D2765-16.pdf

Designation:D276516Standard Test Methods forDetermination of Gel Content and Swell Ratio ofCrosslinked Ethylene Plastics1This standard is issued under the fixed designation D2765;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the 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.1.Scope*1.1 The gel content(insoluble fraction)produced in ethyl-ene plastics by crosslinking is determined by extracting withsolvents such as decahydronaphthalene or xylenes.The meth-ods described herein are applicable to crosslinked ethyleneplastics of all densities,including those containing fillers,andall provide corrections for the inert fillers present in some ofthose compounds.1.2 Test MethodA,which permits most complete extractionin least time,is to be used for referee tests,but two alternativenonreferee Test Methods B and C are also described.TestMethod B differs from the referee test method only in samplepreparation;that is,it requires use of shavings taken at selectedpoints in cable insulation,for example,rather than the groundsample required by the referee test method.Because the shavedparticles are larger,less total surface per sample is exposed tothe extractant,so this test method ordinarily yields extractionvalues about 1 to 2%lower than the referee method.TestMethod C requires that a specimen in one piece be extracted inxylenes at a constant temperature of 110C.At this temperatureand with a one-piece specimen,even less extraction occurs(from 3 to 9%less than the referee test method),this methodpermits swell ratio(a measure of the degree of crosslinking inthe gel phase)be determined.1.3 Extraction tests are made on articles of any shape.Theyhave been particularly useful for electrical insulations sincespecimens can be selected from those portions of the insulationmost susceptible to insufficient crosslinking.1.4 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard.The values stated ineach system may not be exact equivalents;therefore,eachsystem shall be used independently of the other.Combiningvalues from the two systems may result in non-conformancewith the standard.NOTE1This test method is equivalent to ISO 10147,Method B.It isnot equivalent to ISO 10147 in any other measurement or section.1.5 This standard does not purport to address all of thesafety concerns,if any,associated 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 precau-tionary statements are given in Sections 6,9,and 24.2.Referenced Documents2.1 ASTM Standards:2D297 Test Methods for Rubber ProductsChemical Analy-sisD618 Practice for Conditioning Plastics for TestingD883 Terminology Relating to PlasticsD1603 Test Method for Carbon Black Content in OlefinPlasticsD1998 Specification for Polyethylene Upright Storage TanksD3351 Test Method for Gel Count of Plastic Film(With-drawn 2000)3E691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method2.2 ISO Standard:ISO 10147 Pipes and Fittings Made of Crosslinked Poly-ethylene(Pe-X)Estimation of the Degree of Crosslink-ing by Determination of the Gel Content3.Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 gel contentthe percentage by mass of polymerinsoluble in a specified solvent after extraction under thespecified conditions.3.1.2 swell ratiothe ratio of the gel volume in the swollenstate to its volume in the unswollen state.3.2 Terms as shown in Terminology D883 are applicable tothis test method.1These test methods are under the jurisdiction of ASTM Committee D20 onPlastics and are the direct responsibility of Subcommittee D20.15 on ThermoplasticMaterials.Current edition approved Sept.1,2016.Published September 1,2016.Originallyapproved in 1968.Last previous edition approved in 2011 as D2765 11.DOI:10.1520/D2765-16.2For 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 page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.*A Summary of Changes section appears at the end of this standardCopyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 4.Summary of Test Methods4.1 Specimens of the crosslinked ethylene plastic areweighed and then immersed in the extracting solvent at thetemperature specified by the procedure selected and for thetime designated by that procedure.After extraction,the speci-mens are removed,dried,and reweighed as directed.Theamount of material extracted is calculated and,if desired,swellratio determined by the means described in alternative TestMethod C.5.Significance and Use5.1 Many important properties of crosslinked ethylene plas-tics vary with the gel content.Hence,determination of the gelcontent provides a means of both controlling the process andrating the quality of finished products.5.2 Extraction tests permit verification of the proper gelcontent of any given crosslinked ethylene plastic and they alsopermit