ASTM F2042-18

Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part II—Crosslinking and Fabrication

SIGNIFICANCE AND USE
4.1 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.  
4.2 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, F2038. 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 (21 CFR, Parts 210, 211, and 820). 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 ISO 10993–12 and the HIMA Memorandum 7/14/93 for examples of extraction methods).  
1.5 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.  
1.6 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, health, and environmental 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.  
1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2018
• ICS: 11.040.40 - Implants for surgery, prosthetics and orthotics
• Technical Committee: F04 - Medical and Surgical Materials and Devices
• Drafting Committee: F04.11 - Polymeric Materials

Relations

• Replaces: ASTM F2042-00(2011) - Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part II—Crosslinking and Fabrication
• Effective Date: 01-Dec-2018
• Refers: ASTM F748-16 - Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices
• Effective Date: 01-Apr-2016
• Refers: ASTM F2038-00(2011) - Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part I—Formulations and Uncured Materials
• Effective Date: 01-Dec-2011
• Refers: ASTM F748-06(2010) - Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices
• Effective Date: 01-Jun-2010
• Refers: ASTM D792-08 - Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
• Effective Date: 15-Jun-2008
• Refers: ASTM F748-06 - Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices
• Effective Date: 01-Dec-2006
• Refers: ASTM F2038-00(2005) - Standard Guide for Silicone Elastomers, Gels and Foams Used in Medical Applications Part I - Formulations and Uncured Materials
• Effective Date: 01-Mar-2005
• Refers: ASTM F748-04 - Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices
• Effective Date: 01-May-2004
• Refers: ASTM D792-98 - Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
• Effective Date: 10-Dec-2000
• Refers: ASTM D792-00 - Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
• Effective Date: 10-Dec-2000
• Refers: ASTM D955-89(1996) - Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics
• Effective Date: 10-Nov-2000
• Refers: ASTM D955-00 - Standard Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics
• Effective Date: 10-Nov-2000
• Refers: ASTM F2038-00e1 - Standard Guide for Silicone Elastomers, Gels and Foams Used in Medical Applications Part I - Formulations and Uncured Materials
• Effective Date: 10-Jun-2000
• Refers: ASTM F2038-00 - Standard Guide for Silicone Elastomers, Gels and Foams Used in Medical Applications Part I - Formulations and Uncured Materials
• Effective Date: 10-Jun-2000
• Refers: ASTM F748-98 - Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices
• Effective Date: 10-Aug-1998

Overview

ASTM F2042-18: Standard Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part II-Crosslinking and Fabrication provides comprehensive guidelines for the fabrication and processing of silicone elastomers, gels, and foams specifically intended for medical device applications. Developed by ASTM International, this guide addresses safe and effective methods for crosslinking, curing, and evaluating silicone materials that play a critical role in medical implants, devices, and related products.

The standard is designed to educate users-medical device manufacturers, processors, and quality control professionals-on the proper selection and implementation of fabrication techniques. This guide also discusses relevant performance testing, biocompatibility assessment, storage, labeling, and sterilization practices aligned with regulatory and quality requirements.

Key Topics

• Fabrication Methods: Details on process selection for high consistency rubber (HCR), liquid silicone rubber (LSR), and related forms. Explains compression molding, injection molding, transfer molding, extrusion, calendaring, freshening, and other common fabrication approaches.
• Crosslinking Chemistry: Provides guidance on platinum-catalyzed addition cure, peroxide cure, and condensation cure systems-critical for achieving desired physical and mechanical properties in finished silicone products.
• Material Evaluation: Recommends test methods for characterizing physical properties such as tensile strength, durometer hardness, tear strength, density, and compression set, referencing key ASTM standards.
• Biocompatibility: Emphasizes the necessity of biocompatibility testing throughout and after fabrication. Recommends adherence to ISO 10993 and FDA guidance to ensure materials are safe for intended clinical use.
• Sterilization Practices: Outlines best practices for sterilizing silicone materials using ethylene oxide, steam autoclave, and radiation methods, with cautions regarding potential effects on material properties and safety.
• Quality Control: Stresses the importance of batch-to-batch consistency, lot acceptance criteria, and quality programs in compliance with ISO 9001 and Good Manufacturing Practices (GMP).

