gnr mma songkla

7/27/2019 Gnr Mma Songkla

1/15

Rheological, Thermal, and Curing Properties of NaturalRubber-g-Poly(methyl methacrylate)

Charoen Nakason, W. Pechurai, K. Sahakaro, A. Kaesaman

Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of SongklaUniversity, Pattani 94000, Thailand

Received 4 April 2005; accepted 24 June 2005DOI 10.1002/app.22518Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Graft copolymers of NR and PMMA (i.e.,NR-g-PMMA) were prepared with the bipolar redox initia-tion system, using various percentages of molar ratios ofNR/MMA at 95/5, 90/10, 80/20, 70/30, and 60/40. It wasfound that the Mooney viscosity, shear stress, and shearviscosity of the NR-g-PMMA increased with an increase inthe molar ratio of MMA used in the graft copolymerization.

This may be attributed to an increasing trend of the chemicalinteraction between polar functional groups within thegrafted PMMA molecules. Furthermore, a decreasing trendof storage moduli was observed with increasing molar ratiosof MMA. The glass transition temperature was obtainedfrom the tan curves. We found a slightly increasing trendof the Tgs with an increase in molar ratios of MMA used in

the grafting reaction. The NR-g-PMMA was later com-pounded using TBBS as an accelerator. With an increase inmolar ratios of MMA in the grafting reaction, we observedan increasing trend of minimum torque, maximum torque,cure time, and scorch time, but quite similar levels of torquedifference and crosslink density. Furthermore, the tensilestrength of the NR-g-PMMA gum vulcanizate increased

with an increase in molar ratios of MMA, whereas the elon-gation at break decreased. 2005 Wiley Periodicals, Inc. J ApplPolym Sci 99: 16001614, 2006

Key words: natural rubber; rheological properties; thermalproperties; graft copolymer; poly(methyl methacrylate)

INTRODUCTION

Natural rubber (NR) contains 9395% cis-1,4-polyiso-prene.1 It is an unsaturated elastomer with various

good properties, such as high strength, high elonga-tion, and outstanding resilience. However, there are anumber of disadvantages of NR such as high sensitiv-ity to heat and oxidation because of the double bondsin its chains. NR also has low tensile strength, tensilemodulus, and poor creep characteristics unless highlyvulcanized.2 The disadvantages of NR have limited itsapplication in industry. Various types of chemicalmodifications have been performed to increase theuseful properties of the NR. One of the importantmodifications to NR is a preparation of graft copoly-mer of NR with various types of vinyl monomers such

as styrene,35 acrylonitrile,6 stearyl methacrylate/divi-nyl benzene,2 dimethylaminoethyl methacrylate,7 gly-cidyl methacrylate/styrene,8 and methyl methacrylate(MMA).916

The graft copolymerization of NR and MMA hasbeen performed by free-radical polymerization in so-lution9,10 or latex states1116 with various initiation

systems such as benzoyl peroxide,

9

azobis(2-methyl-proprionitrile),9,13 potassium persulfate,16 and redoxinitiators.11,12,14,15

In this work, a graft copolymer of NR and MMAwas prepared with a semibatch emulsion polymeriza-tion technique. A redox initiating system consisting ofan initiator (cumene hydroperoxide, CHP) and coini-tiator (tetraethylene pentamine, TEPA) was used. Thisinitiation system proved to be very effective for theemulsion polymerization of NR latex. That is, it wasnot sensitive to oxygen and worked well in the pres-ence of ammonia.17 Rheological properties of the graftcopolymer (NR-g-PMMA) were later characterized,

using a Mooney viscometer and a capillary rheometer.Dynamic mechanical analysis (DMA) and thermo-gravimetry (TG) were also used to characterize ther-mal properties of the graft copolymers. Furthermore,curing and mechanical properties of the NR-g-PMMAcompounds and vulcanizates were characterized.

EXPERIMENTAL

Materials

The rubber used was low ammonia (LA) concentratedlatex, manufactured by Pattani Industry (Pattani,

Correspondence to: C. Nakason ([email protected]).

Contract grant sponsor: Prince of Songkla University andthe Thailand Research Fund; Contract grant number: RSA4580040.

Journal of Applied Polymer Science, Vol. 99, 16001614 (2006) 2005 Wiley Periodicals, Inc.


