Field of the invention

The present invention belongs to the field of biodegradable water-soluble compositions, which may be used as adhesives or binders. More particularly, the object of the invention is a sodium or potassium silicate composition, which is safe and non-toxic for both the users of the composition and the environment, when evaluated by various criteria.

State of the art

Non-toxic aqueous adhesives for glueing wood, paper and other materials are kn own b a sed o n ve ry different compositions. Increasing environmental consciousness and authoritative requirements have brought about the need for non-synthetic, non-toxic and biodegradable adhesives and binders based on natural materials that is continually growing.

With the aim to develop biodegradable adhesives, components suitable for binders have been combined. E.g. in US2010/006003, there is presented a composition suitable for coating, binding and impregnating, which has beeswax, potassium carbonate, water and sodium silicate. Advantages of the presented composition are improved adhesion, as well as incombustibility.

Sodium silicate based adhesives have traditionally been used in manufacturing cardboard boxes and tubes and paper products. Patent application GB2301598 describes a composition which aims to solve problems of adhesives containing solely sodium silicate, i.e. the brittleness and cracking of dried adhesive and sensitivity to moisture. The above mentioned application presents a mixture of sodium silicate and an aqueous solution of silicone as a solution to these.

Another patent application for the same purpose, EP 0883657, focuses on an adhesive composition containing (in percentages of weight) approximately 0.05- 0.5 % of sodium silicate, 10-47 % of dextrin, approximately 17-70 % of alkali metal sil icate, 0.05-0.5 % of sod ium metaborate octahydrate or sodium metaborate tetrahydrate and 18-62 % of water.

However, in the field, there is a need for a biodegradable adhesive which is not of animal or starch origin. There is further a need for an alternative alkal i metal or al kal ine earth metal sil icate based composition with good adhering qualities. Furthermore, there is a need for compositions which are biodegradable and water- soluble and do not contain organic solvents. There is also a need for boron-free adhesive compositions.

Summary of the invention

Now, the inventors have surprisingly discovered that an al kali metal or alkaline earth metal silicate based composition responds to the above-mentioned needs and further provides additional advantages, when there is alkali metal or alkaline earth metal silicate (e.g. Na2SiO3 or K2S1O3) and sucrose in it. Preferably, the ratio of alkali metal or alkaline earth metal silicate to sucrose is between 4:1 and 7:4, and more preferably between 4:1 and 2:1 .

In its spreadable form , the biodegradable adhesive has a so-called aqueous composition with alkali metal or alkaline earth metal silicate, sucrose, water and optionally one or more additives, and with the ratio of alkali metal or alkaline earth metal silicate to sucrose between 4:1 and 7:4, and preferably between 4:1 and 2:1 . More accurately said aqueous composition contains in percentages of weight: a. approximately 55-68 % of alkali metal or alkaline earth metal silicate b. approximately 24-32 % of sucrose c. approximately 8.1 -9.5 % of water, and d. 0-10 % of one or more additives; and wherein the ratio of al kal i metal or alkal ine earth metal silicate to sucrose is between 4:1 and 7:4, and preferably between 4:1 and 2:1 .

This composition is defined more accurately in claim 1 . Within the scope of the invention, there is also a spreading means that contains composition according to claim 1 .

Defined in dry substance consistency, the composition of a biodegradable adhesive according to the invention contains i. 61 -74 % of alkali metal or alkaline earth metal silicate ii. 25-38 % of sucrose, iii. 0-10 % of one or more additives; wherein the ratio of al kal i metal or alkaline earth metal sil icate to sucrose is between 4:1 and 7:4, and preferably between 4:1 and 2:1 .

Thus this composition resides in products in which adhesive according to the invention has been used and in which the adhesive has dried, i.e. the solvent has evaporated. According to this, there is as an embodiment a paper product that contains composition according to claim 3.

There is yet another embodiment of pulp for manufacturing paper or cardboard, which pulp contains composition according to claim 3.

An embodiment of the invention is a composition in which the additive has been chosen from colourants, flavourings, additives and tracers.

Detailed description of the invention

The object of the invention is a biodegradable adhesive. It is significant for a biodegradable adhesive to contain alkali metal or alkaline earth metal silicate (e.g. Na2SiO3 or K2S1O3) and sucrose, and that the ratio of alkali metal or alkaline earth metal silicate to sucrose is between 4:1 and 7:4, and preferably between 4:1 and 2:1 . A biodegradable ad hesive is su itable for use by spread ing e.g. by a paintbrush, spatula or roller, by spraying or impregnating onto the object being glued. Optionally, it may be spread onto either one or both surfaces being glued . Spreading techniques and means are known to a person skilled in the art.

