ASTM D6746-2008 752 Standard Test Method for Determination of Green Strength of Raw Rubber or Unvulcanized Compounds.pdf

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1、Designation: D 6746 08Standard Test Method forDetermination of Green Strength of Raw Rubber orUnvulcanized Compounds1This standard is issued under the fixed designation D 6746; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye

2、ar 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 This test method covers a method to evaluate a charac-teristic of raw rubber or unvulcanized rubber compounds t

3、hatis designated as green strength. This special strength propertyfor uncured rubbers is an important processing performanceattribute in rubber product manufacturing.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 This s

4、tandard 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.2. Referenced Documents2.

5、1 ASTM Standards:2D 412 Test Methods for Vulcanized Rubber and Thermo-plastic ElastomersTensionD 1349 Practice for RubberStandard Temperatures forTestingD 3182 Practice for RubberMaterials, Equipment, andProcedures for Mixing Standard Compounds and Prepar-ing Standard Vulcanized SheetsD 4483 Practic

6、e for Evaluating Precision for Test MethodStandards in the Rubber and Carbon Black ManufacturingIndustries3. Terminology3.1 Definitions:3.1.1 green strength, na characteristic property of a rawor unvulcanized rubber compound that indicates resistance todeformation when stress is applied.3.1.1.1 Disc

7、ussionThe word “green,” applied to rubber, isa synonym for uncured or unvulcanized rubber.3.1.2 tensile green modulus, nratio of stress to strain of araw or unvulcanized rubber compound that indicates resistanceto tensile deformation or strain.3.1.3 tensile green strength, nyield stress of a raw oru

8、nvulcanized rubber compound that indicates loss of resistanceto tensile deformation or stress.4. Summary of Test Method4.1 The tensile stress-strain characteristics of a dumbbell oranother recommended test piece of raw or unvulcanizedcompounded rubber are determined on a tensile testing ma-chine cap

9、able of maintaining a constant rate of separation ofthe moving jaws.4.2 In evaluating tensile green strength, several types ofstress-strain behavior may be observed, depending on therubber and the composition of the unvulcanized compound.(See Section 5.) Fig. 1 indicates three typical stress-strainc

10、urves. Type 1 has a yield point and exhibits increasing stressbeyond the yield point, culminating in rupture at a higher stressand elongation compared to the initial yield stress. Type 2 alsohas a yield point, but beyond this point follows a decreasingstress behavior as the maximum rupture elongatio

11、n is attained.Type 3 has a stress-strain curve that is similar to cured rubbers,with no elongation beyond the point of rupture at maximum(yield) stress.5. Significance and Use5.1 The stress-strain properties of unvulcanized rubber (ei-ther in a compound or in the raw state) are important to certainp

12、rocessing operations in the rubber industry. These unvulca-nized rubber properties are frequently referred to as “greenstrength,” denoting that the final vulcanization cycle has notyet been achieved.5.2 Green strength is determined primarily by the physicaland chemical characteristics of polymers, s

13、uch as molecularweight, tendency to crystallize, degree of branching, and soforth. It is also related to the compound formulation, particu-larly filler and plasticizer content, and the presence of peptiz-ers. Green strength can be a good indication of processing1This test method is under the jurisdi

14、ction of ASTM Committee D11 on Rubberand is the direct responsibility of Subcommittee D11.12 on Processability Tests.Current edition approved May 1, 2008. Published May 2008. Originallyapproved in 2002. Last previous edition approved in 2003 as D 6746 03.2For referenced ASTM standards, visit the AST

15、M 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.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, Unite

16、d States.FIG. 1 Typical Tensile Stress-Strain CurvesD6746082behavior. It is a particularly important characteristic for allprocessing operations in which elongation predominates.5.3 Green strength is dependent on the test piece prepara-tion, rate of extension, and test temperature. Therefore, asingl

17、e-point method cannot be expected to give correlationbetween green strength and processing behavior over the wholerange of processing conditions.6. Interferences6.1 For reliable test results, it is important that test speci-mens are of accurate dimensions and are free of air inclusions,blisters, and

18、 contaminants.6.2 A defective cutting die or slippage of the test specimenin the clamps may cause incorrect results.6.3 Exposure to excessive ultraviolet radiation or chemicalvapors may also affect the results.7. Apparatus7.1 Tensile Testing MachineThe tensile testing machineshall be in accordance w

