Seeking Silicone and Suppliers

As manufacturers increase their use of liquid injection molding, silicone is becoming a highly desired material.

13 Min Read
Seeking Silicone and Suppliers

MOLDING


Liquid silicone rubber, shown here being removed from a container, is reinforced with silica.

In 1872, John Wesley Hyatt used a rudimentary plastic material called celluloid when he patented the first injection molding machine. Over the next 50 years, manufacturers started employing liquid injection molding (LIM) to mold simple items such as buttons and pocket combs. LIM expanded rapidly during World War II due to demand for inexpensive, mass-produced products. Today, manufacturers use LIM to create a wide array of medical components and products, including hydrocephalus shunts, baby bottle nipples, balloons, gaskets, O-rings, and implantable medical devices. Making the correct material selection for a molded device is more critical than ever.


The complexity of combination medical devices, advanced technology of modern molding equipment, and increasing regulatory concerns all require careful selection of a material and material supplier according to the fit, form, and function of the medical device. More and more, medical device manufacturers are selecting highly specialized silicone elastomers for molding their medical devices. This article examines why silicone has become the material of choice in the healthcare industry, how to examine material options, and how to select the best material supplier.
Silicone Elastomers
Manufacturers often choose silicone for their medical device due to its established pedigree with bioinertness and biocompatibility, minimal concentrations of low-molecular-weight species, favorable physical properties, and the ability to be altered at the polymer level. Silicones entered into healthcare and medical applications in the 1950s after extensive use in the aircraft industry during the previous decade. In 1954, various tissue cultures taken from animals known to be particularly sensitive to foreign materials showed no deviation from their usual growth pattern when exposed to liquid, semisolid, and rubberlike silicone products.1
Silicones have been characterized as biologically and toxicologically inert as a result of this testing.2 Within 20 years, a considerable body of work established that silicone fluids and cross-linked silicone systems did not demonstrate the harmful consequences seen with other materials with subcutaneous, intracutaneous, and intramuscular exposure. Therefore, silicones are used in a variety of applications including implanted medical devices, drug-delivery devices, and disposable medical devices.
Silicone elastomers are ideal for molding because they can match the processing technique being employed—i.e., they can either cure at low temperatures or withstand the high temperatures associated with molding. When choosing the optimal silicone elastomer for an application, a medical device manufacturer must first decide whether to use a high-consistency rubber (HCR) or a liquid silicone rubber (LSR).
When silicone elastomers first appeared, only HCR was marketable due to material property limitations of other materials. An HCR consists of a high-molecular-weight polymer combined with silica to produce a material that can be molded, extruded, or calendered into a useful component. HCRs have a claylike consistency and are primarily formulated in one- or two-part systems (peroxide and platinum catalysts, respectively).

A technician prepares to cut a piece of the high-consistency rubber to weigh for milling.

LSRs became useful to fabricators after the development of the platinum-catalyzed cure system, which allowed silicone material to be pumped into molding equipment. LSRs are reinforced with silica and typically use medium-molecular-weight polymers. They are nonslumping, have a petroleum jelly–like consistency, and can be easily pumped into an injection molding machine to form molded parts. LSRs are routinely formulated in two-part systems in a 1:1 mix ratio, making them user-friendly for LIM applications. Ultimately, the choice between HCR and LSR comes down to the most efficient and cost-effective molding method to manufacture a part.


Molding Methods
The three methods of molding are compression, transfer, and LIM. Both compression and transfer molding are suitable for low-volume production and both methods typically utilize HCRs. Compared with LIM, cycle time for compression or transfer molding parts is longer (about 3 minutes). However, the initial cost of production is relatively lower because they both enable the use of less-expensive equipment and less mold design and prototyping work. When looking at higher annual volumes, LIM provides shorter cycle times (typically under 1 minute), nominal scrap loss, consistent part quality, and minimal risk of contamination. It also reduces labor costs by eliminating material prehandling and minimizing required cleaning.
However, LIM has some disadvantages due to the use of more-complicated molds and the need to balance cycle time with cavitation. The disadvantages of LIM include extensive mold design during prototyping, expensive equipment investment, and large upfront tooling capital expenditures. LIM equipment has traditionally required the use of lower-viscosity LSRs. However, modern LIM equipment can mold with high-viscosity LSRs and even HCRs. When choosing between an LSR or an HCR, medical device manufacturers must consider the material's uncured and cured physical properties.
Cured Properties

Table I. (click to enlarge) A listing of various material categories and their characteristics.

