Browsing all articles by Andrew Cheng, Author at Silicone Recycling

pfpe-greaseIn this article, we’ll be outlining the importance of how grease plays a crucial role in the process of silicone recycling. In the world of silicone recycling, especially in this day and age, there is very little manual labor involved, which means all the hard work is done by industrial machines and tools. Therefore, just like most machines, having a high-quality grease is the key to efficient operations. Below are a few factors why choosing PFPE grease, the best industrial grease on the market is worth the extra cost.

Excellent compatibility

In terms of compatibility, PFPE based grease is by far the superior choice. As silicone often react to petrochemical based greases, it is highly recommended that the machinery and all moving parts involved in silicone recycling should use a more synthetic type of grease. PFPE grease also won’t react to metals, but instead stay put on the surface. PFPE grease is actually so strong, that a special solvent is required to completely remove the grease from a surface. If any other grease is applied to an already PFPE greased surface, the new grease will not be able to stick to the PFPE grease, and eventually fall off.

PFPE grease can only be thickened with PTFE powder, which also serves as an advantage due to the properties of the PTFE. The PTFE powder is actually so fine that it’s not only good for thickening, but also lessens the wear and tear on fast moving parts, such as ball bearings.

Completely chemically inert

All PFPE products, including PFPE greases are known for its inert properties. This means it will never ignite in high temperatures, and that is often a serious issue when choosing greases for high-heat operations. As silicone recycling can sometimes reach extreme temperatures when the silicone needs to be melted and remolded, using PFPE grease is highly recommended for safety and performance reasons.

Another factor to consider is that PFPE grease is the only grease that can withstand extreme humidity, this is not only important for silicone recycling processes, but also any machinery where the grease can be directly exposed to hot steam and other gasses.

Wide temperature range

PFPE grease has the highest temperature range compared to other mainstream greases on the market, handling extreme negative degrees well and perform effectively in high temperatures. Generally, the operating temperature range for a regular PFPE grease is anywhere between -36°C to 288°C (-33°F to 550°F).

In general, PFPE-based greases are very flexible and manufacturers can easily adjust the viscosity of the grease, to fit your requirements. As a general rule of thumb, the higher friction and more demanding tasks usually require a higher viscosity grease. However, always consult with your PFPE grease supplier before placing the order.

1. Introduction

Polydimethylsiloxanes are used in the plastics industry as additives for improving the processing and surface properties of plastics, as well as the rubber phase in a novel family of thermoplastic vulcanizate (TPV) materials. As additives, silicones, and in particular polydimethylsiloxane (PDMS), are used to improve mold filling, surface appearance, mold release, surface lubricity and wear resistance. As rubber portion of a thermoplastic vulcanizate material, the cross-linked silicone rubber imparts novel properties, such as lower hardness, reduced coefficient of friction and improved low and high temperature properties [1].

2. Low viscosity PDMS

Low molecular weight PDMS polymers, with viscosities less than 1000 cSt, are used extensively by the plastics industry as external release agents applied on the mold surface prior to injection molding.

Low viscosity pure PDMS fluids are clear, colorless, and odorless that range in viscosities from 5cSt to 20cSt (centistokes) @ 25°C. They are characterized by their low pour points, low surface tensions, low viscosity change at temperature, excellent lubricity, high dielectric strength, refractive index values between 1.3970 to 1.4000, wide service temperature range and inertness to virtually all substrates [2]

A primary use is as an ingredient in a number of personal care products due to their low surface tension, skin lubricity, high spreadability and non-greasy feel. Applications may include:
antiperspirants, skin creams, skin lotions, suntan lotions, bath oils, hair care products and so on.

Low viscosity PDMS fluids also possess high resistance to shear, high water repellency, low vapor pressure and low reactivity. They are excellent lubricants for plastics, foams and rubbers. Due to their low pour points and low viscosity change at temperature, they are excellent choices for low temperature bath fluids. For instance, Clearco low viscosity pure silicone fluids meet Federal Specification VV-D-1078 “Silicone as Damping Fluid”. They are used in a wide range of military, industrial and avionic gauges, meters, instruments and monitoring systems.

Low viscosity PDMS fluids are exempt from federal VOC regulations, including California (CARB) and OTC regulations. They will not contribute to ozone-depletion and global warming. In addition, they meet RoHS compliancy. The physical and chemical properties are illustrated in table. no.1 below:

TABLE NO.1

Viscosity Specific Gravity Pour Point Flash Point Refractive Index V.T.C Thermal Expansion cc/cc °C Thermal Conductivity g/cal/cm/sec °C
5cSt 0.918 90°C 135°C 1.3970 0.54 0.00109 0.00028
10cSt 0.818 70°C 163°C 1.3990 0.56 0.00108 0.00032
20cSt 0.853 65°C 232°C 1.4000 0.59 0.00107 0.00034

PROPERTIES:

• Low pour points (as low as -90°C)
• Wide service temperature range
• Low surface tension fluids (high spreadability)
• Low viscosity change at temperature (low VTC)
• Refractive index (RI) values between 1.3970 to 1.400
• High dielectric strength
• Newtonian behavior under shear
• Excellent lubricity
• California VOC ,exempt replacement for petroleum based solvents that are not VOC compliant

APPLICATIONS:

• Laboratory bath fluids
• Heat transfer fluids
• Dielectric fluids
• Low viscosity damping fluids
• Turbo generator fluids
• Low viscosity hydraulic fluids
• Refractive index matching fluids
• Low dielectric constant fluids for photonics
• VOC exempt fluids for renewable energies
• Lubricants for plastic parts
• Lubricants for rubber parts

These low viscosity PDMS meet the Federal Specification VV‐D‐1078 “Silicone as damping fluid ” and are used in a wide range of military, industrial and avionic gauges, meters, instruments and monitoring systems.

Super Low Viscosity Pure Silicone Fluids are clear, colorless and odorless linear polydimethylsiloxane fluids that range in viscosities from 0.65cSt (i.e.thinner than water ) to 2cSt (centistokes) @ 25°C. They are characterized by their low pour points, volatile nature (evaporate in open system), stability at low temperature, low surface tensions (high spreadability), low viscosity‐to‐temperature coefficients (viscosity changes little even at extremely low temperatures), low molecular weight, low viscosities (all close to water), high dielectric strength and refractive index values ranging from 1.3750 to 1.3900.Due to their volatility (varying rates of evaporation), low surface tensions (high spreadability) and non-greasy feel, they are used as base fluids, carrying agents and wetting agents in a wide range of personal care products.

TABLE NO.2

Viscosity Specific Gravity Pour Point Flash Point Refractive Index V.T.C Thermal Expansion cc/cc °C Thermal Conductivity g/cal/cm/sec °C
0.65cSt 0.761 -68°C -1°C 1.3750 0.31 0.00134 0.00024
1cSt 0.818 -85°C 39°C 1.3825 0.37 0.00134 0.00024
1.5cSt 0.853 -90°C 63°C 1.3874 0.46 0.00134 0.00025
2cSt 0.873 100°C 75°C 1.3900 0.48 0.00117 0.00026

3. High viscosity PDMS

In order to eliminate an external application during processing, higher molecular weight PDMS materials, with viscosities ranging from 10,000 cSt to 60,000 cSt, have been used as internal additives in thermoplastic polymers to give processing advantages and surface property improvements. Due to the incompatibility between dimethyl siloxanes and most thermoplastics, the PDMS fluid is driven to the surface. A concentration of the PDMS at the surface results in the observed processing and surface property benefits.

