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1. DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM 94 Pressure-Sensitive Adhesives and Adhesive Products 94.1 Introduction ......................................................................94-1 94.2 Adhesives............................................................................94-1 94.3 Adhesive Properties...........................................................94-2 Tack • Peel Adhesion • Shear Resistance • Other Tests • Dynamic Mechanical Analysis 94.4 Products .............................................................................94-5 Tapes • Labels • Other Products 94.5 Processing ..........................................................................94-5 D. Satas* Coating • Drying • Slitting and Die-Cutting Satas & Associates References .....................................................................................94-7 94.1 Introduction Pressure-sensitive adhesives (PSAs) adhere to various surfaces just by application of a light pressure and usually can be easily and cleanly separated. To meet these basic requirements, the adhesive must be sufﬁciently soft in order to be tacky to touch and also have a sufﬁcient cohesive strength to maintain its integrity upon separation.* The invention of rubber-based PSAs is attributed to Dr. Henry Day.1 Such an adhesive consisted of India rubber, pine gum, and other ingredients. The ﬁrst patent was issued to Shecut and Day in 1845.2 The early uses were limited to medical applications, and the progress of further development was slow. The ﬁrst industrial applications of pressure-sensitive tapes appeared only in 1920 and labels in the late 1930s. The technology of PSAs and adhesive products is covered extensively in the Handbook.3 94.2 Adhesives The basis of a PSA is an elastomer made tacky by addition of tackifying resins. Natural rubber was the ﬁrst material used and still is among the most important elastomers for compounding of PSAs. The general composition of such compounded adhesives is as follows. Elastomer, 30 to 60% Tackiﬁer, 40 to 60% Plasticizer, 0 to 30% *Deceased. 94-1 © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 2 Monday, April 25, 2005 12:18 PM 94-2 Coatings Technology Handbook, Third Edition Filler, 0 to 30% Antioxidants, 1 to 2% Other elastomers used widely in addition to natural rubber are styrenic block copolymers, especially those with isoprene midblock. Block copolymers made the basis for successful hot-melt adhesives. Styrene butadiene rubber, polyisobutylene, and butyl rubber are also used. These elastomers must be tackiﬁed with resins compatible with the elastomer forming a tacky material. The resins may be synthetic of petroleum origin (based on C5 or C9 monomers) or they may be based on wood rosin collected from coniferous trees (or from tall oil, a by-product of paper manufacturing). Such rosins are chemically altered to improve their stability. Another category of PSAs is copolymers of acrylic esters. These polymers are inherently tacky and do not require compounding with resins or other additives, although they may be sometimes compounded to enhance or change their properties. They are clear and have a good resistance to oxidation and ultraviolet (UV) light. Acrylic copolymers quickly became the most important materials for pressure- sensitive products. Silicone PSAs are also available. They are expensive and their usage is small, but their usefulness at high and low temperatures is unique and not approached by other polymers. PSAs are available in several forms: as solutions in organic solvents, as aqueous emulsions, as hot- melts, as 100% solids for calendaring or extrusion applications, and as 100% reactive materials for thermal or irradiation curing after application. Solution adhesives are the easiest to apply and are the most versatile in their properties. They are not desirable environmentally, because of solvent emission; but many companies have invested in solvent recovery equipment and can effectively handle solution adhesives. Hot-melt application has the advantages of environmental acceptability, high coating speeds, and less expensive equipment. Hot-melt adhesives are based mainly on block copolymers. Aqueous adhesive emulsions, along with hot-melts, are the fastest growing types. Acrylic adhesives are important as aqueous emulsions, though SBR and natural rubber latexes are also used. The 100% solid adhesives are used in calendaring processes and can be applied by extrusion. There is considerable current interest in 100% reactive adhesives, especially cured by UV radiation, but their future is questionable. 94.3 Adhesive Properties The main properties of PSAs are considered in the trade to be tack, peel adhesion, and shear resistance. Sometimes these properties are thought to constitute the adhesive performance. If under the adhesive performance we understand the adhesive product behavior in its application, then these test values, although important, are not sufﬁcient. 