Thermal analysis unlocks the secrets of elastomers

Transcription

1 4450 CR AN WO O D P AR K W AY C LEVELAN D, OH IO WW W. N SLAN ALYTICAL. C OM Thermal analysis unlocks the secrets of elastomers By Brian Bacher and Michael Walker, NSL Analytical Services, and Alan Riga, Case Western Reserve University Elastomer technology is as much an art as a science. And the constituents of a particular elastomer are often closely held secrets. This situation often puts an end user in a difficult situation. If a particular material is the only one that fits a need, the user is at the mercy of the compounder from a cost and supply standpoint. Elastomers can contain dozens of components at varying concentrations, including polymers, plasticizers, fillers, and stabilizers. Lubricants, antioxidants, flame retardants, and other components can also be present. To help identify the components in an elastomer, a reliable Figure 1 TGA analysis of typical polymers (ref. 1) PVC Polyvinyl Chloride PMMA Polymethyl Methacrylate HDPE High-Density Polyethylene PTFE Polytetrafluoroethylene PI - Polyimide Table 1 applications of DSC (ref. 1) technique to help reverse engineer a material would be a welcome addition to an analyst s arsenal. Such a method would begin to enable a user to determine the components in a material and seek a second, perhaps less costly, source. Reverse engineering also allows companies to duplicate or improve on a specific formulation. In addition, it provides the knowledge needed to understand the nature of the various components of a formulation, the ingredient concentration and how the components and ingredients interact. TGA to the rescue An analytical technique based on Thermal Gravimetric Analysis (TGA) has been developed to determine the constituents of thermoplastic polymers. The analysis has many uses, including: Characterizing competitive products Identifying batch to batch variations Comparative analysis of good and bad samples Investigating possible patent or trade secret infringement TGA measures the change in weight of a material as a function of temperature or time in a specific atmosphere (typically nitrogen or air). These measurements are used to determine the composition of materials and predict their thermal stability (temperature of weight loss or gain). Figure 1 shows PVC to be the least stable and polimide to be the most stable of the group. TGA can determine the thermal and oxidative stability of materials, the composition of multicomponent systems, the moisture content and dehydration, and the decomposition kinetics of materials. In a typical analysis, a 20 mg sample of the material is heated to 600 C in nitrogen then switched to an oxygen atmosphere (for carbon black) up to 850 C. Mass loss is measured at appropriate temperatures as indicated by the first derivative of the TGA curve. Thermoplastic Drugs Starches Chemicals/ Proteins/ Thermoset Elastomer Petroleum Glass Metal Glass Transition Temperature (Tg) X X X X X X X X Glass Transition Size (ΔCp) X X X X X X Melting Temperature (Tm) X X X X X X X Crystallization Temperature (Tc) X X X X X X Crystallinity (J/g not %) X X X X X X Heat Capacity (J/g- C) X X Oxidative Stability (Temp or Time) X X Texturing (process) Temperature X ( C) Curing/Degree of Cure (%) X X Polymorphic Transitions Denaturation/Gelatinization X X 1

2 TGA can be augmented with differential scanning calorimerty (DSC), which measures the differences in heat flow rate between a sample and a reference as both are heated, cooled or held isothermal. This technique provides a great deal of information about the material (table 1). Differences in heat flow occur due to the heat capacity of the sample, which increases with temperature, and at various transitions that occur in the sample as shown in the sample (figure 2). Heat flow rate can be expressed in a variety of units that can also be normalized for the weight of sample used. The plot of heat flow vs. temperature in figure 2 shows exothermic and endothermic transitions in the material. Exothermic transitions such as crystallization, curing and oxidation release heat. Endothermic transitions such as melting, glass transition and evaporation absorb heat. Decomposition can be either endothermic or exothermic. The following ASTM methods can be used in conducting a thermal analysis of an TGA measures the change in weight of a material as a function of temperature or time in a specific atmosphere. elastomeric part: Figure 2 some possible transitions in a DSC curve (ref. 2) ASTM D297 Standard Test Methods for Rubber Products Chemical Analysis ASTM D6370 Standard Test Method For Rubber - Compositional Analysis By Thermogravimetry ASTM E1131 Standard Test Method for Compositional Analysis by Thermogravimetry ASTM D3418 Standard Test Method for Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry Figure 3 TGA sample 1 TGA Residue: 850 C Plasticizer (1.865 mg) Weight (%) RT 400 C (5.562mg) Derivative TGA Polymer 46.51% RT-400 C (16.69 mg) Carbon black Derivative weight (%/ C) 43.25% C Switch from nitrogen to C (10.06 mg) 2

