This month we focus on silanes and more especially sulphur silanes. In the forthcoming month we will focus on vinyl silanes.

Introduction

Silanes have been known for about 50 years.  They were originally used in an industrial scale for adhesives and coatings.  In the 70s silanes found their way into the rubber industry as coupling agents for while filler. During the early 90s, sulphur silanes gained more importance due to the increasing acceptance of the Green Tyre.  The use of silane coupling agents as surface treatments, adhesion promoters or cross -linking agents has been growing significantly over the past decades. When used in sealants, adhesives, coatings, rubbers or even surface primers, silane can provide major improvements in adhesion on various substrates.  

When used as cross-linkers by development technologist, compounders and moulders, silanes will improve abrasion resistance stiffness and heat resistance as well as dynamic properties of the finished products. 

Organofunctional silanes will hydrolyze in the presence of moisture and condense onto the surface of different substrates and fillers, forming chemically covalent bonds between organics and inorganic substrates.

Silane categories

Depending upon the chemical composition, the main categories of silanes available are: sulphur, vinyl, epoxy, isocyanate, amino, chloro, methacryl and thiocyanate.  In the rubber industry, sulphur and vinyl silanes are commonly used, whereas amino, chloro and others are rarely used.

The General Formula for silane

Although the chemical theory is sufficiently researched, the selection of the most suitable silane for specific elastomers and their vulcanization systems requires much know how.  With silanes, not only a physical process takes place inside the mixing chamber, but also at the same time a chemical reaction called silanization.  Here the mixing parameters play an important role.  Certain compound ingredients may promote the silanization or disturb this process, affecting the physical properties of the compound.

Silane products for the Rubber Industry

Deolink TESPT [bis(triethoxysilylpropyl)tetrasulfane] is a preparation with 50% active substance on a polymer/wax carrier system.  It can be used in almost all elastomers which are suitable for sulphur vulcanization, 

Deolink MX is a preparation of a thiocarboysilane with 50% active substance.

In comparison to the tetrasulfane silanes, Deolink MX can be processed over a broad temperature range without the risk of scorch.  It can often be used even at lower dosages without loss of effectiveness.  It has a blocked mercapto group and can be chosen as an alternative to conventional mercapto silanes. The protective group is removed during processing releasing the mercapto silane.  By using Deolink MX, optimized processing properties can be achieved and the unpleasant odour of mercaptanes is avoided.  Deolink MX is suitable for all elastomers which can be cross linked by sulphur vulcanization.

Observing how silanes react

Generally, there are two theories.  The first one describes a direct condensation, whilst the second is based on the pre-hydrolysis of the silane.  Studies, however, are in favour of a two-step reaction with the following pre-reaction:

• Pre-reaction
• Hydrolysis (example: alkoxysilane)

The alcoxy groups of the silicium are subject to hydrolysis. The necessary water is usually available on the surface of the inorganic filler.

Step 1

Connection with the mineral surface (example: silica)

The silanol groups which are formed by the hydrolysis of the silane, condense with the hydroxyl groups of the filler and form stable Si-O-Si bonds by splitting off water.

Step 2

Linking with the polymer matrix (example: mercapto silane)

The organofunctional group (Y) is relevant for the reaction with the polymer. This reactive group must be suitable for the chosen vulcanization system, to enable cross linking during the vulcanization process.

Using this mechanism, it is possible to form a silane bridge through a chemical reaction between the polymer chains and the filler particles. This process results in the formation of a polymer filler network.

Reaction of a silanized filler with the polymer (example: mercapto silane)

Applications for technical rubber compounds

Cables/cable accessories: Improved electrical properties (insulation, water absorption, swelling, dielectrical strength), improved mechanical properties (tensile strength, abrasion).

Roller coverings: Reduced abrasion, improved compression set, improved dynamic properties (heat build up), optimized processing properties.

Sealings/O-rings:  Improved compression set, optimized processing properties, improved mechanical properties, reduced abrasion, for dynamically stressed sealings.

V-Belts/conveyor belts: Reduced abrasion, improved dynamic properties, improved adhesion with the reinforcing fabric

Shoe soles: Reduced abrasion, optimized processing properties, improved dynamic properties (flex cracking resistance)

Moulded articles: Improved dynamic properties, optimized processing properties, improved mechanical properties.

Hoses & tubes: Reduced abrasion, improved mechanical properties, improved adhesion with the reinforcing fabric.

