Rheological Behaviour of Natural Rubber and Its Variant

DEPARTMENT OF CHEMICAL ENGINEERING

RHEOLOGICAL PROPERTIES OF NATURAL RUBBER AND ITS VARIANTS

P16

Group 1 CHEN CHING LIANG HEO ZHI KAI

KEK 070011 KEK 070020

ABSTRACT Natural rubber (NR) skim latex as the by-product from conventional NR centrifugation can be reprocessed for useful applications in the industry. Understanding the rheological behavior of this colloidal dispersions system is vital for the processing and final products qualities. In this paper, the rheological behavior of NR skim latex was investigated. Parameters influencing the rheological properties including effect of shear rate, temperature, concentration, and storage period were discussed in detail. Power Law analysis is used extensively to identify the behavior of skim latex at different conditions. The viscosity of skim latex decreases with increasing shear rate and temperature. This shear-thinning phenomenon indicated pseudoplastic behavior. The extent of pseudoplasticity was analyzed by obtaining pseudoplasticity index from Power law. Arrhenius Law was used to examine the extent of temperature effect on skim latex rheological behavior. Activation energy was identified to have profound influence on the skim latex‟s viscosity and the values were calculated. The increase of skim latex viscosity is relatively slow for skim latex with total solids content below 17%, more rapid between 17-20% and extremely sharp above the latter concentration. The viscosity of the skim latex is also found to decrease as the function of time of storage period.

Keywords: Natural rubber skim latex; rheology; shear rate; pseudoplasticity; storage period

I

ACKNOWLEDGEMENTS We would like to express our gratitude to Prof. Dr. Nik Meriam Sulaiman and Dr Mohamed Kheireddine Aroua, our supervisors for their suggestions, guidance and helpful discussions on the rubber processing process for this research project. Besides, we also thank to Sime Darby Sdn. Bhd for supplying the skim latex samples. We would also like to express our deeply appreciations for Mr. Khairul‟s help on the physical properties measurement. Special thanks go to all of our coursemates and friends who have assisted us throughout this research project. Without their helps, we would not manage to conduct our research smoothly. Again, our sincere gratitude to all involved parties who have contributed in making this research a success.

II

TABLE OF CONTENT 1.

INTRODUCTION ......................................................................................................................... 1 1.1.

Natural Rubber Latex ............................................................................................................. 1

1.2.

Rubber Processing .................................................................................................................. 2

1.3.

Importance of Rheological Behaviour ................................................................................... 3

2.

RESEARCH OBJECTVE ............................................................................................................. 4

3.

LITERATURE REVIEW .............................................................................................................. 5

4.

5.

3.1.

Dependence of Latex Flow on Shearing Rate ........................................................................ 5

3.2.

Dependence of Latex Flow on Concentration ........................................................................ 7

3.3.

Effect of Temperature ............................................................................................................ 9

3.4.

Effect of Surfactants ............................................................................................................. 10

3.5.

Effect of Ammonia Content and Storage Period .................................................................. 11

3.6.

Effect of Electrolytes ............................................................................................................ 12

METHODOLOGY ...................................................................................................................... 13 4.1.

Material Used ....................................................................................................................... 13

4.2.

The Operational and Configuration of the Viscometer ........................................................ 14

4.3.

Standard of Measurement..................................................................................................... 15

4.4.

Rheological Measurement .................................................................................................... 16

RESULTS AND DISCUSSION .................................................................................................. 17 5.1.

Influence of shear rate .......................................................................................................... 17

5.2.

Influence of concentration .................................................................................................... 19

5.3.

Influence of temperature ...................................................................................................... 21

5.4.

Influence of storage period ................................................................................................... 26

5.5.

Pseudoplasticity index .......................................................................................................... 28

6.

CONCLUSIONS ......................................................................................................................... 32

7.

RECOMMENDATION FOR FUTURE WORK ........................................................................ 33

8.

REFERENCES ............................................................................................................................ 34

III

LIST OF TABLES Table 1: General compositions of total solids content(TSC) in NR latex ............................................ 2 Table 2: Typical contents of natural rubber latex ................................................................................. 2 Table 3: Rheological models of NR latex behaviour with reference to the concentration ................... 8 Table 4: Characteristics of NR skim latex. ......................................................................................... 13 Table 5: Summary of the parameters to be investigated and their respective range of study............. 16 Table 6: Physical properties of natural rubber skim latex with different retention time during the membrane processing........................................................................................................... 20 Table 7: Activation energy for skim latex at different shear rate ....................................................... 25 Table 8: Pseudoplasticity index values of natural rubber skim latex with different total solids content at 30, 40 and 50oC. ............................................................................................................... 30

IV

LIST OF FIGURES Figure 1: Molecular structure of cis-1, 4-polyisoprene (monomer of NR latex) .................................. 1 Figure 2: The schematic diagram of the Haake Viscometer VT-550 (Left) and the NV sensor system (Right). ................................................................................................................................. 14 Figure 3: Effect of shear rate on the viscosity of NR skim latex at 30oC. .......................................... 18 Figure 4: Effect of shear rate on the viscosity of NR skim latex at 30oC (for NR skim latex of low concentration) ...................................................................................................................... 18 Figure 5: Formation of network structure by rubber particles in natural rubber skim latex. .............. 19 Figure 6: Effect of temperature on viscosity of skim latex at low shear rate (13.52 s-1) .................... 22 Figure 7: Effect of temperature on viscosity of skim latex at high shear rate (1082 s-1) .................... 22 Figure 8: log η against 1/T for skim latex (at low shear rate of 13.52 s-1).......................................... 23 Figure 9: log η against 1/T for skim latex (at high shear rate of 1052 s-1) ......................................... 23 Figure 8: Effect of storage period and ammonia content on the viscosity of skim latex .................... 27 Figure 9: Effect of storage period and ammonia content on the viscosity of skim latex (for NR skim latex of low concentration) .................................................................................................. 27 Figure 10: Pseudoplasticity index determination from Power Law for skim latex of 20.43% total solids content, measured at 30oC ......................................................................................... 29 Figure 11: Graph shows the pseudoplasticity index values of natural rubber skim latex with different total solids content at 30, 40 and 50oC ................................................................................. 31

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1. INTRODUCTION 1.1. Natural Rubber Latex The term „Latex‟ referred to stable colloidal dispersions of polymers in an aqueous medium. It can also be extended for dispersions of insoluble polymers in non-aqueous medium [1]. The two main classes of rubber latex system are synthetic latex and natural latex. The first is obtained from polymerization process while the latter is harvested from plants. Natural rubber (NR) latex is milky white fluid from rubber tree. Nowadays, most of the rubber trees are from species Hevea brasiliensis of the family Euphorbiaceae and the rheological behaviour of the NR has been studied extensively. Moreover, researchers also investigate the rheological behaviour of latex from other species including Parthenium argentatum and Ficus elastica [2]. NR latex contains both the dispersion of polyisoprene (monomer of rubber) and other nonrubber particles such as proteins, sugars, resins in aqueous serum. The ratio of these two main constituents vary depends on the NR latex source. Generally, the monomer of NR latex, cis-1,4polyisoprene (Figure 1), contributes to more than 90% of the latex solids content with the presence of other compositions such as acetone soluble, nitrogen and ash. Table 1 gives the general total solids content of NR latex [3]. Freshly tapped NR latex will have the following contents with their respective weight percent: water (55-60%), rubber (30-34%), proteins (2-3%), resins (1.5-3.5%), sugar (1.0-2.0%), as well as trace amount of ashes and sterol glycocides. The typical content of NR latex is summarized in Table 2.

