Until recently, one of the largest adhesives markets, pressure sensitive adhesives (PSAs), remained entrenched in the conventional realm of solvent-based, waterborne and hot melt systems. Recent developments of novel liquid rubbers and UV-curable styrenic block copolymers, and further understanding of the effects of resin modifiers, tackifiers and fillers on the UV-curing process, have resulted in UV PSAs breaking through to this large market segment of adhesives.
PSA systems cover a large range of chemistries as outlined as follows:
- Solvent-based solutions of rubber mixtures and acrylic polymers. These systems offer a combination of low-to-moderate cost and the ability to be crosslinked through various mechanisms to create the needed balance of adhesive/cohesive strength combined with thermal and chemical-resistant properties.
- Waterborne acrylic and styrene butadiene rubber (SBR) emulsions and dispersions. These waterborne systems consist of high-molecular-weight dispersions resulting in physical crosslinking upon coalescing or chemically crosslinked networks formed by reactive sites in the resin backbone reacting with suitable crosslinking agents.
- Hot melt rubber and styrene isoprene styrene (SIS) and styrene butadiene styrene (SBS) thermoplastic hot melt materials.
Limitations of Conventional Systems
Solvent and waterborne adhesives share several disadvantages in the increasingly competitive manufacturing area. These disadvantages include: environmental hazards, worker-safety concerns associated with solvents and co-solvents utilized in solvent and waterborne PSAs, high-energy consumption needed to drive off solvents and water, reductions in line speed as coating weights increase, and poor thermal aging for rubber-based materials due to unsaturation in the polymer backbone.Waterborne PSAs show poor moisture resistance caused by high levels of surfactant in the dispersions and emulsions. Hot melt block-copolymer PSAs are thermoplastic materials resulting in poor thermal performance and aging resistance.
UV-curable PSA formulations have been developed to address the deficiencies of conventional solvent-based, waterborne and hot melt systems, resulting in products suitable for freestanding and tape applications. The UV-curing process has also opened up unique possibilities with inline coating and curing of the adhesive directly by the end user.
The Manifest Benefits
Regardless of your direct experience with UV-cure materials, the government may require you to gain expertise in the near future — fast! Southern California, the arbiter of almost all cutting-edge environmental matters, has recently found that ultraviolet/electron beam (UV/ EB) technology is the lowest polluting alternative in many PSA applications. In fact, the state government is so enamored with UV benefits that it has exempted permit applications for industrial processes that include UV materials as a substitute for traditional solvent-based materials. This is no mean saving, given the costs of building or adapting a manufacturing facility.Experience shows that as California goes, so goes national (and soon, via ISO regulations, international) pollution policy. By converting to UV technology, businesses now can avoid what will likely become a major changeover headache in a few years’ time.
At present, the government is opening a brief window, allowing construction and modification of manufacturing lines using UV materials. In the future, regulations will stipulate use of the lowest polluting material; so many companies are making the change today while they still have a choice. Fortunately, there are other immediate benefits from the switchover, as recently cited by RadTech International (Pressure Sensitive Tapes and Labels:
The Clean Air Act Amendments of 1990 and Pollution Prevention Opportunities*). Benefits include:
- Energy savings by eliminating drying ovens associated with traditional curing. UV lamps have lower energy running costs than do drying ovens (greater than 10% savings).
- More-efficient use of manufacturing space. Eliminate drying ovens and you clear expensive square footage. In addition, UV-cure lines are typically shorter than those using traditional oven cures since less noncoated substrate is required for production startup. Lower volume make-ready waste can help trim disposal costs.
- More-efficient manufacturing maintenance. Through the UV-curing mechanism, all of the coating dries, with a negligible, if any, percentage lost in conversion. The plant can eliminate hazardous air pollutants (HAP) and volatile organic compounds (VOC) emissions from the coating steps by converting to a UV-cure material. Cleaning costs are also slashed, since a UV material can sit in a coating reservoir for several days, unlike a solvent-based material. An additional benefit is that UV materials can be compatible with solventborne or waterborne coatings, eliminating the need to convert an entire production process to a UV system.
Current Manufacturing Examples - Coated Abrasives
PSAs are used on coated abrasive products to temporarily bond a large variety of abrasive products to mounting discs (metal) or back-up pads (high-density polyethylene). Conventional rubber-based hot melt adhesives are typically used to mount the discs to the support fixtures. Rubber-based adhesives can be coated onto the abrasive by using a hot melt, slot-dye coater or, in some applications, by transfer lamination of the PSA.The backings of the coated abrasive consist of a variety of materials ranging from casein, acrylic or phenolic size coatings. The poor thermal resistance of hot melts limits use-temperature of the abrasive discs to approximately 100ºC. When the sanding discs are exposed to excessive heat from overuse, the discs typically shear free from the mounting pads. This results in smear or smudging of the part being finished, worker injury caused by flying debris, and lost time caused by replacing the disc and cleaning up residue on the abrasive pads caused by cohesive failure of the PSA.
