Analysing The Processes For Mixing Rubber Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Normally the "recipe" for a rubber compound is given in terms of phr (parts per hundred rubbers by weight). The actual weights used are calculated from the densities of the ingredients, the mixing chamber volume and the fill factor. The fill factor is the fraction of the mixing chamber volume occupied by the compound during mixing. Given that the mixing chamber has a volume of 1.5litres and a fill factor of 0.74 was used, show how the weights given in the table can be calculated from the phr values and densities.

For calculations of the weight that should be used in the formulation, the volume of the components has to be determined. This is assuming that phr = mass.

Where, ρ = Density (kg/m3), M = Mass (kg), V = Volume (m3)

Materials

phr

Density (kg/m3)

Styrene Butadiene Rubber (SBR)

100

920

Zinc Oxide (ZnO)

5

5600

Stearic Acid

1

850

Carbon Black (N330)

40

1800

Antidegradants (AO)

1

1000

Sulphur

1

2100

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS)

1.5

1000

Tetramethylthiuramdisulphide (TMTD)

0.5

1000

SBR:

ZnO:

Stearic Acid:

Carbon Black:

Antidegradant (AO):

Sulphur:

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS):

Tetramethylthiuramdisulphide (TMTD):

Total Volume:

The next step would be to calculate the percentage by weight that the components take up in the chamber:

SBR:

=

=

ZnO:

=

=

Stearic Acid:

=

=

Carbon Black:

=

=

Antidegradant (AO):

=

=

Sulphur:

=

=

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS):

=

=

Tetramethylthiuramdisulphide (TMTD):

=

=

With the percentage by weight of each component calculated, we can then calculate the actual volume that each component takes up:

SBR:

=

=

ZnO:

=

=

Stearic Acid:

=

=

Carbon Black:

=

=

Antidegradant (AO):

=

=

Sulphur:

= =

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS):

=

=

Tetramethylthiuramdisulphide (TMTD):

=

=

Weight of the individual additives component can then be recalculated using the actual volume and density:

SBR:

=

=

ZnO:

=

=

Stearic Acid:

=

=

Carbon Black:

=

=

Antidegradant (AO):

=

=

Sulphur:

=

=

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS):

=

=

Tetramethylthiuramdisulphide (TMTD):

=

=

Table 2: SBR Formulation for Compounding and its weight

Materials

phr

Density (kg/m3)

Percentage (%)

Vol (m3)

Act. Volume(m3)

Weight(g)

Styrene Butadiene Rubber (SBR)

100

920

79.67

0.108

0.884 X 10-3

813.28

Zinc Oxide (ZnO)

5

5600

0.658

0.893 X 10-3

0.0073 X 10-3

40.88

Stearic Acid

1

850

0.867

1.176 X 10-3

0.00962 X 10-3

8.18

Carbon Black (N330)

40

1800

16.23

0.022

0.18 X 10-3

324

Antidegradants (AO)

1

1000

0.738

0.001

0.008 X 10-3

8

Sulphur

1

2100

0.351

0.476 X 10-3

0.003896 X 10-3

8.17

N- Cyclohexyl-2-benzothiazolesulphenamide (CBS)

1.5

1000

1.106

0.0015

0.01227 X 10-3

12.27

Tetramethylthiuramdisulphide (TMTD)

0.5

1000

0.368

0.0005

0.00408 X 10-3

4.08

Question 2:

Determine the amount of powder lost during mixing and express it as a percentage of the batch weight

=

=

=

= 1.10%

=

=

= 2.80%

It was observed from the experiment the weight loss of the compounded product. Compound 1(20 mins mixing time) was observed to have a lower weight loss as compared to compound 2 (4 mins mixing time).

In rubber mixing, there would be a few mechanism of mixing taking place at the same time. They are mastication, dispersion, distribution and incorporation[1]. However without considering the effects of time on mixing, it was observed for some additives, powder forms were used. The usage of powder itself may have resulted in some weight loss. Residue of the powders was observed on the container used to weigh the sample itself. Also, the powders were poured into the mixer using a hopper. This may also result in some loss of powder by the action of pouring. While the amount of powder used was very little in this experiment, the possibility of a powder "cloud" forming above the hopper was possible. The usage of powder was probably the reason why there was some weight loss in compound 1, or even a sample that could be "perfectly mixed".

