RECYCLING OF END-OF-LIFE THERMOPLASTIC COMPOSITE BOATS
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RECYCLING OF END-OF-LIFE THERMOPLASTIC COMPOSITE BOATS
M.E. Otheguya*, A.G. Gibsona, M. Robinsona, E. Findonb, B. Crippsb, A. Ochoa Mendozac,
M.T. Aguinaco Castroc
a
NewRail Research Centre, School of Mechanical and Systems Engineering,
Newcastle University, England, NE1 7RU, UK
b
Small Boat Centre of Excellence, BVT Surface Fleet Portsmouth Ltd, 3/236 PP112,
Military Road, HM Naval Base, Portsmouth, England, PO1 3NH, UK
c
E.U.I.T.Industriales, UPM, Polytechnic University of Madrid,
Ronda de Valencia, 3, 28012 Madrid, Spain
* Corresponding author, m.e.otheguy@ncl.ac.uk
SUMMARY
This paper discusses the recycling of thermoplastic composite materials in the context of
boatbuilding. Work was carried out on the recycling of an experimental thermoplastic composite
rigid inflatable boat, originally built by BVT Surface Fleet and tested in service by the RNLI. It was
found that a range of useful injection moulding materials could be prepared from the hull material of
the craft, demonstrating that structural thermoplastic composites are recyclable in practice as well as
in principle, and confirming that they are a sustainable alternative to thermosets.
1. INTRODUCTION
This paper relates to recycling issues associated with a 8.5m experimental rigid inflatable boat
(RIB), manufactured in thermoplastic (polypropylene-glass) composite to demonstrate the viability
of thermoplastics technology in marine composites. An experimental Atlantic 85 RIB built in
thermoplastic was manufactured by BVT Surface Fleet (VT Halmatic) in 2004 for the Royal
National Lifeboat Institution (RNLI) and had been extensively service tested.
BVT Surface Fleet has been at the forefront of composite boat building in the UK since 1952.
Although BVT provides a new build capability, it is also committed to the provision of through-
life support and disposal of naval ships and small boats. Successfully managing the life cycle of
BVT products is of great importance: the recycling and disposal of composite structures and small
craft is central to this philosophy.
Composite materials have become dominant over five decades of evolution in military and leisure
boatbuilding mainly due to their aesthetic characteristics, low weight and good mechanical
properties. The market for boat hull glass fibres is more than 130,000 tpa and there is expanding
use of carbon and aramid fibre-reinforced materials [1,2]. The boatbuilding field, however, is
predominantly based on thermosetting resins, albeit with a movement away from open mould
processes towards closed mould ones.
The marine composites industry has recently attracted attention in terms of VOC emissions,
carbon footprint and recyclability [3]. As a result, in the EU, it has become desirable to develop
end-of-life strategies that avoid landfill (2004). This follows the example set in the automotive
industry by the End of Life Vehicles Directive (ELVD, 2003), which requires 95% of the mass of
each vehicle manufactured after January 2015 to be reused or recovered. These new rules have led
many car manufacturers to redesign their vehicles to facilitate disassembly, as well as to careful
materials selection to promote recycling and reuse.2. COMPOSITE MATERIALS RECYCLING
The existing technically viable recycling techniques for thermosetting composites comprise
mechanical, thermal and solvent routes (Figure 1).
Figure 1. Recycling processes for thermoplastic and thermosetting composites.
Mechanical recycling has been explored and applied in the recent past, by the ERCOM project and
by Phoenix Fibreglass [4]. Laminates are ground, milled, then used as fillers in new composite
products. Neither of these early initiatives achieved economic viability, due to the lack of suitable
markets for product containing recycled material. There was also the economic problem that the
cost of the recycled filler exceeded that of virgin fillers, such as calcium carbonate.
Thermal processes aim at fibre, material and energy recovery through controlled combustion of the
organic portion of the composite scrap. Combustion with energy recovery makes use of the
calorific value of the resin for heating or energy applications. The key problem is that the glass
fibres, being heated along with the resin, dilute the useful heat that can be extracted. For this
reason, processes, such as cement manufacture have been investigated in which the hot ashes have
some use in the product. Handling of hazardous combustion products is a significant problem.
Fluidised bed processes for composite scrap operate at 400-700ºC and involve blowing air through
a silica sand bed. The fibres are recovered by filtering the hot gas stream emerging from the bed.
