Analysis of Alternatives
ANALYSIS OF ALTERNATIVES
Legal name of applicant(s): Acton Technologies Ltd
Submitted by: Acton Technologies Ltd (jointly developed with Maflon Spa)
Substance: bis(2-methoxyethyl) ether (diglyme): EC 203-924-4: CAS 111-96-6
Use title: Use of bis(2-methoxyethyl) ether (diglyme) as a carrier solvent in the formulation and use of sodium naphthalide etchant for fluoropolymer surface modification whilst preserving article structural integrity (in-house processes). Use of bis(2-methoxyethyl) ether (diglyme) as a carrier solvent in the application of sodium naphthalide etchant for fluoropolymer surface modification whilst preserving article structural integrity (downstream user processes).
Use number: 1 and 2
ANALYSIS OF ALTERNATIVES
CONTENTS
LIST OF ABBREVIATIONS ...... 5
DECLARATION ...... 6
1. SUMMARY ...... 7
2. ANALYSIS OF SUBSTANCE FUNCTION...... 10
2.1. The requirement to modify the surface of fluoropolymers ...... 10 2.1.1 Fluoropolymers ...... 10 2.1.2 Chemistry of surface modification ...... 12 2.1.3 Solvated Electrons in surface modification and the use of sodium metal ...... 13 2.1.4 Radical anions in surface modification and the use of sodium naphthalide ...... 13 2.1.5 The measurement of the extent of surface modification...... 14 2.1.5.1 Change in colour ...... 15 2.1.5.2 Change in surface physical characteristics (surface roughness) ...... 15 2.1.5.3 Change in surface chemical composition ...... 15 2.1.5.4 Change in surface wettability behaviour ...... 16 2.1.5.5 Change in bonding strength after application of an adhesive ...... 19 2.1.5.5.1 Etching and successful surface adhesion ...... 19 2.1.5.6 Industry Standards applied for fluoropolymer etching applications ...... 20 2.1.5.6.1 ASTM Standards ...... 20 2.1.5.6.2 Specific Industry Standards ...... 20
2.2. Technical function of diglyme in the wet chemical treatment fluoropolymer surfaces ...... 22 2.2.1 Chemical functionality ...... 22 2.2.1.1 Solubility of sodium naphthalide complex ...... 22 2.2.1.2 Thermal Stability ...... 23 2.2.1.3 Flash Point ...... 23 2.2.2 Process functionality ...... 23 2.2.2.1 Process Temperature ...... 23 2.2.2.2 Process Reaction Time ...... 24 2.2.2.3 Solvent Viscosity ...... 24 2.2.2.4 Bond strength ...... 24 2.2.2.5 Polymer range ...... 25 2.2.2.6 Tribology of surface requiring etching ...... 25 2.2.2.7 Diglyme recovery and recycling ...... 25 2.2.2.8 In-house versus contract etching services ...... 25
3. ANNUAL TONNAGE...... 27
3.1. Customer Profile for Acton Technologies Ltd FSS Etchant...... 27
4. IDENTIFICATION OF POSSIBLE ALTERNATIVES ...... 28
4.1. List of possible alternatives ...... 28
4.2. Description of efforts made to identify possible alternatives ...... 30 4.2.1 Research and development ...... 30 4.2.2 Data searches ...... 30 4.2.3 Consultations ...... 30
5. SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES ...... 31
5.1. Wet Chemical Treatment Systems ...... 31 5.1.1 Sodium – Ammonia System ...... 31
Use number: 1 & 2 Acton Technologies Ltd 2
ANALYSIS OF ALTERNATIVES
5.1.1.1 Technical feasibility ...... 31 5.1.1.2 Economic feasibility ...... 33 5.1.1.3 Reduction of overall risk due to transition to the alternative ...... 33 5.1.1.4 Availability ...... 34 5.1.1.5 Conclusion on suitability and availability for sodium–ammonia etching alternative ...... 34 5.1.2 Sodium – Naphthalene – Alternative Solvents ...... 34 5.1.2.1 Substance ID and properties ...... 35 5.1.2.2 Technical feasibility ...... 38 5.1.2.2.1 Physico-chemical Properties...... 38 5.1.2.2.2 Toxicological Properties ...... 39 5.1.2.2.3 Process performance ...... 39 5.1.2.3 Economic feasibility ...... 44 5.1.2.4 Reduction of overall risk due to transition to an alternative solvent ...... 44 5.1.2.5 Availability ...... 45 5.1.2.6 Conclusion on suitability and availability for Alternative Solvent ...... 46 5.1.3 Other reductive pre-treatments involving radical anions ...... 47 5.1.3.1 Substance ID and properties ...... 47 5.1.3.2 Technical feasibility ...... 47 5.1.3.3 Economic feasibility ...... 47 5.1.3.4 Reduction of overall risk due to transition to the alternative ...... 47 5.1.3.5 Availability ...... 48 5.1.3.6 Conclusion on suitability and availability ...... 48
5.2. Electrochemical Treatments ...... 48 5.2.1.1 Substance ID and properties ...... 49 5.2.1.2 Technical feasibility ...... 49 5.2.1.3 Economic feasibility ...... 49 5.2.1.4 Reduction of overall risk due to transition to the alternative ...... 49 5.2.1.5 Availability ...... 49 5.2.1.6 Conclusion on suitability and availability for electrochemical treatments ...... 49
5.3. Plasma Treatment ...... 49 5.3.1.1 Plasma treatment description ...... 50 5.3.1.2 Technical feasibility ...... 51 5.3.1.3 Economic feasibility ...... 52 5.3.1.4 Reduction of overall risk due to transition to the alternative ...... 52 5.3.1.5 Availability ...... 52 5.3.1.6 Conclusion on suitability and availability for plasma treatment ...... 53
6. OVERALL CONCLUSIONS ON SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES FOR USE ...... 54
6.1. The use of surface modified fluoropolymers ...... 54
6.2. Overall conclusions on alternatives ...... 54
7. REFERENCES...... 56
TABLES
TABLE 2.1 FLUOROPOLYMER DEFINITION ...... 11 TABLE 2.2 EXAMPLES OF CHANGES IN PTFE SURFACE CHEMISTRY BY CHEMICAL ETCHING ...... 16 TABLE 2.3 RELATIONSHIP BETWEEN CONTACT ANGLE AND SURFACE ENERGY ...... 17 TABLE 3.1 COMPOSITION OF TYPICAL ETCHANT SOLVENT ...... 27 TABLE 4.1 PROCESS CRITERIA MATRIX...... 29 TABLE 5.1 SODIUM AND AMMONIA SUBSTANCE PROPERTIES ...... 31 TABLE 5.2 COST COMPARISON FOR OF PTFE ETCHING USING A SODIUM – AMMONIA SYSTEM ...... 33 TABLE 5.3 SOLVENT CRITERIA MATRIX – PHYSICO-CHEMICAL PROPERTIES ...... 37
Use number: 1 & 2 Acton Technologies Ltd 3
ANALYSIS OF ALTERNATIVES
TABLE 5.4 EFFECT OF SOLVENT STRUCTURE OF SODIUM NAPHTHALIDE EQUILIBRIUM REACTION ...... 40 TABLE 5.5 CORRELATION OF ETCHANT ACTIVITY WITH SOLVENT DIELECTRIC CONSTANT (MARSH, 2006B) ...... 41 TABLE 5.6 BOND STRENGTH AFTER ETCHING PTFE WITH A THF-SODIUM NAPHTHALIDE ETCHANT ...... 42 TABLE 5.7 COMPARATIVE BOND STRENGTH OF ETCHED STANDARD PTFE STRIPS (MARSH, 2006A, B) ...... 42 TABLE 5.8 BOND STRENGTH, PEEL TEST (MAFLON SPA, 2015, UNPUBLISHED LABORATORY RESULTS) ...... 42 TABLE 5.9 COMPARISON OF PTFE SURFACE MODIFICATIONS USING ALTERNATIVE SOLVENTS (MARSH, 2006B) ...... 43 TABLE 5.10 LABORATORY COMPARISON OF PTFE SURFACE MODIFICATIONS USING ALTERNATIVE SOLVENTS (MAFLON SPA, 2015, UNPUBLISHED LABORATORY RESULTS) ...... 43 TABLE 5.11 OTHER COMMERCIAL SOLVENT-BASED FLUOROPOLYMER ETCHANT SYSTEMS ...... 45 TABLE 5.12 SUBSTANCE ID AND PROPERTIES ...... 47
FIGURES
FIGURE 2.1 CHANGE IN COLOUR ON CHEMICAL ETCHING OF PTFE SHEET ...... 15 FIGURE 2.2 CONTACT ANGLE AND WETTABILITY ...... 17 FIGURE 2.3 RELATIONSHIP BETWEEN SURFACE ENERGY AND CONTACT ANGLE FOR ETCHED PTFE SURFACES ...... 18 FIGURE 5.1 COLOUR COMPARISON OF PTFE SHEET ETCHED BY SODIUM NAPHTHALIDE IN THREE DIFFERENT SOLVENTS UNDER PRODUCTION PLANT CONDITIONS ...... 44
Use number: 1 & 2 Acton Technologies Ltd 4
ANALYSIS OF ALTERNATIVES
LIST OF ABBREVIATIONS
AFM Atomic Force Microscopy APT Atmospheric pressure plasma treatment ASTM American Society for Testing and Materials CAS Chemical Abstract Service DNEL Derived No Effect Level DPG-ME Dipropylene glycol dimethyl ether EGDME Ethylene glycol dimethyl ether ETFE Ethylene-tetrafluoroethylene copolymer FEP Fluorinated ethylene-propylene copolymer FSS Fluoroetch Safety Solvent HMPA Hexamethylphosphoramide LPT Low pressure plasma treatment LUMO Lowest Unoccupied Molecular Orbital PFA Perfluoroalkoxy copolymer PTFE Poly(tetrafluoroethylene) PVDF Poly(vinylidene fluoride) SCE Standard Calomel Electrode SET Solvated Electron Transfer SPM Scanning Probe Electroscopy TAPPI Technical Association of the Pulp and Paper Industry TBAT Tetrabutylammonium tetrafluroborate TEGDME Triethylene glycol dimethyl ether THF Tetrahydrofuran XPS X-ray Photoelectron Spectroscopy
Use number: 1 & 2 Acton Technologies Ltd 5
ANALYSIS OF ALTERNATIVES
DECLARATION We, Acton Technologies Ltd, request that the information blanked out in the “public version” of the Analysis of Alternatives is not disclosed. We hereby declare that, to the best of our knowledge as of today February 11th 2016 the information is not publicly available, and in accordance with the due measures of protection that we have implemented, a member of the public should not be able to obtain access to this information without our consent or that of the third party whose commercial interests are at stake.
Signature: Date: February 11th 2016 Place: Adare, Ireland
Terence Neville President Acton Technologies Ltd
Use number: 1 & 2 Acton Technologies Ltd 6
ANALYSIS OF ALTERNATIVES
1. SUMMARY
Bis(2-methoxyethyl)ether (diglyme) is used as a solvent for sodium naphthalide to produce an etchant for the surface modification of fluoropolymers, especially perfluoropolymers such as polytetrafluoroethylene (PTFE), by reductive defluorination in order to increase the surface adhesion properties of such polymers. Diglyme provides sufficient solvation of the radical anion salt to generate the active chemical species in order to promote this reductive defluorination. Other physico-chemical parameters, such as a relatively high flash point, a viscosity similar to that of water and thermal stability during storage and solvent recovery, provide an etchant that can be handled with reduced process risk in a number of process configurations (batch and continuous) to process a range of physical forms of polymer articles (e.g. sheets, seals, tubes etc). Etchants formulated using diglyme as the solvent are used in the surface modification of fluoropolymer components in a range of industrial and medical applications, where the bonding of the fluoropolymer, and PTFE in particular, provides the final bonded manufactured product with the critical non-reactive properties of the virgin fluoropolymer surface and where the specific structural integrity of the components must otherwise be maintained.
Sodium-ammonia etchant systems are commercially available but this methodology is significantly more expensive than the diglyme solvent alternative, is more restricted in its application due to the aggressive and penetrating nature of the generated solvated electron reductant and has a different but significant risk profile in the use of liquid ammonia. As such it can only be implemented at specialist facilities and cannot be used for small scale industrial etching facilities or for components where maintenance of specific structural integrity is required.
Other alternative polar solvents that provide similar sodium napthalide solvation characteristics have been identified and some are also known to be available commercially. However, these solvents either have a similar toxicity profile to diglyme (e.g. monoglyme, triglyme) or pose signficantly greater process risk through lower flashpoint, greater volatility, reduced thermal stability at room and elevated process temperatures or generation of explosive peroxides during solvent recovery and recycling processes (e.g. monoglyme, tetrahydrofuran). Other glyme alternatives also provide additional process restrictions through increased viscosity due either to the extent of solvation of sodium naphthalide (monoglyme) or increased inherent viscosity (triglyme and tetraglyme).
Formulated etchants using either dipropylene glycol dimethyl ether or diethyl glyme have not been demonstrated to produce the same degree of fluoropolymer surface modification in either laboratory of pilot production tests which would allow commercial application of these solvent alternatives.
Other wet chemical methods, including the electrochemical generation of solvated electrons and radical anions, do not provide the same extent of consistency of surface modification, particularly for PTFE, and none have been demonstrated or implemented commercially.
Alternative treatment methods for fluoropolymer surface modification, such as plasma treatments, are available for some fluoropolymers but have not been particularly successful for PTFE. Fully fluorinated polymers are reported to undergo surface modification but this is not necessarily concomitant with required adhesion properties in the final application. In addition, the shelf life of such ‘etched’ surfaces is considerably shorter than that achieved with the solvent etchants and that, combined with the requirement for expensive equipment in a number of configurations to cover the
Use number: 1 & 2 Acton Technologies Ltd 7
ANALYSIS OF ALTERNATIVES range of fluoropolymer articles requiring etching and the resistance of the downstream user market for products etched by this technique, have resulted in limited applications of such techniques.
