The fastest nanocomposits in the world
The new Maserati Quattroporte, one of the most advanced and sophisticated expressions of Italian automotive technology, hides under its bonnet an interesting application of the revolutionary nanocomposits. It is an engine cover, large size but extremely thin, that appears at first sight as soon as the bonnet of the car is opened.
The use of a special nanocomposit polyamide (also called PA NCH, Nylon Clay Hybrid) has allowed Rieter, one of the leading Companies in the production of engine compartment components, to solve several production problems. There were actually so many problems that concerns were raised about the possibility of finding the right material. In order to do so, the following aspects were taken into account:
- dimensional stability: the piece has about a 1200mm length, with a 500 mm depth, and a thickness between 2 and 3 mm, with a requirement for an optimal planarity and precise wheelbase with the joints
- thermal stability: the piece must resist high temperatures created by the power of the engine
- surface aesthetic aspect: the cover makes a strong visual impact, making it necessary to avoid subsequent expensive painting
- rheological properties: due to the big dimension, the material must be able to slide in a controllable way for long trails, with reduced thickness
- process stability: due to the large dimensions of the mould, it was necessary to use a big dimension press with long residence times of the polymer in the cylinder
- chemical resistance: the components located under the engine are in contact with several oils and greases that can reach high temperatures
- weight: especially for sport cars, the weight reduction for every component is extremely important for good performances
After an accurate process of material selection, the choice narrowed to filled and/or reinforced polyamides. These products, although well known by Rieter, left some doubts especially from the sliding and aesthetic point of view.
Maip Group’s technicians then proposed to use a special nanocomposit polyamide made by UBE Japan, the first Company to globally patent nanopolymers. UBE Japan has the most production experience with this unique technology. The selected material was a polyamide base 6, which incorporates only 2% nanofillers, known with the commercial name of UBE Nylon 1015 C2 50.
The results were immediately positive. It was clear that the piece was going to diminish its weight of more than 800 grams with an optimal dimensional stability (the foreseen shrinkage, definitely isotropic, was perfectly confirmed).
As you can see in the PA nanocomposit technical data sheet, the adding of an extremely small percentage of filler (2%) gives to the polymer the thermal and mechanic characteristics of a traditional polyamide filled at 40%, preserving the characteristics of fluidity, aesthetic aspect, and lightness of an unfilled polyamide. (Tab. 1)
DIMENSIONAL STABILITY
All the polyamides, being semi-crystalline, have a quite high shrinkage, and on average are quite anisotropic, that can vary for several reasons. Despite the minimum percentage of charge, the nanocomposit polyamides show a substantial shrinkage decrease (about 20% less) compared to a traditionally unfilled polyamide, even with a reduced anisotropy. Obviously the shrinkage, beyond the moulding parameters and the components’ shape, is linked to the thickness of the piece.
UBE nylon 1015 C2 grade shows a “medium” shrinkage between longitudinal and transversal as expressed in the following table:
THICKNESS AVERAGE SHRINKAGE
Mm 1.5 0.65
2.0 0.80
3.0 1.00
4.0 1.15
With the engine cover it was impossible to consider the values of the longitudinal and transversal shrinkage because of the complexity of the non-symmetric shape which presented empty areas, winglets, and four different injection points. For this reason, considering a 3 mm thickness, it was decided to calculate a 1% shrinkage value for the moulding drawings. As stated before, this value was so precise as to not need any adjustments during the assembling phase. Moreover, the nanocomposit polyamides presented other two important factors for the dimensional stability:
- the coefficient of thermal expansion, in a range of temperatures between -30°C and + 80°C, varies between 67 and 78 mm/mmK, 10-6/°K, while the value of a traditional polyamide 6 is about 80-100.
- the humidity absorption is reduced by almost 30-40% if compared during the same lapse of time. This decreases the dimensional variations in the function of water absorption, as Fig. 1 shows, where a traditional nucleated polyamide 6 is compared to a nanocomposit with 2% of nanofillers. (Fig. 1)
THERMAL STABILITY
For under-engine components, a high value for the distortion temperature is very important.
During the research on nanocomposits, different nanofillers were used (montmorillonite, laponite, saponite, hectorite, fluoromica, vermiculite, megadite, cloisite, etc) and each one influenced in a different way the characteristics of the final polyamide (together with an organic ammonium-alkyd cation which substitutes the inorganic monovalent cation, generally alkaline, [e.g. Na+] and together with the process).
Table 3 highlights the influence of different kinds of silicate on a polyamide 6 in a 4% percentage, and montmorillonite confers the best HDT increase to polyamide 6, taking it from the initial value of 65°C to 152°C.
It is important to highlight that this thermal increase is always valid, as you can see from Fig. 2 which shows the bending form in function of the temperature for the basic polyamide 6 and for the grade 1015 C2. The grade 1015 C2 has a steadily higher value, so much higher that, around 100°C, when compared to the traditional PA, it shows a double value (1000Mpa circa). It is interesting to emphasize that, even after the equilibrium conditioning, the grade 1015 C2 has shown a 940 Mpa value at 100°C and a 860 Mpa value at 120°C.
A similar behaviour can be obtained for the flexural creep resistance, as you can see in Fig. 3, which shows the comparison at 80°C between the two mentioned products; moreover, the 1052 C2 grade has an optimal resistance to thermal degradation at high temperatures for long periods of time, as you can see in Fig. 4 and Fig. 5.