The material's cured physical properties determine the strength, durability, feel, and look of the medical device. First, the appropriate durometer, or hardness, must be determined. LSRs are often available in Shore A durometers of 5–80, or from a very soft, pliable material to relatively rigid. Tensile properties are also important, including elongation, tensile strength, and stress at strain (modulus). Elongation is the percent strain at the point of material rupture, and typical LSRs offer 80–1200% elongation. Tensile strength is the maximum stress at rupture (psi or kappa) of an elastomer, and typical LSRs range from 500 to 1300 psi (3500–9000 kappa). Stress at strain, or modulus, is the amount of stress incurred at a specific elongation. Stress-strain properties of silicones depend on the molecular architecture of the materials. Finally, a manufacturer should consider the material's tear strength, or energy required to tear apart a cut specimen that has a standard geometry. Table I provides a sampling of materials.


After a manufacturer has determined the optimal material for its medical device by examining the material's uncured and cured properties, the next step is to examine the way a material transitions from its uncured state into a cured form. Selecting a material with a long work time (or pot life) can be crucial. Work time is defined as the time available to work with catalyzed silicone at ambient temperature before it becomes unusable due to cross-linking. For LSRs, work time is tested by mixing the two parts and then, at specific intervals, inserting a spatula into the material and slowly withdrawing it, thus elongating the material. An LSR's work time is considered expired when material breaks prior to elongating a minimum of one inch.
Most platinum-cure LSRs have typical work times of up to 72 hours, providing flexibility when manufacturing. A manufacturer can mix the material, load LIM equipment, and use it for multiple shifts or even over a weekend. This not only reduces labor time in terms of having to reload material each shift, but it also reduces the need to purge equipment to prevent material from curing within it.
Cure Profile
For LIM, one of the most important properties to examine is the cure profile of the silicone being considered. Testing a mixed material in an oscillating disk rheometer (ODR) provides an accurate cure profile. With the ODR test, the silicone sample is placed on an oscillating disk that is between two heated platens. The test is typically conducted at a set temperature, and the ODR records the increase in torque response as the silicone begins to cure. This cure profile details the material's rheology and cure rate when heated under stress. Important test values to look at are: minimum torque, or the initial maintained torque value prior to material curing; maximum torque, or the highest maintained torque value after the material curing; scorch time, or the time at which the material exhibits one pound of torque (Ts1) above the minimum torque at the given temperature; and T90, or the time at which the material exhibits 90% of the maximum torque at the given temperature.

Figure 1. (click to enlarge) An ideal rheometry curve for LIM. The darker shaded area represents the <1 minute Δ cure (time it takes to reach T90 after achieving scorch time).

