Toughening of Polyamide-6with a Maleic Anhydride Functionalized Acrylonitrile-Styrene-Butyl Acrylate Copolymer
Zhenguo Liu,?
Yunjiao Deng,?Ye Han,?Ming Chen,§Shulin Sun,?Chunlei Cao,?Chao Zhou,?
and Huixuan Zhang *,?,??Changchun Institute of Applied Chemistry,Graduate School,Chinese Academy of Science,Changchun 130022,China
?Engineering Research Centre of synthetic resin and special ?ber,Ministry of Education,Changchun University of Technology,Changchun 130012,China
§Research Institute,Jilin Petrochemical Company,Jilin132021,China
1.INTRODUCTION Polyamides are an attractive class of engineering polymers because of their excellent strength and sti ?ness,low friction,and chemical and wear resistance.1However,they are highly notch sensitive.In other words,they are often ductile in an unnotched state,but fail in a brittle manner when notched.To improve the notch impact strength,many suitable modi ?ers have been adopted to toughen polyamides,such as elastomers 2?8and core-shell impact modi ?ers.9?18Obviously,the morphology of the resulting polyamide blend is a major factor in determining the ?nal mechanical properties including Izod impact https://www.doczj.com/doc/d918796438.html,pared with other modi ?ers,a remarkable characteristic of core-shell impact modi ?er is that their size is set during the synthesis process and is not susceptible to process.19Consequently,core-shell impact modi ?ers have attracted the attention of many researchers,especially butadiene-g-styrene-co-acrylonitrile (ABS).Since combining solvent resistance,high sti ?ness and strength of polyamide-6(PA-6)and good toughness and surface texture of ABS,PA-6/ABS blends have been of commercial and scienti ?c interest.12?17However,simple blends of these polymers usually can not obtain satisfactory properties,and their phase morphology,which results from their immiscibility or poor interfacial adhesion.Various approaches to promoting the reactive compatibilization of PA-6/ABS have been reported to improve the miscibility of polymer blends and interfacial adhesion between matrix and disperse phase in the literatures,21?24which can be divided into two main strategies.One involves addition of a polymer that is miscible with the styrene-acrylonitrile (SAN)copolymer matrix of the ABS and can react with the amine end-groups of nylon phase.13,15,21,22Another involves grafting of functional groups to the ABS prior to blending with the polyamide.17,18,23,24Compared to the ?rst strategy,the second is simpler.However,PA-6/ABS blends have a fatal drawback stemming from the physical (or chemical)aging of butadiene rubber in ABS because of a double bond in the repeat unit of polybutadiene,which would undergo physical (or chemical)aging caused by ultraviolet (UV)radiation in outdoor applications.As a result,mechanical strength would decline continuously and its color would https://www.doczj.com/doc/d918796438.html,pared
with ABS terpolymer,ASA (butyl acrylate-g-styrene-co-acrylonitrile)terpolymer prepared by grafting copolymerization of styrene and acrylonitrile monomers onto acrylic rubber particles has a similar structure except that the butadiene rubber was replaced by butyl acrylate,which can solve this drawback.However,the lower toughening e ?ciency of ASA has limited its applications.
The focus of this study is to explore a novel weather resistant PA-6/ASA alloy,whose mechanical properties can be comparable with that of PA-6/ABS.In this study,maleic anhydride functionalized acrylonitrle-styrene-butyl acrylate (F-ASA)was prepared in an emulsion polymerization process.During the synthesis of ASA,di ?erent contents of MA monomer were added to the reactive system.The e ?ect of MA content in the F-ASA copolymers on the morphological and mechanical properties was investigated.Scanning electron microscopy (SEM)was used to observe the fracture morphology of the toughened PA-6,and the toughening
mechanisms were also investigated.
Received:December 17,2011
Revised:June 11,2012
Accepted:June 14,2012
Published:June 14,2012
2.EXPERIMENTAL SECTION
2.1.Materials.The PA-6was purchased from Longjiang Plastics Plant,and the concentrations of carboxyl and amine groups are42.6μeq/g and51.2μeq/g.n-Butyl acrylate(BA), styrene(St),and acrylonitrile(AN)used for the polymerization were commercially available from Jilin Chemical,China. Sodium dodecylsulfate(SDS)was used as received.The F-ASA materials were synthesized by the emulsion polymer-ization method in our lab.ASA is a typical core-shell polymer, wherein a soft core made up of the cross-linked poly butyl acrylate(PBA),is grafted by a“shell”of acrylonitrile-styrene (SAN)copolymer.The soft phase of PBA particles induces toughening,while the shell of SAN copolymer allows isolation of the particles from the emulsion by providing a hard coating that keeps the rubbery cores from adhering to one another during the drying process and MAH groups improve compatibility between the modi?ers and the matrix(PA-6). The particle sizes of these materials were determined with dynamic light scattering spectroscopy,DLS(Brookhaven90 Plus Particle Size Analyzer).20
A redox initiator system CHP-Fe2+-EDTA-SFS was used as initiator.Cumene hydroperoxide(CHP,Merck Co.),ethylene diaminetetraacetic acid monosodium salt(EDTA,Ishisu Pharm.Co.Ltd.),ferrous sulfate hyxahydrate(FES,Ishisu Pharm.Co.Ltd.),and sodium formaldehyde sulfoxylate(SFS, Katayama Chemical)were used without further puri?cation. Deionized(DI)water was used in the polymerization.