PROCESSABILITY, WEIGHT, AND AESTHETIC CHARACTERISTICS
As you can see from Fig. 6, the nanocomposit polyamides have a higher crystallization speed than the normal polyamides because the Tcc crystallization temperature increases from about 15°C to beyond 180°C.
This leads to a reduction of the pressing cycles that, moreover, can be reduced if compared to a traditional unfilled polyamide. This is due to the fact that the e-modulus is higher, and it allows considerable weight and pressing cycle time savings.
Then, it is obvious that the advantage becomes more evident if we compare NCH to the typical polyamides employed for under engine applications, such as the mineral or glass fiber filled ones. Depending on the charge and on their percentages, their specific weight varies from 1.36 to 1.55 against 1.15 of a NCH polyamide.
For example, for a polyamide 6 with a 40% traditional mineral charge, which has thermal and mechanical characteristics similar to 1015 C2, the weight-saving is more than 25% (a fundamental advantage in automotive applications).
In this particular case, the products being produced are three because, besides the big, central engine cover, the lateral right and left cover sides are made of PA, too. In total, compared to the mineral PA alternative, there has been a 800 gr. saving for each car.
Another big advantage is, obviously, the easy processability, inside the press, of a polyamide containing 2% against another one containing a 30-40% filler or a rough reinforcement. It is easy to understand that there will be no charge floating, nor superficial roughness, nor dimensional distortions induced by the traditional reinforcements: the aesthetic aspect will be just the same as if an unfilled polyamide were employed!
By adding to these factors, the high fluidity of the neat resin, which is not altered by the nanofillers, obtains a suitable polyamide to fill big dimension parts, even with thin sides, that will guarantee a good copy of the mould surface. Consequently, it will be possible either to have shiny and glossy surfaces if the mold is perfectly polished or very dull surfaces if the mold is adequately photoengraved or embossed.
All of the above shows the possibility to obtain particular pieces, even of medium or big dimensions, with a high aesthetic level. Tests are now being conducted to obtain exterior body parts with class A finishing, because the almost total absence of charges allows further and easier painting or silk-screen processes compared to a charged polyamide.
MECHANICAL CHARACTERISTICS
By comparing the main mechanical characteristics of a traditional polyamide 6 with those of a 2% NCH polyamide (1015 C2) and with those of a polyamide with 40% traditional mineral filler (1013 R), we can clearly see that NCH polyamide takes the best of mechanical properties of the two traditional polymers.
On one side, it has tensile and flexural strength values that are superior to those of an unfilled PA; on the other side, it has improved impact values compared to those of a normally filled PA (beyond double, during the post conditioning Izod test).
Moreover, impact tests were conducted following the Fiat’s standard 50424 to evaluate the behaviour to weight loss. At ambient temperature, and in the same conditions, the values were around 5 J/mm compared to a 0.8 value of a polyamide PA 85.90 (PA66 with 25% circa of the aesthetic silicate).
On the one hand, it is very important to stress one value that is unique for the grade 1015 C2 and that can’t absolutely be found in any other nanocomposit polyamide made by other manufacturers: the elongation at break value. The dry value is 75% and the conditioned is 200%, that is as if there were no charge in the base polymer!
On the other hand, the values for both a traditionally filled polyamide and for other nanocomposit PA are between 2 and 10% for elongation, with the usual problems in case of necessary flexibility, insert plantings, couplings, and so on.
This feature, unique to the grade 1015 C2, is due to UBE nanocomposits exclusive production process.
UNIQUENESS OF THE UBE PROCESS
In recent years, at least four different processes for the preparation of nanocomposits have been studied, as you can see in Fig. 7. The most utilized process is the melt compounding because, technically, it is the most simple, and it allows the use of traditional twin-screw extruders.
UBE approach and technology are completely different and unique worldwide because they are based on three different phases:
- modification of the base silicate (nanoclay)
- dispersion of a nanofiller in a monomer
- polymerization in situ
Modification of the base silicate
We have seen before that there are several kinds of lamellar clay in which each layer is separated from the adjacent layer by an interlayer gallery in which, in order to produce nanoclay, the inorganic cation has to be substituted with an organic cation. In order to do so, cationic surfactants with chemical affinity to fillosilicates (such as w-amino acid) are employed to increase as much as possible the intergallery space.
Research has shown (Fig. 8) that the number of carbon atoms of the aminoacid influenced this increase and that best results were obtained with 12 carbon atoms, which is proper of the amminododecanonic acid (ADA). UBE is the only worldwide producer of ADA, which was already used for the production process of the polyamide 12 UBESTA.
From this research, and from the selection among the various fillosilicates, UBE has produced a special organic montmorillonite modified with ADA.
Dispersion of a nanocharge in a monomer
This organic montmorillonite is dispersed into a monomer solution, which can be caprolactam or a solution of caprolactam and adipic, or yet amminododecanonic acid, in order to further increase the intergallery space by a swelling process that allows an optimal intercalation (Fig. 9).
Polymerization in situ
At this point, the solution with an optimal dispersion of the nanocharges is directly polymerised in the reactor with the base monomer (ex. caprolactam in the case of the mentioned grade 1015 C2) to originate a completely exfoliated polymer where lamellar clays are completely exfoliated and dispersed in an homogeneous way and randomly inside the polymer matrix.
This complete exfoliation is the secret that allows UBE nanocomposits to give the best mechanical and thermal performances yielding to a superior quality.
