Radial block copolymers and adhesives based thereon with improved die-cutting performance

Abstract

Improved adhesives are provided through the use of styrenic radial block copolymers, containing at lest 40 wt % diblock copolymers, the adhesives have improved adhesive properties and a reduced elastic behaviour under die-cutting conditions. Compared to conventional linear block copolymers of similar molecular weight, radial copolymers offer higher holding power and lower melt viscosity for the adhesive. They therefore contribute to an improved balance between processability and end-use properties.

Description

FIELD OF THE INVENTION The present invention is directed to radial block copolymer compositions and pressure-sensitive adhesive compositions based thereon. The pressure-sensitive adhesives are particularly useful in label and tape manufacture. BACKGROUND Radial block copolymers are known and it is also known that during their manufacture up to 20 wt % of the diblock copolymers remain unreacted and are present as diblock copolymer material. These low diblock content radial copolymers have been proposed as components in pressure-sensitive adhesives, where they are used to make during label manufacture, a laminate of a face stock, pressure-sensitive adhesive layer, and a release liner, such as silicone-coated paper, which is passed through an apparatus that converts the laminate into commercially useful labels and label stock. The converting operation processes involve printing, die-cutting, and matrix stripping to leave labels on a release liner, marginal hole punching, perforating,...

AI Technical Summary

Benefits of technology

The present invention therefore provides a radial block copolymer composition and a pressure-sensitive adhesive composition based thereon. The pressure-sensitive adhesive was found to exhibit excellent adhesive properties and convertibility. The adhesive provides the ability to achieve clean rupture of the adhesive layer in processing operations involving cutting through a face stock to the release liner of the laminate. At the same time the adhesive provides excellent adhesive properties at both ambient and reduced temperatures, in particular the adhesive has high shear performance thus increasing flexibility in formulation. The adhesive is particularly suitable for use in labels on both paper and synthetic substrates. The adhesive also has low melt viscosity and may be applied as a hot melt, at low temperature.

The use of the adhesive systems of the present invention has been found to enable improved die-cuttability in the production of labels. The invention therefore further provides the use of a mixture of a radial styrenic block copolymer and a styrenic diblock copolymer comprising 60 wt % to 10 wt % of the radial styrenic block copolymer and 40 wt % to 90 wt % of the styrenic diblock copolymer in label adhesives. Furthermore, we have found that the use of these copolymer mixtures provide adhesives with lower melt viscosity and higher shear resistance.

Problems solved by technology

But these are properties that make the adhesive film difficult to cut or break.

These properties make die-cutting difficult and inconsistent, causing the adhesive lends to form adhesive strings and deposits on the cutting blade. FIG. 1 illustrates a typical die-cutting process.

While the nature of all laminate layers impact convertibility cost, the adhesive layer can limit convertibility ease.

The adhesive layer's viscoelastic nature causes this limitation—in particular its high elasticity prevents it from flowing away from the cut line during die-cutting and also promotes its transfer to cutting blades during cutting.

High adhesive elasticity also causes adhesive stringiness, which hinders matrix stripping as the unwanted facing material is removed after die-cutting.

Good, general-purpose adhesives may exhibit poor convertibility simply because the adhesive is difficult to cleanly sever.

Within a range of speeds, use of a particular adhesive may result in breaking the matrix despite the fact that successful matrix stripping can occur at speeds on either side of the breaking speed.

While these have good die-cutting performance, they require handling large volumes of liquid and subsequent liquid removal.

At low temperature, acrylic-based adhesives perform poorer than hot-melt systems.

While these materials have good adhesive properties when tackified and can be used in hot melts for label production, they do not have optimum die-cutting properties.

Furthermore, their balance of adhesive properties are not optimum.

U.S. Pat. No. 5,663,228 also does not appreciate the importance of the adhesive's elastomeric behavior under die-cutting conditions.

The adhesives have a low diblock content and we have found that this results in an adhesive that is too cohesive and has high elasticity which is detrimental for die cuttability as is shown in Comparative Example 1 which is based on the radial polymer DPX-551 mentioned in European Patent Application 0798358 A1 as a suitable polymer for use in its adhesive formulations.

Another means of reducing coupling efficiency is by the premature addition of a terminator compound.

The three and four arms products of these patents and the commercially available materials suffer from the disadvantages that they do not have optimum rheological properties for use in permanent label adhesives.

These polymers tend to have too high a tensile strength and are harder and too cohesive to be useful in adhesive formulations and in other applications such as sound deadening, shock absorption and polymer modification.

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