Designing with Silicone Materials

This article focuses on the nature of silicones and how this can affect process and the end device. For those readers with a limited knowledge of silicone materials, a brief review is necessary to discuss the variations in those products. The basis of virtually every silicone system is the silicone polymer, a repeating chain of silicon and oxygen molecules. For more information about silicone polymers click (http://www.nusil.com/whitepapers/index.aspx) Polymers can be supplied neat as fluids or can contain reactive species and reinforcing fillers to form greases, adhesives, gels, or elastomer systems. The most common commercial method of producing silicone polymers is via an acid or base catalyzed equilibration reaction of silicone cyclics and endblocking species. The reaction produces a distribution of polymer chain-lengths rather than one discrete polymer chain length. While further processing can narrow the distribution, the result is still variable. Polymer chain length can alter physical properties of adhesives, gels and elastomers – lower chain lengths can increase the system modulus by increasing the amount of crosslinker per unit area and lower the elongation of the system. Reactive species such as catalysts, crosslinkers and reinforcing fillers have some level of variation as well. The multiplication rule of probability statistics essentially rules out achieving the same result time after time when factoring in the variability of these systems. Statistical process control can at best reduce but not eliminate variability associated with these systems. Overall, engineers must realize that silicones can vary from lot to lot, this is evidenced by the allowable range of physical properties on most material certifications. What is the most relevant here is how the variability affects the process and the end device. In our example of a lower polymer chain length described above – two question immediately come to mind: Can the process to mix and dispense adequately handle the lower viscosity? How will the increase in modulus affect the final device? These questions can multiply rapidly as more key physical properties are considered. Other effects such as inter-property relationships must be considered as well (see previous Insight on this topic). Validation processes can go a long way to evaluate these materials. Good modeling programs and selecting product lots that have significant physical property differences within the certified range are just two ways to handle the variability associated with silicone systems. Visit www.nusil.com for even more information on this subject.

Porosity or bubbling

Porosity
Porosity is defined as the state or quality of having bubbles or pores. Although this may be desirable in an application requiring a silicone foam, it most certainly has no benefit in areas regarding molding, extruding, and coating. Certain silicones, when cured without pressurization may exhibit porosity. This not only raises aesthetic issues, but can result in a reduction of physical properties as well.
LSR’s may contain unusually high levels of inhibitor to allow for adequate work times. Because of this, they must be molded under pressure to avoid any chance of bubbling. If porosity is experienced, check the mold gates and injection pressures. Increasing mold temperature is also recommended to achieve a bubble free component.
Porosity in dispersed silicones can be avoided by using a ramp cure schedule, which gradually brings the material up to the ultimate vulcanizing temperature. This allows any residual solvent to be evaporated and avoids the risk of possible bubbles from rapid solvent evaporation.
NuSil Technology has designed a method for predicting porosity in HCR’s. NTTM-073 evaluates high consistency materials for porosity during the initial curing stage. Through simple planning and implementation of consistent processing techniques, porosity can easily be avoided in all fabrication methods. Visit www.nusil.com for more information.

Uncured Silicones - Creping

"Creping" Effects in Silicones


Crepe hardening, or crepe aging, is a phenomenon you may have experienced with LSR’s (http://www.nusil.com/Healthcare-Silicones/restricted/RestrictedProducts.aspx?SCID=131) or HCR’s (http://www.nusil.com/Healthcare-Silicones/restricted/RestrictedProducts.aspx?SCID=129). Your first concern may be that the product is unusable because of its stiffness. However, this creping observed in the uncatalyzed silicone base is completely reversible by mechanical shearing.

The source of creping lies in the polymer/filler interaction. The chemistry of the silica surface contains copious amounts of hydroxyl groups. These OH groups form a direct attraction with the polymer through hydrogen bonding. Excess hydrogen bonding between the polymer and silica can cause stiffness and result in a base that is practically unworkable. Yet a lack of polymer/silica interaction through hydrogen bonding will result in a material with poor properties. The key to maximizing physical properties with the silicone base lies in a balanced treatment of the silica surface. Proper treatment with treating agents will "cap" much of the OH groups, yet leave enough "uncapped" to allow a certain degree of polymer/silica interaction. As a base rests on the shelf over a prolonged period of time, uncured silicone rubbers will form hydrogen bonds with the elastomers components.

