Martin van Duin

Strong decrease in viscosity of nanoparticle-filled polymer melts through selective adsorption

The melt viscosity of poly(propylene) is found to reduce dramatically through the addition of a minute amount of silica nanoparticles. We attribute this unique effect to “selective adsorption of high molar mass polymer chains” on the surface of the nanofillers. This represents a paradigm shift regarding commonly accepted relations between the molar mass and viscosity of molten polymers. This strong viscosity decrease provides major advantages for polymer processing, and materials with improved properties are obtained.

The rubber particle size to control the properties-processing balance of thermoplastic/cross-linked elastomer blends

The influence of the number-averaged rubber particle size (dn) on the mechanical and rheological properties of thermoplastic/cross-linked elastomer blends was studied, with the focus on thermoplastic vulcanizates (TPVs) based on poly(propylene) (PP) and ethylene-propylene-diene (EPDM) rubber. Time-resolved small-angle X-ray scattering measurements revealed that the criteria to obtain tough behavior are very different for TPVs than for traditional rubber-toughened thermoplastics, since the deformation mechanism of TPVs under tensile conditions is dominated by yielding of the semi-crystalline, thermoplastic matrix without the occurrence of matrix crazing and internal rubber cavitation. The formation of interlamellar voids, as occurs in the unfilled thermoplastic during deformation, is more effectively suppressed with decreasing dn, which leads to a significant enhancement of the ultimate tensile properties. Additionally, the decrease in dn leads to an enhanced elastic recovery, but also increases the melt viscosity. These results demonstrate that dn is an important parameter to control the balance between the mechanical properties and the melt processability of blends based on thermoplastics and cross-linked elastomers.

Graft formation and chain scission in blends of polyamide-6 and -6.6 with maleic anhydride containing polymers

The in situ formation of a compatibilizer, consisting of a copolymer of PA grafted onto a maleic anhydride (MA) containing polymer, is essential for the morphology and properties of the corresponding PA blends. In this study four blends, containing PA-6 or PA-6.6 and EPDM-g-MA or poly(styrene-co-maleic anhydride) (SMA; 28 wt % MA), were prepared and characterized. Chemical analyses showed that the amount of PA graft is independent of the blend composition. Going from EPDM-g-MA to SMA the MA content of the original MA-containing polymer increases, which in the corresponding blends results in an increase in the number of PA grafts and a decrease in the length of the PA grafts. In the SMA blends the number averaged molecular weight of the grafted PA is only about 200 g/mol. It is postulated that the water molecule, released upon imide formation at the PA/(MA-containing polymer) interface, hydrolyses a PA amide group, resulting in a new amine end group, which in its turn reacts with the MA-containing polymer, etc. Differential scanning calorimetry shows that the degree of crystallinity of the PA phase is decreased only when the size of the PA phase between the MA-containing polymer domains approaches the PA crystalline lamellar thickness.

The Chemistry of Phenol-Formaldehyde Resin Vulcanization of EPDM: Part I. Evidence for Methylene Crosslinks

Abstract The application of phenol-formaldehyde resins as crosslinking agents is increasing in importance due to the good high temperature properties of the corresponding vulcanizate and the use in thermoplastic vulcanizates. With respect to the chemistry of phenol-formaldehyde cure (reaction mechanism and structure of crosslink) there are still problems that have to be resolved. The reaction products of the phenol-formaldehyde resin curing of EPDM, contain 2-ethylidene norbornene (ENB) as the third monomer, have been studied. Since such an investigation is rather difficult to perform for the polymer system, a low molecular weight model for EPDM was used: 2-ethylidene norbornane (ENBH). Reaction of ENBH and a resole results in scission of the dimethylene ether bridges, i.e. in degradation of the resole into mono-, bis- and terisooctylphenol units. These are consequently converted into products, consisting of two ENBH molecules linked by mono-, bis- and terisooctylphenol units. The solid resole seems to be...

A look inside the extruder : Evolution of chemistry, morphology and rheology along the extruder axis during Reactive Processing and blending

A simple device was recently developed for fast sampling (within a few seconds) of representative melt samples (about 2 g) on a running extruder. An array of such devices has been mounted on a twin-screw extruder. The goal of this study was to de-black-box reactive processing of polymers by studying some typical examples. - Processing of polyolefins in the presence of peroxides: when the polymer is molten and the melt temperature is sufficiently high branching/cross-linking of PE and degradation of PP occurs; the conversion follows a convex profile along the screw axis, which profile is similar to the exponential profile calculated for peroxide decomposition. - Free-radical grafting of maleic anhydride (MA) onto polyolefins: MA grafting onto PE and PP also follows a convex profile with branching/cross-linking as parallel side reaction for PE and degradation for PP; for PE degradation of the formed grafted/cross-linked gel is observed at the end of the extruder. - Reactive blending of PA-6 with EPM-g-MA: within a few seconds the in-situ compatibilization reaction, resulting in PA-6/EPM graft copolymers, is completed and the degree of rubber dispersion has changed from the mm to the sub-μm range, regardless of the MA content of EPM-g-MA and the EPM-g-MA content of the blend; PA degradation occurs along the whole extruder.