Applications

Silicone elastomers, gels, and foams produced under ASTM F2042-18 are broadly used in the medical field, including:

• Implantable Devices: Heart valves, pacemaker leads, cochlear implants, and other long-term or permanent implants.
• Medical Tubing and Catheters: Flexible, biocompatible tubing for fluid transfer and minimally invasive procedures.
• Seals and Gaskets: For syringes, pumps, infusion systems, and diagnostic equipment.
• Protective Covers and Pads: Soft tissue support, scar management products, and wound dressings.
• Components for Single- and Multi-Use Devices: Valves, connectors, and membranes.

By adhering to this standard, manufacturers ensure their medical silicone products meet internationally recognized safety, quality, and performance benchmarks.

Related Standards

• ASTM F2038: Guide for Formulations and Uncured Silicone Elastomers Used in Medical Applications (Part I)
• ISO 9001: Quality Management Systems-Requirements
• ISO 10993-1 & ISO 10993-12: Biological Evaluation of Medical Devices-Parts 1 and 12
• USP Chapters <87>, <88>, <151>, <381>: United States Pharmacopeia monographs on biocompatibility and material testing
• 21 CFR Parts 210, 211, and 820: US FDA regulations for good manufacturing practices
• ASTM Testing Methods: Including D395, D412, D430, D624, D792, D2240, and others related to rubber and elastomers
• ANSI/AAMI ST46, ST79, ST50, ST41: Sterilization and sterility assurance protocols for medical devices

