1、Designation: D2238 92 (Reapproved 2012)1Standard Test Methods forAbsorbance of Polyethylene Due to Methyl Groups1at 1378cm1This standard is issued under the fixed designation D2238; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, t
2、he year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1NOTEReapproved with editorial changes in December 2012.1. Scope1.1 These test methods cover measurement by infraredabs
3、orption spectrophotometry of the 1378.4-cm1(7.255-m)band in polyethylene due to methyl groups. (1, 2, 4-7)2Twotest methods are covered:1.1.1 Test Method A uses compensation with a standardsample film or wedge of known methyl content.1.1.2 Test Method B uses compensation with a wedge ofpolymethylene
4、or a polyethylene of known low methyl content.1.2 These test methods are applicable to polyethylenes ofTypes I (density 910 to 925 kg/m3), II (density 926 to 940), andIII (density 941 to 965).NOTE 1For determination of density, see Specifications D1248.NOTE 2In cases of Type III polyethylene with de
5、nsities greater than0.950 g/cm3, different results are obtained with the two test methods.1.3 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.4 This standard does not purport to address all of thesafety concerns, if any, asso
6、ciated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. Specific hazardsstatements are given in Section 7.NOTE 3There is no known ISO equivalent to this st
7、andard.2. Referenced Documents2.1 ASTM Standards:3D618 Practice for Conditioning Plastics for TestingD1248 Specification for Polyethylene Plastics ExtrusionMaterials for Wire and CableD1505 Test Method for Density of Plastics by the Density-Gradient TechniqueD1898 Practice for Sampling of Plastics (
8、Withdrawn 1998)4E131 Terminology Relating to Molecular SpectroscopyE168 Practices for General Techniques of Infrared Quanti-tative AnalysisE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsIEEE/ASTM SI-10 Standard for Use of the InternationalSystem of Units (SI): The Modern Me
9、tric System3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 absorbancethe logarithm to the base 10 of thereciprocal of the internal transmittance:A 5 LOG101/T! 52LOG10T (1)3.2 Units, symbols, and abbreviations used in this testmethod appear in Terminology E131 or Practice IEEE/
10、ASTMSI-10.4. Significance and Use4.1 When interpreted with the aid of appropriate calibrationdata, either test method can be used to compare the total methylcontents of polyethylenes made by similar processes. Suchinformation can be interpreted in terms of specific alkyl groupswith the aid of data o
11、n infrared absorption at certain otherwavelengths (3).NOTE 4The bias of determination of the concentration of total alkylgroups depends on knowing the concentrations of methyl and ethylbranches present, since these branches have anomalously high absorptivi-ties per group at 1378 cm1(7.25 m).4.2 Know
12、ledge of total methyl groups in polyethylene,when combined with data on molecular weight and on reactiveend groups such as vinyl, can lead to assignment of end-groupstructures and can shed light upon polymerization mechanisms.1These test methods are under the jurisdiction of ASTM Committee D20 onPla
13、stics and are the direct responsibility of Subcommittee D20.70 on AnalyticalMethods (Section D20.70.08).Current edition approved Dec. 1, 2012. Published December 2012. Originallyapproved in 1964. Last previous edition approved in 2004 as D2238 - 92 (2004).DOI: 10.1520/D2238-92R12E01.2The boldface nu
14、mbers in parentheses refer to the list of references at the end ofthese test methods.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary
15、page onthe ASTM website.4The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.3 Data on total methyl groups in polyethylene can becorrelated qualitatively
16、 with certain polymer properties such asmelting point, density, stiffness, and other mechanical proper-ties that are closely dependent on the degree of crystallinity ofthe polymer.4.4 These test methods are especially suitable for research.They have not been tested for use in manufacturing control.5
17、. Interferences5.1 Compensation minimizes interference from methylenegroup absorption bands at 1368 cm1(7.31 m) and 1352 cm1(7.39 m) with the 1378-cm1(7.255 m) methyl deformationband.5.2 In Test Method A residual absorption is often present at1352 cm1after compensation, but this band is believed not