2.2.Preparation of F-ASA.All reactions were seeded under nitrogen atmosphere.F-ASA was prepared via emulsion polymerization.Cross-linked PBA latex was prepared at75°C in advance as described in other work.20Then AN,St,and MAH were polymerized on PBA particles.F-ASA was prepared at65°C in a3L four-neck glass reactor,equipped with a re?ux condenser,a sampling device,a nitrogen inlet,and a two-blade anchor-type impeller coated with PTFE rotating at300rpm, and the reaction took place in an alkaline condition at pH10. First,KOH were mixed together in the glass reactor and stirred
for?ve minutes under nitrogen,and then the azetropic mixture of St/AN(75/25)with CHP were added in a continuous feeding way to the reactor with a feeding speed of1mL/min. After the reaction of St/AN,MAH solution was added to the reactor in the same way.The copolymers were coagulated, washed,and then dried in an oven at65°C for24h before being used.In F-ASAs,the weight ratio of PBA to SAN is50/ 50.F-ASA copolymers were named by their MAH content,that
is,F-ASA2indicates the weight percent to the total weight of St and AN was2%.Table1describes the information about F-ASA materials used in this study.
2.3.Blend Preparation.Before melt mixing,all polymers were completely dried in a vacuum oven at80°C for12h.PA-6/F-ASA blends were prepared using a two-screw extruder(D =30mm,L/D=32).The temperatures along the extruder were all235°C,and the rotation speed of the screw was70 rpm.The extruder produced strips of the blended material from the melt,which were subsequent cooled in a water bath and then palletized.In this work,the rubber content of the PA-6/F-ASA blends was20wt%except where de?ned otherwise. The PA-6/F-ASA blends were dried in a vacuum oven at65°C for24h.Then,injection-molding was carried out to prepare Izod impact specimens and tensile specimens.The temper-atures in the three regions of the injection molding machine were all240°C,the system pressure was52bar and the mold temperature was65°C.
Morphological Properties.The disperse morphology of F-ASA in the blends was characterized by scanning electron microscopy(SEM)(model Japan JSM-5600).A notched sample was freeze-fractured,and the fractured surface was cut at?100°C with a glass knife until a smooth surface was obtained.Then the samples were coated with a gold layer for SEM observation to investigate the dispersion of the modi?er particles(F-ASA).
Mechanical Properties.The notched Izod impact strength of PA-6blends was measured by an XJU-22Izod impact tester at23°C according to ASTM D256.The dimensions of the samples were63.5×12.7×6mm3.The notch of the Izod impact specimens was milled in by a machine with a depth of 2.54mm,an angle of45°,and a notch radius of0.25mm.The tensile tests were carried out using an Instron AGS-H tensile tester at crosshead speed of50mm/min at room temperature according to ASTM D638.For both mechanical tests,?ve samples were tested,and their results were averaged.
3.RESULTS AND DISCCUSION
3.1.Identi?cation of Reactions.The Molau test was conducted in this work to con?rm the compatibilization
reaction between PA-6and the F-ASA interface.Molau25and Illing,26working with emulsions consisting of two immiscible polymer solutions and a grafted copolymer,proposed that the formation of a white,colloidal suspension indicates the emulsifying action of the graft copolymer.In this work,the blend is placed in formic acid,which is a solvent for PA-6and a
Table1.Properties of F-ASA Used in This Work
designation
ratio of
PBA/SAN(w/w)
ratio of
St/AN(w/w)
MAH
content a
(wt%)
PBA
particle size
(μm)
F-ASA05075/2500.26
F-ASA25075/2520.26
F-ASA45075/2540.26
F-ASA65075/2560.26
F-ASA85075/2580.26
F-ASA105075/25100.26
a MAH content referring to the weight percent to the total weight of St and AN.Figure1.Molau test solutions of formic acid and each of the following blends:(a)PA-6/ASA;(b)PA-6/F-ASA2;(c)PA-6/F-ASA8.
