63 Nitrocellulose 631 Preparation 63-1 egies of Substitution» Degce of Polymerization * Types and Grades 63.2. Solubility 632 Solvents and Dilenis Viscosity Elects Bushing » Solution Preparation 63.3 Film Properties. 63-3 Plasticizers Resin» Crow Linkable Coating Sytens»Sasty Daniel M. Zavisza Considerations | Hercules tocorporated Appendix: Typical Properties of RS Nitrocellulose 35 When most people think of nitrocellulose they think of guncotton, a material that was developed for explosives or gum propellant. But they are only partially correct. Nitrocellulose is one of the oldest and ‘most widely used film formers adaptable to a number of uses. Its derived from cellulose, a material from plants, and therefore a renewable source. Soluble nitrocellulose possesses a unique combination of properties such as toughness, durability, solubility, gloss, and rapid solvent release. As the film former in lacquer systems, it affords protective and decorative coatings for wood and metal. In addition, it finds use in flexible coatings for paper, foil and plastic film, printing inks, and adhesives. This chapter brielly covers the properties, uses, and handling procedures for nitrocellulose and the formulations made from it 63.1 Preparation Nitrocellulose isthe common name for the nitration product of cellulose, Other names include cellulose (ei)nitrate and guncotton. The commercial product is made by reacting cellulose with nitric acid, Cellulose is composed of a large number of [Sanhydroglucose units, which are jointed together into a chain. The anhydroglucose units are six-membered rings having three hydroxyl (-OH) groups attached to them. The number of anhydroglucose units in the typical cellulose chain ranges from 500 to 2500 in chemically purified cellulose 63.1.1 Degree of Substitution Nitvic acid can react with these three hydroxyl groups of the anhydroglucose units to form the nitrate ester Fully nitrated cellulose would then be a trinitrate — that is, a nitrate having a degree of substitution of 3. The calculated nitrogen content of such a fully nitrated cellulose is 14.14%. In practice, however, the maximum nitrogen level that can be achieved is 13.89. This corresponds to a degree of substitution of 2.9. At this level, nitrocellulose does not process properties that are useful for coatings use. Film forming properties ae better at degrees of substitution between 1.8 and 2.3. 63163.2 Coatings Technology Handbook, Third Edition 63.1.2 Degree of Polymerization ‘The degree of polymerization of nitrocellulose is the number of anhydroglucose units that are linked together to form one molecule. The more units are linked, the higher the viscosity ofthe nitrocellulose in solution ata given concentration. Commercial nitrocellulose is categorized into grade by the viscosity of a 12.2% solids solution at 25°C in terms of centipoise or the time it takes, n seconds, for a metal ball. of a specified size to fall a measured distance through the solution. The solvent system used is usually 55 parts by weight toluene, 25 parts denatured ethanol, and 20 parts ethyl acetate, 63.1.3. Types and Grades Nitrocellulose is divided into types according to the nitrogen content of the product, which reflects higher or lower degrees of substitution. The lowest commercially useful nitrogen content is 11% (10.0 to 11.2%). ‘This corresponds to a degree of substitution of 1.8 to 2.1. This type is further classified into viscosity grades ranging from 30 t0 35 cp* to 40 to 60 see. These grades are useful in lacquers for paper and foil, low odor lacquers, sealers, fillers, printing inks, and plastics. At this level of nitrogen, nitrocellulose is ‘more tolerant of alcohol than higher nitrogen types. ‘A second type has an average nitrogen content of 11.59 (11.3 to 11.794). Viscosity grades of 0.5 and 5 t06 see are available for use in lacquers for coating cellophane, paper, and textiles. ‘The next type contains an average of 12% nitrogen (11.8 to 12.29) and is available in a large number of viscosity grades, from 18 to 25 ep to 2000 sec. This type is more tolerant of aromatic hydrocarbon solvents, such as toluene, and less tolerant of aliphatic hydrocarbons. This grade is compatible with many resins and has many uses, It is used in coatings for wood, metal, paper, textiles, and foil for lacquer emulsions for wood and metal; and for architectural finishes, adhesives, cements, and inks. Higher nitrogen levels (>13%) find use in the manufacture of gun propellants and explosives. Solubility and viscosity in solution and compatibility of nitrocellulose with a variety of modifiers, such as plasticizers, resins, and pigments, determine its usefulness in preparing lacquers and coatings. The chapter appendix is a table of the typical properties of nitrocellulose containing 12% nitrogen, the type ‘most commonly used for lacquers and coatings. 