In summary, diglyme is the preferred solvent for formulation and use of a sodium naphthalide etchant for perfluoropolymer surfaces as it provides the optimum balance of adequate solvation to maximise the availability of the radical anion reductant, either at or to a limited depth of the perfluoropolymer surface, with process characteristics of relatively low flashpoint, low viscosity and thermal stability, that permits the economic operation of the etching process at elevated temperatures of up to 65⁰C whilst minimising overall process risk.
Alternative solvents or alternative etching technologies do not provide the flexibility of an etchant of sodium naphthalide formulated in diglyme to produce a consistent surface modification of sufficient enhanced wettability, increased surface energy and increased final adhesive bonding strength across the range of critical perfluoropolymer bonding applications that require mandatory attainment of and qualification to end user specification.
Acton Technologies Ltd formulate on diglyme-based fluoropolymer etchant (Fluoroetch Safety Solvent) for downstream customer applications as well as for use in their own contract etching operation for the surface treatment of fluoropolymer articles. . This Analysis of Alternatives has been developed in conjunction with Maflon Spa.
As the formulated etchant is supplied to downstream users, whose use whilst identical technically is considered as a separate use in this Authorisation application, this Analysis of Alternatives is also submitted as the AoA for these downstream user process application.
It should be noted that the terms for the sodium – naphthalene system are referred to as both ‘sodium – naphthalene’ and ‘sodium naphthalide’ and are used interchangeably. The term ‘sodium naphthalide’ is used primarily in this analysis of alternatives as organometallic salt is manufactured and imported by Acton Technologies Ltd.
Justification for Review Period
In conjunction with another authorisation applicant for the same use (Maflon Spa) Acton Technologies Ltd has undertaken an extensive review of both alternative methodologies for perfluoropolymer surface modification and of alternative solvents for the formulation of etching solutions that can deliver the equivalent quality and consistency of bonding characteristics and overall operability. The Chemical Safety Report, submitted as part of this Application for Authorisation, demonstrates that the use of diglyme is adequately controlled for Acton’s own in- house use and for downstream user processes that have been described in detail, with risk characterisation ratios less than 1 for all worker contributing and man via the environment scenarios.
However, Acton Technologies Ltd acknowledges that a) for in-house use adequately control is largely attained through the use of personal protective equipment and b) not all current downstream user applications have been fully documented. Acton Technologies Ltd have identified internal engineering modifications that will be implemented to remove the reliance on PPE and have described measures that will be undertaken prior to the sunset date to ensure that all downstream
Use number: 1 & 2 Acton Technologies Ltd 8
ANALYSIS OF ALTERNATIVES users with non-exempted uses of the formulated etchant product are able to declare and demonstrate that their use is adequately controlled.
Acton Technologies Ltd is therefore requesting a review period of 12 years for the use of bis(2- methoxyethyl) ether (diglyme) as a carrier solvent in the formulation and use of sodium naphthalide etchant for fluoropolymer surface modification whilst preserving article structural integrity (in- house processes) and 12 years for the for the use of bis(2-methoxyethyl) ether (diglyme) as a carrier solvent in the formulation and use of sodium naphthalide etchant for fluoropolymer surface modification whilst preserving article structural integrity (downstream processes)
Use number: 1 & 2 Acton Technologies Ltd 9
ANALYSIS OF ALTERNATIVES
2. ANALYSIS OF SUBSTANCE FUNCTION
2.1. The requirement to modify the surface of fluoropolymers Fluoropolymers were discovered in the late 1930’s with the development of polytetrafluroethylene (PTFE) by Dupont. They are a group of polymers that possess excellent chemical ultra-violet radiation resistance, high temperature resistance, good insulating properties, stability to weathering, low surface energy, low coefficients of friction and low dielectric constants – all properties which arise from the stability of the carbon-fluorine covalent bonding and the unique intra and intermolecular interactions within the polymer matrix (Teng, 2012). Because of the unique physico-chemical properties of this polymer group, they are widely used throughout industry in chemical, electrical and electronic, space, defence, construction, automotive and medical applications. However, the very low surface tension of fluoropolymers results in almost negligible adhesion of other polar materials to the polymer surface. The further use of fluoropolymers in many engineering and technological applications therefore requires some form of surface treatment or modification to enhance surface adhesion. The function of diglyme is as a solvent with an optimum combination of physico-chemical properties in the formulation and use of a chemical reagent that is sufficiently active to achieve such surface modifications in this group of inert polymers.
2.1.1 Fluoropolymers There are two types of fluoropolymers: 1. Perfluoropolymers, in which all the hydrogen atoms in the analogous hydrocarbon polymer structure are replaced by fluorine atoms. Typical examples are: • Poly(tetrafluoroethylene) (PTFE) • Fluorinated ethylene-propylene copolymer (FEP) 2. Partially fluorinated polymers, in which only a proportion of the hydrogen atoms are substituted with fluorine atoms. Typical examples are: • Poly(vinylidene fluoride) (PVDF) • Perfluoroalkoxy copolymer (PFA) Table 2.1 summarises some of the relevant key properties of these fluoropolymers.
Use number: 1 & 2 Acton Technologies Ltd 10
ANALYSIS OF ALTERNATIVES Table 2.1 Fluoropolymer Definition
Polymer CAS Type Fluorine Content Structure Elemental Composition Surface (% w/w, XPS) Energy mJ/m2 Carbon Fluorine Oxygen
Polytetrafluoroethylene 9002-84-0 Fully fluorinated 4H replaced by F 33.3 66.8 0 19.1 PTFE
Polyvinylidene fluoride Partially 24937-79-9 2H replaced by F 51 49 0 23.9 PVDF fluorinated
Polyvinyl fluoride Partially 24981-14-4 1H replaced by F 70.4 28.8 0.8 30.3 PVF fluorinated
Use number: 1 & 2 Acton Technologies Ltd 11
ANALYSIS OF ALTERNATIVES
It is extremely difficult to achieve any adhesion to fully fluorinated polymers, and even partially fluorinated polymers can prove problematical in certain circumstances. There is, therefore, a requirement to modify the surface of fluoropolymers if there is a technical application requirement to achieve adhesion to the polymer surface. The surface pre-treatment process by which such surface modification is achieved depends upon the particular fluoropolymer and sections 2.2, 4 and 5 of this analysis discusses the potential range of alternative surface modification methodologies that have been examined over the past 70 years. However, the primary requirement is to achieve the surface modification of perfluoropolymers. This has required the development of very reactive wet chemical treatment systems in which diglyme has become the primary solvent of choice. This analysis of alternatives therefore focusses upon the surface modification of perfluoropolymers but it should be noted that partially fluorinated polymers are also treated using wet chemical etchants formulated in diglyme. This is further discussed in section 2.2.2.5.
2.1.2 Chemistry of surface modification The primary method for the modification of the surface of perfluoropolymers is by treatment with powerful reducing agents. A possible reaction mechanism for this chemical transformation has been suggested by Brewis and Dahm (2006) as follows:
• Step 1 is postulated to involve electron transfer from the electron source to the fluoropolymer. Elimination of fluoride generates a neutral radical in the polymer. • This neutral radical can then react further to produce new carbon-carbon bonds, resulting in either cross-linking (step 2) or
Use number: 1 & 2 Acton Technologies Ltd 12
ANALYSIS OF ALTERNATIVES
• Acceptance of a further electron by the radical to form a carbanion which either reacts with a protic solvent (HS) to yield carbon-hydrogen bonds or results in the further elimination of fluorine with the formation of carbon-carbon double bonds (step 3).
2.1.3 Solvated Electrons in surface modification and the use of sodium metal Powerful reducing agents are therefore required to promote these reactions on the surface of perfluorinated polymers. One of the most powerful reducing agents that has been used in such surface treatment has been generated through the dissolution of sodium metal in liquid ammonia.
+ (4) Na + NH3 → Na + en(NH3) There are a number of significant drawbacks with the use of this chemical combination (further discussed in section 5.1.1), including the extreme reactivity of sodium, the toxicity of liquid ammonia and the extremely low temperatures at which the working reductant has to be maintained. Therefore, safer methods for handling the very reactive sodium species have been developed.
2.1.4 Radical anions in surface modification and the use of sodium naphthalide Dissolution of sodium metal and naphthalene in certain solvents leads to the generation of another powerful reducing species, the radical anion salt, in which the electron is taken up by the naphthalene ring and can be transferred from there to the electron deficient carbon backbone.
A number of different solvents have been used for the dissolution of the reactant species and generation of the radical anion salt and the properties, advantages and disadvantages of a variety of different solvents, in comparison to diglyme, are examined in detail in sections 2.2 and 5. The formation of radical anions occurs in a single electron transfer process (SET). The electron transferring species are most often the alkali metals (Li, Na, K, Rb, Cs) or the reduction at the cathode of an electrochemical cell. Initially, the electron occupies a LUMO of its acceptor molecule, which subsequently may undergo several types of reactions, such as cleavage, solvation, condensation or protonation. In aromatic systems, some of the resulting radical anions are stable under aprotic conditions. In particular, condensed aromatics systems, like naphthalene, anthracene and perylene are well known for the formation of stable radical anions, which can even be isolated in the form of their alkali metal salts. Radical anions have very powerful reducing properties, the E0 being almost that of the alkali metal itself. For this reason, sodium naphthalide has found considerable interest in organic chemistry as a powerful reducing agent, being able to even reduce the most robust chemical electron receptors under relatively mild conditions. These reducing systems are, almost without exception, a solution of sodium naphthalide in an aprotic solvent.
Use number: 1 & 2 Acton Technologies Ltd 13
ANALYSIS OF ALTERNATIVES
In general, two equilibria have to be considered in the reaction of alkali metals and aromatic hydrocarbons: 1. M + Ar = Ar.-/M+ reduction of the aromatic hydrocarbon 2. (Ar.-/M+)ion-pair = Ar.- + M+ ion-pair dissociation Due to the ability of glycol ethers to chelate the cation, the yield of the reduction of aromatic hydrocarbons (reaction 1) is highest when ethylene glycol ethers are used as solvent. Being an ionic species, the ionic state in solution of the ion-pair of the alkali cation and radical anion strongly depends on the interaction between the two ions (reaction 2). Typically, low polarity aprotic solvents like THF or dioxane favour the formation of tight contact ion pairs whilst in high polarity solvents, like HMPA, the ion pairs are loosely bound. Furthermore, classical factors, such as ion size, polarizability and temperature, have a strong influence on the ionic state, typically the close-contact ion pairs being observed with smaller cations, and small condensed aromatic systems. The dissociation of the ion pair to the loose ions is exothermic. Hence, an increase in temperature, in general, will lead to stronger ion pairing in solution. The reducing properties of aromatic radical ions are governed by the half-wave potential E0 of the respective aromatic system and the electron transfer rate from the radical anion – cation pair to the alkyl halide. As a rule of thumb, the more negative the half-wave potential and the tighter the ion-pair, the higher the electron transfer rate and reducing power will be (although this is not always true in all systems). Sodium naphthalide in this respect forms a unique system with a strong negative half wave potential of approximately -2.5 V vs SCE and the ion pairs being tight in low polarity solvents and exhibits a strong reducing power even to chemically inert C-F bond. Section 2.2 defines the technical function of diglyme as the preferential solvent in the formulation and use as a surface modification agent (etchant) for fluoropolymers. Section 2.2.1.1 discusses further the influence of the solvent on the solvation of the sodium naphthalide radical anion salt.
2.1.5 The measurement of the extent of surface modification A fluoropolymer surface that has been subject to a reducing agent and therefore undergone, to some degree, modification of the surface chemistry will demonstrate the following changes: 1. Change in colour 2. Change in surface physical characteristics (surface roughness) 3. Change in surface chemical composition 4. Change of surface wettability behaviour 5. Change in bonding strength after application of an adhesive The extent to which the treatment penetrates the polymer surface varies and the depth and rate of the defluorination process depends upon the nature and conditions of the solvated radical or radical
Use number: 1 & 2 Acton Technologies Ltd 14
ANALYSIS OF ALTERNATIVES anion salt chemistry employed. Etching can occur at depths of up to 6µm into the polymer surface (Marsh, 2006b).
2.1.5.1 Change in colour Surface etching can lead to polymer discolouration ranging from off-white to yellow to brown to reduction to a black carbonaceous layer. Colour changes are often used as the most useful indication of the extent and regularity of the surface etching process. Figure 2.1 Change in colour on chemical etching of PTFE sheet
Unetched Etched
The extent of colour change is often used in quality control procedures and it is a rapid and convenient method of the assessing the potential extent of surface modification. This method can be standardised, for example, by using ASTM D1535, Standard Practice for Specifying Colour by the Munsell System using Munsell Matte colour standards.
2.1.5.2 Change in surface physical characteristics (surface roughness) Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. Using this technique it is possible to measure the roughness of a surface and to quantify differences in surface roughness achieved by different etching techniques. Surface roughness at this molecular level is reported to correlate with the changes in surface chemistry, wettability and, to a certain extent, surface adhesion.