The fact is, all uncured silicone elastomers will experience some degree of crepe again over an extended period of time. Mechanical shear through milling or pumping will reduce hydrogen bonds, thus reverse the effects of creping, and allow a more consistently processible material. For more information on this topic or for any definitions of the terms used above, please visit http://www.nusil.com/Literature/TR/Silicone%20Terms%20and%20Definitions.pdf

Photonics Related White Papers

White Paper Library

Throughout the year, the NuSil Technology www.nusil.com offers specific white papers available by registering at www.nusil.com/whitepapers . We thought this might be an opportune time to list all of the papers that are available on the website related to photonics. Below is a list of papers with a brief description of the papers contents:

Choosing a Silicone Adhesive and Primer System - More of a guide than a paper, this document touches on the chemistry of silicone adhesives and primers and investigates adhesive / primer combinations on a variety of substrate surfaces

Optical Silicones for Use in Harsh Operating Environments – Presented at Optics East 2004 Philadelphia, Pennsylvania

Terms & Definitions – a glossary of silicone terms

Aerospace Related White Papers

White Paper Library

Throughout the year, the NuSil Technology www.nusil.com makes available specific white papers available by registering at www.nusil.com/whitepapers. We thought this might be an opportune time to list all of the papers that are available on the website. Below is a list of papers with a brief description of the papers contents:

Choosing a Silicone Adhesive and Primer System - More of a guide than a paper, this document touches on the chemistry of silicone adhesives and primers and investigates adhesive / primer combinations on a variety of substrate surfaces

Adhering to Difficult Substrate with Silicone Adhesives – Presented at SAE Montreal, Canada 2003

Accelerating the Cure of Silicone Adhesives – Presented at the International SAMPE Technical Conference 2002 Baltimore, Maryland

Low Outgas Pressure Sensitive Adhesives for Aerospace Applications – Presented at SAMPE Conference 2004 Long Beach, California

Terms & Definitions – a glossary of silicone terms

Healthcare Related White Papers

White Paper Library
Throughout the year, the NuSil Technology makes specific white papers available by registering at www.nusil.com/whitepapers. We thought this might be an opportune time to list all of the papers that are available on the website. Below is a list of papers with a brief description of the papers contents:

Choosing a Silicone Adhesive and Primer System - More of a guide than a paper, this document touches on the chemistry of silicone adhesives and primers and investigates adhesive / primer combinations on a variety of substrate surfaces

Drug Delivery Market Summary – This document outlines the sub-divisions of drug delivery, transdermal, implanted, etc. and suggests applicable silicone materials. Bibliography contains references to other industry publications / documents.

Silicone Adhesives & Primers on Low Surface Energy Plastics and High Strength Metals For Medical Devices – Presented at MEDTEC 2004 Stuttgart, Germany

Silicone as a Material of Choice for Drug Delivery Applications – Presented at the 31st Annual Meeting and Exposition of the Controlled Release Society 2004, Honolulu, HI

Terms & Definitions – a glossary of silicone terms

Visit www.nusil.com for more silicone related information.

Adhesive Film Technology

NuSil Technology, a manufacturer of silicone-based materials for healthcare, aerospace, electronics and photonics, announces the addition of thin film adhesive technology to its family of engineering products (http://www.nusil.com/engineering-silicones)

This film adhesive is designed for ease-of-use in certain bonding and fastening applications and can be customized to each application as standard, optically clear, and electrically and thermally conductive sheets. NuSil’s CV-2680-12, a reinforced, static-dissipative thin film adhesive, represents one of many possibilities in this technology.

Thin films adhesives have distinct advantages over traditional liquid adhesives. Users can die-cut precisely what is needed, and the low-flow characteristics of these adhesives eliminate overflow and many clean-up issues.

Silicone, which is known as an exceptional adhesive in applications in which temperature cycling is an issue, can be useful in aerospace and electronics components and assemblies. In addition, NuSil Technology can customize this adhesive technology for specific properties such as low outgassing and thermal conductivity.

