Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 65

of starch with Quat 188 in aqueous sodium hydroxide at 90 °C were achieved in a twin-
screw extruder (Carr, 1994a), compared to typical reaction efficiencies of 85% for batch
reactions. A similar process has been scaled-up and compared to a computational model
(Berzin et al ., 2007a, 2007b). Oxidized starches having high water solubility were prepared
by reactive extrusion of starch, hydrogen peroxide and ferrous-cupric sulfate catalyst (Wing
and Willett, 1997). Graft copolymers of starch with acrylonitrile using ceric ammonium
nitrate initiator were prepared by twin-screw extrusion; and the conversion of 74% was
similar to that of a batch reaction (Yoon et al . 1994). Graft copolymers of starch with
acrylamide with conversions of 95% and graft efficiencies of 75% were prepared by twin-
screw extrusion using ammonium persulfate as initiator (Finkenstadt and Willett, 2005).
There are numerous examples of starch esters prepared by reactive extrusion, and there have
been many studies on the preparation of starch phosphates by reactive extrusion of dry or
semi-dry mixtures of starch with orthophosphates or metaphosphates (Landerito and Wang,
2005 ; O'Brien et al ., 2009 ; San Martin-Martinez et al ., 2004 ; Seker et al ., 2003 ). Fatty acid
esters of starch, including starch acetates, starch succinates, starch alkenylsuccinates, starch
maleates and starch palmitate, have been prepared by extrusion of starch with anhydrides,
sometimes in the presence of sodium hydroxide or sodium bicarbonate (NaHCO 3 ) as catalyst
(Hannah and Fang, 2002 ; Murua-Pagola et al ., 2009 ; Raquez et al ., 2008 ; Rudnik and
Zukowska, 2004 ; Tomasik et al ., 1995 ; Wang et al ., 1997 ).
Extrusion of starch and starch-polymer blends into biodegradable plastics has been the
subject of extensive research and development over the past twenty years. Worldwide,
starch-based plastics are produced commercially at the rate of about 300 million pounds/
year, and these products represent about 0.1% of the total plastics market (Shen et al ., 2009 ).
Reasons for the current interest in these starch-based materials have been the low cost of
starch, the replacement of dwindling petroleum-based resources with renewable feedstocks,
the biodegradability of these polymers (to alleviate problems associated with the disposal of
plastic waste), and the creation of new markets for farm commodities (Gross and Kalra,
2002). Interest in starch as a plastic has continued despite some problems with the use of
starch as a starting material, for example, poor water resistance, brittleness and low strength.
Examples of some commercially successful starch-based products include starch foam
packing peanuts and protective foam sheet, starch foam plates, and starch-polymer blends
for compost bags and molded articles. Starch-based plastic materials have been the subject
of several reviews (Averous, 2004 ; Averous and Halley, 2009 ; Bastioli, 2001 ; Chivrac et al .,
2009 ; Dubois and Narayan, 2003 ; Janssen and Moscicki, 2009 ; Kalambur and Rizvi, 2006 ;
Shogren et al ., 1993 ; Yu et al ., 2006 ).
Starch foams for protective packaging are typically prepared by extrusion of starch with
15-20% moisture, a nucleating agent such as talc, and 5-20% of an added polymer (Nabar
et al ., 2006 ; Pushpadass et al ., 2008 ; Sjoqvist et al ., 2009 ; Willett and Shogren, 2002 ).
Extrusion temperatures are typically 120-180°C. Water serves as both plasticizer and
blowing agent. The low moisture content ensures that the glass transition temperature of the
starch foam is sufficiently high to prevent collapse after expansion and cooling. The added
polymer serves to enhance expansion and also, in the case of certain hydrophobic polymers
such as PLA and PHBV, to increase water resistance. The latter occurs as a result of
migration of the hydrophobic polymer to the surface of the foam (Willett and Shogren,
2002). Foams made from extruded high amylose starches generally have better expansion
and resilience, due to increased entanglement of the long linear polymer chains. Starch
foams have competed successfully with expanded polystyrene foam cushioning materials
and have captured a significant share of that market.