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Thermoplastic Filament Wound Applications
Thermoplastic Filament Wound Applications Malcolm W. K. Rosenow B.A.Sc, M.A.Sc, P.Eng. FRE Composites Inc., 64, rue Wales St. Andre-est Quebec, Canada Abstract This paper deals with the concept of using a unidirectional fibre reinforced thermoplastic tape, in a filament winding application. The characteristics of the thermoplastic tape and the methods of use, are explained. The example, of an effectively "autofrettaged" fibre reinforced composite pressure vessel without the need to pressurize above the operating pressure of the vessel is described. Introduction Filament winding is a manufacturing method in which continuous resin-impregnated rovings or tows are wound over a rotating mandrel, under controlled tension, in a predetermined geometrical pattern. This manufacturing technique has the capability of varying the wind angle, winding tension of the reinforcement, and resin content (wet winding) in each layer until the desired thickness and resin content are obtained in addition to the required directional strengths. For parts that are simple with repetitive geometries, winding machines are usually controlled by mechanical means such as gears, chains, or cams. Advances in microprocessor based computer control systems have advanced the filament winding process from winding simple linear geometries or even axisymmetric shapes, to include laying the fibre down on concave surfaces, and winding around openings. The state of the art filament winding machines offer six axis of motion, which has greatly expanded the types of shapes that can be filament wound. Figure 1 illustrates a typical state of the art filament winding machine. Filament winding is a very efficient means of making fibre reinforced composites, because the fibres can be laid down automatically, rapidly, and be oriented in the direction of principal stress.
303
304
ADVANCED STRUCTURAL MATERIALS
A variety of fibre reinforcements are available, depending on the cost and desired level of performance. Table 1 lists a selected variety of fibre reinforcements that are commercially available. Blends of fibres can be used to optimize cost, and performance. In general, glass is the least expensive of the fibres, but has the lowest performance level based on its density. Carbon fibers, in general, are the most expensive but have the highest performance level, except for the intermediate modulus grade of carbon fibre(IM), where the price per pound can be less than half the cost of kevlar. Epoxy resin systems are the most widely used matrix resins because of their excellent processibility and physical and mechanical properties. Thermal stability and the glass transition temperature limitations restrict the upper continuous use temperature to about 177°C (350°F). Any increase above the glass transition temperature, results in significant changes in the properties of the cured resin system. Other thermoset resin systems, such as polyimides, have an upper temperature limit of 232°C (450°F) for extended time periods. For applying thermoset resins, wet winding is the most widely used method. In this process the fibres are passed through a heated resin bath to apply the resin. Wet winding is limited to materials with viscosities of 2,000 centipoise, but does offer the most economic means of manufacture. In prepreg winding, the resin is impregnated into the fibres prior to winding using a hot melt or solvent dip process. Prepreg winding can be used with higher viscosity resins such as novolacs, polyimides, and some epoxies. It has the advantage of a longer working life before curing, and the resin content and fibre quality can be determined before winding instead of after. These materials are generally available through companies that specialize in prepreging. Thermoplastic materials are receiving more attention due primarily to their unique properties and processing characteristics. These materials offer better mechanical properties at elevated temperatures, have lower moisture retention, and have better chemical and solvent resistance. The manufacturing process allows for significantly faster fabrication rates because no curing is required, and the fabricated part can be reshaped after the initial fabrication by remelting the resin. The general interest in advanced thermoplastics, is mainly driven by four major advantages, and these are; (1) Thermoplastics provide service operating temperatures greater than 176 °C (350 °F), and under severe environmental conditions. (2) The toughness properties, usually measured by interlaminar shear values (Gic), will provide much better damage tolerance over thermoset resins. (3) The processing technology for thermoplastics provides the potential for low cost manufacturing. (4) Thermoplastics can be easily repaired due to the remelting characteristics.
