Sodium Silicate Adhesives

TAGS:  Waterborne Adhesives      Sealants    

This article was first published in 2006 and is revised in 2021.

Commonly known as "water-glass", soluble sodium silicates are colorless low-cost inorganic materials. When used as adhesives, liquid silicates offer:

• Significantly low cost
• Versatility, and
• Ease of handling

Their main use is to bond porous substrates, such as paper and cardboard where an inexpensive, fast processing adhesive is required.

Other adhesive applications for sodium silicates include bonding of wood, metal foils, or glass to porous substrates, glass fiber insulation bonding, as well as the fabrication of foundry molds and abrasive wheels. Because of their inorganic nature, sodium silicates can be formulated into industrial cements with exceptionally high resistance to temperatures and chemicals.

Sodium silicate adhesives are usually supplied as a viscous water solution. The adhesive bond forms by:

• The evaporation of water and/or
• Chemical reaction

Sodium silicate water solutions are often used directly as an unmodified adhesive, but they can also be blended with polymeric additives to improve specific properties such as toughness. Sodium silicates may also be used as a low-cost, modifying additive in other water-borne adhesives. These adhesives display on a moderate degree of tack, and holding pressure must be applied until the bond is sufficiently dry. Fortunately, sodium silicate solutions dry quickly, and as a result, they can be used in many operations requiring high-machine speeds.

Although the dry adhesive is generally resistant to high temperatures and chemical attacks, it is brittle and somewhat water-sensitive unless completely dehydrated or chemically reacted. Water-resistance can be improved by reacting sodium silicates with a variety of acidic or heavy metal compounds. The addition of sugar, glycerin, and other materials promotes retention of moisture in the film and increases its flexibility, tackiness, and toughness. Kaolin clay is frequently added to raise the viscosity and prevent excessive penetration into porous substrates.

The main advantages of sodium silicate adhesives are:

• Relatively low cost
• Total resistance to combustion, and
• Suitability for fast, waterborne coating application to many porous substrates

Sodium silicates are made from readily available, inexpensive raw materials and, therefore, are low cost in comparison to most adhesives based on synthetic organic polymers. Sodium silicate adhesives are also very resistant to extremely high temperatures. Some can resist temperatures as high as 1100°C. Due to their inorganic nature, they are considered to be completely flame resistant and resistant to microbe or animal attack, and as a result, sodium silicate adhesives are often employed in building & construction.

Let's dive deep to learn about the manufacturing of sodium silicates along with their key properties, applications and safety concerns.

Sodium Silicates – Manufacturing Process and Forms

Sodium silicates are available as either liquid water solutions (1 to 10 poises) or solid powders suitable for use in dry mix adhesives. They are made by fusing varying proportions of sand or silicon dioxide (SiO₂) and soda ash or sodium oxide (Na₂O) as shown in the figure below. The glass that is formed is then dissolved in water or powdered into a fine mesh.


The sodium silicate manufacturing process¹
By varying the SiO₂ to Na₂O ratio and the solids content, sodium silicate adhesive solutions having considerably different properties can be achieved for specific industrial applications. The SiO₂ to Na₂O ratio generally varies from 1.6 to 3.3, and the solids content is generally in the 25 to 65% range. The table below shows the characteristics of various grades of sodium silicate that are commercially available along with their general applications.

Liquid Sodium Silicate Solutions
Weight Ratio (SiO2/Na2O)	Solids Content, weight %	Viscosity @20°C cps	Density, lb/gal	Application
3.25	39.2	8300	11.8	Tube winding, paper board, fiberboard, wallboard, plywood laminating, refractory cement
3.22	37.6	1800	11.6
3.22	38.7	4000	11.8	Fiber drum winding tube winding, concrete and metal casting sealer (diluted)
2.88	42.7	9600	12.3	Tube winding, carton sealing (the solution has greater tack than others)
2.84	43.1	700	12.3	Metal casting sealant
Powdered Sodium Silicate
Weight Ratio (SiO2/Na2O)	Na2O, weight %	SiO2, weight %	H2O, weight %	Density, lb/ft2	Powder Size
3.22	23.1	74.4	0	88	Through 65 mesh
3.22	19.2	61.8	18.5	44	Through 100 mesh
2.40	23.8	57.2	17.5	38	Through 100 mesh
2.00	27.0	54	18.0	46	Through 100 mesh

Typical Properties of Liquid and Powdered Sodium Silicates ^1,2
Liquid silicates can range in viscosity from very fluid to relatively thick products with negligible flow. As a general rule, the sodium silicates solutions with higher SiO₂/Na₂O ratios (2.8-3.3) are used for adhesive products. They are readily available in bulk or drums from manufacturers, such as PQ Corporation and Occidental Chemical Corporation. Solutions should be stored at room temperature to prevent freezing and to provide better wetting of the substrate.

