Microwave welding of high density polyethylene using intrinsically conductive polyaniline.
Plastics and their composites are increasingly being used in structural and commercial applications because of their unique properties (e.g. high specific stiffness and strength) and the flexibility in forming parts. However, the use of these materials is often limited by the ability to join parts and to produce predictable and repeatable high quality joints. This is especially true for welding large structural components. Most existing welding processes are better suited for welding small parts; these would be difficult or impossible to adapt to welding of large structures. Hence the need to develop new welding techniques for welding large parts. In particular microwave welding is very attractive because large scale equipment already exists [Thermex/Thermatron Inc. has built a 7.3 m (24 ft) long by 3 m (10 ft) wide by 3 m (10 ft) high oven.] In addition, schemes for continuous microwave processing are also known (1) and could be used to weld large and small components. In microwave joining, electromagnetic energy-absorbing intrinsically conductive polymers (2-5) or chiral polymer (5, 6) are placed at joint interfaces, thereby locally heating and welding the parts.
During the past 20 years, a new class of electrically conductive polymers, such as polythiophene, polypyrrole, and polyaniline, have been intensively studied. These materials have a unique combination of mechanical and electrical properties, making them very useful for welding, and they provide an opportunity for developing new joining techniques for large structures. In this study, intrinsically conductive polyaniline was chosen because it is relatively inexpensive, it is easy to synthesize and process, and it is stable at room temperature (7). Until recently, most applications of polyaniline have focused on its insulator-to-metal transition properties, such as for rechargeable batteries, integrated circuits (8), electrolytic capacitors, sensors and color-changing windows (9). It can also be used for electromagnetic shielding (10). For plastic joining, intrinsically conductive polyaniline was successfully used in dielectric (11) and microwave (3) welding and preliminary results show that it can also be used in resistance and induction welding.
As in semiconductors, polyaniline conducts electricity through doping (12), which creates partially filled bands through which free moving electrons conduct electricity. The electrical properties of polyaniline are a function of frequency, temperature, morphology, and doping level. For example, microwave conductivity is higher than DC conductivity at low doping levels (13). In the temperature range of 60 to 300 K, the electrical conductivity increases with increasing temperature (14). Increasing the doping level increases the conductivity, and changing the type of dopant used changes the conductivity (4). The molecular structure or morphology of the polyaniline can also affect its conductivity and loss tangent. Stretched films or fibers that have a higher level of crystallinity have a higher electrical conductivity than unstretched films and powders (4). The electrical conductivity of polyaniline can be varied from that of an insulator ([10.sup.-10] S/cm) to that of a conductor ([10.sup.2] S/cm), depending on the processing technique (15). For this work, HCl-doped polyanillne or fully protonated emaraldine hydrochloride salt was used.
In microwave welding, by placing conductive polyaniline at the joint interface, it is possible to locally heat the interface, melt the bulk polymer, and weld the parts. For butt welding of high density polyethylene (HDPE) bars, a conductive composite gasket was molded from a mixture of HCl-doped polyaniline powder and HDPE powder. The gasket was then placed at the joint interface. The gasket absorbs the microwave energy, thereby generating heat at the interface and melting the HDPE in the gasket and in the bars. Under pressure, the molten HDPE in the gasket and bars flows, permitting the formation of intimate contact and enabling intermolecular diffusion. The microwave power is then turned off, allowing the interface to cool and solidify, to terminate the joining process. Generally, the gasket remains at the interface, but under special welding conditions most of the gasket material can be squeezed out.
Many parameters affect the final joint strength of microwave-welded parts, including the electrical and mechanical properties of the gasket, the heating time, the heating pressure, the welding pressure, and the hold time. Previously, the effect of weight fraction of polyaniline on the tensile strength of the composite gasket was studied (3). It was also shown that when the gasket remains at the interface, the joint strength is limited by the gasket strength, but when most of the gasket is squeezed out, the joint strength approaches the tensile strength of HDPE (3).