Adoption of ASTM F2042-18 streamlines the path to regulatory compliance, improves product reliability, and ensures patient safety in the use of silicone-based medical devices.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F2042 − 18
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 responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
1.1 This guide is intended to educate potential users of
mine the applicability of regulatory limitations prior to use.
silicone elastomers, gels and foams relative to their fabrication
Users are also advised to refer to Material Safety Data Sheets
and processing. It does not provide information relative to
provided with uncured silicone components.
silicone powders, fluids, pressure sensitive adhesives, or other
1.7 This international standard was developed in accor-
types of silicone products.
dance with internationally recognized principles on standard-
1.2 Theinformationprovidedisofferedtoguideusersinthe
ization established in the Decision on Principles for the
selection of appropriate processing conditions for specific
Development of International Standards, Guides and Recom-
medical device applications.
mendations issued by the World Trade Organization Technical
1.3 Formulation and selection of appropriate starting mate- Barriers to Trade (TBT) Committee.
rials is covered in the companion document, F2038. This
monograph addresses only the curing, post-curing, and pro- 2. Referenced Documents
cessing of elastomers, gels and foams as well as how the
2.1 ASTM Standards:
resulting product is evaluated.
D395 Test Methods for Rubber Property—Compression Set
D412 Test Methods forVulcanized Rubber andThermoplas-
1.4 Silicone biocompatibility issues can be addressed at
tic Elastomers—Tension
several levels, but ultimately the device manufacturer must
D430 Test Methods for Rubber Deterioration—Dynamic
assess biological suitability relative to intended use. Biocom-
Fatigue
patibilitytestingmaybedoneoncuredelastomerspriortofinal
D624 Test Method for Tear Strength of Conventional Vul-
fabrication,butthemostrelevantdataarethoseobtainedonthe
canized Rubber and Thermoplastic Elastomers
finished device. Data on selected lots of material are only
representative when compounding and fabrication are per- D792 Test Methods for Density and Specific Gravity (Rela-
tive Density) of Plastics by Displacement
formed under accepted quality systems such as ISO 9001 and
current Good Manufacturing Practice Regulations (21 CFR, D813 TestMethodforRubberDeterioration—CrackGrowth
D814 Test Method for Rubber Property—Vapor Transmis-
Parts 210, 211, and 820). Extractables analyses may also be of
interest for investigation of biocompatibility, and the proce- sion of Volatile Liquids
D926 Test Method for Rubber Property—Plasticity and
dures for obtaining such data depend on the goal of the study
(see ISO 10993–12 and the HIMA Memorandum 7/14/93 for Recovery (Parallel Plate Method)
D955 Test Method of Measuring Shrinkage from Mold
examples of extraction methods).
Dimensions of Thermoplastics
1.5 The values stated in SI units are to be regarded as
D1349 Practice for Rubber—Standard Conditions for Test-
standard. The values given in parentheses are mathematical
ing
conversions to inch-pound units that are provided for informa-
D1566 Terminology Relating to Rubber
tion only and are not considered standard.
D2240 Test Method for Rubber Property—Durometer Hard-
1.6 This standard does not purport to address all of the
ness
safety concerns, if any, associated with its use. It is the
F748 PracticeforSelectingGenericBiologicalTestMethods
for Materials and Devices
This guide is under the jurisdiction of ASTM Committee F04 on Medical and
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.11 on Polymeric Materials. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 1, 2018. Published February 2019. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2000. Last previous edition approved in 2011 as F2042 – 00 (2011). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/F2042-18. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2042 − 18
F2038 GuideforSiliconeElastomers,Gels,andFoamsUsed 3. Terminology
in Medical Applications Part I—Formulations and Un-
3.1 The classification of silicone elastomers is based upon a
cured Materials
number of interrelated factors which include the chemical
2.2 Other Biocompatibility Standards:
system used to crosslink the elastomer, the physical character-
FDA Guidance on Use of International Standard ISO
istics of the uncured elastomer, and the methods used to
10993-1, “Biological evaluation of medical devices—Part
fabricate the elastomers.Additional pertinent terms are defined
1: Evaluation and testing within a risk management
in standard D1566.
process”—Guidance for Industry and Food and Drug
3.2 Definitions of Terms Specific to This Standard:
Administration Staff
3.2.1 manufacture—the process which occurs in the suppli-
ISO 10993–1 Biological evaluation of medical devices, Part
er’s facility in which the various components of the elastomer
1: Evaluation and testing within a risk management
are brought together, allowed to interact, and are packaged to
process
provide the uncured elastomer for sale.
