WO2001096491A1

WO2001096491A1 - Pressure sensitive adhesive having high heat resistance and low complex viscosity

Info

Publication number: WO2001096491A1
Application number: PCT/US2001/018525
Authority: WO
Inventors: Jeffrey S. Lindquist; Eugene R. Simmons
Original assignee: H.B. Fuller Licensing & Financing, Inc.
Priority date: 2000-06-09
Filing date: 2001-06-07
Publication date: 2001-12-20
Also published as: AU2001275384A1

Abstract

The present invention is a hot melt pressure sensitive adhesive composition having high heat resistance and a relatively low complex viscosity, methods of use and articles constructed therefrom. The novel adhesive composition is suitable for a variety of high performance pressure sensitive adhesive applications.

Description

PRESSURE SENSITIVE ADHESIVE HAVING HIGH HEAT RESISTANCE AND LOW COMPLEX VISCOSITY

Field of the Invention

The present invention is a hot melt pressure sensitive adhesive composition having high heat resistance and a relatively low complex viscosity, methods of using the adhesive composition and articles constructed therefrom. The novel adhesive composition is suitable for a variety of high performance pressure sensitive adhesive applications.

Background of the Invention

Extrudable grade pressure sensitive adhesives (PSA's) typically have a viscosity ranging from 50,000 to 2,000,000 at 350°F (177°C). Due to their characteristically high viscosity, such_, adhesives are generally compounded and/or applied with an extruder rather than with various other application equipment designed to make and apply hot * melt adhesives having relatively low viscosity. Extrudable grade PSA's are generally used for high performance PSA applications such as appliances, automotive, laser printing labels, wherein high heat resistance is required. The high heat resistance is generally contributed by employing at least about 30 wt-% polymer, since the other lower molecular weight ingredients commonly employed in hot melt PSA's, such as tackifier and oil, typically diminish the heat resistance. The relatively high concentration of polymer results in the elevated viscosity.

High viscosity adhesives are often more difficult to apply and generally do not mechanically penetrate the substrate surface as well as lower viscosity adhesives. Thus, industry would find advantage in lower viscosity extrudable grade pressure sensitive adhesives having very high heat resistance.

Summary of the Invention

The present invention is a hot melt pressure sensitive adhesive composition having a high heat resistance in combination with a relatively low complex viscosity.

The heat resistance, G'/G", as measured by Dynamic Mechanical Analysis, is greater than about 130°C, preferably greater than about 135°C, more preferably greater than about 140°C, even more preferably greater than about 150°C, and most preferably greater than about 160°C.

The complex viscosity (1/tan ø) of the adhesive composition at 177°C is less than about 400,000, preferably less than about 350,000, more preferably less than about 300,000, even more preferably less than about 250,000, and most preferably less than about 200,000. Further, the Brookfield viscosity at 350°F (177°C) is typically at least about 50,000 cps, preferably at least about 75,000 cps, more preferably at least 100,000 cps.

In one embodiment, the hot melt adhesive composition comprises from about 30 wt-% to about 60 wt-% of at least one, preferably styrene-isoprene-styrene (SIS), block copolymer having a diblock content of less than 30%, a styrene content ranging from about 15 wt-% to about 30 wt-%, and a solution viscosity ranging from about 600 cps to about 2000 cps for a 25 wt-% block copolymer solution with respect to the total weight of the block copolymer. The adhesive composition further comprises from about 30 wt- % to about 60 wt-% of a midblock tackifier, and from about 1 wt-% to about 15 wt-% of an endblock tackifier. The endblock resin is an aromatic tackifying resin or a polyphenylene ether polymer having a softening point greater than about 140°C.

