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Cleaning injection molded silicones and other polymers with C[O.sub.2].

For the past 28 years, C[O.sub.2] cleaning technology has been used in the production of high volume and high reliability products. Industries and applications have included aerospace hardware, hard disk drives, medical devices, microelectronic and optoelectronic components, among many others. C[O.sub.2] cleaning technology is used to remove particles, ionic contamination and organic residues from complex components and assemblies, both internal and external surfaces, to improve performance and reliability of the systems within which they are employed. This technology is also used to safely clean precision manufacturing tools and equipment (support hardware) with which these components and assemblies are fabricated, for example precision injection mold surfaces.

One of the earliest applications for C[O.sub.2] cleaning technology is the decontamination of injection molded silicone polymers, first developed at Hughes Aircraft in the early 1980s to prepare silicone rubber components used in commercial satellite electro-optical systems for the rigors of the space environment (ref. 1). Liquid and supercritical carbon dioxide (C[O.sub.2]) based "extraction" cleaning processes were developed to process silicone devices to meet NASA cleaning standards based upon total mass loss (TML) and volatile condensable matter (VCM) (ref. 2). Cleaned components included power cables, vibration dampeners, seals and electrical insulators for multi-pin connectors. Today, liquid, solid, supercritical and plasma C[O.sub.2] cleaning processes have been developed to clean and prepare silicones and many other types of polymers prior to operations such as bonding, coating and precision assembly, for critical operating environments such as cleanrooms, space and biological systems. Examples include flexible electronic cables used in cleanroom manufacturing equipment and implantable drainage tubes.

Silicone rubber contamination

Molded polymers require some form of post-mold cleaning (i.e., scouring) and/or surface treatment (i.e., activation) prior to follow-on manufacturing operations such as precision assembly, coating, plating and adhesive bonding to ensure better bonding, no outgassing or leachable contamination. This is true for molded silicones used in critical applications such as aerospace systems, biomedical devices and electronic cables used in clean environments. Examples of silicone rubbers and other polymers requiring critical cleaning and surface activation are shown in figure 1, and include drainage tubes, electrical conductors, interfacial seals, connector pin insulators, filters, caps and electrical cables. Following conventional liquid silicone rubber (LSR) molding and curing processes, and depending upon the type and amount of additives, cure cycle times and conditions, solidified rubber products can contain up to 10% (by weight) residual unreacted monomer fragments (called oligomers), fillers and other interstitial residues, more simply termed silicone contamination for the purposes of this article. Silicone contamination is generally non-toxic, biocompatible and non-corrosive under standard temperature and pressure conditions (STP). However, the outer surfaces of silicone rubber will continuously present and release residual silicone contamination over time, including so-called medical grade silicones. Trace amounts of silicone contamination migrate to and escape from the surface over time when exposed to non-steady state environmental conditions in accordance with molecular diffusion principles. Residual silicone contamination is not an issue in many industrial and commercial applications, and may in fact be required for the desired performance properties (i.e., antimicrobial, fire suppression). However, silicone contamination, in particular volatile or mobile forms, can be problematic in certain critical applications or systems involving biomedical, aerospace and pharmaceutical devices employed in aqueous, solvent or vacuum environments. For example, silicone oligomers can be extracted from silicone rubbers located within aqueous or solvent environments (considered a leachable contaminant) and volatilized when situated within hot and/or vacuum environments (considered an outgas contaminant).

Examples of situations or environments where silicone contamination poses a potential threat include vacuum systems, thermal systems, the space environment and the human body. Silicone contamination can be a concern when used as seals or septa in containers or vials used to store liquids. Moreover, silicone contamination can present problems during manufacturing processes such as adhesive bonding, coating, plating and welding. Silicone contamination is also a contamination concern in critical manufacturing atmospheres, for example cleanrooms used to fabricate semiconductor chips and hard disk drives.

In spacecraft applications, silicone oligomers will outgas from the pores of the bulk material under the absolute vacuum of space, migrating along internal thermal gradients created by the spacecraft rotation into and away from the sun. This can relocate silicone contamination into or onto critical optoelectronic systems such as navigational or observation devices, causing potential optical surface obscuration. In another example, the performance of heat exchange surfaces can be negatively affected if coated with thin heat-insulating layers of silicone contamination and other forms of volatile condensable matter, or VCM. In pharmaceutical applications, silicone contamination can be leached from container seals or septa by solvent-based diluents (ref. 3). In medical applications, silicone contamination can leach from a silicone rubber device, such as tubing or sleeves, and cause unwanted cellular responses or cellular adhesion problems. Moreover, subsurface silicone contamination will migrate from the interior of the bulk substrate and onto the surface regions during vacuum-based processes such as plasma treatment, interfering with the formation of clean, functionalized surfaces, or shortening the longevity of same. Finally, surfaces containing silicone contamination interfere with adhesive bonding mechanisms, preventing the formation of strong chemical and mechanical adhesive bonds.