ISO 10993-12 Biological evaluation of medical devices—
Part 12: Sample preparation and reference materials
3.2.2 fabrication—the process by which the uncured elasto-
HIMA Memorandum Guidance for Manufacturers of Sili-
mer is converted into a fully vulcanized elastomer of the
cone Devices Affected by Withdrawal of Dow Corning
desired size and shape. This process may occur in the same
Silastic Materials, 7/14/93
facility as the manufacture of the uncured elastomer but is
more typically performed at the facility of a customer of the
2.3 Sterilization Standards:
silicone manufacturer.
ANSI/AAMI ST79 Comprehensive guide to steam steriliza-
tion and sterility assurance in health care facilities
3.2.2.1 injection molding—fabrication of elastomers into
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
forms defined by molds constructed so that the uncured
ISO 10993-7 Biological evaluation of medical devices—
elastomer can be transferred by pumping into the closed mold.
Part 7: Ethylene oxide sterilization residuals
This method requires venting of the mold in some manner.The
ISO 11137-1 Sterilization of health care products—
elastomer may be vulcanized by heating the mold after it is
Radiation Part 1: Requirements for development, valida-
filled but more typically the molding conditions (temperature
tion and routine control of a sterilization process for
and filling rate) are adjusted so that uncured elastomer can be
medical devices
addedtoapre-heatedmoldinwhichitwillthencure.Themold
ISO 11137-2 Sterilization of health care products—
is then opened and the part removed and post-cured, if
Radiation Part 2: Requirements for development, valida-
necessary.
tion and routine control of a sterilization process for
4 3.2.2.2 compression molding—a process in which the un-
medical devices
cured elastomer is placed in an open mold. The mold is closed
2.4 Quality Standards:
and pressure applied to the mold to fill the cavity. Heat is
ISO 9001 Quality Management Systems—Requirements
applied to vulcanize the elstomer, the mold is then opened and
21 CFR 820 Quality System Regulation
the fabricated part is removed.
21 CFR 210 Current Good Manufacturing Practice in
3.2.2.3 freshening—becauseoftheinteractionthatcanoccur
Manufacturing, Processing, Packing or Holding of Drugs:
between the fumed silica and silicone polymers, thick uncured
General
high consistency elastomers can become so stiff over time that
21 CFR 211 Current Good Manufacturing Practice for
they are very difficult to process. To overcome this problem, a
Finished Pharmaceuticals
two–roll mill is used to disrupt this interaction, resulting in a
2.5 Other Standards:
material which is easier to fabricate. This process is called
Dow Corning CTM 0155 (Gel-Like Materials With Modi-
freshening and is typically done immediately before catalyza-
fied Penetrometer)
tion.
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.
mold. The compartment is then closed, pressure is applied to
4473, Jan 24, 1997
transfer the uncured elastomer to the mold, filling the cavity.
Heat and pressure are applied to the mold to vulcanize the
elastomer, the mold is then opened, and the fabricated part is
Available from Food and Drug Administration (FDA), 10903 New Hampshire
removed.
Ave., Silver Spring, MD 20993-0002, http://www.fda.gov.
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
3.2.2.5 extrusion—acontinuousprocessinwhichthemixed,
4th Floor, New York, NY 10036, http://www.ansi.org.
5 uncured elastomer is forced through an orifice having the
Available from Advanced Medical Technology Association, 1200 G St. N.W.
desired cross-sectional profile. The elastomer is then vulca-
Suite 400 Washington, D.C. 20005–3814, http://www.advamed.org.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
nizedbypassingitthrougheitherahotairorradiantheatoven.
Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
The most common application of extrusion processing is the
dodssp.daps.dla.mil.
fabrication of tubing but it can be used to produce other items
AvailablefromMidwestResearchInstitute,425VolkerBlvd.,KansasCity,MO
64110–2299, Ph: [phone]. as well.
F2042 − 18
3.2.2.6 post-cure—the process of subjecting a vulcanized Foams are usually two-part formulations utilizing a platinum-
elastomer to elevated temperature, usually in a hot-air oven, catalyzed addition cure system. They conform as they expand
after its initial fabrication. This process step is done to to irregular surfaces just as gels do to provide intimate contact
complete cross-linking of the object, remove peroxide by- and protection from the environment but are more rigid and
products, and eliminate changes in its physical properties. provide more strength than gels. Since foams are expanded
Post-cure is often necessary when the component is only elastomers, on a weight basis, they are highly crosslinked
partially cross-liked by molding; it is performed in an attempt relativetogels.Mostcureconditionswillresultinaclosedcell
to accelerate the molding process, and increase its output. foam.
3.2.2.7 calendaring—the process of forming an uncured,
4. Significance and Use
mixed elastomer into a thin sheet or film by passing it between