2.1.5.3 Change in surface chemical composition The reduction of the fluoropolymer surface leads to the reduction in fluorine content and carbon: fluorine ratios with a corresponding increase in hydrogen and oxygen content, depending on the nature of the reduction and post-reduction steps employed. These changes in surface chemistry can be quantified by X-ray photoelectron spectroscopy (XPS), in which the solid surface is bombarded with X-rays of known energy and the characteristics of the ejected photoelectrons determined. XPS can be used to study such changes at depths up to 10 nm. Untreated PTFE has a carbon and
Use number: 1 & 2 Acton Technologies Ltd 15
ANALYSIS OF ALTERNATIVES fluorine composition of approximately 40 and 60% respectively. Treated PTFE surfaces can change significantly to >80% carbon, <1% fluorine and 15% oxygen, for example. Table 2.2 Examples of changes in PTFE surface chemistry by chemical etching
Polymer Treatment Colour Surface Composition (% by XPS)
Carbon Fluorine Oxygen
PTFE None White 38.4 61.6 0
THF – Na – Naphthalene PTFE Brown 87.6 0.8 11.6 (10 seconds) Diglyme – Na – PTFE Naphthalene Brown 80.7 1.7 16.3 (30 seconds)
The changes in surface chemical composition have been reported in published papers describing different etchant methodologies (see, for example, Brewis and Dahm, 2005), using XPS as a research tool. It is reported that some companies set standards for surface etching on the basis of changes of surface chemical composition (C:F ratio) but this is not routinely practiced in the industry.
2.1.5.4 Change in surface wettability behaviour One of the key requisites for adequate adhesion is a good level of contact between the mobile adhesive phase and the polymer surface. As a general rule, acceptable bonding adhesion is achieved when the surface energy of a substrate (measured in dynes/cm) is approximately 10 dynes/cm greater than the surface tension of the liquid. In this situation, the liquid is said to “wet out” or adhere to the surface. Surface tension, which is a measurement of surface energy, is the property (due to molecular forces) by which all liquids, through contraction of the liquid surface, tend to bring the contained volume into a shape with the least surface area. The higher the surface energy of the solid substrate relative to the surface tension of a liquid, the better will be its “wettability”, and the smaller the contact angle.
Use number: 1 & 2 Acton Technologies Ltd 16
ANALYSIS OF ALTERNATIVES
Figure 2.2 Contact Angle and Wettability
Many plastics are hydrophobic and are not naturally “wettable”. Pretreatment of the surface to increase adhesion can therefore also be monitored by measurement of the change in surface energy of the solid substrate upon treatment of by measurement of the liquid contact angle. For untreated PTFE surface, the surface energy ranges from 18-22 mJ/m2 (dynes/cm) and the water contact angle with PTFE is approximately 110⁰. The relationship between the surface contact angle and the surface energy of the etched PTFE is described by the following formula: 1 72 0.025 where Es in the surface energy (in dynes per cm) and θ is the contact angle (in degrees). Table 2.3 Relationship between contact angle and surface energy
Material Contact Angle (⁰) Surface energy (dynes/cm) Degree of Etching
PTFE, native 109.2 19.4 Not etched
PTFE, etched 20 – 55 55 – 70 Excellent
PTFE, etched 55 - 60 52 - 55 Good
PTFE, etched 60 - 90 32 – 52 Fair
PTFE, etched >90 <32 Poor
Use number: 1 & 2 Acton Technologies Ltd 17
ANALYSIS OF ALTERNATIVES
Figure 2.3 Relationship between surface energy and contact angle for etched PTFE surfaces
ASTM D7334-08 (2013) specifies the Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement. A semi-quantitative test for assessing the surface energy of plastic films is the ‘Dyne Pen Test’ in which a series of mixtures (of formamide and 2-ethoxyethanol) of incrementally increasing surface tension are applied to a treated surface until a mixture is identified that just wet the surface (no visible contraction of the applied drawn line). The critical surface tension of the surface is estimated from the surface tension of that particular limit mixture. This method is described in the following standards: • ASTM D2578-09: Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films • TAPPI T698: Determination of Wetting Tension of Polyethylene and Polypropylene Films and Coatings (Modified Visking Analytical Technique) • ISO 8296 (2003): Plastics – Film and sheeting – Determination of wetting tension However, it should be noted that contact angle is not the only, or necessarily accurate, indicator of whether the etching process has contributed to the success or failure of the final end user application. This is discussed in more detail below.
Use number: 1 & 2 Acton Technologies Ltd 18
ANALYSIS OF ALTERNATIVES
2.1.5.5 Change in bonding strength after application of an adhesive The final test on the extent of surface modification is the degree to which the adhesion or bondability characteristics have been improved through the specific surface modification treatment. Changes in the adhesion properties of modified polymer surfaces are normally characterised by a prescribed bonding methodology in which the following parameters are controlled: • Surface area to be bonded: e.g. 20 mm wide x 10 mm long • Type of adhesive used: two-part epoxide adhesive; cyanoacrylate adhesive • Type of load test: failure load (Newtons); peel strength (kg/m); joint strength (mPa) Untreated PTFE surfaces (bonded area 20 mm x 10 mm, two-part epoxide adhesive) have failure loads of approximately 420 N (joint strength 2.1 mPa). Bond strength increases by an order of magnitude (to 4260 N, 21.3 mPa) on surface treatment. It has been demonstrated that the evaluation of bond strength for the purposes of comparison of different etching regimes is extremely dependent of the adhesive system used (Marsh, 2006a). Data is presented in section 5.1.2.2.3 to illustrate this. 2.1.5.5.1 Etching and successful surface adhesion This analysis has detailed various methods of monitoring the efficiency and extent of fluoropolymer surface modification as indicators of the subsequent ability of the modified surface to adhere and function adequately in the final end user application. However, there are a number of contributing factors to the success of the final application and the etchant manufacturer/service provider does not necessarily have visibility of the technical reasons for the perceived success or failure of a particular etchant technique for surface modification. There are a number of additional application specific factors that, in addition to the chemical and physical surface modification, contribute to the success of the final bonding application in the manufacture of the perfluoroplymer bonder application. For example, the surface modification parameters (contact angle, wettability, surface roughness) may indicate a particular extent of surface modification but the success of the final bonding of the polymer will also depend on: • Choice of adhesive system employed • End user requirement in terms of the strength of bonding for the specific application • Type of end application testing for adhesive strength (destructive, non-destructive or end point not related to adhesion characteristics). In the automotive sector, for example, bonded perfluoro polymers gaskets on seals and flanges are tested in situ via dynamic integrity testing. The PTFE seal, bonded to a metal substrate, is placed in a rotating jig and the force required to peel off the seal is measured. The company providing either the etchant or etched perfluoro polymer is not necessarily party to any subsequent failure analysis and whether the failure is attributable to a failure in the etching process itself. The surface modification indicators are therefore used primarily for quality control testing of the etching process by the applicability of the etched surface in the final bonding use is application specific.
Use number: 1 & 2 Acton Technologies Ltd 19
ANALYSIS OF ALTERNATIVES
2.1.5.6 Industry Standards applied for fluoropolymer etching applications A number of industry standards are applied in the assessment of the quality and extent of fluoropolymer etching: 2.1.5.6.1 ASTM Standards • ASTM D897-08 (2010): Standard test method for tensile properties of adhesive bond. • ASTM D903-98 (2010): Standard test method for peel or stripping strength of adhesive bonds. • ASTM D1002-10 (2010): Standard test method for apparent shear strength of single-lap- joint adhesively bonded metal specimens by tension loading (metal to metal). • ASTM D2578-09: Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films
• ASTM D5946-09 (2010): Standard test method for corona-treated polymer films using water contact angle measurements. • ASTM D7334-08 (2013): Standard practice for surface wettability of coatings, substrates and pigments by advancing contact angle measurement. 2.1.5.6.2 Specific Industry Standards • Aerospace o SAE Aerospace Material Specification (AMS2491, 2015): Surface Treatment of Polytetrafluroethylene (PTFE). Preparation for Bonding. ° Specifies the engineering requirements for preparing surfaces of PTFE for bonding and properties resulting from that treatment. ° Specifies the use of a solution of sodium or other alkali metal in anhydrous liquid ammonia, THF-naphthalene or naphthalene in other suitable solvents. ° Extent of etching judged on V Extent and uniformity of surface colour change V Tensile strength according to ASTM D897 V Shear Strength according to ASTM D1002 o SAE Aerospace Recommended Practice ARP6167 (2013): Etching of fluoropolymer insulations. o Hamilton Standard HS1801 (1999): Bonding, Fluoroplastics for Surface Preparation of, Specification o Boeing Process Specification P.S. 18165 (2002): Chemical Etching to provide a bondable surface. ° Specifies the use of commercial etchant solutions (such as TetraEtch or FluoroEtch Safety Solvent: see Table 5.8).
Use number: 1 & 2 Acton Technologies Ltd 20
ANALYSIS OF ALTERNATIVES
° Extent of etching judged via a surrogate wettability test (ink beading test). • Automotive o End application testing as described in section 2.1.5.5.1. • Other sectors of use, such as the medical, electronics and wire and cable sector: these sec tors have the own internal specifications, which will be similar to those described above.
Use number: 1 & 2 Acton Technologies Ltd 21
ANALYSIS OF ALTERNATIVES
2.2. Technical function of diglyme in the wet chemical treatment fluoropolymer surfaces Diglyme is used as the solvent of preference for the dissolution of the reaction products of sodium metal and naphthalene (commonly referred to a sodium naphthalide: CAS 3481-12-7; EC 222-460- 3) for the generation of the radical anion salt for the removal of fluorine from perfluoropolymers surfaces. Diglyme is the preferred solvent of choice both for its inherent chemical characteristics and for the subsequent process advantages that it provides for the etching process. These characteristics will be compared with other potential alternative solvents in section 4. These properties are summarised in Tables 4.2 and 4.3.
2.2.1 Chemical functionality
2.2.1.1 Solubility of sodium naphthalide complex Section 2.1.4 described the formation of the sodium naphthalide radical anion salt as follows:
The transfer of a single electron from the sodium metal atom to the naphthalene molecule generates a new aromatic species containing an odd number of electrons and thus possesses a radical nature, the minus sign indicating the presence of a negative charge. The added electron occupies the lowest unoccupied π orbital of the parent hydrocarbon and is delocalised throughout the naphthalene molecule, producing a univalent radical anion. This reaction is a reversible equilibrium and the value of the equilibrium constant depends on the solvent and the temperature. This also determines the nature of cation-anion pairing in the solvent which can be either as • free ions • loose ion pairs • contact (tight) ion pairs or • larger aggregates. Studies in the late 1960s (Shatenshtein et al, 1967 reviewed in Holy, 1974) showed that the highest yield of radical ions was obtained for ethylene glycol ethers because of the ability of these solvents to form comparatively stable five-membered chelate rings. In highly polar solvents free ions are preponderant, in solvents such as dimethyl ether ion pairs are most typical whereas in solvents of low polarity and coordinating ability, such as THF and dioxane, contact ion pairs are the most likely pairing. The mechanism, extent and rate of solvation of the ion species is therefore dependent on the solvent used and will determine the availability of the radical anion for participation in the reductive defluorination of the fluoropolymer surface. The magnitude of the solvation of the metal ion and the naphthalene anion is the principle factor in determining the equilibrium constant for the
Use number: 1 & 2 Acton Technologies Ltd 22
ANALYSIS OF ALTERNATIVES reaction. Rates constants for electron transfer between the naphthalene molecule and its anion range between 106 and 108 liter.mol-1.sec-1 (Stinnett, 1971: Vora, 1972), depending on the solvent. Diglyme is a low molecular ethylene glycol ether and produces a high yield of the sodium metal / naphthalene reaction system. This allows high concentrations of sodium naphthalide, generating the strongly reducing solution suitable as an etching agent for fluorinated polymers like PTFE on a commercial scale. It has been reported (Marsh, 2006b) that there is a relationship between the solvent dielectric constant and the reactivity of the sodium naphthalide complex. Solvents with a dielectric constant of greater that 5.5 are required to achieve the necessary extent of dissolution of the sodium naphthalide (see section 5.1.2.2.3 Process Performance: Availability of the radical anion).
2.2.1.2 Thermal Stability Diglyme is relatively stable over the temperature range at which the etching operation is carried out (20 – 65⁰C). It is stated that thermal decomposition to methyl vinyl ether (CAS 107-25-5; EC 203- 475-4) is of the order of 10% of that associated with monoglyme (CAS 110-71-4; EC EC 203-794- 9) under similar circumstances (Ebnesajjad, 2014). Methyl vinyl ether is an extremely flammable gas (H220, H280, H412). A sodium naphthalide etchant formulated from diglyme has a recommended shelf life of 6 months, but can be stored for up to one year at room temperature without undergoing significant degradation (oral communication; Acton Technologies Ltd and Maflon Spa).
2.2.1.3 Flash Point
Diglyme is a highly flammable liquid (H225) but with a flashpoint (51⁰C) this risk is reduced in comparison with other alternative solvents. The minimisation of flammability hazard during the use of the solvent in etchant formulation and subsequent application is a significant factor in the selection of the solvent.
2.2.2 Process functionality
2.2.2.1 Process Temperature One of the advances in the development of an etchant solvent system that provides an optimum balance of minimising chemical hazard from the components of the etchant system with operational requirements, which provide an economic treatment process, is the rate of the defluorination reaction. As stated above, the selection of diglyme reduces the overall flammability risk, but also allows operation of the etchant process at elevated temperatures (up to 65⁰C), which significantly enhances the rate of the surface defluorination reactions. More radical anion is made available in the diglyme-based etchant at higher temperature (displacement of the equilibrium for radical anion formation, Holy 1973), resulting in reduced etchant times and increased final bond strengths of between 50 and 75% of that obtained at room temperature (Ebnesajjad, 2014). Operation at elevated temperature also reduces solvent viscosity (see section 2.2.2.3).
Use number: 1 & 2 Acton Technologies Ltd 23
ANALYSIS OF ALTERNATIVES
2.2.2.2 Process Reaction Time Etching times are important in the commercial application of etching agents and etchants based on diglyme provide the standard of etching required, as demonstrated by bonding strength (see section 2.2.2.4) after treatment of the fluoropolymer surface for periods ranging from 30 seconds to 5 minutes, depending on the nature process (batch/continuous etc) and the complexity of the surface to be treated.
