Technical Brief: Adhesion with Biopolymers

Sheldon E. Broedel, Jr., Ph.D.
Athena Environmental Sciences, Inc. Baltimore, MD
October 2007

Figure 1. Many organisms in nature produce strong adhesives, such as those that hold this algae to river rocks.

Introduction
Standard glues and adhesives, terms that have now become synonymous for compounds used to bond and coat substrates, are traditionally derived from animal gelatin or synthetic polymers. A new source, biopolymers, has gained increasing interest as an economical and environmentally friendly alternative. In nature, several organisms have the ability to produce biopolymers with strong adhesive properties. The sticking power of mussels and barnacles to a wide variety of surfaces is perhaps the most recognizable. Despite these high bond strengths observed in nature, commercially produced biopolymers are unable to replicate the complexity of the natural biopolymers and their adhesive properties. MagiGlue™ is unique in that it was specifically formulated to imitate the processes that occur in natural bioadhesives.

Natural Bioadhesives
In addition to the sticking power of mussels and barnacles, several types of microbes produce polymeric substances which allow the organism to attach to solid materials in a favorable environment.¹ The bioadhesives made by microorganisms is the familiar slimy material which coats rocks in streams, on beaches, and in the ocean among other places. These bioadhesives tend to have extremely high bond strengths, far superior to commercially produced biopolymers, due to the poor understanding of the complex mixture of polysaccharides and proteins produced by microorganisms. Purified biopolymers do not reproduce this complex mixture and thus do not duplicate the natural adhesive properties. MagiGlue™, on the other hand, is a first generation bioadhesive that was specifically designed based on the principles of how microorganisms produce naturally occurring bioadhesives.

Biopolymers, because of their natural origin and composition, are completely environmentally friendly. They are easily degraded in a variety of natural and man-made settings. Further, biopolymers originate from renewable resources such as plants or the fermentation of microorganisms. While many plastics, resins, and synthetic glues are formed from fossil fuel byproducts, biopolymers are produced using microbes and plants.² These renewable resources offer more consistent long-term options for bioadhesive production, as well as maintaining a more stable cost as the price of crude oil rises. Therefore a glue comprised of biopolymers is completely biodegradable, environmentally safe and economically competitive.

Adhesives/glues work by either 1) dissolving the substrate, 2) interlacing the molecules of the substrate surface or 3) creating a vacuum. No one glue/adhesive is universal in that it will bond every substrate; therefore, one design objective of new glues and adhesives is to maximize the substrates on which they work. The adhesive property of a biopolymer is inherent in its molecular structure and how that structure interacts with the substrate.³ Examples of biopolymers with adhesive properties include several types of polysaccharides and certain proteins.⁴ Often, however, the biopolymers do not form a sufficiently strong bond for use in industrial or consumer applications.⁵ In other cases, the biopolymer may have good adhesive properties but must be used at such high concentrations that the resulting high viscosity makes them impractical.⁶ Those biopolymers which do have strong adhesive properties at low viscosity, such as lignin or fibronectin, have very high production costs due to limitations in extraction, fabrication and purification. Therefore, the use of biopolymers is often limited by low efficiency and/or high cost. The design criteria for the formulation of MagiGlue™ solves many of the limitations of bioadhesives.

An unmet need exists in the adhesives marketplace for an economical, environmentally friendly, and easily reversible glue and coating. Currently, adhesive products that are reversible require hazardous chemical solvents or high temperatures to reverse the adhesive bond and are time-intensive requiring extensive soaking periods that can last for hours to break the bond. The solvents often limit the materials to which an adhesive or coating can be applied as the solvent may have a corrosive effect on the base material. Further, solvents, such as acetone and methylene chloride, are health and environmental hazards. An adhesive that is reversible with water would eliminate many of these disadvantages. Water-reversible glues currently on the market [for example: Dymax™ (Dymax Corporation), Aquabond™ (Aquabond Technologies), and Technikote™ (Questech Services Corporation)] require water at high temperatures to reverse the bond. These temperatures are high enough to present an injury risk. MagiGlue™, by contrast, is comprised of various biopolymers which meets the criteria of a low temperature water reversible adhesive that is economical, environmentally responsible and, importantly, bonds a wide range of substrates.

