An evaluation of cast PVC processing conditions for improved adhesion to polyurethane foam for automotive applications
By Robert Bondar
Introduction
Although the process of laminating cast PVC cover skins over polyurethane foam is well known, it is becoming increasingly clear that failure to account for the skin colour, the roughness of the skin at a nano level and the skin casting temperature can all lead to significant variability in the overall performance of the finished composition.
This article will explain why these parameters are important when trying to create a superior PVC/PU composite, and why the widely used quality control measurements of dyne value and contact angle of cast PVC skins are not good guides to performance in use.
Overview
Cast PVC cover skin is often used for higher quality automotive interior trim, and is frequently used in combination with polyurethane (PU) foam in the production of interior components such as door and instrument panels.
Good adhesion of the PVC skin to the PU foam is critical for good part performance and service life. However, tests of adhesion between PU foam and cast PVC used in the automotive sector have shown wide ranges of results.
At the bottom end of the range, we see undesirably low ply-adhesion forces of less than 50 N/m (0.3 lbf/in). Composites with this behaviour exhibit complete adhesive failure, and are typified by samples where there is no foam adhesion to the PVC skin.
Desirable higher ply-adhesion values are typically greater than 175 N/m (1 lbf/in), and are found in samples that fail the tests by virtually complete cohesive foam tear.
Despite the advancements that have been made in PU foam and vinyl formulations during the past few decades, wide variation in adhesion between foam and cast PVC is still frequently experienced. Commonly, this wide variation in ply-adhesion and adhesive foam failure has been attributed to chemicals in the PU foam or plasticiser migration from the PVC.
To investigate this, we designed experiments to examine PVC casting process variables, along with PU foam moulding, to find the best combination of conditions necessary for improved adhesion between typical PVC skins and PU foam.
Cast skin samples were prepared at the PolyOne laboratory in North Baltimore, Ohio using the standard lab cast skin process. Samples were fused on an electroformed nickel mould with hot air heating to reach the targeted cast temperature, which was confirmed with a surface probe.
Powder mould dwell time was adjusted based on surface temperature to maintain a target skin thickness of 1.0-1.2 mm. Skin samples were demoulded at 45°C following a standard cooling cycle.
Foaming was conducted using high-pressure foam machine mixing in the BASF Urethanes Research Lab in Wyandotte, Michigan. Foamed parts were prepared using a small rectangular mould to simulate a longer foam flow-path similar to that which foam would experience in a closed mould.
Foamed samples were evaluated for foam-to-skin ply-adhesion using standard test methods, 180° peel at 300 mm/min. Typical semi-flexible PU foams were used in these studies.
The experiments were carried out in two groups. The first were designed to discover the effects of mould release, age of PVC resin and casting temperatures (230° and 250°C). These experiments were designated DOE 1.
The second group of experiments, designated DOE 2, were carried out to assess the effects of different moulds, one with turf grain. This had a thicker shell and was slower to heat up and cool down than the other, which had a fawn grain. The second mould featured a thinner shell, and had less thermal inertia, PVC powder colour (tan and black) and three casting temperatures (220°, 230°, and 240°C).
To provide a comparison, samples of an available thermoformable PVC foil from Benecke-Kaliko (BK) were also foamed.
Results and discussion
The initial PVC powder casting experiment, DOE 1, was conducted with the turf-grained laboratory cast skin tool.
After casting, the skins were foam-moulded and cured for five days, before measuring skin-to-foam ply adhesion. Although significant scatter was observed in the foam-to-skin ply-adhesion strength data, it was clear that higher casting temperatures resulted in lower overall adhesion.
Generally, skins cast at the lower casting temperature showed a low incidence of adhesive foam failure and had very good adhesion between the foam and the PVC cast skin.
In our control experiment with PU foam bonded to the BK skin, we observed a complete absence of adhesive foam failure.
While both the cooler cast PVC and BK foil provided excellent adhesion to PU foam, the observed differences prompted further examination. We decided to examine several skin samples both by light microscopy, and ultimately by atomic force microscopy (AFM).
