1、Designation: D7998 15Standard Test Method forMeasuring the Effect of Temperature on the CohesiveStrength Development of Adhesives using Lap Shear Bondsunder Tensile Loading1This standard is issued under the fixed designation D7998; the number immediately following the designation indicates the year
2、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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method concerns bonding and testing of woodadhe
3、sives and related adhesives using small scale tensilelap-shear samples in a manner that emphasizes transientcohesive strength as a function of bonding time and tempera-ture.1.2 Use of thin adherends enables bondlines to be rapidlyheated to elevated temperatures and maintained at thosetemperatures fo
4、r a range of times at a controlled pressurebefore testing.1.3 Optional rapid forced air cooling of bonds after pressingand immediately before testing enables the effect of testingtemperature on transient strength to be evaluated.1.4 Bond overlap distance is specified to ensure that failureoccurs in
5、the bondline rather than in unbonded portions ofadherend strips, and also to minimize the effect of shear stressnon-uniformity along the overlap during tensile testing.1.5 Standard wood or alternative non-standard materialsmust be of specified high quality and uniformity of structureand dimension to
6、 minimize variability of bonding and maxi-mize stress transfer into the bonds during testing.1.6 The effect of wood variability and type, or of theproperties of alternative non-wood materials, on bond strengthdevelopment may be explored using the method.1.7 Optional hermetic sealing of bond overlaps
7、 during theirheated pressing enables the effect of moisture on bonding to beevaluated.1.8 Thermal damage, either of pre-formed bonds or byprolonging bond forming times, may be evaluated as a functionof time and elevated temperature using this test method.1.9 The values stated in SI units are to be r
8、egarded asstandard. No other units of measurement are included in thisstandard.1.10 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 det
9、ermine the applica-bility of regulatory limitations prior to use. Some specifichazards statements are given in Section 10 on Hazards.2. Referenced Documents2.1 ASTM Standards:2D907 Terminology of Adhesives3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 adherend strip, none of
10、two adherend pieces thatconstitute a test specimen.3.2 adhesive spread rate, nthe mass of adhesive appliedper unit surface area of the overlap region of the adherend.3.3 contact heating, nthe transfer of heat energy to the testspecimen by the application of precisely temperature-controlled heads pre
11、ssing on the test specimens surfaces withcontrolled force.3.4 forced air cooling, nthe rapid cooling of the overlapregion of a test specimen after bonding at elevated temperatureby the controlled application of air jets onto both external facesof the overlap portion of the specimen.3.5 test specimen
12、, nthe lap shear sample made up of anadhesive between two pieces of adherend.3.6 thermal damage, nthe decrease in measured bondstrength due to exposure to a specified elevated temperature fora specified time.3.7 transient bond strength, nthe strength of a partiallyformed test specimen bond (sometime
13、s referred to as “greenstrength”) upon being tested.1This test method is under the jurisdiction of ASTM Committee D14 onAdhesives and is the direct responsibility of Subcommittee D14.30 on WoodAdhesives.Current edition approved Sept. 15, 2015. Published November 2015. DOI:10.1520/D7998-15.2For refer
14、enced 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, PO Box C700, West
15、Conshohocken, PA 19428-2959. United States14. Significance and Use4.1 The test method enables strength values for wood andother materials bonded with an adhesive under a range ofcontrolled bonding temperature, time, and pressure conditionsto be evaluated. Bond formation and subsequent testing isaffe
16、cted in a coordinated fashion, and this enables transientstrength values of sets of similar bond types to be exploredwith diverse parameters as independent variables. Principalamong these variables is the temperature at which bonds areformed and the time that selected temperatures are maintainedprio
17、r to testing. The use of controlled methods of adhesiveapplication, the rapid attainment of stable bond formationconditions, and the rapid transition to the bond testing modeenables snapshots of bond strength to be attained as bondsprogress from limited strength (or initial tack) to maximumstrength.
