Thermoplastic Polyurethanes for Instrument Panel Skins 900730
This paper will discuss the development of a Thermoplastic Polyurethane (TPU) product being evaluated for instrument panel applications. Current development includes skins produced via both slush molding and flame spraying using a powder product and sheet vacuum forming. The skins can then be further processed via back foaming (with polyurethane foam).
Current plans are to supply these products fully compounded to match the interior color requirements. Each grade is formulated to meet specific color and/or exposure requirements.
TPU powder products offer the following advantages over current PVC slush molding products:
better high temperature stability
minimal fogging due to plasticizer content (TPU does not contain plasticizer)
TPU does not discolor from contact with polyurethane foam (this is a problem specific to PVC)
TPU is a tougher product (better tear and puncture resistance) and requires a thinner skin: 0.6 mm for TPU vs. 1.3 to 2.5 mm for PVC skins
TPU exhibits better color stability during processing and are thus readily processible with negligible change in color
TPU produces a richer feeling, more leather-like skin compared to PVC
TPU has less of a tendency to decompose at high processing temperatures, thus greatly reducing mold cleaning requirements and corrosion on mold surface
TPU powder offers a new possibility to the automotive industry in their push to develop a higher quality product that will last 5 - 10 years without significantly deteriorating. TPU powder products offer a very good alternative in cost/performance to those products currently available to the automotive industry.
Today both flexible PVC and PVC/ABS alloys are used to manufacture Instrument panel skins. Two manufacturing processes are currently being used: thermoforming from film/sheet stock and sintering or slush molding using PVC powder or plastisol. Both processes have been developed to match the high volume demand.
Due to the increase in windshield size and streamlined angles in today's car designs, the auto interior temperature has been steadily increasing. This has created several problems for the PVC powder products currently being used. The following is a list of the major drawbacks of PVC in today's and tomorrow's care design:
DISCOLORATION DUE TO FOAM CONTACT
DISCOLORATION DUE TO HIGHER TEMPERATURES
DISTORTION AND GRAIN LOSS
EXCESSIVE WINDOW FOGGING
LOSS OF FLEXIBILITY (PLASTICIZER LOSS)
Because of these problems a search was initiated to find a material to replace PVC. With this, polyurethane thermoplastics (TPU's) were taken into consideration. TPU products offer the following advantages:
HIGH TEAR STRENGTH
GOOD PUNCTURE RESISTANCE
THINNER SKINS POSSIBLE
PERMENANT/LONG TERM FLEXIBILITY (NO PLASTICIZER REQUIRED)
HIGHER TEMPERATURE RESISTANCE
TPU's are block copolymers made from a polyester or polyether diol, as the soft phase, and an isocyanate and short chain diol (i.e. butanediol), as the hard phase. Figure 1 shows a schematic of the polymer structure. The flexibility or stiffness of the product can be varied by changing the relative concentration of the hard and soft segments. This property, as measured via tensile modulus, can be varied between 6-7 N/mm2 (870-1000 psi) and 600 N/mm2 (87x103 psi). This is shown graphically in Figure 2.
For applications involving interior automotive parts polyether type TPU's are preferred. In particular those based on polytetrahydrofurane. This is due to its inherent hydrolysis and fungus resistance. Both of which are critical characteristics for this type of application. Polyester type TPU's on the other hand are lacking in both of these key properties.