FIELD OF THE INVENTION

This invention relates to a novel hot melt adhesive composition based on a polymer blend comprising polypropylene polymers, preferably prepared by using single-site catalysts. The adhesive composition has well-balanced adhesion and cohesion properties and finds utility in packaging, product assembly and laminations. The adhesive composition is particularly useful in bonding low surface energy substrates that are often seen in manufacturing a variety of disposable nonwovens hygienic products such as baby diapers, adult incontinent articles and feminine hygiene pads, including elastic attachments.

BACKGROUND OF THE INVENTION

Hot melt adhesives typically exist as a solid mass 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, panty shields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven hygienic products. Other diversified applications have involved paper products, packaging materials, automotive headliners, appliances, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate, often named as the primary substrate. A second substrate, often named as the secondary 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 or a bead 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 to allow the molten hot melt material to flow through the application nozzle smoothly. For most applications, particularly those encountered in food packaging and disposable nonwovens hygienic article manufacturing, bonding of delicate and heat sensitive substrates, such as thin gauge plastic films, is often involved. This imposes an upper limit on coating temperature for hot melt adhesive applications. Today's commercial hot melts are typically formulated to have coating temperature below 200° C., preferably below 150° C., to avoid substrate burning or distortion. Besides directly coating, several indirect or noncontact coating methods, through which a hot melt adhesive can be spray coated with the aid of compressed air onto a substrate from a distance, are also developed. These non-contact coating techniques include conventional spiral spray, Omega™, Surewrap™ and various forms of melt-blown methods. The indirect method, however, requires that the viscosity of the adhesives must be sufficiently low, usually in the range of 2,000 to 30,000 mPa·s, preferably in the range of 2,000 to 15,000 mPa·s, at the application temperature in order to obtain an acceptable coating pattern. 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, which must be determined empirically with application equipment.

Hot melt adhesives are organic materials typically consisting of a polymer, a plasticizer, a tackifying resin, and an antioxidant package. Other ingredients, such as wax, filler, colorant and UV absorber, can also be used to modify the adhesive properties or to provide special attributes. These organic ingredients are prone to heat degradation under the coating conditions of the adhesive. For example, the widely used commercial hot melt adhesive based on styrene-isoprene-styrene (SIS) triblock copolymer, when subjected to 175° C. for 24 hours, can suffer from a viscosity drop of about 50 percent from its original value. A styrene-butadiene-styrene (SBS) based hot melt may cause problems by crosslinking under similar conditions. Crosslinking can result in a dramatic increase in viscosity and may eventually render the adhesive un-flowable by the formation of three dimensional polymer network. The viscosity change is often accompanied by charring, gelling, and formation of skin on top of the molten material. The degradation will inevitably lead to deterioration of the adhesive properties and performance. In addition, they can also cause equipment damage. The rate of degradation is temperature dependent; the higher the temperature, the faster the degradation. Thus, reducing the coating temperature of the adhesive can slow down degradation.

Conventional polyolefins produced by using Ziegler-Natty catalysts such as, for example, low density polyethylene (LDPE), high density polyethylene (HDPE), and istotatic polypropylene (iPP) do not lend themselves to adhesive applications. Ziegler-Natta. (ZN) catalyst systems consist of a pair of catalyst and co-catalyst. The most common of such pairs are TiCl3and Al(C2H5)2Cl, or TiCl4with Al(C2H5)3, Ziegler-Natta catalyst systems are a subject of numerous publications in scientific journals and textbooks and are well known to those skilled in the art. A conventional ZN catalyst system is typically embedded in an inert support and has several catalyst sites, each of which has different activity. This difference in activity causes the formation of polymer molecules with a plurality of molecular weights and composition of copolymer molecules. The polyolefin homopolymers and copolymers produced with ZN catalysts are typically highly crystalline and stiff. This can translate to a hot melt adhesive that is relatively brittle or has poor substrate wetting, poor adhesion, and poor proccessability. Nevertheless, hot melt adhesives containing various types of polyolefin blends are known in various previous patent literatures.

