Modifying polymers for improved toughness

Toughening (or impact modification) can serve a range of needs:

1. General purpose toughening — low levels of modification, such as to avoid conditioning or reduce brittle failures of molded polyamide parts. Toughening achieves reasonable impact strength at room temperature, but does not address low-temperature (<0°C) requirements. Applications typically require only low levels (<10%) of nonreactive modifiers such as DuPont™ Surlyn® ionomers, Fusabond® AEB 560D, or Elvaloy® AC.
2. Low-temperature toughening for medium performance applications — low-temperature flexibility and resistance to break, such as required for many applications in the appliance area. Modifier levels are typically between 5-15%, and usually call for reactive modifiers such as ionomer-terpolymers (Surlyn® 9320, 9020), Fusabond® or Fusabond® AEB 560D.
3. Supertoughness — delivering enough impact strength to avoid failure from high-speed impacts at low temperatures (-30 to -40°C). Typically requires high levels (20-25%) of reactive impact modifier with low Tg, such as Fusabond® or Surlyn® terpolymers with very high termonomer content (e.g., Surlyn® 9320).

For general purpose toughening, Elvaloy® AC or Fusabond® AEB 560D can be used in a variety of polymers, in particular Elvaloy® AC does not provide low temperature impact toughness, but offers wide compatibility and thermal stability even at the process conditions of high-temperature engineering polymers. Thus it can be universally applied in a variety of polymers to provide a low level of toughness improvement and flexibility. At low levels (<5wt% in polyester), Elvaloy® AC can even be more efficient than reactive modifiers such as Elvaloy® due to a better dispersion in the polymer matrix.

EMA-based acrylate copolymers with comonomer content below 25%, such as Elvaloy® 1224 AC, comply with U.S. FDA regulations for direct food contact.

For PVC, and for ABS in particular, Elvaloy® EnBACO and EVACO terpolymers can provide acceptable levels of impact modification while increasing fluidity of the polymers. These terpolymers also are used as polymeric plasticizers for PVC.

For higher levels of toughness, functionalized modifiers should be used.

Functionalized versus nonfunctionized modifiers

There are different types of impact modifiers: functionalized (reactive) and nonfunctionalized. Nonfunctionalized impact modifiers are mainly used in polyolefines such as PP or PE. Other polymers normally require functionalized modifiers.

Functionalization serves two purposes: first, to bond the impact modifier to the polymer matrix; and second, to modify the interfacial energy between polymer matrix and impact modifier for improved dispersion.

Different functionalized ethylene copolymers are available as impact modifiers:

Reactive modifiers for low-temperature toughening and supertough applications

Depending on the polymer matrix and end use, three reactive modifier types are available: Elvaloy® PTW (GMA modified polymers); Fusabond® MAH modified polymers; and Surlyn® ionomer resins.

Elvaloy® PTW is most functional in polyesters at levels above 5 wt% and can be used there to achieve surpertoughness in glass fiber and virgin polyester compounds. The GMA group provides excellent interaction with the polyester polymer. It can be used alone or in conjunction with Elvaloy® AC. Elvaloy® PTW also increases the viscosity of the final compound. It also functions in polyamides.

For toughening polyesters in food contact applications, Elvaloy® AC or Surlyn® should be used. Surlyn® resins offer better and finer distribution of the toughener phase in the matrix, which can provide a final product with a better translucency. Surlyn® also is a glass-clear polymer, imparting no negative influence on the color of the final product.

For polyester toughening, sodium-based Surlyn® grades such as Surlyn® 8920, 8320 and 8020 will be most suitable. Surlyn® 8920 adds benefits by increasing the crystallization speed of polyester and acting as cystallization promotor.

For polyamides, zinc-based Surlyn® grades such as 9020, 9520 and 9320 are generally preferred. However, Surlyn® will not lend itself to the design of really supertough polyamide products. For this purpose Fusabond® will be necessary. Here in particular Fusabond® MN493D and Fusabond® MF416D can be recommended for PA6 and PA66 respectively. For lower levels of toughness also Fusabond® AEB 560D will be suitable.

Fusabond® is the material of choice for most impact modication applications, including supertough polyamides when used at high enough levels (e.g., 25%wt). Fusabond® can also be used in glass fiber reinforced products, with lower levels required to achieve the same level of toughness.

Modifying PBT with Fusabond® MN493D shows advantages over Elvaloy® PTW at modification levels of 20wt% and higher. Using Fusabond® MN493D for polyester modification slightly increases fluidity, an advantage in particular for injection molding applications.

To aid in selecting a polymer modifier for a desired polymer and toughness goal, refer to the Selector Guide.