Intermediate materials

The production of intermediate materials to combine fibers and matrix follows the same principal steps from impregnation, consolidation to solidification as known from the CFRTP component manufacture. The carbon fiber reinforcement for inter-mediate materials covers the whole range of available fiber architectures including spread unidirectional (UD) tows, woven fabrics, non-crimped fabrics (NCF), knit-ted or braided preforms.

Amorphous or semi-crystalline thermoplastic matrix systems for intermediate ma-terials are selected from all application areas that are depicted in Figure 1-1.

PEEK PEKKPEK

PMMA PC PET

PSU

PA6

PPS

PPA

PBT POM

PE

HD-PE PP

Standard thermoplastics Engineering thermoplastics

High-performance thermoplastics

Price, performance

PS ABS

PVC PEI

SAN PA66

Semi-crystalline Amorphous

LD-PE

Figure 1-1 Classification of thermoplastics [8].

In general, thermoplastics are used in form of pellets, ground powder, suspension (with water) or solution (dissolved polymer) to produce intermediate materials.

Amorphous thermoplastics such as polyetherimide (PEI) or polyether sulfone (PES) with high viscosities and no melting point are often processed as powder, solu-tion or suspension [7]. Semi-crystalline thermoplastics such as polyetheretherketone (PEEK), polyphenylene sulfide (PPS) or polyamides (PA6, PA66, PA10T) cannot be dissolved properly due to their high chemical resistance against most solvents.

They are usually processed via melt impregnation or powder-coating [9].

Over the last decades, new types of thermoplastic intermediates appeared simulta-neously to the invention of new manufacturing techniques for thermoplastic com-posites [10]. Considering various forms of intermediates, the production may be divided in the following principal process steps with regard to the thermoplastic matrix [7]:

• Matrix application (suspension, solution, melt, film, powder)

• Heating (oven, infra-red source, calender, nozzle, double-belt press)

• Cooling/calibration (calender, double-belt press)

Due to this large variety, potential manufacturing routes for thermoplastic com-posites are presented in Figure 1-2, starting from reinforcing fibers and matrix to a final part.

Manufacturing technique for shaping and forming

Autoclave, vacuum consolidation, AFP, ATL, filament winding, pultrusion, press forming, thermoforming

PART *FIT: Fibres Imprégnées de

Thermoplastique

Reinforcement (e.g. unidirectional (UD) spread tows,

woven fabrics, NCF, braids)

Thermoplastic matrix (e.g. powder, fiber, film, suspension)

Pre-impregnation

Figure 1-2 Overview of potential manufacturing routes originating from the production of in-termediates for thermoplastic composites, based on [7, 10, 11].

The production of intermediates can be divided into two principal techniques:

pre-impregnation and pre-forming [10]. Pre-impregnation is commonly reached by melt [12–18] or solution impregnation [19–21], and other exotic techniques such as impregnation by using aqueous suspensions [22, 23].

The pre-forming technique brings reinforcement and matrix together in a defined way without impregnation. Here, the impregnation takes place during part

manu-facture by forming or shaping. Using such intermediates, reinforcement and matrix reveal a weak link due to the non-impregnated state leading to a high degree of drapeability that is maintained during lay-up. At the same time, a large flow dis-tance has to be covered in pre-formed intermediates. This makes the wetting as well as the impregnation to the critical phase during component production [10].

In the case of commingled yarns, the matrix is mixed in the form of thermoplastic fibers to reinforcing fibers producing a hybrid yarn that becomes rigid after consol-idation [24]. Another example for pre-formed intermediates is represented by the film-stacking method where thin polymer films and reinforcement layers (fabrics, NCF, spread tows etc.) [10] are consecutively stacked and consolidated in a double-belt press [25].

Using the powder-coating method, polymer powder with particles in the range of 5 to 200 μm is deposited on the reinforcement [10]. The powder deposition can take place in an impregnation bath by using a fluidized bed or a fine suspension of powder particles in a liquid. The powder may also be directly applied by a needle roller or electrostatic deposition. To avoid loss of powder, the fabric, NCF or spread tows coated with powder subsequently pass a heat system. By means of an oven, calender or heater, the powder is surface-fused ensuring sufficient adhesion to the reinforcement without impregnation [7].

In 1983, Ganga [26] patented a special type of powder-coated intermediates: Fibre Impregnée Thermoplastique (FIT). Here, the tows are powder-coated and enclosed by a flexible sheath made of preferably the same thermoplastic as the powder parti-cles. Thus, the powder maintains its position while these intermediates are further processed [27]. In general, FIT and commingled yarns are usually further processed to more complex preforms such as braids, knits, fabrics or three-dimensional pre-forms and consolidated afterwards. Film-stacked and powder-coated intermediates can transform into pre-impregnated intermediates after passing a heat system un-der pressure e.g. in a double-belt press as indicated in Figure 1-2 [7, 28].

The introduced intermediates vary with regard to the degree of impregnation (DOI) and the remaining flow path. Both characteristics determine the type of production technology that can be used for the subsequent processing to a CFRTP compo-nent. Table 1-1 compares the introduced intermediate materials with regard to the initial degree of impregnation (DOIi) before component production along with the remaining flow path, production rate, flexibility in relation to the material avail-ability and equipment costs. Here, the expression “hybrid” designates commingled yarns or FIT bundles. As not every thermoplastic polymer can be spun into a fiber or dissolved the flexibility of hybrid and solvent-impregnated intermediates is con-sidered to be low. Intermediates produced via melt impregnation show the highest DOIi with the lowest remaining flow path.

Table 1-1Comparison of intermediate materials; based on [7].

Intermediate Pre-formed Pre-impregnated

types Powder Film Hybrid Melt Solution

DOIi low medium medium high medium

Remaining flow path high medium medium low medium

Production rate high medium medium medium medium

Flexibility medium medium low high low

Equipment cost medium high low-high medium high