2/15

Thailand). Methyl methacrylate (MMA) was used as amonomer for the preparation of the graft copolymer ofNR and MMA (NR-g-PMMA), manufactured byMerck (Muchen, Germany). The CHP and TEPA usedas redox initiators were manufactured by Fluka Che-mie AG (Buchs, Switzerland). The nonionic surfactant

used to stabilize the latex during graft copolymeriza-tion was Teric N30 (alkylphenol ethoxylate), manufac-tured by Huntsman Corp., Australia Pty Ltd (AscotVale, Vic, Australia). The zinc oxide used as an acti-vator was manufactured by Global Chemical (Samut-prakarn, Thailand). The stearic acid also used as anactivator was manufactured by Imperial Chemical(Pathumthani, Thailand). The sulfur used as a vulca-nizing agent was manufactured by Ajax Chemical(Samutprakarn, Thailand). The tetramethylenethi-uram disulphide (Perkacit, TMTD), N-tert-butyl-2-

benzothiazolesulphenamide (Santocure, TBBS), 2,2-mercaptobenzothiazole (Perkacit, MBT), and 2,2-di-

thiobis(benzothiazole) (Percacit, MBTS) used as curingaccelerators were manufactured by Flexsys (USA).

Preparation of NR-g-PMMA

A semibatch emulsion polymerization technique withbipolar redox initiation system was used to preparethe NR-g-PMMA. The redox initiation with CHP andTEPA was performed as described in our previouswork.14 The NR-g-PMMA was prepared using variouspercentages of molar ratios of NR and MMA (i.e.,NR/MMA) at 95/5, 90/10, 80/20, 70/30, and 60/40.That is, various concentration of MMA monomer at 5,

10, 20, 30, and 40 mol % were studied. The predeter-mined quantities of LA latex and distilled water werefirst added into the main reactor. The mixture wasthoroughly stirred, under a nitrogen atmosphere anda fixed temperature of 50C. The precise quantity ofMMA monomer, nonionic surfactant (Teric N30) anddistilled water were incorporated into a feeding tank,under continuous mixing by mechanical stirrer. Thechemicals in the feeding tank were then delivered intothe main reactor, using a fluid delivery pump at aconstant flow rate of 5 mL/min. The mixture wasallowed to react for 2 h, after all chemicals from thefeeding tank were completely delivered into the mainreactor. The latex was later coagulated and dried in avacuum oven for 24 h. The samples were cut intosmall pieces. The presence of grafted PMMA on theNR backbone was confirmed using a FTIR spectropho-tometer, series 1600 (Perkin-Elmer, USA), on a puri-fied form of the graft copolymer. For rheological, ther-mal, and curing characterization, the graft copolymerswere used without purification.

Rheological characterization

Two types of rheological techniques were used tocharacterize flow properties of the NR-g-PMMA. They

were the Mooney viscometer (viscTECH, shearingdisk viscometer) and a single bore Rosand capillaryrheometer (model RH7). Mooney viscosities of un-modified NR and various types of NR-g-PMMA werequantified at 180C, using a small rotor with a pre-heating time of 1 min and a testing time of 4 min. The

typical type of the Mooney viscosity was classified asMS (1 4), 180C.

Shear flow properties, in terms of shear stress andshear viscosity at various shear rates, were deter-mined using a capillary rheometer over a wide rangeof shear rates (51600 s1) at a test temperature of180C. Dimensions of the capillary die used were a2-mm diameter, 32-mm length, and 180 entrance an-gle with the aspect ratio (L/D) of 16:1. The materialswere first preheated in a barrel for 7 min under apressure of approximately 24 MPa to get a compactmass. The excess molten material was automaticallypurged until no bubbles were observed. The test was

then carried out at various shear rates as set by aprogram via a microprocessor. The pressure dropacross the capillary channel and melt temperaturewere captured via a data acquisition system duringthe test. Equations used to calculate shear stress, shearviscosity, and shear rate were described in our previ-ous publications.18,19

Thermal properties characterization

Two techniques were used to follow the thermal be-havior of the NR-g-PMMA. They were TG and DMA.

The thermogravimetric analysis was carried out in anitrogen atmosphere, at a heating rate of 20C/min, ina Shimadzu TGA-50 thermal analyzer. A constantweight of sample was used in all cases. Thermogramswere recorded from room temperature to 700C. Thetemperature at which the rate of mass loss is at amaximum was determined from the differential ther-mogravimetry (DTG) curve and called the decompo-sition temperature (Td). The DMAs were carried outon a PerkinElmer DMA7 instrument. The experimentwas conducted in tension mode. A temperature/timescan was carried out at temperature ranges of 80C to150C at a frequency of 1 Hz. The heating rate was

2C/min. Liquid nitrogen was used to achieve subam-bient temperature.

