silicone-solutions

Designing with Silicone Materials

2006-08-29T11:13:00.000-07:00

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

2006-08-23T09:42:00.000-07:00

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

2006-08-15T14:50:00.000-07:00

"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

2005-06-20T15:19:00.000-07:00

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

2005-06-20T15:08:00.000-07:00

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

2005-06-20T15:04:00.000-07:00

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

2005-06-08T08:50:00.000-07:00

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)

2005-05-11T11:02:00.000-07:00

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

2005-03-30T13:21:00.000-08:00

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

Rheometry

2005-03-22T14:05:00.000-08:00

Rheometry is an extremely useful tool for determining the flow properties of various materials by measuring the relationships between stress, strain, temperature, and time.
There are several types of rheometers on the market. We use oscillating disc rheometers.

Here’s how they work: Approximately 10 grams of high consistency (platinum or peroxide catalyzed) or liquid silicone rubber (LSR) materials are placed on an oscillating disc and rotor assembly (5° arc) which is preheated to 240°F. A die in the top platen, also at temperature, is quickly lowered such that it comes to rest a few millimeters above the disc, forming a narrow cavity. This action causes the material to flow and fill the cavity between the disc and die.
As the material begins to vulcanize, it produces a torque, or angular force, on the disc. A load transducer on the disc/rotor assembly records these torque changes as changes in voltage. These voltage signals are then sent to a chart recorder (or data acquisition software) where they are displayed on x-y plots as torque (inch pound units) vs. time.

An example of a rheometry trace for a 50 durometer, platinum catalyzed high consistency silicone can be found at http://www.nusil.com/newsletter The top platen is lowered, positioning the die directly above the oscillating disc. The material on the disc now flows into this cavity. The transducer records of torque increases. The material at this time is still effectively at room temperature. The torque is an indication of the viscosity (plasticity) of the material.
Thermal energy is absorbed by the material. Torque decreases to a minimum. The viscosity of the material decreases as the material temperature increases. The chemical reactions associated with crosslinking have not yet begun.

This coordinate is defined as minimum torque

Hydrosilation reactions (platinum systems) or free radical addition reactions (peroxide systems) begin. The material begins to crosslink. The torque increases.

This coordinate is the scorch time.

It is defined as the time at which the torque has increased to 1 in-lb above minimum torque.

Another coordinate of interest is when the material is 90% cured.

This coordinate, identified as T90, is defined as the time at which the torque reaches 90% of its maximum value.

The slope of line between scorch time and T90 is also characteristic of the material.
The last coordinate of interest is when the torque reaches a maximum value.

This scalar value is defined as maximum torque.

We use Rheometry to monitor intermediate processes and to optimize cure rates. Rheometry has also proved to be an extremely useful QA tool. Pass/fail gates for scorch time, T90, and maximum torque used to insure lot-to-lot consistency.

"Great, but what can Rheometry do for me?" you ask. Plenty. Mold makers can make use of scorch time information to determine the allowable fill-time of their molds. Injection gates and runners can then be designed to optimize this process. Similarly, the scorch time/T90 interval is important to process engineers who wish to maximize throughput. Finally, extrusion engineers have found T90 values to be useful in setting HAV temperature and time parameters.

Viscosity

2005-03-18T12:43:00.000-08:00

Viscosity is a term that we in the materials industry typically use to describe the "feel" of uncured silicone materials. Viscosity is a rheological term, and rheology is the science of the deformation and flow of matter. Qualitatively defined, viscosity is defined as the property of a material to resist deformation increasingly with increasing rate of deformation. We measure this property as the shear stress (a component of stress parallel to the area considered) divided by the rate of shear (relative deformation per unit time), often expressed in centipoises or Pascal seconds. NuSil Technology TM-001 is our test method for measuring viscosity in accordance with ASTM D-1084, D-2196. Kinematic viscosity is used for materials with a viscosity of less than 1000 centipoises, where most viscosity measurements at NuSil are conducted with a Brookfield viscometer.

Many terms are used to describe the consistency of silicones and the following is a list of definitions:

Newtonian Fluid : characterized as a constant value for the quotient of the shear stress divided by the rate of shear.
Thixotropy : a decrease in viscosity under constant shear stress or shear rate followed by a gradual recovery when the stress or shear rate is removed.
Pseudoplastic or shear thinning: The reduction of viscosity with an increasing rate of shear.
Dilatancy or shear thickining: The increase of viscosity with an increasing rate of shear.

Viscosity is an extremely important factor in assessing the type of equipment needed to process a silicone system. Factors like polymer chain length, filler loading levels, and polymer-filler interactions can have a significant impact on the viscosity of a silicone system. In the case of a silicone elastomer system, choosing the right system is a balance between the cured physical properties like tensile and elongation and the uncured properties like viscosity and cure temperature.

