Background of the Invention

The present invention pertains to novel, acrylic- based, fumaric-acid-ester interpolymers useful as -.g pressure-sensitive adhesives in which, surprisingly, peel and tack values of the adhesive can be modified without affecting cohesive strength, as measured as shear.

U.S. Patent 2,544,692 to Kuegler et al discloses 0 copolymerization of an acrylic ester with minor amounts of a fumarate di-ester. Emulsion and bulk polymeriz¬ ation are stated as being feasible.

U.S. Patent 2,642,414 to Bauer et al discloses preparation of copolymers of any ester of methacrylic 5 acid and a saturated aliphatic monohydric alcohol and a maleinoid ester of a saturated aliphatic monohydric alcohol of from 4 to 14 carbon atoms, and certain organic acids. The maleinoid esters include dibutyl fumarate. The polymers prepared were used to modify the pour point of hydrocarbon' ^' oils. 0

5

OMPI ΠΌ

Adhesives for hydrophobic polymers containing an interpolymerized amount of a vinyl acetate, a mono- or di-alkyl ester of an alpha, beta ethylenically-unsatu- rated dicarboxylic acid (e.g., dibutyl fumarate) or mixture thereof, are disclosed in U.S. Patent 3,275,589 to Alexander et al as being prepared by an emulsion polymerization process.

In the pressure-sensitive adhesive art, cohesive strength is measured by shear or shear strength, which is the resistance of adhesive joints to shear stresses, and is measured as the force per unit area sheared at failure. Shear is parallel to the adhesive joint.

Peel or peel strength is measured by drawing an adhesive applied to a substrate at some angle to the substrate, usually 90° or 180°. Tack is the resistance offered by the adhesive film to detachment from an adhered surface. It is a measure of stickiness of an adhesive, or the ability to form an instant bond when brought into low-pressure contact with a substrate to which adhesive is to be adhered.

Up until now, it has been generally accepted in the art that cohesive strength may be sacrificed to increase peel and tack values of an adhesive.

Summary of the Invention

The present invention is directed to novel, pres¬ sure-sensitive adhesive systems prepared by emulsion polymerization that contain an interpolymerized amount of at least one dialkyl ester of fumaric acid, which controls peel and tack, and an interpolymerized amount of at least one hard monomer, which independently controls cohesive strength, or shear, of the pressure- sensitive adhesive. /

The copolymers provided as pressure-sensitive adhesives of the instant invention comprise from about 50% to about 95% by weight, based on the total weight of the monomers, of soft monomers, of which up to about 30% by weight, preferably from about 5% to about 30% by weight, based on the total weight of the mono¬ mers, is one or more diester(s) of fumaric acid ^' in which each ester group independently contains from about 2 to about 8 carbon atoms. The balance of the soft monomer(s) is comprised of an alkyl acrylate con¬ taining from 2 to about 10 carbon atoms. The balance of the monomer system is comprised of hard monomers, at least one of which is alkyl methacrylate containing from about 2 to about 6 carbon atoms present in amount up to about 25% by weight, based on the total weight of the monomers. There may be included as a hard monomer, optionally but desirably, at least one unsaturated carboxylic acid containing from about 2 to about 8 carbon atoms present in an amount of from 0% to about 10% by weight, preferably from about 2% to about 7% by weight, based on the total weight of the monomers.

It is presently preferred that the pressure-sensi¬ tive adhesives of this invention contain, on an inter¬ polymerized basis and based on the total weight of the monomers, from about 5% to about 30% by weight dibutyl fumarate, and from about 55% to about 90% by weight 2-ethylhexylacrylate. Methyl methacrylate is present in an amount of up to about 25% by weight, more prefer¬ ably in an amount of from about 10% to about 25% by weight. The interpolymers preferably contain from about 2% to about 7% by weight acrylic acid.

