Polyurethane composites: Technology Overview

Polyurethanes have been a minor player in the composite world. Unsaturated polyesters (UP) have the lion’s share among thermosets. But the outlook for polyurethane is very positive. There are sizable opportunities opening up as increased levels of concern over styrene emissions from unsaturated polyester and vinyl ester systems coincide with the need for fast, automated and consistent production quality.

Historically the material cost of polyurethanes was an issue, but this is now often offset by the increased productivity and/or possibility of slimming down moulded parts thanks to the superior toughness of PU composites compared to their rival technologies.

The versatility of polyurethane chemistry and innovations in fabrication machinery technologies are also combining into a wide array of solutions for composite manufacturers; from low volume serial production to massive production runs.

Physical properties such as tensile strength vary greatly with formulations and combinations of fibre reinforcement systems. Products with relatively lower strength produced using chopped-fibre-reinforced polyurethanes, to truly structural parts using continuous fibre reinforcements are possible.

Amenable chemistry makes it relatively easy to make polyurethane composites reinforced with chopped fibres for a growing list of applications

There are two main fabrication processes for chopped-fibre-reinforced polyurethane composites: long-fibre injection moulding (LFI) and fibre composite spray.

Both rely on the specific reactivity of PU chemistry. The LFI method involves cutting the fibre to length and then wetting with the PU reaction mixture. This is done using a special mixing head, and discharging over an open mould.

The mould then closes and the part is shaped and consolidated under pressure. Microscopic inspection of the composite material shows a morphology characterised by bundles of fibres oriented in random directions in the plane of the part. (see Fig. 1)

It is easy to tune the mechanical properties of the composite by selection of the polyurethane formulation, the type, amount and length of chopped fibre, and, possibly, the use of mineral fillers. The broad range of attainable properties (Table 1) is shown by mineral-filled PU glass-fibres LFI composites which can have a flexural modulus of 14 GPa. This is approaching typical value of polyester SMC.

The LFI fabrication process provides inherent versatility in terms of thickness variation and can be used with integrated stiffening ribs, enabling proper part design for high loading applications. An example of this is in manhole covers (Fig. 2), which are produced up to a thickness of 100 mm to meet load classes up to 400 kN.

For lightweight, non-structural, applications a foamed polyurethane matrix and/or the use of a spacer (i.e. paper honeycomb) can be used advantageously.

For applications requiring a more decorative surface the fibre-reinforced polyurethane layer may be coupled with a thermoformed plastic skin using film-moulding technology or with in-mould coating (IMC) processes. Both approaches can be used to eliminate costly and time-consuming secondary painting operations.

Thermoformed co-extruded skins with an acrylic top layer provide high-gloss Class-A surface with excellent weatherability. For less demanding surface quality applications, the IMC technology such as PU paints, polyurea coating and urethane acrylate gel coats can be used to produce a good finish part with a single mould tooling.

When a thin IMC is used, the application of a solid PU barrier layer is in general recommended between the in-mould paint and the LFI layer, in order to prevent the glass fibres from spoiling the smoothness of the paint surface. (Fig. 3)

The LFI technology is currently used for tractor bonnets and body panels for industrial vehicle; hulls and decks of personal water craft; spa panels, and for architectural or building products like door skins and decking.

The fabrication process typically uses a shuttle mould carrier, is highly automated and allows good control of processing conditions and reduced workforce.

The cycle time depends on the types of surface enhancement layers, the sizes of the parts and the chemical characteristics of the components. New-generation fast-cure PU LFI enable a demould time as low as 2 minutes, making the process suitable for volumes up to roughly 45 000 parts/year, with one tool operating on a two shift-basis. Compared with conventional polyester sheet moulding compound, PU LFI can be processed at far lower mould temperatures and clamping forces. This gives lower energy and investment costs, particularly for large parts.

Fibre composite spray (FCS) products are produced by blowing chopped fibres and added into the PU spray jet exiting the mixing head. The process of simply spraying onto a form liner (without the need of the two mould halves and a clamping unit) is very cost-effective for small series and/or large parts.

Material mechanical properties are definitely lower than with the LFI process, because the process only works with relatively short fibres and because there is no consolidation under pressure.

