar except that different temperature ranges are typically analyzed (<400°C for TD and >400°C for PY). In fact, the PY instrument may be used for TD or vice versa[11,12]. In this work, TD instrumentation was chosen due to the enhanced temperature control in the range of 25-230°C. Typically, TD-GC-MS is used for the analysis of pollutants/undesired chemicals in various non-solid media [5,13,14] and analysis of biological samples [15-17], although some recent reports on this technique have also focused on its use in quantifying additives and impurities in polymeric materials [18-20]. However, little attention has been given to the application of this technique for thermoset materials characterization. In this report, the application of TD-GC-MS will be expanded toward thermoset materials identification, aging behavior characterization, and formulation optimization, with additional background information for these targets provided below.

I.

Materials Identification through Analysis of Pyrolysis Products

As early as 1975, PY-GC-MS analysis of pyrolysis products was recognized as a viable means of identifying polymeric materials [21]. More recently, Fries et al. have reported using this technique to identify microplastics in marine environments [11]. They were able to identify plasticizers, flavoring agents, antioxidants, and the nature of microplastic particles using sample masses below 350 µg. Similarly, Lever et al. have reported TD-GC-MS coupled with TGA pyrolysis as a means to identify thermoplastic materials and study their degradation at high 2

temperatures [9]. Additionally, they recognized the utility of this technique toward analyzing materials which exhibit multi-stage decomposition reactions. Later, Tsytsik et al. applied this technique toward studying the de novo synthesis of dioxins from fly-ash and were able to detect dioxin concentrations in the 10-1000 ppb range [10]. More recently, Duemichen et al. have automated this combinatorial technique to enable rapid identification and quantification of micro-thermoplastic materials for environmental monitoring purposes [8,22-24]. This coupled TGA and TD-GC-MS technique, now known as thermal extraction desorption (TED)-GC-MS, was then used to identify and quantify dissimilar thermoplastics such as polyethylene, polyesters, and polyamides found in soil samples [8,22,23]. Later, this technique was also used to differentiate PE from PP and determine the composition of a corresponding blend [24]. The TED-GC-MS technique is similar to PY-GC-MS except that pyrolysis is performed on a separate instrument where degradation products are captured on a sorbent. One of the advantages here is that if polymeric species are produced during high temperature pyrolysis, these will likely contaminate the TGA instrument which can easily be oxidatively cleaned. Thus, high molecular weight products which may condense or form char in the TD or GC are never introduced into that instrument. As an example of this technique being used to study thermosetting resins, Sullivan et al. utilized TD-GC-MS to provide insight into the degradation of cured and uncured epoxy/phenolic resins at elevated temperatures (180-220°C) [25]. In all of these cases, as well as others [26-28], the general strategy for identifying polymeric materials using GC-MS centers on pyrolysis of the material and characterization of the resulting volatile degradation products. However, this technique has not been explored fully with regard to the identification of thermosetting materials with similar structures, something which has now been accomplished in this study.

II.

Characterization of Chemical Changes in Aged Materials

Although TD-GC-MS has been used for the identification of polymeric materials, both thermoset and thermoplastic, these and related techniques (PY-GC-MS) have only been applied in a few cases to study how materials age. Yang et al. applied PY-GC-MS methods toward simply identifying the oxidative aging signatures in polyolefins (high Mw hydrocarbon fragments) [29], as well as mostly tracking the decrease of antioxidants as a function of material age in nitrile rubber materials [30]. Similarly, La Nasa et al. have examined PY-GC-MS for qualitative detection of oxidative aging in polyurethane foam materials. Material taken from the surface of a polyurethane foam object produced more di-isocyanate monomer when pyrolyzed (306°C) than a sample taken from the bulk of the material [31]. While this data was used to show that the bulk material experiences less oxidation than the surface over long aging times (~49 years) at atmospheric conditions, no quantitative assessment of this aging behavior was reported. Additionally, Gröning et al. have used headspace SPME to identify degradation products in thermo-oxidatively aged nylon 6,6 and discovered a correlation between the appearance of certain off-gassing species and modulus changes in that material [32]. Of course, simple head space gas analysis can also provide complementary insight into mechanistic degradation pathways [33,34]. Other techniques such as IR spectroscopy, oxygen consumption, and mechanical testing have previously been used to determine the oxidative sensitivity of materials, and can offer guidance 3

for material lifetime in use environments [35]. To provide insight into the usefulness of TD-GCMS in the field of polymer aging and degradation, a comparison with other approaches is required. For materials aged in a sealed oxidative atmosphere, the oxidation rate of a material can be quantified based on the decrease in oxygen partial pressure [36-38]. Integrating this rate over time yields the oxidation level of that material (equal to the amount of oxygen that has reacted) as a function of aging time. Usually correlations can then be drawn between this oxidation state and other characteristics of the material. For instance, ATR-IR can be used to quantify the build-up of carbonyl groups or depletion of oxidatively sensitive functional groups (e.g. carbon-carbon double bonds) in materials aged in oxidative environments. These measurements can serve as an accurate indicator for the oxidation state of the material once correlations have been established. Further, increased oxidation levels in thermosets are usually accompanied by changes in mechanical properties, though this correlation varies depending on the polymer type and aging conditions [39-41]. Typically, such correlations are obtained when changes in the physical properties, such as modulus or fracture toughness, occur at higher oxidation levels as is the case for epoxies and other thermosets [39-41]. Additionally, the chemistry by which this oxidation occurs may not always be thermo-oxidation. As an example, Larche et al. have reported that for an acrylic-urethane thermoset material, photo-oxidation is more likely than thermo-oxidation to cause modulus increases during natural aging of the material [39,40]. Such different aging processes may also result in variable scission or crosslinking behaviors affecting volatile formation and therefore potential analysis by thermal desorption methods as discussed later. Given that the following three factors (oxidation state, analytical oxidation signature, and mechanical property of interest) can be correlated for a given material, then data extracted from accelerated aging studies can be extrapolated and time-to-failure can be predicted at a lower temperature [42,43]. The major assumption here is that the high temperature oxidation mechanism does not differ at lower temperatures. If true, then the prediction made by extrapolation of high temperature data will be meaningful. Conversely if this assumption does not hold the high temperature data has little value at operating temperatures. For this reason it is important that the material be monitored over its use, a suitable method may be ATR-IR, to determine the actual oxidation state of the material so that any discrepancies between predictions and observations can be identified. There are instances in which ATR-IR would be limited for the characterization of chemical changes, most notably when aging of materials under inert atmosphere is of interest. However, there are other IR-based techniques that can be applied to pyrolytic aging. As an example, Celina et al. have reported a transmission IR based method for quantifying off-gassing of an epoxy material thermally exposed at 240°C [44]. The benefit of this method is that it allows for the study of off-gassing during pyrolytic aging in a sealed environment while also providing data that could allow the prediction of this behavior at lower temperatures if systematic data for multiple time-temperature exposure conditions are obtained. However, since this volatile analysis can only be used to analyze the headspace in which that material resides, it does not provide any information on the aging state of the remaining material. This illustrates an ongoing need for the development of analytical techniques that can document subtle chemical changes in 4