In its spreadable form , the biodegradabl e ad hesive has a so-called aqueous composition with alkali metal or alkaline earth metal silicate, sucrose, water and optionally one or more additives, and with the ratio of alkali metal or alkaline earth metal silicate to sucrose between 4:1 and 7:4, and preferably between 4:1 and 2:1 . Preferably, in the aqueous composition there are in percentages of weight: a. 55-68 % of alkali metal or alkaline earth metal silicate b. 24-32 % of sucrose, c. 8.1 -9.5 % of water, and d. 0-10 % of one or more additives; and further, in which composition the ratio of alkali metal or alkaline earth metal silicate to sucrose is between 4:1 and 7:4 and preferably between 4:1 and 2:1 .

In certain embodiments, the ratio of alkali metal silicate to sucrose is between 3.5: 1 and 2.5:1 . Most preferably, the ratio of alkali metal silicate to sucrose is approximately 2.9:1 . The inventors have noticed the aqueous composition with such ratio of alkali metal silicate to sucrose to form clear, colourless, stable glue composition with excellent adhesive properties.

According to an embodiment, the so-called aqueous composition is composed of 55-68 % of alkali metal or alkaline earth metal silicate, 24-32 % of sucrose, 8.1 -9.5 % of water, and 0-10 % of an additive.

Alkali metal or alkaline earth metal silicate refers to water-soluble, glass-forming silicate salts of alkali metals. In a composition according to the invention, sodium and potassium silicates are most appropriate because of their commonness and availability. Of the alkaline earth metal silicates magnesium silicates (Mg-silicates) and calcium silicates may be considered for use in the compositions according to the present invention. Of the magnesium silicates, at least synthetic magnesium silicate is known, wherein the molar ratio between magnesium oxide to silicon dioxide is approximately 2:5, and natural hydrated magnesium silicate, hydrated magnesium metasilicate also known as talcum, to which minerals such as alpha quartz, calcite, chlorite, dolomite, magnesite, and phlogopite may be incorporated. Another alkaline earth metal silicate is calcium silicate, which is white powder refined from lime and silicon dioxide. Silicates from other alkaline earth metals, such as beryll ium , strontium , barium and rad ium , are not commercially as attractive as calcium and magnesium silicates, due to the rarity and adverse health effects related to the same. The biodegradable adhesive prepared from magnesium silicate according to the present invention was noticed to possess a characteristic smell.

Instead, the biodegradable adhesive, when prepared from sodium silicate, has a further advantage of being odorless. The alkali metal silicate according to the invention is chosen from sodium silicate (Na2SiOs) and potassium silicate (K2S1O3) or mixtures thereof. Mixtures comprising both alkali metal silicates and alkaline earth metal silicates may be contemplated.

Sod ium sil icate is a common name for the compound sodium metasilicate, Na ₂ SiO3, also known as water glass. K2S1O3 is respectively known. It can be obtained as an aqueous solution or in solid state. Water glass is formed when molten alkali metal carbonate and silicon oxide react. The reaction can be illustrated as in the following

K2CO3 + SiO ₂ → K2S1O3 + CO ₂ or

Na ₂ CO3 + SiO ₂ → Na ₂ SiO3 + CO2

Water glass is manufactured by mixing sodium carbonate or potassium carbonate and quartz into a homogeneous mixture and feeding the mixture into a glass oven. By the effect of heat (> 1200 °C), the carbon dioxide of the carbonate is released and the sodium or potassium oxide reacts with the quartz forming sodium or potassium silicate. Further processing may be made for instance such that the molten glass is drained from the oven to cooling conveyors at which time it solidifies. From the conveyors the solid glass falls into a container for glass and is further transferred to an outdoor storage.

The alkali metal or alkaline earth metal silicate used in the invention (K2S1O3 or Na2SiO3) is preferably of technical standard of purity. Alkali metal or alkaline earth metal silicate suitable for industrial use is commercially available from various chem ical suppl iers in a form suitable for the composition accord ing to the invention. Preferably, alkali metal or alkaline earth metal silicate is used as powder.

