Dr. R. Scott Archibald Dr. Raimondo Baldassarri Dr. Andrea Donghi 8 May 2017

Transcription

1 An Overview of the Similarities and Differences of Cast and Thermoplastic (TPU) Polyurethane Dr. R. Scott Archibald Dr. Raimondo Baldassarri Dr. Andrea Donghi 8 May 2017 When I first joined my company, one of my partial assignments was to provide Technical support to our TPU business. The Genesis of this paper was my initial thoughts as to why was TPU more widely used versus our cast urethane industry especially since, in my understanding, cast urethane has superior properties to TPU. With the cooperation of our TPU experts in Italy, the goal of this paper is to actually test Cast and TPU samples of similar hardness and chemistry to understand both their advantages and differences. Today s talk will start with the chemistry of Cast and TPU and follow up looking at manufacturing with these systems. We will discuss the generic properties of urethane before looking at the round robin physical testing of the cast and TPU samples. Finally, we will look at the dynamic testing of the cast and TPU systems before evaluating the reasons why each technology is used in various applications and markets.

2 Our polyurethane competitors fall into two separate classes of plastic types. Thermoplastic resins are materials that can be reversibly melted and shaped into the final form during the melting process. Including TPU, the thermoplastic groups includes many common plastics such as polyethylene, polypropylene, and Nylon. Cast polyurethane is part of the thermoset class whereby the resin is first formed into the final shape before a non-reversible chemical process cures the resin into its final shape. Cast urethane as a thermoset resin is similar to rubber and epoxy resins. Typically the generation of cast urethane utilizes two steps. In the first step a prepolymer is formed. In this case excess MDI is reacted with a polyol. The prepolymer itself is a diisocyanate end-capped polyol.

3 TPU is generated in one step to form a fully reacted polyurethane usually in a pellet form. The same raw materials as cast urethane are utilized. For TPU, the MDI, polyol, and diol curative are reacted together by reactive extrusion. The hardness is controlled by varying the ratios of the three components. In order to be able to reprocess the TPU pellets manufacturers must use near 100% stoichiometry. In addition, nearly all TPU is MDI based. Polyurethane gains its excellent properties from the phase segregation of hard and soft segments that make up its overall composition. Some studies have measured the actual size of the hard segment. The overall hardness of the final polyurethane is based on the volume ratio of hard segments to soft segments.

4 While the soft segments are made up of the polyol portions of the polyurethane, the hard segments are chemically made up of the hydrogen bonded sections. The hydrogen bonded segments are composed of the reacted diisocyanates and curative sections. The polyurethane elasticity exists because of the stretching of the soft segments under strain. These hard segments can also stretch but they anchor the entire matrix during periods of stress. During the manufacturing process to produce the final formed polyurethane parts, we also see different methodology between cast and TPU. For TPU, dried TPU pellets are re-melted and mixed with pigment and other additives. The molten TPU is then typically molded by extrusion or injection molding. Once the newly formed final part cools, the part can be removed from the mold. Some TPU parts especially when injection molded have built in stresses and require a post curing. One important benefit of TPU manufacturing is that all the scrap TPU can be recycled and reprocessed.

5 Most of you are familiar with manufacturing with cast urethane. The process starts with the prepolymer we discussed previously. The cast urethane manufacturing process brings together the prepolymer and curative in a reactive manner to produce the final polyurethane elastomer. Now that the prepolymer and curative mixed in a reactive manner. The reactive mixture is dispensed into the mold before it cures. After demolding the urethane is typically postcured before the formed part is finished. Finishing can involve removing flashing, grinding, or milling. Manufacturing with a prepolymer allows for much greater flexibility whereby different diisocyanates, curatives, and stoichiometry are allowed. Similar prepolymers can also be blended to produce multiple hardnesses of polyurethane. In general, cast polyurethane provides greater flexibility in the manufacturing process.

6 Once the final parts are manufactured for both TPU and Cast, the reasons for choosing polyurethane are similar. They both provide tough abrasion resistant parts. Similarly both TPU and Cast urethane also have similar deficiencies. These deficiencies are directly related to their chemical composition and hence are similar for both systems. These include high temperature service, chemically reactive environments, and they are higher cost than standard plastic systems.

