1、Designation: D 2238 92 (Reapproved 2004)Standard Test Methods forAbsorbance of Polyethylene Due to Methyl Groups1at 1378cm1This standard is issued under the fixed designation D 2238; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 These test methods cover measurement by infraredabsorption spectrophotometry of the 1378.4-cm1(7.255-m)ba
3、nd 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 or a polyethylene of known low methyl content.1.2 Thes
4、e 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 D 1248.NOTE 2In cases of Type III polyethylene with densities greater than0.950 g/cm3, different results ar
5、e obtained with the two test methods.1.3 The values stated in SI units are to be regarded as thestandard. The values given in brackets are for information only.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the use
6、r 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 similar or equivalent ISO standard covering thesubject matter of these test methods.2.
7、 Referenced Documents2.1 ASTM Standards:3D 618 Practice for Conditioning Plastics for TestingD 1248 Specification for Polyethylene Plastics Molding andExtrusion MaterialsD 1505 Test Method for Density of Plastics by the Density-Gradient TechniqueD 1898 Practice for Sampling of PlasticsE 131 Terminol
8、ogy Relating to Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Quanti-tative AnalysisE 177 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 Metric System3. Terminology3.
9、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 E 131 or Practice E 380.4. Significance and Use4.
10、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 on infrared absorption at certain ot
11、herwavelengths (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 Knowledge of total methyl groups in pol
12、yethylene,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.4.3 Data on total methyl groups in polyethylene can becorrelated qualitatively with certain polymer propertie
13、s 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.1These test methods are under th
14、e jurisdiction of ASTM Committee D20 onPlastics and are the direct responsibility of Subcommittee D20.70 on AnalyticalMethods (Section D20.70.08).Current edition approved July 1, 2004. Published July 2004. Originally approvedin 1964. Last previous edition approved in 1999 as D 2238 - 92 (1999).2The
15、boldface numbers 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 Docume
16、nt Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Interferences5.1 Compensation minimizes interference from methylenegroup absorption bands at 1368 cm1(7.31 m) and 1352 cm1(7.39 m) with the 1378-cm
17、1(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 tocontribute appreciable interference in the measurement of themethyl peak at 1378.4 cm1in samples with very low methylcontent.6. Apparatus6.1 Infra
18、red Spectrophotometer, double beam, with rocksalt prism, and spectral resolution as defined by Condition C inPart III (Spectral Resolution) of the Proposed Methods forEvaluation of Spectrophotometers, or4,56.2 Fourier Transform Instrument, capable of a spectralresolution of at least 2.0 cm1.6.3 Comp
19、ression-Molding Press, small, with platens ca-pable of being heated to 170C.66.4 Metal Plates, approximately 150 by 150 by 0.5 mm withsmooth surfaces.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
20、.1 to 0.5 mm.6.6 Micrometer Calipers, with thimble graduations of 0.001mm.76.7 Mounts, for film specimens with aperture at least 6 by 27mm.7. Hazards7.1 Caution must be used during molding to handle the hotplatens and molds with appropriate gloves for hand protection.8. Sampling8.1 The polyethylene
21、shall be sampled in accordance withPractice D 1898.9. Preparation of Apparatus9.1 The precision obtained using this test method dependsvery markedly upon the condition of the spectrophotometer.Instrument performance should be at least equal to that cited inthe manufacturers specifications for the ne
22、w instrument.Resolution should be checked to assure conformance with 6.1.The linearity of the photometric system should be measured;linearity should not deviate from absolute by more than 4 % ofthe transmittance range of interest. Frequency or wavelength inthe 1430 to 1250-cm1(7 to 8-m) region shoul
23、d be calibrated.NOTE 5For wavelength calibration, it is helpful to record the spec-trum of water vapor upon the spectra of the samples.10. Calibration and Standardization10.1 Check the instrument for resolution and wavelengthaccuracy by checking against known wavelengths and absor-bance for methyl a
24、bsorbance 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 humidity for not lessthan 40 h prior to test in accordance with Procedure A ofPractice D 618, for those tests where conditioning is required.In ca
25、ses of disagreement, the tolerances shall be 61C61.8F and 62 % relative humidity.11.2 Test ConditionsConduct tests in the standard labora-tory atmosphere of 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 disagre