aged thermosets by direct analysis of the solid material. Such changes could be weak crosslinking or scission that may not be easily observed with IR or NMR based methods. In fact, reactions associated with scission (fragmentation) might be easily probed with thermal desorption approaches. Currently we will address how TD-GC-MS can be applied toward detecting signatures of oxidative aging believed to be associated with scission reactions. Evidence for aging processes is obtained by using TD-GC-MS to quantify trapped medium molecular weight volatiles desorbed from a material. Additionally, though not covered in the current work, we believe these analytical approaches could also assist in the characterization of inert atmosphere pyrolytic aging processes, or similarly for hydrolytically sensitive materials. We should note that theoretically, similar analysis could also be done using TGA, but despite the large number of existing studies where ramped TGA experiments are used to examine degradation at high temperatures [45-48], we found no literature where TGA was used to study isothermal pyrolytic sensitivity or quantify unbound molecules as a function of aging time/state.

III.

Optimization of Thermosets Containing Volatile Curatives

Although TD-GC-MS has been applied to the analysis of thermoset materials and their uncured constituents, we have found no reports of it being used to aid in optimization of thermoset material formulations. This is likely due to most thermoset formulations being based on inherently non-volatile constituents. However, anhydride cured epoxies are an interesting exception. Despite that these materials often exhibit high Tg when fully cured, anhydrides are more volatile than most amine curatives, especially aromatic amines which impart a similarly high Tg. Further, because of the chain-growth mechanism by which these formulations cure, materials containing a slight excess of anhydrides will contain unbound curative, whereas their analogous amine cured materials may not, though this has not been reported in the literature. We will discuss how TD-GC-MS can be used to optimize these formulations and minimize initial off-gassing when volatile curatives are used.

2.

Experimental I.

Materials

Araldite® DY-D; 4,4’-diaminodiphenyl sulfone (DDS); Jeffamines® D230, D400, and D2000; Aradur® 9690; Aradur® 917; and Aradur® HY 906 were obtained from Huntsman Corporation. Epon® Su-8, Epon® 828, Eponex® 1510, Epon® 1031, and Epon® 161 were obtained from Hexion Inc. Ancamine® 2049 was obtained from Evonik Industries. Hardener HY 5200 was obtained from Ciba Specialty Chemicals. All chemicals were used as received. Chemical structures for these compounds are given in Fig. 1 and Fig. 2. Amine and novolac cured epoxy materials were formulated with a 1:1 stoichiometry. Phenolic material Plenco® 11956 was obtained from Plenco Plastics Engineering Company and cured using the following temperature schedule: 68°C for 16.7 h, 121°C for 4.7h, 163°C for 2h. Anhydride cured epoxies were formulated with an epoxy:anhydride stoichiometry of 1:0.9 and catalyzed by the addition of 0.5 wt% 1-methylimidazole (1MI,Sigma-Aldrich, 99%) unless otherwise stated, and cured at 90°C for 3 hours followed by 150°C for 1 hour. 5

Epon 828

Eponex 1510 Epon SU-8 Epon 1031 Araldite DY-D Epon 161 Fig. 1. Chemical structures of epoxy resins used in this study.

~90%

~10%

Aradur HY906

HY5200

Aradur 917

Aradur 9690

Jeffamine D230; n≈2.5 D400; n≈6.1 D2000; n≈33

4,4’-diaminodiphenylsulphone (DDS)

Ancamine 2049 (A2049)

Fig. 2. Chemical structures of epoxy curatives used in this study.

II.

TGA Pyrolysis/Thermal Desorption-Gas Chromatography-Mass Spectrometry (TED-GC-MS) for Identification of Thermoset Materials

Materials identification via GC-MS was accomplished in this work by first pyrolyzing the thermoset materials in a Netzch TG 209 F3 Tarsus TGA instrument (Fig. 3) by heating from room temperature to 550°C at a rate of 50°C/min and then maintaining the sample at this temperature for 5 minutes. This was done under 25 mL/min nitrogen purge which was subsequently passed over a silicone sorbent (SorbStar® 2mm x 2.5cm cylinder) allowing the volatiles evolved from the sample to be adsorbed onto its surface. Then, the loaded silicone sorbent was transferred to a thermal desorption tube (Gerstel, I.D.=4mm, O.D.=6mm, 6

Length=17.8 cm (7in)) and loaded into a pre-conditioned thermal desorption chamber. The TDGC-MS instrument consisted of a Gerstel TDS3 thermal desorption system affixed via a cooled injection system (CIS) to the inlet of a Bruker Scion GC-MS 436-GC SQ as illustrated in Fig. 4, with the instrumental parameters given in Table 1. For identification of volatile compounds, mass spectra were compared to those found in the 2005 NIST mass spectrometry database. Gas Flow Sorbent Evolved Volatiles

Sample Heated Microbalance

N2

Fig. 3. Illustration of TGA setup allowing for collection of degradation products on a sorbent cartridge.

Table 1. TD-GC-MS parameters used for identification of materials via TED-GC-MS.

Parameter TD initial temperature TD He flow rate TD heating rate TD desorption temperature, desorption time TD split TD transfer line temperature CIS initial temperature CIS ramp rate CIS final temperature, hold time CIS split GC initial temperature GC He flow rate GC temperature ramp GC final temperature, hold time GC column

Set value 40°C 34.5 mL/min 180°C/min 300°C, 5 min Splitless 350°C -50 12°C/s 300°C, 3 min 10:1 50°C 1.5 mL/min 20°C/min 280°C, 5 min Agilent 19091J-413 HP-5 30m x .320mm x .25mm

7

Fig. 4. Diagram of TD-GC-MS setup.

III.

Cleaning Procedure Between Samples

9

Total Ion Count (TIC) (10 Cts)

The following cleaning procedure was implemented on the TGA and executed between samples: Under an oxygen purge of 30 mL/min, the TGA chamber was heated from room temperature to 900°C at a rate of 50°C/min and held at this temperature for 15 minutes to oxidatively clean the collection environment. The custom-made lid was thoroughly cleaned with an isopropanol-soaked tissue to remove any residue that may have built up during the previous experiment. After this cleaning procedure, a background was collected to determine its effectiveness. Fig. 5 shows the TD-GC-MS spectrum taken from the sorbent used to collect the residual volatiles present after this oxidative cleaning. In this chromatogram, traces of compounds from the previous sample (DGEBA based thermoset) are noticeable, however this signal is low. Further, we recognize that this TD-GC-MS method can cause a small amount of degradation of the sorbent, resulting in the evolution of low molecular weight silicone species which may complicate analysis of silicone containing materials. The typical thermal exposure is only 5 minutes at 300°C, yet we still discarded the sorbent after each use.

