Textile Processing Department,
Sarvajanik College of Engg. & Tech., Surat.

1. Introduction: 1,2,3,4 Polypropylene possess combination of some useful properties such as high tensile strength, low bulk density (lightness), excellent resistance to many chemicals, good rot resistance etc. which makes it an essential component of modern textile market. It satisfies the end use requirements for variety of applications such as:

(i) Upholstery fabrics, tufted and woven carpets, tufted and woven blankets and others constituting a range of household textiles.

(ii) Industrial textiles such as woven and needled paper maker felts, woven and needled filtration fabrics, wool, cotton and linen type fabrics, cable, nets, cords etc.

(iii) Outerwear and bedding

(iv) Webbing, ribbons, net and bandages

(v) Apparels

Indeed, quite often polypropylene fibres are described as non textile fibres because, it can not be dyed from an aqueous bath. Some inherent properties of polypropylene such as maximum hydrophobicity, high crystallinity and stereo regularity in its isotactic form and completely non polar structure makes Polypropylene very difficult to be dyed. Many attempts were made to confer dyeability to isotactic polypropylene.

This paper covers some important features of various studies carried out for imparting dyeability to polypropylene.

2. Various techniques used for dyeing of PP:

The various techniques used for dyeing of PP are:
(i) Spin coloration
(ii) Physical modification
(iii) Chemical modification
(iv) Dyeing of unmodified fibres

2.1 Spin coloration or mass coloration 1, 5, 6:

Organic pigments are primarily used for spin coloration though some inorganic pigments do find the outlet.

Besides titanium dioxide and carbon black pigments, some organic pigments used for spin coloration of polypropylene are classified into groups I - III based on the criteria of fastness requirements, fibre application and polymer processing temperature.

The pigments suitable for spin coloration are commercially available in form of pigment dispersed in carrier, in the form of powder or granules and known as “pigment preparation” or “master batch”. The pigment content of master batch is generally 20-40% in case of organic pigment and about 50% in inorganic pigment.

The pigment stability deciding factors for spin coloration are; heat stability, dispersibility, fastness properties especially light fastness and solubility in polymer melts.

The pigment should withstand the conditions prevailing in the polymer melt i.e., there should not be any colour or other changes.

The changes depend mainly upon spinning temperature. The choice of pigment becomes wider with decreasing spinning temperature. Pigments in group II are preferable at higher processing temperature about 2850C and when good fastness properties are required. Pigments from this group are more preferable for BCF carpet yarns and upholstery materials. Group I pigments are used for low temperature spinning processes. Such pigments are used in production of staple fibre, slit and split fibres.

The pigment should disperse as easily as possible. Good dispersibility allows maximal color strength within short period of time.

The light fastness of shade produced depends not only upon chemical structure of pigment but also upon heat stability and solubility of pigment in polymer melt. From various studies it is generalized that inferior heat stability and good stability can be equated with poor light fastness in fibre. Also the more the temperature, the higher is the solubility of pigment which increases the transparency of coloration.

2.2 Physical Modification :

Physical modification of polypropylene to confer it dyeability includes
i) Melt blending
ii) Blending with polymers and copolymers
iii) Blending with dendritic polymers
iv) Incorporation of metals

2.2.1 Melt blending 7,8,9,10,14:

Melt blending of polypropylene with small percentages of polyester and polystyrene brings about the considerable changes in the crystallinity of polypropylene. The addition of 5% polystyrene reduces the crystallinity as high as 50%. The reduction in crystallinity improves the dyeability of polypropylene. Particularly the disperse dyeability is found to be improved to a great extent.

Polypropylene can be blended with dyeable polymers before melt spinning.
Blending of polypropylene with oligocyclopentadiene imparts disperse dyeability.
Blending with polyamides Nylon 6 & 6, 6 imparts dyeability with acid dyes.
Disperse dyeable polypropylene can also be made by blending with poly (ethylene-covinyl acetate) (EVA), having mechanical properties well enough for textile use. Obviously the dye uptake, light fastness, wash fastness etc. depend upon the proportion of EVA in melt blend.
Disperse Dyeability can also be achieved by blending polypropylene with poly butylene thereptalate ( PBT) while blending with cationic dyeable polyester( CDPET) confer it the dyeability with cationic dyes.

