WO1998002496A1

WO1998002496A1 - Adhesive based on starch

Info

Publication number: WO1998002496A1
Application number: PCT/NL1997/000419
Authority: WO
Inventors: Stephan Henrick Dick Hulleman; Remigius Oene Jules Jongboom; Herman Feil
Original assignee: Instituut Voor Agrotechnologisch Onderzoek (Ato-Dlo)
Priority date: 1996-07-17
Filing date: 1997-07-16
Publication date: 1998-01-22
Also published as: NL1003622C2; AU3465097A

Abstract

A process is described for the preparation of an adhesive based on starch, wherein a biopolymer, which consists to at least 70 % of starch, and 1-40 % by weight water (with respect to starch) are mixed with a hydrolysis catalyst, the mixture is shaped mechanically at a temperature such that the hydrolysis temperature is not exceeded and, before use, the shaped starch is heated to at least the hydrolysis temperature. With this procedure an adhesive of low viscosity is produced which can be applied to the substrate to be glued, such as paper, glass, polyethylene and polypropylene.

Description

Adhesive based on starch

The invention relates to a process for the preparation of a biodegradable thermoplastic adhesive based on starch.

Thermoplastic adhesivcs, that is to say adhcsivcs which arc workable under the influence of heat, are used in large quantities in the packaging and labelling industry for rapid sticking of cardboard, paper and labels. Said thermoplastic adhcsivcs generally consist of polymers based on synthetic monomers, such as polycthylviny] acetate and alcohol, polyurethane, or polyamides. Polymers of this type arc not biodegradable. A large proportion of the thermoplastic adhesives produced is used in products which have a short life cycle, such as packaging. The percentage volume of these products in the waste stream which has to be processed by composting, dumping, incineration or recycling is high. The additional loading of this waste stream with the non-dcgiadablc adhcsivcs is highly undesirable. There is therefore a need for thermoplastic adhcsivcs which arc biodegradable or repulpable, because said adhesivcs have an advantageous effect both on the biological degradation of the waste and on the re-use thereof.

Starch is a polymer which is not only biodegradable but also has adhesive properties. Methods for the preparation of adhesivcs based on starch by means of hydrolysis with enzymes or alkaline or acid additives arc known, for example from WO 95/02646 and EP-A 511916. With these methods hydrolysis takes place during the preparation, a fluid of low viscosity being obtained. Thermoplastic processing of these known starch adhesives is not possible.

On the other hand, thermoplastic adhesives based on starch derivatives, such as starch acetate (US-A 5434201) or other starch esters (such as propionatc, US-A 5360845) are known. Starch derivatives of this type arc thcrmoplastically workable, but they arc poorly, if at all, biodegradable.

A process has now been found for the preparation of a biodegradable thermoplastic adhesive based on starch, wherein a biopolymcr, which consists to at least 70 % by weight of starch, is mixed in the presence of 1-40 % by weight water (with respect to the weight of starch) with a hydrolysis catalyst and the mixture is shaped at a temperature below the hydrolysis temperature of the starch. This shaped precursor can then be used as adhesiv e in that it is heated to a temperature of at least the hydrolysis temperature, after which an adhesive of low viscosity is obtained which, after removal of water, forms a coonnection with the substrate to be glued by heat loss.

The advantage of the process according to the invention compared with the preparation of starch adhesives according to the prior art is that the adhesive can not only be thermoplastically processed but is also biodegradable. The precursor can be marketed and processed in the form of solid material (for example in the form of granules) and is converted to the degradablc adhesive of low viscosity only during use. The precursor can be produced continuously. The starting material for the process according to the invention is a biopolymer which consists to at least 70 % by weight, in particular to at least 80 % by weight (based on solids), of starch. The starch concerned here is starch which has not been chemically modified or has been only slightly chemically modified. In addition, the biopolymer can contain other polymers which are largely also biodegradable, in particular proteins or other polysaccharides. ln this context, said other biopolymcrs, such as the proteins present in wheat or rice flour, caseinates or pectins, can serve as binders. If the biopolymer has been slightly modified, it contains no more than 10 % by weight, in particular no more than 5 % by weight and preferably no more than 2 % by weight of chemically-modified biopolymer units. The starch to be used originates, for example, from potatoes, corn, wheat, rice, tapioca, barley and/or peas. It can also originate from genetically modified crops such as waxy corn and high-amylose corn. The starch can be native (granular) or it can be gelatinised; it can also be physically modified (rolled, milled).