A more recent advancement in the field of PDMS additives is the use of ultra high molecular weight (UHMW) PDMS, with viscosities ranging from 10 to 50 x 106 cSt [3]. Additives are now available with 50 weight percent UHMW PDMS in various thermoplastic carriers and as pellets in order to allow easy addition of the additive directly to the thermoplastic during processing. An important improvement obtained using UHMW PDMS is that the loading of PDMS in the concentrated additive is increased from approximately 20 to 50 weight percent [1]

Ultra high molecular weight PDMS results in the same processing benefits, such as improved mold release, lower extruder torque and easier mold filling as compared to lower molecular weight PDMS, but it eliminates the “bleed-out” that can occur after processing. This benefit is clearly seen when comparing the print adhesion to polypropylene films containing various additives .High viscosity pure silicone fluids are clear, colorless and odorless linear polydimethylsiloxane fluids that range in viscosities from 5,000cSt to 100,000cSt (centistokes) @ 25°C. They are characterized by their low pour points, high flash points, wide service temperature range, low viscosity change at temperature, excellent lubricity, high dielectric strength, refractive index values between 1.3970 to 1.4000 and inertness to virtually all substrates [2]. Their physical and chemical properties are shown in the table no.3 below:

TABLE NO.3

Viscosity Specific Gravity Refractive Index Pour Point Flash Point (open cup) V.T.C Thermal Expansion cc/cc °C Thermal Conductivity g/cal/cm/sec °C
5,000cSt 0.975 1.4035 50°C 315°C 0.61 0.00096 0.00038
10,000cSt 0.975 1.4035 48°C 315°C 0.61 0.00096 0.00038
12,500cSt 0.975 1.4035 46°C 315°C 0.61 0.00096 0.00038
30,000cSt 0.976 1.4035 42°C 315°C 0.61 0.00096 0.00038
60,000cSt 0.976 1.4035 42°C 315°C 0.61 0.00096 0.00038
100,000cSt 0.977 1.4035 41°C 315°C 0.61 0.00094 0.00038

PROPERTIES:

• High flash points
• Low pour points
• Wide service temperature range
• Low viscosity change at temperature (low VTC)
• Refractive index matching fluids for RI value = 1.4035
• High dielectric strength
• High damping action
• High oxidation resistance
• Stable under high rates of pressure
• Stable under high rates of shear

APPLICATIONS INCLUDE:

• Damping fluids for gauges, instrument & electronics
• O‐Ring, gasket & valve lubricant
• Cable lubricant
• Dielectric fluids
• Down‐hole fluids for oil & gas exploration
• Hydraulic fluids
• Flow control fluid
• High pressure fluid
• Refractive index matching fluids
• Refractive index matching fluids for LED and optics
• VOC exempt fluids for renewable energies
• Lubricants for plastic parts
• Lubricants for rubber parts
• Fluid for touch screen technology
• Vibration control fluid
• Calibration fluid

4. Environment and recycling

Low- and high viscosity silicone fluids are exempt from federal VOC regulations, including California (CARB) and OTC regulations. They are HAP‐free and do not contribute to ozone‐depletion and global warming. In addition, they meet RoHS Compliancy.

The Environment Agency’s environmental risk assessment for octa-methyl-cyclo-tetra-siloxane (D4) is based on the methods outlined in the Technical guidance document (TGD) of the European Union (EU) for the risk assessment of new and existing chemicals. The persistence, bioaccumulative, and toxic (PBT) status is assessed, and a ‘quantitative’ risk assessment was made by comparison of exposure with effects [3].The overall conclusions of the risk assessment are the followings:

• No risks from off-site use as an intermediate (both wet and dry processes), from formulation and use in both personal care products and household products and from regional sources of D4 are identified for air, water, sediment and the terrestrial compartments. There are also no risks for humans exposed to D4 via the environment.

• Possible risks are identified from the production and on-site use as an intermediate at the UK production site, and apply to freshwater, freshwater sediment, predators, marine waters, marine sediments, marine and top predators. These conclusions are based on the best information available, but this is limited and hence there is significant uncertainty in the conclusions. The receiving fresh water, which is subject to local Environment Agency regulation, is a relatively non-standard environment and dilution into the sea is possibly higher than the default value used.

• Further information required to reduce these uncertainties should include clarification of the emissions from the production site. This could be statistically analyzed site-specific data on emissions, in compliance with the TGD (i.e..monitoring of the receiving water or further effluent monitoring ).

References
[1] W.Shearer “ Silicones in the plastic industry” /www.dowcorning.com
[2] www.clearcoproducts.com
[3] www.gov.uk/uploads/environmental risk assesment report

1. Introduction

Most polydimethylsiloxane fluids are non-volatile polymeric organosilicon materials consisting of

[- (CH3)2 – SiO- ]n
structural units.

Various PDMS fluids ranging from low to high viscosity are used in a wide range of industrial applications, such as manufacturing textiles, paper and leather goods. Due to this wide range of applications, they can enter the environment in a variety of ways.

2. Applications

PDMS fluids are highly-efficient process aids, able to provide desirable properties at very low concentrations. They often serve as antifoams, softeners, or water repellents. In consumer applications, PDMS fluids can be found in personal-, household- and automotive care products.[1] They are used as conditioners in hair care, softeners in skin care products, additives in polish formulations, water proofers as well as components of other surface treatments. Some PDMS materials are also used as end-products (usually in the industrial market), such as transformer dielectric fluids and heat transfer liquids.

3. Environment and recycling

Since they are non-volatile, PDMS fluids do not evaporate into the atmosphere. Very small amounts of PDMS fluids, which are contained in household products, may be washed from the surfaces to which they have been applied and eventually enter into the soil or a water treatment plant .By instance, personal care products such as shampoos and conditioners are rinsed away after use and consequently the PDMS they contain is carried with water to the treatment site. This treatment site could be a private municipal plant or a septic system . When PDMS fluids are used in industrial applications such as process aids or surface treatments, small quantities can also be found in process water that is carried to the treatment plant. End-use industrial products such as transformer fluids are used in contained applications. These transformer oils are suitable for recycling and are therefore unlikely to enter the environment, except in cases of accidental release.

The fate of PDMS is partly a function of where it enters the environment. A number of studies have shown that PDMS will degrade into lower molecular weight compounds, primarily Me2Si(OH)2 , when in contact with soils [2]. Testing under a variety of representative conditions has confirmed this observation in a wide range of different soils, indicating that the phenomenon is widespread in nature. After only few weeks of soil contact, it has been noted a significant degradation to lower molecular weight compounds. The extent of degradation and the actual rate vary as a function of soil moisture content and clay type. These lower molecular weight degradation products have been shown to further oxidize in the environment, both biologically [3] and abiotically[4] to form naturally-occurring substances : silica, carbon dioxide and water. No effects from PDMS (or its degradation products) have been observed on seed germination, plant growth or plant survival as well as on the plant biomass. Additionally, research has shown no adverse effects from PDMS on terrestrial life forms such as insects or birds, even under highly exaggerated conditions of exposure. Research includes studies on survivability and growth.

Consequently, PDMS fluids pose no known hazard to the environment and they are not classified as hazardous materials. If PDMS fluids should enter the aquatic environment, they attach to particulate matter and are removed from the water column by the natural cleansing process of sedimentation. PDMS fluids do not partition back into the water column and have no detectable Biological Oxygen Demand (BOD).

Bio-concentration does not represent a significant concern with PDMS. Their molecular size renders them too large to pass through biological membranes in fish or other organisms. Specific testing has shown that PDMS is not toxic and does not bio-accumulate in sediment dwelling organisms or various terrestrial species, including earthworms. In water treatment: Household (on-site) septic systems and municipal treatment plants are both designed to facilitate the natural degradation of water by microscopic organisms. Biomass (or “sludge”) is generated by this natural degradation, and must eventually be discarded. In a municipal system, treated sludge is typically incinerated, landfilled or used as fertilizer. In the United States, where on-site septic systems are common, the tank is usually pumped out periodically and the biomass is taken to a water treatment plant. PDMS fluids from personal care and household products enter these treatment systems as tiny dispersed droplets in water. Because the water solubility of these silicone fluids is essentially nil, they attach to suspended materials in water treatment systems and become a minor part of the sludge. Water treatment monitoring and simulation studies have confirmed that PDMS fluids which enter treatment facilities will be almost completely absent from the treated effluent. PDMS does not inhibit the microbial activity by which water is treated. Test levels, far exceeding those expected in the environment, have shown no effect on the activated sludge process, other than the expected benefits of foam control. PDMS loadings had no effect on the operating parameters (pH, solids, sludge volume index and specific oxygen uptake) or physiological activity of the micro-flora in the model activated sludge units. Sludge digestion operating parameters (suspended solids, gas generation, pH) were also unaffected by loadings of up to 100 mg/kg of PDMS. The ultimate fate of sludge-bound PDMS depends on the sludge disposal technique. If the sludge is incinerated, the silicone content converts to amorphous silica, which presents no further environmental consequence when the ash is landfilled. When treated sludge is used as fertilizer, very small levels of PDMS may be introduced to the soil environment, where it is further subject to soil-catalyzed degradation. Similar soil-catalyzed degradation may also occur if sludge-bound PDMS is landfilled. Overall, PDMS has shown no significant environmental effects. For example, Dow Corning maintains an extensive facility in the U.S. dedicated to health and environmental sciences and was a significant contributor to a handbook on the environmental aspects of organosilicon materials.