Compounding and cure characterization of NR-g-PMMA

The NR-g-PMMA prepared with a percentage molarratio of NR/MMA 95/10 was selected to compoundwith various chemicals, as shown in Table I. The in-fluence of various types of accelerators (i.e., TBBS,MBT, MBTS, and TMTD) at the same concentration(i.e., 0.0029 mol/100 g of NR-g-PMMA) was first in-vestigated. The NR-g-PMMA was premasticated for 5

PHYSICAL PROPERTIES OF NR-g-PMMA 1601


3/15

min, using a two-roll mill maintained at 60C. Otherchemicals were later incorporated according to themixing schedule shown in Table II. The rubber com-pounds were sheeted out and conditioned at room

temperature in a closed container for 24 h. Curingcharacteristics were then studied at 160C, using aMonsanto oscillating disk rheometer (model ODR2000) at a 1 arc. The scorch time, cure time, and curerate index [i.e., 100/(cure time scorch time)] werecalculated based on the ODR curve or curing curve.Furthermore, torques of the curing curves were alsoquantified.

The most suitable type of accelerator was later se-lected based on its superior curing characteristics. Var-ious types of NR-g-PMMAs (i.e., prepared with per-centage molar ratios of NR/MMA 95/5, 80/20,

70/30, and 60/40) were again compounded, using thesuitable type of accelerator and other chemicals asindicated in Table I, with a mixing schedule shown inTable II. The curing properties were again studied.

Sample preparation and physical testing

The rubber compounds were compression moldedinto test specimens at 160C according to the respec-tive cure times determined by ODR 2000. Dumb-bellsamples with the length of 115 mm were cut from a2-mm-thick molded sheet. The tensile testing proce-dure was carried out according to Test Method A as

described in ASTM D412. A Hounsfield tensometer(model H 10KS) operating at 500 mm/min was usedto determine the tensile properties in terms of ultimatetensile strength and elongation at break.

Crosslink density

Samples weighing about 0.8 g were cut from curedsheets and immersed in 80 mL of toluene in a darkcontainer for 1 week at room temperature. Swollensamples were blotted with a tissue paper and imme-diately transferred into a weighing container to deter-mine swollen weight. The swollen samples were later

transferred to a vacuum oven and dried at room tem-perature for 24 h. The dried weight was then deter-mined. Crosslink density was calculated using theFloryRehner equation:20

c

1

2Vs

ln(1 vr0) vr

0 vr

0

vr01 / 3

vr0

2

(1)

where c is the crosslink density (mol/m3); Vs is the

molar volume of toluene (1.069 104 m3/mol) at25C; vf

0 is the fraction of rubber in the swollen gel;and is the interaction parameter (0.39 for gum NR).

RESULTS AND DISCUSSION

Mooney viscosity

Figure 1 shows the Mooney viscosity of NR-g-PMMA

prepared using various molar ratios of NR/MMAcompared with the Mooney viscosity of unmodifiedNR (i.e., at a molar ratio of MMA 0%). It can be seenthat the Mooney viscosity of the graft copolymers washigher than that of the unmodified NR (which wascoagulated from the LA latex used as a raw materialfor a preparation of NR-g-PMMA). This may be attrib-uted to higher chemical interaction between moleculesof NR-g-PMMA via polar functional groups (i.e., car-

bonyl group). Higher torque therefore was needed todeform the NR-g-PMMA melt during the Mooney test.The Mooney viscosity also increased with an increasein the molar ratio of MMA or the concentration ofMMA monomer used in the graft copolymerization.This is because of an increase in the grafted PMMA onthe NR molecules with increasing concentrations ofMMA monomer, as in our earlier findings.14 As aconsequence, higher chemical interaction between po-lar functional groups of the grafted PMMA was ob-tained. The force used to move the Mooney viscome-ters disc increased with the increase of grafted PMMAcontent in the graft copolymer molecules.

Shear flow properties of NR-g-PMMA

Plots of apparent shear stress versus apparent shear rateof the NR-g-PMMA prepared with various molar ratios

TABLE IIMixing Schedule

Descriptions Time (min)

Rubber mastication 5Stearic acid 1Zinc oxide 1Wingstay L 1Accelerators 2Sulfur 2Compound finishing 4

TABLE ICompounding Formulation

Ingredients

Quantity (phr)

Fl F2 F3 F4

NR-g-PMMA 100 100 100 100

Stearic acid 0.5 0.5 0.5 0.5ZnO 6 6 6 6TBBS 0.7 MBT 0.5 MBTS 1.0 TMTD 0.7Wingstay L 1 1 1 1Sulfur 3.5 3.5 3.5 3.5