However blending brings about decrease in tensile characteristics which may be controlled by careful selection of blending ratios.

2.2.2 Blending with polymers and copolymers 16:

Efforts to enhance the dyeability have been tended on introducing dye sites in to bulk of the fibre. Alternative practical approach is to physically modify the fibre by introducing a dye fixing additive prior to extrusion. This method requires that the dyed additive particles be microscopically dispersed in the body of fibre so that the whole fibre gives the appearance of being colored. Polymeric compounds containing basic nitrogen atoms may function as dye additives and render dyeability with acid dyes. A truly compatible additive should form a solid solution with polypropylene which is very difficult practically due to very high crystallinity of polypropylene. Additive particles should be interconnected with each other creating network in the polymer matrix. So a certain minimum concentration of additive is necessary to create this network under certain conditions which obviously are related to the fibre/additive compatibility, molecular masses of both PP and additives, blending shear rates, temperature and other influences.

According to one investigation PP could be modified with styrene-amine resin propimid, which can be dyed well with acid dyes in spite of the hydrophobicity of polypropylene.

2.2.3 Blending with dendritic polymers 17:

An extremely high degree of branching is the typical characteristic of the dendritic polymers. The degree of branching of dendrimers is maximal and their structure is perfectly regular. For industrial purposes two synthetic routes are economically feasible which are poly (propylene imine) dendrimer and hyper branched polyester amides.

All dendritic polymers possess typical distinguishing characteristics such as an approximately spherical shape, a high number of reactive end groups, a high number of branching points and the possibility to take up guest molecules between branches. The dendritic polymers compatible with polypropylene were made by reacting their end groups with fatty acids. For this purpose amine terminated poly (propylene imine) dendrimers were converted in to fatty amides, and OH- terminated hyper branched poly (ester amides) in to fatty esters.

The acid dyes combine with fatty amide modified poly (propylene imine) dendrimres. These dendrimers contain a basic tertiary amine group at each branching point form sites for acid dye molecules.

The disperse dyes can be taken up by PP modified with poly (propylene imine) dendrimers and hyper branched poly (ester amides). It offers the suitable conditions for interaction with disperse dyes such as vanderwaals forces, dipole and donor-acceptor forces. Both the uptake and fastness properties of dyed materials are found excellent.

2.2.4 Incorporation of Metals 15:

Organo metallic compounds containing polyvalent transition metals capable of forming chelates with selected dyes have been widely used. Nickel, Aluminium and cobalt when added to polypropylene, act as excellent reaction sites for chelation which provides improved dyeability. The mechanism includes dye adsorption and diffusion followed by chelation in situ with metal in fibre, forming a dye-metal organo complex.

Nickel is preferred most as it contribute better in resistance to light. Most suitable nickel compounds are salts of higher fatty acids which dissolve in polymer easily though, complex compounds based on phenolates, phenosulphides, phenosulphates etc. may also be used. Ni- modified fibres require special dyes for dyeing. The dyes should essentially consist of nitrogen atom so as to react with Ni of modified fibre. On the other hand Cr, Al, and Co can react easily with legands containing oxygen. Ni – modified polypropylene fibre requires disperse mordant dyes for its dyeing.

Ni-modified fibre possess good light, washing and dry cleaning fastness and with wide color range. However it is not free from demerits which include criticalness of dyeing process, risk of uneven dyeing and necessity of special dyes.

2.3 Chemical Modifications:

Chemical modification of polypropylene to impart it the dyeability includes Chlorination, Bromination and Grafting of polypropylene.

2.3.1 Chlorination and Bromination 11, 12, 13, 15:

The well prepared material either fabric or yarn can be chlorinated with sodium hypochlorite solution of concentration 10 g/l at pH 4.5 adjusted with HCl for 45 minutes at room temperature while, bromination can be done by photobromination consisting the use of either bromine gas solution in water or liquid bromine in different proportions of dimethyl Formamide (DMF) and water.

Both chlorination and bromination improves moisture regain with altering the mechanical properties. Both chlorinated and brominated polypropylene possesses good dyeability for basic and cationic dyes. All the cationic dyes show good wash fastness on these modified polypropylenes. However, some dyes show inferior light fastness, some moderate and only Methylene blue, a Thiazine dye exhibits good light fastness. Fastness to other agencies like hypochlorite bleaching, sea water, soda boil etc. also becomes moderate to good.

