Among adhesive chemistries, (meth)acrylics (i.e., methacrylate polymers and acrylate polymers) are one of the most prominent. (Meth)acrylics have evolved as a preferred class of adhesives due to their durability, permanence of properties over time, and versatility of adhesion, to name just a few of their benefits.
Traditionally, adhesives, such as (meth)acrylics, have been provided in organic solvent for subsequent application. Such adhesives are applied to a substrate and the solvent is then removed. Hot-melt adhesives advantageously reduce or eliminate the use of organic solvents in adhesives and their processing. Hot-melt adhesive systems are essentially 100% solid systems. Usually, such systems have no more than about 5% organic solvents or water, more typically no more than about 3% organic solvents or water. Most typically, such systems are free of organic solvents and water. Advantageously, by reducing the use of organic solvents, special handling concerns associated therewith are also reduced.
Hot-melt processable adhesives have a sufficient viscosity upon melting, such that they can be hot-melt processed (e.g., applied to a substrate). By adjusting the processing temperature and components of an adhesive, the viscosity of the adhesive can be readily tailored for application. For high performance applications (i.e., those requiring relatively strong cohesive strength, such as shear holding strength), some method of increasing the cohesive strength of applied hot-melt adhesives is often needed (e.g., post-crosslinking or moisture-curing).
For example, energy sources, such as electron beam (e-beam) or ultraviolet (UV) radiation, are commonly used to chemically crosslink adhesives after application. These methods, however, often require an additional processing step and, thus, result in decreased processing efficiency. Furthermore, e-beam is not always desired because it is expensive and can cause damage to some backings when the adhesive is used in a tape. Similarly, UV-radiation has its limitations as a crosslinking energy source. For example, UV-radiation is often not able to be used effectively for crosslinking relatively thick adhesives due to the need for UV-radiation to penetrate throughout the entire thickness of the adhesive. As such, certain fillers and pigments cannot be used in adhesives when UV-crosslinking is used because they potentially interfere with penetration of UV-radiation therethrough.
Another disadvantage of chemically crosslinking a polymer is the irreversible nature of such chemical crosslinks. Variations in polymer morphology may obviate the necessity for chemical crosslinking of a polymer, however. For example, block copolymers advantageously show phase segregation causing domains to form, which domains serve as physical crosslinking and reinforcement sites. Thus, block copolymers do not require additional steps that involve chemical crosslinking (e.g., post-crosslinking or moisture-curing). However, the degree and type of physical crosslinking may still not be adequate for certain high performance applications that require adhesives with adequate cohesive strength after application.
Molecular structure and morphology of polymers is often dictated by their polymerization methods. It is known that (meth)acrylate block copolymers can be prepared through a variety of living anionic and living free radical polymerization (also referred to as controlled free radical polymerization) techniques.
For example, U.S. Pat. No. 5,686,534 (Bayard et al.) and U.S. Pat. No. 5,677,387 (Bayard et al.) demonstrate the synthesis of homopolymers and block copolymers of (meth)acrylates or other vinyl monomers using living anionic polymerization. Block copolymers described therein include those having primary alkyl acrylate blocks with blocks of methacrylic or secondary or tertiary alkyl acrylate monomer or nonacrylic vinyl monomer. The block copolymers are said to be useful in the manufacture of elastomeric articles (e.g., injection-molded articles) and as additives in pressure-sensitive-adhesive (PSA) formulations.
PCT Publication No. WO 97/18,247 (Carnegie Mellon) describes the synthesis of homopolymers or copolymers through atom (or group) transfer radical polymerization (ATRP (or GTRP)). Copolymers prepared therefrom (including block and random copolymers) are purportedly useful in wide variety of applications, such as adhesives and as thermoplastic elastomers, for example. However, the block copolymers (triblock copolymers derived from monomers of styrene and (meth)acrylate monomers and triblock copolymers derived from monomers of methyl methacrylate and acrylate monomers) described therein are stated to be useful merely as thermoplastic elastomers.
These references do not teach how to make a hot-melt processable PSA utilizing such block copolymers. Yet, a number of references do describe (meth)acrylate block copolymers, some of which can be used in hot-melt processable PSAs. These references, however, typically synthesize the block copolymers using iniferter polymerization. The term iniferter refers to a chemical compound that has a combined function of being a free radical initiator, transfer agent and terminator. Photoiniferters are compounds in which light is used to generate the free radical iniferter species.
