Influence of Epoxide Level and Reactive Blending on Properties of Epoxidized Natural Rubber and...

Influence of Epoxide Level andReactive Blending on Propertiesof Epoxidized Natural Rubberand Nylon-12 Blends

MASWANEE NARATHICHATCenter of Excellence in Natural Rubber Technology, Faculty of Science and Technology,Prince of Songkla University, Pattani, 94000, Thailand

CLAUDIA KUMMERLOWE, NORBERT VENNEMANNFaculty of Engineering and Computer Science, University of Applied Sciences Osnabruck,Albrechtstr. 30, 49076 Osnabruck, Germany

KANNIKA SAHAKARO, CHAROEN NAKASONCenter of Excellence in Natural Rubber Technology, Faculty of Science and Technology,Prince of Songkla University, Pattani, 94000, Thailand

Received: July 28, 2010Accepted: March 26, 2011

ABSTRACT: A simple blend (i.e., blend without curative) of 60/40rubber/nylon-12 with three different types of natural rubber (i.e., air-dried sheetnatural rubber (ADS) and epoxidized natural rubbers (ENR) with 25 and 50 mol%epoxide) was prepared by the melt-mixing process. Influence of different types of

Correspondence to: C. Nakason; e-mail: [email protected].

Contract grant sponsor: Thailand Research Fund through theRoyal Golden Jubilee Ph.D. Program.

Contract grant number: PHD/0138/2548.Contract grant sponsor: DAAD–TRF Project Based Personnel

Exchange Programme.Contract grant number: PPP 2008.

Advances in Polymer Technology, Vol. 31, No. 2, 118–129 (2012)C© 2011 Wiley Periodicals, Inc.

PROPERTIES OF EPOXIDIZED NATURAL RUBBER AND NYLON-12 BLENDS

rubbers and the level of epoxide groups in ENR molecules on the properties ofthe blends were investigated. It was found that all the blends exhibited aco-continuous phase morphology. Furthermore, the ENR/nylon-12 blendsexhibited superior mechanical properties, stress relaxation behavior, thermal andrheological properties, and a finer grain morphology than that of theADS/nylon-12 blend. This may be attributed to the higher interfacial adhesionbetween the ENR and nylon-12 phases via a chemical interaction of epoxidegroups in ENR molecules and polar functional groups in the nylon-12 molecules.Temperature scanning stress relaxation (TSSR) measurement was also performed.Improvement in stress relaxation and thermal resistance of the blends with ENRwas observed. Moreover, the higher temperature coefficient and lower glasstransition temperature (Tg) of the nylon-12 phase in the ENR-50/nylon-12 blendswere found. C© 2011 Wiley Periodicals, Inc. Adv Polym Techn 31: 118–129, 2012;View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20243

KEY WORDS: Epoxidized natural rubber, Nylon, Stress relaxation,Thermal properties

Introduction

B lending of polymers gives rise to new materi-als with tailored properties and performance.

There has been a considerable interest in recent yearsin exploring the potential of polymer blends, whichcombine useful properties of each component andminimize undesired characteristics. Thermoplasticelastomers (TPEs) based on blending of rubbers andthermoplastics are such blends, which exhibit two-phase structures. They have been one of the con-siderable materials, where the required propertiescan be achieved by selecting suitable blend com-ponents and blend ratios.1,2 Generally, most of thepolymer blends are immiscible with poorer me-chanical properties when compared with the par-ent polymer pairs. This relates to their coarse andunstable morphological properties, together withlack their of interfacial adhesion. To avoid thisdrawback, reactive compatibilization has been em-ployed to enhance miscibility, which provides high-performance polymer blends and TPE materials.3−7

It has been well established that the functionalizedpolymer with the proper reactive groups could re-act with the other polar polymer at the interfaceduring a mixing operation. In such a manner, thefunctionalized rubber components have been usedto prepare the reactive blending with polyamide.The epoxy functionalized acrylate rubber (ACM)has been one of the successful examples.8−10

