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A number of industrial projects are ongoing to optimise automotive parts, notably for Class A parts. In many cases, the projects involve optimising formulations as well. Replacing certain formulation components is not always recognised as the obvious way to optimise, however. We have been developing processing additives for all kinds of applications, designing the additives to have a significant influence on performance.
(Published on August-September 2005 – JEC Magazine #19)
BY GERARD REESTMAN, TECHNICAL SERVICE MANAGER, CLOSED MOULD, BYK-CHEMIE GMBH
Over the years we have developed a number of additives that are used in the compound sector. Some of these, such as wetting and dispersing (W/D) additives, are now a staple ingredient of (Class A) LP SMC formulations. Our success has been based on the ability to create formulations with varying quantities of filler, finer filler granulometry, constant impregnation viscosity and a more Newtonian flow of the paste on the SMC line. W/D additives also have a positive influence on the flow behaviour in the mould.
Processing additives are one of our more recent developments, starting with our BYK®-P 9050 for headlamps. Products moulded with standard BMC/DMC formulations and used at high temperatures will cause emission or sublimation of several components. Some of the evaporated material will condense on a cool surface and cause a milky layer, called fogging, which is unacceptable for headlamps, for example, because it reduces the light yield over time. In low-shrink formulations, non-homogeneity is a well-known but undesirable phenomenon after moulding pigmented low-shrink SMC parts. Whereas anti-separation additives prevent the phaseseparation of polystyrene or polymethyl-methacrylate from the unsaturated polyester resin, effects like polarity, loading and flowrelated phenomena still occur even when using an anti-separation additive. These effects are not always visible in the compound, but they are usually very noticeable in the final part.
In the development of BYK®-P 9060, the objective was to have very little or no influence on the thickening, and no influence at all on the cure properties. Apart from improving flow, the new processing additive also had to have excellent mould-release properties to enable the moulder to release the parts from the tool without resorting to the use of stearates, which are the main cause for many of the defects in pigmented SMC. We focused on compression- moulded SMC, of course, but also on injection-mouldable BMC, with the advantage of much better flow. We were able to greatly improve the stability of amongst others grey formulations, which are often used in the electrical industry. The result is very homogeneous pigmented parts.
Although BYK®-P 9060 considerably improved the appearance of LS parts, it was ineffective for LP or Class A parts, since it does not eliminate defects like dull, non-homogeneous surfaces that are no longer acceptable for Class A. We initiated a development programme and were able to establish that (LP) Class A formulations are very sensitive to all kinds of ingredients. The challenge is there!
We started to work on several well-known Class A formulations to check the compatibility of the resin systems with our ingredients. Combining all the results from partners and also internally, we created the new processing additive for LP Class A SMC: BYK®- P 9080.
Some of the results were very interesting, and could constitute a break-through for the industry. We observed direct improvements, such as a better and more homogeneous flow of the compound with good mould-release properties; lower emission for interior parts; improved wetting of surfaces for painting and bonding; better paint adhesion; and less sensitivity to size and placement of the cut SMC in the mould.
To highlight some results: - emission Low emission and odour are important issues when considering the use of SMC parts inside a car. All materials must be stable and contribute neither to emissions nor smell. The industry, resin producers, and other raw material producers are working with compounders and OEMs to lower emissions. Any formulation component that contributes to emissions will be phased out. Our investigations of several formulations showed that using the new processing additive helped to reduce emission and did not contribute to odours.
- surface wetting, painting and bonding Wetting depends primarily upon the surface tension of the liquid and the critical surface tension of the surface to be wetted. As a general rule, the surface tension of the liquid must be lower than or at least equal to the surface tension of the substrate. Improper wetting is to be expected when the surface tension of the liquid is higher than the surface tension of the substrate. Contact angle measurements can be illustrative in demonstrating the effect. Equilibrium geometry of the liquid drop is indicative of the interactions between surface and liquid. Measured contact angles can be used in conjunction with theories published in scientific literature to determine the surface free energy of a solid and characterise solid surfaces regarding wet ability or adhesion properties.
We checked several formulations and found in every case that, whenever we used the new processing additive, the contact angle was reduced and surface wettability was increased.
Bonding tests and paint tests also showed real improvements. On samples prepared with a urethane-based adhesive, we checked the tensile strength in accordance with EN 1465 (10 mm/min) at different temperatures and performed the BMW test 308.1:
Painting is particularly important for Class A parts. In many cases, IMC is also used to prevent paint defects. The processing additive’s chemistry contributed to improve IMC distribution and bonding. Paint studies were performed in several industrial programmes. We checked the adhesion with a Gravelometer and also got very good results.
- moulding experiments The best way to test this new additive is to compare two different SMC formulations produced in the same conditions and using the same moulding technique. One is a standard formulation and the second, a formulation with the additive, but no extra mould-release agents. Evaluating the mouldings with Diffracto, for example, will show the effect.
In creating the BYK®-P 9080, we knew there would be a big influence on the SMC flow and appearance of the moulded part. We performed mouldings in a 30x40-cm plate mould on a press without parallelism. All moulding parameters must be adjusted to avoid difficulties with the SMC’s uniform flow and back-flow. Otherwise, the back-flow will create “mountains” in the Diffracto tests. We checked several different sizes of SMC, cut to cover 10%, 20%, 40%, or 80% of the mould. All packages were placed in the centre of the mould and moulded under the same conditions. Pictures were taken from the screen when the Diffracto was set up for the long-term waviness: it may be seen clearly that the standard SMC formulation is very sensitive as regards the cut size, with a direct effect on the short- and long-term waviness.
The newly developed group of processing additives provides new opportunities for formulators. The use of processing additives can help to eliminate many types of processing problems. In many applications, changing the processing characteristics can open up windows of opportunity, and improved processing always leads to cost reduction.