ASTM F2042-00(2005)
(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
SIGNIFICANCE AND USE
This guide is intended to provide guidance for the specification and selection of fabrication methods for silicones used in medical devices. It also provides guidance relative to testing that might be done to qualify lots of acceptable material, based on desired performance properties.
Silicone manufacturers supplying material to the medical device industry should readily provide information regarding non-proprietary product formulation to their customers either directly or through the US FDA Master File program.
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).
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F2042 – 00 (Reapproved 2005)
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 F2042; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope plastic Elastomers—Tension
D430 Test Methods for Rubber Deterioration—Dynamic
1.1 This guide is intended to educate potential users of
Fatigue
silicone elastomers, gels and foams relative to their fabrication
D624 Test Method for Tear Strength of Conventional Vul-
and processing. It does not provide information relative to
canized Rubber and Thermoplastic Elastomers
silicone powders, fluids, pressure sensitive adhesives, or other
D792 Test Methods for Density and Specific Gravity (Rela-
types of silicone products.
tive Density) of Plastics by Displacement
1.2 Theinformationprovidedisofferedtoguideusersinthe
D813 Test Method for Rubber Deterioration—Crack
selection of appropriate processing conditions for specific
Growth
medical device applications.
D814 Test Method for Rubber Property—Vapor Transmis-
1.3 Formulation and selection of appropriate starting mate-
sion of Volatile Liquids
rials is covered in the companion document, F2038 Part I.This
D926 Test Method for Rubber Property—Plasticity and
monograph addresses only the curing, post-curing, and pro-
Recovery (Parallel Plate Method)
cessing of elastomers, gels and foams as well as how the
D955 Test Method of Measuring Shrinkage from Mold
resulting product is evaluated.
Dimensions of Thermoplastics
1.4 Silicone biocompatibility issues can be addressed at
D1349 Practice for Rubber—Standard Temperatures for
several levels, but ultimately the device manufacturer must
Testing
assess biological suitability relative to intended use. Biocom-
D1566 Terminology Relating to Rubber
patibilitytestingmaybedoneoncuredelastomerspriortofinal
D2240 Test Method for Rubber Property—Durometer
fabrication,butthemostrelevantdataarethoseobtainedonthe
Hardness
finished device. Data on selected lots of material are only
F619 Practice for Extraction of Medical Plastics
representative when compounding, and fabrication are per-
F719 Practice for Testing Biomaterials in Rabbits for Pri-
formed under accepted quality systems such as ISO 9001 and
mary Skin Irritation
current Good Manufacturing Practice Regulations. Extract-
F720 Practice for Testing Guinea Pigs for Contact Aller-
ables analyses may also be of interest for investigation of
gens: Guinea Pig Maximization Test
biocompatibility, and the procedures for obtaining such data
F748 Practice for Selecting Generic Biological Test Meth-
depend on the goal of the study (see F619, the HIMA
ods for Materials and Devices
Memorandum 7/14/93, and USP23, for examples of extraction
F813 Practice for Direct Contact Cell Culture Evaluation of
methods).
Materials for Medical Devices
2. Referenced Documents
F981 Practice forAssessment of Compatibility of Biomate-
rials for Surgical Implants with Respect to Effect of
2.1 ASTM Standards:
Materials on Muscle and Bone
D395 TestMethodsforRubberProperty—CompressionSet
F1905 Practice For Selecting Tests for Determining the
D412 Test Methods for Vulcanized Rubber and Thermo-
Propensity of Materials to Cause Immunotoxicity
F1906 Practice for Evaluation of Immune Responses In
This guide is under the jurisdiction of ASTM Committee F04 on Medical and
Biocompatibility Testing Using ELISATests, Lymphocyte
Surgical Materials and Devices and is the direct responsibility of Subcommittee
Proliferation, and Cell Migration
F4.11 on Polymeric Materials.
F1984 Practice for Testing for Whole Complement Activa-
Current edition approved Mar. 1, 2005. Published March 2005. Originally
´1
tion in Serum by Solid Materials
approved in 2000. Last previous edition approved in 2000 as F2042 – 00 . DOI:
10.1520/F2042-00R05.
