Effect of short-chain architecture on the resulting thermal properties of polypropylene

Highlights

• Facile and novel grafting of short-chain monomers on PP.

• Novel utilization of extrusion approach for grafting reaction.

• Proposal of potentially excellent material for energy applications.

• Enhanced PP thermal stability via in-situ extrusion grafting.

• Outstanding improvement in crystallinity of grafted PP.

Abstract

Polypropylene (PP) is a versatile commodity polymer with a plethora of potential applications, in this work, short-chain branches of anhydride, amine, and various acrylate monomers with varying molecular weights were grafted onto PP using reactive extrusion. It was envisioned that such graft co-polymers can enhance the performance of PP making it a viable material such as water heating applications. The main interest herewith is to evaluate the occurrence of the grafting reaction and its effect on the thermal properties of the resulting graft co-polymers. Fourier Transform Infrared (FTIR), Differential Scanning Calorimeter (DSC), Polarized Optical Microscopy (POM), Thermogravimetric Analysis (TGA), gel content, and rheology tests were conducted to evaluate the effect of grafting on the performance of PP. DSC results showed a novel increase in crystallinity and crystallization temperature indicating a nucleating effect by the grafted chains. This is correlated with TGA and rheology results, where an increase in the temperature of maximum degradation and a reduction in the viscosity proved to be converging confirmations of grafting. The results clearly indicated successful grafting of PP which resulted in improved thermal stability that is essential for use in energy applications.

Introduction

An overall increase in population growth worldwide has led to a consequent increase in economic activities which has resulted in a further intensification in the global energy demand. A substantial portion of this energy demand is electricity, which is predominantly produced by coal-fired power stations especially in South Africa. This renders the continued use of such derived electricity detrimental to planetary homeostasis. To address this challenge, the South African government has since made efforts to reduce electricity consumption by adopting an enabling framework of policies to increase the share of renewable energy in the national energy mix. This includes the installation of solar water heaters and the use of energy-efficient light bulbs [1].

In particular, the installation of solar water heaters was intended to significantly reduce the electricity demand for water heating by 70% as water heating accounts for most of the total residential energy consumption at 40% (see Table 1). On account of entropic energy losses from the multi-step system of electricity generation, transfer and use would fundamentally prove inferior to direct on-site solar heating for household heating of electric geysers vis-a-vis solar geyser. The use of solar water heaters has the potential saving R962 million in underprivileged households, however, the success of these installations has been limited due to technical faults, the non-existence of maintenance plans, lack of proper understanding of the actual savings by households, and no training on the use of solar water heater [2].

In addition to these challenges, the tube in solar collectors used in solar water heaters are made from annealed borosilicate glass, which is extremely fragile due to their fragility, these solar collectors often break due to small hail, or improper handling which can be very costly to replace [3].

In comparison to the metallic and glass materials currently in use, polymeric materials have a tremendous potential to better satisfy the needs of the thermal energy materials market, which might result in a breakthrough in the production of solar thermal energy. Over the years, polymer technology research and development has produced a large number of innovative materials, parts, as well as manufacturing techniques. The vast range of qualities that may be customized to particular needs and applications, the great potential for multifunctional integration, the highly flexible processability, but also automated manufacturing are crucial to the success of plastics. additional total cost decrease as a result of rotor molding and other cost-effective manufacturing, processing, and application techniques [[3], [4], [5], [6]].

The use of polymeric materials in solar water heater production, whether as a storage tank or perhaps a solar collector, has attracted particular interest and offers a wide range of opportunities for lower production costs, whether through the use of less expensive materials or easier manufacturing processes. The use of polymeric materials restricts the anticipated maximum working temperatures for the collectors (stagnation) to values around 80 °C, which are easily attained by any straightforward glazed solar collector: this being a disadvantage in this regard [[6], [7], [8], [9], [10], [11], [12], [13]].

When exposed to high temperatures over an extended period of time, as would be the case with a solar heater collector designed to gather and concentrate solar energy for water heating, polymers are particularly susceptible to losing their properties owing to deterioration. The UV photons in solar radiation make this problem worse [11,12]. It is well known that both heat and UV speed up the aging process by causing chain scission and reduced mechanical characteristics.

In a series of two articles, Kahlen and co-workers have intensively investigated the potential applicability of both engineering and commodity polymers as solar collectors while focusing on the aging behavior at maximum operating conditions [14]. They found that the average molecular mass and strain at break results demonstrated to be suitable in order to detect the chemical aging of polymeric materials while DSC turned out to be sensitive to the detection of physical aging [15].

Amongst commodity polymers, PP is the most encouraging polymeric material contender for solar collectors [14]; while amid engineering polymer materials, impact-modified polyamide 12 (high impact) showed promising results in mechanical properties with exposure to water and air at high temperatures for approximately 2 years, thus is a promising engineering polymer for solar collectors [12]. Furthermore, the time frame of aging studies for polymeric solar collector materials made from PP, polyphenylene sulfide, and polyamide has been significantly reduced to a year while varying the frequency of exposure to temperature, wind speeds, and irradiation [10]. Amongst commodity polymers, research has already been done on the chemical modification of PP using electron beam irradiation – in a solid state [16] and post-reactor chemical reaction-in the melt state [17,18], however, the focus has been on improving the strain hardening and melt strength of the polyolefin.

Research has been done to modify polymers to slow down this aging process [[19], [20], [21], [22], [23]], however, to the best of our knowledge, di(trimethylolpropane) tetraacrylate and trimethyl methylpropane ethoxylate(1EO/OH) methyl ether diacrylate have not been used as monomers in the modification of PP, and there is no study that determines the effect of chemical modification of PP with these monomers on the performance as a thermal energy material.

This work form part of a series of investigations in the development of PP-based thermal energy materials for solar water collectors: Part 1 (the current research) studies the effect of monomer chain architecture on the resulting thermal properties of PP by extrusion graft modification using monomers that have never before been reported in the literature for PP modification. The current study details the process for producing modified PP and describes its thermal characterization as a gauge for the material's potential development through successful grafting. Part 2's projected follow-up work will examine how the monomer architecture affects the mechanical characteristics of PP in this regard. While a later Part 3 will examine the aging characteristics of PP with grafted monomer chains.