Silane Crosslinking Research Articles

Halogen-free flame retardation and silane crosslinking of polyethylenes

Halogen-free flame-retarded and silane crosslinkable polyethylenes have been prepared by a melt process using magnesium hydroxide (MH) as a flame retardant. The effects of silane concentration, peroxide concentration, etc. on the silane grafting on linear low density polyethylenes were investigated. The thermal analysis of the silane crosslinked polyethylenes was performed by thermogravimetry (TG), and its results show that silane crosslinking provides an increase in the thermal stability of polyethylenes. The combustion characteristics of the silane crosslinked polyethylenes containing MH have also been studied using limiting oxygen index (LOI) and the Cone Calorimeter. The rate of heat release and smoke emission of the silane crosslinked polyethylenes decrease, and LOI, the time to ignition and the residue after combustion increase as MH content increases.

Structure‐property relationships of LLDPE—highly filled with aluminum hydroxide

AbstractThe mechanical performance, rheological behavior, and phase morphology of linear low‐density polyethylene (LLDPE) highly loaded with aluminum hydroxide [Al(OH)3] were investigated. It was found that titanate surface‐active agent and ethylene‐vinyl acetate copolymer (EVA) improve the processing and ductile properties of the composite remarkably but are accompanied by the deterioration of the tensile strength. Addition of vinyl triethoxy silane (VTEO) and dicumyl peroxide (DCP) improves the tensile strength of the composite because of the silane crosslinking structure introduced. A synergistic effect of interface modifying and silane crosslinking method in improving mechanical performance of the composite is presented. Phase morphology of the LLDPE/Al(OH)3 composites was studied by means of scanning electron microscopy (SEM) technique. SEM micrographs indicate that a core‐shell type with Al(OH)3 as a core and EVA as a shell is formed in the composite. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2485–2490, 2002

Kinetics of cure, cross link density and adhesion of water-reactive alkoxysilicone sealants

When a silicone sealant is exposed to wet air, a surface skin is formed, which becomes thicker with time. For a 1-part room temperature vulcanization (1-RTV), neutral cure system based on the hydrolysis and condensation reactions of alkoxysilanes, it has been observed that, at constant temperature and humidity, the thickness of the cured layer is initially proportional to the square root of time, but later the gradient of such a plot increases. There are thus two regions of cure, an outer region and an inner one. Swelling in toluene, which has been measured for samples taken at various depths, shows that the cross link density is greater in the outer regions. Single lap joints of aluminium or glass have been cured in these two regions and joints from the outer region are always stronger than the corresponding ones from the inner region. This behaviour has been ascribed to the mobility of low molar mass cross linking and coupling agents. During the initial stages of cure they migrate into the outer region, but once cure has passed into the inner region there is now a paucity of these compounds. Young's modulus and the concentration profile of silane crosslinking agent have also been measured. All observations confirm migration of crosslinking species, and gradient formation for most physical properties in the final joint. Measurement of the water vapour permeability coefficient (P) of the cured siloxane network has shown that P is independent of humidity and stretching of the material. A plot of log P against reciprocal absolute temperature is non-linear with positive slope, showing that the heat of permeation is negative and varies with temperature. This can be explained by the formation of water-clusters. When the network is formed water is absorbed and alcohol is released.

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Moisture crosslinking process for foamed polymers

AbstractAlkoxysilanes can be grafted onto an ethylene homopolymer or copolymer to give a moisture crosslinkable polymer. Moisture crosslinking has many benefits over peroxide‐ or irradiation‐induced crosslinking. Simple, conventional equipment allows processing with lower capital investment and less manufacturing space. The moisture crosslinking process permits conventional thermoplastic processing after the initial grafting process to give greater flexibility in the types of applications for this crosslinking technique. In foaming processes, silane crosslinking is compatible with most types of conventional blowing agents and other additives. The combination of silane crosslinking with metallocene polyethylene technology has given rise to a broad range of crosslinked polyolefin materials, including flexible, rigid, open, and closed cell foams. Good quality foams with very low densities, approaching 1 lb/ft3 (16 kg/m3), are possible. The advantages of silane crosslinking technology have opened many new markets for polyethylene foams.