Using the preferred monomer system, it has been determined that peel and tack are influenced by the concentration of diester of fumaric acid, and increases with fumarate contact. Cohesive strength is indepen- dently controlled by hard-monomer content, and increases as the concentration of this hard monomer increases. The two are mutually-independent controls, which is surprisingly unusual for pressure-sensitive adhesives, since an increase in peel and tack is normally to the sacrifice of cohesive strength. The polymer systems formed in accordance with the instant invention have a glass-transition temperature in the range of from about 30° to about 70°C below the use temperature, preferably from about 50° to about 70 ^β C below the use temperature. Th polymers are formed by emulsion polymerization, preferably by two-stage polymerization, in which all of the fumarate diester, which is a relatively slow reactant, is charged to the reaction vessel where poly¬ merization is initiated. A portion of the monomers may be added prior to initiation of polymerization. The balance of the monomers are added, after initiation of polymerization, over a period of time to ensure that the bulk of the polymerization is copolymerization, as opposed to mixed homopolymerization and copolymeriza- tion.

In the polymer system, the unsaturated carboxylic acid is an effective ingredient for modifying polymer properties, such as cohesive strength, and which can be controlled through crosslinking with a suitable agent, such as chromium acetate.

We have observed that if you replace the fumarate ester(s) with another monomer, the independently-con¬ trolled tack and peel are lost. /

The Drawings

FIG. 1 depicts the effect of dibutyl fumarate on tack, and establishes that dibutyl-fumarate content effectively controls tack. The substrate is vinyl.

FIG. 2 depicts the effect of dibutyl fumarate on peel, and establishes that dibutyl-fumarate content effectively controls peel. The substrate is vinyl.

FIG. 3 illustrates, for the monomer system shown, the influence of methylmethacrylate on cohesive strength (shear), and establishes that its concentra¬ tion can be effectively employed to control the cohesive-strength properties of the polymer. The sub¬ strate is vinyl.

Detailed Description

It has now been surprisingly found that emulsion- polymerized, pressure-sensitive adhesives based on

5 dialkyl esters of fumaric acid, in combination with hard monomers, such as alkyl methacrylate, and at least one soft monomer, such as an alkyl acrylate, provide a pressure-sensitive adhesive in which the hard monomer can be used as a determinant of the cohesive strength i0 of the pressure-sensitive adhesive, with cohesive strength increasing with hard-monomer content, and with mutually-independent control over the peel and tack properties of the adhesive being controlled by the con¬ centration the dialkyl ester of fumaric acid.

,g In particular, pressure-sensitive adhesives of the instant invention are prepared by emulsion polymeriza¬ tion and comprise polymers containing up to about 30% by weight, based on the total weight of the polymer charge of at least one dialkyl ester of fumaric acid,

20 preferably dibutyl fumarate, and up to about 25% by weight of a hard monomer, preferably lower alkyl meth¬ acrylate. It is presently preferred that the hard- monomer content include from 0% to about 10% by weight, more preferably from 2% to about 7% by weight, based on the total weight of the monomers, of an unsaturated

25 carboxylic acid, preferably acrylic acid. The bulk of the monomer system consists of soft monomers, preferably alkyl acrylates, most preferably 2-ethylhexyl acrylate, provided in a concentration of from about 55% to about 94% by weight. In this polymer system, the unsaturated

30 carboxylic acid is provided to enable cohesive-strength modi ication, such as by crosslinking.

35

¹ In the system, cohesive strength is directionally proportional to the concentration of hard monomers, e.g., the lower alkyl methacrylate, and increases with an increase in the concentration of hard monomers. ⁵ Peel strength and tack are independently influenced by the concentration of the di-lower alkyl ester of fumaric acid, and increase with its concentration. Polymers of optimum properties are preferably obtained in a two-step emulsion-polymerization process. ^• *-° A one-step emulsion-polymerization process is also useful, but substantially-lower adhesive values are obtained.

The presently-preferred pressure-sensitive adhe¬ sives prepared in accordance with the instant invention ¹⁵ contain from about 15% to about 30% by weight, based on the total weight of interpolymerized monomers of dibutyl fumarate; from about 55% to about 75% by weight of 2-ethylhexylacrylate; from about 15% to about 25% by weight methylmethacrylate; and from about 2% to about ⁰ 7% by weight acrylic acid.