Critical to the success of the microwave welding process is the ability of the gasket to absorb electromagnetic energy, which has a complex relation to the electrical properties and geometry of the gasket. Therefore, it is important to study the heating of the gasket in a microwave. In general, heat generation in an electromagnetic field is due to many loss mechanisms, including dipolar or polarization, electronic, atomic, Maxwell-Wagner, and ohmic losses (16). Often, all of these losses are grouped together in one effective loss factor (16). In HCl-doped polyaniline, ohmic losses dominate (4). Therefore, in this case the effective dielectric loss factor is
[[Epsilon][double prime].sub.eff] = [Sigma]/[[Epsilon].sub.0][Omega] = [Epsilon][prime] tan [[Delta].sub.eff] (1)
where [[Epsilon][double prime].sub.eff] is the effective loss factor, [Sigma] is the electrical conductivity, [[Epsilon].sub.0] is the dielectric constant of free space, [Omega] is the frequency, [Epsilon][prime] is the relative dielectric constant, and tan [[Delta].sub.eff] is the effective loss tangent. Thus, by measuring the complex dielectric constant, the effective loss tangent and conductivity can be found from Eq 1. To determine the power dissipation in the gasket, it is necessary to determine the electric field distribution in the material. Assuming the electric field in the gasket to be harmonic and of constant magnitude (gasket is small compared with the wavelength and penetration depth), the average power dissipated in the gasket is (16)
[Mathematical Expression Omitted] (2)
where [P.sub.avg] is the average power dissipated, [E.sub.rms] is the root mean square value of the electric field strength in the gasket, and V is the volume of the gasket. It is important to note that the electric field strength in the gasket is related to the conductivity of the gasket. Therefore, increasing the conductivity of the gasket may not necessarily increase the power dissipated. For example, for a perfect conductor, the microwaves are reflected and the internal electric field strength is zero, resulting in no heating (16, 17). Unfortunately, determining the internal field strength in the gasket is very complicated, since its introduction into the microwave cavity alters the field (16). Therefore, the field strength and the internal heat generation rate in the gasket were estimated using calorimetry or adiabatic heating of the gasket while measuring the temperature rise (16).
[E.sub.rms] = [square root of [Rho][C.sub.p]/[Sigma] dT/dt] (3)
where [Rho] is the density, [C.sub.p] is the specific heat, T is temperature, and t is time. This way the heating effectiveness of each type of gasket could be evaluated.
Intrinsically Conductive Polyaniline
A chemical oxidation process was used to make intrinsically conductive polyaniline. Aniline, ([C.sub.6][H.sub.5])N[H.sub.2], was dissolved in 1 M HCl that was then mixed with a solution of 1 M HCl and ammonium peroxydisulfate, [(N[H.sub.4]).sub.2][S.sub.2][O.sub.8], at 5 [degrees] C (15), resulting in a precipitation of "A-B" type polymer (18). The precipitated "cake-like" polyaniline was collected on a Buchner funnel. The dark green polyaniline was then stirred in 1 M HCl, washed and dried under vacuum for 24 hours, resulting in intrinsically conductive polyaniline that is 50% (fully) protonated or fully doped.
HDPE/Polyaniline Composite Gasket
The "cake-like" dried polyaniline was ground into powder by using a mortar and pestle. Different weight percents of polyaniline powder were then mixed with HDPE powder. The mixtures were compression molded in a hot press using a 31.75-mm diameter cylindrical mold. A pressure of 8.27 [+ or -] 2.07 MPa was applied during heating and cooling. The mold was heated to 180 [degrees] C, maintained at this temperature for 6 min and then air cooled back to ambient. Two gaskets with different weight fractions of PANI were made. A mixture of 0.2 g PANI and 0.2 g HDPE powder was used to make a 0.5-mm-thick 50% PANI gasket, and a mixture of 0.24 g PANI and 0.16 g HDPE powder was used to make a 0.5-mm-thick 60% PANI gasket. About 5% of the material squeezed out during molding, after which the round gasket was cut into 6.35 by 6.35 mm squares.
Adiabatic Heating Experiments
For the adiabatic heating experiments, the gasket was suspended in a beaker to minimize convective losses caused by the fan in the microwave. A Luxtron 755 multi-channel fluoroptic thermometer was attached to one side of the gasket. The beaker was then covered with low density polyethylene film. During heating, the temperature was recorded as a function of time. To determine if degradation in the electrical properties of the gasket occurs at elevated temperatures, some gaskets were subsequently heated two additional times.