2 1 0 0

2

4

6

8

10

12

Retention Time (min)

Fig. 5. Background total ion chromatogram (TIC) taken after an oxidative cleaning of the TGA. The high temperature used in the TDS causes some degradation of the sorbent resulting in the release of various dimethylsiloxane oligomers. This spectrum also serves to show the effectiveness of oxidative cleaning of the TGA coupled with solvent based cleaning of the TGA lid. The residual peaks from previous samples (phenolic compounds) are less intense than the peaks from native degradation of the silicone sorbent.

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IV.

TD-GC-MS Characterization of an Oxidatively Aged Epoxy Material

TD-GC-MS may be a potential tool for quantitatively determining the aged state of materials through identification of specific signature molecules that are associated with the aging process, e.g. a scission product, oxidized or similarly functional or structural fragment that is not native to the original material. We therefore examined the accelerated aging of a generic epoxy encapsulation material based on Epon 815 (bisphenol A, BPA based epoxy) cured nearly stoichiometrically with a mixture of Jeffamine D230 and ATBN (amino terminated polybutadiene) that contained approximately 10% ATBN and 17% GMB (glass micro balloons, H2O/1000 from 3MTM) in its formulation. When identifying samples using TED-GC-MS, volatiles from pyrolyzed samples are not quantitatively trapped. Thus, the amount of volatiles injected onto the GC is also not quantitative. To quantitatively determine the material’s oxidation state, we desorbed volatiles directly from aged samples placed in the desorption chamber. Although this allowed us to quantify volatiles which evolved from these samples, much care was taken to ensure that the amount of evolved volatile material was sufficiently low to prevent build-up in the cooled injection system. Therefore, we used TGA to identify a suitable desorption temperature (230°C) which would result in no more than 5% weight loss over 20 minutes for the sample (aged 594 days at 80°C) which we expected contained the highest amount of easily volatilized material. Overload of the GC column was further prevented by ensuring that sample amounts were limited to 1-5 mg. All TD-GC-MS parameters used for analysis of aged epoxy samples are given in Table 2. Table 2. TD-GC-MS parameters used for analysis of oxidation state of materials via TD-GC-MS.

Parameter TD initial temperature TD He flow rate TD heating rate TD desorption temperature, desorption time TD split TD transfer line temperature CIS initial temperature CIS ramp rate CIS final temperature, hold time CIS split GC initial temperature GC He flow rate GC temperature ramp GC final temperature, hold time

Set value 40°C 34.5 mL/min 180°C/min 230°C, 5 min Splitless 250°C -50 12°C/s 300°C, 3 min Splitless 50°C 1.5 mL/min 20°C/min 280°C, 5 min 9

GC column

V.

Agilent 19091J-413 HP-5 30m x .320mm x .25mm

TD-GC-MS Calibration for BPA

Integrated BPA Signal (1010 Cts)

The GC-MS was calibrated for BPA using two BPA solutions (0.01 and 0.001 wt% in THF). Fig. 6 below shows the calibration curve derived by analyzing between 1 and 10 µL of each of these solutions injected into a thermal desorption tube. For this calibration, the cooled injection system (CIS) initial temperature was set at -150°C, and no split was used for the CIS (splitless injection). All other parameters were identical to those used in Table 2. The minimum detection limit of this method was 50 ng (which also means easily ppm level sensitivity for any aged materials) and, as shown by the linear trend in Fig. 6, the BPA signal intensity is directly proportional to the mass of BPA evolved. 7

6

5

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3

0.001 wt% BPA in THF

0.01 wt% BPA in THF

2

1

0 0.0

0.2

0.4

0.6

0.8

1.0

Mass BPA (µ µ g)

Fig. 6. GC-MS calibration curve for BPA. Two BPA solutions were used (green:0.01 wt% and blue:0.001 wt%) allowing calibration over two orders of magnitude of BPA mass. The linear correlation which passes through the origin allows for quantification of BPA by this method.

3.

Identification of Thermoset Materials

Of key interest is the development of a rapid GC-MS based method for the identification of thermoset materials that could be generally applied to all polymeric materials regardless of their thermal stability. As such, TGA was used only as a rapid controlled pyrolysis to simply generate volatiles that could then be identified. We recognize that a more detailed analysis of materials could be achieved using a slower temperature ramp and analyzing the degradation products from each degradation regime should a material exhibit multiple pyrolysis steps. However, the TGA heating rate (50°C/min up to 550°C) was too rapid to provide any insight into multiple degradation steps that may be present in these materials. This TED-GC-MS procedure can be summed up as:

10

1.) Complete pyrolysis of the material under inert atmosphere 2.) Capturing of the pyrolysis products on a sorbent 3.) Analysis of the adsorbed products by TD-GC-MS Fig. 7 shows the TED-GC-MS chromatograms of multiple thermoset materials examined in this manner. Some minor variation in retention time for many of the degradation products was observed, presumably due to a small effect of the absolute loading level. There are some overarching conclusions from this overview with each sub-group of materials discussed in detail later. The degradation products identified for each material can generally be categorized as being characteristic of one component of the formulation. For instance, when Epon 828 is used as the epoxy resin, the presence of phenol, cresol, 4-isopropylphenol, 4-isopropenylphenol, etc. can be detected regardless of what curative is used. Likewise, amine curatives tend to produce fragments which are characteristic of the specific amine regardless of epoxy resin used. Table 3 lists these characteristic degradation species for each resin and curative used in this study.

11

Dy-d/917

1510/5200

Fragments

828_1510/DDS 828/5200

828/A2049

Derivatives

828/906 828_1031/DDS

828/DDS

828/D400_2000

828/D230

SU8/D230 Plenco 11956 Resol 161/9690

0

2

4

6

8

10

Retention Time (min) Fig. 7. Chromatograms of all cured thermoset samples examined in this work. Resins and curatives can be identified based on their specific degradation products.

12

Table 3. Key signature degradation products for curatives and resins studied in this work. Epon 828

Eponex 1510

Epon SU-8

Epon 161

Epon 1031

DY-D

Ancamine 2049 Derivatives

4,4’-DDS

Aradur 5200

Jeffamines Fragments

Aradur 917

Aradur HY906

Unfortunately, some degradation products co-elute with others during GC analysis resulting in a peak with a convoluted mass spectrum which does not match any compound in the reference library. Fig. 8A provides an illustration of a spectrum for a peak where two compounds elute at the same time. In this instance, the convoluted spectrum displayed three peaks which were indicative of trimethylphenol (Fig. 8B), a compound which elutes at the same time as the convoluted peak. By subtracting its spectrum from the convoluted data, we were able to deconvolute the original mass spectrum and match the residual spectrum (Fig. 8C) to dimethylbenzofuran (Fig. 8D).

13

100%

A.) Mass Spectrum with no library match

100%

-

80%

60%

Main peaks for trimethylphenol

40%

20%

B.)

80%

60%

40%

20%

0%

0%

20

40

60

80

100

120

140

160

20

40

60

80

M/z m/z

100%

100

120

140

160

M/z m/z

C.) Residual MS after subtraction of B matches D.

100%

D.)

