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Synthesis and Processing of PEEK for Surgical Implants
Chapter 2
Synthesis and Processing of PEEK for Surgical Implants Steven M. Kurtz Ph.D.
2.1 Introduction Polyaryletheretherketone (PEEK) is a challenge to synthesize and convert into surgical implants. The polymer is chemically inert and insoluble in all conventional solvents at room temperature. Indeed, PEEK can only be completely dissolved using fairly esoteric solvents, such as diaryl sulfones [1]. Although inertness and insolubility are desirable for a biomaterial, these attributes constrain the synthesis and manufacture of PEEK. Fabrication techniques for polyaryletherketone (PAEK) polymers have undergone constant refinement since the preparation of polyetherketoneketone (PEKK) was first described in the 1960s [1e3]. Although many of the details associated with synthesis and processing of PAEKs are proprietary to resin and stock material suppliers, it is important to understand the steps used in the manufacture of raw materials, because these techniques can substantially impact the properties and quality of the stock shapes and molded implant components [4]. Consequently, this chapter summarizes the principal steps used to synthesize and fabricate PEEK implant components. Early studies on PEEK processing tend to emphasize the fabrication of PEEK composites, using carbon and glass fibers [4]. This chapter is focused on the synthesis and processing of neat, unfilled PEEK polymer. We begin by outlining the two main synthesis routes for contemporary PEEK. As a high-temperature thermoplastic, PEEK can be processed using a variety of commercial techniques, including injection molding, extrusion, and compression molding. This chapter provides an overview of the methods for processing unfilled PEEK Biomaterials Handbook. DOI: 10.1016/B978-1-4377-4463-7.10002-8 Copyright Ó 2012 Elsevier Inc. All rights reserved.
PEEK used in biomedical applications. Readers with an interest in PEEK composites may wish to skip ahead to Chapter 3, which covers blending of PEEK with additives.
2.2 Synthesis of PAEKs As noted previously, the polymerization of aryletherketones is a complex and challenging process due to the insolubility of PAEKs in typical solvents. Furthermore, the solvents and high temperatures necessary to carry out successful polymerization of PEEK, such as benzophenone or diphenylsulfone above 300 C, necessitate dedicated plant facilities with rigorous safety procedures (Fig. 2.1). Because of the precautions necessary to carry out safe polymerization, the reactions are typically carried out in batches, as opposed to in a continuous process. All these challenges contribute to the higher cost in producing PAEK polymers, when compared with other thermoplastics. Historically there are two main routes involved in the production of PAEKs. The first method involves linking aromatic ether species through ketone groups, whereas a second method involves linking aromatic ketones by an ether bond. The first method involves an electrophilic reaction and Friedel Crafts acylation chemistry, and the second route involves a nucleophilic displacement reaction.
2.2.1 Electrophilic Routes to PAEK Polymers The inherent solvent resistance and propensity to reach high crystallinity levels prevents PAEK
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Figure 2.1 PEEK production facility, Thornton-Cleveleys, United Kingdom. Source: Victrex.
polymers from being synthesized in common organic solvents. Early attempts to synthesize PEEK in methylene chloride or nitrobenzene produced only low-molecular-weight variants. Work by DuPont using a combination of anhydrous hydrogen fluoride/boron trifluoride succeeded in protonating the carbonyl groups and meant that high-molecular-weight polyetherketone (PEK) became a possibility (Scheme 2.1) [5]. Raychem also reported the synthesis of PAEK polymers using similar reaction conditions in the presence of alkylthiochloroformates. Another electrophilic process exemplified by Ueda and Oda uses methanesulfonic acid (MSA)/ phosphorus pentoxide (P2O5) at low temperatures [6]. Although PEEK produced by this method has a less branched structure than AlCl3-catalyzed systems, it also suffers from high temperature instability and hence cannot be molded or extruded without extensive cross-linking and degradation. Colquhoun and Lewis [7] have described the Friedel Crafts polycondensation of 4-(40 -phenoxyphenoxybenzoic acid) in trifluoromethanesulfonic
acid to form PEEK. This route has only remained of academic interest due to the extremely high cost and corrosive nature of the solvent used (Scheme 2.2). The electrophilic synthesis of PAEK polymers produces materials with reactive end groups such as benzoic acids. Such polymers cannot be processed, without endcapping, due to their high thermal
O O
C
Cl
HF / BF3
O *
O
Scheme 2.1
C
*
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Scheme 2.2
O O
n
O
C
OH
CF3SO3H O *
O
O
C
*
quantities in the finished polymer. The choice of the endcapping agent may therefore significantly alter the leachable and biocompatibility profile of the material.
instability. When the reactive end group materials are subjected to high-temperature processing, the polymer immediately cross-links, producing gels, which cannot be shaped into desired articles. Therefore, PEEK production by electrophilic processes as described earlier has historically had limited commercial success. More recently, a modification to the electrophilic process for manufacturing PAEK polymers has been described. This again involves the polycondensation of 4-(40 -phenoxyphenoxybenzoic acid). However, methanesulfonic acid was used as the reaction solvent in the absence of phosphorus pentoxide, and 1,40 -diphenoxybenzene was used as an endcapping agent [8]. This route permits the manufacture of thermally stable PAEK polymers and has been used in industrial processes (Scheme 2.3). It should be noted that to ensure thermal stability, significant quantities of the endcapping agent are used and as a result may be present in significant
2.2.2 Nucleophilic Routes to PAEK Polymers The nucleophilic route to PAEK polymers provides a straightforward pathway to polymers such as PEEK. Initial attempts to form high-molecularweight PAEKs from the reaction of a dihalobenzophenone and an equivalent bisphenate failed due to the polymer product crystallizing from the sulfolane solvent (Scheme 2.4). Owing to the poor solubility of PEEK, the selection of the synthesis solvent is crucial. Suitable solvents should be thermally stable and inert to phenoxide species. It became apparent that solvents such as benzophenone or diphenylsulfone could be Scheme 2.3
O O
n
C
O
OH
CH3SO3H O *
O
O
C
C6H6-O-C6H6 End Capping Agent O O
C
O
*
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O F
O
C
F
+
KO
C
OK
Sulfolane O *
O
C
*
Scheme 2.4
O F
C
F
HO
+
OH
Diphenylsulfone K2CO3 O *
O
O
C
*
Scheme 2.5
used in the synthesis of PAEK polymers [3]. The inherent instability of bisphenates to oxidation was overcome by the use of hydroquinone and sodium or potassium carbonate to form the bisphenate in situ. Very high temperatures (>300 C) are required to reach high molecular masses, the molecular weight being controlled by a slight excess of difluorobenzophenone, leading to fluorine-terminated chains (Scheme 2.5). This process was patented in 1977 by ICI and sold under the brand “Victrex PEEK,” and this route provided the majority of PEEK polymer used in industrial applications. The establishment of the nucleophilic route to PAEK polymers permitted the investigation of polymer variants by the use of different bisphenols to produce PAEK polymers with various properties, as reported by Attwood et al. [1]. The family of PAEK polymers grew to contain variants such as PEK, PEEK, PEKK, PEKEKK, and so on, with a range of glass transition temperatures (143e160 C) and high crystalline melt temperatures (335e441 C). As the dominant member of the PAEK family of polymers, PEEK is in its “glassy” state at room temperature, as its glass transition temperature occurs about 143 C,
whereas the crystalline melt transition temperature (Tm) occurs around 343 C.