The dyeing with methylene blue also affords considerable photo protective action to the fibre. Obviously the concentration of reagents used for modification, pH of the dye bath are the quality deciding factors.

The attachment of basic dye molecule with modified polypropylene is not through salt formation. According to Agster, under alkaline conditions basic dyes react with chlorine or bromine present in backbone of modified polypropylene and there takes place a formation of covalent bond between fibre and dye molecule.

2.3.2 Graft copolymerization 4, 18,19,23,24,25,26:

Another interesting work attempting to chemically modify polypropylene is use of Grafting technology.

Graft copolymerization of polypropylene fibres with suitable monomer which modify the physico-chemical characteristics of fibre, represent a very effective approach to enhance the dyeability.

Several methods have been used for providing free radical sites on polypropylene molecules which are actually the site for grafting of other monomers.

One method is the chemical initiation i.e., use of conventional peroxide or azo polymerization initiators, while the other is irradiation technique, using U.V.radiation to initiate grafting. Various monomers which could be grafted on polypropylene are:

Acrylic acid (AA), Meth acrylic acid(MAA), Methyl methacrylate(MMA), Meth acrylate(MA), Acrylamine (AAM), Acrylonitrile (AN) etc.

Grafting of Polypropylene with different monomers adds suitable functional groups to confer it dyebility with basic dyes. Some monomers can increase the disperse dyeability by opening up the fibre structure. The R.I.F. values for cationic dyes increase considerably when polypropylene is grafted with Acrylic acid and Meth acrylic acid.

About 180 to 1500 times improvement in dyeability is possible with cationic dyes on grafted polypropylene fibres containing 44 to 66% graft of AA and MAA respectively. The dye uptake of disperse on grafted polypropylene was found comparatively lower.

The properties of grafted polypropylene depends on number of factors such as type of monomer and initiator, monomer and initiator concentration, temperature, time, irradiation dosages, and obviously the percentage graft.

Grafting also brings about the improvement in moisture regain of hydrophobic fibre. However, the improvement in dyebility and moisture regain is accompanied by loss in tensile strength. This loss in tensile strength can however, be controlled by optimizing the various affecting processing conditions. The decrease in strength is partly due to the chain scission and partly due to the hinderance offered by the graft to close packing.

2.4 Dyeing of unmodified polypropylene 20,21,22,23 :

Studies attempting to dye unmodified fibre proves the possibility of dyeing unmodified polypropylene with azoics and acid leuco vat dyes. However the azoics were dropped because of environmental and toxicological concerns and leuco vat acid dyeing of polypropylene was with limited success. Though leuco vat acid dyeing leads to non uniform dyeing in case of yarn dyeing due to filtration problems, it can be used in tumble dyeing of garments to produce wide range of shades on a day to day basis.

Another important development in dyeing of unmodified polypropylene is the development of special type of disperse dyes.

The newly synthesized dye was 1, 4,-bis-[octadecylamine] – 9, 10 anthraquinone(C-18 type). The long chain alkyl groups maximize fixation through dispersion forces as well as provide opportunity for co-crystallization of the dye molecule with fibre.

This C-18 dye shows almost no affinity for nylon or polyester. So C-18 dye is a true disperse dye for polypropylene. The exhaustion level was high and deep shades could be obtained. Quality of dyeing with C-18 is highly dependant upon the choice of dyeing auxiliaries.

3. Conclusion:

Various studies have been carried out to make Polypropylene dyeable with different approaches but each method when compared with other has some advantages and disadvantages. Only the mass coloration technique could be used on commercial scales for coloration of polypropylene in spite of its drawbacks. Wide scope therefore, still lies in doing either the extension work or adopting completely new approach in the direction of conferring dyeability to Polypropylene by referring the details of the past work and to develop a method better than all which would be more acceptable by overcoming the drawbacks of all methods, commercially more viable and with wide suitability. The scope lies in the development of new dyes like C-18, some modification in grafting process so as to minimize the tensile losses, new methods of chemical and physical modification and similarly suitable modifications in other processes can be thought of.


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