3 Figure 4 TGA sample 2 Weight (%) Plasticizer 14.09% RT C (3.279mg) Derivative TGA Polymer 39.21% C (9.124mg) Carbon Black % C (10.06mg) Derivative weight (%/C) Residue: C (0.8017mg) An example analysis Rubber materials from two different suppliers were analyzed with TGA to determine their composition. Results of the analysis are shown in figures 3 and 4. TGA plots indicate where each component part of the sample decomposed. The plots show discernible differences between two sample materials. The biggest differences are in polymer and carbon black content. The discontinuity at 600 C indicates that the materials contain high surface area carbon black. Chemical functional groups in the materials was then determined by Fourier Transform Infrared Analysis (FTIR) after various wet chemistry techniques were employed to separate each component and prepare them for analysis, including acetone extraction per ASTM D297; hexane fraction to identify the plasticizer; benzene (or other solvent) fraction anti-degradant; and pyrolysis. The FTIR analysis identified the two materials as ethylene propylene diene monomer (EPDM). These materials have a fully saturated backbone, making them naturally resistant to ozone and weathering (no double bonds to attack). However, they are susceptible to swelling from oil, or solvent contact. Figure 5 FTIR comparison The identifying wave number peaks for EPDM are 720, 885, 910 and 965 cm -1 (figure 5). While 885 cm -1 is also a peak for natural rubber, further analysis showed that no natural rubber was present. If isoprene/ isobutylene were present, a butyl peak would be evident at 1230 cm -1. Percent transmittance Sample 1 Sample 2 Wave number (cm ¹) 3

4 Figure 6 EPDM glass transition scan In addition to using TGA data to determine the number of components, DSC can be helpful in corroborating information from the FTIR analysis. DSC confirms the number of components by showing the number of thermal transitions. When analyzing rubbers, the analyst looks for dual glass transitions to help determine whether a multicomponent system is present (figure 6) C Tg C(H) C Table 2 thermogravimetry / FTIR analysis Property Sample 1 Sample 2 TGA % Low MW Plasticizer % Polymer = % Carbon Black (CB) % Ash (Inorganic) FTIR Polymer Type EPDM EPDM Plasticizer Type Paraffin Oil Paraffin Oil Suggested Carbon N550 N550 Black Suggested Accelerator Peroxide Peroxide The DCS reveals the glass transitions for the different fractions and the quantitative. Combining results from the various FTIR analyses of the different fractions and the quantitative data from the TGA analysis provides the information shown below in table 2 about the elastomer. Further analysis provides information on the material composition, as shown in table 3. The sulfur level refers to the curing package. A sulfur this low indicates that the materials are likely not sulfur-cured rubbers. Rather, this level of sulfur probably indicates contamination of the carbon black. The acetone extract is the plasticizer. Zinc oxide (ZnO) is needed with a stearic acid to form a fatty acid complex for curing and cross-linking. Ash that is insoluble in hydrochloric acid contains silicon dioxide and clay. The bottom three materials are typical rubber fillers. From this information the rubber compound formula can be reconstructed (table 4). Table 3 compositional analysis Test Sample 1 Sample 2 Method Specific Gravity Displacement % Acetone Wet Chemistry Extract % TGA Ash TGA % Ash Insoluble Wet Chemistry HCl % Sulfur LECO or Ion Chromatography % ZnO ICP-OES % CaCO ICP-OES % Silica 2.63 Not ICP / Detected Wet Chemistry % Clay Wet Chemistry Raw Material Table 4 rubber formulations Concentration (parts per hundred) Sample 1 Sample 2 EPDM Carbon Black Paraffin Oil Stearic Acid 2 2 ZnO Clay Sulfur Silica 5 Accelerator 3 3 Theoretical Gravity Measured Gravity

5 Summary References FTIR was used to verify the identity of the rubber, plasticizer and some additives. Inductively Coupled Plasma/Optical Emission Spectrometry (ICP-OES) was used to determine elemental filler composition. TGA determined the quantities of polymer, plasticizer and carbon black. Various wet chemical methods determined the additives and fillers. 1. A. Riga and R. Collins, Differential Scanning Calorimetry and Differential Thermal Analysis, in the Encyclopedia of Analytical Chemistry, R. A. Myers, Ed., J. Wiley and Sons Ltd., Chichester, England, , A. Riga, J. Cahoon, and W.P. Pan, Characterization of Polymers: Thermal analysis, in Comprehensive Desk Reference of Polymer Characterization and Analysis, Oxford Publishing, and American Chemical society Publication, Washington, DC, Chapter 13, , Figure 7 plasticizer identification Percent transmittance Wave number (cm 1) - Hux # 3571, paraffin oil, analytical grade - Hexane fraction 1 - Hexane fraction 1 About NSL Analytical Services, Inc. A recognized leader in analytical testing, NSL Analytical Services, Inc. is an Independent Commercial Testing Laboratory specializing in Inorganic Elemental Chemical Analysis, Metallurgical and Microscopy Evaluations and Polymer Materials Testing. NSL Analytical helps customers achieve the highest standards of product quality from design to launch by providing accurate, reliable and repeatable materials testing results. Learn how NSL can work with you to solve your product performance challenges. Visit us at NSLanalytical.com/materials-testing-quote or call us at and speak to a technical specialist. 5