Figure 1: Molecular structure of cis-1, 4-polyisoprene (monomer of NR latex)

1

Table 1: General compositions of total solids content(TSC) in NR latex [4]

Compositions

Percent by weight (%)

Cis-1,4-polyisoprene

>90

Acetone soluble

2.5-4.5

Nitrogen

0.3-0.5

Ash

0.2-0.4

Table 2: Typical contents of natural rubber latex

Compositions

Percent by weight (%)

Rubber

30-34

Proteins

2-3

Resins

1.5-3.5

Ashes

0.5-1.0

Sugar

1.0-2.0

Sterol glycosides

0.1-0.5

Water

55-60

1.2. Rubber Processing The harvested filed latex will undergo a series of processing until the final production. Addition of ammonia is the very first step in the processing line in order to preserve the NR latex. Ammoniated filed latex will further be concentrated to around 60% rubber content. Commonly used concentrating 2

methods include centrifugation, creaming and evaporation. Centrifugation is currently dominating the industrial application of NR latex concentrating in Malaysia, accounting to 95% of concentrated products [5]. The centrifuging process produces skim latex (4-6% rubber) as by-product. Processing of skim latex involves the addition of 98% sulfuric acid to recover the rubber content by coagulation. However, the acid will contaminate the serum portion which also contains many hazardous compounds [6]. Discharge of this effluent will cause serious problems to the environment. The membrane technology is able to separate the serum from skim latex, producing concentrated skim latex with approximately 20% of rubber content. This alternative can prevent the contamination of skim serum and offers an environmental friendly rubber processing [5].

1.3. Importance of Rheological Behaviour NR latex is very useful in many industrial applications such as the manufacturing of tires, gloves, balloons etc. Understanding the rheological behaviour of this colloidal dispersions system is vital for the processing of NR latex and final products qualities. The knowledge on the flow behaviour of latex lattices is crucial from both fundamental and applied perspective. At fundamental level, the rheology of the system reveals the various interaction forces between the particles [7]. Five main inter-particle forces identified are van der Waals forces, electrostatic forces, hydrophilic interaction, depletion or exclusion interaction and polymer-polymer interaction [8]. At an applied point of view, the rheological behaviour can serve as a guideline for the manufacturing of various NR latex applications. The knowledge is equally important during the transportation and handling of lattices. The rheological behavior of the NR has been studied extensively [2]. The flow behavior of NR latex is affected by a number of factors including shear rate, temperature, concentration, electrolytes, surface-active agents, blend composition, ammonia content and storage period of field latex [1, 7, 914]. However, limited studies on rheological properties of NR skim latex are available. In this research project, the rheology and structural analysis of NR skim latex is addressed with reference to the influence of shear rate, temperature, concentration ammonia content and storage period. Understanding the effect of these factors will lead to better control on the processing and hence enhance the final products qualities from recovered skim latex.

3

2. RESEARCH OBJECTVE This research is targeted to study the rheological behavior of natural rubber and its variants. Concentrated natural rubber skim latex will be the main focus for this research project. The main objectives of this research are: i.

To determine the rheological behavior of natural rubber skim latex

ii.

To investigate the change of the viscosity of concentrated skim latex on the influence of shear rate

iii.

To investigate the change of the viscosity of concentrated skim latex on the influence of the temperature

iv.

To investigate the effect of storage period on the viscosity of concentrated skim latex

v.

To investigate the effect of concentration (total solid content) on the viscosity of concentrated skim latex

vi.

To investigate the effect of ammonia content on the viscosity of concentrated skim latex

vii.

To analyzed the pseudoplastic behavior of concentrated skim latex using Power Law

viii.

To analyzed the temperature sensitivity of concentrated skim latex using Arrhenius Equation

4

3. LITERATURE REVIEW 3.1. Dependence of Latex Flow on Shearing Rate One of the important elements in the rheological behaviour study for the NR and synthetic rubber lattices is shearing rate. The NR lattices only show non-Newtonian behaviour at high concentration, with the flow known as “pseudoplastic” flow. The deviations from Newtonian behaviour are generally more pronounced in the synthetic lattices compared to the natural one [15]. The viscosities of Newtonian fluids do not change over wide ranges of shear rate. Subsequently, the linear relationship should be attained in the shear stress-shear rate plot for all the Newtonian fluids. There are wide ranges of studies on the influence of shear stress on shear rate for NR lattices, synthetic lattices and the natural-synthetic rubber blends. However, all the following reviews will be confined to the discussion of the relation between rate of shear and shearing stress observed with synthetic lattices and the natural-synthetic rubber blends. One of the earlier study on the flow behaviour of synthetic lattices by Krieger, Maron and Madow [16, 17] showed that the 50:50 butadiene-styrene copolymer latex exhibiting a Newtonian fluid behaviour at the total solid concentration lower than 25% and having a non-Newtonian fluid behaviour at higher concentration. This breaking line of conc. 25% in between the Newtonian and non-Newtonian flow had also been observed in the lattices of high butadiene content emulsified with both rosin and fatty acid soaps [18-20] in Neoprene latex [21] and in latex mixtures [22]. Figure 2 shows a typical shape of flow curves for synthetic latex above conc. 25% solids. It is found out that all of the curves are nearly linear at the concentration just above 25% and start to deviate significantly from linearity when concentration is increased. In the study, it is shown that the latex possesses pseudoplastic behaviour at high concentration. Moreover, the slope for each of the curves becomes constant at sufficiency high shear rate. Later, Maron and Belner [20] had also found that latex above a volume fraction in between volume fraction of 0.28 and 0.54 exhibiting Newtonian flow at very low shear stresses. Ranimol et al. [7] studied the flow properties for the blends of unvulcanised NR and carboxylated styrene butadiene rubber (XSBR). In his study, blend ratio of NR to XSBR had been set at 100, 70, 50, 30 and 0. The reduction in viscosity for all the tested samples was observed with the rise in shear rate, indicating a non-Newtonian behaviour. However, no more changes were 5

