APPLICATION INFORMATIONCOUPLING AGENTS: STRENGTHENING THE WEAKEST LINKIN THE COMPOSITES CHAIN
TS-A 12

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Coupling agents: strengthening the weakest link in the composites chain

Contents
03 Introduction
04 Performance-limiting interfaces
05	General structure and working
mechanism of coupling agents06	Quartz-filled unsaturated polyester
system as an example for a casting
application
07 Carbon fiber reinforced vinyl ester
system as an example for closed
mold applications

08 Glass fiber reinforced epoxy as anexample for ambient cure applications
10 Overview of suitable productcombinations
11 Conclusion

03

Introduction
The increasing importance of wind energy and electromobility
inevitably results in a greater need for lightweight solutions
that simultaneously guarantee high mechanical stability andreliability. Coupling agents build a strong bridge between resin
and reinforcement, enabling more durable and long-lastingmaterials while increasing design freedom.

For additional information
on additives and technical
topics, please contact us:
Thermosets.BYK@altana.com

Note
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open in Adobe Acrobat.

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Coupling agents: strengthening the weakest link in the composites chain

Performance-limiting interfaces
Composites based on thermoset materials have become an
essential part of our daily life. They find application inestablished goods such as sanitary ware or sporting
equipment. Moreover, they are also accompanying
technology changes by giving access to crucial components,for example in wind turbines and electric vehicles.
In many cases, composite materials are comprised of acombination of a cross-linked organic polymer (matrix) andinorganic materials, such as particulate fillers or fibers.
While the inorganic material can also have the function ofa cost-effective filler, it often improves surface quality, flame
retardancy, or abrasion resistance of the final composite.
Fibrous materials (such as glass fibers or carbon fibers) help
to significantly enhance the mechanical properties of thecomposite, such as tensile, flexural, and shear strength,
while contributing to a lower weight of the final partcompared to non-composite materials. Without thisinorganic reinforcement, lightweight materials required for
reduced emissions, fuel efficiency, and renewable energy
would not be possible.

Despite the beneficial nature of the inorganic components
in the composite, the boundary between the matrix (resin)
and the inorganic part may turn out to be the weakest linkof the overall system, limiting its maximum performance.
With respect to this boundary effect, though it may bepossible to improve single components, such as resins,fillers, or fibers, there is a certain limit to the possible
optimization of each individual component. Taking into
consideration the relevance of the organic/inorganic
boundary, an improved link between the resin matrix andthe dispersed phase (fillers, fibers) can result in improved
mechanical properties. An easy way to achieve the desired
coupling are additives that have the ability to cross-link
with the resin and form a bond to the filler or fiber.
These additives are known as coupling agents.

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Coupling agents: strengthening the weakest link in the composites chain

General structure and working mechanism of coupling agents
Coupling agents provide a connecting function between theorganic resin matrix and particulate filler or fiber materials
by establishing strong bonds to the surface of the filler/fiberand cross-linking into the matrix during the thermoset
curing process. This strong connection between the organicand the inorganic components of the formulation results inincreased mechanical properties of the composite material
prepared. To become efficient, the coupling agent needs tobe employed in a system where the weakest boundary of
the composite is the interface between filler/fiber and resin(G. 01). This means a coupling agent cannot prove value if
the material failure is caused by cohesive fracture of theresin or the inorganic component.

In general, a coupling agent contains at least two types offunctional groups:

General working principle of a coupling
agent in a composite material

Working mechanism of a coupling agent
in a thermoset system

Resin(matrix)

Coupling
agent(1) a reactive group which is able to copolymerize with thematrix during the curing process, and(2) a highly affinic group which is able to adhere strongly tothe surface of the particle or the fiber.
For example, in a free-radical curing system (such as anunsaturated polyester or acrylate system), the coupling
agent likely comprises polymerizable double bonds. If thethermoset system is cured via a two-component mechanism(2-pack system, such as most epoxy or polyurethanesystems), the coupling agent may be reactive towards theresin or the hardener.

The general mode of action is shown in G. 02, in which thecomposite system is shown before curing and after curing
is completed. Surface affinic groups of the coupling agent
adhere to the surface of the inorganic component(s) used.
At the same time, the additive has cross-linked into thematrix. Since the coupling agent works as a bridge builder
between matrix and dispersed phase, it is apparent that thisadditive must be designed for a certain combination of
matrix and inorganic phase: There is no universal coupling
agent that will provide benefit for all combinations.