2.2.2.3 Solvent Viscosity The viscosity of the etchant, which is determined by the viscosity of the solvent and the extent of dissolution of the sodium naphthalide in the solvent, is an important factor in the controlled etching of confined areas, such as areas of small diameter and high aspect ratio (e.g holes in printed circuit boards, woven PTFE fabrics etc). Uniform etching is promoted by low viscosity etchants, which are able to flow more easily over the surfaces to be etched. Diglyme has a viscosity similar to that of water which is further reduced at operational temperatures of 50-65⁰C to ensure a much more consistent and uniform etched polymer surface, irrespective of the article tribology. The viscosity of the etchant solution is also important in determining the extent of decontamination of the etched product in the subsequent washing steps. Higher viscosity etchants may result in the contamination of etched surfaces with residual deposits of naphthalene and fluoride residues, which will then require more aggressive cleaning regimes. Control of etchant viscosity in the etchant bath at increased operational temperatures is therefore an important operational parameter in order to balance potential evaporative loss of the solvent against potential changes in solution viscosity. The use of diglyme as a solvent simplifies the management of the etching solution and subsequent downstream cleaning of etched fluoropolymer surfaces
2.2.2.4 Bond strength As discussed above, the requirement of an etchant is to increase the bond strength from 420 N (joint strength 2.1 mPA) for the untreated perfluoropolymer surface to up to 4260 N (21.3 mPa) for a treated surface. Although the specific solvent used in the formulation of the wet chemical etching methods based on sodium naphthalide may not be the sole influence on the extent, reproducibility and consistency of the increase in the bonding strength of the etched surface (see 2.1.5.5.1), the specific properties of the solvent allow these technical process specifications to be achieved using a methodology that: • Minimises the hazards of a system composed of very hazardous components • Allows the use of small scale etching systems, in either batch or continuous process mode, whilst implementing economic risk management measures that ensure adequate process control • Provides an etchant system that is portable and can be implemented with relative ease without requiring a significant capital outlay for specialised technical equipment • Provides access to the most powerful and reproducible etchant system available.
Use number: 1 & 2 Acton Technologies Ltd 24
ANALYSIS OF ALTERNATIVES
2.2.2.5 Polymer range Whilst this analysis of alternatives has focussed on the use of the diglyme formulated etchant solution in the surface modification of perfluoropolymers, and specifically PTFE, it should be noted that the etchant is also used in a number of applications requiring the surface modification of other fluorinated polymers such as FEP, ETFE or PFA. Whilst other types of etchant processes or formulations may provide adequate surface modification for such polymers, the qualification of the use of specific etchants and etchant techniques are subject to the same validation and verification requirements that are discussed later in relation to potential alternatives in section 4.
2.2.2.6 Tribology of surface requiring etching The nature and physical dimensions of the surface to be etched also dictates the strength and aggressiveness of the etching method for surface defluorination. Diglyme based etchants are the etchant of choice in the surface preparation of PTFE skived tapes and sheets of thicknesses down to as low as 0.025 mm (0.25 µm) without prejudicing the integrity and performance of the PTFE performance in subsequent applications (such as lamination of PCB circuit boards). In contrast, for example, ammonia-based etchants are too aggressive and are limited to skived sheets thicknesses of not less than 0.25 mm
2.2.2.7 Diglyme recovery and recycling Diglyme can be easily and economically recovered from spent etchant solution and recycled into the formulation of further etchant, thus reducing the overall annual consumption of the solvent by the applicants. Diglyme is recovered from spent etchant generated by in-house production by both applicants. Acton Technologies Ltd also offers a recovery and recycling service for spent etchant returned from its customer base where the annual turnover of solvent used by the customer makes this economic to do so. In the recent past, this service has been offered to of Acton Technologies Ltd who purchase greater that % of the primary etchant formulated and sold to downstream users. The recovery and recycling of diglyme from spent waste etchant reduces the annual consumption of diglyme by between 55 and 70%. From the mass balance estimates reported in the CSR Acton Technologies Ltd recycled 68% of diglyme used in 2013 and 57% in 2014. Approximately was recycled from customer returned spent etchant. Diglyme is recovered by vacuum distillation at 90⁰C, is thermally stable under these conditions and does not undergo any significant extent of degradation.
2.2.2.8 In-house versus contract etching services Wet chemical etchant technology can either be used in-house or contracted out to contract service organisation with the requisite experience in the etching of fluoropolymers. Organisations requiring the surface modification of fluoropolymers for incorporation into their own products may lack the time, space or expertise for in-house etching and outsource the etching process to third parties that provide a complete line of batch and continuous contract etching services. The use of such third party services offering proprietary techniques optimises the sodium etching process for custom applications, for both small and large production, through the development of application-specific surface modification techniques (including secondary treatments) to provide
Use number: 1 & 2 Acton Technologies Ltd 25
ANALYSIS OF ALTERNATIVES tailored, specific adhesion for each bonding situation. Such customised etching services offer the end customer an economic way of generating a bondable surface with fast turnaround to ISO 9001/14001 certified reliability. The use of diglyme as a solvent provides a greater degree of flexibility in the tailoring of application specific surface modification of fluoropolymers.
Use number: 1 & 2 Acton Technologies Ltd 26
ANALYSIS OF ALTERNATIVES
3. ANNUAL TONNAGE Acton Technologies Ltd is a downstream user of the solvent diglyme at a low annual consumption of metric tonnes per year, depending on the demand for FluoroEtch Safety Solvent (FSS) into which the diglyme is formulated. Acton Technologies Ltd sourced diglyme in 2010 from Ubichem and their upstream supplier Clariant. Since that date, Acton Technologies Ltd have sourced diglyme from Univar who are understood to be supplied by BASF. All purchased diglyme is formulated into the FluoroEtch Safety Solvent which is used both for internal custom etching applications and also sold to downstream users within and outside the EU.
Table 3.1 Composition of Typical Etchant Solvent
Substance CAS Number EC Number % (w/w)
Diglyme 111-96-6 203-924-4 80 – 90
Sodium naphthalide 3481-12-7 222-460-0 10 – 20
3.1. Customer Profile for Acton Technologies Ltd FSS Etchant.
Acton Technologies Ltd defines its downstream users as follows:
• Downstream users that use the FSS Etchant commercial applications: these are defined as those customers who place on an annual basis repeat orders for the FSS etchant and purchase the product in total annual quantities of greater than 20 litres per annum.
• Downstream Users that use the FSS Etchant for research and development purposes: these are defined as those purchase the FSS Etchant in quantities less than 20 litres per annum. Research and Development uses of the FSS Solvent are outside the scope of the REACH Authorisation Application.
Acton Technologies Ltd currently have with commercial applications for the FSS Etchant and these are described in this application as a use distinct from Acton Technologies Ltd own internal use of the product and is described in a separate Chemical Safety Report, which also details how this use will be controlled on successful completion of the Authorisation.
Use number: 1 & 2 Acton Technologies Ltd 27
ANALYSIS OF ALTERNATIVES
4. IDENTIFICATION OF POSSIBLE ALTERNATIVES The surface treatment of plastics and polymers has been the subject of intense research and study over many years. The low surface energy of polymer surfaces results in intrinsically low adhesion without some form of surface treatment. These surface treatments can be broadly classified as follows: • Cleaning • Mechanical treatments • Wet Chemical Treatments • Plasma Treatments, including flame and corona • Photochemical Treatments and Laser treatment Surface changes occur as a result of four distinct processes (cleaning, ablation, cross-linking and surface chemical modification), all of which serve to increase the surface energy of the polymer surface through surface oxidation of the polymer chains. In the case of fluoropolymers, this requires significant defluorination of the polymer surface. For the purpose of this analysis of alternative methodologies for the surface modification of fluoropolymers surface cleaning, for the removal of contaminants, and mechanical abrasion, to roughen the surface and increase contact area for subsequent adhesion, will not be further addressed as they play little part in the chemical modification of fluoropolymeric surfaces.
4.1. List of possible alternatives The following alternative methodologies, which have been identified from both industry knowledge and experience and from a review of publicly available literature sources, are examined in further detail in this document:
• Wet Chemical Treatment
o Generation of solvated ions and radical anions by chemical means ° Sodium – ammonia systems ° Sodium – naphthalene systems in solvents other than diglyme ° Other chemical systems
o Generation of solvated ions and radical anions by electrochemical means ° Indirect ° Direct ° Metal amalgams
• Plasma Treatments
o Plasma o Corona o Flame
Table 4.1 provide a comparative summary of these alternative techniques.
Use number: 1 & 2 Acton Technologies Ltd 28
ANALYSIS OF ALTERNATIVES
Table 4.1 Process Criteria Matrix
Suitability for Polymer Treatment Treatment Suitable for Commercially Process Adhesion Wettability Scale Hazards Type time Temperature complicated shapes available Fully Partially
fluorinated fluorinated Wet Chemical Methods Solvated electrons and radical Seconds to Hazardous Yes Yes <65⁰C Excellent Excellent Portable Yes Yes anion salts minutes solvents Strong aqueous bases No Yes NA 80⁰C Good Good Portable Yes No Strong bases
Other reductive pre-treatments No Yes Hours Unknown Unknown Unknown Portable Yes No Variable
Electrochemical Treatment Methods
Direct Possibly Yes Unknown Room Good Variable Fixed Possibly No
Indirect Possibly Possibly Unknown Unknown Unknown Unknown Fixed Possibly No
Metal amalgams Possibly Possibly Unknown Unknown Unknown Unknown Fixed Possibly No
Plasma Treatments Possibly Yes Variable Room Poor/variable Variable Fixed Poorly Yes
Flame Treatment Methods No Yes Unknown Unknown Unknown Unknown Fixed Yes No
Use number: 1 & 2 Acton Technologies Ltd 29
ANALYSIS OF ALTERNATIVES
4.2. Description of efforts made to identify possible alternatives
4.2.1 Research and development Acton Technologies Ltd have been a leader in the development and application of fluoropolymer etching systems for over 50 years and have a consolidated reputation in the industry. Acton Technologies Ltd have undertaken a range of research and development activities for the assessment of alternative solvents for wet chemical etching (Marsh, 2006a; 2006b), for evaluation of alternative etching methodologies and for the development and validation of etching methodologies in a number of industry sectors.
Maflon Spa has developed its own etching process and, in doing so, has undertaken cooperative development with Acton Technologies Ltd.
4.2.2 Data searches Acton Technologies Ltd have engaged an external consultant to undertake an extensive literature search on the availability of alternative solvents for wet etching and alternative etching techniques.
4.2.3 Consultations Acton Technologies Ltd have undertaken a detailed co-operative research and development programme over the period 1996 – 2006 in order to evaluate the use of other alternative solvents (Marsh, 2006a; 2006b). These studies are referenced in the Analysis of Alternatives.
Acton Technologies Ltd has not undertaken any additional external consultations to identify alternatives except as identified above as part of their own Research and Development programme. The company has consulted with chemical suppliers (Dow, BASF, Brenntag) to evaluate other solvent alternatives that have been made available and which are described in the section 5.1.2.
Use number: 1 & 2 Acton Technologies Ltd 30
ANALYSIS OF ALTERNATIVES
5. SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES
5.1. Wet Chemical Treatment Systems Wet chemical systems are based on the generation of a solvated electron transfer (SET) chemical species which can functionalise a fluoropolymer surface. SET donors are available in different chemistries and the suitability of each of these alternatives is summarised in Table 4.1 and discussed in more detail in the sections below.
5.1.1 Sodium – Ammonia System Solvated electrons can be generated by the chemical reaction of alkali metals in liquid anhydrous ammonia. The most commonly used alkali metal is sodium and this provides very harsh reduction conditions.
Exposure of fluoropolymers to this system often leaves the treated surface black due to the extent of the reduction reactions. Other alkali metals, such as magnesium, can be used to provide more mild reduction conditions but it is reported that electrochemical methods are also required to assist in the dissolution of the magnesium (Zhang et al., 2014). Table 5.1 Sodium and ammonia substance properties
CAS EC CLP Classification
H314: Causes severe burns and eye damage Sodium 7440-23-5 231-132-9 H260: In contact with water releases flammable gases whish may ignite spontaneously
H221: Flammable gas H280: Contains gas under pressure, may explode if heated H314: Causes severe skin burns and eye damage Liquid ammonia, anhydrous 7664-41-7 231-635-3 H331: Toxic if inhaled H400: Very toxic to aquatic life
Contact with liquid may cause cold burns/frostbite
5.1.1.1 Technical feasibility The sodium–ammonia etching system for fluoropolymers was the original surface treatment method for this polymer group. The reagent is prepared by the dissolution of metallic sodium in liquid anhydrous ammonia to give a dark blue solution with a sodium concentration of 0.5–1.0 % (w/w). An exposure of between two and twenty seconds is required to achieve an etched surface, after which the treated article is removed from the etchant bath and ammonia allowed to evaporate from the surface of the etched article. The article is then further cleaned by an ethanol wash. In a continuous process the product is removed from the etchant and transferred to a water wash.
Etching times are dependent upon the freshness of the etching solution and the etchant has a limited lifetime. Etchant solution life span is limited by the rate of evaporation of the anhydrous ammonia and the sodium metal oxidation rate (which itself is accelerated by humidity). As this process is performed under ambient conditions the brief effective pot life can be measured in minutes. To
Use number: 1 & 2 Acton Technologies Ltd 31
ANALYSIS OF ALTERNATIVES enable continuous processing larger volumes of solution (100 litres) need to be available for continuous feed to the point of etching. Etched surfaces vary from black to brown in colour.
There is no recovery of ammonia in a sodium ammonia etching system.
However, the use of these materials requires investment in cryogenic equipment for the handling of liquid ammonia, control of the process environment to restrict the ingress of moisture due to the reactivity of sodium with water, emission control technology to ensure that emissions of ammonia from the process are minimised and control of subsequent effluent discharge carrying high levels of hydroxides and fluorides. As a consequence, this technology alternative can only be offered at fixed commercial installations as a contract service, unless the volume requirement for etching for a particular end-use is sufficient to justify the economics of installation of such technology.
