LASER-MARKABLE PLASTIC LABELS

Abstract: Flexible plastic labels which exhibit excellent ink-printability, excellent laser-imprintability and excellent mechanical properties are produced by incorporating a suitable laser-opaque material into the core layer of a multilayer coextrudate of the type previously used for labeling flexible plastic bottles.

Description: LASER-MARKABLE PLASTIC LABELS BACKGROUND OF THE INVENTION The present invention relates to flexible, plastic labels for use on flexible plastic bottles, which labels are both ink printable and laser-markable. Many commodities are supplied today in squeezable, flexible plastic bottles. Examples of such materials are household cleaning products such as sink and bathtub cleaners, liquid soaps and detergents and the like as well as personal care products such as shampoos, conditioners, lotions, suntan oils and the like. Traditionally, flexible labels for squeezable, flexible plastic bottles have been made from paper coated with pressure sensitive adhesives. More recently, plastics have replaced paper to a large extent for this purpose. Plastic labels tend to exhibit a higher degree of flexibility, squeezability and a greater resistance to water and other chemicals than paper. Accordingly, plastic labels are becoming increasingly popular for use on flexible plastic bottles. Attaching flexible plastic labels to flexible plastic bottles is normally accomplished in one of two ways. In one way, a pressure sensitive adhesive is applied to the label and the label attached to a previously formed bottle by pressure. In the other way, known as IMFL (In Mold Film Label) , the label is placed into the mold used to form the bottle by blow molding and the label incorporated physically into the plastic bottle itself as part of the blow molding operation. In this procedure, a heat-activatable adhesive is normally applied to the label for firmly bonding the label to the bottle body. In actual industrial practice, flexible plastic labels are attached to flexible plastic bottles at high rates of speed. For example, typical industrial applications using pressure sensitive labels can process as many as 200 bottles per minute and even up to 600 bottles per minute. In typical industrial applications for IMFL, 5 to 150 bottles per minute can be made with labels attached. An important property of flexible plastic labels is that they are ink-printable. Ink printability depends both on the physical as well as chemical nature of the label surface to be printed. Accordingly, it is important in producing such labels that the materials and processes used to form the labels give label products having the appropriate surface properties. In addition to surface properties, however, it is also important that labels exhibit appropriate gross mechanical properties. In order that the labels can flex with the bottles to which they are attached, they obviously must exhibit suitable flexibility, at least in one direction (usually the transverse direction) . Moreover, in order that they can be effectively used in the modern, high-speed industrial processes described above, the labels must also exhibit additional properties such as die cutability, matrix stripability, dispensability (i.e., with enough stiffness to be dispensed at high speed from a peel plate or handled for insertion in a mold) and the like. Also, pressure sensitive labels further need to be repositionable, i.e., when misapplied they can be easily peeled off the bottle in a single piece with all the adhesive remaining on the label. In order to meet these requirements, much technology has been developed for the manufacture of flexible plastic labels. In accordance with one such development, the label body is made by coextruding a number of different plastic materials together to form a multilayer coextruded product. A real advantage of this approach is that the main body of the film can be formulated to maximize the desired gross mechanical properties of the label, while the skin layers of the product can be tailored for accepting printing ink, for receiving adhesives or both. A good description of how flexible plastic labels can be tailored to meet a variety of different physical requirements while still retaining ink-printability, both in the case of pressure sensitive adhesive labels and heat sensitive adhesive labels, can be found in U.S. 4,713,273; U.S. 4,888,075; U.S. 4,946,532; U.S. 5,143,570; U.S. 5,186,782 and U.S. 5,242,650, the disclosures of which are incorporated herein by reference. One type of product that is often sold in flexible plastic bottles is pharmaceuticals such as contact lens solutions and various other health care products. In order to conform to various labeling laws and other legal requirements relating to such products, it is often necessary for the manufacturer to mark the product containers with the date of manufacture as well as the lot number of the material being sold. In the past, this has normally been done by marking a preprinted label, i.e. a label which has previously been printed with all of the desired information thereon other than the lot number and date, with the lot number and date immediately before or after the bottle is filled. Typically, this is done either by a thermal transfer process or other conventional ink-printing process. Unfortunately, such printing operations are very time consuming and hence represent a real bottleneck in mass producing individually marked flexible bottles rapidly and efficiently. In order to overcome this drawback, it has been proposed to employ laser-marking technology to mark the date and lot number on individual bottles. In accordance with one such proposal, the date and lot number are marked directly on the bottle body. Most plastics, however, are transparent to laser light, and accordingly, it is necessary if adopting this proposal to incorporate a material which is opaque to laser light into the plastic forming the bottle body. Such materials are well known and exemplified, for example, in U.S. 4,595,647; U.S. 4,654,290; U.S. 4,753,863; U.S. 4,816,374 and U.S. 4,861,620. See also European Patent 0 190 997 as well as Kilp, Laser Marking of Plastics. Ortech International, Mississauga, Ontario, Canada, copyright ANTEC 1991, pp. 1901-1903. Each of these disclosures is also incorporated herein by reference. Another material also known