# The Ability to Control VOC Emissions from Multilayer Building Materials
## 1 Introduction
### State of Knowledge-Emission Modelling
The third group of models provided in the literature (Figure A1, Appendix A) is related to the emission from wet materials. When considering the study of emissions from wet material, it should be emphasised that the generally accepted definition of wet material is a solid, homogeneous material covered with a layer of liquid coating. Furthermore, for such materials, the [PERSON] et al., [35] models were built, which take as a variable the thickness of the liquid's emitting top layer. As demonstrated by testing VOC emissions from liquid coating materials in a small test chamber in accordance with standard ASTM D 5116\(-\)06 [36], the emission profile consists of two stages: an early stage (stage I), during which the material is still quite wet, has a high emission level whichdecays quickly and another dry stage (stage 2) during which the VOCs are released much more slowly [35]. Rapid emission occurs mainly by evaporation from the surface of the material, with internal diffusion relatively negligible at this early stage, while in the subsequent dry stage, internal diffusion becomes the controlling factor. Usually, there is a transitional phase between the two steps, which makes the prediction of VOC emissions from liquid materials more complicated. Studies by [PERSON] et al. [35] and [PERSON] and [PERSON] [28] have shown that the emissions of \"wet\" materials will depend on environmental conditions (e.g., temperature, air velocity, turbulence, humidity and VOC concentration in the air), as well as the physical properties of the material and the substrate (e.g., diffusivity). Since many factors can influence the emission behaviour of \"wet\" materials, testing of emissions by laboratory experimentation is usually necessary and expensive (time-consuming). [PERSON] [37] built an emission model from a single homogeneous layer of wet material that accommodates this layer's changing thickness during emission and considers both internal and external mass transfer resistances through the moving wet shell/air interface. However, the simplifying assumptions introduced, related to the assumption of homogeneity of the wet material layer (constant \(C_{m}\) and \(K_{m}\) values of the wet layer), limit this model's scope of application. A frequently cited work by [PERSON], [PERSON] and [PERSON], [38] presents a model of emission from a moist material with thickness varying during the emission process. This model is not exponential but expressed by a fairly simple differential equation, the variable of which is the thickness of the material layer, but its solution is complicated by the adopted variables (reduction) of the \(C_{m}\) values (VOC concentration in the material) as well as the partition coefficients \(K_{m}\). Therefore, in multilayer systems, the Deng equation for emissions from wet materials would be difficult to use by a difficult mathematical model because the [PERSON], [PERSON] and [PERSON] [38] equation is solved using the generalised integral transform technique (GITT). All the above-mentioned mathematical models of emission processes are troublesome to use, although they were created to predict the rate and mass balance of emissions from building materials of different structures and physical conditions. Moreover, it is also difficult to find a relationship between the models. The experimental VOC emission characteristics are inconvenient for engineering applications due to the relatively slow emission rates under chamber test conditions. Therefore, it remains to simulate emission processes, which is related to the need to develop simpler models. The fourth group of publications (Figure A1, Appendix A) presents analytical models for the simulation of VOC emissions from building materials. For example, the [PERSON] [15] model's mass transfer model parameters have a practical application for understanding the mechanism of VOC emissions' physical processes from building materials. However, these parameters usually involve additional physical parameter measurementsneeded to perform a simulation using the model.