2.3 Nomenclature The literature on PAEK resin is a maze of trade names and producers, which have changed over the years, complicating interpretation of reported data for today’s materials. For researchers interested in deciphering the historical polymer science literature, we provide here a brief primer on the nomenclature of PAEK resins used for industrial purposes as well as for biomaterials (Table 2.1). Resin, when used in this context, refers to the neat, unfilled powder that is created by polymerization, whereas grades are typically characterized by flow characteristics (e.g., for injection molding or compression molding) or based on their filler content (e.g., glass fiber or carbon fiber). Because PAEK polymers are converted using standard thermoplastic processing techniques, such as injection molding, they are generally available as pellets, although powder resin is also available. Stock shapes, such as rods, are also available from producers.
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Table 2.1 Summary of PAEK Materials Related to Implant Use Polymer
Trade Name
Producer
Comments
PEEK
OPTIMA (Biomaterial)
Invibio (subsidiary of Victrex), Thornton-Cleveleys, United Kingdom
Manufacturer and supplier of longterm implantable PEEK in CE and Food and Drug Administrationapproved devices since 1998
Invibio (subsidiary of Victrex), Thornton-Cleveleys, United Kingdom
Available only in experimental quantities (see Chapter 13)
PEK
PEEK
Victrex
Victrex, Thornton-Cleveleys, United Kingdom
Provides PEEK for blood/tissue contact less than 24 h
PEEK
Gatone
Gharda, India
No record of supplier implantation studies. Discontinued for medical use when acquired by Solvay in December 2005
PEEK
Keta-Spire
Solvay Advanced Polymers, LLC
Not available for implant use
PEEK
Zeniva
Solvay Advanced Polymers, LLC
Implantable grade available
PEEK
VESTAKEEP I
Evonik
Implantable grade available
PEKK
PEKK
DuPont, Wilmington, DE
Discontinued for medical use by DuPont
PEKK
OXPEKK
OPM, Enfield, CT
Implantable grade available. Base resins supplied by Cytec [9]
PEKEKK
Ultrapek
BASF, the United States
Discontinued in December 1995
Historically, PAEK materials, including PEEK, have been produced primarily as niche polymers for industrial use, because their cost even today is at least two orders of magnitude more expensive than lowtemperature thermoplastics such as polyethylene. When ICI launched Victrex PEEK in 1987, the primary application targets were not medical. However, Victrex PEEK was used, if not yet supported, for implant applications. The Victrex PEEK business was sold by ICI in 1993, and in 1998, Victrex launched PEEK-OPTIMAÒ for long-term implantable applications. The offering of PEEK-OPTIMA provided a higher specification product and was aimed at addressing the previous failings of PAEK polymers by offering long-term supply assurance agreements in addition to a policy of no change concerning the main characteristics of polymer properties. PEEK-OPTIMA was supported by drug and
device masterfiles and manufactured in compliance with Good Manufacturing Practice. In 2001, Victrex established Invibio Biomaterial Solutions to specifically provide grades of PEEK suitable for long-term implantation. In reviewing the historical PEEK literature, whether for industrial or biomedical applications, the reader should keep in mind that polymers from ICI, Victrex, and Invibio were all produced at the same plant location, although the name of the company has changed since that time. Similarly, the nomenclature for the resin grades has changed over time, but the polymerization technology has remained fundamentally similar (Table 2.2). Today, PEEK biomaterials are designated by the OPTIMA trade name based on their molecular weight, which governs their flow properties in the melt (Table 2.2). The same range of molecular weight of PEEK polymers was also previously available from Victrex.
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Table 2.2 Contemporary and Historical Nomenclature for Medical Grades of PEEK Property
General-Purpose Grade
Medium-Flow Grade
Easy-Flow Grade
Historical Victrex nomenclature
450
381
150
Invibio nomenclature
OPTIMA LT1
OPTIMA LT2
OPTIMA LT3
Melt flow index
3.4
4.5
36.4
Molecular weight (Mn)
115,000
108,000
83,000
LT1 Standard grade LT2 Optimized grade for melt strength and melt viscositydrecommended for tubing LT3 High-flow grade for injection molding thinwalled parts The most commonly used grade for PEEK is OPTIMA LT1, which has flow properties similar to 450. Victrex grades are also designated as PF (fine powder), P (powder), or G (granulated). Powder grades are recommended for compounding, whereas granulated resin is preferred for injection molding. Although powder grades are produced for industrial applications, there are no powder grades commercially available for implantable grade PEEKOPTIMA, only granules. PEEK-OPTIMA undergoes a melt filtration step as a quality control measure to ensure cleanliness and biocompatibility. Following melt filtration, the polymer is granulated into cylindrical pellets (Fig. 2.2). Thus, to obtain powder from PEEK-OPTIMA, it is necessary to mill or grind the granules to obtain the desired particle size. Recently, two new resin manufacturers entered the medical PEEK market. In 2007, Solvay announced an implantable grade of PEEK marketed under the Zeniva trade name. In 2009, Evonik began marketing an implantable grade of PEEK under the VESTAKEEP trade name. However, no reports have been published describing these resins in scientific studies or how they are used in long-term implants. Publication of further details about the performance of Solvay and Evonik materials is anticipated. PAEK alternatives to PEEK are available from Invibio and Oxford Performance Materials (OPM). PEK has been made available by Invibio in experimental quantities as a candidate biomaterial for
tribological testing (see Chapter 13). However, PEK has not been commercialized for implant manufacture. PEKK resins are produced by OPM (Enfield, CT) and have been marketed under the OXPEKK trade name since the company was founded in 2000. Both medical grades and implantable grades of OXPEKK are available. OPM was acquired by Arkema (Colombes Cedex, France) in 2009. Sustainability of biomaterial supply has been a concern with PAEK resins in the 1980s and 1990s. With the exception of PEEK-OPTIMA, which can trace its origins back to 1998, many industrial PAEK materials have been withdrawn from the market, either out of concern for liability, patent infringement, and concerns about viability of a niche market or due to technical difficulties (Table 2.1). Nonetheless, to the extent that biomaterials history is not fully reflected in the literature, Table 2.1 provides some guidance as to the current availability of PAEK materials for industrial and implant use.
2.4 Quality Systems for Medical Grade Resin Production PEEK-OPTIMAÒ biomaterials are manufactured under a Quality Management System certified to ISO 9001:2000 and ISO 13485:2003. Only fully approved raw materials are used at the production stages together with extensive supervision and checks at key production stages. The recording of key parameters is performed at all stages. PEEK-OPTIMA biomaterials are manufactured on a campaign basis, thus enabling the employment of contamination risk reduction procedures. Invibio embraces Good Manufacturing Practice in relation to
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(A)
(B)
Figure 2.3 Clean room production environment for MG PEEK. Photo courtesy of Invibio.
