Subsequent investigations have shown that his analysis was not entirely correct but the chemical has retained its original name. It is a member of the class of compounds known as triazines and may be designated 2 4 6 triamino 1 3 5 triazine. It may also be considered a trimer of cyanamide. It may be prepared from dicyandiamide by heating under pressure in the presence of a diluent such as alcohol or ammonia. The relation of melamine to cyanamide and dicyandiamide is shown in Fig.
The chemistry historical development and methods of production of melamine are given in considerable detail by McClellan Hughes 12 studied crystalline melamine and found it to be a resonance hybrid with the position of the atoms as shown in Fig. Ostrogovich 13 suggested both amino and imino structures in view of the possibility of tautomerism Fig. The amino form is generally used in the discussion of melamine in coating resins because its highmelting point and heat stability suggest this benzenoid structure. It has a molecular weight of and specific gravity of 1. Its solubility in water has been reported by Chapman 14 to be 0.
Terpene Solvents. The terpene solvents are the oldest in use by the paint industry and are obtained from pine trees. They have been replaced by lower cost aliphatic hydrocarbon solvents in many coatings. Their chemical properties make them quite valuable as raw materials for synthetic resins and other compounds.
The chemical structures of important constituents of terpene solvents are shown in Fig. Turpentine is the most widely used terpene solvent its principal use being in house paints and in some varnishes. The production of gum turpentine from the exudation of the pine tree is described. The production of wood turpentine dipentene and pine oil from solvent extraction of pine stumps followed by steam distillation is described.
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Dipentene has a higher boiling point than turpentine and excellent solvent properties. It is also used to retard the skinning of varnishes synthetic resins and enamels. The heavy fractions obtained from the production of wood turpentine are known as pine oil. These fractions consist of terpene tertiary and secondary alcohols plus varying percentages of highboiling terpene hydrocarbons.
Small percentages of phenol ethers and ketones are also present. The polar non polar structure of pine oil makes it suitable for a wide range of uses. It has excellent solvent properties improves the flow of enamels retards skinning is an antifoaming agent and has some bactericidal action. A typical group of terpene alcohol solvents is given in Table 1. As a result they have a higher degree of purity and are used as chemical raw materials as well as solvents.
The specific uses for terpene solvents in varnishes resins and coatings are given in other sections of this book and in Volume II. The physical characteristics of a typical group of commercial terpene solvents are given in Table 2. Hydrocarbon Solvents. The petroleum and coal tar hydrocarbon solvents are used extensively because of their low cost good solvent power for oils and resins and effectiveness as diluents for nitrocellulose lacquers. The petroleum solvents are the lighter fractions obtained by the distillation and fractionation of the crude oil.
Vast improvements have been made in large scale fractionation apparatus with the result that today many grades of hydrocarbon solvents are available as shown in Tables 3 and 4. The coal tar hydrocarbons are obtained by distillation of the material from the coke oven by product recovery process. They include benzene benzol toluene toluol xylene xylol and other aromatic hydrocarbons. The tremendous wartime demand for toluene to make explosives stimulated research to produce it from other sources.
Today more aromatics are produced from petroleum than from coal tar. The commercial hydrocarbon solvents are usually mixtures of closely related compounds and isomers hence the range in distillation temperatures for single solvents shown in Tables 3 and 4. The structures of typical hydrocarbons are shown in Fig. The boiling points increase with increase in molecular weight in a given series but the effect of molecular shape is shown by the first three compounds in Fig. They are isomers of hexane and therefore have the same composition and molecular weight but the boiling point decreases with the shortening of the main carbon chain.
The increase in boiling point of the normal straight chain saturated hydrocarbons or alkanes is shown in Fig. The normal alkanes containing from 5 to 16 carbon atoms in the chain are liquids at room temperature. The solid paraffin wax contains from 18 to 25 carbons per chain and polyethylene contains several hundred carbons. This topic was discussed in Chapter 1 with respect to the secondary valence forces and chain length and their effect on the physical properties. The wide range of properties in hydrocarbon solvents which are available commercially is illustrated in Table 3 with the Amsco solvents of the American Mineral Spirits Co.