comparison between different crosslinked ethyleneplastics,including those containing fillers,provided that,forthe latter,the following conditions are met:5.2.1 The filler is not soluble in either decahydronaphtha-lene or xylenes at the extraction temperature.5.2.2 The amount of filler present in the compound either isknown or will be determined by other means.5.2.3 Sufficient crosslinking has been achieved to preventmigration of filler during the extraction.Usually it has beenfound that,at extraction levels up to 50%,the extractantremains clear and free of filler.5.3 A suitable antioxidant is added to the extractant toinhibit possible oxidative degradation at the extraction tem-peratures.5.4 Before proceeding with this test method,reference shallbe made to the specification of the material being tested.Anytest specimen preparation,conditioning,dimensions,or testingparameters,or a combination thereof,covered in the materialsspecification shall take precedence over those mentioned in thistest method.If there are no material specifications,then thedefault conditions apply.6.Precautions6.1 This test method measures a much larger three dimensional polymer network and is different than that mea-sured by Test Method D3351.6.2 It has been reported that ultra-high molecular weightpolyethylene fails to completely dissolve in this procedure attimes.6.3 Atmospheric pressure affects the boiling temperature ofsolvents.It is possible that laboratories performing these testmethods at altitudes greater than 1500 m(5000 ft)above sealevel will observe lower boiling temperatures for the solventused.If this is observed it shall be noted in the results report.TEST METHOD A(REFEREE TEST METHOD)7.Apparatus7.1 The extraction apparatus shall be of the followinggeneral type,as illustrated in Fig.1:7.1.1 Round-Bottom Flask,equipped with a suitable jointthat has been demonstrated to provide an essentially leak freeconnection.For one or two determinations at one time,a500-mL flask is appropriate.For several determinations at onetime,but not exceeding six,a 2000-mL flask is suitable.7.1.2 Heating Mantle to fit the flask and with sufficientheating capacity to boil decahydronaphthalene.7.1.3 Reflux Condenser with ground-glass or cork joint to fitinto flask.7.1.4 Ring Stand and Appropriate Clamps.7.2 Grinding Equipment,suitable for reducing the sample toa fineness between 30 and 60 mesh.A bench-top laboratorymill is satisfactory,although any procedure which will producea sample of the required fineness without generating excessiveheat is acceptable.7.2.1 U.S.No.30 and U.S.No.60 Sieves.7.2.2 U.S.No.120 Stainless Steel Wire Cloth.7.3 Vacuum Oven,with vacuum source capable of creatinga vacuum of at least 710 mm(28 in.)Hg and equipped with athermometer capable of measuring to 150C.7.4 Analytical Balance,capable of weighing to 0.0001 g.8.Reagents8.1 Decahydronaphthalene(Decalin),anhydrous,boilingpoint 189 to 191C.8.2 Xylenes,ACS reagent grade,boiling point 138 to 141C.8.3 Suitable stabilizers for the solvent include:8.3.1 2,2-methylene-bis(4-methyl-6-tertiary butyl phenol)(CAS#119-47-1)8.3.2 Tetrakis-(methylene-(3,5-di-(tert)-butyl-4-hydrocinna-mate)methane(CAS#6683-19-8)8.3.3 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxy-benzyl)benzene(CAS#1709-70-2)8.3.4 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1h,3h,5h)-trione(CAS#27676-62-6)9.Safety Precautions9.1 Xylenes and decahydronaphthalene are toxic and flam-mable solvents and shall be handled carefully.Use only in aventilated hood.Check the effectiveness of the hood beforestarting the tests.Do not inhale the vapors.Excessive inhala-tion of the vapors has been known to cause dizziness orheadache,or both.In the event of excessive inhalation,seekfresh clean air.10.Test Specimens10.1 At least two specimens each containing 0.300 6 0.015g of ground polymer weighed to the nearest 0.0001 g shall betested.10.2 The test samples shall be ground to a fineness that willpass through a 30-mesh sieve.Shake this sieved material overa 60-mesh sieve and reject any material that passes through.10.3 Test specimens originating from insulation cut fromwire shall be homogeneous and consist of an equal mixture ofsamples representative of insulation nearest to the conductorand samples representative of insulation nearest the outerD2765 162 periphery of the construction unless specific portions of theinsulation are being examined for insufficient crosslinking.11.Procedure11.1 Prepare a specimen holder by cutting a piece of120-mesh stainless steel cloth measuring approximately 80 by40 mm(3 by 112in.).Fold this to form a square measuringapproximately 40 mm(112in.).Fold two sides of this squareclosed by folding the cloth at the edges about 6 to 7 mm(14in.)and stapling those folds.In that