Adhesion to film substrates has been a formulation hurdle for UV chemists for many years. A particularly difficult situation needs to be overcome due to the need to take into account the surface energy and the shrinkage of the UV coating. Coating shrinkage occurs during polymerization when weak van der Waals forces are replaced by strong short covalent bonds between carbon atoms and different monomer units. This causes the UV coating to shrink and pull away from the substrate, which in turn causes the film to fail. Low surface energy substrates can be difficult to wet properly, which can lead to similar failures. When trying to solve these problems, formulators and converters have used a number of mechanical and chemical solutions, including surface treatments on and between the wires, and the use of adhesion promoters.
Film substrates often have low surface energy values, which creates problems for formulators. To solve this problem, surface treatment is employed. Typical surface treatments include corona and flame treatments. These treatment techniques alter the surface chemistry of the film, allowing more functional groups to increase the adhesion potential. However, these treatments have drawbacks.
Corona discharge begins decay immediately after treatment, reprocessing and overtreatment accelerates the rate of decay, causing surface defects, and the ozone generated by the process needs to be neutralized before it is released into the atmosphere. Flame treating the film is not recommended as it results in higher investment costs, possible shrinkage of the film, and presents escalating safety concerns.
Solvent-based adhesion promoters are also considered a viable solution for bonding. Unfortunately, the use of solvents in UV systems can adversely affect polarity and pH, which can affect the UV light absorption spectrum and lead to improper curing. Shrinkage caused by evaporation of volatile compounds is also a problem.
While both of the above solutions improve adhesion performance, specialty co-adhesive resin technologies can also be applied to help formulators achieve proper adhesion with minimal impact on other system properties, namely viscosity. Evaluation of these 100% active materials was carried out in polyester and epoxy acrylate systems to demonstrate rheological effects and possible improvements in adhesion properties to difficult-to-handle plastic substrates.
plastic substrate
Better known as plastic films, synthetic polymers are petrochemically derived nonporous substrates. Due to their weak attraction, they generally have low surface energy and are difficult to wet. If left untreated, these surfaces cannot accept paints or inks and their use will result in poor adhesion.
Common plastic films are composed of thermoplastic polymers, which are further classified into polyolefins, polyesters, vinyl, and natural films.2 The two main types of polyolefins are polyethylene and polypropylene. These films are thin, flexible and do not have any adhesive properties without treatment. In this study, mainly polypropylene plastic was used.
Polypropylene is mainly used in food packaging due to its good strength properties, high clarity and reasonable barrier properties. Its relatively low cost makes it an attractive substrate. Different polypropylene variants are manufactured to provide the specific properties required by formulators. Oriented polypropylene (OPP) offers excellent durability and moisture resistance, while cast polypropylene is heat-sealable and provides abrasion resistance. These various properties allow packaging to meet the most demanding and special requirements.
Adhesion Definition and Mechanism
Adhesion in its simple form can be defined as the tendency of molecules of one material to bond with molecules of another material. For coatings and printing inks, adhesion can be described as the adhesion between the coating/ink and substrate interface. The bond strength that exists at the interface depends on the degree to which various bonding mechanisms are achieved. These mechanisms can be classified as mechanical or chemical.
Mechanical bonding focuses on the interlocking principle created by the coating flowing into the pores and irregular surface structures of the substrate. Penetration and absorption of the coating into these areas creates adhesion. Plastic substrates are generally less absorbent, so mechanical bonding did not play a significant role in this study.
Chemical adhesion focuses on bonding through chemical reactivity with the surface. This can be achieved through two types of bonding: primary valence and secondary valence bonding. Primary bonds are stronger in nature and can be broadly classified into three types: ionic, covalent, and metallic. These bonds focus on sharing or donating electrons between atoms to form a more stable electronic configuration. For secondary bonds classified as van der Waals forces or hydrogen bonds, there is no sharing or transfer of electrons. Therefore, these bonds are weak in comparison.
Adhesion parameter
Parameters affecting adhesion include substrate wetting, substrate pretreatment, adsorption and film formation. Regarding substrate wetting, the coating/ink system needs to be able to form and maintain intimate contact with the substrate surface. Continuous and uniform contact defines good wettability. For this phenomenon to occur, the surface tension of the liquid coating/ink needs to be lower than that of the substrate. Therefore, substrates with high surface energies wet out more easily than substrates with lower values.
Pretreatment of the substrate helps to ensure a homogeneous surface that creates a sufficient number of reactive sites for chemical or mechanical bonding. For example, corona treatment severes molecular bonds on the substrate surface, which allows attachment to free radicals or other particles in the environment. This results in additional surface polar groups which contribute to chemical attraction between substrate and coating/ink. As a second option, flame treatment improves attraction by creating a fixed level of oxidizing species on the membrane surface. This provides more functional groups for bonding.
