Molded silicone rubber automotive parts.

Though not always immediately visible, silicone-rubber parts have become indispensable in modern automotive engineering. Thanks to their versatile properties, they are to be found in the engine compartment, in the drive system, in electrical and electronic systems, around the body and increasingly in the interior; in fact, wherever sealing or covering functions are needed. Silicones also provide protection against aggressive media, even under conditions of extreme heat or cold, and provide an absolutely leak-tight seal between the engine and the cylinder head. They optimize and safeguard the ignition system and protect electric plug-connections. They act as conductors or insulators in highly sensitive areas of ultra-modern automotive electronics. And they serve as sealants and dampers in all areas of the car body.

Innovations in automotive engineering require innovative, high-quality materials that are precisely tailored to specific requirements, materials such as silicone rubbers. Their astonishing versatility derives from their remarkable range of properties (ref. 1):

* Hot-air resistance up to +200 [degrees] C (with stabilizers up to +250 [degrees] C, or up to +300 [degrees] C for brief periods);

* low-temperature flexibility down to -50 [degrees] C;

* high transparency;

* wide choice of color;

* good resistance to many chemicals;

* resistance to light (including UV and X-rays);

* steam resistance;

* outstanding aging behavior;

* outstanding electrical and thermal insulation properties;

* high dielectric strength;

* very good mechanical fatigue resistance;

* excellent mechanical properties over a very wide temperature range;

* biocompatibility;

* no toxic combustion products; and

* simple and economical to process.

Chemical structure and processing

Silicone elastomers are produced by the vulcanization of silicone rubbers. In general, silicone rubbers are classified according to their crosslinking systems (figure 1). Whereas room-temperature-curing grades are usually used as sealants or potting compounds, high-temperature-curing grades are mainly processed into molded articles.

[ILLUSTRATION OMITTED]

Silicone rubbers primarily consist of silicone polymers and fillers (ref. 2). For the production of heat-curing silicone rubbers, the polymers used are linear or branched, vinyl-group-containing polydiorganosiloxanes. Their basic structure, like that of quartz or sand, is the silicon-oxygen bond (ref. 3). The remaining valencies on the silicon atoms are usually occupied with methyl groups (MVQ). Fillers usually comprise fumed silicas with a specific surface area of [is greater than] 125 [m.sup.2]/g, whose silanol groups (3-4.5 Si-OH groups per [mm.sup.2]) can form hydrogen bridges with the oxygen atoms of the polymer. These filler-polymer interactions improve the mechanical properties (ref. 2).

Solid silicone rubbers (HTV) are pasty, non-sag compositions with a viscosity of about 20 kPa s for a polymer chain length of about 10,000 Si-O units. They are supplied as slabstock, strip or profile tape and processed by typical rubber industry techniques such as extrusion, compression or transfer molding, automated injection molding or calendering (ref. 4). Wacker-Chemie has also succeeded in pelletizing HTV compounds. Supplied in this form, the materials can be efficiently and fully automatically fed to, in particular, extruders or injection molding machines by means of pneumatic metering equipment.

Liquid silicone rubbers (LSR) are two-component 1:1 systems with a viscosity of about 10 to 100 Pa s and a chain length of about 1,000 SiO units (ref. 5). They are predominantly processed by liquid injection molding (LIM). LSR compounds are molded by an injection molding process that resembles that used for plastics, which is combined with a meter/mix unit (figure 2) developed specifically for the process (ref. 6).

[ILLUSTRATION OMITTED]

Liquid silicones permit mass production - as required in the automotive industry. Production is more economical than traditional rubber processing (ref. 4) thanks to:

* Lower viscosities;

* short cycle times;

* fully automated production;

* selective crosslinking without the formation of by-products;

* elimination of post-finishing operations; and

* possibility of achieving complex geometries with long flow paths.

High-temperature-curing silicone robbers crosslink by addition across the double bonds of the silicone polymer's vinyl groups. If a peroxide is added to the rubber compound, addition takes place by a free-radical mechanism on decomposition of the unstable molecule. An alternative crosslinking mechanism is hydrosilylation, in which methylhydrogen-siloxanes add across the vinyl groups of the polymers in the presence of a platinum catalyst. The advantage of hydrosilylation is that no by-products are produced on curing (ref. 4).

Applications under the hood

Typical applications under the hood include cylinder-head or radiator gaskets. They require not only high heat resistance and low compression set, but also good resistance to engine oils and antifreezes. Standard silicones may expand and swell in the presence of organic solvents. In general, resistance is in direct correlation to filler content. For applications under the hood, therefore, compounds with inert additive fillers, such as quartz or diatomaceous earths, are used.

Conventional antifreezes contain, in addition to glycols, inorganic mineral acids as corrosion inhibitors. In recent years, there has been a trend towards "OAT antifreezes" with added organic acids. Another growing trend is to raise the maximum radiator temperature from 120 [degrees] C to 140 [degrees] C. Conditions are therefore becoming too hostile for the organic rubbers used until now. This was the spur for the development of a new liquid silicone rubber LR 3023/60, a product that can meet the required conditions with no completely (table 1). A welcome side effect is that the material can also be processed two to three times faster than previous materials.

