Patent Application
20150233058
Adhesives containing starch and dextrine have a major role to play in a wide range of industrial applications, in particular in the packaging industry. Starch and dextrine are used there for manufacturing packing paper and corrugated cardboard for gluing them. Furthermore, other porous substrates are glued with these adhesives.
Starch and dextrine are also very affordable raw materials, easily available and they are frequently used as an aqueous dispersion because both can be used in the form of special formulations both cold and warm. Beyond this, starch and dextrines are used in powders dissolved in water to form a relatively viscous paste. Finally, they harden with water loss.
Although starch and dextrine have been used as adhesives for a very long time, there are a whole series of reasons why these natural adhesives cannot be replaced 100% by synthetic products:
The table below shows the characteristics of some important types of starches:
The proportions of the types of starch amylosis and amylopectin vary depending upon the type and sort of the starch plant. Since amylopectin is primarily needed for industrial utilisation, starch plants with the highest content of amylopectin are preferred. For instance, the genetically modified starch potato sort amflora is presently in the licensing procedure in the EU and its starch consists almost exclusively of amylopectin while other barley sorts whose starch consists 95% of amylopectin are based on conventional methods of cultivation. Both products (starch and dextrine) are what are known as carbohydrate polymers or more precisely the starch is a natural polysaccharide. Only a few plants contain sufficient starch in their leaves, corns or roots, which is why these are the only plants suited for extracting starch from them.
Starch contains two main components, amylosis and amylopectin, where amylosis has long chains while amylopectin has a branched and chain-linked structure. This combination (and therefore this structure) varies from plant to plant. However, one important property is its amylosis content since the greater the proportion of amylosis, the more valuable is the plant for extracting starch.
Amylopectin is soluble in cold water and can be used in this soluble form, while amylosis is non-soluble in water. Instead, it can be dissolved by cooking it in hot water while adding strong alkali under pressure, mostly NaOH or at 150-160° C. In its dissolved form, amylosis forms a dispersion and solid gels at concentrations greater than 2%. Furthermore, amylosis cannot be completely dissolved (at 100%) in water, and even crystalline aggregates form over time in these dispersions with H-bridges. This is a process that is also called retrogradation. This process is responsible for the great fluctuations in the viscosity of adhesives containing starch.
Adhesives containing starch have been manufactured state-of-the-art for the corrugated cardboard industry with the Stein-Hall process since 1930. Starch-based adhesives/starch glues/are needed for manufacturing corrugated cardboard that is a glued lightweight construction made of paper. Therefore, along with paper, starch glue is the most important component for producing corrugated cardboard. The adhesive and gluing together have a substantial impact on the quality of the corrugated cardboard.
Starch glue has the job of firmly bonding the smooth sheets with the corrugated sheets. The deeper the starch glue penetrates into the paper's pores, the better the bond. Beyond this, the product of corrugated cardboard is manufactured in a process where a paper sheet is initially corrugated on heated rollers. Afterwards, a sheet such as this is applied on one side by spreading starch glue on the tips of the flutes. This is then what is known as “one-sided” cardboard, even though there are also “double-sided” cardboards. When manufacturing corrugated cardboard, it is unavoidable for the starch glue to form a fast bond between the paper surfaces. This is the reason why it is very important that the starch glue used has a viscosity that makes it possible on the one hand to apply or spread the adhesive and on the other hand to allow it to be bonded quickly enough. To determine the quality of the bond, the ripping test is used where the smooth sheet is torn off from the corrugated sheet with a piece of corrugated cardboard. The more fibre residues remain on the previous adhesive points, the better it was bonded. We can recognise excellent bonding from the fact that the two paper sheets can only be separated by tearing off the fibre.
Fast conglutination and bonding is supported on the one hand by the high temperature of application and on the other hand by including alkali, NaOH and boron compounds such as boric acid or borax in the combination of adhesives. Borax (that is chemically disodium tetraborate, Na2B4O7) is used to control the stability, rheology and the surface wettability of the starch adhesive during manufacturing and processing. The quantities of borax vary from 1.5-20% atro with reference to the starch used depending upon the applications and type of starches. Adhesive producers and processers have been recently limiting its use to 7% of borax. Viscosity increases as soon as borax with a mixture of starch and alkali is added while stirring and heating and it remains stable over a long period of time. This starch comes from wheat, maize, potatoes, tapioca or peas that are modified naturally, chemically, enzymatically or physically in accordance with the general processes. Common yellow dextrines used in normal production of adhesives containing starch are also suitable just like those that are partially preconglutinated and partially swollen and are used in the normal adhesive processes (such as Stein-Hall).