As used herein, Z—N refers to a Ziegler-Natta catalyst for olefin polymerization.

As used herein, LDPE and HPDE refer to low density polyethylene and high density polyethylene, respectively.

As used herein, iPP refers to isotactic propylene homopolymers or copolymers having predominantly an isotactic chain structure.

As used herein, APAO refers to a class of low molecular weight amorphous propylene homopolymers or copolymers with ethylene or butene typically produced with a Lewis acid catalyst.

As used herein, PB refers to polybutene homopolymers and copolymers.

For example, Trotter et al, in U.S. Pat. No. 4,022,728, describes a hot melt pressure sensitive composition comprising a mixture of amorphous poly-alpha-olefins (APAO), a low molecular weight substantially amorphous elastomer, a liquid tackifier, and a conventional crystalline polypropylene (iPP) in the amount of up to 2% by weight. According to the '728 patent, the composition provides good adhesive properties at low temperatures.

Meyer et al, in U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of low molecular weight PE, low molecular weight iPP, and APAO. These adhesive compositions are said to offer short open times and to be useful for bonding of paraffin modified corrugated board.

Lakshmanan et al, in U.S. Pat. No. 4,761,450, discloses a polymer blend useful as a hot melt adhesive comprising a LDPE, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier, and a low molecular weight polymer consisting of a low molecular weight liquid polybutene, an APAO, and mixtures thereof.

Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer, and a hydrocarbon tackifier.

Blending APAO with polyethylene, polybutene copolymers, or the conventional iPP leads to severe drawbacks. The prior art adhesives containing APAO/PE or APAO/PB blends, such as, for example, those described herein above in U.S. Pat. Nos. 4,120,916 and 4,761,450 tend to have poor compatibility. These adhesives can undergo phase separation during the application process when the hot melt adhesive must be kept in the molten state at high temperature for a prolonged period of time, sometimes for hours or even days. Charring, skinning and gelling can develop rather quickly in the phase separated hot melt adhesives, thereby causing the application equipment to block or plug-up. The incompatibility of such polymer blends also imparts brittleness, optical haziness, poor or no open time, and low bond strength. Although APAO and the conventional iPP blend based hot melts do not have the compatibility problems, they may still suffer from all the other drawbacks herein described above. Moreover, due to high crystallinity and high melting point of the conventional iPP polymers, hot melt adhesives based on APAO/iPP blends tend to be hard and brittle unless the iPP polymer amount is to kept at a very low level, such as, for example, at about or below 2% by weight as disclosed in U.S. Pat. No. 4,022,728. As a result, these adhesives will have poor tensile strength, poor bond strength, and poor impact resistance. Another detrimental effect of iPP is the increased coating temperature. The adhesive must be heated well above the melting point of iPP (ranging from 180 to 200° C.) to reach liquid state. Although the blend of high and low molecular weight atactic polyolefin approach described in U.S. Pat. No. 5,723,546 offers some improvement on tensile properties of APAO, it has not been able to provide sufficient tensile strength and high temperature properties to overcome the deficiencies of sole APAO based hot melts.

The shortcomings in the prior art mentioned above are partially overcome in more recent inventions that are disclosed in U.S. Pat. No. 6,329,468, that teaches the use of semicrystalline flexible polyolefin for hot melt adhesive compositions; in U.S. Pat. No. 7,262,251 that teaches a hot melt adhesive composition based on a random copolymer of isotactic polypropylene (RCP) and a secondary polymer; in US Patent Application Publication US2003/0096896 A1 that describes a hot melt composition comprising a blend of syndiotactic polypropylene (sPP) and APAO; in U.S. Pat. No. 8,383,731 that describes an adhesive blend based on a semicrystalline copolymer of propylene with an alpha-olefin. These compositions, however, all consist of a rigid semicrystalline polymer that is non-uniform in either intramolecular and/or intermolecular compositional distribution, and in the tacticity distribution of molecular chains. It is not the intention of the present invention to dwell on theoretical discussions of polymer property-function relationships, but non-uniformity in the composition and chain structure, coupled with a very broad molecular weight distribution, may be accountable for poor adhesive properties and poor processibility for a hot melt composition. These semicrystalline polymers in the above compositions can have rigid polymer chain structure, which is detrimental to adhesion and application properties of hot melt adhesives containing such polymers. It is extremely difficult, if not impossible, to balance the complex requirements of adhesion, cohesion, low viscosity, broad application temperature range, and applicability by a whole array of application methods.

