Effect of maleic anhydride and liquid natural rubber as compatibilizers on the mechanical properties and impact resistance of the NR-NBR blend

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2004; 15: 209–213

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pat.461

Effect of maleic anhydride and liquid natural rubber as

compatibilizers on the mechanical properties and impact

resistance of the NR-NBR blend

Ahmed Mounir*, Nabila A. Darwish and Adel ShehataNational Institute for Standards, Polymer Testing Laboratory, Tersa Street 12211, P.O. Box 136, Al Haram, Giza, Egypt

Received 24 June 2003; Accepted 30 August 2003

The degree of compatibilization between natural rubber (NR) and acrylonitrile-butadiene rubber

(NBR) was investigated by two different methods. NBR was chemically modified with maleic anhy-

dride in a screw twin mixer with and without reaction initiator, benzoyl peroxide. Also, the effects

of molecular weight of liquid natural rubber (LNR) as a compatibilizer were studied. The degree of

compatibilization between NBR and NR is determined indirectly through measurements of

mechanical properties and impact resistance. The maleic anhydride and benzoyl peroxide concen-

trations influence the mechanical properties and impact resistance of the blends. Also, the mechan-

ical properties of the blends showed that the molecular weight of LNR played an important role in

determing their performance. Copyright # 2004 John Wiley & Sons, Ltd.

KEYWORDS: maleic anhydride; rubber; strength; impact resistance; NR-NBR blend

INTRODUCTION

Polymer blending is an attracive technique for polymer mod-

ifications owing to its economical advantages. Polymer blend

is a well-known strategy and an important topic studied

because of the low cost, and benfits that may be obtained in

blends such as processing new physical and chemical proper-

ties. It is very difficult to obtain a good dispersion of polymer

blends since the components are insoluble in each other, par-

ticularly for combinations of polar polymers, such as polya-

mide or acrylonitrile-butadiene rubber (NBR) with non-polar

polymers such as polystyrene (PS) or natural rubber (NR).

This leads to phase separation manifested as a reduction of

the blends mechanical properties as compared to those of

the polymer alone. Saheb and Jog1,2 have investigated the

compatibility of polybutylene terphthalate (PBT) and polyo-

lefin; the blends are immiscible owing to the difference in

polarities of the component polymers and lack of adhesion

at the surface. Also, similar studies have been reported in

the case of polyamide/polyolefin blend system.3 The

mechanical behavior of the polymer blends is dependent on

the adhesion at the interface for efficient transfer of stress

between the component phases.4 In order to overcome this

short coming effect, efforts have been made such as addition

of reactive compatibilizers. Nah5 has used trans-polyocty-

lene, as a compatibilizer for incompatible NBR, it has been

found that the mechanical and rheological properties of

NR-NBR are fairly weak and unsatisfactory in all propor-

tions. Also, Li and Li6 have studied the effect of carboxylated

polystyrene (CPS) as a compatibilizer on the notched impact

toughness of polyamide/polystyrene blend, PA/PS/CPS.

They found6 that the cracking of the spherical PS domains

occurrs with increasing amounts of CPS, this may be respon-

sible for the gradual decrease in notched impact strength of

the blend with further increase in CPS level. So, in this study,

it has been proposed that two alternative and economical

simple methods be examined to enhance the degree of com-

patibilization between NBR and NR blend and consequently

the mechanical properties of this blend.

The two methods followed in this study involve the

grafting of the rubber with maleic anhydride (MAH) to build

chemical links between NBR and NR. The grafting process is

carried out by a reaction between MAH and NBR rubber,

with the addition of benzoyl peroxide (BPO) as initiator.7

BPO and other peroxides have been used as reaction initiators

and/or crosslinking agents previously.8,9 In the second

method, liquid natural rubber (LNR) is used as a different

compatibilizer to improve the compatibility of NR-NBR

blend and its mechanical properties.