Light microscopy clearly showed that the extruded PVC had a much rougher back surface than a cast PVC skin. AFM confirmed this. It showed that cast skin had an average roughness estimate of ~54 nm for the cast skin sample; the BK skin had an average roughness estimate of ~530 nm (See fig. 1). This was the first indication that surface micro-roughness might contribute to foam adhesion.
In the second PVC powder casting experiment (DOE 2), as with DOE 1, after the skins were cast they were foamed in a small rectangular mould, and cured for five days prior to measuring skin-to-foam ply adhesion. Although there was variability in the results, it became clear that three variables contributed to variations in foam-to-skin adhesion.
We found that cooler PVC casting temperatures produce composites with the best adhesion values, and the lowest levels of adhesive failure.
At the same time, we found that composites produced with PVC cast using the fawn-grained tool – which had a shorter heating cycle than the turf-grained tool – had better overall performance.
It is possible that this is because the turf-grained tool’s longer heating cycle exposes PVC powder to more heat and higher relative temperatures. Samples produced with PVC cast on the fawn-grained tool had higher average ply-adhesion values and less adhesive failure.
We also found that composites produced with black PVC skins exhibited less adhesive failure than those made using tan skins under equivalent processing conditions. This could be due to the pigment package in the PVC. The black skins contain only perhaps 1-2% of pigmenting mixture, whereas the tan skins may contain 5-8% of pigmenting mixture.
A comparison of adhesion failure results for the fawn-grained test samples is shown in fig. 2.
Additionally, attempts were made to better understand how cast PVC skins could be quickly quality control checked after casting to ensure that they could be used to produce composites with good foam adhesion.
During the initial experimentation (DOE 1), the surface tension of the test skins was evaluated with dyne liquids from Accudyne, in 2 dyne increments.
All samples exhibited about the same dyne value (~32 dyne/cm), whether skins have good or poor foam adhesion. Therefore, dyne liquids do not give a good indication of skin-to-PU foam adhesion.
It was also suggested that contact angle measurement might provide more definitive results. To investigate this, the last set of PolyOne test skins were evaluated using a Kruss DSA 100 analyser. The skin samples were tested with both deionised water and ethylene glycol, which is less polar.
Statistical comparison of the different sample responses to a fawn grain reference sample, which was one of the best-case performers in the adhesion studies, indicated that the samples exhibit statistically similar contact angles when evaluated with deionised water. ( = 0.05).
More importantly, when comparing the differences in the average contact angle measurements to their foam ply-adhesion responses, we gained no useful information. Contact angle measurements were also made using ethylene glycol. Similarly, no useful information was acquired.
Finally, in another attempt to understand how changing the PVC casting conditions could alter surface phenomena, samples of these special test skins were examined by atomic force microscopy. This showed that the variability in sample adhesion is associated with material which exudes on to the skins’ back surface from the bulk of the material contained in the foil. This is shown in the images in fig. 3 and fig. 4.
Conclusions
Experimental data indicate that cast skin powders should be processed under the supplier’s recommended casting conditions. Exceeding the recommended skin casting temperature, while giving greater throughput, is likely to result in a much higher incidence of foam-to-skin adhesive failure.
Processors should take the PVC powder’s colour into account when they are evaluating casting process conditions and subsequent adhesion to PU foam to ensure acceptable overall performance. Colour is an important variable, because we found that, depending on the colour, material exudes from the film onto the back which comes into contact the foam.
Foils in some colours, such as tan, seem to exude more than other colours such as black. This may explain the differences in adhesion between skins of different colours processed under the same conditions. Additionally, the surface roughness of the film is an important contributor to adhesion between the film and the foam.
Neither dyne value, nor contact angle measurement of cast PVC skins, appear to provide any benefit as a quality control tool after casting.
Ultimately, casting process conditions must be developed and verified to provide for proper adhesion to the selected PU foam for interior trim applications, and to fulfil customer overall production part requirements.
About the author
Robert Bondar is a Development Leader at BASF Corporation in Poly-urethanes Development, in Wyandotte, MI. Bob is a Chemical Engineer and has been working in both the automotive and chemical Industries for more than 35 years. Bob has co-authored two U.S patents and multiple articles for preparation of automotive interior trim.
This is an edited version of a paper which was presented at UTECH Las Americas, Mexico City, 4-6 April 2017.
