ASTM F2042-00
(Guide)Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part II - Crosslinking and Fabrication
Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part II - Crosslinking and Fabrication
SCOPE
1.1 This guide is intended to educate potential users of silicone elastomers, gels and foams relative to their fabrication and processing. It does not provide information relative to silicone powders, fluids, pressure sensitive adhesives, or other types of silicone products.
1.2 The information provided is offered to guide users in the selection of appropriate processing conditions for specific medical device applications.
1.3 Formulation and selection of appropriate starting materials is covered in the companion document, F 2038 Part I. This monograph addresses only the curing, post-curing, and processing of elastomers, gels and foams as well as how the resulting product is evaluated.
1.4 Silicone biocompatibility issues can be addressed at several levels, but ultimately the device manufacturer must assess biological suitability relative to intended use. Biocompatibility testing may be done on cured elastomers prior to final fabrication, but the most relevant data are those obtained on the finished device. Data on selected lots of material are only representative when compounding, and fabrication are performed under accepted quality systems such as ISO 9001 and current Good Manufacturing Practice Regulations. Extractables analyses may also be of interest for investigation of biocompatibility, and the procedures for obtaining such data depend on the goal of the study (see F619, the HIMA Memorandum 7/14/93, and USP 23, for examples of extraction methods).
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Designation: F 2042 – 00
Standard Guide for
Silicone Elastomers, Gels, and Foams Used in Medical
Applications Part II — Crosslinking and Fabrication
This standard is issued under the fixed designation F 2042; 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 and Density of Plastics by Displacement
D 813 Test Method for Rubber Deterioration—Crack
1.1 This guide is intended to educate potential users of
Growth
silicone elastomers, gels and foams relative to their fabrication
D 814 Test Method for Rubber Property—Vapor Transmis-
and processing. It does not provide information relative to
sion of Volatile Liquids
silicone powders, fluids, pressure sensitive adhesives, or other
D 926 Test Method for Rubber Property—Plasticity and
types of silicone products.
Recovery (Parallel Plate Method)
1.2 The information provided is offered to guide users in the
D 955 Test Method of Measuring Shrinkage from Mold
selection of appropriate processing conditions for specific
Dimensions of Molded Plastics
medical device applications.
D 1349 Practice for Rubber—Standard Temperatures for
1.3 Formulation and selection of appropriate starting mate-
Testing
rials is covered in the companion document, F 2038 Part I. This
D 1566 Terminology Relating to Rubber
monograph addresses only the curing, post-curing, and pro-
D 2240 Test Method for Rubber Property—Durometer
cessing of elastomers, gels and foams as well as how the
Hardness
resulting product is evaluated.
F 619 Practice for Extraction of Medical Plastics
1.4 Silicone biocompatibility issues can be addressed at
F 719 Practice for Testing Biomaterials in Rabbits for
several levels, but ultimately the device manufacturer must
Primary Skin Irritation
assess biological suitability relative to intended use. Biocom-
F 720 Practice for Testing Guinea Pigs for Contact
patibility testing may be done on cured elastomers prior to final
Allergens—Guinea Pig Maximization Test
fabrication, but the most relevant data are those obtained on the
F 748 Practice for Selecting Generic Biological Test Meth-
finished device. Data on selected lots of material are only
ods for Materials and Devices
representative when compounding, and fabrication are per-
F 813 Practice for Direct Contact Cell Culture Evaluation of
formed under accepted quality systems such as ISO 9001 and
Materials for Medical Devices
current Good Manufacturing Practice Regulations. Extract-
F 981 Practice for Assessment of Compatibility of Bioma-
ables analyses may also be of interest for investigation of
terials for Surgical Implantation With Respect to Effect of
biocompatibility, and the procedures for obtaining such data
Materials on Muscle and Bone
depend on the goal of the study (see F 619, the HIMA
F 1905 Practice for Selecting Tests for Determining the
Memorandum 7/14/93, and USP 23, for examples of extraction
Propensity of Materials to Cause Immunotoxicity
methods).
F 1906 Practice for Evaluation of Immunological Re-
2. Referenced Documents
sponses in Biocompatibility Testing Using ELISA Tests,
Lymphocyte Proliferation, and Cell Migration
2.1 ASTM Standards:
F 1984 Practice for Testing Whole Complement Activation
D 395 Test Methods for Rubber Property—Compression
in Scrum by Solid Materials
Set
F 2038 Guide for Silicone Elastomers, Gels and Foams
D 412 Test Methods for Rubber Properties in Tension
Used in Medical applications Part I: Formulations and
D 430 Test Methods for Rubber Deterioration—Dynamic
Uncured Materials
Fatigue
2.2 Other Biocompatibility Standards:
D 624 Test Method for Tear Strength of Conventional
United States Pharmacopeia, current edition (appropriate
Vulcanized Rubber and Thermoplastic Elastomers
monographs may include: <87>, <88>, <151>, <381>)
D 792 Test Methods for Specific Gravity (Relative Density)
FDA Department of Health and Human Services General
This specification is under the jurisdiction of ASTM Committee F04 on
Medical and Surgical Materials and Devices and is the direct responsibility of Annual Book of ASTM Standards, Vol 08.01.
