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Frequently Asked Questions About Silicone

 

 

 

Question: How long has silicone been used in medical devices?

Answer: Medical silicone has been used in medical devices since the 1960’s.

 

Question: Why are liquid silicone molded parts used for medical applications?

Answer: The main reason that liquid medical silicone is used is because it is biologically and toxicologically inert. This means the medical silicone parts will not kill living cells.

 

Question: What is the operating temperature range of medical silicone rubbers?

Answer: The operating temperature range of medical silicone rubber is -180 to 600 degrees F (-118 to 316 degrees C). It should be noted that the temperature has less effect on the physical properties of silicone than any other rubber.

 

Question: Can medical silicone parts be steam sterilized?

Answer: Yes medical silicone parts can be steam sterilized; this includes autoclaving.

 

Question: What is the effect on medical silicone parts that are autoclaved?

Answer: There is no effect on the physical properties when autoclaving medical silicone parts.

 

Question: Will medical grade silicone cause skin irritations?

Answer: Medical grade silicone is hypo-allergenic. Medical silicone is the only known Elastomer that is hypo-allergenic.

 

Question: Who are the major manufacturers of medical silicones?

Answer: The major manufacturers of medical silicone are Wacker Silicones, Dow Silicones, BlueStar Silicones, Shin ETSU silicones, Momentive Silicones, and NuSil Silicones.

 

Question: Who is the major manufacture of Long term implantable grade of medical Silicone?

Answer: NuSil is the major manufacture of Long term implantable grade medical Silicone. Unrestricted medical grade silicone is medical silicone that has no time limit for its use. A typical example is a medical silicone implanted stent that is coated with medical silicone.

 

Question: What are the elongation properties of silicone?

Answer: Silicone elongation can range from 600% up to 800% depending on which material is selected. This also applies to medical silicones.

 

Question: What is the catalyst cure systems used for silicone?

Answer: The best cure system for medical silicone applications is the platinum cure system. This is due to the fact that this cure system does not produce any extractable. Peroxide curing systems produce extractables and therefore, are not suitable for medical applications because they do not pass biocompatibility testing.

 

Question: What is post baking of medical silicone parts?

Answer: Post baking of medical silicone parts is done to assure the medical silicone parts are fully crossed linked. If the medical silicone parts are not fully cross linked there could be an effect on compression set as well as out gassing.

 

Question: Why would you use liquid medical silicone rubber instead of TPE (Thermoplastic Elastomer) for medical applications?

Answer: Platinum cured medical silicone rubbers that are fully cross linked do not produce extractables, but some TPE’s do.

 

Question: What durometers are available in liquid silicone?

Answer: Liquid silicone rubber is typically available from 01 to 80 durometers. Durometer is a measure of the indentation hardness of a material. The procedure for determining indentation hardness of a polymer material can be found in ASTM D2240. There are twelve different durometer types: A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Nevertheless, type A and type D are the most commonly known and used in the industry.

Type A is generally used for soft and flexible polymer materials and type D is used for hard material. Therefore, silicone rubber is generally labeled in type A durometer. Silicone durometer is normally categorized in an increment of 10, but the distribution of the durometer can range from minus to plus 5 durometer from the labeled value; industrial standard allows +/- 5 durometer that includes +/- 2 potential test error.

Nevertheless, the durometer of silicone may be customized to meet the specifications of individual applications. The hardness of a silicone can be modified by changing the crosslink density, filler concentration, or both.

 

Question: What colors are available for medical silicone?

Answer: Medical silicone is available in a wide choice of colors. Color matching is also available.

 

Question: Are there colors that are available for long term medical silicone implants?

Answer: Yes, NuSil Silicone has a selection of colors for medical silicone long term implants.

 

Question: Why use medical silicone for o-rings and gaskets?

Answer: They have the best compression set values of any elastomers for medical applications.

 

Question: What are some of the common applications for liquid medical grade silicones?

Answer: Common applications for liquid medical grade silicone are:

Tubing, Drains, Feeding Tubes, Catheters, Gaskets, Seals, Long and Short Term Implants

 

Question: What is the difference between liquid silicone and gum stock silicone?

Answer: The main difference is that the cure time for liquid silicone is about 1/3 less than gum stock. This equates to lower molding costs.

 

Question: What is silicone gum stock?

Answer: Silicone gum stock is also called High Consistency Rubber (HCR). The consistency is that of a thick dough.

Liquid silicone is the commonly used form for medical applications due to its quicker processing time and it is platinum cured. Liquid medical grade silicone has to be rated for medical applications by the silicone manufacture. Gum stock silicone can also be platinum cured.

 

Question: What is peroxide catalyst cured silicone?

Answer: Peroxide cure systems were used in earlier silicone compounds and is not used for medical silicone applications due to acid residue and out gassing of the peroxide by products.

