Effects of titanate coupling agent on rheological behaviour, dispersion characteristics and...

E�ects of titanate coupling agent on rheological behaviour,dispersion characteristics and mechanical properties of talc

®lled polypropylene

Chuah Ai Waha,*, Leong Yub Choongb, Gan Seng Neonb

aPetronas Research and Scienti®c Services Sdn. Bhd., O� Jalan Ayer Itam, Kawasan Institusi Bangi, 43000 Kajang, Selangor,

MalaysiabDepartment of Chemical Engineering, University of Malaya, 50603, Kuala Lumpur, Selangor, Malaysia

Received 3 July 1998; received in revised form 9 February 1999; accepted 29 March 1999

Abstract

A commercially available titanate coupling agent, LICA 12 from Kenrich Petrochemicals was used to study its

e�ect on talc ®lled polypropylene. It was found that the melt viscosity of the ®lled PP decreased and dispersionquality was improved in the presence of coupling agent. Further investigation on molecular weight distribution(MWD) shows that polypropylene (PP) molecules have undergone some chain scission and coupling reactions

resulting in a broader MWD. Glass transition temperature (Tg) measurement revealed the plasticising e�ect causedby coupling agent, which has imparted changes in morphological character and mechanical properties. Tensile and¯exural properties were observed to decrease whilst impact and elongation properties were improved with the

incorporation of coupling agent in the ®lled system. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction

Polypropylene (PP) ®lled with particulate ®llers are

of increasing interest in both industry and research.

Considerable experimental data can be found in recent

literatures dealing with mechanical properties of calcite

®lled polypropylene [1], compression moulded iPP with

talc [2], and rice husk ash as ®ller in polypropylene [3].

It is generally recognised that the ®ller size, inter-

facial adhesion and surface characteristics of the

composites can greatly a�ect the performance of

®lled system [4]. Among these important factors are:

the bond strength between the di�erent phases, sur-

face properties of particulate, the amount of agglom-

erated particle and the concentration of particles in

polymer system [4]. In a highly ®lled polymer sys-

tem, a major problem is nonuniformity of properties

due to poor dispersion of the ®ller in the matrix.

Therefore, the application of some coupling agents

in particulate ®lled polymer have generally been di-

rected to overcome the dispersion problem and to

enhance the mechanical strength of composites by

improving adhesion across the interface and thus

upgrade the performance of composites [5].

Conventional type of titanate coupling agents have

been found to be very e�ective in modifying rheologi-

cal behaviour by providing lubrication action to the

composites and at the same time, maintain or improve

the mechanical properties [6]. However, the new gener-

ation of titanate coupling agents were reported to be

more user friendly, easier to be incorporated with

higher aspect ratio ®llers and could eliminate ®ller pre-

European Polymer Journal 36 (2000) 789±801

0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057(99 )00123-8

* Corresponding author. Fax: +603-825-9702.

E-mail address: [email protected] (C. Ai Wah).

treatment step during processing, thus are more coste�ective [6].

In this paper we report our ®ndings on the in¯uenceof a titanate coupling agent on rheological behaviourand mechanical properties of talc ®lled polypropylene.

The results are discussed in relation to the change inmolecular weight distribution, particulate ®ller dis-persion and morphology of PP phase.

2. Experimental

2.1. Materials

Homoisotactic polypropylene pellets gradePropelinas 600G, produced by Polypropylene MalaysiaSdn. Bhd., was used in this study. The melt ¯ow index

and density of this PP were 10±11 g/10 min and 0.9 g/cm3, respectively. Two commercial talcs of similarmineralogical composition from Luzenac, Austria,grade A-10 and A-30 were employed. 30% Luzenac

Talc A-10 and 40% of Luzenac Talc A-30 were usedin this study. The properties of Luzenac talcs areshown in Table 1 and Fig. 1(a) and (b). In this case,

the talcs were not chemically pre-treated with surfacemodi®er but a commercial grade of titanate LICA 12Fig. 1(c) was used as a coupling agent in this com-

pounding. Polypropylene was treated with couplingagent by a rotating tumble mixer, prior to compound-ing.

