1、Designation: D7444 11Standard Practice forHeat and Humidity Aging of Oxidatively DegradablePlastics1This standard is issued under the fixed designation D7444; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revisio
2、n. 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 practice indicates how to test the oxidative degra-dation characteristics of plastics that degrade in the environ-ment under
3、atmospheric pressure and thermal and humiditysimulations, only, in the absence of any selected disposalenvironment such as soil, landfill, or compost. This practicedoes not by any extension or extrapolation of data or resultsgenerated indicate that such plastics are suitable for or willdegrade on di
4、sposal in these said environments. It is particu-larly noted that in real world environments such as soil,compost and landfill oxidations, if they occur, will predomi-nantly be under conditions where other interfering ingredientsare present and, in the case of landfill, at sub-atmosphericoxygen conc
5、entrations. This practice, therefore, can only resultin a relative ordering of the potential for oxidation of plasticmaterials under the conditions tested, which are not alwaysreflective of their behavior in a particular real world disposalsystems. Prediction of the oxidation of a plastic under real
6、world disposal conditions is an essential further testing inappropriate methodologies, such as Test Method D5338 forcomposting. No claims can be made for real world behaviorbased on this practice.1.2 This practice is only intended to define the exposureconditions of plastics at various temperatures
7、in air at atmo-spheric pressure under controlled humidity levels for extendedperiods of time. The humidity levels and temperature rangesare selected to be within the variable recorded seasonal ranges(upper and lower levels) generally observed in disposal siteswhere such plastics are discarded. For e
8、xample: soil (15 to 40percent moisture); landfill (35 to 60 percent moisture), andcompost (45 to 70 percent moisture). It is optional to exposethe plastic at zero humidity, if comparison with specifiedhumidity ranges is of interest. Only the procedures for heat andhumidity exposures are specified, n
9、ot the test method orspecimen, necessary for the evaluation of the heat and humid-ity exposure effects. The effect of heat and humidity on anyparticular property is determined by selection of the appropri-ate test method and specimen; however, it is recommended thatPractice D3826 be used to determin
10、e the embrittlement end-point, which is defined as that point in the history of a materialwhen 75 % of the specimens tested have a tensile elongation atbreak of 5 % or less at an initial strain rate of 0.1 mm/mm min.1.3 This practice is used to compare the effects of heat andhumidity at any selected
11、 temperature, such as those found inthe mentioned disposal environments, on the degradation of aparticular plastic by selection of an appropriate test method andspecimen.1.4 This practice is to be used in order to apply selectedexposure conditions when comparing the thermal-aging char-acteristics at
12、 controlled humidity levels of plastic materials asmeasured by the change in some property of interest (that is,embrittlement by means of loss of elongation, molecularweight, disintegration, etc.). It is very similar to PracticeD3045 but is intended for use in evaluating plastics designed tobe oxidi
13、zed easily after use. The exposure times used for thispractice will be significantly shorter than those used forPractice D30451.5 The type of oven used can affect the results obtainedfrom this practice. The user can use one of two methods foroven exposure. Do not mix the results based on one methodw
14、ith those based on the other one.1.6 Procedure A: Gravity-Convection OvenRecommended for film specimens having a nominal thicknessnot greater than 0.25 mm (0.010 in.).1.7 Procedure B: Forced-Ventilation OvenRecommendedfor specimens having a nominal thickness greater than 0.25mm (0.010 in.).1.8 This
15、practice recommends procedures for comparing thethermal and humidity aging characteristics of materials at asingle temperature under dry or selected humidity conditions.Recommended procedures for determining the thermal agingcharacteristics of a material at a series of temperatures andhumidity condi
16、tions for the purpose of estimating time to adefined property change at some lower temperature are alsodescribed. This practice does not predict thermal aging char-acteristics where interactions between stress, environment,temperature, and time control failure.1This practice is under the jurisdictio
17、n ofASTM Committee D20 on Plastics andis the direct responsibility of Subcommittee D20.96 on Environmentally DegradablePlastics and Biobased Products.Current edition approved March 15, 2011. Published March 2011. DOI:10.1520/D7444-11.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700,