Part design can compensate to some extent for lower mechanical properties through multi-layered structures. Different layers may have different levels and types of fibre reinforcement for example. Lightweight multi-layered sandwich structures consisting of reinforced layers and expanded core layers provide adequate strength for many self-supporting non-structural applications. Similar to PU LFI, the FCS process can be coupled with film-moulding technology or in mould coating eliminating costly and time consuming secondary painting operations. (Fig. 4)

Structural-RIM for industrial and automotive applications

Moulded composite parts with continuous fibre reinforcement can be produced with Structural Reaction Injection Moulding (S-RIM). The process involves placing a pre-formed reinforcement in the mould followed by the injection of neat resin. The process enables the production of structural parts characterised by an excellent strength-to-weight ratio and, when desired, highly orthotropic mechanical properties. Among recent applications with glass fibres is worth again mentioning man-hole covers.

Carbon-fibre structural composites are today of high interest to the automotive industry. Such lightweight composites enable the reduction of CO2 emissions and this is driving an unprecedented development rush in innovating fabrication technology and formulation chemistry, both for PU and epoxy.

Several investigations have tried to compare carbon fibre composites made with PU and epoxy, but it is not always straightforward considering the many processing variables and the evolution of formulation chemistries. In general, PU is advantaged in cycle time (demould time as short as 60 seconds) and polymer toughness ; moreover polyurethane compounds enable lower mould temperatures. On the other end, the inherent latent reactivity of epoxy may help the impregnation quality of highly complex parts.

Some automakers have made significant investments in high-pressure epoxy RTM. Therefore, it is most likely the first generation of large series lightweight vehicles will be built using epoxy-carbon fibre composite structural members.

The technology development rush in polyurethane composites is however continuing and considering the performance potential and the economic lever it is believed that carbon-fibre-PU composites will find significant inroads into this industry, where it will provide, a complementary offering to epoxy-carbon-fibre composites, cost-effective solutions for massive production of lightweight vehicles.

Growing demand for Pultruded Polyurethane

Polyurethanes are also making significant inroads in pultrusion and filament winding.

In pultrusion the toughness and strength of PU polymers enables simplification of the fibre lay-out and it may be possible to reduce or eliminate transverse reinforcing fibres. Pultrusion is a continuous process in which impregnated fibre reinforcements are pulled and consolidated through a heated die.

Typical reinforcements are roving and continuous strand mat (CSM). Typically for polyester, vinyl ester, or epoxy pultrusion, the fibre reinforcement consists of layers of roving and CSM. The random nature of the CSM helps give strength in the transverse direction.

Polyurethane resins are tougher so it is possible in some applications to run polyurethane systems exclusively with roving.

One specific example are window profiles which are produced with polyurethane resins using a direct roving configuration. Avoiding use of mats helps increase processing speed particularly for profiles with complex cross-sections.

Polyurethane’s toughness also helps secondary operations. This is because the profiles can accept fasteners without splitting and can be firmly screwed.

More generally, the physical properties of polyurethane  allow the thickness of the profile to be slimmed-down. This gives significant weight reduction for a given specified set of mechanical properties . Some applications, such as sheet-piling for erosion protection, additionally benefit of the better water resistance of PU as compared with unsaturated polyesters.

A closed-impregnation chamber is rather common for PU pultrusion. This is a process where high injection pressure helps to achieve excellent fibre impregnation. Atmospheric wetting devices can be used as well. According to a particular design, fibre tows enter through a backing plate into a first impregnation chamber and subsequently into a second one of decreasing cross section in order to build hydraulic pressure before the entrance in the curing die.

Processing speeds with PU compares well with other resins, typically ranging from 0.5 to 2 m/min depending on the complexity of feeding the reinforcement.

Filament winding is the other process using direct roving. This exploits polyurethanes’ toughness. Today this process is used with polyurethane for one application: the manufacture of high-duty utility poles. In this area, formulation chemistry and impregnation fabrication technology is evolving to minimise the influence of conditions in the working environment conditions on the product consistency.

Interest in PU filament winding is rapidly rising in several geographies, possibly leading in the next future to sizable business opportunities.

About the lead author:

Luigi Bertucelli is Research Leader for Dow Formulated Systems and is located in Correggio (Italy). He joined Dow in 1983 with the acquisition of Corradini Poliuretani. He has been working mainly in the developments of polyurethane rigid foams and composites. Luigi holds a degree in Industrial Chemistry from the University of Parma (Italy, 1978).