Crosslinking systems

Scheme 8. Condensation reaction as the base of moisture condensation crosslinking system.

In this reaction, no by-products are evolved, and the crosslinking reactions occur within the bulk of the material. A typical hydrosilylation crosslinking system is depicted in Scheme 11, where n and y can vary depending on the required viscosity of the uncured formulation and the final targeted properties of the cured adhesive. 

The final physical properties of thermoset polymers depend primarily on the network structure that is developed during cure. Development of improved thermosets has been hampered by the lack of quantitative relationships between polymer variables and final physical properties. The development of a mathematical relationship between formulation and final cure properties is a formidable task requiring detailed characterization of the polymer components, an understanding of the cure chemistry and a model of the cure kinetics, determination of cure process variables (air temperature, heat transfer etc.), a relationship between cure chemistry and network structure, and the existence of a network structure parameter that correlates with physical properties. The lack of availability of easy-to-use network structure models which are applicable to the complex crosslinking systems typical of "real-world" thermosets makes it difficult to develop such correlations. 

A difference between microcrystallite-based ultrastructure and covalently-crosslinked systems is that microcrystallites melt at specific temperatures, allowing the polymer to be fabricated by heating at modest temperatures. Subsequent cooling of the system below the crystallization temperature allows the physical property advantages of the solid state to become manifest. Liquid crystallinity is also possible if some order is retained in the molten state. Crystalline order not only adds mechanical strength, it also provides opportunities for the appearance of other properties that depend on solid state order—such as electronic conductivity. 

Cationic polyacrylamide may be used in the initial treatment stages to promote rapid polymer adsorption (201). Adjustment of the pH may allow deeper penetration of the fluids in an aluminate crosslinking system prior to gelation (202). A process involving injection of alternate slugs of stoichiometrically equivalent amounts of partially hydrolyzed polyacrylamide and Al O ) has been evaluated in the laboratory permeability of sana packs were reduced by more than 96% (203). Mixtures of Al(IIl) and Zr(IV) have also been evaluated as partially hydrolyzed polyacrylamide crosslinkers (204). 

Copolymers of sodium acrylate with sodium 2-acrylamido-2-methylpropane sulfonate (220) or N,N-dimethylacrylamide (221) have been found useful for preparing crosslinked systems that must function at high temperatures and relatively high salinity. 

Muthukumar [44] further investigated the effects of polydispersity, which are important for crosslinking systems. He used a hyperscaling relation from percolation theory to obtain his results. If the excluded volume is not screened, n is related to df by... 

Sample changes during the measurement might cause severe problems. The shear stress response of a crosslinking system exemplifies this nicely ... 

The stress depends on the extent of reaction, p(tf), which progresses with time. However, it is not enough to enter the instantaneous value of p(t ). Needed is some integral over the crosslinking history. The solution of the mutation problem would require a constitutive model for the fading memory functional Gf Zflt, t p(t") which is not yet available. This restricts the applicability of dynamic mechanical experiments to slowly crosslinking systems. 

Off-balancing of stoichiometry by the right amount (depending on crosslinking system) allows preparation of materials near the gel point [66],... 

The purpose of the various Ingredients Is to provide desirable fluid properties at different times 1n the treatment. For example, the crosslinker system may be designed to have a low reaction rate while the fluid 1s at low temperatures (such as those found 1n the mixing tanks and before the fluid has traveled very far Into the fracture). Then as the fluid temperature Increases above a threshold value, the crosslinking rate accelerates and changes the rheology of the fluid to one more desirable 1n the low-shear, but high-temperature, environment of the fracture. 

Depending on the mobility of chain segments, some crystallisation can occur in crosslinked systems. 

A crosslinked system, including units of the diene type, can be considered as formed by linear chains (the initial chains) linked by randomly distributed bridging links. 

Alternative crosslinking systems used to produce the wide variety of factices offered include ... 

Crosslinking systems based on peroxides are usually simple. Unlike with sulphur vulcanisation systems, the addition of ZnO and stearic acid is not necessary, so crosslinking is achieved in many cases simply using a crosslinking peroxide. However, the addition of a coagent may be useful (see below). In some other cases, a combination of various types of crosslinking peroxides may be suitable. 

Antidegradants known to be suitable for use in combination with peroxide crosslinking systems are thiophosphatcs. / -phenylenediamines and modified triazines. 

This method is the only anaerobic non-pressurised system which is suitable for vulcanisation of peroxide crosslinked systems. 

Fig. 1. Preparation of configurational biomimetic imprinted networks for molecular recognition of biological substrates. A Solution mixture of template, functional monomer(s) (triangles and circles), crosslinking monomer, solvent, and initiator (I). B The prepolymerization complex is formed via covalent or noncovalent chemistry. C The formation of the network. D Wash step where original template is removed. E Rebinding of template. F In less crosslinked systems, movement of the macromolecular chains will produce areas of differing affinity and specificity (filled molecule is isomer of template).

Figure 5 shows similar experimental rate data for the DEGDMA/DMPA/TED polymerization. As seen in the case of HEMA, TED addition decreases both the initial rate and the maximum rate of polymerization of DEGDMA. As described earlier, polymerization of DEGDMA results in a highly crosslinked polymer. The autoacceleration effect is characterisitc of highly crosslinked systems as the diffusional limitations reduce the carbon-carbon radical termination kinetic constant... 

Figure 5.5 Calculated dependence of weight fractions of various substructures in the crosslinking system of H3 + I2 type on the initial molar ratio of H-groups to l-groups, ah (a polyurethane system) DC - dangling chains, BC - backbone chains, S - sol (backbone chains are elastically active network chains without dangling chains)...

Randomly branched precursors any crosslinking system before the gel point can potentially be used as a polymer network precursor provided that... 

Epoxies - The term "reactive oligomer" is relatively new but the concept is fairly old. This concept has been used for many years with systems such as epoxies, phenolics, unsaturated esters, cyanates, isocyanates and many other crosslinked systems. An example of a 177°C curing epo y system (Narmco s 5208) which was introduced into the marketplace about 1971 is shown in Eq. 1. 

The effects of formulations containing similar gel content on swell ratio are shown in Fig. 15.6. The results show that at ary specific gel content, the swell ratio reduces with increasing TAC concentration. This confirms the earlier hypothesis that higher TAC concentrations in the crosslinking system produce higher crosshnk density. The results are also in accordance with melt properties where the higher the TAC concentration, the higher the melt modulus. 

---