SMI Labels and Packaging Materials

Sunday, October 5, 2014

Components of Self adhesive Labelstocks and the part they play in label converting.

This is a very large subject and a whole big book can be written on it as exhaustive information is availabel and needs to be dwelled upon. However still I have tried to reconstruct the gist of an article that I wrote in 2002 and have moderately updated it. I hope it benefits all the producers, converters and end users in Labels to understand the self adhesive materials.
The labelstock which is the starting material for self adhesive labels consists primarily of three main components.

1. The facestock: Which can be paper, film, foil, fabric, etc.
2. The adhesive: This is anchored on to the face stock.
3. The release liner or the protective backing: This can be paper or film.

We shall discuss each of the components separately and dwell on their impact in the process of label converting. There was a time here in India, not long ago, when with just one multi-purpose coater you could cater to most of the label printers, making release liners and then coating and laminating with adhesives on the same equipment. Alternatively silicone liners were outsourced but intense competition made in house siliconising an imperative. Though this may still hold good for many a labelstock manufacturer, yet things have changed at the higher end for the better. Specialized equipments for coating the newer silicone technologies, different adhesives like hotmelts, emulsions and solvented systems and other equipments to technically upgrade the labelstocks are finding place in the manufacturing units.

Face stocks: Paper remains the most widely used face material for labels, though in recent times filmic stocks

have gained substantial market shares. The most predominantly used varieties of paper initially were the uncoated matt, the one side or two side coated semi-gloss popularly known as the chromo art paper and the cast gloss also known as mirror coat paper but in recent times semi-gloss or one side coated C1S paper is the most used variety. The usage of woodfree or uncoated paper variety is also substantial but seems limited to plain price labels, mailing labels and VIP labels. With the advent of UV curable inks and varnishes which also provide high gloss attributes to labels, the usage of Mirror coated or cast gloss paper has not grown in labels or has decreased. Moreover because of stiffness of this paper, a self adhesive label may have edge lifting defect at higher ambient temperatures.  It is advisable that for tightly curved surfaces flexible varieties of face stocks be used. Now with customer needs getting to be more specific, it is necessary to use more modified varieties of paper. General requirements of the paper include the surface and mechanical characteristics. Printability is a function of the paper surface quality while mechanical properties like lay flat property, dimensional stability and flexibility, influence the processes of printability due to curling, die-cutting, waste matrix removal and label dispensing. While papers are largely used as they are supplied directly by mills however films need to be modified if not already done so by the suppliers. Modifications of the surface characteristics include top coating, metalizing, laminating, etc. For non contact printing like inkjet on films, a relatively porous surface would be needed requiring special coatings that will make ink dry by absorption. Various pigments, compounds, fillers, binders and primers are used to coat the surface. Different pigments possess different surface energies and ph stabilities, the binders which are vehicles to carry pigments; influence the anchorage of the ink or topcoat, the stiffness, the porosity and ink absorbance. Depending on the requirement of the printing process i.e. offset, letterpress, Flexo, gravure, screen, inkjet, laser, thermal transfer, digital, etc. the top coatings are designed accordingly. The top coatings for paper are mostly done at the paper mills where the paper is produced however some coaters develop their own coatings and coating methods to achieve different qualities in face papers but this is getting to be rare. Sometimes paper is coated with a barrier or a primer on the reverse side to prevent any migration of the plasticizer from the adhesive to the face or for acting as an anchoring agent for the adhesive. However the adhesive technologies have improved to an extent that this process is not largely required now. Other surface improvement techniques include latex impregnation for weather resistance, laminating with films to achieve hi-gloss, varnishing, top coating for frozen food and long life labels, special coatings with colorless leuco dye and acidic colour developer for producing direct thermal printing paper, etc. Due to requirements of producing in line to achieve economies of scale, now paper surface modification at the level of labelstock production is almost not there. Specialised converting companies supply modified paper for thermal transfer printing, direct thermal printing, etc. Labelstocks with textured surfaces are also in use in Europe and USA, especially for wine and gourmet food labeling. Some printers in India have become innovative and use special dies to emboss and deboss paper while converting on their label presses to produce textured effects. For mechanical properties requirements, (a) the strength of the paper is important (high for faster conversion and waste matrix removal requirements and low for security labels to achieve tamper evidence), (b) Bulk and moisture content of the paper influences the dimensional stability. An ideal paper would have 6% humidity. Most widely used face papers are in the range of 70-85 GSM. Today most of mechanical properties are left to mills and labelstock manufacturers just need to select the mill from which to source paper and decide on the grammage of paper that will deliver the desired mechanical strength. The label printers use from films of 15 microns to boards of 300GSM.

