Functions And Processing Methods of Organic Nano-Montmorillonite in The Field of Polymers

Functions and Processing Methods of Organic Nano-Montmorillonite in the Field of Polymers

Introduction to Organic Nano-Montmorillonite

1. Structure of Montmorillonite
Montmorillonite is a natural layered hydrated aluminosilicate mineral. It has a 2:1 crystal structure composed of two layers of silicon-oxygen tetrahedra sandwiching a layer of aluminum-oxygen octahedra. Each sheet layer has a thickness of about 1 nm, with length and width of about 100–1000 nm, and an interlayer spacing of about 1–1.5 nm, making it a natural two-dimensional nanomaterial.
Chemical formula: (Na,Ca)₀.₃₃(Al,Mg)₂Si₄O₁₀₂·nH₂O

2. Inorganic Montmorillonite (Aqueous Bentonite)
The main component of montmorillonite is montmorillonite mineral, with a content generally between 40%–65% in naturally mined ores. Through mining, drying, crushing, and screening of the raw ore, a montmorillonite base material is obtained. This is then dispersed into a slurry, followed by hydrocyclone desanding, centrifugal purification, sodium modification, filtration, drying, and deagglomeration to produce high-purity sodium-based montmorillonite (bentonite).
Typically, montmorillonite particles consist of negatively charged silicate nanosheets that are stacked together due to electrostatic forces. Even under fully hydrated and dispersed conditions, montmorillonite particles are still composed of dozens of stacked sheets. Therefore, inorganic montmorillonite cannot achieve individual nanosheet dispersion in polymer matrices, resulting in low functionality and limited applicability.

3. Organic Nano-Montmorillonite
Due to the negative charge on the montmorillonite crystal layers, cations such as Ca²⁺, Mg²⁺, K⁺, and Na⁺ can be absorbed and exchanged with other organic cations, giving montmorillonite good ion-exchange capacity. Through cation exchange, montmorillonite is intercalated with commonly used quaternary ammonium salt intercalation modifiers such as octadecyltrimethylammonium chloride (1831), octadecyldimethylammonium chloride (1827), hexadecyltrimethylammonium chloride (1631), hexadecylbenzyldimethylammonium chloride (1627), dodecylbenzyldimethylammonium chloride (1227), and distearyldimethylammonium chloride (2HT-75, SM-95).
This intercalation increases the interlayer spacing to 2–4 nm, reduces the material's surface free energy, and changes the surface from hydrophilic to hydrophobic, resulting in organic nano-montmorillonite. This improves its compatibility with polymer matrices, enabling the formation of new types of montmorillonite/polymer nanocomposites, which significantly enhance mechanical properties, thermal stability, gas barrier properties, antimicrobial resistance, UV aging resistance, flame retardancy, and more.

4. Functional Role of Organic Nano-Montmorillonite in Polymers
The nanostructure and morphological characteristics of organic nano-montmorillonite differ from other two-dimensional or three-dimensional inorganic nanoparticles, thereby imparting excellent mechanical, thermal, functional, and other physical properties to polymer/montmorillonite composites.
Existing practical results show that the mechanical properties of polymer/montmorillonite nanocomposites are significantly improved. For example, tensile strength and flexural strength increase by 20%–50%, modulus increases by 1–2 times, friction coefficient and wear resistance improve by 1 time, heat deflection temperature increases by 80–90°C for crystalline polymers (such as PA) and 10–30°C for amorphous polymers, thermal expansion coefficient decreases by about 40%, moisture absorption rate decreases by 50%, dimensional stability improves by 2–5 times, and permeability to water vapor, O₂, CO₂, and UV light is reduced to 1/2–1/5.
During combustion, char formation occurs, significantly delaying the heat release rate and preventing melt dripping, thereby greatly improving flame retardancy. Melt flowability increases, molding shrinkage decreases, and processability improves. The density of the composite material is similar to that of the pure polymer, which is 20%–30% lower than that of polymers modified with conventional inorganic fillers. The material's transparency is also improved to varying degrees.
Therefore, polymer/montmorillonite nanocomposites have become a new generation of high-barrier packaging materials, high-strength lightweight engineering materials, high-flame-retardant insulating electrical materials, and fatigue-resistant high-performance materials.

5. Processing and Application Methods of Organic Nano-Montmorillonite
Montmorillonite must undergo two processes to achieve nanoscale dispersion in polymers. Only when the nanosilicate sheets are thoroughly and uniformly dispersed can their effectiveness be maximized.

Purified montmorillonite is converted into organic nano-montmorillonite through intercalation reaction.
The interlayer spacing of inorganic montmorillonite is about 1–1.5 nm, and its sheets are stacked due to electrostatic forces, making it impossible to disperse individual nanosheets. Only through intercalation reaction (generally requiring 30–40% intercalant) can the interlayer spacing be increased to 2–4 nm, converting it into organic nano-montmorillonite, reducing surface free energy, and changing the surface from hydrophilic to hydrophobic, thereby enabling better dispersion in polymers.

Intercalated montmorillonite must undergo twin-screw shearing to achieve complete nanoscale dispersion.
To achieve optimal dispersion and exfoliation of organic montmorillonite, it is recommended to use a co-rotating twin-screw extruder or BUSS kneader when compounding with thermoplastics.
During extrusion compounding, it is recommended to select a screw configuration with a high length-to-diameter ratio (>40 L/D) and high-dispersive mixing elements. To avoid compaction, it is best to add the organic montmorillonite via a side-feeding device into the already molten polymer.
During dispersion in the twin-screw extruder, care should be taken not to "over-shear" the organic montmorillonite to avoid agglomeration. It is best to use dispersive mixing screw elements. Excessive heat can lead to performance degradation and odor formation. Overheating may also cause smoke at the die, and excessively high shear rates in the extruder can lead to re-agglomeration and reduced performance.
To achieve better dispersion, it is recommended that cable material manufacturers, barrier packaging producers, and heat-resistant modification companies prepare 30–50% organic montmorillonite masterbatches for blending with plastic pellets. Masterbatches can be produced by first internal mixing followed by twin-screw dispersion, or by side-feeding dispersion, with appropriate addition of graft compatibilizers and dispersing aids.