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EP-4737527-A2 - SINGLE COMPONENT UV CURABLE CONFORMAL COATING WITH MOISTURE SECONDARY CURE FUNCTION

EP4737527A2EP 4737527 A2EP4737527 A2EP 4737527A2EP-4737527-A2

Abstract

An isocyanate functional acrylated polyurethane, the isocyanate functional acrylated polyurethane being the reaction product of reactants comprising the following species: a polyol, preferably an amorphous polyol; a diisocyanate, preferably a branched diisocyanate; and a hydroxyalkyl acrylate.

Inventors

  • KINNER, Phillip James
  • DUFFY, ANDREA

Assignees

  • H. K. Wentworth Limited

Dates

Publication Date
20260506
Application Date
20230404

Claims (15)

  1. An isocyanate functional acrylated polyurethane, the isocyanate functional acrylated polyurethane being the reaction product of reactants comprising the following species: a polyol, preferably an amorphous polyol; a diisocyanate, preferably a branched diisocyanate; and a hydroxyalkyl acrylate.
  2. The isocyanate functional acrylated polyurethane of claim 1, wherein the isocyanate functional acrylated polyurethane is a thermoplastic.
  3. The isocyanate functional acrylated polyurethane of claim 1 or claim 2, wherein the isocyanate functional acrylated polyurethane is an elastomer.
  4. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the isocyanate functional acrylated polyurethane has a ratio of acrylate functional groups to isocyanate functional groups of from 2:1 to 1:2.
  5. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the polyol comprises: one or more of hydroxy terminated polybutadiene, hydroxy terminated hydrogenated polybutadiene, hydroxy terminated polyisoprene, hydroxy terminated polyolefins, hydroxy terminated polyfarnesene, hydroxy terminated hydrogenated polyfarnesene, castor oil, hydrogenated castor oil, and dimer acid based polyester polyols (preferably C36), preferably one or more of hydroxy terminated hydrogenated polybutadiene, hydroxy terminated hydrogenated polyfarnesene, hydrogenated castor oil, and dimer acid based polyester polyols; and/or diols selected from one or more of 2-ethyl-1, 3-hexanediol, 1, 4-butanediol, 3-methyl-1, 5-pentanediol, 1, 6-hexanediol, isosorbide, C 18 dimer diol, 2-butyl-2-ethyl-1, 3-propanediol, 2, 2-dimethyl-1, 3-propanediol and 2, 2-diethyl-1 ,3-propanediol, preferably one or more of 2-ethyl-1, 3-Hexanediol, C 18 dimer diol and 2-butyl-2-ethyl-1, 3-propanediol; and/or triols selected from one or more of trimethylolpropane, triethanolamine, diethanolamine, 1, 2, 6-hexanetriol, phloroglucinol (1, 3, 5-trihydroxybenzene), and glycerol, preferably 1, 2, 6-hexanetriol.
  6. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the polyol has a molar mass of from 500 to 3000 gmol -1 , preferably from 1000 to 2000 gmol -1 , more preferably about 2000 gmol -1 .
  7. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the polyol has a functionality of from 1.9 to 2.5 and/or a hydroxyl value of from 50 to 250 mg KOH/g.
  8. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the diisocyanate comprises one or more of 1, 3-Bis(1-isocyanato-1-methylethyl)benzene (TMXDI, tetramethylxylylene diisocyanate), 5-isocyanato-1-(isocyanatomethyl)-1, 3, 3-trimethylcyclohexane (IPDI, Isophorone diisocyanate), 1, 6-diisocyanato-2, 2, 4 (2, 4, 4)-trimethylhexane (TMDI, Trimethylhexamethylene diisocyanate), 1, 6-diisocyanatohexane (HDI, hexamethylene diisocyanate), 4, 4'-diisocyanatodicyclohexylmethane (H 12 MDI) and 1, 5-diisocyanatopentane (PDI, 1, 5-pentamethylenediisocyanate), preferably 1, 6-diisocyanato-2, 2, 4 (2, 4, 4)-trimethylhexane (TMDI, Trimethylhexamethylene diisocyanate).
  9. The isocyanate functional acrylated polyurethane of any preceding claim, wherein the hydroxyalkyl acrylate comprises one or more of 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, and 4-hydroxybutyl methacrylate, preferably 4-hydroxybutyl acrylate.
  10. A UV and moisture curable conformal coating composition comprising the isocyanate functional acrylated polyurethane of any preceding claim.
  11. The composition of claim 10, further comprising: from 25 to 65 wt.% of the isocyanate functional acrylated polyurethane, from 30 to 65 wt.% acrylic monomer, and optionally one or more of: from 3.5 to 5.5 wt.% photoinitiator, from 0.1 to 1 wt.% defoaming agent, from 0.5 to 1.5 wt.% dehydration agent, from 0.2 to 1.5 wt.% wetting agent, from 0.5 to 2.5 wt.% coupling agent, and from 0.1 to 2.5 wt.% rheology modifier.
  12. The composition of claim 10 or claim 11, comprising: from 25 to 65 wt.% of the isocyanate functional acrylated polyurethane, from 30 to 65 wt.% acrylic monomer, from 3.5 to 5.5 wt.% photoinitiator, from 0.1 to 1 wt.% defoaming agent, from 0.5 to 1.5 wt.% dehydration agent, from 0.2 to 1.5 wt.% wetting agent, and from 0.5 to 2.5 wt.% coupling agent.
  13. The composition of claim 11 or claim 12, wherein coupling agent comprises an acrylate or methacrylate functional silane.
  14. A method for conformal coating of a substrate, the method comprising: providing a substrate, preferably wherein the substrate comprises a circuit board or an electronic component, providing the composition of any of claims 10 to 13, depositing the composition over at least a portion of the substrate, and exposing the coated substrate to actinic radiation.
  15. A substrate coated with the composition of any of claims 10 to 13.

