NOVEL CONCEPTS FOR CLASSIC MATERIALS: RADIATION CURING OF

ISOPRENE BASED PRESSURE SENSITIVE ADHESIVES

Anthony J. Berejka

Ionicorp⁺

Cold Spring Harbor, NY

Dr. Ralph W. Looney

Research Associate

Exxon Chemical Company

Feasibility studies indicate that transparent pressure sensitive adhesives can le developed with a balance of tack and holding power via radiation curing using judicious compounding of polyisoprene oligomers and hydrogenated polycyclic tackifiers with an adroit selection of multifunctional acrylates.

Classic pressure sensitive adhesive formulations Contain natural wood derived tackifiers (rosins and polyterpenes) with a natural rubber base. Synthetics like styrene-butadiene rubber and later styrenated block copolymers have sometimes displaced or augmented the use of natural rubber. Likewise, synthetic tackiflers, often based on Ci and other petroleum feed stocks, have been found to be more effective and often teas costly than natural products.

Beginning in the 1960's, specialized synthetic polymers based on acrylate polymerization began to emerge. Here pressure sensitive properties like tack and hold were achieved through careful monomer selection and polymertitation techniques. Molecular weight, alkyl chain length, co-monomer selection, all had to be carefully balanced in order to achieve desired pressure sensitive properties. Acrylate technology has also led to the more recent use of acrylate terminated oligomers to produce radiation curable pressure sensitives. For example, acrylate terminated low molecular weight polyesters, with reactive terminal vinyl functionality, when cured, have shown acceptable balances of properties.

IRRADIATION CURING

Irradiation curing is a widely accepted industrial technique used extensively in the graphic arts and converting areas. Ultra-violet light or low voltage electron beams are used to cure (crosslink) or "dry" liquid systems, like inks or coatings, which have no hydrocarbon solvent or water as a vehicle. Excellent performance properties, like hardness, gloss or release, can be achieved, depending upon the application. With both ultra-violet irradiators and low voltage electron beams, these properties are attained at relatively low exposure or dose levels. This then implies high production throughout.

The ecological advantages of attaining not Just low volatile emissions, but practically no volatile emissions, make radiation curing an excellent process technique for our current environment. Energy savings, enhanced product quality, new product capabilities and the ability to cure materials on thermally sensitive substrates are proven pluses for irradiation processing.

Ultra-violet irradiators and low voltage electron beam accelerators are available from numerous reputable suppliers. Electron beams offer higher through-put capacity, but require a higher capital investment cost. All this equipment has a proven record of industrial safety. Oft times, irradiation curing equipment can be retrofitted onto existing converting lines. The line space needed for either ultra-violet or electron beam processors is significantly less, by almost orders of magnitude, than the space required for older thermal curing or drying ovens.

In summary radiation curing offers:

• High product out-put.

• No volatile emissions.

• low temperature processing.

• Savings on plant floor space.

• Energy savings.

• Enhanced product quality.

• Reliability and safety.

• New product capabilities.

• Cost effectiveness.

METHODS OF TEST

Pressure sensitive adhesives produced via more traditional or by radiation curing techniques can be characterized by a number of industry standard test procedures. For example, resistance -to-peel is a complex test involving backing modulus or flexibility as well as the rheological properties of the adhesive mass in addition to various interfacial interactions. Some skilled in the art tend to allow destructive test results, like resistance-to-peel to fall where they may in favor of more meaningful bond durability tests and bond formation tests like tack.

Of the variety of tack tests, rolling ball tack (ASTM-3121/PSTC-6) is probably the more discriminating. Statistically designed experiments have shown other tack tests, like probe techniques and loop or quick stick methods, to often be related to the complex resistance-to-peel measurements as shown through high coefficients of correlation. The rolling ball, for its simplicity, is quite able to differentiate between the common understanding of tack and materials which exhibits harsh or raspy pressure Sensitive quality.

With pressure sensitives, bond durability is in most cases related to the adhesives' resistance to creep. Here again, a simple traditional method of suspending a weight from a pressure sensitive coated material, wherein the adhesive is adhered to a stainless steel plate, using a controlled contact area, isa good indication of creep resistance. Published results have yet to correlate sophisticated viscoelastic measurements with creep results obtained via vertical hold tests ~TC-7). More demanding in such hold tests is to measure the time a given weight can be supported rather than just the first indications of slippage.

