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An overview of silicone rubber.

After Dow Corning's commercialization of silicone rubber Noun 1. silicone rubber - made from silicone elastomers; retains flexibility resilience and tensile strength over a wide temperature range
synthetic rubber, rubber - any of various synthetic elastic materials whose properties resemble natural 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 Synthetic polymers are often referred to as "plastics", such as the well-known polyethylene and nylon. However, most of them can be classified in at least three main categories: thermoplastics, thermosets and elastomers.  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 elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. . We have gleaned and assimilated this information from a number of resources to provide germane ger·mane  
Being both pertinent and fitting. See Synonyms at relevant.

[Middle English germain, having the same parents, closely connected; see german2.
 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 HCR High Commissioner for Refugees (UN)
HCR Home Condition Report
HCR Health Care Reform
HCR Highway Contract Route (US Postal Service)
HCR High Consistency Rubber
HCR Human Cognitive Reliability
], ready for manufacturing);

* liquid silicone rubber (LIM, LSR 1. (networking) LSR - Label Switching Router.
2. (operating system) LSR - Local Shared Resources.
, typically a two to three part system for automated handling).

Silicone structure versus organics

Organics, i.e., NBR NBR Number
NBR Nightly Business Report (PBS show)
NBR National Business Review (New Zealand weekly business newspaper)
NBR National Bureau of Asian Research
NBR National Board of Review
, SBR SBR - Spectral Band Replication , 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 vulcanization (vŭl'kənəzā`shən), treatment of rubber to give it certain qualities, e.g., strength, elasticity, and resistance to solvents, and to render it impervious to moderate heat and cold.  can occur. At the same time, this area is prone to degradation by UV, ozone, corona and heat.


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 methyl chloride
An explosive gas, CH3Cl, used in organic synthesis and polymerization, as a refrigerant, and an anesthetic.
 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 con·dense  
v. con·densed, con·dens·ing, con·dens·es
1. To reduce the volume or compass of.

2. To make more concise; abridge or shorten.

3. Physics
 to form linear siloxanes and cyclics;

* these linear siloxanes are exposed to KOH KOH
The chemical formula for potassium hydroxide, which is used to perform the KOH test. The tests is also called a potassium hydroxide preparation.

Mentioned in: KOH Test


potassium hydroxide.
 to form cyclic dimethyl di·meth·yl  
An organic compound, especially ethane, containing two methyl groups.
 tetramer tet·ra·mer
A polymer consisting of four identical monomers.


* D4 is polymerized in the presence of a strong base like KOH where the ring opens for chain formation, and chain stoppers stoppers

see stopper pad.
 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 A carbon-carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond – a bond composed of two electrons, one from each of the two atoms.  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 fabrication (fab´rikā´shn),
n the construction or making of a restoration.
 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 phenyl (fĕn`əl), C6H5, organic free radical or alkyl group derived from benzene by removing one hydrogen atom.  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).

American Society for Testing and Materials
 D-1418 abbreviations used are:

* Q--class is for silicone polymers and shall be preceded by substituent substituent /sub·stit·u·ent/ (-stich´u-ent)
1. a substitute; especially an atom, radical, or group substituted for another in a compound.

2. of or pertaining to such an atom, radical, or group.
 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 fluorine (fl`ərēn, –rĭn), gaseous chemical element; symbol F; at. no. 9; at. wt. 18.998403; m.p. −219.6°C;; b.p. −188.14°C;; density 1. , 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 trans·mis·si·ble  
That can be transmitted: transmissible signals.

 and resonant frequency resonant frequency,
n the specific frequency at which an object vibrates.
 from -50 to 65[degrees]C;

* permeability to gases is greater than other polymers;

* dielectric Strength In physics, the term dielectric strength has the following meanings:
  • Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties.
, 500 volts per mil per mil also per mill
Per thousand.

[per + mil (short for Latin m

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

* release or resistance to adhesion;

* thermal ablative ablative (ăb`lətĭv') [Lat.,=carrying off], in Latin grammar, the case used in a number of circumstances, particularly with certain prepositions and in locating place or time. The term is also used in the grammar of some languages (e.g. , 9,000[degrees]F for minutes.

