Mechanical properties, crystallization and degradation of polypropylene due to nucleating agents, fillers and additives Chapter uri icon

abstract

  • Nowadays polypropylene is one of the three most important synthetic polymers worldwide due to its multiple applications due to its mechanical properties, chemical resistance, low cost and processing ease among others. But even with this properties that result very useful, the polypropylene continues being improved via the use of nucleating agents that modify their crystallization, therefore their mechanical properties. The variety of nucleating agents goes from the common and well known dicarboxilic salts which could be used alone or functionalized to various metal oxides improving highly the amount of certain kind of crystal above the others. The main crystalline phases are known as α-, β-, γ-, and mesomorphic or smectic form. The α form is the primary form of polypropylene obtained under normal processing conditions; however, the β-crystal is perhaps the most interesting crystalline phase for certain applications because of its hardness and impact resistance. Therefore, different nucleating agents have been used to promote this type of crystal in iPP composites. In terms of nucleating agents, there are substances that contain aromatic rings, rare earth metals (referred to as WBG), and salts from dicarboxylic acids, such as calcium salts from pimelic and suberic acids. Particularly, calcium salts from pimelic and suberic acids have demonstrated an extremely high efficiency to promote β-crystals without secondary effects. It was reported that the salts from PA were able to induce approximately 90%25 of the β growth and it also was demonstrated that the raw PA is considered an ineffective β-agent. Different substrates have been explored as supports for depositing calcium pimelate, e.g., carbonates, silicates, zeolites, oxides, metallic salts, clays, and carbon nanotubes, which are responsible for more than 90%25 of the β-crystals. β nucleating agents continue as the main route to obtain β-phase in isotactic polypropylene. Different kinds of nucleating agents, such as multiwalled carbon nanotubes modified with calcium pimelate supported on nano CaCO3, monoglycerolates, and potassium salts have been studied. In these works all of them exhibit interesting properties related to the β- phase present in the composites studied. These results increase their applications that range from medical to engineering whit very interesting features in their respective fields. However the polypropylene faces that it’s very chemical resistance has made a very important pollutant along with most of the organic synthetic polymers. An important effort in order to address this problem has been increasing the degradation (either thermodegradation or photodegradation) capacity of the polypropylene via chemical modification of the chain or by the use of fillers and additives that promotes the degradation of the material. Photodegradation involves the natural tendency of most polymers to undergo gradual reaction with atmospheric oxygen in the presence of light. Typically, a photosensitizing agent is employed to accelerate this natural tendency. The mechanism of photodegradation involves the absorption of UV light, which then leads to the generation of free radicals. An auto-oxidation process then occurs which leads to the eventual disintegration of the plastic. It is believed that the instability of polyolefins is brought about by the presence of impurities (such as carbonyl and hydroperoxide groups) that form during the fabrication or processing of the polyolefin products. Degradable polyolefins are designed to oxo-degrade, undergoing changes in chemical structure as a result of oxidation in air, causing the breakdown of the molecules into small fragments that are then bioassimilated. For the reasons cited above we consider important to give a review on the major advances that this polymer has faced over the years in the areas considered before. © 2016 by Nova Science Publishers, Inc.

publication date

  • 2016-01-01