63.2. Solubility 63. ‘The generally used method of formulating nitrocellulose coating systems features volatile solvents that dissolve the nitrocellulose and its modifiers to form a homogeneous system (with the exception of pigments and filles). "The resulting formulation can then be applied to the substrate by one of a number of methods such as brushing, spraying, or curtain coating. The solvent evaporates to leave a solid film on the substrate ‘True solvents are liquids that will dissolve nitrocellulose completely. For 1296 nitrogen nitrocellulose, these are ketones, esters, amides, and nitroparaffins. Some solvents, such as ethyl or isopropyl alcohol, will not dissolve nitrocellulose on their own. They may be added to true solvents without precipitating the nitrocellulose. These are termed “cosolvents” Aliphatic and aromatic hydrocarbons are nonsolvents, ‘Termed “diluents,” they may be added to nitrocellulose solutions in limited amounts without precipitation to lower cost and improve solubility of resin modifiers. Aromatics may usually be added to a greater extent than aliphatics. At the lower nitrogen level of 119, mote hydrocarbon can be added to the nitrocellulose solution without precipitation, 1. Solvents and Diluents “The viscosity of very low molecular weight nitrocellulose is measured and described in centipise. The falling bull method i used for times ofa quarter-second and higher. This results ina change of units used to describe the viscosity gradesNitrocellulose 63.3 Dilution ratio isthe ratio ofthe volume of diluent that can be added to a given volume of nitrocellulose solution in a true solvent before separation of the nitrocellulose takes place. For example, starting with, 8 g of nitrocellulose in 100 m of solvent, butyl acetate the dilution ratio for toluene is 2.75 compared. with 15 for MV&P naphtha (an aliphatic hydrocarbon mixture). Different solvent-diluent combinations will have different dilution ratios and must be measured separately. When ethanol, a cosolvent, is combined with a true solvent, the dilution ratio for a given diluent is higher than with the true solvent alone. Raising the temperature will usually lower the dilution ratio for a diluent-solvent mixture, 63. ‘The viscosity of a polymer solution will normally increase with the solvating power of the solvent at a given concentration, because this allows the polymer to stretch out further and to become entangled. With other polymer molecules mote easily. A poorer solvent forces the polymer to become more compact and to occupy a smaller volume, resulting in a lower solution viscosity. ‘Toluene is a poorer solvent for nitrocellulose than esters or ketones. Up toa point, however, it may be added to solutions of nitrocellulose in true solvents, such as esters on ketones, without precipitating the nitrocellulose. The effect is that the viscosity of the resulting solution is lower for solutions containing toluene than without it atthe same solids level. This technique allows a formulator to maximize the solids level for a given application viscosity. The most common diluents used in this way are toluene and xylene. Solutions prepared of nitrocellulose exhibit a viscosity drop on standing. A solution of half-second grade at 20 w1% solids in an ethanol-toluene-ethyl acetate solvent blend exhibits for example, viscosity of 3.8 see by a falling ball method 1 h after addition of the solvent but 3.5 sec after 24 h. This viscosity loss will stop after a finite time, .2 Viscosity Effects 63.2.3. Blushing Formulating lacquers that contain too great a percentage of rapidly evaporating solvents may result in a White chalky appearance upon drying. The cause ofthis is cooling of the film surface below the atmo spheric dew point by the evaporating solvent, condensing water onto the film. If sufficient moisture