**1. Introduction**

Analytical Pyrolysis technique hyphenated to GC/MS is used to obtain structural information of macromolecules by GC/MS analysis of their thermal degradation products in the absence of oxygen [1–4]. By Py-GC/MS the polymers are converted into lower molecular weight products by the action of heat. The relative proportions of the produced products depend on the composition of the samples, the temperature and the time it is applied. The composition and relative abundance of the pyrolysis products are characteristic for a given polymer and their determination allows the identification of materials that cannot be determined otherwise. This technique can also provide quantitative analysis of polymer structure, including monomer composition, stereochemistry, tacticity, and molecular arrangements in homo and copolymers [5].

Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (RMN) and thermal analysis techniques such as thermogravimetric analysis and differential scanning calorimetry (DSC) are relevant techniques for the analysis of polymers. Py-GC/MS is a complementary technique which provides useful information that allows a deeper characterization and identification of these polymers.

Real cases often deal with the characterization of unknown samples. In these cases, it can be challenging to select appropriate instrument set-up and method

parameter starting points. However, based on the experience, there are some general considerations on how approach method development in the most efficient way.

Py-GC/MS of polymeric materials is a very powerful technique as it allows the identification of polymer components from a complex pyrolyzate mixture using a library search approach [6–21]. Besides, there are commercial pyrolysis libraries that can be a really useful help.

In more complex cases simple mass spectral identification via library (e.g. NIST, Wiley, etc.) comparison will be of limited use. A database approach is very helpful for automating the extraction of meaningful information simplifying the data evaluation task [22]. This approach consists basically of three steps: measuring reference substances to generate a database, measuring unknown or difficult samples and using the database for the identification of these samples.

Using chemometric methods can help extract useful information in very complex samples [23, 24].

Py-GC/MS has a lot of benefits [25, 26] that makes this technique a really powerful one: Pyrolysis of polymeric materials as a sample introduction technique for gas chromatography allows studying materials and compounds that are not suitable for traditional GC/MS analysis; it allows studying polymeric structures from pure systems to multi-block polymers; it requires minimal sample preparation and solvent is not required for most applications, meaning that low concentration monomers, residual solvents, additives and crosslinking agents can be identified without adding additional contaminants.

Most of the instruments allow different pyrolysis techniques [27, 28], such as stepwise pyrolysis (the sample temperature is raised stepwise, and the pyrolysis products are recorded between each step), fractional pyrolysis (A pyrolysis where the sample is pyrolyzed under different conditions in order to investigate different sample fractions), sequential pyrolysis (the same initial sample is repeatedly pyrolyzed under identical conditions: final pyrolysis temperature, temperature-rise time and total heating conditions), cold injection in split or narrow band mode (the pyrolysis products can be transferred to the GC/MS with a cold injection system) or thermochemolysis (thermally assisted hydrolysis and methylation (THM)) to improve the chromatographic analysis of polar functional groups, such as carboxylic acids.

Besides it is possible to work with polymer solutions, subsequently venting the solvent, which enables the highly precise introduction of small amounts of polymer in solution.

Another advantage of this technique is that it requires small amount of sample [25], in the range of micrograms to milligrams, typically with 300 μg of polymer a complete characterization is possible.

The technique has also some limitations [28]. Some of them are: to obtain reliable comparisons between different laboratories reproducible experimental conditions are required; inhomogeneous samples can have variable results; it does not detect most inorganic components; it is a destructive technique.

Quality control in production, product development and forensic science are some of the key areas where py-GC/MS is applicable.

In this chapter some case studies are shown as real examples from the industry to highlight the technique's potential.

### **2. Practical examples**

Some real examples will be presented to show the potential of this technique. The work presented in this section was carried with a pyrolyzer from Gerstel (see *Pyrolysis-GC/MS, A Powerful Analytical Tool for Additives and Polymers Characterization DOI: http://dx.doi.org/10.5772/intechopen.101623*

### **Figure 1.**

*GC/MS with a MultiPurpose sampler (MPS), thermal desorption unit (TDU) and pyrolysis module (PYRO).*

**Figure 2.** *Pyrolysis module with platinum filament, transport adapters and sample holders (Gerstel).*

**Figures 1** and **2**), that can pyrolyze solids and liquids in a temperature range from 350°C up to 1000°C in a very flexible and automatic way [29], and determine thermal decomposition products in the GC/MS. All spectra were obtained under electron impact (EI) conditions. Ionization voltage: +70 eV. The MS was used in the full scan mode.

### **2.1 Chemical characterization of a synthetic foam by Py-GC/MS**

The application of this technique to the analysis of polyurethanes offers information about the isocyanate and the polyester or polyol used in their formulation. Three reaction mechanisms have been proposed to account for the products arising from the thermal or pyrolytic degradation of the urethane portion of a poly(urethane) [30, 31]. The first one is the dissociation of the urethane linkage to form an isocyanate and an alcohol, the second is the formation of a primary amine, an olefin and carbon dioxide (CO2) via a six-membered transition state and the third is the loss of CO2 to produce a secondary amine. The three mechanisms are shown in **Figure 3**.

Methylene diphenyl diisocyanate (MDI) is a diisocyanate that exists in a pure or a polymeric form. The pure one is made up of three positional isomers, shown below (**Figure 4**):

**Figure 3.**

*Three reaction mechanisms proposed for the thermal or pyrolytic degradation of the urethane portion of a poly (urethane).*

### **Figure 4.** *Chemical structures of monomeric and polymeric MDI.*

**Figure 5** shows some possible products produced in the pyrolysis of a polyurethane made with MDI.

The foam was analyzed by py-GC/MS at a pyrolysis temperature of 600°C.

By studying the pyrogram (**Figure 6**) the following conclusions can be obtained: the sample is a polyurethane formulated from a polyether with styreneacrylonitrile (SAN) and MDI, and although none of the three isomers of MDI monomer was detected, its diamine, 4,4<sup>0</sup> -diaminodiphenylmethane (MDA), is present and also some other degradation products, such as p-tolyl-isocianate, and a phenyldiurea. The main identified products are shown in **Table 1**.

*Pyrolysis-GC/MS, A Powerful Analytical Tool for Additives and Polymers Characterization DOI: http://dx.doi.org/10.5772/intechopen.101623*

**Figure 5.** *Possible products structures.*

**Figure 6.** *Py-GC/MS of the PU foam.*


### **Table 1.**

*Identified products.*

Some of the products detected were produced during pyrolysis mediated between the generated radicals, including monomers in the polymer matrix, as well as acrylonitrile and styrene.

Py-GC/MS provides very relevant information on the monomers used in the polyurethane formulation.
