Chiang Mai J. Sci. 2007; 34(1) 79

Chiang Mai J. Sci. 2007; 34(1) : 79-87www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

One Step Reactive and Dynamic Vulcanizate of NR/PA6 BlendsThoranit Navarat*, Manus Seadan, and Suwat RattanapaneFaculty of Science and Technology, Prince of Songkla University, Pattani, 94000, Thailand.*Author for correspondance; e-mail : tnavarat@bunga.pn.psu.ac.th

Received : 20 June 2006Accepted : 22 October 2006.

ABSTRACTRelevant developments in one step reactive blends of immiscible polymer NR/PA6

have shown that the suitable reaction is the combination of free radical and condensationreactions. The NR and PA6 compatible blends have shown how efficient the amphiphilicspecies are throughout the reactive reaction. The induced reactivity in a single screw extrusionis based on a mixture of peroxide (Perkadox 14), reactive monomer (maleic anhydride) andactivators (ZnO and stearic acid). In the case of NR/PA6 60/40 blends, their morphologycan be changed from co-continuous to reversion phase using rubber particles dispersed inPA6 phase. Dynamic vulcanization process has been done on the same single screw extruderusing a phenolic resin (SP-1045) as a vulcanizing agent. The blend’s morphology has a greateffect on physical properties. The finely NR cross-linked dispersed phase blends have goodmechanical properties. It has also shown very good oil resistance and it can be used at hightemperature of 200oC.

Keywords : thermoplastic vulcanizates, complex reactive blend, reactive blend, dynamicvulcanization, NR/PA6 blend.

1. INTRODUCTIONPolyamide is an attractive class of

engineering polymers and has been used fornumerous engineering applications because oftheir excellent tensile properties, chemical andabrasion resistance, high melting point andfatigue resistance. Blends of polyamide withrubber have been extensively studied in orderto obtain new materials with good impactproperties. Maleated rubbers are successfulexamples of these modifications. The maleicanhydride (MA) groups of these rubbers canreact with amine end groups of polyamideand form graft copolymers at rubber-matrixinterface, which reduces interfacial tension andretards particle coalescence during mixing[1,2]. For non-functionalized rubbers, addition

of compatibilizing agents can be an alternativeto improve the blends[3]. Recent developmentsin polyamide-polyolefin blends have shownthe possibility to create a high amount ofamphiphilic species by an in situ reaction atthe interface using the combination of freeradical and condensation reaction of peroxideand maleic anhydride[4-6]. However, in thecase of compatible blend of PP/EPDM thedynamic vulcanizate has an important effecton blend morphology and physicalproperties[7-9].

The purpose of this paper is to investigatean effective method to compatibilize naturalrubber-polyamide blends. Compatibilizationis done by the in situ combined free radical

80 Chiang Mai J. Sci. 2007; 34(1)

and condensation reaction during a singleextruder process. The induced reactivity isbased on a mixture of peroxide, maleicanhydride and activators (ZnO and stearicacid). The effect of dynamic vulcanization ofrubber phase on blend’s morphology andphysical properties is investigated.

2. MATERIALS AND METHODSThe engineering thermoplastic used is the

extrusion grade polyamide6 (ultramid B4)produced from the BASF company, while aman-made air-dried sheet NR from the localarea is selected in this study. The used peroxidePerkadox 14-40P [1,3-bis(1-tert-butyldioxy-1-methyl) benzene], is supported by Akzocompany. Its half life time is about 2.5 minutesat 180oC. Maleic anhydride (MA) is used as abifunctional monomer of free radical andcondensation reactions. NR vulcanizedactivators stearic acid and ZnO are used topromote the reactive blends and the dynamicvulcanized reactions. The curing system usedis phenolic resin (SP-1045). The toluene andformic acid (FA) are used as solvents for NRand PA6, respectively, whereas the acetone andwater are the main solvents for the relativeprecipitation.

