2 research outputs found
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Fluoride degradable and thermally debondable polyurethane based adhesive
We report the one-pot, solvent free synthesis of a stimuli-responsive polyurethane (PU) adhesive. The hard domains within the supramolecular PU network contain a silyl protected phenol ‘degradable unit’ (DU). The DU undergoes rapid decomposition (<30 minutes) upon treatment with fluoride ions which causes depolymerisation of the linear PU adhesive. The mechanism of depolymerisation was investigated in solution using 1H NMR spectroscopy by following the degradation of the polymer in the presence of tetra-butylammonium fluoride (TBAF). In the absence of fluoride ions, the material behaves as a typical thermoplastic adhesive, and underwent four adhesion/separation cycles without loss of strength. The fluoride initiated depolymerisation of the PU adhesive in the solution state was verified by GPC analysis, showing reduction in Mn from 26.1 kg mol−1 for the pristine PU to 6.2 kg mol−1 for the degraded material. Degradation studies on solid samples of the PU which had been immersed in acetone/TBAF solution for 30 minutes exhibited a 91% reduction in their modulus of toughness (from 27 to 2 MJ m−3). Lap shear adhesion studies showed the fluoride responsive PU was an excellent material to join metallic, plastic, glass and wood surfaces. Pull adhesion tests confirmed that immersing the adhesive in TBAF/acetone solution resulted in a reduction in strength of up to 40% (from 160 N to 95 N at break) after drying
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Stimuli-responsive debondable adhesives
Polymeric adhesives are becoming an increasingly important industrial product. They are
routinely used in a wide range of high value and disposable products ranging from bonding
large sections of the interior of new cars to wound dressings that replace stiches; from single
use packaging to safety critical components in the aeronautical industry. Recently, the
introduction of debondable adhesives that break down with an external stimulus has opened up
new markets and applications in the adhesive industry. For the first time, components can be
securely bonded for the working lifetime of the product, then disassembled on demand to allow
for efficient recycling and disposal at the end of the product’s usable life-cycle.
The objective of this study was to design and synthesise new, debondable polymeric adhesives
that can breakdown or depolymerise in response to an external stimulus, such as chemicals,
light or heat. This would ultimately lead to a new class of debond-on-demand adhesive. The
work towards this goal is summarised below.
Chapter 2 reports the design, synthesis and evaluation of a fluoride degradable unit which can
be incorporated within a linear polyurethane (PU) thermoplastic adhesive. Detailed solution
state studies carried out by 1H NMR spectroscopy and gel permeation chromatography before
and after depolymerisation confirmed the efficient degradation of the material in response to
the addition of fluoride ions. Mechanical strength testing on homogenous films together with
rheometric and differential scanning calorimetry studies over multiple heat-cool cycles
confirmed the reversibility of the supramolecular network within the PU. Lastly, adhesion
testing with different material substrates before and after degradation showed the debond-ondemand
nature of this novel material.
Chapter 3 further explores the use of the fluoride degradable unit within two series of linear
polyurethanes that vary in chemical structure as part of efforts to increase the adhesive and
thermal properties over the polyurethane reported in Chapter 2. The first series of adhesives
explores the effect on bonding properties that varying nature of the diisocyanate linkers in the
PU has on the adhesive. The second series of adhesives varies by the nature of the soft segment
within the PU. It was found that introducing low-melting point crystalline regions into the
material results in a polymer that can provide excellent adhesion at lower bonding temperatures
(ca. 60 °C compared to 120 °C). The thermal response and morphology of the two series of
polymers were analysed by variable temperature rheometric analysis, small angle x-ray
spectroscopy and wide angle x-ray spectroscopy. Finally, the adhesives were tested at AWE
Preface
vi
Aldermaston following international standards at different temperatures before and after
degradation with a fluoride source.
Chapters 4 and 5 explore the possibility of producing a fluoride responsive crosslinked material
using two different approaches. The first system was a variant of a reactive adhesive which was
produced by mixing a trifunctional degradable group with a bis(isocyanate) terminated
prepolymer (Chapter 4). Adhesion testing displayed a 28 % strength increase over the linear
polymers, and a 55 % strength loss after treatment with fluoride ions. The second method
explored the possibility of creating star polymers incorporating the degradable groups at the
core. The resulting alcohol terminated branched polymer was then reacted with a commercially
available aromatic diisocyanate to form a crosslinked adhesive. Adhesive testing was carried
out before and after degradation with fluoride ions showed that fluoride ions could penetrate
the crosslinked network resulting in a measurable reduction in bonding strength (approx. 23 %).
Finally, Chapter 6 reports the design and realisation of a novel UV responsive degradable
group. Model compound studies were carried out before and after degradation with a UV light
source (36 W) using 1H NMR and UV/visible spectroscopies and showed rapid degradation
after only 5 minutes irradiation. The UV group was incorporated into a polyurethane adhesive,
which also showed rapid degradation in solution on exposure to UV light. The linear PU proved
to be an excellent hot melt adhesive for glass substrates (bonds strength = 0.43 MPa), which
weakened by up to 86 % after 5 minutes irradiation at 365 nm. However, the mechanism by
which debonding occurs in the solid state in still an area of active investigation