It is quite evident that these DOG carrier systems of silanes have significant advantages in improved protection against hydrolysis through moisture, excellent dispersion and handling, easy incorporation without spots, no formation of agglomerates, no dust development by fines, prolonged storage stability without loss of activity and cost savings to complete use of opened boxes, no disposal of ineffective hydrolized residues. Other added advantages are shelf life, reactivity retention and less storage sensitive.

Sulphur Silanes – Deolink TESPT/Deolink MX

Mixing temperature, filler, silane types and the general compound formulation have a significant influence on the degree of silanization.  What follows are the outcomes of the studies that support the effects of Deolink TESPT and Deolink MX.

The linking of the filler to the rubber matrix provides positive influences in respect to the mechanical – dynamical values and the processing properties, The effects in SBR, EPDM and CR are illustrated below.  Deolink MX is particularly effective in EPDM and CR, since this silane preparation already gives a high efficiency at lower mixing temperatures.  Thereby processing (higher injection volume) and processability (scorch safety) are improved.

Name	Deolink TESPT/TESPT -100		Deolink MX/MX – 100
Description Active substance	Activator for filler Bis(3-triethoxysilylproply) – tetrasulfane (TESPT)		Activator for the filler thiocarboxysilane
Silane content (%) Appearance Analytical values -Total sulphur         (%) ASTM D 1552 (LECO) -Density at 20°C (g/cm³) DIN ISO 787 T10A -Dropping point      (°C) Mettler-apparatus DIN ISO 2176 Dosage in relation to filler                       (%)	50 Yellow pellets 10-13 1.0 72+/-5 Ca. 1-10	100 Yellow liquid 20-26 1.1   – 0.5-5	50 White pellets 3.8-4.8 1.0 115 +/-5 1-8	100 Clear liquid 7.6-9.6 1.0   – 0.5-4
German Food Legislation (BIR recommendation XXI) Storage stability in originally sealed package in cool and dry places Classification and labelling	Not approvedMin. 1 year –		Not approvedMin. 1 year Labelled as irritant (Xi)according to EEC directives
Supply form Deolink TESPT/Deolink MX Supply form Deolink TESPT -100 Supply form Deolink MX – 100	20 kg. in cardboard boxes with PE-inliner in pre weighed packaging available 200 kg steel drums and 1000 I containers 195 kg steel drums

Effects in SBR/Silica

Recipe 4515	Control	Deolink TESPT	Deolink MX
Buna SBR 1502	100	100	100
Silica (BET 175 m²/g)	50	50	50
Naphthenic oil	10	10	10
ZnO	3.5	3.5	3.5
Stearic Acid	1.5	1.5	1.5
PEG 4000	2	2	2
Antioxidant SPH	1	1	1
Deolink TESPT	0	4	0
Deolink MX	0	0	4
MBTS	1	1	1
DPG	1	1	1
Sulphur	2	2	2
Total phr	172	176	176

The most favourable advantages of Deolink MX take effect in EPDM and CR which we will see below

Effects in EPDM/Silica

Recipe 930E	Control	Deolink TESPT	Deolink MX
EPDM Buna EPG 5450	100	100	100
Silica (BET 125 m²/g)	50	50	50
Paraffin oil	20	20	20
ZnO	5	5	5
Stearic Acid	1	1	1
PEG 4000	2	2	2
Deolink TESPT	0	4	0
Deolink MX	0	0	4
MBTS	1	1	1
Deovulc T 4-75	2.8	2.8	2.8
DPG	0.4	0.4	0.4
Sulphur	1	1	1
Deovulc ZBEC -80	0.6	0.6	0.6
Total phr	183.8	187.8	187.8

Deolink MX also enables a higher injection volume.

Effects in CR/Silica

Recipe 248C	Control	Deolink TESPT	Deolink MX
Baypren 110	100	100	100
Silica (BET 125 m²/g)	50	50	50
Ester plasticizer	10	10	10
MgO	4	4	4
Stearic Acid	0.5	0.5	0.5
Controzon GP	2	2	2
Antioxidant ODPA	1	1	1
Deolink MX	0	0	2
Deolink TESPT	0	2	0
MBTS	0.5	0.5	0.5
ZnO	5	5	5
ETU	1	1	1
Total phr	174	176	176

Deolink MX leads to a better processing safety.

Influence of the time of addition

It is recommended that the addition of the silane be together with the (first) dosage of the light filler.  It is not recommended to add the silane at the start of the mixing cycle together with the rubber, as in some cases a deterioration of the physical values has been found.