Synthetic face materials are finding extensive usage, given the requirement of the no label look and the better aesthetics exhibited by the film labels. Consider a shampoo bottle and the harsh environments it has to encounter in its lifespan. The heat, steam, cold, and sometimes dry conditions all exist in the bathrooms. Add to that the continuous squeezing of the bottle, well you can understand that paper will not do the job, it will not stretch with the bottle and return to its shape, you will end up having a wrinkled label, so you really need a film label and that to a flexible one maybe like Polyethylene. To get the right kind of printing the surface properties of the film are of very high importance. The surface properties required include, the surface tension to be higher than 40 dynes. The ideal would be to have an online corona treater. Films generally have better surface finish, good gloss or matt as required, excellent aesthetics and chemical resistance. Film manufacturers now offer improved surface properties by supplying top coated films for better printability. These films come with a print receptive primer coating. This however makes the product more expensive. Labelstock manufacturers do offer pretreated film stocks but the level of treatment falls on extended storage and leads to reduced anchorage of the printed matter. It is for this reason that an online corona treater is recommended. The ideal still would be a film top coated for print receptivity. As regards mechanical properties, the films have generally improved die cutting and faster waste matrix removal provided proper dies are used. The filmic materials require a sharper angle for blades than paper. Take for example LLDPE which is neither foldable nor stiff and difficult to die cut due to increased elasticity. One ends up blaming the release liner even though the problem lies with the selection of the filmic face stock or the flexible die. For automatic label dispensing the film has to be stiff enough and flexible enough to conform to the shape of the container to resist deformation. Today companies are using multilayer construction of films or hybrid films to achieve the correct properties in terms of printability, die-cutting, dispensability and squeezabilty.

Adhesives: As regards adhesives, again here the scenario is fast changing in India. Gone are the days when you could coat only the general purpose permanent acrylic emulsion adhesive and keep servicing all the segments in labels. With surfaces becoming diverse and the usage of labels becoming complex, a label printer needs to offer a wide variety of adhesives namely acrylic emulsions, solvent borne acrylics, Hotmelt adhesives, rubber resin adhesives, etc. with different grades such as permanent, removable, low-tack, medium tack, high tack, delayed action, deep freeze, high temperature resistant, etc.
The selection of adhesive is of prime importance and needs the continued partnership between the labelstock manufacturer, label printer and the label user. Consideration of the substrates, the conditions of application, the converting process, the storage and usage conditions is imperative before deciding on the adhesive. The adhesive cannot be such that it does not anchor on to the face or the substrate on which is to be applied. In either case it will create a non acceptable situation whereby either the face will come off leaving the adhesive on the substrate or the label will not adhere well on the substrate. Another point to consider would be that die-cutting is better in emulsion adhesives as compared to hotmelts. Also the high initial tack of hotmelts make the die-cut adhesive to rebond on keeping making waste matrix removal difficult. Hotmelt adhesives have very high initial tack while emulsion adhesive attain final tack over a period of time, so if a label has been fixed wrongly in case of acrylic adhesive, it can be instantly removed and refixed but it cannot be done so with hotmelt adhesive. As it will bond immediately and removal will amount to a damaged and wasted label. It is for this reason that companies who use large sized labels prefer acrylic emulsion adhesive to have less wastage due to wrongly affixed labels.

Release Liner: These are the most performing part of the labelstock. Unfortunately they land up in the dustbin as soon as the label is dispensed and starts its journey as a part of the final product. The types of substrates being used to produce release liners are:

1. Glassine, which is the most popular in India
2. SCK or super calendered Kraft
3. CCK or Clay coated Kraft
4. Saturated papers or papers impregnated with saturants.
5. Polycoated papers
6. Synthetic liners like BOPP, PET, LDPE, etc

The above mentioned substrates are coated with silicone formulations to become release paper or release liners. The silicone chemistries available and there evolution so far is as follows:

1. Solvented silicone with Tin catalyst and thermal curing: This is a cost effective product having post curing reaction and the cross linking reaction, triggered by the heat in the drying chamber, could go on for days. In this the consistency in release levels is always in question. The usage of this chemistry has reduced substantially in recent times. In fact the technology has virtually become obsolete and is on the verge of being phased out. The usage of Tin catalyst and solvent in the system is a deterrent for any converter due to suspected toxicity of Tin compounds and Toluene. The solvent in the system is a perpetual fire hazard. A small spark due to static build up could cause massive damage by catching fire.

2. Solvented Silicones with platinum catalyst and thermal curing: It is better than the tin system as it has no or little post cure reaction but use of solvents is continuous fire hazard besides being toxic and not friendly to the environment. Many Indian siliconisers had shifted to this system as it could be coated on their existing equipments but due to the problems mentioned more development in this product cannot be expected. I am not aware of this product being used in India by anyone.

3. The solvent less platinum catalyst system with thermal curing: This system came to be implemented in India around 2003 and requires specialized equipment to coat the materials from a 100% solvent free bath. It gives a much enhanced performance with improved surface gloss ensuring increased area of contact for the adhesive, consistent and controlled release levels and is reproducible. Weldon Celloplast Ltd. was one of the first amongst the Indian labelstock manufacturers to report a shift over to solvent free silicone coating in the latter part of 2003. Today all new installations are being made employing this technology.

