Abstract:
The invention relates to a method of marking a substrate comprising treating the substrate with a boron compound and a charrable agent, and, irradiating the areas of the substrate to be marked such that those areas change colour. Marked substrates obtainable by this method are also provided.

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
     This application is a National Stage Application of International Application Number PCT/GB2009/051062, filed Aug. 26, 2009; which claims priority to Great Britain Application No. 0815999.8, filed Sep. 3, 2008 and Great Britain Application No. 0905785.2, filed Apr. 2, 2009; all of which are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a method of marking a substrate comprising treating the substrate with a boron compound and a charrable agent, and to substrates marked using this method. 
     BACKGROUND OF THE INVENTION 
     Laser coding of materials is well-known—see for example, U.S. Pat. No. 5,783,793, U.S. Pat. No. 4,906,813 and also U.S. Pat. No. 5,340,628 which seeks to contain the particles produced by ablation. These methods present a variety of problems, including difficulties in maintenance, line down-time, taint, as well as the need for extraction. 
     WO 02/068205 teaches a method for marking an object using a laser. Suitable additives, including for instance a polyhydroxy compound and a dehydrating compound are provided in a coating on a solid substrate, which is then imaged. Exemplary substrates include foodstuffs. The polyhydroxy agent is typically a sugar, and a metal salt may be used to remove OH groups. 
     WO 2007/045912 describes methods of marking substrates such as paper, card or board. The substrate is coated with a solution of a soluble alkali or alkaline earth metal salt of a weak acid, and areas of the substrate are irradiated such that those areas change colour. 
     In view of the prior art there remains a desire to provide improved marking methods which can be used on a wide variety of substrates. 
     SUMMARY OF THE INVENTION 
     In one aspect the present invention is a method of marking a substrate comprising treating the substrate with a boron compound and a charrable agent; and irradiating the areas of the substrate to be marked such that those areas change colour. 
     In another aspect of the invention there is provided a marked substrate obtainable by the method of this invention. 
     The inventors have found that surprisingly a formulation comprising a boron compound and a charrable agent such as a sugar can be coated onto any substrate, and gives rise to black (rather than unfavourable brown) images when imaged using a CO 2  laser. 
     Laser imaging with a boron compound on a non-polysaccharide containing substrate (such as PET) in the past gave negligible contrast imaging. However, adding a charrable polysaccharide to the boron compound results in a black image at relatively low laser powers—for instance around 20% laser power with a 40W CO 2  laser. This enables marking at a higher speed and marking on thermally sensitive substrates. It also allows imaging through laminates. 
     On polysacharride-containing substrates it has also been found that surprising results are obtainable by including a charrable with a boron compound. Again black images at relatively low powers can be produced, as opposed to brown images at high powers. Without an added charrable a 40W CO 2  laser required 60% power to generate brown marks. However, including a charrable gave black marks at around 20% power. Less power gave greatly improve scuff resistance. 
     We have also found that the formulation used in this invention is surprisingly heat resistant. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Preferred boron compounds for use in this invention are borates, and examples of such compounds include but are not limited to monoborates, diborates, triborates, tetraborates, pentaborates octaborates, metaborates and the like. Further examples of borates are given in “Chemistry of the Elements” by Greenwood and Earnshaw, 2 nd  Edition, Elsevier 1997. Other compounds that form part of the present invention include perborates, boron oxides, and boric acid. 
     Preferred borate compounds are borate salts. Particularly preferred are borate salts formed with alkali and alkaline earth metal cations including: lithium, sodium, potassium, rubidium, cesium, beryllium, calcium, magnesium, strontium, and barium salts. Ammonium and amine salts also form part of the present invention. Transition metals cations can also form salts with borates that form part of the present invention and include iron, copper, cobalt, nickel and zinc. Other metal salts include aluminium borates. The borate salt can have any number of waters of crystallization. 
     Also included are borate esters such as triethyl borate and the like. 
     A particularly preferred boron compound is sodium metaborate. 
     The charrable agent can be any substance that undergoes a charring reacting on heating to yield a contrasting colour. Suitable examples of charrable agents are compounds that typically contain a high content of carbon and oxygen. Preferably the charrable agent is a carbohydrate. Examples of suitable carbohydrates include saccharides, polysaccharides, sugars, polysugars and sugars wherein the carbonyl group has been reduced to a hydroxyl group, to give a sugar alcohol, starches, celluloses, gums and the like. 
