Effects of Soaking and Freezing on Composites MadeFrom Wood-Based Fillers and Biodegradable Plastics

Michael Witt, Ross AndersonScion, Rotorua, New Zealand

Simon PaulyAm Niederwald 13, 64625 Bensheim, Deutschland

Brendan LeeNuplex Resins, London, United Kingdom

Biodegradable plastic composites were subjected toprolonged soaking and freezing treatments to assessthe effects on the mechanical performance. Radiata pineflour and thermomechanical pulp fibers were used asfillers at various addition levels in three different com-mercial polymer matrices. Two were bioderived, oneoil-derived, each with different hydrophobicities. De-pending on the nature of the biodegradable polymermatrix, the rates and extents of water uptake were foundto be either enhanced or reduced by the wood-derivedfillers. Although the higher aspect ratio of the pulp fibersimproved mechanical performance, relative to the woodflour, water uptake was also significantly enhanced insome cases. POLYM. COMPOS., 27:323–328, 2006. © 2006Society of Plastics Engineers

INTRODUCTION

Wood plastic composites (WPCs) are currently a rapidlygrowing market [1, 2]. This trend is being led by the USAwith outdoor applications and, in particular, decking. WPCsare commonly made by mixing wood flour or fibers and avirgin or recycled plastic to produce a composite product. Inthe ideal case, these products can exhibit the benefits of bothraw materials, for example, long service life, freedom frommaintenance, and ability to accept nails.

A common assumption regarding WPCs is that the plas-tic effectively coats the wood particles and that moisture canonly be absorbed into the exposed, uncoated sections of thewooden filler and is not transmitted across the plastic

boundaries [2]. However, this approach may be too simpleand misleading, since all plastics allow some water diffu-sion. A number of factors influence water uptake in WPCs.It can be expected that time frames, filler substitution levels,filler geometries, and the characteristics of the plastic matrixplay a significant role in water-uptake behavior.

Low values for an initial water uptake can often bemisleading, because the time frame for the measurementwas not sufficiently long, and the experimental values didnot represent equilibrium conditions. It is commonly ob-served that water uptake of natural fiber composites in-creases with time and filler substitution level, but Rowell etal. [3] found that with polypropylene/aspen–fiber compos-ites no further water uptake was observed after the first 24 h.It is also commonly observed that water uptake has an effectof deteriorating the mechanical properties, but Karmaker etal. [4] postulated that swelling of jute fibers in a polypro-pylene composite can have positive effects on mechanicalproperties. In some studies it has been found that the use ofcoupling agents do not have an effect on water uptake [5, 6].Biodegradable polymers often are more hydrophilic com-pared to commonly used oil-derived plastics (e.g., polyeth-ylene, polypropylene), and therefore, their water uptake isusually greater [7].

Wood flour is an established filler for WPCs. In ourwork, we also considered wood fibers since they have thepotential to improve mechanical performance. However,such fibers may also provide a fiber-network structure caus-ing significantly enhanced water uptake characteristics, i.e.,wicking [3].

The current work characterized water uptake behaviorwhen using wood-derived fibers instead of the more usualround shaped flour/dust-based fillers in biodegradable plas-tic composites. The experimental conditions were designed

Correspondence to: Michael Witt; e-mail michael.witt@scionresearch.comDOI 10.1002/pc.20212Published online in Wiley InterScience (www.interscience.wiley.com).© 2006 Society of Plastics Engineers

POLYMER COMPOSITES—2006

to simulate outdoor conditions with extended periods of rainand frost to make differences in water uptake more obvious.

EXPERIMENTAL METHODS

All polymers were dried before processing, according tomanufacturer’s recommendations. Biopol D400G fromMonsanto (now Metabolix) is poly(3-hydroxybutyrate-co-hydroxyvalerate) produced by a bacterial fermentation pro-cess. The manufacturer states a melting point of 153°C forthis bioderived polymer. MaterBi Y101U from Novamont isa corn starch–cellulose–plasticizer blend. The manufacturerstates a glass-transition temperature of 105°C. Bionolle3020 from Showa Highpolymer Co. is an oil-derived ali-phatic polyester produced via polycondensation of glycoland aliphatic dicarboxylic acids.

Wood flour was supplied by Kingsland Seeds, Nelson,NZ. It was obtained by grinding radiata pine wood, withmost particles having a particle size between 250 and 500�m. High-temperature thermomechanical pulp (HTMP) fi-bers were made from radiata pine toplog wood chips in apilot plant at our laboratory. The processing conditions werechosen to mimic production of medium-density fiberboard(MDF).

