METHOD OF MAKING A WATER-SENSITIVE SUBSTRATE WATER-RESISTANT AND RIGID IN THE WET STATE, AND COMPOSITION FOR CARRYING OUT THE METHOD

Abstract: A method of making a water-sensitive substrate water-resistant and rigid in the wet state, is disclosed. The substrate is soaked with an aqueous composition comprising a water-soluble alkali silicate, a hardener for the alkali silicate, and a dispersed waxy substance, whereupon the substrate is dewatered and heat-treated. A composition for carrying out this method is also disclosed. The composition is a thermosetting composition comprising water, a water-soluble alkali silicate, a hardener for the alkali silicate, and a waxy substance dispersed in the water.

Description: METHOD OF MAKING A WATER-SENSITIVE SUBSTRATE WATER-RE¬ SISTANT AND RIGID IN THE WET STATE, AND COMPOSITION FOR CARRYING OUT THE METHOD The present invention relates to a method of making a water-sensitive substrate water-resistant and rigid in the wet state, and to a composition for carrying out the method. The sizing and surface treatment of paper or paper products with alkali silicate has been well known for a long time. The alkali silicates utilised in the context are per se water-soluble, for which reason difficulties were at first experienced with the water resistance of the products. However, by adding a so-called hardener, usually in combination with a heat treatment, the alkali silicate can be made sparingly soluble or insoluble in water, whereby products of some water resistance are obtained. Yet, the treatment was intended to impart rigidity to the paper or the paper product. However, no total water resistance is obtained with an alkali silicate layer on both sides of the substrate, even if the layers have been made sparingly soluble in water. The layers are rendered porous and permeable to water because the application of the alkali silicate layers causes the formation of a gel which dries and forms a porous structure, and through the pores water can pene¬ trate into the substrate and soften it so that the rigidi¬ ty disappears and any glue joint attachments are weakened. This is a major problem of packaging materials of paper, board or corrugated board in different forms. One way of providing hydrophobation of a water-sen¬ sitive substrate is to impregnate it with a wax disper¬ sion. This is freguently the case in, for example, the ma¬ nufacture of paper for packaging food products, such as bread. It is, however, extremely difficult to achieve a uniform distribution of the wax particles in the disper¬ sion throughout and across the entire substrate. The wax layer acguires both pores and cracks through which water can penetrate into the unprotected parts of the substrate. Another alternative is to deep-impregnate the sub¬ strate, such as paper, with a hydrophobing agent, but con- ventional paper makes have inferior impregnability, and also in this case pores or cracks arise in the impregna¬ tion. Besides, this technigue is costly. If the paper substrate is to be used for the manufac¬ ture of corrugated board, and one elects to make the paper hydrophobic before the manufacture of the corrugated board, there is obtained, besides a higher cost due to an additional work operation, also an inferior attachment for gluing. It is the object of this invention to provide a method of making a water-sensitive substrate water-resis¬ tant and rigid in the wet state. The invention provides, on the substrate, a layer which is sparingly soluble in water and, at the same time, protects the actual substrate against water. The method according to the invention is character¬ ised in that the surface of the substrate is coated with an agueous composition comprising a water-soluble alkali silicate, a hardener for the alkali silicate, and a dis¬ persed waxy substance, whereupon the substrate is dewa- tered and heat-treated. A further object of the invention is to provide a thermosetting composition for use in a method of making a water-sensitive substrate water-resistant and rigid in the wet state. The thermosetting composition comprises water, a water-soluble alkali silicate, a hardener for the alkali silicate, and a waxy substance dispersed in the water. Substrates suitable for the treatment according to the invention preferably are substrates of paper or board, such as corrugated board, or other packaging materials of paper. By treating the substrate surface with an agueous so¬ lution containing, at the same time, the alkali silicate, a hardener therefor, and a dispersed waxy substance, there is obtained, after drying and hardening, a surface layer of alkali silicate which in its pores has hydrophobic wax particles. In this manner, the pores are stopped up, and furthermore the entire substrate surface becomes water-re¬ pellent and exceedingly anhydrous, such that the substrate is not wetted when exposed to water after hardening. The aging of an alkali silicate layer freguently causes cracks in the layer. By the treatment according to the invention, all the pores in the surface layer of alka¬ li silicate will contain wax particles, and the pores is¬ suing into the cracks formed are thus filled with the waxy substance, whereby both sides of the cracks are hydropho¬ bic and therefore water-repellent. This means that no water can penetrate into the substrate, not even after crack formation. The alkali silicate is thus made sparingly soluble or insoluble in water by hardening with a suitable hardener. The hardener may be any hardener usually employed in the art for alkali silicate hardening. Suitable hardeners are, for example, oxides and carbonates of divalent or poly¬ valent metals, such as zinc, calcium, beryllium, copper, tin, boron, aluminium etc. A presently preferred hardener is zinc oxide which has the advantage that it is inexpen¬ sive and readily available, simultaneously as it reacts with the alkali silicate already at a temperature of about 93-100°C. The zinc oxide is added in an amount as low as about 0.1-10% by weight of the dry solids content of the alkali silicate, and the heat treatment is performed for about 30-60 min. If a less efficient hardener is added, the hardening time must be extended and/or the hardening temperature raised. Hardeners hardening at room tempera- ture may also be employed. Suitable alkali silicates for use in the context of this invention are sodium silicates, Na-0/SiO-,. The Na 2 0 to SiO_, ratio may vary between 1.5 and 4 and preferably is 3.0-3.5. The alkali silicate concentration should lie be- tween 10 and 50%, preferably in the range 32-40%, based on the total weight of the solution. The waxy substance is selected among hydrophobic waxes and paraffins. A preferred wax type is paraffin wax. The concentration of the wax dispersion added to the com- position is immaterial. If, for example, a 40% dispersion of wax in water is used, it is preferred to add 1-40% thereof, based on the weight of the alkali silicate solu¬ tion. Calculated as % by weight dry wax, 0.4-16%, prefer¬ ably 1-4%, is added. The composition preferably contains also a nonionic dispersing agent for the waxy substance. By treating e.g. corrugated board by the method ac¬ cording to the invention, the protection against water will be so good that the corrugated board does not soften after immersion in water for several minutes. Further details of the invention will appear from the following nonlimiting Example. Example 1% by weight zinc oxide, ZnO (powder), and 10% by weight of a 40% by weight paraffin dispersion containing a nonionic dispersing agent were added under agitation to a solution of sodium silicate having a Na 2 0/Si0 2 ratio of 3.3 and 38° Be. A double corrugated board sheet having a width of 33 mm and a height of 45 mm (in the direction of the cor¬ rugations) was immersed in this sodium silicate solution. After draining, the corrugated board sheet was hardened for 30 min. at 93-100°C, whereby the sheet was coated on all sides with a sodium silicate layer, and the pores were stopped up with paraffin. When the sheet had assumed room temperature, it was immersed for 5 min. in tap water. After a further 5 min. at room temperature, the co pressive strength was mea¬ sured. Six different specimens gave compressive strength values between 588 and 785 N (between 60 and 80 kp) for the specimen which, standing on edge, had a width of 30 mm and a height of 45 mm. The corresponding value of a sodium silicate solution containing 5% by weight ZnO and having the same hardening time, but without wax dispersion, is 147 N (15 kp) , and for a sodium silicate solution containing 6% by weight wax dispersion and no hardener, 98.1 N (10 kp). For a dry un¬ treated corrugated board sheet of the same dimensions, the value is 736-765 N (75-78 kp) . This shows that there is obtained in the treated sheet after immersion in water a strength egualling that of a dry untreated sheet, and several times higher than in sheets treated with either alkali silicate and hardener only, or with a wax dispersion only. The test thus shows that a very high synergistic effect is obtained by the treatment with an alkali silicate solution containing both a hardener and a wax dispersion.