Other types of epoxy-functionalized rubbers in-clude glycidylmethacrylate-grafted ethylene propy-

lene diene monomer EPDM (EPDM-g-GMA),4−6,11

poly(ethylene-co-ethyl acrylate) (PEEA),12 and epox-idized natural rubber (ENR).13

Compatibilization of polymer blends has beentypically characterized by dynamic mechanical anal-ysis (DMA) in terms of viscoelastic properties (e.g.,storage modulus and loss modulus, or tan δ).The compatibility of these blends is strongly de-pendent on the microstructural properties of theblends.5,14−16 The recently developed temperaturescanning stress relaxation (TSSR) method has beenused to characterize the thermal–mechanical be-havior of vulcanized rubber and thermoplasticelastomers.17 Stress relaxation is one of the mostimportant phenomena with respect to the utiliza-tion of the properties of TPE materials. The time–temperature dependency of the stress relaxation be-havior obtained from the TSSR measurement hasbeen reported elsewhere.17,18 It has been establishedthat rubber exhibits a rather complex behavior withrespect to temperature changes. The coefficient oflinear expansion (α) of most materials is positive,whereas the α of vulcanized rubber changes frompositive to negative when the strain is increased.This change has been known as a thermoelastic in-version and is observed under the condition of con-stant load at strain ratios (λ) greater than 1.1.19 Withthe constant strain, an increase in the stress (s) withthe temperature (T) is also observed, if the strainratio (λ) exceeds a value of 1.1.20 The coefficient (κ)is defined as the derivative of mechanical stress (σ )with respect to temperature, as shown in Eq. (1).From the well-known theory of rubber elasticity as

Advances in Polymer Technology DOI 10.1002/adv 119


expressed in Eq. (2), σ is proportional to the absolutetemperature (T), 22,23 where ρ is the mass density, λ

is the strain ratio (l/l0), and R is the universal gasconstant. Mc is defined as the average molar mass ofrubber network chains, which is proportional to thereciprocal value of the cross-link density (ν) of thenetwork. From Eq. (3), obtained by the combinationof Eqs. (1) and (2), ν is proportional to κ , the slopeof the stress versus temperature curve at constantstrain, which implies the interfacial reaction of theblend via the reactive functional groups.

κ = (∂σ/∂T)λ,ρ (1)

σ = ρRTMC

(λ − λ−2) (2)

ν = κ

R(λ − λ−2)with ν = ρ

MC(3)

In this work, rubber/nylon-12 blends with a fixedblend ratio of 60/40 were prepared with three typesof rubber (i.e., ADS, ENR-25, and ENR-50). The in-terfacial reaction of the ENR/nylon-12 blends wasproved by the FTIR technique. Furthermore, stressrelaxation, mechanical, rheological, and morpholog-ical properties were investigated.

Experimental

MATERIALS

Three types of natural rubber samples were usedin this study as blend components. They are air-driedsheet (ADS), natural rubber (Khuan Pan Tae FarmerCooperation, Phattalung, Thailand), and epoxidizednatural rubber (i.e., ENR-25 and ENR-50; MuangmaiGuthrie Co., Ltd., Phuket, Thailand). The Mooneyviscosities [ML1 + 4 (100◦C)] of ENR-25 and ENR-50 are 81 and 70, respectively. The other blendcomponent was nonreinforced injection-molding-grade nylon-12, Grilamid L20G with a melting tem-perature of 178◦C [MFI = 125 g/10 min (5 kg,275◦C)], from EMS-Grivory GmbH, Gross-Umstadt,Germany. The polyphenolic antioxidant (Wingstay-L) was manufactured by Eliokem Inc. (Akron, OH).

