Adhesive pressure-sensitive

Pressure sensitive adhesive have been known for a long time. However, the combination of PIB and silicone components is a more recent development. 

Silicone functionalized PIB is particularly suitable as a pressure sensitive adhesive composition in medical applications including transdermal drug delivery applications (83). 

The preparation proceeds in the following way A telechelic alcohol-functional PIB is first reacted with butyllithium in order to form an alcoholate-functional PIB dianion and the latter is then reacted with hexamethylcyclotrisiloxane to generate a living poly(dimeth-ylsiloxane) chain at the end groups of the PIB. In a final step, the remaining anionic ends are capped with trimethylchlorosilane or dimethyldichlorosilane. In this way a block copolymer, is obtained. The pressure sensitive adhesive exhibits (83)  

The possibility of modifying the properties of the pressure sensitive adhesive, such as drug permeability, solubility, adhesiveness, releasibility, and tackiness, and 

Making the pressure sensitive adhesive either transparent or white resulting in an aesthetically-pleasing product. 

Pressure-sensitive adhesives (PSAs) are materials used to bond other materials on their surfaces by the bonding processes of adhesion and cohesion. They have the abihty to adhere instantaneously (also known as tack), resist removal by peel, and hold their position when shear forces are applied. Pressure-sensitive adhesives have been utilized in a range of medical applications from simple pressure-sensitive tapes to implant adhesives. Silicone PSAs are commonly used in transdermal drug delivery systems drug-loaded adhesive 

Pressure sensitive adhesives (PSA) are highly viscous, viscoelastic liquids which adhere to virtually aU surfaces when pressed down gently. They are used in manufacturing easy to apply self-adhesive products, such as labels and tapes. Pressure sensitive adhesives typically have permanent tack and adequate cohesion, so that further curing operations after applying the tape or label are generally unnecessary. 

Until the beginning of the 1970s, pressure sensitive articles were produced mainly by coating from organic solutions. Solvent-based rubber/resin mixtures and comparatively smaller amounts of acrylate solutions were processed almost exclusively. 

The total market demands for pressure sensitive adhesives (tapes and labels) is about 300 000 tons of polymer in North America and about 200 000 tons in Europe. Aqueous polymer dispersions have a share of about 30 % in North America and 40 % in Europe. About 30 % in North America and 45 % in Europe are applied from organic solution while hot melts have a share of 40 % in North America and 15 % in Europe [17,18]. 

The most common application of PSAs is in tape form. In the dry state, PSAs are aggressively and permanently tacky at room temperature and firmly adhere to a variety of dissimilar surfaces, without the need for more than finger or hand pressure. They require no activation by water, solvent, or heat to exert a strong adhesive holding force. 

Most PSAs are based on natural rubber. Rubber by itself has very low tack and adhesion to surfaces thus requires addition of tackifying resins based on rosins, petroleum, or terpenes. Hydrogenated resins are 

The adhesives discussed above are all applied in the solution and hot-melt forms. These are pressure-sensitive at room temperature. These materials may be based on EVA copolymers tackified with various resins and softeners. They produce rather soft adhesives, with poor cohesive strength. Their use is minor, mostly on label stock. Of wider interest are hot-melt adhesives based on the block copolymers of styrene with butadiene or isoprene. Vinyl ether polymers are also used, particularly in medicinal self-adhering plasters or dressings.  

Silicone adhesives are, to a small degree, used in PSAs. These products are based on silicone rubber and synthetic silicone resins. They have excellent chemical and solvent resistance, excellent elevated-temperature resistance, excellent cold-temperature performance, and high resistance to thermal and oxidative degradation. Their disadvantages include lack of aggressive tack and high cost (three to five times as much as acrylic systems).  

PSAs are often supplied to the final consumer coated onto a substrate such as cellophane tape or insulating tapes based on plasticized PVC film. These consist of the backing film, a primer or key coat, and the adhesive. If the product is to be rolled up in tape form, a release coat may be applied to the back of the film to reduce unwind tension when the tape is applied otherwise it is omitted. The adhesive, generally of the types discussed here, is usually applied from an organic solvent. Aqueous 

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Adhesives. Acryhc emulsion and solution polymers form the basis of a variety of adhesive types. The principal use is in pressure-sensitive adhesives, where a film of a very low T (<—20 " C) acrylic polymer or copolymer is used on the adherent side of tapes, decals, and labels. Acrylics provide a good balance of tack and bond strength with exceptional color stabiUty and resistance to aging (201,202). AcryUcs also find use in numerous types of constmction adhesive formulations and as film-to-film laminating adhesives (qv). 

Pressure sensitive adhesives normally are rated in terms of shear holding power, ie, time to fail in minutes under a constant load. 

