dc.description.abstract
PSA (tapes) are permanently sticky adhesives that can be applied by slight pres-sure to a large variety of surfaces. This easy application has led to the wide-spread use of PSA not only in the DIY sector but also in the construction, rail-way vehicle, and automotive industries, where the fire behavior of the materials is of crucial importance. The fire behavior of PSA-bonded materials is therefore a research topic of high interest to industry and academia. Due to the viscoelas-tic state of PSA, it is a challenge to develop well compatible flame retardants that do not bloom out or migrate to the adhesive surface and degrade the adhe-sion between the substrate and the adhesive. To overcome this problem, phos-phorus flame retardants, as a very versatile class of flame retardants that can be modified to meet the specific demands of the polymer matrix, are a good choice for a PSA matrix. A further advantage of phosphorus-based flame retardants is that they are effective at low concentrations, allowing the PSA tapes to keep their mechanical properties. They act in several modes of protection, depending on the chemical environment of phosphorus within the flame retardant and the polymer matrix in which they are used.
The state of art is to develop phosphorus-based flame retardant PSA tapes that perform well in flammability tests and expect them to have beneficial influence in the bonded product, without this relationship being proven. The questions of how different flame retardants work in PSA and PSA tapes and how these flame retardant adhesives influence the burning behavior of bonded materials is elab-orated on in this doctoral dissertation.
In the first step of this dissertation, the fire behavior and pyrolysis of commer-cially available phosphorus-based flame retardant adhesives (tapes) were ana-lyzed and compared to adhesive tapes without flame retardant to gain a basic understanding of how adhesive tapes behave in fire and how flame retardants affect this behavior. Fire behavior analysis was performed in different fire sce-narios and bond designs established in a multi-methodical approach. The reac-tion of the adhesive tape to a small flame was investigated as a free standing object, bonded one-sided onto different substrates, and as sandwich like bonds (substrate/tape/substrate). The interaction between flame retardant, adhesive matrix, carrier, and substrate was explained by systematically examining the pyrolysis of the adhesive, the flammability of the tapes, and the fire behavior of bonded materials in an ignition and a developing fire scenario. Py-GC/MS, TGA FTIR, PCFC and hot-stage FTIR measurements were used to identify the mode of action of the flame retardant and its interaction with the PSA matrix. The flammability tests, UL 94, OI, and the single-flame source test identified the effect of the flame retardant on the flammability of the tape and the effect on the flammability of bonded materials. Measuring and comparing the fire be-havior of the bonded and monolithic materials in the cone calorimeter led to an understanding of how PSA tapes influence the fire behavior of bonded sub-strates. The fire behavior of different monolithic and bonded substrates with a wide spectrum of burning characteristics was investigated to gain information on the substrate specificity of the tape influence.
The second step of the dissertation consisted in preparing differently phospho-rus-based flame retardant PSA and PSA tapes. One predominantly gas phase ac-tive, one condensed phase active and one covalently bonded phosphorus flame retardant were used in a PSA matrix which mainly consisted of poly(n-butyl acrylate). Determining the adhesion and cohesion properties of the PSA tapes guaranteed the possible application as PSA tape. Their flame retardant mecha-nism and decomposition and combustion behavior were analyzed thoroughly in Py-GC/MS, TGA FTIR, hot-stage FTIR and PCFC to gain precise insights into the chemical and physical mechanisms that govern the pyrolysis process. The dif-ferent modes of actions and mechanisms were subsequently connected to the flammability of adhesive tapes which were prepared from the synthesized flame retardant PSA and assessed in UL 94 and OI.
In the last step of this systematic doctoral dissertation, the self-prepared adhe-sive tapes were used to bond different substrates whose fire behaviors were then investigated in different fire scenarios simulating ignition, developing and fully developed fire. This made it possible to connect the pyrolysis mechanism, mode of action, and fire behavior of the free standing adhesive tapes with the fire behavior of bonded materials. Different carriers (AL and PET) were used in different substrates (wood, bisphenol-A polycarbonate, polymethyl methacry-late) to yield in an understanding of the influence of the adhesive tape carrier on the burning behavior of the individual substrates.
