IKV and RWTH show off German polyurethane

Germany’s IKV institute for plastics processing at RWTH University held its biannual plastics technology colloquium in early Spring, including showcasing a number of reinforced polyurethane developments in papers, in the colloquium exhibition area and during visits to its laboratories, David Vink went along and tells us what he found.

Polyurethane composites are growing in popularity, as they offer cost-effective alternatives to other composite matrix materials. The growing market for polyurethane composites is helping to drive innovation and there were three papers presented on new PU production technologies at the February 2016 biannual Plastics Technology Colloquium held in Aachen,

The first PU paper was presented by Hubert Ehbing of the PAT processing & application department at Covestro in Leverkusen, on behalf of Ulrich Liman, senior vice president for Covestro innovation management.

Liman pointed to newer developments such as PU/metal composite hybrids and the use of open reinforcement cores such as paper honeycombs (PHCs). He said that using open structures such as these provide further weight reduction compared with solid composite structures, along with improved acoustic and heat insulation.

PHC cores such as these are used in new Smart ForTwo car roof, produced with Hennecke machinery by FS Fehrer in a composite spray moulding (CSM) process, as well as in Crushcore composite exterior bodywork solutions which automotive supplier Magna showed at the 2014 AVK conference.

Windmill rotor blades

Covestro sees newer opportunities and challenges in the renewable energy sector: here, polyurethane composites can help reduce costs in meeting reduced cost demands through more efficient production of wind turbine rotor blade spars.

Spars of up to 80 m in length are starting to be installed, replacing earlier designs at around 15-20m. Although some carbon fibre reinforced spars have been introduced, Covestro is evaluating a PU resin in a glass fibre based vacuum-infused system. A trial in October 2015 produced a Covestro PU foam-based 45m long spar beam and cover at the DLR German Aerospace composites development centre in Stade.

A special stitch-bonded, non-crimping glass fibre reinforcement from German company Saertex, which has been optimised for resin infusion, was used in the trial on Huebers Verfahrenstechnik Maschinenbau machinery. Saertex’s SAERfoam foam core replaced a conventional balsa wood core. SAERfoam is an ultra light PU, PE or PIR based structural core material with 3D glass bridges.

Although PU has good potential to substitute epoxy resin in such spars, because the low viscosity of polyurethane resins enables them to fully infuse spars more quickly than epoxy resins, and a much faster cure time than epoxy resins, there are challenges which hamper their adoption. Ehbing said, for example: “just one blister in a part can mean a KO for the part”.

Ehbing remains optimistic however, as PU resin brings improved mechanical properties and the less brittle polyurethane matrix means that microgravity is easier to avoid than with other matrix resins in reinforcing systems.

Potentially, he said combining new infusion resins, optimising fibre layout and process modifications have potential for greater productivity with polyurethane, which could significantly reduce costs.

Window profiles

Ehbing said PU is widely used, alongside unsaturated polyester, vinyl-ester and epoxy resins, in pultruded products for niche applications in markets such as sporting goods, cars and health care. But despite this, he added, Covestro has not yet broken into the high volume window frame market with pultruded PU.

Pultruded polyurethane based reinforced composite profiles have a number properties which would make them ideally suited to replacing PVC window frames, Ehbling said.

He continued, pultruded PU with 80% continuous unidirectional glass fibre content has a coefficient of thermal expansion (CLTE) similar to glass and such composites are much stronger and stiffer than conventional rigid PVC window frames, and they can therefore be thinner than PVC frames, Ehbing pointed out.

“So why aren’t we in the large mass window frame market?” he asked, before providing the answer: “We are still too expensive, but we haven’t given up. On which button must I press to replace PVC or aluminium?” Speed is a prime factor in productivity and therefore cost, pultrusion running at up to 2m/min compared with 7m/min for PVC and up to 80m/min for aluminium profile extrusion, he said.

Ehbing said more work is needed on faster curing PU resin systems, functional integration and process automation. Profiles produced at IKV shown at the colloquium feature a compact skin in the same PU resin system as the foam core. Some had functional integration with a track to accommodate window frame seals for example, as well as integrated ribs for additional reinforcement in optimal lightweighting designs.

