Ultrasonic weld properties of heterogeneous polymers Polylactide and poly

Journal of Materials Processing Technology 211 (2011) 1358–1363Contents lists available at ScienceDirectJournal of Materials ProcessingTechnologyj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j m a t p r o t ecUltrasonic weld properties of heterogeneous polymers:Polylactide and poly (methyl methacrylate)Guohong Zhang a ,Jianhui Qiu a ,∗,Liang Shao b ,Mingzhu Liu b ,Min Zhang c ,Yongbo Wu aaDepartment of Machine Intelligence and Systems Engineering,Faculty of Systems Engineering,Akita Prefectural University,Akita 015-0055,Japan bCollege of Chemistry and Chemical Engineering,Lanzhou University,Lanzhou 730000,PR China cInstitute of Chemistry and Chemical Engineering,Shaanxi University of Science and Technology,Xi’an,Shaanxi 710021,PR Chinaa r t i c l e i n f o Article history:Received 3August 2010Received in revised form 1February 2011Accepted 5March 2011Available online 15 March 2011Keywords:PolylactidePoly (methyl methacrylate)Ultrasonic weld Welding strengthInterposed sheet (IPS)a b s t r a c tThis paper aims to investigate the effects of welding conditions and the correlation between welding strengths and interfaces structures on the weld between polylactide (PLA)and poly (methyl methacrylate)(PMMA).Using our proposed ultrasonic weld (USW)method,PLA and PMMA composites (PLA/PMMA)are valid to employ as interposed sheet (IPS)experimentally.There are optimum welding times and welding presses depending on the peak welding strengths.The maximum welding strength can reach as high as 90%of the tensile strength of the matrix PLA,while the welding time is 4s and welding press is 0.6MPa.But excessive welding presses are deleterious to the weld,and it can also bring IPS destroying during the weld work.© 2011 Elsevier B.V. All rights reserved.1.IntroductionUltrasonic welding (USW)for plastics is a joining technique without additional,foreign substances,such as adhesives or sol-vents,with well defined local heating of only the joint area,with short cycle times in the range of just a few seconds and on com-paratively cost effective machines (Truckenmüller et al.,2006).During ultrasonic consolidation process,energy generated from an ultrasonic transducer is transferred to a workpiece through a conotrode in the form of ultrasonic oscillation (Li and Soar,2009).Conventionally,ultrasonic weld (USW)techniques have always been used between only identical polymers (Qiu et al.,2009).Recently,welding immiscible polymers with a supercriti-cal fluid (Wang et al.,2007)is reported.However,welds between heterogeneous materials and substances are largely necessary in various fields and paid much attention to in the latest days (Kocabekir et al.,2008).However,so far there is no method for weld between heterogeneous polymers but almost only adhe-sive way which usually brings the problems such as durability,heat resistance (The Adhesion Society of Japan,2004),poi-sonous character and environmental hormone (Brockmann et al.,2009).∗Corresponding author.Tel.:+81184272134;fax:+81184272134.E-mail address:qiu@akita-pu.ac.jp (J.Qiu).Use our new USW that inserting an interposed sheet (IPS)between the two work pieces (WPs)may solve the problems above.This method (Qiu et al.,2007)is that through the ultrasonic vibra-tion of IPS,the frictional heat can be produced at the interfaces between IPS and WPs and melts the polymer.After the polymers solidify again,the welds are achieved.For many polymer,the mold-ing method are used as injection molding (An and Chen,2008).In our method,IPS is also made by injection molding method,the properties of molding process have been discussed by many researchers (Qiu and Wu,2007).A biodegradable polymer,polylactide (PLA,crystalline)has been used for biomedical applications,such as drug-delivery systems,implant materials for bone fixation,and surgical sutures because of its good biocompatibility and nontoxicity (Zhang et al.,2003).Additionally,PLA is useful for welding with other polymers,espe-cially in the areas in which adhesions are limited (Viljanmaa et al.,2003).Poly (methyl methacrylate)(PMMA)is an amorphous poly-mer which has been widely used because of its good engineering properties.Joints between these two polymers (amorphous and crystalline,transparent and not)are significative and helpful to investigate the weld mechanism.A weld line is not a line as the name suggests but a three dimen-sional region with complex morphology (Merah et al.,2004).In the present paper,we have achieved the weld between PLA and PMMA and the effects of welding conditions and the correlation between welding strengths and interfaces structures (weld line morphology)are investigated.0924-0136/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2011.03.005G.Zhang et al./Journal of Materials Processing Technology211 (2011) 1358–13631359Fig.1.Novel USW principle schematic drawing.2.Materials and experimental methods2.1.Materials