One of the difficulties in modifying silicone surfaces relates to the mobile nature of amorphous polymer molecules. For example, during surface modification, for example oxidation, of a silicone rubber surface, molecular motions can (over a period of time) cause the modified surface to intermingle and diffuse into the polymer matrix. This tendency is most pronounced in silicone elastomers, which have very mobile polymer chains. To overcome this problem, plasma treatments may be used to crosslink and stabilize the polymeric surface. However, within hours or even minutes after plasma treatment, the surface begins to revert back to its original hydrophobic state. Uncrosslinked oligomers and low molecular weight oils begin to migrate to the polymeric surface in accordance with Fick's law of molecular diffusion (migration of polymers from interior regions of high concentration to exterior surface regions having a low concentration). These oils tend to interfere with attachment or grafting of coatings to the polymer surface in the case of plasma coating. These oils also prevent the formation of strong chemical and mechanical adhesive bonds in adhesive bonding applications.

Conventional silicone rubber treatment solutions

There are, generally, three conventional treatment methods used to clean, scour or fully-react silicone substrates, respectively, and these include solvent extraction, thermal vacuum bakeout and enhanced silicone curing processes

Solvent extraction using organic solvents or solvent blends can be a very slow process, and can alter the mechanical properties of certain silicone devices. Extraction with toxic or flammable solvents on a large scale is dangerous and consumes significant amounts of energy. Moreover, residual extraction solvent residues left in the pores of the silicone rubber can be problematic to the function or performance of the extracted device.

An alternative to solvent extraction is thermal vacuum bake-out, or TVB. The TVB process extracts oligomers from the bulk polymer using a high vacuum (i.e., less than 1 x [10.sup.-5] Torr) and heat (i.e., 125[degrees]C) over an extended period of time. TVB extraction proceeds somewhat slowly, for example, up to 80 hours or more, and usually degrades the mechanical properties of the silicone rubber due to accelerated thermal aging. In addition, TVB-treated silicone rubbers can bum or exhibit color change.

An alternative to post-treat methods is more complete vulcanizing or curing to convert most of the silicone monomer into solid polymer. These methods include peroxide (free radical) and platinum (addition) curing techniques. However, these curing processes create constraints, as well. Peroxide curing can lead to microscopic bubbles, surface darkening and a tackier surface (i.e., can pick up more dirt). Platinum curing is a cleaner, but a more expensive treatment process and is typically used only in critical applications such as medical implants, i.e., so-called medical grade silicones. Also, platinum-cured silicones exhibit higher wear rates and spallation relative to peroxide-cured silicones when used in mechanical devices such as pumps. As such, all conventional silicone rubber treatment methods (i.e., post-treatment and curing methods) offer tradeoffs in terms of processing time, life cycle costs and end-product performance.

A newer treatment technique generally unknown or not well understood by molded component manufacturers is carbon dioxide processing, or simply C[O.sub.2] processing. C[O.sub.2] processing uniquely employs one or a combination of solid, liquid, supercritical and plasma C[O.sub.2] chemistries and processes to treat silicones, as well as many other types of substrates requiring dry chemical scouring and/or surface treatment. C[O.sub.2] processing is a robust, lean and green cleaning (and surface treatment) option that can provide distinct economic and performance advantages relative to the conventional molded component treatment processes.

The C[O.sub.2] alternative

C[O.sub.2] processing technology includes the following techniques: Centrifugal C[O.sub.2] immersion-extraction cleaning; C[O.sub.2] composite spray cleaning; and C[O.sub.2] particle-plasma surface modification.