(C)
Enhanced Quality Control procedures and standards, together with extensive testing and product release control, ensure a tight product specification for PEEK-OPTIMA. Test results are verified by dual testing at an independent UKAS-accredited laboratory. Invibio has a “no-change” agreement for the longterm supply of PEEK-OPTIMA biomaterials to assure its specification and production methods over time.
2.5 Processing of Medical Grade PEEK
Figure 2.2 High-resolution optical micrographs of PEEK-OPTIMA LT1 granules (A, B) and representative scanning electron micrograph (C). SEM image courtesy of Josa Hanzlik, Drexel University.
the manufacture of PEEK-OPTIMA biomaterials, including clean room conditions for processing (Fig. 2.3). Process documentation is archived to provide long-term batch and raw material traceability.
Despite the exceptional properties of medical grade PEEK polymers, these materials are processed by traditional plastic processing methods (Table 2.3). medical grade PEEK-based materials exhibit melting temperatures of around 340 C; however, PEEK polymers demonstrate good melt stability and remain workable with most conventional process equipment between 360 and 400 C. Commercial manufacturers of implantable grade PEEK provide detailed guidance and support for all processing techniques. However, for students and researchers, a brief summary of the main techniques is discussed in the
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Table 2.3 Traditional Plastic Processing Methods and Applicability to PEEK Processing Method
Applicable
Injection molding
Yes
Extrusion (profiles, sheet, and monofilament)
Yes
Compression molding
Yes
Powder coating
Yes
following. A comparative plot of melt viscosity versus temperature for a range of conventional polymers in comparison with PEEK-OPTIMA LT1 is shown in Fig. 2.4. Although commercial PEEK materials are supplied nominally dry, the pellet form of the material typically absorbs 0.5% w/w atmospheric moisture. It is therefore recommended that for processing operations such as injection molding, the polymers should be dried to less than 0.02% w/w moisture. Typically, suitable drying of PEEK pellets can be achieved by 3 h of exposure in an air-circulating oven at 150 C. If the oven is only capable of lower temperatures, a longer drying time will be necessary (e.g., 12 h of exposure at 120 C). PEEK-OPTIMA materials are provided in a range of viscosities in relation to the processing technique Figure 2.4 Shear viscosity versus temperature for a range of thermoplastics. Source: Invibio.
used. As mentioned previously in this chapter, PEEKOPTIMA LT1 is recommended for the majority of machining and injection molding of medical device components. PEEK-OPTIMA LT2 demonstrates good melt strength with reduced viscosity and is therefore recommended for the extrusion of thinwalled parts such as tubing. PEEK-OPTIMA LT3 is especially preferred for the injection molding of thin-walled parts. The problem of machine wear is common to all engineering plastics, and it is therefore recommended that screws, dies, and barrels should be hardened to minimize wear, especially when processing fiber-reinforced materials.
2.5.1 Injection Molding Injection molding is an attractive manufacturing technique suitable for mass projection of PEEK implant components (Fig. 2.5). Injection molding is typically performed using pellets or granules, which are poured in a hopper in the machine. The pellets are then automatically introduced into a heated screw assembly that melts and pressurizes the molten polymer, so that it flows into a heated mold. Once the PEEK component has consolidated, it is automatically ejected from the mold, so that a new cycle can take place. Injection molds are optimized and specially designed for each part, taking into account the details of the part geometry and flow and pressure capabilities of the molten polymer injection system.
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Figure 2.5 Representative injection molding system for PEEK. Picture supplied with kind permission of Krauss Maffei Technologies.
Consequently, the cost of the designing and fabricating mold itself represents a significant financial investment for this process. However, after this investment has been made, multiple PEEK parts can be manufactured in their near-final shape with cycle times of the order of minutes. Figure 2.6 shows a tree of tensile test specimens immediately after injection molding. The cylindrical sprue, radial runners, and any extra flashing are trimmed to complete the part manufacturing process. Because of the up-front cost of a suitable mold, injection molding techniques are not suitable for prototyping or low-volume PEEK part production. Most standard reciprocating screw injection molding machines are capable of molding natural IG PEEK materials and also carbon fiber-reinforced PEEK materials (Fig. 2.5). Furthermore, specialized injection molders with extensive experience in manufacturing PEEK-based medical devices exist in Europe and the United States. Injection molding of PEEK typically requires barrel and nozzle temperatures in the region of 400 C. The recommended mold surface temperature for PEEK lies in the range of 175e205 C, and this is extremely important to ensure that molded parts demonstrate uniform levels of crystallinity. Failure to achieve the minimum
Figure 2.6 Injection-molded test specimens, joined by sprue and runners. Injection molding allows multiple PEEK components to be fabricated in near-final shape. High up-front tooling costs are the main drawback of injection molding. Photo courtesy of David Jaekel, Drexel University.
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Component design should also take into account the shrinkage that occurs as crystalline regions form within the cooling polymer melt. Shrinkage is therefore dependent on the level of crystallinity and therefore the mold and polymer melt temperature. If operating under recommended injection molding conditions, commercial grades such as PEEK-OPTIMA can produce consistent parts with dimensional tolerances as low as 0.05%. Commercial manufacturers of implantable PEEK materials provide full guidelines and technical support for molding PEEK medical components.
2.5.2 Extrusion Figure 2.7 Amorphous regions within an injectionmolded part as a result of low mold tool temperatures. Photo courtesy of Invibio.
mold temperature results in rapid cooling of the part, with insufficient time for crystallization, as demonstrated in Fig. 2.7. Additional detailed guidelines for injection molding and mold design for PEEK implants can be obtained from raw material suppliers [10]. It is possible to increase crystallinity by annealing amorphous or low-crystallinity PEEK moldings (Fig. 2.8). However, the process of crystallization may lead to distortion and dimensional changes.
Figure 2.8 Effect of annealing after processing on injection-molded test specimens. The amorphous sample was created by heating to 400 C and then immersed in liquid nitrogen. Image courtesy of David Jaekel, Drexel University.
Extrusion is a manufacturing process for producing long stock shapes, such as rods, sheets, and monofilament fibers (Fig. 2.9). PEEK pellets or granules are typically the starting, raw material for extrusion. Similar to injection molding, the pellets are poured into a hopper that feeds into a heated screw assembly that melts and pressurizes the molten polymer. The molten, pressurized polymer is then forced through a heated die and slowly cools to room temperature along an extrusion line. The extrusion of PEEK can be accomplished using conventional extrusion equipment and dies at similar temperatures as those described for injection molding. The drive motor typically requires a power output of 0.25 HP/kg h. This power requirement is similar to that required for polycarbonate, poly(ethersulfone) (PES), or high-molecular-weight polyolefins. PEEK is a very stable polymer and as such is not sensitive to prolonged exposure (up to 2 h) to temperatures above its melting point. However, for optimal results, residence times should be of the order of 5e10 min. Dead spots, areas of material hold within the barrel, should be avoided. Therefore, careful design of the screw and barrel is necessary. The extrusion of PEEK-OPTIMA stock shapes followed by machining remains the predominant method of manufacturing medical device components. PEEK-OPTIMA can be purchased in rod or plate form ranging from 6 to 150 mm in diameter for rod and 40 mm in plate thickness (Fig. 2.10). In addition to this, thin-walled implantable PEEK tubing is also available from commercial suppliers. The thermal processes involved in extrusion and annealing of stock shapes also result in slight mechanical property variances when compared with
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Figure 2.9 PEEK polymer extruder. Photo kindly supplied by Coperion GmbH, Stuttgart.