Subcommittee F4.11 on Polymeric Materials. Annual Book of ASTM Standards, Vol 13.01.
Current edition approved July 10, 2000. Published October 2000. Available from United States Pharmacopeia 12601 Twinbrook Parkway Rock-
Annual Book of ASTM Standards, Vol 09.01. ville, MD 20852.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
F 2042
Program Memorandum #G95–1, May 1, 1995: Use of istics of the uncured elastomer, and the methods used to
International Standard ISO-10993, Biological Evaluation fabricate the elastomers. Additional pertinent terms are defined
of Medical Devices Part I: Evaluation and Testing in standard D 1566.
ANSI/AAMI 10993–1 Biological Evaluation of Medical
3.2 Definitions:
Devices, Part I: Guidance on Selection of Tests
3.2.1 manufacture—the process which occurs in the suppli-
HIMA Memorandum Guidance for Manufacturers of Sili-
er’s facility in which the various components of the elastomer
cone Devices Affected by Withdrawal of Dow Corning
are brought together, allowed to interact, and are packaged to
Silastic Materials, 7/14/93
provide the uncured elastomer for sale.
2.3 Sterilization Standards:
3.2.2 fabrication—the process by which the uncured elas-
ANSI/AAMI ST46 Good Hospital Practice: Steam Steril-
tomer is converted into a fully vulcanized elastomer of the
ization and Sterility Assurance
desired size and shape. This process may occur in the same
ANSI/AAMI ST41 Good Hospital Practice: Ethylene Oxide
facility as the manufacture of the uncured elastomer but is
Sterilization and Sterility Assurance
more typically performed at the facility of a customer of the
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
silicone manufacturer.
ANSI/AAMI ST29 Recommended Practice for Determin-
3.2.2.1 injection molding—fabrication of elastomers into
ing Ethylene Oxide in Medical Devices
forms defined by molds constructed so that the uncured
ANSI/AAMI ST30 Determining Residual Ethylene Chloro-
elastomer can be transferred by pumping into the closed mold.
hydrin and Ethylene Glycol in Medical Devices
This method requires venting of the mold in some manner. The
AAMI 13409–251 Sterilization of Health Care Products—
elastomer may be vulcanized by heating the mold after it is
Radiation Sterilization—Substantiation of 25kGy as a
filled but more typically the molding conditions (temperature
Sterilization Dose for Small or Infrequent Production
and filling rate) are adjusted so that uncured elastomer can be
Batches
added to a pre-heated mold in which it will then cure. The mold
AAMI TIR8–251 Microbiological Methods for Gamma Ir-
is than opened and the part removed and post-cured, if
radiation Sterilization of Medical Devices
necessary.
2.4 Quality Standards:
3.2.2.2 compression molding—a process in which the un-
ANSI/ASQC Q9001 Quality Systems—Model for Quality
cured elastomer is placed in an open mold. The mold is closed
Assurance in Design, Development, Production, Installa-
and pressure applied to the mold to fill the cavity. Heat is
tion and Servicing
applied to vulcanize the elstomer, the mold is than opened and
21 CFR 820 Quality System Regulation (current revision)
the fabricated part is removed.
21 CFR 210 Current Good Manufacturing Practice in
3.2.2.3 freshening—because of the interaction that can oc-
Manufacturing, Processing, Packing or Holding of Drugs:
cur between the fumed silica and silicone polymers, thick
General (current revision)
uncured high consistency elastomers can become so stiff over
21 CFR 211 Current Good Manufacturing Practice for
time that they are very difficult to process. To overcome this
Finished Pharmaceuticals (current revision)
problem, a 2–roll mill is used to disrupt this interaction,
2.5 Other Standards:
resulting in a material which is easier to fabricate. This process
Dow Corning CTM 0155 (Gel-Like Materials With Modi-
is called freshening and is typically done immediately before
fied Penetrometer)
catalyzation.
Dow Corning CTM 0813 (Gel-Like Materials With One
3.2.2.4 transfer molding—a process in which the mixed,
Inch Diameter Head Penetrometer)
uncured elastomer is placed in a compartment connected to the
PCB Test Methods such as those used for MRI project No.
11 mold. The compartment is then closed, pressure is applied to
4473, 1/24/97,
transfer the uncured elastomer to the mold, filling the cavity.
Biological Performance of Materials: J. Black, Marcel De-
Heat and pressure are applied to the mold to vulcanize the
kker, NY 1992
elastomer, the mold is then opened, and the fabricated part is
removed.