1602 NAKASON ET AL.


4/15

of NR/MMA compared with the unmodified NR (i.e.,pure NR) are shown in Figure 2. It can be seen that theapparent shear stress increased with an increase of shearrate. At the same level of apparent shear rate, the pureNR exhibited the lowest apparent shear stress. Further-more, the apparent shear stress increased with an in-crease in MMA concentration used to prepare the NR-

g-PMMA. That is, among various types of the NR-g-PMMA, the graft copolymer prepared with a percentagemolar ratio of NR/MMA 95/5 showed the lowestapparent shear stress, while a molar ratio of NR/MMA 60/40 exhibited the highest value. The levels of ap-parent shear stress at a given shear rate for various typesof graft copolymer can be ranked according to the per-

centage of molar ratio of NR/MMA as follows: 95/5 90/10 80/20 70/30 60/40. The increasing trendof the apparent shear stress with an increase in molarratio in NR/MMA was the result of polar functionalgroups in the NR-g-PMMA molecules. Therefore, in-creasing the concentration of MMA monomer caused anincreasing trend of the apparent shear stress and

Mooney viscosity (Fig. 1).Figure 3 shows the plots of apparent shear viscosity

versus apparent shear rate for the NR-g-PMMA com-pared with that for pure NR. It was found that theapparent shear viscosity decreased with an increase inapparent shear rate. That is, a shear thinning behaviorwas observed for all sets of the polymers. Further-more, the same phenomenon was observed as in thecases of the apparent shear stress and Mooney viscos-ity. That is, the pure NR exhibited the lowest apparentshear viscosity. Also, the trends of the apparent shearviscosity at a given shear rate increased with an in-

crease in concentration of MMA used in the graftcopolymerization. Figure 4 shows the apparent shearviscosity at constant apparent shear rates of 10, 30, and100 s1. The results confirm the increasing trend ofapparent shear viscosity with the increase in molarratios of MMA or concentration of MMA used in thegraft copolymerization. The increasing levels ofgrafted PMMA on NR molecules were the reason forthe increasing trend.

To clarify the shear thinning behavior of the rubbermelts, the power law equation was applied to therelationship between shear rate and shear stress (i.e.,in Fig. 2).21

Figure 1 Mooney viscosities for various types of NR-g-PMMA compared with that for pure NR.

Figure 2 Relationship between apparent shear rate andapparent shear stress for various types or NR-g-PMMA com-pared with that for pure NR.



5/15

K( )n (2)

where n is the power law index or the flow behaviorindex, is the shear rate, is the shear stress, and Kisthe consistency or viscosity coefficient index.

From the linear relation on a loglog scale, we canget the slope (n) and intercept (K), which are plotted inFigure 5. The n value of the pure NR was higher thanthose of various grades of the graft copolymers. The nvalues decreased with increasing levels of the grafted

Figure 3 Relationship between apparent shear rate and apparent shear viscosity for various types of NR-g-PMMA comparedwith that for pure NR.

Figure 4 Apparent shear viscosity for various types of NR-g-PMMA at constant apparent shear rates of 10, 30, and 100 s1

compared with that for pure NR.

1604 NAKASON ET AL.


6/15

PMMA in the NR-g-PMMA molecules. This indicatesgreater shear thinning in the flow upon increasing themolar ratio of MMA used in the graft copolymeriza-tion. The value of n also reflects the deviation of theflow profiles from uniform parabolic flow patterns(i.e., n 1 for Newtonian flow) to plug-like flowprofiles. The NR-g-PMMA exhibited very low n values(i.e., n 0.25). Therefore, the highly pseudoplasticfluid flowed through the capillary almost as a plugmoving at a uniform speed as the melt was slidingdown against the capillary wall. The consistency in-dex, K, is also shown in Figure 5. By definition, theconsistency is a Newtonian viscosity if n 1 andrelated to the zero-shear viscosity (i.e., shear viscosityat shear rate of zero) of the flowing rubber melt at atest temperature. It was found that the consistencyindex increased with the level of grafted PMMA (i.e.,increasing the molar ratio of MMA). The trends ofconsistency index (i.e., zero-shear viscosity) corre-sponded to the trends of Mooney viscosity (Fig. 1),

apparent shear stress (Fig. 2), and apparent shear vis-cosity (Fig. 3 and 4).

Dynamic mechanical analysis

Thermal transition phenomena of the NR-g-PMMAwere studied using DMA technique. Figure 6 showsdependence of storage modulus on the temperaturefor various types of NR-g-PMMA compared with thatfor pure NR. We see that the storage moduli decreasewith increase in testing temperature. This is attributedto an increase in mobility of the polymer chains withincreasing temperature. The storage modulus at low

temperature of pure NR is greater than those of thegraft copolymers. The storage moduli also decreasedwith an increase in MMA concentration used in thegraft copolymerization. As stated earlier, an increasein MMA concentration or a percentage molar ratio ofMMA caused an increasing level of chemical interac-tion between the NR-g-PMMA molecules. Also in ourprevious work,22 infrared spectroscopy was used tocharacterize the NR-g-PMMA prepared with variouspercentage molar ratios of NR/MMA. The character-istic bands of infrared spectrum of CAO stretchingand COO stretching were observed at 1732 and 1140cm1, respectively. This proves the presence of thegrafted PMMA on the NR molecules. The level ofgrafted PMMA was also quantified using the absor-