19、ith the requirements of Test MethodsD 412. It shall be capable of maintaining a constant rate ofseparation of the jaws at 100 6 10 mm/min or, for specialcases, at a value within the range of 20 to 1000 mm/min. Itshould have means to test the force on the test piece and theelongation by the distance

20、between the gauge marks on thedumbbell. It should be capable of recording the force/elongation curve obtained during the test. If using an automaticextensometer, a non-contacting type of extensometer is pre-ferred.7.2 MoldThe mold shall meet the requirements of Prac-tice D 3182.8. Hazards8.1 There a

21、re no hazards inherent to the techniques de-scribed. Caution should be exercised and the user should beaware of any possible pinch points.8.2 Normal safety precautions, including the use of personalprotective equipment and good laboratory practice, should beobserved when using any equipment. This is

22、 especially truewhen performing tests at elevated temperatures.9. Test Specimen9.1 The preferred test specimen is the dumbbell Die Cspecimen in accordance with Test Methods D 412. Other typesof test specimens may also be used. Since surface conditionscan affect the results, there may be cases where

23、a widerspecimen would be preferred, such as DieAfrom Test MethodsD 412.9.2 Since different types of test specimens do not necessar-ily give the same values, comparison of results from differentspecimen types should be avoided.10. Preparation10.1 Raw rubber and unvulcanized compounds shall besheeted

24、out to approximately 2.5-mm thickness and placed inthe mold with the grain direction oriented so as to have thegrain direction parallel with the length of the dumbbells. It isalso permissible to test against the grain, but this must be notedin the report section. (See Section 15.) Only fresh stock s

25、hall beused.10.2 Polyester film (0.05-mm thickness) shall be placedbetween the mold walls and the rubber in order to promotemold release. The sample shall be compressed for 5 6 1 minat 100 6 2C under a minimum of 2.5 MPa platen pressure,allowed to cool to laboratory temperature for 30 6 2 min undera

26、dequate pressure to keep test specimen flat, then removed.The cooling shall be done out of the press using roomtemperature metal plates.10.3 For some raw rubbers, longer times or higher tempera-tures may be necessary in order to obtain a smooth sheet freefrom porosity. For some compounds, a lower te

27、mperature maybe necessary when there is a danger of scorch at the preferredtemperature.10.4 A sample may also be acquired during processing bysheeting off a mill, calender, or by extrusion and then cut witha die. Special care should be taken to ensure that the sample isfree of trapped air.10.5 Cut f

28、ive test specimens from the sheet, using a suitabledie. (See 9.1.)11. Calibration and Standardization11.1 The tensile testing machine shall be calibrated accord-ing to the procedure in Test Methods D 412.12. Conditioning12.1 The test specimens shall be conditioned at a tempera-ture of 23 6 2C for at

29、 least 16 h and not more than 24 h beforetesting.12.2 Some slow crystallizing polymers, such as polychlo-roprene, may need a longer conditioning period in order toaccurately measure the maximum green strength.13. Procedure13.1 Place the dumbbell in the grips of the testing machinein accordance with

30、the procedure in Test Methods D 412.Adjust the rate of displacement of the moving jaw to 100 6 10mm/min and start the tensile test. If the sample breaks at thegrips, that result shall be discarded and a retest carried out.13.1.1 The preferred rate of separation of the jaws is 100 610 mm/min. In spec

31、ial cases, other rates may be used, but onlytests carried out at the same rate can be compared. Rollover orself-tightening grips may help prevent slippage.13.2 The test shall be carried out on three test specimens. Ifthe standard deviation is greater than 20 % of the mean value,then five specimens s

32、hould be tested.13.3 Unless otherwise specified, the standard test tempera-ture shall be 23 6 2C.13.4 When it is necessary to test at other temperatures, useone of the standard temperatures listed in Practice D 1349.Specimens shall be conditioned at these test temperatures for15 6 2 min prior to tes

33、ting.14. Calculation or Interpretation of Results14.1 Using the typical stress-strain curves given in Fig. 1,determine the yield stress or maximum stress in MPa. Otherparameters may be determined, such as yield elongation (E1)orD6746083the stress at a definite reference elongation corresponding toth

34、e deformation entailed by a subsequent processing operation.The median value is the preferred value to report.14.2 Calculate the tensile stress at any specified elongationas follows:Txxx!5 Fxxx!/A (1)where:T(xxx)= tensile stress at (xxx) % elongation, MPa,F(xxx)= force at specified elongation, MN, a