Table 1 - mechanical properties of LR 3023/60 and aging
behavior after 60 days in various antifreezes

                Initial   Water   Ethy-     Gly-     MB     Dex-Cool
                mecha-             lene    santin   325.0    anti-
                 nical            glycol    G48              freeze
                proper-
                 ties

Hardness,         60        63       62       56      56        52
  durom. A
Tensile           4.5      4.6      5.3      4.3      5.5      4.6
  strength
  (N/
  [mm.sup.2])
Ultimate          300      230      250      280     300       350
  elongation
Weight change             +0.9     -3.3     +3.9    +1.8      -0.6

Test specimen: Press vulcanization, 170 [degrees] C, five minutes

Test method: Hardness (DIN 53505), tensile strength and ultimate
elongation (DIN 53504-S1)

Conditioning: Distilled water, 100 [degrees] C (with reflux), 42d;
ethylene glycol/dist. water = 50:50, 125 [degrees] C (autoclave),
42d; Glysantin G 48 (BASF AG)/dist. water 50:50, 125 [degrees] C
(autoclave), 42d; Mercedes coolant MB 325.0 (Daimler Chrysler AG)/water
= 50:50, 125 [degrees] C (autoclave), 42d; Dex-cool antifreeze (AC
Delco,)/dist. water = 50:50, 125 [degrees] C (autoclave), 42d

Applications in electrical systems

As a high-voltage lead between the distributor and spark plugs, the ignition cable plays a key role, and must therefore be comprehensively protected. It may have a core of electrically conductive silicone rubber, which helps to suppress electromagnetic interference. This obviates the need for an additional interference-suppression resistor in the spark plug. Such cables are manufactured by extruding a solid silicone rubber onto a glass or aramid fiber bundle core. The silicone rubber used is distinguished by a low volume resistivity ([is greater than or equal to] 10 [Omega] cm), low dependency of conductivity on temperature and low water absorption.

Tinned copper strands or the conductive core are sheathed with extruded insulating solid silicone rubbers, which are characterized by high dielectric strength ([is greater than or equal to] 30 kV/mm) and excellent mechanical properties.

To ensure reliable, long-term functionality, the connection between the high-voltage cable and spark plug is also made from silicone rubber. The spark plug boots are made by compression, transfer or injection molding from HTV or LSR, and offer reliable protection against water spray, moisture and dirt. They also have good electrical insulation properties, high heat resistance and no corona effects.

Electrical connectors need permanent protection against moisture. But they must also be easy to assemble. Cable bushings for multipin plugs, single wire seals and radial seals for housing seals are therefore made from self-lubricating liquid silicone rubbers. Self-lubrication is obtained by adding a methylphenylsilicone fluid to the rubber. The silicone fluid disperses homogeneously throughout the rubber, but is incompatible with it (ref. 7). After vulcanization, it diffuses to the surface of the part and forms a thin covering film. This process is highly dependent on time and temperature (figure 3).

[ILLUSTRATION OMITTED]

The fluid film permits simple, fully automated cable assembly by reducing resistance to sticking friction. It also provides lasting protection against corrosive environmental influences.

Applications in car bodies

With increasing quality awareness in the auto industry, noise in the passenger compartment must now be kept to a minimum. Examples of damping elements include exhaust hangers, catalytic converter hangers, resonance dampers and, increasingly, engine bearings (hydraulic bearings).

A significant advantage of silicone rubber is that it withstands high dynamic loads at both extremely high and extremely low temperatures without any significant changes in its damping characteristics. The loss angle, a measure of the damping, can be precisely adjusted to suit requirements by varying the mixture (table 2). Elastosil R 752/50, for example, is suitable for low-modulus damping elements with particularly low rebound resilience (35%) at a loss angle of 13 [degrees].

Table 2 - damping characteristics of selected Elastosil R
and LR silicone rubbers

Elastosil       Loss factor     Loss angle    Specific energy loss
              (% tan [Delta])   ([degrees])      (nJ/[mm.sup.3])

R 401/60 C1                        3.18
R 402/60 C1                        5.63
R 420/60                           6.63
R 4000/60                          5.76
R 4001/60          7.78            4.45       132.85 [+ or -] 7.05
R 4020/60          7.20            4.11       161.96 [+ or -] 10.10
R 4070/60          10.63           6.07       272.99 [+ or -] 13.23
LR 3003/60         5.67            3.24        102.1 [+ or -] 1.3
LR 3005/60         6.40            3.66        97.72 [+ or -] 1.63

The specific energy loss Wv, diss iS the product of [Pi], the dynamic
stress amplitude ([Sigma] A, the dynamic strain amplitude
[[Epsilon].sub.A] and sin [Delta]:

[W.sub.vdiss] = [Pi] [Sigma] A [Epsilon] A sin [Delta]

Test parameters:

Test specimen: Cylinder, height 6 mm, diameter 10 mm

Pre-load: -100N

Amplitude: [+ or -] 0.5%

Frequency: 15 Hz

Temperature: ca. 25 [degrees] C

References

(1.) P. Jerschow, KGK Kautschuk Gummi Kunststoffe, 51 (1998) 6, pp. 410-415.

(2.) A. Tomanek, "Silicones & industry, a compendium for practical use, instruction and reference" (published by Wacker-Chemie GmbH, Munich) Hanser, Munich 1990.

(3.) G. Koerner, M. Schulze and J. Weis (editors), Silicones - Chemistry and Technology, Vulkan, Essen 1991.

(4.) C. Freyer and K. Pohmer, Kunststoffberater (2000) 7/8, pp. 27-30.

(5.) K. Pohmer and H. Steinberger, Silicone Rubbers - Innovative - High Performance - Efficient, in N. Auner and J. Weis (editors) Organosilicon Chemistry IV, Wiley-VCH, Weinheim 2000, pp. 699-709.

(6.) K. Pohmer, G. Schmidt, H. Steinberger, T. Brundl and T. Schmidt, "Liquid silicone rubber (LSR) - properties - processing - applications," Kunststoffe/ plast Europe 87 (1997) 10, pp. 46-48.

(7.) K. Pohmer, GAK - Gummi Fasern Kunststoffe 53 (2000) 10, pp. 709-711.