NANO-EFFECT IN IN SITU NYLON-6 NANOCOMPOSITES
Ying Liang, Scott Omachinski, Jason Logsdon, Jae Whan Cho, Tie Lan
1350 West Shure Drive
Arlington Heights, IL 60004
12-Aminododecanoic acid modified montmorillonite (ADA-MONT) has been incorporated in nylon 6 by in situ polymerization. Mechanical and barrier properties were evaluated for nanocomposites containing 2, 4, 6 and 8 wt.% ADA-MONT. The high aspect ratio of montmorillonite and the interaction between polymer chains and dispersed silicate nanolayers creates a 110% increase in flexural and tensile moduli, and a 175% increase in heat distortion temperature under load (DTUL), at a loading level of 8 wt.% ADA-MONT. In addition, smooth, transparent films were successfully cast using standard techniques and equipment. These films were tested for gas permeation at 65% relative humidity. Oxygen transmission rates (OTR) improve as ADA-MONT addition levels increase. At 8 wt.% addition level, OTR reduction is 80%. This makes nylon 6 nanocomposites particularly appropriate for packaging applications requiring improved barrier.
Nanocomposite technology has been described as the next great frontier of material science. Polymer resins containing well-dispersed layered silicate nanoclays are emerging as a new class of nanocomposites. By employing minimal addition levels (< 10 wt.%) nanoclays enhance mechanical, thermal, dimensional and barrier performance properties significantly. Generally speaking, for every 1 wt.% addition, a property increase on the order of 10% is realized. This loading-to-performance ratio is known as the “nano-effect”.
The most promising candidate of the layered silicates is montmorillonite owing to its natural abundance and high aspect ratio. In the nanocomposite field, nylon 6 is the most studied plastic due to Toyota Research and Development Laboratory’s pioneering work (1) and nylon’s popularity as both a film grade for food packaging, and an engineering grade for injection molded parts. Organic treatment on clay surface serves to enhance compatibility between hydrophilic montmorillonite and hydrophobic nylon. By creating a nanocomposite via in situ polymerization, moduli, DTUL and gas barrier can be significantly improved. The degree of property enhancement meets the criteria of the nano-effect and in some instances exceeds those criteria.
In general, two methods have been employed to disperse montmorillonite nanolayers into nylon 6 matrix. One is in situ polymerization, in which polymerization takes place after mixing monomer or oligomer with organically modified montmorillonite, such as ADA-MONT (1-4). The second method is melt compounding, which adds an organically modified montmorillonite into a polymer melt (5). Based on current literature, in situ formed nanocomposites out-perform melt compounded ones by a significant margin (6). One obvious advantage of in situ polymerization is the tethering effect, which enables the nanoclay’s surface organic chemical, such as 12-aminododecanoic acid (ADA), to link with nylon 6 polymer chains during polymerization, as illustrated in Figure 1.
The tethering effect not only renders the montmorillonite nanolayer compatible with nylon 6, but also delivers surprisingly strong linkage to nylon 6 matrix, which can be illustrated by high mechanical properties. Tethering brings the surface treatment to be part of the nylon molecule itself, thus eliminates the possibility of ADA migration out of nylon 6. FDA has approved the use of in situ nylon 6 nanocomposites containing ADA-MONT for direct food contact applications.
In Situ Polymerization
ADA-MONT is formulated to be dispersed
easily into nylon monomer, e-caprolactam. In situ polymerization is based on the following procedure: swelling ADA-MONT in molten e-caprolactam at 90oC for 2-4 hrs, adding 5% wt/wt 6-aminocaproic acid based on caprolactam, charging the reactor, and polymerizing at 260oC for 10 hrs under agitation and nitrogen blanket. The nanocomposite is then pelletized, washed and dried as normal.
Due to the high surface area of the nanolayers, the caprolactam-containing nanolayers can develop increased viscosity compared with the pure monomer. If not controlled, elevated viscosity will interfere with polymerization. The ADA-MONT (Nanomer® I.24TL) used in this study was developed to minimize viscosity increase. We have measured viscosities at 2, 4, 6, and 8 wt.% ADA-MONT in caprolactam melts at 80oC, and found that in all cases the viscosity of the mixture was low enough to easily transfer the swollen ADA-MONT/molten caprolactam to the batch reactor and conduct a successful polymerization.
Characterization of Nanocomposites
XRD can be used to a limited extent to reveal the intercalation/exfoliation of nanolayers within the polymer matrix (7). As shown in Figure 2, no initial or swollen ADA-MONT peaks were observed by XRD in the nanocomposites. This indicates full exfoliation of the nanolayers in the matrix. TEM confirmed well-exfoliated systems for the nanocomposites. Tethering effect contributes to the exfoliation. It allows some of the polymer chains to grow from the ADA-MONT inner surface where ADA is attached by cation exchange reaction, and the polymer synthesis force helps to bring the ADA-MONT nanolayers further apart, ultimately exfoliating them into the polymer domain.
The nano-effect is quite evident judging from mechanical and barrier properties. As for mechanical properties, in the highest loaded nanocomposite we achieved a 110% increase in flexural and tensile moduli (Figure 3), and a 175% increase in heat distortion temperature under load (DTUL). Detailed data is listed in Table 1. On the other hand, there is no negative effect on notched Izod impact strength from nanocomposite formation.
Barrier Properties (O2 and H2O)
As shown in Figure 4, OTR was reduced 80% with 8 wt.% ADA-MONT. At the same relative humidity level, the absolute OTR of a nylon 6 nanocomposite containing 4 wt.% ADA-MONT is superior to amorphous nylons based on hexamethylene diamine, isophthalic acid and terephthalic acid. With excellent barrier performance and regulatory clearance, nylon 6 nanocomposites have wide applications in food packaging, ranging from single layer and multi layers film to rigid plastic containers.
In addition, the water uptake for the nanocomposites is dramatically decreased. For example, the nanocomposite with 4 wt.% ADA-MONT uptakes around 40% less water after 800 hours immersion than does neat resin. Reduced water permeation allows nanocomposites to maintain better performance compared with neat resin under prolonged water exposure conditions.
High performance in situ nylon 6 nanocomposites with up to 8 wt.% ADA-MONT nanolayer loading have been successfully prepared. The nano-effect has been demonstrated for moduli and barrier properties. These new materials resist moisture uptake, making nylon 6 perform better in high humidity conditions.
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Nanomer® is a registered trademark of Nanocor, Inc.
In situ polymerization, Nylon 6, Nanocomposite, Tethering effect.
Figures and Graphs
Nylon 6 Nanocomposite Formed through in situ Polymerization with ADA-MONT (Nanomer® I.24TL).
Figure 2. XRD of in situ Nylon 6 Nanocomposites.
Figure 3. Dependence of Modulus on Nanoclay Loading.
Figure 4. OTR of Nylon 6 Nanocomposite from In situ Polymerization (65% RH).
Table 1. Mechanical Properties of Nylon 6 Nanocomposites.