title: Morphology control of polystyrene foams with supramolecular additives
Creators: name: Klose, Bastian
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Polystyrene
subject: Foams
subject: Supramolecular additives
subject: Morphology control
subject: Extrusion
subject: Batch
subject: Thermal conductivity
subject: 540

description: In view of the climate change and CO2 reduction, polymer foams are an important material class for insulating and lightweight products. A significant proportion of the world's energy consumption is attributed to heating and cooling of houses and buildings. Therefore, polystyrene foams are widely used for insulation purposes. Heat transfer depends primarily on the density and morphology of the foams. At very low-densities, heat transfer via the solid phase is significantly reduced however the mechanical properties often suffer. Controlling the morphology towards homogeneous, smaller cell sizes, further reduces the heat transfer via the gas phase, as well as via thermal radiation. For this reason, this thesis addresses the morphology control of polystyrene foams using supramolecular foam nucleating agents. The unique feature of supramolecular additives is the ability to self-assemble in polymer melts to supramolecular nanostructures through intermolecular non-covalent interactions. Supramolecular additives must be tailored with respect to the selected polymer in terms of solubility, self-assembly behavior, optimum concentration and applied processing procedure including different processing parameters. Supramolecular additives are used for example as nucleating and clarifying agents for semi crystalline polymers and recently as foam nucleating agents. So far supramolecular foam nucleating agents have been mainly investigated and developed for polypropylene foams. In this thesis the concept of supramolecular nucleating agents is transferred to tailor the morphology of amorphous polystyrene foams. The first part of this thesis deals with C3-symmetric benzenetrisamide (BTA) additives as foam nucleating agents. In addition to a commercially available BTA used as reference additive and benchmark, a second BTA more compatible to the polystyrene matrix was used. The first additive is a 1,3,5-triaminobenzene-based core with three t-butyl side groups. The second additive consists of 1,3,5-benzenetricarboxylic acid with three i-pentyl side groups, resulting in a higher solubility in the polystyrene melt. Self-assembly experiments revealed that both additives can form supramolecular nanofibers in polystyrene melts, whereby those of the second BTA have a significantly smaller diameter. Both BTAs were used in a concentration range of 0.01 - 1.0 wt.% in a temperature-induced batch foaming process. In screening experiments the optimal CO2 concentration, the foaming temperature and the foaming time for the batch process were established. With these conditions, both foams with and without additives were produced and analyzed. The foam morphology of samples with the additive showed that both BTAs were able to nucleate the formation of the foam cells and significantly reduce the cell sizes in contrast to the neat reference foam. By improving the solubility from the first to the second BTA, it was possible to reduce the average cell size with 1.0 wt.% of the BTA by a factor of 4.5 compared to neat polystyrene foams. To verify the concept of the morphology control by supramolecular additives on a larger scale, foam extrusion was employed. Extruded polystyrene foams with the BTA revealed an average cell size of 18 µm corresponding to a reduction by a factor of more than 20 compared to the neat reference foam. The thermal conductivity was reduced about 15 %. Despite this substantial improvement, the limited solubility of the BTA in the polystyrene melt is a drawback. Therefore, the second part of this thesis, deals with a novel class of kinked bisamides as supramolecular foam nucleating agents. Here, the central unit consists of two benzene rings connected by a methylene linker, which increases the solubility in polystyrene. In addition, by varying the substituents on the central unit and the peripheral substituent, the solubility and self-assembly behavior can be tailored. A total of twelve kinked bisamide additives were investigated. Extensive self-assembly studies in xylene as a model solvent for polystyrene showed that, depending on the structure, different supramolecular objects are formed in the form of fibers, ribbons or platelets. A comprehensive investigation of these kinked bisamides in the batch foam process identified two candidates capable of forming foams with a homogenous morphology and an average cell size below 4 µm This equals a cell size reduction of 80 % compared to the neat reference foam. To reveal the industrial potential including the ability to control the foam morphology for this class of additives, foam extrusion was carried out with one selected kinked bisamide consisting of an ethyl substituted central unit and cyclohexane groups in the periphery. Such extruded foams featured a homogeneous morphology with an average cell size of 10 µm and a foam density of 71 kg m-3 by using only 0.5 wt.% of the additive. These low-density polystyrene foams exhibit a reduction of the thermal conductivity by 26 % compared to the neat polystyrene reference foam. In addition, the effect of the kinked bisamide was also investigated in foam injection molding process. Here, it was also possible to reduce the average cell size and improve the homogeneity of the resulting polystyrene foams.