UV-curable coatings, inks, and adhesives have rapidly become a technology of choice for industries that manufacture automobiles, building components, electronics, packaging, and more to improve operational efficiency and sustainability. Compared to water- and solvent-based systems, which can take hours or days to cure fully, UV-curable systems typically cure within a few seconds or minutes. Furthermore, as UV-curable systems do not require solvent carriers, they practically eliminate VOCs, making them more sustainable than solvent-based systems.
UV-Curable Shrinkage Issues
Noting the above, these systems are prone to suffer from performance issues stemming from how quickly photoinitiators achieve UV cure. Initially, monomers and oligomers in a UV-curable system formulation are relatively spread out. Upon curing, however, these components rapidly polymerize and occupy a much smaller volume. This compaction results in a rapid volumetric shrinkage of the coating, ink, or adhesive system. As shrinkage occurs, a mechanical force develops that pulls the polymers away from the substrate, disrupting interfacial contact. Therefore, this shrinkage results in reduced adhesion to substrates. To combat this issue and realize the full benefits of UV-curable systems, BASF offers Lupasol®, an effective ingredient in primers that mitigates shrinkage issues.
BASF Lupasol®
BASF Lupasol® is a line of polyethyleneimines featuring the highest possible amino group density available among commercial polyamines, with a nitrogen-to-carbon ratio of 1:2. These polymers have an extremely high cationic charge density that forms bonds between negatively charged surfaces and negatively charged functional groups present on polymers.
By forming these bonds, Lupasol® creates an extremely powerful primer that strongly secures UV-curable system polymers to substrates and resists adhesion failure during UV curing. The diagram below depicts this mechanism.
A distinct benefit Lupasol® provides is its versatility in the vast substrate types it functions with, which are listed below.
Cellulosic materials (e.g., paper and wood)
Polyolefins (PP, PE, LDPE, and HDPE)
Polyesters
Nylons
Halogenated polymers (PVC and PVDC)
Metals
Lupasol® also works well with acrylate monomers and oligomers, commonly used in UV-curable coatings, inks, and adhesives. The combination of substrates and compatibility with acrylates permits widespread usage across applications such as architectural coatings, industrial coatings, packaging inks, and electronic and automotive adhesives. The following sections provide data on the adhesion and film-formation improvements Lupasol®-based primers make in UV-curable systems.
Plastic and Wood Substrate Test
In the study below, a four-micrometer thick UV-curable coating was applied to five different plastic and wood substrates prepared with and without a primer made using Lupasol® P. The coatings were compared for adhesion levels using the cross-hatch adhesion test and strapping tape, where adhesion was rated from 0 (indicating no removal) to 5 (indicating no adhesion). The results show a significant improvement in adhesion for the PP and PE plastic substrates and a substantial improvement for the wood substrates. The plastic substrate sample test results were particularly notable, as PP and PE are low–surface energy surfaces that are difficult to bond to.
Aluminum Substrate Test
To visually depict the performance improvements Lupasol® provides UV-curable systems, an aluminum panel was prepared with and without applying 1% and 10% Lupasol® P primers. The picture below was taken after a UV-curable coating was applied and cured. Here, Lupasol® provided drastic improvements in mitigating lateral shrinkage of the UV-cured film. Also of note, the relatively low 1% Lupasol® use rate practically achieves the same result as the 10% use rate. The improvement in adhesion and lateral shrinkage mitigation also helped improve the leveling of the coating on the aluminum.
How to Prepare Lupasol® Primers
Lupasol® P primers may be prepared by adding 0.5%–1% of actives in soft water. An alcohol may be added up to 30% by weight to improve drying and wettability. For substrates with particularly low surface energy, up to 0.5% of a nonionic surfactant may be added to help with wettability. A 1% primer solution should be applied at a rate of 1–5 grams/m2, which is equivalent to 10–50 mg/m2 of Lupasol® P. Once applied, the primer should be dried completely using a stream of warm air at or above 50°C. In packaging applications, a Lupasol®-coated primer film and topcoat should be combined under heat and pressure using a heated nip roller between 50°C and 100°C, depending on the material type and thickness.
Summary
The use of UV-curable coatings, inks, and adhesives is increasing in various industries looking to improve sustainability and increase time and energy savings. However, the rapid speed of UV curing can cause issues with adhesion and leveling due to the volumetric shrinkage that occurs during polymerization. Lupasol® P primers solve these issues by significantly increasing the adhesion of acrylate monomers and oligomers to a wide variety of plastic, metallic, and fibrous surfaces used in applications related to aerospace, building and construction, packaging, transportation, and more. Click below to request a sample of Lupasol® P today.
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