Cure Systems For Epdm Rubber Engineering Essay

Ethylene-propylene-diene terpolymers have been widely used in industrial applications because of their excellent resistance against heat, ozone and weathering, as well as their unusual availability of accepting high loading of fillers [1-3]. Reinforcement in the performances of rubber compounds, such as tensile strength, resilience, wear resistance and flex resistance, can be achieved by loading the compounds with particulate fillers. Different grades of carbon black are the well-known conventional fillers used in EPDM rubbers compounds [4]. Progressively, mineral fillers like silica and clay have attracted more attention as they cost less and give less health hazards [4]. But due to the poor silica-rubber bonding, the reinforcement by silica has not been fully exploited[4, 5]. The availability of silanised silica, which is usually obtained by pre-treating silica with bis(3-triethoxysilylpropyl) tetrasulphane (TESPT), a coupling agent, adheres silica to the rubber [6]. Moreover, it is attractive that the sulphur-bearing bifunctional organosilane can also help to produce crosslinks between rubber chains with the presence of accelerators and activators at elevated temperatures, i.e. 140-240°C [5-12]. The presence of TESPT improves the cure process in silanised silica-filled EPDM rubbers with other common vulcanising systems. Though many researchers have made efforts to investigate different cure systems for EPDM rubbers [3, 4, 13-17], the question on the efficiency of cure systems for commercial production remains open. That gives the objective of this project which are as following:

Using different cure systems to crosslink silanised silica-filled EPDM rubber;

Assess efficiency of the cure systems;

Select the most efficient one for curing the rubber.

This literature review first introduces the basic background of EPDM rubber, including composition, chemical structure and corresponding properties and industrial applications in Section 2. Then a brief overview of the formulation of silanised silica-filled EPDM rubber compounds is given in Section 3, followed by the detailed introduction of recent works on fillers and cure systems for silanised silica-filled EPDM rubber in Sections 4, 5 and 6. Finally, the project plan will be discussed.

BASIC BACKGROUND OF EPDM RUBBER

Terpolymerisation of ethylene, propylene and a non-conjugated diene gives EPDM rubber with a saturated ethylene-propylene backbone and unsaturation site in the side group, introduced by diene monomers [17]. Generally, ethylene and propylene monomers are the major components in an EPDM, providing inherently excellent resistance against degradation by heat, light, oxygen, and, in particular, ozone [18]. The small amount of non-conjugated diene monomers place the reactive unsaturation sites available for sulphur vulcanisation or polymer modification chemistry, as the dienes are so structured that only one of the double bonds will polymerise [19].

Figure 1 EPDM ternonomers

The three co-monomers used in industry are present in Figure 1. Each diene monomer incorporates with a different ability of triggering long chain branching or polymer side chains, hence affect the processing and vulcanisation process [20]. The most commonly used termononer is ethylidene norborne (ENB) as it can incorporate easier and has greater reactivity with sulphur vulcanisation [19]. The chemical structure of EPDM with ENB termonomer is illustrated as follows:

Figure 2 Chemical structure of EPDM

A general summary of properties of EPDM rubber is listed in Table 1 below.

Table 1 Properties of EPDM rubbers

Polymer Properties

Mooney Viscosity, ML(1+4, 125°C)

5 to 200

Ethylene Content (wt. %)

45 to 80

Diene Content (wt. %)

0 to 15

Specific Gravity (gm/ml)

0.855 to 0.88

Vulcanisate Properties

Hardness (Shore A Durometer)

30 to 95

Tensile Strength (MPa)

7 to 21

Compression Set B, (%)

20 to 60

Elongation (%)

100 to 600

Useful Temperature Range (°C)

-50 to +160

Tear Resistance

Fair to Good

Abrasion Resistance

Good to Excellent

Resilience

Fair to Good

Electrical Properties

Excellent

EPDM is the fastest growing synthetic rubber owning to its superior ozone and thermal resistance over other diene rubbers and its loading of fillers and plasticisers to an extremely high level [18]. EPDM has found widespread applications in [18]:

Automotive applications, such as seals, hoses and profiles;

Construction applications, such as roof sheeting, profiles, and seals;

Electrical cables and jacketing;

Moulded appliance parts; also is

Blended with other rubbers and thermoplastics.