* when properly compounded, very little out-gasing when measured at [10.sup.-6] torr torr (tōr),
n a unit of pressure equivalent to 0.001316 atmosphere; named after the physicist Torricelli. Also called
mm Hg.

* easily formulated for FDA FDA
Food and Drug Administration

FDA, See Food and Drug Administration.

FDA, the abbreviation for the Food and Drug Administration.
 compliance in food contact application;

* flame resistance to HF and VO specs;

* can be made odorless o·dor·less  
Having no odor.

odor·less·ly adv.

 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 fluor·i·na·tion  
A chemical reaction that introduces fluorine into a compound.
, the heat stability and low temperature resistance are diminished slightly with regard to the PVMQ and VMQ VMQ Virtual Memory Query  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 polymerization

Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same.
 method; and

* branching.

Compounding is very similar to organics. Reinforcing agents, i.e., fumed fume  
1. Vapor, gas, or smoke, especially if irritating, harmful, or strong.

2. A strong or acrid odor.

3. A state of resentment or vexation.

 silicas, are utilized for strength. Precipitated silicas are used for economy and reinforcement selectively based on fabrication methods. Ground quartz, calcium carbonate calcium carbonate, CaCO3, white chemical compound that is the most common nonsiliceous mineral. It occurs in two crystal forms: calcite, which is hexagonal, and aragonite, which is rhombohedral.  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 des·ic·ca·tion
The process of being desiccated.

 of the materials can enhance consistency in processing. Other proprietary additives can be added to influence specific properties Specific properties of a substance are derived from other intrinsic and extrinsic properties (or intensive and extensive properties) of that substance. For example, the density of steel (a specific and intrinsic property) can be derived from measurements of the mass of a steel bar  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 A ballotine is a piece of meat, fish, or poultry that has been boned, stuffed and then rolled and tied into a bundle. It is then usually poached or braised. References , HAV HAV hepatitis A virus.

hepatitis A virus

HAV Hepatitis A virus, see there
 (hot air vulcanization), LCM (Liquid Crystal Monitor) A flat panel display that uses the liquid crystal (LCD) technology. See flat panel display.  (liquid cure media) and IR (infrared);

* molding via injection, transfer and compression methods;

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

* calendering calendering, a finishing process by which paper, plastics, rubber, or textiles are pressed into sheets and smoothed, glazed, polished, or given a moiré or embossed surface. .

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 specific gravity, ratio of the weight of a given volume of a substance to the weight of an equal volume of some reference substance, or, equivalently, the ratio of the masses of equal volumes of the two substances.  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 x $1.25 = $2.00 pound volume cost; and

* Current material C--1.28, 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 splicing /splic·ing/ (spli´sing)
1. the attachment of individual DNA molecules to each other, as in the production of chimeric genes.

2. RNA s.
, etc. Table 8 shows the factors that need to be considered when trying to optimize the cost of your product. These RMA (RealMedia Architecture) See RealMedia.  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 Noun 1. pounds per square inch - a unit of pressure

pressure unit - a unit measuring force per unit area
 (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

Table 2--general physical properties of silicone

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

 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%
Phenyl-vinyl silicone          10%            15%

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

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

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

General purpose                100%
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"


(1.) Wilfred Lynch, "'Handbook of Silicone Rubber Fabrication," Van Nostrand Reinhold, 1978 (ISBN ISBN
International Standard Book Number

ISBN International Standard Book Number

ISBN n abbr (= International Standard Book Number) → ISBN m 
: 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 Silastic /Si·las·tic/ (si-las´tik) trademark for polymeric silicone substances that have the properties of rubber but are biologically inert; used in surgical prostheses.  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 LCCCN Library of Congress Catalog Card Number  67-22772).

(5.) Rubber Handbook for Molder, Extruded, Lathe lathe (lāth), machine tool for holding and turning metal, wood, plastic, or other material against a cutting tool to form a cylindrical product or part. It also drills, bores, polishes, grinds, makes threads, and performs other operations.  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
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