F2038 Guide for Silicone Elastomers, Gels, and Foams
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Used in Medical Applications Part I—Formulations and
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Uncured Materials
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F2042 – 00 (2005)
2.2 Other Biocompatibility Standards: Biological Performance of Materials: J. Black, Marcel De-
United States Pharmacopeia, current edition (appropriate kker, NY 1992
monographs may include: <87>, <88>, <151>, <381>)
FDA Department of Health and Human Services General 3. Terminology
Program Memorandum #G95–1, May 1, 1995: Use of
3.1 The classification of silicone elastomers is based upon a
International Standard ISO-10993, Biological Evaluation
number of interrelated factors which include the chemical
of Medical Devices Part I: Evaluation and Testing
system used to crosslink the elastomer, the physical character-
ANSI/AAMI 10993–1 Biological Evaluation of Medical
istics of the uncured elastomer, and the methods used to
Devices, Part I: Guidance on Selection of Tests
fabricate the elastomers.Additional pertinent terms are defined
HIMA Memorandum Guidance for Manufacturers of Sili-
in standard D1566.
cone Devices Affected by Withdrawal of Dow Corning
3.2 Definitions:
Silastic Materials, 7/14/93
3.2.1 manufacture—the process which occurs in the suppli-
2.3 Sterilization Standards:
er’s facility in which the various components of the elastomer
ANSI/AAMI ST46 Good Hospital Practice: Steam Steril-
are brought together, allowed to interact, and are packaged to
ization and Sterility Assurance
provide the uncured elastomer for sale.
ANSI/AAMIST41 GoodHospitalPractice:EthyleneOxide
3.2.2 fabrication—the process by which the uncured elas-
Sterilization and Sterility Assurance
tomer is converted into a fully vulcanized elastomer of the
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
desired size and shape. This process may occur in the same
ANSI/AAMI ST29 Recommended Practice for Determin-
facility as the manufacture of the uncured elastomer but is
ing Ethylene Oxide in Medical Devices
more typically performed at the facility of a customer of the
ANSI/AAMI ST30 Determining Residual Ethylene Chloro-
silicone manufacturer.
hydrin and Ethylene Glycol in Medical Devices
3.2.2.1 injection molding—fabrication of elastomers into
AAMI 13409–251 Sterilization of Health Care Products—
forms defined by molds constructed so that the uncured
Radiation Sterilization—Substantiation of 25kGy as a
elastomer can be transferred by pumping into the closed mold.
Sterilization Dose for Small or Infrequent Production
This method requires venting of the mold in some manner.The
Batches
elastomer may be vulcanized by heating the mold after it is
AAMI TIR8–251 Microbiological Methods for Gamma Ir-
filled but more typically the molding conditions (temperature
radiation Sterilization of Medical Devices
and filling rate) are adjusted so that uncured elastomer can be
2.4 Quality Standards:
addedtoapre-heatedmoldinwhichitwillthencure.Themold
ANSI/ASQC Q9001 Quality Systems—Model for Quality
is than opened and the part removed and post-cured, if
Assurance in Design, Development, Production, Installa-
necessary.
tion and Servicing
3.2.2.2 compression molding—a process in which the un-
21 CFR 820 Quality System Regulation (current revision)
cured elastomer is placed in an open mold. The mold is closed
21 CFR 210 Current Good Manufacturing Practice in
and pressure applied to the mold to fill the cavity. Heat is
Manufacturing, Processing, Packing or Holding of Drugs:
applied to vulcanize the elstomer, the mold is than opened and
General (current revision)
the fabricated part is removed.
21 CFR 211 Current Good Manufacturing Practice for
8 3.2.2.3 freshening—because of the interaction that can oc-
Finished Pharmaceuticals (current revision)
cur between the fumed silica and silicone polymers, thick
2.5 Other Standards:
uncured high consistency elastomers can become so stiff over
Dow Corning CTM 0155 (Gel-Like Materials With Modi-
time that they are very difficult to process. To overcome this
fied Penetrometer)
problem, a two–roll mill is used to disrupt this interaction,
Dow Corning CTM 0813 (Gel-Like Materials With One
resulting in a material which is easier to fabricate.This process
Inch Diameter Head Penetrometer)
is called freshening and is typically done immediately before
PCB Test Methods such as those used for MRI project No.
catalyzation.
4473, 1/24/97,
3.2.2.4 transfer molding—a process in which the mixed,
uncured elastomer is placed in a compartment connected to the
mold. The compartment is then closed, pressure is applied to
Available from U.S. Pharmacopeia (USP), 12601Twinbrook Pkwy., Rockville,
transfer the uncured elastomer to the mold, filling the cavity.
MD 20852.
Heat and pressure are applied to the mold to vulcanize the
Available from Food and Drug Administration (FDA), 5600 Fishers Ln.,
elastomer, the mold is then opened, and the fabricated part is
Rockville, MD 20857.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St., removed.