Adsorption claims that adhesion develops in the attractive force of contact between the coating/ink film and the substrate. These forces are often divalent and, if classified as hydrogen bonds, are susceptible to co-binders, increasing the number and strength of available contact points.
Almost all desired properties of a coating or ink depend on the quality of the film. Unlike solvent and water-based systems, which rely on evaporation of the carrier system to form a film, UV and 100% solids systems do not benefit from this phenomenon. Therefore, improving its film-forming properties requires substrate modification or formulation enhancement by 100% active materials.
co-adhesive technology
Specialty co-adhesive resin technologies affect many properties in UV systems. Gloss enhancement, improved adhesion, reduced volume shrinkage, and better resistance and protection properties are just some of the key properties influenced by their use.
The bonding potential of silanes depends on their chemical nature (Figure 1). Silanes are activated by hydrolysis and the functional groups have the potential to react with the adhesive itself and the substrate. Chemical and mechanical bonding potentials are possible due to increased functionality and enhanced substrate wetting. Both of these mechanisms make bonding more likely.
Special modified polyester resins are prepared by polycondensation of special purpose carboxylic acids and polyols3 (Figure 2). These resins are resistant to hydrolysis and always contain free carboxyl and hydroxyl groups. These hydrophilic and hydrophobic properties better improve adhesion.
Keto-aldehyde resins are prepared by the aldol condensation of ketones with aldehydes 3 (Figure 3). Alcohol groups are obtained by hydrogenation of ketone groups and, theoretically, this hydroxyl functionality interacts with the polar groups of the treated substrate, which provides adhesion promotion. In turn, there are non-polar groups of untreated substrates that interact with the aromatic functional groups. Taking this into consideration, the groups present provide multiple points of adhesive contact regardless of the surface chemistry of the substrate.
experimental design
In this study, five different plastic substrates were chosen to test the adhesion within the UV polyester system and the UV epoxy acrylate system (Table 1-2). Three 100% active co-binders were selected for testing at loading levels ranging from 3% to 10%.
Co-binder Description:
• Co-binder 1 (for samples A and B):
Aminofunctional Alkoxysilanes
• Co-binder 2 (for sample CE):
Hard ketone aldehyde resin
• Co-binder 3 (for sample FH):
Hard Polyester Resin
Substrate description:
• OPP (Oriented Polypropylene)
• Cast PP (cast polypropylene)
• Corrugated plastic
• Metallized PET (metallized poly
Ethylene terephthalate)
• Unprocessed OPP (Unprocessed Orientation)
polypropylene)
Each sample was prepared by combining all components of the lead formulation into an 8 oz glass jar. The formulation was processed using a Dispermat high speed disperser equipped with 25 mm Cowles blades and a 2 minute mix cycle at 1000 rpm.
Polyester Formulation Results
viscosity
Viscosity was measured on a Haake Rheostress 1 rheometer equipped with a PP25 Ti L03 089 plate and tested at 25°C with a 0.20 mm gap. After one week of storage at room temperature, the viscosity was also tested to confirm the performance of the co-binder.
The initial tack results (Figure 4) show that Sample A has a lower viscosity and Sample B has a higher viscosity compared to the blank. The blank maintained a consistent viscosity over a week, while Sample A experienced a slight increase in viscosity, and Sample B experienced a larger increase over the same period. This suggests that the silane may be overreacting. Furthermore, all samples exhibit near-Newtonian behavior as their viscosities remain relatively constant at different shear rates.
The overall viscosity curve (Figure 5) shows that sample C provides lower viscosity compared to the blank. Sample D had a slightly higher initial tack but more than doubled during storage. For sample E, the viscosity increased significantly in the original and aged samples. All samples show near-Newtonian distributions at different shear rates.
Sample F showed a slight increase in viscosity compared to the blank (Fig. 6). For sample G, the initial and steady viscosity more than doubles. Initial and aged viscosities show that sample H provides significantly higher distribution than the blank. The initial tack curves of all samples were lower than their initial results except the blank.
tape test
Tape Test Adhesion analysis was performed by firmly applying a piece of Scotch® celluloid film tape 610 to the cured coating film and then removing it in one quick stretch. The prints were visually evaluated for the degree of coating removal. The rating scale ranges from 0 to 5, where 5 indicates a uniform film with no delamination and 0 indicates complete delamination. After the 24 hour rest period, a second piece of tape was applied in the same manner and tested. For this study, a passable performance is any performance rated 3 or higher in the initial and stability tests.