observed for the viscosities for all the tested samples at high shear rate. Rubber particles were spread evenly throughout the aqueous phase in the colloidal system. At high shear rate, these rubber particles were pulled apart and the structure of the colloidal system collapsed which result in sharp decrease in viscosity[14]. Varkey et al. [23] in his investigation of the flow characteristic of the NR and styrene butadiene rubber lattices, had evaluated the influence of shear rate on viscosity in detail. It is found that the blends with a low content of NR (lower than 50%) exhibiting Newtonian fluid behaviour. However, the blends with higher concentration of the NR exhibit pseudoplastic behaviour. Highest viscosity is attained by the blend with the content of 70% of NR. Varkey et al. claimed that the microflocculation is being taking place in between the mixed particles at this concentration. However, the viscosity of the blend decline with the raise in shear rate as the network structure formed in between the particles is collapsed at high shear rate. The detail study on microflocculation can be found in the works of Backley and Charnock [24]. They proposed that the exchange of stabilizers had been taking place in between the particles of NR and synthetic rubber and this will result in gain in viscosity for the blends with higher content of NR. Peethambaran et al. [12] examined the effect of the addition of surfactants like casein, polyvinyl alcohol (PVA), sodium alginate and carboxymethylcellulose (NaCMC) to the flow characteristics of the NR in detail under the influence of rate of shear and temperature. With the presence of surfactants, it is found out that the NR will exhibit a higher viscosity and pseudoplastic behaviour. However, the viscosity profiles for the NR with and without the presence of surfactants are likely the same where the viscosity reduced sharply at the rate of shear of 25 sec-1.

6

3.2. Dependence of Latex Flow on Concentration Viscosity of NR latex relies very much on the influence of concentration. The concentration of latex can be represented by indicators such as total solids content (TSC) whereas so studies would apply dry rubber content (DRC) to represent the concentration [1]. TSC is the mass percent of overall nonvolatile components of the latex under specified conditions. International Standard correspond to the determination of TSC is ISO 124:1997(E). Meanwhile, DRC is the mass percent of coagulated latex during colloidal destabilization. It can be determined by ISO 126:1995(E). The TSC and DRC are proportional to each other as the major constituent of solids content in latex is rubber. TSC will most often be slightly higher than DRC. Maron and Krieger [9] confirmed that latex flow depends on concentration to a great extent. They reviewed that the increase of viscosity is rather slow at low TSC (30% solids, α=1.70

Investigation on synthetic latices system; 6

[18]

only appicable for solidscontent above 60%

7

(

)

Applicable for all range of solids content

[9]

8

3.3. Effect of Temperature NR latex is a strong structural network which consisting of huge amount of colloidal particles evenly spread throughout the aqueous medium. Sensitivity of the NR latex to the temperature changes is high where the viscosity decreases sharply with the rise of temperature. This behaviour has significant implication for the polymer processing. There are no published data available on the effect of temperature on the NR latex for the years before 1960 [9]. The only reference that only could be found is the statement made by Madge [26] that the viscosity of the NR latex decreased sharply and most probably exponentially with the temperature. However, there is a detailed study on the influence of the temperature for the viscosity of synthetic rubber (styrene butadiene latex) and it was conducted by Maron and Fok [19]. Investigation on the change in the rheological behaviour for the styrene butadiene was done at the temperature varying from 20°C to 50°C. Lattices consist of two distinct flow units which are nonNewtonian (rubber particles) and Newtonian (aqueous medium). They discovered that there is a close relationship in between the viscosity of the aqueous medium with the viscosity for the whole latex system. The viscosity of the latex decrease with the temperature is mainly due to the decrease of the viscosity in aqueous medium. Furthermore, it is found that the latex deviate from the Newtonian fluid behaviour and independent of the changes in temperature. The latex is only dependant on the changes in concentration and shear stress. [9]. A detailed analysis for these data in terms of Ree-Eyring theory had been given by Maron and Pierce [27]. Varkey et al. [23] examined the flow behaviour for the lattices of the blends of NR and styrene butadiene rubber under the influence of temperature by using Arrhenius equation:

Where η is the viscosity (mPas), η0 represents zero shear viscosity, Ea represent activation energy in kJ/mol and T is the temperature in K. He claimed that better understanding on the influence of the temperature on the lattices can be made with the Arrhenius plot where the activation energy can be obtained for each of the blends. The gradient of the graph of η against 1/T represents the magnitude of the activation energy. The 9

values of the activation energy obtained represent the degree of the sensitivity for the material. It is found out that the blends with a higher content of NR (> 50wt%) has higher value of activation energy which mean that more energy is required to rupture the whole network structure formed in between in the particles in the colloidal system. Arrhenius plot also found in the study of flow characteristic by Ranimol at el. [7] for the blends of unvulcanised NR and carboxylated styrene butadiene rubber. Same approach is used to obtain the value of activation energy for each blend. In his study, the rheological behaviour changes due to the changes in temperature are examined accordingly with the obtained values. Ranimol et al. have the same finding with Varkey et al. that high degree of sensitivity is attained for the blends with a higher content of NR (> 30wt%) . Besides, there are also some reports claiming that that the temperature has a great influence on the colloidal dispersion system [12, 28, 29]. Once the temperature increase, the restriction for the flow units in the network structure is less and the flow units turn to be highly active. The whole system becomes unorganized and collapsed at end.

3.4. Effect of Surfactants Surfactant is a surface active chemical and able to alter the surface characteristic in the aqueous medium [30]. Surface tension and the energies of the interfacial can be efficiently lowered down by the surfactants even in a small amount. Moreover, surfactants possess the behaviour of natural colloid system which are hydrocolloid soluble in water and will result in viscous solutions. In rubber industries, modification of flow behaviour in NR latex is crucial as to ensure that it meet the adopted rubber manufacturing process [31]. Types of commonly used surfactants in rubber processing are sodium carboxymethylcellulose (NaCMC), polyvinyl alcohol (PVA), fatty acid (Lauric soap), sodium alginate and casein. Even though the viscosity of NR latex is great influenced by the presence of surfactants, there is no detailed study reported on effect on surfactants on the flow properties of NR latex under the influence of temperature and shear rate for the years before 1990 [12].

