THERMOPLASTIC COMPOSITES EXPLAINED

Thermoplastic composites are composites that use a thermoplastic polymer as a matrix. These composites can be reinforced with glass, carbon, aramid or metal fibres. A thermoplastic polymer is a long chain polymer that can be either amorphous in structure or semi-crystalline. These polymers are long chain, medium to high molecular weight materials, whose general properties are those of toughness, resistance to chemical attack and recyclability.

Thermoplastic polymers used in thermoplastic composites can be divided into two classes, high temperature thermoplastics and the engineering thermoplastics. The classification is based on the maximum service temperature of the polymers, which in turn is based on the Glass Transition (Tg) temperature. This is the temperature at which the amorphous portion of the polymer changes from a glassy to a rubbery phase on heating. Thermoset polymers may not usefully carry mechanic loads above Tg, but semi-crystalline thermoplastic polymers may carry load above Tg, as only the amorphous phase of the polymer has become rubbery. The crystalline portion of the polymer remains solid until the melt temperature, Tm.

Table 1 shows the most commonly used high temperature thermoplastic polymers for thermoplastic composites.

Table 1 - High Temperature Thermoplastics

Table 2 shows the engineering thermoplastic polymers used in composites.

Table 2 - Engineering Thermoplastics

THERMOPLASTICS VS THERMOSET MATRICES
Thermoset polymers are the matrix of choice for most structural composite materials. The single biggest advantage of thermoset polymers is that they have a very low viscosity and can thus be introduced into fibres at low pressures. Impregnation of the fibres is followed by chemical curing to give a solid structure, which can usually be carried out isothermally. An advantage of thermoplastics is that the moulding can be carried out non-isothermally, i.e. a hot melt into a cold mould, in order to achieve fast cycle times. However, polymerised thermoplastics tend to have melt viscosities between 500 and 1000 times that of thermosets, which necessitates higher pressures, causes processing difficulties and adds expense.

Thermoplastic composite polymers can, however, be readily recycled, an increasingly important issue in many markets, but especially in the automotive sector. For instance, an advanced thermoplastic composite component can be chopped to pellet-size and injection-moulded to yield long-fibre reinforced mouldings, which can in turn be recycled at the end of their life. Thermoset composite materials, on the other hand, can only be ground and used as filler, a process which decreases the value of the composite enormously.

Another advantage of thermoplastic composites are their superior impact and damage resistance properties. Over 90% of polymers used in composites are thermosets, with thermoplastic composites still a niche market, mainly due to the difficulties in processing.

There are also significant environmental issues associated with thermoset processing, as a chemical reaction is necessary to form the solid structure of the polymer. Approximately 65% of thermoset matrices used in structural composites are unsaturated polyesters. Environmental regulations regarding the styrene emissions of unsaturated polyester are affecting the total cost associated with using them. Because of this, many people are willing to consider a substitute for unsaturated polyester at a higher price.

Thermoplastic composites to date have needed high processing pressures, and hence expensive product tooling, as well as significant energy input in heating and cooling the tooling. These disadvantages have in many areas outweighed the advantages of these materials such as ease of recycling and high toughness, and limited their applications. On the other hand, thermoset composites are easier to process, requiring less energy and pressure, but are inherently brittle and cannot be usefully recycled.

A new composites processing technology, known as liquid monomer processing, has been developed. The advantage of the liquid monomer thermoplastics is that they combine the processing advantages of thermoset materials with the mechanical, durability and environmental advantages of thermoplastic polymers. In particular, one of the liquid monomer materials (Cyclics PBT) can be processed isothermally: injected, polymerised, crystallised and de-moulded at the same temperature, but yielding a thermoplastic polymer. The cycle time is therefore only limited by the injection, polymerisation and crystallisation time of the material itself.

