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An overview of silicone rubber.

After Dow Corning's commercialization of silicone rubber in 1942, we have seen the material improved both physically and economically to the point that it can now compete in traditional organic markets and imparts longer life to today's applications demanding greater reliability.

Silicone rubber is like a chameleon in nature. It may be seen, or it may be hidden within its natural surroundings. Silicone is used in just about every imaginable environment. And like the chameleon, it ensures its existence due to its unique chemical make-up.

Various forms of silicone physically touch our everyday life, from shampoo, surfactants in instant coffee, pharmaceutical tubing and automotive gaskets to fishing lures.

There is no other synthetic polymer available that can perform under such an extreme temperature range and retain its flexibility in application. Silicone's ability to be transparent physically is matched by its transparency in our everyday use of it. Silicone rubber's application is only hindered by the limits of one's imagination.

This article will provide a general overview of silicone rubber that may inspire one to change an existing application or create a new one using this versatile elastomer. We have gleaned and assimilated this information from a number of resources to provide germane information necessary to determine if silicone robber is a viable solution to meet current and future needs.

Terminology of silicones

Let's first look at some simple terminology regarding the metal from which the silicone is spawned and the various terms associated with silicone rubber.

* Silicon, Si (the metal);

* silicone rubber (general term);

* silicone gum (pure polymer);

* silicone R-gum or base (contains polymer and fillers for reinforcement);

* silicone compound (heat cured rubber [HCR], ready for manufacturing);

* liquid silicone rubber (LIM, LSR, typically a two to three part system for automated handling).

Silicone structure versus organics

Organics, i.e., NBR, SBR, IR and NR, are polymeric chains with carbon to carbon bonding that may have unsaturation present within the polymer chain. Silicones (figure 1) are formed via repeated alternating silicon to oxygen atoms and contain no unsaturation in their backbone. Unsaturation or a double bond in a organic polymer backbone (figure 2) is an area of chemical activity where vulcanization can occur. At the same time, this area is prone to degradation by UV, ozone, corona and heat.

[FIGURES 1-2 OMITTED]

Synthesis of silicone gum

The synthesis of silicone rubber is quite amazing due to the fact that one of its precursors is silicon metal. The process is briefly described below:

* Sand or silica is reduced to the elemental form of silicon, Si;

* Si is mechanically ground and reacted with methyl chloride in the presence of Cu at 300[degrees]C;

* this results in a combination of methyl chlorosilanes (mono, di and tri);

* dimethylchlorosilanes are separated out via distillation;

* dimethylchlorosilanes are hydrolyzed to form silanols that rapidly condense to form linear siloxanes and cyclics;

* these linear siloxanes are exposed to KOH to form cyclic dimethyl tetramer (D4);

* D4 is polymerized in the presence of a strong base like KOH where the ring opens for chain formation, and chain stoppers are used to terminate the process.

The stronger silicon to oxygen bond strength and the completely saturated backbone and the peroxide curing are the key to providing silicone rubber its resistance to heat and weathering over organics. The silicon-oxygen bond energy is 88-117 kcal/mole versus the typical carbon-carbon bond energy of 83-85 kcal/mole. In addition to the higher bond energy, the larger silicon atom versus the carbon also provides greater free space, which assists in the lower glass transition and greater permeability as opposed to its organic counterparts. The gas permeability may be an advantage or disadvantage, depending on the application.

Vulcanization of silicones

Silicones are traditionally cured with various peroxides in order to optimize their high temperature capabilities. Silicones containing vinyl groups can be cured with sulfur; however, heat stability is jeopardized due to the weak heat-sensitive external sulfur bridges.

Platinum cure systems are employed also and provide other desirable properties over the peroxide curing, including:

* Lower volatility;

* tighter surface cure; and

* ultra fast curing in any media.

Platinum cure systems tend to be a little less heat stable than the traditional peroxide cure equivalents.

Table 1 shows typical peroxides used in the fabrication of silicone rubber. Unlike the sulfur and sulfur donor systems used in traditional organic compounding, peroxides join the polymer chains by joining the pendant groups themselves. There are no weak external sulfur/sulfur complexes bridging the polymers together. Silicone polymers are bridged by hydrogen abstraction and the creation of a strong carbon to carbon bond that is much more heat stable than sulfur/sulfur donor systems.

Types of silicone rubber

Silicone rubber contains various pendant groups attached to the silicon atom to impart various properties, including:

* Methyl silicone rubber (original material commercial product);

* methyl vinyl silicone rubber (general purpose, good compression set);

* phenyl methyl vinyl silicone rubber (low temperature, heat and radiation resistance);

* trifluoropropyl methyl vinyl silicone rubber (combined chemical and temperature resistance range of +375 to -80[degrees]F).