26、ements, thetolerances shall be 61C 61.8F and 62 % relative humid-ity.TEST METHOD AMEASUREMENT OF THEABSORBANCE AT 1378 cm1(7.25 m) BY ASTANDARDSAMPLE COMPENSATION METHOD12. Materials12.1 Aluminum Foil.12.2 Crushed Ice.12.3 Reference Wedge or Films, prepared as described in13.2.1.13. Calibration and
27、Standardization For Test Method A13.1 Calibration of Reference Polymer by a Self-Compensation MethodMold a 0.5 mm film of annealedhigh-density polyethylene, 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 of
28、difference spectra, with the annealed film in the sample beamof the spectrophotometer and each shock-cooled film, in turn,in the reference beam. From a 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
29、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 methyl group content, preferably less than 0.3 foreach 100 carbon atoms. Essentially linear Type III polyethylene withdensity approximately 0.96 g/cm3h
30、as 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. The filmsshall be smooth and free of voids. The shock-cooled fil
31、ms 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 preparation with a4Proceedings, ASTM, Vol 58, 1958, p. 472.5The Pe
32、rkin-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 purpose.6Hydraulic presses that have been found satisfactory for this purpose are madeby Pasadena Hy
33、draulics, Inc., Pasadena, CA, and Fred S. Carver, Inc., Summit, NJ.7Brown and Sharpe micrometer No. 223 RS has been found satisfactory for thispurpose.D 2238 92 (2004)2second aluminum foil and metal plate. Heat the press to 170C.Insert the mold assembly between the press platens. Preheat for15 s, th
34、en 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 pressure, grasp the assembly withpliers, and quickly plunge it into a bucket containing a slurry ofice and water. Carefully remove the film and dry it
35、 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 at 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 av
36、erage thickness on the sample holder. Measure thedensity of small clippings made close to but not in the apertureof the holder. Measure the density according to Test MethodD 1505.13.2.4 Scan the spectrum between 11 and 13 m and rejectany film showing interference fringes.13.2.5 Measure spectra in th
37、e 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 sample beam of the spectrophotometer. Place ashock-cooled film in the reference beam. Set the spectropho-tometer to achieve the resolution specifie
38、d 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 1368 cm1 (7.31 m). 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 a
39、nd 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) 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 i
40、s essential for precise measure-ments.NOTE 8For the Perkin-Elmer Model 21 Spectrophotometer, the fol-lowing settings are generally satisfactory: Response 1 (electrical andmechanical), Suppression 2, Resolution 3 (960 program), electricalbalance to give no drift. For the Beckman Model IR-4 or IR-7 Sp
41、ectro-photometer, slits 0.40 mm, gain to give single beam/double beam ratioabout 1 to 1368 cm1(7.31 m), Response 2 s.13.2.6 Run the spectrum from 1430 to 1250 cm1(7 to 8 m)(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 watervap
42、or 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 line 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.13.3.2
43、 Measure the absorbance of the methyl band at 1378.4cm1(7.255 m). 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:a5A1378/dsts2 dr
44、tr! (2)b5A1304/dsts2 drtr! (3)where:A1378= absorbance at 1378 cm1(7.25 m),A1304= absorbance at 1304 cm1(7.67 m),FIG. 1 Example of Self-Compensation Spectrum of Type III Polyethylene (Method A)D 2238 92 (2004)3ds= density of sample film, g/cm3,dr= density of reference film, g/cm3,ts= thickness of sam
45、ple film, cm, andtr= thickness of reference film, cm.13.3.4 Plot the quantity a as ordinate against b as abscissaon graph paper for each pair of films.13.3.5 Draw the best straight line through the points on thegraph (see Fig. 2). The intercept on the ordinate is theabsorptivity, K81378, in square c
46、entimeters per gram, due to themethyl band at 1378.4 cm1(7.255 m). Let R, the slope of theline, = Da/Db. A value of 0.5 for R at 1378.4 cm1isrecommended.NOTE 10Provided that the 1378.4-cm1(7.255-m) band is normal,the proportionality factor R for the correction is the same for allpolyethylenes.NOTE 1
47、1The 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 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 measur
48、ed absor-bance at 1378.4 cm1, A1378, for absorption by amorphousbands at the same frequency by use of the equationA1378due to methyl groups!5A13782 R3 A1304(4)Record R and the corresponding A1378(due to methylgroups) on each reference film mount.13.4 Factors:13.4.1 Factors, f1378, in methyl groups f
49、or 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 groups calculated as methyl in alkyl groups . C3! 5f1378 .C3!3 K81378(5)13.4.2 The conversion factors, f1378, must be determined foreach spectrophotometer.NOTE 12In the calculations, the Beer-Lambert law is assumed toapply, that is, the absorptivity at 1378 cm1(7.25 m) is proportional tothe concentr