Powder grades differ according to composition, particle sizing, and degree of hydration. The various sodium silicate powders will differ in solution rate according to these properties. Times for a complete solution can range from minutes to hours. Hydrated powders have been especially developed to dissolve quickly in water at room temperature. These products are often used as redispersable adhesives or as dry mixes for field-applied cements.

Potassium silicates (SiO₂/K₂O blends) are also commercially available from many of the same sources that produce sodium silicates. These solutions have similar application and setting properties. However, the potassium silicates have properties (e.g., greater electrical resistance) that are more appropriate for certain applications. Potassium silicates are not used as extensively as sodium silicate adhesives because they have a higher cost.

Mechanism of Setting and Bond Formation

Liquid sodium silicate solutions are converted to solid adhesive bonds by the loss of water or by a chemical setting mechanism. These mechanisms can be applied separately or in combination depending on the requirements of the application.

• Loss of water occurs either by evaporation or by sorption through porous substrate materials.
• Chemical setting is often used to improve properties of the final adhesive film such as early strength development, moisture resistance, and ultimate bond strength.

Liquid sodium silicate products are slightly sticky, but they have only moderate tack compared to many organic pressure sensitive adhesives. As the water evaporates, liquid silicates become progressively tackier and more viscous. Generally, only a small portion of the water (several percent by weight) needs to evaporate before the adhesive exhibits good handling strength. Therefore, sodium silicate adhesives are often employed in applications requiring high machine speeds and fast setting characteristics. The effects of water evaporation on the viscosities of several liquid sodium silicate products are shown in the table below.

Weight Ratio SiO2/Na2O	Viscosity at 20°C (Poises)			Weight Loss for Initial Set, %
	Initial	6% Weight Loss	12% Weight Loss
3.22	1.8	20	2300	13.6
3.22	4.0	120	20000	11.2
2.88	9.6	150	10000	12.0

Effects of Evaporation on the Viscosity of Liquid Sodium Silicate Solutions²
For an equivalent viscosity, the more siliceous (higher SiO₂/Na₂O ratio) systems have faster air-drying times than the lower ratio, higher alkaline grades. However, the lower ratio grades have generally more tack and a higher wetting power. The lower ratio grades may require heat for drying or treatment with chemical setting agents to achieve practical application speeds. Because the low ratio sodium silicate adhesive solutions tend to retain more water, their dried films are usually less brittle than that of their counterparts.

The pH of liquid sodium silicate adhesives is fairly constant because it changes little when the solids content of the sodium silicate solution exceeds 10%. For adhesive products, the pH will range between approximately 11 for the high ratio products and 13 for the lower ratio (more alkaline) products.

Heat curing or chemical reaction is recommended for applications where water resistance is required. Once dehydrated by heat curing, the sodium silicate film has moderately good resistance to moisture. Heat curing must be done in stages to prevent steam from forming within the adhesive film. Usually, the temperature is gradually increased to 100-105°C to slowly remove water and gain viscosity. Final curing is then achieved at a temperature of 150-200°C.

Chemical reactions provide maximum impermeability and water resistance. Sodium silicates can be reacted with acidic or heavy metal compounds to produce solid, insoluble bonds. These compounds are commonly added directly to the liquid sodium silicate solution before application. However, they can also be added to the substrate as a primer or pre-bond treatment, or they can be applied to the finished bond as an after-treatment.

For example, moisture resistance can be significantly improved by applying suitable aluminum salts to substrates such as paper prior to bonding. The use of zinc oxide as a chemical setting agent is especially desirable since, after curing at 100-105°C, it produces a film that is capable of actually shedding water.³

The water-resistance of sodium silicate solution adhesives may also be improved by the addition of fine powders such as zinc oxide or amorphous calcium carbonate. These additives will react slowly with the silicate to form an insoluble mass. Combination of the silicate with acidic materials, such as aluminum sulfate, will result in neutralization of the alkaline silicate to cause gelation and a strong, polymerized form of the silica component. Various chemical setting mechanisms are summarized in the table below.