In another embodiment, the hot melt adhesive composition comprises from about 30 wt-% to about 60 wt-% of a block copolymer component comprising a mixture of two or more block copolymers. The block copolymer component has an average diblock content of less than 30% and an average styrene content ranging from about 15 wt-% to about 30 wt-% with respect to the total weight of the mixture of block copolymers, and from about 30 wt-% to about 60 wt-% of a midblock tackfier. Preferably, the block polymer component comprises a first SIS block copolymer that is substantially free of diblock, having a styrene content of about 20 wt-% or greater and a solution viscosity ranging from about 600 cps to about 2000 cps combined with a second SIS block copolymer having a lower styrene content and higher solution viscosity than the first. Optionally, the adhesive composition may comprise up to about 20 wt-% of a liquid polymer or up to abut 10 wt-% of a plasticizing oil. In another embodiment, the present invention is an article comprising a substrate coated or adhesively bonded with the previously described novel hot melt adhesive composition. The article and coated substrate may be subjected to temperatures in excess of 100°C and maintain sufficient bond strength.

Description of the Drawing Figure 1 depicts the heat resistance and complex viscosity of Examples 1 and 2 as well as various other comparative adhesive examples A through H, not having the unique combination of high heat resistance in combination with a low complex viscosity. Figures 2 and 3 depict temperature sweeps of Examples 1 and 2.

Detailed Description of the Invention

The present invention is a hot melt pressure sensitive adhesive composition having a high heat resistance in combination with a relatively low complex viscosity. The adhesive is useful for a variety of high performance adhesive applications in which high heat resistance is required. Advantageously, the adhesive can be subjected to elevated temperatures ranging from 120°F (49°C) to in excess of 200°F (93°C) without adversely affecting the bond internal strength of the adhesive. hi general, the adhesive composition comprises at least one thermoplastic polymer and at least one tackifying resin. Virtually any thermoplastic polymer(s) and tackifier combination(s) at any concentration may be employed, provided the resulting adhesive composition exhibits the desired high heat resistance in combination with a relatively low complex viscosity. However, in most instances the thermoplastic polymer is present in an amount ranging from about 30 wt-% to about 80 wt-% in the adhesive, whereas the tackifying resin is present in an amount ranging from about 30 wt-% to about 70 wt-%. Preferably, the thermoplastic polymer is a single block copolymer or blend of block copolymers having certain characteristics. The amount of block copolymer preferably ranges from about 30 wt-% to about 60 wt-% and more preferably from about 40 wt-% to about 50 wt-%.

A wide variety of block copolymers may be useful in the present invention including A-B-A triblock structures, A-B diblock structures, (A-B)_n radial block copolymer structures, as well as branched and grafted versions of such, wherein the A endblock is a non-elastomeric polymer block, typically comprising polystyrene and/or vinyl, and the B block is an unsaturated conjugated diene or hydrogenated version thereof, hi general, the B block is typically isoprene, butadiene, ethylene/butylene (hydrogenated butadiene), ethylene/propylene (hydrogenated isoprene), and mixtures thereof. In the adhesive composition of the present invention, block copolymers having an unsaturated midblock such as butadiene and isoprene are preferred with isoprene being the most preferred. Saturated block copolymers are less advantageous due to their difficulty in tackifying the midblock and their high cost. Commercial embodiments include the Kraton® D series block copolymers, available from Shell Chemical Company (Houston, TX), Europrene® Sol T block copolymers available from EniChem (Houston, TX), Nector® block copolymers available from Exxon (Dexco) (Houston, TX), as well as others.

Block copolymers range in A block (styrene or vinyl) content from 0 wt-%, as in the case of multi-arm (EP)n⁸ 100% diblock polymers, to about 50 wt-%. Typically, the non-elastomeric A block concentration (styrene content) ranges from about 5 wt-% to about 45 wt-% with respect to the weight of the block copolymer. In the present invention, the block copolymer preferably has a styrene content ranging from about 15 wt-% to about 30 wt-% with respect to the total weight of the block copolymer. At too low of styrene contents, the adhesive may be too low in cohesive strength; whereas at too high of a styrene content, the adhesive tends to be too stiff, lacking sufficient tack and wet out properties.

Block copolymers range in diblock content from 0%, wherein the block copolymer is 100% coupled, to 100% diblock. Preferable to the present invention are block copolymers having relatively low diblock concentrations. High concentration of diblock tends to disadvantageously reduce the cohesive strength of the adhesive composition. Hence, the diblock content of the block copolymer is less than 40%, preferably less than about 30%, more preferably less than about 20%, even more preferably less than about 10% and most preferably the block copolymer is substantially free of diblock.