Description of MagiGlue™
MagiGlue™ was developed by scientists at Athena Environmental Sciences, Inc. as a proprietary blend of biopolymers in low concentrations with enhancing agents to form a strong reversible adhesive and coating. The adhesive retains the characteristics of biopolymers such as: solubility in water, reversibility, biodegradability, and ability to form a strong bond. By blending different biopolymers, the MagiGlue™ adhesive was made stronger while keeping the biopolymer concentration low and the cost economical. The product is formulated in water and cures into a hardened opaque layer. As a glue, it forms a bond between objects within two to fifteen minutes depending on the substrates and will then cure completely in 24 hours. As a coating it will harden to a thin, hard, off-white layer with strong tensile strength.

Broad Spectrum Substrate Bonding: One design criterion for MagiGlue™ was to define a polymer composite formulation that maximized the range of substrates that it could bond. In the initial screening a qualitative test was employed to simplify the formulation development. For each bonding test, 0.1 ml of a given glue formulation was applied (unless otherwise noted) to one substrate and a second substrate pressed to the first substrate. The bond was allowed to cure for 24 hours before assessing the strength. Reversibility was determined by soaking the test substrates in 2-5 ml of water and recording the time required to dislodge the two substrates as a measure of the ease of reversibility. Both the adherence strength and ease of water reversibility were measured using the qualitative scale shown in Table 1.

The materials used for the formulation development and qualitative testing are listed in Table 2. A range of materials were selected that included paper, plastics, metal, glass and wood. Homologous and heterologous substrate combinations were used. Since the bonding of most homologous substrates was so strong, resulting in the destruction of the substrate before the bond broke, heterologous substrates were used to determine the performance limits of MagiGlue™ for most of the initial characterization work. The primary substrates to which various materials were adhered were glass, polystyrene and painted surfaces. Glass and polystyrene are substrates to which biopolymers typically have poor adherence properties. These materials, therefore, provided a stringent measure of MagiGlue™ performance and permitted the development of an adhesive with higher bonding strengths than most biopolymers as well as many traditional glues. Figure 2 shows the relative bond strengths of several different substrates when adhered to glass or polystyrene. Paper was consistently the better substrate with an average score of 4.7 over 9 experiments. Cotton cloth was the most difficult to bond with a mean score of 1.6. Wood, aluminum, plastic and glass were about equal with average scores between 2 and 3. When adhered to four different painted surfaces, paper and wood gave the highest scores, plastics and metals intermediate scores and cork and cloth the lowest scores (Table 2). In all cases the bond was stable and did not fail unless some amount of force was applied; in particular in one experiment, paper (20 lb bond copier paper) was adhered to glass and has remained stable for more than one year. Importantly, homologous substrate bondings of paper, wood, cloth and polystyrene could not be broken without destruction of the substrate. This precluded quantitative measures of the bond strength using these substrates. Aluminum-aluminum bonding, on the other hand, was less strong and could be used for quantitative comparison measures.

Figures 2 and 3.

Figure 3 shows the average score for water reversal of the bond. Each of the substrates was glued to glass or polystyrene and then water was used to disrupt the bond. There was a direct correlation between bond strength and ease of reversing the bond. Paper adhered to glass or polystyrene (the highest bond strength rating) was more difficult to disrupt than cloth to either substrate (lowest bond strength rating). In no case were the bonds found to be irreversible when water was applied. Residual adhesive remaining on the surfaces of all three substrates, glass, plastic and latex paint, after the various materials were dislodged, were easily cleaned with water without any visible evidence of the surfaces being compromised. Warm water with a mild dilute detergent was the most effective means of removing the residual glue, though room temperature water did work but required a larger volume and longer exposure times.

Box Load Test: MagiGlue™ was used in a box load test to demonstrate its ability to glue cardboard. The test involved gluing the bottoms of two cardboard shipping boxes rated at 65 and 95 lbs., respectively. To two 65 lb. rated boxes, 2.5 or 5 ml of a 30% adhesive solution was applied to the four corners (a total of 10 and 20 ml, respectively) of the two outer most panels of the box bottoms and compressed to the two inner most panels. For the 95 lb. rated shipping box, 4 x 12.5 ml (50 ml total) of a 30% solution was applied as described for the 65 lb. boxes. The adhesive was allowed to cure for 24 hours. The load test was performed by applying incremental amounts of mass to each of the boxes until there was a structural failure when the boxes were lifted 3 feet off of the ground. The results are given in Table 3.

In each case, the box failure occurred at 27.8, 53.2, and 15.8% above the weight rating of respective container. Of the three cases tested, only one showed any signs of a bond failure. This was the test where the amount of adhesive was the lowest volume applied and the adhesive failure was only at one of the four bonding sites. No other adhesive failures were observed.

