Adhesion Promoters for Polymers

How to Improve Adhesion of Polymers?

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 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.

Based on the mode of action, adhesion promoters are of two types:

When adhesion promoters are used to increase the compatibility of two immiscible polymers, they are called Compatibilizers (mode of action).
When adhesion promoters are used to increase the adhesion between a polymeric system and a filler, they are called Coupling Agents (mode of action).

Chemically these are the same materials in both cases.

Adhesion promoters can be reactive or non-reactive. It will depend if they contain a functional group that can react with the substrates they can adhere to or not.

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.

Coupling Agents/Compatibilizers
Reactive	Non-reactive
Reactive compatibilizers/coupling agents will contain reactive groups They essentially chemically interact with the components of the mixture, form a covalent bond and this way reduce or eliminate the repelling effect of the components of the mixture. Reactive groups can be Carboxylic acid groups, Epoxy groups (e.g. glycidylmethacrylate, 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. Nonreactive Products are ethylene Copolymers of ethylene and acrylates (EMA, EEA, EBA) or terpolymers containing also Carbonmonoxide (CO) and/or Vinylacetate (VA).

The purpose of adhesion promoter is to act at the interface to increase the adhesion between two substrates through the reduction of the interfacial tension. They work add active chemical sites to the polymer which is to be enhanced.

Adhesion Promoter Examples

Let’s learn about individual mechanisms as well as chemistries in detail…

Coupling Agent Mechanism

A coupling agent, or better a polymeric coupling agent is a polymer that attaches an inorganic filler to the polymer matrix.

Typical fillers are:

Calcium Carbonate
Glass fibers
Talc, or
Flame retardants such as ATH (Aluminum Trihydrate) or Mg(OH)₂ (Magnesium Hydroxide)

The purpose of adding fillers is either to:

Lower the cost of the polymer (CaCO₃, Talc),
Make it tougher or stiffer (glass fibers, CaCO₃) or,
Make it flame retardant so that it does not burn when it is ignited (ATH, Mg(OH)₂).

In any case, the addition of the filler will reduce the elongation at break, the flexibility and in many cases the toughness of the polymer because the fillers will be present up to very high levels. (E.g. ATH; 20% Polymer, 80% Filler).

The reason for this is that the fillers in most cases are not compatible with the polymers, this means that the fillers do not like the polymers very much and will even repel them.

In order to overcome the drawbacks of the addition of fillers, coupling agents must be added in order to reduce the repellency of the polymers and fillers respectively. Learn how to select appropriate fillers for your thermoplastics here »

As a result, the polymer will like the filler more, the filler will adhere better to the polymer matrix and the properties of the final mixture (e.g. elongation, flexibility, solubility of the filler in the polymer will be enhanced.

PP Homopolymer with 30% Glass Fiber

PP Homopolymer + 30% Glass Fiber (L), 5% Polymeric coupling agent added (R)
Such coupling agents have to be on one hand compatible with the polymer (ideally, they have to be the same chemistry of the polymer) and on the other hand they have to react/interact or better glue to the filler.

Let's explore more about the types of coupling agents:

Organofunctional Silanes as Coupling Agents

The organofunctional silanes were introduced over 50 years ago as coupling agents for fiberglass. They have subsequently proved equally successful in treating mineral fillers. Their success is due to their ability to react with a broad range of fillers and resins. They can be produced in readily dispersible form, with stably attached organic functionalities, and to the generally benign nature of the silicon atom, both from toxicity and polymer degradation aspects.

Silanes have the generic structure:

Y-R-Si-X₃

Where,

X is a hydrolyzable alkoxy group (methoxy, ethoxy or acetoxy) and
Y an organofunctional group (amino-, vinyl-, epoxy-, methacryl- etc.) attached to the silicon by an alkyl bridge, R.

The alkoxy groups reacts with the surface groups of many inorganic fillers. They first react with water to produce the silane triol, releasing alcohol as a by-product. The silanol groups then condense with oxide or hydroxyl groups on the filler surface. Neighboring siloxane chains can interact further to produce a polysiloxane layer at the surface.