4.1 This guide is intended to provide guidance for the
two rolls.
specification and selection of fabrication methods for silicones
3.2.2.8 dispersion—the process of placing an uncured elas-
used in medical devices. It also provides guidance relative to
tomerinasolvent.Thislowerstheviscosityofthematerialand
testing that might be done to qualify lots of acceptable
is usually done to allow the fabrication of thinner films than
material, based on desired performance properties.
can be obtained by calendaring or to form coatings. Following
4.2 Silicone manufacturers supplying material to the medi-
dispersion use, the solvent must be removed either before or
cal device industry should readily provide information regard-
during the vulcanization process. Care must be taken to assure
ing non-proprietary product formulation to their customers
that the solvent is compatible with the elastomer, to prevent
either directly or through the US FDA Master File program.
preferential settling of the components of the formulation by
excessive dilution of the elastomer.
5. Crosslinking Chemistry
3.2.3 one-part elastomer—an elastomer supplied in the
5.1 Silicone elastomers used in medical applications are
uncured form in one package containing all of the formulation
typically crosslinked by one of three commonly used cure
components. It does not require mixing before fabrication.
systems. These involve the platinum-catalyzed addition of a
3.2.4 two-part elastomer—an elastomer supplied in two
silylhydride to an unsaturated site, the generation of free
packages which must be mixed in specified proportions before
radicalsbyaperoxide,orthereactionofaneasilyhydrolyzable
fabrication.
group of silicon.
3.2.5 liquid silicone rubber or low consistency silicone
5.1.1 addition cure—thiscuresystemutilizestheadditionof
rubber (LSR)—an elastomer having a viscosity such that it can
a silylhydride to a site of unsaturation, usually a vinyl group.
be moved or transferred by readily available pumping equip-
As shown in Fig. 1, this reaction is catalyzed by a platinum
ment. LSRs are typically used in injection molding operations.
complex. The catalyst will be present at a level such that the
concentration of platinum is in the range of 5 to 20 ppm but is
3.2.6 high consistency rubber (HCR)—an elastomer having
more typically present at a level of about 7.5 ppm. When
a viscosity such that it cannot be moved or transferred by
multiple silylhydrides are present in the same molecule, for
readily available pumping equipment. These elastomers are
example in a crosslinker molecule, and they react with vinyl
fabricated using high shear equipment such as a two-roll mill
groupsattachedtoasiliconinasiliconepolymer,acrosslinked
and cannot be injection molded. They are typically used in
network results.
compression or transfer molding and extrusion processes.
Elastomers using this cure system are two-part elastomers
3.2.7 room temperature vulcanization (RTV)—a one-part
and are utilized in both LSRs and HCRs. In practice, the
elastomerwhichcuresinthepresenceofatmosphericmoisture.
platinum catalyst, an inhibitor, and vinyl functionality on the
Little, if any, acceleration of cure rate is realized by increasing
silicone backbone are present in one part of the formulation
temperature. Because cure is dependent upon diffusion of
and the crosslinker in the presence of vinyl functionality on the
water into the elastomer, cure in depths of greater than 0.64 cm
silicone backbone is present in the other. These two parts are
is not recommended.
intimately mixed shortly before they are intended to be used.
3.2.8 gel—a lightly crosslinked material having no or rela-
At room temperature a certain amount of working time (time
tively low levels of reinforcement beyond that provided by the
before the crosslink network builds to unacceptable levels) is
crosslinked polymer. Gels are usually two-part formulations
provided to allow time to fabricate the silicone part. Heat is
utilizing a platinum-catalyzed addition cure system. The hard-
then applied to activate the platinum, the crosslinking reaction
ness of the gel can be adjusted within wide limits.The material
occurs, and the elastomer is vulcanized. The amount of
is not usually designed to bear a heavy load but rather to
working time and rate of cure are determined by the amount of
conform to an irregular surface providing intimate contact. As
crosslinker, catalyst, and inhibitors used in the formulation.
aresult,loadsaredistributedoverawiderarea.Thesematerials
may also be used to provide protection from environmental
contaminants.
3.2.9 foam—a crosslinked material which has a component
added to it which generates a volatile gas that creates gas-filled
NOTE 1—Si=Silicon
cells as the material is being vulcanized. This vulcanization
Pt=Platinum
process results in a material with a relatively low density. FIG. 1 Silylhydride Addition Cure Reaction
F2042 − 18
Mixing of LSRs of this type is usually accomplished by
pumping the material in the prescribed ratios through a static
mixer.HCRsareusuallymixedbyplacingtheprescribedratios
of the two parts on a two-roll mill and crossblending until
adequately mixed. Mixing of the two components of the
FIG. 3 Hydrogen Abstraction Reaction