Bond Strength Measurement - General Method: To quantify the bonding strength of MagiGlue™, aluminum coupons were bonded end-to-end with different adhesive compositions. Aluminum was selected because MagiGlue™ and other similar commercial adhesives bonded the material with a bond strength within the measurement range of the force meter employed without destruction of the coupon. Each coupon was 0.25 inches in diameter (0.05 inches²) and 1 inch long with a hole at one end to allow the coupon to be attached to the force meter. The bond strength was measured using a force meter (Mark-10 Model BG1 Force Meter) and defined as the force required to break the bond. The coupons were prepared for bond strength testing by sanding the end opposite the hole with #220 granite sandpaper, cleaning them with soap and water and allowing the coupons to air dry for 24 hours before using. To test an adhesive composition, 10 µl, unless otherwise noted, of the adhesive was applied to the sanded end of one coupon and a second coupon aligned and pressed against the first. The glued coupons were allowed to cure for 24 hours at room temperature (20-25°C) and humidity (30-50% RH). The bond strength was measured by attaching each pair of coupons to the clamps of the force meter and recording the pounds of force needed to break the bond. For each test condition, five sets of coupons were prepared and the average bond breaking force determined.

Comparison to Other Adhesives: The relative bond strength of MagiGlue™ was compared to other commonly used general purpose adhesives and several commercially available bioadhesives. Each adhesive was used to glue pairs of aluminum coupons and the force needed to break the bond determined as described above. Bioadhesives that did not bond the aluminum coupons were excluded from further consideration. This included rice and tapioca starch, wheat starch formulations less than 15% w/v, carageenan, gum arabic, and chitosan. MagiGlue™ exhibited a similar bond strength to white glue and wheat starch (Table 4). All three of these glues had lower bond strengths than wood glue. Interestingly, MagiGlue™ appeared to perform better with a larger application of adhesive as compared to wheat starch where the bond strength appeared to be unaffected by the amount of adhesive applied.

Performance Enhancing:
The above analysis showed that the bond strength of MagiGlue™ is comparable to wheat starch and white glues. To determine if the bond strength could be enhanced, a series of experiments was performed which examined the effects of different polymer cross-linking agents. In the initial screen two different diisocyanates were tested. Diisocyanates are homobifunctional cross-linking reagents that react with free hydroxyl groups to form a urethane linkage. The MagiGlue™ was formulated as 15, 25 or 30% w/v biopolymers in water or alkaline buffer and each of the respective diisocyanates added at 0.5, 1.0, 2.0, 4.0 and 8.0% along with either DABCO or DBTDA catalysts. The mixtures were then applied to the aluminum coupons, cured for 24 hours and tested for bond strength as described above. The bond strength of 25% polymer formulations was increased 1.5 to 2-fold with hexane diisocyanate, but was decreased for 15% polymer mixtures (Table 5). Interestingly, the lower concentration of hexane-diisocyanate appeared to produce a stronger bond than the higher levels did. Poly(propylene glycol) tolylene diisocyanate did not increase bond strength significantly, less than 2-fold for 15% polymer formulations at low concentrations of the diisocyanate. However, in most cases the bond strength was decreased.

Using a 30% polymer formulation, a second set of homobifunctional cross-linking agents was examined. These agents included formaldehyde, gluteraldehyde, and octyl-diisocyante along with poly(propylene glycol) tolylene diisocyanate and hexane-diisocayanate for comparison. Figure 4 shows the relative bond strengths of the different mixtures. From this analysis, none of the agents tested lead to significant increases in bond strength; in fact, most seemed to inhibit the adherence properties of MagiGlue™. The exception being gluteraldehyde which only slightly increased bond strength.

Figures 4 and 5.

A third chemistry used to cross-link biopolymers is sodium tetraborate ("borax"). Figure 5 shows the results of a scouting experiment using different amounts of borax and in combination with other additives. Borax clearly had an enhancing effect on bond strength. Interestingly, 1% (w/v) formulations yielded 4 higher strength bonds than did 2%. Inclusion of a thickening agent, carageenan, or an oxidizer had the greatest effect where the bond strengths were increased 2.5- and 2.0-fold, respectively. In a follow up experiment to further examine the effects of borax on bond strength, borax was added to 0.5, 1.0 or 2.0% with the blending done in water, 0.5N or 1N NaOH. In this experiment, a borax concentration-dependent increase in bond strength (Fig. 6) was observed. However, base catalysis clearly had an adverse effect on bond strength that directly correlated to the hydroxide concentration. In contrast, the addition of an oxidizing agent significantly increased bond strength.

