15
What's Next

As the polymer coating field is wide, it is impossible to discuss all aspects within the restricted number of pages of a single book. Hence, in this last, brief chapter we indicate, first, some general challenges and problems and, second, a few topics that have not been dealt with, but which are nevertheless important. Moreover, we try to look ahead and indicate some options for future developments.

15.1 Generic Problems and Challenges

A polymer coating, like any other material, needs to fulfill a number of requirements. Some of the properties required are generic and nearly independent of the application field. Aspects like proper adhesion, resistance against water (and the contaminants dissolved in water, such as acids), and environmental factors (UV radiation, erosion, etc.) have to be fulfilled as a basic demand. Specific properties, more related to a specific application field, such as transparency (e.g. for optical coatings), gloss and color (e.g. for decorative coatings), and antibiofouling behavior (e.g. for marine and biomedical coatings), need to be tuned specifically. Generally, for any application, multiple properties are relevant, so that always a balance or compromise between the various properties must be sought.

Furthermore, sustainability and durability have become important issues for nearly all polymer coatings. In this respect there are four items that carry weight:

  • The first is toxicity and environment. We have already mentioned the HAPs list and Blaue Engel certification in Chapters 1 and 4, respectively. It is to be expected that more chemicals will be banned for health and/or environmental reasons. For example, isocyanates and fluorine‐containing compounds have become suspect. Another issue is the reduction of environmentally unfriendly materials. An example is polyurethane resins (PUR), often made by using a Sn catalyst. Also these metal–organic compounds are at present under severe scrutiny. Alternatives for existing processes and technologies that use suspect chemicals therefore have to be sought, and alternatives compounds are most welcome.
  • The second is closely related to the first and is the reduction of VOCs. This will strengthen the already existing trend toward waterborne and/or solventless coatings.
  • The third deals with energy. In case coatings have to be crosslinked, thermal curing is still the most frequently used approach. Hence, there is an increasing attention toward low temperature curing. Also the increasing use of UV and, in particular, LED irradiation for crosslinking, can reduce the impact of the curing process.
  • Fourth and finally, sustainability including renewability, and the use of bio‐based or green raw materials can help in this respect.

We discussed in Chapters 3 and 4 petro‐based and bio‐based thermoset chemistries, bio‐based raw materials for renewable thermosets, and options for lower curing temperatures by using co‐crosslinkers and/or catalysts. The interest in bio‐based raw materials is increasing, as is the interest in sustainable technology. The most frequently quoted definition of sustainability is from Our Common Future, also known as the Brundtland Report [1], which states that “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. It contains within it two key concepts: the concept ofneeds, in particular the essential needs of the world's poor to which overriding priority should be given, and the idea oflimitationsimposed by the state of technology and social organization on the environment's ability to meet present and future needs.”.1 As illustrated in Chapter 1, coating technology does have a significant environmental footprint. If we expect to increase the world population by a factor of 2 or so in the next few decades and also assume that wealth spreading (equalizing options for the world population) will increase the footprint by a factor of about 4, it will be clear that to break even one has to reduce the footprint itself by a factor of about 8. This seems fairly impossible to reach by one solution strategy only, and therefore multiple options have to be pursued. The various synthetic compounds in use for coatings all have their general strengths and weaknesses with respect to degradation and mechanical behavior. Table 15.1, modified after [2], provides an overview. These pros and cons of each type of material can be used to advantage for a range of specific problems. As an example, we mention a few specific problems of bio‐based monomers:

  • Availability and purity are typically poor, generally dependent on the scale, time, and site of production. Moreover, some compounds may (or will) compete with the food chain, while the required agricultural area to obtain these compounds may be (very) large.
  • Polarity. The generally larger amount of heteroatoms present in bio‐based monomers than in petro‐based monomers renders them more polar. Consequently, they generally take up more moisture and hydrolyze more easily. Moreover, they do oxidize more easily via hydrogen abstraction followed by O2 addition.
  • Aromatic building blocks from bio‐based sources are less abundant than from petro‐based sources. As bio‐based sources furans and lignin‐derived compounds can be used but bio‐based analogs are often softer, that is, have a lower Tg than their petro‐based originals.