Most recently, Tse et al in U.S. Pat. No. 9,109,143 revealed an adhesive composition containing a blend of two low molecular weight propylene based copolymers having a to weight average molecular weight (Mw) less than 100,000 g/mol. The low molecular weight propylene copolymers of the '143 patent also have low melting point and low crystallinity. The copolymers, primarily directed to sealing of corrugated boxes, have poor cohesive strength and therefore do not lend themselves to an array of demanding applications such as, for example, in elastic attachment for nonwoven hygiene products and auto headliner assembly.

U.S. Patent Application No. 2016/0121014 discloses a disposable absorbent article and an adhesive composition including a first polymer that is propylene-based and has a molecular weight of no greater than about 75,000 and a second polymer selected from a group including propylene based polymers with a molecular weight of at least about 100,000 and styrene block copolymers with a styrene content of no greater than about 20%, where the adhesive composition is alleged to be useful for elastic attachment applications.

Wang et al. in US Patent Application No. 2017/0204306 discloses a hot melt adhesive composition based on a blend of high molecular weight, essentially amorphous propylene polymer and a low molecular weight, semicrystalline propylene polymer, both of which are preferably produced by using a single site catalyst. The composition showed excellent creep resistance for nonwovens elastic attachment applications. For some applications requiring high peel strength (>100 g/in) at low coating weight (such as certain nonwovens construction adhesives), the performance of adhesives of this patent application could be further improved upon.

SUMMARY OF THE INVENTION

Therefore, it would be advantageous to provide a hot melt adhesive that will overcome the shortcomings of the prior art adhesives mentioned above. It is found in the present invention that a polymer composition comprising at least one semicrystalline, low molecular weight (LMW) propylene-based polymer; at least one essentially amorphous, high molecular weight (HMW) propylene-based polymer; and at least one essentially amorphous, LMW propylene-based polymer provides a unique combination of properties which the prior art hot melt systems have failed to offer. Such a composition provides both high creep resistance and high peel strength to a variety of low surface energy substrates such as LDPE films and PP based nonwovens, excellent heat stability, good wet-out property, broad application temperature range, long open time, and good green bond strength. The composition is suitable not only for elastic attachment, but also for many other bonding applications around nonwoven hygienic articles such as bonding waist stretch panels to chassis and attaching landing zone and Velcro ear to waist areas.

In accordance with the present invention, a hot melt adhesive composition comprises the polymer composition comprising the three propylene-based polymers, as well as other constituents typically found in a hot melt adhesive. Preferably, these polymers are produced using single-site catalysts (SSC) and are: at least one essentially amorphous, HMW propylene-based polymer (referred to sometimes herein as “HMW SSC-Amorphous PP polymer”); at least one essentially amorphous propylene, LMW propylene-based polymer (referred to sometimes herein as “LMW SSC-Amorp-PP polymer”); and at least one semicrystalline, LMW propylene-based polymer (referred to sometimes herein as “LMW SSC-Cryst-PP polymer”). The distinct molecular weights of the component polymers give rise to a polymer blend having a trimodal molecular weight distribution. Besides the difference in molecular weights, the SSC-PP polymers in the blend also differ in enthalpy of melting, which is an indirect measure of polymer crystallinity.

The hot melt adhesive composition also includes a tackifier component, a plasticizer component, an antioxidant package, and optionally other additives such as a wax, a filler, a colorant, a UV absorber, and another polymer. The composition is well suited for applications with a variety of direct and indirect coating methods and has a novel combination of properties including low coating temperature, wide coating temperature range, excellent adhesion to low surface energy plastic substrates, high cohesive strength, high shear resistance, high strand hold property, low viscosity, low melting point and superior heat stability. The composition of the present invention is particularly useful for food packaging, product assembly and for assembly of disposable nonwoven articles for bonding of polyethylene and polypropylene films, nonwoven fabrics and elastic strands to each other or to themselves.

