Processing acetal resins

Other aspects of stabilization of acetal resins are briefly discussed under processing and fabrication. Reference 15 provides a more detailed discussion of the mechanism of polymer degradation. 

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

Acetal resins may also be fabricated into rod, slab, and other shapes by profile extmsion. Extmded shapes are frequendy further machined. Parts fabricated by mol ding or extmsion are ammenable to all typical postforrning processes. 

Reference 27 gives a concise overview of the processing and fabrication of acetal resins. 

Scrap and Recycle. Acetal resins can be processed with very Htde waste. Spmes, mimers, and out-of-tolerance parts can, in general, be ground and the resins reused. Up to about 25% of regrind can usually be safely recycled into virgin resin. However, the amount of regrind that can be used in a particular circumstance varies. The appropriate Hterature from the suppHer should be consulted. 

When processed and used according to manufacturer s recommendations, acetal resins present no extraordinary health risks. Before the use of any plastic material, including acetal, the Material Safety Data Sheet (MSDS) appHcable to the grade in question should be consulted. 

If acetal resins are processed at temperatures substantially above those recommended for the particular grade, minor amounts of formaldehyde may be Hberated. Formaldehyde (qv) is a colorless, lacrimatory gas with a pungent odor and is intensely irritating to mucous membranes. The human nose is sensitive to concentrations in the range of 0.1 to 0.5 ppm. The current threshold limit value for formaldehyde is 1 ppm. 

Trioxane and Tetraoxane. The cycHc symmetrical trimer of formaldehyde, trioxane [110-88-3] is prepared by acid-catalyzed Hquid- or vapor-phase processes (147—151). It is a colorless crystalline soHd that bods at 114.5°C and melts at 61—62°C (17,152). The heats of formation are — 176.9 kJ/mol (—42.28 kcal/mol) from monomeric formaldehyde and —88.7 kJ/mol (—21.19 kcal/mol) from 60% aqueous formaldehyde. It can be produced by continuous distillation of 60% aqueous formaldehyde containing 2—5% sulfuric acid. Trioxane is extracted from the distillate with benzene or methylene chloride and recovered by distillation (153) or crystallization (154). It is mainly used for the production of acetal resins (qv). 

Acrylic ESTER POLYMERS Acrylonitrile POLYMERS Cellulose esters). Engineering plastics (qv) such as acetal resins (qv), polyamides (qv), polycarbonate (qv), polyesters (qv), and poly(phenylene sulfide), and advanced materials such as Hquid crystal polymers, polysulfone, and polyetheretherketone are used in high performance appHcations they are processed at higher temperatures than their commodity counterparts (see Polymers containing sulfur). 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

In the mid-1950s a number of new thermoplastics with some very valuable properties beeame available. High-density polyethylenes produced by the Phillips process and the Ziegler process were marketed and these were shortly followed by the discovery and rapid exploitation of polypropylene. These polyolefins soon became large tonnage thermoplastics. Somewhat more specialised materials were the acetal resins, first introduced by Du Pont, and the polycarbonates, developed simultaneously but independently in the United States and Germany. Further developments in high-impact polystyrenes led to the development of ABS polymers. 

Acetal resins may be processed without difficulty on conventional injection moulding, blow moulding and extrusion equipment. The main points to be considered are ... 

Polyethers. Acetal Resins. These stabilized polyoxymethy-lenes were introduced dramatically by DuPont and Celanese as engineering plastics to replace non-ferrous metals. Good mechanical strength, resilience, fatigue-resistance, lubricity, abrasion-resistance, heat distortion temperature, water and solvent-resistance can approach the behavior of metals on a volume basis, while processability, color possibilities, and corrosion-resistance are superior. Major weakness is sensitivity to thermal, oxidative, and ionic degradation. 

Unfomuilated poly(vinyl acetal) resins form hard, unpliable materials which are difficult to process without using solvents or plasticizers. Plasticizers aid resin processing, lower the glass-transition temperature, Th, and can profoundly change other physical properties of the resins. 

Reference "Acetic Acid Process Catalyzed by lonically Immobilized Rhodium Complex to Solid Resin Support," Journal of Chemical Engineering of Japan, Vol. 37, 4, pp. 536-545 (2004)... 

Scheme 4.307],528 If water is an objectionable component or low solubility a problem, THP removal can be accomplished by an acetal exchange process using methanol or ethanol as solvent Scheme 4.308 illustrates the deprotection of an acid-sensitive allylic alcohol using camphor sulfonic add in methanol.448 Pyridi-nium p-toluenesulfonate in methanol or ethanol (pH 3.0) can also be used, but higher temperatures (45-55 C) are required [Scheme 4.309].558 559 Ion exchange resins in methanol cleave THP ethers at room temperature.5601561 Other THP deprotection conditions to beware or exploit are CAN in the presence of pH 8 buffer,562 DDQ in aqueous acetonitrile,563 and Montmorilonite clay in methanol.564... 

As already stated the two-thirds of acetal resins commercialized on the market are represented by copolymers prepared by the cationic process and one-third by homopolymers obtained anionically. The capacity of production can be estimated to be about 450,000 tons per year. The homopolymer is mostly produced by Du Pont under the trade name of Delrin... 

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