4. Solvent less silicone with platinum catalyst and UV curing: It is much quicker but not widely used in India. Some installations have been made in recent times and this is the preferred technology for heat sensitive films. It also eliminates the need for solvents or long heating chambers. Caution again here because the normal UV curing results in inconsistency in release levels. The UV curing of silicone needs to take place in an inert nitrogenous, oxygen free environment. The availability of this silicone is at present also an issue as the silicone has to be imported and is not formulated locally.

5. Solvent less silicone with platinum catalyst and Electron beam curing: It is an improvement on the UV curing system. Again this chemistry has not reached India as yet. There are perhaps just one or two installations at present in the country.

The additional advantages of the shift towards platinum catalysts systems is that the reaction is not post curing. All the curing is done in the heat chamber itself and the reaction comes to a stop when the substrates return to ambient temperatures. Also there exists the additional possibility of achieving different release levels by employing Control release additives.

With evolution of the label usage, selection of the right base paper or liner is also now of utmost importance. In the earlier days Indian label printers, end users and labelstock manufacturers would only want to use glassine based liners. Everything else was considered lower in quality. As the market has matured, CCK (Clay Coated Kraft) are preferred liners for the sheeted labelstock market. The base paper experiences high temperatures in excess of 150 degrees C for thermal curing of silicones and at this temperature the paper loses moisture and shrinks. On adhesive coating and exposure to atmospheric moisture the release paper expands bringing curl to the gummed sheets. This is not acceptable in offset printing. For this reason now CCK is advised because it has a porous back due to which the paper quickly regains moisture to stabilize and impart flatness to the converted sheets. In case of glassine which is a highly calendered paper which will not absorb moisture quickly even when subjected to inline humidification. Glassines are the preferred liners for barcode labels where the transparency is a necessity for sensing the labels and also in auto label dispensing glassine is preferred due to consistent caliper for die-cutting, better release levels and higher tensile strength. All these applications are for labels in roll form. These properties aid higher speeds with lesser web breaks. Similarly other varieties need to be considered for demanding applications.

Die cutting and other issues:
Die cutting is a technical operation transforming web like materials to discrete items like labels. It depends on solid state components of the laminate like face material and the release liner. Their quality and combination are important. For a good clean cut it is necessary to have a uniform caliper, densified paper liner. The thickness and density of the face paper are less important. Best results are obtained by having a highly densified paper, like glassine as liner. However where lay flat properties are of importance it is advisable to use the densified Kraft liners with porous backing. Dimensional stability of the liner is of utmost importance. As mentioned earlier in this article, if a liner is exposed to high temperature without rehumidification while siliconizing it loses its moisture content. This can cause wrinkles or curling. It is always advisable as far as possible to maintain the moisture level at 6-7% for both the face paper and the release paper. Printers often encounter problems like

1. Over cutting: This occurs if the die punctures a little into the release paper, the rest of the paper will split itself causing a thru punch.
2. Undercut: If the die blade cuts only the face and does not go though the adhesive, there is the possibility of labels coming off with the waste matrix. This happens because the adhesive has also cohesive properties due to which the adhesive film stays together. It is important to cut the adhesive layer also.
3. Clean cut: In an ideal situation the die blade should cut through the face paper and the adhesive and come

to stop just before and on the surface of the release paper. Dies employed for die cutting also play an important role in die cutting. For example in case of paper if the angle of the blade is wider the waste matrix removal becomes better. However this may not be applicable for films as for a clean cut a different angle of blade is necessary. This is a very technical issue and needs the close cooperation of the printer and the die manufacturer.
As for face stocks the best cutting is that of paper followed by PET which is quite similar in cutting to Paper and then we have BOPP and PE.

Waste matrix removal is a highly technical operation. It depends on a variety of issues. The strength of the face material is of prime importance and equally important is the release levels of the release liner. But here one has to strike a balance. If the release is too easy the labels may lift with the matrix and in case of computer labels they may come off while passing over rollers and stick to the print head and damage the same. In case the release is too tight the waste matrix will keep breaking, making converting a painful process. Other issues that confront the label printer are tensile strength requirements of release paper for the automatic label dispenser, residual moisture in the adhesive, adhesives tend to lose tack in winters due to hardening of polymers, emulsion adhesives attaining peak performance over a period of time, lack of adhesion to low energy surfaces like HDPE and a host of similar requirements. All these can be addressed effectively when all the three, the labelstock manufacturer the label printer and the label user put their brains together as partners and design the product.

The above article is written by Harveer Singh Sahni, Managing Director, Weldon Celloplast Limited New Delhi. Presented at India Label Show 2002 and updated moderately in October 2014

Note: Print publications are free to reproduce the above article in their magazines by giving due credit to the author.