As used herein, SSC refers to single-site catalysts for α-olefin polymerization.

As used herein, Mw refers to weight average molecular weight of a polymer. Weight average molecular weight is characterized using a high temperature Size Exclusion Chromatograph (SEC) using a polystyrene reference standard.

For the purposes of the present invention, the term essentially amorphous is used to refer to a state wherein a PP based polymer exhibits an enthalpy of melting from 0 J/g to about 30 J/g.

For the purposes of the present invention, the term semicrystalline is used to refer to a state wherein a PP based polymer exhibits an enthalpy of melting above 30 J/g.

As used herein, HMW SSC-Amorp-PP refers to a class of high molecular weight essentially amorphous propylene homopolymers or copolymers produced by using single-site catalysts having a Mw greater than about 100,000 g/mole. The polymers can be completely amorphous showing no melting peaks on a DSC curve, but they can also have a small fraction of crystals that give rise on a DSC curve to a small, but noticeable, melting peak or peaks with associated enthalpy of melting of 30 joules per gram of material (J/g), or less, i.e. from 0 J/g to about 30 J/g.

As used herein, LMW SSC-Amorp-PP refers to a class of high molecular weight essentially amorphous propylene homopolymers or copolymers produced by using single-site catalysts having a Mw less than about 90,000 g/mole. The polymers can be completely amorphous showing no melting peaks on a DSC curve, but they can also have a small fraction of crystals that give rise on a DSC curve to a small, but noticeable, melting peak or peaks with associated enthalpy of melting of 30 joules per gram of material (J/g), or less, i.e. from 0 J/g to about 30 J/g.

As used herein, LMW SSC-Cryst-PP refers to a class of low molecular weight semicrystalline propylene homopolymers or copolymers having a weight average molecular weight (Mw) of about 90,000 g/mole or less, and a distinct melting peak or peaks on a DSC curve with associated enthalpy of melting of 30 joules per gram of material (J/g) or greater, i.e. typically from about 30 J/g to about 100 J/g, more preferably from about 35 J/g to about 80 J/g. The terms “enthalpy of melting”, “enthalpy of fusion”, “heat of fusion” and “heat of melting” are used interchangeably.

As used herein, DSC curve refers to a plot of heat flow or heat capacity versus temperature obtained by using differential scanning calorimetry (DSC) instrument. The test method used to determine these values is ASTM E793-01 “Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning calorimetry”.

As used herein, SSC-PP blend refers to a polymer blend comprising at least one HMW SSC-Amorp-PP polymer, at least one LMW SSC-Amory-PP and at least one LMW SSC-Cryst-PP polymer. The invention contemplates that the adhesive includes only these three polymers and no other polymers or may also contain other polymers, in addition to the other constituents of the adhesive.

As used herein, polymer is “propylene-based” if it is a polypropylene homopolymer or if it is a copolymer with another monomer, such as ethylene, and having at least 50% by weight of propylene.

A polymer composition can be a preformed mixture or blend of the listed polymers or it can represent the particular polymers that are the ingredients of a hot melt composition even if such polymers are not mixed together before being added to the adhesive.

