Acrylic precursor fiber

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 ... 

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. 

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

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... 

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). 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

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

We presently report on a broad search for specific acrylic carbon fiber precursors, which should be stabilized in short time (less than one hour), and yet would give carbon fibers with satisfactory tensile properties. In planning the chemistry of such precursors, it was necessary to take into account the chemical reactions and physical processes going on during the heat treatment. 

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]. 

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... 

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. 

It will be difficult to improve upon acrylic textUe fiber as a potential filament-based precursor at 1.6 per kg. CF at 11 per kg is likely to be accessible through the use of this material, with appropriate modification, using current best-practice low-cost CF conversion equipment. 

If acrylic textile fiber continues to be available and can be successfully converted to LCCF, then thermoformable acrylic pol5uners will not be favored, unless the sale price of the resin is less than 1 per kg. This may encourage capital investment for new precursor forms. 

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. 

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

Fibrillated Fibers. Acrylic fibers are sold in the form of fibrillated pulps for use as highly efficient binders. These fibrillated fibers have a tree-like structure with limbs (fibrils) attached to the main trunk (fiber). The trunk is 20-50-fx diameter and the limbs range from a few microns to snbmicron. The product is generated from a special precursor fiber by intense mechanical action. Commercial examples are CCF from Sterling Fibers, Acri-Pulp from Solntia, and Dolanit lOD from Acordis. 

Carbon fibers are not directly spun but are the product of a complicated aftertreatment. Nowadays, most carbon fibers (90%) are produced from an acrylic precursor. Cellulose rayon is no longer applied as a precursor. Production from pitch has been developed, but is still a small-volume business. 

PAN currently is the most used precursor for carbon fibers. Figure 11.1 shows the main processes for producing carbon fibers by using the PAN route. PAN can be synthesized from acrylonitrile through a radical polymerization process. Dirring the synthesis of PAN, co-monomers, such as methyl acrylate (up to 5%) can be added to improve the processability of PAN-based precursor fibers. The addition of co-monomers also can improve the mecharrical properties of the final carbon fibers by increasing the molecular orientation. 

Itaconic acid is a specialty monomer that affords performance advantages to certain polymeric coatings (qv) (see Polyesters, unsaturated). Emulsion stabihty, flow properties of the formulated coating, and adhesion to substrates are improved by the acid. Acrylonitrile fibers with low levels of the acid comonomer exhibit improved dye receptivity which allows mote efficient dyeing to deeper shades (see Acrylonitrile polymers Fibers, acrylic) (10,11). Itaconic acid has also been incorporated in PAN precursors of carbon and graphite fibers (qv) and into ethylene ionomers (qv) (12). 

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