PREPARATION OF RUBBER/NYLON-12BLENDS

Nylon-12 was first dried at 40◦C for 24 h priorto blending with various types of NR by a two-step mixing process using a Haake Rheocord 600

laboratory internal mixer (Thermo Electron Cor-poration, Karlsruhe, Germany). Two mixing stepswere exploited in this work. The first mixing stepwas performed by mixing natural rubber with1 phr of Wingstay-L at 80◦C and a rotor speed of40 rpm for 4 min before removing from the mix-ing chamber. In the second mixing step, the pre-mixed rubber was again incorporated into the mix-ing chamber and mixed at 160◦C and a rotor speed of80 rpm. After 1 min, the rotor speed was increased to150 rpm. After achieving a temperature of approxi-mately 170◦C, the other blend component, nylon-12,was added. The temperature was abruptly droppedapproximately 10◦C due to the original tempera-ture of nylon-12. The temperature of the blend wasthen gradually raised to 185–195◦C due to the fric-tional heat. The rotor speed was then decreased to80 rpm to reduce the mixing temperature to ap-proximately 190◦C. The mixing was then continuedfor another 7 min. The blend product was even-tually removed from the mixer, cooled down toroom temperature, and pelletized for fabrication andcharacterization.

TEMPERATURE SCANNING STRESSRELAXATION

The TSSR measurement was performed us-ing a TSSR-meter (Brabender GmbH, Duisburg,Germany). The apparatus consists of an electricalheating chamber, where a sample was placed be-tween the two clamps. A thermocouple was placednear the center of the samples to detect the actualtemperature. Uniaxial extension was applied to thesamples via a linear drive unit. A high-quality sig-nal amplifier in combination with a high-resolutionA/D-converter was used to detect and digitize theanalogue signals from a high-resolution force trans-ducer and thermocouple. All signals were trans-ferred to a personal computer. A special softwareprogram was used to treat and evaluate the dataand to control the devices.17,18 During the TSSR mea-surement, constant tensile strain of at least 50% wasapplied to dumbbell test pieces (ISO527, type 5A)with preconditioning for 1 h at 55◦C. During thepreconditioning period, most of the short-time re-laxation processes occur and the quasi-equilibriumstate is reached. The sample was then heated at aconstant heating rate of 2 K/min−1 until the stressrelaxation was completed or the sample was rup-tured. The glass transition temperature (Tg) of nylon-12 phase in the blends was also determined by

120 Advances in Polymer Technology DOI 10.1002/adv


preconditioning at 23◦C for 2 h with a constant ten-sile strain of 25%.

RHEOLOGICAL CHARACTERIZATION

A high-pressure capillary rheometer (GottfertRheo-Tester 2000, Gottfert Werkstoff-PrufmaschinenGmbH, Buchen, Germany) was used to characterizethe shear flow properties of the blends. That is, therelationship between apparent shear stress, appar-ent shear rate (i.e., flow curves), and apparent shearviscosity with an apparent shear rate (i.e., viscositycurves) was studied. The test was carried out over awide range of shear rates (i.e., 10–2000 s−1) at 230◦C.A capillary die with dimensions of 1 mm diameter,20 mm length, and 180◦ entry angle with an aspectratio (L/D) of 20/1 was used. The material was firstprepared by palletizing, and then incorporated intothe rheometer’s barrel and purged until reaching thepressure at approximately 100 MPa. The pressuredrop was then decreased to approximately 10 MPato eliminate bubbles and to get a compact mass. Thematerial was thereafter preheated for 5 min. The testwas then carried out using a set of shear rates in aprogram via a microprocessor. During the test, thepressure drop across the capillary channel and themelt temperature were captured via a data acquisi-tion system. The apparent values of the shear stress,shear rate, and shear viscosity were calculated usinga derivative of the Poiseuille law for capillary flowas follows21:Apparent wall shear stress [Pa]:

τ = R�P2L

(4)

Apparent wall shear rate [s−1]:

∗γ app = 4Q

π R3 (5)

Apparent wall shear viscosity [Pa s]:

ηs = τ∗γ app

(6)

where �P is the pressure drop across the channel(Pa), Q is the volumetric flow rate (m3 s−1), R isthe capillary radius (m), and L is the length of the

capillary (m). The values of R and L are =1 and20 mm, respectively.