Additional heat must be applied after the bottom of the flask is dry to obtain the last drop. It is advantageous in many cases to have the spread in distillation temperatures kept as small as possible but this requires closer fractionation with an increase in cost. In Table 3 the solvent power is indicated by the range of KB values from 34 to 37 for the regular mineral spirits type of solvent to for toluol.
The straight aniline point is used for the aliphatic or mineral spirits ype solvents and the mixed aniline point for the aromatic type as explained previously. The results from the nitrocellulose dilution ratio test are given for the solvents having fast enough evaporation rates and sufficient solvent power to be used as diluents. The test was run with butyl acetate as the true solvent portion.
The values would be different if another solvent were used as explained previously. It will be noted that the aromatics are higher in weight per gallon than the aliphatics. A typical set of characteristics of aromatic solvents as produced by the coal tar industry is given in Table 4. Fire and Explosion Hazard. A comparison of the flash point autoignition temperature and explosive limits in air for a variety of hydrocarbon solvents may be obtained from Table 5. Also given is the solvent vapor density in comparison with air. The volume of solvent vapor at a given temperature may be calculated from the relationship between molecular weight and volume.
These factors are expressed in the following formula. Dyes are intensely colored substances that can be used to produce a significant degree of coloration when dispersed in or reacted with other materials by a process which at least temporarily destroys the crystal structure of the substances. This latter point distinguishes dyes from pigments which are almost always applied in an aggregated or crystalline insoluble from. Modern dyes are products of synthetic organic chemistry. To be of commercial interest dyes must have high color intensity and produce dyeings of some permanence.
The degree of permanence required varies with the end use of the dyed material. All molecules absorb energy over various parts of the electromagnetic spectrum. The characteristic of dye molecules is that they absorb radiation strongly in the visible region which extends from angstroms. Only organic molecules of considerable complexity which contain extensive conjugation systems linked to electron withdrawing and attracting groups give sufficient absorption tinctorial value in the visible region to be useful as dyes.
The shade and fastness of a given dye may vary depending on the substrate due to different interactions of the molecular orbitals of the dye with the substrate and the ease with which the dye may dissipate its absorbed energy to its environment without itself decomposing. The primary use for dyes is textile coloration although substantial quantities are consumed for coloring such diverse materials as leather paper plastices petroleum products and food. The manufacture and use of dyes is an important part of modern technology. Because of the variety of materials that must be dyed in a complete spectrum of hues manufacturers now offer many hundreds of distinctly different dyes.
An understanding of the chemistry of these dyes requires that they be classified in some way. From the viewpoint of the dyer they are best classified according to application method. The dye manufacturer on the other hand prefers to classify dyes according to chemical type. Both the dyer and the dye manufacturer must consider the properties of dyes with relation to the properties of the materials to be dyed. In general dyes must be selected and applied so that color excepted a minimum of change is produced in the properties of the substrate. It is necessary therefore to consider the chemistry of textile fibers as a background for an understanding the chemistry of dyes.
The major uses of dyes are in coloration of textile fibers and paper. The substrates can be grouped into two major classes hydrophobic and hydrophilic. Hydrophilic substances such as cotton wool silk and paper are readily swollen by water making access of the day to the substrate relatively easy. On the other hand the ease of penetration also allows easy removal in aqueous systems and special techniques must be used where a high degree of wet fastness is required.
On the other hand hydrophobic fibers such as the synthetic polyesters acrylics polyamides and polyolefin fibers are not readily swollen by water hence higher application temperatures and smaller molecules are generally required. The polymer chemist has increased the versatility of the newer fibers by incorporating dye sites of a varying nature as needed to achieve dyeability with a predetermined class of dyes. It is now possible to have polyesters acrylics and polyamide fibers which can be dyed with positive basic cationic negative acid anionic or neutral disperse dyes.
These recent developments have allowed the fabric designer to produce materials textiles carpets fabricated in patterns which can be dyed three different colors from one dyebath containing three types of dyes. This concept is called cross dyeing and is becoming increasingly popular as a low cost method of coloration.
Cotton and rayon regenerated cellulose fibers are composed of cellulose in quite pure from. Cellulose lacks significant acidic or basic properties but has a large number of alcoholic hydroxyl groups. It is hydrolyzed by hot acid and swollen by concentrated alkali. When cotton is swollen by concentrated alkali under tension so that the fibers cannot shrink lengthwise it develops a silk like luster. This process is called mercerization.
