High-Temperature Engineering Thermoplastics

High-Temperature Engineering Thermoplastics

8 High-Temperature Engineering Thermoplastics: Polysulfones, Polyimides, Polysulfides, Polyketones, Liquid Crystalline Polymers, and Fluoropolymers 8...

4MB Sizes 0 Downloads 80 Views

8 High-Temperature Engineering Thermoplastics: Polysulfones, Polyimides, Polysulfides, Polyketones, Liquid Crystalline Polymers, and Fluoropolymers 8.1 Introduction High-temperature thermoplastic polymers have made significant inroads in medical device applications during the last 20 years. These materials are characterized by their high heat resistance ( . 200 C); their strength, toughness, and durability; their ability to withstand several cycles and doses of all types of radiation;

their ability to be molded into parts with extremely tight tolerances; their biocompatibility; and their longterm durability. The need for higher-temperature and higher-performance materials has led to the use of aromatic polysulfones (PSUs), aromatic polyimides, aromatic polyketones, and aromatic polysulfides in demanding components and applications like medical trays, surgical and dental instruments, medical electronic

CH3

O O

O

S O

CH3

n

Polysulfone (PSU)

O S

O

O

O

n Polyphenylsulfone (PPSU)

O O

S

n

O Polyethersulfone (PES)

O S

O

O

O

n Polyethersulfone (PES)

Figure 8.1 The structures of polysulfones.

Plastics in Medical Devices. DOI: http://dx.doi.org/10.1016/B978-1-4557-3201-2.00008-2 © 2014 Elsevier Inc. All rights reserved.

173

174

PLASTICS

CH3 n NaO

IN

MEDICAL DEVICES

O ONa

C

+

Cl

S

n Cl

O

CH3

CH3

O O

CH3

+

O

S

2n NaCl

n

O

Figure 8.2 Synthesis of polysulfones via a nucleophilic reaction.

components, drug delivery components, and machined parts. Fluoropolymers have been used in packaging, tubing, insulating materials, endoscopic, endocardial and endotracheal devices, catheter liners, and surgical instruments. High-performance engineering thermoplastics comprise about 8% of all plastics used in medical devices. This chapter will discuss the use of aromatic polysulfones, aromatic polyimides, aromatic polyketones, aromatic polysulfides, and various fluoropolymers.

8.2 Polysulfones (PSUs) Polysulfones are transparent, hydrolytically stable, amorphous thermoplastics. Their hydrolytic stability allows them to be used in applications that require repeated cycles of steam sterilization. Their stiffness, rigidity and toughness, and chemical and high heat resistance make them attractive candidates for the production of high-performance parts and products. They are

Table 8.1 Typical Properties of Polysulfones Property

Unit

Polysulfone (PSU)

Polyether Sulfone (PES)

Polyphenylene Sulfone (PPSU)

Density

g/cc

1.24

1.37

1.29

Light transmittance

%

70

70

80

Water absorption at equilibrium

%

0.5

2

1.2

Glass transition temperature



C

185

220

220

HDT at 0.46 MPa or 66 psi



C

175 185

215 220

215

HDT at 1.8 MPa or 264 psi



C

165 175

195 210

205 210

Softening point/melt temperature



C

185

215

190 200

Tensile strength

MPa

70

83

70

Elongation at break

%

50 100

30 90

60 120

Flexural modulus

GPa

2.7

2.9

2.4

Impact strength, notched, 23 C

J/m

50 70

60 85

690 700

Processing temperature



330-385

345 385

340 395

C

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

175

Thermal Property Comparison of Polysulfones 250 Tg (°C) HDT (°C)

Temperature (°C)

200

150

100

50

0 PSU

PES

PPSU

Figure 8.3 Thermal property comparison of polysulfones (HDT at 1.8 MPa).

inherently flame retardant, with low smoke emission. Applications of polysulfones include medical and surgical trays, dialysis membranes, surgical instruments, provisional trials, and device housings and components. The aromatic polysulfones are the polymers that are used in medical device applications. The three most common polysulfones are standard polysulfones (PSUs) polyether sulfones (PESs), and polyphenyl sulfones (PPSUs). Their structures are shown in Figure 8.1. The ether and alkyl linkages provide processability to these otherwise very high-temperature polymers.

8.2.1 Polysulfone Production Aromatic polysulfones can be produced by the reaction of bisphenol salts with 4,4-dichlorodiphenyl sulfone (4,4-DCDPS) (Figure 8.2). Reactions typically are conducted in high-boiling polar solvents like sulfolane, dimethyl sulfoxide (DMSO), and N-methyl pyrrolidone (NMP) at temperatures ranging from 100 to 250 C [1 3]. A co-solvent like chlorobenzene is also used. Chlorobenzene removes the water formed as an azeotropic mixture. The alkali salt that is formed is filtered and the resulting solution is cooled and poured into a nonsolvent like methanol. The polymer is precipitated, purified, and dried.

Comparison of Polysulfone Properties Impact Resistance 5 4 3

Gamma Sterilization

2

Thermal Stability

1

PSU

0

PES PPSU

Steam Sterilization

Hydrolytic Resistance

Organic Solvent Resistance

Figure 8.4 Property comparison of polysulfones.

176

PLASTICS

IN

MEDICAL DEVICES

Betadine

Lipids

Soaps/ Detergents

Disinfectants

Hydrogen Peroxide

Bleaches

Saline Water

Silicones

Oils/Greases

Ethylene Oxide

IPA

Acetone

MeCL2

MEK

THF

Dilute Acids

Polymer

Dilute Basses

Table 8.2 Chemical Resistance of High-Temperature Engineering Thermoplastics

High temperature thermoplastics Polysulfones

Good Good Fair

Poor Poor Poor Fair

Good Good Good Good Good Good Good Good Good Good

PPS

Fair

LCP

Good Good Good Good Good Good Good Good Good Good Good Good Fair

Good Good Good Good

PEI

Fair

Fair

PAI

Good Fair

PEEK

Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good

Good Good Good Good Good Good Good Good Good Good Fair

Poor Good Poor Poor Poor Good Good Good Good Good Fair

Good Good Good Good Good

Fair

Fair

Good Fair

Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good

All ratings at room temperature

8.2.2 Properties of Polysulfones

8.2.3 Chemical Resistance of Polysulfones

Polysulfones are transparent, amorphous polymers with high strength and high heat resistance. Their glass transition temperatures range from 180 C to 250 C. They are hydrolytically stable and (along with their heat resistance) are very stable to repeated cycles of steam sterilization. Their high aromatic content also makes them resistant to gamma and e-beam radiations. Polysulfones can be injection molded or machined into precision parts. Table 8.1 details their properties. Figure 8.3 compares the thermal properties of polysulfones. Standard polysulfones have slightly lower glass transition and heat deflection temperatures (at 264 psi/1.8 MPa) than PESs and PPSUs. This is because the latter two polymers have a higher aromatic content, which leads to higher heat resistance and stiffness. PPSUs have better toughness than polysulfones and PESs. This is reflected in its higher impact strength and elongation at break. The properties of the three types of polysulfones are compared in Figure 8.4.

Polysulfones are resistant to most aqueous acids and bases; however, they are not resistant to chlorinated organic solvents like chlorobenzene and dichloromethane, and ketones like acetone and methyl ethyl ketone (MEK; see Table 8.2). PPSUs and PESs have slightly better chemical resistance than the bisphenol A containing PSU. This is because polysulfones have an aliphatic isopropylidene group that lowers the chemical resistance of the material compared to the polysulfones with only aromatic groups. In addition, polysulfones will not stain when exposed to disinfectants, even after repeated steam or autoclave sterilizations.

8.2.4 Sterilization of Polysulfones Polysulfones are hydrolytically and thermally stable. They are suitable for both steam and autoclave sterilization. Polysulfones can also be sterilized with ethylene oxide (EtO), gamma, and e-beam radiation (Table 8.3).

Table 8.3 Sterilization of High-Temperature Engineering Thermoplastics Polymer

Steam

Dry Heat

Ethylene Oxide

Gamma Radiation

E-Beam

High temperature thermoplastics Polysulfones

Good

Good

Good

Good

Good

PPS

Good

Good

Good

Good

Good

LCP

Good

Good

Good

Good

Good

PEI

Fair

Fair

Good

Good

Good

PAI

Fair

Fair

Good

Good

Good

PEEK

Good

Good

Good

Good

Good

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

177

Hydrolytic Stability of Polysulfone Resins (Boiling Water Immersion for 10 Days) 120% Tensile Elongation

Percent Property Retention (%)

Tensile Strength 100%

80%

60%

40%

20%

0% PSU

PES

PPSU

Figure 8.5 Hydrolytic stability of polysulfones.

Figure 8.5 shows that they retain over 80% of their tensile strength after immersion in boiling water for 10 days. PSU and PES lose over 80% of the tensile elongation, but PPSU retains 99% of the original value [4]. These changes are reflected in the number of steam sterilization cycles that these materials can withstand (Figure 8.6). PSU and PES can withstand up to 100 cycles of steam sterilization before crazing, cracking, or rupturing. The PPSU, on the other hand, can withstand up to 1,000 cycles of steam sterilization without losing any of its physical and mechanical properties [4].