COMMINGLED THERMOPLASTIC COMPOSITES
The alternative method of processing thermoplastic composites is to introduce the polymer in solid form to the fibres in such a manner that an intimate mixing of the two parts of the composite is achieved, normally in either fibre or powder form. In the fibre commingling process, the reinforcing fibres and the polymer fibres are blended in as fine a manner as possible. The subsequent "hybrid-yarn" is processed into fabrics or other textile forms, and impregnation is achieved by application of sufficient heat and pressure to cause the polymer to flow the short distances around the fibres. Cooling of the impregnated material results in a solid thermoplastic composite.

The main advantages of the commingling route are that the textile preform is now quite drapeable over complex shapes, and is significantly lower in cost than the pre-impregnated tapes. Disadvantages can include higher pressures and longer times to process because of the extra infiltration/consolidation process. Problems can also be associated with excessive fibre movement as the commingled yarns undergo much debulking during the melting process.

TWINTEX™ GLASS-FIBRE REINFORCED POLYPROPYLENE

Twintex™ is a trade name of St Gobain Vetrotex and refers to a commingled fabric of glass and polypropylene fibres. The fibre volume fraction of this material is around 20-25% and the material is processed under vacuum or in a press at around 190°C. The low melt viscosity of the polypropylene at this temperature makes it relatively easy to process under vacuum conditions.

SHEET-FORMING OF THERMOPLASTIC COMPOSITES

Sheet-forming of thermoplastic composites is a process much like sheet-forming of metals or plastics, where a solid composite laminate is heated above its melt temperature and rapidly formed over, or into, a complex-shaped mould. A typical process is shown in Figure 1, where the composite sheet is heated rapidly in an infra-red oven, and then indexed between two cool tools, which close rapidly to form and cool the sheet. The main advantage of this process is that very fast cycle times can be achieved, but it is limited to components with simple or medium shape complexity.

Figure 1: Press-Forming of Thermoplastic Composite Sheets

TAPE-LAYING OF THERMOPLASTIC COMPOSITES

Thermoplastic composites, because of their chemistry, can be rapidly heated and rapidly cooled without any damaging effects to their microstructure. The tape-laying process uses this principle to locally heat and melt, consolidate, and cool a tape of thermoplastic pre-preg, while placing it in position (see Figure 2). Thermoset tape-placement has been developed for many years, with large, seven-axis robotically-controlled fibre-placement systems in operation in many aerospace plants worldwide. The difference between a thermoplastic and a thermoset system is the extra heating, consolidation and cooling equipment, needed with the head. The main advantage of the thermoplastic system is that the product is completely finished once the tape has been laid, whereas the thermoset product must be further bagged and cured in an autoclave.

Figure 2: Tape-Laying of Thermoplastic Composites

OPEN Vs CLOSED MOULDS - LIQUID MOULDING

Advanced thermoset composites have traditionally been hand-laminated and cured under vacuum or in autoclaves. While suitable for low-volume applications in the aerospace sector, recent trends have sought to use closed-mould processes such as liquid moulding (Figure 3 ), where the dry preform is laid between two rigid tools (or between one rigid and one soft tool or bag material), and the liquid thermoset resin injected or infused through the reinforcement. The main advantage of this process for aerospace is that the dimensional tolerances achieved are much better than with autoclaving. The potential for other sectors is in the automation of the process, leading to higher volumes and faster cycle times. Moulding pressures can be low (1-5 bar) and thus light, inexpensive tooling can be used. The process is currently limited to thermoset materials, and their cycle times are relatively long because of the necessity for chemical curing.

Figure 3: Liquid Moulding of a Thermoset Resin into a Dry Fibre Preform

LIQUID MOULDING OF THERMOPLASTICS

The new generation of thermoplastic materials are processed in a water-like state, and thus need much lower pressures, less expensive tooling, and lower energy input, while retaining all of the attractive properties of thermoplastic materials. Examples of the new liquid-moulded thermoplastic composites are Cyclics PBT from Cyclics Corp. (US) and Grilamid PA-12 materials from Ems-Chemie AG (Switzerland) and. End-of-life recycling is easily achieved by chemical means, or by re-melting and use as injection-moulding compounds.