Cellulosics. CeUulosic adhesives are obtained by modification of cellulose [9004-34-6] (qv) which comes from cotton linters and wood pulp. Cellulose can be nitrated to provide cellulose nitrate [9004-70-0] which is soluble in organic solvents. When cellulose nitrate is dissolved in amyl acetate [628-63-7] for example, a general purpose solvent-based adhesive which is both waterproof and flexible is formed. Cellulose esterification leads to materials such as cellulose acetate [9004-35-7], which has been used as a pressure-sensitive adhesive tape backing. Cellulose can also be ethoxylated, providing hydroxyethylceUulose which is useful as a thickening agent for poly(vinyl acetate) emulsion adhesives. Etherification leads to materials such as methylceUulose [9004-67-5] which are soluble in water and can be modified with glyceral [56-81-5] to produce adhesives used as wallpaper paste (see Cellulose esters Cellulose ethers). 

Pressure-Sensitive Adhesives. A pressure-sensitive adhesive, a material which adheres with no more than appHed finger pressure, is aggressively and permanently tacky. It requkes no activation other than the finger pressure, exerts a strong holding force, and should be removeable from a smooth surface without leaving a residue. 

Natural-mbber-based pressure-sensitive adhesives can be cured by standard mbber curatives, eg, sulfur plus an accelerator (see Rubber, natural) ... 

Other Plastics Uses. The plasticizer range alcohols have a number of other uses in plastics hexanol and 2-ethylhexanol are used as part of the catalyst system in the polymerization of acrylates, ethylene, and propylene (55) the peroxydicarbonate of 2-ethylhexanol is utilized as a polymerization initiator for vinyl chloride various trialkyl phosphites find usage as heat and light stabHizers for plastics organotin derivatives are used as heat stabHizers for PVC octanol improves the compatibHity of calcium carbonate filler in various plastics 2-ethylhexanol is used to make expanded polystyrene beads (56) and acrylate esters serve as pressure sensitive adhesives. 

Hydrocarbon resin is a broad term that is usually used to describe a low molecular weight thermoplastic polymer synthesized via the thermal or catalytic polymerization of coal-tar fractions, cracked petroleum distillates, terpenes, or pure olefinic monomers. These resins are used extensively as modifiers in the hot melt and pressure sensitive adhesive industries. They are also used in numerous other appHcations such as sealants, printing inks, paints, plastics, road marking, carpet backing, flooring, and oil field appHcations. They are rarely used alone. 

G-5—G-9 Aromatic Modified Aliphatic Petroleum Resins. Compatibihty with base polymers is an essential aspect of hydrocarbon resins in whatever appHcation they are used. As an example, piperylene—2-methyl-2-butene based resins are substantially inadequate in enhancing the tack of 1,3-butadiene—styrene based random and block copolymers in pressure sensitive adhesive appHcations. The copolymerization of a-methylstyrene with piperylenes effectively enhances the tack properties of styrene—butadiene copolymers and styrene—isoprene copolymers in adhesive appHcations (40,41). Introduction of aromaticity into hydrocarbon resins serves to increase the solubiHty parameter of resins, resulting in improved compatibiHty with base polymers. However, the nature of the aromatic monomer also serves as a handle for molecular weight and softening point control. 

Although most aromatic modified C-5 resins are typically higher softening point resins, certain appHcations, such as adhesives, require lower softening points. Copolymerization of a C-8—C-10 vinyl aromatic fraction with piperylenes in the presence of a C-4—C-8 mono-olefin chain-transfer stream yields resins with softening points ranging from 0—40°C (44). A particular advantage of these Hquid resins is the fact that they eliminate the need for plasticizers or oils in some pressure sensitive adhesive appHcations. 

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Pressure sensitive adhesives typically employ a polymer, a tackifier, and an oil or solvent. Environmental concerns are moving the PSA industry toward aqueous systems. Polymers employed in PSA systems are butyl mbber, natural mbber (NR), random styrene—butadiene mbber (SBR), and block copolymers. Terpene and aUphatic resins are widely used in butyl mbber and NR-based systems, whereas PSAs based on SBR may require aromatic or aromatic modified aUphatic resins. 

Styrenic block copolymers (SBCs) are also widely used in HMA and PSA appHcations. Most hot melt appHed pressure sensitive adhesives are based on triblock copolymers consisting of SIS or SBS combinations (S = styrene, I = isoprene B = butadiene). Pressure sensitive adhesives typically employ low styrene, high molecular weight SIS polymers while hot melt adhesives usually use higher styrene, lower molecular weight SBCs. Resins compatible with the mid-block of an SBC improves tack properties those compatible with the end blocks control melt viscosity and temperature performance. 