The gas can then be combusted in an afterburner to recover heat. Any metallic inserts in the
composite can be recovered from the sand. The glass fibres recovered with this technique are short
and show a strength loss of about 25%. Finally, composites can be heated in the absence of air,
again resulting in short fibres, along with and gaseous and liquid decomposition products from the
resin. Unfortunately, the current glass fibre price makes it unprofitable to recover fibres using
these methods. By contrast, carbon fibre is claimed to be profitably recycled by companies such as
Adherent Technologies Inc. (US) [5, 6] and Recycled Carbon Fibre Ltd. (former Milled Carbon,
UK) with proprietary catalysed processes that use lower temperatures.
Other recycling methods include chemical decomposition with solvents or supercritical fluids [7].
However, the chemicals used can constitute an environmental hazard and the resulting chemical
mixtures require complex processing to recover useable products.Thermoplastic resins offer the possibility of recyclability [8-10]. There has been considerable
interest in their use in boat structures, primarily for reasons of toughness, as well as recyclability.
With real end-of-life structures, such as the hull of a rigid inflatable boat there are several
“contaminant” materials in addition to the composite material. For instance the hull laminate is
painted with a primer-topcoat system, and the internal structure comprises a sandwich
configuration including a balsa core. One of the aims of the present study was to examine the
effect of these contaminants on the properties of the recycled composite.
In the present study it was decided to manufacture mouldable thermoplastic granules by
granulating the hull material and diluting it with additional thermoplastic resin to aid
processability and bring the resin content in line with that of conventional injection moulding
compounds that could be used to produce new parts.
3. EXPERIMENTAL
3.1 MATERIALS
The materials employed in this study were laminates taken from the thermoplastic version of the
Atlantic 85 RIB, Figure 2, and an additional set of purpose-made glass/PP laminates. The main
structural material used in the boat was Twintex T PP 60 1485 woven polypropylene-glass
commingled fabric, containing 60 wt.% glass in the form of 18µm diameter fibres. The hull
surface was painted with an epoxy-based primer, PPG Industries NEXA Autocolor P572-212,
followed by AWLGRIP, an acetate-based G-line Standard Marine topcoat. Treatment with a
primer is usually necessary when coating thermoplastics because of their non-polar nature that
inhibits adhesion.
Figure 2. RNLI rigid inflatable boat, Atlantic 85, in service.
The boat internal structure was a sandwich construction of the Twintex fabric and DIAB Pro-balsa
Standard core material, with a specific gravity of 0.155. The sandwich laminates employed various
thicknesses and numbers of plies, depending on function and position. In addition, a set oflaminates was manufactured for recycling, containing exactly the same products (Batches 1-3). A
further set of laminates (Batch 4) was manufactured with plain Twintex PP 60 to enable an
assessment of the influence of dry paint (Batch 5) on the recycled hull material properties to be
undertaken.
It was desirable to decrease the fibre fraction to facilitate processing into injection moulding
material, as manufacturers would not normally employ material containing more than about
40wt.% glass. A high melt flow index PP homopolymer would normally be chosen for
impregnated wood and glass fibre products. In the present case, the material used was Sasol
HTV145, which had a melt flow rate (MFR) value of 45g/10min. It should be noted that, although
virgin PP was used here, appropriate PP recyclate is available.
For wood and/or glass fibre impregnation the addition of a coupling agent to the thermoplastic
melt is known to improve the fibre-resin adhesion and thus the final mechanical properties. In this
case 2wt.% by of Polybond 3200 maleic anhydride modified PP was added.
3.2 RECYCLING ROUTE AND EQUIPMENT
Panels were removed from the boat with a pneumatic saw and subsequently cut into 15 mm
(approx.) squares, which were fed to, a Homa moulding Granulator Type 01, 4.1 kW. The
resulting granulate was dry blended with virgin PP pellets, and fed into a single screw extruder, for
compounding. There were 5 temperature-controlled zones, set at (from feed to die): 200ºC, 225ºC,
250ºC, 235ºC and 220ºC. The solidified extrudate was again passed through a granulator. The
resulting granules were injection moulded into tensile bars, using a Sandretto HP 40 (40 ton)
machine. The following optimum conditions were used: injection pressure: 300 bar; barrel
temperatures: 220ºC, 230ºC, 240ºC and mould temperature: 65ºC. The resulting dog-bone samples
complied with ISO 527-1:1993.