With their high aromatic content, polysulfones have excellent stability when exposed to gamma and e-beam radiations [5,6]. Figure 8.7a c shows that all three types of polysulfones retain close to 100% of their properties even after exposure to 100 kGy of radiation [4,7].

8.2.5 Polysulfones Biocompatibility Polysulfones are used in filtration membranes, hemodialysis, and implants. The hydrophobic nature of the polysulfones results in “membrane-fouling”;

Steam Autoclave Capability of Polysulfone Resins (132C, 2 bar) 1000

Cycles to Crazing

900

Cycles to Rupture

Number of Cycles

800 700 600 500 400 300 200 100 0 PSU

PES

Figure 8.6 Steam and autoclave sterilization capability of polysulfones.

PPSU

178

PLASTICS

(a)

Gamma Radiation Stability of Polysulfone (PSU)

Percent Tensile Strength Retention (%)

120% 100% 80% 60% 40% 20% 0% 50

75

100

Radiation Dose (kGy) Gamma Radiation Stability of Polyether Sulfone (PES)

(b)

Percent Tensile Strength Retention (%)

120% 100% 80% 60% 40% 20% 0% 40

60

80

Radiation Dose (kGy) Gamma Radiation Stability of Polyphenylene Sulfone (PPSU)

(c)

Percent Tensile Strength Retention (%)

120% 100% 80% 60% 40% 20% 0% 50

75 Radiation Dose (kGy)

Figure 8.7 Gamma radiation of polysulfones (a) PSU, (b) PES, (c) PPSU.

100

IN

MEDICAL DEVICES

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

179

Effect of polysulfone hydrophilicity on cell adsorption 100

Adsorption (%)

80

60

40

20

Osteoblasts Adsorption Fibroblasts Adsorption

0 0

10

20

30

40

50

60

70

80

90

Water contact angle (°)

Figure 8.8 Effect of hydrophilicity on protein adsorption of polysulfone surfaces.

i.e., the adsorption of proteins on the membranes, leading to ineffective filtration. The surfaces can be made hydrophilic by surface treatments using hydrophilic materials like polyvinyl pyrrolidone (PVP). PVP-treated polysulfones are used commercially in various types of dialysis membranes. Figure 8.8 shows the effect of hydrophilicity on the biocompatibility of Polysulfones. A polysulfone was irradiated with ultraviolet (UV) light and subsequently treated with hydrogen peroxide plasma. The contact angles of the surfaces were measured [8]. The lower the contact angle, the more hydrophilic the surface is, resulting in lower adsorption of cells and improved filtration capability. Figure 8.9 shows the biocompatibility of a PVPtreated PES. The normalized white blood cells and

the platelet counts are comparable to the original cell counts [9], again indicating low cell adsorption even after 3 hours of treatment.

8.2.6 Joining and Welding of Polysulfones Polysulfones can be joined by ultrasonic welding, heated tool welding, vibration welding, infrared welding, and spin welding. High-frequency welding techniques are not suitable for polysulfones. Solvent bonding with solvents like NMP, dimethylacetamide (DMAC) and N,N-dimethyl formamide (DMF) can be used. Care should be taken to ensure that there is no environmental stress cracking. Acrylic and epoxy adhesives are also suitable for bonding polysulfones.

Biocompatibility of PVP-treated polyether sulfone (PES)

Normalized counts (%)

120 100 80 60 40 White Blood Cells Platelets

20 0 0

50

100 Treatment time (min)

Figure 8.9 Biocompatibility of polyether sulfones.

150

200

180

PLASTICS

IN

MEDICAL DEVICES

Table 8.4 Medical Device Applications of Polysulfones Application

Requirements

Material

Dialysis membrane

Permeability

PSU, PES

Filtration capability Narrow pore size distribution Biocompatibility Hemocompatibility Low protein adsorption Mechanical strength Gamma sterilization Processability Reusable cases and trays

High impact strength

PPSU

Chemical resistance to disinfectants Repeated steam sterilization Dimensional stability Colorability Thermoformability Connectors

Dielectric properties

PSU

Dimensional stability Impact resistance Chemical resistance Colorability Repeated steam sterilization Dental impression gun

Stiffness and strength

PSU

Dimensional stability Colorability Chemical resistance Repeated steam sterilization Suction jars

Transparency

PSU

Impact resistance Durability Chemical resistance to disinfectants Repeated steam sterilization Dental picks (handles)

Stiffness and strength

PPSU

Dimensional stability Compatibility with silicones Chemical resistance Colorability (Continued )

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

181

Table 8.4 (Continued) Application

Requirements

Material

Provisional trials

Dimensional stability

PPSU

Repeated steam or gamma sterilization Colorability Durability Chemical resistance to soaking, cleaning, and disinfecting agents Medical drawers

Toughness

PPSU

Heat resistance Hydrolytic stability Repeated steam sterilization Dimensional stability Heart valve transportation unit

Biocompatibility

PPSU

Chemical resistance Transparency Colorability Toughness and impact resistance Stiffness and strength

Disposable filter devices

Gamma sterilization

PES

Low protein binding Superior filtration capability Large surface area and low hold-up volume

8.2.7 Polysulfones—Applications The ability of polysulfones to withstand steam and other high-heat sterilizing methods makes them an attractive choice for use in a wide variety of medical products. They are tough, strong, and transparent. Because of these properties, polysulfones are used in products like surgical tool trays, nebulizers, humidifiers, flow controls, instrument housings, dental and surgical instruments, fluid containers, heart valve cases, pacemakers, respirators, blade disposal systems, molding cases for soft contact lenses, microfiltration apparatus, dialysis membranes, and other kinds of lab equipment. Those applications or products that require higher heat utilize PESs and PPSUs (Table 8.4).

8.3 Polyimides Polyimides are amorphous or crystalline hightemperature materials. They have excellent mechanical

and dielectric properties. Polyimides are also selfextinguishing and can be used in applications that require high temperature resistance and flame retardance. Such applications include wire and cable sheathing in aerospace applications, flame-retardant protective equipment, and films. Aromatic polyimides are more commercially viable than the aliphatic polyimides. This is because aromatic polyimides have very high temperature resistance, excellent dielectric properties, and good mechanical properties. Wholly aromatic polyimides are typically not melt processable. They are formed into films and fibers using solutions. Sulfuric acid is a typical solvent that is used in these processes. This section will discuss thermally processable polyimides. Their structures are modified by the incorporation of flexible alkyl or ether links in the polymer backbone. These flexible links lower the melting points of the polymers, which can be processed using processes like injection molding and extrusion.

182

PLASTICS

IN

MEDICAL DEVICES

O

O

CH3

N

N

O

n

O

O

O

CH3

O

O

O

N

N

n O

O (Polyetherimides)

Figure 8.10 Structures of thermally processable aromatic polyetherimides.

Structures of these thermally processable aromatic polyimides are given in Figure 8.10.

synthesized by the reaction of an aromatic diamine and an aromatic diacid or dianhydride (Figure 8.11). Polyetherimides (PEIs) are manufactured by the reaction of a flexible anhydride and 1,3-diamino benzene (Figure 8.12) [10]. The anhydride is based on bisphenol A and has both ether and isopropylidene links. It is these two flexible links that provide

8.3.1 Polyetherimides (PEI) Production Polyimides are typically produced by the reaction of a diacid and a diamine. Aromatic polyimides are O

O

n O

+

O

Ar

n H2N

Ar

O

O Heat (–2n H2O)

O

O

N

N

Ar

Ar n O

O

Aromatic polyimide (Ar = aromatic groups)

Figure 8.11 Production of aromatic polyimides.

NH2

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

183

O

O

NH2

H2N CH3

n O

+ n

O O

O O

O

CH3 Heat (–2n H2O) O

O

CH3

N O O

N n

O O

CH3

Figure 8.12 Production of thermally processable polyetherimides.

the flexibility and melt processability to PEI compared to its wholly aromatic analogs.

8.3.2 Properties of Polyetherimides Polyetherimides are transparent, amorphous, hydrolysis-resistant, high-temperature polymers. Their advantage over other wholly aromatic polyimides is that they are melt processable, so they can be molded or extruded into a variety of shapes, parts, and films. Their high aromatic content also makes them radiation stable. Table 8.5 details the properties of a PEI.

8.3.3 Chemical Resistance of Polyetherimides PEIs are resistant to dilute acids but will degrade and hydrolyze in dilute bases. They are resistant to lipids, cyclic ethers, hydrocarbons, and some ketones (Table 8.2). Polyetherimides, however, are susceptible to stress cracking when exposed to chlorinated solvents, oxidizing agents, and bleach. Figures 8.13a and b show the chemical resistance of PEI when exposed to various solvents under strains of 0.5% and 1.5%, respectively [11]. Figure 8.14 also shows that PEIs are resistant to lipids [11].