Tailored rheology of a metallocene polyolefin through silane grafting and subsequent silane crosslinking

Polymer modification through silane grafting and its subsequent crosslinking allows the rheological properties of a polymer to be tuned from those of a viscous melt to those of a crosslinked elastic network. In this study, a metallocene polyolefin resin is grafted with vinyl trimethoxy silane (VTMS) using dicumyl peroxide (DCP) as the initiator and is subsequently crosslinked in an oxidative environment. Dynamic rheological experiments are conducted to elucidate the effects of DCP and VTMS concentrations on the grafting and ensuing crosslinking processes. We find that the addition of VTMS alone to the polymer produces no grafting. In contrast, the presence of DCP by itself leads to direct crosslinking between polymer chains as suggested by an increase in elastic modulus and complex viscosity. Samples containing both DCP and VTMS undergo silane grafting, with the extent of grafting increasing with increasing DCP concentration. This conclusion is borne out by both rheological and Fourier transform infrared measurements. The grafted samples undergo silane crosslinking only in an oxidative environment and at temperatures equal to or greater than 190 °C. During crosslinking, the samples undergo a transition from a viscous melt with frequency-dependent moduli to a gel exhibiting frequency-independent moduli with the elastic modulus exceeding the viscous modulus. However, the kinetics of crosslinking and the extent of the modulus increase are a function of the DCP concentration, with both exhibiting a maximum at a specific DCP and VTMS combination.

Novel semi-IPN through vinyl silane polymerization and crosslinking within PVC films

Novel semi-IPN (interpenetrating polymer networks) were synthesized through vinyl silane modification of unplasticized poly(vinyl chloride) (PVC) films using relatively low temperatures, relatively high vinyl silane contents, and several different processing routes. A free-radical initiator was used to promote reaction of the vinyl groups, and an aqueous acetic acid solution was used to promote the methoxysilane hydrolysis and condensation (HC) reactions for siloxane crosslink formation. A gel consisting of silane alone was formed prior to the HC process, indicating the formation of a semi-IPN. The gel content following the HC process far exceeded the silane content, indicating a significant amount of PVC was entrapped by the silane network. This conclusion is supported by the homogeneous molecular structure and morphology of the films.

Poly(Dimethylsiloxane) Elastomers from Aqueous Emulsions: II. Mechanical Properties

Abstract Stress—strain measurements in simple uniaxial extension were used to characterize the mechanical properties of the elastomers prepared from poly(dimethylsilxoxane) emulsions as described in the preceding paper. The studies were carried out on the materials in the unswollen state, after they had received different treatments, specifically no aging, aging in the dry state, or aging in the wet (emulsion) state. Increase in silane crosslinker concentration was found to increase nominal stresses and moduli but to decrease extensibility, two changes that parallel the observed decreases in soluble polymer fractions and extents of equilibrium swelling reported earlier. The energy for rupture (“toughness”) frequently stays roughly constant because the decreased extensibility at least partially offsets the increased stresses. The changes in mechanical properties are due both to increased crosslinking and to reinforcing effects from silica generated from the silane, with the latter effect generally being the more important. The mechanical properties are much more affected by aging in the wet state, as opposed to aging in the dry state. For example, wet-aged elastomers generally had values of the nominal stress and modulus that were consistently lower than those for the unaged elastomers. Wet aging appears to be the result of the continual breaking and reforming of siloxane bonds in the aqueous environment of the emulsion, as probably accelerated by the presence of the tin catalyst.

