Brian Vest, Lichang Zhou – SYENSQO
The waterborne coatings industry is increasingly subject to stringent environmental regulations, driving the need for formulations with low VOC content and free of hazardous substances such as fluorocarbon surfactants (FCS). Replacing fluorosurfactants, however, remains a significant challenge, as they deliver critical performance attributes, particularly early-stage blocking resistance during drying. Because fluorosurfactants belong to the PFAS class, which is categorized as substances of very high concern, there is growing pressure to identify safer and more sustainable alternatives.
Background
Block resistance is the capability of a paint, when applied to two surfaces, to not stick upon contact when pressure is applied under various temperature and humidity conditions. For example, good block resistance helps keep a door from sticking to the jamb or a window from sticking to its frame. When two painted surfaces are pressed together, chain diffusion and entanglement can occur from the mobile polymer chain ends, resulting in poor block resistance.
Several factors can influence the blocking resistance of a formulation, such as the polymer Tg, the formulation space, and the type of surface-active additives being used.
Fluorocarbon surfactants (FCS) are commonly used to improve blocking resistance in waterborne formulations by providing a “protective layer” at the surface, thereby preventing polymer-to-polymer entanglement (Fig. 1). However, due to the environmental challenges and regulations associated with fluorocarbon chemistry, formulators need a new additive solution to improve block resistance. The scope of this project was to identify an additive that could be easily used by the formulator, either as a post-add to an emulsion or added directly into the formulation, that could match the block resistance performance of fluorosurfactant chemistry without the environmental concerns.
Experimental
To study the effect of additive chemistry on block resistance, a series of common wetting agent chemistries were chosen based on structure and functionality. The additives were benchmarked against an anionic fluorosurfactant, which is an industry benchmark for block resistance. The additives chosen for the initial screening are shown in Table 1.
Block resistance testing
Early hot-block (1 day) was measured by ASTM D4946-89. Paint films (~50um dry film thickness) were cured for 24h at ~23° C & 50% RH, then placed face-to-face at 50° C under a 1 kg weight for 30 minutes and rated 0-10 with 10 being the best.
Results and discussion
Screening of different surfactant chemistries
A range of common wetting aid chemistries were screened to determine if existing products could reach the early hot block resistance performance of an FCS wetting aid. The 1-day hot block resistance results from the screening study are shown in Figure 2. The wetting aids tested could not reach the early hot block resistance achieved when using the FCS; however, the commercial phosphate ester did show some improvement in block resistance. These results led to a DOE study focused on developing modified phosphate ester structures that would maximize their surface activity at the air/water interface and improve block resistance.
Novel block dditive variables
Three modified phosphate esters (PE#1–#3) were developed with all showing lower static surface tension than the commercially available phosphate ester. The new phosphate esters were evaluated in the same semi-gloss formulation and tested at a reduced use level of 0.2% actives, while the FCS was kept at 0.04% active. PE#2 matched the FCS early hotblock performance; PE#3 improved block modestly, while PE#1 required a higher dosage. After 10-day heat aging at 60 °C, only PE#2 and FCS retained strong hot-block resistance (Tab. 2).
Based on the results, phosphate ester additive PE#2 was chosen for further testing to compare the overall paint performance versus the non-ionic and FCS control paints. The paint containing PE#2 not only improved the early hot block resistance but also matched the performance for other key properties such as scrub resistance, household stain resistance, color acceptance, water resistance, gloss, and stability.
Understanding additive performance
PE#2 was identified as a new, novel additive which can provide excellent early hot block resistance, matching the performance of the fluorosurfactant for 1d hot block. The anti-blocking additive must also have a balance of compatibility within the system, in which the preferential state is at the air/water interface, or the blocking efficiency could decrease over time due to interactions with other interfaces. This effect was seen after aging the paints and retesting hot block resistance (Tab. 2).
By modifying the structure of PE#2, the long-term robustness in block resistance is not an issue. To better understand the surface coverage, the slope of the CMC curve was used to derive information on how the additives are behaving at the air/water interface.
Using the CMC slope, the Acmc (area per surfactant molecule), which is the minimum area occupied by a single amphiphilic molecule at the air/water interface, was calculated (Tab. 3)[2]). Larger Acmc values indicate greater surface coverage and may support better block resistance. Method validity was checked with SLS (Acmc 43 Ų vs literature ~47 Ų) [3].
The calculated Acmc values for the three new phosphate esters are significantly higher than the commercial phosphate ester (Tab. 3). The Acmc of the novel phosphate esters increases as the hydrophobicity decreases. This could be due to different packing or alignment of the molecules at the air/water interface.
Based on the structure modifications, it is suspected that the hydrophobic chains are inclining or laying horizontally at the interface in samples PE#1 and PE#2. From these findings, it was expected that samples PE#1 and PE#2 would have improved block resistance versus the commercial phosphate ester; however, due to the high CMC of PE#1, the use level during screening of 0.2% active in the formulation was not sufficient to improve block resistance. When the use level is increased to 0.5% active ingredient in the formulation, there is a significant boost in block resistance (Fig. 3). When comparing the Amin with the 1-day hot block results, there appears to be a trend towards larger Acmc values and improved block resistance (Fig. 3). PE#2 provides the best balance between use level and block resistance. PE#1 provides excellent early hot block; however, the use level needs to be increased to 0.5% active ingredient in the formulation due to the high CMC, making this less favorable.
Conclusions
The replacement of fluorosurfactants in water-based coatings is challenging, but a novel phosphate ester structure has demonstrated strong early hot block resistance without compromising other properties. A potential correlation between surfactant area (Acmc) and block resistance was observed, although further studies in formulated systems are needed. Among the developed solutions, a modified phosphate ester (PE#2) matched FCS early hot-block resistance without sacrificing key coating properties and remained effective after heat aging. PE#2 performed across multiple resin platforms and bases and can also contribute wetting, reducing the need for other additives.
References
1. 2017 Annual Book of ASTM Standards, vol. 6.02. ASTM international, 2017, pp. 456–457.
2. S Abbott, Surfactant science: principles & practice. Lancaster, Pennsylvania: Destech Publications, Inc, 2017.
3. A. Czajka, G. Hazell, and J. Eastoe, “Surfactants at the Design Limit,” Langmuir, vol. 31, no. 30, pp. 8205–8217, Apr. 2015, doi: https://doi.org/10.1021/acs.langmuir.5b00336.