2.2. Blending conditions

Blending was carried out on a Brabender twin screwcompounder (model DSK 42/7) equipped with a vibra-tory feeder and a screw drive dosing hopper. Talc was

introduced to the system by a vibratory feeder locatedat feeding zone and titanate coated polypropylene pel-

lets were dosed by a screw drive hopper. The barreltemperatures of three zones were 190, 200 and 2008Cfrom feeding to die zone. Compounds were blended at

a screw speed of 80 rpm and extruded through twin, 4mm round dies. The extrudates were air cooled atroom temperature, pulled and pelletised.

2.3. Injection moulding

Test specimens were injection moulded in a multi-

cavity mould of tensile and rectangular bars using a 20ton Battenfeld BA 200 CD Plus machine with aUNILOG 4000 control system (closed-loop control).

Table 2 lists the injection moulding conditionsadopted.

2.4. Melt Flow Index (MFI)

The rheological behaviour of the ®lled PP wasmeasured by MFI in accordance with ASTM D1238-

90b on a Tinius Olsen MFI, model MP957. A loadwith 2.16 kg at 2308C was employed in the measure-ment.

2.5. Gel Permeation Chromatography (GPC)

The molecular weight analysis of talc ®lled polypro-

pylene was carried out using a WATERS GPC model150-C. The samples were dissolved in 1,2,4-trichloro-benzene at 1458C in the presence of antioxidant to pre-

vent any further degradation. Talc particles wereseparated by ®ltering through a 0.45 micron ®lter at1458C. The analysis was carried out at 1458C.

2.6. Transmitted-light microscopy

Filled polypropylene pellets collected from the extru-

der were consolidated by heating at 1808C and press-ing them into uniform thickness ®lm of 15 micron in acompression press. The applied pressure used was 2ton, so as not to in¯uence ®ller dispersion in polypro-

pylene matrix. The specimens were then examined bytransmitted-light microscope model Leitz Laborlux 12Pol S, Leica to study ®ller particle dispersion in the

matrix.

2.7. Scanning electron microscopy

Scanning electron microscopic (SEM) studies offractured tensile samples were carried out on a PhilipsSEM 515 Scanning Electron Microscope. The acceler-

ated voltage used was 25 kV. Samples were sputtercoated with gold, using Polaron SC 510 Series SputterCoaters to increase surface conductivity.

Table 1

Characteristics of Luzenac talc [7]

Properties Luzenac talc

A-10 A-30

Chemical analysis (%)

SiO2 60.72 60.08

MgO 31.29 31.2

Al2O3 0.78 0.71

Fe2O3 0.71 0.72

CaO 1.15 0.77

MnO 0.01 0.01

L.O.I 5.5 5.5

Surface analysis

Surface area (m2/g) 9.3 4.4

Average pore radius (AÊ ) 23.5 44.8

Total pore volume (cm3/g) 70.85 636.35

C. Ai Wah et al. / European Polymer Journal 36 (2000) 789±801790

2.8. Tensile testing

Tensile properties were measured using an InstronUniversal Testing Machine, Model 5565 with a 5 kN

load cell. A video extensometer having view length of100 mm was attached to the machine for the measure-ment of elongation. Tests were conducted in accord-

ance with ASTM D 638-94b using Type I testspecimen dimensions. A crosshead speed of 5 mm/min

was employed and the average value of ®ve specimens

were taken for each sample.

2.9. Flexural testing

The ¯exural tests were carried out on InstronUniversal Testing Machine, Model 5565. A 3 mm

cylindrical surfaces of a three-point loading systemwere used and the support span length was adjusted to

Fig. 1. (a) Particle size distribution of Luzenac talc A-10 and A-30 [7]. (b) Micrograph of Luzenac talc A-10 (magni®cation

2000�). (c) Titanate LICA 12, in liquid form.

C. Ai Wah et al. / European Polymer Journal 36 (2000) 789±801 791

50 mm. Tests were conducted at crosshead speed of 2

mm/min at 238C.