bance ratio of IR peaks at 1732 (OCAO stretch) and835 cm1 (OCOH out-of-plane-bending on cis CAC

bonds). We concluded that the level of OCAO bondsin the graft copolymer molecules increased with anincreasing level of MMA monomers in the grafting

reaction. Therefore, an increasing trend of chemicalinteraction within and between the NR-g-PMMA mol-ecules is obvious because of increasing levels of polargroups (i.e., OCAO and OCOO) in the molecules.This leads to an increasing trend for a formation ofchemical bond such as hydrogen bond. An increase inchain stiffness of the graft copolymer molecules wasthe consequence. Therefore, the storage moduli de-creased with an increase in chain stiffness. The effectof percentage molar ratio of MMA on loss tangent (tan) is shown in Figure 7. It can be seen that the tan showed a peak around the temperature between65C to 50C, this being known as a transition re-

Figure 5 Power law index (n) and consistency index (K) for various types of NR-g-PMMA compared with that for pure NR.



7/15

gion. The glass transition temperature (Tg) can bequantified at a maximum value of each tan curve.The pure NR exhibited the lowest Tg (i.e., 65C). Wealso observed the slight shift of the Tg to higher valuewith an increase in MMA concentration or a percent-age molar ratio of MMA used in the graft copolymer-ization. That is, the Tgs are ordered as follows: NR-g-PMMA with a molar ratio of NR/MMA 60/40

70/30 80/20 90/10 95/5 pure NR, and thedetails are given in Table III. This is attributed to an

increase in a restriction of molecular chains upon in-creasing levels of the grafted PMMA in the NR mole-cule.

Thermal degradation

Thermal degradation of the various types of NR-g-PMMA and unmodified NR (i.e., pure NR) was inves-

tigated using TGA. Figure 8 shows the TGA thermo-grams for various types of rubbers. It can be seen that

Figure 6 Storage moduli (G) of NR-g-PMMA prepared using various molar ratios of NR/MMA in the graft copolymer-ization compared with that of pure NR.

Figure 7 Tan of NR-g-PMMA prepared using various molar ratios of NR/MMA in the graft copolymerization comparedwith that of pure NR.

1606 NAKASON ET AL.


8/15

the degradation of pure NR started and completed atthe lowest temperature, while the NR-g-PMMA pre-pared using a percentage molar ratio of NR/MMA 60/40 exhibited the highest temperatures. There-fore, the starting and completing degradation temper-

ature for various types of rubbers are ordered as fol-lows: pure NR NR-g-PMMA (95/5) NR-g-PMMA(90/10) NR-g-PMMA (80/20) NR-g-PMMA (70/30) NR-g-PMMA (60/40). The degradation temper-ature (Td) of various types of rubbers was evaluatedfrom the main weight loss obtained from DTG curves(not shown here), and the results given in Figure 9. Wefound that the pure NR exhibited the lowest decom-position temperature (i.e., at 364.5C), while varioustypes of NR-g-PMMA showed the Tds at 376.96C,377.32C, 380.63C, 384.92C, and 390.96C for thegraft copolymer prepared with the percentage molarratios of NR/MMA 95/5, 90/10, 80/20, 70/30, and

60/40, respectively. Therefore, it is clear that the Td ofthe pure NR was lower than those of the graft copol-ymers. Furthermore, the Td increased with increasingconcentrations of MMA monomer used in the graftcopolymerization. The increasing trend of Td with in-creasing concentrations of MMA can be described as

increasing levels of chemical interaction between po-lar functional groups present in the rubber molecules.Stronger interaction, as observed from increasing lev-els of the Mooney viscosity (Fig. 1), apparent shearstress (Fig. 2) and apparent shear viscosity (Figs. 3 and4), upon increasing concentrations of MMA causedincreasing decomposition temperatures of the graftcopolymers. This may be attributed to the strongerchemical bonds between the molecules with increas-ing MMA concentration in the grafting reaction. Also,increasing oxygen compounds that exhibited higherresistance to thermal degradation than that of the purehydrocarbon might be another reason for the increas-

ing trend of Tds.