Sucrose is also known as household sugar. It is a disaccharide formed of glucose and fructose. Sucrose is obtained for instance by purifying and enriching from sugar plants such as sugar cane and sugar beet. The recovery processes of sucrose for food use are known to a person skilled in the art. A standard of purity applicable for food use, obtained by crystallising, is suitable for the composition according to the invention. Preferably, sucrose obtained for source material is used in crystalline form.

In certain embodiments, the composition turns clear and permanent. According to an embod iment, bleached sucrose is used, in wh ich case the composition according to the invention is colourless, clear and transparent. Optionally, unbleached sugar may be used, in which case the composition is of a brownish tone and suitable for applications in which colourlessness is not needed or in which the objects to be glued are substantially brownish. Dry substances dissolve in a water solvent. A composition in a completely solu ble form is easily spreadable.

Other advantages have been perceived in embodiments of a composition according to the invention. A composition according to the invention is non-toxic and thus suitable for use in various objects, including food casings. No toxic or harmful gases are evaporated from it in any phase. The composition is safe for both users and the environment.

The composition in its spreadable form has been analysed for its microbiological qualities. Basic alkal i metal silicate as such is known to be an antimicrobial substance. In the spreadable solution also sugar has antimicrobial effect and synergetically, the composition is a difficult growth medium for the most common microbes. In the performed experiment (in the experimental section), in the composition analysed as an aqueous solution, no microbial growth was perceived in the sample which had been produced before the analysis or in the sample which had been stored in room temperature for one month before the analysis. Bearing in mind the life span of products to be glued, the adhesive and products glued by using it has one more advantage. When composted, the composition disintegrates into its components, which are substances otherwise found in the nature. The adhesive or products glued by using it may be disposed of among organic or compostable waste. Should accidents happen, the recovery or disposal of the composition does not cause occupational health or environmental hazards.

Combustion does not form harmful discharges, either. The composition also has fire slowing or flame proof qualities, which is preferable in some embodiments.

For some applications, a biodegradable adhesive according to the invention provides benefits in not being of animal origin. There are also applications in which it is preferable for the adhesive and its object that the adhesive is not of starch origin.

It is in the nature of adhesives that when they are in a spreadable form, they contain solvent that adds to the composition amongst other things qualities dissolving, dispersing, lubricating, assisting spreading and dosing the components of the composition and/or dissolving, softening, activating, roughening or otherwise increasing adherence to the surfaces to be glued. Within the scope of this invention, the solvent is water, H ₂ O. Use of water as solvent instead of for example organic solvents provides benefits: environmental requirements for production, use and waste d isposal are more relaxed, occupational health requirements in production and further use are more relaxed and the regulatory routines easier to handle. Normal freshwater, especially water intended for household use, e.g. tap water according to the usual European standards is applicable for being used in the composition in according to the invention. Microbiological purity is typically sufficient in tap water because the composition as such has antimicrobial effects. If there are requirements for uniform quality, it is preferable to use purified, e.g . filtered water.

When spreading the composition onto the surface of the product it is preferable to use spreading means according to the state of the art. Optionally, the composition may be spread with fingers because as a non-toxic adhesive it is not harmful when touching the skin. In consumer packaging and corresponding use, it is particularly preferable to supply the composition according to the invention in a 20-1000 ml container, which may optionally have a spreading tip. A preferable embodiment is a 40-100 ml container arranged to feed adhesive from the container onto a roller, by rolling which the adhesive is transferred from the container via the roller onto the object being glued. Preferably, the container fits in hand and acts as a handle such that the adhesive does not get onto the user's hands.

Characteristically for adhesives, the adhering qualities improve and the objects to be glued adhere to each other when the solvent evaporates. This is also referred to as curing. In the case of the present invention, the composition changes such that the proportion of water decreases. The water content of the composition being spread substantially diminishes into the same moisture level as in the objects to be glued, typically below 8 %, when glueing papers below 5 %, and in some cases below 1 %. It is clear to a person skilled in the art that the moisture level of many objects may vary influenced by environmental circumstances.

Cured adhesive may be described as a composition the dry substance proportions of which in percentages of weight are: i. 61 -74 % of alkali metal or alkaline earth metal silicate ii. 25-38 % of sucrose, iii. 0-10 % of one or more additives; and wherein the ratio of alkal i metal or alkaline earth metal silicate to sucrose is between 4:1 and 7:4, and preferably between 4:1 and 2:1 .