Related articles can be found at http://www.nusil.com/newsletter

Material Characterization with Optical Absorption

2005-03-17T08:58:00.000-08:00

LED’s and fiber optics require materials that allow a range of visible and non-visible light to pass through relatively uninhibited. If a material absorbs light, the device efficiency is compromised. If your concern is the production and transmission of bright light from LEDs or signal and data integrity of fiber optic systems, it’s critical to select materials that operate at the highest efficiency. When selecting a material, screening techniques can be employed to determine which products will meet the engineers efficiency needs. A simple test, optical absorption versus wavelength can be very useful in this screening process.

Optical absorption is measured to a ± 0.2% transmission loss resolution across the wavelength range of 300 nanometers to 1700 nanometers, with a 2 nanometer resolution, using a grating spectrophotometer. Testing is typically conducted a 25ºC but other testing temperatures are possible. The spectrophotometer generates light via a tungsten lamp which is directed to a grating structure that acts as a prism, separating wavelengths of light. The wavelength range spans from near UV through the visible and near infrared. The separated wavelengths of light are then directed through a quartz test cell containing the material sample. In a double beam spectrophotometer, some light is directed to the detector without passing though the sample as a reference. A detector measures the resulting transmission at specific wavelengths is typically a solid state silicon photodiode or a photomultiplier. Transmittance is calculated by comparing the intensity of light transmitted by the sample (I) to the intensity of light incident on the sample

(Io):
T = I/Io
Absorbance is calculated by the following formula:
A = log (I/Io)

Optical absorption graphs and other optical characterization testing descriptions can be accessed at http://www.nusil.com/literature/press/ds_testservices.pdf

More information on testing and Lightspan brand materials for photonics applications can be accessed at http://www.nusil.com/engineering/photonics/

Healthcare Market Trends

2005-03-16T14:39:00.000-08:00

At NuSil we often ask, what’s next? Emerging trends or technologies often point to what kind of silicone materials will be used in the future. We listed a couple of trends below that will affect our business and our customers businesses.

Cosmetic Surgery – An aging population combined with positive media and strong global economies have resulted in steady demand for these types of surgeries. Growth in cosmetic surgery procedures has grown globally, with hotspots like China averaging 20%.

Medical Devices - Again an aging population combined with technological advances has pushed demand for products in this area. Analysts expect a 7% growth in the medical device arena, cardiovascular devices and ophthalmic devices are expected to have stronger growth numbers. The next generation of devices are expected to communicate with home based computing systems through radio frequency signals. Medical devices will begin to take on a diagnostic role in addition to their therapeutic roles. NuSil's products that are most applicable to this industry are listed at http://www.nusil.com/healthcare/index.aspx

Drug Delivery – The top 200 products in the drug delivery market constituted $31.2 billion or 22% of the top pharmaceutical products market. Patent expirations, patient compliance, and improved delivery technologies will fuel growth in this area. Niche pharmaceutical development will also require the use of drug delivery technologies in the future. NuSil's products that are most applicable to this industry are listed at http://www.nusil.com/healthcare/drug%20delivery/

Cosmeceuticals – New products deliver active agents or pharmaceuticals through the skin. Wound healing agents, anti-inflammatory agents, and barrier enhancing vitamins represent a new list of components in creams, lotions and ointments that are on the market. This area is also characterized by rapid growth and rapid product development. NuSil's products that are most applicable to this industry are listed at http://www.nusil.com/healthcare/cosmetics/index.aspx

Engineering Market Trends

2005-03-16T12:56:00.000-08:00

At NuSil we often ask, what’s next? Emerging trends or technologies often point to what kind of silicone materials will be used in the future. We listed a couple of trends below that will affect our business and our customers businesses.

Projection displays:
Projection displays in mobile entities are power hungry and consume large amounts of battery power. The convergence of the personal computer, telephone, and personal digital assistant has created a demand for sophisticated display systems. While boosting battery power is one option, creating efficiencies within the device is another. Today, these efficiencies are created with CMOS and LED technology. In a past issue of Insight, http://www.nusil.com/newsletters, we talked about the importance of the emerging LED market. The low power requirements of this solid state lighting make it idea for these applications. Looking beyond personal electronics, the goal of LED manufacturers is to boost the brightness of LED’s so that they can continue to add efficiencies to conventional lighting sources. Lumileds Lighting, a high brightness LED manufacturer, retrofitted a commercial LCD television with some high powered LEDs with some success. Pictures can be found in a paper located on their website at www.lumileds.com

Electro-optical systems
The growth of electro-optical systems has increased dramatically. Commercial applications have moved well beyond the disk drive into digital cameras and camera phones. This new growth can challenge existing electronic systems, requiring more precise materials with new properties. Low outgassing materials, used extensively in disk drive systems, should play a greater role in these new systems as engineers look to eliminate contaminates.

Sensors and Detectors:
The growth in this area is tremendous. Automotive and healthcare are a few industries where sensors are integrated into a systems that continuously monitors and diagnoses with the help of computer systems. RFID Tags are gaining widespread acceptance in many aspects of logistics but has tremendous potential. These systems will require the use of electronic systems for communications.