and WPsCrystalline PLA(L-PLA,Unitika Ltd.,Japan)with melting point of170◦C and amorphous PMMA(MD,Mitsubishi Rayon CO.,Ltd., Japan)are used as matrix polymers.Wps of PLA and PMMA are made using an injection molding machine(FANUC Ltd.,ROBOSHOT S2000i100A)in a half shape dumbbell(half of JIS1).The molding conditions for PLA are extrusion temperature(nozzle temperature) of190◦C,extrusion rate of20%,mold temperature of40◦C and cooling time of30s and those of PMMA are extrusion tempera-ture(nozzle temperature)of230◦C,extrusion rate of17%,mold temperature of40◦C and cooling time of30s.2.2.IPS preparationThe materials of IPS are made by PLA and PMMA compos-ite(PLA/PMMA)according to the injection molding.PLA/PMMA is firstly molded as dumbbell(JIS1)with PLA and PMMA at1:1volume ratio.The molding conditions are nozzle temperature of190◦C, extrusion rate of15%,mold temperature of40◦C and cooling time of30s.After molding again to modify its thickness to1mm with the hot-press machine(Imoto Ltd.,molding parameters:tempera-ture of200◦C and heating time of10min),the plastic layers are cut as a rectangle(12mm×8mm×1mm;length×width×thickness) in order to install them on the IPS holder of the apparatus.In the front edge at the middle of the IPS,a3mm diameter hole is drill for fixing in place with a screw.2.3.Ultrasonic weldAs is shown in Fig.1,WPs(made of PLA and PMMA)arefixed on the IPS holders of the slide guide,and the IPS is installed on the IPS holder with the screw.After IPS is laid between the two WPs, welding presses are given and the joints begin.Welding conditions are as follows:frequency of28kHz,amplitude of30␮m,vibrating time from1.0s to5.0s,and welding press from0.1MPa to0.6MPa. Moreover,the length from thefixing point to the welding surface (called the protruded length)is set to10mm to lessen the deflec-tion effect.To get uniform surface roughness of about Ra=0.2␮m, abrasive paper is placed on slide guide in the verticaldirection.Fig.2.Nominal strain versus nominal stress of PLA,PMMA and composite of PLA/PMMA.3.Measurements and characteristics3.1.Tests of tensile and welding strengthsTests of tensile and welding strengths are carried out using a universal tester(Instron Model Series3360)at the crosshead speed of5mm/min and temperature of23◦C±2◦C.The correlation of the load,deformation and the curve of nominal stress and strain can be calculated,and the breaking stress can also be obtained from the peak load.3.2.Rupture surface analysisThe rupture surfaces before and after tensile tests are observed using a scanning electron microscope(SEM;Hitachi,Model S-4300) to analyze the correlation of interfacial morphology and strength. For the SEM observations,specimens are needed to modify a Pt and Au coating on surface.3.3.Surface temperature measurementsIn order to observe the welding process and the change of the interfacial temperature,measurements are made with infrared radiation thermometers(Chino Corp.,Model CPA-SC620),where the frame time is30Hz,and resolving power is640×480Video Format.Together with a FT IR system(ThermaCAM Researcher Pro.2.8),the highest surface temperatures of interfaces can be obtained where the region is chosen in advance.4.Results and discussion4.1.Mechanical properties of PLA,PMMA and PLA/PMMA compositeFig.2plots of nominal strain versus nominal stress of the three materials.The yield strengths of PLA and PMMA are58MPa and 65MPa,respectively,and the nominal strains are all about10%, showing a brittle characteristic.For the PLA/PMMA composite,the yield strength is significantly enhanced to73MPa which is over18% than that of PLA.Moreover,the nominal strain approaches180%, showing a typical ductile characteristic.This indicates that the strength and plasticity of the PLA/PMMA have been much enhanced compared with the matrix polymers.From the SEM photo shown in Fig.3at rupture surface of PLA/PMMA,we can see that the two materials are very compatible,there are only plastic deformations and no phase separation.1360G.Zhang et al./Journal of Materials Processing Technology211 (2011) 1358–1363Fig.3.SEM photo at rupture surface of PLA/PMMA.Fig.4plots the TG curves of the three materials.The decom-position temperatures(the temperature at the weight loss of2%) are close to each other,about320◦C.This suggests that the ther-mal properties of the composite are almost the same as the matrix polymers,and the decomposition temperature has not changed a lot.That is to say,when welding,IPS cannot decompose before the matrixes.The mechanical properties of PLA/PMMA are enhanced very much,especially the change from brittle to ductile.The IPS made of PLA/PMMA will not cause any loss of welding strength.Further,it can also relax the residual stress given after welding and improve welding performance due to its excellent plasticity.Therefore,the following experiments use PLA/PMMA as IPS to weld.4.2.Welding time effects on welding strengthEffects of welding time on welding strength