Sealant Solid Content Testing: Principles, Standards, Methods and Applications

Sealant solid content testing determines the non-volatile fraction of a sealant by driving off volatile components under controlled heating and expressing the residue as a mass percentage. Reliable solids data are essential for batch quality control, formulation development, and VOC compliance across silicone, polyurethane, polysulfide, and hybrid sealant systems. This article explains the test principles, applicable standards such as ISO 3251, ASTM D2369 and GB/T 2793, typical sample types, step-by-step procedures, and common applications.

Technical Overview

Solid content, also referred to as non-volatile matter content, is the percentage by mass of a sealant that remains after all volatile components, such as water, solvents, and low-molecular-weight curing by-products, have been evaporated under specified temperature and time conditions. For most sealant systems, the non-volatile residue represents the polymer network, crosslinking components, fillers, plasticizers, pigments, and non-evaporating additives, and it is one of the most fundamental quality parameters reported on a sealant technical data sheet.

The test is typically performed by a gravimetric oven-drying method in accordance with ISO 3251, ASTM D2369, or GB/T 2793. A weighed specimen is dispersed as a thin film in a tared dish, heated at a defined temperature until constant mass is reached, cooled in a desiccator, and reweighed. The ratio of dry residue mass to original specimen mass, multiplied by one hundred, gives the solid content. Typical values vary widely by chemistry: solventless one-part silicone sealants often exceed 95 percent non-volatile matter, polyurethane sealants typically fall between 90 and 99 percent, polysulfide and hybrid silyl-modified sealants are usually above 90 percent, and water-based acrylic sealants commonly range from 60 to 80 percent.

Accurate solid content determination matters for several reasons. It verifies that production batches match the qualified formulation, detects excessive dilution or volatile loss, supports raw-material incoming inspection, provides a key input for calculating VOC content under regulations such as GB 33372, and helps formulators balance filler loading, cost, and cured properties. Because results are sensitive to specimen mass, drying temperature, film thickness, and weighing technique, standardized procedures and duplicate determinations are mandatory for defensible data.

Testing Samples

Solid content testing applies to virtually every sealant family. Sample handling differs slightly by chemistry because curing mechanism and volatility of by-products influence how the specimen must be prepared and dried.

Silicone Sealants

One-part room-temperature-vulcanizing (RTV) silicone sealants, including neutral-cure alkoxy and oxime types and acetoxy-curing types, contain only a small volatile fraction contributed by curing by-products such as methanol, methyl ethyl ketoxime, or acetic acid. Two-part structural silicone sealants used in curtain-wall glazing are also tested, with the base and curing-agent components evaluated separately or as a freshly mixed specimen depending on the specification.

Polyurethane Sealants

One-part moisture-curing and two-part polyurethane sealants used in construction joints, flooring, and automotive assembly are tested for non-volatile content. Because uncured polyurethane prepolymers react with atmospheric moisture, specimens must be weighed quickly after dispensing and the container resealed immediately after sampling to prevent false results.

Polysulfide Sealants

Polysulfide sealants used in insulated glass units, aircraft fuel tanks, and marine applications are tested component by component. The base compound, which contains polysulfide liquid polymer and fillers, and the manganese dioxide or other curing paste are evaluated separately, since the mixing ratio and solids of each component jointly determine cured density and performance.

Hybrid and Silyl-Terminated Sealants

Hybrid MS polymer and silyl-modified polyether sealants, which cure by moisture-triggered silanol condensation releasing methanol or ethanol, are widely used in construction and industrial assembly. Their non-volatile content is generally high, and testing confirms the absence of unauthorized solvent extension.

Acrylic, Butyl, and Hot-Melt Sealants

Water-based acrylic sealants contain a significant water fraction, making solid content a primary quality indicator for checking solids consistency between batches. Butyl sealants, hot-melt butyl used in insulated glass spacers, and plastic-ram-type sealants approach nearly 100 percent non-volatile matter but are still verified to confirm process consistency and the absence of trapped volatiles.

Testing Items

A standard solid content evaluation of a sealant typically includes the following reported items:

• Total non-volatile (solid) content, expressed as mass percentage of the original specimen.
• Total volatile matter content, calculated as one hundred percent minus the solid content.
• Water content, where required for VOC calculation, determined separately by Karl Fischer titration.
• Duplicate determinations with the individual results, mean value, and observed difference for precision assessment.
• Full documentation of drying temperature, drying time, specimen mass, and the standard method applied, such as ISO 3251, ASTM D2369, or GB/T 2793.

Testing Methods

Oven Gravimetric Method (Reference Procedure)

The reference gravimetric procedure, aligned with ISO 3251, ASTM D2369, and GB/T 2793, is performed as follows:

• Clean and dry a flat-bottomed glass, aluminum, or stainless-steel dish, cool it in a desiccator, and weigh it to 0.1 mg (record as m0).
• Introduce approximately 0.5 g to 1 g of homogenized sealant into the dish, spread it into a thin uniform film, and weigh the dish plus specimen immediately (record as m1). For moisture-reactive sealants, complete the weighing within one minute of dispensing.
• Place the dish in a forced-ventilation oven set at 105 plus or minus 2 degrees Celsius, or at 110 plus or minus 5 degrees Celsius when following ASTM D2369. Heat-sensitive systems may use 80 degrees Celsius with extended drying time or a vacuum oven.
• Dry the specimen for the initial holding period, typically 60 to 180 minutes, then transfer the dish to a desiccator, cool to room temperature, and reweigh (record as m2).
• Repeat drying cycles of approximately 30 minutes until the change in mass between successive weighings is no greater than 1 mg, or the agreed constant-mass criterion, indicating complete removal of volatiles.
• Run the determination in duplicate and report the mean value, discarding and repeating the test if the two results differ by more than the repeatability limit, typically 0.5 percent absolute for well-controlled sealant systems.