As mentioned earlier, powders forms of the additives were used and the understanding of powder and the mechanism of mixing will assist in the understanding weight loss difference between compound 1 and 2. Powder itself will agglomerate and certain forces will be required to break them down.

(1) Incorporation works by deformation of the rubber, resulting in an increased surface area for accepting the additive particles. Relaxation of the rubber would then encapsulate the particle within the rubber matrix. The rubber would then breakdown into smaller pieces and mix with the particles and re-coalescence seals it in the rubber matrix. By reducing the mixing time, some of the powder might still be in an agglomerate form that could exist on the surface or within the matrix. The consequence was that the during removal of the compound from the mould, compound 2 with significantly lower mixing time will result in more weight loss by the powders.

(2) Distribution was also known as "simple mixing" where it will increase the randomness of the particles within the rubber matrix with no changes in size of the particles. Mixing time in this case would have little or no effect as the longer the mixing time, the higher the probability the mixture can be more random. However this does not indicate that there will be any changes in powder loss as a function of time.

(3) Mastication will alter the rheological properties of the elastomer. Two sub mechanism of mastication were in place; (i) mechano-chemical as a result of shear stress, (ii) thermo-oxidative which was due to the oxidative chain scission reaction in the presence of oxygen in elevated temperature.

(4) Dispersive mixing will reduce the powder agglomerate to its aggregate. With lesser mixing time, the particles will exist more in the agglomerate form and resulting in higher weight loss when removed.

The combination of the mixing mechanism will indicate the reason of higher weight loss from insufficient mixing time. The distributive and dispersive mixing mechanism works together to allow the powder particles to be well dispersed and distributed in the rubber matrix. Longer mastication time will allow a rubber with lower viscosity to be produced, allowing better efficiency of the incorporation, dispersive and distributive mixing mechanisms to occur.

Summarizing on the points, the powders loss in compound 1 should be due to handling of powder whereas in compound 2, it is a combination of insufficient mixing and powder handling.

Question 3:

Describe and briefly comment on the appearance of the two compounds in relation to the mixing conditions.

Table 3: Observations of rubber compounded with different mixing time

Mixing Time (mins)

 

4

20

Observations

* Dull/matte Surface

* Glossy Surface

* Less Elastic

* More Elastic

* Rough Surface

* Smooth Surface

* Loses material when

* No loss of materials when

stretched/touched

stretched/touched

 

 

* Harder

* Softer

As discussed earlier, the mechanism of mixings were mastication, incorporation, distribution and dispersion. The differences spotted in Table 2 could probably be the results of either a single mixing mechanism or a synergistic effect of the different mechanisms.

The effect of insufficient mixing time could result in a rubber compound that in which the viscosity was still very high, thus reducing the effectiveness of distributive, dispersive and incorporative mechanisms. With a reducing in those mechanisms, powder might still remain in the matrix or on the surface as agglomerates, resulting in rough and matte surface finishing. Insufficient mixing time also results in lesser incorporation of the additives into the rubber matrix, causing it to lose materials when stretched or touched.

However it should also be noted that longer does not means better rubber. Where the degree of mixing is higher, long mixing time might result in a rubber compound with a lower mechanical property or unsuitable cure property for the application. Mixing time should be controlled at a suitable time where the compound can be sufficiently mixed and yet still yield suitable rubber compounds with appropriate rheological properties for the curing process.

Question 4:

Plot batch temperature against mixing time for both compounds. On the temperature profile indicate when additives were added.

Addition of remaining additives

Addition of SBR and Carbon BlackGraph 1. Plot of Temperature against mixing time for both compounds

From Graph 1, we can observe the temperature profile for both compounds during the mixing process. In compound 1, the remainder of the additives were added at T+15min and compound 2 at T+2min.

The temperature drop occurs at the time where the additives were added. During mixing, shear forces will cause the rubber to have a heat build up and when because rubber does not possesses good heat dissipation properties, the heat will continue to build up. When the additives were added, the temperature drops due to the introduction of air and also the powders where was at room temperature. The particles could probably act as a heat sink to remove some heat away from the rubber and resulting in a small drop. In compound 2, which was compounded immediately after compound 1, the rate of heat build up was faster, probably due to the residual heat from the chamber that lead to the faster heat build up.

Question 5:

Suggest how you can change the mixing conditions to improve quality of the compound.

