Urea Finishing Processes

Urea can be prilled, granulated, flaked, and crystallized. At the present time only prilling and granulation can be considered important. Most new plants that plan to ship internationally utilize granule on because of its far superior handling and storage qualities. Comparative product characteristics are shown in Table 9.3. 

Prilled urea is made by using a spinning bucket, shower heads, or acoustic vibration (19 plants). The most common method is the spinning bucket. Stamicarbon uses its own design for the buckets Snamprogetti and Toyo both use the turtle bucket, which was initially developed by Malcolm Tuttle and Premier Petrochemical Pasadena, 

Texas) in 1964 and 1965. Toyo also will provide the acoustic vibration unit. In prilling, the urea melt is concentrated via vacuum evaporation to 99.8% and fed as quickly as possible into the bucket to minimize biuret formation. The liquid forms drops that then fall down a cylindrical concrete tower that has either induced, forced, or natural draft air flow. The prills solidify and are removed at the bottom by belt conveying to storage. Some plants have a fluidized-bed cooler in the prill tower bottom and others use an in-line cooler before storage. If low biuret product is to be produced, the urea melt from the last decomposition stage of the synthesis plant is first crystallized and the crystals are then melted just before prilling. 

All prill towers have a dust problem, and even those with dust collection systems may have difficulties in meeting environmental standards. Properly designed prill towers utilizing natural draft can usually meet environmental standards without a dust collection system. Since prills are not as strong as granules in either crushing or impact 

5 length, many of the new plants are using granulation 2 rocesses for finishing. 

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Like methylolureas, cycHc ureas are based on reactions between urea and formaldehyde however, the amino resin is cycHc rather than linear. Many cychc urea resins have been used in textile-finishing processes, particularly to achieve wrinkle resistance and shrinkage control, but the ones described below are the most commercially important. They ate all in use today to greater or lesser extents, depending on specific end requirements (see also Textiles, finishing). 

Table 3.17 shows the process economics for urea synthesis. Raw materials and utilities per 1,000 kg of urea are given. The finishing process can be coupled with synthesis, either prilling or granulation, both direct or via crystallization. 

The flammability of polyamide fibres is usually reduced in the finishing process. Rigid yams are dressed by applying urea, thiourea, or melamine/formaldehyde resins. On the surface of flexible polyamide fibres, about 10 per cent of ammonium bromide is fixed by a urea/formaldehyde resin. Specific flame-retardant finishing is not usual. ... 

In the partial-recycle process, part of the off-gas ammonia and carbon dioxide from the carbamate strippers is recycled to the urea reactor. Recycling is accomplished by absorbing the stripper gases in a recycle stream of partially stripped urea effluent, in process-steam condensate, or in mother liquor from a crystallization finishing process. In this manner, the amount of NH3 in off-gas is reduced. Any proportion of the unreacted ammonia can be recycled typically, the amount of ammonia that must be used in some other process is reduced to about 15% of that from a comparable once-through unit. 

The key process unit is an FW Evaporative Scrubbing System (ESS). The raw process condensate from the urea unit is first steam stripped to reduce ammonia content. The stripped process condensate is mixed with a circulating stream of urea solution and enters the distribution system of the ESS, which also acts as a cooling tower. The air from the finishing process (prilling or granulation) is introduced into the bottom of the ESS. Particles entrained in the air are dissolved in the urea... 

Transureidoalkylation plays an important part in fine textile finishing processes and, generally speaking, in lacquer curing. For example, the ability of novolac to cure on the addition of polymethylene urea resins (see Section 26.3) depends on this reaction. 

Finishes based on these reagents are durable to numerous launderings. This finish is primarily used on work clothing. This finishing process, developed by Albright Wilson (now marketed by Rhodia), is known as the PROBAN process, which employs a precondensate of the phosphonium compoimd with urea (125, 126). 

The urea produced is normally either prilled or granulated. In some countries there is a market for Hquid urea—ammonium nitrate solutions (32% N). In this case, a partial-recycle stripping process is the best and cheapest system. The unconverted NH coming from the stripped urea solution and the reactor off-gas is neutralized with nitric acid. The ammonium nitrate solution formed and the urea solution from the stripper bottom are mixed, resulting in a 32—35 wt % solution. This system drastically reduces investment costs as evaporation, finishing (priQ or granulation), and wastewater treatment are not required. 