7 In this summary of the manufacturing section, Cast polyurethane has the advantage of being a liquid system and can easily fill molds without pressure. The molds tend to be smaller and less expensive. Cast has the versatility to use many different chemistries, stoichiometries, hand or machine cast, processing techniques, etc. Once cured the resulting urethane is a thermoset and cannot be easily reprocessed. TPU is less versatile but has the advantage of being a thermoplastic which allows for complete recycling. Though the thermoplastic needs to be processed by melting with an extruder, once the more expensive and intricate molds are in place for injection molding many parts can be cheaply made. Extrusion processes can be used to manufacture for example; wire, cable, hoses, and tubing in a continuous manner. The chemistry of TPU is limited to MDI, 100% theory, and different resins must be used to achieve different hardness ranges. The second part of this talk will focus on the physical testing of similar Cast and TPU polyurethanes. Four systems were evaluated based on different polyol backbones. The backbones include esters, ethers, and polycaprolactones. The physical testing will focus on standard tensile, tear, abrasion, and DMA testing. Round-robin testing were completed with our Italian group to look at both ASTM and DIN standards.

8 For the four polyol systems that were tested, this slide provides a reference for each system. Laripur is our TPU trade name and Imuthane is our Cast trade name. The systems studied were 85A and 95A polyester, 90A Polyether, and 85A Polycaprolactone. The samples were cured with BDO to yield the MDI polyurethanes. The TPU samples were injection molded to form small sheets and buttons. Polyurethane samples were cast in our US labs and the TPU was injection molded in Italy. Samples were exchanged and tested by each group. Our US testing is based on ASTM standards. We all know statistically there are large variations in tensile testing. The tests highlighted in RED show areas where we see large differences in the Cast versus TPU samples in 86A MDI ester systems. We see a large variation in the tensile strength and we see slight cast advantages for rebound and compression set. We also see slight advantages for TPU in split tear and abrasion. For reference, we also included testing data on an 85A TDI ester system.

9 For 95A MDI ester system, we see some similar trends with better rebound, compression set, and abrasion resistance with Cast and better split tear with TPU. For the 90A PTMG MDI ether based urethane systems, we see the same trends with better tensile strength, rebound, compression set, and abrasion resistance with Cast and better split tear with TPU.

10 For the 86A MDI polycaprolactone based urethane systems, we see the same trends with better tensile strength and rebound with Cast and better split tear and in this case compression set with TPU. I mentioned previously that samples were tested in both the US and Italy. We saw similar trends with the DIN European standard testing (instead of ASTM). For the ester systems, the TPU samples showed better tear strength and abrasion resistance. While the cast samples had better compression set.

11 Similarly for the ether and polycaprolactone systems, we see the TPU samples showed better tear strength and abrasion resistance. While the cast samples had better compression set. As we review the physical properties in total, we see some trends from the data. Cast Urethane tends to have better tensile strength and rebound. Cast also has slightly better compression set. Whereas TPU has better tear resistance and slightly better 100% modulus. In general, both Cast and TPU have similar abrasion resistance.

12 Dynamic Mechanical Analysis or DMA is testing methodology that measures the mechanical properties of materials (such as the storage modulus and loss modulus) as the temperature, time and frequency is varied. The lines in green are the storage modulus for the 86A Ester systems. The storage modulus is related to the stiffness or hardness of the material and measures the stored energy. The blue lines are the loss modulus and is related to the heat loss ability of the material. Probably the most important property for predicting the dynamic performance of the material is the Tan Delta listed here as the red lines. We see similar results for all four systems. The TPU is the dashed line and the Cast sample is the solid line. The Tan Delta for the 86A Ester MDI Urethanes is represented by the red lines. Tan Delta is related to both the storage and loss modulus. Tan Delta directly represents the resilience or energy release of the material. The lower the Tan Delta the higher the resilience and the higher the Tan Delta the material has lower rebound and higher energy absorption. The three main features of this curve are the Glass transition temperature and in this case centers around -20 C for both materials. The glass transition temperature (Tg) is the where the cured urethane goes from a stiff glass to a more rubbery material. The second important feature is the critical temperature (Tc). The critical temperature is a temperature where the material can no longer dissipate heat that the material receives from external sources. In this

