WO1984004098A1

WO1984004098A1 - Highly absorbing graft copolymerisate of starch and acrylonitrile

Info

Publication number: WO1984004098A1
Application number: PCT/SE1984/000127
Authority: WO
Inventors: Bengt Gustav Raonby
Original assignee: Bengt Gustav Raonby
Priority date: 1983-04-08
Filing date: 1984-04-09
Publication date: 1984-10-25
Also published as: SE8301955D0

Abstract

A starch derivate with high absorption of aqueous fluids consisting of a water insoluble alkalisalt of a in aqueous alkaline hydrolysis medium hydrolyzed starch-acrylonitrile-graft copolymerisate obtained through graft polymerizing acrylonitrile on granular starch. By using for the hydrolysis a hydrolysis medium containing water, a lower alcohol and a further with water and the alcohol mixable organic solvent, which is more hydrophobic than the alcohol, a non-dusting granulate with a particle size in the range between 0,1 and 1,5 mm can be isolated direct from the hydrolysis medium. The granulate has excellent handling properties to be used as absorption agent for water and aqueous media.

Description

Highly absorbing graft copolymerisate of starch and acrylonitrile

The present invention relates to a starch derivate with high absorption of aqueous fluids consisting of a water insoluble alkali salt of a in aqueous alkaline hydrolysis medium hydrolized starch-acrylonitrile-graft copolymerisate obtained through graft polymerizing acrylonitrile on granular starch, and a process for the production of the starch derivate.

Starch derivates based on starch and acrylonitrile (AN) with high absorption of aqueous media are previously known. US patent 3 661 815 relates to alkali metal salts of graft copolymers of granular starch and acrylonitrile. The saponification is carried out in an alcohol containing medium, preferably in a mixture of methanol, an alkali hydroxide and water. Even if these polymers have a considerable capacity to absorb water and other fluids (a water absorption of about 50-100 g water/gram polymer), this product presents considerable handling problems due to its fine-grained dusting structure. Further, its absorption capacity, sufficiently large per, se has proved to be difficult to utilize due to a tendency of the fine-grained (floury) material, when used as absorption medium, to form after a introductory swelling an agglomerate with swollen surface layer that prevents further fluid absorption.

US patent 3 932 322 discloses an improvement with respect to said problem with grain structure and dusting tendency for carboxylate salts of hydrolized starch-acrylonitrile-graftcopolymers. According to this reference the fine-grained copolymer is mixed with certain oxides. Even if a certain improvement of the end product is obtained thereby, the problem of handling a dusting intermediate product before the mixing step is still awkward.

Lately further modifications of copolymers of starch and acrylomonomers with extremely high water absorption have been produced. US patent 3 997 484 discloses such a composition having a water absorption of more than 1000 times its own weight. The process for its production is, however, relatively complex. The polymerization is carried out on gelatinized starch, which compared to polymerization on granular starch in water suspension implies a more technically complex gel phase reaction on large substrate volume, and moreover several further process steps are required to carry out saponification, acid precipitation, isolation of the water insoluble acid form of the hydrolized polymer, neutralization and drying. US patent 4 134 863 also describes a starch based graft copolymer , which exhibits extremely high water absorption. These polymers are produced by reacting starch with a mixture of acrylonitrile and a certain water soluble monomer, particularly 2-acryloamido-2methylpropane-sulphonic acid.

However, such an extremely high water absorption of e.g. more than 1000 times dry substance weight is hardly required since it cannot be utilized full out for several practical absorption purposes. If for example more and more water is absorbed in those polymer films described in US patent 3 997 484 towards a theoretical value of perhaps 1000 times the film weight, a gel with decreasing cohesive strength and decreasing viscosity is obtained, which attains more and more liquid characteristics. Thus, high water absorption values per se are not a measure of the utility of the composition in most practical applications. Further, these "super absorbing" polymers exhibit far from the same improved absorption characteristics compared with the introductively mentioned alkali salts of PAN-gelatinized starch with respect to absorption of water media of considerable ionic strength, such as urine, other body liquids and the like. The following claim specification of the type of starch polymers here of concern for most absorption purposes can be summarized: a product requiring a simple production process, and which has a sufficient rapid absorption for practical use and a suitable form for industrial handling and practical use. According to the present invention a starch derivate of the kind introductively mentioned has been achieved, which complies with the above mentioned claim specification and is primarily characterized in constituting a granulate directly isolated from a hydr lysis medium used for hydrolyzing the graft copolymer, having a water absorption exceeding 150 times its weight and a bulk density less than 0,4 g/cm³ and being constituted to more than 90 % by weight by porous granule aggregates with a particle size in the range 0,1-1,5 mm.