8.3.4 Polyetherimides Sterilization PEIs can be sterilized by steam, autoclave, ethylene oxide, and high-energy radiation (Table 8.3). Steam

sterilization can be used up to about 100 cycles. Figure 8.15a shows that PEIs can be used up to 600 cycles before cracking [12]. After about 100 cycles, PEI’s properties drop off drastically (Figure 8.15b) [11]. PEIs can be sterilized by ethylene oxide and high-energy radiations like gamma and e-beam without losing their physical and mechanical properties. Table 8.5 Properties of a Polyetherimide Property

Unit

PEI

Density

g/cc

1.27

Water absorption (24 h)

%

0.25

Glass transition temperature



C

215

HDT at (0.46 MPa or 66 psi)



C

210

HDT at (1.8 MPa or 264 psi)



C

201

Softening point



C

220

Tensile Strength at break

MPa

115

Elongation at break

%

60 80

Flexural modulus

GPa

3.5

Impact strength, notched, 23 C

J/m

25 60

Processing temperature



365 375

C

184

PLASTICS

(a)

IN

MEDICAL DEVICES

Chemical Resistance of Polyetherimide at 0.5% Strain

Percent Property Retention (%)

180% 160%

Tensile Strength

140%

Tensile Elongation

120% 100% 80% 60% 40% 20%

pr

Fo

rm

al

in

ol

hl

op

or

an

ite

e oc

Is o

yp

So

di

91

um

%

H

70

%

Be ta d

Et ha

in

no

in Sa l

C

ID

0. 9%

EX

ID

l

e

s Pl u

7 EX

ZO L C

C

EN

on

tro

l

0%

Chemical Resistance of Polyetherimide at 1.5% Strain

(b) Percent Property Retention (%)

180% 160%

Tensile Strength

140%

Tensile Elongation

120% 100% 80% 60% 40% 20% l

in al rm

91

%

Is

Fo

pa ro op

oc yp H

um So

di

no

ite hl

di ta Be

Et 70

%

or

ne

l no ha

lin Sa 9%

ID C

0.

EX

Pl

EX ID C

e

us

7

L ZO EN

C

on

tro

l

0%

Figure 8.13 Chemical resistance of polyetherimides under strain. (a) 0.5% strain, (b) 1.5% strain.

They are stable up to radiation doses of 80 kGy, retaining up to 80% of their properties, after which rapid degradation occurs, as shown in Figure 8.16[11].

8.3.5 Polyetherimides Biocompatibility PEIs did not produce any toxic responses in cell culture testing and also were found to be hemocompatible [13]. Studies using PEI membranes showed similar activity and behavior to tissue culture polystyrene (TCPS) [14]. The levels of alkaline phosphatase (ALP), prostaglandin E2 (PGE2), and transforming growth factor β1 (TGF-β1) cytokines produced by MG63 cells on the prepared PEI membranes were

similar to those on TCPS on either surface of the membrane. Biocompatible grades of PEIs are available.

8.3.6 Joining and Welding of Polyetherimides PEIs can be joined by various welding techniques like heated tool welding, ultrasonic welding, and vibration welding. Solvents like methylene chloride and tetrahydrofuran can be used in solvent bonding PEIs to other plastics like polyvinyl chloride. Most adhesives like epoxies, phenolics, and acrylics can be used to bond PEIs to various plastics. PEI parts can be joined with almost all common assembly methods.

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

185

Lipid Resistance of Polyetherimide at 2% Strain 120% Percent Property Retention (%)

Control

20% Intralipid

100% 80% 60% 40% 20% 0% Tensile Strength

Tensile Elongation

Figure 8.14 Lipid resistance of polyetherimides.

(a)

Effect of Steam Sterilization Under Stress for Polyetherimide

Number of Cycles before breaking or cracking

700 600 500 400 300 200 100 0 0%

0.6%

1.2%

Strain (%) (b)

Effect of Autoclave Sterilization on the Properties of Polyetherimide 120%

Percent Property Retention (%)

100% 80% Unnotched Izod Impact Strength Tensile Strength

60% 40% 20% 0% Control

2000 cycles

3000 cycles

Figure 8.15 Steam sterilization capability of polyetherimides. (a) Number of cycles to breaking under stress, (b) property retention.

186

PLASTICS

IN

MEDICAL DEVICES

Polyetherimide gamma radiation stability Percent property retention (%)

120% 100% 80% 60% 40% Tensile Strength Impact Energy

20% 0% 0

20

40

60

80

100

120

Dose (kGy)

Figure 8.16 Gamma sterilization capability of polyetherimides.

8.3.7 Polyetherimides— Applications PEIs are used in storage trays, drug delivery components, and tubing, as shown in Table 8.6.

8.4 Polyamide-Imides (PAIs) Another thermally processable polyimide is polyamide-imide (PAI), whose structure is shown in Figure 8.17. Like PEIs, the amide link in the polymer chain makes this material melt processable. PAIs have excellent strength and stiffness, wear resistance, and friction properties. They can be used in applications like surgical instruments and instrument components and parts that require tight tolerances and dimensional stability.

8.4.1 Production of PAIs

Glass-reinforced grades provide even more strength, stiffness, and durability for high-performing parts, with tight tolerances and dimensional stability. Table 8.7 details the properties of both unfilled and a 30% glass-reinforced PAI (GF-PAI).

8.4.3 Chemical Resistance and Sterilization of PAIs PAIs are resistant to most dilute acids and bases and most organic solvents (Table 8.2). Strong bases will degrade the polymer. Like PEIs, PAIs are capable of steam sterilization for up to 100 cycles. They can be sterilized by ethylene oxide, gamma, and e-beam radiation. Figure 8.19 shows that PAIs retain over 85% of their properties when exposed to very high doses (1,000 kGy or more) of gamma radiation, far above typical radiation doses encountered in medical device applications [15].

PAIs are produced by the reaction of trimellitic anhydride with an aromatic diamine, as shown in Figure 8.18. The reaction can be conducted in a high-boiling solvent like NMP or dimethyl acetamide or using heat and a vacuum to remove the water formed. The polymer is precipitated (if the synthesis is conducted in solution), purified, and dried.

8.4.4 Joining and Welding of PAIs

8.4.2 Properties of PAIs

The high strength, stiffness, and wear resistance of PAIs make them viable candidates for gears, bearings, pump housings, and hydraulic components, as well as for metal replacement. They are also used for micromolding and production of miniaturized parts due to their high heat resistance and high strength and excellent dimensional stability.

PAIs have the highest strength of any unreinforced polymer with excellent creep resistance. They have good stiffness and strength and excellent wear and friction properties. These polymers are also inherently flame retardant and have good chemical resistance.

Epoxy adhesives are an excellent option for the joining of PAIs. They also can be joined by assembly techniques like snap-fit assemblies and self-tapping screws.

8.4.5 PAIs—Applications

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

187

Table 8.6 Medical Device Applications of Polyetherimides Application

Requirements

Stapler

High mechanical strength EtO and gamma sterilization Biocompatibility Dimensional stability Stiffness and mechanical strength Colorability

Storage, sterilization case

Repeated steam sterilization Colorability Strength and toughness Dimensional tolerance and stability Thin-wall molding capability

Tubing

N

Ar

HN

O

O

n

Polyamide-imide

Figure 8.17 Structures of polyamide-imides.

Table 8.8 details a few medical device applications that use PAIs.

Dimensional stability

Impact resistance Surgical base cover

O

Transparent Flexibility

8.5 Polyphenylene Sulfide (PPS) Polyphenylene sulfides (PPSs) are wholly aromatic sulfides (Figure 8.20). They are semicrystalline and opaque materials with very high heat resistance, high strength and stiffness, and chemical resistance. Glass reinforcement improves the dimensional stability, heat resistance, strength and stiffness, and impact resistance compared to the unfilled material. Medical applications (typically using glass reinforced grades) include surgical instruments and device components and parts that require high dimensional stability, strength, and heat resistance. PPS fibers also are used in medical fabrics and membranes.

High burst strength High heat and chemical resistance Durability Steam, EtO, and gamma sterilization Pipette cans

Transparent Toughness and durability Repeated steam and autoclave sterilization Cleanability

Drug delivery components

Chemical and lipid resistance

8.5.1 Production and Properties of PPSs PPSs are manufactured by the reaction of 1,4dichlorobenzene and sodium sulfide in a solvent (Figure 8.21). The polymer formed is filtered, purified, and dried. PPS can be produced in linear, branched, or cross-linked forms. The linear polymer (with low branching) typically is used in injection molding and extrusion applications. Properties of unfilled and 40% glass-filled PPS (GF-PPS) are given in Table 8.9. The heat distortion temperature (HDT) and the stiffness (flexural modulus) increase significantly with the incorporation of the glass filler.

Transparency Colorability Dimensional stability toughness Moldability

8.5.2 Chemical Resistance of PPSs There is no known solvent for PPS under 200 C. The polymer is resistant to most chemicals,

188

PLASTICS

IN

MEDICAL DEVICES

O

n

O

+

n H2N

NH2

Ar

HO O

O

Heat –2n H2O

O

N

Ar

HN n

O

O

Polyamide-imide H Ar = H2N

H2N

NH2

NH2

,

, H2N

C

NH

, H2N

NH2

O

H

Figure 8.18 Production of polyamide-imide.

acids, and bases (Table 8.2, Figure 8.22a) [16]. Oxidizing agents like concentrated nitric acid will attack and degrade the polymer, as shown in Figure 8.22b [17].

