Bonding of additives to functional polyolefins by reactive blending

by Roberts, Ann Jennifer

This study examined the concept of using a reactive blending process to develop new polymeric additive systems. The objective was to investigate the potential of using a reactive processing technique as a means to bond additives to functional polymers, to create “in situ” bonds between functional groups present on the polymers and those present on the additives. The work is reported in two parts; the first part studied the bonding of colorants to functional polyolefins and the second part investigated the bonding of UV stabilisers to functional polyolefins. The research was completed with the long term objective that the approach should offer alternative additives to conventional non-bonded systems for use in polypropylene. An ethylene ionomer was utilised for the bonding of dyes, this was chosen for its optical clarity and chemical functionality. Polyethylene methacrylic acid (EMAA) ionomers and methine dyes were blended in the melt phase using an internal mixer to produce bright intrinsically colored polymers. Fourier transform infrared spectroscopy (FTIR) in transmission mode was used to assess the bonding of the dye to the ionomer. Bonding resulted through electrostatic interactions between carboxylate groups on the ionomer and cations on the dye molecules. The reactive blending process also resulted in a change in the chromophoric structure of the dye. The bonded system was compared to a system whereby no bonding between the methine dye and polymer was expected. In the later system the methine dye was blended with polyethylene using an internal mixer. From FTIR results no interaction was observed between the dye and polyethylene in this system. This was supported by microscopic analysis that showed that the dye was present in the polyethylene as a dispersion. The second stage of research focussed on the UV stabilisation of polyolefins. A melt reaction was explored between polypropylene functionalised with maleic anhydride (PP-g-MA) and an alkoxyamine hindered amine light stabiliser (NOR-HALS) with hydroxyl functionality. The technology proposed is based upon the reaction between the carboxylic acid groups of maleated polypropylene and hydroxyl groups of a specific NOR-HALS (Tinuvin 152). The efficiency of the modification was assessed using FTIR to verify the esterification reaction between the NOR-HALS and the maleated polypropylene. This reaction resulted in the grafting of a pendant UV stabiliser to the polypropylene through an ester linkage. A twin-screw extruder (TSE) was used to complete this study. A larger quantity of material could be produced using a TSE compared to the colorant system where an internal mixer was used. Samples of the reactively blended materials were exposed to UV radiation for a maximum time period of three hundred hours to assess the resulting stability of the materials. Diffuse reflectance FTIR (DRIFT) spectroscopy and X-ray photoelectron spectroscopy (XPS) provided an effective means to study oxidative degradation. IR spectroscopic measurements were used to determine the effectiveness of HALS in inhibiting the photo-oxidation of maleic anhydride grafted polypropylene. The inhibition was quantified by measuring the formation of carbonyl groups, with and without HALS bonded to the polymer, at fixed exposure times of UV radiation. DRIFT and XPS analysis confirmed that stabilised samples oxidised less, as indicated by the lower carbonyl index values and O1s / C1s ratios. These findings were complemented by results from Charpy impact tests. The mechanical property results indicated that the longevity of the materials with UV stabilisers grafted to them exceeded the PPg- MA system where there was no stabiliser present. Visible spectrophotometry was used to assess the colour of the polymeric samples and change in colour following exposure to UV radiation. Samples with bonded HALS demonstrated greater colour stability than control samples. The microstructure of the polymer surfaces was viewed using scanning electron microscopy (SEM). The polymeric samples demonstrated resistance to crazing when the NOR-HALS were bonded to the polymer. For both the colorant and UV stabiliser areas of research, thermal properties of the materials were assessed using differential scanning calorimetry (DSC). It was found that increasing the additive concentration in the polymer resulted in an increase in the temperature of crystallisation (Tc). Melt flow index can indicate if any change in molar mass had occurred during processing. An increase in melt flow index values (MFI) was observed when additive loading increased which suggested that degradation of the polymer had occurred during processing. In summary, reactive processing showed considerable promise as a means to bond additives to a functional polypropylene.