It is well accepted that polydimethylsiloxane fluids become permanent residents of sediment, but should not exert adverse environmental effects. Polydimethylsiloxanes fluids are very surface active, because the flexible siloxane linkages permit the alignment of the hydrophobic methyl substituents towards the non-polar phase and of the polysiloxane backbone towards the polar phase. The polar medium is generally water. Other polar media to which polydimethylsiloxanes become attached may be textiles, sewage sludge, sediment , hair, algae and so on. In aqueous environments, polydimethylsiloxanes are adsorbed onto sedimenting particles. Also, in the presence of nitrate ions, which exist at various concentrations in the environment, short chain siloxanes are photo-degraded to the level of silicate within days. The stability of the siloxanes, desirable from a technical point of view, makes the siloxanes very persistent, and once released to the environment the said siloxanes remain for many years.

The volatile siloxanes may account for a significant part of the siloxanes used for cosmetics. The main source of releases of siloxanes to the air is volatile siloxanes used in cosmetics, wax, polishes, and to a minor extent in several other applications. Non-volatile silicone fluids used in cosmetics, wax, polishes, cleaning products and for textile applications (softeners) will end up in water and be directed to water treatment plants to a large extent. The cyclic siloxanes and small-chain linear siloxanes are bioconcentrated ( note that bioconcentration factors for long-chained siloxanes have not been assessed). The estimated bioconcentration factors (BCF) of the small siloxanes range from 340 for HMDS to 40,000 for a phenylated trisiloxane (phenyl trimethicone). The small phenylated siloxanes seem to have very high BCF, and the model estimates indicate that these substances are the most toxic for aquatic organisms. The substances were screened using the PBT profiler screening developed by U.S. EPA (U.S. EPA 2003PBT profiler screening ) in order to make a first comparison between the substances as to persistence, bioaccumulation and toxicity. The mentioned profiler uses a procedure to predict persistence, bioaccumulation and toxicity of organic chemicals on the basis of the chemical structure and physical parameters of the substances combined with experimental parameters for substances with a similar structure. The results for six members of the siloxane family predict the highest bioconcentration factors for the two phenyl siloxanes, one order of magnitudes higher than the values for the cyclic siloxanes and two orders of magnitudes higher than the values for the small linear methyl siloxanes. The predicted toxicity is significantly higher (lowest ChV values) for the phenyl siloxanes as well. The predicted half-life is nearly the same for all substances. Using U.S. EPA’s criteria, the screening indicates that all substances are of high concern as to environmental toxicity and that the phenyl siloxanes are considered very bioaccumulative.

The environmental fate and effects of volatile methylsiloxanes (mainly cyclosiloxanes) and polydimethylsiloxane (PDMS) have been reported as follows:
For octamethylcyclosiloxane: 9 of 11 Fish acute LC50 (14 day): rainbow trout 10 ug/l; sheepshead minnow : >6.3 ug/l Daphnia magna acute EC50 (48 h): >15 ug/l; NOEC 15 ug/l Mysid shrimp acute LC50 (96 h): >9.1 ug/l; NOEC 9.1 ug/l.

For PDMS: Daphnia magna NOEC 572 mg/kg.

Physical effects such as surface entrapment have been observed when testing aquatic invertebrates in clean laboratory water, but similar effects are not expected in natural environments where a large variety of other surfaces provide opportunities for deposition.

All materials must be handled in accordance with the local, state and federal regulations. Legislation regarding disposal requirements may differ by country, state and/ or territory. Each user must refer to laws operating in their area. Certain materials must be tracked in some areas. This material may be recycled if unused, or if it has not been contaminated so as to make it unsuitable for its intended use. If it has been contaminated, it may be possible to reclaim the product by filtration, distillation or some other procedures. Shelf life considerations should also be applied in making decisions of this type. It is important to note that the properties of a material may change in use and recycling or reuse may not always be appropriate. It is prohibited to allow wash water from cleaning equipment to enter drains. All wash water for treatment before disposal must be collected. It is advisable to recycle wherever possible or to consult the manufacturer for recycling options. Residues are to be buried or incinerated at an approved site. If possible, it is advisable to recycle containers or dispose them in an authorized landfill.

References:
[1] Health Environment & Regulatory Affairs (HERA) Ref. n° 01-1034A-01 1/4 © Copyright Dow Corning Corp., 1997
[2] RR Buch and D.N. Ingebrigtson, “Rearrangement of Polydimethylsiloxane Fluids on Soil,” Environmental Science and Technology 13, 676 (1979).
[3] R.G. Lehmann, S. Varaprath, C.L. Frye, “Fate of Silicone Degradation Products (Silanols) in Soil,” Environmental Toxicology and Chemistry 13, 1753 (1994).
[4] C.L. Sabourin, J.C. Carpenter, T.K. Leib, J.L. Spivack, “Biodegradation of Dimethylsilanediol in Soils,” Applied and Environmental Microbiology 62, 4352 (1999)

1. Introduction

Polydimethylsiloxane (PDMS) is a commonly used silicon-based organic polymer. Due to its unique mechanical, chemical, and optical properties, it has become integrated into many optical and micro-fluidic devices.

Polydimethylsiloxane can be purchased as a two-part kit. The kit consists of a base and a cross-linking agent. The two parts are in a viscous liquid form until mixed and cross-linking occurs. The cross-linking procedure will occur without other aid once the two parts are mixed. However, the procedure can be greatly accelerated with heat. The mixing ratios and curing procedures used during development determine the mechanical, chemical, and optical properties of the final solid.

2. PDMS Mechanical Properties

When cross-linked, PDMS acts like a rubbery solid. In this state, the polymer does not permanently deform when under stress or strain. Rather, the elastic polymer will return to its original shape when released. The elastic properties of PDMS are highly dependent on the amount of cross-linking agent (often is used methyltrichlorosilane) integrated into the polymer. The higher the concentration of the cross-linking agent, the more solid the final polymer becomes. With little or no cross-linking agent, the polymer will remain a viscous liquid. Since the curing process changes PDMS from a liquid into an elastic solid, PDMS is commonly used in micro-fabrication molds. PDMS has been also used as walls for micro-fluidic channels and as a silicon wafer bonding agent. [1]

3. PDMS Chemical Properties

PDMS is generally considered to be chemically inert and also notably hydrophobic, meaning that water cannot easily penetrate its surface. This property has led extended use of PDMS in micro-fluidics. However, most organic solvents can still penetrate the PDMS surface, limiting its versatility. PDMS has also increasingly been used in extraction processes, where PDMS is used to remove organic contaminants from water for analysis. As organic solvents are absorbed into the polymer, the volume of the polymer must increase, or swell, referred to the volume of the introduced chemicals. The solubility parameter of each chemical determines the amount of swelling that occurs. Neither chemical absorption, nor physical swelling are permanent. The absorbed chemicals can just as easily diffuse out of the polymer as they can diffuse in. The diffusion mechanics are dependent on equilibrium states between the polymer and the surrounding medium. Therefore, absorbed chemicals will remain in the polymer as long as a similar concentration of that chemical exists in the surrounding medium at the PDMS surface. If the concentration in the medium decreases, then diffusion mechanics will cause the absorbed chemical to naturally flow out of the PDMS until a new equilibrium is met.

4. PDMS Optical Properties

PDMS is optically clear at a wide range of wavelengths. In addition, the curing time and temperature used during cross-linking can determine the refractive index (RI) of the bulk. Since the polymer can be easily molded, it has been used to form lenses and waveguides. Also, the effective refractive index and the absorption spectrum of PDMS are changed when organic compounds are physically absorbed into the polymer. These properties have created the basis for several fiber-optic based chemical sensors. Through monitoring changes in refractive index or absorption spectrum, chemical concentrations absorbed into a volume of PDMS may be identified and characterized.

Polydimethylsiloxane (PDMS) fluids are available in a broad range of viscosities and are used in a wide range of applications. Polydimethylsiloxane fluids are known in the beauty and personal care industry by their INCI name, i.e.“dimethicone.” The Dow Corning commercial name of PDMS is XIAMETER®.[ 2 ]

Very-low-viscosity (≤ 2 cSt) polydimethylsiloxane fluids are categorized as volatile methylsiloxanes (VMS). In the United States, VMS fluids are exempt from regulation as volatile organic compounds (VOCs).