10

Later, Peethambaran et al. [12]studied the influence of various types of surfactants on the flow properties of the centrifuged and creamed NR latex concentrates at vary shear rate and temperature (25, 35 and 45 °C). The surfactants which included in his study are PVA, NaCMC, sodium alginate and casein. Each of the surfactants is prepared as 5 % solution in water before adding into the NR latex at the fixed ratio of 1 to 200. In the presence of surfactants, NR latex exhibits a higher viscosity characteristic even at a higher temperature (45°C). It is found out that the effect due the presence of NACMC is significant compared to the other surfactants. The viscosity of the centrifuged NR latex for had been increased by twice with the addition of NaCMC. Moreover, the general characteristic of centrifuged NR latex toward the changes of temperature is altered. The degree of the sensitivity for the NR latex is enhanced where the viscosity of NR latex decrease sharply with the temperature. Increase in pseudoplasticisity in NR latex is also observed with the presence of surfactants in his investigation. Varkey et al. [23] also investigated the effect of the surfactants on the rheological characteristic of the blends of NR latex and styrene butadiene (SBR). It is discovered that the surfactants are capable to reduce the structural buildup in the blends with high content of SBR (> 50 wt%) by impairing the mechanism of microflocculation in between the particles of SBR and NR. Increase in viscosity and pseudoplasticity in the blends with high content of NR latex (> 50 wt%) is reported in his study. In these blends, part of the surfactants is adsorbed on the surface of NR particles while the balance is dispersed in the aqueous medium. Formation of strong structural network in these blends took place as there is a strong interaction in between the surfactant particle in the lattices.

3.5. Effect of Ammonia Content and Storage Period NR latex will coagulate spontaneously when harvested from the tree. Ammonia has been a widely used preservative to prevent latex coagulation for field latex and concentrated latex. It can prevent bacteria action due to pH condition of the latex, hydrolyzed fatty acid esters and stabilize the colloid system. Amount of ammonia added to the latex varies from 0.3-0.8% by weight of the latex. The latex will most often stored for a period of time before sent for further processing, Santipanusopon et al. [13] studied the effect of ammonia content and storage period of field latex on the viscosity of 11

concentrated field latex and skim latex. It is noted that the viscosity of concentrated latex increases with increasing ammonia content as well as longer storage period of field latex. The study indicated that predominant factor of increased viscosity in concentrated NR latex is due to gel formation in the lattices system through cross linking (hydrogen bonding and chemical cross links) of the particles. The prolonged storage period also increase the stability of skim latex.

3.6. Effect of Electrolytes Generally, addition of electrolytes which are compatible with latex will reduce the viscosity of the latex. However, excessive addition of electrolyte may promote agglomeration of the particles which leads to latex coagulation eventually. Typically, lattices are negatively charged, therefore cations will have more profound influence on the rheological behaviour of latex compared to anions. Certain electrolytes function as thickeners as they increase the viscosity of latex when added. Examples of such electrolytes are sodium silicofluoride and polyelectrolytes; sodium or ammonium alginate and sodium acrylate. These agents stimulate the creaming and gelation of lattices [9].

12

4. METHODOLOGY 4.1. Material Used Concentrated NR skim latex was provided by Sime Darby Research Sdn. Bhd., Malaysia. The characteristics of each NR skim latex sample were shown in Table 4. Table 4: Characteristics of NR skim latex.

Sample Notation Properties

Unit S1

S2

S3

S4

S5

S6

Dry rubber content (DRC)

wt%

5.54

6.21

9.87

14.95

19.06

20.14

Total solids content (TSC)

wt%

7.92

9.15

11.85

16.77

20.43

21.54

Ammonia content

wt%

0.46

0.51

0.57

0.50

0.40

0.35

Volatile fatty acid

wt%

0.354

0.367

0.434

0.348

0.319

0.290

pH

-

9.88

9.15

9.80

9.73

9.72

9.66

13

4.2. The Operational and Configuration of the Viscometer Haake Viscometer VT-550 (rotational viscometer model) which equipped with NV sensor system is used for the rheological measurements for all of the samples.

Motor

M d n

Figure 2: The schematic diagram of the Haake Viscometer VT-550 (Left) and the NV sensor system (Right).

The samples to be measured are located in the measuring gap of the sensor system as shown in Figure 2. The rotor will rotate at the pre-defined rotational speeds (n) which is ranged from the scale of 1 to 10. Two speed programs with different rotational speeds are used in the experiments. A resistance force will act against this rotational movement by the measured samples due to the viscosity of the fluids. Thereby, the measuring shaft of the VT-550 is exerted by this braking force which is known as torque value (Md). The values of viscosity η (mPas), shear rate ν (s-1) and shear stress τ (Pa) will be determined by the computer system in VT-550 based on the measured variables of speed, torque and sensor geometry (system factor). An optional temperature control system with water bath and thermocouple is connected to the viscometer. Temperature of the sample can be manipulated by the heating system. The temperature T is demonstrated in °C when temperature control is utilized. All the results are shown on the display of the VT 550.

14

The Viscometer VT 550 is operated with different type of sensor systems which is available in the Hakke program. Certain range of viscosity and shear rate can be achieved by using different sensor system. The viscosity values of the samples (skim latex, skim serum and centrifuged latex) provided by Sime Darby is ranged from maximum 60.0 mPas to minimum 4.0 mPas. The tested samples can be categorized as low viscosity liquid. Sensor System NV is primarily used for the viscosity measurement for low viscosity liquid such as oils, diluted solutions, fruit juices and etc. and working in the medium shear rate range. The sensor has a cup and a bell-shaped rotor as shown in Figure 2. It is classified as a coaxial cylinder sensor system with two gaps for shearing the samples on the inside and on the outside of rotor. Moreover, it can be used together with the temperature vessel. The amount of tested samples should be adjusted as to ensure that tested sample will not cover the upper surface of the inner cylinder during the operation. The bell-shaped rotor must not be contacted by the liquid sample on its top end-face. Thereby, the excess sample must be removed by a pipette or syringe. However, instable flow condition with the samples of low viscosity may distort the measuring results. Hence several readings need to be taken during the rheological measurement in order to get the average viscosity value.

4.3. Standard of Measurement The coaxial cylinder sensors are designed according to DIN 53018 and viscosity is measured based on ISO 3219.