8.5.3 Sterilization of PPSs PPS can be sterilized by steam, autoclave, ethylene oxide, and radiation (Table 8.3). The physical properties are retained after several cycles of steam

Table 8.7 Properties of Unfilled and Reinforced Polyamide-Imides Property

Unit

PAI

30% GF PAI

Density

g/cc

1.42

1.6

Water absorption (24 h)

%

0.3

0.3

Glass transition temperature



C

280

275

HDT at (0.46 MPa or 66 psi)



C

280

280

HDT at (1.8 MPa or 264 psi)



C

275

285

Softening point



C

260

260

Tensile strength

MPa

150 155

160 170

Elongation at break

%

7 20

4

GPa

5

6.9

Impact strength, notched, 23 C

J/m

100 150

65 75

Processing temperature



310 375

315 375

Flexural modulus 

C

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

189

Gamma Radiation Stability of Polyamide-imides Percent Property Retention (%)

120% 100% 80% 60% 40% Tensile Strength Elongation

20% 0% 0

2000

4000

6000

8000

10000

12000

Radiation Dose (kGy)

Figure 8.19 Gamma radiation stability of polyamide-imides.

sterilization and several doses of high-energy radiation (Figure 8.23) [18].

8.5.5 PPS—Applications PPSs are used in surgical instruments, valves and filters, as shown in Table 8.10.

8.5.4 Joining and Welding of PPSs PPS can be joined by vibration welding and ultrasonic welding. Ultrasonic welding produces joints with excellent weld strengths. Unfilled grades weld better than glass-reinforced grades. PPS can be joined with adhesives like epoxies, acrylics, and cyanoacrylates. Typical mechanical fastening techniques can also be used.

8.6 Polyarylether ketones Polyether ether ketones (PEEK) belong to the polymer family of polyaryletherketones. These polymers are semicrystalline, aromatic polymers with ether and ketone links in the main chain. Structures of some polyaryletherketones are given in

Table 8.8 Polyamide-Imide Medical Device Applications Application

Requirements

Material

Micromolded parts for cardiovascular repair procedures

Tight dimensional tolerances

Polyamideimide with added fluoropolymer

Stiffness and strength Radiation sterilization Wear resistance

High-speed rotary micro-component for pumps

Light weight Tight tolerances Dimensional stability Low friction, high wear resistance High temperature resistance Hydrolytic stability Steam and gamma sterilization Micromolding

Polyamideimide

190

PLASTICS

IN

MEDICAL DEVICES

and applications. This section will focus on PEEK, the most widely used of the polyaryletherketones in medical device applications.

S n

Figure 8.20 Structure of polyphenylene sulfide.

8.6.1 Polyaryletherketone Production

Figure 8.24. The aromatic rings provide stiffness, heat resistance, and radiation resistance. The ether and ketone links provide flexibility and thermal processability to these very high-temperature ketones. These polymers have very high temperature resistance, chemical resistance, and exceptional dimensional stability. They are chemically inert, possess very low extractables, and are biocompatible. They are used in several long-term implant applications like spinal and dental implants. Their strength and stiffness allow them to be used in load-bearing applications and surgical instruments that require repeated cycles of sterilization. With the incorporation of fillers and reinforcements like glass and carbon fiber, the mechanical properties of these materials can be tailored to a wide range of performance requirements

Polyarylketones are typically produced by the reaction of an aromatic dihalide (chloride or fluoride) with a di-phenol. PEEK is synthesized by the reaction of difluorobenzophenone and hydroquinone in the presence of sodium hydroxide (Figure 8.25). The solvent is usually a high-boiling aromatic sulfone. The polymer precipitates after reaching a certain molecular weight. This method limits the molecular weight of the polymer and thus affects the polymer’s physical and mechanical properties. A second method is by the Friedel-Crafts acylation method using a catalyst like boron trifluoride, as shown in Figure 8.26 for PEEK. The solvent is hydrofluoric acid and the reaction is conducted at room temperature. The polymer formed, remains in solution till it is precipitated, purified, and dried.

Polyphenylene sulfide

Cl + n Na2S

n Cl

S

+ 2n NaCl n

Figure 8.21 Production of polyphenylene sulfide.

Table 8.9 Properties of Polyphenylene Sulfide (PPS) Property

Unit

PPS

40% GF PPS See section 8.5.1

Density

g/cc

1.35

1.6

Water absorption (24 h)

%

0.01

0.02

Glass transition temperature



C

75 85

75 85

HDT at (0.46 MPa or 66 psi)



C

205

280

HDT at (1.8 MPa or 264 psi)



C

120 130

265

Melting point



C

285

285

Tensile strength at break

MPa

80 95

200

Tensile elongation at break

%

3 8

2

GPa

3.9 4.1

13.8

Impact strength, notched, 23 C

J/m

0.2 0.3

1.1

Processing temperature



300 315

315 345

Flexural modulus 

C

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

191

Chemical resistance of polyphenylene sulfide

(a) Percent Tensile Strength Retention (%)

120%

100%

80%

60%

40%

20%

0% Control

Oil @ 120°C/40 days

10% HCl @ 80°C/180 days

Water @ 95°C/120 days

Chemical Resistance of Polyphenylene sulfide (nonwoven fabric)

(b)

Percent Property Retention (%)

120% 100% 80%

Nitric acid Hydrochloric acid

60%

Sulfuric acid

40% 20% 0% 0

1

2

3

4

5

Concentration (mol/liter)

Figure 8.22 Chemical resistance of polyphenylene sulfide. (a) Oils and chemicals, (b) acids. Gamma Radiation Stability of 40% Glass filled Polyphenylene sulfide Percent Flexural Modulus Retention (%)

120% 100% 80% 60% 40% 20% 0% Control

3000 Radiation Dose (kGy)

Figure 8.23 Gamma radiation stability of polyphenylene sulfide.

5000

6

192

PLASTICS

IN

MEDICAL DEVICES

O O

C n

Polyether ketone (PEK) O O

O

C n

Polyetherether ketone (PEEK)

O

O

O

C

C n

Polyetherketone ketone (PEKK)

O O

O

C

O

O

C

C n

Polyetherketoneetherketone ketone (PEKEKK)

Figure 8.24 Structures of polyaryletherketones.

8.6.2 Properties of Polyaryletherketones Polyaryletherketones are semicrystalline polymers and have very high strength, stiffness, and dimensional stability. They are also resistant to high heat, chemicals, hydrolysis, and high-energy radiation. Polyaryletherketones have excellent electrical properties over a wide range of temperatures. Carbon fiber and glass-reinforced grades provide additional heat resistance, strength, stiffness, and wear resistance. Table 8.11 gives the properties of unfilled PEEK, PEKK and a carbon fiber filled PEEK (CF-PEEK); see Figure 8.24 for acronyms. The higher aromatic content in PEKK and PEKEKK is reflected in their higher glass transition temperatures and melt temperatures compared to PEEK (Figure 8.27).

8.6.3 Chemical Resistance of Polyaryletherketones PEEK is resistant to most chemicals (Table 8.2). Concentrated sulfuric acid will degrade the material over time. Strong oxidizing agents like nitric acid

and halogenated organic solvents like chlorobenzene will swell and degrade the polymer. PEKK is extremely hydrolytically resistant. Figure 8.28 shows the excellent chemical resistance of PEEK to acids, bases, and organic solvents [19]. ONa O C n

+

n F

F

ONa

–2NaF

O O

C

O n

Figure 8.25 Synthesis of PEEK—method 1.

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

O O

C Cl

n O

Catalyst

steam and autoclave sterilization. Figure 8.29 shows that PEEK can retain 100% of its mechanical properties after being exposed to steam for several thousand hours [20,21]. Figure 8.30 shows that PEEK retains 100% of its mechanical properties even after 75 kGy of gamma radiation [20].

8.6.5 Polyaryletherketone Biocompatibility O

O

193

C

O

n

Figure 8.26 Synthesis of PEEK—method 2.

