UV Binders For LiB, Part 3: Synthesis of UV-Curable Urethane Acrylate Co-Polymers for Use in LiB

In this edition of Professor’s Corner, we will focus on a one-pot synthesis of UV urethane acrylate co-polymers from simple building blocks and review final physical properties based on Gel Permeation Chromatography (GPC) and tensile strength, as well as their effect in UV binder electrodes on battery-specific capacity rate at 1C, in Part 4. Herein, we report on our first series of copolymers synthesized to better understand the chemical backbone structure effects based on known materials on final battery performance for UV binders.

Investigating the Effect of Co-Polymer Binder Backbone Structures on Rate Performance

To combine the discharge capacity properties of non-PVDF based binders and provide a ‘green solution’ to the use of the hazardous NMP solvent and PFAS type materials, the design of UV binders by co-polymerization offers a method to improve needed properties of adhesion, mechanical strength and elasticity. This typically is done by incorporation of different functional groups into the backbone structure, such as vinyl (C=C), cyano (-CN), carboxyl groups (COOH) and hydroxyl groups (OH), that leads to improvements in interchain polymer interactions with collectors, active and carbon components of the liB. ^1-3

Experimental Section

In this study, block urethane copolymers were prepared using a combination of hard and soft segments where a branched Polyglycol (PolyG) and OH terminated polybutadiene (PBD) were end capped with OH terminated acrylate functionalities, as summarized in Table 1. The relative ratios of these materials were varied to determine the effect of their structure on rate capacity at 1C of the final coin cell prepared. A pre-polymer backbone was formed utilizing the OH terminated starting materials with an aliphatic isocyanate and an isocyanate end capped with an acrylate moiety to control the degree of crosslinking and MW. The concept was to prepare one UV copolymer vs. encompassing multiple acrylate-based materials as described previously. ^4,5

Four copolymers were prepared in a similar fashion where different ratios of OH terminated Polyglycol (Poly G), OH terminated Polybutadiene (PBD), and OH terminated polyester acrylate (AOH) were added to a reaction vessel, along with tin catalyst with a dry air flow (0.2 sfch) and continuous mechanical stirring at 62-65° C. Chemical structures and abbreviations are given in Table 2. A detailed Experimental Section is available online.

Results and Discussion

The end product backbone urethane structure theoretically could be comprised of different chains of random P and B groups end-capped by an acrylate moiety A and copolymers made up of ADBDPDA units and many others (see Table 2). ⁶ The first stage of the reaction, several types of intermediate products are formed that incorporate P and B moieties and mixtures. In the case where the reaction takes place with IA, several types of products can form, including ADBDPIA.

Table 3 summarizes the variables used for each of the four-block co-polymers prepared as represented by equivalents (eq) with increasing amounts of PBD. For example, the first block copolymer prepared was UA0 having a 1:2:4.8 equivalent of PBD:PolyG:OH-A with total OH:OH acrylate eq ratio of 1.6 and NCO/OH=1.

How Does the Backbone Structure Affect Mw, Mn and Pd Data for Synthesized Block Co-Polymers?

Results for UA1 and UA3 are summarized in Table 4 using THF as the mobile phase and using polymethacrylate as the standard for calibration. For this series, reducing the ratio of OH acrylate/total OH from 3.2 to 1.6 for UA1 and UA2 resulted in doubling of the Mn from 1692 to 3717, with a slight decrease in Mw from 9177 to 8576. By doing so, the number of terminated chains of the polymer due to the OH terminated acrylate is cut in half, resulting in the number average Mn doubling as the total number of end capped polymers is cut in half. Polydispersity was found to be equal to 5, indicating a wide range of chain lengths are within a polymer sample for UA1 that decreased in half to 2.3 for the more monodispersed UA2. The UA3 copolymer was prepared in an identical fashion to the UA2 copolymer with a change in the ratio of PBD:Polyol going from 1:1 equivalent to 3:1 equivalent. This change resulted in the Mn going from 3717 for UA2 to 707 for UA3 while the Mw remained relatively unchanged. One explanation for the dramatic decrease in Mn for UA3 is that the average number of molecules is smaller due to the higher amount of PBD resulting in a smaller number of high molecular weight chains.