EPDM RUBBER FORMULATION

Fillers for EPDM Rubber

Due to the non-crystallising nature of EPDM rubber, reinforcement is required for EPDM rubber, since the mechanical properties of the unfilled rubber are quite poor. Carbon black is the most widely used filler for reinforcing EPDM rubbers, but silica, clay, talc and some other mineral fillers are also used [19]. Progressively, more attention is being paid to silica [1, 2, 4, 15, 16, 21-25]. To achieve full exploitation of reinforcement by reinforcing fillers in EPDM rubbers, carbon black and other fillers must be well dispersed. Good reinforcement can yield EPDM rubbers with high tensile strength, good tear resistance and improved abrasion resistance. Moreover, a well-mixed batch also improves the processability for extrusion, calendaring and moulding [19]. The reinforcing fillers and their effects on EPDM rubbers will be discussed in detail in Sections 4 and 5.

Cure Systems for EPDM Rubber

As mentioned before, the incorporation of unsaturation sites allows the sulphur vulcanising of EPDM rubber. Sulphur cure is the most widely used method, occupying about 80 % of EPDM applications [17]. EPDM rubber can also be vulcanised in a peroxide cure system. Rubber vulcanised by sulphur cure system can accommodate more stress and exhibit higher elongation at break, while the advantage of peroxide cure over sulphur cure is the formation of thermo-stable carbon-carbon bonds instead of thermo-labile sulphur-sulphur bonds, as the dissociation temperature and energy of sulphur-sulphur bonds is lower than that of carbon-carbon bonds [17, 26]. Hence higher effectiveness of heat resistance of EPDM rubber can be obtained by peroxide cure systems. The discussion of cure systems for EPDM rubbers will be unwrapped in Section 6.

Other Additives

Other commonly used additives in EPDM rubber compounds are plasticisers, softeners and processing aids. Naphthenic oils have been the most widely used plasticisers as they have the best compatibility with EPDM rubber and lowest cost. Paraffinic oils are usually used for elevated-temperature applications or in coloured compounds due to the lower volatility and higher UV stability. Stearic acid, zinc stearic and other internal lubricants are often used as processing aids in EPDM rubber compounds. The presence of tackifier or not is dependent on if there is a need for introducing tack as EPDM rubber compounds are inherently not tacky [19].

Different formulations of EPDM rubber compounds result in a variety of applications. A typical recipe for carbon black-filled EPDM rubber for sheeting application is shown in Table 2 below. Tiwari and co-workers [27] studied effect of different treatments of silica on silica-filled EPDM rubber properties and the basic formulation for silanised silica-filled rubber is given in Table 3.

Table 2 Typical recipe for carbon black-filled EPDM sheeting [19]

Components

Amount (phr)

EPDM A

100

N – 347 black

120

Talc

30

PARAFFINIC oil type 103B

95

Zinc oxide

5

Stearic acid

1

MBTS

2.2

TMTD

0.65

TETD

0.65

Sulphur

0.75

Table 3 Basic formulation for silanised silica-filled single EPDM rubber

Components

Amount (phr)

EPDM

100

Silica

50

ZnO

5

Stearic acid

2.0

Silane (TESPT)

4

Sulphur

1.04

N-cyclohexylbenzothiazole-2- sulphonamide (CBS)

1.5

Tetramethylthiuram disulphide (TMTD)

0.8

Zinc dibenzyldithiocarbamate (ZBEC)

1.5

FILLERS USED IN EPDM RUBBER

Carbon Black: A Conventional Filler

Carbon blacks are the most widely used reinforcing fillers in rubber industry since the discovery of their effectiveness of improving the physical and mechanical properties of raw elastomers in 1904 [12]. Different grades of carbon black have been used in EPDM rubbers for industrial applications, such as roof sheeting and automotive profiles and many researchers have studied about the mechanical behaviour of carbon black reinforced EPDM rubbers.

