New Toughening Agents (Micro & Nano) for Structural Adhesives

TAGS:  Epoxy Adhesives      Acrylic Adhesives    

Toughened structural adhesives generally have two distinct phases:

• The larger phase is the base resin
• The minor phase consists of small (micro- and nano-sized) distributed entities

A variety of toughening agents have been used to modify structural adhesives without significantly affecting other properties of the base resin, as flexibilizers or reactive liquid elastomers do.

Modern structural adhesive formulations benefit from tougheners, which operate on a completely different mechanism than flexibilizers. Flexibilized adhesives are molecular blends of polymers in a single-phase system, whereas toughened structural adhesives have discrete particles (sizes on the order of 10^-6 to 10^-9 meters, minor phase) embedded in the resin (larger phase) matrix of the adhesive.

The addition of the second phase significantly improves fracture toughness and impact strength by providing crack pinning and stress distribution mechanisms. Improvements in tensile-shear and peel strength are also usually noted.

A toughening network can be accomplished in several ways, including the addition of reactive elastomeric resins (e.g. CTBN) that form particles during cure¹ and preformed rubber or plastics particles². However, a more productive approach is the addition of micro- to nano-sized particles into the adhesive formulation.

Generally, these modifiers can be classified into several types:

• Inorganic nanoparticles
• Organic nanoparticles (primarily core-shell elastomeric particles)
• Resinous particles (plastic or elastomeric)
• Synergistic blends of nano-sized particles with other toughening agents

Due to dispersing and distribution problems associated with blending nano-sized particles directly into a base polymer, commercial admixtures have been developed where a concentrated amount of the particles are blended into a compatible base resin.

A nanoparticle is usually defined as a particle of matter that is between 1 and 100 nanometers (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions.


Since the typical diameter of an atom is between 0.15 and 0.6 nm, a large fraction of the nanoparticle's material lies within a few atomic diameters from its surface. Therefore, the properties of that surface layer will dominate over those of the bulk material. This effect is particularly strong for nanoparticles dispersed in a medium of different composition since the interactions between the two materials at their interface becomes significant.

Let's review the new micro- and nano-sized tougheners that are commercially available or being developed for use in structural adhesive formulations. This article attempts to describe the value provided by these newer nano-sized toughening agents, the various types that are available to the formulator, and the mechanism by which they work. Starting formulations, processing guidelines, and resulting properties will also be provided here.

#1 Inorganic Nanoparticles

Certain inorganic particulate nano-fillers have been considered as toughening agents for structural adhesives. Surface-modified silica nanoparticles have been commercially available for a number of years.

• These are characterized by a particle size of around 20 nm and with a very narrow particle size distribution.
• They are available as discrete particles and as mixtures in epoxy resins, acrylic monomers, and other common resins used in structural adhesives.

Applications for nano-silica filled epoxy adhesives are dominant in the transportation, electrical and electronic, defense industries.

These functional fillers can be used in adhesive formulations to improve mechanical properties without an increase in viscosity or a decrease in properties, such as:

• Adhesive strength
• Glass transition temperature, Tg

Since these particles are sized in the nano-range, they can also be used in formulating transparent adhesives and coatings.

a. Nano-silica

The use of silicate-based fillers as toughening agents in structural adhesives has been of major interest. The nano-silica particles are amply small, which benefits the dispensing process as they are not filtered out during the process. Nano-silica has been at the forefront of toughened epoxy adhesive formulations for many years⁴.

Most notably silica nanoparticles offer:

• Significant improved fatigue performance
• Increased toughness (fracture energy, fracture toughness, impact resistance)
• Reduced coefficient of thermal expansion
• Improved modulus
• Higher lap-shear and peel strength
• Insignificant or slightly improved effects on Tg of the base resin⁵

The figure below shows the results of the wedge impact test (DIN EN ISO 11343) of a 2K epoxy adhesive on oil-treated automotive steel as the substrate. The impact resistance is increased by an additional 40 - 150%, and lap shear strength and peel strength are improved as well.



Nano-silica toughening is achieved by a combination of plastic shear-yield bonding and debonding of the matrix from the silica nanoparticles, followed by plastic void-growth. The nanoparticles are themselves able to improve the properties of an epoxy resin by increasing the glass transition temperature by about 10°C.

