Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-10-18
2003-11-25
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S487000, C524S445000, C524S494000, C524S451000, C524S515000, C156S081000, C156S155000
Reexamination Certificate
active
06653385
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to novel hot melt adhesive compositions based on polymer blends which consists of stereospecific, predominately crystalline syndiotactic polypropylene homo- and co-polymers (SPP) and amorphous atactic poly-&agr;-olefins (APAO). More particularly, this invention relates to adhesive compositions that find utility in case/carton sealing and in manufacturing nonwoven disposable articles such as diapers and feminine hygiene products. The adhesive compositions are particularly useful as an elastic attachment and construction adhesive in assembly of disposable nonwoven articles.
BACKGROUND OF THE INVENTION
Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating the costly process associated with solvent removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film by using piston or gear pump equipment. In this case, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive, which is typically between 150 and 200° C. For some applications, particularly for manufacturing nonwoven articles, bonding of delicate and heat sensitive substrates, such as thin gauge polyethylene films, is often involved. Direct contact between the film and the die, in these cases, must be avoided to prevent the film from burning or distorting. Several application methods have been developed through which a hot melt adhesive can be spray coated with the aid of compressed air onto a substrate from a distance. These non-contact coating techniques include spiral spray and various forms of melt-blown methods. Direct contact between the coating head and the substrate is thus eliminated. All the coating techniques herein described above are well know to those skilled in the art and commercial equipment is readily available.
The spray coating techniques, however, pose stringent requirements on hot melt adhesives. The viscosity of the adhesives must be sufficiently low, usually in the range of 2,000 to 30,000 cP, preferably in the range of 2,000 to 15,000 cP, at the application temperature. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt. The majority of commercial hot melt products do not lend themselves to spray applications. There are no accepted theoretical models or guidelines to predict sprayability and it must be determined empirically with application equipment.
In accordance with the present invention, it has been found that a mixture comprising a polymer blend of SPP and APAO, a tackifying resin, a plasticizer and, optionally, a synthetic polyolefin wax or petroleum wax provides a sprayable hot melt adhesive composition. The composition has novel combinations of properties including toughness, low or no shrinkage, high cohesive strength, low viscosity, excellent heat stability and good adhesion to a variety of substrates. The composition of the present invention is particularly useful in assembly of disposable nonwoven articles for bonding of polyethylene and polypropylene films, nonwoven fabrics and elastic strands to each other or to themselves.
One of the components in the polymer blend is the syndiotactic polypropylene (SPP) polymers. The SPP polymers useful in this invention are essentially high molecular weight stereospecific propylene homopolymers or copolymers of propylene with other &agr;-olefin monomers such as ethylene, butene-1 or hexene-1. The syndiotactic polymers should not be confused with the conventional crystalline polypropylenes and essentially amorphous atactic poly-&agr;-olefins (APAO). These polymers differ from each other in both structures and properties. It is well know to those skilled in the art that the conventional crystalline polypropylenes have an isotactic molecular chain configuration. The isotactic configuration can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane drawn through the main polymer chain. This type of stereochemistry structure can be illustrated graphically by using the Fisher projection formula as the follow:
Due to its high degree of chain regularity, the conventional isotactic polypropylenes (IPP) are highly crystalline with crystallinity typically greater than 50% and a heat of fusion greater than 70 J/g. They are usually stiff materials having high density and high melting point. Due to the lack of flexibility, an IPP polymer can only be used as a modifier in small amounts, typically around 2% to 5% by weight, in hot melt adhesive formulations. A typical conventional IPP usually has a melt flow rate, which is inversely related to the weight average molecular weight, in the range of 0.5 to 200 g/10 min as measured in accordance with ASTM D-1238 test method.
Another component of the polymer blend comprises an APAO polymer. APAO polymers are a family of essentially amorphous low molecular weight homopolymers of propylene or copolymers of propylene with ethylene or butene or hexene. In contrast to the regular structures in IPP or SPP, APAOs have atactic molecular chains with the methyl groups on the successive monomeric units sterically randomly distributed on both sides of the hypothetical plane through the polymer chain. The stereo configuration of the atactic APAO molecular chain can be illustrated graphically by using the following Fisher projection formula:
The stereo chain structure of SPP is uniquely different from that of IPP and from that of APAO. In contrast to the isotactic chain configuration of IPP and the atactic chain configuration of APAO, the stereochemistry of SPP can be described as having the tertiary methyl groups of successive monomer units along the chain alternatively disposed on each side of the hypothetical plane. The stereo configuration of SPP can be depicted below:
The stereo configuration of polypropylene can also be characterized quantitatively through C-13 NMR. In NMR nomenclature, a “meso” dyad of successive methyl groups on the same side of the plane, as in the case of IPP, is represented by the letter m. A “racemic” dyad of successive methyl groups on the opposite sides of the plane, as in the case of SPP, is represented by the letter r. The percentage of m or r defines the degree of polymer tacticity with the sum of m and r equal to 100%. Thus, a perfect isotactic polypropylene will have 100% m dyad, whereas a perfect syndiotactic polypropylene will have 100% r dyad. This unique stereochemical structure of SPP results in an unusual and desirable combination of physical and mechanical properties such as low density, low melting point, flexibility and toughness.
The syndiotactic polymers used in the present invention preferably have an r value equal to or greater than 70%. The polymers having an r value greater than 80% are more preferred and those having an r value greater than 85% is most preferred. It should be pointed out that the r values of conventional IPPs, in c
Wang Baoyu
Zhang Chongyao
Andrus Sceales Starke & Sawall LLP
Bostik Findley, Inc.
Cheung William
Wu David W.
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