Discovering the world of silicone
Avantor has defined general terms for the NuSil brand of high purity silicones. As a leading formulator of silicone compounds for healthcare, aerospace, electronics and photonics applications, count on NuSil to develop and deliver standard and customized silicone compounds designed to efficiently meet your unique property requirements.
The consistency of silicones, in the uncured state, range from liquid (water-like) to clay-like materials. Silicone consistencies can be reported in flow rates, viscosity, or plasticity measurements.
The stiffening of the uncured silicone elastomer. Other terms that refer to this phenomenon are “crepe aged” and “crepe structuring”. Crepe hardening is caused be chemical interactions (hydrogen bonding) between a silica reinforcing filler and the polymer fraction.. One hydrogen bond by itself is considered a weak bond and can easily be broken but in aggregate they can have considerable effect. Over time, the amount of hydrogen bonding increases and stiffening is observed. Crepe-aged silicones can be re-softened by shearing the material (Stirring or milling).
Cure inhibitors, unlike catalyst poisons, do not prevent the system from curing permanently. When used correctly they can provide very specific control over the reaction mechanism. Work time is a property that is most commonly adjusted with cure inhibitors.
The cure schedule is a combination of temperature and time to which a heat curing silicone material must be exposed for “complete” curing. For the same material, several temperature/time combinations may be selected for applications with time or temperature constraints.
Addition cured silicone elastomers are commonly referred to as platinum catalyzed silicones and are generally two-part systems with each part containing different functional components. These two component systems can be formulated in various ratios, with the most common ratios being 10:1 and 1:1. Generally, the Part A component contains vinyl functional silicones and the platinum catalyst, whereas Part B contains vinyl functional polymer, hydrogen-functional crosslinker, and cure inhibitor. Cure inhibitors are additives used to adjust the cure rate of the system. The cure chemistry involves the direct addition of the Si-H functional crosslinker to the vinyl functional polymer forming an ethylene bridge crosslink. The vulcanization of addition cured silicone elastomers can be heat accelerated. Depending on the specific product, addition cured elastomers can be fully cured at temperatures and times ranging from 10 minutes at 116 oC to 2 minutes at 150 oC. Cure conditions vary with product mass (e.g. A thin sheet of elastomer will cure much quicker than a thick section as the appropriate cure temperature will take longer to achieve in the core of the silicone sample) Special care must be taken to eliminate the presence of contaminants that might have a negative impact on the catalyst (see Poisons below).
Condensation Cure (Acetoxy)
Acetoxy functional systems are typically used in the formulation of one part dispersions, sealants and adhesives. These materials are very effective when cured in thin section and provide good adhesion to most substrates. The cure system consists of hydroxyl-terminated polymers, alkyltriacetoxysilane crosslinkers and a tin catalyst. These one-component products vulcanize when exposed to ambient humidity. Full cure generally occurs in 7-10 days depending on the silicone thickness and humidity level. Attempting to accelerate the cure with heat is not recommended. Acetic acid is liberated during the cure of these products and is evidenced by a vinegar-like odor that disappears when full cure is achieved.
Condensation Cure (Alkoxy)
Alkoxy or alcohol cure systems are typically used in low consistency or pourable elastomers and foams. These materials can be cured in thicker section than acetoxy crosslinked materials and are generally supplied as two component products. The cure system consists of a hydroxyl-terminated polymer, a tetraalkoxysilane crosslinker and a tin catalyst. Like the acetoxy cure system, water vapor (moisture) is required for vulcanization. Unlike the acetoxy system, the leaving group is an alcohol rather than acetic acid and as such it is non-corrosive. These materials also require 7 days or more to reach complete cure depending on humidity and thickness of the silicone section.
Condensation Cure (Oxime)
Oxime cure systems are typically one part, thin section, moisture cure adhesives. The cure mechanism is again the hydroxy end-blocked silicone reacted with an oxime functional crosslinker in the presence of tin catalyst and ambient humidity. The leaving group, methyl ethyl ketoxime, is non-corrosive.
Peroxide cured silicones utilize a peroxide catalyst. This cure chemistry is most commonly used in high consistency rubbers. The cure mechanism can involve vinyl functional, hydride functional and non-functional polymers. The choice of peroxide catalyst is contingent on the vulcanization technique and parameters desired (vinyl specific and non-vinyl specific). The concentration of non-vinyl specific peroxide catalysts is directly proportional to the desired crosslink density of the cured elastomer. Typical cure schedule of non-vinyl specific peroxide catalyzed elastomers is ten minutes at (116°C), followed with a 2-4 hour “post cure”(see discussion below) at (177°C), to remove residual by-products.