ADVANCED STRUCTURAL MATERIALS
305
Thermoplastic resins range from low melt temperature polymers such as Nylon, to high melt temperature polymers such as PEEK (Polyetheretherketone). A small selection of thermoplastic resin properties is outlined in Table 2. These thermoplastic materials are now available in several forms including powder impregnated yarns, comingled yarns, melt impregnated yarns or tows, and slit tape. A typical thermoplastic tape consists of several thousand unidirectional filaments bonded together by a thermoplastic resin, and has a tape width and thickness tailored to a specific processing needs. Generally the tape width used in our applications would be 3.2 mm (.125 inches) by .15 mm (.006 inches) thick. The concept of using thermoplastic impregnated materials in filament winding is simple in principle. The steps required are, guiding the material, preheating the thermoplastic tape, contact point heating of the tape and the bonding surface, consolidating the material, and finally cooling the part. Some of the major obstacles still to be overcome are the higher processing temperatures which can exceed 427°C (800°F), and the higher resin viscosities. A variety of processing parameters must still be optimized before thermoplastic filament winding is as common as thermoset filament winding. Thermoplastic Filament Winding of High Pressure Containers: In addition to the general advantage of the fibre reinforced thermoplastic materials, there are a number of specific or component related advantages. One of these, "autofrettage" fibre reinforced composite pressure vessels without the need to pressurize above the operating pressure will be discussed. Pressure vessels can be categorized according to four types of material classifications. These are all metal containers, hooped wound metal containers utilizing a hooping material that is different than the container, a composite vessel using a thin metal liner such as aluminum, and an all composite container. The subject of this paper is only to deal with a hooped wound metal container that is open ended, similar to a gun barrel. The process of Frettage is a long standing process of reinforcing a metal container subject to internal pressure with a different material. It consisted of reinforcing the cylindrical part with a material that would absorb a portion of the circumferential stress, therefore sharing the load. It was originally applied to gun barrels, and is centuries old. The parts were made of steel and the reinforcement was obtained either by doubling the thickness of the same material, or by forging, or by wire winding. The longitudinal stresses were resisted by the original component. The technique of Autofrettage is a process of pre-stressing or over stressing a hollow cylindrical member beyond the elastic range using internal pressure. With a metal cylinder comprising of two different layers of the same material, that is the base material (inner layer) and the reinforcing layer (outer layer), the resultant structure would be pre-stressed, with the inner layer in compression and the outer layer in tension.
306
ADVANCED STRUCTURAL MATERIALS
This type of system has two limitations. First, it does not offer any weight advantages, and secondly the elastic limit of the reinforcing layer must be much greater than the inner layer to take full advantage of this process. Another technique is to create an interference fit between the two materials by having the outside diameter of the base material greater than the inside diameter of the reinforcing layer. Again, the main limitation to this process is that, the ultimate tensile strength of the reinforcing material is only marginally different from that of the container. A particular recurring problem with filament winding over a metal container using an thermoset epoxy is the separation of the composite section from the metal part after curing. Due to the differences in the thermal coefficients of expansion of the composite and the metal, this problem will always exist. Another problem is the level of stress that is developed in the composite section during the pressuhzation of the metal component. For a given strain level, the stress level developed in the composite, Sc, is very low, compared to the ultimate stress, Scu, of the material as illustrated in Figure 2. As a consequence, the composite material would be stressed far below its ultimate capacity and is an inefficient use of the composite's high stress carrying capabilities. We can take advantage of the differences in elastic elongation of each material, by filament winding under very high tension. To take full advantage of the thermoplastic prepreg tape, the fibres can be loaded under very high tension, and melted on the metal part at the same time. By winding under high tension, a desired level of pre-stressing in the metal component can be achieved, with no risk of the fibres migrating close to the metal component. Filament winding with a liquid resin impregnated system under tension merely gathers all the fibres in close contact with the metal part, and forcing the resin to the outer surface. This creates a two layer coating, rather that an absolute composite structure in which the fibers are individually and uniformly embedded in the matrix to achieve structural integrity. Thermoplastic tapes can be stressed in their present form, unlike thermoset prepregs, because they are in a fully consolidated state. That is, the thermoplastic tape is a composite preform, and when stressed, the fibres and the thermoplastic resin share the load. To utilize the unique characteristics of the thermoplastic tape, the tape is wound under controlled high tension onto the base material. This technique of Frettage as applied to thermoplastics is illustrated in Figure 3. In this case, a pre-stressed liner is obtained through filament winding at very high tension levels. For a given strain level, the stress in the composite section is much higher than would be if it where not wound under very high tension, resulting in a reduced level of stress in the metal section. As a result, the composite section is now contributing significantly to the overall performance to the loading. There are additional advantages for pressure vessels by applying the frettage process with a thermoplastic tape, and they are; 1) Half the thickness of the pressure vessel can be replaced for the same operating pressure resulting in weight savings.
