Acrylate precursor

Carbon and Graphite Fibers. Carbon and graphite fibers (qv) are valued for their unique combination of extremely high modulus and very low specific gravity. Acrylic precursors are made by standard spinning conditions, except that increased stretch orientation is required to produce precursors with higher tenacity and modulus. The first commercially feasible process was developed at the Royal Aircraft Fstablishment (RAF) in collaboration with the acrylic fiber producer, Courtaulds (88). In the RAF process the acrylic precursor is converted to carbon fiber in a two-step process. The use of PAN as a carbon fiber precursor has been reviewed (89,90). 

Peebles, L. H., Carbon fibers from acrylic precursors. In Carbon Fibers Formation, Structure, and Properties. CRC Press, Boca Raton, FL, 1995, pp. 7 26. 

The products of cyclopropanation have been further elaborated to cyclopropane-containing amino acids (42) and Boc-protected amino esters (43) (see Scheme 10.23) [79]. A single recrystallization either before or after deprotection afforded enantiomerically pure material. As the acrylate precursors are made in one step, this sequence provides an efficient route to this important class of conformation-ally locked amino acids. 

One of the most significant steps in the preparation of carbon fibers from acrylic precursor fibers is the oligomerization of the nitrile groups. This reaction has originally been studied in context with the problem of thermal discoloration of PAN (e.g. McCarthney Grassie and McNeill Grassie and Hay It was supposed to lead to a so-called ladder structure ... 

Materials. The polyurethane precursor materials were Adiprene L-lOO (Uniroyal, Inc.), a poly(oxytetramethylene glycol) capped with toluene diisocyanate, eq. mol. wt. 1030 1,4-butanediol (BD) and 1,1,1-trimethylolpropane (TMP) and, as catalyst, dibutyltin dilaurate (DBTDL). Acrylic precursors included n-butyl methacrylate (BMA), washed with 10% aq. NaOH to remove inhibitor tetramethylene glycol dimethacrylate (TMGDM) crosslinker and benzoin sec-butyl ether (BBE) as a photosensitizer. These materials were dried appropriately but not otherwise purified. 

Methyl acrylate precursor. A heterocyclic NH is converted into the NCH2CH2COOMe moiety by the reagent under basic conditions. 

A series of [2.2.1]bicycloheptenyl (norbornene) functional prepol)nners have been prepared via the cycloaddition reaction of cyclopentadiene monomer with corresponding acrylics. When these materials are formulated with an appropriate multifuntional thiol crosslinker and photoinitiator and irradiated, a rapid, exothermic, crosslinking reaction takes place. When the acrylic precursors are organic resins, the derived polymers behave like toughened plastics. The choice of a norbornene functional polydimethylsiloxane precursors gives elastomeric products. 

CROSSLINKED NETWORK MODEL CALCULATIONS Our research has shown that tetrathiol crosslinked norbomene resins form a densely crosslinked, three dimensional network. Recently there has been considerable interest in crosslinked networks from a theoretical and practical point of view (29-31). As part of our study we attempted to analyze the polymer network using the Miller-Macosko formalism as applied by Bauer (32). For the purposes cf this analysis we assumed that the curable formulation was an A2 (ene) B4 (thiol) type system. We also assumed, based on HPLC analysis of EBPA DN and acrylate precursor batches that the norbomene resins was a mixture of oligomers consisting of difunctional olefin (85%) and monofimctional olefin (15%). The thiol crosslinker was assumed to be essentially tetrafunctional. Furthermore, we made the not unreasonable assumption that there would be no thiol-thiol or norbornene-norbornene reactions. In one case, a chain extending... 

The preceding study has demonstrated [2.2.1]bicycloheptenyl functionalized resins can be useful and interesting ene components in photoinitiated thiolene polymerizations. The addition of thiols to the unsaturation of this bicyclic system appears to be rapid and exothermic. The relative rates of this addition compared with allylic derivatives and vinyl ethers are quite favorable. The organic resins can be readily prepared from either polyols, f>olyamines, or acrylic precursors (Figure 10) and the yields are generally quite good. When acrylate esters are used as precursors, the cycloaddition reaction occurs spontaneously and no catalysis of the reaction is necessary. 

Though high specifications and, perversely, high prices have fed the fashionable desire for carbon in premium markets, continued fails in carbon prices could extend the material s market base. Producers (notably Zoltek) that have developed production routes based on alternative cheaply sourced acrylic precursors believe that breaching the psychological 5/lb barrier would transform carbon s prospects. So far price levels of some 7/lb are about the best that can be obtained. 

Fibers spun from polyvinyl alcohol, polybenzimidazoles, polyamides, and aromatic polyamides have been used as carbon fiber precursors. However, at present, the most attractive precursors are made from acrylonitrile copolymers and pitch, and a small amount from rayon. Today more than 95% of the carbon fibers produced for advanced composite applications are based on acrylic precursors. Pitch-based precursors are generally the least expensive, but do not yield carbon fibers with an attractive combination of tenacity (breaking strength, modulus, and elongation as those made from a acrylic precursor fiber). The acrylic precursors provide a much higher carbon yield where compared to rayon, typically 55% versus 20% for rayon, and this translates directly into increased productivity. 