15

4.4. Rheological Measurement The viscosity of the samples will be determined according to the scope of study as shown in Table 5 below. Table 5: Summary of the parameters to be investigated and their respective range of study

No

Parameter Investigated

Range

Remarks

1

Shear rate

Speed Program 1: Speed 1-10 Temperature is fixed at 30oC (10.82, 13.52, 21.64, 27.05, 54.1, 108.2, 270.5, 324, 542, 1082 s-1) Speed Program 2: Speed 1-10 (27.05, 44.90, 75.19, 125.5, 245.0, 349.4, 583.1, 976.1, 1610, 2705 s-1)

2

Temperature

30, 40, 50oC

Shear rate is fixed at 13.52, 1082 s-1

3

Storage period

0, 7, 14, 21, 28, 35 days

Shear rate: 2 Speed Program Temperature: 30oC

4

Concentration (total According to the samples of Shear rate: 2 Speed Program solids content) different retention time provided, Temperature: 30, 40, 50oC ranged from 7.92 – 21.54 %w/w

5

Ammonia content

According to the samples of Shear rate: 2 Speed Program different retention time provided, Temperature: 30, 40, 50oC ranged from 0.35 – 0.65 %w/w

16

5. RESULTS AND DISCUSSION 5.1. Influence of shear rate The rheological behavior of natural rubber skim latex depends on many factors; shear rate is one of the most influencing parameter. The effect of shear rate on natural rubber skim latex is shown in Figure 3 and Figure 4Error! Reference source not found. Figure 3 shows the results for all samples investigated, with total solid content ranges from 7.92-21.54% while Error! Reference source not found. is a magnified view of Figure 3, focusing on the lower total solid content samples, which exhibits much lower viscosity value compare to higher TSC samples.

In general, the

viscosities of natural rubber skim latex decrease with increasing shear rate, indicating a pseudoplastic behavior. However this decreasing trend levels off at a particular shear rate and the viscosity remain constant beyond this point which indicates a Newtonian behavior. This phenomenon can be explained from the perspective of colloidal system network structure. The rubber particles are dispersed in the aqueous medium of skim latex. These particles will form systematic network structure, which support the buildup of molecular structure. Figure 5 shows the formation of network structure in natural rubber skim latex [32]. At low shear rate, this structure will remain intact as they are able to sustain shear stress subjected on it. However as the shear rate increases, the increasing shear stress will destroyed the network gradually and as a result viscosity decreases. Although in general all the samples exhibit decreasing viscosity with increasing shear rate, at some points there are slight increase of viscosity, which happens at high shear rate region. This slight increment may due to the flocculation of latex particles by destabilization of the colloid. It is common for the colloid particles to assemble and form network structures during the measurement. This has caused the slight inconsistency in the results obtained.

17

300 S1 S2

250 Viscosity, η (mPa.s)

S3 S4

200

S5 S6

150

100

50

0 0

500

1000

1500

2000

2500

3000

Shear rate (s-1)

Figure 3: Effect of shear rate on the viscosity of NR skim latex at 30oC.

10 S1

9

Viscosity, η (mPa.s)

S2

8

S3

7

S4

6 5 4 3 2 1 0 0

500

1000

1500

2000

2500

3000

Shear rate (s-1)

Figure 4: Effect of shear rate on the viscosity of NR skim latex at 30 oC (for NR skim latex of low concentration)

18

Figure 5: Formation of network structure by rubber particles in natural rubber skim latex.

5.2. Influence of concentration Viscosity of natural rubber skim latex relies very much on the influence of concentration. The concentration of latex can be represented by indicators such as total solids content (TSC) whereas so studies would apply dry rubber content (DRC) to represent the concentration [1]. TSC is the mass percent of overall nonvolatile components of the latex under specified conditions. In this research project, total solids content will be used as the parameter to indicate skim latex concentration. The determination of total solid contents for each corresponding sample had been carried out by the collaborating company and provided for this research purpose. Natural rubber skim latex is the byproduct from conventional natural rubber latex centrifugation process. The untreated skim latex generally contains 4-6% of dry rubber content (5-8% TSC). The membrane technology is able to separate the serum from skim latex, producing concentrated skim latex with approximately 20% TSC. This is in conjunction with the retention time of skim latex during the membrane processing. The longer the retention time, the higher the total solids content of concentrated skim latex obtained. The range of concentration as provided by collaborating company from real operation is studied under this section and the effect on skim latex rheological behaviour is discussed. Table 6 shows the physical properties of natural rubber skim latex provided with different retention time during the membrane processing.

19

Table 6: Physical properties of natural rubber skim latex with different retention time during the membrane processing.

Retention

Total Solids

Dry Rubber

Ammonia

Volatile Fatty

pH Value

Time (hour)

Content, TSC

Content, DRC

Content (%)

Acid Content

(%)

(%)

0.5

7.92

5.54

0.46

0.354

9.88

1.0

9.15

6.21

0.51

0.367

9.83

1.5

NA

NA

NA

NA

NA

2.0

11.85

9.87

0.57

0.434

9.80

2.5

NA

NA

NA

NA

NA

3.0

16.77

14.95

0.50

0.348

9.73

3.5

NA

NA

NA

NA

NA

4.0

20.43

19.06

0.40

0.319

9.72

4.5

21.54

20.14

0.35

0.290

9.66

(%)

As shown in Figure 3 and Figure 4, the viscosity of natural rubber skim latex increase with increasing total solids content. The increase of viscosity is relatively slow below 17% solids, more rapid between 17-20% and extremely sharp above the latter concentration. At TSC below 16%, the viscosities of examined samples fall below the value of 9mPa.s. An increment from 12% to 17% TSC recorded an increase of about 2mPa.s in maximum measurable viscosity. On the other hand, Slight increment of TSC from 20% to 22% achieves a significant increment of 200 mPa.s in maximum measurable viscosity of the samples.

20

The increment of viscosity is related to the formation of network structure in the rubber particles. Sample with higher TSC has more rubber particles and other lattices content. Therefore the network structure can be formed more easily and stronger. As a result of this more effective agglomeration, the skim latex can resist higher shear stress which in turn exhibits a higher viscosity reading. 5.3. Influence of temperature The viscosity of NR is highly sensitive to temperature. Skim latex, the byproduct from NR centrifuging process possesses the same characteristic like NR toward the temperature variation. This phenomenon will give significant effect on the NR skims latex processing. The viscosity of these colloidal dispersions is greatly affected by the temperature changes [12, 28]. The influences of temperature on the viscosity of NR skim latex at low and high shear rate are illustrated in Figure 6 and Figure 7. In all the cases, the values of the viscosity decrease with temperature. The high TSC skim latex decreases sharply toward high temperature. The free-volume of the NR skim latex increased with temperature. As the result, the strong structural network formed in between the rubber particles in the skim latex is weaken and then collapsed at high temperature condition. The structural network built within these colloidal dispersion system becomes less constrained and less organized since the rubber particles are highly energized at high temperature environment [14]. The reduction in viscosity for skim latex S5 and S6 at high temperature study range of 40-50°C is more significant compared to low temperature study range of 30-40°C. This may be attributed to the collapsed of the structural buildup inside the colloidal dispersion system. The viscosity change profile for the skim latex is likely the same at low and high shear rate. The temperature influence on the low TSC skim latex (≤16.77% w/w) is found insignificant. Normally, skim latex is comprised of 4-6% of rubber content and dispersed freely in large aqueous medium. For NR, the magnitude of the inter-particle forces (Van der Waals forces) that coexisting in between the rubber particles is directly proportional to the number of rubber particles containing inside the skim latex. Thus, skim latex with higher TSC will have a stronger linkage network. The magnitude change for the inter-particle force is insignificant toward the temperature change for low TSC skim latex if comparing to the concentrated skim latex. The rheological behavior for the low TSC skim latex is nearly independent of temperature change. 21

