Cure kinetics of organoclay-unsaturated polyester resin nanocomposites

16 October 2013
Mehdi Poorabdollah, Mohammad Hosain Beheshty, Mohammad Atai, and Mehdy Vafayan
The choice of clay affects homogeneity, curing behavior, and optimal curing conditions.

Unsaturated polyester resins are the most widely used resins in composite industries, and consequently improving their properties would be of great interest. Recently, researchers have studied how nanoparticles can improve properties such as flame retardancy1 or Young's modulus.2An important feature of unsaturated polyester resins is their two-component nature. They contain alkyd chains with a polar structure and also less polar styrene.3 When nanoclays are incorporated into the resin, the ability of the nanoclay layers to absorb each element of unsaturated polyesters (alkyds/styrene) is important. However, nanoclay-dependent kinetic parameters must also be taken into account when calculating the optimal curing cycle. By computing the activation energies while curing, we may find the probable performed reactions. We examined the effect of two different nanoclays on the curing behavior as well as the obtained system and probable reactions.


The variation of activation energy with degree of conversion for neat unsaturated polymer resin (UP), nanocomposites containing UP and Cloisite 30B (UP/30B), and nanocomposites containing UP and Cloisite 10A (UP/10A), calculated by an advanced isoconversional method.3

We used dynamic differential scanning calorimetry to investigate the curing behavior of an unsaturated polyester resin containing 3wt% Cloisite® 10A (UP/10A) and 3wt% Cloisite® 30B (UP/30B) catalyzed with methyl ethyl ketone peroxide as initiator and promoted by cobalt naphthenate as accelerator. Cloisite 10A is a natural montmorillonite modified with 2MBHT (dimethyl, benzyl, hydrogenated tallow, quaternary ammonium). Cloisite 30B is a natural montmorillonite modified with MT2EtOH (methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium). The organic modifier of Cloisite 10A (2MBHT) is less polar than that of Cloisite 30B.

We first calculated the kinetic parameters of the resin curing reactions using a new, advanced isoconversional method.4 The method is based on calculating the temperature (T) integral,

where Eα is the activation energy, and R is the gas constant, over a small temperature interval that corresponds to a small conversion degree (Δα). Results of this method are very accurate in such a way that in many situations reaction mechanisms can be known without performing spectrometry experiments. Figure 1 shows the summary of activation energy computations in different degrees of conversion for the virgin unsaturated polyester resin (UP), resin containing Cloisite 30B (UP/30B), and resin containing Cloisite 10A (UP/10A). The results indicate that incorporating nanoclay reduces the activation energy of the unsaturated polyester curing reaction. The activation energy of UP/10A was reduced by more than that of UP/30B. A lower activation energy raises the reaction rate at lower temperatures, and we attributed this to ligands forming between the nanoclay surfactants and cobalt naphthenate in the resin. Ligand formation was also seen by a change in the resin color when nanoclay was incorporated: UP is red but UP/30B is red-orange, and UP/10A is green-blue.3

The activation energy computation for UP/10A indicated that styrene homopolymerizes in the system, which is probably due to the absorption of styrene by Cloisite 10A platelets.3 This homopolymerization reaction leads to heterogeneity in the system, which may reduce the mechanical properties of cured composite. The type of nanoclay used in such two-component systems should therefore be selected with care. Comparing pre-exponential factors, which indicate the number of collisions of reactionary components, showed interesting results. The pre-exponential factors of UP/10A and UP/30B were significantly lower than that of UP. The reduction in collisions is affected by factors such as viscosity, steric hinderance caused by incorporated nanoclay particles, and styrene absorption by the nanoclay platelets.3

In summary, we have found significant effects of nanoclay on the curing behavior and the homogeneity of UP and its nanocomposite systems. These factors should be taken into consideration when selecting a nanoclay to incorporate. As any heterogeneity reduces the mechanical properties of the cured composite, nanoclays that have a minimal effect on the heterogeneity in the system should be chosen. However, special care is also needed when selecting curing conditions to control the temperature. When the nanoclay is incorporated, the curing reaction rate increases, which in turn raises the temperature of the sample. We are now using segmental relaxation investigation methods to study the structures that result from adding Cloisite 10A and Cloisite 30B nanoclay platelets. Additionally, we are involved in modifying and using spherical nanosilica particles and studying their effects on mechanical properties, kinetic behavior, and segmental relaxation of unsaturated polyester resin.


Authors

Mehdi Poorabdollah
Iran Polymer and Petrochemical Institute

Mehdi Poorabdollah is a PhD candidate.

Mohammad Hosain Beheshty
Iran Polymer and Petrochemical Institute

Mohammad Hosain Beheshty is professor of polymer composites.

Mohammad Atai
Iran Polymer and Petrochemical Institute

Mohammad Atai is associate professor of polymer composites.

Mehdy Vafayan
Iran Polymer and Petrochemical Institute

Mehdy Vafayan is a PhD candidate.


References

  1. A. B. Morgan, Flame retarded polymer layered silicate nanocomposites: a review of commercial and open literature systems, Polym. Adv. Technol. 17, pp. 206-217, 2006.

  2. A. B. Inceoglu and U. Yilmazer, Synthesis and mechanical properties of unsaturated polyester based nanocomposites, Polym. Eng. Sci. 43, pp. 661-668, 2003.

  3. M. Poorabdollah, M. H. Beheshty, M. Atai and M. Vafayan, Cure kinetic study of organoclay-unsaturated polyester resin nanocomposites by using advanced isoconversional approach, Polym. Comp., 2013. Published online in advance of print

  4. S. Vyazovkin, A. K. Burnham, J. M. Criado, L. A. Maqueda, C. Popescu and N. Sbirrazzuoli, ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data, Thermochim. Acta 520, pp. 1-19, 2011.

DOI:  10.2417/spepro.005110