to impart laser markability to various types of plastics is titanium dioxide-coated mica particles. Such materials are sold, for example, under the name AFFLAIR ® by E. Merk Company of Raway, New Jersey and MEARLIN ® Lustre Pigments sold by the Mearl Corporation of New York, New York. Unfortunately, such materials are comparatively expensive. Accordingly, incorporating such materials into the plastic materials used to form the bodies of the flexible bottles is not cost effective. In order to deal with this problem, it has also been proposed to laser imprint the labels used on the bottles rather than the bottles themselves. In accordance with one method of applying this technique, a preprinted label is provided with an ink-printed black field and the lot number and date laser imprinted on the black field by burning off the black ink with the laser. Because registration of the laser image with the black field becomes difficult at the high speeds involved in industrial operations, this technique requires a black field of relatively large size to be effective. This, in turn, is regarded by many as being aesthetically unpleasing and is therefore not desired. Another technique proposed for laser marking flexible plastic labels is to make the labels from a vinyl plastic such as polyvinyl chloride. Carbon dioxide lasers produce high quality, good contrast marks ranging in color from gold to deep orange on polyvinyl chloride films. Unfortunately, many manufacturers also regard this coloration as being aesthetically unacceptable. Also, polyvinyl chloride contains chlorine as well as plasticizers and therefore often produces noxious by-products upon laser bombardment. Accordingly, laser marking of vinyl labels is not attractive. Accordingly, it is an object of the present invention to provide a method for producing laser-markable labels whose backgrounds are transparent or light in color and which can be easily imprinted with easily-readable, high intensity indicia by conventional laser marking technology. In addition, it is a further object of the present invention to provide a method for producing laser-markable labels as described above which does not adversely affect the various other properties of the label such as ink-printability, die cutability, matrix stripability, dispensability, repositionability and the like. It is a still further object of the present invention to provide a method for producing ink-printable, laser-markable, flexible, plastic labels which employs a minimum amount of additives and thus is inexpensive to carry out. It is a still further object of the present invention to provide new flexible, plastic labels for use on flexible plastic bottles which meet all of the foregoing objects as well as to further provide web or sheet stock that can be processed by normal label manufacturing techniques and equipment to provide these labels. SUMMARY OF THE INVENTION These and other objects are accomplished by the present invention which is based on the discovery that flexible plastic labels which exhibit excellent ink-printability, excellent laser-markability and excellent mechanical properties can be produced by incorporating a suitable laser-opaque material into the core layer of a multilayer coextrudate of the type previously used for labeling flexible plastic bottles. Surprisingly, it has been found that by burying the laser-opaque materials in the core rather than in the surface layers of these multilayer coextrudates, neither the surface properties of the product, which control its ink-printability, nor the gross mechanical properties of the product, which control the ability of the product to be manipulated properly in modern high speed equipment, are compromised. At the same time, it has also been found that intense, highly visible contrasting marks can be produced in light-colored or transparent coextrudates made in this manner using conventional laser marking technology even though the laser opaque materials are restricted to the core layer and not present in the product surface where the laser light first impinges on the product. Accordingly, it is possible in accordance with the present invention to produce flexible, plastic labels for affixing to flexible plastic bottles, as well as webs or sheets of flexible plastic coextrudates useful in manufacturing such labels, which exhibit the same excellent combination of ink-printability and other mechanical properties as prior labels and which at the same time are readily laser-markable with conventional laser marking technology. DETAILED DESCRIPTION The present invention utilizes known laser marking technology to impart laser imprinted images to the unique, flexible, plastic labels made in accordance with the present invention. These unique plastic labels are multilayer coextrudates which are produced by coextrusion of at least two different polymer materials to form a product with at least two and preferably at least three distinct polymer layers bonded together. In general, such products are composed of an inner core layer and at least one outer skin layer. Usually, the product will have two outer skin layers, one on each side. One of these outer skin layers is typically intended to be ink printable (hereinafter "printing skin"), and accordingly the material used to form this layer and the manner of its extrusion are selected to maximize its ability to accept and retain printing ink. Usually, the other outer skin (hereinafter "bonding skin") is intended for bonding or facilitating bonding of the label to a bottle. Depending on the chemical nature of the core material, the method of bonding and the chemical nature of the bottle, this means that the bonding skin layer can either be adapted to receive a subsequently applied adhesive or, in fact, may constitute the adhesive itself. For example, where a label having a highly olefinic core layer is intended to be bonded to a bottle with an acrylic latex-based pressure sensitive adhesive, the bonding skin is preferably a material _ which promotes adhesion of the acrylic adhesive to the olefin core, for example, an olefin copolymer containing polymerized vinyl acetate. Where, however, a label having a highly olefinic core layer is intended to be attached by IMFL to a highly olefinic bottle made, for example, from HDPE (high density polyethylene) , the bonding skin layer can itself comprise