Description

The present invention relates to a conformal coating composition, a method for conformal coating of a substrate using the composition and a substrate coated with the composition. Conformal coating material is a thin polymeric film which conforms to the contours of a printed circuit board to protect the board's components. Typically applied at 25 to 250 µm thickness, it is applied to electronic circuitry to protect against moisture, dust, chemicals, and temperature extremes. Coatings can be applied in a number of ways, including brushing, spraying, dispensing and dip coating. Furthermore, a number of materials can be used as a conformal coating, such as acrylics, silicones, polyurethanes and parylenes. Polyurethane coatings are a popular choice for conformal coatings due to their high chemical resistance, flexibility, and hardness. Coatings curable by actinic radiation (e.g., "UV coatings") find use as conformal coatings as such coatings may be cured relatively quickly by exposure to a radiation source, typically UV, including traditional UV (arc and microwave types) and UV-LED light sources. The fast cure allows manufacturers to increase throughput. However, because circuit boards and other electronic components have highly contoured surfaces, such UV coatings, when used as conformal coatings, suffer from a phenomenon whereby areas of the coating which cannot easily be exposed to UV light due to the contours (i.e., the "shadow" areas) remain uncured. US 4,424,252 describes the problem of "shadow cure" and solves such problem using dual cure resins. Such resins cure by two mechanisms: a) exposure to UV light and b) curing by atmospheric moisture. The atmospheric moisture reacts with free isocyanate groups in the dual cure resin forming an amine group(s). The amine groups react with other isocyanate groups to form a polyurea. The curing by atmospheric moisture allows the areas of the coatings in the "shadows" to fully cure. However, dual-cure polyurethane resins have suffered a disadvantage due to their relatively high viscosity. This issue is especially relevant for conformal coatings, due to the contoured shape of the substrate. Low viscosity resins are more favoured from a flowability standpoint, i.e., the low viscosity resins more easily flow around the contours of a circuit board and may more evenly coat such circuit boards. Previous attempts to lower the viscosity of dual-cure polyurethane resins included adding a significant amount of reactive diluent to the dual-cure polyurethane resins. While the addition of reactive diluents served to lower the viscosity of the resin, it also reduced the wt. % of free isocyanate groups in the resin. This, in turn, has the effect of reducing the effectiveness of the moisture cure mechanism especially in shadow areas, which results in a coating with insufficient hardness and/or solvent resistance in such areas. US 9,932,492 describes one-component, dual-cure conformal coating compositions containing an isocyanate-functional urethane acrylate and a polyisocyanate containing allophanate and/or uretdione groups. The compositions are described as providing coatings with high hardness and solvent resistance. Conformal coating compositions are also disclosed in US2014/199491A1, CN109321125A, CN111548726A and CN111471337A. Thus far, conventional conformal coating compositions exhibit an unfavourable change in their properties after aging at elevated temperature. This may result in the conformal coating providing inadequate protection to a circuit board or electronic component when used in a high temperature environment, due to stress cracking. The present invention seeks to tackle at least some of the problems associated with the prior art, or at least to provide a commercially acceptable alternative solution thereto. Further, this invention discloses the construction/fabrication of new assemblies, devices and components using the disclosed materials and processes. In a first aspect, the present invention provides a UV and moisture curable conformal coating composition comprising: from 25 to 65 wt.% isocyanate functional acrylated polyurethane,from 30 to 65 wt.% acrylic monomer,from 3.5 to 5.5 wt.% photoinitiator,from 0.1 to 1 wt.% defoaming agent,from 0.5 to 1.5 wt.% dehydration agent,from 0.2 to 1.5 wt.% wetting agent, andfrom 0.5 to 2.5 wt.% coupling agent. Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. The inventors have surprisingly found that in comparison to conventional conformal coating compositions the composition of the present invention exhibit improved retention of properties during thermal ageing testing at 130 °C. Advantageously, this may result in improved thermal shock resistance compared with existing techn