Inorganic
reinforcement(filler, fiber)
Coupling agent
G. 01

Particleor fiberResin (prepolymer)  

Particleor fiberReactive group of the resin  Hardener (optional)  

Surface affinic group  Resin-reactive group

G. 02

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Coupling agents: strengthening the weakest link in the composites chain

Quartz-filled unsaturated polyester system
as an example for a casting application
Typical applications for quartz-filled unsaturated polyester
systems include polymer concrete, artificial marble, andengineered stone. In these systems, it is desirable to increase
the mechanical performance for a given part or to reduce
the thickness of the construction part while having the samelevel of mechanical properties (material savings, cost reduction).
Both can be achieved by using a coupling agent. As can beseen in G. 03, the flexural strength can significantly beincreased using a dosage as low as 0.2 wt.% (based onfiller). Once the ideal amount of connecting points has beenpromoted by the coupling agent, a further increase of its
dosage cannot result in higher mechanical properties.
This effect can also be visualized via microscopic imaging(G. 04): In the presence of the coupling agent, cavities areminimized, and resin and filler are connected to each otherin a robust manner.

Electron microscopic image of a quartzfilled unsaturated polyester resin after
exposure to hot water in the absence andpresence of BYK-C 8000 coupling agent

Relative improvement of the flexural strength in a quartz-filled unsaturated
polyester system as a function of the dosage level of the coupling agent

Flexural strength increase (%)

In addition to mechanical improvement, the systems
enhanced with the coupling agent can provide additional
benefits beyond increase of mechanical properties. Suchadditional improvements can comprise increased hot waterand chemical resistance. This prolongs the lifetime of aconstruction part and minimizes the formation of blisters
when exposed to humidity and temperature.

60Without additive

50
40
10 µm

3020

With BYK-C 8000

10000,050,10,150,20,250,30,35

Dosage of coupling agent BYK-C 8000 (weight %, based on filler)

G. 03

10 µm

G. 04

07

Coupling agents: strengthening the weakest link in the composites chain

Carbon fiber reinforced vinyl ester system
as an example for closed mold applications
Improvement of mechanical properties
in a chopped carbon fiber reinforced
vinyl ester system

MPa

Flexural strength (MPa)450400

Tensile strength (MPa)+ 18 %

350

Carbon fiber reinforced plastics are used when the need forhigh material strength meets the demand for lightweightproperties of the composite parts. Examples for theseapplications comprise aerospace and general automotive
industry, electromobility, and wind energy applications. In
these industries, composite production is frequently carriedout using, for example, RTM processes, prepregs, pultrusion,or SMC technology in combination with various resin
matrix systems.

300
250

Appropriate coupling agents can provide benefits for
flexural and tensile strength both for chopped fibers andfabrics (G. 05).

+ 30 %

200150In analogy to the particulate filler, the mechanicalimprovement of the fiber reinforced system was visualized byelectron microscopy (G. 06): In the absence of a coupling
agent, only a few resin leftovers can be detected on thefiber surface after material failure; this clearly underlines apoor adhesion between fiber and matrix. In contrast, in thepresence of a suitable coupling agent, significant amountsof resin residues can be seen on the fiber surface. In somecases, when using the coupling agent, the adhesion
between carbon fiber and matrix is so strong that a fiber
fracture occurs (G. 07).

100500Control

BYK-C 8013Control

BYK-C 8013
G. 05

Electron microscopic image of a material
failure resulting in a fiber fracture
instead of an adhesive fracture

The coupling effectNo coupling effect

Adhesive failure

Coupling effect

Cohesive failure in the matrix
G. 06

G. 07

08

Coupling agents: strengthening the weakest link in the composites chain

Glass fiber reinforced epoxy as an example for ambient cure applications

The addition of a coupling agent can also result in improved
fatigue properties. The final part is more reliable and has alonger life span. Coupling agents therefore not only improve
the performance of the overall system but contributesignificantly to its sustainability.

Increase of transverse tensile strength in various glass
fiber reinforced epoxy composites
Transverse tensile strength (MPa)

Mechanical properties of composites are generally improved
by application of fibers. The most commonly used fibrousmaterial to reinforce composites are glass fibers. Examples
for such glass fiber reinforced systems can be found in windenergy, marine, sports and leisure, sanitary, construction,and building parts. As shown for carbon fiber reinforced
materials, the transverse tensile strength can be significantly
improved by the addition of an appropriate coupling agent.
Mechanical improvements found in different resin systems
are shown in G. 08.

6050+ 50 %+ 38 %+ 45 %

403020100
Resin 1 (control)

Resin 1 + BYK-C 8001

Resin 2 (control)

Resin 2 + BYK-C 8001

Resin 3 (control)

Resin 3 + BYK-C 8001
G. 08

Transverse tensile strength of a glass fiber reinforced epoxy resin
before and after aging at elevated temperature
Transverse tensile strength (MPa)As an additional effect to the general improvement, glass
fiber sizings deteriorate with age, leading to an inferior
performance of the final system. Again, the coupling agent
can assist to compensate this aging process, yielding
composites with similar properties such as those based on
virgin fiber material. This effect helps to improve processrobustness for the composite manufacturer thanks to higher
reliability and lower batch variations of the mechanicalproperties of the finished part as well as the final
construction (G. 09).