Attenuated Total ReflectionFourier Transform InfraredSpectroscopy

To elucidate the interfacial reactions between theblend components, each blend component was ex-tracted and then characterized with attenuated totalreflection Fourier transform infrared spectroscopy(ATR-FTIR).5 In this work, the mixed solvent of phe-nol/tetrachloroethane/xylene (40/40/20 w/w/v)was first prepared by mixing phenol (40 part byweight) and tetrachloroethane (40 parts by weight)using a stirring rod at room temperature. The lastcomponent, xylene (20 parts by volume) was then in-corporated into the mixture and continuously stirredfor at least 30 min. The mixed solvent was then usedto extract the NR/nylon-12 blends at room temper-ature. It is noted that phenol is a good solvent fornylon, and a mixture of tetrachloroethane/xyleneis a good solvent for NR and ENR. The blendswere first dissolved in the mixed solvent of phe-nol/tetrachloroethane/xylene for 72 h at room tem-perature. The solution was naturally separated intotwo layers in which the upper layer was the rubbersolution, which thereafter was separated. Ethanolwas added into the rubber solution to precipitate therubber phase. The rubber product was then driedat 40◦C until a constant weight was reached. TheATR-FTIR spectra were then recorded using ThermoNicolet Avatar 360 FTIR (Thermo Electron Corpo-ration (Thermo Nicolet), Madison, WI), which isequipped with a germanium ATR crystal probe with4 cm−1 resolution and 32 scans over a wave numberrange of 4000–400 cm−1.

Thermogravimetric Analysis

Thermal properties of the rubber/PA-12 blendswith different types of rubber were characterized us-ing a Perkin-Elmer (Waltham, MA, USA) STA 6000simultaneous thermal analyzer (thermogravimetrycoupled with differential scanning colorimetry)(TG/DSC). Approximately 10 mg of sample wasadded into an aluminum pan and thereafter heatedfrom room temperature to 800◦C with a heating rateof 10◦C/min−1 under the O2 atmosphere.



Tensile and Hardness Tests

Dumbbell-shaped specimens according to ISO527 (type 5A) were prepared by thermoplas-tic injection molding with a screw diameter of22 mm (BOY 22M, BOY Machines Inc., Ex-ton, PA). Detailed dimensions of the tensiletest specimens are as follows: overall lengthof .75 mm, width at ends of 12.5 ± 1 mm,length of a narrow parallel-sided portion of 25 ±1 mm, width of a narrow parallel-sided portion of4 ± 0.1 mm, small radius of 8 ± 0.5 mm, large ra-dius of 12.5 ± 1 mm, initial distance between grips of50 ± 2 mm, gauge length of 20 ± 0.5 mm, and thick-ness more than 2 mm. The tensile test was performedat 23 ± 2◦C using a Zwick Z 1545 tensile testing ma-chine (Zwick GmbH & Co., KG, Ulm, Germany) ata cross-head speed of 200 mm/min−1. Indentationhardness was tested by a durometer Shore A ac-cording to ISO 868 using a hardness-testing machine(Karl Frank GmbH, Weinheim, Germany).

Morphological Properties

Morphological characterization was performedusing a Leo scanning electron microscope, (modelVP 1450, Leo Co., Ltd., Cambridge, UK). Injection-molded samples were first cryogenically cracked af-ter immersion in liquid nitrogen to avoid any pos-sibility of phase deformation during the crackingprocess. The rubber phases were then extracted byimmersing the fractured surface in toluene at roomtemperature for 3 days. The samples were then driedin an oven at 40◦C for 24 h to eliminate the contam-ination of the solvent. The dried surfaces were goldcoated and examined by using a scanning electronmicroscope at 10 kV.