18、 tocontribute appreciable interference in the measurement of themethyl peak at 1378.4 cm1in samples with very low methylcontent.6. Apparatus6.1 Infrared Spectrophotometer, double beam, with rock saltprism, and spectral resolution as defined by Condition C in PartIII (Spectral Resolution) of the Prop
19、osed Methods for Evalu-ation of Spectrophotometers, or5,66.2 Fourier Transform Instrument, capable of a spectralresolution of at least 2.0 cm1.6.3 Compression-Molding Press, small, with platens ca-pable of being heated to 170C.76.4 Metal Plates, approximately 150 by 150 by 0.5 mm withsmooth surfaces
20、.6.5 Brass Shims, approximately 75 by 75 mm or larger withan aperture in the center at least 25 by 38 mm in a series of atleast five thicknesses from 0.1 to 0.5 mm.6.6 Micrometer Calipers, with thimble graduations of 0.001mm.86.7 Mounts, for film specimens with aperture at least 6 by 27mm.7. Hazards
21、7.1 Caution must be used during molding to handle the hotplates and molds with appropriate gloves for hand protection.8. Sampling8.1 The polyethylene shall be sampled in accordance withPractice D1898.9. Preparation of Apparatus9.1 The precision obtained using this test method dependsvery markedly up
22、on the condition of the spectrophotometer.Instrument performance shall be at least equal to that cited inthe manufacturers specifications for the new instrument.Resolution shall be checked to assure conformance with 6.1.The linearity of the photometric system shall be measured;linearity shall not de
23、viate from absolute by more than 4 % ofthe transmittance range of interest. Frequency or wavelength inthe 1430 to 1250-cm1(7 to 8-m) region shall be calibrated.NOTE 5For wavelength calibration, it is helpful to record thespectrum of water vapor upon the spectra of the samples.10. Calibration and Sta
24、ndardization10.1 Check the instrument for resolution and wavelengthaccuracy by checking against known wavelengths and absor-bance for methyl absorbance bands in the 2851.4 cm1(3.507m) range.11. Conditioning11.1 ConditioningCondition the test specimens at 23 62C (73.4 6 3.6F) and 50 6 5 % relative hu
25、midity for not lessthan 40 h prior to test in accordance with Procedure A ofPractice D618, for those tests where conditioning is required.In cases of disagreement, the tolerances shall be 61C(61.8F) and 62 % relative humidity.11.2 Test ConditionsConduct tests in the standard labora-tory atmosphere o
26、f 23 6 2C (73.4 6 3.6F) and 50 6 5%relative humidity, unless otherwise specified in the test meth-ods or in this specification. In cases of disagreements, thetolerances shall be 61C (61.8F) and 62 % relative humid-ity.TEST METHOD AMEASUREMENT OF THEABSORBANCE AT 1378 cm1(7.25 m) BY ASTANDARDSAMPLE C
27、OMPENSATION METHOD12. Materials12.1 Aluminum Foil.12.2 Crushed Ice.12.3 Reference Wedge or Films, prepared as described in13.2.1.13. Calibration and Standardization For Test Method A13.1 Calibration of Reference Polymer by a Self-Compensation MethodMold a 0.5 mm film of annealedhigh-density polyethy
28、lene, as well as a series of thinner,shock-cooled films of the same polymer over a range ofthickness from 0.1 to 0.4 mm (Note 6). Measure a series ofdifference spectra, with the annealed film in the sample beamof the spectrophotometer and each shock-cooled film, in turn,in the reference beam. From a
29、 graph of absorptivity of the CH3band maximum at about 1378 cm1(7.25 m) as a function ofabsorptivity at 1304 cm1(7.67 m), obtain a corrected valueof absorptivity at 1378 cm1(7.25 m) as well as the slope ofthe graph.NOTE 6The polyethylene used for preparation of reference filmsshould have very low me
30、thyl group content, preferably less than 0.3 for5Proceedings, ASTM, Vol 58, 1958, p. 472.6The Perkin-Elmer Model 21 Spectrophotometer, Beckman IR-4 and IR-7,Hilger H-800, and Grubb-Parsons GS-2A Spectrophotometers are capable of thisdegree of resolution and have been found satisfactory for this purp
31、ose.7Hydraulic presses that have been found satisfactory for this purpose are madeby Pasadena Hydraulics, Inc., Pasadena, CA, and Fred S. Carver, Inc., Summit, NJ.8Brown and Sharpe micrometer No. 223 RS has been found satisfactory for thispurpose.D2238 92 (2012)12each 100 carbon atoms. Essentially l
32、inear Type III polyethylene withdensity approximately 0.96 g/cm3has been found satisfactory for thispurpose (Note 1).13.2 Procedure:13.2.1 From the reference polyethylene, mold three or fourshock-cooled films about 0.5 mm in thickness and a number offilms with thicknesses varying from 0.1 to 0.4 mm.
