ASTM_D_794_-_93.pdf

Designation:D 794 93Standard Practice forDetermining Permanent Effect of Heat on Plastics1This standard is issued under the fixed designation D 794;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(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice is intended to define the conditions fortesting the resistance of plastic sheet,plastic laminated mate-rials,and molded plastics to changes in properties due toexposure at elevated temperatures.Only the procedure for heatexposure is specified,and not the test method or specimen.Theeffect of heat on any property may be determined by selectionof an appropriate test method and specimen.1.2 This standard does not purport to address all of thesafety problems,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.NOTE1There is no similar or comparable ISO standard.2.Referenced Documents2.1 ASTM Standards:D 495 Test Method for High-Voltage,Low-Current,DryArc Resistance of Solid Electrical Insulation2D 618 Practice for Conditioning Plastics and ElectricalInsulating Materials for Testing3D 756 Practice for Determination of Weight and ShapeChanges of Plastics Under Accelerated Service Condi-tions3D 790 Test Methods for Flexural Properties of Unreinforcedand Reinforced Plastics and Electrical Insulating Materi-als3D 883 Terminology Relating to Plastics3D 1870 Practice for Elevated Temperature Aging Using aTubular Oven4E 145 Specification for Gravity-Convection and Forced-Ventilation Ovens53.Terminology3.1 Definitions:Definitions of terms applying to this prac-tice appear in Terminology D 883.4.Significance and Use4.1 Plastic materials exposed to heat may be subject tomany types of physical and chemical changes.The severity ofthe exposures in both time and temperature determines theextent and type of change that takes place.A plastic material isnot necessarily degraded by exposure to elevated temperatures,but may be unchanged or improved.However,extendedperiods of exposure of plastics to elevated temperatures willgenerally cause some degradation,with progressive change inphysical properties.4.2 Generally,short exposures at elevated temperatures maydrive out volatiles such as moisture,solvents,or plasticizers,relieve molding stresses,advance the cure of thermosets,andmay cause some change in color of the plastic or coloringagent,or both.Normally,additional shrinkage should beexpected with loss of volatiles or advance in polymerization.4.3 Some plastic materials may become brittle due to loss ofplasticizers after exposure at elevated temperatures.Othertypes of plastics may become soft and sticky,either due tosorption of volatilized plasticiser or due to breakdown of thepolymer.4.4 The degree of change observed will depend on theproperty measured.Mechanical or electrical properties may notchange at the same rate.For instance,the arc resistance,seeTest Method D 495,of thermosetting compounds improves upto the carbonization point of the material.Mechanical proper-ties,such as flexural properties,see Test Methods D 790,aresensitive to heat degradation and should be included in the testprogram.4.5 Effects of exposure may be quite variable,especiallywhen specimens are exposed for long intervals of time.Factorsthat affect the reproducibility of data are the degree oftemperature control of the enclosure,the type of molding,cure,humidity of the oven room,air velocity over the specimen,andperiod of exposure.Errors in exposure are cumulative withtime.Unless the effect of the molding procedure is a variableunder investigation,transfer or injection molding is recom-mended as the trapped air and volatiles have a better chance toescape.Incompletely cured thermosets tend to blister or crackwhen exposed to high temperatures.Certain materials aresusceptible to degradation due to the influence of humidity inlong-term heat resistance tests.Materials susceptible to hy-drolysis may undergo degradation when subjected to long-termheat resistance tests.1This practice is under the jurisdiction of ASTM Committee D-20 on Plasticsand is the direct responsibility of Subcommittee D20.50 on Permanence Properties.Current edition approved Feb.15,1993.Published April 1993.Originallypublished as D 794 44 T.Last previous edition D 794 82.2Annual Book of ASTM Standards,Vols 08.01 and 10.02.3Annual Book of ASTM Standards,Vol 08.01.4Annual Book of ASTM Standards,Vols 08.02 and 09.01.5Annual Book of ASTM Standards,Vol 14.02.1AMERICAN SOCIETY FOR TESTING AND MATERIALS100 Barr Harbor Dr.,West Conshohocken,PA 19428Reprinted from the Annual Book of ASTM Standards.Copyright ASTMNOTICE:This standard has either been superseded and replaced by a new version o