Coupling of a typical silane (gamma-aminopropyltrimethoxysilane) to a siliceous substrate

Silanes require active sites, preferably hydroxyl groups, on the filler surface for reaction to occur. They can therefore be used to treat all:

Silicate-type fillers
Inorganic metal oxides and hydroxides

Materials successfully treated with these coupling agents include:

ATH
Alumina
Chrome oxide
Hydrous and calcined clays
Glass fiber and beads
Magnesium hydroxides

Mica
Mineral wool
Oxide pigments
Minerals like quartz, silicas, talc, vermiculite and wollastonite
Titanium dioxide

However, silanes do not interact to any significant degree with the ubiquitous calcium carbonate, or with barium sulfate, carbon black or boron compounds, and cannot be used as coupling agents with these fillers.

Once coupled at the filler interface, the reactive Y component allows bonding to the polymer matrix by chemical reaction (grafting, addition, substitution) with active groups on the polymer and/or by physicochemical interactions. Y groups are selected to maximize compatibility with particular resin formulations.

For example:

Methacrylate-functional silanes are most used in unsaturated polyesters
While amino-functional silanes are more broadly applicable in polyamides and polycarbonates, as well as epoxys, urethanes and other systems

In general, the silanes are highly effective coupling agents for polar thermoplastics, thermosets and rubbers but have only a slight interaction with non-polar polymers such as polyolefins. However, silanes are sometimes used as surface modifiers for fillers in PP and PE, providing improved dispersion and reduced water absorption.

Silane coupling agents provide a strong, stable, water- and chemical-resistant bond between filler and resin, which otherwise would interact only weakly.

Benefits of silane treatment:

The superior bonding with the matrix typically:

enhances mechanical and electrical properties
reduces shrinkage
increases weather resistance
lessens or eliminates surface or internal defects

It provides a lower-cost composite with physical properties generally equivalent or superior to the base resin
It permits higher filler loadings
It provides processing advantages such as improved dispersion, improved wet-out between resin and filler, and better rheological (flow) characteristics during mixing and molding - lower mold pressure translates to less energy to compound or extrude.

Organotitanates as Coupling Agents

The organotitanates overcome many of the limitations of silanes as coupling agents for fillers. Like silanes they have four functional groups, but where silanes have only one pendant organic functional Y group, titanates have three.

Titanates provide more effective coupling to the resin

In addition, the mechanism by which they couple to inorganic surfaces differs, they are also suitable for carbonates, carbon black and other fillers that do not respond to silanes.

In addition to improving filler dispersion and enhancing the properties and processing of the composite as with silanes, titanate couplers also act as:

Plasticizers facilitating higher filler loadings, and
Catalysts for a number of reactions in the polymer matrix

Costs for titanate treatment fall in the same range as for silanes

Titanates have the generalized structure:

XO-Ti-(OY)₃

where,

XO- can be a monoalkoxy or neoalkoxy group capable of reacting with the inorganic substrate, and
-OY is the organofunctional fragment

The Y portion can typically contain several different groups to provide interaction with:

Polar and non-polar thermoplastics (e.g. benzyl, butyl) and thermosets (e.g. amino, methacryl)
As well as binder groups such as pyrophosphato or carboxyl that can introduce additional functions to the composite.

Unlike silanes they do not require the presence of water to react.

The titanates fall into several classes:

Monoalkoxy (e.g. isopropoxy) titanates
Neoalkoxy titanates
Chelate (for greater stability in wet environments),
Quat (water-soluble systems), coordinate and cycloheteroatom

They are available in liquid, powder and pellet forms.

As compared to monoalkoxy, the neoalkoxy titanates have a more complex but more thermally stable structure. They were developed for high-temperature applications (above 200°C in the absence of water) such as in-situ addition during thermoplastics compounding and the production of urethane composites. They react via a coordination mechanism with free protons on the filler surface generating no by-product or leaving group. (The full mechanism is not disclosed.)

Get inspired: Thermoplastics Composites Interface Optimization for High Heat Applications

Free protons, unlike the hydroxyl groups needed for silane reaction, are present on almost all three-dimensional particulates, which is claimed to make titanates more universally reactive.

The reaction with free protons generates an organic monomolecular layer at the inorganic surface - in contrast to the polymolecular layers typical with other coupling agents - which in combination with the chemical structure of the titanates creates novel substrate surface energy modifications and polymer phase interactions.