18、 West Conshohocken, PA 19428-2959, United States.1.9 The values stated in SI units are to be regarded as thestandard.1.10 The values stated in inch-pound units are to beregarded as standard. The values given in parentheses aremathematical conversions to SI units that are provided forinformation only
19、 and are not considered standard.NOTE 1There is no known ISO equivalent to this standard.2. Referenced Documents2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingD883 Terminology Relating to PlasticsD1870 Practice for Elevated Temperature Aging Using aTubular Oven3D2436 Specific
20、ation for Forced-Convection LaboratoryOvens for Electrical Insulation3D3045 Practice for Heat Aging of Plastics Without LoadD3593 Test Method for Molecular Weight Averages andMolecular Weight Distribution of Certain Polymers byLiquid Size-Exclusion Chromatography (Gel PermeationChromatography GPC) U
21、sing Universal Calibration3D3826 Practice for Determining Degradation End Point inDegradable Polyethylene and Polypropylene Using a Ten-sile TestD5032 Practice for Maintaining Constant Relative Humid-ity by Means of Aqueous Glycerin SolutionsD5338 Test Method for Determining Aerobic Biodegrada-tion
22、of Plastic Materials Under Controlled CompostingConditionsD5510 Practice for Heat Aging of Oxidatively DegradablePlastics3D6954 Guide for Exposing and Testing Plastics that De-grade in the Environment by a Combination of Oxidationand BiodegradationE104 Practice for Maintaining Constant Relative Humi
23、dityby Means of Aqueous SolutionsE145 Specification for Gravity-Convection and Forced-Ventilation Ovens3. Terminology3.1 DefinitionsThe definitions used in this practice are inaccordance with Terminology D883.4. Significance and Use4.1 Since the correlation between the conditions specified inthis pr
24、actice and actual disposal environments (for example,composting, soil or landfill) has not been determined orestablished, the test results are to be used only for comparativeand ranking purposes in the laboratory. No extrapolation to realworld disposal expectations or predictions are to be made from
25、results obtained by this procedure. Real world evaluations andcorrelations are needed for such claims.4.2 Degradable plastics exposed to heat and humidity aresubject to many types of physical and chemical changes. Theseverity of the exposures in both time, temperature andhumidity level, determines t
26、he extent and type of change thatoccurs. For example, short exposure times at elevated tempera-tures generally serve to shorten the induction period ofoxidatively degradable plastics during which the depletion ofantioxidants and stabilizers occurs. Physical properties, such astensile and impact stre
27、ngth and elongation and modulus,sometimes change during this induction period; however, thesechanges are generally not due to molecular-weight degrada-tion, but are merely a temperature-dependent response, such asincreased crystallinity or loss of volatile material, or both. Theeffects of humidity a
28、re less well understood and are moredifficult to predict and depend on the degradable plasticscharacteristics such as hydrophilicity, polarity and composi-tion.4.3 Generally, short exposures at elevated temperaturesdrive out volatiles such as moisture, solvents, or plasticizers;relieve molding stres
29、ses; advance the cure of thermosets;increase crystallinity; and cause some change in color of theplastic or coloring agent, or both. Normally, additional shrink-age is expected with a loss of volatiles or advance in polymer-ization.4.4 Some plastic materials such as PVC become brittle dueto loss of
30、plasticizers or to molecular breakdown of thepolymer. Polypropylene and its copolymers tend to becomevery brittle as molecular degradation occurs, whereas polyeth-ylene tends to become soft and weak before it embrittles withresultant loss in tensile strength and elongation.4.5 Embrittlement of a mat
31、erial is not necessarily commen-surate with a decrease in molecular weight. Test MethodD3593 is used to characterize any molecular-weight changesthat are suspected to have occurred during thermal exposure.4.6 The degree of change observed will depend on theproperty measured. Different properties do
32、not change at thesame rate. In most cases, ultimate properties, such as breakstrength or break elongation, are more sensitive to degradationthan bulk properties such as modulus.4.7 Effects of exposure can be quite variable, especiallywhen samples are exposed for long intervals of time. Factorsthat a
33、ffect the reproducibility of data are the degree oftemperature control of the enclosure, humidity level of theoven, air velocity over the specimen, and exposure periodwhich are evaluated by this practice. Errors in exposure arecumulative with time; for example certain materials have thepotential to