In addition, the reductant power of this system is often too aggressive for the article/surface to be etched. Too long an immersion time can reduce the adhesion achieved if the surface becomes significantly altered and weakened by too extensive a defluorination process.
The small molecular size of the ammonia system creates an aggressive and deep penetrating behaviour that in many instances makes it ineffective for controlled use. Fluoropolymer skived tapes, sheet or tubes with a wall thickness of less than 0.25 mm cannot be treated in a controlled manner by this etchant.
An example of this would be thin wall fluoropolymer tubing for use in medical catheters. The deep penetration of an ammonia treatment would penetrate fully through the thin wall of this polymer lining, creating pinholes and defeating the required integrity of the medical tubing. In comparison, a diglyme-based sodium etching solution limits the surface treatment only to the outer surface of this tubing, leaving the inside surface of the tubing “virgin” fluoropolymer and thus able to deliver the intended material characteristics of untreated fluoropolymers, i.e. non-stick.
Another example of the over penetration of this treatment is in PTFE coated fiberglass cloth in thin dimensions, where a sodium-ammonia treatment would permeate throughout the fabric and, in the application in the construction of multi-laminate industrial belting, the desired non-stick characteristic would defeated wherever “bleed-through” of the etching solution had occurred.
The adhesion achieved from etching PTFE with a sodium–ammonia solution is stated to be 15% weaker than that observed with a corresponding diglyme system etchant (http://www.polyfluor.nl/en/archive/fluoro-etch--etching-fluid/). In a comparative study carried out for Acton Technologies Ltd by Lehigh University Department of Chemistry, the impact of two commercial etching techniques on the final surface bonding of PTFE strips using identical adhesion regimes was examined (Acton Technologies Ltd, unpublished research). This study demonstrated that a statistically significant increase in final bond (peel) strength was achieved after use of a diglyme-based etchant (FSS) in comparison with a commercial sodium-ammonia etchant based process. However, it is difficult to corroborate this as a more general statement of extent of adhesion because, as stated in section 2.1.5.5.1, there are a number of other factors in addition to the nature of the etchant that define the extent and suitability of promoted adhesion in each specific end user application.
Use number: 1 & 2 Acton Technologies Ltd 32
ANALYSIS OF ALTERNATIVES
5.1.1.2 Economic feasibility The use of a sodium-ammonia etching system is also a less economic alternative, both from the investment required in the equipment to handle the sodium and liquid ammonia reactants and also in the cost of the consumables, as ammonia is not recovered in this process. A comparison of the consumable costs for a ‘standard etching’ process is given in Table 5.2.
Table 5.2 Cost comparison for of PTFE etching using a sodium – ammonia system
Sodium – Ammonia Sodium – naphthalene – diglyme
System Continuous processing of PTFE film, dimensions 0.25 mm x 600 mm x 500 m
Processing time Consumption quoted on hourly basis
Etchant Cost € per litre € per litre
litres litres Etchant carrier lost from process per Diglyme evaporation and carryover Ammonia evaporation, oxidation and hour litres carry over at ambient temperature Diglyme degradation by oxidation
litres To maintain etchant volume and litres Rate of etchant replacement per hour replacement spent etchant, which is To maintain etchant volume sent for diglyme recovery (70% recovery)
€ Cost of replacement etchant € Some of the cost will be offset by diglyme recovery
The use of the sodium–ammonia system is therefore at least a factor of times more expensive than the equivalent diglyme system. Making further allowance for the recovery and recycling of diglyme will increase this comparative cost ratio.
5.1.1.3 Reduction of overall risk due to transition to the alternative The fluoropolymer etching industry moved away from the sodium–ammonia etchant system on the basis of the hazards and economics of the use of metallic sodium and liquid anhydrous ammonia. Although the system can etch a wide range of fluoropolymers there are a number of significant disadvantages:
• The etchant is very aggressive and is not self-limiting, resulting in a surface that may not have the required bonding characteristics.
• There is a limitation in the minimum thickness of materials that can be etched (not less than 0.25 mm). In today’s applications of PTFE in the electronics, aerospace and medical sectors where smaller and lighter components are being developed continuously, this is a significant disadvantage.
Use number: 1 & 2 Acton Technologies Ltd 33
ANALYSIS OF ALTERNATIVES
• The required use of the etchant at very low cryogenic temperatures can result in the distortion of PTFE components
• The health and environmental risks are substantially different from the system which is subject to this authorisation. Transition back to this alternative will not reduce overall risk to health and the environment.
• A sodium–ammonia etchant is not pourable, is difficult to store and handle and has a significant odour.
• Ammonia cannot be recovered and recycled from this use
5.1.1.4 Availability Fluoropolymer etching using a sodium-ammonia system is offered commercially in the European Union from the following service providers:
• Fluorten s.r.l. (Italy) (http://www.fluorten.com/eng) • SA Pirep (France) (http://www.pirep.fr/finitionEN.html) • Fluorocarbon UK Ltd, http://www.fluorocarbon.co.uk/products/solutions/ptfe-sheet-and- tape; • Holscot, http://holscot.com/glossary/fluoroplastic-etching/;
The process is described as ‘the most effective etching medium available’ but also one in which ‘only a few companies in the world are able to run and manage with environmental compliance’, reflecting the aggressive nature of the reduction conditions provided. As indicated above, this is offered primarily as a fixed installation, contracted etching service and is not suitable for smaller scale fluoropolymer etching operations.
5.1.1.5 Conclusion on suitability and availability for sodium–ammonia etching alternative Sodium–ammonia etching is available on a commercial basis for the larger scale custom etching of fluoropolymer surfaces but is of limited application due to the aggressive nature of the etchant itself and the inability to employ the etching technique cost effectively at many downstream users sites where fluoropolymer etching is integrated into the manufacturing process.
As a consequence the sodium-ammonia etching system has not been the primary etchant of choice for PTFE surface modification and is only provided on a contract basis at fixed, bespoke installations where the risks of handling liquid ammonia can be completely assured. A number of historic incidents of the release of ammonia from such systems are known.
5.1.2 Sodium – Naphthalene – Alternative Solvents The dispersion of sodium in naphthalene in a number of solvents to provide a more economic and convenient method of etching fluoropolymers has been described in the literature over the last thirty years. The mechanism by which this is achieved and the generation of the solvated anion which is key to the reduction process is described in greater detail in sections 2.1.2, 2.1.3, 2.1.4 and 2.2.2.1.
Use number: 1 & 2 Acton Technologies Ltd 34
ANALYSIS OF ALTERNATIVES
A number of alternative carrier solvents for the sodium-naphthalene reductant system have been described and commercialized to a limited extent and this section compares the suitability of these solvents based on the criteria developed above for the use of diglyme (see summary Tables 5.2 and 5.3). These solvents are selected primarily on the solubility of sodium napthalide and the generation of the solvated anion. Typical formulations are 1:1 molar ratios of sodium and naphthalene in the appropriate solvent.
5.1.2.1 Substance ID and properties Table 5.3 lists the solvents that are examined as possible alternatives to diglyme in a sodium – naphthalene – solvent system. Alternative solvents have been selected for further examination in this section based on either reports of their use for the formulation of solvents for etching of fluoropolymers or their ability to generate solvated anion species in a sodium – naphthalene system.
Use number: 1 & 2 Acton Technologies Ltd 35
ANALYSIS OF ALTERNATIVES
Table 5.3 Solvent Criteria Matrix – Classification
REACH REACH Common name Chemical name CAS EC Hazard Classification Candidate Annex XIV List
H226 Diglyme Bis(2-methoxyethyl) ether 111-96-6 203-924-4 H360 1B Yes Yes EUH019
H225 Highly flammable liquid or vapour H302 Harmful if swallowed Tetrahydrofuran Tetrahydrofuran 109-99-9 203-728-8 H319 Causes serious eye irritation No No H351 Cat 2 Suspected of causing cancer H335 May cause respiratory irritation 1,2-dimethoxyethane H225 Highly flammable liquid or vapour Monoglyme Ethylene glycol dimethyl ether 110-71-4 203-794-9 H332 Harmful if inhaled No Yes (EGDME) H360 1B May damage fertility or the unborn child
1,2-bis(2-methoxyethoxy)ethane H360Df May damage fertility or the unborn child Triglyme Triethylene glycol dimethyl ether 112-49-2 203-977-3 No Yes EUH019 May form explosive peroxides (TEGDME)
Tetraglyme bis(2-(2-methoxyethoxy)ethyl) ether 143-24-8 205-594-7 H360 1B May damage fertility or the unborn child No No
H225 Highly flammable liquid or vapour H319 Causes serious eye irritation 1,4-dioxane 1,4-dioxane 123-91-1 204-661-8 No No H335 May cause respiratory irritation H351 Suspected of causing cancer
Dipropylene glycol Dipropylene glycol dimethyl ether 111109-77-4 404-640-5 Not classified No No dimethyl ether (DPG-ME)
Diethyl glyme Bis(2-ethoxyethyl) ether 112-36-7 203-963-7 H315 Causes skin irritation No No
Use number: 1 & 2 Acton Technologies Ltd 36
ANALYSIS OF ALTERNATIVES
Table 5.3 Solvent Criteria Matrix – Physico-chemical properties
Boiling Common name Flash Point Density Viscosity Evaporation rate Odour Thermal stability Point
⁰C ⁰C g/ml mPa.sec n-butylacetate = 1 1,010.8 hPA Closed cup 20⁰C 20⁰C
Mild ethereal <10% degradation to methyl vinyl Diglyme 160.16 51 0.95 1.06 0.36 Non-residual ether compared to monoglyme
Tetrahydrofuran 65 -21.2 0.883 0.456 6.3 Ethereal
At temperatures >0⁰C, Ethereal Monoglyme 84.45 2 0.87 0.47 4.99 spontaneously decomposes to Non-residual methyl vinyl ether Mid-ethereal Triglyme 216.25 111 0.99 3.39 <0.001 Non-residual Very mild Tetraglyme 276.49 141 1.012 4.01 <0.001 ethereal Non-residual
1,4-dioxane 101.3 11 1.03 1.19 2.2 Mild, ether-like
Dipropylene glycol dimethyl ether 175 65 0.903 1 0.13 Very mild
Mild Diethyl glyme 188 82.2 0.91 1.4 0.04 Non-residual 1 Some data derived from BASF: High Performance Solvents: Glymes, 1,3 dioxalane and 1,4 dioxane (http://www.standort-ludwigshafen.basf.de/group/corporate/site-ludwigshafen/en/literature- document:/Brand+Ethyl+Glyme-Brochure--High+performance+solvents+Glymes+1+3+Dioxolane+and+1+4+Dioxane-English.pdf)
Use number: 1 & 2 Acton Technologies Ltd 37
ANALYSIS OF ALTERNATIVES
5.1.2.2 Technical feasibility The alternative solvents listed above have all been used or purported to have been used in the formulation of sodium-naphthalene etching solutions, with the exception of dipropylene glycol dimethyl ether and diethyl glyme, which have been tested as potential alternatives by the current applicants.
However, comparison of their physico-chemical and toxicological properties demonstrates that:
• None of the alternative solvents offer any significant advantage in the reduction of risk from use as a formulating solvent, apart from of dipropylene glycol dimethyl ether and diethyl glyme
• Most of the alternatives have the same combination of physico-chemical properties as diglyme in providing the most suitable solvent for this application. Dipropylene glycol dimethyl ether has similar boiling point, flash point and density to diglyme.
The comparison of solvent properties, described below, is summarised in Table 5.3.
5.1.2.2.1 Physico-chemical Properties Flash Point The flash points of tetrahydrofuran (-21.2⁰C), 1,4-dioxane (11⁰C) and monoglyme (2⁰C) are significantly lower than that of diglyme and increase the flammability risk in the use of these solvents for the etching process. Dipropylene glycol dimethyl ether has a flash point of 65⁰C, which is higher than that of diglyme. Boiling Point and Evaporation Rate The boiling points of tetrahydrofuran, 1,4-dioxane and monoglyme are substantially lower than that of diglyme. As a consequence the evaporation rates for these solvents at process temperatures will be significantly higher than diglyme, leading to additional requirements for emission controls. These higher evaporation rates combined with decreased flash points, in comparison with diglyme, present significant handling risks in the application of such solvent-based etchants. Dipropylene glycol dimethyl ether has a boiling point of 175⁰C, which is slightly higher than that of diglyme, and a lower evaporation rate. Thermal Stability Monoglyme undergoes spontaneous decomposition to methyl vinyl ether (CAS 107-25-5, EC 203- 475-4) at temperatures in excess of 0⁰C. Methyl vinyl ether is an extremely flammable gas (H220). The decomposition of diglyme is approximately 10% of the rate on monoglyme and therefore substitution of diglyme with monoglyme substantially increases the process risk through the generation of a flammable gas by-product (Ebnesajjad, 2014). At temperatures above 0⁰C, monoglyme-based etchants will begin this spontaneous decomposition and will consume some of the active etching ingredient, sodium, in the process.