A decisive property of the particles composed by grafted starch granules according to the invention is their open pore structure which results in a relatively low bulk density compared with unmodified starch. This for absorption use desired porosity and the resulting low bulk density create, however, also the above mentioned dusting problem of the above mentioned products. According to the invention it has surprisingly been found that a suitable selection of hydrolysis medium for the hydrolysis of the graft copolymer makes possible the isolation of such aggregates of grafted starch granules, which gives a non-dusting and free-flowing powder with a particle size suitable for practical use and a very rapid liquid absorption.

The crucial process step for achieving the highly absorbing starch derivate with suitable grain structure according to the present invention is thus found to be the hydrolysis of the starch-PANpolymer. It is carried out as at previously known hydrolysis by means of an hydroxide of an alkali metal or NH₄ ⁺ for transforming the nitrile groups to amide groups and carboxylate groups. An ordinary result of the hydrolysis is that about 30 % amide groups and about 70 % carboxylate groups are obtained. It is also known that the hydrolysis medium except water also must contain another solvent to limit the swelling of the hydrolyzed product. A mixture of water and methanol has up to now been the preferred hydrolysis medium. According to the present invention it has now been found that if a hydrolysis medium is used which except water consists of a lower alcohol, such as methanol or ethanol, and a further, with the alcohol and water mixable solvent, which is more hydrophobic than the alcohol, there is obtained an excellent means to control further aggregation of the granules obtained at the polymerization step. Simultaneously, cracking by overswelling of the starch granules in the aggregates is prevented. Among other variables having effect on the aggregation is the stirring required during the hydrolysis. However, the stirring only seems to have a marginal effect on the aggregation compared with the selection of hydrolysis medium.

As the hydrophobic swelling-limiting component of the hydrolysis medium, acetone due to its volatility and low boiling point and relative innoxiousness seems to be the most suitable alternative. Other water-mixable solvents with similar water affinity as acetone can be used for obtaining non-dusting granules of suitable size directly from the hydrolysis medium. One example is methylethylketone. However, at testing the combination water-ethanolmethylethylketone, it has been found that larger amounts of ethanol and methylethylketone compared with methanol and acetone are required to avoid lump formation during the hydrolysis. Since the required amount of alcohol and hydrophobic component seems to be related to molarity, it is realized that amount and costs for these components increase with rising molecular weight. Further, the boiling point generally rises with increasing molecular weight, and recovering of the components from the hydrolysis medium by distillation becomes less favourable with the increasing molecular weight. From these considerations it is clear that a mixture of water, methanol and acetone is a preferred hydrolysis medium.

The granular product obtained directly at the hydrolysis step is suitably isolated from the hydrolysis medium through filtration and/or centrifugation. The filtration is facilitated by the fact that the relatively large aggregated granules do not clog the filter. The granules are suitably washed by means of a mixture of alcohol and acetone in e.g. same proportions as used in the hydrolysis step. At the end of the washing step the acetone proportion can be increased, possibly up to pure acetone, to facilitate the final drying step.

The graft copolymerization is carried out in a way known per se by adding to a stirred suspension of starch granules and acrylonitrile under inert gas atmosphere an initiator system known for this purpose at about 30°C. It should be noted that it is well known e.g. from DOS 2 742 595 belonging to same applicant that other vinyl monomers than AN, such as methacrylonitrile, methylacrylate, methylmethacrylate and acrylamide alone or as monomer mixture can be graft copolymerized with starch in the same way as AN. Therefore such monomers are considered in this context to be equivalent with AN. The amount ratio starch/AN is in the range 1:1,5-1:8 mol/mol, while the range 1:2-1:4 in most cases is favourable from economical point of view (the amount of starch is calculated as mol glucose units). The absorption capacity of the product increases rapidly up to a molar ratio starch/acrylonitrile of about 1:3, while with larger proportions of AN rather a small increase in absorption is obtained.

While US patent 3 661 815 recommends a cerium nitrate-HNO₃-initiator system, a Mn³⁺-pyrophosphate complex is preferably used as initiator in the process according to the present invention.

