New polycaprolactone diols for soft polyurethanes

Soft thermoplastic polyurethanes can win roles in applications where touch and feel are important but maintaining softness can be challenging. Ingevity’s new polyol product line for soft TPUs, the Capa S series, could help.

There is high, but untapped, market demand for soft TPUs with good mechanical properties. They are particularly desirable in applications such as wearable devices like smart watches and fitness trackers, and also automobile dashboards, seating and door panels. These are products where consumers want functional long-lasting products that are also comfortable and aesthetically appealing.

Ingevity has two new polyols, Capa S 22X and Capa S 23Y, that are designed to help formulators produce materials that stay soft, and minimise cracking during a product’s functional lifetime. They could also simplify formulations, and lower production costs.

In applications where softness is required for extended periods, the market is currently dominated by the more expensive fluoroelastomers and silicone elastomers.

Polyurethanes have been able to compete, but need to be plasticised to overcome the problem of cold hardening – the slow crystallisation of the soft polyol blocks of the polyurethane over time.

Although plasticisation is a well-known process used to produce polymeric compounds that are more flexible and softer than their unplasticised homologues, adding plasticisers to formulations can generate a number of challenges. First, plasticisers must be added to the formulation, which can involve an extra processing step and additional cost. Once incorporated into the polymer, plasticisers can migrate from the compound into surrounding materials and the environment. This can lead to unwanted chemical exposure and a gradual stiffening of the part over time.

Keeping polyurethanes soft

By modifying the crystallinity in soft blocks in the finished TPU, we believe Capa S can help fill the gap between plasticised TPUs and premium soft elastomeric materials. Our technology uses carefully branched monomers which act as internal plasticisers for TPUs. Here, the branches can disrupt packing and crystallinity in soft blocks.

Since the soft block chains are more mobile than customary polyols, it is possible to produce polymers that are intrinsically softer than similar polyurethanes. At the same time, the hard blocks retain the crystallinity needed to make injection moulding a viable process. There is a balance between the crystallinity of the soft segment and the ability to process the materials. These Capa polyols are characterised by a lack of by-products, very low acid values and water content, 100% primary hydroxyl content, and a narrow and consistent polydispersity.

This technology is flexible, and a wide range of polyols can be generated using a variety of hydroxyl-functionalised initiators. Additionally, co-monomers can be integrated into the polyol, often leading to synergistic effects. The Capa product portfolio includes materials that can be used as polyurethane soft segments and chain extenders. The ability to tailor the structure of these polyols gives customers the ability to differentiate their products.

Capa S 22X and Capa S 23Y are polycaprolactone diols with a structure tailored for use in polyurethane materials of hardness less than 70 Shore A. They have molecular weights of 2000 g/mol and 3000 g/mol respectively.

Compare and contrast

We compared the effectiveness of two new polyols Capa S 22X and 23Y (Slide 27) against an equivalent grade 2210A from our range and a group of competitive materials including silicone elastomer, flroroelastomer, thermopolastic polyolefin, thermoplastic vulcanisate and a styrene butadiene copolymer.

All of the finished polymers were formulated to generate materials with hardnesses between 50 and 70 Shore A. The TPUs were conditioned for one week at 23°C and 50% relative humidity before testing.

These products have properties in line with the existing Capa range, including extremely low acid values and water content, and a low glass transition temperature (T_g). Like other Capa polyols, the products’ viscosities are low relative to other polyester polyols — a key advantage when handling and processing these materials.There is a strong correlation between cold hardening of polyurethane systems and the polyol component’s melting enthalpy.

The graph above shows that this is between 110-115 J/g for pure polyester or pure polyether diols. This is slightly lower in mixed polyester adipates, while polycaprolactone homopolymers have intermediate values.

The new products have much lower crystallinity and melting enthalpy but, importantly, are not amorphous. Our work found that by achieving melting enthalpy below 40 J/g, materials can retain softness for extended periods.

Graph Q (Slide 30) shows the effect of tuning the crystallinity in Capa S polyols compared with a lower isocyanate formulation and a conventional grade Capa 2201A. This is designed to produce a softer material. Initial hardness was around 60 Shore A, but very quickly hardness increases beyond 70 A and then settles.

Our work shows that the new polyols allow formulations to maintain hardness within 5% of the target value for up to six weeks, whether stored in a conditioning oven or in a refrigerator. This demonstrates that reducing crystallinity in the polyol delays cold hardening of the polyurethane over time.

Physical properties

Despite modifying the polymer structure, the mechanical properties are still comparable or superior to competitive benchmarks such as silicone elastomers, fluroroelastomers, thermoplastic polyolefins (TPOs), thermoplastic vulcanisates(TPVs) and styrene–butadiene copolymers (SBC) (Table B) (Graph 31).

Tensile strength testing put Capa S 22X and Capa S 23Y grades well above the values achieved by the competitive materials. The Shore A 60 formulation based on S 22X failed at 24.9 MPa, while the 50 Shore A S 23Y formulation failed at 14.5 MPa. Only the fluoroelastomer (15.7 MPa) was in the TPU< range. Silicone failed at 7 MPa<, while TPO, TPV and SBC achieved 6 MPa before failure.

The results presented in Table C (Slide 31) show that the tear strength range of 36-43 kN/m for the Capa S 22X and Capa S 23Y formulations are well within the range observed for competitive materials. Silicone elastomers typically have a tear strength of 44 kN/m and SBC 22kN/m in such tests.

Compression set performance is also important. The results of compression set tests under constant deflection for 72 hours at 23°C and 24 hours at 70°C are shown in Graph R. Despite the reduced isocyanate content, 60 Shore A hardness polyurethanes made with Capa S 22X and Capa S 23Y, matched or out-performed harder polyurethanes made with Capa 2201A, a typical soft segment used in such systems.

Ball rebound resilience for all three grades are similar and there is only a slight decline in resilience in the softest materials. Under dynamic conditions, the storage modulus (E’) is lower for softer materials made with Capa 22X and Capa 23Y materials, while the tan delta is higher (see Graph B). When reducing the isocyanate component and increasing the polyol content, a change to the viscous flow and elastic response of the material should be expected. Tan deltais higher for the softer material, but still below 0.1, indicating acceptable performance for an elastomer. The polyurethane made using Capa S 22Y exhibits stable tan deltabetween 0°C and 60-70°C, depending on which of the polyols is used. The rubbery plateau is wider for the formulation based on Capa S 23Y.

Thermal properties

Differential scanning calorimetry (DSC) heating curves show well-defined enthalpy associated with the crystallinity of both the soft and hard segments of the polyurethane for 80 Shore A materials. It is important to maintain the hard segment crystallinity, which is vital for injection moulding.

The DSC cooling profiles show a very distinct crystallisation enthalpy, particularly for harder polyurethanes. Our laboratory polyurethane formulations were solid and demouldable well above ambient temperatures. This suggests that TPUs can be moulded quickly. Softer materials have similar, less distinct profiles, but the crystallisation events are still complete by 25°C.

There may still be some opportunities in achieving viable cycle times in injection moulding. Ingevity has independently verified that producing soft TPU pellets is possible by reactive extrusion, and these have been successfully injection moulded. It is expected that continued optimisation of the formulation and moulding conditions will advance commercialisation of the technology, and provide benefits throughout the TPU value chain.

As well as the physical properties outlined above, Capa S polyols should be useful in applications where the finished product needs to have hydrophobicity, stain resistance, slip resistance, good low temperature performance and long-term transparency when formulated with an appropriate isocyanate.