 

Question: Do liquid silicones have a flame rating?

Answer: Yes, liquid silicone can be flame rated by the manufacture. Typical examples would be UL-94, V-1, V-0, and UL Code 62.

 

Question: Is liquid silicone fungus resistant?

Answer: Liquid silicone is not a nutrient for fungi nor is it adversely affected by fungus or mold.

 

Question: Can liquid silicone be used for food contact and does it meet FDA regulations?

Answer: Liquid silicone can be used for food contact and meets FDA regulations. Liquid silicone must be tested and certified by the liquid silicone manufacture.

 

Question: What are the typical tensile strengths of liquid silicone rubbers?

Answer: Typical tensile strengths for liquid silicone rubber are in the range of 4 to 12 MPA.

 

Question: What type of tear strength does liquid silicone rubbers have?

Answer: Typical tear strength for liquid silicone rubber is between 11-52 kN/m.

 

Question: Is liquid silicone a good electrical insulator?

Answer: Yes, the dielectric properties of liquid silicone are the best of any of the available elastomers. For example, a ½ inch thick piece of liquid silicone has the same dielectric properties as that of 18 inches of air.

 

Question: Will extreme flexing of liquid silicone still retain its original shape?

Answer: Liquid silicone can undergo extreme flexing and still retain its original shape.

 

Question: Will extreme flexing of liquid silicone parts make it lose its physical properties?

Answer: Extreme flexing of liquid silicone will not make it lose any its physical properties.

 

 

Question: What are some applications using medical silicone where flex life is critical?

Answer: Diaphragms, peristaltic pumps, and artificial limbs are some of the applications for medical silicone.

 

Question: What is the coefficient of friction for liquid silicone?

Answer: The coefficient of friction for liquid silicone rubbers ranges from .025 to more than 0.75; Steel is about.10.

 

Question: Does durometer of liquid silicone have an effect on the coefficient of friction?

Answer: The lower the durometer of liquid silicone: the higher the coefficient of friction.

 

 

Question: How does the surface finish of a liquid silicone part affect the coefficient of friction?

Answer: Very smooth – highly polished surfaces increase the coefficient of friction of liquid silicone parts.

Textured surfaces decrease the coefficient of friction of liquid silicone parts. These properties of liquid silicone are helpful in designing of medical silicone parts

 

Question: Are liquid silicone parts Ozone resistant?

Answer: Liquid silicone parts, when tested for resistance to ozone, show excellent stability.

 

Question: What are the permeability gas properties of liquid silicone?

Answer: Liquid silicone gas permeability is approximately 400 times that of rubber.

 

Question: Can gas permeability properties of liquid silicone be changed?

Answer: The gas permeability properties of liquid silicone can be reduced but not increased.

 

Question: What is the range of compression set for liquid silicone?

Answer: The compression set range for liquid silicone is generally between 5% and 30%. Where 5% displays very little permanent deformation and 30% is relatively high permanent deformation. The grade of silicone and the degree of completion of cross linking generally effects the final compression set. If compression set is a major issue for your project, it may be worth the time to post cure (A.K.A. Post Bake) your parts in order to complete cross linking of your parts. Your material supplier should be able to provide the compression set range for their materials.

 

Question: Can medical grade silicone be bonded to thermal plastic parts?

Answer: Medical silicone can be bonded to some thermal plastics such as polycarbonate, nylon, polybutylene terephthalate (PBT), and others. Bonding silicone can be improved with material selection, surface treatments, adhesives, and primers but many of these fail biocompatibility limiting their medical applications. Many thermal plastics don't have enough thermal resistance to withstand over-molding temperatures while others bond poorly with silicone such as polyethylene and polypropylene. Surface treatments like heating or flame, scratching, corona, plasma, and others can assist bonding. adhesives and primers can be effective but they can add significant cost and cause problems with biocompatibility. There are some vendors that make implantable self adhesive silicones, adhesives, and primers but there aren't a lot of options. One additional consideration is that materials (silicone, plastics, etc) from different vendors have different formulations and may result in significant differences in bond strength.

 

Question: How is compression testing done for liquid silicone?

Answer: Compression set measures the permanent deformation of parts that have been subjected to high temperatures for some predetermined amount of time. One standard for compression set used in supplier literature is ASTM D395 A or B. Basically a standard puck is compressed between two plates and subjected to temperatures in the range of 250F at constant force or constant deflection. The sample is allowed to recover for 30 minutes and then measurements are taken. Alternative tests like ISO 815-B heat for 22 hours at 175 degrees C. The test method dictates the calculation but basically the compression set is a measurement of how well the puck returns to the initial thickness, so lower values indicate higher resistance to permanent deformation.