Calculation of Solid Content

The solid content W is calculated as W equals (m2 minus m0) divided by (m1 minus m0), multiplied by 100 percent, where m0 is the mass of the empty dish, m1 is the mass of the dish plus specimen before drying, and m2 is the mass of the dish plus dry residue after constant mass is achieved. Volatile matter is reported as 100 percent minus W. Results are rounded to 0.1 percent unless the governing product specification states otherwise.

Rapid Infrared and Halogen Moisture Analyzers

For in-process quality control, infrared or halogen-lamp moisture analyzers apply the same loss-on-drying principle in a fraction of the time, typically 5 to 15 minutes. The instrument is calibrated against the reference oven method for each sealant chemistry, since radiant heating and automatic endpoint logic can give slightly different values for slow-volatilizing or film-forming products. Rapid analyzers are ideal for production-line screening, while the oven method remains the-equivalent reference for release testing and dispute resolution.

Vacuum and Low-Temperature Drying

Sealants containing temperature-sensitive additives, oxidatively curing components, or high-boiling plasticizers are dried in a vacuum oven at reduced pressure, for example below 5 kPa, at 40 to 80 degrees Celsius. This lowers the evaporation temperature of volatiles and prevents thermal degradation or premature side reactions that would bias the result upward or downward.

Supplementary Techniques

• Karl Fischer titration quantifies water content specifically, which is necessary when calculating VOC content under GB 33372 by subtracting water and exempt compounds from total volatiles.
• Thermogravimetric analysis (TGA) provides a research-grade mass-loss profile across a programmed temperature ramp, useful for distinguishing water, solvent, curing by-products, and polymer decomposition in failure investigations.
• Determination of density and volatiles together allows full VOC calculation, since VOC content in grams per liter equals volatile fraction multiplied by the density of the sealant after correcting for water and exempt solvents.

Applications

Solid content testing serves multiple functions throughout the sealant value chain:

• Production quality control: verifying that every manufactured batch of silicone, polyurethane, polysulfide, or hybrid sealant matches the release specification for non-volatile matter before shipment.
• Incoming material inspection: contractors, glazing fabricators, and OEM assembly plants checking supplied sealant cartridges against declared technical data sheet values.
• Formulation and cost management: monitoring filler loading and solvent or water levels to balance cured performance, sag behavior, and raw-material cost.
• Regulatory compliance: supplying the solids input required for VOC declarations under adhesives and sealants VOC limit standards such as GB 33372 and comparable regional regulations.
• Product certification: generating test reports within accredited third-party laboratories for specification compliance against sealant classification standards such as ISO 11600, ASTM C920, and relevant GB product standards.
• Research and failure analysis: comparing solids before and after storage, transport, or partial cure to diagnose container aging, moisture exposure, or component segregation.

FAQ

What temperature is used for sealant solid content testing?

Most standards specify 105 plus or minus 2 degrees Celsius (ISO 3251, GB/T 2793) or 110 plus or minus 5 degrees Celsius (ASTM D2369). Heat-sensitive sealants may be dried at lower temperatures, typically 40 to 80 degrees Celsius, with longer drying times or under vacuum to avoid thermal degradation.

How large should the test specimen be?

A specimen of approximately 0.5 g to 1 g, spread as a thin film no thicker than about 1 mm, is standard. Excessively thick specimens trap volatiles and give falsely high solids, while very small specimens amplify weighing errors.

What does constant mass mean in this test?

Constant mass is reached when repeated drying and reweighing cycles produce a mass change of no more than 1 mg, or another agreed criterion. Reaching constant mass confirms that all removable volatiles have been driven off before the final calculation.

Why is solid content important for VOC compliance?

VOC content is calculated from total volatiles after subtracting water and exempt compounds. An inaccurate solids value therefore propagates directly into the declared VOC figure, which is why regulatory VOC testing under GB 33372 and similar standards pairs non-volatile content determination with Karl Fischer water analysis.

Do two-component sealants need special handling?

Yes. The base and curing-agent components are usually tested separately to reflect their individual formulations. If the mixed sealant is tested, the specimen must be dispensed, spread, and placed in the oven immediately so the curing reaction proceeds under controlled, repeatable conditions.

What precision can be expected?

Within a single laboratory, duplicate determinations on a homogenized sealant typically agree within 0.5 percent absolute. Between laboratories, reproducibility limits of roughly 1 to 2 percent absolute are realistic and are defined in the governing standard.

Can solid content indicate sealant quality by itself?

No. Solids content confirms the non-volatile fraction but says nothing about cure behavior, adhesion, or mechanical properties. It is one element of a full specification package that also includes workability, movement capability, adhesion-cohesion testing, and durability assessment in accredited third-party laboratories when certification is required.