Silane crosslinking of plasticized poly(vinyl chloride)

In this study the use of three different types of aminosilanes for crosslinking plasticized PVC was investigated. The processing techniques used were milling, followed by press molding and extrusion. The processing conditions were carefully adjusted to minimize premature crosslinking. Crosslinking was carried out by immersing the samples in hot water for 4 h. Gel content was determined by Soxhlet extraction in tetrahydrofuran. Tensile properties were measured at room temperature and at 100°C or 130°C. Bis(γ-trimethoxysilylpropyl)amine was found to be the best of the three silanes investigated for crosslinking plasticized PVC, enabling adequate gel content to be obtained to improve the properties at elevated temperature, while avoiding premature crosslinking. Compounds stabilized with either tin or Ba/Zn possessed adequate thermal stability at 190°C. No changes were seen in the tensile properties at room temperature, but significant improvements were obtained at 100°/130°C. Results obtained using thermomechanical analysis showed increased penetration resistance with increase in gel content. © 1998 John Wiley & Sons, Inc. Adv in Polymer Techn 17: 37–51, 1998

Silane crosslinking of plasticized poly(vinyl chloride)

Etude de la reticulation de formulations de compositions differentes par le 3-(2-aminoethylamino) propyltrimethoxysilane en utilisant deux methodes differentes. Des formulations stabilisees par le stearate de plomb peuvent etre mises en reaction avec l'aminosilane dans une presse a extrusion a vis unique sans reticulation prematuree et sans degradation. L'immersion dans l'eau des produits d'extrusion en presence d'un catalyseur a base d'etain permet d'obtenir des tractions de gel elevees ce qui entraine une amelioration des proprietes de traction a haute temperature

The gel point as reference state: A simple kinetic model for crosslinking polybutadiene via hydrosilation

Abstract Kinetic data can be described in a simple model when using the gel point as reference state. This has been shown for crosslinking data on five high molecular weight polybutadienes (18 100–97 000 g/mol). Experiments were carried out under isothermal conditions with a constant amount of catalyst. Crosslinking took place by a hydrosilation reaction at vinyl sites which were randomly distributed along the backbone of the polybutadiene precursor molecules using the bifunctional aromatic silane p-bis(dimethylsilyl) benzene as connector. FTIR measurements showed that the hydrosilation reaction followed first order kinetics. The critical gel time, tc, was determined by the instant at which power law relaxation behavior was observed (i.e. tan δ = constant) over the two lowest decades in frequency. The main parameters that were observed to affect tc were the stoichiometric ratio, r, and the functionality of the polybutadiene, f. It was observed that the time to reach the gel point decreased with increasing precursor molecular weight (~f) of the polybutadiene. The gel time also decreased when increasing the initial amount of silane crosslinker. We were able to find a relationship between the gel time measured by the CFS technique and r and f by coupling the Flory-Stockmayer gelation theory with a simple first order kinetic curing scheme.

Solvent partitioning in fluorescent polydimethylsiloxane networks

End-linked polydimethylsiloxane networks have been synthesized with 5-dimethylaminonaphthalene-1-sulfonamide (dansyl) chromophores covalently bound to trifunctional silane cross-linkers. The solvatochromic behaviour of the dansyl-tagged cross-linkers, dissolved in solvents and bound in swollen networks, has been investigated. The spectral shifts in one-component solvents are characteristic of the dansyl probe and are explained in terms of dielectric solvent relaxation and solvophobic effects. In binary solvent mixtures, analogous to the swollen network, preferential solvation of the probe by the more polar solvent is observed and explained using a model for dielectric enrichment. Solvent partitioning in swollen networks is stronger than in the binary solutions and is attributed to a combination of dipoledipole interactions and configurational restriction effects. Owing to large spectral shifts measured from networks swollen with small amounts of alcohols, this material may find application as a transducing element in a fibre-optic alcohol sensor.

Silane crosslinking of polyolefins: observations on the tin catalyst employed

FTi.r. has shown that the dibutyltin dilaurate catalyst employed in silane crosslinking of polyolefins exists in true equilibrium. The position of this equilibrium is highly dependant upon the polarity of the surrounding medium, which in turn may have a profound effect on the rate of crosslinking in such systems.

