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Adhesion promoters

Adhesion promoters can be reactive or nonreactive. It will depend on whether they contain a functional group that can react with the substrates they can adhere to or not.

Compatibilizers and Coupling Agents
About compatibilizers we are talking mainly if the substrates or blend components are of polymeric nature; About coupling agents we are talking if one of the components is an inorganic component (filler, metal etc). These definitions are however not well established and many people are using the word compatibilizer or coupling agent for all kinds of applications.

Reactive compatibilizers/coupling agents will contain reactive groups. Reactive groups can be Carboxylic acid groups, Epoxy groups (e.g. glycidyl methacrylate, oxazoline), Maleic anhydride, or others.

Non reactive compatibilizers/coupling agents draw their functionality mainly from their polarity which is introduced by a comonomer. They then represent an intermediate polarity between the 2 components of a mixture or between adhering substrates and the adhesion is assured by Van der Waals forces.

When mixing polymers with other components, be it fillers or even other polymers these two or more components will not necessarily like each other. In most of the cases there will be a repelling force and there will be very poor or even no adhesion. This will occur while mixing or even in many cases also when trying to adhere such components. An analogy of the real world will be mixing of oil and water in your salad sauce. Without mixing the components will separate. An example in the polymer world will be PA and PE

In order to improve adhesion, adhesion promoters can be added. Adhesion promoters that are most easy to handle are polymeric adhesion promoters, which can also be called compatibilizers or coupling agents. They act as surfactants . For example detergent powder as surfactant will ‘compatibilize’ the dirt with the water in the washing machine and facilitate the washing cycle.

When adhesion promoters are used to increase the compatibility of two immiscible polymers, they are called COMPATIBILIZERS.

When adhesion promoters are used to increase the adhesion between a polymeric system and a filler, they are called COUPLING AGENTS.

Chemically these are the same materials in both cases.

The purpose of adhesion promoters

The purpose of adhesion promoters is to act at the interface to increase the adhesion between two substrates through the reduction of the interfacial tension.

When the adhesion promoter is used to increase adhesion between two incompatible polymers it is also called a Compatibilizer.

When the adhesion promoter is used to increase adhesion between a polymer and a filler it is called a Coupling agent.

Compatibilizers or Coupling agents can be reactive or non reactive. In the case that they are reactive they will essentially chemically interact with the components of the mixture, form a covalent bond and this way reduce or entirely eliminate the repelling effect of the components of the mixture.

Compatibilization mechanism of incompatible polymers

The general principle of compatibilization is to reduce interfacial energy between two polymers in order to increase adhesion and also help dispersion. Generally, the addition of compatibilizers also allows finer dispersion, more regular and stable morphologies.

  • 30% PA6 / 70% LLDPE without compatibilizer
  • With the addition of 10% polymeric compatibilizer (Fusabond®)

The addition of compatibilizer generally increases the mechanical performance and surface aspect.

We can distinguish 3 categories of compatibilizers:

  • Block copolymers (non reactive)
  • Reactive functional copolymers (in-situ formation of block copolymer)
  • Non reactive polar copolymers (specific interaction by polarity)

Block copolymers

The principle of compatibilization by block or grafted copolymers is shown in figure 1. The compatibilizer acting in fact like a “surfactant” and will preferably migrate at the interface to reduce surface tension. Red blocks are compatible with polymer A (matrix) and Blue blocks are compatible with polymer B (dispersed phase). The consequence will be a better interfacial adhesion and better dispersion.
Figure1: compatibilization by Block Copolymers
Like a surfactant, block-copolymers have also the tendency to create micelles. The amount of compatibilizer is generally high (sometimes more than 5%).

Moreover, there are not too many block-copolymers commercially available for all polymers and they are generally expensive.

Reactive functional copolymers

The principle of action is to react at the interface to create “in-situ” a grafted block copolymer by reaction between functional groups of the different polymers. The functionalized copolymer is miscible with the matrix and can react with functional groups of the dispersed phase.

Figure 1 illustrates the mechanism of action.
The advantages are :

  • Adjustable reactivity
  • High efficiency
  • Generally cheaper than block copolymers

Malleated polymers
The reactive monomer is generally maleic anhydride. Malleated polymers are among the widest known family of functionalized polymers used as compatibilizer and adhesion promoters. They can be prepared directly by polymerization or by modification during compounding (this process is called reactive extrusion).

Anhydride groups can react with amine groups, epoxy groups and eventually alcohol groups. Figure 2 shows the example of reaction between a malleated polymer and -NH2 end groups of polyamides or Nylon 6,6 in order to compatibilize PA/polyolefin blends.

Malleated resins are also used to :

  • Increase adhesion of plastics to metal.
  • Improve cohesion between a polymer and fillers (ex ATH, wood, mica…)
  • Improve adhesion between polymer and glass fiber in thermoplastics and composites
  • Impact modification

Epoxidized Polymers
Epoxidized polymers are also commercially available. Generally they are mainly modified by glycidyl methacrylate. They are very reactive with NH2, anhydride, acid, alcohol groups. They are recommended to compatibilize polyesters (PET, PBT) and olefinic polymers or elastomers according a mechanism shown in figure 3.

Non reactive polar copolymers

The concept is to reduce interfacial tension and increase the adhesion by creation of a specific polar interaction like hydrogen bonding or Van der Waals forces. The compatibilizer has to be compatible with one phase (generally nonpolar) and has to create specific interactions with the other phase. Figure 1 illustrates the mechanism of action