C[O.sub.2] solvent properties are similar to halogenated solvents, such as Freon 113 and HFE-7100, and hydrocarbon solvents such as n-Hexane ([C.sub.6][H.sub.12]) (figure 2). C[O.sub.2] possesses a Hildebrand solubility parameter in the adjustable range between 14 [MPa.sup.1/2] to 22 [MPa.sup.1/2], depending upon phase, temperature and pressure. C[O.sub.2] can be compressed to a range of liquid-like densities, yet it will retain the diffusivity of a gas with extremely low viscosity. Supercritical and liquid C[O.sub.2] cleaning agent densities may be adjusted between 0.5 g/[cm.sup.3] and 0.9 g/ [cm.sup.3]. Solid phase C[O.sub.2] has a density of 1.6 g/[cm.sup.3], identical to Freon 113. High density provides significant and controllable impact shear stresses of between 10 kPa (i.e., using free C[O.sub.2] particles at low velocity) and 300 MPa (i.e., using C[O.sub.2] pellets at high velocity) when projected against a non-compliant substrate surface. Surface tensions for C[O.sub.2] fluids range from 0 dynes/cm (supercritical) to 5 dynes/cm (liquid). Finally, C[O.sub.2] is easily ionized under low or atmospheric pressures to form energetic treatment plasma (fourth state of matter) containing electrons, ions, UV radiation, ozone and heat. All C[O.sub.2] treatment fluids are compatible with most common silicones and many other types of polymers (refs. 4 and 5).

Practical benefits derived from these unique and broad-based solvent treatment properties include rapid penetration and wetting, hydrocarbon solubility and energetic cleaning, cooling and dry lubrication effects, and surface modification capabilities. The following sections describe several C[O.sub.2] processing technologies which are very useful for treating silicone as well as many molded polymeric products in preparation for assembly, adhesive bonding, plating, coating and other manufacturing operations.

Centrifugal C[O.sub.2] immersion-extraction cleaning

Liquid (as well as supercritical) C[O.sub.2] is a non-toxic solvent that dissolves many types of organic films and oils, and is particularly suited to dissolving and removing unreacted silicone oils. Near-zero or zero (supercritical state) surface tension and low viscosity allows dense phase C[O.sub.2] to penetrate microscopic pores and crevices, delivering solvent cleaning power deep into the interior of silicone rubber. Using patented centrifugal C[O.sub.2] immersion-extraction cleaning processes, contamination is rapidly removed from silicone rubber under both physical (centrifugal pumping and scouring) and chemical cleaning actions. Figure 3 shows a typical immersion-extraction profile for PDMS using a centrifugal C[O.sub.2] cleaning process employing liquid C[O.sub.2] to meet ASTM E 595 outgas performance criteria; in this case, attaining a total mass loss (TML) of 0.77% and collected volatile condensable material (CVCM) of 0.01% (ref. 6) in a 40 minute extraction cycle. As shown in table 1, not only is C[O.sub.2]-extracted silicone rubber cleaner, it exhibits enhanced physical characteristics, as well. Moreover, in a study performed by the author, liquid C[O.sub.2]-extracted fluoro-silicone rubber connector insulators exhibited a 15 fold increase in column-to-column electrical resistance, increasing from 3 gigaohms to 47 gigaohms (ref. 1).

Centrifugal C[O.sub.2] cleaning processes employ either liquid or supercritical C[O.sub.2] and require no special permits, even from the tough SCAQMD or EPA, because it is non-toxic, nonflammable and non-VOC containing. Integrated C[O.sub.2] distillation recycles nearly 100% of the C[O.sub.2] extraction solvent while separating and concentrating silicone oils (and solvent modifiers, if present). Optionally, centrifugal C[O.sub.2] processes can be hybridized with other treatment processes, such as low-pressure plasma, to provide cleaned-surface modifications, bio-burden reduction, or to assist with outgassing and decomposing residual interstitial contaminants.

Any industry approved or MIL-SPEC cleaning solvent additive may be used with the centrifugal C[O.sub.2] process as a (carbonated) prewash-extraction agent or C[O.sub.2] chemistry modifier, providing numerous novel and dry immersion-extraction cleaning chemistries. This capability is very important when a certain type of cleaning-extraction chemistry is needed to mimic a particular solvent environment (polar, non-polar, ionic) in which the processed substrate will be exposed; for example, a specific pharmaceutical drug solvent carrier. Moreover, this capability allows for the continued use of "spec'd in" immersion-extraction cleaning solvent chemistries, but in a safer, more minimal and more robust cleaning process. Moreover, the centrifugal C[O.sub.2] process is available with a vacuum plasma post-treatment capability to provide both cleaning and surface activation in a single, dry treatment process.

For example, aqueous cleaners, deionized rinses and surface activation solvents, such as acetone used to prepare molded PMMA for metal plating operations, can be eliminated. Using centrifugal C[O.sub.2] immersion-extraction cleaning with in-situ vacuum, C[O.sub.2] plasma post-treatment first removes silicone mold release agent and other polymer contaminants from PMMA surfaces using liquid C[O.sub.2], and then activates the clean PMMA surface with C[O.sub.2] plasma to create an ultraclean, micro-etched and chemically activated PMMA surface for better metal plating adhesion.