Additive	Characteristics
Acidic materials (e.g., aluminum sulfate, mineral, and organic acids), sodium bicarbonate and monosodium phosphate, carbon dioxide gas.	Insoluble bonds are formed by the neutralization of the alkaline silicate and subsequent gelation.
Multivalent metal compounds (e.g., calcium chloride, magnesium sulfate, borax, and sodium metaborate).	React with silicate solutions to form coatings or adhesive bonds by precipitation of insoluble silicate compounds. Usually used in concentrations of 5-10% by weight based on the liquid silicate.
Finely divided zinc oxide or sodium silicofluoride.	Provide longer working times and good water resistance. These are used at a concentration of about 7% by weight based on the liquid silicate. Silicofluoride is particularly effective for ambient temperature curing; zinc oxide is generally used with a 100-110°C cure temperature.

Additives Used for Chemical Setting of Sodium Silicate Adhesives²

Value of Sodium Silicate Adhesives

The primary factors for using sodium silicate adhesives are their ready availability, general ease of application, relatively rapid set, consistent properties, and lack of volatile emissions. They also provide a completely inorganic (incombustible) material. The bond is strong, rigid, and in many cases heat and water-resistant. The advantages, disadvantages, and typical applications of sodium silicate adhesives are summarized in the table below.

Advantages	Disadvantages	Example Applications
Low cost Non-flammable Environmentally compliant Resistant to animal attack and microbial growth Resistant to temperatures above 1000°C Resistant to many chemicals Odorless and non-toxic Bondable to many porous (e.g., paper, wood, glass insulation) and nonporous (e.g., metal foil, glass) materials High tensile strength Fast handling strength development	Rigid and relatively brittle (depending on moisture content) Sensitive to moisture (depending on the degree of dehydration and method of setting) The caustic nature of solutions require caution in handling	Adhesive for corrugated board, paper tubing, fiber drums, etc. Foil to paper lamination Masonry sealing Extender for latex adhesives and coatings Binders for fibrous materials (insulation, wallboard, etc.) Binders for palletizing Cements for high temperature, chemically resistant industrial applications Porosity sealant for concrete, metal castings, etc.

Advantages, Disadvantages, and Example Applications of Sodium Silicate Adhesives
The fast-setting properties, that are exhibited when only a small amount of water in liquid silicates is lost, are ideal for applications requiring high machine speeds. This feature is of particular value on corrugating machines, which have been successfully operated at speeds as high as 500 feet per minute with a silicate adhesive.³

When dehydrated to the glass state, soluble sodium silicates are resistant to attack by water; however, continued exposure to high humidity or aqueous solution will destroy it unless it is chemically set as described above. However, at ordinary relative humidity, the silicate bond may contain as much as 20-50% water. The bond becomes stronger as the water content decreases as shown in the figure below. Thus, at high relative humidities, a sodium silicate adhesive film would be more flexible but lower in tensile strength than the same adhesive that was conditioned at lower humidities.


A 3.2 ratio sodium silicate increased rapidly in tensile strength as the water content is reduced

Achieve Flexibility by Adding Additives & Rubbers

Sodium silicate adhesives, even when hydrated, have a low degree of flexibility. However, a moderate degree of flexibility can be achieved by the addition of plasticizers to the silicate solution. Sugar, glycerin, and sorbitol provide water retention and, thereby, increase flexibility. They are generally used in concentrations of 1-5% by weight. These additives usually also reduce viscosity, but at the same time improve tack. Starches and dextrins and the latex of natural or synthetic rubbers have also been used as additives to improve the flexibility of sodium silicate adhesives.