Use number: 1 & 2 Acton Technologies Ltd 38
ANALYSIS OF ALTERNATIVES
Viscosity The viscosities of triglyme and tetraglyme are significantly greater than diglyme and restricts the applicability of the resultant etchant for the surface modification of controlled areas of small diameter or high aspect ratio. Monoglyme-based etchants also have a high viscosity, due to the higher degree of solvation of sodium and naphthalene in this solvent. In combination with increased rates of evaporation, this leads to additional difficulties in the control of etchant viscosity and the subsequent removal of process residues from etched surfaces. Open weave fluoropolymer fabrics or woven glass clothes, for example, should not be etched with monoglyme-based solvents as this higher viscosity leads to lower rates of removal from treated materials which can then give rise to a flammable vapour on evaporation under atmospheric conditions. Dipropylene glycol dimethyl ether has a similar viscosity to diglyme. 5.1.2.2.2 Toxicological Properties Reproductive toxicity Both monoglyme and triglyme are substances of very high concern that have been prioritised on the ECHA Candidate List for the same reproductive hazard as diglyme. Substitution of diglyme with either of these solvents would not therefore reduce the potential reproductive hazard. Tetraglyme is also classified for the same reproductive toxicity end point but is not on the Candidate List at present. Dipropylene glycol dimethyl ether is not classified for this hazardous end-point. Carcinogenicity Both tetrahydrofuran and 1,4-dioxane are classified as suspected carcinogens. 5.1.2.2.3 Process performance Etchant shelf life There are two different solvent-based etching solutions that are commercially available. The monoglyme-based etchant has a manufacturer defined storage life of 6 months when stored at or below 0⁰C. In use at ambient temperatures (20-25⁰C) the etchant shelf life is a maximum of 7 days. THF based etchant is also reported to have a maximum shelf life of 6 months. A diglyme- based etchant, in comparison, has a shelf life of one year at room temperature and can be used both and ambient temperatures and at temperatures up to 60⁰C. Availability of the radical anion In section 2.2.1.1 the chemical mechanism of solvation of the sodium naphthalide complex was discussed. The mechanism, extent and rate of solvation of the ion species is dependent on the solvent used and will determine the availability of the radical anion for participation in the reductive defluorination of the fluoropolymer surface. The magnitude of the solvation of the metal ion and the naphthalene anion is the principle factor in determining the equilibrium constant for the reaction. Rates constants for electron transfer between the naphthalene molecule and its anion range between 106 and 108 liter.mol-1.sec-1 (Stettin, 1971: Vora, 1972), depending on the solvent. There is little data in the public literature detailing the solvation of sodium naphthalide in different solvents. Holy (1973) tabulates the effect of a number of solvents upon the equilibrium constant of sodium naphthalide which gives some indication of the impact of different solvent but does not provide a comparative basis for the solvents considered as alternatives for fluoropolymer etchants.
Use number: 1 & 2 Acton Technologies Ltd 39
ANALYSIS OF ALTERNATIVES
Table 5.4 Effect of solvent structure of sodium naphthalide equilibrium reaction
Equilibrium Solvent Structure CAS EC constant
Diglyme CH3OC2H4OC2H4OCH3 111-96-6 203-924-4 No data
Dimethyl ether CH3OCH3 115-10-6 204-065-8 0.2
Diethyl ether C2H5OC2H5 60-29-7 200-467-2 0.02
Dimethoxyethane (monoglyme) CH3OC2H4OCH3 110-71-4 203-794-9 1.0
Methoxyethoxyethane* (ethylene glycol ethyl methyl CH3OC2H4OC2H5 5137-45-1 225-893-6 1.0 ether)
Methoxypropoxyethane* CH3OC2H4OC3H7 77078-18-3 0.85
Dibutoxyethane* C4H9OC2H4OC4H9 112-48-1 203-976-8 0.2
Dimethoxymethane* CH3OCH3OCH3 109-87-5 203-714-2 0
1,1 dimethoxyethane* CH3O(CH2)2OCH3 534-15-6 208-589-8 1.0
1,1 dimethoxypropane* CH3O(CH2)3OCH3 4744-10-9 225-258-3 0.5
1,1 dimethoxybutane* CH3O(CH2)4OCH3 4461-87-4 - 0.2
1,1 dimethoxypentane* CH3O(CH2)5OCH3 26450-58-8 247-716-1 0.05
*not currently registered under REACH at this time The use of glymes in the solvation of sodium naphthalide is of particular significance as glymes contain multiple ether-type oxygen atoms and flexible alkyl chains that allow them to behave like crown ethers in solvating metal ions such a sodium through oxygen-ion complexing (Tang and Zhao, 2014). The equilibrium constant for sodium naphthalide in tetraglyme is reported at 200-300 M-1 at 27⁰C and forms loose ions pairs (Hoefelmann et al. 1969, reported in Tang and Zhao (2014)). The behavior of sodium in a range of glyme solvents has been studied using sodium iodide as a model system and the type of ion pairing shown to vary from free ions, to loose ion pairings to clusters of ion pairs, depending on the concentration of the salt and the structure of the solvent. The reactivity of free ions and solvent-separated ion pairs is very high compared to that of contact ion pairs (Bakskaran and Muller, 2007).
- + - + - + - + [R M ]n nR M R //M R + M
Aggregated ion Contact ion pairs Solvent separated Free ions pairs ion pairs The flexibility of glymes enables the formation of many stable structures within a narrow energy range with very different geometrical arrangements of ether oxygens. At least four geometries have been identified for metal-glyme complexes of sodium (Tang and Zhao, 2014). Analytical attempts to detect or quantify the ionic species by UV spectroscopy for after the dissolution of sodium naphthalide in tetrahydrofuran, diglyme and dipropylene glycol dimethyl ether were not successful (Marsh, 2006b). However, a relationship between solvent dielectric
Use number: 1 & 2 Acton Technologies Ltd 40
ANALYSIS OF ALTERNATIVES constant and therefore solvent polarity, sodium naphthalide solubility and extent of fluoropolymer surface modification was observed during the course of this study. Table 5.5 Correlation of etchant activity with solvent dielectric constant (Marsh, 2006b) Colour of Organometallic Dielectric Colour of Surface Solvent etched salt Constant solution etched? surface
Diglyme 7.2 Green Yes Dark
Tetrahydrofuran 7.5 Green Yes Medium Sodium naphthalide Dipropylene glycol dimethyl ether 5.5 Green Yes Light
Red-brown Dioxane 2.2 Insoluble No NA complex
From this study it was concluded that the dielectric constant of the solvent needed to be greater than 5.5 in order to adequately solvate the sodium naphthalide. Attempts to improve solvation by the addition of a cosolvent with a high dielectric constant were not successful (Marsh, 2006b). It can therefore be concluded that the solvation of sodium naphthalide will be highly dependent on the nature of the chosen solvent, the degree of solvation and form of complexation which will determine the overall activity of the radical carbanions necessary for fluoropolymer surface defluorination reactions. The unique properties of diglyme, to complex alkali metal ions in solution, make it difficult to find suitable alternatives, which also allow • economic and low risk recycling of the solvent by way of distillation • fine tuning operational reducing power by adjustment of temperature and concentration and • sufficient complexing properties to allow high yields of the alkali metal with naphthalene. Alternatives such as propylene glycols also have complexing properties to alkali metal ions, albeit weaker than that of the ethylene glycols. End group modification of the diethylene glycol ether will leave the complexation capacity greatly intact but the resulting significant reduction in the vapor pressure will make economic distillative workup for solvent recycling more difficult. Process reaction time and bond strength The concentration of the radical anion available in the solvent-sodium naphthalide system will determine both the rate and extent of the surface modification reaction and will therefore determine both the etching process time and the final bond strength that can be achieved. There are no definitive comparative studies comparing the performance of sodium naphthalide in different solvents for the surface modification of the same fluoropolymer surface and with the final modified surface tested with the same bonding system. As described in section 2.1.5.5.1, bonding adhesion tests do not necessarily characterize the extent of fluoropolymer surface modification.
Use number: 1 & 2 Acton Technologies Ltd 41
ANALYSIS OF ALTERNATIVES
Table 5.6 Bond strength after etching PTFE with a THF-sodium naphthalide etchant Solvent Polymer Reaction time Adhesion Reference
THF PTFE 10 seconds 21.4 MpA Brewis et al., 1993 Joint strength Brewis and Dahm, 2004
THF PTFE 10 seconds 4,280 N Brewis and Dahm, 2004 Failure load Bonded area 20 x 10 mm, two part epoxide adhesive THF PTFE (skived tape) 10 seconds 2,420 N Brewis and Dahm, 2004 30 seconds 2,730 N 4 hours 2,760 N Failure load Bonded area 12 x 12 mm, epoxy adhesive
The bond strength achieved with different adhesive systems was reported during a comparison of sodium naphthalide etchants formulated in diglyme and dipropylene glycol dimethyl ether (Marsh 2006a). Table 5.7 Comparative bond strength of etched standard PTFE strips (Marsh, 2006a, b) Loctite Araldite Loctite Spunfab Etchant Cyanoacrylate Epoxy resin Silicone copolyamide Contact Angle adhesive adhesive adhesive adhesive Peel Strength (pli) ⁰
Diglyme – sodium naphthalide 0.35 11.15 9.88 7.75 54-61
Dipropylene glycol dimethyl 1.92 3.01 Not tested Not tested 66 ether – sodium naphthalide Note: Bond strength here is peel strength for test PTFE strips and is quoted in units of lbs per linear inch (pli) All tested etchant formulations were ranked on the basis of average bond strength across four adhesive systems (cyanoacrylate, epoxy resin, silicone and melt-processable polyamide sealant). It was concluded that the diglyme – sodium naphthalide etchant was the most powerful of the formulated etchants tested and, although producing weak bonding with cyanoacrylate adhesives, had the highest average bond ranking and the highest overall rated performance (Marsh, 2006b). Table 5.8 Bond strength, peel test (Maflon Spa, 2015, unpublished laboratory results) Peel Strength Etchant Contact Angle Surface tension (N/cm)
Diglyme – sodium naphthalide 44.4 60.58 2.88
Dipropylene glycol dimethyl ether – sodium naphthalide 80 38.95 2.03
Ethyl glyme - sodium naphthalide 62.3 50.59 1.92
In both independent evaluations, dipropylene glycol dimethyl ether based etchant did not give a suitable bonding strength in comparison with the diglyme based etchant across a range of formulations and adhesives tested. The use of ethyl glyme as solvent also did not produce an etchant of comparable performance in terms of bonding strength.
Use number: 1 & 2 Acton Technologies Ltd 42
ANALYSIS OF ALTERNATIVES
Surface Modification The performance of etching solutions prepared from dipropylene glycol dimethyl ether and diglyme has been compared in experimental work using analytical techniques which determine the extent of chemical and physical modification of the PTFE surface. The results are tabulated below which demonstrates that the extent of etching is both more extensive and more consistent with the diglyme-based solvent system (Table 5.9 and Table 5.10). Table 5.9 Comparison of PTFE surface modifications using alternative solvents (Marsh, 2006b) Dipropylene glycol dimethyl ether Technique based etchant Diglyme based etchant (CAS 111109-77-4)
Wettability – Contact Angle 70⁰ (30 second etch) 35⁰ (30 second etch) (Unmodified PTFE 109⁰) 58⁰ (60 second etch) 48⁰ (60 second etch)
Similar levels of reduction in levels of surface CF which do not correlate with Surface Chemistry by XPS Analysis 2 contact angle data.
Surface topography (roughness) by Surface roughness of 0.5 – 0.8% Surface roughness 2.9-3.0% deviation AFM deviation from smooth from smooth
It was concluded that these test results demonstrated that the extent of surface modification achieved by this alternative etchant formulation as judged by both degree of wettability and surface topography measurements, was not compatible with that required for commercial applications. Table 5.10 Laboratory comparison of PTFE surface modifications using alternative solvents (Maflon Spa, 2015, unpublished laboratory results)
Dipropylene glycol Diethyl glyme based dimethyl ether based Diglyme based etchant Technique etchant etchant (CAS 111-96-6) (CAS 112-36-7) (CAS 111109-77-4)
Wettability – Contact Angle Good Good Good (Unmodified PTFE 109⁰)
Wettability- Dyne Pen Good Good Good testing
Colour of etched surface Brown Brown Brown
Table 5.10 reports the laboratory testing of two alternative solvents by Maflon Spa (2015) in which dip etching of PTFE skived tape was used to assess the extent of etching from sodium naphthalide etchants of the same concentration in three different solvents. In the laboratory a similar level of surface treatment was obtained, as judged by change of surface colour, contact angle measurement and wettability using the dyne pen test to derive a value for surface energy. However,
Use number: 1 & 2 Acton Technologies Ltd 43
ANALYSIS OF ALTERNATIVES when transferred to the production plant for plant trials, significantly worse results were obtained with lower and less consistent colour, less wettability and higher contact angles in comparison with a diglyme-based etchant. Figure 5.1 Colour comparison of PTFE sheet etched by sodium naphthalide in three different solvents under production plant conditions
Diglyme Ethyl Diglyme Dipropylene glycol dimethyl ether
Both Acton Technologies Ltd and Maflon Spa have concluded that although dipropylene glycol dimethyl ether is, theoretically, a potential alternative solvent for the formulation of sodium- naphthalene etchants, the performance of such etchants has not been successful from both laboratory and production pilot trials, for either batch or continuous etching applications. Neither company is, at this time, planning further investment on research and development of this solvent system on the basis of these negative results.
5.1.2.3 Economic feasibility Acton Technologies Ltd refer to the assessment made by Maflon Spa for the potential production cost increase had dipropylene glycol dimethyl ether proved a suitable alternative solvent. Maflon Spa have estimated the increase in overall cost of etching for their continuous PTFE sheet etching process (defined as € per metre) for the use of both dipropylene glycol dimethyl ether and diethyl glyme, should either of these alternative solvents have produced commercially acceptable quality etched products. Maflon Spa have concluded that the continuous production line speed would be reduced by up to 30% and result in an overall increase in cost of between 2 and 4%. However, as the formulated etchants did not produce the required commercial final quality, the replacement of diglyme with these alternative solvents was not pursued.
