Posted in Recycle Silicone

Polydimethylsiloxane

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 :

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)