Posted in Silicone Recycling

High and low viscosity PDMS

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

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

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

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