Rigid polyurethane foams from a polyglycerol-based polyol
Graphical abstract
Introduction
Nowadays, increasingly more information about the use of materials from renewable resources in the production of polyurethane is published. This is related to a decreasing stock of petroleum and law regulations, which stimulate researchers to look for ecological materials and novel technical solutions. As demonstrated in many published studies, materials from renewable resources can almost fully substitute their petrochemical analogs without noticeable deterioration of the product’s properties. Moreover, due to constantly increasing price of petroleum, biomaterials will become more and more competitive in the future because of their relatively low cost. Also, the accessibility of materials from renewable resources is obviously higher as they are regenerated all the time. Bio-based polyols are usually triglycerides of, predominantly, unsaturated fatty acids. Such compounds show lack of functional groups, however present in the structure unsaturated bonds can be effectively converted into hydroxyl groups, by e.g. epoxidation followed by oxirane ring-opening, hydroformylation and hydrogenation, thermal polymerization followed by transesterification or halogen addition and nucleophilic substitution. Research focusing on conversion of triglycerides showing rather low reactivity into polyols, that can be incorporated into polymer technology are very popular nowadays. Among the potential biopolyols, which are presently researched, one can find substances such as, glycerol, polyglycerol, modified and unmodified soybean or castor oils, chestnut oil, palm oils, and sunflower and linseed oils [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
Soybean oils are mainly used in the United States, while palm and coconut oils are mostly applied in Asia. In Europe, the most important are polyols based on canola and sunflower oils. Glycerol and its polymeric form are relatively new materials; their incorporation is connected to another technology based on renewable resources, i.e. the production of biodiesel, which is a fuel based on fatty acid methyl esters (FAME), destined for diesel engines. Glycerol is a by-product of biodiesel production; 100 kg of glycerol is obtained per each tone of biodiesel produced. Definitely, such by-product can be quite easily and efficiently used in the production of polyurethane. According to prevailing trends, this would allow a gradual decrease in the amount of petrochemical materials used in polyurethane industry [12].
Glycerol can be used directly as a polyol or as an excipient in the production of other polyols based on different plant oils. The possibilities to apply glycerol and polyglycerol in PU production are vast, but they still require more research. This paper is an example of such research focusing on the introduction of polyglycerol into the production of polyurethane foams.
Section snippets
Materials
Rigid polyurethane foams were synthesized from Rokopol 551 (oxypropylenated sorbitol), a commercial polyol, which was then partially replaced with two kinds of polyglycerol, i.e. Pole and PGK, which are the products of thermo-catalytic polycondensation of waste glycerol. The properties of the aforementioned polyols are presented in Table 1. The chemical structure of petrochemical polyol and the general structure of polyglycerols are shown in Fig. 1.
Isocyanate used in the reaction was polymeric
Kinetic profile of foaming
The processing times of reference foam and polyurethane–polyglycerol foams are shown in Fig. 2. The results indicate a slight increase in start time and rise time for the foams containing polyglycerol, which is related to the molecular weight of polyglycerols (Table 1) and their viscosity (mainly of polyglycerol PGK). Properties of petrochemical polyol and polyglycerols are presented in Table 1.
Changes in the concentration of unreacted isocyanate groups and the values of foam rise in dependence
Conclusions
The presented results demonstrate that the introduction of polyglycerol into the structure of rigid polyurethane foams allows for obtaining materials that are more thermally stable and have increased compressive strength, while their cellular structure and thermal insulation properties remain practically unchanged. The foams produced with the use of polyglycerol were characterized by increased apparent density and higher compressive strength, which is connected to the differences in molecular
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