Hot melt adhesive based on acrylic block copolymers

HOT MELT ADHESIVE BASED ON ACRYLIC
BLOCK COPOLYMERS

FIELD OF THE INVENTION

[0001] The invention relates to high performance, low viscosity hot melt adhesives comprising acrylic block copolymers.

BACKGROUND OF THE INVENTION

[0002] Typical acrylic pressure sensitive adhesive formulations are copolymers of alkyl ester monomers and a functional monomer such as acrylic acid. These adhesives, however, are generally low in adhesion. While such adhesives are conventionally formulated with low levels of tackifiers (5-30%) to improve their tack and peel, tackification results in loss of heat resistance and poor aging properties. Moreover, acrylics designed for hot melts have typically been poor in shear even without tackification.

[0003] Block acrylics have been synthesized for hot melt applications to improve heat resistance, but have exhibited relatively poor adhesion. This is due to the presence of a hard block phase that stiffens the acrylic to a degree such that grab and tack are insufficient.

[0004] There continues to be a need in the art for improved acrylic polymer compositions that can be used to prepare pressure sensitive hot melt adhesive compositions that can be formulated with sufficient adhesion for applications such as industrial tapes and dermal applications.

[0005] The current invention fulfills this need.

SUMMARY OF THE INVENTION

[0006] The invention provides a hot melt adhesive comprising an acrylic block copolymer, wherein the acrylic block copolymer is present in the composition in amounts of less than about 50% by weight based on the weight of the adhesive composition. The adhesive of the invention will also preferably contain a tackifier and/or diluent. Apreferred diluent for use in the practice of the invention is polypropylene glycol. The adhesives of the invention may advantageously be formulated for low temperature application.

[0007] Block copolymers that may be used in the practice of the invention will generally be multiblock polymers wherein greater than about 50 weight % of the polymer comprises at least 2 hard blocks per molecule. Examples of block copolymers include those having the formula -A-B-Aand (-A-B-)n—Xm. Particularly preferred are block copolymers of the formula -[A1]-[B]-[A2]-. In the above representative formulations, A, Al and A2 each represents a polymer block having a glass transition temperature (Tg) of greater than about 30° C. (also referred to as a hard block) and B represents a polymer block having a Tg of less than about 20° C. (also referred to as a soft block) and X represents a multifunctional coupling agent. Polymer block B will preferably be present in amounts of at least 50 weight %. In a particularly preferred embodiment, Al and A2 is methyl methacrylate and B is n-butyl acrylate.

[0008] The invention also provides a process for bonding a substrate to a similar or dissimilar substrate using the adhesive.

[0009] The invention further provides articles of manufacture comprising the adhesive. The properties of the adhesive makes it particularly useful in the manufacture of industrial tapes, in medical applications (e.g., for dermal applications) and in nonwoven applications.

BRIEF DESCRIPTION OF THE DRAWING
FIGURE

[0010] FIG. 1 shows the rheology of an adhesive comprising 29.1% acrylic block copolymer, 58.4% tackifier and 12.5% of a diluent.

DETAILED DESCRIPTION OF THE
INVENTION

[0011] All references cited herein are incorporated in their entireties by reference.

[0012] It has now been discovered that hot melt adhesives with high shear, aggressive tack and peel and low viscosity can be obtained using acrylic block copolymers, when low levels of acrylic block copolymers are used in the formulated adhesive. The acrylic block copolymers are formulated with different types and amounts of tackifiers and/or diluents to obtain the properties required for a desired end use.

[0013] The adhesives of the invention exhibit unique features that make them useful for a variety of applications. Features such as high heat resistance at low viscosity, high polarity for good adhesion to polar surfaces, high moisture vapor transmission rate and good weatherability. These features make the adhesives of the invention well suited as hot melt adhesives for medical (breathability) and industrial (heat, plasticizer, polar surface adhesion and weathering resistance) applications, as positioning adhesives (e.g., for sanitary napkins) that are breathable, non-staining, and transfer resistant, as bottle labeling adhesives with very low application temperatures and excellent clarity, and as elastic attachment adhesives with high creep resistance and lycra adhesion.

[0014] Adhesives of the invention may be formulated for application at low temperatures. Alow application temperature hot melt adhesive formulation is one that can be applied at temperatures below about 300° E, more preferably at about 250° F. and down to about 200° F.

[0015] Adhesives having high levels of grab and tack, well beyond those of solution acrylics, may be obtained when low polymer levels are used. By low levels of acrylic block copolymers means amounts of less than about 50% by weight based on the weight of the adhesive composition. The block copolymer component will be present at levels of less than 50% by weight of the adhesive composition, more typically at levels of less than about 40% by weight, most preferably at levels of from about 20 to about 35% by weight.

[0016] Acrylic polymer, as used herein, is intended to include those polymers that contain at least one acrylic or methacrylic acid alkyl ester monomer. Block copolymers that may be used in the practice of the invention will generally be multiblock polymers wherein greater than about 50 weight % of the polymer comprise at least 2 hard blocks. Examples of useful block copolymers include those of the formula -A-B-A- and (-A-B-)n—Xm. Particularly preferred are block copolymers of the formula -[A1]-[B][A2]-. In the above representative formulations, A, Al and A2 each represents a polymer block having a glass transition temperature (Tg) of greater than about 30° C, preferably greater than 80° C, most preferably greater than 110° C, as determined by differential scanning calorimetry (DSC), B represents a polymer block having a Tg of less than about 20° C, preferably less than 0° C, most preferably less than -20° C. as determined by DSC, and X represents a multifunctional coupling agent such as silicon tetrachloride, dibromoethane and tris(nonyl phenyl) phospite.