European Patent Publication No. 0 349 270 A2 (Minnesota Mining and Manufacturing Co.) describes the preparation of (meth)acrylate block copolymer PSA compositions and a method of making the same by use of an iniferter as a means of promoting, controlling and terminating polymerization. The resultant reinforced (meth)acrylate block copolymer and tackifier, if used, provides a PSA composition. If a tackifier is used, it is typically in an amount of 0 to 150 parts by weight based on 100 parts by weight of the block copolymer. Shear strength values for tapes prepared from the block copolymers therein and tested according to ASTM D3654 are all reported to be 133 minutes or less. These values are not large enough for many high performance applications.
Japanese Patent Publication HEI No. 10-30078 (Sekisui Chemical Co., Ltd.) describe adhesives prepared from (meth)acrylate block copolymers. The adhesives are purportedly useful as PSAs and hot-melt adhesives. It is taught that the block copolymers can be prepared by different polymerization methods, such as those that use iniferters as initiators, and including anionic polymerization and group transfer polymerization. Additives, such as adhesion-providing resins (e.g., tackifiers), are taught to be used according to need. Exemplifed therein are (A-B-A) triblock copolymers, wherein the A block is derived from methyl methacrylate and the B block is derived from ethyl acrylate. Further publications in related technologies of Sekisui Chemical Co., Ltd. include: Japanese Patent Publication HEI Nos. 9-324,165; 10-25,459; 10-25,460; 10-80,111; 10-80,112; and 10-80,113.
It has been difficult, however, to obtain clean, discrete block configurations in (meth)acrylate copolymers in the past when polymerizing using iniferters. Drawbacks of polymerization methods using iniferters include, for example: blocks having broad molecular weight distributions (i.e., as indicated by a relatively high polydispersity), poorly defined blocks/phase boundaries, relatively impure blocks, and inadequate control of molecular weight during polymerization. These drawbacks may present formulation difficulties and, thus, decrease the overall formulation latitude of such compositions. For example, when iniferters are used for polymerization, resulting block copolymers cannot be highly tackified without detrimentally affecting PSA properties of the composition. Accordingly, Japanese Patent Publication HEI No. 10-25,459 cautions that when the amount of adhesion-providing resin (i.e., tackifier) is greater than 40 parts by weight based on 100 parts by weight of the block copolymer, tackiness of the adhesive is decreased.
As stated above, block copolymers have been found useful in the preparation of PSAs. Furthermore, U.S. Pat. No. 3,239,478 (Harlan); U.S. Pat. No. 3,932,328 (Korpman); U.S. Pat. No. 4,780,367 (Lau); and U.S. Pat. No. 5,296,547 (Nestegard et al.) describe preparation of rubber-type block copolymers to provide PSAs. Some of these references also describe tailorability of the PSAs therein to provide hot-melt adhesives.
To date, typical block copolymers have predominantly utilized rubber-type materials, such as linear or branched polystyrene-polydiene-type materials. A drawback of these materials, however, is that the unsaturated hydrocarbon polydiene segments are often vulnerable to degradation, limiting the weatherability of these polymers. Although hydrogenation of the unsaturated hydrocarbon polydiene segments has improved weatherability of these materials (see, for example, U.S. Pat. No. 5,773,506 (Nestegard et al.)), rubber-based materials have not shown the durability, permanence of properties over time, and versatility of adhesion that are common to (meth)acrylics. Furthermore, the subsequent hydrogenation step decreases processing efficiency of the compositions. Thus, these types of block copolymers are not preferred.
In order to meet consumer demands for high performance applications, hot-melt processable adhesives must possess adequate cohesive strength after application. The increase in cohesive strength cannot come, however, at the expense of hot-melt processability. Furthermore, processing efficiency should not be compromised.
It would also be desirable to provide hot-melt processable adhesives, such as pressure-sensitive-adhesives, with broad formulation latitude. In order to provide broad formulation latitude, it is also desirable that the hot-melt processable adhesives comprise block copolymers having well-defined blocks.