are shown in Fig.5. We can see that the welding strength is generally increased with welding time prolonging when welding press is0.1MPa.When the welding press is set to0.3MPa,the peak welding strength of 47MPa can be obtained at welding time of3s,and when the weld-ing press is set to0.6MPa the peak welding strength of45MPa can be obtained at4s.Then the welding strength decreases as the welding time continue increasing.That is to say,the peak welding strengths can be achieved at the optimum welding times.We can also conclude that no matter how big the welding presses are set, the welding strength is about33MPa at welding time of5s.In addi-tion,with the welding press increasing,the peak strength shouldfirst reach normally.But when the welding press is0.6MPa,ithasFig.4.TG curves of PLA,PMMA,andPLA/PMMA.Fig.5.Effects of welding time on welding strengths.taken longer time(4s)than the one of0.3MPa.The reasons will beexplained in Figs.10and11.The SEM photos of the rupture surfaces of welded inter-faces when welding press is0.3MPa are shown in Fig.6.FromFigs.5and6,due to the short welding time(below3s,Figs.6(a)and(b)),there is only a little heat,the non-welded areas are broadand the strengths are weak.With the welding time increasing,thenon-welded areas shrink and the strengths enhance accordingly.The welding time of3s(Fig.6(c))is critical,the frictional heat riseis large and the peak welding strength can be obtained.The reasonmay be that they rupture in the PLA matrix and the work piecesshow the constriction at the middle part,resulting in the rupturesurface shrinking.Then,when welding times prolong again,due tothe weld for a long time under a high temperature,they are mainlyrupture of IPS(Fig.6(d)).That is to say,excessively long weldingtime is not good for weld.4.3.Welding press effects on welding strengthEffects of welding press on welding strength are shown in Fig.7.Welding strength is enhanced earlier as the welding presses golarger and the peak strength occurs at welding press of0.4MPawhen the welding times are1s and2s.After that,the strengthweakens.When welding time is4s,the strengths are also enhancedwith the press increasing.But from0.4MPa to0.6MPa,the effectturns to be low.Usually,when the welding press increases,the friction isinspired much,heating is acute and the weld can be achieved dueto sufficient melting.When the welding presses are too small,thefrictional heat produces a little and the interfacial temperatures arelow.When the welding presses are too large,the frictional heat isacute accordingly,resulting to the small ultrasonic amplitude andthe interfacial temperatures can also be low.Therefore,the weld-ing press has an optimum value.For the presses of0.3MPa and0.6MPa,the temperatures at1s are low and the strengths are weak.So the press of0.4MPa may be the better press and even though atthe welding time of1s,the welding strengths are strong enough.When the welding areas cover the whole surface of the interface,the strengths can also reach the peak value.Moreover,as the presscontinues to increase,the frictional heat is getting too much whichcause the high temperature and polymer decomposition.Finally,air bubbles are produced in the interfaces,resulting weak weldingstrengths.The rupture surfaces of welded interfaces for welding time of2sare shown in Fig.8(a)and(b)when the welding presses are0.3MPaand0.6MPa,respectively.There is a non-welding part(Fig.8(a))G.Zhang et al./Journal of Materials Processing Technology211 (2011) 1358–13631361Fig.6.Rupture surfaces of welded interfaces when welding press is0.3MPa,with the welding time of:(a)1s,(b)2s,(c)3s and(d)5s.Fig.7.Effects of welding press on welding strengths.due to insufficient heating.Fig.8(b)shows that bubbles are resulted from overheating temperature which affects the welding strengths.The strengths for welding time of4s are obviously enhanced compared with the ones at times of1s and2s.There is no peak strength when the presses go up.But the strengths are sharply enhanced earlier and from0.4MPa the strengths increase mildly. We believe that from0.1MPa to0.4MPa,the strengths become high due to the more frictional heat.But at the welding press of0.4MPa(Fig.9(a)),the welded areas are extended resulting in higher strengths.At the welding press of0.6MPa(Fig.9(b)),the bubbles are caused by too large welding press.Because of the slightly longer welding time of4s,the bubbles mixing with the melted materials are squeezed out of the interfaces.Finally,the interfaces become smooth,so the two strengths(for welding press of0.4MPa and0.6MPa)are all whole surface joint and they are nearly the peak strengths as above.4.4.Analysis of interfacial temperature and weld mechanismThe peak surface temperatures around the interfaces are shown in Fig.10for welding time of5s and welding press of0.3MPa.We have also tested all the welds at various welding time when welding press is0.3MPa.The curves share the same peak temperature and are