Solid Content Determination by Reduced-Pressure (Vacuum) Drying Method: Principles, Procedures and A

Solid content determination by the reduced-pressure drying method is a gravimetric technique used to measure the non-volatile matter fraction of coatings, emulsions, adhesives, food products, and fine chemicals that are sensitive to heat. By drying a weighed specimen in a vacuum oven at a lowered temperature and reduced absolute pressure, the method minimizes thermal degradation and oxidation while accurately quantifying volatile loss. This article explains the principle, apparatus, step-by-step procedure, calculation, applicable standards, and typical applications of the method.

Technical Overview

Solid content (also referred to as non-volatile matter content) is defined as the mass fraction of a sample that remains after all volatile components—water, organic solvents, residual monomers, and low-boiling additives—have been evaporated under specified drying conditions. In the reduced-pressure drying method, the test specimen is heated in a vacuum oven at a temperature significantly lower than that used in conventional atmospheric oven methods. Because the boiling point of a liquid decreases as the ambient pressure is reduced, moisture and solvents can be driven off completely at 40–80 °C instead of 105–110 °C.

This makes the technique particularly valuable for heat-sensitive, thermoplastic, or easily oxidized materials that would crosslink, skin over, decompose, or undergo discoloration at atmospheric drying temperatures. The method is empirical by nature: results are method-dependent and are only comparable when temperature, pressure, drying time, and specimen mass are identical. It is therefore essential that all parties agree on the exact test conditions before comparing data.

Vacuum drying procedures for water and volatile determination are specified in standards such as GB/T 6284 (chemical products, general rules for water content by vacuum drying) and GB 5009.3 (determination of moisture in foods, vacuum drying procedure), while related oven-based non-volatile matter methods are described in ISO 3251 and GB/T 1725 for paints, varnishes, and plastics, as well as in pharmacopoeial loss-on-drying monographs.

Testing Samples

Polymer Emulsions and Aqueous Dispersions

Acrylic, styrene-acrylic, vinyl acetate, and vinyl chloride emulsions; polyurethane dispersions; natural and synthetic rubber latices; and specialty polymer dispersions used as binders. These materials frequently form surface films that trap water, making vacuum drying at moderate temperature with a thin, well-spread specimen layer essential for complete volatile removal.

Adhesives and Sealants

Waterborne adhesives, pressure-sensitive adhesive emulsions, laminating adhesives, construction binders, and primer formulations. Solid content data directly govern formulation ratios, coat weight calculations, and batch-to-batch consistency.

Coatings, Paints, and Inks

Waterborne architectural and industrial coatings, pigment dispersions, printing inks, and varnishes. Many modern water-reducible resins are thermally sensitive, and reduced-pressure drying prevents erroneous results caused by oxidation or thermal crosslinking during the test.

Foodstuffs and High-Sugar Products

Honey, glucose and maltose syrups, jellies, and other high-sugar or viscous food products in which heating at atmospheric pressure causes decomposition, caramelization, or the formation of an impermeable surface crust. The vacuum drying procedure in GB 5009.3 is designed specifically for such matrices.

Pharmaceutical and Fine Chemical Products

Active pharmaceutical ingredients, intermediates, heat-sensitive organic powders, surfactants, and catalyst precursors for which loss on drying under reduced pressure is specified as a release criterion or incoming inspection item.

Battery and Electronic Materials

Lithium-ion battery electrode slurries, ceramic slurries, conductive pastes, and electronic-grade coating formulations, where accurate solids values are critical for coating thickness control, solvent balance, and yield calculations.

Testing Items

• Solid content (non-volatile matter content), expressed as mass fraction of the original sample
• Volatile matter content, obtained by difference (100% minus solid content)
• Water content of solid chemical products determined by the vacuum drying principle
• Loss on drying for pharmaceutical, food, and fine chemical specimens
• Duplicate determinations and method-condition verification (temperature, pressure, drying time)

Testing Methods

Principle

A weighed quantity of the sample is placed in a pre-conditioned weighing vessel and dried in a vacuum oven at a specified temperature and absolute pressure until constant mass is reached. The solid content is calculated from the ratio of the dry residue mass to the original sample mass. All volatile substances removable under the chosen conditions are collectively determined; the method does not distinguish water from organic solvents.

Apparatus

• Analytical balance with a readability of 0.1 mg, calibrated and verified
• Vacuum drying oven with temperature control of approximately ±2 °C, a calibrated vacuum gauge, a dry-gas inlet, and the capability to maintain an absolute pressure typically between 2.7 kPa (20 mmHg) and 20 kPa depending on the governing standard
• Flat weighing bottles, low-form dishes, or aluminum pans with covers; inert glass-fiber pads for spreading liquid samples into a thin film
• Desiccator charged with fresh desiccant (silica gel or phosphorus pentoxide) for cooling specimens without moisture uptake
• Vacuum pump fitted with a cold trap to condense evaporated moisture and solvent and protect the pump

Specimen Preparation

Clean the weighing vessel, dry it to constant mass in a conventional oven, cool it in the desiccator, and weigh it accurately (recorded as m0). Introduce the test portion—typically 1–2 g for emulsions and dispersions, or as specified in the product standard—and spread it into a thin, uniform layer; for low-viscosity liquids a pre-weighed glass-fiber pad may be used as a carrier. Weigh the vessel plus specimen immediately (recorded as m1). For foaming emulsions, allow partial surface drying at ambient temperature or approximately 40 °C before applying vacuum to prevent bumping and splashing.

