ASTM F2038-00
(Guide)Standard Guide for Silicone Elastomers, Gels and Foams Used in Medical Applications Part I - Formulations and Uncured Materials
Standard Guide for Silicone Elastomers, Gels and Foams Used in Medical Applications Part I - Formulations and Uncured Materials
SCOPE
1.1 This guide is intended to educate potential users of silicone elastomers, gels and foams relative to their formulation and use. It does not provide information relative to silicone powders, fluids, and other silicones. The information provided is offered to guide users in the selection of appropriate materials, after consideration of the chemical, physical and toxicological properties of individual ingredients or by-products. This standard offers general information about silicone materials typically used for medical applications. Detail on the crosslinking and fabrication of silicone materials is found in Part II of this standard.
1.2 Fabrication and properties of elastomers is covered in the companion document, F604, Part II. This monograph addresses only components of uncured elastomers, gels and foams.
1.3 Silicone biocompatibility issues can be addressed at several levels, but ultimately the device manufacturer must assess biological suitability relative to intended use.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Users are also advised to refer to Material Safety Data Sheets provided with uncured silicone components.
1.5 Biological and physical properties tend to be more reproducible when materials are manufactured in accordance with accepted quality standards such as ANSI ISO 9001 and current FDA Quality System Regulations/Good Manufacturing Practice Regulations.
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Designation: F 2038 – 00
Standard Guide for
Silicone Elastomers, Gels and Foams Used in Medical
Applications Part I — Formulations and Uncured Materials
This standard is issued under the fixed designation F 2038; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 Sterility Standards:
ANSI/AAMI ST41 Good Hospital Practice: Ethylene Oxide
1.1 This guide is intended to educate potential users of
Sterilization and Sterility Assurance
silicone elastomers, gels and foams relative to their formula-
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
tion and use. It does not provide information relative to silicone
ANSI/AAMI ST29 Recommended Practice for Determin-
powders, fluids, and other silicones. The information provided
ing Ethylene Oxide in Medical Devices
is offered to guide users in the selection of appropriate
ANSI/AAM1 ST30 Determining Residual Ethylene Chlo-
materials, after consideration of the chemical, physical and
rohydrin and Ethylene Glycol in Medical Devices
toxicological properties of individual ingredients or by-
AAMI 13409-251 Sterilization of Health Care Products—
products. This standard offers general information about sili-
Radiation Sterilization—Substantiation of 25kGy as a
cone materials typically used for medical applications. Detail
Sterilization Dose for Small or Infrequent Production
on the crosslinking and fabrication of silicone materials is
Batches
found in Part II of this standard.
AAMI TIRS-251 Microbiological Methods for Gamma Ir-
1.2 Fabrication and properties of elastomers is covered in
radiation Sterilization of Medical Devices
the companion document, F 604, Part II. This monograph
2.3 Quality Standards :
addresses only components of uncured elastomers, gels and
ANSI/ASQC Q9001 Quality Systems—Model for Quality
foams.
Assurance in Design, Development Production, Installa-
1.3 Silicone biocompatibility issues can be addressed at
tion, and Servicing
several levels, but ultimately the device manufacturer must
21 CFR 820 Quality System Regulation (current revision)
assess biological suitability relative to intended use.
21 CFR 210 Current Good Manufacturing Practice in
1.4 This standard does not purport to address all of the
Manufacturing, Processing, Packing or Holding of Drugs;
safety concerns, if any, associated with its use. It is the
General (current revision)
responsibility of the user of this standard to establish appro-
21 CFR 211 Current Good Manufacturing Practice for
priate safety and health practices and determine the applica-
Finished Pharmaceuticals (current revision)
bility of regulatory limitations prior to use. Users are also
advised to refer to Material Safety Data Sheets provided with
3. Terminology
uncured silicone components.
3.1 Additional pertinent definitions can be found in standard
1.5 Biological and physical properties tend to be more
D 1566.
reproducible when materials are manufactured in accordance
3.2 Definitions:
with accepted quality standards such as ANSI ISO 9001 and
3.2.1 silicone polymer—polymer chains having a backbone
current FDA Quality System Regulations/Good Manufacturing
consisting of repeating silicon-oxygen atoms where each
Practice Regulations.
silicon atom bears two organic groups. The organic groups are
2. Referenced Documents typically methyl, but can be vinyl, phenyl, fluorine, or other
organic groups.
2.1 ASTM Standards:
3.2.2 cyclics and linears—low molecular weight volatile
D 1566 Standard Terminology Relating to Rubber
cyclic siloxane species are referred to using the “D” nomen-
F 813 Standard Practice for Direct Contact Cell Culture
clature which designates the number of Si-O linkages in the
Evaluation of Materials for Medical Devices
material (usually D -D ); species from D to D (or more)
4 20 7 40
may be called “macrocyclics”. Linears are straight chain
oligomers that may be volatile or of higher molecular weight,
This specification is under the jurisdiction of ASTM Committee F04 on
Medical and Surgical Materials and Devicesand is the direct responsibility of
Subcommittee F04.11on Polymeric Materials. Available from the American National Standards Institute, 11 West 42nd St.,
Current edition approved June 10, 2000. Published September 2000. 13th Floor, New York NY 10036.
