Rheological Modification of Polypropylene by Incorporation of Long Chain Branches Using UV Radiation (2014)

Modification of polypropylene (PP) is important as it aims at changing the molecular structure of PP and synthesizing different grades of PP for different processing applications. These changes include narrowing of the PP’s molecular weight distribution (MWD) by degradation via β-scission (controlled rheology PP) or formation of long chain branches (LCBs) in the PP backbone (via the formation of macroradicals). However, β-scission should be controlled to prevent severe degradation in the polymer. In addition to β-scission reactions, bimolecular chain combination reactions are necessary to form long chain branched PP (LCBPP). Thus, by adjusting reaction conditions, PP with different molecular structures can be generated.\n\nIn order to initiate any PP modification reaction, PP macroradicals should be formed first by abstracting hydrogens from the PP backbone. In this thesis, UV radiation was used along with photoinitiator (hence, UV photomodification) to abstract hydrogens from PP backbones and initiate their modification. The reaction was conducted in the solid state. It was found that the final PP molecular structure is affected by photoinitiator concentration, radiation time, UV lamp intensity, radiation temperature and type of photoinitiator used. Combinations of these variables that result in LCBPP with improved melt strength were identified. \n\nIn order to characterize the molecular structure of PP, rheological measurements were found to be versatile and reliable tools. Rheological techniques, along with GPC and gel content determination, were utilized in this thesis to characterize the molecular properties of the samples. Number of long chain branches (LCBs), crosslinked structures and strain hardening behavior of LCBPP and degraded PP were compared using these techniques.\n\nAfter detailed analysis of the effects of different radiation variables on PP modification, high benzophenone (BPH) concentration and low lamp intensity were found to be necessary to form LCBs in PP in the solid state. It was also found that more than 5 min radiation time is required to form LCBPP rather than degraded PP, when the thickness of the samples is 1 mm. \n\nOne of the drawbacks in using UV radiation in polymer modification is its limited penetration depth. In order to investigate the effect of UV penetration depth along with UV radiation time, discs with thickness of 1, 2 and 3 mm were radiated for 5, 10 and 15 minutes. It was shown in this thesis that as sample thickness decreased and/or radiation time increased, more LCBs were formed. The limited UV penetration depth in PP solid samples was found to be below 1 mm.\n\nLong radiation time (above 5 min), which is necessary to form LCBPP decreases the potential of this technique for commercialization. Thus, an attempt was made to decrease the required radiation time by using a coagent. Trimethylolpropane triacrylate (TMPTA) is a trifunctional acrylic monomer, which was used as a coagent to reduce degradation by stabilizing the radical center. The effects of coagent concentration, BPH concentration and radiation time on formation of LCBs were studied via rheological measurements and relaxation spectra analysis. Formation of LCBs in the runs was also confirmed by GPC. It was found that an increase in coagent concentration, BPH concentration and radiation time, led to formation of more branches; however, gel content of the samples also increased. Low gel content is required for certain applications, such as packaging; thus, it is important to optimize the conditions to minimize crosslinking in PP. Conditions that result in maximum long chain branching (LCB) in PP while gel content of the runs remained relatively low were found using a central composite design of experiments. Mechanisms were suggested to explain formation of LCBs or crosslinked structures under different processing conditions. \n\nFinally, in order to assist in the commercialization and scale up of the PP photomodification, a method was developed to continuously radiate PP. The modification was carried out on the solidified strand after extrusion from a twin screw extruder. The strand was stretched and folded several times over two parallel rollers. UV radiation was carried out on stretched strands between rollers and radiated PP strands were then collected on a winder. Continuous photomodification was carried out both with and without coagent. By manipulating BPH concentration and radiation time, LCBPP was successfully produced both with and without coagent. In the continuous photomodification of PP, the radiation time required for formation of LCBPP was significantly lower than in batch reactions due to low thickness of the strands. Moreover, post-extrusion stretching of the strands limits chain mobility and restricts β-scission reactions. \n\nIn general, the results obtained in this thesis show that UV radiation can easily be used to modify PP and form different PP grades, ranging from controlled rheology to long chain branched and crosslinked PP.