ADVANCED STRUCTURAL MATERIALS
307
2) The pressure rating can be doubled by hooping over the existing pressure vessel with only marginal weight gain. Now with the introduction of advanced thermoplastic materials this century old process of Frettage has now taken on a new dimension. It is possible to limit the stress levels in the hooping material or the metal canister by altering the thickness or winding tension and therefore meet the design requirements. The thickness of the composite section and the winding tension must be chosen so that the pre-stressing combined with the lowest possible operating temperature is always effective, otherwise, the hooping material will separate from the metal body. Also the stress due to the internal pressure loading and the design temperature must remain within the permissible material limits. Another design tool, allows for optimization of the composite section by choosing the combination of reinforcing fibres and resin to obtain the desired modulus and tensile strength. The modulus of the tape can be varied by using a different reinforcing fibre and/or by varying the thermoplastic resin content. Summary: Thermoplastic filament winding techniques can be utilized to perform tasks that are difficult, or impossible to accomplish with thermoset materials or even metals. At FRE Composites, we have responded to this challenge, and have developed techniques that have enabled us to filament wind with thermoplastic materials. The technique of Frettage, using the exceptional properties of the thermoplastic tape, compared to the properties of the metal is one example. Thermoplastic filament winding is not limited to the over wrapping of metal containers. There are many other applications in which thermoplastics can be used that will not only replace metals, but even the presently used thermosets. Filament winding with thermoplastic reinforced composites is the up and coming technology for this century. References: 1
"Modern Plastics Encyclopedia", McGraw-Hill Publications
2
"Engineered Materials Handbook, Composites", Volume 1, ASM International
ADVANCED STRUCTURAL MATERIALS
AUXILIARY CAfifiiAC€ SPRAW CHOPPER cm TAP{
TCRUIHAi.
Figure 1. Typical computer controlled filament winding machine configuration (Source: Entec)
Comparison of Reinforcing Fibres and Metals
Carbon Fibre Aramid Glass Metals Ultra High High Intermediate Very High High Kevlar 49 E E-CR Aluminum Titanium U.S.A Canadian Alloy Modulus Modulus Modulus Strength S t r e n g t h Alloy 6A1-4V L65
Property Tensile Modulus GPa. psi. x 10A6 Tensile
Strength GPa. psi. x 10A6
Steel 080M46 (BS970:1983)
586 85
345 50
295 43
235 34
235 34
124 18
72 10.5
77 11.2
72 10.5
110 16
207 30
1.86 270
2.2 320
5.6 817
4.1 598
3.5 512
3.6 525
1.4 210
1.5 224
0.46 67
0.93 135
0.62 90
1.96 0.071
1.82 0.066
1.74 0.063
1.8 0.065
1.76 0.064
1.45 0.052
2.62 0.09
2.715 0.098
2.8 0.1
4.5 0.162
7.8 0.281
Density gr/cc lb/in A3 Specfic
Specfic
Modulus GPa/gr/cc 10 A8in
299 12
189 7.6
169 6.8
130 5.2
133 5.3
85 3.5
27 1.2
28 1.1
26 1
24 0.99
26 1.1
Strength GPa/gr/cc 10 A6in
0.95 3.8
1.21 4.8
3.2 13
2.3 9.2
2 8
2.5 10.1
0.53 2.3
0.55 2.3
0.165 0.67
0.21 0.83
0.08 0.32
ADVANCED STRUCTURAL MATERIALS
Table 2
Thermoplastic
Properties
Nylon 6
Melting Temperature Tm crystalline C F Tg amorphous C F
Properties
Polycarbonate
PolyetheretherKetone PEEK
334 633
290 554 88 190
293 560
349-382 660-720
316-329 600-625
66 9,500
70 10,200
66 9,500
300
110
50-110
30,682
51 7,400
62 9,000
91 13,200
Tensile Modulus MPa psi
689 100,000
2,379 345,000
Compressive Modulus MPa psi
172 250,000
2,413 350,000
Flexural Modulus MPa psi
965 140,000
2,344 340,000
Processing Temperature C F
210-220 410-428
Polyphenyl Sulfide PPS
150 302 227-288 440-550
Tensile Strength MPa psi Elongation at break % Tensile Yield Strength MPa psi
3,309 480,000
3,000 435,000
3,792 550,000
ySCU
Stress ^r
Composite
Section
Metal Body
Sm
Sc
J
/
/ V Strain
Figure 2. Metal canister filament wound with a composite material under zero tension.