2.10. Unnotched Izod impact testing

Unnotched Izod impact tests were performed usinga 2.75 J pendulum on a CEAST Impact Tester based

on ASTM D256-73, Method E. The injection mouldedsamples were notched at 458C, using a Notchvis HandDrive Cutter. A total of six specimens were tested foreach sample at 238C to obtain the average impact

value.

2.11. Glass transition temperature measurement (Tg)

Dynamic Mechanical Analysis (DMA) was per-formed using a three-point bending system. Samples

were cut in rectangular form and mounted in thePerkin±Elmer DMA 7e instrument. Temperature ScanMode was applied with a setting frequency of 1 Hz.

The sample was scanned from ÿ100 to 1508C at 58C/min to determine the Tg.

3. Results and discussion

3.1. E�ect on rheological properties

The e�ect of titanate coupling agent on rheologicalbehaviour was studied using a melt ¯ow indexer

(MFI). Normally, the addition of particulate ®llers topolypropylene restricts molecular motion in the matrix,thus imposed resistance to ¯ow. As a result, it gives

lower MFI value as ®ller content increases. However,by adding titanate coupling agent to the ®lled system,the MFI values were observed to increase with respectto its concentration for both 30% and 40% talc ®lled

polypropylene, as illustrated in Fig. 2.Generally, an increase in MFI value indicates better

molecular motion between polymer chains. The poss-

ible reasons for this phenomenon could be molecularchain scission or change in molecular weight distri-bution (MWD), or it might be a lubricating/plasticising

action induced by coupling agent [6]. Gel permeationchromatography (GPC) was used to determine theweight average molecular weight � �Mw� and number

average molecular weight � �Mn� of 30% talc ®lled PP inthe presence of various concentration of couplingagent.Table 3 summarises the molecular weight measure-

ments on the in¯uence of coupling agent and Fig. 3demonstrates their molecular weight distributioncurves.

Table 3 reveals that in the presence of titanate coup-ling agent, the �Mn of ®lled PP is lower than theuncoupled one. As the concentration of coupling agent

increases, the �Mn values decreases. On the other hand,the �Mw of the titanate coupled ®lled PP is noted toincrease with respect to the concentration of couplingagent. The increase in �Mw and the decrease in �Mn has

resulted in a broader molecular weight distribution(MWD) in the system. This seems to suggest that tita-nate coupling agent has played a role in increasing the

number of shorter polymer chains as well as longerones. Fig. 3 depicts that M peak of the MWD curve oftitanate coupled PP was shifted slightly to the right as

compared to the uncoupled one, but this appearance isnot signi®cantly noted in log scale axis. The improve-ment in ¯ow of melt blend is more likely to be due to

Fig. 2. E�ect of titanate coupling agent on MFI of talc ®lled

PP.

Table 3

Summary of molecular weight measurement of talc ®lled PP

in the presence of titanate coupling agent

Coupling agent (wt% ®ller) �Mw � 105 �Mn � 105 �Mw= �Mn

0 2.7 0.39 6.92

0.5 2.85 0.36 7.92

1 3.22 0.33 9.75

Table 2

Injection moulding conditions

Barrel temperature pro®le (feed-nozzle, 8C) 180, 200, 220, 230

Injection pressure (bar) 120

Follow-up pressure 1 (bar) 50

Follow-up pressure 1, time (s) 6

Follow-up pressure 2 (bar) 45

Follow-up pressure 2, time (s) 6

Injection speed (%) 50

Metering stroke (mm) 75

Screw speed (rpm) 121

Mold temperature (8C) 35

Cooling time (s) 25


the plasticising e�ect of the coupling agent which hasdi�used into the polymer matrix to serve as an internalplasticiser [6,8,9].

3.2. E�ect on particulate ®ller dispersion

Figs. 4±8 show the photomicrographs of 30% talc®lled polypropylene specimens containing di�erentlevels of titanate coupling agent. The ®ller employed in

this study was Luzenac Talc A-10. Fig. 4 shows poor®ller particle dispersion in the polypropylene matrix,

where no coupling agent was added to the system.Filler particles were agglomerated as large black spotsunder microscopic examination. Slight improvement of

®ller particle dispersion was observed in Fig. 5, where0.25% of titanate coupling agent was added duringmelt blending. Nevertheless, the optimum ®ller dis-

persion was not achieved since ®ller particles aggrega-tion can still be observed in the micrograph. Additionof 0.5% of titanate as illustrated in Fig. 6 shows sig-

ni®cant improvement of ®ller particle dispersion.Particle dispersion was further improved by the incor-

Fig. 3. Molecular weight distribution curves.