formulation at other than the ratio prescribed by the vendor is
lik
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: F2042 − 00 (Reapproved 2011) F2042 − 18
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
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, F2038 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,compounding and fabrication are performed under accepted quality systems such as ISO 9001 and current Good
Manufacturing Practice Regulations. Regulations (21 CFR, Parts 210, 211, and 820). 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,
ISO 10993–12 and the HIMA Memorandum 7/14/93, and USP 23, 7/14/93 for examples of extraction methods).
1.5 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions
to inch-pound units that are provided for information only and are not considered standard.
1.6 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, health, and environmental 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.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D395 Test Methods for Rubber Property—Compression Set
D412 Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension
D430 Test Methods for Rubber Deterioration—Dynamic Fatigue
D624 Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
D813 Test Method for Rubber Deterioration—Crack Growth
D814 Test Method for Rubber Property—Vapor Transmission of Volatile Liquids
This guide is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee F04.11
on Polymeric Materials.
Current edition approved Dec. 1, 2011Dec. 1, 2018. Published January 2012February 2019. Originally approved in 2000. Last previous edition approved in 20052011 as
F2042 – 00 (2005).(2011). DOI: 10.1520/F2042-00R11.10.1520/F2042-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM 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 − 18
D926 Test Method for Rubber Property—Plasticity and Recovery (Parallel Plate Method)
D955 Test Method of Measuring Shrinkage from Mold Dimensions of Thermoplastics
D1349 Practice for Rubber—Standard Conditions for Testing
D1566 Terminology Relating to Rubber
D2240 Test Method for Rubber Property—Durometer Hardness
F619 Practice for Extraction of Medical Plastics
F719 Practice for Testing Biomaterials in Rabbits for Primary Skin Irritation
F720 Practice for Testing Guinea Pigs for Contact Allergens: Guinea Pig Maximization Test
F748 Practice for Selecting Generic Biological Test Methods for Materials and Devices
F813 Practice for Direct Contact Cell Culture Evaluation of Materials for Medical Devices
F981 Practice for Assessment of Compatibility of Biomaterials for Surgical Implants with Respect to Effect of Materials on
Muscle and Insertion into Bone
F1905 Practice For Selecting Tests for Determining the Propensity of Materials to Cause Immunotoxicity (Withdrawn 2011)
F1906 Practice for Evaluation of Immune Responses In Biocompatibility Testing Using ELISA Tests, Lymphocyte Proliferation,
and Cell Migration (Withdrawn 2011)
F1984 Practice for Testing for Whole Complement Activation in Serum by Solid Materials
F2038 Guide for Silicone Elastomers, Gels, and Foams Used in Medical Applications Part I—Formulations and Uncured
Materials
2.2 Other Biocompatibility Standards:
United States Pharmacopeia, current edition (appropriate monographs may include: <87>, <88>, <151>, <381>)
FDA Department of Health and Human Services General Program Memorandum #G95–1, Guidance onMay 1, 1995: Use of
International Standard ISO-10993, Biological Evaluation of Medical Devices Part I: Evaluation and TestingISO 10993-1,
“Biological evaluation of medical devices—Part 1: Evaluation and testing within a risk management process”—Guidance for
Industry and Food and Drug Administration Staff
ANSI/AAMI 10993–1ISO 10993–1 Biological Evaluationevaluation of Medical Devices, Part I: Guidance on Selection of
Testsmedical devices, Part 1: Evaluation and testing within a risk management process
ISO 10993-12 Biological evaluation of medical devices—Part 12: Sample preparation and reference materials
HIMA Memorandum Guidance for Manufacturers of Silicone Devices Affected by Withdrawal of Dow Corning Silastic
Materials, 7/14/93
2.3 Sterilization Standards:
ANSI/AAMI ST46 Good Hospital Practice: Steam Sterilization and Sterility Assurance
ANSI/AAMI ST41ST79 Good Hospital Practice: Ethylene Oxide Sterilization and Sterility AssuranceComprehensive guide to
steam sterilization and sterility assurance in health care facilities
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
ANSI/AAMI ST29ISO 10993-7 Recommended Practice for Determining Ethylene Oxide in Medical DevicesBiological
evaluation of medical devices—Part 7: Ethylene oxide sterilization residuals
ANSI/AAMI ST30ISO 11137-1 Determining Residual Ethylene Chlorohydrin and Ethylene Glycol in Medical DevicesSteril-
ization of health care products—Radiation Part 1: Requirements for development, validation and routine control of a
sterilization process for medical devices
AAMI 13409–251ISO 11137-2 Sterilization of Health Care Products—Radiation Sterilization—Substantiation of 25kGy as a
Sterilization Dose for Small or Infrequent Production Batcheshealth care products—Radiation Part 2: Requirements for