There are many variables that can be optimized to achieve a better quality compound, listed but not limited to:

(1) Rotor Speed- High rotor speed will result in the evolution of heat. Heat in rubber mastication was not desired as it will result in the additional thermo-oxidative mastication which is the chain scission of rubber chain in the presence of oxygen at elevated temperature. Lower rotor speed can be used to achieve a better quality.

(2) Mix time- Long mixing time may mean higher mechano-chemical mastication via application of shear stress. However, this should be kept at a suitable time whereby the rubber achieves the desired rheological properties for the curing process. An adjustment in mixing time may give a better quality product.

(3) Chamber temperature- Temperature of the rubber is important. As discussed earlier about the thermo-oxidative mastication of rubber. An elevated temperature will affect the rheological properties of the rubber and ultimately the mechanical properties as well. Lower chamber temperature is recommended for better quality products.

(4) Circulatory Cooling- As mentioned earlier, a lower temperature is desired in rubber mixing. Therefore the addition of circulatory cooling to the mixing chamber will keep the rubber temperature low and therefore increasing the quality of the rubber compound.

(5) Ram Pressure- Higher ram pressure will result in a better distributive and dispersive mixing of the rubber compound. High pressure is generally desired for mixing, especially for high viscosity mixes as it will allow the reduction of voids and at the same time increasing shear stress by reduction of slippage. However, it was discovered that a high ram pressure at the later stage of mixing, at the point whereby the viscosity of the rubber changes, a high ram pressure will impede with material flow within the mixing chamber. Switching the ram pressure at suitable points will allow better quality rubber compound.

(6) Fill factor- For good mixing, it was believed that the formation of voids behind the rotor tip and the resultant turbulent flow was necessary. Therefore, the chamber should not be overloaded but yet at the same time, sufficient materials need to be present to allow effective ram pressure. If the chamber was overloaded, it is believed that higher mixing time are required to compensate for the effect. Therefore an optimum fill factor should be used as recommended by the manufacturer for production of better quality rubber compounds.

(7) Sequence of addition of additives- There are two methods of mixing available, conventional mixing and upside-down mixing. Conventional mixing involved the addition of elastomer first for mastication before addition of dry ingredients followed by oils. Upside down mixing mixes all other ingredients except the elastomer first, followed by the elastomer. The second method is a fast method and is effective for compounds with large amount of liquid and large particle size fillers. In our case, it was noted that the rubber and carbon black was added together first. The addition of carbon black at a later stage will allow better mixing after mastication of the rubber and also reduce the temperature slightly. This would probably increase the quality of the rubber compounds.

(8) Type of rotor/mixer-the type of rotor blade will affect the void zone and also the area of mixing. A suitable rotor should be used to achieve a good quality rubber compound.

Session 2 (Rubber Characterisation)

Question 1:

Tabulate Mooney Viscosity and Monsanto Curemeter results.

Table 3: Mooney Viscosity and Monsanto results for Compound 1 and 2

 

 

Compound 1 (150oC)

Compound 2 (150oC)

Compound 2 (160oC)

Compound 2 (170oC)

Mooney Viscosity (MU)(1+4,100oC)

129

67

67

67

Minimum Torque (ML)(dNm)

23.8

16.67

14.28

15.15

Maximum Torque (MH)(dNm)

140.48

104.76

102.38

98.49

Scorch time (ts2)(min)

3.13

5.13

3.52

2.33

Cure time (t95)(min)

15.13

13.86

8.97

5.5

Cure Rate Index (CRI)(min-1)

8.33

11.45

18.34

31.55

dM/dt (dNm/min)

88

28.57

38.1

49.22

Type of curve

 

Marching

Marching

Marching

Equilibrium

Question 2:

Describe and suggest reasons for the effect (or lack of effect) of mixing conditions on the average and spread of Mooney Viscosity values.

Question 3:

Viscosity and cure rate are known to satisfy an Arrhenius-type relation P=Ae-B/T, where P is the property being studied, T is absolute temperature for A and B are constants. Check the validity of this approach for the scorch time data which you have obtained. Estimate the maximum safe processing time for the compound at 100oC. Estimate how long it would be safe to store the mixed compound before further processing.