120 Shear rate 13.52 s-1

S1 S2 S3 S4 S5 S6

Viscosity, η (mPas)

100

80

60

40

20

0 25

30

35

40

45

50

55

Temperature, °C Figure 6: Effect of temperature on viscosity of skim latex at low shear rate (13.52 s-1)

18

S1

Shear rate 1082 s-1

S2

16

S3 S4

14

Viscosity, η (mPas)

S5 S6

12 10 8 6 4 2 0 25

30

35

40

45

50

55

Temperature, °C

Figure 7: Effect of temperature on viscosity of skim latex at high shear rate (1082 s-1)

22

5.0 Shear rate 13.52 s-1

S1 S2 S3 S4 S5 S6

4.5 4.0

log η

3.5 3.0 2.5 2.0 1.5 3.05

3.10

3.15

3.20 1/T x10-3 K-1

3.25

3.30

3.35

Figure 8: log η against 1/T for skim latex (at low shear rate of 13.52 s-1)

3.0 Shear rate 1082 s-1

S1

2.5

S2 S3 S4

2.0

S5

log η

S6

1.5

1.0

0.5

0.0 3.05

3.10

3.15

3.20 1/T

x10-3

3.25

3.30

3.35

K-1

Figure 9: log η against 1/T for skim latex (at high shear rate of 1052 s-1)

23

Skim latex consists of two distinct flow units which are non-Newtonian (NR particles) and Newtonian (aqueous medium). There is a direct relationship in between the viscosity of the aqueous medium with the viscosity of the skim latex system. In Maron and Fox‟s studies [21], it is found that the viscosity of skim latex decreasing with the temperature as the result of the decreased viscosity in the aqueous medium. The activation energy required for the flow of latex is slightly difference with that of pure water [17]. To have a better investigation on the influence of temperature on the viscosity of skim latex, activation energy for each of the skim latex samples is determined by using the following Arrhenius relation:

Where, is viscosity, mPas 0

is zero viscosity, mPas

Ea is activation energy, kJ/mol R is gas constant, kJ/ mol K T is temperature, K Arrhenius plots is constructed for skim latex at high (1082 s-1) and low (13.52 s-1) shear rate as illustrated in Figure 8 and Figure 9 respectively. The logarithm of viscosity, log η is plotted against the reciprocal of temperature, 1/T. The gradient of the graph represents the magnitude of the activation energy, Ea. Activation energy represents the degree of sensitivity of a material toward the temperature change. The calculated activation energy for each skim latex concentration at different shear rate is given in Table 7.

24

It was found that the activation energy for skim latex increase with TSC up to 20.34% w/w. Skim latex S1 has recorded the lowest activation energy. When the TSC increased from 7.92% w/w to 20.43% w/w (S5), the activation energy is increasing gradually toward the highest value. It is apparent that the activation energy is increased as the function of TSC contained in the skim latex. The very high activation energy of the skim latex S5 indicates the high temperature sensitivity of the agglomerated NR particle in skim latex. Table 7: Activation energy for skim latex at different shear rate

Shear

Activation Energy (kJ/mol)

Rate (s-1)

S1

S2

S3

S4

S5

S6

10.82

0.115

6.332

6.451

28.029

67.047

12.916

13.52

0.158

3.008

2.587

30.316

64.948

11.062

21.64

0.019

7.115

5.630

32.875

77.636

14.066

27.05

0.474

7.138

13.553

31.112

74.774

12.148

54.10

0.158

7.949

9.913

22.655

87.164

10.319

108.2

0.167

6.325

8.487

34.485

80.737

8.577

270.5

0.476

5.243

6.855

30.896

84.637

10.327

324.0

0.021

6.272

4.210

35.610

61.576

9.666

542.0

0.103

5.097

6.904

36.578

51.033

9.973

1082

0.084

4.159

7.460

31.505

42.307

14.134

25

5.4. Influence of storage period The relation between the viscosities of skim latex with different ammonia content and TSCs is examined at low shear of 27.05 s-1 for storage period time of 7, 14, 21 and 35 days and illustrated in Figure 10. In the presence of low ammonia content, the viscosity of the high TSC skim latex decreases as the function of storage period time. It is found inconsistent with the results obtained in Sirinapa et al. [13] study on the viscosity change over the time for the concentrated NR field; the viscosity of the concentrated NR field increased with the time regardless of the amount of ammonia. In the presence of 0.35 % w/w ammonia, the viscosity was examined to be 249.8, 96.92, 63.84 and 16.06 mPas respectively for the high TSC (21.54 %w/w) skim latex sample S6. Referring to Figure 11, it can be observed that the viscosity for all the skim latex with TSC lower than 16.77 % w/w increases as the function of storage period time in the presence of ammonia content ranging from 0.50 to 0.65 % w/w. For low TSC skim latex S2, the viscosity obtained from skim latex that in the presence of 0.50 % w/w ammonia for day 7, 14, 21 and 28 was 4.56, 5.35, 9.11 and 11.24 respectively. During the centrifuging process for the NR latex, the same volume amount of skim latex is generated altogether with the concentrated NR latex but high content of hydroxyl group and amine group is discovered in skim latex than the concentrated NR latex which is mainly came from the hydration of phospholipids and proteins[33]. Cross-linking in between the proteins and rubber particles in the colloidal system is mainly established via the hydrogen boding. For the case of concentrated NR, the presence of ammonia simulates the gel formation via the cross-linking built-up by hydrogen bonding and chemical cross links which result in increased viscosity over the extended storage period[13]. The same phenomenon is discovered in the colloidal dispersion system for skim latex. Formation of hydrogen bonding through the proteins and chemical cross-link has significant influence on the viscosity formation for skim latex.

