1、Designation: D7444 18Standard 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. Scope*1.1 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 d
4、isposal 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 con
5、centrations. 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 rea
6、lworld 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
8、example: 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,
9、not 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 determi
10、ne the embrittlementendpoint, which is defined as that point in the history of amaterial when 75 % of the specimens tested have a tensileelongation at break of 5 % or less at an initial strain rate of 0.1mm/mm min.1.3 This practice is used to compare the effects of heat andhumidity at any selected t
11、emperature, 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 c
12、ontrolled 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 oxidize
13、d 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 methodwit
14、h 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 pr
15、actice 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 conditi
16、ons for the purpose of estimating time to adefined property change at some lower temperature are also1This practice is under the jurisdiction ofASTM Committee D20 on Plastics andis the direct responsibility of Subcommittee D20.96 on Environmentally DegradablePlastics and Biobased Products.Current ed
17、ition approved July 1, 2018. Published August 2018. Originallyapproved in 2011. Last previous edition approved in 2011 as D744411.DOI:10.1520/D7444-18.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken,
18、 PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organizatio
19、n Technical Barriers to Trade (TBT) Committee.1described. This practice does not predict thermal aging char-acteristics where interactions between stress, environment,temperature, and time control failure.1.9 The values stated in SI units are to be regarded as thestandard.1.10 The values stated in i
20、nch-pound units are to beregarded as standard. The values given in parentheses aremathematical conversions to SI units that are provided forinformation only and are not considered standard.NOTE 1There is no known ISO equivalent to this standard.1.11 This international standard was developed in accor
21、-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM
22、 Standards:2D618 Practice for Conditioning Plastics for TestingD883 Terminology Relating to PlasticsD3045 Practice for Heat Aging of Plastics Without LoadD3593 Test Method for Molecular Weight Averages/ Distri-bution of Certain Polymers by Liquid Size-ExclusionChromatography (Gel Permeation Chromato
23、graphy GPC)Using Universal Calibration (Withdrawn 1993)3D3826 Practice for Determining Degradation End Point inDegradable Polyethylene and Polypropylene Using a Ten-sile TestD5032 Practice for Maintaining Constant Relative Humidityby Means of Aqueous Glycerin SolutionsD5338 Test Method for Determini
24、ng Aerobic Biodegrada-tion of Plastic Materials Under Controlled CompostingConditions, Incorporating Thermophilic TemperaturesD6954 Guide for Exposing and Testing Plastics that Degradein the Environment by a Combination of Oxidation andBiodegradationE104 Practice for Maintaining Constant Relative Hu
25、midityby 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
26、practice 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 fr
27、omresults 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
28、 the 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 st
29、rength and elongation and modulus,sometimes change during this induction period; however, thesechanges are generally not due to molecular-weightdegradation, but are merely a temperature-dependent response,such as increased crystallinity or loss of volatile material, orboth. The effects of humidity a
30、re less well understood and aremore difficult 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
31、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 polym-erization.4.4 Some plastic materials such as PVC become brittle dueto loss of
32、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
33、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
34、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
35、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
36、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
37、than oxidation.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
38、standard is referenced onwww.astm.org.D7444 1824.8 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 fol
39、lowing exposure. However, since it does notaccount for the influence of stress or environment that isinvolved 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 information, su
40、ch as moisture, soil-type andcomposition, and mechanical-action effects that are consistentwith the 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 exposure temperatu
41、re and humidity).Therefore, for any application of the temperature index to bevalid, the thermal-aging program must duplicate the intendedexposure conditions of the end product. If the plastic materialis exposed in the end use in a manner not evaluated in the agingprogram, the temperature index thus
42、 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 practice can beused f
43、or 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 errors when Arrhenius
44、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 % confidence interval for
45、 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.).5.2.2 Forced-Ventila
46、tion 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 desirable to use a tubular-ovenprocedure, such as described in Specification E145, in accor-dance with Type 1A and Type IIB,
47、with 50 6 10 air changes/hand the requirements for uniformity extended to include therange of test temperatures. Recording instrumentation to moni-tor the temperature and humidity of exposure is recommended.5.3 Specimen RackA specimen rack or frame of suitabledesign to allow ready air circulation ar
48、ound 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 at different temperatures can becontrolled within a narrow range by using saturated aq
49、ueoussolutions 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 determined; 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 comparable to but nogreater than the maximum thickness of the intended
