Damage tolerant laminates with reduced shrinkage

Besides the significantly improved fracture toughness, further important properties of fiber composites can be improved, e.g. inter-laminar shear strength, fiber adhesion and reduced shrinkage. 

Product overview
Improvements to properties
How it works

Product overview [back]

Technical data (no specification)

Exclusively tailored, customer-specific products are also available for special applications.

To modify an existing system, part of the epoxy resin is replaced by Albipox® 1000 or Albipox® 1005 (see also application remarks below). If blending is not possible, the ready-to-use Albipox® 3001 can be employed. 

Improvements to properties [back]

Epoxy resins have a substantial disadvantage: Their brittleness. This disadvantage can be more than compensated by an elastomer modification (so-called "toughening" or impact resistance modification). In contrast to an elastification, the elongation at break of the cured modified resin normally remain under 10 %.

The toughening of epoxy resins proves to be difficult, however. Thus, for example, the use of flexible hardeners or the addition of non-reactive flexibilizers significantly impairs a number of important properties such as tensile strength and modulus, thermal and chemical resistance as well as thermodimensional stability.

These negative effects can be avoided by toughening with copolymers based on reactive elastomers. However, the pure liquid elastomers are only slightly miscible with epoxy resins, if at all.

The different Albipox® grades are reaction products between epoxy resins and an elastomeric copolymer. Hereby, an epoxy resin is reacted with an excess amount of the reactive liquid elastomer. After the reaction, the elastomer molecules are epoxy functional and will be chemically bonded to the resin matrix during curing.
 
Albipox® products are miscible with all epoxy resins in any ratio.

Albipox® products can be used by epoxy resin formulators like a modular system. There are no limitations in respect to the resins and hardeners that can be used.

Figure 1 shows the effect of such a resin modification on the laminate.

Figure 1:  Improvement in the laminate properties through the use of toughened epoxy resins

As the glass transition temperature of the liquid rubber used lies in the range  -40 °C to -50 °C, the significantly improved properties are also found at these lower temperatures.

An additional advantage is the improved processability of the modified laminates, thereby avoiding splintering on mechanical finishing. The shrinkage is also reduced, as the rubber domains can absorb the internal stresses arising during curing.

How it works [back]

During curing, a phase separation of the elastomeric parts occurs regardless the chemical nature of the hardener and the curing temperatures. This results in finely dispersed rubber domains which are homogenously distributed across the resin. As can be seen in Figure 2, the domain size typically is in the range between 0.2 – 4 µm.

For the most part, the rubber domains consist of the relatively long molecules of the elastomer used, and are chemically bonded to the matrix via their epoxy groups at the phase boundary. If a force is now applied to the cured resin system, it can be dissipated uniformly in all directions when encountering a rubber domain.

Figure 2:  Scanning electron microscope images of a rubber-modified epoxy resin

If a tear has already occurred, it is prevented from further tearing: The elastomer particles stretch perpendicular to the direction of tear and are not torn out, as they are bonded chemically to the matrix. Figure 2 shows the finely distributed elastomer particles in the epoxy matrix (see also Figure 3).

Figure 3: Schematic representation of the rubber domain under stress

For further details, please contact our application specialists.