Results and Discussion

TSSR MEASUREMENT

Figure 1 shows normalized force–temperaturecurves of 60/40 natural rubber/nylon-12 blendswith three different types of rubber. It can be seenthat different types of rubber gave different charac-teristics of force–temperature curves, in particular in

FIGURE 1. Normalized force–temperature curves of60/40 rubber/nylon-12 blends using various types of NR(T0 = 55◦C at 50% strain).

the temperature range of 90–170◦C. That is, the ENR-50/nylon-12 blend exhibited a significantly higherrelaxation curve than those of the ENR-25/nylon-12 and ADS/nylon-12 blends. Normalized force inthis case is defined as the quotient F (T)/F0, thatis, the force ratio, where F (T) is the force at tem-perature Tand F0 is the initial force determined atstart temperature T0. The certain characteristic quan-tities including T10, T50, andT90, and the rubber index(RI) were calculated based on the normalized force–temperature curves in Fig. 1.17,18 It is noted that Tx

stands for the temperature at which the force (F ) hasdecreased x% with respect to the initial force (F0).The rubber index is a measure of the rubber-like be-havior of the material, which was calculated fromthe area underneath the force–temperature curvesaccording to the equation given below:17

RI =∫ T90

T0

F (T)F0

dT

T90 − T0(7)

where F (T) is the force at temperature T and F0is the initial force determined at starting temper-ature T0. The T90 is the temperature at which theforce F has decreased 90% with respect to the initialforce F0.

Table I summarizes the TSSR results in terms ofT10, T50, T90, and the rubber index. It is clear thatthe lowest values of T10, T50, and T90 are observedin the ADS/nylon-12 blends. However, the valuesofT10, T50, T90, and RI of ENR-50/nylon-12 blendare slightly higher than that of the ENR-25/nylon-12 blend. This is attributed to the enhancement



TABLE ITSSR Results of Rubber/Nylon-12 Blends Using Various Types of NR

Temperature Coefficient,Samples σ0 (MPa) T10 (◦C) T50 (◦C) T90 (◦C) RI κmax (MPa/K)

ADS/nylon-12 3.77 73.1 109.1 155.7 0.57 0.23 × 10−3

ENR-25/nylon-12 3.72 74.1 110.1 157.9 0.57 2.70 × 10−3

ENR-50/nylon12 3.72 74.2 111.7 159.6 0.58 4.02 × 10−3

in the thermal properties of the blends because ofthe chemical interaction between epoxide groupsin ENR and polar functional groups in nylon-12molecules. As a consequence, the superior stress re-laxation behavior and thermal property of the ENR-50/nylon-12 blend are observed. This result is con-firmed by TGA, as shown in Fig. 2 and Table II. It canbe seen that there is a change in degradation charac-teristics (i.e., Td and Tonset), level of final weight loss,and temperature at which 10% mass loss (T10%) oc-curs in the blends with different types of rubber. Fur-thermore, it is clearly seen that the ADS/nylon-12blend exhibited lower thermal stability than those ofthe ENR-25/nylon-12 and ENR-50/nylon-12 blends,respectively. This corresponds to the superior ther-mal properties of the ENR-50/nylon-12 blend basedon TSSR results shown in Fig. 1. This is attributedto the chemical reactions between epoxide groupsin the ENR molecules and polar functional groups(i.e., NH2, COOH, and NH CO ) in the nylon-12 molecules.8−12 As a consequence, a formation ofin situ graft copolymerization of the ENR and nylon-12 molecules occurred at the interface. The probablereaction mechanism is shown in Scheme 1. Thus, in-terfacial tension reduces, thereby improving interfa-

FIGURE 2. TGA thermograms of rubber/nylon-12blends using various types of NR.

cial adhesion between the rubber and nylon phases.As a result, the blend with ENR exhibits superiorthermal and stress relaxation properties.

In Table I, the temperature coefficient values(κmax) are also determined from the initial part ofthe stress–temperature curves in Fig. 1. The κmax isdefined as the derivative of mechanical stress withrespect to temperature (i.e., Eq. (1)), which is influ-enced by the entropy effect.19,20 Therefore, the κmaximplies the compatibilizing effect of the blends. InTable I, it is seen that the ENR-50/nylon-12 blend ex-hibited the highest κmax value of 4.02 × 10−3 MPa/K.This implied the highest chemical interaction be-tween the ENR-50 and nylon-12 phases. The κmaxvalues of the ENR-25/nylon-12 and ADS/nylon-12blends are 2.70 × 10−3 and 0.23 × 10−3 MPa/K, re-spectively. This confirms the compatibility betweenENR-25 and nylon-12, but poor interfacial adhesionbetween unmodified NR (i.e., ADS) and nylon-12.It is concluded that an increasing level of epoxidegroups in ENR molecules caused a higher κmax valueand hence, compatibility of rubber and nylon-12phases. Therefore, the other benefit of the TSSR mea-surement is to determine the blend compatibility byobserving the slope of the initial part of the stress–temperature curves at low-temperature ranges (i.e.,κmax values).