94.3.1 Tack Tack is the capability of the adhesive to form a bond at low pressure during a short time period. It is measured (most frequently) as the force required to separate such an instantaneous bond formed at low pressure. A subjective measure of tack is experienced by placing a thumb over the adhesive surface and quickly removing it. Tack is tested by several techniques, and it is basically deﬁned by the testing method. Thus, the probe tack test has been designed to approximate thumb tack, the loop tack and quick stick techniques are similar to peel testing, and the rolling ball tack test employs a steel ball rolling on the adhesive surface. While tack is the main property that makes the adhesive pressure sensitive, its numerical determination depends on the testing technique used, and the tack value is not that important in describing the adhesive performance. Figure 94.1 shows the loop tack measurement. 94.3.2 Peel Adhesion Peel adhesion is measured by applying the tape to a standard stainless steel panel and measuring the force required to remove the tape by peeling at 180 or 90 degrees. The peel test gives important data in © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 3 Monday, April 25, 2005 12:18 PM Pressure-Sensitive Adhesives and Adhesive Products 94-3 FIGURE 94.1 Loop tack test. A B C D FIGURE 94.2 Schematic diagram of various peel tests: (A) 180-degree peel; (B) 90-degree peel; (C) drum peel (tape unwind); (D) T-peel. evaluating the adhesive. However, the peel values depend on the test conditions and on the stiffness of the tape backing. We are measuring not only the force required to break the adhesive bond but also the energy expended to deform the backing and the adhesive itself. The test results, therefore, are comparable only to the adhesive applied to the same backing. Figure 94.2 shows the 180° peel test arrangement, and Figure 94.3 shows the peel test results for tapes with various backings and various backing thicknesses. In the case of paper label stock, the failure is paper failure, except in the case of removable adhesives. In label stock, however, the release of silicone-coated liner from the adhesive is important, and the release is tested similarly to the peel test. 94.3.3 Shear Resistance Shear resistance is the only test of the main three tests that measures directly the cohesive strength of the adhesive, and it correlates well with its viscosity. The shear resistance can be measured by recording the © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 4 Monday, April 25, 2005 12:18 PM 94-4 Coatings Technology Handbook, Third Edition 5 A – Mylar Polyester Film B – Aluminum Foil 1145-H19 4 C – Aluminum Foil 1145-0 Peel Force – Lbs. D – Urethane Rubber Film B 3 C 2 A D 1 2 4 6 8 10 12 14 16 18 20 22 Backing Thickness - In × 10−3 FIGURE 94.3 The dependence of peel force on the backing thickness in a 180-degree peel test. [Reprinted from Adhesive Age, 9(8) (1966).] FIGURE 94.4 Shear test. time to failure (cohesive) under some static load. Shear creep is measured by the distance the adhesive has moved in a given time under a static load. Shear resistance shows directly the capability of the adhesive to carry the load, and it is an important factor in predicting the adhesive’s behavior under the load. Figure 94.4 shows the shear test setup. 94.3.4 Other Tests The main information expected from testing is the determination of whether the adhesive performs well in the intended application. The above tests generally do not give such information, and therefore a large number of speciﬁc application tests are used. Special tests are available or are improvised for various applications, such as hospital and ﬁrst aid tapes, electrical tapes, packaging uses, masking and surface protection tapes, and other uses. 94.3.5 Dynamic Mechanical Analysis PSAs are viscoelastic materials, i.e., their properties depend upon the rate of force application. Therefore, it is of interest to investigate the mechanical adhesive properties (such as moduli) at various shear rates. Such data also appear to shed considerable light on the adhesive performance, which mainly depends on the bulk adhesive properties. This test is covered in a separate chapter. © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 5 Monday, April 25, 2005 12:18 PM Pressure-Sensitive Adhesives and Adhesive Products 94-5 94.4 Products PSA-coated products are subdivided into three main groups: tapes, label stock, and other products. Tapes are narrow strips of material with adhesive applied on one side and wound in a roll. Generally no release liner is needed; the adhesive unwinds from the outer side of the backing, which may be coated with a special release coating. Label stock is printable material with the adhesive coating on one side, which is protected by easily removable silicone-coated liner. 94.4.1 Tapes Tapes can be subdivided according to the backing material used: ﬁlm tapes, fabric, paper, foil, foam, etc. Special tape products are double-coated tapes, which have adhesive on both sides of a supporting sheet (ﬁlm, paper, nonwoven fabric, foam), and transfer tapes, which consist of an unsupported adhesive ﬁlm. Double-coated tapes and transfer tapes are protected by a silicone-coated release liner. These tapes are heavily used for product assembly applications and mounting and holding uses. Tapes are also subdivided according to their application. Packaging tapes are the largest single application. This ﬁeld is dominated by OPP ﬁlm tape, but other ﬁlm, paper, and glass ﬁber reinforced tapes are also used. Medical tapes and related products constitute an important product area. Ofﬁce tapes are dominated by matte acetate ﬁlm tape, although cellophane ﬁlm tapes are still used. Electrical tapes include many products starting with black vinyl electrician’s tape and ending with high-temperature ﬁber/ﬁlm tapes with silicone adhesive. The automotive industry consumes many different tapes: electrical harness wraps, paint masking tapes, product (including foam) assembly tapes, and tapes for many other applications. The construction industry requires paint masking tapes, foam tapes for gasketing, tapes for insulation of heating and air conditioning ducts, and many other uses. The appliance industry uses wood grain and other surface decorating tapes, tapes for attaching name plates, foam gasketing tapes, etc. Tapes are used in all industries for many different applications. In most cases, PSAs are used for easier and more convenient application. 