应用文献——聚乙烯氧化诱导时间参考资料

RECERTIFICATION OF THE POLYETHYLENEOXIDATION INDUCTION TIME REFERENCE MATERIAL

*Roger L.Blaine, Ph.D.

TA Instruments,Inc., 109 Lukens Drive, New Castle, DE 19720, USA

and

Alan T. Riga,Ph.D.

TechCon, Inc.,6325 Aldenham Drive, Cleveland, OH 44143, USA

*correspondingauthor

ABSTRACT

Acommercial Oxidation Induction Time (OIT) reference material polyethylene filmis retested six years after its original certification. Using ASTMInternational Standard E1858, the mean value, within laboratory repeatabilityand between laboratory reproducibility were unchanged at 29.3 min, 1.7 and 2.1min, respectively. In addition the new results are reported on aninterlaboratory test for Oxidation Onset Temperature (OOT) for the same materialusing Method A (oxygen purge gas) and C (air purge gas) of ASTM Standard E2009.The mean values are found to be 236.8 and 245.0 °C under oxygen and air,respectively. The within laboratory repeatability standard deviation is 1.1 and0.68 °C, and the between laboratory reproducibility standard deviation is 1.3and 1.4 °C, respectively, for oxygen and air.

INTRODUCTION

In1996, a well-characterized polyethylene sample was proposed as a reference materialfor Oxidation Induction Time (OIT) determinations (1). Subsequently, this OIT ReferenceMaterial was commercialized by TA instruments, New Castle, DE, (part number900319.901) and is in widespread use as a diagnostic and research tool for polyethyleneperformance and the OIT test method. The reference material was used in a numberof interlaboratory test programs including those for ASTM standards E1858 and E2009(2,3). The sample was also examined under a wide variety of conditions by LeconWoo and co-workers at Baxter Healthcare (4, 5, 6).

Inthe original 1996 paper, it was suspicioned that the OIT value of the material wasdecreasing with time due to the reduction in the antioxidant package throughslow “leaching”out. According to the estimates in the early work of Blaine and coworkers, theOIT value should have declined to about 25 minutes by 2002 if the earlier trendcontinued. This was predicted from a series of test programs conducted between1991 and 1995, the data for which is presented in Figure 1. The decrease in OITvalue was uncertain, however, as the trend could be attributable to normalexperimental scatter.

Inan attempt to stabilize the OIT value of the film in its preparation for commercialization,two sheets of the polyethylene film reference material were placed into anenvelope composed of the same polyethylene film. It was thought that the two innersheets of film would then be protected from leaching by the sacrificialenvelope of the same material. The envelope was then placed into a seconddarkened and opaque, brown polyethylene envelope to serve as a light shield andlabeled container.

Thispresent work was undertaken six years after the original certification in an attemptto determine the effectiveness of the stabilization process and to recertifythe value for the OIT Reference Material.

Figure1 – Projected Decay of OIT with Time

EXPERIMENTAL

Allwork was carried out on model Q1000 and 2920 Differential Scanning Calorimetersprovided by TA Instruments. All DSC’s were equipped with auto samplers forprecise sample placement. One each of the Q1000’s was fitted with the FinnedAir Cooling System (FACS), the Refrigerated Cooling System (RCS) and LiquidNitrogen Cooling System (LNCS). All instruments were temperature calibratedwith indium at a heating rate of 10 °C/min according the ASTM InternationalStandard E967 (7). Temperature calibration was then re-performed at 1 °C/min toapproximate the isothermal temperature condition as described in E1858.

5mg pieces of the OIT reference materials were cut from the original film sheet usinga 6.3 mm paper punch. These sample disks were then placed in open DSC plans previouslycleaned in toluene and dichloromethane. OIT was determined using method E1858where the specimen is loaded at ambient temperature and then heated at 20 °C/minto the isothermal test temperature in inert nitrogen. The sample is held atthis test temperature for 5 minutes. The purge gas is then switched fromnitrogen to oxygen at 50 mL/min and the elapsed time clock is set to zero. Thetime to the onset of oxidation is then measured and reported as OIT in minutes.The sample temperature is recorded 5 minutes into this method segment. All workwas carried out at an isothermal test temperature of 200 °C with a minimum of10 replicates from each laboratory resulting in 27 degrees of experimentalfreedom. The interlaboratory results are presented in Table 1.