The first commercially feasible process for converting acrylic fibers to carbon fibers was developed by Walt, Phillips, and Johnson of the Royal Aircraft Establishment (RAE) in collaboration with the acrylic fiber producer, Courtaulds [621]. In the RAE process, the acrylic precursor is converted to carbon fiber in a two-step process [622]. Preoxidation or filament stabilization is carried out in the first stage. The precursor is heated in an oxygen atmosphere under tension at a temperature of approximately 200 250°C, well below its carbonizing temperature (approximately 800°C). At this temperature, the nitrile groups react with each other via a free radical addition process leading to the so-called ladder structure shown in reaction 12.34 [609,621 625]. 

Escale, P., Van Camp, W., Du Prez, F., Rubatat, L., Billon, L., Save, M. Highly structured pH-responsive honeycomb films by a combination of a breath figure process and in situ thermolysis of a polystyrene-block-poly(ethoxy ethyl acrylate) precursor. Polym. Chem 4, 4710-4717 (2013)... 

Acrylic polyols are prepared by the free radical polymerisation of hydroxyethyl acrylate or methacrylate with other acrylic precursors. To introduce rigidity and stiffness into the polymer, the structure must be highly cross-linked. This is obtained by use of polyhydroxyl polyols (triol... 

The rate controlling step in the production of carbon fiber from an acrylic precursor is the oxidation stage and G Gould and his research team looked at ways of speeding up this reaction. Various techniques could be used to catalyze the cyclization of PAN, but because the SAF already contained a catalyst comonomer (itaconic acid), the effects were much smaller than those reported in the literature for other acrylic fibers. One of the most promising was treatment with a Lewis acid, SnCU, which when applied as a solution in diphenyl ether, reduced the residual exotherm of SAF to less than 50 cal in only 6 min, which would normally have taken some 3 h of air oxidation at 220°C to have produced the... 

Rose PG, A study of the physics and chemistry of the preparation of carbon fibres from acrylic precursors. Thesis, University of Aston in Birmingham, Oct 1971. 

Johnson JW, Thorne DJ, Effect of internal polymer flaws on strength of carbon fibres prepared from an acrylic precursor. Carbon, 1, 659-661, 1969. 

Courtaulds introduced, specifically for the manufacture of carbon fiber, a Special Acrylic Fiber (SAF), which was made at the Coventry works using the same dope, but spun on production lines with additional filtration and individual dope spinning pumps, enabling precise d tex control for smaller tows. Courtaulds ceased production of their Special Acrylic precursor (SAF) in 1991. 

Acrylic precursors for the carbon fiber industry originated from companies that were established commercial scale producers of textile grade acrylic fibers. Hence, the manufacturers that could most readily adapt their existing technology to create a precursor grade material have been most successful (Table 4.2). However, some aspects such as dyeability and a tendency to yellow are not important parameters for a carbon fiber precursor but, because that particular polymer formulation was initially used for other textile end uses, the polymer composition could not be changed. As carbon fibers have developed, the market requirement for suitable precursors has increased and new polymers have been developed specifically for the manufacture of carbon fibers. 

A detailed review of acryhc precursors for carbon fibers is given by Gupta et al [19], some precursor examples are discussed by Rajalingam and Radhakrishnan [20], whilst Bajaj and Roopanwal present an overview of the thermal stabilization of acrylic precursors for the production of carbon fibers [21]. 

Gupta AK, Paliwal DK, Bajaj P, Acrylic precursors for carbon fibers, J Macromol Sci Rev Macromol Chem Phys, C31(l), 1-89, 1991. 

If a thermoplastic acrylic precursor process is developed, the main technical issues will be overcoming inherent thermoplasticity in stabilization. Chemical treatments or radiation may be helpful here. 

The acrylic precursor is stabilized by controlled low temperature heating (200-300°C) in air to convert the precursor to a form that can be further heat treated without the occurrence of melting or fusion of the fibers. In order to achieve this end, a slow heating rate must be used to avoid run-away exotherms occurring during the stabilization process, exacerbated by the PAN precursor which is a poor conductor of heat. 

Pn = density (g cm ) of the fiber after the treatment stage Po = density (g cm ) of the acrylic precursor fiber... 

Kiminta [63] investigated the rapid stabilization of acrylic precursors for carbon fibers using NH3. 

Figure 5.37 Density as a function of stabilization time for acrylic precursor fiber containing AN/MA at stabilization temperatures of O, 240°C A, 255°C , 270°C. Source Reprinted from Bajaj P, Roopanwal AK, Polym Sci, 1,368, 1994.

Figure 5.42 Density of carbon fibers as a function of the density of stabilized fibers obtained from an AN/MA acrylic precursor fiber. Source Reprinted with permission from Takaku A, Hashimoto T, Miyoshi T, JAppI Polym Sci, 30, 1565, 1985. Copyright 1985, John Wiley Sons Ltd.

Jain and Abhiraman (1983) [203] used thermal analysis and WAXD to study the effect of annealing acrylic precursor fibers for 2 min at 230° C and 4 min-4 h at 270° C. Annealing in the absence of constraint caused significant shrinkage, an increase in the orientation of the... 

Stereoregularity of the acrylic precursor has been shown by Peebles and Snow to be of no importance to the stabilization reaction [225] and one explanation given by Peebles [225] is a rapid imine-enamine tautomerization of the final cyclized unit causing racemization or scrambling of any chiral centers ... 

Peebles LH Jr., Carbon fibres Structure and mechanical properties, Int Mat Rev, 39(2), 75 92,1994. Bajaj P, Roopanwal RK, Thermal stabUization of acrylic precursors for the production of carbon fibres An overview, JMS Rev Macromol Chem Phys. C37(l), 97 147, 1997. 

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