Preliminary studies of NR/PA6 reactiveblends are done in a Brabender mixer at 240oCby using NR/PA6 (the composition is20/80). Haft of PA6 is introduced into themixer. One minute later all ingredients (rubber,PA6, peroxide and MA) are put and mixedfor 5 minutes. In preparing TPE, whosecomposition of NR/PA6 is 60/40, the

process is continued on a single screwextruder. The rubber is masticated and thenthe peroxide, monomer and activator aremixed on a two roll-mill mixer at 80oC. Whena well distributed mixer is obtained, the PA6is added and then a complex further reactionof NR/PA6 compound (from the two roll-mill mixer) is carried out using a single screwextruder at 240oC for a 5 kg/hr flow rate.The extrudate is quickly cooled with water asit comes out from the extruder die and thencut to granulated forms. The grain of NR/PA6 reactive blends are dried under vacuumat 50oC for 24 hours. The dried reactive blendis physically mixed with the vulcanizationagents, and then in situ dynamic vulcanizedblend is done in the same single screw extruderat the same condition operated. The productis dried and injected into the standard tensilesample by using the plastic injection mouldingmachine. The morphology of the blends ischaracterized by using solvent extractiontechnique and SEM image of fracture surface.The mechanical, thermal and solvent extractionproperties are investigated.

3. RESULTS AND DISCUSSION3.1 Pre-study on Small Mixer

The in situ reactive blends of NR/PA6are pre-studied by using reactivity of peroxide(Perkadox 14) and maleic anhydride. Theseblends are performed in Brabender mixer atthe temperature about 240oC, blending time10 min and rotor speed of 60 rpm. Theblends formulas are shown in table 1 and itsmorphologies are shown in figure 1.

Table 1. Formulation of NR/PA6 reactive blend.

Ingredients / Formulas phr1.1 1.2 1.3 1.4 1.5 1.6

PA6 80 80 80 80 80 80NR 20 - 20 20 20 20MNR - 20 - - - -maleic anhydride - - 1.0 0.5 1.0 2.0Perkadox 14 - - - 0.1 0.1 0.1

Chiang Mai J. Sci. 2007; 34(1) 81

PA6/NR : 80/20 PA6/MNR : 80/20

PA6/NR/MA/Perkadox 14 : 80/20/0.5/0.1

PA6/NR/MA/Perkadox 14 : 80/20/1.0/0.1 PA6/NR/MA/Per : 80/20/2.0/0.1

(a) Formula 1.1 (b) Formula 1.2

(d) Formula 1.3 (c) Formula 1.4

(e) Formula 1.5 (f) Formula 1.6

Figure 1. Morphology of (a) Physical blend, (b) MNR/PA6 and (c)-(f) NR/PA6/peroxide/MA complex reactive blends with various amounts of MA.

The MNR is prepared by MA 1 phr andperoxide 0.1 phr for 10 min reaction time at140oC in Brabender mixer. It clearly shows a

finer rubber dispersion phase and a higherinterfacial adhesion of the compatible blend.The complex reactive blends where the

Figure 1(a) shows non-compatibilitybetween PA6 and NR phase of physicalblends. A rough surface with no effectivebinding to NR matrix is clearly seen. Figure

1(b) shows a good improvement on blendmorphologies of the reactive blends of PA6and MNR contained anhydride groups about0.8 phr.

82 Chiang Mai J. Sci. 2007; 34(1)

peroxide, MA, PA6 and NR are mixed butonly one step processed also give good blendmorphologies as shown in figure 1(c) – 1(f).The complex reactive blends have the sameeffective level of reduction rubber dispersionphase and interfacial adhesion. It can beconcluded that only a small amount of MAand peroxide is needed. Therefore, thecomplex reactivity of peroxide and maleicanhydride is a suitable method for NR and

PA6 blends, the reaction performs in the onlyone step processing.3.1 Complex reactive blends

The NR/PA6 complex reactive blendpreparation needs to be carried out in a singlescrew extruder. The blends are operated at240oC (both barrel and die temperatures), 30rpm of the screw speed at the output rate of5 kg/hr. Table 2 presents the blend formulasand its tensile strength and elongation at break.

Table 2. Recipes for complex reactive blends and mechanical properties.

