What is High-Consistency Rubber?
High-consistency rubber (HCR) is a substance formed by combining a high molecular weight polymer with silica, resulting in a material that can be shaped through molding, extrusion, or calendaring to create useful components.
High-consistency rubber silicone (HCR) is a versatile substance with applications in medical devices and various industries. It is commonly utilized in a diverse range of end-use scenarios, including tubing, balloons, sheeting, and certain molded parts. HCRs are composed of a high molecular weight polymer blended with silica, yielding a silicone that exhibits a clay-like texture when in its uncured state.
HCR In Medical Devices
Silicone is often the preferred choice for medical device manufacturers due to its well-established track record of biocompatibility, a wide range of processing parameters, and excellent physical properties. Initially, one of the earliest applications of HCRs was in the development of implantable hydrocephalus shunts. Since then, the use of HCRs has expanded to encompass a vast array of medical applications, both implantable and non-implantable.
One of the standout features of HCRs is their versatility. They can be processed using various fabrication methods, such as extrusion, calendering, and compression or transfer molding. In comparison to other silicone products like liquid silicone rubber (LSR), HCRs generally exhibit greater strength and possess more robust physical properties.
Furthermore, the proven track record of HCRs in numerous approved implant and non-implant applications adds to their appeal. This extensive heritage makes it easier for medical device manufacturers to consider incorporating HCRs into new devices or developing next-generation iterations of existing devices.
Versatility In HCRS
Understanding the physical properties and process considerations of HCRs, as well as the advantages offered by peroxide- and platinum-catalyzed solutions, is crucial.
Peroxide-catalyzed systems have a distinct advantage in that the curing mechanism, which involves cross-linking the polymers to cure the HCR, is only initiated when the HCR is subjected to heat. This extended work time is highly beneficial for molding or extrusion processes. For larger and more intricate parts, delayed cure initiation allows for sufficient time to fill the mold. Conversely, operations involving smaller and simpler parts may benefit from faster curing to achieve higher throughput. Silicone HCRs can be tailored to adjust the cure rate at a specific temperature for optimal performance. It’s important to note that peroxide-catalyzed systems require a post-curing process to remove residual byproducts.
The “random” cross-linking characteristic of peroxide-cure HCRs imparts unique elastomeric properties that are valuable in applications such as balloons, where “tension set” is a critical attribute. If a balloon needs to be inflated and then return to its original shape upon deflation, a peroxide-cure HCR may offer the best solution. Similarly, in peristaltic pump applications, silicone tubing may undergo cycles of repeated compression and relief.
Medical device manufacturers often select silicone for components and devices due to its established biocompatibility, wide processing parameter ranges, and excellent physical properties.
Platinum-catalyzed systems typically comprise a Part A component containing the platinum catalyst, and a Part B portion containing hydride-functional cross-linkers and cure inhibitors. When these two parts are combined, the HCR retains its consistency before curing for one to two hours. A key advantage of using platinum-catalyzed HCRs is the ability to accelerate the curing process with heat, resulting in faster cure times and increased throughput.
Additionally, platinum-catalyzed HCRs typically yield significantly higher physical properties compared to traditional peroxide-catalyzed HCRs. This characteristic can be highly valuable for specific medical device applications that involve molded or extruded components,
HCR Processing For Medical Device Fabrication
When working with HCRs, it is important to follow specific steps depending on the chosen fabrication methods. For instance, prior to molding or extruding, HCRs require processing on a two-roll mill. This mill effectively softens and blends the material by rotating cylinders in opposite directions at slightly varying speeds, creating shear forces that modify the HCR’s consistency. As a result, the HCR becomes softer and more capable of flowing smoothly through the die or intricate areas of the mold.
If the HCR is a two-part material, Parts A and B must be individually softened on a cooled mill before being combined. This precaution ensures that the milling heat does not trigger premature curing once the two parts are mixed. Similarly, if the HCR is a one-part material, it still needs to be softened on the mill before use.
Different end-use applications require different fabrication methods. For example, when manufacturing a Foley catheter or any medical device incorporating silicone tubing, extrusion is typically the most efficient production method. Once the processing method is determined, additional specific requirements associated with the tubing need to be considered. Factors such as desired properties (e.g., flexibility, rigidity), tensile strength, and modulus will help guide the decision between a peroxide-cure or platinum-cure system.
The broad processing parameters of silicone elastomers make them ideal for molding. In the case of molded products like hydrocephalic shunts, attention must be given to the valve mechanism’s performance within the shunt. Manufacturers aim to select an HCR that reliably offers the appropriate modulus, tension set, and other elastomeric properties to ensure proper valve functionality.
HCRs are frequently calendered into flat sheets that can be die-cut. This method is commonly employed in creating gaskets used as seals in medical devices. By utilizing a peroxide-cure HCR, it is possible to produce self-adhering gaskets. Properly stored, an uncured peroxide HCR sheet has an almost indefinite shelf life. A supplier can fabricate the sheet, package it, and ship it to another manufacturer who will die-cut the gaskets. The gaskets are then pressed in place and cured to provide a complete seal.
Key Factors for Using HCRs
HCRs possess versatile material properties, processing features, and cure options that make them highly suitable for a wide range of medical device applications. When selecting HCRs for a project, there are several factors that device designers and fabricators should take into consideration.
Integration/interaction with other materials: One advantage of HCRs is their ability to incorporate additives into the formulation prior to curing, catering to specific medical device requirements. For instance, device manufacturers may wish to include additives like colorants, radiopaque fillers, antimicrobial agents, or even active pharmaceutical ingredients (APIs) in the device.
It is important to assess how these additives or other materials interact with the silicone and the molding process in terms of chemical and mechanical behavior. If any additives are temperature-sensitive or can adversely interact with the formulation, resulting in incomplete curing, the silicone supplier may offer customized solutions to address these concerns.
Preventing cross-contamination: Platinum-cured HCRs, like other platinum-cured silicones, can be negatively affected by certain chemicals that come into contact with the silicone prior to curing. These contaminants can partially or completely inhibit the platinum-catalyzed cure system. Common inhibitors include sulfur-containing materials (e.g., natural rubber, latex, and neoprene), nitrogen-containing materials (e.g., amines), or organotin-containing materials (e.g., condensation-cured silicones). To prevent contamination, it is essential to follow clean manufacturing practices, including using dedicated instruments like spatulas for handling HCR and thoroughly cleaning all surfaces between uses.
Flexibility in HCRs: Given that each medical device and fabrication process is unique, leading silicone providers offer solutions that provide greater flexibility during the manufacturing process
Visit our website – A1 Silicones to learn more about Silicone Grease