Forming flexible foam

Materials World magazine
,
5 Feb 2012
squeegee mop head

The introduction of nanoclay to polyurethane improves properties such as strength and stability in the resulting foam. Sara Tabandeh from Amirkabir University of Technology in Tehran, Iran, outlines studies into the synthesis process.

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Polyurethanes (PUs) are unique polymeric materials with a wide range of physical and chemical properties, and can be used in applications such as coatings, adhesives, fibres, thermoplastic elastomers and foams. Of all the polyurethane products, flexible PU foam has the highest levels of production and is most widely used as a cushioning material with applications in furniture, vehicles and packaging.

Nanoparticles are used in the foam industry to overcome property disadvantages, such as low mechanical strength, and low thermal and dimensional stability. These may lead to a new class of materials that are lightweight, high-strength and multifunctional. For example, a small amount of well-dispersed nanoparticles in the PU matrix may serve as nucleation sites to facilitate the bubble nucleation process due to the accumulation of gas on the polymer-particle interface and creation of nucleation sites.

Given the above, research has been employed to examine the effects of incorporating nanoclay particles into the foam polymer to see if a more durable substance can be produced.

Studies have shown that nanoparticles can lessen cell size. With the nucleation of more bubbles, less gas is available for bubble growing. Furthermore, nanoparticles can significantly increase the melt viscosity and strain-induced hardening through nanoclay alignment. Therefore, inhibiting cell growth and production of smaller cell sizes is achieved. The research team from Amirkabir University of Technology has been studying the synthesis of flexible PU foam and the effect of clay on the foaming process. The mechanical properties have been investigated and the morphology of nanoclay foams characterised by X-ray diffraction (XRD) and image analysis. The influence of modified nanoclays on the properties of PU nanocomposite foams has also been studied.

Polyurethane-clay preparation

The clay and polyol were dehydrated in an oven at 100°C and 60°C respectively and five set mixtures were prepared (A1, A2, A3, B1, B2). All samples had clay content of 5wt%, except A3, where a content of 3wt% was used. Samples A1, A3 and B1 were stirred at high shear for three minutes, and all samples were stirred on low shear for one hour. Only A2 and B2 were stirred with no shear overnight. Temperature for series A was ambient compared to 60°C for series B.

Applying two driving forces – thermodynamic and mechanical – can improve the degree of exfoliation and this effect was investigated. After cooling, the required amount of water, catalysis and surfactant were added to the polyol mixture in the case of series B and to the single polyol in the case of series A, and stirred until good dispersion of all ingredients was reached. The polyol mixture was combined with a predetermined amount of isocyanate for seven seconds using a high-speed stirrer. The mixtures were immediately poured into a stainless steel mould and kept at room temperature. The foam was then allowed to cure at room temperature for at least one day. A foam sample without clay was also prepared for comparison.

As shown in the graphs of the XRD patterns (see Figures 1 and 2), series A, shows that good dispersion is a result of the OH-groups’ affinity on the clay surface to the MDI chains (NCO groups). Also, in one case of series B, sample B2, no clear peak in the range of 0–10° is observed. Having no clear peak in the latter case can be related to the processing condition. Sufficient time for diffusion of polyol chains into clay layers can be a reason for the good dispersion. In the case of the sample B1 distinct peak at 2_=2.07 is recorded corresponding to 4.26nm. With no time for complete diffusion, the clay layers can only expand (from 1.9nm to 4.26nm) and the exfoliation step cannot be reached. As expected, the conventional polyurethane foam also shows no peak.

Typical scanning electron micrographs of the foams are shown in Figure 3. The smaller cells can be observed in the case of nanofoams, and the cell density has increased. The nucleation of nanoparticles may be important because these particles facilitate the process and serve as nucleation sites in the foam matrix. On the other hand, they can increase the viscosity of the matrix. They also act more like a surfactant as a stabiliser agent. When silicone is added, a surface tension gradient develops along the gas/liquid interface. This surface tension gradient reduces the drainage flow rate, therefore coalescence is decreased. Furthermore, the SEM micrograph shows the anisotropic microstructure of nanofoams.

Open cells

The lower molecular weight build-up, such as urea and urethane polymerisation of a reacting polymer due to the delayed NCO-water reaction, can increase the open-cell content. This delay in gelling reaction can increase drainage rate and open cell content. Since the hydrophilic modifier was placed on the surface of the nanoclay used in this study, a delay in the foaming reaction is expected. The hydrogen bonding between water and OH groups on the nanoclay surface prevents the NCO-water reaction, thus delaying reaction times. As shown in the micro-graphs, presence of nanoclay in the matrix can increase open cell content.

To exclude the density difference of the foam samples, the reduced compressive modulus, such as the compressive divided by the density of the foam sample, have been used to compare the mechanical properties of the PU foams with clay. There is a significant increase in modulus with the addition of clay in foams made with isocyanate dispersion (column chart of compressive modulus shown in Figure 4). Polyol-dispersed foams have increased module but not of the same grade as series A. The modulus can be improved by increasing the content of cells (constant density), therefore, the more cell density (nucleants/cm3), the better the modulus achieved. The curve of modulus versus concentration of nanoclay is demonstrated in Figure 5. As expected, the modulus of the foam has a linear relation with the concentration of clay in the range of investigation.

The thermogravimetric analysis (TGA) test has been carried out in a nitrogen atmosphere at 10°C/min. and the TGA graphs are shown in Figure 6. During ramped heating under nitrogen, such as in a TGA experiment, the degradation process usually passes through three stages. As displayed here, the nanocomposite foams show the same TGA profiles to the same extent as pure foam, but displaced to a higher temperature than conventional foam. Furthermore, additional carbonaceous char is formed. This is due to the barrier effect generated by highly anisotropic (differing material behaviour dependent on direction of interaction) clay platelets, which delays the escape of volatile degradation products from the nanocomposite. Another difference in the TGA profiles is that in the case of nanocomposite foams, two-stage decomposition is observed. Sample A2 shows the highest degradation temperature. The larger weight loss in this case indicates the formation of more organic components on the clay surface. Due to more diffusion of chains into the galleries of nanoclay, more reactive sites on the clay surface may be accessible and react with the products of polyurethane decomposition. Consequently, formation of stable structures has been prevented.

Polyurethane nanocomposite foams were synthesised using organically modified montmorillonite clay (Cloisite 30B). As shown by X-ray Diffraction patterns, the reaction of clay’s OH groups and isocyanate’s NCO groups is a driving force for separation of clay layers into the matrix. This effect is also confirmed by scanning electron micrographs of the foams.

Therefore, improvement in mechanical properties, especially in the case of MDI-dispersed mixtures, has been displayed by the tests. Thermogravimetric analysis was carried out and formation of more carbonaceous char in the presence of nanoclays, due to the barrier effect of the clay layer, has been shown. This indicates that a more effective and durable foam material can be produced.

Further information

Amirkabir University of Technology, 424 Hafez Avenue, Tehran, Iran. Tel: +98-21-6695-4260. Email: tabandeh_usc@yahoo.com Thanks to co-authors Prof. Faramarz Afshar Taromi and Prof. Hosein Nazockdast, and to Urethane Systems Co.