Epoxy Formulations Using Nano-silica Particles

Nanopox (Evonik) products are epoxy resins containing amorphous silica nanoparticles with a spherical shape. Supplied as concentrates, they can be used like standard epoxy resins and be blended with all standard epoxy resins. No special dispersing or mixing equipment is necessary. These resins have a relatively low viscosity (4,000 to 60,000 cps at 25°C), although they contain 40 weight percent silica nanoparticles. In many adhesive formulations, addition levels of 5-15 weight percent Nanopox (which equal 2-6% nano-silica) are sufficient.

Example epoxy formulations using nano-silica particles are shown in the table below:

Epoxy Formulation	Original Formulation	5% SiO2 Formulation	10% SiO2 Formulation	15% SiO2 Formulation
Standard DGEBA epoxy resin (EEW 185)	100	87.5	75	62.5
Nanopox A 410 (EEW 295)	--	12.5	25	37.5
Total mass, all parts	100	100	100	100
Resulting epoxy equivalent weight	185	194	204	215

Formulation Examples for Epoxy Resin Component Modified with Nanopox Product (All Values are Parts by Weight) 

The amount of hardener is reduced in proportion to the new epoxy equivalent weight (EEW) of the resin blend. For some non-stochiometric hardeners like dicyandiamide, the formulator need not change the hardener amount.

Fillers and other ingredients of the formulation can be used as normal. If the viscosity of Nanopox products turns out to be too high for the formulating procedure, one can preheat the product to 60° - 80°C, and the viscosity will be lowered below 10,000 cps.

The rigidity of thermosetting structural acrylic adhesives is similar to that of cured epoxy resins. As a result, they also require the addition of a toughening agent for certain applications.

The introduction of nano-silica into a two-part acrylic has been shown to provide enhancement in the shear and tensile strength of the epoxy composite joints⁷. The shear and tensile strengths of the adhesive joints increased with addition of the nano-silica filler content up to 1.5% by weight, after which they decreased with the addition of more filler. However, the addition of nanoparticles caused a slight reduction in the peel strength of the joints.

• Tg values of the adhesives rose with increasing the nano-filler
• Equilibrium water contact angle was decreased for adhesives containing nanoparticles

Similar toughening mechanisms have been used with photo-curable polymer networks.⁸

b. Carbon Nanotubes and Nanofibers

Carbon nanotubes (CNTs) are tubes made of carbon with diameters typically measured in nanometers.

• They can exhibit remarkable electrical conductivity.
• They also have exceptional tensile strength and thermal conductivity because of their nano-structure and strength of the bonds between carbon atoms.
• In addition, CNTs can be chemically modified.

Carbon nanotubes also often denote multi-wall carbon nanotubes (MWCNTs) consisting of nested single-wall carbon nanotubes (See figure below).



Carbon nanotubes have been found to play a highly effective bridging function across the fracture surface of the bonded joints. It has been found that a maximum improvement of about 60% in the adhesive fracture energy was obtained when the adhesive is toughened with 0.3% by weight of multi-wall carbon nanotubes⁹.

The effect of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on Mode I adhesive fracture energy of double cantilever beam joints of carbon fiber-reinforced laminates bonded with an epoxy adhesive has been studied. It was observed that the presence of carbon nanofillers in the epoxy adhesive results in a significant increase in the propagation value of Mode I adhesive fracture energy with CNTs producing the largest increase.

The toughening mechanisms, analyzed using scanning electron microscopy (SEM), for the two nano-filler systems differed:

• Pull-out with CNFs
• Pull-out and crack bridging with CNTs

Due to the poor dispersion ability and weak interaction between the nanomaterials and the matrix resins, the strengthening effect of nano-fillers is still relatively limited. The presence of van der Waals forces causes the dispersion process to be very ineffective.

• The implementation of surface functionalization has been found to overcome this difficulty.¹⁰
• Sonification during mixing has also been found to provide higher fracture toughness values.¹¹

c. Nano-clay

Nano-clays are nanoparticles of layered mineral silicates. Plate-like montmorillonite (MMT) is the most common nano-clay used in materials applications. Montmorillonite consists of ~1 nm thick aluminosilicate layers surface-substituted with metal cations and stacked in ~10 µm-sized multi-layer stacks.

Depending on surface modification of the clay layers, montmorillonite can be dispersed in a polymer matrix to form polymer-clay nanocomposite. Within the nanocomposite individual nanometer-thick clay layers become fully separated to form plate-like nanoparticles with extremely high aspect ratio.