Ghosh and Chakrabarti [28] reported effects of different amounts of carbon blacks on the physical and mechanical properties, ageing behaviour and conductivity of EPDM rubber compounds and the rheological behaviour of EPDM rubber in extrusion processing. Osanaiye [29] used sinusoidal shear flows to study the effects of carbon black, temperature and shear frequency on dynamic mechanical properties of EPDM rubber compounds. The effects of different amounts of conducting carbon black filler on melt rheology and relaxation behaviour of curative free EPDM rubber by cone plate viscometer was reported by Ghosh and Chakrabarti [30]. Abd-El Salam and co-workers [31] used static and dynamic analysis to study effect of different vulcanising systems on the mechanical properties of butyl rubber/ EPDM general furnace black. Cavdar, S. et al [3] reported a comparative study on mechanical, thermal, viscoelastic and rheological properties of vulcanised carbon black filled EPDM rubber.

There are many more examples of research on other aspects of carbon black reinforced EPDM rubbers. For example, conductive rubbers have been made by adding conductive carbon blacks into EPDM and its blends by Das, N. C. et al [32]. The electrical and mechanical properties have been studied.

Silica: A Novel Filler

Recently, synthetic silica is becoming more popular as reinforcing filler in EPDM rubbers because they have proved to be as effective as carbon blacks [12]. Furthermore, silica offers several advantages over carbon black: in tire treads, a higher wear resistance and better wet-grip with a lower rolling resistance can be obtained by using silica rather than carbon black [1]. Also, silica-filled compounds are very suitable for light colour applications.

Problem and Treatments

The reinforcement of silica in EPDM rubber has not reached the desired level because of the poor silica-EPDM bonding. The surfaces of silica have siloxane and silanol groups, which make the filler acidic and polar [7] while EPDM rubber is non-polar. When the polar silica is mixed with non-polar and olefinic hydrocarbon rubbers, e.g. EPDM, hydrogen-bond interactions between polar siloxane or silanol groups in agglomerates are more likely to occur than the interactions between silica and rubber [1], resulting in poor compatibility of hydrocarbon rubbers with silica. Moreover, the acidic silanol groups interact with the basic accelerators, expanding the cure times to an unacceptable level and lowering the crosslinking density [5]. The polar surface of silica will also tend to absorb moisture and this influences cure and properties of the vulcanised rubber [5]. Additionally, the viscosity increases with increasing amount of silica filler and if the viscosity is too high, the processability will be reduced and excessive wear and tear of the processing machine will take place [5].

However, the availability of specific coupling agents makes the use of silica in EPDM rubber compounds possible. Bifunctional organosilanes are commonly used to improve the compatibility between silica and hydrocarbon rubbers by modifying the surfaces of silica [1].

Silanes and Silanised Silica

Bifunctional silanes can be used to chemically link an organic material to an inorganic substrate. The principle aim of using silanes to react with silica involves reducing ablating hydrophilicity of silica and introducing a new organo – functional groups onto the silica surfaces [1]. In the case of sulphur-cured compounds, sulphur-functional silanes perform best and for peroxide-cured compounds, unsaturated silanes such as vinylsilanes are recommended.

In terms of sulphur-cure systems, the use of bis(3-triethoxysilylpropyl-) tetrasulphane (TESPT) (Figure 3) as a coupling agent is well established, since first introduced in 1991 in a practical application in green tires by Rauline [33]..

Figure 3 Chemical structure of TESPT

TESPT possesses ethoxy groups and tetrasulphane groups. The ethoxy groups react with silanol groups on the silica surfaces via hydrolysis mechanism [33], leading to the strong covalent silica-filler bonding. The tetrasulphane groups are rubber reactive and hence stable rubber-silica bonding can be achieved via sulphur crosslinking. Bis(3-triethoxysilylpropyl)-disulphane (TESPD) was later introduced mainly to overcome the pro-scorching problem of TESPT, as the sulphur-sulphur dissociation energy of TESPD was lower than that of TESPT [33].

The silanisation of silica are usually obtained by two approaches. Silica and silanes are mixed preliminarily at an optimum temperature and reaction time, or, alternatively, they can be mixed in situ during the mixing process. [5]. The latter is the more commonly used method [34]. A good silanisation is required as it yields best reinforcement and reduces compound hardening during storage. A certain amount of water can accelerate the silanisation. The optimal moisture content is suggested to be around 3-6% [34]. The main influences on the in situ silanisation of silica-silane filled compounds are summarised in Figure 4.