different from the time of the temperature begin to low down, where the temperature would turn down once the vibration is over. From Fig.10we see an inflexion at welding time of1s(the surface temperature is about100◦C).After that the temperature increases sharply and from1.5s the temperature reaches the highest value of210◦C until the vibration is stopped(5s)finally.From the results above,we can conclude that when the weld-ing time is below1s,the surface temperatures cannot reachthe Fig.8.Rupture surfaces of welded interfaces when welding time is2s,with the welding presses of:(a)0.3MPa and(b)0.6MPa.1362G.Zhang et al./Journal of Materials Processing Technology211 (2011) 1358–1363Fig.9.Rupture surfaces of welded interfaces under welding time of 4s,with the welding press of:(a)0.4MPa and (b)0.6MPa.melting points of the materials,so the weld cannot be formed.Because these temperatures are the surface temperatures accord-ing to the instrument,the top temperature must be the temperature of melted polymer.So the constant top temperature value is sug-gested that the melted materials are squeezed out of the interfaces continuously.At 2s,the surface temperature has just reached the maximum.The polymers are starting to melt,and the diffusion of molecules is also just beginning,the weld is not the whole surface weld.At 3s,the interfacial parts have melted enough,the bubbles and melted materials are squeezing out of the interfaces,which can help to make the interfaces smooth.At this time,the peak strengths are obtained.If welding time continues prolonging,the surface is covered by the melted materials,and the inner tempera-ture may be much higher,especially at the later weld,the materials may decompose due to the overheating temperature keeping a long time.Finally,the strengths turn lower.This is the reason whytheFig.10.The top surficial temperatures around interfaces under welding time of 5s and welding press of 0.3MPa.Fig.11.The top surficial temperatures around interfaces under welding time of 5s and welding press of 0.6MPa.strengths of specimens welded for 4s and 5s decreasing when the welding press is 0.3MPa.The peak surface temperatures around interfaces are shown in Fig.11for welding time of 5s and welding press of 0.6MPa.Also,the temperatures of weld at other welding times are tested and the peak temperatures are all similar to Fig.11.Further,we see an inflexion at welding time of 1s,like the curve of Fig.10above.The temperatures rise sharply and reach the first peak tempera-ture until about 3s.Then,the temperature reaches the second (the highest)value 210◦C and keep until the vibration stop (5s).From the results above,we can conclude that no matter whether the welding press is (0.3MPa or 0.6MPa),there is one inflexion at welding time of 1s,then the temperature continues to rise.That is to say,the weld cannot be achieved before 1s.Due to the lower interfacial temperature,when welding time prolong,the welding presses are so large that the IPS has been destroyed and the weld cannot achieve due to the friction has been stopped.Therefore,the temperatures decrease when the welding time is about 3s.If the welds stop at 3s,the disfigurements produce easily and remain at the interfaces,resulting to the weak welding strengths.With the welding times turns longer to 4s,the melted polymers have enough time to fill in the space of the interfaces,which prevent the disfigurements remaining.It is indicated that the strengths at 3s are much weaker and temperatures decrease due to the IPS being destroyed which does not enhance the welding strengths.These are the reasons why the peak strengths are at 4s for specimens welded at 0.6MPa compared with 3s for 0.3MPa.5.ConclusionsThe effects of welding conditions and the correlation between welding strengths and interfaces structures on the weld between polylactide (PLA)and poly (methyl methacrylate)(PMMA)are ing our proposed ultrasonic weld (USW)method,PLA/PMMA composites are valid to employ as interposed sheet (IPS)experi-mentally.There are optimum welding times and welding presses depending on the peak welding strengths.The maximum weld-ing strength can reach as high as 90%of the tensile strength of the matrix PLA,while the welding time is 4s and welding press is 0.6MPa.But excessive welding presses are deleterious to the weld,and it can also bring IPS to be destroyed easily during the weld work.G.Zhang et al./Journal of Materials Processing Technology211 (2011) 1358–13631363AcknowledgementsThis work was supported by the Japan Society for the Promotion of Science for the supply by the basic research program C,Grant-in-Aid for Scientific Research,2007–2008.The authors would like to acknowledge the Japan Society for the Promotion of Science for their help.ReferencesAn,C.-C.,Chen,R.-H.,2008.The experimental study on the defects occurrence of SL mold in injection molding.J.Mater.Process.Technol.201,706–709. 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