5.1.2.4 Reduction of overall risk due to transition to an alternative solvent Two of the alternative solvents examined, monoglyme and triglyme, have been prioritised for the REACH Candidate List on the basis of their reproductive toxicity. The increased volatility of monoglyme would potentially increase the risk of exposure. Tetraglyme is also classified for the same endpoint but has not been prioritised to the REACH Candidate List. The lower volatility of triglyme and tetraglyme would reduce the risk of exposure but substitution of one solvent with
Use number: 1 & 2 Acton Technologies Ltd 44
ANALYSIS OF ALTERNATIVES another with the same hazard classification that resulted in the prioritisation of diglyme to REACH Annex XIV is not considered a sensible viable alternative strategy.
There are significant additional safety issues associated with the high vapour pressure and low flash point of monoglyme and tetrahydrofuran as alternative solvents, which has caused the industry to move away from their use over the last twenty years. The formation of peroxides and the increase in process risk during solvent recovery is also a significant issue to take into consideration. Monoglyme is not a solvent that can be recovered easily, due to its thermal instability and can more readily form explosive peroxides, and etchant formulations using this solvent are essentially single use with no recovery.
5.1.2.5 Availability The alternative solvents examined in the sections above are all considered commercially available on the basis that they have all been registered in full at an appropriate tonnage band by at least one EU legal entity under REACH.
Formulated sodium-naphthalene etching solutions, that are either currently commercially available or to which references can be found in an internet search for PTFE etchants, are listed in the following table:
Table 5.11 Other commercial solvent-based fluoropolymer etchant systems
Solvent Supplier Trade Name Monoglyme W.L. Gore Tetra-Etch W.L. Gore produced a tetrahydrofuran-based etchant historically and such an etchant is referred to in much of the literature on sodium-naphthalene based etchant systems. However, the THF-based etchant has not been available commercially for over twenty years Monoglyme Tetra-Etch Products Ltd Tetra-Etch Triglyme Matheson Gas Poly-Etch W Tetraglyme Matheson Gas Poly-Etch Matheson Gas is a United States Supplier Only Safety data sheets are available for these Matheson products but their availability is uncertain (It is understood that they have not been available for over twenty years) Either diglyme or Artilabo International Fluoroplastic-Etch monoglyme PrimeEtch Monoglyme Technetics Group PrimeEtch Plus PrimeEtch II Technetics Group is a United States Supplier Only Diglyme Reltek (US) Bond-It Reltek is a United States Supplier Only Diglyme APC (Scotland) Ltd Bond-Prep Monoglyme Fluortech (US) Not Known Fluortech is a United States Supplier Only Natrex 25 Natrex 19 Monoglyme Fulcrum Chemical (US) Natrex VisReduce Natrex High FP Fulcrum Chemical is a United States Supplier Only Not known Adtech Customised etching service Ammonia and Holscot Customised etching service diglyme based
Use number: 1 & 2 Acton Technologies Ltd 45
ANALYSIS OF ALTERNATIVES
Solvent Supplier Trade Name etchants
5.1.2.6 Conclusion on suitability and availability for Alternative Solvent The selection of an alternative solvent for this use requires an optimum combination of the following physico-chemical and process characteristics:
1. Adequate solvation of sodium naphthalide to maximise the availability of the radical anion at the fluoropolymer surface. This dictates the use of a polar solvent that can solvate the sodium ions through oxygen complexing but in a form that makes sufficient concentrations of the active ions available for the defluorination reaction.
2. Physico-chemical properties of a relatively high flash point and good thermal stability at room temperature to minimise the risk during handling and storage of the formulated etchant.
3. A viscosity close to that of water to ensure that the etchant is able to access and etch all exposed surface in an even manner and to be subsequently removed, together with the process residues, by the subsequent washing steps.
4. Produce an etch that provides the right degree of surface modification in a controlled and even manner, as judged by wettability testing. As illustrated in section 2.2 an acceptable etch would achieve a reduction in the contact angle to <60⁰.
5. Produce an etch that is suitable for subsequent bonding applications for the etched PTFE component, as judged by specific application based adhesion tests.
Selected solvents that have been used in the formulation of commercial etchants, tetrahydrofuran and monoglyme, do produce an acceptable etch across the range of fluoropolymers. However, there are significant additional safety issues associated with the high vapour pressure and low flash point of these two solvents which has caused the industry to move away from their use over the last twenty years. Monoglyme has been prioritised onto the Candidate for reproductive toxicity and THF is suspected carcinogen
The other alternative glymes (triglyme, tetraglyme) that have been reported as alternatives do not produce such a good etch as diglyme, due to the reduced availability of the active components of the etch, and also have a viscosity which would provide additional process difficulties for the removal of the etchant and etchant residues across the range of etched PTFE products. Triglyme has been prioritised onto the Candidate for reproductive toxicity and tetraglyme is classified for the same hazardous end point.
There are two potential alternative glymes, dipropylene glycol dimethyl ether and diethyl glyme, which have similar physicochemical properties to diglyme and which have been demonstrated in the laboratory to produce etchants of reasonable characteristics. However, such formulated etchants have failed to produce the required consistency of results in etching applications on the production scale.
Use number: 1 & 2 Acton Technologies Ltd 46
ANALYSIS OF ALTERNATIVES
Diglyme is recovered in a controlled and cost effective manner from the spent etchant solution via vacuum distillation and subsequently recycled. Recovery and recycling of either THF or monoglyme presents unacceptable process risk because of the low flash points and potential for the generation of explosive peroxides. Spent etchant formulated with diglyme can be either be recycled internally or returned by downstream users to the manufacturer for recovery and disposal.
5.1.3 Other reductive pre-treatments involving radical anions A further chemical system that functions through the generation of an SET mechanism, comparable with the sodium-naphthalene system is the potassium salt of the benzoin dianion in dimethyl sulphoxide (Costello and McCarthy, 1987; Hung and Burch, 1995; Brewis and Dahm, 2005; Zhang et al., 2014).
5.1.3.1 Substance ID and properties Table 5.12 Substance ID and Properties Substance CAS EC CLP Classification
Dimethyl sulphoxide 67-68-5 200-664-3 Not classified
Benzoin 119-53-9 204-331-3 Not classified
Potassium t-butoxide 865-47-4 212-740-3 H228, Flammable solid
H252, Self-heating in large quantities
H314, Causes severe skin burns and eye damage
EUH014, Reacts violently with water
5.1.3.2 Technical feasibility This method for the reduction of PTFE surfaces was undertaken as an academic research project and has never been commercialised at any scale to the applicants’ knowledge. From the literature reports, the method requires extended reaction times of up to 24 hours and reaction temperatures of up to 50⁰C (Castello, 1987; Hung and Burch, 1995; Brewis and Dahm, 2005).
5.1.3.3 Economic feasibility No economic evaluation of this experimental methodology has been undertaken. However, the extended reaction times required for surface modification (from minutes to hours) would make its routine application impractical and significantly reduce etching process throughput.
5.1.3.4 Reduction of overall risk due to transition to the alternative Whilst none of the reactants have a hazard profile similar to diglyme, potential increased process risk from the reactivity of the butoxide would require careful control.
Use number: 1 & 2 Acton Technologies Ltd 47
ANALYSIS OF ALTERNATIVES
5.1.3.5 Availability This methodology has not been tested commercially and its performance against standard sodium- naphthalene etching technologies is unclear.
5.1.3.6 Conclusion on suitability and availability This alternative chemical treatment is not a suitable alternative because it has not been tested commercially to determine whether it is either technically or commercially feasible. Extended reaction times would make commercial implementation impractical.
5.2. Electrochemical Treatments Electrochemical methods for the reduction of PTFE have also been reported in the literature (Brewis and Dahm, 2001; 2005) as the reducing species required can be generated electrochemically by an SET mechanism (Brewis and Dahm, 2001; 2005; Zhang et al, 2014). The appearance of electrochemically treated PTFE is reported to be similar to that obtained in other wet chemical treatments reported above but it is also stated (Brewis and Dahm, 2001) that the nature of the surface modification is fundamentally different in that involves almost the complete conversion of the polymer into carbon in a surface layer up to several microns in thickness A number of electrochemical methodologies have been demonstrated for the surface treatment of PTFE A. Indirect electrochemical pre-treatment Electrochemical generation of tetrabutyl ammonium naphthalenide at a platinum cathode from a solution of tetrabutylammonium tetrafluoroborate (TBAT) and naphthalene in dimethyl formamide was reported to be as effective in the surface modification of PTFE as sodium naphthalenide in tetrahydrofuran (commercial Tetra-Etch).
Electrochemical methods have been used to generate solvated electrons with magnesium counter-ions from a solution of ammonium tetrafluroborate in liquid ammonia (Brace et al, 1997; Brewis and Dahm, 2001; 2005). However, this methodology has the same restrictions and disadvantages as the wet chemistry system based on sodium in anhydrous liquid ammonia.
B. Treatment with metal amalgams Electrochemical reduction and carbonisation of PTFE can also be achieved using alkali metal mercury amalgams, in which direct chemical interaction occurs between the amalgam and polymer surface followed by defluorination reactions to carbonise the surface. To the applicant’s knowledge there are no commercial applications of metal amalgam systems being used for the surface modification of fluoropolymers.
Use number: 1 & 2 Acton Technologies Ltd 48
ANALYSIS OF ALTERNATIVES
C. Direct electrochemical pre-treatment Brewis and Dahm (2005) have also demonstrated the direct electrochemical reduction of PTFE by direct contact on a metal cathode with a PTFE surface under a solution of tetrabutyl ammonium tetrafluoroborate in dimethyl formamide. Bonding strength expressed as failure load (for treated PTFE skived tape in a lap shear test of 12 x 12 mm overlap using an epoxy adhesive) of 3,240 N were obtained electrochemically in comparison with 2,400 – 2,700 N obtained by surface treatment with a sodium naphthalide/tetrahydrofuran etchant.
The applicants are not aware of any commercial applications of electrochemical methods for the surface modification of perfluoropolymers. Such methods have so far only been reported in the academic literature. The applicants have not, therefore, pursued investigation of such methodologies during their own extensive research and development of etching technologies over the last 50 years. Sections 5.2.1.1 through to 5.2.1.4 are not therefore further addressed in this analysis of alternatives.
5.2.1.1 Substance ID and properties Not further considered.
5.2.1.2 Technical feasibility Not further considered.
5.2.1.3 Economic feasibility Not further considered.
5.2.1.4 Reduction of overall risk due to transition to the alternative Not further considered.
5.2.1.5 Availability There are no known commercial applications for the electrochemically mediated surface modification of PTFE.
5.2.1.6 Conclusion on suitability and availability for electrochemical treatments Electrochemical reduction for the surface modification of PTFE is not further examined as there are no known examples of the commercial application of such techniques.
5.3. Plasma Treatment Plasma treatment is a common method for the surface modification of polymers to improve adhesion and wettability characteristics and there has been significant research effort into the development of these techniques for the pretreatment of fluoropolymers in response to the increasing regulatory pressure on the use of many substances used for the wet chemical treatment techniques described above.
Use number: 1 & 2 Acton Technologies Ltd 49
ANALYSIS OF ALTERNATIVES
Plasma treatments, which are all based on dielectric barrier discharge phenomenon, can be categorized as follows: • Flame treatment: this is a commercial process in which the polymer article is passed over an oxidising flame of oxygen-rich hydrocarbon. It is applicable to a number of polymers, particularly polyolefins and polyethylene terephthalates but is not effective in the surface modification of PTFE.
• Corona discharge treatment: corona discharge treatment of polymer films has also been commercially available for many years. A corona discharge gas a high voltage arc in air. When air is ionised, electrons generated collide with the surface material to disrupt molecular bonds, create free radical and generates atomic oxygen which chemically reacts with surface carbon and hydrogen to form polar functional groups on the polymer surface. It makes use of atmospheric pressure air plasma. This treatment is only applicable to a limited number of polymer materials (for example, polyethylene, polypropylene) and objects of simple three dimensional shape. Corona treatment of PTFE is reported to be more difficult (http://www.bde-equipements.fr/images/tigres- publication-32_corona-treatment-of-cable-7b33 ).
• Plasma treatment at reduced pressures (LPT): plasma, in general, consists of a mixture of partially ionised gases that is produced by subjecting a gas at low pressure to a high intensity electric field. The ionised particles are accelerated to energies that are comparable or exceed the bond energies of the polymer surface and impact will promote the structural and chemical modification of the polymer surface. The adhesion of polymer surfaces is improved due to the removal of low molecular mass materials, stabilisation of polymer surfaces by cross-linking, surface roughening and surface functionalization. Commercial plasma treatments are available for perfluoropolymers as described below.
• Plasma treatment at atmospheric pressure (APT): this technique has the advantages of plasma technology as an alternative to LPT and overcomes the short comings of corona treatments. It has been reported to be demonstrated for perfluoropolymers. It is marketed under such proprietary technology brands as ‘Aldyne’ (www.softal.de).
5.3.1.1 Plasma treatment description The major developments in the use of plasma technology for the surface modification of perfluoropolymers have in LPT treatments and these are now available for commercial scale application for the routine treatment of polymer surfaces.
Plasma treatment has the following advantages:
• Able to treat complex tribologies • Do not produce chemical wastes • Can be modified to deliver specific surface modifications • Can be used to treat heat sensitive materials • Processes are controllable through regulation of the process parameters such as power, pressure, gas type and processing time.
Use number: 1 & 2 Acton Technologies Ltd 50
ANALYSIS OF ALTERNATIVES
A variety of gases can be used in the LPT and are characterised as follows: • Inert gas plasma: helium, argon and neon • Oxygen containing plasma: oxygen • Nitrogen containing plasma: nitrogen, ammonia, • Fluorine containing plasma: fluorine • Other plasmas: hydrogen, carbon dioxide
The principle of the technique is that the support gas is excited with electrical energy at low pressure (10-2 to 10-3 mbar) to generate charged particles – positive and negative ions and free radicals. The charge particles are electrically conductive and can be influenced by a magnetic field. They are also intensively reactive and are very versatile in stimulating surface modification through the following mechanisms:
• Removal of contaminating surface materials • Surface reactions between the gas phase and surface atoms and chemical groups • Reactions between surface species to produce functional groups and crosslinking on the polymer surface.