Moreover, if silica is used in a blend with, e.g., carbon black, comparatively more silane is required as silane is less likely to reach the silica surface quantitatively in a given mixing time [34]. In these cases, silanised silica obtained by the pre-treatment is advisable.

Optimum silanisation

And

Short mixing times

Release of ethanol

T ↑and t ↑

Release of ethanol

T↑and t↑

Good silica dispersion

η↑as T↓

Complete coupling reaction T↑ and t ↑

Avoid pre-crosslinking

T ↓ and t ↓

Fast transportation processes

η↓as T ↑

High mobility of the silane; small size

Best rotor and mixing chamber geometry

Figure 4 main influences on the silanisation reaction [34]

Apart from sulphur vulcanisation, the addition of vinylsilanes is usually applied to improve the mechanical properties of peroxide-cured compounds. The general structure of vinylsilanes is shown in Figure 5.

Figure 5 generalised structure of vinylsilanes

In contrast to the fairly high dosage of sulphur-functional silanes in products requiring high mechanical properties, a strong reinforcement can be achieved by the incorporation of only 2 parts by weight Si 225 (VTEO) per silica [34]. Adding more radical initiator or activators can result in higher crosslink densities [34].

However, the applications of silanised silica are mostly focused in natural rubber (NR), styrene butadiene rubber (SBR), and polybutadiene rubber (BR). Very few research works has been published on the effects of silane on EPDM rubbers, but there are still some. Kim [33] reported effect of TESPD on the processability and mechanical properties of EPDM rubber. Taikum and Luginsland [16] studied the role of silane-rubber coupling in sulphur, peroxide and metal oxide curing systems for EPDM rubber. Das et al [4] showed that the presence of TEPST increased the content of bound rubber in silica-filled EPDM compounds, which was critical to the mechanical properties of the rubber.

Other Treatments

Other coupling agents

Besides silane, several other coupling agents have been employed to modify the silica-EPDM bonding. Das et al [4] use bis diisopropyl thiophosphoryl disul¬de (DIPDIS), to modify EPDM rubber instead of silica by two-stage vulcanisation technique. The effects of TAC (Triallyl Cyanorate) as a coupling agent on curing and mechanical properties of silica-filled EPDM rubber were studied by Abtahi and associates [1].

Others methods

Tiwari et al [23] treated the surfaces of silica by plasma-polymerisation with acetylene monomer and one year later, the comparative study of plasma-thiophene and -acetylene coated silica in EPDM reinforcement was reported [27]. Tan and Isayev [22] treated silica using a coaxial ultrasonic extruder and investigated the effects on properties of ultrasound-treated silica on filled EPDM rubber.

Other fillers

In most cases, carbon black and silica are used to reinforce EPDM rubbers. Some other kinds of fillers have been added to EPDM rubber matrix and their effects been investigated, involving montmorillonite (OMMT) nanofiller [35], nano-zinc oxide [36], Sm2O3 [26], short melamine fibres [37], ash/halloysite [38] and so on.

EFFECTS OF FILLERS ON PROPERTIES OF EPDM RUBBER

Effects of Carbon Black

As mentioned before, researchers have studied a lot about the effects of adding carbon black on the mechanical properties of EPDM rubber, showing that the properties were improved significantly [3, 28-32]. Çavdar and associates [3] reported that the Young’s modulus, Shore A hardness, and compression force over deformation ranage increased with increasing content of carbon black, while the elongation at break reduced (Figure 6).

Figure 6 Effects of carbon black content on (a) mechanical properties; and (b) rheological properties of EPDM rubber [3].

The Young’s modulus was most filler content sensitive as the value increased sharply with amount of carbon black. In terms of rheological properties, increasing carbon black content resulted in higher maximum torque and the difference between maximum and minimum torque, which corresponded to relative crosslinking density. The optimum cure time decreased with increasing the filler content.

Considerable research has been done to understand the mechanism of reinforcement. Two main characteristics of active blacks are their surface area and aggregate structure, which determine the static and dynamic in-rubber properties and hence make it possible to tailor the performance of rubber products.