Disodium tetraborate in water and an alkali milieu reacts with starch in the following steps while forming a water-soluble sodium starch borate:
The proposal has been made in U.S. Pat. No. 6,056,816 to include another additive in the starch glue, namely an alkali silicate. Typically, this combination contains 12-35 weight % of starch, 1.5-12 weight % of soluble alkali silicates, 0.25-2.5 weight % of sodium hydroxide and as much as 2 weight % of borax or boric acid. The remainder of this formulation is water that might contain 55-70 weight %. The technical benefits of this composition are supposed to be higher viscosity, highly heat stability and higher solid matter content of as much as 45 weight % of the finished composition, in contrast to 15-35 weight % of solid matter content as per state-of-the-art (also refer to column 4, from line 25).
Although these starch glue formulations are actually very well suited to the fast process of manufacturing corrugated cardboard, there still have been other improvements such as by adding fillers such as calcium carbonate that have also boosted the solid matter content. EP 1 101 809 B1 describes this.
EP 2 363 440 A1 [describes] adding a homopolymer of an α, β-unsaturated acrylic acid as per the general formula: CH2═CR—COOH where R can be a substituent that is selected from H, monovalent alkyl, aryl, or aralkyl radicals or monovalent cyclical alkyl radicals, haloalkyls and cyanoalkyls, with 1-9 C-atoms. Claim 16 states that there is a typical composition of 20-80 parts by weight (GT) starch, 5-50 GT NaOH, 200-250 GT starch powder, 1-25 GT borax and 0.01-5 GT polyacrylic acid or 2-methacrylactic acid additive. This invention also had the purpose of adapting viscosity to manufacturing process (also refer to page 2, lines 35, [0008]).
In spite of these additives, the aforementioned compositions still contain as much as 2 weight % or 25 GT of borax depending upon what the quantity of borax referred to. The Römpp-Online Chemielexikon states that borax should be avoided wherever possible, especially in packaging used with food or for general human consumption. Römpp-Online, 2011 also states that “borax smoke irritates the mucous membranes of the nose and throat”. Consuming greater amounts even causes stomach pain, vomiting, diarrhoea including circulatory insufficiency all the way to a shock. It can cause inflammations if it comes into contact with the skin or eyes, although borax does not attack intact skin; TRGS 900 (2009): workplace concentration 0.5 mg/m3.” This is the reason why it also has to be ensured in all corrugated cardboard works that it does not exceed the maximum workplace concentrations. Therefore, many manufacturers of corrugated cardboard are switching over to “liquid borax”. Contact with the skin is the most frequent means of contact with packaging. A replacement for borax is urgently needed in the packaging or corrugated cardboard industry. Finally, using borax is limited by lower water solubility (it only dissolved 3-5%) and irritating the mucous membranes when used in powder form.
One suggestion has been using freebor as a replacement in starch adhesive formulations. Apparently, this product is a gel and has a stabilising effect on viscosity when adding it to starch glue. Unfortunately, neither the chemical structure nor the formula of this bond has been revealed and there is also a safety data sheet without any data on its chemical nature. The assumption is that it is an acrylate gel.
In any event, there was the urgent need to completely replace the borax in starch glue compounds used in manufacturing paper or corrugated cardboard while still maintaining all of the important properties such as viscosity, solid matter content and pH-value. Beyond this, paper or corrugated cardboard should be provided using a starch glue compound free of borax under simplified process conditions and with short retention times while streamlining the application conditions and energy costs.
The above challenge is met in an invention where paper or corrugated cardboard (produced using a starch glue compound free of borax and containing one dextrine) is provided where the starch glue composition is cross-linked with sodium aluminate in an alkali milieu and where the quantity of sodium aluminate is in the range of 1-35% atro with reference to the use of dextrine. Surprisingly enough, it was found that using the sodium aluminate both in its liquid and sold form guarantees the maintenance of these properties while making the compositions completely free of borax. This is a major benefit in terms of toxicity and industrial hygiene.
Sodium aluminate can be described with the general formula: Na[Al(OH)4] or also Na2Al2O4 because the product contains more than the stoichiometrically required amount of Na20. This is also the reason why the Na2O/Al2O3 ratio is given to characterise it. Furthermore, sodium aluminate is also extracted with large-scale technology by what is known as the Bayer development of bauxite or also by recovering alumnate waste lye arising in industrial processes such as the electrolytic oxidation process of aluminium. In general, sodium aluminates are produced when converting excess sodium hydroxide solution with aluminium salts. Römpp-Online, 2011 states that it is used in the following fashion: “sodium metaaluminate is used for manufacturing soaps, papers, milk glass and paints in the enamel industry, as a caustic in alizarin dyeing, for water softening, fresh water processing, sewage purification by flocculating (in particular in combination with other flocculation agents), as a waterproofing compound and fast hardener in building dams, tunnels and bridges. It also can catalyse reestering vegetable oil to biodiesel. The sodium aluminate added to starch glue composition can preferably have a granulate or powder form.