80%

80%

≈

60%

40%

60%

40%

20%

20%

0%

0% 20

40

60

80

100

120

140

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160

40

60

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100

120

140

160

M/z m/z

M/z m/z

Fig. 8. Illustration of spectral subtraction used for GC peaks with no library match. This mass spectrum is taken from the peak at ~5 minutes for an epoxy phenolic material (161/9690). The initial spectrum (A) does not match any compounds in the library, but exhibits three peaks which are characteristic of trimethylphenol, a compound which, based on previous observations, should also elute near the peak where this spectrum was taken. The spectrum of trimethylphenol (B) is subtracted from the raw spectrum to yield spectrum C. This spectrum closely matches 4,7-dimethylbenzofuran (D), therefore the complex MS spectrum is identified as a combination of trimethylphenol and 4,7-dimethylbenzofuran.

I.

Thermosets Containing Novolacs/Resols

Fig. 9 shows the TED-GC-MS spectra of three materials containing a resin and/or curative formed via the reaction of a phenolic compound (BPA, Phenol, or Cresol) with formaldehyde. While the 161/9690 and SU-8/DDS formulations are cured via addition of a curative to epoxides, Plenco 11956 is a thermoset formed by the base catalyzed reaction of phenol and formaldehyde (resol). Comparing the chromatograms of the Plenco 11956 and 161/9690 formulations highlights the value of this technique for the identification of thermoset materials. Regarding the network structures of these materials, both contain phenol-formaldehyde condensation products, but the 161/9690 formulation also contains epoxy linkages. Thus, it is no surprise that they share many of the same degradation products (phenol, cresol, and dimethylphenol). However, the degradation products for 161/9690 include increased amounts of trimethylphenol which is indicative of the cresol novolac curative as well as trimethylbenzimidazole, as a degradation product of the alkaline catalyst used for this formulation. Additionally, while the Plenco 11956 chromatogram contains some polyphenyl compounds (anthracene etc.), these are absent for the 161/9690 formulation. 14

The pyrolysis of an SU-8/D230 (amine cured epoxy) material released degradation products typically only seen in Epon 828 based materials (isopropyl and isopropenylphenol), along with the polyphenyl compounds indicative of high molecular weight phenolic materials (anthracene etc.). This is expected as the epoxy resin Epon SU-8 is formed from the epoxidation of a BPA based novolac with Mn ~1500 g/mol. Additionally, similar to the 161/9690 formulation, we detect fragments indicative of the curative used in this formulation, an amine functional polypropylene oxide, in the form of polypropylene oxide fragments. The detection of curative signatures normally associated with epoxy materials is seen as the best way to distinguish phenolic based epoxies from purely phenolic materials (formaldehyde or similarly cured) via TD-GC-MS. However, phenolic epoxies are also known to cure via epoxy homopolymerization [49]. For such materials, TD-GC-MS may not be adequate to distinguish them from pure phenolics since no distinct curative is present, and only a small amount of a catalyst is used. However, using ATR-IR, it may be possible to distinguish these based on aromatic substitution and ether signatures. Therefore, both techniques could be complementary to determine the network chemistry (ATR-IR) as well as the material’s constituent monomers (TED-GC-MS).

SU-8/D230 Plenco 11956

HO

OH

161/9690 0

2

4

6

8

Retention Time (min) Fig. 9. TD-GC-MS chromatograms of degradation products from a BPA novolac epoxy cured with an amine (Top), a novolac of phenol (Middle) and an epoxy novolac cured with a cresol novolac (bottom). When these materials are pyrolyzed, their degradation products can be sufficiently specific to identify the material.

II.

Analysis of DGEBA Epoxies Cured by Amines

Amine cured epoxies are commonly used in commercial applications as adhesives, encapsulants, and composite resins. Specifically, diglycidyl ether of bisphenol A (DGEBA) is seen as particularly attractive due to its processability (low viscosity), coupled with high Tg and toughness when cured with an appropriate amine. In this section we will examine DGEBA (Epon 828 in the current work) based formulations which have been cured by various amine curatives. The TED-GC-MS chromatograms of these materials, shown in Fig. 10, all contain 15

degradation products which are characteristic of Epon 828 such as phenol and its methylated, isopropylated, and isopropenylated analogues. Additionally, though not presented here, the presence of BPA is also observed when an activated carbon filled tube is used as the sorbent. In this figure, degradation products which are indicative of the curative used in the formulation are also present. DDS cured materials tend to yield aniline and N,N-dimethylaniline when pyrolyzed at high temperatures. However, we do not observe any sulfone containing species. Conversely, when 828/2049 is subjected to the TED-GC-MS procedure, cyclic aliphatic hydrocarbons are observed which match the aliphatic backbone of Ancamine 2049. However, amine-containing compounds cannot be identified. This trend seems to hold for the other aliphatic and aromatic amines investigated as well. For aliphatic amines, we observe fragments which are indicative of the backbone of the curative, but never an amine-containing fragment. Conversely, for aromatic amines, we usually observe some amine-containing derivative of the curative. This is likely due to the greater stability of the C-N bond for aromatic amines as compared to aliphatic amines [50].

828/DDS

828/A2049

Fragments

828/5200

828/D400_D2000 PPO Fragments

828/D230

0

2

4

PPO Fragments

6

8

Retention Time (min) Fig. 10. GC-MS Total Ion Chromatogram of Epon 828 cured by various aliphatic and aromatic amines. Along with degradation products of the epoxy resin (dashed lines), each material yields fragments of its curative for identification.

The aliphatic amine curatives known as Jeffamines (D230, D400, and D2000 in this study) are amine functionalized polypropylene oxide (PPO) oligomers. These are widely used in commercial applications due to the wide availability of various molecular weight curatives and their slower cure kinetics as compared to other aliphatic amines. When PPO amine cured Epon 16

828 is analyzed by TED-GC-MS, we observe an early eluting peak whose mass spectrum can be matched to a dimer of propylene oxide. Higher molecular weight oligomers appear throughout the chromatogram and coincide with other eluting compounds. The mass spectra of these oligomers, as illustrated in Fig. 11, exhibit a main peak at m/z=59, and so far as the other investigated resins and curatives are concerned, this peak is unique to PPO oligomers. Thus, to determine whether a formulation contains PPO based resin/curative, the 59 m/z extracted ion chromatogram is a key indicator. Dividing the signal intensity of each point in the 59 m/z chromatogram by the corresponding point in the total ion chromatogram (TIC) yields a fractional 59 m/z chromatogram. The chromatograms resulting from this spectral division are presented in Fig. 12 for two materials containing PPO amine curatives (828/D230 and 828/D400+D2000) and one material with a generic aliphatic amine (828/2049). In this figure, the PPO amine cured epoxies exhibit more peaks which contain a significant fraction of PPO oligomers. Thus, this type of analysis can be used to determine the presence of PPO in a formulation; however, there are limitations. Although we can detect the presence of PPO, other PPO based epoxy resins are commercially available as well (e.g. DER 732). We have not examined any formulations containing such epoxies, but we believe it would be difficult to distinguish these from a PPO amine curative. However, it should be noted that PPO epoxy resins are less commonly used, and the detection of PPO in a thermoset material would most likely indicate the presence of PPO amines. Further, although there seems to be an effect of PPO amine molecular weight on peak intensity in the normalized 59 m/z chromatograms of Fig. 12, we have not examined whether this difference is statistically significant, nor if the sample mass during pyrolysis has any contribution. We also employed a fast heating rate which limits the observation of multiple degradation regimes which should be expected for these materials. If a slower heating rate were used, it might be possible to identify the degradation products from each degradation step, and also quantify each step based on the TGA data. Thus, while the current iteration of this method can detect PPO in a thermoset sample, these approaches must be improved to allow for the determination of the molecular weight of the PPO in the formulation.