nonsolvent for ASA and F-ASA,and the Molau test results are shown in Figure 1.As it is shown,the PA-6/F-ASA0blend showed phase separation after mixing,while the PA-6/F-ASA blends showed a milky,colloidal solution,which is undoubtedly attributed to the emulsifying e ?ect of a graft copolymer in situ formed during melt processing.A similar phenomenon was found by ABS-g-GMA/PA-6blends by Sun et al.18
3.2.Morphological Properties.The morphology of PA-
6/ASA blend and PA-6/F-ASA blends with MAH contents ranging from 2%to 10%was investigated by scanning electron microscopy.Figure 2shows the freeze-fractured SEM micro-graphs.As the arrows in Figure 2a showed,the F-ASA2particles clustered obviously in the PA-6matrix,and the interface between PA-6and the modi ?ers was clear,which showed
poor interfacial adhesion between PA-6and F-ASA.With the amount of MAH increasing,the modi ?er particles could disperse uniformly,and the interface between PA-6and the dispersed phase was obscure,which showed that the interfacial adhesion between the two phases improved because
Figure 2.Morphology of the fracture surfaces of PA-6/F-ASA blends:
(a)PA-6/F-ASA2;(b)PA-6/F-ASA8;(c)PA-6/F-ASA10.Figure 3.E ?ect of MAH content on Izod impact strength of PA-6/F-
ASA blends.Figure 4.Tensile behavior of PA-6/ASA and PA-6/F-ASA blends.
Figure 5.E ?ect of rubber content on impact strength of PA-6/F-ASA8.
of the compatibilization reactions between PA-6and F-ASA.And the domain size of the dispersed particles decreased.3.3.Mechanical Properties.The e ?ect of MAH content in F-ASA on Izod impact strength of the PA-6/F-ASA blends is shown in Figure 3.The PA-6/F-ASA0blend still behaved as a brittle material because of the incompatibility of PA-6and ASA.The incorporation of F-ASA2and F-ASA4generated no signi ?cant improvement in toughness;however,when the content of MAH in F-ASA was higher than 8%,a brittle-ductile transition was found and the sample fractured in a ductile mode.Hence,the optimum MAH content was 8%in our work.Figure 4shows the stress ?strain curves of PA-6/F-ASA blends.As shown in the Figure 4,the yield stress is in ?uenced insigni ?cantly by MAH content in the particles.According to the Ishai ?Cohen model,27the tensile yield stress,σyt (Φ)of a composite containing a volume fraction,Φ,of low modulus inclusions can be expressed as follows:σσΦ=?Φ()(0)(1 1.21)yt yt 2/3(I)where σyt (0)is the yield stress of the matrix.Applying this model to blends of PA-6/F-ASA,it can be seen that the volume
fraction of rubber particles in all the PA-6/F-ASA blends are the same,which is the reason why yield stress of PA-6/F-ASA blend varies insigni ?cantly.However,because of the compatibilization reactions between PA-6and F-ASA,the interface adhesion improved with MAH content increasing.Hence,the elongation at break and fracture energy increased as shown in Figure 4.As discussed above,the modi ?er particles (F-ASA)could not disperse uniformly when the MAH content in F-ASA was lower than 8%.Then,there is a distribution of thin and thick ligaments among modi ?er particles.Thin ligaments have a relatively low yield stress since they tend to
be in plane stress,whereas thick ligaments have higher yield stresses because of an incompletely relieved plane strain situation.Local excessive deformation would take place.As a result,the blends would break in a premature mode (low notched impact strength and elongation at break).
The e ?ect of rubber content on the notched impact strength of PA-6/F-ASA8was investigated.As shown in Figure 5,the impact strength of PA-6/F-ASA8increased with rubber content signi ?cantly;and a brittle-ductile transition was found when the rubber content was 20wt %.
In most blends,a discontinuous jump in impact energy at a certain temperature can be observed,which is called brittle-ductile transition (BDT)temperature.Below this temperature,plastic deformation mainly takes place before crack initiation,whereas above the BDT plastic deformation also occurs as the crack propagates.Figure 6shows the e ?ect of temperature on the Izod impact strength of PA-6/F-ASA blends with di ?erent MAH content in the F-ASA copolymers.At lower temperature,all the blends fracture in a brittle mode,and the fractured samples show slightly light stress whitening only near the notch tips.With the temperature increasing,the notched impact strength increases and BDT can be found.As for PA-6/F-ASA2,the Izod impact strength is only about 400J/m even when the temperature is 60°C.The BDT temperature of PA-6/F-ASA4blend is approximately 50°C,which is near the T g of PA-6.With MAH content increasing further,the BDT temperature of PA-6/F-ASA shifts to a lower temperature.The BDT temperature is only about 0°C when the MAH content increases to 8%and 10%.
Figure 6.E ?ect of temperature on Izod impact strength of PA-6/F-ASA blends.Figure 7.Morphology of the fracture surfaces of PA-6/F-ASA at room temperature:(a)PA-6/F-ASA2;(b)PA-6/F-ASA10.
