UV-curable encapsulation of surface—Modified organic pigments for inkjet printing of textiles

https://doi.org/10.1016/j.colsurfa.2014.01.075Get rights and content

Highlights

  • Aqueous dispersion of nano pigment was prepared via milling and ultrasonication.

  • Nanoscale organic pigment was encapsulated into UV-curable resin via miniemulsion.

  • Efficient encapsulation was proven by ultracentrifuge sedimentation, TGA and TEM.

  • Encapsulation of pigment into UV-curable resin improves dispersion stability.

  • Pigmented UV-curable inkjet inks are effective.

Abstract

Aqueous dispersions of nano-scale organic pigments were prepared through ball milling and ultra-sonication in presence of dispersing agents. The well dispersed pigment was encapsulated into UV-curable resin via miniemulsion technique. Effects of molecular structure of organic pigments, type of dispersing agent and time of ball milling were investigated. Results showed that C.I. Pigment Green 7 had better dispersion stability with time compared with other pigments. Ultracentrifuge sedimentation test, shelf life stability, thermal gravimetric analysis (TGA) and transmission electron microscope (TEM) provided supporting evidences for the encapsulation of C.I. Pigment Green 7 into polyester tetra acrylate/1,6-hexandiol dimethacrylate. Zeta potential results proved that UV-curable encapsulation of C.I. Pigment Green 7 intensifies the charge on the surface of the pigment and significantly increases the dispersion stability. This method of modifying organic pigments to be used as pigmented UV-curable inkjet inks was found to be durable and effective in preliminary application in inkjet printing of textiles.

Introduction

Pigments are an important group of colorants that have been extensively used in coating, printing inks, paint and plastic industries or even in color filters for electronics and communication apparatus [1]. This wide range of applications is due to that pigments have many advantages such as photosensitivity, color strength, brilliance and transparency. However, the limited hiding power, poor dispersion ability, and especially the poor weather durability are clearly the obstacles for organic pigments in many applications [2], [3].

Organic pigments are insoluble in the aqueous media; therefore, they are commonly used as dispersions with the addition of dispersing agents [4], [5]. However, conventional pigment dispersions generally have large particles, wide particle size distribution and poor stabilities because particles are able to aggregate easily. Aggregation and coagulation of particles refer to Van Der Waals attractive forces, which limit their application in inkjet printing [6], [7], [8]. Thus, the surface treatment of organic pigment particles is necessary to make them be practically useful.

Shearing forces in mechanical dispersion methods are usually applied to disperse the aggregated particles into separate and fine particles. Nano-scale organic pigments in the range between 50 and 150 nm can be generated through milling or grinding pigments in the presence of dispersing agent [5]. Numerous studies on the preparation, stabilization, and characterization of aqueous well dispersed nano-scale pigment systems have been reported.

Encapsulating pigments with various polymers is a promising approach to improve pigment processing and prevent pigment agglomerations. Moreover encapsulation protects pigments from environmental aggressions like UV radiation and pH [9]. In the last decade, many techniques have been developed for encapsulating organic pigments [10], [11], [12]. It must be noted that a successful encapsulation technique should not impair the original color appearance of pigments but enhance their dispersion stabilities [13].

Among all these technologies, miniemulsion polymerization has received the great attention. This might be because particle nucleation in miniemulsion occurs primarily within the monomer droplets, which are stabilized against coagulation and diffusion degradation. As well as ability to control the size of monomer droplet within a range of 50–500 nm by mechanical shearing thus improve the encapsulation efficiency [14], [15], [16], [17], [18], [19], [20].

The preliminary investigations on encapsulation of nano-scale Pigment Red 122 into a UV-curable system containing polyester acrylate and 1,6-hexanedioldiacrylate using the mini-emulsion technique have been reported by Hakeim et al. Furthermore it was found that using the oligomer; polyester acrylate in the presence of organic pigment could stabilize the mini-emulsion droplets without introducing any other hydrophobes or co-stabilizer in the formulation [20].

In recent years, digital inkjet printing applications on textiles have attracted interest due to its unique characteristics of simplicity, high speed, compatibility with a wide range of substrates, low waste, ability to deposit very small droplets (2–12 pl), low cost, high level of technical development, and high image quality and stability approaching those of photographic prints [21], [22], [23]. Cotton and Viscose fibers are still the most important textile fabrics. They are used worldwide and their consumption has been increasing since years and reached a peak in 2010. However, there remain significant challenges including issues associated with productivity, print quality, and process integration [24], [25].

In this paper, nano-scale organic pigments were prepared using a liquid phase method in the presence of dispersing agents. The well dispersed Nanoscale C.I. Pigment Green 7 was encapsulated via miniemulsion technique into UV-curable system containing 1,6-hexandiol dimethacrylate (monomer) and polyester tetraacrylate (oligomer). The encapsulated pigment was applied in preliminary studies as pigmented UV-curable inkjet inks for variety of fabrics.

Section snippets

Materials

Cotton, Polyester and Viscose fabrics were supplied by Masr El Mahalla co., Egypt. C.L. Pigment Green 7, C.L. Pigment Yellow 17 and C.L. Pigment Orange 5 were supplied from Dye Star, Egypt (Scheme 1). Surfactant used in this work included Anionic dispersing agent; Sodium dodecyl sulphate(SDS), Cationic dispersing agent; Hexadecyl trimethyl ammonium bromide (CTAB) were purchased from Sigma, Egypt. Non-ionic dispersing agent; Egyptol BLM (based on nonyl phenol ethoxlate) was supplied from Starch

Pigment structure influence on dispersion stability

The influence of pigment chemical structure on its dispersion stability level plays an important role in the application of pigments. To investigate the effect of molecular structure of organic pigments, three different organic pigments; C.I. Pigment Green 7, C.I. Pigment Yellow 17 and C.I. Pigment Orange 5 have been investigated in terms of particle morphology and shelf life stability. To correlate the ease of dispersion of the three nano-scale pigments with their structures, pigment

Conclusions

Nanoscale organic pigments with particle size less than 100 nm were successfully prepared by using ball milling and ultrasonication in the presence of dispersing agent. Time of ball milling, type and concentration of dispersing agent were influential parameters and required to be optimized. Encapsulation of Nanoscale organic pigment into UV-curable resin was successfully achieved via miniemulsion technique. The effective encapsulation was confirmed by TEM, TGA, and ultra-centrifuge

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