Table 15.1 Resistance toward various processes for various types of compounds.

Resins Mechanical Hydrolysis Photolysis Oxidation
Epoxy (BPA based) 0 + −/−
Acrylics + + +/+ +/+
Vinylics 0 + +
Polyurethanes (aromatic) + + −/− 0
Polyurethanes (aliphatic) +/+ +/+ 0 +
Polyester (aromatic) +/+ + 0 0
Polyester (aliphatic) +/+ 0 + +
Alkyds 0 0 0 −/−a

−/−, Very low; −, low; 0, neutral; +, good; +/+, very good.

aThe oxidation sensitivity of alkyd coatings is due to the fact that dryers, which promote oxidation, are added.

It follows that bio‐based monomers may provide alternative raw materials, but it seems unlikely that they will replace all monomers.

The problems are not only due to the polymeric components of a coating. For nontransparent coatings the inorganic oxide TiO2 is, arguably, the most important pigment component. It might be useful to indicate clearly that this compound requires a large amount of energy to produce and to disperse in paints. TiO2 is used in many paints, also colored ones. Moreover, the quality of the most promising alternatives, calcium carbonate, zinc oxide, kaolin, and talc, is assessed as significantly lower than that of TiO2. For example, Estebaranz et al. [3] state that actually there are no viable alternatives to TiO2 as colorant additive in plastics. Although moderated by another report [4], also that report clearly points out that none of the current alternatives are comparable with TiO2 at scattering light. Continued research into low carbon footprint alternatives to TiO2 is therefore important to the future sustainability of the paints industry. The Titanium Dioxide Manufacturers Association (TDMA) has undertaken a project to determine the cradle‐to‐gate carbon footprint of the manufacturing processes for titanium dioxide pigments [5]. Their report estimates that in 2012, 5.3 tons CO2 per ton TiO2 product was produced, essentially the same amount as estimated for 2010. Altogether, in view of the large impact, a not too bright prospect.

15.2 What Else?

The selection of applications discussed in Chapters 1214 is obviously limited, and other important application areas do exist. In the present section, we indicate a few of them without trying to be exhaustive.

Generally, one of the first points to mention is the ongoing attempts to increase the lifetime of materials in general and for many coating materials in particular. These efforts do comprise, for example, the realization of better initial properties of raw materials given the requirements for a certain application. Also a better understanding of the mechanisms involved may lead to these better properties, although there is no guarantee. This applies in particular to the rather complex field of weathering, where many highly complex reactions play a role that usually is only limitedly understood.

Another major problem for the areas of weathering is that the mechanisms are typically rather specific for a particular material class, or even for a specific material, thus also limiting the scope of the applicability of solutions found. This implies that if the materials still have to be used, a dedicated effort has to be made to elucidate the mechanisms upon which a possible solution must be based. An example of an extensive investigation on weathering for a specific class of coating materials can be found in a series of papers on the degradation of polyester–polyurethane coatings in the presence and absence of oxygen and/or water, leading to a quite detailed picture of the phenomena involved [6]. The use of molecular and mesoscopic simulations contributed to a large extent to the understanding of the processes involved [7, 8] and the resulting properties [9]. However, such an exercise requires a substantial effort for a specific material under various conditions, and only a few studies of this type are around. A main extension of the durability of coatings is given by the addition of stabilizers. These additives (HALS, UVA) provide, for example, in PUR clear coats a durability increase of more than a factor 2. New stabilizers which are tuned for a better compatibility with, for example, waterborne coatings are being developed. The book by Rabek [10] provides an extensive overview on polymer degradation in general.

