1、Designation: D6746 15Standard Test Method forDetermination of Green Strength and Stress Relaxation ofRaw Rubber or Unvulcanized Compounds1This standard is issued under the fixed designation D6746; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、 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. Scope1.1 This test method covers methods to evaluate a charac-teristic of raw rubber or unvulcanized
3、rubber compounds thatis 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 thiss
4、tandard.1.3 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.2. Refe
5、renced Documents2.1 ASTM Standards:2D412 Test Methods for Vulcanized Rubber and Thermoplas-tic ElastomersTensionD1349 Practice for RubberStandard Conditions for Test-ingD3182 Practice for RubberMaterials, Equipment, and Pro-cedures for Mixing Standard Compounds and PreparingStandard Vulcanized Sheet
6、sD4483 Practice 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 raw orunvulcanized rubber compound that indicates resistance todeformation when stress is applie
7、d.3.1.1.1 DiscussionThe 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 stres
8、s of a raw orunvulcanized rubber compound that indicates loss of resistanceto tensile deformation or stress.4. Summary of Test Method4.1 This test method is divided into two parts. Part Aconsists of measurement of tensile stress and tensile moduluswhile a test specimen of raw or unvulcanized compoun
9、dedrubber is subjected to a constant rate of crosshead separation.Part B consists of subjecting a test specimen of raw orunvulcanized compounded rubber to a 100 % strain conditionand measuring tensile stress after an interval while maintainingthe strained condition.4.2 Part A of this test method con
10、sists of measuring thetensile stress-strain characteristics of a dumbbell or otherrecommended test piece of raw or unvulcanized compoundedrubber are determined on a tensile testing machine capable ofmaintaining a constant rate of separation of the moving jaws.4.3 In evaluating tensile green strength
11、, 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-straincurves. Type 1 has a yield point and exhibits increasing stressbeyond the yield point, culminating in rupture
12、 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 elongation is attained.Type 3 has a stress-strain curve that is similar to cured rubbers,with no elongation beyond th
13、e point of rupture at maximum(yield) stress.4.4 Part B of this test method consists of a dumbbell or otherrecommended test specimen being placed in the grips of atensile machine capable of a constant rate of grip separation,pausing the grip separation when an initial strain condition of1This test me
14、thod is under the jurisdiction of ASTM Committee D11 on Rubberand is the direct responsibility of Subcommittee D11.12 on Processability Tests.Current edition approved Nov. 1, 2015. Published December 2015. Originallyapproved in 2002. Last previous edition approved in 2010 as D6746 10. DOI:10.1520/D6
15、746-15.2For 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.Copyright ASTM International, 100 Barr Harbor Drive, P
16、O Box C700, West Conshohocken, PA 19428-2959. United States1FIG. 1 Typical Tensile Stress-Strain CurvesD6746 152100 % is reached, and reporting the tensile stress after aspecific time interval or the time for a given percentage ofstress decay.5. Significance and Use5.1 The stress-strain properties o
17、f unvulcanized rubber (ei-ther in a compound or in the raw state) are important to certainprocessing operations in the rubber industry. These unvulca-nized rubber properties are frequently referred to as “greenstrength.”5.2 Green strength is determined primarily by the physicaland chemical character
18、istics of polymers, such 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 processingbehavior. It is a
19、 particularly important characteristic for allprocessing operations in which elongation predominates.5.3 Green strength is dependent on the test piecepreparation, rate of extension, and test temperature. Therefore,a single-point method cannot be expected to give correlationbetween green strength and
20、 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 contaminants.6.2 A defective cutting die or slippage of the test specimenin th
21、e 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 with the requirements of Test MethodsD412. It shall be capable of maintaining a
22、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 between the gauge marks on thedumbbell. It should be capable of recording the fo
23、rce/elongation curve obtained during the test. If using an automaticextensometer, a non-contacting type of extensometer is pre-ferred. Part B of this test method also requires that the tensiletesting machine be capable of stopping extension at a specificstrain condition, holding that position, and r
24、ecording force onthe test piece and force decay time after a specified waitinginterval.7.2 MoldThe mold shall meet the requirements of PracticeD3182.8. Hazards8.1 There are no hazards inherent to the techniques de-scribed. Caution should be exercised and the user should beaware of any possible pinch
25、 points.8.2 Normal safety precautions, including the use of personalprotective equipment and good laboratory practice, should beobserved when using any equipment. This is especially truewhen performing tests at elevated temperatures.9. Test Specimen9.1 The preferred test specimen is the dumbbell Die
26、 Cspecimen in accordance with Test Methods D412. Other typesof test specimens may also be used. Since surface conditionscan affect the results, there may be cases where a widerspecimen would be preferred, such as DieAfrom Test MethodsD412.9.2 Since different types of test specimens do not necessar-i
27、ly give the same values, comparison of results from differentspecimen types should be avoided.10. Preparation10.1 Raw rubber and unvulcanized compounds shall besheeted out to approximately 2.5-mm thickness and placed inthe mold with the grain direction oriented so as to have thegrain direction paral
28、lel with the length of the dumbbells. It isalso permissible to test against the grain, but this must be notedin the report section. (See Section 16.) Only fresh stock shall beused.10.2 Polyester film (0.05-mm thickness) shall be placedbetween the mold walls and the rubber in order to promotemold rel