In addition to their higher thermal stability, neoalkoxy titanates offer somewhat enhanced final properties compared to monoalkoxy counterparts

As compared to silanes, titanates:

Are effective with carbonates, carbon black and other fillers (where silanes have no useful reaction)
Can also bond successfully with polypropylene (and other polyolefins) as well as PVC (silanes show little interaction with polyolefins)
Have no unusual handling issues or price barriers

The reactivity of the TiO bond can cause problems with discoloration in the presence of phenols in some instances, but this disadvantage would seem to be more than offset by the positive aspects of titanates as coupling agents. In terms of commercial usage, however, the titanate coupling agents appear to have some way to go to match up to their potential and significantly displace silanes.

Organozirconates Used as Coupling Agents

The chemical structure and applications of the alkoxy zirconates are completely analogous to those of the alkoxy titanates.

The zirconates' main advantage is their greater stability

Unlike titanates they neither discolor body producers in the presence of phenols (with the exception of nitrophenols), nor do they interact with hindered amines (HALS). In unfilled plastics, they also often improve UV stability compared to titanates, and the neoalkoxy variants can provide novel opportunities for coupling fluorinated polymers to metal substrates. Though the cost of manufacturing zirconates has decreased substantially since their introduction in 1986 they are still typically about twice as expensive as titanates.

The most recently developed organometallic coupling agents are the aluminates and zircoaluminates. These are conceptually similar to silanes and titanates and find limited use in highly specialized applications. The aluminates appear to be used primarily in adhesives.

Functionalized Polymers

The functionalized polymers are the most recently developed class of coupling agents.

The coupling concept here is to have substrate reactive groups on molecules of:

The host polymer itself, or
Another polymer compatible with it

This removes the problem of finding polymer reactive functionalities and is particularly attractive for thermoplastic polyolefins. The problem to date seems to have been in producing effective functionalized polymers. This is partly due to the prevalence of siliceous fillers in composite materials. These are most effectively bonded to with alkoxysilanes, but such groups are difficult and expensive to introduce into polymer chains.

The easiest materials to produce are probably acid functionalized polymers, especially those with grafted or copolymerized anhydride groups. Examples are:

Carboxylated polyethylene and polypropylene, and
Maleinized polybutadienes

All of these are commercial products with some application in filled composite materials. The main limitation of these acid functionalized additives is that they are most effective with basic or amphoteric substrates, while the majority of substrates where true coupling is required are siliceous in nature and generally not directly responsive to them. One option is to pre-treat the siliceous filler with an aminosilane, which can then react with the acid functionalized polymer to form an amide linkage. Where functionalized polymers can be used, the effects achieved are typically those of coupling (improved heat distortion temperature, strength, stiffness and abrasion resistance, for example).

Functionalized Polybutadienes - In functionalized polybutadienes, the unsaturated groups can take part in crosslinking processes with various elastomers and polymers such as polymethylmethacrylate. The substrate functionality is usually grafted acid anhydride (mostly maleic anhydride) at up to 25 wt%.

The maleinized polybutadienes (MPDB) are mainly used with calcium carbonates in elastomers. Polybutadienes can also be functionalized with alkoxysilyl groups for use with glass, clays and silicas.

Functionalized Polyolefins - In the case of the functionalized polyolefins, linking to the bulk polymer matrix is postulated as being achieved by entanglement or co-crystallization. Acid and anhydride functionalized PP, PE and EVA forms are available.

Their main current use appears to be with glass fiber (in conjunction with aminosilanes) and mica in polyolefin-based composites. In addition to carbonates and metal hydroxides, these products are also claimed to react with the surface of talc and of cellulosic fillers.

Compatibilization Mechanism of Incompatible Polymers

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

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

We can distinguish 3 categories of compatibilizers:

Compatibilization by Block Copolymers

The principle of compatibilization by block or grafted copolymers is shown in figure below. 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.

Compatibilization by Block Copolymers

Like a surfactant, block-copolymers have also the tendency to create miscelles. The amount of compatibilizer is generally high (sometimes more than 5%).

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

Compatibilization by 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.

Mechanism of Action by Reactive Functional Groups

The advantages are:

Adjustable reactivity
High efficiency
Generally cheaper than block copolymers

Maleated Polymers

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

Anhydride groups can react with amine groups, epoxy groups and eventually alcohol groups. Figure 8 shows the example of reaction between a maleated polymer and -NH2 end groups of polyamides or Nylon 6,6 in order to compatibilize PA/polyolefin blends.

Maleated 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 shows in figure below.

Compatibilization by 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 on the left illustrates the mechanism of action.

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A Comprehensive Guide on Adhesion Promoters for Polymers