Features And Benefits of PDMS

  • Excellent water repellency
  • Good dielectric properties over a wide range of temperatures and frequencies.
  • Low glass transition (Tg) temperature
  • Low surface tension
  • Heat stability
  • Oxidation resistance
  • Very low vapor pressure
  • High flash point
  • Inert, nonreactive

Typical Uses

  • Mechanical fluids
  • Dielectric coolants
  • Insulating and damping fluids for electrical and electronic equipment
  • Release agents
  • Foam control
  • Surface active fluids
  • Lubricants
  • Ingredients for cosmetic and personal care formulations, polishes and specialty chemical products
  • Plastics additives

Most polydimethylsiloxanes are non-volatile organosilicon polymers consisting of
(CH3)2 SiO structural units as shown below :

polydimethylsiloxanes

Polydimethylsiloxane structure, where typically x > 4

Various polydimethylsiloxane fluids are linear, ranging in viscosity from very low to ultrahigh viscosities.

  • PDMS fluids draw strength, stability and flexibility from their siloxane backbone.
  • PDMS fluids gain inertness, lubricity, release properties and water repellency from their attached methyl groups,.

Consequently, they are used in a wide range of industrial applications, such as paper, leather goods or textiles. They often serve as antifoams, softeners or water repellents. [3]

PDMS fluids can also be found in auto motive care products, personal – and household products.

5. Environment and Recycling

Due to their wide range of applications, PDMS fluids can enter the environment in different ways. Since they are non-volatile, PDMS do not evaporate into the atmosphere. In household products, only very small quantities of PDMS fluids can be washed from the surfaces to which they have been applied , eventually into the soil or a water treatment plant. This is the case for personal care products such as conditioners and shampoos, that are rinsed away after use and consequently the PDMS they contain is carried with water to the treatment site. In industrial applications, where PDMS are used as surface treatments or process aids, small quantities may be found in process water too. About 17% of the total polydimethylsiloxane production volume worldwide is used in “ down – the – drain” applications.

End-use industrial products such as transformer fluids are used in contained applications. These are suitable for recycling and unlikely to enter the environment, except in cases of accidental release.

A number of studies have shown that PDMS will degrade into compounds of lower molecular weight when in contact with soils, especially into (CH3)2Si(OH)2 [4]. The phenomenon is widespread in nature, as confirmed by testing under different representative conditions in a wide range of soils. A significant degradation to lower molecular weights was observed after only few weeks of soil contact. The degradation rate and extent vary as a function of soil moisture content and of clay type. It was shown that the resulting degradation products further oxidize in the environment, both biologically and abiotically , in order to form silica, carbon dioxide and water.

It was not observed any effect from polydimethylsiloxane or its degradation products on plant growth, seed germination or the plant biomass [5]

PDMS fluids are not classified as hazardous materials. Specific testing has shown that PDMS is not toxic and does not bioaccumulate in sediment-dwelling organisms. Consequently, PDMS is not relevant for European product labeling.

Water treatment plants and on-site septic systems are both designed to facilitate the natural degradation of this water by microscopic organisms. Biomass ( sludge ) is generated by this degradation and must eventually be discarded. The treated sludge in a municipal system is typically landfilled, incinerated or used as a fertilizer . In on-site septic systems, common in US, the tank is periodically pumped out and the biomass is taken to the water treatment plant.

Usually PDMS fluids resulted from personal care and household products enter into the mentioned treatment systems as tiny dispersed droplets in water. Due to the fact that the PDMS fluids are essentially not soluble in water, they attach to the suspended materials in water systems and therefore become a minor part of the sludge.

PDMS does not inhibit the microbial activity by which water is treated. Also, PDMS loadings had no effect on the operating parameters (such as pH, suspended solids, specific oxygen uptake) or physiological activity of the micro-flora in the activated sludge units. Sludge digestion operating parameters were also unaffected by loadings of up to 100 mg/kg of PDMS [6].

The ultimate fate of sludge-bound PDMS depends on the sludge disposal technique. If the sludge is incinerated, the silicone converts to amorphous silica, which has no further environmental consequence when the ash is landfilled. When treated sludge is used as fertilizer, very small percents of PDMS may be introduced to the soil environment, where it is subject to soil-catalyzed degradation. Consequently, PDMS had shown no significant environmental effects.

References:

[1] www.photonics.byu.edu (Brigham Young University)
[2] Dow Corning/Xiameter – www.xiameter.com
[3] W.Noll “ Chemistry and Technology of Silicones “, New York ( Aug.1996 )
[4] J.C.Carpenter,J.A.Cella,S.B.Dorn “ Study of the degradation of Polydimethylsiloxanes on soil”, Environmental Science and Technology, 29,p.864(1995)
[5] D.A.Tolle, C.L.Frye and others “ Ecological effect of PDMS-augmented sludge amended to agricultural microcosms”, The Science of the Total Environment, 162, p.193 (1995)
[6] R.J.Watts,S.Kong and others, “ Fate and effects of PDMS on pilot and benchtop activated sludge reactors and anaerobic/aerobic digesters”, Water Research, 29, p.2405 (1995)

Silicone oil has many different names such as : silicone fluid, polydimethylsiloxane, dimethicone, dimethylpolysiloxane, dimethyl siloxane, PDMS. From a chemical point of view, probably that polydimethylsiloxane name best describes the silicone oil structure.: Poly (it is a polymer), dimethyl (two methyl (CH3) groups) , siloxane (Silicone and Oxygen).

The chemical structure of polydimethylsiloxane is:

silicone-structure

where n= number dimethylsiloxane units.

The customers of polydimethylsiloxane will be aware that the silicone oil is available in a number of different viscosities. The most common silicone oils are are that of viscosities 100cSt (centistoke), 350cSt and 1000cSt. The number of dimethylsiloxane units depicts the size of the polymer chain which determines the viscosity. The smaller the polymer chain the lower the viscosity (e.g. 100 cSt) , the bigger the polymer chain the higher the viscosity (e.g.1000cSt).

An Allcock product in the Allcosil 200 range has the lowest viscosity equal to 0.65cSt. Allcosil 200/0.65 has only two units, this means that it is a dimer, and not a polymer. The dimer is hexamethyldisiloxane.

The chemical structure of hexamethyldisiloxane is:

hexamethyldisiloxane

It is clear that that the dimer has a structure very similar to the structure of polydimethysiloxane .The polymer chain length also has an effect on the other properties of the oil as shown in Table no.1 below :

Table no.1.

Viscosity, cSt Flashpoint, °C COC Freezing Point,°C Specific Gravity, @ 25°C Surface Tension, mN/m Refractive Index, @ 25°C
0.65 -4 -67 0.760 15.9 1,375
4 40 -85 0.816 17.4 1,382
2 48 -90 0.830 18.1 1,387
3 62 -100 0.900 18.9 1,392
5 136 -100 0.910 19.7 1,397
10 162 -65 0.930 20.1 1,399
20 230 -60 0.950 20.6 1,400
50 280 -55 0.959 20.7 1,402
100 >300 -55 0.965 20.9 1,403
200 >300 -50 0.970 21.0 1,403
300 >300 -50 0.970 21.1 1,403
350 >300 -50 0.970 21.1 1,403
500 >300 -50 0.970 21.1 1,403
1000 >300 -50 0.970 21.2 1,403
5000 >300 -50 0.975 21.4 1,403
10000 >300 -50 0.975 21.5 1,403
12500 >300 -50 0.975 21.5 1,403
30000 >300 -50 0.975 21.5 1,403
60000 >320 -50 0.975 21.5 1,403
100000 >300 -50 0.976 21.5 1,404
300000 >300 -45 0.976 21.5 1,404
1000000 >300 -40 0.976 21.5 1,404

(1000000 cSt, longest polymer chain.)

It may be observed from the table no.1 that the polymer chain length becomes less important in effecting the properties at viscosities higher than 50cSt. According to the FDA Regulation 21 CFR, certain viscosities are food grade. Silicone oils have many uses due to their lubrication, dielectric and water repellent properties.

2. Silicone Emulsion

Silicone Emulsion contains 3 ingredients; polydimethylsiloxane, emulsifier & water. The key ingredient is the emulsifier, which encapsulates the polydimethylsiloxane oil and holds it in suspension.