The molecular weight of a block copolymer is related to its melt index and its solution viscosity at 77°F (25°C) of a given weight of polymer in toluene. The amount of block copolymer employed for determining the solution viscosity depends on the molecular weight. The solution viscosity is reported as a function of neat polymer concentration. For the higher molecular weight block copolymers, the solution viscosity maybe expressed as a function of a 10 wt-% or 15 wt-% block copolymer solution, whereas for more conventional and lower molecular weight block copolymers, a 25 wt-% block copolymer solution is employed. The preferred block copolymers for use in the adhesive composition of the present invention have a moderately high molecular weight. The melt index ranges from about 1 g/10 min. (Condition G) to about 20 g/10 min. and preferably from about 3 g/10 min. to about 10 g/10 min. The solution viscosity (25 wt-% polymer solution) ranges from about 600 cps to about 2000 cps, and preferably from about 800 to 1800 cps. The adhesive composition may comprise a single block copolymer or a blend of block copolymers having these polymer attributes. In the case of block copolymer blends, the polymer component preferably comprises a first SIS block copolymer that is substantially free of diblock, having a styrene content of about 20 wt-% or greater and a solution viscosity ranging from about 600 cps to about 2000 cps combined with a second SIS block copolymer having a lower styrene content and higher solution viscosity than the first. The second block copolymer preferably has a styrene content of about 15 wt-% and a melt index ranging from about 5 g/lOmin to about 20 g/lOmin. The ratio of the first block copolymer to the second block copolymer ranges from about 1.5:1 to about 2.5:1 with 2:1 being the most preferred. As used herein, the term "tackifier" or "tackifying resin" refers to a composition useful for impart tack to the hot melt adhesive composition. ASTM D-1878-61T defines tack as "the property of a material which enables it to form a bond of measurable strength immediately on contact with another surface".

The adhesive composition of the invention comprises from about 20 wt-% to about 70 wt-%, and preferably from about 30 wt-% to about 60 wt-% tackifying resin. At too high of a resin concentration, the adhesive is too stiff exhibiting zippery or slip-stick types peel properties, whereas at too low of a resin concentration, the adhesive lacks sufficient tack properties.

Tackifying resins range from being a liquid at 37°C to having a ring and ball softening point of about 135°C. Solid tackifying resins are present in amounts up to about 65% by weight. However, if liquid tackifϊers are employed, the tackifier concentration may exceed the typically employed concentration. Preferably, a mixture of a midblock tackifying resin and end-block tackifying resin is employed. In the case of SIS block copolymers, the midblock tackifier is a predominantly aliphatic tackifying resin such as branched and unbranched C₅ resins, C₉ resins and dicyclopentadiene based resin, as well as styrenic and hydrogenated modifications of such. Midblock tackifying resins, particularly for SBS also include resins derived from renewable resources such as rosin derivatives including wood rosin, tall oil, gum rosin, rosin esters, natural and synthetic terpenes, and derivatives of such. The Applicant surmises that employing a marginally incompatible resin may contribute to the elevated heat resistance. Accordingly, in some embodiments it is preferred to employ and SIS midblock resin in combination with a SBS block copolymer or vice- versa. The midblock tackifying resin is generally the major tackifying resin, present in amounts ranging from about 30 wt-% to about 60 wt-%.

In contrast to midblock resins, endblock tackifying resins tend to be predominantly aromatic resins (greater than about 40 wt-% of the resin) such as those available from Hercules hie. under the tradename Kristalex® and from Hercules under the tradename Endex®. Endblock reinforcing resins typically have a softening point greater than about 100°C, more preferably the softening point ranges from about 120°C to about 160°C. Polyphenylene ether (PPE) is another useful endblock reinforcing ingredient. Products containing PPE preblended with either a tackifier or a block copolymer are currently available from Arizona and Shell Chemical. The endblock resin tends to be present in the adhesive composition in a minor amount in comparison to the midblock resin, ranging from about 1 wt-% to about 20 wt-% and preferably from about 2 wt-% to about 10 wt-%.