Effects of Silica

Effects of silica on the mechanical properties of EPDM rubber

Without silanes

The effectiveness of silica as reinforcing filler in EPDM rubber was confirmed by Ichzo and co-workers [2] who showed that tensile strength had improved by 500%, tear strength by 400% and elongation at break at 140% by adding 20 phr of precipitated silica. They used silica with different size and demonstrated that an increasing tendency of tensile strength can be achieved when the size of silica particle decreased. The hardness of EPDM rubber increased with the filler loading but it was not particle size dependent. They also found that silica aggregates size distribution affected the mechanical properties and it deserved more attention.

With silanes

Das [4] indicated that the Young’s modulus, tensile strength and crosslinking value of silica-filled EPDM rubbers increased considerably when 1-2 phr of TESPT was added, while the elongation at break decreased, as illustrated in Figure 7 below.

(b)

(a)

(d)

(c)

(f)

(e)Figure 7 Effect of TESPT on the mechanical properties of EPDM rubber compounds: (a) modulus at 100% elongation; (b) modulus at 300% elongation; (c) tensile strength; (d) hardness; (e) elongation at break; (f) crosslinking value [4].

Effect of silica on processing property of EPDM rubber

As mentioned earlier, adding silica to EPDM rubber will make the processing more difficult as the viscosity increases significantly when a big amount of silica is involved [5].

However, the availability of silanes such as TESPT or TESPD weaken the interaction between silica particles as the ethoxy groups in silane react with the surfaces of silica by the silanol groups, leading to a change in interfaces between the polymer-polymer, polymer-silica and silica-silica [33]. Hence, it reduces the viscosity and improves the processability of the rubber compounds [5]. Kim [33] reported that the addition of TESPD to silica-filled EPDM rubber yielded lower Mooney viscosity, heat generation and extrusion pressure build-up through an extruder, which made processing easier.

Effects of silica on thermal property of EPDM rubber

Madani [39] studied the thermal property of gamma radiation cured silica-filled EPDM via thermogravimetric analysis (TGA) and demonstrated that the presence of silica reduced the rate of degradation and the weight loss of vulcanisates. This was due to the improved adhesion between silica and EPDM rubber matrix. He also stated that thermal property of silica-filled EPDM rubber was determined by the loading of filler, filler size and structure, filler-matrix interactions and processing technique.

Effect of silica on the ageing property of EPDM rubber

Planes et al [15, 40] used gamma radiation to age unfilled and filled EPDM rubbers at room temperature and at 80°C to study the influence of silica on the rubber degradation. They evidenced that adding untreated silica accelerated the polymer phase degradation due to the formation of supplementary radicals triggered by silica irradiation. If silane-treated silica was presented, the degradation acceleration was delayed.

Effects of silica on the electrical property of EPDM rubber

Raw EPDM rubber is an insulator with a conductivity of about 10-14 S-1 [39]. It was proved that the addition of inorganic fillers such as silica increased the conductivity of polymer [39]. Madani [39] investigated the variation of dielectric constant () of some cured EPDM and silica-filled EPDM rubbers as a function of frequency and found that was filler content dependent: it increased up to 10 phr, and then decreased with increasing loading. He pointed that the increase was due to the polar groups present on silica surfaces, and that the decrease was due to the increasing system density and the extent of orientation of dipoles.

Effects of dispersion of silica on the properties of EPDM rubber

Filler dispersion has a distinct effect on the properties of rubber compounds. Poor dispersion has a negative effect on rubber properties by creating structural flaws [5]. Polmanteer and Lentz [41] demonstrated that some properties such as tear strength and tensile strength improved as the filler dispersion quality increased after they examined effect of dispersion of silica on the properties of some sulphur-cured rubbers. To obtain a better dispersion of fillers in rubber compounds, increasing mixing time is an efficient method, however, at the cost of lowering the molecular weight of polymer, which leads to the reduction in mechanical properties [5]. The degree of dispersion of filler can be examined by microscopy methods, such as electron microscopy and atomic force microscopy.

CURE SYSTEMS FOR EPDM RUBBER

Sulphur Cure systems

Every rubber product is vulcanised with its own specific cure system, resulting in various properties. As already mentioned, the incorporation of pendant unsaturation sites enables that EPDM rubber to be vulcanised by sulphur plus accelerators. Sulphur cure is the most widely used vulcanising method for curing EPDM rubbers, constituting about 80% of the EPDM applications [17]. Compared with peroxide-cured EPDM rubbers, sulphur-cured rubber compounds are able to accommodate more stress and exhibit higher elongation at break.