59 m/z

100%

59 m/z

100%

192 g/mol

80%

148 g/mol

80%

60%

60%

40%

40%

20%

20%

0%

0% 20

40

60

80

100

120

140

160

20

m/z

40

60

80

m/z

17

100

120

140

160

Fractional 59.1 Ion Intensity

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

Fractional 59.1 Ion Intensity

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

Fractional 59.1 Ion Intensity

Fig. 11. MS spectra of PPO oligomers. The most prominent peak is at 59.1 m/z which is unique for PPO fragments.

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

828/A2049

A

0

2

4

6

8

10

12

828/D230

B

0

2

4

6

8

10

12

828/D400+D2000

C

0

2

4

6

8

10

12

Retention Time (min)

Fig. 12. Fractional 59.1 ion chromatograms of Epon 828 cured by Ancamine 2049 (A), Jeffamine D230 (B), and a 1:1 wt. blend of Jeffamines D400 and D2000 (C). The 59.1 ion chromatogram was normalized by the total ion chromatogram (i.e. the 59.1 peak intensity with time was divided by the corresponding total peak intensity giving its relative fraction).

As shown by the multiple significant peaks in Fig. 12B and Fig. 12C, the Jeffamine curatives tend to produce more distinct degradation products as compared with other amines. Unlike other curatives which are structurally identical throughout, PPO amines are oligomeric in nature. Consequently, there is a distribution of molecular weights, and so degradation products with various molecular weights are certainly expected. Further, since each curative molecule has multiple repeat units, it is likely that the most pyrolytically sensitive linkages are repeated throughout its structure also resulting in a distribution of molecular weights in the degradation fragments. Interestingly, we do not observe monomeric propylene oxide or glycol as might be expected. Either this fragment is not produced during pyrolysis or it is not trapped on the sorbent used in this study. The aromatic curative diethyltoluenediamine (HY 5200) was examined in this study as well due to its low viscosity and slow cure kinetics which make it attractive in applications where such properties are desired. In the TED-GC-MS chromatogram of 828/HY 5200, a peak was observed which did not match any compounds in the reference library (Fig. 13). While we have not identified every fragment detected in this mass spectrum, we believe that it matches N,N,N’,N’-tetramethyldiethyltoluenediamine based on the match between the molecular weight of that compound and the highest molecular weight fragment detected. This is consistent with the observation for DDS as a curative, when a completely methylated fragment (N,Ndimethylaniline) was observed as well. 18

234 234 219 219

Mn=234 Mw=234

100%

80%

174 174 205 189 205 189

60%

40%

20%

0% 0

50

100

150

200

250

m/z Fig. 13. MS spectra of peak from 828/HY 5200 corresponding to tetramethylated aromatic amines. Although the structures for each fragmentation peak have not been identified, the highest molecular weight peak matches the molecular weight of the methylated curative. The formation of this compound is consistent with the observation of dimethylaniline from DDS cured Epon 828.

III.

Analysis of Anhydride Cured Epoxies

Anhydride cured epoxy formulations are attractive due to their low viscosity, high Tg, and tunable cure kinetics which can be altered without negatively affecting physical properties. Typically, this is done by adjusting the catalyst level within a reasonable window. Fig. 14 shows the TED-GC-MS chromatograms for two epoxies cured by anhydrides. In the DY-D/917 material, there is little evidence of the epoxy resin, and it is only possible to identify butanediol as a degradation product. Most notably, a large amount of anhydride curative is dominant. Similarly, the 828/HY 906 formulation displays a significant fraction of anhydride species, as well as other degradation products not seen in amine cured epoxies. Since these formulations were slightly epoxy rich (stoichiometry=1:0.9), these anhydrides cannot be the result of excess anhydride in the material, but must originate through some chain scission reactions. Further, although anhydrides exhibit nearly the same mass spectra as analogous diacids, any misidentification can be ruled out as diacids elute much later. The exact mechanism by which the other non-anhydride products are formed is not known, but we may speculate on the mechanism by which anhydrides are evolved from these materials.

19

DY-D/917

828/HY906 0

2

4

6

8

Retention Time (min) Fig. 14. GC-MS total ion chromatograms of 828/HY 906 and DY-D/917. When these materials are exposed to high temperatures, a substantial portion of the anhydride curative is given off as a result of degradation.

Depolymerization is a known phenomenon for polymers synthesized via a chain-growth reaction [51,52]. This behavior is often referred to as “ceiling temperature behavior” whereby depolymerization becomes more favored than polymerization above the ceiling temperature. Typically, this is only seen in materials produced via chain-growth polymerization as there is no condensate which must be reintroduced to drive depolymerization. Theoretically, this could also be true for other non-condensation addition reactions such as epoxy/amine and polyurethanes, but typically this does not occur as the temperatures required to reverse these reactions are higher than those for competing pyrolysis reactions. Further, since ceiling temperature behavior requires active chain ends be present [51], it usually only occurs if the catalyst used during polymerization (or an analogue of that catalyst) is present when the material is heated to its ceiling temperature. These criteria are met for anhydride cured epoxies as the imidazole catalyst remains in the material after curing. Fig. 15 illustrates the likely mechanism by which depolymerization occurs.

Fig. 15. Likely mechanism for high temperature degradation of epoxy/anhydride materials. This mechanism explains the generation of anhydrides as volatile degradation products in the presence of catalyst residues.

PY-GC-MS was performed by pyrolyzing two 828/HY 906 materials with differing catalyst levels (0.5% and 4%) in the thermal desorption chamber at 200°C to explore the nature of this degradation reaction. This procedure was chosen over TED-GC-MS due to the uncertainty of how quantitatively accurate the latter method is. Performing the pyrolysis in the thermal desorption chamber allows for all evolved volatiles to be captured in the cryo-trap, allowing for 20

quantitative comparison of the two chosen samples. Fig. 16 shows the PY-GC-MS chromatograms of these two samples after 5 min. pyrolysis at 200°C. For both samples, the major degradation species is, surprisingly, methylcyclopentadiene dimer with some evidence of cyclopentadiene/methylcyclopentadiene adducts. Interestingly, the amount of these products evolved during 200°C exposure does not vary with the catalyst loading suggesting that their presence is likely due to reversal of the Diels-Alder reaction by which these anhydrides are formed. Additionally, the curative is detected as well, and the amount evolved during this pyrolysis depends on the catalyst loading. While for this anhydride (Aradur HY 906) the retro Diels-Alder reaction may be a significant degradation route, evidence of ceiling temperature depolymerization (anhydride generation facilitated by remaining catalyst) is also present. The latter mechanism is also expected to be more generally relevant for other anhydride curatives.