A typical emulsifier used in making silicone emulsions is ethoxylated glycol ethers. These emulsifiers have a hydrophobic (dislikes water) and hydrophilic (likes water) part in their structure. The hydrophobic faces the polydimethylsiloxane and the hydrophillic faces the water. This creates a barrier between the water and the polydimethylsiloxane. Here below is represented a picture that shows how an emulsifier works. A surfactant (i.e. surface active agent) is just a type of emulsifier.

emulsion-550

Unfortunately emulsifiers, due to their chemical make-up, are vulnerable to bacteria. Bacteria can digest certain emulsifiers causing the polydimethylsiloxane to float to the top or disrupt the pH enough for non-digestible emulsifiers to split from the oil. Splitting in an emulsion can be seen and smelt. Bacterial growth causes smell and usually the emulsion smells like sour milk.

Emulsions are a key mould release/lubricant and antifoams for many industries. For instance, Allcock biggest selling emulsion is Allcosil 435 FG, which is sold to the food packaging industry. It is food grade and kosher certified.

3. Recycling

Research and review of peer-reviewed scientific studies indicate one should begin to be cautious about silicone for the following reasons [2] .

  • Silicones are not completely inert or chemically unreactive and can release toxic chemicals. They can leach certain synthetic chemicals at low levels, and the leaching is increased with fatty substances, such as oils. Evidence of contamination from silicone was found in wine and edible oil foods. Materials such as aluminium, platinum, magnesium and calcium were found to have leached into food when testing was carried out on silicone bakeware.
  • Fluid silicone studies indicated release of siloxanes, one of which – cylcopentasiloxane – is considered toxic and persistent. This siloxane, also known as D5, is used as a softener in cosmetics, and according to the U.S. Environmental Protection Agency may also be carcinogenic. [3]
  • Silicone tubing commonly used for medical applications has been shown to leach several chemicals, including dioctyl phthalate . DOP Silicone intravenous devices have been shown to leach silicone and cause local inflammation [4].
  • Silicones are likely not completely inert and may cause local inflammation. as suggested in this study on the breakdown of silicone joint implants.[5]
  • Silicone oils have a low recycling rate.
    Silicone does not biodegrade or decompose (certainly not in our lifetimes). It is recyclable, but not likely through a local municipal recycling program. Preferably it should be taken to a specialized private recycling facility.

An alternative process is mentioned for removing silicones present on fibers, yarns or textile sheet materials (substrates), wherein an aqueous preparation is allowed to act on the substrates. Thus, the treated substrates are rinsed to remove the residual silicone and the said preparation contains from 0.5 to 10% by weight of surface-active quaternary ammonium compounds and from 0.5 to 5% by weight of alkali hydroxide, respectively based on the aqueous preparation. The process is suitable, in particular, for the recovery and recycling of fibrous material and silicones from airbags or coated fabric materials from the ready-making of airbags.[6]

Another process is mentioned for recycling a silicone compound, wherein the silicone compound is decomposed by employing an alkyl carbonate and a compound containing active hydrogen group in the presence of a catalyst. Following this procedure, the silicone monomers and/or silicone oligomers are recovered from the decomposed silicone compound. Silicone monomers and/or silicone oligomers are produced without producing any byproduct, and no step for removal by separation is needed in an after process. Therefore the implementation of the said process is claimed to be easy.[7]

TMC Industries uses both centrifugal molecular stills and packed column stills in order to recover silicone oils. Using centrifugal molecular stills, the thin-film, short-path, high-vacuum technology, TMC Industries can restore contaminated vacuum fluids to their original state with exceptionally high yields. Using packed column technology, TMC can purify many fluorocarbon solvents by removing contamination and restoring them to their original high purity. Inert and semi-inert fluids can be reclaimed many times without affecting their performance capabilities.[8]

References:

[1] www.allcocks.co.uk
[2] www.lifewitoutplastic.com
[3] 2005 Report commissioned by the United Kingdom Food Standards Agency : “Chemical migration from silicones used in connection with food-contact materials and articles”
[4] Archives of Disease in Childhood: “Plastic migration from implanted central venous access devices” , a 1999 study
[5] Oral Surgery, Oral Medicine, Oral Pathology : “ Silicone-induced foreign body reaction and lymphadenopathy after temporomandibular joint arthroplasty”, a 2006 study.
[6] US Patent no. 20090126122 A1
[7] US Patent no.6,172,252
[8] www.tmcindustries.com

diffusion-pump-imageAnother great source for recycling of silicone oil comes from oil made for diffusion pumps. A diffusion pump is used for creating high vacuums mainly in industrial applications such as thin film deposition, vacuum furnaces, protective film coatings, semiconductor manufacturing, space simulations, and much more. As diffusion pumps do not have any moving parts, it’s one of the most stable and affordable ways of producing high vacuums. Generally speaking, diffusion pumps can attain vacuums between 10e-6 to 10e-10 torr depending on the size of the pump as well as the type of diffusion pump oil used.

As stated earlier, there are no moving parts in a diffusion pump. The only moving aspect of the pump is the movement of the diffusion pump oil within. A heater is located at the bottom of the pump that heats the oil until it turns into a vapor or gaseous state. We won’t go into depth as to how a diffusion pump works, however, the vapors eventually are turned back into oil and continuously changes between these two states where air molecules are forced outside the system causing a vacuum.

Interestingly, when diffusion pumps were first introduced back in 1915, the diffusion pump oil used was derived from mercury. As mercury is poisonous to humans, this was replaced by oil made from hydro-carbons. However, as mineral based oils tend to thicken and darken when used continuously inside the pump, a problem arises where pumps became often clogged where oil changes were required relatively frequently, every several months.

The newest trend in diffusion pump oil is actually the use of silicone based oils. As silicone offers low vapor pressure and very low degradation when continuously heated and cooled, it became an ideal solution compared to hydro-carbon based oils. Nowadays, inside most diffusion pumps is a silicone based formula.

While degradation of silicone based oils is low, it is still highly recommended that the oil inside the diffusion pump be replaced at least once per year. With all the diffusion pumps being in use in the world, this left a lot of silicone oil for recycling. Luckily, we offer one of the best solutions for recycling diffusion pump oil.

The constant vaporization and condensation of silicone oil actually breaks down the oil on a chemical level. As you continue the use the oil, you’ll notice a loss of efficiency, that is, the rate of vacuum will decrease. The chemical deterioration prevents the oil from being reused as diffusion pump oil. While this is so, we have come up with a brilliant solution. Instead of trying to filter and reuse the oil for diffusion pumps, we’re now able to completely reprocess diffusion pump oil back into standard grade silicone oil that can be used in other industrial applications.

For more information about our recycling process or if you have a source for used diffusion pump oil, please make sure to contact us.

Silicone emulsions are used in both hair and skin care products. The preparation of
stable emulsions results in a silicone oil in a micelle, having a fine particle size. The
preparation of the emulsion requires the use selection of the proper emulsifier pair and more commonly the use of a homogenizer to obtain a stable emulsion.[ 1 ]

When silicone is delivered from a micelle, the energetics of delivery to the substrate is
complicated by: (a) the presence of emulsifiers, (b) the type of emulsifiers, and (c) the
particle size of the silicone. All must be optimized for best performance of the emulsion in the formulation.

Many of the complications of using emulsions for the delivery of silicone to substrates relate to the fact that the silicone is delivered out of a micelle. When surface-active agents are added to water, the first observable effect is that the surface tension at the air water interface drops. As one continues to add surfactant, the critical micelle concentration is reached. At this point micelles are formed. There is an equilibrium between surfactant in the micelle and the surfactant the interface reducing the surface tension. Additionally, the surfactants used have detergency properties. When an emulsion is applied to the skin or hair, the silicone oil is delivered to the substrate that has been wet out by the surfactant at the air water interface. The emulsion breaks and the oil is deposited.

However, the surfactant having emulsification properties re-emulsifies some of the oil. The net result is that silicone ends up both on the substrate, in the wash water. This complex equilibrium results in inefficiency when one uses emulsions.

In addition, emulsions have some inherent shear instability, and freeze thaw instability.
Finally, there are limitations as to the type of additional surfactant that can be added to an
emulsion containing system. If the formulation is shifted too much, the emulsion will break. Care must be exercised in preparing emulsion-based systems. With the proper selection of emulsion and the proper formulation techniques, silicone emulsions can be used in the creation of many emulsions useful in many applications.

