WO2002018512A2

WO2002018512A2 - Hot melt adhesive useful for producing destructive bonds on difficult to bond substrates

Info

Publication number: WO2002018512A2
Application number: PCT/US2001/027284
Authority: WO
Inventors: Jeffrey S. Lindquist; Carlos Briseno
Original assignee: H.B. Fuller Licensing & Financing, Inc.
Priority date: 2000-08-31
Filing date: 2001-08-30
Publication date: 2002-03-07
Also published as: WO2002018512A3; AU2001287014A1

Abstract

The present invention relates to a hot melt adhesive composition comprising a soft thermoplastic polymer component, a tackifying resin component, and a plasticizer that crystallizes in the adhesive. The soft thermoplastic polymer is preferably a highdiblock block copolymer. The adhesive has a balance of various physical properties, including a very long open time and high heat resistance such that destructive bonds can be formed with difficult to bond substrates.

Description

HOT MELT ADHESIVE USEFUL FOR PRODUCING DESTRUCTIVE BONDS ON DIFFICULT TO BOND SUBSTRATES

Field of the Invention

The present invention relates to a hot melt adhesive composition comprising a soft thermoplastic polymer component, a tackifying resin component, and a plasticizer that crystallizes in the adhesive. The soft thermoplastic polymer is preferably a high diblock block copolymer. The adhesive has a balance of various physical properties, including a very long open time and high heat resistance such that destructive bonds can be formed with difficult to bond substrates.

Background of the Invention

U.S. Patent No. 5,624,986 issued to Bunnelle et al. April 29, 1997 relates to a hot melt adhesive composition comprising a plasticizer comprising acycloaliphatic or aromatic ester of a benzene dicarboxylic acid or a cycloaliphatic polyester of aromatic carboxylic acids, which is solid at room temperature, a tackifier, and in most instances a thermoplastic polymer. After application and cooling, the adhesive composition cold flows for a controllable period of time at ambient temperature prior to subsequently increasing in modulus. Example IN contains 20% Kraton 1117, 59.8% Permalyn 305, 20% Benzoflex 352, and 0.2% Irganox 1010. U.S. Patent No. 5,853,864 issued Dec. 29, 1998 to Bunnelle relates to an article having increase resistance to moisture induced delamination. The increased resistance to moisture and increase in bond strength is produced through a novel adhesive that retains sufficient liquidity to fully penetrate the second layer causing the layers to be bonded through intimate contact or physical entrapment. U.S. Patent No. 5,627,229 issued May 6, 1997 to Bunnelle et al. claims a cigarette carton, a pallet comprising an assembly of cartons, and a disposable article having an absorbent layer covered by an outer wrap secured by an effective amount of an adhesive comprising a cyclohexane dimethanol dibenzoate plasticizer. Summary of the Invention

The present invention is a hot melt adhesive composition comprising a relatively soft thermoplastic polymer, a tackifying resin component, and a plasticizer that crystallizes in the adhesive. The soft thermoplastic polymer component preferably has a Shore A hardness of less than about 20, the more preferably less than about 10. The soft thermoplastic polymer component preferably has a melt index of at least 10 g/10 minutes and a melt index of less than about 80 g/10 minutes. The thermoplastic polymer is preferably a high diblock block copolymer having at least 30% diblock. The block copolymer preferably comprises anunconjugated diene midblock, with isoprene being most preferred.

In one embodiment, a high diblock block copolymer is combined with a tackifying resin component comprising at least two tackifying resins wherein at least one of the tackifying resins is polar and the second resin is a hydrocarbon resin having an aromatic content of at least 5 wt-% or greater.

The adhesive composition has a balance of physical properties that are amenable to forming destructive bonds with a variety of difficult to bond substrates. The adhesive composition preferably has a viscosity greater than about 1000 cps at 350°F and preferably greater than about 3,000 cps at 300°F. The adhesive has an open time ranging from about 30 to 180 seconds. The open time is preferably at least 60 seconds, and more preferably at least 90 seconds for a 1/8" bead or 50 gram/m² spiral spray. The adhesive has good heat resistance, having an upper crossover temperature of at least about 50°C or higher after resoldification of the solid plasticizer.

