High pressure vulcanization : crosslinking of diene rubbers without vulcanization agents

Abstract: High pressure vulcanization (HPV) is defined in this thesisas crosslinking of rubber materialsat elevated pressure andtemperature, without the use of any vulcanization agents. Thisis atotally new vulcanization technique, not used in theindustry today. The HPV technique,however, has an interestingpotential since no hazardous chemicals need to be handled,thematerial does not contain any additives. The phenomenon thatcertain rubber materials can be crosslinked in this way wasobserved already in the 1960s, but there has been a lack ofknowledge of the underlying reaction mechanisms, kinetics, theresulting network structure and also the correlation betweenthe molecular structure and the physical properties ofthesematerials. The purpose of this thesis is to increase theknowledge of high pressurevulcanization, with respect toreaction kinetics and mechanisms, network structure andphysical properties.A low cis-polybutadiene was used as a model material forcrosslinking at high pressure (10-290 MPa) and temperature(240-250°C), and comparisons were made withperoxidevulcanizates. Both unfilled and carbon black filledsamples were studied. Nitrile- and styrene-butadienerubber werealso crosslinked successfully. Crosslink density is a centralconcept inthis thesis and was evaluated by swellingmeasurements. The microstructural changesoccurring duringvulcanization were studied by Fourier Transform InfraredSpectroscopy(FTIR) and 13 C solid state nuclear magneticresonance (NMR). The mechanical propertieswere investigated bydynamic mechanical thermal analysis (DMTA), tensile testing,and permanent set and stress relaxation measurements.Theprogress of the reaction proved to be well controlled by thetreatment time, temperature and pressure, resulting inmaterials with widely different properties within onesinglecompound due to the possibility of controlling thecrosslink density during the vulcanizationprocess. The reactionmechanism was shown to be of a radical type, and vinylunsaturations inthe polybutadiene are consumed concurrentlywith the crosslink formation. The effect ofpressure on thereaction is a decrease in molecular mobility, which results inhighercrosslinking reaction rate due to a decrease intermination reactions. When a carbon black filleris introduced,the number of polymer-filler interactions is increased due tothe ability of thepolymer to penetrate the carbon blackstructure under pressure. The mechanical properties areas goodas those of peroxide vulcanizates, and especially theresistance to deformation inservice-use at elevatedtemperatures suggests that the resulting material may be usedin gasketand seal applications. In spite of the difficulties ofmould sealing that arise at elevatedpressures, it was possibleto obtain vulcanized samples by common injection mouldingwithproperties comparable to those obtained by traditionalvulcanization.Keywords:High Pressure Vulcanization, Vulcanization,Crosslinking, Pressure, Elevated Temperature, Polybutadiene,Carbon Black, Rubber-Carbon Black Interactions, CrosslinkDensity, Peroxide Vulcanization, Injection Moulding,Nitrile-Butadiene Rubber.