2 5
Annual Book of ASTM Standards, Vol. 09.01 Available from the Superintendent of Documents, U.S. government Printing
Annual Book of ASTM Standards, Vol. 13.01 Office, Washington, DC 20402.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
F 2038
depending on chain length; they are designated by “M” and or relatively low levels of reinforcement beyond that provided
“D” combinations, where “M” is R Si-O, and D is as explained by the crosslinked polymer. They are usually two-part formu-
above; “R” is usually methyl. (For example, MDM is (CH ) lations utilizing a platinum catalyzed addition cure system. The
3SiOSiOSi(CH ) ). Low molecular weight species are present hardness of the gel can be adjusted within wide limits. The
3 3
in silicone components to varying degrees depending on material is not usually designed to bear heavy loads but rather
process and storage. The levels of macrocyclics that can be to conform to an irregular surface providing intimate contact.
removed from silicone polymers by vacuum, high temperature As a result, loads are distributed over a wider area. These
stripping, or oven post-cure is dependent on the conditions materials may also be used to provide protection from envi-
used. ronmental contaminants.
3.2.3 catalyst—a component of a silicone elastomer formu-
3.2.10.5 foams—are crosslinked materials which have a
lation that initiates the crosslinking reaction when the material component added to them that generates a volatile gas as the
is vulcanized. material is being vulcanized. This results in a material with a
very low density. These are usually two-part formulations
3.2.4 crosslinker or crosslinking agent—a component of a
utilizing a platinum catalyzed addition cure system. They
silicone elastomer that is a reactant in the crosslinking reaction
conform to an irregular surface as they expand to provide
that occurs when an elastomer is vulcanized.
intimate contact and protection from the environment but are
3.2.5 inhibitor—a component of a silicone elastomer added
more rigid and provide more strength than gels. Since foams
to moderate the rate of the crosslinking reaction.
are expanded elastomers, on a weight basis they are highly
3.2.6 filler—a finely divided solid that is intimately mixed
crosslinked relative to gels. Most cure conditions will result in
with silicone polymers during manufacture to achieve specific
a closed cell foam.
properties. The fillers used in silicone elastomers are one of
3.2.11 lot or batch—a quantity of material made with a
two types:
fixed, specified formulation in a single, manufacturing run
3.2.6.1 reinforcing fillers— usually have high surface areas
carried out under specific processing techniques and condi-
and are amorphous in nature such as fumed or precipitated
tions.
silica. Such fillers impart high strength and elastomeric physi-
3.2.12 vulcanization—an irreversible process in which co-
cal properties to the elastomer.
valent chemical bonds are formed between silicone polymer
3.2.6.2 extending fillers— typically have lower surface area
chains. During vulcanization, the material changes from a
and lower cost than reinforcing fillers. They include crystalline
flowable or moldable compound to an elastomeric material
forms of silica and diatomaceous earths. While they provide
which cannot be reshaped except by its physical destruction.
some reinforcement, because they are relatively inexpensive,
3.2.13 types of cure—based upon the cure chemistry em-
they are used primarily to extend the bulk of the silicone.
ployed, silicone elastomers used in medical applications fall
3.2.7 additives—a component of a silicone elastomer used
into one of three categories: condensation cure, peroxide cure,
in relatively small amounts to perform functions such as
and addition cure.
marking, coloring, or providing opacity to the elastomer.
3.2.13.1 condensation cure—these materials liberate an or-
3.2.8 silicone base—a uniformly blended mixture of sili-
ganic leaving group during curing and are normally catalyzed
cone polymers, fillers, and additives which does not contain
by an organometallic compound.
crosslinkers or catalyst.
one-part—material supplied ready to use in an air tight
3.2.9 uncured elastomer—a silicone base which contains
container which cures upon exposure to atmospheric moisture.
crosslinker and/or catalyst but has not been vulcanized.
The material cures from the surface down and cure depths of
3.2.10 silicone elastomer—an uncured elastomer that has
greater than about 0.25 inches (0.635 cm) are not practical.
been subjected to conditions which cause it to become
two-part—material supplied in two separate containers
crosslinked. Elastomers may be either high consistency rub-
which must be intimately mixed in the prescribed proportions
bers, low consistency rubbers, or RTVs. (see below)
shortly before use. Because they do not rely upon dispersion of
3.2.10.1 high consistency rubbers (HCRS)—are materials
atmospheric moisture into the silicone, the cure depth is not
which cannot be pumped by conventional pumping equipment.
limited.