Basically, three types of sulphur crosslinks are used in elastomers, namely, monosuphfidic (C-S-C), disulphidic (C-S2-C) and polysuphidic (C-Sn-C). The crosslink density and the value of n are mainly determined by vulcanising system and process conditions such as cure temperature and time. Over the years three special types of cure systems have been established based on the level of sulphur and the ratio of accelerator-to-sulphur applied. They are:

Efficient vulcanisation (EV) systems,

Semi-efficient vulcanisation (SEV) systems and

Conventional vulcanisation (CONV) systems.

EV systems are characterised by a high ratio of accelerator-to-sulphur or even sulphurless, but containing sulphur-donor instead. They are usually used in vulcanisates which require an extremely high heat and reversion resistance [42]. CONV systems are vulcanisation systems with a low ratio of accelerators to sulphur and they can provide better flex and dynamic properties but worse thermal and reversion resistance. A semi-efficient cure system has an accelerator-to-sulphur ratio in between those of the CONV and EV vulcanisation systems. For SEV systems, optimum levels of mechanical and dynamic properties of vulcanisates with intermediate heat, reversion and flex properties can be obtained [42]. The compositions of CONV, SEV and EV systems are shown in Table 4.

Table 4 the levels of accelerators and sulphur in CONV, SEV and EV systems [42]

Type

Sulphur (phr)

Accelerator (phr)

A/S ratio

CONV

2.0-3.5

1.2-0.4

0.1-0.6

SEV

1.0-1.7

2.5-1.2

0.7-2.5

EV

0.4-0.8

5.0-2.0

2.5-12

Increasing accelerator-to-sulphur ratio results in increased amount of shorter mono- and disulphidic crosslinks. As the dissociation energy of C-C bonds are larger than that of S-S bonds. Vulcanisates obtained by EV and SEV systems possess a better heat and reversion resistance than those cured by CONV systems. The general influences of the type of vulcanisation systems on the structure and properties of the vulcanisates are summarised in Table 5.

Table 5 vulcanisate structure and properties for different cure systems

Features

Cure systems

CONV

SEV

EV

Poly-and disulphidic crosslinks (%)

95

50

20

Monosulphidic crosslinks (%)

5

50

80

Cyclic sulphide (conc.)

High

Medium

Low

Non-sulphidic (conc.)

High

Medium

Low

Reversion resistance

Low

Medium

High

Heat ageing resistance

Low

Medium

High

Fatigue resistance

High

Medium

Low

Heat build up

High

Medium

Low

Tear resistance

High

Medium

Low

Compression set (%)

High

Medium

Low

Moreover, nitrosamine free or safe curing packages were developed for the replacement of cure systems which develop nitrosamines during vulcanisation. N-nitrosamines formed during vulcanisation as condensation products from certain accelerators and nitrous gasses and are carcinogenic [43]. They are generated from some thiuram and dithiocarbamates accelerators, which are known as ultra-accelerators and usually used in EPDM rubber compounding [43]. Traditional ultra-accelerators can be replaced by nitrosamine-free systems, but at expense of high costs.

Almost all conceivable combination of curing ingredients for EPDM rubber compounds have been evaluated over the years [42]. Five typical cure systems are listed in Table 6. The alternative nitrosamine free or safe cure systems are suggested in Table 7.

Table 6 Five cure systems for EPDM rubber [42]

Systems (phr)

Advantages

Disadvantages

System 1

Low cost

Bloom

S 1.5

TMTD 1.5

2-mercaptobenzothiazole (MBT) 0.5

System 2

Excellent physical properties and fast cure

Scorchy and expensive

S 2.0

MBT 1.5

Tellurium diethyl dithiocarbamate (TDEC) 0.8

Dipentamethyl thiuram tetrasulphide (DPTT) 0.8

TMTD 0.8

System 3

Excellent compression set and good heat ageing resistance

Bloom and very high cost

S 0.5

Zinc dibutyldithiocarbamate (ZDBC) 3.0

Zinc dimethyldithiocarbamate (ZDMC) 3.0

4,4’dithiodimorpholine (DTDM) 2.0

TMTD 3.0

System 4

Non-blooming

Cure relatively slow and worse compression set

S 2.0

2,2′-dithiobenzothiaole (MBTS)

