Experiment 4: Measurement of Viscosity of NR Latex

4.1 Objective

4.1.1 To determine the viscosity of the NR latex according to MS 281: Part 15:1992.

4.1.2 To measure the effect of spindle speed and spindle number on viscosity of

natural rubber latex.

4.2 Introduction

Havae Latex which is comprised of rubber particles and a dispersed phase in aqueous

serum known as “viscoids”, exhibits significant viscosity change when treated with

ammonia, sodium sulphite, formaldehyde or distilled water. The NR latex is a highly

specified material and is characterized by properties that are significant to the user.

Viscosity of latex significantly influences processing conditions and final rubber product

properties. Therefore, understanding factors affecting viscosity of latex is beneficial.

(Perez, 1993)

Viscosity is the measure of flow or the internal friction for a liquid system. This friction

becomes apparent when a layer of fluid is made to move in relative to another layer. The

greater the friction, the greater the amount of force require to cause this movement

which is called shear. Shearing occur whenever the fluid is physically moved or

distributed. The higher the viscous fluids, therefore, require more force to move than less

viscous materials. The viscosity of latex can be very complex as the latex is hetero-

phase system. (Athey, 1991)

The Brookfield viscometer is probably one of the most commonly used rotational

viscometers. It consists of a series of spindles which are immersed into a large vat of

fluid and enables scale deflection to be measured for a given rotational speed of the

spindle. A viscosity value can then be found from a table provided by the manufacturers;

however, a corresponding shear rate is not given. This viscosity value is the viscosity of

the Newtonian fluids. This means that Brookfield R.V.T is essentially a quality control

instrument whereby data is given in terms of scale deflection, spindle number and speed

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of rotation rather than a viscometric shear stress-shear rate relationship. (Williams,

1984)

For many industrial applications, it is important to obtain accurate results with the least

expensive and most versatile instrument. Of those mentioned, the Brookfield Synchro-

Lectric Viscometer introduced in 1981 is probably the best available compromise

between accuracy and price. The variety of attachments available allows coverage of a

broad range of shear rate and viscosity. In the older style Brookfield instruments, many

of which are available in laboratories and plants today, a dial deflection (0 to 100) by an

appropriate factor which are obtained from a “Factor Finder” supplied with the instrument

and should only be used for Newtonian fluids since the “dial viscosity” and the apparent

viscosity are identical only in this case. (David& Meyer, 2000)

Latex is a non-Newtonian fluid. The viscosity measured is not an absolute value. Most

latex exhibits decreasing viscosity with increasing shear rate and for this reason any

reference to latex viscosity should include a description of the method of measurement,

the rate of shear (if known) and the test temperature. The viscosity of particular latex is a

function of its origin, including the clone from which it was derived, and of the

composition of the latex serum, its total solids concentration, and the means of

preservation. (Perez, 1993)

Besides that, the reading of the latex viscosity must be taken by clamping both pointer

and scale of the viscometer where the rotation of spindle is stopped. Thus, it is not

suitable to measure the viscosity associated with non-Newtonian flow behavior

continuously. The viscometer also can be affected by turbulence and angle of immersion

of the spindle. (Blackley, 1997)

4.3 Materials

Old natural rubber (NR) latex, Acrylonitrile rubber (NBR) latex and Distilled water

4.4 Equipment

Brookfield Synchro-Lectric viscometer RV 200 Type R, Beaker, Glass rod, Spindle

number 2 and 3 and Filter.

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4.5 Procedure

1. The spindle of number 2 was attached to the motor shaft securely, while the guard

was attached securely to the motor housing of the viscometer.

2. The spindle and guard were inserted carefully into the NR latex sample until the

surface of the sample was at the mid-point of the groove on the spindle shaft. The

procedure was carried out in such a way as to avoid air being trapped,

3. The spindle was placed vertically in the sample and in the centre of the beaker.

4. The rotational speed of the instrument was selected at 5, 10, 20, 50 and 100 rpm

respectively.

5. The viscometer motor was switched on and the equilibrium reading was taken to the

nearest unit scale division.