在取向聚丙烯上进行的测试显示，在初始测试期间，Blank具有差的粘附性，并且在稳定性测试后完全分层（图7）。样品B，E，G和H显着增加粘附性，在测试期间没有分层。由于粘合性能没有增加，样品C不合适。
在浇铸聚丙烯上进行的测试表明，样品E在整体测试中具有很好的性能（图8）。虽然样品F，G和H显示出正的初始结果，但在稳定性测试期间其性能下降。
在瓦楞塑料上进行的测试（图9）表明，所有样品都具有很好的性能优势，并且优于Blank。可以考虑所有样品用于粘合促进。
在金属化聚对苯二甲酸乙二醇酯上进行的测试（图10）显示样品E和F对粘附产生积极影响。虽然与Blank相比，其他样品显示出更好的结果，但可以认为改善可以忽略不计。对于样品C和D，未发现增加的粘合性能。
对未处理的取向聚丙烯进行的测试（图11）显示，与空白相比，样品B和E显着增加了粘合促进。所有其他样品在稳定性测试期间导致完全分层或不能保持性能。
环氧丙烯酸酯配方结果
粘度
环氧丙烯酸酯样品的粘度参数与前面提到的聚酯样品的粘度参数相同。空白和样品A的初始粘度非常低。样品B在初始测试期间具有低粘度，但在稳定性测试后升高。尽管所有样品的粘度均高于空白，但它们保持了可行的粘度曲线（图12）。
与空白相比，样品C，D和E都具有稍高的粘度（图13）。老化粘度略高于初始粘度，但所有样品都保持可行的粘度曲线。
与空白相比，样品F，G和H具有稍高的粘度（图14）。稳定性结果与初始粘度略有不同。但是，所有样品都保持了可行的粘度曲线。
胶带测试
胶带环氧丙烯酸酯样品的测试参数与前面提到的聚酯样品相同。
在取向聚丙烯上进行的测试（图15）显示，空白配方表明一些初始粘合性能，但在更积极的测试期间没有保持。样品B和G显示出整体粘合性增加。与空白相比，样品C显示出粘合性总体降低。
对流延聚丙烯进行的测试（图16）显示，与空白相比，大多数样品确实增加了初始粘合促进。只有样品B和样品H提供了本研究中认为合格的初始和稳定性粘合性能。与空白相比，样品C显示出粘合性总体降低。
在瓦楞塑料上进行的测试表明，与空白相比，所有样品都提供了整体粘合性能的增加（图17）。具体而言，样品BG实现了很好的性能。
在金属化聚对苯二甲酸乙二醇酯上进行的测试表明，Blank没有任何粘合性能（图18）。尽管样品A和G显示出正的初始粘合性能，但样品C是提供可通过的粘合性能的配方。
对未处理的取向聚丙烯进行的测试（图19）显示大多数样品增加了初始粘合力但不提供可通过的稳定性粘合。样品E没有提供任何额外的粘合性能。样品F是提供可行的初始和稳定性粘附结果的样品。
结论
如所证明的，特种共粘合剂技术可以对能量固化系统的粘合促进产生深远的影响。该技术可以帮助配方设计师处理和未经处理的基材的粘合性能。除粘合外，这些技术还可以辅助其他性能，例如基材润湿性，柔韧性和耐受性。重要的是要注意粘度不应该是一个威慑因素，因为它取决于应用并且可以在配制过程中改变。
聚酯配方
样品A，样品C和样品D的结果显示出良好的流变性能，但仅显示出波纹塑料的粘合性改善。样品B显示出合理的粘度，在稳定性期间增加。然而，该样品显示出OPP，波纹塑料和未处理的OPP的粘合性的改善。在样品E的情况下，在稳定性期间粘度不会保持，但是在所有测试的基材上都看到了可行的粘附结果。使用样品F获得了很好的的流变学特性。它也是仅测试的两种材料中的一种，其改善了与金属化PET相比的粘合性能。样品G还提供了可行的流变性能以及OPP，浇铸PP和波纹塑料中的粘合性的改进。最后，样品H没有提供有利的粘度分布，但确实提供了增强的OPP粘附性，
环氧配方
The overall rheology holds true for all samples even though the stability of sample B increases in viscosity. Sample A, Sample D, and Sample E only showed improved adhesion to the corrugated plastic. The results for Sample B show additional adhesion promotion in OPP, cast PP and corrugated plastics. For sample C, positive results were shown in corrugated plastic and metallized PET. Finally, sample FH shows improvement in corrugated packaging while also providing additional adhesion to untreated OPP, OPP and cast PP, respectively. References 1. Moeck, A. (nd). Shrinkage of UV oligomers and monomers. 1. Retrieved February 17, 2018, from http://radtech.org/proceedings/2014/papers/Formulation/Moeck%20-20Shrinkage%20of%20
UV %20Oligomers %20 and %20Monomers.pdf
2. National Association of Printing Ink Manufacturers. NPIRI Handbook of Printing Inks (7th Edition). National Institute of Printing Inks; 2017 p. 633. Evonik AG, TEGO Chemie Service GmbH. TEGO Journal, 4th edition; 2012.pp 109-117.
This article was presented at the RadTech UV & EB Technology Expo & Conference 2018 in Chicago, IL.