These applications include uses as mold release agents, automotive tire gloss compounds, textile softeners, overspray in web offset printing and antifoam compounds.

Dimethicone and dimethiconol emulsions are used commonly in many industrial and personal care applications. All emulsion products comprise (a) water typically at least 40%, (b)silicone (typically 55%) and the remainder surfactant to make an emulsion. The fact that the silicone is contained in an emulsion by necessity requires that the delivery be from a micelle.

Since there is an equilibrium that exists between the silicone on the substrate, like fabric, fiber, metal, rubber, hair or skin, and the silicone in the emulsion, much of the silicone ends up in the wash water. Not only is this very costly and an inefficient use of expensive raw materials, but there are real environmental concerns since the wash water ends up in the sewer. In order to overcome this limitation, silicone surfactants have been developed that provide non-micellular delivery to the substrate.

In pulping, silicone process aids reduce the amount of heat and harsh chemicals required to “cook” the wood chips, which lowers energy and material costs and reduces fiber damage [ 2 ].

Silicone antifoams control foam and improve pulp drainage, which improves process efficiency and reduces bleaching requirements.

Silicone process aids do not contain dibenzodioxin or dibenzofurans and do not form harmful byproducts; they do not add to biological oxygen demand (BOD) in water systems and have proven safe for water treatment operations.

Silicone release coatings give label and tape makers an almost limitless array of substrate, processing, performance, and application options.

Silicone pressure sensitive adhesives adhere reliably to low-energy surfaces. They also withstand extreme temperatures, chemical attack, and long-term exposure to weather and UV light.

Water-based, solvent less, and solvent-reducing silicone formulations help pulp, paper, and label and tape manufacturers worldwide address cost, safety, and environmental protection issues.

Silicone technology for de-inking and micro-“stickies” control make paper recycling easier and more cost effective.

Silicones perform under conditions that would defeat organic (carbon-based) materials, are more effective at lower levels, and provide unique solutions to difficult problems.

Silicone architectural coatings typically last twice as long as acrylic coatings, and silicone building sealants typically last three times as long as urethane sealants.

High-voltage-insulator silicone coatings perform for 10 years or more, while some other protection methods must be reapplied every 18-36 months. Imagine the long-term cost savings of silicone. Adding as little as 1.1¢ worth of silicone to a typical 300-gram hair-conditioning rinse doubles dry combing benefits and increases shine by 20%.

A single silicone paint additive can provide as many as five different performance benefits.

Silicones have been used safely and successfully in personal care products for more than 30 years.

If an organic sealant needs to be cut out and replaced every seven years, the amount of garbage produced and solvents used will be at least three times greater than if a longer-lasting silicone sealant had been used.

Many silicone fluids and elastomers can be recycled.

Silicones help manufacturers eliminate water-wasting process steps and reduce the use of air-polluting solvents.

Silicones help automakers comply with an entire alphabet of environmental laws and regulations ( i.e. RoHS, EPA, CARB LEV (Low-Emission-Vehicle), WEEE, Euro 4, Euro 5, EU law 1999/13/EC and more ).

Silicone Coating Recycling

Belgian recycling specialist RecuLiner has struck up a partnership with Munksjö Group, a Finnish manufacturer of release papers for the pressure sensitive adhesive industry (PSA). Their goal is to develop and promote the recycling of silicone-coated release liners from PSA label end-users into cellulose fibre insulation.[ 3 ]

The fibre used in the thermal and sound insulation of buildings has typically been produced from old newspapers. But silicone-coated release paper material has proved to be an ‘excellent’ substitute material for this purpose, resulting in an even better performing product.

The partnership with Munksjö will promote this new recycling option as part of a program and as a complementary possibility’ to its existing recycling opportunities in paper production. The program was set up and will now ensure free-of-charge collection from many release liner end-users in Belgium, Luxembourg, the Netherlands, France and Germany.

The collaboration with RecuLiner provides a valuable opportunity to increase the number of available recycling options for paper release liners in Europe. It also further broadens the geographical coverage of the recycling program.

Silicone Emulsions Recycling

In the environment, polydimethylsiloxane fluid breaks down into water, carbon dioxide, and minerals already found in the earth’s crust. Silicone emulsions are basically silicone oil, water and an emulsifier. The silicone oil does the lubrication, the water carries the oil and allows for easy dilution of the oil and the emulsifier binds the two together. However, problems can arise in emulsions, the large array of splitting possibilities are shown perfectly in the diagram below:

Schematic-of-mechanisms-leading-to-coalesence-of-an-oil-in-water-emulsion

Taken from: http://www.particlesciences.com/news/technical-briefs/2011/emulsion-stability-and-testing.html

Though there are many technical terms, the origin of all these kinds of splitting are the same. Like milk, Silicone emulsions can ‘go off’ if left in a hot, moist environment. Like milk, Bacteria and fungi can grow that feed on the emulsifier and cause the separation of an emulsion, leading to a ‘lumpy’ consistency.

There are a few precautionary measures that can be taken to ensure that Silicone emulsion doesn’t split:

  • Store the container in a cool, dry area
  • Do not store diluted material for long periods of time
  • Regularly wash out dilution vessels to stop bacteria/fungi carrying over into fresh batches
  • Use Allcosil Stabiliser to increase the lifespan of the Silicone Emulsion
  • Allcock&Sons Ltd. Company is producing specially formulated products which are tailored-made for the digestion and removal of silicone emulsion as follows [ 4 ]
Allcostrip DI-AQUA
Chemically digests cured silicone polymers, making them water solvent and rinsable. This bio-degradable detergent is claimed to effectively emulsify silicone oil, greases and uncured elastomer. Ideal for the removal of the silicone emulsions.

References:

  1. Basic Silicone Chemistry, Anthony O’Lenick, Silicone Spectator, January 2009
  2. www.dowcorning.com
  3. www.reculiner.com and www.munksjo.com
  4. www.allcocks.co.uk

Silicones are the “missing link” between organic and inorganic chemistry and have unique properties that other polymers can not match. By changing the size or structure of the silicone molecule or by adding different compounds to it, one can enhance or change the way it behaves. The secret to silicone’s amazing capabilities lies in its flexible Si-O-Si backbone.

Silicones enable the development of electronic devices that are more powerful, more versatile, more cost effective, and easier to use. They make the cars safer, more reliable, and less costly to maintain.
Household appliances manufactured with silicones are more dependable and require less maintenance.
Silicones protect power transmission equipment from environmental damage and help keep the electricity flowing .They enable also address labels to peel off easily and industrial tapes to stick tightly to difficult surfaces [1].

Silicone fluids, also called silicone oils, or simple silicone are sold by their viscosity and range from 0.65 centistokes to 1,000,000 centistokes. If the product is not made by blending two different viscosity fluids the viscosity is related to molecular weight. The viscosity allows for an approximate calculation of the value of “n” in the formula below [2].

Viscosity 25C
(Centistokes)
Approximate
Molecular Weight
Approximate
“n” Value
5 800 9
50 3,780 53
100 6,000 85
200 9,430 127
350 13,650 185
500 17,350 230
1,000 28,000 375
10,000 67,700 910
60,000 116,500 1,570
100,000 139,050 1,875

Silicone may be adhered to substrate, including fiber, fabric, metal surface, hair and skin by virtue of one or more of the following mechanisms:

(a) Hydrophobicity – When oil is placed into water, it disrupts the hydrogen bonding between the water molecules in the water solution. This disruption is accomplished only
when the energy of mixing is sufficient to break the hydrogen bonds. When the mixing is stopped, the oil is forced out of the water by the re-formation of the hydrogen bonds between water molecules. This phenomenon can be used to deliver of oil to a surface. Silicone fluids are delivered this way.

(b) Ionic Interactions – The charge on the molecule will also have an effect upon the delivery of the oil to the hair or skin. For example, if the oil has a cationic charge on the
molecule, it will form ionic bonds with substrates that contain negative surface charges. The two opposite charges together forms a so-called pair bond.

(c) General Adhesion – If an oil is delivered to the skin or hair penetrates and then polymerizes, there will be an interlocking network of polymer developed. Although not bonded directly to the substrate, this polymer network will adhere to the substrate.

(d) Specific Adhesion – If an oil is delivered to the skin or hair penetrates and then reacts with groups on the hair or skin, there will be a chemical bond between the polymer and
the substrate. This is the strongest and most permanent of the adhesion mechanisms.