The surface modification is confined to the top several tens of nanometers of the polymer surface and does not affect the bulk properties of the polymer.
Oxygen, nitrogen and ammonia are the most common gases used in LPT for perfluoroploymer treatment, causing the generation of free surface radicals, the rupture of covalent bonds and the polarisation of the surface.
5.3.1.2 Technical feasibility Whilst is has been found that perfluoropolymers do not respond as well to plasma treatments as well as other fluoropolymers, the effectiveness of the technique in comparison to wet chemical etching techniques has not been widely reported in the literature. Some reports have stated that the LPT treatment of PTFE does not impart sufficient strong adhesion to the polymer surface, giving about half the bonding strength compared to sodium naphthalide etching. Other published studies have reported that PTFE surfaces undergo high rates of fluorine loss coupled with low extent of oxygen incorporation. Ammonia and nitrogen plasma treatments of PTFE using the Planartron for 50 seconds have been reported to provide significant increases in bonding strength from 0.5 to 5.9 N.mm-2. The applicants have reviewed these application of plasma technologies to the etching of PTFE surfaces. Most plasma systems provide a surface modification that provides considerably lower bond strengths, primarily because the technique primarily improves surface roughness. In addition the shelf life of the treated surface is much shorter than that for the wet chemical sodium naphthalide technique. Atmospheric plasma treated PTFE surfaces have shelf lives of the order of minutes to days and vacuum plasma treatment may extend this shelf life to a number of weeks, The guaranteed shelf life for sodium naphthalide etched surfaces, protected from ultraviolet light and moisture, is one years and will last substantially long. The consequence of this is that treatment by this method would require immediate use of the etched PTFE surface for the bonding applications (data from Acton Technologies Ltd and Maflon Spa).
Use number: 1 & 2 Acton Technologies Ltd 51
ANALYSIS OF ALTERNATIVES
There may be specific applications where plasma mediated etching on perfluoropolymers is the preferred methodology, especially where a colour change on etching is undesirable or where chemical residues may be problematic.
5.3.1.3 Economic feasibility LPT treatments are of higher unit cost due to • Higher capital cost equipment – each discreet material type requires specific equipment for their configuration: for example PTFE film, machined parts or tubing each require at least a different equipment for their material profile and therefore multiple set-ups may be required for differing sizes of the same part types. • The requirement for low pressure systems • Lower productivity throughput Acton Technologies Ltd have made a significant investment in the development of plasma treatment technology for perfluoropolymer surface treatment over the last 20 years but have not seen a return on that investment through the widespread adoption of either the technology or the etched products produced by the technology. Whilst there may be niche markets for this technology for perfluoropolymer surface modification, wet chemical methods will remain the predominant technology because of the ease of use for multiple configurations of surfaces and consistency of performance in a number of validated product areas. Plasma treatments are, however, finding much wider acceptance for the surface modification of partially fluorinated polymers and other polymeric surfaces.
5.3.1.4 Reduction of overall risk due to transition to the alternative The use of plasma treatment would avoid the need for the use of hazardous chemicals.
5.3.1.5 Availability Commercial plasma treatments for the surface modification of PTFE are reported to be available from: • Diener Electronic: (http://www.plasma.de/en/plasmatechnique/etching.html) • Henniker Plasma: (http://plasmatreatment.co.uk/henniker-plasma-technology/plasma- treatments/plasma-surface-activation-to-improve-adhesion/) • Acton Technologies Ltd: Acton Technologies Ltd has developed the most effective and longest lived plasma surface treatment in the market place. This has been deployed in pilot scale processing for fluoropolymer films. However, despite more than ten years of application development, the technique and products produced have not gained market acceptance. Adhesion performance is reported by the downstream users of etched products to be lower and the treatment has been deemed by the market place to not be an adequate replacement for sodium naphthalene etching. As discussed in section 2.1.5.5.1, the downstream user of etched products does not always reveal to the etchant technology supplier the reasons for end application failure and whether this lies with the etchant technology, the adhesion technology or the application characteristics.
Use number: 1 & 2 Acton Technologies Ltd 52
ANALYSIS OF ALTERNATIVES
5.3.1.6 Conclusion on suitability and availability for plasma treatment Brewis and Dahm (2006) concluded that ‘plasma treatments can result in the introduction of substantial quantities of functional groups into fully fluorinated polymers but adhesion levels are moderate at best, probably as a failure to eliminate weak boundary layers’. The development work carried out by Acton Technologies confirms that this is still the market situation today and that there are only limited applications where plasma treatment might be used, for example if there was a requirement for the etched surface not to be coloured (i.e. the etched surface is required to be white) or in applications where there are severe constraints on the presence of possible chemical residues on the etched product. In both cases there would be a compromise in the acceptance of low adhesive properties of the etched surface.
Use number: 1 & 2 Acton Technologies Ltd 53
ANALYSIS OF ALTERNATIVES
6. OVERALL CONCLUSIONS ON SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES FOR USE
6.1. The use of surface modified fluoropolymers Surface treatments of fluoropolymers are utilised in many critical applications where the failure of the bonded fluoropolymer surface is both expensive and would subject the end user to high risk. Some key examples of these uses include:
• medical tubing for use in heart catheters and stroke-treatment catheters • automotive crankshaft sealing applications, • aerospace components • fire-resistant data cable (used to prevent fire-spreading in commercial buildings).
In virtually all of these applications there is extensive, mandatory qualification work performed to specify a particular manufacturing process for each component to be utilized in the overall product.
In each of these instances switching from the current diglyme-based sodium etchant to another treatment, if technically and economically feasible, would require re-qualification of the manufacturing route and the component performance.
For example, it has been reported to the applicant that the automotive Product Part Approval Process (PPAP) required for component qualification has an average cost of €90,000 ($100,000). In the case of an automotive supplier this would mean a requalification cost for each engine platform crankshaft seal and, using a conservative estimate of 30 seals being processed to support 15 different engine platforms, could result in €2,700,000 requalification testing alone for surface treatment substitution. The actual seal count is understood to be significantly higher.
An analogous qualification process is required for the numerous applications where surface treated fluoropolymer tubing is being utilized in many different medical device assemblies. Such re- qualifications could be reasonably estimated at the same €90,000 cost and taken in whole could result in significant costs in testing and specification changes.
Aerospace qualifications could result in many additional requalification requirements with further significant costs.
6.2. Overall conclusions on alternatives Bis(2-methoxyethyl)ether (diglyme) is used as a solvent for sodium naphthalide to produce an etchant for the surface modification of fluoropolymers, especially perfluoropolymers such as polytetrafluoroethylene (PTFE), by reductive defluorination in order to increase the surface adhesion properties of such polymers. Diglyme provides sufficient solvation of the radical anion salt to generate the active chemical species in order to promote this reductive defluorination. Other physico-chemical parameters, such as a relatively high flash point, a viscosity similar to that of water and thermal stability during storage and solvent recovery, provide an etchant that can be handled with reduced process risk in a number of process configurations (batch and continuous) to process a range of physical forms of polymer articles (e.g. sheets, seals, tubes etc). Etchants
Use number: 1 & 2 Acton Technologies Ltd 54
ANALYSIS OF ALTERNATIVES formulated using diglyme as the solvent are used in the surface modification of fluoropolymer components in a range of industrial and medical applications, where the bonding of the fluoropolymer, and PTFE in particular, provides the final bonded manufactured product with the critical non-reactive properties of the virgin fluoropolymer surface and where the specific structural integrity of the components must otherwise be maintained.
Sodium-ammonia etchant systems are commercially available but this methodology is significantly more expensive than the diglyme solvent alternative, is more restricted in its application due to the aggressive and penetrating nature of the generated solvated electron reductant and has a different but significant risk profile in the use of liquid ammonia. As such it can only be implemented at specialist facilities and cannot be used for small scale industrial etching facilities or for components where maintenance of specific structural integrity is required.
Other alternative polar solvents that provide similar sodium napthalide solvation characteristics have been identified and some are also known to be available commercially. However, these solvents either have a similar toxicity profile to diglyme (e.g. monoglyme, triglyme) or pose significantly greater process risk through lower flashpoint, greater volatility, reduced thermal stability at room and elevated process temperatures or generation of explosive peroxides during solvent recovery and recycling processes (e.g. monoglyme, tetrahydrofuran). Other glyme alternatives also provide additional process restrictions through increased viscosity due either to the extent of solvation of sodium naphthalide (monoglyme) or increased inherent viscosity (triglyme and tetraglyme).
Formulated etchants using either dipropylene glycol dimethyl ether or diethyl glyme have not been demonstrated to produce the same degree of fluoropolymer surface modification in either laboratory or pilot production tests which would allow commercial application of these solvent alternatives.
Other wet chemical methods, including the electrochemical generation of solvated electrons and radical anions, do not provide the same extent of consistency of surface modification particularly for PTFE, and none have been demonstrated or implemented commercially.
Alternative treatment methods for fluoropolymer surface modification, such as plasma treatments, are available for some fluoropolymers but have not been particularly successful for PTFE. Fully fluorinated polymers do undergo surface modification but the resulting shelf life of such ‘etched’ surfaces is considerably shorter than that achieved with the solvent etchants and that, combined with the requirement for expensive equipment in a number of configurations to cover the range of fluoropolymer articles requiring etching and the resistance of the downstream user market for products etched by this technique, have resulted in limited applications of such techniques.
In summary, diglyme is the preferred solvent for formulation and use of a sodium naphthalide etchant for perfluoropolymer surfaces as it provides the optimum balance of adequate solvation to maximise the availability of the radical anion reductant, either at or to a limited depth of the perfluoropolymer surface, with process characteristics of relatively low flashpoint, low viscosity and thermal stability, that permits the economic operation of the etching process at elevated temperatures of up to 65⁰C whilst minimising overall process risk.
Alternative solvents or alternative etching technologies do not provide the flexibility of an etchant of sodium naphthalide formulated in diglyme to produce a consistent surface modification of sufficient enhanced wettability, increased surface energy and increased final adhesive bonding
Use number: 1 & 2 Acton Technologies Ltd 55
ANALYSIS OF ALTERNATIVES strength across the range of critical perfluoropolymer bonding applications that require mandatory attainment of and qualification to end user specification.
7. REFERENCES Baskaran, D. and Muller, A.H.E. (2007) Anionic vinyl polymerization. Progress in Polymer Science, 33(2), 173-219
Brace, K., Combellas, C., Dujardin, E., Thiebault, A., Delamar, M., Kanoufi, F. and Shanahan, M.E.R. (1987) Surface modification of halogenated polymers: 1. Polytetrafluoroethylene. Polymer 38 (13), 3295-3305.
Brewis, D.M. and Dahm, R.H. (2001). A review of electrochemical pretreatments of polymers. International Journal of Adhesion and Adhesives 21, 397-409.
Brewis, D.M. and Dahm, R.H. (2005). Adhesion to fluoropolymers. RAPRA Report Volume 16 (3). ISBN-10 1-85957-524-2
Costello, C.A. and McCarthy, T.J. (1987) Surface-selective introduction of specific functionalities onto PTFE. Macromolecules 20 (11), 2819-2828.
Marsh, S. (2006a) fluoropolymer etchant: summary of performance data for the use of solvent for etching of fluoropolymers. Internal Action Technologies Report.
Marsh, S. (2006b) for Acton Technologies Fluoroetch Solvent: summary of laboratory and surface science work conducted . Internal Acton Technologies Ltd Report.
Ebnesajjad, S (2014). Fluoroplastics, Volume 1: Non-Melt Processible Fluoropolymers – The Definitive User’s Guide and Data Book, Volume 1, 2014. eBook ISBN 9781455732005. Elsevier.
Hofelmann, K., Jagur-Grodzinski, J. and Szwarc, M. (1969) Structure and reactivities of ion pairs and free ions studied by electron spin resonance spectroscopy. Effect of coordination with tetraglyme. Journal of the American Chemical Society, 91(17), 4645-4651
Holy, N.L. (1974). Reactions of the Radical Anions and Dianions of Aromatic Hydrocarbons. Chemical Reviews. 74 (2), 243-277
Hung, M-H. and Burch, R.R. (1995) Functionalisation and metallization of fluropolymer surfaces through reduction. Journal of Applied Polymer Science, 55(4), 549-559.
Shatenshtein, A.I., Petroc, E.S. and Yakovleva, E.A (1967). Effect of solvent and metal on the formation equilibria on naphthalene and diphenyl radical anion. Journal of Polymer Science, Part c (16), 1729-1737
Siperko, L.M. and Thomas, R.R. (1989) Chemical and physical modification of fluoropolymer surfaces for adhesion enhancement: a review. Journal of Adhesion Technology, 3(3), 157-173.
Use number: 1 & 2 Acton Technologies Ltd 56
ANALYSIS OF ALTERNATIVES
Stinnett, J. (1971). The reduction of carbonyl compounds by sodium naphthalenide. Master of Science Thesis, Department of Chemistry, Western Kentucky University.
Tang, S. and Zhao, H. (2014) Glymes as versatile solvents for chemical reactions and processes: from laboratory to Industry. RSC Adv., 4(22): 11251-11287
Teng, H. (2012) Overview of the development of the fluoropolymer industry. Applied Sciences, 2, 496-512.
Vora, M. (1972) The reactions of sodium naphthalenide with carbonyl compounds and esters. Master of Science Thesis, Department of Chemistry, Western Kentucky University.
Zhang, N., Samanta, S.R., Rosen, B.M and Percec, V. (2014) Single electron transfer in radical ion and radical-mediated organic materials and polymer synthesis. Chemical Reviews, 114, 5848-5958
Use number: 1 & 2 Acton Technologies Ltd 57