Claims: CLAIMS 1. A method of making a water-sensitive substrate water-resistant and rigid in the wet state, c h a r a c ¬ t e r i s e d in that the surface of the substrate is coated with an agueous composition comprising a water-so¬ luble alkali silicate, a hardener for the alkali silicate, and a hydrophobing dispersed waxy substance, whereupon the substrate is dewatered and heat-treated. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the composition also comprises a nonionic dispersing agent for the waxy substance. 3. A method as claimed in claim 1 or 2, c h a - r a c t e r i s e d in that a sodium silicate is used as alkali silicate. 4. A method as claimed in one or more of the preced¬ ing claims, c h a r a c t e r i s e d in that the sub¬ strate is paper or board. 5. A method as claimed in one or more of the preced¬ ing claims, c h a r a c t e r i s e d in that the waxy substance is wax or paraffin. 6. A method as claimed in one or more of the preced¬ ing claims, c h a r a c t e r i s e d in that the har- dener is selected among oxides or carbonates of Zn, Be, Cu, Sn, B, Ca, Al. 7. A method as claimed in one or more of the preced¬ ing claims, c h a r a c t e r i s e d in that the heat treatment is performed at a temperature of 20-600°C. 8. Thermosetting composition for Imparting water re¬ sistance and rigidity in the wet state to water-sensitive substrates, c h a r a c t e r i s e d in that it com¬ prises water, a water soluble alkali silicate, a hardener for the alkali silicate, and a waxy substance dispersed in the water. 9. A composition as claimed in claim 8, c h a ¬ r a c t e r i s e d in that it also comprises a nonionic dispersing agent for the waxy substance. 10. A composition as claimed in claim 8 or 9, c h a r a c t e r i s e d in that the alkali silicate is a sodium silicate. 11. A composition as claimed in one or more of claims 8-10, c h a r a c t e r i s e d in that the waxy sub¬ stance is wax or paraffin. 12. A composition as claimed in one or more of claims 8-11, c h a r a c t e r i s e d in that the hardener is a metal oxide or a carbonate selected among oxides or car¬ bonates of Zn, Be, Cu, Sn, B, Ca, Al.

Inventors: INGE MATS, INGE, MATS
Applicants: SILIKEMI AB, SILIKEMI AKTIEBOLAG
Application year: 1989
Main classification: D21H1
IPC codes: D21H19, D21H21, D21H25