Silicone fluids react almost exclusively by mechanism (a). To the extent the other mechanisms may be introduced, the more strongly and efficiently the conditioner can be
delivered to substrate. Organo-functional silicone seek in large part to capitalize on these additional mechanisms to provide through and efficient conditioning for hair and skin.

An Amazing Range of Capabilities

Silicones are a huge group of products that show some very useful traits like stability at high temperatures and resistance to age, sunlight, moisture, temperature extremes, and chemicals. Silicones can take many different forms and perform hundreds of different jobs. They can be hard and brittle, or soft and flexible.

Silicones can be liquids or solids, durable or temporary, can adhere or release. These polymers can be hydrophobic (repel water) or hydrophilic (absorb water).Silicones can make things soft, smooth, and silky or hard, rough, and tacky. They can destroy foam or stabilize it.

Proven Performance

Structural silicone sealants installed in buildings around the world in the 1980s are still performing today.

Approximately half of all makeup, hair and skin care, and underarm products introduced today contain silicone.

Silicone finishes are widely recognized as the best materials for increasing the softness of fabrics and enhancing the way they feel.

Silicone defoamers have been used extensively in pulp washing operations worldwide since the early 1990s.

Virtually any electronic device that is powered by batteries or electric current relies on silicones.

Silicone Fluids Recycling

2.1.A method is provided for recycling and treating the silicon wafer, cutting and polishing processes as follows [3 ] : a dewatered filter cake is mixed with water so that the filter cake is diluted to form a working fluid. The water reacts with silicon in the filter cake to produce silicon dioxide and hydrogen. After the hydrogen is extracted for storage, specific gravity separation takes place via water so that silicon carbide and silicon particles are separated for sorting. Then, solid-liquid separation is performed on the remaining working fluid to separate silicon dioxide (solid) from water and polyetyleneglycole (PEG, liquid), before PEG is separated from water. Thus, the useful silicon particles, silicon carbide, silicon dioxide, and PEG are recycled from the filter cake to reduce the total amount of materials. Moreover, as the side product, hydrogen, is of high commercial value, the method also adds value to recycling.

2.2 Recycling silicon wire-saw slurries: separation of silicon and silicon carbide in a ramp settling tank under an applied electrical field:

The growing demand for silicon solar cells in the global market has greatly increased the amount of silicon sawing produced each year. Recycling Si and SiC from sawing is an economical method to reduce this material. A study [ 4 ] reports the separation of Si and SiC using a ramp settling tank. As they settle in an electrical field, small Si particles with higher negative charges have a longer horizontal displacement than SiC particles in a solution of pH 7, resulting in the separation of Si and SiC. The agreement between experimental results and predicted results shows that the particles traveled a short distance to reach the collection port in the ramp tank. Consequently, the time required for tiny particles to hit the tank bottom decreased, and the interference caused by the dispersion between particles and the fluid motion during settling decreased. In the ramp tank, the highest purities of the collected SiC and Si powders were 95.2 and 7.01 wt%, respectively. Using a ramp tank, the recycling fraction of Si-rich powders (SiC < 15 wt%) reached 22.67% (based on the whole). This fraction is greater than that achieved using rectangular tanks. Implications: Recycling Si and SiC abrasives from the silicon sawing is regarded as an economical solution to reduce the sawing materials. However, the separation of Si and SiC is difficult. Compared with the rectangular tanks, the recycling fraction of Si-rich powders using a ramp tank is greater, and the proposed ramp settling tank is more suitable for industrial applications.

References

  1. www.dowcorning.com
  2. Basic Silicone Chemistry, Anthony O’Lenick, Silicone Spectator, January 2009
  3. Patent application number: 2012031274, Jr-Jung Iang (Changhua City, TW) ,2012-12-13
  4. Air Manag Assoc., Tsai TH1, Shih YP, Wu YF. 2013 May;63(5):521-7

1. Introduction

The use of silicones in these applications is much related to cable end terminations or silicone rubber connections made at the end of underground high voltage cables insulated with polyethylene, as well as to silicone insulators for power lines.

Key benefits from silicones are their high electrical resistivity, resistance to environmental degradations and to electrical aging as well as their hydrophobicity, which results in lower assembly and maintenance costs

2. Silicone Cable End Terminations

Modern materials allow pre-assembly and thus avoid problems associated with the use of molten casting material or mistakes made during manual assembly on the construction 60 site. Today cable accessories are completely built at the supplier. Typically they consist of rubber terminations made of different insulating silicone rubbers.

Silicones allows for two types of design:

– Push-on technique where a PE ring acts as a space holder until placement, and using silicone rubbers with hardness from 35 to 50 Shore A

– Cold shrink technique using softer silicone rubbers with hardness from 25 to 35 Shore A Insulation is made without chemical bonding between the termination and the cable, and it relies on the elastomeric characteristics of the silicone termination to exclude any entrapped air, particularly in areas of high electrical field and around the edges at the cable end. The high gas permeability of silicones allows any included air to diffuse out to leave an air-free joint.

Such silicone rubber cable end terminations are produced by rubber injection molding using a silicone high consistency rubber (HCR) or by liquid injection molding using a two-part liquid silicone rubber (LSR).
Silicones provide overall electrical insulation because of their high dielectric strength. In addition to their good resistance to high temperature, UV and ozone, they are hydrophobic and so do not promote surface insulation failures. But more important, specially formulated silicones have been developed to smooth the electrical fields within the connection end and to ensure long-term performance. This is achieved in composite cable terminations either using some electrically-conductive silicone rubbers or, in more modern and smaller accessories, shaped deflectors made from silicone rubbers with medium electrical permittivity.

Silicones are appreciated in cable end terminations because of their resistance to erosion caused by radiation. As silicones do not absorb UV-visible sunlight, they are not prone to chalking or cracking. Such phenomena are typical with organic-based materials and, associated with dirt pickup and humidity, can lead to a significant reduction of insulation properties.

Silicone resistance to so-called “tracking” is also higher than with organic-based insulation materials. Tracking is the formation of electrically-conductive surface paths under intensive electrical surface leaks and discharges. In organic materials, this leads to the formation of carbon-based decomposition products that unfortunately show high conductivity. With silicones, even if poorly designed or not properly assembled, decomposition leads to nonconductive silica, and silicones will meet the highest class of electrical erosion resistance.

3. Silicone Insulators

Another key property is hydrophobicity, particularly for electrical insulators, or devices installed between power lines and supporting structures. Water on an insulator made of a silicone elastomer remains as droplets and does not form a continuous film because of the low surface energy of the silicone elastomer surface . This reduces surface currents on the insulator. Surface hydrophobicity is maintained even after surface discharges or deposition of airborne pollution because of the presence of low molecular weight, unreacted polydimethylsiloxane species in the composition of the silicone elastomers. These species can migrate to the external surface and maintain low surface energy or hydrophobicity . Insulators made of silicone elastomer therefore need little cleaning or maintenance and perform over a long period of time.

When selecting a dielectric fluid, there are a number of criteria that must be contingent upon the application. One of the strongest advantages that a silicone fluid has over a fluid such as mineral oil is its much higher thermal stability, flash points and fire points. This is critical for fluids used in transformers that are located inside or near to a building where flammability is a significant concern. A silicone transformer oil can be subjected to very high temperatures, well above normal transformer operating temperatures, without creating excessive vapor pressure, breaking down or creating corrosive by-products.

Silicones are chemically inert, have good oxidation resistance and are compatible with conventional insulating materials at transformer operating temperatures.

There are two modes of degradation for silicone fluid : thermal breakdown and oxidation. whese temperatures the longer polymer chains will slowly begin to degrade in order to form more volatile cyclic silicone material.
Oxidation of silicone fluid takes place very slowly ( in the presence of oxygen ) at temperatures above 175°C ( 342°F ). When it oxides, silicone fluid polymerizes, gradually increasing in viscosity until gelation occurs. This process occurs without the formation of objectionable acids or sludges. Additionally, the dielectric properties of the longer-chain silicone molecule are similar to the dielectric properties of fresh silicone fluid.

The temperatures at which thermal and oxidative degradation take place are well in excess of the hot-spot temperatures expected in 65°C-rise transformers. In the limited-oxygen atmosphere of sealed transformers, silicone transformer fluid can be used at temperature rises that are above standard rises of other transformer fluids.

The silicone transformer fluid is not expected to degrade in any significant manner over the useful service life of a 65°C-rise transformer.

