a study on epoxidised natural rubber

a study on epoxidised natural rubber

A STUDY ON EPOXIDISED NATURAL RUBBER FOR POSSIBLE

APPLICATION IN TUBELESS TYRE INNER LINER

, Khaw Pei Chin and Che Su Mt Saad Technology & Engineering Division, Rubber Research Institute of Malaysia, Malaysian Rubber Board, 47000 Sungai Buloh, Selangor, Malaysia

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Corresponding author Tel.: +6 (0)3 6156 1121; fax: +6 (0)3 6156 4418;

E-mail address: ywngeow@lgm.gov.my

ABSTRACT

Epoxidised Natural Rubber containing 50 mole % epoxidation (ENR 50) has been shown to have unique set of properties One of these properties is reflected in vulcanisates with excellent air retention characteristics which is comparable to that of butyl rubber A low air permeability characteristic is important for tubeless tyre inner liner to ensure the performance and durability of the tyre during service Other requirements for tyre inner liner compound are good processibility and good physical properties In this paper, ENR 50 is studied and compared with blends of ENR 50 and SMR 20 at different ratios Natural Rubber (NR) possesses good processing properties including high green strength, as well as excellent cured physical properties

Keywords: Epoxidised natural rubber (ENR), ENR 50, tyre, inner liner

1.1 Introduction

Epoxidised Natural Rubber (ENR) is produced by modification of natural rubber (NR) latex In general, the desire % mole epoxidation can be achieved at any level, however only two grades are available commercially These grades are marketed as Ekoprena 25 and Ekoprena 50 by Malaysian Rubber Board for ENR 25 and ENR 50 with epoxide contents of 25 % mole and 50 % mole respectively The epoxidation of

NR has progressively reduced the air permeability ability of NR as the % mole epoxidation increased Studies has found that the air retention characteristic of ENR 50

is as good as halogenated butyl rubber which is an important requirement of tubeless tyre inner liner application1,2

Tyre inner liner is used for air barrier in the inner surface of a tyre to retain the internal tyre pressure Tyre with inflation pressure at optimum level ensure the durability and performance of a tyre, resulting in better performance in term of better fuel economy, handling and ride compared to tyre that is not inflated at optimum pressure Besides that, tyre with optimum pressure can prolongs the tyre carcass and tread lives and contributes to even tread wear According to French tyres manufacturer

Other requirements for tyre inner liner compound are good processibility and physical properties The compound must be easily calendered to give smooth and even thickness during tyre manufacturing process NR is known of possessing good processing characteristics and high green strength Blending NR with ENR 50 provides compound that exhibits the unique qualities of these respective polymers for tyre inner liner application and promote the usage of sustainable materials for tyre compound development

According to International Rubber Study Group, 44% of 22.7 million tonnes of world rubber consumption is source from natural rubber in year 20095 and majority of this rubber is used for the tyre industry Tyre inner liner compound occupied approximately 5% of the total tyre weight In general synthetic rubber such as halogenated butyl rubber (HIIR) is conventionally used for tyre inner liner Replacing part of the tyre components with sustainable materials will greatly reduce the usage of our depleting natural resources and meeting the stringent carbon dioxide gas emission regulations made by policy makers

In this study, ENR 50 is blend with NR to obtain the optimum properties for tubeless tyre inner liner application Further study is carried out to compare ENR 50/

NR blend at 70 : 30 % by weight with Bromobutyl (BIIR) compounds

2.1 Material and Method

2.2 Preparation of ENR/ NR Compounds

Raw rubbers ENR 50 and SMR 20 are obtained from Malaysian Rubber Board All the vulcanisates are prepared according to standard techniques Laboratory Banbury mixer size 1.6 L (BRL600) is used to blend the polymers and chemicals The mixer is set at 110 r.p.m rotor speed and the initial temperature is set at 70oC with 0.7 filled factor The mixing cycle is as follows for ENR 50/ NR blend:

0 min : Add polymers + calcium stearate

½ min : Add powders (zinc oxide, stearic acid, antioxidant, etc.)