Ingredients / Formulas phr2.1 2.2 2.3 2.4 2.5

PA 6 40 40 40 40 40NR 60 60 60 60 60Perkadox 14 - 0.1 0.1 0.15 0.2Maleic Anhydride - 1.0 1.0 1.0 1.0Zinc Oxide - - 1.0 1.0 1.0Stearic Acid - - 1.0 1.0 1.0Mechanical propertiesTensile Strength (MPa) 11.62 9.63 8.22 7.18 8.3Elongation at Break (%) 20 40 90 50 40

The physical blend is the formula 2.1 andthe complex reactive blends are formula 2.2to formula 2.5 with and without activators atvariant peroxide levels. The physical blendgives an extremely low value of 20%elongation at break in correspondence to thepoor interfacial adhesion as shown in figure2(a). All reactive blends show higher valuesof elongation at break than physical blend.Whereas the complex reactive blend with theactivation agents, ZnO and stearic acid(formula 2.3), has a greater elongation at break(90%) with lightly decreased value of tensilestrength. It also provides an easy goingoperation in extrusion. Thus, this complexreactive formulation 2.3 will be furtherexamined for a suitable condition. Thecorrespondent morphology is shown in figure2(b) where one clearly sees a good interfacial

adhesion but a rough co-continuous phase.The fine rubber dispersion in PA6 phase couldbe achieved by dynamic vulcanized processing.

The dynamic vulcanization is taken placein the single screw extruder at the samecondition of blending and the recipes areshown in table 3. The morphology of theblends is characterized by using SEM imageof toluene extracted fracture surface. Themorphology development during dynamicvulcanized blend is studied using four levelsof phenolic resin from 0 to 5 phr and it isshown in figure 2. The rough continuousmorphology of non dynamic vulcanizedblend is changed to the fine continuousmorphology in figure 2(c) and then theinversion phase is taking place with increasingof phenolic resin level up to 2.5 phr in figure2(d).

Chiang Mai J. Sci. 2007; 34(1) 83

Table 3. Recipes for dynamic vulcanise complex reactive blends.

Ingredients / Formulas phr3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

PA 6 40 40 40 40 40 40 40 40NR 60 60 60 60 60 60 60 60Perkadox 14 - 0.1 0.1 0.1 0.15 0.2 0.2 0.2Maleic Anhydride - 1.0 1.0 1.0 1.0 1.0 1.0 1.0Zinc Oxide - 1.0 1.0 1.0 1.0 1.0 1.0 1.0Stearic Acid - 1.0 1.0 1.0 1.0 1.0 1.0 1.0Phenolic resin - - - - - 1.25 2.5 5.0Phenolic resin (Second step) - - 1.25 2.5 5.0 - - -Mechanical propertiesTensile Strength (MPa) 11.62 8.22 11.2 14.1 13.7 12.1 10.9 11.8Elongation at Break (%) 20 90 60 100 100 50 40 45

This cure system has enough time for theblend to modify a conversion phaseprocessing to obtain the fine rubber vulcanizeddispersion phase morphology. But when wetry to blend and make dynamic vulcanizedblend in only one step process, it gives a goodinterface adhesion but contain some largerubber phase as shown in figure 2(f) - 2(h). Itclearly showed that dynamic vulcanized stepwould be done after the reactive blend step.

Four types of blends, the physical blend(formula 3.1), complex reactive blend(formula 3.2), dynamic vulcanized reactiveblend (formula 3.4) and dynamic vulcanizedreactive blend mixed with carbon black arecharacterized (formula 3.4 + carbon black).They are named as formula A, B, C and Drespectively. Formula D consists of 15%carbon black, added in the first step, and 0.6%-antioxident (6PPD), added in the dynamicvulcanized step. Figure 3 presents the solventextraction of those four formulas; A to D. Itcomposes of three regions of the soluble intoluene, the soluble in formic acid and theinsoluble fraction. Formula A indicates thatnearly all of the NR and PA6 phases dissolvein the corresponding solvent. Only tinyamount of NR gel content might be left.

There are approximately 1/4 of PA6 phasethat cannot be dissolved in the cases of thereactive blend and dynamic vulcanized blends(formula B, C and D). For this condition, thePA6 phase cannot be crosslinked, so that this1/4 of PA6 can be considered as the fractionof NR/PA6 block copolymer, created fromthe complex reactive blends. This extremelyhigh amount of created block copolymer canbe confirmed by the dramatic change inmorphology of the complex reactive blend.Extremely high amount of formula C and Drubber crosslinking, indicated as the white areain figure 3, is clearly seen. This iscorresponding to the morphology shown infigure 2, indicated the fine rubber crosslinkingparticles dispersed in the thermoplastic PA6phase.

