SES22251/CAT22251-35 and RTV-4135 are both Shore A35 addition-cure silicone systems, so they occupy a similar balance between mold flexibility and dimensional support. The main difference is not hardness, but how each material balances mechanical performance, processing time, handling and production speed.
Shin-Etsu positions SES22251/CAT22251-35 as a high-strength, high-modulus mold-making silicone with high tensile strength, excellent tear resistance, low shrinkage and easy release. Its published 90-minute working time gives substantially more processing latitude than RTV-4135, which can be valuable for large molds, complex geometries and processes requiring more time for mixing, deaeration and controlled pouring.
The mechanical data are one of the clearest differences. SES22251/CAT22251-35 publishes 7.19 MPa tensile strength and 530% elongation, compared with 4.8±0.5 MPa and 300±50% for RTV-4135. SES22251 therefore offers the higher published tensile strength and elongation, while RTV-4135 reports the higher tear-strength value at 28±0.5 N/mm versus 24 N/mm. These figures are manufacturer-published reference values and should not be treated as a direct laboratory ranking because test methods and conditions may differ.
Processing is another major distinction. SES22251/CAT22251-35 uses a 100:10 mixing ratio by weight, while RTV-4135 uses a 1:1 ratio by weight or volume. RTV-4135 is therefore simpler to batch manually, especially for small quantities, while SES22251 requires more accurate measurement of the catalyst component.
Viscosity also reflects different handling profiles. Shin-Etsu lists approximately 50 Pa·s for the SES22251 base, while MinghuiLink reports approximately 4,500±200 mPa·s for Part A of RTV-4135. Because the manufacturers report viscosity at different measurement points, these values should not be treated as a strict like-for-like comparison. The published data nevertheless indicate that RTV-4135 is the easier-flowing system for routine mixing and pouring.
The cure cycle creates a further production trade-off. SES22251/CAT22251-35 has a published 24-hour cure at 23°C, while RTV-4135 cures in approximately 4–5 hours. SES22251 is therefore better suited to users who benefit from its longer processing window and mechanical profile, while RTV-4135 is more attractive when faster mold turnover is the primary requirement.
Shore A35 itself is a practical middle ground for reusable molds. Compared with softer A20–A30 grades, it provides more body and resistance to deformation, while remaining more flexible during demolding than harder A40 materials. This makes A35 useful for applications where both mold support and controlled flexibility are required.
For users comparing these materials as alternatives, the decision should therefore focus on the actual production constraint. SES22251/CAT22251-35 is more compelling when higher published tensile strength and elongation, longer processing time, low-shrinkage positioning and easy release are important. RTV-4135 is better aligned with simpler 1:1 batching, lower published component viscosity, higher published tear strength and faster mold turnaround.