100% solids mixtures of isoprene oligomers (e.g. Hardman Isolene 400) with tackifylng resins were prepared in a slightly heated kneader mixer, with sufficient heat and shear being used so as to flux the tackifier into the polyisoprene. To attain transparency hydrogenated polycyclic tackifiers were preferred (e.g. Exxon Chemical's Escarez 5380). Subsequent to preparing these blends, different multifunctional acrylate monomers were added at different quantities.

Hand draw downs were made using a Bird applicator to achieve adhesive costing thickness of approximately 50 microns (about 2 miss) Onto 37 micron (1.5 mil) polyester film. These 100% non-volatile pressure sensitive coatings were then subjected to an irradiation source. All of the electron beam irradiation was done using an Energy Sciences Electrocurtain at 250 KeV with the beam current and sample transport speed being adjusted in order to achieve a desired level of dose or beam exposure. Dose is the electron beam energy imparted to the sample and is often expressed in megarads (Mrad) wherein one megarad equals 10 ergs per gram. The under beam atmosphere was adjusted to permit either atmospheric air to be present or to have the air purged and the irradiation to take place in a nitrogen atmosphere.

After being transported in contact with a release film, the test samples were subjected to the rolling ball tack test (ASTM D31-/PSTC-6) and a vertical hold test (modified PSTC-7). In the tack test, the ball roll was recorded in centimeters. For the hold test, a 2.54 by 2.54 centimeter ~ inch by 1 inch) contact area of adhesive to stainless steel plate was used. The time over which either a 500 gram or a 1000 gram weight could be suspended by the adhesive mass was recorded.

Two different molecular weight versions of polyisoprene oligomers derived by the depolymerization of synthetic polyisoprene by Hardman Incorporated (13ellevilie, NJ) were examined:

(to view structures click on link)
POLYISOPRENE

1/2 - POLYBUTADIENE

The tackifier of choice was the lowest softening point grade of a homologous series of hydrogenated or saturated polycyclic hydrocarbons. These water white tackifying resins are produced by the Exxon Chemical Company. The different grades in this series are:

As will be shown in the data below, a blend of two different multifunctional acrylates was used to enhance the radiation cure rate. Initial experiments were conducted using difuncitional acrylates with alkyl mid-sections in order to promote the compatibility of the acrylate monomer with the very non-polar isoprene oligomer and tackifier blends. The high degree of polarity of the commonly used trimethyloipropane triacrylate by itself led to phase separation and cloudiness when incorporated into these transparent and highly non-polar systems.

The high alkyl difunctional monomers examined were: 1/6-hexanediol diacrylate, SR-238 from Sartomer Company, Inc. (Exton, PA) and a difunctional monomer with a long alkyl mid-section of about C₁₅ in length, Sartomer's SR-2000. Of these the SR-2000 exhibited a higher degree of compatibility and no cloudiness in the mixtures studied. Both of these monomers moreover could enhance the compatibility of the more reactive trimethyloipropane triacrylate, Sartomer's SR-351.

(to view structures click on link)
1,6-HEXANEDIOL DIACRYLATE

HIGH ALKYL MID-SECTION DIACRYLATE

TRIMETHYLOPROPANE TRIACRYLATE

Finally, an experiment was conducted to see if the most promising at these isoprene oligomer systems could also be photoinitiated under ultra-violet light. Using Ciba-Geigy's (Oak Brook, IL) liquid photoinitiator, Irgacure 500, a 50/50 mixture by weight of 1-hydroxyceclohexylphenylketone and benzophenone, at 6% by weight, some encouraging results were obtained with the tack and bold properties similar to those noted for electron beam cured systems. The desired transparency and clarity were also maintained.

2:1 blend ratios of the higher molecular weight isoprene oligomer to the Escorez 5380 tackifier were studied using the two alkyl mid-section difunctional acrylates at approximately the Same molar concentrations. In these systems, moderately high electron beam exposure levels of 10 to 15 Mrads were required in order to develop any cohesive strength as measured by creep resistance in the vertical hold test These materials were all irradiated in air.