development, validation and routine control of a sterilization process for medical devices
AAMI TIR8–251 Microbiological Methods for Gamma Irradiation Sterilization of Medical Devices
2.4 Quality Standards:
ANSI/ASQC Q9001ISO 9001 Quality Systems—Model for Quality Assurance in Design, Development, Production, Installation
and ServicingManagement Systems—Requirements
21 CFR 820 Quality System Regulation (current revision)
21 CFR 210 Current Good Manufacturing Practice in Manufacturing, Processing, Packing or Holding of Drugs: General
(current revision)
21 CFR 211 Current Good Manufacturing Practice for Finished Pharmaceuticals (current revision)
2.5 Other Standards:
Dow Corning CTM 0155 (Gel-Like Materials With Modified Penetrometer)
Dow Corning CTM 0813 (Gel-Like Materials With One Inch Diameter Head Penetrometer)
Available from Food and Drug Administration (FDA), 10903 New Hampshire Ave., Silver Spring, MD 20993-0002, http://www.fda.gov.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Available from Advanced Medical Technology Association, 1200 G St. N.W. Suite 400 Washington, D.C. 20005–3814, http://www.advamed.org.
Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://dodssp.daps.dla.mil.
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PCB Test Methods such as those used for MRI projectProject No. 4473, 1/24/97,Jan 24, 1997
Biological Performance of Materials: J. Black, Marcel Dekker, NY 1992
3. Terminology
3.1 The classification of silicone elastomers is based upon a number of interrelated factors which include the chemical system
used to crosslink the elastomer, the physical characteristics of the uncured elastomer, and the methods used to fabricate the
elastomers. Additional pertinent terms are defined in standard D1566.
3.2 Definitions:Definitions of Terms Specific to This Standard:
3.2.1 manufacture—the process which occurs in the supplier’s facility in which the various components of the elastomer are
brought together, allowed to interact, and are packaged to provide the uncured elastomer for sale.
3.2.2 fabrication—the process by which the uncured elastomer is converted into a fully vulcanized elastomer of the desired size
and shape. This process may occur in the same facility as the manufacture of the uncured elastomer but is more typically performed
at the facility of a customer of the silicone manufacturer.
3.2.2.1 injection molding—fabrication of elastomers into forms defined by molds constructed so that the uncured elastomer can
be transferred by pumping into the closed mold. This method requires venting of the mold in some manner. The elastomer may
be vulcanized by heating the mold after it is filled but more typically the molding conditions (temperature and filling rate) are
adjusted so that uncured elastomer can be added to a pre-heated mold in which it will then cure. The mold is thanthen opened and
the part removed and post-cured, if necessary.
3.2.2.2 compression molding—a process in which the uncured elastomer is placed in an open mold. The mold is closed and
pressure applied to the mold to fill the cavity. Heat is applied to vulcanize the elstomer, the mold is thanthen opened and the
fabricated part is removed.
3.2.2.3 freshening—because of the interaction that can occur between the fumed silica and silicone polymers, thick uncured high
consistency elastomers can become so stiff over time that they are very difficult to process. To overcome this problem, a two–roll
mill is used to disrupt this interaction, resulting in a material which is easier to fabricate. This process is called freshening and is
typically done immediately before catalyzation.
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 transfer the uncured elastomer to the mold, filling the cavity. Heat and
pressure are applied to the mold to vulcanize the elastomer, the mold is then opened, and the fabricated part is removed.
3.2.2.5 extrusion—a continuous process in which the mixed, uncured elastomer is forced through an orifice having the desired
cross-sectional profile. The elastomer is then vulcanized by passing it through either a hot air or radiant heat oven. The most
common application of extrusion processing is the fabrication of tubing but it can be used to produce other items as well.
3.2.2.6 post-cure—the process of subjecting a vulcanized elastomer to elevated temperature, usually in a hot-air oven, after its
initial fabrication. This process step is done to complete cross-linking of the object, remove peroxide by-products, and eliminate
changes in its physical properties. Post-cure is often necessary when the component is only partially cross-liked by molding; it is
performed in an attempt to accelerate the molding process, and increase its output.
3.2.2.7 calendaring—the process of forming an uncured, mixed elastomer into a thin sheet or film by passing it between two
rolls.
3.2.2.8 dispersion—the process of placing an uncured elastomer in a solvent. This lowers the viscosity of the material and is
usually done to allow the fabrication of thinner films thatthan can be obtained by calendaring or to form coatings. Following
dispersion use, the solvent must be removed either before or during the vulcanization process. Care must be taken to assure that