They normally must be processed using high shear equipment
3.2.13.2 peroxide cure—one-part formulations vulcanized
such as a two-roll mill and parts are typically fabricated using
by free radicals generated by the decomposition of an organic
compression or transfer molding techniques.
peroxide.
3.2.10.2 low consistency rubbers or liquid silicone rubbers
3.2.13.3 addition cure—two-part elastomers which must
(LSRS)—are normally flowable materials which can be readily
first be mixed together and then cure by addition of a
pumped. They can be mixed by pumping through static mixers
silylhydride to a vinyl silane in the presence of a platinum
and parts can be fabricated using injection molding techniques.
catalyst.
3.2.10.3 RTVs (room temperature vulcanization)— are one-
3.2.14 dispersion—an uncured silicone elastomer dispersed
part elastomers which cure in the presence of atmospheric
in a suitable solvent to allow application of a thin layer of
moisture. Little, if any, acceleration of cure rate is realized by
elastomer to a substrate by either dipping or spraying.
increasing temperature. Because cure is dependent upon diffu-
sion of water into the elastomer, cure in depths greater than
4. Significance and Use
0.25 inches (0.635 cm) is not recommended.
3.2.10.4 gels—are lightly crosslinked materials having no 4.1 This standard is intended to provide guidance for the
F 2038
specification and selection of silicone materials for medical
device applications.
4.2 Silicone manufacturers supplying materials to the medi-
cal device industry should readily provide information regard-
ing non-proprietary product formulation to their customers
either directly, or through the US FDA master file program.
5. Formulation
5.1 Elastomers, gels, and foams shall be manufactured using
FIG. 2
formulations containing combinations of the following raw
materials.
5.1.1 silicone polymer—any polymer of medium or high
metric excess of SiH groups over the amount of unsaturated
molecular weight of the structure shown in Fig. 1 where R is a alkyl groups when the 2 components (parts) of the addition
methyl, an unsaturated alkyl group or a hydroxy group, R is
cure silicone elastomer are mixed together in the manufactur-
generally a methyl or an unsaturated alkyl group but may also er’s recommended ratio.
be a phenyl, trifuoropropyl, or other hydrocarbon radical, and
5.1.3.2 One-part RTVs and two-part addition cure
x and y are integers greater than or equal to zero. At least 2.0 formulations—the crosslinker may be an organosilane mono-
alkenyl groups must exist per chain if R is not a hydroxy group.
mer of the general formula:
5.1.2 catalyst—an organometallic complex of platinum or
R Si OR’ (1)
~ !
x 42x
tin bonded to ligands made of any suitable combination of
elements such as carbon, hydrogen, oxygen, fluorine and where:
R = organic group excluding phenyl
silicon.
OR’ = hydrolyzable group such as alkoxy, acetoxy, ke-
5.1.2.1 platinum—this catalyst may be dispersed in a sili-
toximo, etc.
cone polymer of the structure shown in Fig. 1 having a
5.1.3.3 Peroxide vulcanized elastomers— organic peroxides
viscosity low enough that the resulting dispersion is easily
comprise a third type of crosslinking agent which participates
pourable. Platinum catalysts can be used in the range of 5 to 20
in the crosslinking reaction that does not become directly
ppm of active platinum but typically are present at about 7.5
incorporated into the crosslinked network. Peroxide levels
ppm.
range from less than a percent to as high as a couple of weight
5.1.2.2 tin—one-part condensation cure formulations will
percent in the total formulation. These peroxides decompose, at
typically contain from 0.1 to 0.5 wt percent of an organotin
a rate which is dependent upon the temperature, to form
compound. Two-part condensation cure formulations will typi-
radicals which then abstract hydrogen atoms from some of the
cally contain from 0.5 to 2.0 weight percent organotin com-
alkyl groups attached to the silicone backbone. Recombination
pound. The ligands attached to tin will be some combination of
of these radicals results in the formation of a crosslinked
alkyl groups, alkoxy groups, or the anions of a carboxylic acid.
silicone network. One commonly used peroxide is 2,4,-
5.1.3 Crosslinker or crosslinking agent—
dichlorobenzoyl peroxide. Decomposition of this peroxide
5.1.3.1 Two-part, addition cure formulation—the
results in the formation of small amounts of polychlorinated
crosslinker is a polymer of the structure shown in Fig. 2 where
biphenyls and other catalyst decomposition by-products which
R is generally a methyl or a hydrogen group such as to provide
must be, and are, removed from the cured elastomer during
at least 2.0 SiH groups per chain and x and y are integers
post-curing.
greater than or equal to zero. In order to avoid chain extension,
5.1.4 Filler—a high purity amorphous silica commercially
the functionality of either the vinyl-containing polymer or the
known as fumed or precipitated silica. This silica can be treated
SiH-containing crosslinker must be at lea
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