Detailed Description of the Invention

The adhesive composition employed in the present invention comprises a relatively soft thermoplastic polymer, at least one tackifier, and at least one plasticizing ingredient that recrystallizes or resolidifies. The thermoplastic polymer is sufficiently soft such that the Shore Hardness is less than about 20 and preferably less than about 10. In a preferred embodiment, the thermoplastic polymer is so soft that the Shore A Hardness is too low to measure. Concurrently, the polymer preferably has a tensile strength of at least about 400 psi to insure adequate cohesive bond strength.

The melt index (MI, Condition G) of the thermoplastic component is typically about 10 g/10 min. or greater and more preferably about 20 g/lOmin. Further, to insure the adhesive composition has sufficient cohesive strength, the MI of the thermoplastic polymer component is typically less than 100 g/10 min, preferably less than about 80 g/10min., more preferably less than about 60 g/10min., and most preferably less than about 40 g/lOmin.

The soft thermoplastic polymer is preferably a block copolymer having a substantial concentration of A-B diblock, wherein the A endblock is a non-elastomeric polymer block, typically comprising vinylidene (polystyrene), and the B block is an unsaturated conjugated diene. In the case of A-B-A triblock structures both ends of the block copolymer participate in the formation of styrene domains forming a polymer matrix network. In contrast, diblock block copolymers only have one polystyrene endblock to associate with other styrene blocks and an unassociated midblock "tail". In general, the B block is typically isoprene, butadiene, and mixtures thereof. Unconjugated diene and in particular polyisoprene is the preferred midblock. Commercial embodiments include theKraton® D series block copolymers, available from Shell Chemical Company (Houston, TX), Vector® block copolymers available from Exxon (Dexco) (Houston, TX), as well as others. Physical properties of specific grades of high diblock block copolymers are exemplified in Table I as follows:

Table I

25 wt-% polymer in toluene at 77°F

The block copolymers for use in the adhesive of the invention range in diblock content from about 20% to 100% with respect to the total weight of the block copolymer. In instances wherein the high diblock block copolymer is the sole polymer, the preferred diblock content is at least about 30%, preferably at least about 40%, and more preferably about 50% or higher. Higher diblock content species may also usefully be employed provided the polymer is blended with a second "reinforcing" polymer having a higher modulus (G'). Alternatively, lower diblock block copolymers may also be useful provided the polymer is blended with a lower modulus, "softening" polymer. In the case of polymers blends the kind and amount of each polymer is chosen such that the polymer blend has similar properties to that of desired block copolymer for use as the sole polymer.

The molecular weight of a block copolymer is related to the melt index (MI) as well as the solution viscosity at 77°F (25°C) of a given weight of polymer in solvent. In instances wherein the high diblock block copolymer is the sole polymer, the MI (Condition G) of the block copolymer is preferably about 10 g/10 min. or greater and more preferably about 20 g/10 min. The styrene content of the high diblock block copolymer is typically less than about 30 wt-% with respect to the total weight of the block copolymer and preferably less than about 20 wt-%.

The amount of high diblock block copolymer employed in the adhesive composition is dependent on the physical properties of the polymer selected. For a preferred composition wherein the high diblock block copolymer is the sole polymer in the adhesive composition, the amount of block copolymer ranges from about 15 wt-% to about 50 wt-%, and preferably from about 20 wt-% to about 35 wt-%. In the case of other soft thermoplastic polymers however, the preferred amount may range from 10 wt- % to 80 wt-%.

Although the high diblock block copolymer is preferred, the Applicants surmise that other thermoplastic polymers having a similar Shore A Hardness or storage modulus (G¹) to that of the preferred high diblock block copolymer may also be suitable. Certain species or polymer blends containing amorphous polyalphaolefins; low density homogeneous ethylene/alpha-olefin interpolymers and copolymers andterpolymers ethylene with at least one vinyl esters of a carboxylic acid having a highcomonomer content such as ethylene/vinyl acetate (EVA) ethylene/acrylic acid (EEA) and its ionomers; ethylene/methacrylic acid and itsionomers; ethylene/methyl acrylate (EM A); ethylene/n-butyl acrylate (EnBA) may be suitable.

