SMP Solid Content Testing: Methods, Standards, and Applications

SMP solid content testing determines the non-volatile fraction of silyl-modified polymer (SMP) based adhesives, sealants, and coatings, serving as a key indicator of product quality, formulation consistency, and regulatory compliance. This technical article explains the testing principle, common sample types, applicable standards, laboratory methods, and practical considerations for accurate solid content determination of SMP materials.

Technical Overview

Silyl-modified polymers (SMP), also known as silane-modified polymers or MS polymers, are moisture-curing prepolymers built on polyether or polyurethane backbones and terminated with reactive alkoxy-silane groups. When formulated into adhesives and sealants, SMP systems cure upon exposure to atmospheric moisture, crosslinking into elastic elastomers while releasing small amounts of alcohol. Because high-quality SMP products are typically marketed as solvent-free and isocyanate-free systems, their non-volatile content is expected to approach 100 percent.

Solid content (non-volatile matter) is defined as the percentage of sample mass that remains after volatile components, such as residual solvents, moisture, and released alcohol, are evaporated under specified temperature and time conditions. Accurate solid content measurement is essential for verifying solvent-free claims, confirming batch-to-batch consistency, detecting formulation deviations or contamination, supporting VOC declarations required by environmental regulations, and calculating the true polymer delivery per unit mass in formulation and cost calculations.

Testing Samples

Finished SMP Adhesives and Sealants

Elastic construction adhesives, structural bonding adhesives, and weatherproofing sealants supplied in cartridges, sausages, or pails. These highly filled paste materials are the most common samples submitted for solid content verification.

SMP Primers and Surface Treatments

Primer formulations applied before bonding or sealing often contain carrier solvents. Solid content testing for primers verifies the active residue left on the substrate after solvent evaporation and is directly related to bonding performance.

SMP Raw Materials and Prepolymers

Silane-terminated polyether or polyurethane prepolymers received as incoming materials are tested to confirm purity, detect dilution, and check residual volatile impurities before production use.

SMP-Based Coatings and Membranes

Liquid-applied waterproofing membranes and protective coatings based on SMP chemistry are tested to confirm non-volatile content and compliance with low-VOC requirements.

Testing Items

Depending on the purpose of the analysis, a solid content test program for SMP materials may include the following items:

• Total solid content (non-volatile matter), expressed as mass percentage
• Moisture content, determined separately by Karl Fischer titration
• Volatile organic fraction, including residual solvents and released alcohol
• Ash or filler content, determined by ignition as a complementary check on formulation
• Repeatability and reproducibility verification through duplicate or triplicate determinations

Testing Methods

Oven Drying (Gravimetric) Method

A sample of approximately 1 to 3 grams is spread into a thin film in a pre-weighed aluminum dish, weighed accurately, and dried in a forced-ventilation oven at 105 ± 2 °C for 2 to 3 hours, or until constant mass is achieved. The dish is cooled in a desiccator and reweighed. Solid content is calculated as the ratio of residue mass to initial sample mass, multiplied by 100. This method is simple, economical, and suitable for routine quality control, and it is covered by standards such as ISO 3251, ASTM D2369, and GB/T 2793 for adhesives.

Karl Fischer Titration

Because trace moisture in SMP systems triggers premature crosslinking and affects shelf stability, water content is often measured separately by volumetric or coulometric Karl Fischer titration in accordance with ISO 760 or ASTM D4017. Well-formulated SMP products typically require moisture levels below 0.1 percent.

Rapid Infrared or Halogen Moisture Analyzer

Balances with integrated halogen or infrared heaters provide solid content results within 10 to 20 minutes and are widely used for in-process screening. The heating program must be validated against the reference oven method to ensure equivalent results for SMP materials.

Vacuum Drying Method

For samples containing heat-sensitive additives or volatile plasticizers, drying at a reduced temperature (60 to 80 °C) under vacuum minimizes thermal bias and provides a more selective measure of true volatiles.

Thermogravimetric Analysis (TGA)

TGA uses milligram-scale samples and a programmed temperature ramp to separate volatile loss, polymer decomposition, and inorganic filler residue in a single run. It is a powerful diagnostic tool for failure analysis and formulation studies.

Key practical considerations for SMP testing include avoiding temperatures above approximately 150 °C, which can cause plasticizer loss or thermal degradation and bias results downward, and standardizing dwell time, since the moisture-curing reaction itself converts water into the polymer network rather than evaporating it, while small amounts of alcohol released during cure do escape and contribute to mass loss.

Applications

Solid content testing of SMP materials supports quality control and product development across multiple industries:

• Construction: elastic joint sealants, facade bonding, and structural glazing adhesives
• Flooring: parquet and resilient flooring adhesives with low-emission requirements
• Automotive: direct glazing, trim attachment, and body-in-white bonding systems
• Waterproofing: liquid-applied membranes and protective coatings
• Manufacturing QC: incoming material inspection, batch release, and shelf-life stability monitoring
• Regulatory compliance: VOC content declarations and eco-label certification support

FAQ

What solid content should a qualified SMP adhesive or sealant have?

Solvent-free SMP adhesives and sealants typically show non-volatile content above 98 percent. Values significantly below this level may indicate solvent addition, contamination, or incomplete formulation and warrant further investigation.

Which standards are used for SMP solid content testing?

Commonly applied standards include ISO 3251 and ASTM D2369 for non-volatile content determination, GB/T 2793 for adhesives, and ISO 760 or ASTM D4017 when water content is measured by Karl Fischer titration.

What drying temperature is appropriate?

A temperature of 105 ± 2 °C is standard for most SMP samples. Higher temperatures risk volatilizing plasticizers or degrading the polymer, while lower temperatures extend drying time and may leave residual volatiles.

How much sample is required and what accuracy is expected?

A sample of 1 to 3 grams spread as a thin film is generally sufficient. With proper technique, repeatability within ±0.2 to 0.5 percent is achievable in routine laboratory testing.

Does moisture curing during the test affect the result?

The silane cure reaction consumes water into the crosslinked network, so this portion does not appear as mass loss; however, the alcohol by-product released during curing does evaporate at oven temperatures. Standardized test conditions ensure that results are comparable between laboratories and between batches.