The given Arrhenius equation:

It can be converted to:

Table 4: Cure temperature and scorch time of compound 2

Cure Temperature (°C)

150

160

170

Scorch time (ts2)(min)

5.13

3.52

2.33

1/T (K-1)

0.00236

0.00231

0.00226

ln(ts2)

 

1.63511

1.25846

0.84587

Graph 2. Plot of ln(ts2) against 1/T

From the graph:

A=1.34 X 10-7

B=7390.81

For storage at 23°C,

For storage at 100°C

Question 4:

Compare the cure rate indices you calculated at different temperatures using the theoretical expression with the dM/dt values you measured directly from the same rheometer traces. Is there any trend between these results? Explain your answer.

dM/dt

CRI

Graph 3. Comparison of CRI and dM/dt against temperature

Cure rate index(CRI) was calculated by the following equation:

Where t95 is the cure time (end of curing), ts2 is scorch time (Onset of curing).

A higher CRI value will indicate a higher cure rate and in Graph 3, higher temperature will result in a higher CRI. However from 160°C to 170°C, the rate of cure increased.

The other method, dM/dt, is a measurement of torque per unit time. As rubber cures, more crosslinking will occur and higher torque values will be detected. From Graph 3, the values increased linearly with temperature. Cure rate is expected to increase with temperature as at higher temperature, more energy was input into the rubber compound of the same thickness. Also as the test article is a relatively thin sample, good thermal diffusion was expected and it will be able to absorb the thermal energy more effectively, therefore resulting in higher cure rate with higher temperature.

However comparing the two techniques, the question of which method would be more reliable comes to the mind. CRI takes into consideration the cure rate from scorch time till cure time. However dM/dt only considers the steepest part of the cure, which was a representation of the cure rate at maximum. When a technologist is considering the reliability of both, CRI would probably be more reliable as it takes a wider range into consideration, which is in between scorch time and cure time.

Question 5:

Describe and suggest reasons for differences or similarities in the curemeter results of the compounds you tested.

Comparing both curemeter results obtained for compound 1 and 2 at 150°C, the torque value for compound 1 was higher than compound 2. Reason being was because that compound 1 has a higher mixing time of 20 mins as compare to compound 2 which has a mixing time of 4 mins. A higher mixing time will lead to more mastication, lowering the viscosity by damaging the rubber chain by mechanical shear stress. A longer mixing time was also noted to result in more heat buildup in the rubber and as rubber does not possesses good heat dissipation properties, the heat will result in a thermo-oxidative degradation of the rubber, resulting in an even lower viscosity. The scorch time of compound 2 was higher than compound 1, which could be a result of more uniformed dispersive, distributive and incorporative mixing. Additives were well mixed in this case and allowing most importantly the crosslinking agents and accelerators to be well mixed. This allow a shorter scorch time and shorter optimum cure time in this case.

For compound 2, the maximum torque values were found to be decreasing with higher temperature. It should be noted that during the measurement of the torque values, an oscillating disc was constantly moving and damaging the rubber chains. An increased temperature will further aggravate the effect by thermo-oxidative degradation. Also higher temperature will lead to higher chain mobility and also lower viscosity, therefore decreasing the maximum torque with higher test temperature. At higher temperature, lower scorch time and optimum cure time was observed. This is because the rubber test article is a relatively thin article and it will allow effective heat transfer. A higher temperature therefore reduces the scorch time, optimum cure time and at the same time reducing the CRI and dM/dt.

Question 6:

From your results, suggest the general effect of mixing time on down-stream processing of the compound used in these experiments.

Longer mixing time will result in lower viscosity due to the 2 effects of mastication;(1) the mechano-chemical effect of chain breaking by the applied shear stress and (2) thermo-oxidative chain scission in the presence of heat and oxygen. Long mixing time will result in heat buildup and result in the second effect being more prominent. However the viscosity of the rubber should not be too high as it can be tough to process or even damage the machines. The mixing time should be at an optimum level that allows good mixing of additives into the rubber matrix to allow good mechanical properties and uniform cure. It should not be too long to reduce the viscosity or too short to make it difficult to process. It should be controlled at a suitable range for the desired mechanical and cure properties for the application.

References:

[1] Dr. Ali Ansarifar: Handouts of Rheology and Mixing (MPP217)

[2] J.L.White, Rubber Processing: Technology, Materials and Principles, Hanser Publishers, New York, 1996.

[3] Freakley P.K. , Rubber Processing and Production Organization , Plenum New York,1995

[4] Kenny T.N. (1995) Thesis: Control and Optimisation of Rubber Mixing in an Internal Mixer Loughborough: Loughborough University of Technology

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.