Silicone Rubber Specification FAQ

Silicone Rubber Specification FAQ

Frequently asked questions from procurement engineers and product teams about specifying, qualifying, and sourcing silicone rubber.

Q1: What is the difference between food-grade and medical-grade silicone?

Food-grade silicone complies with FDA 21 CFR 177.2600 (U.S.) and/or LFGB §30/§31 (Germany/EU) for repeated contact with food and beverages. The key criteria are: no harmful extractables under food-simulant conditions, and (for LFGB) no taste/odor transfer to food. Food-grade silicone is used in baking molds, kitchen utensils, food processing seals, and baby products.

Medical-grade silicone meets biocompatibility requirements for contact with body fluids or living tissue. The minimum standard is USP Class VI (three in vivo biological tests). For implant-adjacent or device-contact applications, ISO 10993 biocompatibility evaluation is additionally required. Medical-grade silicone is typically platinum-cured, manufactured in controlled conditions, and supplied with lot-by-lot biocompatibility documentation.

The distinction matters: food-grade silicone is not automatically medical-grade. A baking mold-grade silicone may pass FDA 21 CFR but not USP Class VI biological tests. For medical devices, always request USP Class VI certification and ISO 10993-5 cytotoxicity data.

Q2: Does "platinum cure" automatically mean FDA compliant?

No. Platinum cure (addition cure, hydrosilylation) means the crosslinking reaction uses platinum catalyst and produces no organic decomposition by-products. This gives platinum-cured silicone a cleaner extractable profile than peroxide-cured silicone, making FDA 21 CFR compliance easier to achieve.

However, FDA compliance requires that the entire compound formulation — including fillers, pigments, plasticizers, and processing aids — meets the ingredient requirements of 21 CFR 177.2600. A platinum-cured silicone with non-listed pigments or additives does not automatically comply with FDA. Always request the specific Declaration of Conformity citing 21 CFR 177.2600.

Q3: What sterilization methods are compatible with silicone rubber?

Silicone rubber is compatible with the following standard sterilization methods:

Steam autoclave (121 °C or 134 °C): Excellent compatibility. Silicone withstands hundreds of cycles with minimal property change. The preferred method for reusable silicone medical devices.

Ethylene oxide (EtO) gas: Compatible, but EtO residues can be trapped. Adequate aeration period (12+ hours in ventilated oven) required before use.

Gamma irradiation (25–50 kGy): Compatible. Some reduction in mechanical properties (typically <10% elongation reduction at 50 kGy). Validate per compound formulation.

E-beam irradiation: Compatible, similar to gamma.

Dry heat (160–180 °C): Compatible for HTV grades. May exceed service temperature limits for low-durometer LSR grades.

Hydrogen peroxide vapor (VHP/HPV): Generally compatible. Verify with compound supplier for specific grades.

Not compatible: concentrated acids or oxidizing agents; prolonged immersion in petroleum solvents.

Q4: How do I specify Shore hardness on an RFQ?

On an RFQ, specify Shore hardness with the following elements:

1. Scale: Shore A (for most silicone rubber). Shore D is used for very hard compounds (>90A), but this is rare for silicone.
2. Nominal value: The target hardness (e.g., 50A).
3. Tolerance: The acceptable range (e.g., ±5). For medical and precision applications, specify ±3.
4. Test method: ISO 48-4 (instantaneous reading) or ASTM D2240 (15-second delayed reading). Note that ASTM D2240 values are typically 3–5 Shore A lower than ISO 48-4 instantaneous values for the same material.
5. Test specimen thickness: Minimum 6 mm per ISO 48-4.

Example specification: "Shore hardness per ISO 48-4: 50A ±5 (minimum specimen thickness 6 mm, hardness measured within 1 second of contact)."

Q5: What is compression set and why does it matter?

Compression set (CS) is the permanent deformation remaining in a rubber specimen after it has been held under a defined compression for a defined time at a defined temperature, then released and allowed to recover. It is measured per ASTM D395 or ISO 815 and expressed as a percentage of the original deflection:

CS = 0% means full elastic recovery (ideal, not achievable in practice). CS = 100% means the part took a permanent set equal to the applied compression.

For sealing applications, low compression set is critical: a seal compressed 25% in a groove that develops 30% compression set will have only 5% residual sealing load — likely insufficient for reliable sealing. Platinum-cured, properly post-cured silicone achieves CS <15% at 70 °C/22 hours (ASTM D395 Method B), which is excellent for most sealing applications.

Q6: Can silicone rubber be bonded to other materials?

Yes, with the right approach:

To metals: Use silicone-compatible bonding primers (e.g., organosilane-based primers) applied to the metal surface before molding or assembly. Standard epoxy or acrylic adhesives do not bond well to cured silicone.

To thermoplastics (PC, PA, PBT): Self-adhesive LSR grades bond directly during overmolding without primer. Post-bond peel strength 2–8 N/mm depending on plastic type and surface preparation.

To cured silicone: Silicone-to-silicone bonding uses silicone RTV adhesive or specialized platinum-cure adhesive. Surface must be cleaned of silicone release agents before bonding.

Common failure: Trying to bond silicone with cyanoacrylate (superglue) or standard epoxy — these do not adhere to silicone. Always use silicone-specific adhesives or primer systems.

Q7: What does "post-cure" mean and when is it required?

Post-cure is a secondary heat treatment (typically 200 °C for 2–4 hours in a circulating air oven) applied after the primary molding/cure step. It is required for:

Peroxide-cured compounds: Removes organic decomposition by-products (benzoic acid, cumyl alcohol) from the cured rubber. Post-cure is mandatory for food-contact and medical applications of peroxide-cured silicone.

Mechanical property optimization: Post-cure completes crosslinking and stabilizes Shore hardness and tensile properties at their final values. Silicone parts measured immediately after demolding may be 2–4 Shore A softer than their post-cure values.

Dimensional stability: Post-cure removes the volatile cyclic siloxane oligomers (D4, D5, D6) that can cause shrinkage in service if not removed during cure.

Platinum-cured silicone does not strictly require post-cure for food/medical compliance, but post-cure still improves final mechanical properties and dimensional stability and is recommended.

Contact us to verify specifications and connect with qualified silicone suppliers for your application.