Fig. 4. Photomicrographs of 30% talc/PP without titanate coupling agent.


poration of 0.2% calcium stearate in the ®lled polypro-

pylene matrix, where uniform dispersion was achievedas in Fig. 7. On the other hand, the compound con-taining 1.0% of titanate coupling agent without the ad-

dition of calcium stearate also provides a welldispersed mixture as indicated in Fig. 8.Figs. 9 and 10 are photomicrographs of 40% talc

(Luzenac A-30) ®lled polypropylene. Optimum ®llerparticle dispersion achieved at 0.5% titanate couplingagent and 0.1% calcium stearate as shown in Fig. 10.As can be observed from Table 1, both Luzenac talc

A-10 and A-30 have the characteristic of broad particle

size distribution that could contribute to ease the par-

ticle dispersion in polymer matrix. Generally, Talc A-30, which has larger particle size and smaller speci®csurface area, promotes a better ®ller dispersion in poly-

mer matrix as compared to Talc A-10. Therefore, TalcA-30 required less dispersing aids such as calcium stea-rate to assist ®ller particle dispersion.

3.3. Mechanism of ®ller dispersion

The mechanism of titanate reaction in inorganic ®l-

ler surface is represented in Fig. 11. Titanate coupling

Fig. 5. Photomicrographs of 30% talc/PP/0.25% titanate.



agent reacts with hydroxyl groups at the inorganic ®l-ler surface, resulting in the formation of monomolecu-

lar layer on the inorganic surface to increasecompatibility of ®ller/matrix interface [6].The inorganic ®ller (Luzenac Talc A-10) as shown in

Fig. 1(b) is irregular in shape and agglomerated due to

interparticle force, which is dependent on ®ller surfacearea. The agglomeration of ®llers causes the formationof air voids as illustrated in Fig. 12(a).

In the presence of titanate coupling agent, the dis-persion of ®ller particles in polymer phase is enhanced

by the replacement of water of hydration at the inor-ganic surface of the ®ller, with organofunctional tita-

nate causing inorganic/polymer interface compatible,[6,10] thereby eliminating air voids in the system.Consequently, it results in deagglomeration and moreuniform dispersion in melt blending as showed in Fig.

12(b).The combination of calcium stearate in the ®lled sys-

tem promotes better ®ller particle dispersion. This

additive functions as an internal lubricant to wet theparticles surface by displacing air locked in the par-

Fig. 7. Photomicrographs of 30% talc/PP/0.5% titanate/0.2% calcium stearate.



ticle-melt interface and permitting mechanical energy

(in the form of shear stress developed in blending pro-cess) to be applied to overcome the interparticle forces.

3.4. E�ect on tensile, ¯exural and elongation properties

Application of titanate coupling on talc ®lled poly-propylene has resulted in some changes of mechanical

properties. Figs. 13±15. exhibit the tensile yield stress,

elongation at break and ¯exural strength of 30% and

40% talc ®lled PP.Incorporation of titanate coupling agent resulted in

decrease in yield stress and ¯exural strength. Theelongation at break was observed to increase with

increased concentration of the coupling agent in 30%talc ®lled PP. In the case of 40% talc ®lled PP, therewas no appreciable change in the elongation at break,

with the addition of coupling agent. Apparently,


Fig. 10. Photomicrographs of 40% talc/PP/0.5% titanate, 0.1% calcium stearate.


higher ®ller content would tend to reduce the plasticis-

ing e�ect of the coupling agent.The observed modi®cation in mechanical and ¯ow

properties by the titanate coupling agent is consistent

with the role of the titanate as a plasticiser in the ®lledsystem [6,11]. This is further con®rmed by glass tran-sition temperature (Tg) measurement of titanate

coupled-®lled PP in comparison with the uncoupledone, as shown in Figs. 16 and 17. The coupled ®lledPP shows a Tg of ÿ2.88C as compared to 1.18C for theuncoupled-®lled PP. This observation matches the role

of plasticiser, that lowers the Tg of polymer and conse-quently promotes a change in properties from hardand brittle to those of soft, ¯exible and tough [12].