Figure 2.10 PEEK-OPTIMA rod stock, pellets, and powder.
injection-molded components. This is highlighted graphically in Fig. 2.11.
2.5.3 Compression Molding Compression molding is a manufacturing process for stock shapes, such as plates or thick sheets (Fig. 2.12). A compression molding press consists of
two heated platens. The lower platen contains a recess for the plate or sheet that is charged with resin powder or granules. The platens are then pressed together and heated to consolidate the resin. Compression molding is typically used for lowvolume production, prototyping, and evaluation work or in the production of industrial components with very thick sections. Compared with injection molding, it is a relatively inexpensive process, but cycle times are long and as such it is not suited to high-volume production. The process involves the heating and cooling down of the melt and tooling. Pressure is applied to the melt and maintained during the cool-down phase. This is a time-consuming process, particularly when section thicknesses are large. Compression molding of PEEK requires a heated press, an oven, and the tool, which can be low-grade steel/ metal due to the levels of stresses, shear, and forces involved. There is generally a preference for fine powder grade of PEEK polymer for compression
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Figure 2.11 Graphical representation of the differences between injection-molded and machined PEEK components. Source: Invibio.
molding to avoid granular boundary parts in molded parts. These boundary points may also represent weak points in the components. If controlled cooling over long time periods is practiced, then compression-molded parts typically have higher crystallinity and tensile strength than injectionmolded components. Although, in theory, milled fibers or fillers could be compounded into PEEK for compression molding, this is rarely practiced because of the difficulty in ensuring a uniform fiber distribution and therefore uniform properties in the final component.
2.5.4 Film and Fiber Production
Figure 2.12 PEEK polymer compression molding. The figure shows a 20-ton compression molding press (Rondol Kompress 20 T).
The use of implantable grade PEEK in fiber and film form is a growing area of interest in medical devices because of mechanical performance and inherent purity. The industrial applications of PEEK in these forms have been established for a number of years. Thin sections of PEEK film are produced by extruding polymer using a suitable die and haul-off equipment, which controllably handles and stores the film for secondary operations. PEEK film can be produced in either crystalline or amorphous forms by controlling the temperature of the casting drums in the haul-off equipment. However, as the thickness of the film increases, the production
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2.6 Machining
Table 2.4 Properties of a 0.5-mm PEEK Monofilament Material Property
Typical Value
Energy to break
1.24 J
Tensile load at 2% elongation
1.18 kg
Tensile load at 5% elongation
1.77 kg
Tensile load at 10% elongation
2.66 kg
Tensile load at break
5.81 kg
Elongation at break
22.0%
Knot strength
2.42 kg
of fully amorphous PEEK films becomes more challenging. PEEK monofilament can be produced by extrusion followed by drawing of the PEEK extrudate. Drawing the polymer provides orientation within the fiber prior to heat setting. The resultant monofilament is tough, highly oriented, and has a controlled diameter, which will retain its set form above the glass transition temperature of the material. Typical properties of a PEEK monofilament are shown in Table 2.4. PEEK monofilaments are distributed as multifilament yarns (Fig. 2.13) and can be woven into more complex three-dimensional shapes.
Often for prototype designs or short production runs, it is not economically viable to manufacture an injection molding tool. Under such circumstances, it is common to use compression molding or to machine PEEK-OPTIMA polymer materials to form components. PEEK-OPTIMA polymer may be machined and finished using the same techniques and equipment as for other engineering thermoplastics. However, because of the excellent physical properties and wear characteristics of these materials, it is advisable to use carbide or diamond tipped tools and bits. Machining and finishing operations on polymeric materials are prone to propagating molded-in or residual stresses. Before machining, it is recommended that components formed from PEEK are annealed to relieve stress. During machining or finishing, further stresses may be built up within the material by localized heating at the cutting point. Therefore, if a large amount of machining and finishing is to be carried out on a component, a second annealing procedure may be required. The thermal conductivity of all polymeric materials is lower than that of metals; hence, heat build-up during machining is rapid. It is advisable that a cooling fluid is used to remove some of the heat generated by working the material. Water or air jet cooling is generally recommended for medical grade PEEK polymer-based materials. A summary of the suggested machining conditions for PEEK-OPTIMA has been provided by Invibio [10].
2.7 Summary
Figure 2.13 Spool of technical grade, multifilament PEEK yarn.
As detailed in this chapter, there are various synthetic routes involved in the production of polyketone-based materials. These routes have been developed to overcome the initial challenges in relation to polyketone manufacturing, resulting in the consistent production of polymers for medical applications. It should be noted that the synthetic chemistry used in PAEK manufacture has a major influence on the thermal stability of the polymer in processing operations and on the leachable and volatile content of these polymers. Therefore, the manufacturing route has the potential to alter the biocompatibility of PAEK materials. The progress in manufacturing has been extended to processing where established methods have been developed to
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allow the production of reproducible components from a range of production techniques such as injection molding or machining of stock shapes. The relatively recent addition of a biomedical focus to manufacturing and supply offers device companies an opportunity to use these PEEK-based materials, in implants, with confidence in the quality and security of supply. In the following chapter, we turn to the combination of PEEK with additives to form polymer matrix composites.
Acknowledgments Special thanks to David Jaekel, Drexel University, for editorial assistance and to Chris Espinosa, Exponent, for assistance with figures. This chapter would not have been possible without helpful discussions and insights provided by John Devine, Ph.D., and Craig Valentine, Ph.D., from Invibio.
References [1] T.A. Attwood, P.C. Dawson, J.L. Freeman, L.R. Hog, J.B. Rose, P.A. Staniland, Synthesis
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and properties of polyaryletherketones, Polymer 22 (1981) 1096. [2] W.H. Bonner. US patent 3,065,205. 1962. [3] J.B. Rose. British patent 1,414,421. 1975. [4] D.P. Jones, D.C. Leach, D.R. Moore, Mechanical properties of poly(ether-ether-ketone) for engineering applications, Polymer 26 (1985) 1385e1393. [5] B.M. Marks. US patent 3,442,857. 1969. [6] M. Ueda, M. Oda, Synthesis of aromatic poly(ether ketones) in phosphorus pentoxide/ methanesulfonic acid, Polymer 21 (9) (1989) 673e679. [7] E.Q. Colquhoun, D.F. Lewis, Synthesis of aromatic polyetherketones in trifluoromethane sulfonic acid, Polymer 29 (1988) 1902e1908. [8] D.J. Kemmish. US patent 6,909,015. 2003. [9] R. Leaversuch, Materials close up: demand surge tightens PEEK supply. Available from: Plast. Technol. (7) (2001) http://www.ptonline. com/articles/200107cu200102.html [accessed May 2007]. [10] P.E.E.K. Unfilled, Optima Processing Guide: Invibio Biomaterial Solutions (2009). Available from: http://www.invibio.com [accessed September 7, 2010].
