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Dispersing and Coupling Agents for Filled Plastics

Fillers play an important role in plastics industry to achieve desired strength, dimensional stability and other application requirements. The performance of reinforced polymers depends not only on the filler characteristics, but also on its dispersion and the interactions formed at the polymer-filler interface.

Good filler dispersion in a polymer resin is crucial for achieving desired end application performance.

For effective dispersion of filler, first, it is important to understand about Dispersing and Coupling Agents. Find out how these additives operate at filler/polymer surface and what are the several benefits offered to formulations for improved performance and cost.

Why Dispersing and Coupling Agents are required?


Dispersing and Coupling Agents for Filled PlasticsFor decades, the incorporation of inorganic and organic fillers into a polymer matrix has been of significant industrial importance. These additives are added to develop new composites with desirable properties adapted to specific applications.

Pigments, fillers and other fine-particle solids can be incorporated more easily into plastics compositions via addition of dispersing agents and coupling agents.

Dispersants are used to wet, stabilize and increase loading of pigments and other fillers. They are commonly used in composites and nanocomposites for:

  • Proper dispersion and
  • Enhancing the level of interaction between the filler used and polymer matrix

Hence, dispersants and coupling agents help produce stable suspension which is long enough to be processed, without mechanical stirring and therefore reducing particle aggregates.

  • The energy needed for dispersion is reduced, and the homogeneity and storage-stability of the final products are improved
  • Furthermore, the color strength of pigments increases as a result of the fine dispersion, and their productivity therefore rises
  • At any concentrations, the dispersing and coupling agents can effectively enhance processability, mechanical and aesthetic properties of plastics

View all Commercial Available Grades and Suppliers in Polymer Additives Database

        » Dispersing Agents           » Coupling Agents

This polymer additives database is available to all, free of charge. You can filter down your options supplier, trade names, suitable polymer, applications, and many more dimensions.


Some of the important dispersing and coupling agents supplies include:


Benefits of Using Dispersants in Composites


  • Lower viscosity/improve polymer flow for improved productivity i.e. better mold filling, thinner walled parts (Graph 1)
  • Increase in impact strength
  • High yield strength & elongation to break
  • Dispersing pigments gives higher tinting strength
  • Enhance aesthetics such as gloss and surface finish
  • Blocked filler surface and cannot absorb other additives from the polymer

Effect of Dispersing Agents on Viscosity 

Source: Phantom Plastics

These enhanced properties make plastics material suitable for several applications like:

  • Packaging
  • Consumer Appliances
  • Electronics components
  • Automotive 
  • Aerospace

Mechanism of Action


Adhesion of the polymer to the filler surface is promoted by agents having atleast bifunctional properties. Here one group adheres to the surface of polymer and the other provides adhesion to the polymer. This effectively prevents flocculation of the filler particles.

The general structure of dispersant contains Anchor group (A) which has to chemically bond to the filler surface and Buffer group (B) which separate particles & hence, stops filler particles from sticking together. Dispersants adhere to the particles but have no strong or specific interactions with the surrounding polymer. Dispersants promote homogeneity and prevent defect sites via agglomeration.

Mechanism of Action 

Dispersants promote homogeneity and prevent defect sites via agglomeration
Coupling agents are bifunctional and effectively immobilize filler and polymer chains through A-B-C structure

While the structure of coupling agent features Anchor group (A), a Buffer/Bridge Group (B) & Couplant (C). Coupling agents are essentially short chain hydrocarbon molecules, on end of which is compatible or inter-reactive with the polymer whilst the other end is capable of reacting with the fiber or filler.

They adhere to the particulate matter but they must also adhere to the polymer through chemical bonds or through chain entanglements to provide strength.

Bonding of Dispersants and Couplers 

Dispersants must bond strongly to the filler surface only While Coupling agents bond strongly to filler and surrounding polymer matrix

Usually, the fiber or powder is treated with the coupling agent before incorporation into the polymer and becomes coated with a chemically bound surface layer of agent.

Dispersing & coupling Agents Selection Strategies for Filled Plastics


Types of Dispersants


Dispersing agents optimize the distribution of fillers in compounds. During dispersing process, these additives help to cover the newly formed surface of the aggregates and primary particles. Hence, they avoid agglomeration of particles.

The types of dispersants are similar to the types of coupling agents because in both cases, the chemistry needed to bond the additive to the filler surface is the same.

If the type of coupling agent is not chosen properly, then it will usually act as a dispersant instead. For example, if an organosilane is used that bonds to the filler but the chemistry or reaction conditions do not allow it to bond to the polymer, then it will disperse but not couple. Thus, additives that are labeled as coupling agents do not necessarily couple.