The adhesive of the invention comprises from about 20 wt-% to about 70 wt-% of at least one tackifying resin. Preferably, the adhesive comprises from about 30 wt-% to about 65 wt-%) and more preferably from 40 wt-% to 60 wt-% tackifier. The kind and amount of tackifying resin(s) is chosen such that the resin component has a sufficient concentration of polarity and/or aromaticity such that the open time of the adhesive ranges from about 30 seconds to 3 minutes.

Several polar tackifying resins are derived from renewable resources such as rosin derivatives including wood rosin, tall oil, gum rosin, rosin esters, terpenes and derivatives of such. Aromatic and mixed aliphatic-aromatic petroleum based tackifiers include alpha-methyl styrene resins, and aromatically modified (styrenated) C5 resins, C9 resins, dicyclopentadiene based resins and hydrogenated versions thereof. Tackifying resins range from being a liquid at about 25°C (room temperature) to having a ring and ball softening point of about 150°C, the majority of midblock tackifiers ranging from about 70°C to about 140°C. Solid tackifying resins with a softening point of about 100°C and less are particularly useful.

In a preferred embodiment, a mixture of tackifying resins are employed in the adhesive of the invention. The first resin is a polar tackifying resin, such as a rosin ester having a polarity of less than 100 as measured by Hercules Modified Di-acetone Alcohol (HMD A) and Diacetone Alcohol resin cloud points (DACP), and more preferably less than about 60. The second resin is an aromatically modified hydrocarbon resin having an aromatic content of at least 5 wt-% of the resin, and preferably an aromatic content ranging from at least about 10 wt-% to about 40 wt-%. The ratio of polar tackifying resin to hydrocarbon tackifying resin typically ranges form 1 :4 to 4: 1 and preferably ranges

The hot melt adhesive of the invention comprises from about 5 wt-% to about 40 wt-% of at least one solid plasticizer that recrystallizes in the adhesive. Preferably, the solid plasticizer is employed at an amount ranging from about 10 wt-% to about 30 wt-% The solid plasticizer has a softening point greater than about 60°C and preferably ranging from about 80°C to about 120°C. The preferred solid plasticizer comprises an aromatic carboxylic acid ester of cycloaliphatic polyfunctional alcohol having 2 to 10 hydroxyl groups. Aromatic carboxylic acids that can be used with the cycloaliphatic polyfunctional alcohols to form this class of ester plasticizer compounds typically have at least one aromatic group and at least one carboxyl functional group. Representative acids include benzoic acid, naphthanoic acid and 4-methyl benzoic acid. Specific examples include 1, 4 - cyclohexane dimethanol, and other cycloaliphatic polyfunctional hydroxyl compounds. A representative example is Benzoflex® 352 available from Nelsicol Chemical Corporation (Rosemount, IL). Optionally, an oil or plasticizer such as paraffinic and napthenic oils, mineral oils and liquid polymers including polybutenes may be employed at low concentrations, ranging up to about 10 wt-%. Most preferably, however, the adhesive composition is free of plasticizing ingredients, particularly oils since elevated concentrations of low molecular weight plasticizers tend to reduce the heat resistance and cause substrate staining when exposed to elevated temperatures. Further, waxes may be usefully employed to reduce viscosity as well as reduce surface tack and improve blocking resistance at concentrations from about 2% to about 10%. Useful waxes include paraffin waxes, microcrystalline waxes, Fischer-Tropsch, polyethylene and by-products of polyethylene. The adhesive for use in the method of the invention may further comprise common hot melt adhesive additives including antioxidants, stabilizers, fillers, etc.