the heat-activatable adhesive normally used for this purpose. A homopolymer or copolymer of ethylene or propylene is a good example of an appropriate material for this purpose. The thickness of the inventive labels can vary widely. Typically, they range between 0.5 and 15 mils, more preferably 1 to 10 mils, even more preferably between about 2 and 5 mils, thick. Of this amount, the printing and bonding skins each occupy about 5 to 25 percent of the thickness of the label, more typically about 10 to 15 percent of the thickness of the label, while the core layer occupies the rest. In most applications, manufacture of the inventive labels involves production of a continuous sheet or web of the coextrudate, orientation of the web or sheet usually in a single direction (machine direction) only, and finally cutting or otherwise subdividing the web or sheet into individual, discrete labels. In the case of pressure sensitive adhesive labels, the coextruded web or sheet after orientation is usually laminated to a release liner comprising the pressure sensitive adhesive, a release agent such as a silicone resin and a paper or film backing layer. The laminate so formed is then typically slit longitudinally into strips and the strips wound up on spools, which are stored and/or sold, as desired. When it is desired to ' make and use labels, the laminate strip 'after unwinding from the spool is fed to a printer/die cutter. This machine ink prints the desired graphics on the coextrudate layer and immediately cuts this layer plus attached adhesive into individual labels. In this cutting operation, a small strip of the coextrudate layer is typically left between adjacent labels so that the coextrudate layer after cutting is composed of a plurality of individual, discrete labels plus an intergral matrix of coextrudate material surrounding the individual labels. This matrix is then removed leaving a strip comprising a continuous paper backing layer carrying discrete, physically separated labels thereon, each label comprising an ink- printed coextrudate with attached pressure sensitive adhesive mounted on the backing layer via a silicone release agent. This strip is then fed to an automatic label applying machine which manipulates the strip, for example, by sliding or rolling the strip around a peel plate at high speed, to cause the individual labels to automatically detach from the backing strip and be projected onto suitably placed bottles. In the case of IMFL, the procedure is similar, except that the coextruded web or sheet is not laminated to a release layer. Rather, the web or sheet, after optional winding up into bulk rolls for storage, is slit and subjected to printing/die cutting with the individual labels produced thereby being bundled together in a stack. The blow-molder then loads individual labels from the stack into the label magazine of his blow-molding machine for automatic incorporation into the blow molded bottles as part of the bottle forming operation. In both IMFL and pressure sensitive adhesive labels, it is conventional to employ coextrudates which have been oriented in the longitudinal, or machine, direction only. Typically, orientation is done by stretching the coextrudate while still hot in the machine direction at a stretch ratio of about 2:1 to about 9:1, with stretch ratios of 4:1 to 6:1 being typical. This results in a significant decrease in overall film thickness as well as adjustment of the mechanical properties .of the coextrudate in a known manner. For example, using a 5:1 stretch ratio will reduce the overall extrudate thickness from 16 mils at the extrusion nozzle to approximately 3.2 mils after stretching and will orient the polymer chains in the machine direction to thereby impart considerable stiffness in this direction but not in the transverse direction. Other known methods for orienting polymer films, for example compression orientation or "blowing" a film produced by extrusion through an annular orifice, can be used. The hot-stretched coextrudates produced as described above can also be annealed or "heat set" in accordance with known techniques. Typically, this is done after extrusion and initial chilling of the extrudate by reheating the extrudate to an elevated temperature, for example, 300°F. After coextrusion, orientation, heat setting, etc., the coextrudates can be directly processed into labels. More typically, however, the coextrudates are taken up (i.e. wound around) suitable cores to form rolls of material typically containing 500 to 15,000, preferably 2,000 to 10,000, linear meters of material in the form of continuous sheets or webs. Such rolls, which can be subdivided radially (i.e., cut in planes perpendicular to their axes to form rolls of smaller axial width) or left as is, can be stored, shipped and sold for use as needed. Processes for forming coextrudates in the manner described above for use as flexible labels for flexible bottles are well known in the art and shown, for example, in the above-identified patents. Any such procedure can be used in accordance with the present invention. The coextrudates of the present invention can be formed from any materials commonly employed for making coextrudate flexible plastic labels. For example, a suitable material for making the core layer for many applications in accordance with the present invention is polyethylene of low, medium or high density between 0.915 and 0.965 specific gravity. This is a relatively low cost, extrudable film- forming material whose stiffness is dependent, among other things, on the density selected and whose body and strength are sufficient for most uses. Polyethylene of lower densities, down to a specific gravity of 0.890, may be employed for greater flexibility. A preferred material for the core layer is polypropylene (or a propylene copolymer) having a flex modulus ranging between about 130,000 and 300,000 psi at 73°F., depending on the stiffness desired. Still other preferred materials for forming the core layer comprise copolymers of olefin monomers with ethylenically unsaturated carboxylic acid ester comonomers, such as ethylene-vinyl acetate copolymer, as well as blends of such copolymers with any and all of the other polymers and copolymers described above. Still other preferred materials comprise physical blends of (l) polypropylene or copolymers of polypropylene and