Why Dispersing & Coupling Agents are Required?
Why Dispersing & Coupling Agents are Required?
For 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.
On one hand, dispersants are used to wet, stabilize and increase the loading of pigments and other fillers. While on the other hand, coupling agents are used to provide a stable bond between two otherwise nonbonding and incompatible surfaces. Addition of these additives lead to:
- Proper dispersion – The energy needed for dispersion is reduced
- Enhancing the level of interaction between the filler used and polymer matrix
- Homogeneity and storage-stability of the final products are improved
- The color strength of pigments increases as a result of the fine dispersion, and their productivity therefore rises
- Greater composite strength and longer service life
- At any concentration, the dispersing and coupling agents can effectively enhance processability, mechanical and aesthetic properties of plastics
Hence, dispersants and coupling agents help produce stable suspension which is long enough to be processed, without mechanical stirring and therefore reducing particle aggregates.
They are commonly used in composites, nanocomposites, reinforced and filled plastics. Some benefits of using dispersants in composites include:
- 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
- Higher tinting strength
- Enhance aesthetics such as gloss and surface finish
- Blocked filler surface and cannot absorb other additives from the polymer
Source: Phantom Plastics
These enhanced properties make plastics material suitable for several applications like:
- Packaging
- Consumer Appliances
- Electronics components
- Automotive
- Aerospace
Dispersing & Coupling Agents - Mechanism of Action
Dispersing & Coupling Agents - 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.
While the structure of coupling agent features:
- Anchor group (A),
- Buffer/Bridge Group (B) &
- Couplant (C)
Coupling agents are bifunctional and effectively immobilize filler and polymer chains through A-B-C structure. They 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.
| BASIC STRUCTURE OF DISPERSING AND COUPLING AGENTS |
Dispersant
Anchor – Buffer
Anchor (A) – Adheres to filler
Buffer (B) – Separates particles
|
Coupling agent
Anchor – Bridge – Coupler
Anchor (A) – Adheres to filler
Bridge (B) – Insert spacer group
Coupler (C) – Bonds to polymer via chemistry or entanglements
|
They adhere to the particulate matter but they must also adhere to the polymer through chemical bonds or through chain entanglements to provide strength.
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.
Types of Dispersants and Couplants
Types of Dispersants and Couplants
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.
Silane Dispersing and Coupling Agents
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.
Organometallic Dispersing and Coupling Agents
Polymeric Dispersants and Couplants
Polymeric Dispersants and Couplants
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
Best in Class Dispersants for Fillers
There are so many different types of fillers available and each one requires different dispersing and coupling agents. These fillers include:
| 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 |
Learn how to meet your yield strength, processability, cost and other targets without adding additives or fillers in excess in your filled plastics. Also, make the right selection of your dispersing & coupling agents in order to deliver their best performance.
Access Various Dispersing Agents Suitable for Plastics
View a wide range of dispersing agents available today used for plastics, analyze technical data of each product, get technical assistance or request samples.
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