Five batches of tensile samples were produced, the overall compositions and final specific gravity
(SG), after moulding, being shown in Table 1. Batches 1-3 were all based on the granulated hull
sandwich laminate. Batches 1 and 2 were diluted with virgin PP and Batch 3 contained laminate
only. The main processing difficulty occurred due to the presence of the fluffy lightweight balsa
component, which resulted in some feeding problems, which probably could have been overcome
by using a twin-screw extruder with a specialised augur feed. There were also some initial wet-out
problems with the balsa, which again could probably have been overcome by using more
specialised compounding equipment. Batch 3, with the highest balsa and fibre content, was the
most difficult to process. Despite the compounding difficulties the process resulted, in Batches 1-
3, in granules that were acceptably injection mouldable.
Table 1. Recycled material batches, compositions and specific gravities.
Batch Composition, % in weight S.G.
1 87.6% PP, 9.4% glass, 3.0% balsa wood 0.970
2 53.0% PP, 35.5% glass, 11.4% balsa wood 1.201
3 33.2% PP, 50.5% glass, 16.2% balsa wood 1.390
4 60.3% PP, 39.7% glass 1.204
5 As 4, with 0.7% paint added 1.201
Batch 4, which contained no balsa, was granulated from pure laminated Twintex material (60wt.%
glass) and designed to indicate the maximum achievable properties in recycling. Again, the glass
content was reduced by adding virgin PP at the compounding stage, to achieve a final glasscontent of 40wt.% in the moulding material. Batch 5 was the same with 0.03% of paint residue
added, to ascertain whether this would have a deleterious effect. Both these batches processed
well.
3.3 RESULTS AND DISCUSSION
The specific gravities of Batches 1-3 after moulding were compared with the calculated values
based on the initial SGs of the components, as shown in Table 2. The calculated value was made
assuming an SG of 0.155 for the balsa, as given by the supplier. It can be seen that the actual SG
values are all significantly higher than those calculated, the most probable reason being that the
compounding and injection moulding processes had removed a significant proportion of the air
initially present in the balsa. To test this hypothesis, the SGs were re-calculated, replacing the
balsa SG with a higher value of 1.5, corresponding to the value for wood fibre [11]. This second
estimate would correspond to the situation where most of the air in the balsa was removed during
processing. These values can be seen to agree much more closely with the measured SGs.
Comparison of the values suggests that approximately 65% of the air enclosed in the structure of
balsa wood was probably eliminated in the compounding and moulding processes. Remaining
porosity can be observed by Electron Microscopy, Figure 10.
Table 2. Calculated and measured specific gravity of Batches 1-3.
S.G. S.G. S.G.
Batch
(No air removed) (Measured) (All air removed)
1 0.834 0.970 0.977
2 0.684 1.201 1.249
3 0.620 1.390 1.485
The tensile strength of all the batches of recycled mouldings was measured and compared to the
values expected for injection moulded samples of similar glass content [12, 13]. A number of
interesting results were observed.
Batches 4 and 5, based on pure Twintex diluted with PP, show the highest tensile strength values,
albeit with a small reduction in the case of the paint-contaminated batch (Figure 3).
120 4%
100
Elongation at break, %
Tensile strength, MPa
3%
80 1
1
2
2
60 2% 3
3
4
4
5
40 5
1%
20
0 0%
Figure 3. Tensile strength of recycled injection mouldings Figure 4. Elongation-at-break of recycled injection
(bars show 95% confidence limits). Material batch mouldings (bars show 95% confidence limits). Material
numbers correspond to those in Table 1. batch numbers correspond to those in Table 1.
Both these batches show properties comparable with those expected of conventional PP compounds
of similar glass content. The reduction in the case of the paint contamination may be due to sharp
edged, low strength paint particles. These particles (Figure 11) are probably not well-bonded to the
polypropylene, acting as stress raisers.14 160
12 140
Modulus of elasticity, GPa
120
10
Tensile strength, MPa
1 100
8 2
80
3
6 4 60 Recycled, glass only
5
4 Recycled, glass and paint
40 Recycled, glass and wood
Virgin, ComAlloy® 121,135,143,150
2 20 Virgin, glass only (Thomason), LF
Virgin, glass only (Thomason), SF
0 0
0% 10% 20% 30% 40% 50% 60%
Glass content (% weight)
Figure 5. Modulus of elasticity of recycled injection Figure 6. Tensile strength values for recycled laminates
mouldings (bars show 95% confidence limits). compared to virgin materials (Thomason from [13],
Material batch numbers correspond to Table 1 ComAlloy® from [12]).