26

300 Shear rate 27.05 s-1

250

S1 S2 S3 S4

Viscosity, η (mPas)

200

S5 S6

150

100

50

0

0

5

10

15

20

25

30

35

40

Time (days)

Figure 10: Effect of storage period and ammonia content on the viscosity of skim latex

12 Shear rate 27.05 s-1

Viscosity, η (mPas)

10

8

6 S1

4

S2 S3 S4

2

0

0

5

10

15

20

25

30

35

40

Time (days)

Figure 11: Effect of storage period and ammonia content on the viscosity of skim latex (for NR skim latex of low concentration)

27

5.5. Pseudoplasticity index From the analysis of the effect of shear rate influence on viscosity of skim latex, it shows pseudoplastic behaviour, which is one type of shear-thinning non-Newtonian behaviour. The flow properties if skim latex will be analysed by Power Law:

Where, (

(

)

)

Power Law is widely used to describe the rheological behaviour of non-Newtonian fluids, including shear thinning (pseudoplastic), shear-thickening (dilatant). The type of behaviour can be examined from the value of power law index (n). With n1 is shear-thickening behaviour and n=1 is Newtonian behaviour. In other words, the smaller the index, the more pseudoplastic is the fluid. In contrary, when the index value is approaching unity, the fluid resemblances behaviour of a Newtonian fluid. Determination of pseudoplasticity index from Power Law is as follow:

(

)

28

Plotting a graph of

versus

will give a straight line with slope = n, and intercept =

. Example of pseudoplasticity index determination is shown in Figure 12. The selected example is skim latex with 4hours retention time (TSC=20.43%), viscosity measure at 30oC. The results for pseudoplasticity index determination for each examined skim latex concentration at 30, 40 and 50oC are given in Table 8 and plotted in Figure 13. It is apparent from the values that as the total solids content in natural rubber skim latex increases, the fluid behaves more pseudoplastically. The shear-thinning effect of the skim latex is more obvious with lower pseudoplasticity index, n. Another factor that affects the pseudoplastic behaviour of natural rubber skim latex to a great extent is the effect of temperature. As demonstrated in Figure 13, the pseudoplasticity index values increase when temperature increases, indicating the tendency of the fluid to behave less pseudoplastically and more as Newtonian fluid. The increase of temperature will tend to break down the network structure of the fluid, causing it to be less sensitive towards the change in shear stress. In the case for low TSC skim latex and high temperature, the index values are very close to unity, suggesting that the skim latex rheological behaviour is similar to a Newtonian fluid such as water. 1.6 y = 0.6375x - 0.7495 R² = 0.998

1.4

Log τ (shear stress)

1.2 1 0.8 0.6 0.4 0.2 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Log γ (shear rate)

Figure 12: Pseudoplasticity index determination from Power Law for skim latex of 20.43% total solids content, measured at 30oC

29

Table 8: Pseudoplasticity index values of natural rubber skim latex with different total solids content at 30, 40 and 50oC.

Retention Time (hour)

Temperature Total Solids Content, TSC (%) 30

40

50

0.5

7.92

0.923

0.954

0.981

1.0

9.15

0.910

0.925

0.970

1.5

NA

0.890

0.905

0.954

2.0

11.85

0.875

0.885

0.933

2.5

NA

0.830

0.853

0.915

3.0

16.77

0.812

0.835

0.886

3.5

NA

0.766

0.794

0.853

4.0

20.43

0.638

0.734

0.786

4.5

21.54

0.496

0.592

0.603

30

1.100

1.000 0.5 hour 1.0 hour

Pseudoplasticity Index

0.900

1.5 hour 2.0 hour

0.800

2.5 hour 3.0 hour

0.700

3.5 hour 4.0 hour

0.600

4.5 hour 0.500

0.400 25

30

35

40

45

50

55

Temperature (oC)

Figure 13: Graph shows the pseudoplasticity index values of natural rubber skim latex with different total solids content at 30, 40 and 50oC

31

6. CONCLUSIONS The rheological behavior of the NR skim latex was studied as a function of shear rate, concentration, temperature, storage period and ammonia content. The skim latex exhibits shear-thinning behavior which is often referred as pseudoplastic behavior. In the investigation, the viscosity of skim latex decreases with increasing shear rate. This phenomenon is due to the network structure buildup of the rubber particle. Higher shear rate will tend to destroy the structure, causing the decrease of viscosity. This decreasing trend will level off at a particular high shear rate. Beyond the region, the viscosities of skim latex remain constant which is identical to a Newtonian fluid. In the investigation, skim latex of different concentration is obtained from membrane separation process. Longer retention time during the separation process will give skim latex of higher concentration. Concentration of skim latex has profound influence on the rheological properties. Two parameters are used to indicate the concentration of skim latex, which is the total solids content (TSC) and dry rubber content. The increase of viscosity is relatively slow from for TSC below 17% w/w, more rapid between 17-20% w/w and extremely sharp above the latter concentration. Similar to the effect of shear rate on skim latex flow behavior, the observation is found to be in close relation with the formation of network structure within the skim latex. The TSC indicates the number of particles in the skim latex and with more particle stronger network structure can be formed. In addition, the viscosity of skim latex decreases as temperature rises. This is due to the collapsed of rubber particles cross-links at high temperature. Moreover, analysis on the temperature sensitivity was determined from the activation energy value. These activation energy values were evaluated via the Arrhenius plot. Skim latex with TSC of 20.43 % w/w records the highest activation energy which is more sensitive towards temperature change compared to low TSC skim latex. At high temperature, the skim latex exhibits behavior close to a Newtonian fluid. In addition, the viscosity of skim latex decreased as a function of storage period time in the presence of low ammonia content. Power Law was used to analyze the rheological behavior of skim latex and calculate the pseudoplasticity index. It is concluded that as the TSC in NR skim latex increases, the fluid behaves more pseudoplastically. Increase of temperature has the impact to reduce the pseudoplasticity of skim latex. 32

7. RECOMMENDATION FOR FUTURE WORK This research work was collaboration with Sime Darby Research Sdn. Bhd, Malaysia. Some of the parameters are worth further investigation as they are reported to have strong influence on the flow behavior of natural rubber latex. It is positive that these parameters will affect the rheological behavior of skim latex as well. These include the presence of surfactant and ammonia content. In the samples provided, constant amount of acid laurate was added before the separation process took place. It is recommended that in future work, the rheological behavior of skim latex can be studied as a function of acid laurate concentration. Besides, some of the commonly used surfactants in rubber processing industry such as sodium carboxymethylcellulose (NaCMC), polyvinyl alcohol (PVA), sodium alginate and casein can be used as replacement for acid laurate. The rheological behavior obtained by using different surfactants can serve as a guideline for the skim latex processing as well as the natural rubber processing. The concentration of ammonia is proposed to be another research focus in future. In this preliminary investigation, it is found to have significant influence on the flow properties especially in prolong storage period. The ammonia content shall be varied before it undergoes the membrane separation process and the variation will be traced throughout the membrane separation process. In the investigation of the effect of storage period of skim latex flow behavior, the measurements on the physical and chemical properties of skim latex such as pH value, total solids content and volatile fatty acid content shall be carried out in sync with the rheological measurements. This will give a better picture on the factors that influence the viscosity of skim latex at prolonged storage period.