The present invention is, therefore, directed to hot melt adhesive compositions comprising a polypropylene polymer blend containing three distinct polymers: a semicrystalline LMW SSC-Cryst-PP polymer, an essentially amorphous LMW SSC-Amorp-PP polymer and an essentially amorphous HMW SSC-Amorp-PP polymer. The total amount of the polymer blend herein mentioned comprises from 20 to 80% of the adhesive composition. The adhesive composition comprises, in addition to the SSC-PP blend, a tackifying resin, a plasticizer, and an antioxidant system as the primary ingredients. The compositions of the present invention have taken advantage of complimentary properties among the semicrystalline LMW SSC-Crys-PP polymer, the essentially amorphous LMW SSC-Amorp-PP polymer, and the essentially amorphous HMW SSC-Amorp-PP polymer and have overcome the shortcomings of the prior art polyolefin based hot melt adhesives. Compositions according to embodiments of the present invention provide well balanced properties of creep retention and peel strength, thereby overcoming the deficiency in peel values of prior art compositions based on binary polymer blends. The composition also exhibits high cohesive strength to provide high shear resistance, excellent heat stability, good wet-out properties, broad application temperature range, long open time, good green bond strength, low viscosity, low or no residual tack when set, and suitability with essentially all known hot melt coating methods. In particular, embodiments of the present invention lead to an adhesive composition that is well suited for a variety of spray coating application techniques, such as, for example, spiral spray, Omega™, Surewrap™, melt-blown, Control Coat®, and the like, and non-spray coating application techniques such as, for example, slot coat, V-Slot™, Allegro™ and the like; these coating techniques are well known to those skilled in the art and are not a subject of discussion of the present invention.

It is, therefore, an objective of the present invention to provide a hot melt adhesive composition comprising a polymer blend containing at least one semicrystalline LMW SSC-Amorp-PP polymer, at least one essentially amorphous LMW SSC-Amorp-PP polymer and at least one essentially amorphous HMW SSC-Amorp-PP polymer; all of which are either homopolymers of propylene or copolymers of propylene with an alpha-olefin comonomer preferably prepared by using single-site catalysts (SSC) and have a statistically random comonomer distribution along the polymer chain. The total amount of the polymer blend in the composition of the present invention is from 20% to 80% by weight, preferably from 25% to 60% by weight and most preferably from 30% to 50% by weight.

The semicrystalline LMW SSC-Cryst-PP polymer may be present in embodiments of the invention in an amount of between about 10% and about 40%, preferably between about 15% and about 35%, and most preferably between about 18% and about 25%, by weight and based on the total weight of the polymer composition. The essentially amorphous LMW SSC-Amorp-PP polymer may be present in embodiments of the invention in an amount of between about 5% and about 40%, preferably between about 10% and about 30%, and most preferably between about 18% and about 25%, by weight and based on the total weight of the polymer composition. The essentially amorphous HMW SSC-Amorp-PP polymer may be present in embodiments of the invention in an amount of between about 40% and about 80%, preferably between about 50% and about 70%, and most preferably between about 54% and about 65%, by weight and based on the total weight of the polymer composition. The semicrystalline LMW SSC-Cryst-PP polymer may be present in embodiments of the invention in an amount of between about 2.5% and about 25%, preferably between about 4% and about 20%, and most preferably between about 5% and about 12%, by weight and based on the total weight of the hot melt adhesive composition. The essentially amorphous LMW SSC-Amorp-PP polymer may be present in embodiments of the invention in an amount of between about 2% and about 20%, preferably between about 3% and about 15%, and most preferably between about 5% and about 12%, by weight and based on the total weight of the hot melt adhesive composition. The essentially amorphous HMW SSC-Amorp-PP polymer may be present in embodiments of the invention in an amount of between about 12% and about 40%, preferably between about 15% and about 30%, and most preferably between about 18% and about 26%, by weight and based on the total weight of the hot melt adhesive composition. When various ranges of any constituent are provided herein, the invention contemplates that the composition may contain a range of that constituent extending from a lower limit of a first range to an upper limit of a second range.

A second objective of the present invention is to teach the art of formulating a hot melt adhesive composition containing the polymer blend herein mentioned above in combination with a plasticizer, a compatible tackifier, and antioxidant. The composition can contain optional additives including, but not limited to, a wax, a functionalized polymer, a colorant, an UV absorber, and a filler.

A third objective of the present invention relates to a hot melt composition comprising the polymer blend that has low viscosity ranging from 500 mPa·s to 35,000 mPa·s at 177° C., preferably from 1000 mPa·s to 20,000 mPa·s and most preferably from 2,000 mPa·s to 15,000 mPa·s. Low viscosity is essential for applications involving various spray coating methods.