GC/MS Chromatogram After 5 min. 200°C Exposure

Unknowns

3

9

Normalized Signal (10 Cts/mg)

4

2

4% 1MI

1

0.5% 1MI 0 2

4

6

8

Retention Time (min)

Fig. 16. GC/MS Chromatogram of 828/HY 906 formulations cured with 4% 1MI (red, shifted upward) and 0.5% 1MI (blue). Substituted cyclopentadiene dimers are produced in near equal amounts regardless of catalyst. Anhydride production, however, increases with increasing catalyst loading. Unknowns between 7 and 8 min. retention time contain methylcyclopentadiene as evidenced by the similar MS pattern around 80 m/z.

IV.

Analysis of Epoxy Resins and Their Blends

A main objective for the TED-GC-MS technique is to adequately distinguish between similar materials, with Fig. 17 showing trends for a few epoxy resins. Eponex 1510 is a hydrogenated DGEBA, and as such produces cyclic aliphatic hydrocarbons when pyrolyzed. Distinguishing this resin from Epon 828 is easily possible since these resins do not produce any shared species. Further, when a mixture of Epon 828 and Eponex 1510 is used as the epoxy resin, both compounds can be easily identified as the pyrolysis products from such a mixture contain species characteristic of each resin. In contrast, Epon 828 and Epon 1031 (tetraglycidyl ether of 1,1,2,2tetrakis(p-hydroxyphenyl)ethane) share many of the same characteristic pyrolysis compounds. In fact, the set of characteristic compounds from 828 encompasses those from 1031 with the exception of one compound, dibenz[b,e]oxepin-11(6H)-one (DBOP). However, the pyrolysis of 21

1031 based materials yields low amounts of this compound. Thus, differentiation of a material based on a blend of 828 and 1031 is not easily distinguished from one based solely on 828.

OH

828_1510/DDS 1510/5200

Fragments

828/DDS 1031/DDS

O

O

828_1031/DDS 0

2

4

6

8

10

Retention Time (min) Fig. 17. GC-MS total ion chomatograms of Epon 828, Eponex 1510, a mixture of these, and a mixture of Epon 828 and Epon 1031 cured by amines. Obvious differences between 828 and 1510 are apparent, but a mixture of 828 and 1031 can only be distinguished from 828 through one compound.

4.

Characterization of Epoxy Material Aging Behavior

Glass microballoon (GMB) filled epoxy/amine thermoset materials were examined to determine the utility of TD-GC-MS toward quantitatively determining the aged state of a material in an oxidative environment. These materials were oxidatively aged at temperatures of 80°C to 125°C in a controlled atmospheric environment, with an oxygen consumption rate separately measured using techniques described previously [38,53]. Fig. 18 shows the time shifted integrated oxygen consumption for materials aged at 80, 95, 110, and 125°C. The Arrhenius plot of the resulting shift factors vs 1/T is linear and yields an activation energy (Ea) of ~74 kJ/mol for oxidation. For any study involving oxidative aging, diffusion limited oxidation (DLO) is a potential concern. This phenomenon has been studied in detail [54-57], and occurs if an oxidation gradient is present in the aging material due to the rate of oxidation approaching or surpassing the permeation rate of oxygen into the material. Typically, this behavior becomes more of a concern as the aging temperature increases but can be minimized by the use of thin samples. However, the oxidation rates for epoxy materials are surprisingly high [53], and 22

permeation is low [58], meaning DLO can occur in some circumstances even for thicknesses less than 100 micron [59]. Previously, Gillen et al. derived an equation to define a critical thickness (Lc) of a material under which DLO is not significant (eq. 1) where is the equilibrium oxidation rate, Pox is the oxygen permeability coefficient, p is the oxygen pressure at the surface of the material (13.2 cmHg in Albuquerque NM), ρ is the density of the material (0.65 g/cm3 due to GMB filler), β is a constant that is assumed to be 5 for this class of materials, and αc is a factor which relates to sample geometry (assumed to be 28.5 for β=5) [54]. Given that the and Pox values measured at 125°C were 10-9 mol*g-1*s-1 and 9*10-10*ccSTP/cmHg*s*cm respectively, the critical sample thickness required for DLO to be considered significant would be more than approximately 620 microns. Thus, for the 100 micron thick samples used in this study, DLO did not contribute providing homogeneous samples for the evaluation of TD-GC-MS. This expectation is also consistent with perfect time-temperature superposition and linear Arrhenius behavior for the oxidation behavior of this material, where the integrated oxidation levels for a range of timetemperature conditions are presented in Fig. 18. =

∗ ∗

∗

. ∗ρ∗

Eq. 1

125°C aT=2600

10

95°C aT=340

10-3

85°C aT=100

1

50°C aT=14 23°C aT=1 Ea=74 kJ/mol

10-4

0.1

10-5 0.01 10-6

Percent Oxidation

Oxygen Consumed (mol/g)

10-2

0.001 10-7 100

101

102

103

104

105

106

ShiftedTime Time(days) [t x aT] (day) Fig. 18. Time-temperature superposed oxygen consumption data (previously unpublished). The material exhibited a constant oxygen consumption rate for the temperatures studied, similarly as discussed in related characterizations [48].

These oxidized samples were analyzed by TD-GC-MS to determine what trend, if any, could be detected. For materials aged at 80°C, we noticed that as the material aged, increasing amounts of two compounds, BPA and isopropenylphenol (IPP), were detected. Fig. 19 shows how the relative amounts of these compounds evolve with aging time. Interestingly, materials aged at >80°C did not display IPP in their chromatograms. This is likely due to volatilization of the compound at temperatures over 80°C. As IPP is created at these higher aging temperatures, it is volatilized into the headspace of the aging chamber. Thus, since it is not present in the material when it is analyzed by TD-GC-MS, it is not detected. This is the most feasible reason 23

for the absence of IPP in samples aged at temperatures above 80°C as the melting point of this compound is 84°C, and above this temperature it may display increased vapor pressure. Alternatively, this compound may undergo reactions or polymerization through the isopropenyl group, rendering it non-volatile. This would have the same effect (no detection at higher aging temperatures) and cannot be ruled out. Peak Area/Sample Weight

1012

1011

1010

109

108

BPA Signal IPP Signal

107 10

100

1000

Aging Time (day) Fig. 19. Normalized peak area vs aging time of BPA and isopropenylphenol (IPP) for epoxy samples aged at 80°C.

