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Silicone Rubber Compounding
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Unlike organic polymers, silicone polymer by itself is relatively weak and produces tensile strengths of only 1.0 Mpa when crosslinked. To achieve useful engineering properties, it is necessary to reinforce the polymer by the addition of very fine, high surface fillers which are compatible chemically with the silicone polymer. In addition, functional fluids called process aids are required for adequate shelf life control as well as processability, and curing agents are needed for vulcanization. Figure 4 lists the various formulation ingredients for silicone rubber compounds and their primary function. The following sections will treat these in more detail.
Fillers
The most common reinforcing filler used in silicone rubber compounds is fumed silica which is manufactured by burning silicon tetrachloride in the presence of hydrogen and oxygen. The smoke-like silica particles produced are extremely fine and spherical in shape with surface areas as high as 325 m2/gm. These particles are amorphous but associate in string-like clusters which chemically interact with the Si – O polymer backbone, yielding the desirable reinforcement properties.
Crosslinked silicone rubber compounds containing fumed silica typically have tensile strengths in excess of 10 Mpa, i.e., a factor of ten higher than the pure polymer.
Precipitated silicas made through the acidification and precipitation of sodium silicate can also be used as reinforcing fillers in silicone compounds but usually give weaker mechanical properties compared to fumed silica. In addition, because salt is a by-product of the manufacturing process, compounds based on precipitated fillers tend to be poorer in terms of electrical properties. These compounds are, however, extremely good in terms of low compression set and high resilience, and are more cost effective than their fumed silica counterparts.
Because precipitated silica holds absorbed water on its surface, sponging may occur during curing unless pressure can be maintained on the part. For this reason, precipitated fillers are primarily utilized in compounds intended for molding processes.
The degree of reinforcement of a filler in silicone polymer can be categorized according to its incremental effect on Shore A durometer (Figure 5) which strongly correlates to filler surface area.
In the semi-reinforcing category, diatomaceous earths are often added to improve oil resistance.
A third category of essentially non-reinforcing or extender fillers include ground quartz which reduces cost, reduces shrinkage, and improves thermal conductivity.
Other fillers which are useful in silicone rubber formulations include iron oxide and titania for heat stability, aluminum trihydrate for tracking resistance in insulators, and various carbon blacks for conductivity. (Figure 6)
Figure 4
Formulation
| Component |
Primary Function |
| Silicone Polymer |
Inherent Chemical Properties |
| Reinforcing Filler |
Physical and Rheological Properties |
| Extender Filler |
Cost Reduction and Oil Resistance |
| Process Aid |
Processability and Shelf Life Stability |
| Organic Peroxide |
Crosslinking Agent |
| Metallic Oxide |
Oil and Reversion Resistance |
| Platinum Complex |
Addition Cure Catalyst and/or Flame Retardant |
| Iron Oxide, Titania |
High Temperature Heat Stability |
| Coupling Agent |
Lower Compression Set and Increase Modulus |
| Phenyl Silicone Fluid |
Self-Bleed or Self Lubrication |
| Teflon Powder |
Increase Green Strength |
Figure 5
Fillers Suitable For Use With Silicone Polymers
| Type |
Reinforcement |
Sp. Gravity |
Particle Diameter (Millimicron) |
Surface Area (m2/gm) |
1 Duro Pt. Loading (parts per 100) |
| Fumed Silica |
High |
2.20 |
17 10 |
200 325 |
1 – 1.5 0.5 – 1.0 |
| Precipitated Silica |
High |
2.00 |
22 |
160 |
1 – 1.5 |
| Diatomaceous Earth |
Semi |
2.15 – 2.30 |
3000 |
5 |
2.0 |
| Ground Quartz |
Low |
2.65 |
5000 – 30,000 |
--- |
3.0 – 5.0 |
| Red Iron Oxide |
Low |
4.95 |
1000 |
--- |
--- |
| Titania |
Semi |
3.90 |
300 |
9 |
--- |
Figure 6
Comparative Properties Of Conductive Carbon Blacks For Silicone Rubber Compounding
| |
Acetylene Black (Shawinigan) |
Furnace Black (Ketjen EC) |
Graphite (Lonza KS15) |
| % Volatiles |
0.3 |
1.0 |
0.1 |
| Particle Size (millimicrons) |
420 |
30 |
15,000 |
| BET Surface Area (m2/gm) |
64 |
950 |
14 |
| Degree of Structure |
High |
Medium |
Low |
| Microstructure |
Highly Ordered |
Medium |
Extremely Ordered |
| Degree of Porosity |
Medium |
Very High |
Very Low |
Effect Of Extender Filler On 35 Durometer General Purpose Compound
| Formulation |
A |
B |
C |
D |
E |
| SE6035 |
100 |
100 |
100 |
100 |
100 |
| 5 Micron Minusil |
---- |
25 |
50 |
100 |
150 |
| DBPH-50 |
0.8 |
0.8 |
0.8 |
0.8 |
0.8 |
Press Cured 10' @ 177° C
+ 1 Hour @ 200° C Post Bake
| Shore A |
34 |
38 |
42 |
56 |
66 |
| Tensile, psi |
1320 |
1065 |
950 |
730 |
700 |
| Elongation, % |
780 |
750 |
690 |
500 |
210 |
| Tear Die B, ppi |
95 |
90 |
90 |
95 |
75 |
| Compression Set, % |
35 |
37 |
36 |
38 |
42 |
| Specific Gravity |
1.10 |
1.25 |
1.37 |
1.55 |
1.69 |
Heat Aged 70 Hours @ 225° C
| Shore A Change |
+2 |
+4 |
+4 |
+5 |
+9 |
| Tensile Change, % |
-16 |
-12 |
-6 |
-4 |
-4 |
| Elongation Change, % |
-30 |
-30 |
-28 |
-45 |
-30 |
Process Aids
Process aids, also known as softeners, are reactive silicone fluids which chemically modify the surface of the silica fillers to reduce their association with the silicone polymer. Most process aids are liquids which can either be prereacted with the silica filler in a pretreatment process, or can be introduced during the compounding/mixing phase to effect “in-situ” treatment. In many cases, both techniques are employed.
To understand the need for this critical ingredient, reference is made to (Figure 7). The upper schematic shows the surface of a typical fumed silica particle. Note that the pendant hydroxyl (OH) groups on the filler surface form secondary bonds with the oxygen (O) in the polymer backbone (Center schematic in Figure 7). This is called hydrogen bonding and increases as a function of time so that an uncured compound containing only silicone polymer and untreated fumed silica will harden or structure as it ages. This adversely affects the processability of the compound such that it crumbles and cracks on the mill instead of forming a smooth, continuous band.
Process aids react with the pendant hydroxyl groups, thereby reducing the level of polymer-filler interaction and improving processability and shelf life of the silicone compound. This is shown in the lower schematic of (Figure 7) in which the hydroxyl groups have been replaced with a nonreactive alkoxy group designated by –R. Process aids typically are hydroxyl stopped fluids or chemicals which hydrolyze during compounding to produce hydroxy functional fluids which then react with the filler surface.
In addition to processability improvements, the proper amount of process aids can aid dispersion of extender filler, improve physical properties, and enhance clarity. Too much process aid can adversely impact compression set and high temperature aging.
Figure 7 – Surface Treatment Of Fumed Silica
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