The ASTM D-1418 abbreviations used are:

* Q--class is for silicone polymers and shall be preceded by substituent groups attached to the polymer chain;

* MQ--silicone elastomers having only methyl-containing groups on the polymer chain;

* PVMQ--silicones containing phenyl, vinyl and methyl groups;

* VMQ--silicones containing both vinyl and methyl groups;

* FVMQ--silicones containing fluorine, vinyl and methyl containing groups; and

* PMQ--silicones containing phenyl and methyl groups attached to the polymer chain.

For ASTM D-2000 type and class designations, the first letter designates a temperature range and the second letter indicates a swell resistance to ASTM #3 Oil:

* FC--high strength silicone rubber;

* FE--high strength and higher heat;

* FK--fluorinated silicone rubber; and

* GE--general purpose high temperature silicone rubber.

General properties of silicone rubber

The most amazing characteristic of silicone rubber is its ability to remain flexible over a broad range of temperatures for extended periods of time. Silicone rubber can withstand extreme temperature ranges and maintain its stress-strain properties that exceed that of its synthetic counterparts: -100[degrees] to 315[degrees]C (-150[degrees] to 600[degrees]F).

Table 2 contains a range of general physical properties. The following are other unique characteristics that are associated with this versatile elastomer:

* Radiation resistance--sterilization dosages have a negligible effect;

* vibration resistance--virtually constant transmissibility and resonant frequency from -50 to 65[degrees]C;

* permeability to gases is greater than other polymers;

* dielectric Strength, 500 volts per mil;

* conductivity, < .1 to 15 ohm-cm;

* release or resistance to adhesion;

* thermal ablative, 9,000[degrees]F for minutes.

* when properly compounded, very little out-gasing when measured at [10.sup.-6] torr;

* easily formulated for FDA compliance in food contact application;

* flame resistance to HF and VO specs;

* can be made odorless and tasteless;

* water resistant;

* non-toxic; and

* surgical implants--physiological inertness.

Silicones can be easily compounded and colored to meet a customer's expectations and aesthetic requirements.

At elevated temperatures silicone rubber loses physical strength at a much slower rate than its organic counterparts. At high temperatures, the stress-strain properties can equal those of organics in some instances.

Organics are much more plastic, and upon exposure to temperature they soften and lose their higher ambient stress-strain values. For example, silicone rubber at 150[degrees]C retains up to 75% of its physical strength. Taking a natural rubber compound with a tensile of about 3,000 psi and testing it at the same temperature, we observe that it retains only 15 to 20% of its original value, 600 psi max. You might say that heat is a great equalizer.

Table 3 provides a rough idea of silicone's superiority to other elastomers.

Table 4 demonstrates the elastomer's superiority in its ability to retain its size and shape as a seal at low temperatures.

Albeit the cure systems employed in the vulcanization of silicone are very simple and straightforward, the fabrication method and the final specification dictate which peroxide is most suited for attaining the optimum results in a given application. Questions that must be asked before selecting the right peroxide or peroxides are:

* What type of fabrication is to be employed?

* Will the pH of the fillers or ingredients affect the peroxide?

* Are there metal oxides or other ingredients in the formulation that reduce the efficiency of the peroxide?

* Will the size of the part influence the selection of the peroxide?

* Is compression set critical?

* Is the peroxide vinyl or non-vinyl specific, and is there sufficient vinyl content in the formulation?

* Is high temperature critical?

* Is clarity of the finished product a requirement?

* Is volatility critical?

Fluorosilicones for low temperature and chemical resistance

Fluorosilicones combine both a broad range of chemical resistance along with silicone's inherent ability to withstand a broad range of temperatures. Due to the fluorination, the heat stability and low temperature resistance are diminished slightly with regard to the PVMQ and VMQ types. Other features include:

* Temperature resistance from 375[degrees]F to -80[degrees]F typically;

* durometer range of 35 to 80;

* tensile over 800 psi;

* elongations over 200%;

* wide variety of chemical resistance; and

* good compression set.

Table 5 demonstrates fluorosilicone rubber's resistance to various ASTM fuels. Table 6 compares fluorosilicone to other elastomers known for their chemical resistance, along with how the fluorination improves the performance of general purpose silicone (MQ).

Silicone compounding

A typical compound may have 5 to 12 ingredients in its formulation. Table 7 represents a general silicone formulation. Literally, you can add anything to silicone imaginable to achieve various results. The polymer itself can vary with regard to:

* Vinyl, methyl and phenyl percentages;

* plasticity or molecular weight;

* volatile content;

* polymerization method; and

* branching.