Synthesis and Processing of PEEK for Surgical Implants Steven M. Kurtz Ph.D.
2.1 Introduction Polyaryletheretherketone (PEEK) is a challenge to synthesize and convert into surgical implants. The polymer is chemically inert and insoluble in all conventional solvents at room temperature. Indeed, PEEK can only be completely dissolved using fairly esoteric solvents, such as diaryl sulfones [1]. Although inertness and insolubility are desirable for a biomaterial, these attributes constrain the synthesis and manufacture of PEEK. Fabrication techniques for polyaryletherketone (PAEK) polymers have undergone constant refinement since the preparation of polyetherketoneketone (PEKK) was first described in the 1960s [1e3]. Although many of the details associated with synthesis and processing of PAEKs are proprietary to resin and stock material suppliers, it is important to understand the steps used in the manufacture of raw materials, because these techniques can substantially impact the properties and quality of the stock shapes and molded implant components [4]. Consequently, this chapter summarizes the principal steps used to synthesize and fabricate PEEK implant components. Early studies on PEEK processing tend to emphasize the fabrication of PEEK composites, using carbon and glass fibers [4]. This chapter is focused on the synthesis and processing of neat, unfilled PEEK polymer. We begin by outlining the two main synthesis routes for contemporary PEEK. As a high-temperature thermoplastic, PEEK can be processed using a variety of commercial techniques, including injection molding, extrusion, and compression molding. This chapter provides an overview of the methods for processing unfilled PEEK Biomaterials Handbook. DOI: 10.1016/B978-1-4377-4463-7.10002-8 Copyright Ó 2012 Elsevier Inc. All rights reserved.
PEEK used in biomedical applications. Readers with an interest in PEEK composites may wish to skip ahead to Chapter 3, which covers blending of PEEK with additives.
2.2 Synthesis of PAEKs As noted previously, the polymerization of aryletherketones is a complex and challenging process due to the insolubility of PAEKs in typical solvents. Furthermore, the solvents and high temperatures necessary to carry out successful polymerization of PEEK, such as benzophenone or diphenylsulfone above 300 C, necessitate dedicated plant facilities with rigorous safety procedures (Fig. 2.1). Because of the precautions necessary to carry out safe polymerization, the reactions are typically carried out in batches, as opposed to in a continuous process. All these challenges contribute to the higher cost in producing PAEK polymers, when compared with other thermoplastics. Historically there are two main routes involved in the production of PAEKs. The first method involves linking aromatic ether species through ketone groups, whereas a second method involves linking aromatic ketones by an ether bond. The first method involves an electrophilic reaction and Friedel Crafts acylation chemistry, and the second route involves a nucleophilic displacement reaction.
2.2.1 Electrophilic Routes to PAEK Polymers The inherent solvent resistance and propensity to reach high crystallinity levels prevents PAEK
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Figure 2.1 PEEK production facility, Thornton-Cleveleys, United Kingdom. Source: Victrex.
polymers from being synthesized in common organic solvents. Early attempts to synthesize PEEK in methylene chloride or nitrobenzene produced only low-molecular-weight variants. Work by DuPont using a combination of anhydrous hydrogen fluoride/boron trifluoride succeeded in protonating the carbonyl groups and meant that high-molecular-weight polyetherketone (PEK) became a possibility (Scheme 2.1) [5]. Raychem also reported the synthesis of PAEK polymers using similar reaction conditions in the presence of alkylthiochloroformates. Another electrophilic process exemplified by Ueda and Oda uses methanesulfonic acid (MSA)/ phosphorus pentoxide (P2O5) at low temperatures [6]. Although PEEK produced by this method has a less branched structure than AlCl3-catalyzed systems, it also suffers from high temperature instability and hence cannot be molded or extruded without extensive cross-linking and degradation. Colquhoun and Lewis [7] have described the Friedel Crafts polycondensation of 4-(40 -phenoxyphenoxybenzoic acid) in trifluoromethanesulfonic
acid to form PEEK. This route has only remained of academic interest due to the extremely high cost and corrosive nature of the solvent used (Scheme 2.2). The electrophilic synthesis of PAEK polymers produces materials with reactive end groups such as benzoic acids. Such polymers cannot be processed, without endcapping, due to their high thermal
O O
C
Cl
HF / BF3
O *
O
Scheme 2.1
C
*
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Scheme 2.2
O O
n
O
C
OH
CF3SO3H O *
O
O
C
*
quantities in the finished polymer. The choice of the endcapping agent may therefore significantly alter the leachable and biocompatibility profile of the material.
instability. When the reactive end group materials are subjected to high-temperature processing, the polymer immediately cross-links, producing gels, which cannot be shaped into desired articles. Therefore, PEEK production by electrophilic processes as described earlier has historically had limited commercial success. More recently, a modification to the electrophilic process for manufacturing PAEK polymers has been described. This again involves the polycondensation of 4-(40 -phenoxyphenoxybenzoic acid). However, methanesulfonic acid was used as the reaction solvent in the absence of phosphorus pentoxide, and 1,40 -diphenoxybenzene was used as an endcapping agent [8]. This route permits the manufacture of thermally stable PAEK polymers and has been used in industrial processes (Scheme 2.3). It should be noted that to ensure thermal stability, significant quantities of the endcapping agent are used and as a result may be present in significant
2.2.2 Nucleophilic Routes to PAEK Polymers The nucleophilic route to PAEK polymers provides a straightforward pathway to polymers such as PEEK. Initial attempts to form high-molecularweight PAEKs from the reaction of a dihalobenzophenone and an equivalent bisphenate failed due to the polymer product crystallizing from the sulfolane solvent (Scheme 2.4). Owing to the poor solubility of PEEK, the selection of the synthesis solvent is crucial. Suitable solvents should be thermally stable and inert to phenoxide species. It became apparent that solvents such as benzophenone or diphenylsulfone could be Scheme 2.3
O O
n
C
O
OH
CH3SO3H O *
O
O
C
C6H6-O-C6H6 End Capping Agent O O
C
O
*
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O F
O
C
F
+
KO
C
OK
Sulfolane O *
O
C
*
Scheme 2.4
O F
C
F
HO
+
OH
Diphenylsulfone K2CO3 O *
O
O
C
*
Scheme 2.5
used in the synthesis of PAEK polymers [3]. The inherent instability of bisphenates to oxidation was overcome by the use of hydroquinone and sodium or potassium carbonate to form the bisphenate in situ. Very high temperatures (>300 C) are required to reach high molecular masses, the molecular weight being controlled by a slight excess of difluorobenzophenone, leading to fluorine-terminated chains (Scheme 2.5). This process was patented in 1977 by ICI and sold under the brand “Victrex PEEK,” and this route provided the majority of PEEK polymer used in industrial applications. The establishment of the nucleophilic route to PAEK polymers permitted the investigation of polymer variants by the use of different bisphenols to produce PAEK polymers with various properties, as reported by Attwood et al. [1]. The family of PAEK polymers grew to contain variants such as PEK, PEEK, PEKK, PEKEKK, and so on, with a range of glass transition temperatures (143e160 C) and high crystalline melt temperatures (335e441 C). As the dominant member of the PAEK family of polymers, PEEK is in its “glassy” state at room temperature, as its glass transition temperature occurs about 143 C,
whereas the crystalline melt transition temperature (Tm) occurs around 343 C.
