Surface treatment in finishing of pigments

Excerpt: Surface treatment plays an important role for application of pigments in inks, plastics, paints, and textiles. Pigment finishing encompasses surface treatment during or after synthesis of pigment.

Mr. Pankaj Bet Lecture at ICT Dyes Workshop

Surface treatment in finishing of pigments

Contributed by Dr. K. S. Murthy (Pidilite Industries Ltd)

MARK TWAIN said “Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do, so throw off the bowlines, sail away from safe harbor, and catch the trade winds in your sails. Explore, Dream, Discover.”

invited to deliver a lecture in Dyes Department Workshop on Pigment Finishing and Printing Technology under the Aegis of Technical Education Quality Improvement Program (TEQIP) on 13-14 September, 2016 at K. Venkataraman Auditorium. He had presented an overview of Surface Treatments for Pigments. Prof. N. Sekhar chaired the program attended by faculty, students and a few industry delegates.

entrepreneur with Bet & Bet Industries in August 2013 for dyes manufacturing and formulation and later surface treatment chemicals besides consultancy to a few industries for customised solutions for pigment development for plastics, paints and inks as per export requirements.

Surface treatmentplays an important role for application of pigments in inks, plastics, paints, and textiles. Pigment finishing encompasses surface treatment during or after synthesis of pigment. Dyes (soluble in water except disperse dyes) and Pigments (water insoluble) have wide range of applications and surface treatment is gaining importance. Selection of surfactants is now polymer based unlike HLB based earlier and MNCs (BASF, Clariant, Tego, Air Products, Dai-Ichi Karkaria, Lubrizol) are sought after with appropriate chemistry. Mr. Bet spoke on pigments in general and surface treatment and finishing technique with case studies followed by discussion with questions and answers.

Pigments classified as organic and inorganic, which are incorporated by physical mixing in media in which they are applied. Most of the pigments require mechanical processing for chemical treatment to render them useful. After such finishing they are used in application otherwise they are crude pigments, which may not give the desired results for yield, strength and application. Organic pigments are monoazo, disazo, naphthoic, polycyclic, phthalocyanine, quinacridone, anthraquninine and dioxazine based and High Performance pigments and inorganic pigments such as titanium dioxide, iron oxide, lead oxide, cadmium, Ultramarine blue and chrome green etc.

Manufacturing of pigment entails synthesis and after treatment in some cases is given during the reaction and not after finishing. Earlier plant consisted diazo and coupling kettles and mixing clarifier. Surface treatment is given to the pigment at manufacturing stage like coupling to effect desired chemical structure with synergy yielding the pigment ready for finishing. Azo pigments are prepared by diazotising corresponding aromatic amine in presence of surfactants and coupling reaction in alkaline medium (barring a few in acidic medium). The suspended coupling component consists of particle and surfactants play important role during coupling reaction. Hard particles are formed which are sensitive to heat, light, and temperature. Mr. Bet went on to discuss individual surface treatment for individual product and medium is aqueous or solvent. In case of plastics, different polymer adhesives for different treatment are required depending upon medium.

Before manufacturing, decide on what to manufacture and for which application such as water-base or solvent base, plastic or paint or ink. All have different criteria and application demands are different and hence need to finish pigment accordingly. After treatment given to a pigment for paint may not be useful for plastic or ink because of particle size and physical parameters are different. Pigment finishing plays very important role for application and finishing also changes with application and medium in which it is applied. Offset printing inks are prepared through pigment black, cyan, magenta, yellow.

Benefits of Surface Treatment: Surface treatment is widely used in industry. In the case of pigment emulsion, a medium is needed to go on to the substrate and adhesion is created which is the preparation for application and this is called surface treatment. It is application wise surface treatment.

Methods of Surface Treatment: Physical absorption, chemical absorption for which various types of surfactants are available and pigment doesn’t have activity for anything. Recent development is use of polymers, which have affinity for pigment surface like coating or a sort of attraction. Some companies have developed different surface treatment chemicals and designed the criteria. Surface treatment is given to pigment as crude and powder and also wet cake while manufacturing followed by finishing and drying.