Ziac foil coated with a conductive, pressure-sensitive adhesive is used for repair of other ziac coatings or for imparting corrosion resistance at field sites. The 0.08-mm ziac tape or sheet has a 0.025-mm coaductive adhesive. The laminate is cut to size and pressed tightly to activate the adhesive. Conductive tape can be wrapped around pipe, especially around welds or connections. The corrosion resistance of this material is iatermediate between galvanized or thermally sprayed coatings and zinc-filled paints (21,50). 

Adhesive Transfer Processes. Many polymers, whether dehberately or accidentally, are adhesives, so that much of the adhesive industry can be regarded as a part of the mbber and plastics industry. However, there are several important material-transfer appHcations involving polymer products that are so critically dependent on controlled adhesion that they merit specific mention in that category. They include hot stamping foils, release coatings for pressure-sensitive adhesive products, photocopier materials, transfer coatings, and transfer printing of textiles. 

Sihcone products dominate the pressure-sensitive adhesive release paper market, but other materials such as Quilon (E.I. du Pont de Nemours Co., Inc.), a Werner-type chromium complex, stearato chromic chloride [12768-56-8] are also used. Various base papers are used, including polyethylene-coated kraft as well as polymer substrates such as polyethylene or polyester film. Sihcone coatings that cross-link to form a film and also bond to the cellulose are used in various forms, such as solvent and solventless dispersions and emulsions. Technical requirements for the coated papers include good release, no contamination of the adhesive being protected, no blocking in roUs, good solvent holdout with respect to adhesives appHed from solvent, and good thermal and dimensional stabiUty (see Silicon COMPOUNDS, silicones). 

Rosin ester resins are used extensively in pressure-sensitive adhesives as tackifiers. The adhesive is formulated by blending the resin with a polymer in solution or as aqueous emulsions. Typical compositions may contain about 50% resin. The glycerol or pentaerythritol esters of stabilized rosins are often used because they are stable on aging. 

Pressure-Sensitive Adhesives. SiHcoae PSAs are used primarily ia specialty tape appHcatioas that require the superior properties of siHcoaes, including resistance to harsh chemical environments and temperature extremes (398,399). SiHcone PSAs are also used ia appHcatioas requiring long service Hfe, electrical iasulatioa, and protection from moisture. Another distinctive advantage of siHcone PSAs is their abiHty to wet low surface energy tape substrates such as PTEE. 

R. P. Eckberg, in D. Satas, ed.. Advances in Pressure Sensitive Adhesive Technology-1, Satas Associates, Warwick, R.I., 1992, p. 50. 

Uses ndReactions. Some of the principal uses for P-pinene are for manufacturing terpene resins and for thermal isomerization (pyrolysis) to myrcene. The resins are made by Lewis acid (usuaUy AlCl ) polymerization of P-pinene, either as a homopolymer or as a copolymer with other terpenes such as limonene. P-Pinene polymerizes much easier than a-pinene and the resins are usehil in pressure-sensitive adhesives, hot-melt adhesives and coatings, and elastomeric sealants. One of the first syntheses of a new fragrance chemical from turpentine sources used formaldehyde with P-pinene in a Prins reaction to produce the alcohol, Nopol (26) (59). 

In the adhesives area, thermoplastic, fatty polyamides are used in hot-melt and heat-seal adhesives for leather, paper, plastic and metal. Blends of EDA- and DETA-based polyamides are suggested for use in metal can seam sealants with improved toughness (234) pressure sensitive adhesives have been formulated with DETA-based polyamides (235) and anionic and cationic suspensoid adhesives are used as heat-seal coatings in paper converting (236). PIP and certain PIP derivatives are used with EDA in some appHcations (237). 

Ethyl Vinyl Ether. The addition of ethanol to acetylene gives ethyl vinyl ether [104-92-2] (351—355). The vapor-phase reaction is generally mn at 1.38—2.07 MPa (13.6—20.4 atm) and temperatures of 160—180°C with alkaline catalysts such as potassium hydroxide and potassium ethoxide. High molecular weight polymers of ethyl vinyl ether are used for pressure-sensitive adhesives, viscosity-index improvers, coatings and films lower molecular weight polymers are plasticizers and resin modifiers. 

Another major area of use is in the field of adhesives. The main attractions of the material are the absence of a need for mastication, easy solvation of the polymer, which is supplied in a crumb form, the production of low-viscosity solutions and high joint strength. In conjunction with aromatic resins they are used for contact adhesives whilst with aliphatic resin additives they are used for permanently tacky pressure-sensitive adhesives. In addition to being applied from solution they may be applied as a hot melt. 

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