[0017] The Tg of the acrylic blocks can be determined by differential scanning calorimetry (DSC) conducted at a heating rate of 20.0° C./minute with 5 mg or smaller samples. The Tg is calculated as the midpoint between the onset and endpoint of heat flow change corresponding to the glass transition on the DSC heat capacity heating curve. The use of DSC to determine Tg is well known in the art, and is described by B. Cassel and M. P. DiVito in "Use of DSC To Obtain Accurate Thermodynamic and Kinetic Data", American Laboratory, January 1994, pp 14-19, and by B. Wunderlich in Thermal Analysis, Academic Press, Inc., 1990.

[0018] Suitable Al and A2 polymer blocks include polymers or copolymers derived from acrylic or methacrylic acid alkyl ester monomers such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isobutyl methacrylate, isobornyl acrylate, isobornyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, cyclohexyl methacrylate and combinations thereof.

[0019] Suitable B polymer blocks include polymers or copolymers derived from acrylic or methacrylic acid alkyl ester monomers such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isobutyl acrylate, n-butyl acrylate, n-propyl acrylate, isobutyl acrylate, sec-butyl acrylate, t-butyl acrylate, amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, iso-octyl acrylate, decyl methylacrylate and combinations thereof.

[0020] It is understood that the same acrylic monomers may be included in both the hard and soft blocks, and that one or more other copolymerizable monomers may be used in the preparation of the polymeric blocks. Copolymerizable olefinic monomers include but are not limited to, acrylic acid, methacrylic acid, vinyl esters, vinyl ethers, styrene monomers, and acrylamides and methacrylamides. Other olefinic comonomers may be present in amounts of up to about 25% of each block, preferably less than 10%, provided they do not interrupt the clean phase separation between the hard and soft blocks upon cooling.

[0021] It is particularly preferred to add 1-10% of adhesion promoting comonomers to the soft B block, examples of which would be acrylic acid, N-vinyl-2-pyrrolidone, acrylamide, and hydroxy ethyl acrylate.

[0022] Polymer block B will preferably be present in amounts of at least 50 weight %. In a particularly preferred embodiment, Al and A2 is methyl methacrylate and B is n-butyl acrylate.

[0023] Methods of preparing acrylic block copolymers are known in the art. Block copolymers for use in the practice of the invention may be made by anionic polymerization as described in Japanese Kokai 11-30222617, by free radical polymerization as described by P. Mancinelli, Materiaux et Techniques, March-April 1990, pp. 41-46, by polyfunctional

chain transfer agents such as described by in U.S. Pat. No. 5,679,762, by iniferter polymerization as described in EP 0 349 270 Bl and/or by free radical retrograde precipitation, as described in copending commonly assigned U.S. application Ser. No. 10/045,881. Particularly preferred are acrylic block copolymers prepared by anionic polymerization.

[0024] In addition to the acrylic block copolymers, or blends thereof, the hot melt adhesive compositions of the invention will comprise a compatible tackifier and/or plasticizer. The block copolymer, tackifier and plasticizer will be selected and used in amounts effective to produce the desired properties required for the intended end use.

[0025] Tackifying resins useful in the adhesive compositions of this invention include hydrocarbon resins, synthetic polyterpenes, rosin esters, natural terpenes, and the like. The tackifying agent will generally be present at a level of greater than about 30% by weight of the adhesive composition and preferably at a level of from about 35% by weight to about 80% by weight.

[0026] More particularly, and depending upon the particular base polymer, the useful tackifying resins may include any compatible resins or mixtures thereof such as natural and modified rosins including, for example, as gum rosin, wood rosin, tall oil rosin, distilled rosin, hydrogenated rosin, dimerized rosin, and polymerized rosin; glycerol and pentaerythritol esters of natural and modified rosins, including, for example as the glycerol ester of pale, wood rosin, the glycerol ester of hydrogenated rosin, the glycerol ester of polymerized rosin, the pentaerythritol ester of hydrogenated rosin, and the phenolic-modified pentaerythritol ester of rosin; copolymers and terpolymers of natural terpenes, including, for example, styrene/terpene and alpha methyl styrene/terpene; polyterpene resins having a softening point, as determined by ASTM method E28-58T, of from about 80° C. to 150° C; phenolic modified terpene resins and hydrogenated derivatives thereof including, for example, the resin product resulting from the condensation, in an acidic medium, of a bicyclic terpene and a phenol; aliphatic petroleum hydrocarbon resins having a Ball and Ring softening point of from about 70° C. to 135° C; aromatic petroleum hydrocarbon resins and the hydrogenated derivatives thereof; and alicyclic petroleum hydrocarbon resins and the hydrogenated derivatives thereof. Mixtures of two or more of the above described tackifying resins may be required for some formulations. Also included are the cyclic or acyclic C5 resins and aromatic modified acyclic or cyclic resins.

[0027] Also useful are resins that are substantially aromatic. Examples of such resins can be prepared from any substantially aromatic monomers having a polymerizable unsaturated group. Typical examples of such aromatic monomers include the styrenic monomers, styrene, alphamethyl styrene, vinyl toluene, methoxy styrene, tertiary butyl styrene, chlorostyrene, etc., indene monomers including indene, and methyl indene.

[0028] Various plasticizing agents or diluents may also be present in the composition in amounts of up to about 50% by weight, preferably in amounts of from about 10 to about 45% by weight of the adhesive composition. Suitable diluents will preferably be primarily compatible with the soft (B) block of the acrylic block copolymer. Diluents are liquid or semi-solid materials with a Tg, as determined by DSC,