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.
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.