8.6.4 Polyaryletherketone Sterilization PEEK can be sterilized by steam, autoclave, ethylene oxide, and high-energy radiation (Table 8.3). Its hydrolytic stability and high heat resistance enables the material to go through several hundred cycles of

PEEK continues to grow in the area of orthopedic, trauma, and spinal implants [22,23]. Its high strength, stiffness, biocompatibility, and durability make them extremely attractive candidates for implants. One major application is in spinal implants. The strength and stiffness of a carbon fiber filled PEEK (CF-PEEK) matches the strength and stiffness of cortical bone (Figure 8.31) [24]. This makes carbon fiber filled PEEK an excellent choice for spinal implants. For applications like spinal implants, PEEK must be biocompatible and also be compatible with bone material. PEEK does not deleteriously affect osteoblasts and fibroblasts [25]. Ethanol and chloroform extracts of PEEK were evaluated for mutagenicity and toxicity using the Ames test. PEEK does not induce any mutagenicity or toxicity, as shown in

Table 8.10 Polyphenylene Sulfide (PPS) Medical Device Applications Application

Requirements

Material

Surgical forceps

Dimensional stability

40% GF PPS

Tight tolerance Strength and stiffness Repeated sterilization Low moisture absorption Valves

Strength

PPS

Dimensional stability Tight tolerance Excellent mechanical properties Durability Steam, EtO, or gamma sterilization Chemical resistance Filters

Heat resistance Chemical resistance Durability Gamma and EtO sterilization

PPS

194

PLASTICS

IN

MEDICAL DEVICES

Table 8.11 Properties of Polyaryletherketones Property

Unit

PEEK

PEKK

PEKEKK

30% CFPEEK

Density

g/cc

1.31

1.31

1.3

1.41 1.44

Water absorption (24 h)

%

0.5

, 0.2

, 0.5

0.06

Glass transition temperature



C

145

163

162

145

HDT at 0.46 MPa or 66 psi



C

160







HDT at 1.8 MPa or 264 psi



C

260 280

175

172

280 315

Melting point



C

334

360

387

340

Tensile strength at break

MPa

90 110

110

115

200 220

Elongation

%

20 40

10

20

1 5

Flexural modulus

GPa

4.1

4.55

4.1

13 19

Impact strength, notched, 23 C

J/m

55 65

69

60

54

M100 (R126)

M88



M70 M105

345 390

345 370

375 395

350 400

30 35

25 30

10 25



Hardness rockwell Processing temperature



Degree of crystallinity

%

C

Figures 8.32a d using the hypoxanthine-guaninephosphoribosyl-transferase (HPRT) test. PEEK exhibited no activity compared to that of the negative control. The positive control had a 10- to 100-fold increase in cell production [26]. The Ames V-79 test also showed no mutagenic effects even for concentrations as high as 5.0 µg/ml of PEEK (Figure 8.33). All PEEK test materials were comparable to the negative control, whereas the positive control had a 10-fold increase in mutagenic effects [26].

8.6.6 Joining and Welding of Polyaryletherketones PEEK can be welded by heated tool welding, ultrasonic welding, and laser welding. Due to the high melting points of PEEK, significant amounts of energy must be put into the material during welding in order to achieve good weld strength. Light curing acrylic and cyanoacrylate adhesives work well with PEEK, as do epoxy adhesives.

Thermal Properties of Various Polyaryletherketones 450 400 350

PEEK PEKEKK

Temperature (°C)

PEKK 300 250 200 150 100 50 0 Glass Transition Temperature

HDT @ 1.8 MPa

Figure 8.27 Thermal property comparison of polyaryletherketones.

Melting Point

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

195

Chemical Resistance of PEEK

Percent Property Retention (%)

110%

100% 50% Sulfuric acid

90%

45% Potassium hydroxide Acetone Methy ethyl ketone

80%

Methylene chloride Toluene

70%

Ethanol

60%

50% Modulus

Tensile Strength

Figure 8.28 Chemical resistance of PEEK. Mechanical properties of PEEK after steam sterilization Percent Property Retention (%)

120% 100% 80% 60% 40%

Tensile Strength Flexural Modulus

20% 0% 0

500

1000

1500

2000

2500

3000

3500

4000

Number of Hours of Exposure

Figure 8.29 Steam sterilization capability of PEEK.

8.6.7 Polyaryletherketones— Applications Medical device applications of polyaryletherketones take advantage of their high heat resistance, stiffness, strength, and dimensional stability in addition to their chemical, hydrolytic, and radiation resistance. Medical applications include scalpels, angioplasts, surgical tools, sterilization equipment, and dialysis machine components. For medical instruments that are repeatedly sterilized, the polymer’s ability to withstand heat, chemicals, and radiation is critically important. Trauma, orthopedic, and spinal implants take advantage of its stiffness, strength, wear resistance, and biocompatibility. Applications like prosthetic hips and knees use

PEEK composites. Table 8.12 details some of the applications and their requirements that use PEEK.

8.7 Liquid Crystalline Polymers (LCPs) Liquid crystalline polymers (LCPs) possess some degree of order in the liquid state (i.e., when molten or in a solution). This is achieved by incorporating rigid structural segments into the polymer. These segments will form some order in the liquid state and maintain that order in the solid state. This results in their properties being in between those of liquids and crystalline solids. For example, they can have liquidlike flow properties and solidlike tensile strength and stiffness.

196

PLASTICS

IN

MEDICAL DEVICES

Effect of Gamma Sterilization (75 kGy) on PEEK Properties 110%

Percent Property Retention (%)

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Tensile Strength

Tensile Elongation Control

Impact Strength

Flexural Modulus Gamma Sterilized

Figure 8.30 Gamma sterilization capability of PEEK. (a)

PEEK strength compared to Cortical Bone

Tensile Strength (MPa)

250 200 150 100 50 0

Cortical Bone

Stiffness - Flexural Modulus (GPa)

(b)

CF PEEK

PEEK

Polyethylene

PEEK stiffness compared to Cortical Bone 20 18 16 14 12 10 8 6 4 2 0 Cortical Bone

CF PEEK

PEEK

Polyethylene

Figure 8.31 Properties of carbon fiber filled PEEK compared to cortical bone. (a) Strength comparison, (b) stiffness comparison.

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

Mutagenicity of PEEK with TA 100 S(+9)

180

180

160

160

0

PEEK - Ethanol PEEK - Chlorofom Extract Extract

Mutagenicity of PEEK with TA 98 S(–9)

180

160

160

Revertants per plate

180 140 120 100 80 60

Negative Control

140 120 100 80 60 40 20

20

21

Positive Control

28

1232

40

PEEK - Ethanol PEEK - Chlorofom Extract Extract

Mutagenicity of PEEK with TA 100 S(–9) 200

0

Negative Control

(d)

200

20

Positive Control

0

PEEK - Ethanol PEEK - Chlorofom Extract Extract

97

(c)

40

633

Negative Control

60 20

23

Positive Control

25

0

22

40

80

132

60

100

Positive Control

Negative Control

164

80

120

136

100

140

126

120

136

140

1526

Revertants per plate

200

20

Revertants per plate

(b)

Mutagenicity of PEEK with TA 98 S(+9) 200

1909

Revertants per plate

(a)

197

PEEK - Ethanol PEEK - Chlorofom Extract Extract

Figure 8.32 Hypoxanthine-guanine-phosphoribosyl-transferase (HPRT) toxicity tests for polyether ether ketone (PEEK). (a) TA 98 with metabolic effect (1 S9), (b) TA 100 with metabolic effect (1 S9), (c) TA 98 without metabolic effect (2 S9), (d) TA 100 without metabolic effect (2 S9).

LCPs may be divided into two main categories, depending on how they achieve their liquid-crystalline characteristics. Lytropic LCPs are obtained from solutions; i.e., when polymers are dissolved in solvents. Thermotropic LCPs are produced via heat; i.e., when polymers are heated to a molten state. This section

will deal with thermotropic LCPs. Figure 8.34 shows the formation of a liquid-crystalline phase (i.e., order in the liquid state), when an LCP is melted. The order provides very high flow to these otherwise very stiff polymers. Upon cooling, the order is retained, providing high strength and stiffness to the part.

Ames V–79 Test for PEEK 60

Number of Mutants /105 surviving cells

With metabolic activation (+S9) 50

Without metabolic activation (–S9)

40 30 20 10 0 Positive Control

Negative Control

PEEK (0.5)

PEEK (1.5)

PEEK (5.0)

Figure 8.33 Ames V-79 mutagenicity test for PEEK (numbers in parentheses are the concentration of the PEEK extract expressed in µg/ml).

198

PLASTICS

IN

MEDICAL DEVICES

Table 8.12 Medical Applications of Polyether Ether Ketones Application

Requirements

Material

Spinal cages

Adjustable stiffness

Carbon fiber reinforced PEEK (CF-PEEK)

Strength Load-bearing capability Wear resistance Biocompatibility Dimensional stability and tolerance Gamma sterilization Radiolucency No image artifacts Processability Pins, screws, and plates

High stiffness

CF-PEEK

Mechanical properties Biocompatibility Durability Fatigue strength Dental implants

Adjustable stiffness Mechanical properties Dimensional stability

CF-PEEK, Glass fiber reinforced PEEK (GF-PEEK)

Thermal conductivity Steam sterilization Processability Tri-leaflet heart valve

Hemocompatibility

PEEK

Electrical insulating properties Good dielectric properties Mechanical properties Adjustable stiffness Durability Chemical and stress crack resistance Processability Heat shrinkable tubing

High and low temperature resistance

PEEK

Chemical resistance Flexibility EtO, gamma sterilization Excellent moisture and air barrier Formability (Continued )

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

199

Table 8.12 (Continued) Application

Requirements

Material

Check-valves, pressure relief valves

High temperature resistance

PEEK, GF-PEEK

Chemical resistance Long-term durability Toughness Colorability Corrosion resistant Sterilization

LCPs contain rigid rods or “mesogens” either along the polymer main chain or as a side chain (Figure 8.35). Most commercially available thermotropic LCPs contain these mesogens along the polymer main chain. Due to their orientation in the solid state, LCPs have excellent thermal and dimensional stability and chemical resistance. They are flame retardant, absorb very little moisture, and are impermeable to gases. They flow well in the melt and can be molded into extremely thin-wall parts and extruded into high-strength fibers and films. Applications of LCPs include surgical tools, equipment components and parts, films, and tubing.