polyethylene and (2) ethylene-vinyl acetate (EVA) in weight ratios ranging from 50/50 to 80/20, preferably 55/45 to 65/35. For clear label applications, a physical blend of (1) a copolymer of polypropylene and polyethylene and (2) ethylene-vinyl acetate (EVA) is also preferred. For opaque film labels applications, a preferred core layer is a physical blend of polypropylene and EVA. Polystyrene is also a candidate material for the core layer particularly where a stiffer label is desired. In order to make core layers opaque, various inorganic fillers may be incorporated into the polymer forming the layer. Useful fillers include calcium carbonate, titanium dioxide and blends thereof. Pigments and dyes can also be added for imparting color thereto. Examples of materials found suitable for the skin layers of the inventive labels are materials which are formed predominantly from polyolefins. By being formed "predominantly from polyolefin" is meant that the layer is formed from a homopolymer or copolymer of a polyolefin or blends of such homopolymers and/or copolymers, with the proviso that at least 50% of the polymerized monomers in the layer are polyolefins. Examples of such materials are homopolymers and copolymers of ethylene and propylene such as polyethylene, polypropylene and ethylene/propylene copolymer, copolymers of olefin monomers with ethylenically unsaturated carboxylic acid or ethyleni- cally unsaturated carboxylic acid ester comonomers such as ethylene- vinyl acetate copolymer (EVA) and blends of such homopolymers and copolymers. In many applications, the polymers, copolymers and blends described above in connection with the core layer can be used. Other materials useful for making the outer skin layers of the inventive labels include meltable film-forming substances used alone or in combination such as polyethylene methyl acrylic acid, polyethylene ethyl acrylate, polyethylene methyl acrylate, acrylonitrile butadiene styrene polymer, polyethylene vinyl alcohol, nylon, polybutylene, polystyrene, polyurethane, polysulfone, polyvinylidene chloride, poly¬ propylene, polycarbonate, polymethyl pentene, styrene maleic anhydride polymer, styrene acrylonitrile polymer, ionomers based on sodium, potassium, calcium or zinc salts of ethylene/ methacrylic acid, polymethyl ethacrylates, cellulosics, fluoroplastics, polyacryloni- triles, and thermoplastic polyesters. The preferred materials to use in forming both the core and the skin layers of the inventive coextrudates are basically matters of choice and depend on the particular applications involved. The foregoing patents and publications incorporated herein by reference go into great detail in explaining how to pick particular materials for particular uses and these techniques can also be used in accordance with the present invention to design a particular coextrudate for a particular utility. In accordance with the present invention, flexible plastic labels as described above are made laser-markable by including in the core layer of the label a laser-opaque material. By laser-opaque material is meant any material which will absorb or reflect laser light so as to cause marking of the polymer layer in which the laser-opaque material is contained. Surprisingly, it has been found that the intensity of the marks made as a result of laser marking will not be compromised, and in fact may ' be improved, by restricting the laser- opaque materials to the core of the extrudate rather than in its skins. Moreover, keeping the laser-opaque material out of the skins also has the beneficial effect of not deleteriously affecting the physical properties, particularly the smoothness of the skins or their chemical nature either. Furthermore, by keeping the laser-opaque material in the core only, the coextrudates can be made without adversely affecting the various mechanical properties such as dimensional stability, stiffness, high speed dispensability, die cutability, matrix stripability, repositionability and the like of the label product. Any type of laser-opaque material can be employed in accordance with the present invention. In this connection, there are many materials known for their ability to absorb and/or reflect laser light of different wave lengths and energy densities and, as a result, "interact" with a polymer material in which they are contained to cause a visible mark to form. The type of "interaction," e.g., thermal degradation of the polymer, simple chemical reaction, generation of gas bubbles, etc., varies depending on the type and operation of the laser employed as well as the type of polymer material employed, and accordingly there must be a "match" of the laser-opaque material with the polymer employed as well as the type and operation of the laser employed. In accordance with the present invention, any known laser- opaque material can be employed, so long as it "matches" both the polymer as well as the type and operation of the laser employed. The preferred laser-opaque materials used in accordance with the present invention are solid, particulate materials. Solid particulate materials having a high aspect ratio, particularly those which have a platelet structure, are especially preferred. When particulate materials are used, it is preferable that they have an average particle size from 0.2 to 400, preferably 0.5 to 60, most preferably 1 to 25 microns. Examples of this type of laser-opaque materials for use in the present invention, particularly with respect to C0 2 TEA lasers as discussed below, are kaolin, mica, mixtures of mica and titanium dioxide and wollastonite. Also useful are the various types of laser markable materials shown in the following patents, the disclosures of which are incorporated herein by reference: European Patent 0190997; U.S. 4,595,647; U.S. 4,654,290; U.S. 4,753,863; U.S. 4,816,374 and U.S. 4,861,620. Especially preferred laser-opaque materials are titanium dioxide-coated mica particles. These materials are commercially available from E. Merck Corporation of Hawthorne, New York under the designation AFFLAIR ® and The Mearl Corporation of New York, New York under the designation of MEARLIN ® luster pigments. These materials typically have particle sizes of 1 to 200, preferably 1 to 60, more preferably 1 to 25 microns. The amount of laser-opaque material to be incorporated into the core layer of the inventive coextrudate