Table1 – Interlaboratory Oxidation Induction Time Test Results

RESULTS ANDDISCUSSION

Theresults from the four laboratories were statistically treated using ASTM MethodE691 (8). The mean value was 29.3 minutes with a within laboratory repeatabilitystandard deviation of the ± 1.7 minutes and a between laboratory reproducibilitystandard deviation of ± 2.1 minutes. These values are compared to the 1995 testdate in Table 2. The mean values are different by about 3 % but this is not consideredsignificant based upon the precision of the measurement as evaluated by the Student’sT-test. A comparison of the results shows that the repeatability and reproducibilitystandard deviations are within the statistical limits at the 95 % confidence limitaccording to the statistical F-test.

Thusthe steps taken to stabilize the condition of the OIT material appear successfuland the material may be regarded as unchanged over the 5-year period since theoriginal work.

Table2 – Oxidation Induction Time Comparative Test Results

OXIDATION ONSETTEMPERTURE (OOT)

Asecond set of measurements was made on the OIT Reference Materials – that of theOxidation Onset Temperature (OOT). While the Oxidation Induction Time test is anisothermal time-to-event test, the test for Oxidation Onset Temperature test isa dynamic heating rate test. According to ASTM International Standard E2009,the test specimen is heated from ambient temperature at 10 °C/min in anoxidizing atmosphere. The (extrapolated onset) temperature at which the testspecimen begins to oxidize is taken as the OOT value. Differences in OOT valuemay be used to rank-order dramatic changes (such as different antioxidantpackages) while the companion isothermal OIT test may be used to evaluate themore subtle lot-to-lot variations of a particular formulation.

EXPERIMENTAL

Twointerlaboratory test (ILT) programs were carried out in 2001 to obtain within laboratoryrepeatability and between laboratory reproducibility for the determination of E2009Oxidation Onset Temperature. One study used oxygen as a reactive purge gas (MethodA) and the other used air (Method C). The Oxidation Induction Time Reference Materialwas used as the test specimen in these studies. The results of these ILTs addto the list of reference values for this material.

Inthe first study seven laboratories, using four instrument models from a single instrumentmanufacturer (TA Instruments), determined the OOT value in oxygen in hextuplicateusing E2009 Method A. In the second study six laboratories using four instrumentmodels from a single manufacturer, determined the OOT value in air in heptuplicateusing E2009 Method C. The mean value, repeatability and reproducibility standarddeviation for OOT for the OIT Reference Material are presented in Table 3 with 25and 30 degrees of experimental freedom for oxygen and air purge gases,respectively.

Table3 – Oxidation Onset Temperature Test Results

CONCLUSIONS

TheOxidation Induction Time Reference material available from TA Instruments isstable over the six-year period since its original certification. Thus theexperimental value originally provided and added to in the interim should beconsidered valid. Additionally, Oxidation Onset Temperature values in oxygenand air are added to the certificate for the material.

REFERENCES

1.R. L. Blaine and M. B. Harris, “A Proposed Reference Material for Oxidative InductionTime by Differential Scanning Calorimetry”, Oxidative Behavior of Materials byThermal Analytical Techniques, A. T. Riga and G. H. Patterson (Eds.), SpecialTechnical Publication 1326, American Society for Testing and materials, WestConshohocken, PA, 1997, pp. 193-204.

2.E1858, “Method for Determining Oxidation Induction Time of Hydrocarbons by DifferentialScanning Calorimetry”, American Society for Testing and Materials, WestConshohocken, PA

3.E2009, “Oxidation Onset Temperature of Hydrocarbon by Differential Scanning Calorimetry”,American Society for Testing and Materials, West Conshohocken, PA.

4.S. Y. Ding, M. T. K. Ling, A. R. Khare, L. Woo, “Durability of a Reference MaterialOver Diverse Conditions”, Thermochimica Acta, 2000, 357-358, pp. 147-153.

5.L. Woo, Y. S. Yank, M. T. K. Ling, C. Qin, A Khare, “Flow, Pressure, TemperatureDependence of Oxidation Induction Time Measurements on a Heat Flow DSC”, Proc.56th Ann. Tech. Conf, Soc. Plast. Eng., 1998, 2, pp. 1986-1990.

6.L. Woo, A. R. Khare, C. L. Sandford, M. T. K Ling, S. Y. Ding, “Relevance of HighTemperature Oxidative Stability Testing to Long Term Polymer Durability”, Journalof Thermal Analysis and Calorimetry, 2001, 64, pp. 539-548.

7.E967, “Temperature Calibration of Differential Scanning Calorimeters and DifferentialThermal Analyzers”, American Society for Testing and Materials, WestConshohocken, PA.

8.E691, “Conducting an Interlaboratory Study to Determine the Precision of a TestMethod”, American Society for Testing and Materials, West Conshohocken, PA.

KEYWORDS

differentialscanning calorimeter, polyolefins, oxidative stability, thermoplastic

polymers