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Rubber toughening commonly refers to the process of distributing a rubber microparticle secondary phase in a continuous phase polymer to tune the material’s mechanical properties. By toughening a polymer it is meant that the ability of a polymeric substance to absorb energy and plastically deform without fracture is increased. Considering the significant advantages in mechanical properties that rubber toughening offers, most major thermoplastics are available in rubber-toughening versions; for many engineering applications, material toughness is a deciding factor in final material selection.

The effects of disperse rubber nanoparticles are complex and differ across amorphous and crystalline polymeric systems. Rubber particles toughen a system by a variety of mechanisms such as when particulates concentrate stress causing cavitation or initiation of dissipating crazes. However the effects are seen as two-sided; excess rubber content and debonding between the rubber and polymer can reduce toughness.

The prevalence of a given mechanism is determined by many factors: the continuous polymer phase, the stress, loading speed, and ambient conditions. The action of a given mechanism in a toughened polymer can be studied in a high-resolution microscope. Little work has been done to study the effects of particle size and interfacial adhesion of rubber particles due to the numerous confounds in the science.

Current research focuses on how optimizing the secondary phase composition and dispersion affects mechanical properties of the blend including; fracture toughness, tensile properties, and glass transition temperature.

Toughening Mechanisms
There are many different theories to describe how the dispersed rubber phase may toughen a polymeric substance, most employ methods of dissipating energy throughout the matrix. In 1956, the microcrack theory became the first theory that tried to explain the toughening effect of a dispersed rubber phase in a polymer. Since then, many other theories have been proposed: shear-yielding theory, multiple-crazing theory, shear band/crazing interaction theory, and more recently critical ligament thickness, critical plastic area, voiding and cavitation, damage competition and others.

In the damage competition theory, there are two main assumptions, crazing, microcracks, and cavitation dominate in brittle systems while shearing dominates in the ductile systems. Systems that are in between brittle and ductile will show a combination of these. The damage competition theory defines the brittle-ductile transition as the point at which the opposite mechanism (shear or yield damage) appears in a system dominated by the other mechanism.

Shear yielding will result if rubber particles act as stress concentrators and initiate volume-expansion through crazing, debonding, and cavitation, to halt the formation of cracks. Overlapping stress fields from one particle to its neighbor will contribute to a growing shear-yielding region. The closer the particles are the more overlap and the larger shear-yielding region.

Cavitation is common in epoxy resins and other craze resistant toughened polymers, and is prerequisite to large-scale shearing in notched impact testing. Cavitation requires a minimum particle size of about 200 nanometers. The volume expansion associated with the formation and coalescence of the voids formed in cavitation is the main cause of toughening.

The dominant failure mechanism is usually observed directly using high resolution microscopy techniques. If cavitation or crazing is dominant, one could use tensile dilatometry to measure the extent of the mechanism by measuring volume strain. However, if multiple dilatational mechanisms are present, it is difficult to measure the separate contribution. Shear yielding is a constant volume process and cannot be measure with tensile dilatometry.

Relevant characteristics of the continuous phase
The mechanical failure characteristics of the pure polymeric continuous phase will strongly influence how rubber toughening occurs. When a polymer usually fails due to crazing, rubber toughening particles will act as craze initiators and when it usually fails by shear yielding, the rubber particles will initiate shear bands. It is also possible to having multiple mechanisms come into play if the polymer is prone to failing by multiple stresses equally. Polystyrene and styrene-acrylonitrile are brittle materials that are prone to craze failure while polycarbonate, polyamides, and polyethylene terephthalate (PET) are prone to shear yield failure.

Quality of the continuous phase is of importance. Recycled polyethylene terephthalate from manufactured soft drink bottles shows decreased molecular weight and shows resultant lower melt viscosity and poor mechanical properties. This is a motivation for research into appropriate toughening technique, to make good use of recycled materials.

Secondary phase concentration
Increasing the rubber concentration in a nanocomposite decreases the modulus and tensile strength. In one study, looking at PA6-EPDM blend, increasing the concentration of rubber up to 30 percent showed a linear relationship with the brittle-tough transition temperature, after which the toughness decreased. Suggesting that the toughening effect of adding rubber particles is limited to a critical concentration.

Rubber particle size
A material that is expected to fail by crazing is more likely to benefit from larger particles than a shear prone material, which would benefit from a smaller particle. In materials where crazing and yielding are comparable, a bimodal distribution of particle size may be useful for toughening. At fixed rubber concentrations, one can find that an optimal particle size is a function of the entanglement density of the polymer matrix. The neat polymer entanglement densities of PS, SAN, and PMMA are 0.056, 0.093, and 0.127 respectively. As entanglement density increases, the optimum particle size decreases linearly, ranging between 0.1 to 3 micrometers.

Temperature effects
Temperature has a direct effect on the fracture mechanics. At low temperatures, below the glass transition temperature of the rubber, the dispersed phase behaves like a glass rather than like a rubber that toughens the polymer. As a result, the polymer will fail as if the rubber were not there.

As temperature increases past the glass transition temperature, the rubber phase will increase the crack initiation energy. At this point the crack self-propagates due to the stored elastic energy in the material.

As temperature rises further past the glass transition of the rubber phase, the impact strength of a rubber-polymer composite still dramatically increases as crack propagation requires additional energy input.

Rubber selection and miscibility with continuous phase
In material selection it is important to look at the interaction between the matrix and the secondary phase. For example, crosslinking will promote high strength fibril formation that will toughen the material.

Carboxyl-terminated butadiene-acrylotnitrile (CTBN) is often used to toughen epoxies, but using CTBN alone will increase the toughness at the cost of stiffness and heat resistance. Using ultra-fine full-vulcanized powdered rubber (UFPR) researchers have been able to improve all three, toughness, stiffness, and heat resistance simultaneously. Resetting the stage for rubber toughening with particles smaller than previously thought to be effective.

In applications where high optical transparency is necessary, as is possible with poly(methyl methacrylate) and polycarbonate it is important to find a secondary phase that does not scatter light. To do so it is important to match refractive indices of both phases. Traditional rubber particles do not offer this quality. Modifying the surface of nanoparticles with polymers of comparable refractive indices is an interest of current research.

Epoxy resins
Epoxy resins are a highly useful class of materials used in engineering that can be used to make many types of elements. Some of their many uses include use for adhesives, fiber-reinforced composites, and electronics coatings. Their rigidity and low crack propagation resistance makes epoxies a candidate of interest for rubber toughening research. Research in toughening epoxies has been ongoing for three decades.

Improving polystyrene
Polystyrene generally has good qualities but its low impact resistance at low temperatures can be problematic. The generation of vast quantities of waste rubber from car tires has sparked interest in finding uses for this discarded material. The rubber can be turned into a fine powder, which can then be used as a toughening agent. However, poor miscibility between the waste rubber and polystyrene will cause more deleterious effects than good ones. This problem requires the use of a compatibilizer in order to reduce interfacial tension and prevent coalescence and ultimately to make rubber toughening of polystyrene effective. A polystyrene/styrene-butadiene copolymer acts to increase the adhesion between the dispersed and continuous phases.