products can vary widely. Basically, the minimum amount is that amount which is sufficient to form a visible marking of the desired intensity. The maximum amount, in turn, is usually dictated by economics, amounts over that necessary to produce a mark of a desired intensity being unnecessary. Typically, the amount will be on the order of 0.1 to 10 percent by weight, based on the weight of the material forming the core layer (including any other filler or pigment such as titanium dioxide, calcium carbonate and the like) . More typically, the amount of laser-opaque material will be on the order of about 0.5 to 5 percent by weight. In this regard, it is convenient to refer to the amount of laser opaque material in the core layer of the inventive labels in terms of effective thickness. By "effective thickness" is meant the number obtained ^ by multiplying the thickness of the core layer, measured in mils, times the concentration of the laser-opaque material in the core layer, measured in weight percent expressed as a decimal. Measured in this way, it is preferable that the amount of laser opaque material in the core layer be enough so that the effective thickness thereof is 0.005 to 0.15, more preferably 0.01 to 0.10, even more preferably 0.02 to 0.06. In order to laser imprint the desired image or information onto labels made in accordance with the present invention, the labels are irradiated with laser light containing or embodying the desired information or image therein. As a practical matter, only three basic types of marking lasers are now available commercially. These are eximer lasers based on rare earth gas halides, Nd/YAG systems and pulsed carbon dioxide lasers. Of these, the Nd/YAG systems and carbon dioxide lasers are typically used for plastics. However, eximer lasers have also been used for this purpose. In accordance with the invention, each of these types of lasers can be used, although pulsed carbon dioxide TEA (transverse excited atmosphere) lasers are preferred from the point of view of cost and reliability. The conditions of laser marking vary widely and are dependent on a number of factors such as the identity and amounts of laser-opaque materials in the films, film thickness and the like. In the case of the preferred TEA carbon dioxide laser (wave length 10.6 micrometers -- infrared range), energy densities on the order of 0.8 to 36, preferably 1.8 to 28.8, Joules per square centimeter per pulse at pulse durations of 50 to 1,000, preferably 100 to 300, nanoseconds are appropriate. The particular operating variables of the laser to employ in a particular embodiment of the invention can be easily determined by routine experimentation. As well appreciated by those skilled in the art, the rate in which energy is supplied by the laser should not be so great that the film is destroyed or otherwise unacceptably degraded but yet needs to be enough so that indicia of suitable intensity will be produced. In order to imprint a label by the laser marking technique of the present invention, the laser beam generated by the laser is passed through a suitable stencil containing the desired information to generate an information-containing laser beam. This beam is then focused onto the label to be marked and the label irradiated with the laser light for the imprinting process. Exactly how this is done is well known to those skilled in the art of laser marking, and any conventional procedure for this purpose can be employed in accordance with the present invention. WORKING EXAMPLES In order to more thoroughly illustrate the present invention, the following working examples are presented. In these examples, a hot coextrudate was produced in accordance with the process described in U.S. 5,242,650 with a total thickness of 17.5 mils. The coextrudate was then hot stretched to make a film of 3.5 mils. In each example, the coextrudate was made with two identical skins, each skin layer making up 10% of the total thickness of the coextrudate and the remainder comprising the core. After manufacture, each film was them imprinted with a simulated date and lot code by means of a Blazer 6000 Pulsed Carbon Dioxide Laser made by Lasertechnics Corporation of Albuquerque, New Mexico. The laser beam produced was passed through a mask having a simulated date and lot code about one inch wide and then focused to a reduced size onto the target film to imprint the image thereon. Imprinting was done at different energy levels (3 and 4 Joules per pulse) and different reduction ratios (ratio of mask size to image size) . In these experiments, the laser beam as produced by the laser has an energy density of 0.8 Joules/cm 2 , at a maximum energy of 5 Joules. The energy density of the beam as it strikes the target can be reduced from this value by reducing the energy of the laser or increased by reducing (narrowing) the beam size between the mask and the surface of the target. In the following working examples, the approximate energy densities of the laser beam striking the targets were as follows: ■ Pulse Energy Reduction Ratio Energy Density 4 Joules 2.75:1 5 Joules/cm 2 4 Joules 2.0:1 3 Joules/cm 2 3 Joules 2.75:1 4 Joules/cm 2 3 Joules 2.0:1 2 Joules/cm 2 Two masks were used to optimize the width of the image to give four 0.95" character for the 2:1 size reduction (reduction ratio), and two rows of 0.90" characters for the 2.75:1 size reduction. The larger characters apparently influenced the visual impact of the mark, as the mark contrast seems to be higher for the larger mark than the energy density would predict, as more fully illustrated below." The laser produces silvery-gray laser imprinted marks. The images so produced were visually observed and rated using an arbitrary scale of from 0 (no mark) to 10 (black and very distinct) . The following examples were conducted: Example 1 A coextrudate having the following composition was produced: Table l Weight % Printing Polypropylene homopolymer (HPP) 50 Skin Ethylene-vinyl acetate copolymer (EVA-18% VA) 50 Core Layer Calcium particulate carbonate concentrate 40 (40% particulate CaC0 3 in HPP) Ethylene-vinyl acetate copolymer (EVA-18% VA) 35 Titanium dioxide concentrate 15 (50% Rutile Ti0 2 in HPP) AFFLAIR 110 concentrate 10 (20% Ti0 2 coated mica in HPP) Bonding Polypropylene homopolymer (HPP) 50 Skin Ethylene-vinyl acetate copolymer (EVA-18% VA) 50 The results