4th Floor, New York, NY 10036.
3.2.2.5 extrusion—a continuous process in which the
Available from Advanced Medical Technology Association, 1200 G St. N.W.
mixed, uncured elastomer is forced through an orifice having
Suite 400 Washington, D.C. 20005–3814.
the desired cross-sectional profile. The elastomer is then
Available from Association for the Advancement of Medical Devices, 1110
North Glebe Rd., Suite 220, Arlington, VA 22201–4795.
vulcanized by passing it through either a hot air or radiant heat
AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
oven.The most common application of extrusion processing is
Robins Ave., Philadelphia PA 19111–5094, Attn: NPODS.
the fabrication of tubing but it can be used to produce other
AvailablefromMidwestResearchInstitute,425VolkerBlvd.,KansasCity,MO
64110–2299. items as well.
F2042 – 00 (2005)
3.2.2.6 post-cure—the process of subjecting a vulcanized just as gels do to provide intimate contact and protection from
elastomer to elevated temperature, usually in a hot-air oven, the environment but are more rigid and provide more strength
after its initial fabrication. This process step is done to than gels. Since foams are expanded elastomers, on a weight
complete cross-linking of the object, remove peroxide by- basis, they are highly crosslinked relative to gels. Most cure
products, and eliminate changes in its physical properties. conditions will result in a closed cell foam.
Post-cure is often necessary when the component is only
4. Significance and Use
partially cross-liked by molding; it is performed in an attempt
to accelerate molding process, and increase its output.
4.1 This guide is intended to provide guidance for the
3.2.2.7 calendaring—the process of forming an uncured, specification and selection of fabrication methods for silicones
mixed elastomer into a thin sheet or film by passing it between used in medical devices. It also provides guidance relative to
two rolls. testing that might be done to qualify lots of acceptable
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-
tomerinasolvent.Thislowerstheviscosityofthematerialand
cal device industry should readily provide information regard-
isusuallydonetoallowthefabricationofthinnerfilmsthatcan
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
during the vulcanization process. Care must be taken to assure
5. Crosslinking Chemistry
that the solvent is compatible with the elastomer, to prevent
preferential settling of the components of the formulation by 5.1 Silicone elastomers used in medical applications are
excessive dilution of the elastomer.
typically crosslinked by one of three commonly used cure
systems. These involve the platinum catalyzed addition of a
3.2.3 one-part elastomer—an elastomer supplied in the
uncured form in one package containing all of the formulation silylhydride to an unsaturated site, the generation of free
radicals by a peroxide or the reaction of an easily hydrolyzable
components. It does not require mixing before fabrication.
group of silicon.
3.2.4 two-part elastomer—an elastomer supplied in two
5.1.1 addition cure—this cure system utilizes the addition
packages which must be mixed in specified proportions before
ofasilylhydridetoasiteofunsaturation,usuallyavinylgroup.
fabrication.
As shown in Fig. 1, this reaction is catalyzed by a platinum
3.2.5 liquid silicone rubber or low consistency silicone
complex. The catalyst will be present at a level such that the
rubber (LSR)—an elastomer having a viscosity such that it can
concentration of platinum is in the range of 5 to 20 ppm but is
be moved or transferred by readily available pumping equip-
more typically present at a level of about 7.5 ppm. When
ment. LSRs are typically used in injection molding operations.
multiple silylhydrides are present in the same molecule, for
3.2.6 high consistency rubber (HCR)—an elastomer having
example, in a crosslinker molecule, and they react with vinyl
a viscosity such that it cannot be moved or transferred by
groupsattachedtoasiliconinasiliconepolymer,acrosslinked
readily available pumping equipment. These elastomers are
network results.
fabricated using high shear equipment such as a two-roll mill
Elastomers using this cure system are two-part elastomers
and cannot be injection molded. They are typically used in
and are utilized in both LSRs and HCRs. In practice, the
compression or transfer molding and extrusion processes.
platinum catalyst, an inhibitor, and vinyl functionality on the
3.2.7 RTV (room temperature vulcanization)—a one-part
silicone backbone are present in one part of the formulation
elastomerwhichcuresinthepresenceofatmosphericmoisture.
andthecrosslinkerinthepresenceofvinylfunctionalityonthe
Little, if any, acceleration of cure rate is realized by increasing
silicone backbone is present in the other. These two parts are
temperature. Because cure is dependent upon diffusion of
intimately mixed shortly before they are intended to b
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