It has been postulated that titanate coupling agent

improves the compatibility of inorganic ®ller withpolymer, by enhancing their interfacial adhesion [1]. Inorder to investigate this e�ect and to relate the mech-

anical properties to morphology, the tensile fracturesurfaces of the composites were examined by SEM asillustrated in Fig. 18(a) and (b). Fig. 18(a) displays the

SEM micrographs of the 30% talc ®lled PP while Fig.18(b) shows the morphology of 30% talc ®lled PP with0.5% titanate coupling agent.These micrographs depict that uncoupled ®lled PP

exhibits brittle failure at ®ller/matrix interface. Brittlefailure mode would be contributed by the formation ofvoids due to incompatibility of ®ller/PP and poor dis-

Fig. 11. Mechanism of titanate reaction in inorganic ®ller surface [6].

Fig. 12. Mechanism of ®ller dispersion in PP matrix [6].


persive e�ect of ®ller in the matrix. With the incorpor-ation of 0.5% titanate coupling agent, PP phase exhi-bits long ®brils morphology and ®llers are embedded

in the matrix on the fracture surface. A ductile failureis then observed, which implies an improvedelongation property. It is therefore apparent that tita-

nate coupling agent has an e�ect to modify the surfacecharacteristics of ®ller and improves the elongation atbreak. On the other hand, the plasticising e�ect of tita-

nate might have reduced the intermolecular forces inthe matrix, and consequently increased ¯exibility ofthe composite [13].

3.5. E�ect on impact strength

Fig. 19 shows an improvement of impact strength asthe concentration of coupling agent increases. The gen-

eral trend towards an increase in impact strength withthe incorporation of titanate is in agreement with thereport made by Salvatore J. Monte [6].As can be deduced from our earlier ®ndings, titanate

coupling agent did play a role as a plasticiser. In thiscase, the plasticising e�ect of titanate is operating poss-ibly in conjunction with interfacial adhesion to yield

an increase in toughness. Toughness, however, is themajor factor that controls the impact strength. Duringimpact test, a crack travels through the polymer and

interfacial region. Generally, high ®ller/matrix inter-facial adhesion provides e�ective resistance to crackpropagation during impact test [3].

4. Conclusions

Addition of titanate coupling agent to ®lled PP hasnotably increased the MFI, impact strength andelongation property, and improved ®ller dispersion ofcomposites. Flexural strength and yield stress were

seen to be decreased, for both 30 and 40% composites.GPC measurement showed that the titanate has pro-

Fig. 13. E�ect of coupling agent on yield stress.Fig. 14. E�ect of coupling agent on elongation at break.

Fig. 15. E�ect of coupling agent on ¯exural strength.


Fig. 16. DMA-Tg analysis of 30% talc/PP.


Fig. 17. DMA-Tg analysis of 30% talc/PP/1.5% titanate.


duced a slightly broader MWD of ®lled system. Themorphology and Tg analysis have demonstrated the

role of titanate as a plasticiser in ®lled PP system.0.5% of titanate and 0.2% calcium stearate were

able to produce good dispersion of ®ller in the system,

while maintaining reasonable properties of tensile and¯exural strength. A combination of 0.5% titanate and0.1% calcium stearate is su�cient to provide uniform

®ller dispersion in 40% composite, where Luzenac TalcA-30 is employed in the system.

Acknowledgements

The authors sincerely appreciate the support of

Petronas Research and Scienti®c Services Sdn. Bhd.and thanks to Naintsch Mineralwerke for the supplyof Luzenac Talc.

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Fig. 18. SEM micrographs of tensile fracture surface (magni®-

cation 1000�): (a) PP/talc, (b) PP/talc/0.5% titanate.

Fig. 19. E�ect of coupling agent on impact property.


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