6. The experiment was repeated with NR latex with spindle of number 3.

7. The procedure from 1 to 6 is repeated by using NBR latex.

8. All the readings were obtained and tabulated.

4.6 Result& Discussion

In the study of flow behavior, it is more common to describe the variation of viscosity as

a function of shear rate (spindle speed). The Brookfield device can be used with the

expectation of about 10% precision. Besides, spindle number is another parameter that

will give significance to the experimental result.

0 20 40 60 80 100 1200

200400600800

100012001400

Spindle number 2Spindle number 3

Spindle speed (rpm)

Visc

osity

(mPa

.s)

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Figure 1: Comparison of viscosity of NR latex using spindle number 2 and 3 at different

spindle speed

Figure 1 shows that both NR latices with different spindle number exhibits similar trend

of variation that is decreasing viscosity with the increasing of the spindle speed. Besides,

it can be observed that the viscosity of the NR latex tested by spindle number 3 is higher

than the spindle number 2. Hence, the decreasing viscosity of NR latex with shear rate

indicates that the NR latex exhibit shear thinning effect (pseudo-plastic) as most

polymers solution and melted polymers will do.

The viscosity of the both NR lattices shows the highest value at the slowest spindle

speed (5.0 rpm) used. At the early stage without apparent shearing force, the latex

particle is extensively entangled without the alteration on the molecular chain of the

sample. This can be explained by the shearing force produced at the slowest speed is

finite and insufficient to cause any chain orientation on the latex particles. As the speed

of spindle increase, more shearing force is generated and hence enough to do some

alteration on the molecular chain.

When the spindle shaft is rotated, the latex particles at the molecular level are interfered

by the dragging force applied by the shaft. The shear rate applied on the latex particles

lead to molecular orientation where the chain entanglement of the latex is reduced

dramatically. Subsequently, the slippage between the latex particles are readily to occur

which can increase the average distance between the latex particles. The free volume

between the adjacent particles is now increases causing the latex particles are now

more dispersed than before. At low shear rates, Brownian motion of the rubber latexes

made them to rotate and they interfere strongly with one another so the viscosity is high.

As the shear rate increased, the rubber latices were became deformable and aligned

with the direction of flow, so that they interfere less with one other and the viscosity

decreased.

Figure 2 shows the variation of viscosity as a function of shear rate in NBR latex. When

discussing the viscosity of synthetically made latex, it is different from the natural clone

Havae latex due to difference in molecular structure, molecular weight, molecular weight

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distribution and chain polarity. All these parameters have on effects on the viscosity of

NBR latex.

0 20 40 60 80 100 1200

20

40

60

80

100

120

Spindle number2Spindle number 3

Spindle speed (rpm)

Visc

osity

(mPa

.s)

Figure 2: Comparison of viscosity of NBR latex using spindle number 2 and 3 at different

spindle speed

NBR latex shows the abnormal trend of viscosity change as a function of shear rate.

Although a clear trend for both NBR lattices cannot be made, from Figure 2, it is obvious

to observe that the viscosity of the NBR latex tend to increase at fastest spindle speed.

This can be explained by the NBR latex being shorter molecular weight polymer are

more prone to heat sensitization effect compared to NR latex with higher molecular

weight. This is especially the case where spindle number 2 with larger diameter will

generate more shear. The shearing force that generated on the NBR latex system will

cause the viscosity to decrease until a point where accumulated kinetic energy in the

latex system is sufficient to overcome the repulsive force between latex particles in the

emulsion system. More kinetic energy is generated in the latex system and latex

destabilization is increased. When the latex is destabilized, latex flocculation occurs

where micro-coagulation of latex results in small aggregates of rubber. The occurrence

of the latex aggregates causing the dragging force applied by the shaft and the torque

developed increase. This leads to a significant increase of viscosity.

However, by making comparison, heat sensitization of the NR latex is lower due to

higher molecular weight and broader molecular weight distribution. NR latex particles in

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the latex system that are long and entangled are stable in the thermal condition and

higher kinetic energy is needed to supplied to increase the frequency and violence of the

latex particles collision in order to form small flocculate as NBR latex.

There are some external factors that attribute to the experiment result. The same latex

sample is used throughout the test conducted causing an accumulation of the shearing

force effect. The same latex sample is used to test the effect of every spindle speed

causing the effect from previous spindle speed test will be add up to the effect of next

spindle speed tested. Therefore, the latex should be regularly changed during the

experiment conducted in order to increase the accuracy of the result.