The batches containing the balsa show reduced, though still acceptable, properties. Although the
glass content of Batch 4 (39.7wt.%) is lower than that of Batch 3 (50.5wt.%) the strength is
greater, underlining the negative effect of balsa. Again this could be due to insufficient bonding
with the PP matrix, or to degradation of some of the balsa components. As reported by Jakab et al.,
wood decomposition commences at around 200°C, with decomposition of lignin and
hemicellulose, accompanied by the release of water and formaldehyde [14]. Thus, partly degraded
wood particles could act as stress raisers, initiating failure. Most of the samples containing wood
showed wood particles on fracture surface (Figures 9, 10).
The tensile strengths were compared with values for standard glass/PP materials [12, 13]. Figure 6
shows that the strength values of the recycled samples are lower than those of ‘long fibre’
moulding materials. This is not surprising, since the latter materials, which are manufactured by a
different method, have a fibre length of the order of 1mm, which is significantly larger than in the
present case. However, the strengths of the recyclates can be seen to be comparable with those of
the more widely used ‘short fibre’ materials, which are also produced by extrusion compounding,
and which have an average fibre length of around 200 microns. These results place the properties
of these mixed recycled materials well within the range that would be acceptable in the
marketplace for short fibre moulding compounds.
4% 14
12
Modulus of Elasticity, GPa
3%
10
Elongation at break, %
8
2%
6
Recycled, glass only
Recycled, glass only
Recycled, glass and paint
Recycled, glass and paint 4
1% Recycled, glass and wood
Recycled, glass and wood
Virgin, Borealis® 205,402,306
Virgin, ComAlloy® 143,150
2 Virgin, glass only (Thomason), LF
Virgin, glass only (Thomason), LF
Virgin, glass only (Thomason), SF
Virgin, glass only (Thomason), SF
0% 0
0% 10% 20% 30% 40% 50% 60% 0% 10% 20% 30% 40% 50% 60%
Glass content (% weight) Glass content (% weight)
Figure 7. Elongation-to-break values for recycled Figure 8. Young’s Modulus values for recycled
laminates compared to virgin materials (Thomason laminates compared to virgin materials (Thomason
from [13], ComAlloy® from [12]). from [13], Borealis® materials from [12]).The values of elongation-to-break in Figure 4 show the expected decrease with increasing fibre
content. Again the deleterious effect of the balsa can be seen. Nevertheless the elongation-to-
break values are still in the region of commercial acceptability, as can be seen from Figure 7.
Figures 5 and 8 show that, for all compounds, the Young’s modulus increases with glass content.
Comparing the result for Batch 2 (~35wt.% glass, 11wt.% wood ) to that for Batch 4 (40% glass)
it can be inferred wood particles do have a positive influence on modulus (Figure 5). By contrast,
paint particles seem to have a negative effect, probably because their modulus is similar to that of
PP.
Figure 9. SEM image of a Batch 3 sample fracture Figure 10. Wood chip detail, Batch 3 sample
surface showing an approx. 2mm wide wood chip. (see Figure 9).
Figure 11. Detail of a paint particle on fracture
surface, Batch 5 sample.
6. CONCLUSIONS
The present research has demonstrated the recyclability of thermoplastic-based composites. In
particular we have shown that the hull of a rigid inflatable boat, composed of glass/polypropylene
laminate along with balsa core material and paint, can be recycled by melt processing into
injection mouldable granules which have acceptable properties when processed.
Although both balsa and paint have a deleterious effect on moulded strength and elongation-to-
break the properties achievable in the compounded granules are well-within the region of
commercial interest for reinforced polypropylene moulding materials.These materials could be used either in non-appearance automotive applications, where talc and
glass reinforced PP are currently used. Alternatively, they could be used in decking and wood
imitation applications where wood-reinforced composites are currently being considered.
The Young’s modulus of the recycled materials is comparable to that of conventional short fibre
polypropylene moulding materials. The presence of balsa does have a small positive effect on
modulus.
7. ACKNOWLEDGEMENTS
We would like to acknowledge the support of the European Commission under the MOMENTUM
Research Training Network, Contract No. MRTN-CT-2005-019198.
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