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The energy curing industry is currently undergoing a significant transformation due to the reclassification of Trimethylolpropane Triacrylate (TMPTA). This reclassification, led by the International Agency for Research on Cancer (IARC) and the European Chemicals Agency (ECHA), marks a pivotal change in the formulation options for the energy curing sector, a field historically at the forefront of innovation and sustainability
TMPTA: A long-standing pillar in energy curing
TMPTA has been integral to energy curing formulations for decades, with its presence in patents stretching back to the early 1970s. This monomer’s popularity is attributed to its balanced properties, including substantial crosslinking capabilities without causing brittleness, high reactivity which enhances productivity in printing and coating applications, and low viscosity that aids in achieving a desirable balance of flow and performance. Its compatibility with a wide range of acrylated resins further cemented its status as a key component in a diverse array of formulations. The global market for TMPTA has been substantial, estimated at around 110kT, underscoring its widespread use.
The regulatory shift and its wide-ranging impact
The regulatory landscape for TMPTA began to shift in 2018 when the IARC classified it as “Possibly Carcinogenic to humans.” This classification was based on studies of carcinogenicity in mice and rats, primarily through skin application studies. Despite some debate over the validity of these studies, there has been no change in stance from authorities. Following this, the ECHA updated its chemical regulations, classifying TMPTA as a Class 2 carcinogen, along with aquatic toxicity classifications. In the United States, California’s Prop 65 listed TMPTA as a carcinogen, complicating matters further due to the absence of established exposure limits and the unique enforcement mechanism of Prop 65 through civil litigation.
These changes have profound implications for the industry. A classification as carcinogenic often leads to a gradual phase-out of the substance in many applications. For instance, many consumers of energy-cured wood coatings do not approve components with a CARC.2 classification, effectively excluding TMPTA. The new classification also eliminates TMPTA from use in food-related packaging applications.
Finding a direct replacement for TMPTA is challenging, as no single alternative currently available can match all its benefits in terms of properties and cost. Alternatives like TPGDA, DPGDA, and G3POTA, while considered, differ significantly in performance compared to TMPTA, particularly in reactivity and crosslinking. Acrylates based on pentaerythritol are closer in performance but face their own regulatory and acceptance challenges.
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Alkoxylate-acrylates: a promising path for the industry
Alkoxylate-acrylates offer a viable and versatile alternative as reactive diluents in energy-cured formulations. Companies like Perstorp have a long history of offering a wide range of polyol alkoxylates, specifically tailored as precursors for acrylates used in energy curing.
These monomers can be modified with varying degrees and types of alkoxylation, allowing for the tailoring of key properties such as viscosity, reactivity, glass transition temperature and surface tension.
Alkoxylated polyols are liquid at room temperature, simplifying the production process compared to solid forms like TMP or Penta. The viscosity of these alkoxylates increases with the degree of alkoxylation, a result of chain entanglements. Ethoxylated and propoxylated TMP acrylates, for instance, show a viscosity minimum with 2 to 6 alkylene oxide segments, with propoxylated monomers typically having slightly higher viscosities than their ethoxylated counterparts.
Reactivity, crucial for the durability, appearance and performance of coatings and inks, is significantly influenced by the monomers’ functionality and acrylate concentration. Lower alkoxylation degrees generally lead to higher reactivity. Achieving a thorough cure is essential, especially in applications like food packaging, to prevent the migration of uncured components. Similarly, surface cure is vital for the immediate quality of the coating, affecting appearance, touch, and resistance to damage. Alkoxylation positively impacts surface cure by reducing air inhibition, a common issue in energy curing.
The flexibility and hardness of the resulting coating or ink are also impacted by the degree of alkoxylation. Higher alkoxylation typically enhances flexibility but reduces hardness due to the increased distance between crosslinking points. Additionally, the chemical resistance of the coating is influenced by the crosslinking density and the hydrophilicity of the ethylene oxide groups, with a lower degree of alkoxylation generally leading to better chemical resistance.
A detailed look
Among the alkoxylates, Polyol R3540 stands out as a particularly promising candidate. It offers a performance profile similar to TMPTA, with a good balance of reactivity, crosslinking and low viscosity. While it has a lower degree of crosslinking and reactivity compared to TMPTA, it compensates with higher flexibility and compliance with various safety regulations. To address its limitations, formulators can modify end formulations with high-functional resins or other alkoxylates.
The continuing journey
Perstorp’s commitment to innovation is evident in our ongoing efforts to develop next-generation monomers that could further match TMPTA’s properties. Our goal is to find alternatives that not only replicate TMPTA’s benefits but also align with the industry’s evolving safety and environmental standards.
The reclassification of TMPTA is a significant moment for the energy curing sector, prompting a shift towards safer, high-performance alternatives. Perstorp’s development of alkoxylate-based acrylates, particularly Polyol R3540, offers a path forward in this transition. As the industry adapts to these regulatory changes, support is available for those seeking TMPTA-free formulations, ensuring the continued viability and sustainability of energy curing technologies.
