APPLYING THE RIETVELD METHOD TO MINERAL FILLED POLYPHENYLENE SULFIDE COMPOUNDS

Transcription

1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol APPLYING THE RIETVELD METHOD TO MINERAL FILLED POLYPHENYLENE SULFIDE COMPOUNDS R. W. Morton, D. E. Simon, J. F. Geibe13, J. J. Gislason4 and R. L. Heald4 Phillips Petroleum Company, 22SA PL, Bartlesville, OK DES Consulting, 2409 South Elder Ave., Broken Arrow, OK Chevron Phillips Chemical Company LP, Bartlesville, OK Phillips Petroleum Company, Technical Resources Division, Bartlesville, OK ABSTRACT Rietveld modeling has become a widely used tool in the x-ray diffraction laboratory. Some of the most difficult systems to model are polymeric materials because they can be composed of amorphous, low crystalline and crystalline phases. In addition, engineering plastics like polyphenylene sulfide (PPS) are routinely tilled with amorphous and crystalline minerals. This paper discusses the application of the Rietveld method to model a mineral tilled PPS compound. INTRODUCTION Rietveld modeling of x-ray diffraction (XRD) data is becoming a widely used tool in the modern x-ray laboratory. One of the major advantages of using Rietveld modeling is that non-crystalline phases can be treated in the same manner as crystalline phases. This advantage can be applied to mineral filled polymer compounds where other analytical techniques (including traditional XRD methods) are difficult or impossible to apply. Mineral filled polyphenylene sulfide (PPS) compounds are especially difficult to work with analytically. PPS compounds soften around 550 F and remain viscous & tacky with additional heating until thermal decomposition occurs. PPS calibration standards are difficult and costly to prepare. Traditional XRD techniques are difficult to apply to PPS compounds that usually show preferred orientation plus contain a complex mixture of multiple non-crystalline, low-crystalline and highly crystalline components. This paper shows an approach taken to apply Rietveld modeling to mineral filled polyphenylene sulfide compounds. EXPERIMENTAL Instrumentation: All x-ray diffraction measurements were taken using a Philips XRG generator equipped with a long fine focus copper x-ray source powered at 40 kv & 30 ma; Philips 3020 digital goniometer & Philips 3710 MPD control computer; and a Kevex PSI Peltier cooled silicon detector. The Kevex detector was operated with a Kevex 4601 ion pump

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol controller, Kevex 4608 Peltier current supply, Kevex detector bias, Kevex 4561A pulse processor, and Kevex A single channel analyzer. Software: Diffraction patterns were acquired using Philips APD version 4. lc software. All Rietveld calculations were performed using Material Data, Inc. Riqas version 3. lc software.2 The programs were run under the MS Windows@ 95 operating system using an Intel PentiumB II 300 MHz class personal computer equipped with 128 MB of RAM. Sample Preparation: Polyphenylene sulfide compounds were prepared at Phillips Petroleum Company Plastics Technical Center by weighing the appropriate amounts of PPS powder and mineral fillers to the expected fractional weight. These mixtures were individually dry blended, pelletized, and then injection molded into impact bars having the general dimensions of l/8 inch thick, % inch wide and 5 inches long. Phase Filter Preparation: A phase filter input file was prepared by including non-crystalline PPS, non-crystalline calcium aluminosilicate, low crystalline PPS, crystalline calcite and crystalline anhydrous calcium sulfate in a crystallographic library for Rietveld modeling. The conditions for the major five phases are listed in Table 1. A few additional phases were included in the actual analytical phase filter to cover proprietary trace components. Table 1. Rietveld modeling parameters used in the phase filter. W-LOW U-High V-High W-High m Iso. Temp

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol RESULTS AND DISCUSSION Establishing a phase filter for Rietveld modeling is analogous to preparing a calibration curve for traditional analytical measurements. However, Rietveld modeling utilizes calibration standards to verify the integrity of the model instead of producing a mathematical correlation from the calibration standards. More importantly, Rietveld modeling can be used with a phase filter that includes modified phase structures to resemble the expected state-of-matter of the measured specimens. A phase filter acts like a small search/match library and can contain more structures than those routinely analyzed. The software automatically zeros out the scale factor for phases not present in the pattern. A zero scale factor eliminates their contribution to the final results. This property of a phase filter allows the modeler to include additional components like an internal standard3 or, in the case of production control, have one phase filter cover a number of product formulations. Assembling a phase filter begins with the modeling of the non-crystalline and the low crystalline phases. Figures 1 and 2 show the Rietveld refinement of PPS powder before compounding and a PPS compound containing only calcium aluminosilicate glass fibers. Silicon metal powder was added to the PPS powder in Figure 1 to help eliminate any sample displacement error. The 60 percent crystallinity determined by differential scanning calorimetry for the PPS powder closely matches the 6 1 percent calculated from the data in Figure 1. The refinements used to model the data in Figure 1 were then used to model the data from a PPS compound prepared with 35.0 weight percent calcium aluminosilicate glass fibers. The refined PPS and calcium aluminosilicate structures from Table 1 were combined to calculate the 34.6 weight percent glass fiber content in Figure 2. The low crystalline PPS, non-crystalline PPS, and non-crystalline calcium aluminosilicate phases, as described in Table 1, correctly model their broad features. Without successfully modeling these broad features using crystallographic data, the Rietveld modeling technique would have to incorporate an empirical tit to the data to compensate for their presence. As a consequence, the final analytical method would expend more resources preparing analytical specimens to provide satisfactory accuracy and reproducibility. These additional analytical step(s) used during sample preparation would reduce the effectiveness of the method for production control. Adding the calcium carbonate and calcium sulfate crystalline structures to the phase filter is relatively easy. Figures 3 & 4 show the results found for the 10.0 weight percent calcium carbonate and 10.0 weight percent calcium sulfate blended PPS compounds used as validation standards. Even though no calcium carbonate was blended with the calcium sulfate PPS standard, the phase filter picked up a trace residue of calcium carbonate from the hard water used to wash the PPS. The residual trace (difference between the measured and calculated model) at the bottom of these two figures shows the effect of orientation on the calcium sulfate and calcium carbonate phases. Orientation was not corrected in the Rietveld refinement for any of