Figure 3 shows relaxation spectra of the blendsbased on different types of natural rubber. The re-laxation spectrum, H′(T), was calculated using thefollowing equation17:

H′(T) = −T(

dE(T)dT

)ν=const

(8)E(T) = σ (T)/ε0

where σ (T) is the temperature-dependent stress andε0 is the applied strain (i.e., 50%).

Figure 3 shows two peaks at different tempera-tures. The first peak at approximately 158◦C revealedthe degradation of the rubber phase during the TSSR



TABLE IITGA Results of Rubber/Nylon-12 Blends using Various Types of NR

Samples Tonset (◦C) T10% (◦C) Td (◦C) Weight Loss (%)

ADS/nylon-12 308.4 333.6 340.5 13.02ENR-25/nylon-12 336.4 349.9 358.8 13.54ENR-50/nylon12 339.5 356.6 362.9 13.43

SCHEME 1. Possible grafting reactions of epoxide groups in ENR molecules and functional groups in nylon-12molecules at the interface with (a) acid-end groups, (b) amine-end groups, and (c) amide groups.



FIGURE 3. TSSR relaxation spectra of 60/40rubber/nylon-12 blends with various types of rubber(T0 = 55◦C and 50% strain).

test. It is clear that there is a significant differenceamong different types of natural rubber used. Thatis, the peak value is slightly shifted from 157.5◦Cfor ADS/nylon-12 blend to 158.1◦C for the ENR-25/nylon-12 and to 159.1◦C for the ENR-50/nylon-12 blends. This resulted from an improvement of thethermal resistance of ENRs, which increased with in-creasing the epoxide content. The second peaks areobserved in a range of temperatures of 170–175◦Ccorresponding to Tm of the PA-12 phase. It is seenthat the peak is slightly shifted from approximately171◦C for the ADS/nylon-12 blend to approximately174◦C for the ENR-25/nylon-12 and ENR-50/nylon-12 blends. This is a result of chemical reactionsbetween epoxide groups in ENR molecules and po-lar functional groups in nylon-12 molecules, as de-scribed in Scheme 1. However, there is no significantdifference in locations for the peaks of the ENR-25and ENR-50 blends.

The TSSR relaxation spectra of the blends in atemperature range of 23–60◦C are shown in Fig. 4.Three peaks around 40–45.5◦C are assigned to theglass transition relaxation of the nylon-12 phase inthree types of blends. It is clear that different types ofnatural rubber affected different values of the glasstransition temperature (Tg) of the nylon-12 phase inthe blends. That is, the nylon-12 phase of the ENR-50/nylon-12 blend exhibits the lowest Tg value atapproximately 41.9◦C, followed by the Tg of theENR-25/PA-12 and ADS/nylon-12 blends at 43.7and 45.5◦C, respectively. It can be seen that the ad-dition of epoxide content in the ENR phase causesshifting of the Tg of the nylon-12 phase to a lowertemperature. This indicated a better compatibility

FIGURE 4. TSSR relaxation spectra of 60/40rubber/nylon-12 blends using various types of NR(T0 = 23◦C and 25% strain).

between the ENR and nylon-12 phases. That is, in-corporation of ENR increased the chemical interac-tion between the phases.

Figure 5 shows gradients of the initial part ofstress–temperature curves at low temperatures ofADS/nylon-12 and ENR-50/nylon-12 blends. Thegradients are typically used to calculate the tem-perature coefficient (κmax) values, as described inTable I. It is seen that the initial curve of ADS/nylon-12 blend is a plateau, whereas the curve of ENR-50/nylon-12 blend shows an increasing gradientwith the increasing temperature. A greater gradientof the stress–temperature curve indicates a higherinterfacial reaction leading to a greater κmax value.This is a result of the grafting reactions between theENR and nylon-12 molecules.