Composites were labeled according to the matrix (BP forBiopol, MB for MaterBi, and BN for Bionolle), the fillercontent (0, 20, or 40%), and the type of filler (WF for woodflour, MDF for medium density fibers, which is synony-mous with HTMP fibers).

Methods

Each formulation of fiber or flour was compounded withpolymer in an extruder to produce pelletized feeds for aninjection molder. Compounding was carried out on an OMC19/30 corotating twin-screw extruder with a 19 mm screwdiameter and 30 L/D. Injection molding was carried out ona Boy 15 S injection molder to make rectangular shapes 127� 12.7 � 3.2 mm3.

All molded samples were conditioned before testing at atemperature of (23 � 2)°C and a relative humidity of 50%(ASTM D 618–96) for 2 days after manufacturing and forat least 1 week after water/freeze cycles under the sameASTM conditions. The three-point bending test was carriedout according to ASTM D 790–96a (Method 1) on an

Instron 5566 universal testing machine with a 10 kN loadcell.

The six steps in the water-exposure and freeze-cycle testsare summarized in Table 1.

Some samples were tested straight after soaking (wettesting), while others were reconditioned before testing (drytesting). Steps A, B, and D were intended to test how asuccessively prolonged period of water exposure affects thewet performance. Steps C and F were intended to testwhether there is an irreversible effect after reconditioning.

RESULTS

The plastic and composite samples were characterized bymonitoring weight change during the treatment steps and bymeasuring the flexural properties.

Weight Increase

Pure Bionolle absorbed only small amounts of water and,after each reconditioning, the samples returned to theirinitial weight (Fig. 1). The immersion time did not make aconsiderable difference. When the samples were soaked for24 h, the weight gain was 0.5%. After an additional 14 days,the weight gain was �0.7%.

The Bionolle composites behaved differently in that theynever returned to their original weight after water-soak-reconditioning cycles (Fig. 1). Furthermore, immersion timeplayed a significant role, with the exception of the 40%HTMP composite. These samples reached the same (high)water content after 24 h and 14 days. This suggests adifferent uptake mechanism, perhaps based on a fiber net-work structure within the composite. A network structurewould provide a fast route for water uptake. The 40%wood–flour reached a comparable water content after 14days, but a significantly lower value after short-term expo-sure. Weight changes were similar for the HTMP andwood–flour composites at the 20% substitution level. Thedifference between results for HTMP fibers and wood flourare attributed to the higher aspect ratio of the HTMP fiber,promoting the formation of an interpenetrating network thatcan conduct water into the composite.

Pure Biopol absorbed only small amounts of water(�0.7%) and returned to the initial weight, within the mea-suring error, after reconditioning (Fig. 2). None of theBiopol composites returned to their initial weight.

TABLE 1. Summary of soaking and freezing tests.

Step Result Conditions

Start (moulded samples) A (dry) Conditioned 2 days at 23°C and 50% RHFirst water soak B (wet) Soaked at 20°C and tested wet 20°CFirst freezing step and reconditioning C (dry) 2 days in freezer at �20°C, then 7 days reconditioned (23°C, 50% RH)Second water soak D (wet) Soaked at 20°C and tested wet at 20°CSecond freezing step E (wet) Stored at �21°C, tested wet at 20°CReconditioning F (dry) 7 days at 23°C and 50% RH

324 POLYMER COMPOSITES—2006 DOI 10.1002/pc

The composite of Biopol and 40% HTMP showed aweight increase of nearly 8% after 14 days, compared to 6%for the 40% WF composite (Fig. 2). Weight gains for the20% HTMP and WF samples were �2%. As for Bionolle,the higher HTMP substitution level led to a significantlygreater increase in water uptake. In contrast to Bionolle, thisvalue was not reached in 24 h. The WF composite alsoshowed a nonproportional increase in water uptake goingfrom 20 to 40% substitution in the 14-day soak.

With pure MaterBi and its composites, a complicatingeffect was observed. During the water soak, the matrixmaterial began to dissolve in water. This might have hap-pened in the other systems, but at a level lower than the

detection limit. The relevant difference is that the MaterBiis starch-based and contains a water-soluble plasticizer.

As expected, and different to the other polymers, pureMaterBi showed the highest water uptake, i.e., 22% weightgain after 14 days of immersion in water (Fig. 3). Withincreasing filler substitution levels, lower weight gains werefound, which shows that in this case the fibers are morehydrophobic than the matrix. Because of dissolution ofmatrix material, the weight gain did not increase steadilyover time. After reconditioning, lower weights were foundthan for the initial samples. MaterBi reinforced with 20%HTMP showed a greater weight loss than MaterBi rein-forced with 20% WF after final reconditioning. The differ-

FIG. 1. Weight increases for Bionolle and its composites with wood flour (WF) and MDF fiber.