18、 Derived data may be used to evaluate and comparethe strength development characteristics of diverse types andformulations of adhesive. The method may thus be used to aidin tailoring and matching adhesives to the manufacture ofdiverse bonded products that involve heating.4.2 The method may also be u
19、sed to evaluate the co-dependent effect of temperature and time on the degradation ofsample bonds. Pressing temperatures up to 265C (509F) maybe necessary for such investigations of thermal degradation.Specimens are pressed for a range of times and temperaturesand very shortly thereafter tested eith
20、er at elevated temperatureor immediately following rapid forced air cooling.Alternatively, thermal damage of pre-formed bond samplesmay be evaluated by subjecting them to controlled temperatureand time sequences prior to testing.4.3 The method may also be used to evaluate the effect ofwood type and
21、variability, or of non-wood materials, on bondstrength development.4.4 By hermetically sealing the overlap region of samplebonds during their formation, the method may also be used toevaluate the effect of moisture and other resident volatile fluidson bond strength development.4.5 The method may als
22、o be used to evaluate the effect thatthe temperature at which variously formed bonds are tested hason their strength. Controlled rapid forced air cooling immedi-ately after bond formation but before testing is necessary forsuch investigations. This approach may be employed to ex-plore the thermoplas
23、tic characteristic of thermosetting adhe-sives and also the strength of hot melt adhesives as a functionof pressing and testing temperatures.5. Apparatus5.1 Integrated Bond Forming and Testing Apparatus:5.1.1 Sample Forming FunctionA small laboratory scaleapparatus that has two horizontally opposed
24、temperature-controlled pressing heads measuring approximately 25 by 25mm (1.0 by 1.0 in.) is required. The heads must closesynchronously to a controlled force and dwell time in order toform adhesively bonded specimens in a lap fashion. Thespecimens must be in line with the gripping mechanisms of the
25、apparatus (Fig. 13and Fig. 2) and position stops must beprovided within those mechanisms to ensure accuracy of bondoverlap distance during sample positioning. Temperature con-trol of the pressing heads between ambient and 150C (302F)must be maintained within 61.5C (2.7F). Pressing forcemust be contr
26、ollable between 20 and 800 6 2 N (4.5 to 180 60.45lbf), and press closure from the open position must beachievable within 2 s.5.1.2 Bond Testing FunctionFor measuring transient bondstrengths, the apparatus requires an integrated mechanicalloading function which acts in an axis perpendicular to that
27、ofthe sample forming function. Automated gripping of the endportions of test specimens is necessary to avoid any disruptionof partially formed bonds prior to their being pulled. Theapparatus must be capable of applying tensile testing force at acontrolled rate of 500N/s 6 25 N/s (112 6 5.6lbf/s) up
28、to atleast 1kN (224lbf), and load must be digitally sampled at afrequency of not less than 2 kHz during sample pulling in orderto capture the peak load at bond failure with an accuracy of6 2N (0.45lbf). The apparatus must be capable of affecting thetransition from press opening to the onset of pulli
29、ng within 2 s.Alternatively, if forced air cooling is employed after pressopening according to Alternative Test Method I and 5.1.3, thenthe onset of pulling must be possible within2softhecessationof bond cooling.5.1.3 Rapid Bond Cooling FunctionBonds may be testedafter rapid cooling to ambient tempe
30、rature by employingAlternative Method I, 9.1. A small retractable forced airattachment is required to affect rapid and controlled tempera-ture reduction of test bonds when the pressing heads areretracted at the end of the bond forming stage and immediatelyprior to the bond pulling stage. The transit
31、ion between pressopening and the activation of cooling must be achievablewithin 2 s. The cooling function must be capable of reducingthe bondline temperature so as not to exceed 31C (88F)within 5 s. A possible cooling device is given in Fig. 3.3Manufactured and available from Adhesive Evaluation Sys
32、tems, Inc., 155 SWMadison Ave, Corvallis, OR 97333, .A typical test sample mounted in an apparatus for rapid bond heating andpressing followed by bond pulling.FIG. 1 Typical Test Sample3D7998 1526. Adherend Type and Preparation6.1 Adherend TypeUnless otherwise noted in the report,uniform, planar and
33、 defect-free hard maple (Acer saccharumspp.) or European beech (Fagus sylvatica spp.) of 0.6 to 0.8mm (0.024 to 0.031 in.) thickness is to be used. The selectedthickness must be maintained within 6 0.04 mm (0.0016 in.)and the measured thickness must be reported. The surfaces tobe adhered must be kni