EXPERIMENTAL

MaterialsNR (Malaysian Rubber CV60) within the average molecular

weight of 300 000 g/mol and NBR (AN content 30%) of aver-

age molecular weight 105 400 g/mol were supplied from the

authors laboratory. BPO and MAH were obtained from

Aldrich. LNR of various molecular weights was produced

by photochemical oxidation of NR as follows: degraded NR

of various molecular weights was prepared by using UV

irradiation in toluene solution. The UV source was generated

from the mercury lamp of a 400 W supplied Photochemical

Copyright # 2004 John Wiley & Sons, Ltd.

*Correspondence to: A. Mounir, National Institute for Standards,Polymer Testing Laboratory, Tersa Street 12211, P.O. Box 136,Al Haram, Giza, Egypt.E-mail: [email protected]

Reactors Ltd, UK. The NR was masticated by using a two-roll

mill for 30 min at room temperature prior to dissolving the

rubber in toluene. The concentration of rubber in solution

was 20 wt%. The reduction of the molecular weight over irra-

diation time was measured using gel permeation chromato-

graphy technique (L-6000; Hitachi Co., Ltd, tetrahydrofuran

(THF) as eluent).

Grafting reaction and blending procedureMixtures of BPO and MAH with NBR were fed to the Rheo-

cord twin-screw mixer at 1508C. The amount of MAH used in

the grafting reaction changed from 0, 1, 1.5, 2, 2.5, 3, and

3.5 phr, gMAH/100gNBR. BPO concentrations considered

were 0, 3, 7, and 10 phr with respect to the MAH weight.

The extent of the MAH-NBR reaction is determined by mea-

suring the acid number and percentage of reacted MAH.10

This is obtained by dissolving a gram of grafted rubber in

100 ml of toluene with reflux at 658C for 3 hr. Subsequently

50 ml of water is added and three different phases are formed,

these are: organic, gel and aqueous. The organic phase con-

tains the rubber grafted with MAH, the gel phase contains

the crosslinked rubber, and the aqueous phase contains the

MAH, which did not react, and remains dissolved in water.

The organic phase is titrated with KOH solution in ethanol

using 0.1 N thymol blue indicators, the acid number is

defined as mgKOH/g rubber.10 The blending of NR with

30 phr of NBRg is processed in a twin-screw mixer with a

speed of 50 rpm at 1508C.

The NR-NBR blends of 70:30 ratio(70 g/30 g), by using two

different methods (different compatibilizer), were prepared

at blending temperature 1508C, and speed 50 rpm, blend time

15 min. NR was masticated in the mixer before LNR was

added. NR and LNR were mixed for about 5 min before NBR

was added. The infrared (IR) spectra of NR-NBRg blends

were measured as a cast film on a NaCl window using a

Shimadzu DR-8060, Japan. The notched impact resistance of

NBRg-NR blends was measured using an impact testing

machine, Roell Amsler RKP 50, Germany. The Mooney-

viscosity was measured at 1008C using a Mooney viscometer,

Alpha Technologies MV 2000, Akron, OH, USA. Tensile

strength of NR-NBRg blend was measured using a Zwick

2010/TH2A, Germany.

RESULTS AND DISCUSSION

It is reported that the anionic polymerization of butadiene can

be carried out through 1,4 or 1,2 addition routes. Through 1,2

addition route, branched polybutadiene with vinyl group is

obtained.11 Because NBR contains a mixture of linear and

branched molecules, the grafting of MAH can take place on

either linear chains or chains with vinyl groups. Results of

the percentage of MAH-grafted groups on NBR (gMAH/

gNBR) are shown in Table 1. Percentage of reacted MAH

means the amount of MAH, which react in the organic phase,

as a percentage of the total MAH added (gMAH/g of total

rubber). Percentage grafting means that the percentage of

MAH that reacted with NBR. It can be seen that the number

of grafted molecules varies, as the peroxide amount is

changed. In the last column of Table 1 the percentage grafting

(main chain and branched vinyl groups) is shown. As

observed, the grafting degree of MAH lies approximately

between 0.7 and 0.9 implying similar reaction kinetics.

Results of NBR IR spectrum show a strong peak at

968 cm�1, which it assigned to the presence of a vinyl group.

The change in the magnitude of this peak was monitored by

IR measurements. It was found that the decrease in the mag-

nitude of this peak may be attributed to the reaction with

MAH.