These ingredients may be analysed from the product in which the composition has been used and from which the solvent has evaporated and the adhesive has cured.

According to an embodiment, the cured adhesive may be returned to a so-called aqueous composition by introducing water into the object, especially to the part containing adhesive. In that case the adherence is decreased and the glueing may be renewed.

Optionally, a composition according to claim 1 or 3 may be manufactured such that it contains an additive chosen from colourants, flavourings, fillers and tracers. Depending on the application, only one colourant, flavouring, filler or tracer may be chosen, or multiple colourants, flavourings, fillers or tracers or mixtures thereof in desired proportions may be used. Preferably, the additive being used is non-toxic. Preferably, the additive being used in the composition is water-soluble. In the composition, inert additives, e.g. flavourings of plant origin or mineral flavourings, colourants or fillers are accordingly applicable. For instance so-called vegetable colours obtained from plants or fungi are suitable additives for the composition. For obtaining visual effects, it is possible to mix glitter and other particles in the composition. For a person skilled in the art, choosing additives comes routinely when desired qualities are known. Likewise, a person skilled in the art can choose suitable amount of additives, understanding that e.g. flavourings are used to a lesser extent than fillers.

An object of the invention is pulp for paper production, which pulp contains composition according to claim 3. The pulp typically contains fibre material suspended in water. Mixed in the aqueous solution there may be composition according to the invention 0.01 -20% percentage of weight of the dry substance proportion of the pulp. Use of the composition in the pulp for manufacturing paper improves strength and surface qualities of the manufactured paper or cardboard product.

Advantages are achieved with the embodiment in which the adhesive according to the invention is m ixed into the pul p for paper or cardboard production. An advantage is the biodegradability of the paper manufactured from the pulp if other components in addition to the adhesive according to the invention are also biodegradable. Further advantages are the microbiological purity of the adhesive which is proven by test and anti-microbial qualities. This is particularly preferable when manufacturing processed paper or cardboard products for the use of food industry. An adhesive having anti-microbial activity not only reduces the growth of microbes possibly left from the paper manufacturing environment and their reproducing in the fin ished product or precursor, but also reduces microbe population when the finished product is exposed to the environmental microbe strain during the storage and use. An object of the invention is also a paper product containing composition according to claim 3. A paper product may be obtained like described in the foregoing by mixing a composition according to the invention in the pulp in manufacturing paper or alternatively by spreading or spraying the composition onto the surface or surfaces of the finished paper, paper product or its blank.

Embodiments of the aqueous composition according to the invention are suitable for glueing other objects, as well. For instance usual art and craft materials such as wood and other materials of plant origin, feathers, leather and other materials of animal origin, fabric, plastic etc. are such other objects.

The experimental section

Composition according to the invention was manufactured for the tests without additives, containing sodium silicate, sucrose and water.

Microbiological quality

Three culture tests according to standards were performed in an accredited laboratory for measuring the microbiological quality.

There were two samples of an aqueous composition according to the invention, of which 3175-2 had been stored for one month in room temperature in a bowl with lid not particularly protected from microbe contacts. The other sample 3175-2 had been prepared just before the microbiological analysis. The results are shown in Table 1 .

Table 1 . The results of the microbiological tests.

Analysis 3175-1 3175-2 Unit Method

Sample of Sample of

adhesive adhesive

BioJahu ® The second

version

Total number of < 1 < 1 CFU/ml internal method bacteria (TSA 30 °C 2 days)

Amount of mould + < 1 < 1 CFU/ml internal method yeast (OGYE 20 °C 5 days)

Amount of mould + < 1 < 1 CFU/ml internal method yeast (OGYE 30 °C 2 days)

Est. estimate, < less than, > more than

In a statement, the laboratory confirmed that the microbiological quality of both samples was good.

Good microbiological quality is particularly preferable when the objects to be glued are manufactured for the use of food industry, e.g. food packages. An adhesive having anti-microbial activity not only reduces the growth of microbes possibly left from the paper manufacturing environment and their reproducing in the finished product or precursor, but also reduces microbe population when the finished product is exposed to the environmental microbe strain during the storage and use.

Fire retardant properties

Composition according to the invention was applied onto a sample of paper. The sample and the control paper were exposed to open flame. The burning was monitored visually. The paper containing composition according to the invention burned more slowly than the control paper.