94.4.2 Labels PSAs are increasingly used for labels. Although they are more expensive than labels secured by waterborne or hot-melt adhesives, because a release liner is required, PSA label application equipment is less expen- sive, it is easier and cleaner to run, and the adhesive is quite suitable for plastic surfaces, such as polyethylene containers. PSAs are used for impervious foil labels; they are easier to use for intricate label design, and they are used for transparent ﬁlm invisible labels. The label industry consists of two separate groups. Large manufacturing companies produce label stock: unprinted adhesive-coated large rolls or sheets. These are sold to label manufacturers who print and die-cut the labels. While there are only few label stock manufacturers, there are many label printers, often servicing a limited geographical area. 94.4.3 Other Products PSAs are used for many different products other than tapes and labels. The medical area is a large one for a variety of dressings, transdermal drug delivery systems, and other products and devices that use pressure-sensitive adhesives to secure these devices to the human body. Easily removable stickers (Post- its) have been developed for ofﬁce uses. There are many other ofﬁce uses that employ PSAs. The list of other products is difﬁcult to exhaust, and this area is growing fastest and carries the highest markup. 94.5 Processing PSAs are applied in thin layers on various substrates. Most of the coating is carried out roll-to-roll, i.e., the substrate is supplied as a wound roll, it is unwound, the adhesive is applied to the substrate’s surface, © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 6 Monday, April 25, 2005 12:18 PM 94-6 Coatings Technology Handbook, Third Edition Metering Roll Dams Applicator Roll Backing Roll FIGURE 94.5 Top nip fed three-roll reverse roll coater. the adhesive is dried or otherwise hardened, and then the substrate is rewound into a large roll. Further processing then starts with the material in a large coated roll. 94.5.1 Coating The adhesives are applied by specialized coating equipment. They must be liquid, or sufﬁciently ﬂuid, to form a thin layer in the coating head. Coating equipment is discussed in several books.4,5 Several techniques are used for PSA coating. The reverse roll coater (see Figure 94.5) is the most versatile and frequently used machine for solvent-borne and aqueous adhesives. Knife-over-roll coaters are used for heavier solvent-borne adhesive coaters. The slot oriﬁce coater is the main technique for the application of hot-melt adhesives, and this technique is also useful for the coating of aqueous emulsion adhesives. Calendering is used for some specialized adhesive products. The compounded solid adhesive is squeezed and formed into a thin sheet between heavy calender rolls. Calendering was the main coating technique early in the development of PSA products. 94.5.2 Drying If the adhesive is applied as solution or emulsion, it requires drying before it can be wound up. Forced air convection dryers dominate this application, with less frequently used infrared heating. The web is carried through the oven supported on idlers, sometimes on an apron, or sometimes supported by an air cushion in ﬂoatation ovens. More detailed information on web drying can be found in several books.4,5 No drying is needed in hot-melt coating: cooling is sufﬁcient to solidify the adhesive. UV irradiation is also used as means of solidifying reactive adhesive coatings by the initiation of a polymerization reaction in the reactive oligomer and monomer mixture. 94.5.3 Slitting and Die-Cutting Large rolls produced on the drying equipment are slit to narrow tapes or to wide material in case of label stock to be printed and die-cut. Several slitting techniques are used: shear, score, and razor blade. See Reference 3 for more detailed information on the slitting of PSA products. The label stock is die cut by cutting through the label material and leaving the backup release liner uncut. Rotary die cutters are usually employed. The matrix between the labels is peeled off and the labels are then dispensed as needed. © 2006 by Taylor & Francis Group, LLC
DK4036_book.fm Page 7 Monday, April 25, 2005 12:18 PM Pressure-Sensitive Adhesives and Adhesive Products 94-7 References 1. C. W. Bemmels, “Pressure-sensitive tapes and labels,” in Handbook of Adhesives. I. Skeist, Ed. New York: Van Nostrand Reinhold, 1977. 2. W. H. Shecut and H. H. Day, U.S. Patent 3,965 (1845). 3. D. Satas, Ed., Handbook of Pressure Sensitive Adhesive Technology, 3d ed. Warwick, RI: Satas and Associates, 1999. 4. D. Satas, Ed., Web Processing and Converting Technology and Equipment. New York: Van Nostrand Reinhold, 1984. 5. E. D. Cohen and E. B. Gutoff, Eds., Modern Coating and Drying Technology. New York: VCH Publishers, 1992. © 2006 by Taylor & Francis Group, LLC