Aesthetically textured, hard latex coatings by fast IR-assisted evaporative lithography

Highlights

• A new, simple and inexpensive patterning technique has been developed.

• This process combines IR particle sintering with the concept of evaporative lithography.

• Under optimized conditions a hard textured coating can be obtained within 5 min without the addition of volatile organic compounds, such as coalescing aids.

• The process was used to create novel, textured waterborne coatings to decorate glass bottles.

• The process can be applied on nearly any substrate, and it is suitable for batch processing.

Abstract

Polymer coatings with periodic topographic patterns, repeating over millimetre length scales, can be created from lateral flows in an aqueous dispersion of colloidal particles. The flow is driven by differences in evaporation rate across the wet film surface created by IR radiative heating through a shadow mask. This new process, which we call IR radiation-assisted evaporative lithography (IRAEL), combines IR particle sintering with the concept of evaporative lithography. Here, a series of experiments has been conducted in which the mass of the latex is measured as a function of the exposure time under infrared radiation through a mask. The water evaporation rates and the minimum exposure time required for a dry film are estimated as a function of the power density of the IR emitter. The temperature of the wet film is monitored to avoid overheating and boiling of the water, which will otherwise cause defects. It is demonstrated that textured films can be created on a variety of substrates (plastics, metals, paper and glass), and processing times can be as short as 5 min. We use IRAEL to decorate household goods with an aesthetic coating with the desired texture.

Introduction

For numerous applications, there is a need for patterned surfaces at the micro- and nano-length scales. The topography or texture of a surface has a profound influence on its properties. For instance, the correct length scales of surface structure can impart hydrophobicity [1], alter the adhesion [2], reduce the reflectivity of electromagnetic radiation [3], affect friction and wear [4], and reduce the aerodynamic drag on aircraft and hence decrease fuel consumption [5]. There is a variety of techniques with which a patterned polymeric surface can be prepared, including nanopatterning and micropatterning using a mould [6], solution-casting of polymer films [7], photolithography, and inkjet printing. Inspired by previous reports of the evaporative lithography of dilute, hard nanoparticles [8], we recently developed a new technique called infrared radiation-assisted evaporative lithography (IRAEL) [9], [10]. This technique introduces a new way to create nearly any-desired pattern of surface topography on a waterborne coating made from a latex dispersion. It is a promising new example of a method that can be classified as “controlled evaporative self-assembly” technique [11].

In IRAEL, a mask containing holes of any desired shape (as in Fig. 1a) is used to modulate the evaporation rate across the surface of a wet colloidal film. The surface tension holds the water surface flat. Water must therefore flow to the fast-drying regions to replenish the water that has been lost by evaporation. This lateral flow carries colloidal particles in the direction towards the unmasked regions (Fig. 1b and c). The result is a collection of particles into a pattern defined by the mask, and these particles coalesce to create a textured coating.

In contrast to the initial work on evaporative lithography, the use of infrared radiation in combination with shadow masks offers two key advantages. First, the heat from the radiation increases the evaporation rate locally, so that the technique is practical on realistic time-scales. Secondly, polymer particles are sintered by heating from the IR radiation [12], so that patterned coatings can be made from “hard” polymers, with a glass transition temperature far above room temperature.

The use of radiative heating from infrared (IR) sources has gained greater prominence within the past few decades. There are previous reports of the use of IR radiative heating to speed the water loss from waterborne polymers [13], [14]. For instance, the drying time of aqueous solutions of poly(vinyl alcohol) was reduced from 120 min for convective drying to only 15 min for combined convective and IR radiative drying [13]. There are also reports of using IR-absorbing polymers as thermal transducers to raise the temperature of hot-melt adhesives [15] and polymer films [16] under IR radiation. IR radiative drying processes are attractive because of their energy efficiency. For example, the energy consumed by an IR lamp combined with a convection oven in removing water from foods was 245% less than that used by the oven alone [17].

In our previously reported experiments on IRAEL [9], we used a 250 W IR lamp, and textured coatings were created in times on the order of 30 min. In the present work, we use a carbon medium-wave emitter, leading to faster film formation times. We determined the effects of the key process parameters of the power of the IR emitter and the distance between the IR emitter and the wet film. (The emitter power and its distance from the film both affect the power density of the IR radiation on the film.) We will show how we can create textured, hard and latex coatings (free of volatile organic compounds, VOCs) in times as short as 5 min.