Hybrid pultrusion

A hybrid pultrusion process described in the paper by IKV researcher Peter Schneider, involves glass fibre reinforced PU resin as a core profile, extruded over by a thermoplastic layer that provides functional integration potential and weldability.

Schneider said IKV work aims to balance the different characteristics and processability of these two material classes.

Pultruded PU has high stiffness but poorer impact strength, relatively slow production speeds and fibres show through on the part surface. Thermoplastics, can be processed faster with a more automated process, which gives parts with higher and more consistent surface quality.

A particular restriction and challenge, which Schneider has been working to overcome, is temperature differences. Depending on the type of polymer, thermoplastic extrusion operates at peak temperatures in the range of 180-260°C. This compares with 80-180°C in PU-based pultrusion, with PU resin starting to degrade above 180°C. In production, “worst case estimation” would mean PU being exposed to a constant thermoplastic top layer temperature of 230°C for 7.5s, Schneider said.

Another challenge described by Schneider is matching the viscosities of the thermoplastic and PU joining partners, to avoid deformation of the pultruded profile, despite its PU core being cured. This aim has been achieved, by higher temperature transfer from the thermoplastic into the PU decreasing cure time for the PU, he said.

One feature of the process is use of an injection box for glass fibre roving impregnation, instead conventional tank impregnation. But really key to hybrid thermoplastic/PU pultrusion is a co-extrusion spiral mandrel die, which follows immediately after the pultrusion die.

Using this setup, pultrusion and thermoplastic processes are thermally separated before joining in the die.

Material bonding takes place with the two flow-streams under equilibrated pressure of the two materials. This bonding is achieved through a combination of the thermoplastic shrinking, as well as adhesive bonding between PU and thermoplastic. Schneider said IKV is working on increasing PU viscosity to develop a laminar stream in the die to increase interlayer bond strength, as well as looking for a chemical bonding mechanism.

This work has led the IKV to produce hybrid profile pultrusion within the Inpulse (integrated pultrusion and simultaneous extrusion) project, which started-up in October 2015. That has established that a thermoplastic top layer enables elimination of addition of a release agent to a PU matrix, resulting in improved inter-layer bonding between thermoplastic and PU layers.

IKV has made impact tests on profiles based on Elastocoat C6226/105 PU resin from BASF, co extruded with different thermoplastic layers, including a layer of Lupolen 2420 K from LyondellBasell. Schneider said PU Pultrusion alone based on a PU matrix reinforced with StarRov 907 4800 tex glass fibre rovings from Johns Manville showed characteristic fibre breakage on both profile sides in impact tests.

The IKV work found that coating of the glass reinforced PU substrate with different thermoplastic layers produced different results.

There was little surface damage and no top layer fibre breakage with a co-extruded grade of polyethylene as the top layer, but it delaminated under impact. In contrast, a top layer in the Ultramid B27E grade of polyamide 6 had less top layer surface damage and no top layer delamination in the impact tests.

The 35mm wide, 4mm thick hybrid profiles have been pultruded and coated with thermoplastic top layers at 1.0 m/min line speed, at maximum temperatures of 180°C for the PU core and respectively 205°C (PE top layer) and 260 °C for co-extruded PA top layers.

Although IKV has so far only produced straight hybrid pultruded profiles, when challenged by UTI, Schneider did not rule out future work with curved pultruded PU-based hybrid profiles with use of e.g. the Thomas Technik + Innovation radius pultrusion technology.

One-step wet moulding

In the third PU paper, IKV researcher Regina Riedel talked about one-step production of high-performance PUR-LIT sandwich components using a combination of PU spraying and the liquid or wet resin press moulding process. The sandwich parts produced consist of a lightweight PU foam core and a compact continuous fibre reinforced PU outer layer.

IKV’s one-step process firstly involves preparation of two dry preforms with vacuum support. These are sprayed and impregnated one after another with PU from a high-pressure PU equipment (KraussMaffei RIM-Star MiniDOS 8/8) via a fixed location KM MK 3.0-2K-S mixing and spraying diehead. The preforms are held firmly by vacuum to a perforated plate and brought into position under the diehead by an articulated arm robot, to be sprayed in rows.