is, condensed, the nitrocellulose can precipitate. This blushing problem is worsened during humid weather. ‘The remedy for blushing is to add a slowly evaporating solvent to the solvent blend, This has the effect of retarding the evaporation rate and lowering the cooling effect. Examples of such solvents are isobutyl ketone, methyl amyl acetate, amyl acetate, methyl amyl ketone, and 2-butoxyethanol Blushing or haze formation can also occur when a poor solvent, which has a slow evaporation rate, js used with a fast, good solvent. The good solvent will evaporate frst, causing precipitation of the nitrocellulose by the poor solvent. Too much high boiling hydrocarbon diluent will do this. 63.2.4 Solution Preparation ‘The type of mixing apparatus is usually the preparer’s choice. A vertical tank with propeller or dise agitators works well. Laboratory samples are easily dissolved by tumbling or rolling jats containing the nitrocellulose and solvent. Ifa solvent blend is to be used, it is best to disperse or wet the nitrocellulose With the cosolvent or diluent frst, then add the active solvent portion. This procedure will reduce the time required to effect solution, Solvent blends containing higher ratios of diluent to active solvent will take longer to dissolve the nitrocellulose than those with lower diluent ratios 63.3 Film Properties Film properties of nitrocellulose are affected by the solvents used, the casting technique, the dying conditions, as well as other parameters. Different types of modifier are also used to alter nitrocellulose dry film properties. Nitrocellulose is only one ingredient in lacquers and coatings. The following descrip- tion of modifiers i intended to give a flavor of how nitrocellulose systems may be modified,4 Coatings Technology Handbook, Third Edition 63. Plasticzets are nonvolatile materials added to control flexibility and elongation of a film. Plasicizers should be nonvolatile, colorless, odorless, and tasteless. They should be nontoxic and should provide ‘maximum flexibility with minimum loss in film strength and toughness. They should not destabilize the film chemically, Plasticizets fll into two types: solvent and nonsolvent, Solvent-type plasticizers ate those that exhibit complete miscibility with nitrocellulose in all proportions, Examples are dibutyl phthalate and diisononyl phthalate Nonsolvent-type plasticizers neither dissolve nor cause formation of colloidal nitrocellulose at room temperature. However, they ate compatible with nitrocellulose in solution and in the dry film, Examples of nonsolvent-type plasticizers ate castor oil and polymeric or polyester-type plasticizers. The polymeric plasticizers improve flexibility and have very low volatility, are nonmigeating and nonspewing, and do not leave the film at elevated temperatures 1 Plasticizers 63.3.2 Resins Resins are used in nitrocellulose coating compositions to improve the degeee of film build by increasing the solids content at a given viscosity (ie., spray viscosity). Depth, gloss, and adhesion can also be promoted by added resin, Natural resin, such as shellac, dammas, elem, and mastic, were some of the first resins used to modify nitrocellulose. Most ofthese resins are not film formers in themselves and do not improve tensile strength, flexibility, or elasticity. The use of natural resins is limited. ‘Some synthetic resins now available are designed to have specific properties that make them generally more adaptable for use in lacquers than natural resins. Alkyd resins are widely used. They are prepared. by reacting a polyhydric alcohol, such as pentaerythritol, with a polybasic acd, such as phthalic acid. Alkyd resins modified with drying oils or unsaturated fatty acids are good film formers. Addition of nitrocellulose to these resins accelerates the drying time and often eliminates the need for baking. Phenol-formaldehyde resins have excellent resistance to alcohols, acids, and alkalies. When used with nitrocellulose, however, there may be compatibility problems and poorer color stability. Other useful resins ae acrylic resins, vinyl resins such as polyvinyl butyral and polyvinyl acetate, certain polyamides, epoxies, and low molecular weight polyester. Pigments are added for producing opaque, colored finishes. Because nitrocellulose tends to be degraded in sunlight, some pigments extend the service life of films exposed to sunlight. Certain pigments should be avoided because they show alkaline reactions, which cause nitrocellulose degradation. 