3. Terminology
3.2.2.5 extrusion—a continuous process in which the
3.1 The classification of silicone elastomers is based upon a
mixed, uncured elastomer is forced through an orifice having
number of interrelated factors which include the chemical
the desired cross-sectional profile. The elastomer is then
system used to crosslink the elastomer, the physical character-
vulcanized by passing it through either a hot air or radiant heat
oven. The most common application of extrusion processing is
6 the fabrication of tubing but it can be used to produce other
Available from Food and Drug Administration HFI-40, Rockville, MD 20857.
Available from American National Standards Institute, 11 West 42nd Street, items as well.
13th Floor, New York, NY 10036.
3.2.2.6 post-cure—the process of subjecting a vulcanized
Available from Advanced Medical Technology Association, 1200 G St. N.W.
elastomer to elevated temperature, usually in a hot-air oven,
Suite 400 Washington, D.C. 20005–3814.
after its initial fabrication. This process step is done to
Available from Association for the Advancement of Medical Devices, 1110
North Glebe Rd., Suite 220, Arlington, VA 22201–4795.
complete cross-linking of the object, remove peroxide by-
Available from Standardization Documents Order Desk, Bldg. 4 Section D,
products, and eliminate changes in its physical properties.
700 Robins Ave., Philadelphia PA 19111–5094, Attn: NPODS
Post-cure is often necessary when the component is only
Available from Midwest Research Institute, 425 Volker Blvd., Kansas City,
MO 64110–2299 partially cross-liked by molding; it is performed in an attempt
F 2042
to accelerate molding process, and increase its output. specification and selection of fabrication methods for silicones
3.2.2.7 calendaring—the process of forming an uncured, used in medical devices. It also provides guidance relative to
mixed elastomer into a thin sheet or film by passing it between testing that might be done to qualify lots of acceptable
two rolls. material, based on desired performance properties.
3.2.2.8 dispersion—the process of placing an uncured elas- 4.2 Silicone manufacturers supplying material to the medi-
tomer in a solvent. This lowers the viscosity of the material and cal device industry should readily provide information regard-
is usually done to allow the fabrication of thinner films that can ing non-proprietary product formulation to their customers
be obtained by calendaring or to form coatings. Following either directly or through the US FDA Master File program.
dispersion use, the solvent must be removed either before or
5. Crosslinking Chemistry
during the vulcanization process. Care must be taken to assure
5.1 Silicone elastomers used in medical applications are
that the solvent is compatible with the elastomer, to prevent
typically crosslinked by one of three commonly used cure
preferential settling of the components of the formulation by
systems. These involve the platinum catalyzed addition of a
excessive dilution of the elastomer.
silylhydride to an unsaturated site, the generation of free
3.2.3 one-part elastomer—an elastomer supplied in the
radicals by a peroxide or the reaction of an easily hydrolyzable
uncured form in one package containing all of the formulation
group of silicon.
components. It does not require mixing before fabrication.
5.1.1 addition cure—this cure system utilizes the addition
3.2.4 two-part elastomer—an elastomer supplied in two
of a silylhydride to a site of unsaturation, usually a vinyl group.
packages which must be mixed in specified proportions before
As shown in Fig. 1, this reaction is catalyzed by a platinum
fabrication.
complex. The catalyst will be present at a level such that the
3.2.5 liquid silicone rubber or low consistency silicone
concentration of platinum is in the range of 5 to 20 ppm but is
rubber (LSR)—an elastomer having a viscosity such that it can
more typically present at a level of about 7.5 ppm. When
be moved or transferred by readily available pumping equip-
multiple silylhydrides are present in the same molecule, for
ment. LSRs are typically used in injection molding operations.
example, in a crosslinker molecule, and they react with vinyl
3.2.6 high consistency rubber (HCR)—an elastomer having
groups attached to a silicon in a silicone polymer, a crosslinked
a viscosity such that it cannot be moved or transferred by
network results.
readily available pumping equipment. These elastomers are
Elastomers using this cure system are two-part elastomers
fabricated using high shear equipment such as a two-roll mill
and are utilized in both LSRs and HCRs. In practice, the
and cannot be injection molded. They are typically used in
platinum catalyst, an inhibitor, and vinyl functionality on the
compression or transfer molding and extrusion processes.
silicone backbone are present in one part of the formulation
3.2.7 RTV (room temperature vulcanization)—a one-part
and the crosslinker in the presence of vinyl functionality on the
elastomer which cures in the presence of atmospheric moisture.
silicone backbone is present in the other. These two parts are
Little, if any, acceleration of cure rate is realized by increasing
intimately mixed shortly before they are intended to be used.
temperature. Because cure is dependent upon diffusion of
At room temperature a certain amount of working time (time
water into the elastomer, cure in depths of greater than 0.64 cm
before the crosslink network builds to unacceptable levels) is
is not recommended.
provided to allow time to fabricate the silicone part. Heat is
3.2.8 gel—a lightly crosslinked material having no or rela-
then applied to activate the platinum, the crosslinking reaction
tively low levels of reinforcement beyond that provided by the
occurs, and t
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