The fossil fuel industry has historically been powerful, successful and lucrative for various reasons, globally.
Substances created by this process, for energy, are not merely mined or sourced, but their by-products have resulted in other materials that can be useful. For example, petroleum production results in a number of non-fuel derivatives, which include propylene, formaldehyde, sulfur, and benzene. These chemicals can, in turn, be used in chemical or plastic-making. Collectively known as petrochemicals, they also components of adhesives, which are found in popularly branded products such as 'Scotch' tape.
A new report in the journal, ACS Central Sciences, asserts that there could be a sustainable analog of these petrochemicals and that its source is all around us, and always has been.
The adhesives in question are those of the pressure-sensitive (or PSA) variety, which means that they can stick two surfaces together in a robust and persistent manner, in response to being firmly pressed together.
The PSA market is estimated to be worth 13 billion dollars within the next five years.
PSAs are composed of trifold molecules, the most central of which has rubber-like properties. These ‘middle-block’ molecules are often petrochemicals such as polybutadiene. However, their use actually reduces the adhesive properties of the overall triple polymer.
Therefore, petrochemical-based adhesives can often require additional plasticizers or tackifiers (which is the technical term for a molecule that makes a compound stickier) to enhance their desirable properties. However, the same additives can also reduce the functional life of the PSA.
Sustainable By-Products of Petroleum
A team of researchers from the University of Delaware (Newark) has been investigating the possibility that more natural chemicals can make more effective and eco-friendly middle-block polymers.
The source of these polymers could be lignin found in many tree species. Lignin gives arboreal tissue strength and rigidity. However, it is also composed of subunits (or monomers) that can behave much like the middle-block polymers available today.
Furthermore, the conventional tree-products industry results in huge amounts of lignin (which can be found in nearly any kind of wood scrap or waste) as a by-product. Some companies in North America will veritably give this product away rather than pay for it to go into a landfill.
Therefore, in general, lignin is a sustainable and plentiful source of potential building blocks for PSAs.
There is just one main stumbling block: bulk lignin is super-resilient and resistant to breakdown as a raw material.
However, the Delaware team finally developed a process to reduce lignin into its monomers, which is known as de-polymerization. Additionally, this new process was seen to require relatively low temperatures and fewer steps, making it appreciably cost-effective and feasible.
The monomers were then used to synthesize slightly more complex polymers, one of which was poly-(n)-butyl acrylate (PBA). This polymer was chosen as it had been found to have a reduced effective modulus compared to polybutadiene, and also has a higher molecular weight (28 kg/mol compared to 1.7 kg/mol for polybutadiene).
Therefore, PBA can make a better adhesive, without the need for tackifiers or plasticizers.
The researchers sourced their lignin from poplar trees. (Source: Public Domain)
The researchers also found that another of the acrylate polymers made from lignin subunits made an effective alternative to the other two components of the PSA tri-block.
This molecule, poly-4-propylsyringyl acrylate (or P4pSA) was placed on either side of a PBA unit to make a full lignin-sourced triblock known as 4pSA-b-BA-b-4pSA or SaBSa. This basic polymer could then be extended to make poly-SaBSa as a bulk adhesive.
Poly-SaBSa ended up being about 78% PBA and was comparable to a conventional PSA formulation made of PBA in a poly (methyl methacrylate (or PMMA)) sandwich. The poly-SaBSa had a molecular weight of 66.4 kg/mol and a modulus of 1.15, which was not significantly different from the PMMA-PBA-PMMA polymer, which also had a PBA content of about 77%.
The lignin-derived adhesive was also reported as comparable in terms of ‘stickiness’ and ‘tackiness’ to those found on two commercially-available, popular tape products (Scotch ‘Magic’ and Fisherbrand labeling tape, respectively).
The product was associated with a tack force of 2.5 newtons per centimeter (N/cm) and an average 180-degree peeling force of 3.1 N/cm. A strip of poly-SaBSa could also hold a 500-gram weight (while also sticking to stainless steel from above) for up to 810 minutes.
This time beat that of conventional duct tape (about 200 minutes), electrical tape (500 minutes), and Post-it notes (about 30 seconds), but not Scotch tape (over 10,000 minutes). The poly-SaBSa also beat other emerging potential PSAs that are or could be derived from sustainable sources in these metrics.
This figure illustrates the 180-degree peel and tack force tests, and also displays the p-SaBSa’s performance against conventional tapes. MBM= poly (MMA-b-BA-b-MMA) tape. (Source: S. Wang et al., 2018)
Future of New Products
This project may offer vital evidence that at least some petrochemicals can be replaced by biomass derivatives in the future.
In addition, the Delaware researchers have estimated that they can possibly ‘tune’ their potential PSA’s properties by varying the lignin polymers, or even the wood type producing the lignin.
On the other hand, the lignin depolymerization technique requires solvents such as methanol, which are sourced from the petrochemical industry.
All in all, this is an exciting step forward into a world where important industrial and commercial concerns can operate with less reliance on fossil fuels or their by-products.
Top Image: A basic schematic of how trees are turned into adhesive. (Source: S. Wang et al, 2018)
New process turns wood scraps into tape, 2018, University of Delaware News, https://www.udel.edu/udaily/2018/june/process-makes-tape-from-wood/ , (accessed 10 Jun. 18)
Z. Sun, et al. (2018) Bright Side of Lignin Depolymerization: Toward New Platform Chemicals. Chem Rev. 118:(2). pp.614-678.
S. Wang, et al. (2018) From Tree to Tape: Direct Synthesis of Pressure Sensitive Adhesives from Depolymerized Raw Lignocellulosic Biomass. ACS Cent Sci.