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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Packaging and converting > New pressure sensitive adhesive papers||Update: April 26th 2007|
|Scientific report of the LGP2
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|Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"
In Europe, the pressure sensitive labels market presents about 1 billion Euros, increasing annually by 5 to 7 %. The fabrication of these materials involves the preparation of protective anti-adhesive silicone-based films, which cause significant environmental problems after use. In fact, these films are relatively expensive materials and are very hard to recycle, both strong reasons to discontinue their use.
The main objective of this work is to conceive new pressure sensitive adhesive papers without using the silicone-based protective films. In order to achieve this goal, the adhesive should be microencapsulated, calling upon the use of “hard” shell material. The microcapsules thus obtained will not stick. However, they will be coated onto a paper surface and will break only when submitted to a pressure. After pressing, the capsules liberate their content and the paper will stick.
Coacervation is a term borrowed from colloid chemistry to describe the basic process of capsule wall
formation [Figure 1].
Presently, coacervative encapsulation (or micro-encapsulation) is a three part process:
The microcapsules are usually collected by filtration or centrifugation, washed with an appropriate solvent, and subsequently dried by standard techniques such as spray drying or fluidised bed drying to yield free-flowing, discrete particles. Coacervation may be initiated in a number of different ways. Examples are changing the temperature or pH, or adding a second substance such as a concentrated aqueous ionic salt solution or a nonsolvent.
|Figure 1 - Microcapsules obtained by coacervation
of an industrial adhesive emulsion with chitosan
Coacervation with chitosan Chitosan is a natural polymer, soluble in aqueous acetic acid solution at a pH of about 5. In order to achieve the coacervation process and thus inducing the precipitation of the chitosan macromolecules on the surface of the adhesive-containing micelles, some NaOH is added, aimed at increasing the pH of the reaction medium to about 8.
Different tests were carried out, including model systems like oils, pressure sensitive adhesives synthesised in our laboratory, as well as commercial products. These tests showed the feasibility of micro-encapsulation of such polymers. Nevertheless, more work is needed to optimise the encapsulation process, the micro-capsules themselves, the thickness of their shell, etc. The microcapsules thus obtained were observed by optical microscopy as shown in Figure 1. The capsules are spherical and mononuclear and have a tendency to agglomerate.
The FTIR spectrum [Figure 2] of these microcapsules was recorded and showed peaks representative of both adhesives (core material) and chitosan (shell polymer).
|Figure 2 - FTIR spectrum of microcapsules made by encapsulation
of an industrial adhesive as the core material and chitosan as the shell polymer
The capsule size distribution was analysed using a Malvern Mastersizer. The capsules are stirred in water during 5 minutes. Ultrasonic waves could be used in order to break capsule agglomerates. The data obtained were processed in order to obtain both the distribution by volume and by number. As shown in [Figure 3], the capsules had a small diameter (number average diameter of 0.53μm) but displayed a tendency to agglomerate.
|Figure 3 - Particle size distribution of capsules obtained
by encapsulation of an industrial adhesive with chitosan
Experiments to optimise of the process were already initiated to minimise the agglomeration phenomena. [Figures 4, 5, 6] show the first results obtained by the micro-encapsulation of an industrial adhesive supplied in the form of an emulsion and commonly used as a pressure sensitive adhesive. These tests show that the micro-capsules are spherical and mononuclear, and that their size is close to one micrometer, independent of the shell material used.
|Figure 4 - Capsules obtained by encapsulation
of industrial adhesive with chitosan,
using optimal conditions
|Figure 5 - Capsules obtained by encapsulation
of industrial adhesive with gelatine,
using optimal conditions
|Figure 6 - Particle size distribution of capsules obtained
by encapsulation of an industrial adhesive with different shell materials
Microencapsulation based on the aminoplast route (polycondensation at the interface) was also investigated using a similar procedure as employed when producing microcapsules for carbonless paper: After the preparation of the pressure sensitive adhesive emulsion, melamine-formaldehyde prepolymer was added at an acidic pH. The pH was subsequently increased to basic conditions before heating the reaction mixture in order to build the shell of the capsules. These tests gave also very promising results
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