Water, in an amount (based on the weight of starch) of 1 -40 % by weight, in particular 5-35 % by weight, and also a hydrolysis catalyst are added to the biopolymer. Said hydrolysis catalyst is in general an acid or a base. Examples of such catalysts are inorganic or organic acids such as hydrochloric, phosphoric, sulphuric and nitric acid, oxalic, lactic, citric, acetic and formic acid and the like, and bases such as sodium hydroxide. The amount of catalyst is partly dependent on the strength thereof and on the desired processing temperature and in general will be in the range of from 0.005 to 0.2 mol per kg starch.

The mixture is then thermoplastically processed and shaped at a temperature below the hydrolysis temperature. The hydrolysis temperature, or dcpolymcrisation temperature, is the temperature at which the starch in the mixture is hydroiyscd to an appreciable extent, that is to say shows chain scission. Said temperature can be set in the manner described, for example to between 80 and 165 °C, in particular between 90 and 150 °C. Shaping can be effected, for example, by extrusion in a conventional extrusion device, or by pelleting. In this way granules or another shape, such as strands, sheets a d the like, can be obtained. Shaping can be carried out at a temperature of 20-160 °C. depending on the depolymerisation temperature.

If desired, a plasticiser, such as a polvol, can also be used in the preparation of the adhesive. Suitable plasticiscrs are polyols, polycthcrs, polyesters and compounds containing mixed functional groups. Examples are glycol, propylene glycol,

lcnc glycol, glycerol, neopentylglycol, erythritol, pentaerythritol, sorbitol, polyalkylcnc glycols, such as di- and polyethylene glycol, di- and polypropylene glycol and di- and polyhydroxypropylenc glycol, glycol mono- and diestcrs, glycerol mono- and dicstcrs, citric acid esters and mixtures thereof. An amount of 1-50 % by weight, in particular of 5-35 % by weight plasticiser with respect to the starch is preferred. The plasticiser can be added before or after the shaping step. If necessary, a melt flow accelerator such as a triglyccridc (lipid or phospholipid) with C-^-C_{j g} fatty acids (e.g. castor oil or lecithin) can also be added, e.g. at a level of 0.5-5 % by weight with respect to the biopolymer The hydrolysis temperature is determined by the amount of water, the catalyst used, the presence of a plasticiser and the duration and the conditions of the pretrcatment and of the shaping. The precise location of the hydrolysis tcmpcratuic can be determined with the aid of thermal analysis methods, such as differential scanning calorimctry (DSC). In combination with a determination of the molecular size (HPSEC-MALLS: High Pressure Size Exclusion Chromatography with Multi-Angle Laser Light Detection) it is possible by this means to obtain insight into the molecular degradation as a consequence of hydrolysis and to determine a relationship with the physical properties such as adhesion to paper, glass and polyolefines such as polyethenc.

The thermoplastic adhesivcs which can be prepared according to the invention can be used in products which are suitable for recycling or biological degradation. Such applications are found in particular in the packaging and labelling industry. Example 1

A mixture of 18.06 kg potato starch (including 3.06 kg water), 0.45 kg oxalic acid, 1.51 kg glycerol and 0.9 kg castor oil was extruded at 1 15 °C and shaped into strands. On heating to above the depolymerisation temperature, said strands arc converted to an adhesive of low viscosity. Said adhesive adheres to paper, glass, polycthcnc and polypropene. Example 2

This example illustrates the effect of water on the dcpolymerisation temperature. Potato starch having a water content of 18 % was treated with 2 M HCl for 2 hours at 35 °C. The treated starch was dried to a water content of 14 % by weight (based on dry starch) and mixed with 30 % by weight glycerol (based on dry starch). The water content was then adjusted to a value between 14 and 40 % by weight. The dcpolymerisation temperature fell with increasing amount of water in accordance with the relationship shown below: w_wate w_{dry starch} 0.14 0.20 0.25 0.30 0.35 0.40

Tdepolyπ_jerisanon TO 158 151 142 1 34 1 27 1 21

Example 3: Effect of type and amount of acid on the depolymerisation temperature

In a mixture of potato starch, glycerol and water in a ratio 10 : 3 : 3 (% w/w) amounts of 0-1 % (w/w) of nitric acid (o), oxalic acid (•). citric acid (D). lactic acid (+) or acetic acid (Δ) were added. The dcpolymerisation temperature was determined using

DSC. The depolymerisation temperature as a function of the amount and type of acid is given in figure 1.