Since sodium aluminate dissolves well in water, this makes it possible use it in the liquid and aqueous form, which is enormously beneficial to industrial hygiene because this makes it possible to avoid caustic dust pollution. This aqueous solution has a pH-value of 11-12. Sodium aluminate dissociates in water to Na+ and [Al(OH)4] ions that enter into the following reactions with the starch molecules in an alkali milieu. After forming a water-soluble sodium starch aluminate, they are cross-linked with a second starch molecule in a second step in condensation:
Dextrines (also called starch gum or maltodextrines) are degradable starch products whose molecule size is between those of starch and oligosaccharides. Normally, they are in the form of white or bright yellow powders and they are mainly extracted from wheat, potato and maize starch by dry heating (>150° C.) or with the effect of acid. In nature, dextrine is generated from such things as bacterium macerans and dextrines also occur by enzymatic decomposition of starch by means of amylase (http://de.wikipedia.org/wiki/dextrine). Therefore, paper or corrugated cardboard is preferred that is manufactured using a starch glue compound free of borax and containing one dextrine. This dextrine is a yellow dextrine.
The effect of the starch glue compound free of borax containing one dextrine consists of a very good cold water-soluble property, a short retention time and a fast increase in viscosity where it is only necessary to use a smaller starch cooker (or none at all due to cold water-soluble property of the dextrine) for manufacturing the adhesives. This is in contrast to starch that allows lower procurement/investment costs, savings in operating expenditures, energy savings and simplified and more flexible usage of starch glue composition in production. As already shown, if we compare the aforementioned reactions with those of borax and starch, we see a similar process. This reaction ensures that the starch is converted into a water-soluble aluminate salt while cross-linking. This makes it possible to adjust rheology and the optimum processability of the starch glue when manufacturing corrugated cardboard.
That was in and of itself surprising because of the fact that NaOH (i.e., a very strong alkali additive) was used. However, it is known that NaOH cannot bring about starch cross-linking so that it only regulates the pH-value and acts as an alkaline decomposition agent for starch while changing the gelatinising point. The alkali sodium silicate with NaOH is also not suited to bringing about cross-linking and thus stabilise viscosity without adding borax (refer to U.S. Pat. No. 6,056,816).
As per the invention, sodium aluminate can now be added in aqueous form, but also in the solid form of a starch-water dispersion with thermal decomposition. Solid sodium aluminate is preferred with an Al2O3 content of 53-55% and it is used with the usual grain size of 0.1-0.063 mm and 1.0-0.09 mm or it is used as an aqueous sodium aluminate solution with a 10-45% solids concentration and 5-25% Al2O3 content as a component. Sodium aluminate is also preferred in granulate or powder form in quantities from 1 to 35 weight %, preferably 2-10%, 25-70° C. warm water where the starch is dissolved, added or dispersed with a starch (or dextrine) proportion of as much as 45% atro. We observe an increase in viscosity with the beginning cross-linking reaction depending upon the mixtures. The finished adhesive with low to highly viscous mixtures stays stable for a longer period of time and can be used as starch adhesives for corrugated cardboard and paper production.
Sodium aluminate is used for controlling the stability, rheology and surface moistening of the starch adhesive during manufacture and processing on the surfaces to be adhered. The quantity of sodium aluminate needed as per the invention ranges from 1-35% atro with reference to the starch and/or the use of dextrine. Polyphosphates can be worked into the mixture as liquefier and/or suitable filler systems such as CaCO3 for manufacturing mixtures with a high level of solids concentration and to maintain a mixture with lower viscosity. The utilisation (or possible utilisation) of sodium aluminate both in solid and liquid form offers the user a great deal of freedom in individually developing their own starch glue recipe applying the adhesive levels to be achieved. In comparison, borax only has limited water solubility (only 3-5% in hot water) which is why it always has to be worked in solid form. This is among the well-known problems in industrial hygiene and occupational safety.
The sodium aluminate preferably has a Na2O to Al2O3 ratio in a range of 1.0-1.5. Furthermore, the starch glue compound free of borax has an alkali milieu with a pH-value of 12.5. The starch glue compound free of borax also preferably has solid sodium aluminate before cross-linking with a Al2O3 content of 53-55% and with grain size distribution of 0.1-1 mm in quantities from 1 to 35 weight % (preferably 2-10 weight %) with reference to the starch glue compound free of borax. Beyond this, with cross-linking, the starch glue compound free of borax preferably has a sodium aluminate solution with 10-45 weight % of solids concentration with reference to the proportion of the starch glue compound free of borax and 5-25 weight % Al2O3 content as a component of the starch glue compound free of borax. A final subject of the invention is using the starch glue compound free of borax mentioned above for manufacturing paper or corrugated cardboard. Examples 1-3 below describe manufacturing starch glues for the paper processing industry without restricting the invention:
Starch glue is manufactured from:
Starch glue is manufactured from:
Starch glue is manufactured from:
| Number | Date | Country | Kind |
|---|---|---|---|
| 11009577.5 | Dec 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2012/074790 | 12/7/2012 | WO | 00 |
