Quantification of Polymer Content and Molecular Weight in Modified Bitumen Samples by GPC. Clay T. Enos William F. Gorman

Transcription

1 Quantification of Polymer Content and Molecular Weight in Modified Bitumen Samples by GPC Clay T. Enos William F. Gorman Firestone Building Products Company 525 Congressional Blvd Carmel, IN ABSTRACT High temperature GPC is used to quantify the amount of atactic and isotactic polypropylene contained in modified bitumen roofing formulations. In addition to the total amount of polymer present in the formulation, the molecular weight distribution and ratio of APP to IPP can be extracted by data manipulations with commercially available peak deconvolution software. 347

2 GPC Quant pg 2 INTRODUCTION This paper presents the results of a research project in which high temperature GPC was used to quantify the amount of atactic (APP) and isotactic (IPP) polypropylene used in modified bitumen roofing formulations. Firestone's building products division deals with various roofing systems, including EPDM, thermoplastic membranes, roofing insulation, and modified bitumen. Bitumen, another term for asphalt, is derived primarily from the still bottoms of the oil conversion process. It is most often sold in viscosity grades designated as AC5, AC10, AC20, and so forth. It is a complicated material consisting of many organic compounds and some inorganic ones (usually less than 1%). The way a particular asphalt performs in any given application is very dependent on the particular oil field from which it was derived. At high temperatures bitumen shows good viscous flow, while at lower temperatures it becomes a brittle solid. Consequently, asphalt has limited utility by itself, especially for roofing applications where it must not run off a sloped roof in hot weather, or embrittle and crack in cold weather. As a result, asphalt is usually modified by incorporation of additives. Fillers, such as calcium carbonate, talc, or even glass beads, are often added to reduce the overall cost of a formulation. Fillers such as talc or calcium carbonate sometimes give the additional benefit of absorbing excess oils that may exist in some asphalts. Flame retardants, such as aluminum hydroxide and calcium borate, are added in applications where this property is required. Polymers, such as SBS or a combination of APP & IPP, are added to extend the useful temperature range of the asphalt and to give the product structural integrity. SBS offers advantages where elasticity is required (e.g., an application where the building might be expected to undergo lots of movement). The APP/IPP membranes offer advantages in UV stability since they don't have the high levels of unsaturation that the SBS membranes contain. Irrespective of the polymer type, the influence of the overall level of the polymer on the performance of the final bitumen product is substantial. For the APPflPP formulations, the APP imparts the low temperature flexural properties while the IPP provides the high temperature stability. Consequently it is important to have not only the correct overall polymer content, but also the correct ration of APP to IPP. 348

3 GPCQuant pg 3 Since the polymer level is important, the problem is one of how to quantify it. Though solvent extraction with various filtration steps has been shown capable of separating the various components 1, it is extremely laborious and difficult to accomplish with the precision our laboratory was required to achieve. Since GPC has been used successfully to determine the i amount of polymer in SBS formulations 2, we decided to try this technique for APP/IPP [ samples as well. i EXPERIMENTAL I with All of TCB the polymer/asphalt solvent. The separations four columnwere bank done consisted with of a Waters microstyragel 150C GPC HT 103, operated 104, at 105, 145"C and with a flow rate of 1.0 ml:minute. Sample concentrations were in the 2 mg/ml range and I were filtered through the stainless steel frit system that comes with the instrument. The oxidized asphalt separations were performed in chloroform at 40"C with a similar column bank as above. The software package used for deconvolution was the Windows 95 version of PeakFit from Jandel Scientific. DISCUSSION Figure 1 shows a separation for an SBS/Asphalt mixture obtained with our GPC system. For the case of SBS a quantitative GPC analysis is straight forward since the polymer peak(s) at a retention time near 32mls is well separated from the asphalt peaks of much lower molecular weight. In fact this analysis is easily accomplished in THF rather than TCB since SBS is soluble in THF. Examination of the asphaltic section of the chromatogram displayed in Figure 1 reveals the complex nature of the asphalt. The sharp negative drop in the center of the asphaltic region suggests the chemical composition is different for this molecular weight range, with a refractive index on the opposite side of the solvent as that of the SBS. This suggests that this molecular weight region may have a substantial amount of non-aromatic compounds, but additional work is needed to be able to draw definite conclusions. Typical GPC chromatograms for IPP and APP are shown in Figure 2, and the separation of a mixture of the two polypropylenes with an AC5 asphalt is shown as Figure 3. Obviously a 349