Curing properties of NR-g-PMMA with a molarratio of NR/MMA 90/10

Figure 10 shows curing curves or rheographs (fromODR 2000) of NR-g-PMMA prepared from a percent-age molar ratio of NR/MMA 90/10. Curing char-acteristics of the NR-g-PMMA compounds using var-ious types of accelerators (i.e., TBBS, MBTS, MBT, andTMTD) were examined at 160C for 30 min. Forcerequired to oscillate a rheometers disc or torque was

TABLE IIIGlass Transition Temperature (Tg) of NR-g-PMMA

Prepared Using Various Molar Ratios of NR/MMA inthe Graft Copolymerization Compared With Pure NR

Types of polymerGlass transition

temperature (C)

Pure NR 65.0NR/MMA 95/5 53.7NR/MMA 90/10 52.2NR/MMA 80/20 51.3NR/MMA 70/30 51.0NR/MMA 60/40 50.5

Figure 8 TGA thermograms of NR-g-PMMA prepared using various molar ratios of NR/MMA in the graft copolymeriza-tion compared with that of pure NR.



9/15

recorded as a function of testing time. The torque isnormally proportional to the shear modulus or thestiffness of the rubber compounds. In Figure 10, it can

be seen that the torques for all sets of experiments firstdecreased during the specimen warming up time. Af-ter 2 min, the torque increased abruptly, owing tovulcanization of the rubber component. The torquesthen reached the maximum values (i.e., at 100% vul-canization). The characteristics of the curing curve forthe NR-g-PMMA compounds using various types ofaccelerators include a maximum torque and slightreversion. That is, the curing curve was maximizedand some crosslinking structures were later destroyed

by thermal energy, under the continuous rotor oscil-lation. Different curing behaviors were observed forthe compounds using different types of accelerators.That is, the rubber compound with TBBS exhibited thehighest maximum torque (MH) and torque difference

(MH ML), while the compound with MBT showedthe lowest values (Fig. 11). The torques of the NR-g-PMMA compounded with TMTD and MBTS fall be-tween the compounds using TBBS and MBT. There-fore, the MH and MH ML of various types of NR-g-PMMA compounds can be ranked as follows: TBBS TMTD MBTS MBT. It is therefore concludedthat the NR-g-PMMA compounded with TBBS gavethe highest degree of crosslinking, according to itstorque difference. The crosslinking density of varioustypes of vulcanizates is shown in Figure 12. We foundthat the crosslinking density of the gum vulcanizatesof NR-g-PMMA using various types of acceleratorswas in agreement with the levels of torque difference.

Figure 13 shows the scorch time (ts2) and cure time(t90) of various types of compounds. It was found thatthe NR-g-PMMA compounded with TBBS showed thelowest cure time and a reasonable scorch time (i.e.,

Figure 9 Decomposition temperatures (Td) of NR-g-PMMA prepared using various molar ratios of NR/MMA in the graftcopolymerization compared with that of pure NR and PMMA.

Figure 10 Rheographs of NR-g-PMMA with molar ratio of NR/MMA 90/10, using various types of accelerators.

1608 NAKASON ET AL.


10/15

Figure 11 Torque of NR-g-PMMA with molar ratio of NR/MMA 90/10, using various types of accelerators.

Figure 12 Crosslink density of gum vulcanizate for NR-g-PMMA with a percentage molar ratio of NR/MMA 90/10, usingvarious types of accelerators.



11/15

long enough to store without a high degree ofcrosslinking at storage temperature). The cure time ofthe other types of compounds (i.e., MBT, MBTS, andTMTD compounds) were longer than that of the TBBScompound. Furthermore, the MBTS and TMTD com-

pounds exhibited scorch times similar to the TBBScompound, but the MBT compound showed a shorterscorch time. Figure 14 shows the cure rate index ofvarious types of NR-g-PMMA compounds. We foundthat the TBBS compound showed the highest cure rate

Figure 13 Scorch time and cure time of NR-g-PMMA with molar ratio of NR/MMA 90/10, using various types ofaccelerators.

Figure 14 Cure rate index of NR-g-PMMA with molar ratio of NR/MMA 90/10, using various types of accelerators.

1610 NAKASON ET AL.


12/15

index or rate of vulcanization, while the MBT com-pound showed the lowest value. Other compoundswere ordered as follow: TBBS TMTD MBTS MBT. Therefore, TBBS was chosen as an accelerator

for curing various types of NR-g-PMMA (i.e., at themolar ratio of NR/MMA 95/5, 90/10, 80/20, 70/30,and 60/40) by using the compounding formulation(F1) and mixing schedule as shown in Tables I and II,respectively.

Curing properties of NR-g-PMMA with variousmolar ratios of NR/MMA

Figure 15 shows the influence of molar ratios of MMAused in the graft copolymerization on torques of var-ious types of NR-g-PMMA, as obtained from the ODRtests. It can be seen that the minimum (ML) and max-

imum (MH) torques increased with an increase in apercentage molar ratio of MMA monomer. This may

be attributed to an increase in chemical interaction,and hence stiffness of the molecules due to an increasein the grafted PMMA with increasing MMA concen-tration. However, very similar levels of the torquedifference (i.e., MH ML) were observed. Therefore,the increasing trend of MMA concentration used inthe grafting reaction caused increasing trends of stiff-ness of the rubber vulcanizates but did not have amajor affect on the torque difference. The crosslinkdensity of the gum vulcanizate at various levels ofmolar ratios of MMA monomer was determined. We

found that increasing levels of molar ratios of MMAdid not play a major role on the level of crosslinkdensity. That is, a similar level of crosslink densitywas observed at approximately 50 mol/cm3. There-

fore, the trends of crosslink density corresponded tothe trends of torque difference.