33

8. REFERENCES [1]

J. Sridee, "Rheological Properties of Natural Rubber Latex," Degree of Master of Engineering in Polymer Engineering, Suranaree University of Technology, 2006.

[2]

K. Cornish and J. L. Brichta, "Some Rheological Properties of Latex from Parthenium argentatum Gray Compared with Latex from Hevea brasiliensis and Ficus elastica," Journal of Polymers and the Environment, vol. 10, pp. 13-18, 2002.

[3]

Z. P. Hamza, et al., "Microwave Oven for the Rapid Determination of Total Solids Content of Natural Rubber Latex," International Journal of Polymeric Materials, vol. 57, pp. 918 923, 2008.

[4]

P. Cacioli, "Introduction to latex and the rubber industry," Revue Française d'Allergologie et d'Immunologie Clinique, vol. 37, pp. 1173-1176, 1997.

[5]

D. Veerasamy, et al., "Environment Friendly Natural Rubber Latex Concentration by Membrane Separation Technology," presented at the The Fifth International Membrane Science & Technology Conference, Sydney, Australia, 2003.

[6]

V. T. Abraham, et al., "Electrochemical treatment of skim serum effluent from natural rubber latex centrifuging units," Journal of Hazardous Materials, vol. 167, pp. 494-499, 2009.

[7]

R. Stephen, et al., "Flow properties of unvulcanised natural rubber/carboxylated styrene butadiene rubber latices and their blends," Journal of Applied Polymer Science, vol. 104, pp. 2528-2535, 2007.

[8]

D. Quemada and C. Berli, "Energy of interaction in colloids and its implications in rheological modeling," Advances in Colloid and Interface Science, vol. 98, pp. 51-85, 2002.

[9]

S. H. Maron and i. M. Krieger, "The Rheology of Latex," in RHEOLOGY: Theory and Applications. vol. 3, F. R. Eirich, Ed., ed Brooklyn, New York: Academic Press Inc. Publisher, 1960.

[10]

S. Mitra, et al., "Effect of electron beam-cross-linked gels on the rheological properties of raw natural rubber," Radiation Physics and Chemistry, vol. 77, pp. 630-642, 2008.

[11]

S. Mitra, et al., "Rheological Behavior of Gel-Filled Raw Natural Rubber and StyreneButadiene Rubber with Reference to Gel-Matrix Intermixing," Polymer Engineering and Science, pp. 1050-1062, 2009.

34

[12]

N. R. Peethambaran, et al., "Rheological behavior of natural rubber latex in the presence of surface-active agents," Journal of Applied Polymer Science, vol. 41, pp. 975-983, 1990.

[13]

S. Santipanusopon and S.-A. Riyajan, "Effect of field natural rubber latex with different ammonia contents and storage period on physical properties of latex concentrate, stability of skim latex and dipped film," Physics Procedia, pp. 127-134, 2009.

[14]

J. T. Varkey, et al., "Rheological Behavior of Blends of Natural Rubber and StyreneButadiene Rubber Latices," Journal of Applied Polymer Science, vol. 56, pp. 451-160, 1995.

[15]

F. R. Eirich, "Rheology Theory and Applications." vol. 3, ed: Academic Press Inc., 1960, p. 125.

[16]

I. M. Krieger and S. H. Maron, "Rheology of synthetic latex. I. Test of some flow equations," Journal of Colloid Science, vol. 6, pp. 528-538, 1951.

[17]

S. H. Maron and B. P. Madow, "Rheology of synthetic latex. III. concentration dependence of flow in type III GR-S latex," Journal of Colloid Science, vol. 8, pp. 130-136, 1953.

[18]

S. H. Maron, et al., "Rheology of synthetic latex. II. Concentration dependence of flow in type V GR-S latex," Journal of Colloid Science, vol. 6, pp. 584-591, 1951.

[19]

S. H. Maron and F. Shiu Ming, "Rheology of synthetic latex : V. Flow behavior of lowtemperature GR-S latex," Journal of Colloid Science, vol. 10, pp. 482-493, 1955.

[20]

S. H. Maron and R. J. Belner, "Rheology of synthetic latex VII. Flow behavior of synthetic rubber latex at low shear stress," Journal of Colloid Science, vol. 10, pp. 523-535, 1955.

[21]

S. H. Maron and A. E. Levy-Pascal, "Rheology of synthetic latex : VI. The flow behavior of neoprene latex," Journal of Colloid Science, vol. 10, pp. 494-503, 1955.

[22]

S. H. Maron and B. P. Madow, "Rheology of synthetic latex. IV. Effect of polydispersity on flow behavior," Journal of Colloid Science, vol. 8, pp. 300-308, 1953.

[23]

J. T. Varkey, et al., "Rheological behavior of blends of natural rubber and styrene–butadiene rubber latices," Journal of Applied Polymer Science, vol. 56, pp. 451-460, 1995.

[24]

R. S. C. D.C. Blackley, Journal of Institute of Rubber Industry, vol. 7, p. 113, 1976 1973.

[25]

E. Rhodes and H. F. Smith, "The viscosity of preserved and concentrated latex," The India Rubber Journal, vol. 7, 1939.

[26]

E. W. Madge, Rubber Chemical and Technology, vol. 8, p. 501, 1935.

[27]

S. H. Maron and P. E. Pierce, "Application of ree-eyring generalized flow theory to suspensions of spherical particles," Journal of Colloid Science, vol. 11, pp. 80-95, 1956. 35

[28]

J. Mewis and J. Vermant, "Rheology of sterically stabilized dispersions and latices," Progress in Organic Coatings, vol. 40, pp. 111-117, 2000.

[29]

Y. Ngothai, et al., "Effect of Temperature on the Flow Behavior of Polystyrene LatexGelatin Dispersions," Journal of Colloid and Interface Science, vol. 172, pp. 289-296, 1995.

[30]

D. C. Blackley, HIgh Polymer Lattices vol. I. London: Maclaren & Sons, 1966.

[31]

G. L. Brown and B. S. Garrett, "Latex thickening: Interactions between aqueous polymeric dispersions and solutions," Journal of Applied Polymer Science, vol. 1, pp. 283-295, 1959.

[32]

R. Stephen, et al., "Rheological behavior of nanocomposites of natural rubber and carboxylated styrene butadiene rubber latices and their blends," Journal of Applied Polymer Science, vol. 101, pp. 2355-2362, 2006.

[33]

M. M. Rippel, et al., "Skim and cream natural rubber particles: colloidal properties, coalescence and film formation," Journal of Colloid and Interface Science, vol. 268, pp. 330340, 2003.

36