     Examples include but are not limited to glucose, sucrose, saccharose, fructose, dextrose, lactose, sorbitol, xylitol, pectin, mannitol, manitose, erythritol, galactose, cellobiose, mannose, arabinose, ribose, erythrose, xylose, cyclodextrin, meso-erythritol, pentaerythritol, indulin, dextrin, polydextrose, maltose, maltodextrin of any DE, corn syrups, starch, amylose, amylopectin, pectic acid, cellulose and cellulose derivatives such as such as sodium-CMC, and hydroxypropylcellulose, galactomannans, guar gum, locust bean gum, gum arabic and the like. Other examples of charrable agents include amino acids, amino sugars such as glucosamine, chitin and chitosan, alginates as taught in WO06/129086, gluconates and malonates as taught in WO06/129078, and any of the charrable compounds which undergo an elimination reaction as taught in WO02/068205, such as poly(vinyl alcohol) and poly(vinyl chloride). Further examples of charrable agents are taught in WO08/107345. 
     The boron compound and the charrable agent of the present invention can be applied to the substrate as part of a coating that is applied to the surface of the substrate. This is done by formulating the boron compound and the charrable agent into a, usually liquid, ink formulation. The ink can be water or solvent based. The ink can be applied to the substrate using any printing process such as flexo, gravure, screen printing, off-set, UV curable and flood coating and the like. 
     The boron compound and the charrable agent can also be directly embedded into the substrate. This is done by adding the boron compound and charrable agent into the substrate as it in manufactured (for example, into paper at the sizing stage). 
     The ink formulation or substrate comprising the boron compound and the charrable agent can further comprise other additives. Examples include binders; anti-foam agents; biocides; surfactants; rheology modifiers; colour forming agents which can be inorganic such as molybdates or tungstates (particularly preferred is ammonium octamolybdate), or organic (examples include leuco dyes, diacetylenes and charge transfer agents); acid generating agents such an ‘onium’ type species; base generating agents; UV absorbers; light stabilizing agents; optical brightening agents; traditional dyes and pigments; whitening agents such as TiO 2 ; near infrared absorbing agents such as copper (II) salts (particularly preferred is copper (II) hydroxyl phosphate), non-stoichiometric compounds (particularly preferred are reduced indium tin oxide and reduced zinc oxide), organic NIR dyes/pigments such as N,N,N′,N′-tetrakis(4-dibutylaminophenyl)-p-phenylenediammonium diperfluoroantimonate, conductive polymers such as PEDOT; and Iriodin and Lazerflair type pigments as used in laser welding applications. 
     The irradiation that causes the substrate to change colour is preferably supplied by a laser. The laser can have a wavelength in the range 120 nm to 20 microns. 
     Particularly preferred lasers are CO 2  lasers operating in the mid-infrared region typically with a wavelength of 10.6 microns. Also preferred are NIR lasers operating with a wavelength in the range 780 to 2500 nm. When a NIR laser is employed it is preferable to include into the substrate an NIR absorbing agent as described above, particularly if its absorptivity profile approximately matches the wavelength of the laser. The laser can be a single steered beam, or an array of laser emitters. A suitable non-coherent irradiation source can also be used. The radiation can be monochromatic or broadband. 
     The substrate can be any substrate which requires printed information. Examples include but are not limited to: paper, card, corrugate, board, metals, foils, glass, textiles, wood, leather, plastic films such as PET and PP, plastic parts, foodstuffs and pharmaceutical unit dose preparations. The substrate of the present invention can then also be used to form labels, or primary or secondary packaging. 
     The laser can be used to image on to the substrate human readable text, graphics, logos and devices and machine readable codes such as barcodes and the like. 
     EXAMPLES 
     Example 1 
     An ink formulation was made up as follows: 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Water 
                 35.0 
               
               
                   
                 Sucrose 
                 4.0 
               
               
                   
                 Polydextrose 
                 4.0 
               
               
                   
                 Biocide 
                 0.4 
               
               
                   
                 Anionic surfactant 
                 0.2 
               
               
                   
                 Borax 
                 20.0 
               
               
                   
                 Binder 
                 36.0 
               
               
                   
                 Anti-foam 
                 0.4 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     An ink formulation was made up as follows: 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 % 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Water 
                 35.0 
               
               
                   
                 Maltodextrin 
                 8 
               
               
                   
                 Biocide 
                 0.4 
               
               
                   
                 Non-ionic surfactant 
                 0.2 
               
               
                   
                 Sodium metaborate 
                 20.0 
               
               
                   
                 Binder 
                 36.0 
               
               
                   
                 Anti-foam 
                 0.4 
               
               
                   
                   
               
             
          
         
       
     
     The inks were drawn down on to 50 micron PET film and clear BOPP using an RK-coater and 30 micron K-bar. 
     A CO 2  laser was used to black characters on the substrate including human readable text and machine readable barcodes.