The strategic use of fillers in plastics has long been a key practice to tailor properties such as stiffness, strength, or cost. However, this approach may introduce trade-offs, impacting properties like tensile or impact strength. The intricate dance of properties in filled plastics is heavily influenced by the filler’s size, shape, and its ability to adhere to the polymer matrix.
The Starch Dilemma: Balancing Act in Polymer Enhancement
When starch joins the polymer party without specific interactions, it often triggers a decline in tensile and impact properties, reminiscent of challenges seen in other composites. The culprit is the high energy at the surface of starch granules, creating a less-than-ideal bond with the hydrophobic polymer matrix. This results in a weaker material, courtesy of a reduced load-bearing cross-section.
Navigating the Origins: 1960s and 1970s Exploration
The late 1960s and early 1970s witnessed a surge in interest in using starch as a filler in plastics and rubber. Independent research in England and the US Department of Agriculture’s Northern Regional Research Center explored the incorporation of granular starch into various plastics, including low-density polyethylene (LDPE), polyurethanes, and poly(vinyl chloride) (PVC). The goal was to infuse films with paper-like characteristics.
Biodegradability Concerns: Starch Steps into the Spotlight
Amidst rising concerns about plastic disposal in the 1970s, researchers delved into the potential of starch in creating biodegradable materials. Blending starch with synthetic resin revealed a trade-off, as it reduced material strength and stretchiness but increased susceptibility to microbial attack. Although the PVC matrix remained resistant, the starch filler was anticipated to erode over time.
Evolution in the 1980s: Starch Returns to the Biodegradable Scene
The 1980s witnessed a resurgence of interest in incorporating starch into biodegradable plastics. Researchers aimed to maintain optimal properties while ensuring robust adhesion between starch granules and the plastic matrix. Modifying starch with n-octenyl succinate (NOS) emerged as a game-changer, enhancing properties in linear low-density PE films. NOS-treated starch exhibited superior properties and a slower hydrolysis rate compared to untreated starch.
Copolymers and Coextrusion: Refined Approaches in Starch-Filled Materials
To refine the properties of starch-filled polyolefin materials, researchers explored the use of copolymers in blends with polyethylene (PE). Copolymers of ethylene with methyl acrylate, ethyl acrylate, or butyl acrylate showcased the potential to improve PE films, allowing for higher starch content. Coextrusion of starch-containing films with outer layers featuring oxidative prodegradants emerged as another approach, enabling access to the inner starch-containing layer upon disposal.
Reactive Extrusion and Ionomers: Advancing the Bond Between Starch and Matrix
Innovative processes like “reactive extrusion” leveraged low molecular weight coupling agents, such as maleic anhydride and methacrylic anhydride, to enhance the connection between starch and polyethylene. Treating starch granules with ethylene–acrylic acid ionomers showcased promising results, enhancing the properties of compression-molded PE-starch materials.
Bridging with Biodegradable Polymers: Starch Joins Forces with PHAs
In the pursuit of sustainable solutions, starch found an ally in poly(hydroxy alkanoates) (PHAs), biodegradable polymers produced by microorganisms. Blending starch with PHAs offered a cost-effective approach to creating biodegradable composites. While the addition of starch accelerated the hydrolysis rate, resulting in increased porosity and matrix collapse, coatings like poly(ethylene oxide) (PEO) emerged to enhance mechanical properties.
Future Horizons: Blends and Beyond
The journey of starch in plastics continues to evolve, with researchers exploring blends with copolymers of polyethylene terephthalate (PET) and composites with cellulose esters. Compostable products like fibers and films with starch contents up to 80% by weight have been claimed. Mayer et al. pioneered composites of starch with cellulose esters, and research on starch composites with poly(vinyl alcohol), poly(lactic acid), and polycaprolactone holds promise.
A Delicate Balance: Starch and Polycarbonate Resins
Starch’s addition to polycarbonate resins, even in minute quantities (up to 1% by weight), proves invaluable. It facilitates the creation of films with a low static coefficient of friction, high light transmission, and low haze.
As starch continues to weave its way into the intricate world of plastics, the quest for sustainable, high-performance materials persists. The synergy between starch and polymers unfolds as a journey of innovation, each chapter revealing new possibilities for a more sustainable and adaptable future.