The first step shows that phosphorus flame retardants significantly improve the flammability of the commercially available adhesive tapes. The gas phase active flame retardant resulted in a UL 94 V-2 rating and a large increase (5.3 vol.-%) in OI, and significantly changed the burning behavior in the single-flame source test. Tape-bonded materials behaved substantially different from their mono-lithic counterparts in cone calorimeter measurements, where the adhesive tape mostly acted as an insulating barrier separating the bonded substrate layers. This separation led to new fire risks in PC and increased the FIGRA up to 20% and the PHRR up to 26%. Cone calorimeter measurements also showed that phosphorus flame retardants and carriers must be tailored to each other because they can react and degrade the protective properties of the carrier, as it was the case for the AL carrier in combination with a commercial phosphorus flame re-tardant. The strong enhancing effect of the flame retardant on the flammability of the non-bonded adhesive tapes was not present in adhesive tape-bonded ma-terials where the fire behavior was determined by the substrates.
During the second step of this dissertation, the individual decomposition of the PSA matrix and the flame retardants were analyzed in Py-GC/MS, TGA FTIR and PCFC and detailed decomposition and flame retardant mechanisms were postu-lated. The gas phase active DOB 11 released PO and PO2 radicals at low temper-atures resulting in a V-2 UL 94 rating at small concentrations. The condensed phase active flame retardant, RDP, improved the charring of the PSA tape sur-face, as it is suspected to be a precursor of phosphoric/ polyphosphoric acid. These acids lead to elimination reactions which result in unsaturated structures and finally enhanced char formation. The small amount of RDP had, normalized on the p-concentration and compared to the other flame retardants, the biggest positive effect on the OI of the adhesive tape. DOPO-pentyl-methacrylate alone has a low decomposition temperature (starting at 300 °C) but was covalently bonded to the polymer backbone and therefore decomposed together with the polymer (starting at 350° C). It released PO and PO2 radicals that improved the flammability in all fire tests. In addition to its beneficial influence on the fire behavior, the covalently bonded flame retardant also increased the mechanical properties at elevated temperatures, which is an important parameter for PSA tapes. The pyrolysis and flammability of these different flame retardant types for PSA show that the phosphorus flame retardants have individual advantages and need to be tailored to the matrix and the application.
The third and final step of this doctoral dissertation focusses on the fire behav-ior of self-prepared adhesive tapes with different flame retardant PSA and dif-ferent carriers used to bond substrates representative of automative, railway vehicle, and construction applications. Cone calorimeter measurements showed that it is not the adhesives and the flame retardants in the PSA matrix that result in changes in the fire behavior, but rather the choice of carrier. In the case of PMMA, for example, an AL carrier can improve the fire behavior by acting as a barrier to protect the underlying material, leading to a 25% reduction in MARHE and a 30% reduction in PHRR compared to a PET carrier. The same AL carrier used in PC substrates resulted in a 30% increase of MARHE and FIGRA, indi-cating higher fire hazards and poorer performance in the cone calorimeter test.
To determine the transferability of the results obtained for adhesive tapes to other adhesives, the fire behavior of adhesive tape-bonded materials was com-pared to materials bonded by liquid thin-layer adhesives. Similar effects of the liquid adhesives were obtained in the cone calorimeter, suggesting that the find-ings for the burning behavior of adhesive tape bonds are likely to be general-izable to materials that are bonded by other adhesives.
Overall, a fundamental understanding of the fire behavior of adhesive tapes and bonded materials was generated and the individual effect of different flame re-tardants for PSA tapes was investigated thoroughly. Based on the new findings, the state of the art PSA development is questioned and new, more end applica-tion focused research is suggested. The strong impact of adhesive gaps in several tape-bonded materials provides a promising outlook for future research in-to the fire behavior of materials bonded by other adhesives and using different substrates and applications.
en