ADVANCED STRUCTURAL MATERIALS
310
omposite Body
Stress
Tensile stress in composite at zero pressu
Tensile stresses in the composite and in the metal at working pressures
Compressive stress metal body at zero pressure
Figure 3. Metal canister filament wound with a thermoplastic tape under high tension.
303
304
ADVANCED STRUCTURAL MATERIALS
A variety of fibre reinforcements are available, depending on the cost and desired level of performance. Table 1 lists a selected variety of fibre reinforcements that are commercially available. Blends of fibres can be used to optimize cost, and performance. In general, glass is the least expensive of the fibres, but has the lowest performance level based on its density. Carbon fibers, in general, are the most expensive but have the highest performance level, except for the intermediate modulus grade of carbon fibre(IM), where the price per pound can be less than half the cost of kevlar. Epoxy resin systems are the most widely used matrix resins because of their excellent processibility and physical and mechanical properties. Thermal stability and the glass transition temperature limitations restrict the upper continuous use temperature to about 177°C (350°F). Any increase above the glass transition temperature, results in significant changes in the properties of the cured resin system. Other thermoset resin systems, such as polyimides, have an upper temperature limit of 232°C (450°F) for extended time periods. For applying thermoset resins, wet winding is the most widely used method. In this process the fibres are passed through a heated resin bath to apply the resin. Wet winding is limited to materials with viscosities of 2,000 centipoise, but does offer the most economic means of manufacture. In prepreg winding, the resin is impregnated into the fibres prior to winding using a hot melt or solvent dip process. Prepreg winding can be used with higher viscosity resins such as novolacs, polyimides, and some epoxies. It has the advantage of a longer working life before curing, and the resin content and fibre quality can be determined before winding instead of after. These materials are generally available through companies that specialize in prepreging. Thermoplastic materials are receiving more attention due primarily to their unique properties and processing characteristics. These materials offer better mechanical properties at elevated temperatures, have lower moisture retention, and have better chemical and solvent resistance. The manufacturing process allows for significantly faster fabrication rates because no curing is required, and the fabricated part can be reshaped after the initial fabrication by remelting the resin. The general interest in advanced thermoplastics, is mainly driven by four major advantages, and these are; (1) Thermoplastics provide service operating temperatures greater than 176 °C (350 °F), and under severe environmental conditions. (2) The toughness properties, usually measured by interlaminar shear values (Gic), will provide much better damage tolerance over thermoset resins. (3) The processing technology for thermoplastics provides the potential for low cost manufacturing. (4) Thermoplastics can be easily repaired due to the remelting characteristics.
