The Chemistry of Phenol-Formaldehyde Resin Crosslinking of EPDM as Studied with Low-Molecular-Weight Models: Part II. Formation of Inert Species, Crosslink Precursors and Crosslinks

Abstract

Abstract Phenol-formaldehyde resins (resols) are the preferred crosslinking agents for the dynamic vulcanization of blends of polypropylene and ethylene—propylene—diene rubber (PP/EPDM) for the production of thermoplastic vulcanizates (TPVs). The goals of this study were to elucidate in detail the chemical structures formed using low-molecular-weight model compounds and to elaborate on the chemical mechanism of resol cure of EPDM as presented in our previous study. The use of model compounds for EPDM (2-ethylidenenorbornane [ENBH] and 4-methylheptane), but also for resol (2-hydroxymethylphenol [HMP] and 2,6-di(hydroxymethyl)-p-cresol) allowed the characterization of the chemical structures formed with the aid of gas chromatography/mass spectrometry (GC/MS). In combination with derivatization of unreacted hydroxyls a large variety of species could be distinguished with the general structure of R−phenol-[-CH2−phenol-]n−R′ (n=0, 1 or 2). R and R′ can be inert end groups resulting from degradation or disproportionation, viz. —H, —CH3 or —CHO, reactive end groups, viz. —CH2OH, and products of addition to ENBH (C9H14), viz. —CH2—C9H13. When either R or R′ is —CH2—C9H13 a model crosslink precursor is formed, when both are —CH2—C9H13 a model crosslink is formed. It has been demonstrated beyond doubt that both chroman and methylene bridged species are produced. It is still unclear what parameters determine the selectivity for chroman versus methylene bridged structures (presence of activators, structure of olefin etc.). The two major model crosslink products of HMP and ENBH have also been characterized with nuclear magnetic resonance, which confirmed the chroman and methylene bridged structures and showed that the addition of the phenol predominantly occurs at the least sterically hindered C-8 of ENBH. The yield of the reaction of resol with ENBH is enhanced by temperature increase and/or the addition of SnCl2·2H2O. 4-Methylheptane does not react with HMP. This indicates that in a PP/EPDM based TPV only crosslinking of EPDM via the residual diene unsaturation occurs and that tertiary C-atoms of PP or of the ethylene—propylene rubber (EPM) chain of EPDM are not involved. The high reactivity of ENBH compared with 1-decene or hydrogenated dicyclopentadiene supports a cationic mechanism.