Addition of nano-clay particles to a typical tertiary amine cured polysulfide-modified epoxy adhesive leads to large increase in the single-lap shear strength of aluminum-aluminum joint.¹²

• The nano-clay particles are very well dispersed (near fully exfoliated) in the polymer matrix even at 8% by weight of nano-clay concentration.
• The addition of nano-clay particle results in a significant increase in the elongation at break values along with the marginal increase in the tensile strength.
• The nano-clay provides good ductility to the epoxy-polysulfide adhesive by forming very flexible interface, which leads to less brittle composite having good strength.
• The increase in work to break (Wb) values by the addition of nano-clay, represents the potential for dissipating a greater amount of energy during the bond rupture process of the lap shear test, and hence the adhesive strength increases.

In another study¹³, it was found that the addition of clay (up to 3% by weight) increased the tensile strength, flexural strength, impact strength, and the fracture toughness of epoxy resins. Above 3% by weight of clay produced a reverse effect.

• The epoxy resin used as the matrix was a bisphenol A diglycidyl ether-based resin cured with a polyaminoamide.
• The MMT clay used in this study was Nanomer 1.28E (Nanocor Company).

d. Other Inorganic Nanoparticles

Several other types of nanoparticles have been used and tested as tougheners and reinforcing agents in structural adhesives. These have not been commercialized to the extent as the types of nanoparticles discussed above.

Graphene Nano-fillers

Graphene is a crystalline allotrope of carbon with 2-dimensional properties. Its carbon atoms are densely packed in a regular atomic-scale chicken wire type of pattern (See figure below).



Among various nano-fillers, graphene has recently been employed as reinforcement in epoxy to enhance the fracture related properties of the produced epoxy–graphene nanocomposites.

• Graphene can significantly improve fracture toughness of epoxy at very low volume fraction by deflecting the advancing crack in the matrix.
• It is beneficial up to about 4% by weight and then the beneficial effect drops off with higher concentrations.

As with other nanoparticles, it has been observed that graphene size, weight fraction, surface modification, and dispersion mode have strong influence on the improvement in fracture toughness values.

Nano-scale Zirconium Oxide Nanoparticles

Nano-scale zirconia or zirconium oxide nanoparticles are typically 5-100 nanometers with specific surface area in the 25 - 50 m²/g range. Although, zirconium dioxide possesses outstanding properties, such as high strength, high fracture toughness, excellent wear resistance, high hardness, and excellent chemical resistance, few works can be found on epoxy / zirconia nanocomposites.

• Zirconia nanoparticles have been found to increase the glass transition temperature of epoxy resins up to a filler content of 1% by volume.
• In addition, tensile modulus, stress at break and fracture toughness of bulk adhesives samples were positively affected by the presence of an optimal amount of zirconia nanoparticles.¹⁴

Surface-Modified TiO₂ Nanoparticles

The role of surface-modified TiO₂ nanoparticles on the mechanical and thermal properties of rubber toughened epoxy nanocomposite has been studied.¹⁵

• Functionalized TiO₂ (fTiO₂) was mixed with carboxyl-terminated butadiene (CTBN) toughened epoxy by ultrasonication followed by the addition of curing agent.
• The loss in the modulus of epoxy due to addition of rubber toughener was offset by the addition of nanoparticles.
• One of the significant findings of the work was the observed trend in the fracture energy of the samples:
  
  
  TiO₂ filled CTBN toughened epoxy > TiO₂ filled CTBN toughened epoxy > TiO₂ filled epoxy > CTBN toughened epoxy > Epoxy resin

The findings show promising use of TiO₂ nanoparticles in conjunction with CTBN toughener for improving the toughness of epoxy resin without comprising the tensile strength.

#2 Organic Nanoparticles

a. Core-Shell Elastomeric Particles

Several insoluble, unreactive rubber modifiers, such as core-shell rubbers (CSR) have been used to modify epoxy and thermosetting acrylic systems. Core-shell polymers are structured composite particles consisting of at least two different components: one at the center as a core and surrounded by the second as a shell. This is shown in the figure below:



These preformed dispersions of insoluble rubbers are particularly useful in many adhesive applications. This is because the rubbery-phase volume of the final product is relatively insensitive to variations in curing conditions. The glass transition temperature does not decrease with increasing toughener content up to a certain level. In addition, the modulus of these modified systems can be varied independently of the Tg.¹⁶

Overcoming the Drawback of Reactive Liquid Elastomer

One of the drawbacks of reactive liquid elastomer toughening is the increase in viscosity. This cannot be tolerated in some formulations. But, by using core-shell elastomers as tougheners, the viscosity increase becomes minimal. The typical addition levels are on the order of 10% which results in a substantially improved toughness over:

• A broad temperature range
• Reduced shrinkage
• No or minimal loss of modulus, and Tg

Furthermore, the fatigue performance of the adhesive is increased significantly.