Several types of dispersing and coupling agents include:

  • Organosilanes
  • Organometallics (Titanates, Zirconates, Aluminates…)
  • Unsaturated Acids
  • Acid-Functionalized Polymers
  • Hyperdispersants or Polymeric Dispersants
  • Waxes (Polyethylene, polypropylene, micronized, metallocene…)
  • And so on….

There is no universal dispersing or coupling agent suitable for all filler-polymer systems. Some are most generally applications than other, whilst some are very specific.

Filled Thermoplastics Optimization



Silane Dispersing and Coupling Agents


During silane treatment of a filler or pigment a reaction takes place between the functional groups of the filler or pigment (such as OH groups) and the alkoxy groups of the silane to create a silane functionalized surface.

The surface of the filler can be functionalized to improve compatibility with the polymer matrix via specific interactions or chemical reaction between the polymer and silane organo functional group. The functionality of the silane should be chosen to match the polymer matrix.

Silane treatment also creates a "protective layer" preventing re-agglomeration of the particles.

The use of a silane dispersing agent in a filled thermoplastic, rubber or thermoset formulation results in a number of benefits which ultimately translate into easier processing and/or better product performances…Check Out these benefits in detail

The general formula of an organosilane shows two classes of functionality.

RnSiX(4-n)

Where:

  • R is an Organofunctional Group which enables the coupling agent to bond with organic resins and polymers
  • X is a hydrolyzable group, typically, alkoxy, acyloxy, amine, or chlorine

A silane coupling agent acts as a sort of intermediay which bonds filler surface to polymer matrix. Upon hydrolysis, a reactive silanol group is formed, which can condense with other silanol groups, e.g. those on the surface of silica, siliceous & other fillers (which have hydroxy group on their surface) to form siloxane linkages. This characteristic makes these coupling agents suitable to improve mechanical strength & hardness of composites, enhance adhesion for resin and surface modification.


Applied Plastics Engineering Handbook

Check Out More About Dispersing & Coupling Agents by Chris DeArmitt and Roger Rothon in Applied Plastics Engineering Handbook


Organometallic (Titanates, Zirconates, Aluminates…) Dispersing and Coupling Agents


Organometallic dispersing and coupling agents work as molecular bridges at the interface between inorganic fillers (such as CaCO3, BaSO4, graphite, talc, carbon black, silica and metal oxides) and polymers & mostly employed to improve the flexibility.

Titanate is much popular than both zirconate and aluminate in organometallic coupling agent family.

Organometalic coupling agents mostly bond like covalent, Van der Waals forces and hydrogen bonding. They are suitable for both thermoplastics and thermoset resins. As discussed above, these additives are effective on fillers like carbon black, graphite, barium sulfate and other fillers without hydroxy group.


Hyperdispersants or Polymeric Dispersants


Polymeric dispersants, also known as hyperdispersants, are polymeric materials designed to offer significantly higher levels of performance. They are typically higher molecular weight, and this means they may contain multiple anchoring groups and stabilization chains. They can be tailored to work across a broader range of pigments or fillers and in different media.

In comparison with standard dispersant solutions such as stearate and waxes, hyperdispersants offer unique advantages and performance for thermoplastics, thermosets and liquid colorants due to their specific structure and characteristics.

  • Higher quality – high color strength
  • Improved flexibility
  • Increased productivity and improved processing conditions
Lubrizol Solplus™ Polymeric Dispersants for Plastics


Best, 2nd Best and 3rd Best Dispersant for Various Fillers


There are so many different types of fillers available and each one requires different dispersing and coupling agents. These fillers include:

  • Carbon black
  • Natural and precipitated calcium carbonate
  • Precipitated silica 
  • Aluminum hydroxide 
  • Talc 
  • Kaolin and 
  • … Many more

Filler Type Best Dispersant Second Best Third Best
Calcium Carbonate Succinic anhydride Carboxylic acid Primary amine
Dolomite Sulfonic acid Carboxylic acid Succinic anhydride
Magnesium Hydroxide Succinic anhydride Trichlorosilane Carboxylic acid
Mica Primary amine Trichlorosilane Sulfonic acid
Talc Trichlorosilane - -
Silica Trichlorosilane Sulfonic acid Succinic anhydride
Wollastonite Primary amine Succinic anhydride Carboxylic acid
Titanium dioxide Succinic anhydride Carboxylic acid Trichlorosilane


Commercially Available Dispersant Grades for Polymers






References:
  • tel.archives-ouvertes.fr/tel-01537585/document 
  • Handbook of Fillers for Plastics
  • www.polymerjournals.com/pdfdownload/847467.pdf 
  • phantomplastics.com/ 
  • www.gelest.com/wp-content/uploads/Goods-PDF-brochures-couplingagents.pdf
  • www.shinetsusilicone-global.com/catalog/pdf/SilaneCouplingAgents_e.pdf 
  • Engineering Polymers by R.W. Dyson

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