Similar remarks apply to anticorrosion coatings, although here the relevant aspects are often somewhat more generic. Anticorrosion coatings tend to be rather complex systems and often containing rather toxic compounds, with Cr6+ as the most well‐known one. Significant efforts are made to eliminate these compounds, but the quality of the substitutes remains an issue (for one attempt, see, e.g. [11]). Another approach in anticorrosion coatings is using sacrificial materials, for example, Zn, which is preferentially oxidized so as to protect the underlying (usually steel) substrate. Still another approach is the use of electrically conductive coatings. These coatings use either intrinsically conductive polymers, such as polypyrrole or polyaniline, or composite coatings containing conductive particles conductive particles like carbon black [12]. Moreover, a low permeability of water and a proper ratio of components are important factors to obtain good corrosion resistance (see, e.g. [13]).

Also antifouling, as discussed for marine applications in Chapter 13, is a rather complex field. There is a large biodiversity in fouling species on different length scales, and a solution that will be effective for one condition may not be effective for another. More generally, it will be clear that for these (and other) complex fields, multiple solutions will be required. To paraphrase, one solution does not fit all.

In addition, the efforts to prolong lifetime comprise the implementation of self‐replenishing and/or self‐healing concepts, so that either autonomously or triggered by some treatment, the initial properties are (largely) restored. Chapter 14 dealt with this type of approach. It should be made clear though that damage does change surfaces irreversibly anyway in spite of self‐healing or self‐replenishing mechanisms. Another important effort is involved in the reduction of cleaning frequency and the avoiding of fouling. Creating superhydrophobic surfaces is one approach to provide remedies. Some developments in this area are discussed in Sections 7.3, 13.4, and 14.6.

Proper, efficient application is a prerequisite for a polymer coating. Proper means without defects, and some aspects of the origin of defects and their remedy have been discussed in Section 11.1. However, not discussed to any extent is the efficiency of the various application processes. In particular, conventional spraying is an uttermost inefficient process with losses of up to 75%, that is, the fraction really used material is only about 25%. This not only affects the cost but also is a rather unfriendly process from an environmental point of view. Other processes, such as powder coating and bath coating, have a much higher efficiency.

More generally, cradle‐to‐cradle circularity is an important issue, which is not even closely approached though, in spite of all discussions held.

15.3 What's Next?

“It is difficult to make predictions, especially about the future” is an aphorism, often attributed to the American writer, humorist, entrepreneur, publisher, and lecturer Mark Twain (1835–1910).2 The truth of this statement can be illustrated by predictions made in the past on novel developments. One list of such predictions [15] quotes that briefly after the invention of the personal computer, the statement “There is no reason anyone would want a computer in their home” was made by Ken Olsen, founder of the Digital Equipment Corporation, who apparently missed an opportunity. To be fair, his computers were bigger than many people's homes at the time. Another one is “I predict the internet will go spectacularly supernova and in 1996, catastrophically collapse” by Robert Metcalfe, the inventor of Ethernet cable, worrying that his clever piece of wire would not be able to handle all these data. “Nuclear‐powered vacuum cleaners will probably be a reality in 10 years” is the last one quoted. Alex Lewyt, president of the Lewyt vacuum company, predicted the invention of a device that few people would want to have under their stairs. Maybe that as it is, we nevertheless try to make some remarks about (desired and ongoing) developments and their prospects.

Possibly the most important point is the general trend that coatings do not only have to provide protection and/or color but also should bring another feature to the surface of the material upon which the coating is applied. Here one can think of, for example, the proper feel of a coating, the proper wettability, or the proper conductivity of a coating. In fact, the trend is ongoing to multifunctionality combining still more functions, for example, superhydrophobicity with self‐replenishing.

More efficient use of materials and energy is another trend that is expected to remain for quite some time. For materials a rather important aspect is thus improved quality, that is, less degradation susceptibility, less damage vulnerability, and probably self‐repairing and/or self‐replenishing properties. Considering energy, the use of alternative curing methods, such as radiation curing employing UV or, preferably, visible light from LEDs, can still progress a great deal.