Welding of HDPE
Two 6.35 x 6.35 x 50.8 mm HDPE bars were butt welded across the square ends. The parts together with the gasket in the joint interface were placed in the HDPE fixture shown in Fig. 1. Welding was effected by placing the fixture in a domestic 2.45 GHz microwave oven (Toshiba ERS-5740B 600 watt) that had a digital timer. Pressure was applied by an air cylinder pushing on a HDPE bar [ILLUSTRATION FOR FIGURE 1 OMITTED] through a hole in the oven wall. The heating pressure and the welding pressure were changed by varying the air pressure in the cylinder.
Testing and Evaluation
The tensile strength of the unwelded HDPE and of the welded joint were evaluated on an Instron 4201 testing machine at a displacement rate of 5.08 mm/ min. The structure of the composite gasket and welds were examined using an optical microscope.
RESULTS AND DISCUSSION
Estimating Heat Generation Rate
Adiabatic heating was used to estimate the heat generation rate and the internal electric field strength in the gasket. Figure 2 shows typical temperature histories for adiabatic heating of the 50% PANI and 60% PANI gaskets. To estimate the initial heat generation rate in the gasket, linear regression was used to calculate the temperature rise rate for the temperature range of 30 [degrees] C to 40 [degrees] C. The rule of mixtures was used to calculate the density of the 50% PANI and 60% PANI gaskets to be 950 kg/[m.sup.3] and 960 kg/[m.sup.3] respectively. Bird et al. (19) report the specific heat for HDPE at 35 [degrees] C to be 1810 J/(kg [center dot] [degrees] C). Differential scanning calorimetry was used to measure the specific heat for PANI powder at 35 [degrees] C. Then, the rule of mixtures was used to calculate the specific heat of 50% PANI and 60% PANI to be 1970 J/(kg [center dot] [degrees] C) and 2000 J/(kg [center dot] [degrees] C) respectively. Table 1 shows the calculated initial heat generation rates for three samples of each gasket. Figure 2 and Table 1 show that the initial temperature rise rate and heat generation rate for both gaskets is the same. However, as shown in Fig. 2, as the heating time increases, the temperature rise rate (heat generation rate) and the absolute temperature for the 60% PANI exceeds that of 50% PANI. Therefore, 60% PANI gaskets were used for most of the welding experiments.
Figure 2 shows that thermal runaway (5) is not a problem for these gaskets because at elevated temperatures, the temperature rise rate (heat generation rate) decreases with increasing temperature. This decrease in heat generation rate is probably due to a reduction in the electrical conductivity of PANI at elevated temperatures (20). Figure 3 shows that in subsequent heating of the same 60% PANI gasket, the temperature and temperature rise rate in the gasket are substantially lower than they were during the first heating. This irreversible loss in electromagnetic absorption is probably due to permanent chemical changes, which cause an irreversible loss of conductivity [TABULAR DATA FOR TABLE 1 OMITTED] in PANI at high temperatures (21). Further work is being done to investigate the reversible and irreversible reductions in electrical conductivity of the gasket at elevated temperatures and their effects on microwave welding.
Estimating Electric Field Strength
The adiabatic heating experiments and Eq 3 can also be used to estimate the initial electric field strength in the gasket. To use Eq 3 the electrical conductivity of the gaskets was measured at microwave frequencies and room temperature using a transmission line technique (22). The average electrical conductivity of the 50% PANI and 60% PANI gaskets at 2.45 GHz were measured to be 3.34 S/m and 8.13 S/m, respectively. As shown in Table 1, the estimated electric field strength in the 50% PANI gaskets is substantially higher than the electric field strength in the 60% PANI gaskets. As mentioned earlier, this is to be expected since the electrical conductivity of the 60% PANI gaskets is much higher than that of the 50% PANI gaskets. Therefore, for maximum electromagnetic absorption, there is a trade-off between having a high electrical conductivity and a high electric field strength (see Eq 2). That is the reason that the 50% PANI and 60% PANI have essentially the same initial heat generation rate (see Table 1) while their conductivities differ by more than a factor of two. Thus, there must be an optimal electrical conductivity for maximum electromagnetic absorption. While it is desirable to find this optimal electrical conductivity, it should be remembered that optimal heating is not possible over the whole temperature range because, as was discussed earlier, the conductivity of the gasket changes with temperature.