ZDBC 2.5

TMTD 0.8

System 5

Zinc O,O-dibutylphosphorodithioate (ZBPD) 2.0

TMTD 1.0

N-butylbenzothiazole-2-sulfenamide (TBBS) 2.0

S 1.0

Fast cure and good physical properties

Bloom

Table 7 Some NA free alternatives for the cure systems above [42]

Systems

NA free alternatives

System 1

S 1.5

S 1.3

MBT 0.5

MBT 0.75

TMTD 1.5

CBS 3.8

System 2

S 2.0

S 1.5

MBT 1.5

ZMBT 2.0

TDEC 0.8

ZBEC 0.5

DPTT 0.8

ZBPD 2.0

System 5

ZBPD 2.0

ZBPD 2.5

TMTD 1.0

TBBS 2.0

TBBS 2.0

S 1.0

S 1.2

Besides, an activator, such as zinc oxide, is usually needed in EPDM cure systems to maximise the efficiency of accelerators and chemical bonding between the filler and rubber.

Silanised Silica: a “Crosslinking Filler”

An important issue must be considered regarding the sulphur cure systems for silanised silica-filled EPDM rubber is the fact that the use of sulphur-functional silanes such as TESPT combine silica with sulphur into one single product known as a “crosslinking filler” [6], such as silanised silica. It can not only improve the mechanical properties of rubber, but also can produce crosslinks between the rubber chains at elevated temperatures in the presence of accelerators due to the sulphur-containing groups. Therefore, the vulcanisation process can be achieved without elemental sulphur being present [6]. Research has shown that the mechanical properties of some vulcanisates improved significantly in spite of the reduction in the use of the curing chemicals [6].

It was demonstrated that during the vulcanisation process the formation of both rubber matrix crosslinking network and silica-rubber coupling network occurred simultaneously and did not separate. In the presence of elemental sulphur, the two different crosslinking reactions compete for the added sulphur as the sulphur-functional silanes like TESPT are sulphur acceptor [34]. Therefore the crosslinking structure and the reinforcement are determined by the amounts of silane and sulphur. The influence of adding different amounts of sulphur and silane on the matrix and silica-rubber coupling networks is shown in Figure 8.

Figure 8 effect of the amount of (a) added sulphur and (b) silane (TSEPT) on the crosslinking densities of matrix and silica-rubber coupling [34]

It is evident that increased amount of sulphur enhances the efficiency of the silica-rubber coupling until all the silane is activated. Initially, a small degree of increase in the matrix crosslink density is observed, due to the consumption of free sulphur by the activation of silane. After the full activation of silane, the matrix crosslink density increases much faster. With respect to the increasing amount of silane at a constant amount of sulphur, the total number of silica-rubber bonds increases while the level of the matrix crosslink density reduces, owing to the incorporation of sulphur by the silane [34]. Moreover, in the case of TESPD, a disulphide silane, the effect is more significant. Therefore it can be concluded that changing the amount of elemental sulphur and silane results in different ratios of the matrix and silica-rubber networks [34]. Furthermore, mechanical properties of silanised silica-filled rubber can be predicted because silica-rubber coupling dominates the modulus and hydrophobation lowers hardness values. Some detailed effects of silane on the mechanical performance of silica-filled EPDM rubber have been presented in Section 5.2.1.

Peroxide Cure systems

Overview

Crosslinking with peroxides was first introduced in 1915, but didn’t attract too much attention until the development of fully saturated ethylene-propylene copolymers (EPM) [42]. Many kinds of elastomers can be vulcanised by peroxide cure systems efficiently, including NR, SBR, EPDM, BR, nitrile rubber, Silicones and fluorocarbon elastomers [42]. The unsaturation can improve the efficiency of peroxide vulcanisation [17], due to the higher concentration of allylic hydrogens [42]. The relative efficiency of peroxide crosslinking for different elastomers is:

BR>NR and SBR >NBR>CR>EPDM

The advantages and disadvantages of peroxide cure compared to sulphur vulcanisation are listed in Table 8 [42]:

Table 8 Advantages and disadvantage of peroxide cure compared with sulphur cure

Advantages

Disadvantages

Thermo-stable C-C bonds instead of thermo-labile S-S bonds,

Scorch free storage of compounds,

Simple compound formulation,

Low compression set even at high cure temperature,

It is possible to cure at high temperatures without reversion,

Good electrical properties of cured rubber,

No discolouration of compounds.

