Shrinkage During Stabilization

PAN based fibers develop shrinkage forces during two different stages of stabilization entropic shrinkage in the early stages, and shrinkage due to chemical reaction in the later stages. 

Qualitatively, the same sequence of shrinkage, relaxation and shrinkage (Fig. 8) is also to be expected if the fiber is stabilized dynamically, by continuously travelling through a furnace, which latter generally is not at a constant temperature, but has a characteristic profile, which may be different for different precursors (see Section 4.3.2). 

It is known since time that stretching before (Prescott and Ott ) and/or during (Johnson et al. ) the stabilization treatment improves the mechanical properties of the resulting carbon fiber. Evidently, high molecular order is required. If such order has been obtained in the spinning process by extensive stretching, care must be taken 

---

Epoxies show superior adhesion to metals and glass, and have limited shrinkage during cure. They are used for surface coatings, as adhesives and rc>r laminating to produce plastic tanks, pipe, and aircraft parts, etc (Ref 3, p 445-R). They also have been used in manuf of plastics (Ref 1), construction of rockets St missiles (Ref 2) and in manuf of explosive compositions of improved mechanical and thermal stability (Ref 4)... 

In addition to conferring transparency on these polymers, the amorphous noncrystallizable nature of polysulfones assures minimal shrinkage during fabrication of the resins into finished parts. The absence of crystallinity also assures dimensional stability during the service life of the parts where high use temperatures are encountered. Good dimensional stability is important to many structural and engineering applications. 

The materials studied displayed relatively low glass transition temperatures and therefore poor orientational stability. Future studies should be directed to solving this problem. Additional research is required to improve the efficiency of photoinitiation for materials with strong absorptions in the visible region of the spectrum. Other issues such as strain caused by shrinkage during crosslinking need to be addressed. 

Fig. 8. Shrinkage forces developed by acrylic fibers during stabilization at constant temperature (schematic)...

Table 13. Overall shrinkage force during stabilization standard screening conditions...

Polymer blending also affects the dimensional stability of the HKL-based fibers during the thermostabilization process. The HKL homofibers undergo more than 20% shrinkage during thermostabilization. This fiber deformation affects the utility and the... 

An improved stability is essential in order to prevent shrinkage during use. Such shrinkage may be caused by the relaxation of tensions imposed on fibres and/or yams during manufacture or be due to the inherent shrinkage properties of the raw materials themselves. 

Cast allylics. The ally lie ester resins possess excellent clarity, hardness, and color stability and thus are used to cast them into optical parts. These castings can be either homo or co-polymers. The free radical addition polymerization of the allylic ester presents some casting difficulties such as exotherm control, monomer shrinkage during curing and the interaction between the exotherm, the free radical source, and the environmental heat required to decompose the peroxide and initiate the reaction. 

---