By shifting to a 1:1 isoprene oligomer to tackifying resin blend ratio and to the more commonly used TMPTA, again at comparable molar concentrations with respect to the unsaturated polyisoprene, more encouraging holding power results were obtained at moderate electron beam dose levels. Unfortunately, because of their incompatibility, these blends had to be remixed just prior to coating in order to redisperse the TMP'TA into the pressure sensitive system. Again all the irradiation was carried out in air.

With these compounds, one would have expected a tighter crosslinked system from the higher isoprene level and thus a higher resistance to creep. Surprisingly, the 1:1 ratio of the isoprene oligomer to tackifer gave both good creep resistance and maintained satisfactory rolling ball tack compared to the 2:1 blend ratio at this same dose.

The 1:1 blends of the polyisoprene oligomer (Isolene 400) and the hydrogenated polycyclic tackifier (Escorez 5380) when used with the multifunctional monomer blend (Sartomer SR-351 and SR-2000) and cured via an electron beam in an inert nitrogen atmosphere with doses or energy input as low as 3 Mrads yielded commercially acceptable transparent pressure sensitive adhesives.

Duplicate experiments showed that consistent results could be achieved with this approach.

MARKET POTENTIAL

Through judicious compounding, pressure sensitive adhesives based on 100% solids, non-volatile components can be irradiation cured to yield transparent adhesives which exhibit reasonable commercially acceptable properties. These properties can be achieved using the efficient electron beam process at commercially acceptable dose exposure. Because of their non-polar make-up, these adhesives could find application where existing irradiation curable pressure sensitives based on polar backbones have experienced some market difficulties. Non-polar materials like these exhibit electrical insulating properties and resistance to polar solvents and water.

These feasibility studies have merely pointed out that when properly compounded, classic materials, like polyisoprene (in this case in a low molecular weight form) can yield pressure sensitive adhesives with reasonable balances of tack and hold. Hopefully, formulators will be able to take advantage of these feasibility studies to generate industrially viable products. Such products can be made using the efficient and cost-effective irradiation curing process. Again, with the proper selection of initiators, these systems should also respond to ultra-violet irradiation curing.

FORMULATING OLIGOMERIC MATERIALS

When formulating oligomeric materials for irradiation curing, some general guidelines ought be kept in mind in order to achieve a desired balance of properties.

In compounded pressure sensitives, the development of some properties, like tack, often require some lower molecular weight species, such as tackifier resins. In contrast, creep resistance can be obtained through the use of higher molecular weight materials and through the build up of crosslinks between high polymer chains. Resistance-to-peel often relies on attaining a balance of relatively high tensile properties and high elongation in the final pressure sensitive in order to both enhance the contact area under the nib and at the same time give the adhesive sufficient integrity to be cohesive. In cured pressure sensitives based on oligomeric materials, reflection on the need to achieve a controlled molecular weight between crosslinks would indicate how such balances of properties can be attained.

Wheras, most materials made for irradiation curing are based on sophisticated synthesized oligomers, the intermediate level properties obtained via these formulated materials have relatively low raw material costs. The exemplar formula based on a 1:1 isoprene oligomer to polycyclic tackifier and 9% of an equal blend of the two select monomers has a relatively moderate raw materials cost To this mixing and compounding costs would also have to be added. Because of the viscosity of the base isoprene oligomer, some modest heating will also likely be needed to attain high speed smooth coatings.

This paper is dedicated to the memory of Dr. Ralph Looney whose untimely passing away in the spring of 1986 left many of the goals and ambitions of this program unfulfilled. Dr. Looney, if he were alive, and Mr. Berejka would like to acknowledge the support and encouragement of Dr. Joseph A. Mountain, Manager of Project Development, Adhesives and Sealants Market Sector, Exxon Chemical Company, Houston, Texas. Dr. Mountain recognized the potential for this program in the pressure sensitive adhesives market.

Dr. Ralph Looney was a Research Associate with Exxon Chemical Company in the polymer and specialty chemical field. Ralph was with Exxon for more than twenty years.

Anthony Berejka is an independent consultant experienced in the pressure Sensitive adhesive area and has done extensive work in the field of irradiation processing and technology. He is President of Ionicorp+, a consulting firm, Post Office Box 79, Cold Spring Harbor, New York 11724.