THPC—Amide Process. The THPC—amide process is the first practical process based on THPC. It consists of a combination of THPC, TMM, and urea. In this process, there is the potential of polymer formation by THPC, melamine, and urea. There may also be some limited cross-linking between cellulose and the TMM system. The formulation also includes triethanolamine [102-71-6J, an acid scavenger, which slows polymerization at room temperature. Urea and triethanolamine react with the hydrochloric acid produced in the polymerization reaction, thus preventing acid damage to the fabric. This finish with suitable add-on passes the standard vertical flame test after repeated laundering (80). 

THP—Amide Process. THP has also been made directly from phosphine [7803-5-27] and formaldehyde. The THP so generated contains one less mole of formaldehyde than either THPC or THPOH. It can be used in a THP—amide flame-retardant finish. The pad formulation contains THP, TMM, methylol urea, and a mixed acid catalyst (93—95). 

Ammonia—Gas-Cured Flame Retardants. The first flame-retardant process based on curing with ammonia gas, ie, THPC—amide—NH, consisted of padding cotton with a solution containing THPC, TMM, and urea. The fabric was dried and then cured with either gaseous ammonia or ammonium hydroxide (96). There was Httle or no reaction with cellulose. A very stable polymer was deposited in situ in the cellulose matrix. Because the fire-retardant finish did not actually react with the cellulose matrix, there was generally Httle loss in fabric strength. However, the finish was very effective and quite durable to laundering. 

THPC—Amide—PoIy(vinyI bromide) Finish. A flame retardant based on THPC—amide plus poly(vinyl bromide) [25951-54-6] (143) has been reported suitable for use on 35/65, and perhaps on 50/50, polyester—cotton blends. It is appUed by the pad-dry-cure process, with curing at 150°C for about 3 min. A typical formulation contains 20% THPC, 3% disodium hydrogen phosphate, 6% urea, 3% trimethylolglycouril [496-46-8] and 12% poly(vinyl bromide) soUds. Approximately 20% add-on is required to impart flame retardancy to a 168 g/m 35/65 polyester—cotton fabric. Treated fabrics passed the FF 3-71 test. However, as far as can be determined, poly(vinyl bromide) is no longer commercially available. 

Phosphonium Salt—Urea Precondensate. A combination approach for producing flame-retardant cotton-synthetic blends has been developed based on the use of a phosphonium salt—urea precondensate (145). The precondensate is appUed to the blend fabric from aqueous solution. The fabric is dried, cured with ammonia gas, and then oxidized. This forms a flame-resistant polymer on and in the cotton fibers of the component. The synthetic component is then treated with either a cycUc phosphonate ester such as Antiblaze 19/ 19T, or hexabromocyclododecane. The result is a blended textile with good flame resistance. Another patent has appeared in which various modifications of the original process have been claimed (146). Although a few finishers have begun to use this process on blended textiles, it is too early to judge its impact on the industry. 

Of the various amino-resins that have been prepared, the urea-formaldehyde (U-F) resins are by far the most important commercially. Like the phenolic resins, they are, in the finished product, cross-linked (thermoset) insoluble, infusible materials. For application, a low molecular weight product or resin is first produced and this is then cross-linked only at the end of the fabrication process. 

Thermosets are formed by crosslinking (curing) of reactive linear and branched macromolecules and can be manufactured by polycondensation, polymerization and polyaddition. Thermosets can therefore be processed once only with the application of heat and pressure to form semi-finished products or finished articles and cannot be recovered their processing is irreversible. Amongst the most familiar thermosets are the combinations of formaldehyde with phenol, resorcinol etc. (phenolics), urea, aniline, melamine and similar combinations (aminoplastics). 

Although inorganic salts can provide excellent flame-retardant properties for cellulose, reasonable laundering durability must be incorporated into any finish destined for apparel use. The most successful durable flame retardants for cellulose are based on phosphorous- and nitrogen-containing chemical systems that can react with the fibre or form crosslinked structures on the fibre. The key ingredient of one of these finishes is tetrakis(hydroxymethyl)phosphonium chloride (THPC), made from phosphine, formaldehyde and hydrochloric acid (Fig. 8.11). THPC reacts with urea to form an insoluble structure on cellulose in a pad-dry-cure process (Fig. 8.12). 

A variation on the THPC-urea system was developed to produce finishes with less stiffness and fibre damage (Proban process). A precondensate is prepared by the careful reaction of THPC with urea. This precondensate is padded onto the fabric and the fabric is dried to a specific moisture content ( 15 %). The fabric is then exposed to ammonia vapours in a special reaction chamber, followed by oxidation with hydrogen peroxide (Fig. 8.13). The polymer that forms is primarily located in the lumen of the cotton fibre. The final finish provides durable flame retardancy to cotton with much improved fabric properties. It is important to note... 

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