1 min : Add Carbon Black (CB) and processing oil

2 ½ min : Sweep down feed chute

In all the ENR 50/ NR compounds preparation, insoluble sulphur and accelerators are added later on a Carter 9” x 18” two-roll mill and the compounds are cured to optimum (t95) at 1500C using electrical press The semi-EV ENR 50/ NR tyre inner liner formulation employed in this study is shown in Table 1:

Table 1: ENR 50/ NR Tyre Inner Liner Formulations

Ingredient p.p.h.r

Polymers 100 Calcium Stearate 2

Stearic Acid 2 Carbon Black N660 55 Escorez Resin 5 Tudalen 65 10 Santoflex 13 1 Permanax TMQ 1 Insoluble Sulphur 0.7 TBBS 1.8

The ratios of ENR 50 and NR blends are shown in Table 2 ENR 50 are taken as control Six compounds are prepared and shown in Table 2

Table 2: ENR 50/ NR Blend Ratios Used for Tyre Inner Liner Study

The physical testing procedures are done in accordance to the International Standard Organisation (ISO) or British Standard

2.3 Scanning Electron Microscope (SEM) Analysis

Joel FESEM JSM 6701F model is used in this study for morphology analysis The instrument is operated at 2kV with 15mm working distance The analysis is based on semi-quantitative measurement of element extracted from a specific window size Samples are placed onto the specimen stub and examined by evaporative coating with ultra-thin layer of platinum under high vacumm which provided a conducting layer that permits SEM examinations

2.4 Filler Dispersion Analysis

Reflective Light Microscope disperGrader+TM of Dynisco is used to measure the filler dispersion Samples are cut to generate a ‘fresh face’ for analysis By utilising the

1 2 3 4 5 6

3.1 Results and Discussions

3.2 Processing Properties

Understanding the processing properties of compound are important in the tyre industry to ensure the consistency and quality of the final product Table 3 showed the processing properties of ENR 50/ NR compounds

Table 3: Processing Properties of ENR 50/ NR Compounds

From Table 3, the cure time (t95) decreased with increasing NR contents in general except for compound 4 and compound 6 This may due to the homogeneity of the blends It is observed that NR has an effect on the cure time (t95) and the Mooney Scorch (t5) of the compounds It is noteworthy that in general, the cure time (t95) and the Mooney Scorch (t5) are reduced when NR contents increased

The rheographs of compounds are shown in Figure 1 Compound 1 consists of ENR

50 without any blend with NR showed the lowest minimum torque From the figure, it

is clear that compound 1 is undergoing cure reversion which is not good for tyre application However, it is no longer observed when NR is incorporated into ENR 50 It

is postulated that NR increases the crosslink density in the rubber matrix due to higher double bond density in NR and improves the cure reversion resistance of ENR 50/ NR compounds

Figure 1: Rheographs of Compounds

Processing Properties Compound

1 2 3 4 5 6 Compound Viscosity

(ML 1+4 at 100 oC)

37.7 35.1 36.2 37.1 36.5 37.2 Mooney Scorch, t 5 (Min: Sec t 120 oC) 22:15 21:29 20:51 20:41 20:23 19:52 Rheo Cure, t 95 (Min: Sec) 4:48 4:37 4:32 4:40 4:15 4:18

0

1

2

3

4

5

6

7

Time (Second)

Compound 1 Compound 2 Compound 3 Compound 4 Compound 5 Compound 6

Time (second)

3.3 Scanning Electron Microscope (SEM) Analysis

The physical properties of the blends are largely determined by phase morphology and compatibility These effects are particularly marked with regard to air permeability efficiency, tear strength and elongation at break SEM micrographs of ENR 50/ NR compounds at different blend ratios are presented in Figure 2

Figure 2: SEM Micrographs of Compounds ENR 50/ NR at 100:0 % by weight showed finer morphology with relatively smooth surface compared with the rest of the compounds due to the presence of single polymer

In the 65:35 %, 60:40 % and 50:50 % by weight of ENR 50/ NR compounds, the rough surfaces became apparent and higher number appearance of agglomeration are

3.4 Filler Dispersion Analysis

Reflective light microscope is used to measure the filler dispersion Many

mechanical properties of vulcanisates are directly affected by filler dispersion, i.e

tensile strength and tear strength Figure 3 and Figure 4 showed the average filler agglomeration sizes and percentage of fillers dispersion in the compounds

Figure 3: Average Agglomeration Sizes of Compounds

Figure 4: Percentage of Fillers Dispersion in The Compounds

It is observed that the percentage of CB dispersion decreased when the ENR 50 contents decreased It is also found that the average CB agglomerate size increased when ENR 50 is decreased The observation is consistence with SEM micrographs analysis where higher number of agglomerates are observed when ENR 50 contents decreased The percentage of CB dispersion is above 97% for blending of ENR 50 above 65% by weight in the compound The study postulated that ENR 50 improved the CB dispersion in the blending of ENR 50 and NR