8.7.1 LCPs Production Thermotropic LCPs are typically based on polyesters. They are produced by the reaction of

aromatic acids and aromatic phenols or their derivatives, as shown in Figure 8.36. The entire reaction is a melt reaction where the water or the alcohol is removed via a vacuum. Highmolecular weight polymers can be formed in this method.

8.7.2 LCPs Properties LCPs have very high temperature resistance and excellent thermal stability. The low viscosities in the melt make them very attractive materials for thin-wall applications and parts. Due to the high level of order in the solid state, they possess very high dimensional stability and high strength and rigidity. In addition, they maintain their mechanical properties even at very low temperatures.

Heat/Melt

Solid State

Order

Liquid State

Some Order

Figure 8.34 Schematic of thermotropic liquid crystalline polymers.

Main-chain mesogens

Side-chain mesogens

Figure 8.35 Main-chain and side-chain mesogens in liquid crystalline polymers.

200

PLASTICS

O

IN

MEDICAL DEVICES

O COOH

n H3CCO

O

Heat, vacuum –n CH3COOH

C n

Liquid crystalline polymer COOH

O n

+

COOH

n H3CCO

H3CCO O Heat, vacuum –2n CH3COOH O C O O

C

O n

Liquid crystalline polymer

Figure 8.36 Production of an a liquid crystalline polymer.

LCPs have excellent chemical resistance, low extractables, and are biocompatible. Their flame retardance and dielectric properties make them excellent candidates in electrical and electronic applications. Table 8.13 details the properties of two typical LCPs.

8.7.3 Chemical Resistance and Sterilization of LCPs LCPs are chemically resistant to most acids, bases, and organic solvents (Table 8.2). Strong oxidizing agents and strong bases will degrade the polymer.

Table 8.13 Typical Properties of Liquid Crystalline Polymers Property

Unit

LCP 1

LCP 2

Density

g/cc

1.4

1.6

Water absorption (24 h)

%

0.01 0.02

,0.1

Glass transition temperature



C

110

160 90

HDT at 0.46 MPa or 66 psi



C





HDT at 1.8 MPa or 264 psi



C

105 187

270 300

Melting point



C

212 280

320 360

Tensile strength

MPa

140 182

135

Tensile elongation at break

%

1.7 3.4

1.6

Flexural modulus

GPa

9 12

13.4

Impact strength, notched, 23 C

J/m

3 5

95

Processing temperature



225 295

330 380

C

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

201

Gamma Radiation Stability of Liquid Crystalline Polymers (after 5000 kGy) Percent Property Retention (%)

120% LCP 1

LCP 2

100%

80%

60%

40%

20%

0%

Tensile Strength

Elongation

Flexural Modulus

Figure 8.37 Gamma radiation stability of an a liquid crystalline polymer.

Table 8.14 Liquid Crystalline Polymer Medical Device Applications Application

Requirements

Material

Surgical instrument

Very tight tolerance ( 6 0.0005 in.)

30% GF LCP

Dimensional stability Stiffness and strength—like metals Repeated sterilization Colorability No flash during molding Surgical device control cables

High strength

LCP fibers

Inert, nontoxic Sterilization (EtO, gamma) Surgical tubing, cannulae

Precision dimensions

LCP

Insulating properties High strength Excellent mechanical properties Biocompatibility Chemical resistance Films

Clarity Strength Repeated sterilization Moisture and gas barrier Tear resistance

LCP

202

PLASTICS

Alcohols at elevated temperatures for extended periods of time will also degrade the polymer. Due to their high temperature resistance and hydrolytic stability, LCPs can be sterilized by steam and autoclave (Table 8.3). Their high aromatic content makes them resistant to gamma and e-beam radiations, as shown in Figure 8.37, where 80% or more of the properties are retained after 5,000 kGy of radiation [27].

metal replacement. Their lighter weight, design flexibility, and thin-wall-molding capability provide significant cost reductions compared to metals. They are also used in various parts and components in medical equipment like surgical and dental instruments that require repeated sterilizations. They can also be machined into various high-performance parts. Table 8.14 describes some medical device applications that use LCPs and their requirements.

8.7.4 Joining and Welding of LCPs

8.8 Fluoropolymers

Ultrasonic welding can be used to weld LCPs. As with PEEK, sufficient energy must be supplied to the material for an effective weld. Heated tool welding will cause the LCP to bond to the hot plate. Adhesives like epoxies and acrylics can be used to bond LCPs. Various mechanical fastening methods also can be used.

Fluoropolymers are amorphous or semicrystalline fluorine-containing polymers that can be either aliphatic or aromatic. Most commercially available fluoropolymers are fully fluorinated olefinic (aliphatic-based) materials. Homopolymers contain over 99% fluorine by weight. Copolymers are obtained by the copolymerization of a fully fluorinated monomer (like tetrafluoro-ethylene) with an olefin (typically ethylene). Copolymers thus have a lower amount of fluorine content compared to homopolymers, and their properties can be tailored to meet various

8.7.5 LCPs Applications —Examples With their temperature resistance and high strength and stiffness, LCPs have been used as materials for (a)

F

F

C

C

Cl

F

C

C

n

F

H

F

C

C

n

F

F

n

H

F

Polychlorotrifluoroethylene (PCTFE)

Polytetrafluoroethylene (PTFE)

MEDICAL DEVICES

IN

F

Polyvinylidene fluoride (PVDF) CF3

(b) F

F

C

C

F

CF3

C

C

x

F

F

F

F

C

C

y

F

H

H

F

C

C

F

F

C

C

x

H

H

F

F

Ethylene tetrafluoroethylene copolymer (ETFE)

C

C y

F

F

F

Perfluoro alkoxy copolymer (PFA) H

H

C

C

y

F

O

x

Fluorinated ethylene propylene copolymer (FEP) (c)

F

Cl

F

C

C

x

H

H

y

F

F

Ethylene chlorotrifluoroethylene copolymer (ECTFE)

Figure 8.38 Chemical structures of fluoropolymers. (a) Homopolymers, (b) fully fluorinated copolymers, (c) copolymers with other olefins.

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

(a)

F

F

203

F

F

C

C

R F

F

Cl

F

0 – 100°C 0.7 – 3.5 MPa

n

F F Polytetrafluoroethylene (PTFE)

R

Cl

F

C

C

F

F

n

F

F

Polychlorotrifluoroethylene (PCTFE) H

F

H

F

C

C

H

F

R H

10 – 150°C 1 – 10 MPa

F

n

Polyvinylidene fluoride (PVDF) CF3

(b)

F

F

F

x

+ F

O

CF3

F

F

C

C

F

O

C

C

F

F

R

y

F

F

F

15 – 95°C 0.5 – 3.5 MPa

x

F

y

F

Perfluoro alkoxy copolymer (PFA) F

F

x

CF3

F y

+

F

F

C

C

F

CF3

C

C

R x

F

F

F

F

F

y

F

F

F

Fluorinated ethylene propylene copolymer (FEP)

(c)

H

H

F +

x

F

H

H

C

C

F

F

C

C

R

y

x

H

H

F

F

H

y

F

H

F

Ethylene tetrafluoroethylene copolymer (ETFE) H

Cl

H +

x H

H

F

H

H

C

C

Cl

F

C

C

R

y F

F

60 – 120°C 5 – 10 MPa

x

H

H

y

F

F

Ethylene chlorotrifluoroethylene copolymer (ECTFE)

Figure 8.39 Polymerization methods of fluoropolymers. (a) Polymerization of homopolymers, (b) polymerization of fully fluorinated copolymers, (c) copolymerization with olefins.

204

PLASTICS

performance and application needs. Figure 8.38 shows the structures of fluorine-containing homopolymers and copolymers. The carbon-fluorine bond is a very polar bond with very high bond strength, leading to very low intermolecular attractions. Because of this, fluoropolymers have very low surface energy, a low coefficient of friction and are lipid, water, and stain repellant. The low coefficient of friction makes fluoropolymers very lubricious. Fluoropolymers will

IN

MEDICAL DEVICES

“bloom” to the surface when blended with other polymers due to their low surface energy. Fluoropolymers thus are blended with or coated onto various polymers to render the surfaces lubricious and water repellant. Fluoropolymers have high temperature resistance and have excellent dielectric properties.