Example 4: Effect of amount of water on the depolymerisation temperature

To a mixture of potato starch and glycerol in the ratio 10 : 3 (% w/w) and 0.5% of one of the acids oxalic acid (•), citric acid (D), lactic acid (+) and acetic acid (Δ) water was added so that the ratio of water/potato starch varied between 0.15 and 0.50. The depolymerisation temperature (determined by DSC) as a function of the ratio water/starch and of the type of acid is given in figure 2.

Example 5: Effect of amount of glycerol on the depolymerisation temperature To a mixture of potato starch and water in the ratio 10 : 3 (% w/w) and 0.5%

(w/w) of one of the acids oxalic acid (•), citric acid (D) and lactic acid (+), glycerol was added such that the ratio glycerol/potato starch varied from 0.10 to 0.50. The dcpolymerisation temperature determined by DSC, as a function of the ratio glyccrol/starch and of the various acids is given in figure 3

Example 6: Extrusion of mixtures of potato starch, glycerol and water and measurement of the adhesive strength to paper A mixture of potato starch, glycerol, water, lecithin and oxalic acid in the ratio

100 : 30 : 50 : 3 : 2 (% w/w), having a depolymci isation temperature of 1 15°C, was extruded at a maximum extrusion tempcratuic of 120 and I 0°C The low \ ιscosιty material obtained was dried out to varying amounts of water ( I I 4- 16 4% w/w) Two test strips of 80 g/m² paper were adhered by heating to 1 ()°C at a picssuic of 67 kg/cm² The materials were conditioned at 30, 60 and 90% rclatι\ c humidity at 20°C foi 14 days The adhesion strength was tested using the T-pccltcst according to ASTM D 1876-93 The results are given in the table below

Table: Adhesion strength

T_max (°C) water content ng of glue (%) s

120 ^■ i 1 30 0 4 1 1 100

60 0 5 1 100

90 0 4 0 2 0

16 4 30 0 7 1 2 1 0

60 0 2 1 2 100

90 0 2 0 7 50

150 11 4 30 0 5 1 0 100

60 0 6 ] 0 1 0

90 0 6 0 1 0

16 0 30 0 2 1 1 100

60 0 4 I 0 100

90 0 3 0 4 50

Claims

1. Process for producing an adhesive based on starch, characterised in that a biopolymer, which consists to at least 70 % of starch, and 1-40 % by weight water (with respect to starch) are mixed with a hydrolysis catalyst, the mixture is shaped mechanically at a temperature such that the hydrolysis temperature is not exceeded and in that, before use, the shaped starch is heated to at least the hydrolysis temperature.

2. Process according to claim 1 , wherein the hydrolysis catalyst is an acid.

3. Process according to claim 1 or 2, wherein the biopolymer contains less than 5% of chemically modified monomer units.

4. Process according to any one of claims 1 -3, wherein 1 -50 % by weight, in particular 5-35 % by weight, (based on starch) of a plasticiser, such as a polyol, is also used.

5. Process according to any one of claims 1 -4, wherein shaping is carried out at a temperature of 20-160 °C.

6. Process according to any one of claims 1 -5, wherein, after shaping, the water content is raised or lowered in order to influence the hydrolysis temperature.

7. Process according to any one of claims 1 -6, wherein the hydrolysis temperature is adjusted to a value of between 80 and 165 °C.

8. Process according to any one of claims 1 -7, wherein shaping is carried out by extrusion.

9. Process according to any one of claims 1 -8, wherein the biopolymer contains 0.5-5 % by weight of a triglydcridc melt flow accelerator.