2.3 Nomenclature The literature on PAEK resin is a maze of trade names and producers, which have changed over the years, complicating interpretation of reported data for today’s materials. For researchers interested in deciphering the historical polymer science literature, we provide here a brief primer on the nomenclature of PAEK resins used for industrial purposes as well as for biomaterials (Table 2.1). Resin, when used in this context, refers to the neat, unfilled powder that is created by polymerization, whereas grades are typically characterized by flow characteristics (e.g., for injection molding or compression molding) or based on their filler content (e.g., glass fiber or carbon fiber). Because PAEK polymers are converted using standard thermoplastic processing techniques, such as injection molding, they are generally available as pellets, although powder resin is also available. Stock shapes, such as rods, are also available from producers.
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Table 2.1 Summary of PAEK Materials Related to Implant Use Polymer
Trade Name
Producer
Comments
PEEK
OPTIMA (Biomaterial)
Invibio (subsidiary of Victrex), Thornton-Cleveleys, United Kingdom
Manufacturer and supplier of longterm implantable PEEK in CE and Food and Drug Administrationapproved devices since 1998
Invibio (subsidiary of Victrex), Thornton-Cleveleys, United Kingdom
Available only in experimental quantities (see Chapter 13)
PEK
PEEK
Victrex
Victrex, Thornton-Cleveleys, United Kingdom
Provides PEEK for blood/tissue contact less than 24 h
PEEK
Gatone
Gharda, India
No record of supplier implantation studies. Discontinued for medical use when acquired by Solvay in December 2005
PEEK
Keta-Spire
Solvay Advanced Polymers, LLC
Not available for implant use
PEEK
Zeniva
Solvay Advanced Polymers, LLC
Implantable grade available
PEEK
VESTAKEEP I
Evonik
Implantable grade available
PEKK
PEKK
DuPont, Wilmington, DE
Discontinued for medical use by DuPont
PEKK
OXPEKK
OPM, Enfield, CT
Implantable grade available. Base resins supplied by Cytec [9]
PEKEKK
Ultrapek
BASF, the United States
Discontinued in December 1995
Historically, PAEK materials, including PEEK, have been produced primarily as niche polymers for industrial use, because their cost even today is at least two orders of magnitude more expensive than lowtemperature thermoplastics such as polyethylene. When ICI launched Victrex PEEK in 1987, the primary application targets were not medical. However, Victrex PEEK was used, if not yet supported, for implant applications. The Victrex PEEK business was sold by ICI in 1993, and in 1998, Victrex launched PEEK-OPTIMAÒ for long-term implantable applications. The offering of PEEK-OPTIMA provided a higher specification product and was aimed at addressing the previous failings of PAEK polymers by offering long-term supply assurance agreements in addition to a policy of no change concerning the main characteristics of polymer properties. PEEK-OPTIMA was supported by drug and
device masterfiles and manufactured in compliance with Good Manufacturing Practice. In 2001, Victrex established Invibio Biomaterial Solutions to specifically provide grades of PEEK suitable for long-term implantation. In reviewing the historical PEEK literature, whether for industrial or biomedical applications, the reader should keep in mind that polymers from ICI, Victrex, and Invibio were all produced at the same plant location, although the name of the company has changed since that time. Similarly, the nomenclature for the resin grades has changed over time, but the polymerization technology has remained fundamentally similar (Table 2.2). Today, PEEK biomaterials are designated by the OPTIMA trade name based on their molecular weight, which governs their flow properties in the melt (Table 2.2). The same range of molecular weight of PEEK polymers was also previously available from Victrex.
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Table 2.2 Contemporary and Historical Nomenclature for Medical Grades of PEEK Property
General-Purpose Grade
Medium-Flow Grade
Easy-Flow Grade
Historical Victrex nomenclature
450
381
150
Invibio nomenclature
OPTIMA LT1
OPTIMA LT2
OPTIMA LT3
Melt flow index
3.4
4.5
36.4
Molecular weight (Mn)
115,000
108,000
83,000
LT1 Standard grade LT2 Optimized grade for melt strength and melt viscositydrecommended for tubing LT3 High-flow grade for injection molding thinwalled parts The most commonly used grade for PEEK is OPTIMA LT1, which has flow properties similar to 450. Victrex grades are also designated as PF (fine powder), P (powder), or G (granulated). Powder grades are recommended for compounding, whereas granulated resin is preferred for injection molding. Although powder grades are produced for industrial applications, there are no powder grades commercially available for implantable grade PEEKOPTIMA, only granules. PEEK-OPTIMA undergoes a melt filtration step as a quality control measure to ensure cleanliness and biocompatibility. Following melt filtration, the polymer is granulated into cylindrical pellets (Fig. 2.2). Thus, to obtain powder from PEEK-OPTIMA, it is necessary to mill or grind the granules to obtain the desired particle size. Recently, two new resin manufacturers entered the medical PEEK market. In 2007, Solvay announced an implantable grade of PEEK marketed under the Zeniva trade name. In 2009, Evonik began marketing an implantable grade of PEEK under the VESTAKEEP trade name. However, no reports have been published describing these resins in scientific studies or how they are used in long-term implants. Publication of further details about the performance of Solvay and Evonik materials is anticipated. PAEK alternatives to PEEK are available from Invibio and Oxford Performance Materials (OPM). PEK has been made available by Invibio in experimental quantities as a candidate biomaterial for
tribological testing (see Chapter 13). However, PEK has not been commercialized for implant manufacture. PEKK resins are produced by OPM (Enfield, CT) and have been marketed under the OXPEKK trade name since the company was founded in 2000. Both medical grades and implantable grades of OXPEKK are available. OPM was acquired by Arkema (Colombes Cedex, France) in 2009. Sustainability of biomaterial supply has been a concern with PAEK resins in the 1980s and 1990s. With the exception of PEEK-OPTIMA, which can trace its origins back to 1998, many industrial PAEK materials have been withdrawn from the market, either out of concern for liability, patent infringement, and concerns about viability of a niche market or due to technical difficulties (Table 2.1). Nonetheless, to the extent that biomaterials history is not fully reflected in the literature, Table 2.1 provides some guidance as to the current availability of PAEK materials for industrial and implant use.
2.4 Quality Systems for Medical Grade Resin Production PEEK-OPTIMAÒ biomaterials are manufactured under a Quality Management System certified to ISO 9001:2000 and ISO 13485:2003. Only fully approved raw materials are used at the production stages together with extensive supervision and checks at key production stages. The recording of key parameters is performed at all stages. PEEK-OPTIMA biomaterials are manufactured on a campaign basis, thus enabling the employment of contamination risk reduction procedures. Invibio embraces Good Manufacturing Practice in relation to
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(A)
(B)
Figure 2.3 Clean room production environment for MG PEEK. Photo courtesy of Invibio.