FIG. 2. Weight increases for Biopol and its composites with wood flour (WF) and medium-density fibers(MDF).

DOI 10.1002/pc POLYMER COMPOSITES—2006 325

ence is attributed to an increased accessibility of water tothe interior of the composite, helping to dissolve the matrixcomponents.

Mechanical Testing

All composite materials were significantly affected bythe water soak treatment, which decreased the flexural mod-uli and maximum flexural stresses. This could be due toreduced stiffness of the wet fibers, partial debonding of thefiber–matrix interface and/or softening/plastification of thepolymer matrix, or in the case of MaterBi, dissolution of thematrix material. As the flexural moduli and the maximum

stresses gave the same trends in all experiments, only themoduli are reported here.

The flexural moduli of the pure Bionolle polymer sam-ples were not significantly affected by the water soak/freezecycles (Fig. 4). Dry-tested HTMP composites gave 10–20%higher modulus values compared to the wood flour samples.This was observed for the other matrices also, and is indic-ative of a greater reinforcing effect in using fiber rather thanflour.

After only 1 day of water exposure, a freeze cycle andreconditioning of the Bionolle composites, within the ex-perimental error, we found the same moduli as for the initialmaterial (Fig. 4). After two consecutive water-soak-freeze

FIG. 3. Weight increases for MaterBi and its composites with wood flour (WF) and medium-density fibers(MDF). Data for BP40PF are missing because the high fiber fraction did not allow injection molding ofspecimen.

FIG. 4. Flexural moduli for Bionolle and its composites with wood flour (WF) and medium-density fibers(MDF).

326 POLYMER COMPOSITES—2006 DOI 10.1002/pc

cycles (water soak 1 day and 14 days) performance leveldecreased by about 50% for the wet-measured 40% filledcomposites.

There was no significant influence on the flexural moduli inthe case of pure Biopol samples (Fig. 5). However, in the caseof the Biopol composites, the flexural moduli were signifi-cantly affected by the water soak/freeze cycles. Again, thetested Biopol composites gave 10–20% better performance ifHTMP fibers were used as reinforcement instead of woodflour. All of the wet-tested HTMP composite samples gavecomparable flexural moduli, �3,000 MPa, after the two watersoak/freeze cycles, irrespective of the type of filler or substi-tution level and despite the differences in weight gains de-scribed in Results section. One week of reconditioning in-creased the stiffness, but, in the case of the 40% filledcomposites, not to the initial performance level: The perfor-mance decrease between the initial and reconditioned samples,

after two water soak/freeze cycles, was more pronounced inthe case of the HTMP composite. Thus, the performanceadvantage of the fiber reinforcement degrades after this simu-lated “weathering” to a few percentage only.

Pure MaterBi showed a completely different behaviorthan the other composites (Fig. 6). After the water soak andfreeze cycle and reconditioning the flexural moduli wasslightly higher, probably because of some extraction of theplasticizer or a change in morphology. A further increasewas found after 14 days water soak, but in this case thereproducibility of the results was poor.

Another unexpected behavior for MaterBi was the per-formance under wet testing conditions (Fig. 6). A fairlyconstant flexural modulus of �500 MPa was measured forthe pure polymer and for all the composites. This suggeststhat water dramatically affected the matrix, preventingstress transfer from the matrix to the reinforcement. Using

FIG. 5. Flexural moduli for Biopol and its composites with wood flour (WF) and medium-density fibers(MDF).

FIG. 6. Flexural moduli for MaterBi and its composites with wood flour (WF) and medium-density fibers(MDF).

DOI 10.1002/pc POLYMER COMPOSITES—2006 327

fillers of different aspect ratios and substitution levels thusdid not appear to influence performance.

CONCLUSIONS

The different aspect ratios of wood-derived fillers influ-ence both mechanical performance and water uptake. Flex-ural performance characteristics of the initial compositeswere usually 10–20% higher when wood fibers were usedas reinforcement, rather than wood flour, but the fiber-basedcomposites also showed greater rates of water uptake inmost cases. For some of the highly filled biodegradableplastic composites, the enhanced rate of water uptake wasattributed to a fiber–fiber network.

Prolonged water soaking and freeze cycles significantlyreduced performance characteristics, and only small perfor-mance differences were observed with wet-tested samples.Reconditioning resulted in partial restoration of performance,but usually not to the level of the initial material. With Biopolcomposites, this performance loss was more pronounced withHTMP fibers than with the wood flour. With MaterBi com-posites, the situation was somewhat more complicated because

of the high sensitivity of the matrix to water and a tendency todissolve or partly dissolve over time.

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328 POLYMER COMPOSITES—2006 DOI 10.1002/pc