Besides, there are also another one crucial parameter that can be associated with this

experiment which is the spindle number used. Theoretically, the spindle number used

can influence the viscosity of the latex, where higher viscosity of NR latex can be

obtained by using a higher spindle number during the test. There is a range of spindle

number that has significantly different disc diameter. The diameter of the disc increases

with the lowering of the number of the spindle used; so as the surface area. Thus,

spindle number 2 which has the largest disc diameter has the highest contact surface

area with the NR latex. Such larger contact area can significantly interfere the movement

or the arrangement of the latex particles since higher shearing force is produced during

rotation.

As the shear rate increased, the latex particles become deformable and aligned with the

direction of flow, resulting in the decrease in viscosity. On the other hand, smaller

contact area imparted by the larger spindle number (spindle number 3) will contribute to

a lower shearing force on the latex particles and subsequently exhibit lower shear rate.

As a result, the latex particles cannot be readily orientated or aligned from the chain

entanglement. Furthermore, the longer chains of the latex particles are not broken

sufficiently into shorter chains to increase the flow behaviour of the latex. Thus, the

viscosity of the latex is higher by using the larger spindle number or smaller disc

diameter.

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4.7 Conclusion

In conclusion, the latex viscosity depends upon the speed of the spindle. For NR latex,

the shear thinning effect or pseudo-plastic behavior will be dominant that is the viscosity

of NR latex decrease with spindle speed (shear rate). At high spindle speed, the rubber

chains undergo chain scission more aggressively due to higher shear rate that exerted

on the NR latex. Hence the free volume is increase and the latex particle are more

dispersed, subsequently the viscosity of the NR latex decreases. For NBR latex, the

viscosity tends to decrease gradually and up to the point before latex destabilization due

to heat build-up in the latex system.

Besides, the viscosity of NR latex sample will also decrease with the decreasing spindle

number used. This due to a larger contact area with latex is associated with the

decreasing of spindle number used and more shearing force is able to be produced. As

a result, the movement of particles is significantly interfered and the chain scission of the

latex is more likely to occur. Thus, the higher viscosity of the latex sample can be

observed with large spindle number.

References

(1) Athey, R.D. (1911). Emulsion Polymer Technology. New York: Marcel

Dekker, Inc

(2) Blackley, D.C. (1997). Polymer Latices: Science and Technology Vol 1:

Fundamental Principles. New York: Springer.

(3) David, D.B& Mayer, R.R (1990), Rheology Modifiers Handbook: Practical Use

and Application (Materials and Processing Technology). New York: William Andrew

Publishing

(4) Jean Perez. (1993). Natural Latex: Control and Industrial Procedures. Clinical

Reviews in Allergy, Volume 11, 355-361.

(5) R. W, Williams. (1978). Determination of Viscometer Data from The

Brookfield R.V.T. Viscometer, Volume 18, 345-359.

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Table 1: Viscosity of NR Latex using different spindle speed

Spindle number

Speed Factor Dial Reading Average Viscosity

2 5 80 13 13 13 13 104010 40 18 18 18 18 72020 20 25 25 25 25 50050 8 39.5 39.5 39.5 39.5 316

100 4 52 55 54 54 214.673 5 200 6 6 6 6 1200

10 100 8 8 8 8 80020 50 11 11 11 11 55050 20 16.5 17 17 17 336.7

100 10 24 24 17 22 250

Table 2: Viscosity of NBR Latex using different spindle speed

Spindle number

Speed Factor Dial Reading Average Viscosity

2 5 80 0.5 0.5 0.5 0.5 4010 40 1 1 1 1 4020 20 2.5 2.5 2.5 2.5 5050 8 6.5 6.5 7.5 6.8 54.67100 4 17.5 16 16.5 16.7 66.67

3 5 200 0.5 0.5 0.5 0.5 10010 100 0.5 0.5 0.5 0.5 5020 50 1 1 1 1 5050 20 4 3 4 3.7 73.33100 10 5.5 6 5.5 5.7 56.67

Useful formulae:

Apparent viscosity= Factor× Dial Reading

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