RHEOLOGICAL PROPERTIES

Plots of the apparent shear stress versus shearrate (i.e., flow curves) of the natural rubber/nylon-12 blends prepared using various types of naturalrubber are shown in Fig. 6. It is seen that the shearstress increases with the increasing shear rate. At agiven shear rate, the values of apparent shear stressof the ENR/nylon-12 blends are higher than that ofthe ADS/nylon-12 blend. Furthermore, the appar-ent shear stress of ENR/nylon-12 blends increasedwith increasing level of epoxide groups in the ENRmolecules. Therefore, it can be concluded that theENR-50/nylon-12 blend exhibits the highest flowcurve, whereas the ADS/nylon-12 blend shows thelowest one, and the ENR25/nylon-12 blend providesthe intermediate flow curve.



FIGURE 5. Gradients of the initial slopes of stress–temperature curves of 60/40 rubber/nylon-12 blends used tocalculate the temperature coefficient (κmax) values: (a) ADS/nylon-12 and (b) ENR-50/nylon-12 blends.

Figure 7 shows the plots of apparent shear viscos-ity versus shear rate (i.e., viscosity curves) of natu-ral rubber/nylon-12 blends. A decreasing trend ofshear viscosity with an increase in the shear rate orshear-thinning behavior (i.e., pseudoplasticity) canbe seen. Among the three types of the rubber com-ponents, the blend with ENR-50 exhibits the highestviscosity curve, followed by the blend with ENR-25and the blend with ADS. This confirms the compat-ibilizing effect attributed to the grafting reactions ofepoxide groups in ENR molecules with polar func-tional groups in nylon-12 molecules (Scheme 1).

ATR-FTIR ANALYSIS

Interfacial reactions between the functionalgroups of ENR and nylon-12 were elucidated by

FIGURE 6. Relationship between apparent shear stressand shear rate for rubber/nylon-12 blends at 230◦C.

ATR-FTIR. The rubber component in the blends wasfirst separated by solvent extraction before charac-terizing by ATR-FTIR. Figure 8 shows the infraredspectra of pure ENR-25, pure nylon-12, extractedENR-25/nylon-12, and ENR-50/nylon-12 blends. Itis seen that the characteristic peaks of the extractedblends consist of peaks of the ENR (i.e., symmet-ric stretching vibration of C H in oxirane rings at870 cm−1 and C H out of plane-bending bands at833 cm−1) and the characteristic peaks of nylon-12(i.e., NH-stretching vibration at 3295 cm−1, C Ostretching vibration at 1642 cm−1, and NH-bendingvibration at 1544 cm−1). This confirms the chemi-cal reactions between epoxy groups in ENR-25 andENR-50 with polar functional groups of the nylon-12 molecules. On the other hand, the spectrum of

FIGURE 7. Relationship between the apparent shearviscosity and shear rate for rubber/nylon-12 blends at230◦C.



FIGURE 8. Infrared spectra of the extracted ADS/nylon-12, ENR-25/nylon-12 and ENR-50/nylon-12 blends comparedwith pure ENR-25, ENR-50, ADS, and nylon-12.

the extracted ADS/nylon-12 blend shows a similarspectrum to that of the pure ADS, that is, only astrong absorption peak at 833 cm−1, which is as-signed to =CH–; out-of-plane-bending vibration ofthe isoprene unit is observed in addition to the ab-sorption peaks of C H stretching and bending. Thisindicates that there is no interfacial reaction betweenADS and nylon-12 molecules.