The adhesive may be produced according to known manufacturing techniques. In the case of block copolymer based compositions, the polymer, additives and a sufficient amount of resin is typically added to a sigma-blade mixer to first produce a relatively viscous mixture of such ingredients. Additional resin is then added, slowly at first, to "let-down" the viscous mixture. The lowest viscosity (molecular weight) ingredients such as plasticizers are generally added last. However, it is also common to alternate adding a portion of plasticizer followed by a portion of tackifier.

The molten viscosity of the adhesive composition is generally less than about 15,000 cPs at 350°F (177°C), preferably less than about 10,000 cPs, and more preferably less than about 5,000 cPs. Further the molten viscosity is typically at least about 1000 cPs at 325°F (163°C), and preferably at least about 2,000 cPs. At too low of a viscosity the adhesive is absorbed by porous substrates, whereas at too high of a viscosity the adhesive can be difficult to apply, particularly with a glue gun.

The adhesive may be applied with any of a variety of application techniques including slot coating, spiral spray, melt blown, and bead applicators. The adhesive is applied at a sufficient amount to form a destructive bond. Typically the amount ranges from about 25 to 50 g/m². The adhesive advantageously has a very long open time ranging from about 30 second to 180 seconds. The open time is preferably at least 30 seconds, more preferably at least 60 seconds, and most preferably about 90 seconds or greater. Substrate failure or fiber tearing bonds can be formed with a variety of substrates. Preferably, the bond failure exhibits at least 10% fiber tear, more preferably at least 25% fiber tear, and most preferably about 50% fiber tear or greater. The most preferred compositions are those that produce the desired bond strengths at the longest open times. Once recrystallized, the resulting bond is resistant to heat induced bond failure. The heat resistance is a function of the crossover temperature, the intersection of G' & G" at 10 radians/second and a temperatures greater than 120°F. The crossover temperature is greater than 50°C, preferably greater than about 55°C and more preferably about 60°C or higher.

The preferred adhesive composition cold flows for up to about 24 hours prior to substantially completely recrystallizing or resolidifying. Preferably, the adhesive composition substantially recrystallizes within 8 hours, typically in less than about 4 hours, preferably in less than about 2 hours, more preferably in less than 1 hour and most preferably in about 10 to 30 minutes. The rate of recrystallizing is typically measured with colorimetry. The adhesive is intially transparent. The crystallization of the solid plasticizer cause the adhesive to become opaque and increase in "whiteness". In general, the composition is heated until molten and an 1 lg sample is poured into a circular ring having a diameter of about 4.5 cm that is placed on top of a transparent film. The whiteness is measured as a function of time while cooling at ambient temperature until the color reaches a plateau. "Substantially recrystallizes" refers to reaching about 90% of the color (whiteness) of the plateau. Dynamic Mechanical Analysis techniques may also be employed wherein the modulus (G') as a function of time is measured until reaching a plateau. The rate of recrystallization or resolidification is accompanied by a decrease in tack. Hence, tack testing as a function of time is also indicative of the rate of recrystallization.

The adhesive is useful for bonding a variety of difficult to bond substrates such as various films comprised of polyethylene, ethylene- vinyl-acetate, and block copolymers, various treated fabrics, and particularly nonwovens. In view of the long open time coupled with high heat resistance, the adhesive is also suitable for use for bonding paper and paperboard for various packaging applications, craft and floral applications, insulation bonding of HNAC, for bonding various elastomeric materials, foams and hook as well as for bonding substrates having a low surface energy. Several of these materials are used in the construction of disposable absorbent articles such as disposable diapers, feminine napkins, hospital pads, surgical drapes and gowns, and the like.

The invention is further illustrated by the following non-limiting examples:

Test Methods

1. Viscosity - The molten viscosity of the adhesive is measured with a Brookfield DV-II in accordance with the manufacturer's instructions.

2. Open Time - The adhesive was spiral sprayed at a coat weight of 50 g/m² onto spunbond nonwoven. A second piece of spunbond nonwoven was placed on top at various time intervals. Light hand pressure (about 10 psi) was applied to the laminate. The open time is the maximum amount of time before forming a bond that results in a destructive bond being formed.