Table 2 shows an increase in the relative absorbance. The

relative absorbance is a comparison of absorbance intensities

of one peak at 2930 cm�1, which corresponds to the non-

reacted C–H groups and another peak at 968 cm�1, which

corresponds to the vinyl group. It has been noted that from 1.5

to 2% MAH concentrations, the consumption of vinyl groups

go through a minimum which indicates that the grafting

reaction now takes place on the main chain double bond and

not on the vinyl branches. In contrast, at 1% MAH

concentration the highest consumption of vinyl groups is

shown due to the grafting reaction. For higher MAH

concentrations, the depletion of the vinyl groups increases

once, which implies that the reaction is no longer occurring

on the main chain double bond, since, the percentage of

grafting on NBR as a function of MAH concentration is a

steady increasing function over the whole range of MAH

concentration.

Figure 1 shows the variation of tensile strength and impact

resistance of NR-NBRg blend with 30 phr NBR content, as a

function of MAH concentration at 3% BPO/MAH and

50 rpm. It can be noted that the largest tensile strength was

observed between 0.5–1 phr of MAH, and then decreases

again with an increase in the concentration of MAH up to

2.5 phr. This is may be due to the grafting of MAH takes place

on the vinyl branches. On the other hand, the largest impact

resistance was observed at 2 phr of MAH due to the grafting

takes place on the main chain double bond.

Table 2. Percent relative absorbance (consumption of vinyl

groups) as function of MAH content. Effect of MAH-NBR

grafting reaction on the vinyl group concentrations from peak

at 968 cm�1 in IR spectra

Percentage relativeabsorbance (%)

Maleic anhydride(phr)

Percentage graftingof MAH (%)

3.8 0 0.112.8 0.5 0.214 1 0.35 1.5 0.385 2 0.6

12.6 2.5 0.716 3 0.7916 3.5 0.95

Table 1. Reaction of MAH with NBR

%BPO/MAHPercentage of reached

MAH (%)Percentagegrafting (%)

0 42.41 0.7753 50.68 0.9057 40.55 0.746

10 45.17 0.818

MAH concentration is 2 g/100 g NBR (2 phr).

Copyright # 2004 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2004; 15: 209–213

210 A. Mounir, N. A. Darwish and A. Shehata

Figure 2 shows the variation of Mooney-viscosity of NBRg

and NR-NBRg blends with 30 phr NBRg content as function of

the MAH concentration at 3% BPO/MAH ratio and extrusion

speed of 50 rpm. It was observed that the amount of MAH

affects the viscosity of NBRg and that of the blend

considerably. When the concentration of 2 phr of MAH is

exceeded, the viscosity of NR-NBRg blend decreases

strongly. It indicates that the reaction on the vinyl groups

affects strongly the viscosity of the resulting blend. Appa-

rently an excess of grafted vinyl groups affects the viscosity of

both NBRg and NR-NBRg blend, reducing the magnitude of

the mechanical and impact properties. However, as observed

in Fig. 2 the largest viscosity of the blend shows up at 2 phr of

MAH, when the impact properties achieve the optimum

Figure 1. Variation of tensile strength and impact resistance of NR-NBRg, blend as a function of

MAH concentration.

Figure 2. Variation of Mooney-viscosity of NBRg and NR-NBRg blends as a function of MAH

concentration.

NR-NBR blend 211


value as shown in Fig. 1. The biggest grafting percentage does

not provide the best mechanical properties of the blend due to

an excessive amount of functional group in the blend. This

may affect on the compatibility of the blend and consequently

its mechanical properties.

Figure 3 shows the variation of Mooney-viscosity of the

NBRg and NR-NBRg blends with 30 phr NBR, as function of

BPO concentration at 2 phr of MAH. As shown, at a fixed

amount of MAH, BPO concentration is the main factor that

influences the grafting reaction on the vinyl groups. It can be

seen from Fig. 3 that the viscosity of the blend goes through

maximum at 3% of BPO compared to NBRg, which implies

the presence of a large concentration of non-reacted vinyl

groups, and therefore the reaction takes place on the main

chain. From the results obtained of varying concentrations of

BPO, the peroxide behaves more as an inhibitor than a

promoter of the grafting reaction, and therefore there is a

specific BPO concentration at which maximum grafting is

obtained. In these conditions, the small amount of grafted

vinyl groups in NBR dominate the grafting on the main chain

which induces the highest degree of compatibility with NR.

