Crosslinking and thermal stability

This chapter first summarizes MOM and COC alternative bearing designs and some of the unique risks associated with their use. The history of MOM bearings is particularly noteworthy, because it predates the use of UHMWPE in artificial hip joints. We also review the use of ceramics as a counter face in articulations with UHMWPE. For all practical purposes, however, highly crosslinked UHMWPE remains the most widely used alternative to conventional UHMWPE in orthopedics today. Thus, this chapter also summarizes the development of highly crosslinked and thermally stabilized UHMWPE and describes the characteristics of the most prevalent alternative to conventional UHMWPE in joint arthroplasty. 

Wilkie, C.A. Levchik S.V. Levchik G.F. Is there a correlation between crosslinking and thermal stability in S. Al-Malaika, A. Golovoy, and C.A. Wilkie, Eds., Specialty Polymer Additives Principles and Application. Blackwell Science, Oxford, England, 2001, pp. 359-374. 

Clinical introduction of first-generation highly crosslinked and thermally stabilized UHMWPEs forTHA. 

A series of phosphorus- and bromine-containing FRs were synthesized and studied to understand their role, especially their combined effects. Thus, monocar-danyl phosphoric acid, its bromo derivatives and their formaldehyde condensates and crosslinked products [28,188] were prepared and their properties compared with analogous products made from phenol [28,189]. Table 14 gives the LOI values, char yields (Cy at 600°C), and thermal stability at 50% (T6o) decomposition. 

A fluid loss additive for hard brine environments has been developed [1685], which consists of hydrocarbon, an anionic surfactant, an alcohol, a sulfonated asphalt, a biopolymer, and optionally an organophilic clay, a copolymer of N-vinyl-2-pyrrolidone and sodium-2-acrylamido-2-methylpropane sulfonate. Methylene-bis-acrylamide can be used as a crosslinker [1398]. Crosslinking imparts thermal stability and resistance to alkaline hydrolysis. 

Since the utility of these materials is improved by the incorporation of these reactive functionalities without severely decreasing other favorable properties such as thermooxidative stability and solvent resistance the chemistry of the isoimide isomerization and acetylene crosslinking reactions is of considerable interest. Previous work in our laboratory has shown that these materials, when loaded with metal powders, provide a convenient and effective method of optimizing the electrical conductance and thermal stability of aluminum conductor joints. 

By the addition of glass fibers, textile fibers, or chopped fabrics to crosslinkable polymers molding materials are produced with increased tensile strength, stiffness, and thermal stability compared to the filler-free polymers. The so-called reinforcing fillers, like carbon black, have good adhesion to the matrix due to their nonpolar structure and their characteristic geometry. 

The modification of lignins with chlorophosphazenes allows the manufacture of products characterized by flame resistance and thermal stability. This can be attributed to the aromatic structure of the lignin-phosphazene polymer as well as to the presence of such flame inhibiting elements as phosphorous, nitrogen and sulfur. Other useful properties may also result from this combination. It has previously been reported (8-13) that the modification provides crosslinked products with suitably low chlorine content. This is a consequence of incomplete substitution of the phosphazenes cycles. Additional modification of the reaction products by chemical compounds with reactive hydroxyl or amine groups reduces the unreacted chlorine content and improves product properties (8-13). Some properties of the derivatives obtained are presented in Table I. 

A1(OH)3 HDPE PVC silicone OMCTS silane PDMS increased adhesion to matrix depends on concentration of modifier crosslinking decreases thermal stability, elongation and tensile str. surface hydrophobization resistance to solvent extraction and water 29 46 37... 

CEs are known to react with phenols to form iminocarbonates which eventually lead to polycyanurates with the liberation of more acidic phenol moiety. This can be a method to alter the gel point of the resin, Tg, and thermal stability of the network by co-curing diphenol with CE. Thus, copolymerization of dicyanate with diphenols resulted in polycyanurates with altered network structure and diminished crosslink density [237]. However, an earlier report claims poly(imi-nocarbonate) by reaction of these two in equimolar quantities. The thermoplastic so formed was reported to retain the mechanical properties like a polycarbonate. This approach can produce strong, non-toxic, biodegradable films and molded plastics that are degradable at temperatures above 140 °C [169,238]. Except for a few very early reports [239], the reaction of CE with anhydrides to form poly(iminocarbamates) has not been explored much. 

The radiative-oxidative stability of low density PE, LDPE, with PP blends were found to be more stable than pure PP [Gorelik et al., 1992]. This was explained as due to a decrease in crystallite size and possible interfacial crosslinking. The thermal stability after irradiation was considerably impaired in comparison with untreated material [Minkova et al., 1992]. This was due to the presence of free radicals within the PP component (which itself is significantly more susceptible to thermal degradation after irradiation than LDPE). 

The concept of acid labile crosslinked units in polymers for positive DUV resist was evaluated with poly-(4-hydroxystyrene/4-vinylcyclohexanol) and implemented on a poly-4-hydroxystyrene backbone. The principle to increase the molecular weight by a transacetalization reaction was used to improve acetal protected phenolic resins for advanced chemically amplified resists. The polymer chains were linked via diols with acetal bridges. This type of linkage can be applied to a wide range of phenolic polymer binders. Resolution and thermal stability of the photoresist patterns were improved. Gel permeation chromatography and dissolution rate measurements prove that the crosslinked units remain essentially intact in the unexposed areas, thus enhancing... 

Incorporation of multifunctional POSS into polymer systems has been investigated with different polymers [6,62-66]. In these cases, single-phase polymer networks with POSS molecularly dispersed are often formed. POSS acts as a polyhedral cross-link. But no definite effect of POSS on network properties has been established. Both a decrease [64,65] and no change in Tg [6] were reported. The rubbery modulus increases due to a high crosslink density, and thermal stability increases with POSS content. 

The solvent resistance characteristics of non-crosslinked and crosslinked NR composites reinforced by multi-walled carbon nanotubes (MWCNTs). All the MWCNTs were individually dispersed within the rubber matrix where a three-dimensional cellulation structure was created when the added amount of MWCNTs exceeded 16 wt%. The non-crosslinked MWCNTs-filled rubber composite was not soluble, but swelled in toluene. They found that a continuous three-dimensional structure at the interface between the MWCNTs and the NR was extremely tough and thus contributed to the improvement in the elastic modulus and thermal stability of NR composites. 

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