Drying Procedure

Place the open vessels in the vacuum oven pre-set to the specified temperature—commonly 60 ± 2 °C for heat-sensitive polymer systems, with 40 °C or 80 °C used where the product standard requires. Evacuate the oven gradually to the specified absolute pressure, for example not more than 20 kPa for general moisture removal or approximately 2.7 kPa for tightly bound residual moisture. Maintain these conditions for the specified period, typically 3 hours for the first cycle; a slight continuous bleed of dry air or nitrogen may be used to sweep vapors out of the chamber and to protect oxidation-sensitive samples. At the end of the cycle, slowly admit dry air to release the vacuum, transfer the covered vessels to the desiccator, cool for approximately 30 minutes, and weigh. Repeat drying cycles of about 1 hour until constant mass is achieved, defined in most procedures as a mass change of no more than 0.3 mg (or the limit stated in the applicable standard) between consecutive weighings.

Calculation and Expression of Results

The solid content w is calculated as w = (m2 − m0) / (m1 − m0) × 100%, where m0 is the mass of the empty vessel, m1 is the mass of the vessel plus the specimen before drying, and m2 is the mass after drying to constant weight. The volatile matter content equals 100% minus the solid content. Perform at least two independent determinations and report the arithmetic mean, rounded according to the product standard, together with the drying temperature, absolute pressure, and drying time used. Duplicate results should normally agree within 0.2 to 0.5 percentage points, or within the repeatability limit stated in the governing method.

Quality Control and Interference Control

• Pre-dry all vessels to constant mass and verify desiccant activity before each series
• Verify oven temperature uniformity periodically and record the actual chamber pressure during the run
• Use a thin, uniform specimen layer to prevent solvent or water entrapment beneath a surface skin
• Weigh hygroscopic residues rapidly and consistently, keeping vessels covered whenever possible
• Use an inert gas bleed for oxidation-sensitive samples and a cold trap to avoid moisture back-diffusion
• Analyze a control sample of known solid content alongside routine specimens to monitor method drift

Applications

The reduced-pressure drying method is widely applied across quality control, research, and incoming-inspection workflows. Emulsion and dispersion manufacturers use it to confirm polymerization conversion and batch conformity before release; formulators rely on certified solids values to calculate additive dosages and adjust recipes to target volume solids. Adhesive and coating producers apply the method for in-process control, yield accounting, and customer certification, while food laboratories use the vacuum drying procedure of GB 5009.3 for moisture determination in high-sugar products that cannot tolerate atmospheric oven temperatures. Pharmaceutical laboratories perform reduced-pressure loss-on-drying as part of release testing, and battery material producers use it for electrode slurry control.

The method complements, rather than replaces, other techniques: Karl Fischer titration is preferred when only water must be quantified in the presence of organic volatiles, infrared and halogen moisture analyzers offer rapid shop-floor screening when calibrated against the vacuum method, and conventional oven methods such as those in ISO 3251 and GB/T 1725 remain appropriate for thermally stable coatings and resins. Manufacturers without in-house vacuum drying capability frequently engage accredited third-party laboratories, ensuring that results are traceable, standardized, and acceptable to supply-chain partners.

FAQ

Why choose reduced-pressure drying instead of a normal oven at 105 °C?

At reduced pressure, water and solvents evaporate at much lower temperatures. This allows complete drying at 40–80 °C, avoiding the thermal degradation, crosslinking, oxidation, and crust formation that can distort results for heat-sensitive or thermoplastic samples at atmospheric drying temperatures.

What temperature and vacuum level are typically used?

Common conditions are 60 ± 2 °C with an absolute pressure not exceeding 20 kPa for general applications; deeper vacuum, around 2.7 kPa, is applied when tightly bound residual moisture must be removed. The exact values must always follow the governing product or method standard.

What does "drying to constant weight" mean?

Constant weight is reached when two consecutive drying-and-weighing cycles differ by no more than the limit specified in the method, commonly 0.3 mg for a 0.1 mg readability balance. In practice, an initial 3-hour drying followed by 1-hour repeat cycles achieves constant weight within 3 to 6 hours for most emulsions and dispersions.

How much sample should be tested?

Typical test portions are 1–2 g for polymer emulsions, adhesives, and coatings, spread as thinly as possible. Excessive specimen mass or thickness slows drying and increases the risk of trapped volatiles, biasing the solid content result upward.

Does the method measure water content specifically?

No. The mass loss represents all volatiles removable under the chosen conditions, including water, organic solvents, and residual monomers. When a water-specific result is required, Karl Fischer titration should be used instead of, or alongside, the gravimetric method.

Which standards govern the method?

Vacuum drying procedures are specified in GB/T 6284 for chemical products and GB 5009.3 for foods, while pharmacopoeial monographs define reduced-pressure loss on drying. Related non-volatile matter determinations by oven drying are covered by ISO 3251 and GB/T 1725 for paints, varnishes, and plastics, and ASTM oven-based methods provide equivalent atmospheric counterparts for coatings.

What repeatability can be expected?

Under standardized conditions, duplicate determinations performed by a trained operator in the same laboratory typically agree within 0.2 to 0.5 percentage points absolute. Larger deviations usually indicate non-uniform specimen spreading, unstable oven temperature or pressure, or a sample that reacts during drying, and should trigger an investigation.

Which samples are unsuitable for this method?

Samples containing high-boiling solvents that will not evaporate at the selected temperature and pressure, strongly foaming systems that cannot be controlled by pre-drying, materials that sublime, and products whose residue is highly hygroscopic are poor candidates. In such cases, alternative techniques such as Karl Fischer titration, gas chromatography, or thermogravimetric analysis should be considered.