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ISO 25179: Determination of Solubility of Water-Soluble or Alkali-Soluble Pressure-Sensitive Adhesives
Adhesives — Test Method for PSA Solubility
1. Introduction to PSA Solubility Testing
ISO 25179:2018 specifies a test method for determining the solubility of water-soluble or alkali-soluble pressure-sensitive adhesives (PSAs) by measuring the loss in mass when the adhesive is immersed in water or alkali solution. This test method is a key tool for assessing the recyclability of products with PSA-applied labels, a critical consideration in the packaging industry’s transition toward circular economy principles.
Pressure-sensitive adhesives are ubiquitous in modern packaging — from shipping labels to barcode stickers. However, their presence on paper and cardboard packaging creates significant challenges for recycling. If the adhesive does not dissolve during the repulping process, it remains as contaminant particles in the recycled fiber, reducing paper quality and potentially damaging recycling equipment.
ISO 25179 provides a standardized, quantitative method for evaluating PSA solubility. A high solubility value (above 80%) indicates that labels using this adhesive can be effectively removed during standard paper recycling processes, supporting sustainability goals.
2. Test Method and Apparatus
The test method is straightforward in principle: a coated adhesive specimen is immersed in a specific solvent (water or alkali solution), subjected to controlled agitation at elevated temperature, and the mass loss is measured gravimetrically. However, the standard specifies precise apparatus requirements and procedural controls to ensure reproducibility.
2.1 Key Apparatus Components
| Apparatus | Specification |
|---|---|
| Orbital shaker | Enclosed chamber, heated 40-80 deg C (+/- 2 deg C), 10-100 r/min speed range |
| Glass bottles | 500 ml volume, sealable with caps to prevent vapour escape |
| Test specimen holder | AISI 316 stainless steel, cylindrical drum with clamping bars |
| Drying oven | Forced-air convection, 50-120 deg C (+/- 2 deg C) |
| Analytical balance | Accuracy of 0.0001 g |
The test specimen holder is a particularly critical component. Made of AISI 316 stainless steel with a PTFE lining, it consists of a cylindrical drum and two specimen-clamping bars. This design ensures that the adhesive-coated test specimen is held securely while allowing maximum exposure of the adhesive surface to the solvent during agitation.
2.2 Specimen Preparation
The adhesive is coated onto repulpable kraft paper (80-100 g/m2) at a wet-film thickness of approximately 40 um, dried at 80 deg C for 30 minutes, and cut into specimens measuring 110 mm x 20 mm. Ten or more specimens are prepared for each test to ensure statistical validity.
The standard includes a safety statement about proper waste disposal after testing. Solubilized adhesives and alkali solutions must be disposed of according to local environmental regulations. Some water-soluble adhesives may contain compounds with negative environmental impact if released untreated.
3. Procedure and Expression of Results
The test procedure involves several precisely controlled steps. After preparing and weighing the specimens, each specimen is clamped onto the holder and placed in a bottle containing 350 ml of the appropriate solvent. The bottles are then placed in the preheated shaker chamber at 50 deg C and shaken at 50 r/min.
At specified time intervals, specimens are removed, dried, and weighed. The solubility at each time point is calculated as the percentage mass loss relative to the initial adhesive mass. This time-dependent data reveals both the rate and extent of dissolution.
Results are expressed as the percentage loss in mass of the adhesive coating. The standard recommends testing at multiple time points to generate a solubility curve. Key parameters derived include the initial dissolution rate and the equilibrium solubility value.
4. Engineering Design Insights
For adhesive formulators, ISO 25179 provides critical data for optimizing PSA compositions for recyclability. The balance between adhesive performance (tack, peel strength, shear holding power) and solubility is a classic engineering trade-off. Water-soluble PSAs typically incorporate hydrophilic polymers such as polyvinyl alcohol or acrylic acid copolymers, while alkali-soluble formulations use polymers with acid functional groups that ionize at high pH.
For packaging engineers, the solubility data from ISO 25179 testing informs label material selection for specific recycling streams. A label that performs excellently in the warehouse but cannot be removed during recycling creates downstream costs that may outweigh its initial benefits.
5. FAQs
Q: What types of adhesives are covered by this standard?
A: This standard covers water-soluble and alkali-soluble pressure-sensitive adhesives. It does not cover solvent-soluble adhesives or non-pressure-sensitive adhesives.
A: This standard covers water-soluble and alkali-soluble pressure-sensitive adhesives. It does not cover solvent-soluble adhesives or non-pressure-sensitive adhesives.
Q: Can the test temperature be modified from the standard 50 deg C?
A: Yes. The standard permits other temperatures if the solubility at a specific temperature is of interest. The shaker speed and temperature can also be increased to accelerate the test.
A: Yes. The standard permits other temperatures if the solubility at a specific temperature is of interest. The shaker speed and temperature can also be increased to accelerate the test.
Q: How is the solubility threshold for “recyclable” defined?
A: ISO 25179 does not define pass/fail criteria. The threshold depends on the specific recycling process and paper grade. Generally, solubility above 80% is considered good for standard paper recycling.
A: ISO 25179 does not define pass/fail criteria. The threshold depends on the specific recycling process and paper grade. Generally, solubility above 80% is considered good for standard paper recycling.
Q: What kind of paper substrate should be used?
A: Repulpable kraft paper with a mass per unit area between 80 g/m2 and 100 g/m2. This standard substrate ensures consistent results across different laboratories.
A: Repulpable kraft paper with a mass per unit area between 80 g/m2 and 100 g/m2. This standard substrate ensures consistent results across different laboratories.