are obtained by laser-imprinting their coextrudate in the manner described above. Table 2 Concentra¬ tion of AFFLAIR Visual Intensity of Image at AFFLAIR 110 Effective different energy intensities Example in core Thickness and Reduction Ratios 3 Joules 4 Joules RR RR RR RR 2:1 2.75:1 2:1 2.75:1 1 2% 0.056* 6 7 7 7 * (3.5 mils) x (80%) x (20%) x (10%) = 0.056 Table 2 shows that the laser marking technique as described above produced images having a fairly high degree of contrast or intensity under essentially all the conditions tried in the experiment. This shows that images of good intensity can be produced in -accordance with the present invention, even though the laser-opaque material is buried in the core and not present in the skin layers. Examples 2-6 Example 1 was repeated except that initial film thickness was 15 mils before stretching and final fill thickness was 3 mils after stretching. In addition, the amount of laser-opaque material in the core was. varied from 0 to 3.8 weight percent, based on the weight of the core, to illustrate the effect of varying concentration of this material. The specific compositions of the different layers used in these examples is set forth in the following Table 3. Unless otherwise indicated, the polymers and copolymers used have the same compositions as in Example 1: Table 3 Printing Polypropylene homopolymer 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 (0.25 weight % maleic anhydride) Core Layer Propylene-ethylene random copolymer 40 (3.2 weight % ethylene) 35-X Ethylene-vinyl acetate copolymer (EVA) 20 Titanium dioxide concentrate 5 (50% TiO 2 /50% polypropylene) X Polypropylene-maleic anhydride graft copolymer AFFLAIR 110 concentrate (40% in EVA) Bonding Polypropylene homopolymer 40 Skin Ethylene-vinyl acetate copolymer 50 Polypropylene-maleic anhydride graft polymer 10 The following results were obtained: Table 4 • Equals X/0.4 since AFFLAIR is 40V of the AFFLAIR 110/EVA concentrate The foregoing results show that the contrast of the visual images produced in accordance with the present invention increases proportionally with the effective thickness of the laser-opaque material in the core layer. In addition, these examples show that the images produced appear to be more intense at higher reduction ratios and also at higher laser impulse energy levels. Examples 7-9 Example 1 was repeated except that the coextrudates in Examples 7 and 8 had the composition set forth in the following Tables 5 and 6: Table 5 Printing Polypropylene homopolymer (HPP) 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 Core Layer Propylene-ethylene copolymer 35 Ethylene-vinyl acetate copolymer (EVA) 35 Titanium dioxide concentrate 20 Polypropylene-maleic anhydride graft copolymer 5 AFFLAIR 110 concentrate (20% in EVA) 5 Bonding Polypropylene homopolymer 40 Skin Ethylene-vinyl acetate copolymer 50 Polypropylene-maleic anhydride graft polymer 10 Table 6 Printing AFFLAIR 110 concentrate (20% in HPP) 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 Core Layer Propylene-ethylene copolymer 40 Ethylene-vinyl acetate copolymer (EVA) 35 Titanium dioxide concentrate 20 Polypropylene-maleic anhydride graft copolymer 5 Printing AFFLAIR 110 concentrate (20% in HPP) 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 Bonding AFFLAIR 110 concentrate (20% in HPP) 40 Skin Ethylene-vinyl acetate copolymer 50 Polypropylene-maleic anhydride graft polymer 10 In addition, in Example 9, a single layer extrudate was used, this single layer having the same composition as the core layer of Example 7 and the same overall thickness as the total extrudate thickness of Example 7. The following results were obtained: Table 7 Example Examples 7 and 8 compare the effect of placing the laser-opaque material' in the core rather than in the skin. In this regard, note that the calculations of effective thickness in Example 8 assume that only the outer skin layer is effected by the laser impregnation. In other words, in assessing the visual impact of a laser impregnation, any effect on the lower skin layer is disregarded. Example 7, illustrating the present invention, shows that when the laser-opaque material is present in the core layer at an effective thickness of 0.024, visual images having significant contrast are produced at all operating conditions. On the other hand, when the same amount of laser-opaque material as used in the core of Example 7 is placed in the printing skin of the Example 8 composite, images with little if any contrast are produced at reduction ratios of 2:1, while holes are produced in the printing skin at the higher reduction ratio of 2.75:1. This shows that the images produced by laser marking are not simply a result of the amount of laser opaque material in the system but also depend on where it is located. In addition, this also shows that locating the laser opaque material in the core, rather than in the skin layer which first receives the laser light impinging on the article provides a significantly, and unexpectedly, superior result. To show the effect of not having any skins at all, Example 9 was conducted. In this example, a single layer extrudate having the same thickness as the coextrudates of Examples 7 and 8 was filled with the same concentration of laser opaque material as in the core of the Example 7 coextrudate. This means that the Example 9 extrudate contained a significantly greater overall amount of laser-opaque material than the coextrudate of Example 7. Notwithstanding this greater amount of active ingredient, the images produced in the Example 9 coextrudate have essentially the same visual impact as those of the Example 7 coextrudate. This shows that burying the laser-opaque material in the core and thereby keeping the laser-opaque material out of the skins does not adversely impact the contrast or intensity of the images produced. This, in turn, shows that images of acceptable contrast can be produced with less overall laser-opaque material and further that this advantageous result can be obtained while the outer and inner skin layers are maintained free of this material so that they can retain their smoothness and ink-printability properties. This also means that it is possible to adopt the use of laser marking with opaque materials in such a way that the laser opaque material is kept away from the mechanical equipment involved in processing, such