Dispersion is a term used to describe silicones that are suspended or dissolved in a solvent carrier. The viscosity of the dispersion depends on the properties of the silicone base and the relative ratio of silicone to solvent. The consistency of dispersions range from water-like to thick, minimally flowable liquids. Dispersions can be applied using dipping or spraying methods.
Silicone fluids are silicone polymers of varying molecular weights and functionality. Polymers contain two essential components, repeating siloxane units and endblocking siloxane units. Repeating siloxane units can have dimethyl, trifluoropropylmethyl, phenylmethyl, diphenyl, vinylmethyl or hydrogenmethyl functionality, with dimethyl as the most common siloxane. Endblocking siloxane units terminate polymer chains. Endblocking may contain reactive groups such as vinyl or hydroxy functionality or non-reactive functionality such as trimethyl or dimethyl phenyl. Fluids can be applied to a surface by dipping, spraying, or brushing applications.
Fluorosilicones are silicones that contain varying levels of trifluoropropyl methyl polysiloxane repeating units. Fluorosilicones are available as high consistency elastomers, LSR’s, foams, gels, fluids, and one-part sealants.
Like gels, foams are designed to be conformable. These materials are typically liquid while uncured. Catalyst addition (tin or platinum) rapidly yields a silicone rubber foam at room temperature. A cellular structure is created by the release of hydrogen gas during the curing process. Foams can also be created by the use of certain additives such as ammonium bicarbonate. Such an additive can create foam from a high consistency rubber via the application of heat.
Silicone gels are designed to be soft and conformable. They achieve their gel-like consistency by having less crosslinking than is typical of elastomers and are generally not silica-reinforced. Uncured gels are easily pourable and can be mixed by hand and molded into finished parts.
High Consistency Rubber
A high consistency rubber consists of a high molecular weight polymer combined with silica to produce a material that can be molded, extruded, or calendered into a useful end product. An HCR has the consistency of clay and is primarily formulated in a one or two part system (peroxide and platinum catalysts respectively).
Liquid Silicone Rubber
Liquid silicone rubbers like HCR’s, are reinforced with silica, but typically use a lower molecular weight polymer. LSR’s are non-slump, with mayonnaise like consistency and are often pumped with an injection-molding machine to form a molded part. These two-component materials are mostly formulated in a 1 to 1 mix ratio.
Various material additives are available in concentrated form called masterbatches. Masterbatches are premixed additives in a functional silicone polymer. Examples of additives include pigments, catalysts, and inhibitors.
Medical-grade silicones are specifically designed, manufactured and purified to meet the strictest needs of the healthcare industry. These products are made under applicable cGMP standards in facilities indirectly or directly regulated by US FDA and are typically supported with master access files.
- Maximum continuous operating temperature: The temperature to which a material can be exposed for an unlimited period of time without any damage.
- Maximum intermittent temperature - For a short period (up to several hours) it is possible to exceed the maximum continuous operating temperature without noticeable damage to the material. The maximum intermittent temperature is the temperature under which a weight loss of 5% occurs within five minutes.
- Degradation temperature - The temperature at which molecular deterioration of a material occurs. This temperature is determined by a TGA-analysis (Thermogravimetry analysis).
This measurement determines the hardness of a silicone material by means of a unit-less standard test. Measurements are reported on a type 00(soft), type A(medium), and type D(hard) scales. Within each scale, a higher number translates to a harder material.
A test to determine how long a material stretches, from its state of rest, to the point of rupture. These results are typically given in percentages.
A test to determine how quickly a liquid silicone can be pumped out a specified orifice at a specified pressure. These figures typically are given in grams per minute.
A mechanical property of material used to describe its flexibility or stiffness. It is the force (stress) needed to stretch (strain) a material a given amount (elongation). Modulus can be related to a materials' hardness. Softer materials take less force to stretch a given distance and therefore are in lower modulus than stiffer materials. Modulus is a ratio of stress to strain. It is typically described as the stress (force) required to stretch the material to a given % elongation. Modulus is reported in lbs/in2 (psi) at a specified % elongation.
A test used to determine a materials resistance to the propagation of a cut or tear. These figures are typically given in pounds per inch (ppi), or their metric equivalent of kiloNewton per meter (kN/m).
The amount of force or strain required to break the material. This testing is typically reported in units of pounds per square inch (psi) or their metric equivalent mega Pascals (Mpa).
A test to determine the consistency of silicones that are liquid in nature.
