Aliphatic TPUs for light-stable applications.

The demands made on elastomeric plastics, particularly those destined for applications where appearance is key, are growing ever more rigorous. Many such applications make use of thermoplastic polyurethanes (TPUs), drawing on their excellent properties of abrasion resistance, flexibility, chemical resistance and freedom from plasticizers. At the same time, the market is also demanding/prefers materials that can be brightly colored and retain that color over an extended period of time, or be virtually transparent. The basis of these requirements is explained below, illustrated using practical examples.

History of TPUs for exterior applications

Traditional TPUs have been used in exterior applications for many years. TPUs exhibit good weathering characteristics in general, however, will discolor after exposure to UV light. This has limited TPU use in exterior applications to non-cosmetic parts, parts with limited life expectancy, heavily UV stabilized TPUs, or in colors where color shifting was not apparent (i.e., black or other dark colors). Now, with the advent of aliphatic TPUs, all of the previous color limitations are no longer an issue, and coloring and decorating options once not thought possible are wide open.

The light stability of TPUs hinges on raw materials and techniques used in the production process. If the polymer diols and chain extenders used are predominantly aliphatic (non-benzene-ring-containing) compounds, good color stability under UV light results. If aromatic diisocyanates are used. the influence of light can cause discoloring, even after the addition of UV-stabilizers. Therefore, only the use of an aliphatic diisocyanate will offer the advantage of UV light stability. A comparison of aromatic, aromatic UV-stabilized and aliphatic TPUs during a weathering test in a UV chamber is shown in figure 1.

[FIGURE 1 OMITTED]

The figure clearly shows that both the aromatic and the UV-stabilized aromatic systems display a yellowness value in excess of 40 after 1,000 hours of UV treatment. By comparison, the curve for the aliphatic system is relatively horizontal and the yellowness value, even after 1,500 hours of weathering, remains well below 10.

The yellowing in the aromatic systems can be attributed to the chemical structure of the diisocyanates used, namely 4,4'-methylene-bis(phenylisocyanate) or MDI. The MDI contains a methylene bridge between the two phenyl groups, which can oxidize, thus losing the hydrogen on the methylene bridge. As shown in figure 2, this creates a conjugated system via the two aromatic rings, known as a quinone imide. Although this oxidation has a minimal effect on the physical properties of a TPU, this leads to irreversible yellowing of the TPU.

[FIGURE 2 OMITTED]

Therefore, in order to achieve genuinely good light stability, aliphatic building blocks are required.

Figure 3 shows samples of TPU exposed to UV via the Xenon-WOM accelerated method (six weeks continuous). As is evident, the once all white aromatic TPU plaques discolor to different degrees, depending on addition of UV stabilizers. However, the HDI aliphatic TPU to the left shows virtually no color change.

[FIGURE 3 OMITTED]

Property differences

Property differences among TPUs can be dramatic, depending on the starting raw materials. Traditionally, most comparisons of TPUs are done depending on the many types of polymer diols or polyols (soft segment) that are used in making the urethane polymer. These can be based on ester, ether and carbonate linkage polyols that can be of differing molecular weight and make-up. From these differing materials, you can get varying degrees of flexibility, hydrolysis resistance, fuel/oil resistance and various other properties. Little is talked about the various diisocyanates that are available, since the bulk of TPUs are based on readily available MDI, and thus these properties are assumed as constant. Now that TPUs based on HDI are becoming more available, a property comparison is warranted.

It is important to remember comparing TPUs based either on MDI or HDI will have a minimal effect on the properties that come with the soft segment portion of the polymer. Thus, the improved hydrolysis resistance achieved from use of a polyether polyol will be prevalent no matter what type of isocyanate is used. With that said, what are some of the advantages of HDI versus the more common MDI-based TPUs?

HDI or hexamethylene diisocyanate is a straight chain isocyanate versus an aromatic or cyclical isocyanate such as MDI. This lack of a bulky aromatic backbone allows for the polymer chains to twist and crystallize more randomly, giving the polymer a much higher elongation rate at similar hardness levels. These polymers also exhibit an increased level of "rebound" or "resilience" compared to their aromatic cousins. However, the one feature that clearly stands out is the UV color stability of HDI-based TPUs. The lack of the methylene bridge as described above prevents yellowing of TPU due to simple UV exposure. The results are materials that can be used outdoors or where UV light is prevalent without color shifting or discoloration under normal conditions. Thus, products that were once darkly colored can be colored using bright or translucent colors knowing that these colors will not be dramatically affected by UV light.

As table 1 demonstrates, HDI aliphatic TPUs have a favorable property matrix when compared to more traditional MDI aromatic TPUs. In addition, TPUs based on HDI hold up well under accelerated aging testing and thus perform exceptionally well in outdoor applications. Table 2 demonstrates property retention as affected by several aging tests.

TPUs and transparency

Depending on their morphology, thermoplastic polyurethanes can appear as turbid, translucent or even transparent.

TPUs are formed from three basic components: diisocyanate, chain extender and polymer diol, as depicted in chemical terms in figure 4.

[FIGURE 4 OMITTED]

Starting with these three basic raw materials, the diisocyanate can react with the OH-groups of the polymer diol and also with those of the chain extender. This creates areas that are rich in polymer diol urethane (soft segment) and, correspondingly, regions that are rich in chain extender urethane (hard segment). The soft segments determine such properties of the final TPU as elasticity, low-temperature flexibility and, to a certain extent, the swell behavior of the polymer and hydrolysis. Meanwhile, the hard segments are responsible for hardness, modulus of elasticity, demoldability and behavior at high temperatures (thermal stability). Due to the presence of the hydrogen bonds, these segments tend to form crystals measuring just a few micrometers. As a result of the chemical incompatibility of the hard and soft segments, the phases separate. The dimensions of these hard segment crystals scatter light, creating a cloudy material.

