In this issue topic, we are going to cover biopolymer composites. Generally, most of the plastic materials, bioplastics or not, fulfill the definition of being a composite. Partly, this is owed to the general definition of composites. As soon as at least two phases are present within a condensed matter that are physically and chemically distinct, one can talk of a material composite. One of the most known composites is wood. At the same time, wood or cellulosic fibers are a major common composite material. However, we are going to start more general and we will not go into too much commercial detail.
To a large part, the domain of fillers is governed by geometrical and surface interaction considerations. As far as the surface interactions are concerned, it is required to have a strong adhesion between the carrier resin and the composite material. If the compatibility is not given by nature because of a too-high free surface energy difference (e.g. CaCO3 + PP) the filler is usually coated. This can be seen as like a tie layer in multilayer film extrusion. One possibility is coatings of the filler with fatty acids to span this bridge. For other combinations of materials, other compatibilizers need to be employed.
Coming to the geometry, the shape and hardness of a filler material. Please note that we are now going to consider the shapes of the filler on a micrometer scale looking at individual particles. Many geometries are available: starting from spheres (aluminum silicates) going over cubes (CaCO3) and platelets (talcum), whiskers (wollastonite) to short-cut (glass, natural fibers), long-cut (glass, natural fibers), and endless fibers (carbon fibers). Provided the surface adhesion with the carrier resin is sufficiently good the shape of the filler is decisive in how it influences the material properties. Spherical, or almost spherical fillers (cubic) tend to introduce flexibility and improved impact resistance to otherwise brittle carrier resins. This is why our Carbomax®Bio CaCO3 masterbatches ideally complement the brittleness of our neat PLA resins. A reduction of the brittleness comes along with an increase in elongation at break and a decrease in tensile strength. The reason for this is illustrated in Figure 8. By introducing particles, the continuous phase between the dispersed phase becomes thinner and is, thus, not able to take up less load compared to the non-filled material. The distance “X” is significantly smaller than the distance “Y”. It is important to note that this effect is only observed with filler with an aspect ratio of around 1. Another important aspect is that the finer the particles and the better distributed they are, the less the material is “weakened”.
To realize this Plastrans introduced the specially treated thermoplastic starch NuPlastiQ, its Bioblend masterbatches, and the Carbomax®Bio masterbatches into our portfolio, allowing even and easy incorporation of starch and CaCO3 into carrier resins.
If the aspect ratio of the filler is >>1 the material characteristics change significantly and very often after processing even anisotropic mechanical characteristics are observed. As shown in Figure 9 the shear forces during processing lead to an orientation of the filler into the direction of the shear forces.
This increases the strength of the material in the direction of the imposed force and it increases the flexural modulus of the part. If forces are occurring in a perpendicular direction the tensile strength will be reduced, just as the impact resistance compared to the direction of orientation. An example of such a composite would be glass fiber-reinforced PA5.6, which is available with around 47 % biobased content and is a good substitute for the fossil PA6.6.
The third mentioned shape from above is platelets, which is represented by the talcum example. Talcum platelets have two great characteristics. The platelets themselves are built up by sheets that can slide past each other making talcum less hard compared to CaCO3 and introducing abrasion resistance to the composite. At the same time, talcum functions as an efficient crystallization seed for polyesters like PLA (see blog post about Crystallization Kinetics of semicrystalline Polyesters). Plastrans can support our customers also with this feature by offering the Talcomax®Bio, a talcum masterbatch, to the bioplastics industry.
Finally, the reinforcing material can also chemically or biologically influence the final composite. Considering the thermoplastic starch NuPlastiQ, it supports the formation of biofilms on and inside the composite material (starch blend) and while biodegrading itself it creates beneficial conditions for carrier resins like PLA or PBAT to be biodegraded, as well. Similar observations were made with PLA and PHAs.
Dr. Rudi Eswein
Director of Sustainability