Figure 8.Schematic presentation of the di ?erent sample planes in the fractured impact sample of PA-6/F-ASA10(XZ planes are studied in
this paper).
3.4.Deformation Mechanism.Figure 7shows the digital photographs (the top right corner)and fractured surface of test samples formed during Izod impact tests at room temperature,which showed macroscopic stress-whitening and plastic ?ow,respectively.As shown in Figure 7a,the low impact strength of the PA-6/F-ASA2is clearly re ?ected,and the fracture surface shows the typical characteristics of brittle fracture:a relatively ?at,smooth surface with few signs of deformation;while,as the MAH content increases,an apparent ductile fracture character-istic is clearly visualized as shown in Figure https://www.doczj.com/doc/d918796438.html,pared with PA-6/F-ASA2,macroscopic stress-whitening and plastic ?ow can be found in PA-6/F-ASA10.The matrix yielded apparently and the dispersed phase seriously deformed.Hence,the fracture energy was released e ?ectively,and impact strength was improved signi ?cantly at room temperature.To correlate the external morphology of the ductile surface to the internal deformation mechanisms of the stress-whitening zone,was chosen for SEM analysis.Three scanning electron micrographs of varying distances to the fracture surface were obtained in the XZ planes through the stress-whitening zone of the sample,as de ?ned in Figure 8.To observe the XZ plane,a sharp notch was prepared vertical to the fracture surface only about 1mm away from the stress-whitening zone of the fractured Izod impact specimen.After being submerged in liquid nitrogen for 3h,the specimens were split to investigate deformation mechanisms as shown in Figure 8.The deformed morphology inside the XZ plane is shown in Figure 9.A number of voids much larger than the size of modi ?er particles can be seen below the fracture surface (Figure 9a).These cavitated rubber particles deform so seriously that they cluster together,and the matrix around the cavitated rubber particles have plastically deformed in a similar degree.With increasing the distance to the fracture surface,plastic deformation of the matrix disappears and only a few smaller voids can be found (Figure 9c).The micrographs taken in the XZ planes reveal that the stress-whitening is a result of
cavitation within the rubber particles.And cavitation within the matrix (crazing)is prevented by a high entanglement density of PA-6,and a fatal crack is prevented.The matrix material yielded,which is evident from substantially irreversible deformation of the rubber particles.This phenomenon is consistent with the stress ?strain transition theory proposed by Yee.28The shearing yielding is initiated by the stress concentration associated with rubber particles;consequently,the cavitation of the rubber particles releases the hydrostatic tensile strength and spurs the shear yielding to proceed.So in the PA-6/F-ASA blend systems,shear yielding of the PA-6matrix and cavitation of rubber particles are the major toughening mechanisms.
4.CONCLUSIONS
In this work,a series of maleic anhydride functionalized acrylonitrile-styrene-butyl acrylate (F-ASA)with di ?erent anhydride group content were prepared via emulsion polymer-ization.The functionalized particles were used as impact modi ?ers for PA-6.Molau test identi ?ed the interfacial reactions between the epoxy groups of F-ASA and the amine or carboxyl groups of PA-6.
Morphological observation showed that when the F-ASA
particles with low MAH content (lower than 8wt %)were mixed with PA-6,a poor dispersion of F-ASA was found because of low interfacial adhesion between PA-6and F-ASA.As the MAH content increased from 2to 10wt %,the interfacial adhesion between F-ASA and PA-6increased,and the domain size of the dispersed phase decreased.Mechanical tests showed that the notched impact strength of PA-6was e ?ectively improved by the use of F-ASA.At a rubber content of 20%by weight in PA-6/F-ASA blends,it was found that the impact strength increased with the MAH content.When the MAH content was 8%in F-ASA,the impact strength was 1008J/m.The in ?uence of rubber content on the properties of PA-6/F-ASA8blends was investigated.The results showed that the brittle-ductile transition took place when the rubber content was 20%.Moreover,increasing the MAH content of the blends reduced the brittle-ductile transition
Figure 9.Fracture zone in the XZ plane of the PA-6/F-ASA 10%
blend.The picture is taken from the region (a)immediately under the fracture surface,(b)far away from the fracture surface,and (c)even farther away from the fracture surface.
temperature of the F-ASA/PA-6blends from50°C to about0°C.
SEM was used to study the deformation mechanisms of the F-ASA/PA-6blends,and the results showed that cavitation of rubber particles and shearing yielding of the PA-6were the major deformation mechanisms.
■AUTHOR INFORMATION
Corresponding Author
*E-mail:zhanghx@https://www.doczj.com/doc/d918796438.html,.
Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
The?nancial supports of Natural Science Foundation of China (51073027,50803007),Jilin Provincial Science and Technol-ogy Department(20090142)are gratefully acknowledged.■REFERENCES
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