The affinity of mercerized cotton for dyes is greater than that of untreated cotton. Cotton and rayon fibers are easily wetted by water and afford ready access to dye molecules. Dyeing may takes places by adsorption occlusion or reaction with the hydroxyl groups. It is also possible to make cotton and rayon receptive to a variety of dyes by pretreatment or mordanting with a material capable of binding the dyes. Wool and silk fibers are protein substances with both acidic and basic properties. They are destroyed by strong alkali. Strong acid causes hydrolysis but the process may be controlled to permit dyeing from acidic solutions.
Wool and silk are wetted by water and are dyed with either acid or basic dyes through formation of salt linkages. They may also be dyed with reactive dyes that from covalent bonds with available amino groups. Mordanting is sometimes used to alter the dyeability of wool and slik. Acetylated cellulose fibers differ from cellulose fibers in that they are more hydrophobic and lack large numbers of free hydroxyl groups. The higher the degree of acetylation the more unlike cotton and rayon the acetates become. Strong acid and strong alkali degrade cellulose acetates although the initial attack is slow under moderate conditions because of the difficulty of wetting the fiber.
The triacetate is the most hydrophobic and the most stable. Dyeing of cellulose actetates is effected with dyes of low water solubility which become dissolved in the fiber or by occlusion of dyes formed in situ. Acid basic and reactive dyes cannot be used because of the lack of sites for attachment. Polyamide fibers nylon are synthetic fibers possessing properties somewhat like those of wool and slik. They are more hydrophobic however with only a limited numbers of basic or acidic groups.
Polyamides are degraded by strong acid but may be dyed from acidic dye baths under controlled conditions. Polyamide fibers are dyeable near the boiling point of water with acid dyes that from salt linkages with basic sites. Dyeing by this means is limited by the availability of these sities. Dyes like those used on cellulose acetates i. Polyester fibers are synthetic fibers unlike any produced in nature.
They are hydrophobic and posses good stability to acid and alkali as a result of this hydrophobicity. They are hydrolyzed under sufficiently drastic conditions however. Some polyester fibers lack functional groups others are provided with acidic groups or otherwise modified to make them more hydrophilic. Unmodified polyester fibers are dyed by solution of dyes in the fiber or to a limited extent by occlusion of dyes formed in situ.
Modified polyester fibers may be dyed in these ways or with dyes selected according to the nature of the sites introduced by the modification. Both unmodified and modified polyester fibers must be dyed under vigorous conditions often with the assistance of a swelling agent to open up the fiber. Acrylics fibers are hydrophobic synethetic fibers with excellent chemical stability.
They do not resemble any natural product. The only funcational groups pressent are those introduced for the purpose of providing sites for dyeing. Acrylic fibers are dyed by solution of dyes in the fiber by occlusion of dyes formed in situ and by formation of salt linkages with dyes capable of attachement to sites provided for that purpose. Basic dyes are used on acrylic fibers bearing sulfonic acid groups for examples. Vinyl polymers and copolymers make up a class of fiber forming materials that varies greatly in properties depending on constitution.
Some vinyl fibers are very resistant to degradation by acids. Dyes are selected accoding to the nature of the specific polymer to be dyed. Polyolefin fibers are formed from the products of polymerization of unsaturated compounds of carbon and hydrogen for example propylene. They do not absorb water and are chemically quite inert. They can be dyed with special disperse dyes but are colored best by introducing a colorant into the polymer before the fibers are spun.
Glass fibers are used for special purpose for example where flammable materials cannot be tolerated. They are often colored during manufacture but can be dyed by special techniques which involve the use of surface coatings that have affinity for dyes. Paper is a nonwoven material made up primarily from cellulose of varying degress of refining see chapter Paper may be colored in the pulp as a watery fibrous slurry by either continuous or batch methods. The dyeing process takes place at ambient temperature and the dyes are adsorbed on the pulp by their affinity for the cellulose.
Direct dyes are most commonly used. In continuous coloration the dye solutions are metered directly into a moving stream of pulp.
In batch operations dye is added to a pulper beater or blending chest containing a given quantity of slurry. Paper may also be colored on its surface after the inital sheet is formed pressed and partially dried. This can be done at the size press of the paper machine or color can be carried by a calender roll for heavier sheets. A wide vareiety of low cost dyes can be used for surface coloration. The properties of dyes may be classified as application properties and end use properties. Application properties include solubility affinity and dyeing rate. End use properties include hue and fastness to degrading influences such as light washing heat sublimation and bleaching.
Dyes are selected for acceptable end use properties at minimum expense. Involved application procedures are used only when necessary to acheive unusually good results. It has become common practice to treat dyed textiles with agents designed to improve resistance to shrinking wrinkling and the like. These agents frequently alter the appearnce and fastness of dyes. Stability to after treatments must therefore be considered as an important end use property of dyes.
The amount of dye required to obtain a light shade is usually about 1 per cent of the weight of the fiber heavier shades may require as much as 8 per cent. These values are very approximate since dyes differ in colour strength and are usually sold in diluted form. These amounts of dye are not sufficient in most cases to markedly affect the properties other than color of the fiber. Care must be exercised however to apply the dye under conditions that do not cause fiber degradation. It is obvious from the list above that many basic dyes have about 10 20 times the color value per molecule as the anthraquinone types.
Unfortunately light fastness is in the reverse order the anthraquinones being used where maximum durability to light is needed. The challenge to be dye chemist or engineer is to increase the strength of the light fast dyes or to increase the fastness of the strongest dyes. Dyes are classified according to application method for the convenience of the dyer. The best classification method available is that used in the color index a publication sponsored by the society of dyers and colourists England and the American Association of Textile Chemists and Colorists.
Acid dyes depened on the presence of one or more acidic groups for their attachment to textile fibers. These are usually sulfonic acid groups which serve to make the dye soluble in water. An example of this class is Acid yellow 36 Metanil Yellow. Acid dyes are used to dye fibers containing basic groups such as wool slik and polyamides. Application is usually made under acidic conditions which cause protonation of the basic cause protonation of the basic groups.
It should be noted that this process is reversible. Generally acid dyes can be removed from fibers by washing. The rate of removal depends on the rate at which the dye can diffuse through the fiber under the conditions of washing. For a given fiber the diffusion rate is determined by temperature size and shape of the dye molecules and the number and kind of linkages formed with the fiber. Chrome dyes. A special kind of acid dye used mainly on wool they posses improved fastness when converted to chromium complexes.
A suitable chromium salt is applied to the fiber 1 before the dye 2 at the same time as the dye or 3 after the dye. All these methods are staisfactory but more complicated than is desired. In recent years manufactures have made available dyes in which chromium is already a part of the molecule. These dyes are simpler to apply than the older types and as a consequence are increasing in importance. Cationic dyes become attached to fibers by formation of salt linkages with anionic or acidic groups in the fibers. Basic dyes are those which have a basic amino group which is protonated under the acid conditions of the dyebath.
Cationic dyes can be divided into the three classes which are illustrated. Basic brown 1 Bismark brown is an amino containing dye which is redily protonated under the pH 2 5 conditions of dyeing. Crystal violet Basic violet3 is an example of a cationic dye in which the cationic charge is delocalized by resonance and may be present at any one of the basic centers at any time. These resonance forms of almost equivalent energy are one of the reasons that crystal violet is among the strongest dyes known. This high color value tinctorial strength has important commercial interest in the hectograph copying system.
In this system crystal violet in a wax base is transferred to the back of a typewritten copy sheet. By using paper moistened with alcohol more than good copies may be made from the master. Drycleanings of garments is done in much the same manner as laundering except that organic solvents are used in place of water. As in laundering detergents are added to the solvent to enhance its cleaning quality.
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Other solvent additives are used to give the textile the desired finish. This may be done merely to improve the hand or drape of the textile or chemical additives may be used to acheive water repellency insect repelleney or flame resistance. Drycleaning washes are similar in construction to commercial laundry washers but in drycleaning provision is made for clarifying the solvent for reuse. In laundering the used wash water is discarded this cannot be done with the more expensive drycleaning solvents.
In the drycleaning system the solvent is continuously pumped through the washes and then through some type of after designed to remove all suspended soil. Provision is also made for distillation of the solvent to free it form the solvent soluble soil. The filters also contain activated carbon to absorb dissolved dye which would otherwise build up in the solvent. Other chemical products used in small quantities by drycleaners are formulted to remove stains by local appplicaton to the affected area of the garment.
Only two classes of solvents have proved suitable for drycleaning petroleum fractions and a few halogenated hydrocarbons. All other classes of solvents fail to meet the following eight major requirements of a drycleaning solvent. It must be sufficiently volatile to permit reclamation by distillation and to permit garments to be tried without prolonged heating at excessive temperatures. The major drycleaning solvents used in the U.
With the exceptions of the solvents the chemicals used in drycleaning are sold as brand name formulations and the only tests performed on them are the determination of the amount of detergent in the solvent and the amount of water in a solution of detergent in solvent. However these chemicals are tested to determine how well they perform the function they are designed for according to a number of procedurs developed by the National Insitiute of Drycleaning NID. Much of the drycleaning done in the United States employs a solvent corresponding to a petroleum fraction with a minimum flash point of C.
This solvent has been named Stoddard solvent for W. The first commercial standard for a drycleaning solvent CS3 28 was issued in by the National Bureau of Standards. Table 1 summarizes the current specifications of regular Stoddard solvent.
Today three other petroleum fractions are also broadly termed as Stoddard solvents. These are the F solvent the low end point solvent and the ordoless solvent. This solvent is safer than the regular stoddard solvent. Therefore it may be used in locations where stoddard solvent is prohibited. Also building codes for plants using F solvent are not so rigorous. For example explosion proof motors and other electrical fixtures are not required.
Specifications of F solvent which differ from the specificantions of the regular stoddard solvent are listed in Table 2. Low End Point Solvent. This type of stoddard solvent has a dry point in the range of F compared with F for the regular stoddard solvent.
The result is a rapid drying solvent. There is no specification covering this solvent. It is regaeded as a premium grade because of the fast drying feature. Odorless Solvent. Whereas regular stoddard solvetn is specified to be free of objectionable odor this new class of stoddard solvent is free of all odors. This is acheived by removing or hydrogenating all aromatic compounds.
The solvent also meets all requitements for a nonsmog producing solvent since smog production is related to the aromatic content. Odorless solvent is also regarded as a premium grade of stoddard and is not conered by a separate specification. The term sweet as used in the specification means the opposite of rancid or sour. Although the usual methods of clarifying solvent in a drycleaning plant remove odors that accumulate during continued drycleaning these processes do not always remove dodrs caused by improper refining.
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Therefore the solvent when received from the refinery should be free from undesirable odor. There is nothing to show whether or not a solvent meets the requirement except the opinion of the examining chemist. Many samples of stoddard solvent have a rather strong odor but it is easily removed from the fabric by conventional drying methods. Flash point. The flash point is governed by those portions of the solvent that have the lowest boiling points and are therefore the most volatile. Since these portions evaporate more rapidly than the rest of the solvent the flash point of Stoddard solvent in a drycleaning system gradualy rises with use.
Soaps prespotters or other added materials sometimes contain low flash solvents such as some alcohols that lower the flash point of the solvent and increase the fire hazard. The introduction of even small amounts of methyl ethyl or isopropyl alcohol into a washer lowers the flash point of the solvent below normal room temperatures.
A lighted match held over Stoddard solvent at ordinary room temperatures does not ignite the solvent because the solvent is not giving off enough vapor to form a combustible mixture with the air. If the tempetature of the solvent is raised vapor pressure is increased and the air above the solvent becomes richer in solvent vapors.
Finally a temperature is reached where enough solvent has vaporized to form a combustible mixture with the air If a flame is then introduced above the solvent the vapors will flash. The lowest temperature at which this occurs is called the flash point. Since the flash point of stoddard may not be under F there is no danger of fire from solvent vapors until the temperature of the solvent rises to F or above. The flash point specification is frequently violated.