As used herein, by the term, a "hard monomer", there is meant a monomer which, when homopolymerized, will yield a polymer of a high glass-transition tempera¬ ture, normally a homopolymer having a glass-transition 5 temperature greater than 0°C, preferably greater than about 10°C. Among such monomers there may be mentioned alkyl methacrylates, such as ethylmethacrylates, ethyl- methacrylates, butylmethacrylates, and the like; copoly- merizable vinyl-unsaturated monomers, such as vinyl 0 acetate, vinyl propionate, and the like; and styrenic monomers, such as styrene, methylstyrene, and the like; unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, fumaric /acid, and the like. By the term, "soft monomer", there is a meant a 5 monomer which, when homopolymerized, will yield a polymer of a low glass-transition temperature, i.e..

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l Exemplary are the alkyl acrylates, such as butylacry- late, propylacrylate, 2-ethylhexylacrylate, isooctyl- acrylate, isodecylacrylate, and the like. Presently preferred is 2-ethylhexylacrylate.

5 The polymers prepared in accordance with the present invention, to be functional pressure-sensitive adhesives, must have a net effective glass-transition temperature of from about 30° to about 70°C below the use temperature, preferably from about 50° to about _Q 70°C below the use temperature. Accordingly, the mono¬ mer used in the preparation of polymers of the instant invention are proportioned to provide polymerization products falling within the designated glass-transition temperature. It is for this reason that a soft monomer

•^5 other than a diester of fumaric acid, preferably

2-ethylhexylacrylate, is provided as the bulk of the monomers employed.

Pressure-sensitive-adhesive polymers produced in accordance with the instant invention are preferably

20 prepared by two-stage polymerization. There is initially formed a first emulsified-reaσtion system containing from about 80% to 100% of the total fumarate charge. The balance of the charge of the other of the monomers is formed as a second emulsified-reaction system.

25

Polymerization occurs with a conventional, free- radical catalyst. Thermal and redox systems, and like catalyst systems, may be used. Polymerization may occur at temperatures of up to about 100°C, preferably from ambient to about 50°C. The catalyst concentration

30 will normally range from about 0.1% to about 1%, pre¬ ferably from 0.1% to about 0.5% by weight, based on the total weight of the monomers. Once polymerization is initiated, the balance of the monomers are fed over a period of time ranging from 30 minutes to 30 hours. 35

Polymerization may also occur in one step, in which about 20% of the total charge of mixed monomers is added in proportions in which they appear in the polymer, with the balance added over a period of time. The polymers formed generally have a lower tack, with tack being as much as 50% less than polymers produced according- to the preferred, two-step process.

Examples 1 to 5

There were formed five pressure-sensitive polymers from monomers provided for polymerization in the concen¬ tration shown in Table I. For the polymer of Example 1 to a stirred reaction vessel containing 40 parts-by- weight water, there was charged 0.01 part-by-weight ferric ion, 0.2 part-by-weight ammonium phosphate, 1 part-by-weight ammonium hydroxide, 2.5 parts-by-weight alkylaryl ether disulfonate as a surfactant, and 15 parts-by-weight dibutyl fumarate. There was formed a second emulsified-monomer mix containing 30 parts-by-weight water, 2 parts-by-weight sodium alkylaryl polyether sulfonate as an emulsifier, 62 parts-by-weight 2-ethylhexylacrylate, 18 parts-by- weight methylmethacrylate, 5 parts-by-weight acrylic acid, 0.3 part-by-weight tertiarybutyl hydrogen peroxide, and 0.06 part-by-weight dodecyl mercaptan. A reducing agent of 20 parts-by-weight water and 0.3 part-by-weight sodium formaldehyde sulphoxylate was also provided. While maintaining the reaction charge under a nitrogen atmosphere, the reactor was heated to 40°C. When the reactor temperature reached 40°C, 20% of the emulsified-monomer mix and 10% of the initiator were added. When the temperature reached _/ 45°C, the balance of the emulsified mix was added at the rate of 2 parts per minute. The initiator charge was simul-

taneously introduced at a rate of 0.3 part per minute. Polymerization temperature was maintained in the range of from 45° to 50°C. After the emulsified-monomer mix was completed, the reaction vessel was maintained at 60°C for approximately 1 hour, and the remaining sodium formaldehyde sulfoxylate solution was added at about 0.1 part per minute. After completion of the reaction, reactants were cooled to room temperature.

