Starch grafted with hydrophilic monomers may have the potential to replace many of the current applications of water-soluble polymers [2
]. One example of such applications is superabsorbents based on grafted starch, originally disclosed by the research of the US Dept. of Agriculture [10
], which have been in industrial production for some time within several companies. Their commercial success was however limited. Production difficulties on a large scale may have been one reason for this [19
]. Crosslinking, mainly with polyacrylic acid or other synthetic polymer part of the material, is needed to create a gel-network that can absorb and retain the large amounts of fluid. The critical balance between important product properties—the absorption capacity versus the mechanical strength to retain the absorbed fluid—is mainly determined by the crosslink density. There must be sufficient space between the crosslinks or, when based on starch, the graft attachments, to allow for easy entrance of the water or solute (blood, urine) to be absorbed [20
]. By calculations based on data from literature [10
] one can estimate that average chain lengths between two knotting points in the total network must then be in the order of (much) more than 150–200 molecular units. This means that NAGU
must be also at least 200, and grafted chains must have a size of well over 15,000 Dalton. Many applications of starch based superabsorbent materials including drug delivery agents are discussed in a review from Athawale and Lele [22
3.2. Discussion of the Demands to Other Applications
Beside superabsorbents, many more applications of grafted starches may be possible. The reviews of Jyothi [17
] and Meimoun et al. [2
] give a clear overview. Still, in these or other papers, there are no indications of factual current applications in industry, in spite of a more recently issued patent and published work in a continuous reactor to produce grafted starch products [23
In Table 3
, the most prominent potential applications are listed. There, an assessment about the demands for the size and spacing of the grafts associated to specific applications is presented, based on some literature data but mainly on reasonable assumptions. The terms ‘short’ and ‘long’ are used here, which deserves at least some kind of specification. For example, in Qudsieh et al. [25
] a MWw
range of the grafts between 300,000 and 740,000 is reported. The products with the longest grafts gave the best flocculation performance. On the other hand, for applications such as detergent co-builders, a desirable molecular weight of below 100,000 is mentioned in the patent of Klin et al. [23
]. But, that is a molecule consisting of both synthetic and at least 50% natural polymer. Even if this molecule would have a single graft, it must be shorter than 50,000 Dalton. In Berndl [26
], a grafted chain size of over 72,000 is reported, also for application as a detergent co-builder. To conclude, it can be estimated from this relatively scarce information that for ‘short’ grafts an size range around 50,000 can be regarded, while grafts can be mentioned ‘long’ if they are in the order of ten times larger, >500,000 Dalton.
For application of the grafted product as a thickening agent
, as in textile printing pastes [12
], the presence of homopolymer may be tolerable but it probably do not contribute much to the desired properties. For thickening agents, the establishment of high viscosity must come from long chains as well as from a branched structure. Both the (branched) starch chains and the grafts contribute to this, so the grafts that are formed should be long and widely distributed. This is, very schematically, depicted in the graphical abstract: the product right below. From literature data, desired graft sizes can not be estimated with any accuracy. Still, in the patent of Hamunen et al. [27
] it is reported that starch with grafted side chains causes a higher Brookfield viscosity than carboxymethylcellulose, a product with only monomolecular substitutions. Witono et al. [28
] found that thixotropic behavior of starch in an application as thickening agent can be reduced when starch is grafted with polyacrylic acid side chains [28
]. These are clear indications that the side chains have an important role in the rheological behavior of such products, but to quantify this more research will be needed.
On the contrary, when grafted starch would be used for metal absorption
like in heavy metal ions removal from wastewater, the viscosity should not be too high. In this case, the functionality is probably in the grafts only [5
], so there need to be enough of them to give a good binding capacity. Since the metals ions must be able to penetrate the structure, a relatively open structure seems a reasonable requirement. Witono et al. [29
] emphasize the importance of accessibility.
Application of grafted starches as flocculants
for wastewater treatment, to remove particulate pollutants, has been the subject of many publications, e.g., [4
]. As stated by most researchers, e.g., Qudsieh et al. [25
], this application asks for long dangling chains with sufficient space in between, to allow particles to easily interact with the grafted polymer chains. Beside the physical architecture of a flocculants, other properties such as electrostatic charge are also of importance [4
]. In addition, in this application, some homopolymer may be tolerated although it is not clear whether it contributes to the flocculation performance [30
For use in co-builders
to bind calcium ions in detergent formulations, relatively small molecules are wanted to not cause a high viscosity [23
]. For that purpose, starch may be degraded before the grafting process [26
]. Hence, a high graft frequency must be strived for, to make sure that also smaller starch chains bear grafts. Such grafts should also be short for the same reason.
Several authors [13
] report on the application of the mixture of grafted starch and homopolymer as sizing agent
for textile fibers, e.g., cotton. All the grafted starches that were synthesized and tested, had much better sizing properties than native starches. If no homopolymer removal is needed, this would be a major economic advantage. From the referred literature, one cannot derive clear constraints or demands concerning molecular weight and spacing of the grafts in sizing agents. In the work of Djordjevic et al. [30
] it is stated that the smaller molecules of hydrolyzed grafted potato starch show good sizing performance, but that is only compared to non-grafted starch.