manner,a pouch open at thetop is obtained.Weigh this pouch(W1).11.2 Place the ground and screened sample in the weighedpouch.Weigh the pouch and specimen(W2)to the nearest0.0001 g.Fold over the open side of the pouch and staple theedge to form a cage.Weigh the cage and sample(W3).NOTE2Compaction of specimens has been observed to negativelyaffect the repeatability of this method.Avoid compaction of the specimensand the specimen cages.11.3 Fill a round-bottom flask as described in 7.1.1 withenough solvent to immerse the 120-mesh cage and samplecompletely during extraction.Dissolve 1%an appropriateantioxidant(8.3)in the decahydronaphthalene or xylenes.NOTE3A volume of 350-mL in a 500-mL flask or 1000-mL in a2000-mL flask has been found to be sufficient.11.4 Boil the decahydronaphthalene or xylenes vigorouslyenough to ensure good agitation of the solution.Twenty to 40drops/min from the condenser,while using 1000 g of solvent,is a satisfactory boiling rate.Suspend the cage and specimen inthe solvent so that the bottom edge of the cage is almosttouching the bottom of the flask.A small wire,attached to theAIdentification tag and fine wire attached to cage.BReflux condenser.CRing stand clamp.DWater inlet.EJoint.FLarge-mouth round-bottom flask.GVariable transformer.HWater outlet.IRing stand.JDecahydronaphthalene or xylene.KHeating mantle.L120-mesh wire cage containing the specimen.NOTE1If a slightly higher degree of accuracy is desired(about 1 to 2%)a modified Soxhlet Extractor may be employed wherein the specimen isheld in an extraction thimble.The extraction thimble,with extra-coarse fritted glass disk sealed in,will have 45-mm body diameter,130-mm height orequivalent(height must subsequently be cut to 75 mm).The fritted disk is extra-coarse with fused edge.Glass wool 13 to 19 mm thick(12to34in.)willbe placed on the bottom of the extraction thimble to support one gram 30-60 mesh ground sample which is covered with 13 to 19 mm thick layer of glasswool,a fritted glass disk and a small glass weight.The extraction thimble assembly rests upon the 55-mm portion of the thimble previously cut off,thelatter inserted into the modified Soxhlet Extractor.FIG.1 Extraction ApparatusD2765 163 cage and extending through the reflux condenser has been usedfor suspending the cage.Extract the specimen for 6 h indecahydronaphthalene or for 12 h in xylenes.NOTE4Since most of the extraction occurs within less than thespecified time,the period of extraction may be shortened for control testson the basis of experience.If this is done,the time of extraction must bereported.For referee tests,extraction time is to be as specified by this testmethod.11.5 After extraction,place the cage and specimen imme-diately in a vacuum oven preheated to 150C.Dry thespecimen to constant weight under at least 710 mm(28 in.)Hgvacuum.Cool and weigh(W4).If the compound absorbsmoisture,cool the specimen in a desiccator before weighing.NOTE5It has been reported that drying time sometimes can bematerially shortened if the cage and sample are cooled for 15 min and thenare placed either on a suspended screen or on lint-free absorbent materialto remove excess solvent before being put into the vacuum drying oven.11.6 If extraction tests are regularly made,it is acceptable toleave the apparatus assembled and the extracting solvent(containing inhibitor as directed)reused until it darkens.However,if there is any doubt about the values obtained withreused extractant,the test shall be repeated with fresh extract-ant.NOTE6If the modified Soxhlet apparatus is used,the completethimble assembly containing 1 g of sample is weighed to the nearest60.0001 g before extraction and after vacuum drying.12.Calculation12.1 Calculate the solvent extraction in percent as follows:Extract,%5weight lost during extraction!/(1)weight of original specimen2 weight of filler!5W32 W4!/W22 W1!2 FW22 W1!#31005W32 W4!/1 2 F!W22 W1!#3100where:W1=weight of the pouch(sealed on three sides,one sideopen),W2=weight of the specimen and the pouch(sealed on threesides,one side open),W3=weight of the specimen and the cage,after beingstapled shut,W4=weight of the specimen and the cage after extractionand drying,F=fraction of filler(which must be insoluble in decahy-dronaphthalene or xylenes)in the polyethylenecompound,andgel content=100 percent extract.NOTE7If the fraction of filler in the material is not known,the testmethods described in Test Method D1603 or in Test Methods D297,maybe used to determine filler content.13.Report13.1 Report the following information:13.1.1 Complete identification of the compound,13.1.2 Density of the polyethylene in the compound(usu-ally as identified by its supplier),13.1.3 Weight percent of polyethylene in the compound orfiller content(usually as identified by