The adhesive may optionally contain small quantities of liquid diluents provided the amount is not sufficient to diminish the heat resistance. The amount of liquid diluents than may be employed is dependent on the molecular weight of the diluents. The adhesive composition typically comprises less than 10 wt-% and preferably less than about 5 wt-%) of a plasticizing oil, having a molecular weight (Mw) of less than about 3000 g mole. However, high viscosity liquid polymers such as various polyalphaolefm (polybutene) polymers may be employed at higher concentration ranging up to about 20 wt-% and preferably at amounts of about 10-15 wt-% or less. As is known in the art, various other components can be added to modify the tack, color, odor, etc., of a hot melt adhesive. It is generally preferred that the other components or ingredients should be relatively inert and have negligible effects upon the properties contributed by the blend of block copolymer(s) and tackifying agent(s). Antioxidants and other stabilizing ingredients are also typically added to protect the adhesive from various heat and light induced degradation.

The hot melt adhesives of the present invention are pressure sensitive, exhibiting a storage modulus in the range of about 20°C to about 60°C of less than about 5 X 10⁶ dynes/cm². The adhesive composition of the present invention is ideally suited for a variety of high performance PSA applications wherein high heat resistance is required such as for various labels for appliances, automobiles and laser printing, for example; and the production of various sanitary disposable medical products and medical packaging that are exposed to steam sterilization. Various substrates such as paper, plastic film, and nonwovens are coated with the adhesive. In many embodiments a permanent or semipermanent bond is formed with a substrate or surface. In other embodiments, wherein higher concentrations of liquid diluents are employed, the adhesive maybe a removable grade.

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

Test Methods

1. Dynamic Temperature Steps

The heat resistance, G'/G" and complex viscosity, 1/tan 0, were determined using a Rheometrics 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.

2. Niscosity - The molten viscosity of the adhesive is measured with a Brookfield DN-II in accordance with the manufacturer's instructions. 3. Shear Adhesion Failure Temperature (SAFT) - The adhesive was cast from a solvent blend as a 1 mil film using a Baker Applicator on 2 mil gauge polyester (Mylar) film. The adhesive film was allowed to dry. Release paper was placed over the top of the film and 4 inch squares were then cut. The adhesive coated squares were matted forming a 1" X 4" bond that was cut into 4 samples, each having a 1" X 1" bond. The bonds were conditioned at ambient temperature for 12 hours and placed in a programmable oven with a 500 g weight in a shear mode. The oven was ramped from 25°C to 175°C at a rate of 25°C per hour, automatically recording the temperature at which the bond fails.

4. 180° Peels to Stainless Steel - were determined according to #PSTC-1.

5. Loop Tack - Adhesive coated film were prepared as described in the SAFT test method. The films were cut into 1" X 5" strips and inserted into a Chemsultants International Loop Tack Tester. The Loop Tack tested automatically records the tack value. ' •^'• ^' •

6. Mettler Softening Point - was determined in accordance with ASTM-D3461.

Example 1 A blend of 45 wt-% Kraton® D-l 111, a 22% styrene, linear styrene-isoprene- styrene block copolymer having a melt index of 3 g/10 minutes (solution viscosity of 1200 for a 25 wt-% solution in toluene at 77°F (25°C)) from Shell Chemical Co. (Houston, TX); 37 wt-% Sylvares TR 7115®, a ll 5°C softening point polyterpene resin from Arizona (Panama City, FL); 7 wt-% Endex® 155, an aromatic hydrocarbon resin having a melting point of 155°C from Hercules (Wilmington, DE); 10 wt-% Isolene 40, a 40,000 Mw polyisoprene elastomer from Elementis Performance Polymers (Belleville, NJ); and 1 wt-% Irganox® 1010, a hindered phenolic antioxidant were blended together using an extruder.