Another way to make a more efficient use of materials and energy is to apply less layers. This probably requires a better quality of each layer, in particular with respect to defects.

Alternative application methods can also provide more efficient processes. There is a trend from applying coatings to depositing foils. For the latter process much less energy is required to realize the final system as these foils are glued to the substrate, and energy for curing is no longer required. The process is already applied, typically for large, flat areas, for example, on (large) trucks.

Coatings can also help to improve energy efficiency more generally. Examples are coatings for heat reflection (to keep the inside of a space cooler) or for IR absorption (to reduce energy required to heat the space). The latter coatings could be using either intrinsically absorbing IR radiation polymers or IR absorbing particles.

Society will be benefiting from flame retardant coatings (that contain ingredients to extinguish fires) and intumescent coatings (that provide passive fire protection by producing a highly porous layer upon heating, thereby shielding the substrate for some time from the heat). First, they save lives and, second, they can lead to less exhaust in case of calamities.

Society will also benefit from stimulus‐responsive coatings, to be used as sensors or indicators. Sensors can be used, for example, in wearable electronics (providing response on, e.g. physical activity), in food packaging (providing response about the quality of the food), or in RF‐ID tags. Note though that recycling of such composite materials will be more difficult.

In conclusion and repeating in somewhat different words what has been said in Section 1.2, polymer coatings are rather important for many industrial processes and (societal) applications and carry more application‐oriented weight than their physical weight suggests.

References

  1. 1 http://www.un‐documents.net/our‐common‐future.pdf (accessed 31 November 2017).
  2. 2 van Benthem, R.A.T.M., Evers, L.J., Mattheij, J. et al. (2005). Thermosets. In: Handbook of Polymer Reaction Engineering (ed. T. Meyer and J. Keurentjes), 833. Weinheim, Chapter 16: Wiley‐VCH.
  3. 3 Estebaranz, M. and de Kort, P. (2016). Analysis of Alternatives for Titanium dioxide as Colourant Additive in Plastic Industry. Brussels: Polymer Comply Europe.
  4. 4 Ruszala, M.J.A., Rowson, N.A., Grover, L.M. and Choudhery, R.A. (2015). Int. J. Chem. Eng. Appl.6: 331.
  5. 5 Titanium Dioxide Manufacturers Association (2013). The Carbon Footprint of Titanium Dioxide Pigment. Brussels.
  6. 6 Adema, K.N.S., Makki, H., Peters, E.A.J.F. et al. (2016). Polym. Degr. Stab.123: 13.
  7. 7 Adema, K.N.S., Makki, H., Peters, E.A.J.F. et al. (2016). Polym. Degr. Stab.123: 121.
  8. 8 Adema, K.N.S., Makki, H., Peters, E.A.J.F. et al. (2015). Phys. Chem. Chem. Phys.17: 19962.
  9. 9 Makki, H., Adema, K.N.S., Peters, E.A.J.F. et al. (2016). Polym. Degr. Stab.123: 1.
  10. 10 Rabek, J.F. (1995). Polymer Degradation, Mechanisms and Experimental Methods. London: Chapman and Hall.
  11. 11 Foyet, A., Wu, T.H., van der Ven, L. et al. (2009). Prog. Org. Coat65: 257.
  12. 12 (a) Wu, T. (2014), Conductive nanocomposite coatings for corrosion protection. PhD thesis. Eindhoven University of Technology, Eindhoven.(b) Foyet, A., Wu, T.H., Kodentsov, A. et al. (2013). J. Electrochem. Soc.160: C159.
  13. 13 Foyet, A., Wu, T.H., van der Ven, L. et al. (2009). Prog. Org. Coat64: 138.
  14. 14 https://quoteinvestigator.com/2013/10/20/no‐predict/ (accessed 21 October 2017).
  15. 15 https://www.weforum.org/agenda/2016/10/10‐predictions‐for‐the‐future‐that‐got‐it‐wildly‐wrong/ (accessed 21 October 2017).

Notes

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