The platinum catalyst used in the addition cured silicones is very susceptible to poisoning which negatively affects the cure. These poisoning effects can range from a slight surface tack to a complete failure to cure. Although there are many substances that can cause poisoning of the cure, the following list includes the majority of the most common poisons:
A. Sulfur containing materials
- Buna N
- Natural Rubber
- Sulfur Compounds
B. Organotin containing materials
- Condensation cured silicones
- Acetoxy cured silicones
- Oxime cured silicones
- Plasticized polyvinyl chloride
- Adhesive tapes
- Some amino groups
Curing a small amount of silicone in contact with a questionable material can be an effective way to evaluate potential poisoning effects.
The post-cure serves two purposes:
- Post-curing removes the volatile components and other residuals generated from the decomposition of the peroxide catalyst during vulcanization.
- Post-curing stabilizes and enhances the physical properties of the elastomers. Post-curing is accomplished by heating the vulcanized material in a hot air circulating oven to a predetermined temperature for a length of time.
The time required for post-curing at a given temperature depends upon the rate at which the volatiles can escape from the elastomer, which in turn depends upon the thickness of the part, the exposed surface area and the oven loading.
As an example, a standard ASTM slab (1.9 mm (0.075 in) thick) should be post-cured at 177°C. (351°F) for a minimum of 2 hours or 200°C (392°F) for a minimum of 1.5 hours for peroxide catalyzed elastomer systems. The user must determine post-cure conditions for specific applications.
High consistency materials by nature have a tendency to crepe or “structure” over time. It is recommended that these materials are softened prior to catalyzing and/or molding/extruding. Softening of high consistency materials is most often accomplished with the use of a two-roll mill. For two-part systems, the recommended sequence is to first soften Part B on a cooled two-roll mill followed by the Part A. CAUTION: The temperature of the milled material must be kept as low as possible to give maximum work time. Re-milling may extend the useful life of catalyzed material.
Air entrapped during hand-mixing and naturally dissolved gasses can be removed by vacuum deaeration prior to molding.
Pressure Sensitive Adhesives
Pressure-sensitive adhesives (PSA) impart a tacky surface capable of forming a non-permanent bond. Pressure sensitive adhesives are supplied as silicone dispersions. The volatile carrier solvent typically evaporates quickly, leaving a tacky silicone layer that can be adhered to another object through application of pressure.
Silicone primers are specially formulated silanes supplied in a solvent. Silicone primers are used to improve the bond strength between a silicone rubber or adhesive and another substrate surface (silicone, metals, and certain plastics). The organo-functional component of the primer bonds into the silicone matrix while the hydrolized silicon-functional portion bonds to the inorganic substrate.
A process used to form silicones into thin sheets of uniform thickness. This sheeting may or may not be reinforced with other materials.
A form or “mandrel” is immersed (“dipped” or “dip coated”) in a solvent dispersion of a silicone elastomer. Dip coating is usually a multiple coat process to achieve a specific cured thickness.
High consistency elastomers can be extruded through an unheated die to make rod, tubing and coated wire. High consistency silicones, which can retain a formed geometry in the uncured state, are ideal for this process. Vulcanization with this fabricating technique is normally accomplished by passing the extruded silicone through a horizontal or vertical heated chamber.
Silicone elastomers can be molded into cured configurations by compression, transfer or injection molding processes. Most of these processes can accommodate a range of silicone consistencies for the exception of injection molding. Liquid silicone rubber systems are typically designed for the injection molding process.
To perform in the harsh conditions of outer space, silicones must be highly purified. NASA and the European Space Agency (ESA) require materials to be tested per ASTM E-595 prior to use in space. These materials must meet the specifications outlined in NASA SP-R-0022A and ESA PSS-014-702, with a maximum Total Mass Loss (TML) of 1% and Collected Volatile Condensable Material (CVCM) of less than 0.1%. NuSil considers materials that pass these tests to be of space grade.
Our space-grade materials are offered in two lines: Controlled Volatility (CV) and Ultra Low OutgassingTM (SCV). CV materials meet or exceed the ≤ 1% TML and ≤ 0.1% CVCM requirements; SCV materials exceed the current ASTM E 595 by an order of magnitude, ≤ 0.1% TMLs and ≤ 0.01% CVCM.
The formation of a cross-link network in a silicone system.
Work time is the time period between system catalysis and the time when the material is no longer workable. Work time for flowable silicones may be determined by dipping a micro-spatual below the surface of the material, slowly withdrawing the spatula to a height of more than one inch, and observing the resultant snappy condition. Work time may also be determined by testing the viscosity of a catalyzed sample at specified time intervals. Several factors must be taken into account when determining work time, including the application and physical characteristics of the material being tested.