Effect of Third Monomer Type and Content on Peroxide Crosslinking Efficiency of EPDM

Abstract Peroxide crosslinking of EPDM is commonly applied in rubber practice. Although the presence of a diene termonomer is not a prerequisite for peroxide crosslinking, it does provide a significant increase of the peroxide crosslinking efficiency. Different explanations for the effect of the type and the amount of the third monomer on the peroxide curing efficiency have been put forward; but, a comprehensive overview and an acceptable explanation of all the effects observed are still lacking. In the present paper, this gap is filled by combining results from low-molecular-weight model studies and rheometer experiments with information from the literature. It is shown that peroxide crosslinking of EPDM proceeds via the combination of two EPDM macro-radicals and the addition of an EPDM macro-radical to the residual EPDM unsaturation. The extent to which the latter radical addition occurs, is governed by the amount of the third monomer and by the steric hindrance of the residual unsaturation of the EPDM,...

EPR and modelling studies of hydrogen-abstraction reactions relevant to polyolefin cross-linking and grafting chemistry

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233-453 K. in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 x 10(3)-3.7 x 10(5) dm3 mol-1 s-1 per hydrogen) have been determined for C-H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C-H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.

Self-Crosslinking Rubber/Rubber and Rubber/Thermoplastic Blends: A Review

Abstract Blends of two or more polymers having appropriate reactive groups can be crosslinked through condensation or substitution reactions in the absence of crosslinking chemicals when molded at high temperatures for prolonged times. When at least one of the two polymers is a rubber, such blends are called “self-crosslinking rubber blends.” Self-crosslinking rubber/rubber blends included in this review are binary CSM/ENR, ENR/XNBR, CR/ENR, ENR/Zn-SEPDM, CSM/XNBR and CR/XNBR blends, and ternary CR/XNBR/ENR and CSM/XNBR/ENR blends. Self-crosslinking thermoplastic/rubber blends include binary PVC/XNBR, PVC/ENR, PVC/NBR, PVC/HNBR, PAA/CR and PAA/ENR blends, and a ternary PVC/ENR/XNBR blend. The formation of crosslinks in self-crosslinking blends is manifested in the rise of the rheometer torque with time. Solvent swelling studies and dynamic mechanical analysis support the self-crosslinking behavior of the blends. The extent of crosslinking depends on the amount and reactivity of the functional groups of th...

Mechanical and Elastic Properties of Sulfur Vulcanized EPDM as a Function of Third Monomer Content

Abstract A comparison is made of the vulcanization kinetics as studied with rheometry and of the mechanical and elastic properties for 65 ethylene—propylene—diene rubbers (EPDMs) vulcanized with an activated sulfur system according to the ISO 4097 recipe. The selected 50 commercial EPDMs and 15 development grades provide a broad range of chemical and physical structures, including different third monomer (TM) structures and contents ([TM]). At low [TM] (up to 40 mmol TM/100 g EPDM: for example ∼4.5 wt % 5-ethylidene-2-norbornene, ENB) the mechanical and elastic properties of EPDM are strongly affected by [TM], but at higher [TM] they are more or less unaffected. For fully-cured EPDMs this transition at ∼40 mmol TM/100 g is independent of the TM type (ENB, dicyclopentadiene, DCPD, 1,4-hexadiene, HD, etc.). In previous Fourier transform Raman (FT-Raman) spectroscopy and 1H NMR relaxation time studies it was established that for the ISO 4097 vulcanization system the average length of the sulfur crosslinks is...

Thermoreversible Cross-Linking of Furan-Containing Ethylene/Vinyl Acetate Rubber with Bismaleimide

A proof of principle for the use of Diels–Alder (DA) chemistry as a thermoreversible cross-linking tool for ethylene–vinyl acetate (EVA) rubber is demonstrated using two differently prepared amorphous furan-functionalized EVA rubbers. The first is an EVFM terpolymer of ethylene, vinyl acetate, and furfuryl methacrylate. The second is an EVA-g-furan product, resulting from the reaction of maleated EVA with furfurylamine. Both furan-containing EVA rubbers have been cross-linked with bismaleimide (BM) via a DA coupling reaction to yield final products with similar cross-link density. The BM cross-linked EVFM terpolymer products display rubber properties similar to the ones of peroxide-cured EVA rubbers with similar cross-link densities, whereas the rubber properties of the BM cross-linked EVA-g-furan correspond to those of a rubber with a higher cross-link density. The preparation of the EVA-g-furan was up-scaled to a small internal mixer, which also allowed compounding with carbon black and mineral oil in the same step. Compounding with carbon black results in reinforcement of the EVA rubber (i.e., enhanced strength), and does not interfere with the reprocessing via the retro DA reaction.