DEGREE OF CROSSLINKING

Figure 3.4 Uj versus M" for polyisobutylene samples with three different degrees of crosslinking (T and a constant). [Reprinted with permission from P, J. Flory, Ind. Eng. Chem. 38 417 (1946). Copyright 1946, American Chemical Society.]...

Forced by the necessity to limit the subsequent formaldehyde emission, the UF-resin molar ratio, F/U, has been progressively decreased to very low values. The main differences between UF-resins with high and with low content of formaldehyde, are (1) the reactivity of the resin due to the different content of free formaldehyde, and (2) the degree of crosslinking in the cured network. 

For example, a UF-resin for particleboard at the end of the 1970s would have had a F/U molar ratio of approx. 1.6-1.8. To day a UF-resin for the same application has a molar ratio of between 1.02 and 1.08, but the requirements for the boards, as given in the quality standards, are still the same. The degree of crosslinking of the cured resins as well as the reactivity of the hardening reaction depends on the availability of free formaldehyde in the system. 

The oldest technology involved in the elastomer blending and vulcanization process is essentially a temperature controlled two roll mill as well as internal mixers followed by an optimum degree of crosslinking in autoclave molds (compression, injection, etc.) in a batch process or in a continuous process such as continuously heated tube or radiated tubes. A few examples of laboratory scale preparation of special purpose elastomeric blends is cited here. 

Rubber separators have a relatively low porosity ( 50 - 55 percent) and thus high acid displacement and electrical resistance. Furthermore, they are brittle and for this reason difficult to handle in larger sizes. In order to balance this disadvantage, an adjustment to a lower degree of crosslinking has been attempted the result was a corresponding increase in susceptibility to oxidative attack. 

Crosslinked polymers are rather peculiar materials in that they never melt and they exhibit entropic elasticity at elevated temperatures. The present review on the influence of crosslink density is structured around model polymers of uniform composition but with widely varying numbers of crosslinks. The degree of crosslinking in the polymers was verified by use of the theory of rubber elasticity. 

Covalently crosslinked siloxane containing liquid crystalline networks with elastic properties were prepared 349). In all of the networks liquid crystalline phases of the linear precursors were retained. For low degrees of crosslinking the phase transition temperatures remained nearly unchanged, whereas higher degrees of crosslinking reduced the phase transition temperatures. 

The fracture strength and mode of fracture of a material have been found to be related to a number of characteristics of the polymer molecules of which it is made up. These include, among others, constitution, molar mass, polydispersity, crystallinity, and degree of crosslinking. Other factors which also affect fracture strength and mode of fracture are temperature, strain rate, and geometry of the specimen, all of which are decided upon prior to testing the material. 

A hydrogel is formed by a water-soluble polymer that has been lightly crosslinked. Hydrogels swell as they absorb water but they do not dissolve. The volume expansion is limited by the degree of crosslinking. The minimum number of crosslinks needed to form a three-dimensional matrix is approximately 1.5 crosslinks per chain, and this yields the maximum expansion possible without separation of the chains into a true solution. Thus, a hydrogel may be more than 95% water and, in that sense, has much in common with living soft tissues. 

FIGURE 6 Degree of water absorption (and, therefore, inversely, the degree of crosslinking) by [NP(0CH2CH20CH2CH20CH3)2ln as a function of y radiation dose. (From Ref. 41.)... 

The collagen shield, fabricated from procine scleral tissue, is a spherical contact lens-shaped film whose thickness can be made to vary from 0.027 to 0,071 mm. It has a diameter of 14.5 mm and a base curve of 9 mm. Once the shield is hydrated by tear fluid and begins to dissolve, it softens and conforms to the corneal surface. Dissolution rates can be varied from 2 to as long as 72 hr by exposing the shields to ultraviolet radiation in order to achieve varying degrees of crosslinking. 

Third, the introduction of crosslinks between chains confers insolubility and increased solid state rigidity, often accompanied by improved thermal stability. High degrees of crosslinking confer ceramic-type properties on the solid, whether the backbone atoms are carbon atoms or inorganic species. 

CR Robert, PA Buri, NA Peppas. Effect of degree of crosslinking of water transport in polymer microparticles. J Appl Polym Sci 30 301-307, 1985. 

Thermal quenching of the crosslinking reaction In an actual reactive extrusion process, the degree of crosslinking can be controlled by adjusting the residence time at elevated temperature. 

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