as the die lips of the extrusion die, or the calender rolls used in subsequent processing. This prevents die lines from appearing in the product as well as excessive wear on the die lips and other equipment which would otherwise occur. Incidentally, note that the results obtained in Example 7 are almost identical to those obtained in Example 4 in which the coextrudate had an effective thickness of laser-opaque material of 0.023. This, in turn, shows that "effective thickness" is a meaningful number. Examples 10-14 Examples 2 to 6 were repeated except that the Afflair additive was Afflair 100, rather than Afflair 110. Afflair 100 is slightly larger in particle size, and as can be seen below appears to be slightly less effective. Also, in some of Examples 10-14, a single layer extrudate was produced rather than a multi-layer extrudate, while in one of these examples, Example 14, the extrudate was not oriented after extrusion but was simply produced as cast. The results obtained are set forth in the following Table 8: Table 8 * Oriented in the machine direction only Comparison of Examples 10 and 11 in the above Table 8 shows that providing an extrudate with protective skins as accomplished in accordance with the present invention, does not hurt and also may even prevent burn through at more intense conditions. Note in particular that the composite of Example 11 is essentially the same as Example 10 in terms of the active thickness and active ingredient concentration, the only difference between that in Example 11, protective skins having no laser-opaque material therein are provided. This is significant in that it shows the coextrudated skins can be fine tuned to meet performance criteria without reducing laser-markability at the same additive cost. Comparison of Example 12, in which a single layer extrudate thicker than that of Example 10 and hence having more overall laser- opaque material than in Examples 10 or 11 (0.027 effective thickness rather than 0.0252) shows that the visual impact of this extrudate is no better than that of the coextrudate of Example 11 even though the extrudate of Example 12 has more laser-opaque material. Examples 13 and 14 are comparable in that both have the same effective thickness of laser-opaque material. However, the Example 14 product, which is a single layer extrudate not subjected to orientation, provides a visual impact which is noticeably less intense than that provided by the Example 13 product which is composed of multiple layers having been oriented in the machine direction. This shows that the combination of burying the laser-opaque material in the core and orienting in at least the machine direction facilitates reduction in the amount of expensive laser-opaque material necessary to produce a visually acceptable image. Example 15 The procedure of Examples 2 to 7 was repeated except that the thickness of the coextrudate passing out of the extrusion die was 12.5 mils and the coextrudate so made was stretched to a final product thickness of 2.5 mils. Also, the composition of the individual layers of the coextrudate was changed so that the coextrudate product was a slightly hazy, essentially transparent film. The compositions of the individual layers of the coextrudate are set forth in the following Table 9: Table 9 Printing Polypropylene homopolymer 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 Core Layer Propylene-ethylene copolymer 60 Ethylene-vinyl acetate copolymer (EVA) 32.5 Polypropylene-maleic anhydride graft copolymer 5 AFFLAIR 110 concentrate (40% in EVA) 2.5 Bonding Polypropylene homopolymer 40 Skin Ethylene-vinyl acetate copolymer (EVA) 50 Polypropylene-maleic anhydride graft copolymer 10 The results obtained are set forth in the following Table 10: Table 10 Concentration of AFFLAIR 110 in Active AFFLAIR 110 Visual Intensity of Images in Layer Effective at Different Energy Intensities and Example <"<- i) Thickness Reduction Ratios 3 Joules 4 Joules RR RR RR RR 2*1 2 75*1 2*1 2 75*1 15 1.0 0.020 From Table 10, it can be seen that under all operating conditions, a very high visual impact image was produced even though the effective thickness of the laser-opaque material in the active layer was relatively low. This illustrates that the present invention is particularly effective in providing laser-markable images on transparent labels. Example 16 The following example demonstrates the present invention is readily applicable to making in-mold, machine direction oriented labels. The recipe for an opaque white flexible film with a print layer, and an adhesive layer, is as follows: Table 11 Top Polypropylene homopolymer 50 Ethylene-vinyl acetate copolymer (EVA) 50 Central Propylene-ethylene copolymer 70 Ethylene-vinyl acetate copolymer (EVA) 10 Titanium dioxide concentrate 15 AFFLAIR 110 concentrate (16% in EVA) 5 Bottom Low-density polyethylene (LDPE) 24 Ethylene-vinyl acetate copolymer (EVA) 40 Anti-block concentrate (20% in polyethylene) 36 The above film was coextruded as in previous Examples 2 through 7, but the coextrudate thickness was 20 mils, and stretching was done to produce a film of 4.0 mils. As in the previous examples, the thickness of the top and bottom layers were each 10% of the total, the central layer making up the remaining 80%. Table 12 Concentration of AFFLAIR 110 in Active Visual Intensity of Images at in Layer Effective Different Energy Intensities and Example («t %) Thickness Reduction Ratios 3 Joules 4 Joules RR RR RR RR 2:1 2.75:1 2:1 2.75:1 16 0.8 0.020 2+ Although only a few embodiments of the present invention have been described above, it should be appreciated that many modifications can be made without departing from the spirit and scope of the present invention. For example, although the multi-layer coextrudates of the present invention have been described above in terms of three layers, it should be appreciated that any number of plural layers can be employed. For example, where necessary, suitable tie layers for bonding dissimilar skin and core layers can be employed. In addition, a bonding skin layer may be unnecessary in some instances, particularly wherein a heat sensitive adhesive exhibiting good adhesion to the core layer is or will be employed. All such modifications are intended to be included in within the scope of the present invention, which is to be limited only by the following claims.