ADVANCED STRUCTURAL MATERIALS
305
Thermoplastic resins range from low melt temperature polymers such as Nylon, to high melt temperature polymers such as PEEK (Polyetheretherketone). A small selection of thermoplastic resin properties is outlined in Table 2. These thermoplastic materials are now available in several forms including powder impregnated yarns, comingled yarns, melt impregnated yarns or tows, and slit tape. A typical thermoplastic tape consists of several thousand unidirectional filaments bonded together by a thermoplastic resin, and has a tape width and thickness tailored to a specific processing needs. Generally the tape width used in our applications would be 3.2 mm (.125 inches) by .15 mm (.006 inches) thick. The concept of using thermoplastic impregnated materials in filament winding is simple in principle. The steps required are, guiding the material, preheating the thermoplastic tape, contact point heating of the tape and the bonding surface, consolidating the material, and finally cooling the part. Some of the major obstacles still to be overcome are the higher processing temperatures which can exceed 427°C (800°F), and the higher resin viscosities. A variety of processing parameters must still be optimized before thermoplastic filament winding is as common as thermoset filament winding. Thermoplastic Filament Winding of High Pressure Containers: In addition to the general advantage of the fibre reinforced thermoplastic materials, there are a number of specific or component related advantages. One of these, "autofrettage" fibre reinforced composite pressure vessels without the need to pressurize above the operating pressure will be discussed. Pressure vessels can be categorized according to four types of material classifications. These are all metal containers, hooped wound metal containers utilizing a hooping material that is different than the container, a composite vessel using a thin metal liner such as aluminum, and an all composite container. The subject of this paper is only to deal with a hooped wound metal container that is open ended, similar to a gun barrel. The process of Frettage is a long standing process of reinforcing a metal container subject to internal pressure with a different material. It consisted of reinforcing the cylindrical part with a material that would absorb a portion of the circumferential stress, therefore sharing the load. It was originally applied to gun barrels, and is centuries old. The parts were made of steel and the reinforcement was obtained either by doubling the thickness of the same material, or by forging, or by wire winding. The longitudinal stresses were resisted by the original component. The technique of Autofrettage is a process of pre-stressing or over stressing a hollow cylindrical member beyond the elastic range using internal pressure. With a metal cylinder comprising of two different layers of the same material, that is the base material (inner layer) and the reinforcing layer (outer layer), the resultant structure would be pre-stressed, with the inner layer in compression and the outer layer in tension.
306
ADVANCED STRUCTURAL MATERIALS
This type of system has two limitations. First, it does not offer any weight advantages, and secondly the elastic limit of the reinforcing layer must be much greater than the inner layer to take full advantage of this process. Another technique is to create an interference fit between the two materials by having the outside diameter of the base material greater than the inside diameter of the reinforcing layer. Again, the main limitation to this process is that, the ultimate tensile strength of the reinforcing material is only marginally different from that of the container. A particular recurring problem with filament winding over a metal container using an thermoset epoxy is the separation of the composite section from the metal part after curing. Due to the differences in the thermal coefficients of expansion of the composite and the metal, this problem will always exist. Another problem is the level of stress that is developed in the composite section during the pressuhzation of the metal component. For a given strain level, the stress level developed in the composite, Sc, is very low, compared to the ultimate stress, Scu, of the material as illustrated in Figure 2. As a consequence, the composite material would be stressed far below its ultimate capacity and is an inefficient use of the composite's high stress carrying capabilities. We can take advantage of the differences in elastic elongation of each material, by filament winding under very high tension. To take full advantage of the thermoplastic prepreg tape, the fibres can be loaded under very high tension, and melted on the metal part at the same time. By winding under high tension, a desired level of pre-stressing in the metal component can be achieved, with no risk of the fibres migrating close to the metal component. Filament winding with a liquid resin impregnated system under tension merely gathers all the fibres in close contact with the metal part, and forcing the resin to the outer surface. This creates a two layer coating, rather that an absolute composite structure in which the fibers are individually and uniformly embedded in the matrix to achieve structural integrity. Thermoplastic tapes can be stressed in their present form, unlike thermoset prepregs, because they are in a fully consolidated state. That is, the thermoplastic tape is a composite preform, and when stressed, the fibres and the thermoplastic resin share the load. To utilize the unique characteristics of the thermoplastic tape, the tape is wound under controlled high tension onto the base material. This technique of Frettage as applied to thermoplastics is illustrated in Figure 3. In this case, a pre-stressed liner is obtained through filament winding at very high tension levels. For a given strain level, the stress in the composite section is much higher than would be if it where not wound under very high tension, resulting in a reduced level of stress in the metal section. As a result, the composite section is now contributing significantly to the overall performance to the loading. There are additional advantages for pressure vessels by applying the frettage process with a thermoplastic tape, and they are; 1) Half the thickness of the pressure vessel can be replaced for the same operating pressure resulting in weight savings.