3. Heat Resistance - The heat resistance, G7G" was determined using aRheometrics Scientific Dynamic Mechanical Spectrometer Model # RDS7700. The parallel plates used had a 25 mm diameter and a 1.502 mm gap. The instrument was set to a frequency of 10 rads/sec and temperature sweep was performed from 180°C to -50°C.

Aromatic Content of Tackifying Resins by Proton ΝMR The hydrogen atom percentages or proton content of hot melt tackifying resins can be determined on a relative basis by 'H NMR spectroscopy. A normal proton NMR spectrum of the sample resin is acquired, the proton functional groups are identified (namely, aromatic, olefinic, aliphatic, and other), and the peak integration areas of these groups are measured and ratioed to yield relative hydrogen atom percentages.

(See ASTM D-5292)

EQUIPMENT:

1. Varian Gemini 300 BB spectrometer S/N 2836 2. 5 mm broadband probe S/N 538

3. Gemini software version = 6.3B or later

4. 5.0 mm NMR tubes, Norell 508-UP, Wilmad 528-PP or equivalent

5. 5.0 mm spinner turbine

6. CDC13, 99.5 % D atom enriched or better, containing 0.05 v/v % TMS

The tackifying resin is completely dissolved in CDC13 (approximately 0.025g/mL). About 0.5 to 0.75 mL of the solution is pipetted through a Kimwipe filter into a clean NMR tube. A quantitative proton NMR spectrum with region integration of the sample is acquired and plotted. The basic proton functional groups, namely aromatic, olefinic, aliphatic and perhaps acid, phenolic-ether, or ester-alcohol, are identified from the spectrum. Their exact frequency position is somewhat sample dependent, but withTMS set at 0 ppm the following frequency ranges are typical:

Proton Group PPM Range

Aromatic 8.0 - 5.8

Olefinic 5.8 - 4.3

Aliphatic 3.7 - 0.2

Acid 13 - 10*

Phenolic-ether 4.3 - 3.7 Ester-alcohol 4.3 - 3.7

The integration region values for each of the groups present are averaged, tabulated, and summed, and a relative hydrogen atom percentage is calculated for each class of protons. The total relative H atom % should equal 100.0 +/- 0.1 %

Note - This procedure requires the use of the "particularly hazardous" chemical chloroform-d. Refer to the relevant MSDS for specific information and use appropriate safety and health practices in performing this test.

Example 1 wt-% Tradename Description

25 KratonD1112P 16% styrene, 55% diblock SIS linear block copolymer, MI = 20 g/10 min.- Shell Chemical Company (Houston, TX)

19 Benzoflez 352

500 Viscosity Naphthenic Oil

34 Sylvalite RE lOO 100°C softening point rosin ester tackifying resin Arizona Chemical (Panama City, FL)

14 Wingtack 86 87°C softening point, 30% aromatics C5 tackifying resin - Goodyear

0.75 Irganox 1010 hindered phenol antioxidant - Ciba Geigy

Example 2 Example 2 was the same composition as Example 1 with the exception that

Kraton D 1119 was used in place of Kraton D-l 112. Example 3

Example 3 was the same composition as Example 1 with the exception that Kraton D 1113 was used in place of Kraton D-l 112. Example 3 had a Brookfield viscosity of 5250 cps at 300°F, 3500 cps at 325°F and 2375 cps at 350°F.

The examples were sprayed at 325°F at a coating weight of 50 g/m² to various substrates including polyethylene foam, polypropylene nonwoven, polyethylene film and film/foam laminate. The adhesive coated substrate was bonded to a second substrate at various open times ranging form 30 seconds to 90 seconds. After conditioning the bonds for 1-2 days, the bonds were separated by hand to determine the relative bond strength. Example 1 exhibited 20-25% fiber tear for a 30 second open time, and 10% fiber tear for a 60 second open time. Example 2 exhibited 20% fiber tear for a 30 second open time, and 15-20% fiber tear for a 60 second open time. Example 3 exhibited 50% fiber tear for a 30 second open time, and 55-60% fiber tear for a 60 second open time, and 50% fiber tear for a 90 second open time. The bonds were subjected to elevated temperature ranging up to 100°C for 10-20 hours. After reconditioning at ambient temperature, the bonds were reevaluated and found to maintain their initial bond strength and appearance.

