Improved vacuum thermoforming of polyamide
Thermoforming is one of the most important techniques in polymer processing, and many different variants of the process are used to form a heated thermoplastic sheet into a 3D part. Vacuum thermoforming is a very common variant, where the sheet is drawn over a male or into a female mold by a vacuum between the sheet and the mold. This limits the available pressure for forming and means that a thermoplastic must remain sufficiently stiff during heating to avoid sagging or even destruction of the sheet.1 However, it also has to be soft enough to be formed by the vacuum. This is one reason why it is more difficult to thermoform semi-crystalline polymers.2
Semi-crystalline thermoplastics normally have certain advantages for applications such as engine covers or components for noise protection. For example, polyamide 12 (PA12) has superior electrical properties, high stress cracking resistance, and excellent stability against greases, oils, fuel, and alkaline substances.3 However, like other semi-crystalline thermoplastics and in contrast to amorphous thermoplastics, it shows a sharp drop in modulus at melting temperature. In addition, it is sufficiently stiff over only a narrow temperature range, so it is not suitable for vacuum thermoforming.
Another prerequisite for effective vacuum thermoforming is sufficient elongational viscosity during sheet stretching. Several researchers have found uniaxial elongational viscosities of 105–107Pa·s at forming temperatures for polyethylene (PE) and polypropylene (PP).4–6 Additionally, strain-hardening elongation behavior is important, because a strain-hardening material reaches a more homogeneous wall thickness distribution.5 Strain hardening depends on the molecular weight of a polymer,7 its molecular weight distribution, high molecular weight components, and chain branching.8
One way to modify polymers for vacuum thermoforming is to use high-energy radiation to induce crosslinking. This occurs when radiation-induced polymer radicals recombine, forming covalent bonds between macromolecules. It increases molecular weight, chain branching and, at higher degrees of irradiation, a molecular network forms. Thus, crosslinking should stiffen a polymer sheet for thermoforming. Pausch and Wunsch have demonstrated the general thermoformability of radiation-crosslinked PE by mechanical deep-drawing of specimens below melting temperature.9 Modifying semicrystalline thermoplastics such as polyamides with high-energy irradiation may make them suitable for vacuum thermoforming, which may result in new applications for vacuum thermoformed parts.
We studied the influence of radiation treatment on the thermoformability of extruded, crosslinkable PA12 films. We irradiated PA12 films (product code V-Rilsan-PA12-AECN 0 TL, which is PA12 with 3.0wt% of triallyl isocyanurate) with 66kGy of 5MeV electron beams. We then thermoformed these films using modular male thermoforming molds (see Figure 1). We characterized rheological material behavior during thermoforming with oscillatory plate/plate measurements with an AR2000 rheometer and used an ARES-EVF rheometer for uniaxial elongational rheometry.
The temperature window that allows complete molding of the test geometries for different areal draw ratios is shown in Figure 2. Our experiments showed that radiation-crosslinked PA12 films offer very good thermoformability. The lower temperature limit rises with areal draw ratio, and the upper limit falls. The forming temperature window of irradiated PA12 film is, depending on the areal draw ratio, 40°C to 100°C broad, which shows the high process stability when using crosslinked sheets.
In contrast, we could not thermoform non-irradiated films sufficiently. It was not possible to heat non-crosslinked sheets above their crystalline melting temperature. The polymer is not sufficiently stiff, resulting in excessive sagging that destroys the film. We determined the strain distribution in thermoformed parts by a line grid of 10mm spacing drawn on the sheets. We calculated that the local strains in the radial direction form the line distance measured after thermoforming (in the radial direction). We found that there was no significant influence of the forming temperature on the strain distribution (see Figure 3).
We measured the storage and loss moduli of non-irradiated and irradiated PA12 over a wide temperature range (see Figure 4). Covalent bonds between macromolecules resulted in a much higher storage modulus above crystalline melting temperature for irradiated PA12 compared to the non-crosslinked one. The crosslinked melt behaved elastically and showed a nearly constant modulus up to about 250°C. The reason for the excessive sagging of non-crosslinked PA12 sheet can be seen in the low modulus of less than 103Pa.
The elongational viscosity of polymer melts generally depends on temperature. For crosslinked PA12 there is only a minor influence of measurement temperature on the elongational viscosity (see Figure 5). This explains the independence of the strain distribution in thermoformed parts from the forming temperature (see Figure 3).
Moreover, elongation at break dropped with rising temperature. From 180°C to 220°C, we found temperature has no effect on the resulting elongation at break, but for temperatures above 220°C, specimens fracture at lower strains. This can be explained by thermal and thermo-oxidative degradation effects at the measurement temperature. Further investigations have shown that higher elongations at break can be reached with smaller radiation doses, maintaining the advantage of an extraordinarily robust thermoforming process.10, 11
Our results demonstrate that thermoforming of radiation-crosslinked PA12 is feasible. Irradiation enhances the processability of the polymer and makes a robust vacuum thermoforming process possible. The irradiated polymer has excellent thermoformability, thanks to its modulus during heating and its elongational viscosity during sheet stretching. The radiation-induced crosslinking greatly increases the modulus above melting temperature and allows the polymer to be heated without excessive sagging. This method for modifying semi-crystalline technical thermoplastics, like PA12 or polybutylene terephthalate (commonly known as PBT), offers the potential to make new materials available for vacuum thermoforming technical products such as engine covers or components for noise protection.12 We now intend to explore the use of low doses of radiation. Lower doses are more efficient and might even, if sufficiently low, allow recycling of the material.
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- D. Drummer and A. Seefried, Investigation of thermoformability of e-beam cross linked polyamide 12, J. Plast. Technol. 6 (6), pp. 292-315, 2010.
- D. Drummer and A. Seefried, Thermoformability of radiation cross linked polyamide 12, Proc. Ann. Tech. Conf., pp. 2520-2526, 2011.
- D. Drummer, A. Seefried and M. Fuchs, Irradiation opens up third film dimension, Kunstst. Int'l 100 (11), pp. 45–49, 2010.