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Phase Measured Exoected * Low c :r: yst. PPS 61 wt % -60 wt% Non-C yst. PPS 39wt% -40 wt%.non-crystalline PPS t Residual Pattern h., n A 60.0 Figure 1. Rietveld modeling of PPS powder before compounding showing the expected vs. measured crystallinity. Si metal was used to correct for sample displacement. Sample is an injection molded plate..q %. 2 9 / n Phase Added Meas. Total PPS 63.3 wt% 64.1 wt% Glass Fibers 35.0 wt% 34.6 wt% Low. Crystalline PPS Non-crystalline PPS 60.0 Figure 2. Rietveld modeling of a PPS compound blended with 35.0 wt.% calcium silicate glass fibers.

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Sample is an injection molded plate. Phase Added Meas. Total PPS 53.8 % 50.9 % Glass Fibers 35.0 % 38.0 % Anhydrite NA 0.0 % Calcite 10.0 % 10.3 % Other 1.2 % 0.8 % Low Crystalline PPS NA -Not Added Calcium Aluminosilicate / Calcium Carbonate (Calcite) Figure 3. Rietveld modeling of a PPS compound blended with 35.0 wt.% calcium aluminosilicate glass fibers and 10.0 wt.% calcium carbonate I s I 7 J Low Crystalline PPS 1 Sample is an injection molded plate. Phase Added Meas. Total PPS 53.2 % 54.5 % Glass Fibers 35.0 % 35.0 % Anhydrite 10.0 % 9.5 % Calcite NA 0.2 % Other 1.8 % 0.8 % NA - Not Added Calcium Aluminosilicate Residual Pattern 60.0 Figure 4. Rietveld modeling of a PPS compound blended with 35.0 wt.% calcium aluminosilicate glass fibers and 10.0 wt.% calcium sulfate.

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol the diffraction patterns in this study. The analyzed composition shows a good correlation to the blended value due to the averaging of phase orientation over the entire diffraction pattern. Other validation standards were prepared and analyzed using this phase filter. Concentrations of up to 30 weight percent calcium carbonate and calcium sulfate were measured. Figures 5 & 6 are graphs that show the good relationship between the measured and blended amounts of these two mineral fillers over this concentration range. These results show the effectiveness of using Rietveld modeling with phase filters to analyze more than one product line of PPS compounds Measured Calcium Carbonate, wt.% Figure 5. Graph showing the good relationship and blended calcium carbonate concentrations. between the measured Measured Calcium Sulfate, wt.% Figure 6. Graph showing the good relationship and blended calcium sulfate concentrations. between the measured

8 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol CONCLUSIONS Two types of mineral tilled PPS compounds were analyzed using Rietveld modeling with phase filters. The phase filter contained more phases than the PPS compounds would normally contain and consisted of two non-crystalline, one low crystalline and two crystalline phases. The low crystalline and non-crystalline phases were modeled first to minimize the errors that may impact the crystalline phases. Blended PPS compounds were used to validate the phase filter. Results showed the effectiveness of Rietveld modeling for neutralizing orientation effects and handling multiple non-crystalline components simultaneously. For some applications, Rietveld modeling with a designed phase filter can reduce the cost of developing and maintaining x-ray diffraction methodologies. ACKNOWLEDGEMENTS The authors wish to express their appreciation to Phillips Petroleum Company for their support of this prqject. Also, we would like to thank Mark Woods and the Phillips Petroleum Company Plastic Technical Center for preparing the PPS compounds used in this study. REFERENCES 1. Richardson, J. W., Jr., The Rietveld Method, R. A. Young, Ed., Oxford University Press, New York, 105 (1995). 2. RIOAS rietveld analvsis, Operators Manual, Material Data Inc., Berkley, CA (1999). 3. Jenkins, R., Snyder, R. L., Introduction to Powder DifIi-actometrv, John Wiley & Sons, Inc., New York, (1996).