The GPC chromatographs show 3 distinct peaks for UA2 and UA3 (see Figure 1). For UA2, the three distinct GPC peaks have identical elution times where the center Mw peak at 33 retention is assigned to ADBDA type chains and the shoulder on the LHS at 32 retention assigned to ADPDA type chains. Further support for the PB retention volume assignment comes from the GPC for UA3. For UA3, there is a higher proportion of the center peak vs UA2 that is assigned to the PBD backbone portion for the 3:1 ratio of PBD:Polyol having the structure type of ADPDA. This is somewhat of a simplification of the structure as 10%wt of the D component is comprised of the mono-isocyanate acrylate. The lower shoulder peak on RHS with retention time of 34 is speculated to be comprised of the structure *ADA where the diisocyanate is end-capped with two OH-acrylate moieties.

How Does The Backbone Structure Affect Tensile Properties?

Tensile properties for elongation at break are UA3>UA0 >UA1>UA2, as summarized in Table 5. The average elongation of UA3 is 60.5% for the 3:1 PBD:PolyG ratio indicating a more elastic material than UA1 or UA2. The elongation for UA2 having an OH/OH acrylate ratio of 1.6 was found to be significantly less, 25% less than that of UA1 having twice the number of acrylate groups, 42%, to UV polymerize. The only explanation is that less hydroxyalkyl acrylate in UA2 vs. UA1 results in less flexibility in the backbone structure despite lower double bond crosslinking resulting in a lower elongation. Similarly, the tensile strength varied from 109psi for UA2, 170.5psi for UA3 and highest UA1 at 297psi due to the degree of crosslinking after UV polymerization. UA1 would be expected to have the highest tensile strength based on having a total OH Acrylate/polyol ratio of 3.28 vs 1.62 for UA2 and UA3. The lowest tensile strength of 109psi for UA2 is presumed due to a weaker network structure.

Summary and Conclusions

The author’s company has found that single-pot synthesis of block UV binders for LiB can be designed to influence critical properties, such as tensile properties, that ultimately affect battery performance as determined by high specific capacity (mAh/g) vs. cycles at 1C rate capacity at high loadings.

The four block copolymers prepared were determined to have a range of GPC and tensile properties. Most noted are the properties for 1:1 PBD:PolyG vs. 3:1 PBD:PolyG ratios determined by GPC giving rise to the proposed three different molecular weight distributions representing ADBDA, ADPDA and ADA type backbone structures for soft and hard segments of these materials.

In Part 4, we will review how the backbone chemical structure of these blocked copolymers affects DMA thermogram properties; single vs. multiple Tg domains, Tan Delta, Loss Modulus and Storage Modulus, as well as battery performance (specific capacity mAh/g) when incorporated into UV binder electrode formulations. These UV block copolymers binders, when incorporated into UV binder electrode cathodes and tested in coin cells, were found to display excellent rate performance.

References:

1. Sigel, G. “Professor’s Corner Part I.” UV+EB Technology, v12, No 1, 2025, p36.
2. Sigel, G. “Professor’s Corner Part II.” UV + EB Technology, v11, No 2, 2025, p10.
3. Qin, T. Yang, H. Li, Q, Li, H. Design of functional binders for high-specific-energy lithium-ion batteries: from molecular structure to electrode properties
4. Sigel, G. Bauman, D. Hamann, O, Abbott, P, Hensley, D ,” UV+EB Technology, 9, No. 2, 2^ndQuarter, 2023, pp. 20. “A New Class of UV Curable Binders for Li-Ion Battery Applications.”
5. Sigel, G. RadTech 2024, A New Class of UV Curable Binders for Li-Ion Battery Applications, May 21, 2024.
6. Igor V. Khudyakov, Kenneth W. Swiderski, ‘Structure‐property relations in UV‐curable urethane acrylate oligomers,’ Journal of Applied Polymer Science, January 2006, 99(2) p489.    
7. Definitions of GPC terminology, Research Gate.
8. Christmas, B. K., “Professor’s Corner,” UV+EB Technology, 6, No. 3, 2^ndQuarter, 2020, pp. 12-‘Understanding Glass Transition Temperature: Part
9. Useful discussions with Dr. Byron Christmas and Kristy English in preparation of manuscripts.

Gary A. Sigel, Ph.D.
Senior Principal Scientist, Lab Manager, Miltec UV
gsigel@miltec.com