(C)
Enhanced Quality Control procedures and standards, together with extensive testing and product release control, ensure a tight product specification for PEEK-OPTIMA. Test results are verified by dual testing at an independent UKAS-accredited laboratory. Invibio has a “no-change” agreement for the longterm supply of PEEK-OPTIMA biomaterials to assure its specification and production methods over time.
2.5 Processing of Medical Grade PEEK
Figure 2.2 High-resolution optical micrographs of PEEK-OPTIMA LT1 granules (A, B) and representative scanning electron micrograph (C). SEM image courtesy of Josa Hanzlik, Drexel University.
the manufacture of PEEK-OPTIMA biomaterials, including clean room conditions for processing (Fig. 2.3). Process documentation is archived to provide long-term batch and raw material traceability.
Despite the exceptional properties of medical grade PEEK polymers, these materials are processed by traditional plastic processing methods (Table 2.3). medical grade PEEK-based materials exhibit melting temperatures of around 340 C; however, PEEK polymers demonstrate good melt stability and remain workable with most conventional process equipment between 360 and 400 C. Commercial manufacturers of implantable grade PEEK provide detailed guidance and support for all processing techniques. However, for students and researchers, a brief summary of the main techniques is discussed in the
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Table 2.3 Traditional Plastic Processing Methods and Applicability to PEEK Processing Method
Applicable
Injection molding
Yes
Extrusion (profiles, sheet, and monofilament)
Yes
Compression molding
Yes
Powder coating
Yes
following. A comparative plot of melt viscosity versus temperature for a range of conventional polymers in comparison with PEEK-OPTIMA LT1 is shown in Fig. 2.4. Although commercial PEEK materials are supplied nominally dry, the pellet form of the material typically absorbs 0.5% w/w atmospheric moisture. It is therefore recommended that for processing operations such as injection molding, the polymers should be dried to less than 0.02% w/w moisture. Typically, suitable drying of PEEK pellets can be achieved by 3 h of exposure in an air-circulating oven at 150 C. If the oven is only capable of lower temperatures, a longer drying time will be necessary (e.g., 12 h of exposure at 120 C). PEEK-OPTIMA materials are provided in a range of viscosities in relation to the processing technique Figure 2.4 Shear viscosity versus temperature for a range of thermoplastics. Source: Invibio.
used. As mentioned previously in this chapter, PEEKOPTIMA LT1 is recommended for the majority of machining and injection molding of medical device components. PEEK-OPTIMA LT2 demonstrates good melt strength with reduced viscosity and is therefore recommended for the extrusion of thinwalled parts such as tubing. PEEK-OPTIMA LT3 is especially preferred for the injection molding of thin-walled parts. The problem of machine wear is common to all engineering plastics, and it is therefore recommended that screws, dies, and barrels should be hardened to minimize wear, especially when processing fiber-reinforced materials.
2.5.1 Injection Molding Injection molding is an attractive manufacturing technique suitable for mass projection of PEEK implant components (Fig. 2.5). Injection molding is typically performed using pellets or granules, which are poured in a hopper in the machine. The pellets are then automatically introduced into a heated screw assembly that melts and pressurizes the molten polymer, so that it flows into a heated mold. Once the PEEK component has consolidated, it is automatically ejected from the mold, so that a new cycle can take place. Injection molds are optimized and specially designed for each part, taking into account the details of the part geometry and flow and pressure capabilities of the molten polymer injection system.
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Figure 2.5 Representative injection molding system for PEEK. Picture supplied with kind permission of Krauss Maffei Technologies.
Consequently, the cost of the designing and fabricating mold itself represents a significant financial investment for this process. However, after this investment has been made, multiple PEEK parts can be manufactured in their near-final shape with cycle times of the order of minutes. Figure 2.6 shows a tree of tensile test specimens immediately after injection molding. The cylindrical sprue, radial runners, and any extra flashing are trimmed to complete the part manufacturing process. Because of the up-front cost of a suitable mold, injection molding techniques are not suitable for prototyping or low-volume PEEK part production. Most standard reciprocating screw injection molding machines are capable of molding natural IG PEEK materials and also carbon fiber-reinforced PEEK materials (Fig. 2.5). Furthermore, specialized injection molders with extensive experience in manufacturing PEEK-based medical devices exist in Europe and the United States. Injection molding of PEEK typically requires barrel and nozzle temperatures in the region of 400 C. The recommended mold surface temperature for PEEK lies in the range of 175e205 C, and this is extremely important to ensure that molded parts demonstrate uniform levels of crystallinity. Failure to achieve the minimum
Figure 2.6 Injection-molded test specimens, joined by sprue and runners. Injection molding allows multiple PEEK components to be fabricated in near-final shape. High up-front tooling costs are the main drawback of injection molding. Photo courtesy of David Jaekel, Drexel University.
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Component design should also take into account the shrinkage that occurs as crystalline regions form within the cooling polymer melt. Shrinkage is therefore dependent on the level of crystallinity and therefore the mold and polymer melt temperature. If operating under recommended injection molding conditions, commercial grades such as PEEK-OPTIMA can produce consistent parts with dimensional tolerances as low as 0.05%. Commercial manufacturers of implantable PEEK materials provide full guidelines and technical support for molding PEEK medical components.
2.5.2 Extrusion Figure 2.7 Amorphous regions within an injectionmolded part as a result of low mold tool temperatures. Photo courtesy of Invibio.
mold temperature results in rapid cooling of the part, with insufficient time for crystallization, as demonstrated in Fig. 2.7. Additional detailed guidelines for injection molding and mold design for PEEK implants can be obtained from raw material suppliers [10]. It is possible to increase crystallinity by annealing amorphous or low-crystallinity PEEK moldings (Fig. 2.8). However, the process of crystallization may lead to distortion and dimensional changes.
Figure 2.8 Effect of annealing after processing on injection-molded test specimens. The amorphous sample was created by heating to 400 C and then immersed in liquid nitrogen. Image courtesy of David Jaekel, Drexel University.
Extrusion is a manufacturing process for producing long stock shapes, such as rods, sheets, and monofilament fibers (Fig. 2.9). PEEK pellets or granules are typically the starting, raw material for extrusion. Similar to injection molding, the pellets are poured into a hopper that feeds into a heated screw assembly that melts and pressurizes the molten polymer. The molten, pressurized polymer is then forced through a heated die and slowly cools to room temperature along an extrusion line. The extrusion of PEEK can be accomplished using conventional extrusion equipment and dies at similar temperatures as those described for injection molding. The drive motor typically requires a power output of 0.25 HP/kg h. This power requirement is similar to that required for polycarbonate, poly(ethersulfone) (PES), or high-molecular-weight polyolefins. PEEK is a very stable polymer and as such is not sensitive to prolonged exposure (up to 2 h) to temperatures above its melting point. However, for optimal results, residence times should be of the order of 5e10 min. Dead spots, areas of material hold within the barrel, should be avoided. Therefore, careful design of the screw and barrel is necessary. The extrusion of PEEK-OPTIMA stock shapes followed by machining remains the predominant method of manufacturing medical device components. PEEK-OPTIMA can be purchased in rod or plate form ranging from 6 to 150 mm in diameter for rod and 40 mm in plate thickness (Fig. 2.10). In addition to this, thin-walled implantable PEEK tubing is also available from commercial suppliers. The thermal processes involved in extrusion and annealing of stock shapes also result in slight mechanical property variances when compared with
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Figure 2.9 PEEK polymer extruder. Photo kindly supplied by Coperion GmbH, Stuttgart.