When liquid injection molding, a liquid silicone is often pumped into a heated mold, and because of this, possibly the most important value when considering a material for LIM is the delta (Δ) cure. The Δ cure is defined as the time it takes to reach T90 after achieving scorch time. Having a Δ cure of less than one minute (preferably less than 30 seconds) prevents molding concerns such as partial curing or voids in curing, and it is important to both appearance and porosity. See Figure 1 for an example of an ideal rheometry curve for LIM.
Interaction with Other Materials
OEMs should examine each material's chemical or mechanical behavior with the silicone material and the molding process. For instance, some manufacturers may want to impregnate a silicone with an active pharmaceutical ingredient (API). The silicone can then be molded into a drug-delivery device that releases the API over time. However, if the API is temperature sensitive, i.e., degrades after exposure to high temperatures, this trait may preclude a manufacturer from using high-temperature molding techniques. In such cases, an OEM must select a low- or room-temperature vulcanizing material.
Cure inhibition is another issue that a medical device manufacturer should pay particular attention to during design and design transfer. The cure mechanism of some silicone materials can be permanently inhibited or poisoned, which occurs when adjacent substrates or gases slow down or deactivate a cross-linking reaction. This undesirable effect has earned these chemical agents the trade term poisons. They can lead to unacceptable variations in the manufacturing process and thus the finished product.
Various chemical elements and compounds may retard the platinum catalyst cure systems typically used to cure silicone in medical device applications. Some common materials are amines, amides, or UV-cured adhesives; certain elastomers such as latex and natural rubber; some chlorinated plastics; certain tin or sulfur-based compounds; and various organic materials such as wood, leather, and clay. During the initial design and design transfer process, manufacturers should test candidate materials that may come in contact with the silicone during the manufacturing process.
Platinum Cure Systems
Medical devices containing silicone often use platinum cure systems due to their production floor versatility and the fact that there are no by-products of the cure mechanism. Such systems are most commonly two-part systems, with each part containing different functional components that begin curing once mixed. Generally, one part contains vinyl-functional silicones and a platinum catalyst, and the other part contains a vinyl-functional polymer, a hydride-functional (Si-H) cross-linker, and a cure inhibitor. These cure inhibitors adjust the system's cure rate and are different from the cure poisons previously discussed. The cure chemistry involves the direct addition of the Si-H functional cross-linker to the vinyl-functional polymer, forming an ethylene bridge cross-link. The material's cured consistency can be particularly sensitive to the final cross-link density. Severe cure inhibition can lead to complete cure elimination, and modest cure inhibition can result in a lower final durometer (i.e., the material will be softer than expected). Even weak cure inhibition can cause the silicone material to appear wet at the substrate interface.
End Use
The final critical aspect that manufacturers should examine when making material selection is the medical device's end use. There are typically three end-use applications: long-term, short-term, and disposable. Long-term (unrestricted) materials are for applications in which the device will be used for chronic human implant (>29 days). Short-term (restricted) materials are for applications in which the device will be used for temporary implant (≤29 days). A material may also be used for a disposable medical device. The end use is relevant to the material cost, material characterization, and regulatory support provided by the material supplier.
Selecting a Supplier
Once a manufacturer selects a material, the next step is selecting a supplier. A device manufacturer should select a material supplier that understands the particular needs of medical OEMs and demonstrates experience in and commitment to working with regulatory authorities. Suppliers should have the following attributes:
•A robust quality system certified to ISO 9001:2000. •Experience with FDA's device master files. •U.S. Pharmacopeia, European Pharmacopoeia, and CGMP compliance.

A worker mixes liquid silicone rubber by hand.

In the long run, a medical device manufacturer needs to work with a partner that not only adheres to 21 CFR Part 820 requirements, but also one that can help get the device to market. A material partner that already has an established relationship with regulatory bodies can be an asset in achieving this goal. OEMs should select a material supplier that demonstrates a willingness to collaborate, establish partnerships with their clients, and show shared ownership of projects. Manufacturers should also make sure that a supplier is willing to sign a confidentiality agreement and allow its clients to perform site visits and company audits.
In addition, a material supplier should have an effective design control procedure to define the processes used to work with the material. The supplier should also have an established design transfer procedure to take the product from the design phase to full-scale manufacture. Efficient and thorough design control and design transfer can predict, identify, and prevent potential problems when working with silicone materials.
When working specifically with silicone, it may also be helpful for manufacturers to assess the material supplier's setup on the production floor. For instance, when using LIM equipment, side-by-side tube kits that mix small quantities of two-part materials while dispensing may be used during prototyping, and then larger drum kits may be required during full-scale production. In addition, some medical devices may also require sterilization, so a material supplier may need to sterilize packaging before shipment or package material in a configuration that allows for sterilization.
Conclusion
Making the correct material selection for a molded device is critical. For LIM applications, highly specialized silicone elastomers have become one of the materials of choice in the medical device industry. But OEMs must ensure that they find a supplier that can meet their needs and help get their devices to market. Involving the raw material partner from the onset of a project will lead to a more easily marketed and manufactured medical device.

Alexander Kurnellas is lead technical writer for Nusil Technology LLC (Carpinteria, CA). Robert Umland is national sales manager for the company.


References

1. W Lynch, Handbook of Silicone Rubber Fabrication (New York: Van Nostrand Reinhold, 1978).

2. W Noll, Chemistry and Technology of Silicones (New York: Academic Press, 1968).


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