Hot melt adhesives are defined as adhesives that are applied from the melt and that build both adhesive and cohesive strength upon resolidification and crystallization of one or more components of the adhesive composition. Because hot melt adhesives are applied without solvents, increasing regulation of VOC emissions in recent years has caused an increase in demand for new and better performing hot melt adhesive formulations.
Hot melt adhesive compositions are formulations that include one or more structural or base polymers and, typically, one or more adjuvants. Adjuvants typically include functional materials or diluents such as tackifying resins, plasticizers, fillers, oils and waxes or other low molecular weight polymers. The base polymer is typically the major component of a hot melt adhesive formulation and contributes cohesive strength to the adhesive. Copolymers of ethylene and a mono-olefinically unsaturated polar comonomer such as vinyl acetate, methyl acrylate, acrylic acid and the like, are widely used as the base polymer component in many hot melt adhesive compositions. Another class of hot melt adhesives employs copolymers of ethylene or propylene with one or more relatively nonpolar α-olefins such as 1-butene, 1-hexene, and the like. Hot melt adhesive formulations employing nonpolar polymers, while less common in the industry than their EVA and other similar polar polymer based counterparts, are known. Examples of some such formulations are disclosed in U.S. Pat. Nos. 4,072,735; 5,118,762; and 4,568,713 to name just a few examples.
U.S. Pat. No. 6,747,114 discloses hot melt adhesive formulations employing metallocene polymerized propylene copolymers, wherein the comonomer is ethylene and/or a C4-C20 α-olefin as the base polymer in the formulation. The highly isotactic or syndiotactic propylene content of the metallocene polymerized base polymers imparts crystallizable content to the base polymers; the metallocene polymerized propylene copolymers are called semicrystalline copolymers. As used herein and as applied to semicrystalline copolymers, the term “crystallizable” describes those sequences which are mainly amorphous in the undeformed state, but can crystallize upon stretching, annealing or in the presence of a nucleating agent, such as a crystalline compound or another segment within the polymer. Crystalline content of the solidified semicrystalline copolymers increases the cohesive strength of the hot melt adhesives. Hot melt adhesive formulations based on metallocene polymerized propylene-based semicrystalline copolymers can eventually build sufficient crystalline content over time to achieve good cohesive strength in the formulation.
However, such formulations suffer from the drawback that the slow rate of crystalline content formation, retards cohesive strength formation. For many potential end uses, the rate of crystallization of the crystallizable content of the metallocene polymerized propylene-based semicrystalline copolymers is unacceptably long and results in a significant delay in effective adhesive bonding and reduced productivity. Attempts to accelerate the rate of crystallization have centered on the use of nucleating agents. U.S. Pat. No. 6,747,114 discloses metallocene polymerized propylene-based semicrystalline copolymers hot melt adhesive formulations having about 10 to 25 wt. % of a Fischer-Tropsch wax and/or 5% of a low molecular weight isotactic polypropylene homopolymer in order to affect crystalline content in the semicrystalline copolymer-based hot melt adhesive.
While the nucleation agents described above effectively increase the ultimate degree of crystalline content, as evidenced e.g. by a higher ultimate tensile strength of the adhesive, they do not address the rate of nucleation and crystal formation at a given temperature. Slow rate of crystallization is evidenced by a long effective set time: that is, a relatively long period of time between application of the hot melt adhesive formulation and the ability of the formulation to cause a bond to form between two adherends of sufficient strength for the envisioned application. In fact, the effective set time of the metallocene polymerized propylene-based semicrystalline copolymer hot melt adhesive formulations of the art is too slow for many industrially significant commercial applications.
Additionally, the high level of wax included in some such formulations can result in lower ultimate adhesive strength when the adhesive is completely crystallized. For example, one known application of Fischer-Tropsch waxes is as a mold release. The greater the amount of wax in the formulation, the greater the amount present at the adhesive-substrate interface and the greater its effect upon reduced ultimate adhesion and release.
Thus there is a need in the industry to obtain hot melt adhesive compositions based on metallocene polymerized propylene-based semicrystalline copolymer that builds crystalline content quickly. There is a need in the industry to obtain such adhesives having a low wax content. There is a need for such hot melt adhesive compositions to have the properties of melt rheology, adhesion, and cohesive strength widely associated with conventional hot melt adhesive formulations.