Although we did not detect IPP in materials aged at higher temperatures, the detectable BPA increased with aging time for all temperatures studied as shown by the plots in Fig. 20. These plots are superposable for the temperature range we studied, with minor exceptions which will be discussed later. The Arrhenius plot of the shift factors in Fig. 21 is linear with a calculated Ea of 67.7 kJ/mol which is in close agreement with the Ea obtained by oxygen consumption measurements discussed earlier and also agrees with oxidative Ea’s typically measured for epoxy thermosets as reported elsewhere [53,60]. The agreement between these two techniques documents that the TD-GC-MS analysis detects a representative signature that evolves in parallel with the oxidation state of the material. Complementary to other analytical techniques, such as IR based monitoring of C=O stretch frequency, this technique can potentially provide guidance on the oxidation state of materials of unknown history, or at least provide evidence for chemically driven changes in a material. This could also be hydrolytic or radiatively driven in nature, if access is given to the original material as a reference for comparison.

24

Cts) 10 125°C 110°C 95°C 80°C 4

2

0 1

10

100

1000

BPA Peak Area/Sample Wt. (10

BPA Peak Area/Sample Wt. (1010 Cts)

6

6

125°C 110°C 95°C 80°C

a T=14 a T=6 a T=2.8 a T=1

4

2

0

Aging Time (day)

10

100

1000

Shifted Time [t x aT] (day)

Fig. 20. Trends of BPA detected as a function of aging time for the temperature range we studied and the corresponding t-T superposition.

10

aT

Ea=67.7 kJ/mol

1

2.45

2.50

2.55

2.60

2.65

-3

2.70

2.75

2.80

2.85

-1

1/T (10 *K ) Fig. 21. Arrhenius plot of shift factors shown in Fig. 20. The Ea of 68 kJ/mol is in close agreement with the Ea obtained from oxygen consumption analysis (Fig. 18).

Understanding the limitations of this technique requires insight into how and when the BPA is cleaved from the network. There are two possible sources of BPA; either the BPA is sensitive to pyrolytic release from the crosslinked material at 230°C (bound only once to the network and released by in-situ cleavage during TD), or the BPA exists as an already unbound trapped species which was generated during previous ‘aging’ and is simply desorbed at 230°C. One avenue to distinguish between the origin of BPA is through the use of solvent extraction methods. As shown in Table 4 extracting equivalent samples aged at 80°C with THF reduced the BPA signal by approximately 90% for all samples meaning that BPA must be present as unbound material and most likely evolves during thermal exposure at 230°C via simple desorption and not pyrolysis. Table 4. Comparison of BPA signals with and without THF extraction.

Days

BPA signal per mass BPA signal per mass after % 25

BPA

aged 80°C 14

at (not extracted)

101 182 594

7.37E+9 1.16E+10 5.15E+10

8.19E+8

THF extraction

signal decrease Not distinguishable from 100 noise 6.41E+8 91 1.61E+9 86 5.55E+9 89

Since this technique requires some desorbable compound be present in the aged materials whose abundance is directly related to the material’s aged state, this technique may not be applicable to materials whose major oxidation pathway is oxidative crosslinking or whose degradation products are only small easily volatilized molecules (CO2, H2O, Acetone, etc.). The latter is illustrated by examining the presence of isopropenylphenol in these aged samples as mentioned earlier. Since this compound is volatilized at higher aging temperatures (>80°C), we do not detect it in samples aged at those temperatures. Thus, for this material if BPA had not been present in aged samples, there would have been no chemical marker indicative of the oxidation state of these materials. Additionally, oxidative aging does not always result in scission reactions. As an example, for a variety of thermoset materials under photo-oxidative degradation it was shown that even for materials which do not contain crosslinkable unsaturation, this aging environment often results in densification and hardening of the material associated with the formation of new crosslinks [39,40]. In such cases where a given material does not produce a moderately volatile compound during aging which can be desorbed and detected, the TD-GC-MS technique can be easily converted to a controlled PY-GC-MS by holding the sample at a higher temperature in the desorption chamber. Again, much care should be taken to ensure that the GC is not overloaded. Another weakness of this technique for predictive aging is illustrated by the BPA trend for samples aged at temperatures higher than 95°C. In Fig. 20, we see that the evolved BPA trends for samples aged at 110°C and 125°C are superposable up to 59 and 14 days, respectively. However, past these aging times, the trend deviates based on time-temperature superposition and some absence of expected BPA is evident. This behavior is likely due to volatilization of the BPA (beginning of sublimation) despite its higher melting point of 158°C. This means that quantification of BPA using TD-GC-MS would not be adequate for materials aged at these higher temperatures for extended times. Although we have not examined these options, we believe that this technique would be ideal to detect aging signatures at lower aging temperatures (30-60°C). The sample size in this study was limited to 5 mg, but the TD chamber can hold up to 1 g of sample while maintaining adequate airflow around all parts of the sample. Further, the high sensitivity of the MS detector allows quantification of evolved BPA masses of approximately 50 ng, meaning it is theoretically possible to detect the presence of 50 ppb BPA in a 1 g sample.

26

5.

Epoxy/Anhydride Formulation Optimization

For the formulation of thermosetting resins which contain volatile resins/curatives, it is important that the cure stoichiometry be optimized so that none of the volatile reactant is present in the final material as a remaining unbound molecule. If this happens, the material may off-gas these compounds which may induce compatibility concerns related to corrosion, additional reactions, or toxicological aspects. Anhydride cured epoxies represent an important example of such a formulation as the anhydrides are particularly volatile, and they are also vulnerable to containing residual unreacted curative due to the chemistry by which these materials cure. Whereas these materials cure via a chain-growth mechanism, traditionally used thermoset materials cure via a step-growth mechanism. The differences between these two types of polymerization mechanisms have been discussed in detail elsewhere [51,61]. The most important distinction for the current work is that for materials which cure via step-growth polymerization, molecular weight increases occur homogeneously throughout the material. Conversely, for materials which cure via chain-growth mechanisms and contain a monofunctional monomer, molecular weight increases occur non-homogenously such that at any point in the reaction, some mass fraction of the mono-functional monomer will be incorporated into a high molecular weight polymer, and the rest will exist as completely unreacted monomers. This means that materials which cure via step-growth mechanisms will exhibit a sharp drop in unbound species with increasing conversion, and all species will become bound prior to full conversion. However, chain-growth cured (anhydride-epoxies) materials will exhibit a linear trend of unbound species as a function of conversion and so, even at very high conversions, unreacted volatile species will remain. This difference is illustrated in Fig. 22 and plotted in Fig. 23 using basic reaction probabilities where a difunctional epoxy cured by a monofunctional anhydride is compared to a difunctional epoxy cured by a tetra-functional amine.

Fig. 22. Illustration of network formation at high but incomplete conversion in anhydride cured (left) and amine cured (right) formulations. Because of the ring opening polymerization (chain-growth) mechanism of cure in epoxy/anhydride systems, any unreacted anhydride functionality results in unbound volatile curative.