The traditional disadvantage of thermoplastic resins is that the melts have a high viscosity, typically above 500 Pa.sec, which is much too viscous to infiltrate a high-volume fraction of fibres. Figure 4 illustrates the principles of liquid moulding of thermoplastics. The low viscosity resins now available are injected into the composite as an activated monomer, with a resulting low viscosity. Once infiltrated, polymerisation takes place in-situ, yielding a semi-crystalline thermoplastic composite with all the inherent advantages of toughness, solvent resistance, dielectric strength and recyclability.

Manufacturing of high content continuous fibre reinforced composites by direct impregnation of the fibre bed by a liquid matrix can be industrially applied if each of the following conditions is fulfilled (Connor, SAMPE 99):

(i) The matrix viscosity during the impregnation stage is very low (e.g., < 1Pa.s)
(ii) Once the fibre bed is fully impregnated, the matrix can be solidified chemically (e.g. curing, polymerisation) or physically (e.g. cooling, crystallisation) in a sufficiently short time (order of minutes).
(iii) The final matrix has high enough physical properties to transmit good mechanical stability to the composite part.

Two liquid thermoplastic materials in particular are known to be developed which meet these criteria, APLC-12 and Cyclic PBT/PC. In both cases, the injection material is a pre-activated monomer melt with low viscosities, which polymerises in-situ to form tough, solvent-resistant, semi-crystalline polymer matrices.

Cyclic PBT (CBT™)

Cyclic PBT and PC resin systems were developed by GE in the late 1980s. The technology was purchased from GE, resulting in Cyclics Corporation (http://www.cyclics.com), which is now marketing the Cyclic PBT and PC systems for applications as diverse as automotive, electrical, sports and transportation.

The Cyclics resins are available as either 1-part or 2-part systems (Eder, SAMPE 2001). In the 2-part system, the process starts with powdered cyclic oligomers at room temperature. These are heated to a low viscosity liquid at about 140 C, a catalyst is added (either Tin or Titanate), the melt is then injected into the mould at temperatures above 150 C. Typical processing viscosities are between 17 and 150 mPa.sec. polymerisation follows in-situ, with polymerisation times varying from seconds to minutes. 1-part Cyclics resin systems are also available that are pre-mixed with catalyst, and are solid at room temperature. These are simply heated and injected as with the 2-part system.

An important advantage of the Cyclics technology is that the processing can be carried out at near-isothermal conditions, i.e. the material solidifies and crystallises at the processing temperature. It is not, therefore, necessary to cool the tool in order to de-mould the component, which is an important economic consideration, leading to energy savings and shorter cycle times. This behaviour is due to the fact that the Cyclics resins polymerise at temperatures below both the melt temperature and the crystallisation temperature of the PBT polymer.

Anionically-Polymerised Lactam-12

The first, Anionically-Polymerised Lactam-12 (APLC-12, Connor, SAMPE 99) has been developed by Ems-Chemie AG (http://www.emschem.com) and is close to commercialisation, with first applications in automotive and other transport fields. The material and process results in a PA-12 polymer composite which has major advantages in terms of toughness in particular, when compared to traditional epoxy-based advanced composites. The Composites Research Unit at NUI Galway has particular experience in working with APLC-12 composites and has published research in processing/property relationships for this material (Ó Máirtín 2001).

APLC-12 materials are processed at moulding temperatures between 180 C and 240 C, and the polymerisation time varies from seconds to minutes, depending on the percentage of activator used, and is shorter at higher temperatures. Viscosities less that 0.1 Pa.sec can be achieved with APLC-12 moulding, and as a result, infiltration of high fibre volume-fractions under gravity forces only has been demonstrated. The polymerised composite must be cooled to below 100 C before de-moulding.

Figure 4: Liquid-Moulding of Thermoplastic Composites