Tailoring and Assessment of Polylactic Acid Biopolymers for Use as VOC-Free Adhesives
Abstract
Formaldehyde and its derivatives are so widely used that it can be almost impossible to spend a single day without coming into contact with an end use. The downside however is that exposure to humans is known to cause undesirable health effects. In 2011, the 12th Report on Carcinogens officially classified formaldehyde as a known human carcinogen and this has been reasonably anticipated by toxicologists since 1981. For most humans, the primary source of exposure is contaminated indoor air as the compound is off-gassed from a variety of common household goods and exposure over threshold quantities can result in negative side effects. In this light, the Environmental Protection Agency (EPA) has initiated legislation to reduce or eliminate the use of formaldehyde in consumer products; their primary area of concern being composite wood panels which rely heavily on the use of urea-formaldehyde (UF) adhesive resins at present. Research focused on the development and validation of a non-toxic alternative to formaldehyde in this segment could provide great benefit in reducing unnecessary chemical exposure to individuals both in the home and workplace. Polylactic acid (PLA), a biobased and nontoxic polymer, offers great promise as an adhesive and sealant in other applications. As part of this dissertation, tailored PLA was studied as a possible candidate to replace UF in fabrication of plywood and particleboard composites. A primary challenge in making a compelling argument for manufacturers to consider such a new adhesive system involved abiding by the limitations of their current infrastructure, and for UF resins the standard industry technique relies upon use of spray applied emulsions. This technique is not ideal for PLA-based adhesives without relying on an organic solvent as the fluid carrier, which would counteract the intent of this work. To overcome this challenge a novel dry application method was developed using fine powders of tailored PLA which ensures the adhesive component will remain non-toxic. A range of necessary particulate distributions was established and the technique demonstrated on laboratory scale as an effective means of adhesive dispersion. With such a protocol in place the polymer itself was then studied (both experimentally and via theoretical modeling) for variations in material properties to optimize the system as a whole and find the best combination of strength and friability for each prospective end use. In accomplishing this task high molecular weight PLA was tailored to different structure-property spaces using hydrolysis, thermal degradation, and ionizing radiation. Industry metrics for flexural strength, internal bond and water resistance were used as benchmarks to ensure that the industry relevant range of performance metrics could be met when compared to similar panels produced with commercially available UF resin. Through these trials acceptable ranges of molecular weight, crystallinity, polydispersity, and resin loading were identified and shown viable for the particleboard and plywood sectors. This resultant PLA-based adhesive system and application technology provides a potential drop-in pathway for certain composite wood applications and shows great promise as an environmentally friendly alternative to UF resins.
Degree
Ph.D.
Advisors
Taleyarkhan, Purdue University.
Subject Area
Nuclear physics|Materials science|Wood sciences
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