Advanced UV technology has successfully addressed these problems. UV PSAs can be directly coated onto jumbo rolls of coated abrasive using existing hot melt dispensing and slot-dye coaters followed by inline UV curing. UV-cured material is also supplied in a completely crosslinked, freestanding film on differential slip liner, which can be utilized by transfer lamination directly onto coated abrasive.
Table 1 outlines the performance properties of the material for this converting application. Development is ongoing for direct coating of nonwoven abrasive products.
Roofing-Industry Applications
Solventborne polychloroprene adhesives are commonly used to bond neoprene rubber to ethylene propylene diene rubber (EPDM) foam and metal flashing used for expansion joints on rubber roofs. The increased pressure to move away from solvent-based adhesives resulted in the development of UV alternatives for this unique application.The process parameters for the expansion-joint fabrication required fast dry time, heat and water resistance, and flexibility down to -20ºC. Fielco material is applied directly as a low-temperature-melting PSA to the rubber substrate and cured using a 600-watt/linear inch standard UV system at 60 ft/min.
As the warm, partially cured adhesive exits the UV unit, the EPDM foam is nip-rolled on the tacky adhesive film with subsequent crimping of the metal flashing. The open time is sufficient to allow this inline assembly process, mimicking the high tack level of a solventborne adhesive with residual solvents remaining in the coating.
T-peel adhesion testing showed consistent cohesive tearing of the EPDM foam after both thermal cycling from -20ºC to 100ºC and 48-hour water immersion at 60ºC. In addition, the UV material demonstrated excellent adhesion to copper, tin and galvanized metal used as flashing substrates that typically require a solvent-based primer with the standard chloroprene adhesive.
With this specific UV-curable adhesive, eliminating the need for drying ovens for the solvent-based adhesive decreased the existing footprint of the manufacturing line. All of the existing equipment was utilized, with only a straightforward addition of a standard UV-exposure unit.
Electronics Manufacturing
Double-sided, high-performance tapes are used to bond chemical milling and polishing pads to support fixtures used for integrated circuit planerization and optical-lens polishing. The mounting adhesive must withstand exposure to an acidic polishing slurry (pH 2) and a basic polishing slurry (pH 12) for up to 30 days at a temperature of 54ºC. Adhesive bond-strength stability and zero residues upon removal of the polishing pad were critical for the application.The mounting surfaces for the polishing pads were either stainless steel or marble, depending on the substrate being polished. The adhesion of the PSA to corona-treated Mylar after long-term water exposure was critical. Delamination of the adhesive from the support film results in long downtime while the mounting pad is cleaned. The testing of the current PSA standard in the industry resulted in complete adhesive-layer delamination from supporting Mylar after two weeks in the acidic or basic polishing solution at 54ºC when mounted to stainless steel.
On the other hand, the UV material demonstrated superior performance compared to the standard tape currently used in this application. (See photographs below) Table 2 shows the adhesion performance of both products.
The improved adhesion to corona-treated Mylar may be attributed to the covalent bonding of the PSA to the corona-treated substrate during the high-energy UV exposure. UV material was coated directly onto standard corona-treated Mylar film without secondary-primer application typically used in tape fabrication.
Additionally, hydrolyzable ions were reduced from 35,000 ppm in the current system to less than 500 ppm. This results in lower ionic contamination, which has become an increasing concern with integrated-circuit construction.
Testing is currently under way to investigate direct hot melt PSA application to the nonwoven polishing pads followed by inline UV curing. Improved adhesion to the nonwoven has been observed. This is caused by the hot melt adhesive flowing at approximately 1 mil to 1.5 mil into the porous, nonwoven pad, resulting in a PSA/pad composite-layer post UV-cure.
Crosslink density can be readily adjusted to deliver freestanding PSA films with elongation values in excess of 500% (See photographs at right) and adhesion values ranging from 0.1 to 12 pli (180º peel when bonding Mylar to stainless steel). These properties can be achieved with full cure of all reactive sites. This results in reproducible properties from batch to batch and stable one-pack formulations with long pot-life in processing equipment. Similar adhesion trends have been observed when bonding to untreated hydrophobic materials such as polyethylene and polypropylene with adhesion values ranging from 0.1 to 6 pli. Additionally, as demonstrated in prior information, these bond strengths can be maintained with long-term exposure to water. Ongoing testing is under way to maximize performance to other chemicals.
This performance latitude results in a wide variety of bonding applications, ranging from chemically resistant, temporary-mounting products to high-shear, thermally resistant structural applications.
Combining the:
- Wide range of applications reviewed,
- Environmental friendliness of ultra violet-light-curable PSAs,
- Utilization of existing production equipment and
- Ease of adapting existing processes to UV curing clearly demonstrates both the viability and cost effectiveness of UV PSA technology, and may very well change how PSAs are used in a variety of applications.
For further information, please contact Tony Ring directly at tring@fielco.com, by telephone at 215-674-8700, or through the Fielco Web site at www.fielco.com.