The two prepregs formed in this resin spray prepregging (RSP) process are placed with PU foam in between into a mould for wet compression moulding. They then form continuous fibre reinforced PU facing layers on both sides of the PU when it expands to form a foamed core bonding to the facings.

Although this one-step wet pressing process stage is separate from RSP, overall production is efficient, as both process stages run parallel to each other. IKV uses a press from Mueller Weingarten (now part of Schuler Pressen). The mould from KraussMaffei includes pressure and temperature sensors and its cavities can be heated and cooled by close contour temperature control. A non-woven Spunfab PA 1300 thermoplastic fleece laminate adhesion web from Keuchel Associates in Cuyahoga Falls, Ohio binds the layers in the facings with each other.

IKV uses the one-step process in a research project aimed at PU rigid foam cored sandwich structure development. This is based on continuous fibre reinforced compact PU facing layers, in large series production for IT transport system products. The project aims at 50 vol% fibre content in the facings, corresponding with overall 100 kg/m³ volume weight applying to established structural sandwich elements based on Evonik’s Rohacell polymethacrylimide (PMI) foam.

Riedl said work with 38 vol% fibre content during impregnation followed by wet pressing at 10-25 bar pressure resulted in good laminate quality, with good glass fibre impregnation and low pore levels. A 10 mm thick sandwich analysis shows foam pores penetrating up to 50% into the facings, due to pressure from the core foaming reaction. She believes this effect could be eliminated by venting the mould cavity, so that foaming can proceed without counter pressure until the mould cavity is filled.

Mould temperature of 70°C is around 15°C lower than recommended by Ruhl Puromer when using its two commercially available Puropreg (facing) and Purotherm (core) PU materials. Riedel says this delays the crosslinking reaction of the first prepreg, so it has not advanced significantly by the time the second prepreg contacts the hot tool surface during mould closure. Longer cycle time due to lower temperature can be significantly reduced in future with automatic handling systems, cutting time between insertion of each prepreg in the mould, Riedel suggested.

Riedel say the process has potential to produce sandwich structures in series production without conventional, expensive pre-machined or pre-moulded foamed cores. Part density of 97.4 kg/m³ was obtained with 38 vol% fibre content in the facings. Further work is required to raise facing fibre content to 50 vol% in order to more fully exploit lightweighting potential, reaching the 100 kg/m³ part density target.

IKV will conduct benchmark tests on process economics and mechanical properties with conventionally made sandwich sheets with honeycomb and rigid foam cores. A demonstartor in the form of a suitcase shell should demonstrate potential use of the new process in more complex three dimensional shapes. Aside from KraussMaffei and Ruhl Puromer, other partners in the one step process development are PU spraying specialist Greiner Perfoam and PU parts processor Parat.

On display

IKV also displayed several other PU processes during the colloquium. For example, production of lightweight PU foam parts by inclusion of carbon dioxide in PU to form mould with counter-pressure of up to 22 bar, a process said to be the first ever using CO₂ alone for lightweight PU foam. Densities of 61 kg/m³ and 116 kg/m³ have been achieved in respectively flexible and rigid PU foam with respectively up to 10 wt% and 15 wt% CO₂ content.

Examples were shown of continuous rigid glass and carbon fibre, as well as flexible aramide fibre reinforced PU ducts produced in the PIT-S-RIM projectile-assisted reaction injection moulding process.

The aramid fibre version included flanges injection moulded onto the duct. Automotive media ducts made this way can adapt to e.g. engine movement by combining flexibility with strength and a smooth tight dimensional tolerance interior surface, as well as reducing the number of components needed for assembly.

PU producers Covestro and Ruhl Puromer and braided hose producer Siltex are among PIT S RIM process material development partners. Other partners include fluid assist specialist PME fluidtec, PU parts producer Unnapur Kunststofftechnik, PU machinery producer Frimo, toolmaker Dohlen & Krott and automotive OEM Audi.