Polymeric binder degradation

To remove the polymeric binder, the green body is heated often in an oxidizing atmosphere. During this process the polymer degrades along many possible degradation pathways, which include (1) scission of the main chain, (2) reaction with side chains and substituents, and... 

It is also desirable that the monomer acts as a plasticizer for the polymeric binder. A critical necessity of the polymerized monomer, as with the remaining impol)mierized monomer, is that they readily decompose to gaseous products when the cured formulation is fired during the sintering step. Otherwise, the partially degraded polymer will cause the formation of blisters in the sintered material, resulting in poor hermeticity and other defects. Suitable monomers that can be used alone, or in combination with other monomers, include acrylates and metiiacrylates as shown in Table 7.2 [10]. 

Degradation of Polymeric Propellant Binders , Final Rept, under Contract N00017-74-C-4335 (1975) 39) W. Brenner B. Rugg, Feasi-... 

P is the number of polymer molecules of degree of polymerization n, R is the number of radicals found in a volume V, R is the number of polymer radicals with degree of polymerization n found in a volume, V. For other definitions, please use the nomenclature associated with Table 15.2. Noting equation 15.14, the kinetics of polymer degradation are very complex. Only the most simple mechanisms have been thoroughly researched. These simplified reactions presented in Table 15.2 are sometimes zero order, more frequently first order, and infiiequently second order in polymer mass. These simplified rate expressions are typically used to model binder burnout. 

To reduce internal resistance, electrode pastes contain polymeric binding agents (PTFE, Nafion, PVdF, or PVB) and conductive carbon additives (acetylene black. Super P) [127-129]. Binders increase durability and prevent degradation of the collector-electrode interface over time. However, one downside of using these agents is an increase in internal electrode resistance that reduces device power [31]. Optimal balance of internal resistance and sufficient paste stability leads to loading of about 3 to 5% by weight. 

Composite bipolar plates generally contain polymeric matrices as binder and filler materials such as graphite, carbon black, and others. As the degradation mechanism of these differ significantly they will be discussed separately in the following, starting with the polymeric matrices. 

Attachment of molecules to the surface of a solid filler in polymeric biocomposites affects a variety of innate properties, particularly those related to the surface of (he filler material. An overview of the surface modification techniques and how they alter specific filler properties is outlined in Table 3.3. The attachment of molecules affects the immediate physical and chemical composition of a surface, which can alter secondary surface properties related to surface interactions, such as wetting, zeta potential, suiface solution reactions including dissolution/degradation, as well as cellular interactions. These primary and secondary properties do not necessarily alter how the filler interacts with polymer binders in a biocomposite setting, but these properties can change the inherent overall properties of the resultant filler. 

Modifying the surface of sohd fillers used in polymeric biocomposites controls the surface properties (both primary and secondary), which affects both the mechanical and physical properties of the resultant polymeric biocomposite as well as its ability to remodel in vivo. An overview of the surface-modification techniques and how they alter the resultant biocomposite properties is outlined in Table 3.3. The fundamental theory of composite design is to obtain physical properties that lie between those of the individual components. As previously outlined, a primary motivator to modify the surface of a solid filler is to inaease adhesion between the solid filler and polymer components, and thus the overall mechanical properties of the biocomposite. This observation has been supported by numerous studies citing an increase in tensile properties. Other overall biocomposite properties that are affected by surface modification of filler components include binding to polymer phase, solid-filler incorporation into polymer binder, water uptake, and degradation. 

---