Certain differences in aggregation tendency have been experienced at polymerization with a cerium initiator compared with a Mn³⁺- pyrophosphate-system. At the same molar ratio starch/AN and the same stirring conditions a somewhat larger aggregation effect on the starch granules is obtained with the Mn³⁺-pyrophosphatesystem. The aggregation presumably occurs primarily by a reaction between a growing PAN-chain from one starch granule with PANchains or glucose units in another starch granule. A possible, however not verified explanation to the difference in aggregation tendency is that a Mn³⁺-ion, complexed to 2-3 pyrophosphate groups, constitutes a considerable larger complex compared with the Ce⁴⁺-ion and therefore meets larger resistence against diffusion into the densely packed layers of starch chains in the starch granule. The probability for a higher degree of grafting on the surface of the granules compared with grafting within the granules should therefore increase with the sterical extension of the initiator complex.

The product grains obtained directly from the hydrolysis step according to the invention have open pore structure giving a bulk density in the range 0,20-0,40 g/cm³, preferably 0,25-0,35 g/cm³ and consists to at least 90 % by weight of particles with an average particle diameter in the range 0,1-1,5 mm, preferably 0,2-1 mm. Due to said open pore structure and the grain size of the product, good penetration of liquous media is obtained, which makes possible rapid fluid absorption up to almost the full absorption capacity of the product.

The invention defined in the accompanied patent claims is further illustrated by the following examples.

EXAMPLE 1

Starch-PAN-graft copolymers were prepared with a Mn³⁺-pyrophosphate-system as initiator.

85 g potato starch in granular form and 85 g acrylonitrile (i.e. weight ratio starch/AN 1:1 and molar ratio 1:3) were charged together with 1 liter water acidified with 1 ml concentrated sulphuric acid in a termostatically controlled reaction vessel with stirring equipment. Oxygen was removed from the mixture by alternating nitrogene blast and pressure release. The temperature of the mixture was controlled to 30°C. The reaction was initiated by adding an initiator solution consisting of 0,22 g MnSO₄ · H₂O, 0,05 g KMnO₄ and 0,83 g Na₄P₂O₇ · 10 H₂O in 100 ml water. The reaction was allowed to proceed in two hours at 30°C in nitrogen atmosphere. The mother liquid was sucked off and the reaction vessel was evacuated for boiling away small amounts of unreacted monomers. The residue was washed with a mixture of water and methanol.

EXAMPLE 2-11

Alkaline hydrolysis of the still wet polymer prepared in example 1 was carried out with varying composition of the hydrolysis medium.

In each test about 15 g polymer was charged in a solution of 5,0 g sodium hydroxide in water mixed with varying amounts of methanol and acetone up to a total volume of 150 ml. The tests were carried out in autoclave at 90°C during two hours. The composition of the hydrolysis medium and observations of the aggregation of the hydrolysis product are recorded in table 1 below. The term "fine granules" is used for aggregated product granules of suitable size for direct use as absorption means.

From examples 2-11 it can be concluded that in examples 6, 7, 8 and 9 fine granules without lump formation were obtained, while in example 5 and 10 coarse granules were obtained. The latter two examples constituted borderline cases in which lump formation to some degree could be counteracted through vigorous stirring. In examples 2, 3 and 11 almost total clogging of the product was obtained.

EXAMPLE 12

The hydrolysate prepared in example 7 was filtrated, washed a first time with a mixture of equal parts methanol and acetone and a second time with a mixture of one part methanol and three parts acetone. After that the product was dried and its bulk density was measured to 0,29 g/cm³. A sample of the dried product was also studied by means of SEM-technique, and the result of this study is shown in the accompanied pictures of which: Fig. 1 shows a SEM-picture of a number of granule aggregates at 15 times magnification.

Fig. 2 shows a SEM-picture of a number of granule aggregates at 50 times magnification. Fig. 3 shows a SEM-picture of a number of granule aggregates at 200 times magnification and

Fig. 4 shows a SEM-picture of a number of granule aggregates at 400 times magnification.

Of Fig. 1 and 2 can be seen that the product according to the invention exhibits grains of relatively equal sizes, and the major part of the sample consists of granule aggregates with an average particle diameter of 0,2-0,8 mm. The particle shown in Fig. 3 at 200 times magnification has a length of about 0,4 mm and a width of about 0,2 mm and consists of 200-300 original starch granules. From Fig. 4 showing a granule aggregate at 400 times magnification can be seen that the particle has an open pore structure, and the picture also indicates that the porosity on one hand side is formed through aggregation of the original starch granules, presumably already at the polymerization reaction, and on the other hand side through cracking of individual starch granules caused by limited swelling.