Insulation systems using silicone fluid in combination with solod insulating materials having high-temperature capabilities have shown significantly improved thermal capabilities and longer insulation life ( i.e. insulation system consisting of silicone fluid combined aromatic polyester amide/imide, Nomex paper and glass materials).

The Pure Silicone Fluids are 100% linear Polydimethylsiloxane Fluids (CAS# 63148-62-9) with viscosities measured @ 25C. They contain no additives such as pour-point depressants or heat-stabilizers. In addition, they contain no chlorine or other halogens. Pure Silicone Fluids with viscosity >5cSt are chemically inert, non volatile, thermally stable, have excellent oxidation resistance, and are compatible with conventional insulating materials. Material compatibility and thermal stability are closely related. A large number of materials have been tested for compatibility with silicone transformer oil. Below is a list of materials tested and found suitable in STO-50 Silicone Transformer Oil.**)

Transformer Materials that are compatible with Silicone Transformer Oil:

________________________________________________________________________
*) Silicone in Medium to High Voltage Electrical Applications , E. Gerlach, Dow Corning GmbH, Wiesbaden (Germany),2002
**) www.clearcoproducts.com

1. Introduction

By analogy with ketones, the name “silicone” was given in 1901 by Kipping to describe new compounds of the brut formula R2SiO. These were rapidly identified as being polymeric and actually corresponding to polydialkylsiloxanes. Among them, the most common are polydimethylsiloxanes (PDMS), trimethylsilyloxy terminated with the structure:

The methyl groups along the chain can be substituted by many other groups (e.g., phenyl, vinyl or trifluoropropyl). The simultaneous presence of “organic” groups attached to an “inorganic” backbone gives silicones a combination of unique properties and allows their use in fields as different as aerospace (low and high temperature performance), electronics (electrical insulation), health care (excellent biocompatibility) or in the building industries (resistance to weathering).

2. Synthesis

The main chain unit in PDMS, – (SiMe2O) -, is often shortened to the letter D because, as the silicon atom is connected with two oxygen atoms, this unit is capable of expanding within the polymer in two directions.

In summary, PDMS is obtained from the hydrolysis of dimethyldichlorosilane Me2SiCl2, which leads to a mixture of linear and cyclic oligomers:

Higher molecular weight PDMS is obtained after polymerisation, for example, of the
above cyclics in the presence of an end-blocker such as hexamethyldisiloxane and
catalysed by a strong acid or strong base according to the following reaction:

Using other chlorosilanes, different end-blockers and/or different cyclics leads to many structures including polymers with various functional groups grafted on the polymer chain and/or at the polymer ends (e.g., vinyl, hydrogeno, phenyl, amino alkyl). These can be formulated into solvent-based, emulsion or solventless products.

Reactive polymers can be cross-linked into elastomers using:

– a peroxide-initiated reaction; in particular, if the silicone polymer carries some vinyl
groups

– a condensation reaction; for example, between a hydroxy end-blocked PDMS and an
alkoxysilane, in presence of tin salt or titanium alkoxide as catalyst

– an addition reaction; for example, between a vinyl-functional PDMS and an
hydrogenomethyl dimethyl siloxane oligomer, in presence of a organic platinum complex.

Such polymer, cross-linker and catalyst are formulated with various additives as one-part, ready-to-use products or two-part products to be mixed prior to use and to cure at room temperature or only at elevated temperatures.

3. Physicochemical Properties*)

The position of silicon, just under carbon in the periodic table, led to a belief in the existence of analogue compounds where silicon would replace carbon. Most of these analogue compounds do not exist, or if they do, they behave very differently. There are few similarities between Si-X bonds in silicones and C-X bonds.

Between any given element and silicon, bond lengths are longer than for carbon with this element. The lower silicon electronegativity (1.8) vs. carbon (2.5) leads to a very polarised Si – O bond, highly ionic and with a large bond energy, 452 kJ/mole (108 kcal/mol). The Si – C bond has a bond energy of ±318 kJ/mole (76 kcal/mol), slightly lower than a C – C bond, while the Si – Si bond is weak, 193 kJ/mole (46.4 kcal/mole).

These values partially explain the stability of silicones; the Si-O bond is highly resistant to homolytic scission. On the other hand, heterolytic scissions are easy, as demonstrated by the re-equilibration reactions occurring during polymerization reactions catalysed by acids or bases. Silicon atoms do not form stable double or triple bonds of the type “sp” or “sp” with other elements, yet the proximity of the “d” orbitals allows “dπ-pπ” retro-coordination.

Because of this retro-coordination, trialkylsilanols are more acid than the corresponding alcohols. Yet, the involvement of retro-coordination is challenged.

Another example of the difference between analogues is the tetravalent diphenyldisilanol, (C6H5)2Si(OH)2, which is stable, while its carbon equivalent, a gem-diol, dehydrates. The Si – H bond is weakly polarised and is more reactive than the C – – H bond. Overall, there are few similarities between a silicone polymer and a hydrocarbon polymer.

Silicones display the unusual combination of an inorganic chain similar to silicates and often associated with high surface energy but with side methyl groups that are, on the contrary, very organic and often associated with low surface energy]. The Si – O bonds are strongly polarised and without protection should lead to strong intermolecular interactions. However, the methyl groups, only weakly interacting with each other, shield the main chain.

This is made easier by the high flexibility of the siloxane chain; rotation barriers are low, and the siloxane chain can adopt many conformations. Rotation energy around a CH2–CH2 bond in polyethylene is 13.8 kJ/mol but only 3.3 kJ/mol around a Me2Si-O bond, corresponding to a nearly free rotation. The siloxane chain adopts a configuration that can be idealised by saying that the chain exposes a maximum number of methyl groups to the outside, while in hydrocarbon polymers, the relative backbone rigidity does not allow “selective” exposure of the most organic or hydrophobic methyl groups. Chain-to-chain interactions are low, and the distance between adjacent chains is also higher in silicones.

Despite a very polar chain, silicones can be compared to paraffin, with a low critical surface tension of wetting .Yet because of their low intermolecular forces, PDMS materials remain liquid in a much wider range of molecular weights and viscosities than hydrocarbons.

The surface activity of silicones is displayed in many circumstances:

– Polydimethylsiloxanes have a low surface tension (20.4 mN/m) and are capable of wetting most surfaces. With the methyl groups pointing to the outside, this gives very hydrophobic films and a surface with good release properties, particularly if the film is cured after application. Silicone surface tension is also in the most promising range considered for biocompatible elastomers (20 to 30 mN/m).

– Silicones have a critical surface tension of wetting (24 mN/m), which is higher than their own surface tension. This means that silicones are capable of wetting themselves, a property that promotes good film formation and good surface covering.

– Silicone organic copolymers can be prepared with surfactant properties, with the silicone as the hydrophobic part (e.g., in silicone polyether copolymers).

The low intermolecular interactions in silicones have other consequences:

– Glass transition temperatures are very low (e.g., 146 K for a polydimethylsiloxane compared to 200° K for polyisobutylene, the analogue hydrocarbon); cross-linked PDMS will be elastomeric at RT in the absence of any plasticizers.

– The presence of a high free volume compared to hydrocarbons explains the high solubility and high diffusion coefficient of gas into silicones. Silicones have a high permeability to oxygen, nitrogen and water vapour, even if in this case liquid water is not capable of wetting a silicone surface. As expected, silicone compressibility is also high.

– In silicone, the activation energy to the viscous movement is very low, and viscosity is less dependent on temperature compared to hydrocarbon polymers. Moreover, chain entanglements are involved at higher temperature and contribute to limit the viscosity reduction.

The presence of groups other than methyl along the chain allows modification of some of the above properties:

– A small percentage of phenyl groups along the chain perturbs sufficiently to reduce crystallisation and allows the polymer to remain flexible at very low temperatures. The phenyl groups also increase the refractive index.

– Trifluoropropyl groups along the chain change the solubility parameter of the polymer from 7.5 to 9.5 (cal/cm3 )

These copolymers are used to prepare elastomers with little swelling in alkane or aromatic solvents.
Considering the above-mentioned reasons, many polymeric “architectures” can be prepared of different physical forms (volatile, liquid, viscoelastic, solid) with different functionalities, inert or capable of interacting or reacting with many other compounds. Formulation into convenient products leads to even more products. This explains the wide range of industries where silicones are used.
___________________________________________________________________
*) www.dowcorning.com

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