The reactor charge for Examples 2-5 was 170 parts- by-weight water; ferric-ion, complexed with ethylene- diaminetetraacetic acid, was present in 0.1 part by weight, and ammonium hypophosphate in the amount of 1 part-by-weight ammonia was added to adjust pH to 9. Dowfax ^" * 2AO (40% active), manufactured and sold by Dow Chemical Company, was added in an amount of 30 parts by weight, and total dibutyl fumarate charge added per Table, for a total charge of 500 parts-by-weight total monomer.

A pre-emulsion was formed of 150 parts-by-weight water, 50 parts-by-weight Triton® X-200 (28% active), manufactured and sold by Rohm & Hass Co. , and the balance of the monomers in the proportions shown in Table I, but to supply a total monomer charge of 500 parts by weight less the amount of dibutyl fumarate added to the kettle. There was then added 2 parts-by- weight tertiarybutyl hydrogen peroxide. There was formed as a reducing agent, 2 parts-by-weight sodium formaldehyde sulfoxylate in 150 parts-by-weight water. At a total solids content of 47.6% by weight, the main- tained reaction temperature was about 45°C. The pre- emulsion was added gradually to the reactor over a period of 120 minutes. The reactants were raised to about 60°C for 1 hour. Again, a nitrogen, atmosphere was maintained. The products of reaction were cooled to room temperature after completion of the reaction.

- ^* gJREΛ OMPI

Table II shows the adhesive properties of the uncrosslinked adhesives prepared following the above procedures, using as a substrate G-vinyl, manufactured by Kelex Corporation, Brooklyn, New York, with the adhesive neutralized to a pH of from 6.5 to 7.0.

Table III shows the adhesive values of the poly¬ mers of Examples 2-4 to a Kelex vinyl, with the adhesive neutralized to a pH of 6.5 and 7.0, while Table IV shows the adhesive properties with respect to a Kelex vinyl, with the polymers crosslinked using 0.1% by weight chromium acetate (CrAC3) as the crosslinking agent.

Table V shows the bonding properties of the poly¬ mers of Examples 2-5 to a Kelex vinyl. Crosslinking has a significant affect on cohesive strength.

TABLE I

Example Example Example Example Example 1 2 3 4 5

2-ethyl- hexyl- acrylate 62 55 55 55 55

methyl¬ methacrylate 18 23 18 23 18

acrylic acid 5 2 2 2 2

dibutyl fumarate 15 20 25 20 25 dodecyl mercaptan 0. 14 0. 14 0. 14 0. 04 0.04

TABLE II

Initial Testing on Kelex G-Vinyl, Polymer Uncrosslinked

Example Example Example Example Example

1 2 3 4 5

180° peel, 20' dwell SS,

N/25 mm 17.4 _p 23.0 _S p _S 25 _* *-c * s 19.Op 19.2 _p

looptack, SS, N/25 mm 11.3 _p 16.Op 8.1 _p 14.l _p 11.9 _p

500g RTS

SS, 1/2" x 1/2", hrs. 24 ^{+ 2} - ⁷ c 0.7 _C 7.0 _C ⁴ ' ⁴ c

coating weight, g/m**** 28 28 29 27 27

p = panel failure c = cohesive failure ps = failure with panel staining sps = failure with slight panel staining

XA XA t to Ol Ol o OI o Ol

TABLE I I I

Initial Testing on Vinyl, Adhesive Uncrosslinked

Example Example Example Example 2 3 4 5

180° peel, 20' dwell, SS, N/25 mm 23.5 _S p _S 26.0 _c /p _S 21.0sps 21.4sps

looptack, SS, N/25 mm 16.4. 14.6. 14.4. 14.7.