13 graph, the Tc for the TPU system is about 65 C and the Tc for the Cast system is 121 C. A lower value for the Tan Delta near the Tc allows for better dynamic performance. The Tan Delta value for the TPU system is higher than the Cast system at the Tc. We see similar features in the all the remaining systems. The last transition is related to the melting of the hard segments in the urethane. At this point, the urethane loses all structural strength. The melting begins about 125 C for the TPU material and 150 C for the Cast system. The next slide represents the DMA curve for the 95A Ester MDI systems. TPU is the solid line and Cast is the dashed line. Here we see the Tan Delta graph for the 95A Ester MDI systems. The Tg for the cast system is narrower and is centered at a lower temperature. The Tc for the TPU system is at 88 C versus 140 C for the Cast system. The Tan delta value is again lower for the Cast system.

14 The next slide represents the DMA curve for the 90A Ether MDI systems. TPU is the solid line and Cast is the dashed line. Here we see the Tan Delta graph for the 90A Ether MDI systems. The Tg for the cast system is centered at a lower temperature at -44 C for the Cast system and -10 C for the TPU system. The Tc for the TPU system is at 80 C versus 119 C for the Cast system. Again we see a lower Tan Delta value for the Cast system.

15 The next slide represents the DMA curve for the 86A Polycaprolactone MDI systems. TPU is the solid line and Cast is the dashed line. The final graph highlights the Tan Delta for the Polycaprolactone MDI urethane system. The Tg for the cast system is narrower and is centered at a lower temperature at -30 C for the Cast system and -14 C for the TPU system. The Tc for the TPU system is at 76 C versus 123 C for the Cast system. The Tan delta value is about 0.07 for the TPU and 0.05 for the Cast system. Using the DMA data, I had our mechanical engineer run his tire model from the data in the DMA graph for the 86A Polycaprolactone system. The critical difference in the Tc and Tan Delta yielded a larger performance window for the Cast polycaprolactone system over the TPU system. Using the engineering wheel model, we predict 8 times faster speed for the same load and 5 times the load at the same speed for the Cast system.

16 Cast Urethane has a large performance advantage in dynamic systems where the material generates heat under stress. This is seen with lower Tan Delta, higher Tc, and higher melting temperature. Cast urethane should be considered for any dynamic application. TPU limitations appear to derive primarily from the lower melting temperature required for reprocessing the material. As we have looked at the performance differences between Cast and TPU, we will now look at the Polyurethane markets for clues to application usage. The world-wide market for polyurethane is over $55 Billion dollars.

17 The major polyurethane markets include construction, bedding, etc. But when we look at the TPU and Cast, we see that the TPU market is about two or three times the size of the Cast elastomer market. When we look at the major cast elastomer markets, we see all these applications including tires and wheels, rolls, agriculture, etc. that all have uses in high temperature or dynamic applications. Another advantage of Cast urethane is the flexibility to make smaller production runs for specialized applications including both very small and very large parts. When we look at the major TPU markets (besides the TPU Engineering market), we see applications where toughness and abrasion resistance is needed. The footwear and engineering markets equally make up 70% of the volume. When we consider hoses, tubing, wire, and cable we see applications where continuous processes are used to make these extrusion produced products. Many of the automotive parts are used for abrasion resistant knobs and covers that are easily mass produced with injection molding.

18 Because Cast and TPU have similar toughness and abrasion resistant properties there are other factors that differentiate these materials for their end-uses. We have discussed how cast has the advantage in higher temperature service and dynamic applications. Cast has more flexibility in production where small or large parts can easily manufactured. TPU has advantages for high volume production and the ability to recycle the TPU for repeated use. TPU is produced using extrusion or injection molding. TPU material also tends to be clear and I suspect TPU has a lower material cost once the parts are mass produced and expensive capital inputs are paid off. In review, we have looked at the chemical and production differences of TPU and Cast urethane. We evaluated both TPU and Cast systems by physical testing and DMA. We discussed the reasons why these TPU and Cast perform similarly and we also looked at why each system should be considered for different applications. We also now understand why the markets for both systems have evolved for their end use applications. As cast urethane producers, I hope you now have a better understanding of what types of applications we should concentrate on with Cast urethane. Also if you are looking to displace TPU from an application, how, why, and where you should compete.

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