ADVANCED STRUCTURAL MATERIALS
307
2) The pressure rating can be doubled by hooping over the existing pressure vessel with only marginal weight gain. Now with the introduction of advanced thermoplastic materials this century old process of Frettage has now taken on a new dimension. It is possible to limit the stress levels in the hooping material or the metal canister by altering the thickness or winding tension and therefore meet the design requirements. The thickness of the composite section and the winding tension must be chosen so that the pre-stressing combined with the lowest possible operating temperature is always effective, otherwise, the hooping material will separate from the metal body. Also the stress due to the internal pressure loading and the design temperature must remain within the permissible material limits. Another design tool, allows for optimization of the composite section by choosing the combination of reinforcing fibres and resin to obtain the desired modulus and tensile strength. The modulus of the tape can be varied by using a different reinforcing fibre and/or by varying the thermoplastic resin content. Summary: Thermoplastic filament winding techniques can be utilized to perform tasks that are difficult, or impossible to accomplish with thermoset materials or even metals. At FRE Composites, we have responded to this challenge, and have developed techniques that have enabled us to filament wind with thermoplastic materials. The technique of Frettage, using the exceptional properties of the thermoplastic tape, compared to the properties of the metal is one example. Thermoplastic filament winding is not limited to the over wrapping of metal containers. There are many other applications in which thermoplastics can be used that will not only replace metals, but even the presently used thermosets. Filament winding with thermoplastic reinforced composites is the up and coming technology for this century. References: 1
"Modern Plastics Encyclopedia", McGraw-Hill Publications
2
"Engineered Materials Handbook, Composites", Volume 1, ASM International
ADVANCED STRUCTURAL MATERIALS
AUXILIARY CAfifiiAC€ SPRAW CHOPPER cm TAP{
TCRUIHAi.
Figure 1. Typical computer controlled filament winding machine configuration (Source: Entec)
Comparison of Reinforcing Fibres and Metals
Carbon Fibre Aramid Glass Metals Ultra High High Intermediate Very High High Kevlar 49 E E-CR Aluminum Titanium U.S.A Canadian Alloy Modulus Modulus Modulus Strength S t r e n g t h Alloy 6A1-4V L65
Property Tensile Modulus GPa. psi. x 10A6 Tensile
Strength GPa. psi. x 10A6
Steel 080M46 (BS970:1983)
586 85
345 50
295 43
235 34
235 34
124 18
72 10.5
77 11.2
72 10.5
110 16
207 30
1.86 270
2.2 320
5.6 817
4.1 598
3.5 512
3.6 525
1.4 210
1.5 224
0.46 67
0.93 135
0.62 90
1.96 0.071
1.82 0.066
1.74 0.063
1.8 0.065
1.76 0.064
1.45 0.052
2.62 0.09
2.715 0.098
2.8 0.1
4.5 0.162
7.8 0.281
Density gr/cc lb/in A3 Specfic
Specfic
Modulus GPa/gr/cc 10 A8in
299 12
189 7.6
169 6.8
130 5.2
133 5.3
85 3.5
27 1.2
28 1.1
26 1
24 0.99
26 1.1
Strength GPa/gr/cc 10 A6in
0.95 3.8
1.21 4.8
3.2 13
2.3 9.2
2 8
2.5 10.1
0.53 2.3
0.55 2.3
0.165 0.67
0.21 0.83
0.08 0.32
ADVANCED STRUCTURAL MATERIALS
Table 2
Thermoplastic
Properties
Nylon 6
Melting Temperature Tm crystalline C F Tg amorphous C F
Properties
Polycarbonate
PolyetheretherKetone PEEK
334 633
290 554 88 190
293 560
349-382 660-720
316-329 600-625
66 9,500
70 10,200
66 9,500
300
110
50-110
30,682
51 7,400
62 9,000
91 13,200
Tensile Modulus MPa psi
689 100,000
2,379 345,000
Compressive Modulus MPa psi
172 250,000
2,413 350,000
Flexural Modulus MPa psi
965 140,000
2,344 340,000
Processing Temperature C F
210-220 410-428
Polyphenyl Sulfide PPS
150 302 227-288 440-550
Tensile Strength MPa psi Elongation at break % Tensile Yield Strength MPa psi
3,309 480,000
3,000 435,000
3,792 550,000
ySCU
Stress ^r
Composite
Section
Metal Body
Sm
Sc
J
/
/ V Strain
Figure 2. Metal canister filament wound with a composite material under zero tension.
ADVANCED STRUCTURAL MATERIALS
310
omposite Body
Stress
Tensile stress in composite at zero pressu
Tensile stresses in the composite and in the metal at working pressures
Compressive stress metal body at zero pressure
Figure 3. Metal canister filament wound with a thermoplastic tape under high tension.