Effect of Heating Pressure on Weld Strength
During the initial microwave welding tests, the applied pressure on the samples was kept constant throughout the heating and cooling of the parts, resulting in equal heating and welding pressures. Figure 4 shows that for heating and welding pressures of 0.31 MPa, even long heating times could not improve the average weld strength beyond [similar to]16 MPa, which is only 65% of the bulk strength of HDPE. Viewing the heating process showed that under these heating conditions (heating pressure of 0.31 MPa) the gasket gets squeezed out of the interface as the HDPE in the gasket melts. As the gasket squeezes out, less conductive material remains at the interface, resulting in reduced heating of the interface and in a smaller molten layer in the parts. Therefore, to minimize the squeeze out of the gasket, the parts were brought in contact with the gasket, but no pressure was applied during the heating. Figure 4 shows that for the case of no heating pressure and a welding pressure of 0.31 MPa, the weld strength increases with increasing heating time, and it approaches the bulk strength of HDPE. Therefore, in all subsequent welding tests, no heating pressure was applied.
Effect of Heating Time on Weld Strength
During microwave welding, increasing the heating time results in the development of a thicker molten layer in the parts, which improves the joint strength. The larger molten layer provides more time for intimate contact and diffusion to occur prior to resolidification. Also, the thicker molten layer enables complete squeeze-out of the gasket during the welding stage so that the mechanically weak gasket does not limit the strength of the joint (3). Figure 5 shows the effect of heating time on weld strength for two welding pressures. In both cases, the weld strength increases with increased heating time, and it approaches the strength of the bulk HDPE.
Effect of Welding Pressure on Weld Strength
Figure 5 shows that increasing the welding pressure increases the weld strength for all heating times. The higher welding pressure results in more rapid and complete squeeze-out of the mechanically weak gasket, and it enables the parts to achieve intimate contact at the interface faster. Figure 6 shows that for a constant heating time of 60 sec, increasing the welding pressure substantially improves the weld strength, and for a weld pressure of 0.9 MPa the weld strength is equal to the bulk strength of HDPE. However, as was observed in hot plate welding (23), it is expected that for high welding pressures, the weld strength will decrease because most of the molten polymer would be squeezed out prior to intimate contact and diffusion occurring.
A novel technique for microwave joining HDPE using conductive polyaniline composite gaskets was developed. Adiabatic heating measurements were used to estimate the initial heat generation rate and electric field strength in the gasket. It was found that although 50% PANI gaskets had a much lower electrical conductivity than 60% PANI gaskets, their electric field strength was higher resulting in an initial heat generation rate that was equal to the initial heat generation rate of the 60% PANI gaskets. During welding, it was found that by applying no heating pressure, thicker molten layers could develop in the parts, resulting in stronger welds. Increasing the heating time also increases the thickness of the molten layer, and therefore it also improves the weld strength. Finally, increasing the welding pressure also increased the weld strength because most of the mechanically weaker gasket could be squeezed out, enabling faster intimate contact between the parts. Under the best welding conditions, using a 60% PANI gasket with a heating time of 60 sec and a welding pressure of 0.9 MPa, the weld strength was 24.79 [+ or -] 0.34 MPa, which is 100% of the bulk strength of HDPE. Pressure control during welding appears to be important and requires further modeling and experimental work. In general, microwave welding of HDPE using intrinsically conductive polymers was very successful, and it is expected to have numerous applications for other types of thermoplastics and joints.
This work was sponsored by The Ohio State University Center for Materials Research. Our thanks to Professors E. Newman and L. J. Lee for many helpful discussions and suggestions. Our many thanks to Professor A. J. Epstein and his group for instructions on synthesizing polyaniline, and for providing materials and many useful suggestions. We especially thank J. S. Joo and C. Faisst for helping with the measurements. Finally, our thanks to Professor Alan MacDiarmid and his group for supplying us with materials and help.
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|Title Annotation:||Joining of Plastics and Plastic Composites|
|Author:||Wu, Chung-Yuan; Benatar, Avraham|
|Publication:||Polymer Engineering and Science|
|Date:||Apr 1, 1997|
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