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Physical Blend no phenolic resin

phenolic resin = 1.25 % phenolic resin = 2.5 %

phenolic resin = 5.0 % one step phenolic = 1.25%

one step phenolic = 2.5% one step phenolic = 5.0%

(a) Formula 3.1 (b) Formula 3.2

(c) Formula 3.3 (d) Formula 3.4

(e) Formula 3.5 (f) Formula 3.6

(g) Formula 3.7 (h) Formula 3.8

Figure 2. Dynamic phenolic resin vulcanized morphology evolution (a) physical blend(b) non-vulcanized reactive blend (c)-(e) dynamic vulcanized reactive blend two steps processand (f)-(h)dynamic vulcanized reactive blend one step process.

Chiang Mai J. Sci. 2007; 34(1) 85

A B C D

A B C D Polyamide 6 C

ontent (%)

0

20

40

60

80

100Nat

ural

Rub

ber C

onte

nt (%

)

0

20

40

60

80

100

Rubber CrosslinkSolution in TolueneSolution in Formic Acid

Figure 3. Solvent extraction properties of formula A, B, C and D.

The stress-strain relations of these fourformulas are shown in figure 4. It is clearlypresented that the reactive blend (formula B)is softer than the physical blend. This mightbe caused by the change of reactive blendmorphology and the activator’s (ZnO andstearic acid) softening effect on the rubberphase. The dynamic vulcanized blendimproves not only the tensile strength and

elongation at break but it also makes the blendstiffer. Unusually, the carbon black containedin formula D makes a softening materialcompared with the dynamic vulcanized blendsformula C. This might be caused by anincompatibility of PA and carbon black ormight be caused by the effect of carbon blackon the crosslink level in the rubber particlephase.

Strain (%)0 20 40 60 80 100 120 140

Stre

ss (M

Pa)

0

2

4

6

8

10

12

14

16

Fomula-A Fomula-BFomula-CFomula-D

A

B

C

D12.5

135

Figure 4. Stress and strain relation of NR/PA6 (60/40) blend for formula A (physical blend)formula B (reactive blend), formulas C and D (reactive and dynamic vulcanized blend).

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The results of solvent resistance examina-tion, following the ASTM D 471-98 theswelling standard method, are presented intable 4. A very good solvent resistance is clearly

seen compared to the vulcanized NR. Threetimes oil resistant values of the producedTPEs is higher than that of the vulcanized NR.

Table 4. Oil resistance of the complex reactive NR/PA6 (60/40) TPE.

Oil % SwellingC (3.4) D (3.4+CB) Reference (NR-Cure)*

Isooctane 100 25.54 24.69 56.09Isooctane + Toluene ; 50+50 46.39 41.58 128.00Toluene 100 54.17 47.90 178.57Deisel Fuel 100 17.02 34.65 158.51

* NR 100, ZnO 5, Stearic Acid 2.5, Oil (Deoflow S) 15, TBBS 0.5, Wingstay L 1 and Sulphur 2 phr

The dynamic mechanical properties areanalyzed by using Dynamic MechanicalThermal Analyzer (Rheometric Scientific,DMTA V). Dual Cantilever sampledeformation is tested at the frequency of 1

Hz, the strain control of 1%, and thetemperature ranging from 25-250oC. Theresults are shown in figure 5 for the whiteand the black TPE.

Figure 5. Dynamic properties of the NR/PA6 (60/40).

It shows that the storage modulus ishigher than the loss modulus, which confirmsthat our TPEs are the hard rubber materials.The modulus is rather constant during thetemperature range of 25-200oC and it dropsrapidly at about 200oC. These results confirm

that this provided material can be used up toextremely high temperature of about 200oC.

4. CONCLUSIONSThe compatibility NR/PA6 blends can

be done using induced reactivity in a single

Chiang Mai J. Sci. 2007; 34(1) 87

screw extruder based on a mixture ofperoxide (Perkadox 14), reactive monomer(maleic anhydride) and activators (ZnO andstearic acid). The results confirm that a goodlevel of reciprocal grafting occurred betweenNR and PA6. Their morphologies show agood interfacial adhesion with fine continuousphase of PA6. The TPE is done by vulcanizedrubber phase using the dynamic vulcanizedmethod in the secondary step.

Dynamic vulcanized process is studiedusing phenolic resin (SP-1045) on the singlescrew extruder. It shows a finely NR cross-linked dispersed phase and good mechanicalproperties are obtained. Its tensile strength andelongation at break are about 12.5 MPa and135 %, respectively (figure 4, formula D). Theblend also shows a very good oil resistanceand can be used up to very high temperatureof 200 oC.

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