Claims

Claims:

1. A hot melt adhesive composition comprising: a) from about 15 wt-% to about 50 wt-% of a thennoplastic polymer component having a Shore A Hardness of less than about 20; b) from about 20 wt-% to about 70 wt-% of a tackifier component; and c) 5 wt-% to about 40 wt-% of at least one solid plasticizer.

2. The hot melt adhesive composition of claim 1 wherein the thermoplastic polymer component has a Shore A Hardness of less than about 10.

3. The hot melt adhesive of claim 1 wherein the thermoplastic polymer component has a melt index of at least about 10 g/10 minutes.

4. The hot melt adhesive of claim 1 wherein the thermoplastic polymer component has a melt index of at least about 20 g/10 minutes.

5. The hot melt adhesive of claim 1 wherein the thermoplastic polymer component has a melt index of less than about 80 g/10 minutes.

6. The hot melt adhesive of claim 1 wherein the thermoplastic polymer component has a melt index of less than about 60 g/10 minutes.

7. The hot melt adhesive composition of any one of claims 1 to 6 wherein the thermoplastic polymer component is a single block copolymer having diblock content of greater than about 30%.

8. The hot melt adhesive composition of any one of claims 1 to 6 wherein the thermoplastic polymer component is a single block copolymer having diblock content of greater than about 40%.

9. The hot melt adhesive composition of any one of claims 1 to 6 wherein the thennoplastic polymer component is a single block copolymer having diblock content of greater than about 50%.

10. The hot melt adhesive of any one of claims 1 to 9 wherein the open time is at least about 30 seconds.

11. The hot melt adhesive of any one of claims 1 to 9 wherein the open time is at least about 60 seconds.

12. The hot melt adhesive of any one of claims 1 to 9 wherein the open time is at least about 90 seconds.

13. The hot melt adhesive of any one of claims 1 to 12 wherein the crossover temperature is at least 50°C.

14. A hot melt adhesive composition comprising: a) from about 15 wt-% to about 50 wt-% of at least one block copolymer having a diblock content greater than about 30 wt-% and a melt index of less than about 60 g/lOminutes; b) from about 20 wt-% to about 70 wt-% of a tackifier component; and c) from 5 wt-% to about 40 wt-% of at least one solid plasticizer, wherein said adhesive composition has a viscosity greater than about 1000 cps at 350°F.

15. The hot melt adhesive composition of claim 14, wherein said adhesive composition has a viscosity greater than about 3,000 cps at 300°F.

16. A hot melt adhesive composition comprising: a) from about 15 wt-% to about 50 wt-% of at least one block copolymer having a diblock content greater than about 30 wt-%; b) from about 20 wt-% to about 70 wt-% of a tackifier component comprising at least one polar tackifying resin and at least one hydrocarbon tackifying resin having at least about 5 wt-% aromaticity; and c) from 5 wt-% to about 40 wt-% of at least one solid plasticizer.

17. The hot melt adhesive of claim 16 wherein the hydrocarbon resin has an aromaticity of at least about 10 wt-%.

18. The hot melt adhesive of claims 16 or 17 wherein the ratio of polar tackifying resin to hydrocarbon tackifying resin ranges from 1 :4 to 4: 1.

19. The hot melt adhesive of claims 16 or 17 wherein the ratio of polar tackifying resin to hydrocarbon tackifying resin ranges from 1 : 1 to 3 : 1.

20. The hot melt adhesive of any one of claims 16 to 19 wherein the polar tackifying resin is a rosin ester.

21. An article having at least one substrate bonded with the adhesive composition of any one of claims 1 to 20.

22. The article of claim 21 wherein the substrate is selected from the group consisting of nonwoven, thermoplastic films, elastomeric films, fabric, paper, paperboard, and fastening materials.