Figure 16 shows the influence of a molar ratio ofMMA monomer on scorch time and cure time of theNR-g-PMMA compounds. We observed an increase inscorch and cure time with increasing levels of MMAconcentration used in the grafting reaction. As men-tioned earlier, increase in MMA concentration causedan increase in the grafted PMMA in the molecules.Therefore, a higher level of polar functional groups inthe NR-g-PMMA was obtained. In the compoundingprocedure, the same concentration of an accelerator(i.e., TBBS) was used to prepare the NR-g-PMMAcompounds. Some contents of the TBBS might be ab-sorbed by the polar functional groups of the graftcopolymer because of their polarities. Therefore, theseamounts of accelerator could not perform their func-tion of accelerating the vulcanization process. Hence, alonger time was needed to perform the crosslinkingprocess to reach the optimum curing characteristics.According to our previous observation, we found anincreasing trend of ungrafted PMMA or PMMA ho-mopolymer, with an increase in MMA concentrationused in the grafting reaction.14 In this work, the reac-tion products were compounded without purification

Figure 15 Torque of NR-g-PMMA with various molar ratio of NR/MMA used in graft copolymerization.



13/15

or separation of any other chemicals that remained inthe crude polymer. Therefore, the polar functionalgroups in the PMMA molecule were capable of ab-sorbing some amount of TBBS because of their polar-ity. Therefore, a longer time was needed to completethe vulcanization process. Moreover, a lower cure ratewas observed with an increase in the molar ratio of

MMA monomer used in the graft copolymerization, asshown in Figure 17. This can be attributed to de-creased levels of TBBS, which could function as anaccelerator upon increasing levels of MMA monomeror increasing levels of polar functional groups in theNR-g-PMMA molecules.

Mechanical properties of NR-g-PMMA gumvulcanizates

The influence of molar ratios of MMA used in thegrafting reaction on ultimate tensile strength is shownin Figure 18. It can be seen that the tensile strengthincreased with an increase in the molar ratio of MMA.

This can be described as an increasing trend of thegrafted PMMA on the molecules. This caused an in-creasing trend of stiffness of the material. Therefore,higher force was needed to elongate the specimensprepared from NR-g-PMMA compounds, usinghigher molar ratios of MMA. The ultimate elongationor elongation at break (EB) of the NR-g-PMMA com-pounds prepared using various molars ratio of MMAis shown in Figure 19. We found that the NR-g-PMMAprepared with a molar ratio of MMA 95/5 exhibitedthe highest elongation at break (i.e., 820%), which issimilar to the EB of unmodified NR gum vulcanizate.Increasing levels of molar ratio of MMA caused anabruptly decreasing trend of the elongation at break.We found that the elongation at break of NR-g-PMMAprepared with molar ratios of MMA 30 and 40 wasvery poor. The increasing trend of chain stiffness as anincreasing level of the grafted PMMA was the reasonfor a decreasing trend of elongation at break. There-fore, at higher content of the grafted PMMA, the chainmobility was very much restricted. Rupture of thespecimens was therefore observed instead of elonga-tion, when the NR-g-PMMA gum vulcanizate usinghigh molar ratios of MMA such as 30% and 40% wereused.

Figure 16 Scorch time and cure time of NR-g-PMMA with various molar ratios of NR/MMA used in graft copolymerization.

Figure 17 Cure rate index (CRI) of NR-g-PMMA with var-ious molar ratios of NR/MMA used in graft copolymeriza-tion.

1612 NAKASON ET AL.


14/15

CONCLUSIONS

Graft copolymers of NR and PMMA (i.e., NR-g-PMMA) were prepared using various percentage mo-lar ratios of NR/MMA at 95/5, 90/10, 80/20, 70/30

and 60/40. Rheological properties were characterized.It was found the Mooney viscosity, shear stress, andshear viscosity of the unmodified NR were lower thatthose of the graft copolymers. Furthermore, we found

Figure 18 Tensile strength of NR-g-PMMA vulcauizates with various molar ratios of NR/MMA used in graft copolymer-ization.

Figure 19 Elongation at break of NR-g-PMMA with various molar ratios of NR/MMA used in graft copolymerization.