4 _._... "_ GPCQuant pg 4 traditional GPC analysis is difficult because of the severe overlap of the APP with higher molecular weight asphaltic components. Our laboratory had recently discovered a software package entitled PeakFit by Jandel Scientific. The Windows 95 version contains powerful peak fitting algorithms which allow relatively strai_ht forward peak deconvolution. We decided to use this software in an attempt to deconvolute chromatograms similar to that of Figure 3 into its APP and IPP components. Since both of these polymers have nearly the same drgdc in TCB, simple addition of the two peak areas would allow calculation of the total polymer content, while the ratio of the individual peak areas would give the APP/IPP ratio. We initially tried the software with binary mixtures of APP and IPP, three of which have their chromatograms displayed in Figure 4. Both the APP and IPP were adequately fitted by a 4 parameter log normal distribution. Using the 40% IPP sample as an example, Figure 5 displays the chromatogram of the mixture and the results of the Peak.Fit software deconvolution. Figure 6 is a display of the residuals of the deconvolution showing that the model specified (2 log-normal distributions) was a good one. Table I gives a summary for the various mixtures and the data shows good precision and accuracy. The chromatogram for a low level polymer content sample is shown in Figure 7. The PcakFit analysis resulted in four peaks being found: two for the polymers and two for the asphalt. In order to be able to quantify the total level of polymer, it was necessary to calibrate the refractometer. The calibration curve which resulted from preparing various polymer/asphalt standards is shown in Figure 8. Using this calibration along with the results from PeakFit generated the data shown in Table 2. Again both the precision and the accuracy are good. The Chromatograms in Figure 4 suggest that the amount of IPP in an APP/IPP mixture could be determined by monitoring the M wor M z averages of the total distribution. Figure 9 shows a plot of M_, vs IPP content. We have used this procedure to successfully determine the ratio of APP to IPP. Table 3 shows some examples along with results from a DSC procedure for comparison. Though this procedure worked fairly well, there are a couple of drawbacks as well. First, the refractometer must constantly recalibrated in order to get the accuracy required in our analysis. The second problem is that the asphalt molecular weight distribution changes as it is oxidized (Figure 10). A third problem is that the molecular weight distribution of the polypropy lene'_

5 m_ast no(deviate dramatically from the standards used to calibrate the system. These problems make the analysis of competitive and aged materials tenuous. Also, the technique usually requires running five replicates to obtain the required precision. CONCLUSIONS High temperature GPC analysis of APP/IPP/Asphalt mixtures combined with peak deconvolution allows the determination of total polymer content, polymer molecular weight distribution, and the ratio of APP/APP. Although constant calibration of the refractometer and the running of five replicates makes the procedure a little tedious, it is still much less laborious than solvent extraction/filtration procedures and it does provide good precision. RFERENCES 1. F. Choquet, E. Ista, and P. Cogneau, International Symposium of Roofing Technology, 1991, pg J. Wei, J.C. Shull, M.C. Hawley, and J. Barak, International GPC Symposium Proceedings, 1994, pg

6 GPCQuant pg 6 4 Table 1. Results For APP/IPP Mixtures 1 1 Known Found,, Std. Dev Table 2. Precision Study Known Found Std. Dev. I I I I I Table 3 % IPP From Wt. Avg MW Calibration Known GPC DSC I II I I II II