Compounding is very similar to organics. Reinforcing agents, i.e., fumed silicas, are utilized for strength. Precipitated silicas are used for economy and reinforcement selectively based on fabrication methods. Ground quartz, calcium carbonate and, in some cases, clays are used for enhancing the economies of the finished product. Mold release additives are used to facilitate removal from the mold. Acid acceptors improve compression set and stabilize the material without post curing. Desiccation of the materials can enhance consistency in processing. Other proprietary additives can be added to influence specific properties such as heat stability, dielectric strength, conductivity and adhesion. The overall compounding is simplified due to the fact that the vulcanization is carried out with peroxides and not the multiple components of a sulfur cure system.

Selection of the proper fabrication method and other considerations for competitiveness

The material is very easy to handle due to its low viscosity nature and very versatile with regard to compounding and fabrication. The various means of fabrication are:

* Continuously extruded in Ballotine, HAV (hot air vulcanization), LCM (liquid cure media) and IR (infrared);

* molding via injection, transfer and compression methods;

* wasteless/flashless transfer molding for optimum cost reductions; and

* calendering.

If a part can be extruded and spliced to meet a customer's expectations, this will provide the most economical product for the application. In general, molded parts are typically much more expensive due to the cost of the mold and its maintenance throughout the life of the part. There are some unique fabricators that have transitioned traditionally molded parts to extruded and cut and/or spliced pans by taking the attitude, "We can do that another way." At the same time, there are molders that have minimized their scrap and accelerated cure times to surpass their competition using state-of-the-art methods that employ "wasteless/flashless molding."

Pound volume costs consideration

The dry cost of the material is not the only factor when evaluating a cost reduction. The specific gravity of a material is critical in determining the actual yield of finished product and the pound volume cost of a compound.

Compare the alternative material (A) that costs $1.25 and has a 1.60 specific gravity to the current material (C) you are using that is $1.45 and has a 1.28 specific gravity. The alternative material appears to save the fabricator some money until the pound volume cost is calculated. It is seen in the following that the material is in fact more expensive:

* Alternative material A--1.60 sp.gr. x $1.25 = $2.00 pound volume cost; and

* Current material C--1.28 sp.gr, x $1.45 = $1.856 pound volume cost.

Optimizing the cost of parts

Tolerancing of the finished part

The other factors that influence the cost of a part will be the acceptable tolerances. How much are you willing to pay for higher precision tolerances, and does your application necessitate it? Does the part have to be molded or can it be extruded and finished into the desired form by secondary operations, i.e., cutting, splicing, etc. Table 8 shows the factors that need to be considered when trying to optimize the cost of your product. These RMA categories are subdivided into fixed (individual dimensions) and closure (largest dimension applies to all) dimension tolerances.

The bottom line is: In order to optimize the cost of a silicone part you need to look at multiple aspects in order to ensure that you are obtaining the most economical alternative to suit your final product. This necessitates that the fabricator and customer work hand in hand in a very open manner.

Other forms of silicone rubber

Continuously extruded closed cell or molded open cell

Besides being manufactured in a dense form, silicone rubber can also be expanded to further reduce the pound volume cost of the product. There are a variety of fabrication methods employed to achieve this, but the fabricators that understand and utilize this technology are limited. These manufacturing specialists can assist the engineer in determining what specific form of silicone sponge can enhance the product design. These manufacturers can also help educate you on cellular rubber and supply materials that are customized to meet your individual needs.

Some of the reasons to use expanded sponge profiles are:

* Lower sealing or closure pressures;

* better conformance to irregular surfaces and higher than expected engineered tolerances;

* weight reduction; and

* cost reduction, i.e., replacement of dense profile with closed cell sponge.

Pound volume cost comparisons show:

* $1.00 with a gravity of 1.25 (density of 78.0375 lbs./[ft.sup.3]) = $78.0375 per cubic foot.

* $1.00 with a gravity of .33 (density of 20.6 lbs./[ft.sup.3]) = $20.6 per cubic foot.

The general sponge classification system per ASTM D 1056 standard specification for flexible cellular materials lists types of sponges as:

* Type 1--open cell rubber; and

* Type 2--closed cell rubber (to be classified as closed cell the material cannot absorb a maximum amount of water under a given vacuum and time).

In the classes of sponge rubber, both types, 1 and 2, are divided into four separate classes. They are:

* Cellular rubbers made of natural or synthetic lubber, alone or in any combination where no oil resistance is required;

* cellular rubber made from synthetic rubber having oil resistance with low swells;

* cellular rubber made from synthetic rubber having oil resistance with medium swells; and

* cellular rubber made of synthetic rubber possessing low temperature resistance of -75 to -175[degrees]C, but not possessing oil resistance.