In this case the viscosity is also the highest. It can be also noted

that, the grafted NBRg-NR blend in the presence of 3% BPO

and processed with speed of 50 rpm render a blend with high

viscosity. This property influences positively the mechanical

behavior and the impact resistance of the blend with NR,

10.4 J/m with a rubber content 30 phr, which gives out-

standing increase in the impact strength as compared with

that of NR alone, 5.2 J/m. It can be concluded that the grafting

of MAH on NBR affects the viscosity of NBRg and that of

NBRg-NR. The blend with the largest viscosity obtained the

best impact strength. The viscosity is obtained as a function of

BPO and MAH concentration indicating that with 3% BPO

and 1.5–2 phr MAH, the optimum conditions for the grafting

reaction with NBR are obtained at 50 rpm. The achieved

compatibility in NR-NBR increase the impact resistance from

5.2 to 10.4 J/m. From an economical point of view the

compatibility between NR and NBR could be enhanced by

using cheap and available chemicals such as MAH and BPO,

and an easy method like grafting.

Effects of molecular weight of LNRTable 3 indicates the relationship between UV irradiation

time and molecular weight of irradiated NR. It can be con-

cluded that the molecular weight of NR decreases as the irra-

diation time increases until a constant level is reached at

higher irradiation time.

Figure 4 shows the effects of molecular weights of LNR on

the tensile strength and impact resistance of NR-NBR (70:30)

blends. LNR content used in this blend was 20%. It is

observed that the LNR imparts the lowest tensile strength

and impact at lowest molecular weight that was prepared for

40 hr. At this molecular weight, the LNR added to the blend

acts as a plasticizer, not a compatibilizer. Upon increasing the

molecular weight, LNR begins to act as compatibilizer for the

blends until they reach an optimum molecular weight at

Table 3. Effect of irradiation time on the number-average

molecular weight (Mn) of NR

Irradiation time (hr) Mn� 104

0 30.114 26.108 18.70

10 15.4314 9.0218 6.3240 6.03

Figure 3. Variation of Mooney-viscosity of NBRg and NR-NBRg blends as a function of BPO

concentration at 2 phr of MAH.

212 A. Mounir, N. A. Darwish and A. Shehata


Mn¼ 18.70� 104. The LNR with this molecular weight was

produced by UV irradiation for 8 hr. The blend shows highest

tensile and impact properties, respectively. Upon further

increasing the molecular weight, it is observed that the LNR is

no longer effective, as a compatibilizer. This is due to the

interaction being induced by LNR, is dependent on its

molecular size, as represented by molecular weight. Smaller

molecular sizes LNRs have a larger interphasing area,

resulting in better physical adhesion and wetting properties

for the blend. Furthermore, the molecules are more mobile;

therefore they interact more easily with matrix molecules,

and enhance the mechanical properties of this blend. It can be

concluded that the compatibility enhanced by using econo-

mical techniques like UV and available material like NR. In

comparison the UV method (second method) depends on the

molecular size change in NR that behaves as a good

compatibilizer at high molecular weight and low UV

irradiation. In contrast the first method (grafting method),

the compatibility between NR and NBR improved due to the

reaction between MAH and NBR, in the presence of BPO.

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1980; C18: 109.5. Nah C. Polym. Int. 2002; 51: 245.6. Li H, Li Z. Polym. Int. 1999; 48: 124.7. Sheng J, Liu XL, Yao K. J. Macromol. Sci. Chem. 1990; A27: 2.8. Minoura Y, Ueda M, Mizunuma S, Oba M. J. Appl. Polym.

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Figure 4. Effects of molecular weights of LNR on the tensile strength and impact resistance of

NR-NBR blend.

NR-NBR blend 213


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Effect of maleic anhydride and liquid natural rubber as compatibilizers on the mechanical properties and impact resistance of the NR-NBR blend