Various methods can be used for the targeted production of transparent materials, all with the aim of minimizing light scattering. The smaller the hard segment crystals, the less severe the light scattering effect--this can be achieved, for example, by using crystallization agents (physical approach). The use of other constitutional units that interfere with the crystallization process in the hard segments or improve the compatibility of the hard and soft segments (prevent phase separation) is a further means of creating transparent materials (chemical approach). Since HDI or other straight chain aliphatic materials are only one of several aliphatic diisocyanates available, other aliphatic materials such as cyclical [H.sub.12]-MDI or IPDI can be used to improve initial clarity by introducing a large cyclical backbone which disrupts crystallization. Of course, introduction of a different raw material will bring along different properties.

Figure 5 shows two aliphatic TPUs with differing crystallization behavior caused by "chemical interference," with curve (a) depicting an HDI-based material and curve (b) an [H.sub.12]-MDI-based material. The HDI based material is highly elastic and displays high heat distortion, but is turbid to translucent in appearance due to its crystalline structure. The [H.sub.12]-MDI-based product is less elastic (plastic) and displays lower heat distortion, but is glass-clear on account of its amorphous character.

[FIGURE 5 OMITTED]

New developments in producing HDI grades that are less crystalline and thus more transparent using some of the techniques above are progressing. Look for some of these grades to be commercially available in the near future.

Multi-component molding

An area of rapid growth is in multi-component or soft-touch overmolding techniques. TPUs offer a great advantage when it comes to overmolding, since TPUs inherently bond to a wide variety of thermoplastic substrates and possess a higher property matrix over many other elastomers such as TPVs, SEBS, PVC elastomers and elastomeric olefins. One thing that is apparent when it comes to overmolding articles intended for outdoor use is the prevalence of dark colors used. This is due to several factors, including the color shifting of many of these polymers and the difficulty of cleaning articles made from polymer blends. New aliphatic TPUs open doors for creative coloring of components intended to be exposed to UV and harsh weathering. Brightly colored or translucent effects can now be used to provide creative product distinctions. In addition, due to the pure nature of TPUs and the total absence of plasticizers and fillers, TPUs can be readily cleaned of dirt and oils commonly deposited by the human touch.

In addition, HDI-based TPUs also offer a feel or touch that is unique. Described by some as a velvet-rubber-like feel, this new generation of elastomers can offer a distinct differentiation in the marketplace.

In-mold decoration

In-mold decoration (IMD) or film-insert molding (FIM) involves placing a decorative film in the injection mold and then back-injecting to create a finished decorative part. By using specific combinations of films, such properties as abrasion resistance, diverse color, high depth of color, symbols and transmitted light technology can be brought together in one single product as part of a simple process. A well-known example of IMD in action is the instrument panel of a car.

The facing in IMD applications is the important part in terms of the product's appearance. This is where properties such as scratch resistance, good surface finish, transparency, light stability, moldability and suitability for printing come into their own. Not surprisingly, light-stable, transparent TPUs, which reconcile all of these requirements, are particularly well suited for surface materials produced using IMD.

Decorative watch straps with pre-printed film

Decorative straps for watches are currently being developed using transparent, light-stable TPUs as the surface material. The process used to produce these straps is shown in figure 6. The material used is a polycarbonate film printed using screen printing, which is currently the standard process used to produce temperature-resistant films for IMD processes. The printed film is laminated with a light stable TPU on the printed side, thereby permanently protecting the printed design. The laminated film is then placed in the injection mold and back-injected with an aromatic TPU with a durometer A hardness of 85. Crucial criteria in the use of such materials include:

* Adhesion between the different layers;

* dimensional stability of the molding during back-injection;

* washout/folding of the decorated films;

* thermal/mechanical strength of the decorated film/colors;

* homogeneity of the surface finish.

[FIGURE 6 OMITTED]

IMD--graphics

A further example is based on a method involving fewer steps. These results are based on initial laboratory tests. A finished transparent film made from clear aliphatic TPU is used, printed on the back using the screen printing technique developed by Indigo, a Belgian company. The pigments used in this process need to be thermally and dimensionally stable. The printed TPU film is then placed in a mold and back-injected with an aromatic TPU, resulting in a flexible and protected, colorful end product,

In-mold decoration offers a wealth of design possibilities and is not restricted to the two-dimensional. In fact, the first 3D applications have already been realized. Basically, the sky's the limit. Light-stable, transparent TPUs have a key role to play in surface finish.

Summary

In addition to typical TPU properties such as good abrasion resistance, flexibility over a broad temperature range and good resistance to weathering and chemicals, the latest market demands are for increasingly good light stability and transparency. In order to meet the tough requirements in terms of light stability, aliphatic building blocks are needed. The optical appearance (turbid to transparent) of TPUs is related to the light scattering effect of the phase separation into hard and soft segments. To enhance transparency, either physical or chemical methods can be used. With all these aliphatic TPU material advancements, creating products that are bright and colorful or transparent and protective can become a common occurrence. These new materials can offer competitive advantages since the excellent properties of a TPU can now be offered in a fully cosmetically appealing product, making design innovations and creative schemes simple. In addition, excellent substrate adhesion, a balanced property matrix, the ability to be creatively colored, and its soft-feel characteristics make HDI-based TPUs a natural fit for multi-component or overmolding applications.