MS POLYMER™ and Lignin Fusion: A Sustainable Revolution for Bio-based Adhesives

TAGS:  Sustainability / Natural Adhesives     Sealants    

This is a sponsored article by Kaneka.

KANEKA MS POLYMER™ is an isocyanate-free, low-VOC Silyl-Terminated Polyether (STPE) base resin. It provides well-balanced properties such as:

• durability,
• reactivity, and
• storage stability

In spite of being so advantageous, the rubbery backbone of MS POLYMER™ limits its use in hard coatings. Thus, to keep up with your quest for sustainable and high-performance coatings, the integration of MS POLYMER™ and lignin comes as a promising avenue.

Blending MS POLYMER™ with bio-based lignin fragments can enhance thermal stability, yet developing the ideal lignin-based coating requires optimizing additional properties in terms of excellent adhesion and chemical resistance.

In this article, you will find a stepwise approach to engineering durable, high-performance lignin coatings by functionalizing lignin oligomers and dimers and combining them with additives, MS POLYMER™, and silanes for a desirable hard bio-based coating.

Synergizing Strength by Combining MS POLYMER™ and Lignin

KANEKA MS POLYMER™ is a class of polymers having a long middle polyether section functionalized by silyl terminal groups. In the presence of moisture and a catalyst, MS POLYMER™ cures at room temperature into a rubbery product by hydrolysis and condensation of the reactive silyl groups. They are used in sealants, adhesives, or coatings with a variety of commercial applications. For example, the building and construction, transportation, do-it-yourself, and flooring industries.

MS POLYMER™-based sealants and adhesives have intrinsically good characteristics, such as:

• adhesion on a wide range of substrates,
• flexibility, and
• ease of handling

Moreover, in a recent study, it was shown that their thermal stability can strongly be improved by blending MS POLYMER™ with bio-based, silylated lignin fragments¹. However, as the MS POLYMER™ backbone is soft and rubbery, it is challenging to use it as a base material for hard coatings.

In the search for an alternative backbone to improve the properties of MS POLYMER™-based hard coatings, the use of bio-based materials is a prerequisite for Kaneka. Because of its hard, aromatic structure, lignin is an interesting base polymer for hard coating applications. Moreover, lignin is the second most abundant biopolymer on earth after cellulose and is available as a by-product of the pulping process of the paper industry^2,3,4,5. It is therefore not a surprise that the utilization of lignin as an alternative feedstock in value-added applications, such as bio-additives is attracting growing attention from both academia and industry.

Figure 1: Lignin: Base Polymer for Hard Coating Applications
In this work, functionalized lignin fragments were used as a base material for partly bio-based, hard coatings. They were combined with MS POLYMER™ and other additives to optimize the performance.

Procedure: Stepwise Optimization of Lignin-based Coatings

STEP 1: A lignin oligomer (Mw = 2100 g/mol) and lignin dimer (Mw = 330 g/mol) were functionalized with dimethoxymethyl silyl groups (Figure 2) to obtain moisture curing materials, similar to MS POLYMER™.

Figure 2: Chemical Structure of a Dimethoxymethyl Silyl Functionalized Lignin Oligomer (L) and Dimer (R)
STEP 2: Because the lignin oligomer is a solid material, it was dissolved in bio-based ethyl acetate. The lignin dimer is a low-viscous liquid and was used without solvent.

STEP 3: Depending on the required properties, additives were added to the lignin solution.

STEP 4: After adding a tin-based curing catalyst, the sample was coated on four different substrates (glass, aluminum, zinc-coated steel, and concrete) with a wet thickness of 250 µm.

STEP 5: The coating was first dried at room temperature for 30 minutes and subsequently placed in an oven at 70 °C for 24 hours to ensure full curing. The final thickness of the coating was around 40 µm. Figure 3 (left) shows the appearance of a lignin-based coating.

STEP 6: The adhesion of the coating to the four different substrates was evaluated by a crosshatch test. Scratches in the form of a check pattern were made in the coating with a knife, after which a piece of duct tape was adhered firmly onto the scratched surface and pulled off quickly. Figure 3 shows an example of good (middle) versus bad (right) adhesion of the coating to the substrate.


Figure 3: Appearance of a Lignin-based coating (L) Result of a Crosshatch Test Showing a Good (C) versus Bad (R) Adhesion of the Coating to the Substrate
STEP 7: To assess if the coating can also be applied as a primer, the adhesion of an MS POLYMER™-based sealant to the coating was evaluated by a hand peel adhesion test, by using three different substrates (glass, aluminum, and concrete). The failure mode was either cohesive failure (CF) of the sealant, or adhesive failure (AF) of the bond between the coating and the sealant. In some cases, the coating was pulled off the substrate during the test.

STEP 8: Evaluation of the acetone resistance was done by wiping the coating, applied on a glass substrate, with a tissue wetted with acetone 50 times. When the resistance was good, the coating remained shiny and no streaks were visible. Acetone resistance is an indication of the degree of cross-linking of the coating.

