Crosslinking elastomers

One-part moisture condensation cure. The one-part condensation cure system is a room-temperature vulcanizing (RTV) system that is based on a reactive PDMS polymer that undergoes hydrolysis on contact of air moisture, followed by condensation to yield a crosslinked elastomer. The most common systems [3,12,14,33] are based on the reactions shown in Scheme 5. 

The resulting silanol group of a polymer chain condenses with acetoxy siloxy group of another polymer chain to form a siloxane (Si-O-Si) linkage (Scheme 8). Further similar reactions finally result in a crosslinked elastomer. Acetic acid is... 

TPEs are materials that possess, at normal temperatures, the characteristic resilience and recovery from the extension of crosslinked elastomers and exhibit plastic flow at elevated temperatures. They can be fabricated by the usual techniques such as blow molding, extrusion, injection molding, etc. This effect is associated with certain interchain secondary valence forces of attraction, which have the effect of typical conventional covalent crosslinks, but at elevated temperatures, the secondary... 

When a thermoplastic polyurethane elastomer is heated above the melting point of its hard blocks, the chains can flow and the polymer can be molded to a new shape. When the polymer cools, new hard blocks form, recreating the physical crosslinks. We take advantage of these properties to mold elastomeric items that do not need to be cured like conventional rubbers. Scrap moldings, sprues, etc. can be recycled directly back to the extruder, which increases the efficiency of this process. In contrast, chemically crosslinked elastomers, which are thermosetting polymers, cannot be reprocessed after they have been cured. 

Organic peroxides have been used to crosslink elastomers and plastics for over 50 years. The organic peroxides utilised by the rubber industry reactvery predictably. Most are stable at room temperature and will decompose based on their half-life temperature curves. They can represent a severe hazard, however, if they are stored or used improperly. These issues are reviewed in detail. 4 refs. USA... 

Equation (2.53) is stating that the network modulus is the product of the thermal energy and the number of springs trapped by the entanglements. This is the result that is predicted for covalently crosslinked elastomers from the theory of rubber elasticity that will be discussed in a little more detail below. However, what we should focus on here is that there is a range of frequencies over which a polymer melt behaves as a crosslinked three-dimensional mesh. At low frequencies entanglements... 

By comparison, the synthesis of SIN s involved a castor oil derived crosslinked elastomer and crosslinked polystyrene as... 

When one wants to replace crosslinked elastomers by TPEs, it is necessary to survey the problem with a fresh eye and to redesign the subset where the part is used and redraw it to reduce the strains and integrate a maximum of functions to reduce the end-cost. 

Studies of crosslinked networks of LC polymers have been proposed ( ), and prepared without disruption of the mesophase (11-14). Crosslinked elastomers were shown to retain the mesophase up to certain crosslink densities (15,16). 

This discussion has been concerned primarily with the chemistry of the polymer in filled crosslinked elastomer formulations. Since the purpose of these formulations is to produce a gas with high enthalpy, thermochemistry is important. The heat of combustion of the components and the effect of the nature and molecular weight of the gaseous products are included in several literature references. The increase in enthalpy that can be obtained by adding finely divided metals to the formulations makes the use of these materials desirable in many applications. Their presence has catalyzed many excellent studies on two-phase gas flow particularly during expansion in a nozzle. 

Three common methods of measuring crosslinking (swelling, elastic modulus, and gel point measurements) have recently been critically appraised by Dole (14). A fourth method using a plot of sol + sol against the reciprocal dose has also been used extensively. However, Lyons (23) has pointed out that this relation, even for polyethylenes of closely random distribution, does not have the rectilinear form required by the statistical theory of crosslinking. Flory (19) pointed out many years ago that the extensibility of a crosslinked elastomer should vary as the square root of the distance between crosslinks. More recently Case (4, 5) has calculated that the extensibility of an elastomer is given by ... 

Thus, in alkyd, polyester, and epoxy coatings applied to CRS, phosphatized steel, and aluminum, the use of ZAs APG (aminofunctional) and CPG (carboxy-functional) has allowed for the virtual elimination of blister formation and corrosion after 300 h of salt fog exposure. The use of multifunctional ZAs in a Kraton base adhesive has allowed for a 52% increase in T-peel strength on EPDM rubber when compared with the same adhesive containing aminofunctional silane. Incorporation of mercaptofunctional ZA into crosslinkable elastomers has... 

Recently, solid-state 13C NMR has been widely applied for the characterisation of vulcanised rubber systems [10] with several reviews published covering the solid-state NMR analysis of crosslinked elastomers [11, 12],... 

From this chapter, it is clear that high resolution 13C NMR spectroscopy has made a large number of contributions to our knowledge of the structure of vulcanised crosslinked elastomers as well as the mechanism by which the vulcanisation process occurs. It is anticipated that further NMR measurements of these systems will continue to generate new structural and mechanistic information. 

Mechanical properties of crosslinked elastomers are influenced not only by the volume-average crosslink density but also by network heterogeneity. The influence of structural defects (such as residual sol, dangling chains, chain loops and the heterogeneity of the junction distribution) on the viscoelastic properties and the equilibrium swelling data is still under discussion. Local methods which probe molecular properties are very suitable for the determination of the degree of network heterogeneity [11]. 

A quantitative analysis of the shape of the decay curve is not always straightforward due to the complex origin of the relaxation function itself [20, 36, 63-66] and the structural heterogeneity of the long chain molecules. Nevertheless, several examples of the detection of structural heterogeneity by T2 experiments have been published, for example the analysis of the gel/sol content in cured [65, 67] and filled elastomers [61, 62], the estimation of the fraction of chain-end blocks in linear and network elastomers [66, 68, 69], and the determination of a distribution function for the molecular mass of network chains in crosslinked elastomers [70, 71]. 

New crosslinked elastomers may be formed by self-aggregation of substituted functional units able to form directed hydrogen-bonds. 1,4-PB has been randomly substituted with about 4 mol% urazole units [94], The urazole units form hydrogen-bonded supramolecular, plate-like aggregates which act as effective crosslinking zones, thus creating a thermoplastic elastomer [95, 96, 97]. The system is deuterated either in the PB backbone or in the urazole units, which allows to investigate the molecular dynamics in... 

D. W. van Krevelen, Properties of Polymers, 3rd ed., Elsevier, New York, 1972, pp 44—54. (The data used here, Table 4.3 on p. 44, are titled Molar volumes of rubbery amorphous polymers at 25° C. They need not necessarily represent crosslinked elastomers. We are making an... 

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