Effects of Silane dosage on various fillers

The degree of modification is certainly influenced by the quantity of silane used. An increased quantity of Deolink TESPT releases additional quantities of sulphur, which can influence the vulcanisation process.  Medium active filler (e.g.Clay) often require only reduced silane dosages to provide the maximum reinforcement strength.

Recipe 4515	Control	Deolink TESPT			Deolink MX
Buna SBR 1502	100	100	100	100	100	100	100
Silica (BET 175 m²/g)	50	50	50	50	50	50	50
Naphthenic oil	10	10	10	10	10	10	10
ZnO	3.5	3.5	3.5	3.5	3.5	3.5	3.5
Stearic Acid	1.5	1.5	1.5	1.5	1.5	1.5	1.5
PEG 4000	2	2	2	2	2	2	2
Antioxidant SPH	1	1	1	1	1	1	1
Deolink TESPT	0	2	4	8	0	0	0
Deolink MX	0	0	0	0	2	4	8
MBTS	1	1	1	1	1	1	1
DPG	1	1	1	1	1	1	1
Sulphur	2	2	2	2	2	2	2
Total phr	172	174	176	180	174	176	180

If Deolink TESPT is used in higher dosages, additional quantities of sulphur will be released, which will affect the vulcanisation process.

Effect on SBR/Clay

Recipe 4565	Control	Deolink TESPT
Buna SBR 1502	100	100	100	100
Clay	100	100	100	100
Naphthenic oil	10	10	10	10
ZnO	3.5	3.5	3.5	3.5
Stearic Acid	1.5	1.5	1.5	1.5
PEG 4000	2	2	2	2
Antioxidant SPH	1	1	1	1
Deolink TESPT	0	2	4	8
MBTS	1	1	1	1
DPG	1	1	1	1
Sulphur	2	2	2	2
Total phr	222	224	226	230

In comparison to silica, the use of silane with the medium active clay provides a relatively lower degree of reinforcement. If silanes are used in higher dosages, it should be considered, that additional quantities of sulphur will be released which will affect the vulcanisation process.  The use of Deolink MX leads to similar test results without releasing sulphur.

Influence of different mixing temperatures

The reaction speed of the silanization depends on the processing temperature.  For the effective use of sulphur silanes, a dumping temperature of 130°C – 150°C is recommended. The tyre industry achieves their required silanization effects through a longer mixing process or with higher mixing temperatures of around 160°C.

Typical silica tread compounds are produced in a three-pass procedure to ensure that the silane is used to its best potential.

The technical rubber compounds these processes with several steps are usually not necessary. When using Deolink TESPT, mixing temperatures of >160°C should be avoided to exclude the risk of pre-scorch of the raw compound through the poly-sulfane groups of the silane.  Also, Deolink MX can be used in these cases, as mixing temperature of 170°C are easily possible.

To illustrate the following temperature ranges have been chosen

Open Mill: Mixing temperature 80°C

Banbury: Dumping temperature 115°C

Banbury: Dumping temperature 140°C

Already at 80°C on an open mill both grades provide a silanization of the raw compound and an improvement of the physical properties. At this low temperature, Deolink MX shows an even higher activity than Deolink TESPT.

At the dumping temperature of 115°C, the silanization effects are more significant and again Deolink MX still presents advantages in comparison to conventional sulphur silanes. At a dumping temperature of 140°C abrasion and processing are further improved.

Recipe 4515	Control	Deolink TESPT			Deolink MX
Buna SBR 1502	100	100			100
Silica (BET 175 m²/g)	50	50			50
Naphthenic oil	10	10			10
ZnO	3.5	3.5			3.5
Stearic Acid	1.5	1.5			1.5
PEG 4000	2	2			2
Antioxidant SPH	1	1			1
Deolink TESPT	0	4			0
Deolink MX	0	0			4
MBTS	1	1			1
DPG	1	1			1
Sulphur	2	2			2
Total phr	172	176			176

Conclusion

The above data has only dealt with sulphur silanes. Founded in 1902, D. O. G. are a family-owned company located in Hamburg’s free harbor.  Being a specialist in additives with its own research and development facilities, D. O. G. have developed many liquid silanes and dry liquid blends for the rubber and coating industry. The development of user friendly and environmentally friendly products have been a tradition and long may it continue.

I R Tubes Pvt. Ltd. is a leading specialty chemical suppliers for the chemical industry. Contact I. R. Tubes on [email protected] or Call: 9689927193 for more information