Role of Adhesive Strength in Bonding Silicate to Substrate

The shear strength of sodium silicate adhesives is sufficient for many applications. Extenders in the form of sodium chloride, clay, or talc are occasionally added to:

• Adjust viscosity
• Reduce penetration, and
• Even further reduce material cost

However, higher strengths have been noted for thin, free films. The adhesive strength is often stronger than the substrate (e.g., paperboard, wood, etc.) to which it is attached. Bond strength on the order of 300 psi is possible on maple blocks. Strength values as high as 1500 psi have been reported on some metal substrates and up to 1000 psi on glass substrates.⁴

Maintain Temperature Resistance by Adding Clays & Organic Fillers

The softening point of most sodium silicates is above 650°C. Resistance to even higher temperatures can be achieved by adding clay or other inorganic fillers to the formulation. These sodium silicate formulations will provide exceptional high-temperature resistant coatings for metals or glass and good binders for refractory cement.

Make your selection process easy by getting in-depth knowledge about fillers and extenders, their effects, applications and formulation examples.

Sodium Silicate Adhesives – Formulations and Applications

Sodium silicates adhesive perform most effectively on clean, hydrophilic surfaces. Thus, they are capable of bonding to many porous products, such as paper, wood, and cement, and even to many non-porous substrates, such as metals and glass.

In bonding to metal substrates, the silicate adhesives must wet the substrate well. Thus, a surfactant is often incorporated into the sodium silicate solution. Up to 2% by weight of materials such as sulfonated castor oil, fatty acids, rosin soaps, or naphthalene sulfonic acids have been used.³

Generally, the limiting factor in bonding to metal is the inflexibility of the silicate bond and the difference in thermal expansion coefficient between the silicate and the metal substrate. Thin silicate films have proven best in these applications since they are relatively more flexible than thick coatings.
However, liquid silicates are ideal for porous substrates since impregnation and mechanical interlocking primarily determine adhesion. These substrates can also be laminated to metal films with sodium silicate adhesives. Sodium silicate's major application in the paper conversion field is the production of tubes, cores, and fiber drums. Another popular application of sodium silicate adhesive is to seal corrugated containers before shipping or storing them.

Sodium silicate solutions are widely used for making many kinds of cement. These include cements for acid-proof construction, refractory applications, and binding thermal insulating materials. Cements made with sodium silicates are used for lining and laying refractory units, preparing foundry molds and cores, laying brick in sulfite digester for chemical wood pulps, and the construction of acid-proof masonry. Cements for stoves, chimneys, furnaces, coke ovens, spark plugs and for binding metal to glass and porcelain are often made using sodium silicate-based cements.


Sodium silicate solutions have also been used as an impregnant for metal castings that have a tendency to leave microscopic voids and capillaries at grain boundaries. They are especially useful in sealing the porosity in sand castings. They remain popular in this application due to their excellent performance, low cost, and safety and health properties associated with the material.

Most sodium silicate impregnating sealants are modified with additives to improve the sealing performance. Typical additives are:

• Iron, copper, or aluminum powder
• Kaolin clay
• Powdered graphite
• Powdered vermiculite
• Thickeners and suspending agents

Most sealing processes use vacuum treatment to insure sealing efficiency and reduced treating times.

Use of Sodium Silicate as an Additive to Latex-Based Adhesives

Soluble silicate can be incorporated as an additive into many latex-based adhesives. The sodium silicate becomes part of the total binder solid concentration. The advantages of sodium silicate additions to the formulation include:

• Significant cost savings
• Increased flame retardancy
• More stable formulations
• Increased bond strength
• Increased resistance to heat, light, oxidation, and microbes
• Generally compatible with alkaline (pH above 8.0) aqueous emulsions

Sodium silicates are also employed to adjust the rheology and coating properties of certain latex systems. They are generally compatible with the following polymeric latex materials:

• Styrene-butadiene
• Polystyrene
• Neoprene
• Polyvinyl chloride
• Polyvinyl acetate
• Acrylonitrile copolymer
• Acrylic polymer and copolymers

Environmental and Safety Information

Sodium silicates are made from two of the world's simplest elements – sand and alkali. Thus, sodium silicate adhesives are generally considered to be non-toxic and environmentally harmless.

Depending on their degree of alkalinity, soluble silicate may irritate or burn the skin and eyes on contact. The supplier's instructions for handling and the instructions on the Material Safety Datasheet must be observed. Sodium silicates are completely inorganic and, therefore, do not present hazards of explosion or flammability.

Sodium Silicate Grades for Adhesive Applications

View all the commercially available sodium silicate grades for adhesives, analyze technical data of each product, get technical assistance or request samples.