The glassy shell is introduced to the rubbery core to make handling and working with particles possible. In the absence of the glassy shell, rubber particles can easily stick together and lose their size and shape. Additionally, core-shell morphology allows varying the chemistry of the rubbery core and the glassy shell independently. The rubbery core in these modifiers is usually made of either acrylic or butadiene-based rubbers. The glassy shell is designed based on the chemical structure of the matrix and the desired interface.

With core-shell toughened structural adhesives, the particle size and the volume fraction of the second phase can be well controlled. Preformed core-shell particles can be used with fast curing adhesive systems, whereas reactive liquid elastomers cannot. This is because of the time that it takes for particles to form. The core-shell particles have a somewhat higher price than reactive liquid elastomers. But, this is off-set by their formulation advantages and consistent final properties. Core-shell modifiers have been successfully used in both 1K and 2K systems and are compatible with epoxy resins, acrylic resins, and a variety of curing agents.

Commercial Core-Shell Products for Structural Adhesives

There are several commercial core-shell products that can be used in structural adhesives, composites, and other products requiring toughening. The following are short descriptions of several of these products.

1. ALBIDUR (Evonik) –
• It is a product line consisting of a concentrated dispersion of high-performance core-shell reactive silicone particles (0.1-3.0 µm) in an epoxy resin base.
• Unsaturated polyester resins and vinyl ester resins can also be modified with ALBIDUR®.
• The silicone elastomer particles have a physically bonded organic shell which contains reactive groups.
• The typical addition levels are 10% and result in a substantially improved toughness over a broad temperature range with reduced shrinkage, no or minimal loss of modulus and Tg.


2. PARALOID™ EXL (Dow Chemical) –
• It is a product line of acrylic core-shell modifiers.
• It is claimed to be ideal for epoxy applications which require high impact strength at temperatures as low as -10°C as well as good heat and UV stability.
• Although not marketed specifically as a modifier for adhesives and coatings, Dow claims improved properties with epoxy and other engineering resins as molding compounds.
• PARALOID™ EXL-2300G is a powder form which consists of grains of agglomerated submicronic primary core-shell particles.


3. Zeon 351 (Nippon Zeon Chemicals) –
• It consists of a core believed to be an acrylate or methacrylate polymer having a glass transition of about -30°C or lower and a shell comprising an acrylate polymer or a methacrylate polymer having a glass transition temperature of about 70°C or higher. The product:
  
  • Provides good fracture toughness and compression after impact in epoxy adhesives and composites
  • Possesses good resistance to hot/wet conditions.


• Zeon also has a product line of DuoMod® modifiers which are designed for direct addition to resin systems without the need for solvents.
  • DuoMod® 5045 is pre-crosslinked and carboxyl functional ultra-fine NBR powder. It provides large increases in fracture toughness at loading levels less than 4% by weight.
  • Epoxy film adhesive formulations containing 15 parts per hundred of DuoMod® core-shell tougheners have been evaluated and found to provide improvements in Mode II fracture toughness.¹⁷


4. KaneAce™ MX (Kaneka) –
• This product line consists of concentrates comprised typically of 25% by weight of core-shell elastomers (50 nm) dispersed in various thermosetting resins.
• The liquids are easily poured after heating.
• Also, the individual core-shell domains remain uniformly dispersed during storage and formulating.
• KaneAce™ MX core-shell particles are compatible with typical cold, warm, and hot curing agents for adhesives, coatings, and composites.
• Epoxy adhesives modified with KaneAce™ MX 120 core-shell particles were evaluated for t-peel and lap-shear strength. Table below shows the observations for the same: 



Formulation Component	Control Formulation	MX Toughened Formulation
Part A:
DGEBA epoxy resin A	70	52
Epoxy resin B	20	20
Reactive diluents	10	10
KaneAce™ MX 120	--	24
Part B:
Catalyst	30	30
Properties	Value
Mix ratio Parts A/B	100/30	100/30
Core-shell concentration, %	4.4	4.4



Epoxy Structural Adhesive Formulations (Unmodified and Kane Ace MX Toughened)


KaneAce™ MX 120 is a 25% concentrate of core-shell rubber toughening agent in an unmodified epoxy resin based on DGEBA. The viscosity of KaneAce™ MX 120 is 22,000 cps at 50°C and its epoxy equivalent weight is 243 g/eq.