34、fe-cut and not sanded. Other species andnon-wood materials may be employed, and must be reportedfully. In such cases, thickness must be minimized to ensurerapid heat transfer into bondlines to achieve target tempera-tures up to 150 6 2C (302 6 3.6F) within 15 s, while havingsufficient tensile streng
35、th to ensure failure in the bondlinerather than in the unbonded portions of adherend strips.6.2 Adherend PreparationWood or related material is tobe loosely stacked and equilibrated at 21 6 2C (70 6 3.6F)and 50 6 5 % relative humidity for at least 12 h beforeadherend strips are cut to size. Photo-de
36、gradation due toexposure to sunlight and UV light must be minimized. Thestrips are to be cut from portions of wood veneer having cleargrain to provide strips of 120 6 0.2 mm (4.724 6 0.008 in.)length parallel to the grain 64 and width of 20 mm 6 0.5 mm(0.787 6 0.02 in.) across the grain. The specifi
37、ed precision inlength is required to ensure accuracy of overlap distance whenthe two adherend strips are mounted in the bond forming andtesting apparatus.7. Adhesive Type and Application7.1 Adhesive TypeA wide range of adhesive types may beevaluated using this test method as long as they may be appl
38、iedin an accurate and uniform fashion onto the overlap portion ofadherend strips. The method is well suited to the evaluation ofthe effect of temperature on the strength development ofthermosetting adhesives. It may also be employed with hotmelt adhesives in accordance with Alternative Method III, 9
39、.3.Adhesive type and method of preparation must be reported.7.2 Adhesive Application and Spread RateAwide range ofadhesive spread rates may be employed, depending upon theadhesive type. A spread rate of 50 6 5 g/m2(6.14 lb/1000 ft2)is standard. A standard adhesive application method is notspecified,
40、 but methods could include shielded spraying, contactor transfer printing, spatula, metered droplet deposition anddispersion methods, among others. Care should be taken tomaximize uniformity of adhesive distribution over the bondingarea. The application method employed and spread rate accu-racy achi
41、eved must be reported in accordance with 7.3.7.3 Adhesive Spread Rate Measurement and ReportingWeight gain of adherend strips due to adhesive applicationusing the reported method should be reported. Five randomlyselected strips are to be weighed before and after adhesiveapplication over a measured a
42、rea. Variability in the delaybetween adhesive application and weighing must be minimizedin order to minimize the effect of evaporation on the measure-ment. In any case, this delay must not exceed 10 s. Mean andstandard deviation values of spread rate must be reported in theunits of g/m2(lb/1000ft2).
43、8. Bond Formation and Testing8.1 Bond FormationAdherend pairs are to be mounted inthe bond forming and testing apparatus following the applica-tion of adhesive to the endmost portion of one of the adherendstrips. A non-standard alternative is to apply adhesive to bothadherend strips.Abond overlap of
44、 5 6 0.4 mm (0.197 6 0.016in.) is standard (see 8.1.1). The delay between adhesiveapplication and closure of the heated pressing heads onto theoverlap should be 7 6 3 s in order to minimize the effects ofadhesive pre-cure, penetration and evaporation. Deviationsfrom these limits must be reported, in
45、cluding when openassembly time is an independent variable. A pressing pressureof 2 6 0.2MPa (290 6 29 psi) is standard. This pressurerequires a force of 200 6 20N (50 6 5lbf) on the standard bondwith a 20 by 5 mm (.787 by .196 in.) overlap. Deviations fromthis pressure may be necessary, particularly
46、 to reduce compres-sion when non-standard wood or related materials of reducedhardness are employed or when pressing pressure is anindependent variable. Minimum pressing time must be suffi-cient to ensure the selected temperature is attained at thebondline within 62C (3.8F). For the standard adheren
47、d, thisminimum pressing time is 15 s.FIG. 2 Schematic of the Bonding and Testing ConceptAn attachment for rapid air-jet cooling which may be raised, activated andlowered automatically.FIG. 3 Attachment for Rapid Air-jet CoolingD7998 1538.1.1 Non-standard overlap values must be reported inaccordance
48、with Alternative Method V, 9.5. Care must exer-cised if overlap distances greater than standard are employedsince shear stress non-uniformity may be increased, a signifi-cant bending moment may be induced during pulling, andfailure outside the bond zone may occur. Large overlapdistances may be appro
49、priate to explore tack properties (lowshear strength) of adhesives.8.2 Bond TestingThe transition from press opening at theend of a selected pressing period to the onset of pulling forcerise must be within 2 s. Tensile testing force is to be applied ata rate of 500N/s 6 25N/s (112 6 5lbf/s). Slower rates may beemployed in non-standard tests where the effect of loading rateand duration of load studies are conducted; these methods mustbe reported in accordance with Alternative Method V of 9.5.Load must be digitally samp
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