27

% Curative Unbound

100

80

Epoxy/Anhydride 60

40

20

Epoxy/Amine

0 0

20

40

60

80

100

% Curative Functional Groups Consumed Fig. 23. Plot of unbound curative versus conversion illustrating how cure chemistry affects this relationship. While step-growth chemistries (epoxy/amine) exhibit a steep drop in unbound curative with increasing conversion, this relationship is linear for chain-growth chemistries (epoxy/anhydride) when the curative is mono-functional. For these plots, we assumed that all curative functional groups of a given type are equally reactive.

Normalized Signal

We examined two fully cured 828/917 based formulations by TD-GC-MS, with one material being epoxy rich (1:0.9 stoich.) and one being anhydride rich (1:1.06 stoich.). It should be noted that these materials were powderized to ensure that limited diffusion of unreacted curative did not artificially reduce the amount of curative desorbed or detected. As shown in Fig. 24, with TD-GC-MS unreacted anhydrides are detected in the material which contains excess anhydrides, but not in the sample which is epoxy rich.

1.2 1.0 0.8 0.6 0.4 0.2 0.0

1:1.06 stoichiometry 1:0.9 stoichiometry 2

4

6

8

10

Retention Time (min) Fig. 24. TD-GC chromatograms of epoxy/anhydride materials intentionally mixed with two different stoichiometric ratios.

The optimal stoichiometry at which no unreacted anhydrides are present in the material was determined using TD-GC-MS to analyze a series of anhydride cured epoxy materials ranging from 6% excess anhydride to 10% excess epoxy. As shown in Fig. 25, although we would expect that a 1:1 stoichiometry would be sufficient to allow all anhydrides to react, a slightly epoxy rich stoichiometry is required for anhydrides to not be detected by this technique. The reason for this is not exactly known, but we will speculate on possibilities. Firstly, the functional equivalent weight of Epon 828 varies on the order of +/- 2% based on the usual numbers given in material data sheets. Thus, using an average functional equivalent weight is likely to result in 28

excess anhydrides if the actual equivalent weight is higher (less epoxy groups per gram) than the average. Secondly, epoxy homopolymerization is a known reaction [62], and, though it is not favored in the presence of anhydrides, epoxy/epoxy addition may occur during the copolymerization, but at a lower frequency than the epoxy/anhydride addition. Regardless, the presence of excess anhydrides can be minimized if a sufficiently epoxy rich stoichiometry is used.

Fig. 25. Graph of normalized anhydride peak integration vs. excess anhydride equivalents. Even at a 1:1 target stoichiometry (excess anhydride equivalents=0), unreacted volatile anhydrides are detected as by this method.

An additional observation here relates to the relative reactivities of the anhydrides which make up the commercial blend which we used. Fig. 26 shows a comparison between the GC-MS chromatogram for Aradur 917 and the TD-GC-MS chromatogram of an anhydride rich 828/917 formulation. In the commercial mixture, we see the presence of two anhydrides: tetrahydrophthalic anhydride (THPA) and methyltetrahydrophthalic anhydride (MTHPA). Additionally, the MTHPA is present as two isomers which elute at slightly different times. If all monomers were equally reactive, we would expect that in an anhydride rich formulation, the make-up of the remaining anhydride would be unchanged. However, we have found that only one isomer of MTHPA remains for such a material suggesting that this monomer is less reactive than the other two.

29

Relative Intensity

1.4

Unknown isomer of MTHPA

1.2 1.0 0.8 0.6

Aradur 917

0.4 Anhydride rich formulation

0.2 0.0

5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4

Retention Time (min) Fig. 26. GC chromatograms of commercial anhydride blend Aradur 917 (red) and the anhydride evolved from an anhydride rich formulation (blue). Despite the curative containing multiple anhydride species, only one remains for post-cured anhydride rich formulations.

6.

Conclusions

Thermal desorption (TD) and pyrolysis based GC-MS were examined as improved and complementary analytical methods for general thermoset materials characterization. With an emphasis on materials identification, a more specialized approach which utilizes an additional TGA pyrolysis step, namely TED-GC-MS which has previously been used for thermoplastic materials [23,24], is also attractive for determining the nature of resin and curatives in thermoset formulations. Aliphatic and aromatic constituents are easily distinguished from each other. However, aside from BPA containing materials, distinguishing different aromatic epoxy resins is not as easily accomplished as most compounds released on pyrolysis are identical for these materials. Identifying the amine or anhydride curative in these materials is possible, ideally when the curative is not polymeric in nature. The backbone structure of polymeric curatives such as polyether (PPO) amines can be identified, but determining the molecular weight of this curative requires further method development. Interestingly, when pyrolyzed, anhydride cured epoxies produce a significant fraction of intact anhydride curative along with degradation products indicative of the epoxy used. By gently pyrolyzing these materials directly in the thermal desorption chamber, it was shown that they depolymerize at temperatures above 200°C, and that this depolymerization reaction is accelerated by the trapped catalyst required for their cure. However, we note that 200°C is not claimed to be the “ceiling temperature” of this material, as a polymer’s ceiling temperature is dependent on the concentration of free monomer, and we did not perform a systematic determination of the minimum temperature where this behavior is observed. The broad application of this technique toward identifying primarily epoxy based thermosets serves as an initial demonstration of the utility of this technique. Future work in this area will center on the applicability of PY-GC-MS toward other thermosets.

30

For the field of polymer degradation, we have shown that TD-GC-MS is a useful tool for quantifying the oxidation state of some epoxy materials. For a generic amine cured Epon 815 material, it was possible to detect unbound BPA which increased with aging time and hence oxidation level. The trend of detected BPA versus aging time was superposable over a temperature range from 80-125°C, and the Ea derived agreed with that obtained by oxygen consumption. However, at extended aging times at higher temperatures, the trend became more non-superposable. Although the reasons for this behavior are not yet known, it is likely that at higher temperatures BPA either volatilizes from the material during aging or undergoes some pyrolytic reaction and thus is present at lower than expected levels in these samples. Future work in this area will focus on the use of TD-GC-MS for low temperature analysis of oxidatively and non-oxidatively aged materials during the early stage of degradation. Finally, we have demonstrated the effectiveness of TD-GC-MS toward formulation optimization of thermoset materials that contain volatile curatives such as epoxy/anhydride thermosets. These materials are uniquely challenging in that unbound curative may exist even in “fully cured” materials. Based on analysis of a stoichiometry series, a slightly epoxy rich formulation is required to eliminate unreacted anhydride curative. Additionally, it was shown that for a particular anhydride mixture, the constituent anhydrides were not equally reactive. This resulted in one of the anhydrides present in the commercial blend being overproportionately responsible for off-gassing species in anhydride rich formulations. In summary, polymer fragments can be formed by materials degradation, present in some non-perfect otherwise curable thermoset formulations, or intentionally produced via pyrolytically driven decomposition methods. They have often been overlooked or are rather inaccessible for in-depth material characterization purposes. Thermal desorption methods coupled with GC-MS are the most promising analytical techniques to identify and quantify evolved volatiles for insight into material identity, formulation optimization and degradation pathways, and this study has succeeded to show the underlying similarities among these different analytical challenges.

Acknowledgements Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Note: This paper describes objective technical results and analyses. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: