Alternatives to Water in Textile Processing

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  • Alternatives to Water in Textile Processing

    J . K. SKELLY

    ClBA G I I W (UK) I.td (lay ton Manchester. M I 1 4AK

    Attempts have been made to correlate the use of organic solvents as alternatives to water in textile processing with the solubility parameter concept. Chlorinated h-vdro- carbons prove suitable for scouring and some finishing operations since their solubility parameters w e ideal for such operations. In dyeing, however, the solubility parameter of polyester fibres, dyes and perchloroethylene are very similar, resulting in poor exhaustion o f conven- tional disperse dyes. New solvent dyeing systems and new dyes, with high partition coefficients in polyester fibres, must be developed for solvent dyeing to become of practical consideration. Other solvents have been examined for solvent assisted dyeing. It has been shown that when benzyl alcohol and 2-phenoxy ethanol are used in dyebaths at concentrations at which these solvents are not corn- pletely in solution, both the rate of dyeing and penetration of dye into the fibre can be markedly increased in corn- parison to normal aqueous dyeing with polyester, nylon and acrylic fibres.

    Introduction Great progress has been made in the last decade in the introduction of scouring and thishing processes in which water has been partially or completely replaced by alternative solvents. In general terms, any alternative solvent t o water must give an end result equal t o or better than that achieved by water. If possible, the solvent should improve the efficiency of the process by reducing the time of treatment, energy required and labour costs. In addition, the capital costs must be competitive tor a given output and the system should be ecologically acceptable with, as far as possible, recovery and recycling of the materials used.

    The ready availability and cheapness of water has meant that it has been used almost exclusively and often wastefully in scouring, bleaching, dyeing and finishing operations. Ideally, an alternative solvent to water should have the following properties:-

    - A boiling point of 100- 150C to enable the solvent to be used for established textile fibres.

    - A low specific heat and a low heat of vapourisation to give savings in the energy required for heating and distillation or recovery when required.

    - A high flash point or non-flammability. - Non-toxic both as liquid and as vapour.

    Special equipment is essential to handle non-aqueous solvents, with special seals and high standards of engineering t o avoid loss of solvent vapour and distillation and recycling of

    recovered solvent. The advantages of using alternative solvents to water must be considered in relation t o extra machinery costs needed t o handle non-aqueous systems.

    Classification of Solvents Other properties of solvents, as distinct from purely physical characteristics, which must be considered are such effects as swelling of fibres, resulting in shrinkage or distortion of textiles at the temperatures used in processing. Physical properties of solvents such as dipole moment and dielectric constant, which are a measure of the polarity of a given solvent, d o not in themselves specify its swelling and solubilis- ing capabilities. Other methods of classifying solvents can be considered, for example, by their proton donating properties, where protic solvents such as acids and alcohols have varying degrees of electron accepting characteristics, while aprotic solvents such as amines have electron donating properties. These classifications however are of little value with non-polar hydrocarbons or chlorinated hydrocarbon type solvents and the protic or aprotic characteristics are not necessarily a guide to the suitability of a solvent for different polymers.

    However, a useful method of indicating the compatibility of solvents and polymers has been evolved and is widely used in the paint, printing ink and plastics industries. The theoreti- cal description of the solution process was first proposed by van Larr nearly 50 years ago and has been developed by Hildebrand [ I ] and Scatchard [2]. Hildebrand introduced the concept of a solubility parameter. In this concept each solvent is given a solubility parameter S which characterises its solubility behaviour in relation to other solvents and also to polymers.

    Theoretically the solubility parameter can be defined by the equation

    where CED is described as the cohesive energy density and L , is the latent heat, of vapourisation, R is the gas constant, T the temperature and Vis the molar volume. The solubility parameter may be determined for a solvent or polymer by one of the following methods:-

    (a) Calculation from known physical constants, e.g. the L,-values [3] .

    (b) Calculation from chemical structure by summation of calculated molar attraction constants divided by the molecular weight. This is of particular interest for fibrous polymers [4] .

    (c) By matching the solubility behaviour of the solvent against the solubility behaviour of products with known 6 values. This is also useful for polymers using a range of products of known S value; the mid-point of the range of solvents with which the polymer is most miscible is deduced t o be the 6 of the unknown product.

    JSDC June 1975 177

  • Solubility parameter values are generally quoted at 25C but vary with temperature, e.g. a 7.0"C rise in temperature reduces 6 by 0.1, and therefore when dyeing at 130C a reduction of 1.5 in 6 could be expected [ 5 ] .

    Solubility parameters are given for some solvents of interest in textile processing in Table 1. These values are subject to variation, depending upon the method of derivation, and are taken from different sources [6].

    Burrell [5] has shown that the solubility parameters of solvents in mixtures are approximately additive in proportion to their molar fractions. It is therefore possible to blend two non-solvents having 6s on either side of that of a given polymer, so that the mean 6 is close to that of the polymer. Burrell has shown that such mixtures of non-solvents then become solvents for the polymer.

    In addition to solubility parameter, hydrogen bonding characteristics are also used to classify the solubility properties of solvents. The normal method is to classify solvents into

    , / three hydrogen bonding groups since this characteristic is

    TABLE 1

    Solubility Parameters of Various Solvents

    Generic

    Acid

    Ketones

    Alcohols

    Amides Esters

    Aldehydes

    Amines

    Ethers

    Hydrocarbons

    Chlorinated hydrocarbons

    Specific

    Water

    Acetic Acid

    Acetone Cy clohexanone Cy clopentanone Methyl ethyl ketone Ethyl alcohol Benzyl alcohol 2-phenoxy ethanol 2-methoxy ethanol

    NN-dimethyl formamide Ethylene carbonate Propylene carbonate Butyl salicylate Methyl salicylate Butyl benzoate

    Acetaldehyde Propionaldehyde

    NN-dimethyl ethanolamine Hexylamine

    Diethyl ether

    m-xylene Toluene Benzene Hexane

    Trichloroethylene Perchloroethylene o-dichlorbenzene Carbon tetrachloride

    Solubility parameter (ca~/crn )

    23.4

    13.0

    9.6 10.4 10.5 9.5

    12.8 12.1 11.5 1 I .7

    11.8

    14.7 13.3 9.7

    10.2 9.5

    9.9 9.4

    10.4 8.5

    7.5

    8.9 8.9 9.2 7.3

    9.2 9.3

    10.0 8.6

    extremely difficult to evaluate quantitatively. Table 7- shows the qualitative classification of solvents according to hydrogen bonding properties [7].

    TABLE 2

    Hydrogen Bonding Properties

    Hydrogen bonding strength Solvent types

    Strongly hydrogen bonding Water Carboxylic acids Amides

    Alcohols Ketones

    Moderately hydrogen bondingAmines Ethers Aldehydes Esters

    Chlorinated hydrocarbons

    Glycols

    Poorly hydrogen bonding Hydrocarbons

    The closer together the solubility parameter and hydrogen bonding strength of the solvent and polymer the greater thc swelling effect produced by the solvent on the polymer. Such effects may or may not be desirable in textile processes. For example, chlorinated hydrocarbons which dissolve impurities such as the oils and waxes present on textile materials should have a solubility parameter widely different from that of the fibre being treated in a solvent scouring operation. Alternat- ively in solvent finishing the solubility parameter 01 the polymer applied in the finish should be close to that of the solvent used and if swelling of the textile is not desired the value for the solvent should not be close to the value for the fibre.

    The solubility parameters of different fibrous and other polymers are derived by similar methods to those for solvents and are given in .Table 3 141.

    TABLE 3

    Solubility Parameter of Fibres

    Polymer

    PTFE Polyethylene Natural rubber Polystyrene Polymethylmethacrylate Polyvinylacetate Polyvinylchloride Polyethyleneglycol terephthalate Secondary cellulose acetate Polyacrylonitrile Nylon 66

    Solubility Parameter(s) (cal/cm3 12

    6.2 7.9 -8.1

    9.1 9.0-9.5 9.4 9 . 5 9.7

    10.7

    8.1 5-8.35

    I 0.9 - 1 1.35

    12.75 (1 5.4) 13.6

    178 JSDC June 1975

  • Theoretical Considerations of Solvent Interactions with Fibres While the solubility parameters given in Table 1 are a useful guide to the solubilising action of a given solvent on an amorphous polymer (Table 3). other physical factors also intluence the swelling effect produced on long chain fibrous polymers. In addition t o the chemical structure which influ- ences intermolecular bonding forces, the degree of physical order or disorder of the polymer chains clearly plays an important part in fibrous polymers. Highly ordered fibrous polymers have low