In some cases the refinery may have set the lower limit of the distillation range too low. Such violations usually result in a solvent with a flash point of 98 or F. A more dangerous type of violation however results from careless handling of the solvent. For example stoddard is sometimes transported in tank trucks that were previously used for carrying gasoline and still contain small quanities of gasoline.
Corrosive properties. Improperly refined solvent may contain traces of dissolved free sulfur which can corrode the metals of storage tanks and equipment. Under these conditions corrosion that would be apparent after considerable use of the solvent at room temperature can be seen after only 3 hr. Distillation Range. From the standpoint of a drycleaning solvent there are disadvantages in products containing very low or very high boiling hydrocarbons.
Low boiling hydrocarbons petroleum ether and gasoline cause fires and high evaporation loss high boiling hydrocarbons such as kerosene cause excessive drying time. The distillation range for Stoddard solvent is between and F a range not low enough to cause undue fire hazard and evaporation loss or high enough to prolong the drying time. Excessive nonvolatile matter in the solvent often contributers to odors and lengthens drying time. Because of the high temperatures used a small amount of odorous residue is usually formed during the distillation test.
A sample of the same solvent evaporated on a steam bath where temperature is not raised above F yields a smaller and less odorous residue. If the solvent is given a sulfuric acid treatment in the refinery and not followed by a neutralizing treatment such as with caustic soda it will contain small amounts of sulfuric acid or other acidic materials.
Even small amounts of sulfuric acid are undesirable in a drycleaning solvent as they corrode equipment and damage garments. Fortunately almost without exception drycleaning solvents pass the acidity test. Sulfuric acid has a very high boiling point.
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The presence of this substances in the solvent will be as residue in the flask after the distillation. If a residue of 1 ml remains from distilling ml of solvent any sulfuric acid present is concentarted there times. Thus it is logical to test the residue from distillation for sulfuric acid. Doctor Test. Mercaptans impart to the solvent unpleasant odors which may be absorbed onto the garments during drycleaning. The doctor test is a qualitative method to determine whether the treatment for mercaptans was properly done in the refinary. Sulfur and sodium plumbite are added to the solvent in a test tube.
If mercaptans are present in the solvent the reaction proceeds and the black lead sulfide formed is indicative of a positive test. Sulfuric Acid Absorption Test. This test determines if the solvent contains appreciable amounts of unsaturated hydrocarbons. These would be in the solvent if it was inadequately treated with sulfuric acid during refining.
Since unsaturated hydrocarbons turn ranoid and cause undesirable odors in drycleaned garments it is imperative that they be removed before the solvent leaves the refinery. In the test concentrated sulfuric acid is added in a graduated cylinder to the solvent and shaken. Sulfuric acid reacts with any unsaturated hydrocarbons present and most of the products of the reaction settle into the acid layer thus the volume of the solvent is decreased.
Variations in the strength of commerically available concentrated sulfuric acid cause variations in the sulfuric acid absorption test. Therefore the acid strength must be standardized if reproducibility is desired. Perchloroethylene tetrachloroethylene became an important drycleaning solvent because of its nonflammability which permits its use in places where all types of flammable solvent are either forbidden by codes or inhibited by high insurance rates. Its general properties are given in Table 3 and the specifications proposed by the NID are listed in Table 4. Residual Odor. Any residual odor left in a fabric after treatment in the solvent is objectionable.
Detection of such odors by smelling is more sensitive if the fabric is steamed immediately prior to the test. Soak the swatch in perchlororthylene for 5 min then remove it and hang it to drain dry for about 4 hr. Tumble the swatch in a tumble dryer for 30 min at F. To test for odor grasp the swatch in the center with a forceps hold it in live steam for 5 sec and smell it immediately.
Test an untreated swatch smiultaneously. There should be no discernible difference in odor between the two swatches. Nonvolatile Residue. This test detects the presence of nonvolatile impurities in the solvent. It is determined gravimetrically by evaporating a measured quantity of solvent and weighing the residue as follows. Dry a 4 in. Place it on a steam bath in a hood and add the perchloroethylene to be tested by pipet in two 50 ml portions. Perchloroethylene has a high specific gravity. Add the second portion afted the first is partially evaporated. After the solvent has completely evaporated on the steam bath heat the dish further in an oven at C for I hr then cool it in a desiccator and weigh.
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