29、ease. 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 underadequate pressure to keep test specimen flat, then removed.The cooling shall be done out of the press using roomtemperature metal pla
30、tes.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 temperature maybe necessary when there is a danger of scorch at the preferredtemperature.10.4 A sample may also be acquired during pro
31、cessing 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 five test specimens from the sheet, using a suitabledie. (See 9.1.)11. Calibration and Standardization11.1 The tensile testing machin
32、e shall be calibrated accord-ing to the procedure in Test Methods D412.12. Conditioning12.1 The test specimens shall be conditioned at a tempera-ture of 23 6 2C for at least 16 h and not more than 24 h beforetesting.12.2 Some slow crystallizing polymers, such aspolychloroprene, may need a longer con
33、ditioning period inorder to accurately measure the maximum green strength.D6746 15313. Part A Test Procedure13.1 Place the test specimen in the grips of the testingmachine in accordance with the procedure in Test MethodsD412.Adjust the rate of displacement of the moving jaw to 1006 10 mm/min and sta
34、rt the tensile test. If the sample breaks atthe grips, 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 special cases, other rates may be used, but onlytests carried out at the same rate can be compared. Rollover orself
35、-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 should be tested.13.3 Unless otherwise specified, the standard test tempera-ture shall be 23 6 2C.13.4 When it i
36、s necessary to test at other temperatures, useone of the standard temperatures listed in Practice D1349.Specimens shall be conditioned at these test temperatures for15 6 2 min prior to testing.14. Part B Test Procedure14.1 Place the test specimen in the grips of the testingmachine in accordance with
37、 the procedure in Test MethodsD412.Adjust the rate of displacement of the moving jaw to 1006 10 mm/min and start the test. If the test specimen breaks atthe grips, that result shall be discarded and a retest carried out.14.2 Stop the displacement of the moving jaw when a100 % strain condition is rea
38、ched for the center section of thetest specimen. Begin timing the waiting interval as soon as thejaw separation is stopped. Record the force on the testspecimen after a waiting interval of 30 s or the force decaytime for a specified percentage of stress decay.14.3 The test shall be carried out on th
39、ree test specimens. Ifthe standard deviation is greater than 20 % of the mean value,then five specimens should be tested.14.4 Unless otherwise specified, the standard test tempera-ture shall be 23 6 2C.14.5 When it is necessary to test at other temperatures, useone of the standard temperatures liste
40、d in Practice D1349.Specimens shall be conditioned at these test temperatures for15 6 2 min prior to testing.15. Calculation or Interpretation of Results15.1 Using the typical stress-strain curves given in Fig. 1,determine the yield stress or maximum stress in MPa. Otherparameters may be determined,
41、 such as yield elongation (E1)orthe stress at a definite reference elongation corresponding tothe deformation entailed by a subsequent processing operation.The median value is the preferred value to report.15.2 Calculate the tensile stress at any specified elongationas follows:Txxx!5 Fxxx!/A (1)wher
42、e:T(xxx)= tensile stress at (xxx) % elongation, MPa,F(xxx)= force at specified elongation, MN, andA = cross-sectional area of unstrained specimen, m2.15.3 Evaluate the yield strain as that strain or elongationmagnitude, where the rate of change of stress with respect tostrain goes through a zero val
43、ue.15.4 Calculate the tensile green strength (yield stress) asfollows:Ystress!5 Fy!/A (2)where:Y(stress)= yield stress, that stress level where the yield pointoccurs, MPa,F(y)= magnitude of force at the yield point, MN, andA = cross-sectional area of unstrained specimen, m2.15.5 Calculate the elonga
44、tion (at any degree of extension) asfollows:E 5 100L 2 Lo!#/Lo!(3)where:E = the elongation in percent (of original bench markdistance),L = observed distance between bench marks on the ex-tended specimen, andL(o)= original distance between bench marks (use sameunits for L and L(o).15.6 The maximum el
45、ongation is evaluated when L is equalto the distance between benchmarks at the point of specimenrupture.15.7 Calculate the tensile green modulus at any specifiedelongation as follows:Mxxx!5Txxx!xxx!3100 (4)where:M(xxx)= tensile green modulus at (xxx)% elongation, MPa,T(xxx)= tensile stress at (xxx)
46、% elongation, MPa, andxxx = the specified elongation in percent of original benchmark distance.15.8 If required, the percent stress decay is calculated as:%Dtt!5Tdecaytime tt!T100!3100 (5)where:%D(tt)= Percent decay after specified decay time (tt),T = tensile stress at specified decay time (tt), MPa
47、, andT(100)= the tensile stress at 100 % strain, when the cross-head is stopped for measurement of stress decay.16. Report16.1 Report the following information:16.1.1 Test procedure used (Part A or Part B),16.1.2 Part A tests report median values of yield stress,maximum stress, yield elongation, max
48、imum elongation, andtensile green modulus at a specified elongation,16.1.3 Part B tests report median values of stress at 100 %elongation, and stress after t seconds at 100 % strain (30 s isD6746 154typically used), or ty, the time for y % stress decay with nospecified end to the decay time (75 % st
49、ress decay is typicallyused),16.1.4 Full description of the sample and its origin,16.1.5 Method of preparation of the test specimen, that is,time and temperature of molding, if not at standard conditions,16.1.6 Type and dimensions of test specimen, if not a Die Cdumbbell,16.1.7 Grain direction, if not at the standard condition oftesting with the grain,16.1.8 Date of test,16.1.9 Rate of extension, if not at standard conditions,16.1.10 Temperature and humidity of test room, if not atstandard conditions,16.1.11 Time of conditioning,16.1.12 Temperature of