MECHANICAL PROPERTIES

Effects of different types of natural rubber onmechanical properties of the natural rubber/nylon-12 blends in terms of tensile strength, elongationat break, and hardness are presented in Table III.It is seen that the blends with epoxidized nat-ural rubber improved the tensile strength. Thatis, the ENR-50/nylon-12 blend exhibits the high-

est tensile strength, followed by ENR-25/nylon-12 and ADS/nylon-12 blends, respectively. This isattributed to the compatibility between the twophases as previously discussed. However, the ul-timate elongation of the blends with ENR-25 andENR-50 is lower than 100%, and significantlylower than that of the ADS blend. This might becaused by the interaction between oxirane groupsin ENR molecules and the polar functional groupsthat restricted the extension of the ENR/nylon-12blends.

According to the indentation hardness (Shore A)of the natural rubber/nylon-12 blends with dif-ferent types of natural rubber, a significant differ-ence is exhibited in hardness properties. That is,the ENR-50/nylon-12 blend exhibits higher hard-ness than that of the ENR-25/nylon-12 blend andthe ADS/nylon-12 blend. This might be the result ofthe higher compatibilizing effect.

TABLE IIIEffect of Rubber Types on the Mechanical Properties of Rubber/Nylon-12 Blends

Mechanical Properties ADS/Nylon-12 ENR-25/Nylon-12 ENR-50/Nylon12

Tensile strength (MPa) 13.35 ± 0.04 14.91 ± 0.22 15.81 ± 0.09Elongation at break (%) 96.1 ± 3.8 67.2 ± 2.9 68.7 ± 3.9Hardness (Shore A) 86.5 ± 0.58 88.0 ± 0.76 90.5 ± 1.04



FIGURE 9. SEM micrographs of rubber/nylon-12 blendsusing various types of NR: (a) ADS/nylon-12, (b)ENR-25/nylon-12, and (c) ENR-50/nylon-12 blends(×1000).

MORPHOLOGICAL PROPERTIES

High-resolution SEM micrographs of the naturalrubber/nylon-12 blends with three different typesof rubber are shown in Fig. 9. The rubber phaseis etched by extraction with toluene, and the cryo-genically fractured surfaces are visualized. It can beseen that the blend exhibits a co-continuous struc-ture with dual continuous phases. As might be ex-pected, the ENR/nylon-12 blends exhibit a finergrain morphology than that of the ADS/nylon-12blend because of the compatibilizing effect by thegrafting of epoxide groups in ENR with polar func-tional groups in nylon-12 molecules. It is also seenthat the ENR-50/nylon-12 blend exhibits a finerco-continuous morphology than that of the ENR-25/nylon-12 blend. Thus, an increase in the epoxidecontent of ENR molecules in the blend increasedcompatibility of the blends. This leads to an im-provement in stress relaxation behavior (Fig. 1), ther-mal resistance (Fig. 2), rheological property (Figs. 6,and 7), tensile strength (Table III), and hardness(Table III).

Conclusion

The 60/40 natural rubber/nylon-12 blends withthree different types of natural rubber (i.e., ENR-25, ENR-50, and ADS) were prepared by the melt-mixing process. The effect of types of natural rub-ber and the level of epoxide groups on the ENRmolecules on the properties of the blends (i.e., stressrelaxation behavior, thermal, rheological and me-chanical properties, and morphology) was inves-tigated. It was found that the types of naturalrubber played an important role on the stress relax-ation behavior, thermal, rheological, and mechanicalproperties of the blends. Moreover, according to theTSSR measurement and ATR-FTIR, it was found thatthe in situ grafting reactions of ENR and nylon-12 oc-curred at the interface. Also, the ENR-25 and ENR-50caused an improvement in the stress relaxation be-havior, thermal, rheological, and mechanical proper-ties of the blends because of the chemical interactionbetween the epoxide groups in the ENR moleculesand the polar functional groups of the nylon-12molecules. That is, an increasing level of epoxidegroups resulted in the superior properties of theblends. In addition, the TSSR measurement made itpossible to determine the compatibility of the blends



based on the temperature coefficient (κmax). That is,the ENR-50/nylon-12 blend exhibited the highestκmax values, implying the highest level of compati-bility with the finest co-continuous morphology.

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Influence of Epoxide Level and Reactive Blending on Properties of Epoxidized Natural Rubber and...

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