Figures 6 and 7.

Reversibility Testing: To test whether the addition of borax decreased the water reversibility of the adhesive, MagiGlue™ was blended with 2% borax and the bond strength measured after soaking in water. Two sets of 30 aluminum coupons were bonded with MagiGlue™ and MagiGlue™ plus 2% borax and allowed to cure for 24 hours. The bond strength for five coupons in each group was measured and the remaining coupons were soaked in water at 23°C. At 1, 5, 10, 15 and 20 min. five coupons per group were removed from the water and the bond strength determined. A plot of the bond strength versus time showed a first order exponential decay in bond strength following soaking in room temperature water (Fig. 7). With borax the apparent bond half-life was 1.96 min. as compared to 8.56 min. for the base formulation. These results demonstrated that while borax can substantially increase the adhesive strength of MagiGlue™, it seemingly enhanced the water reversibility properties of the glue as well. The large experimental error in measurement of the bond strength could account for the difference. Nevertheless, sodium tetraborate did increase the bond strength while not interfering with the water reversible properties of the adhesive.

Potential Applications:
MagiGlue™ can be employed as a coating, protective film, or glue. It is suitable for both temporary bonding as well as semi-permanent and permanent bonds in dry conditions. As a glue it allows for temporary or permanent bonding of a variety of surfaces including glass, ceramic, plastics, wood, paper, and metals, or cross-bonding of many of these substances. In addition, MagiGlue™ can be blended with light absorbent chemicals, heavy metals, clay, carbon fiber, or other protectant materials to provide protection for ceramics, silicon, metals and more. It can be used commercially as a surface protectant, for jig mounting, as a non-invasive in-process adhesive or coating, for nanofabrication, for cardboard and paperboard packaging, or as a barrier film, in addition to working well as a craft glue for use at home or school to bond labels, signs, decorations, crafts and art projects.

The reversible and biodegradable properties of MagiGlue™ could be particularly useful in label adhesives or for box assembly, facilitating recycling or other disposal processes. Less effort would be needed in the disassembly of boxes or removal of labels before the product is recycled and would not require the separation of the adhesive from the solubilized cellulose fibers before re-casting into recycled paper products. This could help to reduce recycling time and costs.

MagiGlue™ could also be used in manufacturing in a variety of ways. Etching processes, which require a protective layer, would also benefit from the application of the adhesive as a temporary coating. As an added benefit, the same substance used as a coating can be applied to hold the piece being etched in place during the etching process. Industrial areas where this particular etching application is optimal is in silicon chip processing and etching, and in microcircuitry. MagiGlue™ could also work as a temporary placeholder for machined parts manufacturing. The bond formed by the adhesive is ideal for use in fabrication and modification of parts during a machining process, as it holds well then is removed quickly and easily with no residue.

In the human healthcare and cosmetic industries there is a need for a nontoxic, easily reversible adhesive. The cosmetic industry is currently searching for new adhesives and bonding agents that are human- and environment- friendly. MagiGlue™ could potentially fulfill this role and offers several possibilities for compounding products such as costume cosmetics.⁷ In the medical field the versatile adhesive can also serve as a bandage adhesive and wound dressing.

The areas listed above include only some of the many uses for the described adhesive and coating. The versatile nature of the nontoxic reversible adhesive shows promise in a variety of multi-faceted applications.

References:

1. Sutherland, I. W. 2001. Biofile exopolysaccharides: a strong and sticky framework. Microbiology 147:3-9.
2. Kolybaba, et al., 2004. Recent developments in the biopolymer industry. North Central ASAE/CSAE Conference, Paper Number MB04-301.
3. Byrom, D. 1991. Biomaterials: Novel Materials from Biological Sources. Stockton Press, New York, NY.
4. Kaplan, D. et al., 1994. Naturally occurring biodegradable polymers. In Polymer Systems-Synthesis, Swift, G. and Narayan, R. eds. Hanser Publishing, New York, NY.
5. Morris, C.A. 1987. US Patent 4 981 707; Green, W.M. 1978. U.S. Patent 4 161 545; and Chino, J. 1976. U.S. Patent 4 053 650.
6. Huisman, E.J.T.M. and Vis, R. 1979. U.S. Patent 4 308 289; and Noznick, et al. 1967. U.S. Patent 3,314,800.
7. Consumer Product Testing Co. Allergenic and Toxicology Report on Non-GRAS Listed Component of MagiGlue™. November 2005.