NuSil Technology’s CV-2680-12 is provided in 12-by-12-inch sheets and is reinforced with a fiberglass scrim. More information is available at http://www.nusil.com/engineering-silicones/adhesivefilm. Those interested in custom-formulated adhesive films may contact NuSil Technology directly at http://www.nusil.com or at 805-684-8780

Silicones and Light Emiting Diodes (LED)

LED’s or light emitting diodes is not a relatively new concept. Most calculators have employed this technology in their display systems for some time. Many people are probably aware that the use of LED technology has expanded well beyond the flashing numeral 12:00 on our clock radios. LED’s can be found in massive illuminated billboard ads, flashlights, car taillights, and most notably, in traffic signals where it is estimated that 39 percent of red lights and 29 percent of green lights employ LED technology in the United States. Its thought by many that LED technology will replace conventional lighting; namely, incandescent and fluorescent bulbs, by 2025.
Why the drastic shift in lighting systems? The answer lies in operating efficiency. Incandescent bulbs waste 95 percent of the electricity entering the bulb in heat (who hasn’t burned their fingers changing a bulb). Fluorescent bulbs fare better, converting 20 to 30 percent electricity into light. LED’s convert considerably more >90% of electricity into light. The California Department of Transportation estimates the current replacement of incandescent bulbs in many traffic lights has trimmed $10 million annually from the state’s electric bill. Replacement is also a factor with LED lighting lasting up to five times longer than traditional bulbs.
So what’s the hold up? A few issues still face rapid adoption and commercialization of LED’s. The first is the production of a bright white light. Academic researchers partnered with industry are racing to produce bright white LED’s. The second is the cost efficient production of these LED’s. Traditional LED’s use gallium nitride crystals, grown on sapphire substrates. The growing of numerous crystal layers and the differing chemistries of those layers is a time consuming and variable process. Phosphors are added to the configuration to produce certain colors but can reduce the efficiency of the LED. The current method of producing a bright light LED is too expensive at this point to be a viable replacement for most conventional lighting.
Where is the silicone? Silicone is part of the packaging configuration of an LED. Silicones are essential to the protection of the LED and are considered the material of choice because of the high operating temperatures of LED’s. As was eluded to earlier, its critical that each component of a LED not detract from the efficiency of the system. Silicones employed in LED systems must maintain maximum optical clarity and exhibit the proper refractive index.
NuSil Technology (www.nusil.com) is excited about the long-term prospects of LED lighting and feels the industry will benefit from the refractive index matching capabilities our Lightspan products offer. Visit our website for more information about refractive index matching and other white papers available at www.nusil.com/engineering-silicones/photonics.

Silicones in Drug Delivery

Silicones in Drug Delivery

In 1999 NuSil Technology, http://www.nusil.com , produced an article that investigated the use of antimicrobial agents in silicone elastomer systems and the benefits to the end user. The article explained that the molecular properties of silicone, namely the Si-O bond found in siloxane polymers, created large free volume in silicone elastomer systems. This free volume makes permeation of molecules and compounds possible and positions silicone as the material of choice.

Five years have passed and we find ourselves re-addressing the issue, but on a much larger scale. A quick internet patent search reveals that both tin catalyzed and platinum catalyzed silicones can be of use in drug delivery. Some of these patents list silicones combined with various pharmaceutical agents as useful antibiotics, antidepressants, anxiolitics, antifungals, antivirals, analgesics, and permeable to selected vitamins. With zero order release rates, many of these pharmaceutical infused silicones can release 10 to 500 micrograms of agent per day. Device design and the addition of fatty acid esters, thought to improve the hydrophilicity of the silicone, can enhance release rates. Transmucosal, implantable, and transdermal drug delivery technologies are favored in theory because oral degradation of many compounds and patient compliance are issues currently facing traditional orally administered pharmaceuticals.

The market for drug delivery technologies has been expanding for some time. The fastest growing segments of this market are transmucosal and transdermal delivery technologies. As pharmaceutical agents loose patent protection, pharmaceutical manufacturers look for methods to extend product lifecycles. Novel delivery methods can provide this extension.

For a market summary regarding silicones and drug delivery applications, please contact your NuSil Technology representative or register for this information on our website at: http://www.nusil.com/healthcare/drug%20delivery