Another objective of the present invention is to provide a hot melt adhesive composition for nonwoven elastic attachment applications having a creep retention of at least 80% or higher; a value of 80% (based on the methodology described herein) generally the minimum acceptable by disposable nonwoven hygienic article industry.

The present invention encompasses any application where bonding of similar or dissimilar substrates is involved using a hot melt adhesive at a temperature lower than 200° C., preferably equal to or lower than 160° C. while obtaining good cohesion from the adhesive bond to withstand mechanical stress at low, ambient or elevated temperature, in particular under creep conditions. The present composition is particularly advantageous in bonding printed or coated paperboard having the low surface energy of plastic materials, such as for example, the paper boxes for freezing food packaging, where the boxes are often coated with a moisture barrier material to protect their contents from drying during storage, or printed for aesthetic reasons, or both. For such packaging applications, the present composition allows fiber tear bond strength on coated and printed boxes at low temperatures below the freezing point, where the conventional EVA hot melts fail to work. It is, therefore, another objective of the present invention to provide a hot melt for low temperature packaging application.

Another objective of the present invention is to teach a method for preparing the hot melt adhesive composition involving a batch process.

Another objective of the present invention is to provide a hot melt adhesive composition that has good processibility at low temperatures, below 160° C., including at about 155° C., for applications involving heat sensitive substrates which are unavoidably encountered in nonwovens hygiene absorbent article manufacturing, where thin gauge LDPE films and PP nonwovens fabric are typically used.

Another objective is to teach the art of applying the hot melt composition of the present invention by using various hot melt coating methods and to provide a method of bonding or laminating two or more substrates by first applying the hot melt to the primary substrate and subsequently mating the primary substrate to a secondary substrate.

A further objective of the present invention is to provide a hot melt adhesive composition that is particularly useful for various application in manufacturing of nonwovens hygienic absorbent articles including, but not limited to baby diapers, training pants, adult incontinent articles, feminine pads, panty liners, surgical gowns and poultry absorbent pads, the composition having well balanced properties of tensile strength, toughness, flexibility and adhesion. It exhibits high bond strength to a variety of low surface energy substrates such as LDPE and iPP films, high cohesive strength to hold elastic materials, such as elastic strands, under constant tension, excellent heat stability, good wet-out property, broad application temperature range, long open time, good green bond strength, low viscosity, low or no residual tack when set, and suitability with essentially all known hot melt coating methods.

The objectives of the present invention may be achieved by a hot melt adhesive composition comprising as components thereof a mixture of the following ingredients:a. a polymer composition comprising a semicrystalline LMW SSC-Cryst-PP polymer; an essentially amorphous LMW SSC-Amorp-PP polymer; and essentially amorphous HMW SSC-Amorp-PP polymer, all of which are either homopolypropylene or copolymers of propylene with an α-olefin comonomer, preferably prepared by using SSC catalysts. As an alternative to being made by single site catalysts, one or more of the polymers used herein may have a polydispersibility index of between about 1 to about 4, preferably between about 1.7 to about 3.3;b. a compatible tackifier in the amount of about 15% by weight to about 75% by weight, preferably between about 30% and about 65%, and most preferably between about 35% and about 60%, by weight and based on the total weight of the hot melt adhesive composition;c. about 1% to about 35%, preferably between about 2% and about 25%, and most preferably between about 3% and about 20%, by weight, of a plasticizer, by weight and based on the total weight of the hot melt adhesive composition;d. about 0.1% to about 3%, preferably about 0.2% to about 2%, of a stabilizer or antioxidant, by weight and based on the total weight of the hot melt adhesive composition; ande. optionally about 0% to about 20% by weight, preferably about 0% to about 15% by weight, of a wax.

the components of the composition (which may include further additional components) adding up to 100% by weight. The adhesive composition may contain other components such as filler and/or a colorant and/or a fluorescing agent and/or another polymer that can modify the adhesive properties of the above basic adhesive composition.

