HCR vs. LSR Silicones: Crosslinking and Mechanical Properties
High consistency silicone rubber (HCR) and liquid silicone rubber (LSR) are commonly used silicone elastomers. For engineers who need to compare these two types of materials, this article from Stockwell Elastomerics describes differences in their chemistries and, consequently, their mechanical properties. Research published in RSC Advances (Royal Society of Chemistry) Issue 66, 2015, cited at the bottom of this page, is the basis for this content.
PDMS and Mechanical Properties
Silicone compounds can be based on polydimethylsiloxanes (PDMS), polymers that have many desirable properties but lack mechanical strength. Although PDMS is chemically inert and has good thermal stability, it’s subject to tearing and can’t withstand high elongation or strain. Elongation, a measure of deformation, occurs before a material breaks when subjected to a pulling force, or tensile load. Strain, another measure of material deformation, is the ratio of the change in length to the original length.
To improve the mechanical properties of PDMS, inorganic fillers can be added. Silica, one of the most common materials in the Earth’s crust (and the main ingredient for silicone rubber), is widely available and has excellent reinforcing capabilities. It’s used in both HCR and LSR rubber. With any filler in an elastomeric matrix, the addition of rigid particles creates what the researchers describe as “obstacles to the material’s flow”. This isn’t the only reinforcement mechanism, however, and silica mainly strengthens mechanical properties by bonding with PDMS chains.
HCR and LSR rubber both obtain their end-use properties through crosslinking, a chemical reaction between polymer chains that knits them together. Curing, or vulcanization as it’s also known, is what causes the long polymer chains in the rubber to become crosslinked, or interconnected, both with each other and with filler materials. A catalyst, a chemical substance that accelerates or otherwise changes the conditions for a reaction, plays an important role.
For engineers, crosslinking matters because of its effects on mechanical properties such as elongation at break, strain recovery, and compression set – a measure of a material’s ability to rebound to its original thickness after a compressive stress is removed. Depending on the catalyst there are implications for biocompatibility as well. It’s important to note, however, that while the crosslinking mechanisms described in the research paper are typical, they’re not the only ones.
Basically, cross-linking reinforcements are critical for silicone to have mechanical properties like moderate tear resistance, excellent compression set resistance. Too much cross-linking ends up having detrimental effects to the rubber-like properties however.
High Consistency Rubber (HCR)
HCR rubber is heat-cured and often (but not always) peroxide or platinum-catalyzed. Before cross-linking occurs, HCR is a solid, gum-like material. After vulcanization, it gains it’s elastic mechanical properties. Both LSR and HCR can me made with less or more crosslinks depending on the application.
The main difference between LSR /HCR is the polysiloxane monomer chain length. When HCR is peroxide-catalyzed, however, biomedical applications require post-curing to remove residues. Platinum-catalyzed silicones often do not require post-curing.
Note: Stockwell Elastomerics provides platinum-cured high consistency rubber.
At higher temperatures, HCR can exhibit residual strain. In other words, some amount of permanent deformation may remain even after a load is removed. The researchers posit that this is because of the material’s “long dangling chains” in its chemical structure. HCR can match LSR in terms of strain recovery, but it can take hours for HCR to reach a completely stable state. HCR also has a lower elongation at break and a higher tensile strength up to the point of failure.
Liquid Silicone Rubber (LSR)
Liquid silicone rubber (LSR) is typically addition-cured and platinum-catalyzed. Before crosslinking occurs, it’s a highly viscous liquid, similar to Crisco. After crosslinking, LSR has a more regular network of crosslinks than HCR. In addition, LSR offers greater elasticity and is free from molecular byproducts that would prevent its use in biomedical applications. LSR can be cast, extruded or injection molding and hardens with the application of heat. All processes require specialized equipment specifically meant for LSR processing.
Platinum catalysts are relatively expensive, but only a small amount of them are used during curing. In terms of mechanical properties, HCR actually has a lower compression set than LSR. The lower the compression set percentage, the better the material resists permanent deformation under a given deflection and temperature range. LSR’s elongation at break percentage is twice that of HCR, meaning that LSR can be pulled or stretched much more under a tensile load. With that said, both LSR and HCR based silicones have the best compression set and stress relaxation properties of any elastomer. (More information about compression set and stress release is available in 2 posts from the Elastomerics Blog: Material Selection – Compression Set Resistance and Stress Relaxation and Rubber Compression Set, Stress Relaxation and Creep.)
HCR vs. LSR: Mechanical Properties and Material Selection
This table compares HCR silicone vs. LSR silicone based on data from the research publication.
|HCR vs. LSR|
|Silicone Elastomer||Relative Cost||Catalyst||Compression Set||Tensile Strength||Elongation at Break||Supports|
|HCR||Mid||Peroxide||Excellent||Higher||Lower||Compression Molding Extrusion|
Photos of HCR & LSR
|Raw gum HCR compound prior to heat cure in molding||Raw Part A of LSR compound scooped with a spatula|
Contact Stockwell Elastomerics for HCR vs. LSR Silicones
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Stricher, Arthur & Rinaldi, R. & Barrès, Claire & Ganachaud, Francois & Chazeau, Laurent. (2015). How I met your elastomers: from network topology to mechanical behavior of conventional silicone materials. RSC Adv.. 5. 10.1039/C5RA06965C.