Optimizing the Process Efficiency of Reactive Extrusion in the Synthesis of Vinyltrimethoxysilane-Grafted Ethylene-Octene-Copolymer (EOC-g-VTMS) by Response Surface Methodology

Abstract

Thermoplastic polymers like ethylene-octene copolymer (EOC) may be grafted with silanes via reactive extrusion to enable subsequent crosslinking for advanced biomaterials manufacture. However, this reactive extrusion process is difficult to control and it is still challenging to reproducibly arrive at well-defined products. Moreover, high grafting degrees require a considerable excess of grafting reagent. A large proportion of the silane passes through the process without reacting and needs to be removed at great expense by subsequent purification. This results in unnecessarily high consumption of chemicals and a rather resource-inefficient process. It is thus desired to be able to define desired grafting degrees with optimum grafting efficiency by means of suitable process control. In this study, the continuous grafting of vinyltrimethoxysilane (VTMS) on ethylene-octene copolymer (EOC) via reactive extrusion was investigated. Successful grafting was verified and quantified by 1H-NMR spectroscopy. The effects of five process parameters and their synergistic interactions on grafting degree and grafting efficiency were determined using a face-centered experimental design (FCD). Response surface methodology (RSM) was applied to derive a causal process model and define process windows yielding arbitrary grafting degrees between <2 and >5% at a minimum waste of grafting agent. It was found that the reactive extrusion process was strongly influenced by several second-order interaction effects making this process difficult to control. Grafting efficiencies between 75 and 80% can be realized as long as grafting degrees <2% are admitted.

Synthesis of an Addition-Crosslinkable, Silicon-Modified Polyolefin via Reactive Extrusion Monitored by In-Line Raman Spectroscopy

“…The grafting efficiency for all experiments was between 16.14% and 85.03%. This is well within the expected range of grafting degrees observed with similar grafting reactions performed via reactive extrusion in earlier studies [ 2 , 4 ]. All experiments were included in the model building.…”

“…All three reactive extrusion processes are governed by non-linear factor effects and the same types of second-order interaction effects. For instance, the linear components of the temperature effect on the degree of grafting were in a comparable order of magnitude in all systems: EPM-g-VTMS: 0.10 [ 2 ]; EOC-g-VTMS: 0.10 [ 4 ]; EPM-g-VTMDS: 0.13 (this work), although in [ 4 ] the temperature was varied within a slightly narrower range from 100 °C–220 °C instead of 80 °C to 220 °C. Although most effects were similar, some notable differences should be pointed out.…”

“…Determination of grafting degree and grafting efficiency was performed in an analogous way as described in previous studies on grafting silane coupling agents onto ethylene propylene (EPM) and ethylene-octene copolymer (EOC) polyolefins [ 2 , 4 ]. The grafting degree as a weight percentage (wt%) was calculated according to Equation (1) [ 2 ].…”

“…Grafting efficiency in wt% was calculated using Equation (2). For this, the measured VTMDS grafting degree was divided by the grafting degree theoretically expected under the assumption of 100% conversion [ 2 , 4 ]. …”

“…By grafting organosilanes onto them, an addition-crosslinkable system is produced that allows the formation of covalent crosslinkages via the grafted silane functionalities. Prominent examples include the vinyl trimethoxysilane (VTMS) grafted polyolefines ethylene propylene (EPM-g-VTMS) [ 2 ] and ethylene octene (EOC-g-VTMS) [ 3 , 4 ], co-polymers that combine the performance characteristics of poly-olefinic systems with silicon-containing polymers.…”