Short crosslink

Figure 5.15 illustrates the two general types of crosslink. In Fig. 5.15 a), a short crosslink connects adjacent chains to form a four-armed star , the arms of which are normally unequal in length. In Fig. 5.15 b), a longer crosslink connects two adjacent molecules. When... 

A definite embrittlement is observed in formaldehyde-treated wood that may result from the short crosslinking unit (O—C—O) formed between formaldehyde and fibrils in the cell wall. If the inner carbon unit were longer, there would be more flexibility in this unit and the embrittlement should be reduced. 

Jump-like creep was also studied for UHMWPE films with intentionally varied structural organization of interfaces between fibrils at the expense of their different preparation (gel-casting, crystallization from the melt), various draw ratios X (from 7 to 119), and the special cross-linking of fibrils [315,317]. The fibrils were weakly connected and loosely packed, weakly tied but closely packed, or cross-linked by long molecular segments or connected by short crosslinks, in these samples. As a result, absolutely different jump-like creep rate vs deformation curves and jump sharpness parameter h vs strain dependencies were obtained for these model samples with different interfacial structures. Short interfibrillar crosslinks provided the largest effect on creep behavior. Creep occurred basically through shear of fibrillar structural units relative to one another in an acceleration-deceleration way deceleration was due to slip resistance by some stoppers. ... 

Double bonds and allylic C — C bonds are very reactive with oxygen and ozone, while short crosslinks increase chain stiffness and thermal stability. All structural effects are better evidenced in the thermal studies of olefin copolymers which, with a few exceptions, have lower stabilities than the corresponding homopolymers [170, 175, 176]. 

A typical rubber consists of long chains connected by short crosslinks every few hundred carbon atoms. The chain segments between crosslinks are known as network chains. (These segments are characterized by Me, the molecular weight between crosslinks.) The change in entropy upon stretching a sample containing N moles of network chains is... 

The choice of accelerator will influence the length of crosslinks, but this can be further controlled by adjusting the accelerator sulfur ratio. Increasing this ratio progressively favors shorter crosslinks. This can be alternatively accomplished by using sulfur donors in place of most or all of the elemental sulfur for vulcanization. The thiuram accelerators, particularly the disulfides, and dithiodimorpholine are commonly used for this purpose. The use of a cure system designed to form predominately short crosslinks is referred to as efficient vulcanization (EV). 

This is also known as Bulk Moulding Compound (BMC). It is blended through a mix of unsaturated polyester resin, crosslinking monomer, catalyst, mineral fillers and short-length fibrous reinforcement materials such as chopped glass fibre, usually in lengths of 6-25 mm. They are all mixed in different proportions to obtain the required electromechanical properties. The mix is processed and cured for a specific time, under a prescribed pressure and temperature, to obtain the DMC. 

The second path in Fig. 3 outlines the approach to a more robust tape designed by Drew [21]. Here the milled rubber and filler are combined with tackifiers and other additives/stabilizers in an intensive dispersing step, such as a Mogul or Banbury mixer. Next, a phenolic resin or an alternative crosslinker is added and allowed to react with the rubber crosslinker to a point somewhat short of crosslinking. The compounded mixture is then charged to a heavy duty chum and dissolved in a suitable solvent like mineral spirits. To prepare a masking tape. 

Resistance to common aircraft fluids such as water, salt water, hydraulic fluid and jet fuel is determined by additional shear testing after exposure to these fluids. Since adhesives are typically only exposed at bond edges, are protected by secondary primers and enamels and are not expected to be exposed to these fluids (save for water) for extended periods, exposure time prior to testing is relatively short. Lastly, the adhesive is tested for propensity to creep rupture under load in standard and aggressive environments. This testing indicates whether the polymer is crosslinked sufficiently to resist long-term creep under low load. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

It was concluded that the crosslinking reaction proceeded through initial alkylation of N-7 of adenine by the aziridine ring, followed by guanine N-7 alkylation by the epoxide (Figure 11.12). This seems inconsistent with Armstrong s earlier conclusions, but the very short oligonucleotides used by Saito et al. may influence the DNA alkylation chemistry. 

A conclusion as to the effect of crosslinking on thermal expansion is not possible. Clearly, the polymers with many crosslinks and with short strands expand less than the uncrosslinked materials when heated. However, this effect cannot exclusively be attributed to the presence of crosslinks. It may just as well originate from the increased density of the crosslinked materials which was shown to be responsible for the increase in the moduli. 

The activation volume of the three polymers turned out to be v 2 nm3, independent of their crosslink density. In the crosslinked polymer A the strands are short and about five of them fit into the activation volume. In contrast, one strand of polymer E requires a volume five times larger than the activation volume ... 

In general, coating systems are designed to achieve optimum properties at crosslinking levels short of 100% conversion in order to minimize cure time. The effect of Isocyanate to hydroxy ratio and humidity on crosslink density after a fixed cure time is shown in Figure 4. The fixed cure time has been arbitrarily defined as the time at which a coating with equal Isocyanate and hydroxy functionality reaches 85% conversion in the absence of humidity (H-0). If the initial ratio of isocyanate to hydroxy is less than or equal to 1, the crosslink density drops with increasing humidity. 

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