Figure 2.10 PEEK-OPTIMA rod stock, pellets, and powder.
injection-molded components. This is highlighted graphically in Fig. 2.11.
2.5.3 Compression Molding Compression molding is a manufacturing process for stock shapes, such as plates or thick sheets (Fig. 2.12). A compression molding press consists of
two heated platens. The lower platen contains a recess for the plate or sheet that is charged with resin powder or granules. The platens are then pressed together and heated to consolidate the resin. Compression molding is typically used for lowvolume production, prototyping, and evaluation work or in the production of industrial components with very thick sections. Compared with injection molding, it is a relatively inexpensive process, but cycle times are long and as such it is not suited to high-volume production. The process involves the heating and cooling down of the melt and tooling. Pressure is applied to the melt and maintained during the cool-down phase. This is a time-consuming process, particularly when section thicknesses are large. Compression molding of PEEK requires a heated press, an oven, and the tool, which can be low-grade steel/ metal due to the levels of stresses, shear, and forces involved. There is generally a preference for fine powder grade of PEEK polymer for compression
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Figure 2.11 Graphical representation of the differences between injection-molded and machined PEEK components. Source: Invibio.
molding to avoid granular boundary parts in molded parts. These boundary points may also represent weak points in the components. If controlled cooling over long time periods is practiced, then compression-molded parts typically have higher crystallinity and tensile strength than injectionmolded components. Although, in theory, milled fibers or fillers could be compounded into PEEK for compression molding, this is rarely practiced because of the difficulty in ensuring a uniform fiber distribution and therefore uniform properties in the final component.
2.5.4 Film and Fiber Production
Figure 2.12 PEEK polymer compression molding. The figure shows a 20-ton compression molding press (Rondol Kompress 20 T).
The use of implantable grade PEEK in fiber and film form is a growing area of interest in medical devices because of mechanical performance and inherent purity. The industrial applications of PEEK in these forms have been established for a number of years. Thin sections of PEEK film are produced by extruding polymer using a suitable die and haul-off equipment, which controllably handles and stores the film for secondary operations. PEEK film can be produced in either crystalline or amorphous forms by controlling the temperature of the casting drums in the haul-off equipment. However, as the thickness of the film increases, the production
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2.6 Machining
Table 2.4 Properties of a 0.5-mm PEEK Monofilament Material Property
Typical Value
Energy to break
1.24 J
Tensile load at 2% elongation
1.18 kg
Tensile load at 5% elongation
1.77 kg
Tensile load at 10% elongation
2.66 kg
Tensile load at break
5.81 kg
Elongation at break
22.0%
Knot strength
2.42 kg
of fully amorphous PEEK films becomes more challenging. PEEK monofilament can be produced by extrusion followed by drawing of the PEEK extrudate. Drawing the polymer provides orientation within the fiber prior to heat setting. The resultant monofilament is tough, highly oriented, and has a controlled diameter, which will retain its set form above the glass transition temperature of the material. Typical properties of a PEEK monofilament are shown in Table 2.4. PEEK monofilaments are distributed as multifilament yarns (Fig. 2.13) and can be woven into more complex three-dimensional shapes.
Often for prototype designs or short production runs, it is not economically viable to manufacture an injection molding tool. Under such circumstances, it is common to use compression molding or to machine PEEK-OPTIMA polymer materials to form components. PEEK-OPTIMA polymer may be machined and finished using the same techniques and equipment as for other engineering thermoplastics. However, because of the excellent physical properties and wear characteristics of these materials, it is advisable to use carbide or diamond tipped tools and bits. Machining and finishing operations on polymeric materials are prone to propagating molded-in or residual stresses. Before machining, it is recommended that components formed from PEEK are annealed to relieve stress. During machining or finishing, further stresses may be built up within the material by localized heating at the cutting point. Therefore, if a large amount of machining and finishing is to be carried out on a component, a second annealing procedure may be required. The thermal conductivity of all polymeric materials is lower than that of metals; hence, heat build-up during machining is rapid. It is advisable that a cooling fluid is used to remove some of the heat generated by working the material. Water or air jet cooling is generally recommended for medical grade PEEK polymer-based materials. A summary of the suggested machining conditions for PEEK-OPTIMA has been provided by Invibio [10].
2.7 Summary
Figure 2.13 Spool of technical grade, multifilament PEEK yarn.
As detailed in this chapter, there are various synthetic routes involved in the production of polyketone-based materials. These routes have been developed to overcome the initial challenges in relation to polyketone manufacturing, resulting in the consistent production of polymers for medical applications. It should be noted that the synthetic chemistry used in PAEK manufacture has a major influence on the thermal stability of the polymer in processing operations and on the leachable and volatile content of these polymers. Therefore, the manufacturing route has the potential to alter the biocompatibility of PAEK materials. The progress in manufacturing has been extended to processing where established methods have been developed to
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allow the production of reproducible components from a range of production techniques such as injection molding or machining of stock shapes. The relatively recent addition of a biomedical focus to manufacturing and supply offers device companies an opportunity to use these PEEK-based materials, in implants, with confidence in the quality and security of supply. In the following chapter, we turn to the combination of PEEK with additives to form polymer matrix composites.
Acknowledgments Special thanks to David Jaekel, Drexel University, for editorial assistance and to Chris Espinosa, Exponent, for assistance with figures. This chapter would not have been possible without helpful discussions and insights provided by John Devine, Ph.D., and Craig Valentine, Ph.D., from Invibio.
References [1] T.A. Attwood, P.C. Dawson, J.L. Freeman, L.R. Hog, J.B. Rose, P.A. Staniland, Synthesis
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and properties of polyaryletherketones, Polymer 22 (1981) 1096. [2] W.H. Bonner. US patent 3,065,205. 1962. [3] J.B. Rose. British patent 1,414,421. 1975. [4] D.P. Jones, D.C. Leach, D.R. Moore, Mechanical properties of poly(ether-ether-ketone) for engineering applications, Polymer 26 (1985) 1385e1393. [5] B.M. Marks. US patent 3,442,857. 1969. [6] M. Ueda, M. Oda, Synthesis of aromatic poly(ether ketones) in phosphorus pentoxide/ methanesulfonic acid, Polymer 21 (9) (1989) 673e679. [7] E.Q. Colquhoun, D.F. Lewis, Synthesis of aromatic polyetherketones in trifluoromethane sulfonic acid, Polymer 29 (1988) 1902e1908. [8] D.J. Kemmish. US patent 6,909,015. 2003. [9] R. Leaversuch, Materials close up: demand surge tightens PEEK supply. Available from: Plast. Technol. (7) (2001) http://www.ptonline. com/articles/200107cu200102.html [accessed May 2007]. [10] P.E.E.K. Unfilled, Optima Processing Guide: Invibio Biomaterial Solutions (2009). Available from: http://www.invibio.com [accessed September 7, 2010].