STEP 9: Finally, the hardness of the coating, applied on a glass substrate, was determined by a pencil hardness test.

Performance of a Pure Lignin-based Coating

Table 1, line 1 shows the performance of a coating prepared with the reactive lignin oligomer (lignin oligomer-R), without any other additives. Although the adhesion of the sealant to the coating was already very good, the adhesion of the coating itself to different substrates was rather weak. Also, the acetone resistance was low, indicating that the lignin was not highly cross-linked.

Influence of different additives on performance

To improve the performance, different additives were evaluated.

Addition of reactive lignin dimer

In line 2 in Table 1, a low molar mass reactive lignin dimer (lignin dimer-R) was blended with the reactive lignin oligomer. The addition of the dimer strongly improved the adhesion of the coating by making it slightly softer. Although the dimer contains reactive silyl groups to react with the lignin oligomer during the curing process, the degree of silylation was not high enough to improve the cross-linking of the coating, as can be derived from the unchanged acetone resistance.

Addition of low molar mass trifunctional MS POLYMER™

In line 3 in Table 1, a low molar mass trifunctional MS POLYMER™ was blended with the reactive lignin oligomer. The presence of three reactive silyl groups indeed strongly improved the acetone resistance. The other properties remained unchanged compared to the native lignin coating in line 1.

Addition of adhesion promoter

Silanes are commonly used in coatings to improve adhesion and hardness. Consequently, a blend of the reactive lignin oligomer and aminopropyl trimethoxysilane, a standard adhesion promoter, was evaluated in line 4 in Table 1. The coating indeed showed an improved adhesion to some of the substrates compared to the native lignin coating in line 1. The higher degree of cross-linking also resulted in improved acetone resistance and hardness.

Combining all the above additives

Because each of the three additives showed its own specific advantages, they were all combined into one coating to obtain an optimal performance for each evaluated property. From line 5 in Table 1, it can be concluded that the combination of all additives resulted in a high-performance coating, both in terms of adhesion and chemical resistance.

Moreover, the bio-based content of the coating goes up to 75% when bio-based ethyl acetate is used as a solvent. With conventional ethyl acetate, the bio-based content is a maximum of 28%. When the solvent is not included in the calculation, up to 52% of the solids are bio-based.

		Adhesion Coating to Substrate				Adhesion MS-based Sealant to Coating			Acetone Resistance	Pencil Hardness
		Glass	Al	Zn-coated steel	Concrete	Glass	Al	Concrete	Glass	Glass
		1 (Good) - 5 (Bad)				CF (Good) - AF (Bad)			1 (Good) - 5 (Bad)	N
1	Lignin oligomer-R (100 wt%)	5	5	3	2	CF	CF	CF	4	2, 5
2	Lignin oligomer-R (70 wt%) + Lignin dimer-R (30 wt%)	1	5	1	Not tested	CF	Coating pulled off	CF	4	2, 0
3	Lignin oligomer-R (90 wt%) + MS POLYMER™ (10 wt%)	4	5	3	Not tested	CF	Coating pulled off	CF	1	2, 5
4	Lignin oligomer-R (97 wt%) + Aminopropyl trimethoxysilane (3 wt%)	4	5	1	2	CF	CF	CF	2	4, 0
5	Lignin oligomer-R (58 wt%) + Lignin dimer-R (29 wt%) + MS POLYMER™ (10 wt%) + Aminopropyl trimethoxysilane (3 wt%)	2	5	2	1	CF	Coating pulled off	CF	1	2, 0
Al - Aluminium; Zn - Zinc

Table 1: Performance of the Lignin-based Coatings

Figure 4: The Performance of Lignin-based Coatings Based on the Influence of Different Additives

KANEKA MS POLYMER™ PRODUCT RANGE

View KANEKA MS POLYMER™ grades suitable for adhesives and sealants available in our database. Analyze the technical data of each product, get technical assistance, or request samples.

Conclusion

This study showed the potential of lignin to be used as a base material for bio-based hard coatings or primers. By functionalizing lignin oligomers and dimers with reactive silyl groups, a moisture-curing coating could be obtained.

The addition of an MS POLYMER™ reactive plasticizer and a low amount of silane resulted in a well-crosslinked, chemically resistant system, showing good adhesion to different substrates. Also, the adhesion of an MS POLYMER™-based sealant to the coating was excellent.

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Acknowledgments

This article is part of the CAMBIUM project, a cooperation between Kaneka Belgium and the Flemish Institute for Technological Research (VITO), funded by Flanders Innovation and Entrepreneurship (VLAIO). VITO’s lignin program is part of the Biorizon shared initiative, an industry-driven partnership on bioaromatics (https://www.biorizon.eu/).


DISCLAIMER: All images and tables used in this article are copyright of Kaneka.