Low compounding flexibility;

Lack of flexibility in regulating scorch and optimum cure time

Inferior tensile, tear and flex performance,

Inferior abrasion resistance

Higher cost,

Sensitivity to oxygen during vulcanisation,

Odours of peroxide decomposition products.

Peroxide cure of EPDM

Peroxide-cured EPDM rubbers are being widely used for many years. They are commonly used in window seals, automotive hoses, roof sheeting, tanking lining, electrical insulations, steam hoses, roll coverings mouldings and so on [42].

Mechanism of peroxide cure of EPDM rubber

The mechanism of peroxide cure of EPDM rubber and the subsequent practical consequences have been reviewed by van Duin and co-workers [17, 44, 45]. In the generally accepted mechanism of peroxide cure of EPDM, the major steps are illustrated in Figure 9.

Figure 9 Mechanism for peroxide cure of EPDM [17, 44, 45]

Thermal degradation of the peroxide initiated by process triggered a chain of free-radical reactions, leading to the formation of primary alkoxy (RO•) or secondary alkyl radicals (R•). Then the abstraction of H-atoms from the EPDM polymer yields EPDM macro-radicals (EPDM•). The next step is the combination of two EPDM macro-radicals, or the incorporation of a macro-radical to an EPDM unsaturation. If a coupling agent, such as vinylsilanes mentioned in Section 4.2.1.1 (TAC, trimethylolpropane or m-phenylenbis(maleimide)), is present, the peroxide cure efficiency can be increased, as they can enhance the H-atoms abstraction and the following reactions are repressed [1, 17]:

EPDM• + RO• → EPDM – RO (No crosslinking)

EPDM• →EPDM•’ + (β-Scission)

The issues of the detailed mechanism of peroxide vulcanisation of EPDM rubber remain open, mainly owning to the complexity of the system. Several studies have been conducted to see further understanding of the chemical mechanism of peroxide cure of EPDM [46, 47].

Coupling agents for peroxide cure of silica-filled EPDM rubber

As previously stated, vinylsilanes are usually incorporated in peroxide cure systems for silica-filled EPDM rubber to improve vulcanisation efficiency and mechanical performance of vulcanisates. The effectiveness of TAC as a coupling agent for peroxide vulcanisation of silica-filled EPDM rubber was investigated by Abtahi et al [1] and concluded that TAC coupling agent improved some properties such as tensile strength, abrasion and resilience resistance with a proper formulation. They also used SEM to have a deep insight in the interaction between silica filler and rubber in the presence of TAC and found that the degree of reinforcement was mainly affected by the degree of wettability.

New Developments

Reducing the sulphur content results in vulcanisates with better thermal resistance throughout service life, however, this is at the expense of reduction in mechanical performances such as dynamic fatigue resistance and tear resistance. This compromise can be eliminated by using two additives, namely hexamethylene-1, 6-bisthiosulphate (HTS), a post vulcanisation stabiliser and 1, 3-bis (citraconimidomethyl) benzene, an anti-reversion agent [42].

Vulcanisation techniques have effects on EPDM rubbers. Das [4] studied the efficiency of one-stage and two-stage sulphur vulcanisation techniques of silica- filled EPDM rubbers and reported that modification of EPDM rubber by two-stage vulcanisation technique enhanced the interactions between EPDM rubber and silica.

Furthermore, mixed cure systems can also be employed to enhance the performance of vulcanisates.

PROJECT PLAN

The overall aim of this project is to test some sulphur and peroxide cure systems to cure silanised silica-filled EPDM rubber to produce a rubber with good mechanical properties using a suitable curing system. The project plans are:

Prepare silanised silica-filled EPDM rubber compounds with peroxide and cure the rubber with accelerator and activator via the sulphur in the silane;

Use carbon black and silanised silica nanofillers in EPDM rubber to assess their effects on the mechanical properties of the rubber;

Cure and measure the mechanical properties of filled EPDM rubber to establish effect of the curing systems and fillers on the rubber properties.