Claims: WE CLAIM: 1. A process for forming a visual image on an ink-printable, flexible, plastic label, said label comprising a coextrudate of a core layer and at least one skin layer, said core layer containing a laser- opaque material therein, said skin layer being free of laser opaque material and -further being capable of receiving and retaining an ink- printed image thereon, said process comprising irradiating said coextrudate with laser light in the form of said image to cause said visual image to form in said core layer. 2. The process of claim 1, wherein said at least one skin layer has an ink-printed image thereon. 3. The process of claim 1, wherein the thickness of said core layer is at least 50% of the thickness of said coextrudate. 4. The process of claim 1, wherein the thickness of said at least one skin layer is about 5 to 15% of the thickness of said coextrudate. 5. The process of claim 1, wherein said core layer and said at least one skin layer are formed predominantly from polyolefin. 6., The process of claim 1, wherein said laser-opaque material is titanium dioxide-coated mica. 7. The process of claim 6, wherein the particle size of said laser-opaque material is 0.5 to 200 microns. 8. The process of claim 1, wherein the effective thickness of said laser-opaque material in said core layer is 0.005 to 0.15. 9. The process of claim 1,- wherein said core layer and said at least one skin layer contain copolymerized vinyl acetate. 10. The process of claim 1, where said coextrudate contains a skin layer on each side of said core layer. 11. An ink-printable, flexible, plastic label capable 'of generating a visual image therein when irradiated with laser light, said label comprising a coextrudate of a core layer and at least one skin layer, said core layer containing a laser-opaque material responsive to said laser light in an amount sufficient to cause a visual image to form in the areas of said core layer which are irradiated with said laser light, said at least one skin layer being capable of receiving and retaining an ink-printed image thereon, said at least one skin layer also being free of said laser-opaque material. 12. The label of claim 11, wherein said core layer and said at least one skin layer are formed predominantly from polyolefin. 13. The label of claim 11, wherein the effective thickness of said laser-opaque material in said core layer is 0.005 to 0.15. 14. The label of claim 11, wherein the particle size of said laser-opaque material is 0.5 to 400 microns. 15. The label of claim 11, wherein said laser-opaque material is titanium dioxide-coated mica. 16. The label of claim 11, wherein said core layer and said at least one skin layer contain copolymerized vinyl acetate. 17. The label of claim 11, wherein said coextrudate is oriented in a single direction only. 18. The label of claim 11, wherein said label comprises a first skin layer on one side of said core layer and a second skin layer on the other side of said core layer, said first skin layer being free of said laser-opaque material. 19. The label of claim 18, wherein said second skin layer is also free of said laser-opaque material. 20. The label of claim 19, wherein said second layer includes a heat activated adhesive. 21. The label of claim 19, further comprising a pressure sensitive adhesive attached to said second skin layer. 22. The combination comprising a flexible plastic bottle and the label of claim 11 integrally attached to said bottle. 23. The combination of claim 22, wherein said label is attached to said bottle by means of a pressure sensitive adhesive. 24. The combination of claim 22, wherein said label is attached to said bottle by means of a heat activatable adhesive. 25. A process for affixing a label to a flexible plastic bottle, said label being integrally affixed to said bottle and capable of flexing with said bottle without detachment therefrom, said label bearing an ink-printed image as well as a laser-generated image, said process comprising affixing the label of claim 11 to said bottle after said ink-printed image is applied to said label and thereafter marking said laser-generated image on said label by irradiating said label with laser light embodying said laser-generated image. 26. The bottle produced by the process of claim 25. 27. A coextrudate useful for the manufacture of labels by subdividing said coextrudate into a plurality of said labels, said coextrudate comprising a flexible web or sheet of plastic material, said web or sheet being formed from a core layer and at least one skin layer, said core layer containing a laser-opaque material responsive to said laser light in an amount sufficient to cause a visual image to form in the areas of said core layer which are irradiated with said laser light, said at least one skin layer being capable of receiving and retaining an ink-printed image thereon, said at least one skin layer also being free of said laser-opaque material. ' 28. A roll of material comprising the coextrudate of claim 27 in the form of a continuous web or sheet having a length of at least 500 meters, said web or sheet being wound around itself to form said roll.

Inventors: ADAMS RICHARD CAYLE, KIMES DONALD FRANCIS, ADAMS, RICHARD, CAYLE, KIMES, DONALD, FRANCIS
Applicants: AVERY DENNISON
Application year: 1995
Main classification: A61F13
IPC codes: B41M5, B65C3, B65C9, G09F3