Figure below shows similar properties of KaneAce™ MX 120 modified DGEBA epoxy structural adhesives compared to CTBN modified adhesives.




Comparison of KaneAce™ core-shell rubber (CSR) modified versus unmodified DGEBA epoxy adhesive¹⁸



The core-shell modified adhesives also exhibited improved fracture toughness and durability without sacrificing Tg or other thermal properties.

Related Read: Additives & Polymers for High Temperature Structural Adhesives

b. Polyhedral-Oligomeric-Silsesquioxane (POSS)

POSS's are inorganic silica-like nanocages of 1.5 nm in size that have organic substituents (See figure below).



Inactive organic substituents make the POSS physically compatible with relevant polymers promoting dispersion in a polymer at a molecular level, while substituents that are reactive, promote curing or grafting reactions. There are two methods by which the silica-like cages can be incorporated into the polymer.

• In the first method, the silsesquioxane cages are mechanically dispersed in the organic matrix like a filler without covalent bonding.
• In another method, the silsesquioxane cage is functionalized and covalently linked to the polymer.

In both cases, combined strengthening (shear) and toughening (peel) is realized in addition to higher Tg. It should be emphasized that in the case of the reactive POSS functionalities, toughening is obtained at relatively low concentrations (4% by weight).¹⁹

#3 Nano-sized Resinous Particles

a. Thermoplastic Additives

Various kinds of engineering thermoplastics have also been studied as toughening agents for epoxy and other structural adhesive systems, such as:

• Polyether sulfone
• Polyether imide
• Polyaryl ether ketone, and
• Polyphenylene oxide

They are used either as granulated particles or as polymers dissolved in the liquid epoxy and later precipitated out as second phase particles. In comparison with rubber-modified systems, the use of tough thermoplastic polymers offers better improvement in fracture toughness for higher crosslink density epoxy systems.

Compared to the carboxy-terminated nitrile elastomer additives, the use of thermoplastics has primarily been focused on the aerospace industry, and in composites and coatings rather than adhesives. On a cost per pound basis, the two-phase nitrile additives offer the best combination of property improvement without negative impact.

Disadvantages of Thermoplastics as Tougheners

The thermoplastic additives, however, offer better high temperature performance, but they are more difficult to process as adhesives. As a result, the cost of these adhesives is generally higher than other toughening mechanisms.

• Poor miscibility of the thermoplastic resins
• Poor processability of the final product

Several research efforts have been directed at making a more miscible system. In an effort to toughen an epoxy resin by incorporating engineering thermoplastic units into the main chain, Kun-Soo Lee and coworkers developed epoxy resins containing ether ketone units.²⁰ These resins provided toughness without phase separation at elevated temperatures.

Low molecular weight polyphenylene ether (PPE) resins have also been developed that, unlike its high molecular weight counterparts, are readily miscible with epoxy resins and form low viscosity blends with them.²¹ The cured thermoset product exhibits improved thermal properties, as well as enhanced fracture toughness.

Advantages of Thermoplastics as Tougheners

As was discussed earlier, one drawback of nano-inorganic particles as nanofiller is their poor dispersion that significantly affects the properties of nanocomposites. In order to overcome the shortcomings, organic modification is needed which is complicated and costly.

Elastomeric nanoparticles with a size range of 100 nm in diameter and very high cross-linking degree on surface, have nano-scale effect in polymer modification just like nano-inorganic particles.

• Compared to nano-inorganic particles, they are much easier to disperse into the polymer matrix during blending without any modification. This is because shear force can transfer to the contact surface of the particles easily by making particles elongated.
• Moreover, elastomeric nanoparticles can also assist dispersion of other nanoparticles and ultra-fine additives in polymer matrix.²⁴

The results in one study revealed that for an epoxy adhesive, nano-sized particles can produce nearly 18 times more increase in fracture toughness than what can be achieved using micrometer-sized particles at the same volume fraction of 5% by volume. This huge improvement of adhesives’ fracture toughness by nano-sized particles is similar to that observed in bulk nanocomposites, indicating that the superior toughening mechanism of nano-sized elastomeric particles is equally effective in thin adhesives constrained by stiff adherends.²⁵

Commercially Available Thermoplastic Tougheners

Several micro- and nano-sized thermoplastic particles have been commercialized for use in structural adhesives, composites, and other products requiring toughening. The following are short descriptions of several of these products.

