Pyrogram of a rosin methyl ester resin after pyrolysis at 800 °C. The peak with the highest intensity was determined to be methyl abietate (m/z = 316).
Making it stick
Thermal desorption combined with gas chromatography is a simple yet powerful tool that can quickly extract low levels of volatile organic compounds (VOCs) from a wide range of solid matrices, including polymers and adhesives, providing comprehensive information about VOC content. The addition of a pyrolysis module to the tool kit enables the analyst to see more, looking deeper into the structure of the polymer or adhesive material while using a standard GC/MS system.
Fastening, bonding, joining, and mounting of materials is done in a number of ways. For example, materials used in industrial applications and building materials used in homes and offices are increasingly bonded using adhesive solutions. These replace more traditional fastening methods, such as screws, bolts, rivets and even welding. The reasons that adhesives are making such inroads into these applications are manifold: Adhesive tapes, for example, are flexible, light, and easy to position or even reposition and are largely immune to mechanical impact. They are widely used in the automobile-, electronic-, and paper industries, among professional builders and by do-it-yourself handymen or -women. Adhesive solutions are as varied as the applications they are used for. In many industries, it is critically important to know the composition of the adhesive material used since it can greatly influence the quality of the final product. This information can also be used to identify cheap imitations that can negatively impact revenues and ruin the reputation of a quality brand.
A typical adhesive tape consists of a carrier film, an anchor coating and the adhesive material. If it is a double-sided tape, one side is typically protected with a separating film, called a liner. Depending on the application, different types of liners can be used, such as polyethylene (PE), polypropylene (PP) or polyethylene terephthalate (PET). Carrier films can be made of PE, PP, PET, or of soft or hard poly vinyl chloride (PVC). In addition, various additives are used to tailor material properties to the specific application. These include plasticizers added to produce, for example, soft PVC, anti-aging agents, or anti-blocking agents. The adhesive material is typically produced from polyacrylate, synthetic caoutchouc, or natural caoutchouc, all of which may contain one or more of the following: Adhesive resin, fillers, crosslinking agents, and/or plasticizers. As an example, a double sided tape may be based on a soft PVC carrier with a resinous polyacrylate adhesive layer on both sides coated with a PE film.
Schematic diagram of a
double-sided adhesive tape
(PE = polyethylene, PVC =
Poly vinyl chloride)
If the soft PVC carrier and the PE film are purchased materials, analytical laboratory resources will be needed for quality control to identify the raw materials used. Often, all additives contained in all materials used must be identified in order to ascertain their compatibility with the adhesive, which of course must also undergo compositional analysis. The focus of the product quality control efforts would typically be the determination of the polymer composition and the adhesive resin content. For these and similar tasks, Pyrolysis-GC/MS has proven particularly useful. Thermal fragmentation followed by GC separation and MS determination offers a range of possibilities for qualitative and quantitative analysis as the three following examples show.
Using pyrolysis-GC/MS for polymer analysis
Initially, the PE film used for the double sided tape described above was analyzed. Material testing had revealed that the mechanical properties did not meet requirements – even though the infrared (IR) spectrum for both the front and back side of the tape showed only the absorption bands typical for PE. To clarify the matter, pyrolysis GC/MS was used with the following instrument configuration: A GERSTEL MultiPurpose Sampler (MPS) for automated sample preparation and introduction mounted on top of an Agilent 7890/5975B GC/MSD system. The setup was extended with a GERSTEL Thermal Desorption Unit (TDU) mounted on a GERSTEL Cooled Injection System (CIS 4), PTV-type inlet. The PE film was pyrolyzed at 700 °C and the formed fragments separated using a Restek Stabilwax DA column. Both linear and branched aliphatic hydrocarbons were found. A comparison with a user-generated pyrolysis library of various polymers revealed: Linear aliphatic hydrocarbons typically occur in PE pyrograms; branched aliphatic hydrocarbons, on the other hand, typically occur in polypropylene (PP). Finally, the analysis of a PE/PP reference mixture revealed that the unknown “PE” material was in fact a mixture of PE and PP. Since IR spectra of the front and back of the tape only produced PE adsorption bands, it was concluded that the unknown material was a three layer film with a PP layer between two PE layers.
Using Pyrolysis GC/MS to determine additives in polymers
To help assess, whether an adhesive would be compatible with a PVC carrier film, the plasticizer used in the PVC was determined. A combined analysis procedure of IR spectroscopy and pyrolysis GC/MS was used; in both cases, methods were selected, which didn’t rely on solvent-based extraction. Based on the IR analysis, it was determined that there was polyurethane in the PVC, but no phthalate. The PVC film was then pyrolyzed at 800 °C and the fragments identified. In addition to the hydrochloric acid fragment, which is to be expected when pyrolyzing PVC, a high intensity peak for the hexamethylene diisocyanate (HDI or HMDI) fragment was found. This information made it clear that a polyurethane- based plasticizer, which contained HMDI as isocyanate component, was used for the PVC. Based on experience, the plasticizer was deemed compatible for use with the adhesive.
Quantifying additives with pyrolysis GC/MS
In order to quantify adhesive resin content, in this case rosin aka colophony resin, a pyrolysis GC/MS method based on matrix- matched calibration was developed. The main challenges during method development were to identify and select suitable pyrolysis fragments as markers for individual adhesive resins and to optimize sample preparation in order to achieve adequate reproducibility. Methyl abietate (m/z = 316) was selected as marker for rosin methyl ester (see figure). For hydrated rosin resins, norabietan (m/z = 262) was used as quantification marker. Both methods were developed as Single Ion Monitoring (SIM) methods in order to achieve better signal to noise ratios and higher sensitivity. The sample preparation was optimized as follows: The adhesive material sample was weighed on silicone-coated release paper, i.e. anti-adhesive paper (sample weight: 0.5-1 mg). Then the sample was sprinkled with a fine glass powder, which prevents the sample from adhering to the pyrolysis tube without interfering with the pyrolysis process and without impacting the analysis result. For all samples and calibration standards, duplicate analyses were performed and the mean values used. Calibration standards were generated by adding 5 to 40 % (w/w) rosin resin to resinfree polyacrylate adhesive material made up of n-butyl acrylate, 2-ethylhexyl acrylate and acrylic acid. The calibration standards were pyrolyzed at 800 °C resulting in a calibration curve with a correlation coefficient of 0.9916. The calibrated methods were subsequently used to determine both the rosin resin and the hydrated rosin resin contents in polyacrylate adhesives. Additional hydrated resin types were analyzed to determine whether hydrated rosin glycerol ester resin or other esterbased resins could be analyzed quantitatively. Multiple rosin methyl ester resins from different manufacturers were finally analyzed. For both methods it was found that the rosin resin content could be determined with a standard deviation below 4 %. Given the highly heterogeneous nature of such adhesive resins, the developed pyrolysis GC/MS methods for quantitative determination of resin content are considered sufficiently accurate. Using this method, the rosin resin content of polyacrylate adhesives can be determined for quality control purposes. Further investigations will determine whether the methods can be applied to other matrices such as synthetic caoutchouc materials. It should also be possible to expand the use of the method to determine other resin classes such as polyterpene resins and hydrocarbon resins.
Pyrolysis GC/MS is a versatile technique, which can be used for qualitative as well as quantitative analysis of polymers in general and, as shown in this work, also of adhesive materials. Of particular benefit to the analyst is the very limited effort required for sample preparation as well as the considerable depth of information obtained – even from just a single analysis.
Aylin Meß, Vjaceslav Frank und Andreas Westphal,