34、be degraded due to the influence of humidity ratherthan oxidation in long-term tests and thus give misleadingresults. Materials susceptible to hydrolysis (that is, hydrolyti-cally degradable plastics) undergo degradation when subjectedto long-term thermal tests due to the presence of moisturerather
35、than oxidation.4.8 Do not infer that comparative material ranking is unde-sirable or unworkable. On the contrary, this practice is de-signed to provide information that can be used for suchcomparative purposes after appropriate physical property testsare performed following exposure. However, since
36、it does notaccount for the influence of stress or environment that is2For 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
37、website.3Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.D7444 112involved in most real life applications, the information ob-tained from this practice must be used cautiously by thedesigner, who must inevitably make material choices usingadditional info
38、rmation, such as moisture, soil-type and com-position, and mechanical-action effects that are consistent withthe requirements of the particular application.4.9 It is possible for many temperature indices to exist, infact, one for each failure criterion (time to reach failure isdependent on the expos
39、ure temperature and humidity). There-fore, for any application of the temperature index to be valid,the thermal-aging program must duplicate the intended expo-sure conditions of the end product. If the plastic material isexposed in the end use in a manner not evaluated in the agingprogram, the tempe
40、rature index thus derived is not applicableto the use of the plastic material.4.10 In some situations, a material can be exposed to onetemperature and humidity, for a particular period of time,followed by exposure to another temperature at the samehumidity, for a particular period of time. This prac
41、tice can beused for such applications. The heat-aging curve of the firsttemperature and humidity is derived, followed by derivation ofthe heat-aging curve for the second temperature at the samehumidity, after exposure of samples to the first temperature andhumidity.4.11 There can be very large error
42、s when Arrhenius plots orequations based on data from experiments at a series oftemperatures and humidity are used to estimate time to producea defined property change at some lower temperature. Thisestimate of time to produce the property change or failure mustalways be accompanied by a 95 % confid
43、ence interval for therange of times possible based on the calculation or estimate.5. Apparatus5.1 Provisions for Conditioning, at specified standard con-ditions.5.2 Oven Type:5.2.1 Gravity-Convection OvenRecommended for filmspecimens having a nominal thickness not greater than 0.25mm (0.010 in.).5.2
44、.2 Forced-Ventilation OvenRecommended for speci-mens having a nominal thickness greater than 0.25 mm (0.010in.). When it is necessary to avoid contamination amongspecimens or materials, it is possible that a tubular-ovenprocedure, such as that specified in Practice D1870, will bedesirable. Oven appa
45、ratus shall be in accordance with Speci-fications D2436 and E145, Type 1A and Type IIB, with 50 610 air changes/h and the requirements for uniformity extendedto include the range of test temperatures. Recording instrumen-tation to monitor the temperature and humidity of exposure isrecommended.5.3 Sp
46、ecimen RackA specimen rack or frame of suitabledesign to allow ready air circulation around the specimens.5.4 Relative Humidity Control:5.4.1 The chamber shall be able to control relative humidityto within 65 % when the set point temperature has anoperational tolerance of 61C.5.4.2 Relative humidity
47、 at different temperatures can becontrolled within a narrow range by using saturated aqueoussolutions in accordance with Practice E104 or Practice D5032.6. Test Specimen6.1 The number and type of test specimens required shall bein accordance with the ASTM test method for the specificproperty to be d
48、etermined; this requirement must be met ateach time and temperature and humidity selected. Unlessotherwise specified or agreed upon by all interested parties,expose a minimum of three replicates of each material at eachtime and temperature and humidity selected.6.2 The specimen thickness is to be co
49、mparable to but nogreater than the maximum thickness of the intended article inits application, and its method of fabrication must be the sameas that for the intended application.6.3 All test specimens for a series of temperatures must beof the same age, preferably from the same manufacturing runand date.7. Conditioning7.1 Conduct initial tests in the standard laboratory atmo-sphere as specified in Practice D618, and with the specimensconditioned in accordance with the requirements of the testmethod for determining the specific property or propertiesrequired.7.2 When required,