33、 The filmsshall be smooth and free of voids. The shock-cooled films maybe prepared in the following way: Place the desired brass shimon the aluminum foil on top of one of the metal plates. Placesufficient polymer in the aperture of the shim to fill completelythis aperture after pressing. Cover the p
34、reparation with asecond aluminum foil and metal plate. Heat the press to 170C.Insert the mold assembly between the press platens. Preheat for15 s, then apply pressure slowly until after 30 s the pressure hasreached 30 000 lb. Hold the preparation at this pressure for anadditional 30 s. Release the p
35、ressure, grasp the assembly withpliers, and quickly plunge it into a bucket containing a slurry ofice and water. Carefully remove the film and dry it with a clothor tissue.13.2.2 Anneal several 0.5-mm shock-cooled films by asuitable press or oven technique to obtain an increase in densityat 23C of a
36、t least 0.020 g/cm3.13.2.3 Mount each film on a suitable holder. Measure thethickness in millimeters at three places in the aperture andrecord the average thickness on the sample holder. Measure thedensity of small clippings made close to but not in the apertureof the holder. Measure the density acc
37、ording to Test MethodD1505.13.2.4 Scan the spectrum between 11 and 13 m and rejectany film showing interference fringes.13.2.5 Measure spectra in the range from 1430 to 1250 cm1(7 to 8 m) and record the absorbance of the 1368-cm1(7.31-m) band on each sample as follows: Place an annealedsample in the
38、 sample beam of the spectrophotometer. Place ashock-cooled film in the reference beam. Set the spectropho-tometer to achieve the resolution specified in 8.1 (Note 7, Note8). Set the speed at 0.1 m/min or 17 cm1/min. Set the gain toproduce overshoot of 1 division (1 % of full scale) at 1368cm1(7.31 m
39、).Adjust the electrical balance so that there is nodrift.Adjust the balance control so that no part of the spectrumbetween 1430 and 1250 cm1(7 and 8 m) has more than 90%transmittance. Adjust the 0 % transmittance. Adjust the ab-scissa scale to at least 75 mm/200 cm1(1.1 m).NOTE 7The 1378-cm1(7.25-m)
40、 methyl band has a half-width lessthan 6 cm1(0.03 m) and is thus very sensitive to slit width in prisminstruments. Close control of slit width is essential for precise measure-ments.NOTE 8For the Perkin-Elmer Model 21 Spectrophotometer, thefollowing settings are generally satisfactory: Response 1 (e
41、lectrical andmechanical), Suppression 2, Resolution 3 (960 program), electricalbalance to give no drift. For the Beckman Model IR-4 or IR-7Spectrophotometer, slits 0.40 mm, gain to give single beam/double beamratio about 1 to 1368 cm1(7.31 m), Response 2 s.13.2.6 Run the spectrum from 1430 to 1250 c
42、m1(7 to 8m) (see Fig. 1).13.2.7 Return to 1430 cm1(7 m) at full speed withoutreleasing the drum or paper lock. Record a spectrum of watervapor on the paper below the polyethylene trace.13.2.8 Repeat 13.2.5, 13.2.6, and 13.2.7 for the remainingshock-cooled films.13.3 Calculations:13.3.1 Draw a base l
43、ine to each curve from 1396 to 1330cm1(7.17 to 7.52 m). Draw a second base line from 1330 to1270 cm1(7.52 to 7.87 m) on each spectrum.FIG. 1 Example of Self-Compensation Spectrum of Type III Polyethylene (Method A)D2238 92 (2012)1313.3.2 Measure the absorbance of the methyl band at 1378.4cm1(7.255 m
44、). Measure the absorbance at 1304 cm1(7.67m).NOTE 9Type I polyethylene may show weak bands between 7.3 and8.0 m which interfere slightly in this measurement.13.3.3 Calculate the following quantities for each pair offilms: 5 A1378/dsts2 drtr!(2) 5 A1304/dsts2 drtr!(3)where:A1378= absorbance at 1378 c
45、m1(7.25 m),A1304= absorbance at 1304 cm1(7.67 m),ds= density of sample film, g/cm3,dr= density of reference film, g/cm3,ts= thickness of sample film, cm, andtr= thickness of reference film, cm.13.3.4 Plot the quantity as ordinate against as abscissaon graph paper for each pair of films.13.3.5 Draw t
46、he best straight line through the points on thegraph (see Fig. 2). The intercept on the ordinate is theabsorptivity, K1378, in square centimeters per gram, due to themethyl band at 1378.4 cm1(7.255 m). Let R, the slope of theline, = /. A value of 0.5 for R at 1378.4 cm1is recom-mended.NOTE 10Provide
47、d that the 1378.4-cm1(7.255-m) band is normal,the proportionality factor R for the correction is the same for allpolyethylenes.NOTE 11The center of the 1378-cm1(7.25-m) methyl band isnormally found between 1379.3 and 1378.0 cm1(7.250 and 7.257 m).Variation in position of the band center within this
48、range will give a valueof R between about 0.40 and 0.52, in direct proportionality.13.3.6 For each reference film, correct the measured absor-bance at 1378.4 cm1, A1378, for absorption by amorphousbands at the same frequency by use of the equationA1378due to methyl groups! 5 A13782 R 3A1304(4)Record
49、 R and the corresponding A1378(due to methylgroups) on each reference film mount.13.4 Factors:13.4.1 Factors, f1378, in methyl groups for each 100 carbonatoms, in grams per square centimeter, are used to convertabsorptivity data to methyl groups for each 100 carbon atomsby the following relationship:Methyl groupscalculated as methyl in alkyl groups.C3!5 (5)f1378 .C3!3K137813.4.2 The conversion factors, f1378, must be determined foreach spectrophotometer.NOTE 12