the solvent is compatible with the elastomer, to prevent preferential settling of the components of the formulation by excessive
dilution of the elastomer.
3.2.3 one-part elastomer—an elastomer supplied in the uncured form in one package containing all of the formulation
components. It does not require mixing before fabrication.
3.2.4 two-part elastomer—an elastomer supplied in two packages which must be mixed in specified proportions before
fabrication.
3.2.5 liquid silicone rubber or low consistency silicone rubber (LSR)—an elastomer having a viscosity such that it can be moved
or transferred by readily available pumping equipment. LSRs are typically used in injection molding operations.
3.2.6 high consistency rubber (HCR)—an elastomer having a viscosity such that it cannot be moved or transferred by readily
available pumping equipment. These elastomers are fabricated using high shear equipment such as a two-roll mill and cannot be
injection molded. They are typically used in compression or transfer molding and extrusion processes.
Available from Midwest Research Institute, 425 Volker Blvd., Kansas City, MO 64110–2299.64110–2299, Ph: [phone].
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3.2.7 RTV (room temperature vulcanization)—room temperature vulcanization (RTV)—a one-part elastomer which cures in the
presence of atmospheric moisture. Little, if any, acceleration of cure rate is realized by increasing temperature. Because cure is
dependent upon diffusion of water into the elastomer, cure in depths of greater than 0.64 cm is not recommended.
3.2.8 gel—a lightly crosslinked material having no or relatively low levels of reinforcement beyond that provided by the
crosslinked polymer. Gels are usually two-part formulations utilizing a platinum catalyzed platinum-catalyzed addition cure
system. The hardness of the gel can be adjusted within wide limits. The material is not usually designed to bear a heavy load but
rather to conform to an irregular surface providing intimate contact. As a result, loads are distributed over a wider area. These
materials may also be used to provide protection from environmental contaminants.
3.2.9 foam—a crosslinked material which has a component added to it which generates a volatile gas that creates gas-filled cells
as the material is being vulcanized. This vulcanization process results in a material with a relatively low density. Foams are usually
two-part formulations utilizing a platinum catalyzed platinum-catalyzed addition cure system. They conform as they expand to
irregular surfaces just as gels do to provide intimate contact and protection from the environment but are more rigid and provide
more strength than gels. Since foams are expanded elastomers, on a weight basis, they are highly crosslinked relative to gels. Most
cure conditions will result in a closed cell foam.
4. Significance and Use
4.1 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.
4.2 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.
5. Crosslinking Chemistry
5.1 Silicone elastomers used in medical applications are typically crosslinked by one of three commonly used cure systems.
These involve the platinum catalyzed platinum-catalyzed addition of a silylhydride to an unsaturated site, the generation of free
radicals by a peroxide, or the reaction of an easily hydrolyzable group of silicon.
5.1.1 addition cure—this cure system utilizes the addition of a silylhydride to a site of unsaturation, usually a vinyl group. As
shown in Fig. 1, this reaction is catalyzed by a platinum complex. The catalyst will be present at a level such that the concentration
of platinum is in the range of 5 to 20 ppm but is more typically present at a level of about 7.5 ppm. When multiple silylhydrides
are present in the same molecule, for example,example in a crosslinker molecule, and they react with vinyl groups attached to a
silicon in a silicone polymer, a crosslinked network results.
Elastomers using this cure system are two-part elastomers and are utilized in both LSRs and HCRs. In practice, the platinum
catalyst, an inhibitor, and vinyl functionality on the silicone backbone are present in one part of the formulation and the crosslinker
in the presence of vinyl functionality on the silicone backbone is present in the other. These two parts are intimately mixed shortly
before they are intended to be used. At room temperature a certain amount of working time (time before the crosslink network
builds to unacceptable levels) is provided to allow time to fabricate the silicone part. Heat is then applied to activate the platinum,
the crosslinking reaction occurs, and the elastomer is vulcanized. The amount of working time and rate of cure are determined by
the amount of crosslinker, catalyst, and inhibitors used in the formulation.
Mixing of LSRs of this type is usually accomplished by pumping the material in the prescribed ratios through a static mixer.
HCRs are usually mixed by placing the prescribed ratios of the two parts on a two-roll mill and crossblending until adequately
mixed. Mixing of the two components of the formulation at other than the r
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