500g RTS, SS, 1/2" x 1/2", hrs. 2.5. 1.0, 9.9, 4.4,

coating weight,

28 29 26 29

p = panel failure c = cohesive failure 8 * ps = failure with panel staining sps = failure with slight panel staining

o- n o ro tr- Ol

Ol o Ol O

TABLE IV

Initial Testing on Kelex G-Vinyl

Adhesive Crosslinked with 0.1% Chromium Acetate

Example Example Example Example Example

1 2 3 4 5

180° peel, 20' dwell, SS, N/25 mm 18.Op 21«0 _S p _S 20.6 _S p _S 17.2 _p 17.5 _p

looptack, SS, N/25 mm 11.Op 11. _p 14.7 _p 13.Op 12.2 _p

500g, RTS, SS, 1/2" x 1/2", hrs. 24+ 17.0 _C 21.8 _C 24+ 24+

s coating weight, g/m**** 27 29 30 30 32

p = panel failure c = cohesive failure ps = failure with panel staining sps = failure with slight panel staining

. M Ol

Ol o l o l o

TABLE V

Initial Testing on Kelex G-Vinyl Adhesive Crosslinked with 0.1% Chromium Acetate

Example Example Example Example 2 3 4 5

180° peel, 20' dwell SS, N/25 mm 25.0. 18.8sps 19.7. 17.2,

looptack, SS, N/25 mm 16.1. 10.3. 12.8. 12.7, o\ I

500 g RTS, SS, 1/2" x 1/2", hrs. 24+ 8.5, 24+ 24+

coating weight, g/ ² 31 28 30 30

Example 6

Following the second procedure disclosed for Examples 1 to 5, except that the surfactant was Dowfax 2A1 (40% solids), there was formed an interpolymer comprised of 15 parts-by-weight dibutyl fumarate; 60 parts-by-weight 2-ethylhexylacrylate; 20 parts-by- weight vinyl acetate; and 5 parts-by-weight acrylic acid.

Solid content was 50%, and pH was 4.6. Table VI shows the adhesive properties on Kelex G-Vinyl at a coating weight of 27 -29 g/m^ for uncross- linked and crosslinked adhesives (0.15% by weight chrome acetate), while Table VII shows adhesive properties on silver vinyl for crosslinked adhesive at the chrome-triacetate levels shown.

TABLE VI

Property Uncrossl inked Crosslinked

180 ° peel , N/25 mm 20.1, 20.6 _p

looptack, N/25 mm 14.9, 12.3,

RT 500 shear,

TABLE VII

0.1% 0.2% 0.3%

Property CrAc- _? CrAc- ₃ CrAc ₃

90° peel, N/25 mm, 20 min. lip 10.9 _p 10.5 _p

90° peel, N/25 mm, 24 hr. 15.7 _p ^' 15.3 _p 15.3 _p

looptack, N/25 mm 12.7 _p 12.2 _p 13.6 _p

RT 500 shear. hr. 60 _c 75 _DS 75 _DD

Example 7

A series of emulsion polymerizations were run, employing controlled amounts of the monomers dibutyl fumarate 2-ethylhexylacrylate, methylmethacrylate, and acrylic acid. Fifteen different monomer combinations were selected. Dibutyl fumarate ranged from 0% to 20% by weight; -2-ethylhexylacrylate ranged from 51% to 94% by weight; methylmethacrylate ranged from 0% to

23% by weight; and acrylic acid ranged from 2% to 6% by weight. Reactor pH ranged from 5.2 to 5.6, and polymer pH ranged from 2.8 to 3.0. With dibutyl fumarate as the initial charge, monomer addition ranged over a period of from 85 to 163 minutes. Polymerization temperature ranged from 45° to 50°C. Solids content ranged from 55.2% to 59.8%, with reactor agitation being from 100 to 600 RP . Reaction was run under autogenous conditions. Regressive analysis of the data of the properties of the polymers formed showed a high degree of correlation of adhesion to various substrates, namely, mylar and vinyl.

With reference to the tertiary diagrams of Figs. 1 and 2, valid predictions were obtained from polymer per- formance within the region ^' by the lines: methylmetha¬ crylate - 23; dibutyl fumarate - 20; and 2-ethylhexyl- acrylate - 94. As shown in Fig. 1, peel and tack could be controlled, independent of all other factors, by ^• dibutyl fumarate content, with peel increasing as dibutyl fumarate content of the polymer increased. As shown in Fig. 2, shear values increased, were inde¬ pendently influenced by concentration of methylmetha¬ crylate, and increased with its concentration.

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