8.8.1 Fluoropolymers Production Fluoropolymers are typically produced by the free radical reaction of the monomers. The free radical

Table 8.15 Physical and Mechanical Properties of Some Fluoropolymers Property

Unit

PTFE

PFA

FEP

PVDF

PCTFE

ETFE

ECTFE

Density

g/cc

2.15 2.25

2.15

2.15

1.77

2.1

1.13

1.68 1.70

Water absorption (24 h)

%

, 0.01

, 0.03

, 0.01

, 0.04

, 0.05

, 0.03

, 0.1

Glass transition temperature



C

297



80

235

95

2100

85

HDT at (0.46 MPa or 66 psi)



C

120 125

75

70

121

126

104

90 100

HDT at (1.8 MPa or 264 psi)



C

50 60

50

50

90

65 75

71

65 75

Melting point



C

327

305

260

171

210

260

240

Tensile strength at break

MPa

20 40

27

25 30

30 40

30 40

45

54

Elongation at break

%

250 500

300

325

300 450

100 250

200

250

Flexural modulus

GPa

0.45 0.60

0.7

0.55 0.65

0.6 1.2

1.2 1.5

1.45

1.65

Impact strength, notched, 23 C

J/m

160 190

No break

No break

190 210

100 200

No break

No break

Shore hardness

D50

D60

D56

D75 D85

D75 D85

D75

D75

Dynamic coefficient of friction

0.1

0.2

0.2

0.4

0.4

0.23

0.19

325 335

300 305

255 285

170 180

255 285

265 270

240 245

Processing temperature



Surface energy

nM/m

20





36.7

30.9





Percent of crystallinity

%

90 98

50 80

50 65

30 70

30 70

30 50

50 60

C

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

(a)

205

Coefficient of Friction of Some Fluoropolymers 0.45

Coefficient of Friction

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 PTFE

PFA

ECTFE

FEP

ETFE

PVDF

Surface Free Energy of Some Fluoropolymers

(b)

Surface Free Energy (nM/m)

40 35 30 25 20 15 10 5 0 PTFE

FEP

PCTFE

PVDF

Figure 8.40 Surface properties of some fluoropolymers. (a) Coefficient of friction, (b) surface free energy.

Table 8.16 Comparison of Fluoropolymer Properties Fluoropolymer

Key Properties

Disadvantages

PTFE

Lowest coefficient of friction

Poor radiation resistance

Lowest surface energy

Not melt processable (very high-melt viscosities)

Low temperature stability

Virgin resin has poor mechanical properties

FEP

Chemical resistance Gamma sterilization Low coefficient of friction Low surface energy Biocompatibility Continuous use temperature (up to 200 C) Melt processable (Continued )

206

PLASTICS

IN

MEDICAL DEVICES

Table 8.16 (Continued) Fluoropolymer

Key Properties

Disadvantages

PFA

Good clarity

Poor radiation resistance

Excellent flexibility Good mechanical properties Continuous use temperature (up to 260 C) Chemical resistance Chemically inert PVDF

Good mechanical properties

Attacked by strong bases, amines, esters, and ketones

Chemical resistance Melt processable Continuous use temperature (up to 115 C) PCTFE

Excellent low-temperature properties

Attacked by organic solvents

Excellent barrier properties

Degrades during melt processing (needs stabilizers)

Good chemical resistance Thermal stability ETFE

Excellent toughness and impact resistance Good mechanical properties Excellent chemical and stress crack resistance Gamma sterilization Good dielectric properties Continuous use temperature (up to 150 C)

ECTFE

Low permeability Durability and flexibility Good mechanical properties Abrasion resistance Chemical resistance Temperature resistance (up to 150 C)

initiators used are ammonium persulfate, potassium persulfate, disuccinic peroxide, or other organic peroxides. Free radicals can be generated by UV or gamma radiation as well. Chain transfer reagents are also used to control the molecular weight. Depending upon the fluoropolymer, temperatures can range from 23 C to 150 C, and pressures can range from 0.5 to 10 MPa. Aqueous or organic solvents can be used for the polymerizations. Suspension polymerizations will produce granular material, and emulsion polymerization will produce fine powders. Figure 8.39

details the basic polymerization methods for some fluoropolymers.

8.8.2 Fluoropolymers Properties Fluoropolymers have both low temperature and high heat resistance. They have excellent dielectric properties, are chemically inert, and are melt processable. Medical device applications take advantage of the low coefficient of friction and the inertness (i.e., biocompatibility) of fluoropolymers. Table 8.15 gives the properties of various fluoropolymers. Polytetrafluoroethylene

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

207

(PTFE) has the highest density of all the fluoropolymers. The densities of the ethylene copolymers are much lower and will depend upon the amount of ethylene incorporated into the copolymer. The homopolymers PTFE, polyvinylidenedifluoride (PVDF), and polychlorotrifluoroethylene (PCTFE) have much lower impact strengths compared to all the copolymers. All the fluoropolymers are semicrystalline, with PTFE having the highest percentage of crystallinity. Fluoropolymers have very low surface free energies and will exude or bloom to the surface when blended (in small amounts) with other polymers. The low coefficient of friction provides excellent lubricity to fluoropolymers. Thus, blending fluoropolymers with other polymers that have much higher surface free energies will provide additional lubricity and water repellency to the surfaces to parts and components made from these blends. Figure 8.40 shows that PTFE has the lowest coefficient of friction and the lowest surface free energy compared to all the other fluoropolymers. In contrast, polyethylene has a coefficient of friction of 0.2 and a surface free energy of 35.7 mN/m at 20 C.

The properties of the various fluoropolymers are compared in Table 8.16.

8.8.3 Chemical Resistance of Fluoropolymers Fluoropolymers are very resistant to most chemicals. Depending upon the chemical structure of the fluoropolymer, the chemical resistance to specific types of chemicals will vary. Copolymers with ethylene have lower chemical resistance than wholly fluorinated polymers (Table 8.17).

8.8.4 Sterilization of Fluoropolymers PTFE has marginal performance when exposed to steam and autoclave sterilization and degrades when exposed to gamma and e-beam radiations. Almost all the other fluoropolymers can be sterilized by steam, autoclave, ethylene oxide, and gamma and e-beam radiations (Table 8.18). PTFE is susceptible to degradation when exposed to high-energy radiation [28,29]. Free radicals are

Betadine

Lipids

Soaps/ Detergents

Disinfectants

Hydrogen Peroxide

Bleaches

Saline Water

Silicones

Oils/Greases

Ethylene Oxide

IPA

Acetone

MeCL2

MEK

THF

Polymer

Dilute Basses

Dilute Acids

Table 8.17 Chemical Resistance of Fluoropolymers

Fluoropolymers PTFE

Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good

FEP

Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good

PFA

Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good Good

ECTFE/ETFE

Good Good Fair

PVF/PVF2

Good Good Good Fair

Fair

Fair

Good Good Good Good Good Good Good Good Good Good Good

Good Fair

Fair

Good Good Good Good Good Good Good Good Good Good Good

All ratings at room temperature.

Table 8.18 Sterilization Capabilities of Fluoropolymers Polymer

Steam

Dry Heat

Ethylene Oxide

Gamma Radiation

E-Beam

PTFEa

Fair

Fair

Good

Poor

Poor

FEP

Good

Good

Good

Fair

Fair

PFA

Good

Good

Good

Good

Good

ECTFE/ETFE

Good

Good

Good

Good

Good

PVF/PVDF

Good

Good

Good

Good

Good

Fluoropolymers

a

Radiation stable grades should be considered for gamma and e-beam radiation sterilization

208

PLASTICS

F

F

F

IN

MEDICAL DEVICES

F + F

F

F

F

F

F

Radiation

F

F

F

F

F

Radical formation F

F

F

F

F

F

Radiation

F

F

F

Oxidation and degradation reactions F

+ F

F F F F Radical formation

Figure 8.41 Radiation degradation mechanisms of polytetrafluoroethylene (PTFE).

Effect of E-Beam Radiation on the Thermal Stability of PTFE and FEP 600 PTFE FEP

Onset of Thermal Decomposition (°C)

500 400 300 200 100 0 0

500

1000

1500

2000

2500

3000

3500

4000

4500

Dose (kGy)

Figure 8.42 Effect of e-beam sterilization on polytetrafluoroethylene (PTFE) and FEP.

Effect of Gamma Radiation on the Properties of PTFE Percent Property Retention (%)

140% Tensile Stress Tensile Strain Molecular weight

120% 100% 80% 60% 40% 20% 0% 0

0.5

1 1.5 Radiation Dose (kGy)

Figure 8.43 Effect of gamma sterilization on polytetrafluoroethylene (PTFE).

2

2.5

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

formed either by a carbon-fluorine or a carboncarbon dissociation (Figure 8.41). Degradation most likely occurs via the carbon-carbon chain scission, leading to the unzipping of the polymer chain. Radiation in air will lead to oxidation reactions when the free radicals react with oxygen. Figure 8.42 shows the effect of radiation on the thermal stability of PTFE and fluorinated ethylenepropylene (FEP) [30]. The onset of thermal degradation was measured. Very high radiation doses significantly reduce the onset of thermal decomposition. Figure 8.43 shows that the mechanical properties of PTFE go down rapidly, even after low doses of gamma radiation [31]. The change in molecular weight is a clear indication of the degradation of the polymer. Free radical scavengers and other stabilizers are needed to improve the radiation stability of PTFE. Radiation sterilization is typically not recommended for PTFE.

8.8.5 Fluoropolymers Biocompatibility Fluoropolymers are biocompatible, chemically inert materials with little or no extractables or leachables. They are used in tubing and catheters because they fail to react with surrounding fluids and tissue or cause any thrombogenetic or toxic effects. Fluoropolymers are also hemocompatible for large vascular graft applications. In applications like vascular grafts, where the material is the outer layer of the graft, the materials must be able to interact with endothelial cells for cell attachment, spreading, and proliferation. Surface modifications of PTFE can be conducted to make the graft surface more amenable to cell attachment [32,33].