Each type and class is further divided into a grade that is predicated on firmness or compression-deflection as expressed in pounds per square inch (psi). The material is typically compressed by 25% (to 75% of its original height):

* Grade 0 = 0.5 to 2 psi;

* Grade 1 = 2 to 5 psi;

* Grade 2 = 5 to 9 psi;

* Grade 3 = 9 to 13 psi;

* Grade 4 = 13 to 17 psi; and

* Grade 5 = 17 to 25 psi.
Table 1--peroxides used in vulcanization of
silicone sponge or dense

Type General cure Recommended use in
 temp. [degrees]F cure media

2,4-dichlorobenzoyl 220-250 Hot air, LCM (molten
 salt), Ballotine
Benzoyl 240-280 Molding, steam, LCM,
 HAV, Ballotine
Dicumyl 320-340 Molding, LCM, HAV
t-butyl perbenzoate 290-310 LCM, molding, steam
2,5-bis(t-butyl per- 330-350 Molding, steam
 oxy) 2,5-dimethyl
 hexane

Table 2--general physical properties of silicone
rubber

Durometer ranges 10 to 90
Tensile strength Up to 1,400 psi
Elongation 100 to 1,200%
Tear resistance, Die B 275 ppi max
Bashore resilience 10 to 70
Compression set Unequaled by other elastomers
Temperature ranges -100[degrees]C to 316[degrees]C

Table 3--estimated service life of silicone
rubber

 90[degrees]C (194[degrees]F) 40 years
121[degrees]C (250[degrees]F) 10-20 years
150[degrees]C (300[degrees]F) 5-10 years
200[degrees]C (392[degrees]F) 2-5 years
250[degrees]C (482[degrees]F) 3 months
315[degrees]C (600[degrees]F) 2 weeks

Table 4--typical compression set values

Compression set @ [degrees]C 23[degrees]C -40[degrees]C

General purpose 10% 25%
 silicone
Phenyl-vinyl silicone 10% 15%

Compression set @ [degrees]C -60[degrees]C -80[degrees]C

General purpose 100% 100%
 silicone
Phenyl-vinyl silicone 40% 60%

Compression set @ [degrees]C -100[degrees]C

General purpose 100%
 silicone
Phenyl-vinyl silicone 100%

Table 5--chemical resistance of FVMQ

Fluids Immersion Durometer Volume
 conditions (at 23[degrees]C) change swell (%)

ASTM Fuel B 1 day -7 +21
 7 days -6 +19
 28 days -8 +20
ASTM Fuel C 1 day -8 +22
 7 days -8 +22

Table 6--% swell in various chemicals after
three days at 23[degrees]C

Polymer MQ FVMQ Viton B NBR CR

Benzene 175 27 12 100 290
OS-45 hydraulic fluid 80 13 3 -- --
NaOH (50%) -1 1 0 0 2
ASTM Oil #3 49 4 3 10 --
HCI (concentrated) 5 8 1 11 4

Table 7--typical formulation

Ingredient phr

Silicone base 100
Fumed or precipitated silica 2-5
Ground quartz or CaC[O.sub.3] 25-100
Pigment 0.5-2.0
Heat stabilizers 0.8-2.0
Peroxides 0.8-1.4
Acid acceptors or oil resistance additives 2.0-6.0
Process aids for shelf life and green strength 0.3-2.0

Table 8--factors to consider in optimizing
product cost

 Molding tolerances Extrusion tolerances
 (0.63" to 1.00" (0.63" to 0.98")
High precision fixed) +/-.027"
Precision +/-.006" +/-.039"
Commercial +/-.010" +/-.063"
Basic +/-.016"


References

(1.) Wilfred Lynch, "'Handbook of Silicone Rubber Fabrication," Van Nostrand Reinhold, 1978 (ISBN: 0-442-24962-4). Maurice Morton, "Rubber Technology," Van Nostrand Reinhold, 1987 (ISBN: 0-442-26422-4).

(2.) Dow Corning Fabrication Manual, "Fabricating with Silastic Silicone Rubber," 1990.

(3.) Dow Corning Fabrication Manual, "Designing with Silastic Silicone Rubber," 1991.

(4.) Walter Noll, "Chemistry, and Technology of Silicone," Academic Press, 1968 (LCCCN 67-22772).

(5.) Rubber Handbook for Molder, Extruded, Lathe Cut and Cellular Products published by RMA, Inc.

(6.) American Society for Testing and Materials, Volumes 09.01 and 09.02 (c) 2000 (ISBN 0-8031-2270-5).
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Author:Hamilton, James R., II
Publication:Rubber World
Date:Jun 1, 2003
Words:3086
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