Selection of Surface Treatment: If the product and treatment are designed to provide hydroplhylic in the aqueous phase and solvent phase, give such treatment for pigment in the corresponding phase in aqueous dispersion and not in solvent phase as the nature of surface treatment and surfactant used are different. Hydrophylically you will find hydrophylicity so that it goes into water for aqueous dispersion for use in textile printing and aqueous ink. When it is hydrophobic, solvent is used as solvent based paint. Surface treatment is predetermined and in the development process, manufacturer is aware of diazotisation and coupling besides unit processes such as distillation, sulphonation, bromination, nitration and accordingly what needs to be done for surface treatment. Technical testing of the formulation during product development can guide the process.

Overall performance of surface treatmentis composite of a number of factors. Physical and chemical properties like bulk density, oil absorption and particle size and since pigment is not water soluble, it should be made soluble in the medium in which it is applied to go on to substrate. Pigment is applied on quaternised fibre (pigment dyeing). Coverage of coating—after surface treatment, it is uniformly coated with a surface active agent so coating gives more surface area for reacting with the substrate.

Surfonamine/Huntsman:Batch process is used for azo pigment manufacture and after filtration and milling, follow it up by after treatment or directly in the vessel. In 2007, Huntsman introduced Surfonamine amines as pigment modifiers and dispersant intermediates in terms of reactivity and compatibility. They are ideally suited to the synthesis of unique dispersant molecules and this approach may be made for dispersion and stabilisation of pigment concentrate. Surfonamine B-60 (Hydrophobicity) has Ethylene Oxide (EO) and Propylene Oxide (PO) ratio of 19, HLB value of 2.0 and molecular weight of 600. B-100 has low HLB value and L-100 (Hydrophilicity) with 193 EO/PO and HLB value of 17. HLB (Hydrophilic-Lipophilic) balance values give a measure of the surfactant’s solubility in aqueous solution. Water solubility increases with increasing HLB value. Huntsman were selling huge quantity ( ! 10,000 tons) of surface active agents with high or low HLB and required HLB value like 13-15.

Pigment Yellow: Yellow 12 has demand of 1,000 tons for different inks e.g., newspaper. Diarylide yellow pigments applied with Surfonamine L- 207 amine are optimised for superior storage stability in terms of improved colour strength and viscosity retention. Mono azo pigments are modified to give inks of greater colour strength and lower viscosity.

Pigments have become transparent or less opaque and for a paint, opacity matters and transparency for ink. With conventional surface active agents, viscosity of ink and dispersion are affected. In textile or paint application, viscosity of 500-1000 is required, but for inks it is less than 100 cP.

Critical Pigment Volume Concentration (CPVC): The point at which there is just enough binder to wet pigment particles is called the critical pigment volume concentration. It is important during milling. Dispersion is made with proper viscosity and when surface treated pigment is used, grinding is better and on application it gives right viscosity for the application medium.

Surphonamines have aspects of crystal modification, particle size distribution, particle surface structure and processing of pigment powder to the desired application level especially in terms of drying and milling which comprises finishing. In some cases advantage of pigment in binder system is realised and in textile binding, reduction in viscosity is observed using surface treated pigment. This is not required for textiles where a particular viscosity is maintained and drops considerably when the pigment is surface treated.

Optimum Dosage of Surphonamine: Used at different stages like diazo, coupling and finishing while manufacturing azo pigment to optimise surface treatment. Dispersion is particle size distribution and the mechanism of particle size and dispersibility assume importance. Particle size after surface treatment of the pigment with Surphonamine at different percentage is considerably reduced. In some cases, 5% of amine treated pigment became difficult to check whether it is soluble or not. Optimum dosage of Surphonamine or any surface active agent during the reaction will benefit pigment in terms of hue, tinctorial value, hiding power, transparency, light fastness, and weather fastness. Light and weather fastnesses are important for masterbatch, PVC and plastics and gloss is improved (ink requires high gloss). Solvent migration fastness and flow of pigment is better in the application media for printing ink, coatings, plastics and emulsions. Surphonamine case study is extended to polymers surface active agents at the same level of achievement though chemistry, percentage and dosage have changed. The surfactant chemistry is no longer applicable to pigments ever since polymer chemistry has taken over.

Ms.Pratiksha Pawar

Ms.Ketki Chavan

Dr.Yogesh Ahire

Ms.Sujata Patil

Dr.Dinesh Borole

Mr.Sunil Satav

Best Application AwardDr.L.B.Tiwari received at the hands of Mr.N.K.Parekh.