8.8.6 Joining and Welding of Fluoropolymers Fluoropolymers can be joined, welded, and bonded by various techniques. Due to their low surface energy, care should be taken to identify the right method that will provide the required weld or bond strength for the applications. Adhesives should be chosen carefully to bond the fluoropolymer, which is typically difficult to bond. Table 8.19 summarizes the various methods that can be used for the different types of fluoropolymers.

8.8.7 Fluoropolymers Applications—Examples Fluoropolymers are used in applications ranging from flexible tubing to catheters and vascular grafts.

209

Table 8.19 Joining and Welding Methods for Fluoropolymers Material

Joining and Welding Methods

PVDF

Heated tool welding Vibration welding Spin welding Hot gas welding Infrared welding Solvent welding (polar solvents like dimethylformamide and dimethylacetamide)

PTFE

Infrared welding Adhesive bonding (epoxies, cyanoacrylates, and acrylics)

ECTFE

Hot gas welding

FEP

Hot gas welding

PFA

Hot gas welding Adhesive bonding (cyanoacrylates)

ETFE

Heated tool welding Ultrasonic welding Hot gas welding Adhesives (cyanoacrylates, epoxies) Mechanical joining (snap-fit, selftapping screws, riveting)

PCTFE

Hot gas welding Heat sealing Adhesive bonding (epoxies)

Their chemical inertness, biocompatibility, and range of physical properties allow them to be tailored to many different applications. Table 8.20 lists a few medical device applications, their requirements, and the type of fluoropolymer used.

8.9 Conclusion The temperature resistance of high-temperature engineering thermoplastics compared to other (commodity and engineering) thermoplastics is shown in

210

PLASTICS

IN

MEDICAL DEVICES

Table 8.20 Fluoropolymer Medical Device Applications Application

Requirements

Material

Vascular graft

Biocompatible

PTFE

Smooth surface Chemically inert Low thrombogenicity Tubing

Clarity or translucency

FEP

Flexibility and kink resistance Chemical resistance Biocompatibility Gamma radiation Guiding catheters

Lubricity (low coefficient of friction)

PTFE liners

Chemical resistance and inertness Biocompatibility Sterilization (steam and EtO) Multi-lumen tubing

Tight tolerance ( 6 0.00005 in.)

PTFE, FEP, PFA

Abrasion resistance Lubricity Chemical resistance Chemical inertness Flexibility and kink resistance Biocompatibility Sterilization resistance Heat shrink tubing

Barrier properties

PTFE, FEP

Lubricity Dimensional stability Sterilization (EtO, radiation) IV catheters

Lubricity

FEP, ETFE

Chemical resistance and inertness Biocompatibility Kink resistance Sterilization (EtO, radiation) Packaging

Clarity Gamma, EtO sterilization Barrier properties Sealability Toughness and tear resistance

ECTFE, PVDF

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

211

Continuous Use Temperatures of High Temperature Engineering Thermoplastics compared to other thermoplastics Continuous Use Temperature (°C)

300 250 250 250

260 260

250 200 200

200 150 150 150 150

175 180 160

115 100

100 55

50

80

50

LC P PP SU PT FE PE EK

PP S

PF A

FE P

PE I PE S

PS U

E PV D F PC TF E

ET F

PC

PP

PA 66

PE H D

PV C

-P

0

Figure 8.44 Continuous use temperatures of various thermoplastics.

Table 8.21 High-Temperature Engineering Thermoplastics—Suppliers Material

Supplier

PSUs

Ticona (Udel, Radel) Sumitomo (Sumika Excel) BASF (Ultrason)

PEIs

Sabic Performance Polymers (Ultem, Extem)

PAI

Solvay (Torlon)

PPS

Chevron Phillips (Ryton) Ticona (Fortron) Oxford Polymers

Polyaryletherketones

Victrex (PEEK) Invibio (Optima) Solvay (Ketaspire) Oxford Polymers (PEKK)

LCPs

Ticona (Vectra) Amoco (Xydar) DuPont (Zenite, Thermx) Sumitomo (Sumika Super) Toray (Siveras)

Fluoropolymers

DuPont (PTFE, PFA, FEP, ETFE, PVF) Dyneon (PTFE, PFA, FEP, ETFE) Daikin (PTFE, PFA, FEP, ETFE, ECTFE) Solvay Solexis (PTFE, PFA, ECTFE) Asahi Glass (PTFE, PFA, ETFE) Honeywell (ECTFE)

212

Figure 8.44. Continuous use temperatures of these materials range from 150 C to 260 C. For most other thermoplastics, the continuous use temperatures are under 100 C. High-temperature engineering thermoplastics continue to see a healthy growth rate in medical device applications. Their combination of heat resistance, strength and stiffness, mechanical properties, chemical resistance, and biocompatibility make them viable candidates for parts, components, and devices that require tight dimensional tolerances, excellent dimensional stability, and applications that require long-term durability and strength. Metal replacement is also a fast-growing area for these materials. In addition, they can be sterilized repeatedly by most conventional techniques and are being used in several reusable device applications. These materials tend to cost more than their commodity and engineering thermoplastic counterparts. However, the cost is offset by their ease of processing, durability, and design flexibility.

8.10 High-temperature Engineering Thermoplastics Suppliers Table 8.21 lists major suppliers for high-temperature engineering thermoplastics.

References [1] Johnson RN, Farnham AG, Clendinning RA, et al. J Polym Sci 1967;5:2375 (Part A 1). [2] Hale WF, Farnham AG, Johnson RN, Clendinning RA. J Polym Sci 1967;5:2399 (Part A 1). [3] Johnson RN, Farnham AG. J Polym Sci 1967;5:2415 (Part A 1). [4] Radels Design Guide version 3.2, Advanced Polymers, Solvay. [5] Brown JR, O’Donnell JH. J Appl Polym Sci 1979;23:2763. [6] Brown JR, O’Donnell JH. J Appl Poly Sci 1975;19:405. [7] Udels Design Guide version 2.1, Advanced Polymers, Solvay.

PLASTICS

IN

MEDICAL DEVICES

[8] Kowal J, Czajkowska B, Bulwan E, Blazewicz M, Pamula E. Eur Cells Mater 2004;7(Suppl. 1):59. [9] Hoenich NA, Katopodis KP. Biomaterials 2002;23:3853 8. [10] White DM, Takekoshi T, Williams FJ, Rekkes HM, Donahue PE, Webber MJ. J Polym Sci 1981;19:1635 (Part A). [11] Nazareth DB, Cooper SM. Medical plastics degradation and failure analysis. In: Portnoy RC, editor. Plastics design library; 1998. p. 157 65. [12] Bonnadier J-P. Proceedings medical polymers, Dublin, Ireland, November 2004. p. 13 33. [13] Peluso G, Petillo O, Ambrosio L, Nicolais L. J Mater Sci Mater Med 1994;5:738 42. [14] Tao CT, Young CH. J Memb Sci 2006;269:66 74. [15] Torlons Design Guide, Advanced Polymers T-50246 D 03/07, Solvay 2007. [16] Srinivasan, et al. Proceedings INTC_TAPPI September 6, 2001. [17] Tanthapanichakoon W, et al. Polym Degrad Stab 2006;91:2614 21. [18] Massey LK. The effects of sterilization methods of plastics and elastomers. William Andrew; 2005. [19] Advanced Polymers Technical Bulletin KT50549 R 10/07, Solvay. [20] Sinz I, Green S. Proceedings medical plastics 2003, Copenhagen, Denmark, October 2003. [21] http://www.invibio.com/documents/BROCHURE_ PEEK CLASSIX_Polymer_Brochure.pdf [22] Williams DF, McNamara A, Turner RM. J Mater Sci Lett 1987;6:188. [23] Toth JM, Wang M, Estes BT, Scifert JL, Seim III HB, Turner AS. Attachment and proliferation of osteoblasts and fibroblasts on biomaterials for orthopaedic use. Biomaterials 2006;27 (3):324 34. [24] Stuart G, Roland G, Keith C, Roger T. Proceedings medical plastics 2005, Copenhagen, Denmark, October 2005. [25] Hunter A, Archer CW, Walker PS, Blunn GW. 1995;16(4):287 95. [26] Katzer, et al. Biomaterials 2002;23:1749 59. [27] Linstid HC, Kaslusky A, McChesney CE, Turano M. NPE 2000, Chicago, IL, 20 June, 2000.

8: HIGH-TEMPERATURE ENGINEERING THERMOPLASTICS

[28] Forsythe JS, Hill DJT. Prog Polym Sci 2000;25:101 36. [29] Florin RE. Radiation chemistry of fluorocarbon polymers. In: Wall LA, editor. Fluoropolymers. New York: Wiley; 1972. p. 317 80. [30] Lappan U, Hausler L, Pompe G, Lunkwitz KJ. Appl Polym Sci 1997; 66:2287 91.

213

[31] Fayolle B, Audouin L, Verdu J. Polymer 2003;44:2773 80. [32] Chen M, Zamora PO, Som P, et al. J Biomater Sci Polym Ed 2003;14(9):917 35. [33] Bhat VD, Klitzman B, Koger K, Truskey GA, Reichert WM. J Biomater Sci Polym Ed 1998;9:1117.