Residual monomer in superabsorbent polymers: Effects of the initiating system

Abstract

Minimized residual monomer is an essential requirement particularly in hygienic, pharmaceutical, and food packaging polymer products such as superabsorbent polymers (SAPs). The present article is the first non-patent report on the study of a highly effective polymerization variable (e.g. initiator) on the residual monomer of SAPs based on partially neutralized acrylic acid. Two persulfate systems, i.e. ammonium persulfate (APS)/sodium metabisulfite (SMBS) and APS/tetramethyl ethylenediamine (TMEDA), were examined to initiate the free-radical polymerization at room temperature. It was shown that chromatographically measured residual monomer and swelling capacity was strongly dependent on the type and concentration of the initiator. A kinetic model was also derived to describe the experimental results. Dissociation rate of initiation system was recognized to be a key factor to obtain a SAP with low residual monomer. It was found that, in aqueous solution polymerization, the effect of a slowly dissociating system such as APS/TMEDA on decreasing the residual monomer was much higher than that of a rapidly dissociating system like APS/SMBS. Under selected conditions, residual monomer could be reduced up to 5327 AE 138 and 1715 AE 44 ppm for APS/SMBS and APS/TMEDA initiating systems, respectively.

Minimization of residual monomer content of superabsorbent hydrogels via alteration of initiating system

“…Quicker gelation has negative effect respecting to the residual monomer content. According to our previous study, the higher the gelation time, the lower the residual monomer will be 8. A phase conversion from liquid to solid is occurred at gelation point which causes to decrease probability of collision of free radials to unreacted monomers due to lower diffusion in the semisolid (gelled) medium 8…”

“…There are nearly 1200 nonpatent articles on superabsorbents published during years 1976–2009. However, except for a recent paper,8 none of them investigated effect of fundamental parameters on the SAP residual monomer content as a key characteristic especially for those used in hygienic, medical, pharmaceutical, and food packaging applications.…”

“…Redox pair initiating systems practically favor industrial polymerization processes because they often have no need of heating 2, 4, 14. Pairs of ammonium persulfate (APS)/sodium metabisulfite (SMBS)4, 8, 14 and APS/tetramethyl ethylenediamine (TMEDA)1, 8, 15–17 are the most conventional initiating systems for the superabsorbent hydrogel synthesis. Thermally‐induced single initiators such as APS,18 potassium persulfate (KPS)19 and 4,4′‐azobis(4‐cyanopentanoic acid) (ACPA)20, 21 have also been reported for the SAP synthesis.…”

“…We have started to investigate the effect of different parameters on the residual monomer content in recent years. In a previous article, the effect of redox initiating system type and initiator concentration on the residual monomer content was studied 8. Thermodecompositing initiators are even more conventional than redox initiators for SAP synthesis 22.…”

Superabsorbent hydrogel composites and nanocomposites: A review

“…The amount of residual monomer in any hydrogel product is dependent on the reaction conditions, type and concentration of initiator, monomer, reaction temperature, and so on [23]. For instance, it was found that the rate at which the initiator dissociates can change the concentration of the residual monomer to a great extent [120]. Hydrogel composites containing high concentration of clay may show increased swelling and hence decreased gel strength [80].…”