5045403530
25201510500 days resin (control)

0 days resin + BYK-C 8001

1 week at 40 °C resin (control)

1 week at 40 °C resin + BYK-C 8001
G. 09

Coupling agents: strengthening the weakest link in the composites chain / Glass fiber reinforced epoxy as an example for ambient cure applications

Coupling agents can also improve the mechanical propertiesof radically cured systems. The incorporation of coupling
agents allows the system to be adjusted according to theperformance requirements, even if they exceed the typical
properties of the material (G. 10). Material properties ofstandard UP systems are shifted towards the performanceof VE systems. The mechanical properties of VE systems
can be further optimized.

Transverse tensile strength of various radically cured systems inpresence and absence of a coupling agent
Transverse tensile strength
first crack (MPa)

0960
50
4030
20
100
Unsaturated polyester

Unsaturated polyester+ 2 % BYK-C 8003

Vinyl ester

Vinyl ester+ 2 % BYK-C 8003
G. 10

10Coupling agents: strengthening the weakest link in the composites chain

Overview of suitable product combinations
There is a multitude of resin systems with different curing
mechanisms and characteristics. Most of these can becombined with various fillers and/or fibers. This results in avast number of material compositions and therefore in
numerous possible boundary combinations. To ensure agood bonding link for all these reinforcement/matrix
combinations, a variety of coupling agents represents thetoolbox. The following table summarizes suitable couplingagents, which can be employed in different systems toprovide improved mechanical properties and reduced
quality fluctuation.

Recommended coupling agents for different combinations of resins and reinforcements
Mechanism

Resin systems

SiO2

Radical curing

UP, VE

BYK-C 8000

ATH

Glass fiber

Carbon fiber

BYK-C 8003

For MgO thickenedsystems:
BYK-C 8013
For isocyanate
thickened systems:
BYK-C 8014

Polyaddition

Acrylate

BYK-C 8000Epoxy

BYK-C 8001**BYK-C 8001 is certified according to DNV-GL TA-DNVGL-CP-0089-07147.

BYK-C 8002

BYK-C 8003

BYK-C 8013

BYK-C 8001*
T. 01

11Coupling agents: strengthening the weakest link in the composites chain

Conclusion
A key task for composite manufacturers is the optimizationof mechanical properties. This is often achieved by theenhancement of individual components. Despite the option
to improve single materials, such as resins, fillers, or fibers,
there is a certain limit with respect to the influence of every
specific component on the performance of the final
composite part. In an alternative approach, the link between
the resin matrix and a dispersed phase providing
reinforcement (fillers, fibers) can be optimized. An easy way
to achieve the desired performance increase is the use ofadditives which have the ability to cross-link with the resinand form a bond to the filler or fiber at the same time.
These additives are known as coupling agents; theirstructure needs to be selected according to the chemistryof the matrix and dispersed phase. The coupling agents of
the BYK-C 8000 family solve this task by creating a strong
bridge between resin and reinforcement, giving access tomore durable and long-lasting materials and allowing
higher freedom of design.
Coupling agents must be tailored for a certain matrix/
reinforcement combination by the adjustment of functionalgroups. Their improving properties can be essential toboost cost/performance levels of certain application systems.
Therefore, the use of coupling agents provides accessto both innovative technical solutions and cost advantages
as well.

Coupling agents can be used in a large variety of systems,
among those epoxy resins, unsaturated polyester resins,
vinyl ester resins, and acrylate resins in combination withdifferent kinds of inorganic reinforcements, such as quartz,
aluminium trihydroxide, glass fibers, and carbon fibers.
To cover the versatility of possible matrix/reinforcement
combinations, a variety of coupling agents is available.
Among those, the first coupling agent has recently been
qualified according to the Germanisher Lloyd certification(DNV GL); this allows end users to make use of the coupling
agent without affecting existing certifications.
A multitude of material properties have been demonstrated
to be improved using coupling agents. In general, tensile
strength, flexural strength, elongation at break, and fatigue
are in focus.
Coupling agents can be used at different stages in theprocess, such as (i) during the production of reinforcements(surface treatment of fillers and fibers), (ii) a part of theresin formulation (resin additive use), and (iii) upon
composite production (e.g. as second sizing or directly
before or during application). They can therefore be
employed by all members of the value chain to differentiate
from the state of the art, including manufacturers of fillers
and fibers, resin houses, and composite producers.

12/2022
TS-A 12BYK-Chemie GmbH
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GermanyTel +49 281 670-0
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