6

8

10

12

14

Compound

0

20

40

60

80

100

Compound

3.5 Physical Properties of Compounds

The physical properties of the ENR 50/ NR compounds are shown in Table 4 It is observed that hardness values are approximately the same for ENR 50/ NR compounds

It is also observed that the tear strength increased with the increased of NR contents Table 4: Physical Properties of Compounds

A small volume of discrete phase (NR) may determine the path of tear strength tensile strength and elongation at break Some of the physical properties are inconsistent when NR contents increased Is it postulated that Payne effect6 may have occurred due to stronger filler-filler interaction hence poorer CB dispersion as observed

in filler dispersion analysis and lead to inconsistency of these properties

The air permeation coefficient of the compounds are measured according to ISO

2782 and the results are showed in Figure 6 The lower the air permeation coefficient, the better the tyre inner liner in retaining the tyre pressure As expected, the air permeation coefficient of compounds are dependent on the level of ENR 50 contents in the compounds

Physical Properties Compound

Hardness (IRHD) 56 60 63 63 62 62 Tear Strength (N/mm) 22.7 23.9 26.5 30.8 31.4 41.4 M100 (MPa) 1.46 1.5 1.52 1.56 1.49 1.49 M300 (MPa) 5.76 6.0 5.85 5.88 5.66 5.73 Tensile Strength (MPa) 12.6 13.0 15.0 14.0 14.2 14.0 Elongation at Break (%) 520 513 576 565 574 562

0.0

1.0

2.0

3.0

4.0

5.0

6.0

2 /Pa.s

Compound

The results showed that ENR 50/ NR compound at 70 : 30 % by weight will be generally be good for tyre inner liner with high tensile strength and elongation at break

as well as relatively moderate modulus physical properties and air permeability

3.6 COMPARISON OF BROMOBUTYL AND ENR 50/ NR BLEND

In this study, comparison between ENR 50/ NR blend at 70:30 % by weight and bromobutyl rubber (BIIR) are carried out ENR 50 and NR compounds are prepared as control Table 5 showed the physical properties of the compounds

Table 5: Physical Properties of Compounds

Table 5 showed that BIIR tear strength and elongation at break are higher than ENR 50/ NR compound However, the hardness, tensile strength, M100 and M300 of ENR 50/ NR compound are higher compared with BIIR compound due to higher filler-rubber interaction and crosslink density

The results for air permeation coefficient of BIIR and ENR 50 compounds are comparable The air permeation coefficient of ENR 50/ NR is higher than BIIR compound due to the reduction of ENR 50 contents in ENR 50/ NR blend

However, the air permeation coefficient of ENR 50/ NR at 70 : 30 % by weight is acceptable for tyre inner liner7 Besides, tyre inner liner air permeability does not necessary causes the whole tyre loss its pressure The tyre construction parameters do have effect on the whole tyre inflation pressure loss rate

4.1 CONCLUSIONS

The study showed that the physical properties of ENR 50/ NR blends are largely determined by the phase morphology The average filler agglomerate size increased is

in consistence with the observation through SEM micrographs where relatively higher number of agglomerates are observed The study postulated that ENR 50 improved the dispersion of CB in ENR 50/ NR compound

Blending NR with ENR 50 has improved the processing property of ENR 50/ NR blend In general, the cure time (t95) and the Mooney Scorch (t5) are reduced when NR

70/30

Air Permeation Coefficient

contents increased Incorporation of ENR 50 in NR gives improvements in vulcanisation rates and cure reversion resistance

The tensile strength of ENR 50/ NR compound are higher than BIIR compound However, the tear strength, modulus and air impermeation of ENR 50/ NR is lower than BIIR compound

ENR 50/ NR at 70 : 30 % by weight compound is preferred for tubeless tyre inner liner application in corresponding with the rest of the ENR 50/ NR blend ratios

References

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4 Joachim Neubauer, Get a grip, Tire technology International, 24 – 26

5 International Rubber Study Group Vol 64 (1-3), (July – September 2009)

6 Payne AR A, Note on Conductivity and Molulus of Carbon Black Loaded Rubber,

J Appl Polym Sci., (1965), Vol.9, 1073-1082

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James D MacIsaac Jr., Tire Aging Test – Tire Inner Liner Analysis, National Highway Traffic Safety Administration, (2010)

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(1991), 184-205

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