1. Fortegra (Dow Chemical) epoxy tougheners are suitable for any application where adhesion, corrosion or chemical resistance, and greater mechanical performance is required. The Fortegra product line is based on specially designed self-assembling block copolymers.
• By adding a low concentration (typically <5% by weight) to an epoxy resin, novel nano-scale morphologies can be formed in the liquid state and are for the most part preserved through the curing process.
• These morphologies give rise to significant improvements in toughness (resistance to crack propagation) without increasing the formulation viscosity and sacrificing modulus and Tg of these crosslinked epoxy thermoset systems.²²
• By using block copolymers processing concerns are alleviated because the nano-morphology develops spontaneously upon blending with the uncured resin. Commercial products include various blends with epoxy resins.


2. Virantage™ VW-10300 (Solvay) polyethersulfone is a non-functionalized, high temperature polymeric powder (Tg = 220°C). It has been used successfully in a variety of thermosetting resin systems, including:
   
   • Epoxies
   • Phenolics, and
   • Bismaleimides
Three product grades are available ranging in particle size from 45 µm to 500 µm. Their use to-date has primarily been in aerospace composites.


3. Nanostrength® M22N (Arkema) consists of a central block of poly(butyl acrylate) surrounded by two poly(methyl methacrylate) blocks. It is a self-assembling block copolymer which exhibits medium-high molecular weight and very high polarity with low-to-medium soft phase content.
   M22N additives offer advantages, such as:
   • Impact strength
   • Modulus
   • Glass Transition Temperature (Tg)
   • Chemical resistance
   • Fatigue resistance
   • Lap shear, and
   • Transparency
In DGEBA epoxy systems a high toughening effect has been noticed with a combination of M22N and silica nanoparticles.²³

#4 Combinations of Tougheners to Achieve Synergy

Structural epoxy adhesives toughened with reactive liquid rubbers (CTBN adducts) can be further improved by the addition of silica nanoparticles. This represents one of the latest toughening technologies for structural adhesives.

Formulations for 2K room temperature curing adhesives that contains both reactive elastomer (ATBN) and nano-silica particles are shown in the table below:

Formulation Component	Control	A	B	C	D	E
Part A:
DGEBA epoxy resin	100	96.25	92.5	85.0	70.0	0
Nanopox A (Evonik)	0	6.25	12.5	25.0	50.0	100.0
Part B:
Hypo ATBN 1300x16 (Emerald)	45.8	45.2	44.5	43.9	43.2	27.8
Polypox P 502 curing agent - a blend of n,n-dimethyl-1,3-diaminopropane and polyaminoamide (UPPC, Germany)	91.6	90.4	88.9	87.9	86.5	55.5
Properties	Value
Percent mass parts of SiO2 in total	0	1.05	2.1	4.1	8	21.8
Percent mass parts of ATBN in total	18.1	18.1	18.1	18.1	18.1	18.1
Glass transition temperature, °C	70	67	71	67	75	73
Lap-shear strength, MPa On untreated aluminum alloy On acid etched aluminum alloy	13.4 20.8	19.2 20.9	17.8 22.0	16.7 23.0	16.2 23.2	11.8 20.3
Fracture energy, J/m2	1200	1800	1800	2300	2000	1300
Roller peel, N/mm	3.1	5.1	5.5	4.6	3.8	2.8

As indicated by the resulting property data, the addition of low concentrations of nano-silica particles to a typical rubber-toughened adhesive leads:

• To a synergistic increase in the toughness of the adhesive, and
• Also, to increases in the glass transition temperature and the tensile-shear strength

A concentration of only about 1% to 8% by mass of such nanoparticles is needed to achieve significant improvements. Similar improvements have been noticed in single component heat cured epoxy formulations.

Several researchers have very recently investigated the potential synergistic effect of combining nano-silica particles with core-shell nanoparticles.^27,28 In general these early investigations found a positive effect of nano-silica alone on properties such as tensile strength, modulus, and fracture toughness. However, by adding core-shell particles to the nano-silica modified epoxy, these properties decreased even though the effect on Tg remained approximately constant. These studies suggest the need for additional research in the area of combining nano-silica and core shell elastomers in a structural adhesive formulation.



This article was originally published on May 31, 2017 and updated on May 12, 2020.³