15/15

increasing trends of Mooney viscosity, shear stressand shear viscosity of the NR-g-PMMA with an in-crease in molar ratio of MMA used in the graft copo-lymerization. The chemical interaction between polarfunctional groups in the grafted PMMA was respon-sible for the increasing trends of rheological proper-

ties. The DMA and TGA of the NR-g-PMMA and pureNR were also characterized. We found the pure NRexhibited the highest storage modulus. Also, decreas-ing trends of storage moduli were observed with in-creasing molar ratios of MMA. The glass transitiontemperature was obtained from the tan curves. Wefound a slightly increasing trend of the Tgs with anincrease in a molar ratio of MMA used in the graftingreaction. This is because of an increasing level of chainmobility. The decomposition temperature of varioustypes of rubbers was also determined. It was foundthat the pure NR showed the lowest Td, while the Td ofgraft copolymer increased with an increase in molar

ratios of MMA.The graft copolymer with a percentage molar ratio

of NR/MMA 90/10 was used to study cure char-acteristics using various types of accelerators (i.e.,TBBS, MBT, MBTS, and TMTD). We found that TBBSshowed the highest torque difference (i.e., crosslinkdensity), the shortest cure time, and the highest curerate. Therefore, TBBS was later selected to compoundwith various types of NR-g-PMMA. We observed theincreasing trends of minimum torque, maximumtorque, cure time, and scorch time, but quite the samelevels of torque difference and crosslink density. The

ultimate tensile strength of the gum vulcanizate ofNR-g-PMMA increased with an increase in molar ra-tios of MMA used in the grafting reaction, whereas theelongation at break decreased.

References

1. Kroswitz, J. I.; Concise Encyclopedia of Polymer Science andEngineering; Wiley: London, 1990; p 1015.

2. Zhou, M.-H.; Hoang, T.; Kim, I.-G.; Ha, C.-S.; Cho, W.-J. J ApplPolym Sci 2000, 79, 2464.

3. Asletha, R.; Kumaran, M. G.; Thomas, S. Polym Degrad Stab

1998, 61, 431.4. Tho, N. V.; Kadir, M. O. A.; Hashim, A. S. Rubber Chem Technol2002, 75, 111.

5. Kawahara, S.; Kawazura, T.; Sawada, T.; Isono, Y. Polymer 2003,44, 4527.

6. Okieimen, F. E.; Urhoghide, I. N. J Appl Polym Sci 1996, 59,1803.

7. Oliverira, P. O.; Guimaraes, A.; Cavaille, J.-Y.; Chazeau, L.;Gilbert, R. G.; Santos, A. M. Polymer 2005, 46, 1105.

8. Suriyachi, P.; Kiatkamjonwong, S.; Prasassarakich, P. RubberChem Technol 2004, 77, 914.

9. Cambell, D. S.; Seow, P. K. J Nat Rubber Res 1990, 5, 135.10. Enyiegbulam, M. E.; Aloka, I. U. J Appl Polym Sci 1992, 44, 1841.11. Oommen, Z.; Groeninckx, G.; Thomas, S. J Polym Sci Part B:

Polym Phys 2000, 38, 525.12. Oommen, Z.; Gopinathan, M. R.; Thomas, S. Polym Eng Sci

1996, 36, 151.13. Lehrie, R. S.; Willis, S. L. Polymer 1997, 38, 9937.14. Nakason, C.; Kaesaman, A.; Yimwan, N. J Appl Polym Sci 2002,

87, 68.15. Lee, D.-Y.; Subramanium, N.; Fellows, C. M.; Gilbert, R. G. J

Polym Sci Part A: Polym Chem 2002, 40, 809.16. Thiraphattaraphun, L.; Kiatkamjornwong, S.; Prasasarakich, P.;

Damronglerd, S. J Appl Polym Sci 2001, 81, 428.17. Scheider, M.; Pith, T.; Lambla, M. J Appl Polym Sci 1996, 62, 273.18. Nakason, C.; Kaesaman, A.; Wongkul, T.; Kiatkamjornwong, S.

Plast Rubber Compos 2001, 30, 154.19. Nakason, C.; Kaesaman, A.; Samoh, S.; Homsin, S.; Kiatkam-

jornwong, S. Polym Test 2002, 21, 449.20. Hamed, G. R.; Ratanasom, N. Rubber Chem Technol 2002, 75,

323.

21. Cogswell, F. N. Polymer Melt Rheology: A Guide for IndustrialPractice; Woodhead Publishing Limited: Cambridge; 1981; p 24.

22. Nakason, C.; Kaesaman, A.; Rungvichaniwal, A.; Kiatkamjorn-wong, S. J Appl Polym Sci 2003, 89, 1453.

1614 NAKASON ET AL.

gnr mma songkla

Documents