Role of molecular structure on the silane crosslinking of polyethylene: the importance of resin molecular structure change during silane grafting

The degree of peroxide induced chain extension of polyethylene (PE) as a result of free radical silane grafting proves to be of vital importance to the rate of formation and also the final degree of crosslinking of the PE network structure upon moisture curing. The differences in solid state structure (which determines the moisture diffusion rate into the polymer) and silane grafting efficiency of various PE types have been shown to play an important but a secondary role in the crosslinking of the resins.

Modification of the Interface between Paints and Stainless Steels by Means of an Interphase of Crosslinked Organofunctional Silanes

AbstractThe adhesion of paint systems to stainless steel substrates can be markedly improved by a pretreatment using organofunctional silanes. The paint-metal interface thus obtained can be made more resistant to corrosive attack by adding a silane crosslinker to the hydrolyzed silane solution. The films formed on the metal surface were characterized by Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS).

Characterization of films of organofunctional silanes by TOFSIMS and XPS

The structures of thin films formed by the silanes N-[2-(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane (SAAPS) and γ-aminopropyltriethoxysilane (γ-APS) deposited onto mechanically polished zinc or mild steel from dilute aqueous solutions were determined using time-of-flight (TOF) SIMS and XPS. TOFSIMS gave structural information which was highly complementary to the XPS data. Aspects such as silane condensation and crosslinking, oxidation at elevated temperatures, the formation of metallosiloxane bonds, and incomplete hydrolysis were detected by TOFSIMS by virtue of its high mass resolution and unlimited mass range. The structures of the films were found to be strongly dependent on the nature of the substrate, the deposition conditions, and heat treatment of the films.

Electron beam crosslinking of insulated wire and cable: Process economics and comparison with other technologies

Electron beam crosslinking of wire and cable insulation has gained a secure place in industries in North America, some Asian countries and, partially, in Europe. This has been achieved as a result of several factors which made possible the development of radiation technology: the significant amount of knowledge accumulated on radiation chemistry and physics of polymers, availability of adequate radiation sources matching industrial requirements in throughput, reliability and economics, and the growing market for crosslinked products. The present contribution compares the electron beam technology with several of the most common chemical crosslinking techniques: peroxide, monosil and sioplas (silane) crosslinking. The comparison is based on actual industrial experience.

Study of Curing Processes in PDMS at Various Stoichiometric Ratios by Longitudinal Ultrasonic Waves

ABSTRACTCuring reactions of linear vinyl terminated poly(dimethylsiloxane) with a four functional hydride silane crosslinker at various stoichiometric ratios, were studied by wave propagation at ultrasonic frequencies. Longitudinal wave velocity was found to be sensitive to the covalent endlinking, and gel point in these systems.The observed changes in longitudinal wave velocity during the reaction are governed by changes in the bulk modulus of the system.

Heat Generation and Rubber—Filler Coupling Bonds

Abstract It is well known that heat generation is dependent upon the type of polymer, filler, and other components of the compound. In addition, the state of cure has a large effect on heat buildup. With any given vulcanizate system, network constraints can reduce heat generation to a minimum. These network constraints include crosslinks, as well as coupling bonds between filler particles and the polymer chains. Using peroxide crosslinking of natural rubber and organofunctional silanes with a silica filler, network crosslinks, and filler coupling bonds can be independently varied. From equilibrium swelling, it was possible to determine the quantity of each type of constraint. Network crosslinks and coupling bonds respond to equilibrium swelling in approximately the same way, although the peroxide and silane functionality interact chemically to influence the number of each type of constraint. The rate of heat buildup and the maximum temperature generated were obtained from the Goodrich Flexometer (ASTM—D623). Increasing either network bonds or coupling bonds reduced the rate of heat generation and the heat buildup monotonically to a minimum value dependent upon the elastomer system. Both types of network constraint affected these responses about equally. Comparison of silica and carbon black fillers in SBR demonstrated that the insertion of coupling bonds was an efficient method of reducing heat buildup. Other rubber properties (particularly modulus and abrasion resistance) optimized more readily since their response to the coupling bonds and network crosslinks were quite different.