Effects of conditions in hot-melt coating process on microphase-separated structures and macroscopic deformation in coated layers composed of di- and triblock copolymer blends

Highlights

• The coated layers of block copolymer blends were prepared by the hot-melt coating method.

• The effects of the conditions in the hot-melt coating process on the nanostructure and the macroscopic deformation.　

• It have been revealed by SAXS measurements and the change of the size of the specimen upon releasing it from the substrate.

• The extent of the microscopic (d spacing) elongation was lower than the macroscopic elongation in the machine direction.

• Both of the microscopic and the macroscopic deformation can be simply scaled by the final thickness of the coated layer.

Abstract

Effects of the coating rate, the cooling temperature and the discharge rate in the hot-melt coating process on the microphase-separated structure and the macroscopic deformation have been revealed using blend specimens of MA/MAM where MA and MAM are a poly(methylmethacrylate)-block-poly(n-butylacrylate) diblock copolymer and a poly(methylmethacrylate)-block-poly(n-butylacrylate)-block-poly(methylmethacrylate) triblock copolymer, respectively. It was found that the hot-melt coated specimen had the anisotropic distribution of the spheres where the d spacing of the nanostructure in the machine direction (MD) is longer than that in the transverse direction (TD) and had a lower ordering regularity of the spherical microdomains in the MD due to shear and elongational deformation. The change of the macroscopic size upon the release of the coated layer from the substrate also confirmed the elongation of the soft-segment (poly (n-butylacrylate)) chains in the MD during the hot-melt coating by exhibiting the macroscopic shrinkage in the MD upon the release. However, the extent of the microscopic elongation (d spacing) was lower than the macroscopic elongation in the MD. This implies that the soft-segment chains are relaxed due to pulling-out of the poly (methylmethacrylate) chains from the hard spherical microdomains. As for the effects of the coating rate, the extent of the chain stretching of the soft segment was increased with the coating rate when the discharge rate (the flux of materials provided onto the substrate through the die lip) was kept constant. However, no effects were confirmed when the discharge rate was so controlled to maintain the thickness of the as-coated layer constant. Thus, the effects of the coating rate and the discharge rate were compensated, and the effects on the chain stretching of the soft segment can be recognized simply in terms of the thickness of the as-coated layer. As for the effects of the cooling-roll temperature, it was found that the extent of the chain stretching of the soft segment was decreased with an increase in the cooling-roll temperature.

Introduction

Block copolymers are used as materials for various products utilizing their characteristic mechanical or physical properties [1,2] due to co-existing microphases of respective components [[3], [4], [5], [6], [7], [8], [9]]. Especially, A-B-A type triblock copolymers with a minor hard-segment are well known to exhibit an interesting property, and they can be used as elastomers, since the hard phase plays a role of a physical crosslinking point for the soft phase (matrix) with the bridge conformation of the soft-segment chains. Such a triblock copolymer is suitable for a base polymer of a pressure-sensitive adhesive because it provides the two main functions where the hard phase contributes cohesive property and the soft phase does initial adhesion to the adherend (so-called tackiness). As one of the general technique for producing pressure-sensitive adhesive, a diblock copolymer is usually blended with a triblock copolymer for the purpose of softening, and the blend is compounded with additives such as tackifier, plasticizer and filler [[10], [11], [12], [13], [14], [15], [16]].

There are wide ranges of molding options of block copolymers [17], such as molding from solutions through the solvent evaporation or molding from melt through cooling. As a matter of fact, the block copolymer pressure-sensitive adhesive tapes are produced mainly by the solution coating process or sometime by the hot-melt coating process [18]. Adjusting the viscosity of the adhesive formulation by adding solvent or heating in these processes, it can be coated on the substrate. Although the solution coating process is the most common method for producing adhesive tapes that has been used for a long time in the past, efforts to reduce the amount of organic solvents used in the product manufacturing have been performed due to the demand for reducing the environmental load in the recent year, as a subject in the industry. Adhesive tape manufacturers are required to accelerate the shift of production process from the solution coating to the hot-melt coating method. However, when a layer is coated with the same formulation by the solution coating process or by the hot-melt coating process, more suitable properties for the use as an adhesive are often obtained by the solution coating process. Namely, the adhesive tape prepared by the hot-melt coating has poorer adhesion properties (such as practical fixing and holding power) as compared to that by the solution coating process. In order to clarify the reason, it is important to understand the effects of the conditions of the hot-melt coating process on the nanostructures and macroscopic properties of the coated layer.

O’ Conner et al. have intended to report the difference in the adhesive and physical properties of the coated layers prepared by the solution-coating and by the hot-melt coating processes through comparing the nanostructures in the coated layers [19]. Although the specimens used for the evaluation of the adhesive properties reflected rigorously the coating process conditions, the specimens for evaluations of the physical properties and nanostructures did not reflect the coating process condition because the coating history was deleted and did not remain in the test specimens which were subjected to evaluations of the physical properties and nanostructures. Actually, the test specimens were once coated but were hot pressed before the evaluations. Therefore, it should be stated that the comparisons of the physical properties and the nanostructures between the solution-coating process and the hot-melt coating process were insufficient. In order to clarify the difference between these coating processes, it is necessary to keep the preparation conditions remained in the test specimens by careful treatments of the test specimens in prior to the evaluations. Actually, in our previous paper [20,21] we have treated the test specimens with special cares.

In order to surely understand effects of the coating process on the nanostructures of block copolymer, a model system containing only the block copolymers without any additive should be the first target. In this regard, the effects of the drying temperature in the solution coating process on the microphase-separated structures have been reported in our previous paper [20] for the model specimens composed of di- and triblock copolymer blends as revealed by two-dimensional small-angle X-ray scattering (2D-SAXS). The specimens for a nanostructure analysis were used in the forms of the tape (a block copolymer layer coated on a polyimide film). Note that the polyimide film is thin enough for the X-ray beam to penetrate, which enables the 2D-SAXS measurements without peeling off the polyimide film. On the other hand, for tensile tests (for the evaluation of the mechanical properties), the as-coated layers were utilized by taking a special care to prevent changes in the macroscopic size upon the release from the substrate. Furthermore, it should be noted that the as-coated layers were stacked to obtain thicker film specimens, because the single layer was too thin to handle. For this purpose, a release film (silicone-coated polyethylene terephthalate film) was used as a substrate on which the layer was coated (see our previous paper [20] for more details in the special treatments). Thus, we could closely examine the effects of the coating conditions on the degree of regularity of sphere ordering in the body-centered cubic (BCC) lattice for the coated layer prepared by the solution-coated process. Furthermore, we have analyzed the structural changes of the same block copolymer coated layer during the uniaxial stretching, and the correlation between mechanical properties and structural changes was revealed [21].

The current study concerns the hot-melt coated layers composed of di- and triblock copolymer blends of the same formulation as used in our previous studies for the solution coating [20,21]. Note that the purpose of blending diblock copolymer with triblock copolymer is to make the specimen softer as pressure-sensitive adhesive and furthermore to lower the melt viscosity so that it can be applied in the hot-melt coating process. The purpose of the current study is to reveal the effects of the conditions of the hot-melt coating process on the structures and the mechanical properties. Coated products are actually manufactured in factories under various coating conditions to meet the best balance of the production efficiency and the homogeneous product quality. We have analyzed in detail the effects of the coating conditions (such as the coating rate, cooling-roll temperature, the discharge rate of the melted block copolymer specimen from the die lip, and the thickness of the as-coated layer) on the nanostructures and the macroscopic size changes to estimate microscopic and macroscopic deformations during the hot-melt process.