Plastic transformations

Every three years, Düsseldorf becomes the centre of the world for the plastics industry.

The K Show, a major forum for plastics and rubber manufacturers, sees all the major players presenting their latest products and technologies. K2004, in October, promises to be even larger than its predecessor, and the German majors BASF and Bayer were quick off the mark in presenting their latest innovations.

Although the companies normally focus on the products themselves, the processes used to make them are becoming increasingly important as the links between manufacturing method and properties become clearer.

For example, materials composed of, or reinforced by, fibres exhibit anisotropy - different properties in different directions - if they are formed using a process which orients the fibres. Affected properties can include stiffness, tensile strength and resistance to heat, and when these properties are not uniform, it can be difficult to design moulded parts which will be subject to stress or heat.

BASF's researchers have gone back to the process to solve this problem, developing a system called integrative simulation. This uses different simulation techniques to understand what is happening to the material throughout moulding.

In the first step, the team uses a widely-available simulation package called MOLDFLOW to determine the orientation of the fibres within the part. The software takes into account the viscosity of the molten polymer, the fibre content, injection speed and pressure to simulate the way the material fills up the mould.

The second stage uses a new piece of proprietary software called FIBER. This takes the information from the MOLDFLOW package and combines it with data on the mechanical properties of the fibres and the resin matrix to form a non-linear, anistropic model of the mechanical properties of the part.

The third and final step, a structural analysis of the part, once again uses commercial software, with the finite-element packages LS-DYNA or ABAQUS.

The system earned its stripes through an analysis of a torsion beam made from a nylon/metal composite, explains engineering plastics development engineer Stefan Glaser. The standard simulations always underestimate the stiffness of the beam, he says; this is because they cannot take into account the effect of the reinforcement orientation.

'By contrast,' he says, 'the integrative approach results in a more accurate prediction of the part's flexural response. Simulated and experimental results agree very closely, right up to the failure of the part.'

Another process-related problem occurs in fibre spinning, and is caused by pigment particles. 'In melt spinning, it would be fine if all that pigments did was impart colour,' explains Clemens Karg, head of performance chemicals for plastics.

'Unfortunately, depending on their particle size and structure, and their dispersability, they can also adversely affect the spinning process.'

The most common problems are blocked filters and fibre breaks, which can cause long downtimes and high scrap rates.

BASF's solution was to assist in developing a standard method for measuring the dispersability of a pigment, helping processors decide which pigments are suitable for their processes and which will gum up the works.

'The method determines a characteristic filter-pressure value (FPV), the ratio of pressure increase during extrusion of a coloured polymer melt and the amount of pigment present,' Korg explains. 'Pigments with FPVs of less than 2 are suitable for melt spinning, but in general, the smaller the FPV is, the better.' The method has recently been published as a European standard, and BASF is now producing a range of pigments with a guaranteed FPV of 2 or less.

Bayer MaterialScience, meanwhile, is undergoing transformations of its own. The split of its lower-margin businesses into Lanxess has led to the company looking for new ways to squeeze extra margins out of its remaining businesses, and process improvements are high on the list of priorities for achieving this.

For example, the company's new plant for the polyurethane intermediate TDA (toluene diamine), which opened in Dormagen last December, uses a new high-performance catalyst to maximise yield.

Currently the largest TDA plant in the world, with a capacity of 200,000tpa, the plant also concentrates and recycles the sulphuric acid it uses in addition to nitric acid as part of the nitration process.

Another new plant, the HDI (hexamethylene diisocyanate) facility at Leverkusen, is designed to consume less energy than comparable processes. 'Not only is this an ecological benefit, it is also a significant competitive advantage,' comments Bayer MaterialScience chairman Hagen Noerenberg.

Taking a step upstream, the chlorine used in the manufacture of Bayer's most important products, polyurethane and polycarbonate, is being produced by a new energy-saving process. Known as oxygen depolarised cathode (ODC) technology, the process suppresses the generation of hydrogen by incorporating a fuel cell into the electrolysis equipment.

This allows oxygen to be fed into the process, so water rather than hydrogen forms on the cathode of the electrolysis cell. As a result, the electrolysis can be operated at a lower voltage but the same current density as a conventional process, reducing the energy consumption by 30%. A plant using ODC is currently operating at Bayer's Brunsbüttel site.

Lanxess also has process technologies on its mind, though working in more mature markets, the emphasis is slightly different. 'Basic research and the development of products and applications will be more the exception than the rule,' comments Ulrich Koemm, who will sit on the board of the new company.

'Simplifying and improving processes will be a high priority for Lanxess. It also means we will close plants that are unsatisfactory in terms of quality or costs, in order to increase our competitiveness with modern, cost-effective processes that achieve optimum levels of quality.'

One such improvement is in synthetic rubber, almost definitively a mature market. Lanxess has developed a new grade of butadiene rubber, which is a major component of the tread section in tyres.

A multifunctional coupling agent to crosslink the polymer chains reduces the rolling resistance of the material but increases its abrasion resistance.

According to Hubert Fink, head of semi-crystalline materials, tyres incorporating this new material in their treads can cover 13% more distance than a conventional tyre, and last on average five months longer.