35、ndA = cross-sectional area of unstrained specimen, m2.14.3 Evaluate the yield strain as that strain or elongationmagnitude, where the rate of change of stress with respect tostrain goes through a zero value.14.4 Calculate the tensile green strength (yield stress) asfollows:Ystress!5 Fy!/A (2)where:Y

36、(stress)= yield stress, that stress level where the yieldpoint occurs, MPa,F(y)= magnitude of force at the yield point, MN, andA = cross-sectional area of unstrained specimen,m2.14.5 Calculate the elongation (at any degree of extension) asfollows:E 5 100L Lo!#/Lo!(3)where:E = the elongation in perce

37、nt (of original bench markdistance),L = observed distance between bench marks on theextended specimen, andL(o)= original distance between bench marks (use sameunits for L and L(o).14.6 The maximum elongation is evaluated when L is equalto the distance between benchmarks at the point of specimenruptu

38、re.14.7 Calculate the tensile green modulus at any specifiedelongation as follows:Mxxx!5Txxx!xxx!3 100 (4)where:M(xxx)= tensile green modulus at (xxx)% elongation,MPa,T(xxx)= tensile stress at (xxx) % elongation, MPa, andxxx = the specified elongation in percent of originalbench mark distance.15. Re

39、port15.1 Report the following information:15.1.1 Full description of the sample and its origin,15.1.2 Method of preparation of the test specimen, that is,time and temperature of molding, if not at standard conditions,15.1.3 Type and dimensions of test specimen, if not a Die Cdumbbell,15.1.4 Grain di

40、rection, if not at the standard condition oftesting with the grain,15.1.5 Date of test,15.1.6 Rate of extension, if not at standard conditions,15.1.7 Temperature and humidity of test room, if not atstandard conditions,15.1.8 Time of conditioning,15.1.9 Temperature of test if at other than 23 6 2C,15

41、.1.10 Number of test specimens tested if not three, and15.1.11 Median of all results (tensile green strength (yieldstress), maximum stress, yield elongation, maximum elonga-tion, tensile green modulus at specified elongation).16. Precision and Bias16.1 IntroductionThe interlaboratory test program (I

42、TP)for precision evaluation for both yield stress and yield strainwas conducted in 2003 using the precision procedures andguidelines as described in the newly revised version of PracticeD 4483 (scheduled for publication in 2004) which replaces theprevious version of Practice D 4483. Refer to the new

43、 PracticeD 4483 for additional background on precision terminology.16.2 The precision results as determined by this ITP maynot be applied to acceptance or rejection testing for any groupof materials or products without documentation that the resultsof this precision evaluation actually apply to the

44、products ormaterials tested.16.3 Four rubbers (compounds) were used in the ITP;EPDM, NR, SBR and NBR. Four laboratories participated inthe testing program. Mixed compounds of each rubber weresent to the participating laboratories and in each laboratoryeach compound was milled, test sheets molded and

45、 testspecimens cut for testing. This procedure is essentially equiva-lent to a Type 2 precision where a complete set of operationalsteps are required to generate a test specimen. A full orcomplete Type 2 precision would require that each laboratorymix each compound and conduct the other remaining pr

46、epara-tion steps. Thus the precision as evaluated is designated as amodified Type 2 precision. See Practice D 4483, sec 5.1.5 formore on Type 1 vs Type 2 precision.16.4 The testing was conducted on a test day 1 vs test day2 basis, one day apart. On each test day milling, molding andspecimen cutting

47、were conducted as well as stress-straintesting. As stipulated in the method, three specimens weretested on each day and the mean was used as a test result. Allanalysis was conducted using test results. Due to the smallnumber of participating laboratories, Practice D 4483 precisionanalysis Option 2 w

48、as used to respond to outliers as detected bythe h and k analysis operations. Option 2 uses outlier replace-ment rather than outlier deletion which reduces the degrees offreedom for precision evaluation. See Sections 7 and 8, 9 and10 of Practice D 4483 for more details on outlier detection andon the

49、 AOT outlier replacement operations.16.5 Precision ResultsThe precision results for both yieldstress and yield strain are given in Table 1 on the basis ofascending order of the mean test result value. General state-ments for the use of the precision results are cited below. Theseare given in terms of both the absolute precision, r or R andalso for relative precision (r) and (R).16.5.1 RepeatabilityThe repeatability, or local domainprecision, for each material or rubber for each of the testmethods has been established by the values found in Table 1.D67460