Physical Properties 180° Peel 5 lbs Loop Tack 100 oz SAFT 250°

Example 2

A blend of 12 wt-% Kraton® D-l 111 ; 17 wt-% Kraton® D-4433, an oil-backed linear styrene-isoprene-styrene block copolymer (comprising about 80 wt-% Kraton® D- 1111 and about 20 wt-% oil) having a melt index of 25 g/10 minutes (solution viscosity of 370 cps for a 25 wt-% solution in toluene at 77°F (25°C) from Shell Chemical Co.; 15 wt- % Kraton® D-l 107, a 14% styrene, linear styrene-isoprene-styrene block copolymer having a melt index of 11 g/10 minutes (solution viscosity of 1450 for a 25 wt-% solution in toluene at 77°F (25°C)) from Shell Chemical Co.; 54 wt-% Wingtack® 95, a polyterpene resin having a 95°C melting point from Goodyear Chemical Co. (Akron, OH); 1 wt-% Endex® 155; and 1 wt-% Irganox® 1010 were blended together using an extruder.

^• Physical Properties - '-'P ^• • ^' .^' Mettler Softening Point 320°F P ; -. . ^'

. Niscosity at 375°F 260,000 cps

Niscosity at 400°F 80,000 cps

The heat resistance and complex viscosity of Examples 1 and 2 is depicted in Figure 1. Example 2 was employed to bond film to paper in the preparation of a medical package. The bonded substrates were subsequently steam sterilized at 100°C. The initial bond strength was maintained.

Claims

What is claimed is:

1. A pressure sensitive adhesive composition having a heat resistance of greater than about 130°C and a complex viscosity at 177°C of less than 400,000.

2. The pressure sensitive adhesive composition of Claim 1 wherein said heat resistance is greater than about 135°C.

3. The pressure sensitive adhesive composition of Claim 1 wherein said heat resistance is greater than about 140°C.

4. The pressure sensitive adhesive composition of Claim 1 wherein said heat resistance is greater than about 150°C.

5. The pressure sensitive adhesive composition of Claim 1 wherein said heat resistance is greater than about 160°C.

6. The pressure sensitive adhesive composition of any of Claims 1 -5 wherein said complex viscosity is less than about 300,000.

7. The pressure sensitive adhesive composition of any of Claims 1-5 wherein said complex viscosity is less than about 200,000.

8. A hot melt adhesive composition comprising: a) from about 30 wt-% to about 60 wt-% of at least one block copolymer component comprising one or more block copolymer wherein the block copolymer component has a diblock content of less than 30%, a styrene content ranging from about 15 wt-% to about 30 wt-%, and solution viscosity ranging from about 600 to 2000 cps with respect to the total weight of the block copolymer; b) from about 30 wt-% to about 60 wt-% of a midblock tackifier; and c) from about 1 wt-% to about 15 wt-% of an endblock tackifier.

9. The adhesive composition of Claim 8 further comprising up to 20 wt-% of a liquid polymer.

10. The adhesive composition of Claim 8 or 9 wherein the endblock resin is an aromatic tackifying resin having a softening point greater than about 140°C.

11. The adhesive composition of Claim 8 or 9 wherein the endblock resin is polyphenylene ether polymer having a softening point greater than about 160°C.

12. The adhesive composition of Claim 8 or 9 wherein the block component comprises a SIS block copolymer and the tackifier is compatible with SBS.

13. The adhesive composition of Claim 8 or 9 wherein the block component comprises an SBS block copolymer and the tackifier is compatible with SIS.

14. The adhesive composition of Claim 8 or 9 wherein the polymer component comprises a first SIS block copolymer that is substantially free of diblock, having a styrene content of about 20 wt-% or greater and a solution viscosity ranging from about 600 cps to about 2000 cps and a second SIS block copolymer having a lower styrene content and higher solution viscosity than the first.

15. The adhesive composition of Claim 14 wherein the second block copolymer preferably has a styrene content of about 15 wt-% and a melt index ranging from about 5 g/lOmin to about 20 g/lOmin.

16. The adhesive composition of Claim 14 wherein the ratio of the first block copolymer to the second block copolymer ranges from about 1.5:1 to about 2.5:1.

17. An article comprising a substrate adhesively bonded with the hot melt adhesive of any of the preceeding Claims.