Finally, C[O.sub.2] composite sprays and particle-plasma hybrid surface treatment processes serve as adjuncts or alternatives to centrifugal C[O.sub.2] processes, providing a very robust and selective surface treatment platform capability.

C[O.sub.2] particle-plasma hybrid treatment

A C[O.sub.2] composite spray utilizes a mixture of microscopic scouring solid C[O.sub.2] crystals entrained in a heated clean, dry air stream. A C[O.sub.2] composite spray provides precise control of spray cleaning energy, both mechanical and chemical. Delicate substrate surfaces, including surfaces with very thin metallic or polymeric coatings, or surfaces containing fragile mechanical features (i.e., MEMS), may be cleaned effectively using this technology.

The C[O.sub.2] composite spray cleaning process is analogous to a combination of high pressure solvent spray cleaning and ultrasonic immersion cleaning. Like liquid and supercritical C[O.sub.2], solid phase C[O.sub.2] exhibits halogenated solvent-like chemistry (during impact) which delivers powerful (but controllable) shear stress to surfaces being cleaned. The application of dilute (lean) C[O.sub.2] particles-gas spray compositions having variable velocity produce unique phase-change (solid[right arrow]liquid phase) cleaning action at the substrate surface. Solid-to-liquid phase flushing of the substrate surface washes contaminants from the most complex topography, while unique chemistry and precise process control protect delicate surface features from damage.

Shown in figure 4, the surface scouring and solvent cleaning actions of a C[O.sub.2] composite spray are used in cooperation with atmospheric plasma C[O.sub.2] (and other types of gas plasma) to form a very robust surface cleaning and preparation treatment called C[O.sub.2] particle-plasma cleaning. This process combines electron and/or photon driven surface ablation phenomena comprising an ionizing-heating plasma plume with the simultaneous surface scouring and cleaning actions provided by the C[O.sub.2] composite spray. The C[O.sub.2] composite spray is used to both precisely control surface temperature and cleanliness; the simultaneous removal of heat contamination and processing debris such as oxidation residues, gases and ablated surface particles generated by the atmospheric plasma process. Working in cooperation, atmospheric C[O.sub.2] plasma micro-cracks and chemically alters the surface, while C[O.sub.2] particles and fluids simultaneously vector surface debris and excess heat from the treated surface. Shown in figure 5 is a chart describing the significant improvement in adhesive bond strength achieved for a low surface energy polymer (e.g., LDPE) using the C[O.sub.2] particle-plasma hybrid treatment process. A 15 second particle-plasma treatment improves the acrylic bond shear strength (both cyanoacrylate and light-curing acrylic adhesives) by more than 400%.

Conclusion

Conventional pre-and post-treatment options for molded silicone rubbers pose different constraints in terms of cost of ownership, environmental compliance and material performance. The C[O.sub.2] processing alternative offers a robust platform for processing silicone rubbers for medical, aerospace and pharmaceutical applications. Silicones and other types of molded polymers such as LDPE and PMMA can be precision cleaned and treated in preparation for critical manufacturing processes such as bonding, coating, plating and precision assembly.

References

(1.) "Dense phase carbon dioxide cleaning process: Liquid and supercritical carbon dioxide as cleaning solvents, "D. Jackson, 10th Annual Aerospace Contamination Control Working Group Meeting, C.S. Draper Laboratory, Danvers, MA, May 1987.

(2.) ASTM E 595-93, Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgas-sing in a Vacuum Environment.

(3.) "The leachable challenge in polymers used for pharmaceutical applications, "Rubber World, November 2008.

(4.) "Compatibility of medical-grade polymers with dense C[O.sub.2]," A. Jimenez et al., J. Supercritical Fluids. October 1, 2007, 42 (3): 366-372.

(5.) "Evaluation of the interactions between supercritical carbon dioxide and polymeric materials, "Los Alamos National Laboratories, Report #LA- UR-94-2341.

(6.) ASTM E 595 Outgas Test Report (Silicone Cable Extraction), Pacific Testing Laboratories, Valencia, CA, (Customer/Application Confidential).

by David Jackson, CleanLogix

Table 1--properties of C[O.sub.2]-extracted silicone rubber

Silicone rubber           Before    After
ZZ-R-765
Hardness (A)                   44       48
Tensile strength (PSI)        758      834
Elongation (%)                345      411
Compression (%)               3.4     10.6
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Author:Jackson, David
Publication:Rubber World
Date:May 1, 2013
Words:3050
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