63.3.3. Cross-Linkable Coating Systems Soluble nitrocellulose contains residual hydroxyl groups that may be utilized to prepare coatings that are cross-linked to other vehicle materials through these hydroxyls. Some groups that react with these hydroxyls are methylolamino and alkylated methylolamino groups, isocyanates, and epoxides. Catalyzed nitrocellulose-alkyd-aminoplast systems are systems that will cure by acid catalysis, The nitrocellulose and the alkyd provide the hydroxyl groups. The aminoplast can be an alkylated urea- of smelamine-formaldehyde resin, These systems are catalyzed to cross-link with a strong acid, such as p- toluenesulfonic acid or phenyl acid phosphate. The coatings produced are tough and solvent resistant and find application on kitchen cabinets. Once catalyzed, the system must be used promptly because the solution will gel with time. Nitrocellulose-urethane systems are prepared from the reaction of apolyisocyanate with nitrocellulose ‘The coatings may be toughened by the addition of a polyol such as a polyester, acrylic, vinyl, or alkyd polyol. Each of these polyols will give different properties. Conventional urethane catalysts, such as 2 ‘octoate, may be used to speed up the cross-linking. ©2009 Tyr Ar Gap.Nitrocellulose Epoxy-containing resins may be used; but since the than he usual polyols in epoxy systems, they are not 63.3.4 Safety Considerations 35 available hydroxyls in nitrocellulose are less reactive used extensively for coatings systems. Some basic safety precautions must be observed because nitrocellulose is a very flammable material. It is sold wetted with some material such as ethanol or isopropanol, hich lowers its flammability. It should never be allowed to dry, because then it is extremely flammable. No spark-producing sources, flames, or heat of static electricity sources should come into proxi ily of nitrocellulose. Nitrocellulose containers should be kept tightly closed. Smoking must be prohibited when handling nitrocellulose or its solvents, diluents, or solutions. Safety procedures are contained in National Fire Protection Association (NFPA) Standard 35, Manufacturing Chemists Association Chemical Salety Data Sheet SD-96, the Hazardous Materials Regulations of the U.S. Department of Transportation (009) and the U.S. Occupational S and Health Standards, Part 1910, ety Appendix: Typical Properties of RS Nitrocellulose General Properties Odor of materi “Taste of materi Color of film Clarity of film Physical Properties Bulking valve in solution Specific volume in solution Specific gravity of cast lm Refractive index, principal Light transmission, lower init of substanialy complet cri Properties of Disectrc constant at 25-30°C ‘oH kite I MBe Power ctr at 25-30°C ‘oo Ha kite lectie charge on rubbing with sik None None Waterwhite Excelent of Solid or Fil ‘2.0706 gala 16.26 in 1S 11 3130 Unplsticasd Film 2. 7 35% So ‘Negative “Mechanical Properties of Unplasticized Fil ‘Tense stength at 28°C, 50 relative humidity Elongation at 23°C and 80% relative humidity eit of to 4-i film, MIT double folds wader 200-8 Hardness, Swat of glass Softening point range (Pate) '900-16,.000 psi mu 30-50 0% 155-200°C Solubility and Compatibility Charactrisis Solvents, principal res Resins, compatible types Pasties, compatible types Waxes and tars, compatible types Compatible cellulose derivatives ©2009 Tyr Ar Gap. ster; ketones ether-alcohol Alimost all ‘Almost ll neuding many vegetable its None Ethyl cellulose, cellulose acetate, etiylhydoxyetyl cellulose3.6 Coatings Technology Handbook, Third Edition (Chemica and Physical Properties af Unplasticied Cleat Fil, “Moistuve absorption at 21°C in 24h in 80% relative hunny Water vapor permeability t 21°C Sunlight effect on discoloration Sunlight est on embuitlement Aging fet of cold water fet of hot water General resistance: ‘Acid, weak Acids, song, ‘Ais, weak ‘ales, strong ‘cabols Ketones Esters Hydrocarbons ‘Aromatic Aliphatic its Mineral Animal Vegetable 10% 28 gemenh x 10" Moderate Moderite Poot Poot Parl soluble Soluble Soluble Good Excellent xelent Good Fair to good Source: Nitrocellulose (technical brochure), Hercules Incorporated, Wilmington, DE ©2009 Tyr Ar Gap.