EXAMPLE 13

The product prepared according to example 7 was examined with respect to absorption of water and synthetic urine.

The water absorption was determined by suspending a sample in a certain amount of distilled water, after which the suspension was filtered and the amount of not absorbed water was determined. The amount of retained water was calculated to 276 ± 10 g water/g product.

In the same way the absorption of synthetic urine was determined. The synthetic urine consisted of 2,04 g K₂SO₄, 0,85 g CaCl_{2 ·} 2H₂O, 1,14 g MgSO₄ . 7H₂O, 8,2 g NaCl and 20 g urea in 1000 ml water solution, the pH of which being controlled to 7,0. The amount; retained urine was determined to 44 ± 2 g/g product.

EXAMPLE 14

15 g grafted starch prepared according to example 1 was suspended in a solution of 150 ml ethanol and 126 ml methylethylketone, after which a solution of 5,0 g sodium hydroxide in 30 ml water was added. The hydrolysis was carried out in autoclave at 90°C in 2 hours. The obtained product had the form of coarse granules which after washing with ethanol/methyle thy Ike tone and drying in air constituted a non-dusting granulate.

EXAMPLE 15

The water absorption of the product prepared according to example 14 was determined by suspending a sample in a certain amount distilled water, whereafter the suspension was filtrated and the amount not retained water was determined. The amount retained water was calculated to 256 ± 10 g water/g product . The bulk density was measured to 0,3 g/cm³.

Claims

1. A starch derivate with high absorption of aqueous fluids consisting of a water insoluble alkalisalt of a in aqueous alkaline hydrolysis medium hydrolized starch-acrylonitrile-graft copolymerisate obtained through graft polymerizing acrylonitrile on granular starch at molar ratio starch/acrylonitrile in the range 1:1,51:8, c h a r a c t e r i z e d i n a non-dusting granulate directly isolated from said hydrolysis medium, said granulate having a water absorption exceeding 150 times its weight, a bulk density less than 0,4 g/cm³ and which granulate to more than 90 % by weight being constituted by granule aggregates with a particle size in the range 0,1-1,5 mm.

2. A starch derivate according to claim 1, c h a r a c t e r i z e d i n that it is constituted by a non-dusting granule aggregate with a bulk density less than 0,35 g/cm³ and a water absorption exceeding 250 times its weight.

3. A starch derivate according to anyone of the preceding, claims, c h a r a c t e r i z e d i n that it constitutes to more than 90 % by weight of granule aggregates with a particle size in the range 0,2-1,0 mm.

4. A process for the production of a starch derivate with high absorption of aqueous fluids according to claim 1, at which a suspension of granular starch and acrylonitrile in molar ratio starch/acrylonitrile in the range of 1:1,5-1:8 is reacted by means of adding to the suspension an initiator usable for the purpose, whereafter the so obtained graft copolymerisate is hydrolyzed in a hydrolysis medium being constituted by an aqueous solution of an alkalihydroxide, c h a r a c t e r i z e d i n that the hydrolysis is carried out in a hydrolysis medium which also contains a lower alcohol and a further organic solvent which is mixable with water and said alcohol and which is more hydrophobic than said alcohol.

5. A process according to claim 4, c h a r a c t e r i z e d i n that said further solvent, which is more hydrophobic than said alcohol, is a lower ketone.

6. A process according to claim 5, c h a r a c t e r i z e d i n that said ketone is acetone.

7. A process according to any of the claims 4-6 , c h a r a c t er i z e d in that the hydrolysis medium comprises water, methanol and acetone, the total volume acetone and methol exceeding the water volume and the volume ratio methanol/acetone is between 1:4 and 4:1.

8. A process according to claim 7, c h a r a c t e r i z e d i n that the volume ratio methanol/acetone Is between 1:2 and

2:1.

9. A process according to any of the claims 4-8, c h a r a c t e r i z e d i n that the polymerization reaction between acrylonitrile and the starch granules is carried out by means of an initiator, being constituted by a Mn³⁺-anion-complex.

10. A process according to claim 9, c h a r a c t e r i z e d i n that the initiator consists of a Mn³⁺-pyrophosphatecomplex.